纳米材料表征-王中林5 Scanning Probe Microscopy of Nanoclusters

5Scanning Probe Microscopy of Nanoclusters

Lifeng Chi and Christian Röthig

5.1Introduction

The invention of the scanning tunneling microscopy (STM)[1]presents the begin-ning of a novel class of near-field microscopy with nanometer till atomic resolution in real space.Originally the STM was designed for the surface analysis under ultrahigh vacuum (UHV)conditions on conducting surfaces.It was verified soon that this method could also be used under a variety of environmental conditions such as ambi-ent air [2],water [3],oil [4]and electrolytes [5].These exciting discoveries motivated the further development of a whole family of other scanning probe microscopes (SPM)[6].Examples include scanning force microscopy (SFM)(or atomic force mi-croscopy ,AFM)[7]and its extensions [8]and scanning near-field optical microscopy (SNOM)[9].

The applications of SPM increased rapidly in the last decade.Since the requirement of a conductive surface for STM is not the necessary condition for SFM,the objects for SPM are extended to insulating materials and cover a wide range of surfaces of bulk systems such as metals [10],semiconductors [11],adsorbates on solid surfaces [12],polymers [13],biological materials [14]and organic layered systems [15].Besides the fundamental research of surface structure,surface morphology and surface recon-struction,the SPM can also be used to reveal local physical properties of the sample,e.g.electronic energy spectra by using local tunneling spectroscopy (LTS,or scanning tunneling spectroscopy ,STS),local tribological properties by using frictional force mi-croscopy (FFM),local elasticity by using force modulation microscopy (FMM),local magnetic properties by using magnetic force microscopy (MFM),and local electro-static properties by using electric force microscopy (EFM).Another important approach of SPM techniques is in the field of nano-lithography [16].

In recent years,nanometer-sized clusters of metals and semiconductors have received increasing scientific and technological interest [17].Introducing chemically synthesized or physically evaporated nanoclusters on a variety of surfaces is a recent approach to generate nanostructured surfaces,which are promising for novel sensoric,electronic and photonic devices and are therefore inducing increased research activ-ities.The SPM,especially STM and SFM,can yield valuable information on nanoclus-ters as they may reveal the electronic and geometric structure of the cluster's.Further more,they can serve as ideal tools to characterize cluster covered surfaces since a wide range of materials can be chosen as substrates for cluster deposition.

In the following,we will discuss the fundamental principle of SPM (Section 5.2)and different operating modes used experimentally (Section 5.3).In Section 5.4,se-lected applications of SPM on nanoclusters will be presented,including:l imaging of individual nanoclusters and cluster covered surfaces l local physical properties

l SPM-based local deposition and modification l tunneling spectra of nanoclusters

Characterization of Nanophase Materials .Edited by Zhong Lin Wang

Copyright 2000Wiley-VCH Verlag GmbH

ISBNs:3-527-29837-1(Hardcover);3-527-60009-4(Electronic)