Two-dimensional layered materials: Structure, properties, and prospects for device applications

Thermal release tape assisted mechanical exfoliation of pristine TMD and the performance of the exfoliated TMD saturable absorbers for Q-switched laser generation

Although microscopic techniques have been used to characterize transition metal dichalcogenides (TMDs), direct observation of charge carrier dynamics distribution in TMDs with diverse shapes remains unexplored. Herein, ultrafast pump–probe microscopy (UPPM) is employed to reveal the carrier dynamics distribution in molybdenum disulfide (MoS2) and tungsten disulfide (WS2) monolayer of four shapes: triangular (t‐MoS2), curved triangular (c‐MoS2), triangular (t‐WS2), and hexagonal (h‐WS2). Monitoring the photon transmission T at 1.55 eV after pumping with a photon energy of 3.1 eV, a negative ΔT/T occurs in t‐MoS2 and c‐MoS2, while a positive ΔT/T is detected in t‐WS2 and h‐WS2 after 3–7 ps time evolution. This distinctive behavior is attributed to deep/shallow defects below the conduction band minimum (CBM) in MoS2 and WS2. Spatial‐independent ΔT/T is observed in t‐MoS2 and t‐WS2, while the ΔT/T in c‐MoS2 has a rapid decay of photoexcited carriers at the vertices and curved edges. Additionally, a threefold symmetry of ΔT/T is revealed in h‐WS2, attributed to the dissimilar occupation of defect states near the h‐WS2 CBM. This work paves the way for examining charge carrier dynamics of various shapes of TMDs and provides a unique microscopic method for studying the charge carrier dynamics in emerging TMDs heterostructures.

Two-dimensional layered materials such as graphene and transition metal dichalcogenides have gained a lot of attention because of their distinctive chemical, optical, and electronic properties. Transition metal dichalcogenides can have a tunable bandgap in the range of 1 − 3 eV (visible), which enables their applications in ultrathin field effect transistors, photovoltaics, sensors, and optoelectronic devices. They exhibit an indirect-to-direct band gap transition when thinned down from bulk to a single layer. They show strong photoluminescence, electron–photon interaction, valley pseudospin, nonlinear optical response, and lack of dangling bonds. Many exotic phenomena appear when materials are thinned to nanoscale size because of the quantum confinement effect and the symmetry changes in lower dimensions. The stacking of two-dimensional transitional metal dichalcogenides with a controlled twist angle creates “van der Waals” heterostructures, which has opened a pandora box that allows us to explore new quantum properties. These “van der Waals” structures can host Mott insulating states, topological states, and correlated electron physics. In this dissertation, to explore these intriguing properties, I focused on fabricating bilayers of tungsten disulfide (WS2) with a controlled twist angle. The new transfer method can minimize trapped interfacial impurities, chemical residue cracks, or mechanical damage during the transfer process. Furthermore, I studied the electronic and optical properties of stacked bilayers and the role of twist angle in tuning these properties.

There has been a significant rise in the use of stainless steel structures, especially because of its improved corrosion resistance properties in natural water when compared to carbon steels. However, also there has been reports of corrosion failures in these steels especially at weld points and other heat affected zones, some of which can have large scale socio-economic consequences. Coatings with nanomaterials have been proposed to stabilize the surface and prevent the failure. 2 dimensional (2D) materials e.g. graphene has recently elicited a lot of interest due to their improved corrosion resistance properties. This work investigates the corrosion resistance properties of a commercial grade stainless steel in 3.5 wt. % aqueous NaCl solution, coated with 3 different types of 2D nanomaterials; molybdenum disulfide (MoS2), boron nitride (BN) and tungsten disulfide (WS2). Coatings of these materials on the tested stainless steel were made via drop casting method from a previously prepared slurry of each. The electrochemical behavior of the coated stainless steel was studied with bare stainless steel as control, using open circuit potential and Tafel plots. The MoS2 coated stainless steel displayed a superior corrosion resistance than the BN and WS2 coated stainless steel samples. Scanning electron microscopy (SEM) images demonstrate uniform coating for the MoS2 coated stainless steel sample, which provides a reduced number of defect sites for diffusion of corrosive ions and explains its superior corrosion resistance compared to the other coated samples It is postulated that MoS2 showed a higher corrosion resistance vis-à-vis the bare sample due to its higher open circuit potential as well as a lower corrosion rate.

