J. Phys. Chem. C 2010, 114, 11081–11086
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Directly Probing the Anisotropic Optical Emission of Individual ZnO Nanorods Min Gao,*,† Rui Cheng,† Wenliang Li,† Yanping Li,‡ Xiaoxian Zhang,† and Sishen Xie†,§ Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, People’s Republic of China, State Key Laboratory on AdVanced Optical Communication Systems & Networks, Peking UniVersity, Beijing 100871, People’s Republic of China, and Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China ReceiVed: March 14, 2010; ReVised Manuscript ReceiVed: May 27, 2010
We report on the anisotropic optical emission of individual ZnO nanorods directly measured by angularresolved photoluminescence imaging and spectroscopy with independent excitation and detection optics. A reduced diameter at the subwavelength scale results in degraded waveguiding efficiency and more divergent emission at the top ends of individual ZnO nanorods grown by chemical vapor deposition. The near-bandedge (NBE) emission from the nanorod ends exhibits strong angular dependence in both emission intensity and spectral line shape. A continuous intensity increase and spectral red shift of the NBE emission have been observed as the detection direction varies from the radial direction to the axial direction. The pronounced red shift (>60 meV) is primarily attributed to the transition from free exciton (FX) emission to its 2LO phonon replica (FX-2LO) emission. Our results support an exciton-polariton model for the waveguiding behavior of ZnO nanorods and demonstrate the dominant role of the FX-2LO exciton-phonon interaction in the waveguiding process. We also show that the excitation angle has little influence on the NBE emission but can greatly modify the deep level emissions. 1. Introduction Recently, a growing interest has been attracted by the anisotropic optical properties of chemically synthesized quasione-dimensional (1D) semiconductor nanostructures, for example, nanowires, nanoribbons, and nanorods, which are promising building blocks for future integrated photonics.1,2 The intriguing optical confinement, especially in the radial direction of the nanostructures, plays important roles in the reported optoelectronic nanodevices based on 1D nanostructures, for example, light-emitting diodes, nanolasers, and waveguides.3-7 1D ZnO nanostructures are of particular technological importance8 and, meanwhile, a model system for studying light-matter interaction in a reduced dimension. Besides its well-known wide band gap and large exciton binding energy, ZnO also has a large oscillator strength, indicating strong light-matter interaction and possible existence of exciton-polaritons at room temperature,9 which can be further strongly enhanced in 1D nanostructures. For example, a Rabi splitting of ∼100 or even 164 meV has been reported for ZnO nanowires/nanorods.10 Recently, the exciton-polariton has been suggested to account for the waveguiding behavior in 1D ZnO nanostructures.11 Despite the strong technological and academic interest, systematic studies of the anisotropic optical properties of individual 1D nanostructures are still difficult due to the lack of effective angular-resolved experimental techniques with the resolution and sensitivity of individual nanostructures. Photoluminescence (PL) imaging has been widely used as a straight* To whom correspondence should be addressed. E-mail: mingao@ pku.edu.cn. † Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University. ‡ State Key Laboratory on Advanced Optical Communication Systems & Networks, Peking University. § Chinese Academy of Sciences.
forward technique to demonstrate the anisotropic properties of individual nanostructures.10,12,13 Spatially resolved PL spectroscopy using confocal and scanning near-field optical microscopes has also been employed to study the emission from the ends and the sidewall positions of 1D nanostructures.11-16 For the above measurements, the 1D nanostructures under investigation were mostly dispersed sparsely on flat substrates, and the direction of the luminescence collection was fixed to be perpendicular to the axis of the 1D nanostructures. The substrate influence can be minimized by using low-index porous silica substrates.13 However, the very limited collection efficiency along the axis of 1D nanostructures hinders quantitative studies on the anisotropic optical properties because the parallel orientation along the nanostructure axis is often the direction that matters. In this paper, we report our initial effort in performing angular-resolved photoluminescence imaging and spectroscopy on individual ZnO nanorods with independently adjustable excitation and detection angles relative to the nanorod axis. Strong angular distributions of intensity and spectral line shape of the near-band-edge (NBE) emission have been observed at the top ends of individual suspended nanorods attached to sharp metal tips.17 Our results support an exciton-polariton model for waveguiding in 1D ZnO nanostructures and emphasize the active role of 2LO phonon scattering in the exciton-phonon interaction. A reduced nanorod diameter at the subwavelength scale resulted in a degraded confinement efficiency and more divergent out-emission. Additionally, the excitation angle exhibits very little influence on the NBE emission but greatly varies the deep level emissions. 2. Experimental Section The ZnO nanorods used in this study were synthesized on a sapphire substrate by Au-catalyst-assisted chemical vapor
10.1021/jp102301w 2010 American Chemical Society Published on Web 06/08/2010
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Figure 1. SEM image of the ZnO nanorod array sample grown on a sapphire substrate by CVD.
