This report describes important future research directions in nanoscale science, engineering and technology. It was prepared in connection with an anticipated national research initiative on nanotechnology for the twenty–first century. The research directions described are not expected to be inclusive but illustrate the wide range of research opportunities and challenges that could be undertaken through the national laboratories and their major national scientific user facilities with the support of universities and industry.
The principal missions of the Department of Energy (DOE) in Energy, Defense, and Environment will benefit greatly from future developments in nanoscale science, engineering and technology. For example, nanoscale synthesis and assembly methods will result in significant improvements in solar energy conversion; more energy-efficient lighting; stronger, lighter materials that will improve efficiency in transportation; greatly improved chemical and biological sensing; use of low-energy chemical pathways to break down toxic substances for environmental remediation and restoration; and better sensors and controls to increase efficiency in manufacturing.
The DOE's Office of Science has a strong focus on nanoscience discovery, the development of fundamental scientific understanding, and the conversion of these into useful technological solutions. A key challenge in nanoscience is to understand how deliberate tailoring of materials on the nanoscale can lead to novel and enhanced functionalities. The DOE National Laboratories are already making a broad range of contributions in this area. The enhanced properties of nanocrystals for novel catalysts, tailored light emission and propagation, and supercapacitors are being explored, as are hierachical nanocomposite structures for chemical separations, adaptive/responsive behavior and impurity gettering. Nanocrystals and layered structures offer unique opportunities for tailoring the optical, magnetic, electronic, mechanical and chemical properties of materials. The Laboratories are currently synthesizing layered structures for electronics/photonics, novel magnets and surfaces with tailored hardness. This report supplies numerous other examples of new properties and functionalities that can be achieved through nanoscale materials control. These include:• Nanoscale layered materials that can yield a four-fold increase in the performance of permanent magnets• Addition of aluminum oxide nanoparticles that converts aluminum metal into a material with wear resistance equal to that of the best bearing steel• New optical properties achieved by fabricating photonic band gap superlattices to guide and switch optical signals with nearly 100% transmission, in very compact architectures• Layered quantum well structures to produce highly efficient, low-power light sources and photovoltaic cells• Novel optical properties of semiconducting nanocrystals that are used to label and track molecular processes in living cells • Novel chemical properties of nanocrystals that show promise as photocatalysts to speed the breakdown of toxic wastes • Meso-porous inorganic hosts with self-assembled organic monolayers that are used to trap and remove heavy metals from the environment• Meso-porous structures integrated with micromachined components that are used to produce high-sensitivity and highly selective chip-based detectors of chemical warfare agentsThese and other nanostructures are already recognized as likely key components of 21st century optical communications, printing, computing, chemical sensing and energy conversion technologies.
The DOE is well prepared to make major contributions to developing nanoscale scientific understanding, and ultimately nanotechnology, through its materials characterization, synthesis, in situ diagnostic and computing capabilities. The DOE and its National Laboratories maintain a large array of major national user facilities that are ideally suited to nanoscience discovery and to developing a fundamental understanding of nanoscale processes. Synchrotron and neutron sources provide exquisite energy control of radiation sources that are able to probe structure and properties on length scales ranging from Ångstroms to millimeters. Scanning Probe Microscope (SPM) and Electron Microscopy facilities provide unique capabilities for characterizing nanoscale materials and diagnosing processes. DOE also maintains synthesis and prototype manufacturing centers where fundamental and applied research, technology development and prototype fabrication can be pursued simultaneously. Finally, the large computational facilities at the DOE National Laboratories can be key contributors in nanoscience discovery, modeling and understanding.
In order to increase the impact of major DOE facilities on the national nanoscience and technology initiative, it is proposed to establish several new Nanomaterials Research Centers. These Centers are intended to exploit and be associated with existing radiation sources and materials characterization and diagnostic facilities at DOE National Laboratories. Each Center would focus on a different area of nanoscale research, such as materials derived from or inspired by nature; hard and crystalline materials, including the structure of macromolecules; magnetic and soft materials, including polymers and ordered structures in fluids; and nanotechnology integration. The Nanomaterials Research Centers will facilitate interdisciplinary research and provide an environment where students, faculty, industrial researchers and national laboratory staff can work together to rapidly advance nanoscience discovery and its application to nanotechnology. Establishment of these Centers will permit focusing DOE resources on the most important nanoscale science questions and technology needs, and will ensure strong coupling with the national nanoscience initiative. The synergy of these DOE assets in partnership with universities and industry will provide the best opportunity for nanoscience discoveries to be converted rapidly into technological advances that will meet a variety of national needs and enable the United States to reap the benefits of a technological revolution.
This report describes important future research directions in nanoscale science, engineering and technology. It was prepared by members of the Office of Basic Energy Sciences (BES) Nanoscience/Nanotechnology (N/N) Group  in connection with an anticipated interagency national research initiative titled "Nanotechnology for the Twenty–First Century: Toward a New Industrial Revolution" .
The national initiative, described in the report of the Interagency Working Group on Nano Science, Engineering and Technology (IWGN) , identifies five "high priority" research areas for additional funding beginning in FY2001. The first and largest of these is "long–term science and engineering research leading to new fundamental understanding and discoveries of phenomena, processes and tools for nanotechnology". The IWGN report identifies the major DOE interest in nanotechnology as basic energy science and engineering. It also mentions several modes of R&D support that will be needed by DOE/BES to ensure the success of a nano science and technology initiative. Among these are appropriate capital resources for the national labs; secondary funding of universities for collaborations with DOE labs; funding for the national labs to work with other government agencies, and later with industry; and several new national user facilities focused on nanomaterials. The national nanotechnology initiative is considered highly collaborative and interdisciplinary. It is expected to have substantial long–term impact, with the stated intention of facilitating a technological revolution and ensuring that the United States reaps the benefits.
