ARTICLE pubs.acs.org/JPCC
Enhanced Field-Emission and Red Lasing of Ordered CdSe Nanowire Branched Arrays Guohua Li,†,§ Tianyou Zhai,*,‡ Yang Jiang,*,† Yoshio Bando,‡ and Dmitri Golberg‡ † ‡
School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui, 230009 China International Center for Young Scientists (ICYS) and International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: Ordered CdSe nanowire branched arrays were designed and synthesized while merging two particular structural features within a single nanomaterial. This novel CdSe nanostructure combines a branched structure and ordered single-crystalline character. The stems and branches consist of wurtzite CdSe single crystals. When measuring field-emission properties, the CdSe novel nanostructure demonstrated a low turn-on field at 4.3 ( 0.2 V μm1 for the current densities of 10 μA cm2, high field-enhancement factor (1160 ( 50), and long emission stability. It indicates that the CdSe novel nanostructure could potentially be used as field emitters. The excellent field-emission performance is due to the unique morphology of CdSe, e.g., high structural order, branched structure, perfect single-crystallinity, and tapered nanotips. In addition, red lasing, in a range 700720 nm, of the ordered CdSe nanowire branched arrays were demonstrated. The nature of the observed lasing emission accords with coherent random lasing behavior. A lower lasing threshold was achieved due to the homoepitaxial growth of CdSe nanowire branches on the CdSe microrods stems as well.
1. INTRODUCTION One-dimensional nanostructured materials have attracted prominent attention within the research community due to their special physical properties and promising applications in important fields such as optoelectronics,14 piezoelectrics,5 sensing,6 photovoltaic,7,8 and thermoelectric9applications. In particular, both highly ordered and branched nanowires exhibit unique and/or enhanced electronic and optoelectronic properties. For instance, since a nanolaser made of highly ordered ZnO nanowire arrays was first reported by Yang et al.,1 enormous efforts have been devoted to exploring the lasing properties of different 1D nanomaterials including GaN,10 CdS,11,12 ZnS,13 SnO2,14 ZnxCd1xS,15 and CdSx Se1x.16,17 Nanowires/nanoribbons as optical waveguides have been demonstrated. When exposed to intensive optical excitation, they can produce stimulated emission and lasing. In addition, branched nanostructures are very interesting research objects due to specific physical properties and promising applications in optoelectronic and nanoelectronic devices.1822 For example, branched nanostructures with a high packing density and highly crystalline nanotips significantly enhance field-emission (FE) properties and thus show great promise for such applications.22 Thus far, carbon nanotuebe, ZnO, ZnS, CdS, Si, SiC, and AlN nanostructures have been shown to exhibit high FE-current density at low electric fields.23 However, the studies of FE properties, as well as the lasing characteristics of ordered and branched CdSe nanostructures have been rare. Different CdSe nanostructures (nanowires, nanobelts, and quantum dots) could potentially be used to build diverse electronic and optoelectronic devices including photodetectors, r 2011 American Chemical Society
field-effect transistors, biomolecular labels, laser cavities, solar cells, charge-coupling devices, and light-emitting diodes.2430 The morphology of a nanostructure plays an important role for its physical properties. As depicted in the above paragraph, ordered and branched types of nanomaterials contributed a lot to enhance or generate unique physical properties. Similarly, the morphologies of CdSe nanostructures, when assembled into optoelectronic and electronic devices, are expected to show many new properties. In this paper, ordered CdSe nanowire branched arrays were designed and synthesized while merging two particular structural features within a single nanomaterial. This novel CdSe nanostructure combines a branched structure and ordered single-crystalline character. The stems and branches consist of wurtzite CdSe single crystals. The low turn-on field, 4.3 ( 0.2 V μm1 for the current densities of 10 μA cm2, high field-enhancement factor (1160 ( 50), and long emission stability indicate that these CdSe novel nanostructures could potentially be used as field emitters. In addition, lasing in a red range, 700720 nm, of the ordered CdSe nanowire branched arrays was demonstrated. The nature of the observed lasing emission accords with coherent random lasing behavior. As well as, a lower lasing threshold was achieved due to the homoepitaxial growth of CdSe nanowire branches on the CdSe microrods stems. Received: January 14, 2011 Revised: April 14, 2011 Published: April 22, 2011 9740
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Figure 1. Schematic setup for the power dependent PL meassurement.
