structural colors: from plasmonic to carbon nanostructures

concepts

3

Subwavelength Nanostructures

Structural Colors: From Plasmonic to Carbon Nanostructures

Ting Xu , Haofei Shi , Yi-Kuei Wu , Alex F. Kaplan , Jong G. Ok , and L. Jay Guo*

12

In addition to colorant-based pigmentation, structure is a major contributor to a material’s color. In nature, structural color is often caused by the interaction of light with dielectric structures whose dimensions are on the order of visible-light wavelengths. Different optical interactions including multilayer interference, light scattering, the photonic crystal effect, and combinations thereof give rise to selective transmission or refl ection of particular light wavelengths, which leads to the generation of structural color. Recent developments in nanofabrication of plasmonic and carbon nanostructures have opened another effi cient way to control light properties at the subwavelength scale, including visible-light wavelength selection, which can produce structural color. In this Concept, the most relevant and representative achievements demonstrated over the last several years are presented and analyzed. These plasmonic and carbon nanostructures are believed to offer great potential for high-resolution color displays and spectral fi ltering applications.

1. Introduction

Color is one of the most important attribute of human

visual perception. The pioneering experiments on visible light

and colors can be traced back to the 17th century, when Sir

Isaac Newton observed different transmitted colored bands

when a narrow beam of sunlight struck the face of a glass

prism at an angle. This early experiment implies that a desir-

able color can be obtained by spectrally fi ltering the white

light. Indeed most of the modern colored displays rely on

spectrum fi lters to produce primary colors (e.g., red, green

and blue). Traditionally such spectrum fi lters are based on

colorant pigments that absorb light in certain wavelength

range, which fi nds dominant usage in many display systems

such as liquid crystal displays (LCDs). On the other hand,

color can also be created by light interaction with certain

8 © 2011 Wiley-VCH Vwileyonlinelibrary.com

DOI: 10.1002/smll.201101068

Dr. T. Xu , Dr. H. Shi , Y.-K. Wu , A. F. Kaplan , J. G. Ok , Prof. L. J. Guo , Department of Electrical Engineering and Computer ScienceAnn Arbor, Michigan 48109, USAE-mail: [email protected]

Dr. T. Xu State Key Laboratory of Optical Technologies for MicrofabricationInstitute of Optics and ElectronicsChinese Academy of ScienceChengdu 610209, China

nanostructures that strongly infl uence the light propagation

through the structures. In fact this is frequently observed in

nature, for example, the wings of the Morpho butterfl y (a

special southern American butterfl y family) shine a magnifi -

cent blue color, which was found to be due to surface-relief

volume-diffractive nanostructures on its wings ( Figure 1 ). [ 1 ]

The principle of these photonic nanostructures and the asso-

ciated light management including tunable colors has been

the subject of previous reviews [ 2 , 3 ] and will not be repeated

here. Instead, this concept paper will focus on the color gen-

eration due to light interaction with conductive nanostruc-

tures. Specifi cally various patterned metallic nanostructures

can perform spectrum fi ltering by the excitation of surface

plasmons; and carbon nanotube-based structures can produce

colors by tailoring the structures’ effective refractive index.

2. Color Generation via the Exploitation of Surface Plasmons in Metallic Nanostructures

A general overview is fi rst provided on the most relevant

concepts that are driving the development of structural color

from plasmonic nanostructures. Several representative plas-

monic nanostructures that are capable of fi ltering the color

from the visible spectrum are presented. These metallic

nanostructures could be advantageous for the optical appli-

cations such as spectral fi ltering, imaging and high-resolution

color display.

erlag GmbH & Co. KGaA, Weinheim small 2011, 7, No. 22, 3128–3136


Figure 1 . Morpho butterfl y and the scanning electron microscope (SEM) image of the nanostructures on its wings. Scale bar: 1 mm. Reproduced with permission. [ 1 ] Copyright 1990, OSA.

