Published: July 18, 2011

r 2011 American Chemical Society 1940 dx.doi.org/10.1021/jz2008775 | J. Phys. Chem. Lett. 2011, 2, 1940–1945

LETTER

pubs.acs.org/JPCL

The Growth and Optical Properties of ZnO NanowallsMegan M. Brewster, Ming-Yen Lu, Sung Keun Lim, Matthew J. Smith, Xiang Zhou, and Silvija Grade�cak*

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,United States

bS Supporting Information

Nanowalls are novel two-dimensional nanostructures thathold promise for applications that require networks with

large surface area, such as solar cells1 and gas sensors.2Within thepast decade, the first studies of nanowalls have been reported in avariety of materials systems,3�11 and proof-of-concept gassensors,12�14 nanogenerators,15 and photoconductors16 basedon nanowalls have been recently demonstrated. Of the inorganicmaterials utilized in such electronic devices, ZnO is particularlypromising for its fast electron transport.14,17 ZnO may also begrown in a diversity of nanoscale architectures,17,18 which iscritical for bulk heterojunction solar cells, as the varying distancesbetween nanoscale features may be tuned tomatch energy carrierdiffusion lengths.

Nanowalls may be grown by techniques similar to one-dimen-sional nanowires, a class of related but distinct nanostructures.Notably, the growth of ZnO nanowalls and nanowires may coexistduring a single growth:19�22 for example, nanowalls may form anetwork between nanowires and have been observed to be tallernear nanowires, which suggests the sharing of collected precursorsbetween these two nanostructures.21 Nanowire growth has alsobeen observed to occur after nanowall formation at nanowalljunctions, potentially due to precursor adatom clustering.15 Themorphological evolution of nanowires into nanowalls22 furtherimplies regimes of preferred growth of these one- and two-dimensional architectures. The exact nature of the competingnanostructure growth modes is poorly understood, but a compre-hensive understanding of nanostructure growth mechanisms andthe possibility to select between these two growth modes wouldallow for the rational synthesis of predefined nanostructures.

Here, we study growth of ZnO nanowalls by vapor transportand condensation (VTC) and their morphological evolution

from nanowires by increasing the Zn flux during the growth. Thisgrowth progression is investigated through the analysis of bothgrowth mechanisms and growth kinetics. The growth mechan-ism is found to cause nanoscale structural variations (namely, Aunanoparticles embedded within a ZnO film at the base of thenanowalls near the substrate), which locally affect the opticalproperties of nanowalls.

ZnO nanowalls were grown in a horizontal tube furnace onAu-coated a-sapphire substrates, as described in the Experimen-tal Methods. To understand the effect of the Zn flux on thecompeting growth modes of ZnO nanowalls and nanowires, weperformed growths with various growth times (up to 40min) andinvestigated the resulting morphologies by scanning electronmicroscopy (SEM). At short growth times, ZnO nanowires wereobserved to grow vertically from a ZnO film on the substrate(Figures 1a,b). Nanowires were gradually incorporated into thefilm to form nanowalls by 20min (Figure 1c), and only nanowallsremained by 40 min (Figure 1d). Further, the curvature of thenanowall ridge evolved from a concave shape between neighbor-ing nanowires to a uniform height by 40 min (see respectivefigure insets), and nanowall widths increased from∼24 nm at 20min to ∼45 nm at 40 min.

Nanowall growth was achieved by increasing the source fluxcompared to that utilized for nanowire growth,23 accomplishedby changing the following growth parameters: higher sourcetemperature to increase the temperature-activated Zn equilibri-um pressure, increased Zn source mass (i.e., increased surface

Received: June 29, 2011Accepted: July 18, 2011

ABSTRACT: Nanowalls are novel nanostructures whose networked morphology holdspotential for applications such as solar cells and gas sensors. The realization of such nanowall-based devices depends directly on a comprehensive understanding of the nanowall growth,namely, its competition with nanowire growth and the role of seed particles. We induced amorphological evolution from nanowires to nanowalls by increasing source flux during vaportransport and condensation growth. Nanowall growth kinetics indicates that their morpho-logical dominance was driven by a time-dependent curvature of the nanowall growth facet.Nanowalls have excellent crystalline quality and strong near-band-edge luminescence andwere found to grow by a combination of Au- and nonassisted mechanisms, resulting in Aunanoparticles within 300 nm of the substrate whose positions were associated with the originof green luminescence. These results imply that the growth mechanism causes nanoscalestructural variations, which in turn locally affect the optical properties of nanowalls.

