A non-lithographic method of fabricating high-density arrays of nanometer-scale vertical columns was investigated. The use of oblique deposition techniques allows the fabrication of isolated vertical columns in a single-step evaporation process without the need for either pre- or post-deposition lithographic processing. Extreme oblique incidence deposition with computer controlled substrate motion was utilized to fabricate columns with diameters near 100 nm and densities exceeding 109 columns/cm2. The desired column geometry may be engineered through choice of deposition angle and substrate spin rate. In one potential application of these microstructures, arrays of vertical columns were fabricated from silicon and carbon and tested for field emission characteristics. Further studies were made on the use of ion milling to modify the tips of the nanocolumns in order to improve the field emission properties.

A new approach for the use of porous alumina films as a template for nanofabrication is presented. In this process the porous films are prepared on silicon substrates, simplifying both the template fabrication and subsequent processing, and improving the quality of the films and their surfaces. Structural analysis of the film was carried out. Bismuth and bismuth telluride nanowires were prepared by pressure injection and electrochemical deposition, respectively, in alumina films 5-10 μ thick with parallel ordered pores 40 nm in diameter. The films were also patterned by lithography, offering new opportunities for area-selective anodization of non-planar structures. The new approach offers a straightforward method for the fabrication of arrays of nanostructures and their incorporation into electronic and optical devices.

We report weak localization studies of quantum coherence in metal nanowires with widths as small as 5 nm, demonstrating that structures fabricated at sub-50 nm length scales can reveal coherence phenomena not accessible in larger devices. Through selective etching of cleaved molecular-beam epitaxy (MBE)-grown substrates, we produce precise nanoscale surface relief then used as a stencil for metal deposition. This nonlithographic method of lateral definition allows the fabrication of metal (AuPd) nanowires greater than one micron in length with widths below 5 nm, a previously unexplored size regime in studies of quantum corrections to the conductance of disordered metals. Analyzing magnetoresistance data, we find that the coherence time, Tφ, shows a low temperature T dependence close to quasi-1D theoretical expectations (Tφ ∼ T-2/3 in 5 nm wide wires, while exhibiting a relative saturation as T 0 for wide samples of the same material. Since an externally controlled parameter, the sample geometry, can cause a single material to exhibit both suppression and divergence ofTφ, this finding provides a new constraint on models of dephasing phenomena.

Electrochemical methods have been used to produce three-dimensionally periodic metal meshes and spheres. Nickel is initially deposited into porous opal sheets. The opals themselves consist of close-packed silica spheres, which serve as a template for the growth of the nickel arrays within the void space between SiO2 spheres. Dissolution of the SiO2 spheres results in open, three-dimensionally periodic nickel mesh structures. The metal meshes can then be oxidized in air to produce nonconducting NiO meshes. This results in an inverse template that can be used for the growth of three-dimensionally periodic metal sphere arrays. Details on the preparation and characterization of these materials are presented.

There is an increasing interest in electronics functionality on surfaces which are not planar. This paper examines the critical technologies for fabricating electronic surfaces which have a three-dimensional shape. Two different approaches for achieving such a goal are examined. One can fabricate electronics using conventional technologies on a flat surface, and then after fabrication deform that surface into the desired shape (e.g. a spherical cap). In an alternative approach, one can directly fabricate onto substrates with an arbitrary shape. In this case one must address the issue of pattern formation and transfer on the curved surfaces. The scaling of letterpress printing to micron-scale features on flat and spherically curved surfaces is demonstrated.

A novel pattern transfer technique, based upon direct contact between nanofabricated printheads and amorphous films/substrates is being developed. Transfer of features from printhead to substrate is via an induced crystallization mechanism, with a goal of controlling crystal nucleation at dimensions comparable to the printhead features. To realize such a printing mechanism, appropriate mediums/materials that can be crystallized through contact with the printhead elements must be identified. From a range of candidate materials we focus on Ge and Indium Tin Oxide (ITO) films grown by molecular beam epitaxy or sputtering. The crystallization may be induced directly by heating (thermally), pressure (mechanically), or electrical conduction through the contact points. Crystallization of many amorphous materials is known to be self sustaining (so called “explosive crystallization”), raising the need to identify mechanisms for abruptly stopping crystallization if feature dimensions are to be constrained. A set of amorphous or nanocrystalline samples have been studied during crystallization by a heated printhead fabricated with a Focused Ion Beam (FIB). A simplified continuum model suggests the feasibility of achieving lateral confinement of the induced crystallization. In order to characterize fully the transitional and final substrate states, real-time observation of nanoscale crystallization in the transmission electron microscope (TEM) is in progress. A special TEM goniometer is being developed which incorporates a piezoelectrically driven tip and a heating capability to enable observation of tip-substrate contact at elevated temperatures that should give invaluable insight into crystallization mechanisms.

