Electronic transport in colloidal II-VI nanoplatelets and nanorods
Final Report Abstract
In the frame of the project the nature of the excited state, the carrier dynamics and the mobility in CdSe, CdSe-CdTe, CdTe-CdSe (hetero) nanoplatelets as well as 2D black Phosphorus and 1D trigonal Selenium have been investigated. It is shown that CdSe nanoplatelets have superior absorption cross sections and excited electron-hole pairs in the platelets are present in the form of strongly bound heavy hole excitons with ~ 170 meV binding energy. The exciton polarizability and the transition dipole polarizability were measured. We demonstrated that the recombination dynamics in CdSe nanoplatelets is governed by an LO-phonon bottleneck between excited and ground state excitons. It has been shown that this bottleneck and with it the recombination dynamics in CdSe nanoplatelets can be tuned by the lateral platelet size and temperature independent from the transition energy, which is given by the platelet thickness. This opens up the possibility to tune both independently, in contrast to quantum dots, where transition energy and recombination dynamics are not independent. We further showed that the recombination dynamics can be further altered by the introduction of a type II junction in CdSe-CdTe core-wings platelets, having still good quantum yield. The temperature dependent radiative rates were revealed by a combination of temperature dependent quantum yield and recombination dynamics data. Also we found from our analysis that the excited carriers are still present as excitons at the type II junction. We further measured the related valence and conduction band offsets in good agreement with theory and performed a structural characterization of these junctions. Having started with time resolved microwave conductivity measurements, we measured the temperature dependent carrier dynamics and mobility in 2D black Phosphorous and 1D trigonal Selenium chains. We showed that the recombination dynamics can be radiatively dominated. For excess charge densities of the order of 10^18cm^-3 electrons and holes recombine with near unity radiative yield. The latter offers promising prospects for use of black phosphorus as efficient mid infrared emitter in devices like LEDs or lasers around 4 µm. The mobility and second order radiative recombination rate constant were shown to decrease with increasing temperature, the first due to scattering with phonons, the second as predicted with a T-3/2 dependence. We also studied trigonal Se and showed that in contrast the recombination dynamics is dominated by first order trapping. We showed that the trapping process is diffusion limited and revealed the underlying trap rate distribution. We demonstrate that the carrier mobility in the Se wires is limited by the Se chain length and can reach values of ≈ 650 cm^2 V^−1s^−1 for longer chains of length ∼ 4 µm.
Publications
- Temperature dependent radiative and non-radiative recombination dynamics in CdSe–CdTe and CdTe–CdSe type II hetero nanoplatelets. Phys. Chem. Chem. Phys., 3197 (2015)
R. Scott, A.W. Achtstein et al.
(See online at https://doi.org/10.1039/c5cp06623a) - p-State Luminescence in CdSe Nanoplatelets: The Role of Lateral Confinement and an LO-Phonon Bottleneck. Phys. Rev. Lett. 116, 116802 (2016)
A.W. Achtstein et al.
(See online at https://doi.org/10.1103/PhysRevLett.116.116802) - Radiatively Dominated Charge Carrier Recombination in Black Phosphorus. J. Phys. Chem. C, 13836 (2016)
P. Bhaskar, A.W. Achtstein, L.D.A. Siebbeles et al.
(See online at https://doi.org/10.1021/acs.jpcc.6b04741) - Time-Resolved Stark Spectroscopy in CdSe Nanoplatelets: Exciton Binding Energy, Polarizability, and Field- Dependent Radiative Rates. Nano Lett. 16, 6576 (2016)
R. Scott, A.W. Achtstein, L.D.A. Siebbeles et al.
(See online at https://doi.org/10.1021/acs.nanolett.6b03244)
