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DOI: 10.1002/adma.200702624

Fabrication of Infrared Left-Handed Metamaterials viaDouble Template-Assisted Electrochemical Deposition**

By Hui Liu, Xiaopeng Zhao,* Yang Yang, Qingwu Li, and Jun Lv

Left-handed metamaterials (LHMs) have recently been the

focus of both scientific and engineering communities.[1,2]

Simultaneously possessing negative dielectric permittivity eand magnetic permeability m, LHMs exhibit unique electro-

magnetic properties compared with normal, right-handed

materials while still obeying Maxwell’s equation and not

violating known physical laws.[3] Although obtaining the e< 0

response was relatively easy, the realization of m< 0 response

beyond MHz has been a challenge, owing to the absence of

naturally occurring magnetic materials. In the late 1990s,

Pendry theoretically proposed that LHMs can be realized in a

composite materials form consisting of metallic wires and

split-ring resonators (SRRs) components.[4,5] It was shown that

a grid of thin conducting wires can produce a negative

permittivity near its plasmonic frequency, and that an array of

SRRs can produce a negative permeability in the vicinity of a

certain magnetic resonance frequency vm. Because the electric

resonance frequency of metallic wires and magnetic resonance

frequency of SRRs are mainly dependant on their structural

parameters, left-handed behavior in a given frequency can be

realized if the electric and magnetic resonance frequency drop

into the same region through adjusting the structural

parameters of the metallic wires and SRRs, respectively.

Since the first realization and verification of an artificial LHM

at microwave frequencies in 2000,[6] there have been numerous

studies on various aspects of LHMs, seeking their inner physics

and pursuing possible applications. At the same time, much

effort has been put into the fabrication and investigation of

LHMs at IR and visible light frequencies.[7–11]

The current fabrication of LHMs at IR and visible light

frequencies is mostly depended on physical lithography

(top-down approach), and it is difficult to obtain samples of

larger areas. Because the magnetic resonance frequency of

SRRs depends essentially on its geometrical parameters, LHM

at higher frequencies can be realized by straightforward scaling

of these parameters. Enkrich[12] fabricated an array of gold

SRRs with a 50 nm minimum feature size using standard

electron-beam lithography, and realized negative permeability

[*] Prof.X. P. Zhao, Dr.H. Liu, Y. Yang, Q. W. Li, J. LvDepartment of Applied PhysicsNorthwestern Polytechnical UniversityXi’an 710072 (P.R. China)E-mail: xpzhao@nwpu.edu.cn

[**] We acknowledge support from the National Nature ScienceFoundation of China under Grant No. 50632030, National BasicResearch Program of China under sub-project 2004CB719805 andNPU Foundation for Fundamental Research WO18101.

� 2008 WILEY-VCH Verlag

at 1.5mm wavelength. Unfortunately, it was found that this

scaling breaks down for yet higher frequencies for the SRR.

The reason is that the metal of which the SRR is composed

starts to strongly deviate from an ideal conductor. Further-

more, the combination of these SRRs with metal wires to form

3D structure is very challenging on the nanometer scale. Thus,

there was a hunt for alternative designs for higher frequencies,

such as pairs of metal wires or ‘‘double-fishnet’’ structures

separated by a dielectric spacer.[13,14] These structures

decreased the complexity of the structural cell and simplified

the fabrication process to a certain degree. Grigoreko[15]

fabricated antisymmetric pillar pairs with a height of 80 nm by

using high-resolution electron-beam lithography, and achieved

magnetic resonance at 500 nm. Dolling[16] fabricated silver

‘‘double-fishnet’’ structures with a 100 nm minimum feature

size using standard electron-beam lithography, and obtained

negative refraction at 780 nm. These studies approved the

principle of LHMs at higher frequencies, and promoted the

rapid development of LHMs at IR and visible light frequencies.

However, these top-down approaches based on physical

lithography require large apparatus, and the fabrication

process is complicated and expensive. Additionally,

as-prepared samples fabricated using these approaches are

only about 100mm2, and it is fairly difficult to obtain samples of

larger areas. All of the restrictions mentioned above

tremendously limited the extensive study and applications of

LHMs at IR and visible light frequencies.

In previous studies, our group designed a quasi-periodic

dendritic structure composed of hexagonal SRRs which could

provide a negative effective permeability, and experimentally

realized the subwavelength focusing effect with our LHM

composed of metallic dendritic cells and wire strips at

microwave region[17,18] We also fabricated copper dendritic

structure with negative permeability[19,20] and realized left-

handed behavior at IR frequencies using a composite medium

composed of copper dendritic structure and silver film.[21] The

dendritic structure has a fairly high level of symmetry, and

the geometry of the structure may be carefully designed at the

micro- or nanometer-level by using chemical methods, which is

favorable for the mass production and investigation of LHMs

at IR and visible light frequencies.

