32
Molecular electronic devices
Molecular electronic devices
Break junction measurements of benzene-1,4-dithiol
Mark Reed et al., Science 278 (1997) p. 252.
Evidence that the method is valid• Many devices give the same IV curve.• Controls with no thiol molecule behave differently.
Current-voltage characteristics of benzene-1,4-dithiol
Mark Reed et al., Science 278 (1997) p. 252.
..O V
..2 V
Voltage (V)
..4.4 Vπ*
π
There is noNDR because of the continuumof gold states.
First-principles calculation of transport properties
M. Di Ventra, S.T. Pantelides, N.D. Lang, Phys. Rev. Lett. 84 (2000) p. 979.
The electron wavefunctions and energylevels were determined by solving the Lippman-Schwinger equation iteratively to self-consistency in steady state. This method uses a local density approximation.
The rates of tunneling from the electrodes into the molecular states were determined.
Comparison of experimental data to theory
Experimental data(Reed et al.)
Theory(DiVentri, Pantelides, Lang)
The discrepancies were attributed to the lack of knowledge on how the molecules attach to the gold and the limitations of the model.
Another interpretation of the data
E.G. Emberly, G. Kirezenow, Phys. Rev. B 64 (2001) p. 235412.
IV curves were calculated forthe 3 cases shown to the left.
The bottom one agrees quantitatively with Reed’s data. This isn’t surprising since both electrodes are covered with a SAM before they come together.
Their model can be tested by repeating the experiment with monothiol benzene.
S in hollow siteof Au
S on top of Au
Overlapping molecules
UCLA/HP Labs collaboration
Integrated circuits will molecular scale devices will almost certainly have to be made by self-assembly.
We can imagine making symmetric structures like those shown on the left.
A. Pease, J.F. Stoddart, J.R. Heath et al., Acc. Chem. Res. 34 (2001) p. 433.
Cross bars and redundant architecture
Heath, Kuekes, Williams et al. “A Defect-Tolerant Computer Architecture: Opportunities for Nanotechnology” Science 280 (1998) p. 1716.
Self-assembly usually produces symmetric structures. A circuit can be programmed into a symmetric array after it is assembled.
Since self-assembled circuits will have defects, the architecture must be redundant.
Self-assembled nanotube array
M.R. Diehl, J.R. Heath et al. Angew. Chem. Int. Ed. 41 (2002) p. 353.
They used carbon nanotubes that were shorter than 0.5 µm in length.
They aligned the tubes in an electric field, let them stick to the substrate, rotated the sample and repeated the process.
Scale
500 nm
500 nm
1 µm
metaldeposition
wiredeposition
10 nmepoxy
releaseetch
AlGaAs
GaAs
etch
O2Plasma
Superlattice Superlattice NAnoPatterningNAnoPatterning: SNAP: SNAP
Nick Melosh, James Heath et al. To be published soon
2 ××××1010
1010
4 ××××1010
1.6 ××××1011
6 ××××1011
Two terminal switches
It is clearly easier to attach two contacts to a molecule than three. For thisreason, Heath, Stoddart and coworkers have developed molecules whoseconductivity can be switched by applying a high voltage. The devices areread at low voltage.
Bistable [2]catenanes
Charles Collier, J. Fraser Stoddart, James Heath et al., Science 289 (2000) p. 1172.
• TTF binds to the tetracationic cyclophane more strongly than the NP. If the TTF group is oxidized, it is repelled by the tetracationic cyclophane and the ring rotates.• Changing the conformation of the molecule changes the electrical properties.
TTF tetracationic cyclophane
NP
Hysteresis
The switch can be opened at +2 V, closed at -2 V, and read between 0.1 and 0.3 V and may be recycled many times.
Charles Collier, J. Fraser Stoddart, James Heath et al., Science 289 (2000) p. 1172.
Making monolayers
A. Pease, J.F. Stoddart, J.R. Heath et al., Acc. Chem. Res. 34 (2001) p. 433.
amphiphilic counterion
The catenanes are dispersed on water in a Langmuir-Blodgett trought. A salt with an amphiphilic ion helps the films to form. The film is transferred to a substrate by dip coating.
Device structure
AuTi electrodePhospholipid counterions (wide energy gap)
[2] CatenanesSiO2 (wide energy gap)
Doped polysilicon electrode
The devices are 7 µm by 10 µm in size. Remarkably, very few of them have shorts.
• I think polysilicon is used because it is very stable compared to metals like gold.• I think the thin SiO2 barrier is important because it slows down tunneling between the Si and the catenane, which enables the molecule to be oxidized or reduced.• The Ti probably damages the film, but this doesn’t seem to prevent the device from working.
Hysteretic current-voltage characteristics
There is slightly more currenton the return scan.
Charles Collier, J. Fraser Stoddart, James Heath et al., Science 289 (2000) p. 1172.
Remnant molecular signature of the device
The write voltage was varied in 40 mV steps. Reading was done at -0.2 V
Charles Collier, J. Fraser Stoddart, James Heath et al., Science 289 (2000) p. 1172.
