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Various WS2 used in this study including an (a) exfoliated flake, and (b,c) samples grown via chemical vapor deposition. Gated measurements were made on (c) the processed sample.
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We prepare single-layer WS2 films such that the photoluminescence is from either the neutral exciton or the negatively chargedtrion. While the neutral exciton emission has zero polarization at room temperature, we observe a room temperature optical polarization in excess of 40% for the trion. Using an applied gate voltage, we can modulate the elect...
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We investigate the photoluminescence (PL) polarization of millimeter-scale GaSe films with 100 nm and 150 μm thicknesses by excitation with linearly and circularly polarized light. Although free exciton emission predominates at 140 K, multiple peaks appear at 30 K owing to the recombination with impurity and/or localized states. Despite the circula...
Citations
... Electrostatic doping could induce screening effect, which suppresses the long-range e-h exchange interaction and thus improves the degree of valley polarization and valley-polarized lifetime. [30,[52][53][54] Zhang et al. [55] have reported that the degree of valley polarization in ML-MoTe 2 could be increased to 38% for excitons and 33% for trions through electrical manipulation (Figs. 3(a) and 3(b)). ...
In recent years, valleytronics research based on 2D semiconducting transition metal dichalcogenides have attracted considerable attention. On the one hand, strong spin-orbit interaction allows the presence of spin-valley coupling in this system, which provides spin addressable valley degrees of freedom for information storage and processing. On the other hand, large exciton binding energy up to hundreds of millieletronvolts enable excitons to be stable carriers of valley information. Valley polarization, marked by an imbalanced exciton population in two inequivalent valleys (+K and -K), is the core of valleytronics as it can be utilized to store binary information. Motivated by the potential applications, we present a thorough overview of the recent advancements in the generation, relaxation, manipulation, and transport of the valley polarization in nonmagnetic transition metal dichalcogenide layered semiconductors. We also discuss the development of valleytronic devices and future challenges in this field.
... Thus far we have considered only single particle excitations, and we now turn to the question of whether composite many-body excitations (such as excitons and trions [15]) will inherit this phase structure. If we then write an exciton wave function as ...
Electrons at the band edges of materials are endowed with a valley index, a quantum number locating the band edge within the Brillouin zone. An important question is then how this index may be controlled by laser pulses, with current understanding that it couples exclusively via circularly polarized light. Employing both tight-binding and state-of-the-art time dependent density function theory, we show that on femtosecond time scales valley coupling is a much more general effect. We find that two time separated linearly polarized pulses allow almost complete control over valley excitation, with the pulse time difference and polarization vectors emerging as key parameters for valley control. Our findings highlight the possibility of controlling coherent electronic excitation by successive femtosecond laser pulses, and offer a route towards valleytronics in two-dimensional materials.
... Interestingly, the intervalley scattering of excitons leads to fast depolarization of trions due to the X-T conversion in the opposite valley in an intrinsically doped ML-WSe 2 14 . To manipulate valley polarization, chemical doping 24 and electrical gating 25 were considered to control the X-T conversion in continuous-wave photoluminescence (CW-PL) measurement. However, it is still unclear how the mutual conversion of excitons and trions, especially the interplay of their depolarization processes would affect the valley polarization dynamically. ...
Monolayer 2D semiconductors provide an attractive option for valleytronics due to valley-addressability. But the short valley-polarization lifetimes for excitons have hindered potential valleytronic applications. In this paper, we demonstrate a strategy for prolonging the valley-polarization lifetime by converting excitons to trions through efficient gate control and exploiting the much longer valley-polarization lifetimes for trions than for excitons. At charge neutrality, the valley lifetime of monolayer MoTe2 increases by a factor of 1000 to the order of nanoseconds from excitons to trions. The exciton-to-trion conversion changes the dominant depolarization mechanism from the fast electron-hole exchange for excitons to the slow spin-flip process for trions. Moreover, the degree of valley polarization increases to 38% for excitons and 33% for trions through electrical manipulation. Our results reveal the depolarization dynamics and the interplay of various depolarization channels for excitons and trions, providing an effective strategy for prolonging the valley polarization. Here, the authors devise a strategy for prolonging the valley polarization lifetime in monolayer MoTe2 by converting excitons to trions through gate control, and by taking advantage of the longer valley polarization lifetime of trions.
