Crystal Phase Evolution in Quantum Confined ZnO Domains on

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CRYSTAL GROWTH & DESIGN

Crystal Phase Evolution in Quantum Confined ZnO Domains on Particles via Atomic Layer Deposition

2009 VOL. 9, NO. 6 2828–2834

David M. King, Jianhua Li, Xinhua Liang, Samantha I. Johnson, Melinda M. Channel, and Alan W. Weimer* Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309-0424 ReceiVed January 23, 2009; ReVised Manuscript ReceiVed March 19, 2009

ABSTRACT: The evolution of the crystal phase of quantum confined polycrystalline ZnO films fabricated by atomic layer deposition (ALD) was studied on spherical particle surfaces. A statistical design of experiments was performed to quantify the crystallite size of the primary peaks associated with polycrystalline ZnO, as well as the change in bandgap associated with the small crystal domains. The factors of interest were the number of ALD cycles, the core particle size, and the use of postdeposition annealing temperatures up to 550 °C. The crystallite size of each peak increased almost linearly with the number of cycles, and was further increased via thermal annealing steps. The dimension also increased more rapidly with temperature on smaller radius particles, signifying that grain boundary diffusion along the particle surfaces was facilitated with increased curvature. The shift in the optical bandgap of ZnO nanoshells was correlated to the domain size within the films at each point in the experimental matrix. The blue shift of 0.3 eV dissipated beyond crystallite sizes exceeding ∼10 nm, which was indicative of the successful deposition of quantum confined nanostructures. The precision control afforded by ALD can be used to deposit quantum confined materials on substrates, independent of geometry and morphology. Introduction The modeling of crystallite size evolution during the onset of nanoscale film growth has been well-researched, but few experimental studies have focused on the crystallization behavior of films less than 10 nm in thickness. In addition, few techniques exist that can successfully and reliably deposit materials with precision size control at the nanoscale on two-dimensional (2-D) substrates. Atomic layer deposition (ALD) is one such technique that can deposit materials in the form of islands or films with ångstrom-level thickness control.1 Since its inception in the 1970s, ALD (originally atomic layer epitaxy, or ALE) has been extensively utilized by the electroluminescent flat-panel display and semiconductor industries, which benefit from nanoscale films on 2-D surfaces. Within the past decade, ALD has been utilized to deposit conformal, pinhole-free films on three-dimensional (3-D) micron- and nanosized particle substrates, including metals, ceramics, polymers, and even carbon nanotubes.2-8 Of the metal oxide ALD films that have received the most attention on particle substrates (i.e., Al2O3, ZnO, TiO2 and SiO2), the ZnO ALD process has been the only one that has not resulted in the formation of an amorphous shell, even at low operating temperatures.5,9,10 Powder X-ray diffraction is used here to quantify the crystal phase evolution in polycrystalline ZnO ALD films deposited on spherical SiO2 nanoparticle substrates. Quantum confinement (QC) is an attribute that only nanoscale materials can possess, and typically leads to remarkable optical, thermal, and electron transport properties that are significantly different than those of the bulk counterparts.11-14 The most common materials exhibiting QC are quantum dots, which are typically fabricated in liquid solutions or in the gas phase via molecular beam epitaxy.15,16 The nonlinear optical absorption and emission properties of semiconductor quantum dots can be beneficial for a variety of applications and results in an increased opportunity for the nanoengineering of optoelectronic devices. Multiple exciton generation per absorbed photon has been * To whom correspondence should be addressed. Tel: 1-303-492-3759. Fax: 1-303-492-4341. E-mail: [email protected].

