Self-assembled ultrafine hierarchical ZnS treelike nanoarchitectures

Ultrafine hierarchical tree-shaped nanoarchitectures of ZnS were synthesized by a H₂-assisted thermal evaporation and condensation technique. Morphology and composition of the ZnS deposit were studied by means of field emission scanning electron microscopy, high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. The deposit was found to consist of a layer of oriented submicrorods partly covered by microsheets and randomly oriented submicrowires, and three-dimensional treelike nanoarchitectures grew epitaxially on various submicrorods, microsheets, and submicrowires. The growth of the nanostructures is a spontaneous and self-assembled process. Vapor-solid (VS) growth mechanism is proposed for the formation of the treelike nanostructures because catalyst was not introduced during the synthesis process. This novel hierarchical ZnS nanoarchitecture may offer great potential for applications, including three-dimensional nanoelectronics and high efficient spatial resolved photon detector.     

Atomic-scale structure of Mo6S6 nanowires

We have studied the atomic-scale structure of the Mo6S6 nanowires using scanning tunneling microscopy and spectroscopy (STM and STS) and density functional theory (DFT). A novel synthesis route based on metallic Mo precursors is presented for the selective formation of elementary pure Mo6S6 nanowires. The Mo6S6 nanowires selectively organize as trimer bundles, and each of the Mo6S6 nanowires consists of an electrically conducting Mo backbone dressed with a sulfur exterior cap. The Mo6S6 nanowires may thus be of interest as novel building blocks in nanoelectronics because the Mo6S6 nanowires exist in a robust, singular structural conformation with uniquely defined electrical (metallic) properties.     

Scanning Tunneling Microscopy and Spectroscopy of Novel Silver–Containing DNA Molecules

The quest for a suitable molecule to pave the way to molecular nanoelectronics has been met with obstacles for over a decade. Candidate molecules such as carbon nanotubes lack the appealing trait of self‐assembly, while DNA seems to lack the desirable feature of conductivity. Silver‐containing poly(dG)–poly(dC) DNA (E‐DNA) molecules have recently been reported as promising candidates for molecular electronics, owing to the selectivity of their metallization, their thin and uniform structure, their resistance to deformation, and their maximum possible high conductivity. Ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) of E‐DNA presents an elaborate high‐resolution morphology characterization of these unique molecules, along with a detailed depiction of their electronic level structure. The energy levels found for E‐DNA indicate a novel truly hybrid metal–molecule structure, potentially more conductive than other DNA‐based alternatives.     

接触--纳电子器件的关键

接触是纳电子器件研究中的重要课题.我们以由6个金原子构成的Au6原子线簇为核心工作区的原型纳电子器件为例,利用Green函数方法计算了它的电流-电压特性.计算结果表明,随着Au6原子线簇与两端电极的接触由弱到强,整个纳电子器件的电学特性发生了由以共振隧穿为主要特征的分子导电到量子化电导的巨大变化.因此,接触在很大程度上决定了纳电子器件的电学特性.     

Real time and in situ control of the gap size of nanoelectrodes for molecular devices

Molecular electronics is often limited by the lack of a simple method to fabricate nanoelectrodes with controlled gap size. This is partly attributed to the lack of a real time characterization in the fabrication. Here, we report a new method based on an electron induced deposition process operated in scanning electron microscopy that realizes in situ and real time characterization in the nanoelectrode fabrication; thus the gap size can be controlled easily and precisely. It is a clean and nondestructive process for carbon nanotube (CNT) electrodes. The mechanism is detailed. The nanoelectrodes have a pi-conjugated surface due to the deposition of sp(2)-rich amorphous carbon. As an application, DNA molecules are assembled between the CNT electrodes by pi-stacking interaction for current-voltage measurement. Our result provides a feasible route to prepare nanoelectrodes with controlled gap size, and it will be valuable for current efforts in molecular electronics and nanoelectronics.     

