21 - City University of Hong Kong

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Library of Congress Cataloging-in-Publication Data:

Zhai, Tianyou, 1980-0ne-dimensional nanostructures: principles and applications I Tianyou Zhai, Jiannian Yao.

p. cm. Includes bibliographical references and index.

ISBN 978-1-118-07191-5 (hardback) I. Nanowires. 2. Semiconductors-Materials. 3. One-dimensional conductors. 4. Nanostructured materials. I. Yao Jiannian, 1953- II. Title. TK7874.85.Z43 2013 621.3815-dc23

Printed in Singapore

10 9 8 7 6 5 4 3 2 1

2011049797

21 ONE-DIMENSIONAL NANOSTRUCTURES IN PLASMONICS

XuEFENG Gu AND TENG Qrn

Department of Physics, Southeast University, Nanjing, China

PAULK. CHU

Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, China

21.1 INTRODUCTION

Nanotechnology has been a hot (i.e., an important and controversial) research area for more than 10 years, and rapid development has enabled scientists and engineers to design and synthesize various types of nanostrudures by self-assembly, electron beam lithography, and nano-imprint lithography. One-dimensional (lD) nanostructures, which are defined as structures with two of the dimensions on the order of nanometers or hundreds of nanometers, represent one of the most important classes of materials; examples are nanowires, nanorods, nanotubes, and nanobelts. These nanostructures have attracted immense interest because of their unique and novel properties as well as ease of fabrication.l1- 6l In particular, lD nanostructures play important roles in plasmonics and photovoltaics, and in this chapter, the synthesis and application of common lD nanostructures in these two important areas are reviewed.

Plasmonics, first described in the early 1900s is concerned with the interaction between electromagnetic (EM) radiation and conduction electrons at metallic-dielectric interfaces and has been a hot research area since 2000 following some discoveries in the microwave and terahertz regions. Mainly designated as surface plasmon polaritons (SPPs) and localized surface plasmon polaritons (LSPs), plasmonics-based methods have found spurs many intriguing applications such as ultrafast chips, surface-enhanced

Raman scattering (SERS), and enhanced or quenched fluorescence. SPPs are surface-bound EM waves coupled to electron density oscillations and confined to the metallic interface well below the diffraction limit >...0 /n (where >..0 is wavelength in the vacuum and n is the refractive index of the surrounding environment), thereby overcoming the size incompatibility between optical devices and current microfabrication techniques. [7,s1 It is potentially useful in nextgeneration ultrafast chips that may operate at a speed that is many orders of magnitude greater than that of current ones. LSPs involve, on the contrary, a nonpropagating (localized) excitation of conduction electrons in subwavelength nanostructures when illuminated by electromagnetic (EM) waves. A resonance can arise due to the restoration force exerted by the curved surface of the nanostructures. At the resonant frequencies, the electric fields are highly confined in a very small volume, resulting in significantly enhanced local fields. The enhanced fields interact with near-field molecules and phosphors, thus significantly enhancing the Raman scattering signals and fluorescence.[9- 111 Since the principle of plasmonics-enhanced Raman scattering (or SERS) also applies to fluorescence, we will only discuss SERS in this chapter (Section 21.3). Many nanostructures have been found to support the SPP and LSP modes, and so there are a broad range of potential applications.

In photovoltaics, solar cells convert sunlight to electricity, and the current drive for clean, sustainable energy has prompted extensive research in this areaP2l At present,

One-Dimensional Nanostructures: Principles and Application, First Edition. Edited by Tianyou Zhai, Jiannian Yao. © 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

455


most commercial solar cells are based on crystalline silicon, which does not offer a really high conversion efficiency and is more expensive than some competing materials. Nanowires, nanotubes, and nanobelts have been used in novel solar cells and appear to be alternatives in producing cheaper devices. This will be discussed in Section 21.4.1. In addition, incorporation of metallic ID nanostructures, that is, plasmonic ID nanostructures, to enhance photovoltaics via light scattering and trapping, will be discussed in Section 21.4.2.

21.2 lD PLASMONIC WAVEGUIDES

In the context of plasmonic waveguides, a tradeoff between propagation length and light confinement always exists. Thus, how to design waveguide structures with high confinement without substantially decreasing the propagation length is a major concern. Self-assembled materials such as nanorods have been used to study plasmon propagation, and when highly controllable waveguide functionality is required, more precise lithographic techniques such as electron beam lithography and nanoimprint lithography are called for. Furthermore, coupling between plasmonic waveguides and outside components, for example, external sources or optical fibers, is important to practical applications.

21.2.1 Tradeoff between Light Confinement and Propagation Length

Surface plasmon polaritons propagating along the metal! dielectric interface can be characterized with the propagation constant fJ and lie attenuation length into the dielectric Ilka given by[13l

fJ = ko 8m8d

8m + 8ct

k~ = {32 - k6sa

(21.1)

(21.2)

where k0 is the wavenumber in vacuum, 8ct is the permittivity of the dielectric, and sm is the frequencydependent complex dielectric function of the metal as described by the Drude model

())2

8m(w) = 8 00 - ~ w +iyw

(21.3)

where 8 00 is high-frequency bulk permittivity, y describes dissipation, and wp is the plasma frequency.

Detailed analysis of Eqs. (21.1)-(21.3) reveals that a large propagation length [small Img({J)] usually corresponds to weak light confinement [small Re(kd)].[1 3l Hence, if large light confinement is needed, the propagation length must be sacrificed. This conclusion is valid not only in the simple metal/dielectric configuration but also in most other circumstances. Hence, the tradeoff between propagation length and light localization is always a key issue when designing plasmonic waveguides. The appropriate geometry, materials, and operation frequency must be carefully chosen after comprehensively evaluating the resulting properties.