2.1. Surface Plasmons and the Interaction of Light with Metallic Nanostructures

With the development of nanofabrication and charac-

terization techniques, surface plasmons and related plasmonic

nanostructures have generated considerable interest in recent

years. [ 4–8 ] Surface plasmons are essentially surface-bound elec-

tromagnetic waves coupled with the collective oscillation of

conduction electrons on the metal surface that are excited by

an incident light. By exploiting plasmonic nanostructures, effi -

cient conversion between free-space photons and confi ned plas-

mons can be controlled at a subwavelength scale. This means

one can exploit the geometries of the plasmonic nanostruc-

tures to manipulate light properties, including aforementioned

visible-light wavelength selection and thereby the generation of

structural color. In contrast to the color produced by ordinary

electronic absorption, which is determined by the material’s

specifi c molecular spectrum, plasmon-based structural color

is primarily determined by the geometry parameters of the

plasmonic nanostructures. This characteristic brings in several

advantages for the plasmon-based structural color: 1) different

colors can be obtained by using the same material but only with

different structural parameters, which could greatly simplify the

manufacturing process for multiple colored units; 2) unlike tra-

ditional chemical pigment and dye that are affected by photob-

leaching, the plasmonic nanostructures are chemically stable,

and therefore the structural color is attractive for applications

requiring high light intensities or continuous illuminations;

3) comparing with other dielectric periodic structures such as

photonic crystals [ 9–14 ] or semiconductor nanowires [ 15 , 16 ] which

also can generate structural colors, the highly confi ned surface

plasmons in metallic nanostructures makes the device dimen-

sion more compact. Moreover, the metal in the plasmonic

nanostructures can be used as electrodes and thereby offers

better potential for the electro-optic applications.

2.2. Structural Color from Subwavelength Hole Arrays in Metal Films

In 1998, Ebbesen et al. experimentally demonstrated the

extraordinary optical transmission (EOT) through perforated

periodic subwavelength hole arrays in optically thick metallic

fi lms, [ 17 ] where the transmission is several orders higher than

the value predicted by classic Bethe’s theory. The surface-

plasmon modes excited by the subwavlength hole arrays are

believed to play an essential role in the EOT phenomenon. [ 18–21 ]

© 2011 Wiley-VCH Verlag Gmsmall 2011, 7, No. 22, 3128–3136

The subwavelength hole arrays effi ciently convert incident

light into surface-plasmon modes by scattering, which pro-

vides the necessary momentum conservation. The excited

surface plasmon waves tunneling through the subwavelength

holes scatter again at the exit plane of the hole arrays, and

the resulting propagation waves transmit away from the

structure to the far fi eld.

For the subwavelength hole arrays with square-lattice, the

transmission peak wavelength in the normal incidence can be

approximately calculated from the surface-plasmon disper-

sion relation as: [ 4 ]

8max

∼=P√

i2 + j 2

√gmgd

gm + gd (1)

where P is the square-lattice constant; the indices i and j are

the scattering orders from the hole arrays; gm and gd are the

permittivity for the metal and dielectric medium. This equa-

tion does not consider the presence of holes and the associ-

ated scattering loss, and it neglects the Fano-type interaction

which results in a resonance red shift. Therefore the predicted

transmission peak wavelength is slightly smaller than the real

value. [ 22 ] From Equation 1 , we can see that the transmission

peak wavelength 8max is proportional to lattice constant of

the hole arrays. Therefore the subwavelength hole arrays

in metal fi lm can act as optical fi lters for which the trans-

mitted color can be selected by simply adjusting the lattice

constant. [ 4 , 23 ] Figure 2 a,b show the lowest order transmission

peak ( i, j = 1, 0 in Equation 1 ) of the perforated square-lat-

tice hole arrays on a free-standing 300 nm-thick silver fi lm.

