Size-Dependent Phononic Properties of PdO Nanocrystals Probed by

Article pubs.acs.org/JPCC

Size-Dependent Phononic Properties of PdO Nanocrystals Probed by Nanoscale Optical Thermometry Rizia Bardhan,*,† Holly F. Zarick,† Adam Schwartzberg,‡ and Cary L. Pint§ †

Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee, 37235 United States The Molecular Foundry, Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720 United States § Department of Mechanical Engineering, Vanderbilt University, Nashville, Tennessee, 37235 United States ‡

S Supporting Information *

ABSTRACT: With the advent of novel nanoscale devices, fast and reliable thermal mapping with high spatiotemporal resolution is imperative for probing the characteristics of phonons and evaluating the local temperature at the nanoscale. In this work, Raman spectroscopy is employed as a rapid and noncontact optical thermometry technique to investigate phononic properties of macroscopic assemblies of monodisperse palladium oxide (PdO) nanocrystals. PdO has been extensively employed in high temperature catalytic devices; however, the phonon behavior which determines the thermal stability of PdO remains unexplored thus far. Our study focuses on homogeneous, large-scale assemblies of monodisperse 4 and 10 nm nanocrystals synthesized using colloidal chemistry to understand size-dependent effects on the measured thermal properties. By monitoring the Raman peak shifts, peak broadening, and alterations in peak intensities as a function of laser power and particle concentration, a sizedependent trend is observed attributable to confinement of optical phonons within nanocrystal grain boundaries and laserinduced heating, both influenced by nanocrystal size. This study correlates size-dependent single-particle heating effects with sizedependent interparticle heat transfer under laser irradiation and is enabled by controlled nanocrystal synthesis.

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Conventional routes, such as microheating12,13 and threeomega techniques,14,15 require carefully microfabricated devices with measurements that give resolution of only a few micrometers. Interfacial contact resistances also give rise to significant errors with these methods. Infrared thermometers are commonly employed to extract temperature based on the difference in emission of sample at different temperatures; however, poor temporal and spatial resolution hinders their use in devices with submicrometer and nanometer features. Optical thermometry techniques including Raman, fluorescence, and near-field scanning optical microscopy are noncontact and nondestructive probes and provide the overall thermal history of an optical event with high spatiotemporal resolution.16−20 These techniques enable the measurement of thermal profiles

INTRODUCTION Semiconducting metal oxide nanostructures are ubiquitous in applications across field-effect transistors,1 optoelectronics,2 nanofluidics,3 catalysis,4,5 and energy conversion6,7 and energy storage systems.8,9 While improved device performance with nanoscale semiconductors has driven the miniaturization of device technologies, channel lengths in devices are now consistently smaller than the phonon mean free paths. In such nanoscopic devices, electron−phonon interactions and heat transport pathways are characteristically different from equivalent bulk devices.10 This necessitates a critical understanding of thermal processes at the nanoscale to enable the design of next generation high performance device architectures with low thermal losses and improved efficiency.11 However, despite prodigious research efforts, measuring phonon properties and mapping thermal behavior in nanostructured systems remains challenging due to the limitations of conventional tools applied to probe the local temperature at the nanoscale. © 2013 American Chemical Society

Received: July 12, 2013 Revised: September 18, 2013 Published: September 24, 2013 21558

dx.doi.org/10.1021/jp406916h | J. Phys. Chem. C 2013, 117, 21558−21568

The Journal of Physical Chemistry C

Article

In this work, we synthesized monodisperse PdO nanocrystals of two different sizes, 4 and 10 nm, by colloidal chemistry and examined their thermal behavior and phononic properties using Raman optical thermometry. By monitoring the alterations in the phonon peak as a function of laser power, we measured size-dependent phonon confinement effects and size- and particle density-dependent temperature rise of PdO nanocrystal assemblies due to laser induced heating. The latter of these is made possible by the formation of uniform macroscopic assemblies of PdO nanocrystals with highly monodisperse particle characteristics. By comparing the nanoscale phenomena with bulk PdO and incorporating a simple analytical model, a qualitative understanding of the origin of phonon frequency shifts and peak broadening in the Raman spectrum of PdO nanostructures has been established. Further, we have also derived the phonon lifetimes from the Raman spectra and demonstrated a clear size-dependence.

