- Email: [email protected]
Photoluminescence from Silicon Nanocrystals in Encapsulating Materials
Available online at SciVerse ScienceDirect
J. Mater. Sci. Technol., 2013, 29(3), 221e224
Photoluminescence from Silicon Nanocrystals in Encapsulating Materials Z. Deng, X.D. Pi*, J.J. Zhao, D. Yang State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China [Manuscript received October 15, 2012, in revised form December 11, 2012, Available online 19 January 2013]
Naturally oxidized freestanding silicon nanocrystals (Si NCs) are incorporated in commonly used encapsulating materials to explore the photoluminescent application of Si NCs in device structures such as solid-state lighting light-emitting diodes (LEDs) and solar cells. The quantum yield of Si NCs before the incorporation has reached about 45% at the excitation wavelength of 370 nm without any special surface modification. It is found that medium loadings, e.g., 5 wt% of Si NCs in encapsulating materials help to obtain high external quantum efficiency (EQE) of the mixtures of Si NCs and encapsulating materials. The curing of encapsulating materials significantly reduces EQE. Among all the encapsulating materials investigated in this work, siliconeOE6551 enables the highest EQE (21% at excitation wavelength lex ¼ 370 nm) after curing. Based on current findings, we have discussed the continuous efforts to advance the photoluminescent application of Si NCs. KEY WORDS: Photoluminescence; Silicon nanocrystals; Epoxy; Silicone; Quantum yield; External quantum efficiency
1. Introduction It is well known that the technological importance of silicon nanocrystals (Si NCs) largely originates from their efficient light emission. Very high light emission efficiency (>60%) has been achieved by exquisitely tuning the size and surface of Si NCs[1]. Compared with conventional Si NCs that are present in porous Si[2e4], amorphous matrices (e.g., SiOx, SiNx and SiCx)[5e10] and silicon pillars[11,12], freestanding Si NCs have increasingly gained popularity due to their easily accessible surface and great freedom in the incorporation into all kinds of device structures[13]. By modifying the surface of freestanding Si NCs with 1-dodecene and then exploiting the surface-modified Si NCs in a hybrid nanocrystal organic light-emitting structure, Cheng et al.[14] have recently demonstrated impressively high external quantum efficiencies (up to 8.6%) for the electroluminescence (EL) from Si NCs. It should be noted that high-efficiency electroluminescence (EL) is usually more challenging than highefficiency photoluminescence (PL) for Si NCs because of the difficulty in the carrier transport related to Si NCs[15]. However, in contrast to the progress of Si-NC-based electroluminescent devices only limited work has been carried out to take advantage of the PL from Si NCs in device structures. * Corresponding author. Prof., Ph.D.; Tel.: þ86 571 87953003; Fax: þ86 571 87952322; E-mail address: [email protected] (X.D. Pi). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.01.006
The PL process of Si NCs often concerns the absorption of short-wavelength (< w450 nm) light and the subsequent emission of long-wavelength (600e900 nm) light[16]. Such a process may be called down-shifting, which is critical to phosphor-based solid-state lighting light-emitting diodes (LEDs)[17]. Downshifting should also enhance the performance of solar cells by improving the spectral response of solar cells in the shortwavelength region of sunlight[18e20]. To utilize the PL from Si NCs for phosphor-based solid-state lighting LEDs and solar cells, freestanding Si NCs usually need to be placed in encapsulating materials. In previous work freestanding Si NCs were incorporated into PMMA (poly methyl methacrylate) and spinon glass[21,22]. On one hand, PMMA and spin-on glass are not standard encapsulating materials. On the other hand, the PL efficiency for Si NCs embedded in encapsulating materials and the effect of critical parameters such as the loading of Si NCs in encapsulating materials on the PL from Si NCs have not been investigated. Therefore, it is necessary to systematically study the PL from Si NCs embedded in commonly used encapsulating materials. In this work, two types of silicones and two types of epoxies are selected as the encapsulating materials. The dependence of PL from Si NCs on the loading of Si NCs, excitation wavelengths and encapsulating materials has been investigated. 2. Experimental At least partially hydrogen-passivated freestanding Si NCs were firstly synthesized by means of plasma[19,23,24]. During the synthesis 0.6 sccm (standard cubic centimeter per minute at 0 C and 1.01 105 Pa) silane and 200 sccm argon were flowed into
222
Z. Deng et al.: J. Mater. Sci. Technol., 2013, 29(3), 221e224
a quartz tube, where a pressure of 2.5 102 Pa (2.5 mbar) was maintained. Plasma was produced in the quartz tube by using a radiofrequency (13.56 MHz) power supply coupled with a matching network. A power of 12 W was supplied to the plasma. Si NCs produced in the plasma were collected on a mesh. They were then dispersed in ethanol immediately after the collection with the assistance of ultrasonication. The resulting mixture was contained in a Petri dish, which was stored in air at room temperature. With the increase of time, ethanol evaporated and Si NCs were oxidized, resulting in dry oxidized Si NCs. After w6 months the peak position and quantum yield (QY) of the PL from dry oxidized Si NCs hardly changed. The w6 months old Si NCs were then mixed with four types of encapsulating materials, which were silicone-OE6551 (Dow Corning), silicone-SCR1012 (Shin-Etsu), epoxy-1035 (Wuxi Huilong Electronic Materials) and epoxy-5010 (Wuxi Huilong Electronic Materials). Each of these encapsulating materials was composed of two parts (A and B) with an appropriate ratio specified by its supplier. The percentage of Si NCs in the mixture of Si NCs and each encapsulating material by weight varied from 2 to 13 wt%. Bubbles in the