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Enhanced Thermoelectric Properties by Embedding Fe Nanoparticles into CrN Films for Energy Harvesting Applications
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Enhanced Thermoelectric Properties by Embedding Fe Nanoparticles into CrN Films for Energy Harvesting Applications
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ACS Applied Nano Materials

Cite this: ACS Appl. Nano Mater. 2024, 7, 3, 3428–3435
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https://doi.org/10.1021/acsanm.3c06054
Published January 26, 2024

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Abstract

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Nanostructured materials and nanocomposites have shown great promise for improving the efficiency of thermoelectric materials. Herein, Fe nanoparticles were imbedded into a CrN matrix by combining two physical vapor deposition approaches, namely, high-power impulse magnetron sputtering and a nanoparticle gun. The combination of these techniques allowed the formation of nanocomposites in which the Fe nanoparticles remained intact without intermixing with the matrix. The electrical and thermal transport properties of the nanocomposites were investigated and compared to those of a monolithic CrN film. The measured thermoelectric properties revealed an increase in the Seebeck coefficient, with a decrease of hall carrier concentration and an increase of the electron mobility, which could be explained by energy filtering by internal phases created at the NP/matrix interface. The thermal conductivity of the final nanocomposite was reduced from 4.8 W m–1 K–1 to a minimum of 3.0 W m–1 K–1. This study shows prospects for the nanocomposite synthesis process using nanoparticles and its use in improving the thermoelectric properties of coatings.

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Introduction

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Thermoelectric thin films have shown their potential role in waste heat recovery and microdevice refrigeration. (1) The conversion efficiency of thermoelectric material is often governed by the dimensionless figure of merit, ZT = (S2σ)T/k, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the temperature. (2) Several strategies exist to improve thermoelectric performance, including the search for new materials or optimization of existing ones by doping, alloying, and/or introducing nanoscale effects. (2−5) Nevertheless, the most prominent challenge for enhanced thermoelectricity is decoupling the three physical parameters (S, σ, and κ) which are strongly interdependent and coupled with carrier concentration. (5)
Transition metal nitride (ScN and CrN) systems have shown promise for use as thermoelectric materials. (6) CrN-based materials, in particular, can exhibit both n-type or p-type semiconducting behavior, depending on doping and nitrogen stoichiometry. (7−11) The n-type CrN has a relatively high Seebeck coefficient (60–300 μV K–1), high electrical conductivity (0.6–3 mΩ·cm), and medium to low thermal conductivity (4–6 W m–1 K–1). (6,7,10) Due to the presence of magnetic disorder in CrN, (12) lower thermal conductivities (∼2–3 W m–1 K–1 at room temperature (RT) (6,11)) were reported compared to those of other transition metal nitrides with the same crystallographic structure (ScN, VN, TiN: 10–22 W m–1 K–1 at RT (6,13,14)).
Several theoretical and experimental studies have suggested new horizons for improving the thermoelectric properties by phonon and carrier engineering using nanoparticle (NP)-containing nanocomposite materials. (5,7,15−19) In fact, it was suggested that using soft magnetic, typically Fe-based, NPs dispersed in a semiconductor matrix would (i) reduce the thermal conductivity by inserting defects which combined with a Kondo-like effect can increase phonons scattering, (ii) increase the Seebeck coefficient by electron filtering at the metal/semiconductor interface, (20) and (iii) increase the electrical conductivity by modulation doping at the metal/semiconductor interfaces due to the difference in work functions between matrix and NPs. (15)
(Nano)composite materials are characterized by (nano)fillers embedded in a matrix that can be composed of either the same phase (e.g., SiC/SiC) or a different one. To avoid intermixing of the fillers and the matrix, phases that are immiscible or that interact poorly are often easier to synthesize. For example, nanoinclusions and nanoparticles have been reported to form during synthesis and were shown to improve the thermoelectric properties of the nanocomposites. (7,9,18,21,22) However, challenges remain in synthesizing (nano)composites in which the matrix and (nano)filler react with each other. (23) To date, there are few reports on the design and synthesis of nanocomposite films containing nanosized fillers. In fact, these material systems often contain compounds that would normally intermix when processed using conventional synthesis methods with high temperature under more thermionically favorable conditions. (24,25) Such methods cannot therefore be used to produce tailor-made nanocomposites.
In the present study, we explore an uncommon synthesis strategy─using physical vapor deposition for growing nanocomposite materials composed of Fe-NP in a CrN thermoelectric matrix. Fe NPs were selected to maximize the thermal fluctuation of magnetic moment as it is the strongest soft magnetic single element metal and to improve the thermoelectric properties of CrN. Nanocomposite coatings consisting of sequentially deposited CrN layers and dispersed Fe-NPs were developed in the same deposition chamber without breaking the vacuum during growth. Using high-power impulse magnetron sputtering (HiPIMS), polycrystalline CrN was deposited at room temperature, while the NPs were synthesized using a NP cluster source in a so-called NP gun using a hollow cathode target. To compare the effect of Fe-NPs on the thermoelectric properties of CrN, three coatings were deposited: a CrN film without NPs and two nanocomposite coatings with different proportions of Fe-NPs. The development of the combined HiPIMS/NP gun approach is shown to be very effective for the synthesis of nanocomposites that cannot be obtained otherwise. Finally, the effect of Fe-NP insertion on the structure, morphology, and thermoelectric properties of the coatings was investigated.

Methods

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Figure 1 shows a schematic representation of the nanocomposite coating designs and the deposition chamber with the magnetron assembly and NP source. The CrN/Fe-NP coatings consisted of alternating CrN layers and dispersed Fe-NP layers. With a total coating thickness of 80–95 nm, three different types of samples were deposited: (i) a reference monolithic CrN film, (ii) a nanocomposite coating with two “layers” of Fe-NP named 2xFe-NP/CrN film, and (iii) a nanocomposite coating with four layers of Fe-NP named 4xFe-NP/CrN film (Figure 1a). The thickness of each CrN layer was controlled by tuning the deposition time, where the bottom layer was kept constant at 15 nm and the top layer at 30 nm. The layers of CrN between the Fe NPs were fixed at 30 and 15 nm for the 2xFe-NP/CrN film and 4xFe-NP/CrN film, respectively. The nanocomposites were deposited without intentional heating to minimize diffusion and solubility of Fe into CrN. The lack of temperature during growth was overcome by using HiPIMS and its highly energetic ions sputtered particle which allows to produce high crystalline epitaxial film even without intentional heating. (26)

Figure 1

Figure 1. Schematic representations of (a) the nanocomposite coatings and (b) the deposition chamber with the magnetron assembly and the NP source in a secondary chamber.

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The CrN thin film was deposited by HiPIMS in an ultrahigh-vacuum chamber with a base pressure of 1.3 × 10–6 Pa using a 50 mm diameter Cr target (Kurt J. Lesker, 99.95%). More details on the deposition chamber can be found elsewhere. (27) The chamber was equipped with one magnetron assembly and an NP source (Figure 1b). The magnetron assembly, situated at 135 mm from the substrate, was powered by a HiPIMS unit (HiPSTER 1, Ionautics AB) at an average target power of 50 W, an average peak current of 8.5 A, and a fixed voltage, pulse length, pulse frequency of 500 V, 30 μs, and 700 Hz, respectively. The deposition conditions of the CrN layer mentioned above were chosen to obtain pure NaCl-B1 phase at room temperature.
The NP source consists of a “secondary cylindrical chamber” with a 1 mm orifice directed toward the substrate and placed 250 mm from the substrate. The “secondary cylindrical chamber” with an approximate volume of 0.00225 m3 (dimension: 26 cm × 10.5 cm ϕ) had a differential pumping with a base pressure of 5 × 10–4 Pa (Figure 1b) in which a hollow cathode sputtering system was placed to produce NPs. Note here that the walls of the secondary chamber were left electrically floating to avoid any disruption in the NP synthesis and deposition. The NP source (HiPNano, Ionautics AB) was equipped with a Fe hollow cathode of ϕouter 9 mm/ϕinner 5 mm × 54 mm (Good Fellow, 99.5% purity), and the anode ring was fixed 10 mm away from the hollow cathode. The NP source was powered by a HiPIMS unit (HiPSTER 1, Ionautics AB) at an average power of 20 W, a pulse length of 30 μs, a pulse frequency of 300 Hz, and average peak current of 13 A and voltage of 485 V. The conditions of deposition and anode position in the NP source were selected to obtain Fe-NP of 6–13 nm in diameter. The total NP areal density per micrometer square was kept constant at 800 NP/μm2 in both nanocomposites. The NP areal density in one layer (duration of deposition for one layer) was changed from 400 NP/μm2 (10 min) for the 2xFe-NP/CrN film and 200 NP/μm2 (5 min) for the 4xFe-NP/CrN film. The areal density and size estimation were performed by scanning electron microscopy.
A gas mixture consisting of 16 sccm of Ar and 5 sccm of N2 was inserted in the main chamber, while pure Ar gas was injected through the hollow cathode with a 28 sccm flow. The differential pumping in the NP source was throttled to reach a pressure of 130 Pa in the NP gun and a pressure of 0.6 Pa in the main chamber.
One-sided polished c-plane sapphire substrates with dimensions of 10 × 10 × 0.5 mm3 were used. The substrates were cleaned with acetone and ethanol in ultrasonic baths for 10 min and were blow-dried using N2 before being mounted on the substrate holder, which was kept at room temperature and under constant rotation during depositions.
The crystal structure of the films was analyzed using a PANAnalyical X’pert powder diffractometer in a Bragg–Brentano configuration using a Cu Kα radiation wavelength of 1.5406 Å (45 kV and 40 mA). A nickel filter was used to filter the Kβ signal visible on the substrate peak. The measurement scan was recorded with constant rotation of the sample using a X’celerator detector in 1D scanning line mode: 10°–80° 2θ range, 0.0084° step size, equivalent of 19.7 s/step.
The elemental composition of the films was performed by ion beam analysis using the Pelletron Tandem accelerator (5MV NEC-5SDH-2) at the Tandem Laboratory, Uppsala University, Sweden. Time-of-flight elastic recoil detection analysis (ToF- ERDA) measurements were conducted using a 36 MeV iodine ion (127I8+) beam. The incident beam angle to the target surface normal was 67.5°, while the ToF-telescope and the gas ionization detector were placed at 45° relative to the incident beam direction. The depth profile of the elemental composition was acquired from the ToF-ERDA time energy coincidence spectra using POTKU 2.0 software. (28)
A scanning electron microscope (ZEISS Gemini SEM 650) operated with an acceleration voltage of 2 kV and an in-lens detector was used to observe surface morphologies. Thin lamellae were prepared in a FEI Helios NanoLab dual-beam focused ion beam (FIB) microscope, with Ga+ ions accelerated at 30 kV, using the standard lift out and thinning procedure. All scanning transmission electron microscopy (STEM) analyses were performed on an FEI Tecnai G2 microscope operated at 200 kV. STEM imaging was conducted in high angle annular dark field (HAADF) and was combined with energy-dispersive X-ray spectroscopy (EDS).
The transport properties, i.e., in-plane electrical conductivity, and in-plane Seebeck coefficient were measured by the four-point probe-measurement method in the 25–325 °C temperature range using a NETZSCH SBA 458 nemesis. Before the measurement, the chamber was filled with Ar using the standard procedure; i.e., the vacuum was applied to the chamber via pumping and filled with Ar. This procedure was applied three times to ensure the neutral environment of the chamber. To further evade the risk of pressure accumulation while heating, the chamber was purged with Ar gas at the rate of 50 mL/min. During the measurements, two microheaters generated the temperature gradient in both sample directions (one sample side being heated and cooled, and after that the same applies to another sample side).
A second electrical setup was used to evaluate the resistivity, drift mobility, and Hall carrier concentration at room temperature. This homemade setup consists of a Van der Pauw configuration setup adapted for Hall measurement. A Keithley 2400 SourceMeter and a Keithley 2100 Multimeter were used to track the current values and measure voltages, respectively. A permanent magnet of 0.5 T was used, leading to a magnetic field of 0.485 T at the sample position. The Hall measurement was conducted with a constant current of 40 mA where a Labview program was used to select the different measurement configurations necessary for the Hall measurement using the Van der Pauw configuration.
The thermal conductivity of the films was obtained at room temperature by using modulated thermoreflectance microscopy (MTRM). In this setup, a pump beam (532 nm) delivered by a Cobolt MLD laser, intensity-modulated by an acousto-optical modulator at a frequency of f, is focused on the surface of the sample with an objective lens (N.A. = 0.5). The variation of the surface reflectivity was monitored by a probe laser (638 nm Oxxius laser). A photodiode and a lock-in amplifier record the AC reflectivity component at a frequency of 100 kHz. The specification of the setup is the spatial measurement around the pump beam. The measurement of the reflectivity of the probe on the surface is performed along the x-axis from −10 to +10 μm around the pump beam area. Finally, the amplitude and phase data were fitted according to a standard Fourier diffusion law to extract the thermal conductivity of the CrN and nanocomposite films. (29−32)
The different characterization techniques were performed on the same samples and in the following order: XRD, SEM, in-plane electrical resistivity, and in-plane Seebeck coefficient from RT to 350 °C, modulated thermoreflectance microscopy (MTRM), and finally EDS-STEM. It is important to note that the thermal conductivity and TEM were performed after the electrical characterization measurements which included heating of the samples to up to 350 °C.

Results and Discussion

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Figure 2 presents the Bragg–Brentano θ–2θ XRD patterns collected for the reference CrN film and the two nanocomposite coatings. The peaks from the c-plane sapphire substrate were observed at 2θ values of 20.5° and 41.6°, corresponding to the 0003 and 0006 reflections of Al2O3, respectively. A sharp cutoff feature at around 40° can be observed on the substrate peak and is due to the use of a nickel filter in front of the detector. The two other XRD peaks observed belong to CrN 111 and 200 with 2θ = 37.6° and 2θ = 43.5°, respectively. CrN showed similar features in all films with a preferential orientation along the [111] direction with a minor quantity of grains oriented along the [200] directions. This preferential orientation of NaCl-B1 CrN on c-plane sapphire is not surprising and has been observed previously. (7) The cell parameter of CrN was estimated at 4.15 Å for all coatings and was consistent with the experimental and theoretical values reported for CrN (4.13 Å). (33,34) For the nanocomposite, only the CrN phase was observed, as no peaks from Fe-bcc were detected. That is reasonable due to the small volume and size of the Fe NPs. No significant differences were noticeable between the two nanocomposite films except for the relative intensity of the 111 diffraction peak compared with the 200 diffraction peak. Those differences, observable on a log scale, remain low and may originate from variations in the proportion of preferentially oriented grains in the film. Despite the low temperature of deposition and the relatively low thickness of the film, clear diffraction peaks were detected, revealing the presence of crystalline CrN phase in the film.

Figure 2

Figure 2. θ–2θ XRD patterns of the monolithic reference CrN film and the two nanocomposite coatings with 2 and 4 layers of Fe-NP.

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Table 1 displays the composition of the films measured by ToF-ERDA. All three samples have the same composition within the error bar of measurement and are under-stoichiometric in nitrogen (CrN0.77±0.01). No Fe was detected as the content is below the detection limit of ToF-ERDA. Considering a concentration of Fe-NP of 800 NP/μm2 with a density of 7.87 g/cm3 (bcc-Fe ICDD 00-006-0696) and a density of CrN of 5.89 g/cm3 (NaCl-B1 CrN ICDD 01-074-8390), the percentage of iron was estimated to be 0.2 at. % overall and 0.4 to 0.8 at. % within the Fe layer (average of 10 nm thickness) for 4xFe-NP/CrN and 2xFe-NP/CrN films, respectively. A 3–4 at. % oxygen contamination was detected for all the films which was most probably incorporated during the deposition.
Table 1. Composition of the Film Measured by ToF-ERDA
sampleCr (at. %) ± 0.5N (at. %) ± 0.5O (at. %) ± 0.5CrN1−δ with δ ± 0.01
CrN ref54.842.23.0CrN0.77
2xFe-NP/CrN54.241.84.1CrN0.77
4xFe-NP/CrN54.741.93.4CrN0.77
Figure 3a shows the surface morphology of a test sample produced to determine the NP areal density and estimate the sizes of the deposited NPs. The NPs had sizes ranging from 6 and 13 nm with an areal density of 400 NP/μm2 for a 10 min deposition duration, approximately. The reference film and the two nanocomposite coatings (2xFe-NP/CrN and 4xFe-NP/CrN) are also shown in Figure 3b–d. The reference CrN films exhibited the typical morphology of cubic NaCl B1 structure materials deposited on sapphire, (35,36) which consists of pyramid-shaped grains (10–15 nm in size) consistent with the XRD results and the (111) oriented film. Locally a few different grain orientations were visible with square-shaped or elongated grains corresponding to the secondary (200) orientation. The nanocomposite coatings had characteristics similar to those of the reference sample but with increased numbers of randomly oriented grains. The presence of more square-shaped or elongated grains is consistent with the XRD observations. The presence of Fe NPs could be responsible for disturbing, to some extent, the growth of the CrN coating, as seen by the more pronounced randomly oriented character of the CrN surface morphology compared to the reference sample. Nevertheless, their presence was not enough to drastically change the morphology, such as yielding porous films.

Figure 3

Figure 3. Top surface morphology of (a) test sample for NP for areal density/size estimation. The NPs were deposited on a Cr layer for a duration of 10 min. (b) Monolithic CrN films and the two nanocomposite coatings with (c) 2 and (d) 4 layers of Fe-NP.

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Figure 4 shows the STEM images and EDS maps of two areas of interest from the 4xFe-NP/CrN film (zones 1 and 2). The NPs’ spatial distribution and small size mean that when observing the film’s cross section there will always be overlap between the particles and the matrix, even within a thin TEM specimen. Individual particles are then almost impossible to see using traditional transmission electron microscopy (TEM) dark-field TEM and selected area electron diffraction. However, 6–9 nm sized features were observed in STEM images (Figure 4, zone 1). While zone 1 corresponds to the first NP layer at the bottom part of the film, zone 2 represents an area at the top part of the film where several NPs seemed to locally form a cluster. Subsequent EDS-STEM analyses show that these features have a high Fe content, indicating the presence of intact nanoparticles with no clear differences of N or O along the films compared to the matrix. Note that EDS-STEM is not sensitive to light elements, and distinctions between C, N, and O cannot be resolved, making the interpretation challenging. Nevertheless, the features correspond to nanoparticles of Fe, estimated to range between 6 and 9 nm, which is consistent with the observations of the Fe-NP morphology made by SEM (Figure 3).

Figure 4

Figure 4. STEM analyses of the 4xFe-NP/CrN film in the two zones of interest. Zone 1 is for NPs at the bottom part of the film while zone 2 is for NPs at the top part of the film. (a–f) and (i–n) are EDS maps of the region of interest with the map corresponding to N, O, Fe, Cr, and Al. (g, o) STEM images of the zone of interest; (h, p) superposition of the EDS map on the STEM image.

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Figure 5 displays the thermoelectric properties of the three coatings. In the case of the CrN reference sample, both absolute Seebeck coefficient and electrical conductivity values increase with temperature from −25 to −70 μV/K and from 375 to 625 S/cm, respectively, which is a typical behavior of semiconductors. The low Seebeck coefficient values compared to other studies could be caused by the low thickness of the film and small CrN grains (<100 nm) which constitute the matrix. (7,37−39) In addition, the N-under-stoichiometric composition of the film could also influence the Seebeck coefficient of the reference sample where an under-stoichiometry reduces the Seebeck values. (27) The reported room temperature electrical resistivity values exist in a wide range for CrN layers, ranging over 2.5 orders of magnitude from 1.7 × 10–3 to 3.5 × 10–1 Ω·cm. (38−44) Most studies suggest a strong correlation of the resistivity of the coatings and the crystal quality/morphology of the coating, where higher quality/larger grain reduce the electrical resistivity and increases the Seebeck coefficient. (9,11,37,45) The CrN sample produced in this study was characterized by an electrical resistivity of 2.54 mΩ·cm, which falls within the lower range of resistivities mentioned above.

Figure 5

Figure 5. Temperature-dependent thermoelectric properties: (a) electrical conductivity and (b) Seebeck coefficient measured on the CrN reference coating (filled squares) and the two nanocomposite coatings (2xFe-NP/CrN, empty triangles, and 4xFe-NP/CrN, filled triangles).

