Quantitative analysis of the temperature dependency in Raman active vibrational modes of molybdenum disulfide atomic layers

Unraveling High Thermal Conductivity with In-Plane Anisotropy Observed in Suspended SiP2

The anisotropic thermal transport properties of low-symmetry two-dimensional materials play an important role in understanding heat dissipation and optimizing thermal management in integrated devices. Examples of efficient energy dissipation and enhanced power sustainability have been demonstrated in nanodevices based on materials with anisotropic thermal transport properties. However, the exploration of materials with high thermal conductivity and strong in-plane anisotropy remains challenging. Herein, we demonstrate the observation of anisotropic in-plane thermal conductivities of few-layer SiP2 based on the micro-Raman thermometry method. For suspended SiP2 nanoflake, the thermal conductivity parallel to P-P chain direction (κ∥b) can reach 131 W m-1 K-1 and perpendicular to P-P chain direction (κ⊥b) is 89 W m-1 K-1 at room temperature, resulting in a significant anisotropic ratio (κ∥b/κ⊥b) of 1.47. Note that such a large anisotropic ratio mainly results from the higher phonon group velocity along the P-P chain direction. We also found that the thermal conductivity can be effectively modulated by increasing the SiP2 thickness, reaching a value as high as 202 W m-1 K-1 (120 W m-1 K-1) for κ∥b (κ⊥b) at 111 nm thickness, which is the highest among layered anisotropic phosphide materials. Notably, the anisotropic ratio always remains at a high level between 1.47 and 1.68, regardless of the variation of SiP2 thickness. Our observation provides a new platform to verify the fundamental theory of thermal transport and a crucial guidance for designing efficient thermal management schemes of anisotropic electronic devices.

Probing anharmonic phonons in WS2 van der Waals crystal by Raman spectroscopy and machine learning

Understanding the optothermal physics of quantum materials will enable the efficient design of next-generation photonic and superconducting circuits. Anharmonic phonon dynamics is central to strongly interacting optothermal physics. This is because the pressure of a gas of anharmonic phonons is temperature dependent. Phonon-phonon and electron-phonon quantum interactions contribute to the anharmonic phonon effect. Here we have studied the optothermal properties of physically exfoliated WS₂ van der Waals crystal via temperature-dependent Raman spectroscopy and machine learning strategies. This fundamental investigation will lead to unveiling the dependence of temperature on in-plane and out-of-plane Raman shifts (Raman thermometry) of WS₂ to study the thermal conductivity, hot carrier diffusion coefficient, and thermal expansion coefficient.

Morphological dependent exciton dynamics and thermal transport in MoSe2 films

Thermal transport and exciton dynamics of semiconducting transition metal dichalcogenides (TMDCs) play an immense role in next-generation electronic, photonic, and thermoelectric devices. In this work, we synthesize distinct morphologies (snow-like and hexagonal) of a trilayer MoSe₂ film over the SiO₂/Si substrate via the chemical vapor deposition (CVD) method and investigated their morphological dependent exciton dynamics and thermal transport behaviour for the first time to the best of our knowledge. Firstly, we studied the role of spin-orbit and interlayer couplings both theoretically as well as experimentally via first-principles density functional theory and photoluminescence study, respectively. Further, we demonstrate morphological dependent thermal sensitive exciton response at low temperatures (93-300 K), showing more dominant defect-bound excitons (E_L) in snow-like MoSe₂ compared to hexagonal morphology. We also examined the morphological-dependent phonon confinement and thermal transport behaviour using the optothermal Raman spectroscopy technique. To provide insights into the nonlinear temperature-dependent phonon anharmonicity, a semi-quantitative model comprising volume and temperature effects was used, divulging the dominance of three-phonon (four-phonon) scattering processes for thermal transport in hexagonal (snow-like) MoSe₂. The morphological impact on thermal conductivity (k_s) of MoSe₂ has also been examined here by performing the optothermal Raman spectroscopy, showing k_s ∼ 36 ± 6 W m^-1 K^-1 for snow-like and ∼41 ± 7 W m^-1 K^-1 for hexagonal MoSe₂. Our research will contribute to the understanding of thermal transport behaviour in different morphologies of semiconducting MoSe₂, finding suitability for next-generation optoelectronic devices.

