Patterned, 2-D array of close-packed 4 nm diameter Au clusters on LTG:GaAs: SEM and STM images

Jia Liu, et al, submitted to Appl. Phys. Lett. (2/2000).


Self-assembly techniques can offer the potential to fabricate nanoscale elements without direct use of conventional lithographic techniques, which become expensive and slow when used to define nanoscale features.
The realization of electronic circuits of reasonable functionality and complexity using self-assembly approaches will require the ability to controllably break the symmetry typically found in self-assembled networks or to assemble the building blocks (e.g. nanometer-size clusters) into specific configurations as well as the ability to realize devices with functionality comparable to current semiconductor devices.

Toward the goal of a self-assembly technique which can be used to realize robust electronic devices and circuits, we have demonstrated the fabrication of highly ordered nanoscale elements defined in specific patterns using a guided self-assembly technique on a semiconductor surface. Specifically, we have developed a process that allows high-quality close-packed arrays of Au nanoclusters (5 nm in diameter) to be formed in patterned regions on active GaAs substrates.
The local ordering at the nanometer scale is provided by a chemically driven self-assembly process, while the arbitrary global pattern is defined by the patterned template.

SEM (Scanning electron microscopy) image of patterned GaAs (LTG:GaAs) using stamp-pad technique:

The directed self-assembly fabrication technique was used to form the patterned cluster arrays involves (1) the deposition of an organic tether molecule in pre-defined regions on the semiconductor substrate using a microcontact printing technique, (2) the transfer of a large-area (up to the centimeter length scale) close-packed array of Au nanoclusters onto the substrate, and (3) the removal of nanoclusters in the regions not coated with the tether molecule.
The resulting structure is schematically illustrated here.

In the SEM image below, white region is bare LTG:GaAs and dark region is pattened region of 2-D array of single-crystal Au clusters on LTG:GaAs substrate.
(This SEM image was taken by Prof. D. Janes in the School of Electrical and Computer Engineering, Purdue University.)

click to view high-resolution image

STM image of close-packed array of clusters in patterned region.

When the STM tip was over the Au nanocluster array (dark area in SEM image), hexagonal close-packed ordering was observed. This high resolution image indicates that the interface layer of XYL provides a robust mechanical tethering of the Au nanoclusters to the LTG:GaAs surface and electronically links the Au clusters to the LTG:GaAs surface.
(30 x 30 nm UHV STM image was aquired with a sample voltage of -1.5 V, a set tunneling current of 200 pA, and a scan rate of 2 Hz.)

click to view high-resolution image

Also, see TEM image of monolayer array of Au nanoclusters.

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