Article pubs.acs.org/cm
Plasmon-Enhanced Photoconductivity in Amorphous Silicon Thin Films by Use of Thermally Stable Silica-Coated Gold Nanorods Chi-Sheng Chang and Lewis J. Rothberg* Materials Science Program and Department of Chemistry, University of Rochester, Rochester New York 14627, United States S Supporting Information *
ABSTRACT: Enhancement of solar photovoltaics by integrating cells with metal nanoparticles is of potential interest to reduce the usage of semiconductor material. Appropriately shaped metal particles to optimize spectral response have inadequate thermal stability to withstand standard semiconductor processing. We synthesize silica-capped gold nanorods that maintain nonspherical shapes to over 600 °C and show that they can increase photoconductivity in thin films of amorphous silicon by much more than a factor of 2 across the entire visible spectrum. We also report mechanistic studies of this phenomenon that show that much of this effect is primarily due to strong near-field light concentration rather than scattering as has often been assumed.
■
INTRODUCTION Many promising examples where metal nanoparticles are combined with solar photovoltaic devices to achieve efficiency enhancements or reduced use of material have been reported.1−5 In the case of traditional semiconductor photovoltaic devices, the primary motivation to incorporate small numbers of plasmonic scatterers is to reduce the amount of relatively expensive semiconducting material.6 In the case of organic devices, the main drivers are to increase absorbance in the near-infrared spectral region, where absorption is weak, and to make thinner organic layers more efficient by increasing internal fields and fill factors.3,7,8 In both cases, an attractive feature of metal nanoparticles is that their shapes can be adjusted to move the plasmon resonance to spectral regions that are most leveraged in terms of the semiconductor absorption and solar irradiance spectra.9 Unfortunately, use of nonspherical metal nanoparticles is incompatible with hightemperature processing that frequently accompanies traditional semiconductor manufacturing. For example, fabrication of photovoltaics based on thin films of polysilicon requires annealing temperatures as high as 600 °C, and this causes metal nanoparticles to revert to spherical geometries.10 In the present work, we have synthesized silica-capped gold nanorods that approximately retain their rodlike shape and attendant plasmon resonance when thermally annealed at 600 °C. The capping also enables them to remain electrically inert and isolates the metal so that it does not act as a contaminant in the semiconducting film or the fabrication apparatus. After depositing the capped nanorods on amorphous silicon thin films, we demonstrate more than 2-fold enhancement of photoconductivity throughout the visible spectrum. Much larger enhancements are observed near the absorption edge of the amorphous silicon, where we have engineered the © 2015 American Chemical Society
plasmon resonance of the nanorods. We also present data that address the mechanism by which the metal nanoparticles enhance absorption in the amorphous silicon, suggesting that a substantial fraction of the enhancement comes from localization of the incident optical fields rather than path length enhancement due to scattering.
