Strain effect on the electronic properties of 1T-HfS2 monolayer

We perform first-principles based on the density function theory to investigate electronic and magnetic properties of 1T-HfS₂ monolayer with biaxial tensile strain and compressive strain. The results show that HfS₂ monolayer under strains doesn’t display magnetic properties. When the strain is 0%, the HfS₂ monolayer presents an indirect band gap semiconductor with the band gap is about 1.252 eV. The band gap of HfS₂ monolayer decreases quickly with increasing compressive strain and comes to zero when the compressive strain is above −7%, the HfS₂ monolayer system turns from semiconductor to metal. While the band gap increases slowly with increasing tensile strain and comes to 1.814 eV when the tensile strain is 10%. By comparison, we find that the compressive strain is more effective in band engineering of pristine 1T-HfS₂ monolayer than the tensile strain. And we notice that the extent of band gap variation is different under tensile strain. The change of band gap with strain from 1% to 5% is faster than that of the strain 6–10%. To speak of, the conduction band minimum (CBM) is all located at M point with different strains. While the valence band maximum (VBM) turns from Γ point to K point when the strain is equal to and more than 6%.     

Tunable electronic structure in stained two dimensional van der Waals g-C2N/XSe2 (X = Mo,W) heterostructures

The electronic structure of the strained g-C₂N/XSe₂ (X=Mo, W) van der Waals heterostructures are investigated by first-principles calculations. The g-C₂N/MoSe₂ heterostructure is an indirect band gap semiconductor at a strain from 0% to 8%, where its band gap is 0.66, 0.61, 0.73, 0.60 and 0.33 eV. At K point, the spin splitting is 186, 181, 39, 13 and 9 meV, respectively. For g-C₂N/WSe₂ heterostructures, the band gap is 0.32, 0.37, 0.42, 0.45 and 0.36 eV, and the conduction band minimum is shifted from Г-M region to K-Г region as the strain increases from 0% to 8%. Its spin splitting monotonically decreases as a strain raises to 8%, which is 445, 424, 261, 111 and 96 meV, respectively. Moreover, at a strain less than 4%, the conduction band mainly comes from g-C₂N, but it comes from XSe₂ (X=Mo, W) above 6%. Our results show that the g-C₂N/XSe₂ heterostructures have tunable electronic structures, which makes it a potential candidate for novel electronic devices.     

Effects of in-plane stiffness and charge transfer on thermal expansion of monolayer transition metal dichalcogenide

The temperature dependence of lattice constants is studied by using first-principles calculations to determine the effects of in-plane stiffness and charge transfer on the thermal expansions of monolayer semiconducting transition metal dichalcogenides.Unlike the corresponding bulk material,our simulations show that monolayer MX₂（M = Mo and W;X = S,Se,and Te） exhibits a negative thermal expansion at low temperatures,induced by the bending modes.The transition from contraction to expansion at higher temperatures is observed.Interestingly,the thermal expansion can be tailored regularly by alteration of the M or X atom.Detailed analysis shows that the positive thermal expansion coefficient is determined mainly by the in-plane stiffness,which can be expressed by a simple relationship.Essentially the regularity of this change can be attributed to the difference in charge transfer between the different elements.These findings should be applicable to other two-dimensional systems.     

Photoinduced chirality in a photochromic stilbene-molecule-doped polymeric liquid crystal film

Photoinduced chirality has been demonstrated in thin films of a main-chain polymeric liquid crystal (PLC) system doped with photochromic stilbene molecules by irradiation with circularly polarized light (CPL). CPL-induced chirality was observed in the intrinsically achiral films which were doped with low-molecular-weight stilbene molecules. It was found that the level of photoinduced chirality of the stilbene-molecule-doped system is comparable to that of the azobenzene-molecule-doped system. Moreover, reversible photo-rewriting property for the stilbene-doped system was also demonstrated by alternating irradiation with right- and left-CPL.     

