Tunable Schottky barrier in Janus-XGa2Y/Graphene (X/Y = S, Se, Te;X≠Y) van der Waals heterostructures.

Two-dimensional (2D) Janus materials have attracted significant attention due to their asymmetrical structures and unique electronic properties. In this work, by using the first-principles calculation based on density functional theory, we systematically investigate the electronic properties of 6 types of Janus-XGa2Y/Graphene van der Waals heterostructures (vdWHs). The results show that the Janus-XGa2Y/Graphene vdWHs are connected by weak interlayer vdW forces and can form n-type Schottky contact, p-type Schottky contact or Ohmic contact when the spin-orbit coupling (SOC) is not considered. However, when considering SOC, only the SeGa2S/G and G/SeGa2S vdWHs show n-type Schottky contact, and other vdWHs show Ohmic contacts. In addition, the Schottky barriers and contact types of SeGa2S/Graphene and Graphene/SeGa2S vdWHs can be effectively modulated by interlayer distance and biaxial strain. They can be transformed from intrinsic n-type Schottky contact to p-type Schottky contact when the interlayer distances are smaller than 2.65 Å and 2.90 Å, respectively. They can also be transformed to Ohmic contact by applying external biaxial strain. Our work can provide useful guidelines for designing Schottky nanodiodes, field effect transistors or other low-resistance nanodevices based on the 2D vdWHs.

Structure and reactivity of a water-covered anatase TiO2(001) surface.

We systematically studied water adsorption and oxidation on the unreconstructed TiO2(001) surface using first-principles calculations. Water first adsorbs on the surface in a dissociative state and then in a molecular state, as water coverage increases. The geometric properties of all adsorption structures suggest that the dissociative water molecules can induce stress release of the (001) surface at low coverage, reducing reactivity of the surface and thus leading to molecular adsorption of water on the surface at high coverage. The adsorption energy (or the surface energy) monotonously increases (or decreases) with the increase of the coverage, which further confirms that water, irrespective of its dissociative or molecular state, can improve the stability of the (001) surface and reduce its activity. We deeply investigated the mechanism of the oxygen evolution reaction (OER) on the water-covered (001) surface. A new water-assisted OER pathway is identified on the (001) surface, which includes the sequential transfer of protons from molecular water and surface hydroxyls, and O-O coupling processes. During the OER pathway, the O-O coupling step exhibits the largest thermodynamic energy and highest energy barrier, clarifying that it is the rate-determining step in the whole pathway. Our findings provide new insights into the strong dependence of water adsorption modes on coverage for the anatase TiO2(001) surface and may explain the high oxidation activity of the TiO2(001) surface in aqueous environments typical of TiO2 photocatalysis.

Localized Excitation of Ti(3+) Ions in the Photoabsorption and Photocatalytic Activity of Reduced Rutile TiO2.

In reduced TiO2, electronic transitions originating from the Ti(3+)-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the material's blue color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO2(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti(3+) species is identified at 2.5 ± 0.2 eV above the Fermi level (EF) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below EF, as they both result from the Jahn-Teller induced splitting of the 3d orbitals of Ti(3+) ions in reduced TiO2. Localized excitation of Ti(3+) ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO2 photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO2 through Ti(3+) self-doping. Our work reveals the physical origin of the Ti(3+) related photoabsorption and visible light photocatalytic activity in prototypical TiO2 and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides.

Direct observation of enhanced plasmon-driven catalytic reaction activity of Au nanoparticles supported on reduced graphene oxides by SERS.

Graphene-based nanocomposites have recently attracted tremendous research interest in the field of catalysis due to their unique optical and electronic properties. However, direct observation of enhanced plasmon-driven catalytic activity of Au nanoparticles (NPs) supported on reduced graphene oxides (Au/rGO) has rarely been reported. Herein, based on the reduction from 4-nitrobenzenethiol (4-NBT) to p,p'-dimercaptoazobenzene (DMAB), the catalytic property of Au/rGO nanocomposites was investigated and compared with corresponding Au NP samples with similar size distribution. Our results show that Au/rGO nanocomposites could serve as a good catalytic and analytic platform for plasmon-driven chemical reactions. In addition, systematic comparisons were conducted during power- and time-dependent surface-enhanced Raman scattering (SERS) experiments, which exhibited a lower power threshold and higher catalytic efficiency for Au/rGO as compared to Au NPs toward the reaction.

