|
Abstract:
|
Silicon nanocrystals (NCs ) , especially Si NCs embedded in SiO₂ , have been studied intensely for decades for their potential application in silicon photonics , especially as efficient room temperature light emitters . Despite progress in fabricating photonic devices from Si NCs , the origin of the efficient photoluminescence (PL ) , the electronic and microscopic structure of the nanocrystals , and the structure of the elusive NC /SiO₂ interfaces for the oxide -embedded nanocrystals , remain controversial . Optical spectroscopy provides a powerful noninvasive tool for probing the structure of the Si NCs , including the active buried NC /SiO₂ interfaces of embedded particles . In this thesis work , oxide -embedded and free -standing alkyl -passivated silicon nanocrystals , prepared by different techniques , have been studied by linear and nonlinear optical spectroscopies . Cross -polarized 2 -beam second -harmonic and sum -frequency generation (XP2 -SHG /SFG ) has been applied spectroscopically to study oxide embedded Si NCs of different sizes (3 to 5 nm diameter ) and interface chemistries . The SHG /SFG spectra of silicon nanocrystals (Si NCs ) prepared by implanting Si ions uniformly into silica substrates , then annealing , are compared and contrasted to their spectroscopic ellipsometric (SE ) and photoluminescence excitation (PLE ) spectra . Three resonances - -two close in energy to E₁ (3 .4 eV ) and E2 (4 .27 eV ) critical -point resonances of crystalline silicon (c -Si ) , and a broad resonance intermediate in energy between E₁ and E₂ - -are observed in all three types of spectra . These features are observed in conjunction with a sharp 520 cm⁻¹ Raman peak characteristic of c -Si and an a -Si tail in the Raman spectra . The appearance of bulk -like CP resonances in the parallel PLE , SE and SHG /SFG spectra from Si NCs suggests the basic electronic structure of the bulk c -Si is preserved in nano -particles as small as 3 nm in diameter , albeit with significant size -dependent modification . At the same time , the prominence of a non -bulk -like resonance intermediate in energy between E₁ and E₂ CPs in all three types of spectra demonstrates the important contribution of nano -interfaces to the electronic structure .We also applied Raman spectroscopy to study oxide -embedded and oxide -free alkyl -passivated Si NCs with diameters ranging from 3 nm to greater than 10 nm synthesized by thermal decomposition of hydrogen silsesquioxane (HSQ ) . While oxide matrix complicates the size -dependence of the Raman peak shift for oxide -embedded nanocrystals , the Raman peak of the free -standing alkyl -passivated Si NCs shifts monotonically with NC size . |