Published: February 28, 2011

r 2011 American Chemical Society 586 dx.doi.org/10.1021/jz200129x | J. Phys. Chem. Lett. 2011, 2, 586–591

LETTER

pubs.acs.org/JPCL

Role of Au in the Growth and Nanoscale Optical Propertiesof ZnO NanowiresMegan M. Brewster, Xiang Zhou, Sung Keun Lim, and Silvija Grade�cak*

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

bS Supporting Information

The realization of the full potential of nanoscale materialsdepends directly on the ability to synthesize such materials

with predefined structure and, consequently, functionalities. Ofthe low-dimensional semiconductor materials, ZnO nanowireshave a unique potential in nanoscale optoelectronics due thelarge exciton binding energy of ZnO1,2 and the nanowire one-dimensionality that enables realization of functional devicecomponents.3-5 Moreover, nanowires represent an importantplatform for fundamental studies of the nanoscale size effects onmaterials properties, as their diameters and lengths represent twodistinct length scales that may be adjusted independently. Toachieve the full potential of ZnO nanowires, strict control overnanowire growth, and thus a comprehensive understanding ofnanowire growth mechanisms, is required.4,6

The vapor-liquid-solid (VLS) mechanism first describednanowire growth as assisted by a metallic nanoparticle seed;7 yet,recent developments demonstrate that additional growth me-chanisms may compete or coexist with the VLS mechanism.8 Forexample, there are accounts of the nanoparticle remaining solidthroughout nanowire growth.9 Furthermore, the growth of ZnOnanowires can occur with nanoparticles at either the tip or base ofthe nanowire.10,11 When the nanoparticle remains at the nano-wire base or is intentionally absent (i.e., no metal is introducedprior to growth), it is unclear what is the mechanism of such non-assisted growth processes.4,8

Although Au is a common catalyst choice for nanowiregrowth, as it allows for reduced nanowire growth temperaturesand produces more highly oriented nanowires than other catalystmetals,8 its presence can have significant effects on the physicalproperties of semiconductor nanostructures. For example, in Sinanowires, Au diffusion along nanowire sidewalls during

nanowire growth12 may cause the formation of deep centers thatshorten carrier lifetimes, leading to increased nonradiativerecombination.13 In GaAs/AlGaAs core/shell nanowires, Audiffusion during high-temperature shell deposition modifieselectrical behavior.14 Finally, in ZnO nanowires, excitonic lumi-nescence may be altered by the presence of Au nanoparticles byexcited electron transfer to the Fermi level of the Au nanoparticleand/or the excitation of surface plasmon resonances.15,16

Here, we successfully demonstrate both Au-assisted and non-assisted growth during a single ZnO nanowire growth throu-gh the variation of the oxygen content in the growth chamber anddiscuss the role of the nanoparticle under different growthconditions. By investigating a single nanowire growth, we assi-milate the currently disparate topics of nanowire growth me-chanism, effects of oxygen partial pressure, and nanowire growthrate into a congruent and unique theory about the complex roleof the Au nanoparticle during ZnO nanowire growth. In addition,we show that Au nanoparticles induce nanoscale fluctuations ofthe cathodoluminescence (CL) intensity in ZnO nanowires,underscoring the importance of controlling growth mechanismsand investigating optical properties of individual nanowires17,18

for the development of nanowires with predefined properties.ZnO nanowires were grown in a horizontal tube furnace on

Au-coated a-sapphire substrates, as described in the Experimen-tal Methods section. To understand nanowire morphology, wefirst investigated our ZnO nanowires with electron microscopy.Scanning electron microscopy (SEM) images of ZnO nanowires

Received: January 27, 2011Accepted: February 16, 2011

ABSTRACT: Metallic nanoparticles play a crucial role in nanowire growth and haveprofound consequences on nanowire morphology and their physical properties. Here, weinvestigate the evolving role of the Au nanoparticle during ZnO nanowire growth and itseffects on nanoscale photoemission of the nanowires. We observe the transition from Au-assisted to non-assisted growth mechanisms during a single nanowire growth, withsignificant changes in growth rates during these two regimes. This transition occursthrough the reduction of oxygen partial pressure, which modifies the ZnO facet stabilityand increases Au diffusion. Nanoscale quenching of ZnO cathodoluminescence occursnear the Au nanoparticle due to excited electron diffusion to the nanoparticle. Thus, the Aunanoparticle is critically linked to the nanowire growth mechanism and correspondinggrowth rate through the energy of its interface with the ZnO nanowire, and its presencemodifies nanowire optical properties on the nanoscale.

