Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 37 (2025) 03LT01 (6pp) https://doi.org/10.1088/1361-648X/ad81a1 Letter Isolation and characterisation of monolayer phosphorene analogues Nicolas Gauriot1, Raj Pandya1, Jack Alexander-Webber2 and Akshay Rao1,∗ 1 Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 2 Department of Engineering, University of Cambridge, Cambridge, United Kingdom E-mail: ar525@cam.ac.uk Received 5 June 2024, revised 11 July 2024 Accepted for publication 30 September 2024 Published 25 October 2024 Abstract Atomically thin group IV monochalcogenides or phosphorene analogues are a promising family of materials. Theoretical calculations predict that monolayers (MLs) should be semiconducting, ferroelectric and ferroelastic at room temperature, exhibit large charge mobilities and large non-linear optical response. Yet, experimental studies of these systems are scarce because of the difficulty to produce such MLs. Here we focus on two members of this family: GeSe and SnS. We demonstrate a simple mechanical exfoliation method to produce ML samples on gold substrates. We observe the evolution of the Raman scattering as a function of layers and the anisotropic optical response from reflectance contrast measurements. To the best of our knowledge this is the first report of mechanical exfoliation down to the ML of these materials and the first realisation of ML GeSe. Keywords: phosphorene analogues, exfoliation, ferroelectricity, groups IV monochalcogenides, Raman scattering 1. Introduction 2D materials are an attractive class of materials particularly promising for electronic and optical applications [1]. Often new properties emerge at the atomically thin limit and several phenomena can only be observed in low dimensions. 2D semi- conductors have revealed a particularly rich excitonic physics and have appeared as a fertile platform to study many body physics [2, 3]. Finally, Van derWaals heterostructures allow to explore a whole new space in material design with even richer physics [4, 5]. ∗ Author to whom any correspondence should be addressed. Original content from this workmay be used under the terms of the Creative Commons Attribution 4.0 licence. Any fur- ther distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. To date, only a few tens of 2D crystals have been stud- ied experimentally. However, theoretical calculations suggest that more than 1000 other layered materials could be isol- ated in the monolayer (ML) form, with more than 100 of those being semiconducting [6, 7]. Among this vast library of Van derWaalsmaterials, Group IVmonochalcogenides, GeSe, GeS, SnS and SnSe, (also known as phosphorene analogues) stand out. Numerous theoretical studies suggest that they have markedly different properties from the more commonly stud- ied 2D materials [8–25]. These four compounds have an orthorhombic structure in the bulk (Pnma space group) with puckered layers similar to black phosphorus (figure 1) [26, 27]. They are indirect semi- conductors in the bulk and are expected to have optical band gaps in the visible and near infrared region [8–11] in the ML, and similarly to phosphorene, they should host strongly bound anisotropic excitons [9–13]. Additionally, GeSe should undergo an indirect to direct bandgap crossover at the ML 1 © 2024 The Author(s). Published by IOP Publishing Ltd https://doi.org/10.1088/1361-648X/ad81a1 https://orcid.org/0000-0001-7725-7208 https://orcid.org/0000-0002-9374-7423 https://orcid.org/0000-0003-4261-0766 mailto:ar525@cam.ac.uk http://crossmark.crossref.org/dialog/?doi=10.1088/1361-648X/ad81a1&domain=pdf&date_stamp=2024-10-25 https://creativecommons.org/licenses/by/4.0/ J. Phys.: Condens. Matter 37 (2025) 03LT01 Figure 1. Crystal structure of the MX compounds. (a) 3D representation, (b) top view along the z axis, (c) side view along y axis, (d) side view along x axis. limit [8–11], not unlike the more common transition metal dichalcogenides (TMDCs) [28, 29]. Because of the different electronegativities of the group IV metal (M) and the chalcogen (X) atom, each M–X bond is polarized, and a macroscopic in-plane electric polarisa- tion should emerge for odd number of layers. Importantly the Curie temperature of this ferroelectric phase are predicted to be above room temperature [12, 14, 15]. Evidence for fer- roelectricity was observed in ML SnSe [16] and SnS [17]. Additionally, this electric polarisation should be switchable by strain, making these materials ferroelastic [12, 14, 18]. Large bulk photovoltaic currents have been predicted [19, 20], and large second harmonic susceptibilities, one order of magnitude larger than that of MLMoS2, have been calculated as well [21, 22]. Additionally, predicted