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Scanner de Tunelamento

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Escâner de Tunelamento
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  Do It Yourself –Scanning Tunneling Microscope Kantonsschule WettingenMaturaarbeit 2008/2009Authors Sandro MerkliIvan OvinnikovDominik Wild Advisers Wolfgang MannDr. Thomas Graf   Preface Already half a year before the beginning of the  Maturaarbeit  , we had decidedwhat we wanted to do: to build our own scanning tunneling microscope. Theidea emerged when we heard that the physical assistant at our school, HansulrichSchmutz, had bought components to construct such a device. We thought it tobe an ideal project for several reasons. The practical work would set a contrastto school and it would be an opportunity to experience research and its creativeprocesses. Furthermore, we are all intrigued by sciences and technology.We were aware of the fact that our enterprise did not promise definite success.Due to this circumstance, we set our minimal goal to be the following: measureand derive a function of the tunneling current depending on the distance betweenthe tip and the sample. We are glad to have reached this goal but we didnot succeed to accomplish our ultimate goal of a fully functioning microscope.However, we plan to continue our project and this work lays a solid base forthat. Thus, that we are looking forward to make quick progress. We are alsoparticipating in the Schweizer Jugend Forscht competition which sets a differenttime frame. The deadline set for the competition is a few months later than theMaturaarbeit’s deadline. This makes it possible for us to try and achieve ourfinal goals until then.This paper should give an impression of our work and introduce the reader tothe topic of scanning tunneling microscopy. The first part covers the theoreticalbackground whereas the second part specifically documents our implementationof a scanning tunneling microscope. Both parts have their own introductionwhich will guide the reader. In the third part, we document and interpretthe results we have achieved so far. In addition to this paper, we acquired aninternet domain ( www.stm-diy.ch ). Under this address, we published a summaryof the work we’ve done and contact addresses for questions. For more detailedinformation, we provide a downloads section containing useful schematics.We are greatly indebted to many people and organizations who supportedus in our work. The Paul Scherrer Institute, IBM Zurich, EMPA and Carl ZeissAG all contributed to our project with their generous financial aid. IBM Zurichand the Paul Scherrer Institut both offered their help in technical concerns.The co-operation with the Paul Scherrer Institut turned out to be extremelyhelpful and we owe special thanks to Jan Hovind, Siegfried Ebers, Dr. ThomasJung and Valeri Ovinnikov, Ivan’s father. He spent a lot of time designing thecontrol electronics, a task not fulfillable for us. Finally, we have to expressour gratitude to Hansulrich Schmutz. His experience, explanations and effortwere of inestimable value and confirmed us in our enthusiasm. Martin Merkli,Sandros father, helped a lot in the acquisition of non-standard materials usedfor the mechanical setup, and provided a car, making us a lot more mobile andtime-efficient.  CONTENTS Contents I Theoretical Treatment 4 1 Introduction 52 Quantum Tunneling 63 Piezoelectric elements 10 3.1 The piezoelectric effect . . . . . . . . . . . . . . . . . . . . . . . . 103.2 Scanner Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4 Electronics 11 4.1 Amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 Control Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5 Vibration Isolation 156 Tip Preparation 15 II Documentation 16 7 Initial Approach 17 7.1 Mechanical Build . . . . . . . . . . . . . . . . . . . . . . . . . . . 177.2 Tunneling Current Amplifier . . . . . . . . . . . . . . . . . . . . 177.3 Scanning Electronics . . . . . . . . . . . . . . . . . . . . . . . . . 18 8 Difficulties 21 8.1 Vibration Damping . . . . . . . . . . . . . . . . . . . . . . . . . . 218.2 Acoustic Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218.3 Tunneling Current Amplifier . . . . . . . . . . . . . . . . . . . . 228.4 Control Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . 23 9 Current Design 2410 Future plans 25 10.1 Mechanical Build . . . . . . . . . . . . . . . . . . . . . . . . . . . 2510.2 Control Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . 25 III Experimental Results 26 11 Methods 2612 Results 27 Conclusion 31 2  LIST OF FIGURES List of Figures 1 Schematic diagram of a STM . . . . . . . . . . . . . . . . . . . . 52 STM scan of a graphite surface . . . . . . . . . . . . . . . . . . . 63 Potential energy in a STM . . . . . . . . . . . . . . . . . . . . . . 74 Wavefunction of an electron in the sample . . . . . . . . . . . . . 75 Probability function . . . . . . . . . . . . . . . . . . . . . . . . . 96 Piezoelectric coefficients . . . . . . . . . . . . . . . . . . . . . . . 107 Scanner types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 A number of op-amps . . . . . . . . . . . . . . . . . . . . . . . . 129 Operational amplifier symbol . . . . . . . . . . . . . . . . . . . . 1210 Analog and digital feedback control . . . . . . . . . . . . . . . . . 1411 Our approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1712 Picture of our STM . . . . . . . . . . . . . . . . . . . . . . . . . . 1813 PIC32 Microprocessor Board . . . . . . . . . . . . . . . . . . . . 1914 Voltage converters . . . . . . . . . . . . . . . . . . . . . . . . . . 1915 P-CAD 2006 PCB Designer . . . . . . . . . . . . . . . . . . . . . 2016 Empty Printed Circuit Board . . . . . . . . . . . . . . . . . . . . 2017 Stands for the vibration damping . . . . . . . . . . . . . . . . . . 2118 Sound isolation box . . . . . . . . . . . . . . . . . . . . . . . . . 2219 The second and the third amplifier . . . . . . . . . . . . . . . . . 2320 Schematic of the third amplifier . . . . . . . . . . . . . . . . . . . 2321 The current design of the STM . . . . . . . . . . . . . . . . . . . 2422 The control electronics in the opened box . . . . . . . . . . . . . 2423 Computer-based measurement . . . . . . . . . . . . . . . . . . . . 2624 A good measurement . . . . . . . . . . . . . . . . . . . . . . . . . 2725 Current-position characteristic . . . . . . . . . . . . . . . . . . . 2826 A bad measurement . . . . . . . . . . . . . . . . . . . . . . . . . 303
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