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December 24, 2009

Scientists Create World’s First Molecular Transistor

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Scanning electron microscope image (false color) illustrating a full pattern of the devices. The whole structure was defined on an oxidised Si wafer. The yellow regions show portions of the multi-layered Au electrodes (a thin Au layer with a thickness of ~15 nm; a thick Au layer with a thickness of ~100 nm), and the purple region represents the oxidised Al gate electrode. Au wires broken by the electromigration technique (Fig. 1a, inset) are placed on the top of the bottom-gate electrode. The contact pads to which a connection is made by wire bonding are not visible because they are located far from the active part of the device. Insets display the schematic images of Au-ODT-Au (right) and Au-BDT-Au (left) junctions. This is a conceptual diagram only, as only one molecular junction type at a time can be fabricated with the present process.

A group of scientists has succeeded in creating the first transistor made from a single molecule. The team, which includes researchers from Yale University and the Gwangju Institute of Science and Technology in South Korea, published their findings in the December 24 issue of the journal Nature.

The team, including Mark Reed, the Harold Hodgkinson Professor of Engineering & Applied Science at Yale, showed that a benzene molecule attached to gold contacts could behave just like a silicon transistor.

The researchers were able to manipulate the molecule’s different energy states depending on the voltage they applied to it through the contacts. By manipulating the energy states, they were able to control the current passing through the molecule.




The researchers were able to manipulate the molecule’s different energy states depending on the voltage they applied to it through the contacts. By manipulating the energy states, they were able to control the current passing through the molecule.

“It’s like rolling a ball up and over a hill, where the ball represents electrical current and the height of the hill represents the molecule’s different energy states,” Reed said. “We were able to adjust the height of the hill, allowing current to get through when it was low, and stopping the current when it was high.” In this way, the team was able to use the molecule in much the same way as regular transistors are used.

The work builds on previous research Reed did in the 1990s, which demonstrated that individual molecules could be trapped between electrical contacts. Since then, he and Takhee Lee, a former Yale postdoctoral associate and now a professor at the Gwangju Institute of Science and Technology, developed additional techniques over the years that allowed them to “see” what was happening at the molecular level.

Being able to fabricate the electrical contacts on such small scales, identifying the ideal molecules to use, and figuring out where to place them and how to connect them to the contacts were also key components of the discovery. “There were a lot of technological advances and understanding we built up over many years to make this happen,” Reed said.

There is a lot of interest in using molecules in computer circuits because traditional transistors are not feasible at such small scales. But Reed stressed that this is strictly a scientific breakthrough and that practical applications such as smaller and faster “molecular computers”—if possible at all—are many decades away.

“We’re not about to create the next generation of integrated circuits,” he said. “But after many years of work gearing up to this, we have fulfilled a decade-long quest and shown that molecules can act as transistors.”


Nature: Observation of molecular orbital gating

The control of charge transport in an active electronic device depends intimately on the modulation of the internal charge density by an external node1. For example, a field-effect transistor relies on the gated electrostatic modulation of the channel charge produced by changing the relative position of the conduction and valence bands with respect to the electrodes. In molecular-scale devices a longstanding challenge has been to create a true three-terminal device that operates in this manner (that is, by modifying orbital energy). Here we report the observation of such a solid-state molecular device, in which transport current is directly modulated by an external gate voltage. Resonance-enhanced coupling to the nearest molecular orbital is revealed by electron tunnelling spectroscopy, demonstrating direct molecular orbital gating in an electronic device. Our findings demonstrate that true molecular transistors can be created, and so enhance the prospects for molecularly engineered electronic devices.


25 page pdf with supplemental information

1. Device fabrication and characterization
2. Transfer characteristics of a 1,4-benzenedithiol molecular transistor
3. Low temperature transport and IET spectroscopy measurements
4. Experimental estimation of tunnelling barrier height in molecular junctions
5. LUMO-mediated electron tunnelling through Au-BDCN-Au junction
6. Vibrational mode assignments in IET spectra
7. Linewidth broadening in IET spectra of Au-BDT-Au junction
8. Projected density of states near HOMO levels of phenyl and alkyl molecules
9. Theoretical model on resonantly enhanced IET spectra



Engineers adjusted the voltage applied via gold contacts to a benzene molecule, allowing them to raise and lower the molecule’s energy states and demonstrate that it could be used exactly like a traditional transistor at the molecular level. Credit: Hyunwook Song and Takhee Lee


















a, Representative I(V) curves measured at 4.2 K for different values of VG. Inset, the device structure and schematic. S, source; D, drain; G, gate. Scale bar, 100 nm. b, Fowler–Nordheim plots corresponding to the I(V) curves in a, exhibiting the transition from direct to Fowler–Nordheim tunnelling with a clear gate dependence. The plots are offset vertically for clarity. The arrows indicate the boundaries between transport regimes (corresponding to Vtrans). c, Linear scaling of Vtrans in terms of VG. The error bars denote the s.d. of individual measurements for several devices and the solid line represents a linear fit. Inset, the schematic of the energy band for HOMO-mediated hole tunnelling, where eVG,eff describes the actual amount of molecular orbital shift produced by gating. d, Two-dimensional colour map of dln(I/V2)/d(1/V) (from Fowler–Nordheim plots). Energy-band diagrams corresponding to four different regions (points A–D) are also shown. FN, Fowler–Nordheim tunnelling; DT, direct tunnelling.

Editor's summary

The ultimate in electronic device miniaturization would be the creation of circuit elements consisting of an individual molecule. A single-molecule transistor exploiting the electrostatic modulation of a molecule's orbital energy is a theoretical possibility. Now Hyunwook Song and colleagues report the successful realization of such a device, a proof of concept that should enhance the practical prospects for molecularly engineered electronics


View of James Kushmerick, National Institute of Standards and Technology

Transistors have been made from single molecules, where the flow of electrons is controlled by modulating the energy of the molecular orbitals. Insight from such systems could aid the development of future electronic devices.

Transistors, the fundamental elements of integrated circuits, control the flow of current between two electrodes (the source and drain electrodes) by modifying the voltage applied at a third electrode (the gate electrode). As manufacturers compete to produce ever smaller devices, one logical limit to circuit miniaturization is transistors whose channels are defined by a single molecule...




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