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An In Situ Study of Precursor Decomposition via Refractive Index Sensing in p-Type Transparent Copper Chromium Oxide

Cite this: Chem. Mater. 2022, 34, 7, 3020–3027
Publication Date (Web):March 18, 2022
https://doi.org/10.1021/acs.chemmater.1c03910
Copyright © 2022 The Authors. Published by American Chemical Society
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Abstract

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Oxide semiconductors are penetrating into a wide range of energy, environmental, and electronic applications, possessing a potential to outrun currently employed semiconductors. However, an insufficient development of p-type oxides is a major obstacle against complete oxide electronics. Quite often oxide deposition is performed by the spray pyrolysis method, inexpensive to implement and therefore accessible to a large number of laboratories. Although, the complex growth chemistry and a lack of in situ monitoring during the synthesis process can complicate the growth optimization of multicomponent oxides. Here we present a concept of plasmonic, optical sensing that has been applied to spray pyrolysis oxide film growth monitoring for the first time. The proposed method utilizes a polarization based refractive index sensing platform using Au nanodimers as transducing elements. As a proof of concept, the changes in the refractive index of the grown film were extracted from individual Cu(acac)2 and Cr(acac)3 precursors in real time to reveal their thermal decomposition processes. Obtained activation energies give insight into the physical origin of the narrow temperature window for the synthesis of high performing p-type transparent conducting copper chromium oxide CuxCrO2. The versatility of the proposed method makes it effective in the growth rate monitoring of various oxides, exploring new candidate materials and optimizing the synthesis conditions for acquisition of high performing oxides synthesized by a high throughput cost-effective method.

Introduction

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Transparent conducting oxides (TCOs) are materials possessing a rare combination of two contradicting properties of electrical conductivity and optical transparency in a single material. Exhibiting this remarkable coexistence of properties, TCOs are desired materials in a broad range of applications including solar cells, flat panel displays, touch screens, light emitting diodes, transparent electronics, memristors, and neuromorphic applications
The most common and better performing TCOs are n-type, while the development of high performing p-type TCOs remains an outstanding challenge due to low hole density, localized nature of the valence band derived of O 2p, and large hole effective masses. (1,2) Despite current limitations, p-type TCOs have been already used to selectively extract and transport p-type charge carriers (3) in thin film solar cells (4,5) and organic light emitting diodes and have been also used as a UV-blocking layer to improve the photostability of dye sensitized and perovskite solar cells. (6) These applications are a consequence of the wide band gap of p-type TCOs, their thermal stability, a high work function, and good hole conductivity. (7) To best serve these technologies, p-type TCOs have to display an appropriate valence band structure, well aligned with the high work function of a contact and the valence band (8) of the photoactive layer to effectively suppress diffusion between the electrode material and the photoactive layer.
A class of p-type TCO material with the potential to solve the issue of low performance is copper-based delafossite oxides with a general formula CuMO2 in which M corresponds to a trivalent cation such B, Al, Cr, Ga, Sn In, or Y. The delafossite structure is composed of each Cu atom linearly coordinated with two O atoms, forming an O–Cu–O dumbbell parallel to the c-axis. Every oxygen anion is also coordinated to M3+ cations oriented in a way that the M-centered octahedra form MO2 layers that are parallel to the ab plane. (1) Usually Cu-based delafossites have higher conductivity values yet lower Hall mobility values. (9) This higher conductivity is possibly justified by the hole concentrations produced by a higher density of native acceptor-like defects. (1) One notable exception within the class of copper delafossites is CuCrO2, (10) where the top of the valence band was found to be formed by the Cr 3d states rather than the Cu 3d state. (11−13) This facilitates an insensitivity to the crystalline quality as the conduction is based on a hopping mechanism between CrO2 octahedra, allowing for nanocrystalline or even amorphous p-type TCOs grown at much lower temperatures than conventional copper delafossites.
Previously a very narrow processing window for spray pyrolysis grown, best performing p-type CuxCrO2 (14,15) was reported. In order to analyze the origin of this narrow temperature window for CuxCrO2 synthesis, a concept of plasmonic, optical sensing is applied to spray pyrolysis for the first time, allowing in situ oxide film growth monitoring by probing of the activation energies of the film formation process. The gradual changes of the effective medium between the Au nanoparticles from air to CuxCrO2 during film formation were monitored using a polarization-based in situ optical sensing technique for a variety of growth temperatures. This methodology was tested using two widely employed (16) Cu(acac)2 and Cr(acac)3 precursors, yet the versatility of the technique allows for it to be applied to real time growth monitoring of various oxides. To further explore the tunability of CuxCrO2 films, the influence of temperature and oxygen partial pressure on the film composition and electrical performance was investigated.

Methods

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Gold Nanoparticles Preparation

The Au dimers are made by hole–mask colloidal lithography. (17) To investigate the spray pyrolysis process, substrate temperatures of up to 400 °C are required. To minimize any changes in the Au-dimer shape and size, during the actual measurements all Au-dimer substrates are pretreated by coating with a 2 nm thin Al2O3 film grown by atomic layer deposition (Oxford Instruments, 250 °C, 20 cycles of 20 ms pulse of TMA precursor, each followed by Ar purge and water pulse). Fuctionalized substrates were preheated to 400 °C prior to material deposition and optical sensing.

Spray Pyrolysis Using Medical Nebulizers

p-Type CuxCrO2 oxide films were deposited on standard microscope glass slides (Thermo Scientific 2.5 × 5 cm, 1 mm thick) by spray pyrolysis. In contrast with the commonly employed air-blast nozzle modified setup, we employ a medical grade nebulizer positioned in a small chamber with an optical window, used for in situ monitoring. The same pyrolytic decomposition of the dissolved precursors, sprayed by the stream of carrier gas toward the surface of a substrate, governs the deposition. In contrast to conventional spray pyrolysis, the different nebulization method of the solution allows the formation of high-quality, smooth films at lower substrate temperatures. In a conventional spray pyrolysis system, MAD-0331-B1B Ultrasonic Atomisers were employed; they tend to produce a broad range of droplet sizes ranging from ≈10–30 μm. (18) The average droplet sizes produced by mist chemical vapour deposition (CVD) are found to be around 5.4 μm. (19) The main difference is a reduction in the active cooling from the nebulizing gas, as much lower gas flow rates are required. In addition, the more homogeneous droplet size distribution and the reduced impact damage from larger droplets accelerated onto the film by the high pressure in air blast nebulizers further improve the film morphology.
The liquid precursor solution was prepared by dissolving Cu(acac)2 and Cr(acac)3 in methanol with a 70:30 Cr/Cu ratio, an overall total molarity of 0.03 M, (20) and a gas flow of 4 L/m. The choice of precursors is based on characteristics such as volatility, thermal stability, and reactivity. (21,22) The solution was nebulized by a Hudson RCI Micro Mist nebulizer creating a “mist”, which is transported toward the heated substrate (Watlow CER-1-01-00250 Ultramic ceramic heater). The temperature triggers the evaporation of the solvent while vaporized precursors remain in a gaseous state to form a desired material on the substrate surface. (23) A range of processing temperatures were tested, and the optimum temperature for good conductive CuxCrO2 was found to be 310 °C, approximately 40 °C lower than in our conventional air-blast nebulizer system using the same precursors. (14,15)
The precursor activation energy study was conducted in the same chamber, as it is equipped with a window for in situ optical sensing. Each precursor was studied separately in the temperature range from 230 to 390 °C. The film formation was monitored by observing changes in the localized surface plasmon resonance (LSPR) of anisotropic gold dimers. (24)

X-ray Photoelectron Spectroscopy

The film composition was measured by X-ray photoelectron spectroscopy (XPS) in an Omicron MultiProbe XPS using a monochromized Al Kα source (XM 1000, Ehν = 1486.7 eV). Prior to XPS measurements films were sonicated in acetone and isopropanol for 10 min and then etched via an argon ion gun operated at a low voltage of 750 eV with a sputter current of ≈6 μA and chamber pressure of 1.5 × 10–3 Pa to ensure removal of unreacted precursors. Representative XPS scans are shown in the Supporting Information (SI).

Raman Spectroscopy

Raman spectra of CuxCrO2 samples were recorded using a JY Horiba LabRAM800 confocal micro-Raman setup with motorized positioning stages. The excitation wavelength was 488 nm, power ∼ 10 mW, and the integration time was 5 min. To maximize the Raman signal from the thin films, the underlying substrate was numerically subtracted for each spectrum. Details of the subtraction method were previously reported. (25)

Optical Characterization

Optical properties were measured via UV–visible spectrophotometry (UV–vis) on a PerkinElmer S650 instrument.

Electrical Properties

The carrier activation energy is determined from the four point linear probe measurements using a Keithley 2400 source meter. Seebeck measurements were performed as previously described (13) (see SI).

Thickness Measurements and Modeling

The thickness of each film was determined by X-ray reflection (XRR) (Bruker D8-Discover) as well as by optical modeling of UV–vis transmission and reflectance spectra. By dividing the film thickness with the growth time, the average growth rate for each film was determined in absolute terms of nm/min.

Results and Discussion

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Methodology of In Situ Spray Pyrolysis Growth Monitoring

The synthesis of any ternary material is a complex process. The possibility of monitoring the properties of the films in situ, during the growth, enables the determination of vital information, such as the refractive index of the material and the growth mechanisms. For this reason, reflectance based optical techniques are frequently employed. (26−29) However, in the spray pyrolysis the nebulized solution generates large scattering and droplet absorption along the light path within the chamber, rendering standard intensity-based reflectance measurements challenging.
In order to allow in situ measurements of the spray pyrolysis process, a sensing approach based on polarization based refractive index sensing was adapted. (24) Previously, the method was used for optical biosensing that allowed the analyzation of molecular interactions at the nanoscale. However, the usage of plasmonic nanoparticles for oxide growth sensing has never been performed before. Therefore, a small deposition chamber was specifically designed with an optical window normal to the sample surface as schematically demonstrated in Figure 1. The sample was clamped on a vertically mounted heater with the nebulizer placed directly underneath, allowing the use of low cost medical nebulizers. The gas supply was controlled by mass flow controllers (MFCs) to allow for a controlled variation of the oxygen partial pressure, as well as reproducible gas flow rates into the nebulizer. The entire setup was mounted on an optical alignment stage allowing for adjustment of the reflected beam more precisely. The material was deposited on a glass slide with 10% surface area covered by plasmonic Au nanodimers, all aligned along the same direction. An electron micrograph of a typical surface is shown in the inset in Figure 1. The morphological anisotropy of the plasmonic nanostructure induces an optical anisotropy in the glass substrate, as the light is absorbed differently along and perpendicular to the dimer axis. When linearly polarized light is sent at a normal incidence of 45°, rotated with respect to the dimer axis, the optical anisotropy causes a rotation of the polarization axis of the reflected beam. If light is sent at a normal incidence, the rotation angle of light is proportional to the reflectance anisotropy of the surface using the small angle approximation sin θ = θwhere r is the complex reflectance of the sample along the ith direction; is also known as reflection anisotropy spectroscopy (RAS), and it is a self-normalized quantity that monitors the relative change in intensity between two orthogonal directions. Details and the specific use in refractive index sensing are found elsewhere. (24,30,31)

Figure 1

Figure 1. Schematic of the small spray pyrolysis chamber and the main optical components of the RAS setup. The inset shows the SEM image of Au dimers before Al2O3 capping.

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The strong RAS signal from Au dimers arises from a difference in polarizability along and perpendicular to dimers direction as is schematically demonstrated in Figure 2a. Therefore, the Au nanodimers act as local optical transducers retrieving the optical properties of the TCO growth in situ. In fact, not only do the Au dimers induce a rotation of the polarization angle of the polarized light, but they are extremely sensitive to their surrounding refractive index (n) and the thickness (d) of the deposited film as is shown in Figure 2b. The plasmonic nanostructure can be used to extract indirect information on the spray pyrolysis film characteristics. Increasing the refractive index between the nanoparticles in a subsequent growth run will, in a first order approximation, shift the plasmonic resonance toward the infrared due to an increase of the effective medium surrounding the Au nanoparticles.

Figure 2

Figure 2. RAS signal acquisition from gold dimers (a) before deposition and (b) after deposition.

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To allow for a simple comparison between different spectra, the transient of RAS as a function of time is monitored in the region of largest sensitivity of the spectra, at 1.85 eV (see SI for details on the normalization procedure). Due to the self-normalizing nature of the RAS measurement, variations in total intensity caused by fluctuations and turbulence in the nebulizer mist do not effect the spectra. In principle, the setup also allows for measurements of the total reflectance, yet increased noise levels due to the mentioned fluctuations in the droplets around the sample, as well as condensation of unreacted precursor on the inside of the window, limits the use in the simple reflectance mode.
Figure 3 shows a RAS spectrum of a stabilized localized surface plasmon resonance (LSPR) of an anisotropic gold dimer meta-surface acquired before and after a deposition step Sspec (a), as well as a transient Strans (b) during the deposition. The rise in the RAS signal during the transient measurement shows an increase in material deposited over and between the gold dimers. Normalization of the latter with the spectral slope provides a renormalized value Sn = Strans/Sspec that is proportional to the product of the film growth rate and refractive index, measured for the temperature and precursor combination chosen for investigation. For ultrathin films, all optical methods are only sensitive to the product of the refractive index and thickness (n × d) due to the fundamental nature of light interaction at interfaces. (32,33)

Figure 3

Figure 3. Example of a deposition cycle. (a) RAS spectra of the LSPR meta-surface at 230 °C before and after a deposition cycle using a 0.01 M solution of Cu(acac)2 in methanol. The top panel shows the difference in the two spectra. The shift in the spectral position of the plasmon resonance is evident. In the shaded regions there is a large difference and a nearly linear spectral dependence. Single wavelength, transient measurements during depositions are best taken there. (b) Transient measurement at 1.85 eV taken during growth. The shaded area indicates the time when the gas supply to the nebulizer was on. The top panel shows the DC level of the measurement proportional to the intensity of the reflected light.

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For our analysis we assume that n does not significantly vary for the material formed at the different temperatures. While this is a simplification, and the refractive index of the material may easily vary by 10% within the first 2–10 nm, (34,35) a change in thickness from 1 to 2 nm changed d by 100%. Hence the thickness change will dominate the response. Therefore, our methodology is beneficial in an optimization and fast screening of the synthesis of novel multicomponent oxides as well as in understanding the growth mechanism and chemistry of the spray pyrolysis process.

Precursor Decomposition Screening for Copper Chromium Oxide

Thermal decomposition studies of acetylacetonate precursors have found that the Cu(acac)2 precursor starts to decompose at 200 °C while the corresponding Cr(acac)3 starts to decompose at 250 °C. (36) More recent work has shown that the Cr(acac)3 precursor will have lost at least one (acac) group at 280 °C. (37) However, the precursor decomposition is only the first step in the spray pyrolysis growth process. It then also involves subsequent adsorption of the precursor onto the surface, either in an unreacted form or in a partially decomposed form. The adsorption itself will depend on the surface termination and temperature. Indeed, it is already known that Cr(acac)3 adsorption can be self-limited, preventing a continuous growth at low temperatures. Due to these properties, Cr(acac)3 is used as an atomic layer deposition precursor, where this self-limitation is employed and Cr2O3 is formed with subsequent oxidation steps. (38) In spray pyrolysis the presence of oxygen in the chamber, as well as OH groups and H2O from the further pyrolytic decomposition of the precursors and solvents, can lead to continuous reactions and film formation.
By probing the buildup of the fully formed oxide layer around the plasmonic nanoparticles, it is possible to examine the thermodynamics of the entire spray pyrolysis process. As discussed above, one can determine a normalized slope Sn for each precursor and temperature. Plotting log(Sn) in an Arrhenius type graph vs 1/T shows characteristic differences between the Cr and Cu precursors that are depicted in Figure 4.

Figure 4

Figure 4. Arrhenius type plots of the normalized slope Sn for (a) the Cu(acac)2 precursor (0.01 M solution). (b) Comparison of the Cr(acac)3 precursor (0.015 M solution) measurement to those of Cu(acac)2. Both measurements were performed with 5% oxygen content in the nebulizing gas. The estimated growth rate scale in (b) is given as a rough guide and is based on an assumed linear relationship of the growth rate with molarity and Sn.

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As expected from the thermal decomposition studies, the Cu precursor is more reactive at lower temperatures than the Cr precursor. For the Cr precursor a linear slope is seen, giving an effective activation energy for the Cr2O3 film formation of 0.5 eV. The Cu precursor, in contrast, displays a more complex behavior. At low temperatures the onset of growth is observed, limited by the actual decomposition of the precursor with an activation energy of 1.5 eV. Once the precursor fully decomposes, the kinetically limited regime is reached, where the growth rate depends on the solution molarity and oxygen supply. Increasing the temperature further actually reduces the growth rate significantly. This is characterized by an activation energy for desorption of 0.18 eV. Either the copper suboxides thermally desorb, there is a depletion of the precursor in the gas phase, or the formation of acetic acid in the precursor decomposition at higher temperatures (36) leads to an active etching of the formed Cu2O. (39)
The observed behavior for the decomposition and oxide film formation for the individual Cu and Cr precursors now directly explains the narrow growth window for CuxCrO2. For temperatures lower than the optimum 310 °C, the Cr precursor does not sufficiently decompose and only Cu2O is formed independently of the actual Cu molarity in the solution. Increasing the temperature reduces the Cu incorporation dramatically either by Cu suboxide desorption or by depletion of the precursor in the gas phase (see Figure 4a), while also significantly increasing the Cr incorporation. In order to enlarge the temperature processing window for low cost spray pyrolysis of this specific material, one would need to find a Cr precursor that decomposes more readily at lower temperatures.
Our findings demonstrate that Cr2O3 is only formed substantially above 330 °C, yet the ideal growth temperature for the ternary CuxCrO2 is 310 °C. The low temperature regime (<330 °C) is also likely to be dominated by the adsorption of unreacted Cr(acac)3 rather than actual Cr2O3 formation. We observe no film growth in this range when using higher molarity solutions, in line with self-limited adsorption (38) Therefore, the presence of Cu suboxides must alter the Cr(acac)3 adsorption or on-surface decomposition. In mixed solutions, the in situ optical monitoring is of less use, as it does not distinguish compositions of the growing film. Therefore, to confirm these findings, thicker samples grown for a longer time of 10 min and higher molarity of 0.03 M at 310 °C were considered. The resulting film thicknesses were measured to determine the overall growth rate. Using solutions of 0.03 M Cu(acac)2, the growth rate of 1.5 ± 0.3 nm/min for binary Cu2O was obtained. The growth rate for a corresponding Cr2O3 film using 0.03 M Cr(acac)3 is found to be lower at 0.8 ± 0.2 nm/min. The ratio of 1.8 ± 0.4 between these growth rates is in agreement with the estimation from the in situ study where a ratio of 2 was expected based on the values of Sn at 310 °C. Films grown with a mixture of Cu(acac)2 and Cr(acac)3 (ratio 40/60), while keeping the total molarity at 0.03 M, have a significantly higher growth rate of 2.4 ± 0.4 nm/min. Clearly the presence of Cu suboxides on the surface dramatically increases the Cr incorporation at this temperature, and such an effect has been seen for other ternary oxide growth. (40,41) Indeed the film is almost twice thicker than expected for a pure linear combination of the Cu2O and Cr2O3 growth rate.

