MRI: Development of a New Paradigm for Apertureless Near-field Scanning Optical Microscope

我们建议构建一个具有高放大倍率的最先进的成像材料工具，例如可视化分子群并能够识别它们。传统的光学显微镜，例如放大镜，只能在微米尺度上显示，即人类头发的一小部分。这是由于可见光的衍射极限。原子力显微镜使用尖锐的尖端扫描材料表面，可以在纳米级别（人类头发的一小部分）成像材料，然而，不能提供关于尖端下是哪种材料（金属，聚合物或离子）的信息。将原子力显微镜的高分辨率与光学显微镜所显示的材料识别能力相结合，是材料科学界的一项使命。这种新仪器被称为近场扫描光学显微镜。由于两个相互竞争的因素，这项任务并非微不足道，即在成像部位需要足够的光强度（例如使用大探针），并且需要使探针小/锋利以获得光学分辨率。这项提议将使用一种新方法，该方法来源于我们的发现，即当人们将激光束聚焦在尖端时，特定类型的尖锐探针会发光。发光的尖端为成像和光谱学提供了“点光源”。初步结果证明了产生近场信号的可行性，我们计划完成该仪器的构建，优化其性能并展示其应用。与以往的方法相比，该方法具有光强高、操作简单、分辨率高等优点。我们计划通过表征四类重要材料来展示这种新仪器的应用和能力：含有有机（聚合物）和无机（半导体）成分的材料；多组分无机小颗粒；碳纳米纤维;以及活细胞中的纳米材料。这项新技术的发展将把学生和博士后带到扫描探针显微镜技术及其在材料科学中的应用的前沿。该仪器的完成将增强UCD的光谱成像设施（由PI领导）。我们提出了一种近场扫描光学显微镜（NSOM）的新范式。这个想法源于一项发现，即微制造原子力显微镜（AFM）探针在聚焦激光束激发下表现出光致发光（PL）。这个PL尖端为NSOM成像和光谱学提供了一个“点光源”。激发光束将被聚焦到具有垂直于针尖斧的偏振分量的表面上，这样我们就可以通过AFM针尖获得横向定位和增强。初步结果证明了产生近场信号的可行性，我们计划完成该仪器的构建，优化其性能并展示其应用。这种方法的内在优点包括：(a)具有调谐波长能力的高光子吞吐量；(b)由于PL与激发光束的波长不同，使得近场光信号检测简单；(c)由于具有尖锐探头和有效偏转反馈的无孔径AFM平台，空间分辨率高；(d)操作简单。任何AFM用户都应该能够通过大约一周的快速培训掌握这个NSOM的操作。开发计划包括：(a)设计和建造低机械噪声和高稳定性的AFM/NSOM扫描组件，以获得高空间分辨率（横向10 nm和法线2 nm）；(b)获得真实的NSOM信号和局部光谱信息，为此我们计划修改AFM探针以提高PL效率，建立高近场增强的激发路径和配置，并建立高灵敏度和选择性的近场信号检测。结合NSOM仪器（Liu），纳米纤维和波导（Guo），聚合物纳米复合材料（Patten）和具有新应用的纳米颗粒（Kauzlarich）的专业知识，我们计划使用该NSOM:(a)揭示在细胞焦点粘附时形成的蛋白质复合物配体的纳米结构；(b)研究聚合物-纳米颗粒复合材料的结构和光学性能；(c)测量纳米线和纳米线组件的结构和波导特性；并对单磁芯/金属壳粒子的结构和光学性质进行了表征。这个NSOM的发展应该把学生和博士后带到扫描探针显微镜技术的前沿。学生将有机会学习和掌握最先进的AFM仪器，光学和光信号的检测，低噪声和高稳定性的硬件设计，显微镜电子和NSOM的软件宏。此外，他们还将研究光学激发与AFM尖端之间的局部相互作用，尖端-样品相互作用以及各种材料的对比机制，作为NSOM在材料研究中的应用的初步探索。这个NSOM的完成将增强UCD有组织的研究单位NEAT的光谱成像设施（由PI领导）。拟议的研究项目将促进使用NSOM进行材料表征的进一步应用，以揭示局部结构的地形和功能。

We propose to construct a state-of-the-art tool for imaging materials with high magnification, e.g. visualizing groups of molecules and being able to identify them. Conventional optical microscopes, e.g. a magnifying glass, only allow visualization at micrometer scale, i.e. a fraction of a human hair. This is due to the diffraction limit of visible lights. Using a sharp tip scanning over material surfaces, atomic force microscope enables imaging materials at nanometer level (a small fraction of a human hair), however, provides no information as to what kind of materials (metals, polymers or ions) are under the tip. One mission in the materials science community is to combine the strength of the high resolution in atomic force microscope with the ability to identify materials shown by optical microscope. The new instrument is referred to as a near-filed scanning optical microscope. The task is not trivial due to two competing factors, the need for sufficient light intensity at the imaging site (e.g. using a large probe) and the requirement to make the probe small/sharp to attain optical resolution. This proposal will use a new methods derived from our finding that specific kind of sharp probes glow when one focuses a laser beam at the top tips. The glowing tips provide a "point light source" for imaging and spectroscopy. Preliminary results have demonstrated the feasibility of generating near-filed signals, and we plan to complete the construction of this instrument, to optimize the performance and to demonstrate its applications. Compared with past approaches towards this technique, the proposed method exhibits advantages of high intensity of light, simple to operate, and high resolution. We plan to demonstrate the application and capability of this new instrument by characterization of four classes of important materials: materials containing organic (polymeric) and inorganic (semiconductive) compositions; small inorganic particles with multiple components; carbon