Van der Waals SbSeI Quasi-1D Nano-Columns for Advanced Photonic Applications

There is an increasing interest in novel van der Waals materials such as Sb₂(S,Se)₃ and (Sb,Bi)(S,Se)(Br,I) systems, thanks to their natural ability to form low dimensional structures with exotic properties. This is strongly related to their highly anisotropic crystal structure, which results in unique electrical and optical properties when the materials are correctly oriented. In addition, these semiconductors can cover a plethora of different possible technological applications including sensors, supercapacitors, piezoelectric generators, and solar cells. Amongst this family of largely overlooked materials, (Sb,Bi)(S,Se)(Br,I) pnictogen chalco-halides are highly attractive due to their wide range of possible bandgaps between 1.2 eV up to 2.2 eV, customizable optical properties by using different halides or chalcogenides while retaining their quasi-1D structure, defect-tolerance, and ferroelectric properties. Nevertheless, the progresses in the development of these pnictogen chalco-halide semiconductors are being limited by the complexity to synthesize compounds containing halogens and chalcogens in the same structure, due to the very different elemental vapor pressure.<br/><br/>The first attempts to synthesize these materials have been essentially focused on low temperature (&lt;300 C) and atmospheric pressure solution-based chemical routes, resulting in columnar films with poor crystalline quality and very limited morphology tunability. Aiming towards a more controlled, reproducible and versatile system, in this work SbSeI, one of the most relevant materials amongst this family, has been synthesized by a novel procedure based on the selective iodination of co-evaporated Sb₂Se₃ films at high pressures (between 2 atm up to 20 atm), developing extremely uniform, single-crystal nano-columnar structures. We will show how the thickness, height, and density of these 1D nano-columns can be tuned by finely varying annealing temperature and pressure. The resulting nano-columns were characterized using microscopy imaging techniques, Raman spectroscopy and X-ray diffraction, demonstrating the formation of SbSeI single phase with orthorhombic structure (space group Pnma), and with excellent crystalline quality and compositional homogeneity. To study the potential of this material for different photonic applications, electrical and optoelectronic measurements have been performed on micro-scale devices, connecting nano-columnar crystals to Pt and Au contacts on a SiO₂/Si wafer. Interestingly, a clear diode response has been observed with Au-Au and Au-Pt, which is greatly enhanced under illumination. Memory effect, capacitance-voltage and ferroelectric measurements have also been performed. Kelvin probe microscope analysis has been used to determine its work function, observing that different facets of the crystals yield distinct surface potential, supporting our hypothesis that the quasi-1D growth of the material gives rise to anisotropic transport properties.<br/><br/>Overall, this work presents to the best of our knowledge, the first study of the physical vapor deposition synthesis and characterization of novel SbSeI chalco-halide compound, demonstrating the tunability of the SbSeI nano-columns morphology by varying synthesis pressure and temperature, as well as the potential of low dimensional chalcohalides to be implemented into innovative architectures and advanced functionalities (such as for enhanced carrier collection and light trapping), pointing the way to develop defect-tolerant and ferroelectric absorbers with anisotropic electrical properties for advanced photonic applications.