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Crystal structure illustration and transmission electron microscopy (TEM) investigation of ternary Mo0.5W0.5Se1.9. (a) Crystal structure of Mo0.5W0.5Se1.9 with top (upper image) and side (lower image) view. (b) Optical image of monolayer Mo0.5W0.5Se1.9 exfoliated onto a 300 nm thick Si/SiO2 substrate. (c) High-resolution transmission electron microscopy (HRTEM) image of the few-layered region of the Mo0.5W0.5Se1.9 flake (∼10 layers). HRTEM images indicate disordered atomic configurations for the two Mo and W elements throughout the mixture compositions. (d) Electron diffraction pattern of the alloy crystal. The electron diffraction results demonstrate the single crystalline nature of the alloy.
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Binary transition metal dichalcogenides (TMDCs) in the class MX2 (M = Mo, W; X = S, Se) have been widely investigated for potential applications in optoelectronics and nanoelectronics. Recently, alloy-based monolayers of TMDCs have provided a stable and versatile technique to tune the physical properties and optimize them for potential applications...
Citations
... Alloying is another widely used method to tailor the electronic and magnetic properties of solid state materials for designing specific applications [24][25][26][27][28][29][30][31][32][33][34][35][36]. This has been well established for transition-metal dichalcogenides [34][35][36], e.g., MoS 2−x Se x , Mo 1−x W x S 2 , TiS 2−x Se x , Nb 1−x Re x S 2 , etc. Recently, alloying a fraction of Se into TiS 3 has also been experimentally demonstrated by Agarwal et al. [37]. ...
First-principles density functional theory plus Bethe-Salpeter equation calculations are employed to investigate the electronic and excitonic properties of monolayer titanium trichalcogenide alloys TiS3−xSex (x=1 and 2). It is found that the band gap and exciton binding energy display asymmetric dependence on the substitution of Se for S. While the band gap can be significantly decreased as compared to that of pristine TiS3, the exciton binding energy varies slightly, regardless of the position and concentration of the Se substitution. A negative exciton formation energy is found when the central S atoms are replaced by Se atoms, suggesting a many-body ground state with spontaneous exciton condensation. Our work thus offers insight for engineering an excitonic insulator.
... Nonetheless, a small spin splitting is still survived after considering the role of less strongly coupled chalcogenide orbitals and the higher-order effect of the metal orbital [27][28][29] . Despite the giant splitting in the valence bands, engineering spin-orbit splitting is still strongly desired, which is an indispensable element of spintronics. ...
Monolayer transition metal dichalcogenides (TMDs) with strong spin-orbit coupling combined with broken inversion symmetry, leading to a coupling of spin and valley degrees of freedom, make these materials highly interesting for potential spintronics and valleytronic applications. However, engineering the spin-orbit coupling (SOC) at room temperature as demand after device fabrication is still a great challenge for their practical applications. Here we reversibly engineer the spin-orbit coupling of monolayer MoS2 by laser irradiation under controlled gas environments, where the spin-orbit splitting has been effectively regulated within 120 meV to 200 meV. Furthermore, the photoluminescence (PL) intensity of B exciton can be invertible manipulation over 2 orders of magnitude. We attribute the engineering of spin-orbit splitting to the reduction of binding energy combined with band renormalization, originating from the enhanced absorption coefficient of monolayer MoS2 under inert gases and subsequent the significantly boosted carrier concentrations. Reflectance contrast spectra during the engineering stage provide unambiguous proof to support our interpretation. Our approach offers a new avenue to actively control the SOC strength in TMDs materials at room temperature and paves the way for designing innovative spintronics devices.
... However, in graphene, the controllable staggered lattice potential is experimentally difficult to achieve. Recently, TMDs such as MoS2 have attracted much attention [29,[40][41][42][43][44][45][46][47][48][49]. The structure of the TMDs is similar to that of the honeycomb lattice, and its monolayer has natural broken inversion symmetry and direct band gap, which is proved to be an ideal valleytronic material. ...
Valley degree of freedom in the first Brillouin zone of Bloch electrons offers an innovative approach to information storage and quantum computation. Broken inversion symmetry together with the presence of time-reversal symmetry endows Bloch electrons non-zero Berry curvature and orbital magnetic moment, which contribute to the valley Hall effect and optical selection rules in valleytronics. Furthermore, the emerging transition metal dichalcogenides (TMDs) materials naturally become the ideal candidates for valleytronics research attributable to their novel structural, photonic and electronic properties, especially the direct bandgap and broken inversion symmetry in the monolayer. However, the mechanism of inter-valley relaxation remains ambiguous and the complicated manipulation of valley predominantly incumbers the realization of valleytronic devices. In this review, we systematically demonstrate the fundamental properties and tuning strategies (optical, electrical, magnetic and mechanical tuning) of valley degree of freedom, summarize the recent progress of TMD-based valleytronic devices. We also highlight the conclusion of present challenges as well as the perspective on the further investigations in valleytronics.
