Letter pubs.acs.org/NanoLett
Polarization Tunable, Multicolor Emission from Core−Shell Photonic III−V Semiconductor Nanowires Sudha Mokkapati,* Dhruv Saxena, Nian Jiang, Patrick Parkinson, Jennifer Wong-Leung, Qiang Gao, Hark Hoe Tan, and Chennupati Jagadish Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, A. C. T. 0200, Australia S Supporting Information *
ABSTRACT: We demonstrate luminescence from both the core and the shell of III−V semiconductor photonic nanowires by coupling them to plasmonic silver nanoparticles. This demonstration paves the way for increasing the quantum efficiency of large surface area nanowire light emitters. The relative emission intensity from the core and the shell is tuned by varying the polarization of the excitation source since their polarization response can be independently controlled. Independent control on emission wavelength and polarization dependence of emission from core−shell nanowire heterostructures opens up opportunities that have not yet been imagined for nanoscale polarization sensitive, wavelength-selective, or multicolor photonic devices based on single nanowires or nanowire arrays. KEYWORDS: Core−shell nanowires, quantum efficiency, plasmonic nanoparticles, multicolor emission, III−V semiconductor nanowires
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micrograph of the nanowire. The nanowires are free of twin defects. Figure 1c shows a cross-sectional TEM micrograph of the nanowire. The nanowire has a 230 nm diameter GaAs core, 60−80 nm thick AlGaAs shell, and 5−25 nm thick GaAs cap. The AlGaAs shell passivates the GaAs core and the GaAs cap prevents the oxidation of the AlGaAs shell. We use random (in position and size) arrays of silver (Ag) nanoparticles as plasmonic nanoantennas that couple to the AlGaAs shell in the nanowire to enhance its quantum efficiency and determine its emission response to polarization of an excitation source. To achieve the above functionalities, complex nanoantenna designs can be used. However, only very small volumes (most often single molecule) of emitters can be coupled to the antennae, and the fabrication process is both time- and resource-consuming.11−19 Ours is a simple approach that can be used for large area applications. After nanowire growth, silver (Ag) nanoparticles are deposited on the nanowires using a sputter deposition system. Ag nanoparticles are deposited on all facets of nanowires in the array. The Ag nanoparticles cover the nanowires from tip to the base as seen in Figures 1d and e. Since Ag nanoparticles can be deposited on nanowires standing vertically on the substrate, this approach can be used to fabricate array-based devices like solar cells. Following Ag deposition the nanowires are transferred onto Si substrates for optical characterization. The Ag particles remain intact on the nanowire surface after transfer onto Si substrates
pitaxially grown semiconductor nanowires via the vapor− liquid−solid (VLS) mechanism are ideal candidates for three-dimensional integration of nanoscale optoelectronic devices.1 Most of the heterostructure nanowires that have been demonstrated for device applications emit only from a single bandgap semiconductor layer.2−5 Other layers in the nanowire typically provide carrier confinement/optical confinement/carrier injection functionality. A few reports have demonstrated multicolor emission from a single nanowire by exploiting the quantum confinement effects.6−8 Nanowire emitters also show strong polarization dependence due to their geometry and high index contrast with their surroundings.9 In this paper, we demonstrate multicolor emission from GaAs core-AlGaAs shell nanowires without quantum confinement by coupling them to plasmonic nanoparticles. Our approach can increase the quantum efficiency and control the polarization response of the nanowires. The approach demonstrated here will enable the use of high quantum efficiency nanowires for efficient, integrated multicolor, polarization-selective optoelectronic devices. The nanowires studied here are grown via the vapor−liquid− solid (VLS) mechanism by metal−organic chemical vapor deposition (MOCVD). The nanowire dimensions are controlled by changing the diameter of the gold (Au) catalyst particles and/or the growth time. A two-temperature growth process is employed to grow defect-free and uniform (without tapering) nanowires with zinc blende structure.10 Figure 1a shows a scanning electron microscope (SEM) image of a nanowire grown with 250 nm diameter Au catalyst particle, and Figure 1b shows the transmission electron microscope (TEM) © 2012 American Chemical Society