SECTION: Nanoparticles and Nanostructures

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area of source available for carbothermal reduction), lower tubepressure to increase the Zn diffusion length in the gas, andincreased O2 flow to increase the rate of Zn adatom oxidation.As ZnO nanowalls may be grown by multiple techniques,12,24,25

we have generalized these findings from laboratory-specificparameters to an overall increase in source flux, such that ourresults are applicable to various nanostructure growth techniques(detailed discussion on the influence of the growth parameterson the growth evolution is provided in the Supporting In-formation). Under these conditions, the nanowire density priorto nanowall formation was measured to be nearly 3 times greater(93.0 nanowires/μm2 in Figure 1a) than that under reducedsource flux characteristic for nanowire-only growth (33.8 nano-wires/μm2, see Supporting Information).

To understand the growth evolution fromnanowires to nanowalls,we next considered the growth mechanism of these two nanostruc-tures from Figure 1. The similarity in the as-grown nanowire mor-phology (i.e., growing vertically from a ZnO film) under nanowire23

and nanowall growth conditions (before 20 min) suggests that theirnucleation occurs by the same mechanism; that is, a combination ofAu- and nonassisted mechanisms, similar to previous reports.23,26

First, Zn precursor is efficiently captured by Au nanoparticles andoxidized to form c-oriented ZnO pyramidal nuclei. From the nucleiprecipices, nanowire growth occurs by anisotropic surface energies(Figure 1a inset). The nuclei finally coalesce, forming a film thatnetworks between the nanowires (Figure 1b inset). Thus, themixtureof Au- and nonassisted mechanisms causes Au nanoparticles to befound at or near the substrate/film interface.

At this point, film growth begins to dominate for theformation of nanowalls over nanowires due to the followinggeometrical considerations: the precursor collection area of thefilm becomes similar to that of the nanowire sidewalls, and thefilm height approaches that of the nanowires to allow foradatom diffusion between the nanowire sidewalls and thenanowall growth front (inset, Figure 1c). As will be discussedin detail below, adatoms preferentially incorporate in areas ofconcave curvature (i.e., the nanowall growth front betweennanowires) over those of convex curvature (i.e., nanowire tips).Thus, nanowire growth is slowed significantly as more adatomsincorporate at the nanowall, enhancing nanowall growth (inset,Figure 1d).

To understand the interplay of nanowire and nanowall growthmodes, the growth kinetics of the competing nanostructureswere next analyzed, as illustrated in Figure 2. First, the limitinggrowth steps of nanowires were investigated by measuring theirgrowth rate (height/growth time) as a function of nanowirediameter. The nanowires at 5 min into the growth exhibit amaximum growth rate at a critical diameter of ∼30 nm(Figure 2a). The existence of the critical diameter is indicativeof competition between the Gibbs�Thomson effect (at smallerdiameters) and diffusion-limited growth kinetics (at largerdiameters).27,28 We then fit the nanowire growth rate dL/dt asa function of diameter d with a model that combines these twoprocesses by solving steady-state diffusion equations of adatomprecursors at the limit of low adatom surface diffusivity (due tothe high source flux):27

dLdt

� 1� e2RGT=d

θ

" #1 þ 4λ

d

� �ð1Þ

where θ is the Zn activation energy (which is assumed to be equalbetween the vapor and the nanowire tip), λ is the Zn adatomdiffusion length on the nanostructure sidewalls, and RGT is the

Figure 1. SEM images of nanowalls at 45� tilt after (a) 3, (b) 10, (c) 20,and (d) 40 min growth time. Scale bars are 1 μm. Bottom-left insetsshow evolution of nanowall upper facet curvature, with scale barsrepresenting 200 nm. Bottom-right schematics illustrate the evolutionof nanowire and nanowall morphology, with gold-colored spheresrepresenting Au nanoparticles.

