Scientists from Lucent Bell Labs, building upon their recent breakthrough in molecular-scale transistors, have now fabricated an individually addressable transistor whose channel consists of just one molecule, a feat never previously accomplished.
Last month, the same Bell Labs team – physicist Hendrik Schon and chemists Zhenan Bao and Hong Meng – unveiled a transistor with a single-molecule channel length. But that device could only be fabricated as a matrix of a few thousand molecules that worked in tandem. Now, in a major advance, the team has succeeded in fabricating molecular-scale transistors that can be individually controlled.
The main challenges in making nanotransistors are fabricating electrodes that are separated by only a few molecules and attaching electrical contacts to the tiny devices. The Bell Labs researchers were able to overcome these hurdles by using a self-assembly technique and a clever design.
They carved a notch into a silicon wafer and deposited a layer of gold at the bottom to function as one of the transistor’s three electrodes. Then they dipped the wafer into a solution that contained a mixture of thiol molecules and some inert organic molecules, and let it dry. The purpose of adding the inert molecules was to dilute the concentration of thiols. As the solution evaporated from the wafer, a film exactly one molecule thick was left behind on the gold electrode.
By carefully adjusting the ratio of the thiol to the inert molecules, the scientists were able to statistically ensure that just one active molecule was present in the area on top of the gold electrode. They then deposited another gold electrode on top of this film, while they built the transistor’s third electrode on one side of the silicon notch.
‘It is virtually impossible to attach three electrodes to a microscopically small molecule,’ said Bao. ‘We overcame this problem by letting the molecule find these contacts and attach itself to them, a process called ‘self-assembly.’ ‘
The chemical self-assembly technique is relatively easy and inexpensive and, unlike silicon, does not require clean room technology.
‘Our experiment shows that it is possible to realize transistor action in a single molecule without sophisticated fabrication procedures,’ said Schon.
Using two nanotransistors, the Bell Labs scientists built a voltage inverter, a standard electronic circuit module commonly used in computer chips that converts a ‘0’ to a ‘1’ or vice versa. Though just a prototype, the success of this simple circuit suggests that nanotransistors could one day be used in microprocessors and memory chips, squeezing thousands of times as many transistors onto each chip than is possible today.
David Goldhaber-Gordon, a professor at Stanford University, commented that the Bell Labs scientists ‘have achieved several impressive advances toward nanoelectronics. The fabrication technique is particularly elegant in its simplicity.’