on nanoscopic features and have been recently applied to understand thermal properties of carbon nanotubes,21,22 graphene,23,24 Si nanowires,25−30 and other nanostructured semiconductors.31−33 Micro- and nanostructured palladium oxide (PdO), a small bandgap p-type semiconductor, has been extensively utilized in sensors,34 as photocathode materials for water electrolysis,35 and in many high temperature industrial catalytic devices such as for methane and ethanol oxidation,36,37 CO oxidation,38 and cyclohexene oxidation.39 PdO has a tetragonal crystal structure with space group symmetry of D4h.40 Palladium atoms occupy the D2h sites while oxygen atoms occupy the D2d sites responsible for the allowed phonon modes. When illuminated with a visible laser and probed by Raman spectroscopy, PdO exhibits four distinct phonon modes, B1g (∼ 650 cm−1), Eg (∼445 cm−1), and χ8 (278 and 725 cm−1) modes with B1g being the strongest phonon peak.40,41 While the catalytic properties of bulk PdO single crystals,38 their surface chemistry,42,43 and photoemission studies of their electronic structure44 has been well investigated, the photonic and phononic properties of PdO are currently poorly understood. Weber et al.40,45 investigated the phonon behavior of bulk PdO single crystals with Raman spectroscopy over a decade ago. Nevertheless, how the electronic and phononic characteristics of PdO are altered as the size changes from the bulk to the nanoscale remains elusive to date. Raman spectroscopy has been extensively used to examine the electron−phonon characteristics, monitor phonon decay and lifetimes, and probe light induced heating in nanostructured semiconductors providing a straightforward route to map the temperature rise at the nanoscale.16,46−48 Light-induced heating of nanostructured semiconductor/ metal composites has recently gained tremendous interest for heterogeneous catalysis.49−53 Laser induced heating in semiconductors involves the heating of conduction band electrons by the incident radiation and subsequent transfer of this energy to the lattice due to the thermal imbalance between the “cooler” lattice and “hot” electrons.54,55 The rate of heating is controlled both by the optical absorption of laser light and the thermal conduction in the semiconductor. As the size of a semiconductor is reduced from the bulk to the nanoscale, the thermal conductivity monotonically decreases specifically in the size regime where optical confinement effects dominate.25,56 This gives rise to rapid heating and subsequent melting in nanostructured semiconductors relative to the bulk upon laser illumination. While conceptually well-understood, the experimental investigation of heating in nanostructured metal-oxides with Raman spectroscopy remains poorly characterized. Laser induced heating results in significant Raman peak shifts, peak broadening, and alterations in the peak intensities. By examining the Raman peaks, the thermal profile and phononic properties of metal oxide nanostructures can be systematically characterized. Besides nanoscale heating, Raman spectroscopy also effectively probes phonon confinement effects. As the size of the semiconductor is reduced from the bulk, translational invariance is broken typically when the crystal dimension is in the 1−25 nm regime and a plane wave-like phonon wave function can no longer exist.30,57 Lattice vibrations in a nanocrystal then get reflected at the boundary remaining spatially confined. The well-established phonon confinement model58,59 can be fit to the Raman spectrum of nanocrystals and the shifts in phonon frequency and line-width broadening can be used to interpret phonon confinement effects and phonon lifetimes.

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MATERIALS AND METHODS PdO Nanocrystal Synthesis. The PdO nanocrystals were synthesized by modifying a procedure reported previously60 using a Schlenk line under N2 environment. The 4 nm PdO nanocrystals were synthesized by mixing 0.1 g of palladium acetylacetonate (Pd(acac)2 Aldrich 209015) with 10 mL of trioctylphosphine (TOP, Aldrich 117854) in a N2 glovebox for 30−45 min until all solid dissolved to form a transparent orange solution. The solution was then transferred to a 3-neck flask attached to a Schlenk line and heated to 300 °C in 45 min (∼6 °C/min). The solution turned from orange to dark brown as Pd nanocrystals nucleated in solution. The Pd nanocrystals were kept at 300 °C for 15 min in inert atmosphere and then exposed to air for another 15 min. The nanocrystals were cooled to room temperature, 20 mL of ethanol was added, and nanocrystals were centrifuged at 4000 rpm for 5 min. The supernatant was discarded, the precipitate was resuspended in 6 mL of hexane, and 5 μL of oleylamine (O7805, Sigma) was added to minimize aggregation of nanocrystals. The oleylamine was allowed to bind to the PdO surface for 10 min. The nanocrystals were centrifuged again for 5 min, and the supernatant was retained while the precipitate, which mostly consisted of insoluble solids, was discarded. The nanocrystals were resuspended in 2 mL of hexane and 2 mL of ethanol and centrifuged one additional time and finally redispersed in 10 mL hexane. The 10 nm PdO nanocrystals were synthesized following a similar procedure, except 0.1 g of Pd(acac)2 was mixed in 1 mL of TOP until the solid dissolved and then added to 10 mL of oleylamine and heated to 300 °C in 45 min. Both samples had a final concentration of 1 mg/mL. For long-term stability of nanocrystals, 5 μL oleylamine can be added to the final suspension in hexane. The nanocrystals were characterized with a Philips CM20 TEM. Thermogravimetric analysis (TGA) was performed with a TA Instruments Q5000IR TGA. Nanocrystal size-distributions were analyzed with ImageJ software. PdO Foil Preparation. For comparison, PdO foil was used for bulk measurements. A 0.025 mm thick Pd foil (Sigma, 267120) was treated with oxygen plasma for 2 h followed by heating in a furnace at 350 °C for 1 h to oxidize the surface of the Pd foil to PdO. Raman Measurements. The substrates were prepared for Raman measurements by first performing an ex situ ligand exchange to remove most of the organic surfactants (oleylamine) from the surface of the nanocrystals. Quartz substrates 21559



