Thick MoS2 Research Articles

Crypto primitive of MOCVD MoS2 transistors for highly secured physical unclonable functions

Physically unclonable crypto primitives have potential applications for anti-counterfeiting, identification, and authentication, which are clone proof and resistant to variously physical attack. Conventional physical unclonable function (PUF) based on Si complementary metal-oxide-semiconductor (CMOS) technologies greatly suffers from entropy loss and bit instability due to noise sensitivity. Here we grow atomically thick MoS2 thin film and fabricate field-effect transistors (FETs). The inherently physical randomness of MoS2 transistors from materials growth and device fabrication process makes it appropriate for the application of PUF device. We perform electrical characterizations of MoS2 FETs, collect the data from 448 devices, and generate PUF keys by splitting drain current at specific levels to evaluate the response performance. Proper selection of splitting threshold enables to generate binary, ternary, and double binary keys. The generated PUF keys exhibit good randomness and uniqueness, providing a possibility for harvesting highly secured PUF devices with two-dimensional materials.

Insights into the wear track evolution with sliding cycles of carbon-alloyed transition metal dichalcogenide coatings

This study is aimed towards a better understanding of the tribological performance of the optimized Mo-Se-C dry lubricant coatings. The coatings are deposited with a lower carbon content (< 30 at. %), as compared to the literature. The detailed analysis of the wear track and the tribolayer evolution as a function of the C content and the number of sliding cycles is discussed, using Raman spectroscopy, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) analyses. The coatings are highly compact, homogeneous, with (002) basal planes parallel to the surface as observed by X-ray diffraction (XRD) studies. In unidirectional tests, under ambient conditions, the friction coefficient and the specific wear rate decrease with the number of sliding cycles. After 25,000 cycles, the wear tracks are not fully covered with MoSe2 tribolayer, instead displayed zones of the tribolayer as well as the as-deposited coating. After 100,000 cycles, the wear track is covered by a thick MoSe2 tribolayer. Scanning-TEM (STEM) chemical composition mapping also confirms that MoSe2 tribolayer is dominating during sliding in ambient air. Reciprocating sliding tests are also performed in ambient air, dry N2, and at 200 °C. The transition metal dichalcogenide (TMD) phase is always governing the low friction, with coatings not showing the chameleon behavior claimed in the literature for C-containing TMD coatings.

Nanoscale characterization of unintentional doping of atomically thin layered semiconductors by scanning nonlinear dielectric microscopy

We use scanning nonlinear dielectric microscopy (SNDM) to visualize unintentional carrier doping of few-layer Nb-doped MoS2 mechanically exfoliated on SiO2. SNDM enables imaging of the majority carrier distribution in as-exfoliated samples at the nanoscale. We show that, unlike thick MoS2 layers, atomically thin layers exhibit a p- to n-type transition as the thickness decreases. The level of the observed unintentional n-doping is estimated to be 1×1013 cm−2, in agreement with the results of previous independent studies. In addition, the influence of ultraviolet–ozone treatment on the majority carrier distribution is also investigated. The n-type doping is observed to progress with increasing processing time. SNDM can be readily applied to atomically thin layered semiconductors and will advance understanding of and the ability to predict device characteristics even at an early stage of the fabrication process.

Effect of Cu content in CIGSe absorber on MoSe2 formation during post-selenization process

MoSe2 layers, which would help to form ohmic contact and improve the adhesion, were formed at the Mo/CIGSe interface during the CIGSe preparing process. However, a thick MoSe2 layer could introduce carrier recombination center, high contact resistance, and might result in poor adhesion of CIGSe films, which played a deleterious role in the efficiency of the devices. In this study, the effect of Cu content and Ga diffusion on the formation of MoSe2 layer was investigated. The thick MoSe2 layer tended to form with Cu/(In + Ga) < 0.7, and MoSe2 was difficult to form at the Mo/CIGSe interface when Cu/(In + Ga) > 0.9. Finally, CIGSe thin film with large grains and a thin MoSe2 layer was obtained with the value of Cu/(In + Ga) between 0.8 and 0.9, and an 11.04% efficiency CIGSe solar cell was fabricated with the open circuit voltage of 505 mV.

