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Challenges in nanomaterials design
Progvess m Materrals Srrenre Vol 42. pp 5-21. I997 Pubhshed by Elsewer Science Ltd Prtnted m Great Bntam 0079~6425/97 $32.00
PII:SOO79-6425(97)00005-4
CHALLENGES
IN NANOMATERIALS
A. S. Edelstein,*
J. S. Murday
Naval Research Laboratory,
DESIGN
and B. B. Rath
Washington,
DC, 20375, USA
CONTENTS 1. INTRODUCTION 2. ADVANCES IN FABRICATION/PROCESSING 2.1 Bulk Fabrication/processing 2.2. Film Fabrication/processing 3. ADVANCES IN CHARACTERIZATION 3.1. Tunneling 3.1.1. Scanning tunneling microscopy 3.1.2. Scanning tunneling spectroscopy (STS) 3.2. Field Emission 3.3. Force Microscopy/spectroscopj 3.4. Near-jield Microscopylspectroscopj 4. PROPERTIES 4. I. Mechanical Properties 4.2. Surface Properties 4.3. Electric Transport, Electronic and Optrcal Properties 4.4. Magnetic Properties 5. CONCLUSION ACKNOWLEDGEMENTS REFERENCES
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10 10 10 12 12 13 14 14 16 16 17 17 18 18
1. INTRODUCTION When crystalline dimensions are reduced to nanometre sizes, new structures and properties result. Because of this, considerable effort is being devoted to fabricating nanomaterials to exploit their special properties. Here we present an overview of some aspects of these efforts. The field of nanomaterials is very large and can be considered to include clusters and, in fact, any material having a nanometre dimension. Because of space limitations, no attempt has been made to provide a comprehensive overview. A recent book provided such an overview.“’ Although the choice of materials to be discussed was of necessity subjective, the emphasis will be on selected advances in fabricating and characterizing nanomaterials and their properties. We begin with a discussion of advances in fabrication/processing of nanomaterials.
2. ADVANCES
IN FABRICATION/PROCESSING
2.1. Bulk Fabrication/processing As in all areas of materials research, progress in the study of nanomaterials is dependent on having new materials that are worthy of study. There is a need for different techniques *To whom correspondence
should be addressed. 5
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to fabricate the various kinds of nanomaterial, including bulk material composed of nanometre-sized particles. (2.3)For example, thermomechanical processing (an example is given in Section 4.1) and mechanical attrition can be used to produce bulk nanomaterials. Vapor deposition (chemical, physical, laser, etc.) conditions can be arranged to grow small grains.(4) The same is true for solution deposition (sol-gel, electrochemical, etc.) of precipitates. w The fabrication of free-standing powders of nanometre size has been done through atomization technique,@*‘) and by growth from a supersaturated vapor@+ obtained by limiting the mean free path in an inert gas. Material transformations through reactions using chemical precursors can readily produce nanostructures in bulk quantities. For example, the sol-gel process was employed to make nanostructures.‘“) In this process, glassy structures form in which nanometre-sized voids may contain other materials. Subsequent chemical and physical treatment of these sol gels can lead to bulk quantities of material containing the gel, with or without inclusions, having the desired grain sizes. Thermodynamic drivers for the production of certain chemicals are reasonably well understood, although the kinetic principles leading to a desired morphological structure from chemical reactions are less well understood. This represents an interesting area for research. Control of morphology may lead to highly desired properties. Another fabrication method is known as ‘self-assembly’. This refers to the thermodynamic and kinetic processes that lead to an ordered product. Nanocubes of MO self-assemble in the vapor (‘*)to larger n x n x n cubes with n = 2, 3 and 4. Fullerenes self-assemble into clusters with magic numbers that are probably due to the formation of icosahedra similar to those found for Ar clusters. (14)Another approach of interest is the use of vesicles (self-assembled lipid layers in spherical geometry) to cover nanostructures. This cover prevents coagulation and ripening processes until the latter stages of processing. (Is)Understanding the complex interplay between thermodynamics and kinetics to take advantage of nature’s processes is a challenge. There is a need to achieve predictability. Methods using the self-assembly of molecules(‘6~‘7)are eliciting considerable interest, especially in the biomolecular engineering disciplines.
2.2. Film Fabrication/processing Reducing the size of semiconductor structures is one of the most intensely pursued high technology areas today. Decreasing the device sizes generally increases the speed while diminishing the cost and power dissipation per operation. Device size scales have already been reduced to nanometres in one dimension through the use of molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE). Atomically smooth films can be produced with thicknesses ranging from nanometres to micrometres.“*’ This fine control enables electronic band gap engineering through the fabrication of suitable multilayers or superlattices and represents a highly flexible approach to materials by design.(‘g.20)Current electronic devices (quantum well lasers, high electron mobility transistors, resonant tunneling diodes, etc.) already utilize these nanodimensioned films. Further changes from bulk properties are expected when additional dimensions are decreased; e.g. smaller second (wire) and third (dot) dimensions. The fabrication of these structures with the necessary control of composition/structure/geometry is not readily accomplished.