ß-MnO2 as a cathode material for lithium ion batteries from first principles calculations.

The search for excellent cathodes for lithium batteries is the main topic in order to meet the requirements of low cost, high safety, and high capacity in many real applications. ß-MnO2, as a potential candidate, has attracted great attention because of its high stability and potential high capacity among all the phases. Because of the complexity of ß-MnO2, some fundamental questions at the atomic level during the charge-discharge process, remain unclear. The lithiation process of ß-MnO2 has been systematically examined by first-principles calculations along with cluster expansion techniques. Five stable configurations during the lithium intercalation process are firstly determined, and the electrochemical voltages are from 3.47 to 2.77 eV, indicating the strongly correlated effects of the ß-MnO2-LiMnO2 system. During the lithiation process, the changes in the lattice parameters are not symmetric. The analysis of electronic structures shows that Mn ions are in the mixed valence states of Mn(3+) and Mn(4+) during the lithiation process, which results in Jahn-Teller distortion in Mn(3+)O6 octahedra. Such results uncover the intrinsic origin of the asymmetric deformation during the charge-discharge process, resulting in the irreversible capacity fading during cycling. From the analysis of the thermal reduction of delithiated LixMnO2, the formation of oxygen is thermodynamically infeasible in the whole extraction process. Our results indicate that ß-MnO2 has great potential as a cathode material for high capacity Li-ion batteries.

Observing reduction of 4-nitrobenzenthiol on gold nanoparticles in situ using surface-enhanced Raman spectroscopy.

In this article, reduction of 4-nitrobenzenthiol (4-NBT) on Au nanoparticles (NPs) was characterized using surface-enhanced Raman scattering (SERS). Plasmon-driven chemical transformation from 4-NBT dimering into p,p'-dimercaptoazobenzene (DMAB) has been investigated on the surface of Au NPs. The laser power-dependent SERS spectra of 4-NBT on the surface of Au substrates were studied, and show that the laser power has an influence on the SERS signals of 4-NBT on Au NPs and production of DMAB by a plasmon-driven surface-catalyzed chemical reaction tends to be much easier under relative high laser power. Furthermore, we have used simple and efficient Au substrates (gold NPs with a size around 45 nm) exhibiting both catalytic properties and SERS activities to monitor the catalytic reaction of surface catalytic reaction process with borohydride solution. The experiments prove that the nitro-to-amino group conversion could be completed by borohydride at ambient conditions on Au substrates. Illuminated with high laser power, 4-NBT molecules and already formed DMAB molecules are further reduced into 4-aminobenzenthiol (4-ABT) by the addition of borohydride, While with low laser power 4-NBT molecules are transformed into 4-ABT with DMAB as the intermediate, which proves Au NPs are a mild and promising catalyst. Our studies might be helpful in extending the understanding of chemical reactions of 4-NBT and related research as well as providing a new strategy synthesis of azo dyes and anilines.

In situ identification of crystal facet-mediated chemical reactions on tetrahexahedral gold nanocrystals using surface-enhanced Raman spectroscopy.

Direct monitoring of a metal-catalyzed reaction by surface-enhanced Raman scattering (SERS) is always a challenging issue as it needs bifunctional metal structures that have plasmonic properties and also act as catalysts. Here we demonstrate that the tetrahexahedral (THH) gold nanocrystals (Au NCs) with exposed {520} facets give highly enhanced Raman signals from molecules at the interface, permitting in situ observation of chemical transformation from para-aminothiophenol (PATP) to 4,4'-dimercaptoazobenzene (DMAB). The origin of the intense SERS signals of DMAB is carefully investigated based on the comparison of the SERS spectra of PATP obtained with both the THH Au NCs and the Au nanospheres with the exposed {111} facets. It is elucidated that the high-index {520} facet rather than the localized surface plasmons of the THH Au NCs plays a key role in producing a high yield of the product DMAB which is accompanied by the selective enhancement of the characteristic Raman signals.

