Search

Layered oxides exhibit high ionic mobility and chemical flexibility, attracting interest as cathode materials for lithium-ion batteries and the pairing of hydrogen production and carbon capture. Recently, layered oxides emerged as highly tunable semiconductors. For example, by introducing anti-Frenkel defects, the electronic hopping conductance in hexagonal manganites was increased locally by orders of magnitude. Here, we demonstrate local acceptor and donor doping in Er(Mn,Ti)O$_3$, facilitated by the splitting of such anti-Frenkel defects under applied d.c. voltage. By combining density functional theory calculations, scanning probe microscopy, atom probe tomography, and scanning transmission electron microscopy, we show that the oxygen defects readily move through the layered crystal structure, leading to nano-sized interstitial-rich (p-type) and vacancy-rich (n-type) regions. The resulting pattern is comparable to dipolar npn-junctions and stable on the timescale of days. Our findings reveal the possibility of temporarily functionalizing oxide semiconductors at the nanoscale, giving additional opportunities for the field of oxide electronics and the development of transient electronics in general.

The family of hexagonal manganites is intensively studied for its multiferroicity, magnetoelectric coupling, improper ferroelectricity, functional domain walls, and topology-related scaling behaviors. It is established that these physical properties are co-determined by the cation sublattices and that aliovalent doping can readily be leveraged to modify them. The doping, however, also impacts the anion defect chemistry and semiconducting properties, which makes the system highly sensitive to the synthesis and processing conditions. Here, we study the electronic properties of YMnO3 as function of aliovalent cation doping and thermoatmospheric history, combining density functional theory calculations with thermopower and thermogravimetric measurements. We show that the charge carrier concentration and transport properties can be controlled via both aliovalent cation dopants and anion defects, enabling reversible switching between n-type and p-type conductivity. This tunability is of importance for envisaged applications of hexagonal manganites in, e.g. next-generation capacitors and domain-wall nanoelectronics, or as catalysts or electrodes in fuel cells or electrolyzers. Furthermore, our approach is transferrable to other transition metal oxides, providing general guidelines for controlling their semiconducting properties., Comment: 6 figures

We use density functional calculations to investigate the accommodation and migration of oxygen vacancies in bulk hexagonal YMnO$_3$, and to study interactions between neutral ferroelectric domain walls and oxygen vacancies. Our calculations show that oxygen vacancies in bulk YMnO$_3$ are more stable in the Mn-O layers than in the Y-O layers. Migration barriers of the planar oxygen vacancies are high compared to oxygen vacancies in perovskites, and to previously reported values for oxygen interstitials in h-YMnO$_3$. The calculated polarization decreases linearly with vacancy concentration, while the out-of-plane lattice parameter expands in agreement with previous experiments. In contrast with ferroelectric perovskites, oxygen vacancies are found to be more stable in bulk than at domain walls. The tendency of oxygen vacancies to segregate away from neutral domain walls is explained by unfavorable Y-O bond lengths caused by the local strain field at the domain walls., Comment: 9 pages, 6 figures

Ferroelectric domain walls are attracting broad attention as atomic-scale switches, diodes and mobile wires for next-generation nanoelectronics. Charged domain walls in improper ferroelectrics are particularly interesting as they offer multifunctional properties and an inherent stability not found in proper ferroelectrics. Here we study the energetics and structure of charged walls in improper ferroelectric YMnO$_3$, InMnO$_3$ and YGaO$_3$ by first principles calculations and phenomenological modeling. Positively and negatively charged walls are asymmetric in terms of local structure and width, reflecting that polarization is not the driving force for domain formation. The wall width scales with the amplitude of the primary structural order parameter and the coupling strength to the polarization. We introduce general rules for how to engineer $n$- and $p$-type domain wall conductivity based on the domain size, polarization and electronic band gap. This opens the possibility of fine-tuning the local transport properties and design $p$-$n$-junctions for domain wall-based nano-circuitry., Comment: 10 pages, 6 figures, Supp. Info. available on request