Perfect Absorption by an Atomically Thin Crystal

Optical absorption is one of fundamental light-matter interactions. In most materials, optical absorption is a weak perturbation to the light. In this regime, absorption and emission are irreversible, incoherent processes due to strong damping. Excitons in monolayer transition metal dichalcogenides, however, interact strongly with light, leading to optical absorption in the non-perturbative regime where coherent re-emission of the light has to be considered. Between the incoherent and coherent limits, we show that a robust critical coupling condition exists, leading to perfect optical absorption. Up to 99.6% absorption is measured in a sub-nanometer thick MoSe2 monolayer placed in front of a mirror. The perfect absorption is controlled by tuning the exciton-phonon, exciton-exciton, and exciton-photon interactions by temperature, pulsed laser excitation, and a movable mirror, respectively. Our work suggests unprecedented opportunities for engineering exciton-light interactions using two-dimensional atomically thin crystals, enabling novel photonic applications including ultrafast light modulators and sensitive optical sensing.

Strain and Charge Doping Fingerprints of the Strong Interaction between Monolayer MoS2 and Gold.

Gold-mediated exfoliation of MoS2 has recently attracted considerable interest. The strong interaction between MoS2 and Au facilitates preferential production of centimeter-sized monolayer MoS2 with near-unity yield and provides a heterostructure system noteworthy from a fundamental standpoint. However, little is known about the detailed nature of the MoS2–Au interaction and its evolution with the MoS2 thickness. Here, we identify the specific vibrational and binding energy fingerprints of this interaction using Raman and X-ray photoelectron spectroscopy, which indicate substantial strain and charge doping in monolayer MoS2. Tip-enhanced Raman spectroscopy reveals heterogeneity of the MoS2–Au interaction at the nanoscale, reflecting the spatial nonconformity between the two materials. Micro-Raman spectroscopy shows that this interaction is strongly affected by the roughness and cleanliness of the underlying Au. Our results elucidate the nature of the MoS2–Au interaction and guide strain and charge doping engineering of MoS2.

In-situ Raman analysis of hydrogenation in well-defined ultrathin molybdenum diselenide deposits synthesized through vapor phase deposition

We report on the synthesis, characterization and in-situ Raman spectroscopy analysis of hydrogenation in ultrathin crystalline MoSe2 deposits. We use a controllable vapor phase synthesis method using MoSe2 powder as the only precursor, to fabricate nano- to micro-size few layer thick MoSe2 deposits with tunable number densities on SiO2/Si substrates. We employ this controllable synthesis method to correlate characteristic Raman spectroscopy response of MoSe2 at ca. 242 cm−1 (A1g) and ca. 280 cm−1 (E2g1) with the thickness of the deposits acquired from atomic force microscopy (AFM). We also use this array of well-defined atomically thin MoSe2 deposits to study possible hydrogenation effects on select architectures using in-situ Raman spectroscopy. Interestingly, our analysis indicates that ultrathin MoSe2 deposits with exposed edges show a blue shift of 1–2 cm−1 when exposed to H2 flow at 150–250 sccm for 2–4 hours in a sealed reaction cell. Exposure to Ar flow under same condition reverses the observed shift in the A1g mode of the select MoSe2 deposits. Our measurements provide in-situ evidence for hydrogen adsorption on MoSe2 deposits at room temperature and insight into the possible active sites for hydrogen reactions on layered dichalcogenides at lower dimensions.

A MOS capacitor model for ultra-thin 2D semiconductors: the impact of interface defects and channel resistance

In an edge capacitor based on 2D semiconductors, charge transport in the channel leads to frequency dispersion in capacitance and peaks in conductance, which convolutes information from interface defects (Dit). If not careful, Dit can be grossly misinterpreted. In this study we develop a distributed MOS capacitor model that distinguishes between these two effects. We incorporate charge transport in terms of channel resistance and evaluate the impact of interface defects in few monolayers thick (1 − 5 layers) MoS2 channel. Using this, an optimized MOS capacitor geometry is identified. By comparing the model with experimental data, we obtain exponential defect states ( cm−2 eV−1, σ = 0.12 − 0.08 eV) originating from the conduction band for MoS2 on 4 nm HfO2 and 21 nm SrTiO3, respectively. The model successfully captures the length dependence of MOS capacitors and the static and dynamic behavior of Dit.