Coupling and noise induced spiking-bursting transition in a parabolic bursting model.

The transition from tonic spiking to bursting is an important dynamic process that carry physiologically relevant information. In this work, coupling and noise induced spiking-bursting transition is investigated in a parabolic bursting model with specific discussion on their cooperation effects. Fast/slow analysis shows that weak coupling may help to induce the bursting by changing the geometric property of the fast subsystem so that the original unstable periodical solution are stabilized. It turned out that noise can play the similar stabilization role and induce bursting at appropriate moderate intensity. However, their cooperation may either strengthen or weaken the overall effect depending on the choice of noise level.

Polyhedral AgBr microcrystals with an increased percentage of exposed {111} facets as a highly efficient visible-light photocatalyst.

Synthesis of inorganic single crystals with exposed high-reactivity facets is a desirable target in the catalytic chemistry field. Polyhedral AgBr microcrystals with an increased percentage of exposed high-reactivity {111} facets have been successfully prepared for the first time, and the photocatalytic performance of these microcrystals when used as an AgBr/Ag plasmonic photocatalyst was investigated. The results indicate that the as-prepared sample has high photocatalytic activity and, under the same measurement conditions, the photodegradation rate of methyl orange dye over these microcrystals is at least four times faster than with other shapes of AgBr/Ag microstructure, as well as 20 times faster than with the highly efficient Ag(3)PO(4) photocatalyst. DFT calculations suggest that the AgBr (111) surface is mainly composed of unsaturated Ag atoms and has a relatively high surface energy, both of which are favorable for enhancing the photocatalytic activity of the AgBr/Ag polyhedron photocatalyst. This work not only provides a highly efficient plasmonic photocatalyst of polyhedral AgBr/Ag microcrystals with an increased percentage of exposed high-reactivity AgBr {111} facets, but also demonstrates that the shape and crystalline quality of the exposed facets have an important influence on the photocatalytic activities.

A facile way to rejuvenate Ag3PO4 as a recyclable highly efficient photocatalyst.

Recycling awarded the silver: Ag(3)PO(4), a highly efficient photocatalyst, decomposes to the weak photocatalyst Ag during consecutive photocatalytic cycles. A facile and very mild wet chemical oxidation method was proposed, which involves the cooperation of H(2)O(2) with Na(2)HPO(4), to rejuvenate Ag(3)PO(4) from Ag as a recyclable highly efficient photocatalyst.

Chemical effects in SERS of pyrazine adsorbed on Au-Pd bimetallic nanoparticles: a theoretical investigation.

Chemical enhancement in surface-enhanced Raman scattering (SERS) of pyrazine adsorbed on Au-Pd nanoclusters is investigated by using density functional theory. Changing Pd content in the bimetallic clusters enables modulation of the direct chemical interactions between the pyrazine and the clusters. The magnitude of chemical enhancement is correlated well with the induced polarizability for the complexes with low Pd content, which fails for the complexes with high Pd content. Furthermore, the dependence of chemical enhancement on cluster size and coupling is also described by the induced polarizability. Additionally, the chemical enhancement in the cluster-molecule-cluster junction is found to account for as much as 10(3), which suggests that a chemical mechanism might be more important than previously believed, in particular for Au-Pd bimetallic nanoparticle aggregates.

Highly reproducible surface-enhanced Raman spectra on semiconductor SnO2 octahedral nanoparticles.

Highly reproducible surface-enhanced Raman scattering (SERS) spectra are obtained on the surface of SnO(2) octahedral nanoparticles. The spot-to-spot SERS signals show a relative standard deviation (RSD) consistently below 20 % in the intensity of the main Raman peaks of 4-mercaptobenzoic acid (4-MBA) and 4-nitrobenzenethiol (4-NBT), indicating good spatial uniformity and reproducibility. The SERS signals are believed to mainly originate from a charge-transfer (CT) mechanism. Time-dependent density functional theory (TD-DFT) is used to simulate the SERS spectrum and interpret the chemical enhancement mechanism in the experiment. The research extends the application of SERS and also establishes a new uniform SERS substrate.

