Si Nanostructured Films Obtained by Jet-Cooled

J. Phys. Chem. C 2007, 111, 7251-7255

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Properties of Au/Si Nanostructured Films Obtained by Jet-Cooled Cluster Beam Deposition Giuseppe Compagnini,* Luisa D’Urso, Rosario Sergio Cataliotti, and Orazio Puglisi Dipartimento di Scienze Chimiche, Sezione di Chimica Fisica, UniVersita` di Catania, Viale A. Doria 6, Catania 95125, Italy

Antonino Scandurra Consorzio Catania Ricerche, Lab. Superfici ed Interfasi, Stradale Primosole 50, Catania 95100, Italy

Pietro La Fata Dipartimento di Fisica ed Astronomia, UniVersita` di Catania and CNR-IMM, Stradale Primosole 50, Catania 95100, Italy ReceiVed: December 11, 2006; In Final Form: February 21, 2007

Supersonic jet-cooled Au/Si clusters have been produced using a laser ablation procedure. TOF mass spectra reveal a variety of different species consisting of Au and Si clusters as well as mixed AumSin aggregates with variable n and m numbers. The beam has been deposited to form a nanostructured film whose structure is constituted of gold nanoparticles embedded in an amorphous gold silicide matrix. The optical properties of the deposited layer are sensitive to temperatures as low as 150 °C with a sharp change at around 200 °C.

1. Introduction After the pioneering works by Smalley on transition metals and carbon,1,2 large amounts of different nanostructured materials have been studied by depositing supersonic jet-cooled cluster beams. These span from elemental semiconductors to transition metals, alloys, and semiconductor compounds.3-6 Such a deposition technique, frequently termed low-energy cluster beam deposition (LECBD), has recently increased its relevance as an efficient way to obtain novel structures with controlled, often exotic, properties. It essentially consists in the deposition of preformed and well-characterized clusters embedded in a gas carrier, and it has its key points in the possibility to avoid the clusters’ fragmentation upon impact onto the substrate (clusters’ kinetic energies are in the range 10-100 meV/atom) and in the possibility to obtain aggregates for a large range of materials (elements and compounds) by using suitable sources.7 Melinon et al. have described the formation, deposition, and evolution of silicon8 and gold9 cluster assembled films, comparing their properties with those obtained by other techniques and with theoretical models. In the case of gold, a structure constituted from a stack of clusters has been revealed, depending on the thickness of the deposited layer and also influenced by the interactions between the aggregates. The films are also characterized by the occurrence of large cavities, a common feature for these kinds of materials such as silicon and other deposited layers. In the case of transition metal nanostructured materials, the porosity is particular appealing for all the applications in the catalytic field. Despite the large amount of literature, the attempt to obtain silicides and metal/silicon nanocomposites using the LECBD technique has not been considered in detail yet. Particularly interesting is the possibility to have nanoscale silicides (frequently named nanosilicides), strongly deemed in the integrated circuit industry, which use is a nascent one in * Corresponding author. E-mail: [email protected]. Voice: +39 095 7385077. Fax: +39 095 580138.

the nanoera. Moreover metal/silicon systems such as Au/Si or Ag/Si are basic materials in the so-called plasmonics,10 where the unusual optical dispersion properties of metals near the plasmon resonance enable the excitation of resonant modes in nanostructures, opening the path to truly nanoscale plasmonic optical devices. The aim of this paper is to show the possibility to obtain gold/silicon nanocomposite films by depositing preformed mixed Au/Si clusters from a LECBD source. Here we start with some considerations on the formation of the mixed aggregates detected in flight, later giving a full picture of the deposited material. 2. Experimental Section Cluster beams were generated by using a laser vaporization source similar to those reported by Perez et al.8,9 Briefly, a plasma is created by the ablation of a Nd:YAG laser beam (λ ) 532 nm, pulse duration ) 5 ns, repetition frequency ) 10 Hz) onto a target with 60 atom % Si and 40 atom % Au composition (purchased by Good-Fellow). In the plasma chamber the injection of a pulsed helium stream at high pressure (2-10 bar) permits the cluster growth and produces a supersonic expansion that takes place at the exit of a nozzle (background pressure 10-7 mbar). As already reported, neutral and ionized clusters are produced to a large extent. The negative and positive, naturally ionized clusters are detected by a so-called “orthogonal” time-of-flight mass spectrometer (o-TOF-MS) built up according to the Wiley-McLaren setup (M/∆M ) 500). In our machine, it is possible to govern the time delay at which the generated cluster train is extracted at 90° with respect to their initial propagation direction, thus entering into the TOF drift tube. This delay is considered the most important parameter for the cluster detection and can be changed to have a picture of the generated beam at different times with respect to the formation into the source. In some of the reported experiments this delay time has been varied up to 1400 µs to have a complete