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Microfabrication techniques for electronic structures are commercially viable at the 0.35 urn scale (for example, the Pentium Pro chips). Meanwhile, laboratory techniques are focused on solving problems with fabricating 0.1-0.25 urn features. The fabrication of sub-micrometre or nanometre dimensions in one (wire) and zero (dot) dimensional structures generally utilizes lithographic processes. (*‘,**)Visible/UV light is typically used because it can produce accurate patterns simultaneously (parallel processing) across an entire wafer of 8 to 12 in in diameter; as structures become smaller, visible/UV light lithography becomes diffraction limited. Phase contrast techniques have extended the optical lithography to sub-micrometre dimensions’*‘) and, thereby, have extended visible/UV manufacturing lifetime. Anticipating an ultimate limit to the use of those wavelengths, X-ray techniques are being investigated. (24’ Additionally, high-energy electron/ion beam projection techniques are known to give nanometre resolution exposure to resists.“‘,**) Proximal probes enable lithography with nanometre resolution by using lower energy interactions. (25-27)These techniques can construct material structures atom-by-atom’*‘,**’ or write nanometre-dimensioned patterns.(2”33) An example of a functional nanostructure made with an atomic force microscope is shown in Fig. 1. The figure is an atomic force microscope (AFM) image of a single, atomic-sized, metallic, point-contact, room-temperature, quantum device. An electrically conducting AFM was used to anodically oxidize selected regions (raised light areas) of a 1 urn wide Al wire. A second oxide feature, highlighted by the circle, further constricts the current down to a channel approximately one atom in diameter. Other approaches to the synthesis of nanomaterials are discussed in considerable detail in a recent book.“) Parallel processing is an economic necessity for commercial fabrication. While highenergy electron beam exposures are capable of defining structures of 10 nm, currently this method is neither robust nor fast. Writing with proximal probes is presently also performed in serial fashion, but several attempts are being made to develop microfabricated arrays which might introduce as many as lo4 writing heads per square inch.‘3s-37) A major concern with the proximal probe approach is that the probe itself might become changed or damaged due to contact with the writing surface. As a middle ground between proximity and high-energy electron patterning approaches, microfabrication by moderate voltage (1 kV) electron beam sources are under investigation.‘38,39’ Lithographic masks have been typically made by patterning and etching. An interesting alternative under investigation is nanochannel glass. (40’It is found that, by drawing bundles of concentric glass tubing over multiple cycles, and by carefully arranging the glass bundles in each draw, surprisingly regular arrays of holes can be fabricated in a glass wafer. The dimensions of the holes in these wafers have been as small as 30 nm. Figure 2 shows an example of an array of these channels which might serve either as a template for molecular diffusion to a backing plate (mask) or for deposition of quantum dots/wires into the pores themselves. Resist technology at the micrometre scale has utilized polymers. But for very thin films, self-assembly of supramolecules may be a more effective approach; e.g. with appropriate chemicals such as surfactants, a two-dimensional layer of monomolecular film readily forms at the surface of a liquid. The familiar Langmuir-Blodgett technique captures this film on a solid plate. Self-assembly of remarkably defect-free films onto a substrate directly from a gas or liquid under ambient conditions is possible. Several groups are investigating these materials for application in patterning and processing of nanoscale structures.‘4’-43’
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300 200 Time (Set) Fig. 1. (a) An AFM image of a quantum device fabricated by use of an electrically conducting AFM as described in the text. (b) The graph plots the conductance of four different trials of the device, which were recorded in real time during the fabrication of the point contact. A final conductance of 2ez/h was achieved, which corresponds to a single atomic-sized conducting channel (conductance value of one).co
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Fig. 2. SEM micrograph illustrating the uniform hexagonal array of 0.5 pm diameter channels in a nanochannel glass.
3. ADVANCES
IN CHARACTERIZATION
This section highlights the new tools that have become available for characterizing nanostructured materials, with emphasis on the new class of analytical tools-proximal probes. These are analytical tools in which the separation between the probe and the sample is a few nanometres. c-61 The scanning tunneling microscope (STM) is the most recognized example of a proximal probe. Such probes can be organized into four classes defined by the physical principles on which they operate-tunneling, field emission, force, and near-field. CM) The operating p rinciples and capabilities of these proximal probes are succinctly presented below with copious references (4657)for those desiring more extensive understanding. A Workshop on Industrial Applications of Scanned Probe Microscopy focused on accelerating the already rapid rate at which proximal probes are being incorporated into industrial practices.(58) In addition to the proximal probes, other advances in analytical capability are important. Low-energy electron microscopy with better than 10 nm resolution has been developed by Bauer.‘j9) Ourmazd et al. (60)have combined chemical lattice imaging with vector pattern recognition to extract near-atomic composition profiles by means of transmission electron microscopy (TEM). Konnert and D’Antonio(61) have shown that electron nanodiffraction patterns may be obtained with scanning transmission electron
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microscopy (STEM) beams. These diffraction patterns, which change drastically when the beam is translated a small fraction of its diameter, contain information on the structure of the atoms within the beam to a much higher resolution than the beam dimension. The electron nanodiffraction patterns obtained with a STEM from overlapping regions of an Si (110) sample have been used to reconstruct 1 8, resolution images.(62) With the high-intensity synchrotron X-ray sources, the size of crystallites necessary for X-ray diffraction studies is in the nanometre domain (0.4 l.tm3).(63)
3.1. Tunneling 3.1.1.
Scanning tunneling microscopy (STM)
Scanning tunneling microscopy is based on electron tunneling through the potential barrier between two surfaces positioned about a nanometre apart. The exponential dependence of tunneling current on tip-surface separation imparts the ability to image the surface with nanometre resolution. Valuable resources for scanning tunneling concepts and for the state-of-the-art are STM conference proceedings(45) and several books.(4657) Tunneling contrast mechanisms at clean (UHV) metal and semiconductor surfaces are believed to be reasonably well understood. (5’.40Moreover, tunneling is not constrained to vacuum barriers, and can occur through dielectric fluids. Tunneling tips are utilized in the in situ study of electrochemical phenomena, .(61*65) however, the contrast mechanisms at the solid/liquid interface are not always apparent. The tunneling tip also can be used to image buried interface structures in a technique called ballistic electron emission microscopy (BEEM).‘%’ There have been several efforts at microfabricating an STM apparatus;(36,37)MacDonald and co-workers at Cornell have demonstrated an operating microfabricated STM with 5 MHz resonant frequency and 400 nm x 400 nm scanning range.@‘)The principal drivers to the microfabrication efforts are lithography and non-destructive evaluation (NDE) of integrated circuits. Moreover, these microfabricated proximal probes will enable investigation in highly constrained geometries. An elegant example of STM imaging to assist in understanding semiconductor surface structures at the atomic scale is presented in Fig. 3. The figure compares the experimental and theoretical STM images of Si(5 5 12) which were used to determine how the surface reconstructs. However, meaningful characterization of surface features does not always require atom-level resolution. Many groups utilize the STM for characterization of high-technology nanometre structures such as machined surfaces,(69*‘O)fracture surfaces,(“) Vickers’ imprints, (72)X-ray optical multilayer films,(73) microfabricated electronic device patterns,(74) optical disks(75) and microbridges in superconductivity quantum interference devices (SQUIDS).“@ 3.1.2. Scanning tunneling spectroscopy (STS) In addition to imaging, it is also possible to hold an STM tip over a specific location and measure its tunneling current-voltage characteristics. A tip-sample potential range approaching 4 V is possible before the tunneling approximations are violated. When the tip is biased negative, the tunneling electrons originate in filled states of the tip and tunnel into the empty states of the surface and vice versa for reverse bias. The current versus bias data therefore yield a convolution of the tip and surface electron density-of-states. This
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Fig. 3. The atomic-scale structure of Si(5 5 12). Models are shown of the bulk-truncated (D) and reconstructed surfaces (C). The atoms are shaded to highlight their proposed rearrangement within the reconstruction. The unit cell on the reconstructed surface is indicated (black box). Theoretical (A) and experimental (B) STM images of reconstructed surface are exhibited.‘68J
