Origin of Excitonic Emission Suppression in an Individual ZnO Nanobelt

J. Phys. Chem. C 2008, 112, 10095–10099

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Origin of Excitonic Emission Suppression in an Individual ZnO Nanobelt J. Yang and S. Li* School of Material Science and Engineering, The UniVersity of New South Wales, Sydney, NSW 2052, Australia

Z. W. Li Department of Chemical and Materials Engineering, The UniVersity of Auckland, New Zealand

K. McBean and M. R. Phillips Microstructural Analysis Unit, UniVersity of Technology, Sydney, Broadway, NSW 2007, Australia ReceiVed: April 1, 2008; ReVised Manuscript ReceiVed: April 17, 2008

The near band edge emissions of an individual ZnO nanobelt were investigated by cathodoluminescence spectroscopy, which has unique advantages in higher spatial resolution, orientation, and environmental independence over the conventional photoluminescence spectroscopy. The results show that the presence of a large surface-to-volume ratio is the determinant to suppress the formation of excitons in ZnO nanobelts. Ab initio calculations show that a drastic decrease of density-of-state in the conduction band and increase in the valence band upon size reduction are the key consequence of the large surface-to-volume ratio, revealing the possible fundamental physical origin of exciton suppression. The weak exciton polarity also reduces the likelihood for an exciton to couple with longitudinal phonons. This causes a reduction in the first longitudinal phonon replica intensity and then a complete suppression of the second replica. Understanding the effect of large surface ratio upon the physical properties of ZnO nanomaterials may provide new insights into the fundamental science of nanotechnology for the development of optoelectronics. 1. Introduction In the past decades, zinc oxide (ZnO) has attracted a great amount of research interest due to its unique electronic and photonic properties. It has a large direct band gap (Eg ∼ 3.3 eV at 300 K) and exciton binding energy (∼60 meV), which are the promising features for optoelectronic applications in the range from visible to ultraviolet (UV) wavelengths. ZnO has also been theoretically predicted to preserve room-temperature ferromagnetism when doped with transition metal ions1 that drives enormous research effort into developing ZnO as the host material for diluted magnetic semiconductors. The ZnO nanobelt is one of the major configurations of ZnO nanomaterials.2 Its distinctive morphologies with rectangular cross sections offer a great opportunity to fabricate the nano-optoelectronic devices. Although the photonic properties of ZnO nanobelts have been intensively studied with photoluminescence (PL) spectroscopy,3–6 the reports on the photonic behavior of an individual ZnO nanobelt are very limited. This is because the conventional PL signals on nanomaterials are often acquired as a spatial average of individual light-emitting centers, which can be affected by multiple emissions due to overlapping of materials and other artifacts, such as orientation and size effects, etc. On the other hand, the experimental environment, e.g., oxygen absorption on the sample surface, could also affect the determination of the natural properties of the materials. The present work tackles this problem using cathodoluminescence (CL) spectroscopy to investigate the intrinsic photonic behaviors of an individual nanobelt in a vacuum environment. By comparing the CL spectra acquired from the reference ZnO powder and a particular nanobelt, it is found that free exciton (FX) emission * Corresponding author. Tel.: 61-2-9385-5986. Fax: 61-2-9385-5956. E-mail: [email protected].

in nanobelt is suppressed. This is primarily induced by the large surface-to-volume ratio in nanobelts. Ab initio calculations performed on bulk wurtzite, ZnO [21j0] oriented ultrathin film (UTF), and nanobelt with nonpolar (21j0) and (01j0) facets show a dramatic decrease of density-of-state (DOS) in the conduction band (CB), whereas there is an increase in the valence band (VB) as the crystal size reduces. Such a modification of band structure causes an oscillator strength reduction in VB-CB coupling which would eventually lead to a decrease in exciton emission intensity. 2. Methodologies In this work, 99.999% pure ZnO powder (Kurt J. Lesker) is used as a reference to identify the positions of FX emission of ZnO and its longitudinal optical phonon (LO) replicas from the spectra acquired in 77 and 300 K, respectively. ZnO nanobelts were synthesized as follows: (1) 99.99% pure metallic zinc was deposited on glass substrate by direct current unbalanced magnetron sputtering in 10 mTorr Ar atmosphere for 5 min with a target power density of 1 W/cm2; (2) the deposited films were then oxidized in O2 + 10% H2O atmosphere at a temperature range from 375 to 400 °C for 7.5 h. CL studies were performed using Gatan MonoCL2 system attached in a FEI Quanta 2000 environmental scanning electron microscope (SEM). The CL signals were detected using a Czerny-Turner monochromator with a Peltier-cooled Hamamatsu R943-02 photomultiplier UV-vis detector. Samples were cooled down to liquid nitrogen temperature (77 K) with a Gatan C1002 continuous flow liquid nitrogen cold stage. The CL spectra of the ZnO nanobelt were acquired at 10 kV, whereas the spectra of the reference materials were obtained at a 5 kV acceleration voltage. The beam currents were measured by Faraday cup coupled to the liquid N2 cold

