Dendrimer

Langmuir 2007, 23, 7831-7835

7831

Converting Self-Assembled Gold Nanoparticle/Dendrimer Nanodroplets into Horseshoe-like Nanostructures by Thermal Annealing Amir Fahmi,*,† Anthony D’Ale´o,‡ Rene´ M. Williams,§ Luisa De Cola,| Nabil Gindy,† and Fritz Vo¨gtle⊥ The School of Mechanical, Materials and Manufacturing Engineering, The UniVersity of Nottingham, Nottingham NG7 2RD, United Kingdom, Laboratoire de Chimie de l’ENS Lyon, CNRS UMR 5182, France, Molecular Photonics Group, HIMS, UniVersiteit Van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, Physikalisches Institut, Westfa¨lische Wilhelms-UniVersita¨t, Mendelstrasse 7, D-48149 Mu¨nster 152, Germany, and Kekule´ -Institut fu¨r Organische Chemie und Biochemie der UniVersita¨t Bonn, Gerhard-Domagk-Straβe 1, 53121 Bonn, Germany ReceiVed March 6, 2007. In Final Form: April 24, 2007 A simple and effective nonlithographic method to produce a novel organization of noble metal nanoparticles into horseshoe-like nanostructures via self-assembly is described. The adsorption of Au nanoparticles stabilized with the dendrimer 1,2,3,4,5,6-hexakis[(3′,5′-bis(benzyloxy)benzyl)sulfanylmethyl]benzene (S6G1) on hydrophilic surfaces (native oxide-terminated Si(111)) resulted in the formation of spatially correlated droplet aggregates. Annealing of Au/S6G1 in thin films caused amalgamated droplets to form arrays of horseshoe-like nanostructures with an average size of ∼250 nm and an average height of 13 nm. The mobility and the manner in which the semicapped Au nanoparticles are distributed on the hydrophilic substrate are believed to be the promoters that control the growth of the nucleation to create the horseshoe-like structures. Atomic force microscopy (AFM) measurements demonstrated the changes in height and size of the nanoparticles before and after the annealing process. Oxygen plasma etching was used to remove the S6G1 dendrimer to reveal the orientation of the Au nanocrystals in the nanostructure matrix.

Introduction The design, construction, and control of metallic nanoparticles deposited on large surface areas are all essential elements in the production of novel optical and nanoelectronic devices.1 In particular, nanostructures of noble metal nanoparticles2,3 have received significant attention in the past decade due to their unique optical4 and electrical properties5 which are different from those of bulk.6 * To whom correspondence should be addressed. Telephone: +44 (0) 115 9514066. Fax: +44 (0) 115 951 4140. E-mail: Amir.Fahmi@ nottingham.ac.uk. † The University of Nottingham. ‡ Laboratoire de Chimie de l’ENS Lyon. § Universiteit van Amsterdam. | Westfa ¨ lische Wilhelms-Universita¨t. ⊥ Der Universita ¨ t Bonn. (1) (a) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (b) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (c) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M. C.; Casanove, M. J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286-4289. (d) Kagan, C. R.; Afzali, A.; Martel, R.; Gignac, L. M.; Solomon, P. M.; Schrott, A. G.; Ek, B. Nano Lett. 2003, 3, 119124. (e) Lee, J. O.; Lientschnig, G.; Wiertz, F.; Struijk, M.; Janssen, R. A. J.; Egberink, R.; Reinhoudt, D. N.; Hadley, P.; Dekker, C. Nano Lett. 2003, 3, 113-117. (f) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (g) Schmid, G.; Beyer, N. Eur. J. Inorg. Chem. 2000, 5, 835-837. (h) Bradley, J. S.; Tesche, B.; Busser, W.; Maase, M.; Reetz, M. T. J. Am. Chem. Soc. 2000, 122, 4631-4636. (2) Kanehara, M.; Oumi, Y.; Sano, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 8708-8709. (3) Kim, B.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955-7956. (4) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (5) (a) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (b) Kim, S. H.; Medeiros, R. G.; Ohlberg, D. A. A.; Williams, R. S.; Heath, J. R. J. Phys. Chem. B 1999, 103, 10341. (c) Beverly, K. C.; Sampaio, J. F.; Heath, J. R. J. Phys. Chem. B 2002, 106, 2131. (6) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Banin, U.; Cao, Y. W.; Katz, D.; Millo, O. Nature 1999, 400, 542.

