In 2016, Paul Clarkson was awarded the Rideal Travel Bursary. He travelled to the University of San Diego from July to October 2016 to carry out research into the functionalisation of porous silicon. Here, he tells us about how his visit, working alongside Professor Sailor, has aided his research.
‘My name is Paul Clarkson. I am PhD student at the University of Cambridge as part of the Nano Doctoral Training Centre. The philosophy of the doctoral training centre is to operate between two different departments on a PhD to combine different methods on my research. My primary group is in physical chemistry and my second group is in micro-electronic engineering.
‘My research is on porous silicon waveguides as an optical sensing platform. Silicon can be made porous by electrochemically etching doped silicon with an etching solution that contains hydrofluoric acid (HF) and ethanol in constant current mode. By changing the HF concentration, resistivity of the silicon wafer or the current density of the etch, it is possible to tune the pore size and pore distribution in the silicon wafer as well as the thickness of the porous silicon layer. These pores are classed as microporous, mesoporous or macroporous. I tend to work in the mesoporous region which means that the pore size is between 5nm and 50nm.
‘The porous silicon layer creates a thin film – the region in which the pores are present changes the properties of the silicon. Since bulk silicon has been removed throughout the porous silicon layer, the refractive index of the porous silicon layer decreases. Changing the porosity of the layer allows one to tune the refractive index.
‘I etch multilayers: first a low porosity layer followed by a higher porosity layer resulting in a higher and lower refractive index respectively. I then perform various microelectronic fabrication steps including sputtering, photolithography and reactive ion etching in order to fabricate patterns in the porous silicon layers and thus make waveguides; light can be totally internally reflected into the porous silicon layer of higher refractive index. This can then be used as a chemical sensor; the presence of additional chemicals into the pore network changes the refractive index of the layer and hence changes the refractive index of the waveguide which results in a change of the optical signal.
‘Until recently, my main work was on the fabrication of the devices using the clean room techniques just mentioned. With that achieved, I needed to understand more about the surface chemistry of silicon/silica and how to functionalise and characterise it. Professor Michael Sailor at the University of San Diego California is an expert in this field and he kindly offered to teach me the relevant surface chemistries of porous silicon provided I could find the costs of getting to San Diego and living there. Thanks to the Sir Eric Rideal Travel Award from SCI, I was able to gather the funds necessary in order to go.
‘While in San Diego I learned a lot about functionalising porous silicon/silica using silane based chemistries and how to characterise them with methods including contact angle, raman spectroscopy, ATR-FTIR spectroscopy and the spectroscopic liquid infiltration method SLIM used on porous materials. I cannot thank Professor Sailor or the Rideal award enough for greatly facilitating my research in this field.
‘Since arriving back in Cambridge, I am now combining what I learned in San Diego with the devices I have been fabricating. I am now underway fabricating functionalised waveguides which will serve as a sensor platform to investigate different gases and chemistries by observing how the change in the surface chemistries affect the ability to sense different gasses and chemicals.’
Paul Clarkson
PhD Student
University of Cambridge
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