Valleytronic Materials Research Articles

Strain-tuned electronic and valley-related properties in Janus monolayers of SWSiX2 (X = N, P, As)

Abstract Exploring novel two-dimensional (2D) valleytronic materials has an essential impact on the design of spintronic and valleytronic devices. Our first principles calculation results reveal that the Janus SWSiX2 (X = N, P, As) monolayer has excellent dynamical and thermal stability. Owing to strong spin-orbit coupling (SOC), the SWSiX2 monolayer exhibits a valence band spin splitting of up to 0.49 eV, making it promising 2D semiconductor for valleytronic applications. The opposite Berry curvatures and optical selection rules lead to the coexistence of valley and spin Hall effects in the SWSiX2 monolayer. Moreover, the optical transition energies can be remarkably modulated by the in-plane strains. Large tensile (compressive) inplane strains can achieve spin flipping in the SWSiN2 monolayer, and induce both SWSiP2 and SWSiAs2 monolayers transit from semiconductor to metal. Our research provides new 2D semiconductor candidates for designing high-performance valleytronic devices.

Large valley splitting induced by spin-orbit coupling effects in monolayer Hf3N2O2

Valleytronics is an emerging field of electronics that aims to utilize valley degrees of freedom in materials for information processing and storage. Nowadays, the valley splitting of 2D materials is not particularly large, therefore, the search for large valley splitting materials is very important for the development of valleytronics. This work theoretically predicts that MXene Hf3N2O2 is a 2D material with large valley splitting. It is an indirect bandgap semiconductor with a bandgap of 0.32 eV at the PBE level and increases to 0.55 eV at the HSE06 level. Since Hf3N2O2 breaks the symmetry of spatial inversion, when we consider spin–orbit coupling (SOC), there is a valley splitting at K/K′ of the valence band with a valley splitting value of 98.76 meV. The valley splitting value slightly decreases to 88.96 meV at the HSE06 level. In addition, The phonon spectrum and elastic constants indicate that it is both dynamically and mechanically stable. According to the maximum localization of the Wannier function, it is obtained that the Berry curvature is not zero at K/K′. When a biaxial strain is applied, Hf3N2O2 transitions from metal to semiconductor. With increasing biaxial strain, the valley splitting value increased from 70.13 meV to 109.11 meV. Our research shows that Hf3N2O2 is a promising material for valleytronics.

Spontaneous and tunable valley polarization in two-dimensional single-layer LaCl2

Applying the valley contrasting properties of valleytronic materials to logical operations is the foundation of valleytronic device manufacturing. It is predicted that single-layer (SL) LaCl2 is an ferrovalley material with intrinsic and tunable valley polarization through first-principles calculations. It is a ferromagnetic semiconductor (bandgap 0.767 eV) with roughly 1.0 μ B per unit cell as well as out of plane magnetization, and the Curie temperature is about 149 K. The tight-binding model considering five orbitals as well as next nearest neighboring hopping get a consistent band structure with the first-principles calculation. The valley polarization changes from 40.49 to 98.51 meV under the biaxial strain of 5% ∼ −5%. Therefore, the biaxial strain can be a means to tune the valley polarization. In addition, the valley polarization of the double-layer (DL) structure (∼80 meV) is much greater than that of the SL structure (∼59 meV) due to the increased magnetic moment of the DL structure, indicating the potential tunable character by stacking few layers. We believe that SL LaCl2 has great potential for device manufacturing and application in the field of valley electronics.

Large valley splitting and vacancy-induced valley polarization in two-dimensional WSeNH.

The investigation and manipulation of valley pseudospin in promising two-dimensional (2D) semiconductors are essential for accelerating the development of valleytronics. Based on first-principles, we herein report that the WSeNH monolayer is a potential 2D valleytronic material. It is found that stable 2D WSeNH exhibits a semiconducting character with broken inversion symmetry, forming a pair of energy-degenerate but inequivalent valleys at the K and K' points. Arising from the strong spin-orbit coupling strength governed by the W-dxy/dx2-y2 orbitals, it exhibits a large valley splitting of 425 meV at the top of the valence band, which makes it highly plausible for generating the attractive valley Hall effect. Moreover, both valley splitting and optical transition energy can be efficiently modulated by external strain. Furthermore, we find that a considerable valley polarization of 23 meV can be readily realized in 2D WSeNH by introducing hydrogen vacancies. These findings not only broaden the family of 2D valleytronic materials but also provide alternative avenues for valley manipulation.

