ACS Publications. Most Trusted. Most Cited. Most Read
Locally Phase-Engineered MoTe2 for Near-Infrared Photodetectors
More

OR SEARCH CITATIONS

My Activity
Publications

Figure 1Loading Img
Download Hi-Res ImageDownload to MS-PowerPointCite This:ACS Photonics 2024, 11, 10, 4083-4089
  • Open Access
Article

Locally Phase-Engineered MoTe2 for Near-Infrared Photodetectors
Click to copy article linkArticle link copied!

  • Jan Hidding
    Jan Hidding
    Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
    More by Jan Hidding
  • Cédric A. Cordero-Silis
    Cédric A. Cordero-Silis
    Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
    More by Cédric A. Cordero-Silis
    Orcidhttps://orcid.org/0000-0002-0842-3169
  • Daniel Vaquero
    Daniel Vaquero
    Nanotechnology Group, USAL─Nanolab, Universidad de Salamanca, E-37008 Salamanca, Spain
    More by Daniel Vaquero
  • Konstantinos P. Rompotis
    Konstantinos P. Rompotis
    Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
    More by Konstantinos P. Rompotis
  • Jorge Quereda
    Jorge Quereda
    Departamento de Física de Materiales, GISC, Universidad Complutense de Madrid, E-28040 Madrid, Spain
    More by Jorge Quereda
  • Marcos H. D. Guimarães*
    Marcos H. D. Guimarães
    Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
    *Email: [email protected]
    More by Marcos H. D. Guimarães
    Orcidhttps://orcid.org/0000-0002-8150-4379
Open PDFSupporting Information (1)

ACS Photonics

Cite this: ACS Photonics 2024, 11, 10, 4083–4089
Click to copy citationCitation copied!
https://doi.org/10.1021/acsphotonics.4c00896
Published September 16, 2024

Copyright © 2024 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY 4.0 .

Abstract

Click to copy section linkSection link copied!
High Resolution Image
Download MS PowerPoint Slide

Transition-metal dichalcogenides (TMDs) are ideal systems for two-dimensional (2D) optoelectronic applications owing to their strong light-matter interaction and various band gap energies. New techniques to modify the crystallographic phase of TMDs have recently been discovered, allowing the creation of lateral heterostructures and the design of all-2D circuitry. Thus, far, the potential benefits of phase-engineered TMD devices for optoelectronic applications are still largely unexplored. The dominant mechanisms involved in photocurrent generation in these systems remain unclear, hindering further development of new all-2D optoelectronic devices. Here, we fabricate locally phase-engineered MoTe2 optoelectronic devices, creating a metal (1T′) semiconductor (2H) lateral junction and unveil the main mechanisms at play for photocurrent generation. We find that the photocurrent originates from the 1T′–2H junction, with a maximum at the 2H MoTe2 side of the junction. This observation, together with the nonlinear IV-curve, indicates that the photovoltaic effect plays a major role in the photon-to-charge current conversion in these systems. Additionally, the 1T′–2H MoTe2 heterojunction device exhibits a fast optoelectronic response over a wavelength range of 700–1100 nm, with a rise and fall times of 113 and 110 μs, respectively, 2 orders of magnitude faster when compared to a directly contacted 2H MoTe2 device. These results show the potential of local phase-engineering for all-2D optoelectronic circuitry.

This publication is licensed under

CC-BY 4.0 .
  • cc licence
  • by licence
Copyright © 2024 The Authors. Published by American Chemical Society

Subjects

what are subjects

Keywords

what are keywords

Special Issue

Published as part of ACS Photonics special issue “Rising Stars in Photonics”.

Introduction

Click to copy section linkSection link copied!
Historically, low-dimensional systems have been proposed, fabricated, and studied for enhanced photodetection (1,2) and potential on-chip photonic applications. (3−5) Particularly, two-dimensional (2D) materials beyond graphene have shown high interest due to their strong light-matter interaction and compatibility with silicon photonic new generation devices. (6−8) From the large family of 2D crystals, the group of materials called the transition-metal dichalcogenides (TMDs) have gained much attention in the last two decades due to their versatility, mechanical strength, atomically flat interfaces, and strong absorption at the monolayer limit, making them promising candidates for future (opto)electronic and (opto)spintronic applications. (9) The most commonly studied crystal structure of the TMD family is the hexagonal (2H) phase, for which most TMDs are semiconducting and possess a thickness-dependent band gap. (10,11) Apart from the 2H phase, however, TMDs can present a multitude of different crystallographic phases, such as the semiconducting 3R phase, or the semimetallic 1T, 1T′, and 1Td phases, which possess different symmetries and (opto)electronic properties. To benefit from these different properties in a single device, researchers have recently focused on gaining control of the crystallographic phase of TMDs, allowing them to transform the phase of single TMD crystals at will. (12,13) This new and emerging field is now termed the field of phase-engineering and opens the door to creating on-chip 2D circuitry with 2D metals and semiconductors.
In the literature, multiple methods are used to induce a 2H to 1T′ phase transformation for different TMDs, such as crystal deformation, (14−17) electrostatic doping, (18) chemical doping, (19,20) laser heating, (21) and so forth. In particular, MoTe2 gained much attention as the energy barrier between the 2H and 1T′ phase is the smallest (∼40 meV). (22) This low-energy barrier allows, for moderate laser power, to induce a phase change in MoTe2. It was demonstrated, using X-ray and electron microscopy techniques, that the 2H phase evolves into a 1T′ by thermodynamically driven processes, changing the system into a 2D lateral junction. (21,23,24) It was shown electrically that the Schottky barrier, present when directly contacting the 2H TMD with metallic contacts, is significantly reduced when contacting a 2H TMD via a phase-transformed 1T′ region, (19,21,25,26) with these two-dimensional lateral junctions, approaching the quantum limit for the contact resistance. (27) The possibility of fabricating high-quality contacts for a 2D semiconductor is essential to increase the speed of optoelectronic devices based on these materials.
Apart from electrical characterization, only a few reports explored the benefits of local phase engineering on the optoelectronic performance of TMD devices. (28−30) Lin et al. report an increased responsivity for 2H MoTe2 devices using 1T′ interlayer contacts. (29) However, no scanning photocurrent measurements are performed, which makes it difficult to disentangle the possible microscopic mechanism involved in the photocurrent generation to either the photovoltaic effect (PVE), due to the build in electric field at the Schottky barriers, or the photothermal effect (PTE), due to the different Seebeck coefficients of the 2H and 1T′ regions. (9,31,32)
Here, we perform scanning photocurrent measurements on 1T′–2H MoTe2 heterojunction devices, which allow us to spatially resolve the areas involved in the photocurrent generation, giving insights into the underlying mechanisms involved. First, we phase-transform the sides of an exfoliated 2H MoTe2 crystal to a 1T′ phase using local heating by laser irradiation, which allows us to contact 2H MoTe2 via the semimetallic 1T′ regions. We find a clear nonlinear behavior for the 1T′–contacted 2H region, indicative of a Schottky barrier between the 1T′ and 2H regions. Additionally, using the scanning photocurrent measurements, we clearly observe that the photocurrents are generated at the 1T′–2H junction rather than at the Ti/Au electrodes or the 1T′ region. More specifically, we find that the peak of the photocurrent is generated at the 2H side of the junction, which suggests that the observed photocurrents in the 1T′–2H junction can be attributed to the PVE rather than the PTE. Lastly, we characterize the optoelectronic performance of the MoTe2 photodetector by performing time-resolved and laser power-dependent photocurrent measurements. We find fast rise and fall times of 113 and 110 μs, respectively, over a broad spectral range of 700–1100 nm. By comparing our 1T′–2H MoTe2 photodetector to a 2H MoTe2 diode where the electrodes are directly deposited on the 2H MoTe2 crystal, we are able to show that the temporal response of 1T′–contacted 2H MoTe2 is 2 orders of magnitude faster. This indicates that phase-engineering can be considered another tool for improving the performance of TMD-based optoelectronic devices.

Results and Discussion

Click to copy section linkSection link copied!

Raman Spectroscopy

The device used to perform the optoelectronic measurements is depicted in Figure 1a. The green region is the untreated 2H MoTe2, while the dark green regions, indicated by the dashed white line, were irradiated with a laser to induce the phase transformation from 2H to 1T′ (details of the device fabrication can be found in the Methods section). To confirm the phase transformation, we performed Raman spectroscopy measurements, as depicted in Figure 1b. Before laser irradiation, we observe the in-plane E2g mode at 235 cm–1 and an out-of-plane Ag mode near 174 cm–1, indicative of the 2H MoTe2 phase. After laser irradiation, these peaks are strongly suppressed, and we observe two new peaks at 124 and 138 cm–1, corresponding to the Ag mode of 1T′ MoTe2. This significant change in the Raman spectrum indicates the successful phase transformation of the irradiated regions. To quantify the degree of phase change in our devices, we calculated the spectral weight of each peak in the phase-changed device (see Supporting Information Figure S5 and Table S1), finding values of 0.01 for the peak at ∼232 cm–1 and of 2.14 for the peak at ∼124 cm–1, corresponding to the 2H and the 1T′ phases, respectively. Our results are in agreement with previous reports, which extensively characterized the laser-induced phase change with several techniques additional to Raman spectroscopy, including X-ray diffraction and electron microscopy. (21,23,24)

Figure 1

Figure 1. (a) Optical micrograph of a phase-changed MoTe2 device, where the phase-changed regions are outlined with the white dashed line, while the bright green part is the unaltered 2H MoTe2 region. (b) Raman spectra obtained before (green) and after (purple) the phase transformation, which clearly indicate a successful phase transformation. The spectra before the phase change is multiplied by 3 for clarity. (c) Ids–Vds measurements, as indicated in (a), with Vg ranging from 0 to 50 V, taken at 78 K. The nonlinear IV characteristics show the Schottky behavior. The IV measurement for the two 1T′ regions are depicted in the inset, which clearly show Ohmic behavior. (d) Transfer curve measured with a Vds of 3 V, taken at 78 K, shows a clear n-type behavior.

High Resolution Image
Download MS PowerPoint Slide

Electrical Characterization

After fabricating electrical contacts to the phase-changed region, we electrically characterized the device in a cryostat at 78 K. We swept the drain-source voltage (Vds) and measured the drain-source current (Ids) for the different regions (both 1T′ and the 1T′–2H junction). In Figure 1c, the 2-probe Ids–Vds measurements are shown for the 1T′–2H–1T′ junction at different gate voltages, ranging from 0 to 50 V. We observe a clear nonlinear behavior for the Ids as a function of the Vds, indicative of a Schottky barrier present in our device, which could either be between the Ti/Au contact and the 1T′–MoTe2, or the 1T′–2H junction. To determine this, we performed the Ids–Vds measurement on the phase-transformed 1T′ region only and observe a clear linear behavior showing Ohmic contact between the Ti/Au contacts and 1T′ region, as depicted in the inset of Figure 1c. Therefore, we expect the nonlinear behavior observed in Figure 1c to originate from a Schottky barrier between the 1T′–2H junction. Figure 1d displays the gate transfer curve of the 1T′–2H–1T′ device at Vds = 3 V, showing a clear n-type transistor behavior and a threshold voltage of Vth = 40 V, confirming the semiconducting nature of the 2H phase. For the 1T′ regions, we find a two-probe resistance of 8 and 18 kΩ, which again indicate the successful transformation from the semiconducting 2H MoTe2 phase to the semimetallic 1T′ phase.

Optoelectrical Characterization

To observe the optoelectronic response of the 1T′–2H–1T′ sample, we perform both scanning photocurrent measurements and time-resolved photocurrent measurements, as described in the Methods section. Unless otherwise stated, the optoelectrical measurements were performed at room temperature and high vacuum (1 × 10–6 mbar) conditions. First, the scanning photocurrent measurements are presented, which give more insights into the origin of the photocurrent, after which the time-resolved photocurrent measurements are discussed.
The scanning photocurrent measurements enable us to spatially identify where the photocurrent is generated and thus allow us to check whether the photocurrent originates from the Ti/Au contacts or from the MoTe2 flake itself. When performing the scanning photocurrent measurements, the laser beam is focused and scanned across the sample in a raster-like fashion while recording both the reflection and the generated photocurrent. Figure 2a shows the recorded reflection map with an illumination wavelength of 700 nm, a power of 1 μW, and a full width at half-maximum (FHWM) spot size of 0.70 ± 0.02 μm (see Supporting Information). The contours of the flake and the Ti/Au contacts are clearly visible and highlighted with the white outlines for clarity. It should be noted that the different phases of MoTe2 can also be clearly distinguished, and their junctions are highlighted with the white dashed line. The corresponding photocurrent maps with a Vds of −2, 0, and 2 V are depicted in Figure 2b–d, respectively. We clearly observe that the photocurrent originates locally from the 1T′–2H junction, rather than the Ti/Au contacts, which is the case when directly contacting the 2H MoTe2. This, again, confirms a low Schottky barrier between the Ti/Au and 1T′ MoTe2, indicating a successful phase transformation.

Figure 2

Figure 2. (a) Reflectivity map of the scanning photocurrent measurement of the device depicted in Figure 1a with the corresponding photocurrent map in (b), (c), and (d), taken at RT. The white outlines indicate the position of the flake and Ti/Au contacts, while the white dashed lines indicate the 1T′–2H junctions for clarity. The photocurrent maps are obtained with λ = 700 nm, P = 1 μW, and a Vds of (b) −2, (c) 0, and (d) 2 V. From the photocurrent maps, we can clearly see that the induced photocurrent originates from the 1T′–2H junction rather than from the Ti/Au contacts.

High Resolution Image
Download MS PowerPoint Slide
We observe photocurrents with opposite signs at the two 1T′-2H junctions for Vds = 0 V. This is in line with the expected behavior for two possible mechanisms: photovoltaic effect (PVE) due to a Schottky barrier at the 1T′–2H junction or the photothermoelectric effect (PTE) due to a different Seebeck coefficient of the two MoTe2 phases. A more detailed analysis of the influence of the Vds and the Vg on the photocurrent is shown in the Supporting Information. The two mechanisms are schematically depicted in Figure 3. While both the PVE and the PTE can give rise to photocurrent in the 2H–1T′ interface, we conclude that the PTE contribution should be negligible. For the PTE, local heating due to the laser irradiation causes a temperature gradient (ΔT) which is converted into a voltage difference (VPTE) due to a difference in the Seebeck coefficient between the 1T′ (S1T′) and 2H (S2H) phases. (9,33) By using the maximum-generated photocurrent in our measurements, we calculate a range of unrealistic temperature gradients above 7000 K, indicating that the PTE cannot solely explain the observed photocurrent. A more detailed explanation can be found in the Supporting Information.

Figure 3

Figure 3. (a) Schematic of the photothermoelectric (PTE) and photovoltaic effect (PVE). For the PTE, the laser locally heats the device, which creates a temperature gradient, which via the Seebeck effect causes an induced photocurrent (IPTE). For the PVE, the localized electric field at the Schottky barrier causes a separation of the photoinduced carriers, resulting in IPVE. It should be noted that the two effects produce a photocurrent with the same direction and that the induced photocurrent is opposite on both junctions. (b) Line trace of the photocurrent mapping, as indicated in red in Figure 2c, showing the reflection (white) and negative (purple) and positive (green) photocurrent peaks along the line trace. Both the maximum and minimum photocurrents are obtained in the 2H region, as expected for a localized electric field from a Schottky barrier. The phases are indicated by the purple (1T′) and green (2H) backgrounds.

High Resolution Image
Download MS PowerPoint Slide
On the other hand, for the PVE-driven photocurrent, the localized electric field at the 1T′–2H interface causes the photoinduced electron–hole pairs to separate, resulting in a photocurrent. (33) For our n-type MoTe2, the band alignment is depicted in Figure 3a. The electric field from the Schottky barrier is positioned in the 2H region. By taking a line scan of the reflection map and photocurrent map, indicated by the red line in Figure 2c, we can more accurately determine the position of the photocurrent peak with respect to the 1T′–2H junction. Here, we find that the peak of the photocurrent arises in the 2H region rather than at the 1T′–2H junction, which is in line with the expectation for the PVE. (32) Additionally, we can estimate the depletion region due to the Schottky barrier by using the following equation:
W=2ϵ0ϵrϕbieNd
(1)
where W is the width of the Schottky barrier, e is the electron charge, and ϵ0 is the permittivity of free space. By assuming a donor density Nd of 1011 cm2, (34) a barrier height ϕbi of 60 meV, (29) and a relative ϵr of 12, we estimate W to be ∼2 μm, which corresponds well to the FWHM of 1.5 ± 0.2 μm we find by fitting the positive photocurrent peak with a Gaussian.
Buscema et al. observed a similar photocurrent sign in their scanning photocurrent measurements on an n-type MoS2 photodiode. (35) However, they attribute the induced photocurrent to the PTE, as they observe clear photocurrent generation in the center of their Ti/Au contacts and see a linear Ids–Vds behavior with no indication of a Schottky barrier. In contrast, for our 1T′-2H MoTe2 junctions, we observe a clear nonlinear IV-curve, indicating that the Schottky barrier plays a more important role in our devices. Furthermore, they see a pronounced photocurrent even when exciting below the bandgap of MoS2. Unfortunately, our optoelectronic setup only allows for excitation up to 1100 nm, which is still above the bandgap of MoTe2 (∼1.1 eV ∝ ∼1127 nm for bulk). (36,37) Therefore, we suggest further research to be performed on below-band gap excitation to determine to what extent the PTE is contributing to the observed photocurrent.
To characterize the optoelectronic performance of our MoTe2 photodetector, we perform time-resolved and power-dependent photocurrent measurements, as depicted in Figure 4. By measuring the induced photocurrent versus time, using a chopper to modulate the light on and off (see Figure 4a), we are able to extract the rise (τr) and fall times (τf) of the device, which are defined as the time required for the photocurrent to increase from 10 to 90%, and decrease from 90 to 10% of the maximum photocurrent, respectively. By performing these measurements over a range of different excitation wavelengths, we find that we get short rise and fall times of ∼113 and ∼110 μs, respectively, independent of the wavelength, as depicted in Figure 4b. These response times correspond to a 3 dB frequency of 0.35/τr = 3 kHz, (38) which are close to the performance of graphene/MoTe2/graphene photodetectors. (39) In contrast, when directly contacting the 2H MoTe2 with Ti/Au electrodes, we find a much slower response (over 100-fold slower), as shown in Figure S2c in the Supporting Information. Here, we observe a waveform similar to a “sawtooth” response, indicative of capacitive behavior, at 20 Hz. This is a result of the highly resistive contact with the 2H region of the device, leading to photogating and slow charging of the device. This shows that using the 1T′ regions to contact the 2H MoTe2 increases the temporal response of our MoTe2 photodetector by more than 2 orders of magnitude.

Figure 4

Figure 4. (a) Time-resolved photocurrent, taken at RT, where the photocurrent (purple) in the device is plotted together with the chopper signal (gray) versus time, shows the fast response of our MoTe2 photodetector. The dashed gray rectangles indicate the region used to determine the rise and fall times, as depicted in the inset of (b). (b) Extracted rise (purple) and fall times (green) indicate no wavelength dependence on the fast response for wavelengths ranging from 700 to 1100 nm. The inset shows the rise (purple) and fall (green) curves of the photocurrent from which the rise and fall times are extracted. (c) Power-dependent measurements for different Vds, ranging from −2 to 2 V, with a maximum responsivity of 4.5 × 10–8 A/μW. Here, the responsivity (R) of the device is plotted as a function of the laser excitation power (P) and fitted at high laser excitation power to a power law: R ∝ Pα–1. The measured R for 700 and 1068 nm are indicated by the filled circles and unfilled squares, respectively. (d) Extracted index of the power law (α) from the fitting in (c) versus Vds for the wavelengths 700 nm (purple) and 1068 nm (green).

