Enhanced photoelectrochemical activity of vertically aligned ZnO-coated TiO₂ nanotubes

HuaCai

Supervisor：Jiada Wu

The vertically aligned ZnOcoated TiO₂ (ZnO/TiO₂) NTs were fabricated by electrochemical anodization (EA) of Ti foils followed by atomic layer deposition(ALD) of ZnO. After morphology characterization, photoelectrochemical and electrochemical properties were studied in detail. Photocurrent (PC) density, electrochemicalimpedance spectroscopy (EIS), and flat-band potential weremeasured to understand the suppressed electron-holerecombination and the enhanced photoelectrochemical activity of the hetero-nanostructures composed of TiO₂nanotubes and ZnO coatings and to obtain the improved photoelectrochemical performance.

We also report on a study on the correlation between the photoluminescence and the photoelectrochemical properties of ZnO–TiO₂heterogeneous nanostructure composed of anatase TiO₂nanotubes and wurtziteZnO coatings. The obtained ZnO/TiO₂ NT arrays show higher photoresponse to shorter wavelength light than to longer wavelength light. A reduction in photoluminescence and an enhancement in photoelectrochemical activity are observed for annealed ZnO/TiO₂NT arrays. ZnO/TiO₂NTs with thinner ZnO coatings rather than thicker ZnO coatings have better photoelectrochemical properties. Compared with bare TiO₂NT arrays, an increase in photocurrent of about 50 percent is obtained for the arrays of 450℃ annealed of ZnO/TiO₂NTs with 10-cycle deposition of ZnO coatings under visible illumination with cutoff of 420 nm.Their photoelectrochemical activities are studied through photoelectrochemical and electrochemical characterization. Compared with bareTiO₂NTs, the transient photocurrent enhances to over 1.5-fold for the annealed ZnO-coated TiO₂ NTs under visible illumination. The ZnO-coated TiO₂ NTs also show a longer electron lifetime, a lower charge-transfer resistance and a more negative flat-band potential than the bare TiO₂ NTs, confirming the improved photoelectrochemical activity due to the enhanced charge separation.

References

[1] A. Hagfeldt, G. Boschloo, L.C. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010)6595–6663.

[2] W. Krengvirat, S Sreekantan, A.F.M. Noor, N.Negishi, G. Kawamura, H. Muto,A. Matsuda, Mater. Chem. Phys. 137 (2013) 991–998.

[3] J.H. Lee, J.H. Shin, J.Y. Song, W.F. Wang, R. Schlaf, K.J. Kim, Y. Yi, J. Phys. Chem.C 116 (2012) 26342–26348.

[4] L.K. Tsui, T. Homma, G. Zangari, J. Phys. Chem. C 117 (2013) 6979–6989.

[5] M. Hattori1, K. Noda, T. Nishi, K. Kobayashi, H. Yamada, K. Matsushige, Appl.Phys. Lett. 102 (2013) (043105-1–043105-3).

[6] H. Li, Ji.W. Cheng, S.W. Shu, J. Zhang, L.X. Zheng, C.K. Tsang, H. Cheng,F.X. Liang, S.T. Lee, Y.Y. Li, Small 9 (2013) 37–44.

[7] X.B. Chen, S.H. Shen, L.J. Guo, S.S. Mao, Chem. Rev. 110 (2010) 6503–6570.

[8] H. Wender, A.F. Feil, L.B. Diaz, C.S. Ribeiro, G.J. Machado, P. Migowski, D.E. Weibel, J. Dupont, S.R. Teixeira, ACS Appl. Mater. Interfaces 3 (2011)1359–1365.

[9] A. Kubacka, M. Fernandez-García, G. Colon, Chem. Rev. 111 (2012) 1555–1614.

[10] J.R Jennings, A. Ghicov, L.M Peter, P. Schmuki, A. B Walker, J. Am. Chem. Soc.130 (2008) 13364–13372.