Three-dimensional (3D) III-V semiconductors including indium pnictides are widely used in optoelectronic devices, such as light-emitting diodes, laser diodes and photodetectors, in their bulk or thin-film geometries. On the other hand, two-dimensional (2D) atomic crystals such as graphene, phosphorene, and transition metal dichalcogenides are promising candidates for next generation optoelectronic technologies. Here, we designed a type of III-V indium pnictide 2D material that can be exfoliated and rebuilt from bulk wurtzite structures, which show benign stability and intriguing physical properties, including in-plane ferroelectricity/antiferroelectricity with low transition barriers (0.01-0.31 eV f.u.-1), direct/quasi-direct band gaps (HSE + SOC: 1.498-2.852 eV), ferroelasticity (2.86-11.90% elastic deformation), switchable hidden spin polarization and spin splitting (31 meV), as well as controllable in-plane negative Poisson's ratio (∼-0.51). Our study suggests a new class of optoelectronic materials that combines the advantages of the well-studied 3D III-V semiconductors and 2D atomic crystals and offers a platform to study the interplay of optoelectronic properties with multiferroic, spintronic, and mechanical properties for the development of miniaturized multifunctional optoelectronic devices.

Recently emerged layered transition metal dichalcogenides have attracted great interest due to their intriguing fundamental physical properties and potential applications in optoelectronics. Using scattering-type scanning near-field optical microscope (s-SNOM) and theoretical modeling, we study propagating surface waves in the visible spectral range that are excited at sharp edges of layered transition metal dichalcogenides (TMDC) such as molybdenum disulfide and tungsten diselenide. These surface waves form fringes in s-SNOM measurements. By measuring how the fringes change when the sample is rotated with respect to the incident beam, we obtain evidence that exfoliated MoS2 on a silicon substrate supports two types of Zenneck surface waves that are predicted to exist in materials with large real and imaginary parts of the permittivity. We have compared MoS2 interference fringes with those formed on layered insulator such as hexagonal boron nitride where only leaky modes are possible due to its small permittivity. Interpretation of experimental data is supported by theoretical models. Our results could pave the way to the investigation of surface waves on TMDCs and other van der Waals materials and their novel photonics applications.

CONSPECTUS: After the discovery of graphene and the development of powerful exfoliation techniques, experimental preparation of two-dimensional (2D) crystals can be expected for any layered material that is known to chemistry. Besides graphene and hexagonal boron nitride (h-BN), transition metal chalcogenides (TMC) are among the most studied ultrathin materials. In particular, single-layer MoS2, a direct band gap semiconductor with ∼1.9 eV energy gap, is popular in physics and nanoelectronics, because it nicely complements semimetallic graphene and insulating h-BN monolayer as a construction component for flexible 2D electronics and because it was already successfully applied in the laboratory as basis material for transistors and other electronic and optoelectronic devices. Two-dimensional crystals are subject to significant quantum confinement: compared with their parent layered 3D material, they show different structural, electronic, and optical properties, such as spontaneous rippling as free-standing monolayer, significant changes of the electronic band structure, giant spin-orbit splitting, and enhanced photoluminescence. Most of those properties are intrinsic for the monolayer and already absent for two-layer stacks of the same 2D crystal. For example, single-layer MoS2 is a direct band gap semiconductor with spin-orbit splitting of 150 meV in the valence band, while the bilayer of the same material is an indirect band gap semiconductor without observable spin-orbit splitting. All these properties have been observed experimentally and are in excellent agreement with calculations based on density-functional theory. This Account reports theoretical studies of a subgroup of transition metal dichalcogenides with the composition MX2, with M = Mo, or W and X = Se or S, also referred to as "MoWSeS materials". Results on the electronic structure, quantum confinement, spin-orbit coupling, spontaneous monolayer rippling, and change of electronic properties in the presence of an external electric field are reported. While all materials of the MoWSeS family share the same qualitative properties, their individual values can differ strongly, for example, the spin-orbit splitting in WSe2 reaches the value of 428 meV, nearly three times that of MoS2. Further, we discuss the effect of strain on the electronic properties (straintronics). While MoWSeS single layers are very robust against external electric fields, bilayers show a linear reduction of the band gap, even reaching a semiconductor-metal phase transition, and an increase of the spin-orbit splitting from zero to the monolayer value at rather small fields. Strain is yet another possibility to control the band gap in a linear way, and MoWSeS monolayers become metallic at strain values of ∼10%. The density-functional based tight-binding model is a useful tool to investigate the electronic and structural properties, including electron conductance, of large MoS2 structures, which show spontaneous rippling in finite-temperature molecular dynamics simulations. Structural defects in MoS2 result in anisotropy of the electric conductivity. Finally, DFT predictions on the properties of noble metal dichalcogenides are presented. Most strikingly, 1T PdS2 is an indirect band gap semiconductor in its monolayer form but becomes metallic as a bilayer.