High Resolution Image
Download MS PowerPoint Slide
The response dynamics displayed at the 1T′–2H junction are fast compared to other TMD-based photodetectors. (38,40) More specifically, compared to other MoTe2-based photodetectors, they are 1 order of magnitude faster than the report of Huang et al. on directly contacted 2H MoTe2, (41) and similar to the ones found by Lin et al. in 1T′–contacted 2H MoTe2. (29) On other TMD-based devices, a variety of different response dynamics are reported, with the fastest responses reported for deep UV and mid-IR detectors on graphene/MoTe2/black phosphorus devices, which reach bandwidths of 2.1 MHz. (42)
To determine the responsivity of our devices, we vary the excitation power at a fixed excitation wavelength (700 and 1068 nm). From these power-dependent measurements, we are able to determine the responsivity by R = IPC/P, where IPC is the induced photocurrent, and P is the power of the laser, (9,33) and find a maximum R of 4.5 × 10–8 A/μW with a wavelength of 700 nm and a 2 V bias. The value we find is comparable to other reports on TMD-based photodetectors, ranging approximately from 7.25 × 10–11 A/μW to 9.708 × 10–3 A/μW. (40,41,43,44) For the same wavelength and bias voltage, we also calculate a maximum external quantum efficiency (EQE) of ∼8%. Given that there is commonly a trade-off between fast response and high responsivity in these devices, (38) it is not unexpected that both the EQE and the responsivity of our devices fall within the middle or lower half of reported values. (30,39,45) Additionally, the responsivity of our device is measured with a focused laser spot rather than illuminating the entire device. This could lead to an underestimation of the generated photocurrent, as only a small fraction of the photodetector area is used to generate the photocurrent. Moreover, the capacitance added by the global back-gate can lead to an increase in the response times of our device.
Figure 4c clearly shows a decrease of responsivity with incident excitation power for P > 1.9 μW, which is commonly observed in TMD photodetectors. (46,47) It can be associated with a reduced number of photogenerated carriers available for extraction under high photon flux due to the saturation of recombination/trap states that influence the lifetime of the generated carriers. (48) The responsivity versus laser power can be expressed by a power law R ∝ Pdα–1 for P > 1.9 μW, where P is the laser power, and α is the index of the power law. (41,49) From the fit, we are able to extract α for the two different wavelengths at different Vds values, as shown in Figure 4d. The deviation from the ideal slope of α = 1, where the responsivity does not depend on the laser power, can be attributed to complex processes in the carrier generation, trapping, and electron–hole recombination in MoTe2. (50,51) For the PTE, a value of α ∼ 0.8 is expected, while we find a value of α ∼ 0.25, which indicates again that the PTE is not primarily responsible for the generated photocurrent in our device. (31)

Conclusions

Click to copy section linkSection link copied!
In conclusion, laser-induced phase transformation is a simple, scalable, and reliable methodology to engineer MoTe2 optoelectronic devices. Our results indicate that contacting the 2H region of MoTe2 via a phase-transformed 1T′ region is beneficial for the temporal optoelectronic response of MoTe2-based photodetectors and does not require complex heterostructure fabrication. Our scanning photocurrent measurements and nonlinear IV curves clearly show that the origin of the photocurrent in our devices can be ascribed to the Schottky barrier between the 1T′ and 2H junction, rather than the photothermoelectric effect or Schottky barriers at the Ti/Au electrode-TMD interface. Contacting MoTe2 via the phase-transformed 1T′ region, therefore, allows one to study the intrinsic properties of the TMD rather than the electrode-TMD interactions, beneficial for fundamental research. Additionally, an increase of 2 orders of magnitude in the optoelectronic temporal response is observed when contacting the 2H MoTe2 via the 1T′ regions. This shows that tailoring the crystallographic phase of TMDs locally and altering their optoelectronic response at will can have a profitable effect on the optoelectronic operation. Our results, in combination with the wide variety of phase-engineering techniques and different TMDs available, could lead to a further improved performance of TMD-based optoelectronic devices, leading to more sensitive, faster, and flexible photodetectors.

Methods

Click to copy section linkSection link copied!

Device Fabrication

The 2H MoTe2 flakes are obtained by mechanical exfoliation (bulk crystal supplied by HQ graphene) and transferred onto a Si/SiO2 (285 nm) substrate in a nitrogen environment. Using an optical microscope, the MoTe2 flakes are selected based on their size, thickness, and homogeneous surface. Details of the selection criteria, as well as Figure S4, a discussion on the impact of the flake thickness to the laser-induced phase change, can be found in the Supporting Information. Next, the Raman spectra are obtained with an inVia Raman Renishaw microscope using a linearly polarized laser in backscattering geometry. The excitation wavelength and grating used were λ = 532 nm and 2400 l/mm, respectively. The laser power was ∼100 μW with a diffraction-limited spot of ∼1 μm. Using the same system, the 2H–1T′ phase transformation is performed by selectively illuminating parts of the MoTe2 flake with the 532 nm laser beam in a raster-like fashion, using steps of 500 nm and 0.1 s illumination. We find that a laser power of ≥3.25 mW (laser spot size around 500 nm) is needed to initiate the phase transformation. Finally, using standard lithography techniques, the Ti/Au (5/55 nm) contacts are fabricated on top of the flake by means of electron beam lithography and electron beam evaporation.

Optoelectronic Measurements

The electrical characterization (i.e., IV-sweeps, transfer curves) is performed using Keithleys 2400 and 2450 source measure units at 78 K. For the optoelectronic measurements, a supercontinuum white light laser (NKT Photonics SuperK EXTREME) is used as the illumination source, and the measurements are taken at room temperature. The induced photocurrent is measured in a short-circuit configuration using a Stanford Research Systems SR830 lock-in amplifier, which is referenced to the frequency of the optical chopper. The photocurrents are either measured directly by the lock-in amplifier or converted to a voltage using a home build current preamplifier, which is subsequently measured by the lock-in amplifier. The time-resolved photoresponse of the device, as depicted in Figure 3a, is measured using a chopper and an oscilloscope (Keysight DSOX1204A) at room temperature.

Supporting Information

Click to copy section linkSection link copied!

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphotonics.4c00896.

  • Details over the photothermoelectric temperature gradient calculation; laser spot size calculation and data for 700 and 1064 nm for the reported measurements; equation and additional details for charge carrier mobility and further details of another phase-engineered device, including responsivity and transfer curves; drain-source and gate voltage dependence of the photocurrent; normalized Raman spectra and spectral weight contribution of the 1T′ phase characteristic peaks; laser irradiation power; and flake thickness dependence for phase change (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

Click to copy section linkSection link copied!
  • Corresponding Author
  • Authors
    • Jan Hidding - Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
    • Cédric A. Cordero-Silis - Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands;  Orcidhttps://orcid.org/0000-0002-0842-3169
    • Daniel Vaquero - Nanotechnology Group, USAL─Nanolab, Universidad de Salamanca, E-37008 Salamanca, Spain
    • Konstantinos P. Rompotis - Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
    • Jorge Quereda - Departamento de Física de Materiales, GISC, Universidad Complutense de Madrid, E-28040 Madrid, Spain
  • Author Contributions

    J.H. and C.A.C.-S. contributed equally. J.H. and C.A.C.-S. fabricated the samples, and together with D.V. performed both the electrical and optical measurements under the supervision of M.H.D.G.; K.R. joined to perform measurements and sample characterization on additional samples also under the supervision of M.H.D.G.; J.H. performed the data analysis and, together with C.A.C.-S and M.H.D.G., wrote the paper with comments from all authors.

  • Funding

    This work was supported by the Dutch Research Council (NWO-STU.019.014), the European Union (ERC, 2D-OPTOSPIN, 101076932), the “Materials for the Quantum Age”(QuMat) program (Registration No. 024.005.006) which is part of the Gravitation program financed by the Dutch Ministry of Education, Culture and Science (OCW), the Zernike Institute for Advanced Materials, and the innovation program under grant agreement No. 881603 (Graphene Flagship).

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

Click to copy section linkSection link copied!

The authors acknowledge Prof. M. A. Loi and E. K. Tekelenburg for their help with the Raman measurements and thank J. G. Holstein, H. Adema, H. de Vries, A. Joshua, and F. H. van der Velde for their technical support. Sample fabrication was performed using NanoLabNL facilities.

References

Click to copy section linkSection link copied!

This article references 52 other publications.

