Ruiqin Zhang

Ordered silicon nanowire arrays via nanosphere lithography and metal-induced etching

Two-dimensional silica colloidal crystal template is used to create metal nanohole arrays on a silicon surface, which enables the controlled fabrication of aligned silicon nanowire (SiNW) arrays via metal catalytic etching. By varying the size of silica colloidal crystals, aligned arrays of SiNWs with desirable diameter and density could be obtained. The formation of ordered SiNW arrays is due to selective and anisotropic etching of silicon induced by the silver pattern. The orientation of SiNW arrays is influenced by silver movement in silicon, and the wire axes are primarily along the ⟨100⟩ direction.

Photo and pH stable, highly-luminescent silicon nanospheres and their bioconjugates for immunofluorescent cell imaging.

We report a novel kind of oxidized silicon nanospheres (O-SiNSs), which simultaneously possess excellent aqueous dispersibility, high photoluminescent quantum yield (PLQY), ultra photostability, wide pH stability, and favorable biocompatibility. Significantly, the PLQY of the O-SiNSs is as high as 25%, and is stable under intense UV irradiation and in acidic-to-basic environments covering the pH range 2-12. To our best knowledge, it is the first example of water-dispersed silicon nanoparticles which possess both high PLQY and robust pH stability suitable for broad utility in bioapplications. Furthermore, the O-SiNSs are readily conjugated with antibody, and the resultant O-SiNSs/antibody bioconjugates are successfully applied in immunofluorescent cell imaging. The results show that the highly luminescent and stable O-SiNSs/antibody bioconjugates are promising fluorescent probes for wide-ranging bioapplications, such as long-term and real-time cellular labeling.

A surface-enhanced Raman spectroscopy substrate for highly sensitive label-free immunoassay

Large-scale uniform silicon nanowires (SiNWs) array was fabricated by chemical etching on n-Si(111) wafer. Silver nanoparticles (AgNPs) were loaded on their surfaces. The AgNPs on SiNWs (AgNPs@SiNWs) array exhibit strong surface-enhanced Raman effect. On the substrate surfaces, characteristic Raman signals are generated with trace amount of mouse immunoglobulin G (mIgG), goat-anti-mouse immunoglobulin G (gamIgG), and immune complexes formed from 4ng each of mIgG and gamIgG. The shifted positions and changed intensities in Raman bands indicate the occurrence of immunoreactions. This AgNPs@SiNWs array is a unique substrate for surface-enhanced Raman spectroscopy to show the immune reagents and immunoreactions at higher sensitivity.

Chemical reaction dynamics of barium atom with alkyl bromides

Abstract Laser-induced fluorescence (LIF) has been used to probe the internal energy state of nascent BaBr products formed in the reactions of Ba+BrR (R=C 2 H 5 , C 3 H 7 , C 4 H 9 , C 5 H 11 ). Moreover, these reactions have been theoretically studied in an LEPS potential energy surface with a single-particle approximation for the R group. The theoretical results are in good agreement with the experimental ones.