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Figure 1. Transmission electron microscopy images of (a) 4 ± 0.25 nm PdO nanocrystals and (b) 10 ± 0.8 nm PdO nanocrystals with high magnification image provided in the inset. The scale bar of the inset is 5 nm. (c) Size distributions as obtained from sizing ∼300 particles in TEM of PdO nanocrystals. (d) Thermogravimetric analysis of formic acid (FA) decomposition bound to the nanocrystal surface compared with decomposition of formic acid by itself.

their composition. Transmission electron microscopy micrographs and size-distribution analysis of PdO nanocrystals revealed sizes of 4 ± 0.3 (Figure 1a,c) and 10 ± 1 nm (Figure 1b,d) respectively. This synthetic procedure yields monodisperse spherical nanoparticles of up to ∼10 nm size bound by [111] crystal planes. An increase in the precursor concentration to synthesize larger nanocrystals resulted in polydispersity (Supporting Information, Figure S2a). Other synthetic methods generated cubic-shaped nanocrystals bound by the [100] crystal planes (Supporting Information, Figure S2b). As phonon behavior is inherently different for assemblies of nanoparticles with different shapes or different crystalline facets; we only compare the 4 and 10 nm nanocrystals in this study. Prior to Raman measurements, it is essential to remove the ligands from the nanocrystal surface to minimize interference from the sp3 hybridized carbon peak in the PdO Raman spectra. Ligand exchange was performed ex situ to replace the organic surfactants (oleylamine) on PdO nanocrystals with a small molecule (formic acid) as described previously.61 Formic acid was specifically chosen as it decomposes at ∼150 °C when bound to PdO surface while olyelamine decomposes at ∼350 °C. The PdO nanocrystals retain their shape and size when annealed at 150 °C post ligand-exchange while they are close to melting at 350 °C (not shown). Following ligand exchange, the nanocrystals were annealed in a vacuum oven at 150 °C to remove all formic acid ligands from the surface providing clean PdO nanocrystal

were cleaned with piranha (3:1 H2SO4 (conc.)/H2O2 (30 wt %)) followed by oxygen plasma treatment. A 20 μL aliquot of nanocrystals was drop-cast on the cleaned quartz, dried in ambient air, and baked in the oven at 120 °C for 15 min. A 1 M formic acid (F0507, Sigma) solution in acetonitrile was prepared (304 μL of formic acid in 8 mL of acetonitrile) in a petridish. The quartz substrates were immersed and left in the Petri dish for 45 min, after which the substrates were gently rinsed in acetonitrile and acetone and dried with N2. The substrates were then baked in the oven at 150 °C for 1 h to burn off most of the formic acid and then gently rinsed with acetone and dried with N2. The nanocrystals were stable at 150 °C with no signature of melting. The substrates were then used for Raman measurement. The Raman measurements were performed on a Renishaw inVia Raman microscope and were excited at 532 nm wavelength with a 50× objective which had a 1 μm spot size.