  1. 1
    Miao, J.; Hu, W.; Guo, N.; Lu, Z.; Zou, X.; Liao, L.; Shi, S.; Chen, P.; Fan, Z.; Ho, J. C.; Li, T.-X.; Chen, X. S.; Lu, W. Single InAs Nanowire Room-Temperature Near-Infrared Photodetectors. ACS Nano 2014, 8, 3628– 3635,  DOI: 10.1021/nn500201g
    Google Scholar
    1
    Single InAs Nanowire Room-Temperature Near-Infrared Photodetectors
    Miao, Jinshui; Hu, Weida; Guo, Nan; Lu, Zhenyu; Zou, Xuming; Liao, Lei; Shi, Suixing; Chen, Pingping; Fan, Zhiyong; Ho, Johnny C.; Li, Tian-Xin; Chen, Xiao Shuang; Lu, Wei
    ACS Nano (2014), 8 (4), 3628-3635CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Here the authors report InAs nanowire (NW) near-IR photodetectors having a detection wavelength up to ∼1.5 μm. The single InAs NW photodetectors displayed min. hysteresis with a high Ion/Ioff ratio of 105. At room temp., the Schottky-ohmic contacted photodetectors had an external photoresponsivity of ∼5.3 × 103 AW-1, which is ∼300% larger than that of ohmic-ohmic contacted detectors (∼1.9 × 103 AW-1). A large enhancement in photoresponsivity (∼300%) had also been achieved in metal Au-cluster-decorated InAs NW photodetectors due to the formation of Schottky junctions at the InAs/Au cluster contacts. The photocurrent decreased when the photodetectors were exposed to ambient atm. because of the high surface electron concn. and rich surface defect states in InAs NWs. A theor. model based on charge transfer and energy band change is proposed to explain this obsd. performance. To suppress the neg. effects of surface defect states and atm. mols., new InAs NW photodetectors with a half-wrapped top-gate had been fabricated by using 10 nm HfO2 as the top-gate dielec.
  2. 2
    Huang, W.; Xing, C.; Wang, Y.; Li, Z.; Wu, L.; Ma, D.; Dai, X.; Xiang, Y.; Li, J.; Fan, D.; Zhang, H. Facile Fabrication and Characterization of Two-Dimensional Bismuth(III) Sulfide Nanosheets for High-Performance Photodetector Applications Under Ambient Conditions. Nanoscale 2018, 10, 2404– 2412,  DOI: 10.1039/C7NR09046C
    Google Scholar
    2
    Facile fabrication and characterization of two-dimensional bismuth(III) sulfide nanosheets for high-performance photodetector applications under ambient conditions
    Huang, Weichun; Xing, Chenyang; Wang, Yunzheng; Li, Zhongjun; Wu, Leiming; Ma, Dingtao; Dai, Xiaoyu; Xiang, Yuanjiang; Li, Jianqing; Fan, Dianyuan; Zhang, Han
    Nanoscale (2018), 10 (5), 2404-2412CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Two-dimensional (2D) bismuth(III) sulfide (Bi2S3) nanosheets as non-toxic graphene-like nanomaterials were successfully fabricated by a facile liq. phase exfoliation (LPE) method. A robust photodetector employing a Bi2S3 nanosheet film has been fabricated for the first time via a facile fabrication process on an ITO-coated glass. UV-Vis and Raman spectroscopy techniques were carried out and they confirmed the inherent optical and phys. properties of Bi2S3 nanosheets. Photoelectrochem. (PEC) measurements demonstrate that a significantly higher photocurrent d. (42 μA cm-2) and enhanced photoresponsivity (210 μA W-1), at a lower bias potential in alk. soln., of the Bi2S3 nanosheet-based photodetector are achieved, compared with those of other 2D nanomaterial-based photodetectors under light irradn. Furthermore, the as-prepd. Bi2S3 nanosheet-based photodetector not only exhibits an appropriate capacity of self-driven broadband and high-performance photoresponse but also displays strong long-term stability of the ON/OFF switching behavior without any external protection in alk. solns. Because of facile synthesis via a LPE method, a higher photocurrent d. and photoresponsivity, self-driven performance and long-term stability of the Bi2S3 nanosheet-based photodetector at a lower bias potential in alk. solns., the present work can provide fundamental acknowledgment of the high performance of this new kind of PEC-type 2D nanosheet-based photodetector.
  3. 3
    Ilyas, N.; Li, D.; Song, Y.; Zhong, H.; Jiang, Y.; Li, W. Low-Dimensional Materials and State-of-the-Art Architectures for Infrared Photodetection. Sensors 2018, 18, 4163,  DOI: 10.3390/s18124163
    Google Scholar
    There is no corresponding record for this reference.
  4. 4
    Huang, W.; Zhang, Y.; You, Q.; Huang, P.; Wang, Y.; Huang, Z. N.; Ge, Y.; Wu, L.; Dong, Z.; Dai, X.; Xiang, Y.; Li, J.; Zhang, X.; Zhang, H. Enhanced Photodetection Properties of Tellurium@Selenium Roll-to-Roll Nanotube Heterojunctions. Small 2019, 15, 1900902  DOI: 10.1002/smll.201900902
    Google Scholar
    There is no corresponding record for this reference.
  5. 5
    Rojas-Lopez, R. R.; Brant, J. C.; Ramos, M. S. O.; Castro, T. H. L. G.; Guimarães, M. H. D.; Neves, B. R. A.; Guimarães, P. S. S. Photoluminescence and Charge Transfer in the Prototypical 2D/3D Semiconductor Heterostructure MoS2/GaAs. Appl. Phys. Lett. 2021, 119, 233101,  DOI: 10.1063/5.0068548
    Google Scholar
    5
    Photoluminescence and charge transfer in the prototypical 2D/3D semiconductor heterostructure MoS2/GaAs
    Rojas-Lopez, Rafael R.; Brant, Juliana C.; Ramos, Maira S. O.; Castro, Tulio H. L. G.; Guimaraes, Marcos H. D.; Neves, Bernardo R. A.; Guimaraes, Paulo S. S.
    Applied Physics Letters (2021), 119 (23), 233101CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    The new generation of two-dimensional (2D) materials has shown a broad range of applications for optical and electronic devices. Understanding the properties of these materials when integrated with more traditional three-dimensional (3D) semiconductors is an important challenge for the implementation of ultra-thin electronic devices. Recent observations have shown that by combining MoS2 with GaAs, it is possible to develop high quality photodetectors and solar cells. Here, we present a study of effects of intrinsic GaAs, p-doped GaAs, and n-doped GaAs substrates on the photoluminescence of monolayer MoS2. We observe a decrease in an order of magnitude in the emission intensity of MoS2 in all MoS2/GaAs heterojunctions, when compared to a control sample consisting of a MoS2 monolayer isolated from GaAs by a few layers of hexagonal boron nitride. We also see a dependence of the trion to A-exciton emission ratio in the photoluminescence spectra on the type of substrates, a dependence that we relate to the static charge exchange between MoS2 and the substrates when the junction is formed. Scanning Kelvin probe microscopy measurements of heterojunctions suggest type-I band alignments, so that excitons generated on the MoS2 monolayer will be transferred to the GaAs substrate. Our results shed light on the charge exchange leading to band offsets in 2D/3D heterojunctions, which play a central role in the understanding and further improvement of electronic devices. (c) 2021 American Institute of Physics.
  6. 6
    Zhai, X.-P.; Ma, B.; Wang, Q.; Zhang, H.-L. 2D Materials Towards Ultrafast Photonic Applications. Phys. Chem. Chem. Phys. 2020, 22, 22140– 22156,  DOI: 10.1039/D0CP02841J
    Google Scholar
    6
    A 2D materials towards ultrafast photonic applications
    Zhai, Xin-Ping; Ma, Bo; Wang, Qiang; Zhang, Hao-Li
    Physical Chemistry Chemical Physics (2020), 22 (39), 22140-22156CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Having accomplished progress in the versatile battlefields of optics, electronics, catalysis, etc., two-dimensional (2D) materials are now venturing and excelling in yet another arena of ultrafast photonics, a rapidly developing field encompassing a large range of important applications including optical modulation through optical limiting/mode-locking, photodetectors, optical communications, integrated miniaturized all-optical devices and so on. Our group has been devoted to building the arsenal of 2D materials with large third-order nonlinearities, including transition metal dichalcogenides (TMDs), carbon nitride, single-element materials from Group 15, 2D hybrids and vdW heterostructures. In particular, we explore their origin of nonlinear optical responses from the aspect of excited state dynamics using time-resolved spectroscopic techniques such as femtosecond transient absorption spectroscopy. In this review, we propose the roadmap for screening 2D materials for ultrafast photonics through focusing on the third-order nonlinear optical properties of 2D materials and corresponding applications, and then performing mechanistic investigations via time-resolved spectroscopy and calcns., which in turn provide feedback to further guide the fabrication of 2D materials. We offer our own insights on the future trends for the development and theor. calcns. of 2D materials/devices in the final part of Perspectives.
  7. 7
    An, J.; Zhao, X.; Zhang, Y.; Liu, M.; Yuan, J.; Sun, X.; Zhang, Z.; Wang, B.; Li, S.; Li, D. Perspectives of 2D Materials for Optoelectronic Integration. Adv. Funct. Mater. 2022, 32, 2110119  DOI: 10.1002/adfm.202110119
    Google Scholar
    7
    Perspectives of 2D Materials for Optoelectronic Integration
    An, Junru; Zhao, Xingyu; Zhang, Yanan; Liu, Mingxiu; Yuan, Jian; Sun, Xiaojuan; Zhang, Zhiyu; Wang, Bin; Li, Shaojuan; Li, Dabing
    Advanced Functional Materials (2022), 32 (14), 2110119CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. 2D materials show wide-ranging phys. properties with their electronic bandgaps varying from zero to several eV, offering a rich platform to explore novel electronic and optoelectronic functions. Notably, atomically thin 2D materials are well suited for integration in optoelectronic circuits, because of their ultrathin body, strong light-matter interactions, and compatibility with the current silicon photonic technol. In this paper, an overview of the state of the art of using 2D materials in optoelectronic devices and integration is provided. The optoelectronic properties of 2D materials and their typical electronic and optoelectronic applications including light sources, optical modulators, photodetectors, field-effect transistors, and logic circuits are summarized. The device configurations, operation mechanisms, and device figures-of-merit are introduced and discussed. By discussing the recent advances, future trends, and existing challenges of 2D materials and their optoelectronic devices, this review has provided an insight into the perspectives of 2D materials for optoelectronic integration and may guide the development of this field within the research community.
  8. 8
    Liang, G.; Yu, X.; Hu, X.; Qiang, B.; Wang, C.; Wang, Q. J. Mid-Infrared Photonics and Optoelectronics in 2D Materials. Mater. Today 2021, 51, 294– 316,  DOI: 10.1016/j.mattod.2021.09.021
    Google Scholar
    8
    Mid-infrared photonics and optoelectronics in 2D materials
    Liang, Guozhen; Yu, Xuechao; Hu, Xiaonan; Qiang, Bo; Wang, Chongwu; Wang, Qi Jie
    Materials Today (Oxford, United Kingdom) (2021), 51 (), 294-316CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
    A review. With intriguing properties that are different from the conventional bulk materials, two-dimensional (2D) materials have attracted numerous and widespread research interests, including its applications in photonics and optoelectronics. Devices based on 2D materials have been demonstrated in a wide spectral range, from the UV to the terahertz, and the microwave wavelength range. The mid-IR (MIR) region (about 2-20μm) bears a particular scientific and technol. significance because, for instance, many mols. have their spectral fingerprints and there are atm. transparent windows in this region. Nevertheless, the MIR region remains underdeveloped compared to the visible and the near-IR telecommunication regimes, mainly due to the lack of suitable materials, such as narrow bandgap materials, and proper photonic designs for building high performance optoelectronic devices in this wavelength regime. Therefore, researchers have been exploring the possibility and opportunity of 2D materials to fill up the gap. Here, we review the key recent developments of 2D materials in the MIR photonic and optoelectronic applications, including photodetection, light modulation, surface plasmon polaritons, phonon polaritons, and their nonlinearities and provide an outlook on the challenges and opportunities that lie ahead for MIR optoelectronic research fields with 2D materials.
  9. 9
    Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Van Der Zant, H. S.; Castellanos-Gomez, A. Photocurrent Generation with Two-Dimensional van der Waals Semiconductors. Chem. Soc. Rev. 2015, 44, 3691– 3718,  DOI: 10.1039/C5CS00106D
    Google Scholar
    9
    Photocurrent generation with two-dimensional van der Waals semiconductors
    Buscema, Michele; Island, Joshua O.; Groenendijk, Dirk J.; Blanter, Sofya I.; Steele, Gary A.; van der Zant, Herre S. J.; Castellanos-Gomez, Andres
    Chemical Society Reviews (2015), 44 (11), 3691-3718CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Two-dimensional (2D) materials have attracted a great deal of interest in recent years. This family of materials allows for the realization of versatile electronic devices and holds promise for next-generation (opto)electronics. Their electronic properties strongly depend on the no. of layers, making them interesting from a fundamental standpoint. For electronic applications, semiconducting 2D materials benefit from sizable mobilities and large on/off ratios, due to the large modulation achievable via the gate field-effect. Moreover, being mech. strong and flexible, these materials can withstand large strain (>10%) before rupture, making them interesting for strain engineering and flexible devices. Even in their single layer form, semiconducting 2D materials have demonstrated efficient light absorption, enabling large responsivity in photodetectors. Therefore, semiconducting layered 2D materials are strong candidates for optoelectronic applications, esp. for photodetection. Here, we review the state-of-the-art in photodetectors based on semiconducting 2D materials, focusing on the transition metal dichalcogenides, novel van der Waals materials, black phosphorus, and heterostructures.
  10. 10
    Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805  DOI: 10.1103/PhysRevLett.105.136805
    Google Scholar
    10
    Atomically Thin MoS2. A New Direct-Gap Semiconductor
    Mak, Kin Fai; Lee, Changgu; Hone, James; Shan, Jie; Heinz, Tony F.
    Physical Review Letters (2010), 105 (13), 136805/1-136805/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    The electronic properties of ultrathin crystals of MoS2 consisting of N = 1, 2,...,6 S-Mo-S monolayers were investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photocond. spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by >0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material.
  11. 11
    Manzeli, S.; Ovchinnikov, D.; Pasquier, D.; Yazyev, O. V.; Kis, A. 2D Transition Metal Dichalcogenides. Nat. Rev. Mater. 2017, 2, 17033,  DOI: 10.1038/natrevmats.2017.33
    Google Scholar
    11
    2D transition metal dichalcogenides
    Manzeli, Sajedeh; Ovchinnikov, Dmitry; Pasquier, Diego; Yazyev, Oleg V.; Kis, Andras
    Nature Reviews Materials (2017), 2 (2), 17033CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
    A review. Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of at.-scale thickness, direct bandgap, strong spin-orbit coupling and favorable electronic and mech. properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examd. and their properties are discussed, with particular attention to their charge d. wave, superconductive and topol. phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties.
  12. 12
    Aftab, S.; Shehzad, M. A.; Salman Ajmal, H. M.; Kabir, F.; Iqbal, M. Z.; Al-Kahtani, A. A. Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2. ACS Nano 2023, 17, 17884– 17896,  DOI: 10.1021/acsnano.3c03593
    Google Scholar
    There is no corresponding record for this reference.
  13. 13
    Hu, X.; Zhang, F.; Hu, Z.; He, P.; Tao, L.; Zheng, Z.; Zhao, Y.; Yang, Y.; He, J. Preparation of 1T’- and 2H–MoTe2 Films and Investigation of their Photoelectric Properties and Ultrafast Photocarrier Dynamics. Opt. Mater. 2023, 136, 113467  DOI: 10.1016/j.optmat.2023.113467
    Google Scholar
    There is no corresponding record for this reference.
  14. 14
    Dave, M.; Vaidya, R.; Patel, S. G.; Jani, A. R. High Pressure Effect on MoS2 and MoSe2 Single Crystals Grown by CVT Method. Bulletin of Materials Science 2004, 27, 213– 216,  DOI: 10.1007/BF02708507
    Google Scholar
    There is no corresponding record for this reference.
  15. 15
    Song, S.; Keum, D. H.; Cho, S.; Perello, D.; Kim, Y.; Lee, Y. H. Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain. Nano Lett. 2016, 16, 188– 193,  DOI: 10.1021/acs.nanolett.5b03481
    Google Scholar
    15
    Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain
    Song, Seunghyun; Keum, Dong Hoon; Cho, Suyeon; Perello, David; Kim, Yunseok; Lee, Young Hee
    Nano Letters (2016), 16 (1), 188-193CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We demonstrate a room temp. semiconductor-metal transition in thin film MoTe2 engineered by strain. Redn. of the 2H-1T' phase transition temp. of MoTe2 to room temp. was realized by introducing a tensile strain of 0.2%. The obsd. first-order SM transition improved conductance ∼10 000 times and was made possible by an unusually large temp.-stress coeff., which results from a large vol. change and small latent heat. The demonstrated strain-modulation of the phase transition temp. is expected to be compatible with other TMDs enabling the 2D electronics utilizing polymorphism of TMDs along with the established materials.
  16. 16
    Lin, Y.-C.; Dumcenco, D. O.; Huang, Y.-S.; Suenaga, K. Atomic Mechanism of the Semiconducting-to-Metallic phase Transition in Single-Layered MoS2. Nat. Nanotechnol. 2014, 9, 391– 396,  DOI: 10.1038/nnano.2014.64
    Google Scholar
    16
    Atomic mechanism of the semiconducting-to-metallic phase transition in single-layered MoS2
    Lin, Yung-Chang; Dumcenco, Dumitru O.; Huang, Ying-Sheng; Suenaga, Kazu
    Nature Nanotechnology (2014), 9 (5), 391-396CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Phase transitions can be used to alter the properties of a material without adding any addnl. atoms and are therefore of significant technol. value. In a solid, phase transitions involve collective at. displacements, but such at. processes have so far only been investigated using macroscopic approaches. Here, we show that in situ scanning TEM can be used to follow the structural transformation between semiconducting (2H) and metallic (1T) phases in single-layered MoS2, with at. resoln. The 2H/1T phase transition involves gliding at. planes of sulfur and/or molybdenum and requires an intermediate phase (α-phase) as a precursor. The migration of two kinds of boundaries (β- and γ-boundaries) is also responsible for the growth of the second phase. Furthermore, we show that areas of the 1T phase can be controllably grown in a layer of the 2H phase using an electron beam.
  17. 17
    Shang, B.; Cui, X.; Jiao, L.; Qi, K.; Wang, Y.; Fan, J.; Yue, Y.; Wang, H.; Bao, Q.; Fan, X.; Wei, S.; Song, W.; Cheng, Z.; Guo, S.; Zheng, W. Lattice-Mismatch-Induced Ultrastable 1T-Phase MoS2-Pd/Au for Plasmon-Enhanced Hydrogen Evolution. Nano Lett. 2019, 19, 2758– 2764,  DOI: 10.1021/acs.nanolett.8b04104
    Google Scholar
    17
    Lattice -Mismatch-Induced Ultrastable 1T-Phase MoS2-Pd/Au for Plasmon-Enhanced Hydrogen Evolution
    Shang, Bo; Cui, Xiaoqiang; Jiao, Lin; Qi, Kun; Wang, Yingwei; Fan, Jinchang; Yue, Yuanyuan; Wang, Haiyu; Bao, Qiaoliang; Fan, Xiaofeng; Wei, Shuting; Song, Wei; Cheng, Zhiliang; Guo, Shaojun; Zheng, Weitao
    Nano Letters (2019), 19 (5), 2758-2764CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Metallic 1T-phase transition metal dichalcogenides (TMDs) are of considerable interest in enhancing catalytic applications due to their abundant active sites and good cond. However, the unstable nature of 1T-phase TMDs greatly impedes their practical applications. Herein, we developed a new approach for the synthesis of highly stable 1T-phase Au/Pd-MoS2 nanosheets (NSs) through a metal assembly induced ultrastable phase transition for achieving a very high electrocatalytic activity in the hydrogen evolution reaction. The phase transition was evoked by a novel mechanism of lattice-mismatch-induced strain based on d. functional theory (DFT) calcns. Raman spectroscopy and transmission electron microscopy (TEM) were used to confirm the phase transition on exptl. grounds. A novel heterostructured 1T MoS2-Au/Pd catalyst was designed and synthesized using this mechanism, and the catalyst exhibited a 0 mV onset potential in the hydrogen evolution reaction under light illumination. Therefore, this method can potentially be used to fabricate 1T-phase TMDs with remarkably enhanced activities for different applications.
  18. 18
    Wang, Y.; Xiao, J.; Zhu, H.; Li, Y.; Alsaid, Y.; Fong, K. Y.; Zhou, Y.; Wang, S.; Shi, W.; Wang, Y.; Zettl, A.; Reed, E. J.; Zhang, X. Structural Phase Transition in Monolayer MoTe2 Driven by Electrostatic Doping. Nature 2017, 550, 487– 491,  DOI: 10.1038/nature24043
    Google Scholar
    18
    Structural phase transition in monolayer MoTe2 driven by electrostatic doping
    Wang, Ying; Xiao, Jun; Zhu, Hanyu; Li, Yao; Alsaid, Yousif; Fong, King Yan; Zhou, Yao; Wang, Siqi; Shi, Wu; Wang, Yuan; Zettl, Alex; Reed, Evan J.; Zhang, Xiang
