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Synthesis of bismuth nanocap arrays on quartz substrates and their surface plasmon resonance properties
Journal of Physics and Chemistry of Solids 73 (2012) 604–607
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis of bismuth nanocap arrays on quartz substrates and their surface plasmon resonance properties Gui-Na Xiao, Shi-Qing Man n Department of Electronic Engineering, Institute of Nano-Chemistry, Jinan University, Guangzhou 510632, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2011 Received in revised form 1 December 2011 Accepted 24 December 2011 Available online 31 December 2011
Bismuth nanocap arrays have been prepared by vacuum depositing Bi films onto the surfaces of selfassembled monolayer arrays of SiO2 nanoparticles. The surface morphologies, structures, and optical properties of the obtained samples have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscope (AFM), X-ray diffraction (XRD), and ultraviolet–visible–near infrared (UV–vis–NIR) spectrophotometer. TEM and AFM images indicated that the SiO2/Bi composite nanoparticles were incompletely encapsulated and their surfaces were relatively rough. UV–vis–NIR absorption spectra showed that Bi nanocap arrays had strong and tunable surface plasmon resonance peaks in the visible and near infrared regions, which were dependent dramatically on the relative ratio of the SiO2 core diameter to the Bi cap thickness. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Metals A. Nanostructures B. Chemical synthesis B. Vapour deposition D. Optical properties
1. Introduction Nano-sized metal particles have attracted considerable interest because of their unique surface plasmon resonance (SPR) properties. The SPR is very sensitive to the shape, size, composition, dielectric properties, and surrounding dielectric environment of the metal nanostructures [1]. Metal nanostructures with various shapes have been successfully synthesized, including nanorods [2], nanocubes [3], nanoprisms [4], nanoflowers [5], nanowires [6], nanostars [7], nanocages [8], and nanoshells [9,10]. Metal nanoshells are a new and unique type of composite nanoparticle composed of a dielectric core with a metallic shell, which have a tunable surface plasmon resonance that can be easily regulated by varying the relative dimensions of the core and the shell [11]. It has been reported that the plasmon response of reduced-symmetrical metal nanoshells, such as semishells (nanocups, half-shell, and nanocaps) [12–18], is dependent on the orientation of the nanostructure with respect to the direction and polarization of incident light. Up to now, the research of nanocaps has been mainly focused on noble metals like gold, silver, and copper, as well as alkali metal like aluminum [19–22]. However, the commercialization of these metal nanocaps may be limited by the high cost of Au and Ag and the easy oxidation of Cu and Al. Therefore, it is of great practical significance to exploit semimetal nanocap arrays with low cost, good stability, and tunable surface plasmon resonance.
n
Corresponding author. Tel.: þ86 20 85220658; fax: þ 86 20 85220231. E-mail address: [email protected] (S.-Q. Man).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.12.022
Semimetal bismuth possesses exotic optical and electronic properties due to its highly anisotropic Fermi surface, small carrier effective masses, low carrier densities, high electron mobility, and long mean free path [23]. A variety of Bi nanostructures, such as nanospheres [24], nanorods [25], nanowires [26], and nanocubes [27] have been synthesized in the past. In this paper, bismuth nanocap arrays, consisting of a SiO2 core with a Bi cap, were prepared by nanoscale masking technique combined with vacuum thermal evaporation method. This approach has some advantages such as simplicity of operation, low cost, good control over the core diameter and the cap thickness of the composite nanoparticles.
