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

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 T H E O R Y :

Quantum Mechanically 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

S T M 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 Camera provided by Y. Suganuma

3 cm COLLOIDSAMPLES* 20nm10nm5nm METAL SAMPLES IMAGED: A B C D E F 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) P t / Ir Wire used for scanning tip 0.25mm in diameter * provided by P. Trudeau.

GRAPHITE I = 1.00nA, U= 0.40V average distance from nearest neighbor ~ 0.21 nm Graphite is a layered structure with 6- membered rings of sp 2 hybridized carbon atoms note lattice constants in above diagram

FLAT GOLD BUMPY GOLD 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 Monatomic Steps  2.5Å high Flat Terrace  500nm long

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

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

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

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

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

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