SCANNING TUNNELING MICROSCOPY A NEW CHM 326 LAB

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

THEORY: 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

Quantum Mechanically: Mechanically • a finite probability density (P) exists that an electron can be found in this classically forbidden region: P e -2 KL 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

STM 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

COLLOID SAMPLES* 5 nm 10 nm 20 nm METAL SAMPLES IMAGED: • used for scanning tip • 0. 25 mm in diameter A- Flat Au (111) deposited at 300 °C B- Bumpy Au(111) deposited at room temp. AB C D E F C- Graphite D- 5 nm Colloid on flat Au (111) E- 10 nm Colloid on flat Au (111) F- 20 nm Colloid on flat Au (111) *provided by P. Trudeau. Pt/Ir Wire 3 cm

GRAPHITE • Graphite is a layered structure with 6 membered rings of sp 2 hybridized carbon atoms • note lattice constants in above diagram I = 1. 00 n. A, U= 0. 40 V • average distance from nearest neighbor ~ 0. 21 nm

BUMPY GOLD FLAT GOLD Monatomic Steps 2. 5Å Flat Terrace 500 nm long high • I = 1. 00 n. A, U= 0. 40 V • I = 1. 00 n. A, U= 0. 40 V • evaporated onto mica at room temperature in a vacuum chamber • evaporated onto mica at 300°C in a vacuum chamber

Au (111) Monatomic steps Cross-section ~ 2. 1Å I = 1. 00 n. A, U= 0. 40 V

COLLOID PREPARATION ON AU (111) • I = 0. 048 n. A, U = 1. 00 V • 5 nm colloid covers terrace of flat Au(111)

5 n. M COLLOID ON Au (111) Cross-section ~7. 48 nm ~6. 7 nm ~5. 2 nm I = 1. 00 n. A, U= 0. 40 V

10 n. M COLLOID ON Au (111) Cross-section ~9. 3 nm ~11. 6 nm ~12. 8 nm I = 1. 00 n. A, U = 0. 40 V

20 n. M COLLOID ON Au (111) Cross-section ~18. 5 nm ~18. 7 nm ~20. 1 nm I = 1. 00 n. A, U = 0. 40 V

SPECTROSCOPY: I/V CURVES FOR 5 n. M COLLOID • for small voltages, <1 V, 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(60 Hz) noise