spectroscopic capability has been used to investigate the site-specific, surface-state character of numerous surfaces.(“) STS is limited as a chemical analytical tool since the valence electron density-of-states is too broad for atom-specific identification, unless one already knows a great deal about
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the surface under investigation. There are some efforts to couple electromagnetic spectroscopies for chemical identification and tips for localization.(7*,79) 3.2. Field Emission Significant electron tunneling current occurs only when the tip and surface are close (-0.5-2.0 nm) and at low bias conditions (< 4 V). As the tip and surface separation is increased and/or the bias is raised, electron transfer occurs under the field emission approximation.“‘) Field emission tips are commonly used in scanning electron microscopes where their high brightness, low-temperature operation and small source volume enable high-resolution work. Spot sizes of OS-5 nm are achieved with working potentials in the range OS-50 kV. If the tip shaping techniques used for field ion microscopy are used, then Electron beams from these sharp tips the field emission half-angle can be very small. (8’,82) placed close to the surface (N 10 nm) can have spot diameters in the tens of nanometre range with biases in the range of 10 to 100 V. Compared with conventional scanning electron microscopy (SEM), the lower electron energies give greater surface sensitivity and diminished beam damage. Compared with STM, the higher electron energies enable core electron spectroscopic analysis of the surface species. Microfabricated field emission electron guns(38*39) may enable the construction of very small electron microscopes. It is conceivable that we may place a future microfabricated SEM directly onto a sample for analysis, rather than be required to cut off a piece of the sample for insertion into a vacuum chamber. 3.3. Force Microscopy/spectroscopy A limitation to STM/STS and its field emission cousin is the requirement of a large electron current density. To use these techniques, the sample must be reasonably conducting. To overcome this limitation, Binnig et al. (83)developed the concept of atomic force microscopy (AFM). The basic idea is to measure small deflections at the end of a thin cantilever beam; the beam deflection is induced by the forces between a sample and a tip on the free end of the cantilever. If the beam is either microfabricated or a fine wire, then small spring constants (0.1-100 N m-‘) are possible. For 0.01 nm displacement sensitivity, forces of 10e8 to lo-‘* N are measurable (specially constructed force microscopes have measured as low as 1O-‘5N). These AFM-measurable forces are comparable to the forces associated with chemical bonding, e.g. N lo-’ N for an ionic bond, * lo-” N for a van der Waals’ bond and N lo-‘* N for surface reconstruction. Many variants on force microscopy have been developed(46*53,5s’s6) and several are commercially available. The AFM techniques have been widely employed for probing surface forces and mechanical behavior,‘8”87’ imaging samples with low electrical Under selected conditions in the conductivity,‘88) and for probing biological structures. (89,90) contact mode, i.e. the sample in physical contact with the tip, the AFM has produced images with atomic resolution.(9’) The potential for atomic force microscopic technology to provide revolutionary new insight is illustrated by the recent measurements of Lee et aZ.@“).Utilizing atomic force microscopic concepts, they have directly measured the binding forces associated with DNA molecular recognition. Previously those forces had to be deduced from thermodynamic (ensemble averaged) measurements of binding energies. The left side of Fig. 4 has an
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’”
,’
x
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Fadhesive = 2.3 nN
0
Poly-inosinc (I) linked to surfaces by base-pairing with complementary cytosine (C) Fig. 4. strands (B) As is made 25 nm
20 40 60 80 100 120 140 160 Relative Surface Separation (run)
Force required to uncoil a single poly-inosine strand and break the base-pairing interactions
Atomic force microscope measurement of the binding force between complementary of DNA. (A) Schematic of tip/surface linkage by complementary DNA molecules. the tip is moved closer to the surface, there are very low forces until physical contact (the vertical spike at zero separation). On retracting the tip, the large negative force at separation indicates full chain extension. The maximum force is a direct measure of the DNA bond strength.
illustration of strands of poly(cytosine) (a nucleic acid) covalently attached both to a substrate and to a force microscope tip. When strands of poly(inosine) (a nucleic acid complementary to cytosine) are introduced into the solution immersing the tip and substrate, the two complementary strands can pair as shown in the illustration. This binds the tip and surface together. The right side of Fig. 4 shows the force measured as the tip is first brought into contact with the surface and then withdrawn. As the tip recedes from the surface, initially there is a very low attractive force which increases non-linearly. The low attractive force is due to unraveling of the long poly(inosine) strand. When the strand is fully elongated, the binding force becomes much larger. Further retraction causes the cytosine/inosine linkage to fail and the cantilever is then free to return to its equilibrium position. With some improvement in signal-to-noise ratio, this technique should be able to probe the forces associated with the chain unraveling. Such a capability will be extremely useful for resolving the current debates on protein folding. 3.4. Near-field Microscopy/spectroscopy The most developed variant of near-field microscopy/spectroscopy utilizes the optical spectrum. Optical images with a resolution below the diffraction limit of light can be obtained if a small aperture is located close (near-field) to an object.(46,53)In the visible
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spectrum, features with resolution of about 10 nm laterally and 1 nm vertically can be obtained. It has been shown that it is possible to detect the fluorescence of a single molecule with this technique.(92)
4. PROPERTIES 4.1. Mechanical Properties The mechanical properties of nanostructures such as the strength are dominated by the interfacial properties of these structures. For example, ultimate failure results from crack nucleation and migration, which frequently takes place along interfaces. Because of the large number of interfaces, the bulk properties of nanomaterials such as their strength and toughness are significantly modified. (93)It is well established that materials with smaller grain sizes (down to about 100 A) are stronger. The usual mechanisms of plasticity and failure do not adequately represent a sample largely made up of grain boundaries. The strength of materials can be increased by introducing barriers or inhibitors to delay or reduce the crack migration process. Composites which introduce barriers to slip and crack migration within a matrix operate on this principle. Nanostructured composites offer an increased density of inhibitors to slip crack migration, leading to enhanced mechanical properties. A large percentage of material is in the interfacial layer when the surface layer thickness is comparable to the crystallite size. The fraction of the material in the interface is 3dr/r where dr is the interface thickness of a sphere with radius r. Interfaces in crystalline solids also play a role in high-temperature creep. Since one of the predominant mechanisms of creep, grain boundary sliding, results from migration of grains along the grain boundary, a material that is largely made up of grain boundaries will exhibit greater creep rates leading to superplasticity. This may or may not be desirable. For example, ceramic nanostructures are found to be more malleable, or ‘superplastic’. Thus, the formability of ceramic materials is enhanced with nanostructures. However, metallic nanostructured materials, formed from metals that are malleable when they have micrometre-sized grains, may have unacceptable levels of creep. This is especially likely to be a problem if these metallic nanostructured materials are subjected to temperature excursions. Improved mechanical properties have been realized for selected nanomaterials.(94) Direct attempts with other materials to demonstrate the improvements for structural application have met either with prohibitive cost or technological barriers. Laboratory techniques used to demonstrate the behavior of milligram or gram samples are of value to show feasibility of these new materials; however, to be incorporated into technology, sufficient quantities of material must be produced at a market-acceptable cost. Many investigations of the mechanical behavior of materials require bulk quantities (pounds) of material. The problem of having small quantities of material for testing can be mitigated somewhat by using techniques such as nanoindentation. Obtaining quantitative information by this technique has been difficult but there has been recent progress(95)with the use of spherical indentors. Figure 5 shows a plot of the reduced elastic modulus obtained from nanoindentation measurements with spherical indentors on polystyrene films as a function of film thickness. The reduced modulus is an average
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Film thickness (nm) Fig. 5. Measurements of the elastic moduli of thm. polystyrene films on Si obtained by using spherical indentors. The reduced elastic modulus is an average modulus taking mto account the deformation of the tip and the sample. The bulk value is indicated by the straight hne. The deviation at low film thickness is due to tip deformation.
