Alloy nanoclusters in dielectric matrix

Alloy nanoclusters in dielectric matrix

Giovanni Mattei *

INFM-Dipartimento di Fisica, Universit�aa di Padova, Via Marzolo 8, I-35131 Padova, Italy

Abstract

Sequential ion implantation in dielectric matrix determines three different cluster morphologies: separated systems,

alloy clusters and core–shell clusters. Post-implantation thermal treatments influence the alloy stability as a function of

the annealing atmosphere (oxidizing, inert or reducing). Case studies will be given for the three different struc-

tures � 2002 Elsevier Science B.V. All rights reserved.

PACS: 61.72.Ww; 81.05.Pj; 81.05.Bx; 61.46.þw

Keywords: Ion implantation; Glass composites; Alloys; Nanoclusters

1. Introduction

Composite materials formed by metal or semi-conductor nanoclusters (NCs) in insulating ma-trices have been the object of continuouslygrowing interest due to the peculiar features thatthese systems exhibit in terms of optical, magneticand catalytic properties [1–12]. In particular, avery appealing characteristic from the point ofview of the applications is the ‘‘tunability’’ of theirelectronic properties (for instance, the plasma res-onance or the band gap shift for metallic or semi-conducting NC, respectively) as they are stronglysize dependent, due to the quantum confinementof the electronic wave functions, when the size ofthe cluster becomes comparable to or less than theelectronic mean free path (for metals) or the exci-tonic Bohr radius (for semiconductors). As a re-sult, metallic NCs embedded in glass can increase

the optical third-order susceptibility vð3Þ (andtherefore the related non-linear index of refractionn2) of the composite by several orders of magni-tude, making such systems interesting candidatesto be used as optical switches [2,13]. Similarly,along with the interest for non-linear optical ap-plications, semiconductor NCs have received agreat attention for their size dependent photo-lu-minescence in the visible or near infrared with re-markable applications in optoelectronics as lightemitting devices or amplifiers [14–17]. Moreover,as far as the magnetic properties are concerned,it has been observed that particles with size in thenanometer range exhibit features that are quitedifferent from those of the corresponding bulkmagnets, i.e., the superparamagnetism [11,18], theenhanced coercitivity [11], or the shift of the hys-teresis loops [19]. As a consequence, magnetic NCsare getting importance in recent days because oftheir possible application in magnetic recordingmedia [11] or catalysis [20].

Among different possible synthesis techniques,ion implantation in glass proved to be very

Nuclear Instruments and Methods in Physics Research B 191 (2002) 323–332

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* Tel.: +39-049-827-7045; fax: +39-049-827-7003.

E-mail address: [email protected] (G. Mattei).

0168-583X/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0168-583X(02 )00527-X

suitable in obtaining NC-based composite mate-rials [3,4,21–25], as it can virtually introduce anyelement in any matrix without the constrains ofsolubility limit or high cluster free energy of for-mation suffered by other techniques. Moreover,although the typical cluster size distributions ofion-implanted samples are not so narrow, theflexibility in introducing the desired amount ofdifferent elements with a reasonable control of thelocal concentration is of paramount importance.Indeed, along with the size, the other parameterthat strongly affects the composite properties is theNCs composition. Therefore, with respect to themonoelemental case, alloy-based NCs can add afurther degree of freedom, i.e. the composition, tothe engineering of new materials properties. Ofcourse, before this tunability in the NCs propertiescan be used for actual devices, a careful controlover alloy clusters synthesis and stability has to beperformed, in order to clarify which are the pa-rameters (i.e. implantation conditions, subsequentthermal or laser annealings, etc.) that can promoteseparation (via oxidation, for instance) instead ofalloying of the implanted species.

In particular, this last issue will be addressed inthe present work. Indeed, the composition of theclusters can be varied easily by sequential ion im-plantation in the matrix of two different elements,whose energy and dose can be tailored so as tomaximize the overlap between the implanted speciesand to control their local relative concentration.

