Galvanic-Replacement-Assisted Synthesis of Nanostructured Silver-Surface for SERS Characterization of Two-Dimensional Polymers.

Surface-enhanced Raman scattering (SERS) spectroscopy is a powerful technology in trace analysis. However, the wide applications of SERS in practice are limited by the expensive substrate materials and the complicated preparation processes. Here we report a simple and economical galvanic-replacement-assisted synthesis route to prepare Ag nanoparticles on Cu(0) foil (nanoAg@Cu), which can be directly used as SERS substrate. The fabrication process is fast (ca. 10 min) and easily scaled up to centimeters or even larger. In addition, the morphology of the nanoAg@Cu (with Ag particles size from 30 nm to 160 nm) can be adjusted by various additives (e.g., amino-containing ligands). Finally, we show that the as-prepared nanoAg@Cu can be used for SERS characterization of two-dimensional polymers, and ca. 298 times relative enhancement of Raman intensity is achieved. This work offers a simple and economical strategy for the scalable fabrication of silver-based SERS substrate in thin film analysis.

ErF3 microcrystals controllably deposited in perfluoride glass for upconversion red emission.

To the best of our knowledge, this paper first reports ErF3 microcrystals controllably deposited in perfluoride glass using phase-separation engineering techniques. The sample exhibited strong upconversion red-light emission owing to the small distance between Er3+ ions and low phonon energy (585 cm-1). The sample has a high red/green ratio of up to 18.6, which, to our knowledge, is the highest reported value in Er3+-doped fluoride glass ceramics. Furthermore, the sample has a long fluorescence lifetime (3.18 ms @660 nm), good color saturation (0.6255,0.3707), and good thermal stability (Δ E=0.31e V). Therefore, this sample has the potential for application across multiple fields, such as color display, visible laser, and lighting.

Effects of Inorganic Substitutions and Different Metal Electrode Materials on Electronic Transport Properties of Organic Molecular Devices.

Incorporating inorganic components into organic molecular devices offers one novel alternative to address challenges existing in the fabrication and integration of nanoscale devices. In this study, using a theoretical method of density functional theory combined with the nonequilibrium Green's function, a series of benzene-based molecules with group III and V substitutions, including borazine molecule and XnB3-nN3H6 (X = Al or Ga, n = 1-3) molecules/clusters, are constructed and investigated. An analysis of electronic structures reveals that the introduction of inorganic components effectively reduces the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, albeit at the cost of reduced aromaticity in these molecules/clusters. Simulated electronic transport characteristics demonstrate that XnB3-nN3H6 molecules/clusters coupled between metal electrodes exhibit lower conductance compared to prototypical benzene molecule. Additionally, the choice of metal electrode materials significantly impacts the electronic transport properties, with platinum electrode devices displaying distinct behavior compared to silver, copper, and gold electrode devices. This distinction arises from the amount of transferred charge, which modulates the alignment between molecular orbitals and the Fermi level of the metal electrodes by shifting the molecular orbitals in energy. These findings provide valuable theoretical insights for the future design of molecular devices incorporating inorganic substitutions.

Dual transmission channels at metal-MoS2/WSe2 hetero-bilayer interfaces.

van der Waals heterostructures (vdWHs) open the possibility of creating novel semiconductor materials at the atomic scale that demonstrate totally new physics and enable unique functionalities, and have therefore attracted great interest in the fields of advanced electronic and optoelectronic devices. However, the interactions between metals and vdWHs semiconductors require further investigation as they directly affect or limit the advancement of high-performance electronic devices. Here we study the contact behavior of MoS2/WSe2 vdWHs in contact with a series of bulk metals using ab initio electronic structure calculations and quantum transport simulations. Our study shows that dual transmission paths for electrons and holes exist at the metal-MoS2/WSe2 hetero-bilayer interfaces. In addition, the metal-induced bandgap state (MIGS) of the original monolayer disappears due to the creation of the heterolayer, which weakens the Fermi level pinning (FLP) effect. We also find that the creation of the heterolayer causes a change in the Schottky barrier height (SBH) of the non-ohmic contact systems, whilst this does not occur so easily in the ohmic contact systems. In addition, our results indicate that when Al, Ag and Au are in contact with a MoS2/WSe2 hetero-bilayer semiconductor, a low contact barrier exists throughout the whole transmission process causing the charge to tunnel to the MoS2 layer, irrespective of whether the MoS2 is in contact with the metals as the nearest layer or as the next-nearest layer. Our work not only offers new insights into electrical contact issues between metals and hetero-bilayer semiconductors, but also provides guidance for the design of high-performance vdWHs semiconductor devices.

