Tungsten ditelluride (WTe2), a layered transition-metal dichalcogenide (TMD), has recently demonstrated an extremely large magnetoresistance effect, which is unique among TMDs. This fascinating feature seems to be correlated with its special electronic structure. Here, we report the observation of 6 Raman peaks corresponding to the , , , , and phonons, from the 33 Raman-active modes predicted for WTe2. This provides direct evidence to distinguish the space group of WTe2 from those of other TMDs. Moreover, the Raman evolution of WTe2 from bulk to monolayer is clearly revealed. It is interesting to find that the mode, centered at ~109.8 cm−1, is forbidden in a monolayer, which may be attributable to the transition of the point group from C2v (bulk) to C2h (monolayer). Our work characterizes all observed Raman peaks in the bulk and few-layer samples and provides a route to study the physical properties of two-dimensional WTe2.

Using ferroelectric 0.67Pb͑Mg 1/3 Nb 2/3 ͒O 3 -0.33PbTiO 3 single crystals as substrates, we studied the effects of the ferroelectric poling and the converse piezoelectric effect on the strain state, resistance, insulator-to-metal transition temperature ͑T C ͒, and magnetoresistance ͑MR͒ of La 0.7 Ba 0.3 MnO 3 ͑LBMO͒ thin films. In situ x-ray diffraction measurements indicate that the ferroelectric poling ͑or the converse piezoelectric effect͒ induces a substantial reduction in the in-plane tensile strain in the LBMO film, giving rise to a decrease in the resistance and an increase in T C . The relative changes of the resistance and T C are proportional to the induced reduction in the in-plane tensile strain ͑␦ xx ͒ in the film. The reduction in the in-plane tensile strain leads to opposite effects on MR below and above T C , namely, MR is reduced for T Ͻ T C while MR is enhanced for T Ͼ T C . We discuss these strain effects within the framework of the Jahn-Teller ͑JT͒ electron-lattice coupling and phase separation scenario that are relevant to the induced strain. Similar studies on CaMnO 3 thin films, for which there is no JT distortion of MnO 6 octahedra, show that the resistance of the films also decreases when the tensile strain is reduced, indicating that the resistance change arising from the reduction in Mn-O bond length dominates over that arising from the reduction in Mn-O-Mn bond angle.

counterparts. [2] Therefore, 2D materials are ideal for flexible optoelectronics and have the potential to be used in the next-generation ultrathin electronic and optoelectronic devices. [1] The concept of 2D materials was first realized when graphene was found in 2004. [4] Graphene has attracted extensive attention for its excellent electrical, optical, and mechanical properties. [4][5][6] They have been investigated for various technological applications, including spintronics, sensors, optoelectronics, supercapacitors, and solar cells, etc. [5,7] Besides graphene, other 2D materials, such as h-BN, phosphorene, silicene, germanene, and transition metal dichalcogenides (molybdenum disulfide (MoS 2 ), molybdenum diselenide (MoSe 2 ), tungsten disulfide (WS 2 ), and tungsten diselenide (WSe 2 ), etc.), have been studied extensively in recent years. [1,[8][9][10][11] The thickness of single-layer 2D materials is usually on the order or less than 1 nm. At the same time, their lateral sizes could reach much larger size (from microns to even inches), and 2D materials can be transferred to different substrates before subsequent processing or follow-up measurements for characterizations or device applications.Strain engineering is a promising way to tune the electrical, electrochemical, magnetic, and optical properties of 2D materials, with the potential to achieve high-performance 2D-material-based devices ultimately. This review discusses the experimental and theoretical results from recent advances in the strain engineering of 2D materials. Some novel methods to induce strain are summarized and then the tunable electrical and optical/optoelectronic properties of 2D materials via strain engineering are highlighted, including particularly the previously less-discussed strain tuning of superconducting, magnetic, and electrochemical properties. Also, future perspectives of strain engineering are given for its potential applications in functional devices. The state of the survey presents the ever-increasing advantages and popularity of strain engineering for tuning properties of 2D materials. Suggestions and insights for further research and applications in optical, electronic, and spintronic devices are provided.

We report photoelectric properties of two-dimensional electron gas (2DEG) at an amorphous LaAlO3/SrTiO3 interface. Under visible light illumination (650 nm), an enhancement of electric conductivity is observed over the temperature range from 2 to 300 K. Particularly, a resistance upturn appearing below 25 K, which is further proved to from the Kondo effect, is suppressed by the 650 nm visible light. From the results of light-assisted Hall measurements, light irradiation increases the carrier mobility rather than carrier density in the Kondo regime. It is suggested that light induces the decoherence effect of localized spin states, hence the electron scattering is weakened and the carrier mobility is improved accordingly. Moreover, the enhancement of electrical conductivity by visible light verifies that in-gap states located in the SrTiO3 side of the interface play an important role in the electrical transport of the amorphous SrTiO3-based oxide 2DEG system. Our results provide deeper insight into the photoinduced effects in the 2DEG system, paving the way for the design of optoelectronic devices based on oxides.

operable under a wide range of pH environments for the industrialization and commercialization of electrocatalytic H 2 production. Hydrogen binding energy (HBE) is a key parameter for evaluating the intrinsic activity of hydrogen evolution reaction (HER) electrocatalysts. [3] The intrinsic HER activity is determined by the electronic structure of the catalyst. [4] However, when the proton donor changes during the reaction because of the increasing electrolyte pH-H 3 O + in acidic and H 2 O in neutral/ alkaline media-the adsorption/desorption of interfacial water becomes an additional factor that influences the intrinsic HER activity. Therefore, to achieve optimal HER activity, the electronic structure of the electrocatalysts must be optimized. [5] Among the different strategies reported for tuning the electronic structure of these electrocatalysts, [6] interfacial engineering is the most effective. In this strategy, charge redistribution is induced by the electronic interaction across a heterostructure interface, which tunes the adsorption strength of reaction intermediates. [7] However, the monotonic transport pathway of the interfacial electrons in most heterostructure catalysts limits the simultaneous modulation of the catalyst electronic structure for optimal HBE and water dissociation kinetics at the electrocatalytic sites for HER (particularly in neutral and alkaline media). [8] Designing and synthesizing highly efficient and stable electrocatalysts for hydrogen evolution reaction (HER) is important for realizing the hydrogen economy. Tuning the electronic structure of the electrocatalysts is essential to achieve optimal HER activity, and interfacial engineering is an effective strategy to induce electron transfer in a heterostructure interface to optimize HER kinetics. In this study, ultrafine RhP 2 /Rh nanoparticles are synthesized with a well-defined semiconductor-metal heterointerface embedded in N,P co-doped graphene (RhP 2 /Rh@NPG) via a one-step pyrolysis. RhP 2 /Rh@NPG exhibits outstanding HER performances under all pH conditions. Electrochemical characterization and first principles density functional theory calculations reveal that the RhP 2 /Rh heterointerface induces electron transfer from metallic Rh to semiconductive RhP 2 , which increases the electron density on the Rh atoms in RhP 2 and weakens the hydrogen adsorption on RhP 2 , thereby accelerating the HER kinetics. Moreover, the interfacial electron transfer activates the dual-site synergistic effect of Rh and P of RhP 2 in neutral and alkaline environments, thereby promoting reorganization of interfacial water molecules for faster HER kinetics.