Influence of Surface Modification on the Luminescence of Colloidal

Contrary to expectations, there is no direct correlation between excitonic ... for PVP-capped colloidal ZnO nanocrystals, but this emission was not ex...
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J. Phys. Chem. B 2005, 109, 20810-20816

Influence of Surface Modification on the Luminescence of Colloidal ZnO Nanocrystals Nick S. Norberg and Daniel R. Gamelin* Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195-1700 ReceiVed: June 28, 2005; In Final Form: September 9, 2005

The influence of surface modification on the luminescence of colloidal ZnO nanocrystals is described, with particular emphasis given to factors increasing excitonic emission quantum yields. Changes in nanocrystal size, shape, and luminescence intensities have been measured for nanocrystals capped by dodecylamine (DDA) and trioctylphosphine oxide after different growth times. Green trap emission intensities show a direct correlation with surface hydroxide concentrations. Contrary to expectations, there is no direct correlation between excitonic emission quenching and surface hydroxide concentrations. The nearly pure excitonic emission observed after heating in DDA is attributed to the removal of surface defects from the ZnO nanocrystal surfaces and to the relatively high packing density of DDA on the ZnO surfaces. Rapid, nondispersive ripening of ZnO nanocrystals upon heating in DDA is observed and explained using a colloidal growth model.

I. Introduction The wide band gap (3.4 eV), large exciton binding energy (60 meV), and radiation hardness of ZnO make it an excellent candidate UV light-emitter for use in lasers, light-emitting diodes (LEDs), and other UV light-emitting devices.1 Under the right preparation conditions, pure excitonic emission from ZnO can be achieved, and UV lasing in bulk and nanowire ZnO has been observed.2-4 The size-tunable optical properties of quantumconfined semiconductor nanocrystals (quantum dots) have motivated further investigations into the luminescence of ZnO nanocrystals. The synthesis of other semiconductors as colloidal nanocrystals has opened up possibilities for their uses in many new applications. For example, CdSe quantum dots have been prepared showing band-edge emission at a range of wavelengths in the visible with high quantum yields, given proper surface passivation.5,6 Strongly emitting CdSe quantum dots and related materials have been tested for use in biolabeling experiments,7,8 in LEDs,9,10 or as quantum dot lasers.11 ZnO is particularly attractive for similar applications because of the current interest in UV emitters, but the luminescence of colloidal ZnO quantum dots is usually dominated by visible emission from a trap state.12,13 High UV emission quantum yields have not yet been observed in ZnO nanocrystals, limiting their potential uses. The luminescence spectrum of bulk ZnO typically includes two main features, a narrow emission band in the UV (∼26 000 cm-1) and a broad band in the visible (∼17 000 cm-1) related to lattice defects. The UV band originates from the radiative recombination of a hole in the valence band and an electron in the conduction band (excitonic emission), whereas the defectrelated emission (green emission) has been attributed to recombination of an electron in or near the conduction band with a hole at a defect site, proposed to be an oxygen vacancy.14,15 Several shallow trap states have also been identified by luminescence spectroscopy.16 The large surface-to-volume ratios of ZnO nanocrystyals result in surface defects that greatly influence the luminescence properties. From photoluminescence kinetics studies, green luminescence in ZnO nanocrystals was * Author to whom correspondence should be addressed. Phone: (206) 685-0901. Fax: (206) 685-8665. E-mail: [email protected].

deduced to involve inital trapping of photogenerated holes onto surface defect sites.12 These surface hole traps were invoked to explain the increasing visible emission intensity with decreasing ZnO nanocrystal size.17 Green photoluminescence quantum yields up to 20% were reported for nanocrystals ∼1 nm in diameter.17 A direct relationship was also observed between size and excitonic emission intensity in these same ZnO nanocrystals17 as well as in ZnO powders in the 40-2500 nm diameter size range.18 From studies of calcined powders of ZnO nanocrystals, surface hydroxides were proposed to quench the excitonic emission.19 These experiments highlight the complications caused by surface defects when ZnO nanocrystals showing only strong excitonic emission are desired. There have been some limited successes in preparing colloidal ZnO nanocrystals showing mostly UV emission. We recently reported enhanced excitonic and weak green emission intensities in colloidal ZnO nanocrystals that had been heated in dodecylamine (DDA).20 Emission dominated by a UV band had previously been reported for PVP-capped colloidal ZnO nanocrystals, but this emission was not excitonic and was instead attributed to a PVP-related trap site.21 Increased excitonic and quenched green emission intensities have been observed in colloidal ZnO nanocrystals upon UV irradiation and removal of oxygen, an electron scavenger, from the colloidal suspensions, but the strong green trap emission returned immediately if not continuously irradiated with UV light or once air was reintroduced.22 More recently, the UV-enhancing properties of amine capping ligands was again noted in a report of colloidal ZnO nanocrystals with a tear-drop morphology capped with hexadecylamine that showed only excitonic emission.23 While this paper was under review, two additional reports of UV-emitting ZnO nanocrystals were published.24,25 To the best of our knowledge, however, colloidal ZnO excitonic emission quantum yields have not been reported, and the microscopic factors leading to strong excitonic emission remain poorly understood. In this paper, the influence of surface modification by various surface-capping procedures on the luminescence of colloidal ZnO nanocrystals is described, with particular attention given to factors that increase excitonic emission intensities. Changes in nanocrystal size, shape, and luminescence intensities have

