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Condensed-Matter and Materials Physics: The Science of the World Around Us 6 What New Discoveries Await Us in the Nanoworld? Nanometer-scale materials straddle the border between the molecular and the macroscopic. They are small enough to exhibit quantum properties reminiscent of molecules but large enough for their size and shape to be designed and controlled. Furthermore, many of the atoms in a nanoscale object are on the surface, available to catalyze chemical and biological reactions and altering nearly every material property. For example, nanocrystals of semiconductors can melt at temperatures hundreds of degrees lower than the temperatures at which bulk materials melt, allowing thin films to be recrystallized with a hair dryer instead of a furnace. Carbon nanotubes and quantum dots form single-electron transistors that turn from on to off with the addition of a single elementary charge. The potential of nanoscale materials is almost limitless, but scientists must first overcome two fundamental challenges. The first is physical: How does one generically control the identity, placement, and function of every important atom in a nanoscale solid and then assemble them into real-world systems? The second is conceptual: How does one attack problems too big to be solved by brute force calculation but too small to be tackled by statistical methods? Meeting these challenges will transform the study of nanoscale materials from a frontier science to a mature discipline and will have a revolutionary impact on fields from materials to information and from energy to biology. WHY NANO? Nanoscience and nanotechnology have the potential to revolutionize science and technology in ways that will make the world 50 years from now unrecognizable
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Condensed-Matter and Materials Physics: The Science of the World Around Us compared with today. The field can be seen as the logical continuation and combination of three separate trends in science that are all intersecting at a common point, opening the doors to revolutionary concepts and capabilities. The three trends are these: (1) the continuation of Moore’s law and the nonstop shrinking of electronic circuitry, (2) the rapid advances in molecular biology that have completely changed people’s understanding of life over the past 30 years, and (3) the evolution of chemistry from the study of single atoms and molecules to the fabrication and exploitation of very large complexes such as quantum dots and proteins. This scientific “perfect storm” will change the world profoundly over the coming decades. When scientists and engineers created the first human nanotechnology, the integrated circuit (see Figure 1.5 in Chapter 1), it redefined the modern world. In actuality, it is a very limited technology, focusing on just a few materials (silicon, copper, gallium arsenide, and so forth) patterned by a single class of lithography-based techniques, and aimed at one major goal, the manipulation of electronic information. The goal of nanoscience is to perform the fundamental scientific studies needed to create even more nanotechnologies, ones capable of manipulating matter, energy, and light the way that integrated circuits manipulate electrons. Scientists are thus laying the foundations for the next set of revolutions. Nature shows what is possible (Figure 1.5). Using carbon-based building blocks such as deoxyribonucleic acid (DNA), proteins, and lipids, life creates self-replicating complex structures that can harvest energy, store information, and control matter, from the atomic scale to the macroscale. Will we someday be able to duplicate and improve on the incredible abilities of life? Will we someday be able to build complex, functional (and beautiful) structures from nothing but a patch of dirt and a splash of sunlight? To progress down the path, researchers must face a huge number of challenges. First, they must learn to construct and quantitatively understand the basic nanoscale building blocks, discussed below. For example, the energy levels for an electron spiraling down a nanotube (Figure 6.1) are quantized like those in an atom, but these quantum properties are designable: The levels can be tuned by choosing the diameter and chirality of the nanotube. The next decade will see an explosion of designable nanostructures, along with new techniques to probe them and new ideas to understand them. As researchers master these nano-building blocks, they face an even greater challenge: How does one connect these blocks into larger assemblies, and how does one predict the properties of these assemblies? In other words, how does one create the kind of complex, functional structures such as the nanopore discussed in the following section? At the end of the chapter, the new experimental and theoretical tools necessary to make these revolutions happen are discussed. The kinds of technologies that are being envisioned seem almost like science fiction. As an example, consider the quest to replace the complementary metal oxide
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Condensed-Matter and Materials Physics: The Science of the World Around Us FIGURE 6.1 (Left) Electron spiraling down a nanotube. The nanotube energy levels are quantized, as in an atom, so that every nanotube species has a different fluorescence peak determined by its diameter and chirality, as shown in the right panel. SOURCES: (Left) P.L. McEuen, Cornell University. (Right) R.B. Weisman, Rice University. semiconductor (CMOS) transistor with a device that is a nanometer in scale, suggesting that there may be many more orders of magnitude in microcircuit scaling still ahead of us. In the area of novel materials, nano ultrastrong, low-weight materials will enable more fuel-efficient cars and aircraft that are lighter, stronger, and cheaper. Cancers may be cured by multilayer nanoparticles that become activated by penetrating infrared or terahertz radiation. Personalized medicine will become a reality, allowing each of us to have a map of our own genomes so that doctors can tailor therapeutic solutions to the individual’s makeup. NANOSCALE STRUCTURES: HOW DO WE BUILD THEM? The starting point of nanoscience is its fundamental building blocks—the balls, sticks, and sheets out of which more complex structures will be made. These are analogous to atoms or molecules in traditional condensed-matter and materials physics (CMMP)—the fundamental units from which solids are constructed. These building blocks are examined first, and then their assemblies are explored in later sections. The building blocks are most easily categorized by how they are constructed. The first approach is by carving up larger-sized materials, reducing their size along one or more directions to make quantum wells, wires, and quantum dots. An alternative approach is to grow a nanoscale version of a bulk material to the desired shape. The final category consists of chemically or biologically synthesized molecules or molecular assemblies that have no macroscopic counterpart.
