Measuring charge transport through molecules utline Indirect methods 1. ptical techniques 2. Electrochemical techniques Direct methods 1. Scanning probe techniques 2. In-plane electrodes 3. Break junctions 4. Sandwiched monolayers Mark Reed
Using optical experiments to measure the properties of molecular wires Experiment 1. Excite the donor with a 150-ps-long pulse. 2. Monitor the absorption spectrum of the donor and acceptor to see how fast charge transfer occurs. Results Charge transfer occurs over 4 nm in 10 ps. Mark Ratner et al. Nature 396 (1998) p. 60.
Using electrochemistry to study electron tunneling across molecules Fe (CH 2) n SH ligomethylenes (n=5-18) Fe 1 SH Fe SH ( )n ligophenyleneethynylenes (n=1-4) Fe Fe Fe 2a SH 2b SH 2c SH Fe Fe ligophenylenevinylenes 3a Fe 4 SH SH 3b Fe 5 SH SH Chris Chidsey, Steve Dudek et al. designed these ferroceneterminated molecules to study the rates of electron tunneling.
Chris Chidsey Self-assembled monolayer on gold electrode in contact with aqueous electrolyte solution When the potential on the Au electrode reaches a certain value, the ferrocene is oxidized. With this technique, a second electrode is not needed. Fe e - S S S S S S S S S S Au
10 10 10 8 Data for various bridges Electrons tunnel across the PPV oligomer faster than the phenyleneethynylene oligomer. 10 6 Fe SH n k 0 / s -1 10 4 10 2 β = 0.85 Å -1 Creager, et. al. JACS 1999 121(5) 1059-1064 Fe SH ( )n 10 0 PV PE M Fe (CH 2) n SH 10-2 0 5 10 15 20 25 30 35 40 45 Distance / Å Chris Chidsey
Using an STM tip to see if an individual molecule conducts An insulating SAM is first deposited. The conjugated molecule 1 is then added to fill in the defects. The current spikes in the STM image only appear after 1 is added. D.L. Allara, J.M. Tour, P.S. Weiss et al. Science 271 (1996) p. 1705. Science 292 (2001) p. 2303. The current depends on the spacing between the tip and the molecule and the conductivity of the molecule.
Gold-nanoparticle contacts 1. An octanethiol monolayer was deposited onto a gold surface. 2. Some of the molecules were replaced by 1,8-octanedithiol. 3. The films were soaked in a suspension of gold nanoparticles with a diameter less than 2 nm. The particles formed chemical bonds with the exposed thiol groups. S. M. Lindsay et al. Science 294 (2001), p. 571.
S. M. Lindsay et al. Science 294 (2001), p. 571. IV curves Measurements on over 4000 nanoparticles produced only five distinct families of curves.
S. M. Lindsay et al. Science 294 (2001), p. 571. Five families of curves Curves from the previous slide divided by the integer 1, 2, 3, 4 or 5. It is thought that these curves correspond to cases where the gold makes contact to 1,2,3,4 or 5 dithiol molecules. Histogram of values of a divisor, X, chosen to minimize the variance between any one curve and the fundamental curve.
Effect of contact on IV curves AFM tip makes contact to gold particle, which is bound to the dithiol. AFM tip makes contact to an octane thiol. (The contact force was 6 nn.) The current is severely limited by contact resistance unless the electrode is attached to the molecule with a covalent bond.
The role of contact force with gold nanoparticle without gold nanoparticle When the AFM tip makes contact to a gold nanoparticle, the current that passes through the molecule does not depend on the force that the tip exerts on the substrate. This explains why the IV curves are so reproducible.
In-plane electrodes Au S S Au Challenges 1. The gap between electrodes must be very small. 2. Molecules must be placed in the gap.
http://online.itp.ucsb.edu/online/qhall_c98/dekker/ ne carbon nanotube on seven electrodes CNTs are relatively easy to work with. The electrodes can be patterned by e-beam lithography and the tubes can be draped on top or grown across by CVD.
Controlled fabrication of electrodes with atomic separation 0.01 M KAu(CN) 2 1 M KHC 3 0.2 M KH A 50-400-nm gap is first made with e-beam lithography. The gap-size is then reduced by electrodepositing Au on the electrodes. A.F. Morpurgo, C.M. Marcus, D.B. Robinson, Appl. Phys. Lett. 74 (1999) p. 2084.
A.F. Morpurgo, C.M. Marcus, D.B. Robinson, Appl. Phys. Lett. 74 (1999) p. 2084. Electrodes with atomic separation The SEM that was used could not resolve the gap.
A.F. Morpurgo, C.M. Marcus, D.B. Robinson, Appl. Phys. Lett. 74 (1999) p. 2084 Monitoring the resistance to control the gap size When the electrodes start to touch, the conductance (G) jumps up in quantized steps. Small gaps can be obtained by stopping the etch just before this happens.
Reducing gaps with sputtering A. Bezryadin, C. Dekker, G. Schmid, Appl. Phys. Lett. 71 (1997) p. 1273.