Received: October 11, 2012 Revised: November 2, 2012 Published: November 6, 2012 6428
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bandgap energies. The carriers lose the excess energy and quickly relax to the band edge through carrier−carrier scattering or carrier−phonon scattering. Once the carriers have relaxed to the band-edge, they either recombine radiatively or nonradiatively through bulk or interface/surface defects. Since the GaAs core has a lower bandgap than the AlGaAs shell, the carriers generated in AlGaAs can also be captured into the GaAs core, through diffusion. Each of these processes has a characteristic time scale associated with it. Figure 2b illustrates the different processes and the characteristic times for these processes. The lifetime for capture of carriers from AlGaAs shell into the GaAs core, τcap, is estimated from the carrier drift velocity and the average thickness of the AlGaAs shell (70 nm). Using a drift velocity of 106 cm/s20 for the photoexcited carriers in AlGaAs, τcap is estimated to be ∼7 ps. The radiative recombination lifetime in the AlGaAs shell in the bare nanowires can be calculated using τr(AlGaAs) = 1/BN,21 where B is the radiative recombination coefficient and N is the carrier density and is ∼1 μs for a carrier density of 1 × 1018 cm−3. Since τr(AlGaAs) ≫ τcap in the bare nanowires, the carriers generated in the AlGaAs shell are captured by the GaAs core before they can recombine radiatively, and hence AlGaAs emission is not observed in bare nanowires (Figure 2a) or in any optoelectronic quality GaAs/AlGaAs epitaxial heterostructures at room temperature and low excitation power. A strong AlGaAs PL emission is observed from the nanowires with Ag nanoparticles on the surface. This is only possible because the radiative lifetime of carriers generated in the AlGaAs shell is reduced significantly as a result of strong field localization around the nanowire shell due to the Ag nanoparticles, as shown by the simulation results in Figure 2c. The enhanced field intensities in the nanowire shell are due to an increase in the local density of states (LDOS) which affects the carrier recombination rate in accordance with the Fermi’s Golden rule.22 The quantum efficiency of the GaAs core is controlled by nanowire growth parameters.10,23,24 The Ag nanoparticles do not affect the quantum efficiency of the GaAs core since the enhanced electric field intensities decay as we move toward the core of the nanowire as seen from the right panel of Figure 2c. The AlGaAs shell provides surface passivation to the GaAs core. The carriers injected into the GaAs core are confined to the core due to the presence of the higher bandgap AlGaAs shell. This prevents nonradiative recombination via surface defects of carriers injected into the GaAs core. Increased quantum efficiency in the AlGaAs shell in nanowires coupled to plasmonic nanoparticles does not affect this process. The carriers injected into GaAs are still confined to GaAs and are prevented from nonradiative surface recombination because of the higher bandgap AlGaAs shell. The presence of the plasmonic nanoparticles on the surface of the nanowires only affects the carriers injected into the AlGaAs shell. In bare nanowires, these carriers are captured into the core, while they recombine radiatively in the shell itself in nanowires with Ag nanoparticles on the surface. Figure 3a shows the change in intensity of the photoluminescence peak from the GaAs core and AlGaAs shell, as the polarization of excitation radiation is varied. The points are the experimental data, and the solid lines are (cosine)2 fits to the experimental data. The GaAs core emits strongly when the incident light is linearly polarized along the axis of the nanowire. This behavior is typical of high refractive index semiconductor nanowires and arises due to geometry and
Figure 1. Structural characterization of nanowires: (a) Scanning electron microscope (SEM) image of the nanowire studied in this work. The gold (Au) catalyst particle used to grow the nanowire is seen on the top of the nanowire. (b) Transmission electron microscope (TEM) micrograph showing the twin defect-free nanowires. (c) Cross-sectional TEM micrograph of the nanowires showing the thickness of different layers in the nanowire. (d−e) SEM images of a nanowire after deposition of silver (Ag) nanoparticles. The nanoparticles cover the entire nanowire from the tip (d) to the base (e). (f) SEM image of nanowires after transferring to a silicon (Si) substrate for photoluminescence measurements. The Ag nanoparticles remain on the nanowire surface.