Stress Effects on Temperature-Dependent In-Plane Raman Modes of Supported Monolayer Graphene Induced by Thermal Annealing

The coupling strength between two-dimensional (2D) materials and substrate plays a vital role on thermal transport properties of 2D materials. Here we systematically investigate the influence of vacuum thermal annealing on the temperature-dependence of in-plane Raman phonon modes in monolayer graphene supported on silicon dioxide substrate via Raman spectroscopy. Intriguingly, raising the thermal annealing temperature can significantly enlarge the temperature coefficient of supported monolayer graphene. The derived temperature coefficient of G band remains mostly unchanged with thermal annealing temperature below 473 K, while it increases from −0.030 cm⁻¹/K to −0.0602 cm⁻¹/K with thermal annealing temperature ranging from 473 K to 773 K, suggesting the great impact of thermal annealing on thermal transport in supported monolayer graphene. Such an impact might reveal the vital role of coupling strength on phonon scattering and on the thermal transport property of supported monolayer graphene. To further interpret the thermal annealing mechanism, the compressive stress in supported monolayer graphene, which is closely related to coupling strength and is studied through the temperature-dependent Raman spectra. It is found that the variation tendency for compressive stress induced by thermal annealing is the same as that for temperature coefficient, implying the intense connection between compressive stress and thermal transport. Actually, 773 K thermal annealing can result in 2.02 GPa compressive stress on supported monolayer graphene due to the lattice mismatch of graphene and substrate. This study proposes thermal annealing as a feasible path to modulate the thermal transport in supported graphene and to design future graphene-based devices.

Thermal expansion coefficient of few-layer MoS2 studied by temperature-dependent Raman spectroscopy

The thermal expansion coefficient is an important thermal parameter that influences the performance of nanodevices based on two-dimensional materials. To obtain the thermal expansion coefficient of few-layer MoS₂, suspended MoS₂ and supported MoS₂ were systematically investigated using Raman spectroscopy in the temperature range from 77 to 557 K. The temperature-dependent evolution of the Raman frequency shift for suspended MoS₂ exhibited prominent differences from that for supported MoS₂, obviously demonstrating the effect due to the thermal expansion coefficient mismatch between MoS₂ and the substrate. The intrinsic thermal expansion coefficients of MoS₂ with different numbers of layers were calculated. Interestingly, negative thermal expansion coefficients were obtained below 175 K, which was attributed to the bending vibrations in the MoS₂ layer during cooling. Our results demonstrate that Raman spectroscopy is a feasible tool for investigating the thermal properties of few-layer MoS₂ and will provide useful information for its further application in photoelectronic devices.

Lattice dynamics, optical and thermal properties of quasi-two-dimensional anisotropic layered semimetal ZrTe2

In this study, an investigation was conducted on the vibrational properties exhibited by 2D layered zirconium ditelluride by employing Raman spectroscopy and confirmed by DFT calculation.

Influence of MoS2-metal interface on charge injection: a comparison between various metal contacts

Achieving good contacts is vital for harnessing the fascinating properties of two-dimensional (2D) materials. However, unsatisfactory 2D material-metal interfaces remain a problem that hinders the successful application of 2D materials for fabricating nanodevices. In this study, Kelvin probe force microscopy (KPFM) and other high-resolution microscopy techniques are utilized to characterize the surface morphology and contact interface between MoS₂ and common metals including Au, Ti, Pd, and Ni. Surface potential information, including the contact potential difference ([Formula: see text]) and surface potential difference ([Formula: see text]) of each MoS₂-metal contact, is obtained. By comparing the surface potential distribution mappings with and without illumination, non-zero surface photovoltage (SPV) values and evident shift with amplitudes of 32 mV and 44 mV are observed for MoS₂-Au and Ti, but not for MoS₂-Pd and Ni. The Schottky barrier heights of MoS₂-Au, Ti, Pd, and Ni are roughly evaluated from their I-V curves. Raman spectroscopy is also carried out to ensure more convincing results. All the results suggest that a smoother MoS₂-metal interface results in better charge transport behaviors. Our analysis of the underlying mechanism and experimental findings offer a new perspective to better understand MoS₂-metal contacts and underscore the fundamental importance of interface morphology for MoS₂-based devices.