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RESULTS AND DISCUSSION

Nanocrystal Synthesis and Characterization. Sizedependent phonon behavior was measured on films of monodisperse PdO nanocrystals of two different sizes shown in Figure 1. PdO nanocrystals were synthesized by modifying a procedure described previously60 using a solution-processed colloidal synthesis technique in a N2 environment followed by air exposure. X-ray diffraction (XRD) profiles of PdO nanocrystals (Supporting Information, Figure S1) confirm 21560



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Figure 2. Raman spectra of (a) 4 nm PdO nanocrystals with the phonon modes assigned to the different peaks. (b) Raman spectra of 4 and 10 nm PdO nanocrystals, and bulk PdO foil with 532 nm excitation and 0.17 mW laser power. Phonon mode clearly shows size-dependent peak shift and peak broadening with decreasing size.

located on the surface of nanocrystals increases with decrease in size, the optical phonon softening mechanism gives rise to sizedependent red-shift in the Raman peaks. Additionally, the observed peak shifts and peak broadening can also be attributed to phonon localization induced by O and Pd defects in the crystal lattice formed during the high temperature chemical synthesis of the nanocrystals. Such defect induced confinement effects have been reported for nanostructured TiO265 and ZnO.66 The B1g phonon mode of PdO nanocrystals demonstrate significant red-shift, peak broadening, and changes in the peak intensity with increasing laser power from 0−800 kWcm−2. The shifts in phonon frequency and peak broadening of the 4 nm PdO nanocrystals is shown in Figure 3; similar trends were observed for the 10 nm PdO nanocrystals as well. The

samples for Raman analysis. Decomposition and complete removal of formic acid at 150 °C was confirmed by thermogravimetric analysis (Figure 1e). The PdO bulk was formed by oxygen plasma treatment and heat treatment of a pristine Pd foil. XRD (Supporting Information, Figure S3) and SEM images (Supporting Information, Figure S4) confirmed the formation of PdO. Grain sizes of the bulk PdO foil was determined using the Scherer’s equation (see the Supporting Information) and was ∼273 nm. Size-dependent Raman spectra and optical phonon confinement. When illuminated with a laser excitation source at 532 nm and 0.5 mW power (50X objective, 1 μm spot size), the Raman spectra of the nanocrystals exhibited the B1g (646.5 ± 1.5 cm‑1), Eg (443 ± 2 cm‑1), and χ8 (278 ± 1 and and 725 ± 2 cm‑1) phonon modes (Figure 2a). The PdO nanocrystals and bulk foil showed similar Raman characteristics; however, the phonon modes of nanoscale PdO clearly exhibit a sizedependent red-shift as the size of the nanocrystals decreased from 10 to 4 nm. To investigate the effects of optical phonon confinement at the nanoscale, we focus on the strongest mode, B1g, throughout this paper. The B1g mode was observed at 651 cm−1 for bulk PdO, while it was observed at 648 and 645 cm−1 for the 10 and 4 nm PdO nanocrystals, respectively (Figure 2b). In addition to the red-shift, the peak also demonstrated a size-dependent broadening with decreasing size where the fullwidth at half-maximum (fwhm) of bulk PdO was 12.5 cm−1 which increased to 15.5 and 17.5 cm−1 for the 10 and 4 nm PdO nanocrystals, respectively. This size-dependent peak shift and peak broadening can be explained by the optical phonon confinement effect. The degree of confinement of optical phonons is governed by the dimensionality of the system. Raman spectroscopy typically samples the optical phonons near the Brillouin zone center, where wavevector q ≈ 0.57,62 This q ≈ 0 selection rule applies due to the infinite periodicity of a bulk crystal. However, at the nanoscale due to finite sizes and the presence of grain boundaries, lattice periodicity is interrupted and the q ≈ 0 selection rule is relaxed.33 The optical phonons can therefore get reflected from the grain boundaries and remain confined in the nanocrystal. As the size of the nanocrystal decreases, more phonons are confined within the nanocrystal resulting in “softening” of the surface phonons. This optical phonon softening arises from cohesive energy weakening of atoms on the surface of nanocrystals.63,64 Since the number of atoms

Figure 3. Peak shift and peak broadening of the B1g phonon mode of 4 nm PdO nanocrystals as a function of increasing power. The power densities are indicated. Similar peak shifts and broadening were observed for the 10 nm PdO nanocrystals as well. 21561



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nanocrystals exhibited a strong degree of anharmonicity which increased with decreasing size. The Raman spectra of the nanocrystals were fit to a Gaussian function interpreted by the well-established Gaussian phonon confinement model (PCM) proposed by Richter58 and later generalized by Campbell and Fauchet.59 The model is given by, I(ω) =

