conductance of single molecular junctions
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Conductance of Single Molecular Junctions. Chao-Cheng Kaun 關肇正 Research Center for Applied Sciences, Academia Sinica Department of Physics, National Tsing Hua Universit. March 30, 2008. Outline:. Introduction Why molecular electronics? Comparison with experiments - PowerPoint PPT PresentationTRANSCRIPT
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Conductance of Single Molecular Junctions
Chao-Cheng Kaun 關肇正
Research Center for Applied Sciences,Academia Sinica
Department of Physics, National Tsing Hua Universit
March 30, 2008
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Outline:
1. Introduction
Why molecular electronics?
2. Comparison with experiments
Alkanethiol molecules
3. What is the single-molecule conductance?
An alkanedithiol molecule
4. Spontaneous oscillation of current
A C60 molecule
5. Summary
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Human hair Cells Transistors in Integrated Circuits
BiologicalMacromolecules
Atoms andmolecules
10 m 1 m 100 nm 10 nm 1 nm100 m
Nanotechnology: works at the atomic, molecular and supra-molecular levels, at the 0.1 – 100 nm scale, with fundamentally
new properties.
1. Introduction:
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What’s the problem?
Physical limit:Diffraction of light.
Economical limitation:Too expensive.
45 nm now
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Molecular electronics: A solution 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.
For example ..
Aviram & Ratner, (1974).
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But, there is a big problem:
Some experimentsSome experiments
J.G. Kushmerick NanoLetters ‘03
D. Stewart NanoLetters ‘04
H.B. Weber APL ‘03
S.M. Lindsay Science ‘03
Except….
Most experimental data can not be reproduced by other groups!
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A SAM measurement: Alkanethiol molecular wires. Wold and Frisbie, JACS 123, 5549 (2001)
Rather similar results from other groups: M. Reed et al (2003); Lindsay et al, Nanotechnology, 13, 5 (2002).
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Can we simulate these experimental data from first principles?
Dirac Schrödinger
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A quantum mechanical theory used in physics and chemistry to investigate the electronic structure of many-body systems, in particular atoms, molecules, and the condensed phases.
Density Functional Theory (DFT):
Chemistry 1998
Walter Kohn John A. Pople
University of California Santa Barbara
Northwestern University
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Conventional DFT solves two kinds of problems:
Finite isolated system
Gaussian-03
Periodic systems
VASP
A device is neither finite nor periodic, and is in non-equilibrium
Quantum transport:
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500 times of difference!
Previous modeling:
Science 278, 252 (1997)
PRL 84, 979 (2000)
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How to calculate current?
dEVETh
eVI ff
rlbb
),(2
)(2
DFT plus non-equilibrium Green’s functions: Taylor, Guo, Wang, PRB 63, 245407(2001)-----McGill-Device-CALculator (McDCAL); Brandbyge, et al, PRB 65, 165401(2002)---Transiesta.
Our method:
Landauer formula:
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• Density Functional Theory• LCAO• Pseudopotentials
Electronic structure
Nonequilibrium physics• Full description of electrodes using ab initio self-energies• Non-equilibrium electron distribution using NEGF
HH
Interaction region
Bulk region
Bulk region
Computational modeling
• Calculation of electron current
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Our model:
Au electrodes
Al electrodes
2. Comparison with experiments: Alkanethiol molecules
Kaun & Guo, Nano Lett. 3, 1521 (2003)
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Experimental: average slope (beta) is close to 1
Quantitative agreement with measurements
Slope: ~1.0
Theory
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From alkanethiol to alkanedithiol
• Our calculation: still shows ;
• Our calculated beta is still about 1.0;
• Our is smaller than that of alkanethiol by about a factor of 18.
)exp( nRR on
oR
Experiments so far:
1. Cui et al, J. Chem. Phys. 106, 8069 (2002):
2. Engellkes et al (Frisbie lab) (2003):
57.0
05.1Lee and Reed, J. Phys. Chem (2004):
Xu and Tao, Science (2003):
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3. What is the single-molecule conductance?
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Nature 395, 780 (1998)
Conductance of a Au nanowire:
Nano Lett. 6, 2362 (2006)
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Conductance of a single molecule
J. Tao et al, Science (2003)
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J. Tao et al, JACS (2003); Science (2003)
New measurement on single alkanedithiol molecule
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Previous modeling:
Calculation Experiment
N = 6 G = 0.0025 0.0012
Unit: G0
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Our model:
N = 6 G = 0.0010 0.0012
Unit: G0
Calculation Experiment
N = 8 G = 0.000 13 0.000 25
N = 10 G = 0.000 02 0.000 02
s
s
Kaun & Seideman, Phys. Rev. B 77, 033414 (2008)
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Au surface states and Au-S hybridization (from lead s, pz band)
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4. Spontaneous oscillation of current
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H. Park, et al, Nature (2000)
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The bouncing Bucky ball
H. Park, et al, Nature (2000)
Predictions from calculations
T. Seideman, et al, Chem. Phys. (2002)
Current-driven oscillations:
<Z> the lifetime of resonance
f the C60 mass
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Asymmetric coupling
( L = 26.42 a.u.)
Symmetric coupling
Our model:
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Different locations
Three channels
Transmission spectra:
One induces the motion; the other probes it.
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Current oscillates as the molecule vibrates
Kaun and Seideman, PRL 94, 226801 (2005)
The ac/dc ratio, the power output efficiency, is 0.26 ( L = 26.42 a.u.)
When L = 25.42 a.u., the ratio is 0.07
Only a range of L permits both a large ratio and high average conductance
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Applications:
• A nanoscale generator of a radiation field, thus a THz optoelectronic device.
• A miniature mass spectrometry.
• The direct, time-domain probing of the current-driven dynamics in nanojunctions.
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Experimentally …. Nanotube radio
A. Zettl et al, Nano Lett. 7, 3508 (2007)
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5. Sumary:
• Conductance are quantitative consistent to experimental data
• Contacts play important roles
• Au-S hybridization states dominates the conduction
• Current-driven dynamics can be used to produce oscillating current in molecular junctions
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Acknowledgements:
• Prof. Tamar Seideman Northwestern Univ., USA
• Prof. Hong Guo McGill Univ., Canada
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Alkane has a large HOMO-LUMO gap, ~10eV. The Fermi level is inside the gap, but closer to HOMO.
There is a tiny feature near Fermi level which determines the resistance.
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Contact effect (N=6):
a
b
c
dxy
pz