Subwavelength Optical Resonant Cavity-Induced Enhancement of

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Subwavelength Optical Resonant Cavity-Induced Enhancement of the Near-Band-Edge Emission from ZnO-Core/SnO2-Shell Nanorods Changhyun Jin,† Hyunsu Kim,† Han-Youl Ryu,‡ Hyoun Woo Kim,*,§ and Chongmu Lee*,† †

Department of Materials Science and Engineering and ‡Department of Physics, Inha University, 253 Yonghyeon-dong, Incheon 402-751, Republic of Korea § Division of Materials Science and Engineering, Hanyang University, 17 Haengdang, Seongdong-gu, Seoul 133-791, Republic of Korea ABSTRACT: ZnO-core/SnO2-shell nanorods were fabricated by a two-step process: thermal evaporation of ZnO powders and atomic layer deposition of SnO2. Transmission electron microscopy and X-ray diffraction revealed the cores and shells of the as-prepared core
shell nanorods to be single crystal wurtzitetype ZnO and polycrystalline rutile-type tetragonal SnO2, respectively. Photoluminescence (PL) measurements showed that the intensity of near-band edge (NBE) emission of ZnO nanorods was enhanced significantly by the SnO2 coating. The maximum intensity of NBE emission of the ZnO-core/SnO2shell nanorods obtained with a shell layer thickness of 15 nm was ∼25 times higher than that of the ZnO nanorods. The enhancement of the NBE emission might be due to the combination of the following sources: the giant oscillator strength effect due to subwavelength optical resonant cavity formation in the nanorods, the quantum confinement of photogenerated carriers inside the ZnO cores, the suppression of visible emission and nonradiative recombination due to the formation of a depletion region in the ZnO cores, and the suppression of carrier capture by surface states. In particular, the exceptionally high NBE emission intensity for a specific shell layer thickness of 15 nm was attributed mainly to subwavelength optical resonant cavity formation.

1. INTRODUCTION The search for short wavelength luminescence is driven by two major applications: white light emitting diode (LED) illumination and compact disk (CD)/digital video disk (DVD) with high information storage capacity. White LED illumination can be realized by passing near-ultraviolet (UV) LED through luminescent materials. LEDs also find applications in general illumination, indicator lamps, automotive applications, traffic signals, large displays, LCD backlights, keypads, and camera flash units.1,2 The major advantage of the short-wavelength laser diodes (LD) is that they will increase the information storage capacity of such devices as Blue-ray Disc or HDDVD.3 For similar reasons, laser printing systems will also benefit from short-wavelength devices. Short-wavelength UV light sources also find applications in sterilization, air purification, and biosensors.4 In recent years, the development of GaN-based optoelectronic devices covering a wide spectral region from green to UV has determined a revolution in the lighting and information storage industry. Owing to the introduction of high performance device structures, the GaN LED technology is targeted to replace ordinary white light bulbs for general lighting, with enormous energy savings and environmental benefits.5 In addition, the information storage capacity of CD and DVD has been greatly increased by GaN LD technology. On the other hand, since the late 1990s, ZnO has rapidly emerged as a promising oxide semiconductor for its potential applications in optoelectronic devices emitting UV/blue light, such as light-emitting r 2011 American Chemical Society

diodes (LEDs) and laser diodes (LDs) owing to its direct wide band gap of 3.37 eV at room temperature and low power threshold for optical pumping. ZnO has many advantages over GaN, which is currently used for fabricating short-wavelength optoelectronic devices in industry, such as a large exciton binding energy (60 meV), availability of ZnO substrates, amenability to chemical wet etching, the possibility of low-temperature epitaxial growth, and excellent radiation resistance. Moreover, ZnO is thermally and chemically stable in air.6
8 A variety of techniques have been used to enhance the near-band edge (NBE) emissions and simultaneously to suppress the deep level emissions of ZnO nanostructures. These techniques include thermal annealing in a hydrogen9 or oxygen atmosphere,10 hydrogen11 or argon plasma treatment,12 hydrogen13 or Ga doping,14 and encapsulation of the nanostructures with thin films15
28 or coating them with nanoparticles.29 Particularly, in the case of ZnO 1D nanostructures, encapsulation has been widely studied for the enhancement of their NBE emission.15
29 The capping materials include ceramic materials such as SnO2,15
18 ZnS,19,20 MgO,21
23 Al2O3,24 and ZnCdO,25 metals such as Zn,26 Au,27 Ag,28 Pt,29and polymers such as poly(methyl methacrylate)30 and polyaniline.31 Shi et al.17 reported that Received: January 3, 2011 Revised: March 9, 2011 Published: April 12, 2011 8513

