Size-dependent Electronic Structures Research Articles

Size-Dependent Pt-TiO2 Strong Metal-Support Interaction.

The strong metal-support interaction (SMSI) is one of the most important concepts in heterogeneous catalysis. Herein we report a study of Pt-TiO2 SMSI using Pt/rutile TiO2(110) model catalysts with different Pt particle sizes by means of X-ray photoelectron spectroscopy, ion scattering spectroscopy, and the adsorption of probe molecules. The acquired results unambiguously demonstrate a size dependence of the Pt-TiO2 SMSI, in which SMSI occurs barely between supported Pt clusters and the TiO2(110) substrate but obviously between supported Pt nanoparticles and the TiO2(110) substrate. Such a size-dependent SMSI could be associated with size-dependent electronic structures of the supported Pt particles. These results highlight the sensitivity of the SMSI to the size of supported metal particles, which greatly advances the fundamental understanding.

Efficient charge carrier separation and excellent visible light photoresponse in Cu2O nanowires

Last decade has witnessed a surge of research pertaining to Cu2O nanowires for fields of photocatalysts, sensors, solar cells and rechargeable battery systems owing to their unique physicochemical properties. Their atomic properties, especially the size dependent electronic structures, however, remain unknown. Herein, by combining systematic first-principles calculations with material synthesis, characterization, device fabrication and measurement, we investigated physical properties of Cu2O nanowires and their applications in visible light photodiodes. We explored Cu2O nanowires with triangular and hexagonal cross sections of a series of diameter size, and found that they have favorable formation energies, indicative of facile synthesis. These nanowires have tunable direct band gap ranging from 2.2 to 5.4 eV, and a simple model is derived to effectively predict gap size. Remarkably, obvious spatially separated charge distribution of conduction and valance band edges was observed, ensuring long lifetime of excited electron-hole pairs that may greatly benefit performance of optoelectronic devices. Experimentally, we synthesized well-crystallized Cu2O nanowires and fabricated photodiodes, which exhibit a fast rise time of 9.86 ms, decay time of 27.37 ms, and high responsivity of 10 A/W. We expect these results to shed new light on Cu2O nanostructures for scalable, versatile and high-performance optoelectronic devices.

Size dependent electronic band structures of β- and γ-graphyne nanotubes

In the present paper, density functional theory calculations have been implemented by using Dmol3 to study the electronic band structures of β-graphyne nanotubes (βGyNTs) and γ-graphyne nanotubes (γGyNTs).

Precursor engineering and controlled conversion for the synthesis of monodisperse thiolate-protected metal nanoclusters

In very recent years, thiolate-protected metal nanoclusters (or thiolated MNCs) with core sizes smaller than 2 nm have emerged as a new direction in nanoparticle research due to their discrete and size dependent electronic structures and molecular-like properties, such as HOMO-LUMO transitions in optical absorptions, quantized charging, and strong luminescence. Synthesis of monodisperse thiolated MNCs in sufficiently large quantities (up to several hundred micrograms) is necessary for establishing reliable size-property relationships and exploring potential applications. This Feature Article reviews recent progress in the development of synthetic strategies for the production of monodisperse thiolated MNCs. The preparation of monodisperse thiolated MNCs is viewed as an engineerable process where both the precursors (input) and their conversion chemistry (processing) may be rationally designed to achieve the desired outcome - monodisperse thiolated MNCs (output). Several strategies for tailoring the precursor and the conversion process are analyzed to arrive at a unifying understanding of the processes involved.

Photoelectrochemical analysis of size-dependent electronic structures of gold clusters supported on TiO2

Size-dependent electronic structures around the Fermi level of glutathione-protected gold clusters (0.9-1.4 nm in core diameter) were analyzed on the basis of photoinduced charge separation at the interface between the gold cluster and TiO(2). Electron levels such as HOMO and LUMO were estimated from the dependencies of the photocurrents on the irradiation wavelength and the standard electrode potentials of electron donors employed. The potential of the occupied levels involved in the charge separation under visible or near infrared light shifts negatively as the cluster size increases.

