Letter pubs.acs.org/NanoLett
Enhanced Photothermal Conversion in Vertically Oriented Gallium Arsenide Nanowire Arrays Jaspreet Walia,*,†,‡ Navneet Dhindsa,†,‡ Jeremy Flannery,§ Iman Khodabad,†,‡ James Forrest,‡,§ Ray LaPierre,∥ and Simarjeet S. Saini*,†,‡ †
Department of Electrical and Computer Engineering, ‡Waterloo Institute of Nanotechnology, and §Department of Physics and Astronomy, University of Waterloo, 200 University Ave West, Waterloo, ON N2L 3G1, Canada ∥ Department of Engineering Physics, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4L7, Canada ABSTRACT: The photothermal properties of vertically etched gallium arsenide nanowire arrays are examined using Raman spectroscopy. The nanowires are arranged in square lattices with a constant pitch of 400 nm and diameters ranging from 50 to 155 nm. The arrays were illuminated using a 532 nm laser with an incident energy density of 10 W/mm2. Nanowire temperatures were highly dependent on the nanowire diameter and were determined by measuring the spectral red-shift for both TO and LO phonons. The highest temperatures were observed for 95 nm diameter nanowires, whose top facets and sidewalls heated up to 600 and 440 K, respectively, and decreased significantly for the smaller or larger diameters studied. The diameter-dependent heating is explained by resonant coupling of the incident laser light into optical modes of the nanowires, resulting in increased absorption. Photothermal activity in a given nanowire diameter can be optimized by proper wavelength selection, as confirmed using computer simulations. This demonstrates that the photothermal properties of GaAs nanowires can be enhanced and tuned by using a photonic lattice structure and that smaller nanowire diameters are not necessarily better to achieve efficient photothermal conversion. The diameter and wavelength dependence of the optical coupling could allow for localized temperature gradients by creating arrays which consist of different diameters. KEYWORDS: Gallium arsenide, nanowires, photothermal, Raman scattering
R
efficient optical absorption is the one-dimensional semiconductor nanowire. The increased absorption and resonant waveguiding properties of nanowires are attracting ever increasing attention for a wide variety of applications including lasers,7,8 solar cells,9−12 structural color generation,13−15 imagers,16 and sensors.17−19 Specifically, vertically oriented nanowire arrays are known to have markedly increased optical absorption due to resonant coupling into optical modes.20 Nanowires have also been implicated as efficient thermoelectric materials.21−24 Semiconductor nanowires are known to have thermal conductivities which are dominated by phonon processes, as opposed to electronic processes, which make them an attractive choice for thermoelectric devices.25−28 For example, GaAs in its bulk form has a bulk thermal conductivity of 54 W m−1 K−1, whereas GaAs nanowires have been reported to have thermal conductivity ranging from 8 to 36 W m−1 K−1.24 Silicon nanowires have shown a 100-fold increase in ZT values over bulk silicon.21−23 Computer simulations have also suggested that the thermal conductivity of nanowires can be further reduced by creating nanowires with rough surfaces.29 The thermal conductivity of GaAs is naturally lower than Si,
ecently, there has been interest in nanostructures which efficiently convert incident light into heat for photothermal electrical (PTE) applications. Current applications include PTE devices for energy conversion,1−3 bioimplantable electronics, 4 and laser therapeutic agents for selective elimination of microorganisms.5 In PTE applications, light is converted to heat and then converted to electricity, presenting an additional method to fully utilize solar energy. The thermoelectric figure of merit, ZT = σS2T/κ, determines the efficiency of a given thermoelectric (TE) device. Here, S is the Seebeck coefficient, σ is the electrical conductivity, and κ is the thermal conductivity. Naturally, materials that possess both a high electrical conductivity and a low thermal conductivity are of particular interest for use in thermoelectric devices as this would lead to the highest values of ZT. Additionally, PTE applications must efficiently use incoming light to generate heat. As such, there has been an increased attention toward materials which efficiently absorb light and have low thermal conductivity. For example, enhanced PTE conversion in tilted layered cobaltite thin films was demonstrated by using a carbon nanotube layer to increase optical absorption.6 Another study has used the enhanced absorption of carbon nanotubes to develop a PTE converter for bioelectronic applications,4 which uses a low-power laser source outside of the body to power an implanted device. Another structure that is of interest for © 2014 American Chemical Society
Received: July 16, 2014 Revised: September 9, 2014 Published: September 18, 2014 5820
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Figure 1. (a) Top and (b) 30° tilted view SEM images of 95 nm diameter GaAs nanowires spaced 400 nm apart, arranged in a square lattice.
