Jessica Topple

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.

Room-Temperature Single-Electron Charging Detected by Electrostatic Force Microscopy

We use atomic force microscopy to measure electron addition spectra of individual Au nanoparticles that exhibit Coulomb blockade at room temperature. The cantilever tip charges individual nanoparticles supported on an ultra-thin NaCl film via single-electron tunneling from the metal back electrode. The tunneling is detected by measuring frequency shift and damping of the oscillating cantilever. Finite element electrostatic calculations indicate that the total nanoparticle capacitance is dominated by mutual capacitance to the back electrode.

Kelvin Probe Force Microscopy by Dissipative Electrostatic Force Modulation

We report a new experimental technique for Kelvin probe force microscopy (KPFM) using the dissipation signal of frequency modulation atomic force microscopy for bias voltage feedback. It features a simple implementation and faster scanning as it requires no low frequency modulation. The dissipation is caused by the oscillating electrostatic force that is coherent with the tip oscillation, which is induced by a sinusoidally oscillating voltage applied between the tip and sample. We analyzed the effect of the phase of the oscillating force on the frequency shift and dissipation and found that the relative phase of 90

Layer-by-layer growth of sodium chloride overlayers on an Fe(001)-p(1 × 1)O surface

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Field deposition from metallic tips onto insulating substrates

that causes only the dissipation is the most appropriate for KPFM measurements. The present technique requires a significantly smaller ac voltage amplitude by virtue of enhanced force detection due to the resonance enhancement and the use of fundamental flexural mode oscillation for electrostatic force detection. This feature will be of great importance in the electrical characterizations of technically relevant materials whose electrical properties are influenced by the externally applied electric field as is the case in semiconductor electronic devices.

Photovoltaics at the nanoscale

Ultra-thin NaCl films epitaxially grown on an Fe(001)-p(1 × 1)O surface have been investigated in ultra-high vacuum by non-contact atomic force microscopy and low energy electron diffraction. It has been found that at temperatures below 145 °C NaCl initially grows as monoatomic thick islands on substrate terraces, while at temperatures above 175 °C biatomic thick islands are also formed at substrate step edges. Both types of islands have the same Fe(001)-O[100] [parallel] NaCl(001)[110] orientation, leading to a (4 × 4) superstructure, where the NaCl unit cell is oriented at 45° with respect to the substrate. Interestingly, no c(2 × 2) superstructure with the NaCl unit cell oriented at 0° has been observed. The oxygen on the iron surface promotes layer-by-layer growth, resulting in atomically flat films with 40-60 nm wide terraces at coverages ranging from 0.75 to 12 ML. Such NaCl films are of much higher quality than MgO films grown on Fe(001) and Fe(001)-p(1 × 1)O surfaces and represent a unique epitaxial system of an alkali halide on a pure metallic substrate. The reduced number of defects and the layer-by-layer mode of growth make this system very attractive for applications where an atomically defined tunnel barrier is required to control the properties of a device.

Strain induced dewetting of a molecular system: Bimodal growth of PTCDA on NaCl

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.

Molecular dewetting on insulators

Nanoscale thin film morphology has been identified as an important factor in organic solar cell device func- tionality and efficiency. To better understand the limiting factors, it is important to work at the length scale of these processes. A study of thin films of organic molecules with Kelvin probe force microscopy (KPFM) to observe charge distribution and non-contact atomic force microscopy (NC-AFM) to simultaneously obtain structural information is presented. This allows investigation of the structure-function relationships in molecu- lar photovoltaics at the nanometer scale. PTCDI (3,4,9,10-perylenetetracarboxylic diimide) and CuPc (copper phthalocyanine) are used as organic molecules and are precisely grown on alkali halide substrates.