Natalia Zawadzka1,Tomasz Wozniak2,Marcin Strawski1,Igor Antoniazzi1,Zahir Muhammad3,Weisheng Zhao3,Roman Stepniewski1,Adam Babinski1,Maciej Molas1
University of Warsaw1,Wroclaw University of Science and Technology2,Hefei Innovation Research Institute, Beihang University,3
Natalia Zawadzka1,Tomasz Wozniak2,Marcin Strawski1,Igor Antoniazzi1,Zahir Muhammad3,Weisheng Zhao3,Roman Stepniewski1,Adam Babinski1,Maciej Molas1
University of Warsaw1,Wroclaw University of Science and Technology2,Hefei Innovation Research Institute, Beihang University,3
Hafnium-based transition metal dichalcogenides (TMDs) including hafnium disulphide (HfS<sub>2</sub>) have attracted recently more attention of researchers due to their very effective electrical response [1]. This justifies a need to uncover their basic optical and electrical properties.<br/><br/>Along the line of research, we report on optical emission from HfS<sub>2</sub> bulk crystal grown by chemical vapor transport method. The photoluminescence (PL) is investigated in a broad range of temperatures (5-300 K).<br/><br/>The low-temperature emission consists of a series of well-resolved emission lines apparent in the energy range of 1.3-1.5 eV. In order to identify the observed emission lines, classical correlations between the PL intensity at the energies of different emission lines [2] is analysed. A series of spectra measured at different spots of the sample is used in the analysis. The analysis allows to identify two zero-phonon emission lines in the spectrum which intensities are strongly anticorrelated. The zero-phonon lines are accompanied by several phonon replica. The replica involving accoustic (LA, TA) and optical (E<sub>u</sub>(TO)) phonons and their combinations are identified.<br/><br/>The observed emission is related to excitons (most likely neutral and charged) in HfS<sub>2</sub> bound to a localizing potential due to neutral iodine molecules (I<sub>2</sub>). The I<sub>2</sub> molecules were introduced into the crystal during its growth as iodine is used as a transport agent in the process. The molecules are intercalated in van der Waals gaps between covalently bonded HfS<sub>2</sub> layers. The presence of iodine in the investigated samples was confirmed by secondary ion mass spectroscopy and the sample de-iodization results in the spectrum disappearance which further supports our attribution. Similar low-temperature PL, related to intercalated halogen molecules was previously observed in 2H polytypes of tungsten sulphides/selenides [3].<br/><br/>The space localization of the bound excitons leads to their delocalization in the momentum space. This explains a strong coupling to phonons from a whole Brillouin zone. We note that the energies of phonon replicas in the PL spectra correspond to maxima of the total density of phonon states in bulk HfS<sub>2</sub>, rather than to the energies of phonons from particular high-symmetry points of the Brillouin zone.<br/><br/>In conclusion we present for the first time the low-temperature excitonic emission in bulk HfS<sub>2</sub>. We analyze the observed PL spectrum using classical correlation method and we identify two “families” of lines attributed to neutral and charged excitons bound by a potential due to iodine molecules in van der Waals gaps. Our attribution is supported by the observation of iodine in the investigated samples by means of secondary ion mass spectroscopy.<br/><br/><br/>[1] S. Lukman, et al. Nature Nanotechnology, 15, 675, (2020)<br/>[2] B. Pietka , et al. Phys. Rev. B87, 035310 (2013)<br/>[3] D. Duchenko, et al. Tin Solid Films, 495, 82, (2006)