Browsing by Subject "Transition metal dichalcogenides"

Now showing 1 - 12 of 12

Open Access
Anisotropic electronic, mechanical, and optical properties of monolayer WTe2
(American Institute of Physics Inc., 2016) Torun, E.; Sahin, H.; Cahangirov, S.; Rubio, A.; Peeters, F. M.
Using first-principles calculations, we investigate the electronic, mechanical, and optical properties of monolayer WTe2. Atomic structure and ground state properties of monolayer WTe2 (Td phase) are anisotropic which are in contrast to similar monolayer crystals of transition metal dichalcogenides, such as MoS2, WS2, MoSe2, WSe2, and MoTe2, which crystallize in the H-phase. We find that the Poisson ratio and the in-plane stiffness is direction dependent due to the symmetry breaking induced by the dimerization of the W atoms along one of the lattice directions of the compound. Since the semimetallic behavior of the Td phase originates from this W-W interaction (along the a crystallographic direction), tensile strain along the dimer direction leads to a semimetal to semiconductor transition after 1% strain. By solving the Bethe-Salpeter equation on top of single shot G0W0 calculations, we predict that the absorption spectrum of Td-WTe2 monolayer is strongly direction dependent and tunable by tensile strain.
Open Access
Atomic force microscopy experiments on atomically thin materials
(2020-06) Sheraz, Ali
In 2004, successful isolation of graphene attracted immense attention of scientists because of atomic scale thickness and exotic functionalities. Regardless of graphene’s thickness and extraordinary properties only reason that limits the usage of graphene in electronics is no band gap. But there is a way to open band gap of graphene by introducing defects or applying electric ﬁeld but defects introduction can aﬀect its functionality. So, world moved towards transition metal dichalcogenides (TMDCs), new analogs of graphene with thickness dependent band gap option are promising nominee for potential applications in modern physics and electronics. Besides electronic properties, TMDCs depict excellent mechanical characteristics (in plane elastic modulus, breaking strength/strain and pretension) compared to conventional volumetric counterparts. The objective of this study is to investigate work function and mechanical properties of atomically thin materials using Kelvin probe force microscopy (KPFM) and Nanoindentation modes of Asylum Atomic Force Microscopy (AFM) respectively. Firstly, KPFM experiments were performed on CVD grown Vanadium Sesquioxide V2O3 to map surface potential variation and calculated work function value 4.91 eV. This will help in understanding band alignment, contact resistance and appropriate Schottky barrier height (SBH) by choosing metal contacts with closer work function to V2O3. Secondly by using AFM based nanoindentation we ﬁrst time reported elastic features of metallic TMDCs: 2H-TaS2, 3R-NbS2, 1T-TaTe2 and 1T-NbTe2 with various thickness values suspended over circular holes. Comprehensive measurement was done on 2H-TaS2 and found thickness independent Young’s modulus for 2H-TaS2 is 114 ± 14 GPa, breaking strength 12.6 ± 2.6 GPa corresponds to nominal strain of 11% and ultimate strain of 0.22. Same mechanical features were investigated for other three materials and they also manifested extreme elasticity and high strain values compare to other 2D materials reported so far except graphene. This mechanical analysis of metallic materials will contribute in future ﬂexible nano technological devices (for instance piezo electronics), wearable electronics, resistive coatings in electronic devices, nanoelectromechanical systems (NEMS) and strain sensors.
Open Access
Atomically thin materials
(Springer, 2020) Kasırga, T. Serkan
In this chapter, I will provide a brief overview of atomically thin materials that are formed by layers held together by van der Waals forces or weak covalent bonding. These materials provide a unique and cheap way of studying plethora of phenomena. Perhaps, the relative simplicity of the methods that are commonly used in the studies of two dimensional (2D) materials are one of the main reasons why they attracted attention at this level since the advent of graphene. After an introduction to the properties of 2D materials, I will talk about the methods to obtain 2D materials and conclude the chapter with the possibilities of heterostructures of 2D materials.
Open Access
Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis
(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, Salim
Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).