Nonlinear absorption properties and carrier dynamics in MoS2/Graphene van der Waals heterostructures

Recently, molybdenum disulfide (MoS2) nanosheets have been used in passively mode-locked laser due to excellent saturable absorption (SA) property. To enhance their optical nonlinearity, some heterostructures such as MoS2/Graphene (MoS2/G) have been designed, but the carrier dynamics in the materials have rarely been reported. Here, the nonlinear response of MoS2 and MoS2/G heterostructures nanosheets with different thickness are studied via nanosecond laser Z-scan and femtosecond time-resolved transient absorption (TA) technique. The results reveal that the ground state bleaching take place under intense laser excitation, which would block the further absorption of incident light, causing a strong SA effect of the materials. With increasing the thickness of MoS2, electron diffusion take place between adjacent layers, resulting in a slow recombination of excited carriers and the enhancement of SA effect in MoS2. In MoS2/G, excited electrons could transfer from MoS2 to graphene and the interlayer carrier diffusion in MoS2 could be enhanced, prolonging the decay time of ground state bleaching effect. The slower carrier relaxation time in MoS2 would lead a stronger SA effect for nanosecond laser pulse. This work could provide an effective strategy for designing saturable absorbers in mode-locked lasers using MoS2 heterostructures.

Ultra-thin MoS2 nanosheet for electron transport layer of perovskite solar cells

The structure and the physical dimension of the electron transport layer (ETL) in perovskite solar cell (PSC) determine the carrier transport characteristics to the electrode and then the overall performance of the device. ETL with ultimately thin and single-crystalline should facilitate enhanced photogenerated carrier transport to the electrode, inducing a highly dynamic charge transfer at the interface of ETL and perovskite absorber layer. Here, we demonstrated that a planar few atoms (5 monolayer or 5L) thick MoS2 nanosheets synthesized directly onto the ITO substrate allows a facile interfacial charge transfer and transport as judged from the open circuit voltage (Voc) value, steady-state photoluminescence and external quantum efficiency (EQE) analysis results. We found that the nanosheets crystallinity properties determines the photoelectrical process in the PSC device. The champion device in this study can produce a power conversion efficiency (PCE) as high as 3.36% with a short-circuit current density (Jsc), open circuit voltage (Voc) and fill-factor (FF) of as high as 16.24 mA cm−2, 0.56 V and 0.370, respectively. The champion device also exhibited a reasonable high stability properties that shows it can retain more than 90% of its initial PCE when operated under irradiation of 1 sun at maximum power-point. Due to the single crystallinity and the large area properties, the MoS2 nanosheets prepared using the present method may find extended application as high-performance ETL of perovskite solar cells.

Thickness effect on low-power driving of MoS2 transistors in balanced double-gate fields

In field-effect transistors (FETs), when the thickness of the semiconducting transition metal dichalcogenides (TMDs) channel exceeds the maximum depletion depth, the entire region cannot be completely controlled by a single-gate electric field. The layer-to-layer carrier transitions between the van der Waals interacted TMD layers result in the extraordinary anisotropic carrier transport in the in-plane and out-of-plane directions. The performance of the TMD FETs can be largely enhanced by optimizing the thickness of the TMD channel as well as increasing the effective channel area through which the gate field is delivered. In this study, we investigated the carrier behavior and device performance in double-gate FETs fabricated using a 57 nm thick MoS2, which is thicker than the maximum depletion depth of about 50 nm, and a much thinner 4 nm thick MoS2. The results showed that in the thick MoS2, the gate voltages at both ends formed two independent channels which had no synergistic effect on the device performance owing to the inefficient delivery of the vertical electric field. On the other hand, in the thin MoS2 channel, the double-gate voltages effectively controlled one channel, resulting in twice the carrier mobility and operation in a low electric field region, i.e. below 0.2 MV cm−1.

Modulation of Second-Harmonic Generation in Bulk MoS2 via Excitation Wavelength and Metal Film Thickness

We numerically investigated the impact of the excitation wavelength and metal film thickness on second-harmonic generation (SHG) in bulk MoS2, which originates from the change in optical-path difference and phase difference among the multistage reflected lights during the thin-film interference process. The enhancement factor of SHG for a MoS2 layer on metal-dielectric substrates was calculated using the finite-difference time-domain technique. It was shown that the intensity of SHG was strongly affected by the excitation wavelength and the thickness of the metal film. We also obtained the dependence of the thickness of the MoS2 layer with the strongest SHG on the excitation wavelength. The reflection, transmission, and absorption of MoS2 layers with different thicknesses were studied by changing the metal film thickness of substrates, and the MoS2 thickness with the strongest SHG was found at different excitation wavelengths. We found that changing the metal film thickness has little effect on the color of the MoS2 layer, indicating that the thickness of the MoS2 layer can still be roughly estimated from the color. Therefore, it is inferred that MoS2 layers with the strongest SHG have the potential to be used for optical information storage or in the design of optical devices provided that the excitation wavelength or the thickness of the metal film is selected appropriately.