Recently, several techniques have been used to obtain 2D and 3D nanostructures, including self-assembly,7-11 LangmuirBlodgett techniques,12-14 and electrophoretic deposition.15 Selfassembly is an effective tool to produce ordered nanostructures of inorganic nanoparticles in a host organic matrix. The large ordered nanostructure which results from colloidal particles is often determined by the aggregation of the subunit shapes into superstructures.16,17 Therefore, on surfaces of solid substrates, the self-assembled phase of the spherically shaped nanoparticle common subunits is formed when the liquid containing these nanoparticles evaporates. The various interactions between nanoparticles, substrates, and solvents18 are among the key factors that govern the self-assembly process into large ordered (7) Martin, J. E.; Odinek, J.; Wilcoxon, J. P.; Anderson, R. A.; Provencio, P. J. Phys. Chem. B 2003, 107, 430-434. (8) Zheng, J.; Zhu, Z.; Chen, H.; Liu, Z. Langmuir 2000, 16, 4409-4412. (9) Stoeva, S.; Klabunde, K. L.; Sorensen, C. M.; Dragieva, I. J. Am. Chem. Soc. 2002, 124, 2305-2311. (10) Brown, L. O.; Hutchison, J. E. J. Phys. Chem. B 2001, 105, 8911-8916. (11) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 16901693. (12) James, R. H.; Charles, M. K.; Daniel, V. L. J. Phys. Chem. B 1997, 101, 189-197. (13) Brown, J. J.; Porter, J. A.; Daghlian, C. P.; Gibson, U. J. Langmuir 2001, 17, 7966-7969. (14) Kim, B.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955-7956. (15) (a) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706-709. (b) Yeh, S. R.; Michael, S.; Boris, I. S. Nature 1997, 386, 57-59. (16) (a) Wang, Z. L.; Harfenist, S. A.; Vezmar, I.; Whetten, R. L.; Bentley, J.; Evans, N. D.; Alexander, K. B. AdV. Mater. 1998, 10, 808. (b) Petit, C.; Taleb, A.; Pileni, M. P. AdV. Mater. 1998, 10, 259. (c) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12 (10), 693 (17) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. L.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (b) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (18) (a) Rabani, E.; Egorov, S. A. Nano Lett. 2002, 2, 69. (b) Saunders, A. E.; Shah, P. S.; Park, E. J.; Lim, K. T.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 15969.

10.1021/la700651m CCC: $37.00 © 2007 American Chemical Society Published on Web 06/08/2007

7832 Langmuir, Vol. 23, No. 14, 2007

Figure 1. Schematic representation of S6G1 on the Au nanoparticle.