SECTION: Nanoparticles and Nanostructures

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grown for 20 min show their vertical alignment on the substrateand hexagonal cross sections (Figure 1a), which indicates thattheir growth direction is along the (0002) c-axis of the wurtzitecrystal structure (further discussed below). A thin film connect-ing the bases of the nanowires was also observed, and electronbackscatter diffraction patterns of the film demonstrate that it isalso grown along the c-axis (results not shown), consistent withprevious reports.19 Tilted SEM and scanning transmission electronmicroscopy (STEM) images (Figure 1b and c, respectively) revealthat nanowires have uniform diameters along the nanowire length,with no contrast that would indicate chemical or mass-thicknessfluctuations. The diameters of the nanoparticles are 16.3( 4.0 nmas measured from DF-STEM images. Together with energydispersive X-ray spectroscopy (EDS) line scans (Figure 1d), theseimages also demonstrate that Au nanoparticles are present on thesidewalls of as-grown nanowires. The absence of a change in theslope of the Zn EDS signal instensity at the Au nanoparticlesuggests that there is no Zn alloyed in the nanoparticle at roomtemperature within the detection limit of the EDS technique.14

Similarly, the symmetric Au peak shape implies an absence ofdetectable Au diffusion into the ZnO nanowire.

Next, we varied the growth time to track the evolution of thenanoparticle location and thus its role in ZnO nanowire growth.Representative STEM images of nanowires grown for 5, 10, and20 min are shown in Figure 2a-c. After the first 5 min of growth,the Au appears as a flat particle that covers the entire nanowiretop facet (Figure 2a). Interestingly, shortly after 10 min ofgrowth, the Au nanoparticle changes in both shape and position;it is now found on a nanowire sidewall with a rounded shape,likely to reduce its surface energy (Figure 2b). The nanowire tipalso develops higher-order facets, consistent with a non-assistedgrowth model by anisotropic surface energies.20 (Potential screwdislocation-driven growth was dismissed due to a lack of contrastalong the nanowire growth axis during transmission electronmicroscopy (TEM) investigation that would indicate the pre-sence of this dislocation, as well as the absence of a character-istically protruding tip.21) After this growth stage, the nanowirecontinues to grow without the nanoparticle at the nanowire tip;and at 20 min, the nanowire tip has advanced beyond the positionof the nanoparticle (Figure 2c). Thus, we find that the growth

evolves from Au-assisted to non-assisted, with the transitionoccurring between 9 and 10 min into the 20 min growth.

To further understand the evolution of the nanowire tip andAu nanoparticle facets, as well as that of their interface through-out the growth, we investigated the structure of the Au/ZnOinterface by high-resolution (HR) TEM. HR-TEM images andcorresponding fast Fourier transforms (FFTs) reveal single-crystalline ZnO nanowires that are free of 1D and 2D defects.The tip of a ZnO nanowire grown for 5 min is flat and comprisedmainly of the (0002) plane (Figure 2d) and develops higher-order {0112} planes by 10 min (Figure 2e). The crystallographicalignment Au(111) )ZnO(0002) is closely preserved, with mis-alignment up to 6�. (We assume that this texturing is notproduced during the cooling step.)

It is important to note that this Au nanoparticle evolution isobserved only under certain growth conditions, that is, when thegrowth is preceded by growths with increased oxygen flow (at1.5 sccm, versus the 0.9 sccm flow for these growths). It is knownthat high oxygen partial pressure promotes Au-assisted growthover the competing non-assisted growth;22,23 thus, the likelysource of excess oxygen in the initial stage of our growth thatallowed for Au-assisted growth behavior is the oxygen-rich ZnOdeposit on the reactor sidewalls. The oxygen content in the

Figure 1. Morphological and structural analysis of ZnO nanowires. (a)Plan-view and (b) 15� tilted SEM images of ZnO nanowires ona-sapphire substrates. (c) DF-STEM image of ZnO nanowires. In (b)and (c), solid yellow arrows indicate examples of nanoparticles on thenanowire sidewalls. (d) EDS line scan across the diameter of ananoparticle and nanowire in the direction of the dashed yellow arrow(inset).

