Terrylene-doped biphenyl monocrystals for optical single-molecule spectroscopy

Biphenyl forms at normal pressure and temperatures below 40 K incommensurate crystalline structures with physical properties varying in space on a scale of nanometers. As biphenyl crystals are optically transparent, there is a possibility to study these natural nanostructures optically, by doping the host crystal with nanoscopic probes with optical properties depending on their local environment. It has already been demonstrated that terrylene impurity molecules in polycrystalline biphenyl sample can successfully play the role of such kind of sensitive nanoprobes when studied by the methods of high-resolution laser spectroscopy. We report growing of thin biphenyl monocrystals doped with terrylene molecules at very low concentrations. These sublimation-grown flakes can be studied at liquid helium temperatures using the technique of single-molecule spectroscopy. Compared to polycrystalline biphenyl samples, much higher signals and better signal-to-noise ratio can be achieved in single-molecule spectra. This allows to perform much faster spectral scans to find intensive single-molecule lines even in very dilute samples in spite of a very broad inhomogeneous absorption band of terrylene in biphenyl. Fast scanning also allows observation of single-molecule lines with much better temporal resolution, revealing processes of spectral diffusion occurring at different time scales. This can be helpful in our attempts to learn about the role of the matrix incommensurability in spectral features observed. Extremely high variability of temporal and spectral behaviour of terrylene single-molecule lines is reported, which is unusual for crystalline hosts.