Monolayer transition metal dichalcogenides (TMDs) exhibit high nonlinear optical (NLO) susceptibilities. Experiments on MoS$_2$ have indeed revealed very large second-order ($\chi^{(2)}$) and third-order ($\chi^{(3)}$) optical susceptibilities. However, third harmonic generation results of other layered TMDs has not been reported. Furthermore, the reported $\chi^{(2)}$ and $\chi^{(3)}$ of MoS$_2$ vary by several orders of magnitude, and a reliable quantitative comparison of optical nonlinearities across different TMDs has remained elusive. Here, we investigate second- and third-harmonic generation, and three-photon photoluminescence in TMDs. Specifically, we present an experimental study of $\chi^{(2)}$, and $\chi^{(3)}$ of four common TMD materials (\ce{MoS2}, \ce{MoSe2}, \ce{WS2} and \ce{WSe2}) by placing different TMD flakes in close proximity to each other on a common substrate, allowing their NLO properties to be accurately obtained from a single measurement. $\chi^{(2)}$ and $\chi^{(3)}$ of the four monolayer TMDs have been compared, indicating that they exhibit distinct NLO responses. We further present theoretical simulations of these susceptibilities in qualitative agreement with the measurements. Our comparative studies of the NLO responses of different two-dimensional layered materials allow us to select the best candidates for atomic-scale nonlinear photonic applications, such as frequency conversion and all-optical signal processing.

Triboelectric nanogenerators (TENGs) are advanced energy harvesters that convert mechanical energy in diverse environments into electrical energy via static electrification and electrostatic induction. However, their performance needs to be improved to expand their area of use and become more practical. In this study, we introduced molybdenum disulfide (MoS2) and tungsten disulfide (WS2) as separate additives into polyacrylonitrile (PAN) to produce composite nanofibers via electrospinning, aiming to increase the electrical output of the TENGs. This method increased contact area by narrowing the nanofiber diameters, which is a key factor in enhancing the triboelectric effect. The incorporation of MoS2 or WS2, characterized by high specific surface area, interface polarization, quantum confinement effects, and strong electron acceptance and trapping capabilities, led to a significant increase in the dielectric constant and overall electrical performance of the TENGs. Experimental evaluations, connecting the TENGs to circuits with various resistive loads, determined optimal performance at a load resistance of 4.7 MΩ. In particular, the 5 wt% WS2@PAN & polyvinylbutral (PVB) and 5 wt% MoS2@PAN & PVB TENGs exhibited a remarkable peak power output of 40.5 mW, corresponding to a power density of 25.3 W/m2 and provided open circuit voltage of 1,026 V. The integration of 5 wt% MoS2 or 5 wt% WS2 led to more than a twofold increase in electrical power density compared to pristine PAN. These outcomes demonstrate the significant impact of transition metal dichalcogenides in enhancing the energy conversion efficiency of contact–separation mode TENGs, thereby contributing to the advancement of energy harvesting technology.

The fascinating two-dimensional (2D) materials are being unconsciously applied in various fields from science to engineering, which is benefited from the glamorous physical and chemical properties of mechanics, optics, electronics, and magnetism. The representative 2D thermoelectric/piezoelectric materials can directly convert thermal/mechanical energy into electrical energy, which can resolve the energy issues and avoid further environmental deterioration. The thermoelectric or piezoelectric properties of various 2D materials, such as graphene, hexagonal boron nitride, black phosphorus, transition metal dichalcogenides (TMDs), arsenene, metal carbides and nitrides (MXenes), and so on, have been investigated in detail. Although tremendous progress has been achieved in the past few years, these properties still need to be improved for their practical application by designing new 2D materials, strain engineering, chemical functionalization, etc. In addition to this, in 2D materials, there are many other novel physical properties, such as magnetism, topology, valley, and so on. The combination of thermoelectricity/piezoelectricity with other unique properties may lead to novel device applications or scientific breakthroughs in new physics. Overall, the emergence of 2D thermoelectric and piezoelectric materials has expanded energy conversion research dramatically. By combing this new device concept with the novel 2D materials, original devices should have potential applications in energy harvesting.

