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In situ growth of CuS thin films on functionalized self-assembled monolayers using chemical bath deposition
Journal of Colloid and Interface Science 356 (2011) 726–733
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
In situ growth of CuS thin films on functionalized self-assembled monolayers using chemical bath deposition Yongjuan Lu a,b, Xu Meng a, Gewen Yi a, Junhong Jia a,⇑ a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate School of Chinese Academy of Sciences, Beijing 10049, PR China
a r t i c l e
i n f o
Article history: Received 20 October 2010 Accepted 10 January 2011 Available online 15 January 2011 Keywords: Copper sulfide Chemical bath deposition Self-assembled monolayer Selective deposition
a b s t r a c t Nano-structured CuS thin films were deposited on the functionalized –NH2-terminated self-assembled monolayers (SAMs) surface by chemical bath deposition (CBD). The deposition mechanism of CuS on the –NH2-terminated group was systematically investigated using field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS), UV–vis absorption. The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor with high electrical conductivity (12.3 X/h). The functionalized SAMs terminal group plays a key role in the deposition of CuS thin films. The growth of CuS on the varying SAMs surface shows different deposition mechanisms. On –NH2-terminated surfaces, a combination of ion-by-ion growth and cluster-by-cluster deposition can interpret the observed behavior. On –OH- and –CH3-terminated surfaces, the dominant growth mechanism on the surface is clusterby-cluster deposition in the solution. According to this principle, the patterned CuS microarrays with different feature sizes were successfully deposited on –NH2-terminated SAMs regions of –NH2/–CH3 patterned SAMs surface. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction During past several decades, there has been extensive research in metal chalcogenide thin films due to their wide-range applications in large area photodiode arrays, solar selective coatings, solar cells, photoconductors, flat panel displays, sensors, etc. [1–6]. Copper sulfide is a p-type transparent semiconductor with metal-like electronic conductive at low temperature [7,8], and has chemical sensing capabilities [3]. Copper sulfide thin films maintain transmittance in the infrared region, low reflectance in the visible region and relatively high reflectance in the near infrared region [9], which has been found many applications, including optical filter [10,11], sensor [12], architectural glazes [2], cathode material [13], catalysis [14], surperionic materials and nonlinear optical materials [15]. One of important challenges in the solar cell areas is to find semiconductor material with a suitable band gap and a high absorption coefficient. Since the discovery of CdS/Cu2S heterojunction solar cells in 1954, the use of copper sulfide thin film in solid junction solar energy conversion device is well known [16]. Copper sulfide thin films are interesting absorber materials in thin film so-
⇑ Corresponding author. Fax: +86 931 8277088. E-mail address: [email protected] (J. Jia). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.01.031
lar cell due to the ideal optical characteristics [17]. Furthermore, ternary chalcogenides semiconductor material, such as CuInS2 [18,19], Cu3BiS3 [20–22], CuSbS2 [23] and CuGaS2 [24] are receiving considerable attention as promising thin film absorber in photovoltaic solar cell. CuS is one of the most prevalent minor phases co-existing in these films independent from the producing techniques and play an important role both in the structural and electrical properties of the films. So far, a variety of methods were employed to the deposition of CuS thin films, such as chemical vapor deposition [25], thermal coevaporation [2], successive ionic layer adsorption and reaction (SILAR) [9], photochemical deposition [26], electrochemical method [27] and chemistry bath deposition (CBD) [28–30]. Among these, the chemical bath deposition is an attractive technique for the deposition of semiconductors as it does not require sophisticated instrumentation like vacuum system and other expensive equipments. With CBD method, electrical conductivity of the substrate is not the necessary requirement. The low temperature (30– 80 °C) deposition also avoids oxidation and corrosion of metallic substrates. The better orientations and improved grain structure can be obtained under the easily controlled deposition parameters. Further, it can be easily adapted to fabricate large-area semiconductor thin film [31]. Self-assembled monolayers (SAMs) with varying functionalities have been successfully utilized to direct the assembly of metal
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Fig. 1. FESEM images of CuS thin film on –NH2-terminated SAM surface of time series: (a) less than 10 min, (b) 30 min, (c) 3 h and (d) 10 h, by CBD.
alized self-assembled monolayers with CBD method. The deposition mechanisms of the CuS thin film on the functionalized self-assembled monolayers were investigated and discussed based on the morphology and crystallinity analysis of CuS nanocrystals using FESEM, XRD and XPS. The investigation of optical properties and photoelectrochemical response were also carried out to clarify the structure and composition of CuS thin films. This paper also addressed the tentative growth mechanisms of CuS nanocrystals on various functional SAMs terminal group. Meanwhile, the patterned CuS microarrays with different feature sizes were fabricated based on the proposed principle.
2. Materials and methods 2.1. Materials Fig. 2. Thickness of CuS thin film deposited on –NH2-terminated SAM vs. deposition time.
chalcogenide nanocrystals, such as CdS [32], ZnS and PbS [33,34]. The key point of the success of selective deposition is that organic functionalize groups of SAMs can control crystal heterogeneous nucleation and growth. SAMs can be patterned using a wide range of ways including the UV-photolithography [35–37], micro-contact printing [32,38], nanoimprint lithography (NIL) [39,40]. Researchers have successfully prepared patterned thin films such as TiO2 [41,42], SnO2 [43], ln2O3 [37], Ta2O5 [44], and SrTiO3 [45,46] using patterned SAMs as templates. In this work, we reported the convenient chemical deposition of the transparent p-type conducting CuS thin films on the function-
3-aminopropyl triethoxysilane (APTS, 99%) and octadecyltrichlorosilane (OTS, 95%) were purchased from Aldrich. n-type Si and ITO glass (Grinm Semiconductor Materials Co. Ltd., China) was used for the substrates. The Cu grids (Transmission Electron Microscope) were used as masks for producing the patterned SAMs by UV-irradiation method. All the other reagents were analytical grade and used as received. 2.2. Preparation of functionalized self-assembled monolayers (SAMs) The procedure for preparation of SAMs with –CH3 or –NH2 terminal groups has been detailed in our previous paper [47]. To ensure successful preparation of SAMs, the static contact angles of water on SAMs were 108–110° (–CH3 terminal group) and 50–
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Fig. 4. Optical absorption spectra of CuS thin films at various deposition times.
