Dawn A. Bonnell

Controlled fabrication of nanogaps in ambient environment for molecular electronics

We have developed a controlled and highly reproducible method of making nanometer-spaced electrodes using electromigration in ambient lab conditions. This advance will make feasible single molecule measurements of macromolecules with tertiary and quaternary structures that do not survive the liquid-helium temperatures at which electromigration is typically performed. A second advance is that it yields gaps of desired tunneling resistance, as opposed to the random formation at liquid-helium temperatures. Nanogap formation occurs through three regimes: First it evolves through a bulk-neck regime where electromigration is triggered at constant temperature, then to a few-atom regime characterized by conductance quantum plateaus and jumps, and finally to a tunneling regime across the nanogap once the conductance falls below the conductance quantum.

Helical Wrapping of Single-Walled Carbon Nanotubes by Water Soluble Poly(p-phenyleneethynylene)

Amphiphilic, linear conjugated poly[p-{2,5-bis(3-propoxysulfonicacidsodiumsalt)}phenylene]ethynylene (PPES) efficiently disperses single-walled carbon nanotubes (SWNTs) under ultrasonication conditions into the aqueous phase. Vis-NIR absorption spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM) demonstrate that these solubilized SWNTs are highly individualized. AFM and TEM data reveal that the interaction of PPES with SWNTs gives rise to a self-assembled superstructure in which a polymer monolayer helically wraps the nanotube surface; the observed PPES pitch length (13 +/- 2 nm) confirms structural predictions made via molecular dynamics simulations. This work underscores design elements important for engineering well-defined nanotube-semiconducting polymer hybrid structures.

Direct in situ determination of the polarization dependence of physisorption on ferroelectric surfaces

The ability to manipulate dipole orientation in ferroelectric oxides holds promise as a method to tailor surface reactivity for specific applications. As ferroelectric domains can be patterned at the nanoscale, domain-specific surface chemistries may provide a method for fabrication of nanoscale devices. Although studies over the past 50 yr have suggested that ferroelectric domain orientation may affect the energetics of adsorption, definitive evidence is still lacking. Domain-dependent sticking coefficients are observed using temperature-programmed desorption and scanning surface potential microscopy, supported by first-principles calculations of the reaction coordinate. The first unambiguous observations of differences in the energetics of physisorption on ferroelectric domains are presented here for CH(3)OH and CO(2) on BaTiO(3) and Pb(Ti(0.52)Zr(0.48))O(3) surfaces.

Local impedance imaging and spectroscopy of polycrystalline ZnO using contact atomic force microscopy

A current detection scanning probe technique is developed that quantifies frequency-dependent local transport properties. The approach, referred to as nanoimpedance microscopy/spectroscopy (NIM), is based on impedance spectroscopy with a conductive atomic force microscopy (AFM) tip. NIM is applied to study the quality of a tip/surface contact and transport behavior of individual grains and grain boundaries in polycrystalline ZnO. Impedance spectra were measured in the frequency range 40 Hz to 110 MHz, and the grain boundary properties are studied by nonlinear fitting of experimental data to an equivalent circuit. Two-terminal measurements are performed in the vicinity of a single ZnO grain boundary and the Cole–Cole plot indicates two major relaxation processes attributed to grain boundary relaxation and tip/surface contact.

Quantitative topographic analysis of fractal surfaces by scanning tunneling microscopy

The applicability of models based on fractal geometry to length scales of nanometers is confirmed by Fourier analysis of scanning tunneling microscopy images of a sputter deposited gold film, a copper fatigue fracture surface, and a single crystal silicon fracture surface. Surfaces are characterized in terms of fractal geometry with a Fourier profile analysis, the calculations yielding fractal dimensions with high precision. Fractal models are shown to apply at length scales to 12 A, at which point the STM tip geometry influences the information. Directionality and spatial variation of the topographic structures are measured. For the directions investigated, the gold and silicon appeared isotropic, while the copper fracture surface exhibited large differences in structure. The influences of noise in the images and of intrinsic mathematical scatter in the calculations are tested with profiles generated from fractal Brownian motion and the Weierstrass-Mandelbrot function. Accurate estimates of the fractal dimension of surfaces from STM data result only when images consist of at least 1000–2000 points per line and 1/ f -type noise has amplitudes two orders of magnitude lower than the image signal. Analysis of computer generated ideal profiles from the Weierstrass-Mandelbrot function and fractional Brownian motion also illustrates that the Fourier analysis is useful only in determining the local fractal dimension. This requirement of high spatial resolution (vertical information density) is met by STM data. The fact that fractal models can be used at lengths as small as nanometers implies that continued topographic structural analyses may be used to study atomistic processes such as those occurring during fracture of elastic solids.

