Thermal anisotropy in nano-crystalline MoS2 thin films

Grain boundary and misorientation angle-dependent thermal transport in single-layer MoS2

Grain boundaries (GBs) are inevitable defects in large-area MoS₂ samples but they play a key role in their properties, however, the influence of grain misorientation on thermal transport has largely remained unknown. Here, the critical role of misorientation angle in thermal transport characteristics across 5|7 polar dislocation-dominated GBs in monolayer MoS₂ is explored using nonequilibrium molecular dynamics simulations. Results show that thermal transport characteristics of defective GBs are greatly dictated by the misorientation angle, with "U"-shaped thermal conductance as misorientation angle varying from around 5.06-52.26°, as well as by GB energy, 5|7 dislocation type and the grain size. Such unique thermal transport across GBs is primarily attributed to rising phonon-boundary softening and scattering with increasing dislocation density at GBs or GB energy, as well as an increase in localized phonon modes. The study establishes the fundamental relationship between GB and the thermal properties of single-layer MoS₂ and highlights the vital role of GBs in designing efficient thermoelectric and thermal management transition metal dichalcogenides.

Anisotropic electron-photon-phonon coupling in layered MoS2

Transition metal dichalcogenide, MoS2has attracted a lot of attention recently owing to its tunable visible range band gap, and anisotropic electronic and transport properties. Here, we report comprehensive inelastic light scattering measurements on both chemical vapor deposition grown (horizontally and vertically aligned) flakes, and mechanically exfoliated flakes of single crystal MoS2. We probe the anisotropic optical response by studying the polarization dependence intensity of the Raman active phonon modes as a function of different incident photon energy and flake thickness. Our polarization dependent Raman studies reveal strong anisotropic behavior reflected in the anomalous renormalization of the modes intensity as a function of flake thickness, phonons and photon energy. Our observations reflect the strong anisotropic light-matter interaction in this high crystalline symmetric layered MoS2system, especially for the in-plane vibrations, crucial for understanding as well as future applications of these materials.

Experimental Study on Thermal Conductivity and Rectification in Suspended Monolayer MoS2

Thermal rectification is an attractive phenomenon for thermal management, which refers to a specific behavior in a heat transfer system where heat flow in one direction is stronger than that in the opposite direction under the same conditions. Two-dimensional monolayer molybdenum disulfide (MoS₂) synthesized by chemical vapor deposition (CVD) has exhibited exceptional thermal, optical, and electrical properties due to its special structure; however, the thermal rectification in monolayer MoS₂ is still not achieved by experimental measurement. Here, we successfully transferred monolayer MoS₂ samples with three geometrical morphologies to the suspended microelectrodes by the PMMA approach. Through further heating the suspended microelectrodes with AC power in the opposite directions of these three monolayer MoS₂ samples, we experimentally measured the thermal conductivity and first obtained the thermal rectification of monolayer MoS₂. The rectification coefficients of monolayer MoS₂ with three different geometrical morphologies are 10-13, 11-4, and 69-70%. Moreover, a theoretical model was also applied to discuss the dependence of thermal rectification on the geometrical asymmetry (angle and spacing). The results demonstrate that the monolayer MoS₂ has an obvious thermal rectification phenomenon owing to the asymmetric structure, and it would have great potentials in the application of thermal energy control and management.

Thermal properties of thin films made from MoS2 nanoflakes and probed via statistical optothermal Raman method

A deep understanding of the thermal properties of 2D materials is crucial to their implementation in electronic and optoelectronic devices. In this study, we investigated the macroscopic in-plane thermal conductivity (κ) and thermal interface conductance (g) of large-area (mm2) thin film made from MoS2 nanoflakes via liquid exfoliation and deposited on Si/SiO2 substrate. We found κ and g to be 1.5 W/mK and 0.23 MW/m2K, respectively. These values are much lower than those of single flakes. This difference shows the effects of interconnections between individual flakes on macroscopic thin film parameters. The properties of a Gaussian laser beam and statistical optothermal Raman mapping were used to obtain sample parameters and significantly improve measurement accuracy. This work demonstrates how to address crucial stability issues in light-sensitive materials and can be used to understand heat management in MoS2 and other 2D flake-based thin films.