... A variety of factors influence a TMD's DoCP, including defects, 22 exciton lifetime, [23][24][25][26] dopants, 27 temperature, 28 magnetic fields, 29 excitation wavelength, 30 and carrier density. 31 Despite these advances, manipulating the material deterministically in situ with submicron resolution remains a challenge. Using a back-gated device, the carrier density and DoCP can be tuned real-time, but the effect is not stable and disappears when power is removed. ...
... Using a back-gated device, the carrier density and DoCP can be tuned real-time, but the effect is not stable and disappears when power is removed. 26,31 A laser can be used to dope H2-activated monolayer MoS2 with spatial-selectivity and manipulate the DoCP, but it was only demonstrated at very low-temperatures (4K) and the changes are not reversible. 27 In this work, we demonstrate a method to manipulate the valley polarization and DoCP across a large 39.4% range, using a low-power laser (0.762µW) in ambient and at room temperature. ...
Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS2) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of x161), peak position (38.4meV range), circular polarization (39.4% range), and valley polarization of a Bi2Se3-WS2 2D heterostructure using a low-power laser (0.762uW) in ambient. Changes are spatially confined to the laser spot, enabling submicron (814nm) features, and are long-term stable (>334 days). PL and valley polarization changes can be controllably reversed through laser exposure in vacuum, allowing the material to be erased and reused. Atmospheric experiments and first-principles calculations indicate oxygen diffusion modulates the exciton radiative vs. non-radiative recombination pathways, where oxygen absorption leads to brightening, and desorption to darkening.
... A variety of factors influence a TMD's DoCP, including defects, 22 exciton lifetime, [23][24][25][26] dopants, 27 temperature, 28 magnetic fields, 29 excitation wavelength, 30 and carrier density. 31 Despite these advances, manipulating the material deterministically in situ with submicron resolution remains a challenge. Using a back-gated device, the carrier density and DoCP can be tuned real-time, but the effect is not stable and disappears when power is removed. ...
... Using a back-gated device, the carrier density and DoCP can be tuned real-time, but the effect is not stable and disappears when power is removed. 26,31 A laser can be used to dope H2-activated monolayer MoS2 with spatial-selectivity and manipulate the DoCP, but it was only demonstrated at very low-temperatures (4K) and the changes are not reversible. 27 In this work, we demonstrate a method to manipulate the valley polarization and DoCP across a large 39.4% range, using a low-power laser (0.762µW) in ambient and at room temperature. ...
Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS2) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of ×161), peak position (38.4 meV range), circular polarization (39.4% range), and valley polarization of a Bi2Se3-WS2 2D heterostructure using a low-power laser (0.762 μW) in ambient conditions. Changes are spatially confined to the laser spot, enabling submicrometer (814 nm) features, and are long-term stable (>334 days). PL and valley polarization changes can be controllably reversed through laser exposure in a vacuum, allowing the material to be erased and reused. Atmospheric experiments and first-principles calculations indicate oxygen diffusion modulates the exciton radiative vs nonradiative recombination pathways, where oxygen absorption leads to brightening and desorption to darkening.
... Interestingly, it was found that the intervalley scattering of excitons can lead to fast trion depolarization due to the X-T conversion in the opposite valley in an intrinsically-doped ML-WSe2 (14). In order to manipulate valley polarization, chemical doping (24) and electrical gating (25) were considered to effectively control the X-T conversion in continuous-wave photoluminescence (CW-PL) measurement. However, it is still unclear how the mutual conversion of X and T, especially the interplay of their depolarization processes would affect the valley polarization dynamically. ...