The pH of the bath solution was adjusted to 2.2–2.3 by adding H2SO4 solution (1 M). The substrates were placed vertically to the bottom of the beakers to avoid the effect of gravity. After depositing, the deposited films were rinsed in deionized water and ultrasonically washed to remove the leftover copper sulfide precipitates, and dried with nitrogen gas. Patterned SAMs were produced by irradiating the OTS–SAM surface through a Cu grid mask under UV light for approximately 3 h in air. UV-irradiation causes the formation of silanol regions in exposed areas. Then, the patterned substrate with –CH3terminated group was immersed in the hexane solution of APTS for at least 3 h to assemble APTS on the silanol regions. Finally, the substrate was removed from the solution and ultrasonically washed with hexane for 3 min and dried with high pure nitrogen gas flow, resulting in a patterned –CH3/NH2-terminated SAM surface. Then, the substrate with patterned SAMs was immersed in an aqueous solution of CuSO4, Na2S2O3 and EDTA for the deposition of CuS microarray patterns. 2.4. Characterization The morphologies of the CuS were examined by field emission scanning electron microscope (FESEM, JSM-5600LV, Japan). The
Fig. 3. XRD patterns (a) and XPS spectra of Cu (b) and S (c) for the as-prepared CuS thin film.
52° (–NH2 terminal group), in agreement with the literature [48,49]. 2.3. CuS chemical bath deposition and preparation of CuS microarray patterns CuS thin films were prepared by CBD method. The functionalized substrates were immersed in prepared precursor solution consisted of CuSO45H2O (copper source), EDTA (complexing agent) and Na2S2O3 (sulfur source). The solution temperature was maintained at 70 °C using a thermostatically controlled water bath.
Fig. 5. Time-dependent photoconductor response of CuS thin film on –NH2terminated SAM with on–off switch of light and external bias set at 0.6 V.
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Fig. 6. (a) FESEM images of CuS thin film for 3 h without sonication on –NH2-terminated SAM surface. Corresponding magnification of sphere-like nanocrystallines are shown in the inset. (b) Scheme of mechanism of the CuS sheet-like nanoparticles formation on –NH2-terminated SAM surface. (c) XPS spectra of sample with –NH2-terminated SAM surface deposited less than 10 min at 70 °C.
structure and the phase composition were analyzed by X’pert PRO X-ray diffraction (XRD, Netherlands) with Cu Ka radiation at the scanning speed of 1.2°/min. The chemical states of the elements on the films were determined using a PHI5702 multi-functional X-ray photoelectron spectroscope (XPS, USA). The XPS analysis was conducted at 400 W and pass energy of 29.35 eV, using Al Ka (1486.6 eV) radiation as the excitation source and the binding energy of contaminated carbon (C1s = 284.6 eV) was used as reference. The optical absorption spectra were obtained with a UV–vis spectrophotometer (U-3010, Japan) within the wavelength range of 280–1000 nm. Computer controlled potentiostat (CHI 660d) was used for all PEC experiments. The PEC responses of the samples were measured in an electrochemical cell with a there-electrode system, in which CuS thin film, a platinum wire and saturation mercury electrode were used as the working electrode,
the counter electrode and reference electrode, respectively. A 125 W mercury lamp was used as the light source. The electrolyte, aqueous HClO4 (0.1 M) solutions, was freshly prepared using double deionized water. Then, the electrolyte was put into an ultrasonic bath for 30 min before each experiment in order to decrease the gas solutes in the electrolytes.
3. Results and discussion 3.1. Deposition and structural characterization of CuS thin film on NH2-terminated SAMs To understand the formation process and growth mechanism of CuS thin films, detailed time-dependent evolutions of morphology
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Fig. 7. FESEM images of CuS thin films for 3 h on –CH3- (a) and –OH-terminated SAMs at (b) and (c) after ultrasonic washing for 5 min. (d) XPS spectra of (b) surface.
were studied by FESEM as shown in Fig. 1. The variation of film thickness with deposition time is shown in Fig. 2. Initially, no obvious CuS nanoparticles were observed on the substrates at the first 10 min (Fig. 1a). After that, there is a continuous increase of film thickness with deposition time, while the growth rate decreases gradually. The thickness of the film reaches to 120 nm after 6 h. However, at longer deposition time there is no significant increase in film thickness, which can be attributed to the equilibrium of adsorption and desorption of the preformed CuS film taking place in the deposition solution. This feature is well known in chemically deposited semiconductor thin films [50]. From the FESEM image in Fig. 1b, it can be seen that the film is homogeneous and consists of small spherical nanoparticles and sheet-like nanoparticles after 30 min deposition. However, the film density is so low that surface of the film shows holes and fractures in nanoscale. When the deposition time is prolonged to 3 h, the film becomes denser than before, concomitantly, the nanoparticle shape changed from spherical to sheet-like and the quantity of sheet-like nanoparticles rises expeditiously (Fig. 1c). With further observation, almost all the sheet-like nanoparticles were standing perpendicularly on the substrates. (The inset is the magnified image of Fig. 1c). By the reaction time of 10 h, the continuous and compact thin film consists of uniform sheet-like CuS nanoparticles was obtained (Fig. 1d). For the X-ray diffraction spectra of the prepared CuS thin films (Fig. 3a), the peak at 56° is assigned to the diffraction of Si substrate. The other diffraction peaks could be indexed as pure hexagonal phase CuS with lattice parameters of a = 3.800 Å and c = 16.33 Å, corresponding to the JCPDS card 01-1281 file. On the
basis of the full width at half-maximum of [1 1 0] peak and applying the Debeye–Scherrer equation [28], the average size of the CuS nanocrystal was calculated around 8.65 nm. Therefore, upon the above analysis and review of the reported literatures [51], a possible mechanism of growth orientation of the CuS film is proposed as follows: The c/a ratio of the as-prepared hexagonal CuS is 4.32, which is greater than the ideal value (1.633) [52], implying that the {0 0 0 1} facets have a surface energy lower than those of the 0} and {1 1 2 0} facets [53]. Only appearance of {0 0 0 1} crys{1 0 1 tal facets is helpful to limit the exposure of other higher energy surfaces, which might therefore reduce the exposure of the 0} and {1 1 2 0} facets with relatively higher surface energy {1 0 1 of CuS, as a result, reducing the total energy of the nanocrystals and resulting in the formation of the stable sheet-like nanoparticles. In the XPS spectra of Cu and S on the surface of CuS film (Fig. 3b and c), the peaks of the binding energies of 932.3 eV and 952.2 eV were assigned to Cu in CuS and 162.3 eV was assigned to S in CuS, the binding energy of 168.4 eV is the S in the residual of SO24 on the surface. It can be calculated that the Cu/S ratio is 0.9835, which is less than 1.00 might be due to trace of the residual of SO24 on the surface. Thus, the XPS spectra confirmed that the copper sulfide thin film is composing of pure CuS crystal. 3.2. Optical and photoelectrochemical (PEC) properties The UV–vis absorption spectra of CuS thin films with different reaction times are shown in Fig. 4. The spectrum of the 30 min deposited thin film shows an intense absorption peak at the wavelength of 369 nm and reaches a minimum around at 500 nm. This
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Fig. 9. UV–vis absorbance spectra of different feature size CuS microarrays on ITO conducting glass surfaces: (a) 30 lm, (b) 80 lm and (c) 160 lm.