... For x < x c = 0.2, the PL intensity decreases with rising temperature [see the upper two curves in Fig. 5(a)], whereas an opposite behavior is found for large x, as shown in the lowest two curves. For intermediate W-concentration, however, a non-monotonic temperature dependence emerges (see the orange and pink curves), in very good agreement with recent temperature-dependent PL meas- urement of the monolayer Mo 0.5 W 0.5 Se 1.9 alloy 64 . Hence, for low and high W-concentrations, the Mo 1−x W x Se 2 τ rb = 10 T ps/K τ rbb = τ rb τ rd = 1.0 ns τ skx = 0.01 ps τ skxx = 0.02 ps τ bd = 1.0 ps P = 1.0 kW/cm 2 P 0 = 10.2 kW cm −2 β 0 = 36.0 ...
We report a comprehensive theory to describe exciton and biexciton valley dynamics in monolayer Mo1−xWxSe2 alloys. To probe the impact of different excitonic channels, including bright and dark excitons, intravalley biexcitons, intervalley scattering between bright excitons, as well as bright biexcitons, we have performed a systematic study from the simplest system to the most complex one. In contrast to the binary WSe2 monolayer with weak photoluminescence (PL) and high valley polarization at low temperatures and the MoSe2, that presents high PL intensity, but low valley polarization, our results demonstrate that it is possible to set up a ternary alloy with intermediate W-concentration that holds simultaneously a considerably robust light emission and an efficient optical orientation of the valley pseudospin. We find the critical value of W-concentration, xc, that turns alloys from bright to darkish. The dependence of the PL intensity on temperature shows three regimes: while bright monolayer alloys display a usual temperature dependence in which the intensity decreases with rising temperature, the darkish alloys exhibit the opposite behavior, and the alloys with x around xc show a non-monotonic temperature response. Remarkably, we observe that the biexciton enhances significantly the stability of the exciton emission against fluctuations of W-concentration for bright alloys. Our findings pave the way for developing high-performance valleytronic and photo-emitting devices.
... 2D materials are outstanding candidates for optoelectronic devices due to their unique properties, including the wide response spectrum range, excellent flexibility, and strong light-matter interaction [91,92]. Similarly, due to the generation of interlayer excitons and flexible band engineering, 2D heterostructures have been widely applied in optoelectronics including photodetectors [93,94], photovoltaic devices, and light source devices [95][96][97][98][99][100][101]. ...
Abstract With a large number of researches being conducted on two-dimensional (2D) materials, their unique properties in optics, electrics, mechanics, and magnetics have attracted increasing attention. Accordingly, the idea of combining distinct functional 2D materials into heterostructures naturally emerged that provides unprecedented platforms for exploring new physics that are not accessible in a single 2D material or 3D heterostructures. Along with the rapid development of controllable, scalable, and programmed synthesis techniques of high-quality 2D heterostructures, various heterostructure devices with extraordinary performance have been designed and fabricated, including tunneling transistors, photodetectors, and spintronic devices. In this review, we present a summary of the latest progresses in fabrications, properties, and applications of different types of 2D heterostructures, followed by the discussions on present challenges and perspectives of further investigations.
Semiconductor heterostructures have been the backbone of developments in electronic and optoelectronic devices. One class of structures of interest is the so-called type II band alignment, in which optically excited electrons and holes relax into different material layers. The unique properties observed in two-dimensional transition metal dichalcogenides and the possibility to engineer van der Waals heterostructures make them candidates for future high-tech devices. In these structures, electronic, optical, and magnetic properties can be tuned through the interlayer coupling, thereby opening avenues for developing new functional materials. We report the possibility of explicitly tuning the emission of interlayer exciton energies in the binary–ternary heterobilayer of Mo0.5W0.5Se2 with MoSe2 and WSe2. The respective interlayer energies of 1.516 eV and 1.490 eV were observed from low-temperature photoluminescence measurements for the MoSe2– and WSe2– based heterostructures, respectively. These interlayer emission energies are above those reported for MoSe2/WSe2 (≃1.30–1.45 eV). Consequently, binary–ternary heterostructure systems offer an extended energy range and tailored emission energies not accessible with the binary counterparts. Moreover, even though Mo0.5W0.5Se2 and MoSe2 have almost similar optical gaps, their band offsets are different, resulting in charge transfer between the monolayers following the optical excitation. Thus, confirming TMDs alloys can be used to tune the band-offsets, which adds another design parameter for application-specific optoelectronic devices.