    Nature (London, United Kingdom) (2017), 550 (7677), 487-491CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and phys. properties. Exploring the physics of transitions between these different structural phases in two dimensions may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chem. means; purely electrostatic control over crystal phases through electrostatic doping was recently proposed as a theor. possibility, but has not yet been realized. Here we report the exptl. demonstration of an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe2). We find that the phase transition shows a hysteretic loop in Raman spectra, and can be reversed by increasing or decreasing the gate voltage. We also combine second-harmonic generation spectroscopy with polarization-resolved Raman spectroscopy to show that the induced monoclinic phase preserves the crystal orientation of the original hexagonal phase. Moreover, this structural phase transition occurs simultaneously across the whole sample. This electrostatic-doping control of structural phase transition opens up new possibilities for developing phase-change devices based on atomically thin membranes.
  19. 19
    Kappera, R.; Voiry, D.; Yalcin, S. E.; Branch, B.; Gupta, G.; Mohite, A. D.; Chhowalla, M. Phase-Engineered Low-Resistance Contacts for Ultrathin MoS2 transistors. Nat. Mater. 2014, 13, 1128– 1134,  DOI: 10.1038/nmat4080
    Google Scholar
    19
    Phase-engineered low-resistance contacts for ultrathin MoS2 transistors
    Kappera, Rajesh; Voiry, Damien; Yalcin, Sibel Ebru; Branch, Brittany; Gupta, Gautam; Mohite, Aditya D.; Chhowalla, Manish
    Nature Materials (2014), 13 (12), 1128-1134CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    Ultrathin molybdenum disulfide (MoS2) has emerged as an interesting layered semiconductor because of its finite energy bandgap and the absence of dangling bonds. However, metals deposited on the semiconducting 2H phase usually form high-resistance (0.7 kΩ μm-10 kΩ μm) contacts, leading to Schottky-limited transport. The metallic 1T phase of MoS2 can be locally induced on semiconducting 2H phase nanosheets, thus decreasing contact resistances to 200-300 Ω μm at zero gate bias. Field-effect transistors (FETs) with 1T phase electrodes fabricated and tested in air exhibit mobility values of ∼50 cm2 V-1 s-1, subthreshold swing values <100 mV per decade, on/off ratios of >107, drive currents approaching ∼100 μA μm-1, and excellent current satn. The deposition of different metals has limited influence on the FET performance, suggesting that the 1T/2H interface controls carrier injection into the channel. An increased reproducibility of the elec. characteristics is also obtained with the authors' strategy based on phase engineering of MoS2.
  20. 20
    Ma, Y.; Liu, B.; Zhang, A.; Chen, L.; Fathi, M.; Shen, C.; Abbas, A. N.; Ge, M.; Mecklenburg, M.; Zhou, C. Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe2 and Applications for Devices. ACS Nano 2015, 9, 7383– 7391,  DOI: 10.1021/acsnano.5b02399
    Google Scholar
    20
    Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe2 and Applications for Devices
    Ma, Yuqiang; Liu, Bilu; Zhang, Anyi; Chen, Liang; Fathi, Mohammad; Shen, Chenfei; Abbas, Ahmad N.; Ge, Mingyuan; Mecklenburg, Matthew; Zhou, Chongwu
    ACS Nano (2015), 9 (7), 7383-7391CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe2 are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe2 as active channel materials. Recently, a phase engineering concept of 2D MoS2 materials was developed, with improved device performance. Here, we applied this method to chem. vapor deposition (CVD) grown monolayer 2H-WSe2 and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe2. We have also shown that metallic phase WSe2 can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addn., we fabricated FETs based on these CVD-grown WSe2 flakes with phase-engineered metallic 1T-WSe2 as contact regions and intact semiconducting 2H-WSe2 as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 107 and mobilities up to 66 cm2/V·s for monolayer WSe2. These results further suggest that phase engineering can be a generic strategy to improve device performance for many kinds of 2D TMDC materials.
  21. 21
    Cho, S.; Kim, S.; Kim, J. H.; Zhao, J.; Seok, J.; Keum, D. H.; Baik, J.; Choe, D.-H.; Chang, K. J.; Suenaga, K.; Kim, S. W.; Lee, Y. H.; Yang, H. Phase Patterning for Ohmic Homojunction Contact in MoTe2. Science 2015, 349, 625– 628,  DOI: 10.1126/science.aab3175
    Google Scholar
    21
    Phase patterning for ohmic homojunction contact in MoTe2
    Cho, Suyeon; Kim, Sera; Kim, Jung Ho; Zhao, Jiong; Seok, Jinbong; Keum, Dong Hoon; Baik, Jaeyoon; Choe, Duk-Hyun; Chang, K. J.; Suenaga, Kazu; Kim, Sung Wng; Lee, Young Hee; Yang, Heejun
    Science (Washington, DC, United States) (2015), 349 (6248), 625-628CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Artificial van der Waals heterostructures with two-dimensional (2D) at. crystals are promising as an active channel or as a buffer contact layer for next-generation devices. However, genuine 2-dimensional heterostructure devices remain limited because of impurity-involved transfer process and metastable and inhomogeneous heterostructure formation. The authors used laser-induced phase patterning, a polymorph engineering, to fabricate an ohmic heterophase homojunction between semiconducting hexagonal (2H) and metallic monoclinic (1T') molybdenum ditelluride (MoTe2) that is stable up to 300° and increases the carrier mobility of the MoTe2 transistor by a factor of ∼50, while retaining a high on/off current ratio of 106. In situ scanning TEM results combined with theor. calcns. reveal that the Te vacancy triggers the local phase transition in MoTe2, achieving a true 2-dimensional device with an ohmic contact.
  22. 22
    Duerloo, K.-A. N.; Li, Y.; Reed, E. J. Structural Phase Transitions in Two-Dimensional Mo- and W-Dichalcogenide Monolayers. Nat. Commun. 2014, 5, 4214,  DOI: 10.1038/ncomms5214
    Google Scholar
    22
    Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers
    Duerloo, Karel-Alexander N.; Li, Yao; Reed, Evan J.
    Nature Communications (2014), 5 (), 4214CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Mo- and W-dichalcogenide compds. have a two-dimensional monolayer form that differs from graphene in an important respect: it can potentially have more than one crystal structure. Some of these monolayers exhibit tantalizing hints of a poorly understood structural metal-to-insulator transition with the possibility of long metastable lifetimes. If controllable, such a transition could bring an exciting new application space to monolayer materials beyond graphene. Here we discover that mech. deformations provide a route to switching thermodn. stability between a semiconducting and a metallic crystal structure in these monolayer materials. Based on state-of-the-art d. functional and hybrid Hartree-Fock/d. functional calcns. including vibrational energy corrections, we discover that MoTe2 is an excellent candidate phase change material. We identify a range from 0.3 to 3% for the tensile strains required to transform MoTe2 under uniaxial conditions at room temp. The potential for mech. phase transitions is predicted for all six studied compds.
  23. 23
    Tan, Y.; Luo, F.; Zhu, M.; Xu, X.; Ye, Y.; Li, B.; Wang, G.; Luo, W.; Zheng, X.; Wu, N.; Yu, Y.; Qin, S.; Zhang, X.-A. Controllable 2H-to-1T’ Phase Transition in Few-Layer MoTe2. Nanoscale 2018, 10, 19964– 19971,  DOI: 10.1039/C8NR06115G
    Google Scholar
    23
    Controllable 2H-to-1T' phase transition in few-layer MoTe2
    Tan, Yuan; Luo, Fang; Zhu, Mengjian; Xu, Xiaolong; Ye, Yu; Li, Bing; Wang, Guang; Luo, Wei; Zheng, Xiaoming; Wu, Nannan; Yu, Yayun; Qin, Shiqiao; Zhang, Xue-Ao
    Nanoscale (2018), 10 (42), 19964-19971CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Most two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit more than one structural phase, leading to a no. of remarkable physics and potential device applications beyond graphene. Here, we demonstrated a feasible route to trigger 2H-to-1T' phase transition in few-layer molybdenum ditelluride (MoTe2) by laser irradn. The effects of laser power and irradn. duration were systematically studied in this study, revealing the accumulated heating effect as the main driving force for such a phase transition. By carefully adjusting laser power and irradn. time, we could control the structural phases of MoTe2 as 2H, 2H + 1T', and 1T'. After thermal annealing at a rather low temp., the laser-irradiated MoTe2 showed a completely suppressed 2H component and a more stabilized 1T' phase, demonstrating that the microscopic origin of the irreversible 2H-to-1T' phase transition is the formation of Te vacancies in MoTe2 due to laser local instantaneous heating. Our findings together with the unique properties of MoTe2 pave the way for high-performance nanoelectronics and optoelectronics based on 2D TMDs and their heterostructures.
  24. 24
    Kang, S.; Won, D.; Yang, H.; Lin, C.-H.; Ku, C.-S.; Chiang, C.-Y.; Kim, S.; Cho, S. Phase-controllable laser thinning in MoTe2. Appl. Surf. Sci. 2021, 563, 150282  DOI: 10.1016/j.apsusc.2021.150282
    Google Scholar
    24
    Phase-controllable laser thinning in MoTe2
    Kang, Seohui; Won, Dongyeun; Yang, Heejun; Lin, Chia-Hsien; Ku, Ching-Shun; Chiang, Ching-Yu; Kim, Sera; Cho, Suyeon
    Applied Surface Science (2021), 563 (), 150282CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Laser thinning of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) has been considered a promising method to tune the bandgaps of TMDs via precise control of their thickness. However, the laser irradn. generates numerous chalcogen vacancies, which are known to cause a phase transition in polymorphic TMDs such as MoTe2. Therefore, the delicate control of the thickness and the phase during laser thinning is highly demanded to study the intrinsic properties of few-layered TMDs. Here, we report power-dependent laser thinning and phase control of semiconducting hexagonal MoTe2 (2H-MoTe2). High-resoln. X-ray nano diffraction with synchrotron radiation showed that laser-thinned 2H-MoTe2 with low laser power (<2 mW) retained its hexagonal diffraction patterns with a single crystal orientation. In contrast, a phase transition to monoclinic (1T') MoTe2 occurred during laser thinning at a high laser power level. Confocal Raman spectroscopy and at. force microscopy (AFM) revealed that the low-power laser thinning of 2H-MoTe2 retained the crystal structure whereas high-power laser thinning created considerable amt. of chalcogen vacancies and a phase transition. Power-dependent laser thinning thus provides a promising way to control the thickness and the phase of polymorphic 2D TMDs for next-generation optoelectronic devices.
  25. 25
    Zhang, X. Low Contact Barrier in 2H/1T’ MoTe2 In-Plane Heterostructure Synthesized by Chemical Vapor Deposition. ACS Appl. Mater. Interfaces 2019, 11, 12777– 12785,  DOI: 10.1021/acsami.9b00306
    Google Scholar
    25
    Low contact barrier in 2H/1T' MoTe2 in-plane heterostructure synthesized by chemical vapor deposition
    Zhang, Xiang; Jin, Zehua; Wang, Luqing; Hachtel, Jordan A.; Villarreal, Eduardo; Wang, Zixing; Ha, Teresa; Nakanishi, Yusuke; Tiwary, Chandra Sekhar; Lai, Jiawei; Dong, Liangliang; Yang, Jihui; Vajtai, Robert; Ringe, Emilie; Idrobo, Juan Carlos; Yakobson, Boris I.; Lou, Jun; Gambin, Vincent; Koltun, Rachel; Ajayan, Pulickel M.
    ACS Applied Materials & Interfaces (2019), 11 (13), 12777-12785CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Metal-semiconductor contact has been a crit. topic in the semiconductor industry because it influences device performance remarkably. Conventional metals have served as the major contact material in electronic and optoelectronic devices, but such a selection becomes increasingly inadequate for emerging novel materials such as two-dimensional (2D) materials. Deposited metals on semiconducting 2D channels usually form large resistance contacts due to the high Schottky barrier. A few approaches have been reported to reduce the contact resistance but they are not suitable for large-scale application or they cannot create a clean and sharp interface. In this study, a chem. vapor deposition (CVD) technique is introduced to produce large-area semiconducting 2D material (2H MoTe2) planarly contacted by its metallic phase (1T' MoTe2). We demonstrate the phase-controllable synthesis and systematic characterization of large-area MoTe2 films, including pure 2H phase or 1T' phase, and 2H/1T' in-plane heterostructure. Theor. simulation shows a lower Schottky barrier in 2H/1T' junction than in Ti/2H contact, which is confirmed by elec. measurement. This one-step CVD method to synthesize large-area, seamless-bonding 2D lateral metal-semiconductor junction can improve the performance of 2D electronic and optoelectronic devices, paving the way for large-scale 2D integrated circuits.
  26. 26
    Bae, G. Y.; Kim, J.; Kim, J.; Lee, S.; Lee, E. MoTe2 Field-Effect Transistors with Low Contact Resistance through Phase Tuning by Laser Irradiation. Nanomaterials 2021, 11, 2805,  DOI: 10.3390/nano11112805
    Google Scholar
    26
    MoTe2 Field-Effect Transistors with Low Contact Resistance through Phase Tuning by Laser Irradiation
    Bae, Geun Yeol; Kim, Jinsung; Kim, Junyoung; Lee, Siyoung; Lee, Eunho
    Nanomaterials (2021), 11 (11), 2805CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
    Due to their extraordinary elec. and phys. properties, two-dimensional (2D) transition metal dichalcogenides (TMDs) are considered promising for use in next-generation elec. devices. However, the application of TMD-based devices is limited because of the Schottky barrier interface resulting from the absence of dangling bonds on the TMDs' surface. Here, we introduce a facile phase-tuning approach for forming a homogenous interface between semiconducting hexagonal (2H) and semi-metallic monoclinic (1T') molybdenum ditelluride (MoTe2). The formation of ohmic contacts increases the charge carrier mobility of MoTe2 field-effect transistor devices to 16.1 cm2 V-1s-1 with high reproducibility, while maintaining a high on/off current ratio by efficiently improving charge injection at the interface. The proposed method enables a simple fabrication process, local patterning, and large-area scaling for the creation of high-performance 2D electronic devices.
  27. 27
    Wang, Y.; Chhowalla, M. Making Clean Electrical Contacts on 2D Transition Metal Dichalcogenides. Nature Reviews Physics 2022, 4, 101– 112,  DOI: 10.1038/s42254-021-00389-0
    Google Scholar
    27
    Making clean electrical contacts on 2D transition metal dichalcogenides
    Wang, Yan; Chhowalla, Manish
    Nature Reviews Physics (2022), 4 (2), 101-112CODEN: NRPACZ; ISSN:2522-5820. (Nature Portfolio)
    Abstr.: 2D semiconductors, particularly transition metal dichalcogenides (TMDs), have emerged as highly promising for new electronic technologies. However, a key challenge in fabricating devices out of 2D semiconductors is the need for ultra-clean contacts with resistances approaching the quantum limit. The lack of high-quality, low-contact-resistance P-type and N-type contacts on 2D TMDs has limited progress towards the next generation of low-power devices, such as the tunnel field-effect transistors. In this Expert Recommendation, we summarize strategies and provide guidance for making clean van der Waals contacts on monolayered TMD semiconductors. We also discuss the physics of contacts in 2D semiconductors and prospects for achieving quantum conductance.
  28. 28
    Yamaguchi, H.; Blancon, J. C.; Kappera, R.; Lei, S.; Najmaei, S.; Mangum, B. D.; Gupta, G.; Ajayan, P. M.; Lou, J.; Chhowalla, M.; Crochet, J. J.; Mohite, A. D. Spatially Resolved Photoexcited Charge-Carrier Dynamics in Phase-Engineered Monolayer MoS2. ACS Nano 2015, 9, 840– 849,  DOI: 10.1021/nn506469v
    Google Scholar
    28
    Spatially Resolved Photoexcited Charge-Carrier Dynamics in Phase-Engineered Monolayer MoS2
    Yamaguchi, Hisato; Blancon, Jean-Christophe; Kappera, Rajesh; Lei, Sidong; Najmaei, Sina; Mangum, Benjamin D.; Gupta, Gautam; Ajayan, Pulickel M.; Lou, Jun; Chhowalla, Manish; Crochet, Jared J.; Mohite, Aditya D.
    ACS Nano (2015), 9 (1), 840-849CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A fundamental understanding of the intrinsic optoelectronic properties of atomically thin transition-metal dichalcogenides (TMDs) is crucial for its integration into high performance semiconductor devices. Here, the authors study the transport properties of CVD grown monolayer molybdenum disulfide (MoS2) under photoexcitation using correlated scanning photocurrent microscopy and photoluminescence imaging. The authors examd. the effect of local phase transformation underneath the metal electrodes on the generation of photocurrent across the channel length with diffraction-limited spatial resoln. While max. photocurrent generation occurs at the Schottky contacts of semiconducting (2H-phase) MoS2, after the metallic phase transformation (1T-phase), the photocurrent peak is obsd. toward the center of the device channel, suggesting a strong redn. of native Schottky barriers. Anal. using the bias and position dependence of the photocurrent indicates that the Schottky barrier heights are a few millielectron volts for 1T- and ∼200 meV for 2H-contacted devices. Also a redn. of native Schottky barriers in a 1T device enhances the photoresponsivity by >1 order of magnitude, a crucial parameter in achieving high-performance optoelectronic devices. The obtained results pave a way for the fundamental understanding of intrinsic optoelectronic properties of atomically thin TMDs where ohmic contacts are necessary for achieving high-efficiency devices with low power consumption.
  29. 29
    Lin, D.-Y.; Hsu, H.-P.; Liu, G.-H.; Dai, T.-Z.; Shih, Y.-T. Enhanced Photoresponsivity of 2H-MoTe2 by Inserting 1T-MoTe2 Interlayer Contact for Photodetector Applications. Crystals 2021, 11, 964,  DOI: 10.3390/cryst11080964
    Google Scholar
    29
    Enhanced Photoresponsivity of 2H-MoTe2 by Inserting 1T-MoTe2 Interlayer Contact for Photodetector Applications
    Lin, Der-Yuh; Hsu, Hung-Pin; Liu, Guang-Hsin; Dai, Ting-Zhong; Shih, Yu-Tai
    Crystals (2021), 11 (8), 964CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
    The 2H molybdenum telluride (MoTe2) photodetector structures were made with inserting 1T-MoTe2 interlayer contacts. The optical response properties such as photocond. (PC) spectroscopy, illumination intensity dependent photoresponsivity, frequency dependent photocurrent, and time-resolved photoresponse were carried out in this study. In PC spectra, a much higher photoresponsivity of 2H-MoTe2 were obsd. by inserting 1T-MoTe2 interlayer contact. The frequency dependent photocurrent and time-resolved photoresponse investigations explore the carrier kinetic decay process of MoTe2 with different electrode contact. The Schottky barrier heights (SBH) extd. by thermionic emission theory were also investigated by inserting 1T-MoTe2 interlayer contacts. The results show the potential applicability for photodetection devices based MoTe2 layered transition metal dichalcogenides semiconductors.
  30. 30
    Ding, Y.; Qi, R.; Wang, C.; Wu, Q.; Zhang, H.; Zhang, X.; Lin, L.; Cai, Z.; Xiao, S.; Gu, X.; Nan, H. Broad-Band Photodetector Based on a Lateral MoTe2 1T-2H-1T Homojunction. J. Phys. Chem. C 2023, 127, 20072– 20081,  DOI: 10.1021/acs.jpcc.3c05592
    Google Scholar
    30
    Broad-Band Photodetector Based on a Lateral MoTe2 1T-2H-1T Homojunction
    Ding, Yang; Qi, Renxian; Wang, Chenglin; Wu, Qianqian; Zhang, Haozhe; Zhang, Xiumei; Lin, Liangliang; Cai, Zhengyang; Xiao, Shaoqing; Gu, Xiaofeng; Nan, Haiyan
    Journal of Physical Chemistry C (2023), 127 (40), 20072-20081CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The use of a two-dimensional (2D) layered molybdenum ditelluride (MoTe2) nanosheet in electronic devices can be enhanced by transforming its electrode to a metallic phase, forming a 1T-2H-1T heterogeneous structure. This, however, leads to the formation of a junction between the electrode and the channel which can reduce the effectiveness of the device. To address this issue, this paper proposes a novel method of creating 1T-2H-1T MoTe2 optoelectronic devices using oxygen mild plasma prepn. Transmission electron microscopy testing and first-principles calcns. have confirmed that this treatment results in the loss of some Te atoms, thus lowering their energy bands and creating metallic properties. The elec. and optoelectronic performance testing at varying temps. and wavelengths has revealed improved mobility and responsiveness, with a max. mobility of 30 cm2/V s and a responsivity of 0.6 A/W under a 940 nm laser irradn., being 25 times higher than the intrinsic one. This research provides an efficient phase engineering technique for the accurate and undamaged fabrication of optoelectronic devices using two-dimensional materials.
  31. 31
    Xu, X.; Gabor, N. M.; Alden, J. S.; Van Der Zande, A. M.; McEuen, P. L. Photo-thermoelectric Effect at a Graphene Interface Junction. Nano Lett. 2010, 10, 562– 566,  DOI: 10.1021/nl903451y
    Google Scholar
    31
    Photo-Thermoelectric Effect at a Graphene Interface Junction
    Xu, Xiaodong; Gabor, Nathaniel M.; Alden, Jonathan S.; van der Zande, Arend M.; McEuen, Paul L.
    Nano Letters (2010), 10 (2), 562-566CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We investigate the optoelectronic response of a graphene single-bilayer interface junction using photocurrent (PC) microscopy. We measure the polarity and amplitude of the PC while varying the Fermi level by tuning a gate voltage. These measurements show that the generation of PC is by a photothermoelec. effect. The PC displays a factor of ∼10 increase at the cryogenic temp. as compared to room temp. Assuming the thermoelec. power has a linear dependence on the temp., the inferred graphene thermal cond. from temp. dependent measurements has a T1.5 dependence below ∼100 K, which agrees with recent theor. predictions.