mixture of Si NCs and each encapsulating material were removed by pumping in a vacuum chamber. The mixture of Si NCs and each encapsulating material was then cured in an oven. 110 C/1 h, 150 C/2 h, 80 C/1 h and 80 C/1 h were adopted for silicone-OE6551, silicone-SCR1012, epoxy-1035 and epoxy-5010, respectively. The QY (the ratio of the number of emitted photons to that of absorbed photons) for Si NCs and the external quantum efficiency (EQE, the ratio of the number of emitted photons to that of incident photons) for the mixture of Si NCs and each encapsulating material were obtained by using an integrating sphere in which an light-emitting diode (LED) excitation source was mounted. A charge-coupled device (CCD) spectrometer (model Maya 2000PRO, Ocean Optics) was used to collect light emission from Si NCs. The absorption of Si NCs was also measured by using this CCD spectrometer with incident light from a deuteriumetungsten halogen light source (DH-2000BAL, Ocean Optics). The spectral response of the spectrometer was calibrated with a NIST traceable calibration lamp (Ocean Optics LS-1-CAL). 3. Results and Discussion After re-dispersing w6 months old, dry and oxidized Si NCs in ethanol (10 mg Si NCs per ml ethanol) by means of ultrasonication, the absorption and emission of these Si NCs in the resulting solution were measured. The absorption spectrum is shown in Fig. 1(a). It is clear that Si NCs absorb shortwavelength light better than long-wavelength light in the ultraviolet and visible spectral regions. This is consistent with the dependence of the absorption cross-section of Si NCs on the wavelength of excitation[16]. It is noticed that the current dispersion of Si NCs in ethanol does not lead to transparent solution, indicating the agglomeration of Si NCs in ethanol. The agglomerates of Si NCs may scatter incident light, contributing to the absorbance of Si NCs[25]. Therefore, the absorbance of Si NCs decreases more slowly here than that of surfacefunctionalized Si NCs in transparent solution[26] when the wavelength of incident light increases. Fig. 1(b) shows the results for the subtraction of the PL spectrum for a reference sample (ethanol only) from those for the solution (10 mg Si NCs per ml ethanol) with the excitation wavelengths (lex) of 370, 405 and 465 nm. The peaks of the PL
Fig. 1 (a) Absorption spectrum for Si NCs naturally oxidized for w6 months (Si NCs are dispersed in ethanol with a concentration of 10 mg/ml); (b) the results for the subtraction of the PL spectrum for a reference sample (ethanol only) from those for the solution (10 mg Si NCs per ml ethanol) with the excitation wavelengths (lex) of 370, 405 and 465 nm (the peaks of the PL are around 680e700 nm, slightly red-shifting with the increase in lex); (c) the photoluminescent quantum yields (QYs) of Si NCs at the excitation wavelengths of 370, 405 and 465 nm.
are around 680e700 nm, slightly red-shifting with the increase in lex. The small PL redshift may result from the fact that the decrease of the excitation energy disables the excitation of certain small Si NCs[27]. The QYs of Si NCs excited at different wavelengths are shown in Fig. 1(c). Please note that our system for the measurement of quantum efficiency (a relative error of 10%) is calibrated by using Rhodamine 6G dispersed in ethanol. When the values of lex are 370, 405 and 465 nm, the QYs of Si NCs are 45%, 14% and 7%, respectively. It is clear that the QY significantly decreases as the excitation wavelength increases. This indicates that the photoluminescent application of Si NCs can be best explored in device structures with ultraviolet excitation. At ultraviolet excitation, the current QY is comparable to the highest QY reported for naturally oxidized Si NCs in literature[28]. But in this work Si NCs have not undergone any special surface modification (e.g., SF6 etching). Further work should be carried out to investigate why the long-time atmospheric
Z. Deng et al.: J. Mater. Sci. Technol., 2013, 29(3), 221e224
oxidation of Si NCs in ethanol leads to very efficient light emission from Si NCs. Such investigation will help further to improve the light emission efficiency of Si NCs and move Si NCs closer to practical use. Fig. 2 shows the dependence of EQE on the loading of Si NCs in the encapsulating materials before and after curing when lex is 370 nm. A similar trend for the change of EQE with the loading of Si NCs has been obtained when lex is 405 or 465 nm. EQE initially increases and then decreases with increasing the loading of Si NCs for all the encapsulating materials both before and after curing. Before curing the maxima of EQE appears when the loadings of Si NCs are 5 wt% for silicone-OE6551, siliconeSCR1012 and epoxy-1035. But, for epoxy-5010 the maximum EQE is obtained at 9 wt% of the loading of Si NCs. After curing, EQE is the highest when the loading of Si NCs is 5 wt% for silicone-OE6551. For silicone-SCR1012, epoxy-1035 and epoxy-5010, the Si NC loadings of both 5 and 9 wt% lead to the highest EQE. When the loading of Si NCs is small (e.g., 2 wt%), the absorbance of the mixture of Si NCs and each encapsulating material is small. Therefore, a small Si NC loading weakens the conversion of incident short-wavelength light to emitted longwavelength length, resulting in small EQE. However, for a high Si NC loading (e.g., 13 wt%), the re-absorption between Si NCs may become serious, also resulting in small EQE[29]. Clearly, medium Si NC loadings are preferred when Si NCs are placed in encapsulating materials. Since the Si NC loading of 5 wt% leads to rather high EQE for all the encapsulating materials, the effect of encapsulating materials on EQE was examined at different excitation wavelengths by fixing the loading of Si NCs at 5 wt%. The results are shown in Fig. 3. It is clear that before curing silicones give rise to
Fig. 2 Dependence of EQE on the loading of Si NCs in encapsulating materials before (a) and after (b) curing when the excitation wavelength (lex) is 370 nm.