Fig. 8. SEM images of patterned CuS microarrays on Si substrate surface with different feature sizes: (a) 30 lm, (b) 80 lm and (c) 160 lm.
is associated with the spherical CuS nanoparticles, consistent with the previous reports which concluded that CuS nanocrystals in diameter of 10–20 nm show absorption peaks at 400 nm [17,51]. Here, the blue shift of the maximum absorption was attributed to quantum size effect [54]. It can be seen that the kmax of absorption spectra shift to longer wavelength with the prolonged deposition times, which was due to the band gap decrease with the increase of the particle sizes. Moreover, well-defined absorption features were exhibited, which indicated a very narrow size distribution of CuS nanocrystals [55]. The low absorption between 500 nm and 700 nm of the UV–vis absorption spectra indicated the good transparency of CuS thin film in the visible region. The good transparency is closely related to the small grains, which minimize light absorption and scattering losses. In the Near-IR range, the CuS thin films exhibit a strong and broad absorption. This result is in agreement with Zhang and co-worker’ conclusion [8] for that the covellite CuS also possesses a characteristically broad absorption band beyond 800 nm, which extends as a long absorption tail into
the Near-IR region. The NIR absorption of Cu2 xS is suggested be term of the valence band free carriers which is essentially metallic in character [17]. Meanwhile, CuS thin films exhibit high electrical conductivity (12.3 X/h) through the measurement of square resistance. The high electrical conductivity originates from the large carrier concentration that is correlated with absorption of the CuS film in the NIR range. Therefore, composite films with high elasticity and transparency can be obtained by coating the nano-structured CuS thin film on the organic polymer, which have potential use in electric devices and shielding materials for electromagnetic wave [30]. The photoelectrochemical (PEC) responses of samples were measured to examine the possibility of their application in solar energy cells. I–t characteristics of CuS thin film with different deposition time illuminated intermittently using 365 nm light are shown in Fig. 5. The peak to peak current (Ipk–pk) of CuS thin films undergoes a gradual increase with an increase of deposition time at a bias voltage of 0.6 V. This is similar to the PEC responses of the Bi2S3 thin films [56], in that the thicker films possess effectively larger solar photon absorbing volume and contribute to the increased short circuit current. Moreover, the increase of the cathode photocurrent is consistent with the photo-conducting behavior of p-type semiconductor materials [57]. 3.3. Mechanisms of CBD CuS on –NH2-, –CH3-, –OH-terminated functionalized self-assembled monolayer There have been many reports on CuS synthesized by CBD, but no definitive mechanism has been established on the functionalized SAMs surface. In CBD, synthesis of the CuS thin film is based on the releases of copper and sulfur ion from the precursor solution. To enhance uniformity and quality of the films, disodium salt of EDTA was used as complexing agent to control copper ion concentration. In practice, there are two possible parallel reactions leading solid materials deposited in chemical bath: (i) within the bulk immersed of the solution (cluster-by-cluster) [34], (ii) at the surface, the substrate or adventitious reactions taken place on the vessel surface (ion-by-ion) [58,59]. One reaction predominated over another is governed by the extent of the heterogeneous and homogeneous nucleation, which is closely related to the properties of the surface. In this work, two shapes of crystallites were formed on the –NH2-terminated SAMs: sheet-like and sphere-like nanoparticles (Fig. 6a, the inset is the magnified image of sphere-like
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nanocrystallines). While the large sphere-like nanoparticles can be easily washed off by ultrasonication, but the sheet-like nanoparticles showed good adhesion to the substrate as no peel-off was found. The formation mechanism of sheet-like CuS nanoparticles was proposed as below: The –N in the –NH2 groups of APTS–SAMs can form complexes with Cu2+ in the planting solutions, and then, the heterogenous nucleation reaction incessantly takes place on the interface, and as-formed nanocrytallites are in situ assembled to thin film (see Fig. 6c). This reaction is similar to that of ZnS on –COOH-terminated SAMs [34]. The explanation can also be confirmed by the XPS results (Fig. 6b). Though no any CuS nanoparticles were observed in the FESEM image with the 10 min deposition surface (Fig. 1a), the XPS shows the presence of Cu, O, Si, C, N and trace S, which indicated that the copper ion formed complexes with –NH2-terminated SAMs first before the nucleation in the plating solution. The ion-by-ion growth mechanism can here be applied to explain the formation of the sheet-like nanoparticles and the cluster-by-cluster deposition can be used to explain formation of large sphere-like nanoparticles. For the films deposited on –CH3- and –OH-terminated SAMs, only large sphere-like crystalline are observed on both surfaces (Fig. 7a and b). While the large sphere-like crystallites can be easily removed by ultrasonic washing (Fig. 7c). The XPS result confirmed that there is no presence of Cu element and implied no interaction between the Cu2+ and the –OH- and –CH3-terminated groups in deposition process (Fig. 7d). Accordingly, the dominant mechanism was cluster-bycluster growth for the CuS deposition on –OH- and –CH3-terminated SAMs surfaces. It is suggested that –NH2-terminated SAMs is required in order to obtain CuS thin films with good quality and high electrical conductivity. Moreover, based on the proposed the deposition mechanisms of nano-structured CuS thin film on different terminated SAMs surface, the patterned CuS microarrays with different feature size were successfully fabricated by controlling surface chemical properties (Fig. 8). The patterns were regular over a large area and had relatively clear boundaries and show good selectivity between –NH2-terminated regions and –CH3-terminated regions. In this method, the patterned thin films can be produced in a large surface area for application in the photovoltaic devices and be used to tune the physical and chemical properties of thin films [60]. Optical properties of CuS thin film with different features (30 lm, 80 lm, 160 lm) were investigated and shown in Fig. 9. The results indicated that the feature sizes of CuS patterns can effectively affect the UV–vis absorption performance of the patterned CuS thin films. The possible reasons were discussed in our previous paper [61].
