Frontiers in Crystalline Matter From Discovery to Technology (2009) / Chapter Skim
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2 Science and Technology of Crystalline Systems
2 Science and Technology of Crystalline Systems
Pages 33-94
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From page 33... ...
originate from the unique way in which a single sheet of carbon atoms breaks spatial symmetry. Symmetry is described mathematically through the theory of space groups, which are the set of spatial translations and rotations that leave a crystal structure unchanged.
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From page 34... ...
Graphene is a single sheet of hexagonally ordered carbon atoms. The purely two-dimensional nature of graphene sheets gives rise to an astounding array of new phenomena, among which are the following: • Behavior that mimics the relativistic motion of particles in high-energy accelerators, • New states of matter in the quantum Hall regime (in Chapter 1 of this report, see the subsection entitled "Example in the Area of Thin Films: Gallium Arsenide-Based H eterostructures")
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From page 35... ...
Geim, S.V. Morozov et al., "Electric Field Effect in Atomically Thin Carbon Films," Science, 306, 666 (2004)
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From page 36... ...
Controlling and minimizing defects in crystalline materials also constitute an important path to device innovation. Many next-generation devices for applica tions such as solar energy, solid-state lighting, and novel sensors require crystalline order among nontraditional atomic, molecular, or nanoscale building blocks.
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From page 37... ...
In the major sections immediately following, the committee presents a vision for the future of both bulk and thin-film crystalline systems in the form of three grand challenges. This chapter then concludes by discussing the needs for applied crystal growth in technology development and the role that characterization will continue to have in new crystalline materials discovery.
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From page 38... ...
The Creation of New Crystalline Materials for Energy Production and Conversion Future energy technologies will require dramatic advances in crystalline mate rials, including thermoelectric materials for heat-to-electricity conversion, solar photovoltaic materials for sunlight-to-electricity conversion, novel materials for hydrogen production and storage, new electrode and membrane materials for fuel cells, and affordable catalysts for feedstock-to-fuel conversion. Many such energy applications will require crystalline phases with low parasitic energy loss, such as low-cost solar cells with 50 percent power efficiency.
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From page 39... ...
Characterized by an open structure and large numbers of synthetic combinations, the skutterudites exhibit a wide array of physical properties, including metal-insulator transitions, heavy-fermion superconductivity, and large thermoelectric power figure of merit. The latter property is aided by large voids in the crystal structure that allow large thermal vibrations for the atoms that reside in them -- the atoms rattle around.
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From page 40... ...
and barium (green) atoms providing the spacing for a two dimensional electronic structure.
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Such crystalline structures possessing either chains or planes of interacting atoms modify the flow of energy for magnetic and electronic excitations, creating platforms for useful devices. This lower effective dimensionality can also lead to entirely new ground states.
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Low-Dimensional High-Thermopower Cobaltates The concepts of low dimensionality, geometric frustration of spin order ing, structural and orbital degrees of freedom, correlated electron physics, and quantum fluctuations converge to yield the unexpected physical properties of the layered oxide cobaltates. In a structural family based on hexagonal CoO2 layers, these metallically conducting compounds display thermoelectric coefficients two orders of magnitude larger than those observed in conventional metals.
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From page 43... ...
The synthesis scientist approaches the challenge of creating such new emergent behavior by using different paradigmatic approaches to crystal growth. Geometrically Frustrated Structures One route to optimizing competing interactions is through geometrical frustration of magnetic interactions.
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Cl2 has excited physicists for its indications of a quantum spin liquid and illustrates the potential of searching beyond oxides. Because many of the qualitatively new phases anticipated theoretically are extremely sensitive to defects, experimental progress is often entirely controlled by progress in materials synthesis techniques and in identifying new materials that realize specific lattice symmetries and/or that can be produced in a particularly pure form.
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From page 45... ...
This orbital degeneracy is similar to geometrical frustration-induced spin degeneracy in its close connection to the crystal lattice symmetry, and recent work has provided evidence for both orbital liquid and orbital glass phases in compounds with the spinel crystal structure. Phase-Change Materials When two different structural states are similar in energy, they compete with each other for the right to exist at a given temperature.
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From page 46... ...
