Extended photoresponse of ZnO/CdS core/shell nanorods to solar radiation and related mechanisms

Xu Yang

Supervisor: Jiada Wu

Semiconductor nanomaterials have become a subject of great scientific and technological interests because of the possibility of tailoring properties simply by varying their sizes, shapes and structures as well as by changing the surface atoms. Among various semiconductor nanomaterials, a variety of nanostructured II-VI semiconductors have been synthesized successfully and their properties have been studied extensively. With a wide direct band-gap of 3.37 eV and a large exciton binding energy of 60 meV as well as a high electron mobility of 200 cm²V⁻¹s^−1[1,2], one-dimensional zinc oxide (ZnO) nanostructures including nanowires, nanorods (NRs) and nanoneedles are proposed as photocatalysts for photocatalytic reactions and photoelectrodes for photovoltaic processes due to their large surface areas, short diffusion lengths and high energy conversion efficiencies ^[3,4]. Because of its wide band-gap, however, ZnO itself cannot absorb and utilize visible light, hence has disadvantages when being used for applications in the visible region. An efficient approach to extend the spectral region of photoresponse and enhance the photocatalytic or photovoltaic efficiency of ZnO-based devices is to sensitize the surface of nanostructured ZnO with photosensitizers such as narrower band-gap semiconductors, dye molecules, and metal nanoparticles. Depositing a thin coating of a narrower band-gap semiconductor outside nanosized ZnO provides an approach to permanently sensitize ZnO ^[2,5-8]. Meanwhile, the inorganic semiconductor coatingcan protect nanosized ZnO from surface degradation. Compared with organically sensitized ZnO, therefore, the core/shell structures constructed of ZnO cores and semiconductor shells have stable properties and can tolerate harsh conditions. For an efficient electron transfer from sensitizer to ZnO photocatalyst or photoelectrode and hole transfer from ZnO photocatalyst or photoelectrode to sensitizer and hence a suppressed recombination of photogenerated electrons and holes, in addition, both the conduction and valence bands of the sensitizer must be higher than the corresponding bands of ZnO, constructing a so-called type-II core/shell heterostructure ^[6,8]. Not only an extended photoresponse but also an improved charge separation, and hence an enhanced photocatalytic or photovoltaic efficiency can be expected in such heterostructures. Cadmium sulphide (CdS), another II-VI semiconductor with a direct band-gap of 2.4 eV and high optical absorbance in the region from violet to cyan ^[9-11], is considered to be one of the most suitable visible sensitizers for ZnO. Moreover, CdS has the same structure as ZnO and hence a good compatibility with ZnO. Therefore, CdS is one of the most excellent candidate shell materials to construct core/shell heterostructures with ZnO, including one-dimensional core/shell heterostructures ^[2,12,13] which are most promising for photocatalytic and photovoltaic applications.

We report the study on the influences of shell thickness and core-shell structure on the optical properties of ZnO/CdS core/shell nanorods (NRs) for an elucidation of the mechanisms responsible for extended photo-response. Well aligned ZnO/CdS core/shell NRs were fabricated on indium tin oxide substrates using hydrothermally grown ZnO NRs as the cores and pulsed laser deposited CdS coatings as the shells. The sample structure was characterized by X-ray diffraction and Raman backscattering spectroscopy, revealing the wurtzite structure of both the ZnO cores and CdS shells, and the improvement in the structure after annealing. The optical properties were studied through optical transmittance, absorbance and photoluminescence measurements, showing the optical properties featured with type-II heterogeneous nanostructures constructed from ZnO and CdS. The results provide a support that the optical properties of the CdS covered ZnO NRs are attributed to the suppressed radiative recombination of photogenerated carriers due to the efficient spatial separation of electrons and holes in the nanosized ZnO-CdS heterostructures.

References

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[2] S. Khanchandani, S. Kundu, A. Patra, A. K. Ganguli, J. Phys. Chem. C 116 (2012) 23653-23662.

[3] Y.H. Sung,W.P. Liao, D.W. Chen, C.T. Wu, G.J. Chang, J.J. Wu, Adv. Funct. Mater. 22 (2012)3808-3814. 

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[5] K. Wang, J.J. Chen, W.L. Zhou, Y. Zhang, Y.F. Yan, J Pern, A. Mascarenhas, Adv. Mater. 20 (2008) 3248-3253.

[6] P. Reiss, M. Protiere, L. Li, Small 5 (2009) 154-168.