Figure 2. Evolution of the growth mechanism from Au-assisted to non-assisted. (a-c) DF-STEM images of ZnO nanowires at (a) 5, (b) 10,and (c) 20 min into the growth. (d-f) HR-TEM images and corre-sponding FFTs (insets) of ZnO nanowires and Au nanoparticles for (d)5, (e) 10, and (f) 20 min into the nanowire growth (different nanowiresthan those in a-c). Yellow dashed boxes in the HR-TEM imagesindicate regions used for the FFT analyses, and solid yellow lines indicate{0112} planes. ZnO FFT patterns are indexed in red ([2110] zone axis)and Au in green ([011] zone axis). Measured misalignment between Au(111) and ZnO (0002) is (d) 1.2, (e) 1.5, and (f) 5.7�.

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preconditioned deposit is gradually reduced over the course ofthe 20 min growth due to the equilibration of the deposit,causing the transition from Au-assisted to non-assisted growth.This is further confirmed by the fact that only non-assistedgrowth is observed under identical growth conditions if nodeposit preconditioning is performed and that only Au-assistedgrowth is observed during equilibrated growths at 1.5 sccm ofoxygen flow.

We posit that the stability of the ZnO nanowire surface facetsand the Au mobility are altered by the decreasing oxygen partialpressure (PO2) in the growth chamber, causing the transitionfrom Au-assisted to non-assisted nanowire growth mechanisms.Surface facet stabilities are dependent upon the source ratio dueto competing source flux rates and diffusion lengths.24 ReducedPO2 may stabilize the inclined ZnO facets to favor a roundednanowire tip by increasing the Zn adatom diffusion lengthbeyond the width of the (0002) facet. This allows the relativeflux of Zn adatoms from the (0002) surface to be greater thanthat from relatively more stable higher-order surfaces, such as{0112}, which have fewer dangling bonds.24 The Au nanoparticlerounds to maintain its contact with the decreasing (0002)surface, increasing the Au nanoparticle wetting angle and thusalso the Au/ZnO interface energy.25 This increased interfaceenergy may be relieved by breaking the Au(111) )ZnO(0002)alignment. The subsequent migration of the Au nanoparticlefrom its original position on the ZnO(0002) facet is enhanced bythe reduced PO2.

26 The nanowire continues to grow past 10 min,but now by a non-assisted mechanism, likely enabled by aniso-tropic facet growth rates.27 The nanoparticle then remains on thenanowire {0110} sidewall after 10 min (Figure 2c and f),indicating the location of the transition from Au-assisted tonon-assisted growth mechanisms. That the nanoparticle appearsembedded in the nanowire sidewall in Figure 2c and f corrobo-rates our theory of the transition to non-assisted growth as the(0002) growth front proceeds around the nanoparticle.

To analyze the growth kinetics during the Au-assisted andnon-assisted growth mechanisms, we measured the nanowiregrowth rate as a function of nanowire diameter during these tworegimes. As presented in Figure 3, thinner nanowires grow fasterthan thicker ones for both Au-assisted (5 and 9 min) and non-

assisted (20 min) growth mechanisms, suggesting diffusion-limited nanowire growth;28 thus, all data sets were fit with adiffusion-limited growth kinetics model (see Supporting In-formation). The nucleation step prior to nanowire growth isresponsible for the variation in measured growth rates for Au-assisted growth, namely, between 5 and 9 min data sets. Thus,these data represent underestimates of the true Au-assistedgrowth rate. We calculate the nucleation time to be 4.2 min(see Supporting Information) and, in Figure 3, show the theo-retical Au-assisted growth rate. The growth rate of the Au-assisted mechanism is ∼14 times larger than that of the non-assisted mechanism, indicating that the presence of the Au at thenanowire tip enhances the nanowire growth rate, consistent withprevious reports.26,29

The growth progression depicted in Figures 1-3 suggests thatthe Au nanoparticle does not act as a traditional VLS catalyst butinstead as an efficient capture site for Zn from the vapor phase4 toformZn adatoms on the Au surface. Instead of incorporating intothe Au nanoparticle to form a Au-Zn alloy, Zn adatoms maydiffuse on the Au surface to the ZnO. (Further discussion of theAu phase can be found in the Supporting Information.) Adisproportionate number of these adatoms are incorporated intothe nanowire crystal at the (0002) facet, as dictated by theanisotropic growth rates of various crystal facets. Nanowiregrowth thus proceeds by adatom surface diffusion,30 wherenanowire growth near the nanoparticle (i.e., during Au-assistedgrowth) is faster than that away from the nanoparticle (i.e.,during non-assisted growth) due to the difference in timerequired for adatoms to diffuse the varying lengths from thenanoparticle to the growth front.