When the lattice constant of hole arrays increases from 300

to 550 nm, the transmission peak wavelength changes from

436 to 627 nm, covering primary RGB colors in the visible

spectrum. In addition, by arranging the perforated and unper-

forated subwavelegnth hole arrays in the metal fi lm, arbitrary

colored patterns can be realized (Figure 2 c).

In contrast to the square-lattice arrays, triangular-lattice

hole arrays have improved color fi ltering performances. [ 21 , 24 , 25 ]

Figure 2 d,e show the microscope images of the subwavelength

hole arrays with triangular-and square-lattice, respectively. In

theory, the transmission peak wavelength 8max for the hole

arrays with triangular-lattice can be approximated as: [ 21 ]

8max

∼=P√

43 (i2 + i j + j 2)

√gmgd

gm + gd (2)

where P is the triangular-lattice constant. Comparing with

Equation 1 , we can conclude that the triangular-lattice sub-

wavelength hole arrays will have a larger wavelength interval

between the fi rst two transmission peaks than the square-

lattice hole arrays with the same lattice constant. This means tri-

angular-lattice subwavelength hole arrays can reduce the color

cross-talk and can produce purer color. Such improvement can

be easily seen in the comparison between Figure 2 d and e.

A further note on the color fi lters based on the sub-

wavelength hole arrays is the symmetry of structure. A

free-standing structure described above is diffi cult to fabri-

cate. [ 4 ] However, if the fi lter is fabricated on a glass substrate

but without an index-matching superstrate, the structure

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T. Xu et al.concepts

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Figure 2 . Structural color generated by subwavelength hole arrays. Normal incidence transmission images (a) and spectra (b) for three subwavelength hole arrays. For the blue, green, and red arrays, the square-lattice constants were 300, 450, and 550 nm, respectively, the hole diameters were 155, 180, and 225 nm, and the peak transmission wavelengths 436, 538, and 627 nm. The arrays were made in a free-standing 300 nm-thick silver fi lm. c) Holes in an Ag dimple array generating the colored letters ‘h ν ’ in transmission. Some of the dimples are milled through to the other side so that light can be transmitted. The lattice constants were 550 and 450 nm to achieve the red and green colors, respectively. d,e) Optical microscope transmission images of the subwavelength hole arrays in the Al fi lm with d) triangle lattice e) square lattice. The numbers beside the photographs give the lattice constants of the arrays. a,b) Reproduced with permission. [ 4 ] Copyright 2003, NPG. c) Reproduced with permission. [ 21 ] Copyright 2007, NPG. d,e) Reproduced with permission. [ 25 ] Copyright 2011, AIP.

becomes asymmetric. Since both surfaces on either side of

the hole arrays can support the excitation of surface-plasmon

modes, the asymmetric structures will produce two sets of

peaks in the transmission spectrum. The overlap of two sets

of transmission peak will degrade the purity of the fi ltered

color. Besides, asymmetric structure will reduce the coupling

effi ciency of the surface-plasmon modes on the two surfaces,

which leads to a lower transmission peak. [ 26 ] Therefore, in

most cases, [ 21 , 23–25 ] an index-matching layer such as silica or

oil always covers the whole structure when the device is fab-

ricated on a glass substrate.

2.3. Structural Color from Metal–Insulator–Metal Nanoresonators

Metal–insulator–metal (MIM) geometry offers the ability

to support both photonic and plasmonic modes at visible

© 2011 Wiley-VCH Vwileyonlinelibrary.com

frequency and have been widely investigated for dif-

ferent applications, such as guiding waves at subwave-

length scale, [ 27 , 28 ] concentrating light in the thin-fi lm

photovoltaics, [ 29–31 ] and composing negative-index metamate-

rials. [ 32–33 ] Besides, the top and bottom metal layers of MIM

structure can be integrated as electrodes in a straightfor-

ward manner: they are therefore of considerable interest for

electro-optic applications.

