Optoelectronics and defect levels in hydroxyapatite by first-principles

Abstract

Hydroxyapatite (HAp) is an important component of mammal bones and teeth, being widely used in prosthetic implants. Despite the importance of HAp in medicine, several promising applications involving this material (e.g., in photo-catalysis) depend on how well we understand its fundamental properties. Among the ones that are either unknown or not known accurately, we have the electronic band structure and all that relates to it, including the bandgap width. We employ state-of-the-art methodologies, including density hybrid-functional theory and many-body perturbation theory within the dynamically screened single-particle Green's function approximation, to look at the optoelectronic properties of HAp. These methods are also applied to the calculation of defect levels. We find that the use of a mix of (semi-)local and exact exchange in the exchange-correlation functional brings a drastic improvement to the band structure. Important side effects include improvements in the description of dielectric and optical properties not only involving conduction band (excited) states but also the valence. We find that the highly dispersive conduction band bottom of HAp originates from anti-bonding σ* states along the ⋯OH-OH-⋯ infinite chain, suggesting the formation of a conductive 1D-ice phase. The choice of the exchange-correlation treatment to the calculation of defect levels was also investigated by using the OH-vacancy as a testing model. We find that donor and acceptor transitions obtained within semi-local density functional theory (DFT) differ from those of hybrid-DFT by almost 2 eV. Such a large discrepancy emphasizes the importance of using a high-quality description of the electron-electron interactions in the calculation of electronic and optical transitions of defects in HAp.

Sub-Band Gap Absorption Mechanisms Involving Oxygen Vacancies in Hydroxyapatite

“…In a recent work, we have shown how the quality of the exchange-correlation treatment affects the above properties. 43 We found out that the use of hybrid density functional theory (hybrid-DFT), which mixes a portion of Fock exchange with a (semi-)local exchangecorrelation potential, leads to electronic properties very close to those obtained using highly accurate many-body perturbation theory within the GW framework. While the GW and hybrid-DFT band gaps were close to 7 eV, generalized gradient approximated results underesti-mated this figure by 30%.…”

Physical Fundamentals of Biomaterials Surface Electrical Functionalization

“…This is implemented in the AIMPRO software package [ 32 ], in which calculations of HAp (initial pure ideal and with defects) were performed [ 22 , 28 ]. Another one is the generalized gradient approximated (GGA) approach, originally developed by Perdew, Burke, and Ernzerhof (because named “PBE”) [ 33 ], which turns out to be more general and accurate in many cases and that was also used in HAp calculations [ 27 , 34 , 35 ] using the VASP software package [ 36 , 37 , 38 ].…”

Defects in Hydroxyapatite: structure and properties, calculations from the first principles

“…Те же влияния определялись и для вакансии ОН [5]. Таким образом, в этих исследованиях, как и ранее в [7,8], ясно показано, что любые дефекты, прежде всего такие как вакансия О (разного типа и группы), а также вакансия целой группы ОН, изменяют физические свойства исходных идеальных чистых кристаллов гексагонального ГАП на фактические реальные образцы ГАП, которые обычно используются в медицинских и промышленных применениях, и имеют множество различных дефектов.…”

“…Рассматриваемые различные гибридные функционалы, однако, дают примерно почти такую же диаграмму энергии образования дефектов. Для уровней энергии электронов и оптических свойств лучший результат дает применение именно B3LYP [2,5].…”