Shawn Fostner

Determination of the local contact potential difference of PTCDA on NaCl: a comparison of techniques

There has been increasing focus on the use of Kelvin probe force microscopy (KPFM) for the determination of local electronic structure in recent years, especially in systems where other methods, such as scanning tunnelling microscopy/spectroscopy, may be intractable. We have examined three methods for determining the local apparent contact potential difference (CPD): frequency modulation KPFM (FM-KPFM), amplitude modulation KPFM (AM-KPFM), and frequency shift-bias spectroscopy, on a test system of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on NaCl, an example of an organic semiconductor on a bulk insulating substrate. We will discuss the influence of the bias modulation on the apparent CPD measurement by FM-KPFM compared to the DC-bias spectroscopy method, and provide a comparison of AM-KPFM, AM-slope detection KPFM and FM-KPFM imaging resolution and accuracy. We will also discuss the distance dependence of the CPD as measured by FM-KPFM for both the PTCDA organic deposit and the NaCl substrate.

Templated growth of 3,4,9,10-perylenetetracarboxylic dianhydride molecules on a nanostructured insulator

Nanometre-scale 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) crystallites were produced by trapping the molecules inside monolayer-deep rectangular pits on an alkali halide surface. Noncontact atomic force microscopy was used to measure the crystallite dimensions and lattice structure with molecular resolution. The molecule‐substrate lattice mismatch and island heights, typically three to four PTCDA layers, indicate a stress in the first two layers. One- and two-layer crystallites were only observed in pits with side lengths smaller than 10 nm. (Some figures in this article are in colour only in the electronic version)

Quantized conductance and switching in percolating nanoparticle films.

We demonstrate switching behavior and quantized conductance at room temperature in percolating films of nanoparticles. Our experiments and complementary simulations show that switching and quantization result from the formation of atomic-scale wires in the gaps between particles. These effects occur only when tunnel gaps are present in the film, close to the percolation threshold.

Scanning probe microscopy imaging of metallic nanocontacts

We show scanning probe microscopy measurements of metallic nanocontacts between controlled electromigration cycles. The nanowires used for the thinning process are fabricated by shadow evaporation. The highest resolution obtained using scanning force microscopy is about 3 nm. During the first few electromigration cycles the overall slit structure of the nanocontact is formed. The slit first passes along grain boundaries and then at a later stage vertically splits grains in the course of consuming them. We find that first the whole wire is heated, and later during the thinning process as the slit forms the current runs over several smaller contacts and less power is needed.

Use of an electron-beam evaporator for the creation of nanostructured pits in an insulating surface

We demonstrate a method for creating monatomic-depth rectangular pits of controlled size in an alkali halide surface by using an electron-beam evaporator. Atomic resolution noncontact atomic force microscopy is used to characterize the structure and size distribution of the pits, with mean side lengths ranging from 6.5to20nm. It is also demonstrated that metal nanoparticles can be used to nucleate the growth of pits, resulting in pits with metal nanoparticles inside.

Neuromorphic behavior in percolating nanoparticle films.

We show that the complex connectivity of percolating networks of nanoparticles provides a natural solid-state system in which bottom-up assembly provides a route to realization of neuromorphic behavior. Below the percolation threshold the networks comprise groups of particles separated by tunnel gaps; an applied voltage causes atomic scale wires to form in the gaps, and we show that the avalanche of switching events that occurs is similar to potentiation in biological neural systems. We characterize the level of potentiation in the percolating system as a function of the surface coverage of nanoparticles and other experimentally relevant variables, and compare our results with those from biological systems. The complex percolating structure and the electric field driven switching mechanism provide several potential advantages in comparison to previously reported solid-state neuromorphic systems.

Field deposition from metallic tips onto insulating substrates

The deposition of gold ions from atomic force microscope cantilever tips onto bulk insulating substrates with nearby surface electrodes is discussed. Numerical models of the potential distribution are used to estimate potential barriers for the desorption process. These models indicate deposition height thresholds of 7-10 nm with the tip 20-25 nm from the metallic electrode edge over a KBr surface but greater than 20 nm high for InP/GaAs/InP substrates with a two-dimensional electron gas (2DEG) as the back electrode. Experimental results for the deposition of gold clusters over KBr surfaces near metal electrodes in ultra-high vacuum (UHV) are presented and show promising agreement with calculations of the deposition threshold heights. Deposition of clusters over InP is discussed for comparison and indicates similar trends.

Quantum fluctuations in percolating superconductors: an evolution with effective dimensionality

We investigate percolating films of superconducting nanoparticles and observe an evolution from superconducting to metallic to insulating states as the surface coverage of the nanoparticles is decreased. We demonstrate that this evolution is correlated with a reduction in the effective/dominant dimensionality of the system, from 2D to 1D to 0D, and that the physics in each regime is dominated by vortices, phase slips and tunnelling respectively. Finally we construct phase diagrams that map the various observed states as a function of surface coverage (or, equivalently, normal state resistance), temperature and measurement current.

Percolating transport in superconducting nanoparticle films

Nanostructured and disordered superconductors exhibit many exotic fundamental phenomena, and also have many possible applications. We show here that films of superconducting lead nanoparticles with a wide range of particle coverages, exhibit non-linear V(I) characteristics that are consistent with percolation theory. Specifically, it is found that V∝(I−Ic)a, where a = 2.1 ± 0.2, independent of both temperature and particle coverage, and that the measured critical currents (Ic) are also consistent with percolation models. For samples with low normal state resistances, this behaviour is observable only in pulsed current measurements, which suppress heating effects. We show that the present results are not explained by vortex unbinding [Berezinskii-Kosterlitz-Thouless] physics, which is expected in such samples, but which gives rise to a different power law behaviour. Finally, we compare our results to previous calculations and simulations, and conclude that further theoretical developments are required to e...