10.1021/jp0684877 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/28/2007

7252 J. Phys. Chem. C, Vol. 111, No. 20, 2007

Figure 1. Time-of-flight mass spectra of the negative naturally ionized part of a Au/Si cluster beam seen at different times with respect to the laser ignition: (a) 350 µs; (b) 500 µs; (c) 800 µs. The elapsed time is representative of the various species present in the overall clusters’ traveling train as a function of their residence time inside the ignition chamber.

picture of traveling species from the nozzle to the substrate. In this context it is important to consider that each aggregate’s traveling train is composed of clusters that have been produced under different conditions from the source (essentially at different residence times), and therefore, they are far from being homogeneous. The clusters thus produced have been deposited on suitable substrates at a rate of around 2 nm/min to obtain a layer 100300 nm thick. The deposition has been performed in a 10-9 mbar base pressure deposition chamber, using single-crystalline silicon wafers, highly oriented pyrolytic graphite (freshly cleaved), or fused silica substrates, depending on the characterization to be performed. Depositions have been also carried out onto copper grids suitable for our transmission electron microscopy (TEM) machine (JEOL JEM 2010F). A series of spectroscopic characterizations have been performed on the deposited layers using an X-ray photoelectron spectrometer (Kratos Axis HS) and a UV-vis spectrometer (Perkin-Elmer Lambda 19) to measure the extinction spectra in the range 2002000 nm. 3. Results 3.1. Cluster Beam Mass Spectra. Figure 1 reports three typical negative ion mass spectra obtained with the built-in TOF mass spectrometer. They correspond to three different delay times between the laser shot (plasma ignition) and the cluster detection. One immediately observes that a variety of growth conditions are present during the beam formation because of the different waiting times of the plasma into the source chamber. In other words, since the flux of the seeded clusters through the nozzle lasts several hundreds of microseconds, the properties of the initially formed aggregates are to be considered different from those which are later expelled. The first mass spectrum (Figure 1a) has been obtained after 350 µs. This is just the time necessary for the very first part of the cluster cloud to reach the deflection region. For this reason this spectrum is a picture of the very early stage of the clusters’ formation. It is clear that only a few species are found here, that is Si2, Si4, Au, and simple combinations between them, even though with the current resolution Au and Si7 species can hardly be distinguished. So the presence of Si7 cannot be completely ruled out. At higher delay times the situation drastically changes as reported in Figure 1b. In this case AunSim signals start to

