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Near-infrared optical absorption enhanced in black silicon via Ag nanoparticle-induced localized surface plasmon
Nanoscale Research Letters volume 9, Article number: 519 (2014)
Due to the localized surface plasmon (LSP) effect induced by Ag nanoparticles inside black silicon, the optical absorption of black silicon is enhanced dramatically in near-infrared range (1,100 to 2,500 nm). The black silicon with Ag nanoparticles shows much higher absorption than black silicon fabricated by chemical etching or reactive ion etching over ultraviolet to near-infrared (UV-VIS-NIR, 250 to 2,500 nm). The maximum absorption even increased up to 93.6% in the NIR range (820 to 2,500 nm). The high absorption in NIR range makes LSP-enhanced black silicon a potential material used for NIR-sensitive optoelectronic device.
78.67.Bf; 78.30.Fs; 78.40.-q; 42.70.Gi
The efficiency of the photovoltaic devices and photoelectronic detectors made from crystalline silicon (C-Si) decreases seriously due to its high reflectivity to the visible and near infrared light. In order to overcome the intrinsic disadvantage of silicon, many approaches have been explored. Most of these methods fall into two categories: 1) anti-reflection coating [1, 2] and 2) light-trapping structure, like grating and period structures [3–6]. Among these methods, a simple and feasible way to obtain high absorption is to blacken the surface of crystalline silicon. In essence, black silicon is not a new material, but a surface modification microstructure of the Si material. Without any doping, the absorption of black silicon can be enhanced to over 90% in ultraviolet and visible range. There are different ways to fabricate black silicon, including reactive ion etching (RIE), electrochemical etching, acid etching, etc. [7–11]. Although it is easy to obtain black silicon by the methods mentioned above, the absorption in black silicon decreases sharply over 1,100 nm owing to the bandgap of silicon which is 1.12 eV. This high absorption is due to the micro/nanostructure on silicon, which blackened the surface, but not the change of bandgap. E. Mazur  proposed a new approach that sulfur was doped into silicon material by femtosecond laser pulses in SF6 environment, resulting in the high absorption in black silicon over the whole 250 to 2,500 nm wavelength. This high absorption in black silicon is ascribed to the impurity of bandgap levels induced by the doped chalcogenide [13, 14]. However, the preparation of black silicon using a femtosecond laser is a high-cost and time-consuming process.
Recently, localized surface plasmon (LSP) also has gained wide attention ascribed to its enhancement in optical properties. The metallic nanostructures confine the charge density oscillations to produce the LSP effect. As the frequency of incident light matches the frequency of LSP, the light will be absorbed by the metal significantly. This phenomenon is called localized surface plasmon resonance (LSPR). LSPR improves the absorption capability through the enhancement of electromagnetic field near the nanoparticle or inside the nanoparticle. Meanwhile, the frequency of LSP is sensitive to the shape and size of metallic nanostructures as well as the dielectric constant of the surrounding material. LSP has been widely investigated due to the unique properties. The biosensors and solar cells based on metallic nanoparticles were reported since LSPR enhances the performance of such devices [15–20].
In this paper, we report a unique, efficient, and easy method to use LSP to enhance the absorption of black silicon in NIR. Due to the LSP effect, the black silicon with Ag nanoparticles indicates high absorption and low reflectivity in UV-Vis-NIR (250 to 2,500 nm) wavelength.
The Ag thin film (10 nm) used in this work was deposited on silicon by thermal evaporation after the silicon substrate was cleaned by the RCA procedure. The post annealing was performed by rapid thermal processing at different temperatures (300°C, 400°C, 500°C, 700°C) for 3 min in nitrogen. All samples were etched in etchant acid solution (H2O2/H2O/HF/C2H5OH = 20:8:4:8 volume ratio; concentrations of H2O2 and HF are 30% and 40%, respectively) for 8 min at 35°C. A silicon sample without Ag nanoparticle on its surface was etched by an etchant containing HAuCl4 for 8 min at 35°C, and all samples were kept in ethanol. The surface morphology was characterized by field emission scanning electronic microscopy (SEM); the optical properties were measured using a UV-Vis-NIR spectrophotometer equipped with an integrating sphere detector.
Results and discussion
Figure 1 shows the Ag spherical nanoparticles on silicon surface produced by rapid thermal processing-induced stress. The Ag nanoparticles are much of a size under the same annealing temperature. The size of nanoparticles increases from 10.88 nm to 42.03 nm with the increase of annealing temperature, indicating higher annealing temperature produced larger particles. The space between two particles also depends on the annealing temperature. The result from the SEM images shows wider space between two nanoparticles as the annealing temperature increases.