The reason that nanoscale materials and structures are so interesting is that size constraints often produce qualitatively new behavior. We now understand, in a general way, that when the sample size, grain size, or domain size becomes comparable with a specific physical length scale such as the mean free path, the domain size in ferromagnets or ferroelectrics, the coherence length of phonons, or the correlation length of a collective ground state like superconductivity, then the corresponding physical phenomenon will be strongly affected. Although such changes in behavior can be the dominant effects in nanoscale structures, we still have remarkably little experience or intuition for the expected phenomena and their practical implications, except for electronic systems. The physics, chemistry and biology of phenomena occurring in nanoscale systems is effectively a new subject with its own set of physical principles, theoretical descriptions, and experimental techniques, which we are only in the process of discovering. Thus, there is an urgent need for broadly based investigations of the physical phenomena associated with confined systems, especially in materials and structural contexts where the implications are not at all well understood.
Nanotechnology arises from the exploitation of new properties, phenomena, processes, and functionalities that matter exhibits at intermediate sizes between isolated atoms or molecules (~ 1 nm) and bulk materials (over 100 nm). As opposed to the microscale, the nanoscale is not just another step towards miniaturization, but is a qualitatively new scale. Here quantum and size phenomena are allowed to manifest themselves either at a purely quantum level or in a certain "admixture" of quantum and classical components. At the foundation of nanosystems lie the quantum manifestations of matter that become relevant and measurable. Consequently, instead of being a limitation or an elusive frontier, quantum phenomena have become the crucial enabling tool for nanotechnology .
This report consists of 10 sections plus an Appendix. Each section deals with a particular nanoscale research theme. The themes were selected by the BES N/N Group as an effective means of organizing and explaining a complex and highly interdisciplinary subject. Thematic organization also permits focusing attention on fundamental ideas with broad implications for nanoscale science, and illuminating the most important opportunities, challenges and barriers to progress, as we seek to understand phenomena and processes on the nanoscale and use this knowledge as the basis for a new nanotechnology.
Most sections of this report follow a similar format. First, a brief summary is given of selected BES and national laboratory capabilities and recent research accomplishments, in order to indicate the current status of research on each scientific theme. Next comes a discussion of the important scientific issues and opportunities that can be addressed through the national laboratories and universities. The principal problems and barriers to progress are identified wherever possible. Finally, future research directions and corresponding research needs are outlined, with emphasis on the promise and potential of certain choices. Thus, this report mainly presents ideas regarding questions to be answered and the kinds of things that could be done by BES in order to take a leadership position in nanoscale science, engineering and technology.
In order to unequivocally identify and characterize nanoscale phenomena it is necessary to directly probe the small samples in which they occur. This is problematic, because sub–micron samples produce tiny signals in most relevant physical measurements. Possible ways to address this problem, by developing new, high sensitivity nanoscale instrumentation, are discussed in section 9.
BES is especially well prepared for materials, chemical, biological and engineering  research on the nanoscale because of its substantial investment in major scientific user facilities, among which are several synchrotron x–ray and high–flux neutron sources. The wavelength or energy range of the radiation provided by these sources is ideally suited for the reciprocal or real space characterization of nanoscopic structures. Their high flux makes possible probing either static nanoscale structure or real–time changes. From many other perspectives the match of synchrotron x–ray and neutron sources to nanoscale science is also ideal, as described in section 10. However, the potential exists for much more effective use of these sources by consolidating and better focusing the nanomaterials research effort. This requires an investment in synthesis and characterization facilities that would be focused on specific nanomaterials research areas and associated with existing major photon and neutron sources, which can be accomplished by establishing several Nanomaterials Research Centers at national laboratories. Establishing such Centers also will facilitate a necessary change in the current infrastructure and scientific mode of operation. The research efforts of scientists who are currently trained in different academic disciplines will be focused into interdisciplinary research groups. The Centers will provide the infrastructure support necessary for these scientists, for large numbers of external users, and for the interdisciplinary training of students and postdocs to perform nanoscale research effectively and efficiently. The rationale for establishing Nanomaterials Research Centers is presented in more detail in section 10, and an outline description of possible focal research areas for the Centers is given in the Appendix to this report. These Centers, in association with the principal BES x–ray, neutron and nanofabrication facilities, are expected to be among the most important tools for the national nanotechnology initiative.
1. The members of the BES Nanoscience/Nanotechnology Group are listed in the Preface, and the contributions of some members as section editors for this report also are acknowledged.2. Nanotechnology for the Twenty–First Century: Toward a New Industrial Revolution, report of the Interagency Working Group on Nano Science, Engineering and Technology (IWGN, chaired by M. Roco, National Science Foundation) to be issued September, 1999.3. D. N. Noid, R. F. Tuzun and B. G. Sumpter, "On the Importance of Quantum Mechanics for Nanotechnology," Nanotechnology 8, 119 (1997).4. The relationship of the BES Engineering Research Program (ERP) to nanotechnology follows a path similar to the program's involvement with engineering research for larger–scale systems. It is envisioned that eleven topical areas will fall within the programmatic scope of ERP. These areas, along with understanding the quantum relationships and any thermodynamic, chaotic or nonlinear issues that could influence the underlying nanosystems, will capture the important engineering paradigms to be studied and developed under ERP. No programmatic priority is implied in the following list.• Manufacturing procedures to build tomorrow's nano–machines beyond the current and immediate future silicon–based fabrication technology.• Property characterization (thermal, mechanical, optical, chemical, biological) at the nanoscale.• Modeling of the complex behavior of extremely tiny devices at the fundamental molecular–microscopic– components and assembly levels.• Innovative control methods for nanosystems.• Nanoelectronic computing devices.• Using quantum phenomenology for new communications paradigms.• Reliable packaging of nano–devices.• Innovative solutions for energy redistribution, powering and actuation of the nano–machines.• Micro–instrumentation for on–line diagnostics and control.• Infrastructure for testing procedures suitable for nanodevices (in a manner analogous to what is now done for IC devices).• Diagnostics and manipulation of nano–systems for usage, environmental consideration and ability to construct larger systems from the nano–components.• Friction, lubrication, cracking, failure analysis and nondestructive testing in a nanometer world.
