Ultrafast Electron Transfer in Low-Band Gap Polymer/PbS NCs Blend Films

Wenping Guo

Supervisor: Haibin Zhao

Compared to all-organic heterojunction solar cells, bulk heterojunction (BHJ) hybrid solar cells comprised of low band gap polymer and QDs has attracted significant attention recently, because of the spectral tunability, high dielectric constants and high electron mobility all of which are offered by semiconductor nanocrystals. During the last 15years, the efficiency of hybrid solar cells has been enhanced dramatically, as the design rationale of hybrid systems are being developed  But the physical mechanism of charge generation and charge transfer are still poorly understood, as well as the formation and evolution of charge transfer stats . The electronic coupling between the nanocrystals, their ligands and the polymer chain in the hybrid blends is considered to be responsible for the lower efficiencies of hybrid devices. Some studies have been carried out to investigate the charge transfer in the blends of QDs with low-gap copolymer. For different polymer or QDs, the speed of charge transfer is different, the time range of which is from the order of femtoseconds to the order of nanoseconds  Ultrafast charge transfer dynamics in hybrid blend of the low band gap polymer PDBT and PbS quantum dots are studied by using ultrafast time-resolved transient absorption spectroscopy. For the hybrid blend films pumped at lower excitation density (<10mW), the transient PB signal probed at 950nm is dominated by the polymer exciton recombination. While, for higher excitation density (>10mW), the transient PB signal dynamics dominated by the QD state relaxation effects. Compared with the transient PB signal of the e pristine PDBT film excited at high density, ultrafast electron transfer from PDBT to QDs is observed in the blend film, the time range of which is in the 1ps-10ps. And the electrons generated in the excitation states of PDBT transfer to QDs becomes faster with increasing of number of QDs around PDBT molecule. The PIA signal at the initial times gives the evidence of generation of charge transfer states (CTs) in the hybrid blend films.

References:

[1] R. Zhou, R. Stalder et al., ACS Nano 2013 ,7 , 4846-4854.

[2] M. J. Greaney, J. Araujo et al., Chem. Commun. 2013, 49, 8602-8604.

[3] P. Reiss, E. Couderc et al., Nanoscale 2011, 3, 446-489.

[4] M. J. Greaney, S. Das et al., ACS Nano 2012, 6, 4222-4230.

[5] D. Tsokkou, G. Itskos et al., Nanotechnology 2013, 24, 235707.

Molecular dynamics simulation studies of structural and dynamical properties of rapidly quenched Al

Bo Shen

Supervisor: Songyou Wang

Since the success in synthesizing Au–Si metallic glass (MG) in 1960 [1], there have been a lot of studies devoted to the understanding of the atomic structures and the formation mechanism of MGs [2–4]. Of particular interests are Al-based MGs [5], which are intriguing light-weight alloys with superior mechanical and corrosion properties, and are expected to be good candidates for various engineering applications. However, it is very difficult to archive Al-rich bulk MGs despite continuous and relentless pursuits by many scientists [6,7]. Until now, our knowledge about the structure and formation mechanism of Al-based MGs is still very limited. In particular, the dependence of structural order development on the cooling rate is less understood.

In this paper, the structural and dynamical properties of rapidly quenched Al are studied by molecular dynamics simulations. The pair-correlation function of high temperature liquid Al agrees well with the experimental results. Different cooling rates are applied with high cooling rates leading to glass formation, while low cooling rates leading to crystallization. The local structures are characterized by Honeycutt–Andersen indices and Voronoi tessellation analysis. The results show that for high cooling rates, the local structures of the liquid and glassy Al are predominated by icosahedral clusters, together with considerable amount of face-centered cubic (FCC) and hexagonal close packed (HCP) short-range orders. These short-range order results are further confirmed using the recently developed atomic cluster alignment method. The development of FCC and HCP short-range orders in this system would be the structural origin of the poor glass formation ability of Al and Al-rich alloys. We also studied the structural evolution of a nucleation process in supercooled liquid of Al, and estimated the corresponding critical nucleus size. Finally, the mean square displacement and diffusion constant in the liquid is also computed.