Thermodynamic stability and vibrational anharmonicity of black phosphorene-beyond quasi-harmonic analysis

Thermodynamic stability and vibrational anharmonicity of single layer black phosphorene (SLBP) are studied using a spectral energy density (SED) method. At finite temperatures, SLBP sheet undergoes structural deformation due to the formation of thermally excited ripples. Thermal stability of deformed SLBP sheet is analyzed by computing finite temperature phonon dispersion, which shows that SLBP sheet is thermodynamically stable and survives the crumpling transition. To analyze the vibrational anharmonicity, temperature evolution of all zone center optic phonon modes are extracted, including experimentally forbidden IR and Raman active modes. Mode resolved phonon spectra exhibits red-shift in mode frequencies with temperature. The strong anharmonic phonon-phonon coupling is the predominant reason for the observed red-shift of phonon modes, the contribution of thermal expansion is marginal. Further, temperature sensitivity of all optic modes are analyzed by computing their first order temperature co-efficient (χ), and it can be expressed asB_2g>Ag2>B3g1>B3g2>B_1g>Ag1&B_2u>B_1ufor Raman and IR active modes, respectively. The quasi-harmonicχvalues are much smaller than the SED and experimental values; which substantiate that quasi-harmonic methods are inadequate, and a full anharmonic analysis is essential to explain structure and dynamics of SLBP at finite temperatures.

Light-matter interactions in two-dimensional layered WSe2 for gauging evolution of phonon dynamics

Phonon dynamics is explored in mechanically exfoliated two-dimensional WSe₂ using temperature-dependent and laser-power-dependent Raman and photoluminescence (PL) spectroscopy. From this analysis, phonon lifetime in the Raman active modes and phonon concentration, as correlated to the energy parameter E ₀, were calculated as a function of the laser power, P, and substrate temperature, T. For monolayer WSe₂, from the power dependence it was determined that the phonon lifetime for the in-plane vibrational mode was twice that of the out-of-plane vibrational mode for P in the range from 0.308 mW up to 3.35 mW. On the other hand, the corresponding relationship for the temperature analysis showed that the phonon lifetime for the in-plane vibrational mode lies within 1.42× to 1.90× that of the out-of-plane vibrational mode over T = 79 K up to 523 K. To provide energy from external stimuli, as T and P were increased, peak broadening in the PL spectra of the A-exciton was observed. From this, a phonon concentration was tabulated using the Urbach formulism, which increased with increasing T and P; consequently, the phonon lifetime was found to decrease. Although phonon lifetime decreased with increasing temperature for all thicknesses, the decay rate in the phonon lifetime in the monolayer (1L) material was found to be 2× lower compared to the bulk. We invoke a harmonic oscillator model to explain the damping mechanism in WSe₂. From this it was determined that the damping coefficient increases with the number of layers. The work reported here sheds fundamental insights into the evolution of phonon dynamics in WSe₂ and should help pave the way for designing high-performance electronic, optoelectronic and thermoelectric devices in the future.

Quick Optical Identification of the Defect Formation in Monolayer WSe2 for Growth Optimization

Bottom-up epitaxy has been widely applied for transition metal dichalcogenides (TMDCs) growth. However, this method usually leads to a high density of defects in the crystal, which limits its optoelectronic performance. Here, we show the effect of growth temperature on the defect formation, optical performance, and crystal stability in monolayer WSe₂ via a combination of Raman and photoluminescence (PL) spectroscopy study. We found that the defect formation and distribution in monolayer WSe₂ are closely related to the growth temperature. These defect density and distribution can be controlled by adjusting the growth temperature. Aging experiments directly demonstrate that these defects are an active center for the decomposition process. Instead, monolayer WSe₂ grown under optimal conditions shows a strong and uniform emission dominated by neutral exciton at room temperature. The results provide an effective approach to optimize TMDCs growth.