2. Experimental 2.1. Materials Tetraethyl orthosilicate (TEOS, 99.9%), ammonium hydroxide (NH4OH, 25 wt%), anhydrous ethanol (EtOH, 99.7%), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30 wt%), and bismuth grain (99.9%) were used as received. All chemicals were of analytical grade. Milli-Q water ((Millipore) 418.2 MO cm) was used for all solution preparation and experiments. 2.2. Synthesis of SiO2 nanoparticles The silica nanoparticles were synthesized according to the ¨ Stober method [28]. Briefly, 4 mL of TEOS was added dropwise under vigorous stirring to a mixture of 25 mL of EtOH, 2 mL of
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H2O, and 5 mL of NH4OH, and the reaction was continued for 8 h. After being left to stand overnight, the mixture was then boiled for 2 h at about 70 1C. The nanoparticles were purified by centrifugation and redispersion in anhydrous ethanol for four times to remove any residual reactants. 2.3. Fabrication of self-assembled monolayer arrays of SiO2 nanoparticles Monolayer arrays of SiO2 nanoparticles were deposited onto the quartz substrates by the dip-coating method [29]. 2 cm 2 cm quartz substrates were cleaned by immersion into piranha solution (7:3 concentrated H2SO4:H2O2) for 3 h, followed by sonication in Milli-Q water for 30 min, and then dried for 1 h at 100 1C in an oven to achieve a hydrophilic surface. 2–3 drops of 10 wt% SiO2 colloids were then pipetted onto a clean glass substrate. After holding it immobile for 30 s, the substrate was slowly immersed in a beaker with full ultrapure water, immediately an ordered monolayer was formed on the water surface, and the monolayer was then transferred onto a clean quartz substrate. The samples were allowed to dry naturally. 2.4. Preparation of bismuth nanocap arrays Bismuth thin films with various thicknesses were deposited onto the monolayer arrays of SiO2 nanospheres by a conventional vacuum thermal evaporation method at a chamber pressure of about 5 10 4 Pa, and the evaporation rate was maintained at 0.01–0.03 nm s 1. A quartz crystal microbalance was used to monitor the deposition rate and thickness.
Fig. 1. TEM images of (a) SiO2 nanoparticles and (b) SiO2/Bi composite nanoparticles.
2.5. Characterization Transmission electron microscopy (TEM) images were obtained using a Philips TECNAI 10 transmission electron microscopy. The surface morphologies of SiO2 nanospheres coated substrates were characterized by a Philips XL-30 scanning electron microscopy, operating at an acceleration voltage of 20 kV. An Autoprobe CP atomic force microscope was used to characterize the surface topographies of the self-assembled arrays of SiO2 nanoparticles before and after the deposition of Bi films. X-ray diffraction patterns were recorded on a MSAL XD-2 diffractometer with the Ni-filtered Cu Ka radiation (l ¼0.15418 nm). The ultraviolet–visible–near-infrared (UV–vis–NIR) absorption spectra were measured at normal incidence on a Cary 5000 spectrophotometer using unpolarized light with a probe beam size of about 9 mm2.
3. Results and discussion
Fig. 2. SEM image of a self-assembled monolayer of 400 nm SiO2 nanoparticles.
3.1. Morphological analyses Fig. 1(a) shows the TEM image of SiO2 nanoparticles. It is shown that the prepared SiO2 nanoparticles exhibit a regular spherical shape of 400 nm in diameter and have a good monodispersity. The size distribution of SiO2 nanoparticles is narrow with a relative standard deviation less than 8%. Fig. 1(b) shows the TEM image of SiO2/Bi composite nanoparticles peeled off from the substrate by sonication, the diameter of SiO2 is 400 nm and the thickness of Bi is 40 nm. It can be seen that only the top half of the composite nanoparticles is covered by Bi. SEM micrograph of a self-assembled array of 400 nm SiO2 nanospheres is displayed in Fig. 2. It exhibits a nearly wellordered and close-packed monolayer array, though point defects and stacking faults are observed in certain areas, owing to the
relative inhomogeneous size distribution of the prepared SiO2 nanoparticles. AFM images of a monolayer of SiO2 nanospheres before and after depositing Bi films on top of it are exhibited in Figs. 3 and 4, respectively, and the scanning region is 2 2 mm2. Fig. 3 shows 2D and 3D AFM images of the self-assembled monolayer array of SiO2 nanospheres with the diameter of 400 nm. It is displayed that the surface of SiO2 nanospheres is smooth, and the arrangement of nanospheres is regular, which is in accordance with the SEM image. Fig. 4 shows 2D and 3D AFM images of the bismuth nanocap arrays with a 400 nm SiO2 core and a 25 nm Bi cap. In comparison with Fig. 3, it can be seen that the surface roughness of SiO2 nanoparticles is distinctly increased after the deposition of 25 nm thick Bi film. The substructure topography of the Bi cap is
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Fig. 3. AFM images of a self-assembled monolayer of 400 nm SiO2 nanoparticles. A typical 2D image (a) and 3D image (b).