■
EXPERIMENTAL SECTION
Chemicals. Hydrogen tetrachloroaurate(III) hydrate (HAuCl4· 3H2O, >99.9%), cetyltrimethylammonium bromide (CTAB, ≥99%), silver nitrate (AgNO3, >99.9%), poly(styrenesulfonate) (PSS, MW 70 000), poly(allylamine hydrochloride) (PAH, MW 15 000), poly(vinylpyrrolidone) (MW 10 000), and poly(4-vinylpyridine) (MW 60 000) were purchased from Sigma−Aldrich and used as received. Tetraethyl orthosilicate (TEOS, ≥99.9%) and sodium borohydride (NaBH4, >99.9%) were purchased from Fluka. Milli-Q-grade water was used in all preparations. Synthesis of Gold Nanorods and Silica Coatings. Gold nanorods (AuNRs) were synthesized by the modified seed-mediated growth method reported by Nikoobakht and El-Sayed.11 Briefly, for the AuNR with LSPR (localized surface plasmon resonance) peak at 831 nm, seed solutions were made from an aqueous solution composed of CTAB (0.1 M, 9.5 mL) and HAuCl4 (0.01 M, 0.25 mL). Next, freshly prepared ice-cold NaBH4 (0.01 M, 0.6 mL) aqueous solution was added to the mixture solution, which was then stirred vigorously for 2 min. The seed solution was kept at room temperature for 2 h before use. Growth solution was made by mixing aqueous solutions of CTAB (0.1 M, 40 mL), HAuCl4 (0.01 M, 2 mL), AgNO3 (0.01 M, 0.4 mL), HCl (1.0 M, 0.8 mL), and finally ascorbic acid (0.1 M, 0.32 mL) under gentle mixing. Finally, the seed solution (0.096 mL) was added to the growth solution under gentle stirring for Received: November 10, 2014 Revised: April 14, 2015 Published: April 14, 2015 3211
DOI: 10.1021/cm504086z Chem. Mater. 2015, 27, 3211−3215
Article
Chemistry of Materials
RF power of 75 W, substrate temperature at 300 °C) on borosilicate glass. The thickness of a-Si:H is 100 nm. The interdigitated electrode patterns were made by photolithography, and then aluminum was sputtered with a thickness of 200 nm. The length and width of the electrodes used here are 1775 and 50 μm, respectively, and the interelectrode spacing is 10 μm. Each side has 17 electrodes that are connected to the contact pad. The 50 nm SiO2 spacer used for mechanistic studies was deposited by e-beam thermal vapor deposition (Kurt J. Lesker, PVD 75). Device Characterization. Photoconductivity measurements in aSi:H thin films were conducted by measuring the current under illumination in ambient conditions with a bias voltage of 5 V provided by a sourcemeter (Keithley 2400). The light source was a broadband fiber laser (Fianium SC450) equipped with an acoustic-optic tunable filter for wavelength selection. After measurements on a reference device without nanoparticles, AuNR@SiO2 aqueous solution was drop-cast on top of the a-Si:H thin film to study metal particle effects on the device. In order to achieve a uniform particle distribution, the droplet placed on top of the device was dried out in a fume hood. All photoconductivity data are steady-state values.
10 s and left undisturbed overnight. For AuNRs with LSPR around 617 nm, a similar procedure was used except that we controlled the aspect ratio by using different amounts of the reagents. HAuCl4 (0.01 M, 4 mL), AgNO3 (0.01 M, 0.16 mL), and ascorbic acid (0.1 M, 0.64 mL) were used in the growth solution, and more seed solution (0.192 mL) was used for shorter AuNR. Silica coating of AuNRs was achieved via polyelectrolyte layer-bylayer assembly on the AuNR, followed by hydrolysis and condensation of tetraethyl orthosilicate (TEOS).12 AuNR solution (20 mL) was centrifuged at 8000 rpm for 20 min twice to remove excess CTAB and redispersed into Milli-Q water (10 mL). AuNR solution was added dropwise to 10 mL of PSS (2 mg/mL, 6 mM NaCl, presonicated for 30 min) under vigorous stirring. The stirring was continued for 3 h. After that, excess PSS was removed by centrifugation at 6000 rpm for 20 min twice, and the precipitate was redispersed into Milli-Q water (10 mL). PSS-coated AuNR