  32. 32
    Zhang, Y.; Li, H.; Wang, L.; Wang, H.; Xie, X.; Zhang, S. L.; Liu, R.; Qiu, Z. J. Photothermoelectric and Photovoltaic Effects Both Present in MoS2. Sci. Rep. 2015, 5, 7938,  DOI: 10.1038/srep07938
    Google Scholar
    32
    Photothermoelectric and photovoltaic effects both present in MoS2
    Zhang, Youwei; Li, Hui; Wang, Lu; Wang, Haomin; Xie, Xiaomin; Zhang, Shi-Li; Liu, Ran; Qiu, Zhi-Jun
    Scientific Reports (2015), 5 (), 7938CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    As a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS2) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS2 is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS2. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS2 transistors. The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelec. effect (PTE). In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coeff. obsd. in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equil. with the MoS2 lattice.
  33. 33
    Huo, N.; Konstantatos, G. Recent Progress and Future Prospects of 2D-Based Photodetectors. Adv. Mater. 2018, 30, 1801164  DOI: 10.1002/adma.201801164
    Google Scholar
    There is no corresponding record for this reference.
  34. 34
    HQgraphene., MoTe2 (2H Molybdenum Ditelluride). https://www.hqgraphene.com/MoTe2.php (accessed May 23 2023).
    Google Scholar
    There is no corresponding record for this reference.
  35. 35
    Buscema, M.; Barkelid, M.; Zwiller, V.; van der Zant, H. S. J.; Steele, G. A.; Castellanos-Gomez, A. Large and Tunable Photothermoelectric Effect in Single-Layer MoS2. Nano Lett. 2013, 13, 358– 363,  DOI: 10.1021/nl303321g
    Google Scholar
    35
    Large and Tunable Photothermoelectric Effect in Single-Layer MoS2
    Buscema, Michele; Barkelid, Maria; Zwiller, Val; van der Zant, Herre S. J.; Steele, Gary A.; Castellanos-Gomez, Andres
    Nano Letters (2013), 13 (2), 358-363CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The authors study the photoresponse of single-layer MoS2 field-effect transistors by scanning photocurrent microscopy. Unlike in many other semiconductors, the photocurrent generation in single-layer MoS2 is dominated by the photothermoelec. effect and not by the sepn. of photoexcited electron-hole pairs across the Schottky barriers at the MoS2/electrode interfaces. The authors observe a large value for the Seebeck coeff. for single-layer MoS2 that by an external elec. field can be tuned between -4 × 102 and -1 × 105 μV K-1. This large and tunable Seebeck coeff. of the single-layer MoS2 paves the way to new applications of this material such as on-chip thermopower generation and waste thermal energy harvesting.859.
  36. 36
    Ruppert, C.; Aslan, O. B.; Heinz, T. F. Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals. Nano Lett. 2014, 14, 6231– 6236,  DOI: 10.1021/nl502557g
    Google Scholar
    36
    Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals
    Ruppert, Claudia; Aslan, Burak; Heinz, Tony F.
    Nano Letters (2014), 14 (11), 6231-6236CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Single- and few-layer crystals of exfoliated MoTe2 have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe2 in the monolayer limit displays strong photoluminescence. On the basis of complementary optical absorption results, we conclude that monolayer MoTe2 is a direct-gap semiconductor with an optical band gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near-IR.
  37. 37
    Lezama, I. G.; Arora, A.; Ubaldini, A.; Barreteau, C.; Giannini, E.; Potemski, M.; Morpurgo, A. F. Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2. Nano Lett. 2015, 15, 2336– 2342,  DOI: 10.1021/nl5045007
    Google Scholar
    37
    Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2
    Lezama, Ignacio Gutierrez; Arora, Ashish; Ubaldini, Alberto; Barreteau, Celine; Giannini, Enrico; Potemski, Marek; Morpurgo, Alberto F.
    Nano Letters (2015), 15 (4), 2336-2342CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temp. microreflectance (MR) and temp.-dependent photoluminescence (PL) measurements. The anal. of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions assocd. with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The anal. of this behavior and of all our exptl. observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for expts. of fundamental interest and possible future device applications.
  38. 38
    Yadav, P.; Dewan, S.; Mishra, R.; Das, S. Review of Recent Progress, Challenges, and Prospects of 2D Materials-Based Short Wavelength Infrared Photodetectors. J. Phys. D: Appl. Phys. 2022, 55, 313001,  DOI: 10.1088/1361-6463/ac6635
    Google Scholar
    There is no corresponding record for this reference.
  39. 39
    Zhang, K.; Fang, X.; Wang, Y.; Wan, Y.; Song, Q.; Zhai, W.; Li, Y.; Ran, G.; Ye, Y.; Dai, L. Ultrasensitive Near-Infrared Photodetectors Based on a Graphene-MoTe2-Graphene Vertical van der Waals Heterostructure. ACS Appl. Mater. Interfaces 2017, 9, 5392– 5398,  DOI: 10.1021/acsami.6b14483
    Google Scholar
    39
    Ultrasensitive Near-Infrared Photodetectors Based on a Graphene-MoTe2-Graphene Vertical van der Waals Heterostructure
    Zhang, Kun; Fang, Xin; Wang, Yilun; Wan, Yi; Song, Qingjun; Zhai, Wenhao; Li, Yanping; Ran, Guangzhao; Ye, Yu; Dai, Lun
    ACS Applied Materials & Interfaces (2017), 9 (6), 5392-5398CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    A graphene-MoTe2-graphene vertical van der Waals (vdWs) heterostructure on a SiO2/p+-Si substrate was fabricated by a facile and reliable site-controllable transfer method and applied for photodetection from the visible to near-IR wavelength range. Compared to the layered semiconductor photodetectors reported thus far, the graphene-MoTe2-graphene photodetector has a superior performance, including high photoresponsivity (∼110 mA W-1 at 1064 nm and 205 mA W-1 at 473 nm), high external quantum efficiency (EQE; ∼12.9% at 1064 nm and ∼53.8% at 473 nm), rapid response and recovery processes (a rise time of 24 μs and a fall time of 46 μs under 1064 nm illumination), and free from an external source-drain power supply. The authors have employed scanning photocurrent microscopy to study the photocurrent generation in this heterostructure under various back-gate voltages and found that the 2 Schottky barriers between the graphenes and MoTe2 play an important role in the photocurrent generation. The vdWs heterostructure has a uniform photoresponsive area. The photoresponsivity and EQE of the photodetector can be modulated by the back-gate (p+-Si) voltage. The authors compared the responsivities of thin and thick flakes and found that the responsivity had a strong dependence on the thickness. The heterostructure has promising applications in future novel optoelectronic devices, enabling next-generation high-responsivity, high-speed, flexible, and transparent NIR devices.
  40. 40
    Xiao, H.; Lin, L.; Zhu, J.; Guo, J.; Ke, Y.; Mao, L.; Gong, T.; Cheng, H.; Huang, W.; Zhang, X. Highly Sensitive and Broadband Photodetectors Based on WSe2/MoS2 Heterostructures with van der Waals Contact Electrodes. Appl. Phys. Lett. 2022, 121, 023504  DOI: 10.1063/5.0100191
    Google Scholar
    There is no corresponding record for this reference.
  41. 41
    Huang, H. Highly Sensitive Visible to Infrared MoTe2 Photodetectors Enhanced by the Photogating Effect. Nanotechnology 2016, 27, 445201  DOI: 10.1088/0957-4484/27/44/445201
    Google Scholar
    41
    Highly sensitive visible to infrared MoTe2 photodetectors enhanced by the photogating effect
    Huang, Hai; Wang, Jianlu; Hu, Weida; Liao, Lei; Wang, Peng; Wang, Xudong; Gong, Fan; Chen, Yan; Wu, Guangjian; Luo, Wenjin; Shen, Hong; Lin, Tie; Sun, Jinglan; Meng, Xiangjian; Chen, Xiaoshuang; Chu, Junhao
    Nanotechnology (2016), 27 (44), 445201/1-445201/7CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    A review. Two-dimensional materials are promising candidates for electronic and optoelectronic applications. MoTe2 has an appropriate bandgap for both visible and IR light photodetection. Here we fabricate a high-performance photodetector based on few-layer MoTe2. Raman spectral properties have been studied for different thicknesses of MoTe2. The photodetector based on few-layer MoTe2 exhibits broad spectral range photodetection (0.6-1.55μm) and a stable and fast photoresponse. The detectivity is calcd. to be 3.1 × 109 cm Hz1/2 W-1 for 637 nm light and 1.3 × 109 cm Hz1/2W-1 for 1060 nm light at a backgate voltage of 10 V. The mechanisms of photocurrent generation have been analyzed in detail, and it is considered that a photogating effect plays an important role in photodetection. The appreciable performance and detection over a broad spectral range make it a promising material for high-performance photodetectors.
  42. 42
    Shen, D. High-Performance Mid-IR to Deep-UV van der Waals Photodetectors Capable of Local Spectroscopy at Room Temperature. Nano Lett. 2022, 22, 3425– 3432,  DOI: 10.1021/acs.nanolett.2c00741
    Google Scholar
    There is no corresponding record for this reference.
  43. 43
    Yu, W.; Li, S.; Zhang, Y.; Ma, W.; Sun, T.; Yuan, J.; Fu, K.; Bao, Q. Near-Infrared Photodetectors Based on MoTe2/Graphene Heterostructure with High Responsivity and Flexibility. Small 2017, 13, 1700268  DOI: 10.1002/smll.201700268
    Google Scholar
    There is no corresponding record for this reference.
  44. 44
    Yin, L.; Zhan, X.; Xu, K.; Wang, F.; Wang, Z.; Huang, Y.; Wang, Q.; Jiang, C.; He, J. Ultrahigh Sensitive MoTe2 Phototransistors Driven by Carrier Tunneling. Appl. Phys. Lett. 2016, 108, 043503  DOI: 10.1063/1.4941001
    Google Scholar
    44
    Ultrahigh sensitive MoTe2 phototransistors driven by carrier tunneling
    Yin, Lei; Zhan, Xueying; Xu, Kai; Wang, Feng; Wang, Zhenxing; Huang, Yun; Wang, Qisheng; Jiang, Chao; He, Jun
    Applied Physics Letters (2016), 108 (4), 043503/1-043503/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Transition metal dichalcogenides (TMDs) demonstrate great potential in electronic and optoelectronic applications. However, the device performance remains limited because of the poor metal contact. Herein, we fabricate a high-performance ultrathin MoTe2 phototransistor. By introducing an electron tunneling mechanism, electron injection from electrode to channel is strikingly enhanced. The electron mobility approaches 25.2 cm2 V-1 s-1, better than that of other back-gated MoTe2 FETs. Through elec. measurements at various temps., the electron tunneling mechanism is further confirmed. The MoTe2 phototransistor exhibits very high responsivity up to 2560 A/W which is higher than that of most other TMDs. This work may provide guidance to reduce the contact resistance at metal-semiconductor junction and pave a pathway to develop high-performance optoelectronic devices in the future. (c) 2016 American Institute of Physics.
  45. 45
    Xie, Y.; Wu, E.; Zhang, J.; Hu, X.; Zhang, D.; Liu, J. Gate-Tunable Photodetection/Voltaic Device Based on BP/MoTe2 Heterostructure. ACS Appl. Mater. Interfaces 2019, 11, 14215– 14221,  DOI: 10.1021/acsami.8b21315
    Google Scholar
    45
    Gate-Tunable Photodetection/Voltaic Device Based on BP/MoTe2 Heterostructure
    Xie, Yuan; Wu, Enxiu; Zhang, Jing; Hu, Xiaodong; Zhang, Daihua; Liu, Jing
    ACS Applied Materials & Interfaces (2019), 11 (15), 14215-14221CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Van der Waals heterostructures based on two-dimensional (2D) materials have attracted tremendous attention for their potential applications in optoelectronic devices, such as solar cells and photodetectors. In addn., the widely tunable Fermi levels of these atomically thin 2D materials enable tuning the device performances/functions dynamically. Herein, we demonstrated a MoTe2/BP heterostructure, which can be dynamically tuned to be either p-n or p-p junction by gate modulation due to compatible band structures and elec. tunable Fermi levels of MoTe2 and BP. Consequently, the electrostatic gating can further accurately control the photoresponse of this heterostructure in terms of the polarity and the value of photoresponsivity. Besides, the heterostructure showed outstanding photodetection/voltaic performances. The optimum photoresponsivity, external quantum efficiency, and response time as a photodetector were 0.2 A/W, 48.1%, and 2 ms, resp. Our study enhances the understanding of 2D heterostructures for designing gate-tunable devices and reveals promising potentials of these devices in future optoelectronic applications.
  46. 46
    Lee, H. S.; Min, S.-W.; Chang, Y.-G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S. MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap. Nano Lett. 2012, 12, 3695– 3700,  DOI: 10.1021/nl301485q
    Google Scholar
    46
    MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap
    Lee, Hee Sung; Min, Sung-Wook; Chang, Youn-Gyung; Park, Min Kyu; Nam, Taewook; Kim, Hyungjun; Kim, Jae Hoon; Ryu, Sunmin; Im, Seongil
    Nano Letters (2012), 12 (7), 3695-3700CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We report on the fabrication of top-gate phototransistors based on a few-layered MoS2 nanosheet with a transparent gate electrode. Our devices with triple MoS2 layers exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection. The varied functionalities are attributed to energy gap modulation by the no. of MoS2 layers. The photoelec. probing on working transistors with the nanosheets demonstrates that single-layer MoS2 has a significant energy bandgap of 1.8 eV, while those of double- and triple-layer MoS2 reduce to 1.65 and 1.35 eV, resp.
  47. 47
    Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497– 501,  DOI: 10.1038/nnano.2013.100
    Google Scholar
    47
    Ultrasensitive photodetectors based on monolayer MoS2
    Lopez-Sanchez, Oriol; Lembke, Dominik; Kayci, Metin; Radenovic, Aleksandra; Kis, Andras
    Nature Nanotechnology (2013), 8 (7), 497-501CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Ultrasensitive monolayer MoS2 phototransistors with improved device mobility and ON current are demonstrated. The devices show a max. external photoresponsivity of 880 A W-1 at λ = 561 nm and a photoresponse at 400-680 nm. With recent developments in large-scale prodn. techniques such as liq.-scale exfoliation and CVD-like growth, MoS2 shows important potential for applications in MoS2-based integrated optoelectronic circuits, light sensing, biomedical imaging, video recording and spectroscopy.
  48. 48
    Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Van Der Zant, H. S. J.; Castellanos-Gomez, A. Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors. Nano Lett. 2014, 14, 3347– 3352,  DOI: 10.1021/nl5008085
    Google Scholar
    48
    Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors
    Buscema, Michele; Groenendijk, Dirk J.; Blanter, Sofya I.; Steele, Gary A.; van der Zant, Herre S. J.; Castellanos-Gomez, Andres
    Nano Letters (2014), 14 (6), 3347-3352CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Few-layer black phosphorus, a new elemental two-dimensional (2D) material recently isolated by mech. exfoliation, is a high-mobility layered semiconductor with a direct bandgap that is predicted to strongly depend on the no. of layers, from 0.35 eV (bulk) to 2.0 eV (single layer). Therefore, black phosphorus is an appealing candidate for tunable photodetection from the visible to the IR part of the spectrum. We study the photoresponse of field-effect transistors (FETs) made of few-layer black phosphorus (3-8 nm thick), as a function of excitation wavelength, power, and frequency. In the dark state, the black phosphorus FETs can be tuned both in hole and electron doping regimes allowing for ambipolar operation. We measure mobilities in the order of 100 cm2/V s and a current ON/OFF ratio larger than 103. Upon illumination, the black phosphorus transistors show a response to excitation wavelengths from the visible region up to 940 nm and a rise time of about 1 ms, demonstrating broadband and fast detection. The responsivity reaches 4.8 mA/W, and it could be drastically enhanced by engineering a detector based on a PN junction. The ambipolar behavior coupled to the fast and broadband photodetection make few-layer black phosphorus a promising 2D material for photodetection across the visible and near-IR part of the electromagnetic spectrum.
  49. 49
    Zhang, X.; Liu, B.; Yang, W.; Jia, W.; Li, J.; Jiang, C.; Jiang, X. 3D-Branched Hierarchical 3C-SiC/ZnO Heterostructures for High-Performance photodetectors. Nanoscale 2016, 8, 17573– 17580,  DOI: 10.1039/C6NR06236A
    Google Scholar
    49
    3D-branched hierarchical 3C-SiC/ZnO heterostructures for high-performance photodetectors
    Zhang, Xinglai; Liu, Baodan; Yang, Wenjin; Jia, Wenbo; Li, Jing; Jiang, Chunhai; Jiang, Xin
    Nanoscale (2016), 8 (40), 17573-17580CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The ultra-sensitive photodetection of different wavelengths holds promising applications in high-performance optoelectronic devices and it requires an efficient and suitable semiconductor unit. Herein, we demonstrated the designable synthesis of 3D-branched hierarchical 3C-SiC/ZnO heterostructures by a three-step process and their assembling into an ultrasensitive photodetector. Microstructure analyses using high-resoln. transmission electron microscopy reveal that the hierarchical 3C-SiC/ZnO heterostructure is composed of single-crystal 3C-SiC nanowires as a central stem and numerous well-aligned single-cryst. ZnO nanorods as branch shells. Optoelectronic tests on the 3C-SiC/ZnO heterostructure photodetector verify the outstanding photo-detection performance with an ultrahigh EQE (1.69 × 108%), a superior photoresponsivity (4.8 × 105 A W-1), a very fast response time (a rise time of 40 ms and a decay time of 60 ms), a high photo-dark current ratio of 187.8 and an excellent photocurrent stability and reproducibility, which is significantly advantageous or comparable to those of ZnO and other inorg. semiconductor nanostructure based photodetectors. To understand the excellent photodetection of hierarchical 3C-SiC/ZnO heterostructures, a band-gap energy diagram describing the photogenerated electron transport process is plotted and the corresponding mechanism is discussed. The strategy proposed in the present work will open up more opportunities for the design and boost of ultra-sensitive photodetectors based on semiconductor heterostructures.
  50. 50
    Kind, H.; Yan, H.; Messer, B.; Law, M.; Yang, P. Nanowire Ultraviolet Photodetectors and Optical Switches. Adv. Mater. 2002, 14, 158– 160,  DOI: 10.1002/1521-4095(20020116)14:2<158::AID-ADMA158>3.0.CO;2-W
    Google Scholar
    50
    Nanowire ultraviolet photodetectors and optical switches
    Kind, Hannes; Yan, Haoquan; Messer, Benjamin; Law, Matthew; Yang, Peidong
    Advanced Materials (Weinheim, Germany) (2002), 14 (2), 158-160CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
    Highly sensitive nanowire switches were produced by exploring the photoconducting properties of individual semiconductor nanowires. The ZnO nanowires were grown by a vapor phase transport process and dispersed directly on pre-fabricated Au electrodes to characterize their photoconducting properties. For-terminal measurements of individual ZnO nanowires indicated that they were highly insulating in the dark with a resistivity >3.5 Mωcm. The nanowire resistivity decreased when they were exposed to UV-light with λ <380 nm. The high sensitivity of the nanowire photoconductors was revealed which showed the power dependence of the photoresponse and they also exhibited wavelength selectivity. These characteristics of the photoconductive ZnO nanowires suggested that they are good materials for optoelectronic switches with the dark insulating state as OFF and the UV-exposed conducting state as ON. The nanowires could be reversibly switched between low and high cond. state, thus they could serve as highly sensitive UV-light detectors, chem. and biol. sensors, and switching devices for nanoscale optoelectronic applications.
  51. 51
    Liu, F.; Shimotani, H.; Shang, H.; Kanagasekaran, T.; Zólyomi, V.; Drummond, N.; Fal’ko, V. I.; Tanigaki, K. High-Sensitivity Photodetectors Based on Multilayer GaTe Flakes. ACS Nano 2014, 8, 752– 760,  DOI: 10.1021/nn4054039
    Google Scholar
    51
    High-Sensitivity Photodetectors Based on Multilayer GaTe Flakes
    Liu, Fucai; Shimotani, Hidekazu; Shang, Hui; Kanagasekaran, Thangavel; Zolyomi, Viktor; Drummond, Neil; Fal'ko, Vladimir I.; Tanigaki, Katsumi
    ACS Nano (2014), 8 (1), 752-760CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Optoelectronic devices based on layered materials such as graphene resulted in significant interest due to their unique properties and potential technol. applications. The elec. and optoelectronic properties of nano GaTe flakes as layered materials are described. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of ∼0.2 cm2 V-1 s-1. The gate transistor exhibits a high photoresponsivity of 104 A/W, which is greatly better than that of graphene, MoS2, and other layered compds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.
  52. 52
    Hidding, J.; Cordero-Silis, C. A.; Vaquero, D.; Panagiotis Rompotis, K.; Quereda, J.; Guimarães, M. H. D. Locally Phase-Engineered MoTe2 for Near-Infrared Photodetectors . 2024, 2406.01376, arXiv, https://arxiv.org/abs/2406.01376 (accessed Jun 03, 2024).
    Google Scholar
    There is no corresponding record for this reference.

