Self-assembly of length-tunable gold nanoparticle chains in organic solvents

The one-dimensional coagulation of gold colloidal particles dispersed in organic solvent was investigated with transmission electron microscopy. The results indicate that the length of the nanoparticle chains can be modulated by changing the concentration of the solutions. It was also demonstrated that the wetting of the substrate surface hardly influenced the morphology of the nanoparticle chains, which revealed that the particle chains had been formed in the solution before deposition on the substrates. A general theoretical interpretation is provided to explain the linear coagulation of gold colloidal particles, on the basis of the asymmetrical distribution of the charges absorbed on the surface of the gold colloidal particles, as well as the action of the solvent molecules. Received: 8 April 2002 / Accepted: 1 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-025/361-9983, E-mail: jhliao@seu.edu.cn     

Tuning of electron transport through a moebius strip: Shot noise

We study electron transport through a moebius strip attached to two metallic electrodes by the use of a Green’s function technique. A parametric approach is used based on the tight-binding model to characterize the electron transport through such a bridge system and it is observed that the transport properties are significantly affected by (a) the transverse hopping strength between the two channels and (b) the strip-to-electrodes coupling strength. In this context we also describe the noise power of the current fluctuations, which provide key information about the electron correlation which is obtained by calculating the Fano factor (F). The knowledge of these current fluctuations gives important ideas for the fabrication of efficient molecular devices.     

Logic gates scheme based on Coulomb blockade in metallic nanoclusters with organic ligands

We propose a logic gates scheme based on the electron transfer through metallic nanoclusters linked to organic ligands and discuss theoretically the characteristics needed for practical implementation. As a proof-of-the-concept, we demonstrate the OR, AND and NOT gates and study the performance in terms of temperature, applied voltage, and noise.     

Interaction of polar molecules with a resonant RF electric field: strong deflection of a NO molecular beam

Deflection of a cold supersonic NO beam seeded in He has been observed when these molecules interact with both static and a resonant oscillating electric field. The NO beam splits into two beams each one deflecting about 0.5° towards the positive and negative direction of the Stark field when the employed resonant frequency between the two Stark levels of the NO molecule is 1515 kHz. This deflection angle is about four orders of magnitude higher than the value one would expect from the NO dipole moment and the employed RF field gradient. This phenomenon suggests the possibility of a significant translational motion perpendicular to the beam axis, which is induced by the resonant RF electric field on the cold and high-density supersonic beam.     

Novel states of electrons on a helium film

Three novel states of electrons on a helium film are described. The first is a diplon, an electron trapped by a positive charge located on a substrate beneath the film, the second is a variation of the first leading to the possibility of a two-dimensional metal-insulator transition, and the third is a ripplonic polaron, an electron self-trapped in a surface dimple. From a survey of experimental searches for the ripplonic polaron state I conclude that no polaron transition occurs on films thicker than 14 nm at T>1.5 K.     

Theoretical investigation of the O2 adsorption effect in the electron transport of single Fe-porphyrin molecule

The effect of O₂ adsorption on the electron transport behavior of Fe-porphyrin molecule is investigated by the first-principles computational approach. The current-voltage characteristics of Fe-porphyrin and O₂ adsorbed Fe-porphyrin between gold electrodes are calculated. We find that the conductance of the Fe-porphyrin decreases dramatically upon the adsorption of O₂, which suggests that this system has potential application as a molecular sensor or a switch. This switching-behavior is analyzed from the evolutions of the transmission spectra and the molecular projected self-consistent Hamiltonian states of the molecular systems.     

Weak Gate Effect in 1,3-Benzenedithiol Molecular Device

We introduce a full interaction Hamiltonian method to the generalized quantum chemical approach and apply it to investigate the electron tunneling properties of 1,3-benzenedithiol molecular device. The weak gate effect we calculate is consistent with the experiment. The asymmetric current character mainly comes from the asymmetry of the molecule and the nonlinear responding to the gate electric field.     

Kink and conductance of an organic molecular wire

The effect of a kink on the conductance of a nanoscale polymer molecular wire is theoretically investigated on the basis of the Su–Schrieffer–Heeger (SSH) model combined with the Green's function formalism. It is found that, due to the appearance of kink, the molecular conductance decreases rapidly with the molecular length and reaches near zero when the molecular length is over a critical value. The reason is attributed to the localized effect of kink on the electronic state at the Fermi energy. The effect of electron-lattice coupling strength on the conductance is also discussed.     

Mobility-independent doping in crystalline rubrene field-effect transistors

We report doping effects in an organic semiconductor, crystalline rubrene. Oxygen-related states are introduced (removed) by annealing in oxygen (vacuum), at an elevated temperature. Room temperature stability is found in the resulting effects: (1) about two orders of magnitude increase in carrier density at equilibrium, (2) significant modification of threshold voltages, and (3) an unchanged field-effect mobility in the on-current state. Density of states data are modeled as tunneling from the valence band in the channel region into deep-level acceptors in the adjacent region. These oxygen acceptors are the likely dopant species.     

Negative differential resistance of TEMPO molecules on Si(1 1 1)

Negative differential resistance (NDR) has been observed for individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) molecules on Si(1 1 1) in ultra high vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) measurements at room temperature. NDR effects were observed exclusively at negative bias voltage using an n-type Si(1 1 1) sample. At 77 K no NDR effects were observed, but the I(V) curves were similar in shape to those recorded on bare Si(1 1 1) sites. TEMPO was observed to adsorb preferentially at corner adatom sites of the Si(1 1 1)-7 × 7 structure. Although the Si(1 1 1)-7 × 7 reconstruction was conserved, local defects were frequently observed in the vicinity of the TEMPO adsorbates.     