∫

⎛ − q 2d 2 ⎞ d3q ⎟ exp⎜ ⎝ 2α ⎠ [ω − ω(q)]2 +

Γ0 2 2

( )

(1)

where d is the nanoparticle diameter, ω(q) is the bulk phonon frequency, Γ0 is the bulk fwhm, the parameter α determines how rapidly the wave function decays as one approaches the particle boundary, and q is the wave vector of zone-center optical phonon expressed in units of 2π/a where a is the lattice parameter of bulk PdO (a = 3.0434 Å).67 Gaussian PCM takes into account the contribution of the phonons away from the zone center to the Raman line shape. By fitting the Raman peaks of the nanocrystals by the Gaussian confinement function described above, the phonon peak position, fwhm, and peak intensity were determined (Figure 4). The phonon peaks of the PdO foil were fit to a Lorenztian function. All Raman analysis was performed by appropriate background subtraction. The peak characteristics of bulk PdO is drastically different from the nanocrystals and explicit size-dependent trends are observable. Light-Induced Heating of Nanocrystals. Besides phonon confinement effects, by evaluating the peak shifts, peak broadening, and modifications in peak intensity, the contribution of laser-mediated heating of nanocrystals was also quantified. While the B1g mode of bulk PdO foil shifts a total of 4 cm−1 when the laser power density is increased from 0− 800 kW/cm2, the 4 and 10 nm PdO nanocrystals demonstrated 19 and 15 cm−1 peak shifts, respectively (Figure 4a). Significant modifications were observed in the peak widths as well for the PdO nanocrystals. For the bulk PdO foil the peak widths increased by 2 cm−1 when laser power was increased, however, for the 4 and 10 nm PdO nanocrystals the peak width of B1g mode broadened by 13 and 9 cm−1, respectively (Figure.4b). The behavior of the normalized peak intensity of the B1g mode as a function of increasing laser power was also significantly different between the bulk foil and the PdO nanocrystals. While the bulk foil showed an increase in peak intensity with increasing laser power as expected, the nanocrystal phonon peak intensity increased and reached saturation, and then decreased with increasing laser power (Figure 4c). Such nonintuitive behavior of the nanocrystal phonon peak intensity, and the large peak shifts and peak broadening can be attributed to a combination of optical phonon confinement at the nanoscale, as well as laser-induced heating and subsequent melting of nanocrystals. The melting of the nanocrystals is indicated by a sudden red-shift in phonon frequency, and sudden increase in fwhm of the B1g phonon peak. This occurs at ∼330 kW/cm2 for the 4 nm nanocrystals and ∼415 kW/cm2 for the 10 nm nanocrystals. The melting of the nanocrystals is clearly observable in the normalized peak intensity which increases at lower power densities, reaches saturation near 300−400 kW/cm2, and then steeply decreases. The saturation point is interpreted as the onset of melting, and as the nanocrystals continue to melt, B1g peak intensities decrease and eventually the peak disappears. In bulk PdO, however, the lasermediated heating does not raise the temperature sufficiently to induce melting (Tmelting bulk = 750 °C), thus explaining the

Figure 4. Size dependent (a) peak shifts, (b) peak broadening, and (c) change in normalized intensity as a function of increasing power density of the B1g phonon mode of 4 and 10 nm PdO nanocrystals and PdO foil.

small shifts in peak position, minor peak broadening, and only increase in peak intensity with increasing laser power. We note that the peak shifts and peak broadening are reversible, however, a hysteresis is observed when the power is increased and then decreased (Supporting Information, Figure S5). We also note that, after one cycle of increasing and decreasing power, the results are no longer reversible due to sample damage. The approximate temperature rise at the surface of the nanocrystals and PdO film can be measured by taking a ratio of the stokes and antistokes peak intensities given by ⎡I ⎤ ℏω ln⎢ S ⎥ = ⎣ IAS ⎦ kBΔT 21562

(2)



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Figure 5. (a) Size dependent temperature rise as a function of increasing power density of 4 and 10 nm PdO nanocrystals and PdO foil. (b) Linear fit to temperature until the onset of melting in the nanocrystals, and until temperature equilibration in the bulk film.

