2128 Chem. Commun., 2012, 48, 2128–2130 This journal is c The Royal Society of Chemistry 2012

Cite this: Chem. Commun., 2012, 48, 2128–2130

Real-time observations on crystallization of gold nanorods into spiral or

lamellar superlatticesw

Yong Xie,ab

Yongfei Jia,aYujia Liang,

bShengming Guo,

bYinglu Ji,

cXiaochun Wu,*

c

Ziyu Chen*aand Qian Liu*

b

Received 26th September 2011, Accepted 23rd December 2011

DOI: 10.1039/c2cc15989a

Real-time observations on gold nanorods evolving into spiral or

lamellar superlattices are demonstrated. 2D critical nuclei and

screw dislocations initiate the crystallization process. Kinetics of

the superlattice growth is determined to be similar to that of

classical crystal growth, where three basic modes are involved:

spiral, layer-by-layer and dendritic.

Superstructures formed by colloidal particles such as DNA,1

silica microspheres,2 noble metal nanoparticles3,4 or semi-

conductor nanocrystals5 have found many applications in

enhanced spectroscopy,3 chemical and biological sensors,6

and nano-photonic, -electronic or -plasmonic devices.7 Funda-

mental understanding of their crystallization process is

critically important for achieving rational synthesis to advance

these applications. The crystallization mechanisms of the

nanoparticles have been proposed based on either far-from-

equilibrium effect, such as fluid convection,3 or in-solution

interparticle interactions.4,8 However, dynamic processes of

the nanoparticles, especially the anisotropic nanoparticles,

evolving into the superlattices are lacking, while such processes

are the most convincing evidence for revealing the details of the

nanoparticles crystallization, and are critical to gain insight into

the formation mechanism of the superstructures. Herein, we

present direct, real-time observations of gold nanorods (GNRs)

evolving into spiral or lamellar superlattices using an optical

microscope. During the late stage of droplet drying process, we

find that the superlattices form quickly from nucleation, and

grow to the final formation inside the colloidal solution. And

the crystallization process of the GNRs resembles that of

classical crystal growth, where three basic modes are included,

i.e., spiral growth at screw dislocations, layer-by-layer growth

(LBL), and dendritic growth.9,10 The sizes of the nanorods here

are much larger than that of atoms, which allows easier

observations. And similar dynamic observations in crystal

growth are quite difficult.11 The observations presented here

may expand the crystal growth modes to the realm of nano-

particles crystallization, and to some extent, they provide

support to the dislocation-driven crystal growth mechanism.9

As expected, the drying process of a GNRs droplet induced

the formation of a ‘‘coffee-stain-like’’ pattern,12 where GNRs

crystallization superlattices were observed in the marked region

of Fig. 1a. Fig. 1b presents a typical spiral superlattice, which is

composed of thousands of close-packed, standing GNRs. The

transverse size of the spiral superlattices is about 4 mm and the

vertical size approximates to 0.2 mm (about 3 times the nanorods

height). Furthermore, the superlattice as a whole presents a cone-

shaped profile with a big width-to-thickness ratio (S1, ESIw).

Fig. 1 (a) Low-magnification SEM image of the ‘‘coffee stain’’. The

inset shows the nematic crystallization of the GNRs in the ‘‘coffee

ring’’. The spiral and lamellar superlattices are located mainly at the

dashed line marked region. (b) Full image of the spiral superlattice, the

inset shows partial amplification of the superlattice. (c) Full image of

the lamellar-1 superlattice with standing GNRs, the inset shows partial

amplification of the superlattice and corresponding FFT of the inset

image. (d) Full image of the lamellar-2 superlattice with nematic

GNRs, the inset is partial amplification of the superlattice.

aDepartment of Physics/Key Laboratory of Micro-nanoMeasurement-Manipulation and Physics (Ministry of Education),Beihang University, Beijing 100191, China.E-mail: chenzy@buaa.edu.cn

b Laboratory for Nanodevices, National Center for Nanoscience andTechnology, Beijing 100190, China. E-mail: liuq@nanoctr.cn;Fax: +86-10-6265-6765; Tel: +86-10-8254-5585

c CAS Key Laboratory of Standardization and Measurement forNanotechnology, National Center for Nanoscience and Technology,Beijing 100190, China. E-mail: wuxc@nanoctr.cnw Electronic supplementary information (ESI) available: Detailed experi-mental methods, SEM images of the spiral and laminar-2 superlattices,lateral r and L of the superlattices and SEM images of the goldnanospheres. See DOI: 10.1039/c2cc15989a

