Microstructure-Dependent Nucleation in Atomic Layer Deposition of Pt

Article pubs.acs.org/cm

Microstructure-Dependent Nucleation in Atomic Layer Deposition of Pt on TiO2 Han-Bo-Ram Lee and Stacey F. Bent* Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: The effects of TiO2 microstructure on Pt nucleation and on formation of continuous ultrathin Pt films by atomic layer deposition (ALD) were investigated. Pt was deposited by a metalorganic Pt precursor ((methylcyclopentadienyl)trimethylplatinum) and air as a counter reactant on in situ grown ALD TiO2 surfaces. For the same number of Pt ALD cycles, the Pt surface coverage was found to depend on the thickness of the underlying TiO2. From X-ray diffraction (XRD) analysis, it was found that the amorphous microstructure of asdeposited TiO2 transforms into anatase microstructure because of an annealing effect at the elevated Pt ALD temperature, and that this effect is a function of TiO2 thickness. Transmission electron microscopy revealed that continuous growth of ALD Pt occurs on anatase TiO2 whereas island growth occurs on amorphous TiO2. These results indicate that Pt nucleation is significantly affected by the microstructure of TiO2. Effects beyond surface hydrophilicity, such as a catalytic effect, are needed to explain the different nucleation properties of ALD Pt on TiO2. These results provide insight into initial growth during metal ALD and the effects of surface structural properties on ALD nucleation. KEYWORDS: atomic layer deposition, TiO2, Pt, microstructure, nucleation

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INTRODUCTION Pt atomic layer deposition (ALD) is an actively studied ALD process because of the importance of Pt for various applications, including microelectronics, catalysis and renewable energy, and sensing.1−11 In addition, its excellent conformality and controllability over atomic thickness enables Pt ALD to be used for fabrication of complicated threedimensional (3D) nanostructured devices. Pt ALD has been applied to fuel cells,1−5 photovoltaics,6 chemical sensors,7 metal gate layers,8,9 seed layers,10 and nonvolatile memory.11 For each application mentioned above, except for nonvolatile memory which requires Pt nanoparticles, the fabrication of a continuous thin Pt film is desired, for example, to achieve electrical conductivity. Although in the ideal case, an ALD film is formed in a layerby-layer manner through surface self-saturated reactions, growth characteristics in real systems show deviations from ideality, such as nucleation delay and island growth.12,13 Such nonidealities also apply to Pt ALD. To date, Pt has been deposited on various substrates, including SiO2,8,14−16 ZrO2,8,9 Al2O3,8 TiO2,6,17 STO,2,3 TaNx,10 YSZ,1 C aerogels,18 Ni,19 and Ru,4,5 and noncontinuous formation of Pt during initial growth was observed in many of the reported experiments. Because of the difficulty in deposition of continuous thin Pt films by ALD, some researchers have exploited the island growth mechanism of Pt ALD to fabricate Pt nanoparticles, for applications such as alloying with other metals,5,20 reduction of Pt loading,16,18 and nanocrystal memory.11 Since fast nucleation and high wettability of the substrate by Pt tend to produce flat and continuous Pt films, the interaction © 2011 American Chemical Society

between Pt and the surface species is important during initial growth. Some reports have suggested that surface hydrophilicity and electronegativity may be important factors in determining fast nucleation because a hydrophilic or ionic surface can provide active sites to begin the ALD reaction.15,21,22 Although various researchers have carried out investigations into the effects of surface species on Pt nucleation, many studies have reported only phenomenological growth characteristics such as nucleation delay without providing an explanation of the underlying reasons.11,16,23,24 Therefore, Pt ALD should be investigated on various surfaces with attention paid to the properties of the surface materials to better understand the atomic-level processes dictating Pt nucleation. Recently, Kim et al. reported the growth of ALD Ru on TiO2 with different TiO2 microstructures.21 The authors focused on the catalytic effects of three TiO2 phases on Ru growth, although they did not have direct evidence to prove a catalytic effect. In their report, ALD of Ru on both anatase TiO2 and rutile TiO2 showed a shorter nucleation delay than that on amorphous TiO2. In the present paper, we investigate the effects of substrate microstructure on Pt nucleation and on formation of a continuous Pt film. Pt was deposited using a metalorganic Pt precursor (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3)) and air as a counter reactant on an in situ grown ALD TiO2 surface. A striking change of Pt surface Received: September 14, 2011 Revised: October 19, 2011 Published: December 29, 2011 279

