Nonequilibrium Synthesis of TiO2 Nanoparticle “Building Blocks” for

Letter pubs.acs.org/NanoLett

Nonequilibrium Synthesis of TiO2 Nanoparticle “Building Blocks” for Crystal Growth by Sequential Attachment in Pulsed Laser Deposition Masoud Mahjouri-Samani,*,† Mengkun Tian,‡ Alexander A. Puretzky,† Miaofang Chi,† Kai Wang,† Gerd Duscher,‡,§ Christopher M. Rouleau,† Gyula Eres,§ Mina Yoon,† John Lasseter,† Kai Xiao,† and David B. Geohegan*,† †

Center for Nanophase Materials Sciences, §Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡ Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37966, United States S Supporting Information *

ABSTRACT: Nonequilibrium growth pathways for crystalline nanostructures with metastable phases are demonstrated through the gasphase formation, attachment, and crystallization of ultrasmall amorphous nanoparticles as building blocks in pulsed laser deposition (PLD). Temporally and spatially resolved gated-intensified charge couple device (ICCD) imaging and ion probe measurements are employed as in situ diagnostics to understand and control the plume expansion conditions for the synthesis of nearly pure fluxes of ultrasmall (∼3 nm) amorphous TiO2 nanoparticles in background gases and their selective delivery to substrates. These amorphous nanoparticles assemble into loose, mesoporous assemblies on substrates at room temperature but dynamically crystallize by sequential particle attachment at higher substrate temperatures to grow nanostructures with different phases and morphologies. Molecular dynamics calculations are used to simulate and understand the crystallization dynamics. This work demonstrates that nonequilibrium crystallization by particle attachment of metastable ultrasmall nanoscale “building blocks” provides a versatile approach for exploring and controlling the growth of nanoarchitectures with desirable crystalline phases and morphologies. KEYWORDS: Crystallization by particle attachment, nanoparticle building blocks, pulsed laser deposition, molecular dynamics simulation

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rystallization by particle attachment (CPA)1,2 is a fascinating mechanism for crystal growth where nanoscale amorphous or crystalline nanoparticles act as building blocks to form larger crystals by thermodynamically minimizing their collective free energy.3−5 The mechanisms by which nanoparticles build larger crystalline nanostructures by CPA have been investigated primarily by in situ transmission electron microscopy (TEM) of nanoparticles in liquids and include oriented attachment, recrystallization, or amorphous addition.1−5 The growth mechanisms of crystalline nanostructures on substrates resulting from the physical vapor deposition of nanoparticles formed in the gas phase are far less studied. Recently, we showed that “amorphous” TiO2 nanoparticles synthesized by condensation in pulsed laser vaporization plumes are thermodynamically metastable precursor particles which can transform into crystalline TiO2 nanostructures with novel phases, such as “black TiO2” core−shell nanoparticles, depending on their postannealing treatment.6 This phenomenon follows the Ostwald−Lussac law of phases, which states that the pathway to a final crystalline state evolves by passing through increasingly stable states.7−11 Here we utilize the highly nonequilibrium nature of pulsed laser ablation plasma © 2017 American Chemical Society

thermalization in background gases to controllably condense and deliver nearly pure beams of these ultrasmall amorphous precursor nanoparticles to explore their crystallization by particle attachment into nanostructures with crystalline phases of different metastability and morphology. Pulsed laser deposition (PLD) is typically employed as a synthesis tool for epitaxial thin film growth primarily for its advantages of stoichiometric transfer of material from target to substrate with nonthermal (∼100 eV) kinetic energies, if desired.12,13 For the growth of thin films, background gas pressures during PLD are typically kept below ∼10 mTorr to maintain the flux as primarily atoms, ions, and molecules to facilitate the layer-by-layer growth of epitaxial thin films with control rivaling molecular beam epitaxy. However, when the background gas pressure is high enough to scatter and confine a sufficient density of the atoms and molecules in the ablation plasma plume, clusters and nanoparticles can form by condensation. When sufficient time and Received: March 11, 2017 Revised: June 9, 2017 Published: July 10, 2017 4624

