Temperature plays an important role for semiconductors. Thermal energy can excite electrons from the conduction band into the valence band, dissociate excitons, leads to enhanced nonradiative decay, and promotes the creation of phonons. These lattice vibrations interact with energy and charge carriers and strongly broaden energetic levels. This can make it hard to investigate and understand new materials, especially if there is also a large sample inhomogeneity which leads to a further broadening.
Accordingly, by conducting optical spectroscopy below room temperature, one can gain valuable insight into these novel systems. At low temperatures, PL signals of nanocrystals often strongly increase and narrow spectrally (bottom left). The degree to which this happens is mostly influenced by the degree of carrier-phonon interactions. Thus, by measuring the PL over a large temperature range, many details about these interactions can be determined for materials such as nanocrystals.
To control the temperature of our samples we use a so-called closed-cycle cryostat, which - like a refrigerator - uses a compressing medium (in our case liquid helium) to cool the sample down to only 4K. At these low temperature, PL signals narrow so far that they reach the detection limit of most spectrometers. For example, optical transitions in 2D halide perovskite nanoplatelets are below 500 µeV wide (bottom right). This enable a resolution of the fine structure of single nanocrystals, with which the material can be better understood.
For the temperature-resolved measurements we use a home-built µ-PL setup, with the sample mounted inside a closed-cycle cryostat. Temperatures between 4K and 350K can easily be reached. Excitation occurs via a tunable white-light laser, and optical signals are detected using a spectrometer and a CCD (PL-spectroscopy), an avalanche photodiode (time-resolved PL), or two photodiodes are used to investigate bunching/antibunching.
For more information on low-temperature and temperature-resolved optical spectroscopy, check out some of our publications or contact Moritz Gramlich or Markus Schoger.
Dephasing and quantum beating of excitons in methyl ammonium lead iodide perovskite nanoplatelets
B. J. Bohn, T. Simon, M. Gramlich, A. F. Richter, L. Polavarapu, A. S. Urban, J. Feldmann
ACS Photonics 5 (2017), 648-654