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Cite this: Nanoscale, 2014, 6, 12450 Received 16th August 2014, Accepted 15th September 2014 DOI: 10.1039/c4nr04719b www.rsc.org/nanoscale

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Copper plasmonics and catalysis: role of electron–phonon interactions in dephasing localized surface plasmons† Qi-C. Sun,a Yuchen Ding,a Samuel M. Goodman,a Hans H. Funkea and Prashant Nagpal*a,b,c

Copper metal can provide an important alternative for the development of efficient, low-cost and low-loss plasmonic nanoparticles, and selective nanocatalysts. However, poor chemical stability and lack of insight into photophysics and plasmon decay mechanisms has impeded study. Here, we use smooth conformal ALD coating on copper nanoparticles to prevent surface oxidation, and study dephasing time for localized surface plasmons on different sized copper nanoparticles. Using dephasing time as a figure of merit, we elucidate the role of electron–electron, electron–phonon, impurity, surface and grain boundary scattering on the decay of localized surface plasmon waves. Using our quantitative analysis and different temperature dependent measurements, we show that electron–phonon interactions dominate over other scattering mechanisms in dephasing plasmon waves. While interband transitions in copper metal contributes substantially to plasmon losses, tuning surface plasmon modes to infrared frequencies leads to a five-fold enhancement in the quality factor. These findings demonstrate that conformal ALD coatings can improve the chemical stability for copper nanoparticles, even at high temperatures (>300 °C) in ambient atmosphere, and nanoscaled copper is a good alternative material for many potential applications in nanophotonics, plasmonics, catalysis and nanoscale electronics.

Introduction Copper (Cu) metal is ubiquitous in electronic devices across many size-scales from macroscale electrical wires to microscale electronic interconnects. Copper has one of the highest electrical conductivities (6.5 × 107 S m−1) amongst metals (DC conductivity), even when compared to precious metals like gold a Department of Chemical and Biological Engineering, University of Colorado Boulder, 80303, USA. E-mail: [email protected]; Fax: +1 303 492 8425; Tel: +1 303 735 6732 b Materials Science and Engineering, University of Colorado Boulder, 80303, USA c Renewable and Sustainable Energy Institute (RASEI), University of Colorado Boulder, 80309, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr04719b

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(4.9 × 107 S m−1) or silver (6.6 × 107 S m−1).1 This low resistance to electron transport ( joule heating) combined with its low cost makes copper attractive for electronic applications. In addition, this ease of transporting conduction electrons can be very useful for applications in plasmonics or nanophotonics because joule heating is a major loss mechanism that limits the propagation of surface plasmon polariton (SPP) waves on the metal interfaces.2–4 While these coupled electron–photon SPP waves on metal surfaces have been widely investigated for different applications,5–12 most studies have focused on gold and silver nanomaterials13–17 because of their ease of fabrication and chemical stability. The main drawback of the copper nanoparticles (Cu NPs) is the ease of oxidization (Fig. 1(d)) even at the room temperature, due to high surface areas. Recently, many research groups have made important advances in the chemical synthesis and characterization of different shapes and sizes copper nanostructures,18–27 which enables renewed efforts towards finding alternatives to gold or silver for low-loss and low-cost plasmonic applications. All these properties for copper-based materials, combined with the low material cost, can make them viable candidates for replacing precious metals like gold and silver for applications in nanophotonics,2,13–17 renewable energy,12 sensing,5,11 and catalysis.22,28,29 In this work, we synthesized and investigated different sized and shaped Cu NPs to tune the localized surface plasmon resonance frequencies. High-quality Cu NPs were prepared by the thermal decomposition of a copper(I) acetate precursor.27 Monodisperse colloidal suspensions of Cu NPs (shown in transmission electron micrograph (TEM) and size analysis histogram, Fig. 1(a)) were used for optical characterization and to study their photophysical properties. Fig. 1(b) shows size-dependent shift of the SPP frequency observed in metal NPs.22,23 We selected three representative nanoparticle sizes (with high-crystallinity and size monodispersity), 7.5, 10.8, and 12.2 nm, for the investigation of temperaturedependent photophysical properties. To enhance the chemical stability, prevent surface oxidation of nanoscale copper, and to utilize a surface for easy functionalization, we coated the

