Huanfang Tian

Triggering the electrocatalytic hydrogen evolution activity of the inert two-dimensional MoS2 surface via single-atom metal doping

Electrocatalytic splitting of water is one of the most efficient technologies for hydrogen production, and two-dimensional (2D) MoS2 has been considered as a potential alternative to Pt-based catalysts in the hydrogen evolution reaction (HER). However, the catalytic activity of 2D MoS2 is always contributed from its edge sites, leaving a large number of in-plane domains useless. Herein, we for the first time demonstrated that the catalytic activity of in-plane S atoms of MoS2 can be triggered via single-atom metal doping in HER. In experiments, single Pt atom-doped, few-layer MoS2 nanosheets (Pt–MoS2) showed a significantly enhanced HER activity compared with pure MoS2, originating from the tuned adsorption behavior of H atoms on the in-plane S sites neighboring the doped Pt atoms, according to the density functional theory (DFT) calculations. Furthermore, the HER activity of MoS2 doped with a number of transition metals was screened by virtue of DFT calculations, resulting in a volcano curve along the adsorption free energy of H atoms , which was further confirmed in experiment by using non-precious metals such as Co and Ni atoms doping 2D MoS2 as the catalysts.

Microstructure and structural phase transitions in iron-based superconductors

Crystal structures and microstructural features, such as structural phase transitions, defect structures, and chemical and structural inhomogeneities, are known to have profound effects on the physical properties of superconducting materials. Recently, many studies on the structural properties of Fe-based high-Tc superconductors have been published. This review article will mainly focus on the typical microstructural features in samples that have been well characterized by physical measurements. (i) Certain common structural features are discussed, in particular, the crystal structural features for different superconducting families, the local structural distortions in the Fe2Pn2 (Pn = P As, Sb) or Fe2Ch2 (Ch = S, Se, Te) blocks, and the structural transformations in the 122 system. (ii) In FeTe(Se) (11 family), the superconductivity, chemical and structural inhomogeneities are investigated and discussed in correlation with superconductivity. (iii) In the K0.8Fe1.6+xSe2 system, we focus on the typical compounds with emphasis on the Fe-vacancy order and phase separations. The microstructural features in other superconducting materials are also briefly discussed.

Spontaneous formation of circular and vortex ferroelectric domain structure in hexagonal YMnO3 and YMn0.9Fe0.1O3 prepared by low temperature solution synthesis

We report an experimental study of the domain structure of ferroelectric YMnO3 and YMn0.9Fe0.1O3 using polycrystalline samples prepared by direct hydrothermal crystallisation at 240u2009°C, well below their structural phase transition temperatures. Powder X-ray diffraction shows the expected P63cm space group for both samples with an increase in a and a small decrease in c with Fe incorporation, consistent with an adjustment of MnO5 tilting, while XANES spectra at the Mn and Fe K edges show the oxidation state of both metals are maintained at +3 in the doped sample. High resolution TEM shows that curved stripe, annular and vortex domains can all be observed in the YMnO3 crystals, proving that the structural phase transition is not the only driving force for the occurrence of the annular and vortex domains. Furthermore, the absence of the annular and vortex domains in YMn0.9Fe0.1O3 indicates that the tilting state of MnO5 bipyramids plays an important role in the domain formation. Atomic resolution STEM images...

Circular Photogalvanic Effect in the Weyl Semimetal TaAs

Weyl semimetal (WSM) is expected to be an ideal spintronic material owing to its spin currents carried by the bulk and surface states with spin-momentum locking. The generation of a sizable photocurrent is predicted in non-centrosymmetric WSM arising from the broken inversion symmetry and the linear energy dispersion that is unique to Weyl systems. In our recent measurements, the circular photogalvanic effect (CPGE) is discovered in the TaAs WSM. The CPGE voltage is proportional to the helicity of the incident light, reversing direction if the radiation helicity changes handedness, a periodical oscillation therefore appears following the alteration of light polarization. We herein attribute the CPGE to the asymmetric optical excitation of the Weyl cone, which could result in an asymmetric distribution of photoexcited carriers in momentum space according to an optical spin-related selection rule.

Direct Observation of Magnetic-Ion Off-Centering-Induced Ferroelectricity in Multiferroic Manganite Pr(Sr0.1Ca0.9)2Mn2O7

Ferroelectricity in multiferroic Pr(Sr0.1 Ca0.9)2 Mn2 O7 is found to originate from the off-centering of Mn ions. This polar displacement is energetically stabilized by the cooperative interplay of lattice deformation induced by orbital ordering and oxygen octahedral tilting. This mechanism implies that magnetism and ferroelectricity arise from the same magnetic ions, providing direct evidence for the magnetic-ion off-centering-driven ferroelectricity.