2. EMERGENCE OF PROPERTIES AT MULTIPLE LENGTH AND TIME SCALES
The era of materials technology that we are now entering presents an unprecedented opportunity to understand the emergence of new materials properties on the nanometer scale  and to exploit this knowledge for our benefit. The combination of tools responsible for this opportunity includes the ongoing development of synthesis methods that permit atomic and electronic structure to be controlled down to near the atomic scale, in one through three dimensions; the actual or expected availability of instrumental probes that are capable of characterizing nanoscale materials and structures, as well as their properties, on a wide range of length and time scales; and, the continuing explosive growth of computational power which permits theoretical exploration of structure–properties relationships on scales from the atomic to the macroscopic.
Thus far, the most concentrated effort to apply the tools of synthesis, characterization and modeling, in order to understand the consequences of controlling nanoscale structure, has been in studies of the electronic properties of semiconductors and related materials, driven primarily by their vital importance to information technology . Nevertheless, it is quite clear from this experience that many other novel combinations of properties can be expected to emerge, for a wide range of materials, upon entering the world of the nanoscale.
Effects of Size Constraints
Size constraints alone often produce qualitatively new behavior. For example, if the size of a nanoscale structure becomes less than the characteristic length scale for scattering of electrons or phonons (the mean free path) this can lead to qualitatively new modes of transport for electrical current and/or heat. This effect explains the discovery in the early 1980s of persistent normal (non–superconducting) currents in mesoscale metallic rings , and more recently of the ballistic transport of current in carbon nanotubes . Other examples are the remarkably rich set of new phenomena that arise when the de Broglie wavelength of carriers is constrained by the dimensions of quantum dots [1, 2].
Thermodynamic properties, including collective phenomena and phase transitions such as ferromagnetism, ferroelectricity, and superconductivity, have long been expected to demonstrate substantial changes when structures contain a small number of the relevant particles or when the system size is comparable to the particle size or the coherence length for collective behavior . Systems with component sizes ranging from a few tenths to about ten nanometers lie at the fuzzy boundary between the quantum and classical domains. Such systems are also in the size range where thermal energy fluctuations and Brownian motion can have significant effects.
Contemporary needs in data storage and information technology have stimulated considerable research activity in nanoscale magnetism . One area presenting great opportunities is the magnetism of complex molecules containing tens to hundreds of atoms with localized magnetic moments. Such "molecular magnets" exhibit many unusual properties including tunneling, quantum coherence, and thermo–induced spin crossover transitions. Possible technological applications include the use of magnetic molecules in medicine . Exchange–coupled nanoscale structures also provide a path toward the next generation of powerful permanent magnets . An approach to electronics based on spin, rather than the charge of electron and hole carriers, in nanostructured magnetic devices shows great promise . Ultrathin films (a few atomic layers) yield quasi–two–dimensional magnetic materials, reveal novel magnetic domain physics, and provide a fascinating arena for studying the gradual onset of magnetism .
The mechanical properties of materials also change dramatically as the grain size in polycrystalline materials or the concentration of strain fields approaches the nanometer scale . This was demonstrated recently by nanoindentation experiments  which showed that atomic–height steps on Au single–crystal surfaces have dramatic effects on the yield strength. Computer simulations suggest significantly higher mechanical strength of grain boundaries in nanocrystalline diamond than those in coarse–grained diamond films .
Changes in the strength of nanoscale structural elements, changes in the nature of friction, and changing modes of fluid flow (hence, of lubrication), all will require new design strategies for micromachines. The impact of such changes becomes dramatic as the scale of such machines approaches the nanoscale . Modes of failure also will change, as the scale of devices and machines decreases toward the nanoscale. The causes include different mechanical properties that will modify fracture characteristics; the increased importance of surface tension; and, the enhanced role of diffusion and corrosion at the large surface–to–volume ratios that will occur. Even the notion of the thermodynamic efficiency of a nanoscaled machine or device will differ from its macroscopic, or even microscopic, counterpart because of thermal fluctuations, quantum noise  and changes in the efficiency of nanoscaled engines . This is all largely unknown territory.
Shift of Characteristic Time Scales
This discussion has focussed so far on the emergence of new properties as the characteristic length scale decreases into the nanoscale regime. However, such a length scale change will be accompanied by concomitant changes in the characteristic time scale of physical phenomena. In part, this is no more than the increase in characteristic frequencies that follows from the decreased time required to transit shorter distances at a fixed propagation velocity (for phonons, photons, electrons, etc.). However, genuine changes of regime also will occur. These include an increased importance of surface phonons compared with volume phonons, and ballistic electron transport rather than the scattering–limited transport described by the Boltzmann equation.
Another time–scale phenomenon that will emerge is the increased rate of kinetic processes because of the increased fluctuation rate as the system size decreases and as the reduced dimensionality of important structural features (e.g. surface–to–volume ratio) becomes important or dominant at the nanoscale. The latter effect of course also leads to the increased effectiveness of sensor elements in biological systems. Very large surface–to–volume ratios must at least be taken into account, and possibly exploited, in man–made nanosystems . The shift of characteristic time scales from this variety of mechanisms provides opportunities but will also require the use of instrumentation with improved time resolution and of ultrafast probes.
Modeling and Simulation
The emergence of genuinely new phenomena at the nanoscale creates a great need for theory, modeling and large–scale computer simulation in order to understand the new nanoscale phenomena and regime. The links between the electronic, optical, mechanical, and magnetic properties of nanostructures and their size, shape, topology, and composition are not well understood, although for the simplest semiconductor systems, carbon nanotubes, and similar "elementary" systems there has been considerable progress. However, for more complex materials and hybrid structures even the basic outlines of a theory describing these connections remains to be made.
In nanoscale systems, thermal energy fluctuations and quantum fluctuations are comparable to the activation energy scale of the materials and devices, so that statistical and thermodynamic methods must include these effects adequately. Stochastic simulation methods, as well as computational models incorporating quantum and semiclassical methods, are required to evaluate the performance of nanoscale devices. Consequently, computer simulations, both electronic–structure–based and atomistic, will play a major role in understanding materials at the nanometer scale and in the development "by design" of new nanoscale materials and devices. The vastly increased computational capabilities that will become available through the Scientific Simulation Initiative (SSI) will play an important role and will provide many opportunities for substantial progress in this area. Moreover, by exploiting quantum phenomena that have no classical analogues, it is expected that certain computational tasks can be performed much more efficiently than can be done by any classical computer (see the box on the following page).