Tunability of Copper Chromium Oxide via Oxygen Partial Pressure

To further explore the possibilities of CuxCrO2 tuning, an in-depth analysis of the oxygen role in the film formation was conducted. Figure 5a shows the stoichiometry of bulk films grown from the same solution as a function of oxygen partial pressure. Growing in full nitrogen leads to the formation of an insulating Cu2O film, confirmed via XPS and Raman spectroscopy, while supplying only 2% of oxygen creates the condition for CuxCrO2 formation. Instrumental limitations of the MFCs prevented investigations in the 0–2% range. A further increase in oxygen initially further increases Cr incorporation, and between 5% and 10% oxygen content in the nebulizing gas the film stoichiometry matches the solution stoichiometry. Increasing the oxygen content above 10% inhibits Cr incorporation again, and at the same time films become more resistive with a noticeable increase in carrier activation energy as seen from Figure 5b.

Figure 5

Figure 5. Film composition (a) and carrier activation energy (b) as a function of oxygen content in the nebulizing gas. Each sample was deposited until the full cup of 8 mL is emptied at a temperature of 310 °C.

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As temperature and solution stoichiometry remain unchanged, this suggests that the Cr precursor preferentially adsorbs on the Cu–O terminated surface in oxygen poor conditions. Insufficient oxygen supply prevents the adsorption of Cr precursor at 310 °C as seen in the sole formation of Cu2O in this case. At the same time, an excess of oxygen is detrimental to Cr adsorption as the films get thinner and more Cu rich again (see Figure 5a). This may suggest an overoxidized Cu–O layer possibly creating CuO instead of Cu2O. Indeed, studies of Cu(acac)2 decomposition in atmospheric CVD reactors have shown CuO formation when a carrier gas mixture of 10% O2/N2 has been used, although experiments were done at higher temperatures. (42) Hence, these conditions are likely detrimental to CuxCrO2 formation where Cu is in the same charge state as in Cu2O. At the same time one also needs to prevent the formation of oxygen vacancies and, as discussed above, oxygen is required for the incorporation of the Cr. Both effects combined explain the overall dependency of film stoichiometry and film conductivity on the oxygen content. The optimum oxygen content in the nebulizing gas is therefore highly dependent on the instrument, as in larger chambers more residual oxygen in the surrounding atmosphere can influence the growth.
The dramatic change in film composition upon the change of nebulizing gas from full nitrogen to a gas mixture that contains oxygen is most noticeable in the Raman spectra (shown in Figure 6a,b), where for the nitrogen case strong Cu2O Raman modes are observed, while films containing oxygen only demonstrate very weak and broad modes consistent with the nanocrystalline nature of the CuxCrO2. The expected position for the Eg and A1g Raman modes for crystalline CuxCrO2 are 460 and 710 cm–1, respectively. (43) The increase in oxygen does not change the position yet slightly sharpens the Raman modes, indicating a less defective material. This is consistent with the increased carrier activation energy and higher resistivity of the samples grown in higher oxygen content, as the Cu-vacancy defect structures are crucial for the conductivity in this material. The Cu2O Raman spectra of the nitrogen grown sample is dominated by a second-order line at 218 cm–1 and the TO and LO phonon modes of a typically Raman inactive mode (T1u). The latter was shown to be increased by point defects, specifically Cu vacancies. (44)

Figure 6

Figure 6. Raman spectra of films grown at varying oxygen content in the nebulizing gas: (a) full nitrogen and 2% oxygen and (b) 2%, 9%, and 15% oxygen. Labels indicate expected peak positions for highly crystalline materials. (c) T (solid lines), R (dashed lines), and T + R (dotted lines) of the same films. Changes are dominated by differences in overall film thickness, with 9% being the thickest.

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Figure 6c shows transmission (T) and reflection (R) of CuxCrO2 films deposited at various oxygen partial pressures. While we see variation in transmission and reflection, they are largely originating from the changed film thickness rather than substantial changes in the optical properties of the material.
In conclusion, it was demonstrated for the first time that plasmonic sensing is a viable technique for in situ growth information acquisition in spray pyrolysis. In particular, refractive index sensing by RAS using plasmonic meta-surfaces to monitor the film growth was performed. Analysis of precursor decomposition and film formation for Cu and Cr precursors allowed the origin of the narrow synthesis window of CuxCrO2 to be identified. Moreover, upon supplying both precursors simultaneously, a Cu induced uptake of Cr incorporation is observed. The produced CuxCrO2 films display low roughness, unusual for inexpensive solution process deposition, confirming that medical grade nebulizers create gentle mist CVD-like growth. The alteration of the oxygen content can be effectively employed to further tune the composition and electrical performance of the films. These results expand the understanding of the synthesis mechanism and doping in ternary copper chromium delafossite. Beyond this specific material, the developed real time growth monitoring method will be applicable for many other spray pyrolysis processes and can quickly identify ideal growth conditions for various ternary oxides or assist in finding alternative precursors to further reduce growth temperatures to minimize the thermal budget of thin film synthesis.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.1c03910.

  • Additional information on the in situ optical method employed, as well as measurements and properties of the CuxCrO2 films such as X-ray reflection, details of the electrical characterization, and results (Seebeck, carrier activation energy) as well as representative XPS scans illustrating how the sample stoichiometry was determined (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Kuanysh Zhussupbekov - School of Physics and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland;  Orcidhttps://orcid.org/0000-0003-1909-3270
    • Ruggero Verre - Department of Physics, Chalmers University of Technology, Gothenburg 412 96, Sweden;  Orcidhttps://orcid.org/0000-0001-8337-9009
    • David Caffrey - School of Physics and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland;  Orcidhttps://orcid.org/0000-0001-7870-4428
    • Kyle Shiel - School of Physics, Dublin City University, Dublin 9, Ireland
    • Igor V. Shvets - School of Physics and Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland
    • Karsten Fleischer - School of Physics, Dublin City University, Dublin 9, Ireland;  Orcidhttps://orcid.org/0000-0002-7638-4480
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by an Irish Research Council Laureate Award [Grant Number IRCLA/2019/171] and Science Foundation Ireland [Grant Numbers 12/IA/1264 and 19/US-C2C/3579]. A.Z. and K.Z. would like to thank the Government of the Republic of Kazakhstan under the Bolashak program for Ph.D. funding.

