Surface-enhanced fluorescence from copper nanoparticles on silicon nanowires

Abstract

A method to enhance surface plasmon coupled fluorescence from copper nanoparticles on silicon nanowires is presented. Owing to resonant plasmons oscillation on the surface of Cu/Si nanostructure, the fluorescence peaks of several lanthanide ions (praseodymium ions, Pr³⁺, neodymium ions Nd³⁺, holmium ions Ho³⁺, and erbium ions Er³⁺) were markedly enhanced with the enhancement of maximal 2 orders of magnitude, which was larger than that caused by unsupported Cu nanoparticles. These results might be explained by the local field overlap originated from the closed and fixed copper nanoparticles on silicon nanowires.

Keywords

Cu/Si nanostructure / surface-enhanced fluorescence / lanthanide ions / resonant plasmons oscillation

Introduction

Surface-enhanced fluorescence (SEF) is known for more than a few decades [1]. A metallic particle near a fluorophore can influence both its excitation and emission, eventually leading to significantly enhanced spectral properties [2-5]. The enhancement is suggested to occur via a coupling interaction of the fluorophore and the electric field around the metal particle, which can be induced by incident light or by the excited fluorophore [6-10]. This effect can bring an increase in the quantum yield and improve the efficiency of light absorption [11-13].

Recently, research on the enhanced fluorescence properties of lanthanide ions such as Pr³⁺ and Nd³⁺ [14-17] in low dimensional semiconductors has attracted much interest because of their potential application in optoelectronic devices.

The photoluminescence (PL) properties of lanthanide ions have been investigated for decades [18-20]. An attractive feature of luminescent lanthanide ions is their line-like emission, which results in a high color purity of the emitted light. Actually lanthanide ions have been considered the most important optical activators for luminescent devices.

Emission of light from lanthanide ions is a fundamentally important process with a continuously expanding range of applications in contemporary electronic devices. However, the photon absorption cross section of some trivalent Ln³⁺ ions (Pr³⁺, Nd³⁺, etc) is very small due to the dipole-forbidden nature of the intra^-4f transitions and, thus, the intensity of the PL is relatively low. In order to overcome the small absorption cross section, coupling Ln3+ ions with metal nanoparticles was a valuable strategy.

Metal nanoparticles were ready to agglomerate during their application [21]. To overcome the limitation, in the paper, the Cu/Si nanomaterials were employed in investigating the SEF process of Ln³⁺ (Ln= Pr, Nd, Ho, and Er). It showed that the emission of intrinsically fluorescent Ln³⁺ could be significantly enhanced by Cu/Si nanostructure, which had a greater enhancement than the unsupported Cu nanoparticles. Furthermore the preliminary explanation of excellent SEF based on Cu/Si nanostructure was proposed.

Experiment

Materials

Pr₆O₁₁, Nd₂O₃, Ho₂O₃, Er₂O₃ and CuSO₄·5H₂O were purchased from Alfa Aesar Co. Pr₆O₁₁, Nd₂O₃, Ho₂O₃ and Er₂O₃ powders were dissolved in concentrated nitric acid to make a 0.1 M stock solution. CuSO₄ was prepared to 5.0×10^-3 M with doubly distilled water.

Other reagents were of analytical grade without further purification. Doubly distilled water was used throughout.

Synthesis of silicon nanowires (SiNWs) and modification with Cu nanoparticles

The SiNWs were prepared via a simple thermal evaporation of SiO powder using the oxide-assisted growth method developed by us [22]. The as-synthesized SiNWs (0.008 g) were etched with 5 mL 5% HF aqueous solution for 1 min to get rid of their outer oxide layer, rinsed with distilled water, and then immersed in 15 mL 1×10^-3 M CuSO₄ aqueous solution. Then the yellow SiNWs gradually turned black, which indicated that they were modified with Cu nanoparticles. The total amount of copper on the surface was determined to be 75 µg. Then the Cu/Si nanomaterials were dispersed in 2 mL distilled water under the violent ultrasonic irradiation.

