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Formation of self-assembled Cu-γ-Al2 O 3 nanocompositeT. Bhatia a , M. Aindow a & N. P. Padture aa Department of Metallurgy and Materials Engineering,Institute of Materials Science , University of Connecticut , 97North Eagleville Road, Storrs, Connecticut, 06269-3136, USAPublished online: 14 Nov 2010.

To cite this article: T. Bhatia , M. Aindow & N. P. Padture (2003) Formation of self-assembled Cu-γ-Al 2 O 3 nanocomposite, Philosophical Magazine Letters, 83:2, 135-142, DOI:10.1080/0950083021000056614

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PHILOSOPHICAL MAGAZINE LETTERS, 2003VOL. 83, NO. 2, 135–142

Formation of a self-assembled Cu–c-Al2O3

nanocomposite

T. Bhatiay, M. Aindow and N. P. PadturezDepartment of Metallurgy and Materials Engineering, Institute of Materials

Science, University of Connecticut, 97 North Eagleville Road, Storrs,Connecticut 06269-3136, USA

[Received in final form 17 October 2002 and accepted 21 October 2002]

AbstractWe report here the formation of a novel ‘self-assembled’ three-dimensional

nanocomposite consisting of uniformly sized, evenly spaced spherical Cunanoparticles embedded within a polycrystalline g-Al2O3 matrix. This wasdiscovered serendipitously during the examination of an ion-beam-milled ZrO2

specimen in a transmission electron microscope. This nanocomposite wasdeposited as a coating during ion-beam milling by the co-sputtering of Cu andAl from the ion-mill hardware in the presence of O. The mechanism by which thishighly organized nanocomposite forms is discussed, together with broaderimplications of these types of metal–ceramic nanocomposite and the possibilityof using sputter deposition as a generic method for making them.

Inorganic nanoparticles possess, by virtue of their physical size, novel optical,optoelectronic, electrical, magnetic, biological and catalytic properties (for exampleAlivasatos et al. (1998)). Thus, there is great interest in understanding and exploitingthese size effects by creating nanomaterials with unprecedented properties and poten-tial applications. Inorganic nanoparticles with well-controlled chemistries and sizedistributions can now be synthesized using a wide variety of techniques (Alivasatoset al. 1998, Pileni 1998, Dabbs and Aksay 2000, Gleiter 2000). In order to realizenovel properties, the nanoparticles are organized into well-defined two- or three-dimensional superstructures using self-assembly (Alivasatos et al. 1998, Burmeisteret al. 1998, Aindow et al. 1999, Li et al. 1999, Subramania et al. 1999, Dabbs andAksay 2000, Fink et al. 2002). In some instances, synthesis and self-assembly havebeen combined to form two-dimensional structures (Sun et al. 2000). In most self-assembly approaches the nanoparticles are essentially ‘glued’ together with organics,liquids or weak forces. This renders self-assembled nanomaterials friable, precludingtheir widespread use. Consolidation of self-assembled nanomaterials by sintering, orother means, to make more robust superstructures can result in coarsening, disrup-tion of the self-assembled structure and/or creation of defects (Dabbs and Aksay2000).

In this context, we report here a simple one-step sputter deposition method thatresults in the formation of a three-dimensional self-assembled nanocomposite coat-

Philosophical Magazine Letters ISSN 0950–0839 print/ISSN 1362–3036 online # 2003 Taylor & Francis Ltd

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DOI: 10.1080/0950083021000056614

yPresent address: United Technologies Research Center, 411 Silver Lane, MS 129-24,East Hartford, Connecticut 06108, USA.

zAuthor for correspondence. Email: nitin.padture@uconn.edu.

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ing containing evenly spaced spherical Cu nanoparticles of uniform size embeddedwithin a polycrystalline g-Al2O3-based matrix. The formation of this nanocompositewas discovered serendipitously during the examination of an ion-beam-milled Y2O3-stabilized ZrO2 (YSZ) specimen in a transmission electron microscope. Ion-beammilling is a routine process used to prepare thin ceramic transmission electron micro-scopy (TEM) samples. It is belielved that, during this process, Ar-ion sputtering ofCu and Al hardware, which is present in close proximity to the specimen, resulted inthe formation of the highly organized Cu–Al2O3 nanocomposite. In this letter, thecharacteristics of this unique structure are described and possible mechanisms for theformation of this nanocomposite are discussed.