The samples were etched by an etchant and contained HF and H2O2 after the annealing treatment. Lots of bubbles escaped from the surface once the samples were immersed in the etchant. During the etching process, the surface of the sample changed from a flat, light gray mirror finish, to a visibly matte black or dark gray surface. Meanwhile, the color of the etching solution changed from transparent to light purple.
As shown in Figure 2, the nanopores formed on silicon film causes the black surface of silicon. The size of nanopores depends on the size of Ag nanoparticles. In the same annealing time, the size of nanoparticles produced from RTP process increases with the annealing temperature. The Ag nanoparticle forms the primary cell with Si in contact area when the substrates are immersed in the etchant, where the Ag nanoparticle acts as cathode to reduce H2O2. Si under the noble metal is oxidized by some charge transfer between Ag nanoparticles. The silicon oxides are dissolved by HF and nanopores are produced; more details can be found elsewhere [21, 22]. Si covered by metal nanoparticles dissolves faster than the regions without nanoparticles according to the primary cell principle, resulting in nanopores on the surface . The Ag nanoparticles sink into the bottom of the nanopores in black silicon.
The optical properties of the samples are evaluated by measuring the infrared absorptance through a UV-Vis-NIR spectrophotometer equipped with an integrating sphere detector. The diffuse and specular reflectance (R) and transmittance (T) are measured with a 2 nm step length in the wavelength range of 200 to 2,500 nm to determine the absorptance (A = 1 - R - T). Figure 3 displays the reflectivity and absorption curves of samples annealed at different temperatures.
As shown in Figure 3a, the reflection is obviously suppressed owing to the rough surface produced by Ag nanoparticle-induced chemical etching. The reflection of C-Si is much higher than that of black silicon. For the Ag nanoparticle-embedded black silicon, the size of nanoparticles differ with varying annealing temperature. The difference in reflectivity and absorption is ascribed to the different sizes of Ag nanoparticles and nanostructures. The sample annealed at 700°C indicates the highest reflectivity ascribed to the large-sized nanoparticles produced by the highest annealing temperature used in this work . The average absorption of all Ag nanoparticle-induced black silicon is high, up to 91.8% in the range of 250 to 2,500 nm.Figure 4 shows the absorption curve of black silicon, Ag-embedded black silicon as well as crystalline silicon. The absorption range of Ag-embedded black silicon is obviously extended from UV to NIR wavelength. The additional absorption to wavelength over 1,100 nm can be attributed to the LSP in Ag nanoparticles.
After chemical etching, Ag particles are distributed in black silicon unevenly, and the particles can be approximately regarded as surrounded by silicon material. According to the MIE theory and modified Drude model, the frequency of surface plasmon resonance of a single isolate particle is described as follows:
where is the free electron plasma frequency (for Ag, ω p = 1.37036 × 1016 rad/s). ϵ m is the dielectric function of Si. ϵ1 is the real part of the dielectric function of the Ag nanoparticles. ϵb represents the impact of electron interband transition on the dielectric function. V f is the Fermi velocity (1.4 × 106 m/s for Ag), L is the mean free path of the electron in bulk material, d is the radius of nanoparticles [25–27]. For most metals at room temperature, the plasma frequency is usually in the visible and UV regions, approximately from 3 to 20 eV. For Ag particles (5 nm < d <50 nm) in vacuum, the frequency of plasmon resonance approximately corresponds to 350 nm [28, 29]. When ϵ m or refract index increases, the frequency of plasmon resonance could be shifted to NIR [30, 31].
Furthermore, as shown in Figure 1, particles are close to each other, the interparticle distance is less than a radius of particles, and this situation remains roughly the same after etching , which means interaction between particles cannot be ignored . Considering the interaction between particles, the resonance frequency of particle i can be explained as follows:
where ωsp,i is the resonance frequency of particle i without considering interparticle interaction, θ is the angle between the incident polarization and the axis connecting two particles. V represents the volume of the particle. The second part of the equation describes the interaction between particles. This clearly shows that interaction between particles could reduce the resonance frequency and result in the additional red shift of the plasmon resonance frequency .
As a result, frequency of plasmon resonance in Ag nanoparticles shifts to NIR, mainly because of the interaction between particle size, high refractive index surrounding the material, and interparticle coupling. Besides, the refractive index of black silicon is the gradient  and the Ag nanoparticles are unevenly distributed, which means the plasmon-enhanced absorption peak in NIR can be the superposition of many different peaks. As the incident wavelength increases, the decay rate of the free electrons of the dielectric constant of the silver increases, shortening the life of the plasma and broadening the plasma resonance band . These peaks would overlap with each other. Therefore, the high absorption (over 90%) in NIR range presented in this work is ascribed to the enhancement of LSP induced by nanoparticles embedded into the surface of black silicon. It is different from the high absorption induced by the impurity of the bandgap level in black silicon attributed to chalcogenide doping by the femtosecond laser process [12, 13, 35–37].
