Experimental Studies of Single-Molecule Transistors

Transcription

1 Experimental Studies of Single-Molecule Transistors Dan Ralph group at Cornell University Janice Wynn Guikema Texas A&M University Condensed Matter Seminar January 18, 2006 p.1

2 Cornell Image from Dan Ralph Group Duffield Hall (new home of CNF) p.2

3 Outline Motivation Fabrication Single-atom transistors Coulomb blockade Kondo effect Magnetic electrodes (with C 60 ) Mechanically controlled break junctions Optical experiments p.3

4 Previous Molecular Electronics Work Nanotubes e.g. Dekker (Delft) Crossed wires Williams, Heath Nanopore e.g. Reed (Yale) STM e.g. Weiss (PSU), Ho (UCI) c-afm Cui et al. (ASU), Frisbie (Minn.) mechanical break junction e.g. Devoret (Saclay) p.4

5 Single-Molecule Transistors Why Transistors? With only two-probe measurements, it is hard to characterize the device structure. With a transistor gate, we can shift molecular energy levels systematically, to understand the mechanisms of electron motion. V Source Gate Drain I V g Magnetic electrodes, mechanical adjustability, and using light excitations provide additional experimental knobs that enable controlled, systematic experiments. p.5

6 Fabrication p.6

7 Creating single-molecule transistors Larger features are defined via photolithography Nanowires are generated by e-beam lithography Deposit molecules on electrodes Pass large currents: electromigration induced gap formation. (H. Park et al. APL (1999).) C nm Cobalt p.7

8 Device prep and electromigration A chip with ~30 nanowires is: Cleaned in an oxygen plasma. Molecules of interest are immediately deposited. The chip is cooled to cryogenic temperatures. A nanoscale gap is created using electromigration. In a fraction of the break junctions, a molecule will bridge the gap. Electromigration Movie made by Kirill Bolotin Electromigration in break junctions: H. Park et al. APL 75, 30 (1999). p.9

9 Electromigration junctions 150 nm Before After Tunneling resistance exponential with gap size, d. nanoscale gap R e 2Kd where K = 2meφ h After breaking, the gap width can be estimated from the tunneling resistance. Typically 1-3 nm wide. Relaxation occurs when the devices are warmed to room temperature. # of devices Pt Flexible way to make gated nano - junctions R (Ω) p.10

10 Single-atom transistors work primarily by Jiwoong Park (McEuen), Abhay Pasupathy p.11

11 SH Designer molecules (made by the Abruña group) SH 24 Å N N N N N N Co Co N N N N N N 13 Å HS HS Co 2+ (tpy(ch 2 ) 5 SH) 2 Co 2+ (tpysh) 2 Longer molecule: Coulomb-blockade effects. Shorter molecule: Kondo effect. Related measurements, different molecules: H. Park (Harvard) p.12

12 Coulomb blockade in longer molecule 0.5 Vg = -1.00V Vg = -0.86V Vg = -0.74V Vg = -0.56V Vg = -0.41V ) I (na V Source Silicon Gate Drain I V (mv) V g High resistances: 100 MΩ to ~1 GΩ single electron charging Coulomb blockade up to 150 mev bias (unstable beyond) J. Park, A.N. Pasupathy et al., Nature 417, 722 (2002). p.13

13 Interpreting Coulomb blockade data di/dv conductance map with excited states N N+1 n electrons n+1 electrons (figure from Abhay Pasupathy s thesis) excited states of n+1 (red) excited states of n (blue) ground states (black) p.14

14 Vsd (mv) Excited state spectra of longer molecule Co 3+ Co Vg (V) Vsd (mv) Vg (V) Vsd (mv) Vg (V) Know charge states are 2+ and 3+ from electrochemistry and from Zeeman splitting of 2+ excited state only. Excited states are consistent with vibrational mode calculations for the molecule. Energies too small to be electronic excitations. Related: Bouncing ball modes in C 60 transistors H. Park et al., Nature 407, 57 (2000). J. Park, A.N. Pasupathy et al., Nature 417, 722 (2002). p.15

15 Shorter Co molecule Molecules are self-assembled on unbroken wire. Cooled to low temperatures ( K). Then broken by electromigration. Insertion of short molecule can often be seen directly. I (µa) molecule attaches (artist s conception) V (V) J. Park, A.N. Pasupathy et al., Nature 417, 722 (2002). p.16

16 Kondo effect in short molecule 1.5 T= 1.5 K DOS Jun Kondo di/dv (e 2 /h) 1.0 Kondo resonance V (mv) High conductance Zero bias conductance peak Kondo effect: correlation between electron spin on dot and electrons in electrodes. increased DOS Temperature, magnetic field, applied voltage, all disturb this correlation and decrease conductance. p.17

