| AVS 61st International Symposium & Exhibition | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM-TuP |
| Session: | Electronic Materials and Processing Poster Session |
| Presenter: | Joseph G. Tischler, Naval Research Laboratory |
| Authors: | J.G. Tischler, Naval Research Laboratory E.E. Foos, Naval Research Laboratory D. Placencia, Naval Research Laboratory W. Yoon, Naval Research Laboratory J.E. Boercker, Naval Research Laboratory |
| Correspondent: | Click to Email |
Lead chalcogenides (PbS, PbSe and PbTe) nanocrystals (NC) posses outstanding optical properties such as broad optical absorption from the ultraviolet to the near infrared (NIR), bandgap tunability in the NIR, efficient multiple exciton generation and relatively high quantum yield luminescence. Such properties can be exploited in a variety of optoelectronic applications including biological tags, lasers, photodetectors, LEDs and photovoltaics. Thus, the electron-hole pair (or exciton) ground state electronic structure and dynamics is of particular interest. In particular, it has been reported that the luminescence of PbS nanocrystals is produced by two electronic states. A higher energy state with a lifetime of the order of tens of nanoseconds and a lower energy state with a lifetime of a few microseconds. Competition between these two levels have been reported in dynamics as a function of temperature and nanocrystal size. Furthermore, theses two states have been invoked to explain the seemingly large Stokes shift dependence with nanocrystal size. Although there has been a lot of consensus on these observations, the origin of these states has been highly controversial. Some of the explanations utilized to explain the observed structure and dynamics include: a dark-exciton state, a hybrid state consisting of a trapped electron and hole in the conduction band, a trapped exciton state, an exciton state split-off due to the intervalley interaction, and shallow trap surface states.
In this work, we show that both the Stokes shift and splitting between the two energy states scale as d-3, where d is the diameter of the NC. These findings imply that these two states correspond to the singlet "bright" state and the triplet "dark" state respectively. Furthermore we demonstrate that the Stokes shift is mainly given by the electron-hole exchange energy, and that opposite to what it was previously believed, the exchange energy is determined by the short-range interaction instead of the long-range interaction. Also, from these measurements we determined for the first time the bulk exchange interaction strength constant (J) for both PbSe and PbS.