10.1021/jp0535285 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/18/2005

Surface Modification of ZnO Nanocrystals

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been measured for nanocrystals capped by DDA or trioctylphosphine oxide (TOPO) after different growth times. A clear, direct correlation is observed between green trap luminescence and the presence of surface hydroxides. Contrary to expectation, there is no direct correlation between quenching of the green and enhancement of the excitonic emission intensities. The passivation of surface defects during growth in DDA, aided by the high packing density of this ligand, is suggested to be responsible for the greater excitonic emission intensity observed with this ligand compared to TOPO. Finally, rapid, nondispersive ripening of ZnO nanocrystals observed upon heating in DDA is explained using a colloidal growth model. II. Experimental Section A. Materials. Zinc acetate dihydrate (Zn(OAc)2‚2H2O, 98%, Strem), tetramethylammonium hydroxide (N(CH4)4OH‚4H2O, 97%, Sigma-Aldrich), dodecylamine (DDA, CH3(CH2)11NH2, 98%, Sigma-Aldrich), trioctylphosphine oxide (TOPO, ((CH3(CH2)7)3P(O), 90% and 99%, Sigma-Aldrich), stearyl phosphate (SP, City Chemical), dimethyl sulfoxide (DMSO, 99.7%, Acros), and absolute ethanol (AAPER) were purchased and used as received. B. Sample Preparation. ZnO nanocrystals were prepared as described previously.20,26 In this synthesis, 1.7 equiv of an ethanolic solution of 0.55 M NMe4OH‚4H2O were added dropwise to a 0.10 M Zn(OAc)2‚2H2O solution dissolved in DMSO at room temperature while stirring. The nanocrystals were isolated by precipitation with ethyl acetate and resuspension in ethanol. For measurements on as-prepared nanocrystals, the colloids were subsequently precipitated with heptane and resuspended in either ethanol or DMSO. To cap the nanocrystals with dodecylamine, the ZnO nanocrystals were precipitated with a solution of DDA dissolved in ethanol and resuspended in heptane. These nanocrystals could be washed further by precipitation with ethanol and resuspension in heptane. For heating in DDA, the DDA-precipitated nanocrystals were resuspended in pure DDA (2-3 g of ZnO per 100 mL of DDA). The colloidal suspension then was heated to 180 °C under nitrogen with constant stirring. For capping and heating in TOPO/SP, the nanocrystals precipitated from ethanol were resuspended directly into a warm 90% TOPO/SP solution mixed in a 9:1 ratio, then heated to 180 °C with constant stirring. Stearyl phosphate was added to these solutions to help suspend the ZnO nanocrystals. For ZnO first heated in DDA followed by heating in TOPO, the nanocrystals were resuspended directly into 99% TOPO after precipitation from the DDA solution using ethanol. The nanocrystals in TOPO were then heated to 95 °C for 14 h. ZnO nanocrystals from the DDA, TOPO/SP, and 99% TOPO suspensions were precipitated with ethanol and resuspended in heptane two times before luminescence measurements to remove any excess ligands. C. Physical Characterization. Absorption and luminescence spectra of colloidal ZnO nanocrystals were measured at room temperature in 1 cm × 1 cm cuvettes, and the nanocrystals were suspended in ethanol, heptane, or a 2:1 heptane/ethanol mixture for as-prepared, TOPO-capped, or DDA-capped ZnO nanocrystals, respectively. Absorption spectra were recorded on a Cary 500E (Varian) spectrophotometer, and luminescence spectra were recorded on a JY Fluoromax-2 fluorimeter. All luminescence measurements were taken with the absorbance of the samples at the excitation wavelength less than or equal to 0.2 and were corrected for the instrument response. Quantum yields were calculated in reference to Rhodamine 6G dye (quantum yield of 95% in ethanol). Fourier transform IR (FTIR) spectra