In situ exploration of the thermodynamic evolution properties in the type II interface from the WSe2-WS2 lateral heterojunction

The mutual interaction of the type II heterointerface can be very susceptible to the variation of electron states, introducing differences into the band structure and the band alignment in comparison to their pristine states. Here, the thermal evolution of the exciton transition and electronic properties inside the covalently bonded type II interface of the atomically planar WSe₂-WS₂ lateral heterojunction has been studied. With the aid of luminescence and electronic evolution, it was found that the coupling at the heterointerface is strong, and that the change in the photon-electron transition with temperature is weak. Meanwhile, by employing some quantitative computational methods, the temperature variation of the extracted built-in electric field at the interface is unexpectedly pronounced, resulting from the thermodynamical spanning behaviors of the electrons, as well as the strains generated by the difference in the thermal expansion coefficient between the structural lattice. In addition, the electric contact at the interface shows a negative temperature correlation. The present findings provide a vital contribution to the photo-electron interaction-based application and evaluation paths of the electric contact in two-dimensional transition metal dichalcogenide-based devices.

Cross-Plane Carrier Transport in Van der Waals Layered Materials

The mechanisms of carrier transport in the cross-plane crystal orientation of transition metal dichalcogenides are examined. The study of in-plane electronic properties of these van der Waals compounds has been the main research focus in recent years. However, the distinctive physical anisotropies, short-channel physics, and tunability of cross layer interactions can make the study of their electronic properties along the out-of-plane crystal orientation valuable. Here, the out-of-plane carrier transport mechanisms in niobium diselenide and hafnium disulfide are explored as two broadly different representative materials. Temperature-dependent current-voltage measurements are preformed to examine the mechanisms involved. First principles simulations and a tunneling model are used to understand these results and quantify the barrier height and hopping distance properties. Using Raman spectroscopy, the thermal response of the chemical bonds is directly explored and the insight into the van der Waals gap properties is acquired. These results indicate that the distinct cross-plane carrier transport characteristics of the two materials are a result of material thermal properties and thermally mediated transport of carriers through the van der Waals gaps. Exploring the cross-plane electron transport, the exciting physics involved is unraveled and potential new avenues for the electronic applications of van der Waals layers are inspired.

Temperature dependence of phonon properties in CVD MoS2 nanostructures – a statistical approach

In this paper, we report the results of Raman measurements on various molybdenum disulfide (MoS₂) nanostructures grown by the chemical vapor deposition (CVD) method on a typical Si/SiO₂ substrate.

Electron-Phonon Interaction in Organic/2D-Transition Metal Dichalcogenide Heterojunctions: A Temperature-Dependent Raman Spectroscopic Study

The heterojunctions of organic/two-dimensional transition metal dichalcogenides (TMDs) have the potential to be used in the next-generation optoelectronic and photonic devices. Herein, we have systemically investigated the temperature-dependent Raman spectroscopy to elucidate the phonon shift and thermal properties of the semiconducting TMD nanosheets grafted by a conjugated polymer (PG-MoS₂ and PG-MoSe₂) forming heterojunctions. Our results reveal that softening of Raman modes of PG-TMDs as temperature increases from 77 to 300 K is due to the negative temperature coefficient (TC) and anharmonicity. The TCs of E¹ _2g and A_1g modes of PG-MoS₂ nanosheets and A_1g mode of PG-MoSe₂ were found to be -0.015, -0.010, and -0.010 cm^-1 K^-1, respectively. The origin of negative TCs is explained on the basis of a double resonance process, which is more active in single- and few-layer MoS₂ and MoSe₂. Interestingly, the temperature-dependent behavior of the phonon modes of PG-MoS₂ and PG-MoSe₂ is similar to that of pristine nanosheets. Grafting by conjugated polymer does not affect the electron-phonon (e-p) interaction in the semiconducting (2H-phase) TMDs, hinting the application potential of such materials in field-effect electronic devices.