2. EXPERIMENTAL SECTION Material Synthesis and Characterization. Ordered CdSe nanowire branched arrays were synthesized using a two-step approach as follows. First, random CdSe microrods were synthesized via a vapor-solid (VS) growth mechanism by thermal evaporating CdSe. Then, using the obtained CdSe as substrates, high ordered CdSe nanowire arrays homoepitaxially were grown on those microrods substrates via a vaporliquidsolid (VLS) process. The experiment setup consisted of a horizontal tube furnace with a quartz tube mounted inside and a gas flow control system. In the first step, the CdSe powder (Aldrich, 99.99%) placed in the middle of the quartz tube was evaporated and carried by argon at a temperature of 900 °C and a chamber pressure of 200 Torr. The larger CdSe microrods were obtained after several hours’ reaction. Then the CdSe microrods samples were removed and deposited a 5 nm thick gold film via sputtering. In the second step, hydrogen (5% in volume) mixed with argon (95% in volume) was introduced during the synthesis process at a lower temperature (750 °C) and for a shorter duration (1 h). The CdSe microrods covered by thin gold film were used as the substrate where the secondary CdSe nanowire branches grew. Finally, the products of ordered CdSe nanowire branches arrays were synthesized and collected for further investigations. This synthesis is a homoepitaxial method. The use of homo/heteroepitaxial methods are reported before.13,31 The morphologies and structures of the as-synthesized nanostructures were characterized using X-ray diffraction (XRD, D/MAX2500VL/PC), field-emission scanning electron microscopy (FE-SEM, SIRION 200), and high resolution transmission electron microscopy (HRTEM, JEM-3000F). Field-Emission and Photoluminescence (PL) Lasing Measurements. The field-emission properties were studied at roomtemperature in a high vacuum chamber (2.6 106 Pa). A rod-like copper probe with a cross section of 1 mm2 was used as an anode, and CdSe nanowire arrays served as a cathode. A dc voltage sweeping from 100 to 1100 V was applied to the samples. Regular room temperature photoluminescence was recorded via a Fluorolog Tau-3 Lifetime System. Power-dependent PL measurements were conducted at room temperature using the forth harmonic of a Nd: YAG (yttrium aluminum garnet laser, wavelength, 266 nm) with a 6 ns pulse width as the excitation source. The excitation laser was focused onto a sample with a spot diameter about 200 μm. The PL signal was detected with an UV optical fiber coupled to a 0.5 m spectrograph using a 1200 groove mm1 grating (Acton Research
Figure 2. XRD pattern of branched CdSe architectures. The vertical lines at the bottom line correspond to the standard XRD of wurtzite CdS (JCPDS No. 77-0046);.
Figure 3. (a and b) Typical SEM images of CdSe branched architectures. (c) TEM image of an individual CdSe nanowire with tapered shape and a metal particle on its tip, implying a VLS growth mechanism. (d) HRTEM image and corresponding FFT pattern (inset) of this architecture showing high crystallinity;.
Corp. Spectra Pro 500i) and an intensified charge-coupled device (CCD) detector. The fiber with 1.2 mm diameter was placed at a distance of 5 mm from the nanoobjects, giving effective lightacceptance angles from 0 to 80° and 100 to 180°. The schematic setup for the power dependent PL measurement is shown in the Figure 1.
3. RESULTS AND DISCUSSION To confirm the phase of the product, powder X-ray diffraction (PXRD) is performed on the as-synthesized samples. The XRD pattern of the as-synthesized samples (top trace) and the CdSe powder diffraction pattern from the Joint Committee on Powder Diffraction Standards Card (JCPDS) No. 77-0046 (bottom trace) are both presented in Figure 2. All observed diffraction 9741
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Figure 4. (a) TEM image of a CdSe morphology showing nanobranches on the stem. (b) HRTEM image taken from the interface between a CdSe nanowire branch and its stem. (c) Schematic model for the formation of branched CdSe architectures.