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Compared to the CrN film, the nanocomposites exhibited higher absolute Seebeck values of 50–60 μV/K and lower electrical conductivities of 200–230 S/cm at room temperature, indicating superior performance compared with the monolithic CrN film. In addition, their temperature dependence resembles that of the reference CrN, as all samples exhibited semiconducting behavior with an increase of the electrical conductivity with temperature.
Table 2 lists the electrical and thermal characteristics of the film such as carrier concentration, drift mobility, and thermal conductivity measured at room temperature. The reference sample exhibited a Hall carrier concentration of ∼92 × 1020 cm–3 and a Hall mobility of 0.3 cm2 V–1 s–1. Compared to values reported in the literature ((0.2–7) × 1020 cm–3), the measured values of the carrier concentration are rather high while the Hall mobility falls within the reported values of 0.1–3 cm2 V–1 s–1. (7,9,38,39) For both nanocomposites, the Hall carrier concentration dropped by 1 order of magnitude when the Fe-NPs were embedded into CrN and their electron drift mobilities were 1 order of magnitude higher than the reference sample. While these values differ from the monolithic sample, they fall in the ranges reported in the literature.
Table 2. Room Temperature Thermoelectric Characteristics: Electrical Resistivity, Seebeck Coefficient, Charge Carrier Concentration and Mobility, and Total, Electronic, and Lattice Thermal Conductivities
       thermal conductivity (W m–1 K–1)
samplethickness (nm)resistivity (mΩ·cm) ± 0.05S (μV K–1) ± 2S2σ (μW m–1 K–2)μHall (cm2 V–1 s–1) ± 0.1n (×1020 cm–3) ± 0.1κTotalκeκl
CrN ref802.54–26260.392.14.800.294.51
2xFe-NP/CrN853.28–55922.67.33.600.223.44
4xFe-NP/CrN904.26–49562.26.73.000.172.83
The effect of nanoinclusions on the electrical conductivity has been observed previously on similar material systems. (11) For example, nanoinclusions of metal formation observed in heavy metal (Mo or W) alloyed CrN yielded an increase of the electrical resistivity from 2 mΩ·cm (500 S/cm) to 12 mΩ·cm (83 S/cm). (11) That deterioration of the electrical behavior was attributed to the increase of electron scattering at interfaces with the presence of nanoinclusions of metal in the thermoelectric material.
In the present study, the effect of NPs on the electrical behavior and on the Seebeck coefficient cannot be entirely described. Their insertion led to an increase of the electron mobility and of the Seebeck coefficient and simultaneously reduced the measured Hall carrier concentrations. A decrease in the Seebeck coefficient together with an increase in the electrical conductivity is not surprising and is generally attributed to ionized impurities when the carrier concentration increases. Nevertheless, as compositions of the three films were the same (CrN0.77), it is unlikely that ionized impurities lead to different electrical behaviors of the samples. Moreover, between the two nanocomposites, the maximum Seebeck was observed for the film exhibiting the largest Hall electron mobility and carrier concentration, which was the film containing less Fe-NPs.
The increase in the electron mobility together with the decrease in the Hall carrier concentration could result from energy filtering by internal phases created at the NP/matrix interface. Electron energy filtering can be triggered by energy barriers in nonuniform materials with doping modulation, heterostructures, and composite materials. Enhancement of S and power factor (S2σ) have been observed in such structures and was attributed to the presence of energy barriers. (46−49) In such cases, the low-energy electrons are scattered effectively, while high-energy electrons travel through NPs, resulting in an increase of the electron mobility, a reduction of the Hall carrier concentration, and, hence, the increase of the Seebeck coefficient. (2,46,50)
The measured total thermal conductivity was reduced from 4.8 W m–1 K–1 for a monolithic CrN to a minimum of 3.0 W m–1 K–1 for the 4xFe-NP/CrN film. The lattice thermal conductivity κl was calculated from the difference between the total thermal conductivity κTotal and the electronic part of the thermal conductivity κe, which was deduced according to the Wiedemann– Franz law, κe = LTσ, where L is the Lorenz number (2.44 × 10–8 W Ω K–2) and T is the temperature. In the present case, both electronic and lattice thermal conductivities decrease to contribute to an overall reduction in the total thermal conductivity. It is worth noting that the lattice thermal conductivity represented the major contribution (90%) of the total thermal conductivity reduction and was reduced by 25% for the 2xFe-NP/CrN film and by 40% for the 4xFe-NP/CrN film. The decreased thermal conductivity was attributed to the local interfaces, creating phonon scattering centers between the CrN matrix and Fe-NPs. Using NPs to decrease the thermal conductivity of thermoelectric materials has been reported in the literature, and similar effects, as the reduction observed in this study, were discussed. (18,51−53)
Despite the lower Seebeck values measured for the nanocomposites compared with values reported in the literature, the nanocomposites outperformed the reference CrN film. This suggests that the addition of Fe-NPs could be beneficial for improving the thermoelectric properties of transition metal films. Further investigations are therefore required to identify which nanocomposite design may yield an improved thermoelectric performance.

Conclusion

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A novel approach was employed to produce nanocomposites composed of Fe-NPs dispersed in a CrN matrix. The novel approach involved the use of an NP gun and simultaneous HiPIMS deposition. The presence of Fe-NPs in the CrN matrix was confirmed by STEM and EDS mapping, where the Fe NPs remained intact without noticeable reaction with the matrix, which validates the use of our methodology for synthesizing nanocomposites of miscible material systems. The thermoelectric properties of the nanocomposites were assessed and compared to those of a monolithic CrN reference film. The addition of Fe NPs had an impact on the electrical properties of the films by lowering the Hall carrier concentration, increasing the electron mobility, and yielding an increase of the Seebeck coefficient while decreasing the electrical and thermal conductivities. The changes in electrical behavior have been attributed to the decrease of the Hall carrier concentration accompanied by an increase of electron mobility, while the decrease of the thermal conductivity with an increasing number of Fe-NPs was linked to increased phonon scattering at the newly produced Fe/CrN interfaces.
This study revealed the potential of the nanocomposite strategy using magnetic NPs which could lead to significant improvements in the performance of thermoelectric materials with increased Seebeck coefficient and decreased thermal conductivity.

Author Information

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  • Corresponding Author
  • Authors
    • Daria Pankratova - Department of Engineering Sciences and Mathematics, Luleå University of Technology, 97187 Luleå, Sweden;  Orcidhttps://orcid.org/0000-0002-8305-9926
    • Khabib Yusupov - Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden
    • Alberto Vomiero - Department of Engineering Sciences and Mathematics, Luleå University of Technology, 97187 Luleå, Sweden;  Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Via Torino 155, 30172 Venezia Mestre, Italy;  Orcidhttps://orcid.org/0000-0003-2935-1165
    • Sanath Kumar Honnali - Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden;  Orcidhttps://orcid.org/0000-0001-6215-2454
    • Robert Boyd - Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden;  Orcidhttps://orcid.org/0000-0002-6602-7981
    • Daniele Fournier - Sorbonne Université, CNRS, Institut des NanoSciences de Paris, UMR 7588, Paris 75005, France
    • Sebastian Ekeroth - Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden;  Orcidhttps://orcid.org/0000-0002-2264-7345
    • Ulf Helmersson - Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden
    • Clio Azina - Materials Chemistry, RWTH Aachen University, Kopernikusstraße 10, D-52074 Aachen, Germany
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors acknowledge support from the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971) and the Swedish Research Council (VR) under Project Grant 2021-03826. A.V. acknowledges the Knut and Alice Wallenberg Foundation (Grant No. KAW 2016.0346) and the Kempe Foundation for financial support. Daniel Primetzhofer and Mauricio Sortica from Uppsala University are acknowledged for accelerator operation supported by Swedish Research Council VR-RFI (No. 2019-00191) and the Swedish Foundation for Strategic Research (No. RIF14-0053).

