Selective Nanocrystal Synthesis and Calculated Electronic Structure of All Four Phases of Copper–Antimony–Sulfide

A wide variety of copper-based semiconducting chalcogenides have been investigated in recent years to address the need for sustainable solar cell materials. An attractive class of materials consisting of nontoxic and earth abundant elements is the copper–antimony–sulfides. The copper–antimony–sulfide system consists of four major phases, namely, CuSbS₂ (Chalcostibite), Cu₁₂Sb₄S₁₃ (Tetrahedrite), Cu₃SbS₃ (Skinnerite), and Cu₃SbS₄ (Fematinite). All four phases are p-type semiconductors having energy band gaps between 0.5 and 2 eV, with reported large absorption coefficient values over 10⁵ cm^–1. We have for the first time developed facile colloidal hot-injection methods for the phase-pure synthesis of nanocrystals of all four phases. Cu₁₂Sb₄S₁₃ and Cu₃SbS₃ are found to have direct band gaps (1.6 and 1.4 eV, respectively), while the other two phases display indirect band gaps (1.1 and 1.2 eV for CuSbS₂ and Cu₃SbS₄, respectively). The synthesis methods yield nanocrystals with distinct morphology for the different phases. CuSbS₂ is synthesized as nanoplates, and Cu₁₂Sb₄S₁₃ is isolated as hollow structures, while uniform spherical Cu₃SbS₃ and oblate spheroid nanocrystals of Cu₃SbS₄ are obtained. In order to understand the optical and electrical properties, we have calculated the electronic structures of all four phases using the hybrid functional method (HSE 06) and PBE generalized gradient approximation to density functional theory. Consistent with experimental results, the calculations indicate that CuSbS₂ and Cu₃SbS₄ are indirect band gap materials but with somewhat higher band gap values of 1.6 and 2.5 eV, respectively. Similarly, Cu₃SbS₃ is determined to be a direct band gap material with a gap of 1.5 eV. Interestingly, both PBE and HSE06 methods predict metallic behavior in fully stoichiometric Cu₁₂Sb₄S₁₃ phase, with opening up of bands leading to semiconducting or insulating behavior for off-stoichiometric compositions with a varying number of valence electrons. The absorption coefficient values at visible wavelengths for all the phases are estimated to range between 10⁴ and 10⁵ cm^–1, confirming their potential for solar energy conversion applications

Co_<i>x</i>Cu_1–<i>x</i>Cr₂S₄ Nanocrystals: Synthesis, Magnetism, and Band Structure Calculations

Spin-based transport in semiconductor systems has been proposed as the foundation of a new class of spintronic devices. For the practical realization of such devices, it is important to identify new magnetic systems operating at room temperature that can be readily integrated with standard semiconductors. A promising class of materials for this purpose is magnetic chromium-based chalcogenides that have the spinel structure. Nanocrystals of Co_<i>x</i>Cu_1–<i>x</i>Cr₂S₄ have been synthesized over the entire composition range by a facile solution-based method. While CuCr₂S₄ is a ferromagnetic metal, CoCr₂S₄ is known to be a ferrimagnetic semiconductor. Systematic changes in the lattice parameter, size, and magnetic properties of the nanocrystals are observed with composition. The nanocrystals are magnetic over the entire range, with a decrease in the magnetic transition temperature with increasing Co content. Band structure calculations have been carried out to determine the electronic and magnetic structure as a function of composition. The results suggest that ferrimagnetic alignment of the Co and Cr moments results in a decrease in magnetization with increasing Co concentration

Electron-Beam-Induced Synthesis of Hexagonal 1<i>H</i>‑MoSe₂ from Square β‑FeSe Decorated with Mo Adatoms

Two-dimensional (2D) materials have generated interest in the scientific community because of the advanced electronic applications they might offer. Powerful electron beam microscopes have been used not only to evaluate the structures of these materials but also to manipulate them by forming vacancies, nanofragments, and nanowires or joining nanoislands together. In this work, we show that the electron beam in a scanning transmission electron microscope (STEM) can be used in yet another way: to mediate the synthesis of 2D 1<i>H</i>-MoSe₂ from Mo-decorated 2D β-FeSe and simultaneously image the process on the atomic scale. This is quite remarkable given the different crystal structures of the reactant (square lattice β-FeSe) and the product (hexagonal lattice 1<i>H</i>-MoSe₂). The feasibility of the transformation was first explored by theoretical calculations that predicted that the reaction is exothermic. Furthermore, a theoretical reaction path to forming a stable 1<i>H</i>-MoSe₂ nucleation kernel within pure β-FeSe was found, demonstrating that the pertinent energy barriers are smaller than the energy supplied by the STEM electron beam

Electron-Beam-Induced Synthesis of Hexagonal 1<i>H</i>‑MoSe₂ from Square β‑FeSe Decorated with Mo Adatoms

