Transformed Gold Island Film Improves Light-to ... - ACS Publications

Jul 23, 2008 - In this letter we characterize a second-generation optical plasmon capillary (OPC-II) in which thermally transformed electroless (EL) A...
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J. Phys. Chem. C 2008, 112, 12214–12218

Transformed Gold Island Film Improves Light-to-Heat Transduction of Nanoparticles on Silica Capillaries Wonmi Ahn† and D. Keith Roper*,‡ Department of Materials Science and Engineering, 304 CME, UniVersity of Utah, Salt Lake City, Utah 84112, and Department of Chemical Engineering, 3290 MEB, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: March 21, 2008; ReVised Manuscript ReceiVed: May 29, 2008

Gold (Au) nanoparticles (NP) assembled on inner walls of silica (SiO2) capillaries dissipated g10-fold more heat (g96.92 ( 8.94 vs e9.92 ( 2.06%) from incident photons via localized surface plasmon resonance (LSPR) with a 10-fold faster response time (e8.39 ( 1.05 vs g86.30 ( 8.61 s) than colloidal Au NPs suspended in H2O. Au NP assemblies were created by thermal transformation of gold island film (TGIF) that was electrolessly plated on the inner capillary wall to form an optical plasmon capillary (OPC). Enhanced laser-to-heat transduction of the solid-state Au NP OPC resulted from the ability to tune the LSPR peak to the laser line, elimination of laser- or heat-induced NP aggregation, and enhanced thermal transport of the Au NP assembly on the OPC. This yielded g6.4-fold higher temperature increase per unit incident laser power (157.07 ( 21.68 vs 24.51 ( 4.45 °C watt-1) relative to suspended Au NPs. Thermal behavior of the OPC was predictable using design equations obtained from an overall energy balance and consistent with nanoscale opto-thermal expressions. 1. Introduction Microscale heat generation in a thin metal films is important in integrated circuits and microchips,1 microelectromechanical systems (MEMS)2 and metal-coated microstructures3 including sensors, reactors, microPCR devices,4,5 capillary electrophoresis chips6 and DNA/RNA hybridization imaging arrays.7 Laser wavelengths in NIR,8 visible9 and X-ray10 regions have been used to thermalize the electron gas in metal nanoparticles,9 nanoshells (NS),8 and nanometer-thick-films on fused silica substrates.11 The photon-absorbent localized surface plasmon radiation (LSPR) band can be bleached by subpicosecond laser pulses that broaden the absorbance spectra due to transient nonFermi ‘hot’ electrons.12 Subsequent electron-phonon interactions heat the bulk metal. Heat dissipation via conduction13,14 and radiation15 in the bulk metal has been examined theoretically13 and experimentally.16 High-power photoexcitation of NPs or NSs in suspension produces large temperature increases, but can induce shape fragmentation, coagulation, color change, and bubble formation.17,18 On the other hand, low-intensity continuous-wave (cw) laser irradiation of colloidal metal NPs generates modest temperature increases. Improved opto-thermal heat dissipation via cw laser irradiation of a stable nanoparticle structure to rapidly control predictable temperature increases at submicroscopic length scales would provide clear benefit to MEMS and metal-coated microstructures. We recently reported induction of thermal dissipation via localized surface plasmon resonance (LSPR) in thermally isolated aqueous suspensions of Au NP using cw resonant laser light.19 A linearized energy balance on the system showed equilibrium temperature increased in proportion to laser power (from 0.085 to 0.5 W) and gold NP concentration (ranging from * To whom correspondence should be addressed. Tel: +1 801 585 9185. Fax: +1 801 585 9291. E-mail: [email protected]. † Department of Materials Science and Engineering. ‡ Department of Chemical Engineering.