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Figure 1. (a) Cross section of a nonpolar nanobelt model (looking toward the wurtzite c-axis) (Zn in gray and O in red). (b) Orientation relationships between (21j0) and (01j0) cleavage planes shown in the original wurtzite structure. Both Zn and O atoms are present in each cleavage plane, making all facets of the nanobelt to be nonpolar. (c) Cross section of the optimized structure of the ZnO nanobelt. The first very top layer undergoes significant surface reconstruction with changes of the in-plane Zn-O bond into a buckled configuration, whereas the interior of the nanobelt is much less affected.

stage. All spectra are with system response corrected and fitted with Gaussian peak profiles due to thermal broadening. For ab initio calculations, a nanobelt model was built by cleaving the wurtzite ZnO crystal along two perpendicular planes of (21j0) and (01j0) (Figure 1, parts a and b). Twenty layers of vacuum slabs were added on top of the film along each cleavage direction to construct the supercell structure. Therefore, the nanobelt structure can be considered as a combination of [21j0] and [01j0] oriented UTF. To ensure a proper “beltlike” morphology, the film thickness along the [21j0] direction is chosen to be smaller than that of the [01j0] direction. Figure 1a shows the cross section of the initial nanobelt model built. Both large and small facets are nonpolar with each Zn atom bonded directly to an in-plane O atom. For comparison, a free-standing ZnO UTF along the [21j0] direction model was also built. It was built exactly the same as the nanobelt structure in Figure 1a except that it was not cleaved along the [01j0] direction. Ab initio calculations were performed with the density functional theory based DMol3 package developed by Accelrys.7 Geometry optimization and DOS calculations were performed for bulk, UTF, and nanobelt models with the DNP basis set plus Perdew-Wang correlation function8 within the local density approximation scheme. Real-space global cutoff radii were set for all elements at 3.9 Å. All-electron basis sets were used for all elements.9 Brillouin zone sampling was carried out by choosing the k-point sets within the Monkhorst-Park scheme with grid spacing of 0.05 Å-1. Fermi smearing of 0.005 hartree was applied to improve calculation performance. Density mixing of 0.2 and direct inversion in an iterative subspace were also employed to speed up the self-consistent field convergence. The convergence thresholds for energy, gradient, and displacement are set to 2 × 10-5 hartree and 4 × 10-3 and 5 × 10-3 hartree/Å, respectively. 3. Results and Discussion For the synthesis of ZnO nanobelts, the deposited thin film of metallic zinc is approximately 450 nm thick. It is composed of hexahedron-like particles, which had a multilayered structure with thin platelet extrusions. The oxidation treatment of such a thin film resulted in the formation of ZnO nanobelts with a typical width of ∼250 nm and a length of about 15 µm. It is believed that the growth of ZnO nanobelts was related to the

presence of those thin platelets as the nanobelts were not observed on Zn films with smooth Zn particles. When the asdeposited thin films were exposed to the oxidizing atmosphere, the nanobelts grew from these thin platelet extrusions, which served as the seeds for the further growth of ZnO nanobelts. However, the growth mechanism of the nanobelts was not fully understood, and this nonplanar growth behavior was believed to be related with the following synthesis conditions: (a) the presence of water vapor in the atmosphere, (b) the evaporation and sublimation of Zn into ZnO when interacting with the H2 and H2O vapor at the synthesis temperatures, and finally (c) the unique structure of Zn platelets. Figure 2a shows the CL spectra of ZnO reference acquired at 77 and 300 K, respectively. The spectra acquired at 77 K can be deconvoluted into five individual peaks with peak values of 3.334, 3.282, 3.230, 3.201, and 3.120 eV, respectively. In general, the most dominant peak in bulk ZnO PL spectra at low temperature is associated with either FX or bound exciton (BX) emission at the high-energy range. When the temperature increases, this peak gradually shifts to the positions of the first phonon replica (FX-1LO), and subsequently the second phonon replica (FX-2LO) results in the peak broadening at room temperature.10 Therefore, by comparing these CL spectra acquired at liquid nitrogen and ambient temperatures, the fitted spectra can be identified as the following: (1) the peak of ∼3.33 eV is the general band gap emission of Eg ∼ 3.33 eV at 300 K;11 (2) the peak of 3.282 eV can be assigned as the FX peak with binding energy (BE) of 50 meV; (3) two phonon replicas, the emissions of 3.201 and 3.120 eV, are attributed to FX-1LO and FX-2LO, respectively; (4) oxygen vacancy is responsible for the emission at 3.201 eV.12 In comparison with the CL spectra acquired from the reference materials, three similar peaks were observed in the CL spectra of the ZnO nanobelt, with a width of approximately 250 nm, as shown in Figure 2b. They are the strongest band gap emission at 3.330 eV, the FX peak at 3.282 eV, and the weakest FX-1LO emission at 3.200 eV. Unlike the spectra of reference materials, the strongest FX emission of the reference materials fell into the second intensive emission in the nanobelt. In general, the surface-to-volume ratio of nanomaterials significantly overrules that of the bulk, which

