AlN Monolayer Research Articles

Novel porous aluminum nitride monolayer: a first-principles study

Using ab initio calculations within the density functional theory, we explored the possible structures and properties of porous AlN monolayer materials. Two kinds of porous AlN monolayers (H- and T-) are identified. The phonon dispersion spectra together with the ab initio molecular dynamics simulations demonstrate that these structures are stable. We further show that the H- and T-AlN porous monolayers have well-defined porous nanostructures and high specific surface areas of 2863 m2 g−1 and 2615 m2 g−1 respectively, which is comparable to graphene (2630 m2 g−1), and can be maintained stably at high temperatures (>1300 K). Furthermore, both porous monolayers exhibit semiconductor properties, with indirect band gaps of 2.89 eV and 2.86 eV respectively. In addition, the electronic structures of the porous monolayers can be modulated by strain. The band gap of porous T-AlN monolayer experiences an indirect–direct transition when biaxial strain is applied. A moderate −9% compression can trigger this gap transition. These results indicate that porous AlN monolayers may potentially be used in future optoelectronic and catalyst applications.

Monolayer Nitrides Doped with Transition Metals as Efficient Catalysts for Water Oxidation: The Singular Role of Nickel

Exploration of precious-metal-free catalysts for water splitting is of great importance in developing renewable energy conversion and storage technologies. In this paper, on the basis of density functional theory calculations, we reveal the link between the oxygen evolution reaction (OER) activities and the electronic properties of pure and first-row transition-metal (TM)-doped AlN and GaN two-dimensional monolayers. We find that Ni-doped layers are singularly appealing because they lead to a low overpotential (0.4 V). Early TM dopants are not suited for the OER because they bind the intermediate species OH or O too strongly, which leads to very large overpotentials, or no OER activity at all. The late TM dopants Cu and Zn show less or no OER activity as they bind the intermediate species too weakly. Although in many cases the overpotential can be traced back to an OOH intermediate species being adsorbed too weakly compared to an OH species, the Ni dopant breaks this rule by stabilizing the OOH adsorbant. The stabilization can be correlated with a switch from a high-spin to a low-spin state of the dopant atom. This ability to change spin states offers an exciting ingredient for the design of OER catalysts.

Efficient Band Gap Engineering and Enhanced Optical Absorption of Vertical Germanene/h‐AlN Bilayer Heterostructure: Potential New Electronics and Optoelectronics

In this contribution, the structural, electronic, and optical properties of Ge/AlN heterostructure (HTS) using DFT calculations have been systematically investigated. The binding energies suggest it falling into the range of van der Waals (vdW) HTS. The changing interlayer distances are employed to obtain the modulated band gaps of 3–141 meV, confirming the interface engineering an efficient means to present significant semiconductor properties of the two‐dimensional (2D) devices. The results further demonstrate that both the external electric field (E‐field) and in‐plane strain can efficiently tune the band gaps of the Ge/AlN HTS. The charge transfer at the interface region causes a built‐in electric field (Eint), contributing to the efficient separation of the photoexcited electron–hole pairs and enhancing the carrier mobility in the systems. More interestingly, the presence of the linear evolution of the band gap under electric field indicates its application prospective in pressure sensors. Besides, a semiconductor‐to‐metal transition is observed for the Ge/AlN HTS through the application of an external E‐field, which implies its wide potential application in nano‐electronic and semiconductor devices. Relatively enhanced optical absorption is presented for the Ge/AlN HTS, respectively, in the visible range than isolated AlN monolayer, and in the ultraviolet light zone than Ge monolayer. These results predict enormous significance for the Ge/AlN HTS in the field of energy conversion and functional devices.

Morphology of multilayer AlN/SiN coatings with layers of different thickness

Monolayer AlN coating and multilayer AlN/SiN coatings with single layer thickness of 10 nm and 5 nm were formed using reactive magnetron sputtering. The surface microstructure of these coatings was investigated using atomic force microscopy. Roughness, adhesion forces, and friction coefficient for different layers of coatings were determined.