High-performance MoSe2 homojunction infrared photodetector

Two-dimensional (2D) materials with unique structural and physical properties are urgent to exploit and may be applied in the next-generation electronics and infrared detectors. The homojunction devices are ideal and promising candidates compared to heterojunction devices since there are no interface problems, such as unintentionally induced impurities and charge-trapped sites. In this work, we reported a high-performance MoSe2 homojunction infrared photodetector. MoSe2 homojunction devices directly consist of thick and thin MoSe2. MoSe2 homojunction diodes exhibit different rectified characteristics including forward and reverse rectifications, which are dependent on the back-gate voltage. Remarkably, MoSe2 homojunction photodetectors possess a broadband photoresponse with the wavelength from visible to near-infrared at room temperature. The responsivity of MoSe2 homojunction photodetectors under 940 nm laser illumination is approximately 2.25 A W−1. Additionally, the obtained specific detectivity of MoSe2 homojunction device is over 1010 Jones. Our findings provide an excellent way to fabricate feasible homojunction devices. Meanwhile, homojunction devices also show the wealthy potential in novel electronic and optoelectronic devices.

Tunable light emission from chemical vapor deposited two-dimensional MoSe2 by layer variation and S incorporation

Mono- and few-layer thick MoSe2 and MoSxSe2 − x domains were grown on insulating SiO2/Si substrates by chemical vapor deposition. Scanning electron microscopy and optical contrast images were used to determine the domain size and morphology. The structure, crystallinity, and the thickness (number of layers) of the as-synthesized domains were determined from Raman spectroscopy. The light emission was determined from photoluminescence (PL) spectroscopy. The PL emission started appearing only in domains having four layers or less, with the intensity increasing as the number of layers decreased. The PL peak position varied between 1.48 eV (∼838 nm) for four layers to 1.55 eV (∼800 nm) in the monolayer limit. Sulfur incorporation was done to enable a further tunability of the bandgap. The monolayer bandgap changed from 1.55 eV for MoSe2 to 1.64 eV (∼756 nm) for MoS0.32Se0.68. The other effect of S incorporation was the formation of larger area domains in the alloy as compared to binary MoSe2 with an improvement in the structural properties, thus providing a pathway to improve the properties of two-dimensional semiconductors by mixing of two materials with similar atomic arrangements.

Enhanced visible light photochemical activity and stability of MoS2/Cu2O nanocomposites by tunable heterojunction

Here we report the hydrothermal synthesis of MoS2/Cu2O nanocomposites with tunable heterojunction for enhancing visible light photochemical activity and stability. The conduction and valence band positions of few-layers MoS2 have been tuned to optimize photo-induced charge separation efficiency by regulating the layer thickness of MoS2. The photochemical activity and stability of MoS2/Cu2O nanoparticles are demonstrated by visible light assisted degradation of methyl orange. Among all of the samples, the MoS2/Cu2O nanocomposite assembled with 6000 rpm centrifuged MoS2 has the highest degradation efficiency of 95.8 % after 3 h of visible light illumination, due to the high visible light absorption and optimized electron-hole separation by the tunable heterojunction design. The influence of MoS2 mass ratio was further investigated. At last, for better understanding of the enhanced photochemical activity, a novel charge separation mechanism is proposed based on the band alignment and build-in electric field between Cu2O and MoS2.

Analysis of Transferred MoS2 Layers Grown by MOCVD: Evidence of Mo Vacancy Related Defect Formation

A low-temperature multi-frequency electron spin resonance (ESR) study has been carried out on 1, 3.5, and 6 layer thick MoS2 films, grown by metal organic vapor deposition (MOCVD) and subsequently transferred on SiO2/Si. This reveals the observation of a previously unreported, nearly isotropic signal at g ≈ 1.9998 with corresponding spin center (spin S = ½) densities ranging from ∼6 × 108 cm−2 to ∼5 × 1011 cm−2. The ESR investigation is closely combined with an in-depth analysis by an assortment of other experimental techniques, including atomic force microscopy (AFM), Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), to ultimately result in the assignment of the ESR signal to a defect of intrinsic nature, most likely a Mo vacancy (VMo) related defect located at MoS2 grain edges or boundaries. The oxidation of the 2D material at grain edges and boundaries combined with the applied water-based transfer procedure is demonstrated to play a crucial role in the generation of the newly observed defect, thus advising caution with the currently applied process method. The presented analysis, which combines a variety of experimental techniques, contributes to the fine-tuning of the CVD growth and transfer process of high-quality few-layer MoS2 intended for next-generation nanoelectronic devices.

