The GLAD technique uses the “shadowing effect,” which is a “physical self-assembly” through which some of the obliquely incident atoms may not reach certain points on the substrate due to the concurrent growth of parallel structures (see Fig. 1). Due to the statistical fluctuations in the growth, some rods grow faster in the vertical direction. Due to their higher height, they capture the incident particles, while the shorter rods get shadowed and cannot grow anymore. This leads to the formation of isolated nanostructures as shown in Fig. 1. The shadowing effect can be controlled by adjusting the deposition rate, incidence angle, substrate rotation speed, working gas pressure, substrate temperature, and the initial surface topography of the substrate.
Figure 1: Some of the basic nanostructure geometries produced by GLAD.
There are many advantages to this recently explored nanofabrication technique:
- It is a non-lithographic technique that does not require expensive e-beam lithography or X-ray lithography.
- It is not a chemical self-assembly technique in which often only small-aspect-ratio nanostructures can be fabricated. Instead, GLAD is a physical self-assembly growth technique that can produce three-dimensional nanostructures with a very large aspect ratio and controllable porosity, shape and symmetry, characteristics that are not easily attainable by other techniques.
- The technique is very robust in that practically any material (e.g., Pt, Si, C, W, Ru, etc.) or a combination of materials (e.g. alloys, oxides, compounds) that can be sputter or evaporation deposited can be used for fabrication.