Scanning Tunneling Microscope and Atomic Tunneling Microscope

STM AND AFM..

1. SCANNING TUNNELING MICROSCOPE (STM) ATOMIC FORCE MICROSCOPY

2. AFM invented by Binning and co-workers in 1986. Belongs to the Scanning Probe Microscopy family Scanning Probe Microscopes, can image a surfaces of materials with extreme magnification, so that it is possible to make individual atoms visible. In all cases a surface will be scanned in the x, y direction in very small steps. One example type is the scanning tunneling microscope.

3. AFM invented by Binning and co-workers in 1986. Belongs to the Scanning Probe Microscopy family AFM ,  atomic force microscopy   Contact AFM Non-contact AFM Dynamic contact AFM Tapping AFM BEEM,  ballistic electron emission microscopy CFM,  chemical force microscopy C-AFM,  conductive atomic force microscopy ECSTM  electrochemical scanning tunneling microscope EFM,  electrostatic force microscopy FluidFM ,  Fluidic force microscopy FMM,  force modulation microscopy FOSPM,  feature-oriented scanning probe microscopy KPFM,  kelvin probe force microscopy MFM,  magnetic force microscopy MRFM,  magnetic resonance force microscopy NSOM,  near-field scanning optical microscopy  (or SNOM, scanning near-field optical microscopy ) PFM,  Piezoresponse Force Microscopy PSTM,  photon scanning tunneling microscopy PTMS,  photothermal microspectroscopy /microscopy SCM,  scanning capacitance microscopy SECM,  scanning electrochemical microscopy SGM,  scanning gate microscopy SHPM,  scanning Hall probe microscopy SICM,  scanning ion-conductance microscopy SPSM  spin polarized scanning tunneling microscopy SSM,  scanning SQUID microscopy SSRM,  scanning spreading resistance microscopy SThM ,  scanning thermal microscopy STM ,  scanning tunneling microscopy STP,  scanning tunneling potentiometry SVM,  scanning voltage microscopy SXSTM,  synchrotron x-ray scanning tunneling microscopy SSET  Scanning Single-Electron Transistor Microscopy   Binning et al., Physics Review Letters 1986

4. The principle of the STM is remarkable simple, and can be compared with that of an old-fashioned record player. Just like in a record player, the instrument uses a sharp needle, (a tip), to interrogate the shape of the surface. But, in contrast with a normal record player, the STM tip does not touch the surface. When the distance between the tip and the surface is only a 0.5 to 1 nm (2 atomic diameters). AND applying a sample voltage between tip and surface THEN At these distances the electronic can jump from the tip to the surface or visa versa. The quantum mechanical process  tunneling. (Hence the name of the microscope). The current is always very low between a few pico Amperes and a few nano Amperes. The STM tip is attached to a piezo-electric element. By adjusting the voltage on the piezo element, the distance between the tip and the surface can be regulated. Now while scanning in x,y direction, and keeping the tunneling current constant by moving the tip up and down. An image of the Z-values can be plotted.

5. The Scanning Tunneling Microscope works like a record player…

6. So making STM images: Scanning in the X,y – direction, and keeping the tunneling current constant by some feedback electronics, the tip must be adjust in the z-direction. The Z-values are stored as image. X,Y Scan circuit Piezo Feedback Electronics PC specimen U sample Z It tip

7. In the X, Y scan , every time that the last atom of the tip is precisely above a surface atom, the tip needs to be retracted a little bit, while it has to be brought slightly closer when the tip atom is between the surface atoms. The last step is to visualize the tip trajectories. 15 years ago, for this an x-y plotter was used, (slow print). Now we are abele to make a more perspective color view of the graphite surface. Line scan image of graphite surface. Each bump corresponds to a single carbon atom. The size of the image is only 3 nm  3 nm. 15 years Ago: Now: Perspective color view of Graphite surface “Photo Camera”