4. Conclusions We have successfully prepared transparent p-type conducting CuS thin film on the functionalized self-assembled monolayers using a simple chemical bath deposition. FESEM images show CuS particles are assembled to highly homogenous and adherent films with good transparency and high electric conductivity (12.3 X/h). The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor. I–t characteristics illuminated that the CuS thin film showed a strong PEC response by using intermittent light, which was measured to examine the possible application in solar energy. The growth of CuS on the varying SAMs surface show different deposition mechanisms. On the –NH2-terminated SAMs surfaces, the reaction mechanism of ion-by-ion growth and cluster-by-cluster deposition were proposed for the observed
deposition phenomena. The cluster-by-cluster deposition mechanism is dominated in the nanocrystals growth of CuS on the –OH- and –CH3-terminated SAMs surfaces. Based on the proposed the deposition mechanisms, we have successfully prepared the patterned CuS microarrays with different features and investigated their optical properties. Acknowledgments The authors acknowledge the National Natural Science Foundation of China (Grant Nos. 50705094, 50972148) and ‘‘Hundred Talents Program of Chinese Academy of Sciences’’ (Grant No. KGCX2-YW-804) for providing the financial support. References [1] F.F. Amos, S.A. Morin, J.A. Streifer, R.J. Hamers, S. Jin, J. Am. Chem. Soc. 129 (2007) 14296. [2] A. Bollero, M. Grossberg, B. Asenjo, M.T. Gutierrez, Surf. Coat. Technol. 204 (2009) 593. [3] A.A. Sagade, R. Sharma, J.C. Vyas, Sens. Lett. 7 (2009) 550. [4] A. Antony, K.V. Murali, R. Manoj, M.K. Jayaraj, Mater. Chem. Phys. 90 (2005) 106. [5] Mitzi, D. B.; Copel, M. W. Metal chalcogenide film used in e.g. flat panel display, is prepared by contacting isolated hydrazinium-based precursor of metal chalcogenide and solvent to form solution; applying solution onto substrate; and annealing resultant film. US2007099331-A1; US7618841-B2, 2007. [6] T.A. O’Brien, P.D. Rack, P.H. Holloway, M.C. Zerner, J. Lumin. 78 (1998) 245. [7] S. Erokhina, V. Erokhin, C. Nicolini, Langmuir 19 (2003) 766. [8] F. Zhang, S.S. Wong, Chem. Mater. 21 (2009) 4541. [9] H.M. Pathan, J.D. Desai, C.D. Lokhande, Appl. Surf. Sci. 202 (2002) 47. [10] P.S. Khiew, S. Radiman, N.M. Huang, M.S. Ahamd, J. Cryst. Growth 268 (2004) 227. [11] Rozhin, O.; Ferrari, A.; Milne, W. I. Optical nano-material composition used as index-matching gel for sensor device and optical device such as lens, prism, polarization plate and waveguide facet, contains nano-material(s) and optical coupling gel or optical adhesive. WO2008025966-A1; EP2057211-A1; US2010002324-A1, WO2008025966-A1 C08K-003/00 200852, 2008. [12] A.A. Sagade, R. Sharma, Sens. Actuators B 133 (2008) 135. [13] J.S. Kim, D.Y. Kim, G.B. Cho, T.H. Nam, K.W. Kim, H.S. Ryu, J.H. Ahn, H.J. Ahn, J. Power Sources 189 (2009) 864. [14] G.D. Cody, N.Z. Boctor, J.A. Brandes, T.R. Filley, R.M. Hazen, H.S. Yoder, Geochim. Cosmochim. Acta 68 (2004) 2185. [15] Y.J. Yang, S.S. Hu, J. Solid State Electrochem. 12 (2008) 1405. [16] E. Burstein, Phys. Rev. 93 (1954) 632. [17] Y.X. Zhao, H.C. Pan, Y.B. Lou, X.F. Qiu, J.J. Zhu, C. Burda, J. Am. Chem. Soc. 131 (2009) 4253. [18] K. Das, S.K. Panda, S. Gorai, P. Mishra, S. Chaudhuri, Mater. Res. Bull. 43 (2008) 2742. [19] J.J. Nairn, P.J. Shapiro, B. Twamley, T. Pounds, R. von Wandruszka, T.R. Fletcher, M. Williams, C.M. Wang, M.G. Norton, Nano Lett. 6 (2006) 1218. [20] V. Estrella, M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 18 (2003) 190. [21] F. Mesa, G. Gordillo, T. Dittrich, K. Ellmer, R. Baier, S. Sadewasser, Appl. Phys. Lett. 96 (2010). [22] N.J. Gerein, J.A. Haber, Chem. Mater. 18 (2006) 6297. [23] Y. Rodriguez-Lazcano, M.T.S. Nair, P.K. Nair, J. Cryst. Growth 223 (2001) 399. [24] S.C. Abrahams, J.L. Bernstei, J. Chem. Phys. 59 (1973) 5415. [25] M. Kemmler, M. Lazell, P. O’Brien, D.J. Otway, J.H. Park, J.R. Walsh, J. Mater. Sci.Mater. Electron. 13 (2002) 531. [26] J. Podder, R. Kobayashi, M. Ichimura, Thin Solid Films 472 (2005) 71. [27] R. Cordova, H. Gomez, R. Schrebler, P. Cury, M. Orellana, P. Grez, D. Leinen, J.R. Ramos-Barrado, R. Del Rio, Langmuir 18 (2002) 8647. [28] S.V. Bagul, S.D. Chavhan, R. Sharma, J. Phys. Chem. Solids 68 (2007) 1623. [29] M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 4 (1989) 191. [30] T. Yamamoto, K. Tanaka, E. Kubota, K. Osakada, Chem. Mater. 5 (1993) 1352. [31] A. Goudarzi, G.M. Aval, S.S. Park, M.C. Choi, R. Sahraei, M.H. Ullah, A. Avanes, C.S. Ha, Chem. Mater. 21 (2009) 3738. [32] Y.K. Hwang, S.Y. Woo, J.H. Lee, D.Y. Jung, Y.U. Kwon, Chem. Mater. 12 (2000) 2059. [33] F.C. Meldrum, J. Flath, W. Knoll, Thin Solid Films 348 (1999) 188. [34] P. Lu, A.V. Walker, ACS Nano 3 (2009) 370. [35] E. Cooper, G.J. Leggett, Langmuir 15 (1999) 1024. [36] Y.C. Lin, B.Y. Yu, W.C. Lin, Y.Y. Chen, J.J. Shyue, Chem. Mater. 20 (2008) 6606. [37] Y. Masuda, M. Kondo, K. Koumoto, Cryst. Growth Des. 9 (2009) 555. [38] S.S. Yoon, D.O. Kim, S.C. Park, Y.K. Lee, H.Y. Chae, S.B. Jung, J.D. Nam, Microelectron. Eng. 85 (2008) 136. [39] M.D. Austin, S.Y. Chou, Nano Lett. 3 (2003) 1687. [40] M. Ara, H. Tada, Appl. Phys. Lett. 83 (2003) 578. [41] Y.F. Gao, Y. Masuda, K. Koumoto, Chem. Mater. 16 (2004) 1062. [42] Z.L. He, Z.W. Yu, H.Y. Miao, G.Q. Tan, Y. Liu, Sci. China Ser. E-Technol. Sci. 52 (2009) 137.
