- Email: [email protected]
Self-assembled nanostructure of Au nanoparticles on a self-assembled monolayer
ARTICLE IN PRESS
Ultramicroscopy 105 (2005) 26–31 www.elsevier.com/locate/ultramic
Self-assembled nanostructure of Au nanoparticles on a self-assembled monolayer Satoshi Wakamatsu, Jun-ichi Nakada, Shintaro Fujii, Uichi Akiba, Masamichi Fujihira Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan Received 2 August 2004; received in revised form 18 April 2005
Abstract We investigated self-assembled nanostructure of Au nanoparticles (AuNPs) on a dithiol-inserted self-assembled monolayer (SAM) using scanning tunneling microscopy (STM). Phosphine-stabilized AuNPs were immobilized on the SAM covering an Au(1 1 1) surface by the dithiol anchoring bridges. The phosphine-stabilized AuNPs were transformed to dithiol-stabilized AuNPs by a ligand exchange reaction. The additional phosphine-stabilized AuNPs were attached to the dithiol-stabilized AuNPs on the SAM surface. The dithiol-stabilized AuNPs act as nuclei of stepwise reactions for forming self-assembled nanostructure of AuNPs. We observed a variety of nanostructures of AuNPs using STM. r 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructure; Au nanoparticle; Self-assembled monolayer; Scanning tunneling microscopy
1. Introduction Self-assembled nanostructure including metal nanoparticles and organic molecules as building blocks is one promising route towards nanoscale electronics [1]. Immobilization and connection of metal nanoparticles on surfaces are very important
Corresponding author. Tel.: +81 45 924 5784;
fax: +81 45 924 5817. E-mail address: [email protected] (M. Fujihira).
for fabricating nanoscale electronic devices. Au nanoparticles (AuNPs) have been extensively used as building blocks, because AuNPs can be connected to molecules functionalized with appropriate binding functions such as thiol, amino, and phosphine groups [1–10]. AuNPs immobilized on a self-assembled monolayer (SAM) exposing thiol groups on the surface have been studied by scanning probe microscopy in recent years because of their potential applications in molecular electronic devices [1,2]. Scanning tunneling microscopy (STM) was used to image AuNPs
0304-3991/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2005.06.014
ARTICLE IN PRESS S. Wakamatsu et al. / Ultramicroscopy 105 (2005) 26–31
immobilized on a SAM covering an Au(1 1 1) surface and to observe the Coulomb blockade effect in STM [1]. When the STM tip is positioned over such an AuNP on the dithiol SAM, a doublebarrier tunneling junction is formed by an Au nanoisland sandwiched between the STM tip and the Au substrate. The Au nanoisland is isolated from the two electrodes through vacuum and the SAM. For AuNPs with a core size smaller than 2 nm, single electron tunneling effects such as the Coulomb blockade effect were observed using STM even at room temperature. Conducting atomic force microscopy (AFM) was used to measure electronic conduction through a single dithiol molecule, one end of which was chemically bonded to an AuNP and the other end was chemically bonded to an Au(1 1 1) surface [2]. The single dithiol molecules were isolated and dispersed in an insulating SAM. An AuNP attached to a single dithiol molecule acts as a contact pad for the electrical measurement of the single molecule by conducting AFM. A single molecule is wired into the electrical circuit by contacting a conductive AFM tip to the pad AuNP. The pioneering studies described above were aimed at electrical measurements for simple nanostructure. Toward molecular devices, it is interesting to measure electronic properties of nanostructures including multiple metal nanoparticles and organic molecules. For the convenience of the measurement of the electronic properties, well-defined simple nanostructure including a few AuNPs is required. In this study, we demonstrate nanostructure composed of a few AuNPs connected by dithiol molecules on a SAM surface. We developed the method of making simple self-assembled nanostructure using immobilized AuNPs on a SAM, in which dithiol molecules were inserted in order to anchor the AuNPs via Au–S bonds to the SAM surface. 1,4-benzenedimethanethiol (BDMT) was used as both an anchoring bridge for immobilization of AuNPs to the surface and a tethering bridge for formation of self-assembled nanostructure with AuNPs. STM was used to image the self-assembled nanostructure of AuNPs on the SAM under ultra high vacuum (UHV) condition.
