Probing the properties of size dependence and correlation for tantalum clusters: geometry, stability, vibrational spectra, magnetism, and electronic structure.

A comprehensive investigation on the equilibrium geometry, relative stability, vibrational spectra, and magnetic and electronic properties of neutral tantalum clusters (Ta n , n = 2-17) was performed using density functional theory (DFT). We perform a study of the size dependence and correlations among those descriptors of parameters, and showed these could provide a novel way to confirm and predict experimental results. Some new isomer configurations that have never been reported before for tantalum clusters were found. The growth behaviors revealed that a compact geometrical growth route is preferred and develops a body-centered-cubic (BCC) structure with the cluster size increasing. The perfectly fitted functional curve, strong linear evolution, and obvious odd-even oscillation behavior proved their corresponding properties depended on the cluster size. Multiple demonstrations of the magic number were confirmed through the correlated relationships with the relative stability, including the second difference in energy, maximum hardness, and minimum polarizability. An inverse evolution trend between the energy gap and electric dipole moment and strong linear correlation between ionization potentials and polarizability indicated the strong correlation between the magnetic and electronic properties. Vibrational spectroscopy as a fingerprint was used to distinguish the ground state among the competitive geometrical isomers close in energy. The charge density difference isosurface, density of states, and molecular orbitals of selected representative clusters were analyzed to investigate the difference and evolutional trend of the relative stability and electronic structure. In addition, we first calculated the ionization potential and magnetic moment and compared these with the current available experimental data for tantalum clusters.

Impact of Synthesized MoS2 Wafer-Scale Quality on Fermi Level Pinning in Vertical Schottky-Barrier Heterostructures.

Transition metal dichalcogenide (TMD)-based vertical Schottky heterostructures have recently shown promise as a next generation device for a variety of applications. In order for these devices to operate effectively, the interface between the TMD and metal contacts must be well-understood and optimized. In this work, the interface between synthesized MoS2 and gold or platinum metal contacts is explored as a function of MoS2 film quality to understand Fermi level pinning effects. Raman, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy are used to physically characterize both MoS2 and MoS2/metal interface. Metal/MoS2/metal purely vertical heterostructure cross-point devices were fabricated to explore the injection behavior across the Schottky barrier formed between MoS2 and the metal. The temperature dependence of the device behavior is used to understand injection mechanisms, and modeling is performed to verify the injection mechanisms across the interface barrier. By combining both physical characterization with electrical results and modeling, Fermi level pinning is investigated as a function of macroscopic MoS2 quality. Low-quality MoS2 was found to exhibit much stronger pinning than high-quality films, which is consistent with an observed increase in covalency of the metal/MoS2 interface. Additionally, MoS2 was found to pin gold much more strongly than platinum, which is consistent with an increased covalent interaction between MoS2 and gold. These results show that the synthesis temperature and, therefore, the quality of MoS2 dramatically impacts Fermi level pinning and the resultant current-voltage characteristics of Schottky barrier-mediated devices.

Plasmonic sensor with high figure of merit based on differential polarization spectra of elliptical nanohole array.

Using the difference of the polarization transmission spectra of elliptical nanohole arrays (ENAs), the figure of merit (FOM) of the sensor performance of ENA can be significantly improved, and is inversely proportional to the measurement resolution. By optimizing the aspect ratio of the elliptical holes, Ag thickness, substrate-effect, and adhesive layer, the sensitivity, FOM, and relative sensitivity of the ENA can be improved to be 775 nm RIU-1, 705 RIU-1, and 70.23%, respectively, with an excellent linear dependence on the change of refractive index. Such a high-performance sensor also can be used in monitoring the molecule adsorption and RNA hybridization, revealing a highly localized near-field enhancement. This will benefit the sensing of surface-specific binding events in biologic detection and medical diagnosis.

Ag nanoparticle embedded TiO(2) composite nanorod arrays fabricated by oblique angle deposition: toward plasmonic photocatalysis.

Using a unique oblique angle co-deposition technique, well-aligned arrays of Ag nanoparticle embedded TiO2 composite nanorods have been fabricated with different concentrations of Ag. The structural, optical, and photocatalytic properties of the composite nanostructures are investigated using a variety of experimental techniques and compared with those of pure TiO2 nanorods fabricated similarly. Ag nanoparticles are formed in the composite nanorods, which increase the visible light absorbance due to localized surface plasmon resonance. The Ag concentrations and the annealing conditions are found to affect the size and the density of Ag nanoparticles and their optical properties. The Ag nanoparticle embedded TiO2 nanostructures exhibit enhanced photocatalytic activity compared to pure TiO2 under visible- or UV-light illumination. Ag plays different roles in assisting the photocatalysis with different light sources. Ag can be excited and can inject electrons to TiO2, working as an electron donor under visible light. While under UV illumination, Ag acts as an electron acceptor to trap the photogenerated electrons in TiO2. Due to the opposite electron transfer direction under UV and visible light, the presence of Ag may not result in a greater enhancement in the photocatalytic performance.

α-Fe2O3 nanocolumns and nanorods fabricated by electron beam evaporation for visible light photocatalytic and antimicrobial applications.

Both Fe2O3 thin films and nanorod arrays are deposited using electron beam evaporation through normal thin film deposition and oblique angle deposition (OAD) and are characterized structurally, optically, and photocatalytically. The morphologies of the thin films are found to be arrays of very thin and closely packed columnar structures, while the OAD films are well-aligned nanorod arrays. All films were determined to be in the hematite phase (α-Fe2O3), as confirmed by both structural and optical characterization. Texture measurements indicate that films have similar growth modes where the [110] direction aligns with the direction of material growth. Under visible light illumination, the thin film samples were more efficient at photocatalytically degrading methylene blue, while the nanorod arrays were more efficient at inactivating E. coli O157:H7. The size of the targeted agent and the different film morphologies result in different reactant/surface interactions, which is the main factor that determines photoactivity. Furthermore, an analytic mathematical model of bacterial inactivation based on chemotactic bacterial diffusion and surface deactivation is developed to quantify and compare the inactivation rate of the samples. These results indicate that α-Fe2O3 nanorods are promising candidates for antimicrobial applications and are expected to provide insight into the development of better visible-light antimicrobial materials for food products and processing environments, as well as other related applications.