We report the current-voltage characteristics of gold nanoparticle – biopolymer networks at room temperature. Above a threshold voltage the current-voltage relationship is almost linear. From the current-voltage scaling above threshold we argue that one-dimensional regions of the network dominate the transport. Periodic features in the conductance are found in many samples. Both the threshold voltage and the conductance features occur at voltages much greater than expected for the capacitance of the nanoparticles. Possible explanations for the structure are considered.

The interfaces of organic materials with other solids play important roles in the function of various organic devices such as organic light-emitting diodes (OLEDs), spectral sensitization in photography, organic solar cells, and electrophotography. Also they should be important in future molecular devices, both in the central part of the device and at the connection with outside circuits. However, serious experimental examination of such interfaces has started only recently. In this talk we focus our attention on the organic/metal interfaces, and summarize our understanding about (1) the energy level alignment right at the interface, and (2) possible band bending within an organic layer, mainly using the techniques of UV photoemission spectroscopy (UPS) and Kelvin probe method. As for (1), the formation of electric dipole layer was observed in most organic/metal interfaces, and its origin is discussed. As for (2), recent examination of the existence/absence of band bending in ultrahigh vacuum will be reported. It is also pointed out that there can be much effects of (i) atmosphere at sample preparation and measurements, and (2) chemistry and interdiffusion at the interface.

For molecular electronics, Boltzmann's equation is no longer valid for simulating device characteristics. We resent the first fully ab initio simulation of a molecular device that has already been studied ex erimentally, namely a benzene-1,4-dithiolate molecule between gold electrodes. The theoretical I-V curve has the same overall sha e as the ex erimental curve —reflecting the electronic structure of the molecule in the resence of the electric field — but the absolute value of the current is very sensitive to contact chemistry and geometry. In articular the resence ol a single gold atom between the molecule and the electrode surface reduces the conductance by more than an order of magnitude. Re lacement of the single gold atom by an aluminum atom, whose p orbitals cou le more effectively to the molecule's T orbitals, increases the conductance by about an order of magnitude. We have also studied the olarization effects induced by a third terminal (gate) on the I-V characteristics of the above device. In articular, we have found that current gain due to the gate bias can be achieved at reasonable gate fields. Finally the effect of current-induced forces on the device will be discussed.

For CdSe quantum dots (QDs) produced via high temperature pyrolysis in trioctylphosphine oxide (TOPO), a MnSe precursor such as Mn2(μ-SeMe)2(CO)8appears to be necessary to successfully incorporate low levels of Mn. A simple etching experiment and electron paramagnetic resonance (EPR) measurements reveal that most of the dopant atoms reside in the surface layers of the inorganic lattice. The dopant dramatically affects 113Cd solid state NMR spectra; the observed paramagnetic shift and decreased longitudinal relaxation time reproduce bulk material behavior. Paramagnetic atoms in QDs generate large effective magnetic fields, which implies that magneto-optical experiments can be performed simply by doping. Results from fluorescence line narrowing (FLN) studies on Mn doped CdSe QDs mirror previous findings on undoped QDs in an external magnetic field. Experimental fitting of photoluminescence excitation (PLE) spectra of doped QDs reveals that the effective absorption lineshape contains a new feature which is believed to be a previously unobserved – but theoretically predicted – optically dark fine structure state.

Molecular devices utilizing active self-assembled monolayer (SAM) (containing nitroamine (2′-amino-4-ethynylphenyl-4′-ethynylphenyl-5′-nitro-1-benzenethiolate) and nitro (4-ethynylphenyl-4′-ethynylphenyl-2′-nitro-1-benzenethiolate) redox center) as the active component are reported. Current-voltage measurements of the devices exhibited negative differential resistance at room temperature and an on-off peak-to-valley ratio in excess of 1000:1 at low temperature.

A wet chemical approach to preparing one- and two-dimensional arrays of gold nanoparticles assembled onto a polypeptide (poly-L-lysine) scaffold layer is reported. The electrical properties of biopolymer templated and nontemplated gold nanoparticle samples are compared. The electrical response of nontemplated samples is unstable, likely due to nanoparticle mobility. Biopolymer templating provides a simple, chemical method to immobilize the particles and is found to stabilize the electrical response (Coulomb blockade) of the array. A striking feature of the electrical properties is that the electron transport properties are dominated by transport through one-dimensional chains within the sample. Structural studies by AFM and XPS support the transport findings and provide evidence for the formation of one-dimensional assemblies within the arrays.