Figure 2. (a) Nanowire growth rate as a function of nanowire diameter(red symbols) and the resulting fit with eq 1 (solid line). (b) Nanowireand nanowall height as a function of growth time. Nanowire height wastaken at the maximal diameter and nanowall height as the average of allheights at the given time. Hollow symbols represent an average of at least10 data points, with standard deviation error bars shown, and dashedlines are shown to guide the eye.

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Gibbs�Thomson radius, given by

RGT ¼ 2γΩkBT

ð2Þ

Here, γ is the surface activity (1.74 J/m2 for the (0001) face ofZnO nanorods29), Ω is the molecular volume, kB is the Boltz-mann constant, and T is the temperature. For ZnO at 930 �C,RGT was calculated from known parameters to be 4.96 nm, whileθ and λ were used as fit parameters. Equation 1 assumes theabsence of gradients in supersaturation, chemical potential, andtemperature near the growth front. We find that the modelaccurately explains the kinetic behavior at 5 min (Figure 2a),implying the mixture of the Gibbs�Thomson effect and diffu-sion-limited growth kinetics in this growth stage.

However, the growth kinetics fit becomes increasingly inac-curate with longer growth times because the nanowire growthrate becomes independent of nanowire diameter, coinciding withthe growing prominence of nanowalls. This may be because thesource collection and incorporation areas, which introduce thenanowire diameter dependence in eq 1, become inaccurate;instead, the growth rate becomes dependent upon the densityof nanowires and, eventually, nanowall junctions. Source collec-tion may also occur by surface diffusion and by the sharing ofprecursors collected by nanowalls and nanowires. Most impor-tantly, the assumption of constant chemical potential at thegrowth front becomes inaccurate due to the time-dependentcurvature evolution of nanowalls between nanowires that drivesfilm growth. Whereas nanowire tip curvature is convex in twodimensions, the nanowall curvature is saddled: convex across thenanowall thickness, but concave between nanowall junctions(inset of Figure 1b). The difference in atom chemical potential μon the surface and in the vapor is given by

μðkÞ � μðk ¼ 0Þ ¼ kBT lnPeqðkÞ

Peqðk ¼ 0Þ

" #ð3Þ

where Peq(k) is the vapor pressure in equilibrium with the localregion of the surface with curvature k.30 This force drives depo-sition in concave areas, such as that between nanowall junctions,causing the nanowalls to flatten over time (Figure 1d) and eq 3 tobe a function of growth time. The nanowall curvature evolutionmay also invalidate the assumption of stead-state adatom con-centration. Further, nanowire growth may occur away fromequilibrium,31 such that eqs 1 and 3 may not strictly governthe growth behavior at all growth times.

We then analyzed the competing nature of nanowire andnanowall growth modes by comparing the height of both thenanowalls and nanowires at different growth times, as shown inFigure 2b. Nanowire heights (taken at the critical diameterbetween competing growth kinetics behaviors) were measuredup to 15 min into the growth, beyond which most nanowires hadbeen incorporated into nanowalls and thus could not be mea-sured. These results indicate that nanowire growth rate saturatesbeyond∼10 min. Nanowall height was found to be independentof both neighboring nanowire diameter and nanowall width at allgrowth times, so the average nanowall height was employed.From the measured nanostructure heights, nanowire and nano-wall nucleation times were calculated by extrapolation23 to be 1.7min for nanowires and 2.2 min for nanowalls. That nanowiresnucleate before nanowalls is understood by the shape evolutionof the ZnO nuclei in our proposed growth mechanism: the nuclei

produce small c-plane precipices, which initiates vertical nano-wire growth, prior to coalescing for the initiation of film growth.