Figure 1. (a) Electronic absorption at 300 K (left spectra) and photoluminescence spectra of colloidal ZnO nanocrystals capped with DDA after different growth times in DMSO at room temperature (20, 40 min, 1, 2, and 3 h.). (b) Electronic absorption at 300 K (left spectra) and photoluminescence spectra of colloidal ZnO nanocrystals heated in DDA at 180 °C for 0 (thin solid line), 1, 5 (dotted lines), and 24 h (thick solid line) and in TOPO at 95 °C for 14 h (dashed line). Arrows indicate the change in emission intensity with heating time. The magnification factors for each emission panel relative to the initial green emission are provided for reference.

were collected on ZnO nanocrystals drop-coated from a suspension onto NaCl plates using a Bruker Vector 33 FTIR spectrometer. High-resolution transmission electron microscopy (HRTEM) images were collected at the Pacific Northwest National Laboratories on a JEOL 2010 transmission electron microscope (200 kV) with a high brightness LaB6 filament as an electron source. Cu KR X-ray powder diffraction data were collected on drop-coated thin films of the ZnO nanocrystals using a Rigaku Rotaflex RTP300 diffractometer. The nanocrystal diameters were determined by applying the Scherrer equation to analyze powder X-ray diffraction (XRD) line widths. The capping ligand surface coverage was determined from the average nanocrystal radius, the capping ligand molecular weight, and the weight percent of the ligand on the nanocrystals measured by calcination. III. Results Figure 1a shows absorption and photoluminescence spectra of DDA-capped ZnO nanocrystals removed from the DMSO reaction solution after different times following base addition. The band gap absorption shifted to the red with increasing growth times, indicating an increase in the average nanocrystal diameter while in the quantum confinement regime.13 Two main features dominate the photoluminescence spectra of these ZnO colloids, namely, an intense, broad peak at ∼17 000 cm-1 (green) and a narrower peak at ∼27 000 cm-1 (UV), just below the band gap of the ZnO nanocrystals. Both features exhibited red shifts during growth, similar to the band gap absorption, as described previously.13,17 The UV feature can be identified as excitonic emission from the ZnO nanocrystals by its small Stokes shift (∼1200 cm-1). The green emission has been previously associated with trap states on the nanocrystal surfaces.12 When the nanocrystal sizes increase, a gradual decrease in the green emission intensity and a gradual increase in the UV emission intensity were observed, consistent with previous reports.13,17

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Figure 3. Electronic absorption at 300 K and IR absorption spectra (solid) and photoluminescence spectra (dashed) of ZnO nanocrystals. (a) As-prepared, suspended in DMSO. (b) Heated in 9:1 w/w 90% TOPO/SP at 180 °C for 2 h, suspended in heptane. (c) Heated in 99% DDA at 180 °C for 2 h, suspended in 2:1 heptane/ethanol. (d-f) Same as in parts a-c, on NaCl plates.38

TABLE 1: Data for ZnO Nanocrystals Prepared as in Figures 1b and 2 (DDA) and Figure 3b (TOPO/SP) average nanocrystal diameter (nm)a packing density (molecules/nm2)b relative UV emission intensity a

Figure 2. TEM images (right panels) and size distribution analyses (left panels) of ZnO nanocrystals after heating in DDA at 180 °C for (a) 0, (b) 1, (c) 5, and (d) 24 h and (e) after heating the sample in part d in 99% TOPO at 95 °C for 14 h.

Figure 1b shows room-temperature absorption and photoluminescence spectra of ZnO nanocrystals that were heated in DDA for 0, 1, 5, and 24 h at 180 °C (solid or dotted lines) or stirred in 99% TOPO for 14 h at 95 °C following heating in DDA for 24 h (dashed line). Complete surface exchange of DDA by TOPO was verified by FTIR measurements (Figure S1). After 1 h in DDA, a sharp rise in the UV emission intensity was observed (+350%), and the green emission was almost entirely quenched (-98%). The UV emission intensity continued to increase with continued heating, reaching up to 800% of its initial intensity, while the green emission intensity was reduced to less than 1% of its original magnitude. The photoluminescence quantum yield for the UV emission of the nanocrystals heated in DDA for 24 h was determined to be only Φ ) 2 × 10-4, despite the fact that the trap emission had been reduced by 99.9%. When these same nanocrystals were heated in TOPO, the UV emission intensity was reduced to about the intensity it had before heating in DDA (dashed line in Figure 1b), but the green emission did not return. Figure 2 displays representative TEM images of the ZnO nanocrystals from Figure 1b. Particle size distributions were

