Single Molecule Electronics Stuart Lindsayasu edu Arizona Biodesign

Single Molecule Electronics Stuart. Lindsay@asu. edu Arizona Biodesign Institute Single molecules as a tool for understanding More Single molecules for better understanding

Photosynthesis and single molecule electronics The ASU-Physics-Chemistry-Engineering-Motorola group:

The Program Measure Single Molecule in well-defined conditions Compare with theory (repeat as needed!) ‘Calibrated’ molecules and contacts for devices Make fixed-gap (useful? T-dep? ) devices

The molecule-metal contact problem Many Few-Molecule-Devices have been made but measurements/theories generally do not agree: For example, DNA is: AN INSULATOR (D. Dunlap et al. PNAS 90, 7652, 1993) A SEMICONDUCTOR (D. Porath et al, Nature 403, 635, 2000) A CONDUCTOR (Fink and Schoenberger, Nature 398, 407, 1999) A SUPERCONDUCTOR (A. Y. Kasumov et al. Science 291, 280, 2001)

Making Single Molecule Junctions 1. Self-assembly: well defined geometry, contacts 2. Repeated contact (simpler after answer is known) 3. Fixed nano-gaps: problem of manufacturability

Self-assembled nanojunction (Science 294, 571, 2001) i

Alkanedithiols – STM Images

NI(V )/N (n. A) IV-Curves are integral multiples

Two Models for ‘quantized ‘ data

Histogram of curve multipliers • Find X such that variance from curve to curve is minimized • Over 1000 curves for n=1

IV-Curves of bonded molecules not very stress dependent! Current not stressdependent – through bond? Bonded F I F(h) Mechanical (Nanotechnology 13 5 -14, 2002)

I is closer to theory for bonded molecules Theory Bonded Comparison with electrochemistry: R(C 8)=950 M Mechanical K=105 s-1, EA=21 k. J/m, R(C 8)=300 M

More chain lengths give (V) (J. Phys. Chem. B 106 86098614, 2002 ) (CH 2)10 I=I 0 exp[- (v)z] • Also measure Ohmic region carefully to get (0) • Data do NOT agree with theory! (CH 2)12

Clue: I-V doesn’t fit tunneling model well What if the top contact is not so good? Coulomb Blockade?

Coulomb Blockade: Quantized charge transfer R 1>>h/2 e 2 Coulomb Blockade n n

Coulomb Blockade fits (Removes Beta anomaly) C 8 C 10 C 12 r 1 = 1 M r 8 = 128. 36 M C 8=c 10 =c 12= 0. 085 a. F c 1 = 0. 318 a. F r 10 = 252 M (Theory=0. 08 a. F) (from fitting C 8) r 12 = 875 M (c 1, r 1 fixed) Good agreement for I,

Other Measurements - Carotene - Phenylene-ethylenine oligomers -Technique reveals contacts mobility of Au

Caroteniod (J. Phys. Chem. B, 107, 6162 -6169, 2003) 32 Å TRANS T/C=4: 1 CIS

Simple Tunneling with Coulomb Blockade fits well No ‘free’ parameters a. Measured b R 1 R 2 C 1 C 2 Trans Cis Carotene easily oxidized but vibronic contribution not dominant

Phenylene-ethylynene Oligomers and Negative Differential Resistance Applied Physics Letters 81 3043 -3045 (2002)

Single Molecule NDR 1 1. 8 G 50 G 2 Asymmetry, but molecules NOT oriented? c. f. Reichert et al. PRL 88 2002 Confirms NDR but see nonreversible behavior

Single Molecule Bonding Fluctuations (Science 300 1413, 2002) • “Stochastic switching” reported for • We see the same effect in alkane dithiols • Significant switching with gold sphere attached

Single Molecule Bonding Fluctuations – Property of Au-S • Cannot internal electronic changes • Cannot be top ‘dipping’ into film • Cannot be bond to sphere breaking • Rate increases at annealing temperature • Fluctuations of lower bond 25 60

Conclusions Transport in gold-n-alkane-gold fits ab-initio tunneling calculations well: One molecule, well defined environment. Gold nanocluster introduces Coulomb Blockade Carotene – good agreement with tunneling calculations Phenylene-ethylenine oligomers, confirm NDR. Technique reveals mobility of Au contacts

Break Junctions - Geometry unknown BUT - Calibration available AND - Most common peak appears to be correct - Much easier than self-assembled junctions (Xu and Tao, Science 301, 1221 -1223 2003)

Single Molecule Conductance from Break Junctions PZT 1 2 • Are histogram peaks good data points? • What is the effect of strain? 3

Gold filaments stretch – maximum force ca 1 n. N Xu, Xiao, Tao, JACS 2003

Alkanedithiols: N=6: RC 6=10. 5 MW N=8: RC 8=51 MW N=10: RC 10=630 MW Major peaks are right peaks

a b Fixed Gaps: EBL, Electrochemistry, Electromigration c 200 nm Si. O 2 Au d 200 nm Kubatkin et al. (2003) – (but 3 devices/paper) e Au 500 nm 1 um 2 um

Our Fixed Gaps: 10% “Success Rate”? Molecule free molecular transistor 77 K Current(u. A) 90 80 70 60 50 40 30 20 10 0 -10 -20 -1. 5 Vg=-0. 5 V Vg=-0. 25 V Vg=0. 5 V -1. 0 -0. 5 0. 0 0. 5 1. 0 1. 5 SDbias(V) Atomic-scale control needed!

Summary 1: What we think we know 1. Measurements and theory in good agreement for some (single) molecules 2. (n-alkanes, carotenes, BDT [Tao, nanolets 4 267]) 2. Break junctions can give good data and are much simpler

Summary 2: What we don’t know/Can’t do 1. Single Molecule devices need to be assembled with Atomic Precision! 2. Fluctuations at room temperature? 3. Couplings and molecular vs. contact properties? 4. Redox activity molecules and environment?

ACKNOWLEGEMENTS ASU PHYSICS Xiadong Cui Otto Sankey John Tomfohr Jun Li Jin He ASU Engineering Nongjian Tao ASU Chemistry Ana Moore Tom Moore Devens Gust Xristo Zarate Alex Primak Yuichi Terazano Motorola Gary Harris Larry Nagahara