Ultrafast Upconversion Probing of Lasing Dynamics in Single ZnO

The dynamics of the lasing wavelength dependence on carrier density is also studied and related to the band gap renormalization in the EHP regime...
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J. Phys. Chem. C 2008, 112, 1679-1684

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Ultrafast Upconversion Probing of Lasing Dynamics in Single ZnO Nanowire Lasers Jae Kyu Song,† Ulrike Willer,‡ Jodi M. Szarko,‡ Stephen R. Leone,*,‡ Shihong Li,§ and Yiping Zhao§ Department of Chemistry, Kyunghee UniVersity, Seoul 130-701, Korea, Departments of Chemistry and Physics, and Lawrence Berkeley National Laboratory, UniVersity of California, Berkeley, California 94720, and Department of Physics and Astronomy, UniVersity of Georgia, Athens, Georgia 30602 ReceiVed: July 23, 2007; In Final Form: October 9, 2007

The ultrafast lasing dynamics of single zinc oxide nanowires are studied by time-resolved upconversion of the lasing emission as a function of the ultraviolet excitation intensity. Induction times for stimulated emission in individual nanowires are observed to be 1-5 ps or 3-15 roundtrips for nominal 20 µm long nanowires, depending on the 267 nm excitation intensity. In separate experiments, transient absorption profiles are also obtained for time-delayed 800 and 400 nm pulses to elucidate the carrier dynamics, such as rapid decay (2-3 ps) of the electron-hole plasma (EHP) states at high carrier densities during lasing in the nanowire and the slower exciton decay component (15-60 ps) at lower excitation densities. The dynamics of the lasing wavelength dependence on carrier density is also studied and related to the band gap renormalization in the EHP regime.

1. Introduction One-dimensional nanostructure materials have attracted much interest because of their unique properties for applications in electronics, photonics, and biochemistry.1-3 Zinc oxide (ZnO) has a wide band gap (3.37 eV) and a large excitonic binding energy (60 meV), which have triggered many studies of nanoscience and nanotechnology based on ZnO. This wide band gap semiconductor material is suitable for UV/blue optoelectronic applications such as light-emitting diodes and laser diodes as well as sensors.4,5 The large exciton binding energy affords a stable exciton state even at room temperature for optical applications. In recent years, various one-dimensional ZnO nanostructures, such as nanowires, nanoneedles, and nanoribbons, have been fabricated.6-10 Although much of the research focuses on growth, fabrication, and steady-state emission properties of ZnO, optical properties of ZnO nanowires pertinent to lasing are also investigated.11-15 Recently, several lasing studies report investigations of the temporal behavior of ZnO nanowires to infer the carrier and lasing dynamics in nanostructure lasers.16-21 The carrier dynamics in the excited states have been probed by several methods, including transient absorption techniques to obtain excited-state absorption or stimulated emission. Time-resolved photoluminescence provides information on the radiative dynamics of excited carriers. Emission measurements find that the timeresolved emission in single ZnO nanowires includes a fast component when the excitation intensity is above the stimulated emission threshold.17-19 As the excitation intensity is increased further, the decay time of this fast component becomes very short, which is attributed to the rapid decay of the population inversion required for the stimulated emission.17-19 In addition to the change in decay time, the onset of the fast component depends on the excitation intensity. The fast component shifts * Corresponding author. E-mail: [email protected]. † Kyunghee University. ‡ University of California, Berkeley. § University of Georgia.

to earlier times at high carrier densities and can also be related to the initial stimulated emission onset time at the lasing threshold. The results are explained by the fact that the lasing mechanism changes from exciton-exciton scattering to electronhole plasma (EHP) recombination at higher carrier densities.17-19 Previously, we introduced a pump-stimulated emission probe method, which interrogates the carrier dynamics and lasing dynamics simultaneously in individual spatially selected nanostructure lasers.20,21 In this article, we report investigations of the lasing dynamics of single nanostructure lasers obtained from their time-resolved emission response using an upconversion technique. Spectral as well as temporal changes of the stimulated emission in lasing nanostructures are investigated. The emission wavelengths and time history are derived from the spectrally resolved sum frequency, which provides information about the unperturbed lasing dynamics more directly than the previously used pump-stimulated emission probe method, which injects a hole in the gain population and can study different dynamics. The dependence on excitation intensity is also studied in more detail to map out the dynamics of lasing emission as a function of the carrier density; earlier studies report only limited lasing conditions, i.e., near the threshold and high above the threshold regime.17-19 We also study the ultrafast carrier dynamics during lasing using femtosecond transient absorption techniques. Here, the goal is to study single nanowires to understand the lasing mechanisms of individual nanostructures, since the lasing behavior ultimately depends on distinct parameters, such as surface properties, gain, and material quality of each nanostructure.12 Thus, we seek to interrogate the uniqueness of individual nanolasers, whereas ensemble-averaged information may obscure this detail. 2. Experimental Section ZnO tetrapods and nanowires are grown by a vapor-phase transport process, where Zn powder is heated to 700 °C in a quartz tube furnace with an argon flow of 300 sccm under ambient pressure.20,21 The ZnO nanostructures are sonicated in