superstructures. Other factors include hydrodynamic effects,19 drying kinetics,20 and self-diffusion of the nanocrystals on the substrate.21 Dendrimers have a well-defined chemical composition and molecular size and have a hollow spherical shape.22 This makes such substances23 some of the most suitable organic hosts to encapsulate metallic nanoparticles because they are based on a core that is usually hyperbranched, extending from the core and having terminal active functional groups. The groups of Crooks,24 Esumi,25 and Balogh26 were the first to report the use of dendrimers in the stabilization of metal nanoparticles. Their studies show that the reduction of gold ions in the presence of generation 4(G4) PAMAM dendrimers leads to stable colloidal solutions, rather than macroscopic metal precipitation. This study reports a simple method to produce a large area of ordered Au nanoparticles stabilized with S6G1 dendrimer33 (Figure 1). A thin film of the semicapped Au nanoparticles is adsorbed on a hydrophilic solid substrate, forming a large area of nanodroplets. Furthermore, the nanodroplets are spatially correlated.27 Annealing the Au semicapped nanoparticles with dendrimers, which had been surface-functionalized with sulfur groups at their cores, produced an assembly of nanoscopic hybrid functional aggregates. Thermal annealing of the assembled nanostructures is a central theme in the construction of the supra- and macromolecular assemblies. Magonov et al. recently reported that thermal (19) (a) Maillard, M.; Motte, L.; Ngo, A. T.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 11871. (b) Stowell, C.; Korgel, B. A. Nano Lett. 2001, 1, 595. (20) (a) Redl, F. X.; Cho, K. S.; Murray, C. B.; O’Brien, S. Nature 2003, 423, 968. (b) Sztrum, C. G.; Rabani, E. AdV. Mater 2006, 18, 565. (c) Martin, C. P.; Blunt, M. O.; Moriarty, P. Nano Lett. 2004, 4, 2389. (21) Ge, G.; Brus, L. E. Nano Lett. 2001, 1, 219. (22) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and Dendrons; Wiley-VCH: Weinheim, 2001. (23) Zeng, F.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681-1712. (24) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256. (25) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1998, 14, 3157. (26) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (27) Fahmi, A.; Barraga´n, S. P.; D’Ale´o, A.; Williams, R. M.; van Heyst, J.; De Cola, L.; Vo¨gtle, F. Mater. Chem. Phys. 2007, 103, 361-365. (28) Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P.; Magonov, S. N. Langmuir 2000, 16, 5487. (29) Li, C. J.; Zeng, Q. D.; Liu, Y. H.; Wan, L. J.; Wang, C.; Wang, C. R.; Bai, C. L. ChemPhysChem 2003, 4, 857. (30) Bartels, C. R.; Crist, B.; Graessley, W. W. Macromolecules 1984, 17, 2702-2708. (31) Tanner, R. I. Engineering Rheology; Clarendon: Oxford, 1985. (32) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (33) D’Ale´o, A.; Williams, R. M.; Osswald, F.; Edamana, P.; Hahn, U.; Heyst, J. V.; Tichelaar, F. D.; Vo¨gtle, F.; Cola, L. D. AdV. Funct. Mater. 2004, 14, 1167.

Fahmi et al.

annealing of the dendrimer domains on mica led to a transition from rectangular to hexagonal packing.28 Bai and co-workers demonstrated the effect of annealing temperature on the arrangement of stilbenoid dendrimers (SD12) on HOPG. A wellordered hexagonal nanostructure of a well-ordered parallelogram nanostructure in a close-packed arrangement29 was observed. Also, it has been found that thermal treatment can play a very important role in changing the morphology of amorphous polymers. For instance, if the temperature is higher than the glass transition temperature (Tg), the polymer chains start to relax, resulting in a dramatically modified morphology. Nevertheless, annealing at temperatures lower than Tg keeps the polymer chains mainly stationary, thereby avoiding changing the existing morphology.30,31 In this work, it is observed that thermal annealing has significantly influenced the morphologies of Au/S6G1 nanodroplets in thin films. It was found that the change in the morphology after thermal annealing is due to an increase in the mobility of the Au/S6G1 nanoparticles on the solid hydrophilic substrate. Diffusion induces further interactions between the Au nanoparticles and the substrate to produce a specific distribution of Au/S6G1 particles which promote the formation of horseshoelike nanostructures. In a subsequent step, plasma etching is used to reveal the Au nanoparticles in the organic matrix (first generation dendrimer S6G1), leaving behind regular arrangements of noble metal nanoparticles. The novel aspect of this work is the construction of a large surface area containing arrays of ordered and structured inorganic nanoclusters via self-assembled organic molecules. Furthermore, removing the dendrimer molecules leaves the ordered Au nanodots freely accessible to chemical and physical interaction. Their shape is preserved and is remarkably stable on the substrate. Experimental Section Preparation of Nanoparticle/Dendrimer Films. The nanoparticles were prepared following Brust’s synthesis32 by using the dendrimer 1,2,3,4,5,6-hexakis[(3′,5′-bis(benzyloxy)benzyl)sulfanylmethyl]benzene (S6G1) as a stabilizer33 at a concentration where the sulfur atoms were stoichiometrically matched with gold. Spin-coating was used to spread the colloidal nanoparticles on a wide area of SiO2 wafers. One of the conditions to guarantee uniform distribution of the nanoparticles is to ensure that the colloidal solution wets the substrate surface. The SiO2 surface was then prepared by soaking the sample in a 5:1:1 H2O/H2O2/NH4OH solution for 20 min at 65 °C. The substrate was then rinsed with water several times and dried in nitrogen gas. The H/Si substrate was prepared by etching the native oxide surface of the Si wafer in an HF/NH4F (1:7) solution for 3 min. For easy control of the nanocomposite density on the surface, the rotational speed of the spin coater was kept constant at 3000 rpm. The incubation time of the annealing steps was 3 days (72 h). AFM analysis was performed in air using a Digital Instruments Multimode (Santa Barbara, CA) instrument and Si cantilevers (spring constant ) 40 N/m, resonance frequency ) 150-190 kHz) working in the tapping mode (TMAFM).