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The Journal of Physical Chemistry C SnO2 capping led to the improvement of NBE emission and suppression of deep-level emission from ZnO nanowires. They attributed the enhancement of the NBE emission mostly to the quantum confinement effect of charge carriers in the ZnO cores due to an energy band gap of SnO2 larger than that of ZnO. However, the change of the NBE emission intensity with the SnO2 shell layer thickness cannot be fully explained by the quantum confinement effect. This paper reports the fabrication of ZnO-core/SnO2-shell 1D nanostructures with a very uniform shell layer thickness and the significant enhancement of the NBE emission of the ZnO nanorods by SnO2 coating. In particular, this study examined the dependence of the NBE emission intensity of ZnO-core/SnO2-shell nanorods on the SnO2 shell layer thickness. A new model was proposed to explain the exceptionally intense photoluminescence (PL) of the ZnO-core/ SnO2-shell nanorods with a specific shell layer thickness.

2. EXPERIMENTAL DETAILS The preparation of ZnO-core/SnO2-shell nanorods consists of two steps: thermal evaporation of a mixture of ZnO and graphite powders (ZnO:C = 1:1) in an oxidative atmosphere, and ALD of SnO2. A gold (Au)-coated p-type Si (100) substrate was placed on the top of an alumina boat containing a mixture of ZnO and graphite positioned at the center of a quartz tube furnace. The furnace was heated to 650 °C and maintained at that temperature under a constant total pressure of 1 Torr with a mixture of nitrogen (N2) and oxygen (O2) gas for 1 h. The flow rates of N2 and O2 gas were 30 and 5 standard cubic centimeters per minute (sccm), respectively, giving an O2 partial pressure of 1.5%. The ZnO-core/SnO2-shell nanorod samples were then prepared by ALD of SnO2 thin films on the as-synthesized ZnO nanorods. SnCl4 and H2O, which were used as the Sn precursor and oxidant, respectively, were kept in a canister at 0 °C for SnCl4 and 10 °C for H2O. SnCl4, H2O, and N2 purge gas were fed into the chamber through separate inlet lines and nozzles with a purge occurring after each ALD cycle. This cycle was repeated for many times (for example, 1200 times for a SnO2 layer thickness of 20 nm). The typical pulse times for both the SnCl4 and H2O feeds were 1 s, and the purge time between the reactants was 2 s. The other process parameters for SnO2 deposition used in these ALD processes are as follows: base vacuum = 1.0  10
6 Torr,, chamber pressure = 0.1 Torr, N2 gas flow rate = 100 sccm, and substrate temperature = 350 °C. The morphology and size of the final products were examined by scanning electron microscopy (SEM, Hitachi S-4200) equipped with an energy-dispersive X-ray spectrometer (EDS). High-resolution transmission electron microscopy (HRTEM, Philips CM-200) and selected area electron diffraction (SAED) were carried out at an acceleration voltage of 200 kV. The samples used for characterization were dispersed in absolute ethanol and ultrasonicated before the SEM and TEM observations. Glancing angle (0.5°) X-ray diffraction (XRD) was performed to investigate the phases of the obtained products. Room temperature and low-temperature PL measurements were carried out using a He
Cd laser (325 nm) as the excitation source. 3. RESULTS AND DISCUSSION The ZnO 1D nanostructures synthesized via vapor
liquid
solid (VLS) mechanism were 40
120 nm in the side length of the square cross-section and a few tens to a few hundreds of micrometers in length. The ZnO-core/SnO2-shell 1D nanostructures were also fabricated by coating the ZnO 1D

