1

Shape Change and Color Gamut in Gold Nanorods, Dumbbells and Dog-Bones

By Xiaoda Xu and Michael Cortie*

It is shown here that deviations from prolate ellipsoidal shape have a significant effect on the

optical properties of gold nanorods. Transitions from rods to ‘dumbbell’, or ‘phi’-shaped

particles lead to a shift in the longitudinal plasmon peak in the blue and red directions

respectively. Development of ‘dog-bone’ shapes leads to a red-shift and to the development

of a third peak. A broad and flexible color gamut can be obtained.

Keywords : gold, nanorod, plasmon, optical extinction, gamut

[*] Prof. M.B. Cortie, Mr X. Xu

Institute for Nanoscale Technology, University of Technology Sydney,

PO Box 123, Broadway, NSW 2007, AUSTRALIA

E-mail : michael.cortie@uts.edu.au, Fax. +61-2-9514-7553

[**] This research was supported by mining company AngloGold Ashanti and the Australian Research Council. High performance computing facilities were provided by the Australian Centre for Computing and Communications.

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1. Introduction

Gold and silver nanorods, and their highly anisotropic optical properties, have captured

attention since their discovery in 1987,[1] and have been extensively discussed recently.[2-7] In

general, their optical spectra are characterized by two absorption peaks, corresponding to

plasmon resonances in the transverse and longitudinal directions of the rod. The position of

the peaks may be varied from 550 nm to over 1200 nm.[8, 9] This suggests that the rods may

find applications in chemical analysis (for example surface enhanced Raman spectroscopy),

solar glazing, plasmonics, biomedical diagnostics, and therapeutics.[2]

Nanorods may be made by the physical or electrochemical deposition of Au into cavities in

a suitable hard template, or by chemical reduction of HAuCl4. Both procedures have their

origins in the late 1980s.[1, 10] Although the hard template method is still of interest,[11, 12] the

chemical method[4, 13] is now far more commonly used. This has been under development by

various groups since a serendipitous observation in 1992 that gold nanorods were formed

when AuCl4- was reduced in the presence of cationic surfactants.[14] Excellent rods were

produced in 1997 by electrochemical reduction,[8] but significantly higher yields have since

been obtained by systematic improvements to the electroless reduction of Au3+ onto pre-

existing gold nuclei (‘seeds’).[9, 13, 15] An important discovery was that the aspect ratio of the

rods could be controlled by the addition of small quantities of Ag+ to the growth solution.[9, 16,

17] Today, most interest[15, 18-24] is focused on this surfactant-mediated reduction, although

there is disagreement as to whether the mechanism involves soft templating onto a surfactant

micelle, as claimed by Jana,[4] or whether it is due to the preferential adsorption of surfactant,

and possibly AgBr, onto the {100} and {110} surfaces of Au.[2, 18, 25, 26]

The transverse plasmon resonance of the particles is roughly in the same position as that of

isolated spherical nanoparticles of gold and is not influenced much by the aspect ratio of the

rods. Reportedly, it is only slightly red-shifted as the particle volume increases,[6, 27] and

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slightly blue-shifted as the aspect ratio increases.[28] In contrast, the position of the

longitudinal plasmon resonance is very sensitive to the aspect ratio.[8] However, it has been

occasionally observed that the optical properties are also influenced by additional details of

the rod shape, and in particular by the natural transition from cylindrical to flared, or vice

versa,[22] or cylindrical to ‘dog-bone’,[21, 29] ‘I-shaped’,[30] ‘dumbbell’ or ‘peanut’[27, 29, 31, 32] or

to prolate spheroidal, ‘phi’ or bipyramidal forms.[16, 33, 34] While most workers found that the

rods changed from a cylindrical form to a ‘dumbbell’, ‘dog-bone’ or ‘flared’ morphology

during aging, one group has reported the opposite trend.[22] Furthermore, while most

investigators have approximated their rods as prolate ellipsoids for the purpose of

calculation[3, 6, 28, 32, 35-37] this is obviously a simplification.