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    Complex thermoelectric materials
    Snyder, G. Jeffrey; Toberer, Eric S.
    Nature Materials (2008), 7 (2), 105-114CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    A review of recent advances in the field of complex thermoelec. materials, highlighting the strategies used to improve the thermopower and reduce the thermal cond. Thermoelec. materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy soln. Such a development is contingent on identifying materials with higher thermoelec. efficiency than available at present, which is a challenge owing to the conflicting combination of material traits that are required. Nevertheless, because of modern synthesis and characterization techniques, particularly for nanoscale materials, a new era of complex thermoelec. materials is approaching.
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  3. 3
    Lan, Y. C.; Minnich, A. J.; Chen, G.; Ren, Z. F. Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach. Adv. Funct. Mater. 2010, 20 (3), 357– 376,  DOI: 10.1002/adfm.200901512
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    Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach
    Lan, Yucheng; Minnich, Austin Jerome; Chen, Gang; Ren, Zhifeng
    Advanced Functional Materials (2010), 20 (3), 357-376CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Recently a significant figure-of-merit (ZT) improvement in the most-studied existing thermoelec. materials has been achieved by creating nanograins and nanostructures in the grains using the combination of high-energy ball milling and a direct-current-induced hot-press process. Thermoelec. transport measurements, coupled with microstructure studies and theor. modeling, show that the ZT improvement is the result of low lattice thermal cond. due to the increased phonon scattering by grain boundaries and structural defects. In this article, the synthesis process and the relationship between the microstructures and the thermoelec. properties of the nanostructured thermoelec. bulk materials with an enhanced ZT value are reviewed. It is expected that the nanostructured materials described here will be useful for a variety of applications such as waste heat recovery, solar energy conversion, and environmentally friendly refrigeration.
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    Hasan, M. N.; Wahid, H.; Nayan, N.; Ali, M. S. M. Inorganic thermoelectric materials: A review. Int. J. Energy Res. 2020, 44 (8), 6170– 6222,  DOI: 10.1002/er.5313
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    Inorganic thermoelectric materials: A review
    Hasan, Md. Nazibul; Wahid, Herman; Nayan, Nafarizal; Mohamed Ali, Mohamed Sultan
    International Journal of Energy Research (2020), 44 (8), 6170-6222CODEN: IJERDN; ISSN:0363-907X. (John Wiley & Sons Ltd.)
    A review. Thermoelec. generator, which converts heat into elec. energy, has great potential to power portable devices. Nevertheless, the efficiency of a thermoelec. generator suffers due to inefficient thermoelec. material performance. In the last two decades, the performance of inorg. thermoelec. materials has been significantly advanced through rigorous efforts and novel techniques. In this review, major issues and recent advancements that are assocd. with the efficiency of inorg. thermoelec. materials are encapsulated. In addn., misc. optimization strategies, such as band engineering, energy filtering, modulation doping, and low dimensional materials to improve the performance of inorg. thermoelec. materials are reported. The methodol. reviews and analyses showed that all these techniques have significantly enhanced the Seebeck coeff., elec. cond., and reduced the thermal cond., consequently, improved ZT value to 2.42, 2.6, and 1.85 for near-room, medium, and high temp. inorg. thermoelec. material, resp. Moreover, this review also focuses on the performance of silicon nanowires and their common fabrication techniques, which have the potential for thermoelec. power generation. Finally, the key outcomes along with future directions from this review are discussed at the end of this article.
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  5. 5
    Jia, N.; Cao, J.; Tan, X. Y.; Dong, J. F.; Liu, H. F.; Tan, C. K. I.; Xu, J. W.; Yan, Q. Y.; Loh, X. J.; Suwardi, A. Thermoelectric materials and transport physics. Mater. Today Phys. 2021, 21, 100519,  DOI: 10.1016/j.mtphys.2021.100519
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    Thermoelectric materials and transport physics
    Jia, Ning; Cao, Jing; Tan, Xian Yi; Dong, Jinfeng; Liu, Hongfei; Tan, Chee Kiang Ivan; Xu, Jianwei; Yan, Qingyu; Loh, Xian Jun; Suwardi, Ady
    Materials Today Physics (2021), 21 (), 100519CODEN: MTPAD5; ISSN:2542-5293. (Elsevier Ltd.)
    Thermoelecs. is attractive as a green and sustainable way for harnessing waste heat and cooling applications. Designing high performance thermoelecs. involves navigating the complex interplay between electronic and heat transports. This fundamentally involves understanding the scattering physics of both electrons and phonons, as well as maximizing symmetry-breaking in entropy and electronic transports. In the last two decades, thermoelecs. have progressed in leaps and bounds thanks to parallel advancements in scientific technologies and phys. understandings. Figure of merit zT of 2 and above have been consistently reported in various materials, esp. Chalcogenides. In this review, we provide a broad picture of phys. driven optimization strategies for thermoelec. materials, with emphasis on electronic transport aspect of inorg. materials. We also discuss and analyzes various newly coined metrics such as quality factors, electronics quality factor, electronic fitness function, weighted mobility, and Fermi surface complexity factor. More importantly, we look at the non-trivial interdependencies between various phys. parameters even at a very fundamental level. Moving forward, we discuss the outlook for the potential of 3D printing and device oriented research in thermoelecs. The intuition derived from this review will be useful not only to guide materials selection, but also research directions in the coming years.
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  6. 6
    Eklund, P.; Kerdsongpanya, S.; Alling, B. Transition-metal-nitride-based thin films as novel energy harvesting materials. J. Mater. Chem. C Mater. 2016, 4 (18), 3905– 3914,  DOI: 10.1039/C5TC03891J
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    6
    Transition-metal-nitride-based thin films as novel energy harvesting materials
    Eklund, Per; Kerdsongpanya, Sit; Alling, Bjoern
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (18), 3905-3914CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    A review. The last few years have seen a rise in the interest in early transition-metal and rare-earth nitrides, primarily based on ScN and CrN, for energy harvesting by thermoelectricity and piezoelectricity. This is because of a no. of important advances, among those the discoveries of exceptionally high piezoelec. coupling coeff. in (Sc,Al)N alloys and of high thermoelec. power factors of ScN-based and CrN-based thin films. These materials also constitute well-defined model systems for investigating thermodn. of mixing for alloying and nanostructural design for optimization of phase stability and band structure. These features have implications for and can be used for tailoring of thermoelec. and piezoelec. properties. In this highlight article, we review the ScN- and CrN-based transition-metal nitrides for thermoelecs., and drawing parallels with piezoelectricity. We further discuss these materials as a models systems for general strategies for tailoring of thermoelec. properties by integrated theor.-exptl. approaches.
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  7. 7
    Gharavi, M. A.; Kerdsongpanya, S.; Schmidt, S.; Eriksson, F.; Nong, N. V.; Lu, J.; Balke, B.; Fournier, D.; Belliard, L.; le Febvrier, A.; Pallier, C.; Eklund, P. Microstructure and thermoelectric properties of CrN and CrN/Cr2N thin films. J. Phys. D: Appl. Phys. 2018, 51 (35), 355302,  DOI: 10.1088/1361-6463/aad2ef
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    7
    Microstructure and thermoelectric properties of CrN and CrN/Cr2N thin films
    Gharavi, M. A.; Kerdsongpanya, S.; Schmidt, S.; Eriksson, F.; Nong, N. V.; Lu, J.; Balke, B.; Fournier, D.; Belliard, L.; le Febvrier, A.; Pallier, C.; Eklund, P.
    Journal of Physics D: Applied Physics (2018), 51 (35), 355302/1-355302/9CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
    CrN thin films with an N/Cr ratio of 95% were deposited by reactive magnetron sputtering onto (0001) sapphire substrates. X-ray diffraction and pole figure texture anal. show CrN (111) epitaxial growth in a twin domain fashion. By changing the nitrogen vs. argon gas flow mixt. and the deposition temp., thin films with different surface morphologies ranging from grainy rough textures to flat and smooth films were prepd. These parameters can also affect the CrNx system, with the film compd. changing between semiconducting CrN and metallic Cr2N through the regulation of the nitrogen content of the gas flow and the deposition temp. at a const. deposition pressure. Thermoelec. measurements (elec. resistivity and Seebeck coeff.), SEM, and transmission electron microscopy imaging confirm the changing elec. resistivity between 0.75 and 300 mΩ cm, the changing Seebeck coeff. values between 140 and 230 μ VK-1, and the differences in surface morphol. and microstructure as higher temps. result in lower elec. resistivity while gas flow mixts. with higher nitrogen content result in single phase cubic CrN.
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  8. 8
    le Febvrier, A.; Gambino, D.; Giovannelli, F.; Bakhit, B.; Hurand, S.; Abadias, G.; Alling, B.; Eklund, P. p-type behavior of CrN thin films via control of point defects. Phys. Rev. B 2022, 105 (10), 104108,  DOI: 10.1103/PhysRevB.105.104108
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    8
    P-type behavior of CrN thin films via control of point defects
    le Febvrier, Arnaud; Gambino, Davide; Giovannelli, Fabien; Bakhit, Babak; Hurand, Simon; Abadias, Gregory; Alling, Bjoern; Eklund, Per
    Physical Review B (2022), 105 (10), 104108CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    We report the results of a combined exptl. and theor. study on nonstoichiometric CrN1+δ thin films grown by reactive magnetron sputtering on c-plane sapphire and MgO (100) substrates in an Ar/N2 gas mixt. using different percentages of N2. There is a transition from n-type to p-type behavior in the layers as a function of nitrogen concn. varying from 48 to 52 at. % in CrN films. The compositional change follows a similar trend for all substrates, with a N/Cr ratio increasing from approx. 0.7 to 1.06-1.11 by increasing the percentage of N2 in the gas flow ratio. As a result of the change in stoichiometry, the lattice parameter and the Seebeck coeff. increase together with the increase of N in CrN1+δ; in particular, the Seebeck value coeff. transitions from -50μVK-1 for CrN0.97 to +75μVK-1 for CrN1.1. D. functional theory calcns. show that Cr vacancies can account for the change in the Seebeck coeff., since they push the Fermi level down in the valence band, whereas N interstitial defects in the form of N2 dumbbells are needed to explain the increasing lattice parameter. Calcns. including both types of defects, which have a strong tendency to bind together, reveal a slight increase in the lattice parameter and a simultaneous formation of holes in the valence band. To explain the exptl. trends, we argue that both Cr vacancies and N2 dumbbells, possibly in combined configurations, are present in the films. We demonstrate the possibility of controlling the semiconducting behavior of CrN with intrinsic defects from n to p type, opening possibilities to integrate this compd. in energy-harvesting thermoelec. devices.
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  9. 9
    Biswas, B.; Chakraborty, S.; Chowdhury, O.; Rao, D.; Pillai, A. I. K.; Bhatia, V.; Garbrecht, M.; Feser, J. P.; Saha, B. In-plane Cr2N-CrN metal-semiconductor heterostructure with improved thermoelectric properties. Phys. Rev. Mater. 2021, 5 (11), 114605,  DOI: 10.1103/PhysRevMaterials.5.114605
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    9
    In-plane Cr2N-CrN metal-semiconductor heterostructure with improved thermoelectric properties
    Biswas, Bidesh; Chakraborty, Sourjyadeep; Chowdhury, Ongira; Rao, Dheemahi; Pillai, Ashalatha Indiradevi Kamalasanan; Bhatia, Vijay; Garbrecht, Magnus; Feser, Joseph P.; Saha, Bivas
    Physical Review Materials (2021), 5 (11), 114605CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
    Epitaxial metal-semiconductor heterostructures with suitable Schottky barrier can lead to high thermoelec. figure-of-merit (zT) due to selective filtering of low-energy electrons as well as reduced thermal cond. from phonon scattering at the interfaces. Lattice-matched vertical metal-semiconductor multilayer/superlattices as well as metallic nanoparticles embedded inside semiconducting hosts have been studied intensively to explore their thermoelec. properties. However, development of in-plane metal-semiconductor heterostructures and exploration of their phys. properties have remained elusive primarily due to the growth and fabrication challenges. In-plane heterostructures are expected to be more suitable for planar integration and should exhibit unique properties. In this work, we demonstrate an in-plane Cr2N-CrN metal-semiconductor heterostructure that exhibits an improved thermoelec. power factor. The in-plane heterostructure is deposited by controlling the Cr-flux during deposition that leads to an in-plane phase sepn. between the metallic-Cr2N and semiconducting CrN grains. Temp.-dependent elec. transport exhibits an Arrhenius-type thermal activation behavior with an activation energy of 70 meV, and an in-plane elec. cond. that is about two orders of magnitude higher than that of CrN. The Seebeck coeff. also remained moderately large at -150μV/K at 700K leading to a very large power factor of 2.1mW/mK2 at 700 K.
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  10. 10
    Quintela, C. X.; Rivadulla, F.; Rivas, J. Thermoelectric properties of stoichiometric and hole-doped CrN. Appl. Phys. Lett. 2009, 94 (15), 152103,  DOI: 10.1063/1.3120280
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    Thermoelectric properties of stoichiometric and hole-doped CrN
    Quintela, C. X.; Rivadulla, F.; Rivas, J.
    Applied Physics Letters (2009), 94 (15), 152103/1-152103/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We report the thermoelec. figure of merit of chromium nitride, CrN, and its optimization through hole-doping. CrN is a degenerate semiconductor with large thermoelec. power, reaching -185 μV/K at 420 K. The resistivity can be reduced through hole-doping in the series Cr1-xVxN, keeping a large thermopower. The thermal cond. of CrN is rather low compared to other transition-metal nitrides, reaching its min. value of 1.0 W/m K at 267 K. The largest ZT = 0.04 was measured for Cr0.9V0.1N at room temp. Our results suggest that CrN could be a good starting point for the design of a thermoelec. material with optimal mech. properties. (c) 2009 American Institute of Physics.
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  11. 11
    Quintela, C. X.; Rodríguez-González, B.; Rivadulla, F. Thermoelectric properties of heavy-element doped CrN. Appl. Phys. Lett. 2014, 104 (2), 022103,  DOI: 10.1063/1.4861845
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    11
    Thermoelectric properties of heavy-element doped CrN
    Quintela, C. X.; Rodriguez-Gonzalez, B.; Rivadulla, F.
    Applied Physics Letters (2014), 104 (2), 022103/1-022103/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    CrN was doped with Mo and W to study the effect of heavy elements alloying on its thermoelec. properties. A spontaneous phase segregation into Mo- and W-rich regions was obsd. even at the lowest concns. probed at this work (≈1%). In the particular case of W, this segregation creates nanoinclusions into the Cr1-xWxN matrix, which results in a substantial decrease of the thermal cond. in the whole temp. range compared to undoped CrN. In addn., an increased hybridization of N:2p and 4d/5d orbitals with respect to Cr:3d decreases the elec. resistivity in lightly doped samples. This improves substantially the thermoelec. figure of merit with respect to the undoped compd., providing a pathway for further improvement of the thermoelec. performance of CrN. (c) 2014 American Institute of Physics.
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  12. 12
    Stockem, I.; Bergman, A.; Glensk, A.; Hickel, T.; Kormann, F.; Grabowski, B.; Neugebauer, J.; Alling, B. Anomalous Phonon Lifetime Shortening in Paramagnetic CrN Caused by Spin-Lattice Coupling: A Combined Spin and Ab Initio Molecular Dynamics Study. Phys. Rev. Lett. 2018, 121 (12), 125902,  DOI: 10.1103/PhysRevLett.121.125902
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    12
    Anomalous Phonon Lifetime Shortening in Paramagnetic CrN Caused by Spin-Lattice Coupling: A Combined Spin and Ab Initio Molecular Dynamics Study
    Stockem, Irina; Bergman, Anders; Glensk, Albert; Hickel, Tilmann; Koermann, Fritz; Grabowski, Blazej; Neugebauer, Joerg; Alling, Bjoern
    Physical Review Letters (2018), 121 (12), 125902CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
    We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio mol. dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temp. compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the exptl. obsd. unexpected temp. dependence of the thermal cond. of magnetic semiconductors above the magnetic ordering temp.
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  13. 13
    Rao, D.; Biswas, B.; Flores, E.; Chatterjee, A.; Garbrecht, M.; Koh, Y. R.; Bhatia, V.; Pillai, A. I. K.; Hopkins, P. E.; Martin-Gonzalez, M.; Saha, B. High mobility and high thermoelectric power factor in epitaxial ScN thin films deposited with plasma-assisted molecular beam epitaxy. Appl. Phys. Lett. 2020, 116 (15), 152103,  DOI: 10.1063/5.0004761
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    13
    High mobility and high thermoelectric power factor in epitaxial ScN thin films deposited with plasma-assisted molecular beam epitaxy
    Rao, Dheemahi; Biswas, Bidesh; Flores, Eduardo; Chatterjee, Abhijit; Garbrecht, Magnus; Koh, Yee Rui; Bhatia, Vijay; Pillai, Ashalatha Indiradevi Kamalasanan; Hopkins, Patrick E.; Martin-Gonzalez, Marisol; Saha, Bivas
    Applied Physics Letters (2020), 116 (15), 152103CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Scandium nitride (ScN) is an emerging rock salt III-nitride semiconductor and has attracted significant interest in recent years for its potential thermoelec. applications as a substrate for high-quality epitaxial GaN growth and as a semiconducting component for epitaxial single-cryst. metal/semiconductor superlattices for thermionic energy conversion. Solid-soln. alloys of ScN with traditional III-nitrides such as AlxSc1-xN have demonstrated piezoelec. and ferroelec. properties and are actively researched for device applications. While most of these exciting developments in ScN research have employed films deposited using low-vacuum methods such as magnetron sputtering and phys. and chem. vapor depositions for thermoelec. applications and Schottky barrier-based thermionic energy conversion, it is necessary and important to avoid impurities, tune the carrier concns., and achieve high-mobility in epitaxial films. Here, the authors report the high-mobility and high-thermoelec. power factor in epitaxial ScN thin films deposited on MgO substrates by plasma-assisted mol. beam epitaxy. Microstructural characterization shows epitaxial 002 oriented ScN film growth on MgO (001) substrates. Elec. measurements demonstrated a high room-temp. mobility of 127 cm2/V s and temp.-dependent mobility in the temp. range of 50-400 K that is dominated by dislocation and grain boundary scattering. High mobility in ScN films leads to large Seebeck coeffs. (-175μV/K at 950 K) and, along with a moderately high elec. cond., a large thermoelec. power factor (2.3 x 10-3 W/m-K2 at 500 K) was achieved, which makes ScN a promising candidate for thermoelec. applications. The thermal cond. of the films, however, was found to be a bit large, which resulted in a max. figure-of-merit of 0.17 at 500 K. (c) 2020 American Institute of Physics.
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    Faleev, S. V.; Léonard, F. Theory of enhancement of thermoelectric properties of materials with nanoinclusions. Phys. Rev. B 2008, 77 (21), 214304,  DOI: 10.1103/PhysRevB.77.214304
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    Theory of enhancement of thermoelectric properties of materials with nanoinclusions
    Faleev, Sergey V.; Leonard, Francois
    Physical Review B: Condensed Matter and Materials Physics (2008), 77 (21), 214304/1-214304/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Based on the concept of band bending at metal/semiconductor interfaces as an energy filter for electrons, we present a theory for the enhancement of the thermoelec. properties of semiconductor materials with metallic nanoinclusions. We show that the Seebeck coeff. can be significantly increased due to a strongly energy-dependent electronic scattering time. By including phonon scattering, we find that the enhancement of ZT due to electron scattering is important for high doping, while at low doping it is primarily due to a decrease in the phonon thermal cond.
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    Li, J. Unexpected boost of thermoelectric performance by magnetic nanoparticles. Sci. China Mater. 2017, 60 (10), 1023– 1024,  DOI: 10.1007/s40843-017-9129-y
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    Unexpected boost of thermoelectric performance by magnetic nanoparticles
    Li, Jiangyu
    Science China Materials (2017), 60 (10), 1023-1024CODEN: SCMCDB; ISSN:2095-8226. (Science China Press)
    There is no expanded citation for this reference.
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    Zhao, W.; Liu, Z.; Sun, Z.; Zhang, Q.; Wei, P.; Mu, X.; Zhou, H.; Li, C.; Ma, S.; He, D.; Ji, P.; Zhu, W.; Nie, X.; Su, X.; Tang, X.; Shen, B.; Dong, X.; Yang, J.; Liu, Y.; Shi, J. Superparamagnetic enhancement of thermoelectric performance. Nature 2017, 549 (7671), 247– 251,  DOI: 10.1038/nature23667
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    Superparamagnetic enhancement of thermoelectric performance
    Zhao, Wenyu; Liu, Zhiyuan; Sun, Zhigang; Zhang, Qingjie; Wei, Ping; Mu, Xin; Zhou, Hongyu; Li, Cuncheng; Ma, Shifang; He, Danqi; Ji, Pengxia; Zhu, Wanting; Nie, Xiaolei; Su, Xianli; Tang, Xinfeng; Shen, Baogen; Dong, Xiaoli; Yang, Jihui; Liu, Yong; Shi, Jing
    Nature (London, United Kingdom) (2017), 549 (7671), 247-251CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    The ability to control chem. and phys. structuring at the nanometer scale is important for developing high-performance thermoelec. materials. Progress in this area has been achieved mainly by enhancing phonon scattering and consequently decreasing the thermal cond. of the lattice through the design of either interface structures at nanometer or mesoscopic length scales or multiscale hierarchical architectures. A nanostructuring approach that enables electron transport as well as phonon transport to be manipulated could potentially lead to further enhancements in thermoelec. performance. Here, the authors show that by embedding nanoparticles of a soft magnetic material in a thermoelec. matrix they achieve dual control of phonon- and electron-transport properties. The properties of the nanoparticles-in particular, their superparamagnetic behavior (in which the nanoparticles can be magnetized similarly to a paramagnet under an external magnetic field)-lead to three kinds of thermoelectromagnetic effect: charge transfer from the magnetic inclusions to the matrix; multiple scattering of electrons by superparamagnetic fluctuations; and enhanced phonon scattering as a result of both the magnetic fluctuations and the nanostructures themselves. The authors show that together these effects can effectively manipulate electron and phonon transport at nanometer and mesoscopic length scales and thereby improve the thermoelec. performance of the resulting nanocomposites.
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    Sumithra, S.; Takas, N. J.; Misra, D. K.; Nolting, W. M.; Poudeu, P. F. P.; Stokes, K. L. Enhancement in Thermoelectric Figure of Merit in Nanostructured Bi2Te3 with Semimetal Nanoinclusions. Adv. Energy Mater. 2011, 1 (6), 1141– 1147,  DOI: 10.1002/aenm.201100338
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    18
    Enhancement in thermoelectric figure of merit in nanostructured Bi2Te3 with semimetal nanoinclusions
    Sumithra, S.; Takas, Nathan J.; Misra, Dinesh K.; Nolting, Westly M.; Poudeu, P. F. P.; Stokes, Kevin L.