Cited By

Click to copy section linkSection link copied!
Citation Statements
Explore this article's citation statements on scite.ai
powered by  Scite

This article is cited by 2 publications.

  1. Lingfeng Peng, Jianwei Zou, Jinlin Liu, Hao Liu, Jing Sun, Fan Dang, Ningjiu Zhao, Yongjun Hu, Hailong Chen. Unveiling Defect-Dependent Nonradiative Carrier Dynamics in Few-Layer 2H-MoTe2 by Femtosecond Mid-Infrared Spectroscopy. The Journal of Physical Chemistry C 2025, Article ASAP.
  2. Yuan Gao, Haiyan Nan, Huilin Zuo, Renxian Qi, Zijian Wang, Jialing Jian, Zhengjin Weng, Wenhui Wang, Shaoqing Xiao, Xiaofeng Gu. Enhancing photodetection efficiency in MoTe2/MoS2 van der Waals heterojunctions by modulating the phase regions. Surfaces and Interfaces 2025, 65 , 106511. https://doi.org/10.1016/j.surfin.2025.106511
Download PDF
Go to ACS Photonics
Get e-Alerts
Get e-Alerts

ACS Photonics

Cite this: ACS Photonics 2024, 11, 10, 4083–4089
Click to copy citationCitation copied!
https://doi.org/10.1021/acsphotonics.4c00896
Published September 16, 2024

Copyright © 2024 The Authors. Published by American Chemical Society. This publication is licensed under

CC-BY 4.0 .

Article Views

1171

Altmetric

-

Citations

2
Learn about these metrics

Article Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.

Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.

The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated.

  • Abstract

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 1

    Figure 1. (a) Optical micrograph of a phase-changed MoTe2 device, where the phase-changed regions are outlined with the white dashed line, while the bright green part is the unaltered 2H MoTe2 region. (b) Raman spectra obtained before (green) and after (purple) the phase transformation, which clearly indicate a successful phase transformation. The spectra before the phase change is multiplied by 3 for clarity. (c) Ids–Vds measurements, as indicated in (a), with Vg ranging from 0 to 50 V, taken at 78 K. The nonlinear IV characteristics show the Schottky behavior. The IV measurement for the two 1T′ regions are depicted in the inset, which clearly show Ohmic behavior. (d) Transfer curve measured with a Vds of 3 V, taken at 78 K, shows a clear n-type behavior.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 2

    Figure 2. (a) Reflectivity map of the scanning photocurrent measurement of the device depicted in Figure 1a with the corresponding photocurrent map in (b), (c), and (d), taken at RT. The white outlines indicate the position of the flake and Ti/Au contacts, while the white dashed lines indicate the 1T′–2H junctions for clarity. The photocurrent maps are obtained with λ = 700 nm, P = 1 μW, and a Vds of (b) −2, (c) 0, and (d) 2 V. From the photocurrent maps, we can clearly see that the induced photocurrent originates from the 1T′–2H junction rather than from the Ti/Au contacts.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 3

    Figure 3. (a) Schematic of the photothermoelectric (PTE) and photovoltaic effect (PVE). For the PTE, the laser locally heats the device, which creates a temperature gradient, which via the Seebeck effect causes an induced photocurrent (IPTE). For the PVE, the localized electric field at the Schottky barrier causes a separation of the photoinduced carriers, resulting in IPVE. It should be noted that the two effects produce a photocurrent with the same direction and that the induced photocurrent is opposite on both junctions. (b) Line trace of the photocurrent mapping, as indicated in red in Figure 2c, showing the reflection (white) and negative (purple) and positive (green) photocurrent peaks along the line trace. Both the maximum and minimum photocurrents are obtained in the 2H region, as expected for a localized electric field from a Schottky barrier. The phases are indicated by the purple (1T′) and green (2H) backgrounds.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 4

    Figure 4. (a) Time-resolved photocurrent, taken at RT, where the photocurrent (purple) in the device is plotted together with the chopper signal (gray) versus time, shows the fast response of our MoTe2 photodetector. The dashed gray rectangles indicate the region used to determine the rise and fall times, as depicted in the inset of (b). (b) Extracted rise (purple) and fall times (green) indicate no wavelength dependence on the fast response for wavelengths ranging from 700 to 1100 nm. The inset shows the rise (purple) and fall (green) curves of the photocurrent from which the rise and fall times are extracted. (c) Power-dependent measurements for different Vds, ranging from −2 to 2 V, with a maximum responsivity of 4.5 × 10–8 A/μW. Here, the responsivity (R) of the device is plotted as a function of the laser excitation power (P) and fitted at high laser excitation power to a power law: R ∝ Pα–1. The measured R for 700 and 1068 nm are indicated by the filled circles and unfilled squares, respectively. (d) Extracted index of the power law (α) from the fitting in (c) versus Vds for the wavelengths 700 nm (purple) and 1068 nm (green).

    High Resolution Image
    Download MS PowerPoint Slide

  • References

    This article references 52 other publications.