Here we explicitly consider the effect of these deviations in shape on the optical extinction

spectra of gold nanorods. Samples of rods were synthesized and aged, and their optical

properties simulated using a code based on the discrete dipole approximation. The results are

summarized by a color gamut diagram that shows which parts of the color space can be

conveniently obtained by use of randomly oriented dispersions of these nanoparticles.

(a) (b)

Figure 1. Aging of suspensions of gold nanoparticles. a) Optical extinction spectra to 45 hours, as measured. b) Spectra that have been normalized to eliminate variation due to the volume of gold metal in suspension. The longitudinal plasmon peak initially intensifies and shifts to shorter wavelengths (A to B) during aging of the growth solutions. However at aging times of greater than 2 h (point B) it decreases in significance out to 45 h (point C) while a third peak develops (D).

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2. Results

The changes in the optical properties of nanorods grown by a wet chemical method (see

Experimental section) and aged for various times are shown in Figure 1. In Figure 1a the

actual extinction spectra are recorded. It is evident that the optical density of the suspension

increases steadily during the experiment, and that this is caused by the development of a

broad region of extinction below 350 nm, in combination with three peaks at ~520, 560 and

~700 nm. Some of this increase is simply due to the increased content of metallic gold in

suspension. This factor is evident between 300 and 500 nm in the spectra, where absorption is

mostly due to interband transitions[38-40] and hence in proportion to the volume of gold

present. The true relationship between the remaining peaks can be brought out by normalizing

the data with respect to this part of the spectrum[34], Figure 1b. The changes in the normalized

spectra may now be attributed to changes in shape of the metallic nanoparticles. It is clear that

the peak due to the longitudinal plasmon blue-shifts and increases in strength as the rods age.

The literature is in agreement that the former phenomenon can only occur if the aspect ratio

of the rods is decreasing. In principle, such a decrease can occur by growth around the flanks

of the rods (keeping the length constant) or by spheroidization of the particles (keeping the

volume constant). We shall examine both of these possibilities later.

There are two peaks in the spectrum between 500 and 600 nm. One of these peaks red-

shifts and strengthens as the particles age. Of course spherical gold nanoparticles would have

also produced a plasmon resonance at about 530 nm, and the experimentally observed peak at

this position would in general contain contributions from both effects since some spheres are

present in these suspensions.

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Figure 2. Time dependent changes in the shapes of gold nanorods held in growth solution containing 0.1 mM Ag+. a) After 10 minutes. b) After 20 minutes. c) After 30 minutes. d) After 40 minutes. e) After 60 minutes.

An analysis of 264 particle shapes produced by aging suspensions for 50 minutes provided

values of L, D1 and D2 of 45.6 ± 9.2 nm, 22.6 ± 6.1 nm, and 24.3 ± 6.3 nm respectively,

where the range given is the standard deviation of the measurement, and L is the length, D1

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the diameter in the mid section, and D2 the maximum diameter at the ends. The resulting

aspect ratios, L/D1 and L/D2, were 2.18 ± 0.74 and 2.00 ± 0.63 nm respectively. We will use

these averages here for the purpose of calculating typical optical extinction spectra.

The flanks of the experimental rods are believed to be coated with a bilayer of CTAB,[25]

which has a significantly higher refractive index than water. This will red-shift the

longitudinal resonance. However, we will first calculate the extinction spectra of the rod-like

shapes as surrounded by water, returning later to the effect of the CTAB sheath.

Red-shifting of the longitudinal plasmon resonance as the aspect ratio of the rods increases,

calculated in this case using the traditional ellipsoidal model, is shown in Figure 3a for

particles of equal volume, and in Figure 3b for an ellipsoidal particle of fixed aspect ratio but

variable volume. It can be seen that, although the aspect ratio exerts a dominating influence

on the wavelength of maximum extinction, an increase in volume also causes a red-shift.