Open Access
Microcavity coupled interlayer excitons in MOSE2-WSE2 heterostructures
(2024-08) Atalay, Şeyma Esra
After the discovery of graphene, two-dimensional (2D) materials gained immense attention due to their exceptional mechanical, optical, and electronic properties. One of the well-known family of 2D materials is transition metal dichalcogenides (TMDs). Their electronic bandgap makes a transition from indirect to direct when the monolayer limit is reached, making them an excellent medium for studying many-body interactions in condensed matter and light-matter interactions. Also, stacking monolayer TMDs on top of each other enables the creation of heterostructures (HS). The vertical van der Waals heterostructures made from monolayer TMDs can host two types of excitons: one is the intralayer excitons consisting of the strongly Coulomb-bound electron-hole pair within the same material, and the other one is the interlayer exciton made up by spatially separated electrons and holes located in different layers. Intralayer excitons in TMDs exhibit higher oscillator strengths due to spatial confinement. Furthermore, coupling intra- and interlayer excitons with optical cavities allows for observing anticrossing phenomena between excitons and cavity photons. Consequently, TMDs and their HSs provide a versatile platform for investigating exciton-polariton formation and their associated photophysical properties. This thesis presents a detailed study of the room- and low-temperature photoluminescence (PL) spectroscopy of interlayer excitons (IEX) in near-hexagonal MoSe2-WSe2 HSs. The studied hexagonal boron nitride (hBN) encapsulated heterostructures were fabricated using a combination of three methods: (i) mechanical exfoliation for cleaving the monolayers from bulk material, (ii) dry transfer technique to stack them vertically to achieve the heterostructure and (iii) edge identification method to adjust the twist angle between monolayers during stacking. Fabry-Pérot planar microcavities were fabricated for both room and low-temperature studies with different cavity modes using plasma-enhanced chemical vapor deposition with the aim of studying the interlayer exciton-polaritons. Fabricated cavities were further characterized by transmission electron microscopy and focused ion beam lithography for imaging the alternating layers of distributed Bragg mirrors (DBRs) and their thicknesses. The PL of IEXs in MoSe2-WSe2 heterostructures were measured both at room and low temperatures. Low-temperature PL measurements were conducted using a closed-cycle cryostat integrated into a home-built micro-PL setup. The results indicate that our heterostructures exhibit a near-hexagonal structure. The intensities of the spin-triplet and the spin-singlet states of the IEXs are prominent in the energy range of approximately 1.35-1.42 eV. The observed splitting between the spin-triplet and spin-singlet IEXs was in the range of 27-34 meV. Further investigations into the temperature and pump power dependence of the IEXs revealed that the intensity of the spin-triplet IEX is highly sensitive to temperature variations between 3.5 K and 50 K, becoming less intense at higher temperatures. Although the intensity of the spin-singlet IEX also decreased, it remained detectable at 50 K. With increasing pump power, a blueshift of 15 meV was observed for the spin-triplet IEX, while the spin-singlet IEX exhibited a blueshift of only 3 meV in the same pump power range. This indicates that the density of the spin-triplet state IEX increases more significantly with higher pump power than the spin-singlet state IEX. Additionally, the lifetime of the spin-singlet IEX was measured using Time-Resolved Photoluminescence (TRPL) spectroscopy. The fast and slow decay components were in the range of a few nanoseconds and a few tens of nanoseconds, respectively. Moreover, an approximately 9 meV splitting on the spin-singlet IEX PL emission was observed in one of the studied emitters, which has the same resonance wavelength as the FP cavity. This splitting might be attributed to the interlayer exciton-polariton formation at k|| equals zero. However, the whole angle-resolved PL spectrum should be measured to ensure this emission belongs to the polariton formation.
Open Access
Photophysics of interlayer excitons in TMDC heterobilayers
(2024-05) Durmuş, Mehmet Atıf
Since the first isolation of graphene (a single sheet of graphite) in 2004 and the remarkable discoveries achieved in the following years, there has been an ongoing growth in studies and interest in two-dimensional (2D) materials and their heterostructures. With the constant discovery of new 2D materials over time, their range of material properties has expanded significantly as well, opening up a greater variety of applications and corresponding theoretical and experimental research inquiries for their implementation. The majority of them are classified as layered van der Waals (vdW) materials, and the development of different transfer techniques has made it possible to fabricate vdW heterostructures, which introduced exciting possibilities for the development of quantum technologies. The unique characteristics of the constituent layers, in conjunction with the high carrier mobility characteristic of 2D materials, provide a wealth of opportunities for the development of devices with remarkable properties for various applications. Interest in semiconducting transition metal dichalcogenides (TMDCs) among other 2D materials has grown significantly owing to their exceptional optical, mechanical, and electrical properties, particularly as they exhibit direct bandgaps in atomic layer thicknesses (i.e., monolayers). The vdW heterostructures of monolayer TMDCs have been growing in popularity among researchers over the last decade since they typically exhibit staggered type-II band alignment, which promotes ultra-fast charge transfer between the constituent layers, in turn, leading to the formation of strongly Coulomb-bound electron-hole pairs (i.e., interlayer excitons, IXs) located in different layers. The emission characteristics of the IXs can be regulated by varying the twist angle between the constituent layers of the heterostructures, and a superlattice of single-photon quantum emitters of IXs through their localization to periodic quantum dot-like Moiré potentials can be achieved. The main focus of this dissertation is investigating and understanding the photophysics of IXs of hBN-encapsulated WSe2-MoSe2 heterobilayers, which will allow one to grasp the importance and capabilities they hold for the development of quantum applications. The heterobilayer samples used in this study were fabricated by first isolating the 2D layers using the micromechanical exfoliation method and then vertically stacking them via the dry transfer technique. Low-temperature photoluminescence (PL) spectroscopy methods have been performed on the IX species of the fabricated heterobilayers, including magneto-PL, excitation pump power-dependent PL, temperature-dependent PL, and time-resolved PL (TRPL). Finally, first-order correlation g(1)(τ) measurements in the time domain were performed using a home-built free-space Michelson interferometer. Our results on the effect of Moiré-localization of IXs demonstrate that the well-protection of the localized emitters can lead to prolonged dephasing times (T2 ~ 730 fs). Remarkably, we have also successfully shown the presence of coherent coupling for the first time between the two spin states (spin-singlet and spin-triplet) of IXs of hBN-encapsulated WSe2-MoSe2 heterobilayers by utilizing the quantum beat interferometry in the time domain with resulting dephasing times up to T2 ~ 400 fs. Our results on the dephasing characteristics of IXs can provide important insights into the future of exciton-based device development in quantum photonic and valleytronic applications.