In this paper, instead of chalcogenide-doped black silicon prepared using a femtosecond laser, we report a black silicon with Ag nanoparticles embedded showing high absorption from UV to NIR (250 to 2,500 nm) owing to the enhancement induced by LSP. The LSP-enhanced absorption is high, up to 93.6%, and the average is 91.8% over the 250 to 2,500 nm. The LSP-enhanced black silicon provides an alternative approach to producing high absorption in NIR range through a simple and cheap method.
Diedenhofen SL, Vecchi G, Bauhuis G, Rivas JG: Broadband and omnidirectional anti-reflection coating for III/V multi-junction solar cells. Sol Energ Mat Sol C 2014, 190: 571–595.
Makableh YF, Vasan R, Sarker JC, Nusir AI, Seal S, Manasreh MO: Enhancement of GaAs solar cell performance by using a ZnO sol–gel anti-reflection coating. Sol Energy Mater Sol Cells 2014, 123: 178–182.
Campbell P, Green MA: Light trapping properties of pyramidally textured surfaces. J Appl Phys 1987, 62: 243–249. 10.1063/1.339189
Yu Z, Raman A, Fan S: Fundamental limit of nanophotonic light trapping in solar cells. P Natl Acad Sci USA 2010, 107: 17491–17496. 10.1073/pnas.1008296107
Sai H, Kanamori Y, Arafune K, Ohshita Y, Yamaguchi M: Light trapping effect of submicron surface textures in crystalline Si solar cells. Prog Photovolt Res Appl 2007, 15: 415–423. 10.1002/pip.754
Paetzold UW, Smeets M, Meier M, Bittkau K, Merdzhanova T, Smirnov V, Michaelis D, Waechter C, Carius R, Rau U: Disorder improves nanophotonic light trapping in thin-film solar cells. Appl Phys Lett 2014, 104: 131102. 10.1063/1.4869289
Kolasinski KW: Silicon nanostructures from electroless electrochemical etching. Curr Opin Solid State Mater Sci 2005, 9: 73–83. 10.1016/j.cossms.2006.03.004
Branz HM, Yost VE, Ward S, Jones KM, To B, Stradins P: Nanostructured black silicon and the optical reflectance of graded-density surfaces. Appl Phys Lett 2009, 94: 231121. 10.1063/1.3152244
Kim H, Lee Y, Shin C, Han S, Kim S, Lee YJ, Yi JS: Two-step texture process for high-efficiency crystalline silicon solar cell applications. J Nanosci Nanotechno 2013, 13: 7916–7919. 10.1166/jnn.2013.8122
Gharghi M, Sivoththaman S: Formation of nanoscale columnar structures in silicon by a maskless reactive ion etching process. J Vac Sci Technol A 2006, 24: 723–727. 10.1116/1.2167974
Striemer CC, Fauchet PM: Dynamic etching of silicon for broadband antireflection applications. Appl Phys Lett 2002, 81: 2980–2982. 10.1063/1.1514832
Her TH, Finlay RJ, Wu C, Mazur E: Microstructuring of silicon with femtosecond laser pulses. Appl Phys Lett 1998, 73: 1673–1675. 10.1063/1.122241
Sheehy MA, Winston L, Carey JE, Friend CM, Mazur E: Role of the background gas in the morphology and optical properties of laser-microstructured silicon. Chem Mater 2005, 17: 3582–3586. 10.1021/cm049029i
Carey JE, Crouch CH, Shen M, Mazur E: Visible and near-infrared responsivity of femtosecond-laser microstructured silicon photodiodes. Opt Lett 2005, 30: 1773–1775. 10.1364/OL.30.001773
Acimovic SS, Ortega MA, Sanz V, Berthelot J, Garcia-Cordero JL, Renger J, Maerkl SJ, Kreuzer MP, Quidant R: LSPR chip for parallel, rapid and sensitive detection of cancer markers in serum. Nano Lett 2014, 14: 2636–2641. 10.1021/nl500574n
Cheng XR, Hau BYH, Endo T, Kerman K: Au nanoparticle-modified DNA sensor based on simultaneous electrochemical impedance spectroscopy and localized surface plasmon resonance. Biosens Bioelectron 2014, 53: 513–518.