Work Function Modulation of Molybdenum Disulfide Nanosheets by Introducing Systematic Lattice Strain

Tuning the surface electronic properties of 2D transition metal dichalcogenides such as Molebdenum disulfide (MoS₂) nanosheets is worth exploring for their potential applications in strain sensitive flexible electronic devices. Here in, the correlation between tensile strain developed in MoS₂ nanosheets during swift heavy ion irradiation and corresponding modifications in their surface electronic properties is investigated. With prior structural characterization by transmission electron microscopy, chemically exfoliated MoS₂ nanosheets were exposed to 100 MeV Ag ion irradiation at varying fluence for creation of controlled defects. The presence of defect induced systematic tensile strain was verified by Raman spectroscopy and X-ray Diffraction analysis. The effect of ion irradiation on in–plane mode is observed to be significantly higher than that on out-of-plane mode. The contribution of irradiation induced in-plane strain on modification of the surface electronic properties of nanosheets was analyzed by work function measurement using scanning Kelvin probe microscopy. The work function value is observed to be linearly proportional to tensile strain along the basal plane indicating a systematic shifting of Fermi surface with fluence towards the valence band.

Temperature dependent Raman and photoluminescence of vertical WS2/MoS2 monolayer heterostructures

Heterostructures from two-dimensional transition-metal dichalcogenides MX₂ have emerged as a hot topic in recent years due to their various fascinating properties. Here, we investigated the temperature dependent Raman and photoluminescence (PL) spectra in vertical stacked WS₂/MoS₂ monolayer heterostructures. Our result shows that both E¹_2g and A_1g modes of WS₂ and MoS₂ vary linearly with temperature increasing from 300 to 642K. The PL measurement also reveals strong temperature dependencies of the PL intensity and peak position. The activation energy of the thermal quenching of the PL emission has been found to be equal to 69.6meV. The temperature dependence of the peak energy well follows the band-gap shrinkage of bulk semiconductor.

Delineating the role of ripples on the thermal expansion of 2D honeycomb materials: graphene, 2D h-BN and monolayer (ML)-MoS2

Thermally excited ripples are inevitable in 2D crystals, and they can affect the thermophysical properties of these materials significantly. We delineated the role of ripples on the thermal expansion of 2D honeycomb materials using classical molecular dynamics simulations.

Quantitative Analysis of Temperature Dependence of Raman shift of monolayer WS2

We report the temperature-dependent evolution of Raman spectra of monolayer WS₂ directly CVD-grown on a gold foil and then transferred onto quartz substrates over a wide temperature range from 84 to 543 K. The nonlinear temperature dependence of Raman shifts for both and A_1g modes has been observed. The first-order temperature coefficients of Raman shifts are obtained to be −0.0093 (cm⁻¹/K) and −0.0122 (cm⁻¹/K) for and A_1g peaks, respectively. A physical model, including thermal expansion and three- and four-phonon anharmonic effects, is used quantitatively to analyze the observed nonlinear temperature dependence. Thermal expansion coefficient (TEC) of monolayer WS₂ is extracted from the experimental data for the first time. It is found that thermal expansion coefficient of out-plane mode is larger than one of in-plane mode, and TECs of and A_1g modes are temperature-dependent weakly and strongly, respectively. It is also found that the nonlinear temperature dependence of Raman shift of mode mainly originates from the anharmonic effect of three-phonon process, whereas one of A_1g mode is mainly contributed by thermal expansion effect in high temperature region, revealing that thermal expansion effect cannot be ignored.

