Scanning Tunneling Microscopy Investigation On Metal Nanoclusters

Nanoclusters have attracted wide attentions in recent decades because of the potential applications in a variety of fields,such as solar cells,catalysis,drag delivery,biosensor,medical diagnosis and so on.As one of the most high-resolution imaging techniques in real-space,scanning tunneling microscope has been widely applied in investigations of nanoparticles in the past decades.However,researches based on atomic structure well-known nanoclusters are hardly found.Whereas,large amounts of reports of crystal structures of nanoclusters in the recent years open the door and show the challenges.In this work,we target on the structure already known nanocluters,i.e.Ag374 and Ag44 nanoclusters,and have studied the real-space images,single electron tunneling property,eigen-energy levels as well as the activity of the clusters.We hope this work would give an indication in cluster applications,such as biochemistry,solar cells,catalysis and so on.The whole text is divided to five chapters as below:Chapter 1 shows the principles and the devices of scanning tunneling microscopy,introduction of nanoclusters,the advances of investigations on nanoclusters by STM,as well as the basis and significance of this dissertation.In chapter 2,we apply ambient STM and low temperature STM to probe the assembly of nanoclusters on thiol/dithiol modified Au(111)substrate,as well as the self-assembly of thiol and dithiol on Au(111).This part shows the normal method used for nanocluster assembly in the next chapters.In Chapter 3,by imaging crystallographically determined silver nanoclusters Ag374 protected by a monolayer of tert-butylbenzene thiol(TBBT),we demonstrate here sub-molecular resolution in topography of the thiol monolayer of a 5 nm nanoparticle in STM.And the experimental data is confirmed by "pattern recognition":comparisons to simulated topography images from DFT using the known total structure of the Ag374 nanoclusters as well as to an image database built from STM images of individual TBBT molecules in various orientaions on Au(111)surface.Our work demonstrates a working methodology for investigations of structure and composition of organic monolayers on curved nanoparticle surfaces which helps designing functionalities for nanoparticle-based applications.In chapter 4,the electronic properties of Ag374 and Ag44 nanocluster are discussed.Ag374 nanoclusters show the classical Coulomb Staircases and Coulomb Blockade for metallic particles.Whereas,Ag44 nanoclusters have separated energy levels for nonmetallic clusters.The STS data show an interesting Negative Differential Resistance effect for Ag44 clusters.It would give a significant indication for applications of Ag44/Ag374 nanoclusters on nanodevices.In chapter 5,Ag44/(AuAg)44 nanoclusters are firstly found to own unexpected activity in Au(111)crystal surface.It is willing to decompose to small fractions and self-assemble to a quasi-ordered 2D network on Au(111)surface in the solution environment.Fluorobenzenthiol or benzenedithiol modification layers can resist its activity to some degree.The clusters can even immobilize on the dithiol layers at low temperature.This work is important for the applications of Ag44/(AuAg)44 nanoclusters concerning Au(111)crystal face.In chapter 6,some expectations and thinkings of atomic structure well-known clusters are discussed.We hope that more clusters are being studied by STM in the future.Acting as a kind of "artificial superatom",ligand protected metal nanoclusters would show more to the world.