(Scanning Probe Microscopy)

Transcription

1 (Scanning Probe Microscopy) Ing-Shouh Hwang Institute of Physics, Academia Sinica, Taipei, Taiwan References 1. G. Binnig, H. Rohrer, C. Gerber, and Weibel, Phys. Rev. Lett. 49, 57(1982); and ibid 50, 120(1983). 2. J. Chen, Introduction to Scanning Tunneling Microscopy, New York, Oxford Univ. Press (1993). 3. R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge Dawn Bonnell, "Scanning Probe Microscopy and Spectroscopy: Theory, Techniques, and Applications". Wiley-VCH 2001.

2 Scanning Probe Microscopy A feedback control system is used to maintain a constant tip/surface interaction, which is very sensitive to the distance variation. Scanning Tunneling Microscopy (STM) --- G. Binnig, H. Rohrer et al, (1982) Near-Field Scanning Optical Microscopy (NSOM) --- D. W. Pohl (1982) Atomic Force Microscopy (AFM) --- G. Binnig, C. F. Quate, C. Gerber (1986) Scanning Thermal Microscopy (SThM) --- C. C. Williams, H. Wickramasinghe (1986)) Magnetic Force Microscopy (MFM) --- Y. Martin, H. K. Wickramasinghe (1987) Friction Force Microscopy (FFM or LFM) --- C. M. Mate et al (1987) Electrostatic Force Microscopy (EFM) --- Y. Martin, D. W. Abraham et al (1988) Scanning Capacitance Microscopy (SCM) --- C. C. Williams, J. Slinkman et al (1989)

3 1. All SPMs are based on the ability to position various types of probes in very close proximity with extremely high precision to the sample under investigation. 2. These probes can detect electrical current, atomic and molecular forces, electrostatic forces, or other types of interactions with the sample. 3. By scanning the probe laterally over the sample surface and performing measurements at different locations, detailed maps of surface topography, electronic properties, magnetic or electrostatic forces, optical characteristics, thermal properties, or other properties can be obtained. 4. The spatial resolution is limited by the sharpness of the probe tip, the accuracy with which the probe can be positioned, the condition of the surface under study, and the nature of the force being detected.

4 Different Probes thermocouple tip sample

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6 Scanning Tunneling Microscopy References 1. G. Binnig, H. Rohrer, C. Gerber, and Weibel, Phys. Rev. Lett. 49, 57(1982); and ibid 50, 120(1983). 2. J. Chen, Introduction to Scanning Tunneling Microscopy, New York, Oxford Univ. Press (1993). 3. ) )

7 Classical Tunneling Quantum Mechanics Tunneling current Tunneling current I t I t (V/d)exp(-Aφ 1/2 d) A = (ev) -1/2 Å -1 I t = 10 pa~10na V = 1mV ~ 3V d decreases by 1Å, I t increases by about one order of magnitude.

8 Schematics of STM Current pre-amp ~ 1 V/nA

9 STM Images of Si(111)-(7 7) Empty-state image Filled-state image

10 Atomic Model of Si(111)-(7 7) Top view 26.9 A O 7.7 A o C C C E E E E C 112 E E C C Si adatom Si rest atom with a dangling bond Side view C E E C dangling bond 2nd layer rest atom faulted half unfaulted half

11 Atomic Structure of the Si(001) Surface

12 Atomic Structure of the Pt(001) Surface Surface Science 306, 10 (1994).

13 Si Magic Clusters Physical Review Letters 83, 120 (1999).

14 Nucleation and Growth of Ge at Pb/Si(111) Physical Review Letters 83, 1191 (1999). Japanese Journal of Applied Physics 39, Part 1, No. 7A, 4100 (2000).

15 Ultra-High Vacuum Scanning Tunneling Microscope

16 Inchworm-Type Linear Motor

17 Piezoelectric Scanners Tripod scanner d 31 = S 1 /E 3, S 1 = δx/x, E 3 = V/z d 33 = S 3 /E 3, S 3 = δz/z Tube scanner

18 PZT Materials 1. The piezoelectric effect was discovered by Pierre Curie and Jacques Curie in The piezo materials used in STM are various kinds of lead zirconate titanate ceramics (PZT) --- a mixture of PbZrO 3 and PbTiO 3. It consists of small ferroelectric crystals in random orientations. 3. An advantage for PZT ceramics over single crystal materials is that it can be shaped easily and poled at a desired direction. 4. PZT scanners are widely used in SPM because they have excellent resolution in displacement, high stiffness, and fast response. A major drawback of PZT materials is its lack of accuracy due to many nonlinear characteristics, such as hysteresis, creep, and recoil-generated ringing, which often cause distortion in SPM images. The positioning error of a PZT scanner can be as much as 10-15% of the full scanning range. 5. The piezoelectric constants vary with temperature in a complicated manner, and also with the particular batch of materials by the manufacturer and time (the aging effect). 6. Displacement calibration or compensation is needed for accurate measurement of length or size.