Proposal for valleytronic materials: Ferrovalley metal and valley gapless semiconductor

Proposal for valleytronic materials: Ferrovalley metal and valley gapless semiconductor

Valley Polarized Holes Induced Exciton Polaron Valley Splitting.

Monolayer transition metal dichalcogenide semiconductors are promising valleytronic materials. Among various quasi-particle excitations hosted by the system, the valley polarized holes are particularly interesting due to their long valley lifetime preserved by the large spin-orbit splitting and spin-valley locking in the valence band. Here we report that in the absence of any magnetic field a surprising valley splitting of exciton polarons can be induced by such valley polarized holes in monolayer WSe2. The size of the splitting is comparable to that of the Zeeman effect in a magnetic field as high as 7 T and offers a quantitative approach to extract the hole density imbalance between the two valleys. We find that the density difference can easily achieve more than 1011 per cm2, and it is tunable by gate voltage as well as optical excitation power. Our study highlights the response of exciton polarons to optical pumping and advances understanding of valley dependent phenomena in monolayer transition metal dichalcogenide.

Large valley polarization and high Curie temperature in single layer XI2 (X = Sc, Y)

Ferrovalley materials with spontaneous valley polarization are the key to valleytronics applications. Therefore, it is important to find materials with relatively large valley polarization. In this work, we theoretically designed two novel two-dimensional ferrovalley materials namely XI2 (X = Sc, Y), which have both high Curie temperature and large valley polarization. The Curie temperature of monolayer ScI2 and YI2 are 138 K and 58 K, the large valley polarization of 97 meV and 108 meV are also predicted in monolayer ScI2 and YI2. The anomalous valley Hall effect is also proposed based on the valley contrasting Berry curvature. In addition, strain engineering can effectively modulate the valley polarization and Curie temperature of monolayer XI2 (X = Sc, Y). The Curie temperature of ScI2 increases with increasing strain, and the valley polarization of YI2 increases with increasing strain. Our findings suggest that monolayer XI2 (X = Sc, Y) is a promising material for valleytronics.

Energy-Transfer-Induced Enhanced Valley Splitting of Excitonic Emission of Inorganic CdTe@ZnS QDs in the Presence of Organic J-Aggregates: A Spectroscopic Insight into the Efficient Exciton (Inorganic)–Exciton (Organic) Coupling

An inorganic–organic nanohybrid material comprising an inorganic exciton (CdTe@ZnS) and an organic exciton (J-aggregates) has been synthesized, and subsequently investigations on exciton–exciton coupling between two components of the materials are carried out by employing various spectroscopic techniques. Analysis of the data has revealed very high Förster-type resonance energy transfer (FRET) from the donor quantum dots (QDs) to the acceptor J-aggregates. Interestingly, a clear splitting of the QD emission band during the FRET event is observed, indicating efficient coupling between inorganic and organic excitons. The observation of valley splitting of QD emission by organic J-aggregates, even in the absence of an external field, is new and exciting. More interestingly, the investigations have also shown that the valley splitting of the QD emission can be controlled by regulating the FRET process. In fact, a linear relationship between the energy separation value of the valley splitting and the FRET efficiency between QDs and J-aggregates is obtained. Moreover, in the present hybrid system, the presence of an organic moiety becomes advantageous in the sense that the valley splitting can also be controlled easily through chemical modification of the organic moiety. The outcome of the present investigation essentially demonstrates that the present nanohybrid system can be considered as a potential candidate for the fabrication of valleytronic materials.

Valley physics and anomalous valley Hall effect in single-layer h−MNX ( M=Ti , Zr, Hf; X=Cl , Br)

Recently, the valley degree of freedom of electrons in two-dimensional materials has been attracting growing attention as an information carrier. Here, on the basis of first-principles calculations, we propose a series of unique 2D valleytronic materials in single-layer $h\text{\ensuremath{-}}M\mathrm{N}X$ ($M=\mathrm{Ti}$, Zr; Hf, $X=\mathrm{Cl}$, Br) and systematically investigate their valleytronic properties. The underlying valley-contrasting physics including valley spin splitting and valley-dependent optical selection rules in single-layer $h\text{\ensuremath{-}}M\mathrm{N}X$ is unveiled. Moreover, we find that the intriguing valley polarization in single-layer $h\text{\ensuremath{-}}M\mathrm{N}X$ can be achieved through both circularly polarized light and a ferromagnetic substrate. The substrate-induced valley polarization in SL $h\text{\ensuremath{-}}M\mathrm{N}X$ is stacking dependent and can be enhanced by decreasing the interlayer distance. Our findings thus provide a tantalizing platform for operating the valley index in two-dimensional materials.