10.1021/jp0757513 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008

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Figure 2. Schematic diagram of the experimental setup for timeresolved upconversion of transient emissions. The fundamental 800 nm beam is generated by a regeneratively amplified Ti-sapphire laser (RegA, Coherent, Inc., 250 kHz), which is divided into two. One is tripled for the excitation pulses (267 nm), and the other is delayed and used for the gating pulses (800 nm). The transient emission near 390 nm from the nanowire is collected and guided into a BBO crystal, where the gating pulses are focused for the upconversion. The intensity of the sum-frequency is detected using a photomultiplier tube with a lockin amplifier in synchronism with chopping of the excitation beam.

Figure 1. (a) Far-field image of a single ZnO nanowire emission induced by 267 nm excitation, which is obtained by collecting UV emission using a filter. Strong UV emission is observed from each end facet. The inset shows a scanning electron microscopy (SEM) image of a single ZnO nanowire. Scale bar is 10 µm. (b) Excitation intensity dependence of the emission and lasing in a ZnO nanowire with 267 nm excitation, which are offset for clarity. From bottom to top, the emission spectrum corresponds to increasing excitation intensity. As the excitation intensity increases above the lasing threshold, sharp lasing peaks appear. The lasing becomes red-shifted due to the band gap renormalization induced by the electron-hole plasma (EHP) state.

methanol, where some of the legs of the tetrapods are found to be broken into nanowires. The shapes and lengths of the nanostructures are characterized by scanning electron microscopy (SEM). The diameters of the nanowires are in the range of 200-800 nm and the lengths are 10-30 µm. The SEM image of a nanowire is presented in the inset of Figure 1a. The nanostuctures in methanol are drop-coated on quartz or glass substrates, and only nanowires are optically investigated in this study. For time-resolved emission studies using upconversion techniques,22 an isolated single nanowire is selectively excited through an ultraviolet microscope objective by 267 nm pulses and probed by subsequent upconversion of the emission output, as shown in Figure 2. The lasing and photoluminescence emission near 390 nm from the nanowire is collected through the same objective and guided into a BBO crystal, where the gating pulses (800 nm) are focused for the upconversion with a phase matching condition. The sum-frequency generation profiles produced by the gating pulses and the temporal emission from a single nanowire are monitored by varying the delay time of the gating pulses with respect to the excitation pulses using a variable translation stage. Time-resolved emission results are obtained in 10 fs steps. The cross-correlation between the

excitation and gating pulse is about 300 fs. The generated sumfrequency is spectrally resolved by a monochromator and detected by a photomultiplier tube. The intensity of the sumfrequency is monitored using a lock-in amplifier in synchronism with chopping of the excitation beam (1 kHz), while the repetition rate of femtosecond laser pulses is 250 kHz. The emission wavelengths are derived from the spectrally resolved sum-frequencies. Spectrally integrated sum-frequency signals are also collected to provide a better signal-to-noise ratio for low-intensity emissions from a single nanowire. The details of the apparatus for the transient absorption experiments are described elsewhere.20,21 An isolated single nanowire is selectively pumped through an ultraviolet microscope objective by 267 nm pulses and probed through the same objective by time-delayed 800 or 400 nm pulses. The decrease in the intensity of the 800 nm probe is observed due to transient absorption in the nanowire, whereas the intensity of the 400 nm probe is observed as an increase due to transient stimulated emission in the nanowire. The temporal cross-correlation between the pump and probe pulses is 300 fs. Here, we do not derive quantitative details about the absorption amplitudes in the exciton or EHP regimes, however the readily observable changes of the time histories permit clear assignment to the exciton or EHP dynamics. 3. Results and Discussion 3.1. Time-Integrated Emission Spectra. To study the axial waveguiding of stimulated emissions in ZnO nanowires,11-15 a single ZnO nanowire is excited by 267 nm pulses, and the near UV emissions (360-400 nm) from the nanowire are collected using a filter. The strong emissions from both ends of the nanowire are observed in the far-field image in Figure 1a. This shows the efficient coupling of the UV photoluminescence and lasing in the nanowaveguides. The emission spectra in Figure 1b are selectively obtained from the end of the nanowire using the microscope objective.