demonstrated in quantum dot solar cells, leading to quantum yield efficiencies in excess of 250%.17 It has recently been shown that ALD can yield conformal, pinhole-free TiO2 films with blue-shifted absorbance properties in film thicknesses less than 10 nm.18 A blue-shift in absorbance corresponds to an increase in bandgap relative to the bulk, Eg,bulk, which is a measure of the separation between the valence and conduction band energy levels of a material. The appearance of QC-related effects can begin to appear at physical dimensions similar to or less than the exciton Bohr radius of the material, according to the equation aB ) 4πεrε0p2/ µe2 where εr is the static dielectric constant, ε0 is the permittivity of free space, p ) h/2π, where h is Planck’s constant, µ is the reduced effective mass of the electrons and holes, and e is the charge of an electron.19 The exciton Bohr radius is typically on the order of 1-20 nm, depending on the material, which makes ALD an interesting technique to study QC effects. The thickness-dependent bandgap shift is simply the difference between the bandgap at a given thickness, R, less the bulk bandgap, or ∆Eg ) Eg(R) - Eg,bulk. Bandgap shifts associated with QC can be modeled in accordance with the Brus model, or the effective mass approach.19 The Brus model provides a quantitative interpretation of the blue-shift in bandgap as a function of crystallite dimension and effective mass of the electrons and holes:

∆Eg )

(

)

1 h2 1 1.8e2 + m*h 4πε0εr R 8m0R2 m*e

(1)

where m0 is the rest mass of an electron, and m*e and m*h are the reduced masses of the electrons and holes, respectively. ALD is a vapor-phase deposition technique that can produce conformal, pinhole-free films with Angstrom-level precision. ALD is an analog of chemical vapor deposition (CVD) whereby reactive precursors are administered sequentially into the system, rather than at the same time as in CVD processing. The standard binary CVD reaction for the deposition of ZnO is

10.1021/cg9000939 CCC: $40.75  2009 American Chemical Society Published on Web 04/08/2009

Crystal Phase Evolution in ZnO Domains

Zn(CH2CH3)2 + H2O f ZnO + 2(C2H6)

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(2)

Equation 2 can be separated into two ALD half-reactions:

[S]:OH* + Zn(CH2CH3)2 f [S]:OZn(CH2CH3)*+C2H6 (3) [S]:OZn(CH2CH3)*+ H2O f [S]:OZnOH* + C2H6 (4) Reactions 3 and 4 occur solely on the particle substrate, [S], and are each followed by inert (typically N2 or argon) purge steps to prevent unwanted gas-phase side reactions that are normally inherent to CVD processes. The asterisks represent active surface nucleation sites, and as such, overall coating times depend on total powder surface area to be coated, precursor delivery flow rates, and the reactor configuration. The reaction sequence of eq 3, purge, eq 4, purge consists of one ZnO ALD cycle, corresponding to approximately 0.1-0.2 nm/cycle. The self-limiting nature of this gas-phase deposition process allows for the formation of precision-thickness layers on 3-D substrates. The surface chemistry required for atomic layer growth, film structure, and properties of a wide array of materials has already been studied, but these films are typically on the order of hundreds to thousands of nanometers in thickness.20 This paper presents an experimental study of the structural evolution of precision-thickness polycrystalline ZnO on spherical particle surfaces fabricated using ALD techniques. The observed blue shift in optical absorbance band edge, or bandgap, of the core-shell particles with the crystallite size of the film is indicative that quantum confined nanostructures can be fabricated on surfaces of any geometry using the ALD technique. The crystallite size propagation is studied with respect to the number of ALD cycles, radius of curvature effects of the spherical substrate, and postdeposition thermal annealing steps. A fundamental understanding of crystal phase propagation at the onset of growth is critical to be able to design films, using the ALD technique, that retain their quantum confined behavior. Experimental Section ZnO ALD films were deposited using diethylzinc (DEZ) and concentrated H2O2 (50% in H2O) as precursors at substrate temperatures of 100 °C. The operating temperature and strength of the oxygencontaining species, that is, concentrated H2O2 vs only H2O as in eq 2, were selected based on previous studies that balanced precursor decomposition and liquid bridging effects at high and low temperatures, respectively.5,9 Bulk quantities of spherical SiO2 particles (diameters of 100, 250, and 550 nm), each originally manufactured via the Sto¨ber process, were coated in a scalable fluidized bed reactor (FBR) designed for the delivery of ALD precursors to powder substrates. This reactor configuration has been reported previously, including the use of residual gas analysis to monitor the generation of each reaction product and the subsequent breakthrough of unreacted precursors.7 A schematic of the FBR is shown in Figure 1. Precursors were administered to the system from their room temperature vapor pressure, and flow rates were manually adjusted to the pressure drop appropriate for bed fluidization using needle valves during the first cycle. A clamshell furnace and heating tapes maintained constant temperatures in the reactor and dosing lines, respectively. Bed masses between 5.0 and 60.0 g were loaded into the FBR, and powder fluidization occurred using an N2 flow rate between 10 and 27 sccm. Dosing was automated using Labview software and the duration ranged between 4-10 min for DEZ and 3-8 min for H2O2. A central composite design (CCD) is a useful statistical design tool that can be used to map a surface response over a given number of factors, k. A k-dimensional CCD utilizes five points in each dimension, with a spacing of - k, -1, 0, 1, k. Here, two juxtaposed CCDs, namely, one 2-D CCD (k ) 2) and one 3-D CCD (k ) 3) were employed as a composite statistical plan, as detailed in Figure 2. The