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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 2128–2130 2129

The inset of Fig. 1b further shows that a hillock with an obvious

screw dislocation locates at the top of the superlattice. In

comparison, Fig. 1c and inset show typical lamellar superlattices

(here termed lamellar-1) which are consisted of several mono-

layers, and each monolayer is composed of many standing

GNRs in a close-packed, side-by-side fashion. And the overall

superlattice presents a parallel-steps structure. Fast Fourier

transform (FFT) of Fig. 1c inset shows a hexagonal symmetry

of the superlattice. Fig. 1d shows another kind of lamellar super-

lattices (termed lamellar-2), which are made up of the GNRs in a

nematic order (Fig. 1d inset). These lamellar superlattices can

extend to tens of microns along a preferential orientation. Upon

observation from another side of the superlattices, a rectangular

shape is shown on the whole and a multilayer structure similar to

that of Fig. 1c presented (S2, ESIw).To explore the crystallization process of the superlattices,

we directly observed the nucleation and growth of the super-

lattices during the late stage of droplet drying process using a

conventional optical microscope (Leica DM2500). Evaporation

of the GNRs droplet was artificially controlled in a program-

mable temperature and humidity chamber beforehand in order

to obtain the optimal crystallization conditions (Method part,

ESIw). Fig. 2a–j show the evolution of the spiral superlattices.

Screw dislocations first emerge at the early stage of the spiral

growth as shown in Fig. 2a and f. Such dislocations play a vital

role in the formation of the spiral superlattices. According to

crystal growth theory, the axial screw dislocations generally

provide self-perpetuating steps to enable the spiral crystal

growth due to the lower energy barriers at the step edge.9–11

Likewise, the nanorods also prefer to gather at the sponta-

neously generated dislocations. And once the spiral growth

occurs, the step formed by screw dislocation will not fade away

with the growth of the superlattice, but gradually moves forward

around the axial direction driven by the screw dislocation. In

other words, the closer to the vertex of the step, the greater the

angular velocity of the growth as presented in Fig. 2b–e and g–j,

and video material-S1 (ESIw). Fig. 3a–c graphically show the

screw dislocation formed by the GNRs and the spiral growth

process. Compared with the spiral modes, Fig. 2k–o and p–t

show different crystallization processes, respectively. As shown

in Fig. 2k–o and video material-S2 (ESIw), the lamellar-1 super-

lattice originates from a 2D critical nucleus, or maybe a large

imperfect particle, then a symmetrical growth occurs around

the nucleus. At the late stage of the growth process, obvious

terrace profiles are observed as shown in Fig. 2o and schematic

Fig. 3d–f. Such a growth mode is similar to the LBL mode

in crystal growth.10,11 Fig. 2p–t and video material-S3 (ESIw)show that the lamellar-2 superlattices grow always along some

preferred orientations, such a feature is more similar to the

dendritic mode in crystal growth.10 And the formation condi-

tions of the lamellar-2 superlattices usually require higher

GNRs concentration, which is consistent with the higher super-

saturation in the dendritic crystal growth.9 In addition, the

optical observations of the superlattices often exhibit distinct

colors. Specifically, the olive color in Fig. 2a–o represents the

standing GNRs superlattices; the orange color in Fig. 2p–t

represents the nematic superlattices. The origins for the colors

may be attributed to the varying scattering and resonant

absorption of the incident light due to different GNRs align-

ments.4 Besides, the thickness of the superlattices, the depth of

the superlattices inside the solution, and even the thickness of

the droplet may cause subtle changes in the colors.

Fig. 2 (a)–(j) Growth sequence of the spiral superlattices from initial screw dislocation to final formation during the late stages of the

drying process, where Panel f displays clearly an initial screw dislocation on the GNRs superlattice. (k)–(o) Growth sequence of the

lamellar-1 superlattices from initial 2D critical nucleus to final formation. (p)–(t) Growth sequence of the lamellar-2 superlattices. Scale bars

are 2 mm.