0.0046 to 0.092 g Au per 100 mL solution) in a capillary suspended in medium vacuum according to

[ ]

τs(1 - 10-Aλ)ηT ∆Teq ) I mC

∑

(1)

i p,i

i

where [∆Teq/I] is mean equilibrium temperature increase per incident laser power (°C watt-1), τs is system heat-transfer time constant, Aλ is absorbance of NPs in a sample cell at wavelength λ, ηT is transduction efficiency, and the i terms in ΣimiCp,i represent products of mass, m, and heat capacity, Cp of system components. Equation 1 neglects heat dissipated from light absorbed by a SiO2 capillary itself (Qo) since Qo , ΣimiCp,i(Tmax - Tamb)/τs, e.g., 8.37 × 10-6 W , 0.015 W. Thermally isolated, aqueous suspensions of Au NP in a borosilicate glass capillary exhibited a system heat-transfer time constant of τs ) 86.3 ( 8.61 s and an average temperature increase per watt of ∆Teq/I ) 24.51 ( 4.45 °C watt-1. A 36-nm difference between the Au NP LSPR wavelength, λLSPR)550 nm, and the laser excitation wavelength, λexc)514 nm, yielded a value of (1-10-A) ) 0.40 for the absorbance term in eq 1 before irradiation. It decreased to 0.34 after irradiation due to laserinduced Au NP aggregation. A laser-to-heat transduction efficiency, ηT ) 3.12 ( 0.40%, was measured for Au NP suspensions, due in part to laser-induced Au NP aggregation. Modulating the incident laser power at a rate greater than the frequency of suspended Au NP collisions reduced aggregation and increased laser-to-heat efficiency to (9.92 ( 2.06) %. In this letter we characterize a second-generation optical plasmon capillary (OPC-II) in which thermally transformed electroless (EL) Au island films were assembled directly onto the internal SiO2 surface. This produces a solid-state device with g6.4-fold higher ∆Teq/I ) (157.07 ( 21.68) °C watt-1 under ambient conditions due to (1) enhanced LSPR absorbance due to dense assemblies of Au NP; (2) a tunable LSPR wavelength that gave λLSPR - λexce 17 nm; and (2) 96-99% laser-to-heat efficiency

10.1021/jp802497v CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

Light-to-Heat Transduction of Nanoparticles arising from the stability and enhanced thermal transport of the 3D Au NP assembly. Nanoparticles have been assembled on silicon by deposition using AFM tips, nanolithograph, pulsed lasers using electrochemistry,20 and microwaves,21 by self-assembly on thiols,22 dendrimers23 or other polymers,24 by conjugation to porphyrin polymers, evaporation,25,26 and by resistive27 or thermal28 heating of evaporated thin film. Electroless deposition of Au has the advantage of being able to rapidly coat fragile, 3D or internal surfaces at ambient conditions without requiring conductive substrates or expensive, sophisticated equipment. It has been used to create nanowires,29 nanocrystals,30 nanodomes,31 thin films on self-assembled Au NP colloid monolayers,32–34 and electrodes.35 Effects of plating solution composition, temperature, pH, and reducing agent on deposition rate, stability and resulting crystal structure are known.36–38 Recent attention has been given to EL Au plating by galvanic substitution of less noble metals by gold29,39–43 focused on depositing Au structures like islands and thin films followed by electrical,44 thermal,45 or flame46 annealing to modify surface properties. We recently characterized thermal transformation of gold island films (TGIF) to Au NP assemblies on SiO2, varying EL deposition time39,47,48 and thermal annealing time and temperature to produce monomodal ensembles of NPs from (9.5 ( 4.0) to (266 ( 22) nm at densities ranging from (2.6 × 1011) to (4.3 × 108) particles cm-2.49 Effects of particle sizing and interparticle spacing measured by AFM and SEM on photoluminescence and spectral extinction intensity, LSPR wavelength and fwhm measured by transmission UV spectroscopy (T-UV) were examined. Here we extend this development to consider enhanced thermal features of these stable Au NP ensembles on SiO2 in an OPCII.

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12215

Figure 1. Absorbance spectra in arbitrary units (a.u.) and color changes of OPC-II after EL Au plating (A, dash-dot), and after thermal treatments at 250 °C for 90 min. (B, dash) and at 350 °C for 180 min (C, solid). Inset shows absorbance in OPC-I before (D, solid) and after (E, dot) six-periods of laser exposure at I ) 0.085, 0.15 and 0.5 W.

