12 research outputs found
Structural properties of Silicon-Germanium and Germanium-Silicon Core-Shell Nanowires
No full textCore-shell nanowires made of Si and Ge can be grown experimentally with excellent control for different sizes of both core and shell. We have studied the structural properties of Si/Ge and Ge/Si core-shell nanowires aligned along the [110] direction, with diameters up to 10.2 nm and varying core to shell ratios, using linear scaling Density Functional Theory (DFT). We show that Vegard's law, which is often used to predict the axial lattice constant, can lead to an error of up to 1%, underlining the need for a detailed ab initio atomistic treatment of the nanowire structure. We analyse the character of the intrinsic strain distribution and show that, regardless of the composition or bond direction, the Si core or shell always expands. In contrast, the strain patterns in the Ge shell or core are highly sensitive to the location, composition and bond direction. The highest strains are found at heterojunction interfaces and the surfaces of the nanowires. This detailed understanding of the atomistic structure and strain paves the way for studies of the electronic properties of core-shell nanowires and investigations of doping and structure defects
Band alignment and scattering considerations for enhancing the thermoelectric power factor of complex materials : the case of Co-based half-Heusler alloys
No full textProducing high band and valley degeneracy through aligning of conducting electronic bands is an effective strategy to improve the thermoelectric performance of complex band-structure materials. Half-Heuslers, an emerging thermoelectric material group, has complex band structures with multiple bands that can be aligned through band engineering approaches, giving us an opportunity to improve their power factor. Theoretical calculations to identify the outcome of band engineering usually employ detailed density functional theory for band-structure calculations, but the transport calculations are kept simplistic using the constant relaxation time approximation due to the complications involved with detailed scattering physics. In this work, going beyond the constant relaxation time approximation, we perform an investigation of the benefits of band alignment in improving the thermoelectric power factor under different density of states dependent scattering scenarios. As a test case we consider the Co-based p-type half-Heuslers TiCoSb, NbCoSn, and ZrCoSb. First, using simplified effective mass models combined with Boltzmann transport, we investigate the conditions of band alignment that are beneficial to the thermoelectric power factor under three different carrier scattering scenarios: (i) the usual constant relaxation time approximation, (ii) intraband scattering restricted to the current valley with the scattering rates proportional to the density of states as dictated by Fermi's golden rule, and (iii) both intra- and interband scattering across all available valleys, with the rates determined by the total density of states at the relevant energies. We demonstrate that the band-alignment outcome differs significantly depending on the scattering details. Next, using the density functional theory calculated band structures of the half-Heuslers we study their power factor behavior under strain induced band alignment. We show that strain can improve the power factor of half-Heuslers, but the outcome heavily depends on the curvatures of the bands involved, the specifics of the carrier scattering mechanisms, and the initial band separation. Importantly, we also demonstrate that band alignment is not always beneficial to the power factor. In addition, we show that the band structure itself can undergo changes as the bands are aligned in practice, which further affect the band alignment optimization. Our work illustrates the importance of going beyond the constant relaxation time approximation, as well as understanding how the band structure of each material behaves when considering band alignment
Thermoelectric Power Factor Under Strain-Induced Band-Alignment in the Half-Heuslers NbCoSn and TiCoSb
No full textHot Electron Engineering in Nano Structures
No full textWhen electromagnetic
radiation is applied to a nanoparticle, scattering, absorption, and transmission of this radiation can take
place. The absorbed radiation (photons) will increase the kinetic and potential
energy of electrons inside the particle, pushing them into excited states. These
excited electrons with high energy are not in thermal equilibrium with the
lattice of the nanoparticle
and hence are referred to as hot electrons.
Hot electrons are
injected into the surrounding media if their energies are high enough to cross over the energy barrier at the interface
between the nanoparticles and the surroundings. This results in generation of a photocurrent, which is found useful in many opto electronic applications. Therefore,
hot electron generation and injection is a field of high scientific interest.
Localized surface
plasmons (LSPs) generated in metallic nanoparticles via optical excitation create an enhanced electric field inside the
nanoparticle, aiding the hot electron generation process. In smaller metallic nanoparticles, absorption is much more dominant than scattering and a larger proportion
of excited electrons
end up in higher energy levels, making them ideal for hot
electron injection related applications.