solution was added dropwise to 10 mL of PAH (2 mg/mL, 6 mM NaCl, presonicated for 30 min) under vigorous stirring. Stirring was continued for 3 h. The resulting solution was centrifuged once at 6000 rpm for 20 min to remove excess PAH, and the precipitate was redispersed into Milli-Q water (10 mL). PSS/ PAH-coated AuNR (10 mL) was added to 10 mL of PVP (4 mg/mL) and the mixture was stirred overnight. The PVP-coated AuNR solution was centrifuged at 6000 rpm for 20 min, and the precipitate was redispersed into Milli-Q water (0.4 mL). Then it was added dropwise to 2 mL of 2-propanol under vigorous stirring. To AuNR solution were added 0.92 mL of water and 2.86 mL of ammonia solution [3.84 vol % ammonia (33 wt % in water) in 2-propanol] under vigorous stirring. Finally, 0.8 mL of a solution of TEOS in 2-propanol (0.97 vol % TEOS) was added under gentle stirring. The reaction was allowed to continue for 2 h. The thickness of the resulting silica shell is ∼12 nm. These silica-coated AuNRs (AuNR@SiO2) were centrifuged at 8000 rpm for 20 min, and the resulting precipitate was redispersed into water or ethanol for use. Self-Assembly of Gold Nanorods on Glass Substrates. AuNR and AuNR@SiO2 were deposited onto poly(4-vinylpyridine) (PVP) -modified quartz or silicon wafers.13 The quartz and silicon wafers were first cleaned in piranha solution [3:1 (v/v) H2SO4 and H2O2] for 30 min and rinsed with Milli-Q water. Caution: Piranha solution is a powerf ul oxidizing agent and reacts violently with organic compounds. It should be handled with extreme care. The substrates were submerged in 1% PVP (10 mg/mL) aqueous solution overnight. The substrates were rinsed with ethanol three times to remove weakly bound PVP. PVPmodified substrates were annealed at 120 °C in the ambient atmosphere for a few hours. After that, the substrates were submerged in AuNR or AuNR@SiO2 solution for the desired deposition time. Thermal Annealing. Quartz substrates or silicon wafers with AuNR or AuNR@SiO2 were thermally annealed in a tube furnace (Thermolyne 2100) in ambient environment. The temperature was raised from room temperature to the desired temperature (400, 500, or 600 °C) in 10 min. When the temperature reached the desired value, the substrates were annealed for 1 h. After that, the substrates were allowed to cool down to room temperature, which usually required 3−4 h. Characterization. Transmission electron microscopy (TEM) was carried out with a Hitachi 7650 transmission electron microscope operating at an acceleration voltage of 80 kV. For sample preparation, 6 μL of AuNR or AuNR@SiO2 suspension was placed on carboncoated copper grids and allowed to air-dry. Approximately 200 particles were measured to obtain the average size and size distribution. Scanning electron microscopy (SEM) imaging was performed on an Auriga compact FIB-SEM. Extinction spectra of the AuNR solutions were obtained on a Lambda 950 UV/vis/near-IR spectrophotometer. The absorption spectrum of a-Si:H thin film was measured by using a integrating sphere with a Lambda 950 UV/vis/ near-IR spectrophotometer. By measuring reflectance R% and transmittance T% of a-Si:H, the absorption of a-Si:H can be obtained by A% = 1 − R% − T%. Hydrogenated Amorphous Silicon and Interdigitated Electrode Fabrication. Hydrogenated amorphous silicon was made by plasma-enhanced chemical vapor deposition (AME P 5000 PECVD,
■
RESULTS AND DISCUSSION Optical Characterization. Figure 1 presents TEM images of gold nanorods with CTAB surfactant (left) and after growth
Figure 1. Transmission electron micrographs of (a) CTAB-coated gold nanorods and (b) silica-capped gold nanorods. The scale bar is 50 nm. (c) Corresponding extinction spectra illustrating the longitudinal and transverse plasmon resonances. Gold nanorods are 65 ± 7 nm in length and 16 ± 3 nm in diameter. The silica caps are 12.7 ± 0.7 nm.