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    With potentials to outperform traditional semiconductors, oxide semiconductors are penetrating into a wide range of energy, environmental, and electronic applications. However, an insufficient library of p-type oxides is a major obstacle against complete oxide electronics. To identify promising p-type oxides, we carry out a large-scale computational screening over 17,700 oxygen-contg. compds. using the hydrogen descriptor that was shown to select the p-type oxide reliably. To enable high-throughput screening, we use in the initial stage coarse but simple descriptors that correlate with the hydrogen descriptor. Through a hierarchical screening procedure, we find 156 p-type oxides with the band gap larger than 1.1 eV, discovering promising candidates such as NaNbO2 and La2SiO4Se and also new types of p-type oxides. Furthermore, we classify the identified p-type oxides according to the valence-band character and reveal chem. principles underlying the p-type dopability.
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  3. 3
    Dunlap-Shohl, W. A.; Daunis, T. B.; Wang, X.; Wang, J.; Zhang, B.; Barrera, D.; Yan, Y.; Hsu, J. W. P.; Mitzi, D. B. Room-temperature fabrication of a delafossite CuCrO2 hole transport layer for perovskite solar cells. Journal of Materials Chemistry A 2018, 6, 469– 477,  DOI: 10.1039/C7TA09494A
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    3
    Room-temperature fabrication of a delafossite CuCrO2 hole transport layer for perovskite solar cells
    Dunlap-Shohl, Wiley A.; Daunis, Trey B.; Wang, Xiaoming; Wang, Jian; Zhang, Boya; Barrera, Diego; Yan, Yanfa; Hsu, Julia W. P.; Mitzi, David B.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (2), 469-477CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Delafossite oxides are promising hole transport layer (HTL) candidates for perovskite solar cells, due to their wide band gap, favorable energy band alignment relative to the perovskite absorber and simplicity of processing. In this paper, we investigate the properties of CuCrO2 (CCO) delafossite films using an integration of exptl. and computational techniques. Phase-pure CCO films are deposited at room temp. by spin-casting suspensions of hydrothermally-synthesized nanoparticles, for use in a glass/ITO/CCO/CH3NH3PbI3/C60/BCP/Ag device structure. Although d. functional theory (DFT) calcns. predict an elevated hole effective mass along certain crystallog. directions, the nearly isotropic shape and small size of the CCO nanoparticles preserves transport properties within the films by randomizing particle orientation, preventing these unfavorable directions from dominating the film cond. Exptl. measurements confirm that the CCO films display appropriate optical and elec. properties for use as an HTL, in good agreement with the DFT calcns. Solar cells made using these films exhibit stabilized power conversion efficiencies exceeding 14%, with only minor hysteresis in the current-voltage characteristics.
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  4. 4
    Wang, J.; Lee, Y.-J.; Hsu, J. W. P. Sub-10 nm copper chromium oxide nanocrystals as a solution processed p-type hole transport layer for organic photovoltaics. Journal of Materials Chemistry C 2016, 4, 3607– 3613,  DOI: 10.1039/C6TC00541A
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    4
    Sub-10 nm copper chromium oxide nanocrystals as a solution processed p-type hole transport layer for organic photovoltaics
    Wang, Jian; Lee, Yun-Ju; Hsu, Julia W. P.
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (16), 3607-3613CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    We report the synthesis of CuCrO2 nanocrystals, a p-type transparent conducting oxide, and their application as an efficient hole transport layer (HTL) for org. photovoltaic (OPV) devices. A nanometer-sized mixt. of Cu and Cr oxide/hydroxide is synthesized using microwave-assisted heating. With a 550 °C post-annealing treatment in N2, <10 nm CuCrO2 nanocrystals are successfully synthesized. XRD, XPS, EDAX, PESA, UV-vis spectrometry, and Kelvin probe technique are applied to confirm the delafossite phase, optical transmission, and p-type characteristics. Methanol is found to be a good solvent to disperse these nanocrystals for forming a smooth and transparent film. In comparison with the previously reported CuGaO2 HTL, the reduced film roughness enables the CuCrO2 HTL to produce highly efficient thin active layer OPV devices. UV-ozone treatment on the CuCrO2 HTL is found to increase the fill factor. Drift-diffusion modeling, energy level measurements, and XPS results reveal that the device improvement is not due to the reduced injection barrier, but due to an improved CuCrO2 cond. arising from the formation of Cu2+ species.
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  5. 5
    Gil, B.; Kim, J.; Yun, A. J.; Park, K.; Cho, J.; Park, M.; Park, B. CuCrO2 Nanoparticles Incorporated into PTAA as a Hole Transport Layer for 85 °C and Light Stabilities in Perovskite Solar Cells. Nanomaterials 2020, 10, 1669,  DOI: 10.3390/nano10091669
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    5
    CuCrO2 nanoparticles incorporated into PTAA as a hole transport layer for 85°C and light stabilities in perovskite solar cells
    Gil, Bumjin; Kim, Jinhyun; Yun, Alan Jiwan; Park, Kimin; Cho, Jaemin; Park, Minjun; Park, Byungwoo
    Nanomaterials (2020), 10 (9), 1669CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
    High-mobility inorg. CuCrO2 nanoparticles are co-utilized with conventional poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) (PTAA) as a hole transport layer (HTL) for perovskite solar cells to improve device performance and long-term stability. Even though CuCrO2 nanoparticles can be readily synthesized by hydrothermal reaction, it is difficult to form a uniform HTL with CuCrO2 alone due to the severe agglomeration of nanoparticles. Herein, both CuCrO2 nanoparticles and PTAA are sequentially deposited on perovskite by a simple spin-coating process, forming uniform HTL with excellent coverage. Due to the presence of high-mobility CuCrO2 nanoparticles, CuCrO2/PTAA HTL demonstrates better carrier extn. and transport. A redn. in trap d. is also obsd. by trap-filled limited voltages and capacitance analyses. Incorporation of stable CuCrO2 also contributes to the improved device stability under heat and light. Encapsulated perovskite solar cells with CuCrO2/PTAA HTL retain their efficiency over 90% after ~ 900-h storage in 85°C/85% relative humidity and under continuous 1-sun illumination at max.-power point.
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  6. 6
    Zhang, H.; Wang, H.; Zhu, H.; Chueh, C.-C.; Chen, W.; Yang, S.; Jen, A. K.-Y. Low-Temperature Solution-Processed CuCrO2 Hole-Transporting Layer for Efficient and Photostable Perovskite Solar Cells. Adv. Energy Mater. 2018, 8, 1702762,  DOI: 10.1002/aenm.201702762
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    Qin, P.-L.; Lei, H.-W.; Zheng, X.-L.; Liu, Q.; Tao, H.; Yang, G.; Ke, W.-J.; Xiong, L.-B.; Qin, M.-C.; Zhao, X.-Z. Copper-Doped Chromium Oxide Hole-Transporting Layer for Perovskite Solar Cells: Interface Engineering and Performance Improvement. Advanced Materials Interfaces 2016, 3, 1500799,  DOI: 10.1002/admi.201500799
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  8. 8
    Fleischer, K.; Norton, E.; Mullarkey, D.; Caffrey, D.; Shvets, I. V. Quantifying the Performance of P-Type Transparent Conducting Oxides by Experimental Methods. Materials 2017, 10, 1019,  DOI: 10.3390/ma10091019
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    8
    Quantifying the performance of P-type transparent conducting oxides by experimental methods
    Fleischer, Karsten; Norton, Emma; Mullarkey, Daragh; Caffrey, David; Shvets, Igor V.
    Materials (2017), 10 (9), 1019/1-1019/14CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
    Screening for potential new materials with exptl. and theor. methods has led to the discovery of many promising candidate materials for p-type transparent conducting oxides. It is difficult to reliably assess a good p-type transparent conducting oxide (TCO) from limited information available at an early exptl. stage. In this paper we discuss the influence of sample thickness on simple transmission measurements and how the sample thickness can skew the commonly used figure of merit of TCOs and their estd. band gap. We discuss this using copper-deficient CuCrO2 as an example, as it was already shown to be a good p-type TCO grown at low temps. We outline a modified figure of merit reducing thickness-dependent errors, as well as how modern ab initio screening methods can be used to augment exptl. methods to assess new materials for potential applications as p-type TCOs, p-channel transparent thin film transistors, and selective contacts in solar cells.
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  9. 9
    Williamson, B. A. D.; Buckeridge, J.; Brown, J.; Ansbro, S.; Palgrave, R. G.; Scanlon, D. O. Engineering Valence Band Dispersion for High Mobility p-Type Semiconductors. Chem. Mater. 2017, 29, 2402– 2413,  DOI: 10.1021/acs.chemmater.6b03306
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    9
    Engineering Valence Band Dispersion for High Mobility p-Type Semiconductors
    Williamson, Benjamin A. D.; Buckeridge, John; Brown, Jennilee; Ansbro, Simon; Palgrave, Robert G.; Scanlon, David O.
    Chemistry of Materials (2017), 29 (6), 2402-2413CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The paucity of high performance transparent p-type semiconductors was a stumbling block for the electronics industry for decades, effectively hindering the route to efficient transparent devices based on p-n junctions. Cu-based oxides and subsequently Cu-based oxychalcogenides were heavily studied as affordable, earth-abundant p-type transparent semiconductors, where the mixing of the Cu 3d states with the chalcogenide 2p states at the top of the valence band encourages increased valence band dispersion. The authors extend this mixing concept further, by using quantum chem. techniques to study ternary copper phosphides as potential high mobility p-type materials. The authors use hybrid d. functional theory to examine a family of phosphides, namely, MCuP (M = Mg, Ca, Sr, Ba) which all possess extremely disperse valence band maxima, comparable to the dispersion of excellent industry std. n-type transparent conducting oxides. As a proof of concept, the authors synthesized and characterized powders of CaCuP, showing that they display high levels of p-type cond., without any external acceptor dopant. Lastly, the role of Cu-coordination in promoting valence band dispersion and provide design principles for producing degenerate p-type materials are discussed.
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  10. 10
    Lunca-Popa, P.; Botsoa, J.; Bahri, M.; Crêpelliére, J.; Desgardin, P.; Audinot, J.-N.; Wirtz, T.; Arl, D.; Ersen, O.; Barthe, M.-F. Tuneable interplay between atomistic defects morphology and electrical properties of transparent p-type highly conductive off-stoichiometric Cu-Cr-O delafossite thin films. Sci. Rep. 2020, 10, 1416,  DOI: 10.1038/s41598-020-58312-z
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    10
    Tuneable interplay between atomistic defects morphology and electrical properties of transparent p-type highly conductive off-stoichiometric Cu-Cr-O delafossite thin films
    Lunca-Popa, Petru; Botsoa, Jacques; Bahri, Mounib; Crepelliere, Jonathan; Desgardin, Pierre; Audinot, Jean-Nicolas; Wirtz, Tom; Arl, Didier; Ersen, Ovidiu; Barthe, Marie-France; Lenoble, Damien
    Scientific Reports (2020), 10 (1), 1416CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
    Off-stoichiometric copper chromium delafossites demonstrate the highest values of elec. cond. among the p-type transparent conducting oxides. Morphol. and structural changes in Cu0.66Cr1.33O2 upon annealing processes are investigated. Chained copper vacancies were previously suggested as source of the high levels of doping in this material. High resoln. Helium Ion Microscopy, Secondary Ion Mass Spectrometry and Transmission Electron Microscopy reveal a significant rearrangement of copper and chromium after the thermal treatments. Furthermore, Positron Annihilation Spectroscopy evidences the presence of vacancy defects within the delafossite layers which can be assigned to the Cu vacancy chains whose concn. decreases during the thermal process. These findings further confirm these chained vacancies as source of the p-type doping and suggest that the changes in elec. conductivities within the off-stoichiometric copper based delafossites are triggered by elemental rearrangements.
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  11. 11
    Yokobori, T.; Okawa, M.; Konishi, K.; Takei, R.; Katayama, K.; Oozono, S.; Shinmura, T.; Okuda, T.; Wadati, H.; Sakai, E. Electronic structure of the hole-doped delafossite oxides CuCr1–xMgxO2. Phys. Rev. B 2013, 87, 195124,  DOI: 10.1103/PhysRevB.87.195124
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    11
    Electronic structure of the hole-doped delafossite oxides CuCr1-xMgxO2
    Yokobori, T.; Okawa, M.; Konishi, K.; Takei, R.; Katayama, K.; Oozono, S.; Shinmura, T.; Okuda, T.; Wadati, H.; Sakai, E.; Ono, K.; Kumigashira, H.; Oshima, M.; Sugiyama, T.; Ikenaga, E.; Hamada, N.; Saitoh, T.
    Physical Review B: Condensed Matter and Materials Physics (2013), 87 (19), 195124/1-195124/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We report the detailed electronic structure of a hole-doped delafossite oxide CuCr1-xMgxO2 (0 ≤ x ≤ 0.03) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calcns. within the local-d. approxn. +U (LDA + U) scheme. Cr/Cu 3p-3d resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr 3d character with a minor contribution from the Cu 3d through Cu 3d-O 2p-Cr 3d hybridization, having good agreement with the band-structure calcns. This indicates that a doped hole will have primarily the Cr 3d character. Cr 2p PES and L-edge XAS spectra exhibit typical Cr3+ features for all x, while the Cu L-edge XAS spectra exhibited a systematic change with x. This indicates now that the Cu valence is monovalent at x = 0 and the doped hole should have Cu 3d character. Nevertheless, we surprisingly obsd. two types of charge-transfer satellites that should be attributed to Cu+ (3d10) and Cu2+ (3d9) like initial states in Cu 2p-3d resonant PES spectrum of at x = 0, while Cu 2p PES spectra with no doubt shows the Cu+ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu 4s state, but also finite Cu 3d, 4s-Cr 3d charge transfer via O 2p states in the ground-state electronic configuration.
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  12. 12
    Norton, E.; Farrell, L.; Callaghan, S. D.; McGuinness, C.; Shvets, I. V.; Fleischer, K. X-ray spectroscopic studies of the electronic structure of chromium-based p -type transparent conducting oxides. Phys. Rev. B 2016, 93, 115302,  DOI: 10.1103/PhysRevB.93.115302
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    12
    X-ray spectroscopic studies of the electronic structure of chromium-based p-type transparent conducting oxides
    Norton, E.; Farrell, L.; Callaghan, S. D.; McGuinness, C.; Shvets, I. V.; Fleischer, K.
    Physical Review B (2016), 93 (11), 115302/1-115302/8CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)
    The valence band structure of p-type transparent oxides-cryst. MgxCr2-xO3 and nanocryst. CuxCrOy-is analyzed as a function of incoming photon energy. The valence band of both p-type transparent conducting oxides shows striking similarities to measurements on cryst. CuCrO2:Mg with all films showing that chromium states compose the top of the valence band, suggesting that the valence-band structure is dominated by the presence of the Cr-O6 octahedra. A comparison of the valence band between the best performing p-type, cryst. CuCrO2:Mg, with cryst. MgxCr2-xO3 and nanocryst. CuxCrOy shows that the chromium 3d states are fixed irresp. of changes in long-range crystallog. order. This indicates little spatial overlap between adjacent Cr 3d states. This further confirms the conduction mechanism via hopping for chromium based p-type TCOs as the Cr 3d states are localized within the Cr-O6 octahedra.
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  13. 13
    Farrell, L.; Fleischer, K.; Caffrey, D.; Mullarkey, D.; Norton, E.; Shvets, I. V. Conducting mechanism in the epitaxial p -type transparent conducting oxide C r2 O3:Mg. Phys. Rev. B 2015, 91, 125202,  DOI: 10.1103/PhysRevB.91.125202
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    13
    Conducting mechanism in the epitaxial p-type transparent conducting oxide Cr2O3:Mg
    Farrell, L.; Fleischer, K.; Caffrey, D.; Mullarkey, D.; Norton, E.; Shvets, I. V.
    Physical Review B: Condensed Matter and Materials Physics (2015), 91 (12), 125202/1-125202/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    Epitaxial p-type transparent conducting oxide (TCO) Cr2O3:Mg was grown by electron-beam evapn. in a mol. beam epitaxy system on c-plane sapphire. The influence of Mg dopants and the oxygen partial pressure were investigated by thermoelec. and elec. measurements. The conduction mechanism is analyzed using the small-polaron hopping model, and hopping activation energies have been detd., which vary with doping concn. in the range of 210-300 ± 5 meV. Films with better cond. were obtained by post-annealing. The effect of post-annealing is discussed in terms of a crystallog. reordering of the Mg dopant. The highest Seebeck mobilities obtained from thermoelec. measurements are of the order of 10-4 cm2V-1s-1. We investigate the fundamental properties of a Mg dopant in a high cryst. quality epitaxial film of a binary oxide, helping us understand the role of short range crystallog. order in a p-type TCO in detail.
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  14. 14
    Farrell, L.; Norton, E.; Smith, C. M.; Caffrey, D.; Shvets, I. V.; Fleischer, K. Synthesis of nanocrystalline Cu deficient CuCrO2-a high figure of merit p-type transparent semiconductor. Journal of Materials Chemistry C 2016, 4, 126– 134,  DOI: 10.1039/C5TC03161C
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    14
    Synthesis of nanocrystalline Cu deficient CuCrO2 - a high figure of merit p-type transparent semiconductor
    Farrell, Leo; Norton, Emma; Smith, Christopher M.; Caffrey, David; Shvets, Igor V.; Fleischer, Karsten
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (1), 126-134CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    The delafossite structured CuCrO2 system is known as one of the best performing p-type transparent conducting oxides. The details of a low temp. facile growth method for CuCrO2 is described. The dependence of the growth on the precursors, the temp. and oxygen partial pressure were examd. The decompn. routes are crit. to obtain the best performing films. The thermopower and elec. measurements indicate p-type films with cond. ranging from 1-12 S cm-1 depending on the growth conditions. This p-type cond. is retained despite the nanocrystallinity of the films. The figure of merit of these films can be ≤350 μS, which is the best performing p-type TCO by soln. methods to date. The optical properties are also studied using ellipsometry and UV-visible spectroscopy.
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  15. 15
    Farrell, L.; Norton, E.; O’dowd, B. J.; Caffrey, D.; Shvets, I. V.; Fleischer, K. Spray pyrolysis growth of a high figure of merit, nano-crystalline, p -type transparent conducting material at low temperature. Appl. Phys. Lett. 2015, 107, 031901,  DOI: 10.1063/1.4927241
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    15
    Spray pyrolysis growth of a high figure of merit, nano-crystalline, p-type transparent conducting material at low temperature
    Farrell, L.; Norton, E.; O'Dowd, B. J.; Caffrey, D.; Shvets, I. V.; Fleischer, K.
    Applied Physics Letters (2015), 107 (3), 031901/1-031901/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The authors demonstrate a low temp. (≈345°) growth method for Cu deficient CuCrO2 performed by spray pyrolysis using metal-org. precursors and a simple air blast nozzle. Smooth films were grown on glass substrates with a highest cond. of 12 S/cm. The most conductive samples retain transparencies >55% resulting in a figure of merit ≤350 μS, which is the best performing p-type transparent conducting material grown by soln. methods to date. Remarkably, despite the nano-crystallinity of the films, properties comparable with cryst. CuCrO2 are obsd. No postannealing of the films is required in contrast to previous reports on cryst. material. The low processing temp. of this method means that the material can be deposited on flexible substrates. As this is a soln. based technique, it is more attractive to industry as phys. vapor deposition methods are slow and costly in comparison. (c) 2015 American Institute of Physics.
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  16. 16
    Sánchez-Alarcón, R. I.; Oropeza-Rosario, G.; Gutierrez-Villalobos, A.; Muro-López, M. A.; Martínez-Martínez, R.; Zaleta-Alejandre, E.; Falcony, C.; Alarcón-Flores, G.; Fragoso, R.; Hernández-Silva, O. Ultrasonic spray-pyrolyzed CuCrO2thin films. J. Phys. D: Appl. Phys. 2016, 49, 175102,  DOI: 10.1088/0022-3727/49/17/175102
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    16
    Ultrasonic spray-pyrolyzed CuCrO2 thin films
    Sanchez-Alarcon, R. I.; Oropeza-Rosario, G.; Gutierrez-Villalobos, A.; Muro-Lopez, M. A.; Martinez-Martinez, R.; Zaleta-Alejandre, E.; Falcony, C.; Alarcon-Flores, G.; Fragoso, R.; Hernandez-Silva, O.; Perez-Cappe, E.; Laffita, Yodalgis Mosqueda; Aguilar-Frutis, M.
    Journal of Physics D: Applied Physics (2016), 49 (17), 175102/1-175102/7CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
    In this paper the optical, structural and elec. properties of CuCrO2 thin films deposited by ultrasonic spray pyrolysis at temps. from 400 to 600 °C in steps of 50 °C are presented. Copper and chromium acetylacetonates were chosen as sources of Cu and Cr, resp., and N,N-dimethylformamide was used as the solvent. X-ray results confirmed that the films as deposited showed the CuCrO2 phase without any post-deposition thermal annealing. The surface morphol. was obsd. to be mirror like, and as the films were deposited at different temps., they gradually revealed the presence of small crystallites. The best film's optical percentage transmission (in the visible region), about 58%, was obtained in films deposited at 450 °C, and the highest band gap energy (3.17 eV) was measured in films deposited at 400 °C. The elec. properties of the films were obtained by the Hall effect. A hole concn. in the range 1019-1021 cm-3, cond. as high as 35 S cm-1, and mobility lower than 1 cm2 V-1 s-1 were obtained in the films. p-type cond. was confirmed using the hot point probe arrangement, and the Seebeck coeff. was estd. The hole cond. is thought to be due to excess oxygen in the films. Finally, the min. energy required to transfer carriers from acceptor level to the valence band in the films was estd. by impedance spectroscopy.
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    Fredriksson, H.; Alaverdyan, Y.; Dmitriev, A.; Langhammer, C.; Sutherland, D. S.; Zäch, M.; Kasemo, B. Hole-Mask Colloidal Lithography. Adv. Mater. 2007, 19, 4297– 4302,  DOI: 10.1002/adma.200700680
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    Hole-mask colloidal lithography
    Fredriksson, Hans; Alaverdyan, Yury; Dmitriev, Alexandre; Langhammer, Christoph; Sutherland, Duncan S.; Zaech, Michael; Kasemo, Bengt
    Advanced Materials (Weinheim, Germany) (2007), 19 (23), 4297-4302CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Hole-mask colloidal lithog. represents a truly versatile and simple bottom-up nanofabrication method based on colloidal self-assembly lithog. patterning. The technique provides an effective means of patterning vast surface areas with diverse functional nanoarchitectures. Examples include arrays of nanodisks, oriented elliptical nanostructures, (binary) nanodisk pairs, nanocones on extended surfaces and nanodisks embedded in a surrounding matrix.
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    Levitsky, I.; Tavor, D. Improved Atomization via a Mechanical Atomizer with Optimal Geometric Parameters and an Air-Assisted Component. Micromachines 2020, 11, 584,  DOI: 10.3390/mi11060584
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    Hallberg, C. J.; Lysaught, M. T.; Zmudka, C. E.; Kopesky, W. K.; Olson, L. E. Characterization of a human powered nebulizer compressor for resource poor settings. BioMedical Engineering OnLine 2014, 13, 77,  DOI: 10.1186/1475-925X-13-77
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    Characterization of a human powered nebulizer compressor for resource poor settings
    Hallberg Christopher J; Lysaught Mary Therese; Zmudka Christopher E; Kopesky William K; Olson Lars E
    Biomedical engineering online (2014), 13 (), 77 ISSN:.
    BACKGROUND: Respiratory disease accounts for three of the ten leading causes of death worldwide. Many of these diseases can be treated and diagnosed using a nebulizer. Nebulizers can also be used to safely and efficiently deliver vaccines. Unfortunately, commercially available nebulizers are not designed for use in regions of the world where lung disease is most prevalent: they are electricity-dependent, cost-prohibitive, and not built to be reliable in harsh operating conditions or under frequent use.To overcome these limitations, the Human Powered Nebulizer compressor (HPN) was developed. The HPN does not require electricity; instead airflow is generated manually through a hand-crank or bicycle-style pedal system. A health care worker or other trained individual operates the device while the patient receives treatment.This study demonstrates functional specifications of the HPN in comparison with a standard commercially available electric jet nebulizer compressor, the DeVilbiss Pulmo-Aide 5650D (Pulmo-Aide). METHODS: Pressure and flow characteristics were measured with a rotameter and pressure transducer, respectively. Volume nebulized by each compressor was determined by mass, and particle size distribution was determined via laser diffraction. The Hudson RCI Micro Mist nebulizer mouthpiece was used with both compressors. RESULTS: The pressure and flow generated by the HPN and Pulmo-Aide were: 15.17 psi and 10.5 L/min; and 14.65 psi and 11.2 L/min, respectively. The volume of liquid delivered by each was equivalent, 1.097 ± 0.107 mL (mean ± s.e.m., n = 13) for the HPN and 1.092 ± 0.116 mL for the Pulmo-Aide. The average particle size was also equivalent, 5.38 ± 0.040 micrometers (mean ± s.e.m., n = 7) and 5.40 ± 0.025 micrometers, respectively. CONCLUSIONS: Based on these characteristics, the HPN's performance is equivalent to a popular commercially available electric nebulizer compressor. The findings presented in this paper, combined with the results of two published clinical studies, suggest that the HPN could serve as an important diagnostic and therapeutic tool in the fight against global respiratory health challenges including: tuberculosis, chronic obstructive pulmonary disease, asthma, and lower respiratory infections.
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  20. 20
    Norton, E.; Farrell, L.; Zhussupbekova, A.; Mullarkey, D.; Caffrey, D.; Papanastasiou, D. T.; Oser, D.; Bellet, D.; Shvets, I. V.; Fleischer, K. Bending stability of Cu0.4CrO2─A transparent p-type conducting oxide for large area flexible electronics. AIP Advances 2018, 8, 085013,  DOI: 10.1063/1.5027038
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    20
    Bending stability of Cu0.4CrO2-A transparent p-type conducting oxide for large area flexible electronics
    Norton, E.; Farrell, L.; Zhussupbekova, A.; Mullarkey, D.; Caffrey, D.; Papanastasiou, D. T.; Oser, D.; Bellet, D.; Shvets, I. V.; Fleischer, K.
    AIP Advances (2018), 8 (8), 085013/1-085013/7CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
    The current best performing p-type transparent conducting oxides are typically highly cryst. materials, deposited at high temps., and hence incompatible with the drive to low cost flexible electronics. We investigated a nanocryst., copper deficient CuxCrO2, deposited at low temps. upon a flexible polyimide substrate. The as-deposited film without post annealing has an elec. cond. of 6Scm-1. We demonstrate that this p-type transparent oxide retains its excellent elec. cond. under tensile strain, withstanding more than one thousand bending cycles without visible cracks or degrdn. in elec. properties. In contrast, compressive strain is shown to lead to an immediate redn. in cond. which we attribute to a de-lamination of the thin film from the substrate. (c) 2018 American Institute of Physics.
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    Hu, X.; Schuster, J.; Schulz, S. E.; Gessner, T. Surface chemistry of copper metal and copper oxide atomic layer deposition from copper(ii) acetylacetonate: a combined first-principles and reactive molecular dynamics study. Phys. Chem. Chem. Phys. 2015, 17, 26892– 26902,  DOI: 10.1039/C5CP03707G
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    Surface chemistry of copper metal and copper oxide atomic layer deposition from copper(II) acetylacetonate: a combined first-principles and reactive molecular dynamics study
    Hu, Xiao; Schuster, Joerg; Schulz, Stefan E.; Gessner, Thomas
    Physical Chemistry Chemical Physics (2015), 17 (40), 26892-26902CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Atomistic mechanisms for the at. layer deposition using the Cu(acac)2 (acac = acetylacetonate) precursor are studied using first-principles calcns. and reactive mol. dynamics simulations. The results show that Cu(acac)2 chemisorbs on the hollow site of the Cu(110) surface and decomps. easily into a Cu atom and the acac ligands. A sequential dissocn. and redn. of the Cu precursor [Cu(acac)2 → Cu(acac) → Cu] are obsd. Further decompn. of the acac ligand is unfavorable on the Cu surface. Thus addnl. adsorption of the precursors may be blocked by adsorbed ligands. Mol. hydrogen is found to be nonreactive towards Cu(acac)2 on Cu(110), whereas individual H atoms easily lead to bond breaking in the Cu precursor upon impact, and thus release the surface ligands into the gas-phase. On the other hand, water reacts with Cu(acac)2 on a Cu2O substrate through a ligand-exchange reaction, which produces gaseous H(acac) and surface OH species. Combustion reactions with the main byproducts CO2 and H2O are obsd. during the reaction between Cu(acac)2 and ozone on the CuO surface. The reactivity of different co-reactants toward Cu(acac)2 follows the order H > O3 > H2O.