Open Access
A theoretical study of strained monolayer transition metal dichalcogenides based on simple band structures
(2019-10) Aas, Shahnaz
This doctoral thesis deals with optoelectronic and geometric band properties of two-dimensional transition metal dichalcogenides (TMDs) under applied strain. First, we analyze various types of strain for the K valley optical characteristics of a freestanding monolayer MoS2, MoSe2, WS2 and WSe2 within a two-band k p method. By this simple bandstructure combined with excitons at a variational level, we reproduce wide range of available strained-sample photoluminescence data. According to this model strain affects optoelectronic properties. Shear strain only causes a rigid wavevector shift of the valley without any alternation in the bandgap or the effective masses. Also, for exible substrates under applying stress the presence of Poisson's effect or the lack of it are investigated individually for the reported measurements. Furthermore, we show that circular polarization selectivity decreases/increases by tensile/compressive strain for energies above the direct transition onset. TMDs in addition to their different other attractive properties have rendered the geometric band effects directly accessible. The tailoring and enhancement of these features by strain is an ongoing endeavor. In the second part of this thesis, we consider spinless two and three band, and spinful four band bandstructure techniques appropriate to evaluate circular dichroism, Berry curvature and orbital magnetic moment of strained TMDs. First, we establish a new k p parameter set for MoS2, MoSe2, WS2 and WSe2 based on recently released ab initio and experimental band properties. For most of these TMDs its validity range extend from K valley edge to several hundreds of millielectron volts for both valence and conduction band. We introduce strain to an available three band tight-binding Hamiltonian to extend this over a larger part of the Brillouin zone. Based on these we report that by applying a 2:5% biaxial tensile strain, both the Berry curvature and the orbital magnetic moment can be doubled compared to their unstrained values. These simple bandstructure tools can be suitable for the device modeling of the geometric band effects in strained monolayer TMDs.
Open Access
Thermal conductivity measurements in nanosheets via bolometric effect
(IOP Publishing Ltd, 2020) Çakıroğlu, Onur; Mehmood, Naveed; Çiçek, Mert Miraç; Rasouli, Hamid Reza; Durgun, Engin; Kasırga, T. Serkan; Aikebaier, Aizimaiti
Thermal conductivity measurement techniques for materials with nanoscale dimensions require fabrication of very complicated devices or their applicability is limited to a class of materials. Discovery of new methods with high thermal sensitivity are required for the widespread use of thermal conductivity measurements in characterizing materials' properties. We propose and demonstrate a simple non-destructive method with superior thermal sensitivity to measure the in-plane thermal conductivity of nanosheets and nanowires using the bolometric effect. The method utilizes laser beam heating to create a temperature gradient, as small as a fraction of a Kelvin, over the suspended section of the nanomaterial with electrical contacts. Local temperature rise due to the laser irradiation alters the electrical resistance of the device, which can be measured precisely. This resistance change is then used to extract the temperature profile along the nanomaterial using thermal conductivity as a fitting parameter. We measured the thermal conductivity of V2O3 nanosheets to validate the applicability of the method and found an excellent agreement with the literature. Further, we measured the thermal conductivity of metallic 2H-TaS2 for the first time and performed ab initio calculations to support our measurements. Finally, we discussed the applicability of the method on semiconducting nanosheets and performed measurements on WS2 and MoS2 thin flakes.
Open Access
Thickness dependence of solar cell efficiency in transition metal dichalcogenides MX2 (M: Mo, W; X: S, Se, Te)
(Elsevier, 2020) Özdemir, Burak; Barone, V.
