Shape-Dependent Plasmonic Response and ... - ACS Publications

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Shape-Dependent Plasmonic Response and Directed Self-Assembly in a New Semiconductor Building Block, Indium-Doped Cadmium Oxide (ICO) Thomas R. Gordon,† Taejong Paik,† Dahlia R. Klein,† Gururaj V. Naik,∥ Humeyra Caglayan,§ Alexandra Boltasseva,∥ and Christopher B. Murray*,†,‡ †

Department of Chemistry, ‡Department of Materials Science and Engineering, and §Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ∥ School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: The influence of particle shape on plasmonic response and local electric field strength is well-documented in metallic nanoparticles. Morphologies such as rods, plates, and octahedra are readily synthesized and exhibit drastically different extinction spectra than spherical particles. Despite this fact, the influence of composition and shape on the optical properties of plasmonic semiconductor nanocrystals, in which free electrons result from heavy doping, has not been well-studied. Here, we report the first observation of plasmonic resonance in indium-doped cadmium oxide (ICO) nanocrystals, which exhibit the highest quality factors reported for semiconductor nanocrystals. Furthermore, we are able to independently control the shape and free electron concentration in ICO nanocrystals, allowing for the influence of shape on the optical response of a plasmonic semiconductor to be conclusively demonstrated. The highly uniform particles may be self-assembled into ordered single component and binary nanocrystal superlattices, and in thin films, exhibit negative permittivity in the near infrared (NIR) region, validating their use as a new class of tunable low-loss plasmonic building blocks for 3-D optical metamaterials. KEYWORDS: Plasmonics, shape effects, transparent conducting oxide, nanocrystal superlattices, metamaterials, indium-doped cadmium oxide copy (SERS),5 LSPR/SPR sensing,6,7 photothermal therapy,8,9 and plasmon-enhanced fluorescence.10−12 A series of recent papers have reported tunable LSPR at NIR and mid-IR frequencies resulting from semiconductor nanocrystals.13−18 A plasmonic resonance is observed upon introduction of sufficient free carriers into the nanocrystals, either as a result of atomic vacancies or through doping with aliovalent cations. The LSPR frequency is dependent on the plasma frequency (ωp) of the material, which is determined by materials properties such as carrier concentration and carrier

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trong resonances which appear at optical frequencies for metal nanoparticles result from the resonant oscillation of free electrons on the surface of the particles and are referred to as localized surface plasmon resonances (LSPRs). By altering the shape of the metallic particles, one can sensitively manipulate the observed resonant frequencies throughout the UV and visible and even into the NIR.1−3 Highly anisotropic shapes, such as rods or plates, may display multiple modes, while sharply faceted shapes are desirable due to the large electric field enhancements observed at tips and edges.3,4 The focusing properties of plasmonic particles have been harnessed to enable a number of new technologies and sensitive measurement tools, such as surface-enhanced Raman spectros© 2013 American Chemical Society

Received: April 3, 2013 Revised: May 7, 2013 Published: May 23, 2013 2857

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Figure 1. TEM images of octahedral ICO nanocrystals produced at 300 °C using (a) 4 mmol and (b) 3 mmol OLAC and (c) SEM image of sample shown in b. TEM image of spherical nanocrystals produced at reflux using (d) 5 mmol and (e) 3 mmol OLAC. (f) The unit cell of rock-salt type indium-doped cadmium oxide (ICO).

Figure 2. SAXS and WAXS patterns for (a) 51 nm (tip-to-tip) octahedral ICO nanocrystals dispersed at 10 wt % in polyvinyl butyral and (b) 8 nm spherical ICO nanocrystals dispersed in toluene and loaded into a glass capillary.

mobility.19 Unlike in the case of metals in which ωp is typically considered an intrinsic property of the material, through adjustment of dopant concentrations, ωp can be readily tuned in semiconductors.19 This opens the possibility of producing low-loss plasmonic nanocrystals with tunable LSPR frequency to substitute for metals, which typically have high losses at nearand mid-infrared frequencies.20 On the other hand, poor particle uniformity and the difficulty of manipulating both particle shape and dopant concentration independently has made correlation of particle morphology with optical response difficult, although some early observations of shape effects have been made.21−24 As expected, all of the semiconducting nanocrystals recently found to support LSPR in the NIR are known to form highly conductive thin films. Two of the materials systems, ITO and aluminum-doped zinc oxide (AZO), belong to the well-known family of d10 metal based transparent conducting oxides (TCOs), which are technologically important for applications in photovoltaics and displays.14,16 There has been a resurgence of interest in cadmium oxide (also d10) based TCO films, which were previously highlighted as having excellent TCO character-

