Hydride-ion-doped mayenite, 12CaO·7Al2O3 (C12A7:H-), undergoes a permanent insulator-to-conductor conversion under ultraviolet irradiation: hydride ions substituted into the sub-nanometre cages of the nanoporous calcium-aluminate framework are photoionized, releasing electrons that keep the material conducting at room temperature long after the light is removed. Since the 2002 discovery, the microscopic mechanism has remained a hypothesis, and neither the primary photochemical dynamics, the fate of the liberated hydrogen, the quantitative dose-conductivity relation, the maximum conductivity of the optical route, nor the transferability of the concept to other main-group metal oxides has been fully established. This problem asks for (i) direct measurement of the H- photoionization dynamics and the experimental identification of the hydrogen reaction pathway (H2 in cages versus OH- bound to the framework), (ii) determination of the nature, spatial extent and mobility of the photogenerated electronic states, (iii) a quantitative map connecting hydride concentration, ultraviolet dose and persistent conductivity, including its ceiling and ambient-temperature stability, and (iv) demonstration of the light-induced hydride-to-electron conversion in further main-group oxide hosts. First-principles and kinetic studies have substantially constrained the mechanism, but a direct measurement of the primary photochemical event and of the hydrogen fate is still missing.

Background

Transparent conducting oxides combine visible transparency with electrical conduction and are indispensable in displays, solar cells and low-emissivity coatings. The established route is aliovalent cation substitution in oxides of transition or post-transition metals, the archetype being tin-doped In2O3 (ITO); this route does not work for the abundant transparent oxides of main-group metals such as CaO and Al2O3, which remain deep insulators. Finding a way to make main-group oxides conduct, and in a way that can be patterned by light, was the motivation of the originating experiment.

The host material is mayenite, 12CaO·7Al2O3 (written C12A7), a refractory constituent of Portland cement clinker. Its crystal structure is a nanoporous framework with stoichiometry [Ca24Al28O64]4+ per unit cell, built of corner-sharing Ca-O and Al-O polyhedra and containing twelve sub-nanometre cages (about 0.4 nm inner diameter). Two cages per unit cell are, on average, occupied by 'free' O2- anions that do not belong to the framework and balance its positive charge; these encaged anions can be exchanged for other monovalent species (O-, OH-, H-, and even electrons), which makes mayenite a chemically versatile 'anion host'. When the encaged anions are electrons, the compound is an electride, a crystal in which electrons themselves occupy anion lattice sites; the cage-wall potential then acts as the electron trap.

In the originating work (Hayashi, Matsuishi, Kamiya, Hirano and Hosono, Nature 419, 462-465, 2002), hydride ions (H-) were incorporated into the cages by a thermal treatment in hydrogen gas. Subsequent irradiation with ultraviolet light converted the transparent insulator into an electronic conductor persisting after irradiation ceased, with a moderate room-temperature conductivity of roughly 0.3 S/cm and only about one per cent visible absorption loss in 200-nm-thick films. The proposed mechanism was that each cage H- is photoionized, releasing an electron into the cage network; the neutralized hydrogen was proposed to recombine into molecular H2, while the freed electrons occupy cage states described by a simple 'electron in coupled potential wells' model, related to the F+ colour centre (an electron trapped at an anion-vacancy site, known from alkaline-earth oxides; in C12A7 the empty cage plays the role of the anion vacancy), and conduction was ascribed to hopping of electrons between cages. The authors explicitly suggested that the same concept - light-induced generation of persistent conduction in an insulating main-group oxide - applies to other main-group metal oxides, for optical writing of conducting wires and high-density optical memory.

Two adjacent results frame the problem. First, the same group showed by chemical (non-optical) routes that the cages can be filled with high-density electron anions, producing the C12A7 electride whose conductivity is ultimately metallic; the persistent conductor made by light therefore sits between the insulating hydride and the fully electron-loaded electride. Second, persistent photoconductivity - a light-induced conducting state that survives after illumination - was previously known mainly in compound semiconductors, not in stable, wide-band-gap refractory oxides, and the mechanism proposed for C12A7:H- has never been established by direct experiment. The unresolved question is the microscopic mechanism and quantitative capability of this light-induced hydride-to-electron conversion.

Problem Statement

Establish, by direct experiment and quantitative theory, the microscopic mechanism and the process limits of the ultraviolet-light-induced insulator-to-conductor conversion of hydride-ion-doped mayenite, 12CaO·7Al2O3 (C12A7:H-). The components of this single objective are: (1) determine the primary photochemical dynamics - the H- photoionization event in the cage, the charge states and reaction pathway of the liberated hydrogen (recombination to molecular H2 in the cages versus capture by the framework as OH-, via H0 and H+ intermediates), and the sequence of reactions that leaves behind the persistent electron population; (2) determine the nature and spatial extent of the photogenerated electronic states (localized cage-centred states hopping between cages versus a partially delocalized cage-conduction-band state) and the associated carrier mobility and hopping paths; (3) establish a quantitative map connecting hydride-ion concentration, ultraviolet photon dose and the resulting persistent conductivity, including the maximum conductivity and mobility reachable by the optical route and the long-term stability of the conductive state under ambient conditions (the map must at minimum account for the influence of defect chemistry and co-encaged anions, and may additionally treat the influence of grain boundaries and film morphology); and (4) test the authors' suggestion that the same light-induced hydride-to-electron conversion can be realized in other main-group metal oxides with cage or channel structures. Components (1)-(3) jointly establish the mechanism of the conversion in C12A7:H-, and (4) establishes its generality in at least one further main-group oxide host of cage or channel structure.

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