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Nanotechnology

(Mark Andrews, Roland Bennewitz, Aashish Clerk, Peter Grutter, Hong Guo, Michael Hilke, Andrew Kirk, David Plant, and Mark Sutton)

Our current understanding of physical principles operating at the nanoscale is having a major impact in the elucidation of novel phenomena in materials, and in the development of new devices that take advantage of properties that are unique to this scale. Research at MIAM ranges from fundamental studies of material properties at the nanoscale and in reduced dimensionality, to applications in optoelectronics, photonics, biosensors, and electronics.

A natural frontier concerns the study of physical properties at the limit of miniaturization: the atomic scale. This refers to either the direct manipulation of single atoms, or the use of artificial atoms obtained by confining a few electrons in a metallic or semiconducting structure. For example, recent studies have shown the existence of new elementary quasi particles in the two dimensional electron gas (confined in a semiconducting heterostructure). They have an effective fractional electric charge and obey fractional statistics; both properties derive from their character as composite Fermions [M. Hilke, ”Noninteracting electrons and the metal-insulator transition in two dimensions with correlated impurities”. Phys. Rev. Lett. 91, 226403 (2003)]. By reducing the dimension even further to one dimension, strong electron-electron interactions prevent the description of the electron gas as a Fermi gas or a Fermi liquid, and it becomes a so called Luttinger liquid instead. One of the most surprising properties of a Luttinger liquid is the separation of charge and spin, i.e., the charge and the spin become independent of each other. Research at MIAM concerns novel electronic properties that appear when electron-electron interactions, disorder, and quantum phase coherence are all simultaneously relevant in systems of reduced dimensions and dimensionalities.

Leading edge research in Nanotechnology cannot be carried out at present without simultaneously pushing the limits of available instrumentation. A vigorous program of research at MIAM concerns the development of instrumentation that can operate at the absolute limits imposed by nature. As an example, an instrument that would allow one to directly detect the spin of a single proton is currently under development. The device would find application in the field of quantum measurement and also open the door to direct structure determination of complex organic molecules by performing NMR with single atomic resolution and sensitivity. A second example concerns atomic force microscopy to detect charging of quantum dots at the single electron level [R. Stomp, H. Guo, P. Grutter, et al., “Detection of single-electron charging in an individual InAs quantum dot by noncontact atomic-force microscopy”. Phys. Rev. Lett. 94, 056802 (2005)]. As a third example, we mention high intensity X-ray synchrotron sources to measure in situ, time resolved, X-ray diffraction with a time resolution of a few milliseconds. This affords a detailed analysis of the temporal evolution of structure in systems far from equilibrium [A. Fluerasu, M. Sutton, et al., “X-ray intensity fluctuation spectroscopy studies on phase-ordering systems”. Phys. Rev. Lett. 94, 055501 (2005)]. More in the immediate future, the tools and techniques are being utilized in research in Nanoelectronics, both in terms of the development of new, molecular based, components, the design of new logic and switching units, and their integration with nanoscale contacts and circuitry. Large challenges remain in the elucidation of charge transport at the molecular level, dissipation, and the integration into functional circuits [R. Stomp, P. Grutter, et al., “Detection of single-electron charging in an individual InAs quantum dot by noncontact atomic-force microscopy”. Phys. Rev. Lett. 94, 056802 (2005)].

Because of the smallness of the systems considered, and the simplicity of the basic building blocks, there has been a large degree of convergence between experimentation in Nanoelectronics and theoretical modeling of both nanoscopic transport and device operation. However, for example, one notes that current models and experimental measurement of the conduction of a simple molecule such as C60 only agree to within a factor of 10. The discrepancy is likely to be due to the unknown atomic structure of the contact leads (necessary as input to the simulation) and perhaps due to incomplete modeling. At MIAM, theoretical research focuses on the development of first principles, computational descriptions of transport at the molecular scale. At this scale, averaging implicit in the description of transport in macroscopic systems is not relevant, and one needs to develop a new paradigm to describe conduction through single molecules and their coupling to the environment, all under conditions of strict nonequilibrium. This is currently being explored by a combination of density functional theory with Keldysh nonequilibrium Green's functions [J. Taylor, H. Guo et al., “Ab initio modeling of quantum transport properties of molecular electronic devices”. Phys. Rev. B 63, 245407 (2001)].

