Please tell us a little about your educational background and early research.

I studied chemistry at Belarusian State University in my native city of Minsk. The educational standards in this top-tier Belarusian University were very high, and I am proud to meet scientists graduated from this University who are now successfully working all over the world in different fields of the natural sciences. After getting my diploma in Chemistry from this University, I spent four more years there working on my Ph.D., which was on synthesis and optical properties of silver nanoparticles and clusters, and their applications in silver halide photography as catalytic centers.

“We try to get light out of semiconductor nanocrystals, understand the physics behind that, and use it for different applications.”

During my Ph.D. work, I came in contact with nanocrystals in general, getting familiar with publications of Arnim Henglein’s school, among others, and doing syntheses not only of silver nanoparticles, but also of semiconductor chalcogenides. It was just natural for me to look for a postdoc position in Henglein’s group: this worked in a sense that I received an offer from Horst Weller, Henglein’s co-worker, to join his newly established group at the University in Hamburg.

I went there in 1995, and since that time my research has become focused on semiconductor nanocrystals. As these tiny objects possess plenty of properties interesting to photonics and optoelectronics, I collaborated more and more with physicists, and finally joined Jochen Feldmann’s group at the Physics Department of the Ludwig Maximilians University in Munich in 2002.

  What drew you to this particular field of study?

At my institute in Minsk, generations of scientists in the ‘70s and ‘80s were active in the field of so-called "scientific photography," studying the role of silver clusters in silver halide photographic layers. At the time, no one denoted them as "nanoparticles," but rather as clusters, or ultra-fine silver particles. I became involved in these activities during my Ph.D. work and then shifted my attention to semiconductor nanocrystals, mainly because of their fascinating luminescence properties.

  What would you say is the main focus of your research?

We try to get light out of semiconductor nanocrystals, understand the physics behind that, and use it for different applications. This all naturally starts with developing syntheses of brightly emitting nanocrystals, both for the visible and near-infrared spectral regions, and includes their surface functionalization. This is followed by advanced optical studies of nanocrystals, which we are perfectly equipped for in our Munich labs, and the use of them as building blocks for creation of functional hybrid structures (in particular, in combination with polymers).

These are nanocrystals which we synthesize in water using short-chain thiol molecules as capping ligands, and they are highly emissive as synthesized, with quantum yields of up to 60%, compared with the 3-5% we were able to synthesize in 1996, when the first paper on these nanocrystals appeared (Rogach AL, et al., "Synthesis and characterization of thiol-stabilized CdTe nanocrystals," Ber. Bunsen-Ges. Phys. Chem. 100: 1772-8, 1996). The synthesis of CdTe nanocrystals is very straightforward and can be easily scaled up; it has been adapted by plenty of groups worldwide. Complex structures like nanowires and nanosheets have been created out of spherical CdTe nanocrystals.

Another very important property of these nanocrystals is their exceptionally accessible surface chemistry determined by the easy choice of thiol capping ligands. The fact that they are water-soluble as synthesized allows us to use the so-called layer-by-layer assembly method to create functional polymer-CdTe nanocrystal composites, both on planar substrates and on colloidal spheres. Cascaded energy transfer and whispering gallery mode related studies have been done on these superstructures, to name a few. Furthermore, these brightly luminescent, water-soluble nanocrystals with a feasible surface chemistry are perfectly suitable for biological applications, the topic in which we are becoming active at the moment.

Indeed, this is the third-most-cited paper in Nano Letters. It was the first publication resulting from the Ph.D. work of Dmitri Talapin, our exceptionally skilled Ph.D. student in Hamburg. Strong luminescence of nanocrystals is a prerequisite for their use in light-emitting devices and for biological imaging; another important point is that the nanocrystals have to be as monodisperse as possible in order to provide a highly saturated color (narrow emission spectrum).

In this particular publication we showed that, by introducing an additional component (hexadecylamine) into the reaction mixture, it is possible to synthesize very monodisperse CdSe cores and passivate them with a shell of ZnS in the same reaction solution, which leads to core-shell CdSe/ZnS nanocrystals with 50% photoluminescence quantum efficiency and the narrow photoluminescence spectra. The synthetic procedure proposed in this publication is used by a lot of groups—this is one of the reasons for its frequent citation.

  Are there any practical applications that have come about or will arise as a result of your research?

Up to now we were mostly concentrated on fundamental nanocrystal-related research; on the other hand, our results point to the possibility of several practical applications of nanocrystals, in particular for lighting and in photovoltaics. These are the areas in which we are getting really active at the moment, in collaboration with several companies. We also work together with biologists on using semiconductor nanocrystals for advanced cell imaging—I see a lot of potential here.

When I joined the field in the beginning of the ‘90s, synthesizing further and further materials (primary noble metals and II-VI semiconductors) with sizes in the range of 1 to 10 nm and looking at their fascinating size-dependent optical properties was a matter of fun by itself. At present, reliable methods of synthesis of almost any kind of metals and semiconductors provide us with nanocrystals with the desired size, shape, and composition. The issue of surface functionalization of nanocrystals on demand has been advancing very much as well, but still keeps the attention of scientists.

The direction of progress in the nanocrystals field is definitely determined by application-related considerations. In 10 years we might see semiconductor nanocrystals widely used as components of solar cells, and as widely accepted imaging agents in biology. I’m also very confident that the focus of research will shift to the hybrid structures utilizing different functional components, like magnetic and luminescent nanocrystals at the same time.

Dr. Andrey Rogach
Photonics and Optoelectronics Group
Department of Physics and CeNS
Ludwig-Maximilians-Universitaet Muenchen
Munich, Germany