deposition (CVD) at 910 °C.18 Figure 1 shows an SEM image of the as-grown array of ZnO nanorods. A typical nanorod used this study consists of three parts: a smooth upper part with a uniform diameter, a pedestal with a much larger diameter, and a short transition area in between.17,18 Figure 1 also shows that the upper parts of the nanorods usually have a hexagonal shape and flat top end facets. Previous X-ray diffraction and transmission electron microscopy results have demonstrated a [0001] (c axis) growth direction of the nanorods. An FEI XL30 SFEG scanning electron microscope was used in this study for high-magnification observation of the as-grown samples and nanomanipulation. The SEM is equipped with four Kleindiek nanomanipulators that operate sharp tungsten (W) tips at nanometer-scale accuracy. Using the tungsten tips and electron-beam-induced carbon deposition, we attached individual nanorods to the W tips.17 Such individual suspended nanorods were then transferred to a confocal micro-PL system consisting of an Olympus optical microscope, a 325 nm cw He-Cd laser, and an Acton SP2358 spectrometer. In this study, we only measure those nanorods with the pedestals removed. Figure 2a shows a schematic setup for the angular-dependent photoluminescence measurements. A five-dimension adjustable stage is used to mount the W tip to which the individual nanorod is attached. Thus, the nanorod can be aligned to any orientation and still be positioned at the center of the viewing area of the objective lens. The 325 nm laser beam was coupled to a fiber controlled by another multidimensional manipulator and then focused by a lens to form a uniform illumination on the ZnO nanorod. The excitation density was ∼3 W/cm2. The PL emission from the nanorod is collected by a Mitutoyo long working distance UV objective lens and then analyzed by the spectrometer. Meanwhile, PL images can be recorded by a highsensitivity cooled CCD camera (PI MicroMAX 1300Y). A narrow band filter with nominal band pass from 370 to 420 nm (2.95-3.35 eV) is placed in front of the CCD to select the NBE emission for PL imaging. The inset of Figure 2a shows the measured transmission curve of the filter. The long working distance of the objective lens (10 or 20.5 mm, depending on the lens type) enables large tilting angles of the nanorod and the laser beam relative to the optical axis of the microscope. Figure 2b shows the orientation relationships among the ZnO nanorod, optical fiber, and objective lens schematically. The original point is set to the center of the viewing area of the objective lens. The z coordinate axis is defined to be along the optical axis of the objective lens, and the y axis is perpendicular to z axis and the direction of the incident laser beam. The angle θ describes the detection angle between the nanorod axis and the z axis, whereas the angle φ is the excitation angle between the laser beam and the nanorod axis. As a nanorod is aligned along the z axis, the excitation angle φ can
Figure 2. (a) Schematic experimental setup of angular-dependent PL imaging and spectroscopy measurements. The inset shows the measured transmission curve of the band-pass filter. (b) Magnified schematic showing the orientation relationship among the nanorod, the objective lens, and the laser beam.
be varied from 60° to 100° in our setup. Because the objective lens is fixed, the excitation angle and the detection angle can be adjusted by tilting the nanorod and the fiber. Using SEM images as references, the orientation of the nanorod can be determined at an accuracy of (3° routinely. The application of the individual suspended nanostructure attached to sharp metal tips can effectively avoid the substrate effect and enable the emission measurement along the nanorod axis. For PL spectroscopy, the roles of the objective lens and the fiber described above can be switched; that is, the objective lens is used for excitation while the fiber for spectral collection. Thus, localized excitation and a higher excitation density can be achieved. In addition, the small semicollection angle (∼1° in our setup) of the fiber can provide a higher angular resolution. In contrast, the semicollection angles for the PL imaging and spectroscopy performed by the objective lens in this study are 15° and 12°, respectively. However, the fiber lacks confocal capacity and cannot distinguish the luminescence from the sidewall positions and the ends. It is possible to overcome the drawbacks of the above two setups by replacing the fiber optics with improved optics. 3. Results and Discussion 3.1. Angular-Dependent PL Imaging of Individual ZnO Nanorods. To carry out angular-dependent PL measurements on an individual suspended nanorod, we vary the detection angle θ by rotating the nanorod in the yz plane and keeping the excitation angle φ constant (90°); that is, the incident laser beam is along the x axis (see Figure 2b). Panels a and b in Figure 3 are SEM images of two individual suspended ZnO nanorods with diameters of 520 and 290 nm for the uniform upper parts.