Applying pressure, high magnetic or electric fields, or extreme temperatures can suppress one of these states while enhancing the other. For basic science, extreme environments offer the possibility of continuously tuning proper ties to access unique critical parts of phase space that may be inaccessible through materials synthesis without introducing disorder.
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From page 47... ...
The number of combinations allowed between atoms from different classes is large, however, particularly when different crystal structures are taken into account. In the above example of magnetoelectric multifunctional materials, the search for new compounds also requires synthesis and characterization of single crystals, given the need for the intrinsic response of the dielectric as well as the need to access anisotropic structures.
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From page 48... ...
New approaches to control and alter magnetism with an electric field, or, conversely, altering a ferroelectric state with a magnetic field, suggest novel thin-film multiplayer struc tures. In the schematic example shown in Figure 2.7, control of magnetism with an electric field requires a coupling mechanism at the interface that inherently breaks time reversal symmetry, which is required to switch the state of magnetization.
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The use of magnetoelectric coupling in multiferroic spintronics would enable the manipulation of spin magnetism not by slow and weak-coupling magnetic fields but by fast and strong-coupling electric fields.
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Conventional Semiconductor Heterostructures Three decades of research and development yielded practical devices such as the field-effect transistor, the quantum-well laser, and the quantum-cascade laser. They also provided insights into fundamental science from unusual quantum states such as the quantum Hall effects and exciton Bose-Einstein condensates.
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. Important developments in terahertz acoustics are based primarily on compound semiconductors using mature epitaxial growth techniques, such as molecular-beam epitaxy (MBE)
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52 Frontiers in C rys ta l l i n e M at t e r FIGURE 2.8 Schematic illustration of a device incorporating a resonant cavity for acoustic phonons inside an optical cavity, thereby enhancing interaction between sound and light. SOURCE: Reprinted by permission from Macmillan Publishers Ltd: Nature, Adapted from J.M.
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Thin-film growth is most closely associated with a level of control not attainable with bulk-crystal growth techniques. Challenges in thin-film growth are addressed in the section below
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. entitled "Grand Challenge 3: Evolution in the Capacity to Create Crystalline Mate rials by Design." In this subsection, challenges are discussed for the growth of bulk-crystalline material, both in the highly exploratory phase of new materials development and in the growth of large single crystals.
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From page 55... ...
in Japan in the 1990s expanded capabilities for oxide crystal growth to larger crystals, lower defect densities, and more complex stoichiometries. However, as is the case for most other advances in crystal growth technology, this development did not increase the ability to leverage human resources as have advances in measurement techniques.
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From page 56... ...
SOURCE: Reprinted from U.S. figure 2-11.eps Department of Energy, Office of Science, Report of the Basic Energy Sciences Workshop on Solar bitmap Energy Utilization, April 18-21, 2005, Basic Research Needs for Solar Energy Utilization, Figure 30, available at http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf.
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From page 57... ...
While the economics of Si PVs are subject to the demand for high-purity material for use in integrated circuits and energy costs for producing high-purity crystalline Si, much research is devoted to improving the trade-off between the efficiency and cost of growth and the purity and crystallinity of ingots grown by the traditional Czochralski process. Most of the innovation in crystal growth is in non-ingot-based technologies such as Si ribbon growth, which circumvents the cost of slicing ingots to produce wafers.
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From page 58... ...
Organic-based materials are potentially attractive candidates for PV appli cations because of their low formation energies and associated low fabrication cost. Typical fabrication processes involve evaporation or spin deposition from a solution of organic molecules; in contrast, most Si PVs are fabricated from zone refined single crystals.
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From page 59... ...
have the potential to reduce electricity use for indoor lighting by 50 percent, making this area a very important focus for energy reduction. Compound semiconductors that emit light are generally singlecrystal materials grown as thin films at very high purity using layer-by-layer growth processes such as MBE or metal-organic chemical vapor deposition.
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Over the past 30 years, research on strongly correlated electron materials, such as superconducting
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Both the understanding of ultimate performance and the search for alternate materials require single crystals. For example, the highest-Tc materials contain light ions whose location and motion are difficult to ascertain using x-ray diffraction and require neutron scattering.