Reference

[1] W. Klement, R.H. Willens, P.O.L. Duwez, Nature 187 (1960) 869–870.

[2] M.H. Cohen, D. Turnbull, Nature 203 (1964) 964.

[3] D.B. Miracle, Nat. Mater. 3 (2004) 697–702.

[4] H.W. Sheng, W.K. Luo, F.M. Alamgir, J.M. Bai, E. Ma, Nature 439 (2006) 419–425.

[5] A. Inoue, Prog. Mater. Sci. 43 (1998) 365–520.

[6] Y. He, S.J. Poon, G.J. Shiflet, Science 241 (1988) 1640–1642.

[7] B.J. Yang, J.H. Yao, J. Zhang, H.W. Yang, J.Q. Wang, E. Ma, Scr. Mater. 61 (2009) 423–426.

Effect of Fabrication Errors on Beam

Homogenizing Property on Microlens Array

Luolan Lv

Supervisor: Xiangchao Zhang

In micro-optics, fabrication errors with dimensions ranging from the macroscale to the nanoscale, including roughness, waviness, forming, roundness, have distinct influence on their properties. Furthermore, it is common that the designer raises an over rigorous request on fabrication, which may be unnecessary. Therefore, it is necessary to study the functional relations between fabrication errors and properties, in order to come up with some reference to standards for manufacturing.

Microlens array used to map of a Gaussian input beam into flat-top homogeneous beam is a typical type of micro-optical component. It can be applied to illumination, optical coupling, sensors, welding dissimilar materials, etc.

In this work, we have investigated three kinds of fabrication errors on microlens array, analyzed their influence on the output beam in flat-top shape, and evaluated their correlation. The fabrication errors include roughness, waviness, and the radius of rounding off between adjacent lens units due to the certain size of diamond cutter. The properties of flat-top light spot include uniformity error, and efficiency.

Through analysis on the results, we have concluded that the radius of rounding off has little effect on uniformity error and window efficiency. It is because that the region in a lens unit where the lens is imperfectly fabricated occupies only 1% of the whole unit at most. What significantly affect the quality of beam homogenizing are roughness and waviness.

By least squares fitting, we have obtained the functions of roughness and waviness on uniformity error and window efficiency, respectively. The window efficiency expression indicates that roughness and waviness act in exponential function, and window efficiency is much more sensitive to roughness than waviness. The uniformity error expression is more complicated. When roughness and waviness are small, the value of exponential function term is approximately equal to 1, so the parabola term is dominant, which is consistent with the modulate results. As the increasing of roughness and waviness, the attenuation of exp term restrains the parabola term. Consequently, the term including k₂ plays a leading hole.

Using Spearman’s correlation study, we have shown the correlations between different parameters. Radius of rounding off presents very weak correlation to efficiency and uniformity error. Roughness is very strongly correlate to window efficiency. Compared with roughness, waviness is much more sensitive to uniformity error.

Our results can empower designer and fabricator in the set of appropriate tolerance while manufacturing a microlens lens used to homogenize beams.

References:

[1] Bhushan, Bharat. "Biomimetics: lessons from nature–an overview." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 367.1893 (2009): 1445-1486.

[2] 杨国光.微光学与系统[M].浙江：浙江大学出版社，2007：1-14.

[3] Zappe, Hans P. Fundamentals of Micro-optics. Vol. 90. Cambridge: Cambridge University Press, 2010.

[4] Dickey, Fred M., Scott C. Holswade, and David L. Shealy, eds. Laser beam shaping applications. CRC Press, 2005.

[5] Voelkel, Reinhard, and Kenneth J. Weible. "Laser beam homogenizing: limitations and constraints." Optical Systems Design. International Society for Optics and Photonics, 2008.

[6] Pfadler, Sebastian, Frank Beyrau, and Alfred Leipertz. "Application of a beam homogenizer to planar laser diagnostics." Optics express 14.22 (2006): 10171-10180.

[7] ISO 25178-2. Geometrical Product Specifications (GPS) -- Surface texture: Areal - Part 2: Terms, definitions and surface texture parameters: International Organization for Standardization, 2012.

Time:  6:30 pm, Thursday, 2014.11.27

Location:Optical Building. Room 525