We study the influence of surface passivating ligands on the optical and structural properties of zinc blende CdSe nanoplatelets. Ligand exchange of native oleic acid with aliphatic thiol or phosphonic acid on the surface of nanoplatelets results in a large shift of exciton transition energy for up to 240 meV. Ligand exchange also leads to structural changes (strain) in the nanoplatelet's core analysed by wide-angle X-ray diffraction. By correlating the experimental data with theoretical calculations we demonstrate that the exciton energy shift is mainly caused by the ligand-induced anisotropic transformation of the crystalline structure altering the well width of the CdSe core. Further the exciton reduced mass in these CdSe quantum wells is determined by a new method and this agrees well with the expected values substantiating that ligand-strain induced changes in the colloidal quantum well thickness are responsible for the observed spectral shifts. Our findings are important for theoretical modeling of other anisotropically strained systems and demonstrate an approach to tune the optical properties of 2D semiconductor nanocrystals over a broad region thus widening the range of possible applications of A^IIB^VI nanoplatelets in optics and optoelectronics.

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This doctoral thesis reports studies on effects of temperature and density on the mobility and recombination dynamics of charge carriers in low-dimensional van der Waals materials.@en

Trigonal selenium is a semiconducting van der Waals solid that consists of helical atomic chains. We studied the mobility and decay dynamics of excess electrons and holes moving along the selenium chains. Excess charge carriers were generated by irradiation of powdered selenium with 3 MeV electron pulses. Their mobility and decay via trapping or recombination was studied by time-resolved microwave conductivity measurements as a function of temperature. The mobility of charge carriers along the Se chains is at least ca. 0.5 cm2·V-1·s-1 at room temperature. Charges decay predominantly by trapping at defects. The appreciable mobility, together with the potential for large-scale production of Se wires by liquid exfoliation, makes this material of great interest for use in nanoelectronics.@en

Tin disulfide (SnS2) is a two-dimensional semiconducting van der Waals material with an indirect band gap. We measured the mobility and recombination dynamics of charge carriers as a function of temperature and charge density. Excess electrons and holes were generated by pulsed irradiation with 3 MeV electrons. The charge carriers were probed by time-resolved microwave conductivity measurements. The mobility and decay pathways of the charge carriers were determined by a global kinetic rate equation model including decay of charges by recombination and trapping. We found high mobilities for electrons and holes near 100 cm2 V−1 s−1. The mobility decreases at higher temperature, which is typical for bandlike transport. The second-order recombination rate constant is found to be thermally activated with an activation energy close to the energy difference of the direct and indirect band gap of SnS2. We demonstrate that the radiative recombination is reaction-limited and takes place via the Γ-point after thermal excitation of electrons from the M-point to the Γ-point, while a phonon emission-related recombination between the indirect band gap (M-point electrons and Γ-point holes) has no relevant contribution to the population decay. The observed effects result in an unusual increase of radiative electron−hole recombination constant with temperature. @en

Trigonal tellurium is a small band gap elemental semiconductor consisting of van der Waals bound one-dimensional helical chains of tellurium atoms. We study the temperature dependence of the charge carrier mobility and recombination pathways in bulk tellurium. Electrons and holes are generated by irradiation of the sample with 3 MeV electrons and detected by time-resolved microwave conductivity measurements. A theoretical model is used to explain the experimental observations for different charge densities and temperatures. Our analysis reveals a high room temperature mobility of 190 ± 20 cm² V^-1 s^-1. The mobility is thermally deactivated, suggesting a band-like transport mechanism. According to our analysis, the charges predominantly recombine via radiative recombination with a radiative yield close to 98%, even at room temperature. The remaining charges recombine by either trap-assisted (Shockley-Read-Hall) recombination or undergo trapping to deep traps. The high mobility, near-unity radiative yield, and possibility of large-scale production of atomic wires by liquid exfoliation make Te of high potential for next-generation nanoelectronic and optoelectronic applications, including far-infrared detectors and lasers.

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We show that black phosphorus is a highly efficient infrared emitter. To study the carrier dynamics, excess electron-hole pairs were generated in bulk black phosphorus by irradiation with 3 MeV electron pulses. The transient microwave conductivity due to excess charges was measured as a function of time for different initial charge densities at temperatures in the range 203-373 K. A new global analysis scheme, including the treatment of intrinsic carriers, is provided, which shows that the recombination dynamics in black phosphorus, a low bandgap semiconductor, is strongly influenced by the presence of intrinsic carriers. The temperature dependence of the charge mobility and charge carrier decay via second-order radiative recombination is obtained from modeling of the experimental data. The combined electron and hole mobility was found to increase with temperature up to 250 K and decrease above that. Auger recombination is negligible for the studied densities of excess electron-hole pairs up to 2.5 × 1017 cm-3. For this density the major fraction of the excess electrons and holes undergoes radiative recombination. It is further inferred that for excess charge densities of the order of 1018 cm-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.@en