Table 1. Phonon Lifetime of the B1g Mode of PdO Nanocrystals and Bulk PdO at Low Power Density and at the Power Density Where Melting Occurs for the Nanocrystalsa

a

size (nm)

low power density (kW/cm2 × 103)

Γ (cm‑1)

τ (10‑12 s)

high power density (kW/cm2 × 103)

Γ (cm‑1)

τ (10‑12 s)

4 10 bulk

0.0861 0.0861 0.0861

17.5 15.5 12.5

0.303 0.342 0.424

0.339 0.416

22.6 21.8

0.235 0.243

The corresponding FWHM is also shown.

where IS is the intensity of the stokes Raman signal, IAS is the intensity of antistokes Raman signal, ℏ is reduced Planck’s constant, ω is the angular frequency given by ω = 2πf where f is the Raman peak frequency, kB is Boltzmann’s constant, and ΔT is the increase in temperature relative to room temperature. By measuring stokes and antistokes intensities of the B1g phonon mode, the temperature rise as a function of increasing power density was analyzed (Figure 5a). The temperature rise on the PdO foil is significantly lower than that of the PdO nanocrystals and a clear size-dependent trend is observable by laser induced heating. While a maximum temperature of ∼100 °C is observed at 800 kW/cm2 for the PdO foil, both nanocrystal sizes demonstrate a temperature rise to 350−400 °C. The trends observed in the temperature profile of the nanocrystals is similar to those observed in the normalized peak intensity. The temperature continues to increase and reaches a maximum near 300−400 kW/cm2 before decreasing. This behavior confirms that the 4 nm PdO nanocrystals melt near 350 °C, and the 10 nm nanocrystals melt near 400 °C. As the nanocrystals melt and coalesce with neighboring nanocrystals, there is an effective increase in nanocrystal size which consequently decreases the overall heating efficiency. SEM micrographs of nanocrystals assemblies before and after laser-induced melting are shown in the Supporting Information (Figure S6). A reduction in heating characteristics explains the melting-induced decrease in temperature at higher laser power densities. The significantly higher temperature profiles in the nanocrystals relative to bulk PdO is attributable to both confinement of optical phonons at the nanoscale in PdO nanoparticles and lower thermal conductivity of nanocrystal assemblies relative to bulk.25,56 Due to the lower thermal conductivity of PdO nanocrystal assemblies, heat dissipation occurs slower resulting in a large temperature rise with increasing laser power. Conversely, the higher thermal conductivity in bulk PdO enables rapid heat dissipation resulting in lower equilibrium temperatures than the nanocrystal assemblies at a given laser power.

In bulk systems, laser induced heating should be a linear function with increasing laser power. Such a linear behavior in the temperature rise is observable in the PdO foil as a function of power density (Figure 5b). The linear function for bulk PdO was T(°C) = 0.00022P + 22.81 where P is the laser power (kW/cm2). Before the onset of melting, the measured temperature rise in the nanocrystal assemblies also exhibit a linear trend with laser power, with a slope that depends upon nanocrystal size. For the 4 nm PdO nanocrystals, the linear function was T(°C) = 0.001P + 18.07, and for the 10 nm nanocrystals was T(°C) = 0.00083P + 9.16. The melting temperatures of nanocrystals (Tm) can be correlated to their size given by Tm = TBe

−2ΔSB 3R ⎡⎣( DD0 ) − 1⎤⎦

(4h )

where TB is bulk melting temperature, ΔSB is the bulk vibrational entropy of melting, R is the gas constant, D is nanoparticle size, and D0 is the critical size where all atoms of the particle are located on the surface. For nanoparticles, D0 = 3h where h is the atomic diameter (PdO, h ≈ 0.203 nm).68 This model correlating nanocrystal size to their melting behavior was first proposed by Shi.69,70 Later, Jiang et al. extended the model to size-dependent first order and second order phase transformations in various systems.70−74 We have provided a semiquantitative fit to our experimental results (Supporting Information, Figure S7) demonstrating that this simple analytical model can be used to extrapolate the melting temperatures of PdO nanocrystals of variable sizes. This sizedependent trend in phononic properties and corresponding melting temperatures demonstrates that as crystallite size is reduced to the nanoscale, which consequently results in an increase in the surface to volume ratio, the combined effect of the increase in number of surface atoms and surface phonon softening results in a decrease in the melting temperature relative to bulk. 21563



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Figure 6. (a) Illustration of heating effects in nanocrystals of different particle density. A color bar representing low to high temperature scale is shown. Arrows indicate phonon dissipation. (b) Peak shift, (c) changes in fwhm, and (d) rise in temperature as a function of power density of the B1g mode of 10 nm PdO nanocrystals of variable concentrations. (e) Linear fit to temperature until the onset of melting in the nanocrystals of varied concentrations. The particle concentrations are provided in panel b.