CrAs(0 0 1)/AlAs(0 0 1) heterogeneous junction as a spin current diode predicted by first-principles calculations

We report on first-principles calculations of spin-dependent quantum transport in a CrAs(0 0 1)/AlAs(0 0 1) heterogeneous junction and predict a strong diode effect of charge and spin current. The minority spin current is absolutely inhibited when the bias voltage is applied to the terminals of both CrAs and AlAs. The majority spin current is inhibited when the bias voltage is applied to the terminal of CrAs and “relaxed” when the bias voltage is applied to the terminal of AlAs. The charge and spin current diode are promising for reprogrammable logic applications in the field of spintronics.     

A longitudinal stern-gerlach atomic interferometer

A new magnetic field configuration has been used in the mixing and elongating regions of the longitudinal Stern-Gerlach interferometer. This configuration has proven to considerably improve the performances of the interferometer. An analysis in terms of the vector model of a spin 1 particle is presented.Laboratoire associé au CNRS, URA 282     

Two Possible Configurations for Silver-C60-Silver Molecular Devices and Their Conductance Characteristics

Coherent electronic transport properties of silver-C60-silver molecular junctions in different configurations are studied using hybrid density function theory. The experimentally measured current flows of （760 molecules adsorbed on the silver surface are well reproduced by theoretical calculations. It is found that the current-voltage characteristics of the molecular junctions depend strongly on the configurations of the junctions. Transmission spectra combined with density of states can help us to understand in depth the transport properties. Different kinds of electrode construction are also discussed. With the help of the calculation, two possible configurations of silver-C60-silver molecular junctions are suggested.     

Effects of Contact Atomic Structure on Electronic Transport in Molecular Junction

Based on nonequilibrium Green＇s function and first-principles calculations, we investigate the change in molecular conductance caused by different adsorption sites with the presence of additional Au atom around the metal- molecule contact in the system that benzene sandwiched between two Au（111） leads. The motivation is the variable situations that may arise in break junction experiments. Numerical results show that the enhancement of conductance induced by the presence of additional Au is dependent on the adsorption sites of anchoring atom. When molecule is located on top site with the presence of additional Au atoms, it can increase molecular conductance remarkably and present negative differential resistance under applied bias which cannot be found in bridge and hollow sites. Furthermore, the effects of different distance between additional Au and sulfur atoms in these three adsorption sites are also discussed.     

Ti-pumped Mo X-ray laser at 24 Å

We propose using the Ti XXI He- emission line to resonantly photo-pump Li-like Mo (Z=42) and produce lasing on several n=5 n=4 transitions near 24 Å. The continuum emission from Ti photoionizes the Mo to the Li-like isoelectronic sequence. Gains of 1–2 cm^–1 are calculated on these lines before the inclusion of Stark broadening which lowers the gain by as much as a factor of two.     

Superradiant transitions between high-lying levels of Sr in an external electric field

Superradiant transitions were observed between high lying levels in Sr. The transitions react very sensitively to an external electric field. Superradiant cascades or one transition in a superradiant branching can be suppressed. The square root of the high of a superradiant pulse decreases with the second power of the electric field strength. This behaviour can be explained by calculations.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Squeezing Effect of a Nanomechanical Resonator Coupled to a Two-Level System: an Equilibrium Approach

The squeezing effect of a nanomechanical resonator coupled to a two-level system is studied by variational calculations based on both the displaced-squeezed-state （DSS） and the displaced-oscillator-state （DOS）. The stable region of the DSS ground state at both T = 0 and T ≠ 0 and the corresponding squeezing factor are calculated. It is found that when the resonator frequency lies in （kBT,△）, where A is the tunnelling splitting of the two-level-system in the presence of dissipation, tunnelling splitting of a DSS ground state decreases with the temperature, while tunnelling splittihg of a DOS ground state increases with the temperature in low temperature region. This opposite temperature dependence can help to distinguish between the DSS and DOS ground state in the experiment.     

CPT Magnetometer with Atomic Energy Level Modulation

We propose and experimentally investigate a coherent population trapping state based magnetometer prototype with ^87Rb atoms. Through modulating Zeeman sublevels with an ac magnetic field, not only a phase sensitive detection scheme suitable for miniature magnetometer is realized, but also the detection resolution of magnetic field intensity could be improved by a factor of two. Our study result indicates that it is a promising low power consumption miniature sensitive low magnetic field sensor offering spatially resolved measurement at the sub-millimetre level.     

Bridging of lateral nanoelectrodes with a metal particle chain

Received: 4 September 1998/Accepted: 9 September 1998     

Stabilization and Shift of Frequency in an External Cavity Diode Laser with Solenoid-Assisted Saturated Absorption

A simple method to realize both stabilization and shift of the frequency in an external cavity diode laser （ECDL） is reported. Due to the Zeeman effect, the saturated absorption spectrum of Rb atoms in a magnetic field is shifted. This shift can be used to detune the frequency of the ECDL, which is locked to the saturated absorption spectrum. The frequency shift amount can be controlled by changing the magnetic field for a specific polarization state of the laser beam. The advantages of this tunable frequency lock include low laser power requirement, without additional power loss, cheapness, and so on.