Fig. 4. AFM images of bismuth nanocap arrays with a 400 nm SiO2 core and a 25 nm Bi cap. A typical 2D image (a) and 3D image (b).
essentially dependent upon the metal evaporation rate and thickness [30]. 3.2. Structural properties Fig. 5 shows the X-ray diffraction patterns of (a) bismuth film with thickness of 40 nm, and bismuth nanocap arrays with a 40 nm thick Bi cap and SiO2 core diameters of (b) 400 nm and (c) 300 nm. It is observed that the Bi film exhibits strong diffraction peaks at 2y equals to 23.341, 46.661, and 72.221, which are assigned to (003), (006), and (009) planes of the Bi hexagonal phase, respectively [31], and a small peak at 28.121 attributed to the (012) plane of polycrystalline Bi is also observed. A similar behavior has been reported for the Bi films prepared by pulsed laser deposition [32]. Compared with the Bi film, the two asprepared nanocap arrays exhibit an additional weak diffraction peak at 38.861 that corresponds to the (104) plane. However, the peak at 72.221 cannot be observed for the nanocap arrays with 300 nm SiO2, and a typical amorphous halo pattern at around 221 which may be attributed to SiO2 is exhibited. 3.3. Optical properties Fig. 6(a) shows the UV–vis–NIR absorption spectra of bismuth nanocap arrays with a 40 nm thick Bi cap and SiO2 core diameters ranging from 200 to 400 nm. As a reference, the absorption spectrum of a flat bismuth film of the same thickness (40 nm) is
Fig. 5. X-ray diffraction patterns of (a) a 40 nm thick bismuth film and (b, c) bismuth nanocap arrays with a 40 nm thick Bi cap and SiO2 core diameters of 400 and 300 nm, respectively.
showed. The flat Bi film exhibits a small peak at about 400 nm and a broad peak at around 1310 nm, respectively, which are assigned to the excitation of the propagating surface plasmon. However, the absorption spectra of bismuth nanocap arrays appear to be different. The lmax values are located at around 856, 1133, 1237, and 1381 nm for the Bi nanocap arrays with SiO2 core diameters of 200, 300, 350, and 400 nm, respectively, and
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607
4. Conclusions Using the self-assembled monolayers of SiO2 nanoparticles as templates, bismuth nanocap arrays were fabricated by depositing Bi films on top of the templates via vacuum thermal evaporation. TEM, SEM, AFM, XRD, and UV–vis–NIR spectroscopy were used to characterize the SiO2 nanoparticles and the SiO2/Bi composite nanoparticles. It was found that Bi nanocap arrays had tunable surface plasmon resonance peaks ranging from the visible to the near-infrared region, which were red-shifted with increasing the diameter of SiO2 core or decreasing the thickness of Bi cap. Our results indicate that Bi nanocap arrays have potential application in the fields of optical biosensors and photonic band gap materials.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 60477015), Natural Science Foundation of Guangdong Province (Grant no. 9151063201000018), and Team Project of Guangdong Province Natural Science Foundation (Grant no. 05200555). References
Fig. 6. UV–vis–NIR absorption spectra of bismuth nanocap arrays (a) with the same Bi cap thickness of 40 nm and different SiO2 core diameters, and (b) with the same SiO2 core diameter of 300 nm and different Bi cap thicknesses.