Template synthesized Cu-Se microstructures as resonant tunneling diodes

We report on resonant electron tunneling through Cu-Se microstructure fabricated by direct current electro-deposition in templates formed by etching the tracks of heavy ions in polymer (polycarbonate) foils. Negative differential resistance has been observed in the I-V curves of the so fabricated array of Cu-Se microstructures. Results show that electrochemical deposition in the pores of nuclear track filters can be very effective way of fabricating resonant tunneling diodes. SEM results show that the diameter of these aligned microstructures are consistent with the diameter of the templates used.     

Site-Specific Synthesis of Silica Nanostructures on DNA Origami Templates

DNA origami has been widely investigated as a template for the organization of various functional elements, leading to potential applications in many fields such as biosensing, nanoelectronics, and nanophotonics. However, the synthesis of inorganic nonmetallic nanomaterials with predesigned patterns using DNA origami templates has seldom been explored. Here, a novel method is reported to site-specifically synthesize silica nanostructures with designed patterns on DNA origami templates. The molecular dynamic simulation confirms that the positively charged silica precursors have a stronger electrostatic affinity to protruding double-stranded DNA (dsDNA) than DNA origami surfaces. The work describes a novel strategy to fabricate silica nanostructures with nanoscale precision. Moreover, the site-specific silicification of DNA nanoarchitectures expands the scope of customized synthesis of inorganic nonmetallic nanomaterials.     

Synthesis and characterization of two dimensional graphene lamellae based PAn nanocomposites

Novel room temperature synthesis method for graphene lamellae (GL) has been developed by using wet electrochemical route. Cyclic voltammetry and current transient studies were performed to explore the possibility of periodic precursor intercalation in the graphite. The synthesis parameters have been optimized and discussed. The GL sample quality and its mono-layered nature has been examined by Raman spectroscopy and scanning tunneling microscopy. Polyaniline (PAn) nanocomposite was prepared by in-situ doping/mixing of the two dimensional GL material through a novel electrochemical oxidative polymerization method. I-V characteristics of as prepared GL-PAn nanocomposite exhibits low resistance per unit length. The as prepared GL-PAn nanocomposites further investigated using optical absorption spectroscopy show GL dependent bulk heterostructure charge transfer properties. Prospects of GL-PAn nanocomposites in photovoltaic conversion have also been discussed.     

Synthesis of flower-like silver nanoarchitectures at room temperature

Novel flower-like silver nanoarchitectures were synthesized via a facile and environmentally benign route in the presence of citric acid and ascorbic acid. The flower-like structures are composed of nano-petals of ca. 20 nm in thickness. The products were characterized with X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The growth mechanism of flower-like silver nanoarchitectures involves a film-fold process. Some crucial factors affect the nanocrchitectures growth, such as, pH, the concentration of citric acid, and the concentration of ascorbic acid, have also been discussed.     

Ferroelectric gated electrical transport in CdS nanotetrapods

Complex nanostructures such as branched semiconductor nanotetrapods are promising building blocks for next-generation nanoelectronics. Here we report on the electrical transport properties of individual CdS tetrapods in a field effect transistor (FET) configuration with a ferroelectric Ba(0.7)Sr(0.3)TiO(3) film as high-k, switchable gate dielectric. A cryogenic four-probe scanning tunneling microscopy (STM) is used to probe the electrical transport through individual nanotetrapods at different temperatures. A p-type field effect is observed at room temperature, owing to the enhanced gate capacitance coupling. And the reversible remnant polarization of the ferroelectric gate dielectric leads to a well-defined nonvolatile memory effect. The field effect is shown to originate from the channel tuning in the arm/core/arm junctions of nanotetrapods. At low temperature (8.5 K), the nanotetrapod devices exhibit a ferroelectric-modulated single-electron transistor (SET) behavior. The results illustrate how the characteristics of a ferroelectric such as switchable polarization and high dielectric constant can be exploited to control the functionality of individual three-dimensional nanoarchitectures.     

