10170
J. Phys. Chem. C 2007, 111, 10170-10174
ARTICLES Synthesis of Au-C60 Cluster Materials Helge Kro1 ger,† Inga Gerhards,† Velimir Milinovic´ ,† and Petra Reinke*,‡ II. Physikalisches Institut, UniVersita¨t Go¨ttingen, Friedrich-Hund-Platz 1, 37077 Go¨ttingen, Germany, and Department of Materials Science and Engineering, UniVersity of Virginia, 116 Engineer’s Way, CharlottesVille, Virginia 22904 ReceiVed: September 6, 2006; In Final Form: February 21, 2007
We have investigated the formation of Au clusters in a C60 fullerene matrix (binary cluster material) with the aim to produce densely packed arrays of ultrasmall Au clusters with sizes below 4 nm. We achieved the formation of a binary cluster material by co-deposition of Au and C60, and no segregation of the gold was observed. The Au concentration throughout the film was observed with Rutherford backscattering spectroscopy and reflects the deposition rates measured for the single particle fluxes. It was determined by high-resolution TEM that the films show a narrow size distribution of the Au clusters. The size ranges from 1.6 to about 3.8 nm, and the size distribution is determined by the Au concentration. For low Au concentrations of less than 5 atom %, the maximum of the size distribution was positioned at about 2 nm with a full width at halfmaximum of 0.5 nm. The gold appears to have a strong influence on the growth mechanism of the films, as evidenced by a dramatic reduction in the mean roughness of the films with increasing Au content, which was observed with atomic force microscopy. For high gold concentrations, the surface roughness decreases to less than half the radius of a C60 molecule. The competition between surface and volume cluster growth and the role of the Au cluster-fullerene interaction in the film growth are discussed.
Introduction Nanoscale materials have unique properties, which differ in many aspects from the corresponding bulk materials.1-5 Metal and semiconductor clusters are regarded as particularly interesting nanoscale building blocks and have the potential to play an important role in such diverse areas as electronics, optical applications, and catalysis. In the size regime of a few atoms up to a few hundred atoms, quantum size effects, electron confinement, and the number of atoms control the behavior and stability of the cluster. Dramatic changes in cluster properties can thus be observed as a function of cluster size. The synthesis of size-selected and stable cluster arrays is therefore one of the central goals in the development of new cluster-based materials. The cluster formation in a fullerene matrix resembles in some ways exohedral doping, where foreign atoms are incorporated in selected sites of the fullerene lattice but outside of the fullerene cage. The highest dopant concentration was achieved for Na11C60.6 The exohedrally doped fullerenes are also known as fullerides and are novel materials with intriguing new properties like the superconductivity of K3C60.7 It has been shown that these ionic fullerides with alkalis and some earth alkalis are thermodynamically stable.8 The stability of fullerides with other elements and transition metals remains an open question, albeit thermodynamic considerations indicate that the large cohesive energies favor segregation, and thus, only a kinetically driven synthesis route is feasible. The formation of * To whom correspondence should be addressed. † Universita ¨ t Go¨ttingen. ‡ University of Virginia.
Ti, Yb, and Nb fullerides has indeed been achieved by rapid condensation of the reactants from the gas phase,10-13 which suggests the possibility of creating a range of fullerides, embedding ultrasmall clusters in the fullerene matrix and incorporating a wide range of elements. In recent experiments, we have investigated the possibility of employing the highly corrugated surface of a fullerene thin film to grow silicon and gold clusters.14,15 Even composite films containing silicon clusters were created by co-deposition of C60 and silicon.16 These studies also indicate that there is a fundamental difference in the way that these two elements behave in a fullerene environment. The fullerene surface has the potential to function as a template for the formation of silicon cluster arrays, whereas this is not the case for Au clusters. The formation of other cluster-fullerene compounds has also been reported for Ag-C60,25,26 which was synthesized by codeposition, and Al-C60,27 formed by the in-diffusion of surface Al into the C60 matrix. In the case of gold, it may be possible to use the fullerene molecules as a matrix in which to grow densely packed nanosize Au clusters. Although Au and Ag are chemically quite similar, the cohesive energy of Au is significantly larger, which will likely affect the cluster size distribution. In the present study, we expand the surface experiment for gold cluster growth15 to the synthesis of 3D composite films by co-deposition of C60 and Au. For this we have created composite samples with different concentrations of Au. The cluster-size distribution as a function of Au content was investigated, and the observation of film surface morphology with atomic force microscopy allows conclusions about the growth mechanism of these composite films.