Phonon Lifetimes. In addition to quantifying the temperature rise in nanocrystal assemblies, the peak broadening in the Raman spectra can also be used to evaluate the phonon lifetimes. The phonon lifetime, τ, can be derived from the fwhm of PdO Raman spectra via the energy-time uncertainty relation

1 ΔE = = 2πc Γ τ ℏ

dependent decrease in phonon lifetimes is observed with decreasing nanocrystal size attributed to a phonon confinement effect. However, the phonon lifetime at the onset of melting is very similar for the two nanocrystal sizes. If the phonon decay was largely contributed by anharmonic decay, we would expect a much shorter phonon lifetime at the onset of melting for the 4 nm nanocrystals relative to the 10 nm nanocrystals due to stronger confinement of optical phonons within smaller grain boundaries. These results also suggest that this synthetic procedure generates relatively similar levels of defect density and impurities in the nanocrystals of different sizes. Particle Density Dependent Raman Spectral Characteristics. The thermal profiles of nanocrystals are also tunable by altering the density of nanocrystals in the film. Figure 6a shows an illustration of heating effects and phonon dissipation in nanocrystals of different particle density. At low nanoparticle density, upon illumination, particles heat up and surface temperature increases (shown in pink). At higher particle density, nanoparticles illuminated with the same laser power generate intense surface temperature which dissipates to surrounding nanoparticles. In these experiments, the particle

(3)

where ΔE is the uncertainty in the energy of the phonon mode, ℏ is the Planck constant, and Γ is the fwhm of the Raman peak in units of cm−1. The phonon lifetimes of PdO nanocrystals are shown in Table 1 at low power density and at the onset of melting at high power density. Phonon lifetime is primarily controlled by two mechanisms: (1) anharmonic decay of the phonons into two or more phonons, and (2) disruption of the translational symmetry of the crystal due to the presence of impurities and defects.57 It is typically difficult to separate the contribution of each mechanism; however, the values in Table 1 suggest that phonon decay at the onset of melting likely occurs due to impurities and defects in nanocrystals. At low power density, corresponding to a temperature rise of ∼55 °C, a size21564



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density of 10 nm PdO nanocrystals was varied between 1 × 105 to 3 × 105 particles/μm2 and peak shifts and peak broadening were measured as a function of increasing laser power density. The nanocrystal density was varied by drop-casting a higher concentration of nanocrystals on the substrate and gravimetrically analyzing the density (see the Materials and Methods). The enhanced peak shifts (Figure 6b) peak broadening (Figure 6c) of the B1g phonon frequency when nanoparticle density is increased demonstrates rapid heating in multilayered nanocrystal ensembles with increasing laser power density. The onset of melting in the nanocrystals is notable by a sudden shift in the phonon frequency as well as sudden peak broadening which occurs at different power densities depending on the nanoparticle density. The melting approximately occurs at ∼275 kWcm−2 for the highest particle density and ∼415 kWcm−2 for the lowest particle density. A ratio of stokes and antistokes intensities of the B1g phonon mode elucidates a clear trend in the temperature rise in the 10 nm PdO nanocrystals with increasing particle density (Figure 6d). While the melting temperature remains in the range of 390 ± 3 °C for all nanoparticle concentrations, melting occurs at lower laser power with increasing nanoparticle densities. This is attributable to both an increased optical cross section for laser absorption with larger particle densities and “hot” junctions formed between PdO nanocrystals in close proximity in the film. Collectively, these effects yield an enhanced temperature rise for the most dense nanoparticle assemblies that lead to melting under illumination at lower laser power. Before the onset of melting the measured temperature increase in the nanocrystals show a linear behavior and an increase in the slope with higher particle density (Figure 6e). For the lowest nanoparticle density (1E5) the linear function is T(°C) = 0.000835P + 9.16, for the next nanoparticle density (2E5) the linear function was T(°C) = 0.00112P +13.84, and for the highest density (3E5) T(°C) = 0.00139P + 14.83. In addition to heat generated by optical absorption, the heat conduction through the nanoparticle film also contributes to the total temperature rise. The total temperature rise may be understood as T ∝ QCQA where QA is the heat generated by light to heat conversion and QC is the heat generated by conduction. QC of a material is directly proportional to the thermal conductivity (kω) of the material. Therefore, by correlating experimentally observed temperature rise to kω, the heat conduction across the film can be understood. The temperature rise (T) resulting from a cluster of nanocrystals is inversely proportional to kω given by25,75

T∝

Figure 7. Temperature increase showing a linear relationship with the particle concentration of the 10 nm PdO nanocrystals. Results are shown at various power densities.

