Nanoparticle Dispersion on Reconstructed Carbon Nanomeshes

Nov 10, 2004 - A nanoporous template which can be used for the preparation of monodispersed metal nanoparticles can have wide-ranging applications in ...
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Langmuir 2004, 20, 10779-10784

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Nanoparticle Dispersion on Reconstructed Carbon Nanomeshes Wei Chen,† Kian Ping Loh,*,† Hai Xu,‡ and Andrew Thye Shen Wee‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore, and Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore Received June 15, 2004. In Final Form: August 19, 2004 A nanoporous template which can be used for the preparation of monodispersed metal nanoparticles can have wide-ranging applications in the catalyzed growth of single-walled nanotubes, as well as the preparation of energetic, nanostructured ferromagnetic particle arrays. Here, we found that a honeycomblike carbon nanomesh with periodically arranged pores of ∼2-nm dimension could be fabricated on the reconstructed 6H-SiC(0001) surface. The carbon nanomesh arises from the periodic arrangement of segregated carbon clusters on the 6H-SiC surface to form a highly regular, nanoporous film. The carbon nanomesh can be dynamically structured to control the periodicity and depth of the pores by annealing in a vacuum. We evaporated cobalt on the surface of the nanomesh and investigated the diffusion and agglomeration behavior of cobalt clusters using in situ scanning tunneling microscopy. It is found that monodispersed Co nanoclusters that resist aggregation up to a temperature of 500 °C can be fabricated on this template.

Channel-confined deposition involving the growth of dimensional nanotubes in ordered, alumina templates has attracted intense research interests in nanotechnology. Besides the utilization of the channels in the porous template to direct the growth of nanomaterials, the wellordered nanoporous template can present unique surfaces for controlling the lateral arrangement of deposited nanoparticles as a result of the selected binding of these particles on boundary edge sites. For this reason, there has been intense interest recently in the fabrication and application of surface templates with nanosized pores, that is, nanotemplates, to control the assembly of nanostructures. The various strategies for generating nanotemplates include the formation of two-dimensional networks using supramolecular organization on surfaces and1-6 the generation of strain-relief patterns by the deposition of material with a lattice constant different from that of the substrate,7 as well as the formation of superstructures arising from atomic reconstructions on metal or semiconductor surfaces.8-11 Theobald et. * Corresponding author. E-mail: [email protected] (K. P. Loh). † Department of Chemistry, National University of Singapore. ‡ Department of Physics, National University of Singapore. (1) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. 2000, 39, 1230. (2) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. Rev. Lett. 2001, 87, 096101. (3) Spillmann, H.; Dmitriev, A.; Lin, N.; Messina, P.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2003, 125, 10725. (4) Barth, J. V.; Weckesser, J.; Trimarchi, G.; Vladimirova, M.; De Vita, A.; Cai, C.; Brune, H.; Gu¨nter, P.; Kern, K. J. Am. Chem. Soc. 2002, 124, 7991. (5) De Feyter, S.; Gesquie`re, A.; Klapper, M.; Mullen, K.; De Schryver, F. C. Nano Lett. 2003, 3, 1485. (6) Berner, S.; de-Wild, M.; Ramoino, L.; Ivan, S.; Baratoff, A. Gu¨ntherodt, H.-J.; Suzuki, H.; Schlettwein, D.; Jung, T. A. Phys. Rev. B 2003, 68, 115410. (7) Brune, H.; Giovannini, M.; Bromann, K.; Kern, K. Nature 1998, 394, 451-453. (8) Wu, K. H.; Fujikawa, Y.; Nagao, T.; Hasegawa, Y.; Nakayama, K. S.; Xue, Q. K.; Wang, E. G.; Briere, T.; Kumar, V.; Kawazoe, Y.; Zhang, S. B.; Sakurai, T. Phys. Rev. Lett. 2003, 91, 126101. (9) Li, J. L.; Jia, J. F.; Liang, X. J.; Liu, X.; Wang, J. Z.; Xue, Q. K.; Li, Z. Q.; Tse, J. S.; Zhang, Z. Y.; Zhang, S. B. Phys. Rev. Lett. 2002, 88, 066101. (10) Lai, M. Y.; Wang, Y. L. Phys. Rev. B 2001, 64, 241404.