0h

1h

24 h

TOPO/SP

8.6 6.7 1

12.7 6.7 3.5

14.1 6.7 5.8

8.0 2.9 0.96

Determined from XRD. b Determined from weight percent.

calculated by measuring the nanocrystal diameters in the TEM images for >200 nanocrystals from each sample. After 1 h of heating in DDA, the nanocrystals had grown from 7.4 to 12.4 nm in diameter and changed in shape from pseudospherical to mostly hexagonal, the latter assembling on the TEM grid in a hexagonal two-dimensional (2-D) superlattice. After longer heating times, the nanocrystal growth was much slower, with average diameters increasing only up to 13.7 nm after 24 h, and the nanocrystals became more polydisperse in size and shape. The hexagonal shapes of the nanocrystals became less pronounced, yet the nanocrystals still remained faceted on most sides. Mild heating of the ZnO nanocrystals from Figure 2d in TOPO led to nanocrystals with further increased diameters and shapes approaching spherical. Absorption and photoluminescence spectra of ZnO nanocrystals as-prepared (part a), after 2 h of heating in a 9:1 ratio of TOPO/SP at 180 °C (part b), and after 2 h of heating in DDA at 180 °C (part c) are presented in Figure 3. FTIR spectra in the OH- stretching region of the same nanocrystals are displayed on the right side of Figure 3 (parts d-f). The green trap emission intensity was mostly quenched after heating in TOPO/SP and DDA. The decrease in the green emission intensity followed the decrease in the OH- stretch absorption intensity very closely, and the sample heated in DDA showed the lowest green emission and OH- stretch absorption intensities. Table 1 presents data collected for another batch of ZnO nanocrystals heated in DDA using the same procedures as used for Figures 1b and 2 and for ZnO nanocrystals heated directly in TOPO/SP, as in Figure 3b, for 24 h. The average nanocrystal diameter, calculated by applying the Scherrer equation27 to nanocrystal powder diffraction spectra, increased substantially after 1 h of heating in DDA but only slightly after longer heating times, consistent with the data in Figure 2. Longer heating times yielded only a gradual change in diameter. The nanocrystals heated in TOPO/SP did not change much from their starting size. The DDA surface coverage remained constant at 6.7 DDA/ nm2, regardless of heating times or nanocrystal diameters,

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Figure 4. Relative integration of UV (circles) and green (squares) emission intensities after heating ZnO nanocrystals in DDA at 180 °C (solid) and in 99% TOPO at 95 °C (open) (from Figure 1b). Emission intensities are plotted relative to the UV emission at 0 h. The UV and green emission intensities are fit to first-order rate constants of 0.44 h-1 (solid line) and 4.0 h-1 (dashed line), respectively.

whereas the TOPO/SP nanocrystals exhibited about half the packing density of DDA (2.9 TOPO/nm2). IV. Analysis and Discussion A. Relationship between Defects and ZnO Nanocrystal Emission. The effect of growth on the luminescence of ZnO nanocrystals can be observed in Figure 1a. The red-shifting band gap absorption indicates nanocrystal growth in the quantumconfined regime for ZnO (r < ∼7 nm).13 The increase in UV emission intensity and the decrease in green emission intensity with increasing size, observed previously,13,17 suggest that both intensities are influenced by the surface chemistry of the ZnO nanocrystals. A similar trend is seen for the green and UV emission intensities after heating the nanocrystals in DDA for increasing lengths of time (Figure 1b), as shown by the relative integrated intensities of each feature graphed in Figure 4. The green and UV emission intensities do not follow the same rates of change, however, with almost 98% of the original green emission quenched after only 1 h of heating in DDA, whereas the UV emission continues to increase in intensity through 24 h of heating. The changes in emission intensities approximately follow first-order kinetics with rate constants of 0.44 h-1 for the UV emission (dashed line in Figure 4) and 4.0 h-1 for the green emission (solid line in Figure 4). This order of magnitude difference clearly demonstrates that the UV emission is quenched in ways that do not lead to green trap emission. The absence of a direct correlation between green and UV emission in the ZnO nanocrystals is confirmed by the observation that heating the DDA-capped nanocrystals showing the highest UV emission in TOPO quenched the UV emission without returning the green emission. We therefore conclude that other nonradiative decay pathways not related to the green emission are primarily responsible for the absence of strong UV emission in colloidal ZnO nanocrystals. A direct correlation between the green luminescence and the surface OH- is implicated by the data in Figure 3. As observed in Figures 3a-c, the green emission of the as-prepared nanocrystals is mostly quenched after just 2 h of heating in TOPO/ SP or DDA at 180 °C. The decreased OH- stretch absorption intensity (Figures 3d-f) follows the reduction in green luminescence quantitatively. Previous work on calcined nanocrystalline ZnO powders showed a similar relationship between the green emission intensity and hydroxide absorption, but the hydroxide absorption was also inversely correlated with the UV emission and consequently was credited with quenching the UV emission.19 Because these previous measurements were performed on calcined nanocrystalline powders, quantitative comparison of luminescence and IR absorption intensities was not possible. The quantitative comparison in Figure 3 clearly