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Figure 1. (a) SEM images the SnO2-coated ZnO nanorods prepared by a two-step process: thermal oxidation of ZnO powders at 900 °C for 1 h and ALD of SnO2 at 350 °C for 1200 cycles. Inset, enlarged SEM image of a typical SnO2-coated ZnO nanorod with faceted surfaces. (b) Lowmagnification TEM image of a typical as-synthesized ZnO-core/SnO2shell nanorod.

Figure 2. (a) High-magnification TEM image of the interface region of the core and shell of a typical as-synthesized ZnO-core/SnO2-shell nanorod and (b) corresponding SAED pattern.

nanostructures with SnO2 by atomic layer deposition (ALD). Figure 1a shows a SEM image of the as-prepared ZnO-core/ SnO2-shell 1D nanostructures. Most of the nanostructures had a rod-like morphology with faceted surfaces and a square crosssection (inset). ZnO 1D nanostructures with similar faceted surface morphologies (cuboid ZnO nanorods) obtained by thermal evaporation of the mixture of ZnO and graphite powders were reported previously.32 The 1D nanostructures will be called nanorods hereafter. The nanorods were ∼100 to ∼180 nm in width and a few tens of micrometers in length, as shown in Figure 1a. The structure and crystallinity of the nanorods were characterized further by TEM. Figure 1b shows a low-magnification TEM of a typical ZnO-core/SnO2-shell nanorod, indicating that the thickness of the SnO2 shell layer in the core
shell nanorod is ∼30 nm and extremely uniform throughout the length of the nanorod, as expected from the ALD technique employed for SnO2 shell layer formation. Figures 2a and 2b show a bright-field HRTEM image and corresponding diffraction pattern of a typical annealed ZnOcore/SnO2-shell nanorod, respectively. The selected area electron diffraction (SAED) patterns were recorded with the incident electron beam parallel to the [2110] direction. The HRTEM image showed that the core and shell of a typical core
shell nanorod were single crystal wurtzite-type face-centered cubic ZnO with lattice constants of a = 0.3241 nm and c = 0.5187 nm (P63mc, JCPDS No. 79-0205) and polycrystalline (but locally monocrystalline) rutile-type tetragonal SnO2 with lattice constants of a = 0.4738 nm and c = 0.3187 nm (P42/mnm, JCPDS No. 41-1445), respectively. Besides the strong spotty pattern for 8514

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Figure 3. XRD patterns of the nanorods, SnO2 thin films and ZnOcore/SnO2-shell nanorods prepared by the two-step process and subsequent thermal annealing at 65 °C for 1 h.

Figure 4. Room-temperature PL spectra of the as-synthesized ZnOcore/SnO2-shell nanorods with different SnO2 shell layer thicknesses. Inset, normalized PL intensity (I/I0) versus SnO2 shell layer thickness.

single crystal ZnO, several dim circles with several weak spots on them for SnO2 was observed (Figure 2b), which suggests that the SnO2 shells are polycrystalline. The ZnO nanorods appear to have grown along the [101] lattice direction. The interplanar spacings of the core and shell regions were approximately 0.52 and 0.33 nm, corresponding to the (0001) plane of wurtzite-type ZnO and the (112) plane of tetragonal rutile-type SnO2, respectively. Figures 3a
c show XRD patterns of the as-synthesized ZnO nanorods, SnO2 thin films, and ZnO-core/SnO2-shell nanorods, respectively. The XRD pattern of the SnO2 thin films was shown just as a reference for comparison. In the XRD pattern of the core
shell nanorods, besides the reflection peaks from single crystal wurtzite-type ZnO, which are marked by dark squares, four characteristic peaks of SnO2, which are marked by open triangles, were identified and indexed to the (101), (200), (111), and (310) planes for the rutile-type tetragonal SnO2 shell layers. These results are consistent with the TEM diffraction patterns. Figure 4 shows the room-temperature PL spectra of the assynthesized ZnO nanorods and SnO2-coated ZnO nanorods. The uncoated ZnO nanorod sample (corresponding to a SnO2 shell layer thickness of 0 nm) shows relatively weak UV emission at ∼377 nm corresponding to NBE emission along with a broad