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2. Experimental We used phosphine-stabilized AuNPs and BDMT as building blocks for nanostructures. Hexanethiol (HT) is used as a SAM matrix on Au(1 1 1) for isolation of the BDMT molecules. BDMT molecule is expected to be an anchoring bridge between the phosphine-stabilized AuNP and the Au(1 1 1) surface. HT and BDMT were purchased from Aldrich and used without further purification. The details of the synthesis of the phosphine-stabilized AuNP (Au101(PPh3)21Cl5) were described in Ref. [11]. The metal core size of the AuNPs is estimated to be about 2 nm from STM topography and UV–visible absorption spectroscopy. The HT SAM containing isolated BDMT molecules was prepared as follows [12–15]. First, a SAM matrix of HT was prepared by immersing the Au(1 1 1) substrate, which were prepared by thermal evaporation of Au onto freshly cleaved mica, into a 1 mM ethanol solution of HT for 24 h or longer. Second, after thorough rinsing the HT SAM matrix with ethanol, it was immersed in a 1 mM toluene solution of BDMT for 1 min. BDMT molecules were inserted in the HT SAM on Au(1 1 1) to distribute thiol termini on the SAM surface. The phosphine-stabilized AuNPs were attached to the top thiol termini of BDMT by immersing the dithiol-inserted SAM in a 6 mg/l toluene solution of phosphine-stabilized AuNPs for 5 s. In this way, the AuNPs were immobilized and isolated on the SAM surface via Au–S bonds. We made simple nanostructure of the phosphine-stabilized AuNPs by self-assembly using the immobilized AuNPs as nuclei of the nanostructure. The AuNPs-modified SAM described above was immersed in a 1 mM toluene solution of BDMT for 1 min. The dispersed phosphine-stabilized AuNPs on the SAM surface were transformed to BDMT-stabilized AuNPs by a ligand exchange reaction [16,17]. After thorough rinsing the sample with toluene, it was immersed in a 6 mg/l toluene solution of phosphine-stabilized AuNPs for 5 s again. Additional phosphinestabilized AuNPs were attached to the BDMTstabilized AuNPs on the SAM surface. Namely, the additional AuNPs were connected by the
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S. Wakamatsu et al. / Ultramicroscopy 105 (2005) 26–31
BDMT molecules on the BDMT-stabilized AuNPs as tethering bridges for nanostructure formation. The connected AuNPs formed a variety of nanostructures by the self-assembling process. STM images were obtained using a JEOL JSPM4500S/TM-51021 UHV STM with a chemically etched W tip in a constant current mode at room temperature under UHV condition (o 5 108 Pa).
3. Results and discussion The BDMT molecules were inserted in defect sites of the HT SAM to distribute thiol groups on the SAM surface. At the defect sites, the BDMT molecules can approach the underlying Au atoms so that the sulfur head groups can chemisorb to the Au surface. One thiol group of BDMT was bonded chemically to the Au(1 1 1) substrate and the other was exposed on the SAM surface. Fig. 1(a) shows an STM image of the HT SAM including inserted BDMT molecules. Some bright spots corresponding to the inserted BDMT molecules appeared in the STM image. The heights of protrusions for BDMT are from 0.15 to 0.30 nm in STM topography. The phosphine-stabilized AuNPs were immobilized and isolated on the SAM surface via the Au–S bond between the Au core of the AuNP and the thiol group of BDMT at the top of the SAM. Fig. 1(b) shows an STM image of the AuNPs on the BDMT-inserted SAM surface. Nearly monodispersed spherical AuNPs were distributed on the
Fig. 1. (a) An STM image (35 nm 35 nm in size) of the HT SAM including BDMT molecules. Some bright spots, highlighted by white circles, correspond to the inserted BDMT molecules. This image was obtained with a W tip using a constant current mode (50 pA) at the sample bias voltage of +1.0 V. (b) An STM image (100 nm 100 nm in size, V s ¼ þ3 V, I t ¼ 30 pA) taken after the attachment of phosphine-stabilized AuNPs to the top thiol termini of BDMT. (c) A topographic cross section of the attached AuNPs. The topographic cross section was extracted from the white line in (b). The STM heights measured on hundreds of the phosphinestabilized AuNPs, which include the phosphine ligand shells covering the Au cores, were from 2.0 to 3.0 nm.
