Effects of Experimental Conditions on the Morphologies, Structures and Growth Modes of Pulsed-Laser-Deposited CdS Nanoneedles

Hui Li

Supervisor: Jiada Wu/Ning Xu

Nowadays semiconductor nanomaterials like nanowires, nanorods and nanotubes, have aroused great interest in material science and applications owing to their unique characteristics different from film or bulk materials.

CdS, as a direct bandgap (2.4 eV) II-VI compound semiconductor, has good optical and electrical properties, which give it potential applications in light-emitting-diodes, light sensors, photocatalysts, windows of solar cells as well as absorbers and electrodes of hybrid solar cells [1-3]. Compared to CdS thin films, the CdS nanostructures such as nano-particles, nanowires and nanoneedles have higher optoelectronic sensitivities and efficiencies for these devices due to their large surface areas and possible quantum confinement effects [1-3]. There have been many methods for preparing CdS nanowires like electrochemical deposition [4], solution process [5], chemical and physical vapor deposition, etc.. Among the methods, pulsed laser deposition (PLD) is a simple and efficient way to synthesize multicomponent compounds such as II-VI semiconductors [6,7]. It’s easy to control the growth rate and avoid materials from pollution as a result of the adjustable frequency of pulsed laser and the good directivity of laser-ablated plasma [6,7].

In our previous work, the CdS nanoneedles have been grown successfully using the PLD method [8] and the growth modes of vapor-liquid-solid (VLS) and vapor-solid (VS) have been suggested [8,9]. In this article, the effects of the substrate temperature and the laser pulse energy on the growth of CdS nanoneedles were studied in detail. Both the VLS and VS growth modes of CdS nanoneedles were further confirmed experimentally. The transformation from VLS to VS growth modes along with the growth of the CdS nanoneedles was discussed.

References:

[1] Jang JS, Li W, Oh SH, Lee JS: Fabrication of CdS/TiO2 nano-bulk composite photocatalysts for hydrogen production from aqueous H2S solution under visible light. Chem Phys Lett 2006, 425(4): 278-282.

[2] Zhu G, Su FF, Lv T, Pan LK, Sun Z: Au nanoparticles as interfacial layer for CdS quantum dot-sensitized solar cells. Nanoscale Res Lett 2010, 5(11): 1749-1754.

[3] Lee JC, Lee W, Han SH, Kim TG, Sung YM: Synthesis of hybrid solar cells using CdS nanowire array grown on conductive glass substrates. Electrochem Commun 2009, 11(1): 231-234.

[4] Suh JS, Lee JS: Surface enhanced Raman scattering for CdS nanowires deposited in anodic aluminum oxide nanotemplate. Chem Phys Lett 1997, 281(4): 384-388.

[5] Yang J, Zeng JH, Yu SH, Yang L, Zhang YH, Qian YT: Pressure-controlled fabrication of stibnite nanorods by the solvothermal decomposition of a simple single-source precursor. Chem Mater 2000, 12(10): 2924-2929.

[6] Ryu YR, Zhu S, Han SW, White HW, Miceli PF, Chandrasekhar HR: ZnSe and ZnO film growth by pulsed-laser deposition. Appl Surf Sci 1998, 127: 496-499.

[7] Park JW, Rouleau CM, Lowndes DH: Heteroepitaxial growth of n-type CdSe on GaAs (001) by pulsed laser deposition: studies of film–substrate interdiffusion and indium diffusion. J Cryst Growth 1998, 193(4): 516-527.

[8] Chen L, Fu XN, Lai JS, Sun J, Ying ZF, Wu JD, Xu N: Growth of CdS Nanoneedles by Pulsed Laser Deposition. J Electron Mater 2012, 41(7): 1941-1947.

[9] Lai JS, Chen L, Fu XN, Sun J, Ying ZF, Wu JD, Xu N: Effects of the experimental conditions on the growth of crystalline ZnSe nano-needles by pulsed laser deposition. Appl Phys A-Mater 2011, 102(2): 477-483.

