H. GLEITER纳米材料综述

NANOSTRUCTURED MATERIALS:BASIC CONCEPTS ANDMICROSTRUCTUREpH.GLEITERForschungszentrum Karlsruhe,Institute of Nanotechnology,D-76021Karlsruhe,Germany(Received 1June 1999;accepted 15July 1999)Abstract ÐNanostructured Materials (NsM)are materials with a microstructure the characteristic length scale of which is on the order of a few (typically 1±10)nanometers.NsM may be in or far away from ther-modynamic equilibrium.NsM synthesized by supramolecular chemistry are examples of NsM in thermo-dynamic equilibrium.NsM consisting of nanometer-sized crystallites (e.g.of Au or NaCl)with di erent crystallographic orientations and/or chemical compositions are far away from thermodynamic equilibrium.The properties of NsM deviate from those of single crystals (or coarse-grained polycrystals)and/or glasses with the same average chemical composition.This deviation results from the reduced size and/or dimen-sionality of the nanometer-sized crystallites as well as from the numerous interfaces between adjacent crys-tallites.An attempt is made to summarize the basic physical concepts and the microstructural features of equilibrium and non-equilibrium NsM.#2000Acta Metallurgica Inc.Published by Elsevier Science Ltd.All rights reserved.Keywords:Nanostructured materials;Chemical stability;Thermodynamics;Mechanical properties;Micro-structure1.BASIC CONCEPTS1.1.Categories of nanostructured materialsOne of the very basic results of the physics and chemistry of solids is the insight that most proper-ties of solids depend on the microstructure,i.e.the chemical composition,the arrangement of the atoms (the atomic structure)and the size of a solid in one,two or three dimensions.In other words,if one changes one or several of these parameters,the properties of a solid vary.The most well-known example of the correlation between the atomic structure and the properties of a bulk material is probably the spectacular variation in the hardness of carbon when it transforms from diamond to parable variations have been noted if the atomic structure of a solid deviates far from equilibrium or if its size is reduced to a few intera-tomic spacings in one,two or three dimensions.An example of the latter case is the change in color of CdS crystals if their size is reduced to a few nano-meters [1].The synthesis of materials and/or devices with new properties by means of the controlled manipu-lation of their microstructure on the atomic level has become an emerging interdisciplinary ®eld based on solid state physics,chemistry,biology and materials science.The materials and/or devices involved may be divided into the following three categories [2].The ®rst category comprises materials and/or devices with reduced dimensions and/or dimension-ality in the form of (isolated,substrate-supported or embedded)nanometer-sized particles,thin wires or thin ®lms.CVD,PVD,inert gas condensation,var-ious aerosol techniques,precipitation from the vapor,from supersaturated liquids or solids (both crystalline and amorphous)appear to be the tech-niques most frequently used to generate this type of microstructure.Well-known examples of technologi-cal applications of materials the properties of which depend on this type of microstructure are catalysts and semiconductor devices utilizing single or multi-layer quantum well structures.The second category comprises materials and/or devices in which the nanometer-sized microstructure is limited to a thin (nanometer-sized)surface region of a bulk material.PVD,CVD,ion implantation and laser beam treatments are the most widely applied procedures to modify the chemical compo-sition and/or atomic structure of solid surfaces on a nanometer scale.Surfaces with enhanced corrosion resistance,hardness,wear resistance or protectiveActa mater.48(2000)1±291359-6454/00/$20.00#2000Acta Metallurgica Inc.Published by Elsevier Science Ltd.All rights reserved.PII:S 1359-6454(99)00285-2/locate/actamatpThe Millennium Special Issue ÐA Selection of Major Topics in Materials Science and Engineering:Current status and future directions,edited by S.Suresh.coatings (e.g.by diamond)are examples taken from today's technology in which the properties of a thin surface layer are improved by means of creating a nanometer-sized microstructure in a thin surface region.An important subgroup of this category are materials,the surface region of which are structured laterally on a nanometer scale by ``writing''a nano-meter-sized structural pattern on the free surface.For example,patterns in the form of an array of nanometer-sized islands (e.g.quantum dots)con-nected by thin (nanometer scale)wires.Patterns of this type may be synthesized by lithography,by means of local probes (e.g.the tip of a tunneling microscope,near-®eld methods,focussed electron or ion beams)and/or surface precipitation processes.Processes and devices of this sort are expected to play a key role in the production of the next gener-ation of electronic devices such as highly integrated circuits,terrabit memories,single electron transis-tors,quantum computers,etc.In this paper we shall focus attention on the third category of bulk solids with a nanometer-scale microstructure.In fact,we shall focus on bulk solids in which the chemical composition,theatomic arrangement and/or the size of the building blocks (e.g.crystallites or atomic/molecular groups)forming the solid vary on a length scale of a few nanometers throughout the bulk.Two classes of such solids may be distinguished.In the ®rst class,the atomic structure and/or the chemical composition varies in space continuously throughout the solid on an atomic scale.Glasses,gels,supersaturated solid solutions or implanted materials are examples of this type (cf.Fig.1).In many cases these types of solids are produced by quenching a high-temperature (equilibrium)struc-ture,e.g.a melt or a solid solution to low tempera-tures at which the structure is far away from equilibrium.In the last two decades a second class of ma-terials with a nanometer-sized microstructure has been synthesized and studied.These materials are assembled of nanometer-sized building blocksÐmostly crystallitesÐas displayed in Fig.2.These building blocks may di er in their atomic structure,their crystallographic orientation and/or their chemical composition.If the building blocks are crystallites,incoherent or coherent interfaces may be formed between them,depending on the atomic structure,the crystallographic orientation and/or the chemical composition of adjacent crystallites.In other words,materials assembled of nanometer-sized building blocks are microstructurally hetero-geneous consisting of the building blocks (e.g.crys-tallites)and the regions between adjacent building blocks (e.g.grain boundaries).It is this inherently heterogeneous structure on a nanometer scale that is crucial for many of their properties and dis-tinguishes them from glasses,gels,etc.that are microstructurally homogeneous (cf.Figs 1and 2).Materials with a nanometer-sized microstructure are called ``Nanostructured Materials''(NsM)orÐsynonymouslyÐnanophase materials,nanocrystal-line materials or supramolecular solids.In this paper we shall focus on these ``Nanostructured Materials''and use this term exclusively.The synthesis,characterization and processing of such NsM are part of an emerging and rapidly growing ®eld referred to as nanotechnology.R&D in this ®eld emphasizes scienti®c discoveries in gen-eration of materials with controlled microstructural characteristics,research on their processing into bulk materials with engineered properties and tech-nological functions,and introduction of new device concepts and manufacturing methods.1.2.E ects controlling the properties ofnanostructured materialsAs the properties of solids depend on size,atomic structure and chemical composition,NsM exhibit new properties due to one or several of the follow-inge ects.Fig.1.Two-dimensional model of an Al 2O 3glass[3].Fig.2.Two-dimensional model of a nanostructured ma-terial.The atoms in the centers of the crystals are indi-cated in black.The ones in the boundary core regions arerepresented as open circles [13].2GLEITER:NANOSTRUCTURED MATERIALS1.2.1.Size e ects.Size e ects result if the charac-teristic size of the building blocks of the microstruc-ture (e.g.the crystallite size,Fig.2)is reduced to the point where critical length scales of physical phenomena (e.g.the mean free paths of electrons or phonons,a coherency length,a screening length,etc.)become comparable with the characteristic size of the building blocks of the microstructure.An example is shown in Fig.3.If the thickness of the layers of a superlattice is comparable with the wavelength of the electrons at the Fermi edge,dis-crete energy levels for electrons and holes are formed in the quantum wells.Such size e ects mod-ifying the mechanical and optical properties are dis-played in Figs 7(a)and (b).1.2.2.Change of the dimensionality of the system.If a NsM consists of thin needle-shaped or ¯at,two-dimensional crystallites (cf.Fig.6),only two or one dimension of the building blocks becomes com-parable with the length scale of a physical phenom-enon.In other words,in these cases the NsM becomes a two-or one-dimensional system with respect to this phenomenon.1.2.3.Changes of the atomic structure.Changes in the atomic structure result if a high density of inco-herent interfaces (Fig.2)Ðor other lattice defects such as dislocations,vacancies,etc.