Here we describe the fabrication of electrostatic motors by liquid embossing, a contact stamp-based method of patterning liquids with sub-micron resolution. We also demonstrate AFM nano-assembly which can produce sub-40 nm dots and lines by transferring either liquid or solid material from a reservoir to a deposition area. Both of these non-lithographic patterning techniques are applicable to nanocrystal, organic, and polymeric solutions and liquids.

Recently, there has been an immense interest in the fabrication of composite nanoparticles, such as core-shell structural materials [1-6]. These core-shell composite particles may show special properties different from those of the core or shell particles. Thus, Au or Ag nanoparticcles were coated by silica. The silica shell not only stabilizes the colloidal Au nanoparticles, but also influences their optical property [3,6]. Furthermore, Halas et al. [5] have demonstrated that by varying the ratio of gold-shell to silica-core, the plasmon optical resonance of the Au shell and silica core strongly shifts across the visible spectrum and into the infrared region. Core-shell particles can be used as building blocks for different devices including tunable Photonic-Band-Gap (PBG) structures. Herein, we report on the fabrication of such particles with emphasis on the preparation of the Au nano-shells coated polystyrene (PS) with a diameter of about 700 nm.

Several studies to date have probed structural phase transitions in quantum dots (QDs) at high pressure. At low pressure (< 1 GPa), the optical properties of solvated nanomaterials are modulated by pressure induced electronic level tuning, particularly for surface and trap states. In fact, low pressure studies on solvated CdSe QDs may provide insight into the participation of surface hole traps and electron traps on the excited state optical properties in these materials. We report findings of QD size dependent pressure coefficients and postulate that trap state tuning, surface reconstruction events, and electron-hole exchange interactions may play a role in the low-pressure regime.

A procedure was developed for large-scale fabrication of nanometer-sized structures of single crystalline silicon with well-defined dimensions and shapes. Near-field optical lithography was used to define the nanostructures in a thin film of positive-tone photoresist with an elastomeric phase mask. The nanostructures were then transferred into the underlying silicon-on-insulator (SOI) substrate through a reactive ion etching (RIE) process. With this method, we can routinely generate silicon nanostructures ∼130 nm in lateral dimension. They can be supported on the surface of a solid substrate as a patterned array, or released into a freestanding form. The lateral dimension of these silicon structures could be further reduced to as small as ∼40 nm using stress-limited oxidation at elevated temperatures. The flexibility of this approach was demonstrated by fabricating nanoscale wires, rods, rings, and interconnected triangles of silicon. Using a two-step exposure method, the silicon nanowires can be precisely “cut” into silicon nanorods with specific lengths.

Electrons traverse two-dimensional nanocrystal arrays by sequential tunneling between neighboring nanocrystals. Analysis of array conductance at zero bias-voltage gives information about underlying nanocrystal uniformity, as well as the relevant single-electron charging energy. We discuss low-temperature measurements of two-dimensional self-assembled superlattices composed of 10 nanometer diameter cobalt nanocrystals, with ∼2 nm inter-nanocrystal spacing.

We report the fabrication and characterization of the self-assembled structure of tri-n-octylphosphine oxide (TOPO)-capped CdSe nanocrystals in binary polymer phase separated films of polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(2-vinylpyridine) (P2VP). These structures are formed via demixing of CdSe nanocrystals and binary polymer during spin coating. The nanocrystals preferentially segregate to the PS-rich phase in phase separated PS/PMMA and to the P2VP-rich phase in phase separated PS/P2VP, as shown by optical microscopy and photoluminescence images. For CdSe/PS/PMMA, we attribute the driving force for CdSe segregating to the PS-rich domain to the stronger attractive interaction of TOPO on PS with respect to PMMA due to polarity, and in the case of CdSe/PS/P2VP, segregating to the P2VP-rich domain to the interaction such as coordinate bonds being formed between pyridine groups and surface Cd atoms.

Arrays were self-assembled by evaporating suspensions of 4 nm FePt or 8 nm Fe nanoparticles. The monolayers had a hexagonal close packed (hcp) structure, but the multilayer structure varied. To identify the multilayer structures, transmission electron microscopy (TEM) images were compared with phase contrast image simulations. The results showed that Fe could be grown as both hcp and face-centered cubic (fcc), or fcc-like, structures. The results of image analysis of the FePt arrays were consistent with fcc structures.

Ni nanowires were grown in highly-ordered anodic alumina templates using pulsed electrodeposition. This technique yields completely metal-filled alumina membranes. The magnetic behavior of 100 nm period arrays of Ni nanowires with a length of 1 μ and different diameters has been characterized using SQUID magnetometry and magnetic force microscopy. Reducing the diameter from initially 50 to 25 nm while keeping the interwire distance constant leads to increasing coercive fields from 600 Oe to 1200 Oe and to increasing remanence from 30% to 100% of the hysteresis. The deposition of Ni65Fe35 gave a further improvement of the coercive fields up to 1350 Oe.

