Martin Vácha

Persistent room-temperature phosphorescence (RTP) under ambient conditions is attracting attention due to its strong potential for applications in bioimaging, sensing, or optical recording. Molecular packing leading to a rigid crystalline structure that minimizes nonradiative pathways from triplet state is often investigated for efficient RTP. However, for complex conjugated systems a key strategy to suppress the nonradiative deactivation is not found yet. Here, the origin of small rates of a no

We investigated exciton migration, trapping and emission processes occurring within a single conjugated polymer molecule by means of superresolution fluorescence localization microscopy. This methodology allowed us to locate the spatial distribution of emitting sites within single chains with nanometre precision. The study was done on individual poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) molecules with average molecular weights ranging from 215,000 to 1,440,000 and wi

Organic-inorganic halide perovskites have emerged as promising materials for next-generation solar cells. In nanostructured form also, these materials are excellent candidates for optoelectronic applications such as lasers and light-emitting diodes for displays and lighting. While great progress has been achieved so far in optimizing the intrinsic photophysical properties of perovskite nanocrystals (NCs), in working optoelectronic devices, external factors, such as the effects of conducting envi

We map the location of emitting sites within a single conjugated polymer molecule poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] with nanometre accuracy by means of a single-molecule imaging technique.

We present a study of single molecule linewidth broadening of terrylene in dodecane, tetradecane, and hexadecane matrices. The Shpolskii bulk absorption spectra exhibit increase of the bandwidths and site complexity with increasing n-alkane chain length. Single molecule lines are broadened above the lifetime limit even at 1.6 K in all three solvents. The linewidth distributions suggest the existence of relaxing two-level systems coupled to the molecular transitions. Spectral diffusion was observ

We present a simple and straightforward method for determining absolute spatial orientations of transition dipole moments of single fluorescent molecules. Far-field polarization microscopy provides angles of the dipole moments projected in the plane of the sample. Optical field near total internal reflection surfaces has a strong component perpendicular to the sample and, for a given in-plane angle, provides unambiguous orientation of the molecular dipole moment. Experimentally, both excitation

Single-molecule spectroscopy (SMS) of a dual fluorescent flapping molecular probe (N-FLAP) enabled real-time nanoscale monitoring of local free volume dynamics in polystyrenes. The SMS study was realized by structural improvement of a previously reported flapping molecule by nitrogen substitution, leading to increased brightness (22 times) of the probe. In a polystyrene thin film at the temperature of 5 K above the glass transition, the spectra of a single N-FLAP molecule undergo frequent jumps

We study the effect of Zn on the photophysical properties of a family of group I-III-VI nanocrystals (NCs), namely in solid solutions of (AgIn)_xZn_2(1-x)S₂ (ZAIS). We focus on the comparison of the photoluminescence (PL) properties of ZAIS NCs of comparable sizes and different amounts of Zn. This approach helps us to decouple the effects of size and varying chemical composition of the NCs which both influence the PL properties. We show that in the presence of Zn n

Polyfluorenes are conjugated polymers that show strong blue emission and as such have been explored for potential applications in light-emitting devices. However, heat treatment, prolonged exposure to air, or extended operation in electroluminescent devices can lead to an appearance of parasitic green emission that degrades the material performance. This phenomenon has been extensively studied over the past two decades, and two main and conflicting explanations, i.e., oxidation and formation of

We developed a method to determine full three-dimensional orientation distribution of individual molecules based on wide-field defocused fluorescence imaging. Excitation efficiencies of out-of-plane oriented molecules were improved dramatically by illuminating molecules with multiple laser beams. Our high throughput approach allowed us to obtain unbiased statistical distributions of orientations of doped molecules in spin-coated polymer thin films. We found thickness- and glass transition temper

Ternary I-III-VI semiconductor nanocrystals have been explored as non-toxic alternatives to II-VI semiconductors for optoelectronic and sensing applications, but large photoluminescence spectral width and moderate brightness restrict their practical use. Here, using single-particle photoluminescence spectroscopy on nanocrystals of (AgIn)xZn2(1-x)S2 we show that the photoluminescence band is inhomogeneously broadened and that size distribution is the dominant factor in the broadening. The residua

Ring expansion metathesis polymerisation (REMP) has proven to be a viable approach to prepare high purity cyclic polymers. Macrocyclic polymers with a fully conjugated defect free backbone are of particular interest as these polymers have no end groups that can act as charge traps. In this work soluble macrocyclic poly(<i>p</i>-phenylenevinylene)s ( <b><i>c</i>PPV</b>s) have been prepared directly <i>via</i> the REMP of substituted paracyclophanedienes. Single-molecule spectroscopy of the two to

Highly reflecting nanostructures of pseudoisocyanine J aggregates in a thin polymer film are prepared. Simultaneous atomic force microscope and reflection microscopic studies show a high degree of correlation between structural and optical properties on submicrometer scales. As a result of strong coupling between light and the aggregate excitonic states, optical modes evolve in the polariton-like reflectance spectra of the aggregate nanostructures which function as self-assembled optical microca