of uniform silica coating shells around 12 nm thick (right) as described in the Experimental Section. The aspect ratio of the rods is ∼4 and in aqueous solution they exhibit transverse plasmon resonances around 510 nm, near the plasmon resonance for spherical gold nanoparticles. The elongation into rod-shaped particles also endows them with longitudinal plasmon resonances in the near-infrared (∼830 nm for CTAB capping and ∼880 nm with silica caps). The thicker silica shell gives the surroundings a higher effective dielectric constant and causes a red shift. Thermal Stability. The gold nanorods and capped nanorods were dispersed on a substrate for annealing studies by the method of Malynych et al.13 To accomplish that, we treated quartz and silicon wafer substrates with PVP [poly(vinylpyridine)], which behaves as an ambipolar coating that can electrostatically attract the CTAB coating on the nanoparticle via the lone pair on the pyridyl group or form 3212
DOI: 10.1021/cm504086z Chem. Mater. 2015, 27, 3211−3215
Article
Chemistry of Materials hydrogen bonds between the pyridyl group and hydroxide on the silica shells. The extinction spectra and morphology of films made by deposition of Au-NR@CTAB onto quartz/silicon wafers, and how those properties change with thermal annealing of the films, are documented in Figures 2 and 3, respectively. Figures 4 and 5 present analogous data for the films made by deposition of Au-NR@SiOx
Figure 5. Scanning electron micrographs of particles in films of silicacapped gold nanorods before and after annealing in vacuum at 400, 500, and 600 °C. These correspond to the samples whose extinction spectra are presented in Figure 4.
Figure 2. Extinction spectra and photographs of films of CTAB-coated gold nanorods before and after annealing in the ambient environment at 400, 500, and 600 °C. Corresponding scanning electron micrographs are presented in Figure 3.
First, we note that longitudinal resonances in the extinction spectra on quartz are substantially red-shifted and broadened relative to those in water (cf. Figure 1), and we ascribe most of that change to particle aggregation, as is evident in the scanning electron micrographs, causing multiple particle plasmon resonance.14,15 End-to-end assembly tends to red-shift the longitudinal plasmon resonance by effectively increasing the nanoparticles’ aspect ratio but leave the transverse resonance unaffected. Conversely, side-to-side assembly tends to blue-shift the plasmon resonance by effectively reducing the rods’ aspect ratio but red-shift the transverse plasmon resonance. The scanning electron micrographs show that both types of aggregation are observed and explain the observed changes in extinction relative to the isolated nanorods in water. The behavior with annealing differs dramatically for the CTAB-coated and silica-coated samples, however. Previous work indicates that CTAB-coated gold nanorods transform into spheres at 250 °C,16 and the data of Figure 3 confirm that outcome for treatments at 400 °C and higher. Shape changes in the CTAB-coated gold nanorods are due to melting of the gold, which reduces its surface tension by adopting a spherical geometry. In contrast, Figure 5 shows that the silica-coated particles approximately retain their oblong shapes even for treatments as high as 600 °C. The results are summarized in Table 1. Even the silica-capped nanorods tend toward lower aspect ratios (from ∼4.4 prior to annealing to ∼3.2 at 400 °C and ∼2.9 at 600 °C) with heating and suffer some blue shift of their plasmon resonance. The highly enhanced thermal stability with oxide capping is presumably due to the rigidity of the silica encapsulation, which slows down the surface diffusion of gold
Figure 3. Scanning electron micrographs of particles in films of CTAB-coated gold nanorods before and after annealing in the ambient environment at 400, 500, and 600 °C. These correspond to the samples whose extinction spectra are presented in Figure 2.
Table 1. Silica-Coated Gold Nanorod Dimensions before and after Thermal Annealing Figure 4. Extinction spectra and photographs of films of silica-capped gold nanorods before and after annealing in the ambient environment at 400, 500, and 600 °C. The corresponding transmission electron micrographs are presented in Figure 5.