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Fig. 1 (a) Left: size histogram for 10.8 nm Cu NPs. Right: TEM image of 10.8 nm Cu NPs. Note that the size refers to the diameter for all sized NPs. (b) Optical extinction spectra for 7.5 and 12.2 nm Cu NPs. Upper inset: size histogram for 7.5 nm Cu NPs. Lower inset: size histogram for 12.2 nm Cu NPs. The sampling number is ∼300 in each case. (c) The refractive index of medium as a function of the layer number of alumina. (d) Left: change in optical extinction as a function of time for 10.8 nm Cu NPs without the Al2O3 protective ALD coating, at room temperature in ambient atmosphere. Right: extinction spectra at different temperatures for 10.8 nm Cu NPs with the alumina coating.

Cu NPs with a thin aluminium-oxide (Al2O3) shell. Aluminiumoxide was chosen due to its transparency, low-refractive index (good optical property), and high thermal and mechanical stability. Since surface roughness can scatter/dephase SPP waves,2–4 we used an atomic layer deposition (ALD) technique to form thin, smooth, conformal Al2O3 shells around differentsized Cu NPs (Cu core/Al2O3 shell) to prevent surface oxidation during photophysical measurements at different temperatures (300–600 K) in ambient conditions. We measured the change in refractive index (n) of the medium surrounding the Cu NPs (Fig. 1(c)) using the spectral shift in plasmon resonance Ωp frequency (ωsp ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ) where ωsp is the SPP frequency 1 þ 2n2 and Ωp is the bulk plasma frequency (= 7.589 eV for Cu)4 during the ALD coating. To study the effect of NP shape on plasmon resonance and selective catalysis, we also synthesized Cu nanorods and investigated their catalytic properties. Different-shaped Cu NPs can tune the plasmon resonances due to their anisotropic shape, and a large surface-to-volume ratio can enable broad range of applications in sensing and catalysis.

Experimental Synthesis of copper nanoparticles and nanorods All chemicals were used as received from Sigma-Aldrich and handled in a nitrogen-filled glovebox. NPs with sizes, between ∼7 and 12 nm, were synthesized by varying the reaction time and the amount of surfactants. For the synthesis of 7.3 nm

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NPs, copper(I) acetate (3 mmol) and trioctylamine (11 mL) were mixed under sonication. This mixture was transferred to a three-neck flask and tetradecylphosphonic acid (3 mmol) was added. The mixture was first held under vacuum for 20 min, and then heated quickly under nitrogen at 200 °C for 50 min. Then the mixture was heated further at 270 °C for 50 min. After cooling under nitrogen, the colloidal Cu nanocrystals were collected. In a typical synthesis of Cu nanorods,30 50 mL of 16 mM Cu(OH)42− solution was prepared by dissolving 0.2 mg CuSO4·5H2O in 50 mL of 3 M NaOH. CTAB (1.1 mg) was added to the solution and stirred vigorously for 30 minutes at 50 °C. Then hydrazine hydrate (1.5 mL) was added into the solution, and the resulting mixture was maintained at room temperature for 1 h. After the reaction was complete, the resulting product was collected, washed several times using absolute ethanol and distilled water, centrifuged and dried under vacuum at room temperature. While Cu nanorods are produced in high yields, some other shaped nanoparticles are also obtained during this synthesis, including two-dimensional copper nanosheets. TEM characterization TEM images were obtained on a Philips CM100 electron microscope operated at 100 kV, from dilute dispersions drop-cast onto carbon-coated TEM grids. To avoid unwanted oxidation of Cu surfaces, the Cu NPs were coated by 50-layer alumina shell using an atomic layer deposition technique (ALD).31 Atomic layer deposition We used a colloidal suspension of Cu NPs in hexane, to deposit a uniform thin layer of NPs (to ensure uniform ALD coating on all NPs) on a glass substrate, in an air-free glove box. The substrate was then loaded in an ALD-reactor inside the glove-box, and the sealed reactor was attached to the ALD deposition setup, without exposing the Cu NPs to ambient atmosphere. The Al2O3 layers were deposited in a static vacuum apparatus by sequentially exposing the Cu NPs to alternating cycles of (trimethyl aluminium) Al(CH3)3 and water (H2O) vapour sequentially.32 To initiate the ALD deposition of alumina layers without removal of ligands, we seeded the first alumina layer on Cu NP surface by using longer (∼2 minutes) alternating cycle of trimethyl aluminium followed by evacuation, and a longer exposure to water vapour. The base pressure for the setup was 1 millitorr, and the setup was evacuated for 5 minutes between the cycles to ensure no residual gases were left in the chamber. The optical extinction spectrum of the NPs was monitored (using the glass windows) during the deposition, to study the change in refractive index and the potential application of these NPs for refractive index sensing. Optical spectroscopy The UV-Vis absorption spectrum was measured on an Ocean Optics USB4000 spectrometer with resolution of ∼0.1 nm. For our variable temperature studies, we employed a closed-