223
Fig. 3 Dependence of EQE on excitation wavelength for different encapsulating materials at the Si NC loading of 5 wt%.
higher EQE than epoxies for all the excitation wavelengths. After curing only silicone-OE6551 gives rise to higher EQE than epoxies. Silicone-SCR1012 basically leads to EQE similar to that enabled by epoxies, especially when the excitation wavelength is long (e.g., 465 nm). EQE is significantly reduced after curing. For example, EQE for 5 wt% Si NCs in silicone-OE6551, silicone-SCR1012, epoxy-1035 and epoxy-5010 at the excitation wavelength of 370 nm is reduced by 56%, 103%, 61% and 27%, respectively (Fig. 3). It is well known that surface conditions may seriously impact the optical behavior of Si NCs that are usually < 5 nm together with quantum confinement[10,3033]. In the current work Si NC cores are w3.5 nm[34]. The thickness of native silicon oxide at the NC surface is < 1.5 nm[35]. After the incorporation of these oxidized Si NCs into encapsulating materials, the chemistry of the very thin silicon oxide at the NC surface may be modified. Different strains at the NC surface may also be introduced by the formation of NC/encapsulant interfaces. It is clear that after curing the surface conditions of Si NCs in all the encapsulating materials deteriorate, weakening the light emission from Si NCs. We would like to point out that the surface conditions of Si NCs embedded in these commonly used encapsulating materials are rather complicated. In the future more efforts should be made to address this issue in the exploration of the incorporation of Si NCs in device structures. The curing temperature is the highest and the curing time is the longest for silicone-SCR1012 in this work. This leads to the fact that the worst surface-condition deterioration occurs to Si NCs in silicone-SCR1012. Therefore, the curing-induced reduction of EQE is the largest for Si NCs in siliconeSCR1012 (Fig. 3). It is suggested that high curing temperature and long curing time should be avoided to achieve high EQE for
224
Z. Deng et al.: J. Mater. Sci. Technol., 2013, 29(3), 221e224
Si NCs placed in encapsulating materials. After curing, siliconOE6551 enables the highest EQE at all the excitation wavelengths (21% at lex ¼ 370 nm, 16% at lex ¼ 405 nm and 6% at lex ¼ 465 nm) among all the encapsulating materials. Clearly, these values are not high enough for practical use. On one hand, the QY of freestanding Si NCs needs to be promoted toward 100%. On the other hand, novel means should be taken to control the surface condition of Si NCs embedded in encapsulating materials, disabling the encapsulation-induced deterioration of the radiative recombination of Si NCs. 4. Conclusion In summary, the PL from Si NCs mixed with commonly used encapsulating materials has been investigated. Before the mixing, the QY of Si NCs which have only been naturally oxidized may reach 45% at the excitation wavelength of 370 nm. Medium loadings, (e.g., 5 wt%) of Si NCs in encapsulating materials are preferred for the high EQE of the resulting mixtures of Si NCs[20] and encapsulating materials. The curing of encapsulating materials leads to the significant reduction of EQE. Among all the encapsulating materials investigated in this work, siliconeOE6551 enables the highest EQE (21% at lex ¼ 370 nm) after curing. This work may contribute to the exploration of the photoluminescent application of Si NCs in device structures. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 50902122 and 50832006). Partial support from R&D Program of Ministry of Education of China (No. 62501040202), Innovation Team Project of Zhejiang Province, China (No. 2009R50005), Basic Funding for Research at Zhejiang University, China (No. 2011FZA4005), and Major Scientific program of Zhejiang Province, China (No. 2009C01024-2) is acknowledged. REFERENCES [1] D. Jurbergs, E. Rogojina, L. Mangolini, U. Kortshagen, Appl. Phys. Lett. 88 (2006) 233116. [2] R.C. Fang, Q.S. Li, J.B. Cui, Chin. Phys. Lett. 9 (1992) 438e440. [3] X.L. Wu, S.J. Xiong, D.L. Fan, Y. Gu, X.M. Bao, G.G. Siu, M.J. Stokes, Phys. Rev. B 62 (2000) R7759eR7762. [4] J. Heitmann, F. Müller, M. Zacharias, U. Gösele, Adv. Mater. 17 (2005) 795e803. [5] X.D. Pi, O.H.Y. Zalloum, A.P. Knights, P. Mascher, P.J. Simpson, J. Phys.: Condens. Matter 18 (2006) 9943e9950. [6] K. Chen, X. Huang, J. Xu, D. Feng, Appl. Phys. Lett. 61 (1992) 2069e2071.