DOI: 10.1021/acs.nanolett.7b01047 Nano Lett. 2017, 17, 4624−4633

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Nano Letters

Figure 1. In situ diagnostics of plume evolution as a function of pressure. (a) In situ ICCD images of the laser ablation plume luminescence at the indicated arrival times of the plume front at a room-temperature substrate 5 cm away from the target for different background oxygen pressures (20 ns to 1 μs exposures, false color, maximum intensity for each image provided). (b) In situ ion probe voltage (across 50 Ω resistor) at d = 5 cm as a function of background gas pressure, showing plume splitting. For pressures 100 mTorr. The 200 mTorr curve utilized a 500 Ω resistor and is scaled by 10× to show a weak third component of the plume. (c−g) Cross-sectional SEM images showing the morphology of the samples deposited at d = 5 cm for various pressures, with dense films transforming to nearly pure nanoparticle architectures after the second component in the ion probe voltage disappears at 200 mTorr.

to explore the synthesis of nanostructured films with novel phases and morphologies determined by the competition between kinetics and thermodynamics. Results and Discussion. Pulsed laser vaporization of a solid target of TiO2 in low-pressure background gases was used to synthesize ultrasmall (2−5 nm diameter) amorphous nanoparticles of TiO2 by gas phase condensation, under conditions similar to those described recently in our previous work.6 These amorphous ultrasmall nanoparticles (UNPs) can serve as precursors to transform during annealing treatments into larger crystalline nanostructures, such as the core−shell “black-TiO2” nanoparticles in that study.6 Here we employ time-resolved in situ diagnostics to adjust the thermalization dynamics of the ablation plume for different background gas pressures and deposition distances to not only synthesize gasphase condensed nanoparticles but also deposit them selectively on heated substrates to explore the dynamic crystallization of larger TiO2 nanostructures. In situ diagnostics, including gated intensified charge couple device (ICCD) array imaging, ion probe time-of-flight current measurements, and laser-induced fluorescence spectroscopy, were employed to adjust the plume conditions for the onset of nanoparticle formation according to previous studies.16,34 As shown by the ICCD images and ion probe waveforms in Figure 1a in vacuum, the KrF-laser ablation (248 nm, 1 J cm−2, 1 Hz) of TiO2 results in a rapidly expanding, forward-directed plasma plume containing atoms, ions, and molecules ejected from the target with high kinetic energies (up to 100 eV for the leading edge of the plasma plume traveling at 2 cm/μs velocity). With increasing background O2 pressure, the arrival time of the leading edge of the visible plasma plume to the indicated 5 cm position increases to longer delays, as the plume becomes

temperature are afforded (e.g., as in a hot oven environment), time-resolved in situ laser spectroscopy and imaging diagnostics have revealed how these nanoparticles and clusters can selfassemble in the gas-phase into larger nanostructures, such as single-wall carbon nanotubes from condensed Ni/Co and carbon nanoparticles.9,14−19 Alternatively, nanoparticles that have just formed in the ablation plume can be deposited at room temperature to construct mesoporous nanoparticle films for the fabrication of hierarchical electrode architectures (primarily of TiO2, but also WO3, ITO, etc.) for dye-sensitized solar cells,20−23 reflective coatings,24 photocatalysis,25,26 and other applications.24,27−30 In these cases, the nanoparticle films are typically postannealed to form sintered, hyperbranched, mesoporous architectures of polycrystalline nanowires. However, a few reports have shown that nanoparticle aggregates formed at relatively high pressures (e.g., 10 Torr), in pulsed laser ablation plumes can be delivered as the feedstock for the direct synthesis of crystalline nanostructures on hot substrates, such as single-crystal ZnO nanorods, in a process termed nanoparticle-assisted PLD (NAPLD).31−33 Here, with time-resolved in situ diagnostics, high-resolution electron microscopy observations of nanoparticle crystallization and sintering, and molecular dynamics modeling, we reveal the nonequilibrium conditions for the gas-phase synthesis and sequential PLD of ultrasmall amorphous nanoparticles, and their crystallization by particle attachment at elevated temperatures for the direct formation of crystalline nanostructures. We use TiO2 as a well-known model system and demonstrate how interesting crystalline phases, such as TiO2(B) can be directly synthesized. The pulsed laser deposition of amorphous ultrasmall nanoparticles and their dynamic crystallization by particle attachment demonstrated here is a promising technique 4625