al.12 reported the formation of a C60 honeycomb network by the self-assembly of organic molecules on Ag/Si(111)-x3 × x3R30° surface. Boron nitride nanomesh has been fabricated recently by Corso and co-workers13 by decomposing borazine molecules on the Rh(111) surface at high temperatures; the edges of the boron nitride nanomesh can be decorated by C60 molecules to form an ordered C60 honeycomb array. Our aim is to identify a chemically inert nanotemplate for the preparation of monodispersed metal nanoparticles. These can have wide-ranging applications in the catalyzed growth of single-walled nanotubes, as well as the preparation of energetic, nanostructured ferromagnetic particle arrays. To prepare homogeneously sized metal nanoparticles with dimensions of several nanometers, the nanotemplate must be highly ordered and consist of pores with dimensions of the same order. Our strategy is to create these templates on binary compound substrates where one of the elements shows a tendency to segregate on the surface at elevated temperatures to form ordered reconstructions. We found that carbon nanoclusters segregate on the 6H-SiC surface when it was annealed to 1100 °C to form a highly periodic, honeycomb-like film which resembles a nanomesh.14-16 The question we ask next is whether the carbon nanomesh is useful for the preparation of monodispersed metal nanoparticles. Thus, the diffusion and agglomeration of cobalt at different temperatures was studied on this nanomesh using scanning tunneling microscopy (STM). The experiments were carried out in a multichamber ultrahigh vacuum (UHV) system with a base pressure of 1 × 10-10 Torr, allowing in situ transfer between facilities (11) Kotlyar, V. G.; Zotov, A. V.; Saranin, A. A.; Kasyanova, T. V.; Cherevik, M. A.; Pisarenko, I. V.; Lifshits, V. G. Phys. Rev. B 2002, 66, 165401. (12) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (13) Corso, M.; Auwa¨rter, W.; Muntwiler, M.; Tamai, A.; Greber, T.; Osterwalder, J. Science 2004, 303, 217. (14) Xie, X. N.; Wang, H. Q.; Wee, A. T. S.; Loh, K. P. Surf. Sci. 2001, 478, 57. (15) Johansson, L. I.; Owman, F.; Ma¨rtensson, P. Phys. Rev. B 1996, 53, 13793. (16) Owman, F.; Ma¨rtensson, P. Surf. Sci. 1996, 369, 126.

10.1021/la048530m CCC: $27.50 © 2004 American Chemical Society Published on Web 11/10/2004

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Figure 1. 15 × 15 nm2 STM filled state images of carbon nanomesh at different bias: (a) VT ) 2.5 V and (b) VT ) 2.0 V. (c) LEED image of carbon nanomesh, showing a 6x3 × 6x3R30° reconstruction. Electron beam energy ) 70 eV.

for electron beam evaporation and surface analysis. Surface analytical techniques available include variable temperature STM (Omicron VT-STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED).14,17,18 To prepare the carbon nanomesh, a 6H-SiC substrate polished on the (0001) face was first coated with several layers of silicon by evaporating silicon from a resistively heated silicon wafer. The siliconenriched SiC sample was annealed at 1100 °C in an UHV until a 6x3 × 6x3R30° reconstruction pattern could be observed by LEED; we found that this corresponded to the formation of the carbon nanomesh observed in STM. Co was directly deposited on the carbon nanomesh at room temperature by electron beam evaporation with a deposition rate of ∼0.2 Å/min. The chamber pressure was below 1 × 10-8 Torr during Co deposition. All thermal processing in our UHV chamber was performed using direct resistive heating. Figure 1a,b show the STM filled state topographies of this nanomesh at different sample bias. The morphology resembles a hexagonal honeycomb. Analysis of the surface structure using LEED reveals an ordered 6x3 × 6x3R30° reconstruction in Figure 1c, which can be attributed to an incommensurate overlayer of carbon atoms on the 6H(17) Chen, W.; Xie, X. N.; Xu, H.; Wee, A. T. S.; Loh, K. P. J. Phys. Chem. B 2003, 107, 11597. (18) Ong, J.; Tok, E. S.; Xu, H.; Wee, A. T. S. Appl. Phys. Lett. 2002, 80, 3406.

