Morphology of Au nanoparticles formed by magnetron sputtering

Morphology of Au nanoparticles formed by magnetron sputtering: ellipsoids and rings S. Chattopadhyay, S. Sarkar, A. Datta* and P. Chakraborty Ultraviolet (UV) visible spectroscopy, atomic force microscopy and grazing incidence X-ray reflectivity have been used to study morphology of Au nanoparticles grown by direct current (DC) magnetron sputter deposition on hard (glass) and soft amorphous (polystyrene films on quartz) substrates. Au nanoparticles are found to be ellipsoidal showing an increase in ellipticity e [;a/b, a(b)5semimajor (semiminor) axis] with decrease in polystyrene film thickness from 250 to 20 nm, where b remains almost invariant around 3 nm. They sit on top of the film with the semimajor axes roughly parallel to film surface. On glass, the Au film was probed at different stages of growth. After an initial period (1 min) of spheroid nanoparticle formation by dewetting, the coverage was complete (as observed from Au optical spectra) and partially wetting islands appeared after 2 min on the Au covered glass surface. After 5 min, these islands formed rings resembling quantum rings. The rings broke up again into islands after 10 min. Keywords: Morphology, Au nanoparticles, Polystyrene, Film, Sputtering, UV-Vis spectra, AFM, X-ray reflectivity

Introduction Shape transitions in nanoparticles and their control play important roles in nanomaterials research.1 In nanocomposites nanoparticle-matrix interactions are expected to determine this shape transition. Owing to the dominance of surface states, nanoparticles of a kind of material such as metal may exert forces that are different from those exerted by the corresponding bulk material on the surrounding matrix.2 Such novel interactions may be further modified if the matrix is confined and may give rise to better control of the shape transition. Metal nanoparticles with different shapes have generally been grown using chemical routes.3,4 While some of these routes have achieved remarkably monodisperse nanoparticle size and shape, most of them are restricted to producing spherical nanoparticles. The specificity of a chemical reaction, which achieves this monodispersity, is also a hindrance to shape and size changes. Physical vapour deposition (PVD) processes such as magnetron sputter deposition, on the other hand, can be tuned to produce nanoparticles in a wide range of shapes and sizes. Understanding of specific mechanisms of nanoparticle growth by PVD processes would lead to growth of nanoparticles with well defined and tunable sizes and shapes at the same time. It is also important to investigate the initial stages of growth of a metal film that finally leads to a specific [e.g. (111)] orientation. Studies on such ultrathin metal films have been an area of considerable interests for quite Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700 064, India *Corresponding author, email alokmay.datta@saha.ac.in

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some time.5,6 Recent studies on growth of ultrathin films have opened up another aspect of the growth process. This is the evolution of patterns owing to growth.7,8 Formation of metallic nanoparticles and their shape and size transformations are some natural steps in this growth process and the patterns evolving during growth of metal films needs to be studied in detail to investigate the role of nanoparticles in determining the evolution. We have studied the shape transition of Au nanoparticles deposited by direct current (DC) magnetron sputtering on glass and polystyrene films with thickness varying from 20 to 250 nm spin coated on fused quartz. We have studied the samples with optical spectroscopy, grazing incidence x-ray reflectivity and atomic force microscopy (AFM). The results of our studies are presented here.

Experimental Au nanoparticles on glass slides, cleaned by an RCA process, were grown in a Pfeiffer PLS500 DC magnetron sputtering unit at a target power of 25 W. The gas flow rate was very slow at 2.8 sccm. The chamber was flushed twice with Ar before deposition in order to ward off impurities present inside the growth chamber. The base pressure was 561026 mbar. Samples were deposited for durations of 30 s and 1, 2, 5 and 10 min in order to carry out a systematical study of the growth process. Samples were studied using reflection optical spectroscopy with a Cintra 10e Spectrometer working within the range of 190 to 700 nm and contact mode AFM with a Park Scientific scanning probe microscope. For deposition on the soft substrate polystyrene (PS) [Mw5560 900 and Rg (the radius of gyration of the polymer)521.5 nm] films were prepared from toluene solutions by spin coating on fused quartz substrates. The thickness of the films was ß 2006 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 1 August 2005; accepted 14 November 2005 DOI 10.1179/174329406X108834

Chattopadhyay et al.