Monolayer 2D semiconductors provide an attractive option for valleytronics due to the valley-addressability by helicity-specific light beam. But the short valley lifetime for excitons have hindered potential valleytronic applications. In this paper, we demonstrate a strategy for prolonging the valley lifetime by converting excitons to trions through effective gate control and by taking advantage of much longer valley lifetime for trions than for excitons. In continuous-wave experiments, we found the valley polarization increases as gate voltage is tuned away from the charge neutrality, with the degree of valley polarization increased from near zero to 38 % for excitons and to 33 % for trions. This is the first successful observation of valley-polarization in MoTe2 without a magnetic field. In pump-probe experiments, we found that the intervalley scattering process of excitons is significantly suppressed as gate voltage is tuned away from charge neutrality, with scattering time from 0.85 ps to ~ 2.17 ps. In contrast, the intervalley scattering rate for trions increases due to increased availability of partner charges for trion spin flipping, with scattering time from 1.39 ns down to ~100 ps away from charge neutrality. Interestingly, our results show that, despite the accelerated intervalley scattering, the trion polarization degree increases due to polarized trion generation from the exciton-to-trion conversion overtaking the intervalley trion scatterings. Importantly, the efficient exciton-to-trion conversion changed the dominant depolarization mechanisms. As a result, the valley lifetime is dramatically improved by 1000 times from excitons to trions at the charge neutrality. Our results shed new light into the depolarization dynamics and the interplay of various depolarization channels for excitons and trions and provide an effective strategy for prolonging the valley polarization.
... Theoretical and experimental studies have demonstrated the long-range e-h exchange interactions govern the intervalley scattering process of exciton by the Maialle-Silva-Sham mechanism and result in a short valley lifetime (ps) [53,57,89]. Screening effect induced by electrostatic doping can efficiently suppress the long-range e-h exchange interactions in ML-TMDs and give rise to larger τ v and ρ, while has no effect on τ e [90][91][92][93]. Figures 4(a) and (b) clearly show the measured ρ and τ v of ML-WSe 2 increase as the carrier density becomes larger [23]. ...
Recently, the emerging conceptual valley-related devices have attracted much attention due to the progress on generating, controlling, and detecting the valley degree of freedom in the transition metal dichalcogenide (TMD) monolayers. In general, it is known that achieving valley degree of freedom with long valley lifetime is crucial in the implementation of valleytronic devices. Here, we provide a brief introduction of the basic understandings of valley degree of freedom. We as well review the recent experimental advancement in the modulation of valley degree of freedom. The strategies include optical/magnetic/electric field tuning, moiré patterns, plasmonic metasurface, defects and strain engineering. In addition, we summarize the corresponding mechanisms, which can help to obtain large degree of polarization and long valley lifetimes in monolayer TMDs. Based on these methods, two-dimensional valley-optoelectronic systems based on TMD heterostructures can be constructed, providing opportunities for such as the new paradigm in data processing and transmission. Challenges and perspectives on the development of valleytronics are highlighted as well.
... This kind of material exhibits very special physical properties, such as chemical stability [13], low impurity [14], no dangling bonds [12], electrostatic integrity [15] and high thermal stability [16]. Moreover, strong photoluminescence emission indicates its characteristic of a direct band gap semiconductor [17][18][19][20][21], leading to fabricating nano-scale optoelectronic devices such as photodetectors [22], light-emitting devices [10,23], or solar cells [24]. ...
Ambipolar field-effect transistor (FET) devices based on two-dimensional (2D) materials have been attracted much attention due to potential applications in integrated circuits, flexible electronics and optical sensors. However, it is difficult to tune Fermi level between conduction and valence bands using a traditional SiO 2 as dielectric layer. Here, we employed the lithium-ion conductive glass ceramic (LICGC) as the back-gate electrode in a monolayer WS 2 FET. The effective accumulation and dissipation of Li ⁺ ions in the interface induce a wide tune of Fermi level in the conducting channel by electron and hole doping, which show an ambipolar transport characteristics with threshold voltages at 0.9 V and −1.3 V, respectively. Our results provide an opportunity for fabricating ultra-thin ambipolar FET based on 2D materials.