Here, we report a sandwich structural LHM of centimeter

area which is composed of an indium tin oxide (ITO) slice, zinc

oxide dielectric spacer, and silver dendritic cell array. Through

adjusting the chemical reaction parameters in the self-assembly

process, we could carefully control the geometry of the

dendritic structure and modulate its magnetic resonance

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Scheme 1. Schematic illustration of the fabrication of silver dendritic cell arrays. I) Fabrication of a 2Dpolystyrene (PS) colloidal crystal template, II) electrochemical deposition of ZnO nanoparticles, III) aporous ZnO template after removal of the PS microspheres, and IV) fabrication of the silver dendriticcell array.

frequency, and thereby realize left-handed behavior at infrared

frequencies. The flat focusing effect of this sandwich-structured

LHM further proved its left-handed property. Avoiding to

fabricate negative permittivity and permeability materials

separately and combining these two on the nanometer scale,

this method merely required the fabrication of a silver

dendritic cell array on a conducting substrate, which provides

a facile, lower-cost method to fabricate LHMs of larger areas at

IR and even visible light frequencies.

Figure 1. a,b) Optical and field-effect (FE)-SEM images of the surface topography of the colloidal crystaltemplate. c,d) FE-SEM images of the surface topography of the porous ZnO template after removal of thePS microspheres. e) FE-SEM image of the silver dendritic cell array. f) Higher-resolution image of a single Agdendritic structure.

The overall procedure of

fabricating silver dendritic cell

array is represented in Scheme

1.[22] First of all, a polystyrene

(PS) colloidal crystal template

on the ITO substrate was

assembled by the film-transfer

technique.[23] Afterwards, a

porousZnO templatewas fabri-

cated by using the electroche-

mical deposition method.[24,25]

In this step, the diameter and

the depth of the bowl-like

structure of the ZnO template

were controlled by the deposi-

tion time. To obtain the

compact ZnO template, an

appropriate concentration of

Zn(NO3)2 and applied voltage

are also necessary. After depo-

sition of ZnO, the PS micro-

Adv. Mater. 2008, 20, 2050–2054 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhe

spheres are dissolved in CH2Cl2, and

the porous ZnO template is obtained.

Finally, silver dendritic structures are

synthesized through the electrochemi-

cal deposition method in the center of

the bowl-like structure of the ZnO

template. In order to obtain an appro-

priate silver dendritic structure, poly-

glycol (PEG)-20000 is used as disper-

sing and structure-directing agent.[26] In

this step it is important that the space

between the anode and cathode

decreases to a certain distance. Other-

wise, the silver dendritic structures

would grow randomly on both the

surface and bottom of the bowl-like

structures of ZnO template.

Figure 1 shows optical and scanning

electron microscopy (SEM) images of

the overall fabrication process. Figure

1a and b demonstrates a typical PS

monolayer template on ITO substrate.

In the center of Figure 1b, there is a

vacancy through which the PS mono-

layer template can be clearly identified.

Using this PS monolayer as the primary

template, we can obtain the porous ZnO template by the

electrochemical deposition, as reported in Reference [27].

Figure 1c and d shows SEM images of the as-prepared ZnO

template after removal of the PS template. The low-

magnification SEM image in Figure 1c indicates that the

ZnO template with large domain periodicity is obtained.

Figure 1d shows that the diameter of the bowl-like structures is

fairly uniform in comparison with that of the PS microspheres.

Because no calcination is involved, the surface of the ZnO

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Figure 2. FE-SEM images of the samples fabricated at different PEG-20000 concentrations. a–f) PEG-20000 concentrations of 0mM, 0.6mM, 1.2mM, 2.4mM, 3.0mM, and 3.6mM, respectively.

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template composed of small ZnO nanocrystals is tremendously

smooth. Figure 1e and f shows typical results for silver dendritic

cell arrays deposited in the ZnO template. It can be seen that a

silver dendritic structure appears at the bottom of each

bowl-like structure. Outside the bowl-like structures, there are

no silver dendritic structures or particles. Particularly, in the

vacancies that are not occupied by ZnO, indicated by the white

arrows in Figure 1e, there are no silver particles or dendritic

structures. This shows that the formation of the dendritic

structure exhibits a location-selective property. The magnified

SEM image in Figure 1f proves that this dendritic structure

begins to grow from the center of the bowl-like structure, and

gradually filled in the whole bowl-like structure along its

internal wall. Most of these dendritic structures have a

two-level structure, with a whole size near 1.5mm and branch

Figure 3. a) Optical image of an as-prepared sandwich-structured sample. b) Transmission spectra ofdifferent silver structural arrays. Curves a–f corresponded to the different silver structural arrays exhibitedin Figure 2a–f, respectively. c) Curves a–c represent the transmission spectra of the copper dendriticstructure, silver dendritic cell array, and porous ZnO template, respectively.

width of about 80 nm. The low-

er-resolution image in Figure 1e also

indicates that all of these dendritic

structures present an order hexago-

nal periodicity as the result of the

localized effect of ZnO template.