Most of this is due to the increased scattering that occurs as particle volumes increase. In

Figure 3c we explore the effect of varying the basic shape from ellipsoidal to right-cylindrical

while fixing the aspect ratio and volume to that of a hemispherically-capped cylinder with

L=45.6 nm and D1=22.5 nm. The strong effect that particle shape has on extinction spectra,

even at fixed aspect ratio and volume, is a surprising result. Evidently, the maximum

development of the longitudinal plasmon resonance would be achieved in a right cylinder,

while an ellipsoid is the least effective shape.

In Figures 3d and 3e we explore the range of shapes that can be produced by inflating or

deflating the ends of the rods, while maintaining rotational symmetry and keeping the L and

D1 at 45.6 and 22.5 nm respectively. It is evident that when L and D1 are fixed, the initial

effect of the two shape changes mentioned is simply to reduce the intensity of the longitudinal

peak, while also red-shifting it very slightly. However, strong blue-shifting occurs in Figure

3e once the dumbbells ends fuse together. This is in congruence with Gou,[21] who found that

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growth at the ends of the rods initially caused the longitudinal plasmon peak to broaden, with

blue-shifting and growth of this peak only occurring later on in the aging process. The

explanation offered was that the CTAB bilayer had initially shielded the flanks of the rods, so

that growth occurred at the ends and moved inwards. These results of our simulations show

that the position of the longitudinal plasmon of particle is more strongly influenced by the

ratio L/D1, rather than L/D2, a point noted already by Zweifel.[22] Finally, we examine the

development of a dog-bone shape with L=45.0 and D1 = 22.0 nm in Figure 3f. The strong red-

shift induced by this change is an unexpected result, as is the appearance of a third peak in the

vicinity of 560 nm.

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(a) (b)

(c) (d)

(e) (f)

Figure 3. Calculated influences of geometry on the optical properties of gold nanorods. a) Different aspect ratios but equal volumes with a0=20 nm in each case. b) Equal aspect ratios of 3.0, but different effective radii (volume). c) Prolate ellipsoid, hemispherical-capped cylinder and right cylinder compared at L/D1= 2.03 and equal volumes corresponding to a0=20 nm. d) Transition from cylinder to phi shape at L=45.6 nm and D1=22.5 nm. e) Transition from cylinder to dumbbell at L=38 nm and D1=16 nm. f) Transition from cylinder to dog-bone at L=45.6 nm and D1=22.5 nm.

Having determined some of the effects that deviations from the ideal shape have, it is

instructive to consider various scenarios for the time-dependent aging that takes place in the

growth solution. These scenarios are shown in Figure 4. In Figure 4a we consider a

hypothetical growth series that starts with a gold nanorod seed, starting at L=17.5 nm and

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D1=5 nm. This grows into a rod with L=45.5 nm and D1=28.4 nm, developing dog-bone lobes

as it does so. The resulting spectra are characterized by an initial blue-shift of the longitudinal

resonance caused by the decreasing aspect ratio, followed by an arrest in this trend as the red-

shifting lobes of the dog-bone develop. In Figure 4b we consider the possibility that no

accretion of new material takes place after some limiting time, so that all further shape

change is by spheroidization,[19, 21, 22] driven by surface diffusion of Au atoms and the

lowering of surface energy that would result when {111} surfaces replace the higher energy

{100} and {110} flanks. This transition has been observed in cases where the rods have

been heated by laser radiation,[16, 41, 42] and would be a constant volume process. The result of

imposing this sequence of geometries is a strong blue-shift of the longitudinal peak, and a

concurrent reduction in its intensity. It is also possible[27, 31, 32] that the shape change that

occurs in the rods could be due to accretion of Ag. In Figure 4c we show what would happen

when the basic gold rod shape develops an accretion of Ag metal on its ends. In this case, it is

evident that the silver lobes generate a plasmon resonance at about 380 nm, in agreement with

experiments in which Ag has been deliberately accreted onto gold rods.[27, 32] However,

experimental spectra generated by the usual techniques employed to make gold nanorods

show no signs of such a peak, indicating that such accretions of Ag do not form under the

normal synthesis conditions. Finally, in Figure 4d we consider the effect that immersion into

a medium of CTAB has on the optical properties of a dog-bone rod with L=45.6, D1=22.5 nm,

and D2=32 nm. A dielectric constant for CTAB of 2.1 was used for these calculations.[43] It is

evident that the calculated spectrum is very similar in form to that measured in the rods after

120 minutes.