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cycle helium cooled cryostat below 300 K and a resistive-heater above 300 K. Raman spectroscopy was carried out using the excitation wavelength at 543.5 nm and 632.8 nm, respectively. The single NP spectrum was collected by using a home-built confocal setup using an inverted microscope (AXIO-Observer, Zeiss), and silicon photodetectors. Using transparent indexed TEM grids, we measured the optical extinction spectrum and later characterized the morphology of NPs deposited on the grid. Electromagnetic field simulation The interaction of electromagnetic radiation with Cu NPs was simulated using the DDSCAT software. This code is a popular implementation of the DDA (Discrete Dipole Approximation) method originally developed by Purcell and Pennypacker.33 The numerical method divided the analysed Cu particle in dipoles, and the electric field radiated by the particle was calculated taking into account interaction between all of these dipoles. All DDSCAT simulations were run on nanoHUB.34 Catalytic reaction Hydrogenations were conducted in a tubular packed bed flow reactor at 190 °C and atmospheric pressure. Helium was bubbled through furfural (Sigma-Aldrich) to achieve furfural gas-phase mole fraction yfurfural = 0.016 and mixed with H2 at a 25 : 1 molar ratio of H2 to furfural. The amount of catalyst used was 52.1 mg Cu nanorods. The reaction was run for 150 min; selectivity and conversion at the end of this period are reported here.

Results and discussion In order to test the stability of Cu NPs, we measured the optical extinction of 10.8 nm NPs, with and without alumina coating, as a function of time. The oxidation on the surface of NPs changes the refractive index, and small changes (monolayer oxidation) can be easily monitored using the frequency of the surface plasmon resonance. Since surface plasmon resonance is extremely sensitive to changes in surface chemistry (copper metal to copper oxide dielectric here), we monitored the extinction spectra of Cu NPs to study surface oxidation. As shown in the optical spectra on the left in Fig. 1(d), Cu NPs without protective ALD coating oxidized quickly, even at room temperature in ambient atmosphere, resulting in a clear change in resonance frequency and intensity of the extinction spectrum ∼2.1 eV. However, the Cu NPs with ALD coating showed no change in optical extinction and no measurable surface oxidation. Moreover, even upon heating these ALD coated Cu NPs to 573 K in ambient atmosphere, the nanoparticle plasmon resonance frequency remained unchanged, indicating the high chemical stability of the ALD coated NP surface. The small temperature dependence of the intensity (Fig. 1(d), right spectrum) indicates changes in electron– phonon interactions and will be explained based on the