SAM. A topographic cross section extracted from the STM image along the white line is shown in Fig. 1(c). The line passes over two isolated AuNPs attached to the SAM surface. STM heights
ARTICLE IN PRESS S. Wakamatsu et al. / Ultramicroscopy 105 (2005) 26–31
measured on hundreds of the phosphine-stabilized AuNPs immobilized on the SAM were from 2.0 to 3.0 nm. By immersing the resulting SAM including the phosphine-stabilized AuNPs into the toluene solution of BDMT, each phosphine ligand shell covering the Au core was then replaced by a BDMT ligand shell. The BDMT-stabilized AuNPs immobilized on the SAM may act as nuclei for making further nanostructures. By immersing the SAM with the BDMT-stabilized AuNPs into the toluene solution of phosphine-stabilized AuNPs, the phosphine-stabilized AuNPs resulted in AuNPs attached to the BDMT-stabilized AuNPs nuclei on the SAM surface. This self-assembling process leads to simple self-assembled nanostructure of the AuNPs on the SAM surface. Figs. 2(a)–(f) show STM images of selfassembled two-dimensional (2D) nanostructure of molecularly bridged (i.e. tethered) AuNPs on the same sample. We observed a variety of interesting nanostructures of the AuNPs. Arrangement of AuNPs in the 2D nanostructure is clearly seen in these STM images. Schematic representations of the arrangement of the AuNPs in Figs. 2(a)–(f) are shown in Figs. 2(g)–(l), respectively. We cannot yet control the number and the arrangement of the AuNPs in the self-assembled nanostructure. In Fig. 2, two, three, four, five, and seven AuNPs are connected by the BDMT tethering bridges to an anchored AuNP nucleus. In Figs. 2(a) and (b), it is difficult to determine which AuNP was anchored to the Au(1 1 1) substrate by the BDMT molecular bridge. However, in Figs. 2(c)–(f), we can assume the anchored AuNP. In each schematic representation, only the grey circle touches all the rest surrounding AuNPs. It suggests that an AuNP corresponding to a grey circle seems to be anchored initially on the SAM surface. The anchored AuNP is stabilized by the BDMT ligand shell and the surrounding particles are stabilized by the phosphine ligand shell. In Fig. 2(d), the nanostructure looks like the four-leaf clover shape. Fig. 2(e) shows an uncompleted hexagonal structure. In Fig. 2(f), completed hexagonal structure was observed. In this way, we can easily make the various self-assembled nanostructures of the AuNPs.
29
Fig. 2. (a)–(f) STM images (25 nm 25 nm in size, V s ¼ þ3 V, I t ¼ 30 pA) of self-assembled 2D nanostructure of the tethered AuNPs. (g)–(l) Schematic representations of the arrangement of the AuNPs in the STM images of (a)–(f), respectively. In each schematic representation, a grey circle corresponds to the anchored AuNP to the Au(1 1 1) substrate by the BDMT anchoring bridge. In (a) and (b), it is difficult to determine which AuNP was anchored to the Au(1 1 1) substrate.
