Bimetallic non-alloyed NPs for improving the broadband optical absorption of thin amorphous silicon substrates
© Tan et al.; licensee Springer. 2014
Received: 23 February 2014
Accepted: 3 April 2014
Published: 13 April 2014
We propose the use of bimetallic non-alloyed nanoparticles (BNNPs) to improve the broadband optical absorption of thin amorphous silicon substrates. Isolated bimetallic NPs with uniform size distribution on glass and silicon are obtained by depositing a 10-nm Au film and annealing it at 600°C; this is followed by an 8-nm Ag film annealed at 400°C. We experimentally demonstrate that the deposition of gold (Au)-silver (Ag) bimetallic non-alloyed NPs (BNNPs) on a thin amorphous silicon (a-Si) film increases the film's average absorption and forward scattering over a broad spectrum, thus significantly reducing its total reflection performance. Experimental results show that Au-Ag BNNPs fabricated on a glass substrate exhibit resonant peaks at 437 and 540 nm and a 14-fold increase in average forward scattering over the wavelength range of 300 to 1,100 nm in comparison with bare glass. When deposited on a 100-nm-thin a-Si film, Au-Ag BNNPs increase the average absorption and forward scattering by 19.6% and 95.9% compared to those values for Au NPs on thin a-Si and plain a-Si without MNPs, respectively, over the 300- to 1,100-nm range.
KeywordsOptics at surface Surface plasmon resonance Antireflection Scattering light
The deposition of metallic NPs (MNPs) on thin films has attracted great interest due to the ability of such NPs to enhance the optical absorption and scattering through the light-stimulated resonance of the conduction electrons within the NPs. Gold (Au) and silver (Ag) NPs have been widely used to (i) improve the absorption and reduce the total reflection of solar cells[1–4] and photodiodes, (ii) enhance the emission of light-emitting diodes (LEDs)[6, 7], and (iii) increase the sensitivity of biosensors.
One of the key features of depositing MNPs onto the surface of optoelectronic devices is the ability of these NPs to control the localized surface plasmon resonance (LSPR) peak within a wavelength range of interest by simply varying the MNP type, size, shape, and spacing, and also by altering the dielectric medium surrounding the MNPs[2, 9]. Various metal NP structures, such as single MNPs of various shapes (e.g., nanorod, triangular, sphere, star, etc.), bimetallic core-shell NPs, and bimetallic alloy NPs, have been proposed for controlling the LSPR peak of optoelectronic devices. However, for such NP structures, light-stimulated resonance can only occur at specific wavelengths within a narrow wavelength range. MNP-based structures having a narrow LSPR range are impractical for applications requiring broadband absorption, such as photovoltaic and optical telecommunications.
Motivated by the above-mentioned challenges, we propose in this paper the use of Au-Ag bimetallic non-alloyed NPs (BNNPs) to overcome the problem of narrowband absorption of single-type metal NPs; further, we experimentally demonstrate that such BNNPs exhibit LSPR peaks at 437 and 530 nm and enhance the average forward scattering ten times when deposited onto a glass substrate; when deposited on a 100-nm-thin a-Si film, the Au-Ag BNNPs increase the average absorption and forward scattering of the film by more than 85% over the wavelength range of 300 to 1,100 nm. We also verify that the lower total reflection is achieved only in Si films, because the bottom side of the Au-Ag BNNPs blocks the light reflected off the Si thin film/substrate interface and confines it within the Si film, whereas for a glass substrate, Au-Ag BNNPs significantly scatter the incident light, leading to higher total reflection.