10.1021/jp065812h CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007
Synthesis of Au-C60 Cluster Materials
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10171
Experimental Section Thin films composed of fullerenes (C60) and gold (Au) were deposited onto p-type silicon, polished quartz, and copper TEM grids to accommodate the requirements for different analytical methods. The samples were prepared in a UHV chamber at a base pressure of 1 ×10-10 mbar by co-deposition of C60 and Au. The fullerenes were deposited from a BN crucible that was heated by a tungsten filament wire, which was wrapped around the crucible. The deposition rate was monitored by a quartz crystal monitor and for the fullerenes lay in the range of 0.20.5 nm/min. Gold was evaporated from an electron beam evaporator (EBE), and the deposition rate was between 0.008 and 0.06 nm/min. Four different concentrations were used: 1.5 atom % Au, 5 atom % Au, 11 atom % Au, and 25 atom % Au. Thin films for each experiment were deposited onto all three kinds of substrates. For this, on the one side of the sample holder, a silicon substrate was mounted together with a TEM grid. On the other side of the sample holder, a quartz substrate was mounted. Before starting a deposition, the deposition rate of the two elements was determined by employing a quartz crystal monitor. The deposition rate of Au was determined prior to switching on the fullerene source. Subsequently, a mechanical shutter interrupted the Au beam and the fullerene deposition rate was established. The fullerene source was a broad beam source and mechanical shuttering proved to be inefficient. The sample was therefore exposed to a pure fullerene beam in the early stage of deposition until the evaporation rate of the C60 was fully stabilized. At this point, the shutter to the gold beam was opened, and a composite beam reached the substrate. Also, there usually was a slight drop in the fullerene deposition rate of about 10% during the deposition of the sample. Final film thicknesses lay in the range of 20-30 nm. Information about the Au clusters in the fullerene matrix was gained by high-resolution transmission electron microscopy (HRTEM) on a Philips CM 200-UT equipped with a field emission electron source. The samples for these experiments were deposited on commercially available copper TEM grids that are covered with a 20 nm film of amorphous carbon. Information about the surface topography of the composite films was obtained from the samples on silicon substrates by atomic force microscopy (AFM) in a Digital Instruments MMAFM-2 microscope, and the data were analyzed with the Nanoscope III software from Digital Instruments. Rutherford backscattering spectroscopy (RBS) was used to determine the distribution of the gold throughout the sample depth for the samples deposited on Si. RBS was performed at the Go¨ttingen heavy ion implanter IONAS,17 using 900 keV He2+ ions and a total charge of 9 µC. Two silicon surface barrier detectors positioned at 165° to the beam are used. The element/ isotope concentration profiles were deduced using the IBA Data Furnace software.18 Results and Discussion The samples on silicon were investigated with RBS to determine whether segregation of the gold occurs. The TEM analysis provides a projection of the cluster size distribution on the image plane and is insufficient to exclude segregation of Au at the surface or at the Si-fullerene interface. The top part of Figure 1 shows the Au section of the RBS spectrum of the 25 atom % Au sample and a fit to the data points. The distribution of Au throughout the sample is obtained from the fit as described in the experimental section, and the results are depicted in the lower half of Figure 1. The error of the fit is approximately (2.5 %. At a depth of 350 × 1015 at/cm2 the
Figure 1. Top: Au section of the RBS Spectrum of the sample containing 25 atom % Au. The line connecting the data points is the fit. Bottom: element distribution of the RBS spectrum fit as a function of the depth.
Si-substrate begins. The distribution reflects the deposition sequence described in the experimental section, and a lower Au concentration is observed closer to the substrate interface, which can be deduced from the shallower slope on the lefthand side of the Au peak. After the initial increase in Au concentration, the Au percentage in the sample stays relatively constant at about 25 atom %, indicated by the almost flat top of the Au peak in the top part of Figure 1. These two-thirds of the film contain approximately 75-80% of the Au in the film. The relatively high oxygen concentration is attributable to the incorporation of water and other environmental gases in the film due to an extended exposure to air. The RBS causes irradiation damage and was therefore performed at the end of the sample characterization, about 8 weeks after the deposition. The samples were kept in darkness in between measurements, therefore preserving the integrity of the fullerene molecules during storage. In previous experiments, in situ deposition and analysis of C60-Au layers with X-ray photoelectron spectroscopy (XPS) at comparable base pressures have shown that the films are nearly oxygen free (concentration