Two-dimensional layered materials[1] have been of great interest due to their unique physico-chemical properties and wide range of applications. Wide interest in exfoliated 2D layered materials such as graphene[2] and transition metal dichalcogenides (TMDs) motivated scientists to explore similar layered materials and, phosphorene, a layered counterpart of black phosphorous[3], has attracted lot of attention in recent times. The high air sensitivity and chemical reactivity of mono/few-layered phosphorene remain a big challenge in its synthesis and further applications. Here, a novel strategy for controlled synthesis and in situ surface functionalization of phosphorene quantum dots (PQDs) using a single step electrochemical exfoliation process from bulk black phosphorous crystals is demonstrated. Further, our results prove promising potential of phosphorenes as technologically viable OER electrodes for water splitting devices. Keywords: Phosphorene; Electrochemical synthesis; Oxygen Evolution Reaction References [1] S. Z. Butler, S. M. Hollen, L. Cao, Y. Cui, J. A. Gupta, H. R. Gutiérrez, T. F. Heinz, S. S. Hong, J. Huang, A. F. Ismach, E. Johnston-Halperin, M. Kuno, V. V. Plashnitsa, R. D. Robinson, R. S. Ruoff, S. Salahuddin, J. Shan, L. Shi, M. G. Spencer, M. Terrones, W. Windl, J. E. Goldberger, ACS Nano 2013, 7, 2898-2926. [2] A. K. Geim, Science 2009, 324, 1530. [3] L. Kou, C. Chen, S. C. Smith, The Journal of Physical Chemistry Letters 2015, 6, 2794-2805.

We have studied the formation of near-field fringes when sharp edges of materials are imaged using scattering-type scanning near-field optical microscope (s-SNOM). The materials we have investigated include dielectrics, metals, a near-perfect conductor, and those that possess anisotropic permittivity and hyperbolic dispersion. For our theoretical analysis, we use a technique that combines full-wave numerical simulations of tip-sample near-field interaction and signal demodulation at higher orders akin to what is done in typical s-SNOM experiments. Unlike previous tip-sample interaction near-field models, our advanced technique allows simulation of the realistic tip and sample structure. Our analysis clarifies edge imaging of recently emerged layered materials such as hexagonal boron nitride and transition metal dichalcogenides (in particular, molybdenum disulfide), as well as traditional plasmonic materials such as gold. Hexagonal boron nitride is studied at several wavelengths, including the wavelength where it possesses excitation of phonon-polaritons and hyperbolic dispersion. Based on our results of s-SNOM imaging in different demodulation orders, we specify resonant and non-resonant types of edges and describe the edge fringes for each case. We clarify near-field edge-fringe formation at material sharp boundaries, both outside bright fringes and the low-contrast region at the edge, and elaborate on the necessity of separating them from propagating waves on the surface of polaritonic materials.

Layered transition-metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), have attracted considerable interest as alternatives to platinum in hydrogen evolu...

Graphene has been widely studied for its many extraordinary properties, and other two-dimensional layered materials are now gaining increased interest. These excellent properties make thin layer materials very attractive for integration into a wide variety of technologies, particularly in flexible optoelectronic devices. Therefore, gaining control over these properties will allow for a more focused design and optimisation of these possible technologies. Through the application of mechanical strain it is possible to alter the electronic structures of two-dimensional crystals, such as graphene and transition metal dichalcogenides (e.g. MoS2), and these changes in electronic structure can alter their behaviour. In this perspective we discuss recent advances in the strain engineering of thin layer materials, with a focus on using Raman spectroscopy and electrical transport to investigate the effect of strain as well as the effect of strain on the chemical functionalisation of graphene.

Exploring the effects of boron nitride coating on the thermal stability and photoluminescence properties of molybdenum disulfide nanospheres