In addition to such ab–initio and large–scale computational methods, there also is a continuing need for phenomenological methods such as Ginzburg–Landau models and mean–field methods to aid in understanding and to develop intuition. There are great opportunities for all scales of theory and modeling in understanding behavior at the nanoscale, but perhaps the greatest challenge and opportunity will be in those transition regions where nanoscale phenomena are just beginning to emerge from the macroscopic and microscale, regimes which are describable by bulk properties plus the effects of interfaces and lattice defects. Quantum Computing and Communications at the Nanoscale
The possibility of transferring quantum information between arbitrarily remote locations (i.e., quantum teleportation ) is one of the cornerstones of the emerging field of quantum communications [18,19]. Spectacular refinements of the experimental techniques and equipment make it possible for rather modest laboratories to perform "purely quantum" experiments  with far reaching epistemological, scientific, and practical consequences .
Figure 2.1: Femtosecond laser system used in teleportation experiments
Entanglement is the key to a new realm of quantum phenomena that, until very recently, could not be observed, analyzed, or utilized [22–24]. It is an inherently quantum (i.e. non–classical) property that offers reasonable and consistent explanations to the violation of Bell's inequalities or Schrödinger's famous cat paradox. Quantum–mechanical analysis yields predictions of non–classical behaviors such as amplitude–phase squeezing or "apparent violation of causality" in experiments involving entangled states. Moreover entanglement offers solid prospects for implementing secure (quantum) cryptography, quantum–computing algorithms, and quantum teleportation.
Measurements performed on such quantum systems would unequivocally demonstrate non–local behavior with unprecedented practical relevance . For instance, if a signal beam (entangled part of the wave function to the 'target') is sent to one person and the idler beam (entangled part to the 'detector'), one person can modulate their own locally received beam and the other can observe the results of the modulation via the visibility parameter. A message sent in this fashion is secure from interception because an eavesdropper lacks the unmodulated beam, which is the 'receiver' beam. If an eavesdropper managed to intercept the unmodulated beam, the person intending to perform the modulation would know it by performing a simple measurement before modulating.
In other applications, intervention on one side cannot be dismissed as a causal influence on the other. In some sense, this implies "faster–than–light" (FTL) communication, where no energy or particles are travelling faster than light, but the wave function is . The dramatic miniaturization in computer technology over the past few decades is rapidly reaching the point at which we will be forced to use quantum physics to describe elementary computational operations . Thus, at the nanoscale the very theory describing what computers can do must be revised.
Initially, researchers in quantum computing tried to understand how the basic operations of a conventional computer might be accomplished using quantum mechanical interactions. However, it was soon realized that quantum physics offered something genuinely new . By exploiting delicate quantum phenomena that have no classical analogues, it is possible to do certain computational tasks much more efficiently [29,30] than can be done by any classical computer. Moreover, these same quantum phenomena allow unprecedented tasks to be performed such as breaking supposedly unbreakable codes, generating true random numbers, and communicating with messages that betray the presence of eavesdropping. In summary, quantum computing and communications are expanding the foundations of information processing in a way that is consistent with quantum physics – the most accurate model of reality that is currently known. References
1. G. Timp, editor, Nanotechnology, AIP Press, Springer–Verlag (New York, 1999).2. Special Issue on Quantum Dot Structures, Japanese Journal of Applied Physics, Vol. 38, No. 1B (1999).3. L. L. Sohn, L. P. Kouenhoven, and G. Schoen, editors; Mesoscopic Electron Transport, Kluwer (Dordrecht, 1996).4. S. Frank, P. Poncharal, Z. L. Wang, and W. A. de Heer, "Carbon Nanotube Quantum Resistors," Science 280, 1744 (1998).5. Terrell L. Hill, Thermodynamics of Small Systems, Dover (New York, 1994).6. E. Coronado, P. Delhaes, D. Gatteschi, and J. S. Miller, editors: Molecular Magentism: From Molecular Assemblies to Devices, NATO AISI Series E. Vol. 321, Kluwer (Dordrecht, 1996).7. D. Givord and M. F. Rossingol, "Coercivity," p. 210, in Rare Earth Permanent Magnets, J. M. D. Coey, editor, Oxford University Press (Oxford, 1997).8. G. A. Prinz, "Magnetoelectronics," Science 282, 1660 (1998).9. J. Harris and D. Awschalom, "Thin films squeeze out domains," Physics World 19, (1999).10. Special Issue on "Mechanical Behavior of Nanostructured Materials," MRS Bulletin, Vol. 24, No. 2, February 1999.11. T. A. Michalske and J. E. Houston, "Dislocation Nucleation at Nano–Mechanical Contacts," Acta Mater. 46, 391 (1998).12. P. Keblinski et al., "Amorphous Structure of Grain Boundaries and Grain Junctions in Nanocrystalline Silicon by Molecular–Dynamical Simulation," Acta Mater. 45, 987 (1997); D. Wolf et al., "Phonon–Induced Anomalous Specific Heat of a Nanocrystalline Model Material by Computer Simulation," Phys Rev. Lett. 74, 4686 (1995).13. J. Knight, "The Dust Mite's Dillema," New Scientist, April 1999, p. 41.14. J. Travis, "Making Light Work of Brownian Motion," Research News, Science 267, 1593, 1995; B. G. Levi, "Measured steps advance the understanding of molecular motors," Search and Discovery, Physics Today, 19, (1995).15. R. McGraw and R. LaViolette, "Fluctuations, temperature, and detailed balance in classical nucleation theory," J. Chem. Phys. 102, 8983 (1995).16. G. Adam and M. Delbrück, "Reduction of Dimensionality in Biological Diffusion Processes," p. 198, in Structural Chemistry and Molecular Biology, A. Rich and N. Davidson, editors, W. H. Freeman (San Francisco, 1968).17. D. Bouwmeester, J.–W. Pan, K. Mattle, M. Eibi, H. Weinfurter, and A. Zeilinger, "Experimental Quantum Teleportation," Nature 390, 575 (1997).18. Special Issue: Quantum Communications, J. Mod. Optics 41 (1994).19. O. Hirota, and C.M. Caves, Quantum Communication, Computing, and Measurement, Plenum Press (1997).20. W. Tittel, et al., "Long–distance Bell–type tests using energy–time entangled photons," Phys. Rev. A 59, 4150 (1999). 21. A. Watson, "Entangled Trio to Put Nonlocality to the Test," Science 283, 1429 (1999).22. A. Einstein, B. Podolsky, and N. Rosen, "Can Quantum Mechanical Description of Physical Reality be Considered Complete?" Phys. Rev. 47, 777 (1935).23. J. S. Bell, "On the Einstein–Podolsky–Rosen Paradox," Phys. 1, 195 (1964); also, "Speakable and Unspeakable in Quantum Mechanics," Cambridge University Press (1988).24. D. Bouwmeester, et al., "Observation of Three–Photon Greenberger–Horne–Zeilinger Entanglement," Phys. Rev. Lett. 82, 1345 (1999).25. T. P. Spiller, "Quantum Information Processing: Cryptography, Computation, and Teleportation," Proc. IEEE 84, 1717 (1996).26. D. M. Greenberger, "If one could build a macroscopic Schrodinger Cat state, one could communicate superluminally," Physica Scripta T 76, 57 (1998).27. R. Feynman, "Quantum Mechanical Computers," Optics News 11, 11 (1985).28. A. Steane, "Quantum Computing," Rept. Prog. Phys. 61, 117 (1998).29. H. K. Lo, S. Popescu, and T. Spiller, "Introduction to Quantum Computation and Information," World Scientific (1998).30. G. Berman, et al., "Introduction to Quantum Computers," World Scientific (1998).