⎛ P ⎞1 σ T ∝ C ⎜ ⎟ , where C = 4πR ⎝ kω ⎠ N

and C is a constant for a nanocrystal of known size and absorption cross-section. This analysis suggests that if temperature rise is plotted as a function of nanocrystal particle density for a range of power densities, a linear relationship will be established and the slope will be inversely correlated to the thermal conductivity. To demonstrate this theoretical analysis, we plotted the experimentally obtained temperatures (Figure 6c) as a function of PdO particle density at various power densities. The PdO nanocrystal size was kept constant at 10 nm. The temperature vs particle density yields a linear behavior as shown in Figure 7. The slope of the linear function increases with increasing power densities as expected, but this also indicates that the thermal conductivity of the PdO nanocrystal film also improves with increasing particle density. Enhancement of thermal conductivity has been demonstrated for increasing nanoparticle density and nanoparticle clustering.76,77 This suggests that, with an increase in nanoparticle density, the heat conduction across the nanoparticle film which is directly proportional to the thermal conductivity is improved as well. These experimental results relating temperature rise to power density enables us (i) to probe the nanocrystal film phononic behavior at different laser powers as a function of nanocrystal size, (ii) to record the complete thermal history of a photothermal event, and (iii) to monitor the thermal stability of the nanocrystals. The particle density dependent study exemplifies competing effects of laser-induced heating, sizedependent phonon dissipation in nanocrystal assemblies, and thermal stability of PdO nanocrystal monolayer and multilayer films. This study can be straightforwardly generalized to other nanostructured semiconductors and can be applied as a simple, noninvasive measurement approach to understand phasechange phenomena in nanocrystals and phononic-thermal properties of collective nanoscale structures.

QA 4πNRkω

(5)

where R is the radius of each nanocrystal and N is the particle density. If we assume the entire incident laser light was absorbed by the PdO nanocrystals and converted to heat, then QA is proportional to the laser power density (P) and absorption cross-section (σ) of the nanocrystal. Then T may be expressed as

T∝

Pσ 4πNRkω

(7)

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CONCLUSIONS

In conclusion, this study provides a detailed understanding of the size-dependent phononic properties of PdO nanocrystals using Raman spectroscopy optical thermometry. This approach provides high spatial and temporal resolution without requiring any tedious device fabrication for thermal measurements. The PdO nanocrystal assemblies studied here show a sizedependent red-shift and peak broadening in the B1g phonon mode relative to bulk. This is attributable to both optical

(6)

In our experiments (Figure 7), since R and corresponding σ of each PdO nanocrystal does not change, only particle density, N, is varied, T can be rewritten as, 21565



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phonon confinement at the nanoscale and collective heating of nanoparticle assemblies. This occurs as a function of both laser power and nanoparticle density. By examining the sizedependent and particle density-dependent trends of the phonon behavior in the nanocrystals, a systematic understanding of the thermal profile and thermal stability of the nanocrystals has been established. Raman based optical thermometry is demonstrated as a highly versatile approach that can be generalized to study light-matter interactions, photothermal effects, and phonon confinement in a range of semiconductor and dielectric materials. This technique can also be applied to understand the thermal conductivity of materials at the nanoscale with known bulk thermal conductivities.25 We further expect this study to be highly relevant to industrial processes where nanostructured PdO is extensively utilized for high temperature catalysis. Light induced heating is a promising approach in catalysis enabling miniaturization of reactor designs by simply replacing a bulky heat source with a diode laser with a relatively small footprint.

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ASSOCIATED CONTENT

S Supporting Information *

Additional SEM/TEM images of PdO nanocrystals and SEM images of PdO nanocrystal films before and after laser-induced morphological changes. XRD analysis of PdO nanocrystals and PdO bulk, grain size determination, and analytical model correlating size to melting temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Vanderbilt start-up funds and NSF-EPSCoR (EPS1004083). HFZ acknowledges support from the Department of Education for a Graduate Assistance in Areas of National Need (GAANN) Fellowship under grant number P200A090323. Characterization aspects of this research were conducted in part at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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Size-Dependent Phononic Properties of PdO Nanocrystals Probed by

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