Size-dependent electronic structures of boron carbonitride (BC 2N) nanotubes. A DFT approach

The electronic structures and physical properties of zigzag BC 2N ( n,0; n = 4–10) and armchair BC 2N ( n, m; n = m = 4–10) nanotubes (type III) are studied by using density functional theory with the generalized gradient approximation. From a comparison of the binding energies, it is inferred that in the large diameter BC 2N nanotubes, the zigzag form is thermally more stable than the armchair form. BC 2N nanotubes (with the exception of (4,0) which is conductor) are gapless semiconductors. Depend on the chirality index, the zigzag forms of BC 2N nanotubes have narrower band gap than the armchair form. Semiconductor character in the studied BC 2N nanotubes is due to contribution of p electrons in the Fermi level. Mulliken population analyses show that significant amounts of electron charge are transferred between atoms; which suggests the existence of polar covalent bonds in the BC 2N nanotubes.

First-principles study on ZnO nanoclusters with hexagonal prism structures

Size-dependent ground state energies and electronic structures of ZnO nanoclusters with wurtzite structures are investigated using density-functional theory calculations. After structural optimizations, the initial Zn–O double layers merge into single layers. Particularly, a phase transition from the four-coordinate wurtzite to the six-coordinate rocksalt structure is found in the ZnO cluster with 48 atoms, which indicates that clusters with too many dangling bonds will significantly change their configurations during the relaxations. The calculations also show that on the whole both the energy gap and the binding energy approach to the corresponding bulk values with increasing cluster size.

Theoretical Study of Differences between Surface and Bulk Electronic States in Cu Clusters

The size-dependent electronic structures of metal clusters Cun (n=2-20) have been calculated using density functional theory method. The results reveal that their electronic properties are almost the same as bulk material if the cluster size larger than a critical value. The properties can be understood by a surface-noncrystalline-layer model that composed of an interior crystalline-like core and an outer surface noncrystalline layer.

EELS plasmon studies of silver/carbon core/shell nanocables prepared by simple arc discharge

A method for the fabrication of large quantities of high quality silver nanocables encapsulated in carbon nanotubes (Ag@C) using a hydrogen arc is presented. A growth mechanism based on the generation of poly-aromatic hydrocarbons by the hydrogen arc is proposed. The size-dependent electronic structures of the resultant materials are investigated using electron energy loss spectroscopy (EELS). The surface plasmon and bulk-excitation character observed by EELS are discussed. As the diameter of Ag@C nanocable decreases, the surface and bulk plasmons of the silver core shift to lower energy and the peaks broaden. Measurements of electrical conductivity exhibits a liner current–voltage character with a conductivity of 0.5×104 S/cm for the nanocable structure.

Size-dependent electronic structures of ZnO nanowires

ZnO nanowires with a diameter distribution from 20 to 100 nm were fabricated by a simple thermal evaporation process. Two different types of nanowire (circular versus hexagonal cross section) were obtained by controlling the growth conditions. The size-dependent electronic structures of both types of samples were investigated using valence electron energy-loss spectroscopy. Both the common features (surface plasmon, bulk plasmon width) and the differences (O 2s interband transition, bulk plasmon energy) between the two types of samples are discussed. The experimental results strongly suggest that ZnO nanowires with hexagonal cross section and low-index terminating surfaces are of high electronic quality, even at 20 nm diameter, and can thus serve as effective building blocks for optoelectronic nanodevices.

Size-quantized CdS films in thin film CuInS2 solar cells

Semiconductor quantum dots and wires are the subject of great interest, mainly due to their size-dependent electronic structures, in particular increased band gap and therefore optoelectronic properties. We have electrodeposited films of size-quantized CdS (∼4 to 5 nm cross section by 15 nm height) as a buffer layer on CuInS2. The resulting CuInS2/CdS thin-film solar cells gave increased photocurrents and higher light-to-electricity conversion efficiencies (>11%) than those made with conventional nonquantized CdS films. This was due mainly to the increased band gap of the quantized CdS, allowing more light to reach the active CuInS2 layer.