The interference and resonance of the optical modes

within the MIM resonators can be used to fi lter the white

light into individual colors. For example, lateral Fabry-Perot

resonators can be formed if input and output nanoslits are

inserted in the MIM waveguide ( Figure 3 a). [ 34 ] By proper

design of the depth and location of the input and output slits,

the resonator can preferentially couple to different waveguide

modes and select any of the primary colors. Different separa-

tion lengths between the input and output slits produce dif-

ferent exhibited colors at the output slits. The typical length



Figure 3 . Structural color from the MIM nanoresonators. a) A schematic of the MIM geometry used with input and output slits placed on the top and bottom of the waveguide. The fi ltered color is mainly determined by the spacing between input and output slits which is labeled d sep . b) Optical microscope images of the MIM waveguide resonator. This structure consists of 400 nm of silver as the top layer, 500 nm of silicon nitride as the dielectric, and 400 nm of gold as the bottom layer. Devices 1–3 have d sep = 2.5, 3.5, and 4 μ m, respectively. c) Schematic of the MIM stack resonator arrays. Inset is the SEM image of the fabricated device. Scale bar, 1 μ m. d) Optical microscopy image of the plasmonic spectroscope illuminated with white light. Scale bar: 2 μ m. a,b) Reproduced with permission. [ 34 ] Copyright 2009, ACS. c,d) Reproduced with permission. [ 35 ] Copyright 2010, NPG.

of these MIM resonators is about several micrometers. As

shown in Figure 3 b, the resonators consist of a 500 nm-thick

silicon nitride free-standing membrane sandwiched between

a 400 nm-thick gold layer on top and a 400 nm-thick silver

layer on the bottom. Since two metal claddings of the MIM

resonators can be used as electrodes, the electro-optic mate-

rial such as lithium niobate, liquid crystals, or electro-optic

polymers, can be used as the middle layer to make active

devices. An applied electric fi eld can modulate the refractive

index of the insulator layer and thereby change the output

color, which yields the active tunable color fi lter.

The limitation of the MIM resonator color fi lters

described above is their low transmission effi ciencies, because

only a very small portion of the incident light can couple out

through the nanoslits. Even tiling several resonators together,

the overall transmission effi ciency is lower than 0.2%, not

suitable for practical applications. Another type of MIM res-

onator color fi lter capable of fi ltering white light into indi-

vidual colors across the entire visible spectrum with high

resolution and high transmission effi ciency simultaneously

is introduced. [ 35 ] This resonator fi lter is designed as the sub-

wavelength periodic MIM stack arrays, as shown in Figure 3 c.

Each stack is composed by a 100 nm-thick Zinc Selenide

(ZnSe) layer sandwiched between two 40 nm-thick aluminum

(Al) claddings. The basic principle of this device is to use

MIM resonator arrays to realize the photon–plasmon–photon

conversion effi ciently at specifi c resonant wavelength.


For the TM-polarized waves (electric-fi eld is per-

pendicular to the Al grating direction), the MIM stack

resonators support the surface-plasmon modes whose

mode momentum is larger than the free space photon

momentum. Therefore the stack array should have a sub-

wavelength period so that scattering from bottom Al

grating can provide momentum matching for incident light

coupling into surface-plasmon modes. The top Al grating,

having the same period as the bottom one, serves to scatter

the confi ned plasmon modes back to propagation waves

and transmit to far-fi eld. The relation between the reso-

nant wavelength and stack array period can be obtained by

solving the surface-plasmon dispersion equation. Arbitrary

color in the visible spectrum can be fi ltered with this struc-

ture by selecting the proper stack array period. Figure 3 d

shows the optical microscopy images of the seven square-

shaped color fi lters illuminated by the TM-polarized white

light. The fi lters have the stack period ranging from 200 to

360 nm, corresponding to violet to red colors. The stack

arrays show the expected fi ltering behavior with absolute

transmission effi ciency about 60% around the resonant

wavelengths, which are several orders of magnitude higher

than those of MIM resonator fi lters in reference [ 34 ] . This

transmission effi ciency is comparable with the prevailing

colorant-based fi lter used in an LCD panel, but the thick-

ness of this MIM resonator device is 1–2 orders of magni-

tude thinner than that of the colorant one.

