Research interests – The homepage of Łukasz Kłopotowski

Colloidal quantum dots produced from non-toxic and earth-abundant elements

The emission wavelength of colloidal nanocrystals, i.e., the color of light they emit, is controlled by the nanocrystal size. (Photo by Antipoff. Published under CC BY-SA 3.0 license.)

Colloidal quantum dots or nanocrystals are tiny chunks of semiconductors which can be synthesised in a liquid. The synthesis offers a relatively facile control over the properties of these dots. For example, the absorption and emission spectra can be easily tuned by changing the nanocrystal size, in turn controlled by the synthesis time. The easy liquid-processability of these nanostructures make them super-interesting for a wide array of applications. They can be used, e.g., as biomarkers — allowing to track the position of particular entities in living organisms, as sensitizers of solar cells — enhancing light harvesting for energy conversion, or as light detectors to work in outer space. Other applications include light emitters in panels and lasers, as elements of flexible/wearable electronics, and as biosensors.

For group I-III-VI compound nanocrystals, the origin of the large Stokes shift and the linewidth of the emission spectrum are currently poorly understood.

I am currently investigating the properties of ternary colloidal quantum dots built from group I-III-V elements. These materials, such as CuInS₂ or AgInS₂ are expected to be less toxic than lead or cadmium compounds and contain elements of high Earth abundance. However, their properties are poorly understood and the synthetic procedures are not well established. Using advanced optical spectroscopies, in particular studies of single nanocrystals, I aim at obtaining a state of the art understanding of light absorption and emission processes.

Spectroscopy at low temperatures and in magnetic fields

My research background is rooted in magneto-spectroscopy of condensed matter. We use tools of spectroscopy at low temperatures and in magnetic fields (sometimes up to 70 T!) to uncover the effects invisible at room temperature and zero field. In particular, these studies allow us to access the fine structure of excited states of the nanostructures we study.

Single particle spectroscopy

The majority of research on colloidal nanostructures is done at the ensemble level, most often in solution. This method hides effects particular to single particles, such as blinking, spectral diffusion, ability to emit photons on demand. In my group, we develop techniques to study these hidden effects. Our goal is to develop high throughput, multiplexed approaches which would allow to draw robust conclusions on the entire ensemble, but providing insights at the single particle level.

Time-resolved X-ray absorption spectroscopy

A schematic sketch of an optical pump/X-ray probe experiment on a molecular solution (Copyright European XFEL / Tobias Wüstefeld)

The recent developments of free electron lasers offer an unprecedented possibility of probing the material properties with elemental specificity. We employ the techniques of optical pump — X-ray probe absorption spectroscopy to gain an in-depth knowledge of novel materials.