as shown in Figure 1f. Since single nanowires can be isolated with Ag nanoparticles intact on the surface, this approach is also suitable for single nanowire devices. Figure 2a shows the photoluminescence spectra from a single bare nanowire and a nanowire with Ag nanoparticles at 300 K. The spectrum from the bare nanowire shows a single peak at ∼1.4 eV corresponding to band edge emission from the GaAs core. The spectrum from the nanowire with Ag nanoparticles shows two peaks, one at 1.4 eV and one peak centered at 1.75 eV. The peak at 1.4 eV is from band to band recombination in the GaAs core. The peak at 1.75 eV is from the AlGaAs shell. The position of this peak can be tuned by controlling the Al composition in the shell. This is the first report of simultaneous emission from the lower bandgap core and higher bandgap shell of a high quality core−shell nanowire, which is promising for the realization of multicolor optoelectronic devices such as nanowire lasers/LEDs. In the experiments described here, carriers are generated in the nanowires through excitation with 522 nm laser radiation. Since the excitation energy is higher than the bandgap of both GaAs and AlGaAs, carriers are generated in the AlGaAs shell and in the GaAs core with energies much higher than their 6429
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Figure 2. Photoluminescence study of the nanowires: (a) Room temperature photoluminescence (PL) emission from a bare nanowire and a nanowire with silver (Ag) nanoparticles on the surface. (b) Schematic showing the different recombination processes that could occur in different layers of the nanowire together with their associated carrier lifetimes. (c) Calculated electric field intensity at 1.75 eV (AlGaAs emission) in a bare nanowire (left) and a nanowire with a Ag nanoparticle on the surface (right). Light is incident on the nanowire in the z-direction and is polarized along the x-axis. The nanowire and the Ag nanoparticle orientation with respect to these axes is shown in the inset.
Figure 3. Polarization study of luminescence from the nanowire: (a) Variation in the room temperature photoluminescence (PL) counts from the GaAs core and AlGaAs shell of the nanowire, as a function of the polarization of excitation radiation. The insets show how the relative intensity of the GaAs emission and the AlGaAs emission peaks can be tuned by varying the polarization of incident radiation. (b) Numerical results for recombination rate (Γ) enhancement for a dipole emitter 10 nm away from the semiconductor−Ag nanoparticle interface. Results shown are for a nanoparticle radius of 50 nm. The solid black line is for a dipole oriented along the radial direction of the nanowire, and the dash−dot blue line is for a dipole oriented along the axis of the nanowire.
refractive index contrast between the nanowires and their surroundings.9 The AlGaAs shell, on the other hand, emits strongly when the excitation light is polarized along the radial direction of the nanowire. This behavior is atypical of zinc blende nanowire emission and is only possible when the AlGaAs emission is coupled to the Ag nanoparticles. Figure 3b shows the numerically calculated recombination rate (Γ) enhancement expected for a dipole emitter oriented along the axial or radial direction of the nanowire. The dipole emitter is placed 10 nm away from the semiconductor−Ag nanoparticle interface, that
is, in the AlGaAs shell. The recombination rate enhancement factor for a radial dipole is ∼8 times larger than the enhancement expected for an axial dipole due to stronger coupling between the Ag nanoparticle and the dipole emitter for radial orientation.25 Stronger coupling for radial dipole orientation results in stronger emission when the AlGaAs shell is excited with a laser with its polarization axis along the radius of the nanowire. The insets in Figure 3a show how the relative intensities of the GaAs and AlGaAs emission peaks can be tuned by changing the polarization of the excitation beam. The GaAs emission 6430