19 Hysteresis of a PZT Actuator Hysteresis curves of an open-loop piezo actuator for various peak voltages Response of a PZT translator to a 1 V, 200 Hz triangular drive signal Creep of open-loop PZT motion

20 Topographic Images of AFM Square Grating Without correction With correction in the x-direction Jap. J. of Appl. Phys. 45, 3B,

21 Vibration Isolation Power spectra of a tunneling signal Vibration isolation Tunneling assembly Calculated transfer function

22 Vibration Isolation Requirement in vibrational noise: < 0.01Å in z, and < 0.1Å in x and y. 1. Building vibrations Hz. 2. Acoustic noise 10-10k Hz 3. Table/chamber resonances Hz 4. The design strategy is to reduce the resonance frequency of the vibration isolation system as low as possible, increase the resonance frequency of the tunneling assembly. 5. An effective way to damp the low-frequency vibrations is to suspend the microscope on long tension wires or levitate the table/chamber system on air legs. The performance can further be improved by increasing the mass of the system. 6. The microscope itself can be suspended with tension spring. 7. Vibrations in the medium-frequency can be damped by mounting the scanner assembly on a stack of materials of different elastic moduli or to suspend the scanner on tension springs with eddy current damping.

23 Tip Shape Effects

24 Si(111)-(7x7) Artifacts of the Tip Pb/Si(111)

25 Artifacts of the Tip

26 STM Feedback Control PID Control I Control Integrator I t G V I Error signal V I -V r G 1 G 1 (V I V r ) C High voltage amplifier V r R V Z A AV Z Z piezo ADC Computer

27 Theory of STM From one-dimensional tunneling problem tunneling current (ev<< φ ) 1 V A 2 I exp φ S S ( ev ) 1 2 A = A 0 1

28 I T S 2 e = h μν f Tunneling Current ( E )[ 1 f ( E + ev )] M δ ( E E ev ) π 2 μ ν μν μ ν where f (E) is Fermi function E is the energy of state μ,where μ and ν run over all the states of the μ M μν tip and surface, respectively. is tunneling matrix element M μν ( * ψ ψ ψ ) 2 h d s ψ μ ν ν μ * 2m where ψ μ is the wave function, and the integral is over any plane in the barrier region. I = I T I S S T 2 ( E) ρ ( E + ev ) M ( E) [ f ( E) f ( E + ev )] = A' ρ de T S ρ S ρ T where and are the densities of states in the sample and the tip, respectively.

29 Electronic Structures at Surfaces Not Tunneling Empty-State Imaging Tunneling Filled-State Imaging

30 I A' ρ T Tunneling Current 2 ( E) ρ ( E + ev ) M ( E) [ f ( E) f ( E + ev )] S Transmission probability of the electron 1 ( E ) = exp A φ S M 2 Usually, we assume ρ is featureless (ie. ρ const. T T ), and the sample electronic states dominate the tunnel spectra. de However, the tips might have effect on the tunnel spectra, if 1. we have atomically sharp tips,or 2. the tip has picked up a foreign atom.

31 Tunneling Barrier

32 In the low-voltage limit I Vρ ~ ; Case І -----metals where ρ ( ~ r ; E ) is the surface density of states of the sample at the center of the S t F tip( ), r~t ρ S S ( r E ) ρ ( E ) F is the density of states of the tip at the Fermi level and is often regarded as a constant. t ( ~ 2 r ; E) ψ ν ( ~ r ) δ ( E E) ρ t ( E F ) ν t F ν The contour followed by the tip is a contour of constant Fermi-level density of states of the sample, measured at the center of curvature of the tip.

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34 Example -----Semiconductor 1.Tip-negative 2.Tip -positive Science 234, 304 (1986).

