Two-dimensional materials, such as graphene, transition metal dichalcogenides, and phosphorene, can be used to construct van der Waals multilayer structures. This approach has shown potentials to produce new materials that combine novel properties of the participating individual layers. One key requirement for effectively harnessing emergent properties of these materials is electronic connection of the involved atomic layers through efficient interlayer charge or energy transfer. Recently, ultrafast charge transfer on a time scale shorter than 100 fs has been observed in several van der Waals bilayer heterostructures formed by two different materials. However, information on the transfer between two atomic layers of the same type is rare. Because these homobilayers are essential elements in constructing multilayer structures with desired optoelectronic properties, efficient interlayer transfer is highly desired. Here we show that electron transfer between two monolayers of MoSe2 occurs on a picosecond time scale. Even faster transfer was observed in homobilayers of WS2 and WSe2. The samples were fabricated by manually stacking two exfoliated monolayer flakes. By adding a graphene layer as a fast carrier recombination channel for one of the two monolayers, the transfer of the photoexcited carriers from the populated to the drained monolayers was time-resolved by femtosecond transient absorption measurements. The observed efficient interlayer carrier transfer indicates that such homobilayers can be used in van der Waals multilayers to enhance their optical absorption without significantly compromising the interlayer transport performance. Our results also provide valuable information for understanding interlayer charge transfer in heterostructures.Keywords: electron transfer; transient absorption; transition metal dichalcogenide; two-dimensional material; van der Waals interface;

•NdNiO3 thin film has been prepared by pulsed laser deposition on LaAlO3 (001) substrates.•The temperature of insulator-metal transition can be tuned by the thin film thickness.•The level of NiO6 distortion changed by thin film thickness is revealed by Raman spectroscopy.•Optical conductivity study shows the carriers variation in the process of phase transition.NdNiO3 thin film has been prepared by pulsed laser deposition on LaAlO3 (001) single crystalline substrate. Temperature-dependent resistivity measurement shows a sharp metal-insulator transition in such thin film. The phase transition temperature can be tuned from 90 K to 121 K by changing the thickness of thin film. The structure evolution during phase transition is studied by Raman spectroscopy. Optical conductivity reveals that the variation carrier density in the process of phase transition. The results of structural, electrical and optical studies provide useful insights to understand the mechanism of metal-insulator transition of NdNiO3 thin film.

We report a comprehensive study on the effects of rhenium doping on optical properties and photocarrier dynamics of MoS2 monolayer, few-layer, and bulk samples. Monolayer and few-layer samples of Re-doped (0.6%) and undoped MoS2 were fabricated by mechanical exfoliation, and were studied by Raman spectroscopy, optical absorption, photoluminescence, and time-resolved differential reflection measurements. Similar Raman, absorption, and photoluminescence spectra were obtained from doped and undoped samples, indicating that the Re doping at this level does not significantly alter the lattice and electronic structures. Red-shift and broadening of the two phonon Raman modes were observed, showing the lattice strain and carrier doping induced by Re. The photoluminescence yield of the doped monolayer is about 15 times lower than that of the undoped sample, while the photocarrier lifetime is about 20 times shorter in the doped monolayer. Both observations can be attributed to diffusion-limited Auger nonradiative recombination of photocarriers at Re dopants. These results provide useful information for developing a doping strategy of MoS2 for optoelectronic applications.

•Pd–Pt, Pd and Pt nanoparticles/graphene were prepared by a simple chemical route.•CO gas was selected as a reductant and capping agent.•Higher methanol oxidation activity was observed with Pd:Pt atomic ratio of 1/1.7.•Superior specific ORR activity and better tolerance to CO poisoning was observed.Bimetallic PdPt alloy nanoparticles on graphene oxide (GO) have been prepared by a simple and facile chemical route, in which the reduction of metal precursors is carried out using CO as a reductant. Structural and morphological characterizations of GO/PdPt composites are performed using X-ray diffraction, X-ray photoelectron spectroscopy analysis and transmission electron microscopy. It is found that PdPt bimetallic nanoparticles are successfully synthesized and uniformly attached on the graphene sheets. The electrocatalytic and electrochemical properties of GO/PdPt composites including methanol oxidation reaction (MOR), oxygen reduction reaction (ORR) and methanol tolerant oxygen reduction reaction (MTORR) are studied in HClO4 aqueous solution. A significant improvement in the electrocatalytic activities is observed by increasing the atomic ratio of Pt in PdPt bimetallic alloys compared to the freestanding Pd nanoparticles on GO. The prepared GO/PdPt composites with an (Pd:Pt) atomic ratio of 40:60 exhibits higher methanol oxidation activity, higher specific ORR activity and better tolerance to CO poisoning. The results can be attributed to the collective effects of the PdPt nanoparticles and the enhanced electron transfer of graphene.