Results and Discussion The morphology and the molecular orientation of the Au/ S6G1 nanoparticle agglomerates have been studied using AFM measurements. Figure 2a shows an AFM image of a 40 µm by 40 µm area of large ordered two-dimensional arrays of S6G1/Au nanodroplets on a Si/SiO2 substrate. The image demonstrates the distribution of the Au nanoparticles on the solid substrate. Detailed cross-sectional measurements performed on a large number of isolated features show an average height of 33 nm and width of 190 nm. High magnification AFM images reveal a minor amount of smaller Au nanoparticles impact-distributed between the large

Horseshoe-like Au Nanostructures

Figure 2. (a) Shows 40 µm AFM image of large ordered two dimensional arrays of S6G1/Au nanoparticles on Si/SiO2 substrate. Inset: higher magnification of the ordered 2D arrays. (b) Representative TMAFM images of thin films of S6G1/semicapped Au nanoparticles on H/Si(111). Inset: higher resolution and contrast enhanced topographic showing the morphology of the aggregates on the hydrophobic surface.

nanoparticles. Observations showed that almost all of the surface (of the large and small gold nanoparticles) is roughly covered with the dendrimer molecules. It is believed that the use of toluene as a solvent in the formation of the thin film is the reason for the induction of some aggregation between the dendrimer molecules encapsulating the Au nanoparticles on the hydrophilic substrate to form the large particle size.18 The dewetting effect of toluene on a hydrophilic substrate (Si/SiOx) is responsible for producing nonuniform evaporation, leading to a concentration gradient of Au/S6G1 on the substrate. This concentration gradient translates into a variety of sizes of nanodroplets. Consequently, this could explain the small size of the Au/S6G1 nanoclusters impact-distributed between the larger nanodroplets. This model implies that, under the experimental conditions used (characterized by particle size, solvent, and substrate type), the size of the Au nanodroplets is expected to vary and is a function the concentration of the particles34 in the solution used. This explanation is an attempt to rationalize the results of a great number of experiments performed using different solvents and a variety of dissimilar types of substrates. (34) (a) Seemann, R.; Herminghaus, S.; Jacobs, K. Phys. ReV. Lett. 2001, 86, 5534-5537. (b) Seemann, R.; Herminghaus, S.; Jacobs, K. J. Phys.: Condens. Matter 2001, 13, 4925-4938.