other small absorption peaks centered at 438, 616, 650, and 728 nm are also observed, respectively. Fig. 6(b) shows the UV–vis–NIR absorption spectra of bismuth nanocap arrays with a 300 nm diameter SiO2 core and Bi cap thicknesses ranging from 20 to 40 nm. The dipole plasmon resonance absorption peaks are located at 694, 679, 642, and 616 nm for the Bi nanocap arrays with Bi cap thicknesses of 20, 25, 30, and 40 nm, respectively. As the thickness of Bi cap increases, the SPR band intensity increases obviously, but the position of the longitudinal plasmon resonance absorption peak centered at around 1100 nm does not show any significant change. Bismuth nanocap arrays and silver films deposited on twodimensional PS colloidal crystal reported by Farcau and Astilean [33] have the same structure, so their optical properties should be similar. According to this literature, it is deduced that the observed absorption peaks of bismuth nanocap arrays may be attributed to the coupling between the surface plasmon resonances of the truncated tetrahedral Bi nanoparticles and the surface plasmon modes localized at the edge of the Bi caps. From Fig. 6, it is shown that the absorption peaks of bismuth nanocap arrays are shifted towards longer wavelength with increasing the diameter of SiO2 core or decreasing the thickness of Bi cap. When the cap thickness is fixed, as the core diametercap thickness ratio is varied between 5 and 10, the SPR band at shorter wavelength is shifted from 438 nm to 728 nm. When the core diameter is fixed, a less shift (78 nm) is occurred as the ratio increases from 7.5 to 15. Therefore, besides the core–cap ratio, the overall size of the composite nanoparticle has strong influence on surface plasmon resonance absorption bands.
[1] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2003). 668-667. [2] S. Pal, L.E. Depero, I. Alessandri, Nanotechnology 21 (2010) 425701. 8pp. [3] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176–2179. [4] L.J. Sherry, R.C. Jin, C.A. Mirkin, G.C. Schatz, R.P. Van Duyne, Nano Lett. 6 (2006) 2060–2065. [5] B.K. Jena, B.K. Mishra, S. Bohidar, J. Phys. Chem. C 113 (2009) 14753–14758. [6] S.-C. Lin, S.-Y. Chen, Y.-T. Chen, S.-Y. Cheng, J. Alloys Compd. 449 (2008) 232–236. [7] C.L. Nehl, H.W. Liao, J.H. Hafner, Nano Lett. 6 (2006) 683–688. [8] M. Rycenga, K.K. Hou, C.M. Cobley, A.G. Schwartz, P.H.C. Camargo, Y.N. Xia, Phys. Chem. Chem. Phys. 11 (2009) 5903–5908. [9] S.J. Oldenburg, R.D. Averitt, S.L. Westcott, N.J. Halas, Chem. Phys. Lett. 288 (1998) 243–247. [10] N.V. Long, T.D. Hien, T. Asaka, M. Ohtaki, M. Nogamia, J. Alloys Compd. 509 (2011) 7702–7709. [11] B. Mondal, S. Saha, J. Mater. Sci. 46 (2011) 5153–5159. [12] Y. Zhang, A. Barhoumi, J.B. Lassiter, N.J. Halas, Nano Lett. 11 (2011) 1838–1844. [13] J. Ye, P. Van Dorpe, W. Van Roy, K. Lodewijks, I. De Vlaminck, G. Maes, G. Borghs, J. Phys. Chem. C 113 (2009) 3110–3115. [14] J. Ye, N. Verellen, W. Van Roy, L. Lagae, G. Maes, G. Borghs, P. Van Dorpe, ACS Nano 4 (2010) 1457–1464. [15] J. Ye, C. Chen, L. Lagae, G. Maes, G. Borghs, P. Van Dorpe, Phys. Chem. Chem. Phys. 12 (2010) 11222–11224. [16] P. Van Dorpe, J. Ye, ACS. Nano 5 (2011) 6774–6778. [17] J.B. Lassiter, M.W. Knight, N.A. Mirin, N.J. Halas, Nano Lett. 9 (2009) 4326–4332. [18] C. Charnay, A. Lee, S.-Q. Man, C.E. Moran, C. Radloff, R.K. Bradley, N.J. Halas, J. Phys. Chem. B 107 (2003) 7327–7333. [19] J. Liu, A.I. Maaroof, L. Wieczorek, M.B. Cortie, Adv. Mater. 17 (2005) 1276–1281. [20] G.-N. Xiao, S.-Q. Man, Chem. Phys. Lett. 447 (2007) 305–309. [21] H. Takei, J. Vac. Sci. Technol. B 17 (1999) 1906–1911. [22] J. Liu, B. Cankurtaran, G. McCredie, M.J. Ford, L. Wieczorek, M.B. Cortie, Nanotechnology 16 (2005) 3023–3028. [23] C.F. Gallo, B.S. Chandrasekhar, P.H. Sutter, J. Appl. Phys. 34 (1963) 144–152. [24] J.L. Wu, H.M. Yang, H. Li, Z. Lu, X.L. Yu, R. Chen, J. Alloys Compd. 