Theoretical prediction of valley spin splitting in two-dimensional Janus MSiGeZ4 (M = Cr and W; Z = N, P, and As).

With the exploration of valleytronic materials in MA2Z4 structures, larger valley spin splitting has become a hot topic of research. Based on first-principles calculations, we predicted six valleytronic 2D (two-dimensional) Janus MSiGeZ4 (M = Cr and W; Z = N, P, and As) materials. The valley spin splitting value of WSiGeZ4 (Z = N, P, and As) can reach more than 400 meV, which is favorable for the practical application of valleytronics. Two-dimensional WSiGeZ4 (Z = N, P, and As) materials are dynamically and mechanically stable and have an abundance of electronic properties. The two-dimensional Janus WSiGeZ4 (Z = N, P, and As) structures comprise both direct and indirect bandgap semiconductor materials. Among them, WSiGeN4 is an indirect bandgap semiconductor material with a bandgap of 1.654 eV and WSiGeP4 is a direct bandgap semiconductor material. The valley spin splitting originates from the symmetry breaking of the material structure and the spin-orbit coupling effect of the transition metal, which is manifested as the Berry curvature. In particular, the Berry curvature of 2D Janus WSiGeP4 and WSiGeAs4 is as high as 300 Bohr2, which is quite large. The W atom has more d-orbital electrons than the Cr atom, and the SOC (spin-orbit coupling) effect is stronger; thus, the valley spin splitting value CrSiGeZ4 of WSiGeZ4 is more than 300 meV, which is quite large. In addition, the bandgap and valley spin splitting of WSiGeZ4 (Z = N, P, and As) can be significantly modulated by applying a biaxial strain. Our study shows that WSiGeZ4 (Z = N, P, and As) has great potential for valleytronic applications.

Correlation-driven topological phase transition in 2D valleytronic materials: a mini-review.

Electronic correlation combined with spin-orbit coupling (SOC) may have a significant impact on the physical properties of two-dimensional (2D) transition metal magnetic compounds. Moreover, magnetic anisotropy (MA) is very important in determining magnetic, ferrovalley (FV) and topological properties of these 2D systems. Based on a density-functional theory (DFT) + U approach, it is found that the electronic correlation can induce topological phase transition in some special 2D valleytronic materials (for example FeCl2 and VSi2P4) with out-of-plane MA, and a novel valley-polarized quantum anomalous Hall insulator (VQAHI) and half-valley-metal (HVM) can be produced. These topological phase transitions are connected with a sign-reversible Berry curvature and band inversion between dxy/dx2-y2 and dz2 orbitals. However, for in-plane MA, the FV and nontrivial topological properties will be suppressed. For a given material, the correlation strength is fixed, but these novel electronic states and topological phase transitions can still be exhibited by strain in practice. The mini-review sheds light on the possible role of correlation effects in some special 2D valleytronic materials.

Realization of valley-spin polarized current via parametric pump in monolayer

Monolayer is a typical valleytronic material with valley-spin locked valence bands. We numerically investigate the valley-spin polarized current in monolayer via adiabatic electron pumping. By introducing an exchange field to break the energy degeneracy of monolayer , the top of its valence bands is valley-spin polarized and tunable by the exchange field. A device with spin-up polarized left lead, spin-down polarized right lead, and untuned central region is constructed through applying different exchange fields in the corresponding regions. Then, equal amount of pumped currents with opposite valley-spin polarization are simultaneously generated in the left and right leads when periodically varying two pumping potentials. Numerical results show that the phase difference between the pumping potentials can change the direction and hence polarization of the pumped currents. It is found that the pumped current exhibits resonant behavior in the valley-spin locked energy window, which depends strongly on the system size and is enhanced to resonant current peaks at certain system lengths. More importantly, the pumped current periodically oscillates as a function of the system length, which is closely related to the oscillation of transmission. The effects of other system parameters, such as the pumping amplitude and the static potential, are also thoroughly discussed.