Three types of cluster arrangements can inprinciple be obtained by such procedure (see Fig.1): (i) separated clusters, (ii) alloy-based clusters or(iii) mixed composition (core–shell) clusters. Dif-ferent factors can drive the system toward one ofthese structures, among them: the miscibility of theimplanted elements, their chemical interactionwith the host matrix, and the creation of radiation-induced defects, which can influence the nucleationand growth of the clusters. In the present work, wefollowed a multistep process combining sequentialion implantation and subsequent thermal annea-lings in different (inert, oxidising or reducing) at-mospheres. In these experiments, the formation ofalloy NCs was evidenced (also in bulk-immiscibleelements like CuACo) as well as the influence ofannealing atmosphere on the formation of alloy or

separated monoelemental systems. Results will bepresented for metallic (AuACu, AuAAg, PdACu,PdAAg, NiACo, CuACo) or semiconducting(GaN, InN, Ag2S) embedded NCs (see Table 1).

2. Experimental

Sequential ion implantations with different ele-ments were performed mostly on silica substrates,

Fig. 1. Possible outcomes from sequential ion implantation of

elements A and B: (a) separated A, B systems; (b) A–B alloy

clusters; (c) core(B)–shell(A) clusters.

324 G. Mattei / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 323–332

but also on quartz and sapphire, at INFN–INFMIon Implantation Laboratory of INFN–LegnaroNational Laboratories. The current densities weremaintained lower than 2 lA/cm2 and ion beamenergies were chosen to get the same projectedrange for the implanted species (typical energieswere up to 200 keV in order to modify a sub-sur-face layer of 100/200 nm of the host matrix and iondoses ranged from 1 � 1016 ions/cm2 up to 4 � 1017

ions/cm2). Ion-implanted slides were then heattreated in a conventional furnace at differenttemperatures and times in air, N2 or in a H2AN2

gas mixture. The samples were studied by severalcomplementary techniques, in particular trans-mission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS), X-rayphotoelectron spectroscopy (XPS), Rutherfordbackscattering spectrometry (RBS), secondary ionmass spectroscopy (SIMS). Optical characteriza-tion has been performed by linear absorption inthe UV–Vis spectrum (OD) and photo-lumines-cence (PL). Magnetic measurements (hysteresisloops, ZFC–FC curves) were performed also. Inparticular we would like to stress the strict inter-play between optical and structural techniques foreffective alloy detection: in the case of noble metalclusters the surface plasmon resonance (SPR) inthe visible is a clear fingerprint of nanoparticlesformation [13]. In the case of noble metals alloythe SPR resonance is located in between those ofthe pure elements, and is triggered by the complexinterplay between the modified free electrons andinterband absorptions. This can be seen in Fig. 2,which shows a comparison between OD spectra

either simulated with the Mie theory [13,26] for 3nm clusters of pure Au, Ag and Au0:4Ag0:6 alloy insilica (Fig. 2(a)), or measured for analogous sys-tems [27] in ion implanted silica (Fig. 2(b)). Linearabsorption measurements can therefore give thefirst indication of possible alloy formation. Nev-ertheless, in systems containing transition metals(PdAAg, CoANi,. . .) such a simple technique is nolonger effective as interband transitions completelymask the SPR peak, resulting in a structurlessabsorption, which hinders any unambiguousidentification of the alloy. In such cases, one has torely on structural techniques like TEM (SAEDand EDS) or EXAFS to establish alloy formation.

3. Alloy systems

Generally, the criterion valid for bulk systemsof miscibility of the two elements as a constrain for

Fig. 2. Comparison between a simulation based on the Mie

theory of the optical absorption in the UV–Vis range for 3 nm

clusters of pure Au, Ag and Au0:4Ag0:6 alloy in silica (Fig. 2(a)),

with the experimental OD of the same systems in ion implanted

silica (Fig. 2(b)).

Table 1

Nanostructure of investigated systems obtained by sequential

ion implantation without subsequent annealing

System Nanostructure

AuACu [10] Alloy

AuAAg [29] Alloy

PdAAg Alloy

PdACu Alloy

CuANi [37] Alloy

NiACo [12] Alloy

CoACu [12] Alloya

AgAS [35] Core–shell

GaAN [36] Separated

InAN Separated

a hcp and fcc phase coexistence.

