Real-Time Observation of Nanosecond Liquid ... - ACS Publications

Nov 12, 2012 - Thomas LaGrange,. †. Bryan W. Reed,. †. Geoffrey H. Campbell,. † and Philip D. Rack*. ,‡,§. †. Condensed Matter and Material...
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Real-Time Observation of Nanosecond Liquid-Phase Assembly of Nickel Nanoparticles via Pulsed-Laser Heating Joseph T. McKeown,† Nicholas A. Roberts,‡ Jason D. Fowlkes,§ Yueying Wu,‡ Thomas LaGrange,† Bryan W. Reed,† Geoffrey H. Campbell,† and Philip D. Rack*,‡,§ †

Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California 94550, United States Materials Science and Engineering Department, The University of Tennessee, Knoxville, Tennessee 37996, United States § Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

S Supporting Information *

ABSTRACT: Using pump−probe electron microscopy techniques, the dewetting of thin nickel films exposed to a pulsed nanosecond laser was monitored at tens of nanometers spatial and nanosecond time scales to provide insight into the liquidphase assembly dynamics. Thickness-dependent and correlated time and length scales indicate that a spinodal instability drives the assembly process. Measured lifetimes of the liquid metal are consistent with finite-difference simulations of the laserirradiated film and are consistent with estimated and observed spinodal time scales. These results can be used to design improved synthesis and assembly routes toward achieving advanced functional nanomaterials and devices.

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mechanism is described as rupturing of the film by amplification of unstable surface thermal fluctuations, which eventually results in nanodroplets that are spatially well organized. As has been shown,26−28 when the second derivative of the excess free energy (φ) of a thin film system with respect to film thickness (h) is negative, unstable perturbation modes can propagate and a spinodal instability may occur. Based on the analysis of excess free energy, spinodal dewetting morphologies have been investigated for different metal films.4,19,22−24 Among these studies, two typical morphological pathways of spinodal dewetting have been observed: the socalled bicontinuous structures and circular holes. One characteristic of the spinodal process is that both pathways result in correlated particle spacings that are set by the fastest growing surface perturbation wavelengths, in contrast to randomly distributed droplets formed via nucleation (unless a patterned substrate is templated to induce correlated heterogeneous nucleation events). To this end, several studies4,19,22−24 have correlated the spatial characteristics of the final film morphology with the initial film thickness in order to estimate an effective interface potential between the liquid film and substrate. This potential significantly influences the spinodal breakup. Heretofore, the nanosecond liquid-phase dewetting dynamics have been inferred largely by ex situ observations of the

INTRODUCTION The synthesis and organization of functional nanomaterials via bottom-up self- and directed assembly represents a critical challenge for the future of nanoscience, as well as for the expanding interest in structures with mesoperiodicity to obtain two- and three-dimensional (2D and 3D) arrangements with mesoscale functionality at reduced cost and higher efficiency.1 Generating arrays of nanoparticles with controlled size and spatial distributions is key to this challenge, and processes that exploit morphological instabilities offer the potential to attain these fine-scale spatially correlated structures. There has been long-standing interest in the capillarity and surface tension effects on morphological evolution in various materials systems, dating back to the work of Plateau2 and Rayleigh.3 Recently, pulsed-laser-induced dewetting of two-dimensional films,4−10 one-dimensional lines and rings,10−13 and lithographically patterned nanostructures14,15 has demonstrated that understanding and controlling thin-film and Rayleigh-Plateau instabilities improves the ability to create organized metallic nanoparticle ensembles. Assembled particles have also been shown to “jump” or eject from one substrate and transfer to another by controlling the energetics and dynamics of various laser-melted nanostructures.16−18 The spinodal dewetting of liquid metal thin films has been studied in detail4,6,19−25 and shows promise as a self-assembly process for synthesizing large-area correlated nanoparticle ensembles. The spinodal dewetting mechanism dominates the dewetting of thin liquid metal films, generally with film thicknesses ranging from one to tens of nanometers, and the © 2012 American Chemical Society