Photonic crystallization of two-dimensional MoS2 for stretchable photodetectors

Low temperature synthesis of high quality two-dimensional (2D) materials directly on flexible substrates remains a fundamental limitation towards scalable realization of robust flexible electronics possessing the unique physical properties of atomically thin structures. Herein, we describe room temperature sputtering of uniform, stoichiometric amorphous MoS2 and subsequent large area (>6.25 cm2) photonic crystallization of 5 nm 2H-MoS2 films in air to enable direct, scalable fabrication of ultrathin 2D photodetectors on stretchable polydimethylsiloxane (PDMS) substrates. The lateral photodetector devices demonstrate an average responsivity of 2.52 μW A-1 and a minimum response time of 120 ms under 515.6 nm illumination. Additionally, the surface wrinkled, or buckled, PDMS substrate with conformal MoS2 retained the photoconductive behavior at tensile strains as high as 5.72% and over 1000 stretching cycles. The results indicate that the photonic crystallization method provides a significant advancement in incorporating high quality semiconducting 2D materials applied directly on polymer substrates for wearable and flexible electronic systems.

2D MoS2 -Based Nanomaterials for Therapeutic, Bioimaging, and Biosensing Applications

Molybdenum disulfide (MoS₂ ), a typical layered 2D transition metal dichalcogenide, has received colossal interest in the past few years due to its unique structural, physicochemical, optical, and biological properties. While MoS₂ is mostly applied in traditional industries such as dry lubricants, intercalation agents, and negative electrode material in lithium-ion batteries, its 2D and 0D forms have led to diverse applications in sensing, catalysis, therapy, and imaging. Herein, a systematic overview of the progress that is made in the field of MoS₂ research with an emphasis on its different biomedical applications is presented. This article provides a general discussion on the basic structure and property of MoS₂ and gives a detailed description of its different morphologies that are synthesized so far, namely, nanosheets, nanotubes, and quantum dots along with synthesis strategies. The biomedical applications of MoS₂ -based nanocomposites are also described in detail and categorically, such as in varied therapeutic and diagnostic modalities like drug delivery, gene delivery, phototherapy, combined therapy, bioimaging, theranostics, and biosensing. Finally, a brief commentary on the current challenges and limitations being faced is provided, along with a discussion of some future perspectives for the overall improvement of MoS₂ -based nanocomposites as a potential nanomedicine.

Record Low Thermal Conductivity of Polycrystalline MoS2 Films: Tuning the Thermal Conductivity by Grain Orientation

We report a record low thermal conductivity in polycrystalline MoS₂ obtained for ultrathin films with varying grain sizes and orientations. By optimizing the sulfurization parameters of nanometer-thick Mo layers, five MoS₂ films containing a combination of horizontally and vertically oriented grains, with respect to the bulk (001) monocrystal, were grown. From transmission electron microscopy, the average grain size, typically below 10 nm, and proportion of differently oriented grains were extracted. The thermal conductivity of the suspended samples was extracted from a Raman laser-power-dependent study, and the lowest value of thermal conductivity of 0.27 W m^-1 K^-1, which reaches a similar value as that of Teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains in similar proportion. Analysis by means of molecular dynamics and finite element method simulations confirm that such a grain arrangement leads to lower grain boundary conductance. We discuss the possible use of these thermal insulating films in the context of electronics and thermoelectricity.

Tuning thermal conductivity in molybdenum disulfide by electrochemical intercalation

Thermal conductivity of two-dimensional (2D) materials is of interest for energy storage, nanoelectronics and optoelectronics. Here, we report that the thermal conductivity of molybdenum disulfide can be modified by electrochemical intercalation. We observe distinct behaviour for thin films with vertically aligned basal planes and natural bulk crystals with basal planes aligned parallel to the surface. The thermal conductivity is measured as a function of the degree of lithiation, using time-domain thermoreflectance. The change of thermal conductivity correlates with the lithiation-induced structural and compositional disorder. We further show that the ratio of the in-plane to through-plane thermal conductivity of bulk crystal is enhanced by the disorder. These results suggest that stacking disorder and mixture of phases is an effective mechanism to modify the anisotropic thermal conductivity of 2D materials.