    Advanced Energy Materials (2011), 1 (6), 1141-1147CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    The effect of Bi (semimetal) nanoinclusions in nanostructured Bi2Te3 matrixes is investigated. Bismuth nanoparticles synthesized by a low temp. solvothermal method are incorporated into Bi2Te3 matrix phases, synthesized by planetary ball milling. High d. pellets of the Bi nanoparticle/Bi2Te3 nanocomposites are created by hot pressing the powders at 200 °C and 100 MPa. The effect of different vol. fractions (0-7%) of Bi semimetal nanoparticles on the Seebeck coeff., elec. cond., thermal cond. and carrier concn. is reported. Our results show that the incorporation of semimetal nanoparticles results in a redn. in the lattice thermal cond. in all the samples. A significant enhancement in power factor is obsd. for Bi nanoparticle vol. fraction of 5% and 7%. We show that it is possible to reduce the lattice thermal cond. and increase the power factor resulting in an increase in figure of merit by a factor of 2 (from ZT = 0.2 to 0.4). Seebeck coeff. and elec. cond. as a function of carrier concn. data are consistent with the electron filtering effect, where low-energy electrons are preferentially scattered by the barrier potentials set up at the semimetal nanoparticle/semiconductor interfaces.
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  19. 19
    Theja, V. C. S.; Karthikeyan, V.; Assi, D. S.; Roy, V. A. L. Insights into the Classification of Nanoinclusions of Composites for Thermoelectric Applications. Acs Applied Electronic Materials 2022, 4 (10), 4781– 4796,  DOI: 10.1021/acsaelm.2c00617
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    19
    Insights into the Classification of Nanoinclusions of Composites for Thermoelectric Applications
    Theja, Vaskuri C. S.; Karthikeyan, Vaithinathan; Assi, Dani S.; Roy, Vellaisamy A. L.
    ACS Applied Electronic Materials (2022), 4 (10), 4781-4796CODEN: AAEMBP; ISSN:2637-6113. (American Chemical Society)
    A review. Thermoelec. composites are known for their enhanced power conversion performance via interfacial engineering and intensified mech., structural, and thermal properties. However, the selection of these nanoinclusions, for example, their type, size effect, vol. fraction, distribution uniformity, coherency with host, carrier dynamics, and phys. stability, plays a crucial role in modifying the host material thermoelec. properties. In this Review, we classify the nanoinclusions into five types: carbon allotropes, secondary thermoelec. phases, metallic materials, insulating oxides, and others. On the basis of the classification, we discuss the mechanisms involved in improving the ZT of nanocomposites involving redn. of thermal cond. (κ) by phonon scattering, improving the Seebeck coeff. (α) via energy filtering effect and the elec. cond. (σ) by carrier injection or carrier channeling. Comprehensibly, we validate that adding nanoinclusions with high elec. and low thermal cond. as compared to the matrix material is the best way to optimize the interlocked thermoelec. parameters. Thus, collective doping and nanoinclusions in thermoelec. materials is the best possible soln. to achieve a higher power conversion efficiency equiv. to other renewable energy technologies.
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  20. 20
    Lee, E.; Ko, J.; Kim, J. Y.; Seo, W. S.; Choi, S. M.; Lee, K. H.; Shim, W.; Lee, W. Enhanced thermoelectric properties of Au nanodot-included BiTe nanotube composites. J. Mater. Chem. C 2016, 4 (6), 1313– 1319,  DOI: 10.1039/C5TC03934G
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    20
    Enhanced thermoelectric properties of Au nanodot-included Bi2Te3 nanotube composites
    Lee, Eunsil; Ko, Jieun; Kim, Jong-Young; Seo, Won-Seon; Choi, Soon-Mok; Lee, Kyu Hyoung; Shim, Wooyoung; Lee, Wooyoung
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (6), 1313-1319CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    Herein, the authors report on a scalable synthesis of Au nanodot (Au-ND)/Bi2Te3 nanotube (BT-NT) nanocomposites by the bottom-up synthesis of hybrid raw materials and subsequent spark plasma sintering, and their thermoelec. properties were systematically compared with those of Au-doped Bi2Te3 compds. The Au nanodots were included as seeds and co-crystd. in the crystal growth of BT-NTs, which were well-dispersed in the Bi2Te3 matrix as nanoinclusions (10-20 nm). The thermoelec. performance (ZT) of the Au-ND/BT-NT nanocomposite is enhanced by ∼67%, compared to pristine Bi2Te3 due to electron energy filtering and phonon scattering effects in the presence of embedded Au-NDs. The resulting compd. showed an enhanced power factor (23.0 × 10-4 W m-1 K-2 @ 440 K, 27% improvement) and a reduced lattice thermal cond. (0.47 W m-1 K-1 @ 440 K, 22% redn.). The peak ZT value of the present compd. (0.95 @ 480 K) is larger than that of n-type single cryst. Bi2(Te,Se)3, which is one of the highest among the reported values for n-type Bi2Te3-based materials synthesized using a soft chem. route.
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  21. 21
    Shu, R.; Han, Z.; Elsukova, A.; Zhu, Y.; Qin, P.; Jiang, F.; Lu, J.; Persson, P. O. A.; Palisaitis, J.; le Febvrier, A.; Zhang, W.; Cojocaru-Miredin, O.; Yu, Y.; Eklund, P.; Liu, W. Solid-State Janus Nanoprecipitation Enables Amorphous-Like Heat Conduction in Crystalline Mg(3) Sb(2) -Based Thermoelectric Materials. Adv. Sci. (Weinh) 2022, 9 (25), e2202594  DOI: 10.1002/advs.202202594
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  22. 22
    Bueno Villoro, R.; Wood, M.; Luo, T.; Bishara, H.; Abdellaoui, L.; Zavanelli, D.; Gault, B.; Snyder, G. J.; Scheu, C.; Zhang, S. Fe segregation as a tool to enhance electrical conductivity of grain boundaries in Ti(Co,Fe)Sb half Heusler thermoelectrics. Acta Mater. 2023, 249, 118816,  DOI: 10.1016/j.actamat.2023.118816
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    22
    Fe segregation as a tool to enhance electrical conductivity of grain boundaries in Ti(Co,Fe)Sb half Heusler thermoelectrics
    Bueno Villoro, Ruben; Wood, Maxwell; Luo, Ting; Bishara, Hanna; Abdellaoui, Lamya; Zavanelli, Duncan; Gault, Baptiste; Snyder, Gerald Jeffrey; Scheu, Christina; Zhang, Siyuan
    Acta Materialia (2023), 249 (), 118816CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
    Complex microstructures are found in many thermoelec. materials and can be used to optimize their transport properties. Grain boundaries in particular scatter phonons, but they often impede charge carrier transfer at the same time. Designing grain boundaries in order to offer a conductive path for electrons is a substantial opportunity to optimize thermoelecs. Here, we demonstrate in TiCoSb half Heusler compds. that Fe-dopants segregate to grain boundaries and simultaneously increase the elec. cond. and reduce the thermal cond. To explain these phenomena, three samples with different grain sizes are synthesized and a model is developed to relate the elec. cond. with the area fraction of grain boundaries. The elec. cond. of grain interior and grain boundaries is calcd. and the at. structure of grain boundaries is studied in detail. Segregation engineering in fine-grained thermoelecs. is proposed as a new design tool to optimize transport properties while achieving a lower thermal cond.
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  23. 23
    Steinhoff, M. K.; Holzapfel, D. M.; Karimi Aghda, S.; Neuß, D.; Pöllmann, P. J.; Hans, M.; Primetzhofer, D.; Schneider, J. M.; Azina, C. Ag Surface and Bulk Segregations in Sputtered ZrCuAlNi Metallic Glass Thin Films. Materials 2022, 15 (5), 1635,  DOI: 10.3390/ma15051635
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    23
    Ag Surface and Bulk Segregations in Sputtered ZrCuAlNi Metallic Glass Thin Films
    Steinhoff, Michael K.; Holzapfel, Damian M.; Karimi Aghda, Soheil; Neuss, Deborah; Poellmann, Peter J.; Hans, Marcus; Primetzhofer, Daniel; Schneider, Jochen M.; Azina, Clio
    Materials (2022), 15 (5), 1635CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
    We report on the formation of Ag-contg. ZrCuAlNi thin film metallic glass (nano)composites by a hybrid direct-current magnetron sputtering and high-power pulsed magnetron sputtering process. The effects of Ag content, substrate temp. and substrate bias potential on the phase formation and morphol. of the nanocomposites were investigated. While applying a substrate bias potential did not strongly affect the morphol. evolution of the films, the Ag content dictated the size and distribution of Ag surface segregations. The films deposited at low temps. were characterized by strong surface segregations, formed by coalescence and Ostwald ripening, while the vol. of the films remained featureless. At higher deposition temp., elongated Ag segregations were obsd. in the bulk and a continuous Ag layer was formed at the surface as a result of thermally enhanced surface diffusion. While microstructural observations have allowed identifying both surface and bulk segregations, an indirect method for detecting the presence of Ag segregations is proposed, by measuring the elec. resistivity of the films.
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  24. 24
    Moya, J. S.; Lopez-Esteban, S.; Pecharromán, C. The challenge of ceramic/metal microcomposites and nanocomposites. Prog. Mater. Sci. 2007, 52 (7), 1017– 1090,  DOI: 10.1016/j.pmatsci.2006.09.003
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    24
    The challenge of ceramic/metal microcomposites and nanocomposites
    Moya, Jose S.; Lopez-Esteban, Sonia; Pecharroman, Carlos
    Progress in Materials Science (2007), 52 (7), 1017-1090CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
    A review. It is increasingly being recognized that new applications for materials require functions and properties that are not achievable with monolithic materials. The combination of dissimilar materials for these new applications creates interfaces whose properties and processing need to be understood before they can be applied com. In the present review paper we try to emphasize the important role and challenges of ceramic/metal micro/nanocomposites in the new technologies. In this respect we will study and review the exotic effects of metal particles embedded into matrix ceramics due to the dissimilar properties of the components, percolation laws, and the nature of the interfaces. From an electromagnetic point of view we have underlined the enormous enhancement of permittivity in the proximity of the percolation threshold, assocd. with an induced soft mode similar to para-ferroelec. transition. From a mech. standpoint, the synergic effect of nanometer size, clustering addressed by the percolation theory and ceramic/metal interface features produces an unexpected enhancement in the hardness of the composite giving rise to superhard materials.
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  25. 25
    Rane, A. V.; Kanny, K.; Abitha, V. K.; Thomas, S. Chapter 5 - Methods for Synthesis of Nanoparticles and Fabrication of Nanocomposites. In Synthesis of Inorganic Nanomaterials; Mohan Bhagyaraj, S., Oluwafemi, O. S., Kalarikkal, N., Thomas, S., Eds.; Woodhead Publishing: 2018; pp 121– 139.
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  26. 26
    Villamayor, M. M. S.; Keraudy, J.; Shimizu, T.; Viloan, R. P. B.; Boyd, R.; Lundin, D.; Greene, J. E.; Petrov, I.; Helmersson, U. Low temperature (Ts/Tm < 0.1) epitaxial growth of HfN/MgO(001) via reactive HiPIMS with metal-ion synchronized substrate bias. J. Vac. Sci. Technol. A 2018, 36 (6), 061511,  DOI: 10.1116/1.5052702
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    26
    Low temperature (Ts/Tm < 0.1) epitaxial growth of HfN/MgO(001) via reactive HiPIMS with metal-ion synchronized substrate bias
    Villamayor, Michelle Marie S.; Keraudy, Julien; Shimizu, Tetsuhide; Viloan, Rommel Paulo B.; Boyd, Robert; Lundin, Daniel; Greene, Joseph E.; Petrov, Ivan; Helmersson, Ulf
    Journal of Vacuum Science & Technology, A: Vacuum, Surfaces, and Films (2018), 36 (6), 061511/1-061511/11CODEN: JVTAD6; ISSN:0734-2101. (American Institute of Physics)
    Low-temp. epitaxial growth of refractory transition-metal nitride thin films by means of phys. vapor deposition has been a recurring theme in advanced thin-film technol. for several years. In the present study, 150-nm-thick epitaxial HfN layers are grown on MgO by reactive high-power impulse magnetron sputtering (HiPIMS) with no external substrate heating. In the present expts., a neg. bias of 100 V is applied to the substrate, either continuously during the entire deposition or synchronized with the metal-rich portion of the ion flux. Two different sputtering-gas mixts., Ar/N2 and Kr/N2, are employed in order to probe effects assocd. with the noble-gas mass and ionization potential. The combination of x-ray diffraction, high-resoln. reciprocal-lattice maps, and high-resoln. cross-sectional transmission electron microscopy analyses establishes that all HfN films have a cube-on-cube orientational relationship with the substrate,. In distinct contrast, layers grown in Kr/N2 with the substrate bias synchronized to the metal-ion-rich portion of HiPIMS pulses have much lower mosaicity, no measurable inert-gas incorporation, and a hardness of 25.7 GPa, in good agreement with the results for epitaxial HfN(001) layers grown at Ts = 650°C (Ts/Tm = 0.26). The room-temp. film resistivity is 70μΩ cm, which is 3.2-10 times lower than reported values for polycryst.-HfN layers grown at Ts = 400°C. (c) 2018 American Institute of Physics.
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  27. 27
    le Febvrier, A.; Landälv, L.; Liersch, T.; Sandmark, D.; Sandström, P.; Eklund, P. An upgraded ultra-high vacuum magnetron-sputtering system for high-versatility and software-controlled deposition. Vacuum 2021, 187, 110137,  DOI: 10.1016/j.vacuum.2021.110137
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    27
    An upgraded ultra-high vacuum magnetron-sputtering system for high-versatility and software-controlled deposition
    le Febvrier, Arnaud; Landaelv, Ludvig; Liersch, Thomas; Sandmark, David; Sandstroem, Per; Eklund, Per
    Vacuum (2021), 187 (), 110137CODEN: VACUAV; ISSN:0042-207X. (Elsevier Ltd.)
    Magnetron sputtering is a widely used phys. vapor deposition technique. Reactive sputtering is used for the deposition of, e.g, oxides, nitrides and carbides. In fundamental research, versatility is essential when designing or upgrading a deposition chamber. Furthermore, automated deposition systems are the norm in industrial prodn., but relatively uncommon in lab.-scale systems used primarily for fundamental research. Combining automatization and computerized control with the required versatility for fundamental research constitutes a challenge in designing, developing, and upgrading lab. deposition systems. The present article provides a detailed description of the design of a lab-scale deposition chamber for magnetron sputtering used for the deposition of metallic, oxide, nitride and oxynitride films with automated controls, dc or pulsed bias, and combined with a coil to enhance the plasma d. near the substrate. LabVIEW software (provided as Supplementary Information) has been developed for a high degree of computerized or automated control of hardware and processes control and logging of process details.
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  28. 28
    Ström, P.; Primetzhofer, D. Ion beam tools for nondestructive in-situ and in-operando composition analysis and modification of materials at the Tandem Laboratory in Uppsala. Journal of Instrumentation 2022, 17 (04), P04011,  DOI: 10.1088/1748-0221/17/04/P04011
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    Pottier, L. Micrometer Scale Visualization of Thermal Waves by Photoreflectance Microscopy. Appl. Phys. Lett. 1994, 64 (13), 1618– 1619,  DOI: 10.1063/1.111856
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    Micrometer scale visualization of thermal waves by photoreflectance microscopy
    Pottier, L.
    Applied Physics Letters (1994), 64 (13), 1618-19CODEN: APPLAB; ISSN:0003-6951.
    A novel approach of photoreflectance microscopy is proposed that provides a direct visualization of the phase contour lines of the thermal wave. The method is applicable to (possibly heterogeneous) samples of mediocre polish. In a locally homogeneous region it yields the local thermal diffusivity.
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    Li, B. C.; Roger, J. P.; Pottier, L.; Fournier, D. Complete thermal characterization of film-on-substrate system by modulated thermoreflectance microscopy and multiparameter fitting. J. Appl. Phys. 1999, 86 (9), 5314– 5316,  DOI: 10.1063/1.371520
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    30
    Complete thermal characterization of film-on-substrate system by modulated thermoreflectance microscopy and multiparameter fitting
    Li, Bincheng; Roger, J. P.; Pottier, L.; Fournier, D.
    Journal of Applied Physics (1999), 86 (9), 5314-5316CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Modulated thermoreflectance microscopy is applied to a complete thermal characterization of a thin film of gold (78 nm) or YBaCuO (300 nm) on a LaAlO3 substrate. The phase profile, measured at several modulation frequencies covering an appropriate range, is fitted with a rigorous thermal diffusion model. This leads to a simultaneous estn. of the thermal diffusivities of the film and the substrate, as well as of the thermal film/substrate boundary resistance. The estd. values for the gold film sample are, resp., 4.3 × 10-6 m2s-1 (substrate diffusivity), 1.0 × 10-4 m2s-1 (film diffusivity), and 1.0 × 10-8 m2KW-1 (thermal boundary resistance), while for the thermally anisotropic YBaCuO film sample are, 4.1 × 10-6 m2s-1, 3.5 × 10-6 m2s-1 (in-plane diffusivity), and 8.0 × 10-8 m2KW-1, resp.
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    Frétigny, C.; Duquesne, J. Y.; Fournier, D.; Xu, F. Thermal insulating layer on a conducting substrate. Analysis of thermoreflectance experiments. J. Appl. Phys. 2012, 111 (8), 084313,  DOI: 10.1063/1.3702823
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    Thermal insulating layer on a conducting substrate. Analysis of thermoreflectance experiments
    Fretigny, C.; Duquesne, J.-Y.; Fournier, D.; Xu, F.
    Journal of Applied Physics (2012), 111 (8), 084313/1-084313/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Thermoreflectance expts. are sensitive to the thermal properties of thin layers deposited on substrates (cond. and diffusivity). However, retrieving these properties from exptl. data remains a difficult issue. The case of a conducting layer deposited on an insulating substrate was studied previously. We present here a math. and exptl. anal. of the thermoreflectance response in the opposite case: an insulating layer on a conducting substrate. We show theor. that cond. and diffusivity can be detd. independently thanks to a comparison with the substrate. The method is applied to expts. performed on a silicon substrate covered with a thin layer deposited by sputtering a titanium target. (c) 2012 American Institute of Physics.
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    Frétigny, C.; Roger, J. P.; Reita, V.; Fournier, D. Analytical inversion of photothermal measurements: Independent determination of the thermal conductivity and diffusivity of a conductive layer deposited on an insulating substrate. J. Appl. Phys. 2007, 102 (11), 116104,  DOI: 10.1063/1.2818102
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    Analytical inversion of photothermal measurements: Independent determination of the thermal conductivity and diffusivity of a conductive layer deposited on an insulating substrate
    Fretigny, Christian; Roger, Jean Paul; Reita, Valerie; Fournier, Daniele
    Journal of Applied Physics (2007), 102 (11), 116104/1-116104/3CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The long distance behavior of the surface temp. wave in a thermoreflectance microscopy expt. is established for a conductive layer deposited on an insulating substrate. At large distance from the point source, heat is confined, so the amplitude decrease is lower than for a bulk sample. From the slopes which appear on the phase and on the log scale amplitude, a procedure is proposed to ext., sep., the thermal diffusivity and cond. of the layer, taking into account data obtained at different modulation frequencies. Exptl. results are presented which confirm the validity of the method.
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    Alling, B.; Marten, T.; Abrikosov, I. A. Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN. Phys. Rev. B 2010, 82 (18), 184430,  DOI: 10.1103/PhysRevB.82.184430
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    Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN
    Alling, B.; Marten, T.; Abrikosov, I. A.
    Physical Review B: Condensed Matter and Materials Physics (2010), 82 (18), 184430/1-184430/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We use first-principles calcns. to study the effect of magnetic disorder and electron correlations on the structural and thermodn. properties of CrN. We illustrate the usability of a special quasirandom structure supercell treatment of the magnetic disorder by comparing with CPA calcns. and with a complementary magnetic sampling method. The need of a treatment of electron correlations effects beyond the local d. approxn. is proven by a comparison of LDA+U calcns. of structural and electronic properties with exptl. results. When magnetic disorder and strong electron correlations are taken into account simultaneously, pressure- and temp.-induced structural and magnetic transitions in CrN can be understood.
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    Kerdsongpanya, S.; Sun, B.; Eriksson, F.; Jensen, J.; Lu, J.; Koh, Y. K.; Nong, N. V.; Balke, B.; Alling, B.; Eklund, P. Experimental and theoretical investigation of CrScN solid solutions for thermoelectrics. J. Appl. Phys. 2016, 120 (21), 215103,  DOI: 10.1063/1.4968570
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    Experimental and theoretical investigation of Cr1-xScxN solid solutions for thermoelectrics
    Kerdsongpanya, Sit; Sun, Bo; Eriksson, Fredrik; Jensen, Jens; Lu, Jun; Koh, Yee Kan; Nong, Ngo Van; Balke, Benjamin; Alling, Bjoern; Eklund, Per
    Journal of Applied Physics (Melville, NY, United States) (2016), 120 (21), 215103/1-215103/10CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    The ScN- and CrN-based transition-metal nitrides have recently emerged as a novel and unexpected class of materials for thermoelecs. These materials constitute well-defined model systems for investigating mixing thermodn., phase stability, and band structure aiming for property tailoring. Here, we demonstrate an approach to tailor their thermoelec. properties by solid solns. The trends in mixing thermodn. and densities-of-states (DOS) of rocksalt-Cr1-xScxN solid solns. (0 ≤ x ≤ 1) are investigated by first-principles calcns., and Cr1-xScxN thin films are synthesized by magnetron sputtering. Pure CrN exhibits a high power factor, 1.7 × 10-3 W m-1 K-2 at 720 K, enabled by a high electron concn. thermally activated from N vacancies. Disordered rocksalt-Cr1-xScxN solid solns. are thermodynamically stable, and calcd. DOS suggest the possibility for power-factor improvement by Sc3d orbital delocalization on Cr3d electrons giving decreasing elec. resistivity, while localized Cr3d orbitals with a large DOS slope may yield an improved Seebeck coeff. Sc-rich solid solns. show a large improvement in power factor compared to pure ScN, and all films have power factors above that expected from the rule-of-mixt. These results corroborate the theor. predictions and enable tailoring and understanding of structure-transport-property correlations of Cr1-xScxN. (c) 2016 American Institute of Physics.
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  35. 35
    le Febvrier, A.; Tureson, N.; Stilkerich, N.; Greczynski, G.; Eklund, P. Effect of impurities on morphology, growth mode, and thermoelectric properties of (1 1 1) and (0 0 1) epitaxial-like ScN films. J. Phys. D: Appl. Phys. 2019, 52 (3), 035302,  DOI: 10.1088/1361-6463/aaeb1b
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    35
    Effect of impurities on morphology, growth mode, and thermoelectric properties of (111) and (001) epitaxial-like ScN films
    Le Febvrier, Arnaud; Tureson, Nina; Stilkerich, Nina; Greczynski, Grzegorz; Eklund, Per
    Journal of Physics D: Applied Physics (2019), 52 (3), 035302/1-035302/11CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
    ScN is an emerging semiconductor with an indirect bandgap. It has attracted attention for its thermoelec. properties, use as seed layers, and for alloys for piezoelec.application. ScN and other transition metal nitride semiconductors used for their interesting elec. properties are sensitive to contaminants, such as oxygen or fluorine. In this present article, the influence of depositions conditions on the amt. of oxygen contaminants incorporated in ScN films were investigated and their effects on the elec. properties (elec. resistivity and Seebeck coeff.) were studied. Epitaxial-like films of thickness 125 ± 5 nm to 155 ± 5 nm were deposited by DC-magnetron sputtering on c-plane Al2O3, MgO(1 1 1) and r-plane Al2O3 at substrate temps.ranging from 700°C to 950°C. The amt. of oxygen contaminants in the film, dissolved into ScN or as an oxide, was related to the adatom mobility during growth, which is affected by the deposition temp.and the presence of twin domain growth. The lowest values of elec. resistivity of 50μO cm were obtained on ScN(1 1 1)/ MgO(1 1 1) and on ScN(0 0 1)/r-plane Al2O3 grown at 950°C with no twin domains and the lowest amt. of oxygen contaminant. At the best, the films exhibited an elec. resistivity of 50μO cm with Seebeck coeff. values maintained at -40μV K-1, thus a power factor estd. at 3.2 × 10-3 W m-1 K-2 (at room temp.).