modulus taking into account the deformation of the tip and the modulus. One sees quantitative agreement between the literature value and the reduced modulus determined by the nanoindentation measurements on thicker films for which tip deformation is less important. By controlling the thermomechanical processing history of a sample, the grain size may be modified to a surprising degree. A notable example of the application of this approach is the nanostructured composite composed of COWC.‘~~’This material exhibits a hardness that makes it attractive for machine cutting tools. Rapid solidification with controlled processing conditions and chemical composition produces grain structures over a wide range, from being similar to glasses to submicrometre-size crystallites. This is not a simple process to control. Rapid cooling rates may not result in a nanostructured material, but slow cooling rates always produce relatively large grain sizes. The admixture of nanostructures with a matrix material (organic, metallic or ceramic) leads to a variety of composite materials having superior properties. The discovery of carbon tubules having nanometre dimensions’97’ indicates the vitality and continued opportunity for the unexpected discovery of new nanometre-sized materials. These nanotubes of carbon appear to have surprising mechanical properties. They may offer improved material properties once researchers successfully demonstrate cost-effective methods of synthesis and fabrication. The ability to fabricate microstructures using lithographic techniques has been extended to the fabrication of machines consisting of moving structures, gears. levers, etc., A 10 urn gear, however, takes considerable expertise characteristic of larger devices. (98,99) to fabricate and control. The community of micromechanical electromechanical machines (MEMS) is demonstrating a great deal of ingenuity for fabricating mechanical devices and sensors (on a chip) using lithographic techniques. This innovative field has demonstrated cost-effective alternatives for a number of sensors.
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An inherent property of nanostructures is the reduced variation in surface roughness as well as crystallographic texture. Whatever the material, particularly when grown from the vapor phase, a surface is usually uniform only to the dimensions of the grains or sub-units from which it is composed. Materials with smaller crystallites can lead to improved machinability and surface finish which, for hard to machine materials, provides a significant fabrication advantage. (‘O”)This feature of the nanocrystalline aggregates is particularly important for diamond films. The recent discovery of new processes to prepare diamond without the usual application of high temperature/pressure has offered many new opportunities. (lo’)Nanostructures of diamond crystals adhering to each other and to a surface have become recognized as an important adjunct to modifying the properties of surfaces. Nanocrystalline diamond films can be used for many industrial applications without major polishing requirements. Research is underway to formulate advanced composites with diamond as a second-phase particulate.
4.3. Electric Transport, Electronic and Optical Properties
The electronic and optical properties of nanomaterials differ from those extrapolated from larger dimensions. Potential barriers along the transport direction or as boundary conditions introduce major perturbations which modify the waves and can lead to new phenomena. These effects are important for structures having dimensions of 10 nm or less. Resonant tunneling transistors (RTTs) have been proved to be high-speed devices with a useful negative transconductance,(‘02) and are likely to become important commercially. These devices are fabricated by using the close tolerances of atomically smooth layers grown by molecular beam epitaxy, and are referred to as vertical structures (the transport is perpendicular to the plane of the surface). The negative transconductance exhibited with RTTs can be demonstrated with many alternative geometries, including those with fingers serving as a transistor gate, which produces an alternating potential energy field to the migrating charge carriers. Confinement effects modify the electron energy levels much as the quantized energy levels for a particle in a box. The band gap is typically increased in smaller structures, causing a blue shift. (‘03)Some nanomaterials are lasing media with high gain and/or have large non-linear susceptibility coefficients. Usually the dimensions of these nanostructures are such that light is scattered minimally at the boundaries, hence optical transmission through composites can be high; in which case, the composites appear as transparent (or colored) materials. However, by using the nanochannel glass discussed above, it is possible to observe two-dimensional photonic band behavior in the visible.““’ Another phenomenon, the Coulomb blockade, is due to the discrete nature of electric charge. As charge carriers migrate along nanostructures, they charge capacitors which are so small that the presence of a single electron has a substantial influence on the voltage across the capacitance. This, in turn, influences the transport of additional charge carriers to that location in space; hence strong correlation effects are observed at low temperatures with these small structures.“Os’ There have been some reports of possible effects at room temperature,“06’ when charge migration across a small dielectric (such as a molecule between a tunneling tip and a surface) is observed.
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H(kOe) Fig. 6. The longitudinal (II) and transverse (I) magnetoreslstance, p. of a granular Co,,Ag,,, annealed at 500 K. (From 13”.p. 368.)
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The electric transport properties of nanomaterials also can be unusual. Doped nanophase ZnO(B,Bi,Co,Cu,Sb,Sn) with 3-10 nm grains has demonstrated varistor behavior for voltages of up to 30 kV cm-‘.(‘07’ 4.4. Magnetic Properties
Magnetic properties of nanostructures likewise provide a fertile ground for new discoveries. Thin layers of magnetic materials such as iron, in conjunction with chalcogenides in intervening layers, show evidence of high anisotropies and internal fields perpendicular to the plane, just the opposite of that expected from other materials. These materials show a large resistance change as the magnetic field is changed (magnetoresistance),“‘*’ and appear to be promising for improved detectors in recording devices. Some multilayer(‘W’ and granular samples”“, ‘I’)exhibit giantic magnetoresistance. Figure 6 shows an example of this phenomenon. These materials are being studied intensively for use in magnetic recording read heads. They also exhibit important properties required for non-volatile memory devices. Additional, unexpected magnetic behavior appears in other materials. The interplay between magnetic effects and transport properties introduces an interesting potential for magnetically driven electronic devices. 5. CONCLUSION In summary, the advancing frontier of nanoscience and nanotechnology appears to offer exciting scientific challenges and possibilities. The rapid progress in nanomaterials is clearly evidenced in the exponential increase in published work during the last five years. Many related programs will be needed to maintain this progress and to investigate the
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validity of hypotheses, many of which are difficult to test. Often it is only by applying several techniques that one can understand the structure and properties of a new nanomaterial. If present nanomaterial processing techniques are refined and new ones developed, the interplay of new ideas in this frontier will be attractive for some time to come.