Multiple strategies of improving photocatalytic water splitting efficiency in 2D arsenic sesquichalcogenides.

Improving the solar-to-hydrogen efficiency has always been a significant topic in the field of photocatalysis. Based on first-principles calculations, herein, we propose multiple strategies to improve the photocatalytic properties of 2D arsenic sesquichalcogenides for full water splitting. The new configurations As2STe2 and As2SeTe2 monolayers, derived from the As2Te3 monolayers by surface modification, are manifested to be typical infrared-light driven photocatalysts. Notably, under the built-in electric field, As2STe2 and As2SeTe2 monolayers can fulfil overall water splitting and the predicted solar-to-hydrogen efficiencies even reach up to 36.19% and 29.36%, respectively. The Gibbs free energy calculations indicate that the OER can be successfully driven under light irradiation. In addition, the overpotentials can provide most of the energy for HER when illuminated, especially for As2STe2 with the . In addition, both As2S3 and As2Se3 monolayers are capable of satisfying the conditions for photocatalytic water splitting. Furthermore, the band gaps of As2Se3 and As2S3 can dramatically be narrowed by increasing the number of layers and doping, respectively. These findings provide a theoretical basis for As2X3 monolayers to achieve efficient photocatalytic water splitting.

Tunable Dirac states in doped B2S3 monolayers.

Two-dimensional (2D) Dirac materials have been a research hotspot due to their intriguing properties, such as high carrier mobility and ballistic charge transport. Here, we demonstrate that the B2S3 monolayer with a hexagonal structure, which has been reported as a photocatalyst, can be tuned to new 2D Dirac materials by doping atoms. The Young's modulus can reach 65.23 N m-1, indicating that the monolayer can be used as a buffer materials. The electronic structures of the pristine B2S3 monolayer show that some Dirac points appear but do not occur exactly on the Fermi level (EF). Fortunately, we find that the Dirac cone can be tuned to the EF by doping C, N, or Sn atoms. The C-doped B2S3 monolayer can be a half-metallic Dirac material, which has significant potential application in spintronics. For N- and Sn-doped B2S3 monolayers, the typical kagome bands are formed near the EF, which arise from three molecular orbitals hybridized by B, S, and N (Sn) atoms. These outstanding properties render the doped B2S3 monolayers promising 2D Dirac materials for future nanoelectronic devices.

Imaging of electron transition and bond breaking in the photodissociation of H2+ via ultrafast X-ray photoelectron diffraction.

We theoretically investigate the photodissociation dynamics of H2+ using the methodology of ultrafast X-ray photoelectron diffraction (UXPD). We use a femtosecond infrared pulse to prompt a coherent excitation from the molecular vibrational state (v = 9) of the electronic ground state (1sσg) and then adopt another time-delayed attosecond X-ray pulse to probe the dynamical properties. We have calculated photoionization momentum distributions by solving the non-Born-Oppenheimer time-dependent Schrödinger equation (TDSE). We unambiguously identify the phenomena associated with the g - u symmetry breakdown in the time-resolved photoelectron diffraction spectra. Using the two-center interference model, we can determine the variation in nuclear spacing with high accuracy. In addition, we use a strong field approximation (SFA) model to interpret the UXPD profile, and the SFA simulations can reproduce the TDSE results in a quantitative way.

Novel 2D B2S3as a metal-free photocatalyst for water splitting.

Metal-free semiconductors with desirable characteristics have recently gained great attention in the field of hydrogen generation. The non-metal material B2S3has two phases, hexagonal B2S3(h-B2S3) and orthorhombic B2S3(o-B2S3), which compose a novel class of 2D materials. Bothh-B2S3ando-B2S3monolayers are direct semiconductors with bandgaps of 2.89 and 3.77 eV by the Heyd-Scuserria-Ernzerhof (HSE) function, respectively. Under appropriate uniaxial strain (1%), the bandgap ofh-B2S3can be decreased to 2.8 eV. The carrier mobility can reach 1160 cm2V-1s-1, supporting the fast migration of photo-induced carriers. Most importantly, the band edges of bothh-B2S3ando-B2S3cover the reduction and oxidation levels for water splitting. We explore the process of photocatalytic water splitting onh-B2S3monolayers by analyzing the feasibility of the decomposition of H2O and the generation of H2. The results indicate that the special mesoporous structure of B2S3is helpful for photocatalytic hydrogen production. The new nanomaterial, B2S3, offers great promise as a metal-free photocatalyst due to its tunable bandgaps, its useful band edges, and its other excellent electronic properties.