[7] M. Wang, D. Li, Z. Yuan, D. Yang, D. Que, Appl. Phys. Lett. 90 (2007) 131903. [8] X.D. Pi, P.G. Coleman, R. Harding, G. Davies, R.M. Gwilliam, J. Appl. Phys. 95 (2004) 8155. [9] A. Rodriguez, J. Arenas, J.C. Alonso, J. Lumin. 132 (2012) 2385e 2389. [10] J. Wang, V. Suendo, A. Abramov, L. Yu, P. Roca i Cabarrocas, Appl. Phys. Lett. 97 (2010) 221113. [11] H.J. Xu, X.J. Li, Opt. Express 16 (2008) 2933e2941. [12] I. Sychugov, R. Juhasz, J. Valenta, J. Linnros, Phys. Rev. Lett. 94 (2005) 087405. [13] J.G.C. Veinot, Chem. Commun. (2006) 4160e4168. [14] K.Y. Cheng, R. Anthony, U.R. Kortshagen, R.J. Holmes, Nano Lett. 11 (2011) 1952e1956. [15] J. Nelles, D. Sendor, F.-M. Petrat, U. Simon, J. Nanopart. Res. 12 (2010) 1367e1375. [16] D. Kovalev, J. Diener, H. Heckler, G. Polisski, N. Künzner, F. Koch, Phys. Rev. B 61 (2000) 4485e4487. [17] J. Lim, S. Jun, E. Jang, H. Baik, H. Kim, J. Cho, Adv. Mater. 19 (2007) 1927e1932. [18] E. Klampaftis, D. Ross, K.R. McIntosh, B.S. Richards, Sol. Energy Mater. Sol. Cells 93 (2009) 1182e1194. [19] X.D. Pi, Q. Li, D.S. Li, D. Yang, Sol. Energy Mater. Sol. Cells 95 (2011) 2941e2945. [20] X.D. Pi, L. Zhang, D. Yang, J. Phys. Chem. C 116 (2012) 21240e 21243. [21] W. Lu, Y. Bian, H. Liu, L. Han, W. Yu, G. Fu, Mater. Lett. 64 (2010) 1073e1076. cek, A. Slaoui, J.C. Muller, Thin Solid Films 451e452 [22] V. Svr (2004) 384e388. [23] U. Kortshagen, J. Phys. D-Appl. Phys. 42 (2009) 113001. [24] L. Mangolini, E. Thimsen, U. Kortshagen, Nano Lett. 5 (2005) 655e659. [25] Z.C. Holman, U.R. Kortshagen, Nano Lett. 11 (2011) 2133e2136. [26] J.A. Kelly, J.G.C. Veinot, ACS Nano 4 (2010) 4645e4656. [27] X.M. Zhang, D. Neiner, S.Z. Wang, A.Y. Louie, S.M. Kauzlarich, Nanotechnology 18 (2007) 095601. [28] R.W. Liptak, B. Devetter, J.H. Thomas III, U. Kortshagen, S.A. Campbell, Nanotechnology 20 (2009) 035603. [29] V. Wood, M.J. Panzer, J. Chen, M.S. Bradley, J.E. Halpert, M.G. Bawendi, V. Bulovic, Adv. Mater. 21 (2009) 2151e2155. [30] E. Ramos, B.M. Monroy, J.C. Alonso, L.E. Sansores, R. Salcedo, A. Martínez, J. Phys. Chem. C 116 (2012) 3988e3994. [31] Y.S. Ma, X.D. Pi, D. Yang, J. Phys. Chem. C 116 (2012) 5401e 5406. [32] K. Seino, F. Bechstedt, P. Kroll, Nanotechnology 20 (2009) 135702. [33] R. Wang, X.D. Pi, D. Yang, J. Phys. Chem. C 116 (2012) 19434e 19443. [34] X.D. Pi, R.W. Liptak, S.A. Campbell, U. Kortshagen, Appl. Phys. Lett. 91 (2007) 083112. [35] A.R. Stegner, R.N. Pereira, R. Lechner, K. Klein, H. Wiggers, M. Stutzmann, M.S. Brandt, Phys. Rev. B 80 (2009) 165326.