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Fig. 2 (a, b) Variable temperature absorbance spectra of 7.5 and 10.8 nm Cu NPs with alumina ALD coating. (c) Quality factor of 7.5 and 10.8 nm Cu NPs as a function of temperature. Right axis shows the corresponding dephasing time. (d) Inverse dephasing time for 7.5 and 10.8 nm Cu NPs as a function of temperature.

photophysical model developed here, for studying plasmon dephasing for localized surface plasmons. The high chemical stability for ALD coated Cu NPs can provide an important advance for developing an alternative for gold and silver plasmonics. To understand the plasmon decay mechanism and photophysics following the excitation of localized surface plasmon waves in these NPs (using light), we conducted temperature dependent measurements of plasmon dephasing. Our model for the dephasing time was consistent over different sizes investigated here. Since 12.2 nm Cu nanoparticles have a broader size distribution (right hand inset of Fig. 1(b)) complicating the analysis, we show detailed investigations for two smaller sized nanoparticles: 7.5 and 10.8 nm. Using a Gaussian function to fit the lower photon energies in the extinction spectra (due to interband and absorption due to coating at higher energies) for different sized ALD coated Cu NPs with temperature (Fig. 2(a) and (b)), we extracted the ωsp quality factor for plasmon resonance (Q ¼ , where Γ is the Γ full width at half maximum (FWHM) of resonance frequency, Fig. 2(c)) which quantifies the quality of the resonant oscilh lation (frequency spread), and dephasing time (τ ¼ , Fig. 2 Γ (d)) for the optically excited surface plasmons. Using these two figure of merits for the localized surface plasmon resonance generated in Cu NPs (Fig. 2(c) and (d)), we analysed the interaction of these coupled electron–photon oscillations with phonons, NP surface, electrons, and damping due to interband transitions in copper. A strong SPP resonance is observed at ∼2.1 eV, and the resonance frequency only shows a small temperature dependence (Fig. S1 in ESI†), which is an intrinsic property of metal.35 The intensity and peak width changes (Fig. S2 in ESI†) are due to different photophysical events following excitation of these

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Using eqn (1), we can decouple individual contribution using the theoretical model. First we began by separating the bound and free electron contributions to the SPP resonance. By using the complex dielectric function ε(ω) as sum of interband and free electron contribution:39 εðωÞ ¼ εf ðωÞ þ εb ðωÞ;

Fig. 3 (a) Optical extinction spectra for single Cu NPs (∼5 nm and 10 nm). (b) Peak position of single Cu NPs as a function of quality factor.

where εb(ω) is the interband contribution and εf(ω) is described by the well-known free-electron or Drude model:1,39 εf ðωÞ ¼ 1 

resonant waves. The contribution of the individual decay mechanism for the damping or dephasing of SPP resonance can be decoupled using Matthiessen’s rule:1,36 1 1 1 1 1 1 1 ¼ þ þ þ þ þ τ τee τinter τeph τsurface τradiative τdistro 1 þ : τdefect

ð1Þ

This expression allows consideration of microscopic effects including the intrinsic electron–electron (τe–e), interband contribution (τinter), electron–phonon (τe–ph), electron-surface (τsurface), radiative relaxation (τradiative), size distribution contribution (τdistro), and defect (τdefect) scattering. The total dephasing time (τ) is determined by summing inverse lifetimes, representing their relative contributions to plasmon decay or dephasing for each mechanism. To decouple these different interactions which affect the localized SPP resonance, we began by separating the size distribution contribution. First, we measured the single nanoparticle spectra for different sized Cu NPs, using a home-built confocal setup (Fig. 3, details in Experimental) at room temperature. Using the single NP measurements, the FWHM was much smaller than that in the ensemble measurements (Fig. 1(b)) due to inhomogeneous linewidth broadening in NPs (from size dispersion). Using the FWHM of single ωsp NPs, we estimated the quality factor Q ¼ with SPP Γ resonance frequency (or NP size), as shown in Fig. 3(b). Since the peak position has a linear dependence with the particle diameter D,36 the linear variation of the quality factor D 37 (where ρ is a theory-dependent parameter) agrees Q ρ  vF well with the theoretical models (eqn (7)).36 Using the spectra for single nanoparticle measurements, we can estimate the plasmon scattering 1/τ as 2.90 × 1014 and 2.83 × 1014 for 7.5 and 10.8 nm Cu NPs, respectively. Comparing these results for the ensemble data (6.23 × 1014 and 6.06 × 1014 for 7.5 and 10.8 nm Cu NPs) at 300 K, by decoupling the single NP values, 1 we got size distribution contributions ( , 3.33 × 1014 and τdistro 3.23 × 1014 for 7.5 and 10.8 nm Cu NPs). Since size distribution contribution is temperature independent,38 we can obtain the effective single NP variable temperature plasmon scattering (1/τsingle) by subtracting these values from ensemble variable temperature data (Fig. 2(d)).