Figure 5. Energy level diagram depicting nonradiative (NR; dashed arrows) and radiative (thin solid arrows) recombination routes for the ZnO exciton. DDA capping eliminates surface OH- and suppresses green emission.

distinguishes between the effects of hydroxides on green trap emission and their lack of significant effect on excitonic emission. The luminescence data in Figures 1 and 3 are in general accord with the phenomenological model for ZnO nanocrystal luminescence proposed by Meijerink et al.12 We therefore adopt this model as a basis for analyzing our own data. In this model, surface sites are proposed to be responsible for all relaxation processes other than excitonic emission due to the large surfaceto-volume ratios of the nanocrystals. Figure 5a summarizes the possible exciton decay routes described by Meijerink.12 According to the model, the relaxation of an exciton can proceed through one of three routes: (i) The electron in the conduction band and hole in the valence band can recombine radiatively to produce excitonic emission (with rate Texc). (ii) A hole from the valence band can be trapped on the surface by an O2- site producing O- (TNR2), which can either generate green emission if the hole tunnels to a singly ionized oxygen vacancy (Tgreen) or relax nonradiatively otherwise (TNR3). (iii) An electron can be trapped on the surface by O-, followed by hole trapping by O2- and subsequent nonradiative recombination (TNR1). In our adaptation of the model, we generalize iii to include any process that traps either type of charge carrier on the surface but does not lead to green emission (TNR1). This generalization allows for the possibility that DDA binding introduces new nonradiative decay routes, for example. According to Figure 5a, green emission results only when a hole is trapped on the surface by O2- (TNR2). From the close relationship between the green luminescence and OH- absorption (Figure 3), we propose the identity of the O2- surface trap to be surface OH-. As OH- is displaced from the surface by DDA or TOPO, the surface hole traps are removed, and the green emission is quenched (TNR2 f 0 in Figure 5a). Nanocrystals heated in DDA are therefore described by the diagram in Figure 5b. The large green emission intensities observed when OH- is present suggest that hole trapping on the ZnO nanocrystal surfaces is fast (large TNR2) and contributes to the low excitonic emission quantum yields. Even with removal of OH- and green

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emission (Figure 5b), the excitonic emission quantum yield is still low, showing that TNR1 must also be large and comparable in magnitude to TNR2. This conclusion is emphasized by estimates of the nonradiative decay rate TNR1. A previous report on annealed bulk ZnO described an excitonic emission quantum yield of 0.016 with a decay lifetime of 90 ps.28 The excitonic emission quantum yield and the rates of various decay pathways, T, are related phenomenologically as described by eq 1.

Φexc )

Texc Texc + Tother

(1)

Texc is the rate of excitonic emission, and Tother is the effective rate for relaxation of the exciton through processes other than excitonic emission. From this expression, the experimental quantum yield and the overall rate of decay of the excitonic state in bulk (Texc + Tother), a value of Texc ) 2 × 108 s-1 is estimated. According to Figure 5a, the phenomenological quantum yield for excitonic emission in ZnO nanocrystals can be described by eq 2.