On the spectral difference between electroluminescence and photoluminescence of Si nanocrystals: a mechanism

study of electroluminescence

Dong-Chen Wang

Supervisor：Ming Lu

       Light emission of Si is of vital importance for integrated optoelectronics and photonics based on modern Si technologies. The sample of Si nanocrystals embedded in SiO2, or Si-nc:SiO2, is now a promising material for Si light emission, due to its stable light emission, robust structure, and the feature of optical gain. Si light emissions mainly refer to photoluminescence and electroluminescence of Si-nc. As compared to the PL of Si-nc, additional processes of carrier injection and transport exist for the EL, which make the EL process more complicated than the PL one. In fact, the origin of the EL of Si-nc has not been fully understood yet.

       Spectral shift, especially blueshift, in peak position of electroluminescence (EL) spectrum of Si nanocrystal (Si-nc) with respect to its photoluminescence (PL) counterpart has been often observed. Explanations for the spectral difference are different for different EL mechanisms adopted. To gain a relevant picture of the EL process, in this work, we analyze three EL mechanisms that are mainly applied nowadays, i.e., the model of defect light emission, that of band-filling, and that of Si-nc size selection by the carrier energy. Different Si-nc samples and working conditions are designed and their EL and PL emissions monitored according to the predictions of the three models. It is concluded that the observed EL is mainly of Si-nc-related origin. The experimental results are more consistent with the model of Si-nc size selection

       Meanwhile，practical Si light-emitting devices are indispensably important in the field of Si photonics. However, how to efficiently increase the brightness of Si LED, still remains a great challenge. The bottleneck problem lies basically in the poor charge transport within the device. A number of approaches have been proposed to improve the situation, such as increasing Si-NC density, preparing surface nanostructures, and designing novel charge transport channels. Since the charge injection region makes up a part of the whole charge transport route, the enhancement of charge transport there should be equally important for achieving high brightness Si-NC LEDs. We propose an approach of field effect on the enhancement of charge transport in the charge injection regions within Si-NC LEDs. The fields are built at the interface between the active layer and the introduced i-type Si layer, and that between the p-type Si substrate and the introduced Al₂O₃ layer, for enhancements of electron and hole transports, respectively. The interfacial fields are so managed that their directions are the same as that of the forwardly biased electric field.

References

[1] G. R. Lin, Y. H. Pai, C. T. Lin, C. C.Chen, Appl. Phys. Lett. 2010, 96, 263514.

[2] K. Y.Cheng, R.Anthony, U. R.Kortshagen, R. J. Holmes, Nano Lett. 2011, 11, 1952.

[3] L. Ding, M. B.Yu, X. G.Tu, G. Q.Lo, S.Tripathy, T. P. Chen, Opt. Express 2011, 19, 2729.

[4] R. J.Anthony, D. J.Rowe, M.Stein, J.Yang, U. Kortshagen, Adv. Func. Mater. 2011, 21, 4042.

[5] B. G. Lee, D. Hiller, J. W. Luo, O. E. Semonin, M. C. Beard, M. Zacharias, P. Stradins, Adv. Func. Mater. 2012, 22, 3223.

[6] D. Li, Y. B. Chen, Y. Ren, J.Zhu, Y. Y.Zhao, M. Lu, Nanoscale Res. Lett. 2012, 7, 200.

[7] D. Li, Y. B. Chen, M. Lu, Mater. Lett. 2012, 89, 11.

[8] D. C.Wand, J. R.Chen, J. Zhu, C. T. Lu, M. Lu, J. Nanopart. Res. (in press) DOI: 10.1007/s11051-013-2063-x.