Lasing Dynamics in Single ZnO Nanowire Lasers When the excitation intensity reaches the lasing threshold (30 µJ/cm2 for this structure), a sharp spectrally resolved lasing peak appears, while other emissions such as photoluminescence and amplified spontaneous emission still contribute to the emission spectrum. As the excitation intensity is further increased, the intensity of lasing emission increases superlinearly, and a second lasing peak in the long wavelength region appears. When the carrier density increases further, the exciton becomes destabilized and an EHP state is formed. In the EHP regime, the band gap usually decreases.23-25 Therefore, the emissions are redshifted in the EHP regime and the lasing is also red-shifted with increasing excitation intensities, as shown in Figure 1b. 3.2. Lasing Dynamics in a Single Nanolaser. Time-resolved upconversion measurements of the photoluminescence and stimulated emission are carried out to characterize the timescales of the lasing emission dynamics. Time-resolved emission obtained by the upconversion technique for an individual nanowire is presented in Figure 3 as a function of the 267 nm excitation intensity (25-120 µJ/cm2). As with the timeintegrated emissions, the time-resolved emissions strongly depend on the excitation intensity. At an excitation intensity of 25 µJ/cm2, the emission shows a single-exponential decay with a time constant of 25 ps, as observed in the inset of Figure 3a. The time constant agrees with the decay time of exciton states in ZnO thin films, when stimulated emission due to excitonexciton scattering is not observed.26,27 Thus, this component is assigned as the photoluminescence from the exciton recombination. As the excitation intensity increases, a new component appears with a superlinear increase of emission intensity, which is the lasing emission. We note that this component has a delayed onset compared to the photoluminescence. The delay time of the onset is as long as 5 ps at an excitation intensity of 35 µJ/cm2 (the lasing threshold for this structure), which is much longer than the onset of photoluminescence. Despite the delayed onset, this strong component has a fast rise and decay time compared to the photoluminescence. When the excitation intensity is further increased, the rise and decay of this component becomes faster. Thus, the full-width at halfmaximum (fwhm) duration-time becomes shorter (3.2 ps at an excitation intensity of 35 µJ/cm2 compared to 1.5 ps at an excitation intensity of 120 µJ/cm2). Various onset times for the emissions in ZnO nanowires are reported in other studies.17-19 Two mechanisms could be responsible. The first could be a delayed onset of lasing attributed to the time to form the emissive states such as a high concentration of excitons in the exciton-exciton scattering regime and the electrons and holes in the EHP regime.17-19 The second is the optical time to threshold, or induction time, for the buildup of the optical gain in the nanowire for stimulated emission.28 The induction time for the optical stimulated emission would become shorter at higher carrier densities. Naturally, then, the optical lasing due to the exciton-exciton scattering mechanism will have a longer delay time than the lasing in the EHP regime. To consider these two mechanisms, we first discuss the time scale to form excitons or the EHP. Below the threshold of the stimulated emission, the photoluminescence from exciton states appears nearly immediately upon excitation.18,19,26,27 On the other hand, the stimulated emission from the exciton-exciton scattering has a delay time, because it takes time to form a high concentration of excitons.17-19,29 The buildup of the EHP takes place in 1 ps for a cooling process from hot carriers to a quasithermalized system.24,25 In our experiments, the delay time of the optical stimulated emission onset is continuously reduced

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Figure 3. (a) Time-resolved emission profiles obtained by the upconversion technique as a function of excitation intensity. The thin line at time zero shows a cross-correlation between the excitation pulse and gating pulse. The inset shows time-resolved emission profiles at low excitation intensities, which are offset for clarity. Stimulated emission appears at various delay times on top of the relatively prompt photoluminescence. (b) Time-resolved emission profiles in another nanowire obtained by the upconversion technique as a function of excitation intensity. The inset shows time-resolved emission profiles at low excitation intensities, which are offset for clarity. This nanolaser has a lower lasing threshold and a shorter induction time of the lasing than that in (a) possibly due to a longer length of the nanowire.