A number of configurations such as chains of nanoparticles, nanowires, and nanostripes have been utilized to study this tradeoffY4 - 16l Hybrid ID nanostructures have also been proposed to further optimize the propagation length without significantly compromising light confinement. In addition, nanowires have been exploited as the efficient coupling component in conventional photonic waveguides to incorporate plasmonic waveguides into chips. These topics will be reviewed in this chapter, but it should be noted that other waveguide configurations such as wedges, V grooves, and nanoslots[17 - 19J are not included because they are not ID nanostructures and thus are beyond the scope of this chapter.

21.2.2 Surface Plasmon Polariton (SPP) Propagation along Nanoparticle Chains

Maier et al. first reported experimental evidence of SPPs propagating along nanorod chains in 2003 _[1 5l Figure 21. I a

Dye

.. Intensity

. ~ ~ Si

f5 ~

(b)

._. Far-field detection

Figure 21.1 (a) SEM image of a 100 x 100 µm nanorod array cons1strng of Ag plasmon waveguides. The sample is fabricated by electron beam lithography and the liftoff technique. (b) Schematic experimental setup illustrating excitation by the tip of a near-field scanning optical microscope, plasmon propagation, and emission from the dye. (Reproduced from Nat. Mater. 2003, 2, 229. Copyright © 2003, Nature Publishing Group_ll5l)

shows a scanning electron micrograph (SEM) of the sample prepared by electron beam lithography and liftoff on ITOcoated quartz slides. The plasmon waveguides consist of Ag nanorods with a dimension of 90 x 30 x 30 nm and an intersurface- spacing of 50 nm between adjacent particles. The linearly aligned nanorods are excited locally by the tip of a near-field scanning optical microscope, and emission from a dye located hundreds of nanometers away is observed (Figure 21.lb). This phenomenon is attributed to near-field coupling between closely spaced nanorods. Despite considerable light confinement, the dissipation is significant. The propagation length of the EM energy is estimated to be as small as ~0.5 µm, which is not large enough for practical applications. Hence, research on the use of nanoparticles to guide SPPs has been hindered, and most research is now focusing on EM energy transport through nanowires or configurations based on nanowires.

21.2.3 SPP Propagation along Nanowires

Unidirectional plasmon propagation along metallic nanowires was reported by Dickson et al. as early as 2000.l16l Using template-directed electrosynthetic techniques, 20-nm-diameter Au and Ag nanorods are made with lengths ranging from 1 to 15 µm (Figure 21.2a). SPPs on these nanorods are excited via coupling to the evanescent waves at the glass surface when total internal refraction appears to match the wavevector with that of SPPs. Figure 21.2b,d depicts optical microscopic images of a 4.7-µm-long Au nanorods illuminated by 532 and 820 nm light, respectively. Although light is coupled to the SPP mode everywhere along the long axis of the nanorod, only at the input end is the coupling efficiency highest during 532 nm light illumination. When illuminated by 820 nm light, a high coupling efficiency is observed on both ends. However, a large coupling efficiency is observed from both ends of Ag nanorods during both 532 nm and 820 nm light illumination as shown in Figures 21.2c,e. Propagation of the frequency- and metal- dependent SPPs along the nanowires is expected, and it has also been demonstrated that unidirectional SPP propagation with a length of > 10 µm is significantly larger than the propagation length 0.5 µm in nanorod chains. However, the degree of light confinement in the vicinity of the nanowire is not discussed.

21.2.4 Hybrid Waveguiding Nanostructures

Many hybrid nanostructures exhibit large light localization useful to device integration with a reasonable propagation length. One good example is the structure reported by Oulton et al. in 2008.[2Dl This approach integrates plasmonics with dielectric waveguides. Here, a high-refractiveindex dielectric waveguide is separated from the metal

lD PLASMONIC WAVEGUIDES 457

surface by a nanoscale dielectric gap (Figure 21.3). The "capacitor-like" formed structure allows for subwavelength (ranging from A. 21400toA.2140) transmission in the dielectric regions and a propagation length between 40 and 150 µm. The significance of the work is that it is totally compatible with current semiconductor fabrication techniques and, although theoretical, their results can be readily verified by experiments.

As mentioned above, semiconductor plasmonics is of great interest because of the ease of fabrication. Dai et al. conducted a theoretical investigation on a siliconbased plasmonic waveguide with a metal nanocap.l21l At the telecommunication frequency (A.= 1550 nm), light is predicted to be confined well below the diffraction limit (50 x 5 nm) while the loss is relatively small so that the propagation length can reach several tens of wavelengths. Another example of a hybrid plasmonic waveguide is described by Wu et al.[221 This conductor-gap-silicon waveguide consists of a thin Si02 layer sandwiched between a gold layer and a Si layer. The waveguide is fabricated on a silicon-on-insulator substrate (340 nm silicon on I µm oxide). A 50-nm Si02 layer is first deposited by plasma-enhanced chemical vapor deposition (PECVD), and electron beam lithography is employed to define the plasmonic waveguides to a nominal width of 200 nm followed by deposition of Si and Au. With careful selection of geometric parameters, the propagation length can reach 40 µm, which is large enough in interconnect applications. Again, the most desirable feature is the compatibility with complementary metal oxide semiconductor fabrication processes.