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  36. 36
    Tan, S.; Zhang, X.; Wu, X.; Fang, F.; Jiang, J. Comparison of chromium nitride coatings deposited by DC and RF magnetron sputtering. Thin Solid Films 2011, 519 (7), 2116– 2120,  DOI: 10.1016/j.tsf.2010.10.067
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    Comparison of chromium nitride coatings deposited by DC and RF magnetron sputtering
    Tan, Shuyong; Zhang, Xuhai; Wu, Xiangjun; Fang, Feng; Jiang, Jianqing
    Thin Solid Films (2011), 519 (7), 2116-2120CODEN: THSFAP; ISSN:0040-6090. (Elsevier B.V.)
    Chromium nitride coatings were deposited by DC and RF reactive magnetron sputtering on AISI 304 stainless steels without substrate heating. A Cr2N phase was formed in the RF sputtered coatings with a low N2 flow content ranging within 30-50%. A NaCl type CrNx phase was obtained by DC magnetron sputtering with different N2 flow contents. The coating hardness increased with the increase of the N2 flow content. When the coatings deposited with the same N2 flow content were compared, the hardness of the RF sputtered CrNx was higher than that of the DC sputtered CrNx, which was mainly due to the distinct difference between the dense structure (RF process) and the porous structure (DC process). The RF sputtered CrNx coatings showed an excellent adhesion strength as compared to the DC sputtered coatings. By selecting the deposition method and optimizing the N2 flow content, CrNx coatings with a preferred microstructure could be obtained, which would be a candidate material for research and applications in nano-science.
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  37. 37
    Quintela, C. X.; Podkaminer, J. P.; Luckyanova, M. N.; Paudel, T. R.; Thies, E. L.; Hillsberry, D. A.; Tenne, D. A.; Tsymbal, E. Y.; Chen, G.; Eom, C. B.; Rivadulla, F. Epitaxial CrN thin films with high thermoelectric figure of merit. Adv. Mater. 2015, 27 (19), 3032– 7,  DOI: 10.1002/adma.201500110
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    Epitaxial CrN Thin Films with High Thermoelectric Figure of Merit
    Quintela, Camilo X.; Podkaminer, Jacob P.; Luckyanova, Maria N.; Paudel, Tula R.; Thies, Eric L.; Hillsberry, Daniel A.; Tenne, Dmitri A.; Tsymbal, Evgeny Y.; Chen, Gang; Eom, Chang-Beom; Rivadulla, Francisco
    Advanced Materials (Weinheim, Germany) (2015), 27 (19), 3032-3037CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In this Communication, we demonstrate that rock salt CrN shows intrinsic lattice instabilities that suppress its thermal cond. Using ab initio calcns., we detd. that the origin of these instabilities is similar to that obsd. in IV-VI compds. with RB states. Through the fabrication of high-quality epitaxial (OOl) CrN thin films we report a =250% increase in the zT at room temp. compared* to bulk CrN. These results, along with its high thermal stability, resistance to corrosion, and exceptional mech. properties, make CrN a promising n-type material for high-temp. TE applications.
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  38. 38
    Zhang, X. Y.; Chawla, J. S.; Howe, B. M.; Gall, D. Variable-range hopping conduction in epitaxial CrN(001). Phys. Rev. B 2011, 83 (16), 165205,  DOI: 10.1103/PhysRevB.83.165205
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    Variable-range hopping conduction in epitaxial CrN(001)
    Zhang, X. Y.; Chawla, J. S.; Howe, B. M.; Gall, D.
    Physical Review B: Condensed Matter and Materials Physics (2011), 83 (16), 165205/1-165205/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Epitaxial CrN(001) layers, grown by d.c. magnetron sputtering on MgO(001) substrates at growth temps. Ts = 550-850°, exhibit electronic transport that is dominated by variable-range hopping (VRH) at temps. <120 K. A transition from Efros-Shklovskii to Mott VRH at 30 ± 10 K is well described by a universal scaling relation. The localization length decreases from 1.3 nm at Ts = 550° to 0.9 nm for Ts = 600-750°, but increases again to 1.9 nm for Ts = 800-850°, which is attributed to changes in the d. of localized states assocd. with N vacancies that form due to kinetic barriers for incorporation and enhanced desorption at low and high Ts, resp. The low-temp. transport data provide lower limits for the CrN effective electron mass of 4.9me, the donor ionization energy of 24 meV, and the crit. vacancy concn. for the metal-insulator transition of 8.4 × 1019 cm-3. The room temp. cond. is dominated by Hubbard band states near the mobility edge and decreases monotonically from 137 Ω-1 cm-1 for Ts = 550° to 14 Ω-1 cm-1 for Ts = 850° due to a decreasing structural disorder, consistent with the measured x-ray coherence length that increases from 7 to 36 nm for Ts = 550 to 850°, resp., and a carrier d. that decreases from 4 × 1020 to 0.9 × 1020 cm-3, as estd. from optical reflection and Hall effect measurements. The absence of an expected discontinuity in the cond. at ∼280 K suggests that epitaxial constraints suppress the phase transition to a low-temp. orthorhombic antiferromagnetic phase, such that CrN remains a cubic paramagnetic insulator over the entire measured temp. range of 10-295 K. These results contradict previous exptl. studies that report metallic low-temp. conduction for CrN, but support recent computational results suggesting a band gap due to strong electron correlation and a stress-induced phase transition.
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  39. 39
    Muhammed Sabeer, N. A.; Pradyumnan, P. P. Augmentation of thermoelectric power factor of p-type chromium nitride thin films for device applications. Materials Science and Engineering: B 2021, 273, 115428,  DOI: 10.1016/j.mseb.2021.115428
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    39
    Augmentation of thermoelectric power factor of p-type chromium nitride thin films for device applications
    Muhammed Sabeer, N. A.; Pradyumnan, P. P.
    Materials Science & Engineering, B: Advanced Functional Solid-State Materials (2021), 273 (), 115428CODEN: MSBTEK; ISSN:0921-5107. (Elsevier B.V.)
    The transformation of the n-type chromium nitride (CrN) thin films into p-type by intrinsic point defects using reactive radio frequency magnetron sputtering is explored in this report. The formation of the acceptor point defects with increasing nitrogen pressure transformed the n-type cond. of CrN into p-type along with variation of preferred orientation from (1 1 1) to (2 0 0). To improve the mobility of holes and hence to decrease the resistivity, the CrN thin films with preferred orientation along (2 2 0) was fabricated by precisely tuning the sputtering parameters. The root cause behind the redn. in resistivity of CrN thin films with orientation of crystallites was explored by surface charge d. mapping using conductive at. force microscopy. The redn. in resistivity resulted in the enhancement of thermoelec. power factor and the p-type CrN thin film showed a power factor of 24.43μW/m K2with Seebeck coeff. of 301μV/K and resistivity of 3.71 mΩ m at RT.
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  40. 40
    Sanjinés, R.; Banakh, O.; Rojas, C.; Schmid, P. E.; Lévy, F. Electronic properties of Cr1–xAlxN thin films deposited by reactive magnetron sputtering. Thin Solid Films 2002, 420–421, 312– 317,  DOI: 10.1016/S0040-6090(02)00830-1
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    Electronic properties of Cr1-xAlxN thin films deposited by reactive magnetron sputtering
    Sanjines, R.; Banakh, O.; Rojas, C.; Schmid, P. E.; Levy, F.
    Thin Solid Films (2002), 420-421 (), 312-317CODEN: THSFAP; ISSN:0040-6090. (Elsevier Science B.V.)
    XPS and elec. resistivity measurements have been performed to investigate the electronic properties of Cr1-xAlxN films. The films, deposited by reactive magnetron sputtering, crystallize in the fcc. rock salt-type of structure (B1-type) in a wide compn. range of 0 ≤ x ≤ 0.63. The elec. resistivity was measured from 50 to 320 K. The elec. resistivity at 320 K increases with increasing Al content, and the temp. coeff. of the resistivity is always neg. A detailed study of XPS valence band (VB) spectra shows that the substitution of Cr atoms by Al atoms leads to local modifications of covalent-ionic bonds between N 2p and Cr 3d orbitals. The variations of the VB structure due to the changes in the chem. compn. correlate with the elec. and mech. properties.
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  41. 41
    Gall, D.; Shin, C. S.; Spila, T.; Odén, M.; Senna, M. J. H.; Greene, J. E.; Petrov, I. Growth of single-crystal CrN on MgO(001):: Effects of low-energy ion-irradiation on surface morphological evolution and physical properties. J. Appl. Phys. 2002, 91 (6), 3589– 3597,  DOI: 10.1063/1.1446239
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    41
    Growth of single-crystal CrN on MgO(001): Effects of low-energy ion-irradiation on surface morphological evolution and physical properties
    Gall, D.; Shin, C.-S.; Spila, T.; Oden, M.; Senna, M. J. H.; Greene, J. E.; Petrov, I.
    Journal of Applied Physics (2002), 91 (6), 3589-3597CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    CrN layers, 0.5 μm thick, were grown on MgO(001) at Ts = 570-775° by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition in pure N2 discharges at 20 mtorr. Layers grown at Ts ≤ 700° are stoichiometric single crystals exhibiting cube-on-cube epitaxy: (001)CrN‖(001)MgO with [100]CrN‖[100]MgO. At higher temps., N2 desorption during deposition results in understoichiometric polycryst. films with N fractions decreasing to 0.35, 0.28, and 0.07 with Ts = 730, 760, and 775°, resp. The surface morphologies of epitaxial CrN(001) layers depend strongly on the incident ion-to-metal flux ratio JN2+/JCr which was varied between 1.7 and 14 with the ion energy maintained const. at 12 eV. The surfaces of layers grown with JN2+/JCr = 1.7 consist of self-organized square-shaped mounds, due to kinetic roughening, with edges aligned along orthogonal 〈100〉 directions. The mounds have an av. peak-to-valley height 〈h〉 = 5.1 nm and an in-plane correlation length of 〈d〉 = 0.21 μm. The combination of at. shadowing by the mounds with low adatom mobility gave nanopipes extending along the growth direction. Increasing JN2+/JCr to 14 leads, due to increased adatom mobilities, to much smoother surfaces with 〈h〉 = 2.5 nm and 〈d〉 = 0.52 μm. Correspondingly, the nanopipe d. decreases from 870 to 270 μm-2 to <20 μm-2 as JN2+/JCr is increased from 1.7 to 6 to 10. The hardness of dense CrN(001) is 28.5 ± 1 GPa, but decreases to 22.5 ± 1 GPa for layers contg. significant nanopipe densities. The CrN(001) elastic modulus, 405 ± 15 GPa, room-temp. resistivity, 7.7 × 10-2 Ω cm, and relaxed lattice const., 0.4162 ± 0.0008 nm, are independent of JN2+/JCr.
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    Inumaru, K.; Koyama, K.; Imo-oka, N.; Yamanaka, S. Controlling the structural transition at the Néel point of CrN epitaxial thin films using epitaxial growth. Phys. Rev. B 2007, 75 (5), 054416,  DOI: 10.1103/PhysRevB.75.054416
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    Controlling the structural transition at the Neel point of CrN epitaxial thin films using epitaxial growth
    Inumaru, Kei; Koyama, Kunihiko; Imo-oka, Naoya; Yamanaka, Shoji
    Physical Review B: Condensed Matter and Materials Physics (2007), 75 (5), 054416/1-054416/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Chromium nitride (CrN) films were epitaxially grown on α-Al2O3(0001) and MgO (001) substrates by pulsed laser deposition at 973 K under nitrogen radical irradn., and the structural change of the films was investigated at around the Neel temp. of CrN (∼270 K) by temp.-controlled x-ray diffraction expts. Bulk cubic CrN is known to show monoclinic distortion below the Neel temp. The CrN film grown on MgO(001) with the CrN(001) plane parallel to the substrate surface, exhibited a clear structural change at around 260 K. On the other hand, on α-Al2O3(0001) substrates, the CrN phase grew with its (111) planes parallel to the substrate surface, and showed no structural change at the Neel temp. The different orientation of the epitaxial films can explain the different behavior of the films: The structural transition of bulk-CrN causes large variations in the interat. distances and bond angles on the (111) plane, but varies little on the (001) plane. In the case of thin films, the α-Al2O3(0001) substrate surface could prevent the (111)-oriented film from distorting its structure by fixing atom positions on the CrN(111) interfaces of the film. In accordance with the structural behavior of the films, the (111)-oriented CrN film on α-Al2O3(0001) showed no anomaly in its metallic cond. around the Neel temp. On the other hand, the (001)-oriented CrN on MgO showed a steep increase in elec. cond., accompanied by a lattice distortion below the Neel point. These results highlight an example that epitaxy could be used to control the existence of structural transitions, further accompanied by an antiferromagnetic ordering, which is closely related to the electronic properties of materials.
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    Constantin, C.; Haider, M. B.; Ingram, D.; Smith, A. R. Metal/semiconductor phase transition in chromium nitride (001) grown by rf-plasma-assisted molecular-beam epitaxy. Appl. Phys. Lett. 2004, 85 (26), 6371– 6373,  DOI: 10.1063/1.1836878
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    Metal/semiconductor phase transition in chromium nitride(001) grown by rf-plasma-assisted molecular-beam epitaxy
    Constantin, Costel; Haider, Muhammad B.; Ingram, David; Smith, Arthur R.
    Applied Physics Letters (2004), 85 (26), 6371-6373CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Structural and electronic properties of stoichiometric single-phase CrN(001) thin films grown on MgO(001) substrates by radio-frequency N plasma-assisted mol.-beam epitaxy, are investigated. In situ room-temp. scanning tunneling microscopy clearly shows the 1×1 at. periodicity of the crystal structure as well as long-range topog. distortions which are characteristic of a semiconductor surface. This semiconductor behavior is consistent with ex situ resistivity measurements over the range 285 K and higher, whereas below 260 K, metallic behavior is obsd. The resistivity-derived band gap for the high-temp. region, 71 meV, is consistent with the tunneling spectroscopy results. The obsd. electronic (semiconductor/metal) transition temp. coincides with the temps. of the known coincident magnetic (para-antiferro) and structural (cubic-orthorhombic) phase transitions.
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    Biswas, B.; Chakraborty, S.; Joseph, A.; Acharya, S.; Pillai, A. I. K.; Narayana, C.; Bhatia, V.; Garbrecht, M.; Saha, B. Secondary phase limited metal-insulator phase transition in chromium nitride thin films. Acta Mater. 2022, 227, 117737,  DOI: 10.1016/j.actamat.2022.117737
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    Secondary phase limited metal-insulator phase transition in chromium nitride thin films
    Biswas, Bidesh; Chakraborty, Sourjyadeep; Joseph, Anjana; Acharya, Shashidhara; Pillai, Ashalatha Indiradevi Kamalasanan; Narayana, Chandrabhas; Bhatia, Vijay; Garbrecht, Magnus; Saha, Bivas
    Acta Materialia (2022), 227 (), 117737CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
    Chromium nitride (CrN) is a well-known hard coating material that has found applications in abrasion and wear-resistant cutting tools, bearings, and tribol. applications due to its high hardness, high-temp. stability, and corrosion-resistant properties. In recent years, CrN has also attracted significant interest due to its high thermoelec. power factor, and for its unique and intriguing metal-insulator phase transition. While CrN bulk single-crystals exhibit the characteristic metal-insulator transition accompanied with structural (orthorhombic-to-rocksalt) and magnetic (antiferromagnetic-to-paramagnetic) transition at ∼260-280 K, observation of such phase transition in thin-film CrN has been scarce, and the exact cause of the absence of such transitions in several studies is not well-understood. In this work, the formation of the secondary metallic Cr2N phase during the growth is demonstrated to inhibit the observation of metal-insulator phase transition in CrN thin films. When the Cr-flux during deposition is reduced below a crit. limit, epitaxial and stoichiometric CrN thin film is obtained that reproducibly exhibits the phase transition. Annealing of the mixed-phase film inside reducing NH3 environment converts the Cr2N into CrN, and a discontinuity in the elec. resistivity at ∼277 K appears, which supports the underlying hypothesis. Demonstration of the inhibited metal-semiconductor phase transition in CrN due to the presence of secondary Cr2N phase is similar to the previous finding of the substantial change in its mech. hardness and redn. in thermoelec. properties. A clear demonstration of the origin behind the controversy of the metal-insulator transition in CrN thin films marks significant progress and would enable its nanoscale device realization.
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    Gharavi, M. A.; Gambino, D.; le Febvrier, A.; Eriksson, F.; Armiento, R.; Alling, B.; Eklund, P. High thermoelectric power factor of pure and vanadium-alloyed chromium nitride thin films. Mater. Today Commun. 2021, 28, 102493,  DOI: 10.1016/j.mtcomm.2021.102493
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    High thermoelectric power factor of pure and vanadium-alloyed chromium nitride thin films
    Gharavi, M. A.; Gambino, D.; le Febvrier, A.; Eriksson, F.; Armiento, R.; Alling, B.; Eklund, P.
    Materials Today Communications (2021), 28 (), 102493CODEN: MTCAC7; ISSN:2352-4928. (Elsevier Ltd.)
    Chromium-nitride based materials have shown unexpected promise as thermoelec. materials for, e.g., waste-heat harvesting. Here, CrN and (Cr,V)N thin films were deposited by reactive magnetron sputtering. Thermoelec. measurements of pure CrN thin films show a low elec. resistivity between 1.2 and 1.5 × 10-3 Ωcm and very high values of the Seebeck coeff. and thermoelec. power factor, in the range between 370-430 μV/K and 9-11 × 10-3 W/mK2, resp. Alloying of CrN films with small amts. (less than 15%) of vanadium results in cubic (Cr,V)N thin films. Vanadium decreases the elec. resistivity and yields power-factor values in the same range as pure CrN. The D. functional theory calcns. of sub-stoichiometric CrN1-δ and (Cr,V)N1-δ show that nitrogen vacancies and vanadium substitution both cause n-type cond. and features in the band structure typically correlated with a high Seebeck coeff. The results suggest that slight variations in nitrogen and vanadium content affect the power factor and offers a means of tailoring the power factor and thermoelec. figure of merit.
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    Bahk, J. H.; Bian, Z. X.; Shakouri, A. Electron energy filtering by a nonplanar potential to enhance the thermoelectric power factor in bulk materials. Phys. Rev. B 2013, 87 (7), 075204,  DOI: 10.1103/PhysRevB.87.075204
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    Electron energy filtering by a nonplanar potential to enhance the thermoelectric power factor in bulk materials
    Bahk, Je-Hyeong; Bian, Zhixi; Shakouri, Ali
    Physical Review B: Condensed Matter and Materials Physics (2013), 87 (7), 075204/1-075204/13CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We present a detailed theory on electron energy filtering by the nonplanar potential introduced by dispersed nanoparticles or impurities in bulk materials for enhancement of the thermoelec. power factor. When electrons with energies below a certain cut-off energy are prevented from participating in conduction through the material, the Seebeck coeff. and thus the thermoelec. power factor can be drastically enhanced. Instead of using planar heterostructures which require elaborate epitaxial techniques, we study embedded nanoparticles or impurities so that the conservation of lateral momentum limiting electron transport at heterointerfaces is no longer a limiting factor. Based on the Boltzmann transport equations under the relaxation time approxn., the optimal cut-off energy level that maximizes the power factor is calcd. to be a few kBT above the Fermi level, and is a function of the scattering parameter, Fermi level, and temp. The maximized power factor enhancement is quantified as a function of those parameters. The electronic thermal cond. and Lorenz no. are also shown to be suppressed by the electron filtering to further enhance the thermoelec. figure of merit. We find that the power factor of PbTe at 300 K could be enhanced by more than 120% when the cut-off energy level is 0.2 eV or higher and the carrier d. higher than 5× 1019 cm-3. Finally we propose the use of distributed resonant scatterings to partially realize the nonplanar electron filtering in bulk materials.
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    Raz, T.; Edelman, F.; Komem, Y.; Stölzer, M.; Zaumseil, P. Transport properties of boron-doped crystallized amorphous Si1–xGex films. J. Appl. Phys. 1998, 84 (8), 4343– 4350,  DOI: 10.1063/1.368697
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    Transport properties of boron-doped crystallized amorphous Si1-xGex films
    Raz, T.; Edelman, F.; Komem, Y.; Stolzer, M.; Zaumseil, P.
    Journal of Applied Physics (1998), 84 (8), 4343-4350CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Transport properties of crystd. amorphous Si1-xGex films, having different Ge content (x) and highly doped with boron were studied. The films were deposited by mol. beam at room temp. and subsequently annealed in vacuum at different temps. between 500 and 900 °C for 1 h. The microstructure of the crystd. Si1-xGex films was characterized by means of transmission electron microscopy, x-ray diffraction, and SEM. Measured transport properties included Hall hole concn. (pH), Hall mobility (μH), elec. cond. (σ), and the Seebeck coeff. (S), from which the "power factor" (S2σ) was evaluated. The results obtained for the Hall mobility of the Si1-xGex films are discussed on the basis of the carrier trapping model. The trapping state d. at the grain boundaries increases with increasing B concn., although it is not significantly dependent on Ge content. Consequently, the mobility energy barrier decreases with increasing B concn. and increasing Ge content. It was found that in all the studied Si1-xGex films, independent of x, the predominant scattering mechanism changes from acoustic phonon scattering to ionized impurity scattering with increasing the boron concn. from 5×1018 to 5×1020 cm-3. In addn., the Si1-xGex films demonstrate high elec. cond. as well as a high Seebeck coeff., after 1 h annealing at 600-800 °C, and thus exhibit a high "power factor" of the order of 6 μW/cm K2. Thus, these films have potential applications in thin-film thermoelec. devices.
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    Kucherov, Y.; Hagelstein, P.; Sevastyanenko, V.; Brown, H. L.; Guruswamy, S.; Wingert, W. Importance of barrier layers in thermal diodes for energy conversion. J. Appl. Phys. 2005, 97 (9), 094902  DOI: 10.1063/1.1886273
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    Importance of barrier layers in thermal diodes for energy conversion
    Kucherov, Yan; Hagelstein, Peter; Sevastyanenko, Victor; Brown, Harold L.; Guruswamy, Sivaraman; Wingert, Wayne
    Journal of Applied Physics (2005), 97 (9), 094902/1-094902/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    Very high thermal to elec. conversion efficiencies were reported for thermal diode structures in which a thin n-type emitter layer is formed on the hot side of a thick near-intrinsic thermoelec. semiconductor. The figure of merit derived from direct measurements of elec. parameters and heat flow was higher by as much as a factor of 8. The question of what phys. mechanisms are involved is of interest. The authors have conjectured that the short-circuit current injection in these expts. is due to a 2nd-order thermionic injection mechanism. Alternatively, the open-circuit voltage may be generated due to the presence of a p-type blocking layer between the emitter and the near-intrinsic bulk region. A p-type blocking layer is required for the effect, and the dependence of conversion efficiency on the blocking layer concn. and width was studied. The results are generally consistent with calcns. based on a nonlocal generalized Onsager-type transport model.
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Cite this: ACS Appl. Nano Mater. 2024, 7, 3, 3428–3435
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  • Abstract