SiC previously.14,15 The chemical composition of this nanomesh had been investigated previously using electron energy loss spectroscopy and Auger electron spectroscopy and was found to be a carbon-rich phase distinct from silicon carbide.14,15 A STM line scan of this nanomesh in Figure 2b reveals periodic “pores” that are 1.8 ( 0.2 nm in diameter and 0.15 ( 0.01 nm in depth. The pore size of this nanomesh could be enlarged by annealing the SiC substrate at 1100 °C for a longer time (20 min). The changes in the surface corrugations of the carbon nanomesh after different annealing times can be judged from the line scan profiles displayed in Figure 2a,b. The line profile shows a doubling in amplitude and a change in periodicity from 1.8 to 2.5 nm; therefore, the surface corrugation is effectively enlarged after prolonged annealing, suggesting a dynamic surface that can be restructured. The mechanism of the restructuring during prolonged annealing can be explained by the cumulative segregation of carbon atoms on the surface which accompanies the evaporation of silicon atoms from the bulk. After the collapse of several layers of SiC on the surface, these are transformed into a highly ordered carbon nanomesh which adheres epitaxially to the underlying SiC. To evaluate the use of the carbon nanomesh for the fabrication of nanostructures, Co was directly deposited on the nanomesh at room temperature by electron beam evaporation. By controlling the evaporation time, the

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Figure 2. (a) 15 × 15 nm2 STM filled state images of carbon nanomesh annealed at 1100 °C for a longer time (VT ) 2.5 V); (b) line profile 1 for Figure 2a and line 2 for Figure 1a, showing increase in surface corrugation after annealing.

Figure 3. 16 × 16 nm2 STM filled state images of the carbon nanomesh with (a) 0.1-Å Co coverage and (b) 0.2-Å Co coverage; (c) line profile 1 for Figure 3a and line 2 for Figure 1a (clean surface). VT ) 2.5 V.

coverage could be varied from 0.1 to 3.0 Å. At the initial stages (0.1-Å coverage), the STM image as shown in Figure

3a revealed that the Co adsorbed on the corner sites of the nanomesh. The white protrusion in Figure 3a is due to

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Figure 4. 60 × 60 nm2 STM filled state images of the carbon nanomesh with Co coverage at (a) 0.1, (b) 0.5, (c) 1.0, and (d) 2.0 Å (VT ) 2.0 V, I ) 0.20 nA). The corresponding histograms showing cluster diameter versus number of clusters are shown below.

the adsorbed Co nanocluster. In situ XPS of the surface confirmed surface coverage by Co. Figure 3c compares the surface profile of the Co-deposited (line 1) surface with that of the carbon nanomesh (line 2). A line scan of the Co nanoclusters showed that they are 1.4 ( 0.1 nm in diameter and 0.17 ( 0.01 nm in depth, as shown in Figure 3c. Judging from line 1 in Figure 3c, the nanomesh

supercells that were not covered by the Co nanoclusters maintained a similar surface corrugation to that of the bare nanomesh sample (line 2 in Figure 3c). This indicates that the periodicity of the carbon nanomesh was retained after the deposition of Co. This observation shows that the carbon nanomesh is a stable structure, as opposed to surface structures that are prone to reconstruction after

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Figure 5. 250 × 250 nm2 STM filled state images of the carbon nanomesh with Co coverage at (a) 2.50 and (b) 3.0 Å (VT ) 2.0 V, I ) 0.20 nA).

Figure 6. 100 × 100 nm2 STM filled state images of 2 Å Co deposited on the carbon nanomesh and annealed at (a) 200, (b) 500, (c) 800, and (d) 1000 °C (VT ) 2.0 V, I ) 0.20 nA).

the adsorption of submonolayer coverage of metal particles. By increasing the coverage to 0.2 Å (Figure 3b),

more Co nanoclusters were found to adsorb on the corner sites of the nanomesh. Therefore, the corner sites of the