istics.25−28 Highly transparent indium-doped cadmium oxide (ICO) thin films may be produced with conductivities 2−5 times higher than that of commercial ITO films and near metallic n-type carrier densities (1021 cm−3).27 In addition, tindoped cadmium oxide (SCO) films were recently prepared with the highest electrical conductivities and carrier mobilities of any TCO.28 Highly transparent and electrically conductive TCOs have been introduced as a category of low-loss plasmonic building blocks, making cadmium oxide based materials a promising, yet unexplored, system.29 In this report, we describe the first synthesis of indium cadmium oxide (ICO) nanocrystals and their unique plasmonic optical properties. ICO nanocrystals are produced through a high temperature surfactant assisted approach, modified from a previous synthesis of cadmium oxide nanocrystals.30 In Figure 1, transmission electron microscopy (TEM) images of ICO nanocrystals are shown, illustrating the control of shape and high level of monodispersity achievable with the given system. The morphology of the nanocrystals is readily tuned through modification of the reaction temperature and oleic acid (OLAC) concentration. Large octahedral nanocrystals are 2858

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Figure 3. (a) Solution phase spectra of ICO nanocrystals dispersed in CCl4 for five levels of % atomic doping of In. The fwhm or Γ is indicated for the 16.2% doped sample. (b) Plots of (αhν)2 vs photon energy (eV) for five samples of spherical ICO nanocrystals. Open circles are measured data, dotted lines are linear fits, and the legend shows the extracted direct bandgap energies.

formed at 300 °C, and by adjusting the oleic acid concentration from 2 to 4 mmol, the size may be tuned (Figure 1a−c, Figure S1). Refluxing the reaction solution at roughly 315 °C results in much smaller, spherical nanocrystals, the size of which is also tunable depending on oleic acid concentration (Figure 1d,e). These results can easily be interpreted based on previous mechanistic studies, wherein higher reaction temperatures result in faster thermal decomposition, yielding more initial nuclei and thus a larger number of smaller sized crystallites.31 HRTEM results indicate the nanocrystals are singlecrystalline and adopt the Fm3̅m rock salt crystal structure typical of cadmium oxide crystallites (Figure 1d inset). The crystal structure and monodispersity observed by TEM is confirmed through X-ray scattering studies, both small angle (SAXS) and wide angle (WAXS) (Figure 2). Together, the techniques are complementary, as SAXS is highly sensitive particle size distribution, WAXS is sensitive to interatomic spacings and crystal structure, while both are sensitive to particle morphology.32 In combination with Debye function simulations, an atomistic model consistent with TEM and X-ray scattering can be constructed. The X-ray fittings indicate that the octahedral nanocrystals are roughly 51.0 ± 3.6 nm in length (tip-to-tip), while the spherical nanocrystals are 8.0 ± 1.0 nm, in excellent agreement with measurements from TEM. Thus, we have produced monodisperse plasmonic nanocrystals which may act as model systems to test the influence of particle shape on optical response. A strong plasmonic resonance is observed in the highly uniform ICO nanocrystals in the NIR range, and the wavelength maximum of the plasmonic resonance (λmax) is tunable from 1.8 to 3.5 μm depending on the indium concentration (Figure 3a, Figure S2, Table S1). Using the current synthetic method, the doping concentration of octahedral ICO nanocrystals is limited to less than 6% indium as the shape uniformity suffers above this concentration, while spherical particles can be produced up to 20% indium with marginal loss of uniformity. The plasmonic resonance is observed to be much more sensitive to indium concentration at low doping levels in both spherical and octahedral particles (Figure 4a). At very high doping levels, electrons may be trapped or scattered by In centers, resulting in saturation of the free-electron concentration, as suggested in the case of ITO nanocrystals.14 The increasing indium concentration also results in an increase in the optical bandgap of the nanocrystals

