Interfacing Molecules to Electronic Materials

Artificial Enzymes Hydrogen generation, photovoltaics Steinberg-Yfrach et al. Nature 392, 479 (1998)

1. Make functional molecules 2. Wire molecules to electrodes Steps to bio/molecular electronics: 3. Make them function on electrodes like they do in solution 4. Make economically-viable devices

Test Case: Single Molecule Switch made from Oligo Aniline Insulator i V Conductor -2e - Insulator -2e - Single Molecule Switch?

Molecular electronics vs. solution charge transfer chemistry Charge transfer in nature in solution + ions. Charge transfer in molecular electronics electrode to electrode – No water/ions

1 2 LUMO HOMO CHARGED Why solvent + ions matter e-e- ENERGY e-e- LANDAUER MARCUS

Charge Transfer in DNA Barnett et al., Science (2001)

The Challenge Need to measure single molecule conductance in a conducting solution with independent control of charge state. How to do this?

Repeated break junction Xu and Tao, Science 301, (2003)

Wiring Single Molecules Reliably Cui et al. Science (2001) Xu and Tao, Science (2003) GOOD NEWS: 7000:1 G range – worst G theor /G meas is 3.3 BAD NEWS: All Landauer theory

Operating probes in electrolyte Rev. Sci. Instrum. 60, (1989) (DNA - Xu et al. Nanoletts ) Insulating layer

Controlling ion gradients/electric fields at an electrode surface Bigger ion gradient = Bigger electric field at molecule

Measuring transport as a function of oxidation state LOCAL FIELD SURFACE FIELD

Electrochemical Current Surface Potential, E S, V vs. Ag Molecular Conductance (nS) G=G MAX - a(E S -b) 2 InsulatorConductorInsulator TIP-SUBSTRATE V FIXED AT 50mV Ofer et al. JACS , 1990

Neutral molecule Current (nA) Tip-substrate bias (V) (NO IONS) FIX E S, VARY TIP FIELD Oxidized molecule E S =0.4V (H 2 SO 4 )

G=G max -a(E S -b) 2 E S (V) =E S -  V  =1.4 E S =0.3V E S =0.25V Tip-substrate bias (V) Current (nA) MOLECULE FIELD = SURFACE FIELD ± TIP FIELD

We have a two terminal switch!

Current (nA) V ts (Volts) E S (V vs. Ag Wire) --- But it will need more than one molecule: Bias sweeps Potential sweeps

Summary Made a low-voltage switch based on chemical knowledge, get NDR Probe role of fluctuations Roadmap for going from chemistry to molecular electronics