electron microscopy (SEM) images of the 95 nm diameter arrays, where uniform diameters and pitch between the nanowires is seen, along with their vertical nature. Raman Measurements. Confocal micro-Raman spectra were measured using a Jobin-Yvon HR800 spectrometer in the backscattering configuration, with a spectral resolution of ∼0.5 cm−1. A 532 nm excitation laser was used and focused down to a 100 μm2 spot using a 0.35 NA 20× objective. The incident power was 1 mW, and the peak intensity on the focused spot was 10 W/mm2. The highest possible laser power was used such that the smallest diameter sample remained at room temperature. Each spectrum was recorded using an exposure time of 45 s and summed over 15 accumulations. The incident light field was polarized along one of the photonic lattice directions. Results and Discussion. Raman spectra for GaAs nanowires with diameters ranging from 50 to 155 nm are shown in Figure 2. A Raman spectrum was also measured from the unpatterned area between the nanowire arrays corresponding to the etched bottom GaAs surface and is also shown in Figure 2 (labeled as “bulk”). In the backscattering geometry, the Raman selection rules for zinc blende GaAs forbid
making it an attractive material choice for nanowire-based PTE applications. In this paper, the photothermal potentials of vertically etched zinc blende GaAs nanowires arranged in square lattices are examined by measuring temperature increases using microRaman spectroscopy. The diameter of the nanowires was varied between 50 and 155 nm in 15 nm increments. The nanowire temperature increased in a resonant fashion to a maximum value of 600 K at the top facets for the 95 nm diameter sample and to 440 K at the sidewalls for the same sample. In contrast, 50 nm diameter GaAs nanowires remained at room temperature, and 155 nm diameter nanowires only heated up to 350 K. The increased heating effect is attributed to resonant coupling of the incident light field into the HE11 (hybrid electric) mode in the cylindrical nanowire waveguide and is confirmed using computer simulations. To our knowledge, this is the first time diameter-dependent photothermal effects have been reported for nanowires. The findings demonstrate that a photonic lattice consisting of vertically oriented GaAs nanowires allows for further optimization of the photothermal effect. Additionally, the strong diameter dependence allows for the creation of localized temperature gradients by creating arrays consisting of different diameters, which can be used for new PTE devices that use planar temperature gradients rather than the longitudinal gradients normally studied. Experimental Details. Fabrication. The GaAs nanowires were fabricated on a (100) GaAs wafer using a top-down approach involving inductively coupled plasma reactive ion etching (ICP-RIE). The fabrication process has been described in detail previously.30 Briefly, a thin layer of PMMA 950A3 was spin-coated onto the GaAs wafer, followed by baking at 373 K in an oven. A mask pattern was written using electron beam lithography, using a RAITH150-TWO operating at 20 kV, and was developed in a methyl isobutyl ketone (MIBK) and isopropyl alcohol (IPA) solution for 30 s. An Al/Cr (20/30 nm) layer was deposited at room temperature and low pressure using an Intlvac e-beam evaporator. The mask was then created by lift-off using acetone. The mask protected the substrate from etching, resulting in the formation of nanowires. Etching was performed in an Oxford PlasmaTherm ICP-RIE system using a gas mixture of Cl2 (20 sccm) and N2 (5 sccm) at a pressure of 10 mTorr. The RF power level was 500 W, and the ICP power level was 60 W, resulting in an etch rate of about 10 nm/s at 20 °C. The nanowires were etched to a height of 1.0 μm, with eight different diameters ranging from 50 to 155 nm in 15 nm increments. The nanowires were arranged in a square lattice with a 400 nm pitch. The overall lattice sizes were 100 μm × 100 μm. Figure 1 shows top view and side view scanning
Figure 2. Raman spectra covering the 220−305 cm−1 spectral range for eight different diameters of GaAs nanowires ranging from 50 to 155 nm. The Raman spectrum from the bulk starting wafer is also shown. For all nanowires, scattering from the LO mode at 291 cm−1 and the TO mode centered at 267 cm−1 are observed, which are consistent with scattering from the zinc blende crystal structure. The SO mode centered at 286 cm−1 is also present in all nanowire samples. 5821