Synthesis of unsupported Cu nanoparticles

Unsupported Cu nanoparticles were prepared by reducing CuSO₄ solution directly to Cu nanoparticles with the method reported before [23].

Characterization

The as-prepared products were characterized by X-ray diffraction (XRD), which was carried out on a Philips X’pert PRO MPD diffractometer with Cu Kα radiation (λ = 0.15406 nm). A scanning rate of 0.05^o·s^-1 was applied to record the pattern in the 2θ range of 25°-80°. The morphology of the SiNWs was characterized by scanning electron microscopy (SEM, FEI Co., model Quanta-200) with an energy dispersive X-ray (EDS) spectrum. Fluorescence spectra were measured with a Fluoromax-4 spectrofluorimeter.

Results and discussion

Synthesis and modification of SiNWs

The SEM image in Fig. 1 reveals that the sample takes the shape of a wire, which is smooth and uniform with length up to several micrometers. The elemental analysis of SiNWs shows no peaks of other elements except Si and O, indicating a high purity of the products.

Figure 2(a) shows the XRD pattern of the as-prepared SiNWs, and all diffraction peaks can be indexed as the cubic phase of Si. The cell parameter is calculated to be a = 0.5435±0.0054 nm, which is in agreement with the value of face-centered cubic silicon a = 0.5430 nm (Joint Committee of Powder Diffraction Standards, JCPDS card No. 27-1402). The XRD pattern of SiNW-supported Cu nanoparticles is shown in Fig. 2(b). No other characteristic peaks are observed except Cu and Si. The XRD pattern demonstrates that the SiNWs have been modified with Cu nanoparticles. The calculated cell parameter of Cu is a = 0.3616±0.0003 nm, which is in agreement with the reported value, a = 0.3615 nm (JCPDS card No. 04-0836).

Enhancement fluorescence experiment of Cu/Si nanostructure

In Fig. 3, the Ln³⁺ (Ln= Pr, Nd, Ho, and Er) displayed weak fluorescence emission at the peaks of 595 nm, 604 nm, 640 nm of Pr³⁺, 592 nm of Nd³⁺ upon excitation at 470, and 550 nm of Ho³⁺, 533 nm of Er³⁺ upon excitation at 430 nm, which were attributed to be the 4f inter-level ¹I₆ →³F₃ (595 nm), ¹D₂→³H₄ (604 nm) and ³P₀→³F₂ (640 nm) of Pr³⁺, ²F_7/2 →²P_1/2 (592 nm) of Nd³⁺, ⁵G₂→⁵I₅ (550 nm) of Ho³⁺ and ⁴S_3/2→⁴I1_5/2 (533 nm) transitions of Er³⁺.

Figures 4(a), (c), and (e), show the fluorescence spectra of Pr³⁺ at the different concentrations with the increment of Cu/Si amount (50 μL each time). As can be seen, the fluorescence intensity increases gradually with the addition of Cu/Si nanomaterials. After that, the intensity keeps steady with further growth of the Cu/Si. In addition, the enhancement decreased (from 167-fold to 12-fold at 640 nm and 30-fold to 9-fold at 604 nm) when Pr³⁺ concentration changed from 0.01 to 0.05 M. Moreover, the peak at 604 nm dominated gradually and finally resulted in the mergence of the peak about 595 nm.

To make a comparison, unsupported Cu nanoparticles were used. It can be observed from Figs. 4(b), (d), and (f) that the fluorescence intensity of Pr³⁺ was also enhanced with the addition of the unsupported Cu nanoparticles. The enhancement was about 59-fold to 8-fold at 640 nm and 9-fold to 6-fold at 604 nm, respectively. The fluorescence emission increased to the maximum and then keeps steady, which was in agreement with the Cu/Si nanostructure. However, the enhancement factor was smaller than that caused by Cu/Si nanostructure.