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Condensed-Matter and Materials Physics: The Science of the World Around Us Biological examples include lipid membranes; linear, information-carrying structures such as DNA; and protein functional elements. Historically, these approaches have lived in separate disciplines: electrical engineering for lithography; materials science, physics, and solid-state chemistry for growth; and chemistry for molecular synthesis. A key goal in the coming decade will be to train scientists who are capable of combining all of these approaches. Patterning at the Nanoscale: Lithography and Self-Assembly Modern lithographic techniques allow researchers to pattern materials into devices with dimensions down to approximately 30 nanometers (nm), approximately 100 atoms across. These techniques have been the foundation of the modern integrated circuit, creating devices such as the transistor shown in Figure 1.5 in Chapter 1. This relentless miniaturization has allowed engineers to pack the equivalent complexity of a city landscape on a fingertip-sized chip. These same lithographic and patterning procedures are now being applied to the manufacture of a wide range of nanomechanical, microfluidic, and nanobiological devices. The major challenges facing lithography are many, some of which are discussed in later sections. The first is to reproducibly create structures at 10 nm and below. At this scale, the intrinsic fluctuations in the material—the size of an individual polymer molecule in the resist or the detailed properties of the surface of a material—become important. Figure 6.2 shows one application that requires precisely controlled features at the nanometer-scale—a nanopore in a silicon wafer designed to sequence DNA. The DNA passing through the pore blocks it, resulting in a change in the conductance through the nanopore that depends on the chemical identity of the base pairs in the pore. This novel approach to sequencing DNA could revolutionize biology and health care, but it is currently beyond the range of existing lithographic technology. A second challenge is to be able to pattern a wide variety of nanoscale materials. Lithographic processes were initially developed for “tough” inorganic materials such as silicon, metals, and glass that can withstand harsh solvents and high temperatures. However, most organic and biological materials cannot withstand the processing conditions used for standard lithography. New techniques are being developed, such as stamping and dip pen-nanolithography, to directly write such soft molecules such as lipids, DNA, and proteins. There have been notable successes already, such as lithography for the creation of DNA and protein arrays, and the impact has been extraordinary. Scientists are still at the beginning of this revolution that applies the power of nanofabrication to problems in chemistry and biology. Self-assembly involves the organization of a group or subunits, such as atoms, molecules, or particles, into a larger aggregate, or structure, characterized by a length scale many times the size of the individual units. Forces that “drive” self-assembly
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Condensed-Matter and Materials Physics: The Science of the World Around Us FIGURE 6.2 Artist’s rendition of a nanopore fabricated in a silicon wafer used for studying DNA. The nanopore is created by a combination of lithography and controlled annealing. SOURCE: Courtesy of C. Dekker, Delft University of Technology. are varied, depending on the system, and include van der Waals, electrostatic, capillary, hydrogen-bonding, and other types of thermodynamic, noncovalent forces. The organization of quantum dots and/or nanoparticles into crystalline order, the formation of micellar and other mesoscale structures by surfactant systems, and the formation of block copolymer systems are examples of self-assembly. The self-assembly of copolymers (amorphous, rod-coil, and so forth), synthesized with varying functionalities (electronic, optical, magnetic, and so forth), results in the formation of various geometric structures (spheres, cylinders, lamellae, and so forth) on nanometer length scales determined by the size of the molecules.