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In situ growth of CuS thin films on functionalized self-assembled monolayers using chemical bath deposition Yongjuan Lu a,b, Xu Meng a, Gewen Yi a, Junhong Jia a,⇑ a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate School of Chinese Academy of Sciences, Beijing 10049, PR China
a r t i c l e
i n f o
Article history: Received 20 October 2010 Accepted 10 January 2011 Available online 15 January 2011 Keywords: Copper sulfide Chemical bath deposition Self-assembled monolayer Selective deposition
a b s t r a c t Nano-structured CuS thin films were deposited on the functionalized –NH2-terminated self-assembled monolayers (SAMs) surface by chemical bath deposition (CBD). The deposition mechanism of CuS on the –NH2-terminated group was systematically investigated using field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS), UV–vis absorption. The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor with high electrical conductivity (12.3 X/h). The functionalized SAMs terminal group plays a key role in the deposition of CuS thin films. The growth of CuS on the varying SAMs surface shows different deposition mechanisms. On –NH2-terminated surfaces, a combination of ion-by-ion growth and cluster-by-cluster deposition can interpret the observed behavior. On –OH- and –CH3-terminated surfaces, the dominant growth mechanism on the surface is clusterby-cluster deposition in the solution. According to this principle, the patterned CuS microarrays with different feature sizes were successfully deposited on –NH2-terminated SAMs regions of –NH2/–CH3 patterned SAMs surface. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction During past several decades, there has been extensive research in metal chalcogenide thin films due to their wide-range applications in large area photodiode arrays, solar selective coatings, solar cells, photoconductors, flat panel displays, sensors, etc. [1–6]. Copper sulfide is a p-type transparent semiconductor with metal-like electronic conductive at low temperature [7,8], and has chemical sensing capabilities [3]. Copper sulfide thin films maintain transmittance in the infrared region, low reflectance in the visible region and relatively high reflectance in the near infrared region [9], which has been found many applications, including optical filter [10,11], sensor [12], architectural glazes [2], cathode material [13], catalysis [14], surperionic materials and nonlinear optical materials [15]. One of important challenges in the solar cell areas is to find semiconductor material with a suitable band gap and a high absorption coefficient. Since the discovery of CdS/Cu2S heterojunction solar cells in 1954, the use of copper sulfide thin film in solid junction solar energy conversion device is well known [16]. Copper sulfide thin films are interesting absorber materials in thin film so-
⇑ Corresponding author. Fax: +86 931 8277088. E-mail address: [email protected] (J. Jia). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.01.031
lar cell due to the ideal optical characteristics [17]. Furthermore, ternary chalcogenides semiconductor material, such as CuInS2 [18,19], Cu3BiS3 [20–22], CuSbS2 [23] and CuGaS2 [24] are receiving considerable attention as promising thin film absorber in photovoltaic solar cell. CuS is one of the most prevalent minor phases co-existing in these films independent from the producing techniques and play an important role both in the structural and electrical properties of the films. So far, a variety of methods were employed to the deposition of CuS thin films, such as chemical vapor deposition [25], thermal coevaporation [2], successive ionic layer adsorption and reaction (SILAR) [9], photochemical deposition [26], electrochemical method [27] and chemistry bath deposition (CBD) [28–30]. Among these, the chemical bath deposition is an attractive technique for the deposition of semiconductors as it does not require sophisticated instrumentation like vacuum system and other expensive equipments. With CBD method, electrical conductivity of the substrate is not the necessary requirement. The low temperature (30– 80 °C) deposition also avoids oxidation and corrosion of metallic substrates. The better orientations and improved grain structure can be obtained under the easily controlled deposition parameters. Further, it can be easily adapted to fabricate large-area semiconductor thin film [31]. Self-assembled monolayers (SAMs) with varying functionalities have been successfully utilized to direct the assembly of metal
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Fig. 1. FESEM images of CuS thin film on –NH2-terminated SAM surface of time series: (a) less than 10 min, (b) 30 min, (c) 3 h and (d) 10 h, by CBD.
alized self-assembled monolayers with CBD method. The deposition mechanisms of the CuS thin film on the functionalized self-assembled monolayers were investigated and discussed based on the morphology and crystallinity analysis of CuS nanocrystals using FESEM, XRD and XPS. The investigation of optical properties and photoelectrochemical response were also carried out to clarify the structure and composition of CuS thin films. This paper also addressed the tentative growth mechanisms of CuS nanocrystals on various functional SAMs terminal group. Meanwhile, the patterned CuS microarrays with different feature sizes were fabricated based on the proposed principle.
2. Materials and methods 2.1. Materials Fig. 2. Thickness of CuS thin film deposited on –NH2-terminated SAM vs. deposition time.
chalcogenide nanocrystals, such as CdS [32], ZnS and PbS [33,34]. The key point of the success of selective deposition is that organic functionalize groups of SAMs can control crystal heterogeneous nucleation and growth. SAMs can be patterned using a wide range of ways including the UV-photolithography [35–37], micro-contact printing [32,38], nanoimprint lithography (NIL) [39,40]. Researchers have successfully prepared patterned thin films such as TiO2 [41,42], SnO2 [43], ln2O3 [37], Ta2O5 [44], and SrTiO3 [45,46] using patterned SAMs as templates. In this work, we reported the convenient chemical deposition of the transparent p-type conducting CuS thin films on the function-
3-aminopropyl triethoxysilane (APTS, 99%) and octadecyltrichlorosilane (OTS, 95%) were purchased from Aldrich. n-type Si and ITO glass (Grinm Semiconductor Materials Co. Ltd., China) was used for the substrates. The Cu grids (Transmission Electron Microscope) were used as masks for producing the patterned SAMs by UV-irradiation method. All the other reagents were analytical grade and used as received. 2.2. Preparation of functionalized self-assembled monolayers (SAMs) The procedure for preparation of SAMs with –CH3 or –NH2 terminal groups has been detailed in our previous paper [47]. To ensure successful preparation of SAMs, the static contact angles of water on SAMs were 108–110° (–CH3 terminal group) and 50–
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Fig. 4. Optical absorption spectra of CuS thin films at various deposition times.