TEM specimens were prepared by firstly cutting and grinding small pieces ofYSZ (about 3mm in diameter and about 100 mm thick) followed by glueing thespecimen on to a Cu specimen support disc 3mm in diameter. The specimens weresubsequently ion-beam milled in a Gatan model 691 precision ion-polishing system(PIPS) for 1 h. Typical operating PIPS conditions were as follows: vacuum, about10�4 Torr; specimen rotation speed about 8 revmin�1; two Ar-ion-beam sources, eachat 5 kV; beam angle relative to the specimen plane, about 78; nominal specimentemperature, 150–2008C (R. Mitro 2002, Gatan Inc. unpublished work). The sampleswere examined in Philips EM420T and Tecnai F20 transmission electron micro-scopes operating at accelerating voltages of 100 and 200 kV respectively. This latterinstrument is equipped with a field emission source and an Oxford Instruments ISISultrathin-window Inca energy-dispersive X-ray spectrometry (EDXS) system forhigh-spatial-resolution chemical microanalysis.

Figure 1 (a) is a bright-field transmission electron micrograph from a nanocom-posite region showing regularly dispersed nanoparticles of uniform size. The averageparticle diameter and the interparticle spacing (centre to centre) were determined tobe 7:1� 0:7 nm and 9:7� 0:8 nm respectively using routine image analysis (a total ofover 40 particles were considered). Each nanoparticle has different contrast, indicat-ing random crystal orientations. Variation in contrast within nanoparticles suggeststhe presence of a substructure. The arrows in figure 1 (a) point to examples of over-lapping nanoparticles. Faint contrast between nanoparticles suggests that the matrixphase is polycrystalline. Figure 1 (b) is a high-camera-length SAD pattern from thearea in figure 1 (a), showing clearly the presence of a diffracted ring with a highlynon-uniform intensity around the transmitted spot (centre). The radius of this ringcorresponds to a periodicity of about 11 nm, which compares favourably with themeasured interparticle spacing of 9.7 nm. Also visible in figure 1 (b) are diffuse spotsof very low intensities (white arrows) which could be a part of a second ring repre-senting second-order reflections. An optical Fourier transform of the photomicro-graph in figure 1 (a) is presented in figure 1 (c). This also shows two concentric ringsaround the transmitted spot, as expected of a photomicrograph containing featuresof uniform size, shape and distribution. In both figure 1 (b) and figure 1 (c) thecircumferential variations in intensities within the rings (arrows) suggests orienta-tional order in the material.

Each nanoparticle seen in figure 1 (a) is actually a column of nanoparticlesstacked above one another. For most of the nanoparticles in figure 1 (a) the electronbeam is aligned with the column axes; a few columns are slightly misaligned asrevealed by faint contrast around some of the particles (arrows in figure 1 (a)).This is further confirmed by specimen-tilting experiments. Figure 2 is a lower-magnification transmission electron micrograph of the nanocomposite, showing

136 T. Bhatia et al.

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the near-perfect alignment (centre region) and misalignment of nanoparticle columns(splaying) with respect to the electron beam (regions in the bottom left and top rightcomers). The latter results in the appearance of nanoparticle ‘strings’, a feature thatcan also arise from the bending of the TEM sample. Rings observed in the conven-tional SAD pattern of the area in figure 2 (inset) reinforce the notion that the crystalorientations in the nanocomposite (nanoparticles and the matrix) are random. TheSAD pattern does not correspond to any known polymorph of ZrO2; however, wenote that in separate scanning electron microscopy studies it was found that the vastmajority of the specimen consists of ZrO2. Therefore, in order to determine thechemical make-up of the nanoparticles and the areas in between them, a series of

Formation of self-assembled Cu–�-Al2O3 nanocomposite 137

Figure 1. (a) Bright-field TEM image of the nanocomposite. The arrows indicate examplesof overlapping nanoparticles. (b) High-camera-length selected-area diffraction (SAD)pattern from the area in (a) showing one diffracted ring clearly. The white arrowsindicate very-low-intensity diffuse spots as part of a possible second diffracted ring.(c) Fourier transform of the photomicrograph in (a) showing two rings. The arrowsindicate regions of relatively higher intensities (darker) in the first ring.