Excitation mechanism of A1g mode and origin of nonlinear temperature dependence of Raman shift of CVD-grown mono- and few-layer MoS2 films

MoS₂ films are grown on SiO₂/Si substrates by chemical vapor deposition. The vibrational properties of optical phonons of mono-, bi- and multilayer MoS₂ are studied by Raman scattering spectroscopy over temperature range from 90 to 540 K with 514.5 nm and 785 nm lasers. The Raman peaks of E2g1 and A_1g modes are observed simultaneously for mono-, bi- and multilayer MoS₂ with 514.5 nm laser, but only the Raman peak of E2g1 mode is seen for monolayer MoS₂ as 785 nm laser is used, revealing electron-phonon exchange excitation mechanism of A_1g mode for the first time. The Raman shifts of E2g1 and A_1g modes present obvious nonlinear temperature dependence. A semi-quantitative model is used to fit the nonlinear temperature dependence of Raman shifts which matches well to experimental data. Meanwhile, the fitting results reveal that the nonlinear temperature dependence of Raman shifts of E2g1 mode mainly originates from three-phonon anharmonic effect, while one of A_1g mode is contributed by stronger three- and weaker four-phonon anharmonic effects cooperatively but two contributions are comparable in intensity.

Physical vapor deposition synthesis of two-dimensional orthorhombic SnS flakes with strong angle/temperature-dependent Raman responses

Anisotropic layered semiconductors have attracted significant interest due to the huge possibility of bringing new functionalities to thermoelectric, electronic and optoelectronic devices. Currently, most reports on anisotropy have concentrated on black phosphorus and ReS2, less effort has been contributed to other layered materials. In this work, two-dimensional (2D) orthorhombic SnS flakes on a large scale have been successfully synthesized via a simple physical vapor deposition method. Angle-dependent Raman spectroscopy indicated that the orthorhombic SnS flakes possess a strong anisotropic Raman response. Under a parallel-polarization configuration, the peak intensity of Ag (190.7 cm(-1)) Raman mode reaches the maximum when incident light polarization is parallel to the armchair direction of the 2D SnS flakes, which strongly suggests that the Ag (190.7 cm(-1)) mode can be used to determine the crystallographic orientation of the 2D SnS. In addition, temperature-dependent Raman characterization confirmed that the 2D SnS flakes have a higher sensitivity to temperature than graphene, MoS2 and black phosphorus. These results are useful for the future studies of the optical and thermal properties of 2D orthorhombic SnS.

Probing thermal expansion coefficients of monolayers using surface enhanced Raman scattering

A non-destructive method has been proposed to probe thermal expansion coefficients of the monolayer materials using surface-enhanced Raman spectroscopy.

Lighten the Olympia of the Flatland: Probing and Manipulating the Photonic Properties of 2D Transition-Metal Dichalcogenides

Following the adventures of graphene, 2D transition metal dichalcogenides (TMDs) have recently seized part of the territory in the flatland. Branched by different components of metals and chalcogenides, the families of 2D TMDs have grown rapidly, in which the semiconductive ones have shown colorful photonic properties. By tuning the atomic components and reducing the thickness or planar size of the layers, one can manipulate the optical performance of 2D TMDs, e.g., the intensity, angular momentum, and frequency of the emitted light, or toward ultrafast nonlinear absorption. As a powerful optical method, the Raman characteristics of 2D TMDs have been successfully used to explore their lattices and electronic structures. Along with the maturing of 2D TMDs, their hybrids play an important role. The unique photonic properties of 2D van der Waals heterostructures and 2D alloys are introduced here. Apart from the group VI TMDs, future prospects are identified to harness the optical properties of other 2D TMDs and the related investigations of their hybrids are underway.

Temperature dependent phonon shifts in few-layer black phosphorus

Atomically thin two-dimensional (2D) sheets of black phosphorus have attracted much attention due to their potential for future nanoelectronic and photonics device applications. Present investigations deal with the temperature dependent phonon shifts in a few-layer black phosphorus nanosheet sample prepared using micromechanical exfoliation on a 300 nm SiO2/Si substrate. The temperature dependent Raman spectroscopy experiments were carried out on a few-layer black phosphorus sample, which depicts softening of Ag(1), B2g, and Ag(2) modes as temperature increases from 77 to 673 K. The calculated temperature coefficients for Ag(1), B2g, and Ag(2) modes of the few-layer black phosphorus nanosheet sample were observed to be -0.01, -0.013, and -0.014 cm(-1) K(-1), respectively. The temperature dependent softening modes of black phosphorus results were explained on the basis of a double resonance process which is more active in an atomically thin sample. This process can also be fundamentally pertinent in other promising and emerging 2D ultrathin layer and heterostructured materials.

