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Chemical Communications

www.rsc.org/chemcomm Volume 46 | Number 34 | 14 September 2010 | Pages 6189–6392

COMMUNICATIONSilvija Gradečak et al.Synthesis and thermal responsiveness of self-assembled gold nanoclusters

FEATURE ARTICLEDario Braga et al.The growing world of crystal forms

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6246 Chem. Commun., 2010, 46, 6246–6248 This journal is c The Royal Society of Chemistry 2010

Synthesis and thermal responsiveness of self-assembled gold

nanoclustersw

Shenqiang Ren, Sung-Keun Lim and Silvija Gradecak*

Received 10th June 2010, Accepted 14th July 2010

DOI: 10.1039/c0cc01829e

A simple and versatile approach was developed to generate

hierarchical assemblies of ultra-small gold nanocluster thin films

using the combination of galvanic reaction and a block copolymer

coordinated with gold complex. Variation of the temperature

allows effective control over the optical response of these stimuli-

responsive organic-nanocluster hybrid structures.

Noble metal-atomic clusters are composed of a small number

of atoms, with diameters typically ranging from sub-nanometre

to approximately 2 nm.1 They have molecule-like characteristics

and exhibit unique and size-dependent optical, electronic, and

magnetic properties.2,3 For example, due to their enhanced

luminescence over a broad range of absorption wavelengths,4

metal nanoclusters are ideal candidates for light-harvesting

applications.5,6 Controlled synthesis and assembly of the noble

metal nanocluster and their manipulation via external stimuli

would enable design of functional devices based on these

unique properties. Several methods of the metal cluster

synthesis have been developed so far;2,7,8 however, the design

of the functional and responsive ultra-small metal nanoclusters

has been impeded by the lack of simultaneous control over

their synthesis and spatial organization in a film-on-substrate

geometry, as well as by the challenges in incorporating external

stimuli to generate stable responsive nanocluster composites.

In this communication, we describe the templated synthesis

of gold (Au) nanoclusters embedded in copolymer matrix

using the combination of galvanic reaction and self-assembled

block copolymer (BCP). Strong selectivity and homogeneous

spatial distribution of as-synthesized Au nanoclusters in one

block was achieved by selective gold precursor loading, while

BCPs enabled thermally-responsive and fully reversible optical

behaviour of the resulting Au nanocluster composites. In

contrast to previous approaches of using BCP micelles

coordinated with a metal complex9 or using galvanic displace-

ment combined with other assembly techniques,10 both of

which result in synthesis of noble metal nanoparticles, our

method simultaneously combines these two syntheses. In this

way, we control the Au reduction rate and thus synthesize self-

assembled and size-selective nanoclusters.

BCPs are versatile platforms to achieve ordered nanoscale

hybrid materials because of their tendency to self-assemble in

periodic structures of tens of nanometres.11�13 To direct

position- and size-controlled synthesis of Au nanoclusters, in

this work we used polystyrene-b-poly(4-vinylpyridine)

(PS-b-P4VP) BCP with the molecular weights of PS and

P4VP being 19 kg mol�1 and 5.2 kg mol�1, respectively. Au

was loaded selectively into the P4VP domains by dissolving

PS-b-P4VP with HAuCl4 at a ratio of Au/pyridyl of 0.3

(PS-b-P[4VP(HAuCl4)0.3]) in 0.5% (w/w) toluene/tetrahydrofuran

(THF) solution. During this process, anionic complexes AuCl4�

bind to a protonated pyridine moiety due to electrostatic

interactions. A monolayer of the mixture was then spin-coated

on GaAs(111) substrate (Fig. 1a). Atomic force microscopy

(AFM) images of the spun-cast film revealed that no assembly

occurred in the pristine state (Fig. 1b). After THF solvent

annealing (Fig. 1c), the quasi-hexagonal assembly was

observed in the resulting thin film sample, as confirmed by

AFM imaging and the corresponding fast Fourier transform

(Fig. 1d). The sample was then immersed in 1% hydrofluoric

acid (HF) solution for 30 seconds. The HF is selectively

delivered to the semiconductor surface via the hexagonally

ordered spherical P4VP/HAuCl4 cores and the consequent

chemistry of the galvanic displacement is based upon a simple

electrochemical reaction in which sufficiently oxidizing Au(III)

