College of Chemistry and Molecular Engineering Peking University Email: Phone: 62759394,Scanning Probing Microscopy (SPM),Yuanhua Shao,扫描探针显微镜及其应用,Outline: 1. Introduction 2. Instrumentation 3. Theory 4. Applications,REFERENCES: 1. R.M.Wightman and D.O.Wipf, in Electroanalytical Chemistry, Vol:15, (A.J.Bard, Ed.), Marcel Dekker, New York, 1988, p.267 2. A.J.Bard, F.Fan, J.Kwak and O.Lev, Anal.Chem., 1989, 61, 132 1989, 61, 1221. 3. M.V.Mirkin, Anal.Chem., 1996, 68, 177A 4. M.V.Mirkin, Mikrochim.Acta., 1999, 130, 127 5. A.J.Bard, F.Fan and M.V.Mirkin, in Electroanalytical Chemistry, Vol:18 (A.J.Bard, Ed.), Marcel dekker, New York, 1993, p.243 6. 扫描力显微术，白春礼，田芳和罗克，科学出版社，2000,Figure. (a) Representation of tunneling between tip and sample atoms. Shaded portions denote electron distributions. (b) Tip attached to three Piezo elements used to position the tip and scan it across a surface,STM,itun = (constant) V exp (-2x) = V/Rtun,Where V is the tip-substrate bias, x is the distance between tip and surface, 1 -1, And Rtun is the effective resistance of the tunneling gap, typically 109 to 1011 ohms.,Figure. Cell for electrochemical STM. Upper: schematic diagram. Lower: Nanoscope III cell, top view.,Figure. STM images of HOPG. Gray-scale images at low resolution (left) and higher resolution topographic plot (right),Figure. (a) STM images of (A-D) mixed adsorbed layers Of PP and FePP on HOPG. Taken with a wax-coated Pt Tip in 0.05 M NaBO Solution containing FePP and PP in the ratio of (A)0:1 (B)1:4 (C) 4:1 and (D) 1:0. The HOPG substrate potential Was 0.41 V vs. SCE; the tip/ Substrate bias was 0.1 V; And the tunneling current was 30 pA. (b)(E-H) The Corresponding cyclic Voltammograms for A-D, Respectively, at a sweep rate Of 0.2 V/s.,AFM,Figure. Electrochemical cell for AFM for Nanoscope III.,Scanning Electrochemical Microscopy(SECM),Scanning electrochemical microscopy(SECM) was introduced in 1989 by Bard and co- workers at the University of Texas at Austin who published a series of papers describing the instrumentation theory, principles and applications of the technique. SECM is a novel in-situ electrochemical technique with its spatial resolution between optical microscopy and STM. Since its introduction, it has provided considerable insights into, and an understanding of, localized surface reactivity at a variety of solid/liquid interfaces ranging from biomaterials, polymers and minerals to electrode surfaces. Ion and electron transfer processes occurring at the liquid/liquid interface have also been studied.,2. Instrumentation A. Basic Apparatus The basic SECM apparatus consists of four parts: tip position controller; electrochemical cell( including tip, substrate, counter and reference electrodes); bipotentiostat and data acquisition system B. Microprobes The information obtainable from SECM measurements depends mostly on the type and size of the used microprobe. (1) Amperometric tips: solid UMEs, micro- to nano-meter disk UMEs (2) Potentiometric tips: Ion-selective microelectrodes (3) Dual-functional tips: Antimony (Sb), dual-channel tips (4) Tips based on Charge-transfer across liquid/liquid interface: electron transfer, ion transfer processes(Micropipettes),iT, = 4nFDCa,3. Theory A. Modes of Operation The SECM can be used in a variety of ways, e.g., as an electrochemical tool to study heterogeneous and homogeneous reactions, as an imaging device (microscope), and for microfabrication. These applications make use of different modes of the SECM operation. (1) Amperometric feedback mode: (2)Generation/Collection mode and potentiometric measurements: (3) Penetration Mode: (4) Ion transfer feedback mode:,B. Kinetic Measurements and Imaging Surface Reactivity There are two important advantages of SECM for heterogeneous kinetic measurements: (1)high mass-transfer rate allowing one to study fast reactions under steady-state conditions; (2)availability of powerful means for probing the mechanism and physical localization of interfacial reaction(High Spatial resolution).,The mass transfer rate in SECM is a function of the tip-substrate distance: d a, m D/a d a, m D/d D = 1x10 -5 cm 2/s , d=0.1m, m =1cm/s 10cm/s High Spatial Resolution: about 30 50 nm limit for conducting substrates and 2nm for insulating substrates,3. Theory of the SECM The considerable complexity of SECM theory is due to the combination of a cylindrical diffusion to the micro-tip electrode with a thin-layer diffusion space. The general solution of the diffusion problem for an uncomplicated quasi-reversible non-steady-state process in SECM was obtained as a system of two-dimensional integral equations. Two limiting cases, a diffusion-controlled process and one with totally irreversible kinetics, were treated numerically. These results and the theory for complex processes. Including homogeneous chemical stages and adsorption-desorption kinetics, have been reviewed.,The theory for steady-state (time-independent) processes is simpler. The knowledge of