Figure 1. Schematic of the fluidized bed reactor.

Figure 2. Schematic of the juxtaposed experimental designs. two factors in the 2-D CCD were the number of ALD cycles and thermal annealing post-treatment temperature, and the additional factor in the 3-D CCD was the core particle size. The secondary 2-D CCD was used in conjunction with the primary 3-D CCD (rotated by 45° to minimize runs) in order to expand the visibility of the crystal propagation to a lower number of cycles. Overall, ZnO was deposited using 10, 20, 45, 62, 70, 80, 98, and 115 cycles, and thermal annealing post-treatments were performed at temperatures of 100 (as-deposited), 209, 325, 441, and 550 °C. The core SiO2 particle diameters available were 100, 250, and 550 nm, which effectively pulled the ( 3 points into the faces of the cube along that dimension of the 3-D experimental matrix. The main response of interest was the crystallite size of the ZnO deposited on particles, but of secondary importance was the optical bandgap shift of ZnO ALD films with respect to thickness, temperature and core particle size. With the assistance of in situ mass spectrometry, the actual number of cycles administered can be determined. For instance, during the 45 cycle coating run on 250 nm spheres, it was observed that the DEZ was depleted after 40 cycles based on ex situ data analysis. Similarly, the 80 cycle run on 250 nm spheres actually received the equivalent of 62 cycles, which was not detrimental from a comparative perspective, since the 100 and 550 nm spheres also were coated with that exact number of cycles, in accordance with the plan. All other experimental runs were successfully coated by the planned number of ALD cycles. The final analysis of variance was performed using the actual number of cycles administered, rather than solely relying on the planned number for each condition. A powder X-ray diffractometer (XRD, PAD5, Scintag, Inc.) was used to compare changes in crystallinity due to ALD film growth and thermal annealing post-treatments. A Cu X-ray source was used, operating at KR ) 1.540562 Å. XRD data were obtained over a (2θ) range of 30-40° at 0.5°/min, to span the range over which the three most predominant peaks ( ) , and ) associated with ZnO appear. The Cauchy distribution, eq 5, was used to fit each peak:

f(θ) )

[

γ 1 2 2 π (θ - θ ) + γ

]

(5)

where θ is the center line of a given peak and γ is a fitting parameter. A summation of the three somewhat overlapping peaks was

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Figure 3. (a) STEM image of a ZnO-coated 550 nm SiO2 sphere; (b) STEM image of a ZnO-coated 100 nm SiO2 sphere; TEM image of a ZnO-coated 250 nm SiO2 sphere. used for a final fit. The sum squared error between the experimental data and the sum of the individual distributions was minimized to optimize the composite fit in order to determine the full width at halfmaximum (fwhm) for each individual peak. The fwhm was used to calculate crystallite size in each dimension (d) using the Scherrer equation:

