Nano-ARPES Brings Anisotropic van der Waals Alloys into Focus

Alloying has long been one of the primary tools of bandgap engineering, enabling the tuning of crystal structure, lattice parameters, and electronic structure of 3D and 2D/layered semiconductors. Among the latter, alloying can play a key role in tailoring the properties of tin monochalcogenides, a class of van der Waals semiconductors of interest for optoelectronics, thermoelectrics, ferroelectrics, and valleytronics.

In this study, we investigated the synthesis and properties of large flakes of the anion substitution alloys SnSe_1-xS_x. Single-crystalline flakes across a wide composition range were obtained systematically by repeated growth from the same mixed (SnS, SnSe) powder precursor. Combined experiment and theory show full miscibility across all compositions, along with tunable lattice constants, bandgaps, and vibrational modes.

Among the distinguishing properties of tin monochalcogenides is a highly anisotropic crystal structure of the individual layers that make up the van der Waals crystals. Polarized Raman spectroscopy showed that this structural anisotropy is reflected in anisotropic vibrational modes. To investigate the effects on the electronic structure, density functional theory calculations of the effective band structure of SnSe_1-xS_x alloys were combined with measurements of the occupied electronic bands using angle-resolved photoelectron spectroscopy (ARPES). The measurements were performed on individual flakes of ~20 μm size (Fig. 1a) using the unique nano-ARPES capability of the Spectromicroscopy beamline (3.2L) at Elettra, which delivered linearly polarized soft-X-ray radiation (photon energy 74 eV) into a 0.6 μm spot. After establishing the in-situ sample preparation, high-quality valence band maps were obtained (Fig. 1b).

Figure 1.  (a) Characteristic single crystalline few-layer SnSe_1-xS_x alloy flakes. (b) Nano-ARPES band structure map with superimposed calculated effective band structure for an SnSe_1-xS_x alloy with sulfur content x = 0.65 (dots). (c) ARPES constant energy maps obtained at different energies below the valence band maximum (E = 0 eV). The inset illustrates the polarization of the incident synchrotron radiation (pol) relative to the chalcogen (S, Se) p-orbitals.

While the fundamental bandgap is indirect, the existence of direct transitions in non-degenerate valleys along orthogonal (X- and Y-) directions in reciprocal space has raised interest in tin monochalcogenides for valleytronics, where the valley occupancy is used to process and store information. Calculations of the effective band structure of SnSe_1-xS_x alloys show this valley configuration across the entire range of compositions, with systematic shifts in the band-edge energies as the primary effect of alloying. Nano-ARPES measured on individual SnSe_1-xS_x alloy flakes validated the calculations and provided further insight into the anisotropic electronic structure of the semiconducting van der Waals alloys. Figure 1b illustrates the results for the example of an SnSe_0.35S_0.65 alloy, showing the measured valence band dispersion along high-symmetry directions in the Brillouin zone in comparison with the calculated effective band structure of the alloy. The map shows a close correspondence to the calculated band structure along the Γ-Y direction, whereas the valley along the Γ-X line shows no measurable intensity. Constant energy maps, presented in Fig. 1c, also prominently show the Y-valley band edge but no detectable intensity from the X-valley. This finding is explained via the orbital character of the valence band edges and the polarization of the exciting X-ray beam. In particular, the X-valley valence band maximum has mostly chalcogen p_x orbital character while the Y-valley maximum is dominated by S/Se p_y orbitals. Photon polarization selection rules stipulate that the signal from p-orbitals is maximized for incident light with polarization along their principal axis. In Fig. 1, the polarization was aligned closely with p_y and nearly perpendicular to p_x, which explains the large intensity of the Y-valley and absence of signal from the X-valley. This finding directly confirms the persistence of an anisotropic electronic structure across SnSe_1-xS_x alloys. 

Given the broad interest in tin monochalcogenides for applications such as thermoelectrics, optoelectronics, and valleytronics – so far pursued mostly in the endpoint compounds SnS and SnSe – and the ability to tune structure and electronic properties by alloying, the present study on the synthesis and properties of high-quality SnSe_1-xS_x alloy flakes opens up avenues for materials design in support of both existing and emerging applications of anisotropic monochalcogenide van der Waals semiconductors.

P. Sutter¹, A. Barinov², H.-P. Komsa³, P. Ghimire⁴, L. Wu⁵, Y. Zhu⁵, K. Kisslinger⁶, and E. Sutter⁴

¹ Department of Electrical & Computer Engineering, University of Nebraska-Lincoln, Lincoln, USA
² Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
³ Microelectronics Research Unit, University of Oulu, Oulu, Finland
⁴ Department of Mechanical & Materials Engineering, University of Nebraska-Lincoln, Lincoln, USA
⁵ Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, USA
⁶ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, USA

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