Johan Hofkens

Photoluminescence Blinking of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods Induced by Surface Traps

Photoluminescence (PL) of organometal halide perovskite materials reflects the charge dynamics inside of the material and thus contains important information for understanding the electro-optical properties of the material. Interpretation of PL blinking of methylammonium lead iodide (MAPbI3) nanostructures observed on polycrystalline samples remains puzzling owing to their intrinsic disordered nature. Here, we report a novel method for the synthesis of high-quality single-crystal MAPbI3 nanorods and demonstrate a single-crystal study on MAPbI3 PL blinking. At low excitation power densities, two-state blinking was found on individual nanorods with dimensions of several hundred nanometers. A super-resolution localization study on the blinking of individual nanorods showed that single crystals of several hundred nanometers emit and blink as a whole, without showing changes in the localization center over the crystal. Moreover, both the blinking ON and OFF times showed power-law distributions, indicating trapping–detrapping processes. This is further supported by the PL decay times of the individual nanorods, which were found to correlate with the ON/OFF states. Furthermore, a strong environmental dependence of the nanorod PL blinking was revealed by comparing the measurements in vacuum, nitrogen, and air, implying that traps locate close to crystal surfaces. We explain our observations by proposing surface charge traps that are likely related to under-coordinated lead ions and methylammonium vacancies to result in the PL blinking observed here.

Excited State Probing of Supramolecular Systems on a Submicron Scale

Imaging techniques, such as Confocal Fluorescence Microscopy (CFM) or Near-field Scanning Optical Microscopy (NSOM) [1, 2] are essential techniques to study complex heterogeneous organic thin films by mapping their optical properties. They are complementary techniques having different advantages and disadvantages [3]. CFM is relatively easy to apply and combines a lateral resolution approaching λ/2 with the possibility of layered imaging in the z-direction. In contrast, NSOM has significantly better spatial resolution and offers simultaneous optical and topographic images. Although confocal microscopy has been mostly used for biological applications, the technique has proven to be useful, for example, in the study of colloids [4,5], polymer blends [6] and liquid crystals [7,8].

The power of single molecule microscopy: from nanoparticle investigations to microbiome analysis

Optical microscopy has been a tool of choice ever since van Leeuwenhoek used Hookes microscope to observe biological specimens. Chief among its advantages is the fact that imaging is noninvasive. In combination with the straightforward sample preparation and general convenience, optical microscopes remain essential to many aspects of modern-day research.

Multiparametric Detection of Fluorescence Emitted from Individual Multichromophoric Systems

While few years ago the main goal in room temperature single molecule fluorescence spectroscopy (SMS) was to visualize individual molecules, nowadays experiments are designed such that multiparametric observation of different fluorescence characteristics of the investigated molecular systems is allowed. The simultaneous observation of fluorescence characteristics such as spectral peak position, fluorescence decay times, polarization properties or wavelength integrated intensity of fluorescence during the survival time of the investigated molecules leads to a more detailed picture of the molecular states and environment changes experienced by the probed molecules. In this contribution a diffraction-limited scanning stage confocal microscope set-up allowing real time multiparametric observation of fluorescence detected from single molecules immobilized in thin polymer matrix is described. By using pulsed excitation in combination with burst integrated fluorescence lifetime (BIFL) type detection, the simultaneous acquisition of fluorescence spectra, wavelength integrated fluorescence intensity time traces and time-resolved decay curves is demonstrated for synthetic as well as biological systems. By using continuous wave excitation and BIFL type detection, two dimensional fluorescence intensity time traces containing photons resolved in time with an accuracy of 50 ns and carrying polarization information can be recorded from individual immobilized molecules. In combination with specific analysis procedures, the SMS set-up is proved to be a suitable tool for the identification of different emitting species as well as for monitoring dynamical processes at the single molecule level.