Tip preparation for usage in an ultra-low temperature UHV scanning tunneling microscope

This work deals with the preparation and characterization of tungsten tips for the use in UHV low-temperature scanning tunneling microscopy and spectroscopy (STM and STS, respectively). These specific environments require in situ facilities for tip conditioning, for further sharpening of the tips, as well as for reliable tip characterization. The implemented conditioning methods include direct resistive annealing, annealing by electron bombardment, and self-sputtering with noble gas ions. Moreover, results from in situ tip characterization by field emission and STM experiments were compared to ex situ scanning electron microscopy. Using the so-prepared tips, high resolution STM images and tunneling spectra were obtained in a temperature range from ambient down to 350 mK, partially with applied magnetic field, on a variety of materials.     

Characterization of nanometer-sized, mechanically exfoliated graphene on the H-passivated Si(100) surface using scanning tunneling microscopy

We have developed a method for depositing graphene monolayers and bilayers with minimum lateral dimensions of 2-10?nm by the mechanical exfoliation of graphite onto the Si(100)-2 × 1:H surface. Room temperature, ultrahigh vacuum tunneling spectroscopy measurements of nanometer-sized single layer graphene reveal a size-dependent energy gap ranging from 0.1 to 1?eV. Furthermore, the number of graphene layers can be directly determined from scanning tunneling microscopy topographic contours. This atomistic study provides an experimental basis for probing the electronic structure of nanometer-sized graphene which can assist the development of graphene-based nanoelectronics.     

NMR studies of artificial double-crossover DNA tiles

This report documents the design and characterization of DNA molecular nanoarchitectures consisting of artificial double crossover DNA tiles with different geometry and chemistry. The Structural characterization of the unit tiles, including normal, biotinylated and hairpin loop structures, are morphologically studied by atomic force microscopy. The specific proton resonance of the individual tiles and their intra/inter nucleotide relationships are verified by proton nuclear magnetic resonance spectroscopy and 2-dimensional correlation spectral studies, respectively. Significant up-field and down-field shifts in the resonance signals of the individual residues at various temperatures are discussed. The results suggest that with artificially designed DNA tiles it is feasible to obtain structural information of the relative base sequences. These tiles were later fabricated into 2D DNA lattice structures for specific applications such as protein arrangement by biotinylated bulged loops or pattern generation using a hairpin structure.     

Sculpting nanoelectrodes with a transmission electron beam for electrical and geometrical characterization of nanoparticles

A method to produce metal electrodes with a gap of a few nanometers with a highly focused electron beam in a transmission electron microscope (TEM) is described. With this method the electrical and geometrical characterization of the same particle is possible. The I-V characteristics of a gold particle trapped between such electrodes showed the expected single-electron tunneling behavior, with a Coulomb gap corresponding to the geometry of the particle as observed with high-resolution TEM.     

Operating mechanisms of organic bistable devices containing ZnO nanoparticles embedded in a poly-4-vinyl-phenol layer

Scanning electron microscopy images showed that self-assembled ZnO nanoparticles were created inside a poly-4-vinyl-phenol (PVP) layer. Current-voltage (I-V) measurements on the Al/ZnO nanoparticles embedded in a PVP layer/indium tin oxide (ITO)/glass device fabricated by using a simple spin coating method at 300 K showed an electrical hysteresis behavior, indicative of an essential feature for a bistable device. The data fitting results of the I-V curves showed that the carrier transport mechanisms at low and high voltages were attributed to the space charge limited current and the Fowler-Nordheim tunneling processes, respectively. Possible operating mechanisms for the memory effects in the Al/ZnO nanoparticles embedded in a PVP layer/ITO devices are described on the basis of the I-V results.     