dx.doi.org/10.1021/cm202764b | Chem. Mater. 2012, 24, 279−286

Chemistry of Materials

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coverage is observed on different thicknesses of TiO2. From a microstructure analysis using X-ray diffraction (XRD) and transmission electron spectroscopy (TEM), it is found that the microstructure of TiO2 changes from amorphous to anatase with increasing film thickness because of an annealing effect at the elevated substrate temperature used during Pt ALD, and that nucleation of ALD Pt occurs more rapidly on anatase than on amorphous TiO2. By comparing with Pt ALD on a control SiO2 surface, the effects of TiO2 microstructure combined with surface hydrophilicity on Pt nucleation were studied.

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EXPERIMENTAL DETAILS

A custom-made ALD reactor controlled by LabVIEW software was used for the current study. A showerhead inlet and vacuum pumping lines were connected to the top and bottom of the chamber, respectively, and the substrate was placed on a 4-in. diameter substrate heater. MeCpPtMe3 and air were used for the Pt precursor and coreactant, respectively. The Pt precursor was contained in a glass bubbler, and its temperature was kept at 70 °C to obtain a proper vapor pressure. N2 was used for the carrier gas and purging gas, and flow rates controlled by a mass flow controller were fixed at 30 sccm for both purposes. The substrate temperature (Ts) for most Pt ALD experiments here was 300 °C. Further information on the chamber configurations and Pt ALD process can be found elsewhere.25 Two kinds of surfaces were employed as a substrate for Pt ALD. For routine controls, Si(001) substrates with native oxide were cleaned by piranha cleaning and used as a standard OH-terminated SiO2 surface. The nonpiranha-treated sample was cleaned by sequentially dipping in acetone, isopropanol, and deionized water. For the TiO2 studies, TiO2 ALD was carried out in the same reactor immediately prior to Pt ALD. TiO2 was deposited on the Si(001) substrate by using TiCl4 and water as a precursor and a coreactant, respectively, at Ts = 100 °C for various numbers of cycles. After TiO2 ALD, Ts was elevated to 300 °C for Pt ALD without a vacuum break. Field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) were used for analysis of surface morphology, and the chemical composition analysis was performed by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). For the analysis of Pt surface coverage following Pt ALD, surface analysis software with a graphical modification function (Image SXM ver. 1.92) was used, and the detailed process is described in the Supporting Information. The microstructure of Pt and TiO2 films were analyzed by XRD and TEM. Ellipsometry and SEM were used to measure film thicknesses of TiO2 and Pt, and the resistivity of Pt was calculated from the thickness and sheet resistance measured by a 4-point probe. Surface hydrophobicity was analyzed by static water contact angle measurement.

Figure 1. Contact angle pictures of SiO2 surface (a) without and (b) with piranha cleaning. SEM images of ALD Pt deposited for 200 cycles on SiO2 surface (c) without and (d) with piranha cleaning.