Vertical GaN nanocolumns grown on graphene intermediated with a thin AlN buffer layer

We report on the self-assembled growth of high-density and vertically-oriented n-doped GaN nanocolumns on graphene by radio-frequency plasma-assisted molecular beam epitaxy. Graphene was transferred to silica glass, which was used as the substrate carrier. Using a migration enhanced epitaxy grown AlN buffer layer for the nucleation is found to enable a high density of vertical GaN nanocolumns with c-axis growth orientation on graphene. Furthermore, micro-Raman spectroscopy indicates that the AlN buffer reduces damage on the graphene caused by impinging active N species generated by the radio-frequency plasma source during the initial growth stage and nucleation of GaN. In addition, the grown GaN nanocolumns on graphene are found to be virtually stress-free. Micro-photoluminescence measurements show near band-edge emission from wurtzite GaN, exhibiting higher GaN bandgap related photoluminescence intensity relative to a reference GaN bulk substrate and the absence of both yellow luminescence and excitonic defect emission. Transmission electron microscopy reveals the interface of GaN nanocolumns on graphene via a thin AlN buffer layer. Even though the first few monolayers of AlN on top of graphene are strained due to in-plane lattice mismatch between AlN and graphene, the grown GaN nanocolumns have a wurtzite crystal structure without observable defects. The results of this initial work pave the way towards realizing low-cost and high-performance electronic and optoelectronic devices based on III-N semiconductors grown on graphene.

Effect of defects on adsorption characteristics of AlN monolayer towards SO2 and NO2: Ab initio exposure

Density functional theory (DFT) calculations have been employed to explore the adsorption properties of pristine and vacancy-defective AlN monolayer towards SO2 and NO2 in terms of their adsorption energies and electronic structures. The calculations with van der Waals effect are performed to study the binding mechanism of pristine and vacancy-defective AlN nanosheet with SO2 and NO2. Three types of vacancy defects are considered including Al-monovacancy, N-monovacancy and divacancy. The adsorption energies of vacancy-defective AlN monolayer towards SO2 and NO2 are found to be much higher than that of pristine nanosheet. The equilibrium adsorption sites and configurations of the gas molecules on AlN nanosheets are determined. The Bader charge transfer analysis reveals that a considerable amount of charge is transferred from the vacancy-defective AlN nanosheets to the gases leading to the increase of adsorption energies. The results of difference charge densities, band structures, electronic density of states and work function further give insight into adsorption energies. The dramatical changes of electronic band structures after gas interaction suggest the excellent adsorption properties of vacancy-defective AlN monolayer towards SO2 and NO2.

First-principles prediction of ferromagnetism in transition-metal doped monolayer AlN

Two-dimensional (2D) magnetic materials with large spin polarization and high Curie temperature (Tc) are essential and indispensable for quantum computation, logic and memory operations, and spintronic devices at nanoscale. In this study, using the first-principles calculations based on the spin polarized density functional theory with the generalized gradient approximation (GGA) plus Hubbard-U corrections (GGA + U), we investigated the magnetic and electronic properties of 2D AlN sheets doped with different atoms of 3d transition metal (TM) elements. Our calculations demonstrate that these TM-doped 2D monolayer AlN sheets are magnetic semiconductors with integer magnetic moment. The magnetic moments of these doped systems originate mainly from the TM atoms, following the Hund's rule and crystal field theory. Furthermore, we find that the magnetic coupling in the Mn- and Ni-doped 2D AlN sheets is ferromagnetic, while the magnetic coupling in the other TM-doped 2D AlN sheets is anti-ferromagnetic. For the ferromagnetic and semiconducting TM-doped monolayer AlN sheets, the calculated Curie temperatures are above 400 K, exceeding that (155 K) of the most-studied dilute magnetic GaMnAs material. Our studies suggest that the Mn- and Ni-doped 2D monolayer AlN sheets would show very promising applications in the spintronic device at nanoscale.