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(a) (b)

(c) (d)

Figure 4. Effects of simulated aging and of composition on the optical properties of rods. a) Development from cylindrical nucleus with a0=3 nm to dog-bone with a0=19 nm, showing how longitudinal peak will initially blue-shift as the aspect ratio decreases but that development of dog-bone lobes arrests this tendency. b) Spheroidization under constant volume conditions with a0=15.4 nm, such as might occur under laser irradiation. c) Case when rod interior is Au but lobes are comprised of Ag (cf. Figure 4f for pure gold rods which had the same dimensional properties). d) Red-shifting effect from immersing dog-bone of 45 nm length in a medium of CTAB showing affinity to experimental spectrum measured at 120 minutes.

The color of suspensions or coatings of gold nanorods may be important in some potential

applications, such as solar screening. The ‘gamut’ of a particular color-producing technology

is defined as the range of possible colors that it can produce. However, no study seems to

have been made previously of the gamut of gold nanorods, an omission which we remedy

here by mapping the colors of suspensions of the rods to the a* and b* color coordinates of

the CIE-LAB system. Calculated colors of gold nanorod suspensions are mapped in Fig. 5a

for different concentrations and aspect ratios, while Figure 5b shows values measured during

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various growth experiments. It is evident that gold nanorods display a surprisingly wide color

gamut, from purple, through blue, to red depending on their shape. An important

consideration is that the longitudinal peak of gold nanorods with short aspect ratios is in the

visible region. The resulting extinction of part of the upper visible makes the color of such

gold nanorods purple or blue. During growth and/or increase in concentration, the color starts

from light pink, becoming purple and then dark blue. The experimental data of Figure 5b

shows that at some stage a pronounced inflection occurs, evidently as the result of one or

more of the phenomena that we have discussed above, and that are separately illustrated in

Figure 5c.

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(a)

(b)

(c)

Figure 5. Color gamut of gold nanorods. Simulations and measurements are for 0.5 mM Au. a) Gamut of rods of fixed volume but varying aspect ratio at different optical densities in water. Solid lines correspond to various cuvette thicknesses (p=1 mm, q=3 mm, r=5 mm, s=7 mm), dashed lines to particular shapes. b) Development of color in experimental suspensions of gold nanorods. c) Color gamut of simulated shapes in Figures 3 b), d) to f), and 4 a) for a suspension of 3 mm thickness.

Rods of aspect ratios greater than 2 have a quite different behavior. Their longitudinal

plasmon resonance is close to or even in the near-infrared. In this case, that peak has little

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influence on the color perceived by the human eye, and coatings or suspensions of the

nanorods consequently display a grey-purple color.

3. Discussion

While most workers in this field have found the presence of Ag+ to be helpful to produce

nanorods,[2, 5, 9, 13, 34, 44] others have produced rods without it.[15, 18] We have confirmed that

additions of 0.05 - 0.2 mM Ag+ to the growth solution were required in order to provide a

high yield of rods, and that the aspect ratio is controlled by nominal Ag+ content. Excessive

contents of Ag (0.2 mM Ag+ or above) led to the rate of rod formation becoming very slow, a

point noted by Jana.[4] However, the mechanism by which Ag exerts its effect on the reaction

is still uncertain. It might be expected from a consideration of solubility products and solution

redox potential that it would be present in these mixtures as particles of AgBr rather than the

free ion or the metal.[32, 44] This has led to proposals that the AgBr particles absorb selectively

onto the flanks of the rod thereby passivating them,[2, 19] or that the role of Ag+ is to

scavenge[32] Br- and Cl

-, which would otherwise interfere with rod formation. Alternatively,

given the high Br- content of the growth solution, perhaps the CTAB-coated flanks of the rods

are rendered inert by an electrostatically bound layer of AgBr2- ions. Nevertheless, although