uncoated SiO2 surface, respectively. On the SiO2 surface without piranha cleaning (less hydrophilic surface, Figure 1C), Pt is partially deposited and the calculated apparent coverage (see Supporting Information) is 54%. In contrast, the surface of SiO2 cleaned by the piranha process (hydrophilic surface) is almost fully coated with the Pt film, as shown in Figure 1D. The apparent coverage of Pt in Figure 1D is 83%. In previous reports, the reaction chemistry of Pt ALD using the same precursors is shown to be composed of two halfreactions: adsorption of precursor onto the surface, and oxidative reaction of adsorbed Pt precursor with O2.14,28 For the former reaction, surface oxygen species are important because the Pt precursor is chemisorbed on the surface through reaction with surface oxygen. Therefore, it can be inferred that an increase in the number of hydroxyl groups by piranha cleaning provides an increase in the number of chemisorption sites for Pt precursor molecules, resulting in a higher Pt nucleation rate and faster closure of the SiO2 surface by the Pt film. Figures 2A and 2B show plan-view SEM images of ALD Pt on a piranha-cleaned SiO2 surface and on an ALD TiO2 surface, respectively, after 100 cycles of Pt ALD. In this experiment, TiO2 was deposited for 400 cycles by ALD, and the measured thickness was 28.1 nm by ellipsometry. On the SiO2 surface, Pt is not a continuous film but consists of particles as shown in Figure 2A. In contrast to the SiO2 surface, an almost continuous Pt film was observed on the ALD TiO2 surface as shown in Figure 2B. The calculated apparent coverages of these Pt samples were 16% and 96% for Pt on the SiO2 and TiO2 surfaces, respectively. The thickness of Pt on TiO2 was around 9 nm measured by SEM. In previous reports, island growth of ALD Pt on the SiO2 surface was typically observed during the initial growth stage, which is consistent with our observation in Figure 2A.6,8,14−16 The apparent coverage of Pt on TiO2 is much higher than that on SiO2 for the same number of ALD cycles. Moreover, the coverage of the 100-cycle ALD Pt film on TiO2 in Figure 2B is also higher than the film shown in Figure 1D, which was deposited using twice the number of ALD cycles (200 cycles). These results indicate that the reaction of Pt ALD is more facile on the TiO2 surface than on the SiO2 surface.

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RESULTS AND DISCUSSION It is well-known that a SiO2 surface cleaned by the piranha process shows very high surface hydrophilicity because of an increase in the number of surface hydroxyl groups.26,27 Figures 1A and 1B show contact angle pictures of SiO2/Si(001) substrates without and with piranha cleaning, respectively. The measured water contact angles were 38.1° and 2.8° on SiO2 surfaces without and with piranha cleaning, respectively. From the XPS analysis, we did not observe a significant difference in chemical state between piranha and nonpiranha cleaned surfaces. Usually, the silicon oxide surface has hydrophilic properties and typical contact angle values are in the range of 30° to 40°. The lower water contact angle achieved after piranha cleaning reflects an even more hydrophilic surface related to the higher number of surface hydroxyl groups. SEM plan-view images of these two substrates after 200 cycles of Pt ALD are presented in Figures 1C and 1D, in which the bright and dark contrast areas correspond to deposited Pt and the 280



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Figure 2. SEM images of ALD Pt deposited for 100 cycles on (a) piranha-cleaned SiO2 surface and (b) ALD TiO2 surface.

To further understand this phenomenon, the growth characteristics of Pt ALD were investigated for different TiO2 thicknesses. Figures 3A, 3B, and 3C show plan-view SEM images of ALD Pt on ALD TiO2. TiO2 ALD was carried out for 100, 200, and 300 cycles, yielding film thicknesses of 9.7, 17.4, and 21.6 nm, respectively, as measured by ellipsometry. The Pt ALD process was fixed at 100 cycles for these samples. The SEM image reveals several individual Pt nuclei and a cluster composed of small Pt nuclei on the thinnest TiO2 film, shown in Figure 3A. With increasing TiO2 thickness (Figure 3B), several small and large Pt clusters are observed across the surface. In Figure 3C, the entire surface is nearly covered with ALD Pt. At lower numbers of TiO2 ALD cycles (below 100 cycles), no Pt nucleation was detected by the SEM analysis. The apparent coverages for Figure 3A, 3B, and 3C were 5%, 22%, and 69%, respectively. It is clearly seen that the growth of Pt is changed on substrates with different thicknesses of TiO2. We now consider possible explanations for the TiO2 thickness-dependence. One possible reason for the different Pt nucleation behavior on surfaces treated with fewer TiO2 ALD cycles is that the underlying SiO2 surface may not be fully covered by TiO2. However, we can rule out this possibility based on the following observations. The growth rate of ALD TiO2 ALD measured by ellipsometry is approximately 0.7 Å/ cycle. In addition, it is known that ALD TiO2 from TiCl4 and water has no nucleation delay on the SiO2 surface.29,30 Hence, it is expected that after 100 cycles the surface will be well covered by TiO2. This expectation is confirmed by the AES data, in which Ti signal was observed from the entire surface covered with 100 cycles of TiO2 film (Supporting Information, Figure S2), with no Si signal detectable, indicating that the substrate is fully covered by TiO2 after 100 cycles. Another possible explanation is that the surface composition of the TiO2 is different as a function of thickness. Since the growth reaction in ALD should occur only on the substrate surface, substrates