Received: September 11, 2012 Revised: October 30, 2012 Published: November 12, 2012 17168

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and a total energy of 4.14 μJ. The simulation considers a 10-nm-thick nickel film on a 15-nm-thick silicon nitride membrane. The reflectivity and effective thicknesses were compensated for, as the laser was exposed at 45° to the substrate normal. The thermophysical properties of nickel and silicon nitride are considered, and a suppressed melting temperature of ∼1676 K for 10-nm-thick nickel is assumed.39 Zero optical absorption in the silicon nitride is assumed. Radiative heat loss is included at the top of the nickel film. Thermal conduction at the system boundaries is not considered due to the low pressure in the TEM.

resolidified metal quenched after different liquid lifetimes,29 as unlike polymeric systems where the dynamics based on viscosity and surface energy are accessible in the second, minute, or even hour time scales, thin-film metal dewetting uniquely has nanoscale length and time scales due to low viscosity and high surface energy. Herminghaus et al. used in situ reflectometry to measure the liquid lifetime of pulsed-laser irradiated gold films,4 and several others5,22−24 have calculated the time−temperature profiles and liquid lifetimes using both analytical and finite element or differencing models. Liquid nickel has an equilibrium contact angle with silicon nitride of ∼120°30 (consistent with a 104° ± 10° angle measured via tilted scanning electron microscope (SEM) images of resultant dewet nanoparticles; Supporting Information, Figure S1), and therefore, liquid nickel thin films can be expected to dewet silicon nitride substrates. To gain further insight into the assembly dynamics, we present initial results from in situ nanoscale (time and length) observations using the dynamic transmission electron microscope (DTEM)31−38 of the dewetting of a 10-nm-thick nickel film that has been pulsedlaser melted.

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RESULTS AND DISCUSSION Figure 1 shows dynamic TEM images of a 10-nm-thick Ni film region with multiple time delays after exposure to a pulsed laser

EXPERIMENTAL SECTION

Experimental. Nickel films (10, 8, 6, and 4 nm thick) were dc magnetron sputter deposited directly onto 15-nm-thick silicon nitride TEM membranes (Ted Pella, Inc., Redding, CA). The 10 nm nickel films were then exposed to a variety of energies using a 1064 nm wavelength, 12 ns pulsed laser source with a Gaussian beam profile (1/ e2 diameter of 135 ± 5 μm), and an adequate melt threshold was observed using a total deposited energy of ∼4.1 μJ. Additionally, the 4, 6, and 8 nm films were exposed to 5.3, 4.7, and 4.1 μJ pulses, respectively, to accommodate the limited absorption depth of the films and compensate for the variation in the ratio of the Ni film thickness to that of the silicon nitride membranes. Subsequent to determining appropriate laser conditions for the nickel melting and self-assembly, the 10-nm-thick nickel films were laser-heated and in situ observations of the dewetting process were conducted in the DTEM operating in both conventional TEM and pulsed-electron modes at 200 kV. The DTEM is a modified JEOL 2000FX TEM that incorporates two nanosecond pulsed lasers to drive both the specimen and a photocathode,31 allowing pump−probe experiments with single-shot imaging and diffraction capabilities at high spatiotemporal resolution. Dynamic processes in the specimen are initiated with a 12 ns full-width at half-maximum (fwhm) pulse-width neodymium-doped yttrium− aluminum−garnet (Nd:YAG) laser (1064 nm wavelength); a neodymium-doped yttrium−lithium−fluoride (Nd:YLF) laser that is frequency quintupled to 211 nm stimulates a photoemission electron source to generate >2 × 109 electrons in a 15 ns pulse for imaging and diffraction. The time delay between the pump and probe laser pulses can be controlled from nanoseconds to hundreds of microseconds with a timing jitter of ±1 ns. The time scale is the time between the peak of the electron pulse relative to the peak in the sample laser pulse. Further details on the configuration of the DTEM can be found in prior publications.32−35,38 Imaging of the resultant nanoparticle arrays was conducted on a Philips CM300 FEG TEM equipped with a Gatan Imaging Filter (GIF). Simulations. The thermal model is a one-dimensional finite difference model based on the Fourier heat diffusion equation. The model uses a 1 nm spatial resolution and a time step of 1 fs to capture both the thermal absorption of the laser energy and the temporal profile of the laser pulse. The laser pulse is treated as a time and spatially dependent thermal generation term based on the absorption depth of nickel at the 1064 nm wavelength. The laser is simulated with a Gaussian profile in time with a 12 ns pulse width at fwhm. To estimate the temperature in the radial dimension, additional simulations were run where the local fluence was estimated by assuming a Gaussian profile in space with a 135 μm diameter at 1/e2