J. Mater. Sci. Technol., 2013, 29(3), 221e224
Photoluminescence from Silicon Nanocrystals in Encapsulating Materials Z. Deng, X.D. Pi*, J.J. Zhao, D. Yang State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China [Manuscript received October 15, 2012, in revised form December 11, 2012, Available online 19 January 2013]
Naturally oxidized freestanding silicon nanocrystals (Si NCs) are incorporated in commonly used encapsulating materials to explore the photoluminescent application of Si NCs in device structures such as solid-state lighting light-emitting diodes (LEDs) and solar cells. The quantum yield of Si NCs before the incorporation has reached about 45% at the excitation wavelength of 370 nm without any special surface modification. It is found that medium loadings, e.g., 5 wt% of Si NCs in encapsulating materials help to obtain high external quantum efficiency (EQE) of the mixtures of Si NCs and encapsulating materials. The curing of encapsulating materials significantly reduces EQE. Among all the encapsulating materials investigated in this work, siliconeOE6551 enables the highest EQE (21% at excitation wavelength lex ¼ 370 nm) after curing. Based on current findings, we have discussed the continuous efforts to advance the photoluminescent application of Si NCs. KEY WORDS: Photoluminescence; Silicon nanocrystals; Epoxy; Silicone; Quantum yield; External quantum efficiency
1. Introduction It is well known that the technological importance of silicon nanocrystals (Si NCs) largely originates from their efficient light emission. Very high light emission efficiency (>60%) has been achieved by exquisitely tuning the size and surface of Si NCs[1]. Compared with conventional Si NCs that are present in porous Si[2e4], amorphous matrices (e.g., SiOx, SiNx and SiCx)[5e10] and silicon pillars[11,12], freestanding Si NCs have increasingly gained popularity due to their easily accessible surface and great freedom in the incorporation into all kinds of device structures[13]. By modifying the surface of freestanding Si NCs with 1-dodecene and then exploiting the surface-modified Si NCs in a hybrid nanocrystal organic light-emitting structure, Cheng et al.[14] have recently demonstrated impressively high external quantum efficiencies (up to 8.6%) for the electroluminescence (EL) from Si NCs. It should be noted that high-efficiency electroluminescence (EL) is usually more challenging than highefficiency photoluminescence (PL) for Si NCs because of the difficulty in the carrier transport related to Si NCs[15]. However, in contrast to the progress of Si-NC-based electroluminescent devices only limited work has been carried out to take advantage of the PL from Si NCs in device structures. * Corresponding author. Prof., Ph.D.; Tel.: þ86 571 87953003; Fax: þ86 571 87952322; E-mail address: [email protected] (X.D. Pi). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.01.006
The PL process of Si NCs often concerns the absorption of short-wavelength (< w450 nm) light and the subsequent emission of long-wavelength (600e900 nm) light[16]. Such a process may be called down-shifting, which is critical to phosphor-based solid-state lighting light-emitting diodes (LEDs)[17]. Downshifting should also enhance the performance of solar cells by improving the spectral response of solar cells in the shortwavelength region of sunlight[18e20]. To utilize the PL from Si NCs for phosphor-based solid-state lighting LEDs and solar cells, freestanding Si NCs usually need to be placed in encapsulating materials. In previous work freestanding Si NCs were incorporated into PMMA (poly methyl methacrylate) and spinon glass[21,22]. On one hand, PMMA and spin-on glass are not standard encapsulating materials. On the other hand, the PL efficiency for Si NCs embedded in encapsulating materials and the effect of critical parameters such as the loading of Si NCs in encapsulating materials on the PL from Si NCs have not been investigated. Therefore, it is necessary to systematically study the PL from Si NCs embedded in commonly used encapsulating materials. In this work, two types of silicones and two types of epoxies are selected as the encapsulating materials. The dependence of PL from Si NCs on the loading of Si NCs, excitation wavelengths and encapsulating materials has been investigated. 2. Experimental At least partially hydrogen-passivated freestanding Si NCs were firstly synthesized by means of plasma[19,23,24]. During the synthesis 0.6 sccm (standard cubic centimeter per minute at 0 C and 1.01 105 Pa) silane and 200 sccm argon were flowed into
222
Z. Deng et al.: J. Mater. Sci. Technol., 2013, 29(3), 221e224
a quartz tube, where a pressure of 2.5 102 Pa (2.5 mbar) was maintained. Plasma was produced in the quartz tube by using a radiofrequency (13.56 MHz) power supply coupled with a matching network. A power of 12 W was supplied to the plasma. Si NCs produced in the plasma were collected on a mesh. They were then dispersed in ethanol immediately after the collection with the assistance of ultrasonication. The resulting mixture was contained in a Petri dish, which was stored in air at room temperature. With the increase of time, ethanol evaporated and Si NCs were oxidized, resulting in dry oxidized Si NCs. After w6 months the peak position and quantum yield (QY) of the PL from dry oxidized Si NCs hardly changed. The w6 months old Si NCs were then mixed with four types of encapsulating materials, which were silicone-OE6551 (Dow Corning), silicone-SCR1012 (Shin-Etsu), epoxy-1035 (Wuxi Huilong Electronic Materials) and epoxy-5010 (Wuxi Huilong Electronic Materials). Each of these encapsulating materials was composed of two parts (A and B) with an appropriate ratio specified by its supplier. The percentage of Si NCs in the mixture of Si NCs and each encapsulating material by weight varied from 2 to 13 wt%. Bubbles in the mixture of Si NCs and each encapsulating material were removed by