TABLE 1: Physical and Optical Features of OPC-I and II

2. Experimental Methods Briefly, internal walls of borosilicate glass capillaries were sensitized by a tin (Sn2+) solution and activated by ammoniacal AgNO3 which was galvanically replaced by Au reduced from a solution of gold sulfite, Na3[Au(SO3)2], inserted into an open end of the capillary, incubated for 15 s and removed by absorption onto a Kimwipe. Sensitization, activation and galvanic replacement steps were repeated at least three times, alternating the end at which solution was inserted to increase EL uniformity. Excess metal residue was removed between successive gold plating steps by washing the internal walls of the capillary with 25 °C distilled, deionized and degassed water (D3-H2O). Au NPs were then formed on SiO2 of several OPCII samples by subjecting them to successive heating at temperatures that included 250, 350, and 500 °C. Heating and cooling ramp rates at these temperatures were 15, 15 and 25 min, respectively. The furnace was purged with a N2 gas for ∼10 min before heating in the first two thermal processes, and continuous circulation of N2 gas was maintained throughout the 500 °C heating at flow rate of 3.52 standard liters per minute (SLPM). OPC-II in Figure 1 was heated at 250 and 350 °C, whereas OPC-II in Table 1 was subjected to an additional heat treatment at 500 °C (see discussion for details). The OPC-II was placed at right angles to a cw Ar-Ion laser beam (514 nm BeamLok 2060, Spectra-physics, Mountain View, CA) in an ambient environment. The laser beam was collimated to 2.0 mm using a 10x objective lens, which decreased incident laser power by (26.55 ( 8.17) %, and mechanically chopped at 6 × 103 rpm using a 60-slot chopper, which decreased incident laser power 50%. Decreases in incident laser power were measured using a power meter (Laser Precision Corp.).

Laser power values in Figure 2 represent actual incident values after correcting for these decreases. Dynamic temperature changes were monitored using a thermocouple (TC) and recorded by a digital thermometer (Omega HH509R, Stamford, CT). Collimated laser beam and TC were adjusted to the OPCII surface, and the heating was performed with and without H2O injected into the OPC-II to evaluate effects of local refractive index and thermal conductivity on laser-to-heat transduction. 3. Results and Discussion Figure 1 shows spectral changes of an OPC-II as it experiences successive thermal treatments to transform Au island films (TGIF) obtained via EL plating into solid-state Au NP assemblies on inner SiO2 capillary walls. A dark blue/purple hue in (A) appeared after EL plating, but before thermal transformation, corresponding maximum absorbance at 776 nm of EL

12216 J. Phys. Chem. C, Vol. 112, No. 32, 2008

Figure 2. Temperature in OPC-II increases from an ambient value of 20.1 °C to equilibrium values by laser powers of 0.004W (at 30 s), 0.008W (at 60 s), 0.011W (at 120 s), 0.0155W (at 180 s) and 0.0184W (at 240 s). Air cell OPC-II (open circle) was filled with water afterward (filled circle). An empty control capillary was not gold plated (open triangle). Inset shows temperature vs time profiles of OPC-I as laser power intensities increased from 0.085 (solid), 0.15 (dash) to 0.5 W (dash-dot).