[7] S. Cho, J.W. Jang, S.H. Lim, H.J. Kang, S.W. Rhee, J.S. Lee, K.H. Lee, J. Mater. Chem. 21 (2011) 17816-17822.

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[12] P. Kundu, P.A. Deshpande, G. Madras, N. Ravishankar, J. Mater. Chem. 21 (2011) 4209-4216.

[13] C.M. Li, T. Ahmed, M.G. Ma, T. Edvinsson, J.F Zhu, Appl. Catal. B 138-139 (2013) 175-183.

High-power narrow-bandwidth PPMgLN optical parametric oscillator with a volume Bragg grating

Yuefeng peng

Supervisor: Deyuan Shen

Mid-IR lasers in the 3-5mm wavelength region have many applications, such as atmospheric pollution monitoring, laser ranging, optical spectroscopy, medical diagnosis, military countermeasures, remote monitoring of the special environment and so on[1-3]. A narrow-band, widely tunable mid-IR source is of great appeal because of its potential use in molecular detection systems that are sensitive and specific. To work on these applications, a spectral narrowing mechanism is usually introduced to the oscillation system. Several mechanisms have been studied to achieve that purpose, e.g., the use of wavelength selective elements such as the etalon or grazing-incident grating in the cavity[4-10].

In this work, we have investigated high-power narrow-bandwidth optical parametric oscillator (OPO) with a volume Bragg grating. OPOs can offer a unique combination of compact configuration, high peak power, good beam quality, wide wavelength tunability and power scalability. With the development of advanced pumping source and nonlinear crystal technology, OPO can offer wide tuning ranges from ultraviolet to far infrared and from CW to ultra-fast femtosecond pulse durations. The quasi-phase-matching (QPM) can utilize the largest non-linear coefficient of crystals along with advantages of high gain and high efficiency, avoid the limit of the polarization direction of the laser beam. In theory, QPM is able to achieve phase matching in the entire transmission range of crystals. The most commonly used quasi-phase-matching nonlinear material is periodically poled MgO-doped LiNbO₃ crystal (PPMgLN) , since the doping MgO can significantly enhance PPLN crystal’s photorefractive damage threshold and effectively reduce its coercive field[11]. Regrettably, the bandwidth of the generated waves in the conventional PPMgLN OPO is rather broad. The most common efforts to achieve narrow-bandwidth utilized additional wavelength selective elements. With the development of the high-quality large-size periodically poled nonlinear crystal, high-power and high pulsed energy tunable narrow-bandwidth mid-IR lasers based on quasi-phase-matching can be achieved.

Theoretical analysis of PPMgLN OPO is presented including wavelength tuning, threshold, efficiency, acceptable input parameters, acceptable output parameters and so on. A high-power and narrow-bandwidth optical parametric oscillator based on PPMgLN with a volume Bragg grating (VBG) is also presented. The pump source was a Q-switched polarized 1064.4nm Nd:YAG laser with a bandwidth of 0.06nm and the maximum output power of 184W at 10kHz. The beam polarization matched the e→e+e interaction in PPMgLN, thus the maximal nonlinear coefficient d₃₃ (27.4pm/V) was available and walk-off could be avoided. By reducing the bandwidth of the signal laser with a VBG and utilizing a narrow-bandwidth pump laser, a narrow-bandwidth idle laser was obtained. When the pump power was 184W and the temperature of the PPMgLN crystal was 92℃, the average output power of 72.4W was obtained. The 1.679μm signal and 2.907μm idler laser output powers were 39.2W and 33.2W, respectively. The 1.679μm signal bandwidth (~1nm) of the free-running OPO was suppressed to less than 0.02nm by using a VBG as the spectral filter, and the theoretical analyzed bandwidth of 2.907μm idler laser was less than 0.5nm. The measured bandwidth of the idle laser was 0.86nm. The idler laser bandwidth cannot be accurately measured because of the resolution limits of the optical spectrum analyzer. By adjusting the temperatures of the VBG and PPMgLN, the continuously tunable ranges of 3nm of 1.68μm and 9nm of 2.91μm were achieved.

References

[1] M. van Herpen, S.L. Hekkert, S.E. Bisson, and F.J.M. Harren, Proc. SPIE 2002, 4762, 16.

[2] G.G. Ren and Y.N. Huang, Laser & Infrared 2006, 36, 1.

