Research interests – The homepage of Łukasz Kłopotowski

Colloidal quantum dots 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-precessability 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.

The understanding of nanocrystal properties is best developed for cadmium and lead chalcogenides, i.e., CdX and PbX compounds, where X = S, Se, or Te. Applications of these materials in everyday devices is, however, problematic since cadmium and lead ions are toxic. Also, the constituent elements (apart from sulphur) are expensive since their abundance in the Earth crust is low. Therefore, it is necessary to develop alternative, non-toxic materials built from Earth-abundant elements.

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.

Rare-earth-doped upconverting nanoparticles

Rare earth ions have been studied as luminescence activators in different material systems and are now employed in many widely used devices, e.g, Er-doped optical fibers or Nd-doped YAG lasers. Moreover, rare earth dopants can also convert infrared photons to visible photons by a process known as upconversion. This ability is investigated in view of a plethora of applications ranging from optoelectronics (RGB displays, security barcoding, optical storage), via photovoltaics (as solar light converters) to medicine (bio-detection, bio-imaging, photodynamic therapy, nano-thermometry).

I am interested in manipulating upconversion by external stimuli (such as magnetic field) or by fabrication of hybrid nanostructures (e.g., with plasmonic nanoparticles). On the one hand, these studies enrich our understanding of the upconversion process. On the other, they can provide an additional handle to tune the light emission for specific needs.

Transition metal dichalcogenides

Electrons in semiconductors can be used as carriers of information in novel data processing schemes. One of the proposals is to exploit electron spin to encode and read-out information. Spin is an intrinsic momentum carried by the electron, but it is difficult to control it in semiconductors: the interaction with local fields easily randomizes an electron spin polarization and, hence, destroys any information imprinted in it.

Semiconducting transition metal dichalcogenides (or TMDs) are a family of materials with a common formula of MX2, where M = W or Mo and X = S or Se. These semiconductors, as graphite, consist of layers which can be easily peeled off. (This makes them ideal lubricants. If you follow ski-jumping, where lubricating is obviously important, you can see commercials of LiquiMoly. The Moly comes from molybdenum in MoS2.) In the same way you can obtain graphene from graphite, you can obtain a single layer of MX2 from a thicker crystals. Electrons in these monolayers have an additional degree of freedom related to the energy states they occupy. These states, labeled as valleys, are similar to spin and can be used as carriers of information. Valleys are expected to be more robust than spin, as the interaction with the environment should not destroy the valley polarization.

Schematic of a trilayer heterostructure comprised of a MoSe₂ monolayer sandwiched between two monolayers of MoS₂.

Currently, I am interested in the properties of TMD heterostructures. The monolayers act like Lego blocks and can be stacked one on top of the other. This property provides a handle to tailor their properties to specific needs. Since monolayers of different TMDs posses slightly different lattice constants, stacking them results in a moire pattern, which spatially modulates the energy landscape. This property can be exploited to fabricate arrays of single photon emitters, which is an important goal of quantum communication. Monolayer TMDs can also be stacked with other layered materials, e.g., two-dimensional perovskites. Such an approach is beneficial for increasing the light absorption and could be exploited in wearable, flexible solar cells. Actually, each TMD heterostructure is a totally new material with properties sensitive to the stack architecture providing a rich field for investigations.