charge mobilities can be 1 or 2 orders of magnitude higher than in TMDCs [11, 23]. Finally, a one dimensional strongly correlated system should emerge in twisted bilayer GeSe [24, 25]. Despite these extraordinary predictions, experimental stud- ies on these systems are scarce [30]. This is because the simple mechanical exfoliation that has allowed the rapid exploration of the other 2D materials does not yield good results for this group of materials. SnSe MLs of∼100 nm in width, produced via molecular beam epitaxy on graphene, have been reported [16] and flakes of tens of µm have been synthesized through a combination of chemical vapour deposition and Nitrogen etching [31]. For SnS, µm-sized MLs have been produced by pulse laser deposition [17] and few hundred nm ML flakes were isolated via liquid phase exfoliation [32]. To the best of our knowledge, there are no other reports of ML phosphorene analogues. Here we focus on two of these materials: GeSe and SnS. We demonstrate the isolation of few µm large MLs of GeSe and SnS via metal assisted mechanical exfoliation. We then study the evolution of the Raman scattering as a function of number of layers. Finally, we measure the optical anisotropy of the layers through reflectivity measurements. 2. Results and discussion We obtain atomically thin layers of GeSe and SnS from bulk crystals via a modified gold assisted mechanical exfoliation as summarised in figure 2(a) [33]. (see methods for details). Figures 2(b) and (e) presents representative samples obtained with this method. Atomic force microscopy (AFM) height measurements (figures 2(c) and (f)) show that the ML thick- ness can be reached this way. We measure a thickness of 6.5 A for ML GeSe and 6.3 for ML SnS. We also observed a lin- ear evolution of the optical contrast with the number of layers (figures 2(d) and (g)) confirming the identification of the num- ber of layers. This alsomeans that optical contrasts can be used as a rapid and simple way of identifying the number of layers, in a similar manner as for other 2D materials [34]. Raman spectroscopy has been established as a powerful technique to characterize 2D materials [35]. In figure 3, we present the evolution of the Raman spectrum as a function of number of layers for GeSe (figure 3(a)) and SnS (figure 3(b)). We label the different vibrational modes according to their symmetry, following previous reports [35–37]. For bulk SnS, we observe 4 prominent modes (Ag: 94 cm−1, 191 cm−1, 218 cm−1, B3g:163 cm−1). For thin flakes, we observe the appearance of an extra mode at 284 cm−1 which we identify as a B2g mode. For bulk GeSe, we observe 3 modes (Ag:81 cm−1, 187 cm−1, B3g: 149 cm−1). Similarly to SnS, we observe an extra mode for thin flakes at 222 cm−1, which we also attribute to B2g symmetry. We extract the transition frequency of each of these modes via a line shape analysis and present the evolution of the frequency shift in figures 3(c) and (d). In both materials, as the number of layer decreases, we observe that some modes blueshift while some redshift. This type of evolution is com- monly observed in other 2D materials [38]. While the fre- quencies of vibrational modes are expected to decrease as a layered material is thinned down [39], on insulating substrate, it was shown that the unusually strong coulomb interaction 2 J. Phys.: Condens. Matter 37 (2025) 03LT01 Figure 2. Metal assisted exfoliation of group IV monochalcogenides. (a) Schematic of the exfoliation process. (b) Optical micrograph of a monolayer and few layer GeSe sample. (c) AFM height profile along the white line in (a). (d) Optical contrast as a function of layer number as indicated in (b). Solid line is a linear fit to the data. (e) Optical micrograph of a monolayer and few layer SnS sample. (f) AFM height profile along the white line in (e). (g) Optical contrast as a function of layer number as indicated in (e). Solid line is a linear fit to the data. in atomically thin samples, can lead to an increase of some mode frequency [40]. Surprisingly, for GeSe not all modes have monotonous evolution with the number of layers. From 2 to 1 layer, we observe a blue shift of the B3g mode at 149 cm−1 and the Ag mode at 187 cm−1. In contrast, the B2g mode at 222 cm−1 seems to red shift from 2 to 1 l. This non monoton- ous evolution of the modes’ frequencies could suggest a strong interaction with the metallic substrate, which would lead to screening of the coulomb interaction as observed previously in MoS2 ML on Au [41]. Finally, we observe the anisotropic optical response of the thin films. For both materials we observe a resonance in the reflectance contrast near 2.4 eV, with a strong polar- ization dependence. Figure 4(b) shows the evolution