pumping in a vacuum chamber. The mixture of Si NCs and each encapsulating material was then cured in an oven. 110 C/1 h, 150 C/2 h, 80 C/1 h and 80 C/1 h were adopted for silicone-OE6551, silicone-SCR1012, epoxy-1035 and epoxy-5010, respectively. The QY (the ratio of the number of emitted photons to that of absorbed photons) for Si NCs and the external quantum efficiency (EQE, the ratio of the number of emitted photons to that of incident photons) for the mixture of Si NCs and each encapsulating material were obtained by using an integrating sphere in which an light-emitting diode (LED) excitation source was mounted. A charge-coupled device (CCD) spectrometer (model Maya 2000PRO, Ocean Optics) was used to collect light emission from Si NCs. The absorption of Si NCs was also measured by using this CCD spectrometer with incident light from a deuteriumetungsten halogen light source (DH-2000BAL, Ocean Optics). The spectral response of the spectrometer was calibrated with a NIST traceable calibration lamp (Ocean Optics LS-1-CAL). 3. Results and Discussion After re-dispersing w6 months old, dry and oxidized Si NCs in ethanol (10 mg Si NCs per ml ethanol) by means of ultrasonication, the absorption and emission of these Si NCs in the resulting solution were measured. The absorption spectrum is shown in Fig. 1(a). It is clear that Si NCs absorb shortwavelength light better than long-wavelength light in the ultraviolet and visible spectral regions. This is consistent with the dependence of the absorption cross-section of Si NCs on the wavelength of excitation[16]. It is noticed that the current dispersion of Si NCs in ethanol does not lead to transparent solution, indicating the agglomeration of Si NCs in ethanol. The agglomerates of Si NCs may scatter incident light, contributing to the absorbance of Si NCs[25]. Therefore, the absorbance of Si NCs decreases more slowly here than that of surfacefunctionalized Si NCs in transparent solution[26] when the wavelength of incident light increases. Fig. 1(b) shows the results for the subtraction of the PL spectrum for a reference sample (ethanol only) from those for the solution (10 mg Si NCs per ml ethanol) with the excitation wavelengths (lex) of 370, 405 and 465 nm. The peaks of the PL
Fig. 1 (a) Absorption spectrum for Si NCs naturally oxidized for w6 months (Si NCs are dispersed in ethanol with a concentration of 10 mg/ml); (b) the results for the subtraction of the PL spectrum for a reference sample (ethanol only) from those for the solution (10 mg Si NCs per ml ethanol) with the excitation wavelengths (lex) of 370, 405 and 465 nm (the peaks of the PL are around 680e700 nm, slightly red-shifting with the increase in lex); (c) the photoluminescent quantum yields (QYs) of Si NCs at the excitation wavelengths of 370, 405 and 465 nm.
are around 680e700 nm, slightly red-shifting with the increase in lex. The small PL redshift may result from the fact that the decrease of the excitation energy disables the excitation of certain small Si NCs[27]. The QYs of Si NCs excited at different wavelengths are shown in Fig. 1(c). Please note that our system for the measurement of quantum efficiency (a relative error of 10%) is calibrated by using Rhodamine 6G dispersed in ethanol. When the values of lex are 370, 405 and 465 nm, the QYs of Si NCs are 45%, 14% and 7%, respectively. It is clear that the QY significantly decreases as the excitation wavelength increases. This indicates that the photoluminescent application of Si NCs can be best explored in device structures with ultraviolet excitation. At ultraviolet excitation, the current QY is comparable to the highest QY reported for naturally oxidized Si NCs in literature[28]. But in this work Si NCs have not undergone any special surface modification (e.g., SF6 etching). Further work should be carried out to investigate why the long-time atmospheric
Z. Deng et al.: J. Mater. Sci. Technol., 2013, 29(3), 221e224
oxidation of Si NCs in ethanol leads to very efficient light emission from Si NCs. Such investigation will help further to improve the light emission efficiency of Si NCs and move Si NCs closer to practical use. Fig. 2 shows the dependence of EQE on the loading of Si NCs in the encapsulating materials before and after curing when lex is 370 nm. A similar trend for the change of EQE with the loading of Si NCs has been obtained when lex is 405 or 465 nm. EQE initially increases and then decreases with increasing the loading of Si NCs for all the encapsulating materials both before and after curing. Before curing the maxima of EQE appears when the loadings of Si NCs are 5 wt% for silicone-OE6551, siliconeSCR1012 and epoxy-1035. But, for epoxy-5010 the maximum EQE is obtained at 9 wt% of the loading of Si NCs. After curing, EQE is the highest when the loading of Si NCs is 5 wt% for silicone-OE6551. For silicone-SCR1012, epoxy-1035 and epoxy-5010, the Si NC loadings of both 5 and 9 wt% lead to the highest EQE. When the loading of Si NCs is small (e.g., 2 wt%), the absorbance of the mixture of Si NCs and each encapsulating material is small. Therefore, a small Si NC loading weakens the conversion of incident short-wavelength light to emitted longwavelength length, resulting in small EQE. However, for a high Si NC loading (e.g., 13 wt%), the re-absorption between Si NCs may become serious, also resulting in small EQE[29]. Clearly, medium Si NC loadings are preferred when Si NCs are placed in encapsulating materials. Since the Si NC loading of 5 wt% leads to rather high EQE for all the encapsulating materials, the effect of encapsulating materials on EQE was examined at different excitation wavelengths by fixing the loading of Si NCs at 5 wt%. The results are shown in Fig. 3. It is clear that before curing silicones give rise to
Fig. 2 Dependence of EQE on the loading of Si NCs in encapsulating materials before (a) and after (b) curing when the excitation wavelength (lex) is 370 nm.
