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Enhanced infrared absorption with dielectric nanoparticles Mark S. Anderson Citation: Applied Physics Letters 83, 2964 (2003); doi: 10.1063/1.1615317 View online: http://dx.doi.org/10.1063/1.1615317 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/83/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhancement of surface phonon modes in the Raman spectrum of ZnSe nanoparticles on adsorption of 4- mercaptopyridine J. Chem. Phys. 140, 074701 (2014); 10.1063/1.4865136 High-mobility enhancement-mode 4 H -SiC lateral field-effect transistors utilizing atomic layer deposited Al 2 O 3 gate dielectric Appl. Phys. Lett. 95, 152113 (2009); 10.1063/1.3251076 Near-field radiative heat transfer enhancement via surface phonon polaritons coupling in thin films Appl. Phys. Lett. 93, 043109 (2008); 10.1063/1.2963195 Surface enhanced infrared absorption by coupling phonon and plasma resonance Appl. Phys. Lett. 87, 144102 (2005); 10.1063/1.2077838 Infrared properties of silicon nanoparticles J. Appl. Phys. 97, 084306 (2005); 10.1063/1.1866475 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.210.2.78 On: Wed, 26 Nov 2014 11:04:24

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Page 1: Enhanced infrared absorption with dielectric nanoparticles

Enhanced infrared absorption with dielectric nanoparticlesMark S. Anderson Citation: Applied Physics Letters 83, 2964 (2003); doi: 10.1063/1.1615317 View online: http://dx.doi.org/10.1063/1.1615317 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/83/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Enhancement of surface phonon modes in the Raman spectrum of ZnSe nanoparticles on adsorption of 4-mercaptopyridine J. Chem. Phys. 140, 074701 (2014); 10.1063/1.4865136 High-mobility enhancement-mode 4 H -SiC lateral field-effect transistors utilizing atomic layer deposited Al 2 O 3gate dielectric Appl. Phys. Lett. 95, 152113 (2009); 10.1063/1.3251076 Near-field radiative heat transfer enhancement via surface phonon polaritons coupling in thin films Appl. Phys. Lett. 93, 043109 (2008); 10.1063/1.2963195 Surface enhanced infrared absorption by coupling phonon and plasma resonance Appl. Phys. Lett. 87, 144102 (2005); 10.1063/1.2077838 Infrared properties of silicon nanoparticles J. Appl. Phys. 97, 084306 (2005); 10.1063/1.1866475

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Enhanced infrared absorption with dielectric nanoparticlesMark S. Andersona)

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, M/S 125-109Pasadena, California 91109

~Received 9 June 2003; accepted 4 August 2003!

Enhanced infrared absorption is demonstrated for anthracene coating polar dielectric nanoparticlesof silicon carbide and aluminum oxide. An enhancement factor greater than 100 was measured nearthe surface of silicon carbide particles. This is the result of the enhanced optical fields at the surfaceof the particles when illuminated at the surface phonon resonance frequencies. This phononresonance effect is analogous to plasmon resonance that is the basis of surface enhanced infraredabsorption and surface enhanced Raman scattering. The results have implications for near-fieldmicroscopy, the characterization of nano-optical devices, and chemical sensing. In addition, themethodology used for surface phonon analysis of particles is useful for simulating comet andinterstellar dust spectra. ©2003 American Institute of Physics.@DOI: 10.1063/1.1615311#

Locally enhanced optical fields near the surface of illu-minated nanoparticles and nanostructures are well-established phenomena and form the basis of important ana-lytical methods. These include surface enhanced Ramanscattering, surface enhanced infrared absorption~SEIRA!and enhanced pinhole transmission.1–3 In addition, the localfield enhancement at probe tips in scanning probe micro-scopes is used for high spatial resolution optical and Ramanmicroscopy.4–7 In these examples, the locally surface en-hanced electric fields result from a plasmon resonance con-dition of certain metallic nanoparticles and structures. In ad-dition to plasmon resonance, there are other small particleresonance conditions that yield enhanced electric fields nearthe surface of illuminated particles.8 These include phononresonance and polariton resonance. Recently, Hillenbrand di-rectly measured the locally enhanced fields of illuminatedSiC nanostructures by combining infrared spectroscopy withnear-field microscopy.9 Near-field effects may also be ob-served by far-field reflectance measurements when particlesare coated with absorbing molecules that experience the in-creased electric field at the particle surface. This is the basisof SEIRA where the near-field enhancement of the electricfields increases the infrared absorption of molecules coatingthe surface of certain metallic particles.

