Magneto - Excitons in New Emerging Materials
Event details
| Date | 06.05.2015 |
| Hour | 10:00 |
| Speaker |
Prof. Paulina Plochocka, Centre National de la Recherche Scientifique Bio: Paulina Plochocka is currently chargé de recherche de 1ère classe in Laboratoire National des Champs Magnétiques Intenses Toulouse, CNRS. She received her MSc degree in Physics from Department of Physics, Warsaw University, Poland, in June 2000, Ph.D. cum laude from Department of Physics, Warsaw University, Poland in November 2004 and Habilitation diriger des recherches from Université Joseph Fourier, Grenoble, France in 2011. From 2004 to 2006 she was holding Postdoctoral fellowship in Weizmann Institute of Science, Israel, in the group headed by Professor Israel Bar Joseph and from 2006 till 2011 she was Postdoctoral fellow in Grenoble High Magnetic Field Laboratory, France holding Marie-Curie fellowship. She joined LNCMI- Toulouse in 2011. Se works on magneto optical properties of two dimensional semiconductors as well as single nano objects. |
| Location | |
| Category | Conferences - Seminars |
The talk will focus on the electronic properties of the excitons in new emerging materials as atomically thin transition metals dichalcogenides and solid-state perovskite investigated by magneto optics.
Reducing dimensionality of the dichalcogens from 3D to 2D leads to new interesting optical properties e.g. opening a direct gap in the visible range. Moreover, the physics of 2D semiconductors is known to be extremely rich once additional carriers are introduced into the system. In consequence, optical spectra show the existence of not only neutral excitons but also of charged ones. Hence, the ability to control the exciton charge state in semiconductor structures, which emit light at room temperature and in the visible range, is expected to open many possibilities for optoelectronics applications. I will discuss the possibilities of controlling the density of 2D carriers in atomically thin layers of tungsten disulfide (WS2) and tungsten diselenide (WSe2) by simple below- or above-band gap. I will demonstrate that the ratio between the concentrations of charged and neutral excitons can be controlled by inter- or intraband illumination. This is direct proof that we can control the density of 2D carriers in a single layer of WS2 using optical methods and opens a new venue for opto-electronic applications of this material. Additionally I will discuss the results of the optical spectroscopy in high magnetic fields B < 65 T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe2, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe2 where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model.
In the end I will discuss the results of the measurement of the Exciton Binding Energy and effective masses for Charge carriers in Organic-Inorganic Tri-halide Perovskites. Solid-state perovskite-based solar cells have made a dramatic impact on emerging PV research with efficiencies of over 17% already achieved but to date the basic electronic properties of the perovskites such as the electron and hole effective masses and the exciton binding energy are not well known. We have measured both for methyl ammonium lead tri-iodide using magneto absorption in very high magnetic fields up to 150T showing that the exciton binding energy at low temperatures is only 16 meV, a value three times smaller than previously thought and sufficiently small to completely transform the way in which the devices must operate. Landau level spectroscopy shows that the reduced effective mass of 0.104 me is also smaller than previously thought. We also observe Landau levels in the high temperature as used for device production, which has a very similar effective mass and the analysis suggests an exciton binding energy which is even smaller than in the low temperature phase.
Reducing dimensionality of the dichalcogens from 3D to 2D leads to new interesting optical properties e.g. opening a direct gap in the visible range. Moreover, the physics of 2D semiconductors is known to be extremely rich once additional carriers are introduced into the system. In consequence, optical spectra show the existence of not only neutral excitons but also of charged ones. Hence, the ability to control the exciton charge state in semiconductor structures, which emit light at room temperature and in the visible range, is expected to open many possibilities for optoelectronics applications. I will discuss the possibilities of controlling the density of 2D carriers in atomically thin layers of tungsten disulfide (WS2) and tungsten diselenide (WSe2) by simple below- or above-band gap. I will demonstrate that the ratio between the concentrations of charged and neutral excitons can be controlled by inter- or intraband illumination. This is direct proof that we can control the density of 2D carriers in a single layer of WS2 using optical methods and opens a new venue for opto-electronic applications of this material. Additionally I will discuss the results of the optical spectroscopy in high magnetic fields B < 65 T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe2, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe2 where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model.
In the end I will discuss the results of the measurement of the Exciton Binding Energy and effective masses for Charge carriers in Organic-Inorganic Tri-halide Perovskites. Solid-state perovskite-based solar cells have made a dramatic impact on emerging PV research with efficiencies of over 17% already achieved but to date the basic electronic properties of the perovskites such as the electron and hole effective masses and the exciton binding energy are not well known. We have measured both for methyl ammonium lead tri-iodide using magneto absorption in very high magnetic fields up to 150T showing that the exciton binding energy at low temperatures is only 16 meV, a value three times smaller than previously thought and sufficiently small to completely transform the way in which the devices must operate. Landau level spectroscopy shows that the reduced effective mass of 0.104 me is also smaller than previously thought. We also observe Landau levels in the high temperature as used for device production, which has a very similar effective mass and the analysis suggests an exciton binding energy which is even smaller than in the low temperature phase.
Practical information
- General public
- Free
Organizer
- Prof Andras Kis
Contact
- Kis Andras <[email protected]>