The morphology of the final nanowall structure and thestructural effects of the complex growth mechanisms were nextinvestigated on the nanoscale by electronmicroscopy in Figure 3.(Bulk morphological and optical investigations are summarizedin the Supporting Information.) A plan-view SEM image ofnanowalls grown for 40 min (Figure 3a) depicts their randomlynetworked structure and vertical alignment on the substrate.High-resolution (HR) transmission electron microscopy (TEM)of a nanowall section indicates a crystalline structure free of one-and two-dimensional defects that was confirmed by larger-areabright field (BF) TEM and accompanying selected area diffrac-tion (SAD) of a nanowall cross-section (Figure 3c). SADpatterns show that the film remains single crystalline throughoutthe entire thickness, whereas more diffuse spots near thesubstrate indicate increased nanowall thickness in this region.Further investigation of the film revealed the presence of par-ticles close to the substrate interface, which were identified asAu by energy dispersive X-ray spectroscopy (EDS) linescans(Figure 3d). That the Au nanoparticles are found within theZnO film, rather than strictly at the substrate surface, confirmsthat the film grew by a combination of Au- and nonassistedmechanisms.

The presence of Au nanoparticles has been reported to haveprofound consequences on the electrical and optical propertiesof individual nanostructures,23,32�36 so we next investigated the

Figure 3. (a) Plan-view SEM image of ZnO nanowalls after 40 min ofgrowth showing the cross-sectional morphology. Scale bar is 1 μm. (b)HR-TEM indicates a highly crystalline ZnO nanowall structure free ofone- and two-dimensional defects. Scale bar is 5 nm. (c) Cross-sectionBF-TEM of a nanowall with corresponding SAD patterns along the[2110] zone axis indicating single-crystalline nanowalls and the film atthe substrate/nanowall interface. Scale bar is 2 μm. (d) EDS linescanacross a nanoparticle found near the film/substrate interface, similar tothat shown in inset in the direction of the dashed arrow. Inset scale bar is150 nm.

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cathodoluminescence (CL) of a thicker cross-sectional sample toenable a stronger CL signal. BF scanning transmission electronmicroscopy (STEM) and corresponding CL spectra collectedat 108 K (Figure 4a,b) reveal strong near-band-edge (NBE)(3.31 eV) and weak, broad visible wavelength (1.8�2.5 eV)emissions. Although the low CL spectral signal strength isinherent to the CL-STEM technique, we were able to identifythe spatial distribution and origins of different CL emissions bypanchromatic and monochromatic emission images (Figure 4c).From the panchromatic images, strong CL intensity variationswere observed, and their origin was identified as beingmostly dueto thickness variations (horizontal and vertical CL intensitystriations are addressed in the Supporting Information). Mono-chromatic images indicate a spatial distribution of individualemissions: while the NBE emission follows the overall panchro-matic intensity distribution, the visible emission is localized nearthe film�substrate interface. More specifically, 2.25 eV emissionwas observed only at base of the film, whereas 1.91 eV lumines-cence originates from the sapphire substrate.37 CL intensity mayalso be modulated by dislocations (see Supporting Information),but the overall number of dislocations observed was insignificant,so we exclude them from further discussion.

To determine the origins of both the NBE and visible CL, weinvestigated the trends of CL emission intensities and nanoparticle

density as a function of distance from the substrate surface(Figure 4d). The 3.31 eV emission was found to decrease withthe increasing density of Au nanoparticles near the substratesurface, consistent with our previous work on ZnO nanowires.23

The 2.25 eV emission was found to be maximal closer to thesubstrate/film interface than the 3.31 eV emission (∼400 nm vs∼800 nm, respectively), and is not directly related to the Aunanoparticle density. Strong CL quenching is observed at allwavelengths near the substrate surface, likely due to the presenceof surface states from interface defects that cause nonradiativecombination, yet this quenching begins further from the substratefor 3.31 eV (∼100 nm) than for 2.25 eV (∼40 nm) emissions.

Broad green emission in ZnO has been suggested to originatefrom point defects (VO, Zni, VZn, and surface defects38) and Audoping.39We expect an insignificant change in the ZnO emissionprofile due to Ga40 and Al41 doping, which may have been causedby the Ga ion beam during FIB sample preparation and diffusionfrom the sapphire substrate during the high-temperature growth,respectively.