Strain-tunable electronic structure and anisotropic transport properties in Janus MoSSe and g-SiC van der Waals heterostructure.

The van der Waals heterostructures (vdWHs) create a multi-purpose platform to design unique structures for efficient photovoltaic and optoelectronic applications. In this study, on the basis of the first-principles calculations, we present a type-II semiconducting MoSSe/g-SiC vdWH with a moderate bandgap value of 1.31 eV. In particular, the large conduction band offset of 1.18 eV and valence band offset of 0.90 eV are distinguished, which can act as powerful driving forces to promote interlayer charge transfer. Moreover, MoSSe/g-SiC vdWH possesses high carrier mobilities and anisotropic transport properties with a larger transport current along the zigzag direction. More importantly, tensile strain can transform indirect into direct band gap and enhance the visible-light absorption while sustaining type-II band alignment. These results reveal the new physical nature of MoSSe/g-SiC vdWH and demonstrate its practical application potential in photovoltaics and optoelectronic nanodevices.

Surface functional group modification induced partial Fermi level pinning and ohmic contact at borophene-MoS2 interfaces.

Large Schottky barrier at the electric contact interface drastically hinders the performance of two-dimensional (2D) semiconductor devices, because of which it is crucial to develop better methods to achieve the ohmic contact. Recently, a new field effect transistor (FET) device was constructed by the popular 2D channel material MoS2 and an electrode material borophene was detected theoretically, but the large Schottky barrier still existed. Hence, we used surface functional groups modification on the borophene surface to regulate this Schottky barrier, based on ab initio electronic structure calculations and quantum transport simulations. Our study shows that this method makes it possible to obtain tunable metal work functions in a wide range, and the ohmic contact can still be realized. Although van der Waals (vdW) contacts were observed at all the interfaces between the 2D borophene-based metals and the monolayer MoS2, the Fermi level pinning (FLP) effect was still obvious, and existed in our proposed system with the ohmic contact. Moreover, we also discuss the origin of the FLP with varying degrees. It was found that the interface dipole and metal-induced gap states (MIGS) would be responsible for the FLP of vertical and lateral directions, respectively. More precisely, we find that the size of MIGS is dependent on the relative orientation between the functional group and metal-MoS2 interface. This work not only suggests that surface functional group modification is effective in forming ohmic contact with MoS2, but also holds some implication in the fundamental research on metal-semiconductor contacts with the vdW type.

Theoretical Simulations of Heavy-Atom Kinetic Isotope Effects in Aliphatic Claisen Rearrangement.

The aliphatic Claisen rearrangement of allyl vinyl ether has attracted great interest for its broad applications in chemical synthesis and biosynthesis. Although it is well agreed that this reaction proceeds via a concerted, "chair-like" transition state, certain inconsistencies of kinetic isotope effect (KIE) data between experimental measurements and theoretical simulations or between independent experiments indicate that the nature and mechanism of this important reaction need to be investigated in more detail. In order to verify two independent sets of experimental data, we present theoretical calculations on heavy-atom KIE values of alipahtic Claisen rearrangement, using our recently developed path-integral method with the second-order Kleinert's variational perturbation theory, which goes beyond the traditional method for computing KIE values by employing the Bigeleisen equation. Amazingly, the results demonstrate that both sets of experimental measurements are correct, while the inconsistency originates from the fact that the aliphatic Claisen rearrangement undergoes similar but different mechanisms in gas and solution phases. Different experimental conditions will alter the actual reactant state by tuning the population distribution of reactant conformers. According to the comparison between experimental and theoretical results, a more clear reaction mechanism of aliphatic Claisen rearrangement is revealed.

[A new lupane-type triterpenoid from Dichroa hirsuta].