ACKNOWLEDGEMENTS Useful conversations with W. Tolles, R. Tonucci, and B. Shanabrook the Office of Naval Research are gratefully acknowledged.
and the support of
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28. Atomic and Nanometer-Scale
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PII:SOO79-6425(97)00005-4
CHALLENGES
IN NANOMATERIALS
A. S. Edelstein,*
J. S. Murday
Naval Research Laboratory,
DESIGN
and B. B. Rath
Washington,
DC, 20375, USA
CONTENTS 1. INTRODUCTION 2. ADVANCES IN FABRICATION/PROCESSING 2.1 Bulk Fabrication/processing 2.2. Film Fabrication/processing 3. ADVANCES IN CHARACTERIZATION 3.1. Tunneling 3.1.1. Scanning tunneling microscopy 3.1.2. Scanning tunneling spectroscopy (STS) 3.2. Field Emission 3.3. Force Microscopy/spectroscopj 3.4. Near-jield Microscopylspectroscopj 4. PROPERTIES 4. I. Mechanical Properties 4.2. Surface Properties 4.3. Electric Transport, Electronic and Optrcal Properties 4.4. Magnetic Properties 5. CONCLUSION ACKNOWLEDGEMENTS REFERENCES
5 5 5 6 9
10 10 10 12 12 13 14 14 16 16 17 17 18 18
1. INTRODUCTION When crystalline dimensions are reduced to nanometre sizes, new structures and properties result. Because of this, considerable effort is being devoted to fabricating nanomaterials to exploit their special properties. Here we present an overview of some aspects of these efforts. The field of nanomaterials is very large and can be considered to include clusters and, in fact, any material having a nanometre dimension. Because of space limitations, no attempt has been made to provide a comprehensive overview. A recent book provided such an overview.“’ Although the choice of materials to be discussed was of necessity subjective, the emphasis will be on selected advances in fabricating and characterizing nanomaterials and their properties. We begin with a discussion of advances in fabrication/processing of nanomaterials.
2. ADVANCES
IN FABRICATION/PROCESSING
2.1. Bulk Fabrication/processing As in all areas of materials research, progress in the study of nanomaterials is dependent on having new materials that are worthy of study. There is a need for different techniques *To whom correspondence
should be addressed. 5
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to fabricate the various kinds of nanomaterial, including bulk material composed of nanometre-sized particles. (2.3)For example, thermomechanical processing (an example is given in Section 4.1) and mechanical attrition can be used to produce bulk nanomaterials. Vapor deposition (chemical, physical, laser, etc.) conditions can be arranged to grow small grains.(4) The same is true for solution deposition (sol-gel, electrochemical, etc.) of precipitates. w The fabrication of free-standing powders of nanometre size has been done through atomization technique,@*‘) and by growth from a supersaturated vapor@+ obtained by limiting the mean free path in an inert gas. Material transformations through reactions using chemical precursors can readily produce nanostructures in bulk quantities. For example, the sol-gel process was employed to make nanostructures.‘“) In this process, glassy structures form in which nanometre-sized voids may contain other materials. Subsequent chemical and physical treatment of these sol gels can lead to bulk quantities of material containing the gel, with or without inclusions, having the desired grain sizes. Thermodynamic drivers for the production of certain chemicals are reasonably well understood, although the kinetic principles leading to a desired morphological structure from chemical reactions are less well understood. This represents an interesting area for research. Control of morphology may lead to highly desired properties. Another fabrication method is known as ‘self-assembly’. This refers to the thermodynamic and kinetic processes that lead to an ordered product. Nanocubes of MO self-assemble in the vapor (‘*)to larger n x n x n cubes with n = 2, 3 and 4. Fullerenes self-assemble into clusters with magic numbers that are probably due to the formation of icosahedra similar to those found for Ar clusters. (14)Another approach of interest is the use of vesicles (self-assembled lipid layers in spherical geometry) to cover nanostructures. This cover prevents coagulation and ripening processes until the latter stages of processing. (Is)Understanding the complex interplay between thermodynamics and kinetics to take advantage of nature’s processes is a challenge. There is a need to achieve predictability. Methods using the self-assembly of molecules(‘6~‘7)are eliciting considerable interest, especially in the biomolecular engineering disciplines.
2.2. Film Fabrication/processing Reducing the size of semiconductor structures is one of the most intensely pursued high technology areas today. Decreasing the device sizes generally increases the speed while diminishing the cost and power dissipation per operation. Device size scales have already been reduced to nanometres in one dimension through the use of molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE). Atomically smooth films can be produced with thicknesses ranging from nanometres to micrometres.“*’ This fine control enables electronic band gap engineering through the fabrication of suitable multilayers or superlattices and represents a highly flexible approach to materials by design.(‘g.20)Current electronic devices (quantum well lasers, high electron mobility transistors, resonant tunneling diodes, etc.) already utilize these nanodimensioned films. Further changes from bulk properties are expected when additional dimensions are decreased; e.g. smaller second (wire) and third (dot) dimensions. The fabrication of these structures with the necessary control of composition/structure/geometry is not readily accomplished.