DOI: 10.1021/acs.nanolett.7b01047 Nano Lett. 2017, 17, 4624−4633

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Nano Letters

Figure 2. Plume conditions for nanoparticle formation and deposition. (a) Intensified CCD array imaging of visible plasma plume luminescence at the indicated times following laser ablation (20 ns to 5 μs exposures, false color, normalized peak intensities relative to 14 000 counts of 40 μs image). Dashed lines at d = 0 (target) and 6 cm (substrate). (b) R−t plot of the plasma plume front (defined as the point of 5% peak intensity). The propagation is best described (blue line) by the a = −αυ2 drag model, where υ = υ0/(1 + αυ0t); R = α−1 ln(1 + αυ0t), where α = 0.91 cm−1 and υ0 = 5.1 cm/μs. By comparison the linear a = −αυ drag model fit {red dashed curve, where R = (υ0/α)[1 − exp(−αt)], α = 0.21 μs−1} predicts that the plume slows to stop at R = 5.4 cm. (c, d) Cross-sectional and top view SEM images of room-temperature TiO2 nanoparticles deposited onto a Si substrate at 200 mT oxygen background pressure and 5 cm away from the target. (e, f) TEM images showing aggregates of (∼3 nm) nanoparticles deposited onto the TEM grids.

shown in Figure 1c−g. The morphology of the samples changes dramatically and continuously from dense films in vacuum to fluffy, fractal aggregates of nanoparticles at 200 mTorr. With the attenuation of the high kinetic energy species and the emergence of a comparable “second” component of the ion flux in Figure 1b, the films become less dense, rough, and columnar as in Figure 1d,e. After the elimination of the “fast” component, and during the elimination of the “second” component between 120 and 200 mTorr, the films become nearly entirely composed of aggregates of ultrasmall nanoparticles as shown in Figure 1f,g. As shown in the images of Figure 2a and the distance−time (R−t) plot of the leading edge of the visible plume in Figure 2b, the forward-directed progression of the visible plasma plume essentially stops after d = 5.4 cm. A linear drag model, a = −αυ, that relates the plume deceleration a to its velocity υ with a drag coefficient, α, results in the red dashed curve fit in Figure 2b. However, the plume, in fact, continues to diffusively expand more slowly in all directions as fit by a more accurate a = −αυ2 drag model (blue curve), where υ = υ0/(1 + αυ0t), υ0 is the initial velocity of the visible ablation plasma, and α is the drag coefficient. Following the disappearance of the visible plasma plume, we have previously shown that nanoparticles nucleate and grow in the gas phase from this thermalized, diffusively expanding plume material.16,34 Emission spectroscopy measurements confirmed that the weak emission in the long-lived fluorescence observed in Figure 2a at 200 mTorr can be attributed principally to oxide bands, as we have observed previously.16 Over hundreds of microseconds to milliseconds of time, previously we utilized a time-delayed second laser sheet beam to show how ground state oxide species (via laserinduced fluorescence) disappear and nanoparticle aggregates become visible (via Rayleigh scattering) in the thermalized plume region between target and substrate.16 Here, in similar measurements at high laser repetition rates, we were able to observe Rayleigh scattering from gas-suspended aggregated nanoparticles that were organized into bands near the substrate resulting from multiple laser shots, a phenomenon that will be subject of a separate report. However, at the low repetition

