Controlling Dopant Distributions and Structures in Advanced Semiconductors

The suitability of silicon for micro and sub-micro electronic devices is being challenged by the aggressive and continuous downscaling of device feature size. New materials with superior qualities are continually sought-after. In this thesis, defects are examined in two sets of silicon alternate materials; germanium (Ge) and III-V semiconductors. Point defects are of crucial importance in understanding and controlling the properties of these electronic materials. Point defects usually introduce energy levels into the band gap, which influence the electronic performance of the material. They are also key in assisting mass transport. Here, atomistic scale computational methods are employed to investigate the formation and migration of defects in Ge and III-V semiconductors. The behaviour of n-type dopants coupled to a vacancy in Ge (known as E-centres) is reported from thermodynamic and kinetic points of view, revealing that these species are highly mobile, consequently, a strategy is proposed to retard one of the n-dopants. Further, the electronic structure of Ge is examined and the changes induced in it due to the application of different types of strain along different planes and directions. The results obtained agree with established experimental values regarding the bands transition from indirect to direct under biaxial strain. This is used to support further predictions, which indicate that a moderate strain parallel to the [111] direction can efficiently transform Ge into a direct band gap material, with a band gap energy useful for technological applications. Vacancies and antisites in III-V semiconductors have been studied under various growth and doping conditions. Results presented in this thesis help predict and explain the stability of some defects over a range of growth conditions. This, together with knowledge of the kinetics of migration of Ga and As/Sb vacancies is used to explain the disparities in self-diffusion between GaAs and GaSb.Open Acces

Sc and Nb dopants in SrCoO3 modulate electronic and vacancy structures for improved water splitting and SOFC cathodes

SrCoO3 is a promising material in the field of electrocatalysis. Difficulties in synthesising the material in its cubic phase have been overcome by doping it with Sc and Nb ions [Mater. Horiz. 2015, 2, 495-501]. Using ab initio calculations and special quasi random structures we undertake a systematic study of these dopants in order to elucidate the effect of doping on electronic structure of the SrCoO3 host and the formation of oxygen vacancies. We find that while the overall electronic structure of SrCoO3 is preserved, increasing the Sc fraction leads to a decrease of electrical conductivity, in agreement with earlier experimental work. For low Sc and Nb doping fractions we find that the oxygen vacancy formation increases relative to undoped SrCoO3. However, as the dopants concentration is increased the vacancy formation energy drops significantly, indicating a strong tendency to accommodate high concentration of oxygen vacancies and hence non-stoichiometry. This is explained based on the electronic instabilities caused by the presence of Sc ions which weakens the B-O interactions as well as the increased degree of electron delocalization on the oxygen sublattice. Sc dopants also shift the p-band centre closer to the Fermi level, which can be associated with experimentally reported improvements in oxygen evolution reactions. These findings provide crucial baseline information for the design of better electrocatalysts for oxygen evolution reactions as well as fuel-cell cathode materials

Boosting oxygen evolution reaction by activation of lattice‐oxygen sites in layered Ruddlesden‐Popper oxide

Emerging anionic redox chemistry presents new opportunities for enhancing oxygen evolution reaction (OER) activity considering that lattice-oxygen oxidation mechanism (LOM) could bypass thermodynamic limitation of conventional metal-ion participation mechanism. Thus, finding an effective method to activate lattice-oxygen in metal oxides is highly attractive for designing efficient OER electrocatalysts. Here, we discover that the lattice-oxygen sites in Ruddlesden-Popper (RP) crystal structure can be activated, leading to a new class of extremely active OER catalyst. As a proof-of-concept, the RP Sr3(Co0.8Fe0.1Nb0.1)2O7-δ (RP-SCFN) oxide exhibits outstanding OER activity (eg, 334 mV at 10 mA cm−2 in 0.1 M KOH), which is significantly higher than that of the simple SrCo0.8Fe0.1Nb0.1O3-δ perovskite and benchmark RuO2. Combined density functional theory and X-ray absorption spectroscopy studies demonstrate that RP-SCFN follows the LOM under OER condition, and the activated lattice oxygen sites triggered by high covalency of metal-oxygen bonds are the origin of the high catalytic activity.This work was financially supported by the Australian Research Council (Discovery Early Career Researcher Award No. DE190100005)

Unraveling the Factors Behind the Efficiency of Hydrogen Evolution in Endohedrally Doped C-60 Structures via Ab Initio Calculations and Insights from Machine Learning Models

Understanding the origins of catalytic activity (or inactivity) in nanostructures allows for the rational design of cheap and durable catalysts. Here, consistent and comprehensive ab initio screening of endohedrally doped fullerenes as potential catalysts for hydrogen evolution reactions is performed. By examining variations in the electronic structure of the carbon atoms in the presence of the dopant, and by relying on machine learning algorithms, the origin of enhanced activity in fullerenes can be underpinned. The effect is attributed to the formation of free radicals by weakening the CC double bonds. A number of electronic descriptors are discussed which can be fed into machine learning models to efficiently and reliably predict catalytic activities. This allows for a generalization of trends and a predictive ability that could be applied to other fullerene structures

Facile CO Oxidation on Oxygen-functionalized MXenes via the Mars-van Krevelen Mechanism

Ab initio screening of 2D MXenes was performed to identify potentially active candidates for CO oxidation. Unlike singleatom catalysts or metal nanoclusters on reducible metal oxides, we find that a number of oxygen terminated MXenes can intrinsically facilitate the CO oxidation via lattice oxygen participation, also known as the Mars-van Krevelen mechanism. The promising candidates Cr3C2O2, V3C2O2, Sc3C2O2 and Mo3C2O2 are predicted to have very favorable reaction steps, and as they are not made from any noble metals, they have the potential of offering an alternative to the currently used noble metal alloy catalysts. This work offers design strategies based on linking intrinsic electronic properties of the sorbent to its ability to participate in lattice mediated CO oxidation

