Yucheng Huang

Spectroscopic Distinctions between Two Types of Ce3+ Ions in X2-Y2SiO5: A Theoretical Investigation

The Ce(3+) ions occupying the two crystallographically distinct Y(3+) sites both with C1 point group symmetry in the X2-Y2SiO5 (X2-YSO) crystal are discriminated by their spectroscopic properties calculated with ab initio approaches and phenomenological model analyses. Density functional theory (DFT) calculations with the supercell approach are performed to obtain the local structures of Ce(3+), based on which the wave function-based embedded cluster calculations at the CASSCF/CASPT2 level are carried out to derive the 4f → 5d transition energies. From the ab initio calculated energy levels and wave functions, the crystal-field parameters (CFPs) and the anisotropic g-factor tensors of Ce(3+) are extracted. The theoretical results agree well with available experimental data. The structural and spectroscopic properties for the two types of Ce(3+) ions in X2-YSO are thus distinguished in terms of the calculated local atomic structures, 4f → 5d transition energies, and spectral parameters.

First-principles study of Ce-doped Y3Al5O12 with Si–N incorporation: electronic structures and optical properties

Incorporation of Si–N into Ce-doped Y3Al5O12 (YAG:Ce) has previously been shown to give distinct lower-energy emission but with stronger thermal quenching than the typical yellow YAG:Ce emission. Here, we investigate geometric and electronic structures of Ce and Si–N co-doped YAG with first-principles methods, to gain microscopic insight into effects of Si–N addition on electronic structures and optical properties of Ce3+. Hybrid density functional theory (DFT) calculations reveal that the Si–N prefers to be substituted for the tetrahedral Al(tet)–O sites with a random distribution, among which the nearest-neighbor (NN) SiAl(tet)–NO substitutions with the NO coordinated to Ce3+ result in a slight upward shift of the 4f1 ground-state level with respect to the host valence band. Wave function-based CASSCF/CASPT2 calculations at the spin–orbit level show that the NN SiAl(tet)–NO substitutions induce a redshift of the lowest energy Ce3+ 4f(1) → 5d(1) transition, in agreement with experimental observations. The redshift originates from an increase in the 5d crystal field splitting and a decrease in the 5d centroid energy of Ce3+ in comparable magnitude. Combining these results, we find that the energy separation between the lowest Ce3+ 5d(1) level and the host conduction band minimum (CBM) remains largely unchanged upon the NN SiAl(tet)–NO substitution, thus excluding the thermal ionization of the 5d electron as the underlying mechanism for the temperature quenching of the lower-energy Ce3+ emission. This finding also suggests that the thermally activated crossover from the 5d(1) to the 4f1 states could be responsible for the luminescence quenching, which is also consistent with present calculated results.

Methane dehydrogenation on Au/Ni surface alloys – a first-principles study

We present density-functional theory calculations of the dehydrogenation of CHx (x = 1–4) on Au-alloyed Ni(211) surfaces, where the Au atoms are substituted on the Ni surface with the ratio of Au atoms to the total stepped Ni atoms being 1:4, 1:2 and 3:4, respectively. To evaluate the role of Au at the step-edge on the process of methane dehydrogenation, CHx adsorption and dissociation on a pure Ni(211) surface is also conducted. Our results show that Au addition weakens the adsorbate–substrate interaction. With the increase of the Au concentration, the binding energies of CHx gradually decrease and correlate well with the number of Au atoms on each model. On the Ni(211) surface, methane experiences a successive dehydrogenation process at the step-edge site in which carbon is eventually formed. As Au is introduced, the relative formation rate of carbon is greatly hampered even with a small amount of Au addition, while an appropriate amount of Au modification on the Ni catalyst has little effect on the activity of the CHx dissociation. Finally, we also demonstrate that the active center for CHx dissociation is dynamic with the variation of the Au concentration.

