Structural, electronic, and optical properties of three types Ca3N2 from first-principles study

CONTEXT

To find the potential value of Ca3N2 in the field of optoelectronics, the physical properties of Ca3N2 will be analyzed. It can be concluded from the electronic properties that the Ca-N bonds of α-Ca3N2 are more stable than those of δ-Ca3N2 and ε-Ca3N2. The dielectric function, reflectivity function, and absorption function of three types of Ca3N2 were accurately calculated, and it was concluded that α-Ca3N2, δ-Ca3N2, and ε-Ca3N2 have greater transmittance for visible light and exhibit optical transparency in the near-infrared frequency domain. Combined with the high hardness, strong bonding, high melting point, and wear resistance of Ca3N2, Ca3N2 can be used as a new generation of window heat-resistant materials. The α-Ca3N2, δ-Ca3N2, and ε-Ca3N2 are indirect, direct, and indirect narrow bandgap compounds, respectively, that is, δ-Ca3N2 is more suitable for luminescent materials than α-Ca3N2 and ε-Ca3N2. α-Ca3N2 and δ-Ca3N2 have high reflective properties in the ultraviolet region and can be used as UV protective coatings. All three Ca3N2 materials can be used industrially to synthesize photovoltaic devices that operate in the ultraviolet region.

METHODS

Based on the first-principles of density functional theory calculations, the structures, electronic properties, and optical properties of α-Ca3N2, δ-Ca3N2, and ε-Ca3N2 were calculated. The calculation results show that although the α-Ca3N2, δ-Ca3N2, and ε-Ca3N2 have similar electronic structures, some phases have better properties in some aspects.

Study of pnictides for photovoltaic applications

For the transition into a sustainable mode of energy usage, it is important to develop photovoltaic materials that exhibit better solar-to-electricity conversion efficiencies, a direct optimal band gap, and are made of non-toxic, earth abundant elements compared to the state-of-the-art silicon photovoltaics. Here, we explore the non-redox-active pnictide chemical space, including binary A₃B₂, ternary AA'₂B₂, and quaternary AA'A''B₂ compounds (A, A', A'' = Ca, Sr, or Zn; B = N or P), as candidate beyond-Si photovoltaics using density functional theory calculations. Specifically, we evaluate the ground state configurations, band gaps, and 0 K thermodynamic stability for all 20 pnictide compositions considered, besides computing the formation energy of cation vacancies, anion vacancies, and cation anti-sites in a subset of candidate compounds. Importantly, we identify SrZn₂N₂, SrZn₂P₂, and CaZn₂P₂ to be promising candidates, exhibiting optimal (1.1-1.5 eV) hybrid-functional-calculated band gaps, stability at 0 K, and high resistance to point defects (formation energies >1 eV), while other possible candidates include ZnCa₂N₂ and ZnSr₂N₂, which may be susceptible to N-vacancy formation. We hope that our study will contribute to the practical development of pnictide semiconductors as beyond-silicon light absorbers.

Modulating the Combinatorial Target Power of MgSnN2 via RF Magnetron Sputtering for Enhanced Optoelectronic Performance: Mechanistic Insights from DFT Studies

The unique structural features of many ternary nitride materials with strong chemical bonding and band gaps above 2.0 eV are limited and are experimentally unexplored. It is important to identify candidate materials for optoelectronic devices, particularly for light-emitting diodes (LEDs) and absorbers in tandem photovoltaics. Here, we fabricated MgSnN₂ thin films, as promising II-IV-N₂ semiconductors, on stainless-steel, glass, and silicon substrates via combinatorial radio-frequency magnetron sputtering. The structural defects of the MgSnN₂ films were studied as a function of the Sn power density, while the Mg and Sn atomic ratios remained constant. Polycrystalline orthorhombic MgSnN₂ was grown on the (120) orientation within a wide optical band gap range of ∼2.20-2.17 eV. The carrier densities of 2.18× 10²⁰ to 1.02 × 10²¹ cm^-3, mobilities between 3.75 and 2.24 cm²/Vs, and a decrease in resistivity from 7.64 to 2.73 × 10^-3 Ω cm were confirmed by Hall-effect measurements. These high carrier concentrations suggested that the optical band gap measurements were affected by a Burstein-Moss shift. Furthermore, the electrochemical capacitance properties of the optimal MgSnN₂ film exhibited an areal capacitance of 152.5 mF/cm² at 10 mV/s with high retention stability. The experimental and theoretical results showed that MgSnN₂ films were effective semiconductor nitrides toward the progression of solar absorbers and LEDs.