with increasing carrier density (Figure 3), suggesting that a process other than the time to form the emissive states is relevant. In other words, the delay time shows a dependence on the excitation intensity, and it cannot be attributed only to the formation time of the emissive states. In addition, a delay time of the stimulated emission (5 ps at 35 µJ/cm2) compared to our observed relatively prompt photoluminescence of the excitons (25 and 35 µJ/cm2) is too long to attribute only to a time to form a high density of exciton states. Therefore, the results suggest that there must be a significant contribution due to the optical “induction time” for stimulated emission, implying that the observed delay time is the combination of the formation time of the emissive states and the optical induction time. To achieve lasing, spontaneously emitted photons are guided and amplified by the stimulated emission process along the nanowires. The induction time of lasing is the minimum duration time for the photons to propagate over the gain length to achieve

1682 J. Phys. Chem. C, Vol. 112, No. 5, 2008 an optical gain that is large enough to be observed as stimulated emission. Near the threshold, the gain length can be on the order of 10-3 m, leading to an induction time of a few picoseconds.28 On the other hand, the gain length for the stimulated emission is reduced to the order of 10-4 or 10-5 m at higher carrier densities, so that the induction time can be reduced. The induction time at the highest excitation intensity (120 µJ/cm2) is observed to be about 1 ps (Figure 3a), which means that the gain length of such a nanowire laser is about 120 µm. At first glance, the required gain length in the nanowires seems to be larger than expected, when compared to the gain coefficient of bulk ZnO (2 × 103 cm-1).25 However, the nanolaser has only low reflectivity at each end facet (20%),12 which increases the required gain length. Therefore, a value of 3∼4 roundtrips, which can be estimated from the observations in Figure 3a for a nominal 20 µm long nanowire at high excitation intensity, is nearly the minimum number of roundtrips for the optical gain process to achieve observable lasing in a nanolaser. On the other hand, the induction time of the lasing at an excitation intensity of 35 µJ/cm2 is about 5 ps (Figure 3a), implying that it takes at least 15 roundtrips for lasing, assuming that the time to form the high concentration of exciton states is relatively prompt. Figure 3b shows the time-resolved emissions in another nanowire as a function of the 267 nm excitation intensity. Possibly due to a longer length (∼25 µm), this nanolaser has a lower lasing threshold (30 µJ/cm2), because the optical gain process experiences fewer reflections from the low reflectivity end facets.30 In addition, the induction time of the lasing at an excitation intensity of 35 µJ/cm2 is a little shorter (3.5 ps) than that in Figure 3a, implying that a value of 8∼9 roundtrips is enough roundtrips for the optical gain process of this structure at the excitation intensity of 35 µJ/cm2. 3.3. Carrier Dynamics in a Single Nanolaser. In order to understand the relationship between the emissions and excited carriers, time-resolved transient absorption pump-probe experiments are also carried out. The temporal absorption of the probe pulse can be used to map out the excited carrier dynamics in the nanowires. Transient absorption profiles acquired for 800 nm probe pulses on a single nanowire are shown as a function of 267 nm pump intensity (15-75 µJ/cm2) in Figure 4a; the experimental signals are inverted and offset for clarity. When the pump intensity is low (15 µJ/cm2) and no lasing occurs, the transient absorption profile shows a single-exponential decay with a time constant of 25 ps, which agrees with the photoluminescence of the excitons at low excitation intensity (25 µJ/ cm2) in Figure 3a. As the pump intensity is increased, a fast decay component appears and its intensity becomes larger, while the exciton component does not seem to be affected significantly. The fast component is due to the EHP.20,21,24,25 Above a pump intensity of 30 µJ/cm2, stimulated emission occurs for this structure, and the decay of the EHP component becomes faster as the pump intensity increases (2.5 ps at a pump intensity of 45 µJ/cm2 to 2 ps at a pump intensity of 75 µJ/cm2). The high density of carriers in the EHP state has a high probability to experience multiple interactions such as the carrier-carrier scattering, carrier-phonon scattering, and Auger processes, which enhance a faster depopulation of the excited carriers.24,25 In addition, the strong stimulated emission accelerates the rate of carrier depopulation at high carrier densities, which also induces a shorter lifetime of the EHP at high pump intensities. Therefore, the lifetimes of the EHP in the lasing nanowires are found to be shorter than those in the non-lasing nanowires and bulk powders (3-5 ps), although the lifetimes of the EHP also depend