... This spin-orbit (SO) splitting increases the spin-valley coupling, 7 which has also been confirmed experimentally with optical techniques. 10,[12][13][14][15][16] For instance, Hanbicki et al. 16 modulated continuously the polarization of the trion emission from 20 to 40% by applying the gate voltages in WS 2 monolayer with circularly polarized light, subsequently suppressed the intervalley scattering by the electron capture of trion. Zeng et al. 15 measured that the A-B splitting of the hot luminescence peak (B) and prominent band edge peak (A) in WS 2 and WSe 2 monolayer samples was ∼0.4 eV. ...
... This spin-orbit (SO) splitting increases the spin-valley coupling, 7 which has also been confirmed experimentally with optical techniques. 10,[12][13][14][15][16] For instance, Hanbicki et al. 16 modulated continuously the polarization of the trion emission from 20 to 40% by applying the gate voltages in WS 2 monolayer with circularly polarized light, subsequently suppressed the intervalley scattering by the electron capture of trion. Zeng et al. 15 measured that the A-B splitting of the hot luminescence peak (B) and prominent band edge peak (A) in WS 2 and WSe 2 monolayer samples was ∼0.4 eV. ...
Large spin-orbit splitting in the conduction band minimum (CBM) of monolayer transition metal dichalcogenides (TMDs) is in great demand for suppressing the intervalley scattering. Here we propose a new scheme to significantly enhance the spin-orbit splitting at the K point in the CBM of WS2 monolayer, via the n-p co-doping of fluorine and group VA elements (N, P, As and Sb). Based on the first-principles calculations, a giant spin-orbit splitting of 101.86 meV is theorized in the F-Sb co-doped system. This is evidenced to originate from the enhanced spin-orbit interaction, intimately related to the strengthened trigonal prismatic ligand field and the increased asymmetric surface charge. The giant spin-orbit splitting in the CBM can strongly suppress the intervalley scattering, which will enhance the spin-valley coupling and is beneficial for longer spin and valley lifetimes. This theoretical work provides a key to designing the high-performance monolayer TMD-based spintronic devices.
... Therefore, taking advantage of the spin-orbit coupling in order to explore intervalley scattering is feasible [28]. In experiments, by optically pumping a TMD with circularly polarized light and analyzing the subsequent photoluminescence for right and left helicity, SOC has been exploited to great effect in MoS 2 [12,[29][30][31] and WS 2 [32][33][34], and to some extent in MoSe 2 [35,36] and WSe 2 [37][38][39] to obtain some information on intervalley scattering. Recently, Srivastava et al [15] and Xu et al [16] have confirmed the possibility of manipulating valley pseudospin via the valley Zeeman effect in order to control the valley degree of freedom in ML WSe 2 . ...
The high-quality monolayer (ML) WSe2 leads to the possibility of an efficient manipulation of the spin and valley degrees of freedom. Here considering valley Zeeman field, which refers to the strongly anisotropic lifting of the degeneracy of the valley pseudospin degree of freedom using an external magnetic field, we investigate the spin and valley transport properties in the ML WSe2 quantum structure with circularly polarized light. Without valley Zeeman field, it is obviously found that the valley-dependent transmission can be well modulated by the orientation of circularly polarized light. The valley-dependent transmission can be filtered by the right () circularly polarized light, while the perfect filtering effect of K valley-dependent transmission occurs when the left () circularly polarized light irradiates. In the presence of valley Zeeman field, the valley-dependent transmission presents more interesting phenomena by controlling the orientation of circularly polarized light. When the circularly polarized light is applied, the valley polarization can be turned 100% to −100% with the enhancement of the effective energy about circularly polarized light. However, irradiating the circularly polarized light only 100% valley polarization can be obtained, which is independent of the effective energy of circularly polarized light and the height of electric barrier. Moreover, the results show that the valley polarization can also be from 100% to −100% in this device by adjusting the valley Zeeman field coupled with electrical field. Thus a two-way switch is formed in this quantum structure. These intriguing features indicate that the ML WSe2 structure is a promising candidate for future valleytronics devices, for valley switch, valley filter.