21.2.5 Enhanced SPP Coupling between Nanowires and External Devices

Knight et al. have proposed that nanoparticles can serve as nanoantennas to couple visible light to SPPsP3l The Ag nanowires prepared by a room temperature chemical technique[24l have a mean diameter of 220 nm and lengths ranging from 1 to 25 µm. Visible light is focused onto individual nanowire-nanoparticle junctions to excite propagating plasmons. However, this coupling configuration requires light focusing, which is difficult to realize under many circumstances in which direct coupling between SPPs and light sources is necessary.

Akimov et al. have demonstrated efficient coupling between CdSe quantum dot emitters and SPPs on Ag nanowires,l25l which is schematically illustrated in Figure 21.4. The Ag nanowires with a thickness of 102 ± 24 nm are prepared by a solution-phase polyol method with modifications for surface passivation. The samples are produced by spinning chemically synthesized quantum dots onto a glass substrate, covering them with a 30-nm-thick layer of polymethylmethacrylate (PMMA),


Figure 21.2 (a) Photograph of 20-nm-diameter Au and Ag rods. The two labeled rods are typical of those used in experiments. (b) Optical microscopic image of a 4.7-µm-long Au rod exposed to through-prism total internal refraction illumination at 532 nm. Scattering at the rod input is strong but absent at the distal tip [scale bar= 1 µm in (b)-(e)]. (c) Image of the same Au rod under total internal refraction illumination at 820 nm. (d,e) A 4.7-µm-long Ag rod illuminated at 532 and 820 nm, respectively. (Reproduced from J. Phys. Chem. B 2000, 104, 6095. Copyright © 2000, American Chemical Society.l16l)

and then depositing dry wires on top. Finally, the sample is coated with a thick layer of PMMA. The nearest distance between Ag nanowires and CdSe quantum dots, which is determined by the thickness of the PMMA layer and quantum dot shell radius, is about 35 nm. Coupling of emission from quantum to SPP mode is accomplished by light that is reemitted from the nanowire ends. A unique feature of this coupling configuration is that by optimally controlling the separation between the quantum dots and nanowires, spontaneous emission from quantum dots can

be enhanced considerably. This piece of work in enhanced source-plasmon coupling is of importance to SPP-based signal processing.

With regard to SPP coupling to dielectric photonic waveguides, unlike traditional methods, which can excite SPPs on only one nanowire at a time,£26 - 281 Pyayt et al. have successfully integrated multiple Ag nanowires with polymer optical waveguides for nanoscale confinement and guiding of light on a chip by placing nanowires perpendicularly to the polymer waveguide with one end

1D NANOSTRUCTURES IN SURFACE-ENHANCED RAMAN SCATTERING 459

Figure 21.3 Hybrid optical waveguide in which the dielectric (GaAs, cc = 12.25) cylindrical nanowire is separated from a metallic half-space of the metal (silver, cm= -129 + 3.3 i) by a nanoscale dielectric gap (Si02 , cd = 2.25). All values are at the telecommunications wavelength of 1550 nm. (Reproduced from Nat. Photonics 2008, 2, 496. Copyright © 2008, Macmillan Publishers Limited. [lOJ)

Figure 21.4 Radiative coupling between quantum dot emitters and conducting nanowires. A coupled quantum dot can spontaneously emit either photons into free space or guided surface plasmons on the nanowire. The key is to control the respective rates. (Reproduced from Nature 2007, 450, 402. Copyright © 2007, Nature Publishing Group_l25l)

close to the maximum light intensity inside the polymer (Figure 21.5).l29l Theoretical simulation demonstrates that light can be coupled to multiple nanowires from the same waveguide and changing the light polarization results in different coupling efficiency. Ag nanowires have been produced by adjusting the concentrations of Fe(II) and Fe(III) in the polyol reduction of silver nitrate_[3DJ The fabrication process involves spin coating and curing of a 5-µm solgel layer on a silicon wafer, depositing a diluted solution containing Ag nanowires on the solgel, rinsing excess solution, and spin coating SU-8 onto the nanowire. It is then prepattemed to ensure that one part of the Ag

Figure 21.5 The authors' approach, which couples light to a nanowire by guiding light into the plane of the substrate with a polymer waveguide. The end of the nanowire inside the polymi:t is close to the maximum light intensity to facilitate coupling. (Reproduced from Nat. Nanotechnol. 2008, 3, 660. Copyright e 2008, Macmillan Publishers Limited.[291)

nanowires is embedded in SU-8 but the residual part is not. Experimental observation confirms the simulation results and corroborates the superiority of the design over other coupling configurations concerning parallel coupling of light into various nanowires.

21.3 lD NANOSTRUCTURES IN SURFACE-ENHANCED RAMAN SCATTERING

21.3.1 Surface-Enhanced Raman Scattering

When a photon with energy h vL interacts with a molecule, its energy is shifted by the characteristic energy of the molecular vibration h vM via inelastic scattering. If the molecule is in the ground state, the photon energy is reduced and a vibrational mode is excited; otherwise the photon can gain energy while the molecule undergoes a transition to a lower energy state. After scattering, two spectral lines located symmetrically about the incoming photon frequency can be observed, one at the frequency vL - vM (Stokes scattering) and the other at vL + vM (antiStokes scattering). This scattering effect is known as Raman