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ð2Þ

ω2

Ωp 2 ;  iωγ 0

ð3Þ

where Ωp is the bulk plasma frequency and γ0 is the damping constant. The interband contribution, εb(ω), of the dielectric function is described by the simple semi-quantum model resembling the Lorentz result for insulators:1,39 εb ðωÞ ¼

k X j¼1



f j ωp;j  ; ωj 2  ω2 þ iωγ j

ð4Þ

where ωp is the plasma frequency associated with interband transitions, k is the number of oscillators with frequency ωj, oscillator strength fj, and damping γj. Using this model, we can estimate the contribution of interband absorption of ωsp 3 1 Cu using ¼ ε2 ðωsp Þ,39 where, ε2 is the imaginary part τinter Ωp 2 of the optical dielectric function given as, pffiffiffiffiffiffiffiffiffiffiffiffiffiffi E  E0 1  ,40 here, E0 is the difference in ε2 ¼ E  EF E2 1 þ exp kB T energy between the top of the flat d band and the bottom of the partly filled parabolic conduction band, and EF is the Fermi energy (EF = 7 eV for Cu).1 In the temperature range of our measurements, the temperature dependent term took a unit at the plasmon resonance, thus, we can estimate the interband contribution using the parameters for Cu at room temperature.39 After separating the interband contributions in Cu for dephasing SPP waves using the Drude–Lorentz model, we decoupled the contribution of phonon scattering using the temperature dependent quality factor (Q) and dephasing time (τ, Fig. 4(a) and (b)) for different sized Cu NPs. The abrupt change in dephasing time at 343 K corresponds to the Debye temperature of Cu (Θ = 343.5 K).1 Because the time scale for these decay mechanisms is very short (∼0.1–10 femtoseconds) for direct measurements using ultrafast spectroscopy, the temperature dependent dephasing time can provide a reliable estimate of the relative contributions. The expression for the relaxation time of electron–electron scattering can be written as:38,41 1 τee

¼

  2  π 3 ΣΔ hω 2 ; ð k T Þ þ B  EF 12h 2π

ð5Þ

where EF is the Fermi energy, Σ is the Fermi-surface average of scattering probability, kB is Boltzmann constant, ħ is Planck constant, and Δ is the fractional Umklapp scattering.

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Below Debye temperature Θ, we can estimate the electron– phonon scattering contribution using a solid-state Debye model which considers the partial occupation of phonon modes,38,41 1 τeph

" #  5 ð Θ 4 T 1 2 T z þ4 ¼ dz ; z τ0 5 Θ 0e 1

ð6Þ

where τ0 is a constant for a given metal (τ0 = 2.39 × 10−14 s for Cu).44 Using eqn (6), we can estimate the electron–phonon contribution for scattering or dephasing SPP waves below the Debye temperature. Therefore, the green curve in Fig. 4(e) displays that the effective number of electron–phonon scattering events which approaches unity above Debye temperature because all phonon modes can propagate. For spherical NPs, the surface scattering of free electron scattering effects can be expressed as:36 1 vF ¼ 2g ; τsurface D

Fig. 4 (a, b) Calculated results (green line) compared to inverse dephasing time for 7.5 and 10.8 nm Cu NPs, respectively. (c, d) Main scattering mechanisms as a function of temperature for 7.5 and 10.8 nm Cu NPs. (e) Phonon density of state contributes to the electron–phonon scattering (green line) for different sized Cu NPs (7.5 and 10.8 nm shown here), and the time scale for surface scattering (red and blue lines) as a function of temperature for 7.5 and 10.8 nm Cu NPs.