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At an applied science level, since the optimum superconducting materials (layered cuprates) are highly anisotropic from a structural perspective, it is essential to understand the details of their crystal growth in order to optimize the manufacturing processes.
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Needed Crystal Growth Capability for Energy Conversion and Storage Energy conversion and storage are representative areas in which both the development of new crystalline materials and the optimization of existing mate rials are needed in order to advance important technologies.
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Such crystal growth research is also relevant to solid-state lighting applications. Some of the requirements mentioned above for a path to large-scale production apply to organic-based semiconductors.
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Grand Challenge 3: Evolution in the Capacity to create Crystalline Materials by Design This section presents Grand Challenge 3: that of developing the capability of designing new materials from first principles to meet specific technological needs. Long a dream of scientists and engineers, such "materials-by-design" approaches are becoming increasingly possible through rapid advances in theory and model ing, coupled with continuous increases in computational power.
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However, to fully realize the potential offered through materials by design, further advances must be made in a number of computational areas. Two such areas -- the development of more-refined methods for predicting crystal structure and the simulation of crystal growth -- are addressed here.
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The valence states now fix the energy and length scales of the electronic structure because the pseudopotential describes only these states. As a result, very simple basis functions, such as plane waves and Gaussian shapes, or simple grids can be used to describe the electronic states.
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From page 69... ...
Simulating Crystal Growth The growth of bulk single crystals remains one of the most challenging and astonishing technical endeavors of materials processing. For example, electronicgrade Si grown by the Czochralski method is one of the purest and most perfect materials ever humanly produced.
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From page 70... ...
Nevertheless, advances in theory and modeling have led to and will continue to lead to important advances in crystal growth. Bulk-crystal growth encompasses a wide variety of physical phenomena that occur over a vast range of length scales, making it among the most challenging of industrial processes to model.
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From page 71... ...
The understanding gained from more realistic, multiscale models for crystal growth will ultimately lead to the ability to link crystal structure and properties with growth conditions and the macroscopic factors that influence them. Areas of Success in Creating Materials by Design Having discussed some of the theoretical and computational challenges that must be addressed in achieving future success in creating materials by design, the discussion now turns to some of the areas where success has already been reached.
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From page 72... ...
This issue is extraordinarily difficult to treat theoretically, given the absence of any periodicity in lattice mismatches. A particularly important methodological development for modeling device behavior has been the rigorous framework for applying finite electric fields within density functional theory, and its extension to metal-insulator heterostructures and interfaces.
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From page 73... ...
Materials with High Strength and Toughness: The Next Generation of Steels An area in which the materials-by-design philosophy has already paid handsome dividends is the field of structural alloys, particularly alloy steels. Several decades ago, DOE made a strategic commitment to the development of nextgeneration alloy steels having the desirable combination of ultrahigh strength and high fracture toughness, along with high corrosion resistance in chemically and thermally hostile environments.
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From page 74... ...
Single crystals are likely to be of great value in this area. Radiation Sensors Illicit trade in nuclear materials on world markets poses a long-term inter national security threat and has created an urgent need for instrumentation that can rapidly, reliably, and inexpensively detect the x-ray and gamma-ray spectra of such materials.
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From page 75... ...
None of these materials has yet produced results comparable to those achieved with ZnCdTe. Requirements created by nuclear threats for very large numbers of radiation detectors with 1 percent energy resolution at 1 MeV operating at or above room temperature demand new approaches.
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From page 76... ...
This extremely tight coupling between materials purification, crystal growth, and detector fabrication formed the recipe for success. Decoupling Electron and Phonon Transport: The Search for High-Efficiency Thermoelectrics Since the discovery of electricity, research on charge transport in materials has pushed the extremes of electrical conductivity, which now spans more than 20 orders of magnitude.
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matching energy scales. Challenges at Morphological Length Scales Today, various processes (bulk-crystal growth, thin-film growth)
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From page 78... ...
mean that high-quality crystal growth processes, such as the Czochralski method, are not applicable. A good example is SrTiO3, which is an important substrate material for oxide heterostructure growth.
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From page 79... ...
Applied Crystal Growth for Technology Development Grand Challenge 2, the Creation of New Crystalline Materials for Energy P roduction and Conversion, addresses a specific technology area. Crystalline materials are, however, crucially important in many other technologies.