scattering. Raman scattering reveals many characteristics of a substance not easily measurable by direct methods and has found various applications in spectroscopy, sensing, and detection. Moreover, when a Raman active molecule is located close to noble metal nanostructures on a substrate, the Raman signal is enhanced, by a factor of "'1014 . One factor responsible for this enhancement is the modified Raman cross section, which contributes to


only a small part, of "'"'102 • Another more important factor is the so-called hotspots where the local electric field is drastically enhanced, resulting from excitation of LSPs. Theoretically, the contribution by the enhanced electric field is proportional to IE10cl4 /IE0 i4 , and as such, the Raman signal is greatly boosted. SERS has garnered increasing interest because of its high sensitivity and specificity. Most present research focuses on how to produce metallic nanostructures where molecules can be appropriately and predictably located in the hotspots. Anther hot topic is the production of reproducible devices. In this section, the fabrication and use of ID nanostructures such as nanowires, nanorods, and nanotubes in SERS are reviewed, but applications in related topics such as biosensing, imaging, and so on are not included.

21.3.2 Nanowires in Surface-Enhanced Raman Scattering

Lee et al. have reported a simple strategy to place analyte molecules in hotspots between closely spaced nanowires leading to intense SERS enhancement. There are highly reproducible positions for the analyte molecules in the junction between neighboring tips of Ag nanowires that are fabricated in porous aluminum oxide and then exposed by controlled dissolution of the alumina matrix.l31l We have performed Raman experiments using benzenethiol as the probe in two ways:

1. Adding Analyte and Then Etching: In this former route, before etching and in the early etching stages, the benzenethiol-exposed Ag nanowires protrude only slightly above the alumina matrix and the SERS signal is weak. However, after etching for "-'270 s, a sufficient part of the nanowire is freed from its matrix to allow the tips of the nanowires to bend toward each other and form closely interacting bundles. By trapping and automatically positioning the analyte molecules preadsorbed on the tips of the nanowires

in the junctions between neighboring tips, the Raman signal is enhanced significantly. After etching for 450 s, the signal becomes weak again and finally disappears.

2. Etching and Then Adding Analyte: In this method, the maximum signal achieved is comparable to that obtained using method 1. The results suggest a promising future for highly controlled and intense SERS signal generation.

Another example of using nanowires to engineer hotspots can be found in the paper by Kang et al. [32l The freestanding single-crystalline Ag and Au nanowires synthesized by a vapor-phase method possess an atomically smooth surface and can be manipulated by a homemade nanomanipulator one by one to form crossed or parallel nanowire pairs_[33l The nanomanipulator consists of a tungsten tip with a diameter of "'"'100 nm on the end mounted on a 3D piezoelectric stage used to softly touch and transfer the as-synthesized nanowires to the glass substrate. Figure 21.6 depicts the optical microscopy and SEM images of a crossed nanowire pair. In order to obtain the Raman spectra from cresyl blue, 10 µ,L of 10-4 M ethanol is spincoated on the nanowire cross system and a 514.5-nm argon laser with a power of 150 µ, W is employed to excite samples. Figures 21.7a-c displays optical microscopic images of the Ag nanowire cross when the green laser spot is focused on the crossing point of two nanowires, a single nanowire, and the glass substrate, respectively. The corresponding Raman spectra are shown in Figure 21.7d. The observation that the Raman signal is significantly enhanced only when the laser is focused on the cross junction provides unequivocal proof of the importance of the hotspots in this system. The results correlate well with finite-difference time-domain simulations.

Fabrication of large-area (wafer-scale) nanowire-based SERS substrates have been reported by Vo-Dinh et alJ34l The first step involves deep ultraviolet lithography and chemical etching to form silicon nanowires over a 150-mm

Figure 21.6 (a,b) Optical microscopic images of the fabrication procedure of crossed Ag nanowires by using a nanomanipulator (details described in the text); (c) SEM image of the same crossed Ag nanowires. (Reproduced from J. Phys. Chem. C 2009, 113, 7492. Copyright© 2009, American Chemical Society.[32l)


(a)

------- ------- -- (b)

-----......... -------(c) 600 1200

Raman shift (cm-1)

1800

Figure 21.7 (a-c) Optical microscopic images of crossed Ag nanowires. The green laser is focused onto the crossing point of the Ag nanowire, single Ag nanowire, and glass substrate, respectively. (d) Corresponding SERS spectra of brilliant cresyl blue at the laser spots in (a-c). (Reproduced from J. Phys. Chem. C 2009, 113, 7492. Copyright © 2009, American Chemical Society. r3z1)

wafer. Atomic-layer deposition is then conducted to deposit a conformal hafnium oxide layer, and finally, the nanowire substrate is coated with a thin film of gold or silver by electron beam deposition. The SEM and transmission electron microscopic (TEM) cross-sectional images obtained from the fabricated triangle-shaped bare and gold-coated silicon nanowires are shown in Figure 21.8. The maximum intensity of the Raman signal increases as the spacing between silicon nanowires decreases and can be attributed to enhanced interwire coupling (Figure 21.8c). Other SERS substrates based on nanowires have also been reported, and they include nanowires combined with nanocubes,l35l

gold-coated ZnO nanowires,r36l and vertically aligned silicon nanowires. r37J Readers are encouraged to peruse recent papers in this area for more information.

21.3.3 Nanorods in Surface-Enhanced Raman Scattering

In the context of LSPs or EM-enhanced Raman scattering, nanorods are of interest because of their high aspect ratio, which is similar to that of nanowires. Common methods used to produce aligned nanorods include the use of anodic aluminum oxide (AAO) template and oblique-angle deposition (OAD).