Figures 5(a), 6(a), and 7(a) show the fluorescence spectra of other Ln³⁺ at the concentration of 0.05 M (Ln=Nd, Ho, and Er) in the presence of Cu/Si nanomaterials. An enhancement factor of 127-fold for Nd³⁺, 59-fold for Ho³⁺ and 52-fold for Er³⁺ could be obtained. However, in Figs. 5(b), 6(b) and 7(b), the enhancement was only about 45-fold, 25-fold and 21-fold for Nd³⁺, Ho³⁺ and Er³⁺ respectively when unsupported Cu nanoparticles were employed.

The above results indicated that the enhancement of Cu/Si nanostructure is better than that of the unsupported Cu nanoparticles, which drew our interest.

One explanation might result from the overlap of enhanced local fields. The essential difference between these nanomaterials is that Cu/Si nanostructure has silicon carrier, The excellent SEF feature of Cu/Si nanostructure might be attributed to the unique fearures of H-SiNWs (SiNWs after HF-treatment) in anchoring metal NPs (e.g., Au, Ag, Pd, Cu) firmly and in a dispersed fashion on SiNWs surfaces [24,25]. The Cu/Si nanostructure is kept from congregating and growing larger because the small Cu nanoparticles are fixed by the SiNWs and the interparticle overlap might lead to larger field enhancement factors [26].

The unsupported Cu nanoparticles also have SEF feature. However, they are apt to agglomerate and may gradually grow large during the SEF process. Large particles might meet stronger steric hindrances in the couplings, whereas Cu nanoparticles immobilized on the SiNWs are kept from growing large; and the distances among the particles were close enough to favor the enhanced fields’ overlapping, which caused great SEF effect.

Conclusions

In conclusion, the Cu/Si nanostructure was prepared using SiNWs as a carrier, which has the advantage of easy surface modification with various metals. The as-fabricated Cu/Si nanostructure was comparatively stable and was able to prevent Cu nanoparticles from aggregating, and thus enhanced the fluorescence of Ln³⁺ dramatically. The enhancement of Cu/Si nanomaterials are larger than that on unsupported Cu nanoparticles. This result might be attributed to the large local plasmons oscillation resulted from the interparticle overlap on the SiNWs. This easy fabrication of Cu/Si nanostructure with excellent properties enables itself to find wider applications.

References

[1] Chance R R, Prock A, Silbey R. Molecular fluorescence and energy transfer near interfaces. Advances in Chemical Physics, 1978, 37: 1-65

[2] Ray K, Badugu R, Lakowicz J R. Metal-enhanced fluorescence from CdTe nanocrystals: a single-molecule fluorescence study. Journal of the American Chemical Society, 2006, 128(28): 8998-8999

[3] Lakowicz J R. Plasmonics in biology and plasmon-controlled fluorescence. Plasmonics, 2006, 1(1): 5-33

[4] Aslan K, Holley P, Geddes C D. Metal-enhanced fluorescence from silver nanoparticle-deposited polycarbonate substrates. Journal of Materials Chemistry, 2006, 16(27): 2846-28525.

[5] Ray K, Chowdhury M H, Lakowicz J R. Aluminum nanostructured films as substrates for enhanced fluorescence in the ultraviolet-blue spectral region. Analytical Chemistry, 2007, 79(17): 6480-6487

[6] Mertens H, Koenderink A F, Polman A. Plasmon-enhanced luminescence near noble-metal nanospheres: Comparison of exact theory and an improved Gersten and Nitzan model. Physical Review B: Condensed Matter and Materials Physics, 2007, 76(11): 115123-1-115123-12

[7] Lakowicz J R. Radiative decay engineering: biophysical and biomedical applications. Analytical Biochemistry, 2001, 298(1): 1-24

[8] Zhang J, Fu Y, Lakowicz J R. Emission behavior of fluorescently labeled silver nanoshell: enhanced self-quenching by netal nanostructure. Journal of Physical Chemistry C, 2007, 111(5): 1955-1961

[9] Lakowicz J R. Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission. Analytical Biochemistry, 2005, 337(2): 171-194

