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Interfacing Molecules to Electronic Materials.

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Presentation on theme: "Interfacing Molecules to Electronic Materials."— Presentation transcript:

1 Interfacing Molecules to Electronic Materials

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

3 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

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

5 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

6 1 2 LUMO HOMO CHARGED Why solvent + ions matter e-e- ENERGY e-e- LANDAUER MARCUS

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

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

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

10 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

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

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

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

14 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

15 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 )

16 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

17 We have a two terminal switch!

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

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

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