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    Viguié, J. C.; Spitz, J. Chemical Vapor Deposition at Low Temperatures. J. Electrochem. Soc. 1975, 122, 585– 588,  DOI: 10.1149/1.2134266
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    Chemical vapor deposition at low temperatures
    Viguie, J. C.; Spitz, J.
    Journal of the Electrochemical Society (1975), 122 (4), 585-8CODEN: JESOAN; ISSN:0013-4651.
    Coatings of Fe2O3 [1309-37-1], SnO2 [18282-10-5], In2O3 [1312-43-2], Cr2O3 [1308-38-9], V2O3 [1314-34-7], Pd [7440-05-3], and Ru [7440-18-8] were prepd. by deposition from aerosol droplets of solns. of the corresponding acetylacetonates at atm. pressure and <500°. The structure of the coatings depended on the temp., flow of soln. and carrier gas, app. geom., and size of droplets. The variables were analyzed for deposition of Fe2O3. Piezoelec. transducers produced droplets with diam. 1-4 μ, compared with 5-50 for pneumatic transducers.
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    Marchand, P.; Hassan, I. A.; Parkin, I. P.; Carmalt, C. J. Aerosol-assisted delivery of precursors for chemical vapour deposition: expanding the scope of CVD for materials fabrication. Dalton Transactions 2013, 42, 9406,  DOI: 10.1039/c3dt50607j
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    Aerosol-assisted delivery of precursors for chemical vapour deposition: expanding the scope of CVD for materials fabrication
    Marchand, Peter; Hassan, Iman A.; Parkin, Ivan P.; Carmalt, Claire J.
    Dalton Transactions (2013), 42 (26), 9406-9422CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    A review. The prodn. of thin films of materials has become the attention of a great deal of research throughout academia and industry worldwide owing to the array of applications which use them, including electronic devices, gas sensors, solar cells, window coatings and catalytic systems. While a no. of deposition techniques are in common use, CVD is an attractive process for the prodn. of a wide range of materials due to the control it offers over film compn., coverage and uniformity, even on large scales. Conventional CVD processes can be limited, however, by the need for suitably volatile precursors. Aerosol-assisted (AA)CVD is a soln.-based process which relies on the soly. of the precursor, rather than its volatility and thus vastly extends the range of potentially applicable precursors. AACVD offers extra means to control film morphol. and concurrently the properties of the deposited materials. In this perspective the AACVD process, the influence of deposition conditions on film characteristics and a no. of materials and applications to which AACVD was found beneficial are discussed.
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    Verre, R.; MacCaferri, N.; Fleischer, K.; Svedendahl, M.; Odebo Länk, N.; Dmitriev, A.; Vavassori, P.; Shvets, I. V.; Käll, M. Polarization conversion-based molecular sensing using anisotropic plasmonic metasurfaces. Nanoscale 2016, 8, 10576– 10581,  DOI: 10.1039/C6NR01336H
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    Polarization conversion-based molecular sensing using anisotropic plasmonic metasurfaces
    Verre, R.; Maccaferri, N.; Fleischer, K.; Svedendahl, M.; Odebo Laenk, N.; Dmitriev, A.; Vavassori, P.; Shvets, I. V.; Kaell, M.
    Nanoscale (2016), 8 (20), 10576-10581CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Anisotropic media induce changes in the polarization state of transmitted and reflected light. Here we combine this effect with the refractive index sensitivity typical of plasmonic nanoparticles to exptl. demonstrate self-referenced single wavelength refractometric sensing based on polarization conversion. We fabricated anisotropic plasmonic metasurfaces composed of gold dimers and, as a proof of principle, measured the changes in the rotation of light polarization induced by biomol. adsorption with a surface sensitivity of 0.2 ng cm-2. We demonstrate the possibility of miniaturized sensing and we show that exptl. results can be reproduced by anal. theory. Various ways to increase the sensitivity and applicability of the sensing scheme are discussed.
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    Caffrey, D.; Zhussupbekova, A.; Vijayaraghavan, R. K.; Ainabayev, A.; Kaisha, A.; Sugurbekova, G.; Shvets, I. V.; Fleischer, K. Crystallographic characterisation of ultra-thin, or amorphous transparent conducting oxides-the case for Raman spectroscopy. Materials 2020, 13, 267,  DOI: 10.3390/ma13020267
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    Crystallographic characterisation of ultra-thin, or amorphous transparent conducting Oxides-the case for Raman spectroscopy
    Caffrey, David; Zhussupbekova, Ainur; Vijayaraghavan, Rajani K.; Ainabayev, Ardak; Kaisha, Aitkazy; Sugurbekova, Gulnar; Shvets, Igor V.; Fleischer, Karsten
    Materials (2020), 13 (2), 267CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
    The electronic and optical properties of transparent conducting oxides (TCOs) are closely linked to their crystallog. structure on a macroscopic (grain sizes) and microscopic (bond structure) level. With the increasing drive towards using reduced film thicknesses in devices and growing interest in amorphous TCOs such as n-type InGaZnO4 (IGZO), ZnSnO3 (ZTO), p-type CuxCrO2, or ZnRh2O4, the task of gaining in-depth knowledge on their crystal structure by conventional X-ray diffraction-based measurements are becoming increasingly difficult. We demonstrate the use of a focal shift based background subtraction technique for Raman spectroscopy specifically developed for the case of transparent thin films on amorphous substrates. Using this technique we demonstrate, for a variety of TCOs CuO, a-ZTO, ZnO:Al), how changes in local vibrational modes reflect changes in the compn. of the TCO and consequently their electronic properties.
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    Bajaj, J.; Irvine, S. J. C.; Sankur, H. O.; Svoronos, S. A. Modeling of in situ monitored laser reflectance during MOCVD growth of HgCdTe. J. Electron. Mater. 1993, 22, 899– 906,  DOI: 10.1007/BF02817503
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    Modeling of in situ monitored laser reflectance during MOCVD growth of mercury cadmium telluride
    Bajaj, J.; Irvine, S. J. C.; Sankur, H. O.; Svoronos, Spyros A.
    Journal of Electronic Materials (1993), 22 (8), 899-906CODEN: JECMA5; ISSN:0361-5235.
    An effective way to in situ monitor the metalorg. chem. vapor deposition (MOCVD) of HgCdTe/CdTe/ZnTe on GaAs or GaAs/Si substrates is presented. Specular He-Ne laser reflectance was used to in situ monitor the growth rates, layer thickness, and morphol. for each layer in the grown multilayer structure. In situ monitoring has enabled precise measurements of ZnTe nucleation and CdTe buffer layer thicknesses. Monitoring the constancy of reflectance during the thicker CdTe buffer growth where absorption in the CdTe reduces reflectance to just the surface component has led to optimum buffer growth ensuring good quality of subsequently grown GhCdTe. During the interdiffused multilayer process (IMP) HgCdTe growth, because multiple interfaces are present within the absorption length, a periodic reflectance signal is maintained throughout this growth cycle. A theor. model was developed to ext. IMP layer thicknesses from in situ recorded exptl. data. For structures that required the growth of a layer band gap HgCdTe cap layer on top of a smaller band gap active layer, in situ monitored reflectance data allowed detn. of alloy compn. in the cap layer as well. Continuous monitoring of IMP parameters established the stability of growth conditions, translating into depth uniformity of the grown material, and allowed diagnosis of growth rate instabilities in terms of changes in the HgTe and CdTe parts of the IMP cycle. A unique advantage of in situ laser monitoring is the opportunity to perform "interactive" crystal growth, a development that is a key to real time MOCVD HgCdTe feedback growth control.
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    Stafford, A.; Irvine, S. J. C.; Hess, K. L.; Bajaj, J. The use ofin situlaser interferometry for MOCVD process control. Semicond. Sci. Technol. 1998, 13, 1407– 1411,  DOI: 10.1088/0268-1242/13/12/013
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    The use of in situ laser interferometry for MOCVD process control
    Stafford, A.; Irvine, S. J. C.; Hess, K. L.; Bajaj, J.
    Semiconductor Science and Technology (1998), 13 (12), 1407-1411CODEN: SSTEET; ISSN:0268-1242. (Institute of Physics Publishing)
    This paper reports on the technique of single-wavelength laser interferometry for remote monitoring of the thin-film growth of semiconductors by metal-org. CVD (MOCVD). By online fitting of factors governing the complex refractive index of a growing layer, the quality of epitaxial growth can be monitored and early identification of a degrdn. in film growth identified. For example, sub-bandgap 633 nm (HeNe) radiation was used to monitor the pyrolytic and photo-assisted growth of several films of ZnTe on GaAs. Fitting of the effective extinction coeff., keff, at each turning point (peaks and troughs) in the interferogram revealed that different growth mechanisms are dominant under photo-assisted conditions compared to strictly pyrolytic conditions. We propose a variation on the virtual-interface approach for the math. treatment of a dielec. stack for the real-time fitting of complex reflectance interferograms from multilayers. Using this model, interferograms for vertical cavity surface emitting laser structures were theor. generated and are in excellent agreement with the exptl. interferograms recorded by Killeen and coworkers. Finally data are presented which demonstrate the use of laser interferometry for process control on an industrial reactor.
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    Ng, T. B.; Han, J.; Biefeld, R. M.; Weckwerth, M. V. In-situ reflectance monitoring during MOCVD of AlGaN. J. Electron. Mater. 1998, 27, 190– 195,  DOI: 10.1007/s11664-998-0385-8
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    In-situ reflectance monitoring during MOCVD of AlGaN
    Ng, T.-B.; Han, J.; Biefeld, R. M.; Weckwerth, M. V.
    Journal of Electronic Materials (1998), 27 (4), 190-195CODEN: JECMA5; ISSN:0361-5235. (Minerals, Metals & Materials Society)
    This reports in-situ optical reflectance monitoring during the metalorg. chem. vapor deposition (MOCVD) growth of (Al)GaN. In addn. to the well-known thin film interference effect which enables a real-time detn. of growth rate, it is shown that several insights about the MOCVD growth process can be gained by using this simple yet powerful technique. Illustrations from a variety of applications for in-situ reflectance monitoring, specifically the study of growth evolution, the control of alloy fractions, and the use of growth rate to gauge surface kinetics and gas injection will be reported.
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    Nakamura, S. Analysis of Real-Time Monitoring Using Interference Effects. Jpn. J. Appl. Phys. 1991, 30, 1348– 1353,  DOI: 10.1143/JJAP.30.1348
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    Analysis of real-time monitoring using interference effects
    Nakamura, Shuji
    Japanese Journal of Applied Physics, Part 1: Regular Papers, Short Notes & Review Papers (1991), 30 (7), 1348-53CODEN: JAPNDE; ISSN:0021-4922.
    The interference effect, which is measured as a temp. oscillation by a narrow optical band-pass pyrometer during GaN growth in metalorg. chem. vapor deposition (MOCVD), was analyzed by calcg. the transmittance and the reflectance. The results of the calcns. showed that the attenuation of the oscillation amplitude with increasing thickness was not caused by the absorption of the growing layer. To explain this attenuation, the thickness fluctuation within the measured area is proposed. The thickness fluctuation and the growth rate of the growing layer are reflected on the trace of transmittance using the present real-time monitoring technique, which observes the interference effect.
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    Verre, R.; Fleischer, K.; McGilp, J. F.; Fox, D.; Behan, G.; Zhang, H.; Shvets, I. V. Controlled in situ growth of tunable plasmonic self-assembled nanoparticle arrays. Nanotechnology 2012, 23, 035606,  DOI: 10.1088/0957-4484/23/3/035606
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    Controlled in situ growth of tunable plasmonic self-assembled nanoparticle arrays
    Verre, R.; Fleischer, K.; McGilp, F.; Fox, D.; Behan, G.; Zhang, H.; Shvets, I. V.
    Nanotechnology (2012), 23 (3), 035606/1-035606/9CODEN: NNOTER; ISSN:1361-6528. (Institute of Physics Publishing)
    Self-assembled silver nanoparticle (NP) arrays were produced by deposition at glancing angles on transparent stepped Al2O3 templates. The evolution of the plasmonic resonances has been monitored using reflection anisotropy spectroscopy (RAS) during growth. It is demonstrated that the morphol. of the array can be tailored by changing the template structure, resulting in a large tunability of the optical resonances. In order to ext. detailed information on the origin of the measured dichroic response of the system, a model based on dipolar interactions has been developed and the effect of tarnishing and morphol. dispersion addressed.
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    Verre, R.; Fleischer, K.; Sofin, R.; McAlinden, N.; McGilp, J.; Shvets, I. In situ characterization of one-dimensional plasmonic Ag nanocluster arrays. Phys. Rev. B 2011, 83, 125432,  DOI: 10.1103/PhysRevB.83.125432
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    In situ characterization of one-dimensional plasmonic Ag nanocluster arrays
    Verre, R.; Fleischer, K.; Sofin, R. G. S.; McAlinden, N.; McGilp, J. F.; Shvets, I. V.
    Physical Review B: Condensed Matter and Materials Physics (2011), 83 (12), 125432/1-125432/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    One-dimensional Ag nanoparticle arrays were grown on step-bunched vicinal Al2O3 in ultrahigh vacuum using deposition at a glancing angle. The structures grown showed a strong optical anisotropy in the visible region of the spectrum. The optical anisotropy was measured in situ using reflection anisotropy spectroscopy. Relevant optical properties were detd. as a function of deposition angle and Ag thickness. A simple phenomenol. model was developed to reproduce the features seen in the spectra. With this model it was possible to use the inhomogeneous broadening as a guide to the nanoparticle dispersion.
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    Plieth, W.; Naegele, K. Über die bestimmung der optischen konstanten dünnster oberflächenschichten und das problem der schichtdicke. Surf. Sci. 1977, 64, 484– 496,  DOI: 10.1016/0039-6028(77)90058-9
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    Determination of the optical constants of thin surface layers and the problem of layer thickness
    Plieth, W. J.; Naegele, K.
    Surface Science (1977), 64 (2), 484-96CODEN: SUSCAS; ISSN:0039-6028.
    The Fresnel reflection coeffs. of a phase boundary disturbed by values of the optical consts. differing from the values in the adjacent phases (boundary phase) were derived. The derivation on the basis of Maxwells' theory uses integral boundary conditions. The deviations from the ideal reflection properties, caused by a bouncary phase with uniaxial symmetry, are proportional to the difference of the dielec. consts. (for the tangential component) and to the difference of the reciprocal values of the dielec. consts. (for the normal component) to the value in the undisturbed case, integrated over the boundary phase. An optical thickness of the boundary phase can not be detd. by means of ellipsometry of reflection spectroscopy.
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    Jellison, G. E.; Modine, F. A. Optical nature of interface layers: a comparative study of the Si–SiO_2 interface. J. Opt. Soc. Am. 1982, 72, 1253,  DOI: 10.1364/JOSA.72.001253
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    Optical nature of interface layers: a comparative study of the silicon-silicon dioxide interface
    Jellison, G. E., Jr.; Modine, F. A.
    Journal of the Optical Society of America (1982), 72 (9), 1253-7CODEN: JOSAAH; ISSN:0030-3941.
    A comparative study is made of 4 methods for treating the graded nature of the Si-SiO2 interface in the anal. of ellipsometry data: (1) the stratified-media model, in which the total no. of layers is large (>10); (2) the 3-boundary approxn., in which the interface is treated as a single, homogeneous layer; (3) the 2-boundary approxn., in which the effects of the interface are incorporated into an effective oxide-layer thickness; and (4) the integral-equation method, approximated to 1st order, of W.J. Plieth and K. Naegele (1977). The Si-SiO2 transition layer is assumed to have optical functions that vary monotonously between those of Si and SiO2 in 6 different ways. In each of the 6 cases, the ellipsometry parameters ψ and Δ are computed for the air-SiO2 interface bulk-Si system using the 4 methods stated above. The results illustrate the sensitivity of ellipsometry to the interface layer and reveal that the interface layer must be properly taken into account for accurate anal. The 3-boundary approxn. is not necessarily more accurate than a judiciously applied 2-boundary model and the 1st-order approxn. of P. and N. introduces sizable errors at short wavelengths for the Si-SiO2 interface.
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    Hu, J.; Wang, J.; Wei, Y.; Wu, Q.; Zhang, F.; Xu, Q. Effect of film growth thickness on the refractive index and crystallization of HfO2 film. Ceram. Int. 2021, 47, 33751– 33757,  DOI: 10.1016/j.ceramint.2021.08.286
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    Effect of film growth thickness on the refractive index and crystallization of HfO2 film
    Hu, Jianping; Wang, Jian; Wei, Yaowei; Wu, Qian; Zhang, Fei; Xu, Qiao
    Ceramics International (2021), 47 (23), 33751-33757CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
    A series of Hafnium dioxide (HfO2) thin films with nominal thickness of 5-350 nm were reactively deposited on silicon(100)substrates at 200°C using electron-beam evapn. Based on the measurement and characterization techniques of spectroscopic ellipsometry, X ray diffraction, SEM and at. force microscopy, the effect of growth thickness on the refractive index and crystn. of HfO2 film grown under the same conditions was studied. The results indicate that the change of refractive index of HfO2 film is closely related to the change of microstructure i.e., grain size and crystn. which change obviously with the increasing growth thickness. The av. refractive index at 633 nm decreases about from 1.97 to 1.84 with the increase of the av. size about from 0.5 nm to 7∼8 nm and the crystallinity from zero to 74% when the thickness of HfO2 film increases from 5 nm to 350 nm. Meanwhile, a crit. growth thickness of HfO2 film, somewhere on order of 130 nm, is confirmed at which the film transforms from an amorphous to a polycryst. monoclinic structure. The sharp change of microstructure near the crit. thickness value of HfO2 film leads to the abrupt change of optical properties of HfO2 film, such as the turning behaviors of the refractive index curve. The correlation between phase transformation, size and orientation of crystallite, packing d. and hence the refractive index is established. The possible interpretations are proposed to well understand the underlying mechanism of the evolution of optical properties in HfO2 film-forming process.
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    Copper oxide ALD from a Cu(I) β-diketonate: detailed growth studies on SiO2 and TaN
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    ECS Transactions (2009), 25 (4, Atomic Layer Deposition Applications 5), 277-287CODEN: ECSTF8; ISSN:1938-5862. (Electrochemical Society)
    The at. layer deposition (ALD) of copper oxide films from [(nBu3P)2Cu(acac)] and wet oxygen on SiO2 and TaN has been studied in detail by spectroscopic ellipsometry and at. force microscopy. The results suggest island growth on SiO2, along with a strong variation of the optical properties of the films in the early stages of the growth and signs of quantum confinement, typical for nanocrystals. In addn., differences both in growth behavior and film properties appear on dry and wet thermal SiO2. Electron diffraction together with transmission electron microscopy shows that nanocryst. Cu2O with crystallites < 5 nm is formed, while upon prolonged electron irradn. the films decomp. and metallic copper crystallites of ∼ 10 nm ppt. On TaN, the films grow in a linear, layer-by-layer manner, reproducing the initial substrate roughness. Satd. growth obtained at 120°C on TaN as well as dry and wet SiO2 indicates well-established ALD growth regimes.
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    Journal of Physical Chemistry (1958), 62 (), 1098-1101CODEN: JPCHAX; ISSN:0022-3654.
    The order of decreasing heat stability for the acetylacetonates, based on the evolution of gaseous products, was Na(I) > Cr(III) > Al(III) > Ni(II) > Cu(II) > Fe(III) > Co(II) > Co(III) > Mn(III). Acetone and CO2 were the major decompn. products for all the acetylacetonates. For some of the chelates, acetylacetone and CH4 were also among the principal decompn. products. A no. of other gases were also present in low concn.
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    Journal of Thermal Analysis and Calorimetry (2019), 135 (6), 3463-3470CODEN: JTACF7; ISSN:1388-6150. (Springer)
    In earlier work, we studied the phase transitions of a no. of metal(III) acetylacetonates using thermal anal. (DSC) and coupled that with solid-state single crystal X-ray structures. From these studies, we found it necessary to explore the complexes of Al(III), Cr(III), Mn(III), Fe(III) and Co(III) in more detail. In the process of examg. the extant data on the tris-metal acetylacetonates, we discovered that there were a broad range of applications in which those substances seem to have interesting and unusual properties, including thermal and radiation sensitivity, resulting in the formation of radicals which were documented to be useful in the initiation and promotion of polymn., etc. Thus, we decided to combine our synthetic and crystn. experience with thermal studies (e.g., DSC, MS and TG) in an attempt to document the nature of metal tris-acetylacetonates and their thermalization products between room temp. and their decompn. temps.
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    Atomic layer deposition (ALD) is a vital gas-phase technique for at.-level thickness-controlled deposition of high-quality thin films on various substrate morphologies owing to its self-limiting gas-surface reaction mechanism. Here we report the ALD fabrication of thin films of the semiconducting CuCrO2 oxide that is a highly prospective candidate for transparent electronics applications. In our process, copper 2,2,6,6-tetramethyl-3,5-heptanedionate (Cu(thd)2) and chromium acetyl acetonate (Cr(acac)3) are used as the metal precursors and ozone as the oxygen source. Smooth and homogeneous thin films with an accurately controlled metal compn. can be deposited at of 240-270°; a post-deposition anneal at 700-950° in an Ar atm. then results in well cryst. films with a delafossite structure. Elec. transport measurements confirm the p-type semiconducting behavior of the films. The direct bandgap is detd. from UV-vis spectrophotometric measurements to be 3.09 eV. The obsd. transmittance is greater than 75% in the visible range.
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    The removal of copper oxide using acetic acid at low temps. was investigated. Acetic acid removes a variety of copper oxides, including cuprous oxide, cupric oxide, and cupric hydroxide without attacking the underlying copper film. The removal of these oxides was detd. by XPS. Acetic acid can tolerate up to 4 vol. % water diln. without hindering the oxide removal while producing an oxide-free surface. However, if a deionized water rinse is performed after an acetic acid treatment, a surface film of cupric hydroxide forms immediately. An acetic acid treatment at 35°C without a water rinse removes the native copper oxide and produces an oxide-free, streak-free copper surface.
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    SiO2-TiO2 thin films were deposited by metalorg. CVD using an alkoxide source. At 680°, the deposition rate curve showed parabolic behavior and the refractive index increased linearly from 1.45 to 2.35 with increasing Ti tetraisopropoxide (TTIP) ratio. Each oxide component in the film was sepd. anal. and its effective deposition rate, in the composite thin film, was calcd. to analyze the deposition mechanism of the mixed sources. A Lorentz-Lorenz model was used to attain the compn. of the film for each component sepn. Effective SiO2 deposition from tetraethylorthosilicate4 (TEOS) showed parabolic behavior with increasing TTIP ratio, while the effective TiO2 deposition did not. In addn., TTIP lowered the apparent activation energy of SiO2 deposition significantly from ∼40 to ∼10 Kcal/mol. From this, TTIP enhanced the TEOS decompn., which results in the anomalous deposition behavior in composite films. A more reactive TTIP mol. acting as a free radical reaction initiator was suggested as a mechanism for enhancement of the process.
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    We present the deposition of amorphous Ti-Si-O thin films by an aerosol chem. vapor deposition process at atm. pressure. We used di-acetoxi-di-butoxysilane, tetrabutoxysilane, and titanium(IV) n-butoxide as precursors, and the deposition temps. ranged from 475 to 675°C. During the deposition process a chem. reaction between the different precursors took place. We found a synergetic mutual influence of the precursors and a coupled activation of the growth process between Ti and Si. The obtained thin films were characterized by various techniques: SEM, at. force microscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, IR spectroscopy, ellipsometry, and XPS. Our films were amorphous TiO2-SiO2 mixts. with a refractive index between 1.45 and 2.1 depending on the Si/Ti ratio in the films. We established a correlation between the XPS results and the refractive index of the films, allowing us to control the film compn. as a function of the deposition conditions.
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    Journal of Alloys and Compounds (2015), 647 (), 1028-1034CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    Transparent conductive CuCrO2 oxide films were prepd. on sapphire substrates by the sol-gel method using copper (II) acetate monohydrate and chromium (III) nitrate hydrate as raw materials. The highly c-axis orientation and optical transparency (60-80%) in the visible region were obtained. The microstructure, vibration modes, optical and elec. transport properties as a function of annealing temps. have been systematically discussed. With increasing annealing temp., the cryst. quality and the c-axis orientation of CuCrO2 films were improved, which induced the increase of Raman phonon amplitude with the decrease of the linewidth. But the film compactness and d. decrease with the increase of av. particle size. To study the intrinsic mechanism of the optical response behavior, a three-phase layered structure (air/film/substrate) was applied to simulate the transmittance spectra of CuCrO2 films. Three electronic transitions can be uniquely assigned and it is found that the direct transitions are the dominant effect for CuCrO2 optical properties. The temp. dependence of elec. cond. was studied and the elec. conduction mechanisms of the thermal activation behavior and the three-dimensional variable range-hopping transport were obsd. In addn., the cond. of CuCrO2 films decreases with increasing annealing temp. induced by its microstructure and the grain boundary scattering. Thus, the film annealed at 600 °C has the largest elec. cond. of 0.49 S m-1 at room temp. The present results could be crucial and provide theor. support for future applications of p-type semiconductor and optoelectronic devices.
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    Correlation of intrinsic point defects and the Raman modes of cuprous oxide
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    Physical Review B: Condensed Matter and Materials Physics (2014), 90 (4), 045203CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The Raman spectrum of cryst. Cu2O taken off resonance is reproducible and independent of the growth method and conditions employed. But, in contrast to most other cryst. materials, the Raman spectrum of Cu2O is dominated by IR active and silent lattice modes rather than by the only Raman allowed phonon mode. We show that this unusual behavior is most likely caused by the presence of copper vacancies in the so-called split configuration, a point defect particular to Cu2O. The redn. of symmetry due to the presence of point defects may lift the Raman selection rules and may introduce Raman activity for phonon modes that are Raman forbidden in the case of perfect crystal symmetry. Based on this group theor. consideration, we predict the angle dependence of the Raman intensities of all Cu2O one-phonon modes at k = 0 for rotation about the (100) direction caused by the presence of various intrinsic point defects. Of all intrinsic defects in question, only the presence of the copper vacancy in the split configuration introduces Raman activity for all Cu2O extended phonon modes obsd. in expt. and is consistent with the angle-dependent measurements. Our study underlines the special role of the split vacancy in Cu2O.
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  • Abstract