G. Mattei / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 323–332 325

alloying is not so stringent in the case of NCs, dueto the incomplete onset of the bulk propertieslinked to the large number of atoms at the surface,making a cluster more similar to a molecular thanto a massive system [28]. This leads to new possiblealloy phases, which may be thermodynamicallyunfavoured in the bulk. In the case of noble metalbased systems (AuACu, AuAAg, PdAAg andPdACu) perfect miscibility is expected from thebulk phase diagrams and in fact sequentially as-implanted samples exhibit direct alloying[10,27,29]. We now briefly review some of the mostrelevant results obtained by the Padova group onthese systems, whose structural and compositionalfeatures are summarized in Table 2.

3.1. Au-based alloy

Fused silica slides were sequentially implantedat room temperature with Au (190 keV) and Cu(90 keV) or with Au (190 keV) and Ag (130 keV)ions at the same dose of 3 � 16 ions/cm2 to obtainthe same projected range of about 70 nm [10,27].

In the case of AuCu system, a first indication ofalloy formation is given by the OD spectra: theSPR for as-implanted and annealed in H2 is at 550nm, i.e. in between those of Au (530 nm) and Cu(570 nm). Indeed, cross-sectional TEM analysispointed out that the as-implanted sample exhibitsspherical clusters with average diameter�DD ¼ 3:8 � 1:6 nm (mean value � standard devia-tion of the distribution). Selected area electrondiffraction (SAED) analysis indicated a single f.c.c.phase with lattice constant a ¼ 0:3958ð15Þ nm, i.e.an AuxCu1�x alloy with x ¼ 0:67ð3Þ. Energy-dis-persive X-ray microanalysis (EDS), XPS andEXAFS measurements showed that part of the Curemained in the matrix, and only with an anneal-ing in reducing atmosphere it is driven in the alloyclusters promoting a compositional change(x ¼ 0:50ð3Þ) and a structural phase transitionfrom f.c.c. to tetragonal. On the contrary anneal-ing in air induced alloy dissolution: clusters of Auwere found together with very large aggregates ofcrystalline CuO at the sample surface. We followedthis alloy decomposition performing a short (15

Table 2

Summary of the TEM results on some alloy systems

Sample Annealing conditions Sizea (nm) Structure (SAED) x alloy Lattice constant (nm)

AuCu

AuCu – 3:8 � 1:6 AuxCu1�x––f.c.c. 0.67(3) a ¼ 0:3958ð15ÞAuCuH H2, 900 �C, 1 h 8:7 � 2:5 AuxCu1�x––tetrag. 0.50(3) a ¼ 0:3960ð12Þ,

c ¼ 0:3670ð12ÞAuCuA Air, 900 �C, 1 h 33 � 20 AuxCu1�x––f.c.c. +

CuO (tenorite)

0.97(3) a ¼ 0:4060ð12Þ

AuCuHA H2, 900 �C, 1 h þ air,

900 �C, 15 min

28 � 17 AuxCu1�x––f.c.c. +

Cu2O (cuprite, cubic)

0.93(3) a ¼ 0:4051ð12Þ

AuAg

AuAg – AuxAg1�x––f.c.c.

AuAgA Air, 800 �C, 1 h 9:1 � 5:8 AuxAg1�x––f.c.c. a ¼ 0:4079ð12Þ

PdAg

PdAg – 3:7 � 4:3 PdxAg1�x––f.c.c. 0.50(5) a ¼ 0:3987ð10ÞPdAgH H2, 900 �C, 1 h 4:4 � 3:4 PdxAg1�x––f.c.c. 0.80(3) a ¼ 0:3929ð10ÞPdAgA Air, 900 �C, 1 h 11:5 � 6:9 PdxAg1�x––f.c.c. 0.99(3) a ¼ 0:3888ð10Þ

PdCu

PdCu – 1:5 � 0:6 PdxCu1�x––f.c.c. 0.75(5) a ¼ 0:3824ð10ÞPdCuH H2, 900 �C, 1 h 9:8 � 9:5 PdxCu1�x––f.c.c. 0.58(5) a ¼ 0:3775ð10ÞPdCuA Air, 900 �C, 1 h �20 PdxCu1�x––f.c.c. Variable

a Average diameter � standard deviation of the TEM experimental size distribution.