Switching the device
The device was repeatedly opened at 2 V, tested at 0.1 V, closed at -2 V and tested at 0.1 V.
The junction resistance can be varied by a factor of 4.
Charles Collier, J. Fraser Stoddart, James Heath et al., Science 289 (2000) p. 1172.
Control Samples
The following Langmuir films do not exhibit the behavior just described:
• eicosanoic acid• DMPA (A salt with alkyl tails that is used to help the switchable molecules form a Langmuir film)• the tetracationic cyclophane without the ring that has the TTF and NP• a catenane with a ring that has 2 TTF groups instead of 1 TTF and 1 NP
TTF tetracationic cyclophane
NP
These control experimentssupport the notion thatrotation of the ring changes the IV characteristics.
This subject is still controversial.
Reed and Tour’s switching mechanism
Motivation: NO2 is an electron acceptor. NH2 is an electron donor. Applying an electric field might rotate the center ring, which would reduce the conductivity of the molecule.
Another motivation: If the molecule can be reduced or oxidized, its conductance could change.
J. Chen, M.A. Reed, A.M. Rawlett, J.M. Tour: Science 286 (1999) p. 1550Appl. Phys. Lett 77 (2000) p. 1224.
Negative differential resistance
They claim that the current turns on when the molecule is reduced and turns off when it is reduced for the second time. The exact mechanisms are not yet understood.Hysteris curves have not yet been published.
Science 286 (1999) p. 1550
T = 60 K
Time dependence
M.A. Reed et al. Appl. Phys. Lett. 78 (2001) p. 3735.
The device was “turned on” by applying a high voltage pulse. It was thenprobed over a period of time at a lower “read” voltage. The values of the voltage were not published.
Control experiments
Au-alkanethiolate-Au < 1 pA
Au-SiN-Au < 1 pA
Same molecule without the NH2 and NO2 groups No NDR
Same molecule with just the NO2 side group. NDR observed
I haven’t seen any reports on using a symmetric molecule with two NO2 groups to see how important it is to have a dipole moment.
STM studies of switching molecules
Z.J. Donhauser, D.L. Allara, J.M. Tour, P.S. Weiss et al. Science 292 (2001) p. 2303.
Topographic STM images of 3’, inserted in a dodecanethiol SAM. The vertical scale is 8.5 Å. The images were acquired at a sample bias of -1.0 V and a current of 1.0 pA.
B. The molecule in its “on” state.
D. The molecule in its “off” state.
5 nm
Molecules studied by STM
Z.J. Donhauser, D.L. Allara, J.M. Tour, P.S. Weiss et al. Science 292 (2001) p. 2303.
Switching
A. 150 nm x 150 nm topographic STM image of 2’ inserted into a dodecanethiol SAM.
B. The height versus time for the molecule shown in the box.
Z.J. Donhauser, D.L. Allara, J.M. Tour, P.S. Weiss et al. Science 292 (2001) p. 2303.
Hypothesis
If the switching is a result of a conformational change in the molecule, thenthe structure of the SAM should be important. SAMs with larger defects should enable the molecules to switch faster.
The size of the defects can be varied by changing the amount of time that a SAM is given to form or by annealing it in a vapor of the SAM molecule.
The molecules do switch faster in more “defective” SAMs.
Height distribution for 2’
Z.J. Donhauser, D.L. Allara, J.M. Tour, P.S. Weiss et al. Science 292 (2001) p. 2303.
The on-off ratio appears to be higher in the “5-min” film. This might not be the case. It is possible that they missed many of the “off” molecules in the 24-hr film because they never saw them in the “on” state and didn’t notice them.
Occ
uren
ces
Apparent height (Å)
Conducting moleculesin “24-hour” SAM
Conducting molecules in “5-min” SAM
Switching the molecules with an electric field
A. Image of four 2’ molecules in the “on” state.B. Image just after the tip was placed 2 nm from the surface and held at 4 V. All four molecules were put into the “off” state.C. Image after 74 minutes. One molecule has switched back into the “on” state.
All images were taken with a bias of 1.0 V and a tunneling current of 0.7 pA.
Z.J. Donhauser, D.L. Allara, J.M. Tour, P.S. Weiss et al. Science 292 (2001) p. 2303.
Their conclusions
The local environment has a crucial role in mediating stochastic switching.
It appears that an electric field can be used to change the conformation of the molecules.
They don’t emphasize the role of oxidizing or reducing molecules to switch them.
Z.J. Donhauser, D.L. Allara, J.M. Tour, P.S. Weiss et al. Science 292 (2001) p. 2303.
Summary of Molecular Electronics
• Some very interesting and potentially useful device behavior has been observed.
• The behavior is not yet fully understood. You should be skeptical of some of the interpretations of the data that have been published.
• Most of the results have not been reproduced in independent labs.
• We have a long way to go to figure out how to put large numbers of molecular devices into integrated circuits or memory cells.