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    Figure 1

    Figure 1. Schematic of the small spray pyrolysis chamber and the main optical components of the RAS setup. The inset shows the SEM image of Au dimers before Al2O3 capping.

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    Figure 2

    Figure 2. RAS signal acquisition from gold dimers (a) before deposition and (b) after deposition.

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    Figure 3

    Figure 3. Example of a deposition cycle. (a) RAS spectra of the LSPR meta-surface at 230 °C before and after a deposition cycle using a 0.01 M solution of Cu(acac)2 in methanol. The top panel shows the difference in the two spectra. The shift in the spectral position of the plasmon resonance is evident. In the shaded regions there is a large difference and a nearly linear spectral dependence. Single wavelength, transient measurements during depositions are best taken there. (b) Transient measurement at 1.85 eV taken during growth. The shaded area indicates the time when the gas supply to the nebulizer was on. The top panel shows the DC level of the measurement proportional to the intensity of the reflected light.

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    Figure 4

    Figure 4. Arrhenius type plots of the normalized slope Sn for (a) the Cu(acac)2 precursor (0.01 M solution). (b) Comparison of the Cr(acac)3 precursor (0.015 M solution) measurement to those of Cu(acac)2. Both measurements were performed with 5% oxygen content in the nebulizing gas. The estimated growth rate scale in (b) is given as a rough guide and is based on an assumed linear relationship of the growth rate with molarity and Sn.

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    Figure 5

    Figure 5. Film composition (a) and carrier activation energy (b) as a function of oxygen content in the nebulizing gas. Each sample was deposited until the full cup of 8 mL is emptied at a temperature of 310 °C.

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    Figure 6

    Figure 6. Raman spectra of films grown at varying oxygen content in the nebulizing gas: (a) full nitrogen and 2% oxygen and (b) 2%, 9%, and 15% oxygen. Labels indicate expected peak positions for highly crystalline materials. (c) T (solid lines), R (dashed lines), and T + R (dotted lines) of the same films. Changes are dominated by differences in overall film thickness, with 9% being the thickest.