peaks are indexed to CdSe, which has a hexagonal wurzite structure with the lattice constant of a = 4.29, b = 0.701 (JCPDS card: 77-0046). This confirmed the structure of assynthesized products to be the hexagonal wurtzite CdSe. The sharp and strong diffraction peaks suggest that the as-prepared products are well crystallized as well. The morphology of the CdSe nanostructures was examined by means of a field-emission scanning electron microscope (FE-SEM). Figure 3a,b shows the representative FE-SEM images of as-synthesized CdSe products. The ordered high-density CdSe nanowire branched arrays homoepitaxially grow out of the CdSe microrod stems. Compared to branched nanomaterial architectures reported previously,19,28 the density and order of the present nanowire branches are higher. A typical length of the synthesized CdSe nanowire branches is about 1015 μm after 1 h reaction. Transmission electron microscopy (TEM) further confirms that the nanostructures are composed of a single-crystalline CdSe phase. Figure 3c shows a nanowire; this branch was removed from the CdSe main stem.The branches exhibit a tapered shape and their diameters range from ∼50 to ∼100 nm. The high resolution TEM (HRTEM) image (Figure 3d) of a representative nanowire clearly shows lattice fringes of a single-crystal in the [0001] direction. The observed spacings are measured to be 0.70 and 0.37 nm; these correspond to the (0001) and (0010) lattice spacings of CdSe, respectively. The fast Fourier transform (FFT) (Figure 3d inset) patterns of all nanowires exhibit a single-crystalline character with no splitting of the reciprocal lattice reflections along the nanowire lengths, which indicates the apparent high-quality single-crystalline nature of the structures. TEM was also used to further investigate the morphology of the novel CdSe nanostructures. A representative TEM image (Figure 4a) shows the CdSe branches and a stem nanostructure. The branches are perpendicular to the stem. A HRTEM image taken from the interface of a branch and a stem is presented in Figure 4b, which demonstrates the same CdSe lattice fringes for both structural domains. Thus the CdSe nanowire branches grew
Figure 5. EDS spectra of a CdSe nanowire body (a) and its head (b).
homoepitaxially out of the CdSe microrods (stems). Despite the Au catalyst drop off by the mechanical vibration or run out in the growth process in most of case, we occasionally find the Au nanocluster at the head of nanowire (Figure 3c), which indicates that the growth of the epitaxial nanowires has been governed by the VLS mechanism.A schematic model (Figure 4c) depicts the growth process of the ordered CdSe nanowire branched arrays. Since the CdSe microrods were covered with a very thin gold film, this deposited film may melt and form low-melting-point alloy liquid droplets existing together with the coming CdSe vapor on the surface of CdSe microrods under high temperature. When the concentration of CdSe in the alloy reaches the supersaturation, CdSe phase precipitates at the interface between the liquid alloy droplet and surface of CdSe microrods. Then the nanowire branches formed. The energy dispersive spectra (EDS) for a nanowire body and its catalytic particle head are presented in Figure 5 a,b, respectively. For the spectrum of the nanowire body, only Cd, Se, and Cu peaks were detected, the Cu peaks originated from the TEM grid. Moreover, the atomic ratio of the Cd and Se elements is approximated as 1:1 within the instrumental error for standard quantitative analysis. While in the nanowire head case, the Au peaks were observed along with the Cd, Se, and Cu peaks. This observation confirmed that the nanowire growth followed the VLS growth mechanism.32 So far, the studies on the FE properties of CdSe nanostructures have been rather limited compared to other popular field 9742
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suggests that the presently synthesized CdSe nanostructures with low turn-on and threshold fields can be used as practical field emitters. It is important to note that the turn-on field of the present CdSe branched nanostructures is lower than that of the CdSe nanotetrapods (22, 9, and 4 V μm1).33 The lower turn-on field in our case is believed to be due to the special branched geometry and perfect crystal order of the fabricated CdSe nanostructures.22 The FE currentvoltage characteristics were further analyzed using the FN equation23 J ¼ ðAβ2 E2 =jÞ expð Bj3=2 =βEÞ
ð1Þ
or lnðJ=E2 Þ ¼ lnðAβ2 =jÞ Bj3=2 =βE 6
Figure 6. Field-emission properties of the branched CdSe architectures: (a) FE current density versus the applied field (J-E) curve with a turn-on field of 4.3 ( 0.2 V μm1 and a threshold field of 6.3 ( 0.3 V μm1 for the current densities of 10 and 1 mA cm2, respectively. The inset is a FowlerNordheim (FN) plot corresponding to (a) and the straight line is a linear fit of the ln(J/E2)(1/E) plot. (b) A stable emission current of the branched CdSe architectures over 14 h.