Figure 3. SEM images of ALD Pt deposited for 100 cycles on different thicknesses TiO2 formed by (a) 100, (b) 200, and (c) 300 cycles of TiO2 ALD.

with different surface composition may produce different growth characteristics in ALD. However, from the XPS spectra, there is no significant difference in chemical composition between TiO2 films deposited for 100, 200, 300, and 400 cycles (Supporting Information, Figure S3). So, in terms of chemical state of the surface, the TiO2 surfaces are identical irrespective of thickness once TiO2 is deposited for over 100 cycles. A third possible reason for the different Pt nucleation behavior is surface roughness. Roughness can affect nucleation of ALD films because high roughness leads to larger effective surface areas, resulting in an increase in the number of nucleation sites.31−33 Our measurements of the as-deposited TiO2 films using AFM analysis show, however, that there is no significant increase in surface roughness between the different thickness TiO2 films (see Figure 7). Instead, we will show in the following analysis that the TiO2 thickness dependence stems from changes in the microstructure of TiO2 f rom amorphous to anatase with increasing f ilm thickness because of annealing ef fects, where nucleation of ALD Pt occurs more rapidly on anatase than on amorphous TiO2. Figure 4A and 4B show depth profiles of ALD Pt deposited for 100 cycles on two different thicknesses of ALD TiO2, 17.4 and 28.1 nm, respectively. The atomic percent ratio of Ti to O 281



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Figure 5. XRD spectra of ALD Pt deposited for 100 cycles on different thicknesses of ALD TiO2.

increase on both Pt nucleation and the TiO2 microstructure, TiO2 films were annealed at 300 °C for a time equivalent to the total Pt deposition time, but no Pt ALD was performed. XRD results following this experiment are shown in Figure 6. Similar

Figure 4. AES depth profile data of 100 cycle Pt ALD deposited on (a) 200 and (b) 400 cycles of TiO2 ALD.

is 1:2 in both samples, indicating the formation of stoichiometric titanium dioxide film. The Pt signal was only observed within the first few sputtering cycles from the surface in Figure 4A, whereas the Pt signal in Figure 4B was detected for over 1 min of sputtering time. Although the sputtering rates were not calibrated for each Pt and TiO2 material, it is clear that the amounts of Pt deposited on the two thicknesses of TiO2 films were different, a result consistent with the observation from the SEM images. In addition, no interdiffusion is observed at the interface between Pt and TiO2. Therefore, the stoichiometry of ALD TiO2 is not changed with changing thickness and there is no further reaction between the Pt layer and the TiO2 surface after deposition. The stacked films of ALD Pt on ALD TiO2 were also analyzed by XRD. The XRD analysis was carried out before and after Pt ALD with increasing ALD TiO2 thicknesses as shown in Figure 5. Before Pt ALD, there was no observable peak from the sample. After Pt deposition, however, a strong peak at 39.7° was observed for Pt grown on at least 300 cycles of ALD TiO2. This feature was assigned to the Pt(111) diffraction peak. The Pt(111) XRD peak was likely not observed for the Pt samples formed on thinner TiO2 surfaces because the Pt coverage on those surfaces was too low, consistent with SEM results. In addition to the Pt(111) peak, a small XRD peak was detected at 25.2°, corresponding to anatase TiO2(101), and the intensity of this peak increased with increasing TiO2 thickness. Interestingly, this TiO2 diffraction feature appeared only af ter Pt ALD. These observations indicate that there is a change of the TiO2 microstructure during Pt ALD, and that the change is related to the TiO2 thickness. The substrate temperatures used in the current experiments for TiO2 and Pt ALD were 100 and 300 °C, respectively. To carry out Pt ALD, the temperature was elevated from 100 to 300 °C immediately after TiO2 ALD. Thus, it is likely that the higher temperature plays a role in changing the TiO 2 microstructure. To explore the effects of the temperature