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Microfabrication techniques for electronic structures are commercially viable at the 0.35 urn scale (for example, the Pentium Pro chips). Meanwhile, laboratory techniques are focused on solving problems with fabricating 0.1-0.25 urn features. The fabrication of sub-micrometre or nanometre dimensions in one (wire) and zero (dot) dimensional structures generally utilizes lithographic processes. (*‘,**)Visible/UV light is typically used because it can produce accurate patterns simultaneously (parallel processing) across an entire wafer of 8 to 12 in in diameter; as structures become smaller, visible/UV light lithography becomes diffraction limited. Phase contrast techniques have extended the optical lithography to sub-micrometre dimensions’*‘) and, thereby, have extended visible/UV manufacturing lifetime. Anticipating an ultimate limit to the use of those wavelengths, X-ray techniques are being investigated. (24’ Additionally, high-energy electron/ion beam projection techniques are known to give nanometre resolution exposure to resists.“‘,**) Proximal probes enable lithography with nanometre resolution by using lower energy interactions. (25-27)These techniques can construct material structures atom-by-atom’*‘,**’ or write nanometre-dimensioned patterns.(2”33) An example of a functional nanostructure made with an atomic force microscope is shown in Fig. 1. The figure is an atomic force microscope (AFM) image of a single, atomic-sized, metallic, point-contact, room-temperature, quantum device. An electrically conducting AFM was used to anodically oxidize selected regions (raised light areas) of a 1 urn wide Al wire. A second oxide feature, highlighted by the circle, further constricts the current down to a channel approximately one atom in diameter. Other approaches to the synthesis of nanomaterials are discussed in considerable detail in a recent book.“) Parallel processing is an economic necessity for commercial fabrication. While highenergy electron beam exposures are capable of defining structures of 10 nm, currently this method is neither robust nor fast. Writing with proximal probes is presently also performed in serial fashion, but several attempts are being made to develop microfabricated arrays which might introduce as many as lo4 writing heads per square inch.‘3s-37) A major concern with the proximal probe approach is that the probe itself might become changed or damaged due to contact with the writing surface. As a middle ground between proximity and high-energy electron patterning approaches, microfabrication by moderate voltage (1 kV) electron beam sources are under investigation.‘38,39’ Lithographic masks have been typically made by patterning and etching. An interesting alternative under investigation is nanochannel glass. (40’It is found that, by drawing bundles of concentric glass tubing over multiple cycles, and by carefully arranging the glass bundles in each draw, surprisingly regular arrays of holes can be fabricated in a glass wafer. The dimensions of the holes in these wafers have been as small as 30 nm. Figure 2 shows an example of an array of these channels which might serve either as a template for molecular diffusion to a backing plate (mask) or for deposition of quantum dots/wires into the pores themselves. Resist technology at the micrometre scale has utilized polymers. But for very thin films, self-assembly of supramolecules may be a more effective approach; e.g. with appropriate chemicals such as surfactants, a two-dimensional layer of monomolecular film readily forms at the surface of a liquid. The familiar Langmuir-Blodgett technique captures this film on a solid plate. Self-assembly of remarkably defect-free films onto a substrate directly from a gas or liquid under ambient conditions is possible. Several groups are investigating these materials for application in patterning and processing of nanoscale structures.‘4’-43’
Progress in Materials Science
(a)
20 10 0
(b)
8
300 200 Time (Set) Fig. 1. (a) An AFM image of a quantum device fabricated by use of an electrically conducting AFM as described in the text. (b) The graph plots the conductance of four different trials of the device, which were recorded in real time during the fabrication of the point contact. A final conductance of 2ez/h was achieved, which corresponds to a single atomic-sized conducting channel (conductance value of one).co
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Fig. 2. SEM micrograph illustrating the uniform hexagonal array of 0.5 pm diameter channels in a nanochannel glass.
3. ADVANCES
IN CHARACTERIZATION
This section highlights the new tools that have become available for characterizing nanostructured materials, with emphasis on the new class of analytical tools-proximal probes. These are analytical tools in which the separation between the probe and the sample is a few nanometres. c-61 The scanning tunneling microscope (STM) is the most recognized example of a proximal probe. Such probes can be organized into four classes defined by the physical principles on which they operate-tunneling, field emission, force, and near-field. CM) The operating p rinciples and capabilities of these proximal probes are succinctly presented below with copious references (4657)for those desiring more extensive understanding. A Workshop on Industrial Applications of Scanned Probe Microscopy focused on accelerating the already rapid rate at which proximal probes are being incorporated into industrial practices.(58) In addition to the proximal probes, other advances in analytical capability are important. Low-energy electron microscopy with better than 10 nm resolution has been developed by Bauer.‘j9) Ourmazd et al. (60)have combined chemical lattice imaging with vector pattern recognition to extract near-atomic composition profiles by means of transmission electron microscopy (TEM). Konnert and D’Antonio(61) have shown that electron nanodiffraction patterns may be obtained with scanning transmission electron
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Progress in Materials Science
microscopy (STEM) beams. These diffraction patterns, which change drastically when the beam is translated a small fraction of its diameter, contain information on the structure of the atoms within the beam to a much higher resolution than the beam dimension. The electron nanodiffraction patterns obtained with a STEM from overlapping regions of an Si (110) sample have been used to reconstruct 1 8, resolution images.(62) With the high-intensity synchrotron X-ray sources, the size of crystallites necessary for X-ray diffraction studies is in the nanometre domain (0.4 l.tm3).(63)
3.1. Tunneling 3.1.1.
Scanning tunneling microscopy (STM)
Scanning tunneling microscopy is based on electron tunneling through the potential barrier between two surfaces positioned about a nanometre apart. The exponential dependence of tunneling current on tip-surface separation imparts the ability to image the surface with nanometre resolution. Valuable resources for scanning tunneling concepts and for the state-of-the-art are STM conference proceedings(45) and several books.(4657) Tunneling contrast mechanisms at clean (UHV) metal and semiconductor surfaces are believed to be reasonably well understood. (5’.40Moreover, tunneling is not constrained to vacuum barriers, and can occur through dielectric fluids. Tunneling tips are utilized in the in situ study of electrochemical phenomena, .(61*65) however, the contrast mechanisms at the solid/liquid interface are not always apparent. The tunneling tip also can be used to image buried interface structures in a technique called ballistic electron emission microscopy (BEEM).‘%’ There have been several efforts at microfabricating an STM apparatus;(36,37)MacDonald and co-workers at Cornell have demonstrated an operating microfabricated STM with 5 MHz resonant frequency and 400 nm x 400 nm scanning range.@‘)The principal drivers to the microfabrication efforts are lithography and non-destructive evaluation (NDE) of integrated circuits. Moreover, these microfabricated proximal probes will enable investigation in highly constrained geometries. An elegant example of STM imaging to assist in understanding semiconductor surface structures at the atomic scale is presented in Fig. 3. The figure compares the experimental and theoretical STM images of Si(5 5 12) which were used to determine how the surface reconstructs. However, meaningful characterization of surface features does not always require atom-level resolution. Many groups utilize the STM for characterization of high-technology nanometre structures such as machined surfaces,(69*‘O)fracture surfaces,(“) Vickers’ imprints, (72)X-ray optical multilayer films,(73) microfabricated electronic device patterns,(74) optical disks(75) and microbridges in superconductivity quantum interference devices (SQUIDS).“@ 3.1.2. Scanning tunneling spectroscopy (STS) In addition to imaging, it is also possible to hold an STM tip over a specific location and measure its tunneling current-voltage characteristics. A tip-sample potential range approaching 4 V is possible before the tunneling approximations are violated. When the tip is biased negative, the tunneling electrons originate in filled states of the tip and tunnel into the empty states of the surface and vice versa for reverse bias. The current versus bias data therefore yield a convolution of the tip and surface electron density-of-states. This
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Fig. 3. The atomic-scale structure of Si(5 5 12). Models are shown of the bulk-truncated (D) and reconstructed surfaces (C). The atoms are shaded to highlight their proposed rearrangement within the reconstruction. The unit cell on the reconstructed surface is indicated (black box). Theoretical (A) and experimental (B) STM images of reconstructed surface are exhibited.‘68J
