Scanning Probe Microscopy (SPM) setup & instrumentation

Transcription

1 Scanning Probe Microscopy (SPM) setup & instrumentation Jan Knudsen The MAX IV laboratory & Division of synchrotron radiation research K (Sljus) March 27,

2 What do you need for SPM?

3 What do you need for SPM? specific interaction between probe tip and sample sample requirements & preparation probe tip scanner / positioning device control unit / image acquisition isolation against external noise suitable conditions (ultrahigh vacuum, biological environment, low temperature, magnetic / electric fields, ) tip preparation piezo elements feedback electronics electromagnetic screening vibrational damping

4 Sample requirements & preparation: Cleaning methods Ion sputtering Annealing Ion sputtering: sample bombardment with high-energtic ions (typically Ar + with several kev) removal of the top-most surface layers Annealing: desorption of (oxide) layers crystallization of the material possible under different atmospheres Sample cleavage: creating an atomically clean cleavage surface inside/outside ultrahigh vacuum HOPG

5 Sample requirements & preparation: Ultrahigh vacuum At ambient conditions, surfaces are covered by oxide layers, water films, and contaminants For atomic resolution, ultrahigh vacuum (UHV) with p<<1x10-6 P (10-8 mbar) is essential! Kinetic theory of gases (Nk B T = pv): At a pressure p = 1x10-4 P (10-6 mbar), each atom of the sample surface is in average hit by a restgas atom once a second! UHV technology: special connections between chambers, positioning devices, feedthroughs, valves, pumps, only suitable materials combination of different mechanical and electrical pumps baking at >100 C against water films

6 Tip preparation: General The resulting image is always a convolution of the tip shape and the actual surface structure! scanline probe tip with microtips tip material: metal wire for STM stiff, crystalline material for AFM glas fibres for SNOM surface tip preparation: cutting etching sputtering annealing sharp, clean, stable

7 Tip preparation STM tip: cutting or etching a suitable wire, typically Au (very conductive, chemically inert), PtIr (very stable, chemically rather inert), or W (stable, easy to handle, cheap) for W tips: Oxide must be removed by etching, sputtering, or heating in UHV

8 Probe tip: Tip preparation Scanning Probe Microscopy (STM) 10 µm screw 9 V Tip assembly 10% KOH Nice Nice Fair Bad Stainless wire Pyrex beaker E. Laegsgård Århus University Threshold: ma

9 Scanner / Positioning Devices Photo: W tip mounted to a tip holder close to a Si sample clamped in a sample holder positioning and scanning of the SPM tip in two stages: 1. coarse movement / tip-sample approach mechanically or with piezoelectric device controlled by hand or by electronics 2. scan movement piezo tripod / tube control electronics / PC

10 Scanner/positioning device: Piezoelectric actuators Piezoelectric material, usually lead zirconium titanate (PZT): expands or contracts under applied voltage Based on images of known surfaces calibrate the piezoelectric response to convert [Volts] to [nm] The response is continuous down to atomic lengths scales Unfortunately, the response is highly non linear (hysteresis) Very stable high voltage supply needed oxygen titanium figure: S. Becker, TU Berlin

11 Piezoelectric charge coefficient [m/v] Piezoelectric materials x j = d ij E i Applied field [V/m] Strain [m/m]

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13 Highly non linear response A: The dipoles of all the grains will eventually align to the electric field as optimally as is possible and the distortion of the grains will approach a physical limit. B: Reoriented dipoles. As the field gets smaller, the dipoles relax into less ideal orientations and strain decreases at a faster rate. C: As the field becomes negative the dipoles are forced away from their original orientation. At a critical point they completely reverse direction and the piezo actuator becomes polarized in the opposite direction. Piezo creep: L(t) = L t=0.1 s 1 + γ log Scanning area t 0.1 s D: After polarization reversal, the piezo expands again until it reaches its physical strain limit. Piezo creep is observed as bend ato lines / step edges in STM images