Anisotropic Optical Emission of ZnO Nanorods
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Figure 3. SEM images and angular-resolved PL images of two individual suspended nanorods with diameters of 520 (a, c) and 290 nm (b, d), respectively. The detection angle θ is indicated for each PL image. The scale bars in (a) and (b) are 2 µm.
Both nanorods have a similar length of ∼11 µm. The corresponding angular-resolved NBE PL images of the two nanorods (Figure 3c,d) show that the NBE emission has a strong tendency to emit out from the end facets for both nanorods, indicating a pronounced waveguiding behavior along the c axis. Figure 4a compares the radial intensity profiles across the top end facets in the 0° PL images (Figure 3c,d) for the two nanorods. The full widths at half-maximum (fwhm) of the line profiles are ∼1.2 and ∼0.9 µm for the 520 and 290 nm nanorods, respectively, which are enlarged compared with the diameters determined by SEM due to the optical aberrations of the micro-PL system. In Figure 4b, we draw the intensity maxima of the top ends in the NBE PL images (Figure 3c,d) versus the detection angle θ for the two nanorods. For an easier understanding, the θ < 0° half is also plotted by considering a symmetric distribution, which is also applied to the angular distributions shown in Figures 7a and 8. In good focus condition, the angular distribution of the intensity maxima is found to be basically the same as that of the integrated intensity of the top end facets. Figure 4b shows that the intensity maximum drops abruptly from 0° to 30° for the 520 nm nanorod, but much slower for the 290 nm nanorod, resulting in a considerably more divergent NBE emission for the smaller diameter nanorod compared with the larger one. The angular distributions in Figure 4b can be well fitted by Gaussian functions with full widths at half-maximum (fwhm) of ∼45° and ∼90°, respectively, which can be considered as a relative comparison of the emission angles. However, exceptions exist for this general dependence of emission divergence on nanorod diameter, especially in those nanorods with irregular-shaped top ends. For example, we also observed an ∼100° fwhm for a nanorod of 580 nm in diameter. It should be noticed that the relative large collection angle in our experimental setup tends to slightly overestimate the emission angle, whereas the nonuniformities of the band-pass filter transmission (Figure 1a) and the CCD quantum efficiency can lead to a much underestimated emission angle, as discussed later in the paper. In this study, we mainly measure nanorods with diameters close to the wavelength of the NBE emission (∼390 nm). Our results show a general trend that the optical confinement is
Figure 4. (a) Radial intensity profiles of the top facets in the PL images of the two individual nanorods at θ ) 0°. (b) Angular distributions of the emission intensity at the nanorod top ends in the angular-resolved PL images shown in Figure 3c,d. (c) Comparison of the intensity line profiles along the nanorod lengths in the θ ) 60° PL images shown in Figure 3.
degraded with decreasing diameter at subwavelength scale. This conclusion is based on the following experimental results. First, more pronounced light leakage can usually be observed on the sidewalls of smaller diameter nanorods. For example, the SEM images indicate a smoother bottom region with considerably fewer surface steps for the 290 nm rod compared with the 520 nm one shown in Figure 3, but the 290 nm nanorod exhibits higher light leakage in the bottom part. Second, the emission intensity ratio between the top ends and the sidewall positions decreases for smaller nanorods even though the relative intensity of the top end tends to increase at a large detection angle for smaller nanorods due to the more divergent emission (Figure 4b). Figure 4c compares the line profiles across the length of the nanorods in the PL images taken at θ ) 60°, showing a smaller ratio for the 290 nm nanorod (2.8) compared with the 520 nm nanorod (5.3). In addition, a considerably weaker emission intensity from the top facet is usually observed for smaller diameter nanorods, though the excitation power/volume is roughly proportional to 1/d (d is the nanorod diameter). Figure 5 shows two-dimensional finite-difference time-domain (FDTD) simulation results of 390 nm light propagation inside two 11 µm nanorods with diameters of 520 and 290 nm. To simplify the simulation process, a spherical cross-section is applied and the excitation positions are set to be at the bottom (left) end (Figure 5a,c) and at the center of the nanorods (Figure 5b,d). Figure 5e,f compares the energy distributions at 1 µm away outside the top (right) end. For both excitation positions, a more divergent exit emission is observed for the 290 nm
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Figure 6. Angular-dependent micro-PL spectra taken at different detection angles from the top end of the individual suspended ZnO nanorod with a diameter of 520 nm. The vertical dotted lines mark the FX, FX-1LO, and FX-2LO emission energies.