Lithium and sodium adsorption properties of two-dimensional aluminum nitride

In this work the lithiation and sodiation properties of 2-dimensional (2D) AlN sheets are studied from density functional theory (DFT) simulations. 2D AlN showed theoretical specific capacity of 500.8 and 385.3 mA h g−1, maximum open circuit voltage of 1.49 and 1.86 V and diffusion barriers 0.40 and 0.15 eV, for Li and Na adsorption respectively. The calculations show 2D AlN as a possible alternative as anode material in Li-ion and Na-ion batteries. Further the high specific capacity and small diffusion barriers for Na atoms can make 2D AlN useful in supercapacitors. The change in carrier transport properties due to Li/Na adsorption on monolayer AlN can also be useful in chemical/bio-sensors and nanoelectronics devices.

Improved visible-light absorbance of monolayer MoS2 on AlN substrate and its angle-dependent electronic structures

In this paper, we performed density functional theory (DFT) calculations to investigate the geometric structures, electronic structures and visible-light absorbance of MoS2/AlN heterostructure based on van der Waals interaction. The calculated formation energy indicated that the designed MoS2/AlN heterostructure could be experimentally prepared. The Mo-N stacked MoS2/AlN heterostructure exhibited more considerable optical absorption in visible-light region than MoS2 and AlN monolayers. More interestingly, the band gaps were sensitive to strain, which led to an obvious shift of optical absorption spectra along the direction of the infrared region. The two most energetically favorable twisted MoS2/AlN heterostructures (Mo-N and Mo-HAl) had similar band structures, which were different from the non-twisted MoS2/AlN heterostructure. With different rotation angles, their band structures all exhibited an indirect band gap and almost had the same values of indirect band gaps, indicating that the indirect band gaps had no clear dependence on rotation angles.

Chemical and substitutional doping, and anti-site and vacancy formation in monolayer AlN and GaN.

We investigated the effects of chemical/substitutional doping, hydrogenation, and anti-site and vacancy defects on the atomic, optoelectronic and magnetic properties of AlN and GaN monolayers. Upon doping of selected atoms, AlN and GaN monolayers can acquire magnetic properties, and their fundamental band gaps are modified by the localized gap states. Spin-polarized gap states broaden into bands at patterned coverage of adatoms, whereby half-metallic or magnetic semiconducting properties can be attained. Specific adatoms adsorbed to Ga atoms break the nearest vertical Ga-N bonds in the GaN bilayer in the heackelite structure and result in changes in the electronic and atomic structure. While adjacent and distant pairs of anion + cation vacancies induce spin polarization with filled and empty gap states, anti-site defects remain nonmagnetic; but both defects induce dramatic changes in the band gap. Fully hydrogenated monolayers are stable only for specific buckled geometries, where one geometry can also lead to an indirect to direct band gap transition. Also, optical activity shifts to the ultra-violet region upon hydrogenation of the monolayers. While H2 and O2 molecules are readily physisorbed on the surfaces of the monolayers with weak van der Waals attraction, they can be dissociated into constituent atoms at the vacancy site of the cation. Our study performed within density functional theory shows that the electronic, magnetic and optical properties of AlN and GaN monolayers can be tuned by doping and point defect formation in order to acquire diverse functionalities.

Study of Thin Film Coatings by IR Phonon-Polariton Spectroscopy Methods

The methods of vibrational spectroscopy in far and near fields are used for studying thin (nanometer thick) AlN and MgO films on sapphire. It is shown that in the case of monolayer AlN films formed by the method of sapphire nitridation, the frequencies of longitudinal optical phonons of the film decrease in comparison with those of a single crystal, but the frequencies of transverse optical phonons increase. This means that the crystal structures of the film and the surface of the sapphire substrate modify. The solid solution (Al2O3)1 – x(AlN) x of variable composition appears in the transition layer (at the interface); the parameters of the AlN crystal cell change smoothly. In the case of the epitaxial growth of MgO films on sapphire, during the first stage, most likely, only the crystal structure of the sapphire surface is modified because the frequencies of the optical phonons of MgO films and the single crystal are the same. With an increase in the thickness of the MgO film, tensile stresses are accumulated in it, changing the frequencies of the optical phonons.