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On the other hand, because the TE-polarized waves (the

E-fi eld is parallel to the Al grating direction) do not support

the excitation of surface-plasmon modes, there is no obvious

light conversion process. As a result, the TE-polarized inci-

dent light is strongly refl ected. This indicates that the fi ltered

light by such MIM resonator is naturally polarized, making

it attractive for direct integration in LCD without a separate

polarizer layer. [ 36 ]

2.4. Structural Color from Other Plasmonic Nanostructures

Besides the subwavelength hole arrays and MIM nano-

resonantors, there are several other relevant plasmonic

nanostructures capable of color fi ltering in the visible spec-

trum. For example, the wavelength-fi ltering principle of the

subwavelength hole arrays can be easily developed to the

2 © 2011 Wiley-VCH Vwileyonlinelibrary.com

Figure 4 . Structural color from a subwavelength bull’s eye structure anstructure in a 300 nm-thick Ag fi lm on a glass substrate. b) Transmissifor groove periods varying from 400 to 800 nm. c) Schematic of the me(d) and spectra (e) for three square arrays of metal resonant waveguide gand 450 nm, respectively, with 0.25 duty-cycles. The thicknesses of the respectively. a,b) Reproduced with permission. [ 39 ] Copyright 2008, NPG.

structures where single aperture is surrounded by the peri-

odic grooves, i.e., a bull’s eye structure, as shown in Figure 4 a.

Here the whole structure acts as an antenna to effi ciently

couple the incident light into surface-plasmon modes at a

resonant wavelength, which is also determined by the groove

period as in Equation 1 for the normal-incidence illumina-

tion. This make the electromagnetic fi elds at the surface

become intense above the aperture, resulting in extremely

high transmission effi ciency for the hole at this resonant

wavelength. [ 37 , 38 ] In other words, arbitrary color in the visible

spectrum can be obtained only by tuning the groove period,

as shown in Figure 4 b. [ 39 ] Although the transmission spectrum

also can be modulated by the other structural parameters,

such as groove depth, width, and hole shape, [ 40 ] controlling

groove period is the simplest and most effi cient way.

Another relevant structure is the metallic resonant

waveguide grating (MRWG) [ 41–44 ] (Figure 4 c), which is


d the metal resonant waveguide grating. a) SEM image of the bull’s eye on spectra for bull’s eye structures with the same dimensions as in (a) tal resonant waveguide grating. Normal incidence transmission images ratings for blue, green, and red colors. The grating periods are 300, 350,

silica buffer layer and silicon nitride waveguide layer are 50 and 100 nm,


derived from the dielectric resonant waveguide grating

(DRWG). DRWG is an optical element which consists of

a dielectric grating and waveguide structure that allows

light diffracted by the grating to couple into the waveguide

modes. [ 45 , 46 ] Because the dielectric medium is always loss-

less and transparent, transmission spectrum of the DRWG is

notch-type with extremely narrow resonance band, typically

smaller than 1 nm. This characterization makes it very useful

in the signal processing and biosensing applications. However,

in the MRWG structure, the subwavelength grating is made

of metal rather than dielectric medium and therefore it can

excite the surface plasmons and couple with the waveguide

modes to realize color fi ltering. In contrast to DRWGs, the

MRWG structure has a wider resonance bandwidth due to

the metal loss, about several tens of nanometers. Such band-

width is suitable for the purity of the fi ltered color required

for optical applications, as shown in Figure 4 d,e. Moreover,

the resonance bandwidth in MRWG fi lter can be tuned by

changing the thickness of the buffer layer, which would fur-

ther increase its practicality. [ 47 ]