Coupling of ferroelectric and valley properties in 2D materials

Two-dimensional (2D) valleytronic materials are both fundamentally intriguing and practically appealing to explore novel physics and design next-generation devices. However, traditional control means such as optic pumping or magnetic field cannot meet the demands of modern electron devices for miniaturization, low-dissipation, and non-volatility. Thus, it is attractive to combine the ferroelectric property with valley property in a single compound. In this paper, the recent progress of ferroelectric-valley coupling is reviewed. First, we briefly recall the development of valleytronics in the past several years. Then, various structures demonstrating ferroelectric-valley coupling, including heterostructures and intrinsic materials, are introduced. Subsequently, we describe ferroelectric-valley coupling in sliding and adsorption system and the unconventional ferroelectricity in the moiré system. Finally, we discuss the research status and outlook. We hope that this perspective will be helpful to bridge the gap between valleytronics and ferroelectrics in 2D materials and inspire further exciting findings.

K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe2

In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe2. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.

Phonon-Limited Valley Polarization in Transition-Metal Dichalcogenides.

The ability to selectively photoexcite at different Brillouin zone valleys forms the basis of valleytronics and other valley-related physics. Symmetry arguments combined with static lattice first-principles calculations suggest an ideal 100% valley polarization in transition-metal dichalcogenides under circularly polarized light. However, experimental reports of the valley polarization range from 32% to almost 100%. Possible explanations for this discrepancy include phonon-mediated transitions, which would place a fundamental limit to valley polarization, and defect-mediated transitions, which could, in principle, be reduced with cleaner samples. We explore the phonon-mediated fundamental limit by performing calculations of phonon-mediated optical absorption for circularly polarized light entirely from the first principles. We also use group theory to reveal the microscopic mechanisms behind the phonon-mediated excitations, discovering contributions from several individual phonon modes and from multiphonon processes. Overall, our calculations show that the phonon-limited valley polarization is around 70% at room temperature for state-of-the-art valleytronic materials including MoSe_{2}, MoS_{2}, WS_{2}, WSe_{2}, and MoTe_{2}. This fundamental limit implies that sufficiently pure transition-metal dichalcogenides are ideal candidates for valleytronics applications.

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe2.

We report the observation of QΓ intervalley exciton in bilayer WSe2 devices encapsulated by boron nitride. The QΓ exciton resides at ∼18 meV below the QK exciton. The QΓ and QK excitons exhibit different Stark shifts under an out-of-plane electric field due to their different interlayer dipole moments. By controlling the electric field, we can switch their energy ordering and control which exciton dominates the luminescence of bilayer WSe2. Remarkably, both QΓ and QK excitons exhibit unusually strong two-phonon replicas, which are comparable to or even stronger than the one-phonon replicas. By detailed theoretical simulation, we reveal the existence of numerous (≥14) two-phonon scattering paths involving (nearly) resonant exciton-phonon scattering in bilayer WSe2. To our knowledge, such electric-field-switchable intervalley excitons with strong two-phonon replicas have not been found in any other two-dimensional semiconductors. These make bilayer WSe2 a distinctive valleytronic material with potential novel applications.

Valley-filling instability and critical magnetic field for interaction-enhanced Zeeman response in doped WSe2 monolayers

Carrier-doped transition metal dichalcogenide (TMD) monolayers are of great interest in valleytronics due to the large Zeeman response (g-factors) in these spin-valley-locked materials, arising from many-body interactions. We develop an ab initio approach based on many-body perturbation theory to compute the interaction-enhanced g-factors in carrier-doped materials. We show that the g-factors of doped WSe2 monolayers are enhanced by screened-exchange interactions resulting from magnetic-field-induced changes in band occupancies. Our interaction-enhanced g-factors g* agree well with experiment. Unlike traditional valleytronic materials such as silicon, the enhancement in g-factor vanishes beyond a critical magnetic field Bc achievable in standard laboratories. We identify ranges of g* for which this change in g-factor at Bc leads to a valley-filling instability and Landau level alignment, which is important for the study of quantum phase transitions in doped TMDs. We further demonstrate how to tune the g-factors and optimize the valley-polarization for the valley Hall effect.