High-power 2.9µm tunable mid-infrared laser

Yuefeng Peng

Supervisor: Deyuan Shen

Mid-IR lasers in the 3-5mm wavelength region have many applications, such as military countermeasures, remote monitoring of the special environment, and so on[1-2]. Because of their high repetition rate, high stability and compact configuration, the mid-IR solid-state lasers play an important role in the field of mid-IR countermeasures. OPOs can offer a unique combination of high peak power, good beam quality, wide wavelength tunability and power scalability. With the development of good quality and large size periodically poled nonlinear crystal technology, a high average power or high energy tunable laser would be obtained by a quasi-phase-matched (QPM) optical parametric oscillator (OPO) [3-7].

The experimental results of a high-power 2.9mm tunable laser are presented on a quasi-phase-matched single-resonated optical parametric oscillator in PPMgLN pumped by a 1064nm laser. Theoretical analyses of the PPMgLN wavelength tuning are presented. In QPM OPO, the wavelength tuning methods are commonly temperature tuning, period tuning, and angle tuning. The fine precision and continuousness of wavelength tuning can be achieved by temperature tuning. The combination of grating period tuning and temperature tuning can construct widely and continuously wavelength tunable laser sources. The experimental setup is a single-resonated optical parametric oscillator (SROPO) pumped by a 1064nm MOPA laser of an elliptical beam. The 1064 nm laser with high power and good beam quality was obtained by optimizing the laser parameters. The beam polarization matched the e-ee interaction in a 3mm´5mm´50mm PPMgLN with a grating period of 31.2mm@25℃. The PPMgLN was staged in an oven with the temperature range up to 200℃, which is conveniently used to adjust and control the crystal operating temperature.

When the crystal was operated at 125℃ and the pump power was 280W with the repetition rate of 20kHz, average output power of 58.7W at 2.9mm were obtained with a slope efficiency of 23.8%. The variation of the output power was less than 1.2%. The pulse width of 2.9mm idler laser is about 70ns. The beam quality of 2.9mm idler laser was measured by measuring the size of laser spot using the knife-edge method at a different location and hyperbolic fitting the measured data. The beam quality M² factors of 2.9mm idler laser were 2.1 and 5.8 in the parallel and perpendicular directions, respectively. The wavelength tunability of 2.6-3.1mm can be achieved by adjusting the temperature of a 31.2mm period PPMgLN crystal from 200℃ to 30℃, which basically accorded with the theoretic calculation. When the pump power was 280W, the mid-IR laser output power of the OPO varies with the temperature of PPMgLN in the range of 55-63W. We think such a difference may be caused by the difference of the mid-IR laser output wavelength, the error of measurement, the laser loss for the different wavelength output and so on. The comparison experiment using the signal oscillation instead of the idler oscillation was also studied. When the PPMgLN crystal was operated at 125°C, average output power of 71.6W at 2.9mm was achieved for 280W of pump. The output power of 2.9mm laser from the signal resonated OPO was higher than the idler resonated configuration because of its less oscillation loss for the idler laser. However, the beam quality of the 2.9mm laser was not as good as the idler resonated configuration, because the mode size of the idler resonated OPO was larger than the signal resonated OPO.

References

[1] Maarten van Herpen, Sacco te Lintel Hekkert, Scott E.Bisson, Frans J.M. Harren, Proc. SPIE 2002, 16, 4762.

[2] Ren Guoguang and Huang Yunian, Laser & Infrared 2006, 6, 36.

[4] Yuefeng Peng, Weimin Wang, Xingbing Wei, Deming Li, Optics letters 2009, 2897, 34.

[5] Peng Yuefeng, Xie Gang, Chinese Journal of Lasers 2009, 1815, 36.

[6] Da-Wun Chen and Todd S. Rose, Conference on Lasers & Electro-Optics (CLEO) 2005, 1829.

