Abstract
Many materials used in energy devices or applications suffer from the problem of electron–hole pair recombination. One promising way to overcome this problem is the use of heterostructures in place of a single material. If an electric dipole forms at the interface, such a structure can lead to a more efficient electron–hole pair separation and thus prevent recombination. Here we model and study a heterostructure comprised of two polymorphs of Fe2O3. Each one of the two polymorphs, α-Fe2O3 and ε-Fe2O3, individually shows promise for applications in photoelectrochemical cells. The heterostructure of these two materials is modeled by means of density functional theory. We consider both ferromagnetic as well as anti-ferromagnetic couplings at the interface between the two systems. Both individual oxides are insulating in nature and have an anti-ferromagnetic spin arrangement in their ground state. The same properties are found also in their heterostructure. The highest occupied electronic orbitals of the combined system are localized at the interface between the two iron-oxides. The localization of charges at the interface is characterized by electrons residing close to the oxygen atoms of ε-Fe2O3 and electron–holes localized on the iron atoms of α-Fe2O3, just around the interface. The band alignment at the interface of the two oxides shows a type-III broken band-gap heterostructure. The band edges of α-Fe2O3 are higher in energy than those of ε-Fe2O3. This band alignment favours a spontaneous transfer of excited photo-electrons from the conduction band of α- to the conduction band of ε-Fe2O3. Similarly, photo-generated holes are transferred from the valence band of ε- to the valence band of α-Fe2O3. Thus, the interface favours a spontaneous separation of electrons and holes in space. The conduction band of ε-Fe2O3, lying close to the valence band of α-Fe2O3, can result in band-to-band tunneling of electrons which is a characteristic property of such type-III broken band-gap heterostructures and has potential applications in tunnel field-effect transistors.
Highlights
At the interface between two different materials one can o en observe new emergent physical properties and phenomena which are not found in the individual materials.[1,2] For example, LaAlO3 and SrTiO3 both are insulating materials, but in a heterostructure, the interface of these systems was found to be metallic.[3]
Where E3+a is the density functional theory (DFT) total energy of the 3- and a-Fe2O3 heterostructure; N3 and Na are the number of formula units of bulk 3Fe2O3 and a-Fe2O3, respectively, in the heterostructure; m3 and ma are the chemical potential of bulk 3-Fe2O3 and a-Fe2O3 per formula unit, respectively; Esurf is the sum of the surface energy of the top and bottom surfaces of the heterostructure system; n is the number of O atoms in excess/de cient (+/À) relative to the 3-Fe2O3 and a-Fe2O3 stoichiometry; mO is the chemical potential of oxygen vapour and taken as half of the chemical potential of oxygen molecule and A is the interface area
Paper surface made due to the slabs of 3-Fe2O3 and a-Fe2O3, respectively, by using the equation[25,27] as
Summary
At the interface between two different materials one can o en observe new emergent physical properties and phenomena which are not found in the individual materials.[1,2] For example, LaAlO3 and SrTiO3 both are insulating materials, but in a heterostructure, the interface of these systems was found to be metallic.[3]. Heterostructures are proven to show a great amount of reduction of electron–hole recombination by separating the two charges.[36,37,38] The energy band alignment of the two materials at the interface of the heterostructure of BiFeO3/3-Fe2O3 is such that it facilitates the separation of electron–hole pairs.[39] Very recently, epitaxial thin lms of a-Fe2O3 was grown on multiferroic 3-Fe2O3 supported on SrTiO3 as a substrate for a possible application as a 4-resistive state multiferroic tunnel junction (MFTJ).[40] Since heterojunctions of semiconductors, insulators or semiconductor–insulator junctions show unique electronic and magnetic properties For this reason, we have explored the heterostructure of two semiconducting oxides, namely the two different polymorphs of Fe2O3. The 2 Â FeO2 units are removed from the bottom surface of a-Fe2O3 in order to make a perfectly coordinated interface consisting of only octahedrally coordinated Fe-atoms
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