As the particle
dimensions are reduced to nanoscale and become comparable to the wavelength of the electron wave function, the energy
levels of electrons become highly discreet and geometrically dependent. The
efficiency and magnitude of the hot electron injection process depends on the energy
spectrum of the electrons, which is determined based on the shape and size of
the nanoparticle. In this thesis, the
shape and size dependent hot electron generation and injection behaviour of metallic nanoparticles, considering the
quantized motion of the conduction electrons, are studied. The results of this study are used to
design a hot electron based all-optical direction-switching device, which
is extremely useful in nanoscale electronic circuitry.
The shape and size
dependence of the electron energy levels cause the dielectric function of the nanoparticle to be geometry dependent as the
dimensions of the particle are reduced. When analysing hot electron
generation and other optical properties of nanoparticles, the dielectric function is a key
input. Therefore, it is important to obtain a realistic dielectric function based
on electron excitations at different frequencies, considering the shape as well as
the size of the particle by following a quantum-mechanical approach. Therefore, as a
final part of this research, the quantum confinement effects on the
frequency-dependent dielectric function of metallic nanoparticles and its influence on hot
electron generation are studied.
This thesis is
organized as follows. An introduction to hot electrons in nanoparticles and the objectives of this research are presented in Chapter
1, followed by a literature review of the mathematical techniques used to
model hot electron generation and injection in Chapter 2. Chapters 3 and 4 present
theoretical analyses of shape- and size-dependent hot electron behaviour and
design guidelines for nanorods and nanotubes, respectively. A design of a novel
all-optical hot electron based current-direction-switching device (CDSD) is presented
in Chapter 5, while Chapter 6 discusses the effects of quantum confinement
on the permittivity of a nanoparticle and how it effects hot electron generation.
Chapter 7 presents a summary of contributions and suggestions for future research
Impact of the scattering physics on the power factor of complex thermoelectric materials
No full textWe assess the impact of the scattering physics assumptions on the thermoelectric properties of five Co-based p-type half-Heusler alloys by considering full energy-dependent scattering times vs the commonly employed constant scattering time. For this, we employ density functional theory band structures and a full numerical scheme that uses Fermi's golden rule to extract the momentum relaxation times of each state at every energy, momentum, and band. We consider electron-phonon scattering (acoustic and optical), as well as ionized impurity scattering, and evaluate the qualitative and quantitative differences in the power factors of the materials compared to the case where the constant scattering time is employed. We show that the thermoelectric power factors extracted from the two different methods differ in terms of (i) their ranking between materials, (ii) the carrier density where the peak power factor appears, and (iii) their trends with temperature. We further show that the constant relaxation time approximation smoothens out the richness in the band structure features, thus limiting the possibilities of exploring this richness for material design and optimization. These details are more properly captured under full energy/momentum-dependent scattering time considerations. Finally, by mapping the conductivities extracted within the two schemes, we provide appropriate density-dependent constant relaxation times that could be employed as a fast first-order approximation for extracting charge transport properties in the half-Heuslers we consider
Erratum: “Impact of the scattering physics on the power factor of complex thermoelectric materials” [J. Appl. Phys. 126, 155701 (2019)]
No full textMaterial descriptors for the discovery of efficient thermoelectrics
No full textThe predictive performance screening of novel compounds can significantly promote the discovery of efficient, cheap, and nontoxic thermoelectric (TE) materials. Large efforts to implement machine-learning techniques coupled to materials databases are currently being undertaken, but the adopted computational methods can dramatically affect the outcome. With regards to electronic transport and power factor (PF) calculations, the most widely adopted and computationally efficient method is the constant relaxation time approximation (CRT). This work goes beyond the CRT and adopts the proper, full energy and momentum dependencies of electron–phonon and ionized impurity scattering to compute the electronic transport and perform PF optimization for a group of half-Heusler alloys. Then, the material parameters that determine the optimal PF based on this more advanced treatment are identified. This enables the development of a set of significantly improved descriptors that can be used in material screening studies, which offer deeper insights into the underlying nature of high-performance TE materials. We have identified nvεr/Do2mcond as the most useful and generic descriptor, a combination of the number of valleys, the dielectric constant, the conductivity effective mass, and the deformation potential for the dominant electron–phonon process. The proposed descriptors can accelerate the discovery of new efficient and environment-friendly TE materials in a much more accurate and reliable manner, and some predictions for very high-performance materials are presented