We observe that the 2.25 eV emission exists closer to thesubstrate surface than the NBE emission, supporting the notionthat the 2.25 eV emission is caused by the presence of Aunanoparticles through formation of the point defects above. Thesteady decrease in NBE intensity with increasing nanoparticledensity suggests CL quenching due to excited electron transfer,23

whereas the activation of surface plasmons would lead to theenhancement of NBE emission.33 However, the 2.25 eV emissionis not quenched with increasing nanoparticle density, implyingthat these photons are not consumed by the excitation of surfaceplasmons on the Au nanoparticle surfaces. These results implythat the ZnO film near the nanowall/substrate interface, whichemits both visible and NBE emission due to the presence of Aunanoparticles, must be considered when rationally synthesizingnanostructures of predefined functionality. For example, nano-wall-based optoelectronic devices whose functionalities dependupon NBE emission, such as lasers or light-emitting diodes,would mostly utilize the upper regions of the ZnO nanowalls,∼1μm above the nanowall/substrate interface.

In conclusion, we directly analyzed the evolution of ZnOnanowires into nanowalls by investigating the growth mechan-isms and kinetics at various growth times. Nanowire growthpreceded nanowall growth, and exhibited both Gibbs�Thomsonand diffusion-limited kinetics behaviors. A competition betweennanowall and nanowire growth modes was observed, wherenanowall growth dominated through a dynamic chemical poten-tial at the growth front due to changing nanowall curvaturebetween nanowires. The combination of Au- and nonassistedgrowth mechanisms resulted in Au nanoparticles at and near thesubstrate, producing nanoscale variations in optical properties.Strong NBE emission was observed from the entire nanostruc-ture, although its quenching and the appearance of visibleemission was detected at the base of the nanowalls, correlatingwith the presence of Au nanoparticles. Our results show thatthese intricacies of the nanostructure growth mechanism haveprofound consequences on the local optical properties on thenanoscale and must be considered for future ZnO nanowallapplications.

’EXPERIMENTAL METHODS

ZnO nanowalls were grown by carbothermal reduction by theVTC technique similar to our previously reported growth of

Figure 4. (a,b) BF-STEM image (a) and corresponding CL spectrum(b) of a representative ZnO nanowall. (c) Panchromatic and mono-chromatic CL emission images, in the same region as image a. Clock-wise from the upper left: panchromatic, 3.31 eV, 1.91 eV, and 2.25 eVintensity maps. Scale bars for images a and c are 2 μm. (d) Integratedintensities of panchromatic, 3.31 eV (signal multiplied by 5), and 2.25 eVemission (signal multiplied by 50 and offset by 3.5� 107 counts), in thearea denoted by the dashed box in the direction of the arrow in image c,as a function of distance from substrate. Nanoparticle density, deter-mined from BF-TEM images, is overlaid in the same graph.

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ZnO nanowires,23 but with significantly increased source flux(see Supporting Information). The source (4 g of 1:1 mass ratioZnO/C powders) was held at 1000 �C and the substrates(solvent-cleaned a-sapphire, coated with 1 nm Au film byelectron-beam evaporation) was held at 930 �C with a flow of10 sccm O2 gas (>99.994% pure). A constant 200 sccm flow ofultrahigh purity Ar gas and a pressure of 80 Pa were maintainedthroughout the growth. SEM was performed on as-grownsubstrates with a Helios Nanolab 600 operating at 5 kV. Nano-wall cross sections were prepared using the same instrumentoperating in the dual-beam focused ion beam (FIB) mode, andSiO2 and Pt layers (1.5 μm and 200 nm thick) were deposited byplasma-enhanced chemical vapor deposition and FIB beforemilling to protect the nanowalls during sample preparation andto reduce charging, respectively. Nanowall cross sections andindividual nanowalls were investigated by HR-TEM and BF-STEMwith a JEOL 2010F field emission TEM/STEM operatingat 200 kV. Compositional characterization was performed by anaccompanying EDS detector. Electronic band structure wasdirectly correlated with nanoscale structure by CL-STEM usinga JEOL 2011 TEM/STEM equipped with a Gatan MonoCL3CL+ system.42

’ASSOCIATED CONTENT

bS Supporting Information. Description of experimentalmethods utilized to promote ZnO nanowall growth. Presenta-tion of bulk morphological and optical properties of ZnOnanowalls. Discussion of effects on CL homogeneity due tonanoscale variations in sample thickness and the presence of dis-locations. This material is available free of charge via the Internetat http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: gradecak@mit.edu.