In this letter, the enhanced infrared absorption near polardielectric particles using aphonon-resonance mechanism ispresented. This demonstrates that when polar dielectric par-ticles, smaller than the wavelength of illumination, arecoated with infrared absorbing molecules there is an en-hanced absorption near the particle surface. This occurs atthe surface phonon-resonance frequencies that are alsoknown as the particle surface modes. This is distinct from thebroad far-field phonon-induced ‘‘Restrahlen’’ reflectivityband.9 In addition to enhanced infrared absorption, this workalso demonstrates a method for analyzing surface modes forparticles suspended in a liquid medium.

The theory of particle surface modes and enhanced in-frared absorption will be briefly summarized for the SiC and

Al2O3 nanoparticles. Aravind and Miteu suggested enhancedinfrared absorption with silicon carbide and they have pro-vided theoretical analysis of the near-field enhancementproperties of particles using a sphere plane model.8 There isno exact theory for describing the surface modes in irregu-larly shaped particles of SiC and Al2O3 used in this study.However, it is useful to consider the enhanced optical field atthe surface of small spheres. This can be calculated using anelectrostatic approach for particle diameters much smallerthan the illumination wavelength.10,11 The enhancement fac-tor is the ratio of the surface field (ELOC) to the far-fieldillumination (EIN). The enhancement (ELOC/EIN) is propor-tional to the polarizabilitya given by ELOC/EIN}a, witha54pa3 («p2«m)/(«p12«m). Wherea is the particle di-ameter,«p is the dielectric function of the particle, and«m isthe dielectric function of the surrounding medium.12 The so-called Frohlich resonance frequency occurs at Re@«p(v)#522«m. Phonons, plasmons, or excitons can fulfill this nec-essary resonance condition for maximum local field enhance-ment. Stronger and sharper optical resonance is observed forsurface phonons because they have weaker damping thansurface plasmons.9 A more detailed particle analysis showsellipsoid particles have three resonance modes and imbeddedparticles have shifted Frohlich resonance frequency depend-ing on the dielectric function of the surrounding medium.11

Shape and aggregation effects on the surface phonon modesfor alpha Fe2O3 and SiC particles show that irregular par-ticles would have a broader region of enhanced absorptionthan spherical particles.11,13,14

The surface mode frequencies~where enhancement oc-curs! are near the bulk absorption frequency of the particles.8

This poses an experimental problem that was solved in thiswork. Both SiC and Al2O3 have strong molecular absorptionthat could potentially overwhelm any enhanced signal fromabsorbed molecules. Simply concentrating the particles re-sults in saturation at the absorption maximum. The solutionto the problem was to optimize particle concentration bycovering a mirror surface with approximately a half mono-layer of particles. An attenuated total reflectance cell, de-scribed below, was also used to control the path length anda!Electronic mail: [email protected]

APPLIED PHYSICS LETTERS VOLUME 83, NUMBER 14 6 OCTOBER 2003

29640003-6951/2003/83(14)/2964/3/$20.00 © 2003 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 141.210.2.78

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permitted particles dispersed in liquids and particle films tobe analyzed. These methods avoid grinding the particles fordispersion that could disrupt adsorbed layers.