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Fig. 3. (a) Schematic representation of the trajectory of the STM tip. (b) A topographic cross section extracted from the white line in the STM image in Fig. 2(d). The diameter of the center AuNP is about 2 nm.
to the Au cores between the thiol groups and the phosphine groups. The BDMT-stabilized AuNPs, which have reactive surfaces to the Au cores, acted as nuclei of stepwise reactions for forming selfassembled nanostructure of AuNPs. Additional phosphine-stabilized AuNPs were tethered to the BDMT-stabilized AuNPs nuclei by immersing the SAM with the BDMT-stabilized AuNPs into a toluene solution of phosphine-stabilized AuNPs. In this way, we can make simple self-assembled nanostructure on the SAM surface. Various simple arrangements of the immobilized AuNPs were clearly observed using STM. Controlling the number and the arrangement of the AuNPs in the self-assembled nanostructure is now under investigation. The electronic properties of the simple nanostructure including a few AuNPs will be presented shortly.
Acknowledgment Here, we focus on the four-leaf clover shaped nanostructure in Fig. 2(d). In this image, center nanoparticle looks smaller than surrounding nanoparticles. However, the apparent diameters of the AuNPs in the STM image are broadened by convolution of the tip shape as illustrated schematically in Fig. 3(a). The small diameter of the center nanoparticle is the actual diameter of the AuNPs. The diameter of the AuNP is about 2 nm by measuring the cross section in Fig. 3(b).
4. Conclusions We developed easy process for making selfassembled nanostructure of AuNPs. BDMT molecules were inserted in a preformed HT SAM on Au(1 1 1) to distribute thiol termini on the SAM surface. Phosphine-stabilized AuNPs were attached to the thiol termini of the BDMT molecules on the SAM surface, because thiol groups are more reactive to Au cores than phosphine groups. The anchored phosphine-stabilized AuNPs were transformed to BDMT-stabilized AuNPs by a ligand exchange reaction. This ligand exchange reaction was caused by the difference of reactivity
This work was supported by a grant-in-aid for Creative Scientific Research on ‘‘Devices on molecular and DNA levels’’ (No. 13GS0017) from the Japan Society for the Promotion of Science.
References [1] M. Dorogi, J. Gomez, R. Osifchin, R.P. Andres, R. Reifenberger, Phys. Rev. B 52 (1995) 9071. [2] X.D. Cui, A. Primak, X. Zarate, J. Tomfohr, O.F. Sankey, A.L. Moore, T.A. Moore, D. Gust, G. Harris, S.M. Lindsay, Science 294 (2001) 571. [3] S. Chen, Adv. Mater. 12 (2000) 186. [4] F.L. Leibowitz, W. Zheng, M.M. Maye, C.-J. Zhong, Anal. Chem. 71 (1999) 5076. [5] E.W.L. Chan, L. Yu, Langmuir 18 (2002) 311. [6] F.P. Zamborini, J.F. Hicks, R.W. Murray, J. Am. Chem. Soc. 122 (2000) 4514. [7] M. Giersig, P. Mulvaney, J. Phys. Chem. 97 (1993) 6334. [8] M. Brust, D. Bethell, D.J. Schiffrin, C.J. Kiely, Adv. Mater. 7 (1995) 795. [9] A. Doron, E. Katz, I. Willner, Langmuir 11 (1995) 1313. [10] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607. [11] W.W. Weare, S.M. Reed, M.G. Warner, J.E. Hutchison, J. Am. Chem. Soc. 122 (2000) 12890.
ARTICLE IN PRESS S. Wakamatsu et al. / Ultramicroscopy 105 (2005) 26–31 [12] L.A. Bumm, J.J. Arnold, M.T. Cygan, T.D. Dunbar, T.P. Burgin, L. Jones II, D.L. Allara, J.M. Tour, P.S. Weiss, Science 271 (1996) 1705. [13] S. Wakamatsu, U. Akiba, M. Fujihira, Colloids Surf. A 198–200 (2002) 785. [14] S. Wakamatsu, S. Fujii, U. Akiba, M. Fujihira, Nanotechnology 14 (2003) 258.