8. 8 Scanning Tunneling Microscopes Monitors the electron tunneling current between a probe and a sample surface What is electron tunneling? Classical versus quantum mechanical model Occurs over very short distances Scanning Probe Tip and surface and electron tunneling

9. 9 STM Tips Tunneling current depends on the distance between the STM probe and the sample Tip Surface Tunneling current depends on distance between tip and surface

10. 10 x 10 6 x 10 8 x 10 8 STM Tip How do you make an STM tip “one atom” sharp? Let’s Zoom In! e-

11. 11 Source: http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/stm_animated.gif Putting It All Together The human hand cannot precisely manipulate at the nanoscale level Therefore, specialized materials are used to control the movement of the tip How an STM works (click to play or see URL below)

12. 12 Challenges of the STM Works primarily with conducting materials Contamination Physical (dust and other pollutants in the air) Chemical (chemical reactivity)

13. 14 Atomic Force Microscopes (AFMs) Monitors the forces of attraction and repulsion between a probe and a sample surface The tip is attached to a cantilever which moves up and down in response to forces of attraction or repulsion with the sample surface Movement of the cantilever is detected by a laser and photodetector Laser and position detector used to measure cantiliver movement

14. 15 AFM Tips The size of an AFM tip must be carefully chosen Interatomic interaction for STM (top) and AFM (bottom). Shading shows interaction strength. STM tip AFM tip

15. 16 The AFM Specialized materials are again used to manipulate materials at the nanoscale level

16. 17 So What Do We See? Sources: http://www.almaden.ibm.com/vis/stm/blue.html http://www.asylumresearch.com/ImageGallery/Mat/Mat.shtml#M7 Nickel from an STM ZnO from an AFM

17. 18 Xenon atoms Carbon monoxide molecules Source: http://www.almaden.ibm.com/vis/stm/atomo.html And What Can We Do? Using STMs and AFMs in Nanoscience Allows atom by atom (or clumps of atoms by clumps of atoms) manipulation as shown by the images below

18. In Atomic Force Microscopy (AFM) Signal origin from short-range forces between the tip and the sample: (van der Waals, capillary, electrostatic) 1 nm lateral resolution and 0.1 nm depth resolution C ontact mode Tapping mode

19. AFM COMPONENTS feedback

20. Cantilevers thickness Length Cantilever dimensions determine how easy to bend it is.

21. Image quality depends on tip size and shape contact point Tip trace courtesy of Duncan Sutherland

22. Feedback: Deflection of cantilever Contact Mode AFM Detector Laser Cantilever Tip Z X,Y Not too affected by humidity Operation in liquid Damage to soft samples Simple In contact mode, the tip is mounted onto the end of a flexible cantilever and raster scans the surface of the sample. The tip-surface interaction deflects the cantilever, which gives information about the surface topography.   Samples can be analyzed in air, liquids or vacuum. Resolution in liquid and vacuum is increased because of the absence of strong capillary forces due to a thin liquid film on all samples in air.   Unfortunately , biological samples are challenging to study in contact mode because they are generally soft, weakly bound to the surface, and damaged easily .

23. Piezoelectric material drives oscillations 10-100nm Cantilever oscillates can be driven at a resonant frequency ~10-500 KHz Feedback: O scillation amplitude The surface acts to damp the resonance Tapping Mode AFM Detector Slide courtesy of Duncan Sutherland

24. Delay related to tip surface interaction Phase imaging in tapping mode Phase shift Two regions on the surface with different Tip- surface interactions Slide courtesy of Duncan Sutherland

25. Things AFM cannot do: Image very rough samples Image something inside another material Image something that cannot be deposited on a solid material Things AFM is particularly suitable for: Imaging hard nanostructures Imaging soft nanostructures ( proteinfibers , polymers) Imaging single molecules on a flat substrate SUMMARY AFM is a member of the scanning probe family. It uses a sharp tip on the end of a flexible cantilever t o ‘feel’ the sample surface.