Figure 6. XRD spectra of different thicknesses ALD TiO2 after annealing at 300 °C.

to Figure 5, a small anatase TiO2 peak was found at 25.2° after annealing, with its intensity increasing with increasing TiO2 thickness. Therefore, the transformation of TiO2 microstructure from amorphous to anatase is attributed to annealing effects at the relatively high substrate temperature of the Pt ALD process. TiO2 has three crystalline phases, anatase, rutile, and brookite, and among these, anatase and rutile structures have been observed in thin film systems.29 Typically, TiO2 deposited by ALD at temperatures around 100 °C has an amorphous structure.34 For example, in a previous report on TiO2 ALD using TiCl4 and water, the microstructure of ALD TiO2 was amorphous below 165 °C and anatase at 165 °C−350 °C, and the rutile structure was only observed above 350 °C.29 Similarly, using other Ti metal complex precursors, anatase TiO2 films were detected above 250 °C and amorphous TiO2 films observed at lower temperature.30,35 In the previous reports, the transformation of TiO2 microstructure by postdeposition annealing showed a similar trend as with the current results. The amorphous TiO2 deposited at 150 °C by ALD was changed into anatase after annealing at 450 °C.36,37 282



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These literature results support the assertion that the TiO2 film can convert from amorphous to anatase at the elevated temperatures used for Pt ALD. Notably, the transformation of microstructure by annealing is evident in thicker TiO2 films but is not significant in thinner TiO2 films. In Figure 6, the intensity of the anatase TiO2 peak (25.2°) of the thicker film is much larger than that of the thinner film. These results are consistent with literature reports. For example, it was reported that using a spray-coating method, amorphous TiO2 was formed below 50 nm of thickness while anatase TiO2 was observed at thicknesses above 50 nm.38 Howitt et al. also reported the effects of TiO2 thickness on the transformation from amorphous to anatase.39 They found islands of anatase TiO2 crystallites for thin films and a fully crystallized anatase TiO2 for a thick film using TEM, and explained the effects by the inability of a thin TiO2 film to accommodate the density differences associated with the phase transformation. Similarly, some researchers reported that a nanocrystalline anatase-embedded amorphous TiO 2 was formed when thin TiO2 was deposited on nanostructures, such as anodic aluminum oxide.36,37 In light of these reports, we can summarize our experimental results as follows. The asdeposited ALD TiO2 has an amorphous microstructure irrespective of thickness, because of the low deposition temperature. Upon annealing, a more substantial microstructure transformation from amorphous to anatase occurs in thick TiO2 films than in thin TiO2 films, because of the welldocumented effects of thickness on the phase transformation. These conclusions are confirmed by TEM analysis of ALD Pt on different thickness TiO2 (vide infra). Figure 7 shows the ratio of rms roughness (measured by AFM) to TiO2 ALD film thickness before and after annealing. Figure 8. Cross sectional TEM images of 100 cycles ALD Pt deposited on 400 cycles TiO2 in (a) low and (b) high magnification.

ALD Pt on the thicker ALD TiO2 sample. A continuous Pt layer is observed atop the TiO2, and the thicknesses of Pt and TiO2 are measured at ∼10 nm and ∼30 nm, respectively, in Figure 8A. Polycrystalline microstructure is clearly observed in both the Pt and the TiO2 film regions in the magnified TEM image collected at the interface between Pt and TiO2 (Figure 8B). Consistent with XRD and SEM results, the TEM images show that ALD TiO2 is fully transformed into a polycrystalline anatase phase and that the Pt layer is continuously formed. In contrast, different results are observed for the same number of Pt ALD cycles on thinner TiO2 films, as shown in Figure 9. The Pt layer reveals noncontinuous island growth, and its thickness is approximately 3 nm in Figure 9A. The thickness of the TiO2 layer below Pt is ∼7 nm, and TiO2 has an amorphous microstructure. The TiO2 thickness measured by TEM is smaller than that by ellipsometry probably because of deviation of the refractive index from its bulk value. In the magnified TEM image of Figure 9B, small TiO2 nanocrystallites are observed underneath the Pt islands. The presence of anatase TiO2 underlying the Pt islands for the thinner films in Figure 9B is an interesting observation for which we suggest two possible explanations. One possibility is that the TiO2 crystallites formed before Pt ALD, followed by preferential nucleation of Pt on the TiO2 crystallites. The other possibility is that during Pt ALD, the Pt deposition reactions and/or the presence of Pt causes a local transformation of amorphous TiO2 into the crystallites. We believe that the