Within the temperature range of our measurements, the photon energy ħω is much larger than the thermal energy kBT. For example, ħω is over 40 times higher than kBT at 573 K (the highest measured temperature in our experiments). Thus, the temperature dependence of electron–electron scattering is negligible in this investigation. Since the electron–phonon scattering is negligible at low temperature, we simply took the low temperature relaxation time (τe–e = 2.1 × 10−11 s at 10 K42) as the bulk value in eqn (1). By subtracting the interband and electron–electron contribution from the dephasing time, only surface, radiative, defect and electron–phonon scattering contributions remained. The radiative relaxation can be simplified  ω2 e2 εm 1=2 N 2h as, γ r ¼ .38 where e is electron charge, m is 3mc3 effective electron mass, εm is the dielectric constant of the medium, N is the number of free electrons in the particle, and c is the speed of light. This expression indicates that the radiative relaxation rate is independent of temperature. According the work of Sönnichsen’s et al. (1/160 fs−1 calculated for gold nanorods),43 the radiative contribution can be neglected compared to electron–phonon, surface, and defect scattering contributions.

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ð7Þ

where g is the surface factor, vF is the electron Fermi velocity (1.57 × 106 m s−1 for Cu),1 and D is the diameter of Cu NPs. Since Fermi velocity is temperature-independent,1 by assuming the NPs have the same thermal expansion as the bulk copper45,46 and subtracting the electron–phonon scattering contribution (green curves in Fig. 4(c) and (d)) below Θ in 7.5 nm Cu NPs, we can obtain g = 0.35 that is in good agreement with the value (g = 0.45) in ref. 36. Since the surface factor is only determined by the particle shape, the 10.8 nm Cu NPs should have the same g value. Thus, we can estimate the surface scattering contribution for both cases (red curves in Fig. 4(c) and (d)). By subtracting the surface scattering contribution, only electron–phonon scattering remained for 7.5 nm Cu NPs, and electron–phonon and defect scattering remained for 10.8 nm Cu NPs. Since phonon scattering dominates the change in dephasing time above the Debye temperature Θ (Fig. 4(a) and (b)), above Θ the electron–phonon scattering has linear 1 temperature dependence, ¼ AT,41,47 where A is a constant τeph proportional to the heat capacity (Cv, constant at high temperatures) because all phonon modes can propagate.1 This dependence can be understood using the Heisenberg uncertainty principle, ΔEΔt ≥ ħ, where the thermal energy Ethermal(= CvT ) can be used to estimate the time scale of phonon damping 1  Ethermal . Using a simple linear fit, we extracted the τeph effective slope A for different sized Cu NPs, which is proportional to the factor (3NkB). Therefore, we get the slope A7.5 nm = 3.08 × 1011 s−1 K−1 for 7.5 nm Cu NPs and slope A10.8 nm = 9.33 × 1011 s−1 K−1 for 10.8 nm Cu NPs. Since the measured slope (A) above the Debye temperature is proportional to Cv ∼ 3NkB (both with classical Debye and our A10:8 nm ¼ 3:03 model) the ratio between different Cu NP sizes A7:5 nm This journal is © The Royal Society of Chemistry 2014