Ag+ cannot be reduced by ascorbic acid under the usual near-neutral or slightly acidic rod-

growth conditions,[9, 31] (due to the fact that any AuCl4- present is an oxidant[44] with respect

to Ag0 ) it certainly is well known that ascorbate can reduce Ag at pH greater than 7.[31] This

may indicate that only a small change in conditions is necessary to produce an deposit of

perhaps a monolayer of Ag0 on the gold nanoparticles. Liu and Guyot-Sionnest[34] have

shown clearly how such a monolayer might preferentially form by underpotential deposition

on the {110} surfaces of a gold rod, so a role for Ag0 can not be ruled out yet.

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Either way, it has been found that these rods always contain a few % of silver, [19, 44]

suggesting that the mechanism does require some type of preferential deposition of an Ag-

containing species. Furthermore, it follows that once all available Ag is used up in this

manner, further growth of the rods will be unconstrained by the passivated sheath, causing a

transition from cylindrical to dog-bone morphologies to occur- an observation seemingly

born out in the experimental observations in the literature.[27]

In a practical sense, the main importance of the Ag+ seems to be that it increases the yield

and aspect ratio of nanorods, and also that it appears to cause the transition from cylindrical to

dog-bone morphologies.[27] The situation in the absence of silver is confusing, with some[15,

22] finding no flared shapes, while others did.[27] There have even been suggestions that the

lobes of the dog-bones or dumbbells[22, 31] might be Ag itself but ther workers[21, 27] have

shown that the lobes are most likely due to reduction of excess Au left over in the

surrounding liquid.

Comparison of the various scenarios in Figure 5 suggests that the changes in the spectra

shown in Figure 1 are the result of an interplay between a decrease in aspect ratio (which

blue-shifts the peak), the concurrent development of dumbbell or dog-bone lobes and

increased volume (both of which red-shift the longitudinal peak), and the loss of rotational

symmetry (which causes the development of a third peak). A decrease in aspect ratio might

be due to spheroidization at constant volume or to accretion of Au on the flanks, with the

evidence indicating that both phenomena can occur under appropriate conditions.

However, some of the other explanations for blue-shifting proposed in the literature are

unlikely. The accretion of Ag can be ruled out, at least under the experimental conditions that

we have employed. The possibility that the rods do not change in shape at all, but rather that

the effect is caused by the de-sorption of the CTAB bi-layer, with an attendant decrease in the

refractive index of the immediate environment of the rod can also be ruled out. This is

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because we have found that the application of two or three centrifuge-and-wash cycles does

not reposition the longitudinal plasmon peak. Another possible explanation, raised by Jana,[4]

is that the blue-shift is caused by a transition in particle shape from rod to platelet, with the

onset of new quadrupole resonances. However we have shown that, while such an effect can

explain the appearance of an additional peak at about 560 nm, it would produce a red-shifting

of the longitudinal resonance. Finally, there has been a claim in the literature[45] that, as time

goes by the longer and larger rods precipitate out, hence decreasing the average aspect ratio

(and optical density) of the suspension. This would obviously be accompanied by a blue-shift

in the spectra. However, this must be false too both because we have not observed such

precipitation.

5. Conclusions

In summary, although the aspect ratio is the dominant factor controlling the optical

extinction spectrum of gold nanorods, we have shown that volume and shape of particle are

surprisingly important as well. Our work provides the first systematic examination of these

additional factors. An increase in volume generally red-shifts the longitudinal plasmon

resonance, as does the development of a two-lobed dog-bone shape. The development of

dumbbell or phi shapes at a fixed aspect ratio causes a decrease in the intensity of the

longitudinal resonance without substantial change in its position. However, once the end-

spheres of a dumbbell-shaped rod fuse, the result is a rapid decrease in aspect ratio and

consequent blue-shift in plasmon resonance. This outweighs any red-shifting induced by

increased volume. Although Ag can accrete in principle onto gold rods, this would produce

optical spectra unlike those ordinarily observed. The most probable growth trajectory when

gold nanorods are produced by Ag-assisted seeding is by the deposition of gold onto micelle-

encapsulated seed rods of high aspect ratio, followed by a strong growth phase with a

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concurrent decrease in aspect ratio (which would cause detachment of the rod from any initial

micelle) and, finally, the development of two-fold dog-bone symmetry. This ontogeny defines

the color gamut of gold rods, at least up to aspect ratios of 2.5.