min) annealing in air to the already H2-annealedsample [30]: TEM analysis showed that most of theclusters were composed of two interpenetratedparts, one made of an Au-rich AuxCu1�x alloy (x ¼0:93ð3Þ) and the other made of Cu2O (see Fig.3(d)). Such phase exhibited an optical absorption

band at about 620 nm, related to the observedAuACu2O core–shell like structure. Therefore,alloy decomposition takes place with the progres-sive oxidation of Cu, which is extracted by oxygenfrom the alloy clusters: it is interesting to note thatfrom SAED analysis it turned out that these twophases (AuxCu1�x alloy and Cu2O) share the samecrystallographic structure (cubic) and same axesorientations, as if the f.c.c. alloy cluster was a sortof structural template for the out-coming oxide.From these results it is clear that sequential Au/Cuion implantation is able to give directly alloyedAuACu colloids. A possible mechanism for thealloy formation is an enhanced diffusion of copperin small gold clusters [28,31] during the implanta-tion. Subsequent annealing in hydrogen can beused to increase the average size of the clusterswith a change in the structural ordering of thealloy by driving copper atoms (dispersed in thematrix) toward the already formed clusters, in-creasing their size and shifting the Au/Cu con-centration toward the nominal 1:1 ratio. On thecontrary, annealing in oxidizing atmosphere pro-motes a separation of the two species, assisted by achemical interaction between copper and oxygen.Another point that is worth noting is that, withrespect to single Au ion implantation, the secondimplant of Cu introduces a correlated diffusionbetween Au and Cu atoms during thermal annea-lings. This can be seen in Fig. 3, which reports acomparison between single Au implant (Fig. 3(a)and (b)) and sequential Au/Cu implant (Fig. 3(c)and (d) same dose and energy) before and afterannealing. Not only the cluster’s size increases buta remarkable in-depth diffusion of both Cu and Auis present, as some alloy NCs are visible well be-yond the implantation range. This is in agreementwith our previous work on correlated diffusion ofAu in presence of oxygen [32].

Also in the case of sequential implanted Au andAg in silica (at the same dose 3 � 1016 ions/cm2),the resulting structure is an AuxAg1�x alloy (Fig.4). In this case OD spectra show indeed a singleSPR resonance near 500 nm (as expected from theabove reported Mie-type simulations for AuAgalloy), which shifts to 475 nm upon annealing, ir-respective from the atmosphere (Fig. 2(b)). Al-loying in such system has been also demonstrated

Fig. 3. Cross-sectional bright-field TEM images of ion-im-

planted silica substrates: (a) single implant Au (190 keV, dose

3 � 1016 ions/cm2); (b) sample in (a) after annealing in air 900

�C 3 h; (c) sequential Au (190 keV) and Cu (90 keV) at the same

dose 3 � 1016 ions/cm2; (d) sample in (c) after annealing at 900

�C first in H2AN2, 1 h and then in air, 15 min. Note the different

scale marker in (a) and (b) with respect to (c) and (d).


by FEG-TEM EDS measurements on single clus-ters (indicating that both Au and Ag were present)and by EXAFS analysis. The reduced interactionbetween Ag atoms and oxygen, with respect to Cu,makes the annealing less effective in inducingseparation of the alloy obtained in the as-implanted samples. Moreover, only minor differ-ences can be evidenced between air and H2

annealing, as can be seen from the OD data re-ported in Fig. 2(b).

