Small Polarons in Two-Dimensional Pnictogens: A First-Principles Study

Recent Progresses of Polarons: Fundamentals and Roles in Photocatalysis and Photoelectrocatalysis

Photocatalysis and photoelectrocatalysis are promising ways in the utilization of solar energy. To address the low efficiency of photocatalysts and photoelectrodes, in-depth understanding of their catalytic mechanism is in urgent need. Recently, polaron is considered as an influential factor in catalysis, which brings researchers a new approach to modify photocatalysts and photoelectrodes. In this review, brief introduction of polaron is given first, followed by which models and recent experimentally observations of polarons are reviewed. Studies about roles of polarons in photocatalysis and photoelectrocatalysis are listed in order to provide some inspiration in exploring the mechanism and improving the efficiency of photocatalysis and photoelectrocatalysis.

Modifying the electronic and magnetic properties of the scandium nitride semiconductor monolayer via vacancies and doping

In this work, the effects of vacancies and doping on the electronic and magnetic properties of the stable scandium nitride (ScN) monolayer are investigated using first-principles calculations. The pristine monolayer is a two-dimensional (2D) indirect-gap semiconductor material with an energy gap of 1.59(2.84) eV as calculated using the GGA-PBE (HSE06) functional. The projected density of states, charge distribution, and electron localization function assert its ionic character generated by the charge transfer from the Sc atoms to the N atoms. The monolayer is magnetized by a single Sc vacancy with a total magnetic moment of 3.00μB, while a single N vacancy causes a weaker magnetization with a total magnetic moment of 0.52μB. In both cases, the magnetism originates mainly from the atoms closest to the defect site. Significant magnetization is also reached by doping with acceptor impurities. Specifically, a total magnetic moment of 2.00μB is obtained by doping with alkali metals (Li and Na) in the Sc sublattice and with B in the N sublattice. Doping with alkaline earth metals (Be and Mg) in the Sc sublattice and with C in the N sublattice induces a value of 1.00μB. In these cases, either magnetic semiconducting or half-metallicity characteristics arise in the ScN monolayer, making it a prospective 2D spintronic material. In contrast, no magnetism is induced by doping with donor impurities (O and F atoms) in the N sublattice. An O impurity metallizes the monolayer; meanwhile, F doping leads to a large band-gap reduction of the order of 82%, widening the working regime of the monolayer in optoelectronic devices. The results presented herein may introduce efficient methods to functionalize the ScN monolayer for optoelectronic and spintronic applications.

Atomic-scale manipulation of single-polaron in a two-dimensional semiconductor

Polaron is a composite quasiparticle derived from an excess carrier trapped by local lattice distortion, and it has been studied extensively for decades both theoretically and experimentally. However, atomic-scale creation and manipulation of single-polarons in real space have still not been achieved so far, which precludes the atomistic understanding of the properties of polarons as well as their applications. Herein, using scanning tunneling microscopy, we succeeded to create single polarons in a monolayer two-dimensional (2D) semiconductor, CoCl₂. Combined with first-principles calculations, two stable polaron configurations, centered at atop and hollow sites, respectively, have been revealed. Remarkably, a series of manipulation progresses - from creation, erasure, to transition - can be accurately implemented on individual polarons. Our results pave the way to understand the physics of polaron at atomic level, and the easy control of single polarons in 2D semiconductor may open the door to 2D polaronics including the data storage.

Spectroscopic signatures of nonpolarons: the case of diamond

Polarons are quasi-particles made from electrons interacting with vibrations in crystal lattices. They derive their name from the strong electron-vibration polar interactions in ionic systems, that induce spectroscopic and optical signatures of such quasi-particles. In this paper, we focus on diamond, a non-polar crystal with inversion symmetry which nevertheless shows interesting signatures stemming from electron-vibration interactions, better denoted "nonpolaron" signatures in this case. The (non)polaronic effects are produced by short-range crystal fields, while long-range quadrupoles only have a small influence. The corresponding many-body spectral function has a characteristic energy dependence, showing a plateau structure that is similar to but distinct from the satellites observed in the polar Fröhlich case. We determine the temperature-dependent spectral function of diamond by two methods: the standard Dyson-Migdal approach, which calculates electron-phonon interactions within the lowest-order expansion of the self-energy, and the cumulant expansion, which includes higher orders of electron-phonon interactions. The latter corrects the nonpolaron energies and broadening, providing a more realistic spectral function, which we examine in detail for both conduction and valence band edges.