Kessentini S, Barchiesi D, D'Andrea C, Toma A, Guillot N, Fabrizio ED, Fazio B, Maragó OM, Gucciardi PG, Chapelle MLDL: Gold dimer nanoantenna with slanted gap for tunable LSPR and improved SERS. J Phys Chem C 2014, 118: 3209–3219. 10.1021/jp409844y
Nakayama K, Tanabe K, Atwater HA: Plasmonic nanoparticle enhanced light absorption in GaAs solar cells. Appl Phys Lett 2008, 93: 121904. 10.1063/1.2988288
Pillai S, Catchpole KR, Trupke T, Green MA: Surface plasmon enhanced silicon solar cells. J Appl Phys 2007, 101: 093105. 10.1063/1.2734885
Schaadt DM, Feng B, Yu ET: Enhanced semiconductor optical absorption via surface plasmon excitation in metal nanoparticles. Appl Phys Lett 2005, 86: 063106. 10.1063/1.1855423
Huang Z, Geyer N, Werner P, De Boor J, Gösele U: Metal‒assisted chemical etching of silicon: a review. Adv Mater 2011, 23: 285–308. 10.1002/adma.201001784
Chartier C, Bastide S, Lévy-Clément C: Metal-assisted chemical etching of silicon in HF–H2O2. Electrochim Acta 2008, 53: 5509–5516. 10.1016/j.electacta.2008.03.009
Zhang T, Zhang P, Li SB, Li W, Wu ZM, Jiang YD: Black silicon with self-cleaning surface prepared by wetting processes. Nanoscale Res Lett 2013, 8: 1–5. 10.1186/1556-276X-8-1
Younkin RJ: Surface Studies and Microstructure Fabrication Using Femtosecond Laser Pulses, Ph.D. Harvard University, the Division of Engineering & Applied Sciences: Thesis; 2001.
Johnson PB, Christy RW: Optical constants of the noble metals. Phys Rev B 1972, 6: 4370. 10.1103/PhysRevB.6.4370
Smith JB, Ehrenreich H: Frequency dependence of the optical relaxation time in metals. Phys Rev B 1982, 25: 923. 10.1103/PhysRevB.25.923
Perenboom J, Wyder P, Meier F: Electronic properties of small metallic particles. Phys Rep 1981, 78: 173–292. 10.1016/0370-1573(81)90194-0
Noguez C: Surface plasmons on metal nanoparticles: the influence of shape and physical environment. J Phys Chem C 2007, 111: 3806–3819. 10.1021/jp066539m
Noguez C: Optical properties of isolated and supported metal nanoparticles. Opt Mater 2005, 27: 1204–1211. 10.1016/j.optmat.2004.11.012
Xu G, Tazawa M, Jin P, Nakao S, Yoshimura K: Wavelength tuning of surface plasmon resonance using dielectric layers on silver island films. Appl Phys Lett 2003, 82: 3811–3813. 10.1063/1.1578518
Beck FJ, Polman A, Catchpole KR: Tunable light trapping for solar cells using localized surface plasmons. J Appl Phys 2009, 105: 114310. 10.1063/1.3140609
Liu Z, Wang H, Li H, Wang X: Red shift of plasmon resonance frequency due to the interacting Ag nanoparticles embedded in single crystal SiO2 by implantation. Appl Phys Lett 1998, 72: 1823–1825. 10.1063/1.121196
Mertens H, Verhoeven J, Polman A, Tichelaar FD: Infrared surface plasmons in two-dimensional silver nanoparticle arrays in silicon. Appl Phys Lett 2004, 85: 1317–1319. 10.1063/1.1784542
Truong VV, Scott GD: Optical properties of aggregated noble metal films. J Opt Soc Am 1977, 67: 502–510. 10.1364/JOSA.67.000502
Su YJ, Li SB, Wu ZM, Yang YJ, Jiang YD, Jiang J, Xiao ZF, Zhang P, Zhang T: High responsivity MSM black silicon photodetector. Mat Sci Semicon Proc 2013, 16: 619–624. 10.1016/j.mssp.2012.11.008
Su YJ, Zhang P, Jiang J, Li SB, Wu ZM, Jiang YD: Absorption enhancement of near infrared in Te doped nanoporous silicon. J Mater Sci-Mater El 2013, 24: 2197–2201. 10.1007/s10854-013-1079-4
Li SB, Jiang YD, Wu ZM, Wu J, Ying ZH, Wang ZM, Li W, Salamo G: Origins of 1/f noise in nanostructure inclusion polymorphous silicon films. Nanoscale Res Lett 2011, 6: 281. 10.1186/1556-276X-6-281
This work was partially supported by the National Science Foundation of China via Grant Nos. 61204098, 61371046, 61421002, and 61101030, the National Higher Education Institution General Research and Development Funding under Grant No ZYGX2013J063, and the China Postdoctoral Science Foundation under Grant No. 2014 M552330.
The authors declare that they have no competing interests.
PZ carried out the experiments and drafted the manuscript. SL and CL participated in the design of the experiments and revised the manuscript. XW, ZW, YJ, and ZC participated in the experimental design and coordination. All authors read and approved the final manuscript.