emitters such as carbon nanotubes, ZnO, ZnS and CdS nanostructures. Actually, there has been only a single paper devoted to the study of FE of CdSe nanostructures.33 Our FE measurements show that the CdSe nanowire branched arrays are in fact excellent field emitters. Figure 6 illustrates the FE current density, J, as a function of the applied field, E, for a JE plot (Figure 6a) and a ln (J/E2)(1/E) plot (the inset of Figure 6a) at an anodecathode separation of 150 μm. It is found that the current density exponentially increases with the field increase. The turn-on field at a current density of 10 μA cm2 is 4.3 ( 0.2 V μm1 and the threshold field at a current density of 1.0 mA cm2 is 6.3 ( 0.3 V μm1. These values are comparable to those mentioned in the previous reports on carbon nanotubes, ZnO, ZnS, CdS, and Si nanostructures, for example, the turn-on field of the present branched CdSe nanostructures can be compared to that of carbon nanotubes (5.4 V μm1),34 CdS nanostructures (12.235 and 1.436 V μm1), ZnS nanostructures (3.8,37 3.55,19 and 2.3922 μm1), ZnO nanostructures (11 V μm1),38 Si nanostructures (13 V μm1),39 and ZnS-In heterostructures (5.43 V μm1).40 This
2
ð2Þ 3/2
where A = 1.54 10 A eV V , B = 6.83 10 eV V μm1, J is current density, E is the applied electric field, β is the field-enhancement factor, and j is the work function of the emitting material, which is 5.22 eV for CdSe.41 The FN plot of (ln(J/E2)) versus (1/E) shows a linear relationship, indicating that the FE behavior obeys the FN theory, i.e. the electrons tunnel through the potential barrier from the conduction band to vacuum. The line slope of ln(J/E2) versus 1/E (inset of Figure 6a) can be employed into eq 2, thus β can be estimated to be 1160 ( 50 at a working anode-sample distance of 150 μm. This β is much higher than previously reported values of CdSe nanotetrapods (218, 556, and 946).33 Generally, the fieldenhancement factor (β) is related to the crystal structure, tip geometry, aspect ratio of a nanostructure, and spatial distribution of emitting centers. As seen in Figure3, the CdSe nanowire branches all have the tapered nanotips, well-aligned and separated from each other. This can result in a greater field-enhancement value during the FE measurements. In addition, the perfect single crystal characteristics of the CdSe nanowire arrays positively influenced the β value. We also believe that there is a multifold FE enhancement; first the primary applied field is enhanced by a stem which acts as a substrate for the secondary branches. A stronger field at the bottom of the branches is equivalent to a higher applied bias, which, in turn, is further enhanced by the 1D branch. Such multifold FE enhanced mechanism was introduced by Gautam et al.40 Zhang et al. addressed the field enhancement is due to a tip-on-tip geometry for ZnO/CNT array structure as well.39 Furthermore, FE stability measurements on CdSe branched nanostructures were performed by keeping a current density at 1.5 mA cm2 over 14 h. As shown in Figure 6b, there are no current degradations or notable fluctuations during this period. Such long emission stability assures the practical applications in field-emitters. Based on the above-discussed results and compared with other CdSe nanostructures,33 the present CdSe field emitters demonstrate excellent performances such as low turn-on and threshold voltages, high field-enhancement factor, and good stability. It is also notable that the CdSe nanowire branched arrays field emitters can rival previously reported carbon nanotubes, Si, AlN, CdS, ZnS, and ZnO nanostructures.19,23 The room-temperature PL spectrum of CdSe nanowire branch arrays was recorded under a Xe lamp using an excitation wavelength of 525 nm. As shown in Figure 7a, a strong emission peak centered at ∼710.2 nm (1.746 eV) is observed, which is ascribed to the near band-edge emission of the CdSe nanowire. The bandgap (Eg) of wurtzite CdSe is 1.738 eV42 at room temperature, which is slightly smaller compared to the observed 9743
