Band-tail states and the localized-to-extended transition in amorphous diamond

Understanding Twist-Angle-Dependent Carrier Lifetimes in MoSSe Bilayer

Two-dimensional (2D) materials with tunable interlayer interactions hold immense potential for optoelectronic and photocatalytic applications. Understanding the dependence of carrier dynamics on twist angle in Janus bilayers is essential, as it directly impacts device efficiency. This study employs time-dependent density functional theory (TD-DFT) and nonadiabatic molecular dynamics (NAMD) to investigate twist-angle-dependent carrier dynamics in Janus MoSSe bilayers with type-II band alignment. Simulations reveal ultrafast charge transfer times of approximately 70 and 500 fs, largely independent of the twist angle due to multiple intermediate states. In contrast, the electron-hole recombination times depend strongly on twist angles, extending up to 133 ns for twisted configurations (21.8° and 38.2°) compared to 57 ns in high-symmetry bilayers (0.0° and 60.0°). Structural randomness in twisted bilayers weakens interlayer interactions, reducing nonadiabatic coupling and coherence time, which collectively prolong carrier lifetimes. These findings offer valuable guidance for designing 2D materials for high-efficiency photovoltaics and long-durable photocatalysts.

Inter-Oligomer Interaction Influence on Photoluminescence in Cis-Polyacetylene Semiconductor Materials

Semiconducting conjugated polymers (CPs) are pivotal in advancing organic electronics, offering tunable properties for solar cells and field-effect transistors. Here, we carry out first-principle calculations to study individual cis-polyacetylene (cis-PA) oligomers and their ensembles. The ground electronic structures are obtained using density functional theory (DFT), and excited state dynamics are explored by computing nonadiabatic couplings (NACs) between electronic and nuclear degrees of freedom. We compute the nonradiative relaxation of charge carriers and photoluminescence (PL) using the Redfield theory. Our findings show that electrons relax faster than holes. The ensemble of oligomers shows faster relaxation compared to the single oligomer. The calculated PL spectra show features from both interband and intraband transitions. The ensemble shows broader line widths, redshift of transition energies, and lower intensities compared to the single oligomer. This comparative study suggests that the dispersion forces and orbital hybridizations between chains are the leading contributors to the variation in PL. It provides insights into the fundamental behaviors of CPs and the molecular-level understanding for the design of more efficient optoelectronic devices.

On the Structure of Oxygen Deficient Amorphous Oxide Films

Understanding defects in amorphous oxide films and heterostructures is vital to improving performance of microelectronic devices, thin-film transistors, and electrocatalysis. However, to what extent the structure and properties of point defects in amorphous solids are similar to those in the crystalline phase are still debated. The validity of this analogy and the experimental and theoretical evidence of the effects of oxygen deficiency in amorphous oxide films are critically discussed. The authors start with the meaning and significance of defect models, such as "oxygen vacancy" in crystalline oxides, and then introduce experimental and computational methods used to study intrinsic defects in amorphous oxides and discuss their limitations and challenges. To test the validity of existing defect models, ab initio molecular dynamics is used with a non-local density functional to model the structure and electronic properties of oxygen-deficient amorphous alumina. Unlike some previous studies, the formation of deep defect states in the bandgap caused by the oxygen deficiency is found. Apart from atomistic structures analogous to crystal vacancies, the formation of more stable defect states characterized by the bond formation between under-coordinated Al ions is shown. The limitations of such defect models and how they may be overcome in simulations are discussed.