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[15] S. Wakamatsu, S. Fujii, U. Akiba, M. Fujihira, Nanotechnology 15 (2004) S137. [16] L.O. Brown, J.E. Hutchison, J. Am. Soc. 119 (1997) 12384. [17] M.G. Warner, S.M. Reed, J.E. Hutchison, Chem. Mater. 12 (2000) 3316.
Ultramicroscopy 105 (2005) 26–31 www.elsevier.com/locate/ultramic
Self-assembled nanostructure of Au nanoparticles on a self-assembled monolayer Satoshi Wakamatsu, Jun-ichi Nakada, Shintaro Fujii, Uichi Akiba, Masamichi Fujihira Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan Received 2 August 2004; received in revised form 18 April 2005
Abstract We investigated self-assembled nanostructure of Au nanoparticles (AuNPs) on a dithiol-inserted self-assembled monolayer (SAM) using scanning tunneling microscopy (STM). Phosphine-stabilized AuNPs were immobilized on the SAM covering an Au(1 1 1) surface by the dithiol anchoring bridges. The phosphine-stabilized AuNPs were transformed to dithiol-stabilized AuNPs by a ligand exchange reaction. The additional phosphine-stabilized AuNPs were attached to the dithiol-stabilized AuNPs on the SAM surface. The dithiol-stabilized AuNPs act as nuclei of stepwise reactions for forming self-assembled nanostructure of AuNPs. We observed a variety of nanostructures of AuNPs using STM. r 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructure; Au nanoparticle; Self-assembled monolayer; Scanning tunneling microscopy
1. Introduction Self-assembled nanostructure including metal nanoparticles and organic molecules as building blocks is one promising route towards nanoscale electronics [1]. Immobilization and connection of metal nanoparticles on surfaces are very important
Corresponding author. Tel.: +81 45 924 5784;
fax: +81 45 924 5817. E-mail address: [email protected] (M. Fujihira).
for fabricating nanoscale electronic devices. Au nanoparticles (AuNPs) have been extensively used as building blocks, because AuNPs can be connected to molecules functionalized with appropriate binding functions such as thiol, amino, and phosphine groups [1–10]. AuNPs immobilized on a self-assembled monolayer (SAM) exposing thiol groups on the surface have been studied by scanning probe microscopy in recent years because of their potential applications in molecular electronic devices [1,2]. Scanning tunneling microscopy (STM) was used to image AuNPs
0304-3991/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2005.06.014
ARTICLE IN PRESS S. Wakamatsu et al. / Ultramicroscopy 105 (2005) 26–31
immobilized on a SAM covering an Au(1 1 1) surface and to observe the Coulomb blockade effect in STM [1]. When the STM tip is positioned over such an AuNP on the dithiol SAM, a doublebarrier tunneling junction is formed by an Au nanoisland sandwiched between the STM tip and the Au substrate. The Au nanoisland is isolated from the two electrodes through vacuum and the SAM. For AuNPs with a core size smaller than 2 nm, single electron tunneling effects such as the Coulomb blockade effect were observed using STM even at room temperature. Conducting atomic force microscopy (AFM) was used to measure electronic conduction through a single dithiol molecule, one end of which was chemically bonded to an AuNP and the other end was chemically bonded to an Au(1 1 1) surface [2]. The single dithiol molecules were isolated and dispersed in an insulating SAM. An AuNP attached to a single dithiol molecule acts as a contact pad for the electrical measurement of the single molecule by conducting AFM. A single molecule is wired into the electrical circuit by contacting a conductive AFM tip to the pad AuNP. The pioneering studies described above were aimed at electrical measurements for simple nanostructure. Toward molecular devices, it is interesting to measure electronic properties of nanostructures including multiple metal nanoparticles and organic molecules. For the convenience of the measurement of the electronic properties, well-defined simple nanostructure including a few AuNPs is required. In this study, we demonstrate nanostructure composed of a few AuNPs connected by dithiol molecules on a SAM surface. We developed the method of making simple self-assembled nanostructure using immobilized AuNPs on a SAM, in which dithiol molecules were inserted in order to anchor the AuNPs via Au–S bonds to the SAM surface. 1,4-benzenedimethanethiol (BDMT) was used as both an anchoring bridge for immobilization of AuNPs to the surface and a tethering bridge for formation of self-assembled nanostructure with AuNPs. STM was used to image the self-assembled nanostructure of AuNPs on the SAM under ultra high vacuum (UHV) condition.