’ACKNOWLEDGMENT

This work was supported in part by The Center for Excitonics,an Energy Frontier Research Center funded by the U.S. Depart-ment of Energy, Office of Science, Office of Basic EnergySciences under Award Number DE-SC0001088, and in part byMRSEC Program of the National Science Foundation underaward number DMR-0819762. M.B. acknowledges the NationalDefense Science and Engineering Graduate Fellowship support.We thank Jose Lobez, Professor Geoffrey Beach, Dr. ShiahnChen, and Dr. Yong Zhang for their technical support.

’REFERENCES

(1) Mao, S. S.; Chen, X. Selected Nanotechnologies For RenewableEnergy Applications. Int. J. Energy Res. 2007, 31, 619–636.(2) Wright, J. S.; Lim, W.; Norton, D. P.; Pearton, S. J.; Ren, F.;

Johnson, J. L.; Ural, A. Nitride and Oxide Semiconductor Nanostruc-tured Hydrogen Gas Sensors. Semicond. Sci. Technol. 2010, 25,024002–024008.(3) Wu, Y.; Qiao, P.; Chong, T.; Shen, Z. Carbon Nanowalls Grown

by Microwave Plasma Enhanced Chemical Vapor Deposition. Adv.Mater. 2002, 14, 64–67.(4) Ng, H. T.; Li, J.; Smith, M. K.; Nguyen, P.; Cassell, A.; Han, J.;

Meyyappan, M. Growth of Epitaxial Nanowires at the Junctions ofNanowalls. Science 2003, 300, 1249.

(5) Wu, J.-J.; Yu, C.-C. Aligned TiO2 Nanorods and Nanowalls.J. Phys. Chem. B 2004, 108, 3377–3379.

(6) Yu, T.; Zhu, Y. W.; Xu, X. J.; Shen, Z. X.; Chen, P.; Lim, C. T.;Thong, J. T. L.; Sow, C. H. Controlled Growth and Field-EmissionProperties of Cobalt Oxide Nanowalls. Adv. Mater. 2005, 17,1595–1599.

(7) Qi, J. Q.; Chen, W. P.; Lu, M.; Wang, Y.; Tian, H. Y.; Li, L. T.;Chan, H. L. W. Fabrication of Copper Ferrite Nanowalls on CeramicSurfaces by an Electrochemical Method. Nanotechnology 2005, 16,3097–3100.

(8) Gautam, U. K.; Vivekchand, S. R. C.; Govindaraj, A.; Rao,C. N. R. GaS and GaSe Nanowalls and Their Transformation toGa2O3 and GaN Nanowalls. Chem. Commun. 2005, 3995–3997.

(9) Feng, X.; Li, Y.; Liu, H.; Li, Y.; Cui, S.; Wang, N.; Jiang, L.; Liu,X.; Yuan, M. Controlled Growth and Field Emission Properties of CuSNanowalls. Nanotechnology 2007, 18, 145706–145705.

(10) Liu, D.; Zhang, Q.; Xiao, P.; Garcia, B. B.; Guo, Q.; Champion,R.; Cao, G. Hydrous Manganese Dioxide Nanowall Arrays Growth andTheir Li+ Ions Intercalation Electrochemical Properties. Chem. Mater.2008, 20, 1376–1380.

(11) Varghese, B.; Reddy, M. V.; Yanwu, Z.; Lit, C. S.; Hoong, T. C.;Subba Rao, G. V.; Chowdari, B. V. R.; Wee, A. T. S.; Lim, C. T.; Sow, C.-H.Fabrication ofNiONanowall Electrodes forHigh Performance Lithium IonBattery. Chem. Mater. 2008, 20, 3360–3367.

(12) Kim, S.-W.; Park, H.-K.; Yi, M.-S.; Park, N.-M.; Park, J.-H.; Kim,S.-H.; Maeng, S.-L.; Choi, C.-J.; Moon, S.-E. Epitaxial Growth of ZnONanowall Networks on GaN/Sapphire Substrates. Appl. Phys. Lett.2007, 90 (033107), 1–3.

(13) Wang, X.; Ding, Y.; Li, Z.; Song, J.; Wang, Z. L. Single-CrystalMesoporous ZnO Thin Films Composed of Nanowalls. J. Phys. Chem. C2009, 113, 1791–1794.