In an atomically thin-film/dielectric-substrate heterostructure, the elemental physical properties of the atomically thin-film are influenced by the interaction between the thin-film and the substrate. In this article, utilizing monolayer MoS2 on LaAlO3 and SrTiO3 substrates, as well as SiO2 and Gel-film as reference substrates similar to previously reported work [Nano Res, 2014, 7, 561], we systematically investigate the substrate effect on the photoluminescence of monolayer MoS2. We observed significantly substrate-dependant photoluminescence of monolayer MoS2, originating from substrate-to-film charge transfer. We found that SiO2 substrate introduces the most charge doping while SrTiO3 introduces less charge transfer. Through the selection of desired substrate, we are able to induce different amounts of charge into the monolayer MoS2, which consequently modifies the neutral exciton and charged exciton (trion) emissions. Finally, we proposed a band-diagram model to elucidate the relation between charge transfer and the substrate Fermi level and work function. Our work demonstrates that the substrate charge transfer exerts a strong influence on the monolayer MoS2 photoluminescence property, which should be considered during device design and application. The work also provides a possible route to modify the thin-film photoluminescence property via substrate engineering for future device design.

•Amorphous and crystalline LaLuO3 thin films were grown by PLD.•The infrared phonon modes are kept in the crystalline thin film but reduced in the amorphous thin film.•The amorphous thin film has different local structure with crystalline thin film.•The changes of phonon modes and dielectric constant are closely related to the local structure.The infrared phonon modes and local structure of amorphous and crystalline LaLuO3 thin films were studied by infrared spectroscopy and X-ray absorption fine structure (XAFS). The infrared spectroscopy shows that the crystalline thin film keeps the main infrared phonon modes and preserves the static dielectric constant as crystal bulk sample. But the infrared phonon modes of amorphous thin film reduce and cause the static dielectric constant to decrease. The result is closely related to the local structure as revealed by XAFS. The crystalline thin film has the similar local structure as crystal bulk sample, while the amorphous thin film presents obvious change. The different local structure of the amorphous thin film results in the change of phonon modes and static dielectric constant.

Crystallization from memorized ordered melt (MOM) of isotactic polybutene-1 (iPB-1) is investigated with in situ Fourier transformation infrared microspectroscopic imaging and wide- and small-angle X-ray scattering. After being partially melted at high temperature, a small portion of form I crystal of iPB-1 recovers back when the temperature is lowered. This is different from a disordered melt, which crystallizes into form II directly. The recovery of form I crystal is attributed to the presence of MOM, which may keep the conformation order of form I crystal. Experimental evidence show that MOM possesses three characteristics, namely: (i) associating with the preserved form I; (ii) occupying only a small portion of the melt in the partially melted sample; (iii) being stable at high temperature. All this experimental evidence suggests that MOM locates at the boundary of crystal and melt. This is in line with the physical picture of nonclassical nucleation and growth theory or the multistage approach, where a partially ordered melt layer locates at the boundary of crystal and melt.

The crystal–crystal transition of isotactic polybutene-1 (iPB-1) from form II to I at room temperature is investigated with in-situ Fourier transform infrared spectroscopy (FTIR), Fourier transformation infrared microspectroscopic imaging (FTIRI) and synchrotron radiation scanning X-ray micro-diffraction (SR-μSXRD). The transition rate from form II to I shows a non-monotonic correlation with crystallization temperature on samples isothermally crystallized at different single temperatures. An abnormal spatial distribution of transition rate is observed in samples prepared with a two-step crystallization approach, in which samples crystallized at high temperature and then were quenched to low temperatures for further crystallization. A maximum transition rate occurs around the edge of large spherulites formed at high temperature, which cannot be interpreted by the effect of crystallization temperature alone. The accelerated transition rate in this region is attributed to internal stress, where an intermediate state, revealed with SR-μSXRD, may be the structural origin for the fast transition rate from form II to I.