Surface Passivation and Transfer Doping of Silicon Nanowires

One-dimensional nanomaterials are expected to play a key role in future nanotechnology, in addition to providing model systems to demonstrate the unique characteristics of nanoscale effects. Silicon nanowires (SiNWs) in particular are potentially very attractive, given the central role of Si in the semiconductor industry, and are being extensively studied. A SiNW for use in nanodevices is composed of three sections: SiNW core, surface passivant, and adsorbates or interface compounds. A unique way to modulate the transport properties of SiNWs could depend on the individual sections. Volume doping is a conventional method to control conductivity. In volume doping, impurity atoms are introduced into the crystal lattice in the SiNW core by an in situ process during growth, 5] ion implantation, and related methods. However, volume doping for SiNWs has inherent disadvantages, such as poor controllability and destructive processing. Interestingly, the conductivity of amorphous Si films was found to be sensitive to adsorbates, which indicates the importance of the surface of low-dimensional systems in determining the electrical properties of materials. The large surface-to-volume ratio of SiNWs could potentially be important in influencing their transport properties. Its effect could be exploited through SiNW functionalization. Indeed, recent studies of SiNW-based chemical sensors 10] find strong conductivity responses of SiNWs to environmental conditions. Other relevant observations include conductivity modification by adsorbents in the hydrogen-terminated (H-terminated) surfaces of diamond crystals, conductivity determination by surface states in nanoscale thin silicon-on-insulator (SOI) systems, and conductivity enhancement of hydrogenated SiNWs in air and recovery through vacuum or gas purging. Thus, the possibility to modulate the conductivity of SiNWs using surface effects is promising. The ease of such an approach, economically and nondestructively, would offer a unique advantage for use of SiNWs in device fabrication. However, the success of this approach will depend on its controllability and repeatability, and most importantly on the understanding of the mechanisms of the surface effect on SiNWs. Herein, we present a new doping approach, namely surface passivation doping, built on the known surface transfer doping and based on extensive first-principles theoretical investigations and systematic experiments on the surface effects of SiNWs. We also elucidate the involved mechanism and provide better understanding to predetermine the electrical properties of nanomaterials. Surface hydrogen termination is a natural consequence of the hydrogen fluoride treatment of SiNWs. To reveal the role of hydrogen termination in conductivity, we first performed first-principles calculations based on density functional theory (DFT) with an efficient SIESTA code. 15] We adopted popularly used basis sets with double zeta and polarization functions and the Lee–Yang–Parr functional of generalized gradient approximation. We collected atomic charges from a Mulliken population analysis based on DFT calculation, which gave a reasonable charge distribution, as verified using a water molecule ( 0.46 j e j charges on the oxygen atom and 0.23 j e j charges on each hydrogen atom). Interestingly, we obtained extra charges of 0.06 j e j on average on each surface hydrogen atom of the H-terminated SiNWs (H-SiNWs). Clearly, the partial negative charge on the hydrogen atom is due to the higher electronegativity of the hydrogen atom compared to that of the silicon atom (2.2 vs. 1.9). This partial electron transfer from the silicon core to the surface hydrogen is negligible for bulk silicon but is significant for surface-dominated SiNWs whose carrier concentration could be considerably modified, as is estimated below. Assuming each surface silicon atom is terminated by two hydrogen atoms on average, we can calculate the total number of electrons trapped on the terminating hydrogen atoms using Equation (1):

Size dependence of energy gaps in small carbon clusters: the origin of broadband luminescence

Abstract The visible broadband luminescence from carbon-related films has recently been attributed to the band-tail states caused by the variations in the energy gap of individual sp 2 carbon clusters due to their difference in size and/or shape. In this paper, these band-tail states are classified into two parts: localized and confined. The localized states result from the structural deviation from graphite-like configuration, and the associated luminescence may be described by using the conventional theory for amorphous materials. The confined states are generated due to the existence of stable graphite-like local structures with various sizes and are the main factor for giving efficient, room-temperature luminescence. Our calculations of a series of small hexagonal carbon clusters with first-principle and semi-empirical methods demonstrate that the energy-gap distribution, due to the difference in size, is considerably broad, which may explain the broadband feature of luminescence. Calculations for some tetrahedral clusters were also made for comparison.

First Unsymmetrical Bisfullerene, C121: Evidence for the Presence of Both Homofullerene and Methanofullerene Cages in One Molecule

