We report a new phase in the binary SnS system, obtained as highly symmetric nanotetrahedra. Due to the nanoscale size and minute amounts of these particles in the synthesis yield, the structure was exclusively solved using electron diffraction methods. The atomic model of the new phase (a = 11.7 Å, P2(1)3) was deduced and found to be associated with the rocksalt-type structure. Kramers-Kronig analysis predicted different optical and electronic properties for the new phase, as compared to α-SnS.

We report on the synthesis of the newly discovered cubic phase of tin monosulfide π-SnS and compare its properties to the well-known phase of tin monosulfide, α-SnS.

We present the atomic arrangement of 64 atoms within a simple cubic unit cell crystalline structure of lattice constant 11.6 Å, observed in tin sulfide (SnS) thin films. Thin films of 260 or 550 nm in thickness were deposited at 17°C from a chemical bath containing tinIJII) chloride and thioacetamide. The X-ray diffraction (XRD) patterns of these thin films are consistent with those of a simple cubic structure of lattice constant 11.600 ± 0.025 Å (as-prepared) or 11.603 ± 0.007 Å (after 400°C heating). The said recently discovered "π-SnS" structure was adopted from previous reports, using the present, newly acquired experimental data to obtain the atomic positions. This structural assignment unravels a puzzle originated by inconsistencies among the XRD patterns of some SnS thin films and nanocrystals prepared via certain chemical routes, and the zinc blende, rock salt or pseudo-tetragonal structures previously assigned to them. In addition to its relevance as a stable solar cell material, salient features of this SnS polymorph arising from its lack of centro-symmetry are discussed.

A new nanometric cubic binary phase of the tin mono-selenide system, π-SnSe, was obtained as cube shaped nanoparticles. Its structure and atomic positions were adopted from previously reported π-SnS (P2 1 3, a 0 = 11.7 Å). The proposed structure model of π-SnSe, with 64 atoms per unit cell, was refined against experimental X-ray diffraction using Rietveld method (a 0 = 11.9702(9) Å; R p = 1.65 R wp = 2.11). The optical properties of this new cubic SnSe phase were characterized by Raman and optical absorption spectroscopies. The optical band gap was assessed to be indirect, with E g = 1.28 eV (in the near infrared), compared to E g = 0.9 eV (indirect) and 1.3 eV (direct) for the conventional orthorhombic phase of α-SnSe. Raman spectroscopy indicated significant phonon restraining, which is likely to be beneficial for thermoelectric applications. Since the new cubic phase belongs to a class of non-centrosymmetric crystals, interesting and potentially useful properties may arise. Density functional theory calculations have been applied in order to validate phase stability and evaluate the energy bandgap. These results, together with the recently discovered cubic phase of π-SnS, confirm the existence of a new class of nanoscale materials in the tin chalcogenide system.

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Epitaxial thin films of cubic tin monosulfide (π-SnS), a recently discovered new binary phase, were deposited from solution on GaAs substrates and on GaAs with intermediate PbS layers.

Ammonium chloride ligands reduce surface energies, bind preferably to the cubic π-phase and destabilize the orthorhombic phase of SnS.

Cubic π-phase monochalcogenides (MX, M = Sn, Ge; X = S, Se) are an emerging new class of materials that has recently been discovered. Here, their thermodynamic stability, progress in synthetic routes, properties, and prospective applications are reviewed. The thermodynamic stability is demonstrated through density functional theory total energy and phonon spectra calculations, which show that the π-phase polytype is stable across the monochalcogenide family. To date, only π-phase tin monochalcogenides have been observed experimentally while π-phase Ge-monochalcogenides are predicted to be stable but are yet to be experimentally realized. Various synthetic preparation protocols of π-SnS and π-SnSe are described, focusing on surfactant-assisted nanoparticle synthesis and chemical deposition of thin films from aqueous-bath compositions. These techniques provide materials with different surface energies, which are likely to play a major role in stabilizing the π-phase in nanoscale materials. The properties of this newly discovered family of semiconducting materials are discussed in comparison with their conventional orthorhombic polymorphs. These could benefit a number of photovoltaic and optoelectronic applications since, apart from being cubic, they also possess characteristic advantages, such as moderately low toxicity and natural abundance.