Two-dimensional valleytronic semiconductor with spontaneous spin and valley polarization in single-layer Cr2Se3

Two-dimensional (2D) valleytronic materials, exhibiting valley-contrasting physics, are both fundamentally intriguing and practically appealing to be used in nanoscale devices. In this work, through first-principles calculations and model analysis, we identify single-layer ${\mathrm{Cr}}_{2}{\mathrm{Se}}_{3}$ as a promising 2D valleytronic semiconductor. We reveal that single-layer ${\mathrm{Cr}}_{2}{\mathrm{Se}}_{3}$ is a ferromagnetic semiconductor and harbors valley features. In contrast to most previously reported similar systems suffering from a natural in-plane magnetization, it favors the robust out-of-plane magnetization. This, combined with the broken inversion symmetry and strong spin-orbit coupling strength, renders that a sizable valley polarization occurs spontaneously in single-layer ${\mathrm{Cr}}_{2}{\mathrm{Se}}_{3}$, thus facilitating the observation of intriguing anomalous valley Hall effect. More remarkably, by employing strain and ferroelectric substrate, the on-off switching of the valley polarization is realized in single-layer ${\mathrm{Cr}}_{2}{\mathrm{Se}}_{3}$. The underlying physics is discussed in detail. Our work provides a tantalizing candidate for realizing and manipulating the valley and spin physics.

Using ferroelectric polarization to regulate and preserve the valley polarization in a HfN2/CrI3/In2Se3 heterotrilayer

How to effectively regulate and preserve valley polarization is very important for the development of valleytronic devices in practical applications. Here, we propose a scheme of using ferroelectric polarization reversal to control valley polarization and, further, using ferroelectric nonvolatility to stably preserve the regulated valley state. We design a trilayer heterostructure, which is composed of vertically stacked two-dimensional (2D) valleytronic material ${\mathrm{HfN}}_{2}$, 2D ferromagnetic material ${\mathrm{CrI}}_{3}$, and 2D ferroelectric material ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$. Through first-principles simulation, we find that the polarization reversal of ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ can change the orientation of the easy magnetization axis for ${\mathrm{CrI}}_{3}$, and the change of magnetic moment direction causes the change of valley polarization for ${\mathrm{HfN}}_{2}$. When ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ is polarized upward, the easy magnetization axis of ${\mathrm{CrI}}_{3}$ is the $z$ axis, and the valley polarization is 15 meV, while when ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ is polarized downward, the easy magnetization axis of ${\mathrm{CrI}}_{3}$ is changed to the $x\text{\ensuremath{-}}y$ plane, and the valley polarization is changed to zero. Then we study the ferroelectric bistability of the ${\mathrm{HfN}}_{2}/{\mathrm{CrI}}_{3}/{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ heterotrilayer and propose a method to preserve different valley states using ferroelectric nonvolatility.

Gradient rhenium doping enabled tunable anisotropic valleytronic material based on monolayer molybdenum disulfide

Two-dimensional molybdenum disulfide (MoS2) possesses premium electrical, optoelectrical and valleytronic properties to develop future nano-electronic and optoelectronic devices. Valleytronic material platform with anisotropic response is highly desired to develop polarization sensitive valleytronic devices. In this work, we acquire monolayer MoS2 with both valley degree of freedom and anisotropy by introducing Re doping during chemical vapor deposition growth. The atomic substitution with Re element induces prominent n-type doping. Electrical tests under both dark and light illumination show that 5 at% Re-doped MoS2 achieves the optimum electrical and optoelectrical performances, corresponding to 27 and 35 times improved carrier mobility and photoresponsivity than pristine MoS2, respectively. Second harmonic generation (SHG) and polarization Raman spectroscopy (PRS) indicate increased anisotropic optical response of Re doped MoS2 as Re concentration increases: maximum anisotropic ratios of 1.6 and 1.7 have been achieved for SHG and PRS measurements at different doping concentrations, respectively. Circularly polarized PL measurement demonstrates that the Re-doped MoS2 preserves valley dependent optical response. Our theoretical calculations show that the anisotropic response is due to the strain induced by the dopants, so that the 2H lattice structure optical helicity dependent valley selection rules are largely preserved. The work opens promising venue toward polarization sensitive valleytronic material platform.