3. MANIPULATION AND COUPLING OF PROPERTIES AT THE NANOSCALE
A. Current Status
Research on the optical, electronic, and transport properties of semiconductors at the nanoscale is experiencing astonishing growth. The fundamental properties of nanoscale semiconductor structures can be dramatically altered by controlling their size and shape without changing their composition. When the electrons and holes in semiconductors are confined to dimensions less than their de Bröglie wavelength (typically 1–30 nm) quantum mechanical size effects appear. The carrier confinement can be in one dimension (quantum films or quantum wells), two dimensions (quantum wires), or three dimensions (quantum dots, QDs, or nanocrystals). The most dramatically affected electronic properties include: (a) a large blue–shift of the absorption edge with decreasing size, which can be several electron–volts for semiconductor nanocrystals, resulting in absorption or photoluminescence that can be tuned to range from near–UV through the visible to the near IR; (b) conversion of optical spectra from continuous bands to discrete lines; (c) concentration of oscillator strengths in fewer allowed transitions, resulting in greatly enhanced non–linear optical properties; (d) enhanced photoredox and photocatalytic properties due to higher redox potentials of photogenerated electrons and holes; (e) conversion of direct bandgap semiconductors to indirect bandgap semiconductors and vice versa; (f) slowed relaxation and cooling of photogenerated hot carriers, resulting in reduced electron–phonon interactions; (g) greatly enhanced excitonic transitions at room temperature; (h) an optical absorption coefficient that increases with the inverse–cube of the nanocrystal radius.
In the newly–coined field of "nano–optics" the wavefunction of a QD is tailored for a specific response. Electronic and optical properties of QDs have been observed that closely resemble those of simple atomic systems, including sharp line spectra and relatively simple nonlinear optical signatures . In recent experiments employing wavefunction engineering techniques, the excitonic wave function in a single QD was manipulated and monitored on a time scale short compared to the loss of quantum coherence, by controlling the phase and polarization of two–picosecond optical pulses . These experiments have extended the concept of coherent control in semiconductor nanostructures to the limit of a single quantum system in a zero–dimensional QD, and provide the basic tools to create more complex wavefunctions in future such "solid state atoms".
Ballistic Electron Devices
One of the challenges of electron physics is to create devices that allow controlled charge and energy transport at nanometer length scales. Such nanoscale quantum devices may be components in electrical circuits or they could provide sources of stimulated coherent radiation. They might be fabricated from semiconductor wafers, clusters of atoms, carbon fibers, or even from biological molecules. Ballistic electron devices comprise one class of quantum devices that hold great promise for future communications applications. The size and energy at which they operate directly determines their characteristic switching times. For example, present–day nanoscale electron waveguides are typically fabricated from semiconductor materials, operate at very low temperatures, and typically have picosecond switching times. The recent discovery of conduction quantization in carbon nanotubes [3,4] provides hope that such devices might eventually be fabricated to operate with far higher switching speeds at room temperature.
The dynamics of ballistic electron propagation can be studied using the two–dimensional electron gas (2DEG) that can be created at the interface of two semiconductor materials such as GaAs and AlGaAs. Electron channels and cavities of variable shape can be formed at such an interface by means of gates placed on the outer surface of the semiconductor wafer. The observation  that electron conductance is quantized for electron currents passing between point contacts separated by approximately 100 nm showed conclusively that the wave nature of electrons plays a critical role at the nanoscale. At temperatures of 1K or lower, the electron–phonon interaction can be neglected and the electron motion becomes ballistic. The shape of the confining walls in the two–dimensional plane can be controlled and strongly influences the waveguide conductance .
Much of the research effort on electron waveguides until now has focused on the effect of quantum chaos on conductance, both in the presence and absence of external magnetic fields. Waveguide cavities used in 2DEG experiments typically have widths and lengths of order 100–1000 nm. In this regime, enough propagating modes are present that semiclassical methods (such as periodic orbit theory) can be used to make predictions about the electron conductance. Numerical and some laboratory work  has shown that the electron conductance has qualitatively different behavior depending on whether the waveguide cavities give rise to integrable or chaotic behavior. One of the important discoveries in quantum chaos research is that if a quantum chaotic system has a classical counterpart that is fully chaotic, then it exhibits universal statistical behavior in qualitative agreement with the predictions of random matrix theory . Recently, a possible connection has been made between these predictions for the effects of chaos and disorder in electron waveguides. However, as the size of electron waveguides becomes closer to the nanoscale we enter the quantum regime, where far less is known about the electron dynamics.