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Condensed-Matter and Materials Physics: The Science of the World Around Us Combinations of self-assembly and patterning enable the control of the structure of materials over different length scales. Block copolymers, which possess highly ordered structures such as cylinders, with high lateral regularity can be placed on surfaces. The patterns can serve as templates for etching and for growth or synthesis to enable building structures such as nanowires. Controlling Growth at the Nanoscale Fabrication by lithography has been likened to building a bridge by carving it out of a block of steel. An alternative approach is to directly grow the nanoscale structure of interest in the desired size and shape. The past decade has seen tremendous progress in this area. First, researchers have created new types of heterogeneous layered materials in which the properties of the interfaces are controlled with atomic precision, as shown in Figure 6.3. Stunning progress has been made in the growth of nanowires and quantum dots. In the past decade, researchers have created transistors, p-n junctions, heterostructures, lasers, and so forth, in one-dimensional semiconductor wires. Quantum dots grown by a variety of techniques are now used in many applications, from solar cells to biological markers. Another important class of building blocks is based on an intrinsically two-dimensional material, graphene (also discussed in Chapter 2), consisting of a sheet of carbon atoms bonded in a honeycomb-like pattern. These sheets can be rolled to create nanotubes (Figure 6.1) or wrapped to create C60 and related small molecules. The properties of carbon nanotubes have received tremendous attention over the past decade, but a major roadblock looms for applications. There are still no protocols to grow precisely positioned, structurally identical carbon nanotubes of a desired length, diameter, and chirality. Similarly, techniques to create single-sheet graphene layers are in their infancy. A revolution in the understanding and control of growth is needed to reliably create structures with atomically precise characteristics and move these remarkable new materials from the laboratory to the marketplace. Molecular and Biological Building Blocks The building blocks discussed above typically consist of one or a few types of atoms arranged in a rigid, usually periodic fashion. Chemistry and biology provide building blocks that are much more varied, and they can readily mix different functions. Nature, in particular, plays a very different game. Its structures are much more dynamic, assembling and disassembling in response to small changes in their environment. For example, the double-stranded DNA helix unzips at modest temperatures, separating into individual strands that are key to DNA’s ability to duplicate itself. Furthermore, the sheer diversity of chemical and biological
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Condensed-Matter and Materials Physics: The Science of the World Around Us FIGURE 6.3 Crystalline inorganic material building blocks. (a) A complex oxide multilayer. (b) Semiconductor nanocrystal. (c) Nanowire heterostructures. SOURCES: (a) Courtesy of H.Y. Hwang, University of Tokyo. (b) Courtesy of Y. Zhu, Brookhaven National Laboratory. (c) Courtesy of P. Yang, University of California at Berkeley. building blocks is staggering. For example, viruses can present literally millions of peptides that can specifically attach to almost any surface, biological or inorganic. This power has recently been used to recognize, bind to, and nucleate the growth of nanoscale building blocks. The inherent functionality and diversity of these building blocks can be directly used, but they can also be taken as inspiration. From the point of view of biology, current inorganic building blocks are exceedingly primitive. The future challenge is to create nanoscale building blocks with built-in functionality—ones that can move, change, and assemble in specific ways. The way that nature does this is astounding—constructing machines out of a linear string of elements that folds
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Condensed-Matter and Materials Physics: The Science of the World Around Us itself into useful shapes. Researchers are nowhere near being able to perform such subtle and difficult feats, but they are learning how to co-opt nature’s toolbox, as shown in Figure 6.4. Researchers have developed algorithms that can design DNA sequences that will assemble into arbitrary two-dimensional shapes. The assembly is massively parallel, in this case creating 50 billion smiley faces in a single test tube. STUDYING NANOSTRUCTURE BUILDING BLOCKS: THE ATOMIC PHYSICS OF NANOSCIENCE Understanding the properties of individual atoms lies at the heart of the physics and chemistry of solids. Similarly, a thorough and complete understanding of the properties of nanoscale building blocks is central to the field of nanoscience and FIGURE 6.4 Rationally designed DNA assembly of a complex nanoscale structure. SOURCE: Courtesy of P.W.K. Rothemund and N. Papadakis, California Institute of Technology.
