John T. Woodward

Removing drift from scanning probe microscope images of periodic samples

Thermal drift is frequently encountered when imaging with scanning probe microscopes. The drift skews real space images and distorts the reciprocal space lattice vectors. A settling time of up to 2 h is generally required to achieve relatively drift free images at the high magnifications needed for molecular or atomic resolution. We demonstrate a simple method to extract accurate lattice parameters from periodic samples which compensates for drift to first order (approximately constant drift rate), dramatically shortening the necessary waiting time. The method is based on averaging the apparent reciprocal lattice vectors corresponding to consecutive scans obtained in opposite scanning directions.

Electrochemical Properties of Nanocrystalline Cadmium Stannate Films

Electrochemical properties of sol-gel nanocrystalline cadmium tin oxide electrodes (CTO) were investigated in 0.2 M potassium chloride buffered at pH 7.4 with phosphate. Films were found to he n-type degenerate semiconductors with charge carrier levels from 10 19 to 10 22 cm 3 depending on the thermal aftertreatment. X-ray diffraction analysis was used to reveal the appearance of the cubic cadmium stannate (Cd 2 SnO 4 ) phase at annealing temperatures above 600°C, and to indicate the extent of this dominant phase above 750°C. The flatband potential (E FB . ) of the film electrodes, as determined from capacitance measurements, was found to be around +0.25 V at pH 7.4. Electrochemical activity toward ten redox processes in the range -0.45 V < E < 0.45 V was investigated, and standard electron transfer rate constants were estimated from ac impedance measurements. The dominant factor in the charge-transfer rate on CTO electrodes is the bulk film charge carrier concentration. It was found that the charge-transfer rates were dependent on the separation of the redox carrier formal potential (E of ) from the CTO flatband potential. The slowest rates (10 5 cm s -1 were found for redox couples where E of E FB . For charge transfer from redox couples where E of is away from E FB , the rates can be several orders of magnitude greater and it is thought that the density of states in the conduction hand is rate limiting.

Atomic force microscope imaging of molecular aggregation during self-assembled monolayer growth

Abstract ‘Self-assembled’ monolayers of amphiphilic surfactant molecules form spontaneously on solid surfaces by exposure to dilute solutions of the adsorbate molecules. Using a combination of atomic force microscopy (AFM), infrared spectroscopy, and wettability studies, we find that these monolayers form via a mechanism that includes nucleation, growth, coalescence, etc. of densely packed submonolayer islands of the long-chain organic molecules. AFM experiments permit direct observations of the size and shape of these islands. Molecules that attach to each other covalently and irreversibly (such as alkyltrichlorosilanes) form islands that are fractal with a morphology consistent with 2D diffusion-limited aggregation. Molecules that interact with each other via softer van der Waals interactions form rounded islands, suggesting that the island shapes relax via a continuous process of 2D desorption and re-adsorption. In situ AFM measurements allow a quantitative analysis of island nucleation and growth rates as well as determination of the island size distribution as a function of coverage. In the growth regime, the nucleation and growth rates have a power law behavior consistent with a simple point island model of 2D cluster growth. The exponents are consistent with a critical nucleus of two molecules and the 2D diffusion coefficient corresponds to a ‘hopping time’ of about 1 μs. In the aggregation regime, the island size distributions are shown to scale with a single evolving length scale in accordance with the dynamical scaling approximation.