Ðis incorpor-ated.The cores of lattice defects represent a con-strained state of solid matter di ering structurally from (unconstrained)crystals and/or glasses.As a consequence,a solid containing a high density of defect cores di ers structurally from a defect-free solid with the same (average)chemical composition.The boundaries in Fig.2represent an example of this e ect:the mis®t between adjacent crystalliteschanges the atomic structure (e.g.the average atomic density,the nearest-neighbor coordination,etc.)in the boundary regions relative to the perfect crystal (cf.Section 2.1.3).At high defect densities the volume fraction of defect cores becomes com-parable with the volume fraction of the crystalline regions.In fact,this is the case if the crystal diam-eter becomes comparable with the thickness of the interfaces,i.e.for crystal sizes on the order of one or a few nanometers as is the case in NsM.1.2.4.Alloying of components (e.g.elements)that are immiscible in the solid and/or the molten state.The following cases of this type of immiscible com-ponents in NsM may be distinguished:soluteatomsFig.3.Energy-band diagrams for undoped GaAs±Al x Ga 1Àx As superlattices showing conduction and valence-band edges with heterostructure potential wells at x 0X 3,D E c 9300meV X The horizontal lines represent quantum-well discrete energy levels for electrons and holes con®ned in the GaAs layers[5].Fig.4.Schematic model of the structure of nanostructured Cu±Bi and W±Ga alloys.The open circles represent the Cu or W atoms,respectively,forming the nanometer-sized crystals.The black circles are the Bi or Ga atoms,respect-ively,incorporated in the boundaries at sites of enhanced local free volume.The atomic structure shown was deduced from EXAFS and X-ray di raction measurements[6].GLEITER:NANOSTRUCTURED MATERIALS 3(Fig.4)with little solubility in the lattice of the crystallites frequently segregate to the boundary cores (e.g.the free energy of the system in several alloys is reduced if large solute atoms segregate to the boundary core).The second case of nanostruc-tured alloys results if the crystallites of a NsM have di erent chemical compositions.Even if the con-stituents are immiscible in the crystalline and/or molten state (e.g.Fe and Ag),the formation of solid solutions in the boundary regions of the NsM has been noticed (Fig.5)[7].Finally,it may be pointed out that NsM are by no means limited to polycrystalline materials con-sisting of the type displayed in Fig.2.In semicrys-talline polymers,nanometer-sized microstructures are formed that consist of crystalline and non-crys-talline regions di ering in molecular structure and/or chemical composition (Figs 19and 20).Polymeric NsM will be discussed in Sections 2.1.8and 2.1.9.NsM synthesized by supramolecular chemistry result if di erent types of molecular building blocks are self-assembled into a large var-iety of one-,two-or three-dimensional arrays (Figs 23±27).NsM of this type will be considered in Section 2.2.The remarkable potential the ®eld of NsM o ers in the form of bulk materials,composites or coating materials to optoelectronic engineering,magnetic recording technologies,micro-manufacturing,bioen-gineering,etc.is recognized by rge-scale programs,institutes and research networks have been initiated recently on these and other topics in the United States,Japan,EC,China and other countries.In order to keep this article within the length required,it will be limited to considering the micro-structure of equilibrium and non-equilibrium NsM.In other words,nanostructured devices,carbon-based nanostructures (e.g.fullerenes,nanotubes),high surface area (nanometer-sized)materials,sus-pensions of nanometer-sized crystals,thin ®lms and materials with nanostructured surface regions will not be discussed.Concerning recent review articles on NsM we refer to Refs [2,4,8±13].2.MICROSTRUCTURESMaterials with nanometer-sized microstructures may be classi®ed according to their free energy into equilibrium NsM and NsM far away from thermo-dynamic equilibrium which will be called ``non-equilibrium NsM''.2.1.Non-equilibrium Nanostructured Materials Non-equilibrium NsM are materials composed of structural elementsÐmostly crystallitesÐwith a characteristic size (at