Approach: One method that eliminates z-motion artifacts is to acquire the data in constant-height mode (CHM) by scanning the aperture at a set height above the average surface plane. We have developed a method of acquiring NSOM data that allows for the construction of three types of images from one data set: topographic, CGM, and CHM. Instead of collecting optical data at a fixed aperture-sample gap at each XY position, optical data is collected along a controlled scan taken normal to the surface at each XY position. This produces cubes of data as shown in Figure 1a.

Results and Future Plans: The cube in Figure 1a shows the optical features of 80 nm gold particles immobilized on a silanized glass substrate. The cube covers a 1460 nm x 1460 nm x 140 nm region adjacent to the sample surface with topographic changes over this area of 90 nm. The grayscale represents the measured optical intensity. Z-motion artifacts are identified by analyzing the optical intensity for a given image as a function of the sample topography. The CGM image in Figure 1c shows many of the features present in the topographic image, Figure 1b. Z-motion artifacts often cause a direct correlation between optical and topographic features, and it is expected that many of these features are artifacts. In the CHM image shown in Figure 1d the majority of the features observed in Figure 1c are no longer present, indicating that they resulted from z-motion artifacts. The prominent features observed for each particle in Figure 1d consist of a dark center spot surrounded by two lighter lobes. This is the factual NSOM image. We are currently developing an improved (reduced acquisition time) algorithm for the acquisition of artifact-free NSOM images as well as data analysis tools for evaluating CGM images for artifacts.