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(6) Qian, F.; Brewster, M.; Lim, S. K.; Ling, Y.; Greene, C.; Laboutin, O.; Johnson, J. W.; Gradečak, S.; Cao, Y.; Li, Y. Nano Lett. 2012, 12 (6), 3344−3350. (7) Fontcuberta i Morral, A.; Spirkoska, D.; Arbiol, J.; Heigoldt, M.; Morante, J. R.; Abstreiter, G. Small 2008, 4 (7), 899−903. (8) Hiruma, K.; Tomioka, K.; Mohan, P.; Yang, L.; Noborisaka, J.; Hua, B.; Hayashida, A.; Fujisawa, S.; Hara, S.; Motohisa, J.; Fukui, T. J. Nanotechnology 2012, 2012. (9) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Science 2001, 293 (5534), 1455−1457. (10) Joyce, H. J.; Gao, Q.; Tan, H. H.; Jagadish, C.; Kim, Y.; Zhang, X.; Guo, Y.; Zou, J. Nano Lett. 2007, 7 (4), 921−926. (11) O’Carroll, D. M.; Hofmann, C. E.; Atwater, H. A. Adv. Mater. 2010, 22 (11), 1223−1227. (12) Muskens, O. L.; Giannini, V.; Sánchez-Gil, J. A.; Gómez Rivas, J. Nano Lett. 2007, 7 (9), 2871−2875. (13) Bakker, R. M.; Yuan, H.-K.; Liu, Z.; Drachev, V. P.; Kildishev, A. V.; Shalaev, V. M.; Pedersen, R. H.; Gresillon, S.; Boltasseva, A. Appl. Phys. Lett. 2008, 92 (4), 043101. (14) Qiu, T.; Kong, F.; Yu, X.; Zhang, W.; Lang, X.; Chu, P. K. Appl. Phys. Lett. 2009, 95 (21), 213104. (15) Kühn, S.; Håkanson, U.; Rogobete, L.; Sandoghdar, V. Phys. Rev. Lett. 2006, 97 (1), 017402. (16) Bharadwaj, P.; Novotny, L. Opt. Express 2007, 15 (21), 14266− 14274. (17) Anger, P.; Bharadwaj, P.; Novotny, L. Phys. Rev. Lett. 2006, 96 (11), 113002. (18) Kinkhabwala, A.; Yu, Z.; Fan, S.; Avlasevich, Y.; Mullen, K.; Moerner, W. E. Nat. Photon. 2009, 3 (11), 654−657. (19) Tam, F.; Goodrich, G. P.; Johnson, B. R.; Halas, N. J. Nano Lett. 2007, 7 (2), 496−501. (20) Göbel, E. O.; Jung, H.; Kuhl, J.; Ploog, K. Phys. Rev. Lett. 1983, 51 (17), 1588−1591. (21) Ahrenkiel, R. K. Minority-Carrier Lifetime in III-V Semiconductors. In Semiconductors and Semimetals; Elsevier: New York, 1993; Vol. 39. (22) Novotny, L.; Hecht, B. Principles of nano-optics; Cambridge University Press: New York, 2008. (23) Chang, C.-C.; Chi, C.-Y.; Yao, M.; Huang, N.; Chen, C.-C.; Theiss, J.; Bushmaker, A. W.; LaLumondiere, S.; Yeh, T.-W.; Povinelli, M. L.; Zhou, C.; Dapkus, P. D.; Cronin, S. B. Nano Lett. 2012, 12, 4484−4489. (24) Jiang, N.; Parkinson, P.; Gao, Q.; Breuer, S.; Tan, H. H.; WongLeung, J.; Jagadish, C. Appl. Phys. Lett. 2012, 101 (2), 023111. (25) Mertens, H.; Koenderink, A. F.; Polman, A. Phys. Rev. B 2007, 76 (11), 115123. (26) Rau, U. Phys. Rev. B 2007, 76 (8), 085303. (27) Miller, O. D.; Yablonovitch, E.; Kurtz, S. R. IEEE J. Photovoltaics 2012, 2 (3), 303−311.
peak is comparable to the AlGaAs emission peak when excitation is polarized along the axis of the nanowire while AlGaAs emission is much stronger when polarization is along the radius of the nanowire. To summarize, we have demonstrated (i) multicolor, polarization tunable emission from photonic nanowires and (ii) increase in quantum efficiency from a large surface area, high index semiconductor nanowire by coupling it to plasmonic nanoparticles. The demonstration of polarization tunable, simultaneous emission from the core and shell of single photonic nanowires with independent control on the quantum efficiency of the different bandgap emitters opens up the potential for compact, integrated, multifunctional, wavelengthselective or multiwavelength, polarization selective single nanowire optoelectronic devices such as multicolor lasers, multicolor polarized LEDs, or polarization-selective photodetectors. The integration of several functional devices in a single nanowire will miniaturize current nanowire device technology and increase the density of device integration. The increase in quantum efficiency of the semiconductor nanowire due to coupling to plasmonic nanoparticles may lead to low threshold lasing in photonic nanowires. Since our simple fabrication approach can be used to fabricate single nanowires or nanowire arrays coupled to plasmonic nanoparticles, this approach is applicable to the realization of both single nanowire based integrated devices or nanowire array based devices. The increase in quantum efficiency or in other words the increase in radiative recombination rate without significant increase in the nonradiative recombination rate in photonic nanowires has significant consequences for array based devices like nanowire solar cells leading to higher efficiencies.26,27
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ASSOCIATED CONTENT
S Supporting Information *
Details of experimental set-ups and numerical calculations. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Prof. Leigh Smith for discussions. We acknowledge the Australian National Fabrication Facility (ANFF) for access to facilities and the National Computational Infrastructure (NCI) for providing the computational resources used for this work. We acknowledge the Australian Research Council (ARC) for financial support.
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