Functionalized gold nanoparticles as nanosensor for sensitive and selective detection of silver ions and silver nanoparticles by surface-enhanced Raman scattering.

We have developed a surface-enhanced Raman scattering (SERS) nanosensor firstly for Ag ions and Ag nanoparticles detection based on 2-mercaptoisonicotinic acid (2MNA)-functionalized Au nanoparticles. Ag(+) can coordinate with 2MNA resutling in a variation of its SERS spectrum, which is used as a criterion to determine Ag(+) in a solution. This sensor exhibits a detection limit no more than 25 nM and has a high selectivity against other metal ions. More importantly, it can be directly applied in real sample detection.

In situ identification of the adsorption of 4,4'-thiobisbenzenethiol on silver nanoparticles surface: a combined investigation of surface-enhanced Raman scattering and density functional theory study.

We investigated the configuration characteristic and adsorption behavior of 4,4'-thiobisbenzenethiol (TBBT) on the surface of silver nanoparticles (NPs). Under different conditions and preparation processes, several possible surface species were produced including single-end adsorption on a silicon wafer, double-end adsorption and bridge-like adsorption. Although consisting of the same molecule and nano material, different adsorption systems exhibited different spectral characteristics in the surface-enhanced Raman spectroscopy (SERS). A density functional theory (DFT) study further verified the corresponding adsorption states. The combined SERS-DFT study provided a framework towards investigating and designing adsorption systems at a molecular level, indicating the potential use in applications such as nano-sensors.

A DFT investigation of surface-enhanced Raman scattering of adenine and 2'-deoxyadenosine 5'-monophosphate on Ag20 nanoclusters.

We present a detailed analysis of the surface-enhanced Raman scattering (SERS) of adenine and 2'-deoxyadenosine 5'-monophosphate (dAMP) adsorbed on an Ag(20) cluster by using density functional theory. Calculated Raman spectra show that spectral features of all complexes depend greatly on adsorption sites of adenine and dAMP. The complexes consisting of adenine adsorbed on the Ag(20) cluster through N3 reproduce the measured SERS spectra in silver colloids, and thus demonstrated that adenine interacts with the silver surface via N3. We also investigate the SERS spectrum of adenine at the junction between two Ag(20) clusters and demonstrate that adenine can bind to the clusters through N3 and the external amino group, while dAMP can be adsorbed on the cluster in an end-on orientation with the ribose and phosphate groups near to or away from the silver cluster. In contrast to the adenine-Ag(20) complexes, the dAMP-Ag(20) complexes produce new and strong bands in the low- or high-wavenumber region of the Raman spectra, due to vibrations of the ribose and phosphate groups. Furthermore, the spectrum of dAMP bound to the Ag(20) cluster via N7 approaches the experimental SERS spectra on silver colloids.

Internal noise-sustained circadian rhythms in a Drosophila model.

Circadian rhythmic processes, mainly regulated by gene expression at the molecular level, have inherent stochasticity. Their robustness or resistance to internal noise has been extensively investigated by most of the previous studies. This work focuses on the constructive roles of internal noise in a reduced Drosophila model, which incorporates negative and positive feedback loops, each with a time delay. It is shown that internal noise sustains reliable oscillations with periods close to 24 h in a region of parameter space, where the deterministic kinetics would evolve to a stable steady state. The amplitudes of noise-sustained oscillations are significantly affected by the variation of internal noise level, and the best performance of the oscillations could be found at an optimal noise intensity, indicating the occurrence of intrinsic coherence resonance. In the oscillatory region of the deterministic model, the coherence of noisy circadian oscillations is suppressed by internal noise, while the period remains nearly constant over a large range of noise intensity, demonstrating robustness of the Drosophila model for circadian rhythms to intrinsic noise. In addition, the effects of time delay in the positive feedback on the oscillations are also investigated. It is found that the time delay could efficiently tune the performance of the noise-sustained oscillations. These results might aid understanding of the exploitation of intracellular noise in biochemical and genetic regulatory systems.