27
2. Experimental We used phosphine-stabilized AuNPs and BDMT as building blocks for nanostructures. Hexanethiol (HT) is used as a SAM matrix on Au(1 1 1) for isolation of the BDMT molecules. BDMT molecule is expected to be an anchoring bridge between the phosphine-stabilized AuNP and the Au(1 1 1) surface. HT and BDMT were purchased from Aldrich and used without further purification. The details of the synthesis of the phosphine-stabilized AuNP (Au101(PPh3)21Cl5) were described in Ref. [11]. The metal core size of the AuNPs is estimated to be about 2 nm from STM topography and UV–visible absorption spectroscopy. The HT SAM containing isolated BDMT molecules was prepared as follows [12–15]. First, a SAM matrix of HT was prepared by immersing the Au(1 1 1) substrate, which were prepared by thermal evaporation of Au onto freshly cleaved mica, into a 1 mM ethanol solution of HT for 24 h or longer. Second, after thorough rinsing the HT SAM matrix with ethanol, it was immersed in a 1 mM toluene solution of BDMT for 1 min. BDMT molecules were inserted in the HT SAM on Au(1 1 1) to distribute thiol termini on the SAM surface. The phosphine-stabilized AuNPs were attached to the top thiol termini of BDMT by immersing the dithiol-inserted SAM in a 6 mg/l toluene solution of phosphine-stabilized AuNPs for 5 s. In this way, the AuNPs were immobilized and isolated on the SAM surface via Au–S bonds. We made simple nanostructure of the phosphine-stabilized AuNPs by self-assembly using the immobilized AuNPs as nuclei of the nanostructure. The AuNPs-modified SAM described above was immersed in a 1 mM toluene solution of BDMT for 1 min. The dispersed phosphine-stabilized AuNPs on the SAM surface were transformed to BDMT-stabilized AuNPs by a ligand exchange reaction [16,17]. After thorough rinsing the sample with toluene, it was immersed in a 6 mg/l toluene solution of phosphine-stabilized AuNPs for 5 s again. Additional phosphinestabilized AuNPs were attached to the BDMTstabilized AuNPs on the SAM surface. Namely, the additional AuNPs were connected by the
ARTICLE IN PRESS 28
S. Wakamatsu et al. / Ultramicroscopy 105 (2005) 26–31
BDMT molecules on the BDMT-stabilized AuNPs as tethering bridges for nanostructure formation. The connected AuNPs formed a variety of nanostructures by the self-assembling process. STM images were obtained using a JEOL JSPM4500S/TM-51021 UHV STM with a chemically etched W tip in a constant current mode at room temperature under UHV condition (o 5 108 Pa).
3. Results and discussion The BDMT molecules were inserted in defect sites of the HT SAM to distribute thiol groups on the SAM surface. At the defect sites, the BDMT molecules can approach the underlying Au atoms so that the sulfur head groups can chemisorb to the Au surface. One thiol group of BDMT was bonded chemically to the Au(1 1 1) substrate and the other was exposed on the SAM surface. Fig. 1(a) shows an STM image of the HT SAM including inserted BDMT molecules. Some bright spots corresponding to the inserted BDMT molecules appeared in the STM image. The heights of protrusions for BDMT are from 0.15 to 0.30 nm in STM topography. The phosphine-stabilized AuNPs were immobilized and isolated on the SAM surface via the Au–S bond between the Au core of the AuNP and the thiol group of BDMT at the top of the SAM. Fig. 1(b) shows an STM image of the AuNPs on the BDMT-inserted SAM surface. Nearly monodispersed spherical AuNPs were distributed on the
Fig. 1. (a) An STM image (35 nm 35 nm in size) of the HT SAM including BDMT molecules. Some bright spots, highlighted by white circles, correspond to the inserted BDMT molecules. This image was obtained with a W tip using a constant current mode (50 pA) at the sample bias voltage of +1.0 V. (b) An STM image (100 nm 100 nm in size, V s ¼ þ3 V, I t ¼ 30 pA) taken after the attachment of phosphine-stabilized AuNPs to the top thiol termini of BDMT. (c) A topographic cross section of the attached AuNPs. The topographic cross section was extracted from the white line in (b). The STM heights measured on hundreds of the phosphinestabilized AuNPs, which include the phosphine ligand shells covering the Au cores, were from 2.0 to 3.0 nm.