ions (in the form of AuCl4�) are reduced by GaAs substrate.9

The resulting Au nanoclusters connected by P4VP chain

possess the same quasi-hexagonal packing as that of the parent

BCP (Fig. 1e and f). AFM revealed that the surface of Au

loaded films is comprised of quasi-hexagonal arrays of small

localized depressions in the individual P4VP cores (Fig. 1f).

The depressions are likely formed during the volume shrinkage

that occurs from the original HAuCl4 complex to Au metal

nanoclusters reduced by galvanic displacement at the

P4VP microdomain.

Fig. 1 Schematic illustration (upper row) and corresponding AFM

height images (lower row) of Au nanocluster composite synthesis using

BCP and galvanic reaction. Scale bar is 50 nm, identical for all AFM

images. (a and b) PS-b-P4VP doped with Au complex spun cast onto

GaAs(111). (c and d) Self-assembled thin film after solvent annealing. Inset

in (d) is the fast Fourier transform of the corresponding AFM image.

(e and f) The same sample after HF treatment. Au nanoclusters form in

P4VP domains embedded in PS matrix due to the galvanic displacement.

Massachusetts Institute of Technology, Department of MaterialsScience and Engineering, 77 Massachusetts Avenue, Cambridge,MA 02139, USA. E-mail: gradecak@mit.eduw Electronic supplementary information (ESI) available: Fullexperimental details, scanning transmission electron microscopy, highresolution TEM and its corresponding FFT, chemical analysis andUV-Vis absorbance spectrum. See DOI: 10.1039/c0cc01829e

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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This journal is c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 6246–6248 6247

Plan-view transmission electron microscopy (TEM) of the

self-assembled PS-b-P4VP(HAuCl4)0.3 nanocomposite after

galvanic reaction (Fig. 2a) shows that these films are

comprised of a quasi-hexagonal arrays of circular P4VP

domains with 15 nm diameters and 25 nm spacing, which

correlates well with the AFM observations. Each domain is

uniformly filled with Au nanoclusters (ESIw, Fig. S1 and S2),

and the size of each nanocluster is significantly smaller

(B1.0 � 0.2 nm) than the corresponding P4VP domain size.

The crystalline nature and the overall diameter of as-prepared

Au nanoclusters were confirmed using high-resolution TEM

(Fig. 2a, inset). Notably, the sizes of the Au nanoclusters can

be modulated through variations in the deposition conditions,

such as immersion time and Au complex concentration.