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    Figure 1

    Figure 1. Schematic representations of (a) the nanocomposite coatings and (b) the deposition chamber with the magnetron assembly and the NP source in a secondary chamber.

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    Figure 2

    Figure 2. θ–2θ XRD patterns of the monolithic reference CrN film and the two nanocomposite coatings with 2 and 4 layers of Fe-NP.

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    Figure 3

    Figure 3. Top surface morphology of (a) test sample for NP for areal density/size estimation. The NPs were deposited on a Cr layer for a duration of 10 min. (b) Monolithic CrN films and the two nanocomposite coatings with (c) 2 and (d) 4 layers of Fe-NP.

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    Figure 4

    Figure 4. STEM analyses of the 4xFe-NP/CrN film in the two zones of interest. Zone 1 is for NPs at the bottom part of the film while zone 2 is for NPs at the top part of the film. (a–f) and (i–n) are EDS maps of the region of interest with the map corresponding to N, O, Fe, Cr, and Al. (g, o) STEM images of the zone of interest; (h, p) superposition of the EDS map on the STEM image.

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    Figure 5

    Figure 5. Temperature-dependent thermoelectric properties: (a) electrical conductivity and (b) Seebeck coefficient measured on the CrN reference coating (filled squares) and the two nanocomposite coatings (2xFe-NP/CrN, empty triangles, and 4xFe-NP/CrN, filled triangles).