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with functions of the product of the wavevector and cylinder radius R. The prime notation denotes the first derivative of the Bessel function. For GaAs, values for dielectric constant, ε0 = 12.85, and dynamic dielectric constant, ε∞ = 10.88, are generally used in the calculations.31 If the surrounding medium is taken to be air (εm = 1), then one obtains a value of 286 cm−1 for the frequency of surface phonons for a nanowire diameter of 50 nm at room temperature. Again, this value is in excellent agreement with the experimentally measured value of 286.3 cm−1. Therefore, it can be concluded from the fitting parameters associated with each peak that the 50 nm diameter sample did not experience any heating effects from the Raman laser, and the nanowire temperature is 300 K for this sample. A similar analysis was performed for the other nanowire diameters. The center frequency, scattering intensities, and FWHM of the TO and LO modes as a function of diameter are plotted in Figure 4. A highly diameter-dependent nonmonotonic red-shift and spectral broadening are seen for both modes. The maximum red-shift and broadening are observed for the 95 nm diameter sample, which also has the highest Raman scattering intensity. It has been previously shown that the local temperature increase of a GaAs nanowire can be determined from the spectral shift of the peak corresponding to scattering from the TO or LO phonon by using the following linear relation:34,35
scattering from the tranverse optical (TO) phonon centered at 267 cm−1 for the (100) GaAs starting wafer, and only the longitudinal optical (LO) phonon is observed in the corresponding Raman spectrum. The etching process introduces surface roughness and crystalline damage along the nanowire sidewalls and relaxes this selection rule, and scattering by TO phonons can be observed.31 The area of the etched wafer where the nanowires are not present only shows one spectral feature corresponding to the LO phonon. The lack of TO scattering indicates that the bottom wafer surface is still smooth after the etching process,31 and therefore the presence of TO scattering is from the nanowire sidewalls. The Raman spectra from vertically etched nanowires show three distinct spectral features for all of the diameters. In all cases peaks corresponding to scattering from TO phonons at 267 cm−1 and LO phonons at 290 cm−1 are seen as well as a third peak corresponding to scattering from surface optical phonons (SO).31−33 As can be readily seen in Figure 2, the intensity of the LO and TO phonons is highly dependent on the diameter of the nanowires. Diameter-dependent red-shifts in the LO and TO phonons and concomitant broadening of the Raman lines are also observed. In order to accurately determine the peak position and full width at half-maximum (FWHM) of the lines, the Raman spectra were deconvolved into three Lorentzian peaks, and a representative fit for the 50 nm sample is shown in Figure 3. The TO and LO modes for the 50 nm diameter
dυ/dT = 0.016 cm−1 K−1
Calculated temperatures from the TO and LO phonons are also plotted in Figure 4 using eq 3. Since the nanowires are vertically oriented with respect to the light source, it is expected that the top facets will heat up more. Scattering from the TO and LO modes is thought to be from the sidewalls and top facets of the nanowires, respectively (as evidenced by the presence of the TO mode only after nanowire sidewalls are created). It is therefore expected that each nanowire will have a different local temperature on the top facet and sidewalls depending on the nanowire diameter. The dependence of the local nanowire temperature on the nanowire diameter is clearly seen (Figure 4b,c) and demonstrates the use of photonic modes in achieving higher light-induced temperatures. In previous studies involving heating in nanowires, the highest local temperatures are usually observed for the lowest nanowire diameters for a given input energy flux due to reduced thermal conductivities as the diameter decreases.24,29 In contrast, this study confirms that by using vertically oriented GaAs nanowires with a diameter of 95 nm, more heat can be generated for the same amount of input light at a wavelength of 532 nm, and therefore smaller nanowires are not necessarily better. Specifically, temperature increases approaching 300 K at the top facets and 140 K at the sidewalls are realized for the diameter of 95 nm. As a comparison, a temperature increase of 50 K was observed in single wall carbon nanotubes embedded in polydimethylsilicate substrate for an incident intensity of 318 mW/mm2.4 The large Raman scattering enhancement is an indication of strong electric field confinement within the nanowires, leading to higher temperatures. The resonance diameter was observed in this study because of the small increments in diameter used, as opposed to the larger increments studied in previous works. Previous Raman scattering studies on vertically oriented crystalline silicon nanowires arranged in square lattices36,37 have not shown any local heating, even at laser input intensities an order of magnitude higher than the one used in this study. This result
Figure 3. Spectral deconvolution of the Raman spectrum (dark gray trace) for the 50 nm diameter sample. A good fit (solid black line) is achieved by modeling the LO, TO, and SO modes as independent Lorentzian functions (solid gray lines).