4. Experimental

HCl, HNO3, hexadecyltrimethylammoniumbromide (CTAB), potassium borohydride (KBH4), L-

ascorbic acid and fine gold were sourced from diverse suppliers. All chemicals were used as-received.

All H2O used was purified by double-distillation. HAuCl4 solution was prepared by dissolving pure

gold in aqua regia (HCl: HNO3 3:1 v/v). The method may be found elsewhere.[46] The gold nanorods

were prepared by a modified version of the seed-mediated growth method.[9, 13] A brief summary is

given here.

A ‘seed’ solution was prepared by mixing 0.10 ml of ~0.02 M KBH4 (the concentration is not exact

because this substance decomposes over time) with 10.0 ml aqueous solution containing 0.5 mM

HAuCl4 and 0.2 M CTAB by vigorous stirring. The resulting brown-yellowish solution was used

within 30 minutes of mixing. A ‘growth’ solution was prepared by adding 1.0 ml of 0.05 M L-ascorbic

acid to 100.0 ml aqueous solution containing 0.2 M CTAB and 0.5 mM HAuCl4. AgNO3 was added as

0.0, 0.5, 1.0, 2.0 mL of 10 mM stock solution to give final concentrations ranging from 0 to 0.25 mM.

Growth of gold nanorods was initiated by adding 12 µL of seed solution to the growth solution.

Thereafter, the blue to red colour of the gold nanorod colloid developed, taking from tens of minutes

to a few hours before becoming stable. The reaction was carried out at room temperature. Kinetics

were found to depend upon pH, with negligible growth at a value of 2.0, and usable kinetics at 4.5 or

12.0. The gold nanorods colloid was centrifuged at 10,000 rpm for 25 minutes, and the supernatant

liquid was tipped off carefully without disturbing the precipitate. The remaining material was mixed

with 100 mL double-distilled water and spun again. The concentration of gold nanorods in the final

product is 50 – 100 times that of the original colloid. The morphologies of gold nanorods were

characterized using a LEO scanning electron microscope (SEM) with in-lens images taken at 2- 30

17

kV. The optical properties of the colloids were inspected using a Cary 3E UV/visible

spectrophotometer. A resolution of 0.5 nm, a 5 nm/s of scanning rate and a spectral bandwidth of 2 nm

were used to obtain the visible-IR transmission spectra.

The optical properties of gold nanorods were numerically simulated using the DDSCAT code of

Draine and Flatau.[47, 48] This is based on the discrete-dipole approximation method (DDA). The

advantage of DDA is it can in principle be applied to any shape of particle. It is an approximation

method which converts the continuum target into a finite array of polarizable points, which acquire

dipole moments in response to the local electric fields. The applicability and accuracy of the

DDSCAT code has been extensively verified, e.g.[6, 7] For convenience the optical properties here are

expressed in terms of an extinction efficiency, Qext, which is the applicable optical cross-section

normalized by the actual geometric cross-sectional area of the particle.

Much of the prior work in respect of the calculation of the optical properties of rods has taken the

shape as being that of a prolate elliptical spheroid. However, this is an approximation since real rods

exhibit a range of morphologies ranging from cylindrical, flared, ellipsoidal, and bi-spherical. In the

present work these shapes have been explicitly simulated by means of a computer program that

generated the desired shape in digital form for use in the DDSCAT program.

The calculations were performed for the case of gold rods suspended in water, with complex

refraction indices of gold and water.[49] Arrays of 12,000 or more dipoles were applied in the

calculations, considerably more than required to satisfy the program’s requirements regarding

accuracy and convergence.