Clearly, the Au nanoparticles play a complex role in the growthof ZnO nanowires. As previous studies have noted significantconsequences of Au on the physical properties of nanowires, wenext investigated the effects of the Au nanoparticle on the opticalproperties of ZnO nanowires. Room-temperature photolumines-cence (PL)measurements on as-grown substrates (Figure 4a, redcurve) illustrate near-band-edge (NBE) emission at 380.3 nm(3.238 eV) with a full width at half-maximum of 159 meV. Thestrong, narrow NBE peak, as well as the absence of emission inthe visible range, confirms the high crystalline quality of the ZnOnanowires and the absence of Au doping.31 Although PL of theas-grown substrates alone indicates no consequences of the Aunanoparticle on bulk optical properties, inclusion of nanoscaleCL-STEM measurements18 (Figure 4b, with the correspondingDF-STEM image in Figure 4c) demonstrates emission quench-ing in individual nanowires of up to 30% compared to that of thenanowire body near the Au nanoparticle (Figure 4b).

We then explored the origin of the CL quenching by inten-tionally decorating ZnO nanowires with Au nanoparticles of asimilar diameter (15 nm) to that found on the nanowire sidewalland measuring their effects on the NBE intensity. We decoratedeach nanowire with ∼0-2 aggregates of 1-8 nanoparticles(Figure 4a inset) such that nanoparticle coverage was less than9% of the total nanowire surface area, producing insignificantphoton scattering due to reflection off of the Au nanoparticles.We performed PL measurements under the same experimentalconditions and in the same location before and after Aunanoparticle decoration from 11 neighboring sites within 5 μmof each other to confirm the PL intensity trend and determine thestandard deviation of our measurements. We observe a 32.10 (0.06% decrease in NBE emission intensity after nanoparticledecoration (Figure 4a, green curve), where the intensity is

Figure 3. Growth rate of ZnO nanowires at 5 (red), 9 (green), and 20min (blue). Data were grouped for clarity (hollow symbols), and theerror bars represent standard deviations. The full data sets were fit with adiffusion-limited growth model28 (solid curves). The theoretical Au-assisted growth rate is shown by the dashed black line.

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calculated by integrating the area under the peak and thestandard deviation is calculated from that of the before and aftermeasurements. (The peak of the PL intensity blueshifts by 29meV, likely due to binding of aqueous ions in the weakly acidiccolloid solution to nanowire surface states.32)

As our PL and CL studies indicate that the nanoscale opticalquenching is due to the presence of the Au nanoparticles, wefurther examined the CL quenching profile. The integratedintensities of the CL and DF-STEM of a nanowire (dashedyellow boxes in Figure 4b and c, in the direction of the dashedyellow arrows) are depicted in Figure 4d. The CL quenching thatwe observe is most likely due to excited electron transfer from theZnO conduction band to the Au nanoparticle15 as we haveeliminated all other possible causes of CL inhomogenieties thro-ugh our earlier investigations: DF-STEM images (Figures 1c,2a-c, and 4a) are free of contrast variation that would indicatevariations in composition,33 HR-TEM images (Figure 2d-f)confirm the absence of 1D and 2D defects,34 and no periodic CLfluctuations are observed (Figure 4b) that would designate astanding electromagnetic wave due to resonant cavities.35

We note that our quenching trend due to electron transfer is incontrast to a report by Liu et al.;16 however, in addition to allnanowires investigated by CL, all 11 sites probed using PL alsoconsistently indicate quenching behavior. Furthermore, calcula-tions based on the geometry of the CL intensity profile confirm

that a∼30-60% decrease in total emission intensity is expectedfor 1-2 nanoparticle aggregates along the nanowire length,consistent with our PL measurements. The range of CL quench-ing was approximated from the intensity dip at the Au nanopar-ticle location to intensity peaks in either direction along thenanowire length, totaling 142 nm. Thus, we approximate theexcited electron diffusion length to be ∼100 nm at room tem-perature, which is on the order of other estimates.36 Our directcorrelation of reduced CL intensity with the Au nanoparticlelocation provides striking evidence of nanoscale optical emissionquenching by excited electron transfer. By comparing theSchottky energy barriers formed at the ZnO/Au interface duringAu-assisted and non-assisted growth mechanisms (1.06 and0.67 eV, respectively37,38), we expect less luminescence quench-ing at the ZnO/Au interface during Au-assisted growth, as thelarger Schottky barrier will reduce the number of excitedelectrons diffusing from the ZnO conduction band to the AuFermi level.39 This conclusion illustrates the importance of tailor-ing the nanowire growth mechanism for desired optical proper-ties as Au-assisted growth may be more favorable for applicationsrequiring reduced luminescence quenching and/or faster growthrates. In addition, this comparison of microscale and nanoscaleoptical properties highlights the importance of investigatingoptical properties of individual nanowires.