A Langmuir-Blodgett monolayer of γ-(n-hexadecyl)quinolinum tricyanoquinodimethanide, C16H33Q-3CNQ (1), sandwiched between Al electrodes, shows asymmetric DC electrical conductivity between 370 K and 105 K. The enhanced forward current is attributable to rectification of electrical current by a single molecule, and is explained by an electronic transition from a high-moment zwitterionic ground state to a low-moment undissociated first excited state.

We report on polysilane, a synthetically accessible μ-conjugated silicon-catenated polymer with such semiconducting properties as photoconductivity, high hole drift mobility, and electroluminescence, as a candidate for molecular wire. We apply the “end-graft” technique, which we developed for fixing individual polymer chains on a substrate surface, to a semiflexible polysilane. The end-grafted semiflexible polysilane extended more than 500 nm across the Si(111) surface, and was clearly observed by means of atomic force microscopy (AFM).

The deposition chemistry of hexanethiol-stabilized gold nanoclusters self-assembled on Au and SiO2 surfaces by reaction with a series of α,ω-alkanedithiols is studied using X-ray photoelectron spectroscopy and electrical measurements. Testing of micron-scale devices shows that high conductivity is achieved only for a narrow range of dithiols and with a sharp efficiency threshold. Nanoscale devices exhibit similar behavior though with much evidence of “granularity” in the deposition.

This paper reports on some of the recent advances in the development and testing of molecular-scale electronic devices, devices that may ultimately be the basis of a molecular-based computer. These advances include: the synthesis of molecules which will perform specific electronic functions, self-assembled monolayer formation of rigid-rod conjugated oligomers, novel methodologies for the design and testing of molecular electronic devices, and the observance of negative differential resistance (NDR) through a molecule.

Te-doped Bi nanowires with a 40 nm wire diameter have been successfully synthesized in anodic alumina templates by the pressure injection technique. Due to the unique semimetal-semiconductor transition that occurs in Bi nanowires, these systems exhibit a rather different temperature dependence in transport properties from their bulk counterparts. An improved theoretical model of this unique 1D nanowire system is developed based on the band structure of bulk bismuth. The temperature dependence of resistance for Bi nanowire arrays have been studied experimentally for various Te dopant concentrations and the results are compared with theoretical predictions.

Size-monodisperse, stable 15 Å diameter silicon nanocrystals were synthesized in significant quantities using supercritical octanol as a capping ligand. The silicon nanocrystals exhibit an indirect band gap with discrete electronic transitions in the absorbance and photoluminescence excitation (PLE) spectra. The octanol-capped clusters show efficient blue band-edge photoemission with a luminescence quantum yield of 23 % at room temperature.

Transport properties are reported for Bi nanowires, prepared by the filling of an alumina template with molten Bi. The temperature dependence of the resistance is presented for such arrays of Bi nanowires with diameters in the 40 to 200nm range. The data are understood qualitatively on the basis of a model for a quantum-confined system. Finally, a 4-point measurement is performed on an individual Bi nanowire prepared by using an electron beam lithography technique. Techniques for handling the practical issues of non-ohmic contacts and wire burn-out are given. The physical significance of the final results of the measurements are discussed in light of various scattering mechanisms in the nanowire.

We describe a procedure for dispersing electrochemical etched silicon into a colloid of ultra-small (∼1 nm) silicon nano particles. The particles are then reconstituted on a Si substrate by gentle evaporation from a volatile colloid, forming thin films. When the colloid or the film is excited with UV radiation at 355 nm, strong blue emission band at 390 nm is observable with the naked eye in room light. Two-terminal I-V spectra of the deposited film, taken using a scanning tunneling microscope (STM), show a single regular conductance resonance at a spacing of 0.25-0.30 eV, consistent with single electron charging. However, under light irradiation, we observe a second regular resonance, for negative tip biasing less than -1 eV, that modulates the charging structure at ∼ 1.0 eV period, consistent with the particle's quantum energy level spacing. This paves the way for operation of high-temperature Si-based single electron devices.

A model is given for the transport of electromagnetic energy through structures that consist of chains of closely spaced metal nanoparticles. This transport relies on the near-field electrodynamic interaction between metal particles that sets up coupled plasmon modes. The model predicts that the propagation is coherent and the group velocities can exceed typical saturation velocities of electrons in semiconductors (≈ 105 m/s). Furthermore, it is shown that in these structures propagation of energy around sharp corners (bending radius << the wavelength of visible light) is possible at high efficiency. This efficiency is a strong function of the frequency and polarization direction of the plasmon mode. The operation of a plasmon switch is modeled, in which plasmon waves can be switched or modulated. Finally, suggestions are given for the choice of metal particle and host material.