Conductive graphitic carbon nitride as an ideal material for electrocatalytically switchable CO2 capture

Good electrical conductivity and high electron mobility of the sorbent materials are prerequisite for electrocatalytically switchable CO2 capture. However, no conductive and easily synthetic sorbent materials are available until now. Here, we examined the possibility of conductive graphitic carbon nitride (g-C4N3) nanosheets as sorbent materials for electrocatalytically switchable CO2 capture. Using first-principle calculations, we found that the adsorption energy of CO2 molecules on g-C4N3 nanosheets can be dramatically enhanced by injecting extra electrons into the adsorbent. At saturation CO2 capture coverage, the negatively charged g-C4N3 nanosheets achieve CO2 capture capacities up to 73.9 × 1013 cm−2 or 42.3 wt%. In contrast to other CO2 capture approaches, the process of CO2 capture/release occurs spontaneously without any energy barriers once extra electrons are introduced or removed, and these processes can be simply controlled and reversed by switching on/off the charging voltage. In addition, these negatively charged g-C4N3 nanosheets are highly selective for separating CO2 from mixtures with CH4, H2 and/or N2. These predictions may prove to be instrumental in searching for a new class of experimentally feasible high-capacity CO2 capture materials with ideal thermodynamics and reversibility

Unveiling hidden charge density waves in single-layer NbSe2 by impurities

We employ ab initio calculations to investigate the charge density waves in single-layer NbSe2, and we explore how they are affected by transition metal atoms. Our calculations reproduce the observed orthorhombic phase in single-layer NbSe2 in the clean limit, establishing the energy order between three different distorted structures, two consisting of triangular Nb-Nb clusters and a third, energetically unfavored, consisting of hexagonal Nb-Nb clusters. Such energy order, in agreement with known experimental work, is reversed by the adsorption of Co and Mn, which favor the formation of hexagonal Nb-Nb clusters; this CDW structure is indeed allowed from a symmetry point of view but hidden in pure single layers because it is at a higher energy. The other adsorbates, K and Ga, still favor one of the triangular Nb-Nb cluster, while suppressing the other. We report how the energy difference between such distorted structure varies with these adsorbates. Furthermore, transition metals induce magnetism and favor the reduction of the symmetry of the charge density distribution. ©2018 American Physical Societ

p‑Doped Graphene/Graphitic Carbon Nitride Hybrid Electrocatalysts: Unraveling Charge Transfer Mechanisms for Enhanced Hydrogen Evolution Reaction Performance

Recently, hybrid electrocatalyst systems involving an active layer of <i>g</i>-C₃N₄ on a conductive substrate of N-doped graphene (<i>g</i>-C₃N₄@NG) have been shown to achieve excellent efficiency for the hydrogen evolution reaction (HER) [e.g., Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Z. Nat. Commun. 2014, 5, 3783]. We demonstrate here through first principle calculations examining various hybrid <i>g</i>-C₃N₄@MG (M = B, N, O, F, P. and S) electrocatalysts that the N-doped case may be regarded as an example of a more general modulation doping strategy – by which either electron donating or electron withdrawing features induced in the substrate can be exploited to promote the HER. Despite the intrinsically cathodic nature of the HER, our study reveals that <i>all</i> of the graphene substrates have an increasingly electron withdrawing influence on the <i>g</i>-C₃N₄ active layer as H atom coverage increases, modulating binding of the H atom intermediates, the overpotential, and the likely operational coverage. In this context, it is not surprising that p-doping of the substrate can further enhance the effect. Our calculations show that B is the most promising doping element for <i>g</i>-C₃N₄@MG (M = B, N, O, F, P, and S) electrocatalysts due to the predicted overpotential of 0.06 eV at full coverage and a large interfacial adhesion energy of −1.30 eV, offering prospects for significant improvement over the n-dopant systems such as <i>g</i>-C₃N₄@NG that have appeared in the literature to date. These theoretical results reveal a more general principle for rational design of hybrid electrocatalysts, via manipulation of the Fermi level of the underlying conductive substrate

Borophene as a Promising Material for Charge-Modulated Switchable CO₂ Capture

Ideal carbon dioxide (CO₂) capture materials for practical applications should bind CO₂ molecules neither too weakly to limit good loading kinetics nor too strongly to limit facile release. Although charge-modulated switchable CO₂ capture has been proposed to be a controllable, highly selective, and reversible CO₂ capture strategy, the development of a practical gas-adsorbent material remains a great challenge. In this study, by means of density functional theory (DFT) calculations, we have examined the possibility of conductive borophene nanosheets as promising sorbent materials for charge-modulated switchable CO₂ capture. Our results reveal that the binding strength of CO₂ molecules on negatively charged borophene can be significantly enhanced by injecting extra electrons into the adsorbent. At saturation CO₂ capture coverage, the negatively charged borophene achieves CO₂ capture capacities up to 6.73 × 10¹⁴ cm^–2. In contrast to the other CO₂ capture methods, the CO₂ capture/release processes on negatively charged borophene are reversible with fast kinetics and can be easily controlled via switching on/off the charges carried by borophene nanosheets. Moreover, these negatively charged borophene nanosheets are highly selective for separating CO₂ from mixtures with CH₄, H₂, and/or N₂. This theoretical exploration will provide helpful guidance for identifying experimentally feasible, controllable, highly selective, and high-capacity CO₂ capture materials with ideal thermodynamics and reversibility