Bulk transition metal dichalcogenides are indirect gap semiconductors with optical gaps in the range of 0.7–1.6 eV, which makes them suitable for solar cell applications. In this work, we study the electronic structure, optical properties, and the thickness dependence of the solar cell efficiencies of MX2 (M: Mo, W; X: S, Se, Te) with density functional theory and GW + BSE. Through this analysis, we find a change in solar cell efficiency trends at slab thicknesses of 3 μm. For thin films solar cells (thicknesses smaller than 3 μm), the tellurides present the highest efficiencies (about 20% for a 100 nm thick slab). In contrast, for thicknesses greater than 3 μm, our results indicate that a maximum solar cell efficiency can be achieved in WS2. For instance, a 100 μm slab of WS2 presents a solar cell efficiency of 36.3%, making this material a promising candidate for solar cell applications.
Open Access
Trion-mediated förster resonance energy transfer and optical gating effect in WS2/hBN/MoSe2 heterojunction
(American Chemical Society, 2020) Hu, Z.; Hernández-Martínez, P. L.; Liu, X.; Amara, M. R.; Zhao, W.; Watanabe, K.; Taniguchi, T.; Demir, Hilmi Volkan
van der Waals two-dimensional layered heterostructures have recently emerged as a platform, where the interlayer couplings give rise to interesting physics and multifunctionalities in optoelectronics. Such couplings can be rationally controlled by dielectric, separation, and stacking angles, which affect the overall charge or energy-transfer processes, and emergent potential landscape for twistronics. Herein, we report the efficient Förster resonance energy transfer (FRET) in WS2/ hBN/MoSe2 heterostructure, probed by both steady-state and timeresolved optical spectroscopy. We clarified the evolution behavior of the electron−hole pairs and free electrons from the trions, that is, ∼59.9% of the electron−hole pairs could transfer into MoSe2 by FRET channels (∼38 ps) while the free electrons accumulate at the WS2/hBN interface to photogate MoSe2. This study presents a clear picture of the FRET process in two-dimensional transition-metal dichalcogenides’ heterojunctions, which establishes the scientific foundation for developing the related heterojunction optoelectronic devices.
Open Access
Universal photoluminescence enhancement/suppression at the vertical van der Waals metal-semiconductor interfaces
(2024-01) Shakir, Hafiz Muhammad
Monolayers of transition metal dichalcogenides are considered the prospects for optoelectronic devices and photoluminescence (PL) is one of the key parameters to observe the performance and efficiency of such devices. The PL characteristics of monolayers of semiconducting transition metal dichalcogenides (TMDCs) have been consistently reported to be suppressed in the presence of metal, a phenomenon observed through direct metal evaporation or annealing of heterostructures in prior studies. These methods often resulted in a significant negative charge transfer which creates metal-induced gap states (MIGS) and Fermi level pinning (FLP). These MIGS and FLP provide nonradiative pathways to the excited electrons causing a huge suppression in PL intensity. To address this challenge, we explore heterostructures with a van der Waals gap between the metal and semiconductor surfaces. This design reduces the nonradiative relaxation pathways, allowing for more controlled charge transfer due to the van der Waals gap and the modulation of Schottky barrier height (SBH). The SBH for electrons increases with increasing metal work function and hence provides direct control of charge injection type and magnitude to monolayers of TMDCs. Our research presents a universal methodology for controlling the PL intensity of TMDCs by strategically utilizing the van der Waals gap and tailoring the work function of the interfacing metal. This investigation not only unveils a novel approach to prevent PL quenching but also opens avenues for optimizing opto-electronic devices. By carefully selecting metallic and semiconducting materials, this work offers a pathway to enhance device performance and precisely regulate output characteristics in optoelectronic applications.
Open Access
Validation of inter-atomic potential for WS2 and WSe2 crystals through assessment of thermal transport properties
(Elsevier, 2018) Mobaraki, Arash; Kandemir, A.; Yapıcıoğlu, H.; Gülseren, Oğuz; Sevik, C.
In recent years, transition metal dichalcogenides (TMDs) displaying astonishing properties are emerged as a new class of two-dimensional layered materials. The understanding and characterization of thermal transport in these materials are crucial for efficient engineering of 2D TMD materials for applications such as thermoelectric devices or overcoming general overheating issues. In this work, we obtain accurate Stillinger-Weber type empirical potential parameter sets for single-layer WS2 and WSe2 crystals by utilizing particle swarm optimization, a stochastic search algorithm. For both systems, our results are quite consistent with first-principles calculations in terms of bond distances, lattice parameters, elastic constants and vibrational properties. Using the generated potentials, we investigate the effect of temperature on phonon energies and phonon linewidth by employing spectral energy density analysis. We compare the calculated frequency shift with respect to temperature with corresponding experimental data, clearly demonstrating the accuracy of the generated inter-atomic potentials in this study. Also, we evaluate the lattice thermal conductivities of these materials by means of classical molecular dynamics simulations. The predicted thermal properties are in very good agreement with the ones calculated from first-principles.