Figure 5. (a-d) FDTD calculation of the electric field of 390 nm light propagation inside two 11 µm long nanorods 520 (a, b) and 290 nm (c, d) in diameter. The excitation positions are set at the bottom (left) ends (a, c) and at the center (b, d), respectively. (e, f) Comparisons of the energy distribution at 1 µm away outside the top (right) ends of the two nanorods for the excitations at the left end (e) and at the center (f), respectively. The curves are normalized according to their integrated intensities.
nanorod, which matches the experimentally determined larger emission divergence (Figure 4b). The simulation also shows that the confinement efficiency of the 290 nm nanorod is ∼48.9% of that of the 520 nm nanorod. 3.2. Angular-Dependent PL Spectroscopy of Individual ZnO Nanorods. As shown in Figure 2a, a PL spectrum and a PL image can be acquired from the individual nanorod at certain orientations (Figure 3) simultaneously. Figure 6 shows the angular-dependent NBE emission spectra from the top end of the 520 nm nanorod shown in Figure 3. With the increasing detection angle, the intensity of the NBE emission decreases continuously and the spectral weight shifts to a higher energy. In Figure 7a, we plot the integrated intensities of the NBE emission spectra versus the detection angle. The angular distribution is well fitted by a Gaussian with an fwhm of ∼65°, which is considered as a more accurate estimation of the emission angle at the nanorod top end compared with the PL imaging result (45°). The dramatically underestimated value deduced from the PL imaging is believed to be mainly caused by the nonuniform transmission or response of the optical components (e.g., the band-pass filter and the imaging lenses) and the CCD camera across the photon energy range of interest (3-3.35 eV for the NBE emission). For example, both the bandpass filter transmission (Figure 1a) and the CCD quantum efficiency are considerably less sensitive in the higher-energy range (>3.2 eV for the filter and >3.1 eV for the CCD). According to the angular-dependent spectral line shapes shown in Figure 6, the spectra at higher detection angles have considerably larger spectral weights at the higher-energy range, indicating that the nonuniformity of the imaging system results in decreased detection efficiency with the increasing detection angle. Thus, a narrower angular distribution, that is, a smaller estimated emission angle, may be deduced. The above results also demonstrate the less quantitative nature of the normal PL imaging technique, considering that the sensitivity nonuniformities are difficult to calibrate due to the absence of the knowledge on the spectral line shapes. In addition, it might be
Figure 7. (a) Angular dependence of the integrated intensity of the NBE emission spectra (between 2.95 and 3.35 eV) shown in Figure 6a. The distribution is fitted with a Gaussian. (b-h) Spectral fitting of the NBE emission spectra shown in Figure 6 using Gaussian peaks of FX, FX-1LO, FX-2LO, and FX-3LO. The spectra are normalized according to their maximum intensities.
worth mentioning that the experimentally determined emission angles in this study are often within (15° of the simple diffraction model of a planar wave by a hole, which gives an emission angle R ) 1.22λ/d (λ ≈ 390 nm for ZnO nanorods) and may be used as a rough estimation. During the sample preparation, a nanorod can be cleaved at the upper part or at the transition region. Our previous results have shown that the upper parts without transition regions are high-quality Fabry-Pe´rot cavities,16 resulting in the overlapping of the cavity modes and the NBE emission. In the present study, we only measure those nanorods with remaining transition regions that have gradually varying diameters and step structures.17 Our previous results showed that the step structures in the transition region can cause considerable light leakage,17 and thus, cavity resonance modes can be ignored. In addition, the very weak excitation intensity used in this study can effectively avoid the influence of the exciton-exciton scattering on the spectral line shape. Thus, the room-temperature NBE emission of ZnO nanorods is simply dominated by a free exciton (FX) and its LO phonon replicas.19-21 Figure 7b-h demonstrates the curve-fitting procedure of the angular-dependent NBE emission spectra using an FX and its first-third-order LO phonon replicas. The systematically varying spectral line shape with
Anisotropic Optical Emission of ZnO Nanorods
Figure 8. (a) Angular distributions of the integrated intensities of the FX, FX-1LO, FX-2LO, and FX-3LO components of the spectra shown in Figure 7. (b) Angular distributions of the percentages of the different components in the integrated NBE emission intensities.