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of frequencies close to the maximum of the gain spectrum.1 When the excitation intensity exceeds the threshold, a sharp main peak emerges in the emission spectrum (Figure 7b, top trace). The fwhm of this sharp peak is about 4 nm, which is nearly 10 times smaller than the fwhm of the spontaneous emission peak below the threshold. Additional several sharp peaks are observed around the main peak between 700 to 720 nm, which indicates the appearance of lasing action. Those lasing emissions are all in the range of red light spectrum. Such resonant emission is preferentially detected for the nanowires with their axes parallel to the optical fiber as a result of the high directionality of the lasing emission in them. The nature of the observed lasing emission accords with random lasing behavior with coherent feedback.47 Such coherent lasing action was also observed in ZnO nanoparticle47 and CdSe nanowires.48 It is notable, the lasing threshold of the branched CdSe nanowire arrays is around 80 kWcm2. This value is comparable to the reported values on other nanomaterials, including CdS nanowires (40 kWcm2),11 CdS0.4Se0.6 nanoribbons (35 kWcm2),49 Zn0.83Cd0.17S nanoribbons (130 kWcm2),15 and ZnO nanowire (40 kWcm2).1 However, the threshold is lower compared to the CdSe nanobelts (100 kWcm2)29 and CdSe nanowires (296 kWcm2) epitaxially grown out from muscovite mica substrate;It is believed the substrate play an important role. In our case, the CdSe nanobranches homoepitaxially grew on the CdSe stems, thus they have a same refractive index. In contrast, the other CdSe nanowire/ribbons are normally grown on different materials with different refractive index, which probably decrease the random coherent lasing factor. Therefore, the threshold of the CdSe nanowires on a substrate made of different materials may increase markedly.
Figure 7. (a) Room-temperature PL spectrum for as-synthesized CdSe nanostructures obtained with an excitation wavelength of 525 nm from a Xe lamp. (b) PL spectra under Nd: YAG (266 nm) irradiation for assynthesized nanostructures. Main panel: under different excitation powers. Inset: corresponding PL intensity versus input power density showing linear dependency and revealing the threshold for lasing.
value. This blue shift phenomenon for the 1D structure may be caused by a strain effect during the PL experiment. Such explanation has widely been accepted in the pre-existing literature.4345 Moreover, the relative narrow and clear peak with a full width at half-maximum (fwhm) of 30 nm indicates the high-quality optical property of the synthesized CdSe nanowire arrays. It is noteworthy that no defect-related emission peak was found during the PL measurements, which was consistent with the observations of the defect-free structures during HRTEM imaging (Figure 3d). In order to study the possible stimulated emission from the ordered CdSe nanowire arrays, the power dependent emission was examined. The PL spectra of CdSe nanostructures obtained under different excitation powers using a Nd: YAG laser (266 nm) are presented in Figure 7b. At lower intensity, the spectra (Figure 7b, the black and red traces) are broad and featureless, centered at ∼710 nm (1.746 eV) and with a fwhm of ∼43 nm. In this regime, the light is emitted isotropically along the nanowire, and the output optical intensity depends linearly on the excitation intensity, in accord with the spontaneous emission characters.46 As the excitation power density increases, the emission peak narrows due to the preferential amplification