Plasmon-induced electrical conduction in molecular devices.

Metal nanoparticles (NPs) respond to electromagnetic waves by creating surface plasmons (SPs), which are localized, collective oscillations of conduction electrons on the NP surface. When interparticle distances are small, SPs generated in neighboring NPs can couple to one another, creating intense fields. The coupled particles can then act as optical antennae capturing and refocusing light between them. Furthermore, a molecule linking such NPs can be affected by these interactions as well. Here, we show that by using an appropriate, highly conjugated multiporphyrin chromophoric wire to couple gold NP arrays, plasmons can be used to control electrical properties. In particular, we demonstrate that the magnitude of the observed photoconductivity of covalently interconnected plasmon-coupled NPs can be tuned independently of the optical characteristics of the molecule-a result that has significant implications for future nanoscale optoelectronic devices.

Structure of the Reduced TiO2(110) Surface Determined by Scanning Tunneling Microscopy

The scanning tunneling microscope has been used to image a reduced TiO2(110) surface in ultrahigh vacuum. Structural units with periodicities rangng from 21 to 3.4 angstroms have been clearly imaged, demonstrating that atomic resolution imaging of an ionic, wide band gap (3.2 electron volts) semiconductor is possible. The observed surface structures can be explained by a model involving ordered arrangements of two-dimensional defects known as crystallographic shear planes and indicate that the topography of nonstoichiometric oxide surfaces can be complex.

Scanning tunneling microscopy and spectroscopy of oxide surfaces

Considerable progress has been made using scanning tunneling microscopy (STM) and tunneling spectroscopy (STS) to examine the atomic structures and properties of transition metal oxide surfaces. The surfaces are found to be very sensitive to thermochemical history; consequently a large variety of surface structures have been observed. This paper reviews the results to date of STM on single crystal transition metal oxides (excluding superconductors) and, while mentioning ambient analyses, will emphasize ultra high vacuum and atomic scale structural information of single crystal surfaces. First, a summary of salient features of oxide bulk and surface geometric and electronic structures is provided as a framework within which to discuss the STM results. The principles of STM and STS are reviewed with extensive discussion of special considerations for analyses of oxides, including effects of band bending and image interpretation. Variations of surface stoichiometry are illustrated with results on TiO2 and SrTiO3, for which the most data exist. The yet unresolved controversy regarding the basis of contrast in STM images of oxides is introduced explicitly into discussion of these results. Recent work on vanadyls and tungstate bronzes is also presented. First observations of the structures of local defects on surfaces, including vacancies, dopants, steps, and domain boundaries, are illustrated with results from ZnO, NiO, SrTiO3, and TiO2. Finally, the few studies of surface reactions are considered, including those of oxides with metals, with reducing gases, and with organic molecules.

Temperature dependence of polarization and charge dynamics on the BaTiO3(100) surface by scanning probe microscopy

Variable-temperature atomic force microscopy, piezoresponse force microscopy (PFM), and scanning surface potential microscopy were combined to determine the temperature response of polarization and screening charge on BaTiO3(100) surfaces. The ferroelectric-domain induced surface corrugations and piezoelectric response decrease with temperature and disappear at the Curie temperature. The temperature dependence of the PFM contrast is explained within the framework of the Ginzburg–Devonshire theory with the effect of a dielectric tip-surface gap taken into account. The temperature dependence of the surface potential contrast is ascribed to the interplay between the release of the screening charges with temperature and their slow relaxation. The results indicate that surface potential polarity is reversed relative to that expected from polarization orientation on BaTiO3 in ambient.

Quantification of topographic structure by scanning probe microscopy

Several mathematical approaches for quantifying the three-dimensional topographical structure from scanning probe microscopy images are evaluated. Variational, i.e., scale-dependent, roughness based on root-mean-square roughness, Fourier deconvolution, and the two-dimensional autocovariance function are compared for surfaces with widely varying character in order to develop criteria for accurate quantification. Thermally evaporated gold, a calibration grid, polycrystalline Si3N4, and silicon fracture surfaces serve as models for these techniques. The role of image artifacts on each approach is detailed.