SnS2 nanotubes: a promising candidate for the anode material for lithium ion batteries

First-principles calculations were employed to investigate the adsorption and diffusion of lithium atoms (Li) on various SnS2 nanostructures, i.e., bulk, bilayer, monolayer, nanoribbons and nanotubes. Our results show that on the SnS2 bulk and bilayer, Li adsorption is more stable than the counterparts of the monolayer, nanoribbons and nanotubes, but the diffusion is unfavorable. Although the SnS2 monolayer can greatly increase the mobility of Li, its adsorption strength is relatively weak with respect to other nanostructures. When cutting the monolayer into one-dimensional zigzag nanoribbons, the binding energies of Li do not increase, leading to them being excluded as an electrode material for Li-ion batteries. Interestingly, when rolling the monolayer into one-dimensional nanotubes, the adsorption strength is enhanced and the diffusion of Li atoms becomes kinetically favorable. Therefore, SnS2 nanotubes would be expected to be a very promising anode material in Li-ion batteries.

First‐Principles Study on Doping of SnSe2 Monolayers

Doping is a vitally important technique that can be used to modulate the properties of two-dimensional materials. In this work, by using first-principles density functional calculations, we investigated the electrical properties of SnSe2 monolayers by p-type/n-type and isoelectronic doping. Substitution at Sn/Se sites was found to be easy if the monolayer was grown under Sn-/Se-poor conditions. Substitutions at Sn sites with metallic atoms (e.g. Ga, Ge, In, Bi, Sb, Pb) resulted in positive substitution energies, which indicated that they were not effective doping candidates. For substitutions at Se sites with nonmetallic atoms, no promising candidates were found for p-type doping (e.g., N, P, As). Among these, N and As showed positive substitution energies. Although P had a negative substitution energy under Sn-rich conditions, it introduced trap states within the band gap. For n-type doping (e.g., F, Cl, Br), all the calculated substitution energies were negative under both Sn- and Se-rich conditions. Br was proven to be a promising candidate, because the impurity introduced a shallow donor level. Finally, for isoelectronic doping (e.g., O, S, Te), the intrinsic semiconducting features of the SnSe2 monolayer did not change, and the contribution from the impurity to the states near the band edge increased with the atomic number.

Edge-, width- and strain-dependent semiconductor–metal transition in SnSe nanoribbons

First-principles calculations were employed to explore the electronic properties of SnSe nanoribbons. Our results showed that a semiconductor–metal or metal–semiconductor transition can be realized in SnSe nanoribbons by controlling the edge shape, width parameter and different levels of strain. It was found that the transition always occurs in SnSe nanoribbons with a zigzag edge (ZNR). With the width parameter of ZNRs less than 8, their optimized structures are perfect (P-structures) without any Sn–Se bond breaking and the electronic calculations demonstrate they display a metallic character. However, the structures became deformed (D-structures) with some Sn–Se bond rupture when tensile strain was applied, accompanied by the transition occurring from metal to indirect band gap semiconductors. On the other hand, compressive strain cannot induce the metal–semiconductor transition as ZNRs still keep their P-structures. With the width parameter greater than or equal to 8, ZNRs change to D-structure exhibiting a semiconductive feature at equilibrium state. The semiconductor–metal transition cannot be induced through applying tensile strain while a certain extent of compressive strain can trigger it. The localized–delocalized partial charge distribution of the conduction band minimum near the strained domains can be used to explain the metal–semiconductor or semiconductor–metal transition in SnSe ZNRs. Our results suggest that the SnSe ZNRs have potential applications in nanoelectromechanical sensors and switches, which will promote further experimental investigations on SnSe and other fascinating graphene-like metal chalcogenides.

First-principles study on intrinsic defects of SnSe

The formation energies and electronic properties of intrinsic defects of SnSe, including two vacancies (VSn and VSe), two interstitials (Sni and Sei) and two antisites (SnSe and SeSn), are investigated by using density functional theory (DFT) calculations. The results indicate that, due to a relatively low formation energy as well as a desirable ultra-shallow transition energy level, VSn can act as an effective source for p-type conduction under both Sn- and Se-rich conditions, which implies that SnSe is a native p-type semiconductor. On the other hand, a native n-type conduction is unlikely to be realized due to the absence of effective intrinsic sources. In addition, all the three types of intrinsic defects are not capable of inducing magnetism.

