Slow Electron Cooling Dynamics of Highly Luminescent CdS x Se 1-x Alloy Quantum Dot Partha Maity, Tushar Debnath and Hirendra Nath Ghosh Abstract Ultrafast Electron cooling dynamics of highly luminescent oleic acid caped CdS x Se 1-x alloy quantum dot (QD) is investigated by femtosecond transient absorption studies and found to be much slower as compared to pure CdSe and CdS QDs. 1 Introduction Alloy nanocrystals [1, 2] quantum dots are important class of composite materials due to their enormous applications like photovoltaic performances [3], coherent emitter, biological imaging, plasmon wave guide and magneto optical devices etc. The physical and optical properties of the alloy semiconductor quantum dots depend on both size of the nanocrystals as well as the composition of the constituents. As a result composition of nanocrystals plays an extra degree of freedom towards selecting enviable properties for nanostructure designing purpose. The size dependent band structure of the semiconductor quantum dots appear due to their strong confinement which is governed by the size quantization effect (SQE). Thus, by changing the composition of the constituents, one can achieve next contrivance for altering physical and optical properties of the nanocrystals.To obtain higher ef ficiency in quantum dot based solar cell, it is important to separate the electron and hole pair before their exciton-exciton annihilation which occur in sub picoseconds time scale. Higher emission lifetime and emission quantum yield for alloy QD is reported in literature [4, 5], however no reports are available on charge carrier dynamics in ultrafast time scale. In the present investigation we are reporting optical and photo-physical P. Maity Á T. Debnath Á H.N. Ghosh (&) Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India e-mail: hnghosh@barc.gov.in P. Maity e-mail: pmaity@barc.gov.in © Springer International Publishing Switzerland 2015 K. Yamanouchi et al. (eds.), Ultrafast Phenomena XIX, Springer Proceedings in Physics 162, DOI 10.1007/978-3-319-13242-6_67 275