8 research outputs found
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, CuSbS<sub>2</sub> (Chalcostibite), Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> (Tetrahedrite), Cu<sub>3</sub>SbS<sub>3</sub> (Skinnerite), and Cu<sub>3</sub>SbS<sub>4</sub> (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 10<sup>5</sup> cm<sup>–1</sup>. We have for the first
time developed facile colloidal hot-injection methods for the phase-pure
synthesis of nanocrystals of all four phases. Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> and Cu<sub>3</sub>SbS<sub>3</sub> 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 CuSbS<sub>2</sub> and Cu<sub>3</sub>SbS<sub>4</sub>, respectively). The synthesis
methods yield nanocrystals with distinct morphology for the different
phases. CuSbS<sub>2</sub> is synthesized as nanoplates, and Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> is isolated as hollow structures,
while uniform spherical Cu<sub>3</sub>SbS<sub>3</sub> and oblate spheroid
nanocrystals of Cu<sub>3</sub>SbS<sub>4</sub> 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 CuSbS<sub>2</sub> and Cu<sub>3</sub>SbS<sub>4</sub> are
indirect band gap materials but with somewhat higher band gap values
of 1.6 and 2.5 eV, respectively. Similarly, Cu<sub>3</sub>SbS<sub>3</sub> 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 Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> 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
10<sup>4</sup> and 10<sup>5</sup> cm<sup>–1</sup>, confirming
their potential for solar energy conversion applications
Co<sub><i>x</i></sub>Cu<sub>1–<i>x</i></sub>Cr<sub>2</sub>S<sub>4</sub> 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<sub><i>x</i></sub>Cu<sub>1–<i>x</i></sub>Cr<sub>2</sub>S<sub>4</sub> have been synthesized
over the entire composition range by a facile solution-based method.
While CuCr<sub>2</sub>S<sub>4</sub> is a ferromagnetic metal, CoCr<sub>2</sub>S<sub>4</sub> 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>‑MoSe<sub>2</sub> 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>-MoSe<sub>2</sub> 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>-MoSe<sub>2</sub>). 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>-MoSe<sub>2</sub> 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>‑MoSe<sub>2</sub> 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>-MoSe<sub>2</sub> 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>-MoSe<sub>2</sub>). 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>-MoSe<sub>2</sub> 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