A first-principles study of gas adsorption on monolayer AlN sheet

The monolayer AlN is one of graphene-like 2D material. On the basis of density functional theory (DFT) calculations, the adsorption of CO2, CO, H2, N2, CH4, O2 and NO on the monolayer AlN was investigated. Due to the dipole moment of the Al-N bond, the gas molecules can be adsorbed directly on the AlN sheet. This avoids the problem about formation of metal clusters using metal decorated carbon nanostructures as gas storage materials. Among all the gas molecules, only CO2 on the AlN sheet has strong interaction with adsorption energy of 0.91 eV. While other gas molecules bind to the AlN sheet much weakly with the adsorption energy being less than 0.5 eV. Due to the charge transfer between gas molecules and the AlN sheet, the AlN sheet can be used to gas sensors for CO2, CO, H2, O2 and NO. Moreover, the AlN sheet can be taken potential applications as sorbent materials for CO2 separation and capture from gas mixture.

Hydrogenation Induced Carrier Mobility Polarity Reversal in Monolayer AlN

Two‐dimensional (2D) materials promote the development of nanoelectronic devices, which requires candidate systems with both a high carrier mobility and a moderate electronic bandgap. We present a first principles calculation of the intrinsic carrier mobilities of pristine (1L‐AlN) and hydrogenated (1L‐AlN‐H2) monolayer AlN. Numerical results reveal that 1L‐AlN shows a hole‐dominated ultra‐large carrier mobility (up to 5277 cm2 V−1 s−1). Upon full hydrogenation (1L‐AlN‐H2), the polarity of carrier mobility is reversed from hole dominated to electron dominated. This tunable polarity of intrinsic carrier mobility indicates monolayer AlN as a promising candidate for future nanoelectronics.

Point defects and composition in hexagonal group-III nitride monolayers: A first-principles calculation

In this paper, we systematically investigate the structural, electronic and magnetic properties of hexagonal group-III nitride monolayers with point defects and alloying on the basis of first-principles calculations. Six typical point defects including three vacancies and three antisites are modeled in pure AlN, GaN and InN monolayers. The defect-induced modifications of band gaps and magnetic properties are demonstrated. The vacancy of nitrogen, with lowest formation energy, metalizes the semiconducting nitride monolayers. The defects losing single group-III atom introduce net magnetic moment to the systems, while others maintain non-magnetic. For ordered alloy monolayers, the AlGaN and InGaN systems are taken into consideration. The compositional variation is achieved by atomic substitution in supercells with different sizes. We find that the lattice constant and cohesive energy follow good linear relation with concentration while a slight bowing effect is observed for the band gap. These results provide a development in defective and alloy nitride monolayers and extend the potential applications.

A comparative study on magnetic properties of Mo doped AlN, GaN and InN monolayers from first-principles