spectroscopic capability has been used to investigate the site-specific, surface-state character of numerous surfaces.(“) STS is limited as a chemical analytical tool since the valence electron density-of-states is too broad for atom-specific identification, unless one already knows a great deal about
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the surface under investigation. There are some efforts to couple electromagnetic spectroscopies for chemical identification and tips for localization.(7*,79) 3.2. Field Emission Significant electron tunneling current occurs only when the tip and surface are close (-0.5-2.0 nm) and at low bias conditions (< 4 V). As the tip and surface separation is increased and/or the bias is raised, electron transfer occurs under the field emission approximation.“‘) Field emission tips are commonly used in scanning electron microscopes where their high brightness, low-temperature operation and small source volume enable high-resolution work. Spot sizes of OS-5 nm are achieved with working potentials in the range OS-50 kV. If the tip shaping techniques used for field ion microscopy are used, then Electron beams from these sharp tips the field emission half-angle can be very small. (8’,82) placed close to the surface (N 10 nm) can have spot diameters in the tens of nanometre range with biases in the range of 10 to 100 V. Compared with conventional scanning electron microscopy (SEM), the lower electron energies give greater surface sensitivity and diminished beam damage. Compared with STM, the higher electron energies enable core electron spectroscopic analysis of the surface species. Microfabricated field emission electron guns(38*39) may enable the construction of very small electron microscopes. It is conceivable that we may place a future microfabricated SEM directly onto a sample for analysis, rather than be required to cut off a piece of the sample for insertion into a vacuum chamber. 3.3. Force Microscopy/spectroscopy A limitation to STM/STS and its field emission cousin is the requirement of a large electron current density. To use these techniques, the sample must be reasonably conducting. To overcome this limitation, Binnig et al. (83)developed the concept of atomic force microscopy (AFM). The basic idea is to measure small deflections at the end of a thin cantilever beam; the beam deflection is induced by the forces between a sample and a tip on the free end of the cantilever. If the beam is either microfabricated or a fine wire, then small spring constants (0.1-100 N m-‘) are possible. For 0.01 nm displacement sensitivity, forces of 10e8 to lo-‘* N are measurable (specially constructed force microscopes have measured as low as 1O-‘5N). These AFM-measurable forces are comparable to the forces associated with chemical bonding, e.g. N lo-’ N for an ionic bond, * lo-” N for a van der Waals’ bond and N lo-‘* N for surface reconstruction. Many variants on force microscopy have been developed(46*53,5s’s6) and several are commercially available. The AFM techniques have been widely employed for probing surface forces and mechanical behavior,‘8”87’ imaging samples with low electrical Under selected conditions in the conductivity,‘88) and for probing biological structures. (89,90) contact mode, i.e. the sample in physical contact with the tip, the AFM has produced images with atomic resolution.(9’) The potential for atomic force microscopic technology to provide revolutionary new insight is illustrated by the recent measurements of Lee et aZ.@“).Utilizing atomic force microscopic concepts, they have directly measured the binding forces associated with DNA molecular recognition. Previously those forces had to be deduced from thermodynamic (ensemble averaged) measurements of binding energies. The left side of Fig. 4 has an
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,’
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Fadhesive = 2.3 nN
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Poly-inosinc (I) linked to surfaces by base-pairing with complementary cytosine (C) Fig. 4. strands (B) As is made 25 nm
20 40 60 80 100 120 140 160 Relative Surface Separation (run)
Force required to uncoil a single poly-inosine strand and break the base-pairing interactions
Atomic force microscope measurement of the binding force between complementary of DNA. (A) Schematic of tip/surface linkage by complementary DNA molecules. the tip is moved closer to the surface, there are very low forces until physical contact (the vertical spike at zero separation). On retracting the tip, the large negative force at separation indicates full chain extension. The maximum force is a direct measure of the DNA bond strength.
illustration of strands of poly(cytosine) (a nucleic acid) covalently attached both to a substrate and to a force microscope tip. When strands of poly(inosine) (a nucleic acid complementary to cytosine) are introduced into the solution immersing the tip and substrate, the two complementary strands can pair as shown in the illustration. This binds the tip and surface together. The right side of Fig. 4 shows the force measured as the tip is first brought into contact with the surface and then withdrawn. As the tip recedes from the surface, initially there is a very low attractive force which increases non-linearly. The low attractive force is due to unraveling of the long poly(inosine) strand. When the strand is fully elongated, the binding force becomes much larger. Further retraction causes the cytosine/inosine linkage to fail and the cantilever is then free to return to its equilibrium position. With some improvement in signal-to-noise ratio, this technique should be able to probe the forces associated with the chain unraveling. Such a capability will be extremely useful for resolving the current debates on protein folding. 3.4. Near-field Microscopy/spectroscopy The most developed variant of near-field microscopy/spectroscopy utilizes the optical spectrum. Optical images with a resolution below the diffraction limit of light can be obtained if a small aperture is located close (near-field) to an object.(46,53)In the visible
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spectrum, features with resolution of about 10 nm laterally and 1 nm vertically can be obtained. It has been shown that it is possible to detect the fluorescence of a single molecule with this technique.(92)
4. PROPERTIES 4.1. Mechanical Properties The mechanical properties of nanostructures such as the strength are dominated by the interfacial properties of these structures. For example, ultimate failure results from crack nucleation and migration, which frequently takes place along interfaces. Because of the large number of interfaces, the bulk properties of nanomaterials such as their strength and toughness are significantly modified. (93)It is well established that materials with smaller grain sizes (down to about 100 A) are stronger. The usual mechanisms of plasticity and failure do not adequately represent a sample largely made up of grain boundaries. The strength of materials can be increased by introducing barriers or inhibitors to delay or reduce the crack migration process. Composites which introduce barriers to slip and crack migration within a matrix operate on this principle. Nanostructured composites offer an increased density of inhibitors to slip crack migration, leading to enhanced mechanical properties. A large percentage of material is in the interfacial layer when the surface layer thickness is comparable to the crystallite size. The fraction of the material in the interface is 3dr/r where dr is the interface thickness of a sphere with radius r. Interfaces in crystalline solids also play a role in high-temperature creep. Since one of the predominant mechanisms of creep, grain boundary sliding, results from migration of grains along the grain boundary, a material that is largely made up of grain boundaries will exhibit greater creep rates leading to superplasticity. This may or may not be desirable. For example, ceramic nanostructures are found to be more malleable, or ‘superplastic’. Thus, the formability of ceramic materials is enhanced with nanostructures. However, metallic nanostructured materials, formed from metals that are malleable when they have micrometre-sized grains, may have unacceptable levels of creep. This is especially likely to be a problem if these metallic nanostructured materials are subjected to temperature excursions. Improved mechanical properties have been realized for selected nanomaterials.(94) Direct attempts with other materials to demonstrate the improvements for structural application have met either with prohibitive cost or technological barriers. Laboratory techniques used to demonstrate the behavior of milligram or gram samples are of value to show feasibility of these new materials; however, to be incorporated into technology, sufficient quantities of material must be produced at a market-acceptable cost. Many investigations of the mechanical behavior of materials require bulk quantities (pounds) of material. The problem of having small quantities of material for testing can be mitigated somewhat by using techniques such as nanoindentation. Obtaining quantitative information by this technique has been difficult but there has been recent progress(95)with the use of spherical indentors. Figure 5 shows a plot of the reduced elastic modulus obtained from nanoindentation measurements with spherical indentors on polystyrene films as a function of film thickness. The reduced modulus is an average
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Film thickness (nm) Fig. 5. Measurements of the elastic moduli of thm. polystyrene films on Si obtained by using spherical indentors. The reduced elastic modulus is an average modulus taking mto account the deformation of the tip and the sample. The bulk value is indicated by the straight hne. The deviation at low film thickness is due to tip deformation.