increasingly more spatially confined due to collisions between the ejected target atoms, ions, and molecules and those of the background gas. These collisions snowplow the background gas molecules forward, forming a weak “shock front” that is visible due to the bright fluorescence resulting from the collisional processes within the confined plasma.35,36 However, a fast, “dark” component of the plume not visible in the imageshigh kinetic energy atoms and ions that have penetrated the background gas without collisionsis also present during some of these conditions, as revealed by the ion probe current waveforms in Figure 1b. The saturated ion probe current (measured by voltage across 50 ohms, detector area = 1 mm2) waveforms represent a measure of the relative flux (Nυ, where N and υ are ion density and velocity, respectively) of positive ions penetrating through the background gas at each time. The magnitude of the peak current (and also the overall integrated charge) arriving to the ion probe drops exponentially with pressure at a given distance (and drops exponentially with distance at a given pressure) in accordance with a scattering model with a hard sphere collision cross section,34,35,37 which for the data of Figure 1b yields the value of 1.7 × 10−16 cm2. As shown in Figure 1b, this “fast” component of the plume arrives with nearly the identical velocity distribution as in vacuum and eventually separates from the time-of-flight velocity distribution that corresponds to the visible, “slowed” component of the plume. This “plume splitting” phenomenon has been explained in the context of a scattering model, which describes the shape of the two waveforms as composite distributions of target atoms undergoing discrete numbers of collisions en route to the substrate.34,37 As the pressure increases in Figure 1, the ion probe reveals that all of the “fast” component (high kinetic energy) ions have disappeared after ∼100 mTorr and that between 100 and 200 mTorr the ions corresponding to the slowed “second” component also have been attenuated to 50% of the Ti atoms have already formed octahedral oxygen bonds.

of 19 200 atoms) as shown in Figure 6a at 600 °C. Our MD simulations reveal that the amorphous nanoparticle attaches within t ∼ 10 ps, with some interdiffusion of its atoms into the topmost layer of the TiO2(B) crystal. Starting from a metastable amorphous nanoparticle with 8% TiO6 octahedra, the nanoparticle then proceeds to crystallize such that within t ∼ 2 ns as shown in Figure 6b, >50% of the Ti atoms have formed octahedral oxygen bonds. Importantly, the contact with the substrate crystal induces 30% more octahedral bonds than for nanoparticles that were isolated and exposed to the same temperature increase. Although demonstrated in one specific configuration, the results clearly indicate that at the relatively low temperatures involved in our experiments, amorphous nanoparticles can rapidly attach and achieve significant crystallization induced by the substrate within nanoseconds. To estimate the time scales for integration of the attached nanoparticles into the nanorod at our temperatures, which we refer to as sintering, experimentally we note that we could find crystalline nanoparticles that had not integrated fully into the nanorod tip following PLD at 1 Hz at 400 °C, as shown in Figure 4. However, for depositions at 600 °C and above, we found smooth nanorod top surfaces for 1 Hz laser repetition rates, indicating that nanoparticles become sintered and incorporated in those time scales. Both Seto et al.49 and Kobata et al.50 experimentally studied the gas-phase sintering of TiO2 nanoparticles in floating aggregates and derived scaling relationships based upon a surface diffusion neck growth model51 from their data that are used today for comparison with results of MD simulations.44 Sintering was found to follow an Arrhenius temperature dependence, and the sintering time for particles was found to scale as d4, where d is the nanoparticle diameter.49,50 Using the experimental relationship from Seto summarized in ref 44 (where it should be noted that, based on MD simulations, it is predicted that smaller nanoparticles deviate from that relationship to sinter much faster than the 10−100 nm particles used in ref 49), we can estimate upper limits for the sintering time of the 3 nm diameter TiO2 nanoparticles in our study of 0.1 ms at 800 °C, 70 ms at 600 °C, and 2100 s at 400 °C. These estimates support the hypothesis that nanoparticles deposited at 600 and 800 °C can sequentially attach and sinter into the evolving nanorods within the 1000 ms interpulse arrival time of particles in our PLD experiments, in agreement with our observations. However, also in agreement with our experiment, due to the steep Arrhenius behavior, the nanoparticles at 400 °C are predicted to not become fully integrated at the top of the nanorods during this time, yet may sinter to be completely integrated nearer the bottom of the nanorods during the ∼1800 s time of the deposition.