Excitonic Emission Suppression in a ZnO Nanobelt

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Figure 2. (a) CL spectrum of pure ZnO powder acquired at room temperature and 77 K with 24 nA beam current at 1 nm band-pass. Five peaks can be identified by fitting the spectrum with Gaussian peak profiles. The room-temperature luminescence peak shifts strongly toward the first phonon replica. (b) CL spectrum of a ZnO nanobelt acquired at 77 K with 5.4 nA beam current at 1 nm band-pass. Three peaks can be identified by fitting the spectrum with Gaussian peak profiles. The inset shows an SEM micrograph of ZnO nanobelts.

Figure 3. Illustration of different degrees of wave function (WF) confinements within the dielectric potential of a nanobelt: the hole WF is more confined within the nanobelt, whereas the electron WF is spread out, causing the weak oscillator strength of the exciton within the nanobelt.

makes the properties of nanomaterials become surface dominated. The nanobelt surface, in this case, can be considered as an energy barrier when a high-dielectric oxide nanobelt is surrounded in vacuum. The quasiparticle wave function (WF) for an exciton is generally constructed as a product of electron and hole WFs, of which the overlap integral determines the exciton oscillator

strength.13 Electrons with a lower effective mass m*e often experience a lower dielectric barrier than holes of larger effective mass m*h, which implies that WF of holes are more confined within the nanobelt compared with electrons14 (Figure 3). Such a WF confinement effect would be very significant in direct band gap materials such as ZnO, in which electron-hole pairs are created

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Figure 4. DOS comparison across bulk ZnO, UTF, and nanobelt.

directly at the highly symmetrical Γ point in the Brillouin zone. Those excited electrons and holes are of negligible linear momentum; thus, their wavelengths would be much larger than the widths and thicknesses of the nanobelts. As a result, excitons in the ZnO nanobelt are less polarized and of lower oscillator strength, causing a suppression of FX emission. Nevertheless, the appearance of the weak exciton emission may be associated with the relative large scale along the longitudinal axis of the nanobelt. It was reported that organic coating on a nanobelt surface could facilitate the dissociation of the exciton.15 This leads to an enhancement of UV photoconductivity of the ZnO nanobelt, suggesting that surface potential plays an important role for exciton formation in nanomaterials. On the other hand, different from PL excitation which can probe the anisotropic optical properties by using polarized lights, the interaction of electrons with solids in CL is totally random confined within the electron-solid interaction volume. Therefore, the contribution of crystal anisotropy to our CL spectra is considered to be insignificant. Our ab initio DOS calculations on bulk ZnO, UTF, and nanobelt also reveal similar indicative results on the origin of exciton emission suppression in ZnO nanobelts. From Figure 4, it is clear that the DOS of the VB increases for 2 orders of magnitude, whereas both the DOS of the CB and its energy range reduce substantially when the dimensionality of ZnO reduces. This implies that the CB in the nanobelt is less capable to accommodate the excited electrons from the VB. The competition between band gap and FX emission can be considered as the following: in the former case, electrons relax directly back to the VB from the CB, whereas in the latter case, the excited electron needs to first bind to a neighboring hole (in real space) forming a quasiparticle before they are annihilated. As a result, the time for an excited electron to reside on the CB would be longer in the latter case compared with the former one. In the consequences of constant excitations of the nanobelt, with far more electrons to be excited to the CB