    1. 1
      Miao, J.; Hu, W.; Guo, N.; Lu, Z.; Zou, X.; Liao, L.; Shi, S.; Chen, P.; Fan, Z.; Ho, J. C.; Li, T.-X.; Chen, X. S.; Lu, W. Single InAs Nanowire Room-Temperature Near-Infrared Photodetectors. ACS Nano 2014, 8, 3628– 3635,  DOI: 10.1021/nn500201g
      1
      Single InAs Nanowire Room-Temperature Near-Infrared Photodetectors
      Miao, Jinshui; Hu, Weida; Guo, Nan; Lu, Zhenyu; Zou, Xuming; Liao, Lei; Shi, Suixing; Chen, Pingping; Fan, Zhiyong; Ho, Johnny C.; Li, Tian-Xin; Chen, Xiao Shuang; Lu, Wei
      ACS Nano (2014), 8 (4), 3628-3635CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Here the authors report InAs nanowire (NW) near-IR photodetectors having a detection wavelength up to ∼1.5 μm. The single InAs NW photodetectors displayed min. hysteresis with a high Ion/Ioff ratio of 105. At room temp., the Schottky-ohmic contacted photodetectors had an external photoresponsivity of ∼5.3 × 103 AW-1, which is ∼300% larger than that of ohmic-ohmic contacted detectors (∼1.9 × 103 AW-1). A large enhancement in photoresponsivity (∼300%) had also been achieved in metal Au-cluster-decorated InAs NW photodetectors due to the formation of Schottky junctions at the InAs/Au cluster contacts. The photocurrent decreased when the photodetectors were exposed to ambient atm. because of the high surface electron concn. and rich surface defect states in InAs NWs. A theor. model based on charge transfer and energy band change is proposed to explain this obsd. performance. To suppress the neg. effects of surface defect states and atm. mols., new InAs NW photodetectors with a half-wrapped top-gate had been fabricated by using 10 nm HfO2 as the top-gate dielec.
    2. 2
      Huang, W.; Xing, C.; Wang, Y.; Li, Z.; Wu, L.; Ma, D.; Dai, X.; Xiang, Y.; Li, J.; Fan, D.; Zhang, H. Facile Fabrication and Characterization of Two-Dimensional Bismuth(III) Sulfide Nanosheets for High-Performance Photodetector Applications Under Ambient Conditions. Nanoscale 2018, 10, 2404– 2412,  DOI: 10.1039/C7NR09046C
      2
      Facile fabrication and characterization of two-dimensional bismuth(III) sulfide nanosheets for high-performance photodetector applications under ambient conditions
      Huang, Weichun; Xing, Chenyang; Wang, Yunzheng; Li, Zhongjun; Wu, Leiming; Ma, Dingtao; Dai, Xiaoyu; Xiang, Yuanjiang; Li, Jianqing; Fan, Dianyuan; Zhang, Han
      Nanoscale (2018), 10 (5), 2404-2412CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Two-dimensional (2D) bismuth(III) sulfide (Bi2S3) nanosheets as non-toxic graphene-like nanomaterials were successfully fabricated by a facile liq. phase exfoliation (LPE) method. A robust photodetector employing a Bi2S3 nanosheet film has been fabricated for the first time via a facile fabrication process on an ITO-coated glass. UV-Vis and Raman spectroscopy techniques were carried out and they confirmed the inherent optical and phys. properties of Bi2S3 nanosheets. Photoelectrochem. (PEC) measurements demonstrate that a significantly higher photocurrent d. (42 μA cm-2) and enhanced photoresponsivity (210 μA W-1), at a lower bias potential in alk. soln., of the Bi2S3 nanosheet-based photodetector are achieved, compared with those of other 2D nanomaterial-based photodetectors under light irradn. Furthermore, the as-prepd. Bi2S3 nanosheet-based photodetector not only exhibits an appropriate capacity of self-driven broadband and high-performance photoresponse but also displays strong long-term stability of the ON/OFF switching behavior without any external protection in alk. solns. Because of facile synthesis via a LPE method, a higher photocurrent d. and photoresponsivity, self-driven performance and long-term stability of the Bi2S3 nanosheet-based photodetector at a lower bias potential in alk. solns., the present work can provide fundamental acknowledgment of the high performance of this new kind of PEC-type 2D nanosheet-based photodetector.
    3. 3
      Ilyas, N.; Li, D.; Song, Y.; Zhong, H.; Jiang, Y.; Li, W. Low-Dimensional Materials and State-of-the-Art Architectures for Infrared Photodetection. Sensors 2018, 18, 4163,  DOI: 10.3390/s18124163
      There is no corresponding record for this reference.
    4. 4
      Huang, W.; Zhang, Y.; You, Q.; Huang, P.; Wang, Y.; Huang, Z. N.; Ge, Y.; Wu, L.; Dong, Z.; Dai, X.; Xiang, Y.; Li, J.; Zhang, X.; Zhang, H. Enhanced Photodetection Properties of Tellurium@Selenium Roll-to-Roll Nanotube Heterojunctions. Small 2019, 15, 1900902  DOI: 10.1002/smll.201900902
      There is no corresponding record for this reference.
    5. 5
      Rojas-Lopez, R. R.; Brant, J. C.; Ramos, M. S. O.; Castro, T. H. L. G.; Guimarães, M. H. D.; Neves, B. R. A.; Guimarães, P. S. S. Photoluminescence and Charge Transfer in the Prototypical 2D/3D Semiconductor Heterostructure MoS2/GaAs. Appl. Phys. Lett. 2021, 119, 233101,  DOI: 10.1063/5.0068548
      5
      Photoluminescence and charge transfer in the prototypical 2D/3D semiconductor heterostructure MoS2/GaAs
      Rojas-Lopez, Rafael R.; Brant, Juliana C.; Ramos, Maira S. O.; Castro, Tulio H. L. G.; Guimaraes, Marcos H. D.; Neves, Bernardo R. A.; Guimaraes, Paulo S. S.
      Applied Physics Letters (2021), 119 (23), 233101CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      The new generation of two-dimensional (2D) materials has shown a broad range of applications for optical and electronic devices. Understanding the properties of these materials when integrated with more traditional three-dimensional (3D) semiconductors is an important challenge for the implementation of ultra-thin electronic devices. Recent observations have shown that by combining MoS2 with GaAs, it is possible to develop high quality photodetectors and solar cells. Here, we present a study of effects of intrinsic GaAs, p-doped GaAs, and n-doped GaAs substrates on the photoluminescence of monolayer MoS2. We observe a decrease in an order of magnitude in the emission intensity of MoS2 in all MoS2/GaAs heterojunctions, when compared to a control sample consisting of a MoS2 monolayer isolated from GaAs by a few layers of hexagonal boron nitride. We also see a dependence of the trion to A-exciton emission ratio in the photoluminescence spectra on the type of substrates, a dependence that we relate to the static charge exchange between MoS2 and the substrates when the junction is formed. Scanning Kelvin probe microscopy measurements of heterojunctions suggest type-I band alignments, so that excitons generated on the MoS2 monolayer will be transferred to the GaAs substrate. Our results shed light on the charge exchange leading to band offsets in 2D/3D heterojunctions, which play a central role in the understanding and further improvement of electronic devices. (c) 2021 American Institute of Physics.
    6. 6
      Zhai, X.-P.; Ma, B.; Wang, Q.; Zhang, H.-L. 2D Materials Towards Ultrafast Photonic Applications. Phys. Chem. Chem. Phys. 2020, 22, 22140– 22156,  DOI: 10.1039/D0CP02841J
      6
      A 2D materials towards ultrafast photonic applications
      Zhai, Xin-Ping; Ma, Bo; Wang, Qiang; Zhang, Hao-Li
      Physical Chemistry Chemical Physics (2020), 22 (39), 22140-22156CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Having accomplished progress in the versatile battlefields of optics, electronics, catalysis, etc., two-dimensional (2D) materials are now venturing and excelling in yet another arena of ultrafast photonics, a rapidly developing field encompassing a large range of important applications including optical modulation through optical limiting/mode-locking, photodetectors, optical communications, integrated miniaturized all-optical devices and so on. Our group has been devoted to building the arsenal of 2D materials with large third-order nonlinearities, including transition metal dichalcogenides (TMDs), carbon nitride, single-element materials from Group 15, 2D hybrids and vdW heterostructures. In particular, we explore their origin of nonlinear optical responses from the aspect of excited state dynamics using time-resolved spectroscopic techniques such as femtosecond transient absorption spectroscopy. In this review, we propose the roadmap for screening 2D materials for ultrafast photonics through focusing on the third-order nonlinear optical properties of 2D materials and corresponding applications, and then performing mechanistic investigations via time-resolved spectroscopy and calcns., which in turn provide feedback to further guide the fabrication of 2D materials. We offer our own insights on the future trends for the development and theor. calcns. of 2D materials/devices in the final part of Perspectives.
    7. 7
      An, J.; Zhao, X.; Zhang, Y.; Liu, M.; Yuan, J.; Sun, X.; Zhang, Z.; Wang, B.; Li, S.; Li, D. Perspectives of 2D Materials for Optoelectronic Integration. Adv. Funct. Mater. 2022, 32, 2110119  DOI: 10.1002/adfm.202110119
      7
      Perspectives of 2D Materials for Optoelectronic Integration
      An, Junru; Zhao, Xingyu; Zhang, Yanan; Liu, Mingxiu; Yuan, Jian; Sun, Xiaojuan; Zhang, Zhiyu; Wang, Bin; Li, Shaojuan; Li, Dabing
      Advanced Functional Materials (2022), 32 (14), 2110119CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. 2D materials show wide-ranging phys. properties with their electronic bandgaps varying from zero to several eV, offering a rich platform to explore novel electronic and optoelectronic functions. Notably, atomically thin 2D materials are well suited for integration in optoelectronic circuits, because of their ultrathin body, strong light-matter interactions, and compatibility with the current silicon photonic technol. In this paper, an overview of the state of the art of using 2D materials in optoelectronic devices and integration is provided. The optoelectronic properties of 2D materials and their typical electronic and optoelectronic applications including light sources, optical modulators, photodetectors, field-effect transistors, and logic circuits are summarized. The device configurations, operation mechanisms, and device figures-of-merit are introduced and discussed. By discussing the recent advances, future trends, and existing challenges of 2D materials and their optoelectronic devices, this review has provided an insight into the perspectives of 2D materials for optoelectronic integration and may guide the development of this field within the research community.
    8. 8
      Liang, G.; Yu, X.; Hu, X.; Qiang, B.; Wang, C.; Wang, Q. J. Mid-Infrared Photonics and Optoelectronics in 2D Materials. Mater. Today 2021, 51, 294– 316,  DOI: 10.1016/j.mattod.2021.09.021
      8
      Mid-infrared photonics and optoelectronics in 2D materials
      Liang, Guozhen; Yu, Xuechao; Hu, Xiaonan; Qiang, Bo; Wang, Chongwu; Wang, Qi Jie
      Materials Today (Oxford, United Kingdom) (2021), 51 (), 294-316CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
      A review. With intriguing properties that are different from the conventional bulk materials, two-dimensional (2D) materials have attracted numerous and widespread research interests, including its applications in photonics and optoelectronics. Devices based on 2D materials have been demonstrated in a wide spectral range, from the UV to the terahertz, and the microwave wavelength range. The mid-IR (MIR) region (about 2-20μm) bears a particular scientific and technol. significance because, for instance, many mols. have their spectral fingerprints and there are atm. transparent windows in this region. Nevertheless, the MIR region remains underdeveloped compared to the visible and the near-IR telecommunication regimes, mainly due to the lack of suitable materials, such as narrow bandgap materials, and proper photonic designs for building high performance optoelectronic devices in this wavelength regime. Therefore, researchers have been exploring the possibility and opportunity of 2D materials to fill up the gap. Here, we review the key recent developments of 2D materials in the MIR photonic and optoelectronic applications, including photodetection, light modulation, surface plasmon polaritons, phonon polaritons, and their nonlinearities and provide an outlook on the challenges and opportunities that lie ahead for MIR optoelectronic research fields with 2D materials.
    9. 9
      Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Van Der Zant, H. S.; Castellanos-Gomez, A. Photocurrent Generation with Two-Dimensional van der Waals Semiconductors. Chem. Soc. Rev. 2015, 44, 3691– 3718,  DOI: 10.1039/C5CS00106D
      9
      Photocurrent generation with two-dimensional van der Waals semiconductors
      Buscema, Michele; Island, Joshua O.; Groenendijk, Dirk J.; Blanter, Sofya I.; Steele, Gary A.; van der Zant, Herre S. J.; Castellanos-Gomez, Andres
      Chemical Society Reviews (2015), 44 (11), 3691-3718CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Two-dimensional (2D) materials have attracted a great deal of interest in recent years. This family of materials allows for the realization of versatile electronic devices and holds promise for next-generation (opto)electronics. Their electronic properties strongly depend on the no. of layers, making them interesting from a fundamental standpoint. For electronic applications, semiconducting 2D materials benefit from sizable mobilities and large on/off ratios, due to the large modulation achievable via the gate field-effect. Moreover, being mech. strong and flexible, these materials can withstand large strain (>10%) before rupture, making them interesting for strain engineering and flexible devices. Even in their single layer form, semiconducting 2D materials have demonstrated efficient light absorption, enabling large responsivity in photodetectors. Therefore, semiconducting layered 2D materials are strong candidates for optoelectronic applications, esp. for photodetection. Here, we review the state-of-the-art in photodetectors based on semiconducting 2D materials, focusing on the transition metal dichalcogenides, novel van der Waals materials, black phosphorus, and heterostructures.
    10. 10
      Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805  DOI: 10.1103/PhysRevLett.105.136805
      10
      Atomically Thin MoS2. A New Direct-Gap Semiconductor
      Mak, Kin Fai; Lee, Changgu; Hone, James; Shan, Jie; Heinz, Tony F.
      Physical Review Letters (2010), 105 (13), 136805/1-136805/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      The electronic properties of ultrathin crystals of MoS2 consisting of N = 1, 2,...,6 S-Mo-S monolayers were investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photocond. spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by >0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material.
    11. 11
      Manzeli, S.; Ovchinnikov, D.; Pasquier, D.; Yazyev, O. V.; Kis, A. 2D Transition Metal Dichalcogenides. Nat. Rev. Mater. 2017, 2, 17033,  DOI: 10.1038/natrevmats.2017.33
      11
      2D transition metal dichalcogenides
      Manzeli, Sajedeh; Ovchinnikov, Dmitry; Pasquier, Diego; Yazyev, Oleg V.; Kis, Andras
      Nature Reviews Materials (2017), 2 (2), 17033CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
      A review. Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of at.-scale thickness, direct bandgap, strong spin-orbit coupling and favorable electronic and mech. properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examd. and their properties are discussed, with particular attention to their charge d. wave, superconductive and topol. phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties.
    12. 12
      Aftab, S.; Shehzad, M. A.; Salman Ajmal, H. M.; Kabir, F.; Iqbal, M. Z.; Al-Kahtani, A. A. Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2. ACS Nano 2023, 17, 17884– 17896,  DOI: 10.1021/acsnano.3c03593
      There is no corresponding record for this reference.
    13. 13
      Hu, X.; Zhang, F.; Hu, Z.; He, P.; Tao, L.; Zheng, Z.; Zhao, Y.; Yang, Y.; He, J. Preparation of 1T’- and 2H–MoTe2 Films and Investigation of their Photoelectric Properties and Ultrafast Photocarrier Dynamics. Opt. Mater. 2023, 136, 113467  DOI: 10.1016/j.optmat.2023.113467
      There is no corresponding record for this reference.
    14. 14
      Dave, M.; Vaidya, R.; Patel, S. G.; Jani, A. R. High Pressure Effect on MoS2 and MoSe2 Single Crystals Grown by CVT Method. Bulletin of Materials Science 2004, 27, 213– 216,  DOI: 10.1007/BF02708507
      There is no corresponding record for this reference.
    15. 15
      Song, S.; Keum, D. H.; Cho, S.; Perello, D.; Kim, Y.; Lee, Y. H. Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain. Nano Lett. 2016, 16, 188– 193,  DOI: 10.1021/acs.nanolett.5b03481
      15
      Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain
      Song, Seunghyun; Keum, Dong Hoon; Cho, Suyeon; Perello, David; Kim, Yunseok; Lee, Young Hee
      Nano Letters (2016), 16 (1), 188-193CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We demonstrate a room temp. semiconductor-metal transition in thin film MoTe2 engineered by strain. Redn. of the 2H-1T' phase transition temp. of MoTe2 to room temp. was realized by introducing a tensile strain of 0.2%. The obsd. first-order SM transition improved conductance ∼10 000 times and was made possible by an unusually large temp.-stress coeff., which results from a large vol. change and small latent heat. The demonstrated strain-modulation of the phase transition temp. is expected to be compatible with other TMDs enabling the 2D electronics utilizing polymorphism of TMDs along with the established materials.
    16. 16
      Lin, Y.-C.; Dumcenco, D. O.; Huang, Y.-S.; Suenaga, K. Atomic Mechanism of the Semiconducting-to-Metallic phase Transition in Single-Layered MoS2. Nat. Nanotechnol. 2014, 9, 391– 396,  DOI: 10.1038/nnano.2014.64
      16
      Atomic mechanism of the semiconducting-to-metallic phase transition in single-layered MoS2
      Lin, Yung-Chang; Dumcenco, Dumitru O.; Huang, Ying-Sheng; Suenaga, Kazu
      Nature Nanotechnology (2014), 9 (5), 391-396CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Phase transitions can be used to alter the properties of a material without adding any addnl. atoms and are therefore of significant technol. value. In a solid, phase transitions involve collective at. displacements, but such at. processes have so far only been investigated using macroscopic approaches. Here, we show that in situ scanning TEM can be used to follow the structural transformation between semiconducting (2H) and metallic (1T) phases in single-layered MoS2, with at. resoln. The 2H/1T phase transition involves gliding at. planes of sulfur and/or molybdenum and requires an intermediate phase (α-phase) as a precursor. The migration of two kinds of boundaries (β- and γ-boundaries) is also responsible for the growth of the second phase. Furthermore, we show that areas of the 1T phase can be controllably grown in a layer of the 2H phase using an electron beam.
    17. 17
      Shang, B.; Cui, X.; Jiao, L.; Qi, K.; Wang, Y.; Fan, J.; Yue, Y.; Wang, H.; Bao, Q.; Fan, X.; Wei, S.; Song, W.; Cheng, Z.; Guo, S.; Zheng, W. Lattice-Mismatch-Induced Ultrastable 1T-Phase MoS2-Pd/Au for Plasmon-Enhanced Hydrogen Evolution. Nano Lett. 2019, 19, 2758– 2764,  DOI: 10.1021/acs.nanolett.8b04104
      17
      Lattice -Mismatch-Induced Ultrastable 1T-Phase MoS2-Pd/Au for Plasmon-Enhanced Hydrogen Evolution
      Shang, Bo; Cui, Xiaoqiang; Jiao, Lin; Qi, Kun; Wang, Yingwei; Fan, Jinchang; Yue, Yuanyuan; Wang, Haiyu; Bao, Qiaoliang; Fan, Xiaofeng; Wei, Shuting; Song, Wei; Cheng, Zhiliang; Guo, Shaojun; Zheng, Weitao
      Nano Letters (2019), 19 (5), 2758-2764CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Metallic 1T-phase transition metal dichalcogenides (TMDs) are of considerable interest in enhancing catalytic applications due to their abundant active sites and good cond. However, the unstable nature of 1T-phase TMDs greatly impedes their practical applications. Herein, we developed a new approach for the synthesis of highly stable 1T-phase Au/Pd-MoS2 nanosheets (NSs) through a metal assembly induced ultrastable phase transition for achieving a very high electrocatalytic activity in the hydrogen evolution reaction. The phase transition was evoked by a novel mechanism of lattice-mismatch-induced strain based on d. functional theory (DFT) calcns. Raman spectroscopy and transmission electron microscopy (TEM) were used to confirm the phase transition on exptl. grounds. A novel heterostructured 1T MoS2-Au/Pd catalyst was designed and synthesized using this mechanism, and the catalyst exhibited a 0 mV onset potential in the hydrogen evolution reaction under light illumination. Therefore, this method can potentially be used to fabricate 1T-phase TMDs with remarkably enhanced activities for different applications.
    18. 18
      Wang, Y.; Xiao, J.; Zhu, H.; Li, Y.; Alsaid, Y.; Fong, K. Y.; Zhou, Y.; Wang, S.; Shi, W.; Wang, Y.; Zettl, A.; Reed, E. J.; Zhang, X. Structural Phase Transition in Monolayer MoTe2 Driven by Electrostatic Doping. Nature 2017, 550, 487– 491,  DOI: 10.1038/nature24043
      18
      Structural phase transition in monolayer MoTe2 driven by electrostatic doping
      Wang, Ying; Xiao, Jun; Zhu, Hanyu; Li, Yao; Alsaid, Yousif; Fong, King Yan; Zhou, Yao; Wang, Siqi; Shi, Wu; Wang, Yuan; Zettl, Alex; Reed, Evan J.; Zhang, Xiang
      Nature (London, United Kingdom) (2017), 550 (7677), 487-491CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and phys. properties. Exploring the physics of transitions between these different structural phases in two dimensions may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chem. means; purely electrostatic control over crystal phases through electrostatic doping was recently proposed as a theor. possibility, but has not yet been realized. Here we report the exptl. demonstration of an electrostatic-doping-driven phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe2). We find that the phase transition shows a hysteretic loop in Raman spectra, and can be reversed by increasing or decreasing the gate voltage. We also combine second-harmonic generation spectroscopy with polarization-resolved Raman spectroscopy to show that the induced monoclinic phase preserves the crystal orientation of the original hexagonal phase. Moreover, this structural phase transition occurs simultaneously across the whole sample. This electrostatic-doping control of structural phase transition opens up new possibilities for developing phase-change devices based on atomically thin membranes.
    19. 19
      Kappera, R.; Voiry, D.; Yalcin, S. E.; Branch, B.; Gupta, G.; Mohite, A. D.; Chhowalla, M. Phase-Engineered Low-Resistance Contacts for Ultrathin MoS2 transistors. Nat. Mater. 2014, 13, 1128– 1134,  DOI: 10.1038/nmat4080
      19
      Phase-engineered low-resistance contacts for ultrathin MoS2 transistors
      Kappera, Rajesh; Voiry, Damien; Yalcin, Sibel Ebru; Branch, Brittany; Gupta, Gautam; Mohite, Aditya D.; Chhowalla, Manish
      Nature Materials (2014), 13 (12), 1128-1134CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      Ultrathin molybdenum disulfide (MoS2) has emerged as an interesting layered semiconductor because of its finite energy bandgap and the absence of dangling bonds. However, metals deposited on the semiconducting 2H phase usually form high-resistance (0.7 kΩ μm-10 kΩ μm) contacts, leading to Schottky-limited transport. The metallic 1T phase of MoS2 can be locally induced on semiconducting 2H phase nanosheets, thus decreasing contact resistances to 200-300 Ω μm at zero gate bias. Field-effect transistors (FETs) with 1T phase electrodes fabricated and tested in air exhibit mobility values of ∼50 cm2 V-1 s-1, subthreshold swing values <100 mV per decade, on/off ratios of >107, drive currents approaching ∼100 μA μm-1, and excellent current satn. The deposition of different metals has limited influence on the FET performance, suggesting that the 1T/2H interface controls carrier injection into the channel. An increased reproducibility of the elec. characteristics is also obtained with the authors' strategy based on phase engineering of MoS2.
    20. 20
      Ma, Y.; Liu, B.; Zhang, A.; Chen, L.; Fathi, M.; Shen, C.; Abbas, A. N.; Ge, M.; Mecklenburg, M.; Zhou, C. Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe2 and Applications for Devices. ACS Nano 2015, 9, 7383– 7391,  DOI: 10.1021/acsnano.5b02399
      20
      Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe2 and Applications for Devices
      Ma, Yuqiang; Liu, Bilu; Zhang, Anyi; Chen, Liang; Fathi, Mohammad; Shen, Chenfei; Abbas, Ahmad N.; Ge, Mingyuan; Mecklenburg, Matthew; Zhou, Chongwu
      ACS Nano (2015), 9 (7), 7383-7391CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe2 are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe2 as active channel materials. Recently, a phase engineering concept of 2D MoS2 materials was developed, with improved device performance. Here, we applied this method to chem. vapor deposition (CVD) grown monolayer 2H-WSe2 and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe2. We have also shown that metallic phase WSe2 can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addn., we fabricated FETs based on these CVD-grown WSe2 flakes with phase-engineered metallic 1T-WSe2 as contact regions and intact semiconducting 2H-WSe2 as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 107 and mobilities up to 66 cm2/V·s for monolayer WSe2. These results further suggest that phase engineering can be a generic strategy to improve device performance for many kinds of 2D TMDC materials.
    21. 21
      Cho, S.; Kim, S.; Kim, J. H.; Zhao, J.; Seok, J.; Keum, D. H.; Baik, J.; Choe, D.-H.; Chang, K. J.; Suenaga, K.; Kim, S. W.; Lee, Y. H.; Yang, H. Phase Patterning for Ohmic Homojunction Contact in MoTe2. Science 2015, 349, 625– 628,  DOI: 10.1126/science.aab3175
      21
      Phase patterning for ohmic homojunction contact in MoTe2
      Cho, Suyeon; Kim, Sera; Kim, Jung Ho; Zhao, Jiong; Seok, Jinbong; Keum, Dong Hoon; Baik, Jaeyoon; Choe, Duk-Hyun; Chang, K. J.; Suenaga, Kazu; Kim, Sung Wng; Lee, Young Hee; Yang, Heejun
      Science (Washington, DC, United States) (2015), 349 (6248), 625-628CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Artificial van der Waals heterostructures with two-dimensional (2D) at. crystals are promising as an active channel or as a buffer contact layer for next-generation devices. However, genuine 2-dimensional heterostructure devices remain limited because of impurity-involved transfer process and metastable and inhomogeneous heterostructure formation. The authors used laser-induced phase patterning, a polymorph engineering, to fabricate an ohmic heterophase homojunction between semiconducting hexagonal (2H) and metallic monoclinic (1T') molybdenum ditelluride (MoTe2) that is stable up to 300° and increases the carrier mobility of the MoTe2 transistor by a factor of ∼50, while retaining a high on/off current ratio of 106. In situ scanning TEM results combined with theor. calcns. reveal that the Te vacancy triggers the local phase transition in MoTe2, achieving a true 2-dimensional device with an ohmic contact.
    22. 22
      Duerloo, K.-A. N.; Li, Y.; Reed, E. J. Structural Phase Transitions in Two-Dimensional Mo- and W-Dichalcogenide Monolayers. Nat. Commun. 2014, 5, 4214,  DOI: 10.1038/ncomms5214
      22
      Structural phase transitions in two-dimensional Mo- and W-dichalcogenide monolayers
      Duerloo, Karel-Alexander N.; Li, Yao; Reed, Evan J.
      Nature Communications (2014), 5 (), 4214CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Mo- and W-dichalcogenide compds. have a two-dimensional monolayer form that differs from graphene in an important respect: it can potentially have more than one crystal structure. Some of these monolayers exhibit tantalizing hints of a poorly understood structural metal-to-insulator transition with the possibility of long metastable lifetimes. If controllable, such a transition could bring an exciting new application space to monolayer materials beyond graphene. Here we discover that mech. deformations provide a route to switching thermodn. stability between a semiconducting and a metallic crystal structure in these monolayer materials. Based on state-of-the-art d. functional and hybrid Hartree-Fock/d. functional calcns. including vibrational energy corrections, we discover that MoTe2 is an excellent candidate phase change material. We identify a range from 0.3 to 3% for the tensile strains required to transform MoTe2 under uniaxial conditions at room temp. The potential for mech. phase transitions is predicted for all six studied compds.
    23. 23
      Tan, Y.; Luo, F.; Zhu, M.; Xu, X.; Ye, Y.; Li, B.; Wang, G.; Luo, W.; Zheng, X.; Wu, N.; Yu, Y.; Qin, S.; Zhang, X.-A. Controllable 2H-to-1T’ Phase Transition in Few-Layer MoTe2. Nanoscale 2018, 10, 19964– 19971,  DOI: 10.1039/C8NR06115G
      23
      Controllable 2H-to-1T' phase transition in few-layer MoTe2
      Tan, Yuan; Luo, Fang; Zhu, Mengjian; Xu, Xiaolong; Ye, Yu; Li, Bing; Wang, Guang; Luo, Wei; Zheng, Xiaoming; Wu, Nannan; Yu, Yayun; Qin, Shiqiao; Zhang, Xue-Ao
      Nanoscale (2018), 10 (42), 19964-19971CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Most two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit more than one structural phase, leading to a no. of remarkable physics and potential device applications beyond graphene. Here, we demonstrated a feasible route to trigger 2H-to-1T' phase transition in few-layer molybdenum ditelluride (MoTe2) by laser irradn. The effects of laser power and irradn. duration were systematically studied in this study, revealing the accumulated heating effect as the main driving force for such a phase transition. By carefully adjusting laser power and irradn. time, we could control the structural phases of MoTe2 as 2H, 2H + 1T', and 1T'. After thermal annealing at a rather low temp., the laser-irradiated MoTe2 showed a completely suppressed 2H component and a more stabilized 1T' phase, demonstrating that the microscopic origin of the irreversible 2H-to-1T' phase transition is the formation of Te vacancies in MoTe2 due to laser local instantaneous heating. Our findings together with the unique properties of MoTe2 pave the way for high-performance nanoelectronics and optoelectronics based on 2D TMDs and their heterostructures.
    24. 24
      Kang, S.; Won, D.; Yang, H.; Lin, C.-H.; Ku, C.-S.; Chiang, C.-Y.; Kim, S.; Cho, S. Phase-controllable laser thinning in MoTe2. Appl. Surf. Sci. 2021, 563, 150282  DOI: 10.1016/j.apsusc.2021.150282
      24
      Phase-controllable laser thinning in MoTe2
      Kang, Seohui; Won, Dongyeun; Yang, Heejun; Lin, Chia-Hsien; Ku, Ching-Shun; Chiang, Ching-Yu; Kim, Sera; Cho, Suyeon
      Applied Surface Science (2021), 563 (), 150282CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Laser thinning of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) has been considered a promising method to tune the bandgaps of TMDs via precise control of their thickness. However, the laser irradn. generates numerous chalcogen vacancies, which are known to cause a phase transition in polymorphic TMDs such as MoTe2. Therefore, the delicate control of the thickness and the phase during laser thinning is highly demanded to study the intrinsic properties of few-layered TMDs. Here, we report power-dependent laser thinning and phase control of semiconducting hexagonal MoTe2 (2H-MoTe2). High-resoln. X-ray nano diffraction with synchrotron radiation showed that laser-thinned 2H-MoTe2 with low laser power (<2 mW) retained its hexagonal diffraction patterns with a single crystal orientation. In contrast, a phase transition to monoclinic (1T') MoTe2 occurred during laser thinning at a high laser power level. Confocal Raman spectroscopy and at. force microscopy (AFM) revealed that the low-power laser thinning of 2H-MoTe2 retained the crystal structure whereas high-power laser thinning created considerable amt. of chalcogen vacancies and a phase transition. Power-dependent laser thinning thus provides a promising way to control the thickness and the phase of polymorphic 2D TMDs for next-generation optoelectronic devices.
    25. 25
      Zhang, X. Low Contact Barrier in 2H/1T’ MoTe2 In-Plane Heterostructure Synthesized by Chemical Vapor Deposition. ACS Appl. Mater. Interfaces 2019, 11, 12777– 12785,  DOI: 10.1021/acsami.9b00306
      25
      Low contact barrier in 2H/1T' MoTe2 in-plane heterostructure synthesized by chemical vapor deposition
      Zhang, Xiang; Jin, Zehua; Wang, Luqing; Hachtel, Jordan A.; Villarreal, Eduardo; Wang, Zixing; Ha, Teresa; Nakanishi, Yusuke; Tiwary, Chandra Sekhar; Lai, Jiawei; Dong, Liangliang; Yang, Jihui; Vajtai, Robert; Ringe, Emilie; Idrobo, Juan Carlos; Yakobson, Boris I.; Lou, Jun; Gambin, Vincent; Koltun, Rachel; Ajayan, Pulickel M.