Gu et al. have investigated the enhancement effects in adjacent Ag nanorods produced on the AAO template. r3s1

The template is prepared by a two-step anodization process that includes anodization of a clean aluminum sheet in 0.5 M sulfuric acid at l0°C and a constant applied

voltage of 25 V for 20 h after electropolishing, dipping the anodized sheet in an aqueous solution of phosphoric acid and chromic acid for 16 h at 60°C, a second anodization process for 3 min under the same conditions, and a pore-widening process. A voltage with a frequency of 200 Hz and amplitude of 16.60 V is applied to an ethanol solution containing 0.05 M AgN03 at 5°C to produce Ag nanorods in the pores. The resulting thin films are immersed in another ethanol solution containing 1.0 x 10-3 M benzenethiol (the probing molucule) for 20 h, washed with ethanol, and dried prior to the Raman measurement. The experimental procedures for the joint nanorods are the same. The single and joint Ag nanorods produce similar extinction spectra if their total lengths are the same. Maximum Raman scattering enhancement is observed from a junction system of two nanorods with the same size and a total length of 62 nm. It corresponds to the optimal length of a single Ag nanorod at the 632.8 nm excitation laser wavelength. The experiments define the hotspots in the junction system and reveal that the enhancement factor is 3.9 x 109 , which is 4 times higher than that of a 62-nm single nanorod. We have also reported a convenient nanotechnique for fabrication of highly ordered Ag nanorods using the AAO template.r391

The fabrication process involves the growth of the AAO, immersion, and slight heating of the porous template in a saturated silver nitrate solution for about 30 min, drying at 80°C to evaporate the solvent, and heating of the saltfilled template at 500°C for 30 min to decompose the salt


B

--Gold pad region

40000 ~ I -e- 15 nm spacing between nanowires

-+-- 40 nm spacing between nanowires

30000

20000

10000

01----~-...~ ....... --~--~--~--+~--<,__~..,_~+-,----~

800 900 1000 1100 1200 1300 1400 1500 1600 1700 Raman shift (cm-1)

Figure 21.8 (a) Cross-sectional SEM showing fabrication of a lD array of triangular silicon nanowires; (b) cross-sectional TEM of a gold-coated silicon nanowire SERS substrate; (c) SERS signals from p-mercaptobenzoic acid (pMBA) molecules on goldcoated silicon nanowire substrates for different spacings between neighboring nanowires in the one-dimensional nanowire array. The SERS substrate chips are coated with pMBA molecules by dipping the chips in an ethanol solution containing pMBA. (Reproduced from J. Phys. Chem. C 2010, 114, 7480. Copyright © 2010, American Chemical Society.l34l)

into metallic silver (Figure 21.9). Employing rhodamine 6G (R6G) as the probing molecule, the Raman spectra are significantly enhanced as a result of interrod coupling, which produces highly localized and intense electric fields. The AAO membrane protects the Ag nanorods from contamination until use. Liao et al. filled the pores of the PAA membrane and the hotspots with gold. Owing to the good uniformity, AAO is one of the most promising SERS templates for producing highly ordered and reproducible SERS substrates.

Oblique-angle deposition (OAD) is another widely used method for producing sensitive and uniform SERS substrates.l40- 42J The size, shape, and density of the nanostructures can be controlled by adjusting the deposition conditions such as the vapor deposition angle, temperature,

duration, and rate of deposition. Driskell and coworkers have systematically investigated the optical properties, surface morphology, crystallinity, and Raman scattering enhancement of the Ag nanorod substrates prepared by the OAD technique. r43l The typical SEM images of the Ag nanorod arrays deposited on a bare glass slide and on 500-nm-thick Ag film are depicted in Figure 21.10. The nanorods are dispersed quite uniformly on the substrates. The TEM images of individual nanorods reveal that the nanorods are not perfectly cylindrical and lead to large Raman enhancement. Using trans-1,2-bis(4-pyridyl) ethene as the probe molecule at an excitation wavelength of 785 nm, the SERS response reaches a maximum of "-'5 x 108 when the length approaches 868 nm. The SERS response is fairly stable across the whole substrate with a relative standard deviation of 10%. Because of the stability of the substrate, no significant SERS intensity decrease is observed after storage in a vacuum bag for 2 months. A gold-coated Si nanorod array produced by OAD has been reported by Fan et al.f44l After Si nanorod deposition, the Ag thin film is deposited. This structure also contains a highly reflective coating before Si nanorods and Ag nanoparticles are introduced. Some difficult issues such as low enhancement, short shelf life, and poor uniformity can be partly overcome using this method. Liu et al. apply OAD to investigate different SERS responses when the probe molecules adsorbed on different sites of the SERS substrate.r45l The SERS enhancement factor observed from the surface of the nanorods is 50-200 times higher than that of the Ag thin film underneath the nanorod array. However, the polarization-dependent enhancement factor cannot be explained simply by the enhanced local fields only, and we have developed a theoretical model that yields results that agree semi-quantitatively with experimental data.

Tunable SERS has been reported by Alexander et al.l46l We have obtained direct evidence of the interparticledistance-dependent SERS response by controlling the gap between the Au nanorods via strain control of the elastomeric substrate. The work allows tracking. of the distance dependence and avoids enhancement factor reproducibility issues due to morphological differences in disparate nanoparticle pairs. The concept opens the door for future investigation of highly reproducible SERS substrates.