FWHM )

Gλ d cos(θ)

(6)

with G taken to be 0.9; λ is the X-ray source wavelength and all θ values are in radians. Thermal annealing was performed in a clamshell furnace open to the atmosphere. The temperature was increased to the desired set point at a rate of 10 degrees per minute, followed by a 45 min dwell time before returning to room temperature. Bulk elemental analysis was performed on both uncoated and coated powders by inductively coupled plasma s atomic emission spectroscopy (ICP-AES) using an Applied Research Laboratories ICP-AES 3410+. Concentrated H2SO4 (5 M, 95 °C) was used to fully digest the powders, with 0.5% Triton X-100 surfactant to enhance particle dispersion and minimize diffusion resistances. A diffuse transmittance analyzer (Labsphere UV-1000S, Labsphere Inc.) was used to measure the UV absorbance of SiO2 cores with varying ZnO ALD thicknesses. A 5.0 wt% dispersion of each powder was vortexed into a 50:50 butylene glycol/water mixture. The resulting dispersion was applied to hydrated synthetic skin samples (VITROSKIN, IMS Corp.) at a surface concentration of 2.0 mg/cm2. Measurements were made over the wavelength range of 290-450 nm. The uncoated SiO2 spheres did not absorb any energy over the wavelength range. The bandgap for each sample was obtained by following the slope of (Rhν)2 vs (hν) to zero. The experimentally determined crystallite size of the peak of each sample was used to report bandgap shifts observed on particle substrates.

Results and Discussion ALD is a layer-by-layer vapor-phase growth technique that has shown to effectively deposit conformal films on primary particle surfaces for materials that are amorphous as deposited. Independent of temperature, ZnO ALD has been observed to deposit as polycrystalline with a larger surface roughness than amorphous ALD films, though nonetheless conformal. The discovery that quantum confined nanostructures can be fabricated using the ALD technique was significant, but requires additional studies pertaining to crystallite propagation with respect to film thickness and thermal annealing post-treatments. To first ensure that polycrystalline ZnO ALD films grew conformal to primary particle surfaces, electron microscopy was used to observe the films. Figure 3a,b represent scanning transmission electron microscopy (STEM) images of 550 and 100 nm core SiO2 spheres, respectively, coated by 80 ZnO cycles via ALD. STEM is a Z-contrast technique and the Zn in ZnO shows up much brighter than the Si in SiO2. Each of these images shows that the ZnO films were conformal as deposited. Localized “hot-spots” were visible, which can be attributed to

Figure 4. ZnO content with particle size and number of cycles measured using ICP-AES. Trend lines represent the average linear growth rate on each core particle size, and are curved due to the geometrical contributions of linear growth off a spherical surface.