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stronger deep-level (DL) emission band centered at approximately ∼580 nm. In contrast, the coated nanorods exhibit more intense NBE emission and considerably weaker DL emission. Intense NBE emission is obviously essential in the realization of high quality UV optoelectronic devices, such as LEDs and LDs. NBE emission is associated with the excitons bound to shallow donors, whereas DL emission is associated with oxygen vacancyrelated defects, such as singly ionized oxygen vacancies, which can easily form recombination centers. The substantial enhancement of NBE emission from the ZnO nanorods by SnO2 coating can be explained by combining the following four sources: (1) Giant oscillator strength effect due to two-dimensional (2D) subwavelength optical resonant cavity formation in the cross-section of SnO2-coated ZnO nanorods, which may be the main cause for the enhancement. (2) Suppression of not only the tunneling of charge carriers but also the reabsorption of emitted photons: The photogenerated carriers are confined inside the ZnO core because the larger band gap of the SnO2 shell (Eg,SnO2 = 3.54 eV, Eg,ZnO = 3.37 eV)33 suppresses the tunneling of charge carriers21 from the ZnO core to the SnO2 shell (Figure 5) and the emitted photons are also confined in the ZnO core because they are not readily reabsorbed by the SnO2 shell with a lower refractive index22 (nSnO2 = 1.75, nZnO = 1.93).34 These confinement effects, however, may not be so strong because ZnO and SnO2 have similar energy band gap and refractive index. (3) Suppression of visible emission and nonradiative recombination: A depletion region exists in the ZnO core.35,36 The transition process for visible emission and nonradiative recombination may be suppressed in the depletion region where the Fermi energy level is lower than the energy levels of visible emission related defects (oxygen vacancies and Zn interstitials) and nonradiative transition related defects. (4) Suppression of the capture of carriers by surface states: Unless otherwise the surfaces were passivated, the photogenerated carriers would be readily captured by the surface states because of the high specific surface area of the ZnO nanorod and the short relaxation time for carriers tunneling from the interior of the ZnO nanorod to the surface.37 Consequently, the band-edge emissions would be quenched in nanomaterials. In the core
shell nanorod sample synthesized in this study, the surface state density is believed to be sufficiently low because the surfaces of the ZnO nanorods were well passivated by the SnO2 shell layer. The inset in Figure 4 shows that the NBE emission intensity of the ZnO nanorods depends on the coating layer thickness strongly. The highest intensity of NBE emission from the ZnOcore/SnO2-shell nanorods was obtained with a SnO2 shell layer thickness of 15 nm. The NBE emission intensity of the nanorods for a coating thickness of 15 nm was ∼25 times higher than that of the uncoated nanorods (inset). This exceptionally intense NBE emission for a specific SnO2 shell layer thickness of 15 nm can be explained mainly by the giant oscillator strength effect due to the optical resonant cavity formation among the above four sources. A 2-D optical resonant cavity is believed to be formed between the SnO2coated ZnO nanorod surfaces and air as shown in Figure 6, because the difference in refractive index between ZnO and SnO2 is quite small (nSnO2 = 1.75, nZnO = 1.93). The ZnO nanorods are 8515

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Figure 5. Energy band diagram of isolated ZnO adjacent to isolated SnO2 (left). EO, EC, EV, EF, and qχ denote the vacuum level, conduction band edge, valence band edge, Fermi level, and electron affinity, respectively. e
and hþ denote electron and hole, respectively. Energy diagram of the ZnO-core/ SnO2-shell nanorod system at thermal equilibrium (right). The diagram is not drawn to scale. The EFs of ZnO2 and SnO2 were assumed to be nearly the same before contact of the two materials.