Figure 7. Ratio of rms roughness to thickness of ALD TiO2 with different thickness before and after annealing.

The ratio before annealing is slightly decreased with increasing number of ALD TiO2 cycles, reflecting the smooth surface of amorphous TiO2. After annealing, however, the percent roughness of ALD TiO2 deposited for 100 cycles was significantly increased to 30%. This percentage roughness drops rapidly with increasing TiO2 thickness according to Figure 7. The data indicate that the effects of annealing on the ALD TiO2 morphology are more significant in thinner films than in thicker films. These results will be discussed below together with the transformation of microstructure as a function of TiO2 thickness. Two samples of ALD Pt, one deposited on a thinner (100 cycles, 9.7 nm) TiO2 layer and one deposited on a thicker (400 cycles, 28.1 nm) TiO2 layer, were selected for further analysis by HR-TEM. Figure 8 shows cross sectional TEM images of 283



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To understand the differences between the anatase and the amorphous films, we first consider hydrophilicity. Surface hydrophilicity is an important property of the substrate for facilitating formation of a continuous ALD Pt layer on the SiO2 surface, as shown in Figure 1. To investigate the effect of hydrophilicity of the ALD TiO2 surface, its surface hydrophilicity was measured before and after annealing. Before annealing, the contact angles of all the ALD TiO2 films were almost constant within the range of 70°−80°, irrespective of the number of TiO2 ALD cycles (Figure 10). All of the contact

Figure 10. Contact angle of ALD TiO2 with different thickness before and after annealing.

angles decreased to below 50° after annealing. Moreover, the new postanneal contact angles are a strong function of TiO2 thickness. The postanneal film has a contact angle at a maximum for the 200 cycle film and exhibits the smallest contact angle (26.3°) for the thickest TiO2 film (400 cycles). It has been reported that anatase and rutile microstructures have more hydrophilic surfaces than does the amorphous phase.40,41 Ye et al. investigated the effects of annealing on TiO2 hydrophilicity.40 They observed a similar trend with our results after annealing and explained three main factors affecting hydrophilicity with increasing annealing temperature: removal of surface contaminants and transformation of microstructure act in concert to increase hydrophilicity, while change of film porosity acts to decrease hydrophilicity.40 In their work, they concluded that the microstructure factor was dominant. In our case, the transformation of microstructure from amorphous to anatase is also the main reason for the formation of a hydrophilic surface, and the trend toward lower water contact angles for thicker films reflects the greater anatase component in such films. We speculate that the lower contact angle observed at 100 cycles of TiO2 ALD is due to the effects of roughness on the hydrophilicity, since the thinner annealed film had the highest percent roughness.42 Despite the increase in hydrophilicity for thicker TiO2 films, hydrophilicity alone is not sufficient to explain the dependence of Pt ALD on TiO2 film thickness. First, although the water contact angle of TiO2 at 400 cycles is significantly lowered after annealing, the value is still higher than the 2.8° measured for the piranha-cleaned SiO2 shown in Figure 1 for which Pt coverage is not as complete as on TiO2. Therefore, the high nucleation rate of Pt on TiO2 cannot be completely explained by the surface hydrophilicity. Rather than only surface hydrophilicity, the influence of the substrate material on the reaction chemistry of ALD Pt must be significant. Besides surface hydrophilicity, we propose that the catalytic activity of TiO2 may influence the different nucleation rates on anatase and amorphous TiO2. Generally, it is well-known that

Figure 9. Cross sectional TEM images of 100 cycles ALD Pt deposited on 100 cycles TiO2 in (a) low and (b) high magnification.