For this work, a Fourier transform infrared spectrometer~FTS 6000, Digilab, Ma! was used at 4 cm21 resolution witha linearized HgCdTe detector. This was equipped with anattenuated total reflectance~ATR! liquid cell attachment.15,16

The ATR technique allows liquids or powder films to beexamined with a controlled path length governed by the ATRparameters. A biconical diffuse reflectance attachment~Ana-lect, Irvine, Ca! was used for reflection–absorption measure-ments. Beta SiC particles were used~99.8%! of approxi-mately 1 mm size ~Alfa Aesar!. The Al2O3 particles were99.98%, gamma alpha, of 0.01–0.02 micron size range~Alpha Aesar!. The particles were dispersed in 1,1,2 trichlo-rotrifluoroethane~Freon TF! by sonication.17 This suspensionwas evaporated on a polished stainless-steel mirror or theATR crystal. Evaporation left a light haze of approximatelyhalf monolayer particle coverage for the SiC. Repeated rins-ing with dichloromethane did not significantly displace theparticles. To the particle-coated substrates, a layer of anthra-cene was deposited from a dichloromethane solution. Reflec-tance measurements were made on the anthracene-coatedparticle substrates or the anthracene-coated substrates with-out particles. The corresponding substrates uncoated with an-thracene were used as the reference spectra. The conven-tional diffuse reflectance spectrum of SiC in potassiumbromide and the aluminum oxide ATR spectrum are shownin Fig. 1. These reference spectra show the absorption maxi-mum that is used to estimate the position of the near-fieldsurface modes of the respective powders.

The initial set of experiments measured the enhancedinfrared absorption of anthracene adsorbed on SiC andAl2O3 particles. Figure 2 shows the spectrum of the anthra-cene coated on the SiC–mirror substrate compared to theconventional reflectance–absorption spectrum of anthracene.The largest enhancement is at the SiC phonon resonancemaximum near 946 cm21. This is close the phonon reso-nance maximum measured directly in the near field byHillenbrand.9 Figure 3 shows similar results for anthracenedeposited on an Al2O3 coating an ATR crystal. The strongestenhancement is observed for the 724 cm21 anthracene peak

near the expected Al2O3 surface mode at approximately727 cm21.

Next, SiC dispersed in a Freon TF solution was exam-ined. Figure 4 shows the liquid ATR cell spectrum of SiCdispersed in Freon TF with pure Freon TF as the backgroundreference spectrum. The Freon TF has relatively uniform ab-sorption in the region of the SiC surface modes. Therefore,the enhanced absorption reveals the frequency distributionSiC surface modes without any overlapping Freon TF peaks.There is a broad absorption band centered near 937 cm21

and there are additional weaker peaks at 914 cm21 and986 cm21. The SiC particle surface modes are distinct andthis can be attributed to very little aggregation of the par-ticles. Studying the particles suspended in solution by theATR method improves the ability to observe the distributionof the surface phonon modes. The surrounding medium cansignificantly shift the frequency of the surface modes de-pending on the dielectric function of the surrounding me-dium. For example, the calculated shift for SiC spheres isover 70 cm21 from air to KBr media.11 This work gives sur-

FIG. 1. The bulk infrared spectra of the particles are shown for the SiCparticles as diffuse reflectance in KBr~a! and the powder ATR spectrum forthe Al2O3 particles~b!. The absorption maximum for SiC is 946 cm21 andfor Al2O3 is 727 cm21. The absorption maximum gives an approximatelocation of the near-field particle surface mode frequencies.

FIG. 2. The enhanced infrared spectrum of the anthracene coated on theSiC–mirror substrate~line! is compared to the conventional reflectance-absorption spectrum of anthracene on a stainless steel mirror~shaded bars!.There is an enhancement maximum (2.53) of the anthracene peak near theSiC surface phonon resonance near 946 cm21. The enhanced region is broadand extends to lower frequencies. The spectra are slightly baseline correctedand normalized to the 3049 cm21 aromatic CuH stretch peak that is awayfrom the SiC surface modes.

FIG. 3. The infrared spectrum of the anthracene coated Al2O3 on an ATRcrystal ~line!. There is an enhancement of the 725 cm21 anthracene peak.The Al2O3-coated ATR crystal was run as the background spectrum. Theconventional ATR spectrum of anthracene is shown for comparison~shadedbars!. The spectra are slightly baseline corrected and normalized to the3049 cm21 aromatic CuH stretch peak that is away from the Al2O3 surfacemodes.

2965Appl. Phys. Lett., Vol. 83, No. 14, 6 October 2003 Mark S. Anderson

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face resonance modes in Freon TF that could be used tocalculate particle surface modes for other media.

