Techniken der Oberflächenphysik (Techniques of Surface Physics)

Transcription

1 Techniken der Oberflächenphysik (Techniques of Surface Physics) Prof. Yong Lei & Dr. Yang Xu Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: Office: Heisenbergbau (Gebäude V) 202, Unterpörlitzer Straße 38 (tel: 3748) Vorlesung: Mittwochs (G), 9 10:30, C 108 Übung: Mittwochs (U), 9 10:30, C 108

2 Characterization of surfaces An appropriate characterization will play a crucial role in determining various surface structures and their properties (especially for nanosurfaces. Three broadly approved aspects of characterization are 1. Morphology 2. Crystalline structure 3. Chemical analysis

3 SEM: Scanning Electron Microscope; STM/AFM: Scanning Tunneling Microscope/Atomic Force Microscope TEM: Analytical Transmission Electron Microscope X-Ray: X-ray Morphology; IP: Image Processing; LM: Lightweight Morphology; RBS: Rutherford Backscattering Spectrometry (Kelsall et al., Nanoscale science and technology. 2005)

4 Techniques of Surface Physic ---STM and AFM(SPM)

5 Visible by eyes m 10-3 ~~~ mm Optical microscope 10-6 ~~~ µm SEM and TEM 10-9 ~~~ nm SPM

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7 Scanning Tunneling Microscope The tunneling phenomenon in quantum mechanics: an electron's penetration of an energy barrier, even though the electron's energy is below the height of barrier. The tunneling situation:

8 Scanning Tunneling Microscope tunneling phenomenon:

9 Gerd Binnig Heinrich Rohrer (born 20 July 1947) German physicist (born June 6, 1933) Swiss physicist They shared half of the 1986 Nobel Prize in Physics with for the design of the scanning tunneling microscope (STM).

10 STM

11 The principle of STM Probe Sample

12 The structure of STM I t ~ e -2kd

13 The manipulation of STM

14 Constant current image (topography) of an antiferromagnetic atomic layer iron on W(001) with defects and atoms.

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16 1. Atomic imaging The Application of STM Nickel (110) Platinum (111)

17 High performance STM image showing atomic resolution on Si(111) 7nm x 7nm cobalt sulfide "nanoflower" structure synthesized on a Au(111) surface 9nm x 9nm

18 2. Manipulation of single atoms and single molecules

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20 Lateral manipulation: Transfer of atoms/molecules along surface using attractive/ repulsive forces between tip and atom/molecule. Vertical manipulation: Transfer of atoms/molecules between surface and STM tip using electronic/ vibration excitation by tunneling. Desorption: Similar to vertical manipulation, but desorption of atom/molecule into surrounding gas phase.

21 positioning 48 iron atoms into a ring in order to "corral" surface state electrons and force them into "quantum" state.

22 3. Single-molecular chemical reactions Dissociation: Selective bond breaking within a molecule by tunneling processes. Synthesis: Selective bond formation between two molecular units using lateral manipulation followed by electronic/vibration excitation.

23 Schematic of bond formation in Au-PTCDA switch: In non-bonded state, atom and molecule are both negatively charged and stabilized by repulsive interaction (a). By tunneling out of molecular resonance, PTCDA is neutralized, and electrostatic repulsion is weakened (b). This makes Au atom moving towards molecule and form the bond.

24 4. Construct molecular-level electronic device Terbium atom (red) is sandwiched between two organic molecules (grey and blue) to form a single-molecule magnet.

25 The advantages and disadvantages of STM Advantages: a 3D profile of a surface, allows to detect many features, including roughness, defects, and to determine size and conformation of molecules. Other advantages: obtain much more details than other microscopes, for a better understanding of research topics at molecular level. STM is versatile, it can be used in ultra-high vacuum, air, water and other liquids, and gas. can be operated in temperatures from zero Kelvin up to a few hundred degrees (Celsius).

26 Disadvantages: It is difficult to use STM effectively. It is a very specific technique that requires a lot of skill and precision. STM requires very stable and clean surfaces, excellent vibration control and sharp tips. STM only can be used to scan conducting samples which are not easily oxidized. STM uses highly specialized equipment that is expensive.

27 Atomic Force Microscope (AFM) Binnig, Quate and Gerber invented the first atomic force microscope

28 The structure of AFM Position Sensing Part Position Sensing photodetector Force Sensing Part Feedback circuit

29 The principle of AFM When a tip is close to sample, typically two forces operate: Coulombic and van der Waals interactions. The combination of these interactions results in a force-distance curve Coulombic Interaction: strong, short range repulsive force between tip and sample. This repulsion increases as the distance decreases. Van der Waals interactions: long range attractive force, which become obvious at distance of down to 10 nm.

30 As tip is brought towards the sample, van der Waals forces cause attraction. As tip gets closer to the sample this attraction increases. However at very small separations the repulsive coulombic force becomes dominant. The repulsive force causes the cantilever to bend when the tip is getting closer to the surface.