14 Where are piezo electric materials used? Pickup Piezo stack Piezo electrical ignitor

15 Scanner tube Scan movement by piezo elements: The first STM used a piezo tripod. Nowadays typically piezo tubes are used. typical scan range µm Tripod scanner Problems due to piezo scanning: non-linearity of the tube scanner hysteresis of piezo response creep of the piezo element thermal drift Scanner tube

16 Scanner/positioning device: Approaching techniques Piezo voltage time Inchworm Slip/stick stepper

17 Control unit & Image acquisition The experimental setup: Two ways of forming the STM image: a) is most often used! a) constant-current mode: I constant, feedback loop active, Z variation measured b) constant-height mode: Z constant, feedback loop idle, I variation measured

18 Analyzing STM images Scanning Probe Microscopy (STM) Z and XY calibration from known surfaces. Tabulated at:

19 Vibrational isolation I challenge: ~ 1 µm mechanical vibrations of buildings + walking + shouting < 1 Å = 1x10-4 µm resolution of STM Solution: Special buildings (massive baseplates) Actively damped feet STM suspended to soft springs Stiff, compact scan unit Eddy current damping Omicron STM Tuma STM Thomas Michely University of Cologne

20 Spring damping figure: M. Dähne, TU Berlin Omicron STM SPECS STM

21 Eddy current damping Eddy current damping by moving massive metals relative to a magnetic field: A varying magnetic flux creates an electric field (Faraday s law of induction) The electric field creates a current The current creates a magnetic field with opposite direction Eddie current brakes at an ICE3 high-speed train and a rollercoaster (photos wikipedia)

22 The resonance frequency of the STM limits the scanning speed. Drift compensation Scanning Probe Microscopy (STM) Solution: Small and rigid Drift compensation Wait until the STM reached thermal equilibrium and the piezo crep is at minimum Active drift compensation Drift removal of STM movies Å 2

23 Fast-scanning examples Scanning Probe Microscopy (STM) Pt(110)-(1 2) + Pt atoms Cu(110) -> CuO 64 images/sec.

24 Low temperature STM SPECS Aarhus STM (90 K 400 K) Zener diode for counterheating VT Omircron STM (25 K 1500 K)

25 World-leading mk (low-t) STM Rev. Sci. Instr. 81, STM operating at 10 mk

26 World-leading mk (low-t) STM

27 High temperature STM Scanning Probe Microscopy (STM) Problem: Drift Leiden University Phys. Rev. Lett. 104, (2010) SPECS 90K 1000 C

28 (Å 2 ) Scanning Probe Microscopy (STM) Can we scan at high pressures? Electron mean free path: 4kT P Question: What is the maximum pressure we can scan in? Notice the Å 2 unit of the total cross section e - e - e - e - e - High voltage for piezo elements can cause problems!

29 The Au/Ni(111) alloy in high pressure of CO 0.33 ML RT 0.33 ML RT + anneal to 800 K, 10 min 0.33 ML RT + anneal to 800 K, 10 min mbar RT 50 x 50 Å x 3000 Å x 800 Å x 1000 Å 2 Exposure of high pressure of CO The Au/Ni(111) phase separates into single and double layer Au-clusters located on Ni(111) Vestergaard et al., PRL, 95, (2005)

30 The kinetics of the phase separation 1000 x 1000 Å 2 13 mbar CO Step flow [Å] min Pressure [mbar] Step flow rate [Å/s] Dosing time [sec] Ni(s)+4CO(g) Ni(CO) 4 (g) Ni(111) in 658 mbar RT for 15 min Vestergaard et al., PRL, 95, (2005) 2000 x 2000 Å 2

31 Mini-reactor STM O 2 Rasmussen P. et al., Rev. Sci. Inst. 68, 3879 (1998) CO CO 2 Hendriksen B.,et al., PRL, 89, (2002)

32 Conclusions tip preparation sharp and stable tip piezo positioning scanning coarse movement control unit / image acquisition constant-current mode constant-height mode vibrational isolation spring damping Eddy current damping Fast scanning STM High temperature STM Low temperature STM High pressure STM