Two-dimensional (2D) materials have generated interest in the scientific community because of the advanced electronic applications they might offer. Powerful electron beam microscopes have been used not only to evaluate the structures of these materials but also to manipulate them by forming vacancies, nanofragments, and nanowires or joining nanoislands together. In this work, we show that the electron beam in a scanning transmission electron microscope (STEM) can be used in yet another way: to mediate the synthesis of 2D 1<i>H</i>-MoSe₂ from Mo-decorated 2D β-FeSe and simultaneously image the process on the atomic scale. This is quite remarkable given the different crystal structures of the reactant (square lattice β-FeSe) and the product (hexagonal lattice 1<i>H</i>-MoSe₂). The feasibility of the transformation was first explored by theoretical calculations that predicted that the reaction is exothermic. Furthermore, a theoretical reaction path to forming a stable 1<i>H</i>-MoSe₂ nucleation kernel within pure β-FeSe was found, demonstrating that the pertinent energy barriers are smaller than the energy supplied by the STEM electron beam

Directed Atom-by-Atom Assembly of Dopants in Silicon

The ability to controllably position single atoms inside materials is key for the ultimate fabrication of devices with functionalities governed by atomic-scale properties. Single bismuth dopant atoms in silicon provide an ideal case study in view of proposals for single-dopant quantum bits. However, bismuth is the least soluble pnictogen in silicon, meaning that the dopant atoms tend to migrate out of position during sample growth. Here, we demonstrate epitaxial growth of thin silicon films doped with bismuth. We use atomic-resolution aberration-corrected imaging to view the as-grown dopant distribution and then to controllably position single dopants inside the film. Atomic-scale quantum-mechanical calculations corroborate the experimental findings. These results indicate that the scanning transmission electron microscope is of particular interest for assembling functional materials atom-by-atom because it offers both real-time monitoring and atom manipulation. We envision electron-beam manipulation of atoms inside materials as an achievable route to controllable assembly of structures of individual dopants

Directed Atom-by-Atom Assembly of Dopants in Silicon

The ability to controllably position single atoms inside materials is key for the ultimate fabrication of devices with functionalities governed by atomic-scale properties. Single bismuth dopant atoms in silicon provide an ideal case study in view of proposals for single-dopant quantum bits. However, bismuth is the least soluble pnictogen in silicon, meaning that the dopant atoms tend to migrate out of position during sample growth. Here, we demonstrate epitaxial growth of thin silicon films doped with bismuth. We use atomic-resolution aberration-corrected imaging to view the as-grown dopant distribution and then to controllably position single dopants inside the film. Atomic-scale quantum-mechanical calculations corroborate the experimental findings. These results indicate that the scanning transmission electron microscope is of particular interest for assembling functional materials atom-by-atom because it offers both real-time monitoring and atom manipulation. We envision electron-beam manipulation of atoms inside materials as an achievable route to controllable assembly of structures of individual dopants

Directed Atom-by-Atom Assembly of Dopants in Silicon

The ability to controllably position single atoms inside materials is key for the ultimate fabrication of devices with functionalities governed by atomic-scale properties. Single bismuth dopant atoms in silicon provide an ideal case study in view of proposals for single-dopant quantum bits. However, bismuth is the least soluble pnictogen in silicon, meaning that the dopant atoms tend to migrate out of position during sample growth. Here, we demonstrate epitaxial growth of thin silicon films doped with bismuth. We use atomic-resolution aberration-corrected imaging to view the as-grown dopant distribution and then to controllably position single dopants inside the film. Atomic-scale quantum-mechanical calculations corroborate the experimental findings. These results indicate that the scanning transmission electron microscope is of particular interest for assembling functional materials atom-by-atom because it offers both real-time monitoring and atom manipulation. We envision electron-beam manipulation of atoms inside materials as an achievable route to controllable assembly of structures of individual dopants

Directed Atom-by-Atom Assembly of Dopants in Silicon

The ability to controllably position single atoms inside materials is key for the ultimate fabrication of devices with functionalities governed by atomic-scale properties. Single bismuth dopant atoms in silicon provide an ideal case study in view of proposals for single-dopant quantum bits. However, bismuth is the least soluble pnictogen in silicon, meaning that the dopant atoms tend to migrate out of position during sample growth. Here, we demonstrate epitaxial growth of thin silicon films doped with bismuth. We use atomic-resolution aberration-corrected imaging to view the as-grown dopant distribution and then to controllably position single dopants inside the film. Atomic-scale quantum-mechanical calculations corroborate the experimental findings. These results indicate that the scanning transmission electron microscope is of particular interest for assembling functional materials atom-by-atom because it offers both real-time monitoring and atom manipulation. We envision electron-beam manipulation of atoms inside materials as an achievable route to controllable assembly of structures of individual dopants