1 Optical spectra (190–700 nm) of Au on glass deposited for different durations of time

varied from y1Rg to y12Rg by changing the concentration of the solution and the angular velocity, using a calibration curve obtained previously. Au was sputtered on PS films of different thickness using the same DC magnetron for 5 s at 25 W (0.064 A, 410 V, 10 sccm Ar gas flow and 4.761023 mbar pressure). Transmission optical spectra of the Au embedded films with thickness varying from y3Rg to y12Rg were recorded in transmission mode in the range 400–700 nm. An 18 kW rotating Cu anode generator (Enraf Nonius FR 591) and Optix Microcontrole spectrometer9 were used to collect X-ray reflectivity data of films with thicknesses y1Rg, y3Rg and y6Rg.

Results and discussion Au on glass Investigation of the colour of the films after deposition suggests that nanoparticles have been formed for films deposited for short durations, i.e. the initial stages of film. This is found to be true through an ultraviolet (UV)-Vis spectroscopic study of the films.10 Results of optical spectroscopy (Fig. 1) suggest that, for the 30 s and 1 min deposited films, Au nanoparticles have been formed. Nanoparticles sizes were found using c52vF /d, where c is full width at half maximum of the peak, vF is Fermi velocity of Au and d is the diameter of the Au nanoparticles.10 The transition from the nanoparticle nature of films to the bulk nature occurs for the film deposited for 2 min. After this, the films are almost bulk in nature from the spectroscopic point of view. However, there is a persistent weak peak at ,670 nm, which must correspond to some nanostructure. AFM results are shown in Fig. 2. For the film obtained after a 30 s deposition, the coverage is ,67%. The coverage increases with the deposition time. For the film obtained after 1 min of deposition, large circular shaped discs are found to form on the film surface. The diameter of the discs as obtained from AFM is , 600 nm. The height of these discs is found to be , 9 nm. The disc density on the substrate surface is ,17 mm22. It is also observed that on each of these discs there is an island with a diameter of ,50 nm and a height of about 5 to 6 nm. For the film deposited for 2 min, distinctly separated dot-like islands are found to form on the Au film. The diameter of these islands is ,50 nm while the height is ,6 nm. As we go over the film deposited for

Morphology of Au nanoparticles formed by magnetron sputtering

5 min, we clearly observe rings forming on the film surface. Typical diameter of these rings is ,250 nm. The rim width of the ring is ,50 nm while the height is ,5.3 nm. Upon longer deposition, these structures are lost and we ultimately observe more or less a smooth film as the deposition time is increased. Therefore, it is clear that some islands or dots are formed along with larger discs in the initial stages of film growth and while the discs eventually form a layer of Au on glass, these islands coalesce to form rings and thereafter break down and smoothen as the film grows. It is to be noted that the persistent peak in the optical spectra (670 nm) is red-shifted with respect to the peak position corresponding to nanoparticles with the dimensions obtained from AFM studies. This and the persistence of these islands with non-zero contact angles with respect to the underlying Au layer, lead us to suggest that the islands have a composition slightly different from the Au layer and in fact, they are nanoshells of Au.11

