In collaboration with Dr. Philippe Fauchet and Dr. Benjamin Miller we are developing porous silicon for various biotechnology applications. Porous silicon is prepared by anodic electrochemical etching of a single crystal silicon wafer using an electrolyte containing hydrofluoric acid (HF) (Fig. 1a). Pore diameter, porosity and pore channel morphology are controlled by varying etch parameters including wafer doping level and type, current density, and electrolyte formulation. Pore diameters typically range between 20-100 nm but pores exceeding several microns are also possible (Fig. 1b) . Pore channels grow oriented along the <100> crystal axis (perpendicular to the surface plane for a Si(100) wafer). Channel morphology can vary from smooth walls to being filled with branching silicon nanostructures (Fig. 1c).
Figure 1a. Image shows Teflon etch cell with Pt wire cathode (black) and Anode (Red)
Figure 1b SEM images demonstrating a range of pore diameters
|Figure 1c SEM images pore channel morpohology.|
Wafer-Supported 1D-Photonic Band Gap Optical Sensors
Photonic bandgap devices are multilayer structures where the spatial periodicity is on the same order of the probe light frequencies (visible, NIR, IR etc). When the optical thickness (OT) of each layer is tuned to 1/4 λ, under white light illumination, constructive interference occurs such that certain frequencies are highly reflected (eg. Bragg mirror). In Fig. 2 below λ is the operating frequency of the device; d is the layer thickness and μ is refractive index (RI). Breaking the symmetry of the periodic structure forms a microcavity and the defect layer allows certain frequencies of light to transmit. Because the RI of a PSi layer depends on its porosity, a photonic band gap structure is easily fabricated by applying a constant current for a fixed time and cycling between different current densities.
The frequencies of light reflected and their % reflection depend on the porous morphology, the number of layers in the device, the RI of substances that fill the layers and the device operating frequency. When a biological or chemical receptor, tethered to the internal surface area of the sensor, binds target, a change in porosity occurs. This causes a change in OT which is recorded as a shift in the frequencies of light reflected . Highly reflective devices are shown in the Fig. 3 below (left) for devices with air in the pores. Displacing air with a fluid of higher RI (eg. water, ethanol etc.) changes the effective RI causing a color change (movie). This phenomenon is the fundamental operating principle behind porous silicon chemical and biological sensors.