Band edge photoluminescence of undoped and doped TlInS2 layered crystals

Intrinsic photoluminescence (PL) of undoped and of B-, Ag- or Er-doped TlInS2 layered single crystals was investigated by confocal spectroscopy. It is found that position and intensity of PL spectral peak at 2.4 eV is vastly dependent on the excitation light incidence and polarization relative to crystallographic directions. For the normal incidence to the layer plane k||c, a significant Stokes shift between the PL peak in respect to exciton absorption energy is highlighted. The shift increases to 80 meV around the crystal phase transformations region of 200 K. We show that presence of B and Ag impurity do not diminish the intrinsic PL emission while Er atoms incorporation enhances it substantially and modifies a fine excitonic line structure in the ferroelectric phase at low T. The obtained results imply out-of-layer plain oriented 2D excitons in TlInS2 existing at various crystal phases which are influenced by related interlayer stresses.