21.3.4 Nanotubes in Surface-Enhanced Raman Scattering

Nanotubes for SERS substrates can be produced using a combination of templates and shadow deposition. For example, Dickey et al. fabricated arrays of metal and metal oxide nanotubes and determined the SERS response.r47l AAO is used as the template, and the fabrication configuration is shown in Figure 21.11. The SEM images of the Pt nanotubes are shown in Figure 21.12. In


' Dip in AgN03 solution

Dry ~

1. ~~::~'.,,~ec2~:~~~~~'.~~t

and dissolve alumina template partially

Figure 21.9 Schematic illustration of the fabrication of Ag nanorod arrays. (Reproduced from Physica B 2009, 404, 1523. Copyright© 2008, Elsevier B. VP9l)

Figure 21.10 Top-view SEM images of the Ag nanorod array substrates: (a) Ag nanorods deposited on a bare glass slide and (b) Ag nanorods deposited onto a 500-nm Ag film. (Reproduced from J. Phys. Chem. C 2008, 112, 895. Copyright© 2008, American Chemical Society.l43l)

this technique, the height and outer dimensions of the nanotubes can be adjusted by varying the deposition angle and pore diameter of the AAO template. The Raman spectrum obtained from the self-assembled monolayer benzenethiolate using the as-prepared Au nanotube substrates reveals an enhancement factor of ~5 x 105 .

Another piece of work that uses the AAO template

to produce nanotube SERS substrates rs described by Huh et al.[481.

Li et al. have reported a recyclable Au-coated Ti02 nanotube SERS substrate made from a ZnO template (Figure 21.13). [ 491 The process includes preparation of large-area ZnO nanorod arrays, immersion of the sample into the solution for 3 h to form Ti02 , and etching of


!

20-200 nm

(a) Mount membrane on a stage

~ !:{ ; : t: F-r! r i: ~ ; ! : : ; I ! Stage : ! ~ : ! : : : J !

1 i ~ i : i : : I I .I • ~ • f t -* • j l .... If l .. • I

:lJllLlLL'j

! (b) Orient membrane at angle, a, with respect to line from stage to source

! (c) Center stage over

evaporation source

(d) Rotate stage while evaporating metal

Axis of rotation

i Evaporation source

1 (e) Remove from stage and dissolve membrane

~ ...... Figure 21.11 Procedures for the fabrication of nanotube arrays: (a) an AAO membrane is mounted onto a stage; (b,c) the stage is centered directly above the evaporation source at an incident angle a (defined as the angle between the axis of rotation and a line drawn from the source to the AAO membrane); (d) metal is evaporated onto the membrane-the edges of the pores of the membrane cast shadows onto the pores; (e) the coated membrane is immersed in 1 M NaOH for 1 h to completely etch the AAO membrane and yield an array of nanotubes connected by a continuous backing of the same materials. (Reproduced from ACS Nano 2008, 2, 800. Copyright © 2008, American Chemical Society.l47l)

the ZnO nanorods. The sample possesses a unique feature in that the nanotube arrays self-clean by photocatalytic degradation of the target molecules under ultraviolet irradiation. Hence, the sustainability, superior sensitivity, stability, and reproducibility render the technique useful in SERS-based applications. Sun et al. have reported a highly sensitive SERS substrate made from superaligned carbon nanotubes (SACNT).[SOJ By cross-stacking several layers of SACNT films, they observed that square nanoholes formed naturally (Figure 21.14a). Afterward, Ag nanoparticles are evaporated onto the nanotube grids to obtain the SERS substrate (Figure 21.14). Using 1 µM R6G, the Raman spectra acquired from the Ag-carbon nanotube, Ag-Si wafer, and carbon-nanotube-only substrates are as shown in Figure 21. l 4c. Considerable enhancement is apparent. However, the Ag nanoparticles are not very uniform, and

the interparticle distance is relatively large. To overcome this obstacle, we have inserted a silicon bugger layer to alter the interfacial properties between Ag and carbon nanotubes. Measurements show that the substrates are highly sensitive and the enhancement is attributed to the large adsorption area of cross-stacked SACNT films and abundant hotspots.

21.4 PLASMONIC lD NANOSTRUCTURES IN PHOTOVOLTAICS

In order to overcome the high price of thick crystalline silicon, alternative materials and device configurations are being explored to produce thin-film solar cells. Two concepts are particularly interesting with regard to lD nanostructures: (1) use of ID nanostructures such as nanowires, nanotubes, and nanobelts as the building

Figure 21.12 SEM images [with increasing magnification from (a) to (c)] of the Pt nanostructures prepared by evaporation at incident angle a = 45°. The walls of these tubes are ~200 nm tall and ~ 10 nm thick. The average diameter of the tubes (over an area of 100 µm2) is 193 ± 29 nm. (Reproduced from ACS Nano 2008, 2, 800. Copyright© 2008, American Chemical Society.[471)

elements (easy fabrication, high efficiency, and low cost) and (2) employing metallic ID plasmonic nanostructures to improve the photovoltaic efficiency by folding the optical path, increasing scattering and light trapping in the device, or increasing the effective absorption cross section.

PLASMONIC 1D NANOSTRUCTURES IN PHOTOVOLTAICS 465

21.4.1 Solar Cells with lD Nanostructures as Building Elements

Coaxial p-i(intrinsic)-n silicon nanowires were introduced by Tian et al. as efficient solar cells and nanoelectronic power sources.[S!J The p-type silicon nanowire cores are synthesized by the nanocluster-catalyzed vapor-liquid-solid method. The intrinsic and phosphorousdoped n-type silicon layers are deposited onto the cores. The major advantage of the core-shell architecture is that carrier separation takes place in the radial instead of the longer axial direction. The carrier collection distance is smaller than or comparable to the minority diffusion length. thereby giving rise to high efficiency (3.4% under solar equivalent illumination) and less stringent requirements on materials quality.

