For the morphological control of nanostructures during bottom-up growth several techniques are used.
GLAD
Glancing angle deposition (GLAD) is a technique that uses a flow of atoms from gas phase to impinge on a substrate surface under an oblique angle. Physical vapor deposition under conditions of obliquely incident flux and limited atom diffusion results in a film with a columnar microstructure. These columns will be oriented toward the vapor source and substrate rotation can be used to sculpt the columns into various morphologies.
Glancing angle deposition (GLAD) is an advanced bottom-up nanostructuring technique developed by Michael Brett’s group at the University of Alberta, Canada. GLAD provides precision engineering of nanostructures via control over macroscopic geometry during deposition.
Deposition modulation allows for real-time fabrication of previously unachievable hybrid architectures during bottom-up growth. For example, with modulation of deposition rate and substrate orientation during growth, branches can be placed on selected regions of the trunk, and the trunk diameter can be controlled.
GLAD on flat substrates involves a nucleation process yielding layers of distributed columns. Periodic arrays of columns can be grown by patterning the substrate with seed particles prior to deposition. Here the patterned sites will function as nucleation sites for the columns since the shadowing effect will suppress growth on the surrounding substrate.
Columnar structure formation
The origin of the columnar structure characteristic of GLAD films can be discussed in terms of nucleation processes and structure zone models.
As deposition continues, the columnar structures are influenced by atomic-scale ballistic shadowing and surface diffusion. Competitive growth is observed where the tallest columns grow at the expense of smaller features. The column shape evolves during growth, and power-law scaling behavior is observed as shown in both experimental results and theoretical simulations. Due to the porous nature of the films and the increased surface area, a variety of chemical applications and sensor device architectures are possible. Because the GLAD process provides precise nanoscale control over the film structure, characteristics such as the mechanical, magnetic, and optical properties of the deposited film may be engineered for various applications. Depositing onto prepatterned substrates forces the columns to adopt a planar ordering, an important requirement for photonic crystal applications.
VLS-GLAD
GLAD technique can be combined with vapour liquid solid (VLS) to grow indium tin oxide (ITO) nanowhiskers. This technique named VLS-GLAD allows advanced control over ITO nanowhisker morphology and enables the researchers to tailor bulk optical and electrical properties of ITO nanowhisker films.
The ITO nanowhisker structure resembles a tree, with branches like a trunk growing orthogonal to the primary growth direction. VLS-GLAD technique enables control over number density, diameter and branching of ITO nanowhiskers and provides ability to scientists to shape the branch architecture. ITO nanowhiskers find application in high surface area electrodes, gas sensors, protein molecule sensors and UV light sources.
Applications
The VLS-GLAD technique can improve the control over the morphology of nanowhiskers and the ability to fabricate hybridized nanostructures from the bottom-up. The method for nanostructuring of materials is applied in the fabrication of photovoltaics and fuel cells, sensor and analytical devices utilizing porous materials, and nanoscale chiral thin films for photonics and photonic crystals.