Ran E. Abutbul

New nanocrystalline materials: a previously unknown simple cubic phase in the SnS binary system.

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.

Synthesis and properties of nanocrystalline π-SnS – a new cubic phase of tin sulphide

We report on the synthesis of the newly discovered cubic phase of tin sulfide π-SnS and compare its properties to the well-known phase of tin sulfide, α-SnS. Shape control was achieved by the variation of synthesis parameters, resulting in cubic, rhombic dodecahedral and tetrahedral shapes of the π-SnS nanoparticles. X-ray diffraction provided authentication of the proposed model and refined determination of the lattice parameter a = 11.595 A. Raman spectroscopy showed a substantial shift towards higher energies and peak splitting for π-SnS. Optical absorption spectroscopy indicated an indirect band gap of 1.53 eV, in good agreement with density functional theory (DFT) calculations indicating a band gap greater than that of α-SnS. DFT total energy calculations show that the π-SnS phase is energetically similar to α-SnS, and is significantly more stable than the hypothetical ideal rocksalt structure of SnS.

Crystal structure of a large cubic tin monosulfide polymorph: an unraveled puzzle

We present the atomic arrangement of 64 atoms within a simple cubic unit cell crystalline structure of lattice constant 11.6 A, 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 tin(II) 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 A (as-prepared) or 11.603 ± 0.007 A (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 nanocrystalline binary phase: synthesis and properties of cubic tin monoselenide

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 (P213, a0 = 11.7 A). The proposed structure model of π-SnSe, with 64 atoms per unit cell, was refined against experimental X-ray diffraction using Rietveld method (a0 = 11.9702(9) A; Rp = 1.65 Rwp = 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 Eg = 1.28 eV (in the near infrared), compared to Eg = 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.

A new cubic prototype structure in the IV–VI monochalcogenide system: a DFT study

A new cubic binary phase was recently discovered in the IV–VI monochalcogenides: tin monosulfide and tin monose-lenide. Here, we explored the possible materials design space for this phase across groups IV–VI. The structure and properties of this new phase, the π phase, including mechanical and thermodynamic stability and band gap width, were studied using density functional theory across the monochalcogenide systems of groups IV–VI. The structure of the new π phase was interpreted in terms of a distorted rock-salt structure, and the bonding between the 64 atoms of the unit cell was elucidated. It was found that the π phase is mechanically stable in germanium sulfide and germanium selenide and unstable in the tellurides. The energy differences between the π phase and the stable orthorhombic structures were very small, suggesting that the π phase could be thermodynamically accessible in monochalcogenide nanoparticles. The band gaps of the new phase were found to range from 1.0–1.4 eV, which may be attractive for a variety of photovoltaic and photosensing applications, in particular for GeS, which uniquely has a direct band gap in the π phase.

Prediction of the stability of the rhombohedral phase in IV–VI monochalcogenides and its origin

The rhombohedral monotellurides, GeTe and SnTe, are non-centrosymmetric materials with ferroelectric behavior and potential applications in thermoelectricity and spintronics. In a previous computational study [E. Segev, U. Argaman, R. E. Abutbul, Y. Golan and G. Makov, CrystEngComm, 2017, 19, 1751–1761], it was found that the rhombohedral phase may also be the thermodynamically most stable phase in SnSe and GeSe at low temperatures. In the present study, we explore the mechanical and thermodynamic stability of the rhombohedral phase in these systems and its enhancement as a function of pressure and temperature using density functional theory calculations. Within the region of stability, we examine the structure, bonding and physical properties of the rhombohedral phase. This phase is a distorted rock-salt phase forming a non-spherical lone-pair with an opposite directionality to the chemical bond. The monochalcogenides in this phase exhibit ferroelectric behavior, and we calculate the electric polarization. Finally, the transition from a rhombohedral to rock-salt phase at high pressures is examined and found to be second-order at zero Kelvin.

Mapping Charge Distribution in Single PbS Core – CdS Arm Nano-Multipod Heterostructures by Off-Axis Electron Holography

We synthesized PbS core-CdS arm nanomultipod heterostructures (NMHs) that exhibit PbS{111}/CdS{0002} epitaxial relations. The PbS-CdS interface is chemically sharp as determined by aberration corrected transmission electron microscopy (TEM) and compared to density functional theory (DFT) calculations. Ensemble fluorescence measurements show quenching of the optical signal from the CdS arms indicating charge separation due to the heterojunction with PbS. A finite-element three-dimensional (3D) calculation of the Poisson equation shows a type-I heterojunction, which would prevent recombination in the CdS arm after optical excitation. To examine charge redistribution, we used off-axis electron holography (OAEH) in the TEM to map the electrostatic potential across an individual heterojunction. Indeed, a built-in potential of 500 mV is estimated across the junction, though as opposed to the thermal equilibrium calculations significant accumulation of positive charge at the CdS side of the interface is detected. We conclude that the NMH multipod geometry prevents efficient removal of generated charge carriers by the high energy electrons of the TEM. Simulations of generated electron-hole pairs in the insulated CdS arm of the NMH indeed show charge accumulation in agreement with the experimental measurements. Thus, we show that OAEH can be used as a complementary methodology to ensemble measurements by mapping the charge distribution in single NMHs with complex geometries.

Surface energies and nanocrystal stability in the orthorhombic and π-phases of tin and germanium monochalcogenides

A new nanocrystalline cubic binary π-phase was recently discovered in the tin and germanium monosulfide and monoselenide systems. The structure and surface energies of the (100) and (010) surfaces of the orthorhombic phase and the (100) and (111) surfaces of the π phase were studied using density functional theory across several monochalcogenide systems. The thin film electronic band structures were calculated, demonstrating band gaps of ∼1 eV which are slightly lower than in the bulk. Surface states were found to form for the π phase but not for the orthorhombic phase surfaces. Additionally, the surface atomic structure and the chemical bonding in the vicinity of the surface were analyzed. It was found that the orthorhombic phase is stabilized as (010) platelets as observed experimentally, whereas the π phase cannot be stabilized without additional surface modifications.

π-Phase Tin and Germanium Monochalcogenide Semiconductors: An Emerging Materials System

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.