least in one direction)of a few nanometers (Fig.2).In other words,non-equili-brium NsM are inherently heterogeneous on a nano-meter scale consisting of nanometer-sized building blocks separated by boundary regions.The various types of non-equilibrium NsM di er by the charac-teristic features of their building blocks (e.g.crystal-lites with di erent or identical chemical composition,di erent or identical atomic structure,di erent or identical shape,size,etc.).However,the size,structure,etc.of the building blocks are not the only microstructural features distinguishing di erent NsM.In fact,the boundary regions between them play a similar role.The chemical composition,atomic structure,thickness,etc.of the boundary regions are equally crucial for the proper-ties of NsM (e.g.Figs 2,4and 5).In other words,even if the building blocks,e.g.the crystallites of two NsM,have comparable size,chemical compo-sition,etc.,the properties of both NsM may deviate signi®cantly if their interfacial structures di er.Di erent interfacial structures may result if the two NsM have been synthesized by di erent procedures.For example,nanocrystalline Ni (crystal size about 10nm,density about 94%)prepared by consolida-tion of Ni powder exhibited little (<3%)ductility whereas nanocrystalline Ni (similar grain size and chemical composition)obtained by means of an electro-deposition process could be deformed exten-sively (>100%).The major di erence noticed between both materials was the energy stored in the interfacial regions suggesting di erent interfacial structures (cf.also Section 2.1.7).Numerous other examples emphasizing the signi®cance of the micro-structure for the properties of NsM may be found in the literature [4±13].One of the technologically attractive features of non-equilibrium NsM is the fact that theirmicro-Fig.5.Schematic model of nanocrystalline Ag±Fe alloys according to the data of Mo ssbauer spectroscopy.The alloys consist of a mixture of nanometer-sized Ag and Fe crystals (represented by open and full circles,respectively).In the (strained)interfacial regions between Ag and Fe crystals,solid solutions of Fe atoms in Ag crystallites,and Ag atoms in the Fe crystallites are formed although both components are immiscible in the liquid as well as in the solid state.Similar e ects may occur in the grain bound-aries between adjacent Fe and Ag crystals [7].4GLEITER:NANOSTRUCTURED MATERIALSstructure (and properties)can be manipulatedÐas in all non-equilibrium systemsÐby the mode of preparation.This allows a wide variety of micro-structures (and hence properties)to be generated.Naturally,the other side of the coin is that any technological application of NsM is only possible if one is able to fully characterize and control their microstructure,and if the correlation between their properties and their microstructure is well under-stood so that NsM with controlled properties can be produced reproducibly.This is one of the reasons for focussing a large portion of this article on the microstructure of NsM.2.1.1.Classi®cation of Nanostructured Materials.Let us ®rst consider non-polymeric NsM .Non-poly-meric NsM consisting of nanometer-sized crystal-lites and interfaces may be classi®ed [2]according to their chemical composition and the shape (dimen-sionality)of their microstructural constituents (boundary regions and crystallites;Fig.6).According to the shape of the crystallites ,three cat-egories of NsM may be distinguished:layer-shaped crystallites,rod-shaped crystallites (with layer thick-ness or rod diameters in the order of a few nano-meters),and NsM composed of equiaxed nanometer-sized crystallites.Depending on the chemical composition of the crystallites,the three categories of NsM may be grouped into four families .In the most simple case (®rst family,Fig.6),all crystallites and interfacial regions have the same chemical composition.Examples of this family of NsM are semicrystalline polymers (consisting of stacked crystalline lamellae separated by non-crys-talline regions;®rst category in Fig.6)or NsM made up of equiaxed nanometer-sized crystals,e.g.of Cu (third category).NsM belonging to the sec-ond family consist of crystallites with di erent chemical compositions (indicated in Fig.6by di er-ent thickness of the lines used for hatching).Quantum well (multilayer)structures are probably the most well-known examples of this type (®rst category).If the compositional variation occurs pri-marily between crystallites and the interfacial regions,the third family of NsM is obtained.In this case one type of atoms (molecules)segregates preferentially to the interfacial regions so that the structural modulation (crystals/interfaces)is coupled to the local chemical modulation.NsM consisting of nanometer-sized W crystals with Ga atomssegre-Fig.6.Classi®cation schema for NsM according to their chemical composition and the dimensionality (shape)of the crystallites (structural elements)forming the NsM.The boundary regions of the ®rst and second family of NsM are indicated in black to emphasize the di erent atomic arrangements in the crystallites and in the boundaries.The chemical composition of the (black)boundary regions and the crystallites is identical in the ®rst family.In the second family,the (black)boundaries are the regions where two crystals of di erent chemical composition are joined together causing a steep concentrationgradient [2].GLEITER:NANOSTRUCTURED MATERIALS 5gated to the grain boundaries (Fig.4)are an example of this type (third category).An interesting new type of such materials was recently produced by co-milling Al 2O 3and Ga.It turned out that this procedure resulted in nanometer-sized Al 2O 3crys-tals separated by a network of non-crystalline layers of Ga [14].Depending on the Ga content,the thick-ness of the Ga boundaries between the Al 2O 3crys-tals varies between less than a monolayer and up to about seven layers of Ga.The fourth family of NsM is formed by nanometer-sized crystallites (layers,rods or equiaxed crystallites)dispersed in a matrix of di erent chemical composition.Precipitation-hardened alloys belong in this group of NsM.Nanometer-sized Ni 3Al precipitates dis-persed in a Ni matrixÐgenerated by annealing a supersaturated Ni±Al solid solutionÐare an example of such alloys.Most high-temperature ma-terials used in jet engines of modern aircraft are based on precipitation-hardened Ni 3Al/Ni alloys [cf.Fig.7(a)].The NsM considered so far consisted mostly of crystalline components.However,in addition,NsM are known in which one or all constituents are non-crystalline.For example,semicrystalline polymers consist of alternating (nanometer thick)crystalline and non-crystalline layers (cf.Figs 19and 20).The various types of microstructures that may be formed in polymeric NsM will be discussed in Section 2.1.8.Other NsM consisting of a crystalline and a non-crystalline structural component are par-tially crystallized glasses and nanocrystalline metal nitrides,carbides of the type M n N,M n C metal Ti ,Zr,Nb,W,V)embedded in an amor-phous matrix,e.g.a Si 3N 4matrix.Metal nitrides embedded in amorphous Si 3N 4have been prepared by high-frequency discharge,direct current dis-charge or plasma-induced chemical vapor depo-sition [15].The remarkable feature of these materials is their hardness which seems to be com-parable with or higher than that of diamond.Elastic image forces are argued to require a very high stress to force dislocations to cut through the nanometer-sized nitride crystallites.This high stress may,however,not lead to fracture because any crack formed in one of the crystallites is suggested to be stopped by the ductile amorphous Si 3N 4matrix surrounding the cracked crystallite.Another family of technologically interesting NsM consisting of nanometer-sized crystallites embedded in an amorphous matrix is nanocrystalline magnetic ma-terials.They are derived from crystallizing amor-phous ribbons of (Fe,B)-based metallic glasses.Their microstructure is characterized by 10±25nm-sized grains of a b.c.c.-a -FeX phase consuming about 70±80%of the total volume.This phase is homogeneously dispersed in an amorphous matrix.The two families of alloys showing the best per-formance characteristics are Fe±Cu±Nb±B±Si (FINEMET)and Fe±Zr±Cu±B±Si (NANOPERM).``Finemet''alloys have a saturation induction of about 1.2T and their properties at high frequencies are comparable with the best Co-based amorphous metals.The outstanding features of ``Nanoperm''alloys are the very low losses exhibited at low fre-quencies `100Hz o ering potential for appli-cations in electrical power distribution transformers.Spinodally decomposed glasses represent NsM in which all constituents are non-crystalline.Finally,crystalline or non-crystalline materials containing a high density of nanometer-sized voids (e.g.due to the a -particle irradiation)are NsM,one component of which is a gas or vacuum.A well-known example of a NsM with a void-type structure is porous Si.Porous Si has attracted considerable attention because of its strong photoluminescence in visible light.Two fundamental features of bulk Si limit its use in optoelectronic devices:the centrosymmetric crystal structure prevents a linearelectro-opticalFig.7.