Φexc )

Texc Texc + TNR1 + TNR2

(2)

Through the use of Φexc ) 2 × 10-4 for the ZnO nanocrystals heated in DDA for 24 h and TNR2 f 0 when the green emission is quenched (Figure 5b), TNR1 ≈ 9 × 1011 s-1. This value represents an average, and it is likely that smaller, weaker UVemitting nanocrystals will have larger values of TNR1. Although Texc has been predicted to increase when ZnO nanocrystals are quantum-confined (up to >1010 s-1),29 the two nonradiative relaxation processes are both faster (TNR1 and TNR2 > 1012 s-1) and will therefore dominate the nanocrystal exciton decay kinetics in the absence of effective defect passivation. B. Origins of Increased UV Emission Intensity. Several experiments were performed in attempts to clarify the microscopic source of the phenomenological nonradiative decay parameter TNR1 in eq 2. Although the data in Figure 1a suggest a correlation between UV emission intensity and nanocrystal size, the TEM images in Figure 2 reveal that the size and faceting of the ZnO nanocrystals heated in DDA are not directly correlated with the excitonic emission intensities. Heating in DDA for 1 h produced a rapid increase in size, hexagonal faceting, and excitonic emission intensity (Figures 1b, 2a-b, and 4) of the ZnO nanocrystals. Longer heating times yielded marginal additional growth and loss of hexagonal faceting, but the excitonic emission intensity continued to increase. More significantly, the nanocrystals heated in TOPO displayed the largest average diameters of the samples studied (Figure 2e), but the UV emission from these nanocrystals was only 13% of the same nanocrystals before heating in TOPO. In addition, the DDA-capped nanocrystals formed a more varied selection of shapes at longer heating times (Figure 2d), no longer showing predominantly hexagonal structures, but excitonic emission intensities continued to increase. The excitonic emission intensity must therefore depend more on the microscopic structure of the ZnO surfaces than on crystallite size or shape. The growth of ZnO nanocrystals in DDA is proposed to play an important role in increasing the ZnO excitonic emission intensity. Although not correlated with UV emission intensity, the hexagonal facets observed in Figure 2b do indicate that growth of ZnO in DDA reduces the number of surface defects. These facets represent crystal orientations whose growth velocities are the slowest. Such facets are generally the lowest energy

crystal planes, in this case the ZnO {101h0} planes, implying that these faceted ZnO surfaces are uniform and mostly free of defects. The nanocrystals retained faceted surfaces throughout growth in DDA even as they became less hexagonal in shape (Figures 2c-d). We therefore conclude that growth in DDA facilitates formation of uniform, low-energy ZnO nanocrystal surfaces with decreased surface defect densities and in this way increases the ZnO nanocrystal UV emission intensities. Details of the ZnO growth in DDA are discussed in the next subsection. The high packing density of DDA on the ZnO nanocrystal surfaces is also proposed to make a significant contribution to increasing the ZnO excitonic emission intensity. According to Table 1, DDA packs at least twice as densely on ZnO nanocrystal surfaces as TOPO does, regardless of heating times. This difference in packing density can be attributed to the monoalkyl chain of DDA, which requires a smaller volume than the bulkier trialkyl chains of TOPO and therefore allows closer approach of the coordinating functional groups on the nanocrystal surfaces. A similar trend has been observed with CdSe nanocrystals, where the small capping ligand pyridine packs more densely than the bulkier TOPO ligand.30 Photoluminescence studies of CdSe nanocrystals and bulk CdSe surfaces have also shown a direct correlation between higher capping ligand packing densities and increased band gap luminescence intensities.31-33 Without dense surface coverage (as in the case of TOPO), many unpassivated surface defects remain that quench the ZnO excitonic emission and contribute to the low overall luminescence intensities observed in TOPO-capped ZnO nanocrystals. The surface passivation by DDA is evidently far from complete, however. The excitonic emission quantum yield is still only ca. 2 × 10-4 for the nanocrystals heated in DDA for 24 h, indicating that the nanocrystals still have unpassivated surface defects that continue to quench their emission. For comparison, the room-temperature excitonic emission quantum yield for the bulk ZnO film mentioned previously was 1.6 × 10-2,28 and the observations of UV lasing in ZnO2-4 imply high quantum yields. In these samples, the surface-to-volume ratios were either extremely small (bulk ZnO)2,4 or the surfaces were highly ordered (nanowires),3 and in all cases samples were prepared or annealed at higher temperatures than is possible for colloidal nanocrystals (>400 vs 180 °C). When the average density of surface-exposed Zn2+ in ZnO (12.1 Zn2+/nm2 assuming a Zn2+-rich surface) and the average packing density of TOPO (2.9 TOPO/nm2) are compared, TOPO is estimated to bind to ca. 24% of available surface Zn2+ ions. This surface coverage is similar to that found for TOPO on CdSe nanocrystals (22-35%).30,32 Similarly, from the average packing density of DDA (6.7 DDA/nm2), DDA is estimated to bind to ca. 55% of the available surface Zn2+ ions, resulting in a lower-energy surface with fewer unpassivated Zn2+ ions but still far from complete surface passivation. The long heating times at moderately high temperatures (180 °C) may also anneal out some defects from the ZnO nanocrystals. Annealing of defects would explain the continued increase in nanocrystalline UV emission with heating even after the green emission is quenched and with no changes in the DDA packing density. High-temperature growth of colloidal semiconductor nanocrystals has been suggested to have an annealing effect and to result in nanocrystals with fewer lattice defects.34 Such thermal annealing is evidently not the sole factor enhancing UV emission, as shown by the weak UV emission of TOPO-capped nanocrystals regardless of annealing time. We propose that the relatively high solubility of ZnO in DDA facilitates surface