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agrees well with the number of atoms in the NPs N 10:8 nm ¼ 2:99, confirming consistency. N 7:5 nm To evaluate the role of defect scattering for 10.8 nm Cu NPs, we subtracted the electron–phonon scattering and obtained a fit for the defect scattering contribution during analysis. The fitting curve is shown in Fig. 4(d) (blue curve). The contribution of defect scattering for the 10.8 nm NPs was similar to the effect of surface scattering as a function with temperature (Fig. 4(d) and (e)). The combined model (green curves in Fig. 4(a) and (b)) for contributions of individual decay mechanisms (Fig. 4(c)–(e)) is shown for dephasing of localized surface plasmons) reasonably matched the experimental SPP dephasing time for different sized Cu NPs (blue and red scatters in Fig. 4 (a) and (b)). The model fit for 10.8 nm Cu NPs was better than the 7.5 nm Cu NPs because we assumed a perfectly smooth surface for the small NPs. Larger number of defects in larger nanoparticles can be attributed to polycrystallinity, rough surfaces, and growth-induced defects (during Ostwald ripening). Using ALD coating of alumina, we deposited up to 50 layers on the NP surface. To estimate the sensitivity of surface plasmons with change in local dielectric constant (Fig. 1(c)), we monitored the change in optical extinction as a function of growth alumina layer. As we plotted the spectral shift as a function of refractive index of medium, n (Fig. 5(a)), we extracted the slope with refractive index change (228 nm RIU−1). This high sensitivity of Cu NPs to change in refractive index of the surrounding medium indicates potential for applications of ALD coated Cu NPs for nanoplasmonic sensors.5,48 We also tested the surface-enhanced Raman scattering (SERS)

Fig. 5 (a) Spectral shifts as a function of the refractive index of medium surrounding Cu NPs. The calculated spectral sensitivity is 228 nm RIU−1 (RIU = refractive index unit). The solid green line shows a fitted linear curve. (b, c) 3D electric field (squared) simulations of 7.5 and 10.8 nm Cu NPs, respectively. The maximum field enhancements obtained were 9.3 and 6.5 for 7.5 and 10.8 nm NPs, respectively. The brown sphere in the center indicates Cu NP. (d) Raman scattering spectra for single monolayer of the capping ligand (tetradecylphosphonic acid) in 7.5 and 10.8 nm Cu NPs. The excitation wavelength was 543.5 nm.

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using Cu NPs for sensing applications. Fig. 5(b) and (c) show the three-dimensional (3D) simulated electric field intensities (E2) induced by the excitation of surface plasmons. Nominally, Raman scattering signals from single monolayer of molecules cannot be measured due to small scattering cross-sections. However, since the overall Raman scattering enhances scales as E4,49 we obtained SERS enhancement of 86 and 42 for 7.5 and 10.8 nm Cu NPs, respectively. Using the excitation wavelength (543.5 nm shown in Fig. 5(d)), we observed Raman spectra from vibrations of the capping ligand such as PvO stretching and (CH2)n twisting50 due to the strong surface enhancement by the SPP resonance of Cu NPs3 because the Raman signal of the single layer molecule is barely detected.49 As a consistency check, both calculated and experimental Raman enhancement ratio of 7.5 and 10.8 nm Cu NPs gave ∼2, for vibrations of the capping ligand such as PvO stretching and (CH2)n twisting. Since metallic copper exhibits high selectivity and promise for application as nanocatalysts,22,28,29 several groups have focused on the role of surface plasmon waves in improving the conversion efficiency. Since the generation of surface plasmon waves, or hot-carrier electron waves, leads to charge carriers with higher temperature than the lattice, these opticallyexcited polariton waves can be used to reduce the activation energy required for catalytic conversion. Therefore, at the same temperature, a larger fraction of optically-excited hot-carriers would have enough energy to overcome the activation barrier for a reaction, potentially increasing the conversion efficiency. We investigated the dehydration of Furfural using Cu nanorods as a nanocatalyst. Fig. 6(a) shows absorbance of Cu nanorods with two surface plasmon resonances at ∼2.1 and ∼1.4 eV, which were related to the transverse and longitudinal modes, respectively. The quality factors of two modes were 6.9 and 26, respectively. The quality factor for transverse value was close to the expected value, from the position of plasmon resonance

Fig. 6 (a) Optical absorbance of Cu nanorods. Inset: TEM image of Cu nanorods. (b) Two possible reaction pathways: Furfural hydrogenation and decarboxylation. (c) Kinetic plots of selectivity (left axis) and conversion (right axis) as a function of time for shaped Cu NPs (nanorods).