Electronic structure and charge-density wave transition in monolayer VS2

Vanadium disulfide (VS₂) attracts elevated interests for its charge-density wave (CDW) phase transition, ferromagnetism, and catalytic reactivity, but the electronic structure of monolayer has not been well understood yet. Here we report synthesis of epitaxial 1T VS₂ monolayer on bilayer graphene grown by molecular-beam epitaxy (MBE). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that Fermi surface with six elliptical pockets centered at the M points shows gap opening at low temperature. Temperature-dependence of the gap size suggests existence of CDW phase transition above room temperature. Our observations provide important evidence to understand the strongly correlated electron physics and the related surface catalytic properties in two-dimensional transition-metal dichalcogenides (TMDCs).     

Giant two‐photon absorption in monolayer MoS2

Strong two‐photon absorption (TPA) in monolayer MoS₂ is demonstrated in contrast to saturable absorption (SA) in multilayer MoS₂ under the excitation of femtosecond laser pulses in the near‐infrared region. MoS₂ in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO₂/Si by employing the seeding method through chemical vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS₂ possesses a nonsaturation TPA coefficient as high as ∼(7.62 ±0.15) ×10³ cm/GW, larger than that of conventional semiconductors by a factor of 10³. As a result of TPA, two‐photon pumped frequency upconverted luminescence is observed directly in the monolayer MoS₂. For the multilayer MoS₂, the SA response is demonstrated with the ratio of the excited‐state absorption cross section to ground‐state cross section of ∼0.18. In addition, the laser damage threshold of the monolayer MoS₂ is ∼97 GW/cm², larger than that of the multilayer MoS₂ of ∼78 GW/cm².

     

Electronic and magnetic properties of Mn-doped WSe2 monolayer under strain

Electronic and magnetic properties of Mn-doped WSe₂ monolyer subject to isotropic strain are investigated using the first-principles methods based on the density functional theory. Our results indicate that Mn-doped WSe₂ monolayer is a magnetic semiconductor nanomaterial with strong spontaneous magnetism without strain and the total magnetic moment of Mn-doped system is 1.038μ_B. We applied strain to Mn-doped WSe₂ monolayer from -10% to 10%. The doped system transforms from magnetic semiconductor to half-metallic material from −10% to −2% compressive strain and from 2% to 6% tensile strain. The largest half-metallic gap is 0.450 eV at −2% compressive strain. The doped system shows metal property from 7% to 10%. Its maximum magnetic moment comes to 1.181μ_B at 6% tensile strain. However, the magnetic moment of system decreases to zero sharply when tensile strain arrived at 7%. Strain changes the redistribution of charges and arises to the magnetic effect. The coupling between the 3d orbital of Mn atom, 5d orbital of W atom and 4p orbital of Se atom is analyzed to explain the strong strain effect on the magnetic properties. Our studies predict Mn-doped WSe₂ monolayers under strain to be candidates for thin dilute magnetic semiconductors, which is important for application in semiconductor spintronics.     

Giant enhancement of photoluminescence emission in monolayer WS2 by femtosecond laser irradiation

Monolayer transition metal dichalcogenides have emerged as promising mat erials for opt oelectTonic and nanophotonic devices.However,the low photoluminescence(PL)quantum yield(QY)hinders their various potential applications.Here we engineer and enhance the PL intensity of monolayer WS₂by femtosecond laser irradiation.More than two orders of magnitude enhancement of PL intensity as compared to the as-prepared sample is determined.Furthermore,the engineering time is shortened by three orders of magnitude as compared to the improvement of PL intensity by continuous-wave laser irradiation.Based on the evolution of PL spectra,we attribute the giant PL enhancement to the conversion from trion emission to exciton,as well as the improvement of the QY when exciton and trion are localized to the new-formed defects.We have created microstructures on the monolayer WS₂based on the enhancement of PL intensity,where the engineered structures can be stably stored for more than three years.This flexible approach with the feature of excellent long-term storage stability is promising for applications in information storage,display technology,and opto electronic devices.     