[3] R. Tuttle, Aerospace Daily & Defense Report 2004, 210, 6.

[4] M. Henriksson, L. Sjöqvist, V. Pasiskevicius, and F. Laurell, Conference on Lasers & Electro-Optics (CLEO) 2008, CTuY4.

[5] P. Koch, F. Ruebel, M. Nittmann, T. Bauer, J. Bartschke, and J.A. L’huillier, Conference on Lasers & Electro-Optics (CLEO) 2010, CThY6.

[6] Y.H. Chen, J.W. Chang, C.H. Lin, W.K. Chang, N. Hsu, Y.Y. Lai, Q.H. Tseng, R. Geiss, T. Pertsch, and S.S. Yang, Optics Letters 2011, 36, 2345.

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[11] O. Gayer, Z. Sacks, E. Galun, and A. Arie, Appl. Phys. 2008, 91, 343.

Influences of doping Ti element on the properties of Ge₂Sb₂Te₅ by Ab initio calculation and experiments

Shuai Cheng

Supervisor: Jing Li

Chalcogenide alloys are widely used for data storage applications such as rewritable optical disk and phase change random access memory (PRAM). Among these alloys, Ge₂Sb₂Te₅(GST)^[1], which has a stable hexagonal structure and a meta-stable face centered cubic (fcc) structure, exhibits the best performance in fast phase-change speed, stability of the amorphous phase and good reversibility between amorphous and crystalline phases. In the actual application, the Ti adhesion layer^[2] or TiNelectrode^[3] are commonly adopted in a PRAM cell and TiO₂ can be used as dielectric layer in an optical disk. Ti element might diffuse into the GST core cell during cycling and affect the storage performance directly.

In this work, ab initio calculation and experiments are both adopted to research the effects with the introduction of Ti element in GST. The starting GST structure is a supercell containing 35 atoms plus 4 vacant positions^[4]and when Ti is doped, the positions of Ge, Sb, v can be substitution. In the aspect of experiment, GST films were deposited on Si (100) substrates with deposition power at 100W and that of Ti target was 25W.

In calculation, the 2.9 at.% dopant of Ti will not influence the crystal symmetry of GST. However, the volume of supercell and lattice parameters become smaller after doping, which agrees with the results of XRD. From the density of states, nature at the Fermi level (E_F) has changed, which suggests that the Ti-doped GST might present some metallic properties.The experimental optical properties are determined by spectroscopic ellipsometry, and no matter in crystalline state or amorphous state, after doping the reflectivity has decreased which is corresponded with the calculated oneand previous report^[5], especially when the substitution is the position of Ge element it has reduced nearly 10%.The reflectivity contrast (C_R)curves of both calculated and experimental pure GST are higher that that of Ti-doped GST, which means that introducing Ti element has brought down the signal-to-noise ratio (SNR) and generated negative effects on optical storage.

Reference:

[1] H.-K. Lyeo, D. G. Cahill, B.-S. Lee, J. R. Abelson, M.-H. Kwon, K.-B. Kim, S. G. Bishop and B.-k. Cheong, “Thermal conductivity of phase-change material Ge₂Sb₂Te₅”, Appl. Phys. Lett.89 , 151904 (2006).

[2] C. Cabral, K. N. Chen, L. Krusin-Elbaum and V. Deline,“Irreversible modification of Ge₂Sb₂Te₅ phase change material by nanometer-thin Ti adhesion layers in a device-compatible stack”,Appl. Phys. Lett.90 , 051908 (2007).

[3] G. W. Burr, M. J. Breitwisch, M. Franceschini, D. Garetto, K. Gopalakrishnan, B. Jackson, B. Kurdi, C. Lam, L. A. Lastras, A. Padilla, B. Rajendran, S. Raoux and R. S. Shenoy, “Phase change memory technology”,J. Vac. Sci. Technol. B28, 223-262 (2010).

[4]Z. Sun, J. Zhou and R. Ahuja, “Structure of Phase Change Materials for Data Storage”, Phys. Rev. Lett.96, 055507 (2006).

[5] S. J. Wei, H. F. Zhu, K. Chen, D. Xu, J. Li, F. X. Gan, X. Zhang, Y. J. Xia and G. H. Li, “Phase change behavior in titanium-doped Ge₂Sb₂Te₅ films”, Appl. Phys. Lett.98, 231910 (2011).

Time: 6:30 pm, Thursday, 2014.11.06

Location: Optical Building. Room 525