Besides the propagating surface plasmons as discussed

above, the localized surface plasmon (LSP) is another typ-

ical type of collective electron density oscillation found in

isolated metal nanostructures, which has been studied exten-

sively over the past decade due to its potential utility as the

backbone for a number of photonic technologies capable of

controlling light at nanoscopic dimensions. [ 48 , 49 ] Numerous

theoretical and experimental studies have established that

the localized surface plasmon resonance (LSPR) is deter-

mined by the shape, size, interparticle distance, dielectric

environment, and material composition of the constituent

nanoparticles. [ 50–54 ] For example, Figure 5 shows that light

absorption and scattering control over the whole visible

band can be easily obtained by tuning the thickness of an

array of Au nanosquares. [ 55 ] From this fi gure, it can be clearly

seen that there is obvious color variation with different

Au nano squares’ thickness. The shape, size, and spacing of

the nanosquares can be well defi ned by using a large area

nanoimprint mold derived from two 1D gratings with desired

linewidth and period, the details of which can be found in

reference. [ 55 ]


Figure 5 . Structral color from LSPR effect. a) Photograph of a Au samdistinct areas of structures that differ in metal thickness as indicatedmicrograph of the nanosquare array is shown as an inset. Scale bar: 35nanoparticle array with varying nanostructure height, demonstrating ligthe whole visible band. Reproduced with permission. [ 55 ] Copyright 2008

3. Structural Color from Carbon Nanotubes and Generating Perfect Black

Although most of the previous plasmonic nanostructures

are capable of producing various colors at visible frequency,

they work at resonance condition and thus provide relatively

narrow bandwidth. Carbon nanotubes (CNTs), allotropes of

carbon with a cylindrical nanostructure, [ 56 ] are an alternative

type of conductive optical material for structural color appli-

cations. Actually, the optical properties of CNTs have been

extensively studied in recent years, specifi cally on absorption,

photoluminescence, and related topics for broadband applica-

tions. [ 57 , 58 ] In this section, we will review the recent progress

of enhanced color saturation and the ‘perfect black’ gener-

ated by CNTs.

Since the 1960s, it was well known that anodic alu-

minum oxide (AAO) fi lm can produce a bright color in

the visible-light range due to the interference of light. [ 59 ]

The color is bright but its saturation is very low. To make

the color pure and highly saturated, colored substances

have to be infi ltrated into the AAO fi lm nanochannels, but

they suffer from fading. Recently, it was demonstrated that

when a thin AAO fi lm is uniformly coated with carbon by

chemical vapor deposition (CVD), the CNT-coated fi lm

exhibits a vivid and tunable interference color with much

higher saturation than the pristine AAO fi lm. [ 60 ] Figure 6 a

shows the schematic of the pristine AAO fi lm and the CNT-

coated one, and Figure 6 b shows their corresponding

photographs with pattern consists of three partly over-

lapped circles. Results show that the refl ectance of the

carbon-coated AAO fi lms is lowered in such a way that

the saturation of interference color is much enhanced,

namely, the interference color becomes purer with the

carbon coating. As colors of the CNTs and AAO composite

thin fi lms are mainly determined by the interference band

with the maximum refl ectance in the visible region, color

tuning of the CNTs and AAO composite thin fi lms can be

attained by varying the AAO fi lm thickness in the anodiza-

tion process. In addition, by applying wet chemical etching

to different areas of the AAO fi lm for different durations,

different colors can be produced from the patterned CNTs

bH & Co. KGaA, Weinhei

ple possessing four by the labels. SEM 0 nm. b) Au square ht extinction across , Wiley.

and AAO composite thin fi lm. [ 61 ] Such

beautiful CNT-coated AAO fi lms sup-

ported on an Al substrate are also useful

for decorative purposes because they

are dye-free and there is no problem of

fading. The high-throughput fabrication

of AAO fi lms and improved mechanical

stability should be addressed in future

research for practical applications.

