Dateline: August 1, 2001

A major economic motivation for the development of nanotechnology is provided by the electronics industry [1] which steadily miniaturizes [2] and integrates [3] electronic components to produce devices ranging from computer chips [4] to instruments [5]. Nanotechnology naturally seeks to find molecules and aggregates that can perform the basic functions of larger electronic components and integrate (or assemble) them into atomically precise [6] molecular electronic devices [7].

Solid-state electronic logic has the advantage that it is an established industry, although it may not necessarily the best method for molecular scale computation. Entirely new computational architectures that might take advantage of quantum coherence are expected to provide revolutionary computational abilities in the future [8]. As our traditional methods for fabrication of silicon microelectronic devices improve, they are able use less and less electrons per logical operation. Thus, it may not be long until it becomes economically feasible to integrate top-down [9] semiconductor devices with bottom-up [10] molecular electronics, thus paving the way from classical to quantum computation.

Numerous molecules that can function as molecular scale wires [11], switches [12], transistors [13], and much more [7] have been characterized both by molecular simulations [14] and experiments [15]. Most of these molecular components have already been synthesized and characterized with traditional chemical methods, thus providing a supply of components that can be mass produced on the order of Avagadro's number (1 mole = 6*10²³ molecules). So, you might ask: why do our computers still have only millions of transistors per chip? Major obstacles keeping molecular electronics from being feasible today can be divided into two categories: integration and interfacing.

Integration of the components involves molecular assembly, also known as supramolecular chemistry. Rather than patterning two dimensional semiconductor surfaces with doped regions, as is currently staple in silicon-based microelectronics, molecular electronics would have to assemble discrete molecular components into functional circuitry. Although tools such as scanning probe microscopy have demonstrated the possibility of manipulating single molecules, parallel methods of integration such as self-assembly and directed assembly are more promising aproaches [16]. While traditional chemistry and current methods of self-assembly result in imperfect circuitry [17], improvements in nanoscale chemistry are rapidly advancing [18].

Interfacing involves reading and writing to the molecular electronic devices. Being so small, there must be a way for us to communicate with the device by writing our input and reading the output. There are numerous methods that might be used for interfacing molecular electronic devices with our current microtechnology. Optical methods for interfacing seem to be one of the more promising aproaches. For instance, Martini et. al. [19] has demonstrated that electron transfer reactions can be controlled with femtosecond laser pulses. With lasers being one of the more coherent tools for reading and writing, optical interfacing may provide a convenient method for interacting with quantum devices [20].

I find DNA-based electronics to be a model system for molecular electronics [21]. The reason for this choice is manifold. DNA is well characterized & ubiquitous. DNA contains information in the nucleotide sequence. DNA conducts one dimensionally depending on that sequence. Numerous proteins already exist that can process and assemble DNA molecules. In addition to the molecular electronic applications, DNA-based electronics research can also qualify as medical research and thus progress in controlling the assembly of DNA is rapidly advancing [22].

Giese et. al. has helped to decode the sequence dependent electronic properties of DNA, explaining seemingly contradictory observations as a combination of short-distance tunneling and long-distance hopping [23]. With our increasing understanding of single electron transfers within and between individual molecules such as DNA, it is likely only a matter of time before a lucky researcher discovers the right parameters to cheaply integrate moles of addressable molecular circuits.

[1] Electrons: from Microscopy to Lithography
The electron is a most convenient particle (or wave) for microscopy and lithography. Whether they are being focused via e-beams or tunneled through STM tips, without the electron there would be no nanotech.

[2] Moore's Law
An emperical trend in the microelectronics industry for the number of circuits per chip to double roughly every 18 months.

[3] Mechanical VLSI
Over the years, the integration of electronic logic gates has increased from "small-scale integration" (SSI, less than 10 gates) to medium-scale integration (MSI, 10-100 gates) to large-scale integration (LSI, 100-5000 gates) to today's very-large scale integration (VLSI, with more than 5000 gates).

[4] Semiconductor Fabrication
Computer chips (and the silicon based transistors within them) are rapidly shrinking according to a predictable formula. Simple shrinking of circuits works for now, but novel methods for chip fabrication will eventually be needed.

[5] Seeing Things Smaller than Light
Scanning Probe Microscopy (SPM) is a recently developed family of techniques that can be used to produce images of nanoscale surfaces with resolution reaching down to the sub-angstrom level.

[6] There's Plenty of Room at the Bottom
'Another thing we will notice is that, if we go down far enough, all of our devices can be mass produced so that they are absolutely perfect copies of one another. We cannot build two large machines so that the dimensions are exactly the same. But if your machine is only 100 atoms high, you only have to get it correct to one-half of one percent to make sure the other machine is exactly the same size---namely, 100 atoms high!' - Richard P. Feynman

[7] C. Joachim, J. K. Gimzewski, A. Aviram, Electronics using hybrid-molecular and mono-molecular devices (2000) Nature 408, 541 - 548.

[8] Quantum Computer - A computer that exploits quantum mechanical phenomena such as superposition and entanglement.

[9] Top-Down - Molding, carving and fabricating small materials and components by using larger objects such as our hands, tools and lasers.

[10] Bottom-Up - Building (or designing) larger, more complex objects by integration of smaller building blocks or components.

[11] Nanotechnology with Carbon Nanotubes - Columns, pipes, bearings and springs are important for architecture, plumbing and machinery. Carbon nanotubes are molecular cylinders that are rapidly extending our ability to manufacture molecular-scale devices.

[12] DNA Mechanics - Several methods are available for using DNA a nanomechanical control system. The latest research into the mechanical properties movement of DNA is discussed with references, in particular, in vivo implications of supercoiling as an ionic switch.

[13] Nanotube Electronic Devices - Energy gaps, symmetry breaking, molecular transistors and constructive deconstruction are a few of the issues involved in nanotube electronics. Recent research has increased our understanding of one-dimensional wires.

[14] Molecular Simulation - Computer models of atoms, molecules and nanostructures provide the theory behind nanoscience.

[15] Superconductivity in DNA - It insulates, it conducts, it superconducts. This molecule does it all! Experiments have demonstrated that DNA exhibits rare superconducting properties similar to those of carbon nanotubes.

[16] Interdisciplinary Assembly: Nanomachines vs Entropy - The mechanism behind self-assembly is governed by thermodynamics, that is the assembled state is of lower energy than the unassembled state, and the pathway from the starting material to the assembled product is "downhill" on the energy landscape.

[17] J. R. Heath, P. J. Kuekes, G. S. Snider, R. S. Williams, A Defect-Tolerant Computer Architecture: Opportunities for Nanotechnology, (1998) Science, 280, 5370, 1716-1721.

[18] Nanoscale Chemistry - While chemistry deals with molecules in a statistical sense, nanotechnology deals with them as discrete entities, each requiring special attention.

[19] I. B. Martini, E. R. Barthel, B. J. Schwartz, Optical Control of Electrons During Electron Transfer (2001) Science, 293, 5529, pp. 462-465.

[20] Near Field Optics - Optical tunneling has been used to surpass two theoretical limits of light, namely the "speed limit" and the "far-field diffraction limit." Here is a report on the current state of near field optics as presented at NFO-6.

[21] DNA Mediated Energy Transfer - The ability of DNA to transfer electrons through the center of the double helix has been established by recent experiments. As biologists figure out what this means for medicine, nanoscientists are using DNA to assemble nanoelectronic devices.

[22] C. Mao, T. H. LaBean, J. H. Relf and N. C. Seeman, Logical computation using algorithmic self-assembly of DNA tripple-crossover molecules (2000) Nature 407, 493 - 496. Abstract