a higher concentration of active sites that were likely present on the original particle surfaces. Figure 3c shows a standard TEM image of an ∼18 nm ZnO ALD film coated onto a 250 nm SiO2 sphere. The self-limiting nature of ALD reactions allows the use of subsaturating exposures to maximize precursor utilization, with the only effect being a decrease in the stochastic film growth rate. The resulting measured ZnO content is shown in Figure 4 with the number of cycles for the three particle sizes used. The ALD growth rates were calculated using the theoretical mass increase per cycle for concentric, fully dense (i.e., pinhole-free) ZnO shell growth on each given core size. The lines represent the average growth rate measured across each substrate, which were 0.7, 1.6, and 1.6 Å/cycle for the 100, 250, and 550 nm core sizes. The low growth yield observed on the 100 nm spherical substrates was attributed to the use of extremely small batch sizes, which rendered fluidization impractical. During ALD processes on particles in well-fluidized particle beds, the turnover rate of particles circulating throughout the bed is extremely high. At the start of the dosing process, all precursor-particle interactions occur at the base of the reactor. Throughout the dose, the “reaction front” moves upward in the reactor as the number of unreacted surface sites decreases. Throughout this time, only the reaction product is observed from residual gas analysis. The net result is the eventual “breakthrough” of unreacted precursor, where the breakthrough time can be measured using the mass spectrometer, and be subsequently used for feed-forward process control.7 This plug-flowlike behavior has been observed in nearly all particle ALD processes, independent of the ALD film material or the material, size or morphology of the particles.7 The only artifact that can vary widely is the surface conversion, defined as the number active sites reacted per total number of active sites, attained at the point of breakthrough. For poorly fluidized beds, any gasphase bubbling or channeling that occurs will allow unreacted precursors to reach the top of the bed faster than in a wellfluidized case, leading to a lower conversion when the unreacted precursor is first detected. The net result of a subsaturating exposure prior to breakthrough would be a decrease in stochastic growth rate per cycle, rather than a fundamental difference between surface functionality across particle sizes. For ALD processes on flat surfaces, subsaturating exposures are typically not problematic due to the much lower surface areas to be coated. Across the experimental design of interest, powder XRD was performed on the as-deposited films from 10 to 115 cycles, corresponding to thicknesses of ∼1.5-18 nm, grown on 250 nm spheres. A plot showing the crystal phase evolution with

Crystal Phase Evolution in ZnO Domains

Figure 5. Powder XRD spectra for films as deposited at 100 °C on 250 nm spheres.

the number of cycles can be seen in Figure 5. A slight deviation from a straight-line amorphous response was observed for the 10 cycle material, but it is clear that the crystal structure was not well-defined at this thickness. For the most part, the fwhm of each of the three major ZnO peaks increased relatively linearly with the number of cycles. The Cauchy distribution fit quite well with each of the three peaks, and the summation of the three fitted lines was able to be matched to the composite powder XRD response in all cases. It was interesting to note that the center line of each fitted peak did not change with film thickness, which suggested that unit cell strain was not an issue with polycrystalline atomic-scale domain growth on the curved amorphous substrate. For the as-deposited cases (independent of core size), the lengths of the and dimensions of the ZnO domains were relatively linear, as shown in Figure 6a. For a visual comparison, the crystallite sizes in the and dimensions for the as-deposited ∼18.6 nm ALD film (calculated from the ICP-AES ZnO content) on 250 nm spheres were 11.5 and 4.7 nm, respectively. These dimensions can be compared with the TEM image in Figure 3c, which does support the calculated ALD thickness of ∼18.6 nm. ZnO films deposited at higher temperatures prefer to grow in the close-packed hexagonal wurtzite structure and perpendicular to the surface, or completely in the direction, as it is thermodynamically favored. Lower temperature growth studies have reported the occurrence of the peak, and it is predominantly accepted that increasing the oxidizer strength assists the transition of the preferred growth orientation from the to the direction.21-23 The directional growth ratios were relatively steady and repeatable across the number of cycles deposited, signifying that precision crystallite size control is possible as long as process control measures are put into place to maintain constant deposition conditions. The crystallite volume (Figure 6b) was also calculated using the square of the dimension times the dimension. The domain volume followed a normal cubic growth pattern, as expected. It is interesting to note that although the ZnO mass increased much more slowly on the 100 nm spheres than the 250 or 550

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Figure 6. (a) Crystallite size variation in the and dimensions and (b) total calculated domain volume with number of ALD cycles. All data points shown are from the as-deposited case for all core particle sizes.

