Peter Bjerre Jensen

Highly efficient luminescence from a hybrid state found in strongly quantum confined PbS nanocrystals.

We report that high quality PbS nanocrystals, synthesized in the strong quantum confinement regime, have quantum yields as high as 70% at room temperature. We use a combination of modelling and photoluminescence up-conversion to show that we obtain a nearly monodisperse size distribution. Nevertheless, the emission displays a large nonresonant Stokes shift. The magnitude of the Stokes shift is found to be directly proportional to the degree of quantum confinement, from which we establish that the emission results from the recombination of one quantum confined charge carrier with one localized or surface-trapped charge carrier. Furthermore, the surface state energy is found to lie outside the bulk bandgap so that surface-related emission only commences for strongly quantum confined nanocrystals, thus highlighting a regime where improved surface passivation becomes necessary.

Unconventional photoluminescence upconversion from PbS quantum dots

The authors report a type of photoluminescence upconversion that is directly attributable to the strong quantum confinement of charge carriers in PbS nanocrystals and does not involve the usual mechanism of thermally populated intermediate states. Absorption, emission, and excitation spectroscopy, combined with a simple spectral model, reveals that the upconversion process is consistent with single-photon absorption by an extremely broad single nanocrystal absorption line. This type of upconversion is a result of the time-energy uncertainty principle, indicating unusually rapid dephasing in this system, and stands in direct contrast to the mechanisms proposed for upconversion in other types of nanocrystal.

Bistable switching between low and high absorbance states in oleate-capped PbS quantum dots.

We report hysteretic photophysical behavior over a 110 K temperature range in freshly prepared colloidal suspensions of PbS/oleic acid nanocrystals. A bistable regime consisting of both low and high absorbance states of the PbS nanocrystals is observed between 215 and 240 K. The change in absorbance is significant (increasing by at least a factor of 5) and is shown to be entirely nonexcitonic in origin. The absorbance change is correlated with a thermal hysteresis in the photoluminescence quenching rate associated with a sudden switching of the quenching behavior upon cooling below 220 K. Most surprisingly, these effects are temporary, resulting in either the complete destruction of the smallest nanocrystals or stabilization of the larger nanocrystals after many weeks. We attribute this anomalous behavior to a structural phase change in the oleate surface ligands. This phenomenon thus illustrates the potentially large effect that surfaces can have on photophysical properties of PbS nanocrystals. Furthermore it opens up the possibility of surface engineering the photophysical properties of these materials for different applications such as photovoltaics.

ReaxFF modelling of enzyme catalysis

Modelling of chemical systems has many advantages compared to what people normally think chemistry is (that is synthesis). Computers can model complicated chemical reactions much faster, than the time an ordinary experiment would take. In this project the so-called reactive force field, ReaxFF, which is capable describing chemical reactivity orders of magnitude faster than the current state-of-the-art methods, is developed. The method could be used to study chemical reactions and improve them thereby lowering the amount of experiments needed, which is time-saving, reduces the costs and is environmentally friendly due to lower use of chemicals and reduced waste treatment.