Three-dimensional nanoporous silicon (PSi), with inherently large surface areas, tunable pore sizes, film thicknesses, and effective refractive indices, has been utilized as a platform for the detection of biomolecules and high-dose radiation. A brief overview of the fabrication and characterization of the nanoporous framework is presented for novel applications that benefit from such sponge-like, high surface area devices. For many of these applications, it is necessary to ensure that the PSi surfaces are well-passivated and stabilized for subsequent conjugation with linker molecules and for emitters to maintain their emissive properties post-integration with the porous matrix. We present a detailed analysis of the influence that varied levels of interfacial oxide (SiOx) growth has on the optical properties of quantum dots (QDs) immobilized within the PSi thin-films. Reflectance spectroscopy, continuous wave photoluminescence (CWPL) and time-resolved photoluminescence (TRPL) studies provide a comprehensive understanding of the complex QD exciton dynamics at the PSi/SiOx-QD interfaces. The gradual conversion of PSi thin-films into fully-oxidized porous silicon oxide (PSiO2) thin-films is shown to significantly suppress non-radiative recombination pathways of photogenerated QD excitons and achieve almost a five-fold increase in QD exciton lifetimes. This conversion of PSi into PSiO2, a wide bandgap nanoporous material, also circumvents loss of QD emission due to absorption by PSi based devices. Future avenues of research into PSi based devices will be presented based on analyzing the optical scattering response of nanoscale PSi annular rings fabricated over PSi Bragg mirrors via dark field microscopy.
The formation of resonant photonic structures in porous silicon leverages the benefit of high surface area for improved molecular capture that is characteristic of porous materials with the advantage of high detection sensitivity that is a feature of resonant optical devices. This review provides an overview of the biosensing capabilities of a variety of resonant porous silicon photonic structures including microcavities, Bloch surface waves, ring resonators, and annular Bragg resonators. Detection sensitivities > 1000 nm/RIU are achieved for small molecule detection. The challenge of detecting molecules that approach and exceed the pore diameter is also addressed.
The effects of X-ray and gamma irradiation on the optical properties of CdTe/CdS quantum dots (QDs) immobilized in a functionalized porous silicon film have been investigated via continuous wave and time-resolved photoluminescence measurements. Carrier lifetimes of the QDs and photoluminescence intensities decrease with increasing exposure dose from 500 krad(SiO<sub>2</sub>) to 16 Mrad(SiO<sub>2</sub>).
We demonstrate the use of colloidal quantum dots (QDs) as refractive index signal amplifiers for the dual-mode, optical
detection of biotin in streptavidin-functionalized porous silicon (PSi) biosensors. The nanoporous silicon host matrix
was first analyzed to determine the relationship between different formation conditions, the resulting pore size
distributions, and the efficiency with which different sized target molecules infiltrate and bind to the pore walls. Non-specific
detection of glutathione conjugated with QDs was then demonstrated using PSi films with different average
pore diameters. The specific detection of small biotin molecules was confirmed when the QD-biotin conjugates resulted
in a six-fold increase in the reflectance fringe shift and a distinctive fluorescence spectrum upon exposure to an
optimized, streptavidin-functionalized PSi sensor. A biotin detection limit of 0.5 fg/mm<sup>2</sup> is achievable.
Conference Committee Involvement (1)
Energy Harvesting and Storage: Materials, Devices, and Applications IX
14 April 2019 | Baltimore, Maryland, United States