nm spheres (i.e., the basis for the 0.7 vs 1.6 Å/cycle calculated growth rates), the crystallite size increase with respect to number of cycles proceeded identically across all core sizes. This would suggest that the primary cause for the halving of the growth rate was due to the lack of amorphous ZnO depositing between crystalline domains on the 100 nm cores. This concept, coupled with the notion that subsaturating exposures were used throughout the process, suggests that a fundamental difference must exist between active sites that reside on crystalline surfaces and those on amorphous surfaces. It is well-known that lattice matching facilitates the ALD of one material onto another; materials that are typically amorphous as deposited at a given temperature can be forced into a crystalline phase if the substrate lattice dimensions along the plane of growth are similar to those of the film material. These results strongly suggest that ZnO deposition preferentially deposits on crystalline domains first, after which the remaining active surface sites are filled in by amorphous ZnO. The subsaturating exposures (based on total surface area) here were sufficient to saturate the surfaces of the crystalline domains, but not those that yielded amorphous growth. With the ALD process, reactions occur between surface functional groups and precursor ligands. The two most common thin film growth mechanisms are the Volmer-Weber (V-W) and Frank-van der Merwe (F-M), corresponding to island growth only and conformal film growth only, respectively.24 The Stranski-Krastanov (S-K) growth mechanism is a combination of both of these. ALD is a monolayer-by-monolayer technique that ideally operates under the F-M mode, with selflimiting behavior preventing additional growth within a cycle even after supersaturating exposure. At the atomic scale however, the nucleation site density plays a pivotal role in determining whether the F-M mode can be claimed or if V-W growth actually occurs that forms a film after a given number of cycles. The number of cycles that would be required to define a continuous film depends at a minimum on the reactivity of

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the precursors at the operating temperature, and the steric hindrance between adjacent precursor molecules at the operating pressure. It is simpler to visualize the actual ALD growth process following the S-K mode, albeit with a self-limiting growth constraint under supersaturating conditions. It has been shown on flats and particles that DEZ decomposition can be problematic at higher temperatures, which is accentuated when using extended dosing times necessary for high total surface area saturation.5,9 Low operating temperatures are required to mitigate DEZ decomposition effects. For a supersaturating DEZ exposure during which precursor decomposition is not permissible, either one or two ligands of the DEZ molecule will react with surface functional groups. If one ligand reacts, which is typically the case during a self-limiting ZnO ALD process, the ethylzinc molecule will have rotational freedom parallel to the surface, allowing itself to seek a stable configuration. The dose/purge times were significantly longer in this study than studies on flats due to the amount of total surface area, typically 4 or 5 orders of magnitude greater, being coated during a single batch process. With increasing dose time, it is more likely that surface molecules will orient themselves in an increasingly stable configuration throughout each half-reaction, relative to the surrounding chemisorbed molecules. This allowable ordering time is likely the reason why the crystallite size, despite the favorable propagation in all three dimensions, was repeatable with respect to the number of ALD cycles. For the as-deposited cases of ZnO ALD, the predictability of crystallite size lends itself to precision control of any size-specific phenomena, such as quantum confinement at small dimensions, or optical reflectivity at larger length-scales. Analogous ZnO ALD studies that include XRD spectra for relatively thick films tend to exhibit extremely sharp peaks, which was different from the spectra in Figure 5.21-23,25 The primary cause of this difference is the presence of small crystallite size films in this study, relative to films typically on the order of hundreds of nanometers to microns in thickness. Although, since the natural growth of the domains (in all directions) was linear with respect to number of cycles, it can be extrapolated that after thousands of cycles, the polycrystalline properties of the ZnO nanoshells would approach those reported in other studies for thick ZnO ALD films on flat surfaces. Since increasing crystallite size corresponds to a decrease in the fwhm, in accordance with eq 6, peaks for thicker films are inherently expected to become narrower and of higher intensity. One other factor that may lead to general peak broadening is the fact that this stochastic process is being applied to an innumerable amount of individual nanosized particles, which may lead to slight deviations in crystallite size and/or film thickness across the entire sample. This notion becomes less problematic if fully saturating exposures are used, but as was the case in this study, a subsaturating dosing regime was utilized for smaller beds due to poor fluidization and premature breakthrough of the unreacted precursor. It must not be overlooked that the relative presence of each peak can be tuned based on the growth conditions, as discussed previously. Ultimately, it is anticipated that if the experimental window from this study would be extended to the film thickness window utilized by other research groups for crystallography studies of ZnO ALD films on flat surfaces, the domain size data from the former would approach that of the latter. To understand whether postdeposition thermal annealing would force crystal propagation in a direction normal to the surface, as observed in results from other studies, annealing temperatures up to 550 °C in air were selected.21,23,25 The XRD

King et al.