Compagnini et al. appear with a progression in which AunSi4 clusters are preferred. This could be due to the initial presence of a high Si4 species yield, in agreement with recent studies which claim that these Si clusters show much higher stability relative to that of their neighbors.11 In this context, several literature reports appeared in the past few years regarding the stability of metalsemiconductor alloy clusters. For instance, it has been shown that semiconducting species X4 (X ) C, Si) can be stabilized to cubic structures by alloying with transition metals such as gold.12 The same authors also observed that cubic clusters such as Au4Si4 have large binding energies and large HOMO-LUMO gaps. Although in our plasma system there is a high fraction of Si4 aggregates, we have not observed any magic number related to the formation of Au4Si4, and we believe that once Si4 is formed, gold could behave as a hydrogen atom in its bonding to silicon. This has been also recently suggested by Kiran et al.13 investigating SiAun, Si2Aun, and Si3Aun aggregates by using density functional theory and photoelectron spectroscopy. Moreover, jet-cooled Au/Si clusters have been also studied by Scherer,14 who reports the only available mass spectrometric analysis on these systems, essentially in agreement with our findings. The picture emerging from mass spectrometric data allows to state that the produced seeded clouds are far from homogeneous and tend to have a number of different and time-evolving chemical structures which depend on the clusters’ elapsed time into the source before being ejected. We believe that such a warning must be considered in each experiment involving the formation of a cluster-assembled thin film. In the particular case of our work, we expect that the final film is composed of a number of different silicides with embedded pure gold aggregates. 3.2. Nature of the Deposited Layer. The chemical bond between gold and silicon has been discussed with detail in the literature for different systems. In most of these studies, it has been shown that an intermixed AunSim skin is always found on top and/or around metal islands deposited on embedded into semiconductor matrices.15-17 Even though the Au/Si phase diagram has a simple eutectic without stable intermediate compounds,18 electron spectroscopies (photoelectron, Auger, and energy loss) studies19 have revealed that during agglomeration processes the Au/Si interface cannot be explained merely as the separation between two immiscible components. In these regions a chemical interaction between the two elements takes place with the formation of different metastable phases having compositions in the range Au2Si to Au7Si and possessing different electronic properties. On the other hand, several nonequilibrium processes, such as laser annealing or ion irradiation, performed on gold-silicon interfaces and thin films, have shown the formation of bulk (crystalline or amorphous) silicide structures, stable at room temperature.20,21 Regarding the structure of our sample, a first important indication comes from the analysis of the TEM images reported in Figure 2. These show the presence crystalline clusters embedded in an amorphous matrix whose nature is difficult to investigate by diffraction techniques only. Energy-filtered TEM analysis of the samples reveals the inclusion of a small amount of silicon inside each cluster and a higher concentration outside the particles in the as-deposited sample (see left side of Figure 2a). Moreover thermal annealings up to 250 °C (see Figure 2b) leave the TEM images almost unchanged with a mild decrease of the size distribution widths. In any case the average cluster size (around 2 nm) is consistent with the presence of particles composed of a few hundreds atoms

Au/Si Nanostructured Films

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Figure 2. TEM images of the (a) as-deposited and (b) 250 °C annealed Au/Si film (right side). The left side shows the corresponding silicon chemical maps obtained by energy filtering with brighter regions having a higher amount of silicon atoms. A typical high-resolution image of a gold cluster is also reported in part (a) and the clusters’ size distributions are shown as insets in the bright field images.

and leads to the conclusion that the clusters are formed after the beam deposition due to the coalescence of small in-flight aggregates. In Figure 2a we also reported the high-resolution image of a typical single-crystalline cluster. The measured lattice parameters of 2.36 and 2.04 Å for the (111) and (200) lattice fringes, respectively, are consistent with the gold lattice spacing. Figure 3 reports some of the observed photoelectron spectra obtained for the as-deposited mixed Au/Si cluster beams. The Au 4f doublet and the valence band region (essentially Au 5d signals; see Figure 3b) are compared with those measured for a bulk gold sample. The (calibrated) binding energies of the doublet (Au 4f5/2, Au 4f7/2) are shifted toward higher values in the deposited material. An increase of the fwhm from 1.0 eV (bulk gold) to 1.7 eV (deposited layer) and a narrowing of the valence band (5d electrons) are observed too. Sarkar18 has shown that the formation of gold silicide at a gold-silicon interface can be well-studied by looking at the Au 4f doublet’s chemical shift with a direct correlation between the shift and the composition of the silicide. In our case if we measure the overall chemical shift (0.3 eV) with respect to gold bulk values, we can estimate an average composition of about Au4Si, which is very close to the known eutectic composition.18 Nevertheless these spectral line shapes can be also explained as the result of a convolution between a bulklike (unshifted) gold fraction coupled with a silicide phase richer in silicon. If this is the case, the deconvolution reported in the same Figure 3a should lead to a silicide composition of Au5Si2. In both cases the excess of precursor silicon with respect to the average sample composition (60 Si atom %, in the original target) should nucleate to form a mixed silicon amorphous phase.