The pH of the bath solution was adjusted to 2.2–2.3 by adding H2SO4 solution (1 M). The substrates were placed vertically to the bottom of the beakers to avoid the effect of gravity. After depositing, the deposited films were rinsed in deionized water and ultrasonically washed to remove the leftover copper sulfide precipitates, and dried with nitrogen gas. Patterned SAMs were produced by irradiating the OTS–SAM surface through a Cu grid mask under UV light for approximately 3 h in air. UV-irradiation causes the formation of silanol regions in exposed areas. Then, the patterned substrate with –CH3terminated group was immersed in the hexane solution of APTS for at least 3 h to assemble APTS on the silanol regions. Finally, the substrate was removed from the solution and ultrasonically washed with hexane for 3 min and dried with high pure nitrogen gas flow, resulting in a patterned –CH3/NH2-terminated SAM surface. Then, the substrate with patterned SAMs was immersed in an aqueous solution of CuSO4, Na2S2O3 and EDTA for the deposition of CuS microarray patterns. 2.4. Characterization The morphologies of the CuS were examined by field emission scanning electron microscope (FESEM, JSM-5600LV, Japan). The
Fig. 3. XRD patterns (a) and XPS spectra of Cu (b) and S (c) for the as-prepared CuS thin film.
52° (–NH2 terminal group), in agreement with the literature [48,49]. 2.3. CuS chemical bath deposition and preparation of CuS microarray patterns CuS thin films were prepared by CBD method. The functionalized substrates were immersed in prepared precursor solution consisted of CuSO45H2O (copper source), EDTA (complexing agent) and Na2S2O3 (sulfur source). The solution temperature was maintained at 70 °C using a thermostatically controlled water bath.
Fig. 5. Time-dependent photoconductor response of CuS thin film on –NH2terminated SAM with on–off switch of light and external bias set at 0.6 V.
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Fig. 6. (a) FESEM images of CuS thin film for 3 h without sonication on –NH2-terminated SAM surface. Corresponding magnification of sphere-like nanocrystallines are shown in the inset. (b) Scheme of mechanism of the CuS sheet-like nanoparticles formation on –NH2-terminated SAM surface. (c) XPS spectra of sample with –NH2-terminated SAM surface deposited less than 10 min at 70 °C.
structure and the phase composition were analyzed by X’pert PRO X-ray diffraction (XRD, Netherlands) with Cu Ka radiation at the scanning speed of 1.2°/min. The chemical states of the elements on the films were determined using a PHI5702 multi-functional X-ray photoelectron spectroscope (XPS, USA). The XPS analysis was conducted at 400 W and pass energy of 29.35 eV, using Al Ka (1486.6 eV) radiation as the excitation source and the binding energy of contaminated carbon (C1s = 284.6 eV) was used as reference. The optical absorption spectra were obtained with a UV–vis spectrophotometer (U-3010, Japan) within the wavelength range of 280–1000 nm. Computer controlled potentiostat (CHI 660d) was used for all PEC experiments. The PEC responses of the samples were measured in an electrochemical cell with a there-electrode system, in which CuS thin film, a platinum wire and saturation mercury electrode were used as the working electrode,
the counter electrode and reference electrode, respectively. A 125 W mercury lamp was used as the light source. The electrolyte, aqueous HClO4 (0.1 M) solutions, was freshly prepared using double deionized water. Then, the electrolyte was put into an ultrasonic bath for 30 min before each experiment in order to decrease the gas solutes in the electrolytes.
3. Results and discussion 3.1. Deposition and structural characterization of CuS thin film on NH2-terminated SAMs To understand the formation process and growth mechanism of CuS thin films, detailed time-dependent evolutions of morphology
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Fig. 7. FESEM images of CuS thin films for 3 h on –CH3- (a) and –OH-terminated SAMs at (b) and (c) after ultrasonic washing for 5 min. (d) XPS spectra of (b) surface.
were studied by FESEM as shown in Fig. 1. The variation of film thickness with deposition time is shown in Fig. 2. Initially, no obvious CuS nanoparticles were observed on the substrates at the first 10 min (Fig. 1a). After that, there is a continuous increase of film thickness with deposition time, while the growth rate decreases gradually. The thickness of the film reaches to 120 nm after 6 h. However, at longer deposition time there is no significant increase in film thickness, which can be attributed to the equilibrium of adsorption and desorption of the preformed CuS film taking place in the deposition solution. This feature is well known in chemically deposited semiconductor thin films [50]. From the FESEM image in Fig. 1b, it can be seen that the film is homogeneous and consists of small spherical nanoparticles and sheet-like nanoparticles after 30 min deposition. However, the film density is so low that surface of the film shows holes and fractures in nanoscale. When the deposition time is prolonged to 3 h, the film becomes denser than before, concomitantly, the nanoparticle shape changed from spherical to sheet-like and the quantity of sheet-like nanoparticles rises expeditiously (Fig. 1c). With further observation, almost all the sheet-like nanoparticles were standing perpendicularly on the substrates. (The inset is the magnified image of Fig. 1c). By the reaction time of 10 h, the continuous and compact thin film consists of uniform sheet-like CuS nanoparticles was obtained (Fig. 1d). For the X-ray diffraction spectra of the prepared CuS thin films (Fig. 3a), the peak at 56° is assigned to the diffraction of Si substrate. The other diffraction peaks could be indexed as pure hexagonal phase CuS with lattice parameters of a = 3.800 Å and c = 16.33 Å, corresponding to the JCPDS card 01-1281 file. On the
basis of the full width at half-maximum of [1 1 0] peak and applying the Debeye–Scherrer equation [28], the average size of the CuS nanocrystal was calculated around 8.65 nm. Therefore, upon the above analysis and review of the reported literatures [51], a possible mechanism of growth orientation of the CuS film is proposed as follows: The c/a ratio of the as-prepared hexagonal CuS is 4.32, which is greater than the ideal value (1.633) [52], implying that the {0 0 0 1} facets have a surface energy lower than those of the 0} and {1 1 2 0} facets [53]. Only appearance of {0 0 0 1} crys{1 0 1 tal facets is helpful to limit the exposure of other higher energy surfaces, which might therefore reduce the exposure of the 0} and {1 1 2 0} facets with relatively higher surface energy {1 0 1 of CuS, as a result, reducing the total energy of the nanocrystals and resulting in the formation of the stable sheet-like nanoparticles. In the XPS spectra of Cu and S on the surface of CuS film (Fig. 3b and c), the peaks of the binding energies of 932.3 eV and 952.2 eV were assigned to Cu in CuS and 162.3 eV was assigned to S in CuS, the binding energy of 168.4 eV is the S in the residual of SO24 on the surface. It can be calculated that the Cu/S ratio is 0.9835, which is less than 1.00 might be due to trace of the residual of SO24 on the surface. Thus, the XPS spectra confirmed that the copper sulfide thin film is composing of pure CuS crystal. 3.2. Optical and photoelectrochemical (PEC) properties The UV–vis absorption spectra of CuS thin films with different reaction times are shown in Fig. 4. The spectrum of the 30 min deposited thin film shows an intense absorption peak at the wavelength of 369 nm and reaches a minimum around at 500 nm. This
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Fig. 9. UV–vis absorbance spectra of different feature size CuS microarrays on ITO conducting glass surfaces: (a) 30 lm, (b) 80 lm and (c) 160 lm.