Interface characterization of epitaxial Fe/MgO/Fe magnetic tunnel junctions

Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect in epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs), measured magnetoresistance (MR) ratios of about 200% at room temperature (RT) have been reported in MgO-based epitaxial MTJs. Recently, a MR ratio of about 600% has been reported at RT in MgO-based MTJs prepared by magnetron sputtering, using amorphous CoFeB as the ferromagnetic electrode. These MTJs show great potential for application in spintronic devices. Fully epitaxial MTJs are excellent model systems that enhance our understanding of the spin-dependent tunneling process as the interface is well defined and can be fully characterized. Both theoretical calculations and experimental results clearly indicate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystal MgO barrier, especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE). Surface X-ray diffraction, Auger electron spectroscopy, X-ray absorption spectra, and X-ray magnetic circular dichroism techniques have been reported previously for interface characterization. However, no consistent viewpoint has been reached on the interfacial structures (such as FeO layer formation at the bottom Fe/MgO interface), and it is still an open issue. In this article, our recent studies on the interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and spin-dependent tunneling spectroscopy, will be presented.     

CoS-carbon nanotube heterostructure: one-step synthesis and optical properties

This article reports on the synthesis, characterization, and optical properties of a cobalt sulphide (CoS) quantum dot (QD)-decorated multi-walled carbon nanotube (MWCNT) heterostructure. A novel one-pot chemical-solution route has been used for the in situ synthesis of a CoS-decorated MWCNT heterostructure without disturbing the inherent structure of the MWCNTs. The synthesized heterostructure has been extensively characterized by scanning electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier transform infrared spectrometry. The UV-absorption and fluorescence spectral properties of pristine MWCNTs are significantly improved after heterostructure formation with CoS-QDs.     

Modelling current transport through DNA (deoxyribonucleic acid) molecules using equivalent circuits

The current transport properties of DNA molecules are of considerable interest. The key reason for this appears to be linked to the universality of DNA molecules in living organisms, their self-assembly properties, and potential applications as nanoscale devices. The modelling of the I-V characteristics of a DNA molecule using equivalent circuits is reported. The advantages of the proposed model are that non-linear current behaviour can be included together with potential piece-wise solutions. The model includes the use of transistors to mimic current discontinuities at transition points. The simulated results closely resemble measured I-V curves and do not invoke resonant tunneling which contradicts observed temperature dependences. An equivalent-circuit model which includes the use of active devices is shown to be effective way to mimic non-linear current transport in biological molecules.     

Nano and micro structural studies of thin films of ZnO

Zinc oxide thin films grown by sol–gel and RF sputtering methods have been characterized. The characterization techniques used involve ellipsometry, optical absorption, scanning tunneling microscopy, scanning and transmission electron microscopy. The films grown by sol–gel spin method which followed zinc acetate route exhibited a smoother texture than the films, which were deposited by using zinc nitrate route. The later type of films showed a dendritic character. Nano-structured fine grains of size ranging from 20 to 60 nm were observed with zinc nitrate precursor film. Individual grains show a sharp contrast with different facets and boundaries. Crystal planes and lattice parameters calculated by electron diffraction and X-ray diffraction are quite close and in agreement with the reported values in literature. Scanning tunneling microscopy has been used for measuring the average roughness of the surface and estimating the lattice constants. The STM studies of RF sputtered films, although showing a ZnO structure, exhibited a disturbed lattice. This was presumably due to the fact that after deposition the films were not annealed. Nanographs of 2D and 3D view of atomic positions of ZnO have been presented by using scanning tunneling microscopy.     

Synthesis of Multilayer Graphene Films on Copper by Modified Chemical Vapor Deposition

In this paper, large-area uniform multilayer graphene films were synthesized on copper in one growth route by modified low pressure chemical vapor deposition (LPCVD) method by introducing an assembly into the conventional LPCVD method. Scanning electronic microscopy, optical microscopy, Raman spectroscopy, ellipsometry, and transmission electron microscopy were used to characterize the graphene films. The results showed that the graphene films were multilayer. And there are about six layers with good continuity and uniformity. Meanwhile, the growth mechanism was illustrated by a growth model based on the analysis of the effects of the introduced assembly on the generation of the activated carbon atoms and on the catalysis of Cu molecules.