Figure 3. X-ray photoelectron spectra in the (a) Au 4f core and (b) Au 5d valence band regions. The spectroscopic features of the as-deposited sample are compared with those obtained under the same experimental conditions for a bulk gold surface. A deconvolution is reported in the case of the Au 4f7/2 signal indicating two possible contributions that can contribute to the increase and shift of the bandwidth.

One has to be aware that photoelectron spectroscopy data alone are not sufficient to clarify the sample structure. This because a similar spectroscopic picture could be obtained when nanosized gold particles with average size below 2 nm are

7254 J. Phys. Chem. C, Vol. 111, No. 20, 2007 formed. In this case it is widely known22 that binding energies (BEs) and line widths determined by XPS are strongly influenced by the changes in the initial electronic state and final relaxation toward a different situation when going from bulk metal to atoms. Generally, the increase of the BEs is attributed to a cluster charging. As recently proposed by Zhang et al.,22 a BE increase indicates that the d charge at the Au sites of the nanoparticle is depleted relative to the bulk. On the other hand, an increase of the fwhm, which is also accompanied by a systematic increase of the asymmetry in the higher binding energy side of the Au 4f peak, can be attributed to the increase in the surface/bulk atomic ratio and the presence of more localized unoccupied d states at the Fermi level. The decrease of nanoparticle size below 6 nm induces also a rearrangement of the 5d states with a valence-band narrowing and a lowering in the apparent spin-orbit splitting of the two Au 5d3/2 and Au 5d5/2 signals which is correlated to the above-mentioned effect on the core levels. The observed VB bandwidth (5.1 eV for the Au/Si sample and 5.6 eV for the gold foil) and the increase in the Au 4f fwhm are consistent with the presence of 1.5-2 nm gold nanoparticles embedded in the silicon cluster assembled material.23 Since the spectroscopic features presented up to now cannot rule out the presence of silicides, we decided to carry out an optical investigation of the deposited layer to confirm the presence of metallic clusters having sizes compatible with the above-discussed electronic features and share light on the real nature of the mixed Au/Si matrix. 3.3. Thermal Annealing and Optical Properties of the Deposited Layer. Among the most important features of metal semiconductor nanocomposite materials, the optical properties and in particular the optical extinction spectra play a dominant role. The existence of a plasmon resonance due to the nanosized nature of the metal is generally observed with a strong dependence on the nature of the metal, the optical properties of the host matrix, and a number of nanoparticles’ geometrical features such as the size and the shape.24-26 The possibility of an easy tuning of this optical feature is highly desirable for practical applications of the produced material in the field of optics and plasmonics. In general, noble metal nanoparticles exhibit an optical absorption band (the so-called plasmon resonance signal) in the near-UV/vis region, classically described as the oscillation of the electron gas cloud with respect to the ionic background. Even though the theory of the extinction and scattering of small metallic particles is well-known since the Mie model,24 it is always difficult to correlate the spectral features with the microscopic sample’s structure in terms of size/shape of the particles and optical properties of the host matrix. Figure 4 shows the transmittance spectrum for the asdeposited sample in the NIR extended gold plasmon resonance region. Particularly interesting is the absence of any plasmon feature. This fact has to be carefully interpreted in terms of the interplay between the clusters’ features (size, shape, optical properties) and the properties of the host matrix. One interpretation can be given if one consider the formation of a delocalized resonance due to a strong plasmon coupling which shifts continuously toward the infrared region.27 Many recent literature results have shown that this situation arise for interparticle distances very much lower that those observed in our samples.28,29 Specifically, it has been shown that the plasmon resonance shift remains confined to less than 50 nm if the separation between two gold particles is more than 1 nm. For these reasons and driven by the already shown TEM images, we discard this

Compagnini et al.