3.2. Pd-based alloy

After exploring the effect of Cu and Ag in Au-based alloys, we started recently a new set of ex-periments aimed at investigating the role played byAu, substituting it with Pd which, as Au, is not soreactive with either the matrix or oxygen. Fusedsilica slides were sequentially implanted at roomtemperature with Pd (130 keV) and Ag (130 keV)or with Pd (130 keV) and Cu (90 keV) ions at thesame dose of 3 � 1016 ions/cm2 to obtain the sameprojected range as in the Au-based alloys. ODspectra do not evidence the Ag SPR peak at about400 nm, suggesting the formation of metal alloyNCs. Such interpretation is supported by TEManalyses. SAED indicates a dominant f.c.c. phasewith a lattice constant a ¼ 0:3987ð10Þ nm, showingthat an intermetallic alloy has been formed (ex-perimental values for bulk f.c.c. Pd and Ag areaPd ¼ 0:38898 nm and aAg ¼ 0:40862 nm, respec-tively). After annealing in air at 800 �C for 5 h(Fig. 1(b)), SAED analysis evidenced an f.c.c.phase with a lattice parameter a ¼ 0:3946ð10Þ nm,corresponding to a value of x ¼ 0:71ð3Þ, indicatinga loss of silver, as already observed in silica sam-ples, containing silver NCs, prepared by sol–gelmethod [33]. The average PdAg NC diameter afterair annealing increased to a value of 8:4 � 5:8 nm.The cluster size evolution may be interpreted interms of size-related cluster instability [32], with atransfer of atoms from smaller to larger clusters.After annealing in non-oxidizing atmosphere(H2AN2 mixture), the composite exhibits a mor-phology similar to the as-implanted sample. Clus-ter average size is slightly larger (4:4 � 3:4 nm),with an enrichment of Pd in the alloy clustercomposition, as indicated by diffraction analysis.

Fig. 4. (a) Cross-sectional bright-field TEM images of se-

quentially ion-implanted silica with Au (190 keV) and Cu (90

keV) at the same dose 3 � 1016 ions/cm2 and annealed in air at

800 �C for 1 h; (b) SAED pattern showing a single f.c.c. Debye–

Scherrer pattern of AuAg alloy; (c) EDS spectrum on a single

cluster containing both Au and Ag.


Similarly to the PdAg case, also in PdACu se-quentially implanted samples, alloy NCs are ob-served either in the as-implanted or in the H2AN2

annealed samples, with a remarkably different sizedistribution. As for PdAAg system, annealing inoxidizing atmosphere gives rise to an alloy de-composition faster than in the H2AN2 annealedsamples, as already observed in the case of AuCualloy NCs [10], and with the migration of Cu at-oms towards the surface.

3.3. CoANi alloy

We have investigated the CoANi phase diagramby performing sequential ion implantation in silicaof Co and Ni at the same energy of 180 keV(Rp � 150 nm) with different doses in order to havea constant total Co þ Ni dose (15 � 1016 ions/cm2

or 30 � 1016 ions/cm2). For the 1:1 Co:Ni ratio, weperformed also sequential implants at two energies(180 and 70 keV) for each element to have a flatterconcentration profile for a total dose of 40 � 1016

ions/cm2 (sample CoNi40). All the samples inves-tigated exhibit CoxNi1�x alloy NCs. In this caseOD spectra are not useful for alloying detectiondue to the above-mentioned damping of the SPRdue to interband transitions [13]. SAED analysiswas able to monitor a phase transition from f.c.c.to h.c.p. as the Co content in the system is greaterthan 70%, in agreement with bulk CoNi alloy. It isinteresting to note that similar CoNi alloy systemobtained by our group with the sol–gel route, ex-hibited at all the CoNi ratios the f.c.c. structure[34]. As the lattice parameters of the f.c.c. phasesof Co and Ni differ by a quantity that is at the limitof SAED quantification for nanoclustered systems(mostly due to the size-dependent broadening ofthe diffraction peaks), we performed also EDScompositional analysis with a sub-nanometerelectron probe on single clusters which demon-strated the presence of both Co and Ni on a singlecluster. In Fig. 5(a) a BFTEM image of theCoNi40 sample (implanted with ‘‘flat’’ concentra-tion profile) is reported: the SAED pattern in Fig.5(b) exhibits a single alloy f.c.c. phase with latticeparameter a ¼ 0:3533ð12Þ nm. The compositionalEDS spectrum obtained on a single particle is

Fig. 5. (a) Cross-sectional bright-field TEM images of se-

quentially ion-implanted silica with Co (180 þ 70 keV) and Ni

(180 þ 70 keV) with a concentration ratio of 1:1 for a total dose

40 � 1016 ions/cm2; (b) SAED pattern showing a single f.c.c.

Debye–Scherrer pattern of CoNi alloy; (c) EDS spectrum on a

single cluster containing both Co and Ni.


reported in Fig. 5(c), showing both Co and Ni onthe same cluster.