Colours in the Commission Internationale de I’E Clairage (CIE) colour model were directly

calculated from spectra using appropriate tables,[50] after converting either measured absorbance or

calculated Qeff to transmission. The colours of the calculated spectra were calculated for the case of a

suspension containing 0.5 mM Au, a standard white light source, and various optical path lengths. In

the CIE LAB system the L* coordinate refers to the luminance of the color, the a* measures the green-

18

to-red tendency of the color, and the b* the blue-to-yellow tendency. The white point is given by a*=

b* = 0. The color gamut in Figure 5 is viewed from above, i.e. the L coordinate has been suppressed.

References

[1] M. J. Tierney, C. R. Martin, J. Phys. Chem. 1989, 93, 2878.

[2] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E. Hunyadi, T.

Li, J. Phys. Chem. B 2005, 109, 13857.

[3] J. Perez-Juste, I. Pastoriza-Santos, L. M. Liz-Marzan, P. Mulvaney, Coord. Chem. Rev.

2005, 249, 1870–1901.

[4] N. R. Jana, Small 2005, 1, 875.

[5] C. J. Murphy, T. K. Sau, A. Gole, C. J. Orendorff, MRS Bulletin 2005, 30, 349.

[6] A. Brioude, X. C. Jiang, M. P. Pileni, J. Phys. Chem. B 2005, 109, 13138.

[7] K. L. Kelly, E. Coronado, L. L. Zhao, G. C. Schatz, J. Phys. Chem. B 2003, 107, 668.

[8] Y.-Y. Yu, S.-S. Chang, C.-L. Lee, C. R. C. Wang, J. Phys. Chem. B 1997, 101, 6661.

[9] B. Nikoobakht, M. A. El-Sayed, Chem. Mater. 2003, 15, 1957.

[10] J. Wiesner, A. Wokaun, Chem. Phys. Let. 1989, 157, 569.

[11] C. R. Martin Chem. Mater. 1996, 8, 1739.

[12] B. M. I. Van der Zande, M. R. Bohmer, L. G. J. Fokkink, C. Schonenberger, J. Phys.

Chem. B 1997, 101, 852.

[13] N. R. Jana, L. A. Gearheart, C. J. Murphy, J. Phys. Chem. B 2001, 105, 4065.

[14] K. Torigoe, K. Esumi, Langmuir 1992, 8, 59.

[15] J. Perez-Juste, L. M. Liz-Marzan, S. Carnie, D. Y.C.Chan, P. Mulvaney, Adv. Func.

Mater. 2004, 14, 571.

[16] S.-S. Chang, C.-W. Shih, C.-D. Chen, Langmuir 1999, 15, 701.

[17] N. R. Jana, L. A. Gearheart, C. J. Murphy, Adv. Mater. 2001, 13, 1389.

19

[18] B. D. Busbee, S. O. Obare, C. J. Murphy, Adv. Mater. 2003, 15(5),, 414.

[19] T. K. Sau, C. J. Murphy, Langmuir 2004, 20, 6414.

[20] A. Gole, C. J. Murphy, Chem. Mater. 2005, 16, 3633.

[21] L. Gou, C. J. Murphy, Chem. Mater. 2005, 17, 3668.

[22] D. A. Zweifel, A. Wei, Chem. Mater. 2005, 17, 4256.

[23] J.-Y. Chang, H. Wu, H. Chen, Y.-C. Lingb, W. Tan, Chem. Comm. 2005, 1092.

[24] A. J. Mieszawska, F. P. Zamborini, Chem. Mater. 2005, 17, 3415.

[25] B. Nikoobakht, M. A. El-Sayed, Langmuir 2001, 17(20), 6368.

[26] E. Leontidis, K. Kleitou, T. Kyprianidou-Leodidou, V. Bekiari, P. Lianos, Langmuir

2002, 18, 3659.

[27] J. H. Song, F. Kim, D. Kim, P. Yang, Chemistry - A European Journal 2005, 11, 910

[28] J. Zhua, L. Huang, J. Zhao, Y. Wang, Y. Zhao, L. Hao, Y. Lu, Mater. Sci. Engng. B

2005, 121, 199.