SAM. A topographic cross section extracted from the STM image along the white line is shown in Fig. 1(c). The line passes over two isolated AuNPs attached to the SAM surface. STM heights
ARTICLE IN PRESS S. Wakamatsu et al. / Ultramicroscopy 105 (2005) 26–31
measured on hundreds of the phosphine-stabilized AuNPs immobilized on the SAM were from 2.0 to 3.0 nm. By immersing the resulting SAM including the phosphine-stabilized AuNPs into the toluene solution of BDMT, each phosphine ligand shell covering the Au core was then replaced by a BDMT ligand shell. The BDMT-stabilized AuNPs immobilized on the SAM may act as nuclei for making further nanostructures. By immersing the SAM with the BDMT-stabilized AuNPs into the toluene solution of phosphine-stabilized AuNPs, the phosphine-stabilized AuNPs resulted in AuNPs attached to the BDMT-stabilized AuNPs nuclei on the SAM surface. This self-assembling process leads to simple self-assembled nanostructure of the AuNPs on the SAM surface. Figs. 2(a)–(f) show STM images of selfassembled two-dimensional (2D) nanostructure of molecularly bridged (i.e. tethered) AuNPs on the same sample. We observed a variety of interesting nanostructures of the AuNPs. Arrangement of AuNPs in the 2D nanostructure is clearly seen in these STM images. Schematic representations of the arrangement of the AuNPs in Figs. 2(a)–(f) are shown in Figs. 2(g)–(l), respectively. We cannot yet control the number and the arrangement of the AuNPs in the self-assembled nanostructure. In Fig. 2, two, three, four, five, and seven AuNPs are connected by the BDMT tethering bridges to an anchored AuNP nucleus. In Figs. 2(a) and (b), it is difficult to determine which AuNP was anchored to the Au(1 1 1) substrate by the BDMT molecular bridge. However, in Figs. 2(c)–(f), we can assume the anchored AuNP. In each schematic representation, only the grey circle touches all the rest surrounding AuNPs. It suggests that an AuNP corresponding to a grey circle seems to be anchored initially on the SAM surface. The anchored AuNP is stabilized by the BDMT ligand shell and the surrounding particles are stabilized by the phosphine ligand shell. In Fig. 2(d), the nanostructure looks like the four-leaf clover shape. Fig. 2(e) shows an uncompleted hexagonal structure. In Fig. 2(f), completed hexagonal structure was observed. In this way, we can easily make the various self-assembled nanostructures of the AuNPs.
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Fig. 2. (a)–(f) STM images (25 nm 25 nm in size, V s ¼ þ3 V, I t ¼ 30 pA) of self-assembled 2D nanostructure of the tethered AuNPs. (g)–(l) Schematic representations of the arrangement of the AuNPs in the STM images of (a)–(f), respectively. In each schematic representation, a grey circle corresponds to the anchored AuNP to the Au(1 1 1) substrate by the BDMT anchoring bridge. In (a) and (b), it is difficult to determine which AuNP was anchored to the Au(1 1 1) substrate.