The chemical constituents from methanol extract of Dichroa hirsuta were separated by silica gel and Sephadex LH-20 column chromatography,high pressure preparative liquid chromatography( HPLC) and recrystallization. Their structures were elucidated by NMR and MS. Nine compounds were obtained and their structures were identified as 3ß,21α-O-diacetyl-lup-9( 11)-en-7ß-ol( 1),( Z)-methyl p-hydroxycinnamate( 2),cis-p-coumaric acid ethyl ester( 3),( E)-methyl p-hydroxycinnamate( 4),trans-p-coumaric acid ethyl ester( 5),4( 3 H)-quinazolinone( 6),7-hydroxycoumarin( 7),hydrangenol( 8) and thunberginol C( 9). Compound 1 is a new lupane-type triterpenoid,and compounds 1-5,8-9 were firstly isolated from this plant. Dual reporter assay results showed that compounds 2-5 could activate the Nrf2-ARE signaling pathway.

Control of photodissociation of the NaI molecule via pulse chirping.

A dynamic pump control scheme is proposed to manipulate the predissociation process of NaI molecules in different reaction channels. A linearly chirped pulse is used to excite the NaI molecule, and a time-delayed infrared pulse is employed to modify the molecular potentials in the coupling zone. The predissociation branching ratio of the product from two channels can be controlled by tuning the chirp rate with a proper range of delay times. Furthermore, an additional ultrafast photoionization step is adopted to monitor the wave packet evolution and probe the possible modifications of the electronic potential under the influence of a chirped pump field to reveal the physical mechanism behind the control. Aulter-Townes splitting is observed at a proper chirp rate, and the dressed-state population can be controlled via pulse chirping.

Adsorption of gas molecules on a manganese phthalocyanine molecular device and its possibility as a gas sensor.

A theoretical investigation of the gas detection performance of manganese(ii) phthalocyanine (MnPc) molecular junctions for six different gases (NO, CO, O2, CO2, NO2, and NH3) is executed through a non-equilibrium Green's function technique in combination with spin density functional theory. Herein, we systematically studied the adsorption structural configurations, the adsorption energy, the charge transfer, and the spin transport properties of the MnPc molecular junctions with these gas adsorbates. Remarkably, NO adsorption can achieve an off-state of the Mn spin; this may be an effective measure to switch the molecular spin. In addition, our results indicate that by measuring spin filter efficiency and the changes in total current through the molecular junctions, the CO, NO, O2, and NO2 gas molecules can be detected selectively. However, the CO2 and NH3 gas adsorptions are difficult to be detected due to weak van der Waals interaction between these two gases and central Mn atom. Our findings provide important clues to the application of nanosensors for highly sensitive and selective based on MnPc molecular junction systems.

Geometric phase effects on photodissociation dynamics of diatomics.

We investigated the effect of the geometric phase (GP) on photodissociation dynamics at a light-induced conical intersection (LICI) through exact quantum dynamical calculations. By taking the one-photon photodissociation of H 2 + ionic molecules as an example, we explored the conditions wherein the LICI associated GP affects dissociation dynamics. We found that GP leads to a phase shift between the angular distributions of GP included and GP excluded photofragments. This effect is more pronounced when the energy of the initial vibrational level is above the energy of the LICI point.

Effects of temperature, genetic variation and species competition on the sensitivity of algae populations to the antibiotic enrofloxacin.

Primary producers are amongst the most sensitive organisms to antibiotic pollution in aquatic ecosystems. To date, there is little information on how different environmental conditions may affect their sensitivity to antibiotics. In this study we assessed how temperature, genetic variation and species competition may affect the sensitivity of the cyanobacterium Microcystis aeruginosa and the green-algae Scenedesmus obliquus to the antibiotic enrofloxacin. First, we performed single-species tests to assess the toxicity of enrofloxacin under different temperature conditions (20°C and 30°C) and to assess the sensitivity of different species strains using a standard temperature (20°C). Next, we investigated how enrofloxacin contamination may affect the competition between M. aeruginosa and S. obliquus. A competition experiment was performed following a full factorial design with different competition treatments, defined as density ratios (i.e. initial bio-volume of 25/75%, 10/90% and 1/99% of S. obliquus/M. aeruginosa, respectively), one 100% S. obliquus treatment and one 100% M. aeruginosa treatment, and four different enrofloxacin concentrations (i.e. control, 0.01, 0.05 and 0.10mg/L). Growth inhibition based on cell number, bio-volume, chlorophyll-a concentration as well as photosynthetic activity were used as evaluation endpoints in the single-species tests, while growth inhibition based on measured chlorophyll-a was primarily used in the competition experiment. M. aeruginosa photosynthetic activity was found to be the most sensitive endpoint to enrofloxacin (EC50-72h =0.02mg/L), followed by growth inhibition based on cell number. S. obliquus was found to be slightly more sensitive at 20°C than at 30°C (EC50-72h cell number growth inhibition of 38 and 41mg/L, respectively), whereas an opposite trend was observed for M. aeruginosa (0.047 and 0.037mg/L, respectively). Differences in EC50-72h values between algal strains of the same species were within a factor of two. The competition experiment showed that M. aeruginosa growth can be significantly reduced in the presence of S. obliquus at a density ratio of 75/25% M. aeruginosa/S. obliquus, showing a higher susceptibility to enrofloxacin than in the single-species test. The results of this study confirm the high sensitivity of cyanobacteria to antibiotics and show that temperature and inter-strain genetic variation may have a limited influence on their response to them. The results of the competition experiment suggest that the structure of primary producer communities can be affected, at least temporarily, at antibiotic concentrations close to those that have been measured in the environment.