Abstract:The structures and energetics of carbon bridged C60 clusters (C 60)nCm have been studied by simulated annealing technique within the tight-binding molecular-dynamics. The “sp2 addition” ball-and-chain dimers exhibit odd-even alternations over the number of chain atoms, with the dimers containing even chain atoms more stable against dissociation than their immediate neighbors containing odd chain atoms. In addition to the usual “sp2 addition” dimers, a pentagon-linked C121 isomer and a hexagon-linked C122 isomer are also found to be stable. Based on our tight-binding calculations, trimers and larger clusters can be simply regarded as being made up of independent or weakly interacting dimers, if the C-C60 joints on a single cage are not too close to each other. Large C60 clusters connected by chains each containing only one or two carbon atoms have similar stability to that of constituent dimers, indicating the possibility to form stable C60-carbon polymers.We report the isolation and characterization of the bisfullerene C 121, the first all-carbon molecule to contain a homofullerene (also called a fulleroid) cage. This unsymmetrical isomer of C121 ,along with a symmetrical isomer of C121 and C122, were obtained by thermolysis of a mixture of C60CBr2 and C60 and separated by high performance liquid chromatography. The predominant isomer of C121 has a spiro carbon atom bridge that connects to one of the cages through an open (5.6) ring junction (i.e., between a pentagon and a hexagon) and to the other through a closed (6.6) ring junction. Ab initio calculations indicate that the unsymmetrical structure is more stable than either of the symmetrical alternatives, with the bridging carbon atom attached to both C60 cages through closed (6.6) ring junctions or attached to each cage through open (5.6) ring junctions. Experimental evidence for the unsymmetrical structure comes from the 13 C NMR and UV/vis spectra. Electrochemical reduction of this bisfullerene shows three pairs of distinct, reversible peaks that correspond to each of the first three reductions of the (60)fullerene cages. This is consistent with the presence of a homofullerene unit and shows the similarity of the redox behavior to that of C60. The first reduction potential of C121 is slightly shifted toward more positive values than that of (60)fullerene.

Structural properties of hydrogenated silicon nanocrystals and nanoclusters

The structures of fully and partially hydrogenated Si nanocrystals and nanoclusters are studied by geometric optimizations and molecular dynamics simulations based on an empirical tight-binding approach. It is shown that the structural properties of the hydrogen saturated Si nanocrystals have little size effect, contrary to their electronic properties. The surface relaxation is quite small in the fully hydrogen saturated Si nanocrystals, with a lattice contraction of 0.01 to 0.02u200aA residing in the outermost two or three layers. Inside the hydrogenated Si nanocrystals, there is only very small strain (lattice expansion) of the order 10−4 to 10−3, in agreement with the x-ray diffraction measurement. The fully hydrogenated Si nanocrystals are the most stable structures compared to those partially hydrogenated. For the smaller SimHxu200a(m⩽151) nanocrystals, removing up to 50% of the surface terminating H atoms only causes distortions to the crystal structure, with the basic tetrahedral structural features still ...

An effective scheme for selecting basis sets for ab initio calculations

An effective scheme for selecting economical basis sets forab initio calculations has been proposed for wide-range systems based on the analysis of different functions in the currently used basis sets. Accordingly, the selection of the basis sets should be made according to the different properties and real chemical surrounding of the atoms in the systems. For normal systems, the size and level of the basis sets used for the descriptions of the constituent atoms should be increased from left to right according to the position of the atom in the periodic table. Moreover, the more the atom is negatively charged, the more the basis functions and suitable polarization functions and diffuse functions should be utilized. Whereas, for the positively charged atoms, the size of basis set may be reduced. It is not necessary to use the polarization and diffuse functions for the covalently saturated atoms with normal valence states. However, for the system involving hydrogen-bonding, weak interactions, functional groups, metallic bonding with zero valence or low positive valence, and other sensitive interactions, the polarization and diffuse functions must be used. With this scheme, reliable calculation results may be obtained with suitable basis sets and smaller computational capability. By detailed analysis of a series of systems, it has been demonstrated that this scheme is very practical and effective. This scheme may be used in Hartree-Fock, M0ller-Plesset and density functional theoretical calculations. It is expected that the scheme would find important applications in the extensive calculations of large systems in chemistry, materials science, and life and biological sciences.

Theory of the charge-transport properties of naphthyl diamine used in organic light-emitting devices

The electronic structures of a prototypical electroluminescent molecule, N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), with various extra charges (+6 to −6 atomic units), have been theoretically studied by means of the PM3 and ab initio molecular orbital theories as well as density functional theory in combination with a decomposition of the density of states. It was found that, under positive charging, the essential distribution feature of the molecular orbitals at constituent atoms in the NPB molecule can still favor carrier transport, but cannot do so under negative charging. By explaining the efficient hole-transporting property of NPB, the present study elucidates the potential of the theoretical approach for the selection of optimum carrier-transporting organic materials.