former explanation is most likely based on the following observations. First, it is known from the studies of the thicker films that nucleation is facile on the anatase form of TiO2. Moreover, the AFM results of Figure 7 show that after anneal (but before Pt ALD) there is a large increase in rms roughness for the thin (100 cycle) TiO2 film that is not observed in thicker films. The presence of small crystallites of TiO2 in an amorphous matrix, as seen in Figure 9B, can lead to a rougher surface than either amorphous or anatase alone, consistent with the AFM results.39 Consequently, we propose the following sequence to explain the observation of TiO2 nanocrystallites residing underneath the deposited Pt: (1) a small number of anatase crystallites begin to form in thinner amorphous TiO2 films during annealing to 300 °C (2) the crystallites lead to an increase in the percent roughness (3) ALD Pt is preferentially formed on anatase TiO2 crystallites over the amorphous TiO2. Previous reports in the literature provide indirect evidence consistent with our observations.17,23 A much lower nucleation rate of ALD Pt on thin TiO2 (below 10 nm), leading to an overall thickness of Pt below 1 nm for 80 cycles of Pt ALD, was reported by Elam et al.23 In contrast, large numbers of ALD Pt nuclei were reported on an anatase/rutile mixed powder even after 20 cycles by Zhou et al.17 On the basis of the results of our current study, we can attribute the low nucleation of Pt in the report by Elam et al. to the difficulty of microstructure transformation in the TiO2 because of its insufficient thickness, and the high nucleation of Pt in the report by Zhou et al. to the enhancement of Pt nucleation by the anatase microstructure. 284



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anatase TiO2 has higher catalytic activity than amorphous TiO2.43−45 So, the anatase TiO2 surface may more readily catalyze reaction by Pt precursor molecules, leading to an increased adsorption rate of the precursors on the surface, resulting in fast nucleation. The anatase TiO2 surface may also catalyze some decomposition of the Pt precursor. The fact that the thickness of deposited Pt in Figure 8A is larger (10 nm) than the expected thickness calculated based on the growth rate (5 nm) may be explained by such catalytic effects, because if catalytic activity affects the dissociation of the precursor, the adsorption may not be self-limiting. Although several reports have also suggested effects of catalytic activity on initial growth as well as nucleation, detailed experimental evidence for such an effect has not yet been presented.21,46 Therefore, further research focused on the catalytic effects and surface properties of TiO2 on ALD nucleation is ongoing.

CONCLUSIONS The growth characteristics of Pt ALD were investigated on SiO2 and ALD TiO2 surfaces. Pt was deposited by ALD using MeCpPtMe3 and air as a precursor and reactant, respectively. Pt was deposited in situ on ALD TiO2, and different nucleation effects in ALD Pt were observed as a function of TiO2 thickness, with more continuous Pt growth occurring on thicker TiO2 films. These results are explained by the presence of enhanced Pt nucleation on anatase versus amorphous TiO2, with thicker TiO2 films yielding more anatase content. From XRD analysis, it was found that the amorphous microstructure of as-deposited TiO2 was transformed into anatase microstructure because of an annealing effect during the Pt ALD process. Thicker films formed more anatase upon annealing. Pt nucleation occurred preferentially on an anatase TiO2 surface compared to an amorphous TiO2 surface. TEM images confirmed continuous growth of ALD Pt on anatase TiO2 and island-like growth on amorphous TiO2. In addition to surface hydrophilicity, other effects such as catalytic phenomena must be considered to explain the differences in nucleation in ALD Pt on TiO2. These results are useful in helping elucidate initial growth of metal ALD and the effects of surface properties on ALD nucleation. ASSOCIATED CONTENT

S Supporting Information *

SEM images and AES mapping images together with computermodified images for apparent coverage analysis, AES mapping of Ti signal for different thickness TiO2 samples, and XPS spectra of different thickness TiO2 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy Hydrogen, Fuel Cells, and Infrastructure Program through the National Renewable Energy Laboratory under Contract No. DE-AC36-08-GO28308. The studies of TiO2 phase behavior and its effect on nucleation are supported by the Department of Energy under Award Number DE-SC0004782. 285



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