PHYSICAL REVIEW B 100, 094301 (2019) Edge phonons in layered orthorhombic GeS and GeSe monochalcogenides H. B. Ribeiro, 1 S. L. L. M. Ramos, 2 L. Seixas, 1 C. J. S. de Matos, 1 , * and M. A. Pimenta 3 , † 1 MackGraphe - Graphene and Nanomaterials Research Center, Mackenzie Presbyterian University, 01302-907 São Paulo, Brazil 2 Centro de Tecnologia em Nanomateriais e Grafeno (CTNano), Universidade Federal de Minas Gerais, 30161-970 Belo Horizonte, Brazil 3 Departamento de Física, UFMG, 30123-970 Belo Horizonte, Brazil (Received 15 November 2017; revised manuscript received 20 May 2019; published 3 September 2019) Germanium sulfide (GeS) and germanium selenide (GeSe) are layered orthorhombic crystals whose structure bears a strong resemblance with that of black phosphorus and, additionally, are expected to exhibit high piezoelectricity in the few layer domain. In this work, we investigate the Raman properties of exfoliated GeS and GeSe and show that their edges exhibit unusual polarized Raman features that were first observed in black phosphorus. The results include the activation in the spectra of otherwise not allowed modes at the edges of the sample, depending on the crystallographic direction of the edge and the polarization configuration used in the measurements. These features are attributed to atomic rearrangements at the crystal terminations, as well as their impact on phonon symmetries, similar to the case of black phosphorus. Our conclusions are further corroborated by using density functional theory and suggest that edge rearrangements, which will have an impact on the mechanical, electronic, and chemical properties of devices, is a general phenomenon of orthorhombic layered structures. DOI: 10.1103/PhysRevB.100.094301 I. INTRODUCTION The exfoliation of graphite down to the monolayer thick- ness [1] has led to a drastic increase in the interest in the properties of layered materials, such as the hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMD), and black phosphorus (BP) [2]. Particularly, BP is a semicon- ducting layered allotrope of phosphorus with a puckered orthorhombic structure and has drawn attention due to its interesting anisotropic physical properties [3–5] and its direct band gap that is readily tuned with the number of layers [6–8]. The group-IV monochalcogenide semiconductors GeS, GeSe, SnS, and SnSe [9] share the same layered puckered orthorhombic structure as BP. They also belong to point group D 2h , but with eight atoms per unit cell instead of four (space group Pnma, no. 62). In particular, GeS has a direct band gap of ∼1.65 eV [10–12] and GeSe has an indirect band gap of ∼1.14 eV [11,13]. Very recently, GeSe crystals were thinned down to a mono- layer through the use of a laser and a transition from indirect to direct band gap identified for thicknesses of less than three layers [14]. While this result will contribute to future technological applications of monochalcogenides, the produc- tion of large pristine few-layer crystals is still a challenge. Moreover, while the anisotropic behavior of bulk crystals is well reported [15,16] and the impact of dimensionality reduction on the electronic structure has been theoretically predicted [9], detailed experimental characterization is still lacking. Unlike BP crystals, both GeS and GeSe crystals are stable when left in air, which makes them promising materials * cjsdematos@mackenzie.br † mpimenta@fisica.ufmg.br for the fabrication of a wide variety of devices, such as solar cells [17], tunnel field effect transistors for ultralow energy switching applications [18], photocatalysts for water splitting [19], and piezoelectric devices [20–22]. Along with the properties that arise due to the reduction of the dimensionality, the edges of layered materials have proved to be an interesting object of study due to their distinct magnetic, electronic, and optical properties [23–27], with im- portant consequences for applications in optical and electronic devices. It has been recently reported that BP edges present an anomalous phonon behavior [28]. Using polarized Raman spectroscopy, modes that in bulk are symmetry forbidden at certain polarization configurations are observed at zigzag and/or armchair edges. The presence of such features was attributed to atom rearrangements at the crystal edges, which affect the atom displacements associated with vibrations. In this paper, we report on a polarized Raman spectroscopy study performed at the edges of exfoliated GeS and GeSe crystals. The results show the same type of symmetry break in the selection rules of Raman active modes observed in BP. Considering the fact that group-IV monochalcogenides and BP share the same crystal structure, the features observed here are also assigned to atomic rearrangements at the edges. Our experimental results are further corroborated by density functional theory (DFT) calculations showing that atomic rearrangements indeed take place at the crystal’s edges. II. METHODS GeS and GeSe crystals were obtained from HQ graphene with purity of >99.995%. The bulk crystals were exfoliated by using the common micromechanical exfoliation method, using an adhesive tape, and then transferred onto a silicon substrate with a 100-nm-thick silicon nitride layer (Si/Si 3 N 4 ). 2469-9950/2019/100(9)/094301(8) 094301-1 ©2019 American Physical Society