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Condensed-Matter and Materials Physics: The Science of the World Around Us is an emerging area in atomic, molecular, and optical (AMO) physics.1 It is also a necessary first step toward technological applications. The understanding of the basic properties of nanoscale structures has grown extraordinarily over the past decade. Many of the basic phenomena have been carefully worked out. In small conductors, the charge of a single electron can regulate electron flow, resulting in the single-electron transistor. The optical properties of nanocrystals, nanowires, and quantum wells can be continuously and carefully controlled. Similarly, the mechanical, magnetic, and thermal properties of individual nanostructures have been explored. The coming decade will see this basic understanding pushed to the limits, addressing cases in which simple ideas break down, such as when the correlations between electrons in a nanostructure become dominant. Physicists will also explore the quantum manipulation of spins, states, and even entire physical structures. Finally, researchers will begin to engineer nanosystems that combine functions—for example, ones that can absorb a photon and convert it efficiently into electrical or chemical energy (see Chapter 3). Quantum Manipulation One major goal of nanoscience is to controllably and coherently manipulate information stored in quantum states; this capability is important for applications ranging from controlling chemical reactions to quantum computing (see Chapter 7). Already, scientists can create quantum dots and add electrons to them one by one, as shown in Figure 6.5. Each dot can serve as a quantum bit, and researchers have already demonstrated that the quantum state can be read out or transferred to a neighboring dot coherently. These are initial steps on the way to a scalable quantum computer, but enormous hurdles remain to be overcome. One is controlling the sources of “decoherence,” the unwanted interaction of a quantum bit with its environment. A second, more fundamental hurdle, is that of understanding the nature of the ground states and correlations between interacting electrons, which can open up the possibility of new types of quantum objects for manipulation, from Cooper pairs to fractionally charged quasi-particles with exotic quantum properties (see Chapter 2). Another fascinating quantum realm is that of quantum “mechanics,” that is, the properties of nanomechanical systems. Researchers are on the verge of measuring the quantum fluctuations of a mechanical beam, demonstrating that the quantum rules of behavior can apply to objects consisting of millions of atoms. Also of interest are the interactions between different kinds of quantum systems—single electrons interacting with a mechanical resonator, or a single quantum dot coupled 1 National Research Council, Controlling the Quantum World: The Science of Atoms, Molecules, and Photons, Washington, D.C.: The National Academies Press, 2007.
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Condensed-Matter and Materials Physics: The Science of the World Around Us FIGURE 6.5 (Left) Lithographically patterned coupled quantum dot molecule formed in a GaAs/AlGaAs heterostructure. Using voltages applied to the gates, a double-well potential is created in which individual electrons can be trapped in the left- or right-hand well. The charge state of the molecule is read out using the quantum point contact to the right of the confining gates as a sensor. (Right) Measured “charge stability diagram” of the molecule, where the pairs of numbers (a,b) indicate the number of electrons in the left- and right-hand wells. The slanted lines indicate places where the charge state changes. SOURCE: Courtesy of C.M. Marcus, Harvard University. Adapted from J.R. Petta, A.C. Johnson, J.M. Taylor, E.A. Laird, A. Yacoby, M.D. Lukin, C.M. Marcus, M.P. Hanson, A.C. Gossard, “Coherent Manipulation of Coupled Electron Spins in Semiconductor Quantum Dots,” Science 309, 2180 (2005) and J.R. Petta, A.C. Johnson, A. Yacoby, C.M. Marcus, M.P. Hanson, and A.C. Gossard, “Pulsed-Gate Measurements of the Singlet-Triplet Relaxation Time in a Two-Electron Double Quantum Dot,” Phys. Rev. B 72, 161301 (2005). to the quantized light modes of a microfabricated structure. Researchers are at the beginning of a new era in quantum manipulation; the next decade will be decisive in determining how far and how fast this new field can grow. Controlling Light: Nano-Optics Since the wavelength of visible light is on the scale of 1 micron, nano-optics sounds like a contradiction in terms. However, researchers have recently shown that a variety of cleverly designed nanostructures can confine or guide light on a scale smaller than previously thought possible. Using either dielectric waveguides on a silicon chip or metal structures whose plasma excitations guide light, researchers are bringing light down to the nanoscale for applications ranging from biomolecular detection to information processing. Also, researchers have recently used nanowires, nanotubes, and quantum dots to create and detect light on the nanoscale. The ultimate goal is to perform detection and information processing at the single-photon limit.