Structural Properties and ELNES of Polycrystalline and Nanoporous Mg3N2

Nanoporous, high-purity magnesium nitride (Mg3N2) was synthesized with a liquid ammonia-based process, for potential applications in optoelectronics, gas separation and catalysis, since these applications require high material purity and crystallinity, which has seldom been demonstrated in the past. One way to evaluate the degree of crystalline near-range order and atomic environment is electron energy-loss spectroscopy (EELS) in a transmission electron microscope. However, there are hardly any data on Mg3N2, which makes identification of electron energy-loss near-edge structure (ELNES) features difficult. Therefore, we have studied nanoporous Mg3N2 with EELS in detail in comparison to EELS spectra of bulk Mg3N2, which was analyzed as a reference material. The N-K and Mg-K edges of both materials are similar. Despite having the same crystal structure, however, there are differences in fine-structural features, such as shifts and absences of peaks in the N-K and Mg-K edges of nanoporous Mg3N2. These differences in ELNES are attributed to coordination changes in nanoporous Mg3N2 caused by the significantly smaller crystallite size of 2-6 nm compared to the larger (25-125 nm) crystal size in a bulk material.

Synthesis of magnesium-nitrogen salts of polynitrogen anions

The synthesis of polynitrogen compounds is of fundamental importance due to their potential as environmentally-friendly high energy density materials. Attesting to the intrinsic difficulties related to their formation, only three polynitrogen ions, bulk stabilized as salts, are known. Here, magnesium and molecular nitrogen are compressed to about 50 GPa and laser-heated, producing two chemically simple salts of polynitrogen anions, MgN4 and Mg2N4. Single-crystal X-ray diffraction reveals infinite anionic polythiazyl-like 1D N-N chains in the crystal structure of MgN4 and cis-tetranitrogen N44- units in the two isosymmetric polymorphs of Mg2N4. The cis-tetranitrogen units are found to be recoverable at atmospheric pressure. Our results respond to the quest for polynitrogen entities stable at ambient conditions, reveal the potential of employing high pressures in their synthesis and enrich the nitrogen chemistry through the discovery of other nitrogen species, which provides further possibilities to design improved polynitrogen arrangements.

Tunable Light Emission through the Range 1.8-3.2 eV and p-Type Conductivity at Room Temperature for Nitride Semiconductors, Ca(Mg1-xZnx)2N2 (x = 0-1)

The ternary nitride CaZn₂N₂, composed only of earth-abundant elements, is a novel semiconductor with a band gap of ∼1.8 eV. First-principles calculations predict that continuous Mg substitution at the Zn site will change the optical band gap in a wide range from ∼3.3-1.9 eV for Ca(Mg_1-xZn_x)₂N₂ (x = 0-1). In this study, we demonstrate that a solid-state reaction at ambient pressure and a high-pressure synthesis at 5 GPa produce x = 0 and 0.12 and 0.12 < x ≤ 1 polycrystalline samples, respectively. It is experimentally confirmed that the optical band gap can be continuously tuned from ∼3.2 to ∼1.8 eV, a range very close to that predicted by theory. Band to band photoluminescence is observed at room temperature in the ultraviolet-red region depending on x. A 2% Na doping at the Ca site of Ca(Mg_1-xZn_x)₂N₂ converts its highly resistive state to a p-type conducting state. Particularly, the x = 0.50 sample exhibits intense green emission with a peak at 2.45 eV (506 nm) without any other emission from deep-level defects. These features meet the demands of III-V group nitride and arsenide/phosphide light-emitting semiconductors.

Design and research of a self-activated orange magnesium boron nitride phosphor with its application in W-LEDs

A self-activated defect-related orange magnesium boron nitride phosphor with novel W-LED potential.

The superior desorption properties of MgCl2-added ammonia borane compared to MgF2-added systems—the unexpected role of MgCl2 interacting with [NH3] units

MgCl₂ unexpectedly interacts with [NH₃] units in NH₃BH₃, analogous to the amine complex of Mg(NH₃)_xCl₂, resulting in superior desorption.

Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping

Aluminum nitride (AlN) is used extensively in the semiconductor industry as a high-thermal-conductivity insulator, but its manufacture is encumbered by a tendency to degrade in the presence of water. The propensity for AlN to hydrolyze has led to its consideration as a redox material for solar thermochemical ammonia (NH3) synthesis applications where AlN would be intentionally hydrolyzed to produce NH3 and aluminum oxide (Al2O3), which could be subsequently reduced in nitrogen (N2) to reform AlN and reinitiate the NH3 synthesis cycle. No quantitative, atomistic mechanism by which AlN, and more generally, metal nitrides react with water to become oxidized and generate NH3 yet exists. In this work, we used density-functional theory (DFT) to examine the reaction mechanisms of the initial stages of AlN hydrolysis, which include: water adsorption, hydroxyl-mediated proton diffusion to form NH3, and NH3 desorption. We found activation barriers (Ea) for hydrolysis of 330 and 359 kJ/mol for the cases of minimal adsorbed water and additional adsorbed water, respectively, corroborating the high observed temperatures for the onset of steam AlN hydrolysis. We predict AlN hydrolysis to be kinetically limited by the dissociation of strong Al-N bonds required to accumulate protons on surface N atoms to form NH3. The hydrolysis mechanism we elucidate is enabled by the diffusion of protons across the AlN surface by a hydroxyl-mediated Grotthuss mechanism. A comparison between intrinsic (Ea = 331 kJ/mol) and mediated proton diffusion (Ea = 89 kJ/mol) shows that hydroxyl-mediated proton diffusion is the predominant mechanism in AlN hydrolysis. The large activation barrier for NH3 generation from AlN (Ea = 330 or 359 kJ/mol, depending on water coverage) suggests that in the design of materials for solar thermochemical ammonia synthesis, emphasis should be placed on metal nitrides with less covalent metal-nitrogen bonds and, thus, more-facile NH3 liberation.

Computational studies of gas-phase Ca3P2 and Ca6P4

The electronic and molecular structures of Ca3P2 and Ca6P4 are investigated using high-level ab initio methods. The lowest energy structure for Ca3P2 is found to be a Jahn-Teller distorted triplet. An excited-state singlet is found with various post HF methods; however, DFT incorrectly predicts a closed shell singlet to be the ground state. For the Ca6P4 system, both DFT and ab initio methods give consistent relative energies. The computational results demonstrate that the energetics are very sensitive to the size of the Ca basis set. Enhancing the Ca basis sets with additional s and p valence functions significantly affects the calculated energies.

Synthesis and characterization of a new ternary imide-Li2Ca(NH)2

The ternary imide Li(2)Ca(NH)2 was successfully synthesized by dehydrogenating a mixture of LiNH(2) and CaH(2) at a molar ratio of 2:1 in a stream of purified argon at 300 degrees C. A powder X-ray diffraction measurement revealed that Li(2)Ca(NH)2 was of the trigonal anti-La(2)O(3) structure (space group Pm1) with lattice constants of a = 3.5664(3)A and c = 5.9540(8) A. Ca occupied the 1b site (0, 0, 1/2), Li occupied the 2d site (1/3, 2/3, 0.8841(22)), and N occupied the 2d site (1/3, 2/3, 0.2565(15)). Nuclear magnetic resonance and X-ray absorption fine structure analyses demonstrated that each Li ion was coordinated with four imide ions and each Ca ion was coordinated with six imide ions.

Carbon nanotubes as nanoreactors for fabrication of single-crystalline Mg3N2 nanowires

Due to fast decomposition of Mg3N2 in the presence of water in the atmosphere (Mg3N2+6H2O-->3Mg(OH)2+2NH3), the synthesis of single-crystalline Mg3N2 nanowires has been a challenge. Here, we demonstrate that carbon nanotubes may serve as nanoreactors for a simple thermal reaction process resulting in the first fabrication of high-quality, large-yield, single-crystalline Mg3N2 nanowires. The Mg3N2 nanowires are homogeneously sheathed over their entire lengths with very thin graphitic carbon tubular layers, which effectively prevent their decomposition (even when the samples are put into water or exposed to atmosphere for several months). We have systematically analyzed for the first time the Mg3N2 nanomaterial by means of transmission electron microscopy (TEM), high-resolution TEM, and electron diffraction. Successful fabrication of carbon sheath protected Mg3N2 nanowires may promote further experimental studies on their crystal structures and properties.