Influence of metal-MoS2 interface on MoS2 transistor performance: comparison of Ag and Ti contacts

In this work, we compare the electrical characteristics of MoS2 field-effect transistors (FETs) with Ag source/drain contacts with those with Ti and demonstrate that the metal-MoS2 interface is crucial to the device performance. MoS2 FETs with Ag contacts show more than 60 times higher ON-state current than those with Ti contacts. In order to better understand the mechanism of the better performance with Ag contacts, 5 nm Au/5 nm Ag (contact layer) or 5 nm Au/5 nm Ti film was deposited onto MoS2 monolayers and few layers, and the topography of metal films was characterized using scanning electron microscopy and atomic force microscopy. The surface morphology shows that, while there exist pinholes in Au/Ti film on MoS2, Au/Ag forms a smoother and denser film. Raman spectroscopy was carried out to investigate the metal-MoS2 interface. The Raman spectra from MoS2 covered with Au/Ag or Au/Ti film reveal that Ag or Ti is in direct contact with MoS2. Our findings show that the smoother and denser Au/Ag contacts lead to higher carrier transport efficiency.

Synthesis and defect investigation of two-dimensional molybdenum disulfide atomic layers

CONSPECTUS: The unique physical properties of two-dimensional (2D) molybdenum disulfide (MoS2) and its promising applications in future optoelectronics have motivated an extensive study of its physical properties. However, a major limiting factor in investigation of 2D MoS2 is its large area and high quality preparation. The existence of various types of defects in MoS2 also makes the characterization of defect types and the understanding of their roles in the physical properties of this material of critical importance. In this Account, we review the progress in the development of synthetic approaches for preparation of 2D MoS2 and the understanding of the role of defects in its electronic and optical properties. We first examine our research efforts in understanding exfoliation, direct sulfurization, and chemical vapor deposition (CVD) of MoS2 monolayers as main approaches for preparation of such atomic layers. Recognizing that a natural consequence of the synthetic approaches is the addition of sources of defects, we initially focus on identifying these imperfections with intrinsic and extrinsic origins in CVD MoS2. We reveal the predominant types of point and grain boundary defects in the crystal structure of polycrystalline MoS2 using transmission electron microscopy (TEM) and understand how they modify the electronic band structure of this material using first-principles-calculations. Our observations and calculations reveal the main types of vacancy defects, substitutional defects, and dislocation cores at the grain boundaries (GBs) of MoS2. Since the sources of defects in two-dimensional atomic layers can, in principle, be controlled and studied with more precision compared with their bulk counterparts, understanding their roles in the physical properties of this material may provide opportunities for changing their properties. Therefore, we next examine the general electronic properties of single-crystalline 2D MoS2 and study the role of GBs in the electrical transport and photoluminescence properties of its polycrystalline counterparts. These results reveal the important role played by point defects and GBs in affecting charge carrier mobility and excitonic properties of these atomic layers. In addition to the intrinsic defects, growth process induced substrate impurities and strain induced band structure perturbations are revealed as major sources of disorder in CVD grown 2D MoS2. We further explore substrate defects for modification and control of electronic and optical properties of 2D MoS2 through interface engineering. Self-assembled monolayer based interface modification, as a versatile technique adaptable to different conventional and flexible substrates, is used to promote significant tunability in the key MoS2 field-effect device parameters. This approach provides a powerful tool for modification of native substrate defect characteristics and allows for a wide range of property modulations. Our results signify the role of intrinsic and extrinsic defects in the physical properties of MoS2 and unveil strategies that can utilize these characteristics.