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Condensed-Matter and Materials Physics: The Science of the World Around Us Probing Molecular Machines Biological nanomachines (see Chapter 4) accomplish remarkable tasks, from reading and writing information in a single molecule to converting sunlight to energy. How do these machines accomplish their objectives? New techniques to tug on individual molecules using optical tweezers or to measure the light coming out of a single molecule using ultrasensitive fluorescence microscopy are answering these questions. For example, consider the virus shown in Figure 6.6. How much force is required to pack the DNA inside the capsid shell? Studies using optical tweezers to tug on the DNA as it is drawn into the capsid head provide the answer. The virus’s rotary motor exerts 60 piconewton-scale forces that compress the DNA to 6000 times smaller than its normal volume. The internal pressure thus generated is later used to launch the DNA into the cell targeted by the virus. The next 10 years should see explosive growth in the kinds of techniques capable of exploring biological functioning on the nanoscale (see Chapter 11), leading to a revolution in understanding of the detailed physical operation of biological FIGURE 6.6 Artist’s conception of DNA being packed inside a virus capsid by a molecular motor. SOURCE: D.E. Smith, S.J. Tans, S.B. Smith, S. Grimes, D.L. Anderson, and C. Bustamante, “The Bacteriophage Φ29 Portal Motor Can Package DNA Against a Large Internal Force,” Nature 413, 748-752 (2001).
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Condensed-Matter and Materials Physics: The Science of the World Around Us machinery. This research is critical for the life sciences and health industry (for example, understating the mechanisms of viral infection). Equally important, studies of biomolecular nanotechnology will help to understand the physical principles, design rules, and structural motifs for creating new kinds of nanoscale functional entities. Combining Different Properties The next 10 years will see major advancements in structures in which different nanoscale properties interact. Notable examples have already been demonstrated. In small magnetic devices, torques exerted by spin-polarized electrical currents have been developed as a new way to switch ultrasmall magnetic bits. Single-electron transistors have been used as readouts to measure the quantum properties of mechanical oscillators and electromagnetic resonators. The optical absorption of nanoparticles and carbon nanotubes has been used to deliver local pulses of heat to destroy cancer cells. These important first steps point the way to a rich new science and technology, moving beyond the paradigm of controlling and making use of one property at a time. For example, new complex oxide materials promise tremendous integrated control over electronic, magnetic, and optical properties. Again, biology is the master to which researchers can aspire (see Chapter 4). The absorption of photons triggers a complex cascade of electronic, chemical, and physical rearrangements that result in the production of ion gradients and energy-storing molecules. Can researchers too understand and control nanoscale processes with similar sophistication, and furthermore can they expand the suite of available materials far beyond biology’s limited choices? ASSEMBLING THE BLOCKS: THE CONDENSED-MATTER PHYSICS OF NANOSCIENCE The previous sections looked at the building blocks of nanoscience, their fabrication, and their properties. This section turns to assemblies of these building blocks—the condensed-matter physics of nanostructures. This subject has the potential to produce truly revolutionary results, allowing researchers to create designer solids with both practical and scientific applications. There are two central questions to address: How does one build the assemblies? What properties will they have? Ordered Arrays Detailed models of atoms arranged on periodic lattices comprise the central paradigm of condensed-matter physics. They allow CMMP researchers to understand everything from electrical and thermal conductivity to the color of solids.
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Condensed-Matter and Materials Physics: The Science of the World Around Us Both the lattice structure and the nature of the atom(s) in the lattice combine to determine its properties, but the choices are limited by the elements in the periodic table. Individual nano-objects with carefully tailored properties promise to act as “artificial atoms” and to vastly expand the repertoire. However, creating periodic solids composed of nanoscale objects has proven to be quite a challenge. With atoms, all the individual subunits are identical, and they readily build extended, nearly perfect arrays. Only recently have objects such as nanocrystals been synthesized with the level of uniformity needed to create such well-ordered “artificial” crystals. Figure 6.7 shows a binary solid created from mixing two different types FIGURE 6.7 Well-ordered binary crystals formed from two nanocrystal subunits. In this structure, 13 6-nm-diameter PbSe semiconductor nanocrystals (quantum dots) arrange themselves, with each 11-nm Fe2O3 magnetic nanocrystal a close-packed AB13 lattice. This new magneto-optic “metamaterial” captures the novel properties of both nanocrystal systems and allows exploration of new phenomena that emerge from the interaction of the nanoscale building blocks. The repeat unit in this colloidal crystal contains almost 5 million atoms. SOURCE: Courtesy of C.B. Murray, University of Pennsylvania.