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high-resolution EDXS line profiles were collected from regions exhibiting the highlyorganized nanostructures.

Figures 3 (a)–(d) are EDXS line scans showing the variation in signal forwindows centered on the Cu Ka, Al Ka, OKa and Zr La lines respectively alongthe trace depicted in figure 3E. It is evident from figures 3 (a)–(c) that the that theconcentration of Cu is relatively high inside the particles, while Al and O are moreconcentrated between the particles. We are thus able to surmise that this nanocom-posite consists of Cu particles in an aluminium oxide matrix. This is further con-firmed by indexing of the SAD ring pattern from the nanocomposite regionunambiguously to fcc Cu and g-Al2O3 (figure 4). The lack of any significant varia-tion in Zr counts with probe position (figure 3 (d)) and the absence of any diffractionrings corresponding to crystalline ZrO2 in the SAD patterns (inset of figure 2, andfigure 4) lead us to conclude that the Zr signals are probably due to secondaryexcitations in the adjacent ZrO2. Incorporation of Zr into the metallic nanoparticlesis not expected since the equilibrium solubility of Zr in Cu is less than 0.12 at.%(Lundin et al. 1953). Some incorporation of Zr into the regions between the nano-particles is possible, but that would result in a systematic variation of Zr contentwith probe position, which is not observed in figure 3 (d). Figure 5 (a) summarizes theobserved structure of this unique ‘self-assembled’ Cu–Al2O3 nanocomposite, which,

138 T. Bhatia et al.

Figure 2. Bright-field TEM image of the nanocomposite at a lower magnification illustratingthe effect of inclination of nanoparticle columns with respect to the electron beam. Inthe centre of the photomicrograph the nanoparticle columns are well aligned withrespect to the electron beam. In the top right and bottom left corner regions, nano-particle ‘strings’ are observed because of splaying of the nanoparticle columns withrespect to the electron beam. We note that the latter region is thicker and hence the‘strings’ exhibit stronger contrast. The inset shows the corresponding SAD pattern.

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to our knowledge has not been reported before. As shown in figure 5 (b), we inferthat this nanocomposite exists as a coating on the ZrO2. Note that this type of Cu–Al2O3, nanocomposite was observed in at least two different specimens that wereion-beam milled under identical conditions.

Since the original ZrO2 thin films from which these samples were produced aredevoid of Cu and Al, we now examine the source of the Cu and Al in this nano-composite. Figure 5 (c) shows a schematic diagram of the ion-beam-milling arrange-ment. In this particular case, the thin ZrO2 specimen (about 100 mm thick) was gluedto a Cu specimen support, which was attached to the ion-beam-milling machine

Formation of self-assembled Cu–�-Al2O3 nanocomposite 139

Figure 3. High-resolution EDXS line scans of four elements ((a) Cu Ka; (b) Al Ka; (c) O Ka;(d) Zr La) across a trace between two nanoparticles as depicted in the photomicrograph(e). The numbers to the left in (a)–(d) represent full-scale counts.

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holder using Al clamps. It is believed that the low near-grazing angle (approximately78) of the 5 keV Ar-ion beams used to ion-beam mill the specimen caused the co-sputtering of Cu and Al on to the specimen. In a related experiment, a fresh specimenthat was not mounted on a Cu support was ion-beam milled under otherwise iden-tical conditions. TEM observations on that specimen showed no indication of thepresence of the ‘self-assembled’ Cu–Al2O3 nanocomposite. Because of the relativelylow amount of oxygen present in the chamber (a result of the low vacuum and/orsputtering of ZrO2), preferential oxidation of the more reactive Al species is likely tooccur, resulting in the Cu–Al2O3 nanocomposite. This assertion is based on previousobservations in an internally oxidized Cu–Al alloy that showed the presence of Cuand disordered g-Al2O3 (Ernst et al. 1991). Formation of the g-Al2O3 polymorph,over the more common a-Al2O3, is expected of nanocrystalline alumina (McHale etal. 1997). Although there has been great interest in the sputtering of metal nanopar-ticles into ceramics and glasses in general (for example Girardeau et al. (2001)), andof Cu in Al2O3 in particular (Akai et al. 1996, Beszeda et al. 1998, Kishimoto et al.2001), the remarkable uniformity in size, shape and dispersion of the metal nano-particles observed here has not been reported previously.