Roles of external noise correlation in optimal intracellular calcium signaling.

The dynamics of a minimal calcium model, which is subjected to white noise or colored noise, was investigated. For white noise, coherence of noise-induced calcium oscillations reached a maximum at an optimal noise intensity, characterizing coherence resonance. Higher resonance peaks could be observed at lower noise intensity when a control parameter is tuned to approach a bifurcation point. For colored noise, a maximal coherence of the oscillations was found for suitable values of both the intensity and the correlation time. Moreover, the coherence of the oscillations exhibited two maxima at two values of noise intensity (correlation time) for appropriate noise correlation time (intensity). In addition, a quantitative description of the effects of noise correlation time on the resonance behavior was presented. The resonance behavior, which is induced either by white noise or colored noise, was interpreted by terms of height and relative width of a spectral peak.

A DFT study on surface-enhanced Raman spectroscopy of aromatic dithiol derivatives adsorbed on gold nanojunctions.

A computational study on aromatic dithiol derivatives (HS-Ar-X-Ar-SH, X=O, S, Se, NH, CH2, NN, CHCH, CC) interacting with gold cluster(s) was presented to investigate the chemical enhancement mechanism related to surface-enhanced Raman spectroscopy (SERS) for molecular junctions. Density functional theory (DFT) were performed on derivatives molecules as well as their single-end-linked (SEL) or double-end-linked (DEL) complexes for geometric, spectra, electronic and excitation properties, leading to discussions on dominant factor during SERS process. The resulted enhancement factors of SEL and DEL complexes exhibited specific dependency on linking atom or functional group between two phenyls, which was in accordance with the variation of polarizabilities and molecule-cluster transition energy.

Internal signal transmission in one-way coupled excitable system: noise and coupling effects.

We study the spatiotemporal dynamics of a one-way coupled FitzHugh-Nagumo system of twenty neurons, which is subject to external noise at the first neuron. It is shown that noise-induced oscillation (NIO) triggered at the first neuron is propagated along the chain with noise suppression, such that a rather "regular" signal is obtained at the last neuron, which can provide a mechanism for the creation of the informative signal in a neural network. Coherence resonance or coherence biresonance behavior appears in the transmission of NIO at appropriate coupling and the information flow in each neuron can be simultaneously optimized at the optimal value of noise.

Theoretical study of coupling p-aminothiophenol to hydroazo- and azo-adducts on Au(111).

Aminothiophenol/Au(111) has been adopted as an exemplary model in plasmonics research, including surface-enhanced Raman spectroscopy, due to its high plasmonic-induced spectral-signal enhancement. The present work was aimed at clarifying whether aminothiophenol on Au(111) is chemically stable in the absence of any photo- and plasmonic-induced effects. Briefly, first-principles calculations were employed to track the detailed mechanism of oxidative coupling of p-aminothiophenol (PATP) to its azo-adduct with an N = N bond, i.e., p,p'-dimercaptoazobenzene (DMAB). Our results show the following: first, in the presence of adsorbed O2, PATP fractures its N-H bond and transfers the hydrogen to a nearby oxygen. This pathway is more favorable than the transfer of H to Au, but the activation barrier of 0.9 eV is still too high for the reaction to occur in the absence of thermal-, photo-, or plasmonic-activation. If this bar can be lifted, two such dehydrogenated PATP can couple themselves to form an adduct with a N-N bond, i.e., p,p'-dimercaptohydroazobenzene (DMHAB), and this reaction is exoergic with an energy barrier of 0.57 eV. Again, this step is slow in the absence of moderate thermal activation or photo-/plasmonic-activation. Finally, dehydrogenation of DMHAB gives the azo-adduct of DMAB, and this reaction is spontaneous, with no energy barrier.