The size distributions of Au nanoclusters increase with

P4VP(HAuCl4) loading concentration. From TEM images,

the average nanocluster sizes were measured to be 1.0 � 0.2,

1.3 � 0.2, 1.7 � 0.2 and 2.2 � 0.3 nm for loading concentrations

of 0.3, 0.5, 0.8, and 1, respectively (Fig. 2). According to the

‘‘magic numbers’’ for Au nanoclusters,14 the sizes of the

resulting sub-2-nm clusters closely match the sizes of Au

nanoclusters corresponding toB38, 75 and 146 Au atoms.15,16

In our case, the observed size distribution within a sample can

be due to the nature of our synthesis or caused by intrinsic

issues of TEM imaging. Electron beam irradiation can obfuscate

the boundary between cluster and polymer matrix during

TEM imaging and we estimate uncertainty in the size

measurement to be one or two lattice fringes, typically around

0.2 nm. In addition to the nanocluster size control, the

Au complex concentrations have a critical effect on their

spatial distribution in response to observed BCP morphological

changes (Fig. 2a–d). As HAuCl4 loading in the P4VP domain

is increased, the transition from hexagonal (Fig. 2a and b),

gyroid (Fig. 2c) to lamellar morphology (Fig. 2d) is observed

due to the modification of the copolymer block volume ratio.17

Since the Au nanoclusters are not covalently linked to the

P4VP block, bonding between the Au nanoclusters and the

pyridine can be either strengthened or weakened by external

stimuli to adjust the arrangement and architecture of the

polymer chain, and in turn transform the spatial distribution

and ordering of the embedded Au nanoclusters. This brings

about unique opportunities to create responsive materials

where external stimuli can be used to alter the spatial distribution

of Au nanoclusters and the macroscopic properties of the

nanocluster assemblies. We used thermally-responsive properties

of PS-b-P4VP BCP to change the dimensionality and overall

arrangements of the Au nanocluster assemblies. Fig. 3a shows

in situ thermally responsive UV-Vis spectra of the 1.3 nm Au

nanocluster composite. Distinct onsets of absorption bands at

about 480 nm, 565 nm, 640 nm, and 700 nm were observed at

room temperature (Fig. 3a, inset) and these closely match the

characteristic optical absorption features of the classical Au

nanocluster core with 75 atoms (B1.3 nm).18 They may be

interpreted as arising from intraband (between levels in quantized

sp band) or interband (d band to sp band) transitions or a

mixture of the two.19,20 An additional absorption band at

520 nm was also observed, but further studies are needed to

clarify its origin. An interesting feature was observed when the

sample was heated from 298 to 473 K: the 640 nm absorption

band blue shifted by 13 nm, and the process was reversible

upon cooling back to 298 K (Fig. 3b). When the nanocomposite

is heated above the glass transition temperature of each block,

it firstly causes extension of the PS coil-block because of its

lower glass transition temperature (373 K) than that of P4VP

(476 K). Further increase of the temperature reduces the

stiffness of the P4VP(Au nanoclusters) comb-block and leads

to a structural rearrangement of Au nanoclusters and their

relative distance. Therefore, the observed thermally responsive

absorption is likely caused by the interaction between pyridine

and Au nanoclusters and/or the inter-nanocluster distance

evolution during the heating process.21 In the cooling cycle,

a reverse process is observed. Thus, using temperature as a

switch it is possible to direct Au nanocluster distribution.

This process occurs within minutes and is fully reversible.

The optical absorption spectra of clusters synthesized using

different loading concentrations demonstrate the corresponding

nanocluster size-dependent behaviour. Ultimately, a surface

Fig. 2 Bright-field TEM images of Au nanocluster composites and

the corresponding Au nanocluster size distributions as a function

of the xHAuCl4: P4VP concentration. (a) x = 0.3; (b) x = 0.5;

(c) x = 0.8; (d) x = 1. Insets show high resolution TEM images of

representative Au nanoclusters observed within each P4VP domain.

Same scale-bars for all bright-field (50 nm) and high-resolution TEM

images (1 nm). (e) Au nanocluster size distributions correspond to

images (a)–(d).

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6248 Chem. Commun., 2010, 46, 6246–6248 This journal is c The Royal Society of Chemistry 2010

plasmonic band emerges once the Au core diameter exceeds

2 nm (ESIw, Fig. S4).Our synthesis approach is versatile and allows the hybrid

system to respond to external stimuli, temperature in this

study, providing a simple way of controlling the local

environment of the metal nanoclusters, the packing density

of the nanoclusters and the intercluster separation distance

and ordering. It is applicable to a wide range of noble metal

nanoclusters and substrates, opening new routes for

responsive device fabrication. In addition, the Au nanocluster

arrays maintain their hexagonal assemblies on GaAs even after

the removal of polymer with oxygen plasma (ESIw, Fig. S5),and therefore can be used for applications that require particle

ordering without polymer matrix (for example, particle-

mediated nanowire growth).9

In conclusion, we have developed a simple yet robust

approach to generate thermally responsive self-assembled Au

nanocluster composite. The gold complex was in situ

selectively reduced to Au nanoclusters in the P4VP domain

and temperature-dependent absorption properties of the

nanocomposite were demonstrated. The versatility of this

approach lies in the metal nanocluster ligation with BCP that

allows the system to respond to external thermal stimuli, thus

enabling the non-disruptive approach to generate functional

devices.