Figure 4. Extinction spectra around the gold plasmon resonance region for as-deposited and annealed samples up to 350 °C.

possibility, leaving the spectrum of the as-deposited sample to a number of further considerations to be discussed. As a matter of fact, below 450 nm a marked absorption indicates the certain presence of metallic gold: in this region transition metals show absorption due to proper interband contributions. It is known that the dielectric function in the UV-vis-NIR can be written as a combination of an interband term, accounting for the response of the d electrons (in the case of gold), and a Drude term, considering the free conduction electrons only. These terms can be modeled through a series of Drude-Lorentz oscillators in such a way to obey to the Kramers-Kronig relations. If one observes the dielectric response of gold, it is straightforward to conclude that the interband contribution is always heavier than the free electron one thus giving a signal even though the Drude term is negligible. Moreover, the plasmon signal is strongly influenced by the dielectric function of the environment surrounding the nanaparticles. Since the TEM data do not show any appreciable difference in terms of size, shape, and distribution of the aggregates, we are induced to consider this last aspect. First of all, the extinction spectrum of the as-deposited sample cannot be merely described by the presence of (few) small gold clusters because an evident increase in extinction is also observed at high wavelength (above 600 nm) suggesting that the amorphous matrix has a metallic behavior too. Huber et al.20 have shown that the variation of the optical and electrical properties in laser melt quenched Au/ Si alloys can be interpreted in terms of two transitions: a metalsemiconductor transition of silicon and a metal-nonmetal transition of gold, occurring at low and high silicon concentration, respectively. In this case our data support the first hypothesis. A further investigation of the extinction data for the sample annealed at different temperatures shows strong differences when treatments as low as 150 °C are performed. The first straightforward change is the appearance and the increase in intensity of a plasmon resonance feature. However, up to 180200 °C the optical extinction background remains the same with two evident absorptions below 400 nm and above 600 nm. In view of the previously reported TEM data, the growth of a plasmon resonance can be only ascribed to a chemical modification of the sample. Annealing treatments above 200 °C cause a blue shift of the plasmon feature from about 650 to 610 nm and the sharp decrease of the width from 320 to 200 nm. The behavior is summarized in Figure 5, strengthening that these changes suddenly happen within a small annealing temperature range. At the same time, the background optical response changes into a semiconductor-like behavior (insulator-like) which is transparent at high wavelengths. This is indeed a proof that the properties

Au/Si Nanostructured Films

J. Phys. Chem. C, Vol. 111, No. 20, 2007 7255 A.S. gratefully acknowledge Giuseppe Francesco Indelli for the technical support. References and Notes

Figure 5. Behavior of the position and width of the plasmon band appearing after the sample’s annealing. The data show a sharp transition around 200 °C. The behavior of the resistivity curve as reported by ref 21 for an ion beam produced gold silicide has been plotted for considerations.

of the layer are deeply changing in this temperature range. In this respect, it has been observed by Tsaur21 that amorphous Au/Si alloys show a sudden variation toward the equilibrium two-phase system (gold and silicon) when annealed at around 180 °C. The same Figure 5 reports the data from ref 21, obtained by measuring the resistivity of an ion beam produced Au5Si2 layer as a function of the annealing temperature. Apart from a mild resistivity change observed below 100 °C, a sharp variation of this quantity is found just in the same temperature range in which our samples show a variation of the optical properties. Such a sharp change is attributed to the decomposition of the silicide into an equilibrium two-phase Au and Si mixture. The picture emerging from these considerations allows us to indicate the presence of a metallic-like silicide matrix in the as-deposited sample in which small crystalline particles are embedded. This matrix easily convert into a two phase system when annealed at temperatures around 180 °C, with a shift of the surface plasmon resonance and a modification of the its optical and electrical properties. Indeed, the blue shift of the surface plasmon resonance structure is essentially due to a change in the optical properties of the matrix. This technique seems to be a very nice way to produce size confinement and metallic phases embedded in mixed metallicnonmetallic matrices; the relevant phases have peculiar optical properties and plasmonic effects, useful in semiconductor technology applications. Acknowledgment. We gratefully acknowledge the CNRIMM (Catania site) for the TEM images. G.C., L.D., O.P., and