基于磁圆二向色谱的单层MoS_2激子能量和线宽温度依赖特性

以二硫化钼(MoS_2)为代表的过渡金属硫属化物属于二维层状材料,样品可以薄至单层.单层MoS_2是一种直接带隙半导体,在纳米逻辑器件、高速光电探测、纳米激光等领域具有广阔的应用前景.在实际应用中,温度是影响半导体材料能带结构和性质的主要因素之一.因此研究单层二维材料能带的温度依赖特性对理解其物理机理以及开展器件应用具有重要的意义.目前,在广泛采用的测量单层MoS_2反射谱的研究中,激子峰往往叠加在一个很强的光谱背底上,难以准确分辨激子的峰位和线宽.基于自行搭建的显微磁圆二向色谱系统,研究了单层MoS_2在65—300 K温度范围内的反射谱和磁圆二向色谱,结果表明磁圆二向色谱在研究单层材料激子能量和线宽方面具有明显的优势.通过分析变温的磁圆二向色谱,得到了不同温度下的A,B激子的跃迁能量和线宽.通过对激子能量和线宽的温度依赖关系进行拟合,进一步讨论了声子散射对激子线宽的影响.     

Effect of strain on electronic and magnetic properties of n-type Cr-doped WSe2 monolayer

Using first-principles calculations based on the density functional theory, we study the effect of strain on the electronic and magnetic properties of Cr-doped WSe₂ monolayer. The results show that no magnetic moment is induced in the Cr-doped WSe₂ monolayer without strain. For the Cr substitutions, the impurity states are close to the conduction bands, which indicate n-type doping occurs in this case. Then we applied strain (from −10% to 10%) to the doped system, and find that a little magnetic moment is induced with tensile strain from 6% to 9% and negligible. We find that the influence of strain on the magnetic properties is inappreciable in Cr-doped WSe₂. Moreover, the tensile strain appears to be more effective in reducing the band gap of Cr-doped WSe₂ monolayer than the compressive strain.     

Monte Carlo Simulation Study on Domain Formation in the Photoinduced Low-Spin High-Spin Transition

The domain formation process of the photo-induced cooperative low-spin (LS) to high-spin (HS) conversion phenomena are studied based on the Monte Carlo simulation method. The existence of the threshold light intensity and the incubation period are demonstrated. It is pointed out that the nucleation barrier of HS domain is surmounted by the fluctuation caused by the repetition of excitation and de-excitation. By defining an effective temperature for this fluctuation, the time scale of incubation period is estimated following the usual domain formation theory. It grossly accounts the Monte Carlo (MC) results in three-dimensional cases. The phase separation of HS and LS domain is reproduced by considering the volume striction effect. The step wise abrupt conversion is ascribed to the de-pinning of the volume striction.     

吡咯俘精酸酐的光致各向异性研究

将有机光致变色化合物—吡咯取代俘精酸酐掺杂于PMMA中形成薄膜.在紫外光照射下,薄膜由无色态转换为呈色态.用650 nm线偏振激光照射薄膜,在由呈色态转变为无色态时产生光致各向异性.对633 nm的探测光具有正单轴晶体特性,光轴方向平行于激发光振动方向,光致二向色性率(D_⊥-D_∥)可达0.2,光致双折射率(n_∥-n_⊥)可达2×10^-3.实验还测量了光致各向异性与曝光量的特性曲线,发现最佳曝光量为13～20 J/cm²;理论分析了其原因.这些结果为俘精酸酐材料在光信息处理方面的应用提供了实验数据.     

Selectively strong molecular adsorption on boron nitride monolayer induced by transition metal substrate

We studied adsorption of several molecules (CO, CO₂, H₂O, N₂O, NO, NO₂, and O₂) on hexagonal boron nitride (h-BN) monolayers supported on transition metal (TM) surfaces, using density functional calculations. We observed that all the molecules bind very weakly on the pristine h-BN, with binding energies in the range of 0.02–0.03 eV. Interestingly, however, when h-BN is supported on the TM surface, NO₂ and O₂ become strongly chemisorbed on h-BN, with binding energies of >1 eV, whereas other molecules still physisorbed, with binding energies of ～0.1 eV at most. The electron transfer from TM to p_z states of h-BN played a substantial role in such strong bindings of NO₂ and O₂ on h-BN, as these molecules possess unpaired electrons that can interact with p_z states of h-BN. Such selective molecular binding on h-BN/TM originates from the peculiar distribution of the spin-polarized highest occupied and lowest unoccupied molecular orbitals of NO₂ and O₂. Strong molecular adsorption and high selectivity would make the h-BN/TM system possible for a variety of applications such as catalysts and gas sensors.     