223
Fig. 3 Dependence of EQE on excitation wavelength for different encapsulating materials at the Si NC loading of 5 wt%.
higher EQE than epoxies for all the excitation wavelengths. After curing only silicone-OE6551 gives rise to higher EQE than epoxies. Silicone-SCR1012 basically leads to EQE similar to that enabled by epoxies, especially when the excitation wavelength is long (e.g., 465 nm). EQE is significantly reduced after curing. For example, EQE for 5 wt% Si NCs in silicone-OE6551, silicone-SCR1012, epoxy-1035 and epoxy-5010 at the excitation wavelength of 370 nm is reduced by 56%, 103%, 61% and 27%, respectively (Fig. 3). It is well known that surface conditions may seriously impact the optical behavior of Si NCs that are usually < 5 nm together with quantum confinement[10,3033]. In the current work Si NC cores are w3.5 nm[34]. The thickness of native silicon oxide at the NC surface is < 1.5 nm[35]. After the incorporation of these oxidized Si NCs into encapsulating materials, the chemistry of the very thin silicon oxide at the NC surface may be modified. Different strains at the NC surface may also be introduced by the formation of NC/encapsulant interfaces. It is clear that after curing the surface conditions of Si NCs in all the encapsulating materials deteriorate, weakening the light emission from Si NCs. We would like to point out that the surface conditions of Si NCs embedded in these commonly used encapsulating materials are rather complicated. In the future more efforts should be made to address this issue in the exploration of the incorporation of Si NCs in device structures. The curing temperature is the highest and the curing time is the longest for silicone-SCR1012 in this work. This leads to the fact that the worst surface-condition deterioration occurs to Si NCs in silicone-SCR1012. Therefore, the curing-induced reduction of EQE is the largest for Si NCs in siliconeSCR1012 (Fig. 3). It is suggested that high curing temperature and long curing time should be avoided to achieve high EQE for
224
Z. Deng et al.: J. Mater. Sci. Technol., 2013, 29(3), 221e224
Si NCs placed in encapsulating materials. After curing, siliconOE6551 enables the highest EQE at all the excitation wavelengths (21% at lex ¼ 370 nm, 16% at lex ¼ 405 nm and 6% at lex ¼ 465 nm) among all the encapsulating materials. Clearly, these values are not high enough for practical use. On one hand, the QY of freestanding Si NCs needs to be promoted toward 100%. On the other hand, novel means should be taken to control the surface condition of Si NCs embedded in encapsulating materials, disabling the encapsulation-induced deterioration of the radiative recombination of Si NCs. 4. Conclusion In summary, the PL from Si NCs mixed with commonly used encapsulating materials has been investigated. Before the mixing, the QY of Si NCs which have only been naturally oxidized may reach 45% at the excitation wavelength of 370 nm. Medium loadings, (e.g., 5 wt%) of Si NCs in encapsulating materials are preferred for the high EQE of the resulting mixtures of Si NCs[20] and encapsulating materials. The curing of encapsulating materials leads to the significant reduction of EQE. Among all the encapsulating materials investigated in this work, siliconeOE6551 enables the highest EQE (21% at lex ¼ 370 nm) after curing. This work may contribute to the exploration of the photoluminescent application of Si NCs in device structures. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 50902122 and 50832006). Partial support from R&D Program of Ministry of Education of China (No. 62501040202), Innovation Team Project of Zhejiang Province, China (No. 2009R50005), Basic Funding for Research at Zhejiang University, China (No. 2011FZA4005), and Major Scientific program of Zhejiang Province, China (No. 2009C01024-2) is acknowledged. REFERENCES [1] D. Jurbergs, E. Rogojina, L. Mangolini, U. Kortshagen, Appl. Phys. Lett. 88 (2006) 233116. [2] R.C. Fang, Q.S. Li, J.B. Cui, Chin. Phys. Lett. 9 (1992) 438e440. [3] X.L. Wu, S.J. Xiong, D.L. Fan, Y. Gu, X.M. Bao, G.G. Siu, M.J. Stokes, Phys. Rev. B 62 (2000) R7759eR7762. [4] J. Heitmann, F. Müller, M. Zacharias, U. Gösele, Adv. Mater. 17 (2005) 795e803. [5] X.D. Pi, O.H.Y. Zalloum, A.P. Knights, P. Mascher, P.J. Simpson, J. Phys.: Condens. Matter 18 (2006) 9943e9950. [6] K. Chen, X. Huang, J. Xu, D. Feng, Appl. Phys. Lett. 61 (1992) 2069e2071.
