Break junctions in liquid for molecular electronics L. Grüter, R. Huber,, M. Calame & Ch. chönenberger University of Basel and CCR 'anoscale cience (witzerland) anopain, March 2005, Barcelona, pain
Financial support: CCR anoscale science (www.nccr-nano.org) wiss ational cience Foundation O, EF O (elf-organised anotructures)
Outline Motivation: liquid gating of single molecules Break Junction set-up in liquid Characterization: contact regime tunneling regime Measurements with molecules Conclusions
Context: Motivation tudy the electrical properties of single molecules potentially useful in molecular electronics 1) Contacting single molecules: Mechanically Controllable Break-Junctions (MCBJ) z e Atomic contacts Control of the size gap in the nanometric scale. reduction factor: e: elongation of the bridge z: vertical displacement r = e/z 1/10 5
Context: Motivation tudy the electrical properties of single molecules potentially useful in molecular electronics 1) Contacting single molecules: Mechanically Controllable Break-Junctions (MCBJ) z e e: elongation of the bridge ending group z: vertical which displacement forms covalent bonds with gold Atomic contacts Control of the size gap in the nanometric scale. reduction factor: r = e/z 1/10 5
Motivation 2) earch for a switch: single molecular FET Gating (Field Effect Transistor) J. Park et al. ature. 417 722 (2002) a) Back-gating gate electrode: very close and small [A. R. Champagne et al. ano Lett. 5 305 (2005)] b) Electrochemical-gating Trap molecules within a MCBJ in liquid tudy the behavior of MCBJ in liquid M. Krüger et al. Appl. Phys. Lett. 78 1291 (2000)
MCBJ set-up Liquid cell (250 µl) rod controlled by a stepper motor (resolution in z of 3 nm) 24mm phosphorus-bronze polyimide 70 x 100 nm
Contact regime: opening UP: Opening (breaking) the junction 6 2 G0 = 2 e / h = 7.75 10 5 Gold junctions z G/G 0 5 4 3 2 1 0 DMO toluene 20 30 40 50 60 z (µm) opening counts (arb.u.) tatistics in 100 opening curves per environment vacuum air H 2 O DMO octane toluene 0 1 2 3 4 5 G/G 0 o significant effect of the enviroment
Tunneling regime I (A) DOW: Closing the junction Below 1G 0 : tunneling 10-7 Junction 83a vacuum V = 0.1 V toluene DMO 10-8 air DCM octane 10-9 Tunneling: ln z ( I ) = A B z B = 2r 2mφ φ = barrier height / h r = x/z (z =vertical displacement, x =size of the gap) 0 10 20 30 40 z (µm) closing
B (µm -1 ) 0.6 0.4 0.2 0.0 vac Tunneling: tol DMO air Tunneling regime Junction 83a DCM B = 2r 2mφ /h oct caling where r = x/z is affected by plastic deformation of substrate (tensile strength reached at z ~ 1 mm) Forcing φ vac = 3.5-5 ev r~ 5 x 10-5 Reproducible variation of the tunneling response with the environment B (µm -1 ) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 (B/B air ) 2 Toluene 71b 71c 73c 74a 83a 3 2 1 0 vac tol DMO air 83b 83a DCM 83b 74a 73c oct
Molecules H Zn Zn C60 (F. Diedrich, ETHZ, witzerland) Diporphyrin (F. Diederich, ETHZ) CH 3 Ac Ac O O C 5 H 11 C 5 H 11 TTF (J. Becher, ourthern, Denmark) TTF (. Martín, Madrid, pain)
Contacting molecules add molecules in solution z apply voltage (~ 0.1 V), open a 3 nm gap in the junction (molecule + end group ~ 1.5 nm) z bring the electrodes near, without closing completely Current (µa) 1.0 DMO 0.8 0.6 0.4 0.2 0.0 0 100 200 300 z (µm) closing counts (arb. units) H DMO 0.00 0.02 0.04 0.06 G/G 0
olvent dependence H (V=0.2 V) 1.0 0.8 C60 in DMO In DMO C H 3 O I (µa) 0.6 0.4 CH 3 0.2 0.0 0 100 200 300 z (µm) H 1.0 C60 in Toluene 0.8 In toluene CH 3 I (µa) 0.6 0.4 0.2 0.0 0 300 600 900 z (µm)
Conclusions Break-Junction setup can be operated in liquid (electrochemical gating, environment control, allowance of chemical reactions) The change of the environment has a reproducible effect on the tunneling response through the open junction. First measurements with contacted molecules in liquid have been performed. The signal due to the presence of molecules also depends on the environment..