One reason for the need to understand the quantum influence on conductance is the possibility of developing more energy–efficient and much higher density electronic circuitry at the nanoscale. It has been shown that integrable waveguides may have better electron–trapping ability than chaotic waveguides . It appears that the conductance in integrable systems can be shut down and electrons stored for longer periods. Waveguides formed at semiconductor interfaces (by means of gates on the outer surface) have leads and cavities which are soft, and the dynamics of these systems is more integrable than chaotic. Indeed, recent work on the conductance of softwall waveguides with mixed dynamics has shown fractal fluctuations in the conductance as a weak magnetic field is varied . Resonances give rise to waveguide capacitance in the presence of high–frequency electromagnetic driving fields, allowing enhanced power absorption . An important problem about which little is currently known is the effect that Coulomb interactions between electrons have on the electron conductance in the fully quantum regime. A related development, based on the nonlinear charge distribution in capacitively coupled conductors, is the possibility of quantum scanning capacitance microscopy at the nanoscale .
Dramatic quantization and other effects also occur in magnetism and magnetic materials at the nanoscale. The ability to control thin–film growth at the near–atomic level to form epitaxial and heteroepitaxial structures has recently been extended to magnetic nanostructures, including metallic, oxide, and semiconducting phases [12,13]. This progress is driven both by the intriguing physics of these materials and by the $50B per year magnetic storage industry ($150B per year if magnetic recording tape, video, etc. are included). Materials advances have opened up entirely new avenues of study. To illustrate, theoretical limits suggest that a near doubling of the energy product of hard magnets may be possible with an optimum nanoscale configuration of Fe–Co, Nd–Fe–B, or Nd–Fe–Al bulk metallic glass [14,15]. However, putting this on firm footing requires a better understanding of coercivity development and domain wall pinning in nanocomposite hard magnets, how coercivity is developed in bulk metallic glasses, and extensive microstructural evaluation to determine the makeup and locations of atomic clusters that contribute to the magnetic remanence.
Thermal transport properties of nanostructured materials have received relatively little attention in the past decade. It is well known that polycrystalline materials exhibit lower thermal conductivity than low–defect single crystals of the same material. Several investigators realized recently that this could result in significantly reduced thermal conductivities in nanostructured materials such as yttria–stabilized zirconia (YSZ), which could lead to improvements for applications such as thermal barrier coatings [16,17]. Reduced thermal conductivities are expected because of a reduction in the phonon mean free path due to grain boundary scattering. Indeed, recent research has determined that the thermal conductivity is reduced approximately two–fold at room temperature in 10–nm grain–sized YSZ, compared to coarse–grained or single crystal YSZ . While several groups are now investigating the thermal properties of nanostructured materials, there is currently no detailed understanding of nanostructure–thermal properties relationships. Unexplored at present, but perhaps even more interesting, is the effect of grain size on the thermal properties of high–conductivity phonon–based conductors such as diamond.
In contrast to the reduced thermal conductivity expected for nanostructured thin films or coatings, opportunities exist for increasing thermal transport rates in fluids by suspending nanocrystalline particles in them. These "nanofluids" have recently been shown to exhibit substantially increased thermal conductivities and heat transfer rates compared to fluids that do not contain suspended particles . However, there is again no real understanding of the mechanisms by which nanoparticles alter thermal transport in liquids. Multibillion–dollar industries, including transportation, energy, electronics, textiles, and paper, employ heat exchangers that require fluids for efficient heat transfer. If researchers can improve these fluids, then manufacturers can make heat exchange systems smaller and lighter, and they can reduce the amount of pumping energy and heat–transfer fluid required.
Energy Conversion and Transport
Energy conversion and transport in nanostructures impacts a variety of fields and applications. Although energy conversion and transport at macroscales is relatively well understood, it is not clear how it occurs at the nanoscale. For example, it is well known that thermoelectric refrigerators and engines are not as efficient as other energy conversion devices because heat conduction by phonons is too high in thermoelectric materials. There is partial and currently controversial evidence that nanostructuring can both improve electron transport [21–23] and reduce phonon transport [24–27]. However, it is not clear how the phonon transport is actually reduced. Similarly, the mechanisms for friction, and for the control of friction, at the nanoscale are not understood (see the box following "Future Research Directions" below).
Photonic crystals have emerged as unique structures with the capability to manipulate the flow of light energy as well as its coupling with excited electronic states. Many novel applications of these structures have been proposed in the areas of optoelectronics and photocatalysis [28–31]. To create photonic crystals operating at optical wavelengths the smallest feature sizes must be of the order of 100 nm, clearly in the realm of nanotechnology. State–of–the–art semiconductor technology [32,33] has recently been used to fabricate three–dimensional photonic–bandgap crystals at 1.5 µm and 10 µm wavelengths, using stacks of microfabricated polysilicon bars. Further development of lithography techniques (such as two–photon methods) at smaller length scales will be needed to extend this technology to visible wavelengths.
There has been rapid progress recently in the development of alternative approaches  to fabricate photonic crystals by using self–assembled systems that order spontaneously  at nanoscale dimensions. Organic or inorganic spheres or block–polymers have been used as the starting material. The photonic crystals fabricated by such techniques consist of porous air cavities in an interconnected dielectric background. The wavelength of the photonic band gap is controlled by both the spatial periodicity and properties of the dielectric crystal. Further development in self–assembly techniques should allow assembling these structures into integrated device systems at low cost .
Great advantages are expected from these structures, including the control of radiative properties and lifetimes that in turn can control chemical reactions and catalysis. One novel phenomenon is the localization of light in a disordered photonic crystal, in analogy to the localization of electrons in disordered systems . Such photonic–crystal structures have immense potential for a large variety of optoelectronics devices. Another remarkable recent development with interesting potential applications is the demonstration of organic photonic–crystal lasers .
