NER: Chemically Modified Nanotube Tips for Selective Imaging with Scanning Tunneling Microscopy

本提案是应《纳米科学与工程》(NSF 00-119)征集而提交的。纳米技术的最大挑战之一是纳米物体成像技术的发展。这项研究发展了在分子和原子水平上观察表面的方法，并具有化学选择性。它基于扫描隧道显微镜(STM)尖端的使用，这些尖端与感兴趣的样品表面进行化学作用。STM使表面分析发生了革命性的变化，因为即使在空气和液体中，它也可以以原子分辨率进行成像，而许多其他分析方法都无法做到这一点。然而，有限的化学识别能力，即区分不同类型的原子或官能团，是传统扫描隧道显微镜的一个弱点。这个问题可以通过允许STM尖端与样品进行化学作用来解决。最近，研究表明，用自组装单分子膜或聚吡咯修饰金尖端可以选择性地识别形成氢键的官能团。初步结果表明，该方法还能够区分具有不同空间取向的官能团，并能够区分不同的金属原子。具有化学修饰尖端的扫描隧道显微镜的工作原理类似于电分析化学中的化学修饰电极。在前一种情况下，电子在STM针尖和样品之间转移，而在后一种情况下，电子转移发生在传感器电极和样品溶液中的分子之间。在这两种情况下，电子传递需要电子供给侧和接受侧的电子波函数的重叠。在后一种情况下，电极的化学修饰被用来控制选定的氧化还原反应。在STM的情况下，通过化学针尖修饰对电子转移的类似增强导致对表面图像中选定的官能团或原子的选择性识别。要观察样品上的单个官能团或原子，化学修饰的针尖一次只能与样品的一个官能团发生化学作用。不幸的是，电化学腐蚀和化学修饰的金属尖端在分子水平上是尖锐的，不能生产出具有高重复性的尖端。因此，样品与迄今使用的化学修饰针尖上的几个相互作用部位之间的化学作用经常同时发生，从而影响分辨率。为了获得非常高的分辨率，该项目探索使用化学修饰的碳纳米管作为扫描隧道显微镜的尖端。碳纳米管非常适合于化学修饰的STM尖端。碳纳米管呈圆柱形，直径通常在0.8到15纳米之间。这些非常小的直径提供了非常细长和原子尖锐的尖端。此外，在扫描隧道显微镜成像的典型条件下，形成碳纳米管的共价连接碳原子的刚性排列导致了极大的刚性。在这个项目中，碳纳米管将以各种方式进行化学修饰，并用于成像众所周知的测试样品。将研究化学修饰的碳纳米管针尖区分测试样品的不同官能团和原子的能力，并研究决定分辨率的实验参数。STM在常压液体和气体中表征样品的独特能力，加上化学选择的能力，应该使这项技术成为纳米科学的一种非常通用的工具。潜在的现实应用是，例如，纳米设备、自组装结构、催化表面或电分析传感器表面的表征，以及对化学反应和生物过程的原位观察。

0103476BuhlmannThis proposal was submitted in response to the solicitation "Nanoscale Science and Engineering " (NSF 00-119)One of the great challenges of nanotechnology is the development of techniques for imaging of nanoobjects. This research develops the methodology to observe surfaces at the molecular and atomic level with chemical selectivity. It is based on the use of scanning tunneling microscope (STM) tips that chemically interact with the sample surfaces of interest. STM has revolutionized surface analysis because it allowsimaging with atomic resolution even in air and liquids, where many other analysis methods fail. However, the limited ability for chemical recognition, i. e., for discrimination between different types of atoms or functional groups, is a weakness ofconventional scanning tunneling microscopy. This problem can be solved by allowing an STM tip to interact chemically with a sample.Recently, it has been shown that the modification of gold tips with self-assembled monolayers or polypyrrole can be used to selectively recognize functional groups that form hydrogen bonds. Preliminary results have shown that this method is also able to distinguish between functional groups that have different spatial orientations and to differentiate different metal atoms. The working principle of STMwith chemically modified tips resembles that of chemically modified electrodes in electroanalytical chemistry. While in the former case electrons are transferred between the STM tip and sample, the electron transfer in the latter caseoccurs between the sensor electrode and a molecule in the sample solution. In both cases, an overlap of the electronic wave functions of the electron-donating and accepting side is required for the electron transfer.In the latter case, the chemical modification of electrodes is used to control selected redox reactions. In the STM case, an analogous enhancement of electron transfer by chemical tip modification results in selective recognition of selected functional groups or atoms in a surface image.To observe individual functional groups or atoms on a sample, a chemically modified tip must interact chemically only with one functional group of the sample at a time. Unfortunately, electrochemically etchedand chemically modified metal tips that are sharp at the molecular level cannot be produced with high reproducibility. Consequently, chemical interactions between the sample and several interaction sites on chemically modified tip used so far often occur simultaneously, impairing the resolution. To obtain very high resolution, this project explores the use of chemically modified carbon nanotubes as STM tips. Carbon nanotubesare ideally suited for chemically modified STM tips. Carbon nanotubes have a cylindrical shape with diameters that are typically between 0.8 and 15 nm. These extraordinarily small diameters provide for very slender and atomically sharp tips. Also, the rigid arrangement of the covalently linked carbon atoms that form a carbon nanotube results in great stiffness under conditions that are typical for STM imaging.In this project, carbon nanotubes will be chemically modified in various ways and used to image well-understood test samples. The ability of chemically modified carbon nanotube tips to distinguish between different functional groups and atomsof the test samples will be investigated, and the experimental parameters determining the resolution will be studied.The unique ability of STM to characterize samples at atmospheric pressure in liquids and gases, combined with the capability for chemical selec-tivity, should make this technique a very generaltool for nanosciences. Potential real-life applications are, for example, the characterization of nanodevices, self-assembled structures, catalytic surfaces, or electroanalytical sensor surfaces, as well asthe in-situ observation of chemical reactions and biological processes.***