Raman Studies of Mass-Selected Metal Clusters
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RAMAN STUDIES OF MASS-SELECTED METAL CLUSTERS Kenneth Andrew Bosnick A Thesis su bmitted in conformity with the requirements for the degree of Doctor of Philosophy in Experimental Physical Chemistry Graduate Department of Chemistry University of Toronto @ Copyright by Kenneth Andrew Bosnick 2000 National Library Bibliothèque nationale 1*1 of Canada du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. rue Wellington OîtawaON KlAW OnawaN KlAW Canada canada The author has granted a non- L'auteur a accordé une licence non exclusive licence aliowuig the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or sel1 reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la fome de microfiche/film, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des exîraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. Sputtering a metal target under high vacuum conditions with 15 mA of 25 keV Ar' ions produces cationic metal clusters, which are extracted and collimated into a beam using standard ion optics. A particular cluster nuclearity is selected from the beam by a Wein filter, CO-depositedon an aluminum paddle with a matrix gas (Ar or CO) at cryogenic temperatures, and nectralized. Once enough clusters accumulate, Raman spectra are excited by various lines of an Ar' laser. The scattered light is dispersed ont0 a charge-coupled-device detector using a three-grating spectrorneter. Strong bands at -165 cm-', assigned as "breathing" modes, dominate the Raman spectra of Ag,, Ag,, and Ag, deposited in Ar. These bands fall close in frequency to that of srnall length scale vibrations in solid silver, indicating that the bonding between the atoms in these clusters already approximates that of bulk silver. Comparison of the Ag, and Ag, spectra with theoretically calculated ones reveals that the structure of Ag, is planar trapezoid and Ag, is tricapped tetra hedron. The Raman spectra of Ag, and Fe, in an Ar rnatrix show these both to be dynamic Jahn-Teller molecules. All of the bands in the Ag, spectrum are account- ed for using a linear plus quadratic Jahn-Teller coupling model. The Fe, spectrurn cannot be fit using this model, probably due to the high spin state of the cluster (S=4). A new derivation of the matrix elements for the linear plus quadratic Jahn-Teller coupling model, based on operator methods, is presented. Deposition of Ag,, Ag,,, Ag,,, and Ag,, in CO and collection of the Raman spectra in the v(C0) region show that these clusters enhance the Raman scatter- ing of CO by a factor of a few hundred with a strong dependence on cluster size. The results are interpreted as the maximum possible enhancement by the Chem- ical Mechanism of Surface Enhanced Raman Scattering. Acknowledgements Martin Moskovits supervised and guided the production of this Thesis and the scientific results embodied within it. Much of the work was done in co- operation w ith Tom Haslett, who also built the mass-selected cluster machine with assistance from Stephan Fedrigo. Wai-To Chan and Rene Fournier did the density functional theory calculations for Ag, and Ag,. iii Table of Contents Acknowledgements ......................................... iii Table of Contents .......................................... iv List of Tables .............................................vi List of Figures ............................................. vii List of Acronyms ........................................... viii 1. Introduction 1. 1 Atomic Clusters ................................... 1-1 1.2 Matrix-Grown Cluster Experiments ...................... 1-2 1.3 Cluster-Beam Experirnents ........................... 1-3 2 . Experimental Apparatus 2.1 Overview of the Cluster Machine ....................... 2-1 2.2 Cluster Source .................................... 2-2 2.3 Beam and Mass Selection ............................ 2-3 2.4 Typical Ag Cluster Distribution .........................2-5 2.5 Cluster Deposition .................................. 2-6 2.6 Spectroscopy .....................................2-7 2.7 CO Deposition Rate Measurement ....................... 2-8 3 . Raman of Ag,, Ag.. and Agg 3.1 Resonant Raman Spectra of Agsf Ag.. and Agg .............. 3-1 3.2 "Photochemical" Processes ............................ 3-3 3.3 Structures of Ags and Ag. ............................3-5 3.4 Breathing Modes and the Agg Spectrum ................. 3-13 3.5 Comparison of Silver Cluster Raman and Bulk Silver Properties . 