[10] Bjerneld E J, Földes-Papp Z, Käll M, Rigler R. Single-molecule surface-enhanced raman and fluorescence correlation spectroscopy of horseradish peroxidase. Journal of Physical Chemistry B, 2002, 106(6): 1213-1218

[11] Aslan K, Lakowicz J R, Geddes C D. Rapid deposition of triangular silver nanoplates on planar surfaces: application to metal-enhanced fluorescence. The Journal of Physical Chemistry B, 2005, 109(13): 6247-6251

[12] Zhang Y X, Aslan K, Previte M J R, Geddes C D. Metal-enhanced fluorescence from copper substrates. Applied Physics Letters, 2007, 90(17): 173116

[13] Baluschev S, Yu F, Miteva T, Ahl S, Yasuda A, Nelles G, Knoll W, Wegner G. Metal-enhanced up-conversion fluorescence: effective triplet-triplet annihilation near silver surface. Nano Letters, 2005, 5(12): 2482-2484

[14] Zhuo S J, Shao M W, Cheng L, Que R H, Zhuo S J, Ma D D D, Lee S T. Surface-enhanced fluorescence of praseodymium ions (Pr³⁺) on silver/silicon nanostructure. Applied Physics Letters, 2010, 96(10): 103108-1-103108-3

[15] Ahrens B, Eisenschmidt C, Johnson J A, Miclea P T, Schweizer S. Structural and optical investigations of Nd-doped fluorozirconate-based glass ceramics for enhanced upconverted fluorescence. Applied Physics Letters, 2008, 92(6): 061905

[16] Aisaka T, Fujii M, Hayashi S. Enhancement of upconversion luminescence of Er doped Al₂O₃ films by Ag island films. Applied Physics Letters, 2008, 92(13): 132105

[17] Capobianco J A, Boyer J C, Vetrone F, Speghini A, Bettinelli M. Optical spectroscopy and upconversion studies of Ho³⁺-doped Bulk and Nanocrystalline Y₂O₃. Chemistry of Materials, 2002, 14(7): 2915-2921

[18] Bünzli J C G. Benefiting from the unique properties of lanthanide ions. Accounts of Chemical Research, 2006, 39(1): 53-61

[19] Tissue B M. Synthesis and luminescence of lanthanide ions in nanoscale insulating hosts. Chemistry of Materials, 1998, 10(10): 2837-2845

[20] Hasegawa Y, Wada Y, Yanagida S. Strategies for the design of luminescent lanthanide(III) complexes and their photonic applications. Journal of Photochemistry and Photobiology C, Photochemistry Reviews, 2004, 5(3): 183-202

[21] Zhang J, Malicka J, Gryczynski I, Lakowicz J R. Surface-enhanced fluorescence of fluorescein-labeled oligonucleotides capped on silver nanoparticles. Journal of Physical Chemistry B, 2005, 109(16): 7643-7648

[22] Shao M W, Shan Y Y, Wong N B, Lee S T. Silicon nanowire sensors for bioanalytical application: glucose and hydrogen peroxide detection. Advanced Functional Materials, 2005, 15(9): 1478-1482

[23] Lisiecki I, Pileni M P. Synthesis of copper metallic clusters using reverse micelles as microreactors. Journal of the American Chemical Society, 1993, 115(10): 3887-3896

[24] Shao M W, Cheng L, Zhang X H, Ma D D D, Lee S T. Excellent photocatalysis of HF-treated silicon nanowires. Journal of the American Chemical Society, 2009, 131(49): 17738-17739

[25] Tsang C H A, Liu Y, Kang Z H, Ma D D D, Wong N B, Lee S T. Metal (Cu, Au)-modified silicon nanowires for high-selectivity solvent-free hydrocarbon oxidation in air. Chemical Communications, 2009, (39): 5829-5831

[26] Gunnarsson L, Bjerneld E J, Xu H, Petronis S, Kasemo B, Käll M. Interparticle coupling effects in nanofabricated substrates for surface-enhanced Raman scattering. Applied Physics Letters, 2001, 78(6): 802-804

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