(a)Flow stress of Ni±13at.%Ni alloys as a function of the size of the Ni 3Al precipitates.(b)Photoluminescence spectra of nanocrystalline ZnO with di erent crystal sizes in comparison with thebulk material.The detection wavelength was 550nm [2].6GLEITER:NANOSTRUCTURED MATERIALSe ect.Hence,Si cannot be used for light modu-lation.Secondly,the band gap of Si is indirect and lies in the infrared region Eg H1X170eV).As a con-sequence,Si was considered unsuitable for light-emitting technologies.In1990it was reported that porous Si could luminesce.Two lines of thought were put forward to explain the e ect:quantum con®nement and luminescence of chemical com-plexes attached to the free surface of the silicon crystallites.The quantum con®nement model pro-poses the carriers in porous Si to be con®ned to microcrystallites with a size of1±4nm formed due to the porosity of the Si.The chemical complexes capable of luminescing in the observed spectral range were proposed to be siloxene compounds,a complex of Si,H and O.Recently,``hybrid models'' were discussed where both,the interior and the sur-face of the porous Si are involved in the photolumi-nescence.Obviously,the model of a non-equilibrium NsM considered so far(Fig.2)is highly simpli®ed in thesense that it is based on a hard-sphere approach. Nonetheless,two characteristic features of a NsM are already borne out by this approach:the nano-meter-sized crystallites are expected to exhibit size and/or dimensionality e ects for the reasons given in the previous paragraph(Section1.2).Moreover, several properties(e.g.di usion,internal friction, etc.)of a NsM should be controlled by the presence of a high density of grain and/or interphase bound-aries.Indeed,these kinds of e ects have been revealed by a large number of experimental studies in recent years(see,e.g.[52]).For the sake of brev-ity,this paper will be limited to discussing only one or very few experiments as a representative example for each case.2.1.2.Size e ects.Figure7(a)shows the depen-dence of the¯ow stress of a nanocomposite consist-ing of nanometer-sized Ni3Al crystallites dispersed in a matrix made up of a NiAl solid solution.The total volume fraction of Ni3Al crystallites is the same for all Ni3Al crystal sizes shown in Fig.7(a); the only parameter varied is the size of the Ni3Al crystallites.Figure7(b)displays the blue shift in the luminescence spectra as a function of the crystal size for nanocrystalline ZnO(consisting of consoli-dated ZnO crystals separated by grain boundaries). The blue shift is a quantum size e ect.If the crys-tallite size becomes comparable or smaller than the de Broglie wavelength of the charge carriers gener-ated by the absorbed light,the con®nement increases the energy required for absorption.This energy increase shifts the absorption/luminescence spectra towards shorter wavelengths(blue). Another e ect related to the reduced size of the crystallites in NsM concerns the atomic structure of the interfaces.More precisely,the question:is the atomic structure of the interfaces between nano-meter-sized crystallites di erent from the structure of the interfaces between crystals of in®nite size (same chemical composition,orientation,relation-ship,etc.)?So far,only a few speci®c cases have been studied experimentally and theoretically by means of molecular dynamics computations.The results of these studies may be summarized as fol-lows:in metallic NsM,the low-temperature atomic structure of the boundaries of a NsM di ers from the structure of the boundaries in a coarse polycrys-tal primarily by the rigid body translation.The deviating rigid body relaxation of both types of boundaries results from the di erent constraints in both materials:in coarse-grained polycrystals adja-cent crystallites are free to minimize the boundary energy by a translational motion relative to one another(called rigid body relaxation).In a NsM the constraints exerted by the neighboring nano-meter-sized crystallites limit the rigid bodyparison of the vibrational density of states g(n)for nanocrystalline Cu(crystal size8.2A,solid line) with those for a Lennard±Jones glass(molecular-dynamics simulation,dash±dotted line)and for the perfect f.c.c.crystal(dashed line);n is the phonon frequency[29].parison of the temperature dependence of the total free energy of nanocrystalline Cu(for di erent grain sizes as indicated in the®gure)with a Lennard±Jones glass[29]and the perfect crystal.GLEITER:NANOSTRUCTURED MATERIALS7。