In conclusion, we observe a transition in growth mechanismsduring the growth of ZnO nanowires from Au-assisted to nonas-sisted growth due to decreasing PO2, indicating that the Aunanoparticle does not act as a traditional VLS catalyst. Initiallyhigh PO2 promotes Au-assisted growth, but the continual reduc-tion of PO2 throughout the growth allows for a transition to non-assisted growth by changing the ZnO facet stabilities and thus theAu/ZnO interface stability, as well as increasing Au diffusion.Although bulk PL indicates high optical emission quality, thepresence of the Au nanoparticle causes nanoscale optical emis-sion quenching due to excited electron transfer from the ZnOconduction band to the Au nanoparticle. We approximate theexcited electron diffusion length to be ∼100 nm at roomtemperature. It is clear that PO2 has significant effects on thegrowth mechanism of ZnO nanowires, which in turn dictatesnanoscale optical properties, emphasizing the significance of therational synthesis of ZnO nanowires for the realization of theirfull potential in single-nanowire electronic devices and in funda-mental studies.

’EXPERIMENTAL METHODS

ZnO nanowires were grown by vapor transport and condensa-tion in a horizontal tube furnace (see Supporting Information).The source (1:1 mass ratio of ZnO/C powders) was held at950 �C, and the substrates (solvent-cleaned a-sapphire, coatedwith 1 nm Au by evaporation) were held at 930 �C with a flow of0.9 sccm of O2 gas (>99.994% pure). A constant flow of 35 sccm ofultra-high-purity Ar gas and a pressure of 210 Pa were maintainedthroughout the growth. Next, nanowire morphology was investi-gated by electron microscopy. SEM was performed on as-grownsubstrates with a Helios Nanolab 600 dual-beam focused ion beammilling system operating at 5 kV. Individual nanowires wereinvestigated by HR-TEM and DF-STEM, with a JEOL 2010F fieldemission TEMoperating at 200 kV. Compositional characterizationwas performed by an accompanying EDS detector.

Bulk optical properties of ZnO nanowires were investigatedas-grown and on SiO2/Si substrates by PL with λlaser = 262 nm

Figure 4. Bulk and nanoscale optical properties of ZnO nanowires. (a)Room-temperature PL collected before and after decoration of nano-wires with Au nanoparticles. Inset: SEM image indicating nanoparticlecoverage. (b) Panchromatic CL and (c) corresponding DF-STEM ofindividual nanowires, with solid yellow arrows indicating CL quenchingnear nanoparticles. Dashed yellow boxes indicate integration dimen-sions of CL and STEM line scans along the nanowire axis in the directionof the dashed yellow arrow for (d). (d) Profiles of CL (b) andDF-STEMintensities (c), with the extent of CL quenching indicated by the dashedyellow lines in (b).

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focused to a power density of 63 W/cm2. Nanoscale opticalproperties were investigated by CL-STEM at room temperaturein a JEOL 2011 TEM by an accompanying Gatan MonoCL3CLþ system.18

’ASSOCIATED CONTENT

bS Supporting Information. Additional description and dia-gram of the growth of ZnO nanowires by vapor transport andcondensation. Details of the fitting of the growth rate with adiffusion-limited model. Discussion of the phase of the Aunanoparticle. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

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

’ACKNOWLEDGMENT

This work was supported in part by the The Center forExcitonics, an Energy Frontier Research Center funded by theU.S. Department of Energy, Office of Science, Office of BasicEnergy Sciences, under Award Number DE-SC0001088 and inpart by MRSEC Program of the National Science Foundationunder Award Number DMR-0819762. M.B. acknowledges theNational Defense Science and Engineering Graduate Fellowshipsupport. We thank Professor Geoffrey Beach, Dr. Shiahn Chen,Dr. Yong Zhang, and Malorie Landgreen for their technicalsupport.

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