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Figure 6. Relative particle growth rate (dr/dt) as a function of nanocrystal radius assuming an infinite diffusion layer. The particle size distribution increases in the broadening zone and decreases in the focusing zone.

annealing in this solvent and, when combined with the relatively high packing density of DDA on the ZnO surfaces, gives rise to the pure UV excitonic emission of DDA-capped ZnO nanocrystals. The high solubility of ZnO in DDA also gives rise to unusual growth patterns, as described below. C. Nondispersive Ripening of ZnO in DDA. There is significant growth of the nanocrystals during the first hour of heating in DDA (an almost doubling in average nanocrystal diameter, 4.5 times increase in the average nanocrystal volume) that was not observed using TOPO/SP under analogous conditions. Some insight into the chemistry of DDA can be gained from the TEM data in Figure 2. Remarkably, the nanocrystal size dispersion remains relatively constant during this period of rapid growth (Figures 2a and 2b, standard deviation ) 14%). Nondispersive growth is contrary to what would be expected from Ostwald ripening, in which the nanocrystal size distribution increases as dissolution of smaller nanocrystals provides the nutrient for growth of larger ones. Nondispersive ripening of ZnO nanocrystals in DDA can be explained with reference to the Gibbs-Thomson equation (eq 3)35,36

[ ]

Sr ) Sb exp

2σVm rRT

(3)

where Sr and Sb are the solubility of the nanocrystal and bulk solid, respectively, σ is the specific surface energy, Vm is the molar volume, r is the particle radius, R is the gas constant, and T is the temperature. According to eq 3, smaller nanocrystals are more soluble than larger nanocrystals. For diffusion-limited growth, where 2σVm/rRT , 1, the growth rate can be described by eq 435,36

1 1 1 1 dr )K + dt r δ r* r

(

)(

)

(4)

where K is a constant proportional to the monomer diffusion rate and specific surface energy, δ is the thickness of the diffusion layer, and r* is the radius at which the solubility of the nanocrystal equals the monomer concentration (i.e., zero growth rate). The growth rate, dr/dt, is depicted as a function of the dimensionless parameter r/r* in Figure 6 (assuming δ f ∞). Since about 80% of the 7.4 nm diameter nanocrystals would have to completely dissolve to increase the average nanocrystal diameter to 12.4 nm, ZnO is proposed to be highly soluble in DDA (large S). When the ZnO nanocrystals are initially suspended in DDA at 180 °C, there is an equilibrium jump because of the increased ZnO solubility in hot DDA relative to ethanol. At this stage, the solubility of the nanocrystals is much higher than the monomer concentration, so r , r*, and most of the nanocrystals are dissolving (dr/dt < 0) (point A of Figure