mesoporous, hyperbranched, polycrystalline electrodes used in dye-sensitized solar cells and other applications. This data supports the conclusion that ultrasmall nanoparticles deposit and become integrated sequentially, and not as aggregates, to enable the evolution of nanostructures at higher temperatures into smooth columnar shapes. A similar integration of amorphous TiO2 nanoparticles into crystalline TiO2 nanorods has been reported during the atomic layer deposition of TiO2 by the group of Wang et al.10,43 However, in that work, metastable amorphous nanoparticles were observed to grow from within an ALD-deposited ∼10nm-thin amorphous film on an existing crystalline nanorod, and their crystallization was observed within this percolating film to form anatase, then rutile phases according to the Ostwald− Lussac law.10 Nanoparticles within this shell were observed to migrate at higher temperatures (600 °C) to the lower energy facets at the top of the nanorod, where they aligned and attached by the process of oriented attachment, a process witnessed for the first time by vapor-phase attachment. The mobility and ability to reorient was thought to have been facilitated by the presence of hydroxyl groups from the H2O cycles in the ALD process, indicating that surface functional groups in addition to the metastable amorphous shell are key parameters to allow the orientation and integration of nanoparticles during vapor phase CPA.10 The gas-phase CPA process described here appears to be very similar; however, the metastable nanoparticles are condensed in the background gas within the thermalized laser ablation plume and are delivered directly to the tip of the nanorod, foregoing the need for migration before crystallization and attachment. Some mobility of the particles may result from their residual kinetic energy and also acoustic forces imparted by the weak shock front propagating through the snowplowed background gas. The time in our process for the nanoparticles to attach, crystallize, and sinter to become integrated within the nanorod is the subject of ongoing experimental and computational studies. Prior work using molecular dynamics methods to simulate the collision of two ∼3 nm TiO2 nanoparticles has indicated that particles with different initial phases can attach and recrystallize into a common phase as quickly as several nanoseconds at 1200 °C.44−47 However, achieving the necking and coalescence of the two particles within relatively lengthy ∼150 ns timeframes required accelerated MD methods at very high temperatures (1527 °C).44 To gain insight into the processes and time scales (t) involved in our experiments at much lower temperatures, we performed MD simulations based on the Matsui−Akaogi potential describing interatomic interactions48 for a 3 nm amorphous nanoparticle containing 627 atoms arriving at a TiO2(B) nanocrystal surface (consisting 4630

DOI: 10.1021/acs.nanolett.7b01047 Nano Lett. 2017, 17, 4624−4633

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Nano Letters Note also that, based upon the d4 dependence of the sintering time, 10 nm nanoparticles should sinter ∼120 times more slowly at a given temperature than ultrasmall (∼3 nm) nanoparticles. As the ultrasmall nanoparticles sinter within the aggregate chains to form much larger crystallites, their evolution slows to achieve a terminal particle diameter, as in Figure 5. This explains why sequential deposition of individual, ultrasmall nanoparticles as “building blocks” is advantageous over the deposition of aggregates. In conclusion, the conditions for pulsed laser deposition (PLD) can be adjusted to selectively form and deliver ultrasmall (∼3−5 nm) amorphous TiO2 nanoparticles in the gas phase as “building blocks” for dynamic crystallization into nanostructured films with metastable phases and interesting morphologies. The nanoparticles condense in the decelerating laser plasma plume due to background gas collisions and can be deposited at distances beyond the atomic and molecular component of the plume due to the longer ballistic range predicted for individual nanoparticles. The in situ plume diagnostics, TEM observations of nanoparticle sintering, and associated modeling support the conclusion of crystallization by particle attachment (CPA) from sequentially deposited, individual, ultrasmall nanoparticles as the mechanism for the growth of crystalline nanostructures at the 1 Hz repetition rates and temperatures used in these experiments. Several CPA mechanisms are possible for the rapid integration of the UNPs into the crystalline nanostructure, depending on whether the individual amorphous UNPs crystallize during the penetration of the gas-phase thermal gradient near the heated substrate surface. These range from addition of amorphous nanoparticles and crystallization from the growing nanocrystal surface, to oriented attachment, Ostwald ripening, and nonoriented attachment of crystalline nanoparticles followed by recrystallization. The adjustment of conditions in PLD for the deposition of UNPs as “building blocks” of crystalline nanostructures appears quite general and can be used to explore the synthesis of not only a variety of crystalline oxide nanostructures, but also other materials, such as two-dimensional metal chalcogenides.52,53 This report indicates the promise of exploration and exploitation of ultrasmall nanoparticles as intermediate “building blocks” in the formation of nanostructured films with novel phases. Methods. Pulsed Laser Deposition and in Situ Diagnostics. Gas phase condensation in PLD was used for synthesizing TiO2 UNPs as building blocks for the CPA process (Figure S1 in Supporting Information). A 2 in. TiO2 pellet (Praxair Company), composed of rutile powder, was used as the ablation target in this experiment. A pulsed KrF (248 nm, 1 J cm−2, 1 Hz) laser was used for the ablation of the targets in oxygen background gas to produce a spatially confined laser ablation plasma plume that thermalized over a 5 cm stopping distance, and resulted in the synthesis of UNPs with a diameter of 2−5 nm. A 1-in.-diameter stainless steel heater (HeatWave Laboratories, Inc.) was placed d = 5 cm away and parallel to a 5-cm-diameter TiO2 target in a cylindrical chamber (50 cm inner diameter, 36 cm tall) to deposit the nanoparticles at various substrate temperatures. The gas pressure was controlled with a mass flow controller (Ar 99.995%, 0−500 sccm) and a downstream throttle valve. The heater temperature was controlled by an integrated thermocouple to ±2 °C. The plume imaging was performed with an ICCD camera (Princeton Instruments, PI-MAX) with variable gating (minimum 5 ns). The f4.5 Nikon camera lens was positioned