Carbon nanotube (CNT)-based solar cells have been reported by several groups (e.g., Refs. 52 and 53). In Chen's work,[531 a directed array of monolayer singlewalled carbon nanotubes (SWNTs) is nanowelded onto two asymmetric metal electrodes with low and high workfunctions. This results in a strong built-in electric field in the SWNTs for ef?cient separation of the photogenerated electron - hole pairs. In the experiments, the SWNTs synthesized by catalytic chemical vapor deposition are first dispersed in chloroform and aligned onto the source (Pd) and drain (Al) electrodes patterned on a silicon wafer with a 500-nm-thick thermal oxide. An ultrasonic nanowelding technique is then adopted to bond the SWNTs to the metal electrodes. The metallic SWNTs are selectively burned off and a 10-nm Si02 film is deposited onto the device. Schematic illustrations of the fabrication, SEM images of the SWNT bundle array bridging the electrodes after the ultrasonic nanowelding, and welded ends are displayed in Figure 21.15. The power conversion efficiency values are 12.6% and 5.1 % under illumination power of 8.8 W/cm2

and 100 mW/cm2, respectively. Zhang et al. introduced a carbon nanotube (CNT) on

the CdSe nanobelt Schottky junction structure in their solar cell design.[541 Since CdSe nanoparticles can be grafted onto the CNT to form hybrid structures with tunable morphology and strong electronic interaction, solar cells with an efficiency of ::;0.72% are demonstrated. Macroscopic and single-crystalline CdSe nanobelts with widths of 50-150 µ,m are synthesized by chemical vapor deposition. The process involves preparation of a glass substrate with a 100-nm-thick Al electrode pattern by thermal evaporation, picking out single CdSe nanobelts from the as-grown clusters, depositing them onto the glass slide with one end anchored on the Al electrode, and placement of the CNT film on the other end of the nanobelts. The process is illustrated in Figure 21.16. The CNTcoated CdSe nanobelt Schokkty junction solar cells show open-circuit voltages of 0.5-0.6 V. Although the energy


Substrate ZnO nanorod arrays (ZRA)

Ti02 nanotube arrays (TT A)

Au/Ti02 nanotube arrays (AuITTA)

'

ZnO seed .ZnO dissolution I > !· >

..., _____ Growing ZRA Ti02 deposttion

AuffiA-3

Figure 21.13 Schematic illustration of the fabrication process of the Au-coated Ti02 arrays. (Reproduced from Adv. Funct. Mater. 2010, 20, 2815. Copyright© 2010, Wiley-VCR Verlag GmbH & Co. KGaA, Weinheim.E49l)

:; .i -"=' ·c;; c Q)

:g

500

CNT ~:frTO

1000 1500 2000 Raman shift (cm-1)

(c)

Figure 21.14 (a) TEM image of the square nanoholes formed by cross-stacking several layers of SACNTs; (b) TEM image of the Ag-CNT grid SERS substrate; (c) Raman spectra of R6G on AgCNT grid, Ag-Si wafer, and CNT-only grid. The inset is the SEM image of a Ag-Si wafer for comparison with (b). (Reproduced from Nano Lett. 2010, 10, 1747. Copyright© 2010, American Chemical Society_E50l)

conversion efficiency is relatively low compared to other state-of-the-art designs, there is room for improvement. Other ID nanostructure-based solar cell designs have also been reported, for example, oligo- and polythiophene/ZnO hybrid nano wire solar cells, [551 CuinSe2 nanowire array on conducting glass substrate synthesized by using ZnO nanorod arrays as sacrificial templates using an economical solution method, [551 and direct growth of three-dimensional highly regular single nanopillar arrays of optically active semiconductors on aluminum substrates. [571

21.4.2 Plasmonic lD Nanostructures for Improved Photovoltaics

Thin-film solar cells have attracted much attention due to the lower materials cost. A plasmonic nanostmcture is one of the most promising candidates to serve as the light scattering-trapping agent to reduce the physical thickness of current solar cells while maintaining the optical thickness. Three configurations for incorporating plasmonic structures to improve energy conversion are illustrated in Figure 21.17. As shown in Figure 21.17a, metallic nanoparticles can be used as the subwavelength scattering elements to couple and trap freely propagating plane waves from the sun into the absorbing semiconductor thin film by folding the light into a thin absorber layer. As shown in Figure 21.17b, metallic nanostmctures can be used as the nanoantennae to couple the plasmonic near field to the semiconductor, thereby increasing the effective absorption cross section. As shown in Figure 21.17 c,

CONCLUSION AND OUTLOOK 467

Figure 21.15 Structure of SWNT photovoltaic microcells based on nanowelded metal (A)/nanotube/metal (B) junctions: (a) schematic diagram of the SWNT photovoltaic cells; (b) SEM image of the SWNT bundle array with the Pd and Al electrodes; (c,d) SEM images of the SWNT bundle ends nanowelded onto the metal electrodes. (Reproduced from Small 2008, 4, 1313. Copyright© 2008, Wiley-VCR Verlag GmbH & Co. KGaA, Weinheim.[53l)

a corrugated metallic film fabricated on the backside of a photovoltaic absorber layer can couple sunlight into the SPP modes supported at the metal-semiconductor interface as well as guided modes in the semiconductor slab, at which point light is converted to photocarriers in the semiconductor. [SSJ