(14) Lee, C.-H.; Kim, Y.-J.; Lee, J.; Hong, Y. J.; Jeon, J.-M.; Kim, M.;Hong, S.; Yi, G.-C. Scalable Network Electrical Devices Using ZnONanowalls. Nanotechnology 2011, 22, 055205–055206.

(15) Kumar, B.; Lee, K. Y.; Park, H.-K.; Chae, S. J.; Lee, Y. H.; Kim,S.-W. Controlled Growth of Semiconducting Nanowire, Nanowall, andHybrid Nanostructures on Graphene for Piezoelectric Nanogenerators.ACS Nano 2011, 5, 4197–4204.

(16) Fang, F.; Zhao, D. X.; Li, B. H.; Zhang, Z. Z.; Zhang, J. Y.; Shen,D. Z. The Enhancement of ZnO Nanowalls Photoconductivity InducedBy CdS Nanoparticle Modification. Appl. Phys. Lett. 2008, 93 (233115),1–3.

(17) Schmidt-Mende, L.; MacManus-Driscoll, J. L. ZnO - Nanos-tructures, Defects, and Devices. Mater. Today 2007, 10, 40–48.

(18) Kashif, M.; Akhtar, M. N.; Nasir, N.; Yahya, N. Versatility ofZnO Nanostructures. Adv. Struct. Mater. 2010, 219–244.

(19) Yin, Z.; Chen, N.; Dai, R.; Liu, L.; Zhang, X.; Wang, X.; Wu, J.;Chai, C. On the Formation ofWell-Aligned ZnONanowall Networks byCatalyst-Free Thermal Evaporation Method. J. Cryst. Growth 2007,305, 296–301.

(20) Zhang, X.; Shan, X.; Zhang, J.; Chen, L.; Xu, J.; You, L.; Yu, D.In Situ Study of Epitaxial Growth of ZnO Nanowires at the Junctions ofNanowall Networks on Zinc Particles. Micron 2009, 40, 302–307.

(21) Shi, J.; Grutzik, S.; Wang, X. Zn Cluster Drifting Effect forthe Formation of ZnO 3D Nanoarchitecture. ACS Nano 2009, 3,1594–1602.

(22) Fan, H. J.; Zacharias, M. Manipulation of Crawling Growth forthe Formation of Sub-millimeter Long ZnO Nanowalls. J. Mater. Sci.Technol. 2008, 24, 589–593.

(23) Brewster, M. M.; Zhou, X.; Lim, S. K.; Grade�cak, S. Role of Auin the Growth and Nanoscale Optical Properties of ZnO Nanowires.J. Phys. Chem. Lett. 2011, 2, 586–591.

(24) Cao, B. Q.; Matsumoto, T.; Matsumoto, M.; Higashihata, M.;Nakamura, D.; Okada, T. ZnO Nanowalls Grown with High-PressurePLD and Their Applications as Field Emitters and UVDetectors. J. Phys.Chem. C 2009, 113, 10975–10980.

(25) Pradhan, D.; Leung, K. T. Vertical Growth of Two-Dimen-sional Zinc Oxide Nanostructures on ITO-Coated Glass: Effects of

1945 dx.doi.org/10.1021/jz2008775 |J. Phys. Chem. Lett. 2011, 2, 1940–1945

The Journal of Physical Chemistry Letters LETTER

Deposition Temperature and Deposition Time. J. Phys. Chem. C 2008,112, 1357–1364.(26) Fan, H. J.; Werner, P.; Zacharias, M. Semiconductor Nanowires:

From Self-Organization to Patterned Growth. Small 2006, 2, 700–717.(27) Dubrovskii, V. G.; Sibirev, N. V.; Cirlin, G. E.; Soshnikov, I. P.;

Chen, W. H.; Larde, R.; Cadel, E.; Pareige, P.; Xu, T.; Grandidier, B.;Nys, J. P.; Stievenard, D.; Moewe, M.; Chuang, L. C.; Chang-Hasnain,C. Gibbs�Thomson and Diffusion-Induced Contributions to theGrowth Rate of Si, InP, and GaAs Nanowires. Phys. Rev. B 2009,79, 205316–205317.(28) Fr€oberg, L. E.; Seifert, W.; Johansson, J. Diameter-Dependent

