First-Principles Modeling of Charge Transport in Molecular Junctions

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1 First-Principles Modeling of Charge Transport in Molecular Junctions Chao-Cheng Kaun Research Center for Applied Sciences, Academia Sinica Department of Physics, National Tsing Hua University September 6, 2010

2 Outline: 1. Introduction Why molecular electronics? 2. Conductance of single-molecule junctions Alkanedithiol [S-(CH 2 ) n -S] Alkanediisothiocyanate [SCN-(CH 2 )n-ncs] 3. Current-Driven Desorption at the Interface Cyclopentene on Si(100) 4. Summary

3 1. Introduction:

$Physical limit: Diffraction of$

6 Transistors in Integrated Circuits 32 nm now What s the problem? Physical limit: Diffraction of light. Economical limitation: Too expensive.

7 Molecular electronics: A solution The main idea: use molecules to create analogues of today s IC chips. Because molecules are small and can form structures by self-assembly. Aviram & Ratner, (1974). For example..

8 Previous measurement and modeling: Science 278, 252 (1997) 500 times of difference! PRL 84, 979 (2000)

9 As the size of a device goes down, physics change Top Channel Length, L 1 mm 0.1 mm Macroscopic dimensions < L > S. Datta, RQMP presentation (2006) Bottom 10 μm 1 μ m 0.1 μm 10 nm 1 nm 0.1 nm Atomic dimensions

10 Ohm s law:? Coherence length: Elastic mean free path: Ballistic regime:

11 Our method: How to calculate current? Landauer formula: I ( V b ) 2e h 2 T ( E, V b ) f l f r de DFT plus non-equilibrium Green s functions: Taylor, Guo, Wang, PRB 63, (2001); Waldron, Haney, Larade, MacDonald, Guo, PRL 96, (2006) Nanodcal ( & ATK

12 Conventional DFT solves two kinds of problems: Finite isolated system Gaussian-03 Periodic systems VASP Quantum transport: A device is neither finite nor periodic, and is in nonequilibrium

Computational modeling Interaction region Bulk

Functional Theory (GGA) LCAO Pseudopotentials

electrodes using ab initio self-energies

13 Computational modeling Interaction region Bulk region Bulk region Electronic structure Density Functional Theory (GGA) LCAO Pseudopotentials Nonequilibrium physics Full description of electrodes using ab initio self-energies Non-equilibrium electron distribution using NEGF H Calculation of electron current

14 Outline: 1. Introduction Why molecular electronics? 2. Conductance of single-molecule junctions Alkanedithiol [S-(CH 2 ) n -S] Alkanediisothiocyanate [SCN-(CH 2 )n-ncs] 3. Current-Driven Desorption at the Interface Cyclopentene on Si(100) 4. Summary

15 Conductance of a Au nanowire: Nature 395, 780 (1998) Nano Lett. 6, 2362 (2006)

16 Conductance of a single molecule N. J. Tao et al, Science (2003)

17 Measurement on single alkanedithiol molecules R n R o exp( n) 1.05 N. J. Tao et al, JACS (2003); Science (2003)

18 Experimental results N. J. Tao et al, Faraday Discuss. 131, 145 (2006)

19 Previous modeling: Calculation Experiment N = 6 G = Unit: G 0

Our model: s s Calculation Experiment N = 6 G = 0.0010 0.0012 N = 8 G = 0.000 13 0.

20 Our model: s s Calculation Experiment N = 6 G = N = 8 G = N = 10 G = Kaun & Seideman, Phys. Rev. B 77, (2008) Unit: G 0

21 Contact effect (N=6): a b dxy c pz Kaun & Seideman, Phys. Rev. B 77, (2008)

22 New experimental results: N. J. Tao et al, JACS, 128, 2135 (2006)?

23 Quantitative agreement? Muller, Phys. Rev. B, 73, (2006)

24 alkanediisothiocyanate [SCN-(CH 2 ) n -NCS] alkanedithiol [S-(CH 2 ) n -S]

25 Origin of high- and low-conductance traces in alkanediisothiocyanate single-molecule contacts Luzhbin & Kaun, Phys. Rev. B 81, (2010)

26 Luzhbin & Kaun, Phys. Rev. B 81, (2010)

27 Outline: 1. Introduction Why molecular electronics? 2. Conductance of single-molecule junctions Alkanedithiol [S-(CH 2 ) n -S] Alkanediisothiocyanate [SCN-(CH 2 )n-ncs] 3. Current-Driven Desorption at the Interface Cyclopentene on Si(100) 4. Summary

28 2. Current-Driven Desorption at the Interface: Cyclopentene on Si(100) Molecular electronic devices + silicon microelectronic technology The stability of organic molecules on semiconductors must be established.

29 Previous studies: benzene bound to Si(100) with π- orbital character Low-lying ionic resonances S. Alavi, et al., PRL 85, 5372 (2000). Saturated organic/silicon systems offer stability with respect to current-induced failure of silicon-based molecular electronics. S. N. Patitsas et al., Surf. Sci. 457, L425 (2000).

30 Desorption of cyclopentene from Si(100) Experimental results -2V, 0.1 na Elevated sample bias (threshold voltage: -2.5 and 3.5) -2V, 0.1 na N. L. Yoder et al., PRL 97, (2006); Current-Driven Phenomena in Nanoelectronics, chapter 7 (Pan Stanford, 2010)

31 Cyclopentene on Si(100): A saturated molecule Why threshold voltages is so small (-2.5 V and 3.5 V)?

32 A second question in experimental results Yield = N*e/(I*t) The yield is a factor of lower than for benzene/si(100) or chlorobenzene/si(111). A new avenue for desorption dynamics!

Our model: HOMO LUMO Cyclopentene -2.49 6.90 Cyclopentene+Si -2.00 2.

33 Our model: HOMO LUMO Cyclopentene Cyclopentene+Si Hybridization introduces new states into the gap N. L. Yoder et al., PRL 97, (2006); Current-Driven Phenomena in Nanoelectronics, chapter 7 (Pan Stanford, 2010)

34 PDOS peaks and the localized orbitals: The positive ion lifetime 94 fs The negative ion lifetime 257 fs

35 Geometries of cyclopentene on a Si 9 H 12 cluster: Neutral molecule Positive molecule Negative molecule

36 4. Sumary: Conductance are quantitative agree with experimental data The HC and LC may comes from the geometric configurations of electrodes Hybridization introduces new states into the gap leading to lower threshold voltages New desorption pathways are found in a cyclopentene/silicon system

37 Acknowledgements: Dr. A. Sen, Dr. D. A. Luzhbin N. L. Yoder, R. Jorn, T. Seideman and M. C. Hersam, Northwestern Univ., USA Prof. Y.-H. Tang, Dr. W. S. Su, Dr. K. Bagci, J.-H. Chen, C.-C. Su, S.-T. Pi, E. Chien, S.-T. Lin, C.-H. Hsu, Y.-C Chen, M. Lin, and Prof. C.-S. Tang.

Computational Modeling of Molecular Electronics. Chao-Cheng Kaun

Computational Modeling of Molecular Electronics Chao-Cheng Kaun Research Center for Applied Sciences, Academia Sinica Department of Physics, National Tsing Hua University May 9, 2007 Outline: 1. Introduction