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  • References

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      With potentials to outperform traditional semiconductors, oxide semiconductors are penetrating into a wide range of energy, environmental, and electronic applications. However, an insufficient library of p-type oxides is a major obstacle against complete oxide electronics. To identify promising p-type oxides, we carry out a large-scale computational screening over 17,700 oxygen-contg. compds. using the hydrogen descriptor that was shown to select the p-type oxide reliably. To enable high-throughput screening, we use in the initial stage coarse but simple descriptors that correlate with the hydrogen descriptor. Through a hierarchical screening procedure, we find 156 p-type oxides with the band gap larger than 1.1 eV, discovering promising candidates such as NaNbO2 and La2SiO4Se and also new types of p-type oxides. Furthermore, we classify the identified p-type oxides according to the valence-band character and reveal chem. principles underlying the p-type dopability.
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      Dunlap-Shohl, W. A.; Daunis, T. B.; Wang, X.; Wang, J.; Zhang, B.; Barrera, D.; Yan, Y.; Hsu, J. W. P.; Mitzi, D. B. Room-temperature fabrication of a delafossite CuCrO2 hole transport layer for perovskite solar cells. Journal of Materials Chemistry A 2018, 6, 469– 477,  DOI: 10.1039/C7TA09494A
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      Delafossite oxides are promising hole transport layer (HTL) candidates for perovskite solar cells, due to their wide band gap, favorable energy band alignment relative to the perovskite absorber and simplicity of processing. In this paper, we investigate the properties of CuCrO2 (CCO) delafossite films using an integration of exptl. and computational techniques. Phase-pure CCO films are deposited at room temp. by spin-casting suspensions of hydrothermally-synthesized nanoparticles, for use in a glass/ITO/CCO/CH3NH3PbI3/C60/BCP/Ag device structure. Although d. functional theory (DFT) calcns. predict an elevated hole effective mass along certain crystallog. directions, the nearly isotropic shape and small size of the CCO nanoparticles preserves transport properties within the films by randomizing particle orientation, preventing these unfavorable directions from dominating the film cond. Exptl. measurements confirm that the CCO films display appropriate optical and elec. properties for use as an HTL, in good agreement with the DFT calcns. Solar cells made using these films exhibit stabilized power conversion efficiencies exceeding 14%, with only minor hysteresis in the current-voltage characteristics.
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      Wang, J.; Lee, Y.-J.; Hsu, J. W. P. Sub-10 nm copper chromium oxide nanocrystals as a solution processed p-type hole transport layer for organic photovoltaics. Journal of Materials Chemistry C 2016, 4, 3607– 3613,  DOI: 10.1039/C6TC00541A
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      4
      Sub-10 nm copper chromium oxide nanocrystals as a solution processed p-type hole transport layer for organic photovoltaics
      Wang, Jian; Lee, Yun-Ju; Hsu, Julia W. P.
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (16), 3607-3613CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      We report the synthesis of CuCrO2 nanocrystals, a p-type transparent conducting oxide, and their application as an efficient hole transport layer (HTL) for org. photovoltaic (OPV) devices. A nanometer-sized mixt. of Cu and Cr oxide/hydroxide is synthesized using microwave-assisted heating. With a 550 °C post-annealing treatment in N2, <10 nm CuCrO2 nanocrystals are successfully synthesized. XRD, XPS, EDAX, PESA, UV-vis spectrometry, and Kelvin probe technique are applied to confirm the delafossite phase, optical transmission, and p-type characteristics. Methanol is found to be a good solvent to disperse these nanocrystals for forming a smooth and transparent film. In comparison with the previously reported CuGaO2 HTL, the reduced film roughness enables the CuCrO2 HTL to produce highly efficient thin active layer OPV devices. UV-ozone treatment on the CuCrO2 HTL is found to increase the fill factor. Drift-diffusion modeling, energy level measurements, and XPS results reveal that the device improvement is not due to the reduced injection barrier, but due to an improved CuCrO2 cond. arising from the formation of Cu2+ species.
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    5. 5
      Gil, B.; Kim, J.; Yun, A. J.; Park, K.; Cho, J.; Park, M.; Park, B. CuCrO2 Nanoparticles Incorporated into PTAA as a Hole Transport Layer for 85 °C and Light Stabilities in Perovskite Solar Cells. Nanomaterials 2020, 10, 1669,  DOI: 10.3390/nano10091669
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      CuCrO2 nanoparticles incorporated into PTAA as a hole transport layer for 85°C and light stabilities in perovskite solar cells
      Gil, Bumjin; Kim, Jinhyun; Yun, Alan Jiwan; Park, Kimin; Cho, Jaemin; Park, Minjun; Park, Byungwoo
      Nanomaterials (2020), 10 (9), 1669CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
      High-mobility inorg. CuCrO2 nanoparticles are co-utilized with conventional poly(bis(4-phenyl)(2,5,6-trimethylphenyl)amine) (PTAA) as a hole transport layer (HTL) for perovskite solar cells to improve device performance and long-term stability. Even though CuCrO2 nanoparticles can be readily synthesized by hydrothermal reaction, it is difficult to form a uniform HTL with CuCrO2 alone due to the severe agglomeration of nanoparticles. Herein, both CuCrO2 nanoparticles and PTAA are sequentially deposited on perovskite by a simple spin-coating process, forming uniform HTL with excellent coverage. Due to the presence of high-mobility CuCrO2 nanoparticles, CuCrO2/PTAA HTL demonstrates better carrier extn. and transport. A redn. in trap d. is also obsd. by trap-filled limited voltages and capacitance analyses. Incorporation of stable CuCrO2 also contributes to the improved device stability under heat and light. Encapsulated perovskite solar cells with CuCrO2/PTAA HTL retain their efficiency over 90% after ~ 900-h storage in 85°C/85% relative humidity and under continuous 1-sun illumination at max.-power point.
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    6. 6
      Zhang, H.; Wang, H.; Zhu, H.; Chueh, C.-C.; Chen, W.; Yang, S.; Jen, A. K.-Y. Low-Temperature Solution-Processed CuCrO2 Hole-Transporting Layer for Efficient and Photostable Perovskite Solar Cells. Adv. Energy Mater. 2018, 8, 1702762,  DOI: 10.1002/aenm.201702762
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      Qin, P.-L.; Lei, H.-W.; Zheng, X.-L.; Liu, Q.; Tao, H.; Yang, G.; Ke, W.-J.; Xiong, L.-B.; Qin, M.-C.; Zhao, X.-Z. Copper-Doped Chromium Oxide Hole-Transporting Layer for Perovskite Solar Cells: Interface Engineering and Performance Improvement. Advanced Materials Interfaces 2016, 3, 1500799,  DOI: 10.1002/admi.201500799
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    8. 8
      Fleischer, K.; Norton, E.; Mullarkey, D.; Caffrey, D.; Shvets, I. V. Quantifying the Performance of P-Type Transparent Conducting Oxides by Experimental Methods. Materials 2017, 10, 1019,  DOI: 10.3390/ma10091019
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      8
      Quantifying the performance of P-type transparent conducting oxides by experimental methods
      Fleischer, Karsten; Norton, Emma; Mullarkey, Daragh; Caffrey, David; Shvets, Igor V.
      Materials (2017), 10 (9), 1019/1-1019/14CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
      Screening for potential new materials with exptl. and theor. methods has led to the discovery of many promising candidate materials for p-type transparent conducting oxides. It is difficult to reliably assess a good p-type transparent conducting oxide (TCO) from limited information available at an early exptl. stage. In this paper we discuss the influence of sample thickness on simple transmission measurements and how the sample thickness can skew the commonly used figure of merit of TCOs and their estd. band gap. We discuss this using copper-deficient CuCrO2 as an example, as it was already shown to be a good p-type TCO grown at low temps. We outline a modified figure of merit reducing thickness-dependent errors, as well as how modern ab initio screening methods can be used to augment exptl. methods to assess new materials for potential applications as p-type TCOs, p-channel transparent thin film transistors, and selective contacts in solar cells.
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    9. 9
      Williamson, B. A. D.; Buckeridge, J.; Brown, J.; Ansbro, S.; Palgrave, R. G.; Scanlon, D. O. Engineering Valence Band Dispersion for High Mobility p-Type Semiconductors. Chem. Mater. 2017, 29, 2402– 2413,  DOI: 10.1021/acs.chemmater.6b03306
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      9
      Engineering Valence Band Dispersion for High Mobility p-Type Semiconductors
      Williamson, Benjamin A. D.; Buckeridge, John; Brown, Jennilee; Ansbro, Simon; Palgrave, Robert G.; Scanlon, David O.
      Chemistry of Materials (2017), 29 (6), 2402-2413CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The paucity of high performance transparent p-type semiconductors was a stumbling block for the electronics industry for decades, effectively hindering the route to efficient transparent devices based on p-n junctions. Cu-based oxides and subsequently Cu-based oxychalcogenides were heavily studied as affordable, earth-abundant p-type transparent semiconductors, where the mixing of the Cu 3d states with the chalcogenide 2p states at the top of the valence band encourages increased valence band dispersion. The authors extend this mixing concept further, by using quantum chem. techniques to study ternary copper phosphides as potential high mobility p-type materials. The authors use hybrid d. functional theory to examine a family of phosphides, namely, MCuP (M = Mg, Ca, Sr, Ba) which all possess extremely disperse valence band maxima, comparable to the dispersion of excellent industry std. n-type transparent conducting oxides. As a proof of concept, the authors synthesized and characterized powders of CaCuP, showing that they display high levels of p-type cond., without any external acceptor dopant. Lastly, the role of Cu-coordination in promoting valence band dispersion and provide design principles for producing degenerate p-type materials are discussed.
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    10. 10
      Lunca-Popa, P.; Botsoa, J.; Bahri, M.; Crêpelliére, J.; Desgardin, P.; Audinot, J.-N.; Wirtz, T.; Arl, D.; Ersen, O.; Barthe, M.-F. Tuneable interplay between atomistic defects morphology and electrical properties of transparent p-type highly conductive off-stoichiometric Cu-Cr-O delafossite thin films. Sci. Rep. 2020, 10, 1416,  DOI: 10.1038/s41598-020-58312-z
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      10
      Tuneable interplay between atomistic defects morphology and electrical properties of transparent p-type highly conductive off-stoichiometric Cu-Cr-O delafossite thin films
      Lunca-Popa, Petru; Botsoa, Jacques; Bahri, Mounib; Crepelliere, Jonathan; Desgardin, Pierre; Audinot, Jean-Nicolas; Wirtz, Tom; Arl, Didier; Ersen, Ovidiu; Barthe, Marie-France; Lenoble, Damien
      Scientific Reports (2020), 10 (1), 1416CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
      Off-stoichiometric copper chromium delafossites demonstrate the highest values of elec. cond. among the p-type transparent conducting oxides. Morphol. and structural changes in Cu0.66Cr1.33O2 upon annealing processes are investigated. Chained copper vacancies were previously suggested as source of the high levels of doping in this material. High resoln. Helium Ion Microscopy, Secondary Ion Mass Spectrometry and Transmission Electron Microscopy reveal a significant rearrangement of copper and chromium after the thermal treatments. Furthermore, Positron Annihilation Spectroscopy evidences the presence of vacancy defects within the delafossite layers which can be assigned to the Cu vacancy chains whose concn. decreases during the thermal process. These findings further confirm these chained vacancies as source of the p-type doping and suggest that the changes in elec. conductivities within the off-stoichiometric copper based delafossites are triggered by elemental rearrangements.
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    11. 11
      Yokobori, T.; Okawa, M.; Konishi, K.; Takei, R.; Katayama, K.; Oozono, S.; Shinmura, T.; Okuda, T.; Wadati, H.; Sakai, E. Electronic structure of the hole-doped delafossite oxides CuCr1–xMgxO2. Phys. Rev. B 2013, 87, 195124,  DOI: 10.1103/PhysRevB.87.195124
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      11
      Electronic structure of the hole-doped delafossite oxides CuCr1-xMgxO2
      Yokobori, T.; Okawa, M.; Konishi, K.; Takei, R.; Katayama, K.; Oozono, S.; Shinmura, T.; Okuda, T.; Wadati, H.; Sakai, E.; Ono, K.; Kumigashira, H.; Oshima, M.; Sugiyama, T.; Ikenaga, E.; Hamada, N.; Saitoh, T.
      Physical Review B: Condensed Matter and Materials Physics (2013), 87 (19), 195124/1-195124/8CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We report the detailed electronic structure of a hole-doped delafossite oxide CuCr1-xMgxO2 (0 ≤ x ≤ 0.03) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calcns. within the local-d. approxn. +U (LDA + U) scheme. Cr/Cu 3p-3d resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr 3d character with a minor contribution from the Cu 3d through Cu 3d-O 2p-Cr 3d hybridization, having good agreement with the band-structure calcns. This indicates that a doped hole will have primarily the Cr 3d character. Cr 2p PES and L-edge XAS spectra exhibit typical Cr3+ features for all x, while the Cu L-edge XAS spectra exhibited a systematic change with x. This indicates now that the Cu valence is monovalent at x = 0 and the doped hole should have Cu 3d character. Nevertheless, we surprisingly obsd. two types of charge-transfer satellites that should be attributed to Cu+ (3d10) and Cu2+ (3d9) like initial states in Cu 2p-3d resonant PES spectrum of at x = 0, while Cu 2p PES spectra with no doubt shows the Cu+ character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing not only the Cu 4s state, but also finite Cu 3d, 4s-Cr 3d charge transfer via O 2p states in the ground-state electronic configuration.
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    12. 12
      Norton, E.; Farrell, L.; Callaghan, S. D.; McGuinness, C.; Shvets, I. V.; Fleischer, K. X-ray spectroscopic studies of the electronic structure of chromium-based p -type transparent conducting oxides. Phys. Rev. B 2016, 93, 115302,  DOI: 10.1103/PhysRevB.93.115302
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      12
      X-ray spectroscopic studies of the electronic structure of chromium-based p-type transparent conducting oxides
      Norton, E.; Farrell, L.; Callaghan, S. D.; McGuinness, C.; Shvets, I. V.; Fleischer, K.
      Physical Review B (2016), 93 (11), 115302/1-115302/8CODEN: PRBHB7; ISSN:2469-9950. (American Physical Society)
      The valence band structure of p-type transparent oxides-cryst. MgxCr2-xO3 and nanocryst. CuxCrOy-is analyzed as a function of incoming photon energy. The valence band of both p-type transparent conducting oxides shows striking similarities to measurements on cryst. CuCrO2:Mg with all films showing that chromium states compose the top of the valence band, suggesting that the valence-band structure is dominated by the presence of the Cr-O6 octahedra. A comparison of the valence band between the best performing p-type, cryst. CuCrO2:Mg, with cryst. MgxCr2-xO3 and nanocryst. CuxCrOy shows that the chromium 3d states are fixed irresp. of changes in long-range crystallog. order. This indicates little spatial overlap between adjacent Cr 3d states. This further confirms the conduction mechanism via hopping for chromium based p-type TCOs as the Cr 3d states are localized within the Cr-O6 octahedra.
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    13. 13
      Farrell, L.; Fleischer, K.; Caffrey, D.; Mullarkey, D.; Norton, E.; Shvets, I. V. Conducting mechanism in the epitaxial p -type transparent conducting oxide C r2 O3:Mg. Phys. Rev. B 2015, 91, 125202,  DOI: 10.1103/PhysRevB.91.125202
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      13
      Conducting mechanism in the epitaxial p-type transparent conducting oxide Cr2O3:Mg
      Farrell, L.; Fleischer, K.; Caffrey, D.; Mullarkey, D.; Norton, E.; Shvets, I. V.
      Physical Review B: Condensed Matter and Materials Physics (2015), 91 (12), 125202/1-125202/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Epitaxial p-type transparent conducting oxide (TCO) Cr2O3:Mg was grown by electron-beam evapn. in a mol. beam epitaxy system on c-plane sapphire. The influence of Mg dopants and the oxygen partial pressure were investigated by thermoelec. and elec. measurements. The conduction mechanism is analyzed using the small-polaron hopping model, and hopping activation energies have been detd., which vary with doping concn. in the range of 210-300 ± 5 meV. Films with better cond. were obtained by post-annealing. The effect of post-annealing is discussed in terms of a crystallog. reordering of the Mg dopant. The highest Seebeck mobilities obtained from thermoelec. measurements are of the order of 10-4 cm2V-1s-1. We investigate the fundamental properties of a Mg dopant in a high cryst. quality epitaxial film of a binary oxide, helping us understand the role of short range crystallog. order in a p-type TCO in detail.
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    14. 14
      Farrell, L.; Norton, E.; Smith, C. M.; Caffrey, D.; Shvets, I. V.; Fleischer, K. Synthesis of nanocrystalline Cu deficient CuCrO2-a high figure of merit p-type transparent semiconductor. Journal of Materials Chemistry C 2016, 4, 126– 134,  DOI: 10.1039/C5TC03161C
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      14
      Synthesis of nanocrystalline Cu deficient CuCrO2 - a high figure of merit p-type transparent semiconductor
      Farrell, Leo; Norton, Emma; Smith, Christopher M.; Caffrey, David; Shvets, Igor V.; Fleischer, Karsten
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2016), 4 (1), 126-134CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      The delafossite structured CuCrO2 system is known as one of the best performing p-type transparent conducting oxides. The details of a low temp. facile growth method for CuCrO2 is described. The dependence of the growth on the precursors, the temp. and oxygen partial pressure were examd. The decompn. routes are crit. to obtain the best performing films. The thermopower and elec. measurements indicate p-type films with cond. ranging from 1-12 S cm-1 depending on the growth conditions. This p-type cond. is retained despite the nanocrystallinity of the films. The figure of merit of these films can be ≤350 μS, which is the best performing p-type TCO by soln. methods to date. The optical properties are also studied using ellipsometry and UV-visible spectroscopy.
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    15. 15
      Farrell, L.; Norton, E.; O’dowd, B. J.; Caffrey, D.; Shvets, I. V.; Fleischer, K. Spray pyrolysis growth of a high figure of merit, nano-crystalline, p -type transparent conducting material at low temperature. Appl. Phys. Lett. 2015, 107, 031901,  DOI: 10.1063/1.4927241
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      15
      Spray pyrolysis growth of a high figure of merit, nano-crystalline, p-type transparent conducting material at low temperature
      Farrell, L.; Norton, E.; O'Dowd, B. J.; Caffrey, D.; Shvets, I. V.; Fleischer, K.
      Applied Physics Letters (2015), 107 (3), 031901/1-031901/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The authors demonstrate a low temp. (≈345°) growth method for Cu deficient CuCrO2 performed by spray pyrolysis using metal-org. precursors and a simple air blast nozzle. Smooth films were grown on glass substrates with a highest cond. of 12 S/cm. The most conductive samples retain transparencies >55% resulting in a figure of merit ≤350 μS, which is the best performing p-type transparent conducting material grown by soln. methods to date. Remarkably, despite the nano-crystallinity of the films, properties comparable with cryst. CuCrO2 are obsd. No postannealing of the films is required in contrast to previous reports on cryst. material. The low processing temp. of this method means that the material can be deposited on flexible substrates. As this is a soln. based technique, it is more attractive to industry as phys. vapor deposition methods are slow and costly in comparison. (c) 2015 American Institute of Physics.
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    16. 16
      Sánchez-Alarcón, R. I.; Oropeza-Rosario, G.; Gutierrez-Villalobos, A.; Muro-López, M. A.; Martínez-Martínez, R.; Zaleta-Alejandre, E.; Falcony, C.; Alarcón-Flores, G.; Fragoso, R.; Hernández-Silva, O. Ultrasonic spray-pyrolyzed CuCrO2thin films. J. Phys. D: Appl. Phys. 2016, 49, 175102,  DOI: 10.1088/0022-3727/49/17/175102
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      16
      Ultrasonic spray-pyrolyzed CuCrO2 thin films
      Sanchez-Alarcon, R. I.; Oropeza-Rosario, G.; Gutierrez-Villalobos, A.; Muro-Lopez, M. A.; Martinez-Martinez, R.; Zaleta-Alejandre, E.; Falcony, C.; Alarcon-Flores, G.; Fragoso, R.; Hernandez-Silva, O.; Perez-Cappe, E.; Laffita, Yodalgis Mosqueda; Aguilar-Frutis, M.
      Journal of Physics D: Applied Physics (2016), 49 (17), 175102/1-175102/7CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
      In this paper the optical, structural and elec. properties of CuCrO2 thin films deposited by ultrasonic spray pyrolysis at temps. from 400 to 600 °C in steps of 50 °C are presented. Copper and chromium acetylacetonates were chosen as sources of Cu and Cr, resp., and N,N-dimethylformamide was used as the solvent. X-ray results confirmed that the films as deposited showed the CuCrO2 phase without any post-deposition thermal annealing. The surface morphol. was obsd. to be mirror like, and as the films were deposited at different temps., they gradually revealed the presence of small crystallites. The best film's optical percentage transmission (in the visible region), about 58%, was obtained in films deposited at 450 °C, and the highest band gap energy (3.17 eV) was measured in films deposited at 400 °C. The elec. properties of the films were obtained by the Hall effect. A hole concn. in the range 1019-1021 cm-3, cond. as high as 35 S cm-1, and mobility lower than 1 cm2 V-1 s-1 were obtained in the films. p-type cond. was confirmed using the hot point probe arrangement, and the Seebeck coeff. was estd. The hole cond. is thought to be due to excess oxygen in the films. Finally, the min. energy required to transfer carriers from acceptor level to the valence band in the films was estd. by impedance spectroscopy.