Growth Rate of InAs Nanowires. Phys. Rev. B 2007, 76, 153401–153404.(29) Kim, M.; Hong, Y. J.; Yoo, J.; Yi, G.-C.; Park, G.-S.; Kong, K.-j.;

Chang, H. Surface Morphology and Growth Mechanism of Catalyst-Free ZnO and MgxZn1�xO Nanorods. Phys. Status Solidi 2008, 2,197–199.(30) Balluffi, R. W.; Allen, S. M.; Carter, W. C. Kinetics of Materials;

John Wiley & Sons, Inc.: Hoboken, NJ, 2005.(31) Dick, K. A. A Review of Nanowire Growth Promoted by Alloys

and Non-alloying Elements With Emphasis on Au-Assisted III�VNanowires. Prog. Cryst. Growth Charact. Mater. 2008, 54, 138–173.(32) Tambe, M. J.; Ren, S.; Grade�cak, S. Effects of Gold Diffusion on

n-Type Doping of GaAs Nanowires. Nano Lett. 2010, 10, 4584–4589.(33) Lin, H. Y.; Cheng, C. L.; Chou, Y. Y.; Huang, L. L.; Chen, Y. F.;

Tsen, K. T. Enhancement of Band Gap Emission Stimulated by DefectLoss. Opt. Express 2006, 14, 2372–2379.(34) Liu, K.; Sakurai, M.; Liao, M.; Aono, M. Giant Improvement of

the Performance of ZnO Nanowire Photodetectors by Au Nanoparti-cles. J. Phys. Chem. C 2010, 114, 19835–19839.(35) Cheng, C.W.; Sie, E. J.; Liu, B.; Huan, C. H. A.; Sum, T. C.; Sun,

H. D.; Fan, H. J. Surface Plasmon Enhanced Band Edge Luminescence ofZnONanorods By Capping Au Nanoparticles. Appl. Phys. Lett. 2010, 96(071107), 1–3.(36) Cao, B. Q.; Liu, Z. M.; Xu, H. Y.; Gong, H. B.; Nakamura, D.;

Sakai, K.; Higashihata, M.; Okada, T. Catalyst/Dopant-Free Growth ofZnO Nanobelts With Different Optical Properties From NanowiresGrown Via a Catalyst-Assisted Method. Cryst. Eng. Commun. 2011,13, 4282–4287.(37) Mogilevsky, R.; Nedilko, S.; Sharafutdinova, L.; Burlay, S.;

Sherbatskii, V.; Boyko, V.; Mittl, S. Luminescence Study of GrownSapphire: From Starting Material to Single Crystal. Phys. Status Solidi C2009, 6, S179–S183.(38) Djuri�si�c, A.; Leung, Y. H. Optical Properties of ZnO Nanos-

tructures. Small 2006, 2, 944–961.(39) Gruzintsev, A. N.; Volkov, V. T.; Khodos, I. I.; Nikiforova, T. V.;

Koval’chuk, M. N. Luminescent Properties of ZnO Films Doped withGroup-IB Acceptors. Russ. Microelectron. 2002, 31, 200–205.(40) Ko, H. J.; Chen, Y. F.; Hong, S. K.; Wenisch, H.; Yao, T.; Look,

D. C. Ga-Doped ZnO Films Grown on GaN Templates By Plasma-AssistedMolecular-Beam Epitaxy.Appl. Phys. Lett. 2000, 77, 3761–3763.(41) He, H. P.; Tang, H. P.; Ye, Z. Z.; Zhu, L. P.; Zhao, B. H.; Wang,

L.; Li, X. H. Temperature-Dependent Photoluminescence of Quasia-ligned Al-Doped ZnO Nanorods. Appl. Phys. Lett. 2007, 90 (023104),1–3.(42) Lim, S. K.; Brewster, M.; Qian, F.; Li, Y.; Lieber, C. M.;

Grade�cak, S. Direct Correlation between Structural and Optical Proper-ties of III�V Nitride Nanowire Heterostructures with NanoscaleResolution. Nano Lett. 2009, 9, 3940–3944.