In order to obtain unique information of Ganoderma lucidum spores, FTIR microspectroscopy was used to study G. lucidum spores from Anhui Province (A), Liaoning Province (B) and Shangdong Province (C) of China. IR micro-spectra were acquired with high-resolution and well-reproducibility. The IR spectra of G. lucidum spores from different areas were similar and mainly made up of the absorption bands of polysaccharide, sterols, proteins, fatty acids, etc. The results of curve fitting indicated the protein secondary structures were dissimilar among the above G. lucidum spores. To identify G. lucidum spores from different areas, the H1078/H1640 value might be a potentially useful factor, furthermore FTIR microspectroscopy could realize this identification efficiently with the help of hierarchical cluster analysis. The result indicates FTIR microspectroscopy is an efficient tool for identification of G. lucidum spores from different areas. The result also suggests FTIR microspectroscopy is a potentially useful tool for the study of TCM.Graphical abstractIn order to obtain unique information of Ganoderma Lucidum spores, FTIR microspectroscopy was used to study G. Lucidum spores from Anhui Province (A), Liaoning Province (B) and Shangdong Province (C) of China.Highlights► G. Lucidum spores from three producing areas were studied by FTIR microspectroscopy. ► IR spectra and the secondary derivative IR spectra of G. Lucidum spores were studied. ► Curve fitting was used to study the protein secondary structure of G. Lucidum spores. ► H2956/H2919, H1742/H1640, H1078/H1640 of the above G. Lucidum spores were compared. ► FTIR microspectroscopy can be used to distinguish the above G. Lucidum spores.

In order to obtain unique information of the single gastric cancer cell and investigate gastric cancer further at the cellular and subcellular level, synchrotron infrared (SR-IR) microspectroscopy and imaging was used to study human gastric adenocarcinoma cell line SGC7901 and normal gastric mucosal epithelial cell line GES1. When we compared the infrared micro-spectra of the SGC7901 cells with those of the GES1 cells, the peak height ratios of 1121 cm−1/1020 cm−1 (RNA/DNA) of the SGC7901 cells were significantly higher than those of the GES1 cells, which might be useful for distinguishing malignant cells from normal cells. Furthermore, chemical imaging reveals that the amide II/the amide I ratio of the SGC7901 cell is lower than that of the GES1 cell in the most intense absorption region of vasPO2-. The result indicates the study of the single gastric cancer cell at subcellular level by synchrotron infrared microspectroscopy and imaging can provide more detailed information which is very helpful for the research and diagnosis of gastric cancer.Highlights► Single SGC7901 and GES1 cells were studied by SR-IR microspectroscopy and imaging. ► H1121/H1020 is significantly higher in SGC7901cells than that in GES1 cells. ► Chemical image of the amide II/the amide I of SGC7901is different from that of GES1. ► The differences are useful for distinguishing malignant cells from normal cells.

The physical origin of the static dielectric constant and its relationship with lattice dynamics of La2Hf2O7are studied by combining infrared spectroscopy and density functional perturbation theory (DFPT). Both La and Hf show obvious effective charge anomaly which is attributed to the hybridization between 2p states of the oxygen and 5d states of the cations, indicating a mixed covalent–ionic bonding between the cations and the oxygen. The dielectric response is determined by seven infrared phonon modes and the static dielectric constant extracted from infrared reflection spectrum is in close agreement with DFPT calculation. Both experiment and theory reveal that most of the contributions to the static dielectric constant are dominated by three infrared phonon modes at 137, 172 and 297 cm−1. Two of them (172 and 297 cm−1) are from the displacements of oxygen atoms inside HfO6 octahedra and the other one (137 cm−1) is from the bending of La2O′ chain. This result indicates that the origin of the static dielectric constant of La2Hf2O7 is directly connected with the two interpenetrating sub-networks of pyrochlore structure (HfO6 octahedra and La2O′ chain).Highlights► The physical origin of the static dielectric constant of La2Hf2O7 is studied. ► The static dielectric constant is related to the La2O′ chain and HfO6 octahedron. ► The effective charge anomaly is attributed to the hybridization between O-2p and 5d cations.

The physical origin of the static dielectric constant and its relationship with lattice dynamics of La2Hf2O7are studied by combining infrared spectroscopy and density functional perturbation theory (DFPT). Both La and Hf show obvious effective charge anomaly which is attributed to the hybridization between 2p states of the oxygen and 5d states of the cations, indicating a mixed covalent–ionic bonding between the cations and the oxygen. The dielectric response is determined by seven infrared phonon modes and the static dielectric constant extracted from infrared reflection spectrum is in close agreement with DFPT calculation. Both experiment and theory reveal that most of the contributions to the static dielectric constant are dominated by three infrared phonon modes at 137, 172 and 297 cm−1. Two of them (172 and 297 cm−1) are from the displacements of oxygen atoms inside HfO6 octahedra and the other one (137 cm−1) is from the bending of La2O′ chain. This result indicates that the origin of the static dielectric constant of La2Hf2O7 is directly connected with the two interpenetrating sub-networks of pyrochlore structure (HfO6 octahedra and La2O′ chain).Highlights► The physical origin of the static dielectric constant of La2Hf2O7 is studied. ► The static dielectric constant is related to the La2O′ chain and HfO6 octahedron. ► The effective charge anomaly is attributed to the hybridization between O-2p and 5d cations.