Au on PS films Figure 3 shows the optical absorption spectrum of Au sputtered 1Rg thick PS film. The peak in the visible region (y500 nm) is a clear indication of Au nanoparticles formation. Sizes of nanoparticles were found using the above mentioned expression.10 Measurements were carried out to determine d as a function of PS film thickness. It was found that d showed a sudden but small increase from 3.0 to 4.8 nm when the PS thickness was reduced below 10Rg. However, it was also observed (Fig. 3) that below this thickness of the PS film, the visible spectrum of Au nanoparticles shows two peaks. This corresponds to two radii for each particle, indicating an ellipsoidal shape of the Au nanoparticles with ellipticity e5a/b (a is the major diameter and b is the minor radius) in the confined PS matrix.1,12 Plots of a and b (Fig. 4a) and e (Fig. 4b) as a function of PS film thickness are direct proofs of a shape transition of Au nanoparticles from spherical to ellipsoidal as the PS film decreases in thickness. The values of a and b for nanoparticles in the y3Rg and y6Rg thick films are given in Table 1. X-ray reflectivity was used to determine the spatial disposition of these ellipsoidal nanoparticles with respect to the film and study the Au/PS interface. Reflectivity profiles for the three Au sputtered PS films on quartz are presented in Fig, 5a, with film thickness of y1Rg, y3Rg and y6Rg in the bottom, middle and top panels, respectively. These films show root-mean-square (rms) surface roughness of ,0.5 nm. Electron density profiles (EDPs) obtained from the excellent fits (continuous lines) using the Parratt method of analysis13 are shown in Fig. 5b along with the electron densities of bulk PS (rPS) and Si (rSi) as visual guides and reflectivities scaled for clarity. The bulk electron density of Au is not given, as that would have scaled down the EDP to a level where the details would be obscured. The EDPs for the films with three different thicknesses are presented in the same order as the reflectivity profiles. The precise thicknesses and electron densities of the Au nanoparticle layer and the underlying layers and the interfacial width associated with each layer are shown in Table 1. The EDPs in Fig. 5b show that the sputtered Au is almost entirely on top of the PS film. Table 1

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a 20 s; b 1 min; c 2 min; d 5 min 2 Atomic force microscopic images of Au on glass deposited for different time durations

summarises the spectroscopic and X-ray reflectivity results for the three Au implanted films. From the table it is clear that in all films the effective width of the Au nanoparticle bearing layer is almost the same and

matches with b, the minor diameter of the nanoparticles. The Au nanoparticles therefore sit on top of the PS films with the major axes parallel to the film surface and the magnitude of their minor axes is determined by dosage of sputtering.

Conclusions

3 Optical spectra of Au nanoparticles on 1Rg thick PS film

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We have studied the effect of confinement of amorphous matrix on nanoparticle matrix interaction and consequent changes in nanoparticle shape using optical spectroscopy and grazing incidence X-ray reflectivity. Specifically, we have deposited Au by magnetron sputtering on hard (glass) and soft (PS films with varying thickness spin coated on fused quartz substrates) host matrices. We have studied the shape of Au nanoparticles formed by spectroscopy and AFM and the depth distribution of the nanoparticles in the films as well as the morphology and composition of the underlying PS using X-ray reflectivity. In the case of the hard matrix, islands appeared on the Au nanodiscs, persisted on subsequent Au layer and formed rings resembling quantum rings, as deposition progressed from 1 to 5 min. The rings broke up again



4 Variation of (a) Au nanoparticle dimensions and (b) Au nanoparticle ellipticity with PS thickness

5 (a) reflectivity profiles and (b) EDPs of Au implanted PS films of 1Rg, 3Rg and 6Rg thickness, from bottom to top Table 1 Combined results from X-ray scattering and spectroscopy Au nanoparticle

Au nanoparticle layer

PS film thickness, nm

Major dia. (a), nm

Minor dia. (b), nm

Thickness, nm

Electron density (nm23)

Interface width, nm

Thickness, nm

Electron density

Interface width, nm

20.3 78.7 121.6

– 6.2 6.8

– 3.2 3.1

1.8 2.2 2.4

1830 1505 1110

0.65 0.54 0.56

0 0.9 1.9

0 742 644

0 0.7 0.5

into islands after 10 min. On the soft matrix, Au nanoparticles are observed to make a transition from a spherical to an ellipsoidal shape as the polymer thickness is reduced and the ellipticity is enhanced with further lowering of film thickness. The nanoparticles are formed on top of the films with their major axes parallel to the film surface and the magnitude of their minor axes determined by sputter dosage.

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PS/Au mixed layer

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