Distinctive in-Plane Cleavage Behaviors of Two-Dimensional Layered Materials

Mechanical exfoliation from bulk layered crystal is widely used for preparing two-dimensional (2D) layered materials, which involves not only out-of-plane interlayer cleavage but also in-plane fracture. Through a statistical analysis on the exfoliated 2D flakes, we reveal the in-plane cleavage behaviors of six representative layered materials, including graphene, h-BN, 2H phase MoS2, 1T phase PtS2, FePS3, and black phosphorus. In addition to the well-known interlayer cleavage, these 2D layered materials show a distinctive tendency to fracture along certain in-plane crystallography orientations. With theoretical modeling and analysis, these distinct in-plane cleavage behaviors can be understood as a result of the competition between the release of the elastic energy and the increase of the surface energy during the fracture process. More importantly, these in-plane cleavage behaviors provide a fast and noninvasive method using optical microscopy to identify the lattice direction of mechanical exfoliated 2D layered materials.

Large-area, continuous and high electrical performances of bilayer to few layers MoS2 fabricated by RF sputtering via post-deposition annealing method

We report a simple and mass-scalable approach for thin MoS2 films via RF sputtering combined with the post-deposition annealing process. We have prepared as-sputtered film using a MoS2 target in the sputtering system. The as-sputtered film was subjected to post-deposition annealing to improve crystalline quality at 700 °C in a sulfur and argon environment. The analysis confirmed the growth of continuous bilayer to few-layer MoS2 film. The mobility value of ~29 cm(2)/Vs and current on/off ratio on the order of ~10(4) were obtained for bilayer MoS2. The mobility increased up to ~173-181 cm(2)/Vs, respectively, for few-layer MoS2. The mobility of our bilayer MoS2 FETs is larger than any previously reported values of single to bilayer MoS2 grown on SiO2/Si substrate with a SiO2 gate oxide. Moreover, our few-layer MoS2 FETs exhibited the highest mobility value ever reported for any MoS2 FETs with a SiO2 gate oxide. It is presumed that the high mobility behavior of our film could be attributed to low charged impurities of our film and dielectric screening effect by an interfacial MoOxSiy layer. The combined preparation route of RF sputtering and post-deposition annealing process opens up the novel possibility of mass and batch production of MoS2 film.

Anisotropy Enhancement of Thermal Energy Transport in Supported Black Phosphorene

Thermal anisotropy along the basal plane of materials possesses both theoretical importance and application value in thermal transport and thermoelectricity. Though common two-dimensional materials may exhibit in-plane thermal anisotropy when suspended, thermal anisotropy would often disappear when supported on a substrate. In this Letter, we find a strong anisotropy enhancement of thermal energy transport in supported black phosphorene. The chiral preference of energy transport in the zigzag rather than the armchair direction is greatly enhanced by coupling to the substrate, up to a factor of approximately 2-fold compared to the suspended one. The enhancement originates from its puckered lattice structure, where the nonplanar armchair energy transport relies on the out-of-plane corrugation and thus would be hindered by the flexural suppression due to the substrate, while the planar zigzag energy transport is not. As a result, thermal conductivity of supported black phosphorene shows a consistent anisotropy enhancement under different temperatures and substrate coupling strengths.

Hydrogenation of Penta-Graphene Leads to Unexpected Large Improvement in Thermal Conductivity

Penta-graphene (PG) has been identified as a novel two-dimensional (2D) material with an intrinsic bandgap, which makes it especially promising for electronics applications. In this work, we use first-principles lattice dynamics and iterative solution of the phonon Boltzmann transport equation (BTE) to determine the thermal conductivity of PG and its more stable derivative, hydrogenated penta-graphene (HPG). As a comparison, we also studied the effect of hydrogenation on graphene thermal conductivity. In contrast to hydrogenation of graphene, which leads to a dramatic decrease in thermal conductivity, HPG shows a notable increase in thermal conductivity, which is much higher than that of PG. Considering the necessity of using the same thickness when comparing thermal conductivity values of different 2D materials, hydrogenation leads to a 63% reduction in thermal conductivity for graphene, while it results in a 76% increase for PG. The high thermal conductivity of HPG makes it more thermally conductive than most other semiconducting 2D materials, such as the transition metal chalcogenides. Our detailed analyses show that the primary reason for the counterintuitive hydrogenation-induced thermal conductivity enhancement is the weaker bond anharmonicity in HPG than PG. This leads to weaker phonon scattering after hydrogenation, despite the increase in the phonon scattering phase space. The high thermal conductivity of HPG may inspire intensive research around HPG and other derivatives of PG as potential materials for future nanoelectronic devices. The fundamental physics understood from this study may open up a new strategy to engineer thermal transport properties of other 2D materials by controlling bond anharmonicity via functionalization.