What is unique about the method used here is that oxidation and depositionoccur simultaneously, which appears to be key to obtaining the uniformity in size,shape and distribution of Cu nanoparticles in an Al2O3 matrix. In most other studiesof metal–ceramic nanocomposites, the materials are formed by either co-sputteringof the metallic and the ceramic constituents, or ion-implanting the metal into theceramic. As such, the deposited material consists initially of a fine distribution ofmetallic and/or ceramic ‘clusters’, and subsequent microstructural development isdominated by coarsening of this initial distribution. In our case, however, the threemain elemental species (Cu, Al and O) arise from different sources, namely the Cugrid, the Al clamps and the ZrO2 substrate. Under these circumstances, one mightexpect the initial deposit to consist of a more intimate mixture of these three species.

140 T. Bhatia et al.

Figure 4. SAD pattern recorded from the nanocomposite region that is indexed unambigu-ously to Cu and g-Al2O3. The presence of diffraction rings, instead of spots, confirmsthe random crystal orientations of the Cu nanoparticles and the polycrystalline natureof the g-Al2O3 matrix.

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If phase separation were to occur by the nucleation and growth of Cu nanoparticleswithin this mixture, then O might be rejected into the surrounding material untilsome critical concentration is reached whereupon the Al2O3 would form. This wouldaccount for the observed spherical morphology of the Cu nanoparticles and the localuniformity in their size. Moreover, since the relative amount of the metallic Cu phasein the nanocomposite exceeds 40 vol.%, one might expect some degree of structuralorder and alignment on purely geometric grounds. We note that, although the

Formation of self-assembled Cu–�-Al2O3 nanocomposite 141

Figure 5. (a) Schematic representation of the three-dimensional nanocomposite with col-umns of spherical Cu nanoparticles embedded in a polycrystalline g-Al2O3 matrix.(b) The nanocomposite exists as a coating on the ZrO2 specimen, as depicted in thiscross-sectional schematic diagram. (c) Schematic diagram of the Ar-ion-beam-millingset-up. The Cu disc is 3mm in diameter. The angle of the ion beam is near grazing withrespect to the specimen plane; one beam source is above the specimen plane and theother is symmetrically below. The Al-clamped Cu disc rotates while the ion-beamsources remain stationary. (Not to scale.)

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nominal temperature of the specimen is 150–2008C (R. Mitro 2002, Gatan Inc.unpublished work) the effective non-equilibrium temperature is likely to be muchhigher under ion bombardment, promoting such diffusive processes. Further work isrequired to test this hypothesis and thus to elucidate further the mechanisms bywhich this remarkable nanocomposite forms.

Having thus discovered a potential new route to making ‘self-assembled’ metal–ceramic nanocomposites, we are attempting to reproduce this process in a controlledevaporation–deposition chamber. If successful, this appears to be a versatile methodfor making a wide variety of nanocomposites containing a range of different metal-nanoparticle–ceramic-matrix combinations. The use of such nanocomposites toexploit novel nanosize-dependent properties holds considerable promise for arange of optical, optoelectronic, magnetic, biological and catalytic applications,and as templates to grow other ordered nanostructures.

ACKNOWLEDGEMENTS

The authors would like to thank Professor I. P. Jones, Director of the RegionalElectron Microscopy Centre at the University of Birmingham, England for access tothe FEI Tecnai F20 transmission electron microscope and Dr T. S. Rong for assis-tance with the operation of this instrument. We are grateful to Mr L. McCurdy ofthe University of Connecticut for his technical assistance. We are also grateful toProfessor M. Gell and Professor E. H. Jordan of the University of Connecticut, andto Dr L. T. Kabacoff of the US Office of Naval Research for fruitful discussions.This research was supported by the US Office of Naval Research under contractN00014-98-C-0010.

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