plated Au island thin films that were visualized with scanning electron microscopy (SEM).49 The violet color in (B) appeared after heating at 250 °C (90 min), corresponding to maximum absorbance at 592 nm of a mixture of Au island thin films and Au NP observed by SEM. The red hue in (C) obtained after heating at 350 °C (180 min) corresponded to maximum absorbance of Au NP at 539 nm. Additional heating at 500 °C (20 min) changed the color to pink, blue-shifted maximum absorbance to 519 nm and decreased the full width at halfmaximum (FWHW) of the plasmon band and decreased extinction intensity 18% (data not shown), suggesting the Au NP assembly became more uniform but less dense. Spectra of OPC-II shown in Table 1 were similar but with lower maximum absorbance. Variations in Au NP density of OPC-II shown in Table 1 arose from nonhomogeneous tin and silver deposition. Homogeneity was improved by alternating insertion direction of each aqueous solution to produce more uniform gold thin films as shown in Figure 1. Average NP density in counts cm-2 of OPC-II shown in Table 1 was (∼4.29 × 1010) after the final thermal treatment by comparison with T-UV and SEM results from Au NP assembled via TGIF on flat quartz slides.49 Evaporation of EL plating solution left more concentrated Au NP at the point where meniscus formation occurred at the top and bottom of the capillary (the ‘coffee ring’ effect). A trapped air bubble produced the small white dot at the right edge of the capillary in Table 1. The inset to Figure 1 shows LSPR wavelength red-shifted and extinction intensity decreased in OPC-I after six periods of laser irradiation at I ) 0.085, 0.15 and 0.5 W as a result of aggregation between NPs. Figure 2 shows temperature changing vs time in OPC-II as laser power (I) was increased stepwise from I ) 0.004 W at 30 s, to 0.008 W at 60 s, 0.011 W at 120 s, 0.0155 W at 180 s, and finally to 0.0184 W at 240 s. An uncoated, empty capillary showed no temperature increase at corresponding I increases (open triangles). Temperature in air- (open circle) and water(filled circle) filled OPC-II capillaries increased from an ambient value of 20.1 °C to maximum equilibrium values between 20.6 and 23.4 °C. The difference between average temperatures measured in an air and water cell was negligible. The inset in Figure 2 shows temperature increasing from an ambient value of 25.5 to 27.7, 30.0 and 35.5 °C at incident laser powers of 0.085, 0.15 and 0.5 W, respectively, in a first-generation OPC-I that contained Au NP colloid in a thermally isolated chamber.

Ahn and Roper Temperature increase per unit watt, ∆Teq/I, and equilibration rate are significantly higher in OPC-II than OPC-I, g6.4-fold. Higher scatter in OPC-II temperature data above 21.3 °C is attributable to the open environment. Table 1 compares physical, thermal and spectroscopic features of OPC-I and OPC-II. We characterized each cell with an overall energy balance,49 by introducing a dimensionless temperature, θ ≡ (Tamb - T)/(Tamb - Tmax), scaled using maximum, Tmax, and ambient, Tamb temperatures, and a microscale thermal equilibrium time constant for the system τs ≡ (ΣimiCp,i)/hA, where the i terms in ΣimiCp,i represent products of mass, m, and heat capacity, Cp of i system components including borosilicate sample cell and gold NP film for an empty cell, ΣimiCp,iair ) 0.0286 J/K, with an additional water term in a filled cell, ΣimiCp,iwater ) 0.0522 J/K, h is heat transfer coefficient and A is heat transfer area. Values of τs were estimated as the reciprocal slope of a plot of ln(1 - θ) versus time, t, using the dynamic equation, θ ) 1 - exp(-t/τs). This approach also yields the equilibrium expressions in eq 1. It is valid when τs is much greater than internal equilibration times within the cell. For OPCII, internal equilibration times, τint ≈L2/R, were estimated for borosilicate glass as τint,glass ) 0.011 s using L ) 0.1 mm (half the cell wall thickness) and Rglass ) 8.8 × 10-7 m2s-1, and for water as τint,water ) 0.075 s using L ) 0.1 mm (half the sample thickness) and Rwater ) 1.34 × 10-7 m2s-1. Both values are more than 10-fold smaller than τs values in Table 1 that were measured for OPC-II from temperature profiles in Figure 1, which justifies this approach. Adequate characterization of photon-plasmon heat transfer in OPC-II using eq 1 and the linearized energy balance under nonvacuum conditions illustrates the utility of this approach in ambient conditions anticipated for metal-coated microstructures such as optoelectronic circuits, microfluidics, laboratory-on-chips and MEMS. The time constant for thermal equilibration measured for OPC-II averaged between 4.97 and 8.39 s. This was ten to seventeen times faster than for OPC-I. From the definition of τs and measured values of m, and A and Cp we estimated heat transfer coefficients values for OPC-II in an open, ambient environment 8.24-8.97-fold higher than h values in OPC-I. Transduction efficiency, was estimated by rearranging eq 1 to solve for ηT and subtracting heating from laser light incident on the glass cell, Qo ) 5.4 × 10-4 I in the denominator. Radiative and conductive area for heat transfer was, A ∼100 mm2. Resonant absorbance, Aλ, was measured using a UV-vis spectrophotometer at laser wavelength, λ ) 514 nm at each air and water cells, respectively: Aλair ) 1.196 and Aλ,water ) 1.266. Values of optothermal laser-to-heat transduction efficiency in OPC-II (96.92 ( 8.94 - 99.22 ( 2.51%) estimated using eq 1 and summarized in Table 1 increased g10-fold compared to values estimated for OPC-I (3.12 ( 0.40 - 9.92 ( 2.06%). We attribute relatively rapid thermal dynamics and large laserto-heat transduction in OPC-II to (1) dense solid-state Au NP assemblies that enhanced LSPR absorbance; (2) a tunable LSPR wavelength that gave λLSPR - λexc e 17 nm; (3) proximity of assembled Au NP to SiO2; and (4) optothermal stability of Au NP assembled on SiO2. Spherical NP separated by distances smaller than incident wavelength exhibit enhanced intraparticle Forster energy transfer due to dipole-dipole coupling that may be increased by contributions of surface waves.50 Below, we compare temperature increase per unit incident power for Au NP in assemblies and suspensions to estimate the degree of energy transfer enhancement. Rapid energy transfer from Au NP to adjacent NP also reduces photobleaching, increasing the time-averaged value of Aλ during irradiation by the cw laser