498 (2010) L8–L11. [25] J. Reppert, R. Rao, M. Skove, J. He, M. Craps, T. Tritt, A.M. Rao, Chem. Phys. Lett. 442 (2007) 334–338. [26] Y. Bisrat, Z.P. Luo, D. Davis, D. Lagoudas, Nanotechnology 18 (2007) 395601. (6 pp). [27] W.Z. Wang, B. Poudel, Y. Ma, Z.F. Ren, J. Phys. Chem. B 110 (2006) 25702–25706. ¨ [28] W. Stober, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62–69. [29] X.D. Wang, C.J. Summers, Z.L. Wang, Nano Lett. 4 (2004) 423–426. [30] M. Litorja, C.L. Haynes, A.J. Haes, T.R. Tensen, R.P. Van Duyne, J. Phys. Chem. B 105 (2001) 6907–6915. [31] N.Z. El-Sayed, Vacuum 80 (2006) 860–863. [32] A. Dauscher, M.O. Boffoue, B. Lenoir, R. Martin-Lopez, H. Scherrer, Appl. Surf. Sci. 138–139 (1999) 188–194. [33] C. Farcau, S. Astilean, J. Opt. A: Pure Appl. Opt. 9 (2007) S345–S349.
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis of bismuth nanocap arrays on quartz substrates and their surface plasmon resonance properties Gui-Na Xiao, Shi-Qing Man n Department of Electronic Engineering, Institute of Nano-Chemistry, Jinan University, Guangzhou 510632, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 October 2011 Received in revised form 1 December 2011 Accepted 24 December 2011 Available online 31 December 2011
Bismuth nanocap arrays have been prepared by vacuum depositing Bi films onto the surfaces of selfassembled monolayer arrays of SiO2 nanoparticles. The surface morphologies, structures, and optical properties of the obtained samples have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscope (AFM), X-ray diffraction (XRD), and ultraviolet–visible–near infrared (UV–vis–NIR) spectrophotometer. TEM and AFM images indicated that the SiO2/Bi composite nanoparticles were incompletely encapsulated and their surfaces were relatively rough. UV–vis–NIR absorption spectra showed that Bi nanocap arrays had strong and tunable surface plasmon resonance peaks in the visible and near infrared regions, which were dependent dramatically on the relative ratio of the SiO2 core diameter to the Bi cap thickness. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Metals A. Nanostructures B. Chemical synthesis B. Vapour deposition D. Optical properties
1. Introduction Nano-sized metal particles have attracted considerable interest because of their unique surface plasmon resonance (SPR) properties. The SPR is very sensitive to the shape, size, composition, dielectric properties, and surrounding dielectric environment of the metal nanostructures [1]. Metal nanostructures with various shapes have been successfully synthesized, including nanorods [2], nanocubes [3], nanoprisms [4], nanoflowers [5], nanowires [6], nanostars [7], nanocages [8], and nanoshells [9,10]. Metal nanoshells are a new and unique type of composite nanoparticle composed of a dielectric core with a metallic shell, which have a tunable surface plasmon resonance that can be easily regulated by varying the relative dimensions of the core and the shell [11]. It has been reported that the plasmon response of reduced-symmetrical metal nanoshells, such as semishells (nanocups, half-shell, and nanocaps) [12–18], is dependent on the orientation of the nanostructure with respect to the direction and polarization of incident light. Up to now, the research of nanocaps has been mainly focused on noble metals like gold, silver, and copper, as well as alkali metal like aluminum [19–22]. However, the commercialization of these metal nanocaps may be limited by the high cost of Au and Ag and the easy oxidation of Cu and Al. Therefore, it is of great practical significance to exploit semimetal nanocap arrays with low cost, good stability, and tunable surface plasmon resonance.