[7] Y.Hirano, S.Yamamoto, H.Taniguchi, Conference on Lasers & Electro-Optics (CLEO) 2001, 579.

Magnetic field-effects in OLED devices

Student: JunqiangShao

Supervisor: GangNi

Organic magnetoresistance (OMAR) is a recently observed[1-3]large, low-field magnetoresistive effect (up to 10% at 10 mT and 300 K) in organic light-emitting diode structures. Similar effects have also been observed in various measurements ranging from electroluminescence to photoconductivity. OMAR poses a significant scientific puzzle since it is the only known example of large room-temperatureMagnetoresistance in nonmagnetic materials with the exception of very-high-mobility materials. The exact mechanism causing OMAR is currently not known with certainty. Three kinds of models based on spin-dynamics induced by hyperfine interaction have recently been suggested as possible explanations of OMAR:

(i) Electron-hole pair (EHP) mechanism models based on concepts borrowed from the so-called magnetic field-effects in radical pairs. In this model the spin-dependent reaction between oppositely charged polarons to form an exciton (“recombination”) is of central importance[4].

(ii) The triplet-exciton polaron quenching (TPQ) model that is based on the spindependent reaction between a triplet exciton and a polaron togive an excited singlet ground state (i.e., the “quenching” of the triplet exciton by the polaron).

(iii) The bipolaron mechanism that treats the spin-dependent formation of doubly occupied sites (bipolarons) during the hopping transport through the organic film[5].     

Whereasmechanisms (i) and (ii) are excitonic in nature, the bipolaron mechanism can exist also in unipolar devices. We anticipate that the quantitative modeling of OMAR will yield sensitive tests of our understanding of organic semiconductor devices. At present, however, any analysis of OMAR experiments is plagued by ambiguity: experiments must be devised that will allow one to distinguish between the three mechanism mentioned above. Specifically, if model (i) is correct, then measurements of OMAR allow determination of the singlet:triplet ratio in OLEDs, whereas if _ii_ is correct it will yield insights into the physics of triplet excitons, and finally if _iii_ is correct OMAR can be used to test our understanding of charge and spin transport as well as bipolaron formation. In the present paper we will study OMAR in tris-(8-hydroxyquinoline-hydroxyquinoline) aluminum (Alq3) devices with different electrode materials to put the three models of OMAR to a test.

We present magnetoconductivity and magnetoluminescence measurements in sandwich devices made from the ᴨ-conjugated molecule tris-8-hydroxyquinoline aluminum (Alq3) and demonstrate effects of more than 10% and 5% magnitude, respectively. These effects are known to be caused by hyperfine coupling in pairs of

paramagnetic species, and it is often assumed that these are electron-hole pairs. However, we show that the very large magnitude of the effect contradicts present knowledge of the electron-hole pair recombination processes in electroluminescent ᴨ -conjugated molecules and that the effect persists even in almost hole-only devices.

References

[1]Y. Iwasaki, T. Osasa, M. Asahi, M. Matsumura, Y. Sakaguchi,and T. Suzuki, Phys. Rev. B 74, 195209(2006).

[2]P. Desai, P. Shakya, T. Kreouzis, W. P. Gillin, N. A. Morley, andM. R. J. Gibbs, Phys. Rev. B 75, 094423 (2007).

[3]Y. Wu, Z. Xu, B. Hu, and J. Howe, Phys. Rev. B 75, 035214(2007).

[4] J. D. Bergeson, V. N. Prigodin, D. M. Lincoln, and A. J. Epstein,Phys. Rev. Lett. 100, 067201 (2008).

[5]P. A. Bobbert, T. D. Nguyen, F. W. A. van Oost, B. Koopmans,and M. Wohlgenannt, Phys. Rev. Lett. 99, 216801 (2007).

Time: 6:30 pm, Thursday, 2014.12.18

Location: Optical Building. Room 525