Local spectroscopy. N. Witkowski W. Sacks

Transcription

1 Local spectroscopy N. Witkowski W. Sacks

2 Outlook 1. STM/STS theory elements a. history of STM and basic idea b. tunnel effect c. STM/STS 2. Technology a. STM design : vibration and thermal drift b. STM mechanics : course approach, scanning c. STM electronics : signal generation, detection d. Tip preparation e. Design examples 3. Applications a. Surface structure : metals and semiconductors b. Local electron spectroscopy : charge density wave and superconductors c. Inelastic tunnelling spectroscopy : vibration modes of molecules d. Atom and molecule manipulation

3 a. STM Design Vibrations Isolate from external vibations 1 Hz building vibrations due to people walking around etc Hz building vibrations due to ventilation, appliances etc khz external sounds, human voice Internal STM frequency 10kHz Damping systems Air support tables Springs with heavy masses Foucault current damping systems Stack of steel plates separated with viton balls..

4 a. STM Design Example 1rst stage roof 2nd stage 1rst stage : spring UHV chamber STM 2rd stage One dimension harmonic oscillator simulation 3rd stage : pneumatic foot Tansmission factor 2nd stage : pneumatic foot

5 b. STM Mechanics Scanner Piezoelectric ceramics x,y and z motions Tripod : accurate but fragile Tube : most commonly used l Δ l = d Bimorph 31 V h Unimorph disks : cheap but low frequency (2 khz) d 31 ~1-3 Å/V /02_chapter2.pdf;jsessionid=D23499F8BBF8F2960B1FD606CE118E2B?hosts=

6 b. STM Mechanics Non linear effects Hysteris X-Y-Z coupling Creeping effect Ageing Thermal drift Left to right image Ag(111) 130 x 90 nm 2 Right to left image Si(100)-2x1 HOPG FFT hapter2.pdf;jsessionid=d23499f8bbf8f2960b1fd606ce118e2b?hosts=

7 b. STM Mechanics Course approach Piezo-drivers Manuel approach (few mm from surface) Automatic approach (few Å) FFT

8 c. STM Electronics Feedback controle Signal compared to a reference generates an error tension Amplification of error tension with I gain (G I ) and P gain (G P ) 8 Vt = 10 ( Ω) I t FFT (Amp.) %20compleet%20afstudeerverslag%20Mathies.pdf

9 c. STM Electronics Feedback control G P : proportional gain V FB ( t) = G P e( t) Reference position ->e(t)=0 V FB (t)=0 systematic error Large G p value small steady error rapid approach to reference position Small G p value large steady error slow approach to the reference position G I : integral gain «memory» effect V FB ( t) = G e( t) + G e( t dt P I ) FFT 1- G P too large : instability 2- G P too small with respect to G I 3- optimized values for G P a,d G I %20compleet%20afstudeerverslag%20Mathies.pdf

10 d. Tip preparation Platinium-Iridium tip Pt-Ir wire Ø 0.25 mm Cut with scissors application Air or liquid measurements Advantage : low oxidation process FFT

11 d. Tip preparation Tunsgten tip W wire Ø 0.25 mm Electrochimical etching Successif annealing application Vacuum Advantage : every kind of surfaces FFT I. Ekvall et al, Meas. Sci. Technol. 10 (1999) A.-S. Lucier, PhD thesis, 2004, Montreal, Canada

12 d. Tip preparation Dy/W110 film Other tips Magnetic tips : CoCr, Dy, Ni, Cr Superconducting tip : Nb, Pb, MgB 2 Semiconducting tip : SiC, GaAs, InAs Single carbon nanotube topographique di/dv L. Berbil-Bautista, Phys. Rev. B 76, A. Kohen, et al Phys. Rev. Lett. 97, (2006) H. Konishi et al, Rev. Sci. Inst. 78,

13 d. Tip preparation Tip artefacts Multiple tips tip surface image

14 e. Examples of design STM in liquid potentiodynamic STM scan mode Phenyloctane on HOPG D. B. Amabilino, J. Phys.: Condens. Matter 20, (2008) TE tip electrode; CE counterelectrode; WE working electrode; RE reference electrode. M. Wilms et al, Rev. Sci. Inst. 70 (1999) 3641 adsorption and desorption of a sulfate/bisulfate adlayer on Cu(111) in sulfuric acid electrolyte.

15 e. Examples of design STM under vacuum : RT MnAs / GaAs(111) (Marangolo et al) Pt atoms adsorb onto Ge(001) surface Mainly topography at RT due to lack of stability Oguzhan Gurlu, Applied Physics Letter Volume 83, Number 22

16 e. Examples of design STM at variable, low, very low temperature FePc / HOPG at 70 K Variable : N 2 or He cooling, T min =20K Cu(111) at 165 K Problem : the tip and the surface are not at the same temperature J.Åhlund thesis, Uppsala 2007 E. Wahlström, Chalmers University of Techn., Goteborg, Sweden

17 e. Examples of design STM at variable, low, very low temperature STM image of trans- BETBPP overlayer on Au (111) surface prepared by spray-jet technique. Image size = 80 x 80 nm, Ugap = V, IT = 22 pa, Constant current mode. STS of Pt clusters on HOPG at different surface temperatures at 68 K and 6 K. For highresolution STS the sample has to be cooled with liquid helium. The series of STM images of Si(111)7x7 illustrates the extremely low drift of 1 atom/h. (Total time for series 6:40 hours at 5K)

18 e. Examples of design STM at variable, low, very low temperature Co atoms/ Cu (111) at 4K a, b, Topographs showing the e = 1/2 (a) and e = (b) ellipse each with a Co atom at the left focus. c, d, Associated di/d V difference maps H. C. Manoharan, Nature 403,

19 e. Examples of design STM at high temperature Variable : N 2 or He cooling, T min =20K Array of STM images illustrating the thermal evolution of submonolayer Pt on Ru(0001). The various coverages (deposited at about room temperature) were: (A) θpt=0.15 ML, (B) 0.60 ML and (C) 0.90 ML. Note, that the image size of annealed layers is 2000 Å 2000 Å while it is 1000 Å 1000 Å for the layers as grown. Typical tunneling parameters were UBias 0.5 V and ITunnel 1 na. Problem : very high thermal drift U. Käsberger Surface Science 540, Issue 1, 10 August 2003, Pages 76-88

20 e. Examples of design STM under magnetic field Scanning Tunneling Spectroscopy on MgB2 at low Temperatures and High Magnetic Fields D. Roditchev et al