n
Corresponding author. Tel.: þ86 20 85220658; fax: þ 86 20 85220231. E-mail address: [email protected] (S.-Q. Man).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.12.022
Semimetal bismuth possesses exotic optical and electronic properties due to its highly anisotropic Fermi surface, small carrier effective masses, low carrier densities, high electron mobility, and long mean free path [23]. A variety of Bi nanostructures, such as nanospheres [24], nanorods [25], nanowires [26], and nanocubes [27] have been synthesized in the past. In this paper, bismuth nanocap arrays, consisting of a SiO2 core with a Bi cap, were prepared by nanoscale masking technique combined with vacuum thermal evaporation method. This approach has some advantages such as simplicity of operation, low cost, good control over the core diameter and the cap thickness of the composite nanoparticles.
2. Experimental 2.1. Materials Tetraethyl orthosilicate (TEOS, 99.9%), ammonium hydroxide (NH4OH, 25 wt%), anhydrous ethanol (EtOH, 99.7%), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30 wt%), and bismuth grain (99.9%) were used as received. All chemicals were of analytical grade. Milli-Q water ((Millipore) 418.2 MO cm) was used for all solution preparation and experiments. 2.2. Synthesis of SiO2 nanoparticles The silica nanoparticles were synthesized according to the ¨ Stober method [28]. Briefly, 4 mL of TEOS was added dropwise under vigorous stirring to a mixture of 25 mL of EtOH, 2 mL of
G.-N. Xiao, S.-Q. Man / Journal of Physics and Chemistry of Solids 73 (2012) 604–607
605
H2O, and 5 mL of NH4OH, and the reaction was continued for 8 h. After being left to stand overnight, the mixture was then boiled for 2 h at about 70 1C. The nanoparticles were purified by centrifugation and redispersion in anhydrous ethanol for four times to remove any residual reactants. 2.3. Fabrication of self-assembled monolayer arrays of SiO2 nanoparticles Monolayer arrays of SiO2 nanoparticles were deposited onto the quartz substrates by the dip-coating method [29]. 2 cm 2 cm quartz substrates were cleaned by immersion into piranha solution (7:3 concentrated H2SO4:H2O2) for 3 h, followed by sonication in Milli-Q water for 30 min, and then dried for 1 h at 100 1C in an oven to achieve a hydrophilic surface. 2–3 drops of 10 wt% SiO2 colloids were then pipetted onto a clean glass substrate. After holding it immobile for 30 s, the substrate was slowly immersed in a beaker with full ultrapure water, immediately an ordered monolayer was formed on the water surface, and the monolayer was then transferred onto a clean quartz substrate. The samples were allowed to dry naturally. 2.4. Preparation of bismuth nanocap arrays Bismuth thin films with various thicknesses were deposited onto the monolayer arrays of SiO2 nanospheres by a conventional vacuum thermal evaporation method at a chamber pressure of about 5 10 4 Pa, and the evaporation rate was maintained at 0.01–0.03 nm s 1. A quartz crystal microbalance was used to monitor the deposition rate and thickness.
Fig. 1. TEM images of (a) SiO2 nanoparticles and (b) SiO2/Bi composite nanoparticles.