(Figure 3b) referred to as a Burstein shift, which has been noted in ICO thin films, and results in reasonably high visible light transparency relative to undoped CdO.27,33 The bandgap of the nanocrystals shifts from 2.97 to 3.34 eV with increasing In concentration. The ability to synthesize ICO nanocrystals of two morphologies with high uniformity over the same concentration range allows for the shape dependent optical properties to be unambiguously investigated. A clear shape dependence of the optical response is apparent in the ICO nanocrystals, as depicted in Figure 4a,b. Using the synthetic techniques described above, spherical nanocrystals 12 nm in diameter and two samples of octahedral nanocrystals 68 and 132 nm in length (tip-to-tip) are prepared with very similar doping concentrations (0.64−1.43% In). It is important to note that these three particles are sufficiently subwavelength in size (0.94) and prescribed interparticle distances. In addition to forming single component assemblies, spherical ICO nanocrystals of 8 nm with a LSPR at 1975 nm have been selfassembled with 5 nm lead selenide (PbSe) quantum dots to form large-area MgZn2-type binary nanocrystal superlattices (BNSLs) (Figure 5c−f). BNSLs offer a platform in which the interaction between plasmonic particles can be readily tuned, depending on the structure of the BNSL and chemical composition of the building blocks, as recently demonstrated in our group using gold nanocrystals.45 In addition, such assemblies have been recently predicted to facilitate optical magnetism and thus potentially are the basis for negative index metamaterials.38,39,46 Given the number of known uniform nanocrystal building blocks and distinct BNSL structures, thousands of subwavelength metamaterials could be selfassembled using these uniform ICO particles, each with its own unique interactions.47 We have described the synthesis and optical properties of indium-doped cadmium oxide nanocrystals with tunable morphology. The uniformity of the nanocrystals, as confirmed by X-ray scattering, allows for the influence of shape on the plasmonic response to be observed unequivocally. ICO nanocrystals also possess the highest quality factors reported for plasmonic semiconductor nanocrystals which are comparable to traditional metals such as silver and gold. Spectroscopic ellipsometry of films of ICO nanocrystals reveals negative permittivity in the NIR, indicating the strength of the resonance in low-loss ICO nanocrystals. This observation validates the use of as-synthesized nanocrystals as building blocks for optical metamaterials, without the need for ligand exchange strategies. Both spherical and octahedral nanocrystals are shown to self-assemble into 3-D superlattice structures with long-range order, which may act as the basis for the design of new negative index metamaterials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 1-215-898-0588. Fax: 1-215-573-6229. Author Contributions

T.R.G. and D.R.K. synthesized the nanocrystals and performed optical characterization. T.R.G. performed X-ray scattering simulations. T.P. measured X-ray scattering patterns and selfassembled the nanocrystals. T.R.G. and T.P. collected the electron microscopy images. G.V.N. and A.B. collected and interpreted ellipsometric data and performed extraction of optical constants. H.C. simulated nanocrystal extinction with FDTD. T.R.G., T.P., and C.B.M. discussed results and prepared the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.R.G. was supported by the National Science Foundation through the Nano/Bio Interface Center at the University of Pennsylvania Grant Number DMR08-32802. T.P., H.C., G.V.N., A.B., and C.B.M. acknowledge support from the Office of Naval Research (ONR) Multidisciplinary University Research Initiative (MURI) on Optical Metamaterials through award N00014-10-1-0942. D.R.K. is supported by the Roy and Diana Vagelos Scholars Program in the Molecular Life Sciences. C.B.M. is also grateful for the support of the Richard Perry University Professorship. Dr. Matteo Cargnello, Dr. Xingchen Ye, and Benjamin Diroll are acknowledged for enlightening discussions. We thank Dr. Douglas Yates for the support at the Penn Regional Nanotechnology Facility, Dr. David Vann for support in collecting ICP-OES data, and Dr. Paul Heiney for support in collecting SAXS data at the MAXS Facility.

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REFERENCES

(1) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857−70. (2) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025−102. (3) Lu, X.; Rycenga, M.; Skrabalak, S. E.; Wiley, B.; Xia, Y. Annu. Rev. Phys. Chem. 2009, 60, 167−92. (4) Ye, X.; Jin, L.; Caglayan, H.; Chen, J.; Xing, G.; Zheng, C.; DoanNguyen, V.; Kang, Y.; Engheta, N.; Kagan, C. R.; Murray, C. B. ACS Nano 2012, 6, 2804−2817. (5) Nie, S.; Emory, S. R. Science 1997, 275, 1102−1106. (6) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267−97. (7) Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G. Chem. Rev. 2008, 108, 494− 521. (8) El-Sayed, I. H.; Huang, X.; El-Sayed, M. A. Cancer Lett. 2006, 239, 129−35. (9) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578−86. (10) Chen, Y.; Munechika, K.; Ginger, D. S. Nano Lett. 2007, 7, 690−696. (11) Ming, T.; Chen, H.; Jiang, R.; Li, Q.; Wang, J. J. Phys. Chem. Lett. 2012, 3, 191−202. (12) Saboktakin, M.; Ye, X.; Oh, S. J.; Hong, S.-H.; Fafarman, A. T.; Chettiar, U. K.; Engheta, N.; Murray, C. B.; Kagan, C. R. ACS Nano 2012, 6, 8758−66. (13) Zhao, Y.; Pan, H.; Lou, Y.; Qiu, X.; Zhu, J.; Burda, C. J. Am. Chem. Soc. 2009, 131, 4253−61. (14) Kanehara, M.; Koike, H.; Yoshinaga, T.; Teranishi, T. J. Am. Chem. Soc. 2009, 131, 17736−7.