of the reflection contrast near 2.4 eV as the angle of the linear polarisation is rotated for 4-layer GeSe. The peak is well approximated by two Lorentzians with transition energies at 2.41 eV and 2.47 eV. For each angle of the polarisation, we extract the oscillator strength of the two transitions to form the polar plot in figure 4(c). This clearly shows that these two transitions are linearly polarised and perpendicular to one another. Figure 4(a) shows the reflectance contrast for the two extreme polarisation angles 0◦ and 90◦ for 4 to 1-layer Gese. Figures 4(d)–(f) shows the corresponding data for SnS. 3. Conclusion We have presented a simple method to produce atomically thin GeSe and SnS down to the ML. Given their similar exfoli- ation energies [30] GeS and SnSe should be easily isolated in the same way. We hope that this work allows to explore experimentally the large number of exciting properties that have been theoretically predicted for this group of materials. It is important to point out that the samples prepared here are on a conducting gold substrate. This will be beneficial for measurement such as scanning tunnelling microscopy, Kelvin probe microscopy or piezoforce microscopy. These measure- ments could be performed to investigate the ferroelectricity that is expected in these materials, and all require a conducting 3 J. Phys.: Condens. Matter 37 (2025) 03LT01 Figure 3. Evolution of the Raman spectrum with the number of layers for (a) GeSe and (b) SnS. (c) Frequency shift of the Raman modes with respect to the bulk frequency for each of the GeSe modes labelled in (a). (d) Equivalent graph for the SnS modes of (b). substrate. However, the preparation of MLs on insulating sub- strate is also highly desirable. Wet etching is a natural route to remove the gold layer, however the apparent sensitivity of these materials to air and moisture makes this step more challenging than for TMDC. Nevertheless, other metals could be used and therefore a large number of metal/etchant combin- ation could be investigated. Alternatively, exfoliation of Van der Waals materials assisted by Al2O3 as well as by epoxies has already been demonstrated [42, 43], suggesting that the gold layer could be replaced by a dielectric to directly obtain MLs on an insulating substrate. 4. Methods 4.1. Sample preparation Samples were prepared via a modified gold assisted exfoliation [33] Bulk crystals (HQgraphene) are first exfo- liated on tape (Nitto), a 100 nm gold film is then evaporated on the freshly exfoliated crystals in a thermal evaporator. A glass slide is then glued on the gold filmwith an epoxy (Epotek 301, cured at room temperature for 24 h). Finally, the tape is peeled off, revealing thin layers of GeSe/SnS on the gold/e- poxy/glass stack. Samples prepared for optical measurements were processed in an inert environment (<0.5 ppm O2 and H2O) and encapsulated with a glass slide before measurement in air. 4.2. Optical spectroscopy Raman spectra were acquired with a Renishaw Invia confocal setup using an air-cooled Ar-ion 514.5 nm continuous wave (CW) laser via 100× objective in a backscattering geometry. The raman signal was dispersed with a 2400 line mm−1 grat- ing on a CCD camera. For each layer number spectra were collected on at least 5 different spots and averaged. Reflectance spectroscopy was performed on a home build setup. The samples were illuminated by a halogen lamp (Thorlabs) via a 100× objective (Mitutoyo). The reflected light was collected by the same objective and dispersed in a fiber coupled spectrometer (Thorlabs). 4 J. Phys.: Condens. Matter 37 (2025) 03LT01 Figure 4. Anisotropic optical response. (a) Reflectance contrast spectra along the two crystal axes for 4 to 1 layer GeSe. (b) Reflectance spectra of a 4-layer GeSe flake for different angle of the polarisation measured with 10◦ interval from 0◦ to 180◦ and offset for clarity. (c) Oscillator strength of the two transitions in (b) extracted from a Lorentzian fit. (d) Reflectance contrast spectra along the two crystal axes for 4 to 1 layer SnS. (e) Reflectance spectra of a 4-layer SnS flake for different angle of the polarisation measured with 10◦ interval from 0◦ to 180◦ and offset for clarity. (f) Oscillator strength of the two transitions in (e) extracted from a Lorentzian fit. Data availability statement The data that support the findings of this study are openly available at the following URL/DOI: 10.17863/CAM.106627. Acknowledgments This work has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement Number 758826). 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Introduction 2. Results and discussion 3. Conclusion 4. Methods 4.1. Sample preparation 4.2. Optical spectroscopy References