Micrographs taken after different amounts of sputtering They sputtered Pt onto the electrodes and then imaged the sample several times. In this way they were able to stop the sputtering when the gap was at 4 nm. A. Bezryadin, C. Dekker, G. Schmid, Appl. Phys. Lett. 71 (1997) p. 1273.
K.Liu et al (IBM Yorktown), Appl. Phys. Lett 80 (2002) p. 865. E-beam lithography Gaps as small as 4 nm have been made by overexposing e-beam resists. 10- nm gaps can be made reproducibly.
H. Park, A.P. Alivisatos, P.L. McEuen, Appl. Phys. Lett. 75 (1999) p. 301. Creating gaps with electromigration 1-nm gaps can be creating by passing such high current densities through gold wires that the gold atoms migrate.
Placing molecules in gaps If a molecule has groups on both ends that can chemically bond to the electrodes, then molecules can bridge the gap. It is possible, however, that multiple molecules can bridge the gap.
Electrostatic trapping +++ --- +++ --- When an a voltage is applied between the two electrodes, the intense electric field between the two tips induces a dipole in the nanoparticle. The dipole is then pulled into the gap where the electric field is the strongest. 4.5 V was applied for a few seconds to position the 17-nm Pd particle shown above. A. Bezryadin, C. Dekker, G. Schmid, Appl. Phys. Lett. 71 (1997) p. 1273.
Self limiting nce one conducting nanoparticle or molecule bridges the gap, the applied voltage drops across the resistor (R s ) instead of the gap. Consequently, the the electric field between the electrodes is greatly reduced and additional particles are not attracted.
Generality Electrostatic trapping has also been used to trap carbon nanotubes, 1 5-nm-long conjugated molecules 2 and DNA. 3 1 A. Bezryadin, C. Dekker, J. Vac. Sci. Technol. B 5 (1997) p. 793. 2 A. Bezryadin, C. Dekker, G. Schmid, Appl. Phys. Lett. 71 (1997) p. 1273. 3 D. Porath, et al. Nature 403 (2000) p. 635.
http://www.eng.yale.edu/reedlab Mechanically controllable break junctions The gap displacement is 100 to 1000 times smaller than the piezo displacement. Sub-Å gaps are stable.
Microfabricated break junction C. Zhou, C.J. Muller and M. A. Reed Appl. Phys. Lett. 67 (1995) p. 1160
SEM of break junction C.J. Muller and M. A. Reed, Science 272 (1996) p. 1901. Appl. Phys. Lett. 67 (1995) p. 1160.
Appl. Phys. Lett. 67 (1995) p. 1160 Current versus gap distance The gap size is proportional to the piezo voltage. It was cycled back and forth. The data shows that the current depends exponentially on the gap distance and that the break junction is very stable. These experiments are used to calibrate the break junctions so that the distance between the electrodes can be known.
Bridging a break junction This schematic may not be accurate! Mark Reed et al., Science 278 (1997) p. 252.
Reproduciblity of IV measurements Three independent conductance curves offset for clarity show that the measurements are reproducible. ccasionally the conductance is twice the normal value. In this case, there are probably two molecules bridging the gap. Mark Reed et al., Science 278 (1997) p. 252.
Sandwiching monolayers between films
How can electrodes be deposited on both sides of a SAM without introducing a short? The electrodes should be smaller that 50 nm in size.
Putting electrodes on SAMs 1. E-beam lithography and plasma etching are used to make a pore with a 30 nm opening. 2. Gold is evaporated into the pore. 3. A SAM is deposited from solution on the gold. 4. The second electrode is deposited. This is done at low T to avoid damaging the SAM. Chongwu Zhou, Mark Reed et al., Appl. Phys. Lett. 71 (1997) p. 611.
Evaporating at an angle to avoid shorting Chung-Chen Kuo, Thomas Mallouk, Thomas Jackson et al. reported at the 2002 Device Research Conference that shorting can be avoiding by depositing electrodes at approximately 150 K (at lower temperatures water condenses on the substrate even though the pressure in the chamber is less than 10-7 Torr) evaporating at an angle so that the atoms don t bombard the monolayer directly. In their abstract they claim that 1000 µm 2 devices had no shorts.
Mercury drop measurements To avoid the possibility of damaging the film by bombarding it with hot metal atoms, one can use a mercury drop as the top electrode.
Hendrik Schön s fantasy self-assembled monolayer field-effect transistors (SAMFETs) This was a fraud. IBM Yorktown spent millions to reproduce it and showed that it does not work. The monolayer is 1-2 nm thick. The Si 2 layer supposedly was 4-5 nm thick.
Review articles on molecular electronics James Tour, Molecular electronics. synthesis and testing of components, Acc. Chem. Res. 33 (2000) p. 791-804. (highlights the synthesis) Mark Reed, Prospects for molecular-scale electronics, MRS Bulletin, February 2001, p. 113-120. (highlights the measurements) C. Joachim, J.K. Gimzewski, A. Aviram Electronics using hybrid-molecular and mono-molecular devices, Nature 408 (2000), p. 541-8. R.L. Carroll, C.B. Gorman, The Genesis of Molecular Electronics Angew. Chem. Int. Ed. 41 (2002) p. 4378.