Figure 1. Dynamic TEM (15 ns exposure times) time-delay sequence of images for a 10-nm-thick nickel film after filtering and brightness and contrast equalization.

with a total deposited energy of 4.1 μJ. As described in the Experimental Section, the DTEM time labels for images and diffraction patterns are the delays between the arrival times of the peaks of the specimen pump laser pulse and the electron probe pulse at the specimen. Both pulse widths are ∼15 ns, and thus, each exposure (time-resolved diffraction pattern or image) is a time-averaged snapshot over 15 ns. Thus, for |t| < 10 ns, the two pulses overlap such that the sample is being rapidly heated during the electron exposure. Furthermore, for clarity, each image represents a different area of the deposited film that was laser treated and imaged under identical experimental conditions but at different time delays; thus, some pulse-topulse variability is anticipated. For the images in Figure 1, the lower left corners of the images are approximately the laser center. The images were filtered to remove noise and irrelevant intensity variations using a 3 × 3 median filter followed by a local brightness and contrast equalization filter using a Gaussian kernel with an rms width of 33 pixels in the x- and y-directions. The raw images are provided in the Supporting Information, Figure S2. We attribute the observed liquid-phase nickel nanoparticle self-assembly process to a spinodal dewetting process. To determine whether the spinodal mechanism is the appropriate mechanism that we are observing in our experimental regime, a series of films were deposited on silicon nitride membranes with varying thickness (h0 = 4, 6, 8, and 10 nm) and each was exposed to a single pulse at the same 4.1 μJ beam energy. In addition to this constant-energy series, in separate experiments, 17169

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Figure 2. (a) TEM bright-field images of the resultant nickel nanoparticle distributions after a single 15 ns, 4.1 μJ pulsed laser exposure. The initial film thickness is noted in the upper right-hand corner. (b) Plot of the measured correlated nanoparticle spacing versus film thickness (circle data points) and the best fit of the data (thin line) for the effective Hamaker constant of 3.8 × 10−18 J. Also plotted on the right y-axis (square data points) is τm for the maximum spinodal instability time scale as a function of thickness.

the 4 and 6 nm films were exposed to higher energies since the thicknesses were below the absorption depth; the calculated energies necessary to yield peak temperatures comparable to that of the 10 nm film were 5.3 and 4.7 μJ for the 4 and 6 nm films, respectively (see the Supporting Information). Subsequent to irradiating the films on the nitride membranes, we imaged the nanoparticle distributions as a function of thickness and laser beam radius. Figure 2a shows a series of TEM images of the central laser spot region for different thicknesses, and Figure 2b is a plot of the most common nanoparticle spacing at the center region of the irradiated area based on a spatial correlation function (SCF) analysis versus film thickness. Following the procedure and disjoining pressure model, including both short- and long-range forces, outlined in Wu et al.,10 Figure 2b also shows the best fit (rms error ∼ 13 nm) of the particle spacing versus film thickness data, which yields an effective Hamaker constant of 3.8 × 10−18 J for the van der Waals interaction of the liquid nickel surface and solid silicon nitride film. This value is the same order of magnitude as other experimentally determined liquid metal−solid Hamaker values.12,19,24,40 Assuming the peak in the SCF is proportional to the wavelength of maximum growth (λm) we can estimate the maximum perturbation growth rate (σm) by10 σm =