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Figure 4. (a) Transient absorption profiles at the probe wavelength of 800 nm as a function of pump intensity. Transient absorption profiles are inverted and offset for clarity. (b) Transient stimulated emission profiles at the probe wavelength of 400 nm as a function of pump intensity. Transient profiles are offset for clarity.

on the excitation intensities and the defect densities. Because the exact defect densities cannot be estimated in nanowires and in powders examined in our experiments, the direct probing of the lasing effect on the EHP lifetimes was not possible. We also note that the transient absorption profiles in the lasing nanowires do not exhibit a superlinear amplitude dependence on the pump intensity, as found in the emission dynamics (Figure 3). However, the exact dependence of the transient absorption signal amplitudes on the carrier density in the EHP and exciton regimes is not known at this time. The slow exciton time decay is weakly dependent on the pump intensity, noting that the exciton state is formed after depopulation of the higher carrier density EHP state. However, the decay time of the exciton state also varies among the individual nanowires studied, and the decay takes on values in the range of 15-60 ps. The difference in the exciton decay times might be related to the defect densities through processes such as surface-mediated trapping, which would influence the lifetime of the carriers. Generally, the surface-mediated defect density is dependent on the surface-to-volume ratio and concentrations of impurities.12 The diameters of the nanowires in this study are distributed between 200 and 800 nm, which can change the surface-to-volume ratio and defect densities and, thus, alter the

Lasing Dynamics in Single ZnO Nanowire Lasers exciton lifetimes. The exciton lifetimes obtained in this study are comparable to a previous study on ZnO nanowires (30-70 ps),17 whose diameters are in the range of 100-250 nm. The transient absorption profiles obtained at another probe wavelength help map out the carrier dynamics more clearly. The transient changes in stimulated emission20,21 by illumination with a time-delayed probe wavelength of 400 nm follow a biexponential decay with time constants of 2 and 15 ps at the pump intensity of 75 µJ/cm2 (Figure 4b), which virtually agrees with the transient profile at the probe wavelength of 800 nm (Figure 4a), although these results are not for the same nanowire. As the pump intensity decreases, the intensity of fast component becomes smaller and its lifetime becomes longer, 3 ps at the pump intensity of 45 µJ/cm2, whereas the slow component is weakly dependent on the pump intensity. This result supports the interpretation that the fast and the slow components are the EHP dynamics and the exciton dynamics in the nanolasers, respectively. 3.4. Band Gap Dynamics in a Single Nanolaser. The band gap renormalization and its recovery are closely related to the degree of EHP, which occurs at high carrier densities. Because the normal band gap is recovered upon the decay of the carrier density, the band gap dynamics is affected by the carrier dynamics.21,24,25 In order to study the band gap dynamics, we investigate the broadened lasing at high carrier density rather than the sharp lasing peaks at lower density. As discussed above, the lasing emission increases superlinearly and becomes redshifted due to the band gap renormalization in the EHP (Figure 5a), as the excitation intensity increases. At an excitation intensity of 150 µJ/cm2, the longitudinal modes of the lasing are barely noticeable and the lasing peak is much broader, whereas two longitudinal modes are clearly observable at the excitation intensity of 90 µJ/cm2. Possibly amplified spontaneous emission in nanowires is enhanced at high excitation intensity despite its overall lower optical gain than the lasing12,18,19 and that amplified spontaneous emission broadens the lasing peakwidth. In addition, during lasing, transverse modes with a higher threshold due to a higher loss and/or lower gain can appear at high excitation intensity (high carrier density).12,31 Because each transverse mode has its own longitudinal modes based on the respective cavity lengths, the lasing emission is broadened. The peak position of the lasing emission at an excitation intensity of 150 µJ/cm2 is first red-shifted about 30 meV with a time constant of 1 ps after the excitation pulse, as shown in Figure 5b. A cooling process from hot carriers to a quasithermalized system builds up the EHP, and the band gap is renormalized in the EHP.24,25 Therefore, this time constant suggests that it is related to the buildup time of the EHP. The red-shift recovers with a time constant of 2 ps, which is similar to the results found in the peak shift of EHP luminescence dynamics in ZnO thin films,24 implying the recovery to the normal band gap with the decrease of the carrier density due to lasing and other loss processes. Therefore, this time-dependent shift of the lasing wavelength peak position shows the band gap renormalization due to the EHP upon ultrafast excitation and its recovery in the operating nanolaser. We note that the recovery time to the normal band gap is a little shorter than that in luminescence dynamics in the nonlasing ZnO thin films,24 which implies that the lasing results in a faster deactivation of the carrier density in EHP. However, a direct comparison may not be possible without more information about the defect densities in the nanowires. It also should be noted that the dynamics of the band gap is similar to that of the EHP state observed by transient absorption, which supports the result that