Localized surface plasmon achieved via ultrasonic irradiation and postannealing of AgNO₃ on SiO₂ and its enhancement of photoluminescence from Si nanocrystals

Zhiquan Zhou

Supervisor: Ming Lu

 Surface plasmon resonance (SPR) has been utilized to enhance Raman scattering in molecular adsorption studies since 1970s.¹ In recent years, SPR has found other applications in the fields such as sub-wavelength light transmission,^2,3 sharp-angle wave guiding,⁴ enhanced light emission,⁵ enhanced photocatalysis⁶ and solar cell efficiency improvement.⁷ So far, various approaches to generating SPs have been proposed,^2-13 and cost-effective and modern technique compatible ones are required.¹³     

In this work, we propose a facile method to produce SPR on the surface of SiO₂ via ultrasonically irradiating quartz or Si nanocrystal-doped SiO₂ (or Si-NC:SiO₂ for short) in pure AgNO₃ solution, followed by postannealing the samples at elevated temperatures. Ultrasonic irradiation provides nucleation sites for the forthcoming clustering of Ag atoms that are thermally decomposed. The Ag clusters are readily controlled in density and size. The intensities of SPR-induced absorption spectra for various postannealing temperatures correlate well with the formation and growth of Ag clusters whose sizes are in the order of 10^1-2 nm. Applying the SPR to the surface of Si-NC:SiO₂ thin film yields an enhancement of photoluminescence (PL) of Si-NCs due to the localized SP field coupling with the exciting dipole moment of Si-NC. A maximal 3.6-fold PL enhancement occurs for the sample with postannealing temperature of 400°C.

   In our experiment, quartz substrates are ultrasonically irradiated within AgNO₃ solution, followed by postannealing in nitrogen atmosphere at elevated temperatures between 200 and 800°C. Absorption spectra induced by localized surface plasmon (SP) of Ag particles with sizes in the order of 10^1-2 nm are observed after postannealing. The most prominent absorption occurs at 400°C. With the increasing postannealing temperature, AgNO₃ on SiO₂ experiences processes of decomposition, Ag cluster formation, Ag cluster ripening, and Ag oxide nano-ring formation. The localized SP field couples to Si nanocrystal-doped SiO₂ thin film, and yields enhancement of photoluminescence of Si nanocrystals, with a maximal 3.6-fold enhancement achieved for T_a = 400°C.

Refrences

¹  A. Campion, and P. Kambhampati, Chem. Soc. Rev. 27, 241(1998).

² H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, Science 297, 820(2002).

³ Y.-T. Chang, Y.-H. Ye, D.-C. Tzuang, Y.-T. Wu, C.-H. Yang, C.-F. Chan, Y.-W. Jiang, and S.-C. Lee, Appl. Phys. Lett. 92, 233109(2008).

⁴ F. J. Rodriguez-Fortuno, G. Marino, P. Ginzburg, D. O’conner, A. Martinez, G. A. Wurtz, and A. V. Zayats, Sceince 340, 328(2013).

⁵ C. Huh, C.-J. Choi, W. Kim, B. K. Kim, B.-J. Park, E.-H. Jang, S. H. Kim, and G. Y. Sung, Appl. Phys. Lett. 100, 181108(2012).

⁶ W. Hou, and S. B. Cronin, Adv. Func. Mater. 23, 1612(2013).

⁷ X. Chen, B. Jia, J. K. Saha, B. Cai, N. Stokes, Nano Lett. 12, 2187(2012).

⁸ J. P. Zhao, M. Lu, Z. Y. Chen, J. W. Rabalais, Appl. Phys. Lett. 81, 3626(2002).

⁹ R. A. Pala, J. White, and M. L. Brongersma, Adv. Mater. 21, 3504(2009).

¹⁰ C. H. Lu, C. C. Lau, Y. L. Lai, Y. L. Li, and C. P. Liu, Adv. Func. Mater. 21, 4719(2011).

¹¹ P. Spinelli, M. Hebbink, F. Lenzmann, and A. Polman, Nano Lett. 11, 1760(2011).

¹² E. Moulin, J. Sukmanowski, M. Schulte, A. Gordijn, F. X. Royer, and H. Stiebig, Thin Solid Films 516, 6813(2008).

¹³ C. He, L. Liu, and J. Wei, Ultrason. Sonochem. 21, 542(2014).

Time:  6:30 pm, Thursday, 2014.9.25

Location:Optical Building. Room 525