consistently below 0.5 nm. Previously, we have compared Fourier transforms of AFM images with those of the resultant nanoparticles, and the correlated length scales revealed in the AFM was orders of magnitude smaller than the resultant nanoparticle spacings.40 Furthermore, based on TEM images of the as-deposited 10- and 6-nm-thick films, the grain size of the as-deposited films was on the order of 10 nm and thus not correlated to the resultant nanoparticle spacings (Supporting Information, Figure S3). We also estimated the time scales of solid-state dewetting based on Mullins42,43 and modified by Favazza et al.29 (for details of the solid-state dewetting analysis, see the Supporting Information). The estimated time scale for a spinodal instability for the 10 nm film just below the melting temperature (solid state) was determined to be approximately 4 orders of magnitude longer (millisecond range) than just above the melting temperature (liquid state), which, as will be shown, is well beyond the simulated time−temperature time scales. Thus, “thermal grooving”42,43 associated with grain boundaries or surface roughness is not expected to be operative. Finally, to determine if significant mass loss occurred during the pulsed laser heating, we determined the effective nanoparticle volume from a TEM image near the center and near the edge of the pulsed laser region for each film thickness (see Supporting Information). No statistically relevant change was observed for the center versus the edge images, and Table 1 reveals that the calculated mass loss is ≤21%, which is comparable to our experimental error of approximately ±1 nm. Thus, no appreciable evaporation or substrate diffusion is believed to have occurred. Since the nanosecond pulsed laser used to expose the nickel films has a Gaussian energy profile, the intensity varies as a function of radial position and thus can reveal interesting temporal information. Figure 3 shows conventional bright-field TEM images as a function of radial position for the 10-nm-thick film taken approximately every 3−5 μm (for comparable images

16πγh0 3 3μλm 4

where γ is the liquid surface energy, for which we used 1.7 J/ m2,30 and μ is the liquid viscosity, with a value of 4.76 × 10−3 Pa·s.41 Furthermore, the spinodal instability time scale is simply τm = 1/σm. Figure 2b also shows the consequent spinodal time scale as a function of original film thickness on the right y-axis. To confirm that the particle spacing is not attributed to the initial thin-film microstructure, we performed atomic force microscopy on as-deposited nickel films on silicon nitride and determined that the surface roughness for all the films was 17170

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Table 1. Original Thickness, Calculated Effective Thickness after Laser Irradiation, and Calculated Mass Ratio of the Nickel after and before Laser Irradiation laser energy (μJ)

h (nm)

hc (nm)

hc /h

4.1 4.1 4.1 4.1 4.7 5.3

10 8 6 4 6 4

10.0 6.9 4.8 3.4 4.8 3.6

1.00 0.87 0.79 0.84 0.80 0.89

of the 4 nm film, see Supporting Information, Figure S6). Figure 3a provides images beyond the saturated nanoparticle region, showing intermediate stages of hole formation and hole coalescence. The images reveal that individual holes grow, merge with neighboring holes, and form pseudolinear liquid wires or rivulets that can subsequently break up via the Rayleigh−Plateau instability to form nanoparticle arrays with correlated length scales. The outermost holes could be attributable to random nucleation events as described previously.4,19 Based on the contrast of nanoparticles in TEM images, it is evident that the majority are polycrystalline; however, single crystal particles are also observed. Figure 4 shows higher magnification images of individual nanoparticles from the dewet 10 nm film, revealing the three distinct types of observed particle morphologies: polycrystalline, single crystalline, and twinned single crystalline. Twinning is observed in both polycrystalline and single-crystalline particles, consistent with rapid solidification. Zone-axis selected-area diffraction (SAD) patterns are inset in Figure 4b and c, both recorded along the [110] zone-axis, further showing the particles are single crystals. The diffraction pattern in Figure 4c clearly shows the {111}type twin planes, as expected for fcc nickel. Time-resolved SAD patterns were also recorded as a function of time to complement the dynamic real-space imaging and estimate the nickel liquid lifetime. Figure 5a shows a series of SAD patterns taken at various delay times relative to the specimen pump laser’s interaction with the specimen (including as-deposited and postlaser-pulse diffraction patterns using a long exposure time relative to that of the time-resolved diffraction experiments). The simulated diffraction pattern for