Similar to SRR, the magnetic

resonance frequency of the dendri-

tic structure is closely related to its

structural parameters.[18] To obtain

the magnetic resonance in the given

frequency, such as IR frequencies

in this case, one should design the

dendritic structure with suitable

structural parameters. Fortunately,

we can easily control the structural

parameters of the silver dendritic

structure by adjusting the concen-

tration of PEG-20000 during the

electro-deposition process. Accord-

ing to the diffusion-limited aggrega-

www.advmat.de � 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tion (DLA)model of fractal growth,

the formation of a metal dendritic

structure is thought to be associated

with a diffusion-limited particle

aggregation process,[28] that is, small

particles could grow onto large

particles via a process known as

Ostwald ripening, and then these

large particles further grow into

dendritic structures with wider

branches. In this case, as a result

of the stronger steric hindrance

and complexation effect of the

PEG-20000, Agþ is localized on

the PEG chain-like structure, thus

the reductive rate of Agþ and

aggregation of silver particles are

reduced. Furthermore, as a tem-

plate agent, Agþ is absorbed to the

PEG chain-like structure and forms

many 1D action locations, the number of PEG chain-like

structures increased at the given volume along with the

increase of the concentration of PEG-20000, and the branches

of silver dendritic structure are getting more and more

exiguous. Figure 2 shows SEM images of the silver nanos-

tructures fabricated at different concentrations of PEG-20000.

Figure 2a demonstrates that metal particles with a diameter of

about 1mm are obtained when PEG-20000 is absent in the

electrolyte. When the concentration of PEG-20000 is 0.6mM,

1.2mM, 2.4mM, and 3.0mM, respectively, silver dendritic

structures are obtained in the ZnO bowl-like structure (Fig.

2b–e), and the branches of the dendritic structure get more and

more exiguous with increasing PEG-20000 concentration.

However, only the silver nanoparticles with the diameter about

30 nm can be achieved when the concentration of PEG-20000

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increased to 3.6mM (Fig. 2f). Thus, the geometry of the

dendritic structure can be fine-tuned by changing the

concentration of PEG-20000, which is beneficial to the

realization of magnetic resonance in the given frequencies.

The final products are 1� 2 cm2 microchips consisting of an

indium tin oxide slice, zinc oxide dielectric spacer, and the

silver cell array, as shown in Figure 3a. The transmission

spectra of the samples with different structures were measured

by Fourier-transform IR spectrophotometry with an unpolar-

ized beam. Figure 3b shows transmission spectra of the six

different silver structural arrays exhibited in Figure 2.

When the concentration of PEG-20000 is 0.0 and 0.6mM

respectively, the as-prepared silver particle or unobvious

dendritic cell array exhibit no obvious response in the given

frequencies (curves a and b in Fig. 3b). The transmission

spectrum shows a clearly pass band with a relative magnitude

about 6% near the 1.85mm region when the concentration of

PEG- 20000 change to 1.2mM (curve c). In comparison, there is

no obvious response when the concentration of PEG-20000 is

increased to 2.4mM (curve d) although the silver dendritic

Figure 4. a) Schematic illustration of the experimental setup for flat lens focusing. b) Measured result offield amplitude at 1.85mm wavelength with the IR acceptor moving along the x direction. c) 3D view ofschematic (b). d) Measured intensity distribution along the transverse (Y) at the image plane (x¼ 20mm)with and without the silver dendritic cell array.

structure is also obtained. In spite of

the silver dendritic structure with

more exiguous branches that is

achieved when the concentration of

PEG-20000 is 3.0mM, the transmis-

sion spectrum of the sample (Fig. 2e)

presents a stop band (curve e) in

the given frequencies. Similarly, the

sample in Figure 2f also shows a stop

band at longer wavelength (curve f).

Obviously, through selecting appro-

priate concentrations of PEG-20000,

the silver dendritic cell array can

clearly present a pass band in the

given frequencies.

In a previous study,[21] we fabri-

cated a copper dendritic structure on

a filter paper substrate which exhib-

ited a stop band at 1.92mm wave-

length, shown as curve a in Figure 3c.

