@article{gautam_barrette_mai_yan_zhang_danilov_you_ade_gundogdu_2015, title={Direct Optical Observation of Stimulated Emission from Hot Charge Transfer Excitons in Bulk Heterojunction Polymer Solar Cells}, volume={119}, ISSN={["1932-7447"]}, DOI={10.1021/acs.jpcc.5b06557}, abstractNote={Charge transfer excitons (CTEs) play an important role in semiconducting polymer-based optoelectronic applications. In organic photovoltaics, they are an intermediate step between tightly bound excitons and free charges. Although CT state energies at the interface of bulk heterojunction organic solar cells have been reported using quantum chemical calculations and by sensitive external quantum efficiency (EQE) measurements, direct optical observation of CT states was limited to relaxed, low energy, CT levels. Here we used polarization anisotropy transient absorption experiments to measure emission from high-energy CT levels. These experimental methods provide means to study high energy CT state dynamics in BHJs with controlled molecular orientations and complement theoretical calculations of interfacial CT state energies.}, number={34}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Gautam, Bhoj R. and Barrette, Andy and Mai, Cong and Yan, Liang and Zhang, Qianqian and Danilov, Eygeny and You, Wei and Ade, Harald and Gundogdu, Kenan}, year={2015}, month={Aug}, pages={19697–19702} }
 @article{mai_semenov_barrette_yu_jin_cao_kim_gundogdu_2014, title={Exciton valley relaxation in a single layer ofWS2measured by ultrafast spectroscopy}, volume={90}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.90.041414}, DOI={10.1103/physrevb.90.041414}, abstractNote={We measured the lifetime of optically created valley polarization in single layer WS2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS2. One process involves direct scattering of excitons from K to K' valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Gamma valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Gamma and K valleys.}, number={4}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Mai, Cong and Semenov, Yuriy G. and Barrette, Andrew and Yu, Yifei and Jin, Zhenghe and Cao, Linyou and Kim, Ki Wook and Gundogdu, Kenan}, year={2014}, month={Jul} }
 @inproceedings{barrette_mai_yu_semenov_jin_kim_cao_gundogdu_2014, title={Ultrafast valley relaxation dynamics in single layer semiconductors}, volume={9198}, booktitle={Ultrafast nonlinear imaging and spectroscopy ii}, author={Barrette, A. and Mai, C. and Yu, Y. F. and Semenov, Y. and Jin, Z. H. and Kim, K. W. and Cao, L. Y. and Gundogdu, K.}, year={2014} }
 @article{mai_barrette_yu_semenov_kim_cao_gundogdu_2013, title={Many-Body Effects in Valleytronics: Direct Measurement of Valley Lifetimes in Single-Layer MoS2}, volume={14}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl403742j}, DOI={10.1021/nl403742j}, abstractNote={Single layer MoS2 is an ideal material for the emerging field of "valleytronics" in which charge carrier momentum can be finely controlled by optical excitation. This system is also known to exhibit strong many-body interactions as observed by tightly bound excitons and trions. Here we report direct measurements of valley relaxation dynamics in single layer MoS2, by using ultrafast transient absorption spectroscopy. Our results show that strong Coulomb interactions significantly impact valley population dynamics. Initial excitation by circularly polarized light creates electron-hole pairs within the K-valley. These excitons coherently couple to dark intervalley excitonic states, which facilitate fast electron valley depolarization. Hole valley relaxation is delayed up to about 10 ps due to nondegeneracy of the valence band spin states. Intervalley biexciton formation reveals the hole valley relaxation dynamics. We observe that biexcitons form with more than an order of magnitude larger binding energy compared to conventional semiconductors. These measurements provide significant insight into valley specific processes in 2D semiconductors. Hence they could be used to suggest routes to design semiconducting materials that enable control of valley polarization.}, number={1}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Mai, Cong and Barrette, Andrew and Yu, Yifei and Semenov, Yuriy G. and Kim, Ki Wook and Cao, Linyou and Gundogdu, Kenan}, year={2013}, month={Dec}, pages={202–206} }