Thickness-dependent photoelectric properties of MoS2/Si heterostructure solar cells

In order to obtain the optimal photoelectric properties of vertical stacked MoS2/Si heterostructure solar cells, we propose a theoretical model to address the relationship among film thickness, atomic bond identities and related physical quantities in terms of bond relaxation mechanism and detailed balance principle. We find that the vertical stacked MoS2/Si can form type II band alignment, and its photoelectric conversion efficiency (PCE) enhances with increasing MoS2 thickness. Moreover, the optimal PCE in MoS2/Si can reach 24.76%, inferring that a possible design way can be achieved based on the layered transition metal dichalcogenides and silicon.

Direct Observation of Monolayer MoS2 Prepared by CVD Using In-Situ Differential Reflectance Spectroscopy

The in-situ observation is of great significance to the study of the growth mechanism and controllability of two-dimensional transition metal dichalcogenides (TMDCs). Here, the differential reflectance spectroscopy (DRS) was performed to monitor the growth of molybdenum disulfide (MoS2) on a SiO2/Si substrate prepared by chemical vapor deposition (CVD). A home-built in-situ DRS setup was applied to monitor the growth of MoS2 in-situ. The formation and evolution of monolayer MoS2 are revealed by differential reflectance (DR) spectra. The morphology, vibration mode, absorption characteristics and thickness of monolayer MoS2 have been confirmed by optical microscopy, Raman spectroscopy, ex-situ DR spectra, and atomic force microscopy (AFM) respectively. The results demonstrated that DRS was a powerful tool for in-situ observations and has great potential for growth mechanism and controllability of TMDCs prepared by CVD. To the best of the authors’ knowledge, it was the first report in which the CVD growth of two-dimensional TMDCs has been investigated in-situ by reflectance spectroscopy.

Edge Contact for Carrier Injection and Transport in MoS2 Field-Effect Transistors.

The contact properties of van der Waals layered semiconducting materials are not adequately understood, particularly for edge contact. Edge contact is extremely helpful in the case of graphene, for producing efficient contacts to vertical heterostructures, and for improving the contact resistance through strong covalent bonding. Herein, we report on edge contacts to MoS2 of various thicknesses. The carrier-type conversion is robustly controlled by changing the flake thickness and metal work functions. Regarding the ambipolar behavior, we suggest that the carrier injection is segregated in a relatively thick MoS2 channel; that is, electrons are in the uppermost layers, and holes are in the inner layers. Calculations reveal that the strength of the Fermi-level pinning (FLP) varies layer-by-layer, owing to the inhomogeneous carrier concentration, and particularly, there is negligible FLP in the inner layer, supporting the hole injection. The contact resistance is large despite the significantly reduced contact resistivity normalized by the contact area, which is attributed to the current-crowding effect arising from the narrow contact area.

Controlled Plasma Thinning of Bulk MoS2 Flakes for Photodetector Fabrication.

The electronic properties of layered materials are directly determined based on their thicknesses. Remarkable progress has been carried out on synthesis of wafer-scale atomically molybdenum disulfide (MoS2) layers as a two-dimensional material in the past few years in order to transform them into commercial products. Although chemical/mechanical exfoliation techniques are used to obtain a high-quality monolayer of MoS2, the lack of suitable control in the thickness and the lateral size of the flakes restrict their benefits. As a result, a straightforward, effective, and reliable approach is widely demanded to achieve a large-area MoS2 flake with control in its thickness for optoelectronic applications. In this study, thick MoS2 flakes are obtained by a short-time bath sonication in dimethylformamide solvent, which are thinned with the aid of a sequential plasma etching process using H2, O2, and SF6 plasma. A comprehensive study has been carried out on MoS2 flakes based on scanning electron microscopy, atomic force microscopy, Raman, transmission electron microscopy, and X-ray photoelectron microscopy measurements, which ultimately leads to a two-cycle plasma thinning method. In this approach, H2 is used in the passivation step in the first subcycle, and O2/SF6 plasma acts as an etching step for removing the MoS2 layers in the second subcycle. Finally, we show that this technique can be enthusiastically used to fabricate MoS2-based photodetectors with a considerable photoresponsivity of 1.39 A/W and a response time of 0.45 s under laser excitation of 532 nm.