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carbon nanomesh act as stable adsorption sites for Co nanoclusters at room temperature. The STM images of the deposited nanoclusters are shown in parts a (0.1 Å of Co), b (0.5 Å of Co), c (1.0 Å of Co), and d (2.0 Å of Co) of Figure 4. The corresponding histograms showing the cluster size versus numbers of clusters are attached below the respective STM images. From 0.5-Å coverage onward, the maximum cluster size is constant at ∼3.5 nm and does not change with coverage. After the deposition of Co to a thickness of 2.0 Å, a full coverage of monodispersed Co nanoclusters (∼3.5 nm in diameter) is obtained, as shown in Figure 4d. No aggregation of these Co clusters is observed. According to classical nucleation and growth theory, the bigger cluster should grow at the expense of the smaller one as the coverage increases to minimize the surface free energy. We believe that the limit in maximum attainable cluster size is due to the confinement effect of the nanoporous template on the nanocluster, while the narrow dispersion of cluster size observed here is attributable to constrained diffusion on the porous surface characterized by narrow channels joining the corner sites. Because there is a welldefined pattern of nanocluster adsorption on the periodically arranged corner sites, the diffusion of the smaller atoms to the next-nearest-neighbor clusters occupying such sites leads to a rapid equilibration in cluster size due to the short diffusion lengths of the atoms on the nanoporous surface. Such a nucleation pattern is markedly different from that of metal deposition on graphite or sapphire surfaces. On these surfaces, the metal atoms nucleate to form islands with a rather heterogeneous lateral size distribution in the submonolayer regime.19-21 The formation of monodispersed Co nanoclusters in this case is quite similar to the formation of self-organized nano-islands on dislocation networks formed by the second Ag atomic layer on Pt(111), where surface dislocations constrain the diffusion of atoms.7 At increasing coverage, a second layer of Co nucleates on the existing monolayer. The size confinement effected (19) Carrey, J.; Maurice, J.-L.; Petroff, F.; Vaure`s, A. Phys. Rev. Lett. 2001, 86, 4600. (20) Bifone, A.; Casalis, L.; Riva, R. Phys. Rev. B 1995, 51, 11043. (21) Goldoni, A.; Baraldi, A.; Comelli, G.; Esch, F.; Larciprete, R.; Lizzit, S.; Paolucci, G. Phys. Rev. B 2000, 63, 035405.

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by the nanomesh is lost due to the screening by the first layer of Co. As a result, hexagonal islands of about 1.9 nm in height and 20 nm in width began to nucleate as shown in Figure 5 in accordance with classical nucleation and growth model. Therefore, the coverage of the Co must be carefully controlled if a monodispersion of the cluster size is desired. The diffusion of the as-deposited Co at different temperatures was investigated by heating the carbon nanomesh that was preadsorbed with a full coverage of monodispersed Co of about 3.5 nm. Below 500 °C, the adsorbed Co nanoclusters resisted agglomeration and remained remarkably stable. This is clear evidence of the suppressed diffusion of atoms on the nanomesh. After annealing to 500 °C for 20 min, the Co clusters increased in size to 6.8 ( 0.5 nm in diameter and 1.7 ( 0.1 nm in depth. The STM image in Figure 6b shows that the clusters remain homogeneous in size. The increase in cluster sizes is accompanied by a decrease in the cluster density. Further annealing at higher temperatures resulted in the evaporation of the Co nanoclusters from the carbon nanomesh, as shown in Figure 6c,d. Following the complete desorption of Co nanoclusters from the surface, the underlying nanomesh could be imaged clearly by STM. The fact that the nanomesh could be regenerated readily suggests no reaction between the nanomesh and Co. The carbon nanomesh template also prevented cobalt silicide formation during the annealing procedures. In conclusion, we have demonstrated the application of a carbon nanomesh fabricated on 6H-SiC for the preparation of monodispersed Co nanoclusters. Detailed STM investigation reveals that Co atoms prefer to adsorb on the corner sites of this carbon nanomesh at the early stages. Monodispersed Co nanoclusters with a narrow size distribution of 3.5 nm in diameter and 0.33 nm in height could be obtained. The carbon nanomesh is effective in isolating the Co nanoclusters, and agglomeration is prevented up to a temperature of 500 °C. To realize the practical usefulness of the nanomesh, future research must be directed at transferring or fabricating the carbon nanomesh on a wide range of substrates. LA048530M