Polarized Band‐Edge Emission and Dichroic Optical Behavior in Thin Multilayer GeS

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (1 of 6) 1600814 and optical absorption behaviors of GeS nanosheets have been studied and proposed for energy applications,[16,17] however, the light-emitting property of a thin multilayer GeS was not yet explored. Herein, in-plane optical anisotropy of band-edge emission of a multilayer GeS stripe (t ≈ 40 nm) is characterized by using polarized micro-photoluminescence (μPL) measurement with polarization angles ranging from 0 to 90° with respect to the multilayer’s longest crystal edge (b axis). Light emission from multilayer GeS is completely forbidden at θ = 0° (E||b), and fully allowed (intense light) at θ = 90° (E||a), which obeys a dichroic Malus law[18] for the emission peak at 1.622 eV. Multilayer GeS behaves like a dichroic light emitter with linear polarization along the multilayer’s a axis. To understand the band-edge characteristic of the GeS multilayer, polarized thermoreflectance (PTR) measurement by using microscope white-light guiding was also implemented. The selection rule of the PTR spectra also reveals that the lower band-edge transition EA = 1.622 eV is only allowed with E||a polarization while another higher energy transition, EB = 1.732 eV, appears merely along the E||b polarization in the multilayer. In-plane anisotropy of the c plane GeS multilayer occurs with linear polarization along a and b axes. The energy position of the EA transition matches well with the main band-edge emission observed by μPL, while the EB transition may probably originate from valence-band splitting (dominated by the S p orbital) of the multilayer GeS. The structural and optical anisotropy of the multilayer GeS is discussed here. Layered single crystals of IV–VI GeS with different areas, sizes, and thicknesses were grown by using the chemical vapor transport (CVT) method[19] with iodine as the transport agent. The crystals were prepared from their elements (Ge: 99.999% pure and S: 99.999%) by reaction at 600 °C for two days in evacuated quartz ampoules. Sulfur was added in 1 mol% excess with respect to the stoichiometric mixture of the constituent elements. About 10 g of the elements, together with an appropriate amount of transport agent (I2 about 10 mg cm−3), were introduced into a quartz ampoule (22 mm OD, 17 mm ID, 20 cm length), which was then cooled with liquid nitrogen, evacuated to 10−6 Torr and sealed. The mixture was slowly heated to 600 °C and maintained at this temperature for two days. The temperature was then altered from 600 °C (heating zone) → 520 °C (growth zone) with a gradient of –4 °C cm−1 for crystal growth. The reaction was left 240 h to produce large single crystals. The as-grown GeS crystals are dark red and have a shiny surface with an area of up to 5 mm2 and thickness of up to 100 μm. The crystals are mostly like ribbon or stripe-shape layered crystals (see Figure S1 in the Supporting Information). The powder and single-crystal X-ray diffraction (XRD) results of GeS shown in Figure S1 reveal an orthorhombic phase. The lattice constants are a = 4.36 Å, b = 3.67 Å, and c = 10.53 Å, Polarized Band-Edge Emission and Dichroic Optical Behavior in Thin Multilayer GeS