4. CONCLUSIONS To conclude, we developed a route to synthesize ordered CdSe nanowire branched arrays which homoepitaxially grow on the larger CdSe microrod stems. The stems and branches both consist of wurtzite single-crystalline CdSe. These novel nanostructures combined the two merits of ordered and branched morphologies and demonstrate marked field-enhanced phenomena and red lasing properties. FE measurements of the CdSe nanostructures show a low turn-on field of 4.3 ( 0.2 V μm1, a high field-enhancement factor of 1160 ( 50, and long emission stability. The excellent FE performance is due to the unique morphology of CdSe, e.g. high structural order, branched structure, perfect single-crystallinity, and tapered nanotips. In addition, red lasing during photoluminescence measurements, within a range of 700 to 720 nm, was observed. The nature of the observed lasing emission accords with coherent random lasing behavior. A lower lasing threshold was achieved due to the homoepitaxial growth of CdSe nanowire branches on the CdSe microrods stems. The results indicate that the present CdSe novel nanostructures may become valuable for not only fundamental studies but also in practical device applications. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected]. Present Addresses §
201 Gilman Hall, Department of Chemical Engineering and Biomolecular Engineering, UC Berkeley, Berkeley, CA 94720 9744
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’ ACKNOWLEDGMENT The authors greatly appreciate valued discussion with Dr. Brian Bush in Department of EECS at UC Berkeley. This work was financially supported by the National High Technology Research and Development Program of China (No.2007AA03Z301), the Natural Science Foundations of China (Nos. 61076040, 20771032, and 60806028), the National Basic Research Program of China (No.2007CB9-36001). T.Y.Z. thanks the supporting by the World Premier International Research Center (WPI) initiative on Materials Nanoarchitectonics (MANA), MEXT, Japan. G.H.L. thanks the financial supporting from China Scholarship Council (CSC-2008669013). ’ REFERENCES (1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (2) Colli, A.; Tahraoui, A.; Fasoli, A.; Kivioja, J. M.; Milne, W. I.; Ferrari, A. C. ACS Nano 2009, 3, 1587. (3) Cheng, W. Y.; Chen, W. T.; Hsu, Y. J.; Lu, S. Y. J. Phys. Chem. C 2009, 113, 17342. (4) Lei, Y. L.; Liao, Q.; Fu, H. B.; Yao, J. N. J. Am. Chem. Soc. 2010, 132, 1742. (5) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (6) Fan, Z. Y.; Ho, J. C.; Jacobson, Z. A.; Razavi, H.; Javey, A. Proc. Natl. Acad. Sci. 2008, 105, 11066. (7) Peng, K. Q.; Wang, X.; Wu, X. L.; Lee, S. T. Nano Lett. 2009, 9, 3704. (8) Zhu, J.; Hsu, C. M.; Yu, Z. F.; Fan, S. H.; Cui, Y. Nano Lett. 2010, 10, 1979. (9) Hochbaum, A. I.; Chen, R. K.; Delgado, R. D.; Liang, W. J.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. D. Nature 2008, 451, 163. (10) Johnson, J. C.; Choi, H. J.; Knutsen, K. P.; Schaller, R. D.; Yang, P. D.; Saykally, R. J. Nat. Mater. 2002, 1, 106. (11) Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (12) Agarwal, R.; Barrelet, C. J.; Lieber, C. M. Nano Lett. 2005, 5, 917. (13) Jiang, Y.; Zhang, W. J.; Jie, J. S.; Meng, X. M.; Zapien, J. A.; Lee, S. T. Adv. Mater. 2006, 18, 1527. (14) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. D. Science 2004, 305, 1269. (15) Liu, Y. K.; Zapien, J. A.; Shan, Y. Y.; Geng, C. Y.; Lee, C. S.; Lee, S. T. Adv. Mater. 2005, 17, 1372. (16) Kwon, S. J.; Choi, Y.-J.; Park, J.-H.; Hwang, I.-S.; Park, J.-G. Phys. Rev. B 2005, 72, 205312. (17) Li, G. H.; Jiang, Y.; Wang, Y.; Wang, C.; Sheng, Y. P.; Jie, J. S.; Zapien, J. A.; Zhang, W. J.; Lee, S. T. J. Phys. Chem. C 2009, 113, 17183. (18) Yin, L. W.; Bando, Y.; Zhu, Y. C.; Li, M. S.; Li, Y. B.; Golberg, D. Adv. Mater. 2005, 17, 110. (19) Fang, X. S.; Bando, Y.; Shen, G. Z.; Ye, C. H.; Gautam, U. K.; Costa, P. M. F. J.; Zhi, C. Y.; Tang, C. C.; Golberg, D. Adv. Mater. 2007, 19, 2593. (20) Bierman, M. J.; Lau, Y. K. A.; Kvit, A. V.; Schmitt, A. L.; Jin, S. Science 2008, 320, 1060. (21) Watt, J.; Young, N.; Haigh, S.; Kirkland, A.; Tilley, R. D. Adv. Mater. 2009, 21, 2288. (22) Chen, Z. G.; Cheng, L. N.; Xu, H. Y.; Liu, J. Z.; Zou, J.; Sekiguchi, T.; Lu, G. Q.; Cheng, H. M. Adv. Mater. 2010, 22, 2376. (23) Fang, X. S.; Bando, Y.; Gautam, U. K.; Ye, C.; Golberg, D. J. Mater. Chem. 2008, 18, 509. (24) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (25) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. Adv. Mater. 2000, 12, 1102.