[29] C. Wang, T. Wang, Z. Ma, Z. Su, Nanotechnology 2005, 16, 2555–2560.

[30] Y. Niidome, K. Nishioka, H. Kawasaki, S. Yamada, Chem. Comm. 2003, 18, 2376.

[31] C.-C. Huang, Z. Yang, H.-T. Chang, Langmuir 2004, 20, 6089.

[32] M. Liu, P. Guyot-Sionnest, J. Phys. Chem. B 2004, 108(19),, 5882.

[33] M. B. Mohamed, K. Z.Ismail, S. Link, M. A. EI-Sayed, J. Phys. Chem. B 1998, 102,

9370.

[34] M. Liu, P. Guyot-Sionnest, Journal of Phys. Chem. B 2005, 109, 22192.

[35] C.-D. Chen, Y.-T. Yeh, C. R. C. Wang, J. Phys. Chem. Solids 2001, 62(9-10), 1587.

[36] S. Link, M. B. Mohamed, M. A. El-Sayed, J. Phys. Chem. B 1999, 103, 3073.

[37] B. M. I. Van der Zande, L. Pages, R. A. M. Hikmet, A. Van Blaarderen, J. Phys. Chem.

B 1999, 103, 5761.

20

[38] T. Ung, L. M. Liz-Marzan, P. Mulvaney, Colloids and Surfaces A: Physicochemical and

Engineering Aspects 2002, 202, 119.

[39] M. Quinten, J. Cluster Sci. 1999, 10, 321.

[40] J. Garcia-Sole, L. E. Bausa, D. Jaque, An Introduction to the Optical Spectroscopy of

Inorganic Solids, John Wiley & Sons, Chichester 2005.

[41] S. Link, Z. L. Wang, M. A. El-Sayed, J. Phys. Chem. B 2000, 104, 7867.

[42] S. Link, C. Burda, B. Nikoobakht, M. A. El-Sayed, J. Phys. Chem. B 2000, 104, 6152.

[43] B. Yan, Y. Yang, Y. Wang, J. Phys. Chem. B 2003, 107, 9159.

[44] F. Kim, J. H. Song, P. Yang, J. Am. Chem. Soc. 2002, 124(48), 14316.

[45] A.-S. A.-M. Al-Sherbini, Colloids and Surfaces A: Physicochem. Eng. Aspects 2004,

246, 61–69.

[46] X. Xu, M. Stevens, M. B. Cortie, Chem. Mater. 2004, 16, 2259.

[47] B. T. Draine, P. J. Flatau, J. Opt. Soc. Am. A 1994, 11, 1491.

[48] B. T. Draine, P. J. Flatau, User Guide to the Discrete Dipole Approximation Code

DDSCAT 6.1 2004, accessed January 2005.

[49] J. H. Weaver, H. P. R. Frederikse, Optical Properties of Selected Elements, CRC Press,

Boca Raton 2001.

[50] Standard Practice for Computing the Colors of Objects by Using the CIE System,,

American Society for Testing and Materials (ASTM), 2001.

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Figure 1. Aging of suspensions of gold nanoparticles obtained in a growth solution containing 0.1 mM Ag+. a) Optical extinction spectra to 45 hours, as measured. b) Spectra that have been normalized to eliminate variation due to the volume of gold metal in suspension. The longitudinal plasmon peak initially intensifies and shifts to shorter wavelengths (A to B) during aging of the growth solutions. However at aging times of greater than 2 h (point B) it decreases in significance out to 45 h (point C) while a third peak develops (D).

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Table of contents graphic

Xiaoda Xu and Michael Cortie

Shape change and color gamut in gold nanorods, dumbbells and dog-bones

The effect on the optical properties of gold nanorods of deviations in shape from prolate

ellipsoid to right cylindrical, dumbbell, phi and dogbone-like is examined. A broad and

flexile color gamut can be obtained.