length TEM AuNR@CTAB SEM AuNR@SiO2 400 °C 500 °C 600 °C 3213
64.6 62.8 50.8 49.5 48.8
± ± ± ± ±
7.7 8 6.3 6.4 6.7
width 15.7 14.6 16.2 16.3 17.2
± ± ± ± ±
2.8 2 1.9 2.3 2
aspect ratio 4.2 4.4 3.2 3.0 2.9
± ± ± ± ±
0.7 0.7 0.5 0.5 0.5
DOI: 10.1021/cm504086z Chem. Mater. 2015, 27, 3211−3215
Article
Chemistry of Materials
Silica-capped nanorods like those in Figure 4, but with lower aspect ratio where the plasmon resonance matches the a:Si−H absorption edge, were deposited on structures without (Figure 6a) and with (Figure 6b) the 50 nm SiO2 spacer. Nanorod coverage was controlled to be ∼10% as determined by SEM measurements. Control experiments with uncapped nanorods at similar coverage (not shown) led to low electrical resistance of the devices and very poor performance under illumination. In the cases with capped nanorods, the photoconductivity was linear with incident power. The resulting photoconductivity enhancement as a function of incident wavelength is reported in Figure 6c, along with the fraction of light absorbed by the amorphous silicon film in Figure 6d. We presume that the enhancement is due to changes in the amount and spatial distribution of charge photogeneration and that the charge mobility is unaffected by AuNR@SiO2 deposition. The photoconductivity is substantially enhanced and the enhancement roughly tracks the plasmon resonance. The enhancement is several times higher in the case without the spacer, suggesting that field concentration by the nanoparticles plays a substantial role. It is particularly noteworthy that the photoconductivity enhancements in the case without a spacer are larger than a factor of 2, greater than the amount by which the absorption of the amorphous silicon film could be enhanced since it is already 50% absorbing in the visible region of the spectrum. Since the interdigitated electrodes are on top of the photoconductive layer, absorption near the surface is more effective in generating photocarriers in regions of high field where the carriers are separated and collected more efficiently. Thus, absorption enhancement near the surface is particularly valuable, and this observation also tends to support a field enhancement mechanism. The fact that carrier generation near the surface is so critical also probably explains why the absolute photocurrents are considerably lower in the devices with an oxide spacer even in the absence of metal nanorods.
atoms from the ends of the rods toward the center of the rods.17,18 Nevertheless, we have never observed cracking of the silica. Apparently, the silica shell can provide better thermal stability and also effectively sequester the gold.19,20 Our results are consistent with pulsed laser heating of silica-capped gold nanorods, although those experiments were done in aqueous solution where cooling may be rapid.20 Charge Photogeneration Enhancement. The relatively good shape preservation, excellent capping integrity, and ability to maintain near-infrared plasmon resonance bode well for applications using the silica-capped gold with standard semiconductor processing. As an illustration, we have studied whether the silica capped gold particles can enhance photoconductivity in very thin (semitransparent) amorphous silicon films, and if so, via what mechanism. One can imagine circumstances where the enhancement is predominantly due to scattering by the nanoparticles that helps to couple light into the semiconductor at oblique angles and thereby increase absorption by increasing effective path length.1 Alternatively, it is possible that the nanoparticles increase the absorption by localizing the energy of the incident light, as has been demonstrated in fluorescence enhancement experiments.21,22 Figure 6 presents the results of experiments designed to
■
CONCLUSIONS Small concentrations of oxide-capped metallic nanorods can substantially increase charge photogeneration and change its spatial distribution in thin semiconducting films. This phenomenon can in principle be used to reduce the amount of semiconducting material needed in photovoltaics. On the basis of spacer experiments, we conclude that the primary mechanism for enhancement of the photocurrent was increased absorption near the semiconductor surface due to local enhancement of the optical field by the nanoparticles. Even under substantial heating (600 °C for 30 min), the capped nanorods retain high aspect ratio and a longitudinal plasmon resonance in the near-infrared. Using capped nanorods may make it possible to integrate plasmonic enhancement into photovoltaics based on polycrystalline semiconductors that require high-temperature processing. At the same time, there is some promise to reduce semiconductor material usage that is a nonnegligible fraction of fabrication cost. It will be interesting to see whether it is feasible to embed plasmonic particles near the p−n junction in such devices, with the intent of localizing absorption where it can be most effective.
Figure 6. Photoconductivity divided by incident power versus wavelength for a:Si−H films under interdigitated electrodes. (a) With and without silica-capped gold nanorods deposited on top. (b) With and without silica layer. (c) Ratio of photoconductivity with and without silica-capped gold nanorods. (d) Fraction of light absorbed by a:Si−H film in the absence of spacer or gold nanorod.
investigate enhancement and discern the mechanism. A thin film (∼100 nm) of amorphous silicon on glass was fabricated, and lithographically defined interdigitated electrodes with 10 μm spacing made from aluminum were deposited on top. In one set of devices, a 50 nm SiO2 spacer was made on top of the structure via e-beam evaporation, the idea being to differentiate between increased effective path length, where we expect the spacer to make little difference, and absorption enhancement, where we expect the spacer would considerably reduce the enhancement.
