Nonlinear Spectroscopy of Silicon Nano-Interfaces

技术：本项目主要利用非线性光学方法，特别是二次谐波产生(SHG)和和频产生(SFG)光谱，研究嵌入在非晶态氧化物材料和邻近Si(001)和Si(111)表面台阶边缘的纳米硅(NCs)。非线性光学光谱被用来探测这些材料的电子结构、化学修饰和飞秒动力学。实验将扩展到多电子伏光谱范围，使用光学参数放大器来获得界面态光谱。镶嵌硅纳米晶体的研究包括对纳米晶界面态之间辐射和非辐射光学跃迁的综合研究，这些光学跃迁是由Si=O双键、Si悬挂键、非晶态Si过渡区以及应变的Si-Si和Si-O键引起的，这对于理解纳米晶体的发光是至关重要的。由于球形硅纳米晶复合材料具有反转对称性，它们没有传统的电偶极倍频响应，但具有允许的光学四极倍频响应，对界面化学钝化高度敏感。在硅台阶边的研究中，倍频将与反射各向异性光谱(RAS)相结合，并使用简化的键超极化率模型进行分析，以获得键特定的光谱。非技术性：该项目解决了材料科学中具有高度技术相关性的热门领域的基础研究问题。这项研究有望对硅纳米晶界面的结构和性质以及结晶硅表面的台阶产生亟需的基本了解。该项目的成功将有助于未来基于纳米晶体的发光和闪存设备以及整个纳米技术的进步。通过这个项目，研究生和本科生将接受对美国工业和技术至关重要的领域的培训。该项目还包括与德克萨斯大学圣安东尼奥分校合作的教育和外联活动，该分校的拉美裔学生入学率非常高。

Technical: This project is to study silicon nanocrystals (NCs) embedded in amorphous oxide materials and step edges at vicinal Si(001) and Si(111) surfaces mainly using nonlinear optical methods, particularly second harmonic generation (SHG) and sum-frequency generation (SFG) spectroscopy. The nonlinear optical spectroscopy is used to probe electronic structures, chemical modifications, and femtosecond dynamics of these materials. The experiments will be extended to a multi-electron-volt spectral range using an optical parametric amplifier to obtain interface state spectra. The research on embedded Si nanocrystals includes a comprehensive study of both radiative and nonradiative optical transitions between nanocrystals' interface states arising from, e.g., Si=O double bonds, Si dangling bonds, the amorphous Si transition region, and strained Si-Si and Si-O bonds, which are crucial for understanding luminescence of nanocrystals. Because composites of spherical silicon nanocrystals possess inversion symmetry, they lack a conventional electric-dipole SHG response, but possess an allowed, optically quadrupolar SHG response that is highly sensitive to interface chemical passivation. In studies of silicon step edges, SHG will be combined with reflectance-anisotropy spectroscopy (RAS), and analyzed with a simplified bond hyperpolarizability model to achieve bond-specific spectra.Non-technical: The project addresses basic research issues in a topical area of materials science with high technological relevance. The research is expected to yield much needed basic understanding on structures and properties of silicon nanocrystal interfaces and steps at crystalline silicon surfaces. The success of the project would help in the future progress of nanocrystal-based light-emitting and flash-memory devices and nanotechnology in general. Through this project, graduate and undergraduate students will receive training in a field of critical importance to U.S. industry and technology. The project also includes education and outreach activities in collaboration with the University of Texas - San Antonio campus, which has very high Hispanics student enrollment.