2.5. Characterization Transmission electron microscopy (TEM) images were obtained using a Philips TECNAI 10 transmission electron microscopy. The surface morphologies of SiO2 nanospheres coated substrates were characterized by a Philips XL-30 scanning electron microscopy, operating at an acceleration voltage of 20 kV. An Autoprobe CP atomic force microscope was used to characterize the surface topographies of the self-assembled arrays of SiO2 nanoparticles before and after the deposition of Bi films. X-ray diffraction patterns were recorded on a MSAL XD-2 diffractometer with the Ni-filtered Cu Ka radiation (l ¼0.15418 nm). The ultraviolet–visible–near-infrared (UV–vis–NIR) absorption spectra were measured at normal incidence on a Cary 5000 spectrophotometer using unpolarized light with a probe beam size of about 9 mm2.
3. Results and discussion
Fig. 2. SEM image of a self-assembled monolayer of 400 nm SiO2 nanoparticles.
3.1. Morphological analyses Fig. 1(a) shows the TEM image of SiO2 nanoparticles. It is shown that the prepared SiO2 nanoparticles exhibit a regular spherical shape of 400 nm in diameter and have a good monodispersity. The size distribution of SiO2 nanoparticles is narrow with a relative standard deviation less than 8%. Fig. 1(b) shows the TEM image of SiO2/Bi composite nanoparticles peeled off from the substrate by sonication, the diameter of SiO2 is 400 nm and the thickness of Bi is 40 nm. It can be seen that only the top half of the composite nanoparticles is covered by Bi. SEM micrograph of a self-assembled array of 400 nm SiO2 nanospheres is displayed in Fig. 2. It exhibits a nearly wellordered and close-packed monolayer array, though point defects and stacking faults are observed in certain areas, owing to the
relative inhomogeneous size distribution of the prepared SiO2 nanoparticles. AFM images of a monolayer of SiO2 nanospheres before and after depositing Bi films on top of it are exhibited in Figs. 3 and 4, respectively, and the scanning region is 2 2 mm2. Fig. 3 shows 2D and 3D AFM images of the self-assembled monolayer array of SiO2 nanospheres with the diameter of 400 nm. It is displayed that the surface of SiO2 nanospheres is smooth, and the arrangement of nanospheres is regular, which is in accordance with the SEM image. Fig. 4 shows 2D and 3D AFM images of the bismuth nanocap arrays with a 400 nm SiO2 core and a 25 nm Bi cap. In comparison with Fig. 3, it can be seen that the surface roughness of SiO2 nanoparticles is distinctly increased after the deposition of 25 nm thick Bi film. The substructure topography of the Bi cap is
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Fig. 3. AFM images of a self-assembled monolayer of 400 nm SiO2 nanoparticles. A typical 2D image (a) and 3D image (b).
Fig. 4. AFM images of bismuth nanocap arrays with a 400 nm SiO2 core and a 25 nm Bi cap. A typical 2D image (a) and 3D image (b).
essentially dependent upon the metal evaporation rate and thickness [30]. 3.2. Structural properties Fig. 5 shows the X-ray diffraction patterns of (a) bismuth film with thickness of 40 nm, and bismuth nanocap arrays with a 40 nm thick Bi cap and SiO2 core diameters of (b) 400 nm and (c) 300 nm. It is observed that the Bi film exhibits strong diffraction peaks at 2y equals to 23.341, 46.661, and 72.221, which are assigned to (003), (006), and (009) planes of the Bi hexagonal phase, respectively [31], and a small peak at 28.121 attributed to the (012) plane of polycrystalline Bi is also observed. A similar behavior has been reported for the Bi films prepared by pulsed laser deposition [32]. Compared with the Bi film, the two asprepared nanocap arrays exhibit an additional weak diffraction peak at 38.861 that corresponds to the (104) plane. However, the peak at 72.221 cannot be observed for the nanocap arrays with 300 nm SiO2, and a typical amorphous halo pattern at around 221 which may be attributed to SiO2 is exhibited. 3.3. Optical properties Fig. 6(a) shows the UV–vis–NIR absorption spectra of bismuth nanocap arrays with a 40 nm thick Bi cap and SiO2 core diameters ranging from 200 to 400 nm. As a reference, the absorption spectrum of a flat bismuth film of the same thickness (40 nm) is
Fig. 5. X-ray diffraction patterns of (a) a 40 nm thick bismuth film and (b, c) bismuth nanocap arrays with a 40 nm thick Bi cap and SiO2 core diameters of 400 and 300 nm, respectively.