Although there are many reports on the use of metallic nanoparticles to increase the efficiency of solar cells and light emitting diodes,l59 - 64l those on the use of ID nanostructures to enhance photovoltaics are more scarce. One promising candidate to bridge the gap is patterned square metallic nanowires. On one hand, the nanowires can serve as near-field antennas to increase the absorption cross section. On the other hand, by stacking of nanowire layers with different widths, heights, and interwire distances, sunlight can be coupled into different SPP modes, allowing energy conversion of various wavelengths to take place simultaneously. Hu et al.'s work may provide good guidance on the investigation of the role of nanowires in improved photovoltaics. By using lithographic techniques such as electron beam lithography and shadow deposition, nanowires can be patterned on the device without damaging the effective structures.[651 Nanotubes can also be vertically aligned to the planar solar cell substrate to serve as light reflectors and increase the optical path many times in an effort to enhance the energy conversion efficiency.

21.5 CONCLUSION AND OUTLOOK

In this chapter, the applications of ID nanostructures to plasmonics, including plasmonic waveguides, surfaceenhanced Raman scattering/fluorescence, nanotube/nanowire/nanorod-based solar cells, and how plasmonics can significantly enhance photovoltaics, were reviewed. Among the various configurations, long-range, high-confinement hybrid ID nanostructures play an important role in enhancing next-generation ultrafast chips. Nanowires are important to both SPP propagation and efficient SPP source/fiber coupling. In SERS, many kinds of ID nanostructures have been explored in order to develop large-area, cheap, highly sensitive, and reproducible manufacturing schemes. Photovoltaics is attracting the interest of both scientists and the general population because of its sustainable energy initiative and immense potential. Organic and inorganic nanotubes, nonobelts, and nanowires have been used to fabricate economical and high-conversion-efficiency solar cells. However, there have been very few reports in the literature on the incorporation of ID metallic (plasmonic) nanostructures to enhance the solar conversion efficiency via near-field coupling. Since plasmonics, photovoltaics, and nanotechnology are all fledgling fields, future interdisciplinary exploration is expected to continue and make significant imoads to science, technology, and society.


(a)

(b) • r. --f---Ion!~~;~~ <t>[CN1] <l>{CdSe) ~Ee

Ee , '

~~~ I I ---EF -EF ---r:-EF .. -: ;

~--kl__ Ev Ev -- i, ___ _

-,CdSe CNT r.r1~" CNT , NBs

(c)

Al

••

Figure 21.16 Fabrication and characterization of CdSe and CNT Schottky junctions. (a) Schematic illustration of a nanobelt (deposited on a glass substrate) covered by a CNT film on one end and contacted by an Al electrode on the other end. The arrows show the flow direction of charge carriers (holes and electrons) when the device is illuminated. The overlapping area between the CNT film and the CdSe nanobelt constitutes the interface for hole-electron separation and acceleration toward different directions. (b) Band diagram of the CNT-CdSe interface showing formation of a Schottky junction at the contact (c) Optical image of the CNT-CdSe-Al device consisting of a 3-mm-long nanobelt, a transparent CNT film covering about two-thirds of the nanobelt length, and a 100-nm-thick Al electrode on the left The dashed line shows the edge of CNT film that covers the entire area on the right side of the dashed line. The CNT film tends to bunch slightly to form dark strands during transfer. However, most of the overlapping area between the CNT film and the CdSe nanobelt is uniform. (d) Microscopic illustration of the interface area between the CNT film and nanobelt where holes are directed along the CNT network and electrons are transported through the CdSe to the other side. (e-g) SEM images of the device showing a highly pure, uniform CNT network coated tightly around the CdSe nanobelt on the top surface (e) and near the nanobelt side edges (f,g), as indicated in (c). (Reproduced from Nano Lett. 2010, 10, 3583. Copyright © 2010, American Chemical Society.f54l)

' ' ' ' ' '

REFERENCES 469

Figure 21.17 Plasmonic light trapping geometries in thin-film solar cells. (a) Light trapping by scattering from metal nanoparticles at the surface of the solar cell. Light is preferentially scattered and trapped in the semiconductor thin film by multiple and high-angle scattering, causing an increase in the effective optical pathlength in the cell. (b) Light trapping by the excitation of localized surface plasmons in metal nanoparticles embedded in the semiconductor. The excited near-field particle creates electron-hole pairs in the semiconductor. (c) Light trapping by excitation of surface plasmon polaritons at the metal/semiconductor interface. A corrugated metal back surface couples light to the surface plasmon polariton or photonic modes that propagate in the plane of the semiconductor layer. (Reproduced from Nat. Mater. 2010, 9, 205. Copyright© 2010, Macmillan Publishers Limited.l58l)

ACKNOWLEDGMENTS

This work was jointly supported by the National Natural Science Foundation of China, under Grants 50801013 and 51071045; Natural Science Foundation of Jiangsu Province, China, under Grant BK2009291; Specialized Research Fund for the Doctoral Program of Higher Education, under Grant 200802861065; Excellent Young Teachers Program of Southeast University; Hong Kong Research Grants Council (RGC) General Research Fund (GRF) Grant CityU 112510; City University of Hong Kong Strategic Research Grant (SRG) 7008009; and City University of Hong Kong Direct Allocation Grant 9360110.

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21 - City University of Hong Kong

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