Furthermore, the combination of this

dendritic structure with the silver film

presented a pass band and realized

left-handed behavior in the same

region. Utilizing parameter design,

our group has theoretically and

experimentally proven that a 2D

isotropic LHM based on dendritic

cells, which simultaneously provided

negative permittivity and per-

meability, can exhibited a distinct

left-handed effect in the microwave

region.[18,29] Similarly, in this case, we

have prepared the silver dendritic cell

array on the ITO substrate using

double template-assisted electroche-

Adv. Mater. 2008, 20, 2050–2054 � 2008 WILEY-VCH Verla

mical deposition and have directly obtained a left-handed pass

band (Curve b in Fig. 3c) through adjusting the dendritic

geometry. This fabrication process will provide a facile,

lower-cost method to fabricate LHMs of larger areas at IR

and even visible light frequencies.

To further prove the left-handed properties of the pass band

in this case, an experiment is performed to check the possibility

of slab focusing by our sandwich-structured LHM. Figure 4a

shows a schematic picture of the focusing experimental setup.

An IR light source with the light spot of about 150mm in

diameter is employed as a source. The power distribution of the

transmission light through the sandwich structural lens with an

area of 2 cm2, is recorded by the IR acceptor along the x and y

directions. Themeasurement results at 1.85mmwavelength are

shown in Figure 4b–d. When the distance between the source

and LHM lens is 1mm, at the other side a clear point image was

focused near the lens (Fig. 4b and c), and the distance between

the LHM lens and focusing point with maximum light intensity

is about 20mm. Figure 4d exhibits that no focusing effect is

observed when the silver dendritic cell array is not prepared.

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In conclusion, we have fabricated a sandwich-structured

LHM covering a centimeter area, composed of an indium tin

oxide slice, zinc oxide dielectric spacer, and silver dendritic cell

array,by using a double template-assisted electrochemical

deposition method. Through adjusting of the concentration

of PEG-20000, the geometry of dendritic structure can be

carefully controlled at the 10 nm level, and the magnetic

resonance frequency of silver dendritic cell array can be

modulated easily. When the concentration of PEG-20000 is

1.2mM, the caliber andwhole thickness of the as-prepared ZnO

bowl-like structure cell are 2.2mm and 0.8mm, respectively.

The whole size of the as-prepared dendritic structure is 1.5mm

with a branch width of about 80 nm. Transmission results show

that the sandwich structure presents a pass band at 1.85mm.

Focusing experiments confirm that a clear image appeared at

the other side of the sandwich-structured LHM lens, and that

the distance between the lens and the point with maximum

light intensity is about 20mm. Avoiding to fabricate negative

permittivity and permeability material separately and even

combine two of them on the nanometer scale, this method

merely require fabrication of silver dendritic structural array

on conducting substrate, providing a facile, lower-cost method

to fabricate LHMs of larger areas at IR and even visible light

frequencies.

Experimental

Polystyrene (PS, 2.1mm in diameter) suspensions were synthesizedby using an emulsifier-free emulsion polymerization technique. Glasssubstrates were cleaned according to previously published procedures[30]. A 1� 2 cm2 sized ordered colloidal crystal monolayer wasprepared on the ITO substrates by a film-transfer technique. Theelectrolyte used for the synthesis of the porous ZnO template was a0.2mM Zn(NO3)2 aqueous solution. The PS monolayer-covered ITOsubstrate and a cleaned zinc sheet were used as working and auxiliaryelectrode, respectively. The distance between the working electrodeand auxiliary electrode was about 3 cm. Electro-deposition was carriedout at 70 8C at a dc voltage of 1.2V. After deposition, the PSmonolayerwas removed by dissolution in CH2Cl2, and the porous ZnO templatewas obtained and used for the following process. The electrolyte, whichwas used for the synthesis of the silver dendritic structure, was amixture of an aqueous solution of AgNO3 and PEG-20000. The porousZnO-covered ITO substrate and a cleaned silver sheet were used asworking and auxiliary electrode, respectively. The distance betweenthe working electrode and auxiliary electrode was about 0.5mm. Theelectro-deposition was carried out at 3 8C with a dc voltage of 0.8V.After deposition, the as-prepared sample was cleaned in distilled waterand dried with N2, and the silver structural array was obtained [22].

The as-prepared samples were examined by field-emission scanningelectron microscopy (FE-SEM, JEOL-6700F). IR transmissionspectra were recorded at room temperature on an FT/IR-400plusFourier-transform IR spectrophotometer in the wavelength range of1.2–2.6mm. The focusing experiment was performed on a home-builtfocusing apparatus. In the focusing experiment, the wavelength regionof infrared source ranged from 0.8 to 3.0mm, and the infrared acceptorfrom 0.8 to 2.8mm. The distance between the infrared acceptor and

www.advmat.de � 2008 WILEY-VCH Verlag GmbH

LHM lens was controlled by a 3D micropositioner; the shift-step was5mm in the measurement.

Received: October 19, 2007Revised: January 3, 2008

Published online: April 22, 2008

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