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Several areas of opportunity for research in crystalline materials for next-generation technologies are reviewed in the following pages. Next-Generation Crystalline Materials for Future Information Technology The earlier section entitled "Grand Challenge 1: The Development of Next Generation Crystalline Materials -- New States of Matter and New Materials -- for Future Information and Communications Technologies" discussed a wealth of phenomena with potential for integration into future information systems.
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. Experiments performed on sheets one atomic layer in thickness peeled from single crystals of graphite suggest that graphene holds the possibility of becoming the electronic material of the future.
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82 Frontiers in C rys ta l l i n e M at t e r FIGURE 2.18 Nonlinear figure of merit for several crystalline materials used in infrared frequency conversion. SOURCE: Courtesy of Peter Schunemann, BAE Systems.
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From page 83... ...
New Growth Techniques The development of new crystal growth techniques requires a focused research effort, independent of efforts that use established techniques to vary chemical composition. New techniques usually strive to produce crystals that are purer, larger, or of a type that cannot be accessed using standard methods (e.g., growth at high pressure)
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From page 84... ...
Thus, co-joined FET structures provide a novel route to altering the electronic properties of crystalline matter, and new FET methods will likely be developed in the future. • Related to the interest in FET structures, interfaces in crystalline hetero structures are now seen as a well-established type of crystalline matter for studying new electronic states, not only in covalent semiconductors but also in ionic oxides and van der Waals bonded organics.
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From page 85... ...
Future directions in crystal growth techniques will depend strongly on the classes of materials that define the mainstream topics in condensed-matter science. New compounds are constantly being discovered, and novel approaches to producing large single crystals of these compounds are often required.
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National Facilities for Materials Characterization While bulk experiments carried out in a single-investigator laboratory are typi cally completed first, full understanding of new materials often requires the use of national facilities that probe matter on the atomic scale. Table 2.1 provides an overview of major federally funded facilities for materials research.
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From page 87... ...
d The Center for Microanalysis of Materials at the University of Illinois at Urbana-Champaign has not been designated a Basic Energy Sciences user facility since FY 2005; hence no current statistics are available. e User information regarding the National High Magnetic Field Laboratory is from the 2007 Annual Report for NHMFL, available at http://www.magnet.fsu.edu/mediacenter/publications/reports/annualreport-2007.pdf.
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Such information is critical for understanding physical properties, from electronic transport to thermal conductivity. Thin films and nanosize crystalline materials will be incorporated into many future technologies, from quantum computers to medical therapies.
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From page 89... ...
, and thus 100 T fields enable the probing of electronic interactions of 100 K in strength. The availability of such fields enables the study of novel electronic phenomena such as critical phases in the quantum regime, including new fractional quantum Hall states; characterization of high-temperature superconductors for both basic science and large-currentcarrying applications; and novel phases in magnetic materials such as spin ice and low-effective-dimensionality materials.
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From page 90... ...
With further improvements in these areas at both the NIST Center for Neutron Research and at the Oak Ridge National Laboratory, the capacity for single-crystal experiments will continue to grow dramatically in the coming decade. While single-crystal neutron scattering is seldom the first experiment to be conducted on a new material, such experiments are often necessary in order to understand and control new materials properties.
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From page 91... ...
of the materials discovery that initiated the research. SOURCE: Data provided by Peter Gehring, NIST Center for Neutron Research.
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From page 92... ...
Two specific examples of crystal growth as the rate-limiting factor for scientific progress are provided below. The first of these examples is that almost two decades ago, neutron scatter ing experiments uncovered a spectacular spin resonance in the superconducting state of YBa2Cu3O6+δ.
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From page 93... ...
ing letter to Nature reporting a spin resonance in the superconducting state of Bi2Sr2Ca2CuO8+δ had been cited 190 times since its publication in 1999. A second example of research limited by crystal growth capabilities is the recent effort to understand charge and spin dynamics in NaxCoO2.yH2O.
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From page 94... ...
the production of high-quality samples with the appropriate morphology and dimensions for advanced characterization. Increased emphasis on the discovery and growth of novel crystalline materials is needed to realize the potential of facilities for new science and for materials-based applications in technologies ranging from information to energy.
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