Epitaxial Growth of Lattice-Mismatched Core-Shell TiO2 @MoS2 for Enhanced Lithium-Ion Storage

Core-shell structured nanohybrids are currently of significant interest due to their synergetic properties and enhanced performances. However, the restriction of lattice mismatch remains a severe obstacle for heterogrowth of various core-shells with two distinct crystal structures. Herein, a controlled synthesis of lattice-mismatched core-shell TiO2 @MoS2 nano-onion heterostructures is successfully developed, using unilamellar Ti0.87 O2 nanosheets as the starting material and the subsequent epitaxial growth of MoS2 on TiO2 . The formation of these core-shell nano-onions is attributed to an amorphous layer-induced heterogrowth mechanism. The number of MoS2 layers can be well tuned from few to over ten layers, enabling layer-dependent synergistic effects. The core-shell TiO2 @MoS2 nano-onion heterostructures exhibit significantly enhanced energy storage performance as lithium-ion battery anodes. The approach has also been extended to other lattice-mismatched systems such as TiO2 @MoSe2 , thus suggesting a new strategy for the growth of well-designed lattice-mismatched core-shell structures.

Site Selective Doping of Ultrathin Metal Dichalcogenides by Laser-Assisted Reaction

Laser-assisted phosphorus doping is demonstrated on ultrathin transition-metal dichalcogenides (TMDCs) including n-type MoS2 and p-type WSe2 . Temporal and spatial control of the doping is achieved by varying the laser irradiation power and time, demonstrating wide tunability and high site selectivity with high stability. The laser-assisted doping method may enable a new avenue for functionalizing TMDCs for customized nanodevice applications.

Two dimensional atomically thin MoS2 nanosheets and their sensing applications

The extraordinary properties of layered graphene and its successful applications in electronics, sensors, and energy devices have inspired and renewed interest in other two-dimensional (2D) layered materials. Particularly, a semiconducting analogue of graphene, molybdenum disulfide (MoS2), has attracted huge attention in the last few years. With efforts in exfoliation and synthetic techniques, atomically thin films of MoS2 (single- and few-layer) have been recently prepared and characterized. 2D MoS2 nanosheets have properties that are distinct and complementary to those of graphene, making it more appealing for various applications. Unlike graphene with an indirect bandgap, the direct bandgap of single-layer MoS2 results in better semiconductor behavior as well as photoluminescence, suggesting its great suitability for electronic and optoelectronic applications. Compared to their applications in energy storage and optoelectronic devices, the use of MoS2 nanosheets as a sensing platform, especially for biosensing, is still largely unexplored. Here, we present a review of the preparation of 2D atomically thin MoS2 nanosheets, with an emphasis on their use in various sensing applications.

Measurement of Lateral and Interfacial Thermal Conductivity of Single- and Bilayer MoS2 and MoSe2 Using Refined Optothermal Raman Technique

Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides (TMDCs) have attracted extensive interest in recent years, motivating investigation into multiple properties. In this work, we demonstrate a refined version of the optothermal Raman technique to measure the thermal transport properties of two TMDC materials, MoS2 and MoSe2, in single-layer (1L) and bilayer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ∼40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different radial positions. For 1L MoS2 and MoSe2, the room-temperature thermal conductivities are 84 ± 17 and 59 ± 18 W/(m·K), respectively. For 2L MoS2 and MoSe2, we obtain values of 77 ± 25 W and 42 ± 13 W/(m·K). Crucially, the interfacial thermal conductance is found to be of order 0.1-1 MW/m(2) K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices.

Photo-sensitivity of large area physical vapor deposited mono and bilayer MoS2

We present photosensitivity in large area physical vapour deposited mono and bi-layer MoS2 films. Photo-voltaic effect was observed in single layer MoS2 without any apparent rectifying junctions, making device fabrication straightforward. For bi-layers, no such effect was present, suggesting strong size effect in light-matter interaction. The photo-voltaic effect was observed to highly direction dependent in the film plane, which suggests that the oblique deposition configuration plays a key role in developing the rectifying potential gradient. To the best of our knowledge, this is the first report of any large area and transfer free MoS2 photo device with performance comparable to their exfoliated counterparts.