nanofibers; and nanomaterials in living cells. The development of this new technique should bring students and postdocs to the forefront of scanning probe microscopy technology and its applications in materials science. The completion of this instrument will enhance the Spectral Imaging Facility (led by the PI) at UCD. We propose a new paradigm for near-field scanning optical microscopy (NSOM). The idea derives from a finding that microfabricated atomic force microscopy (AFM) probes exhibit photoluminescence (PL) upon excitation by a focused laser beam. This PL tip provides a "point light source" for NSOM imaging and spectroscopy. The excitation beam will be focused onto the surface with polarization component perpendicular to the tip axe, as at such we attain laterally localization and enhancement by the AFM tip. Preliminary results have demonstrated the feasibility of generating near-filed signals, and we plan to complete the construction of this instrument, to optimize the performance and to demonstrate its applications. The intrinsic advantages of this approach include: (a) high photon throughput with the ability to tune wavelength; (b) simplicity in detection of near-field optical signals because the PL exhibits different wavelength from the excitation beam; (c) high spatial resolution due to the apertureless AFM platform with sharp probes and effective deflection feedback; and (d) simplicity in operation. Any AFM users should be able to master the operation of this NSOM with a quick training of ca. one week. Development plan includes: (a) design and construction of a low mechanical noise and high stability AFM/NSOM scanning assembly to attain high spatial resolution (10 nm in lateral and 2 nm in normal directions); and (b) attaining true NSOM signal and local spectroscopy information, for which we plan to modify AFM probes to improve the PL efficiency, to build the excitation path and configuration for high near-field enhancement, and to build a high sensitivity and selectivity detection of near field signals. Combining expertise of NSOM instrumentation (Liu), nanofibers and wave guides (Guo), polymer nanocomposite materials (Patten) and nanoparticles with novel applications (Kauzlarich), we plan to use this NSOM for: (a) revealing the protein complex formed at the cell focal adhesion on nanostructures of ligands; (b) investigating the structure and optical property of polymer-nanoparticle composite materials; (c) measuring the structure and wave-guide property of nanowires and nanowire assemblies; and characterizing the structure and optical property of single magnetic core / metal shell particles. The development of this NSOM should bring students and postdocs to the forefront of scanning probe microscopy technology. Students will have a chance to learn and master the skills for the instrumentation of state-of-the-art AFM, optics and detections of optical signals, hardware design for low noise and high stability, electronics for microscopy, and software macros for NSOM. In addition, they will also investigate local interactions between optical excitation and AFM tip, tip-sample interaction, and contrast mechanism for a variety of materials as the initial exploration for NSOM applications in materials research. The completion of this NSOM will enhance the Spectral Imaging Facility (led by the PI) at UCD's organized research unit known as NEAT. The proposed research projects will facilitate further applications of using NSOM for material characterization to reveal the topographic as well as the functionality of the local structures.