Developments in the nascent field of "optical matter" have demonstrated the production of highly organized, multidimensional states of combined matter and light fields . By using crossed laser beams to create carefully controlled standing wave patterns, microscopic structures such as polystyrene spheres or more recently, various alkali atoms, can be trapped in the resulting arrays of potential wells to form "optical crystals" [40–42]. The methods employed are an outgrowth of the optical molasses techniques developed for ultrahigh precision atomic spectroscopy at micro–Kelvin temperatures . Optical crystals have demonstrated unusual binding, coupling, and energy–quantization properties, and they similarly offer possibilities for precision spectroscopy.
Optical matter has been considered by T. Hänsch and others to be an entirely new state with inherently different properties than conventional states of matter [40–42]. To date, optical–matter experiments have utilized only visible–light wavelengths. However, the groundwork has been laid for extension of the methods and theory to the nanometer length scale. The principal means to achieve this would be by preparation of intense coherent x–ray standing–wave fields using perfect–crystal optics. The precursors for this technology have already been used for surface structure determination  and have been proposed for photo–assisted materials growth at nanometer length scales . The anomalous propagation of on–resonance standing–wave fields in crystals (Borrmann effect) may also find application. .
Role of Theory and Simulation
The optical, electronic, magnetic and mechanical properties of nanostructures depend sensitively on their size, shape, composition and outer–shell structure. The links between these properties and degrees of freedom are incompletely understood but are crucial to their eventual technological application. Establishing this link is the main challenge for theory in the field of nanostructures.
Quantum confinement provides a means to control the optical properties of nanostructures via their size, but different shapes can dramatically alter nanostructure properties even at constant size. Composition (e.g., InxGa1–xAs) changes the confinement potential and the interfacial strain, thus also altering the properties. The outer–shell structure determines how much of the wave function is contained in the nanostructure and how much "spills out", which also can affect the electronic properties. Size, shape, composition, and outer–shell structure are controlled during growth or subsequent processing of the nanostructure. For example, one can grow spherical or pyramidal InAs dots via colloidal and Stranski–Krastanov methods, and the outer shell can be organic (e.g., TOPO) or inorganic (GaAs). Sizes can be controlled via colloidal chemistry. Similar variability in size, shape, composition and outer–shell structure is afforded by many other growth methods.
The current status of nanostructure theory is as follows:
(a) The theory of electronic and optical properties includes the effective–mass theory and its k•p generalization, tight–binding, and full–pseudopotential methods. These approaches are able to treat the link between size and shape and the nanostructure optical and electronic properties with various levels of precision and accuracy. Recent applications include InP, CdSe, and InAs free–standing and embedded QDs.(b) The theory of magnetism and spin in nanostructures is in its infancy. Most theories do not yet treat the spin variable and the magnetic moments.(c) The theory of thermodynamics and stability in nanostructures is also incomplete. Methods are available for small nanostructures (less than 100 atoms), for example the local density approximation, but the methods known for larger systems are of lower quality.
B. Key Scientifc Questions
Many questions remain unanswered concerning the coupling of properties to our advantage at the nanoscale. For instance, we are pressed to inquire:
(a) How do QDs couple electronically when formed into arrays and how does order versus disorder affect the coupling? Does the electronic structure change once the QDs are spatially organized, e.g. vertically versus laterally? What is the maximum possible inter–dot conductivity? Can "minibands" be formed analogous to those that are formed in quantum–film superlattices? (b) How does the ability of a nanostructure to accept carriers affect its properties ("Coulomb blockade")? Is carrier transport between QDs characterized by hopping, and can good carrier transport (e.g. photoconductivity and photovoltaic effects) be obtained? (c) Can magnetism be instilled in a non–magnetic material via quantum size effects, and what controls the spin lifetime? (d) How does size quantization affect the kinetics of photoinduced electron transfer between semiconductors and molecules? (e) Can a QD of an indirect–gap material (Si, Ge, GaP) become optically direct? (f) Can useful non–linear optical devices for optical computing, information storage and transport be achieved with QDs or other nanostructured materials? (g) Can phonon transport be reduced by using nanostructured superlattice materials, and can nanostructuring improve the thermoelectric figure of merit by improving electron transport?
Closely related materials questions that must be addressed include:
(a) How does the shape of the nanostructure affect its properties? (b) How are the photocatalytic properties affected? (c) Are there surface states in nanostructures, and what is the effect of the surface structure on their properties? (d) Can one alter the optical response of a nanostructure by changing its outer–shell structure (e.g. by passivation)? (e) Is the solubility of dopants in nanostructures reduced or enhanced relative to the bulk? Can QDs be doped?
Many of the above questions can be addressed convincingly using theoretical tools that are available now or under construction. This will establish a direct link with experiments and open the way for nanostructured design of targeted properties. One immediate hurdle is to develop theoretical tools to describe accurately the electronic structure of large QDs (104 – 106 atoms).
Ballistic electron devices, in particular, test the limits of theoretical knowledge because they involve small systems where boundary conditions may play a dominant role and the wave nature of the electron is of fundamental importance. In these systems the interplay of electric and magnetic interactions is not yet fully understood. The devices themselves generally are open systems involving electron–decay processes and tunneling phenomena. Multi–photon or multi–phonon interactions also may be important. Their dynamics may be chaotic or regular, or may involve a mixture of chaotic and resonant behavior. The mechanisms governing electron transport in 2DEGs still are not completely understood. At temperatures of the order of 1K the interaction of electrons with phonons is negligible. However, the electrons, whose motion is largely phase–coherent, still experience long–range Coulomb interactions, random electric and magnetic fields due to impurities, and may exhibit manifestations of chaos due to boundary effects. The electrons are thus sensitive to their local environment, which depends critically on the fabrication methods and materials used. Electronic conduction at low temperatures is also affected by resonances determined by the shape of the waveguide channels and cavities, as well as the materials used. Ballistic electron devices operate at the interface between quantum mechanics and thermodynamics, two disciplines that have not yet been reconciled when dealing with small numbers of particles. An understanding of all of these effects in the fully quantum regime is essential, if we are to understand electrical circuits that function at the nanometer scale.