Figure 6. Schematic diagram showing the formation of the twodimensional optical resonant cavity in a typical ZnO-core/SnO2-shell nanorod with a square cross-section. The ZnO core width and the SnO2 shell layer thickness are ∼110 and ∼15 nm, respectively.

well-faceted single crystals. The ZnO-core/SnO2-shell nanorods have faceted surfaces (inset in Figure 1a) because the SnO2 layers formed on the faceted ZnO nanorod surfaces by MOCVD are uniform in thickness. Therefore, the surfaces of the square crosssection of each nanorod may serve as mirrors. The resonance condition in a rectangular cavity with a length of a and a height of b can be written as38 vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u" 2  2 # u neff l m ð1Þ ¼t þ 2a 2b λ where λ is the light wavelength in vacuum and neff is the effective refractive index of nanorod materials. l and m are mode numbers in the x and the y direction, respectively. Since the cross-section of the fabricated nanorod has an almost square shape, a ≈ b. Therefore, for the lowest-order-mode where l = m = 1, the length of the nanorod and the wavelength has the following relation from eq 1. λ a ¼ pffiffiffi 2neff

ð2Þ

Assuming that neff is simply a volume average of the refractive index of the ZnO core and the SnO2 shell, the length a of the nanorod is calculated to be ∼140 nm from eq 2 for the wavelength of 377 nm. The length of the square cross-section of nanorods measured from the TEM images ranges from ∼100 to ∼180 nm.

Therefore, it appears that the calculated cavity length of ∼140 nm falls well in the range of the measured ones. There have been many reports on the enhancement of the PL intensity over the range of wavelengths due to optical resonant cavity formation, e.g., in annealed silicon-on-insulator structures (∼37 times in intensity),39 Er-doped Si/SiO2 resonant cavities (∼5 times)],40 and the CdSe pillars fabricated by a combination of electron-beam lithography and electrochemical deposition.41 In particular, it is worth noting that the length (∼140 nm) of the cavities formed in the nanorods are much smaller than the wavelength of the laser used as the light source in the PL measurement (325 nm). Subwavelength lasing due to optical resonant cavity formation, which is similar to the subwavelength oscillator strength effect found in this work, has been reported for several times recently, e.g., in metallic-coated nanocavities, such as a semiconductor heterostructure encapsulated in a thin gold film,42 full three-dimensional surface plasmon polariton cavity,43 and a semiconductor nanodisc with a silver nanopan cavity.44 Figures 7a
d show the low-temperature PL spectra of the ZnO nanorods and ZnO/SnO2 nanorods. The free exciton (FX), neutral donor exciton (D°X), neutral acceptor exciton (A°X), and longitudinal optic (LO) phonon replica emissions dominate the spectra of the ZnO nanorods at all temperatures from 10 to 250 K. The FX, D°X, and A°X, and LO emissions, also dominate the spectra of the SnO2-coated ZnO nanorods at temperatures ranging from 10 to 250 K, even if FX emission were not observable in the spectra of the SnO2-coated ZnO nanorods at 250 K. A comparison of the spectrum of the ZnO nanowires with that of the ZnO/SnO2 nanorods at 10 K shows that SnO2 coating caused a 0.058, 0.058, 0.054, 0.054, and 0.054 eV blue-shift in the FX, D°X, A°X, FX-LO, and D°X-LO emissions of the ZnO nanorods, respectively. The fact that all the FX, D°X, A°X, FXLO, and D°X-LO emissions were shifted simultaneously by the SnO2 coating suggests that the shift does not originate from a specific point defect with a relatively low concentration but from a far higher density of defects, such as impurity dopants or surface states, on the surface of the nanorods with a high specific surface area. The blue shift may have two possible sources: Sn doping and the passivation of surface states. Regarding Sn doping, there are several reports on the influence of Sn doping on the PL properties of ZnO nanostructures. According to previous reports, the UV emission peak of ZnO doped with Sn was blue-shifted due to the Burstein
Moss effect under a certain doping level but redshifted due to narrowing of the band gap, i.e., ‘band gap renormalization effect’ above the doping level with increasing 8516