3-13 Raman of Ag3 and Fe3 4.1 Resonant Raman Spectre of Ag3 and Fe3 .................. 4-1 4.2 "Photochernical" Processes ............................ 4-3 4.3 Introduction to the Jahn-Teller Problem .................. 4-4 4.4 Operator Methods in the E x e 3ahn-Teller Problem .......... 4-5 4.5 Jahn-Teller Fits to the Ag3 and Fe3 Spectra ............... 4-10 5 . Cluster Enhanced Raman Scattering 5.1 Ag, .Ag5. / CO Raman Spedra ........................ 5-1 5.2 Enhancement Calculations ............................ 5-5 5.3 Normal Modes and the Enhancement Mechanism ............ 5-6 Conclusions .......................................... 6-1 References ..............................................R-1 3- 1 . Comparison of experimental and calculated vibrational data for the low-energy Ag5 isomers .............................3-8 3.2 . Sum man/ of experimental and calculated vibrational data for the Ag, structures .................................. 3-10 4.1 . Jahn-Teller fit to the resonant Raman spectrum of Ag, ........... 4-11 4.2 . Two possible fits of the linear plus quadratic Jahn-Teller model to the Fe, Raman spectrum .............................. 4-13 5.1 . Summary of estimates of deposited cluster size distribution ........ 5-3 5.2 . Data from cross-section enhancement experirnents .............. 5-4 5.3 . Results of enhancement calculations for AgJCO), ............... 5-5 List of Figures 2.1 . The mass-selected cluster machine .......................... 2-1 2.2 . Cluster source ........................................ 2-3 2.3 . Beam and mass selection ................................ 2-3 2.4 . Typical silver cluster mass distribution ........................ 2-5 2.5 . Deposition and spectroscopy.............................. 2-6 2.6 . The principle of thin-film interferometry ...................... 2-9 2.7 . Measurement of CO deposition rate by thin-film interferometry ...... 2-9 3.1 . Resonant Raman spectrum of Ag, in argon .................... 3-2 3.2 . Resonant Raman spectra of Ag,. Ag.. and Ag9 in argon ............ 3-3 3.3 . Time evolution of Ag5 Raman spectnim with laser irradiation........ 3-4 3.4 . Time evolution of Ag3 fluorescence and Ag5 Raman............... 3-4 3.5 . Competing low-energy Ag5 isomers .......................... 3-6 3.6 . DFT predicted Ag5 Raman spedra ........................... 3-8 3.7 . Competing low-energy Ag. isomers .......................... 3-9 3.8 . DFT predicted Ag. Raman spebra ........................... 3-9 3.9 . FG matrix method fit Ag. Raman spebra ..................... 3-11 4.1 . Resonant Raman spectrum of Ag, in argon .................... 4-1 4.2 . Resonant Raman spectrum of Fe, in argon .....................4-2 4.3 . Energy levels for the linear E x e Jahn-Teller model.............. 4-9 4.4 . Energy levels for the linear plus quadratic Jahn-Teller mode1....... 4-10 5.1 . Silver clusters deposited for CERS experiments................. 5-2 5.2 . Estimated distributions of cluster nuclearities deposited ........... 5-3 5.3 . Raman spectra of Ag,, Ag,,. Ag3.. and Ag5. in carbon monoxide...... 5-4 5.4 . Measured Raman scattering enhancements .................... 5-6 List of Acronyms CCD cha rge-coupled device CERS cluster en hanced Raman scattering CM chemical mechanism DF dispersed fluorescence DFT density functional theory DTB distorted trigonal bipyramid EAB electronic absorption bands EMM electromagnetic mechanism ESR electron spin resonance FG (refers to the and matrices) rnR Fourier transform infra-red GAT gas-phase aggregation technique HOPG highly-oriented pyrotytic graphite IR infra-red LA longitudinal acoustic LAT laser ablation technique PBP pentagonal bipyramid PFI-ZEKE pulsed field ionization - zero electron kinetic energy PT planar trapezoid R2PI resonant two photon ionization SERS surface enhanced Raman scattering SPA surface plasmon absorption ST sputtering technique TA transverse acoustic TCT trica pped tetrahedron UV ultra-violet viii Larger than atomic dimers yet smaller than nanoparticles, atomic clusters are aggregates with between a few and a few hundred atoms. They exhibit prop- erties unique from those of the corresponding solid state material and indeed from those of nanometre scale particles. For the smallest cluster sizes, the prop- erties Vary somewhat erratically as the size is increased one atom at a time. In this size range, adding just one more atom to a cluster can greatly alter its prop- erties, although this is not always the case. For example, some cluster sizes are exceptionally stable compared with clusters of just one more atom, and this leads to so called