Figure 4. Bright-field TEM images of individual nanoparticles from the dewet 10 nm film showing (a) polycrystalline, (b) singlecrystalline, and (c) twinned single-crystalline particles. The inset diffraction patterns in (b,c) further illustrate that the particles are single crystals, and the twinning is evident in the diffraction pattern in (c). Both diffraction patterns were recorded in the [110] zone axis. The scale bar in (a) applies to all three images.

polycrystalline nickel is included in the long-exposure asdeposited diffraction pattern. The conventional diffraction pattern from the as-deposited film shows broad diffraction

Figure 3. Bright-field TEM images as a function of the radial position in the pulsed laser region of a 10 nm nickel film. Images progress from the outer radius toward the center moving from left to right across a row. Images in (a) were captured at a higher magnification to reveal more detail in the outer heat-affected zone and (b) at lower magnification for the saturated nanoparticle region. The scale bars in the far right images are applicable to all images in the row. 17171

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Figure 5. (a) Diffraction patterns showing the time evolution of the film as it dewets the substrate. The initial diffraction pattern recorded from the as-deposited film shows the simulated polycrystalline Ni diffraction pattern with the experimental pattern, recorded with a 1 s exposure time. The time evolution was followed in 5 ns intervals using 15 ns exposure times. (b) Radially averaged intensity as a function of reciprocal lattice vector, G, for all diffraction patterns shown in (a). (c) Plot of the fwhm of the peak centered around the {111} peak position (4.92 nm−1) as a function of delay time between the specimen pump laser and electron probe pulse.

rings due to finite crystal-size effects, indicating that the film is nanocrystalline, which is consistent with bright-field images of the as-deposited film (see Supporting Information, Figure S3). After the film has dewet, the diffraction pattern shows spotted rings due to the larger-grain polycrystalline morphology. While there appears to be a slight {111} preferred orientation in the as-deposited thin film,44−46 there is no preferred orientation or texture observed in the dewet film. Inspection of the time-resolved diffraction patterns in Figure 5a reveals changes in the structure of the nickel film indicative of melting and resolidification.47 There is a noticeable change in the short-range order at times as short as the first 10 ns (−5 ns diffraction pattern) after interaction of the pump laser with the specimen, evidenced by a broadening of the {111} diffraction ring and a disappearance of higher-index diffraction rings. Figure 5b shows radially averaged patterns of the experimental

data in Figure 5a that more clearly illustrate the dynamics of the laser melting process. Following the progression of the peaks centered about the {111} and {200} lattice spacings, a broadening of both peaks is observed beginning at around −5 ns. The {111} peak continues to broaden and becomes diffuse with time, while the intensity of the {200} peak decreases with time, disappearing altogether by 5 ns, at which time the diffraction pattern consists of a single diffuse diffraction ring that is characteristic of a liquid, indicating that the region of the film from which the diffraction pattern was obtained is completely melted. Intensity in the {200} peak begins to return at ∼25 ns. This broadening of the {111} diffraction feature (4.92 nm−1) is shown in Figure 5c, which plots the fwhm of the {111} peak as a function of time. The fwhm values were obtained by subtracting the background from the radially averaged patterns and fitting the {111} peak to a 17172

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temporal information: the film melts first in the center and there is a progressive delay in melting as a function of radius, as captured by the images in Figure 1 and consistent with the thermal simulations. As a final point, the spinodal time scale (26 ns) of the 10-nm-thick film determined by the thicknessdependent correlated nanoparticle spacing is consistent with (1) the experimentally determined liquid lifetime from the temporal diffraction patterns, (2) the dynamic TEM images, and (3) the simulated liquid lifetime.