Tuning electronic and magnetic properties of SnSe2 armchair nanoribbons via edge hydrogenation

First-principles calculations were carried out to investigate the electronic and magnetic properties of SnSe2 armchair nanoribbons (ANRs) via edge hydrogenation. Interestingly, at different hydrogenation degrees, SnSe2 ANRs exhibit versatile electronic and magnetic properties, i.e., from nonmagnetic-semiconductors to magnetic-semiconductors or nonmagnetic-metals. Through the analysis from spatial spin distribution and density of states, these transitions are well interpreted. Moreover, the relative stabilities of these ANRs were evaluated by the thermodynamic phase diagram where the Gibbs free energies as a function of the chemical potential of the H2 molecule at different temperatures were plotted. Our results show that hydrogenation is a well-controlled way to modify the physical properties of SnSe2 ANRs. Through controlling chemical potential or partial pressure of H2, the different hydrogenation degrees of ANRs are thermodynamically stable, thus, one can arbitrarily steer their electronic and magnetic properties. The diverse electronic phases and magnetic properties endow the hydrogenated SnSe2 ANRs with potential applications in nanoelectronic devices.

Tuning the Carrier Confinement in GeS/Phosphorene van der Waals Heterostructures

Van der Waals (vdW) heterostructures, which have the advantage of integrating excellent properties of the stacked 2D materials by vdW interactions, have gained increasing attention recently. In this work, within the framework of density functional theory calculations, the electronic properties of vdW heterostructure composed of phosphorene (BP) in black phosphorus phase and GeS monolayer are systematically explored. The results show that the carriers are not separated for both lattice-match and lattice-mismatch heterostructures. For the lattice-match heterostructure, it is found that changing monolayer of GeS to bilayer can increase the energy difference of valence band offsets between GeS and BP, thus realizing electron-hole separation. For the lattice-mismatch heterostructure, altering the layer distance can transform the heterostructure into a typical type-I alignment, but applying the electric field or doping with 2, 3, 5, 6-tetrafluoro-7, 7, 8, 8-tetracyanoquinodimethane (F4TCNQ) can make it display a perfect desirable type-II alignment, where holes migration and electrons transfer are revealed to account respectively for the phenomenon of carrier separation. It is believed that the work would greatly enlarge the potential application of the BP-based heterostructures in photoelectronics and further stimulate the investigation enthusiasms on other fashionable heterostructures and even unassuming heterostructures in which the charming electronic properties can be modulated to emerge by various general methods.

Complete Separation of Carriers in the GeS/SnS Lateral Heterostructure by Uniaxial Tensile Strain

The strategy of forming lateral heterostructures by stitching various two-dimensional materials overcomes the limitations due to the restricted properties of single-component materials. In this work, by using first-principles calculations, the electronic properties of GeS/SnS lateral heterostructures, together with the effect of strain, were systematically investigated. The results showed that with increasing tensile strain along the zigzag direction the band gap displays an extremely interesting variation: it linearly increases in the beginning until 2.4% strain (region I), then remains nearly constant until 5.7% (region II), and finally linearly decreases within the tensile limit (region III). Meanwhile, the electronic properties successively change from quasi-type II alignment to direct band gap to type II alignment with complete carrier separation. Analysis of the densities of states and partial charge densities indicates that the band gap increase in region I is due to the change in the orbital contributions to the states of the conduction band minimum (CBM) from Sn-pz to Sn-px, whereas the band gap decrease in region III is caused by an increasingly loose distribution of antibonding electrons at the CBM. Moreover, it was found that the changes in the orbital constituents from Sn-pz to Sn-px in the CBM and from S-px to S-py in the valence band maximum are responsible for the indirect-direct and direct-indirect band gap crossovers at the junctions of regions I and II and regions II and III, respectively. Finally, through calculations of the carrier concentrations on the basis of deformation potential theory, electrons and holes are demonstrated to be largely separated with the enhancement of strain, and the predicted electron mobilities in the armchair direction at 7% strain are as high as 5860-11 220 cm2 V-1 s-1. We believe our work may lead to potential applications for GeS-SnS heterostructures in electronics, optoelectronics, and straintronics.