1. Towards Single Molecule Electronics
Can a single molecule behave like a diode, transistor (switch), memory ?
If that’s possible, how long will the molecule last ?
First, let’s look at many molecules acting in parallel.
Nitzan and Ratner, Science 300, 1384 (2003); Heath and Ratner, Physics Today, May 2003, p. 43

2. HP Molecular Memory
40 nm line width, 40 Gbit/inch2

3. Crossbar Memory Architecture
Molecular Memory
DRAM
Output: Stored Data
Input: Address
Crossbar Memory Architecture
MRAM
(Magnetic Random Access Memory) 1
A crossbar memory consists of a matrix, where the rows carry the input signal containing the address of a stored bit and the columns carry the output signal with the content of the bit (1 or 0). Row and column number uniquely identify the location of the bit. In a standard silicon DRAM (Dynamic Random Access memory), the input signal opens the gate, such that the charge stored in the capacitor flows out through the channel into the column line.
In a molecular memory and a MRAM, the bit is not stored as charge. Instead, the resistance of the junction point (either high or low) defines a 1 and 0.

4. HP Molecular Memory
Rotaxane molecules switch between high and low resis- tance by receiving a voltage pulse.
The blue ring can shuttle back and forth along the axis of the rotaxane molecule, between the green and red groups.

5. Collier et al., Science 289, 1172 (2000).
HP Molecular Memory
Change the resistance between low and high by voltage pulses.
Is the resistance change really due to the rotaxane ring shuttling back and forth? Other molecules exhibit the same kind of switching.
One possible model is the creation and dissolution of metal filaments which create a short between the top and bottom electrodes. (Some-thing like that happens in batteries).
(Many Molecules)
Collier et al., Science 289, 1172 (2000).

6. Other Molecular Switches
Large On-Off Ratios
Chen et al., Science 286, 1550 (1999)

7. Robert F. Service, Science 302, 556 (2003).

8. Synthesis of a Rotaxane Molecule
Amabilino and Stoddart, Chemical Reviews 95, 2725 (1995).

9. Some Fancy Molecules Rotaxane Catenane Pretzelane Handcuffcatenane
Personally, I prefer a pretzelane over a handcuffcatenane.
Catenane Pretzelane Handcuffcatenane

10. Data Storage via the Oxidation State of a Molecule
Electrochemistry

11. Self-Organizing Memory + Data Processor
People have been thinking about how to combine memory with logic (= a microprocessor) in a molecular device.
Self-assembly is the preferred method. It generates errors, though. They need to be absorbed by a fault-tolerant architecture (e.g. in the HP Teramac)
Quantum
Dot
Molecular Switch
Heath et al., Science 280, 1716 (1998)

12. Local Hotspots Appear after Switching
The latest from HP on how molecules switch.
Miao et al., Phys. Rev. Lett. 101, (2008)

13. “Conductivity”
of DNA
Tunneling at short distances (independent of temperature)
Hopping at large distances (thermally activated)
Berlin et al., Chem. Phys. 275, 61 (2002)

14. Using a Single Molecule

15. Using a Single Molecule
Coulomb Blockade Magnetic (Kondo) Resonance
at the Fermi level (zero voltage)
Park et al., Nature 417, 722 (2002)

16. Using a Few Molecules
Observe tunneling through 1, 2, 3, 4, 5 alkanethiol molecules
Cui et al., Science 294, 571 (2001)

17. A Molecular Transistor
(a) Structures of the long and short linked cobalt coordinated terpyridine thiols used as gate molecules. (b) A topographic AFM image of the gold electrodes with a gap. (c) A schematic representation of the assembled single atom transistor.

18. Break Junctions
At the beginning of single molecule electronics, break junctions were very popular: Just crack a thin Au wire open in a vice and adjust the width of the crack with piezos (as in STM). Then pour a solution of molecules over it.
Alternatively, one can burn out the thinnest spot of a thin Au wire by running a high current density through it (using the effect of electromigration).
These days, many try to achieve a well-defined geometry using a STM or AFM, with a well-defined atom at the end of the tip and another well-defined atom at the surface as con-tacts to a single molecule.
A schematic representation of Reed and Tour’s molecular junction containing a benzene-1,4-dithiolate SAM that bridges two proximal gold electrodes.

19. Conductance through a C60 Molecule
Distance dependence tells whether it is tunneling (exponential decay) or quantum conductance through a single or multiple orbitals (G0).
Kröger et al., J. Phys. Condend. Matter 20, (2008)

20. Many Ideas for Single Molecule Devices
Heath and Ratner, Physics Today, May 2003, p. 43