Light-to-Heat Transduction of Nanoparticles

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12217 calculated from Au NP absorbance (A) by Beer’s Law, (110-A)I ) 0.3484, and inversely with laser spot size (σI ) 7.07 × 10-6 m2), NP concentration ([Au] ) 920 g m-3), Au heat capacity (CpAu ) 0.129 J g-1 K-1), and cell path length (l ) 0.2 mm), according to -A I

∆T - 10 ) [ I(∆t) ] ) (1σ [Au]C l I

I

Figure 3. Equilibrium temperature increase (∆T eq, °C) per incident laser power (I, W) as a function of I in OPC-I (diamond) at I ) 0.085, 0.15 and 0.5 W, and in OPC-II (triangle) at I ) 0.004, 0.008, 0.011, 0.0155 and 0.0184W.

chopped at 6 × 10-3 s-1, which increased the absorbance term in Eq 1 for OPC-II relative to OPC-I. Solving the coupled dipole approximation (CDA) to the exact T-matrix method for calculating effects of coupled multipoles51 shows absorbance of 20nm Au NP assembled on SiO2 increases as (i) interparticle spacing decreases and (ii) surrounding refractive index increases.52 Consistent with the model, extinction efficiency of OPC-II increased upon increasing refractive index from 1.0 to 1.33 by adding H2O. Absorbance maximum of 1.211 was measured in an air cell of OPC-II, compared with a maximum absorbance of 0.328 for Au NP at 920 g of Au m-3 in OPC-I. Increasing Au NP density blue-shifted absorbance spectra of assembled Au NP, producing less than 17-nm differences between excitation and LSPR frequencies as summarized in Table 1. This permitted the absorbance term, (1-10-A), in eq 1 to remain g0.9 for OPC-II. Adding H2O red-shifted the LSPR wavelengths. Rapid thermal equilibration between assembled Au NP and contiguous SiO2 suggests estimating internal equilibration times for borosilicate glass and water of τint,glass ) 0.011 s, and τint,water ) 0.075 s, respectively, as outlined above. Equilibration time within an Au NP is < 10-9 s.53 By comparison, thermal diffusion length between neighboring Au NP in OPC-I was LD ) rp(Fp/xAu)1/3 ∼ 275.8 nm, where rp (particle radius) ) 10 nm, Fp (particle density) ) 19.3 g cm-3 and xAu (Au concentration) ) 920 g Au m-3, which increased equilibration time in OPC-I. We estimated internal equilibration for OPC-I at τint ) 4.5 s 19 using L ) 2 mm (half the cell width) and Rglass ) 8.8 × 10-7 m2s-1. The resulting ratio of τintOPC-I/τintOPC-II ∼ 60 is consistent with the ratio of measured time constants for OPC-I and OPCII g10. While absorbance at Au NP LSPR wavelength (ALSPR) for OPC-I decreased from 0.328 before laser irradiation to 0.248 afterward due to laser-induced aggregation, ALSPR for OPC-II remained constant. Ratio of absorbance values at λLSPR to λexc was closer to 1 in OPC-II, which indicates that OPC-II generates higher energy conversion with a given source of energy. Figure 3 shows mean equilibrium temperature increase (∆Teq, °C) per incident laser power (I, W) as a function of I was 6.4-fold higher for OPC-II (triangles) than OPC-I (diamonds). As temperature increases, thermal conductivity (K) decreases54 and heat capacity (Cp) increases55 for Au and SiO2. This results in ∆T /I of OPC-II increasing ∼36% as I increases from 0.004 to 0.0184 W in Figure 3. ∆T /I for OPC-I remains relatively constant as I increases because K and Cp of H2O dominate the series of resistance terms. Temperature in suspended Au NP relative to incident laser energy in OPC-I, [∆T /I∆t]I, increases in proportion to relative energy absorbance

