Enhancement of anisotropic conductivity, elastic and dielectric
constants in a liquid crystal-gold nanorod system
S. Sridevi1, S. Krishna Prasad1, Geetha G. Nair1, Virginia D’Britto2 and B.L.V. Prasad2
1
Centre for Liquid Crystal Research, Jalahalli, Bangalore 560013, India
2
National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
Supplementary Information
1. Preparation of the AuNR particles
The Au nanorods (Au NRs) were prepared following the seed mediated procedure reported
by Murphy and co-workers by optimizing the conditions suitably [1]. In the first stage of seed
preparation, 5 mL of 0.1 M CTAB (Cetyltrimethylammonium bromide) solution was added
to 5 mL of 5x10-4 M AuCl4 solution and stirred for 5 min. To this orange solution, 0.6 mL of
0.1 M aqueous NaBH4 was added and vigorously stirred for 10 min. during which NaBH4
reduces the Au3+ ions to give a light brown color solution containing gold nanoparticle seeds
capped with CTAB. In the next stage a growth solution was prepared where 5 mL of 0.1 M
CTAB solution was added to a mixture of 0.1 M HAuCl4 solution (50 L) and 5 mL of
deionised water (Milli Q). This mixture was stirred for 5 min., resulting in an orange solution.
To this solution 70 L of 0.1 M ascorbic acid solution was added and the stirring continued
until the orange solution became colourless. In the final stage of preparation, 0.1 mL of 1x103
M AgNO3 solution was added to the growth solution, and subsequently 25 L of the above
mentioned seed solution was added under stirring conditions. The stirring was continued until
the colorless solution starts attaining bluish violet color. This suggests the formation of some
anisotropic particles or aggregated spherical particles. However, we could conclude that the
particles formed are mainly rod like, using TEM imaging (Figure SI-1) as well as by
measuring the optical absorption spectrum (top inset of Figure SI-1). The latter displays a
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double peak profile, which can be attributed to the transverse and longitudinal surface
plasmon resonance expected for the anisotropic shaped rod-like particles. There is a small
distribution in the nanorod size, with an average width of 14 nm and about two-thirds of the
particles having an aspect ratio in the region 3-4, as seen from the histogram given below
(Figure SI-2). The good definition of the particles is seen from the high resolution image of
absorbance (a.u)
single particle (bottom inset of Figure SI-1).
400
600
800
1000
1200
wavelength (nm)
50 nm
50 nm
5 nm
Figure SI-1: Transmission electron micrograph of the prepared gold nanoparticles, showing
that a majority of them have a rod-like shape. Inset (top) depicts the optical absorption
spectrum having a double peak profile consistent with the expected transverse and
longitudinal surface plasmon resonance. Inset (bottom) shows that the prepared particles
have a good definition.
Optical spectra were recorded with a UV-Visible-NIR Spectrophotometer (JascoV570) with a resolution of 2 nm. The nanorod dispersions were taken in a quartz cuvette and
the spectra were recorded. Transmission Electron Microscopy analysis was performed on a
FEI model Technai G2 microscope operated with an accelerated voltage of 300kV. TEM
grids were prepared by placing a drop of sample on carbon coated copper grids.
2
40
35
% particles
30
25
20
15
10
5
0
0
1
2
3
4
5
6
7
8
9
10
Aspect ratio
Figure SI-2
2. Preparation of the LC-AuNR composites
To prevent the aggregation of the AuNR particles they were stabilized in water till the time of
preparation of the composites. Just prior to such a preparation, a quantity of the solution of
AuNR in water, estimated to yield the required amount of the particles in the composite, was
taken in a vial and water evaporated from it. To ensure a good dispersion of LC, the particles
and the weighed amount of 8CB were put into copious amount of hexane and sonicated for
30 minutes. Subsequently the solvent was evaporated and the mixture allowed to dry over an
extended duration of one hour.
3. Procedure for electrical measurements
The electrical measurements were performed with samples sandwiched between two indium
tin oxide coated glass plates pre -treated with a silane solution or a unidirectionally rubbed
polyimide layer to realize homeotropic or planar alignment of the molecules respectively.
Mylar strips, placed outside the electrically active area, defined the thickness of the cell. The
sample cells were placed inside either a hot stage from Mettler, or a home-made copper block
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heater. The measurements of the dielectric constant and electrical conductivity were carried
out using an Impedance analyser (HP 4194A).
Reference
[1] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E. Hunyadi, and
T. Li, J. Phys. Chem. B, 109, 13857 (2005).
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