Crucial Role of S8-Rings in Structural, Relaxation, Vibrational and Electronic Properties of LiquidSulfur close to the λ Transition

Liquid sulfur has been studied by density-functional based molecular-dynamics simulations at different temperatures ranging from 400 K up to 700 K across the well-documented λ-transition. Structure models containing either a majority of Sn chains or S8 rings are considered and compared to experimental data from X-ray scattering. The comparison suggests a liquid structure of a majority of 2-fold sulfur at low temperature, dominated by S8 rings that open progressively upon temperature increase. Typical features associated with such rings are analyzed and indicate that they contribute to a specific third correlating distance in the pair correlation function and to a contribution at low wavevector k in reciprocal space. The vibrational properties of liquid sulfur are also considered and indicate a contribution at 60 meV that is associated with both chains and rings, albeit the latter lead to a more intense peak at this wavenumber. The underlying network structure also impacts the dynamic properties of the melts which display enhanced dynamic heterogeneities when S8 rings are present. The analysis of the electronic Kohn-Sham energies shows insulating character with a gap of about ≃2.0 eV, albeit the presence of localized mid-gap states is acknowledged that can be associated, in part, with the presence of S6rings.

Revealing the intrinsic nature of the mid-gap defects in amorphous Ge2Sb2Te5

Understanding the relation between the time-dependent resistance drift in the amorphous state of phase-change materials and the localised states in the band gap of the glass is crucial for the development of memory devices with increased storage density. Here a machine-learned interatomic potential is utilised to generate an ensemble of glass models of the prototypical phase-change alloy, Ge₂Sb₂Te₅, to obtain reliable statistics. Hybrid density-functional theory is used to identify and characterise the geometric and electronic structures of the mid-gap states. 5-coordinated Ge atoms are the local defective bonding environments mainly responsible for these electronic states. The structural motif for the localisation of the mid-gap states is a crystalline-like atomic environment within the amorphous network. An extra electron is trapped spontaneously by these mid-gap states, creating deep traps in the band gap. The results provide significant insights that can help to rationalise the design of multi-level-storage memory devices.

Ab initio computer simulations of non-equilibrium radiation-induced cascades in amorphous Ge2Sb2Te5

Ion irradiation corresponds to a process that involves the production of non-equilibrium cascades in the host material, and the atomistic modelling of such events in glasses is challenging. Here, non-equilibrium cascades in amorphous Ge₂Sb₂Te₅ phase-change memory material have been investigated by means of first-principles molecular-dynamics simulations. A stochastic boundary-conditions approach is employed to treat the thermal nature of the cascades and drive the modelled system back to equilibrium in a natural way, while four different initial thermal-spike energies are considered. A comprehensive analysis of the cascade evolution is presented with respect to the kinetic profile and the dynamics of the cascade inside the glass structure. The modelling results show that the instantaneous maximum kinetic energy decays rapidly with time, and that the time-scale of the ballistic phase of the cascade inside the glass model is very short. The quality of the implemented approach is validated through a comparison of the calculated structure factor for the modelled glasses with experimental data from the literature. Analysis of the bonding for all the species in the glass structure highlights particular structural modifications in the connectivity of the amorphous network due to the simulated cascade.

Intrinsic charge trapping in amorphous oxide films: status and challenges

We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states stemming from the disorder of amorphous metal oxide networks. We start from presenting the results for amorphous (a) HfO₂, chosen due to the availability of highest purity amorphous films, which is vital for studying their intrinsic electronic properties. Exhaustive photo-depopulation spectroscopy measurements and theoretical calculations using density functional theory shed light on the atomic nature of electronic gap states responsible for deep electron trapping observed in a-HfO₂. We review theoretical methods used for creating models of amorphous structures and electronic structure calculations of amorphous oxides and outline some of the challenges in modeling defects in amorphous materials. We then discuss theoretical models of electron polarons and bi-polarons in a-HfO₂ and demonstrate that these intrinsic states originate from low-coordinated ions and elongated metal-oxygen bonds in the amorphous oxide network. Similarly, holes can be captured at under-coordinated O sites. We then discuss electron and hole trapping in other amorphous oxides, such as a-SiO₂, a-Al₂O₃, a-TiO₂. We propose that the presence of low-coordinated ions in amorphous oxides with electron states of significant p and d character near the conduction band minimum can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides. Finally, we demonstrate that bi-electron trapping in a-HfO₂ and a-SiO₂ weakens Hf(Si)-O bonds and significantly reduces barriers for forming Frenkel defects, neutral O vacancies and O^2- ions in these materials. These results should be useful for better understanding of electronic properties and structural evolution of thin amorphous films under carrier injection conditions.