Fig. 8. SEM images of patterned CuS microarrays on Si substrate surface with different feature sizes: (a) 30 lm, (b) 80 lm and (c) 160 lm.
is associated with the spherical CuS nanoparticles, consistent with the previous reports which concluded that CuS nanocrystals in diameter of 10–20 nm show absorption peaks at 400 nm [17,51]. Here, the blue shift of the maximum absorption was attributed to quantum size effect [54]. It can be seen that the kmax of absorption spectra shift to longer wavelength with the prolonged deposition times, which was due to the band gap decrease with the increase of the particle sizes. Moreover, well-defined absorption features were exhibited, which indicated a very narrow size distribution of CuS nanocrystals [55]. The low absorption between 500 nm and 700 nm of the UV–vis absorption spectra indicated the good transparency of CuS thin film in the visible region. The good transparency is closely related to the small grains, which minimize light absorption and scattering losses. In the Near-IR range, the CuS thin films exhibit a strong and broad absorption. This result is in agreement with Zhang and co-worker’ conclusion [8] for that the covellite CuS also possesses a characteristically broad absorption band beyond 800 nm, which extends as a long absorption tail into
the Near-IR region. The NIR absorption of Cu2 xS is suggested be term of the valence band free carriers which is essentially metallic in character [17]. Meanwhile, CuS thin films exhibit high electrical conductivity (12.3 X/h) through the measurement of square resistance. The high electrical conductivity originates from the large carrier concentration that is correlated with absorption of the CuS film in the NIR range. Therefore, composite films with high elasticity and transparency can be obtained by coating the nano-structured CuS thin film on the organic polymer, which have potential use in electric devices and shielding materials for electromagnetic wave [30]. The photoelectrochemical (PEC) responses of samples were measured to examine the possibility of their application in solar energy cells. I–t characteristics of CuS thin film with different deposition time illuminated intermittently using 365 nm light are shown in Fig. 5. The peak to peak current (Ipk–pk) of CuS thin films undergoes a gradual increase with an increase of deposition time at a bias voltage of 0.6 V. This is similar to the PEC responses of the Bi2S3 thin films [56], in that the thicker films possess effectively larger solar photon absorbing volume and contribute to the increased short circuit current. Moreover, the increase of the cathode photocurrent is consistent with the photo-conducting behavior of p-type semiconductor materials [57]. 3.3. Mechanisms of CBD CuS on –NH2-, –CH3-, –OH-terminated functionalized self-assembled monolayer There have been many reports on CuS synthesized by CBD, but no definitive mechanism has been established on the functionalized SAMs surface. In CBD, synthesis of the CuS thin film is based on the releases of copper and sulfur ion from the precursor solution. To enhance uniformity and quality of the films, disodium salt of EDTA was used as complexing agent to control copper ion concentration. In practice, there are two possible parallel reactions leading solid materials deposited in chemical bath: (i) within the bulk immersed of the solution (cluster-by-cluster) [34], (ii) at the surface, the substrate or adventitious reactions taken place on the vessel surface (ion-by-ion) [58,59]. One reaction predominated over another is governed by the extent of the heterogeneous and homogeneous nucleation, which is closely related to the properties of the surface. In this work, two shapes of crystallites were formed on the –NH2-terminated SAMs: sheet-like and sphere-like nanoparticles (Fig. 6a, the inset is the magnified image of sphere-like
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nanocrystallines). While the large sphere-like nanoparticles can be easily washed off by ultrasonication, but the sheet-like nanoparticles showed good adhesion to the substrate as no peel-off was found. The formation mechanism of sheet-like CuS nanoparticles was proposed as below: The –N in the –NH2 groups of APTS–SAMs can form complexes with Cu2+ in the planting solutions, and then, the heterogenous nucleation reaction incessantly takes place on the interface, and as-formed nanocrytallites are in situ assembled to thin film (see Fig. 6c). This reaction is similar to that of ZnS on –COOH-terminated SAMs [34]. The explanation can also be confirmed by the XPS results (Fig. 6b). Though no any CuS nanoparticles were observed in the FESEM image with the 10 min deposition surface (Fig. 1a), the XPS shows the presence of Cu, O, Si, C, N and trace S, which indicated that the copper ion formed complexes with –NH2-terminated SAMs first before the nucleation in the plating solution. The ion-by-ion growth mechanism can here be applied to explain the formation of the sheet-like nanoparticles and the cluster-by-cluster deposition can be used to explain formation of large sphere-like nanoparticles. For the films deposited on –CH3- and –OH-terminated SAMs, only large sphere-like crystalline are observed on both surfaces (Fig. 7a and b). While the large sphere-like crystallites can be easily removed by ultrasonic washing (Fig. 7c). The XPS result confirmed that there is no presence of Cu element and implied no interaction between the Cu2+ and the –OH- and –CH3-terminated groups in deposition process (Fig. 7d). Accordingly, the dominant mechanism was cluster-bycluster growth for the CuS deposition on –OH- and –CH3-terminated SAMs surfaces. It is suggested that –NH2-terminated SAMs is required in order to obtain CuS thin films with good quality and high electrical conductivity. Moreover, based on the proposed the deposition mechanisms of nano-structured CuS thin film on different terminated SAMs surface, the patterned CuS microarrays with different feature size were successfully fabricated by controlling surface chemical properties (Fig. 8). The patterns were regular over a large area and had relatively clear boundaries and show good selectivity between –NH2-terminated regions and –CH3-terminated regions. In this method, the patterned thin films can be produced in a large surface area for application in the photovoltaic devices and be used to tune the physical and chemical properties of thin films [60]. Optical properties of CuS thin film with different features (30 lm, 80 lm, 160 lm) were investigated and shown in Fig. 9. The results indicated that the feature sizes of CuS patterns can effectively affect the UV–vis absorption performance of the patterned CuS thin films. The possible reasons were discussed in our previous paper [61].