Plasmonic pumping of excitonic photoluminescence in hybrid MoS2-Au nanostructures

We report on the fabrication of monolayer MoS2-coated gold nanoantennas combining chemical vapor deposition, e-beam lithography surface patterning, and a soft lift-off/transfer technique. The optical properties of these hybrid plasmonic-excitonic nanostructures are investigated using spatially resolved photoluminescence spectroscopy. Off- and in-resonance plasmonic pumping of the MoS2 excitonic luminescence showed distinct behaviors. For plasmonically mediated pumping, we found a significant enhancement (∼65%) of the photoluminescence intensity, clear evidence that the optical properties of the MoS2 monolayer are strongly influenced by the nanoantenna surface plasmons. In addition, a systematic photoluminescence broadening and red-shift in nanoantenna locations is observed which is interpreted in terms of plasmonic enhanced optical absorption and subsequent heating of the MoS2 monolayers. Using a temperature calibration procedure based on photoluminescence spectral characteristics, we were able to estimate the local temperature changes. We found that the plasmonically induced MoS2 temperature increase is nearly four times larger than in the MoS2 reference temperatures. This study shines light on the plasmonic-excitonic interaction in these hybrid metal/semiconductor nanostructures and provides a unique approach for the engineering of optoelectronic devices based on the light-to-current conversion.

Electrical transport properties of polycrystalline monolayer molybdenum disulfide

Semiconducting MoS2 monolayers have shown many promising electrical properties, and the inevitable polycrystallinity in synthetic, large-area films renders understanding the effect of structural defects, such as grain boundaries (GBs, or line-defects in two-dimensional materials), essential. In this work, we first examine the role of GBs in the electrical-transport properties of MoS2 monolayers with varying line-defect densities. We reveal a systematic degradation of electrical characteristics as the line-defect density increases. The two common MoS2 GB types and their specific roles are further examined, and we find that only tilt GBs have a considerable effect on the MoS2 electrical properties. By examining the electronic states and sources of disorder using temperature-dependent transport studies, we adopt the Anderson model for disordered systems to explain the observed transport behaviors in different temperature regimes. Our results elucidate the roles played by GBs in different scenarios and give insights into their underlying scattering mechanisms.

Dependence of coupling of quasi 2-D MoS2 with substrates on substrate types, probed by temperature dependent Raman scattering

This work reports a study on the temperature dependence of in-plane E and out-of-plane A1g Raman modes of single-layer (1L) and bi-layer (2L) MoS2 films on sapphire (epitaxial) and SiO2 (transferred) substrates as well as bulk MoS2 single crystals in a temperature range of 25-500 °C. For the films on the transferred SiO2 substrate, the in-plane E mode is only weakly affected by the substrate, whereas the out-of-plane A1g mode is strongly perturbed, showing highly nonlinear, sometimes even non-monotonic, temperature dependence on the Raman peak shift and linewidth. In contrast, for the films on the epitaxial sapphire substrate, E is affected more significantly by the substrate than A1g. This study suggests that the 2-D film-substrate coupling depends sensitively on the preparation method, and in particular on the film morphology for the transferred film. These findings are vitally important for the fundamental understanding and application of this quasi 2-D material that is expected to be supported by a substrate in most circumstances.

Temperature dependent phonon shifts in single-layer WS(2)

Atomically thin two-dimensional tungsten disulfide (WS2) sheets have attracted much attention due to their potential for future nanoelectronic device applications. We report first experimental investigation on temperature dependent Raman spectra of single-layer WS2 prepared using micromechanical exfoliation. Our temperature dependent Raman spectroscopy results shows that the E(1)2g and A1g modes of single-layer WS2 soften as temperature increases from 77 to 623 K. The calculated temperature coefficients of the frequencies of 2LA(M), E(1)2g, A1g, and A1g(M) + LA(M) modes of single-layer WS2 were observed to be -0.008, -0.006, -0.006, and -0.01 cm(-1) K(-1), respectively. The results were explained in terms of a double resonance process which is active in atomically thin nanosheet. This process can also be largely applicable in other emerging single-layer materials.