showed. The flat Bi film exhibits a small peak at about 400 nm and a broad peak at around 1310 nm, respectively, which are assigned to the excitation of the propagating surface plasmon. However, the absorption spectra of bismuth nanocap arrays appear to be different. The lmax values are located at around 856, 1133, 1237, and 1381 nm for the Bi nanocap arrays with SiO2 core diameters of 200, 300, 350, and 400 nm, respectively, and
G.-N. Xiao, S.-Q. Man / Journal of Physics and Chemistry of Solids 73 (2012) 604–607
607
4. Conclusions Using the self-assembled monolayers of SiO2 nanoparticles as templates, bismuth nanocap arrays were fabricated by depositing Bi films on top of the templates via vacuum thermal evaporation. TEM, SEM, AFM, XRD, and UV–vis–NIR spectroscopy were used to characterize the SiO2 nanoparticles and the SiO2/Bi composite nanoparticles. It was found that Bi nanocap arrays had tunable surface plasmon resonance peaks ranging from the visible to the near-infrared region, which were red-shifted with increasing the diameter of SiO2 core or decreasing the thickness of Bi cap. Our results indicate that Bi nanocap arrays have potential application in the fields of optical biosensors and photonic band gap materials.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 60477015), Natural Science Foundation of Guangdong Province (Grant no. 9151063201000018), and Team Project of Guangdong Province Natural Science Foundation (Grant no. 05200555). References
Fig. 6. UV–vis–NIR absorption spectra of bismuth nanocap arrays (a) with the same Bi cap thickness of 40 nm and different SiO2 core diameters, and (b) with the same SiO2 core diameter of 300 nm and different Bi cap thicknesses.
other small absorption peaks centered at 438, 616, 650, and 728 nm are also observed, respectively. Fig. 6(b) shows the UV–vis–NIR absorption spectra of bismuth nanocap arrays with a 300 nm diameter SiO2 core and Bi cap thicknesses ranging from 20 to 40 nm. The dipole plasmon resonance absorption peaks are located at 694, 679, 642, and 616 nm for the Bi nanocap arrays with Bi cap thicknesses of 20, 25, 30, and 40 nm, respectively. As the thickness of Bi cap increases, the SPR band intensity increases obviously, but the position of the longitudinal plasmon resonance absorption peak centered at around 1100 nm does not show any significant change. Bismuth nanocap arrays and silver films deposited on twodimensional PS colloidal crystal reported by Farcau and Astilean [33] have the same structure, so their optical properties should be similar. According to this literature, it is deduced that the observed absorption peaks of bismuth nanocap arrays may be attributed to the coupling between the surface plasmon resonances of the truncated tetrahedral Bi nanoparticles and the surface plasmon modes localized at the edge of the Bi caps. From Fig. 6, it is shown that the absorption peaks of bismuth nanocap arrays are shifted towards longer wavelength with increasing the diameter of SiO2 core or decreasing the thickness of Bi cap. When the cap thickness is fixed, as the core diametercap thickness ratio is varied between 5 and 10, the SPR band at shorter wavelength is shifted from 438 nm to 728 nm. When the core diameter is fixed, a less shift (78 nm) is occurred as the ratio increases from 7.5 to 15. Therefore, besides the core–cap ratio, the overall size of the composite nanoparticle has strong influence on surface plasmon resonance absorption bands.
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