Figure 7. Powder XRD spectra for films coated on 250 nm SiO2 spheres, heat treated at 325 °C.

Figure 8. Powder XRD spectra of 40 cycle ALD films on 250 nm SiO2 spheres, heat treated at 325 and 550 °C.

spectra for films coated on 250 nm SiO2 spheres, and subsequently thermally annealed at 325 °C, are shown in Figure 7. XRD spectra showing the effect of thermal annealing to 550 °C on the crystal propagation measured in the 40 ALD cycle films (on 250 nm spheres) is depicted in Figure 8. In all cases, thermal annealing post-treatments increased the crystallite size both parallel and perpendicular to the substrate surfaces. The degree of thermal annealing induced growth in the direction with core size had a very distinct trend in that growth was much greater for higher radius of curvature substrates (i.e., smaller particle diameter) than lower. A surface plot detailing the inverse relationship between crystallite size and core size across the range of temperatures used is shown in Figure 9. This correlation suggests that grain boundary interference is reduced when propagating along a higher radius of curvature substrate. A similar effect has been observed when comparing ALD growth rates on particles relative to those on flat surfaces.2,3,7 Particle ALD yields higher growth rates since the radius of curvature decreases steric hindrance between adsorbed precursor molecules, effectively increasing the nucleation site density for a given surface area. A schematic of these effects are drawn in Figure 10. Figure 10a shows the increased nucleation site density allowable at smaller particle sizes. The primary feature of adsorbing precursors that dictates how closely packed they can remain at a given operating pressure, while still being stable, is the effective precursor size, which is based on the size of the attached ligands. The outward-pointing ends of the ligands in Figure 10a are each equivalently spaced across

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Figure 9. Surface plot of crystallite size modulation in the direction with annealing temperature, and its effect over various core sizes. The crystallite sizes reported are relative to the as-deposited case.

Figure 11. SEM images depicting grain growth of 40 cycle ZnO ALD films on 250 nm spheres (a) as-deposited and after thermal annealing steps at (b) 325 °C and (c) 550 °C.

Figure 10. Graphical depictions of the effect of decreasing particle size on (a) the allowable increase in nucleation site density through decreasing precursor ligand interactions; and (b) the increase in allowable grain boundary diffusion distance between equivalently separated adjacent grains.

the three surfaces. The increased radius of curvature allows the adsorption points to be closer to one another on each surface, resulting in an increased nucleation site density. The schematic in Figure 10b depicts two equivalently spaced domains that are adjacent to one another on an arbitrary surface. For the flat surface, grain growth tangent to the surface forces the grain boundaries to approach one another rapidly. Once the direct interface is formed, additional energy is required to overcome the activation barrier necessary for the two adjacent grains to merge. However, for substrates with an increased radius of curvature, the allowable grain diffusion length is greater, prior to adjacent domains coming into contact. It was unable to be determined in this study if once an interface was formed between adjacent grains whether growth in the dimension would become more preferred, prior to the coalescence of the grains. SEM images were taken of the 40 cycle ZnO ALD films in the as-deposited case, as well after the 325 and 550 °C heat treatment steps, as shown in Figure 11. For the as-deposited case in Figure 11a, the ZnO film is relatively smooth, with a few isolated grains protruding from the surfaces. Energy dispersive spectroscopy was used in situ to verify that the smooth film was ZnO rather than SiO2. After thermal annealing to 325 °C (Figure 11b), the film began to roughen with most individual grains visible and approximately equal to the size of the isolated grains in Figure 11a (∼10 to 20 nm). After thermal annealing to 550 °C (Figure 11c), significant grain growth was observed to the extent that grains from adjacent particles grew together. The thermal annealing steps were performed in a horizontal clamshell furnace in a ZrO2 boat, and as such the particles were in constant, static contact with one another. Had the thermal annealing been performed in a fluidized bed, the grains from adjacent particles would not have grown together

