Point Defects in Two-Dimensional Indium Selenide as Tunable Single-Photon Sources

In the past few years remarkable interest has been kindled by the development of nonclassical light sources and, in particular, of single-photon emitters (SPE), which represent fundamental building blocks for optical quantum technology. In this Letter, we analyze the stability and electronic properties of an InSe monolayer with point defects with the aim of demonstrating its applicability as an SPE. The presence of deep defect states within the InSe band gap is verified when considering substitutional defects with atoms belonging to group IV, V, and VI. In particular, the optical properties of Ge as substitution impurity of Se predicted by solving the Bethe-Salpeter equation on top of the GW corrected electronic states show that transitions between the valence band maximum and the defect state are responsible for the absorption and spontaneous emission processes, so that the latter results in a strongly peaked spectrum in the near-infrared. These properties, together with a high localization of the involved electronic states, appear encouraging in the quest for novel SPE materials

First-Principles Calculations of Exciton Radiative Lifetimes in Monolayer Graphitic Carbon Nitride Nanosheets: Implications for Photocatalysis

In this work, we report on the exciton radiative lifetimes of graphitic carbon nitride monolayers in the triazine-based (gC3N4-t) and heptazine-based (gC3N4-h) forms, as obtained by means of ground-state plus excited-state ab initio calculations. By analyzing the exciton fine structure, we highlight the presence of dark states and show that the photogenerated electron-hole (e-h) pairs in gC3N4-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC3N4-h in photocatalysis and makes it attractive for the emerging field of exciton devices. Although very long intrinsic radiative lifetimes are an important prerequisite for several applications, pristine carbon nitride nanosheets show very low quantum photoconversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC3N4-t and gC3N4-h layers, which shows a type-II band alignment and looks promising for achieving net charge separation

Active Surface Structure of SnO2 Catalysts for CO2 Reduction Revealed by Ab Initio Simulations

Tin oxide (SnO2) is an efficient catalyst for the CO2 reduction reaction (CO2RR) to formic acid; however, the understanding of the SnO2 surface structure under working electrocatalytic conditions and the nature of catalytically active sites is a current matter of debate. Here, we employ ab initio density functional theory calculations to investigate how the selectivity and reactivity of SnO2 surfaces toward the CO2RR change at varying surface stoichiometry (i.e., reduction degree). Our results show that SnO2(110) surfaces are not catalytically active for the CO2RR or hydrogen evolution reaction, but rather they reduce under an applied external bias, originating surface structures exposing few metal tin layers, which are responsible for formic acid selectivity

Doped ordered mesoporous carbons as novel, selective electrocatalysts for the reduction of nitrobenzene to aniline

Ordered mesoporous carbons (OMCs) doped with nitrogen, phosphorus or boron were synthesised through a two-step nanocasting method and studied as electrocatalysts for the reduction of nitrobenzene to aniline in a half-cell setup. The nature of the dopant played a crucial role in the electrocatalytic performance of the doped OMCs, which was monitored by LSV with a rotating disk electrode setup. The incorporation of boron generated the electrocatalysts with the highest kinetic current density, whereas the incorporation of phosphorus led to the lowest overpotential. Doping with nitrogen led to intermediate behaviour in terms of onset potential and kinetic current density, but provided the highest selectivity towards aniline, thus resulting in the most promising electrocatalyst developed in this study. Density functional theory calculations allowed explaining the observed difference in the onset potentials between the various doped OMCs, and indicated that both graphiticN and pyrdinic N can generate active sites in the N-doped electrocatalyst. A chronoamperometric experiment over N-doped OMC performed at -0.75 V vs. Fc/Fc(+) in an acidic environment, resulted in a conversion of 61% with an overall selectivity of 87% to aniline. These are the highest activity and selectivity ever reported for an electrocatalyst for the reduction of nitrobenzene to aniline, making N-doped OMC a promising candidate for the electrochemical cogeneration of this industrially relevant product and electricity in a fuel cell setup

Facilely synthesized nitrogen-doped reduced graphene oxide functionalized with copper ions as electrocatalyst for oxygen reduction

Nitrogen-doped reduced graphene oxide is successfully synthesized and functionalized with hydroxylated copper ions via one-pot microwave-assisted route. The presence of cationic Cu coordinated to the graphene layer is fully elucidated through a set of experimental characterizations and theoretical calculations. Thanks to the presence of these hydroxyl-coordinated Cu2+ active sites, the proposed material shows good electrocatalytic performance for the oxygen reduction reaction, as evidenced by an electron transfer number of almost 4 and by high onset and half-wave potentials of 0.91 V and 0.78 V vs. the reversible hydrogen electrode, respectively. In addition, the N-doped Cu-functionalized graphene displays a superior current retention with respect to a commercial Pt/C catalyst during the stability test, implying its potential implementation in high-performance fuel cells and metal-air batteries

A quantum-mechanical study of the adsorption of prototype dye molecules on rutile-TiO2(110): a comparison between catechol and isonicotinic acid

In this work we present a theoretical investigation of the attachment of catechol and isonicotinic acid to the rutile-TiO2(110) surface. These molecules can be considered as prototypical dyes for use in Gra¨tzel type dye sensitised solar cells (DSCs) and are often employed as anchoring groups in both organic and organo-metallic sensitisers of TiO2. Our study focuses on determining the lowest energy adsorption mode and discussing the electronic properties of the resultant hybrid interface by means of density functional theory (DFT) calculations using the hybrid exchange (B3LYP) functional. We find that both molecules adsorb dissociatively at the TiO2 surface giving a type II (staggered) heterojunction. Compared to isonicotinic acid, catechol, due to the greater hybridisation of its molecular orbitals with the states of the substrate, is seen to enhance performance when employed as an anchoring group in dye sensitised solar cells

Unravelling electrocatalytic properties of metal porphyrin-like complexes hosted in graphene matrices

Carbon-based materials are a promising class of catalysts for oxygen and carbon dioxide reduction reactions to value-added chemicals. Here we study the electrocatalytic properties of nitrogen doped graphene structures hosting four pyrrolic rings able to coordinate metal ions in a porphyrin-like configuration. The analysis is carried out by means of density functional theory (DFT) with hybrid functionals, employed to estimate the overpotentials of oxygen reduction reaction to water and hydrogen peroxide, as well as carbon dioxide reduction to carbon monoxide and formic acid. The competing hydrogen evolution reaction is also studied. We predict that Co- and Mn-doped structures exhibit low overpotentials for oxygen reduction to water, with the concurrent suppression of hydrogen peroxide production in the Mn case. Carbon dioxide reduction to formic acid is instead favored by Ti and Mn doping with overpotentials lower than 1 V, while hydrogen evolution reaction is disfavored

Unravelling some of the structure-property relationships in graphene oxide at low degree of oxidation

Graphene oxide (GO) is a versatile 2D material whose properties can be tuned by changing the type and concentration of oxygen-containing functional groups attached to its surface. However, a detailed knowledge of the dependence of the chemo/physical features of this material on its chemical composition is largely unknown. We combine classical molecular dynamics and density functional theory simulations to predict the structural and electronic properties of GO at low degree of oxidation and suggest a revision of the Lerf–Klinowski model. We find that layer deformation is larger for samples containing high concentrations of epoxy groups and that correspondingly the band gap increases. Targeted chemical modification of the GO surface appears to be an effective route to tailor the electronic properties of the monolayer for given applications. Our simulations also show that the chemical shift of the C-1s XPS peak allows one to unambiguously characterize GO composition, resolving the peak attribution uncertainty often encountered in experiments