The electronic transport properties of zigzag silicene nanoribbon slices with edge hydrogenation and oxidation.

First principles calculations are performed to study the transport properties of H or H2 edge-hydrogenated zigzag silicene nanoribbon slices with 6 zigzag chains (6ZSiNR) as well as OH or O edge-oxidized 6ZSiNR slices connected with H-terminated 6ZSiNR electrodes. We mainly focus on two configurations: symmetric edge modification and asymmetric edge modification. It is found that these configurations show distinctly different transport behaviours under bias voltages, depending on whether their structures satisfy c2 symmetry operation along the central axis. In addition, the effects of various functional groups on the electronic transport are investigated; comparison of the current magnitudes indicates that the H group has the strongest effect, followed by the OH group, the O group, and the H2 group. This difference is revealed to be related to the coupling interaction between the edge groups of the ZSiNR slices and the H groups of the ZSiNRs electrodes, as well as the transmission channels around the Fermi level.

Spin transport properties in lower n-acene-graphene nanojunctions.

A series of n-acene-graphene (n = 3, 4, 5, 6) devices, in which n-acene molecules are sandwiched between two zigzag graphene nanoribbon (ZGNR) electrodes, are modeled through the spin polarized density functional theory combined with the non-equilibrium Green's function technique. Our theoretical results show that for n-acene molecules ranging from anthracene to hexacene, the spin-polarized electronic states near the Fermi level can be induced by the spin-polarized ZGNR electrodes, which strengthen gradually to facilitate the electronic transport. A nearly 100% spin filtering ratio and a dual-orientation spin-rectifying effect are observed in a wide range of bias voltage. Importantly, an over 8000% giant magnetoresistance is obtained in the low bias range from -0.1 V to +0.1 V. Moreover, negative differential resistance behaviors are detected in these devices. The potential mechanisms of these intriguing phenomena are proposed and these findings would be instructive for the design and synthesis of high-performance graphene-based spin-related devices.

Rectification inversion in oxygen substituted graphyne-graphene-based heterojunctions.

Current rectification is found in oxygen-substituted zigzag graphyne nanoribbon/hydrogen-terminated zigzag graphene nanoribbon heterostructure junctions, from the application of nonequilibrium Green's function formalism combined with density functional theory. This behavior could be tuned by varying the number and location of oxygen atoms in the zigzag graphyne nanoribbon parts, and the rectification direction could be reversed due to the parity limitation tunneling effect. Moreover, an obvious negative differential resistance behavior is found and may be explained by two different mechanisms.

Two-Dimensional Conjugated Polymers: a New Choice For Organic Thin-Film Transistors.

Organic thin-film transistors (OTFTs) as a vital component among transistors have shown great potential in smart sensing, flexible displays, and bionics due to their flexibility, biocompatibility and customizable chemical structures. Even though linear conjugated polymer semiconductors are common for constructing channel materials of OTFTs, advanced materials with high charge carrier mobility, tunable band structure, robust stability, and clear structure-property relationship are indispensable for propelling the evolution of OTFTs. Two-dimensional conjugated polymers (2DCPs), featured with conjugated lattice, tailorable skeletons, and functional porous structures, match aforementioned criteria closely. In this review, we firstly introduce the synthesis of 2DCP thin films, focusing on their characteristics compatible with the channels of OTFTs. Subsequently, the physics and operating mechanisms of OTFTs and the applications of 2DCPs in OTFTs are summarized in detail. Finally, the outlook and perspective in the field of OTFTs using 2DCPs are provided as well.