Langmuir, Vol. 23, No. 14, 2007 7833

Thermal Annealing of the Nanoparticle/Dendrimer Films. The morphologies of dendrimer-stabilized gold (S6G1/Au) in thin films on SiOx were studied at various annealing temperatures in the range of 100-160 °C. The AFM images demonstrate that the structure changed dramatically, depending on the annealing temperature. For instance, annealing the thin film at 120 °C for 24 h increased the nanoparticles size significantly as well as the orientation of the nanoparticles on the substrate. Figure 3 shows that the nanoparticles formed in lacelike structures of 380 nm width and 8 nm height. Indeed, the small nanoparticles disappeared between the large nanoparticles. It seems that, at 120 °C, the mobility of the nanoparticles increases on the solid substrate which gives the nanoparticles the opportunity to coalesce to form relatively large particles. The large nanoparticles then start connecting together to form the lacelike structures. It is interesting to note that the height of the Au/S6G1 nanoparticles drastically changes with increasing annealing temperature. For instance, after spin coating at room temperature, the height of the nanoparticles was greater than 32 nm. Increasing the temperature to 120 °C can result in a remarkable drop in the Au nanoparticle height to 8 nm. Moreover, at higher temperatures, the size of the Au/S6G1 nanoparticles increased to values approximately twice their size at room temperature. To examine this mechanism, one control experiment was performed: a thin film of S6G1/Au semicapped nanoparticles was deposited onto H/Si(111). The inset in Figure 2b is a high magnification AFM image of the aggregates on a highly hydrophobic surface (H/Si (111)). On a highly hydrophobic surface (H/Si(111)), The semicapped Au nanoparticles aggregate so that the average height and diameter of the clusters are much lower (∼5 nm and ∼100 nm, respectively). A Comparison of parts a and b of Figure 2 clearly shows that the substrate chemistry plays a key role in determining the overall “conformation” of the dendrimer/ nanoparticle aggregates. The size of the nanoparticle aggregate and the average separation distance vary significantly between hydrophilic and hydrophobic surfaces due to their different wetting characteristics. There is a clear preference for a higher degree of wetting on the H/Si(111) substrate. The semicapped Au nanoparticles with the S6G1 dendrimer are more hydrophobic and therefore attempt to minimize the contact area with the substrate surface during spreading on the hydrophilic (Si/SiO2) substrate. As shown in Figure 2a, the clusters of Au/S6G1 particles aggregate together to form nanodroplets of 32 nm in height. Annealing at 120 °C raises the mobility of the Au nanoparticles and drives the Au nanoparticles to relax to a reduced height and form a thin layer (Figure 3). An unexpected nanostructure was observed at an annealing temperature of 140 °C as shown in Figure 4. The nanoparticles amalgamated to form horseshoe-like structures with an outer diameter of 250 nm and a height of 13 nm. The horseshoe-like structures covered a large-scale area with a distance between them of more than 600 nm with randomly oriented horseshoe openings. A few of the resulting structures were of the discontinuous ring type with a diameter of ∼300 nm and a consistent height of 13 nm. These results are comparable to the results obtained by Nolte and co-workers35 on the formation of ringlike structures of porphyrin molecules. Rings were observed when solutions of porphyrins were allowed to evaporate fast, followed by drainage of the remaining solution. They explained the formation of the rings by the nucleation of holes (2D gas bubbles) in a liquid film that does not wet the substrate. This followed a theoretical model to explain the assembly of (35) Albertus, P. H.; Schenning, J.; Fransicus, B.; Benneker, G.; Hubertus, P.; Geurts, M.; Liu, X. Y.; Nolte, R. J. M. J. Am. Chem. Soc. 1996, 118, 8549-8552.

7834 Langmuir, Vol. 23, No. 14, 2007

Fahmi et al.

Figure 3. AFM height image of Au/S6G1 nanoparticles after annealing at 120 °C on the Si/SiO2 substrate and cross section of the image.

Figure 4. AFM height image of Au/S6G1 on the Si/SiO2 substrate after annealing at 140 °C. Insets: 3D horseshoe structures and higher magnification image of a cross section of the horseshoe structure.

nanoparticles into ring-shaped structures (Ohara and Gelbart36). They argued that micrometer-sized rings formed from holes nucleating in volatile, wetting thin liquid films containing the nanoparticle. Recently, Pileni and co-workers demonstrated the formation of a variety of microstructures, including polygonal networks of nanocrystals and rings, which were attributed to thermocapillary flow during evaporation.37 To investigate the mechanisms that drive the morphological change from nanodroplets into horseshoe-like nanostructures, two different control experiments were performed. The first involved preparing several samples of the semicapped Au nanoparticles with the S6G1 dendrimer as described previously. These were annealed at temperatures close to 140 °C for 72 h. As expected, horseshoe-like nanostructures were observed. A second set of samples was prepared using different solvents to obtain different distributions of the nanodroplets on the hydrophilic substrate (SiOx). The AFM images taken after annealing demonstrate that, at high annealing temperatures (g135 °C), the semicapped Au nanoparticles start to diffuse on the SiO2 substrate. At the lower annealing temperatures, the semicapped Au nanoparticles build nuclei that are Au/Au and/or dendrimer/ dendrimer faced (Figure 5b). Raising the temperature to 140 °C, the concomitant increase in mobility causes the nuclei to form (36) (a) Ohara, P. C.; Heath, J. R.; Gelbart, W. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1077. (b) Ohara, P. C.; Gelbart, W. M. Langmuir 1998, 14, 3418-3424. (37) (a) Maillard, M.; Motte, L.; Ngo, A. T.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 11871-11877. (b) Maillard, M.; Motte, L.; Pileni, M. AdV. Mater. 2001, 13, 200-204.