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2130 Chem. Commun., 2012, 48, 2128–2130 This journal is c The Royal Society of Chemistry 2012

To quantify the growth behavior of the superlattices, we

study the dependence of lateral radius of curvature r (spiral

and lamellar-1) on growth time, and the dependence of length

L (lamellar-2) on growth time (Fig. 3g), considering that the

rate of lateral growth is important and usually a function of

r in crystal growth theory.10 Here, the values of r and L are

obtained directly from the experimental results as shown in

S3 (ESIw). Within the investigated time of 0–100 s, the radii of

the spiral and lamellar-1 superlattices increase linearly with

time as shown in Fig. 3g (black, red and green marks). The

rate of lateral growth of the spiral-1, spiral-2 and lamellar-1

superlattices are about 0.046, 0.029 and 0.067 mm s�1, respec-

tively. On average, the lamellar-1 superlattice has a faster

rate of lateral advance than the spirals. For the lamellar-2

superlattices, the rate of lateral advance also tends to a linear

behavior (about 0.094 mm s�1) as shown in Fig. 3g, blue

marks. Moreover, the shape of the gold nanoparticles can

also affect the formation of the superlattices. A real-time

observation and SEM imaging on GNRs with a larger aspect

ratio and gold nanospheres indicate that the superlattices can

still form for the longer GNRs (length = 62.3 � 7.7 nm),

but no crystallization can be found for the gold nanospheres

(S4, ESIw). The shape-dependent crystallization may be attrib-

uted that the nanorods have larger interactive areas than the

nanospheres, making them easier to assemble together under an

appropriate solution environment. According to the existing

explanations, high surfactant concentrations (cetyltrimethyl-

ammonium bromide, CTAB) should play an important role

in the formation of the GNRs superlattices.4 In the late stage of

the droplet evaporation, the concentration of the surfactant will

reach a high value, typically leading to two effects: (1) the

increase of free ions dissociated from the CTAB molecules will

result in a strong shielding of the surface charge of the GNRs,

thereby decreasing the electrostatic repulsion among the GNRs;

(2) the CTAB molecules can self-organize into large micelles,

leading to the remarkable depletion attraction.8 Both can promote

the formation of the GNRs superlattices. Besides, the high GNRs

concentrations can further increase the probability of formation of

the crystallization.

In summary, we have achieved direct, real-time observations

of the GNRs crystallization process. Spiral and lamellar

superlattices composed of hexagonal close-packed GNRs have

been found. The growth processes of the superlattices are

proved to be similar to that of the crystal growth, in which

atoms always gather together around a screw dislocation or a

2D critical nucleus at supersaturated atmospheres. To the best

of our knowledge, such dynamic observations of the free

GNRs evolving into the superstructures are first reported.

And the real-time and real-space observations may provide a

simple and robust way to reveal various kinetic processes of

other nanoparticles.

This work is supported by NSFC (10974037, 61006078),

National Basic Research Programs of China (2010CB934102,

2011CB932802), International S&T Cooperation Program

(2010DFA51970) and Eu-FP7 Project (No. 247644).

Notes and references

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7 F. Li, D. Josephson and A. Stein, Angew. Chem., Int. Ed., 2011,50, 360; Q. Liu, Y. Cui, D. Gardner, X. Li, S. He and I. Smalyukh,Nano Lett., 2010, 10, 1347; S. Khatua, W. Chang, P. Swanglap,J. Olson and S. Link, Nano Lett., 2011, 11, 3797.

8 K. J. M. Bishop, C. E. Wilmer, S. Soh and B. A. Grzybowski,Small, 2009, 5, 1600; D. Baranov, A. Fiore, M. van Huis,C. Giannini, A. Falqui, U. Lafont, H. Zandbergen, M. Zanella,R. Cingolani and L. Manna, Nano Lett., 2010, 10, 743.

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10 I. Markov,Crystal Growth for Beginners: Fundamentals of Nucleation,Crystal Growth, and Epitaxy, World Scientific Pub. Co. Pte. Ltd.,Singapore, 2nd edn, 1995; J. C. Brice, The Growth of Crystals fromLiquids, North-Holland Pub. Co., London, 1973.

11 T. N. Thomas, T. Land, W. Casey and J. DeYoreo, Phys. Rev.Lett., 2004, 92, 216103; T. N. Thomas, T. Land, T. Martin,W. Casey and J. DeYoreo, J. Cryst. Growth, 2004, 260, 566.

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Fig. 3 (a)–(f) Schematic diagram of the superlattice growth with

time, shows that the spiral and lamellar-1 superlattices grow around

an initial screw dislocation or an initial 2D critical nucleus, respec-

tively. (g) Dependence of superlattice radius of curvature r or length L

on growth time, indicating a linear rate of lateral growth of the

superlattices. The solid lines represent the linear fitting.

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