The enhancement factor will now be considered. Boththeoretical and experimental works indicate that enhancedabsorption occurs less than 0.025 microns from the particlesurface.8,9 Therefore, the 2.5 enhancement factor that wasdirectly measured for the 1.4 micron anthracene film resultsfrom enhancement of less than 1.8% of total film thickness~Fig. 2!. Thus, the localized enhancement factor can bebounded to greater than 100 for the SiC particles at946 cm21. This is a conservative estimate based on the an-thracene thickness~1.4 microns!, 50% particle coverage ofthe mirror substrate, and by bounding the field-enhanced re-gion to less than 0.025 microns from the particle surface.

The peaks in the enhanced regions have a ‘‘tailing’’asymmetrical shape. This effect is also seen in SEIRA spec-tra of molecules absorbed on metals.18 This peak asymmetrymay be from the anthracene peaks not being centered on thesurface modes. The result is that one side of the peak is moreenhanced. The 950 cm21 anthracene peak is close to the SiCsurface mode maximum and shows the largest enhancementand peak symmetry. The spectra of molecules adsorbed onSiC nanoparticles have increased contrast in the peak ratios.Scattering effects and detector nonlinearity were ruled out ascausing changes in peak ratios.19,20

The enhanced infrared absorption due to phonon reso-nance has been demonstrated for molecules adsorbed on thesurfaces of silicon carbide and aluminum oxide nanopar-ticles. This enhanced infrared absorption by polar dielectricparticles has potential applications for trace molecular analy-

sis and studying of the phonon resonance properties of na-nometer scale optical circuits and similar devices based onsilicon carbide structures. Fabricated structures or porousSiC substrates could provide more stability and control of thesurface modes. This approach may provide better sensitivityin spectroscopic measurements of catalyst systems. Finally,the particles dispersed in a liquid and measured by ATR maybe useful for the analysis of comet and interstellar dust ana-logs.

The work described in this article was carried out at theJet Propulsion Laboratory, California Institute of Technol-ogy, through an agreement with the National Aeronautics andSpace Administration. The author would like to thank EricWong for his useful suggestions. The author would also liketo thank Andre Yavrouian, Gary Plett, Greg Bearman, DarrellJan, and Jeremy Menella for their support and advice.

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10H. Frohlich,Theory of Dielectrics~Clarendon, Oxford, 1949!.11C. Bohren and D. Huffman,Absorption and Scattering of Light by Small

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~1998!.15T. Henning and H. Mutschke, Spectrochim. Acta, Part A57, 815 ~2001!.16The ATR attachment was from CIC Photonics, Albuquerque NM using an

AMTIR-1 crystal. For a review of ATR of particles, see A. R. Hind, S. K.Bhargava, and A. McKinnon, Adv. Colloid Interface Sci.93, 91 ~2001!.

17The sonication time was 5 min in a Branson 2200 sonicator unit with a 1min settling time. This settling in the Freon would tend to give a smallerparticle size fraction.

18A. E. Bjerke and P. R. Griffiths, Appl. Spectrosc.56, 1275~2002!.19Diffuse reflectance scattering tends to compress peak intensity. This peak

ratio compression was experimentally verified by examination of anthra-cene on a thin particle layer of KBr on a mirror. For further explanation ofdiffuse reflectance effects, see M. P. Fuller and P. R. Griffiths, Appl. Spec-trosc.34, 533 ~1980!.

20The absorbance of the samples was less than 0.25 and this is in the linearregion of the detector. For more on detector linearity effects, see R. L.Richardson, Jr., H. Yang, and P. R. Griffiths, Inst. Phys. Conf. Ser.430, 428~1998!.

FIG. 4. The liquid cell ATR spectrum of silicon carbide dispersed in FreonTF with pure Freon TF as the reference spectrum. This is in the infraredregion where the Freon TF has uniform absorbance. The enhanced absorp-tion, therefore, corresponds to the surface modes of the SiC particles.

2966 Appl. Phys. Lett., Vol. 83, No. 14, 6 October 2003 Mark S. Anderson

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