a specific nucleation distribution, which is dependent on the original distribution of the nanodroplates on the hydrophilic substrate. The nuclei distribution and the amount of material (the number of nuclei on the substrate) are the driving forces which control the growing direction in forming the horseshoe structures. It is believed that the number of small ring structures observed (see Figure 4) is due to the heterogeneity in the mobility of the nuclei on the hydrophilic substrate. To further understand the influence of annealing temperature on the resulting morphology, several samples were prepared and annealed for 72 h at temperatures of 100, 110, 130, and 150 °C. The AFM images taken after annealing showed no morphology change at annealing temperatures less than 120 °C. It seems that the energy required to release Au/S6G1 from the hydrophilic substrate was not reached. Annealing at 150 °C hastened the diffusion process and resulted in deviation from the horseshoelike morphology. It can therefore be concluded that the horseshoe structure forms at a specific energy level below which diffusion is not noticeable and above which the resulting structure is somewhat deformed. This paper represents preliminary findings of the experiments conducted to date and offers some insights that may help to explain some aspects of our experimental observations. It must be emphasized that the interaction between Au/S6G1 and the substrate is not completely understood at present. However, this initial study can offer some guidance regarding the experimental conditions that are conducive for the assembly of the Au nanoparticles in specific conformations on basic surfaces. Ultimately, to investigate the potential application of this effort, important questions arise: Where are the Au nanoparticles embedded in the horseshoe-like nanostructures? Do the Au nanoparticles take up the same contour as the structures? If so, what is the extent/contribution of the Au nanoparticles in the horseshoe-like nanostructure matrix? To answer these questions, it was necessary to remove the organic molecules (dendrimers) without destroying the nanostructures. Plasma etching is one of the most important clean techniques and is extensively used to remove the organic material,38 as the process can be readily controlled and reproduced.39 Oxygen-plasma etching was used to reveal the Au nanoparticles in the horseshoe-like nanostructure. Figure 6 shows an AFM image and a cross section of the Au nanoclusters after removing the dendrimer molecules. The AFM image demonstrates that the Au nanocrystals are formed in a discontinuous horseshoe nanostructure. The cross section analysis of the AFM image confirms nanostructures with 100 nm width and 7 nm height. A comparison made between the structures before and after the etching process proves that the Au nanoparticles are dispersed (38) Liang, Z.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176-3183. (39) Hua, F.; Shi, J.; Lvov, Y.; Cui, T. Nano Lett. 2002, 2, 1219.

Horseshoe-like Au Nanostructures

Langmuir, Vol. 23, No. 14, 2007 7835

Figure 5. Schematic representation of the mechanism of the formation of the horseshoe structures during the annealing process. The affinity of the dendrimer molecules to each other and the surface energy of the Au nanoparticles endorse the structure formation.

Figure 6. AFM height images of Au nanoparticles on the Si/SiO2 substrate and the horseshoe structures after plasma etching. Inset cross section of the structure after plasma etching.

in the dendrimer matrix and support the structure formation on the solid substrate. Furthermore, the Au nanocrystals occupy more than 40% of the structure volume, and the rest is combined dendrimers. The dendrimers exist in between and roughly cover the Au nanocrystals (Figure 5).

Conclusion A simple nonlithographic preparation technique to generate a novel nanostructure via annealing of self-assembled noble nanoparticles on hydrophilic substrate surfaces is reported. The semicapped Au nanoparticles with first generation dendrimer S6G1 molecules were prepared following Brust’s methodology using S6G1 as a stabilizer. A thin film of ordered nanoparticles (Au/S6G1) was formed on a large area of the hydrophilic solid substrate (Si/SiO2). Annealing at 140 °C caused a dramatic change in the structure from nanodroplets to a horseshoe-like nanostructure. It was found that at 140 °C the diffusion process of the semicapped Au

nanoparticles was controlled via the specific interaction between the dendrimer molecules and the hydrophilic substrate. This is believed to be the main influence which controls the nucleation of the structure into a specific distribution on the surface. The resulting novel nanostructure could be a promising candidate for nanoelectronic devices, and it may lead to further nanoscale technological developments. Acknowledgment. The authors thank the European Commission for funding the SUSANA Research and Training Network (Supramolecular Self-Assembly of Interfacial Nanostructures) Project No. HPRN-CT-2002-00185. We thank David Taylor for helpful assistance. Supporting Information Available: Schematic formula of the S6G1 dendrimer. This material is available free of charge via the Internet at http://pubs.acs.org. LA700651M