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    17. 17
      Fredriksson, H.; Alaverdyan, Y.; Dmitriev, A.; Langhammer, C.; Sutherland, D. S.; Zäch, M.; Kasemo, B. Hole-Mask Colloidal Lithography. Adv. Mater. 2007, 19, 4297– 4302,  DOI: 10.1002/adma.200700680
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      17
      Hole-mask colloidal lithography
      Fredriksson, Hans; Alaverdyan, Yury; Dmitriev, Alexandre; Langhammer, Christoph; Sutherland, Duncan S.; Zaech, Michael; Kasemo, Bengt
      Advanced Materials (Weinheim, Germany) (2007), 19 (23), 4297-4302CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Hole-mask colloidal lithog. represents a truly versatile and simple bottom-up nanofabrication method based on colloidal self-assembly lithog. patterning. The technique provides an effective means of patterning vast surface areas with diverse functional nanoarchitectures. Examples include arrays of nanodisks, oriented elliptical nanostructures, (binary) nanodisk pairs, nanocones on extended surfaces and nanodisks embedded in a surrounding matrix.
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      Levitsky, I.; Tavor, D. Improved Atomization via a Mechanical Atomizer with Optimal Geometric Parameters and an Air-Assisted Component. Micromachines 2020, 11, 584,  DOI: 10.3390/mi11060584
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      Hallberg, C. J.; Lysaught, M. T.; Zmudka, C. E.; Kopesky, W. K.; Olson, L. E. Characterization of a human powered nebulizer compressor for resource poor settings. BioMedical Engineering OnLine 2014, 13, 77,  DOI: 10.1186/1475-925X-13-77
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      19
      Characterization of a human powered nebulizer compressor for resource poor settings
      Hallberg Christopher J; Lysaught Mary Therese; Zmudka Christopher E; Kopesky William K; Olson Lars E
      Biomedical engineering online (2014), 13 (), 77 ISSN:.
      BACKGROUND: Respiratory disease accounts for three of the ten leading causes of death worldwide. Many of these diseases can be treated and diagnosed using a nebulizer. Nebulizers can also be used to safely and efficiently deliver vaccines. Unfortunately, commercially available nebulizers are not designed for use in regions of the world where lung disease is most prevalent: they are electricity-dependent, cost-prohibitive, and not built to be reliable in harsh operating conditions or under frequent use.To overcome these limitations, the Human Powered Nebulizer compressor (HPN) was developed. The HPN does not require electricity; instead airflow is generated manually through a hand-crank or bicycle-style pedal system. A health care worker or other trained individual operates the device while the patient receives treatment.This study demonstrates functional specifications of the HPN in comparison with a standard commercially available electric jet nebulizer compressor, the DeVilbiss Pulmo-Aide 5650D (Pulmo-Aide). METHODS: Pressure and flow characteristics were measured with a rotameter and pressure transducer, respectively. Volume nebulized by each compressor was determined by mass, and particle size distribution was determined via laser diffraction. The Hudson RCI Micro Mist nebulizer mouthpiece was used with both compressors. RESULTS: The pressure and flow generated by the HPN and Pulmo-Aide were: 15.17 psi and 10.5 L/min; and 14.65 psi and 11.2 L/min, respectively. The volume of liquid delivered by each was equivalent, 1.097 ± 0.107 mL (mean ± s.e.m., n = 13) for the HPN and 1.092 ± 0.116 mL for the Pulmo-Aide. The average particle size was also equivalent, 5.38 ± 0.040 micrometers (mean ± s.e.m., n = 7) and 5.40 ± 0.025 micrometers, respectively. CONCLUSIONS: Based on these characteristics, the HPN's performance is equivalent to a popular commercially available electric nebulizer compressor. The findings presented in this paper, combined with the results of two published clinical studies, suggest that the HPN could serve as an important diagnostic and therapeutic tool in the fight against global respiratory health challenges including: tuberculosis, chronic obstructive pulmonary disease, asthma, and lower respiratory infections.
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    20. 20
      Norton, E.; Farrell, L.; Zhussupbekova, A.; Mullarkey, D.; Caffrey, D.; Papanastasiou, D. T.; Oser, D.; Bellet, D.; Shvets, I. V.; Fleischer, K. Bending stability of Cu0.4CrO2─A transparent p-type conducting oxide for large area flexible electronics. AIP Advances 2018, 8, 085013,  DOI: 10.1063/1.5027038
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      20
      Bending stability of Cu0.4CrO2-A transparent p-type conducting oxide for large area flexible electronics
      Norton, E.; Farrell, L.; Zhussupbekova, A.; Mullarkey, D.; Caffrey, D.; Papanastasiou, D. T.; Oser, D.; Bellet, D.; Shvets, I. V.; Fleischer, K.
      AIP Advances (2018), 8 (8), 085013/1-085013/7CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
      The current best performing p-type transparent conducting oxides are typically highly cryst. materials, deposited at high temps., and hence incompatible with the drive to low cost flexible electronics. We investigated a nanocryst., copper deficient CuxCrO2, deposited at low temps. upon a flexible polyimide substrate. The as-deposited film without post annealing has an elec. cond. of 6Scm-1. We demonstrate that this p-type transparent oxide retains its excellent elec. cond. under tensile strain, withstanding more than one thousand bending cycles without visible cracks or degrdn. in elec. properties. In contrast, compressive strain is shown to lead to an immediate redn. in cond. which we attribute to a de-lamination of the thin film from the substrate. (c) 2018 American Institute of Physics.
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      Hu, X.; Schuster, J.; Schulz, S. E.; Gessner, T. Surface chemistry of copper metal and copper oxide atomic layer deposition from copper(ii) acetylacetonate: a combined first-principles and reactive molecular dynamics study. Phys. Chem. Chem. Phys. 2015, 17, 26892– 26902,  DOI: 10.1039/C5CP03707G
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      Surface chemistry of copper metal and copper oxide atomic layer deposition from copper(II) acetylacetonate: a combined first-principles and reactive molecular dynamics study
      Hu, Xiao; Schuster, Joerg; Schulz, Stefan E.; Gessner, Thomas
      Physical Chemistry Chemical Physics (2015), 17 (40), 26892-26902CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Atomistic mechanisms for the at. layer deposition using the Cu(acac)2 (acac = acetylacetonate) precursor are studied using first-principles calcns. and reactive mol. dynamics simulations. The results show that Cu(acac)2 chemisorbs on the hollow site of the Cu(110) surface and decomps. easily into a Cu atom and the acac ligands. A sequential dissocn. and redn. of the Cu precursor [Cu(acac)2 → Cu(acac) → Cu] are obsd. Further decompn. of the acac ligand is unfavorable on the Cu surface. Thus addnl. adsorption of the precursors may be blocked by adsorbed ligands. Mol. hydrogen is found to be nonreactive towards Cu(acac)2 on Cu(110), whereas individual H atoms easily lead to bond breaking in the Cu precursor upon impact, and thus release the surface ligands into the gas-phase. On the other hand, water reacts with Cu(acac)2 on a Cu2O substrate through a ligand-exchange reaction, which produces gaseous H(acac) and surface OH species. Combustion reactions with the main byproducts CO2 and H2O are obsd. during the reaction between Cu(acac)2 and ozone on the CuO surface. The reactivity of different co-reactants toward Cu(acac)2 follows the order H > O3 > H2O.
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      Viguié, J. C.; Spitz, J. Chemical Vapor Deposition at Low Temperatures. J. Electrochem. Soc. 1975, 122, 585– 588,  DOI: 10.1149/1.2134266
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      Chemical vapor deposition at low temperatures
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      Journal of the Electrochemical Society (1975), 122 (4), 585-8CODEN: JESOAN; ISSN:0013-4651.
      Coatings of Fe2O3 [1309-37-1], SnO2 [18282-10-5], In2O3 [1312-43-2], Cr2O3 [1308-38-9], V2O3 [1314-34-7], Pd [7440-05-3], and Ru [7440-18-8] were prepd. by deposition from aerosol droplets of solns. of the corresponding acetylacetonates at atm. pressure and <500°. The structure of the coatings depended on the temp., flow of soln. and carrier gas, app. geom., and size of droplets. The variables were analyzed for deposition of Fe2O3. Piezoelec. transducers produced droplets with diam. 1-4 μ, compared with 5-50 for pneumatic transducers.
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      Marchand, P.; Hassan, I. A.; Parkin, I. P.; Carmalt, C. J. Aerosol-assisted delivery of precursors for chemical vapour deposition: expanding the scope of CVD for materials fabrication. Dalton Transactions 2013, 42, 9406,  DOI: 10.1039/c3dt50607j
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      Aerosol-assisted delivery of precursors for chemical vapour deposition: expanding the scope of CVD for materials fabrication
      Marchand, Peter; Hassan, Iman A.; Parkin, Ivan P.; Carmalt, Claire J.
      Dalton Transactions (2013), 42 (26), 9406-9422CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      A review. The prodn. of thin films of materials has become the attention of a great deal of research throughout academia and industry worldwide owing to the array of applications which use them, including electronic devices, gas sensors, solar cells, window coatings and catalytic systems. While a no. of deposition techniques are in common use, CVD is an attractive process for the prodn. of a wide range of materials due to the control it offers over film compn., coverage and uniformity, even on large scales. Conventional CVD processes can be limited, however, by the need for suitably volatile precursors. Aerosol-assisted (AA)CVD is a soln.-based process which relies on the soly. of the precursor, rather than its volatility and thus vastly extends the range of potentially applicable precursors. AACVD offers extra means to control film morphol. and concurrently the properties of the deposited materials. In this perspective the AACVD process, the influence of deposition conditions on film characteristics and a no. of materials and applications to which AACVD was found beneficial are discussed.
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      Verre, R.; MacCaferri, N.; Fleischer, K.; Svedendahl, M.; Odebo Länk, N.; Dmitriev, A.; Vavassori, P.; Shvets, I. V.; Käll, M. Polarization conversion-based molecular sensing using anisotropic plasmonic metasurfaces. Nanoscale 2016, 8, 10576– 10581,  DOI: 10.1039/C6NR01336H
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      Polarization conversion-based molecular sensing using anisotropic plasmonic metasurfaces
      Verre, R.; Maccaferri, N.; Fleischer, K.; Svedendahl, M.; Odebo Laenk, N.; Dmitriev, A.; Vavassori, P.; Shvets, I. V.; Kaell, M.
      Nanoscale (2016), 8 (20), 10576-10581CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Anisotropic media induce changes in the polarization state of transmitted and reflected light. Here we combine this effect with the refractive index sensitivity typical of plasmonic nanoparticles to exptl. demonstrate self-referenced single wavelength refractometric sensing based on polarization conversion. We fabricated anisotropic plasmonic metasurfaces composed of gold dimers and, as a proof of principle, measured the changes in the rotation of light polarization induced by biomol. adsorption with a surface sensitivity of 0.2 ng cm-2. We demonstrate the possibility of miniaturized sensing and we show that exptl. results can be reproduced by anal. theory. Various ways to increase the sensitivity and applicability of the sensing scheme are discussed.
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      Caffrey, D.; Zhussupbekova, A.; Vijayaraghavan, R. K.; Ainabayev, A.; Kaisha, A.; Sugurbekova, G.; Shvets, I. V.; Fleischer, K. Crystallographic characterisation of ultra-thin, or amorphous transparent conducting oxides-the case for Raman spectroscopy. Materials 2020, 13, 267,  DOI: 10.3390/ma13020267
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      Crystallographic characterisation of ultra-thin, or amorphous transparent conducting Oxides-the case for Raman spectroscopy
      Caffrey, David; Zhussupbekova, Ainur; Vijayaraghavan, Rajani K.; Ainabayev, Ardak; Kaisha, Aitkazy; Sugurbekova, Gulnar; Shvets, Igor V.; Fleischer, Karsten
      Materials (2020), 13 (2), 267CODEN: MATEG9; ISSN:1996-1944. (MDPI AG)
      The electronic and optical properties of transparent conducting oxides (TCOs) are closely linked to their crystallog. structure on a macroscopic (grain sizes) and microscopic (bond structure) level. With the increasing drive towards using reduced film thicknesses in devices and growing interest in amorphous TCOs such as n-type InGaZnO4 (IGZO), ZnSnO3 (ZTO), p-type CuxCrO2, or ZnRh2O4, the task of gaining in-depth knowledge on their crystal structure by conventional X-ray diffraction-based measurements are becoming increasingly difficult. We demonstrate the use of a focal shift based background subtraction technique for Raman spectroscopy specifically developed for the case of transparent thin films on amorphous substrates. Using this technique we demonstrate, for a variety of TCOs CuO, a-ZTO, ZnO:Al), how changes in local vibrational modes reflect changes in the compn. of the TCO and consequently their electronic properties.
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      Bajaj, J.; Irvine, S. J. C.; Sankur, H. O.; Svoronos, S. A. Modeling of in situ monitored laser reflectance during MOCVD growth of HgCdTe. J. Electron. Mater. 1993, 22, 899– 906,  DOI: 10.1007/BF02817503
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      Modeling of in situ monitored laser reflectance during MOCVD growth of mercury cadmium telluride
      Bajaj, J.; Irvine, S. J. C.; Sankur, H. O.; Svoronos, Spyros A.
      Journal of Electronic Materials (1993), 22 (8), 899-906CODEN: JECMA5; ISSN:0361-5235.
      An effective way to in situ monitor the metalorg. chem. vapor deposition (MOCVD) of HgCdTe/CdTe/ZnTe on GaAs or GaAs/Si substrates is presented. Specular He-Ne laser reflectance was used to in situ monitor the growth rates, layer thickness, and morphol. for each layer in the grown multilayer structure. In situ monitoring has enabled precise measurements of ZnTe nucleation and CdTe buffer layer thicknesses. Monitoring the constancy of reflectance during the thicker CdTe buffer growth where absorption in the CdTe reduces reflectance to just the surface component has led to optimum buffer growth ensuring good quality of subsequently grown GhCdTe. During the interdiffused multilayer process (IMP) HgCdTe growth, because multiple interfaces are present within the absorption length, a periodic reflectance signal is maintained throughout this growth cycle. A theor. model was developed to ext. IMP layer thicknesses from in situ recorded exptl. data. For structures that required the growth of a layer band gap HgCdTe cap layer on top of a smaller band gap active layer, in situ monitored reflectance data allowed detn. of alloy compn. in the cap layer as well. Continuous monitoring of IMP parameters established the stability of growth conditions, translating into depth uniformity of the grown material, and allowed diagnosis of growth rate instabilities in terms of changes in the HgTe and CdTe parts of the IMP cycle. A unique advantage of in situ laser monitoring is the opportunity to perform "interactive" crystal growth, a development that is a key to real time MOCVD HgCdTe feedback growth control.
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      Stafford, A.; Irvine, S. J. C.; Hess, K. L.; Bajaj, J. The use ofin situlaser interferometry for MOCVD process control. Semicond. Sci. Technol. 1998, 13, 1407– 1411,  DOI: 10.1088/0268-1242/13/12/013
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      The use of in situ laser interferometry for MOCVD process control
      Stafford, A.; Irvine, S. J. C.; Hess, K. L.; Bajaj, J.
      Semiconductor Science and Technology (1998), 13 (12), 1407-1411CODEN: SSTEET; ISSN:0268-1242. (Institute of Physics Publishing)
      This paper reports on the technique of single-wavelength laser interferometry for remote monitoring of the thin-film growth of semiconductors by metal-org. CVD (MOCVD). By online fitting of factors governing the complex refractive index of a growing layer, the quality of epitaxial growth can be monitored and early identification of a degrdn. in film growth identified. For example, sub-bandgap 633 nm (HeNe) radiation was used to monitor the pyrolytic and photo-assisted growth of several films of ZnTe on GaAs. Fitting of the effective extinction coeff., keff, at each turning point (peaks and troughs) in the interferogram revealed that different growth mechanisms are dominant under photo-assisted conditions compared to strictly pyrolytic conditions. We propose a variation on the virtual-interface approach for the math. treatment of a dielec. stack for the real-time fitting of complex reflectance interferograms from multilayers. Using this model, interferograms for vertical cavity surface emitting laser structures were theor. generated and are in excellent agreement with the exptl. interferograms recorded by Killeen and coworkers. Finally data are presented which demonstrate the use of laser interferometry for process control on an industrial reactor.
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      Ng, T. B.; Han, J.; Biefeld, R. M.; Weckwerth, M. V. In-situ reflectance monitoring during MOCVD of AlGaN. J. Electron. Mater. 1998, 27, 190– 195,  DOI: 10.1007/s11664-998-0385-8
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      In-situ reflectance monitoring during MOCVD of AlGaN
      Ng, T.-B.; Han, J.; Biefeld, R. M.; Weckwerth, M. V.
      Journal of Electronic Materials (1998), 27 (4), 190-195CODEN: JECMA5; ISSN:0361-5235. (Minerals, Metals & Materials Society)
      This reports in-situ optical reflectance monitoring during the metalorg. chem. vapor deposition (MOCVD) growth of (Al)GaN. In addn. to the well-known thin film interference effect which enables a real-time detn. of growth rate, it is shown that several insights about the MOCVD growth process can be gained by using this simple yet powerful technique. Illustrations from a variety of applications for in-situ reflectance monitoring, specifically the study of growth evolution, the control of alloy fractions, and the use of growth rate to gauge surface kinetics and gas injection will be reported.
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      Nakamura, S. Analysis of Real-Time Monitoring Using Interference Effects. Jpn. J. Appl. Phys. 1991, 30, 1348– 1353,  DOI: 10.1143/JJAP.30.1348
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      Analysis of real-time monitoring using interference effects
      Nakamura, Shuji
      Japanese Journal of Applied Physics, Part 1: Regular Papers, Short Notes & Review Papers (1991), 30 (7), 1348-53CODEN: JAPNDE; ISSN:0021-4922.
      The interference effect, which is measured as a temp. oscillation by a narrow optical band-pass pyrometer during GaN growth in metalorg. chem. vapor deposition (MOCVD), was analyzed by calcg. the transmittance and the reflectance. The results of the calcns. showed that the attenuation of the oscillation amplitude with increasing thickness was not caused by the absorption of the growing layer. To explain this attenuation, the thickness fluctuation within the measured area is proposed. The thickness fluctuation and the growth rate of the growing layer are reflected on the trace of transmittance using the present real-time monitoring technique, which observes the interference effect.
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      Verre, R.; Fleischer, K.; McGilp, J. F.; Fox, D.; Behan, G.; Zhang, H.; Shvets, I. V. Controlled in situ growth of tunable plasmonic self-assembled nanoparticle arrays. Nanotechnology 2012, 23, 035606,  DOI: 10.1088/0957-4484/23/3/035606
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      Controlled in situ growth of tunable plasmonic self-assembled nanoparticle arrays
      Verre, R.; Fleischer, K.; McGilp, F.; Fox, D.; Behan, G.; Zhang, H.; Shvets, I. V.
      Nanotechnology (2012), 23 (3), 035606/1-035606/9CODEN: NNOTER; ISSN:1361-6528. (Institute of Physics Publishing)
      Self-assembled silver nanoparticle (NP) arrays were produced by deposition at glancing angles on transparent stepped Al2O3 templates. The evolution of the plasmonic resonances has been monitored using reflection anisotropy spectroscopy (RAS) during growth. It is demonstrated that the morphol. of the array can be tailored by changing the template structure, resulting in a large tunability of the optical resonances. In order to ext. detailed information on the origin of the measured dichroic response of the system, a model based on dipolar interactions has been developed and the effect of tarnishing and morphol. dispersion addressed.
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      Verre, R.; Fleischer, K.; Sofin, R.; McAlinden, N.; McGilp, J.; Shvets, I. In situ characterization of one-dimensional plasmonic Ag nanocluster arrays. Phys. Rev. B 2011, 83, 125432,  DOI: 10.1103/PhysRevB.83.125432
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      In situ characterization of one-dimensional plasmonic Ag nanocluster arrays
      Verre, R.; Fleischer, K.; Sofin, R. G. S.; McAlinden, N.; McGilp, J. F.; Shvets, I. V.