This work was supported by Eni S.p.A. under the Eni-MIT

Alliance Solar Frontiers Program. The authors also acknowledge

access to Shared Experimental Facilities provided by MIT

Center for Materials Science Engineering supported in part by

MRSEC Program of National Science Foundation under

award number DMR-0213282.

Notes and references

1 S. Fedrigo, W. Harbich and J. Buttet, Phys. Rev. B, 1993, 47,10706.

2 J. P. Wilcoxon and B. L. Abrams, Chem. Soc. Rev., 2006, 35,1162.

3 T. Y. Olson and J. Z. Zhang, J. Mater. Sci. Technol. (Shenyang,People’s Repub. China), 2008, 24, 433.

4 J. Zheng, P. R. Nicovich and R. M. Dickson, Annu. Rev. Phys.Chem., 2007, 58, 409.

5 J. D. Aiken and R. G. Finke, J. Mol. Catal. A: Chem., 1999, 145, 1.6 O. M. Bakr, V. Amendola, C. M. Aikens, W. Wenseleers, R. Li,L. Dal Negro, G. C. Schatz and F. Stellacci, Angew. Chem., Int.Ed., 2009, 48, 5921.

7 R. C. Jin, Nanoscale, 2010, 2, 343.8 L. Shang and S. J. Dong, Chem. Commun., 2008, 1088.9 M. Aizawa and J. M. Buriak, Chem. Mater., 2007, 19, 5090.10 C. Tseng, M. Tambe, S. Lim, M. Smith and S. Gradecak,

Nanotechnology, 2010, 21, 165605.11 W. A. Lopes and H. M. Jaeger, Nature, 2001, 414, 735.12 S. Q. Ren, R. M. Briber andM.Wuttig, Appl. Phys. Lett., 2009, 94,

113507.13 B. J. Kim, J. J. Chiu, G. R. Yi, D. J. Pine and E. J. Kramer,

Adv. Mater., 2005, 17, 2618.14 T. P. Martin, Phys. Rep., 1996, 273, 199.15 T. G. Schaaff, M. N. Shafigullin, J. T. Khoury, I. Vezmar,

R. L. Whetten, W. G. Cullen, P. N. First, C. GutierrezWing,J. Ascensio and M. J. JoseYacaman, J. Phys. Chem. B, 1997, 101,7885.

16 H. Tsunoyama and T. Tsukuda, J. Am. Chem. Soc., 2009, 131,18216.

17 Z. H. Li, H. Sai, S. C. Warren, M. Kamperman, H. Arora,S. M. Gruner and U. Wiesner, Chem. Mater., 2009, 21, 5578.

18 E. Gutierrez, R. D. Powell, F. R. Furuya, J. F. Hainfeld,T. G. Schaaff, M. N. Shafigullin, P. W. Stephens andR. L. Whetten, Eur. Phys. J. D, 1999, 9, 647.

19 M. Zhu, C. M. Aikens, F. J. Hollander, G. C. Schatz and R. Jin,J. Am. Chem. Soc., 2008, 130, 5883.

20 M. M. Alvarez, J. T. Khoury, T. G. Schaaff, M. N. Shafigullin,I. Vezmar and R. L. Whetten, J. Phys. Chem. B, 1997, 101, 3706.

21 Y. Negishi, K. Nobusada and T. Tsukuda, J. Am. Chem. Soc.,2005, 127, 5261.

Fig. 3 The absorbance and differential absorbance (inset) of Au

nanocluster composite (0.5HAuCl4: P4VP) as a function of temperature.

(a) The nanocomposite was heated from 298 K to 473 K and cooled

down to 298 K at a heating/cooling rate of 5 K min�1. (b) The Au

nanocluster composite absorption shift during heating and cooling

recycle. The schematic figure shows the change in the dimensionality

and overall arrangement of Au nanocluster composite.

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