Materials Chemistry: Surface Functionalisation
Material science, for many applications, is focused on 2D and 3D nanomaterials. The properties of materials, as their dimensions are decreased, are often enhanced or augmented by their small size. However, as the dimensions of these materials decrease below a certain size range the surface:bulk ratio increases dramatically. Surfaces are unstable largely due to the fact that the surface atoms are not fully coordinated. They also represent platforms for the adsorption of adventitious matter. For both these reasons the development of chemistries that will passivate uncoordinated surface atoms, while creating a controlled barrier at the surface:air interface is highly advantageous.
Passivation is one of the more important applications of surface functionalisation and can be achieved using self-assembled monolayers (SAMs). There are, however, a plethora of other applications which can benefit from the ability to be able react molecules with surface to form monolayers, e.g. monolayer doping; controlling the wetting of a surface; receptors for sensing applications; electrical passivation etc. A novel vapour phase process is available which is suitable for the functionalisation of suspended or tightly pitched nanostructures.
Finally, it should be noted that the expertise exists to chemically functionalise and characterise a broad variety of materials including metals (e.g. Au, Pt, Cu), semiconductors (e.g. Si, Ge, TiO2), III-V’s (e.g. GaN, InAs), etc. It is also possible to provide surface functionalised metal nanoparticles.
Self-assembled monolayers; passivation, sensors, transistors, metal nanoparticles, III-V, silicon, germanium, surfaces, surface chemistry, biomolecules.
Sample sizes of up to 15 mm2 can be processed using liquid phase chemistry and samples with dimensions of up to 10×50mm can be processed using a novel vapour phase process.
- Nano for Quantum Technologies
- Disruptive Devices
Key Enabling Capability
In general the formation of SAMs on surfaces is achieved through liquid-phase chemistry. In most case this suffices with excellent results being achieved. However, recently, we became aware of a significant problem when attempting to apply SAMs to Si nanowires that had been released from the underlying oxide with the intention of creating suspended nanowires for gate-all-around devices. The process for releasing involved etching of the underlying oxide using a solution of aqueous HF (hydrofluoric acid). The oxide was effectively etched but all the nanowire arrays had stuck together as a result of the capillary forces experienced by them during the aqueous etching step.
It became apparent that in the absence of a process that would eliminate this issue that the project could not proceed. To circumvent this issue, a reactor that could sit in a tube furnace was designed and built. This reactor was connected to a gas flow source and a pump system that allowed the molecular precursors to pass in a vapour over the samples. The results achieved using this system exceeded every expectation yielding significantly higher quality SAMs then its liquid-phase counterpart. This process is currently being rolled out to encompass a variety of applications.
The applications are multifold for formation of SAMs, a few are examples listed below:
- Controlled wetting
- Sensor development
- Doping, including non-line-of-sight doping