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  • References

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      Thermoelecs. is attractive as a green and sustainable way for harnessing waste heat and cooling applications. Designing high performance thermoelecs. involves navigating the complex interplay between electronic and heat transports. This fundamentally involves understanding the scattering physics of both electrons and phonons, as well as maximizing symmetry-breaking in entropy and electronic transports. In the last two decades, thermoelecs. have progressed in leaps and bounds thanks to parallel advancements in scientific technologies and phys. understandings. Figure of merit zT of 2 and above have been consistently reported in various materials, esp. Chalcogenides. In this review, we provide a broad picture of phys. driven optimization strategies for thermoelec. materials, with emphasis on electronic transport aspect of inorg. materials. We also discuss and analyzes various newly coined metrics such as quality factors, electronics quality factor, electronic fitness function, weighted mobility, and Fermi surface complexity factor. More importantly, we look at the non-trivial interdependencies between various phys. parameters even at a very fundamental level. Moving forward, we discuss the outlook for the potential of 3D printing and device oriented research in thermoelecs. The intuition derived from this review will be useful not only to guide materials selection, but also research directions in the coming years.
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      We report the results of a combined exptl. and theor. study on nonstoichiometric CrN1+δ thin films grown by reactive magnetron sputtering on c-plane sapphire and MgO (100) substrates in an Ar/N2 gas mixt. using different percentages of N2. There is a transition from n-type to p-type behavior in the layers as a function of nitrogen concn. varying from 48 to 52 at. % in CrN films. The compositional change follows a similar trend for all substrates, with a N/Cr ratio increasing from approx. 0.7 to 1.06-1.11 by increasing the percentage of N2 in the gas flow ratio. As a result of the change in stoichiometry, the lattice parameter and the Seebeck coeff. increase together with the increase of N in CrN1+δ; in particular, the Seebeck value coeff. transitions from -50μVK-1 for CrN0.97 to +75μVK-1 for CrN1.1. D. functional theory calcns. show that Cr vacancies can account for the change in the Seebeck coeff., since they push the Fermi level down in the valence band, whereas N interstitial defects in the form of N2 dumbbells are needed to explain the increasing lattice parameter. Calcns. including both types of defects, which have a strong tendency to bind together, reveal a slight increase in the lattice parameter and a simultaneous formation of holes in the valence band. To explain the exptl. trends, we argue that both Cr vacancies and N2 dumbbells, possibly in combined configurations, are present in the films. We demonstrate the possibility of controlling the semiconducting behavior of CrN with intrinsic defects from n to p type, opening possibilities to integrate this compd. in energy-harvesting thermoelec. devices.
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      Biswas, B.; Chakraborty, S.; Chowdhury, O.; Rao, D.; Pillai, A. I. K.; Bhatia, V.; Garbrecht, M.; Feser, J. P.; Saha, B. In-plane Cr2N-CrN metal-semiconductor heterostructure with improved thermoelectric properties. Phys. Rev. Mater. 2021, 5 (11), 114605,  DOI: 10.1103/PhysRevMaterials.5.114605
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      In-plane Cr2N-CrN metal-semiconductor heterostructure with improved thermoelectric properties
      Biswas, Bidesh; Chakraborty, Sourjyadeep; Chowdhury, Ongira; Rao, Dheemahi; Pillai, Ashalatha Indiradevi Kamalasanan; Bhatia, Vijay; Garbrecht, Magnus; Feser, Joseph P.; Saha, Bivas
      Physical Review Materials (2021), 5 (11), 114605CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)
      Epitaxial metal-semiconductor heterostructures with suitable Schottky barrier can lead to high thermoelec. figure-of-merit (zT) due to selective filtering of low-energy electrons as well as reduced thermal cond. from phonon scattering at the interfaces. Lattice-matched vertical metal-semiconductor multilayer/superlattices as well as metallic nanoparticles embedded inside semiconducting hosts have been studied intensively to explore their thermoelec. properties. However, development of in-plane metal-semiconductor heterostructures and exploration of their phys. properties have remained elusive primarily due to the growth and fabrication challenges. In-plane heterostructures are expected to be more suitable for planar integration and should exhibit unique properties. In this work, we demonstrate an in-plane Cr2N-CrN metal-semiconductor heterostructure that exhibits an improved thermoelec. power factor. The in-plane heterostructure is deposited by controlling the Cr-flux during deposition that leads to an in-plane phase sepn. between the metallic-Cr2N and semiconducting CrN grains. Temp.-dependent elec. transport exhibits an Arrhenius-type thermal activation behavior with an activation energy of 70 meV, and an in-plane elec. cond. that is about two orders of magnitude higher than that of CrN. The Seebeck coeff. also remained moderately large at -150μV/K at 700K leading to a very large power factor of 2.1mW/mK2 at 700 K.
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    10. 10
      Quintela, C. X.; Rivadulla, F.; Rivas, J. Thermoelectric properties of stoichiometric and hole-doped CrN. Appl. Phys. Lett. 2009, 94 (15), 152103,  DOI: 10.1063/1.3120280
      10
      Thermoelectric properties of stoichiometric and hole-doped CrN
      Quintela, C. X.; Rivadulla, F.; Rivas, J.
      Applied Physics Letters (2009), 94 (15), 152103/1-152103/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We report the thermoelec. figure of merit of chromium nitride, CrN, and its optimization through hole-doping. CrN is a degenerate semiconductor with large thermoelec. power, reaching -185 μV/K at 420 K. The resistivity can be reduced through hole-doping in the series Cr1-xVxN, keeping a large thermopower. The thermal cond. of CrN is rather low compared to other transition-metal nitrides, reaching its min. value of 1.0 W/m K at 267 K. The largest ZT = 0.04 was measured for Cr0.9V0.1N at room temp. Our results suggest that CrN could be a good starting point for the design of a thermoelec. material with optimal mech. properties. (c) 2009 American Institute of Physics.
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    11. 11
      Quintela, C. X.; Rodríguez-González, B.; Rivadulla, F. Thermoelectric properties of heavy-element doped CrN. Appl. Phys. Lett. 2014, 104 (2), 022103,  DOI: 10.1063/1.4861845
      11
      Thermoelectric properties of heavy-element doped CrN
      Quintela, C. X.; Rodriguez-Gonzalez, B.; Rivadulla, F.
      Applied Physics Letters (2014), 104 (2), 022103/1-022103/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      CrN was doped with Mo and W to study the effect of heavy elements alloying on its thermoelec. properties. A spontaneous phase segregation into Mo- and W-rich regions was obsd. even at the lowest concns. probed at this work (≈1%). In the particular case of W, this segregation creates nanoinclusions into the Cr1-xWxN matrix, which results in a substantial decrease of the thermal cond. in the whole temp. range compared to undoped CrN. In addn., an increased hybridization of N:2p and 4d/5d orbitals with respect to Cr:3d decreases the elec. resistivity in lightly doped samples. This improves substantially the thermoelec. figure of merit with respect to the undoped compd., providing a pathway for further improvement of the thermoelec. performance of CrN. (c) 2014 American Institute of Physics.
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    12. 12
      Stockem, I.; Bergman, A.; Glensk, A.; Hickel, T.; Kormann, F.; Grabowski, B.; Neugebauer, J.; Alling, B. Anomalous Phonon Lifetime Shortening in Paramagnetic CrN Caused by Spin-Lattice Coupling: A Combined Spin and Ab Initio Molecular Dynamics Study. Phys. Rev. Lett. 2018, 121 (12), 125902,  DOI: 10.1103/PhysRevLett.121.125902
      12
      Anomalous Phonon Lifetime Shortening in Paramagnetic CrN Caused by Spin-Lattice Coupling: A Combined Spin and Ab Initio Molecular Dynamics Study
      Stockem, Irina; Bergman, Anders; Glensk, Albert; Hickel, Tilmann; Koermann, Fritz; Grabowski, Blazej; Neugebauer, Joerg; Alling, Bjoern
      Physical Review Letters (2018), 121 (12), 125902CODEN: PRLTAO; ISSN:1079-7114. (American Physical Society)
      We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio mol. dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temp. compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the exptl. obsd. unexpected temp. dependence of the thermal cond. of magnetic semiconductors above the magnetic ordering temp.
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    13. 13
      Rao, D.; Biswas, B.; Flores, E.; Chatterjee, A.; Garbrecht, M.; Koh, Y. R.; Bhatia, V.; Pillai, A. I. K.; Hopkins, P. E.; Martin-Gonzalez, M.; Saha, B. High mobility and high thermoelectric power factor in epitaxial ScN thin films deposited with plasma-assisted molecular beam epitaxy. Appl. Phys. Lett. 2020, 116 (15), 152103,  DOI: 10.1063/5.0004761
      13
      High mobility and high thermoelectric power factor in epitaxial ScN thin films deposited with plasma-assisted molecular beam epitaxy
      Rao, Dheemahi; Biswas, Bidesh; Flores, Eduardo; Chatterjee, Abhijit; Garbrecht, Magnus; Koh, Yee Rui; Bhatia, Vijay; Pillai, Ashalatha Indiradevi Kamalasanan; Hopkins, Patrick E.; Martin-Gonzalez, Marisol; Saha, Bivas
      Applied Physics Letters (2020), 116 (15), 152103CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Scandium nitride (ScN) is an emerging rock salt III-nitride semiconductor and has attracted significant interest in recent years for its potential thermoelec. applications as a substrate for high-quality epitaxial GaN growth and as a semiconducting component for epitaxial single-cryst. metal/semiconductor superlattices for thermionic energy conversion. Solid-soln. alloys of ScN with traditional III-nitrides such as AlxSc1-xN have demonstrated piezoelec. and ferroelec. properties and are actively researched for device applications. While most of these exciting developments in ScN research have employed films deposited using low-vacuum methods such as magnetron sputtering and phys. and chem. vapor depositions for thermoelec. applications and Schottky barrier-based thermionic energy conversion, it is necessary and important to avoid impurities, tune the carrier concns., and achieve high-mobility in epitaxial films. Here, the authors report the high-mobility and high-thermoelec. power factor in epitaxial ScN thin films deposited on MgO substrates by plasma-assisted mol. beam epitaxy. Microstructural characterization shows epitaxial 002 oriented ScN film growth on MgO (001) substrates. Elec. measurements demonstrated a high room-temp. mobility of 127 cm2/V s and temp.-dependent mobility in the temp. range of 50-400 K that is dominated by dislocation and grain boundary scattering. High mobility in ScN films leads to large Seebeck coeffs. (-175μV/K at 950 K) and, along with a moderately high elec. cond., a large thermoelec. power factor (2.3 x 10-3 W/m-K2 at 500 K) was achieved, which makes ScN a promising candidate for thermoelec. applications. The thermal cond. of the films, however, was found to be a bit large, which resulted in a max. figure-of-merit of 0.17 at 500 K. (c) 2020 American Institute of Physics.
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    14. 14
      Williams, W. S. The thermal conductivity of metallic ceramics. Jom-Journal of the Minerals Metals & Materials Society 1998, 50 (6), 62– 66,  DOI: 10.1007/s11837-998-0131-y
      There is no corresponding record for this reference.
    15. 15
      Faleev, S. V.; Léonard, F. Theory of enhancement of thermoelectric properties of materials with nanoinclusions. Phys. Rev. B 2008, 77 (21), 214304,  DOI: 10.1103/PhysRevB.77.214304
      15
      Theory of enhancement of thermoelectric properties of materials with nanoinclusions
      Faleev, Sergey V.; Leonard, Francois
      Physical Review B: Condensed Matter and Materials Physics (2008), 77 (21), 214304/1-214304/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Based on the concept of band bending at metal/semiconductor interfaces as an energy filter for electrons, we present a theory for the enhancement of the thermoelec. properties of semiconductor materials with metallic nanoinclusions. We show that the Seebeck coeff. can be significantly increased due to a strongly energy-dependent electronic scattering time. By including phonon scattering, we find that the enhancement of ZT due to electron scattering is important for high doping, while at low doping it is primarily due to a decrease in the phonon thermal cond.
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    16. 16
      Li, J. Unexpected boost of thermoelectric performance by magnetic nanoparticles. Sci. China Mater. 2017, 60 (10), 1023– 1024,  DOI: 10.1007/s40843-017-9129-y
      16
      Unexpected boost of thermoelectric performance by magnetic nanoparticles
      Li, Jiangyu
      Science China Materials (2017), 60 (10), 1023-1024CODEN: SCMCDB; ISSN:2095-8226. (Science China Press)
      There is no expanded citation for this reference.
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    17. 17
      Zhao, W.; Liu, Z.; Sun, Z.; Zhang, Q.; Wei, P.; Mu, X.; Zhou, H.; Li, C.; Ma, S.; He, D.; Ji, P.; Zhu, W.; Nie, X.; Su, X.; Tang, X.; Shen, B.; Dong, X.; Yang, J.; Liu, Y.; Shi, J. Superparamagnetic enhancement of thermoelectric performance. Nature 2017, 549 (7671), 247– 251,  DOI: 10.1038/nature23667
      17
      Superparamagnetic enhancement of thermoelectric performance
      Zhao, Wenyu; Liu, Zhiyuan; Sun, Zhigang; Zhang, Qingjie; Wei, Ping; Mu, Xin; Zhou, Hongyu; Li, Cuncheng; Ma, Shifang; He, Danqi; Ji, Pengxia; Zhu, Wanting; Nie, Xiaolei; Su, Xianli; Tang, Xinfeng; Shen, Baogen; Dong, Xiaoli; Yang, Jihui; Liu, Yong; Shi, Jing
      Nature (London, United Kingdom) (2017), 549 (7671), 247-251CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      The ability to control chem. and phys. structuring at the nanometer scale is important for developing high-performance thermoelec. materials. Progress in this area has been achieved mainly by enhancing phonon scattering and consequently decreasing the thermal cond. of the lattice through the design of either interface structures at nanometer or mesoscopic length scales or multiscale hierarchical architectures. A nanostructuring approach that enables electron transport as well as phonon transport to be manipulated could potentially lead to further enhancements in thermoelec. performance. Here, the authors show that by embedding nanoparticles of a soft magnetic material in a thermoelec. matrix they achieve dual control of phonon- and electron-transport properties. The properties of the nanoparticles-in particular, their superparamagnetic behavior (in which the nanoparticles can be magnetized similarly to a paramagnet under an external magnetic field)-lead to three kinds of thermoelectromagnetic effect: charge transfer from the magnetic inclusions to the matrix; multiple scattering of electrons by superparamagnetic fluctuations; and enhanced phonon scattering as a result of both the magnetic fluctuations and the nanostructures themselves. The authors show that together these effects can effectively manipulate electron and phonon transport at nanometer and mesoscopic length scales and thereby improve the thermoelec. performance of the resulting nanocomposites.
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    18. 18
      Sumithra, S.; Takas, N. J.; Misra, D. K.; Nolting, W. M.; Poudeu, P. F. P.; Stokes, K. L. Enhancement in Thermoelectric Figure of Merit in Nanostructured Bi2Te3 with Semimetal Nanoinclusions. Adv. Energy Mater. 2011, 1 (6), 1141– 1147,  DOI: 10.1002/aenm.201100338
      18
      Enhancement in thermoelectric figure of merit in nanostructured Bi2Te3 with semimetal nanoinclusions
      Sumithra, S.; Takas, Nathan J.; Misra, Dinesh K.; Nolting, Westly M.; Poudeu, P. F. P.; Stokes, Kevin L.
      Advanced Energy Materials (2011), 1 (6), 1141-1147CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
      The effect of Bi (semimetal) nanoinclusions in nanostructured Bi2Te3 matrixes is investigated. Bismuth nanoparticles synthesized by a low temp. solvothermal method are incorporated into Bi2Te3 matrix phases, synthesized by planetary ball milling. High d. pellets of the Bi nanoparticle/Bi2Te3 nanocomposites are created by hot pressing the powders at 200 °C and 100 MPa. The effect of different vol. fractions (0-7%) of Bi semimetal nanoparticles on the Seebeck coeff., elec. cond., thermal cond. and carrier concn. is reported. Our results show that the incorporation of semimetal nanoparticles results in a redn. in the lattice thermal cond. in all the samples. A significant enhancement in power factor is obsd. for Bi nanoparticle vol. fraction of 5% and 7%. We show that it is possible to reduce the lattice thermal cond. and increase the power factor resulting in an increase in figure of merit by a factor of 2 (from ZT = 0.2 to 0.4). Seebeck coeff. and elec. cond. as a function of carrier concn. data are consistent with the electron filtering effect, where low-energy electrons are preferentially scattered by the barrier potentials set up at the semimetal nanoparticle/semiconductor interfaces.
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    19. 19
      Theja, V. C. S.; Karthikeyan, V.; Assi, D. S.; Roy, V. A. L. Insights into the Classification of Nanoinclusions of Composites for Thermoelectric Applications. Acs Applied Electronic Materials 2022, 4 (10), 4781– 4796,  DOI: 10.1021/acsaelm.2c00617
      19
      Insights into the Classification of Nanoinclusions of Composites for Thermoelectric Applications
      Theja, Vaskuri C. S.; Karthikeyan, Vaithinathan; Assi, Dani S.; Roy, Vellaisamy A. L.
      ACS Applied Electronic Materials (2022), 4 (10), 4781-4796CODEN: AAEMBP; ISSN:2637-6113. (American Chemical Society)
      A review. Thermoelec. composites are known for their enhanced power conversion performance via interfacial engineering and intensified mech., structural, and thermal properties. However, the selection of these nanoinclusions, for example, their type, size effect, vol. fraction, distribution uniformity, coherency with host, carrier dynamics, and phys. stability, plays a crucial role in modifying the host material thermoelec. properties. In this Review, we classify the nanoinclusions into five types: carbon allotropes, secondary thermoelec. phases, metallic materials, insulating oxides, and others. On the basis of the classification, we discuss the mechanisms involved in improving the ZT of nanocomposites involving redn. of thermal cond. (κ) by phonon scattering, improving the Seebeck coeff. (α) via energy filtering effect and the elec. cond. (σ) by carrier injection or carrier channeling. Comprehensibly, we validate that adding nanoinclusions with high elec. and low thermal cond. as compared to the matrix material is the best way to optimize the interlocked thermoelec. parameters. Thus, collective doping and nanoinclusions in thermoelec. materials is the best possible soln. to achieve a higher power conversion efficiency equiv. to other renewable energy technologies.
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    20. 20
      Lee, E.; Ko, J.; Kim, J. Y.; Seo, W. S.; Choi, S. M.; Lee, K. H.; Shim, W.; Lee, W. Enhanced thermoelectric properties of Au nanodot-included BiTe nanotube composites. J. Mater. Chem. C 2016, 4 (6), 1313– 1319,  DOI: 10.1039/C5TC03934G
      20
      Enhanced thermoelectric properties of Au nanodot-included Bi2Te3 nanotube composites
      Lee, Eunsil; Ko, Jieun; Kim, Jong-Young; Seo, Won-Seon; Choi, Soon-Mok; Lee, Kyu Hyoung; Shim, Wooyoung; Lee, Wooyoung
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (6), 1313-1319CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      Herein, the authors report on a scalable synthesis of Au nanodot (Au-ND)/Bi2Te3 nanotube (BT-NT) nanocomposites by the bottom-up synthesis of hybrid raw materials and subsequent spark plasma sintering, and their thermoelec. properties were systematically compared with those of Au-doped Bi2Te3 compds. The Au nanodots were included as seeds and co-crystd. in the crystal growth of BT-NTs, which were well-dispersed in the Bi2Te3 matrix as nanoinclusions (10-20 nm). The thermoelec. performance (ZT) of the Au-ND/BT-NT nanocomposite is enhanced by ∼67%, compared to pristine Bi2Te3 due to electron energy filtering and phonon scattering effects in the presence of embedded Au-NDs. The resulting compd. showed an enhanced power factor (23.0 × 10-4 W m-1 K-2 @ 440 K, 27% improvement) and a reduced lattice thermal cond. (0.47 W m-1 K-1 @ 440 K, 22% redn.). The peak ZT value of the present compd. (0.95 @ 480 K) is larger than that of n-type single cryst. Bi2(Te,Se)3, which is one of the highest among the reported values for n-type Bi2Te3-based materials synthesized using a soft chem. route.
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    21. 21
      Shu, R.; Han, Z.; Elsukova, A.; Zhu, Y.; Qin, P.; Jiang, F.; Lu, J.; Persson, P. O. A.; Palisaitis, J.; le Febvrier, A.; Zhang, W.; Cojocaru-Miredin, O.; Yu, Y.; Eklund, P.; Liu, W. Solid-State Janus Nanoprecipitation Enables Amorphous-Like Heat Conduction in Crystalline Mg(3) Sb(2) -Based Thermoelectric Materials. Adv. Sci. (Weinh) 2022, 9 (25), e2202594  DOI: 10.1002/advs.202202594
      There is no corresponding record for this reference.
    22. 22
      Bueno Villoro, R.; Wood, M.; Luo, T.; Bishara, H.; Abdellaoui, L.; Zavanelli, D.; Gault, B.; Snyder, G. J.; Scheu, C.; Zhang, S. Fe segregation as a tool to enhance electrical conductivity of grain boundaries in Ti(Co,Fe)Sb half Heusler thermoelectrics. Acta Mater. 2023, 249, 118816,  DOI: 10.1016/j.actamat.2023.118816
      22
      Fe segregation as a tool to enhance electrical conductivity of grain boundaries in Ti(Co,Fe)Sb half Heusler thermoelectrics
      Bueno Villoro, Ruben; Wood, Maxwell; Luo, Ting; Bishara, Hanna; Abdellaoui, Lamya; Zavanelli, Duncan; Gault, Baptiste; Snyder, Gerald Jeffrey; Scheu, Christina; Zhang, Siyuan
      Acta Materialia (2023), 249 (), 118816CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
      Complex microstructures are found in many thermoelec. materials and can be used to optimize their transport properties. Grain boundaries in particular scatter phonons, but they often impede charge carrier transfer at the same time. Designing grain boundaries in order to offer a conductive path for electrons is a substantial opportunity to optimize thermoelecs. Here, we demonstrate in TiCoSb half Heusler compds. that Fe-dopants segregate to grain boundaries and simultaneously increase the elec. cond. and reduce the thermal cond. To explain these phenomena, three samples with different grain sizes are synthesized and a model is developed to relate the elec. cond. with the area fraction of grain boundaries. The elec. cond. of grain interior and grain boundaries is calcd. and the at. structure of grain boundaries is studied in detail. Segregation engineering in fine-grained thermoelecs. is proposed as a new design tool to optimize transport properties while achieving a lower thermal cond.
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    23. 23
      Steinhoff, M. K.; Holzapfel, D. M.; Karimi Aghda, S.; Neuß, D.; Pöllmann, P. J.; Hans, M.; Primetzhofer, D.; Schneider, J. M.; Azina, C. Ag Surface and Bulk Segregations in Sputtered ZrCuAlNi Metallic Glass Thin Films. Materials 2022, 15 (5), 1635,  DOI: 10.3390/ma15051635
      23
      Ag Surface and Bulk Segregations in Sputtered ZrCuAlNi Metallic Glass Thin Films
      Steinhoff, Michael K.; Holzapfel, Damian M.; Karimi Aghda, Soheil; Neuss, Deborah; Poellmann, Peter J.; Hans, Marcus; Primetzhofer, Daniel; Schneider, Jochen M.; Azina, Clio
      Materials (2022), 15 (5), 1635CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
      We report on the formation of Ag-contg. ZrCuAlNi thin film metallic glass (nano)composites by a hybrid direct-current magnetron sputtering and high-power pulsed magnetron sputtering process. The effects of Ag content, substrate temp. and substrate bias potential on the phase formation and morphol. of the nanocomposites were investigated. While applying a substrate bias potential did not strongly affect the morphol. evolution of the films, the Ag content dictated the size and distribution of Ag surface segregations. The films deposited at low temps. were characterized by strong surface segregations, formed by coalescence and Ostwald ripening, while the vol. of the films remained featureless. At higher deposition temp., elongated Ag segregations were obsd. in the bulk and a continuous Ag layer was formed at the surface as a result of thermally enhanced surface diffusion. While microstructural observations have allowed identifying both surface and bulk segregations, an indirect method for detecting the presence of Ag segregations is proposed, by measuring the elec. resistivity of the films.
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    24. 24
      Moya, J. S.; Lopez-Esteban, S.; Pecharromán, C. The challenge of ceramic/metal microcomposites and nanocomposites. Prog. Mater. Sci. 2007, 52 (7), 1017– 1090,  DOI: 10.1016/j.pmatsci.2006.09.003
      24
      The challenge of ceramic/metal microcomposites and nanocomposites
      Moya, Jose S.; Lopez-Esteban, Sonia; Pecharroman, Carlos
      Progress in Materials Science (2007), 52 (7), 1017-1090CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
      A review. It is increasingly being recognized that new applications for materials require functions and properties that are not achievable with monolithic materials. The combination of dissimilar materials for these new applications creates interfaces whose properties and processing need to be understood before they can be applied com. In the present review paper we try to emphasize the important role and challenges of ceramic/metal micro/nanocomposites in the new technologies. In this respect we will study and review the exotic effects of metal particles embedded into matrix ceramics due to the dissimilar properties of the components, percolation laws, and the nature of the interfaces. From an electromagnetic point of view we have underlined the enormous enhancement of permittivity in the proximity of the percolation threshold, assocd. with an induced soft mode similar to para-ferroelec. transition. From a mech. standpoint, the synergic effect of nanometer size, clustering addressed by the percolation theory and ceramic/metal interface features produces an unexpected enhancement in the hardness of the composite giving rise to superhard materials.
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    25. 25
      Rane, A. V.; Kanny, K.; Abitha, V. K.; Thomas, S. Chapter 5 - Methods for Synthesis of Nanoparticles and Fabrication of Nanocomposites. In Synthesis of Inorganic Nanomaterials; Mohan Bhagyaraj, S., Oluwafemi, O. S., Kalarikkal, N., Thomas, S., Eds.; Woodhead Publishing: 2018; pp 121– 139.
      There is no corresponding record for this reference.
    26. 26
      Villamayor, M. M. S.; Keraudy, J.; Shimizu, T.; Viloan, R. P. B.; Boyd, R.; Lundin, D.; Greene, J. E.; Petrov, I.; Helmersson, U. Low temperature (Ts/Tm < 0.1) epitaxial growth of HfN/MgO(001) via reactive HiPIMS with metal-ion synchronized substrate bias. J. Vac. Sci. Technol. A 2018, 36 (6), 061511,  DOI: 10.1116/1.5052702
      26
      Low temperature (Ts/Tm < 0.1) epitaxial growth of HfN/MgO(001) via reactive HiPIMS with metal-ion synchronized substrate bias
      Villamayor, Michelle Marie S.; Keraudy, Julien; Shimizu, Tetsuhide; Viloan, Rommel Paulo B.; Boyd, Robert; Lundin, Daniel; Greene, Joseph E.; Petrov, Ivan; Helmersson, Ulf
      Journal of Vacuum Science & Technology, A: Vacuum, Surfaces, and Films (2018), 36 (6), 061511/1-061511/11CODEN: JVTAD6; ISSN:0734-2101. (American Institute of Physics)
      Low-temp. epitaxial growth of refractory transition-metal nitride thin films by means of phys. vapor deposition has been a recurring theme in advanced thin-film technol. for several years. In the present study, 150-nm-thick epitaxial HfN layers are grown on MgO by reactive high-power impulse magnetron sputtering (HiPIMS) with no external substrate heating. In the present expts., a neg. bias of 100 V is applied to the substrate, either continuously during the entire deposition or synchronized with the metal-rich portion of the ion flux. Two different sputtering-gas mixts., Ar/N2 and Kr/N2, are employed in order to probe effects assocd. with the noble-gas mass and ionization potential. The combination of x-ray diffraction, high-resoln. reciprocal-lattice maps, and high-resoln. cross-sectional transmission electron microscopy analyses establishes that all HfN films have a cube-on-cube orientational relationship with the substrate,. In distinct contrast, layers grown in Kr/N2 with the substrate bias synchronized to the metal-ion-rich portion of HiPIMS pulses have much lower mosaicity, no measurable inert-gas incorporation, and a hardness of 25.7 GPa, in good agreement with the results for epitaxial HfN(001) layers grown at Ts = 650°C (Ts/Tm = 0.26). The room-temp. film resistivity is 70μΩ cm, which is 3.2-10 times lower than reported values for polycryst.-HfN layers grown at Ts = 400°C. (c) 2018 American Institute of Physics.
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    27. 27
      le Febvrier, A.; Landälv, L.; Liersch, T.; Sandmark, D.; Sandström, P.; Eklund, P. An upgraded ultra-high vacuum magnetron-sputtering system for high-versatility and software-controlled deposition. Vacuum 2021, 187, 110137,  DOI: 10.1016/j.vacuum.2021.110137
      27
      An upgraded ultra-high vacuum magnetron-sputtering system for high-versatility and software-controlled deposition
      le Febvrier, Arnaud; Landaelv, Ludvig; Liersch, Thomas; Sandmark, David; Sandstroem, Per; Eklund, Per
      Vacuum (2021), 187 (), 110137CODEN: VACUAV; ISSN:0042-207X. (Elsevier Ltd.)
      Magnetron sputtering is a widely used phys. vapor deposition technique. Reactive sputtering is used for the deposition of, e.g, oxides, nitrides and carbides. In fundamental research, versatility is essential when designing or upgrading a deposition chamber. Furthermore, automated deposition systems are the norm in industrial prodn., but relatively uncommon in lab.-scale systems used primarily for fundamental research. Combining automatization and computerized control with the required versatility for fundamental research constitutes a challenge in designing, developing, and upgrading lab. deposition systems. The present article provides a detailed description of the design of a lab-scale deposition chamber for magnetron sputtering used for the deposition of metallic, oxide, nitride and oxynitride films with automated controls, dc or pulsed bias, and combined with a coil to enhance the plasma d. near the substrate. LabVIEW software (provided as Supplementary Information) has been developed for a high degree of computerized or automated control of hardware and processes control and logging of process details.
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    28. 28
      Ström, P.; Primetzhofer, D. Ion beam tools for nondestructive in-situ and in-operando composition analysis and modification of materials at the Tandem Laboratory in Uppsala. Journal of Instrumentation 2022, 17 (04), P04011,  DOI: 10.1088/1748-0221/17/04/P04011
      There is no corresponding record for this reference.
    29. 29
      Pottier, L. Micrometer Scale Visualization of Thermal Waves by Photoreflectance Microscopy. Appl. Phys. Lett. 1994, 64 (13), 1618– 1619,  DOI: 10.1063/1.111856
      29
      Micrometer scale visualization of thermal waves by photoreflectance microscopy
      Pottier, L.
      Applied Physics Letters (1994), 64 (13), 1618-19CODEN: APPLAB; ISSN:0003-6951.
      A novel approach of photoreflectance microscopy is proposed that provides a direct visualization of the phase contour lines of the thermal wave. The method is applicable to (possibly heterogeneous) samples of mediocre polish. In a locally homogeneous region it yields the local thermal diffusivity.
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    30. 30
      Li, B. C.; Roger, J. P.; Pottier, L.; Fournier, D. Complete thermal characterization of film-on-substrate system by modulated thermoreflectance microscopy and multiparameter fitting. J. Appl. Phys. 1999, 86 (9), 5314– 5316,  DOI: 10.1063/1.371520
      30
      Complete thermal characterization of film-on-substrate system by modulated thermoreflectance microscopy and multiparameter fitting
      Li, Bincheng; Roger, J. P.; Pottier, L.; Fournier, D.
      Journal of Applied Physics (1999), 86 (9), 5314-5316CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Modulated thermoreflectance microscopy is applied to a complete thermal characterization of a thin film of gold (78 nm) or YBaCuO (300 nm) on a LaAlO3 substrate. The phase profile, measured at several modulation frequencies covering an appropriate range, is fitted with a rigorous thermal diffusion model. This leads to a simultaneous estn. of the thermal diffusivities of the film and the substrate, as well as of the thermal film/substrate boundary resistance. The estd. values for the gold film sample are, resp., 4.3 × 10-6 m2s-1 (substrate diffusivity), 1.0 × 10-4 m2s-1 (film diffusivity), and 1.0 × 10-8 m2KW-1 (thermal boundary resistance), while for the thermally anisotropic YBaCuO film sample are, 4.1 × 10-6 m2s-1, 3.5 × 10-6 m2s-1 (in-plane diffusivity), and 8.0 × 10-8 m2KW-1, resp.
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    31. 31
      Frétigny, C.; Duquesne, J. Y.; Fournier, D.; Xu, F. Thermal insulating layer on a conducting substrate. Analysis of thermoreflectance experiments. J. Appl. Phys. 2012, 111 (8), 084313,  DOI: 10.1063/1.3702823
      31
      Thermal insulating layer on a conducting substrate. Analysis of thermoreflectance experiments
      Fretigny, C.; Duquesne, J.-Y.; Fournier, D.; Xu, F.
      Journal of Applied Physics (2012), 111 (8), 084313/1-084313/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Thermoreflectance expts. are sensitive to the thermal properties of thin layers deposited on substrates (cond. and diffusivity). However, retrieving these properties from exptl. data remains a difficult issue. The case of a conducting layer deposited on an insulating substrate was studied previously. We present here a math. and exptl. anal. of the thermoreflectance response in the opposite case: an insulating layer on a conducting substrate. We show theor. that cond. and diffusivity can be detd. independently thanks to a comparison with the substrate. The method is applied to expts. performed on a silicon substrate covered with a thin layer deposited by sputtering a titanium target. (c) 2012 American Institute of Physics.
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    32. 32
      Frétigny, C.; Roger, J. P.; Reita, V.; Fournier, D. Analytical inversion of photothermal measurements: Independent determination of the thermal conductivity and diffusivity of a conductive layer deposited on an insulating substrate. J. Appl. Phys. 2007, 102 (11), 116104,  DOI: 10.1063/1.2818102
      32
      Analytical inversion of photothermal measurements: Independent determination of the thermal conductivity and diffusivity of a conductive layer deposited on an insulating substrate
      Fretigny, Christian; Roger, Jean Paul; Reita, Valerie; Fournier, Daniele
      Journal of Applied Physics (2007), 102 (11), 116104/1-116104/3CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The long distance behavior of the surface temp. wave in a thermoreflectance microscopy expt. is established for a conductive layer deposited on an insulating substrate. At large distance from the point source, heat is confined, so the amplitude decrease is lower than for a bulk sample. From the slopes which appear on the phase and on the log scale amplitude, a procedure is proposed to ext., sep., the thermal diffusivity and cond. of the layer, taking into account data obtained at different modulation frequencies. Exptl. results are presented which confirm the validity of the method.
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    33. 33
      Alling, B.; Marten, T.; Abrikosov, I. A. Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN. Phys. Rev. B 2010, 82 (18), 184430,  DOI: 10.1103/PhysRevB.82.184430
      33
      Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN
      Alling, B.; Marten, T.; Abrikosov, I. A.