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Condensed-Matter and Materials Physics: The Science of the World Around Us of nanocrystals. This development opens up the way for artificial nanoscale solids whose properties reflect the coupled states of the individual nano building blocks. These metamaterials will have electric optical, thermal, and magnetic properties carefully tailored both for real-world applications and as a laboratory to test theories of condensed-matter physics. For example, optical metamaterials may make possible lenses with resolution beyond the standard limits of wave diffraction. So far, these properties have only been partially achieved—creating metamaterials that will have these properties over the visible spectrum will be a major challenge for the coming decade. Arbitrary Structures The periodic structures described above are among the simplest examples of nanoscale assemblies. Of equal interest are nonperiodic geometries, from the relatively simple nonrepeating linear structure of a DNA molecule to the full-blown complexity of an integrated circuit or a cell. Lithography offers one flexible route to achieve complexity, but it is subject to the materials and dimensional limitations discussed above. Another approach is to use the lock-and-key properties of chemical or biological molecules as a kind of smart glue to link nano-objects to each other or to attach them to specific locations in a lithographic environment. Recent examples include two-dimensional DNA templates that can be a breadboard to which other nano-objects are attached. Another possibility is to assemble the nano-objects using external forces. Researchers have developed techniques using holographic optical traps to manipulate simultaneously hundreds of micron-scale beads and to control, cut, solder, and place individual nanowires. Similarly, circuits to create electric or magnetic forces have been used to manipulate and assemble nano-objects. All of these, however, are difficult to push to the true nanoscale. Researchers need revolutionary new approaches to assembling arbitrary patterns at the nanoscale reproducibly, routinely, and reliably. SMALL PROBES AND BIG IDEAS: CRITICAL NEEDS FOR A NANO FUTURE The past decade has seen tremendous progress in the field of nanoscience, but in a very real sense researchers are still poking around in the dark. They still do not have adequate tools to see and feel at the nanoscale—to determine the positions and function of all the elements (atoms, electrons, electric and magnetic fields) in a nanoscale solid. Similarly, they do not have a complete set of intellectual tools to interpret what they see. Their needs range all the way from simple models that capture the essence of phenomena to detailed simulations that can accurately predict behavior and guide experiment (see Chapter 11).
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Condensed-Matter and Materials Physics: The Science of the World Around Us Better Eyes One of the most pressing needs is techniques to see where all the atoms are and what they are doing inside a nanostructure. While in some cases this is now possible (scanning tunneling microscopy of surface atoms is a prime example), in general the nanoscientist must infer what his or her sample looks like and how it operates on the basis of a limited amount of direct evidence. Is it possible to do better? For example, can electron or x-ray diffraction measurements be performed on individual nano-objects, from nanotubes to proteins? Particularly needed are new ways to look at the workings of nanoscale structures in their natural environment (e.g., in liquids). Recommendations detailed in Chapter 11 will help realize these needs and those detailed below. An example of recent progress is shown in Figure 6.8: Optical images with much higher resolution than the wavelength of light FIGURE 6.8 Photoactivated localization microscopy—PALM—yields nanometer-resolution fluorescence images of biological structures. SOURCE: Courtesy of H. Hess, Howard Hughes Medical Institute.
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Condensed-Matter and Materials Physics: The Science of the World Around Us are obtained by a new kind of fluorescence microscope where individual fluorophores are turned on one by one and their positions accurately measured. Improved Sensing More than knowing where all the atoms are, researchers also want to know their charge, local magnetic properties, chemical identity, and so forth. While scanning probe techniques have provided revolutionary images of the properties of nanostructures, this has only scratched the surface, so to speak. What is needed is the continued development of tools that can probe the local properties in new ways. A recent success, over 10 years in the making, was the first detection of a single electron spin using an atomic force microscope. It took years of concentrated effort by world-class scientists to bring this to fruition. It illustrates the importance of long-term commitments to research, as well as the potential problems with the changing landscape of the industrial research laboratories. A Greater Understanding The final pressing need is for an increased understanding of the fundamental properties of nanostructures, as well as knowledge of the most appropriate design rules for creating nanoscale systems. First, a set of simple paradigms to describe nanoscale phenomena is needed. Many of these models are now well developed— single-electron charging, conductance quantization, and so forth—but more remain to be discovered and sorted out. For example, how does one understand systems in which the nuclei in a structure no longer move much more slowly than the electrons, as in a usual solid? How does one cope with nanostructures in which the motions of the electrons are strongly correlated and the independent particle picture breaks down (see Chapter 2)? Another pressing need is for a set of new analytic and computational tools to allow researchers to address ever-more-complex nanoscale systems (see Chapter 11). This is important in order to continue the evolution of the field from a descriptive to a predictive discipline. A huge part of the challenge of dealing with a complex system is in integrating the separate computational and calculational techniques used to describe different aspects of the problem—the combination of approaches that address the electronic, structural, and optical properties in a unified way, for example.