Controlled optical properties of GaSb/InGaAs type-II quantum dots grown on InP substrate

Effect of GaSb/InGaAs quantum dots (QDs) morphology and In composition in InGaAs interlayer on the optical properties of GaSb/InGaAs QD material system was studied in this work. The interband transition of GaSb QD and type-II transition from the InGaAs conduction band to the GaSb hole level were observed. It was found that both QD morphology and In composition in InGaAs interlayer had an influence on the optical properties of GaSb/InGaAs QD material system. Note that the linear relationship exists between In composition in InGaAs interlayer and photoluminescence peak positions, which indicates the structural characteristics of the InGaAs matrix can be used as an important tool to adjust the optical properties of GaSb/InGaAs QD material system. This experimental result shows that the optical properties of GaSb/InGaAs type-II QD material system can be more precisely controlled than the widely studied GaSb/GaAs QD material system.

Two-coupled structural modulations in charge-density-wave state of SrPt2As2 superconductor

Transmission electron microscopy (TEM) study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave (CDW) state below TCDW ≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1 = 0.62a* on the a*−b* plane and q2 = 0.23a* on the a*−c* plane. The atomic displacements occur along the b-axis direction for q1 and along the c-axis direction for q2, respectively. Moreover, the correlation between q1 and q1 can be generally written as q1 = (q2 + a*)/2 in the CDW state, suggesting the presence of essential coupling between q1 and q2. A small fraction of Ir doping on the Pt site in Sr(Pt1–xIrx)2As2 (x ≤ 0.06) could moderately change these CDW modulations and also affect their superconductivities.

Microstructure and oxidation states in multiferroic Lu2(Fe,Mn)3O7

Microstructural features and oxidation states in multiferroic Lu2Fe2.1Mn0.9O7 have been investigated by neutron diffraction,transmission electron microscopy(TEM), and x-rayabsorption near edge spectroscopy. Our experimental results demonstrate the disordered distribution of Mn atoms in the FeO double (W) layers, indicating that structural modulations originate from the charge disproportionation, which is evidenced by the mixed-valence states of Fe ions.TEM measurements clearly reveal Mn doping induced chemical inhomogeneity and local distortions which are associated with the charge disproportionation in the W layers. In addition, the polarization is discussed in association with the lattice distortions and charge disproportionation.

Hidden CDW states and insulator-to-metal transition after a pulsed femtosecond laser excitation in layered chalcogenide 1T-TaS2−xSex

Ultrafast laser excitation of electronic crystals reveals hidden quantum states with new polaron cluster ordering. The hidden (H) quantum state in 1T-TaS2 has sparked considerable interest in the field of correlated electron systems. Here, we investigate ultrafast switches to stable H charge density wave (H-CDW) states observed in 1T-TaS2−xSex, with x = 0 and 0.5 crystals, upon excitation with a single femtosecond laser pulse. In situ cooling transmission electron microscopy observations, initiated by a single femtosecond laser pumping with a low fluence, reveal a clear transition from a commensurate CDW phase (qC) to a new CDW order with qH = (1 − δ)qC for the H-CDW state (δ = 1/9) accompanied by an evident phase separation. H-CDW domain relaxation then occurs and yields a stable metallic phase under a high-fluence excitation. Furthermore, electrical resistivity measurements show that the notable drop in x = 0 and 0.5 samples associated with the appearance of H-CDW states depend on laser fluence and temperature. These results potentially provide a new perspective on the photodoping mechanism for the emergence of H-CDW states in the 1T-TaS2−xSex family.

Defect effects on spatiotemporal evolution of photoinduced martensitic transition in MnNiSn

Martensitic transition and reverse transition in ferromagnetic shape memory alloy MnNiSn contain a variety of structural dynamic features accompanied directly by atomic motions and micro-domain alterations. To investigate the effects of crystalline defects on the dynamical structural phase transitions, we use ultrafast transmission electron microscopy (UTEM) to directly image the rapid structural phase transition in MnNiSn initiated by femtosecond laser pulses. Via high spatiotemporal resolution images, we reveal the pinning effect by the grain boundary on the phonon-driven martensitic transition after fs-laser pulse excitations, and the structural oscillation is also observed as driven by coherent acoustic phonons that start at the sites of the grain boundary and propagate with the speed of sound. These results elucidate the roles of crystallographic defects in the dynamical processes of martensitic transition and highlight the unprecedented capability of UTEM for direct imaging lattice motions with nanometer spatial and picosecond temporal resolutions.Martensitic transition and reverse transition in ferromagnetic shape memory alloy MnNiSn contain a variety of structural dynamic features accompanied directly by atomic motions and micro-domain alterations. To investigate the effects of crystalline defects on the dynamical structural phase transitions, we use ultrafast transmission electron microscopy (UTEM) to directly image the rapid structural phase transition in MnNiSn initiated by femtosecond laser pulses. Via high spatiotemporal resolution images, we reveal the pinning effect by the grain boundary on the phonon-driven martensitic transition after fs-laser pulse excitations, and the structural oscillation is also observed as driven by coherent acoustic phonons that start at the sites of the grain boundary and propagate with the speed of sound. These results elucidate the roles of crystallographic defects in the dynamical processes of martensitic transition and highlight the unprecedented capability of UTEM for direct imaging lattice motions with nano...