[7] M. Wang, D. Li, Z. Yuan, D. Yang, D. Que, Appl. Phys. Lett. 90 (2007) 131903. [8] X.D. Pi, P.G. Coleman, R. Harding, G. Davies, R.M. Gwilliam, J. Appl. Phys. 95 (2004) 8155. [9] A. Rodriguez, J. Arenas, J.C. Alonso, J. Lumin. 132 (2012) 2385e 2389. [10] J. Wang, V. Suendo, A. Abramov, L. Yu, P. Roca i Cabarrocas, Appl. Phys. Lett. 97 (2010) 221113. [11] H.J. Xu, X.J. Li, Opt. Express 16 (2008) 2933e2941. [12] I. Sychugov, R. Juhasz, J. Valenta, J. Linnros, Phys. Rev. Lett. 94 (2005) 087405. [13] J.G.C. Veinot, Chem. Commun. (2006) 4160e4168. [14] K.Y. Cheng, R. Anthony, U.R. Kortshagen, R.J. Holmes, Nano Lett. 11 (2011) 1952e1956. [15] J. Nelles, D. Sendor, F.-M. Petrat, U. Simon, J. Nanopart. Res. 12 (2010) 1367e1375. [16] D. Kovalev, J. Diener, H. Heckler, G. Polisski, N. Künzner, F. Koch, Phys. Rev. B 61 (2000) 4485e4487. [17] J. Lim, S. Jun, E. Jang, H. Baik, H. Kim, J. Cho, Adv. Mater. 19 (2007) 1927e1932. [18] E. Klampaftis, D. Ross, K.R. McIntosh, B.S. Richards, Sol. Energy Mater. Sol. Cells 93 (2009) 1182e1194. [19] X.D. Pi, Q. Li, D.S. Li, D. Yang, Sol. Energy Mater. Sol. Cells 95 (2011) 2941e2945. [20] X.D. Pi, L. Zhang, D. Yang, J. Phys. Chem. C 116 (2012) 21240e 21243. [21] W. Lu, Y. Bian, H. Liu, L. Han, W. Yu, G. Fu, Mater. Lett. 64 (2010) 1073e1076. cek, A. Slaoui, J.C. Muller, Thin Solid Films 451e452 [22] V. Svr (2004) 384e388. [23] U. Kortshagen, J. Phys. D-Appl. Phys. 42 (2009) 113001. [24] L. Mangolini, E. Thimsen, U. Kortshagen, Nano Lett. 5 (2005) 655e659. [25] Z.C. Holman, U.R. Kortshagen, Nano Lett. 11 (2011) 2133e2136. [26] J.A. Kelly, J.G.C. Veinot, ACS Nano 4 (2010) 4645e4656. [27] X.M. Zhang, D. Neiner, S.Z. Wang, A.Y. Louie, S.M. Kauzlarich, Nanotechnology 18 (2007) 095601. [28] R.W. Liptak, B. Devetter, J.H. Thomas III, U. Kortshagen, S.A. Campbell, Nanotechnology 20 (2009) 035603. [29] V. Wood, M.J. Panzer, J. Chen, M.S. Bradley, J.E. Halpert, M.G. Bawendi, V. Bulovic, Adv. Mater. 21 (2009) 2151e2155. [30] E. Ramos, B.M. Monroy, J.C. Alonso, L.E. Sansores, R. Salcedo, A. Martínez, J. Phys. Chem. C 116 (2012) 3988e3994. [31] Y.S. Ma, X.D. Pi, D. Yang, J. Phys. Chem. C 116 (2012) 5401e 5406. [32] K. Seino, F. Bechstedt, P. Kroll, Nanotechnology 20 (2009) 135702. [33] R. Wang, X.D. Pi, D. Yang, J. Phys. Chem. C 116 (2012) 19434e 19443. [34] X.D. Pi, R.W. Liptak, S.A. Campbell, U. Kortshagen, Appl. Phys. Lett. 91 (2007) 083112. [35] A.R. Stegner, R.N. Pereira, R. Lechner, K. Klein, H. Wiggers, M. Stutzmann, M.S. Brandt, Phys. Rev. B 80 (2009) 165326.