Nanostructures form the web on which a number of electronic devices are built. Therefore, it is important to analyse and study their electrical structures. The subsequent sections provide a detailed analysis of data collected by studying delicate samples using the PeakForce TUNA method.
Characterization of Nanostructures
The topography and the current map obtained by the PeakForce TUNA method applied on carbon nanotubes that are connected to the conductive pads placed on top of SiO2/Si substrate is represented in Figure 1.
Figure 1. PeakForce TUNA images (a) topography (b) current map of carbon nanotubes lying flat on a SiO2/Si sample. Images were taken on Bruker’s Dimension® Icon® AFM in ambient conditions, with an SCM-PIT probe (spring constant ~4N/m), 5micron scan at a DC sample bias of 500mV. Sample courtesy of Prof. Hague, Rice University.
The topographic image shows all the nanotubes clearly, which implies that all of them are conductors connected to the conductive pads. The image also revealed densely packed nanoparticles, which probably are residues formed during the formation of the sample. The conductivity of these particles cannot be analyzed as they are not connected to the conducting pads. This point is confirmed by their absence in the current map. A variation in the conductivity was observed which could be attributed to their presence on or along the tubes. Although the nanotubes are delicate, they can be pushed with the AFM tip (for Contact Mode AFM) as the substrate is hard. When using PeakForce TUNA, the SCM-PIT (platinum-iridium coated) tip can be tolerated for extended hours without the substrate eroding it.
In order to do a comparative study, the same sample was imaged using the Torsional TUNA method. It was observed that the conductivity trace was much wider, which could be due to the lateral dithering of the AFM probe during use. Figure 2a and b present the PeakForce TUNA images of carbon nanotubes mat which is vertical and multi-walled and placed on a conductive substrate.
Figure 2. PeakForce TUNA images (a) topography 50nm scale (b) peak current map (1 nA scale) of a vertical multi-walled carbon nanotube mat on a conductive substrate. Images were taken on Bruker’s MultiMode 8 AFM in ambient, with SCM-PIT probe (spring constant ~4N/m), 1ìm scan at a peak force of 10nN, and DC bias of -1V. TR-TUNA images (c) topography scale 100nm (d) current map (scale 1nA) for comparison.
The image shows end caps of the nanotubes. In the current map, conductivity was not shown by all the multi-walled nanotubes, rather different bundles showed variations in conductivity. This variation could be due to the difference in the way in which the nanotubes are connected or the effect of the capping on the tubes. When Contact Mode was used in the imaging no stable images were obtained; torsional TUNA gave a current image that differed from that obtained from PeakForce TUNA. The TR TUNA images represented many discontinued spots on single tubes which maybe due to the lateral twist that cause intermittent electrical contact with the surface.
Selection of Probe in PeakForce TUNA
While choosing the right PeakForce TUNA probe, the spring constant and the conductive coating material are important factors to be considered. Bruker’s latest probes are designed for use with soft delicate samples. The probes are coated with gold (Au) having spring constants in the range of 0.4N/m. The SCM-PIT probes have a platinum-iridium coating and spring constant of approximately 3N/m and are suitable for working with fragile samples like loosely bound nanostructures. For organic cell characterization, silicon probes which are coated with a low-work function metal are most suitable.
The PeakForce TUNA method when implemented using Bruker’s Peak Force Tapping technology is capable of producing a high- bandwidth, low-noise current amplifier design with high-gain features. PeakForce TUNA method scores over all other AFM methods in being able to work with fragile samples. The AFM imaging achieved using this method is of high resolution and accuracy. Moreover, the ScanAsyst algorithm that comes along with PeakForce TUNA simplifies the optimization of the AFM’s scan parameters. This method allows the Peak Force QNM (quantitative nanomechanical) also to be mapped, which provides details on the electrical information along with the topography. The Bruker’s glove box is an added feature, which takes care of handling air-sensitive samples properly.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
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