4. Core–shell systems

In some cases, sequential ion implantation givesrise to ‘‘mixed’’ clusters, in the sense that bothelements are present on the same cluster but eachone maintaining its structure in a core–shell ar-rangement.

4.1. AgAS

Fused silica slides were sequentially implantedat room temperature with Ag (65 keV) and S (30

keV) ions at a dose of 5 � 1016 and 2 � 1016 ions/cm2, respectively, to obtain the same projectedrange of about 40 nm [35]. In Fig. 6(a) a cross-section bright-field view of the sample implantedfirst with Ag and then with S ions is reported,showing a spherical core–shell clusters. In Fig.6(b), a high-resolution image shows the latticefringes arising from the shell (made of Ag2S in theacanthite form) superimposed on the core com-posed of Ag. It is interesting to note that no mixedclusters are obtained upon reversing the implantorder: performing first S and then Ag implant,only Ag clusters are detected while S takes part tothe formation of thiosilicate species [35].

5. Separated systems

The last class of samples obtained by sequentialion implantation is that in which the two im-planted elements do not chemically interact (orhave a negligible interaction) and constitute twoseparate sub-systems in the matrix.

5.1. Group III nitrides

As an example, we have investigated GaANand InAN systems in which ‘‘alloying’’ (in a broadsense, we should speak about compound forma-tion) is only achieved after thermal annealing inNH3 at 900 �C [16]. In Fig. 7(a) the cross-sectionalBFTEM image of a silica slide sequentially im-planted with Ga (120 keV, 6 � 1016 ions/cm2) andN (30 keV, 7:5 � 1016 ions/cm2) is reported. XPSresults [36] indicated that the spherical, amorphous(by SAED) clusters in zone A are made of metallicGa (their size distribution is reported in Fig. 7(b)),whereas the grainy contrast in zone B is due toGaOxNy or Ga oxides. Therefore, in order to ob-tain GaN nanocrystals (irrespective of the sub-strate investigated, i.e. silica, quartz or sapphire)sequential ion implantation must be followed byannealing in NH3 gas at 900 �C. GaN was formedby reaction of implanted Ga with NH3 combustionproducts and/or via conversion of Ga oxide/oxy-nitrides. A blue shift of the near band-edge photo-luminescence (quantum confinement effect) wasobserved for GaN nanocrystals with size �2–3 nm,

Fig. 6. Cross-sectional TEM images of a silica sample im-

planted with Ag and S: (a) bright-field showing the contrast

between the Ag core and the Ag2S shell; (b) high-resolution

image showing the lattice planes of the Ag2S shell.


present in all the substrates. In the case of InANsystem, work is in progress to find the best an-nealing conditions (temperature and atmosphere)to promote compound formation starting from thesequentially as implanted sample, in which theresult of N implant is to destroy the In clustersformed after the In implant.

6. Conclusions

Sequential ion implantation, as a technique forthe synthesis of mixed (alloyed or core–shell)clusters, has been illustrated with some case studiesinvolving either metallic or semiconducting sys-tems. This technique is able to give in a large va-riety of systems direct alloying: subsequentthermal (or laser) annealing can be used to modifyeither the clusters size distribution or the alloycomposition, due to the selective interaction of theannealing atmosphere with the alloy components.In particular, we demonstrated that annealing inoxidising atmosphere is more effective in the alloydecomposition with respect to annealing per-

formed in reducing or inert atmospheres. The richphenomenology gained in these complex systemswill hopefully give a valuable contribution to thecomprehension of the mechanisms responsible foralloy formation and stability.

Acknowledgements

I would like to gratefully acknowledge the workand the collaboration of the people at the INFMUnits of Padova (P. Mazzoldi, E. Borsella, G. DeMarchi, C. de Julian Fernandez, C. Maurizio, S.Padovani, C. Sada), Venezia (G. Battaglin, E.Cattaruzza, F. Gonella) and Trento (A. Miotello,A. Quaranta). Thanks are also due for the tech-nical assistance to M. Parolin at INFN-INFM IonImplantation Laboratory-Legnaro. This work hasbeen partially supported by M.U.R.S.T. (Univer-sity and Research Ministry) within a NationalUniversity Research Project.

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Alloy nanoclusters in dielectric matrix

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