      Physical Review B: Condensed Matter and Materials Physics (2011), 83 (12), 125432/1-125432/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      One-dimensional Ag nanoparticle arrays were grown on step-bunched vicinal Al2O3 in ultrahigh vacuum using deposition at a glancing angle. The structures grown showed a strong optical anisotropy in the visible region of the spectrum. The optical anisotropy was measured in situ using reflection anisotropy spectroscopy. Relevant optical properties were detd. as a function of deposition angle and Ag thickness. A simple phenomenol. model was developed to reproduce the features seen in the spectra. With this model it was possible to use the inhomogeneous broadening as a guide to the nanoparticle dispersion.
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      Plieth, W.; Naegele, K. Über die bestimmung der optischen konstanten dünnster oberflächenschichten und das problem der schichtdicke. Surf. Sci. 1977, 64, 484– 496,  DOI: 10.1016/0039-6028(77)90058-9
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      Determination of the optical constants of thin surface layers and the problem of layer thickness
      Plieth, W. J.; Naegele, K.
      Surface Science (1977), 64 (2), 484-96CODEN: SUSCAS; ISSN:0039-6028.
      The Fresnel reflection coeffs. of a phase boundary disturbed by values of the optical consts. differing from the values in the adjacent phases (boundary phase) were derived. The derivation on the basis of Maxwells' theory uses integral boundary conditions. The deviations from the ideal reflection properties, caused by a bouncary phase with uniaxial symmetry, are proportional to the difference of the dielec. consts. (for the tangential component) and to the difference of the reciprocal values of the dielec. consts. (for the normal component) to the value in the undisturbed case, integrated over the boundary phase. An optical thickness of the boundary phase can not be detd. by means of ellipsometry of reflection spectroscopy.
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      Jellison, G. E.; Modine, F. A. Optical nature of interface layers: a comparative study of the Si–SiO_2 interface. J. Opt. Soc. Am. 1982, 72, 1253,  DOI: 10.1364/JOSA.72.001253
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      Optical nature of interface layers: a comparative study of the silicon-silicon dioxide interface
      Jellison, G. E., Jr.; Modine, F. A.
      Journal of the Optical Society of America (1982), 72 (9), 1253-7CODEN: JOSAAH; ISSN:0030-3941.
      A comparative study is made of 4 methods for treating the graded nature of the Si-SiO2 interface in the anal. of ellipsometry data: (1) the stratified-media model, in which the total no. of layers is large (>10); (2) the 3-boundary approxn., in which the interface is treated as a single, homogeneous layer; (3) the 2-boundary approxn., in which the effects of the interface are incorporated into an effective oxide-layer thickness; and (4) the integral-equation method, approximated to 1st order, of W.J. Plieth and K. Naegele (1977). The Si-SiO2 transition layer is assumed to have optical functions that vary monotonously between those of Si and SiO2 in 6 different ways. In each of the 6 cases, the ellipsometry parameters ψ and Δ are computed for the air-SiO2 interface bulk-Si system using the 4 methods stated above. The results illustrate the sensitivity of ellipsometry to the interface layer and reveal that the interface layer must be properly taken into account for accurate anal. The 3-boundary approxn. is not necessarily more accurate than a judiciously applied 2-boundary model and the 1st-order approxn. of P. and N. introduces sizable errors at short wavelengths for the Si-SiO2 interface.
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      Hu, J.; Wang, J.; Wei, Y.; Wu, Q.; Zhang, F.; Xu, Q. Effect of film growth thickness on the refractive index and crystallization of HfO2 film. Ceram. Int. 2021, 47, 33751– 33757,  DOI: 10.1016/j.ceramint.2021.08.286
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      Effect of film growth thickness on the refractive index and crystallization of HfO2 film
      Hu, Jianping; Wang, Jian; Wei, Yaowei; Wu, Qian; Zhang, Fei; Xu, Qiao
      Ceramics International (2021), 47 (23), 33751-33757CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
      A series of Hafnium dioxide (HfO2) thin films with nominal thickness of 5-350 nm were reactively deposited on silicon(100)substrates at 200°C using electron-beam evapn. Based on the measurement and characterization techniques of spectroscopic ellipsometry, X ray diffraction, SEM and at. force microscopy, the effect of growth thickness on the refractive index and crystn. of HfO2 film grown under the same conditions was studied. The results indicate that the change of refractive index of HfO2 film is closely related to the change of microstructure i.e., grain size and crystn. which change obviously with the increasing growth thickness. The av. refractive index at 633 nm decreases about from 1.97 to 1.84 with the increase of the av. size about from 0.5 nm to 7∼8 nm and the crystallinity from zero to 74% when the thickness of HfO2 film increases from 5 nm to 350 nm. Meanwhile, a crit. growth thickness of HfO2 film, somewhere on order of 130 nm, is confirmed at which the film transforms from an amorphous to a polycryst. monoclinic structure. The sharp change of microstructure near the crit. thickness value of HfO2 film leads to the abrupt change of optical properties of HfO2 film, such as the turning behaviors of the refractive index curve. The correlation between phase transformation, size and orientation of crystallite, packing d. and hence the refractive index is established. The possible interpretations are proposed to well understand the underlying mechanism of the evolution of optical properties in HfO2 film-forming process.
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      Waechtler, T.; Roth, N.; Mothes, R.; Schulze, S.; Schulz, S. E.; Gessner, T.; Lang, H.; Hietschold, M. Copper Oxide ALD from a Cu(I) beta-Diketonate: Detailed Growth Studies on SiO2 and TaN. ECS Trans. 2009, 25, 277– 287,  DOI: 10.1149/1.3205062
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      Copper oxide ALD from a Cu(I) β-diketonate: detailed growth studies on SiO2 and TaN
      Waechtler, Thomas; Roth, Nina; Mothes, Robert; Schulze, Steffen; Schulz, Stefan E.; Gessner, Thomas; Lang, Heinrich; Hietschold, Michael
      ECS Transactions (2009), 25 (4, Atomic Layer Deposition Applications 5), 277-287CODEN: ECSTF8; ISSN:1938-5862. (Electrochemical Society)
      The at. layer deposition (ALD) of copper oxide films from [(nBu3P)2Cu(acac)] and wet oxygen on SiO2 and TaN has been studied in detail by spectroscopic ellipsometry and at. force microscopy. The results suggest island growth on SiO2, along with a strong variation of the optical properties of the films in the early stages of the growth and signs of quantum confinement, typical for nanocrystals. In addn., differences both in growth behavior and film properties appear on dry and wet thermal SiO2. Electron diffraction together with transmission electron microscopy shows that nanocryst. Cu2O with crystallites < 5 nm is formed, while upon prolonged electron irradn. the films decomp. and metallic copper crystallites of ∼ 10 nm ppt. On TaN, the films grow in a linear, layer-by-layer manner, reproducing the initial substrate roughness. Satd. growth obtained at 120°C on TaN as well as dry and wet SiO2 indicates well-established ALD growth regimes.
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      von Hoene, J.; Charles, R. G.; Hickam, W. M. Thermal decomposition of metal acetylacetonates mass spectrometer studies. J. Phys. Chem. 1958, 62, 1098– 1101,  DOI: 10.1021/j150567a019
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      Thermal decomposition of metal acetylacetonates. Mass-spectrometer studies
      Von Hoene, Joan; Charles, Robert G.; Hickam, William M.
      Journal of Physical Chemistry (1958), 62 (), 1098-1101CODEN: JPCHAX; ISSN:0022-3654.
      The order of decreasing heat stability for the acetylacetonates, based on the evolution of gaseous products, was Na(I) > Cr(III) > Al(III) > Ni(II) > Cu(II) > Fe(III) > Co(II) > Co(III) > Mn(III). Acetone and CO2 were the major decompn. products for all the acetylacetonates. For some of the chelates, acetylacetone and CH4 were also among the principal decompn. products. A no. of other gases were also present in low concn.
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    37. 37
      Lalancette, R. A.; Syzdek, D.; Grebowicz, J.; Arslan, E.; Bernal, I. The thermal decomposition and analyses of metal tris-acetylacetonates: Free radical formation from Al, Cr, Mn, Fe and Co complexes. J. Therm. Anal. Calorim. 2019, 135, 3463– 3470,  DOI: 10.1007/s10973-018-7598-8
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      The thermal decomposition and analyses of metal tris-acetylacetonates - Free radical formation from Al, Cr, Mn, Fe and Co complexes
      Lalancette, Roger A.; Syzdek, Dougles; Grebowicz, Janusz; Arslan, Evrim; Bernal, Ivan
      Journal of Thermal Analysis and Calorimetry (2019), 135 (6), 3463-3470CODEN: JTACF7; ISSN:1388-6150. (Springer)
      In earlier work, we studied the phase transitions of a no. of metal(III) acetylacetonates using thermal anal. (DSC) and coupled that with solid-state single crystal X-ray structures. From these studies, we found it necessary to explore the complexes of Al(III), Cr(III), Mn(III), Fe(III) and Co(III) in more detail. In the process of examg. the extant data on the tris-metal acetylacetonates, we discovered that there were a broad range of applications in which those substances seem to have interesting and unusual properties, including thermal and radiation sensitivity, resulting in the formation of radicals which were documented to be useful in the initiation and promotion of polymn., etc. Thus, we decided to combine our synthetic and crystn. experience with thermal studies (e.g., DSC, MS and TG) in an attempt to document the nature of metal tris-acetylacetonates and their thermalization products between room temp. and their decompn. temps.
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      Tripathi, T. S.; Niemelä, J.-P.; Karppinen, M. Atomic layer deposition of transparent semiconducting oxide CuCrO2 thin films. Journal of Materials Chemistry C 2015, 3, 8364– 8371,  DOI: 10.1039/C5TC01384D
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      Atomic layer deposition of transparent semiconducting oxide CuCrO2 thin films
      Tripathi, T. S.; Niemela, Janne-Petteri; Karppinen, Maarit
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2015), 3 (32), 8364-8371CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      Atomic layer deposition (ALD) is a vital gas-phase technique for at.-level thickness-controlled deposition of high-quality thin films on various substrate morphologies owing to its self-limiting gas-surface reaction mechanism. Here we report the ALD fabrication of thin films of the semiconducting CuCrO2 oxide that is a highly prospective candidate for transparent electronics applications. In our process, copper 2,2,6,6-tetramethyl-3,5-heptanedionate (Cu(thd)2) and chromium acetyl acetonate (Cr(acac)3) are used as the metal precursors and ozone as the oxygen source. Smooth and homogeneous thin films with an accurately controlled metal compn. can be deposited at of 240-270°; a post-deposition anneal at 700-950° in an Ar atm. then results in well cryst. films with a delafossite structure. Elec. transport measurements confirm the p-type semiconducting behavior of the films. The direct bandgap is detd. from UV-vis spectrophotometric measurements to be 3.09 eV. The obsd. transmittance is greater than 75% in the visible range.
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      Chavez, K. L.; Hess, D. W. A Novel Method of Etching Copper Oxide Using Acetic Acid. J. Electrochem. Soc. 2001, 148, G640,  DOI: 10.1149/1.1409400
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      A novel method of etching copper oxide using acetic acid
      Chavez, K. L.; Hess, D. W.
      Journal of the Electrochemical Society (2001), 148 (11), G640-G643CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The removal of copper oxide using acetic acid at low temps. was investigated. Acetic acid removes a variety of copper oxides, including cuprous oxide, cupric oxide, and cupric hydroxide without attacking the underlying copper film. The removal of these oxides was detd. by XPS. Acetic acid can tolerate up to 4 vol. % water diln. without hindering the oxide removal while producing an oxide-free surface. However, if a deionized water rinse is performed after an acetic acid treatment, a surface film of cupric hydroxide forms immediately. An acetic acid treatment at 35°C without a water rinse removes the native copper oxide and produces an oxide-free, streak-free copper surface.
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      Lee, S.-M.; Park, J.-H.; Hong, K.-S.; Cho, W.-J.; Kim, D.-L. The deposition behavior of SiO[sub 2]–TiO[sub 2] thin film by metalorganic chemical vapor deposition method. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 2000, 18, 2384,  DOI: 10.1116/1.1287154
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      The deposition behavior of SiO2-TiO2 thin film by metalorganic chemical vapor deposition method
      Lee, Si-Moo; Park, Jeong-Hoon; Hong, Kug-Sun; Cho, Woon-Jo; Kim, Dong-Lae
      Journal of Vacuum Science & Technology, A: Vacuum, Surfaces, and Films (2000), 18 (5), 2384-2388CODEN: JVTAD6; ISSN:0734-2101. (American Institute of Physics)
      SiO2-TiO2 thin films were deposited by metalorg. CVD using an alkoxide source. At 680°, the deposition rate curve showed parabolic behavior and the refractive index increased linearly from 1.45 to 2.35 with increasing Ti tetraisopropoxide (TTIP) ratio. Each oxide component in the film was sepd. anal. and its effective deposition rate, in the composite thin film, was calcd. to analyze the deposition mechanism of the mixed sources. A Lorentz-Lorenz model was used to attain the compn. of the film for each component sepn. Effective SiO2 deposition from tetraethylorthosilicate4 (TEOS) showed parabolic behavior with increasing TTIP ratio, while the effective TiO2 deposition did not. In addn., TTIP lowered the apparent activation energy of SiO2 deposition significantly from ∼40 to ∼10 Kcal/mol. From this, TTIP enhanced the TEOS decompn., which results in the anomalous deposition behavior in composite films. A more reactive TTIP mol. acting as a free radical reaction initiator was suggested as a mechanism for enhancement of the process.
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      Hodroj, A.; Deschanvres, J.-L.; Gottlieb, U. Growth of Amorphous Ti–Si–O Thin Films by Aerosol CVD Process at Atmospheric Pressure. J. Electrochem. Soc. 2008, 155, D110,  DOI: 10.1149/1.2806790
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      Growth of Amorphous Ti-Si-O Thin Films by Aerosol CVD Process at Atmospheric Pressure
      Hodroj, Abbas; Deschanvres, Jean-Luc; Gottlieb, Ulrich
      Journal of the Electrochemical Society (2008), 155 (2), D110-D114CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      We present the deposition of amorphous Ti-Si-O thin films by an aerosol chem. vapor deposition process at atm. pressure. We used di-acetoxi-di-butoxysilane, tetrabutoxysilane, and titanium(IV) n-butoxide as precursors, and the deposition temps. ranged from 475 to 675°C. During the deposition process a chem. reaction between the different precursors took place. We found a synergetic mutual influence of the precursors and a coupled activation of the growth process between Ti and Si. The obtained thin films were characterized by various techniques: SEM, at. force microscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, IR spectroscopy, ellipsometry, and XPS. Our films were amorphous TiO2-SiO2 mixts. with a refractive index between 1.45 and 2.1 depending on the Si/Ti ratio in the films. We established a correlation between the XPS results and the refractive index of the films, allowing us to control the film compn. as a function of the deposition conditions.
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      Nasibulin, A. G.; Shurygina, L. I.; Kauppinen, E. I. Synthesis of nanoparticles using vapor-phase decomposition of copper(II) acetylacetonate. Colloid J. 2005, 67, 1– 20,  DOI: 10.1007/s10595-005-0041-4
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      Synthesis of nanoparticles using vapor-phase decomposition of copper(II) acetylacetonate
      Nasibulin, A. G.; Shurygina, L. I.; Kauppinen, E. I.
      Colloid Journal (2005), 67 (1), 1-20CODEN: CJRSEQ; ISSN:1061-933X. (Pleiades Publishing, Inc.)
      A review (38 refs.). Exptl. data on the synthesis of cryst. Cu, Cu2O, and CuO nanoparticles obtained earlier by the vapor-phase decompn. of copper(II) acetylacetonate (Cu(acac)2) were systematized and generalized. Studies were performed using a laminar flow reactor at atm. pressure within the ranges of precursor partial vapor pressure Pprec = 0.06-44 Pa and reactor temp. from 432 to 1216°C. The decompn. of Cu(acac)2 was studied in an inert nitrogen atm. and in the presence of various reagents (water vapors, H2, O2, and CO). The compn. of synthesized particles varied from pure copper to its oxides (Cu2O and CuO) depending on exptl. conditions and used reagents. A semi-empirical kinetic model was proposed for describing the product dynamics. The hypothesis on the predominant role of copper dimers in a particle's growth was stated. It was established that the compn. of products is detd. by the surface reactions on growing particles and is dependent on the ratio between the concns. of the gaseous reagents. Calcd. phase diagrams of the products of Cu(acac)2 decompn. in the presence of various reagents were in good agreement with exptl. data. The proposed method of construction of the phase diagram of decompn. products can be employed for other systems. It was established that, upon the Cu(acac)2 decompn. in the presence of CO, carbon nano-onions were formed in addn. to copper nanoparticles.
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      Han, M.; Wang, J.; Deng, Q.; Wang, J.; Li, W.; Zhang, P.; Li, C.; Hu, Z. Effect of annealing temperature on structural, optoelectronic properties and interband transitions of CuCrO2 nanocrystalline films prepared by the sol–gel method. J. Alloys Compd. 2015, 647, 1028– 1034,  DOI: 10.1016/j.jallcom.2015.06.173
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      Effect of annealing temperature on structural, optoelectronic properties and interband transitions of CuCrO2 nanocrystalline films prepared by the sol-gel method
      Han, Meijie; Wang, Jun; Deng, Qinglin; Wang, Junyong; Li, Wenwu; Zhang, Peng; Li, Chuanqing; Hu, Zhigao
      Journal of Alloys and Compounds (2015), 647 (), 1028-1034CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      Transparent conductive CuCrO2 oxide films were prepd. on sapphire substrates by the sol-gel method using copper (II) acetate monohydrate and chromium (III) nitrate hydrate as raw materials. The highly c-axis orientation and optical transparency (60-80%) in the visible region were obtained. The microstructure, vibration modes, optical and elec. transport properties as a function of annealing temps. have been systematically discussed. With increasing annealing temp., the cryst. quality and the c-axis orientation of CuCrO2 films were improved, which induced the increase of Raman phonon amplitude with the decrease of the linewidth. But the film compactness and d. decrease with the increase of av. particle size. To study the intrinsic mechanism of the optical response behavior, a three-phase layered structure (air/film/substrate) was applied to simulate the transmittance spectra of CuCrO2 films. Three electronic transitions can be uniquely assigned and it is found that the direct transitions are the dominant effect for CuCrO2 optical properties. The temp. dependence of elec. cond. was studied and the elec. conduction mechanisms of the thermal activation behavior and the three-dimensional variable range-hopping transport were obsd. In addn., the cond. of CuCrO2 films decreases with increasing annealing temp. induced by its microstructure and the grain boundary scattering. Thus, the film annealed at 600 °C has the largest elec. cond. of 0.49 S m-1 at room temp. The present results could be crucial and provide theor. support for future applications of p-type semiconductor and optoelectronic devices.
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      Sander, T.; Reindl, C. T.; Giar, M.; Eifert, B.; Heinemann, M.; Heiliger, C.; Klar, P. J. Correlation of intrinsic point defects and the Raman modes of cuprous oxide. Phys. Rev. B 2014, 90, 045203,  DOI: 10.1103/PhysRevB.90.045203
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      Correlation of intrinsic point defects and the Raman modes of cuprous oxide
      Sander, T.; Reindl, C. T.; Giar, M.; Eifert, B.; Heinemann, M.; Heiliger, C.; Klar, P. J.
      Physical Review B: Condensed Matter and Materials Physics (2014), 90 (4), 045203CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The Raman spectrum of cryst. Cu2O taken off resonance is reproducible and independent of the growth method and conditions employed. But, in contrast to most other cryst. materials, the Raman spectrum of Cu2O is dominated by IR active and silent lattice modes rather than by the only Raman allowed phonon mode. We show that this unusual behavior is most likely caused by the presence of copper vacancies in the so-called split configuration, a point defect particular to Cu2O. The redn. of symmetry due to the presence of point defects may lift the Raman selection rules and may introduce Raman activity for phonon modes that are Raman forbidden in the case of perfect crystal symmetry. Based on this group theor. consideration, we predict the angle dependence of the Raman intensities of all Cu2O one-phonon modes at k = 0 for rotation about the (100) direction caused by the presence of various intrinsic point defects. Of all intrinsic defects in question, only the presence of the copper vacancy in the split configuration introduces Raman activity for all Cu2O extended phonon modes obsd. in expt. and is consistent with the angle-dependent measurements. Our study underlines the special role of the split vacancy in Cu2O.
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    • Additional information on the in situ optical method employed, as well as measurements and properties of the CuxCrO2 films such as X-ray reflection, details of the electrical characterization, and results (Seebeck, carrier activation energy) as well as representative XPS scans illustrating how the sample stoichiometry was determined (PDF)


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