modulus taking into account the deformation of the tip and the modulus. One sees quantitative agreement between the literature value and the reduced modulus determined by the nanoindentation measurements on thicker films for which tip deformation is less important. By controlling the thermomechanical processing history of a sample, the grain size may be modified to a surprising degree. A notable example of the application of this approach is the nanostructured composite composed of COWC.‘~~’This material exhibits a hardness that makes it attractive for machine cutting tools. Rapid solidification with controlled processing conditions and chemical composition produces grain structures over a wide range, from being similar to glasses to submicrometre-size crystallites. This is not a simple process to control. Rapid cooling rates may not result in a nanostructured material, but slow cooling rates always produce relatively large grain sizes. The admixture of nanostructures with a matrix material (organic, metallic or ceramic) leads to a variety of composite materials having superior properties. The discovery of carbon tubules having nanometre dimensions’97’ indicates the vitality and continued opportunity for the unexpected discovery of new nanometre-sized materials. These nanotubes of carbon appear to have surprising mechanical properties. They may offer improved material properties once researchers successfully demonstrate cost-effective methods of synthesis and fabrication. The ability to fabricate microstructures using lithographic techniques has been extended to the fabrication of machines consisting of moving structures, gears. levers, etc., A 10 urn gear, however, takes considerable expertise characteristic of larger devices. (98,99) to fabricate and control. The community of micromechanical electromechanical machines (MEMS) is demonstrating a great deal of ingenuity for fabricating mechanical devices and sensors (on a chip) using lithographic techniques. This innovative field has demonstrated cost-effective alternatives for a number of sensors.
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An inherent property of nanostructures is the reduced variation in surface roughness as well as crystallographic texture. Whatever the material, particularly when grown from the vapor phase, a surface is usually uniform only to the dimensions of the grains or sub-units from which it is composed. Materials with smaller crystallites can lead to improved machinability and surface finish which, for hard to machine materials, provides a significant fabrication advantage. (‘O”)This feature of the nanocrystalline aggregates is particularly important for diamond films. The recent discovery of new processes to prepare diamond without the usual application of high temperature/pressure has offered many new opportunities. (lo’)Nanostructures of diamond crystals adhering to each other and to a surface have become recognized as an important adjunct to modifying the properties of surfaces. Nanocrystalline diamond films can be used for many industrial applications without major polishing requirements. Research is underway to formulate advanced composites with diamond as a second-phase particulate.
4.3. Electric Transport, Electronic and Optical Properties
The electronic and optical properties of nanomaterials differ from those extrapolated from larger dimensions. Potential barriers along the transport direction or as boundary conditions introduce major perturbations which modify the waves and can lead to new phenomena. These effects are important for structures having dimensions of 10 nm or less. Resonant tunneling transistors (RTTs) have been proved to be high-speed devices with a useful negative transconductance,(‘02) and are likely to become important commercially. These devices are fabricated by using the close tolerances of atomically smooth layers grown by molecular beam epitaxy, and are referred to as vertical structures (the transport is perpendicular to the plane of the surface). The negative transconductance exhibited with RTTs can be demonstrated with many alternative geometries, including those with fingers serving as a transistor gate, which produces an alternating potential energy field to the migrating charge carriers. Confinement effects modify the electron energy levels much as the quantized energy levels for a particle in a box. The band gap is typically increased in smaller structures, causing a blue shift. (‘03)Some nanomaterials are lasing media with high gain and/or have large non-linear susceptibility coefficients. Usually the dimensions of these nanostructures are such that light is scattered minimally at the boundaries, hence optical transmission through composites can be high; in which case, the composites appear as transparent (or colored) materials. However, by using the nanochannel glass discussed above, it is possible to observe two-dimensional photonic band behavior in the visible.““’ Another phenomenon, the Coulomb blockade, is due to the discrete nature of electric charge. As charge carriers migrate along nanostructures, they charge capacitors which are so small that the presence of a single electron has a substantial influence on the voltage across the capacitance. This, in turn, influences the transport of additional charge carriers to that location in space; hence strong correlation effects are observed at low temperatures with these small structures.“Os’ There have been some reports of possible effects at room temperature,“06’ when charge migration across a small dielectric (such as a molecule between a tunneling tip and a surface) is observed.
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H(kOe) Fig. 6. The longitudinal (II) and transverse (I) magnetoreslstance, p. of a granular Co,,Ag,,, annealed at 500 K. (From 13”.p. 368.)
film
The electric transport properties of nanomaterials also can be unusual. Doped nanophase ZnO(B,Bi,Co,Cu,Sb,Sn) with 3-10 nm grains has demonstrated varistor behavior for voltages of up to 30 kV cm-‘.(‘07’ 4.4. Magnetic Properties
Magnetic properties of nanostructures likewise provide a fertile ground for new discoveries. Thin layers of magnetic materials such as iron, in conjunction with chalcogenides in intervening layers, show evidence of high anisotropies and internal fields perpendicular to the plane, just the opposite of that expected from other materials. These materials show a large resistance change as the magnetic field is changed (magnetoresistance),“‘*’ and appear to be promising for improved detectors in recording devices. Some multilayer(‘W’ and granular samples”“, ‘I’)exhibit giantic magnetoresistance. Figure 6 shows an example of this phenomenon. These materials are being studied intensively for use in magnetic recording read heads. They also exhibit important properties required for non-volatile memory devices. Additional, unexpected magnetic behavior appears in other materials. The interplay between magnetic effects and transport properties introduces an interesting potential for magnetically driven electronic devices. 5. CONCLUSION In summary, the advancing frontier of nanoscience and nanotechnology appears to offer exciting scientific challenges and possibilities. The rapid progress in nanomaterials is clearly evidenced in the exponential increase in published work during the last five years. Many related programs will be needed to maintain this progress and to investigate the
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validity of hypotheses, many of which are difficult to test. Often it is only by applying several techniques that one can understand the structure and properties of a new nanomaterial. If present nanomaterial processing techniques are refined and new ones developed, the interplay of new ideas in this frontier will be attractive for some time to come.
ACKNOWLEDGEMENTS Useful conversations with W. Tolles, R. Tonucci, and B. Shanabrook the Office of Naval Research are gratefully acknowledged.
and the support of
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