Au p

(2)

Rapid internal equilibrium in Au NP arises from energy exchange between phonons and electrons characterized by phonon-electron coupling factor, G ) π4(neνsκ)2/18K, where ne is the number density of electrons per unit volume, νs is the speed of sound, κ is Boltzmann constant and K is thermal conductivity.56 This factor couples (1) heating of the electron gas by photon absorbance, Ce(∂Te/∂t) ) 3 · (K3Te)-G(Te-Tl)+S, with (2) heating of the metal lattice by electron-lattice diffusion, Cl(∂Tl/∂t) ) G(Te-Tl), where C is volumetric heat capacity, S is laser source energy, and subscripts e and l standing for electron and metal lattice, respectively. Temperature in Au NP assemblies relative to incident laser energy in OPC-II, [∆T /I∆t]II, increases in proportion to relative NP absorbance, (1-10-A)II ) 0.9465, and inversely with laser spot size (σII ) 3.5 × 10-6 m2) as well as number density (NNP ) 1.28 × 1010 NP cm-2 in a water cell), volume (VNP ) 4.19 × 10-24 m3), mass density (FNP ) 19.3 g cm-3) and heat capacity (CpAu) of 20-nm NPs according to -A II

∆T [ I(∆t) ] ) kσ (1N -V10 F ) C II

II

Au NP NP NP p

(3)

where a factor k g 1 in the denominator represents the ratio of the area of assembled Au NP heated by incident laser light due to enhanced energy transfer relative to the area of assembled Au NP in the laser spot. Taking a ratio of eqs 2 and 3 and substituting corresponding physical values gives 7.8 ) 29/k. This suggests enhanced energy transfer extends to an area ∼3.7 times the size of the laser spot. Combining eqs 1, 2 and 3 gives

ηTII ηTI

)

(hA)IIσI[Au]l kσII(hA)INNPVNPFNP

(4)

which yields a value of ηTII /ηTI ∼15.8 after substitution. Comparing this with the experimentally measured value of ηTII /ηTI ) ∼10 indicates the contribution of enhanced energy transfer in dense Au NP assemblies to improved laser-to-heat transduction in OPC-II. 4. Conclusions In summary, expressions for thermal transport in nanoscale systems indicate that order-of-magnitude improvements in laserto-heat transduction using dense Au NP assemblies on SiO2 arise from resonant near-field interactions that increase LSPR absorbance at laser excitation frequency, and from enhanced energy transfer which increases transduction efficiency. Time-scale and spectroscopic analysis, respectively, showed rapid NP-SiO2 thermal transport and stability of assembled Au NP to laser excitation and heating yielded faster thermal dynamics and higher equilibrium temperatures in turn. Rapid measured temperature increases of (157.07 ( 21.68) °C watt-1 were predictable using a linearized energy balance. These advances offer significant potential for improved optoplasmonic heating in MEMS, sensors, and laboratory-on-chip devices as well as in thermal therapies and thermal imaging applications using

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