■
ASSOCIATED CONTENT
S Supporting Information *
One figure showing extinction spectrum of silica-coated gold nanorods and one table listing gold nanorod dimensions. This 3214
DOI: 10.1021/cm504086z Chem. Mater. 2015, 27, 3211−3215
Article
Chemistry of Materials material is available free of charge via the Internet at http:// pubs.acs.org/page/cmatex/submission/index.html.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work has been supported by New York State Energy Research and Development Authority Award (18809) and NSF Grant DMR-1105355. We thank Dr. Alan Raisenen for fabrication of amorphous silicon, Dr. Shih-Kai Ni for fabrication of interdigitated electrodes, and Brian McIntyre for technical help on SEM.
■
REFERENCES
(1) Atwater, H. A.; Polman, A. Nat. Mater. 2010, 9, 205−213. (2) Rand, B. P.; Peumans, P.; Forrest, S. R. J. Appl. Phys. 2004, 96, 7519−7526. (3) Jankovic, V.; Yang, Y.; You, J.; Dou, L.; Liu, Y.; Cheung, P.; Chang, J. P.; Yang, Y. ACS Nano 2013, 7, 3815−3822. (4) Pan, S.; Rothberg, L. J. Proc. SPIE 2007, 6641, No. 664109. (5) Ferry, V. E.; Munday, J. N.; Atwater, H. A. Adv. Mater. 2010, 22, 4794−4808. (6) Pillai, S.; Catchpole, K. R.; Trupke, T.; Green, M. A. J. Appl. Phys. 2007, 101, No. 093105. (7) Lu, L.; Luo, Z.; Xu, T.; Yu, L. Nano Lett. 2013, 13, 59−64. (8) Chen, X.; Zhao, C.; Rothberg, L.; Ng, M.-K. Appl. Phys. Lett. 2008, 93, 123302. (9) Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. Chem. Rev. 2011, 111, 3669−3712. (10) Aberle, A. G. Thin Solid Films 2006, 511−512, 26−34. (11) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957− 1962. (12) Pastoriza-Santos, I.; Pérez-Juste, J.; Liz-Marzán, L. M. Chem. Mater. 2006, 18, 2465−2467. (13) Malynych, S.; Luzinov, I.; Chumanov, G. J. Phys. Chem. B 2002, 106, 1280−1285. (14) Yum, S.; Park, Y.-K.; Kim, S. K.; Park, S. Anal. Chem. 2007, 79, 8584−8589. (15) Jain, P. K.; Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 18243−18253. (16) Petrova, H.; Perez Juste, J.; Pastoriza-Santos, I.; Hartland, G. V.; Liz-Marzan, L. M.; Mulvaney, P. Phys. Chem. Chem. Phys. 2006, 8, 814−821. (17) Khalavka, Y.; Ohm, C.; Sun, L.; Banhart, F.; Sonnichsen, C. J. Phys. Chem. C 2007, 111, 12886−12889. (18) Wang, Y.; Teitel, S.; Dellago, C. Nano Lett. 2005, 5, 2174−2178. (19) Radloff, C.; Halas, N. J. Appl. Phys. Lett. 2001, 79, 674−676. (20) Chen, Y.-S.; Frey, W.; Kim, S.; Homan, K.; Kruizinga, P.; Sokolov, K.; Emelianov, S. Opt. Express 2010, 18, 8867−8878. (21) Lakowicz, J. R. Anal. Biochem. 2001, 298, 1−24. (22) Pan, S.; Rothberg, L. J. J. Am. Chem. Soc. 2005, 127, 6087−6094.
3215
DOI: 10.1021/cm504086z Chem. Mater. 2015, 27, 3211−3215