      Physical Review B: Condensed Matter and Materials Physics (2010), 82 (18), 184430/1-184430/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We use first-principles calcns. to study the effect of magnetic disorder and electron correlations on the structural and thermodn. properties of CrN. We illustrate the usability of a special quasirandom structure supercell treatment of the magnetic disorder by comparing with CPA calcns. and with a complementary magnetic sampling method. The need of a treatment of electron correlations effects beyond the local d. approxn. is proven by a comparison of LDA+U calcns. of structural and electronic properties with exptl. results. When magnetic disorder and strong electron correlations are taken into account simultaneously, pressure- and temp.-induced structural and magnetic transitions in CrN can be understood.
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    34. 34
      Kerdsongpanya, S.; Sun, B.; Eriksson, F.; Jensen, J.; Lu, J.; Koh, Y. K.; Nong, N. V.; Balke, B.; Alling, B.; Eklund, P. Experimental and theoretical investigation of CrScN solid solutions for thermoelectrics. J. Appl. Phys. 2016, 120 (21), 215103,  DOI: 10.1063/1.4968570
      34
      Experimental and theoretical investigation of Cr1-xScxN solid solutions for thermoelectrics
      Kerdsongpanya, Sit; Sun, Bo; Eriksson, Fredrik; Jensen, Jens; Lu, Jun; Koh, Yee Kan; Nong, Ngo Van; Balke, Benjamin; Alling, Bjoern; Eklund, Per
      Journal of Applied Physics (Melville, NY, United States) (2016), 120 (21), 215103/1-215103/10CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      The ScN- and CrN-based transition-metal nitrides have recently emerged as a novel and unexpected class of materials for thermoelecs. These materials constitute well-defined model systems for investigating mixing thermodn., phase stability, and band structure aiming for property tailoring. Here, we demonstrate an approach to tailor their thermoelec. properties by solid solns. The trends in mixing thermodn. and densities-of-states (DOS) of rocksalt-Cr1-xScxN solid solns. (0 ≤ x ≤ 1) are investigated by first-principles calcns., and Cr1-xScxN thin films are synthesized by magnetron sputtering. Pure CrN exhibits a high power factor, 1.7 × 10-3 W m-1 K-2 at 720 K, enabled by a high electron concn. thermally activated from N vacancies. Disordered rocksalt-Cr1-xScxN solid solns. are thermodynamically stable, and calcd. DOS suggest the possibility for power-factor improvement by Sc3d orbital delocalization on Cr3d electrons giving decreasing elec. resistivity, while localized Cr3d orbitals with a large DOS slope may yield an improved Seebeck coeff. Sc-rich solid solns. show a large improvement in power factor compared to pure ScN, and all films have power factors above that expected from the rule-of-mixt. These results corroborate the theor. predictions and enable tailoring and understanding of structure-transport-property correlations of Cr1-xScxN. (c) 2016 American Institute of Physics.
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    35. 35
      le Febvrier, A.; Tureson, N.; Stilkerich, N.; Greczynski, G.; Eklund, P. Effect of impurities on morphology, growth mode, and thermoelectric properties of (1 1 1) and (0 0 1) epitaxial-like ScN films. J. Phys. D: Appl. Phys. 2019, 52 (3), 035302,  DOI: 10.1088/1361-6463/aaeb1b
      35
      Effect of impurities on morphology, growth mode, and thermoelectric properties of (111) and (001) epitaxial-like ScN films
      Le Febvrier, Arnaud; Tureson, Nina; Stilkerich, Nina; Greczynski, Grzegorz; Eklund, Per
      Journal of Physics D: Applied Physics (2019), 52 (3), 035302/1-035302/11CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
      ScN is an emerging semiconductor with an indirect bandgap. It has attracted attention for its thermoelec. properties, use as seed layers, and for alloys for piezoelec.application. ScN and other transition metal nitride semiconductors used for their interesting elec. properties are sensitive to contaminants, such as oxygen or fluorine. In this present article, the influence of depositions conditions on the amt. of oxygen contaminants incorporated in ScN films were investigated and their effects on the elec. properties (elec. resistivity and Seebeck coeff.) were studied. Epitaxial-like films of thickness 125 ± 5 nm to 155 ± 5 nm were deposited by DC-magnetron sputtering on c-plane Al2O3, MgO(1 1 1) and r-plane Al2O3 at substrate temps.ranging from 700°C to 950°C. The amt. of oxygen contaminants in the film, dissolved into ScN or as an oxide, was related to the adatom mobility during growth, which is affected by the deposition temp.and the presence of twin domain growth. The lowest values of elec. resistivity of 50μO cm were obtained on ScN(1 1 1)/ MgO(1 1 1) and on ScN(0 0 1)/r-plane Al2O3 grown at 950°C with no twin domains and the lowest amt. of oxygen contaminant. At the best, the films exhibited an elec. resistivity of 50μO cm with Seebeck coeff. values maintained at -40μV K-1, thus a power factor estd. at 3.2 × 10-3 W m-1 K-2 (at room temp.).
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    36. 36
      Tan, S.; Zhang, X.; Wu, X.; Fang, F.; Jiang, J. Comparison of chromium nitride coatings deposited by DC and RF magnetron sputtering. Thin Solid Films 2011, 519 (7), 2116– 2120,  DOI: 10.1016/j.tsf.2010.10.067
      36
      Comparison of chromium nitride coatings deposited by DC and RF magnetron sputtering
      Tan, Shuyong; Zhang, Xuhai; Wu, Xiangjun; Fang, Feng; Jiang, Jianqing
      Thin Solid Films (2011), 519 (7), 2116-2120CODEN: THSFAP; ISSN:0040-6090. (Elsevier B.V.)
      Chromium nitride coatings were deposited by DC and RF reactive magnetron sputtering on AISI 304 stainless steels without substrate heating. A Cr2N phase was formed in the RF sputtered coatings with a low N2 flow content ranging within 30-50%. A NaCl type CrNx phase was obtained by DC magnetron sputtering with different N2 flow contents. The coating hardness increased with the increase of the N2 flow content. When the coatings deposited with the same N2 flow content were compared, the hardness of the RF sputtered CrNx was higher than that of the DC sputtered CrNx, which was mainly due to the distinct difference between the dense structure (RF process) and the porous structure (DC process). The RF sputtered CrNx coatings showed an excellent adhesion strength as compared to the DC sputtered coatings. By selecting the deposition method and optimizing the N2 flow content, CrNx coatings with a preferred microstructure could be obtained, which would be a candidate material for research and applications in nano-science.
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    37. 37
      Quintela, C. X.; Podkaminer, J. P.; Luckyanova, M. N.; Paudel, T. R.; Thies, E. L.; Hillsberry, D. A.; Tenne, D. A.; Tsymbal, E. Y.; Chen, G.; Eom, C. B.; Rivadulla, F. Epitaxial CrN thin films with high thermoelectric figure of merit. Adv. Mater. 2015, 27 (19), 3032– 7,  DOI: 10.1002/adma.201500110
      37
      Epitaxial CrN Thin Films with High Thermoelectric Figure of Merit
      Quintela, Camilo X.; Podkaminer, Jacob P.; Luckyanova, Maria N.; Paudel, Tula R.; Thies, Eric L.; Hillsberry, Daniel A.; Tenne, Dmitri A.; Tsymbal, Evgeny Y.; Chen, Gang; Eom, Chang-Beom; Rivadulla, Francisco
      Advanced Materials (Weinheim, Germany) (2015), 27 (19), 3032-3037CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In this Communication, we demonstrate that rock salt CrN shows intrinsic lattice instabilities that suppress its thermal cond. Using ab initio calcns., we detd. that the origin of these instabilities is similar to that obsd. in IV-VI compds. with RB states. Through the fabrication of high-quality epitaxial (OOl) CrN thin films we report a =250% increase in the zT at room temp. compared* to bulk CrN. These results, along with its high thermal stability, resistance to corrosion, and exceptional mech. properties, make CrN a promising n-type material for high-temp. TE applications.
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    38. 38
      Zhang, X. Y.; Chawla, J. S.; Howe, B. M.; Gall, D. Variable-range hopping conduction in epitaxial CrN(001). Phys. Rev. B 2011, 83 (16), 165205,  DOI: 10.1103/PhysRevB.83.165205
      38
      Variable-range hopping conduction in epitaxial CrN(001)
      Zhang, X. Y.; Chawla, J. S.; Howe, B. M.; Gall, D.
      Physical Review B: Condensed Matter and Materials Physics (2011), 83 (16), 165205/1-165205/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Epitaxial CrN(001) layers, grown by d.c. magnetron sputtering on MgO(001) substrates at growth temps. Ts = 550-850°, exhibit electronic transport that is dominated by variable-range hopping (VRH) at temps. <120 K. A transition from Efros-Shklovskii to Mott VRH at 30 ± 10 K is well described by a universal scaling relation. The localization length decreases from 1.3 nm at Ts = 550° to 0.9 nm for Ts = 600-750°, but increases again to 1.9 nm for Ts = 800-850°, which is attributed to changes in the d. of localized states assocd. with N vacancies that form due to kinetic barriers for incorporation and enhanced desorption at low and high Ts, resp. The low-temp. transport data provide lower limits for the CrN effective electron mass of 4.9me, the donor ionization energy of 24 meV, and the crit. vacancy concn. for the metal-insulator transition of 8.4 × 1019 cm-3. The room temp. cond. is dominated by Hubbard band states near the mobility edge and decreases monotonically from 137 Ω-1 cm-1 for Ts = 550° to 14 Ω-1 cm-1 for Ts = 850° due to a decreasing structural disorder, consistent with the measured x-ray coherence length that increases from 7 to 36 nm for Ts = 550 to 850°, resp., and a carrier d. that decreases from 4 × 1020 to 0.9 × 1020 cm-3, as estd. from optical reflection and Hall effect measurements. The absence of an expected discontinuity in the cond. at ∼280 K suggests that epitaxial constraints suppress the phase transition to a low-temp. orthorhombic antiferromagnetic phase, such that CrN remains a cubic paramagnetic insulator over the entire measured temp. range of 10-295 K. These results contradict previous exptl. studies that report metallic low-temp. conduction for CrN, but support recent computational results suggesting a band gap due to strong electron correlation and a stress-induced phase transition.
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    39. 39
      Muhammed Sabeer, N. A.; Pradyumnan, P. P. Augmentation of thermoelectric power factor of p-type chromium nitride thin films for device applications. Materials Science and Engineering: B 2021, 273, 115428,  DOI: 10.1016/j.mseb.2021.115428
      39
      Augmentation of thermoelectric power factor of p-type chromium nitride thin films for device applications
      Muhammed Sabeer, N. A.; Pradyumnan, P. P.
      Materials Science & Engineering, B: Advanced Functional Solid-State Materials (2021), 273 (), 115428CODEN: MSBTEK; ISSN:0921-5107. (Elsevier B.V.)
      The transformation of the n-type chromium nitride (CrN) thin films into p-type by intrinsic point defects using reactive radio frequency magnetron sputtering is explored in this report. The formation of the acceptor point defects with increasing nitrogen pressure transformed the n-type cond. of CrN into p-type along with variation of preferred orientation from (1 1 1) to (2 0 0). To improve the mobility of holes and hence to decrease the resistivity, the CrN thin films with preferred orientation along (2 2 0) was fabricated by precisely tuning the sputtering parameters. The root cause behind the redn. in resistivity of CrN thin films with orientation of crystallites was explored by surface charge d. mapping using conductive at. force microscopy. The redn. in resistivity resulted in the enhancement of thermoelec. power factor and the p-type CrN thin film showed a power factor of 24.43μW/m K2with Seebeck coeff. of 301μV/K and resistivity of 3.71 mΩ m at RT.
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    40. 40
      Sanjinés, R.; Banakh, O.; Rojas, C.; Schmid, P. E.; Lévy, F. Electronic properties of Cr1–xAlxN thin films deposited by reactive magnetron sputtering. Thin Solid Films 2002, 420–421, 312– 317,  DOI: 10.1016/S0040-6090(02)00830-1
      40
      Electronic properties of Cr1-xAlxN thin films deposited by reactive magnetron sputtering
      Sanjines, R.; Banakh, O.; Rojas, C.; Schmid, P. E.; Levy, F.
      Thin Solid Films (2002), 420-421 (), 312-317CODEN: THSFAP; ISSN:0040-6090. (Elsevier Science B.V.)
      XPS and elec. resistivity measurements have been performed to investigate the electronic properties of Cr1-xAlxN films. The films, deposited by reactive magnetron sputtering, crystallize in the fcc. rock salt-type of structure (B1-type) in a wide compn. range of 0 ≤ x ≤ 0.63. The elec. resistivity was measured from 50 to 320 K. The elec. resistivity at 320 K increases with increasing Al content, and the temp. coeff. of the resistivity is always neg. A detailed study of XPS valence band (VB) spectra shows that the substitution of Cr atoms by Al atoms leads to local modifications of covalent-ionic bonds between N 2p and Cr 3d orbitals. The variations of the VB structure due to the changes in the chem. compn. correlate with the elec. and mech. properties.
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    41. 41
      Gall, D.; Shin, C. S.; Spila, T.; Odén, M.; Senna, M. J. H.; Greene, J. E.; Petrov, I. Growth of single-crystal CrN on MgO(001):: Effects of low-energy ion-irradiation on surface morphological evolution and physical properties. J. Appl. Phys. 2002, 91 (6), 3589– 3597,  DOI: 10.1063/1.1446239
      41
      Growth of single-crystal CrN on MgO(001): Effects of low-energy ion-irradiation on surface morphological evolution and physical properties
      Gall, D.; Shin, C.-S.; Spila, T.; Oden, M.; Senna, M. J. H.; Greene, J. E.; Petrov, I.
      Journal of Applied Physics (2002), 91 (6), 3589-3597CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      CrN layers, 0.5 μm thick, were grown on MgO(001) at Ts = 570-775° by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition in pure N2 discharges at 20 mtorr. Layers grown at Ts ≤ 700° are stoichiometric single crystals exhibiting cube-on-cube epitaxy: (001)CrN‖(001)MgO with [100]CrN‖[100]MgO. At higher temps., N2 desorption during deposition results in understoichiometric polycryst. films with N fractions decreasing to 0.35, 0.28, and 0.07 with Ts = 730, 760, and 775°, resp. The surface morphologies of epitaxial CrN(001) layers depend strongly on the incident ion-to-metal flux ratio JN2+/JCr which was varied between 1.7 and 14 with the ion energy maintained const. at 12 eV. The surfaces of layers grown with JN2+/JCr = 1.7 consist of self-organized square-shaped mounds, due to kinetic roughening, with edges aligned along orthogonal 〈100〉 directions. The mounds have an av. peak-to-valley height 〈h〉 = 5.1 nm and an in-plane correlation length of 〈d〉 = 0.21 μm. The combination of at. shadowing by the mounds with low adatom mobility gave nanopipes extending along the growth direction. Increasing JN2+/JCr to 14 leads, due to increased adatom mobilities, to much smoother surfaces with 〈h〉 = 2.5 nm and 〈d〉 = 0.52 μm. Correspondingly, the nanopipe d. decreases from 870 to 270 μm-2 to <20 μm-2 as JN2+/JCr is increased from 1.7 to 6 to 10. The hardness of dense CrN(001) is 28.5 ± 1 GPa, but decreases to 22.5 ± 1 GPa for layers contg. significant nanopipe densities. The CrN(001) elastic modulus, 405 ± 15 GPa, room-temp. resistivity, 7.7 × 10-2 Ω cm, and relaxed lattice const., 0.4162 ± 0.0008 nm, are independent of JN2+/JCr.
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    42. 42
      Inumaru, K.; Koyama, K.; Imo-oka, N.; Yamanaka, S. Controlling the structural transition at the Néel point of CrN epitaxial thin films using epitaxial growth. Phys. Rev. B 2007, 75 (5), 054416,  DOI: 10.1103/PhysRevB.75.054416
      42
      Controlling the structural transition at the Neel point of CrN epitaxial thin films using epitaxial growth
      Inumaru, Kei; Koyama, Kunihiko; Imo-oka, Naoya; Yamanaka, Shoji
      Physical Review B: Condensed Matter and Materials Physics (2007), 75 (5), 054416/1-054416/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Chromium nitride (CrN) films were epitaxially grown on α-Al2O3(0001) and MgO (001) substrates by pulsed laser deposition at 973 K under nitrogen radical irradn., and the structural change of the films was investigated at around the Neel temp. of CrN (∼270 K) by temp.-controlled x-ray diffraction expts. Bulk cubic CrN is known to show monoclinic distortion below the Neel temp. The CrN film grown on MgO(001) with the CrN(001) plane parallel to the substrate surface, exhibited a clear structural change at around 260 K. On the other hand, on α-Al2O3(0001) substrates, the CrN phase grew with its (111) planes parallel to the substrate surface, and showed no structural change at the Neel temp. The different orientation of the epitaxial films can explain the different behavior of the films: The structural transition of bulk-CrN causes large variations in the interat. distances and bond angles on the (111) plane, but varies little on the (001) plane. In the case of thin films, the α-Al2O3(0001) substrate surface could prevent the (111)-oriented film from distorting its structure by fixing atom positions on the CrN(111) interfaces of the film. In accordance with the structural behavior of the films, the (111)-oriented CrN film on α-Al2O3(0001) showed no anomaly in its metallic cond. around the Neel temp. On the other hand, the (001)-oriented CrN on MgO showed a steep increase in elec. cond., accompanied by a lattice distortion below the Neel point. These results highlight an example that epitaxy could be used to control the existence of structural transitions, further accompanied by an antiferromagnetic ordering, which is closely related to the electronic properties of materials.
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    43. 43
      Constantin, C.; Haider, M. B.; Ingram, D.; Smith, A. R. Metal/semiconductor phase transition in chromium nitride (001) grown by rf-plasma-assisted molecular-beam epitaxy. Appl. Phys. Lett. 2004, 85 (26), 6371– 6373,  DOI: 10.1063/1.1836878
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      Metal/semiconductor phase transition in chromium nitride(001) grown by rf-plasma-assisted molecular-beam epitaxy
      Constantin, Costel; Haider, Muhammad B.; Ingram, David; Smith, Arthur R.
      Applied Physics Letters (2004), 85 (26), 6371-6373CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Structural and electronic properties of stoichiometric single-phase CrN(001) thin films grown on MgO(001) substrates by radio-frequency N plasma-assisted mol.-beam epitaxy, are investigated. In situ room-temp. scanning tunneling microscopy clearly shows the 1×1 at. periodicity of the crystal structure as well as long-range topog. distortions which are characteristic of a semiconductor surface. This semiconductor behavior is consistent with ex situ resistivity measurements over the range 285 K and higher, whereas below 260 K, metallic behavior is obsd. The resistivity-derived band gap for the high-temp. region, 71 meV, is consistent with the tunneling spectroscopy results. The obsd. electronic (semiconductor/metal) transition temp. coincides with the temps. of the known coincident magnetic (para-antiferro) and structural (cubic-orthorhombic) phase transitions.
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    44. 44
      Biswas, B.; Chakraborty, S.; Joseph, A.; Acharya, S.; Pillai, A. I. K.; Narayana, C.; Bhatia, V.; Garbrecht, M.; Saha, B. Secondary phase limited metal-insulator phase transition in chromium nitride thin films. Acta Mater. 2022, 227, 117737,  DOI: 10.1016/j.actamat.2022.117737
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      Secondary phase limited metal-insulator phase transition in chromium nitride thin films
      Biswas, Bidesh; Chakraborty, Sourjyadeep; Joseph, Anjana; Acharya, Shashidhara; Pillai, Ashalatha Indiradevi Kamalasanan; Narayana, Chandrabhas; Bhatia, Vijay; Garbrecht, Magnus; Saha, Bivas
      Acta Materialia (2022), 227 (), 117737CODEN: ACMAFD; ISSN:1359-6454. (Elsevier Ltd.)
      Chromium nitride (CrN) is a well-known hard coating material that has found applications in abrasion and wear-resistant cutting tools, bearings, and tribol. applications due to its high hardness, high-temp. stability, and corrosion-resistant properties. In recent years, CrN has also attracted significant interest due to its high thermoelec. power factor, and for its unique and intriguing metal-insulator phase transition. While CrN bulk single-crystals exhibit the characteristic metal-insulator transition accompanied with structural (orthorhombic-to-rocksalt) and magnetic (antiferromagnetic-to-paramagnetic) transition at ∼260-280 K, observation of such phase transition in thin-film CrN has been scarce, and the exact cause of the absence of such transitions in several studies is not well-understood. In this work, the formation of the secondary metallic Cr2N phase during the growth is demonstrated to inhibit the observation of metal-insulator phase transition in CrN thin films. When the Cr-flux during deposition is reduced below a crit. limit, epitaxial and stoichiometric CrN thin film is obtained that reproducibly exhibits the phase transition. Annealing of the mixed-phase film inside reducing NH3 environment converts the Cr2N into CrN, and a discontinuity in the elec. resistivity at ∼277 K appears, which supports the underlying hypothesis. Demonstration of the inhibited metal-semiconductor phase transition in CrN due to the presence of secondary Cr2N phase is similar to the previous finding of the substantial change in its mech. hardness and redn. in thermoelec. properties. A clear demonstration of the origin behind the controversy of the metal-insulator transition in CrN thin films marks significant progress and would enable its nanoscale device realization.
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    45. 45
      Gharavi, M. A.; Gambino, D.; le Febvrier, A.; Eriksson, F.; Armiento, R.; Alling, B.; Eklund, P. High thermoelectric power factor of pure and vanadium-alloyed chromium nitride thin films. Mater. Today Commun. 2021, 28, 102493,  DOI: 10.1016/j.mtcomm.2021.102493
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      High thermoelectric power factor of pure and vanadium-alloyed chromium nitride thin films
      Gharavi, M. A.; Gambino, D.; le Febvrier, A.; Eriksson, F.; Armiento, R.; Alling, B.; Eklund, P.
      Materials Today Communications (2021), 28 (), 102493CODEN: MTCAC7; ISSN:2352-4928. (Elsevier Ltd.)
      Chromium-nitride based materials have shown unexpected promise as thermoelec. materials for, e.g., waste-heat harvesting. Here, CrN and (Cr,V)N thin films were deposited by reactive magnetron sputtering. Thermoelec. measurements of pure CrN thin films show a low elec. resistivity between 1.2 and 1.5 × 10-3 Ωcm and very high values of the Seebeck coeff. and thermoelec. power factor, in the range between 370-430 μV/K and 9-11 × 10-3 W/mK2, resp. Alloying of CrN films with small amts. (less than 15%) of vanadium results in cubic (Cr,V)N thin films. Vanadium decreases the elec. resistivity and yields power-factor values in the same range as pure CrN. The D. functional theory calcns. of sub-stoichiometric CrN1-δ and (Cr,V)N1-δ show that nitrogen vacancies and vanadium substitution both cause n-type cond. and features in the band structure typically correlated with a high Seebeck coeff. The results suggest that slight variations in nitrogen and vanadium content affect the power factor and offers a means of tailoring the power factor and thermoelec. figure of merit.
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    46. 46
      Bahk, J. H.; Bian, Z. X.; Shakouri, A. Electron energy filtering by a nonplanar potential to enhance the thermoelectric power factor in bulk materials. Phys. Rev. B 2013, 87 (7), 075204,  DOI: 10.1103/PhysRevB.87.075204
      46
      Electron energy filtering by a nonplanar potential to enhance the thermoelectric power factor in bulk materials
      Bahk, Je-Hyeong; Bian, Zhixi; Shakouri, Ali
      Physical Review B: Condensed Matter and Materials Physics (2013), 87 (7), 075204/1-075204/13CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We present a detailed theory on electron energy filtering by the nonplanar potential introduced by dispersed nanoparticles or impurities in bulk materials for enhancement of the thermoelec. power factor. When electrons with energies below a certain cut-off energy are prevented from participating in conduction through the material, the Seebeck coeff. and thus the thermoelec. power factor can be drastically enhanced. Instead of using planar heterostructures which require elaborate epitaxial techniques, we study embedded nanoparticles or impurities so that the conservation of lateral momentum limiting electron transport at heterointerfaces is no longer a limiting factor. Based on the Boltzmann transport equations under the relaxation time approxn., the optimal cut-off energy level that maximizes the power factor is calcd. to be a few kBT above the Fermi level, and is a function of the scattering parameter, Fermi level, and temp. The maximized power factor enhancement is quantified as a function of those parameters. The electronic thermal cond. and Lorenz no. are also shown to be suppressed by the electron filtering to further enhance the thermoelec. figure of merit. We find that the power factor of PbTe at 300 K could be enhanced by more than 120% when the cut-off energy level is 0.2 eV or higher and the carrier d. higher than 5× 1019 cm-3. Finally we propose the use of distributed resonant scatterings to partially realize the nonplanar electron filtering in bulk materials.
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    47. 47
      Raz, T.; Edelman, F.; Komem, Y.; Stölzer, M.; Zaumseil, P. Transport properties of boron-doped crystallized amorphous Si1–xGex films. J. Appl. Phys. 1998, 84 (8), 4343– 4350,  DOI: 10.1063/1.368697
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      Transport properties of boron-doped crystallized amorphous Si1-xGex films
      Raz, T.; Edelman, F.; Komem, Y.; Stolzer, M.; Zaumseil, P.
      Journal of Applied Physics (1998), 84 (8), 4343-4350CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Transport properties of crystd. amorphous Si1-xGex films, having different Ge content (x) and highly doped with boron were studied. The films were deposited by mol. beam at room temp. and subsequently annealed in vacuum at different temps. between 500 and 900 °C for 1 h. The microstructure of the crystd. Si1-xGex films was characterized by means of transmission electron microscopy, x-ray diffraction, and SEM. Measured transport properties included Hall hole concn. (pH), Hall mobility (μH), elec. cond. (σ), and the Seebeck coeff. (S), from which the "power factor" (S2σ) was evaluated. The results obtained for the Hall mobility of the Si1-xGex films are discussed on the basis of the carrier trapping model. The trapping state d. at the grain boundaries increases with increasing B concn., although it is not significantly dependent on Ge content. Consequently, the mobility energy barrier decreases with increasing B concn. and increasing Ge content. It was found that in all the studied Si1-xGex films, independent of x, the predominant scattering mechanism changes from acoustic phonon scattering to ionized impurity scattering with increasing the boron concn. from 5×1018 to 5×1020 cm-3. In addn., the Si1-xGex films demonstrate high elec. cond. as well as a high Seebeck coeff., after 1 h annealing at 600-800 °C, and thus exhibit a high "power factor" of the order of 6 μW/cm K2. Thus, these films have potential applications in thin-film thermoelec. devices.
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    48. 48
      Kucherov, Y.; Hagelstein, P.; Sevastyanenko, V.; Brown, H. L.; Guruswamy, S.; Wingert, W. Importance of barrier layers in thermal diodes for energy conversion. J. Appl. Phys. 2005, 97 (9), 094902  DOI: 10.1063/1.1886273
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      Importance of barrier layers in thermal diodes for energy conversion
      Kucherov, Yan; Hagelstein, Peter; Sevastyanenko, Victor; Brown, Harold L.; Guruswamy, Sivaraman; Wingert, Wayne
      Journal of Applied Physics (2005), 97 (9), 094902/1-094902/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      Very high thermal to elec. conversion efficiencies were reported for thermal diode structures in which a thin n-type emitter layer is formed on the hot side of a thick near-intrinsic thermoelec. semiconductor. The figure of merit derived from direct measurements of elec. parameters and heat flow was higher by as much as a factor of 8. The question of what phys. mechanisms are involved is of interest. The authors have conjectured that the short-circuit current injection in these expts. is due to a 2nd-order thermionic injection mechanism. Alternatively, the open-circuit voltage may be generated due to the presence of a p-type blocking layer between the emitter and the near-intrinsic bulk region. A p-type blocking layer is required for the effect, and the dependence of conversion efficiency on the blocking layer concn. and width was studied. The results are generally consistent with calcns. based on a nonlocal generalized Onsager-type transport model.
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    49. 49
      Martin, J.; Wang, L.; Chen, L.; Nolas, G. S. Enhanced Seebeck coefficient through energy-barrier scattering in PbTe nanocomposites. Phys. Rev. B 2009, 79 (11), 115311,  DOI: 10.1103/PhysRevB.79.115311
      49
      Enhanced Seebeck coefficient through energy-barrier scattering in PbTe nanocomposites
      Martin, J.; Wang, Li; Chen, Lidong; Nolas, G. S.
      Physical Review B: Condensed Matter and Materials Physics (2009), 79 (11), 115311/1-115311/5CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Resistivity, Seebeck coeff., and Hall measurements were performed on densified nanocryst. composite materials of undoped and Ag-doped PbTe nanocrystals to investigate the phys. mechanisms responsible for Seebeck coeff. enhancement in nanocryst. systems. The unique temp. dependence of the resistivity and mobility for these PbTe nanocomposites suggests that grain-boundary potential barrier scattering is the dominant scattering mechanism. We propose that carrier trapping in the grain boundaries forms energy barriers that impede the conduction of carriers between grains, essentially filtering charge carriers with energy less than the barrier height. These nanocomposites therefore demonstrate an enhanced Seebeck coeff. as compared to single crystal or polycryst. PbTe at similar carrier concns.
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    50. 50
      Gayner, C.; Amouyal, Y. Energy Filtering of Charge Carriers: Current Trends, Challenges, and Prospects for Thermoelectric Materials. Adv. Funct. Mater. 2020, 30 (18), 1901789,  DOI: 10.1002/adfm.201901789
      50
      Energy filtering of charge carriers: Current trends, challenges, and prospects for thermoelectric materials
      Gayner, Chhatrasal; Amouyal, Yaron
      Advanced Functional Materials (2020), 30 (18), 1901789CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Exhaustive attempts are made in recent decades to improve the performance of thermoelec. materials that are utilized for waste heat-to-electricity conversion. Energy filtering of charge carriers is directed toward enhancing the material thermopower. This paper focuses on the theor. concepts, exptl. evidence, and the authors' view of energy filtering in the context of thermoelec. materials. Recent studies suggest that not all materials experience this effect with the same intensity. Although this effect theor. demonstrates improvement of the thermopower, applying it poses certain constraints, which demands further research. Predicated on data documented in literature, the unusual dependence of the thermopower and cond. upon charge carrier concns. can be altered through the energy filtering approach. Upon surmounting the phys. constraints discussed in this article, thermoelec. materials research may gain a new direction to enhance the power factor and thermoelec. figure of merit.
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    51. 51
      Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Cubic AgPb(m)SbTe (2+m): bulk thermoelectric materials with high figure of merit. Science 2004, 303 (5659), 818– 21,  DOI: 10.1126/science.1092963
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      Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit
      Hsu, Kuei Fang; Loo, Sim; Guo, Fu; Chen, Wei; Dyck, Jeffrey S.; Uher, Ctirad; Hogan, Tim; Polychroniadis, E. K.; Kanatzidis, Mercouri G.
      Science (Washington, DC, United States) (2004), 303 (5659), 818-821CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The conversion of heat to electricity by thermoelec. devices may play a key role in the future for energy prodn. and utilization. However, in order to meet that role, more efficient thermoelec. materials are needed that are suitable for high-temp. applications. We show that the material system AgPbmSbTe2+m may be suitable for this purpose. With m = 10 and 18 and doped appropriately, n-type semiconductors can be produced that exhibit a high thermoelec. figure of merit material ZTmax of ∼2.2 at 800 K. In the temp. range 600 to 900 K, the AgPbmSbTe2+m material is expected to outperform all reported bulk thermoelecs., thereby earmarking it as a material system for potential use in efficient thermoelec. power generation from heat sources.
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    52. 52
      Yamasaka, S.; Nakamura, Y.; Ueda, T.; Takeuchi, S.; Sakai, A. Phonon transport control by nanoarchitecture including epitaxial Ge nanodots for Si-based thermoelectric materials. Sci. Rep 2015, 5 (1), 14490,  DOI: 10.1038/srep14490
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      Phonon transport control by nanoarchitecture including epitaxial Ge nanodots for Si-based thermoelectric materials
      Yamasaka, Shuto; Nakamura, Yoshiaki; Ueda, Tomohiro; Takeuchi, Shotaro; Sakai, Akira
      Scientific Reports (2015), 5 (), 14490CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      Phonon transport in Si films was controlled using epitaxially-grown ultrasmall Ge nanodots (NDs) with ultrahigh d. for the purpose of developing Si-based thermoelec. materials. The Si/Ge ND stacked structures, which were formed by the ultrathin SiO2 film technique, exhibited lower thermal conductivities than those of the conventional nanostructured SiGe bulk alloys, despite the stacked structures having a smaller Ge fraction. This came from the large thermal resistance caused by phonon scattering at the Si/Ge ND interfaces. The phonon scattering can be controlled by the Ge ND structure, which was independent of Si layer structure for carrier transport. These results demonstrate the effectiveness of ultrasmall epitaxial Ge NDs as phonon scattering sources, opening up a route for the realization of Si-based thermoelec. materials.
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    53. 53
      Peng, J.; Fu, L.; Liu, Q.; Liu, M.; Yang, J.; Hitchcock, D.; Zhou, M.; He, J. A study of Yb0.2Co4Sb12–AgSbTe2nanocomposites: simultaneous enhancement of all three thermoelectric properties. J. Mater. Chem. A 2014, 2 (1), 73– 79,  DOI: 10.1039/C3TA13729E
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      A study of Yb0.2Co4Sb12-AgSbTe2 nanocomposites: simultaneous enhancement of all three thermoelectric properties
      Peng, Jiangying; Fu, Liangwei; Liu, Qiongzhen; Liu, Ming; Yang, Junyou; Hitchcock, Dale; Zhou, Menghan; He, Jian
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (1), 73-79CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The single-filled skutterudite Yb0.2Co4Sb12 was long known as a promising bulk thermoelec. material. In this work, we adopted a melting-milling-hot pressing procedure to prep. nanocomposites that consist of a micrometer-grained Yb0.2Co4Sb12 matrix and well-dispersed AgSbTe2 nanoinclusions on the matrix grain boundaries. Different wt. percentages of AgSbTe2 inclusions were added to optimize the thermoelec. performance. We found that the addn. of AgSbTe2 nanoinclusions systematically and simultaneously optimized the otherwise adversely inter-dependent elec. cond., Seebeck coeff. and thermal cond. In particular, the significantly enhanced carrier mobility led to a ∼3-fold redn. of the elec. resistivity. Meanwhile the abs. value of Seebeck coeff. was enhanced via the energy filtering effect at the matrix-nanoinclusion interfaces. Moreover there is a topol. crossover of the AgSbTe2 inclusions from isolated nanoparticles to a nano-plating or nano-coating between 6% and 8% of nanoinclusions. Above the crossover, further addn. of nanoinclusions degraded the Seebeck coeff. and the elec. cond. Meanwhile, the addn. of nanoinclusions generally reduced the lattice thermal cond. As a result, the power factor of the 6% sample was ∼7 times larger than that of the nanoinclusion-free sample, yielding a room temp. figure of merit ZT ∼ 0.51.
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