Since many properties of nanodevices are related to quasibound states of electrons, it is also important to understand their dynamics. A case in point is the decaying states of electron waveguides. For waveguides with long straight leads and a rectangular cavity, the states with a continuous spectrum in the leads correspond to the continuous field states, while the quasibound states of the cavity correspond to unstable states. The case where a discrete energy of the bare states in the cavity is close to the minimum energy of the continuous spectrum of the leads results in interesting nonlocal phenomena associated with "super–luminality." Since nonlocality is a quantum characteristic, this situation also can provide an interesting experimental test of fundamental problems in quantum mechanics. Moreover, electron waveguides can provide an experimental measurement of the magnitude of the "dressing" of the excited unstable state. This has a bearing on the decoherence problem as it relates to nanoscale quantum computers because quantum computation is reliable only for time scales during which one can neglect decoherence.
C. Future Research Directions
The future applications of nanoscale devices are both exciting and boundless. The greatest near–term payoff is likely to be the development of ultrahigh–speed and efficient electronic, optical, and magnetic devices for communications, switching, computing, and storage, and their Friction and Control of Friction at the NanoscaleNano–devices and the new ultra small technology are expected to significantly improve the performance of robots, computers, communication, and other electro/optical/mechanical devices. However, friction imposes a significant limitation in usage of these tiny devices. As a manifestation of the nano in the macro, hundreds of millions of dollars will be lost as a result of wear, friction, breakdowns, and wasted energy. Research to control friction at the nanoscale will result in highly improved performance in the macro world and can produce significant economic savings .
Traditional lubrication involves the use of organic substances whose functional groups can adsorb onto polar surfaces to form a closed–packed arrangement of almost perpendicularly oriented lubricant chains. However, nano–machine lubricant selection is complicated by new considerations. Due to the built–in–place nature of nano–mechanics, lubrication by the conventional means of hidden and contacting surfaces is prohibitive. Fluid lubricants may also introduce capillary and viscous shear mechanisms, which result in energy dissipation. Research will span the following topics:
Mechanisms for Friction and Lubrication at the Nano–Scale. Despite great progress made through the past half century, many basic issues in fundamental tribology such as origin of friction and failure of lubrication have remained unsolved. Moreover, current reliable knowledge related to friction and lubrication is mainly applicable to macroscopic systems and machinery and, most likely, will be of only limited use (if at all) in nano–systems. When the thickness of the lubrication film is of the same order as the molecular or atomic size, the behavior of the lubricant becomes significantly different . Understanding the mechanisms of friction, lubrication, and other interfacial phenomena at atomic and molecular scales will provide designers and engineers the required tools and capabilities to control and monitor friction, reduce unnecessary wear, and predict mechanical faults and failure of lubrication in nano–devices [49,50].
Electromechanical and Optical Control of Friction. Friction can be manipulated by applying small adjustments (perturbations) to accessible elements and parameters of the sliding system . This requires apriori knowledge of the strength and timing of the perturbations, similar in concept to the "residual statics" perturbation problem in seismic imaging, where global optimization techniques have shown great promise . In addition, surface roughness and thermal noise are significant factors in making decisions about control on the micro and nano–scales. Research to study the response of the sliding system and lubricating thin films to small mechanical, electrical, and optical excitations will result in identifying better ways to control friction and to create a novel type of electromechanical lubricant at the nano–scale.
Control of Chaos in Friction. In many situations, the behavior of the lubricant and the sliding surfaces is chaotic. Thus control of chaos and targeting it toward desired behaviors is an important issue for lubrication and friction. Techniques for controlling chaos have been under systematic investigation since 1990 . The main issue is to develop efficient and robust methods to selectively drive low and high dimensional chaotic dynamics by using tiny external perturbations . Both feedback and non–feedback techniques have to be adjusted and applied to control chaos in friction . Control of chaos in spatially extended systems has not yet reached a high applicability level. Thus, research in this direction is very significant.
Quantum Control of Friction. When the size of sliding surfaces decreases to the nano–scale, quantum effects play an important role. Quantum control of friction is significantly different in concept and methods from classical control, and is much less understood. The main difficulties associated with developing efficient quantum control techniques are: (i) insufficient understanding of many–particle systems; (ii) lack of exact solutions for large many–body systems; (iii) inaccessibilty/collapse under measurements of the state function; and (iv) lack of robust algorithms for two–point boundary value problems. A focused effort to develop algorithms for quantum control of friction is required.
Computational Aspects. Research on friction and lubrication is highly interdisciplinary and probes the very foundations of physics, chemistry, mathematics, mechanical, electrical and chemical engineering, and computer science. Due to the high cost of experiments, computational modeling and simulations  to support and predict experimental findings will play a significant role. Developing new, accurate, fast, and stable methods to solve large systems of nonlinear differential equations will be one of the crucial issues in research on friction at the nanoscale. incorporation into highly integrated and massively parallel systems. Current trends toward ever–increasing circuit complexity, miniaturization, and reduced power consumption in the information technology arena can be expected to accelerate, and the drive to develop and commercialize these nanotechnologies will become paramount. But related developments that are further afield at present, such as quantum computing and cryptography, will also profit from a comprehensive and sustained nanoscience research program.
Many other examples can be found to illustrate the close link between fundamental studies of nanoscale phenomena and their technological applications. Consider interlayer magnetic coupling in multilayer structures. Although initial studies of magnetic coupling in multilayers were motivated primarily by scientific curiosity, as giant magnetoresistance (GMR) and the concomitant phenomenon of oscillatory interlayer magnetic coupling were discovered, the technological relevance of these discoveries resulted in products within an extraordinarily short ten–year period. Future developments that can be anticipated include nonlinear magneto–optical effects in magnetic multilayers and nanostructures that may lead to a new generation of magnetic sensors, recording media, and imaging methods.
Another area which can be expected to benefit significantly from the development of nanostructures exhibiting novel geometries and optical coupling is inexpensive mass–production of high–resolution optical components. "Optical matter" methods may prove instrumental to organize and assemble them on the nanoscale. These devices will be capable of focusing and guiding not only microwave and visible light, but shorter wavelengths from the ultraviolet down to the x–ray region. Diffraction–limited performance at the nanoscale requires nanometer wavelengths, i.e. x–rays. Such x–ray optical elements operating at unprecedented (<>