nanowire sample are well fit to Lorentzian line shapes with center frequencies of ωTO = 291 cm−1 and ωLO = 267.5 cm−1 and spectral widths of 3 and 3.3 cm−1, which are in excellent agreement with the values for bulk GaAs crystals at T = 300 K. The SO peak was centered at 286 cm−1 with a FWHM of 10 cm−1. A well-accepted expression for the SO frequency at room temperature as a function of cylinder size and dielectric constant is given by31 ⎛ ωnk ⎞2 ε0 − εmρnk ⎜ ⎟ = ε∞ − εmρnk ⎝ ωTO ⎠
(1)
where ρnk = [K n′(kR )In(kR )]/[K n(kR )In′(kR )]
(3)
(2)
ωnk is the surface phonon frequency of order n for a wavevector k,⃗ and Kn and In are the modified Bessel functions of order n, 5822
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Figure 4. (a) Peak intensities for LO, TO, and SO phonons as the nanowire diameter is changed. Spectral position of the (b) TO and (c) LO phonon frequencies (shown as open circles) in GaAs nanowires with diameters ranging from 50 to 155 nm. The corresponding temperature (shown as solid squares) calculated using the linear relationship in eq 3. (d) FWHM for LO and TO phonon Raman lines with changing diameter of nanowires. FWHM of SO phonon was consistently found to be ∼10 cm−1 over the diameters studied.
and 150 nm are shown in Figure 5a−c. From the figure alone, it is clearly evident that the 100 nm diameter nanowire sample generates the highest electric field enhancement as well as a strongly guided HE11 mode.20,30 The large electric field enhancement is also responsible for the large Raman scattering enhancement of the TO phonon mode seen in Figure 3, where a factor 14 increase in Raman scattering intensity is observed for the 95 nm sample relative to the other diameters studied. This result is similar to our previous work on the Raman enhancement properties of vertically etched silicon (Si) nanowire arrays, where it was demonstrated that Raman scattering intensity from the first-order phonon mode in the diamond lattice of Si can be increased by a factor of 15 by tuning both the diameter and pitch of the nanowires in the square lattice.37 Figure 5d plots optical absorptance for an incident wavelength of 532 nm, in both 500 nm and 1 μm long GaAs nanowires, for various diameters ranging from 50 to 175 nm. Maximum absorption is seen for the 95 nm diameter sample, in agreement with the experimental results, since the amount of heat is directly proportional to the amount of absorption. Interestingly, it was also determined that most of the optical power is absorbed within the first 500 nm. This suggests that the length of the nanowires need not be much longer than 500 nm and also presents another optimization factor. It is instructive to note that the absorption for 155 nm diameter nanowires is larger than for the 140 nm diameter sample, as evidenced by the slightly higher temperature. The larger diameters can support higher order modes increasing the absorption within the nanowires.
