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A. Orozco , E. Kwan, A. Dhirani Department of Chemistry, University of Toronto

SCANNING TUNNELING MICROSCOPY: A NEW CHM 326 LAB. A. Orozco , E. Kwan, A. Dhirani Department of Chemistry, University of Toronto. T H E O R Y :. Assume : a small voltage is applied across the two metals the potential energy in the gap (U) is greater than the energy of the electron (E)

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A. Orozco , E. Kwan, A. Dhirani Department of Chemistry, University of Toronto

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  1. SCANNING TUNNELING MICROSCOPY: A NEW CHM 326 LAB A. Orozco, E. Kwan, A. Dhirani Department of Chemistry, University of Toronto

  2. T H E O R Y : • Assume: • a small voltage is applied across the two metals • the potential energy in the gap (U) is greater than the energy of the electron (E) • Classically: • the gap is an insulator, therefore this region is forbidden • U > E, thus, no current flows

  3. Quantum Mechanically: • a finite probability density (P) exists that an electron can be found in this classically forbidden region: P  e-2KL where • K~ 1Å-1 for typical metals • a quantum “tunneling” current flows when a bias voltage is applied between the two metals • tunnel current grows exponentially as the gap size decreases • CHM 326 LAB:PURPOSE • practical application of quantum mechanics to modern microscopy • preparation and investigation of nanostructures

  4. S T M Camera provided by Y. Suganuma • The basic concept of an STM involves scanning a sample surface with a sharp tip • a bias voltage must then be applied between the tip and sample in order to promote a tunneling current • an STM can thus provide three dimensional, real space images of surfaces at high spatial resolution

  5. COLLOID SAMPLES* 5nm 10nm 20nm • Pt/Ir Wire • used for scanning tip • 0.25mm in diameter METAL SAMPLES IMAGED: A- Flat Au (111) deposited at 300 °C B- Bumpy Au(111) deposited at room temp. C- Graphite D- 5nm Colloid on flat Au (111) E- 10nm Colloid on flat Au (111) F- 20nm Colloid on flat Au (111) A B C D E F 3 cm *provided by P. Trudeau.

  6. GRAPHITE • Graphite is a layered structure with 6-membered rings of sp2 hybridized carbon atoms • note lattice constants in above diagram • I = 1.00nA, U= 0.40V • average distance from • nearest neighbor ~ 0.21 nm

  7. BUMPY GOLD FLAT GOLD Monatomic Steps 2.5Å high Flat Terrace  500nm long • I = 1.00nA, U= 0.40 V • evaporated onto mica at room • temperature in a vacuum chamber • I = 1.00nA, U= 0.40V • evaporated onto mica at 300°C in a vacuum chamber

  8. Au (111) Monatomic steps Cross-section ~ 2.1Å I = 1.00nA, U= 0.40V

  9. COLLOID PREPARATION ON AU (111) • I = 0.048 nA, U = 1.00V • 5nm colloid covers terrace of flat Au(111)

  10. 5 nM COLLOID ON Au (111) Cross-section ~7.48nm ~6.7nm ~5.2nm I = 1.00nA, U= 0.40 V

  11. 10 nM COLLOID ON Au (111) Cross-section ~9.3nm ~11.6nm ~12.8nm I = 1.00nA, U = 0.40 V

  12. 20 nM COLLOID ON Au (111) Cross-section ~18.5nm ~18.7nm ~20.1nm I = 1.00nA, U = 0.40 V

  13. SPECTROSCOPY: I/V CURVES FOR 5 nM COLLOID • for small voltages, <1V, the I-V curves appear linear (ohmic behavior) • for larger voltages, I-V curves appear exponential-like (as expected) • N.B: jagged appearance of curve is most likely due to mechanical or electrical(60Hz) noise

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