46 cm away from the plume center, outside the chamber, looking at the plume through a 2-in. × 8-in. Suprasil window. The exposure time and f-stop were varied from 5 ns to 5 ms and 32 to 4.5, respectively. The ion probe time-of-flight waveforms (voltage measured across 50 ohms, detector area = 1 mm2) were acquired as a function of pressure and distance during the ablation process. TEM Characterization of Nanoparticle Sintering. For observation of crystallization dynamics in TEM, amorphous TiO2 nanoparticles were laser-synthesized and deposited directly onto lacey carbon TEM grids at room temperature, forming aggregated nanoparticle networks. The grids were then transferred onto a heating stage and inserted into the TEM for high-resolution electron microscopy (FEI Titan S aberrationcorrected TEM-STEM, acceleration voltage of 200 kV). The crystallization and sintering dynamics of a small cluster of nanoparticles that were overhanging an empty grid hole were observed in situ at different temperatures using video capture at 25 frames/second and 8 ms exposure time for each frame. The high-resolution TEM of nanoparticles deposited by PLD at high temperatures and then cooled to room temperature was performed with a Zeiss Libra 200 MC TEM at an acceleration voltage of 200 kV, and also with a fifth-order aberrationcorrected STEM Nion UltraSTEM 200. Raman Spectroscopy and XRD Characterizations. A Renishaw Raman spectroscopy system (532 nm excitation source focused onto the samples through a 100×, 0.8 NA objective lens) was used to obtain the Raman spectra of the samples. An X-ray diffractometer (Panalytical X’Pert MPD Pro) with Cu-Ka radiation (λ = 1.54050 Å) was used to acquire XRD patterns (Figure S2 in Supporting Information).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01047. Schematic representation of the PLD setup, XRD data, Raman spectra, and room temperature TEM images of the nanoparticle (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Masoud Mahjouri-Samani: 0000-0002-6080-7450 Mengkun Tian: 0000-0003-2790-7799 Miaofang Chi: 0000-0003-0764-1567 Gyula Eres: 0000-0003-2690-5214 Mina Yoon: 0000-0002-1317-3301 Kai Xiao: 0000-0002-0402-8276 David B. Geohegan: 0000-0003-0273-3139 Author Contributions

M.M-S. performed the synthesis experiments. M.T, M.C., and G.D. carried out the TEM studies and analysis. A.A.P., D.B.G., and M.M-S. participated in spectroscopic diagnostic experiments. M.Y. and J.L. performed MD simulations and analysis. All of the authors contributed to data analysis, discussed the results, wrote, and commented on the manuscript. 4631

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Nano Letters Notes

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This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paidup, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Synthesis science including PLD, in situ plume diagnostics, XRD, TEM analysis, SEM and AFM studies, and MD simulations (M.M.-S., M.T., D.B.G., C.M.R., A.A.P., G.D., K.W., K.X., G.E.) were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division and performed in part as a user project at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The authors gratefully acknowledge the assistance of A. Lupini on the Z-STEM measurements. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under contract no. DE-AC02-05CH11231.

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DOI: 10.1021/acs.nanolett.7b01047 Nano Lett. 2017, 17, 4624−4633