Another important color produced by

the CNT forest is ‘perfect black’, which is

diffi cult to obtain through other methods.

In 2008, it was discovered that a thin

coating comprised of low-density arrays

of loosely vertically-aligned carbon nano-

tubes, absorbs more than 99.9% percent of

light over the entire visible frequency. [ 62 ]

The key to this perfect black was to

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Figure 6 . Brilliant and tunable color from carbon-coated thin anodic aluminum oxide fi lms. a) Schematic of the pristine AAO fi lm and the carbon-coated one. b) A photograph of the pristine patterned AAO fi lm (left) and its carbon-coated one (right). The pattern consists of three partly overlapped circles. Reproduced with permission. [ 60 ] Copyright 2007, AIP.

create a long and extremely porous vertically aligned carbon

nanotube array with certain surface randomness, therefore

minimizing light refl ection and maximizing absorption simul-

taneously. It’s interesting to note that the refl ectance of such

nanotube arrays are two orders of magnitude lower than that

of the glassy carbon, which is remarkable because both sam-

ples are made up of the same element. [ 63 , 64 ]

The origin of this perfect black is from its sparse nanos-

tructures that give an effective refractive index very close to

that of air. The impedance-matched interface allows light to

enter the CNT forest without signifi cant refl ection or scat-

tering, and the conductive nature of the CNTs causes com-

plete absorption of the entered light. Figure 7 a shows the

SEM image of a low-density, vertically aligned multiwalled

CNT forest carpet, whose volume ratio is about 1% with

average CNT diameter of 10 nm and spacing of 100 nm

that grown by plasma-enhanced chemical vapor deposition

(PECVD) process. The complex refractive index is retrieved

using a multioscillator model to fi t both experimental refl ec-

tion and transmission data simultaneously, [ 65 , 66 ] where

Kramers–Kronig constrained variational analysis is used to

ensure the consistence of the retrieved refractive index. The

obtained real and imaginary parts of the refractive index are

shown in Figure 7 b, which exhibits considerable uniform value

in the entire visible band (e.g., n eff = 1.040 + i 0.01 at 633 nm),

and therefore unambiguously demonstrated its extremely

low effective refractive index at broadband frequency range,

which agrees well with effective medium theory of e eff = fe CNTs (1-f)e + Air where f is the volume ratio of CNTs. [ 67 ] As

a result, the refl ection at air and CNT forest interface is less

than 0.1% for a suffi ciently thick CNT forest. Meanwhile, the

absorption inside the CNT forest is very effective. For 30 μ m

thick CNT carpet, the absorption is more than 99.5%; it is

also easy to grow a CNT forest with a thickness of hundreds

of micrometers.

4 © 2011 Wiley-VCH Verlag GmbH & Co. KGaAwileyonlinelibrary.com

Finally we point out an interesting

application by exploiting the perfect

absorption characteristics of a low-

density CNT forest. By coating such a

perfect black material on an arbitrary

shaped object, the 3D object will appear

as a 2D black sheet and all the geometric

information disappears. In this case the

CNT forest acts as a perfect magic black

cloth that can completely conceal the 3D

structure of the object. [ 68 , 69 ] As a dem-

onstration, an arbitrarily shaped object

was fabricated on a 500 μ m thick silicon

substrate by focused ion beam (FIB)

milling. In this case a tank pattern of

65 μ m × 22.5 μ m in size (SEM image in

Figure 7 c) was made and its refl ection

image was taken under an optical micro-

scope illuminated by unpolarized broad-

band visible light (Figure 7 f). To cover

the object with the perfect black CNT

forest, a 60 μ m-thick CNT forest was

grown on top of the whole silicon sample

and therefore follows the profi le of the

original tank object (Figure 7 d). To get the CNT carpet, fi rst

a 300 nm-thick SiO 2 layer is deposited on the silicon sample

by plasma-enhanced chemical vapor deposition, and then

1 nm-thick Fe catalyst layer is deposited by electron beam

evaporation. The sample is loaded in a single-zone tube fur-

nace, which is heated to 775 ° C under the gas mixture of

C 2 H 4 /H 2 /He. [ 70 ] An optical image of the object covered with

the CNT carpet was taken again. Figure 7 g shows that the

tank completely disappears and the surface looks exactly

the same as a fl at CNT sheet. As further proof, a control

experiment was performed where a rectangle mark around

the tank was made by FIB milling that removed the CNT.