17 Kondo signatures in shorter Co Peak height decreases logarithmically around T K Peak splits as a function of magnetic field. Kondo temperatures vary from < 1 K to > 50 K. J. Park, A.N. Pasupathy et al., Nature 417, 722 (2002). p.18

18 Magnetic electrodes work primarily by Abhay Pasupathy p.19

19 Break junctions with magnetic electrodes 50 nm gate 0.38 R (M? ) C AP Ni-Ni JMR = 19% Nickel electrodes Au contacts 1 µm 0.32 P B (mt) Tunneling magneto-resistance of a bare Ni point contact Pasupathy et al., Science 306, 86 (2004). p.20

20 Effect of magnetic electrodes on C 60 Ni C 60 Ni ferromagnetic sample Ni-C 60 -Ni Au electrodes sample B=0 B=0 parallel electrodes 1.5 K B=10T (no splitting for B=0) Splitting of zero-bias Kondo peak due to exchange interaction Cannot be due to magnetic field. 5 mev splitting would require 50 T. p.21

21 Splitting of the Kondo peak large splitting when parallel reduced splitting when antiparallel large magneto-resistance (up to - 80%) theory for (C) Good agreement with theory of Kondo effect with magnetic electrodes J. Martinek et al., PRL 91, (2003). Pasupathy et al., Science 306, 86 (2004). p.22

22 Mechanically controlled break junctions work primarily by Alex Champagne, Abhay Pasupathy & most recently Josh Parks p.23

23 Gated mechanical break junctions Total displacement = 5 Å with stability better than 1 pm. p.24

24 Model of single-molecule transistor dot C s C d Source Drain V G s Gate G d C G I V G The capacitances (C) and the tunneling rates (G) change with displacement of the electrodes. (figure from Jiwoong Park s thesis) p.25

25 Mechanical and gate control with C 60 x 0 x Å sample 1 With increasing displacement: x 0 x Å sample 2 conductance decreases capacitance ratios C s,d /C g decrease degeneracy gate voltage may shift Champagne et al., Nano Letters 5, 305 (2005). p.26

26 Optical experiments work primarily by Janice Guikema & most recently Jacob Grose p.28

27 Molecular Switch ON closed open At t=0 lamp turned on (546 nm). OFF But open closed did not work when attached to gold (excited state of open form quenched by the gold) p.31

28 Intensity requirements for photocurrent I photo = ηep E γ a = ηe( ΙA) hc λ I photo is current, η is quantum efficiency, E γ is photon energy, P a is absorbed power and P a = IA, where I is incident light intensity, and A is absorption cross section. γ Minimum detectible current: I min ~ 10 fa = A (62422 electrons/sec, or one electron every 16 µs.) e - Example, λ = 500 nm E γ = 2.5 ev, with η=100% P a = ΙA = I photo ηe E γ = ( A)( 2.5 V) = 25 fw diameter A = area minimum intensity 1 nm 0.8 nm 2 3 W/cm 2 10 nm 79 nm 2 32 mw/cm nm 7900 nm mw/cm 2 p.35

29 Field enhancement by antennas Bow-tie antenna for microwaves Grober, Schoelkopf & Prober, APL 70, 1354 (1997). d = 1 cm Best antenna shape (mid-ir) Crozier et al., JAP 94, 4632 (2003). r = 120 nm θ = 30º Resonance Small angle Small tip radius p.36

30 Field enhancement by antennas Nano-antennas for optical wavelengths Schuck et al., PRL 94, (2005). Smallest gaps gave >10 3 intensity enhancement two-photon excited photoluminescence λ = 830 nm excitation collected nm Our effective absorption cross-section may be greatly increased due to antenna effect of the electrodes. 150 nm p.37

31 Future of these devices Will single- molecule electronics be useful? -- Hard to say. Challenges: No gain Devices not stable at room temperature yet Relatively slow speeds (long RC times) How to wire up many devices correctly? At this stage, the main goal is scientific exploration. p.46

32 Summary It is possible to make electrical contact to single molecules. With transistor gate can shift the molecules energy levels systematically. made single-atom transistors from Co based molecules Coulomb blockade and vibrational excitation in long molecule Kondo effect in short molecule Can study effect of spin transport through single-molecule transistor using magnetic electrodes (e.g. in C 60 ) Mechanically controlled break junctions allow tuning of the coupling between molecule and electrodes. With optical set-up can study effect of light on single-molecule transistors. p.47

33 Acknowledgements Professor Dan Ralph (Physics, Cornell) Kirill Bolotin Alex Champagne Jacob Grose Ferdinand Kuemmeth Josh Parks Abhay Pasupathy Professor Hector Abruña (Chemistry, Cornell) Samuel Flores-Torres Jay Henderson Geoff Hutchison Professor Paul McEuen (Physics, Cornell) Ken Bosnick Nathan Gabor Jiwoong Park Funding: NSF, DARPA, and ARO p.48