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(Fig. 3(b)). The inset of Fig. 6(a) shows the TEM image of Cu nanorods with an average diameter of 25–30 nm, an average length of 150–250 nm, which gives a length-to-diameter (L/D) ratio of 6–10. The longitudinal quality factor was larger than the transverse mode because of smaller spatial confinement, reduced surface scattering in the longitudinal direction of Cu nanorods compared to the transverse direction, and lower losses due to interband transitions in copper (large spectral separation between longitudinal plasmon mode and interband copper transitions). To test the potential of large surface-areato-volume ratio shaped Cu NPs (mainly nanorods) for selective catalysis, we used these nanocatalysts for furfural hydrogenation reaction. There were two possible reaction pathways: furfural hydrogenation and decarboxylation as shown in Fig. 6(b).51 Even though copper NPs demonstrate modest conversion efficiencies (green curve, Fig. 6(c)), the important challenge here was to control the reaction selectivity for the production of the valuable biofuel products methylfuran and furfuryl alcohol, as opposed to the less desirable furan and THF. Extremely high selectivity for production of desired products (methylfuran and furfuryl alcohol) and negligible undesired side reactions, shown in Fig. 6(c), indicates that Cu nanorods can be used as highly selective nanocatalyst. Generation of highly-efficient surface plasmon waves can be investigated to enhance the conversion efficiency for this biofuel reaction. Furthermore, Fig. 6(c) showed the conversion of the reaction decreased with increasing reaction time, likely due to the surface oxidation. Since the SPP waves can also induce hydrogen reduction on Cu NP surface,28 surface plasmon waves can find further applications in the field of copper nanoparticle catalysis. Therefore, these different size and shaped Cu NPs demonstrate huge potential for broad range of applications from nanoplasmonics and sensing to catalysis.

Conclusions In summary, we successfully prevented the surface oxidation of Cu NPs using a thin, smooth, conformal ALD coating with alumina. The measurements of the quality factor of SPP resonance and dephasing time with temperature, along with our theoretical model decouples the relative contributions of different photophysical mechanism in scattering or dephasing localized SPP waves on Cu NPs. We also show that the electron– phonon scattering plays a dominant role in dephasing localized surface plasmons, although inherent damping in copper due to interband transitions and electron scattering (or Joule heating) also contributes substantially. Exciting surface plasmon modes in shaped Cu NPs like nanorods reduced the spectral overlap and hence losses due to interband transitions in copper metal. This analysis should help in understanding the scattering or dephasing mechanism, and design of lowloss localized surface plasmon modes in other metal NPs too. Using the chemically stable Cu NPs developed here, we demonstrated their applications for refractive index sensing, SERS enhancement, and single nanoparticle spectroscopy. We

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also investigated the selective catalysis using Cu nanorods. These findings will provide valuable insights for the development of nanoscale copper for nanophotonics, plasmonics, catalysis, and nanoscale electronics.

Acknowledgements This work was supported by start-up funds from University of Colorado, and National Science Foundation CAREER award CBET-1351281. The authors would like to thank Amanda N. Bader and Gordana Dukovic for allowing use of their glovebox, Joanna Atkin and Markus B. Raschke for help with Raman spectroscopy, and Simon H. Pang and J. Will Medlin for help with catalysis.

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Nanoscale, 2014, 6, 12450–12457 | 12457

Copper plasmonics and catalysis: role of electron-phonon interactions in dephasing localized surface plasmons.

Copper metal can provide an important alternative for the development of efficient, low-cost and low-loss plasmonic nanoparticles, and selective nanoc...
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