The optical image now clearly shows the rectangle mark,

but the tank pattern inside the mark remains perfect black,

as shown in Figure 7 e,h. Since the incident visible light is

broadband and unpolarized, such perfect black from a CNT

forest is promising for various kinds of applications such as

effi cient solar-energy conversion, [ 71–73 ] photo-detectors, [ 74 , 75 ]

and display-related devices.

4. Discussion and Outlook

High resolution, slim dimension, and low power consump-

tion are always the invariable goals for the development of

display technologies. Typically, human eyes have a resolution

limit of about 80 μ m at 35 mm. [ 76 ] These plasmonic and carbon

nanostructures can be used to build colored ‘super-pixels’ that

are only several micrometers in the lateral dimension so that

much smaller than the resolution limit of the human eyes for

the application of ultrahigh resolution displays. Besides, they

have a longitudinal thickness that is about 1–2 orders of mag-

nitude thinner than that of chemical pigment ones, which can

further reduce the device dimension and thereby increase

the integration density. Also, color fi lter consisted by the 1D

, Weinheim small 2011, 7, No. 22, 3128–3136


Figure 7 . Perfect black from a carbon nanotube forest. a) SEM image of carbon nanotube forest’s cross section. b) Experimentally retrieved refractive index of CNT forest from measured transmission and refl ection spectrum. c) SEM image of a 65 μ m × 22.5 μ m tank pattern fabricated by FIB on a silicon substrate (image (a), taken at a tilt angle of 45 degrees); with the whole tank sample surface covered by a 60 μ m-thick CNT forest (d); and with a rectangular mark around the tank by removing the rectangular CNT layer using FIB (e). The corresponding optical refl ection images taken under broadband visible illumination of the fabricated tank object (f), CNT forest covered tank sample (g), and the rectangular mark surrounding the tank (h).

metallic grating structures can act as the polarizer simultane-

ously, which could not only benefi ts the applications in LCD

by eliminating the need of a separate polarizer layer, but

also can be used for extracting polarimetric information in

spectral imaging. [ 35 , 39 ] Furthermore, the metal and carbon in

nanostructures can be directly used as electrodes and thereby

offers better electro-optic modulation [ 34 ] and photovoltaic

self-powering [ 31 , 71–73 ] in the system.

Although the plasmonic and carbon nanostructures have

these intrinsic advantages which can potentially fulfi ll the

requirements for the development of display technology,

there is still a long way to go before fi nal commercializa-

tion. The next step for the research on the nanostructured

color devices will mainly focus on two aspects: fi rst, the fur-

ther improvement of their optical performances, including

the transmission and refl ection effi ciencies, the purity of fi l-

tered color, and the incident angle-independency. Second,

the development of a more effi cient, high-throughout nano-

fabrication method to realize mass production with low costs.

© 2011 Wiley-VCH Verlag Gmbsmall 2011, 7, No. 22, 3128–3136

We believe these nanostructured color devices will open up

a colored future for the next generation high-resolution dis-

play, spectral imaging, and relevant applications.

Acknowledgements

The authors thank NSF, AFOSR, and ETR fund from the University of Michigan for the fi nancial support. T.X. acknowledges partial sup-port from China Scholarship Council (CSC).

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Received: May 31, 2011 Revised: July 1, 2011Published online: September 20, 2011


structural colors: from plasmonic to carbon nanostructures

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