4. Conclusions We have successfully prepared transparent p-type conducting CuS thin film on the functionalized self-assembled monolayers using a simple chemical bath deposition. FESEM images show CuS particles are assembled to highly homogenous and adherent films with good transparency and high electric conductivity (12.3 X/h). The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor. I–t characteristics illuminated that the CuS thin film showed a strong PEC response by using intermittent light, which was measured to examine the possible application in solar energy. The growth of CuS on the varying SAMs surface show different deposition mechanisms. On the –NH2-terminated SAMs surfaces, the reaction mechanism of ion-by-ion growth and cluster-by-cluster deposition were proposed for the observed
deposition phenomena. The cluster-by-cluster deposition mechanism is dominated in the nanocrystals growth of CuS on the –OH- and –CH3-terminated SAMs surfaces. Based on the proposed the deposition mechanisms, we have successfully prepared the patterned CuS microarrays with different features and investigated their optical properties. Acknowledgments The authors acknowledge the National Natural Science Foundation of China (Grant Nos. 50705094, 50972148) and ‘‘Hundred Talents Program of Chinese Academy of Sciences’’ (Grant No. KGCX2-YW-804) for providing the financial support. References [1] F.F. Amos, S.A. Morin, J.A. Streifer, R.J. Hamers, S. Jin, J. Am. Chem. Soc. 129 (2007) 14296. [2] A. Bollero, M. Grossberg, B. Asenjo, M.T. Gutierrez, Surf. Coat. Technol. 204 (2009) 593. [3] A.A. Sagade, R. Sharma, J.C. Vyas, Sens. Lett. 7 (2009) 550. [4] A. Antony, K.V. Murali, R. Manoj, M.K. Jayaraj, Mater. Chem. Phys. 90 (2005) 106. [5] Mitzi, D. B.; Copel, M. W. Metal chalcogenide film used in e.g. flat panel display, is prepared by contacting isolated hydrazinium-based precursor of metal chalcogenide and solvent to form solution; applying solution onto substrate; and annealing resultant film. US2007099331-A1; US7618841-B2, 2007. [6] T.A. O’Brien, P.D. Rack, P.H. Holloway, M.C. Zerner, J. Lumin. 78 (1998) 245. [7] S. Erokhina, V. Erokhin, C. Nicolini, Langmuir 19 (2003) 766. [8] F. Zhang, S.S. Wong, Chem. Mater. 21 (2009) 4541. [9] H.M. Pathan, J.D. Desai, C.D. Lokhande, Appl. Surf. Sci. 202 (2002) 47. [10] P.S. Khiew, S. Radiman, N.M. Huang, M.S. Ahamd, J. Cryst. Growth 268 (2004) 227. [11] Rozhin, O.; Ferrari, A.; Milne, W. I. Optical nano-material composition used as index-matching gel for sensor device and optical device such as lens, prism, polarization plate and waveguide facet, contains nano-material(s) and optical coupling gel or optical adhesive. WO2008025966-A1; EP2057211-A1; US2010002324-A1, WO2008025966-A1 C08K-003/00 200852, 2008. [12] A.A. Sagade, R. Sharma, Sens. Actuators B 133 (2008) 135. [13] J.S. Kim, D.Y. Kim, G.B. Cho, T.H. Nam, K.W. Kim, H.S. Ryu, J.H. Ahn, H.J. Ahn, J. Power Sources 189 (2009) 864. [14] G.D. Cody, N.Z. Boctor, J.A. Brandes, T.R. Filley, R.M. Hazen, H.S. Yoder, Geochim. Cosmochim. Acta 68 (2004) 2185. [15] Y.J. Yang, S.S. Hu, J. Solid State Electrochem. 12 (2008) 1405. [16] E. Burstein, Phys. Rev. 93 (1954) 632. [17] Y.X. Zhao, H.C. Pan, Y.B. Lou, X.F. Qiu, J.J. Zhu, C. Burda, J. Am. Chem. Soc. 131 (2009) 4253. [18] K. Das, S.K. Panda, S. Gorai, P. Mishra, S. Chaudhuri, Mater. Res. Bull. 43 (2008) 2742. [19] J.J. Nairn, P.J. Shapiro, B. Twamley, T. Pounds, R. von Wandruszka, T.R. Fletcher, M. Williams, C.M. Wang, M.G. Norton, Nano Lett. 6 (2006) 1218. [20] V. Estrella, M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 18 (2003) 190. [21] F. Mesa, G. Gordillo, T. Dittrich, K. Ellmer, R. Baier, S. Sadewasser, Appl. Phys. Lett. 96 (2010). [22] N.J. Gerein, J.A. Haber, Chem. Mater. 18 (2006) 6297. [23] Y. Rodriguez-Lazcano, M.T.S. Nair, P.K. Nair, J. Cryst. Growth 223 (2001) 399. [24] S.C. Abrahams, J.L. Bernstei, J. Chem. Phys. 59 (1973) 5415. [25] M. Kemmler, M. Lazell, P. O’Brien, D.J. Otway, J.H. Park, J.R. Walsh, J. Mater. Sci.Mater. Electron. 13 (2002) 531. [26] J. Podder, R. Kobayashi, M. Ichimura, Thin Solid Films 472 (2005) 71. [27] R. Cordova, H. Gomez, R. Schrebler, P. Cury, M. Orellana, P. Grez, D. Leinen, J.R. Ramos-Barrado, R. Del Rio, Langmuir 18 (2002) 8647. [28] S.V. Bagul, S.D. Chavhan, R. Sharma, J. Phys. Chem. Solids 68 (2007) 1623. [29] M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 4 (1989) 191. [30] T. Yamamoto, K. Tanaka, E. Kubota, K. Osakada, Chem. Mater. 5 (1993) 1352. [31] A. Goudarzi, G.M. Aval, S.S. Park, M.C. Choi, R. Sahraei, M.H. Ullah, A. Avanes, C.S. Ha, Chem. Mater. 21 (2009) 3738. [32] Y.K. Hwang, S.Y. Woo, J.H. Lee, D.Y. Jung, Y.U. Kwon, Chem. Mater. 12 (2000) 2059. [33] F.C. Meldrum, J. Flath, W. Knoll, Thin Solid Films 348 (1999) 188. [34] P. Lu, A.V. Walker, ACS Nano 3 (2009) 370. [35] E. Cooper, G.J. Leggett, Langmuir 15 (1999) 1024. [36] Y.C. Lin, B.Y. Yu, W.C. Lin, Y.Y. Chen, J.J. Shyue, Chem. Mater. 20 (2008) 6606. [37] Y. Masuda, M. Kondo, K. Koumoto, Cryst. Growth Des. 9 (2009) 555. [38] S.S. Yoon, D.O. Kim, S.C. Park, Y.K. Lee, H.Y. Chae, S.B. Jung, J.D. Nam, Microelectron. Eng. 85 (2008) 136. [39] M.D. Austin, S.Y. Chou, Nano Lett. 3 (2003) 1687. [40] M. Ara, H. Tada, Appl. Phys. Lett. 83 (2003) 578. [41] Y.F. Gao, Y. Masuda, K. Koumoto, Chem. Mater. 16 (2004) 1062. [42] Z.L. He, Z.W. Yu, H.Y. Miao, G.Q. Tan, Y. Liu, Sci. China Ser. E-Technol. Sci. 52 (2009) 137.
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