the detection angle leads to a straightforward and unambiguous determination of the positions and widths of the different components at an accuracy of (1 meV. The positions/fwhms of the FX, FX-1LO, and FX-2LO emissions used in the fitting process are 3.308 eV/63 meV, 3.241 eV/66 meV, and 3.181 eV/73 meV, respectively. In addition, it can be observed that a small deviation exists between the fitted curve and the experimental data near 3.4 eV and increases with the intensity of the FX component, which is attributed to the contribution of the excited exciton states. In Figure 8a, we plot the integrated intensities of the fitted FX, FX-1LO, FX-2LO, and FX-3LO as functions of the detection angle. The FX-nLO (n ) 1, 2, 3) emissions exhibit strong angular dependence and increase monotonically from the perpendicular orientation to the parallel orientation, especially near θ ) 30-45°. In contrast, the FX emission remains relatively stable and shows a rather isotropic nature (see also Figure 6), which is consistent with the spatially resolved PL measurements, indicating that the FX emission is not guided during the waveguiding process.11,16 For those nanorods with very small diameters (for example, the 290 nm nanorod), the measured angular-dependent PL spectra show similar trends described above. However, due to the very small top facets compared to the smallest collection area of the confocal microscope and the reduced waveguiding efficiency, the spectra are strongly influenced by the sidewall emission. Figure 8b shows the variation of the percentages of the integrated intensities of fitted FX, FX-1LO, FX-2LO, and FX3LO with the detection angle. It is demonstrated clearly that the dramatic change of the spectral line shape (Figure 6) is mainly caused by the largely varying FX emission (from 7.9% to 37.6%) and FX-2LO phonon replica (from 20.5% to 40.5%) in opposite directions. Though the FX-1LO are the strongest component between 30° and 75°, its weight in the overall NBE emission keeps relatively constant between 32.5% and 36.4%. The FX-3LO emission (9.4-15.4%) shows a similar trend to the FX-2LO but has very limited contribution to the spectral line shape and remains relatively constant for a wide range. In ZnO nanorods, the excitons may exist in exciton-polariton form due to the strong photon-exciton interaction in ZnO
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Figure 9. (a) PL spectra taken at different excitation angles (φ ) 60, 90, and 100°). The intensities have been corrected by a factor of cos(90° - φ). (b) Spectral fitting of the deep level emission shown in (a) using two Gaussian peaks.
nanostructures.10,11 In addition, ZnO is a material with efficient LO phonon-exciton coupling. Thus, the polaritons may relax from the exciton-like part to photon-like part of the lower polariton branch (LPB) via LO phonon scattering. The resulted photon-like polaritons possess large and well-defined c-component wavevectors and can be confined to propagate along the nanorod axis with a very small damping.11,22 Thus, the waveguided emission at the nanorod end exit should have a narrower angular distribution. In Figure 8a, the FX-2LO emissions exhibit an fwhm of 56°, considerably narrower compared with FX1LO (64°) and the total NBE emission (65°, Figure 7a), indicating a larger c component for the wavevector of the FX2LO emission. Further considering the obvious transition from FX to FX-2LO emission in the spectral line shape variation (Figure 7b), we can conclude that the polariton relaxation in the waveguiding process is dominated by the 2LO phonon scattering. The FX-1LO emission is not found to be active in the waveguiding process of ZnO nanorods (Figure 7a,b). The relatively strong FX-1LO recombination in ZnO, forbidden by the parity conservation,23 is mainly attributed to the absence of an inversion center in the wurtzite structure and crystal imperfections.19-21,23 It can also be seen in this study that the NBE emission line shape of the same individual nanorod can vary dramatically with the detection angle. The apparent emission energy (i.e., the center of the NBE emission) shifts by >60 meV in Figure 6. As the nanorod diameter reaches subwavelength scale, the waveguiding efficiency may decrease with the decreasing diameter, which can, in turn, lead to reduced exciton-phonon coupling and considerable blue shift of the apparent NBE emission in the diameter range much larger than the exciton Bohr radius (∼2 nm for ZnO).18 3.3. Influence of the Excitation Condition. The influence of the excitation condition has been investigated as the detection angle θ is set to be 0°; that is, the nanorod axis is aligned along the axis of the objective lens and the luminescence from the top end is measured. Figure 9a shows the comparison of PL spectra at different excitation angles (φ ) 60, 90, and 100°). The lengths of the nanorods (typically