suggests that lower thermal conductivities are seen in GaAs nanowires as compared to Si nanowires. The resonant temperature behavior suggests that local temperatures can be tuned by simply adjusting the nanowire diameter around the 95 nm diameter for the input wavelength of 532 nm. More generally, for each optical wavelength, there exists a certain resonant diameter. One can potentially realize lateral temperature gradients by tuning the nanowire diameter allowing one to make planar PTE devices. Optical coupling into vertically arranged nanowires also has important implications for next-generation GaAs-based PTE devices, where it is important to maximize the amount of heat that can be generated from the photon flux which is used to power the device as the optimal diameter(s) will depend on how well the incident wavelength(s) couple(s) into the nanowires. To gain further insight into potential GaAs nanowire array design, the waveguiding effect of GaAs nanowires, and the effect this has on the local electric field intensity, was examined using finite difference time domain (FDTD) simulations involving the complete structure. The 1 μm long nanowires were oriented in the z direction on top of a semi-infinite GaAs substrate, and a 532 nm plane wave polarized along the x direction was used to simulate the incident light field. Periodic boundary conditions were used in x and y directions so that the array can be simulated by a single unit cell. Perfectly matched layer (PML) boundary conditions were also employed to reduce spurious reflections. Values for the optical constants, n = 4.13 and k = 0.34, for GaAs at 532 nm were used.38 Simulated irradiance distributions for three nanowire diameters of 50, 100, 5823
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Figure 5. Irradiance distribution for three different nanowire diameters of (a) 50, (b) 100, and (c) 150 nm. The distributions were taken at a cross section 100 nm into the nanowire. (d) Absorptance at a wavelength of 532 nm in nanowires of different diameters. The black squares represent absorption over a length of 1 μm, while the circles represent absorption within the first 500 nm of the nanowire.
wavelength span from 450 to 800 nm. For diameters larger than 150 nm, a second absorption peak occurs due to excitation of higher order modes. This suggests that using larger diameter nanowires could potentially generate more photothermal activity than those examined in this paper and remains a topic of future interest. Conclusion. The heating behavior of vertically etched nanowires was examined using Raman spectroscopy. A nonmonotonic behavior in the nanowire temperature as a function of diameter was observed. The smallest diameter sample consisting of 50 nm diameter nanowires remained at room temperature, and as the diameter was increased to 95 nm, the nanowire temperature on the top facet and surface sidewalls increased to 600 and 440 K, respectively. As the diameter was further increased to 155 nm, a decrease in temperature to 330 K occurs, therefore demonstrating that the temperature can be tuned via nanowire diameter. The increased heating is attributed to the vertical nature of the nanowires, which effectively creates an array of dielectric waveguides, into which the incident light field can couple, resulting in the strong diameter-dependent heating effect. The electric field intensity within the nanowire also depends on the nanowire diameter as evidenced by FDTD simulations, and therefore the increased heat generation can be attributed to waveguiding effects. For any nanowire-based PTE device it is of paramount importance to maximize the amount of heat that is generated from a given incident photon flux. To this end, this study has shown that vertically oriented GaAs nanowires arranged in square lattices are able to generate various amounts of heat for a given nanowire diameter and incident wavelength. For the first time, these findings demonstrate that GaAs nanowires arranged in a photonic lattice can be used to optimize the photothermal effect. The diameter-dependent nature of the effect opens up a
As mentioned earlier, the optimal diameter for maximum temperature change will be dependent on the incident wavelength. For applications involving either single or multiple wavelengths, or broadband absorption, simulated electric field intensities or absorption profiles could be used to determine the feasibility of a given design for a PTE device. Once again, using FDTD simulations, the optical absorption for 1 μm long GaAs nanowires at different incident wavelengths (up to the bandgap of GaAs) and for various nanowire diameters (50−250 nm) was calculated to create a contour plot in Figure 6. For each 1 nm change in incident wavelength, the optimal diameter shifts by 0.29 nm in a near linear relationship over a wide
Figure 6. Simulated absorptance for 1 μm long nanowires for different incident wavelengths and diameters. The nanowires were arranged in a square lattice with a pitch of 400 nm. From the contour plot it is seen that larger diameter nanowires (>150 nm) are more effective at absorbing wavelengths below 500 nm, whereas smaller diameter nanowires are more effective for absorbing wavelengths above 500 nm. 5824
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new avenue for PTE devices using localized temperature gradients. In particular, it may be possible to realize PTE devices that use planar temperature gradients, rather than longitudinal ones.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail
[email protected] (J.W.). *E-mail
[email protected] (S.S.S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was generously supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation, and the Ontario Ministry of Research and Innovation. Nanowires were fabricated in the Quantum Nanofab at the University of Waterloo. The infrastructure used for this work would not be possible without the significant contributions of the Canada Foundation for Innovation, the Ontario Ministry of Research & Innovation and Industry Canada. Their support is gratefully acknowledged.
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