Yang et al. but fewer states to accommodate them, it would be kinetically more favorable for the luminescent centers to undergo a direct CB to VB transition, rather than excitonic emissions. The interplay between VB and CB DOS as size reduces can be attributed to the surface effects. It is clear that a large fraction of atoms reside on the surface as the size reduces. In tetrahedrally bonded wurtzite structures, each surface atom would have one less bond compared with its bulk counterpart. This leaves this unbonded electron to be of more atomic character than the others. It leads to an increase in the numbers of atom-like orbitals. Such an increase is the primary reason to cause the corresponding increases in the DOS of the VB, thus decreasing the DOS of the CB which reflects antibonding characters in solids. On the other hand, from Figure 1c, it can be seen that surface reconstruction causes the in-plane Zn-O dimer to reside on a buckled out-of-plane configuration. This would also break the ordinary tetrahedral crystal field symmetry which prevents proper overlapping of atomic orbitals upon bond formations. It is believed that the distortion in crystal field symmetry could reduce the crystal field splitting resulting in more orbitals to remain below the Fermi level retaining their atomistic character. This is true for Zn 3d states, where the double DOS peaks for d states in bulk ZnO at around -5 to -3 eV gradually merge into a single asymmetric broad peak for the nanobelt (Figure 4). In order to understand the subtle interaction between electrons and a nanostructured solid, such as ZnO nanobelts, the surface effects on the relative intensities of LO phonon replicas are investigated. In the reference spectra, two phonon replicas (3.230 and 3.120 eV) are observed, whereas in the nanobelt spectra, however, only one weak peak at 3.198 eV is discernible, and it can be assigned to FX-1LO. This exhibits that the second phonon replica is completely suppressed. The reduced polarity of excitons in nanobelts may be the main reason for the reduction of exciton-LO phonon coupling strength, thus resulting in a weak FX-1LO emission intensity. A similar effect of suppression of exciton-LO phonon coupling has also been observed in ZnO quantum dots.16 The CL spectra for both reference materials and nanobelts give an estimation of exciton BE of ∼50 meV and LO phonon energy quanta of ∼80 meV; the first value is lower than the generally accepted value of 60 meV,11 whereas the latter one is higher (reported values in the range of 71-73 meV10), which were measured by PL. These discrepancies could be contributed by the differences of excitation mechanisms for CL and PL. The exciton-polariton mechanisms of luminescence are schematically illustrated as Figure 5. In PL, the incident photons of excitation are of negligible linear momentums; thus, all transitions are vertical. The FX peak is originated from a vertical relaxation of excitons from the vacuum edge to the ground state on the lower polariton branch (LPB), shown as A to B in Figure 5. However, the electrons used for excitation in CL are of considerable linear momentum; thus, momentum transfer from electron to exciton during exciton relaxation is possible. This may cause the exciton to settle down on point C along the LPB, resulting in a smaller BE as shown in the figure. On the other hand, high-energy excitons created in both reference materials and nanobelts would further relax and accumulate at the bottleneck region of low group velocities dω/dk on the LPB (around point D in Figure 5).17 It is believed that this relaxation process in PL is accompanied by emission of LO phonons, leading toward the first LO phonon replica of FX emission (B to D in Figure 5). The preferential accumulation of the exciton at the bottleneck is an intrinsic property of a material and

Excitonic Emission Suppression in a ZnO Nanobelt

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Figure 5. Possible discrepancy on the exciton binding and LO phonon energies measured in PL and CL illustrated in the exciton-polariton picture: (red) PL FX peak caused by direct exciton relaxation (A to B) with no momentum transfer from the photon; (green) CL FX peak caused by exciton relaxation accompanied by linear momentum transfer from high-energy electrons (A to C). The resultant CL FX peak thus gives a slightly lower FX binding energy (BE) than that measured from PL. The FX-1LO peak is formed, however, in both cases, preferential accumulation of exciton-polariton density around the bottleneck region of the lower polariton branch (D) leads to a larger estimation of LO phonon energy in CL compared with PL.

independent of experimental technique. Therefore, the C to D relaxation in CL gives a higher estimation of LO phonon energy. It is possible that this relaxation is also accompanied by other particle-scattering events in the solid that lead toward a larger estimation of LO phonon energy quanta and broad phonon replica peak in CL. The nature of the other scattering processes is not the topic of this work, and it is not discussed here. 4. Conclusion In conclusion, the CL of a ZnO nanobelt was characterized at 77 K and ambient temperature to compare with the CL spectra of reference materials. The results indicate that the strong FX emission is suppressed in nanobelts at 77 K due to their large surface-tovolume ratio. It was suggested that the surfaces, which act as energy barriers, can effectively confine holes within the nanobelts but not electrons, leading to a low exciton overlap integral within the nanobelt and hence the weak excitonic emissions and the corresponding phonon replicas. Ab initio calculations of electron DOS for bulk, UTF, and nanobelts reveal drastic decreases in CB DOS but increases in VB DOS. This supports our finding on the surfaceinduced suppression of excitonic emission in nanobelts. Discrepancies of FX binding energies and LO phonon quantas measured by CL and PL may be associated with the momentum transfer from incident electrons to excitons causing the latter to relax to a higher energy ground state.

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