Fascinating nanostructured porous materials have recently been synthesized by evaporation-driven coassembly of ceramic precursors and amphiphiles. To expand our understanding of this process, we examine the influence of interfaces on self-assembly process using equilibrium lattice-based Monte Carlo simulations of ternary amphiphile-solvent mixtures. The simulations are able to predict the existence of all significant lyotropic mesophases, including lamellae, hexagonal closepacked cylinders, and cubic phases such as the gyroid. In the presence of walls that attract the majority solvent, the amphiphiles are confined to a smaller region of space, and experience a higher local concentration than the bulk concentration. This can lead to early transitions between mesophases. On the other hand, when the surface repels the majority solvent, amphiphiles tend to adsorb at the walls, and the local effective concentration in solution is lower. This can delay mesophase formation. When the amphiphile concentration is high enough that mesophases form in the bulk solution, however, either type of strongly attracting wall will align the mesophase (lamellae or hexagonal channels) parallel to the walls. In contrast, neutral walls (with no preferential interaction with either component) align mesophases perpendicular to themselves, which could be an interesting route to pores aligned normal to a film.

Alumina formed by the electrochemical anodization of bulk aluminum has a regular porous structure [1]. Sub-100 nm pores with aspect ratios as high as 1000:1 can easily be formed [2] without elaborate processing. Anodization of aluminum thus provides the basis for the inexpensive, high throughput microfabrication of structures with near vertical sidewalls [2]. In this work we explore the patterned anodic oxidation of deposited aluminum thin films, facilitating the integration of this technique with established microfabrication tools. An anodization barrier of polymethylmethacrylate (PMMA) is deposited onto 300 nm thick aluminum films. The barrier film is subsequently patterned and the exposed aluminum anodized in a 10% sulfuric acid solution. Barrier patterning techniques utilized in this study include optical exposure, ion-beam milling and nano-imprint lithography. Sharp edge definition on micron scale patterns has been achieved using optical methods. Extension of this technique to smaller dimensions by ion-beam milling and nano-imprint lithography is presented. We further report on the observation of contrast reversal of anodization with very thin PMMA barriers, which provides a novel means of pattern transfer. Potential applications and challenges will be discussed.

The electrostatic surface potential of self-assembled monolayers (SAMs) of aliphatic and aromatic thiols has been measured using electrostatic force microscopy. The variation of the surface potential of chemisorbed alkanethiols, with respect to bare Au(111), is observed to increase with increasing chain length. The trend is similar to that observed in the literature. A preliminary theoretical model, based on treating the monolayer as a sheet of dipoles, has been used to calculate the surface potential of alkanethiols. Similar measurements on several aromatic thiols, with a symmetric and non-symmetric molecular structure, reveal that non-symmetric systems have significantly higher potential (≥ 170mV) than the symmetric molecules.

We have investigated the local oxidation of an aluminum film to fabricate aluminum and aluminum oxide masks on Si and SiGe substrates. The local oxidation is made by negatively biasing the probe of an atomic force microscope operating in contact mode. The masks are defined by removing the unwanted material using highly selective etching solutions at room temperature. The produced aluminum-based masks can withstand reactive ion etching processes using fluorinated gases mixtures. We report examples of sub-100 nm pattern transfer on the substrate using the AFM fabricated masks. Preliminary observations suggest to use the sputtered aluminum films for which the anodization is found more efficient than for e-beam evaporated aluminum.

In this paper we present a new theoretical method for modeling electron transport through nanostructures under non-equilibrium conditions. The electronic structure of the nanostructures are modeled from first principles and are described selfconsistently under the non-equilibrium conditions by means of a Green's function technique. The method is used to calculate the electron transport through benzene-dithiolate connected to two gold chains.

A technique to cut out small pieces of samples directly from chips or wafer samples in a focused ion beam (FIB) system has been developed. A deep trench is FIB milled to cut out a small, wedge-shaped portion of the sample from the area of interest A micromanipulator with tungsten (W) probe is employed for lifting the micro-sample. The lifted micro-sample is then mounted on a carrier to prepare electron transparent thin foil specimens for transmission electron microscope (TEM) observation. We have also developed a method for site-specific TEM specimen preparation. In this method, FIB system and TEM/scanning transmission electron microscope (STEM) equipped with secondary electron (SE) detector are employed. An FIB–TEM/STEM compatible specimen holder has also been developed so that a specimen can be milled in the FIB system and observed in a TEM/STEM without remounting the specimen. STEM and scanning electron microscopy (SEM) images are used for locating a specific site on a specimen. SEM image observation at an accelerating voltage of 200kV enabled us to observe not only surface structures but also inner structures near the surface of a cross section with depth of field of around 1 micrometer. The STEM image allows the observation of inner structures of 3-5 micrometer thick specimens. Milling of a specimen by FIB and observation of the milled sample by SEM and STEM are alternately carried out until an electron transparent thin foil specimen is obtained. The position accuracy of the method in TEM specimen preparation is approximately 100nm.