6). The large nutrient concentration that results swings r/r* to a higher value (point B of Figure 6) where smaller nanocrystals grow faster than larger nanocrystals, causing a “focusing” of the size distribution.35,36 This description accounts for the preservation of a narrow size distribution in Figure 2b despite a 450% increase in average nanocrystal volume relative to Figure 2a. We note that to obtain such nondispersive growth without adding additional nutrients, the solubility of the ZnO nanocrystals in DDA must be very sensitive to size in the starting nanocrystal size range (i.e., dSr/dr is steep when r < 5 nm). The absence of small (r < 4 nm) ZnO nanocrystals in the TEM images of Figures 2b-d supports this assertion. Equation 3 predicts Sr will increase only 1.6 times with a decrease in nanocrystal radius by two. That change does not seem sufficiently large to account for the absence of small nanocrystals in the TEM images of Figures 2b-d. These data suggest a dependence of σ on nanocrystal radius, a phenomenon observed recently for metal nanoparticles with r < ∼3 nm.37 Although normally assumed to be constant, σ was shown to increase significantly as nanoparticle radii decreased.37 As the ripening proceeds (longer heating times in DDA), the monomer concentration again decreases, causing an increase in r* (a decrease in r/r*). This shifts the growth mechanism from the “focusing zone” to the “broadening zone” (point C in Figure 6). Experimentally, typical Ostwald ripening is observed to dominate the nanocrystal growth mechanism at longer growth times (Figures 2c-d), where increased size distributions are observed. The nondispersive ripening of the ZnO nanocrystals plays an important role in reducing the number of surface defects, as previously suggested by the observation of hexagonal nanocrystals (Figure 2b). The high solubility of ZnO in DDA responsible for nondispersive ripening likely also facilitates surface reconstruction to form low-energy surfaces devoid of high-energy defects. The fact that strong excitonic emission is not observed even in these highly faceted hexagonal ZnO nanocrystals leads us to conclude that the {0001} planes (perpendicular to the hexagonal faceted planes) are responsible for most of the active surface trap states contributing to TNR1. This conclusion is consistent with the high excitonic emission quantum yields (required for UV lasing) observed in ZnO rods of similar diameters grown in the 〈0001〉 direction.3 Indeed, it is the high reactivity of the metastable polar {0001} faces that is responsible for anisotropic growth in the 〈0001〉 direction. V. Conclusions Colloidal ZnO nanocrystals showing purely excitonic UV emission were prepared by heating crystals formed at low temperatures in dodecylamine. A direct correlation is observed between the presence of surface OH- and green trap luminescence, but contrary to expectations, there is no direct correlation between the green and UV emission intensities. Analysis of relative green and excitonic emission intensities and the excitonic emission quantum yield for the ZnO nanocrystals demonstrates that two qualitatively different nonradiative decay pathways quench the excitonic emission at comparable rates, only one of which leads to green emission. The relatively high solubility of ZnO nanocrystals in DDA is proposed to assist in eliminating surface defects. The removal of nanocrystal surface defects during growth in DDA and the higher packing density of DDA on the ZnO surfaces, but not nanocrystal size or shape per se, are thus suggested to be largely responsible for the increased UV emission intensities of DDA-capped ZnO nanocrystals relative to TOPO-capped nanocrystals. Although pure

20816 J. Phys. Chem. B, Vol. 109, No. 44, 2005 UV excitonic emission could be achieved, the highest excitonic emission quantum yields observed in these colloidal ZnO nanocrystals (Φexc ) 2 × 10-4) are still 100 times smaller than in bulk, indicating that surface trap states still dominate the excitonic relaxation. To achieve strongly UV-emitting colloidal ZnO nanocrystals for potential UV-emitting applications, further work on effective surface passivation is required, and the {0001} lattice planes have been suggested as the primary targets for passivation. Acknowledgment. This work was funded by the National Science Foundation (Grant No. DMR-0239325). The authors are grateful to NSF-IGERT at the University of Washington for graduate support (N.S.N.), the Research Corporation, the Dreyfus Foundation, Professor Bart E. Kahr (University of Washington) for use of the fluorimeter, and Dr. Chongmin Wang (Pacific Northwest National Laboratory) for valuable assistance with the TEM measurements. TEM data were collected at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Office of Biological and Environmental Research of the Department of Energy (DOE), located at Pacific Northwest National Laboratory, operated for the DOE by Battelle. Supporting Information Available: Electronic absorption (300 K) and FTIR spectra of ZnO nanocrystals after 24 h of heating in DDA and after these same nanocrystals were heated in 99% TOPO. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Look, D. C. Mater. Sci. Eng., B 2001, 80, 383. (2) Bagnall, D. M.; Chen, Y. F.; Zhu, Z.; Yao, T.; Koyama, S.; Shen, M. Y.; Goto, T. Appl. Phys. Lett. 1997, 70, 2230. (3) 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. (4) Tang, Z. K.; Wong, G. K. L.; Yu, P.; Kawasaki, M.; Ohtomo, A.; Koinuma, H.; Segawa, Y. Appl. Phys. Lett. 1998, 72, 3270. (5) Qu, L. H.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 2049. (6) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1, 207. (7) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (8) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (9) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316.

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