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Figure 7. PL spectra of (a) as-synthesized ZnO nanorods in a temperature range from 10 to 250 K, (b) as-synthesized ZnO nanorods at 10 K, (c) assynthesized ZnO-core/SnO2-shell nanorods in a temperature range from 10 to 250 K, and (d) as-synthesized ZnO-core/SnO2-shell nanorods at 10 K.

the Sn doping concentration.45 Heavy Sn doping was also reported to produce the appearance of strong green emission in doped ZnO nanowires].46 The diffusion of Sn atoms from the SnO2 shell into the ZnO core may occur during the SnO2 deposition process. As a result of Sn diffusion, the ZnO core very near the interface will be doped with Sn. However, the diffusion of Sn atoms is rather unlikely because the substrate temperature of 350 °C in the ALD process for SnO2 coating is too low for the appreciable diffusion of Sn atoms to occur considering that the melting point of SnO2 is as high as 1630 °C. The lack of broadening of the UV emission peak but instead a blue shift in the UV emission peak by the SnO2 coating also supports the insignificant Sn doping. On the other hand, the capturing of carriers by surface states abundant at the surface of nanowires with a high specific surface area would be suppressed substantially by the passivation of ZnO nanorods with SnO2. Therefore, the blue-shift in UV emission was attributed to the surface-state passivation effect.

4. CONCLUSIONS The NBE emission of ZnO nanorods was enhanced considerably by SnO2 coating. The highest NBE emission of the ZnOcore/SnO2-shell nanorods was obtained for a ∼15 nm thick SnO2 coating layer. The enhanced NBE emission is attributed to the following: the giant oscillator strength effect due to subwavelength optical resonant cavity formation in the nanorods, the confinement of photogenerated carriers inside the ZnO cores due to the larger band gap of the neighboring SnO2 shells, the suppression of visible emission and nonradiative recombination due to the formation of a depletion region in the ZnO cores, and the suppression of carrier capture by surface states due to passivation of the ZnO cores by the SnO2 shells. In particular,

the exceptionally intense NBE emission of the core
shell nanorods is attributed mainly to two-dimensional subwavelength optical resonant cavity formation in the edge-length directions of each nanorod of the above four possible sources. In addition, the SnO2 coating caused a 0.058, 0.058, 0.054, 0.054, and 0.054 eV blue-shift in FX, D°X, A°X, FX-LO, and D°X-LO emissions of the ZnO nanorods, respectively, through the surface-state passivation effect.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (C.L.);[email protected] (H.W.K.).

’ ACKNOWLEDGMENT This study was supported by the Korea Science and Engineering Foundation through ‘2007 National Research Lab Program’. ’ REFERENCES (1) Schubert, E; Kim, J. Science 2005, 308, 1274. (2) Crawford, M. IEEE J. Sel. Top. Quantum Electron. 2009, 15, 1028. (3) Kozaki, T.; Nagahama, S.; Mukai, T. Proc. SPIE 2007, 6485, 648503. (4) Shur, M.; Gaska, R. IEEE Trans. Electron Devices 2010, 57, 12. (5) Krames, M.; Shchekin, O.; Mueller-Mach, R.; Mueller, G.; Zhou, L.; Harbers, G.; Craford, M. J. Display Technol. 2007, 3, 160. (6) Nakamura, S.; Mukai, T.; Senoh, M. Appl. Phys. Lett. 1994, 64, 1687. (7) Nakamura, S. Science 1998, 281, 956. (8) Li, B. S.; Liu, Y. C.; Zhi, Z. Z.; Shen, D. Z.; Lu, Y. M.; Zhang, J. Y.; Fan, X. W. J. Cryst. Growth 2002, 240, 479. 8517

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