ASSOCIATED CONTENT

S Supporting Information *

Experimental details; TEM tilt series of ICO octahedra; optical absorbance of ICO octahedra; FDTD simulations of nanocrystal extinction; optical absorption of ICO in different solvents illustrating shift of LSPR with refractive index; table of nanocrystal doping levels, concentrations, and extinction coefficients; table of the fwhm (Γ) and quality factors for spherical ICO nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. 2862

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(15) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Nat. Mater. 2011, 10, 361−366. (16) Buonsanti, R.; Llordes, A.; Aloni, S.; Helms, B. A.; Milliron, D. J. Nano Lett. 2011, 11, 4706−10. (17) Zhao, Y.; Burda, C. Energy Environ. Sci. 2012, 5, 5564−5564. (18) Rowe, D. J.; Jeong, J. S.; Mkhoyan, K. A.; Kortshagen, U. R. Nano Lett. 2013, 0−5. (19) West, P. R.; Ishii, S.; Naik, G. V.; Emani, N. K.; Shalaev, V. M.; Boltasseva, A. Laser Photon. Rev. 2010, 4, 795−808. (20) Boltasseva, A.; Atwater, H. A. Science 2011, 331, 290−291. (21) Hsu, S.-W.; On, K.; Tao, A. R. J. Am. Chem. Soc. 2011, 133, 19072−5. (22) Hsu, S.-W.; Bryks, W.; Tao, A. R. Chem. Mater. 2012, 24, 3765− 3771. (23) Chou, L.-W.; Shin, N.; Sivaram, S. V.; Filler, M. A. J. Am. Chem. Soc. 2012, 134, 16155−16158. (24) Kriegel, I.; Rodríguez-Fernández, J.; Wisnet, A.; Zhang, H.; Waurisch, C.; Eychmüller, A.; Dubavik, A.; Govorov, A. O.; Feldmann, J. ACS Nano 2013, 10.1021/nn400894d. (25) Nozik, A. Phys. Rev. B 1972, 6, 453−459. (26) Coutts, T. J.; Young, D. L.; Li, X.; Mulligan, W. P.; Wu, X. J. Vac. Sci. Technol. A 2000, 18, 2646−2646. (27) Wang, A.; Babcock, J. R.; Edleman, N. L.; Metz, A. W.; Lane, M. A.; Asahi, R.; Dravid, V. P.; Kannewurf, C. R.; Freeman, A. J.; Marks, T. J. Proc. Natl. Acad. Sci. 2001, 98, 7113−6. (28) Yan, M.; Lane, M.; Kannewurf, C. R.; Chang, R. P. H. Appl. Phys. Lett. 2001, 78, 2342−2342. (29) Naik, G. V.; Kim, J.; Boltasseva, A. Opt. Mater. Express 2011, 1, 1090−1090. (30) Choi, D.-H.; Jeong, G.-H.; Kim, S.-W. Bull. Korean Chem. Soc. 2011, 32, 3851−3852. (31) Shevchenko, E. V.; Talapin, D. V.; Schnablegger, H.; Kornowski, A.; Festin, O.; Svedlindh, P.; Haase, M.; Weller, H. J. Am. Chem. Soc. 2003, 125, 9090−101. (32) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545−610. (33) Burstein, E. Phys. Rev. 1954, 93, 632−633. (34) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668−677. (35) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410− 8426. (36) Fafarman, A. T.; Hong, S.-H.; Caglayan, H.; Ye, X.; Diroll, B. T.; Paik, T.; Engheta, N.; Murray, C. B.; Kagan, C. R. Nano Lett. 2013, 13, 350−7. (37) Sönnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J. Phys. Rev. Lett. 2002, 88, 1−4. (38) Shalaev, V. M. Nat. Photonics 2007, 1, 41−48. (39) Smith, D. R.; Pendry, J. B.; Wiltshire, M. C. K. Science 2004, 305, 788−92. (40) Manthiram, K.; Alivisatos, A. P. J. Am. Chem. Soc. 2012, 134, 3995−3998. (41) Liu, X.; Atwater, M.; Wang, J.; Huo, Q. Colloids Surf., B 2007, 58, 3−7. (42) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Nature 2010, 466, 474−477. (43) Jiao, Y.; Stillinger, F.; Torquato, S. Phys. Rev. E 2009, 79, 041309−041309. (44) Henzie, J.; Grünwald, M.; Widmer-Cooper, A.; Geissler, P. L.; Yang, P. Nat. Mater. 2011, 10, 1−7. (45) Ye, X.; Chen, J.; Diroll, B. T.; Murray, C. B. Nano Lett. 2013, 13, 1291−7. (46) Alaeian, H.; Dionne, J. A. Opt. Express 2012, 20, 15781−96. (47) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55−9.

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