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(26) Woo, W. K.; Shimizu, K. T.; Jarosz, M. V.; Neuhauser, R. G.; Leatherdale, C. A.; Rubner, M. A.; Bawendi, M. G. Adv. Mater. 2002, 14, 1068. (27) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538. (28) Jiang, Y.; Zhang, W. J.; Jie, J. S.; Meng, X. M.; Fan, X.; Lee, S. T. Adv. Funct. Mater. 2007, 17, 1795. (29) Pan, A. L.; Liu, R. B.; Zhang, Q. L.; Wan, Q.; He, P. B.; Zacharias, M.; Zou, B. S. J. Phys. Chem. C 2007, 111, 14253. (30) Dai, G. Z.; Zhang, Q. L.; Peng, Z. W.; Zhou, W. C.; Xia, M. X.; Wan, Q.; Pan, A. L.; Zou, B. S. J. Phys. D-Appl. Phys. 2008, 41. (31) Zhou, W. C.; Pan, A. L.; Li, Y.; Dai, G. Z.; Wan, Q.; Zhang, Q. L.; Zou, B. S. J. Phys. Chem. C 2008, 112, 9253. (32) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (33) Zhao, L. J.; Pang, Q.; Yang, S. H.; Ge, W. K.; Wang, J. N. Phys. Lett. A 2009, 373, 2965. (34) Li, C.; Zhang, Y.; Mann, M.; Hiralal, P.; Unalan, H. E.; Lei, W.; Wang, B. P.; Chu, D. P.; Pribat, D.; Amaratunga, G. A. J.; Milne, W. I. Appl. Phys. Lett. 2010, 96. (35) Zhai, T. Y.; Fang, X. S.; Bando, Y. S.; Liao, Q.; Xu, X. J.; Zeng, H. B.; Ma, Y.; Yao, J. N.; Golberg, D. ACS Nano 2009, 3, 949. (36) Pan, H.; Poh, C. K.; Zhu, Y.; Xing, G.; Chin, K. C.; Feng, Y. P.; Lin, J.; Sow, C. H.; Ji, W.; Wee, A. T. S. J. Phys. Chem. C 2008, 112, 11227. (37) Lu, F.; Cai, W. P.; Zhang, Y. G.; Li, Y.; Sun, F. Q.; Heo, S. H.; Cho, S. O. Appl. Phys. Lett. 2006, 89. (38) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 3648. (39) Shang, N. G.; Meng, F. Y.; Au, F. C. K.; Li, Q.; Lee, C. S.; Bello, I.; Lee, S. T. Adv. Mater. 2002, 14, 1308. (40) Gautam, U. K.; Fang, X. S.; Bando, Y.; Zhan, J. H.; Golberg, D. Acs Nano 2008, 2, 1015. (41) Gryadil, I. A.; Dovgoshei, N. I.; Bentsa, V. M.; Chepur, D. V.; Otrokh, P. I.; Povkhan, T. I. Russ. Phys. J. 1970, 13, 2. (42) Mei, J. R.; Lemos, V. Solid State Commun. 1984, 52, 785. (43) Kuykendall, T.; Pauzauskie, P. J.; Zhang, Y. F.; Goldberger, J.; Sirbuly, D.; Denlinger, J.; Yang, P. D. Nat. Mater. 2004, 3, 524. (44) Kipshidze, G.; Yavich, B.; Chandolu, A.; Yun, J.; Kuryatkov, V.; Ahmad, I.; Aurongzeb, D.; Holtz, M.; Temkin, H. Appl. Phys. Lett. 2005, 86. (45) Shan, C. X.; Liu, Z.; Hark, S. K. Appl. Phys. Lett. 2007, 90. (46) Zimmler, M. A.; Bao, J.; Capasso, F.; Muller, S.; Ronning, C. Appl. Phys. Lett. 2008, 93. (47) Cao, H.; Zhao, Y. G.; Ho, S. T.; Seelig, E. W.; Wang, Q. H.; Chang, R. P. H. Phys. Rev. Lett. 1999, 82, 2278. (48) Chen, R.; Utama, M. I. B.; Peng, Z. P.; Peng, B.; Xiong, Q. H.; Sun, H. D. Adv. Mater. 2011, 23, 1404. (49) Pan, A. L.; Liu, R. B.; Wang, F. F.; Xie, S. S.; Zou, B. S.; Zacharias, M.; Wang, Z. L. J. Phys. Chem. B 2006, 110, 22313.
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