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Fig. 3. (a) Schematic representation of the trajectory of the STM tip. (b) A topographic cross section extracted from the white line in the STM image in Fig. 2(d). The diameter of the center AuNP is about 2 nm.
to the Au cores between the thiol groups and the phosphine groups. The BDMT-stabilized AuNPs, which have reactive surfaces to the Au cores, acted as nuclei of stepwise reactions for forming selfassembled nanostructure of AuNPs. Additional phosphine-stabilized AuNPs were tethered to the BDMT-stabilized AuNPs nuclei by immersing the SAM with the BDMT-stabilized AuNPs into a toluene solution of phosphine-stabilized AuNPs. In this way, we can make simple self-assembled nanostructure on the SAM surface. Various simple arrangements of the immobilized AuNPs were clearly observed using STM. Controlling the number and the arrangement of the AuNPs in the self-assembled nanostructure is now under investigation. The electronic properties of the simple nanostructure including a few AuNPs will be presented shortly.
Acknowledgment Here, we focus on the four-leaf clover shaped nanostructure in Fig. 2(d). In this image, center nanoparticle looks smaller than surrounding nanoparticles. However, the apparent diameters of the AuNPs in the STM image are broadened by convolution of the tip shape as illustrated schematically in Fig. 3(a). The small diameter of the center nanoparticle is the actual diameter of the AuNPs. The diameter of the AuNP is about 2 nm by measuring the cross section in Fig. 3(b).
4. Conclusions We developed easy process for making selfassembled nanostructure of AuNPs. BDMT molecules were inserted in a preformed HT SAM on Au(1 1 1) to distribute thiol termini on the SAM surface. Phosphine-stabilized AuNPs were attached to the thiol termini of the BDMT molecules on the SAM surface, because thiol groups are more reactive to Au cores than phosphine groups. The anchored phosphine-stabilized AuNPs were transformed to BDMT-stabilized AuNPs by a ligand exchange reaction. This ligand exchange reaction was caused by the difference of reactivity
This work was supported by a grant-in-aid for Creative Scientific Research on ‘‘Devices on molecular and DNA levels’’ (No. 13GS0017) from the Japan Society for the Promotion of Science.
References [1] M. Dorogi, J. Gomez, R. Osifchin, R.P. Andres, R. Reifenberger, Phys. Rev. B 52 (1995) 9071. [2] X.D. Cui, A. Primak, X. Zarate, J. Tomfohr, O.F. Sankey, A.L. Moore, T.A. Moore, D. Gust, G. Harris, S.M. Lindsay, Science 294 (2001) 571. [3] S. Chen, Adv. Mater. 12 (2000) 186. [4] F.L. Leibowitz, W. Zheng, M.M. Maye, C.-J. Zhong, Anal. Chem. 71 (1999) 5076. [5] E.W.L. Chan, L. Yu, Langmuir 18 (2002) 311. [6] F.P. Zamborini, J.F. Hicks, R.W. Murray, J. Am. Chem. Soc. 122 (2000) 4514. [7] M. Giersig, P. Mulvaney, J. Phys. Chem. 97 (1993) 6334. [8] M. Brust, D. Bethell, D.J. Schiffrin, C.J. Kiely, Adv. Mater. 7 (1995) 795. [9] A. Doron, E. Katz, I. Willner, Langmuir 11 (1995) 1313. [10] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607. [11] W.W. Weare, S.M. Reed, M.G. Warner, J.E. Hutchison, J. Am. Chem. Soc. 122 (2000) 12890.
ARTICLE IN PRESS S. Wakamatsu et al. / Ultramicroscopy 105 (2005) 26–31 [12] L.A. Bumm, J.J. Arnold, M.T. Cygan, T.D. Dunbar, T.P. Burgin, L. Jones II, D.L. Allara, J.M. Tour, P.S. Weiss, Science 271 (1996) 1705. [13] S. Wakamatsu, U. Akiba, M. Fujihira, Colloids Surf. A 198–200 (2002) 785. [14] S. Wakamatsu, S. Fujii, U. Akiba, M. Fujihira, Nanotechnology 14 (2003) 258.
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