Theoretical research is also conducted on the mesoscopic range, intermediate between the microscopic and macroscopic scales. One considers electronic systems that are much larger than the size of a typical atom, but are nonetheless strongly influenced by non-classical (quantum mechanical) effects. Such effects directly follow from the behavior of particles on the atomic or sub-atomic length scale but manifest themselves on a much larger length scale. Often there is a challenging interplay among quantum effects, the presence of disorder, and inter particle interactions. Examples of mesoscopic systems currently under investigation include carbon nanotubes and micron-sized superconducting metallic grains, both relevant as current microelectronic technology pushes towards smaller and smaller devices. Among them, a particularly intriguing application concerns the direct exploitation of the quantum nature of mesoscopic systems for information processing, perhaps leading to the creation of a solid-state quantum computer. A crucial challenge in this case that is under active investigation understanding and controlling mesoscopic electron noise. The existence of noise affects the ability to read out information from these systems, and can result in a complete loss of the quantum behavior, the main motivation for the development of such a system [A. Clerk and S. Girvin, “Shot noise of a tunnel junction displacement detector”. Phys. Rev. B 70, 121303 (2004)].

A completely different, but complementary research avenue which is central to the development of nanotechnology concerns the field of nanomechanics: The mechanical response and the nature of friction and dissipation at the nanoscale. Generic issues such as lubrication, friction, and wear are well understood at the macroscopic scale, but they acquire special relevance for the design, maintenance, and operation of nanoscale devices. Macroscopic friction is statistical in nature, and governed by the formation, distortion, and rupture of a very large number of very small, nanometre-scale contacts. Scanning force microscopy, which can image surfaces down to their atomic structure and can detect forces that are one hundred times smaller than those forces that bind atoms into solids, is being used to study friction at the atomic scale. Therefore these studies will yield detailed information on the microscopic mechanisms leading to friction, and how deviations from macroscopic average behavior emerge at the nanoscale when only a small number of microscopic contacts may be involved in friction forces [A. Socoliuc, R. Bennewitz, et al., “Transition from stick-slip to continuous sliding in atomic friction: Entering a new regime of ultralow friction”. Phys. Rev. Lett. 92, 134301 (2004)].

Beyond the studies just described that center on the electronic or mechanical properties of materials at the nanoscale, one finds research at the intersection of electronics and optics (optoelectronics) to take advantage of matter-radiation interaction at the nanoscale, and bio-photonics, the convergence of radiation and biological systems for nanoscale applications. Polymers are currently being considered to produce integrated optics components and devices such as biophotonic sensors, amplifiers, gratings, multiplexers and other structures that may be useful for optical computing [D. Blanc, M. Andrews, et al., “Self-processing of surface-relief gratings in photosensitive hybrid sol-gel glasses”. Advanced Materials 11, 1508 (1999)]. Guided wave optics are used in the investigation of optical self writing in which an optical wave inscribes its own waveguide into the host medium allowing one to probe photoinduced anisotropy, optical self-trapping, self-lensing, spatial soliton formation, and optical induction of chirality in isotropic polymers [K. Saravanamuttu and M. Andrews, “Visible laser self-focusing in hybrid glass planar waveguides”. Opt. Lett. 27, 1342 (2002)]. Biopolymers and photo-imaging glasses are synthesized from organic and inorganic polymers to build micron sized optical circuit to create an “optical chemical bench” on a chip (OCB), similar to “lab on a chip” developed in the biotechnology world, except that in this case the polymer is used to make the optical circuit that ends up interrogating itself with guided laser light.

Once characteristic scale and diffractive features become smaller than the wavelength of light, new effects and possibilities emerge. The use of sub wavelength structured materials is being investigated in a variety of contexts, including their incorporation into planar waveguides, photonic bandgap devices for routing and demultiplexing, the use of strong and deep gratings for vertical waveguide integration, and the use of sub-wavelength materials in order to improve the performance of free space micro optical systems. By achieving material control below the wavelength of light, the underlying theme of the research becomes that of wavefront engineering: the manipulation of optical wavefronts in order to achieve desired outcomes. In some cases this requires the use of refractive micro-optical elements (such as a microlenses), in others one uses diffractive elements (for example computer generated holograms), sub-wavelength structured elements (zero-order gratings) and photonic bandgap structures. Often, one also interfaces these systems with micro-electromechanical systems (MEMS) to steer and control light, and designs, for example, MEMS elements such as micro-mirrors, actuators, or micro-motors [J. Wen, A. Kirk, et al., “Analysis of the performance of a MEMS micromirror”, IEEE Trans. Magnetics 40, 1410 (2004)]. Research also focuses on developing technologies for direct application in telecommunications, sensing, and biology. Activities include, for example, the design and fabricating of optoelectronic-VLSI circuits, technologies that combine optical inputs/outputs with high speed processing electronics, high speed electro-optic switching devices for all photonic networks, and distributed optical and electrical clocking systems for high speed switching and computing systems [M. Venditti, D. Plant, et al., “Design and test of an optoelectronic-VLSI chip with 540-element receiver-transmitter arrays using differential optical signaling”. IEEE J. of Selected Topics in Quantum Electronics 9, 361 (2003)].

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