31 The manipulation of AFM Two scanning processes (contact mode): Constant-height scan Constant-force scan

32 Three AFM imaging modes: 1. Contact AFM < 0.5 nm probe-surface separation 2. Tapping mode AFM nm probe-surface separation 3. Non-contact AFM nm probe-surface separation

33 1. Contact AFM tip contacts the sample surface. The photo detector monitors the changing of cantilever deflection and the force is calculated using Hooke s law: F = k x (F = force, k = spring constant, x = cantilever deflection) The feedback circuit adjusts the probe height to maintain a constant force and deflection on the cantilever, i.e. deflection setpoint. Constant-force scan

34 2. Tapping mode AFM (Intermittent contact ) cantilever oscillates at (or slightly below) its resonant frequency. Oscillation amplitude ranges in nm. Tip lightly touch ( taps ) on sample surface during scanning. Resonant frequency of cantilever dependent on tip/surface separation. Oscillation decreases when tip is closer to surface. Hence changes of oscillation amplitude are used for measuring tip/surface separation. Feedback circuit adjusts probe height to maintain a constant amplitude of oscillation. i.e. the amplitude setpoint. Constant-height scan

35 Tapping mode in air: A small piezoelectric crystal on AFM tip holder makes the cantilever oscillate up and down. Tapping mode in liquids: Tapping mode operation in liquids is a very useful tool for biologists.

36 3. Non-contact AFM cantilever oscillate near sample surface, but does not contact it. Oscillation is at slightly above resonant frequency. Van der Waals force decrease resonant frequency. This decrease of resonant frequency causes oscillation amplitude to decrease. Normally adsorbed fluidic layer is much thicker than the dominant region of van der Waals force - probe is either out of range of van der Waals force, or becomes trapped in fluidic layer. Therefore noncontact mode AFM works best under ultra-high vacuum conditions.

37 The Properties of the different operation modes in AFM.

38 Advantages and Disadvantages of AFM Modes Contact Mode Tapping Mode Noncontact Mode Advantage - High scan speeds - Atomic resolution possible - Easier scanning of rough samples with large changes in vertical topography. - High lateral resolution (1 to 5 nm). - Lower forces, less damage to soft samples in air. - Almost no lateral forces. - Both normal and lateral forces are minimised, good for measuring very soft samples - Can get atomic resolution in UHV environment Disadvantage Lateral forces distort image Combination of forces reduces spatial resolution and cause damage to soft samples. Slower scan speed than contact mode Slower scan speed than tapping and contact modes Lower lateral resolution, limited by tip-sample separation. Usually only applicable in extremely hydrophobic samples with a minimium fluidic layer.

39 1. Imaging The application of AFM AFM 3D image of a detail of the free surface of an artificial pattern The figure illustrates 800 nm wide and 10 nm high Pd/Fe/Pd thin film dots fabricated using electron lithography.

40 PMMA spheres scanning range 45x45 μm NCAFM image of the Ge/Si(105) surface, 4.2 nm x 4.2 nm AFM image of human plasma sample (fibrinogen)

41 2. Measuring forces (and mechanical properties) at nanoscale Illustration of an AFM tip measuring the force to move a cobalt atom on a crystalline surface. (Credit: Image courtesy of IBM)

42 Single-molecule force microscope uses a target molecule at AFM tip end to probe surface molecules with a strong attractive force. Force measurements are mapped as an image.

43 3. As a nanoscale tool bending, cutting and extracting soft materials (Polymers, DNA, nanotubes), at nano-scale grab and hold a nanoparticle in a position Manipulating nanotube on Si substrate. AFM tip creates Greek letter "theta" from a 2.5 μm long nanotube

44 A single nanotube (red) on an insulating substrate (SiO2, green) is manipulated in a few steps onto a W film thin wire (blue), finally stretched across an tungsten oxide barrier (yellow).

45 The advantage and disadvantage of AFM Advantages : 1) high-resolution 3-D surface images 2) not require special sample treatments (no sample's destruction) 3) Usually not require vacuum (operate both in air and liquid); 4) could be used for organic materials Disadvantages: 1) image size is much smaller than that of electron microscopes; 2) slow scanning rate, unlike an electron microscope which does it in almost real-time. 3) tip convolution -- not true sample topography 4) expensive tips

46 Tip convolution----tip Related Artifacts protrusions (dots) appear wider, depressions (pores) appear narrower than the reality.

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48 AFM Section Analysis of the nano-discs, the average height and size of the nano-discs are approximately 1.5 and 80 nm, respectively.

49 AFM Section Analysis of the nano-hemisphere. To accurately reflect the shape of the nanoparticles, we used the same dimension scale for the horizontal and vertical coordinates. The average height and base diameter of the nanohemispheres are approximately and 75 nm, respectively.

50 AFM Section Analysis of the nano-hemiellipsoids. To accurately reflect the shape of the nanoparticles, we used the same dimension scale for the horizontal and vertical coordinates. The average height and base diameter of the nanohemiellipsoids are approximately and 65 nm, respectively.

51 AFM Section Analysis of the nano-conics. To accurately reflect the shape of the nanoparticles, we used the same dimension scale for the horizontal and vertical coordinates. The average height and base diameter of the nano-conics are approximately and 60 nm, respectively.

52 Thank you and have a nice day!