Fabrication of BNNP nanostructures
Summary of NP size distributions, spacing between particles, and surface densities
NP diameter range in nm (mean)
NP to NP distance range in nm (mean)
Number of NPs on 245 × 169 nm (percentage of area coverage by NPs)
Au NPs on glass
9.6 to 352.4 (130.5)
90.0 to 318.2 (193)
Ag NPs on glass
7.8 to 111.7 (48.2)
45.4 to 118.2 (63.8)
AuAg NPs on glass
7.8 to 254.4 (124.5)
16.2 to 109.3 (43.8)
118.2 to 272.3 (180.9)
36.3 to 90.9 (58.48)
36.3 to 181.9 (61.2)
Au NPs on a-Si
13.5 to 162.4 (108.5)
90.0 to 363.4 (198)
Ag NPs on a-Si
5.5 to 111.7 (52.2)
3.3 to 109 (62.2)
AuAg NPs on a-Si
37.6 to 105.1 (100.5)
7.8 to 126.8 (60.76)
127.3 to 290.1 (201.0)
45.5 to 118.2 (70.0)
36.3 to 145.5 (105)
Results and discussion
Solar weighted absorption enhancement of Au NPs, Ag NPs, and Au-Ag BNNPs on thin a-Si substrates
Solar weighted absorption (%)
SWA enhancement compared to plain a-Si (%)
AuNPs on a-Si
AgNPs on a-Si
AuAg BNNPs on a-Si
We have presented a new approach to the fabrication of Au-Ag BNNPs, which can enhance the absorption of thin a-Si films through interparticle coupling and anti-reflection. A simple modified two-step evaporation process, enabling the deposition of Au-Ag bimetallic non-alloyed NPs using conventional micro-fabrication processes, has been described. Isolated Au-Ag bimetallic NPs with uniform size and spacing distribution have been deposited over large areas of glass and thin a-Si substrates. Experimental results have shown that the extinction bands of both the Au NPs and the Ag NPs deposited on glass substrates are narrow (200 to 400 nm) and that these materials exhibit resonance peaks at 565 and 435 nm, respectively. We also found that the Au-Ag BNNPs display two LSPR peaks at 437 and 540 nm; they have higher overall absorption coefficients. It was also shown that the average absorption and forward scattering of the Au-Ag BNNPs on thin a-Si increased by 19.6% and 95.9% compared to those values for Au NPs on thin a-Si and plain a-Si without MNPs, respectively, over the 300- to 1,100-nm range. These results will find application in Si photovoltaics and optical telecommunications.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0017606). The authors also wish to thank Chan Il Yeo for his precious discussion on SWA.
- Atwater HA, Polman A: Plasmonics for improved photovoltaic devices. Nat Mater 2010, 9: 205–213. 10.1038/nmat2629View Article
- Catchpole KR, Polman A: Plasmonic solar cells. Opt Express 2008, 16: 21793–21800. 10.1364/OE.16.021793View Article
- Temple TL, Mahanama GDK, Reehal HS, Bagnall DM: Influence of localized surface plasmon excitation in silver nanoparticles on the performance of silicon solar cells. Sol Energy Mater Sol Cells 1978, 2009: 93.
- Schaadt DM, Feng B, Yu ET: Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles. Appl Phys Lett 2005, 86: 063106. 10.1063/1.1855423View Article
- Stuart HR, Hall DG: Island size effects in nanoparticle-enhanced photodetectors. Appl Phys Lett 1998, 73: 3815–3817. 10.1063/1.122903View Article
- Okamoto K, Niki I, Shvartser A, Narukawa Y, Mukai T, Scherer A: Surface-plasmon-enhanced light emitters based on InGaN quantum wells. Nat Mater 2004, 3: 601–605. 10.1038/nmat1198View Article
- Yang KY, Choi KC, Ahn CW: Surface plasmon-enhanced energy transfer in an organic light-emitting device structure. Opt Express 2009, 17: 11495–11504. 10.1364/OE.17.011495View Article
- Anker JN, Hall WP, Lyandres O, Shah NC, Zhao J, Van Duyne RP: Biosensing with plasmonic nanosensors. Nat Mater 2008, 7: 442–453. 10.1038/nmat2162View Article
- Bohren C, Huffman DR: Absorption and Scattering of Light by Small Particles. New York: Wiley; 1983.
- Rodríguez-González B, Burrows A, Watanabe M, Kiely CJ, Marzán LML: Multishell bimetallic AuAg nanoparticles: synthesis, structure and optical properties. J Mater Chem 2005, 15: 1755–1759. 10.1039/b500556fView Article
- Shibata T, Bunker BA, Zhang Z, Meisel D, Vardeman CF, Gezelter JD: Size-dependent spontaneous alloying of Au-Ag nanoparticles. J Am Chem Soc 2002, 124: 11989–11996. 10.1021/ja026764rView Article
- Baba K, Okuno T, Miyagi M: Resonance wavelengths of silver-gold compound metal island films. J Opt Soc Am B 1995, 12: 2372–2376. 10.1364/JOSAB.12.002372View Article
- Müller CM, Mornaghini FCF, Spolenak R: Ordered arrays of faceted gold nanoparticles obtained by dewetting and nanosphere lithography. Nanotechnology 2008, 19: 485306. 10.1088/0957-4484/19/48/485306View Article
- Abràmoff MD, Magelhaes PJ, Ram SJ: Image processing with ImageJ. Biophotonics Int 2004, 11: 36–42.
- Saeta PN, Ferry VE, Pacifici D, Munday JN, Atwater HA: How much can guided modes enhance absorption in thin solar cells? Opt Express 2009, 17: 20975. 10.1364/OE.17.020975View Article
- Moharam MG, Gaylord TK: Rigorous coupled-wave analysis of planar-grating diffraction. J Opt Soc Am 1981, 7: 811–818.View Article
- NREL's AM 1.5 standard data set. http://rredc.nrel.gov/solar/spectra/am1.5/
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.