SDW in the 2D Compound CuFeTe2

In a previous work (ICAME'97) we presented the Mössbauer results for a non-stoichiometric sample of the quasi-two-dimensional (2D) dichalcogenide CuFeTe₂, where a Spin Density Wave (SDW) ground state with T _SDW=256±15 K was proposed. Here we report the study of the magnetic and electric properties determined by magnetic susceptibility, Mössbauer spectroscopy and resistance measurements, of an almost stoichiometric sample prepared by the vertical Bridgman growth technique. The SDW behavior is supported by the results obtained by the following different techniques: Magnetic susceptibility: A magnetic transition is observed at T _SDW=308 K with a Pauli paramagnetic behavior above this temperature. Mössbauer effect: The shape of the spectra and the thermal evolution of the hyperfine field are characteristic of the SDW's in quasi-2D systems. Electrical resistance: There is a metal–semiconductor transition along the layers as the temperature decreases indicating the opening of a gap at the Fermi level.     

Polarized photoluminescence spectroscopy in WS2, WSe2 atomic layers and heterostructures by cylindrical vector beams

Due to the large exciton binding energy, two-dimensional (2D) transition metal dichalcogenides (TMDCs) provide an ideal platform for studying excitonic states and related photonics and optoelectronics. Polarization states lead to distinct light-matter interactions which are of great importance for device applications. In this work, we study polarized photoluminescence spectra from intralayer exciton and indirect exciton in WS₂ and WSe₂ atomic layers, and interlayer exciton in WS₂/WSe₂ heterostructures by radially and azimuthally polarized cylindrical vector laser beams. We demonstrated the same in-plane and out-of-plane polarization behavior from the intralayer and indirect exciton. Moreover, with these two laser modes, we obtained interlayer exciton in WS₂/WSe₂ heterostructures with stronger out-of-plane polarization, due to the formation of vertical electric dipole moment.     

Manipulating the Exciton Dynamics in a MoS2/WS2 Heterobilayer with a Si/Au Nanocavity

Manipulating the exciton dynamics in a heterobilayer (HB) composed of two transition metal dichalcogenides (TMDCs) is important in the development of photonic/plasmonic devices based on TMDC HBs. Here, the realization of such a manipulation in a MoS₂/WS₂ HB is reported by using a Si/Au hybrid nanocavity composed of a Si nanoparticle and an Au film, which is manifested in the modification in the photoluminescence (PL) of the embedded MoS₂/WS₂ HB. It is shown that a transition from PL quenching to PL enhancement can be achieved by adjusting the diameter of the Si nanoparticle, which modifies the plasmon resonance supported by the Si/Au nanocavity. More interestingly, it is demonstrated that the enhancement factor can be manipulated by shifting the exciton/trion resonance close to or far away from the plasmon resonance by simply increasing the laser power. It is revealed that the manipulation is realized by effectively controlling the strain and Purcell effects induced by the Si/Au nanocavity. A PL enhancement factor as large as ≈187 in the MoS₂/WS₂ HB at a high laser power is observed. The findings suggest the potential applications of dielectric-metal hybrid nanocavities in the manipulation of the exciton dynamics in TMDC HBs and the development of novel plasmonic devices.     

Charge density wave states in phase-engineered monolayer VTe2

Charge density wave (CDW) strongly affects the electronic properties of two-dimensional (2D) materials and can be tuned by phase engineering. Among 2D transitional metal dichalcogenides (TMDs), VTe$_{2}$ was predicted to require small energy for its phase transition and shows unexpected CDW states in its T-phase. However, the CDW state of H-VTe$_{2}$ has been barely reported. Here, we investigate the CDW states in monolayer (ML) H-VTe$_{2}$, induced by phase-engineering from T-phase VTe$_{2}$. The phase transition between T- and H-VTe$_{2}$ is revealed with x-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) measurements. For H-VTe$_{2}$, scanning tunneling microscope (STM) and low-energy electron diffraction (LEED) results show a robust $2\sqrt 3 \times 2\sqrt 3 $ CDW superlattice with a transition temperature above 450 K. Our findings provide a promising way for manipulating the CDWs in 2D materials and show great potential in its application of nanoelectronics.