      ACS Applied Materials & Interfaces (2019), 11 (13), 12777-12785CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Metal-semiconductor contact has been a crit. topic in the semiconductor industry because it influences device performance remarkably. Conventional metals have served as the major contact material in electronic and optoelectronic devices, but such a selection becomes increasingly inadequate for emerging novel materials such as two-dimensional (2D) materials. Deposited metals on semiconducting 2D channels usually form large resistance contacts due to the high Schottky barrier. A few approaches have been reported to reduce the contact resistance but they are not suitable for large-scale application or they cannot create a clean and sharp interface. In this study, a chem. vapor deposition (CVD) technique is introduced to produce large-area semiconducting 2D material (2H MoTe2) planarly contacted by its metallic phase (1T' MoTe2). We demonstrate the phase-controllable synthesis and systematic characterization of large-area MoTe2 films, including pure 2H phase or 1T' phase, and 2H/1T' in-plane heterostructure. Theor. simulation shows a lower Schottky barrier in 2H/1T' junction than in Ti/2H contact, which is confirmed by elec. measurement. This one-step CVD method to synthesize large-area, seamless-bonding 2D lateral metal-semiconductor junction can improve the performance of 2D electronic and optoelectronic devices, paving the way for large-scale 2D integrated circuits.
    26. 26
      Bae, G. Y.; Kim, J.; Kim, J.; Lee, S.; Lee, E. MoTe2 Field-Effect Transistors with Low Contact Resistance through Phase Tuning by Laser Irradiation. Nanomaterials 2021, 11, 2805,  DOI: 10.3390/nano11112805
      26
      MoTe2 Field-Effect Transistors with Low Contact Resistance through Phase Tuning by Laser Irradiation
      Bae, Geun Yeol; Kim, Jinsung; Kim, Junyoung; Lee, Siyoung; Lee, Eunho
      Nanomaterials (2021), 11 (11), 2805CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
      Due to their extraordinary elec. and phys. properties, two-dimensional (2D) transition metal dichalcogenides (TMDs) are considered promising for use in next-generation elec. devices. However, the application of TMD-based devices is limited because of the Schottky barrier interface resulting from the absence of dangling bonds on the TMDs' surface. Here, we introduce a facile phase-tuning approach for forming a homogenous interface between semiconducting hexagonal (2H) and semi-metallic monoclinic (1T') molybdenum ditelluride (MoTe2). The formation of ohmic contacts increases the charge carrier mobility of MoTe2 field-effect transistor devices to 16.1 cm2 V-1s-1 with high reproducibility, while maintaining a high on/off current ratio by efficiently improving charge injection at the interface. The proposed method enables a simple fabrication process, local patterning, and large-area scaling for the creation of high-performance 2D electronic devices.
    27. 27
      Wang, Y.; Chhowalla, M. Making Clean Electrical Contacts on 2D Transition Metal Dichalcogenides. Nature Reviews Physics 2022, 4, 101– 112,  DOI: 10.1038/s42254-021-00389-0
      27
      Making clean electrical contacts on 2D transition metal dichalcogenides
      Wang, Yan; Chhowalla, Manish
      Nature Reviews Physics (2022), 4 (2), 101-112CODEN: NRPACZ; ISSN:2522-5820. (Nature Portfolio)
      Abstr.: 2D semiconductors, particularly transition metal dichalcogenides (TMDs), have emerged as highly promising for new electronic technologies. However, a key challenge in fabricating devices out of 2D semiconductors is the need for ultra-clean contacts with resistances approaching the quantum limit. The lack of high-quality, low-contact-resistance P-type and N-type contacts on 2D TMDs has limited progress towards the next generation of low-power devices, such as the tunnel field-effect transistors. In this Expert Recommendation, we summarize strategies and provide guidance for making clean van der Waals contacts on monolayered TMD semiconductors. We also discuss the physics of contacts in 2D semiconductors and prospects for achieving quantum conductance.
    28. 28
      Yamaguchi, H.; Blancon, J. C.; Kappera, R.; Lei, S.; Najmaei, S.; Mangum, B. D.; Gupta, G.; Ajayan, P. M.; Lou, J.; Chhowalla, M.; Crochet, J. J.; Mohite, A. D. Spatially Resolved Photoexcited Charge-Carrier Dynamics in Phase-Engineered Monolayer MoS2. ACS Nano 2015, 9, 840– 849,  DOI: 10.1021/nn506469v
      28
      Spatially Resolved Photoexcited Charge-Carrier Dynamics in Phase-Engineered Monolayer MoS2
      Yamaguchi, Hisato; Blancon, Jean-Christophe; Kappera, Rajesh; Lei, Sidong; Najmaei, Sina; Mangum, Benjamin D.; Gupta, Gautam; Ajayan, Pulickel M.; Lou, Jun; Chhowalla, Manish; Crochet, Jared J.; Mohite, Aditya D.
      ACS Nano (2015), 9 (1), 840-849CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A fundamental understanding of the intrinsic optoelectronic properties of atomically thin transition-metal dichalcogenides (TMDs) is crucial for its integration into high performance semiconductor devices. Here, the authors study the transport properties of CVD grown monolayer molybdenum disulfide (MoS2) under photoexcitation using correlated scanning photocurrent microscopy and photoluminescence imaging. The authors examd. the effect of local phase transformation underneath the metal electrodes on the generation of photocurrent across the channel length with diffraction-limited spatial resoln. While max. photocurrent generation occurs at the Schottky contacts of semiconducting (2H-phase) MoS2, after the metallic phase transformation (1T-phase), the photocurrent peak is obsd. toward the center of the device channel, suggesting a strong redn. of native Schottky barriers. Anal. using the bias and position dependence of the photocurrent indicates that the Schottky barrier heights are a few millielectron volts for 1T- and ∼200 meV for 2H-contacted devices. Also a redn. of native Schottky barriers in a 1T device enhances the photoresponsivity by >1 order of magnitude, a crucial parameter in achieving high-performance optoelectronic devices. The obtained results pave a way for the fundamental understanding of intrinsic optoelectronic properties of atomically thin TMDs where ohmic contacts are necessary for achieving high-efficiency devices with low power consumption.
    29. 29
      Lin, D.-Y.; Hsu, H.-P.; Liu, G.-H.; Dai, T.-Z.; Shih, Y.-T. Enhanced Photoresponsivity of 2H-MoTe2 by Inserting 1T-MoTe2 Interlayer Contact for Photodetector Applications. Crystals 2021, 11, 964,  DOI: 10.3390/cryst11080964
      29
      Enhanced Photoresponsivity of 2H-MoTe2 by Inserting 1T-MoTe2 Interlayer Contact for Photodetector Applications
      Lin, Der-Yuh; Hsu, Hung-Pin; Liu, Guang-Hsin; Dai, Ting-Zhong; Shih, Yu-Tai
      Crystals (2021), 11 (8), 964CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
      The 2H molybdenum telluride (MoTe2) photodetector structures were made with inserting 1T-MoTe2 interlayer contacts. The optical response properties such as photocond. (PC) spectroscopy, illumination intensity dependent photoresponsivity, frequency dependent photocurrent, and time-resolved photoresponse were carried out in this study. In PC spectra, a much higher photoresponsivity of 2H-MoTe2 were obsd. by inserting 1T-MoTe2 interlayer contact. The frequency dependent photocurrent and time-resolved photoresponse investigations explore the carrier kinetic decay process of MoTe2 with different electrode contact. The Schottky barrier heights (SBH) extd. by thermionic emission theory were also investigated by inserting 1T-MoTe2 interlayer contacts. The results show the potential applicability for photodetection devices based MoTe2 layered transition metal dichalcogenides semiconductors.
    30. 30
      Ding, Y.; Qi, R.; Wang, C.; Wu, Q.; Zhang, H.; Zhang, X.; Lin, L.; Cai, Z.; Xiao, S.; Gu, X.; Nan, H. Broad-Band Photodetector Based on a Lateral MoTe2 1T-2H-1T Homojunction. J. Phys. Chem. C 2023, 127, 20072– 20081,  DOI: 10.1021/acs.jpcc.3c05592
      30
      Broad-Band Photodetector Based on a Lateral MoTe2 1T-2H-1T Homojunction
      Ding, Yang; Qi, Renxian; Wang, Chenglin; Wu, Qianqian; Zhang, Haozhe; Zhang, Xiumei; Lin, Liangliang; Cai, Zhengyang; Xiao, Shaoqing; Gu, Xiaofeng; Nan, Haiyan
      Journal of Physical Chemistry C (2023), 127 (40), 20072-20081CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The use of a two-dimensional (2D) layered molybdenum ditelluride (MoTe2) nanosheet in electronic devices can be enhanced by transforming its electrode to a metallic phase, forming a 1T-2H-1T heterogeneous structure. This, however, leads to the formation of a junction between the electrode and the channel which can reduce the effectiveness of the device. To address this issue, this paper proposes a novel method of creating 1T-2H-1T MoTe2 optoelectronic devices using oxygen mild plasma prepn. Transmission electron microscopy testing and first-principles calcns. have confirmed that this treatment results in the loss of some Te atoms, thus lowering their energy bands and creating metallic properties. The elec. and optoelectronic performance testing at varying temps. and wavelengths has revealed improved mobility and responsiveness, with a max. mobility of 30 cm2/V s and a responsivity of 0.6 A/W under a 940 nm laser irradn., being 25 times higher than the intrinsic one. This research provides an efficient phase engineering technique for the accurate and undamaged fabrication of optoelectronic devices using two-dimensional materials.
    31. 31
      Xu, X.; Gabor, N. M.; Alden, J. S.; Van Der Zande, A. M.; McEuen, P. L. Photo-thermoelectric Effect at a Graphene Interface Junction. Nano Lett. 2010, 10, 562– 566,  DOI: 10.1021/nl903451y
      31
      Photo-Thermoelectric Effect at a Graphene Interface Junction
      Xu, Xiaodong; Gabor, Nathaniel M.; Alden, Jonathan S.; van der Zande, Arend M.; McEuen, Paul L.
      Nano Letters (2010), 10 (2), 562-566CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We investigate the optoelectronic response of a graphene single-bilayer interface junction using photocurrent (PC) microscopy. We measure the polarity and amplitude of the PC while varying the Fermi level by tuning a gate voltage. These measurements show that the generation of PC is by a photothermoelec. effect. The PC displays a factor of ∼10 increase at the cryogenic temp. as compared to room temp. Assuming the thermoelec. power has a linear dependence on the temp., the inferred graphene thermal cond. from temp. dependent measurements has a T1.5 dependence below ∼100 K, which agrees with recent theor. predictions.
    32. 32
      Zhang, Y.; Li, H.; Wang, L.; Wang, H.; Xie, X.; Zhang, S. L.; Liu, R.; Qiu, Z. J. Photothermoelectric and Photovoltaic Effects Both Present in MoS2. Sci. Rep. 2015, 5, 7938,  DOI: 10.1038/srep07938
      32
      Photothermoelectric and photovoltaic effects both present in MoS2
      Zhang, Youwei; Li, Hui; Wang, Lu; Wang, Haomin; Xie, Xiaomin; Zhang, Shi-Li; Liu, Ran; Qiu, Zhi-Jun
      Scientific Reports (2015), 5 (), 7938CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      As a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS2) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS2 is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS2. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS2 transistors. The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelec. effect (PTE). In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coeff. obsd. in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equil. with the MoS2 lattice.
    33. 33
      Huo, N.; Konstantatos, G. Recent Progress and Future Prospects of 2D-Based Photodetectors. Adv. Mater. 2018, 30, 1801164  DOI: 10.1002/adma.201801164
      There is no corresponding record for this reference.
    34. 34
      HQgraphene., MoTe2 (2H Molybdenum Ditelluride). https://www.hqgraphene.com/MoTe2.php (accessed May 23 2023).
      There is no corresponding record for this reference.
    35. 35
      Buscema, M.; Barkelid, M.; Zwiller, V.; van der Zant, H. S. J.; Steele, G. A.; Castellanos-Gomez, A. Large and Tunable Photothermoelectric Effect in Single-Layer MoS2. Nano Lett. 2013, 13, 358– 363,  DOI: 10.1021/nl303321g
      35
      Large and Tunable Photothermoelectric Effect in Single-Layer MoS2
      Buscema, Michele; Barkelid, Maria; Zwiller, Val; van der Zant, Herre S. J.; Steele, Gary A.; Castellanos-Gomez, Andres
      Nano Letters (2013), 13 (2), 358-363CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The authors study the photoresponse of single-layer MoS2 field-effect transistors by scanning photocurrent microscopy. Unlike in many other semiconductors, the photocurrent generation in single-layer MoS2 is dominated by the photothermoelec. effect and not by the sepn. of photoexcited electron-hole pairs across the Schottky barriers at the MoS2/electrode interfaces. The authors observe a large value for the Seebeck coeff. for single-layer MoS2 that by an external elec. field can be tuned between -4 × 102 and -1 × 105 μV K-1. This large and tunable Seebeck coeff. of the single-layer MoS2 paves the way to new applications of this material such as on-chip thermopower generation and waste thermal energy harvesting.859.
    36. 36
      Ruppert, C.; Aslan, O. B.; Heinz, T. F. Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals. Nano Lett. 2014, 14, 6231– 6236,  DOI: 10.1021/nl502557g
      36
      Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals
      Ruppert, Claudia; Aslan, Burak; Heinz, Tony F.
      Nano Letters (2014), 14 (11), 6231-6236CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Single- and few-layer crystals of exfoliated MoTe2 have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe2 in the monolayer limit displays strong photoluminescence. On the basis of complementary optical absorption results, we conclude that monolayer MoTe2 is a direct-gap semiconductor with an optical band gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near-IR.
    37. 37
      Lezama, I. G.; Arora, A.; Ubaldini, A.; Barreteau, C.; Giannini, E.; Potemski, M.; Morpurgo, A. F. Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2. Nano Lett. 2015, 15, 2336– 2342,  DOI: 10.1021/nl5045007
      37
      Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2
      Lezama, Ignacio Gutierrez; Arora, Ashish; Ubaldini, Alberto; Barreteau, Celine; Giannini, Enrico; Potemski, Marek; Morpurgo, Alberto F.
      Nano Letters (2015), 15 (4), 2336-2342CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temp. microreflectance (MR) and temp.-dependent photoluminescence (PL) measurements. The anal. of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions assocd. with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The anal. of this behavior and of all our exptl. observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for expts. of fundamental interest and possible future device applications.
    38. 38
      Yadav, P.; Dewan, S.; Mishra, R.; Das, S. Review of Recent Progress, Challenges, and Prospects of 2D Materials-Based Short Wavelength Infrared Photodetectors. J. Phys. D: Appl. Phys. 2022, 55, 313001,  DOI: 10.1088/1361-6463/ac6635
      There is no corresponding record for this reference.
    39. 39
      Zhang, K.; Fang, X.; Wang, Y.; Wan, Y.; Song, Q.; Zhai, W.; Li, Y.; Ran, G.; Ye, Y.; Dai, L. Ultrasensitive Near-Infrared Photodetectors Based on a Graphene-MoTe2-Graphene Vertical van der Waals Heterostructure. ACS Appl. Mater. Interfaces 2017, 9, 5392– 5398,  DOI: 10.1021/acsami.6b14483
      39
      Ultrasensitive Near-Infrared Photodetectors Based on a Graphene-MoTe2-Graphene Vertical van der Waals Heterostructure
      Zhang, Kun; Fang, Xin; Wang, Yilun; Wan, Yi; Song, Qingjun; Zhai, Wenhao; Li, Yanping; Ran, Guangzhao; Ye, Yu; Dai, Lun
      ACS Applied Materials & Interfaces (2017), 9 (6), 5392-5398CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      A graphene-MoTe2-graphene vertical van der Waals (vdWs) heterostructure on a SiO2/p+-Si substrate was fabricated by a facile and reliable site-controllable transfer method and applied for photodetection from the visible to near-IR wavelength range. Compared to the layered semiconductor photodetectors reported thus far, the graphene-MoTe2-graphene photodetector has a superior performance, including high photoresponsivity (∼110 mA W-1 at 1064 nm and 205 mA W-1 at 473 nm), high external quantum efficiency (EQE; ∼12.9% at 1064 nm and ∼53.8% at 473 nm), rapid response and recovery processes (a rise time of 24 μs and a fall time of 46 μs under 1064 nm illumination), and free from an external source-drain power supply. The authors have employed scanning photocurrent microscopy to study the photocurrent generation in this heterostructure under various back-gate voltages and found that the 2 Schottky barriers between the graphenes and MoTe2 play an important role in the photocurrent generation. The vdWs heterostructure has a uniform photoresponsive area. The photoresponsivity and EQE of the photodetector can be modulated by the back-gate (p+-Si) voltage. The authors compared the responsivities of thin and thick flakes and found that the responsivity had a strong dependence on the thickness. The heterostructure has promising applications in future novel optoelectronic devices, enabling next-generation high-responsivity, high-speed, flexible, and transparent NIR devices.
    40. 40
      Xiao, H.; Lin, L.; Zhu, J.; Guo, J.; Ke, Y.; Mao, L.; Gong, T.; Cheng, H.; Huang, W.; Zhang, X. Highly Sensitive and Broadband Photodetectors Based on WSe2/MoS2 Heterostructures with van der Waals Contact Electrodes. Appl. Phys. Lett. 2022, 121, 023504  DOI: 10.1063/5.0100191
      There is no corresponding record for this reference.
    41. 41
      Huang, H. Highly Sensitive Visible to Infrared MoTe2 Photodetectors Enhanced by the Photogating Effect. Nanotechnology 2016, 27, 445201  DOI: 10.1088/0957-4484/27/44/445201
      41
      Highly sensitive visible to infrared MoTe2 photodetectors enhanced by the photogating effect
      Huang, Hai; Wang, Jianlu; Hu, Weida; Liao, Lei; Wang, Peng; Wang, Xudong; Gong, Fan; Chen, Yan; Wu, Guangjian; Luo, Wenjin; Shen, Hong; Lin, Tie; Sun, Jinglan; Meng, Xiangjian; Chen, Xiaoshuang; Chu, Junhao
      Nanotechnology (2016), 27 (44), 445201/1-445201/7CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      A review. Two-dimensional materials are promising candidates for electronic and optoelectronic applications. MoTe2 has an appropriate bandgap for both visible and IR light photodetection. Here we fabricate a high-performance photodetector based on few-layer MoTe2. Raman spectral properties have been studied for different thicknesses of MoTe2. The photodetector based on few-layer MoTe2 exhibits broad spectral range photodetection (0.6-1.55μm) and a stable and fast photoresponse. The detectivity is calcd. to be 3.1 × 109 cm Hz1/2 W-1 for 637 nm light and 1.3 × 109 cm Hz1/2W-1 for 1060 nm light at a backgate voltage of 10 V. The mechanisms of photocurrent generation have been analyzed in detail, and it is considered that a photogating effect plays an important role in photodetection. The appreciable performance and detection over a broad spectral range make it a promising material for high-performance photodetectors.
    42. 42
      Shen, D. High-Performance Mid-IR to Deep-UV van der Waals Photodetectors Capable of Local Spectroscopy at Room Temperature. Nano Lett. 2022, 22, 3425– 3432,  DOI: 10.1021/acs.nanolett.2c00741
      There is no corresponding record for this reference.
    43. 43
      Yu, W.; Li, S.; Zhang, Y.; Ma, W.; Sun, T.; Yuan, J.; Fu, K.; Bao, Q. Near-Infrared Photodetectors Based on MoTe2/Graphene Heterostructure with High Responsivity and Flexibility. Small 2017, 13, 1700268  DOI: 10.1002/smll.201700268
      There is no corresponding record for this reference.
    44. 44
      Yin, L.; Zhan, X.; Xu, K.; Wang, F.; Wang, Z.; Huang, Y.; Wang, Q.; Jiang, C.; He, J. Ultrahigh Sensitive MoTe2 Phototransistors Driven by Carrier Tunneling. Appl. Phys. Lett. 2016, 108, 043503  DOI: 10.1063/1.4941001
      44
      Ultrahigh sensitive MoTe2 phototransistors driven by carrier tunneling
      Yin, Lei; Zhan, Xueying; Xu, Kai; Wang, Feng; Wang, Zhenxing; Huang, Yun; Wang, Qisheng; Jiang, Chao; He, Jun
      Applied Physics Letters (2016), 108 (4), 043503/1-043503/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Transition metal dichalcogenides (TMDs) demonstrate great potential in electronic and optoelectronic applications. However, the device performance remains limited because of the poor metal contact. Herein, we fabricate a high-performance ultrathin MoTe2 phototransistor. By introducing an electron tunneling mechanism, electron injection from electrode to channel is strikingly enhanced. The electron mobility approaches 25.2 cm2 V-1 s-1, better than that of other back-gated MoTe2 FETs. Through elec. measurements at various temps., the electron tunneling mechanism is further confirmed. The MoTe2 phototransistor exhibits very high responsivity up to 2560 A/W which is higher than that of most other TMDs. This work may provide guidance to reduce the contact resistance at metal-semiconductor junction and pave a pathway to develop high-performance optoelectronic devices in the future. (c) 2016 American Institute of Physics.
    45. 45
      Xie, Y.; Wu, E.; Zhang, J.; Hu, X.; Zhang, D.; Liu, J. Gate-Tunable Photodetection/Voltaic Device Based on BP/MoTe2 Heterostructure. ACS Appl. Mater. Interfaces 2019, 11, 14215– 14221,  DOI: 10.1021/acsami.8b21315
      45
      Gate-Tunable Photodetection/Voltaic Device Based on BP/MoTe2 Heterostructure
      Xie, Yuan; Wu, Enxiu; Zhang, Jing; Hu, Xiaodong; Zhang, Daihua; Liu, Jing
      ACS Applied Materials & Interfaces (2019), 11 (15), 14215-14221CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Van der Waals heterostructures based on two-dimensional (2D) materials have attracted tremendous attention for their potential applications in optoelectronic devices, such as solar cells and photodetectors. In addn., the widely tunable Fermi levels of these atomically thin 2D materials enable tuning the device performances/functions dynamically. Herein, we demonstrated a MoTe2/BP heterostructure, which can be dynamically tuned to be either p-n or p-p junction by gate modulation due to compatible band structures and elec. tunable Fermi levels of MoTe2 and BP. Consequently, the electrostatic gating can further accurately control the photoresponse of this heterostructure in terms of the polarity and the value of photoresponsivity. Besides, the heterostructure showed outstanding photodetection/voltaic performances. The optimum photoresponsivity, external quantum efficiency, and response time as a photodetector were 0.2 A/W, 48.1%, and 2 ms, resp. Our study enhances the understanding of 2D heterostructures for designing gate-tunable devices and reveals promising potentials of these devices in future optoelectronic applications.
    46. 46
      Lee, H. S.; Min, S.-W.; Chang, Y.-G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S. MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap. Nano Lett. 2012, 12, 3695– 3700,  DOI: 10.1021/nl301485q
      46
      MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap
      Lee, Hee Sung; Min, Sung-Wook; Chang, Youn-Gyung; Park, Min Kyu; Nam, Taewook; Kim, Hyungjun; Kim, Jae Hoon; Ryu, Sunmin; Im, Seongil
      Nano Letters (2012), 12 (7), 3695-3700CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We report on the fabrication of top-gate phototransistors based on a few-layered MoS2 nanosheet with a transparent gate electrode. Our devices with triple MoS2 layers exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection. The varied functionalities are attributed to energy gap modulation by the no. of MoS2 layers. The photoelec. probing on working transistors with the nanosheets demonstrates that single-layer MoS2 has a significant energy bandgap of 1.8 eV, while those of double- and triple-layer MoS2 reduce to 1.65 and 1.35 eV, resp.
    47. 47
      Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497– 501,  DOI: 10.1038/nnano.2013.100
      47
      Ultrasensitive photodetectors based on monolayer MoS2
      Lopez-Sanchez, Oriol; Lembke, Dominik; Kayci, Metin; Radenovic, Aleksandra; Kis, Andras
      Nature Nanotechnology (2013), 8 (7), 497-501CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Ultrasensitive monolayer MoS2 phototransistors with improved device mobility and ON current are demonstrated. The devices show a max. external photoresponsivity of 880 A W-1 at λ = 561 nm and a photoresponse at 400-680 nm. With recent developments in large-scale prodn. techniques such as liq.-scale exfoliation and CVD-like growth, MoS2 shows important potential for applications in MoS2-based integrated optoelectronic circuits, light sensing, biomedical imaging, video recording and spectroscopy.
    48. 48
      Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Van Der Zant, H. S. J.; Castellanos-Gomez, A. Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors. Nano Lett. 2014, 14, 3347– 3352,  DOI: 10.1021/nl5008085
      48
      Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors
      Buscema, Michele; Groenendijk, Dirk J.; Blanter, Sofya I.; Steele, Gary A.; van der Zant, Herre S. J.; Castellanos-Gomez, Andres
      Nano Letters (2014), 14 (6), 3347-3352CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Few-layer black phosphorus, a new elemental two-dimensional (2D) material recently isolated by mech. exfoliation, is a high-mobility layered semiconductor with a direct bandgap that is predicted to strongly depend on the no. of layers, from 0.35 eV (bulk) to 2.0 eV (single layer). Therefore, black phosphorus is an appealing candidate for tunable photodetection from the visible to the IR part of the spectrum. We study the photoresponse of field-effect transistors (FETs) made of few-layer black phosphorus (3-8 nm thick), as a function of excitation wavelength, power, and frequency. In the dark state, the black phosphorus FETs can be tuned both in hole and electron doping regimes allowing for ambipolar operation. We measure mobilities in the order of 100 cm2/V s and a current ON/OFF ratio larger than 103. Upon illumination, the black phosphorus transistors show a response to excitation wavelengths from the visible region up to 940 nm and a rise time of about 1 ms, demonstrating broadband and fast detection. The responsivity reaches 4.8 mA/W, and it could be drastically enhanced by engineering a detector based on a PN junction. The ambipolar behavior coupled to the fast and broadband photodetection make few-layer black phosphorus a promising 2D material for photodetection across the visible and near-IR part of the electromagnetic spectrum.
    49. 49
      Zhang, X.; Liu, B.; Yang, W.; Jia, W.; Li, J.; Jiang, C.; Jiang, X. 3D-Branched Hierarchical 3C-SiC/ZnO Heterostructures for High-Performance photodetectors. Nanoscale 2016, 8, 17573– 17580,  DOI: 10.1039/C6NR06236A
      49
      3D-branched hierarchical 3C-SiC/ZnO heterostructures for high-performance photodetectors
      Zhang, Xinglai; Liu, Baodan; Yang, Wenjin; Jia, Wenbo; Li, Jing; Jiang, Chunhai; Jiang, Xin
      Nanoscale (2016), 8 (40), 17573-17580CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      The ultra-sensitive photodetection of different wavelengths holds promising applications in high-performance optoelectronic devices and it requires an efficient and suitable semiconductor unit. Herein, we demonstrated the designable synthesis of 3D-branched hierarchical 3C-SiC/ZnO heterostructures by a three-step process and their assembling into an ultrasensitive photodetector. Microstructure analyses using high-resoln. transmission electron microscopy reveal that the hierarchical 3C-SiC/ZnO heterostructure is composed of single-crystal 3C-SiC nanowires as a central stem and numerous well-aligned single-cryst. ZnO nanorods as branch shells. Optoelectronic tests on the 3C-SiC/ZnO heterostructure photodetector verify the outstanding photo-detection performance with an ultrahigh EQE (1.69 × 108%), a superior photoresponsivity (4.8 × 105 A W-1), a very fast response time (a rise time of 40 ms and a decay time of 60 ms), a high photo-dark current ratio of 187.8 and an excellent photocurrent stability and reproducibility, which is significantly advantageous or comparable to those of ZnO and other inorg. semiconductor nanostructure based photodetectors. To understand the excellent photodetection of hierarchical 3C-SiC/ZnO heterostructures, a band-gap energy diagram describing the photogenerated electron transport process is plotted and the corresponding mechanism is discussed. The strategy proposed in the present work will open up more opportunities for the design and boost of ultra-sensitive photodetectors based on semiconductor heterostructures.
    50. 50
      Kind, H.; Yan, H.; Messer, B.; Law, M.; Yang, P. Nanowire Ultraviolet Photodetectors and Optical Switches. Adv. Mater. 2002, 14, 158– 160,  DOI: 10.1002/1521-4095(20020116)14:2<158::AID-ADMA158>3.0.CO;2-W
      50
      Nanowire ultraviolet photodetectors and optical switches
      Kind, Hannes; Yan, Haoquan; Messer, Benjamin; Law, Matthew; Yang, Peidong
      Advanced Materials (Weinheim, Germany) (2002), 14 (2), 158-160CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
      Highly sensitive nanowire switches were produced by exploring the photoconducting properties of individual semiconductor nanowires. The ZnO nanowires were grown by a vapor phase transport process and dispersed directly on pre-fabricated Au electrodes to characterize their photoconducting properties. For-terminal measurements of individual ZnO nanowires indicated that they were highly insulating in the dark with a resistivity >3.5 Mωcm. The nanowire resistivity decreased when they were exposed to UV-light with λ <380 nm. The high sensitivity of the nanowire photoconductors was revealed which showed the power dependence of the photoresponse and they also exhibited wavelength selectivity. These characteristics of the photoconductive ZnO nanowires suggested that they are good materials for optoelectronic switches with the dark insulating state as OFF and the UV-exposed conducting state as ON. The nanowires could be reversibly switched between low and high cond. state, thus they could serve as highly sensitive UV-light detectors, chem. and biol. sensors, and switching devices for nanoscale optoelectronic applications.
    51. 51
      Liu, F.; Shimotani, H.; Shang, H.; Kanagasekaran, T.; Zólyomi, V.; Drummond, N.; Fal’ko, V. I.; Tanigaki, K. High-Sensitivity Photodetectors Based on Multilayer GaTe Flakes. ACS Nano 2014, 8, 752– 760,  DOI: 10.1021/nn4054039
      51
      High-Sensitivity Photodetectors Based on Multilayer GaTe Flakes
      Liu, Fucai; Shimotani, Hidekazu; Shang, Hui; Kanagasekaran, Thangavel; Zolyomi, Viktor; Drummond, Neil; Fal'ko, Vladimir I.; Tanigaki, Katsumi
      ACS Nano (2014), 8 (1), 752-760CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Optoelectronic devices based on layered materials such as graphene resulted in significant interest due to their unique properties and potential technol. applications. The elec. and optoelectronic properties of nano GaTe flakes as layered materials are described. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of ∼0.2 cm2 V-1 s-1. The gate transistor exhibits a high photoresponsivity of 104 A/W, which is greatly better than that of graphene, MoS2, and other layered compds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.
    52. 52
      Hidding, J.; Cordero-Silis, C. A.; Vaquero, D.; Panagiotis Rompotis, K.; Quereda, J.; Guimarães, M. H. D. Locally Phase-Engineered MoTe2 for Near-Infrared Photodetectors . 2024, 2406.01376, arXiv, https://arxiv.org/abs/2406.01376 (accessed Jun 03, 2024).
      There is no corresponding record for this reference.

  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphotonics.4c00896.

    • Details over the photothermoelectric temperature gradient calculation; laser spot size calculation and data for 700 and 1064 nm for the reported measurements; equation and additional details for charge carrier mobility and further details of another phase-engineered device, including responsivity and transfer curves; drain-source and gate voltage dependence of the photocurrent; normalized Raman spectra and spectral weight contribution of the 1T′ phase characteristic peaks; laser irradiation power; and flake thickness dependence for phase change (PDF)


    Terms & Conditions

    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.