Three-dimensional self-assembled hybrid silicon nanostructures for high sensitivity SERS chemical and biosensing applications

Jeffery Alexander Powell

doi:10.32920/ryerson.14652153

Abstract

Raman spectroscopy is a powerful tool for detection of chemical and bioanalytes but lacks enhancement required to detect these analytes at the ultrahigh sensitivity needed for many applications. Surface enhanced Raman Scattering is a technique by which an analyte signal can become greatly enhanced and, near single molecule sensitivity, is achievable. Currently, SERS-based detection platforms currently rely on noble metal nanostructures as primary enhancing sources for the detection of chemical and bioanalytes but have significant limitations in terms of reproducibility and biocompatibility. Recent research has shown that semiconductors have the ability to exhibit SERS enhancing characteristics that can potentially supplant the use of noble metals without the limitations associated with noble metal nanomaterials. This thesis presents, the generation of three-dimensional self-assembled hybrid silicon nanostructures though a laser-ion plume formation mechanism. These Si nanostructures exhibit high sensitivity SERS enhancement characteristics which can be applied for chemical and biosensing applications. In this thesis, the Raman enhancing characteristics of the hybrid Si nanostructures are examined and correlated to the unique physical morphology and material chemistry of these nanostructures. These Si nanostructures are shown to be comprised of individual Si nanospheroids that have fused to form a highly 3D nanoweb-like self-assembled nanostructures. It is also shown that these nanospheroids are composed of both amorphous and polycrystalline sub-regions, which can only be generated within an ion-plume formed by a femtosecond pulsed laser. By programming the laser, the nanostructure morphology and hybrid nature can be manipulated and optimized. These Si nanostructures are shown to be highly sensitive as SERS platforms for chemical analytes. In addition, it is shown that with the application of noble metal nanoparticles on the surface of the 3D hybrid silicon nanowebs structures, an additional enhancement boost can be optimized for the detection of chemical molecules. With this, the dual contribution to the SERS sensitivity from both the primary Si nanostructures and the secondary noble metal nanostructures can be used to detect the presence of a biomolecule analyte is shown. To delve deeper into how these hybrid Si nanostructures cause SERS enhancement of bioanalytes, the Si ion interactions within the laser-ion plume were manipulated to induce quantum-scale defects within the hybrid Si nanospheroids. By creating both an inert and oxygenated laser-ion plumes the formation of sub-nanograins within the nanospheroids and sub-nanovoids on the nanospheroid surface is shown to significantly enhance the detection of bioanalyte signal for multiple biomolecules which act as signals for various diseases. Based on the results in this thesis, it has been proven that Si-based nanostructures have the capacity to be used as sole SERS enhancing sources for chemical and biomolecule analytes.

Highlights

1.1 Raman SpectroscopyMany techniques currently exist for identifying and classifying inorganic and organic materials based on a unique spectral signature whether it be based on electrochemical [1-1], piezoelectric [1-2], pyroelectric [1-3] or optical[1-4] phenomenon
The increase in sensitivity to an analyte originates from the enhancement of electric field caused by SPR which is transferred to an analyte molecule resulting in a larger cross-section of Raman scattered photons. [2-14] Noble metals have been predominantly used as surface enhanced Raman scattering (SERS) materials since they have a well-established property of exhibiting surface plasmon resonance (SPR) in the visible and near-infrared (NIR) spectrum, the wavelength range for most Raman lasers[2-15]
The unique nanomaterial created with the ultrafast femtosecond laser is a material that, according to literature, cannot be formed using any other fabrication technique; lasers with longer pulse widths are unable to create this material because only a laser with femtosecond pulse width is able to cause such high temperatures that when the pulses strike the silicon surface, Si atoms are immediately ionized and form an ion plume above the silicon surface without the loss of energy to heating of the substrate. [2-41] Figure 2-2 is schematic showing the laser ion plume formation mechanism with Scanning electron microscopy (SEM) images of formed nanoweb structures

Summary

Introduction

1.1 Raman SpectroscopyMany techniques currently exist for identifying and classifying inorganic and organic materials based on a unique spectral signature whether it be based on electrochemical [1-1], piezoelectric [1-2], pyroelectric [1-3] or optical[1-4] phenomenon. For biosensing applications employing surface enhanced Raman scattering (SERS), the use of nanostructured noble metal materials have been extensively researched due to the well-established surface plasmon resonance (SPR) enhancement effect unique to these nanomaterials[5-1]. These noble metal nanostructures have been fabricated using various techniques including chemical synthesis [5-2], nanolithography[5-3], and physical deposition[5-4] which have been modified and optimized to excite the detection of various bioanalytes; noble metal nanorods[5-5, 5-6], nanoparticles[5-7], and nanowires[5-8] have all shown SERS detection of biomolecules due to the SPR effect. The most studied of these linked resonance mechanisms is charge transfer resonance, which is the process of transferring electrons from a semiconductor to an analyte molecule and vice versa and is dependent on their bandgap and highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMOLUMO) energy difference respectively[5-24]

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