Abstract:Binary transition metal dichalcogenides (TMDCs) in the class MX 2 (M = Mo, W; X = S, Se) have been widely investigated for potential applications in optoelectronics and nanoelectronics. Recently, alloybased monolayers of TMDCs have provided a stable and versatile technique to tune the physical properties and optimize them for potential applications. Here, we present experimental evidence for the existence of an intermediate alloy state between the MoSe 2 -like and the WSe 2 -like behavior of the neutral excito… Show more
“…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 measurement 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 ternary monolayer alloys show MoSe 2 and WSe 2 like behaviors, respectively, while a transition from W-based to Mo-based behaviors occurs at intermediate x values.Figure 5PL intensity of the bright exciton channel I b = n b / τ rb in monolayer Mo 1− x W x Se 2 alloy excited with a right circularly polarized continuous wave laser.…”
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.
“…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 measurement 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 ternary monolayer alloys show MoSe 2 and WSe 2 like behaviors, respectively, while a transition from W-based to Mo-based behaviors occurs at intermediate x values.Figure 5PL intensity of the bright exciton channel I b = n b / τ rb in monolayer Mo 1− x W x Se 2 alloy excited with a right circularly polarized continuous wave laser.…”
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].…”
HIGHLIGHTS • The controllable fabrication methods, the unique properties, and relative applications of 2D heterostructures were summarized. • The generation and detection of interlayer excitons in 2D heterostructures with type II band alignment indicate a longer lifetime and larger binding energy than intralayer excitons. • The advances in magnetic tunneling junctions based on 2D heterostructures can be applied in spintronic devices to realize spin filtering. 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.
“…The latter is most pronounced in MoSe 2 and is usually attributed to excitons bound to defects (X D ). [36,37] As commonly known, WSe 2 is associated with a wide range of excitonic features at low temperature, as discussed in recent publications. [78][79][80][81][82][83] Here, we focus on the exciton and trion peaks which are separated by an energy difference of Δ E ≈ 25-27 meV [84][85][86] in all the three materials.…”
Section: Resultsmentioning
confidence: 77%
“…[25][26][27] Inherently known from conventional III-V semiconductor alloying [28,29] is the ability to engineer material properties by varying concentrations of constituent atoms in order to realize extended functionality. Similarly, TMDs can be alloyed via the transition metal [30][31][32][33][34][35][36][37][38] or the chalcogen [39][40][41][42][43] thereby tuning electronic and optical properties to target specific applications. For instance, in the alloy Mo x W 1−x Se 2 exploited in our study, the optical bandgap can be continuously tuned from 1.56 eV (reached at x = 0.79) to 1.65 eV (reached at x = 0).…”
mentioning
confidence: 99%
“…Thus, in addition to modifying the optical band gap in TMD alloys (via the transition metal or chalcogen), not only can their spin-orbit coupling be manipulated [59] but also their spin-valley attributes. [36][37][38] This is because the binary compounds with different transition metals not only inherit a spin-orbit coupling of different size but also of different sign. Consequently, alloying the transition metal can be used to tune the valley dynamics to the needs of a specific application by adjusting the energy difference between the inter-and the intravalley excitons.…”
to play a crucial role as essential building blocks in future high-tech devices. [1][2][3][4][5][6][7][8] Already in the late 1960s, early studies on the structural and electronic properties of bulk TMDs had started and are detailed in a report by Wilson and Yoffe. [9] TMDs are layered materials which combine a transition metal (M: Mo, W, Ti, Zr, etc.) and a chalcogen (X: S, Se, or Te) in the general formula MX 2 with one layer of M atoms sandwiched between two layers of X atoms. [10] Group VIB TMDs with a 2H structural phase (e.g., MoSe 2 ) are the most explored representatives of such systems. They are characterized by an intrinsic bandgap within the visible and near-infrared regions. [11] Furthermore, the material system exhibits a transition from an indirect to a direct bandgap located at the K or K′ points of the hexagonal Brillouin zone, linked to a decrease in the number of layers from the bulk crystal to a monolayer. [12,13] Moreover, such materials also possess a strong spin-orbit interaction due to the presence of a relatively heavy transition metal along with huge exciton binding energies [14][15][16] resulting from a strong Coulomb interaction and a lack of dielectric screening. These properties lead to a valence-band splitting that strongly affects Alloying semiconductors is 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 Mo x W 1−x Se 2 , the exciton photoluminescence is measured in a broad temperature range between 10 and 200 K. In contrast to the binary materials MoSe 2 and WSe 2 , 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 disorderinduced effects is vital for prospective TMD alloy-based devices.
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