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Figure 5. (a) The emission spectra from a single nanowire as a function of excitation (267 nm) intensity. When the carrier density increases to the electron hole plasma (EHP) regime, the lasing peaks become redshifted due to band gap renormalization. At an excitation intensity of 150 µJ/cm2, the lasing peak is broadened and the longitudinal modes of the lasing are barely noticeable, whereas two longitudinal modes are clearly observable at the excitation intensity of 90 µJ/cm2. (b) Timeresolved emission profiles obtained by the upconversion technique. The emission spectra are spectrally resolved in order to study the dynamics of the band gap change. The line shows the shift of the peak maxima and thus the band gap change with time.

the fast component at high carrier density is due to the EHP state (Figure 4). This close relationship shows that the dynamics of the carrier density can therefore be translated to the dynamics of the band gap shift. However, the decay times of the carrier density of the EHP and the band gap shift are not to be same, because the degree of the band gap renormalization is not linear with carrier density.32 4. Conclusions The ultrafast lasing dynamics of single ZnO nanowires are studied by a time-resolved upconversion technique as a function of the excitation intensity. The optical induction time of lasing is strongly dependent on excitation intensity. The lasing dynamics is strongly affected by the gain of the stimulated emission and, thus, the carrier densities, so that the minimum roundtrips for lasing threshold emission change. The lasing dynamics are compared to the carrier dynamics observed by transient absorption methods. The fast decay component of excited carriers at a high carrier density is due to the EHP states, whereas the slow component is attributed to exciton states. The EHP component in the carrier dynamics is found to have a close

1684 J. Phys. Chem. C, Vol. 112, No. 5, 2008 relationship with the band gap dynamics, which appears as a renormalization of the band gap and its recovery to the normal band gap during lasing. Acknowledgment. The initial lasing experiments were supported by an exploratory grant from the National Science Foundation, ECS-0210106. Construction of the laboratories, additional equipment, and recent support were provided by the Director, Office of Science, Office of Basic Energy Sciences, Materials Science Division, U.S. Department of Energy under Contract No. DE-AC05CH11231. S.H.L. and Y.P.Z. were supported by the National Science Foundation under Contract No. ECS-0304340. J.K.S. was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund, KRF-2006-331C00145). References and Notes (1) Melosh, N. A.; Boukai, A.; Diana, F.; Gerardot, B.; Badolato, A.; Petroff, P. M.; Heath, J. R. Science 2003, 300, 112. (2) Tong, L.; Gattass, R. R.; Ashcom, J. B.; He, S.; Lou, J.; Shen, M.; Maxwell, I.; Mazur, E. Nature 2003, 426, 816. (3) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 61. (4) Tsukazaki, A.; Ohtomo, A.; Onuma, T.; Ohtani, M.; Makino, T.; Sumiya, M.; Ohtani, K.; Chichibu, S. F.; Fuke, S.; Segawa, Y.; Ohno, H.; Koinuma, H.; Kawasaki, M. Nat. Mater. 2005, 4, 42. (5) Huang, X.-J.; Choi, Y.-K. Sensor. Actuators, B 2007, 122, 659. (6) Li, Y. B.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2004, 84, 3603. (7) Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J.; Shim, H. W.; Suh, E. K.; Lee, C. J. Chem. Phys. Lett. 2002, 363, 134. (8) Zapien, J. A.; Jiang, Y.; Meng, X. M.; Chen, W.; Au, F. C. K.; Lifshitz, Y.; Lee, S. T. Appl. Phys. Lett. 2004, 84, 1189. (9) Choy, J. H.; Jang, E. S.; Won, J. H.; Chung, J. H.; Jang, D. J.; Kim, Y. W. Appl. Phys. Lett. 2004, 84, 287. (10) Wang, X.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (11) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897.

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