(1) See for instance: Rohlfing, R. B.; Cox, D. M.; Kaldor, A. J. Chem. Phys. 1984, 81, 3322. (2) See for instance: Hopkins, J. B.; Langridge, P. R. R.; Morse, M. D.; Smalley, R. E. J. Chem. Phys. 1983, 78, 1627. (3) D’Urso, L.; Scalisi, A. A.; Altamore, C.; Compagnini, G. J. Mater. Res. 2006, 21, 1638. (4) Dupuis, V.; Favre, L.; Stanescu, S.; Tuaillon-Combes, J.; Bernstein, E.; Perez, A. J. Phys.: Condens. Matter 2004, 16, S2231. (5) Siciliano, P.; Taurino, A. M.; Toccoli, T.; Pallaoro, A.; Iannotta, S.; Milani, P. Chem. Sens. 2004, 20, 368. (6) Bower, J. E.; Jarrold, M. F. J. Chem. Phys. 1992, 97, 8312. (7) Compagnini, G.; D’Urso, L.; Puglisi, O. Mater. Sci. Eng., C 2006, 26, 1082. (8) Melinon, P.; Keghelian, P.; Prevel, B.; Perez, A.; Guiraud, G.; LeBrusq, J.; Lerme, J.; Pellarin, M.; Broyer, M. J. Chem. Phys. 1997, 107, 10278. (9) Bardotti, L.; Prevel, B.; Melinon, P.; Perez, A.; Hou, Q.; Hou, M. Phys. ReV. B 2000, 64, 2835. (10) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. AdV. Mater. 2001, 13, 1501. (11) Li, S. F.; Gong, X. G. J. Chem. Phys. 2005, 122, 174311. (12) Li, S. F.; Xue, X.; Jia, Y.; Zhao, G.; Zhang, M.; Gong, X. G. Phys. ReV. B 2006, 73, 165401. (13) Kiran, B.; Li, X.; Zhai, H.; Wang, L. J. Chem. Phys. 2006, 125, 133204. (14) Scherer, J. J.; Paul, J. B.; Collier, C. P.; O’Keefe, A.; Saykally, J. J. Chem. Phys. 1995, 103, 9187. (15) Chang, J. F.; Young, T. K.; Yang, Y. L.; Veng, H. Y.; Chang, T. C. Mater. Chem. Phys. 2004, 83, 199. (16) Wu, J. S.; Dharo, S.; Wu, C. T.; Chan, K. H.; Chan, Y. F.; Chen, L. C. AdV. Mater. 2002, 14, 1847. (17) Sarkar, D. K.; Bra, S.; Dhara, S.; Nair, K. G. M.; Narasimhan, S. V.; Chowdhury, S. Appl. Surf. Sci. 1997, 120, 159. (18) Selected Values of the thermodynamic properties of binary alloys; Jossey-Bass Publishers: London, 1981. (19) Callari, L.; Sancrotti, M.; Braicovich, L. Phys. ReV. B 1984, 30, 4885. (20) Huber, E.; von Allmen, M. Phys. ReV. B 1983, 28, 2979. (21) Tsaur, B. Y.; Mayer, J. W. Philos. Mag. A 1981, 43, 345. (22) Zhang, P.; Sham, T. K. Phys. ReV. Lett. 2004, 92, 109602. (23) Compagnini, G.; Scalisi, A. A.; Puglisi, O. Surf. Sci. 2006, 600, L1. (24) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (25) Amendola, V.; Polizzi, S.; Meneghetti, M. J. Phys. Chem. B 2006, 110, 7232. (26) Scalisi, A. A.; Compagnini, G.; Puglisi, O. J. Appl. Phys. 2003, 94, 7874. (27) Kreibig, U.; Vollmer, M. Optical properties of metal clusters; Springer: Berlin, 1995. (28) Danckwerts, M.; Novotny, L. Phys. ReV. Lett. 2007, 98, 026104. (29) Romero, I.; Aizpurua, J.; Bryant, G. W.; Garcia de Abajo, F. J. Opt. Express 2006, 14, 9988.