Intrinsic electron trapping in amorphous oxide

We demonstrate that electron trapping at intrinsic precursor sites is endemic in non-glass-forming amorphous oxide films. The energy distributions of trapped electron states in ultra-pure prototype amorphous (a)-HfO₂ insulator obtained from exhaustive photo-depopulation experiments demonstrate electron states in the energy range of 2-3 eV below the oxide conduction band. These energy distributions are compared to the results of density functional calculations of a-HfO₂ models of realistic density. The experimental results can be explained by the presence of intrinsic charge trapping sites formed by under-coordinated Hf cations and elongated Hf-O bonds in a-HfO₂. These charge trapping states can capture up to two electrons, forming polarons and bi-polarons. The corresponding trapping sites are different from the dangling-bond type defects responsible for trapping in glass-forming oxides, such as SiO₂, in that the traps are formed without bonds being broken. Furthermore, introduction of hydrogen causes formation of somewhat energetically deeper electron traps when a proton is immobilized next to the trapped electron bi-polaron. The proposed novel mechanism of intrinsic charge trapping in a-HfO₂ represents a new paradigm for charge trapping in a broad class of non-glass-forming amorphous insulators.

Theoretical modeling of charge trapping in crystalline and amorphous Al2O3

The characteristics of intrinsic electron and hole trapping in crystalline and amorphous Al₂O₃ have been studied using density functional theory (DFT). Special attention was paid to enforcing the piece-wise linearity of the total energy with respect to electron number through the use of a range separated, hybrid functional PBE0-TC-LRC (Guidon et al 2009 J. Chem. Theory Comput. 5 3010) in order to accurately model the behaviour of localized states. The tuned functional is shown to reproduce the geometric and electronic structures of the perfect crystal as well as the spectroscopic characteristics of Mg_Al hole centre in corundum α-Al₂O₃. An ensemble of ten amorphous Al₂O₃ structures was generated using classical molecular dynamics and a melt and quench method and their structural characteristics compared with the experimental data. The electronic structure of amorphous systems was characterized using the inverse participation ratio method. Electrons and holes were then introduced into both crystalline and amorphous alumina structures and their properties calculated. Holes are shown to trap spontaneously in both crystalline and amorphous alumina. In the crystalline phase they localize on single O ion with the trapping energy of 0.38 eV. In amorphous phase, holes localize on two nearest neighbour oxygen sites with an average trapping energy of 1.26 eV, with hole trapping sites separated on average by about 8.0 Å. No electron trapping is observed in the material. Our results suggest that trapping of positive charge can be much more severe and stable in amorphous alumina rather than in crystalline samples.

Atomistic origin of urbach tails in amorphous silicon

Exponential band edges have been observed in a variety of materials, both crystalline and amorphous. In this Letter, we infer the structural origins of these tails in amorphous and defective crystalline Si by direct calculation with current ab initio methods. We find that exponential tails appear in relaxed models of diamond silicon with suitable extended defects that emerge from relaxing point defects. In amorphous silicon (a-Si), we find that structural filaments of short bonds and long bonds exist in the network, and that the tail states near the extreme edges of both band tails are also filamentary, with much localization on the structural filaments. We connect the existence of both filament systems to structural relaxation in the presence of defects and of topological disorder.

The structure of electronic states in amorphous silicon

We illustrate the structure and dynamics of electron states in amorphous Si. The nature of the states near the gap at zero temperature is discussed and especially the way the structure of the states changes for energies ranging from midgap into either band tail (Anderson transition). We then study the effect of lattice vibrations on the eigenstates, and find that electronic states near the optical gap can be strongly influenced by thermal modulation of the atomic positions. Finally, we show the structure of generalized Wannier functions for amorphous Si, which are of particular interest for efficient ab initio calculation of electronic properties and forces for first principles dynamic simulation.