35 Scanning Tunneling Spectroscopy STM provides atomic-scale topographic information, and atomic-scale electronic information. However, the mixture of geometric and electronic structure information often complicates interpretation of observed feature. Several spectroscopic modes: 1. Voltage-dependent STM imaging. 2. Tunneling I-V curves, di/dv. Density of state (DOS) di/dv

36 Pb/Si(111) +1.8 V +2.4 V T 4 site V Filled-state Surface Science 257, 259 (1991)

37 Scanning Tunneling Spectroscopy (STS) 1. The tunneling transmission probablility T is a smooth, monotonically increasing function of V. Thus dt/dv contributes a smooth background. Therefore, structure in di/dv can usually be assigned to changes in the state density. 2. Extracting quantitative information about the sample density of states is difficult because the density of states of the tip and the tunneling transmission probability T are almost unknown. 3. Normalization of di/dv by I/V was proposed to minimize the distance dependence of the tunneling probability (or the voltage dependence of the tunneling barrier).

38 Single-Wall Carbon Nanotubes

39 Electronic Structure of Single-Wall Nanotubes 1. Armchair nanotubes (n,n) metallic 2. Zigzag nanotubes (n,0) metallic, when n=3q semiconducting, otherwise 3. Chiral nanotubes (n,m) metallic, when m=n+3q

40 Electronic Structure of Single-wall Nanotubes Nature 391, 59 (1998).

41 Atomic Manipulation with STM Nature 344, 524 (1990) Xe on a Ni surface

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43 Quantum Correl M.F. Crommie et al., Science 262, 218 (1993).

44 Energy-Dependent Friedel Oscillations V t = + 10 mv V t = - 10 mv

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46 Phys. Rev. Lett. 85, 2777 (2000)

47 Site Hopping of O 2 on Si(111)-7x7 O 2 molecule starts to hop between neighboring adatom sites at temperature about 300ºC. 1. I.-S. Hwang, R.-L. Lo, and T.T. Tsong, Physical Review Letters 78, 4797 (1997). 2. I.-S. Hwang, R.-L. Lo, and T.T. Tsong, Surface Science 399, 173 (1998).

48 Jumping rate R = N/t = R o exp(-e a /k B T) Arrhenius relation Arrhenius Plot for the Hopping from a Center Site to an Adjacent Center Site

49 Continuous-Time Scanning

50 Site Hopping of O 2 Molecule on Si(111)-(7x7) t = 0s t = 683s t = 730s t = 742s STM images B i I * * i I f B f Atomic model Si adatom Si rest atom Potential diagram ev B i ev ev I * * i I f B f I.-S. Hwang, et al., Phys. Rev. Lett. 78, 4797 (1997)

51 Silicon Magic Clusters on Si(111) Surfaces 480 Sample bias of 1.5V

52 Time interval 4.6 s Sample bias of 1.5V 450

53 The Decay Processes of 2D Islands 450Å 380Å Sample bias -2V 450

54 Decomposition of a Si Island into Magic Clusters Sample bias 1.5V 460

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56 Si Magic Clusters on Si(111)-(7 7) C E E C E C C E C E E C 1. The spacing between the protrusions is ~3.8Å, which is much larger than the Si-Si bond length (2.3Å). 2. The magic number is estimated to be 9 to 15. faulted half unfaulted half sample bias 1.5 V +1.8 V +1.2 V

57 Pb islands on Pb/Si(111) Deposit Pb on Pb/Si(111) at T~200K 3 ---Single thickness Type II IC phase(1) Type I 3 Sample bias : +2V Ref : PRL 90 (2003)

58 I 120K Two-dimensional Ag cluster arrays on quantum Pb islands 100nm x 170nm

59 Deposit Ag on Pb islands at T~175K

60 The size of the nanopucks (a) 37 Hex. 37

61 PRL 97, (2006) Size distribution of Ag nanopucks

62 Stability of Ag nanopucks

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64 Reversible Surface Phase Transition Pb/Si(111)

65 Size Effects on the Pb-Covered Si Islands 200 K Scale bar: 5 nm 267 K I.S. Hwang, et al., Phys. Rev. Lett. 93, (2004)

66 Transition Temperature vs. Size 199 K

67 Temporal Fluctuations Near the Transition Temperature Slow scan direction Image size: 34 nm 38 nm 266 K

68 Slow scan direction

69 Temporal Fluctuations Near the Transition Temperature 260 K Image size: 40 nm 40 nm

70 Electrochemical Scanning Tunneling Microscopy Angew. Chem. Int. Ed. 40, 1162 (2001).