Figure 12. Quanum confined bandgap shift across all ZnO ALD films in the experimental matrices with respect to the experimentally measured crystallite size of each. Inset image represents measurement geometry; the solid line is the predicted shift based on the Brus model for ZnO.

due to the constant cycling of powders and the dynamic aggregation behavior of nanoparticles in FBRs. The UV absorbance of ZnO ALD films on bulk quantities of spherical submicron sized SiO2 spheres was measured using the diffuse transmittance technique. This technique offers two advantages over a standard UV-vis spectrophotometer. First, scattering is neglected due to the presence of a reflective integrating sphere that redirects all scattered light back through the sample. Second, transient absorbance decreases due to particle settling are not encountered, and thus higher solids loadings may be achieved. The calculated bandgap of each film is plotted relative to the measured crystallite size in the direction in Figure 12. All data points in the juxtaposed experimental matrices are included on this plot. It is clear that an increase, or blue shift, in bandgap was present for films thinner than ∼10 nm, which approximately corresponds to the theoretical length beyond which quantum confined effects in ZnO are lost. The solid line in Figure 12 represents the calculated blue shift predicted by the Brus model, eq 1, using literature values for the ZnO-specific properties (εr ) 3.7, m*) e 0.45).26 Thus, independent of whether a given 0.24, m*) h crystallite size in the dimension was obtained via ALD coating after a given number of cycles, or through thermal

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annealing steps, the optical response shown here matches the theory of quantum electrodynamics for confined domains. Conclusions A comprehensive study of the crystal propagation behavior of ZnO ALD films during the onset of growth has been performed. ZnO was deposited using DEZ and H2O2 as precursors at 100 °C on SiO2 spheres with diameters of 100, 250, and 550 nm. The growth was smooth and conformal on primary particle surfaces. A lower than expected growth rate was observed after coating the 100 nm core particles, which was attributed to the use of subsaturating precursor exposures. Nearly linear crystallite size increases in both the and dimensions were observed with the number of ALD cycles, independent of core particle size. Since the domain growth was identical across particle sizes, it was inferred that the subsaturating exposures were adequate for complete reaction with active sites on crystalline surfaces, but insufficient to react with those on amorphous surfaces. The low operating temperature allowed for the increased presence of the peak, although the peak was promoted due to the use of the strong oxidizing agent. Thermal annealing post-treatments promoted crystal growth in all directions. The peak was omnipresent over the experimental design range used here, a result that differed from annealing studies performed on flat substrates by other groups. During the normal volumetric crystallite growth, it was observed that grain growth along the particle surface (i.e., in the dimension) was accentuated on smaller radius substrates. This indicated that substrate morphology must also be taken into consideration when designing nanostructures with precise domain sizes. The absorbance band edge of the films showed an increasing blue shift with respect to decreasing crystallite size, an attribute indicative of quantum confined effects. An FBR or a highthroughput particle coating reactor apparatus can be used to coat bulk quantities of particles with coatings that contain quantum confined domains. The precision control capability of the ALD technique, coupled with the ability to predict crystallite size for given growth conditions, allows for tunable bandgap nanomaterials that could prove to be suitable for next generation photovoltaic, thermoelectric, and/or optoelectronic devices. Acknowledgment. The authors thank Fred Luiszer (University of Colorado) for ICP-AES analysis and Dr. Casey Carney for assistance with the XRD analysis. This work was sponsored in part by the National Science Foundation (Grant No. DMI-0420046), the NASA Graduate Student Researchers Program (Grant No. NNX07AP77H) and the Department of Education Graduate Assistance in Areas of National Need program. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of these government organizations.

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