Lorentzian profile. The initial fwhm of the {111} peak in the asdeposited film is 0.36 nm−1 and reaches a maximum of 1.15 nm−1 at 10 ns, after which it begins to decrease as the film resolidifies. The diffraction data and analysis in Figure 5 indicate a liquid lifetime of ∼20 ns (±7.5 ns half width of the electron beam pulse width). To complement the dynamic TEM imaging and diffraction of the pulsed-laser-induced self-assembly process, one-dimensional (film thickness dimension) finite difference simulations were used to generate a series of time−temperature profiles for the 10-nm-thick nickel film supported on the 15 nm silicon nitride. Multiple simulations were performed at different power densities to mimic the radial energy profile of the Gaussian laser beam (see Experimental Section for details and Supporting Information, Figure S7, for complementary 4 and 6 nm simulations). Figure 6 shows a series of time−temperature

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CONCLUSIONS In summary, we have for the first time captured the nanoscale time and spatial dynamics of the liquid-phase self-assembly of a 10-nm-thick nickel film suspended on a silicon nitride membrane into a nanoparticle array. The correlated time and length scales of the assembly process suggest that assembly is induced via a spinodal thin-film instability. Dynamic TEM imaging captures the temporal dynamics of the spinodal dewetting evolution and time-resolved electron diffraction analysis confirms the metallic liquid lifetimes, determined to be ∼25 ns. Complementary finite-difference modeling of the laser-irradiated films agrees well with the measured liquid lifetime and the observed radial profiles of the TEM images that are generated as a result of the Gaussian laser profile.

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ASSOCIATED CONTENT

S Supporting Information *

SEM image of dewet nickel film (Figure S1) from which contact angles were obtained, raw DTEM images (Figure S2) prior to filtering and contrast equalization, grain size calculation (Figure S3), estimation of the growth rates for a spinodal instability (Figure S4), calculated mass loss during experiments (Figure S5), dewet film morphology as a function of radial position for a 4-nm film (Figure S6), finite difference simulations (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. Simulated nickel time−temperature profiles for the 10-nmthick nickel film as a function of radial position. The left y-axis showing the temperature corresponds to the three radial positions, while the right y-axis showing the irradiance corresponds to the Gaussian laser pulse. The dashed gray line indicates the suppressed melting temperature of a 10-nm-thick Ni film (∼1676 K).

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profiles as a function of radius. While we do not want to overanalyze the comparison between the experimental observations and the thermal simulations due to their simplicity, the simulated results are consistent with our interpretation of the experimental results. The thermal simulations have a slightly lower liquid lifetime in the center (∼9.5 ns) than measured by the dynamic SAD (20 ns) and estimated spinodal lifetime (26 ns); however, the agreement is reasonable. Additionally, though the simulated melt radius (∼5 μm) is smaller than the experimentally observed liquid zone of 20−30 μm as indicated by low-magnification images of the irradiated region consisting of saturated nanoparticles, the radial decay in the temperature due to the Gaussian laser spot is also consistent with the observed radial morphological changes observed in the in situ and ex situ imaging. Finally, the simulated time that melting starts (∼10 ns) is in good agreement with the experimentally observed SAD liquid phase onset of ∼5 ns (±7.5 ns). Finally, we return to the temporal images in Figure 1. The 0 and 5 ns images do not show any hole or particle formation while the diffraction pattern and thermal simulation suggest that the exposed region of the film is melted at 5 ns. The 15 ns image reveals saturated nanoparticles in the center ∼5 μm radius and progressive hole formation at larger radius. The 20 ns image nicely shows that the saturated nanoparticles extend to ∼10 μm and the hole formation extends to larger radius. Thus, the radial profile of the images also provides interesting

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Telephone: 865-974-5344. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.D.F, P.D.R., and Y.W. acknowledge support from the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division for supporting the portions of this work related to understanding the liquid phase assembly dynamics and analysis. N.A.R was supported by an ASEE/NSF fellowship. J.D.F., P.D.R., and N.A.R. also acknowledge that the nickel sputter deposition was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. Work at Lawrence Livermore National Laboratory was performed under the auspices of the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract DE-AC52-07NA27344.

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