Low carrier mobility, closely associated with the formation of localized states, is the major bottleneck of utilizing the unique quantum transport properties in transition metal dichalcogenides (TMDCs). Here, we demonstrate an effective method to quantify the localization energy based on the temperature-dependent spectral variation of photoluminescence (PL) in pristine and hexagonal boron nitride (h-BN) encapsulated monolayer (ML) WSe2. Considering the protecting capability of h-BN against contamination and degradation, while not affecting the electronic structure as an insulating dielectric, the localization energy was comparatively extracted out of PL spectra in pristine and encapsulated ML WSe2. In pristine ML WSe2, two distinctive energy traces were resolved with an energy difference of about 17 meV, which was associated with the localized state revealed below 200 K. Clear evidence for the carrier localization was also evident in the integrated PL intensity trace with temperature as the trace from pristine ML clearly deviates from the dark-exciton-like behavior of ML WSe2, violating the spin selection rule of the lowest exciton state. In clear contrast, the temperature dependency of the h-BN encapsulated ML WSe2 in PL spectra matches well with the typical Varshni formula of free excitonic peaks and the integrated intensity trace of thermally populated spin subbands. Our study suggests that the h-BN encapsulation could suppress the carrier localization channels by avoiding surface oxidation due to air exposure and could provide insights into how one could preserve the excitonic features in TMDC materials and devices.
In the present investigation, Sb0.1Mo0.9Se2 single crystals belonging to semiconductor-layered transition metal di-chalcogenides (SLTMDCs) are grown by direct vapor transport (DVT) technique in a dual-zone furnace. The surface topographic properties of exfoliated crystals are studied under an optical microscope and scanning electron microscopy (SEM). The SEM analysis reveals the presence of hexagonal particles. The direct optical energy band gap of exfoliated crystal, 1.44 eV, is obtained by UV–visible spectroscopy. The photoluminescence (PL) and Raman spectroscopy are employed to elucidate the excitonic mechanism and vibrational properties. The semiconducting behavior of exfoliated crystals is examined by high-temperature resistivity. Heterostructure devices of thin-film SnSe2/Sb0.1Mo0.9Se2 crystal are prepared to study its rectifying nature. The diode parameter such as ideality factor, saturation current, and barrier potential is evaluated by Ln I–V technique. The photoconduction property of the fabricated device along the ab-plane is recorded under a polychromatic source of intensity 30 mW cm−2. The photo-detecting properties such as responsivity (149 mA W−1) and detectivity (108 Jones) are determined to evince the excellent photodetection property of detector at 800 mV and 1200 mV bias voltages, respectively.
Alloying semiconductors are often used to tune the material properties desired for device applications. The price for this tunability is the extra disorder caused by alloying. In order to reveal the features of the disorder potential in alloys of atomically thin transition-metal dichalcogenides (TMDs) such as MoxW1−xSe2, the exciton photoluminescence is measured in a broad temperature range between 10 and 200 K. In contrast to the binary materials MoSe2 and WSe2, the ternary system demonstrates non-monotonous temperature dependences of the luminescence Stokes shift and of the luminescence linewidth. Such behavior is a strong indication of a disorder potential that creates localized states for excitons and affects the exciton dynamics responsible for the observed non-monotonous temperature dependences. A comparison between the experimental data and the results obtained by Monte Carlo computer simulations provides information on the energy scale of the disorder potential and also on the shape of the density of localized states created by disorder. Statistical spatial fluctuations in the distribution of the chemically different material constituents are revealed to cause the disorder potential responsible for the observed effects. A deeper understanding of the disorder-induced effects is vital for prospective TMD alloy-based devices.
Monolayer transition metal dichalcogenides, manifesting strong spin-orbit coupling combined with broken inversion symmetry, lead to coupling of spin and valley degrees of freedom. These unique features make them highly interesting for potential spintronic and valleytronic applications. However, engineering spin-orbit coupling at room temperature as demanded after device fabrication is still a great challenge for their practical applications. Here we reversibly engineer the spin-orbit coupling of monolayer MoS2 by laser irradiation under controlled gas environments, where the spin-orbit splitting has been effectively regulated within 140 meV to 200 meV. Furthermore, the photoluminescence intensity of the B exciton can be reversibly manipulated over 2 orders of magnitude. We attribute the engineering of spin-orbit splitting to the reduction of binding energy combined with band renormalization, originating from the enhanced absorption coefficient of monolayer MoS2 under inert gases and subsequently the significantly boosted carrier concentrations. Reflectance contrast spectra during the engineering stages provide unambiguous proof to support our interpretation. Our approach offers a new avenue to actively control the spin-orbit splitting in transition metal dichalcogenide materials at room temperature and paves the way for designing innovative spintronic devices.