First-principles calculations are performed to comparatively study the structural, electronic structures and magnetic properties of Mo doped AlN, GaN and InN monolayers (MLs). After Mo atom doping, the semiconducting GaN and InN MLs transform to metal, while the AlN ML keeps semiconducting with a reduced gap. Total magnetic moments of 1.0 and 0.54 µB, which mainly arising from the localized Mo 4d states, are induced by doping in AlN and InN MLs, respectively, while the doped GaN ML is still nonmagnetic. Nevertheless, the excessive localization and strongly ionic character of the Mo-4d states in AlN ML directly impedes the magnetic coupling, leading to a paramagnetic ground states. A similar case is observed in Mo atoms doped InN ML. The firm N-Mo interaction prevent the impurity states permeating out the range of N-Mo pair, resulting in a quick vanishing of ferromagnetic coupling as the distance between two Mo atoms increasing. All configurations of Mo atoms doped GaN ML in this paper are room temperature ferromagnetic. Spin polarized itinerant electrons mediate the magnetic interaction between two Mo atoms. Increasing the Mo concentration may stabilize the FM state and produce a higher Curie temperature. Our calculations show that GaN nanosheets with Mo atoms doped may be a nice candidate for future spintronic devices. And we conclude that a appropriate magnitude of localization (or delocalization) is what the key point to produce room temperature ferromagnetism from this comparative study.

Tuning electronic properties of fully hydrogenated AlN nanosheets by external electric field: A van der Waals density functional study

Abstract In this paper, the structural and electronic properties of two dimensional (2D) fully hydrogenated AlN nanosheets have been investigated by density functional theory computations with van der Waals (vdW) correction. The results demonstrate that there exists strong hydrogen bonding between the nanosheets. Especially, fully hydrogenated AlN monolayer and bilayer nanosheets both have an indirect band gap, irrespective of stacking pattern and thickness. The band gap of fully hydrogenated AlN monolayer and bilayer can be flexibly reduced by applying an external electronic field (E-field), resulting in a semiconductor → metal transition. The results provide many useful insights for the wide applications of AlN nanosheets in electronics and optoelectronics.

Half-metallic and magnetic properties of AlN nanosheets doped with nonmagnetic metals: A first-principles study

We theoretically studied the structural, electronic, and magnetic properties in the two-dimensional (2D) AlN nanosheets (AlNNSs) doped with nonmagnetic (NM) atoms X(=Mg, Ca, Zn, and Sr), based on first-principles calculations. The structure relaxations show Mg and Zn atoms locate in the 2D AlNNS plane, while the Ca and Sr atoms lie out of it. The results based on GGA-PBE scheme show that all the doped AlN monolayers (ML) are half-metallic. Further, results within HSE06 scheme show that Mg-, Zn- and Sr-doped AlN ML remain half-metallic while Ca-doped case changes into magnetic semiconductor. Each dopant induces a total magnetic moment of 1.0μB per supercell which mainly stemmed from the spin-polarized holes resided on the three nearest-neighboring N atoms. Calculations illustrate that ferromagnetic (FM) states are energetically stable when two X atoms separate far away from each other, while anti-ferromagnetic (AFM) states are energetically favorable when two X atoms locate at adjacent lattice sites. The long-range FM coupling and AFM one are attributed to the strong p-d/p-p interaction and the virtual hopping mechanism, respectively. Remarkably, because of the participation of the delocalized Zn-d orbitals in Zn-doped case, strong p-d/p-p interactions gain supremacy in the competition between virtual hopping mechanism and p-d/p-p interaction, resulting in FM coupling even two Zn atoms locate at close crystal lattice. Calculations show the two-X-doped AlNNSs have FM states with Curie temperatures (Tc) higher than 600K, indicating that X-doped AlN systems are promising candidates for spintronic devices in the future.

Electronic and magnetic properties in Mn‐doped IIIA‐nitride monolayers

Based on first‐principles calculations plus Hubbard U, we have studied the electronic structure and magnetic properties of Mn‐doped IIIA‐nitride monolayers. The substitution of Mn for Al or Ga atom induces a total magnetic moment of 4.00 µB per dopant, independent of the choice of functional. The doped AlN system is half‐metallic at GGA + U (<5 eV) level, but is a magnetic semiconductor at the HSE06 and GGA + U (≥5 eV) levels. The doped GaN system is a magnetic semiconductor at these two levels. The Mn‐doped InN converts into metal without magnetism, due to the robust p–d hybridization between the Mn‐3d and N‐2p states and the delocalized sd states. The Mn atoms have a clear clustering tendency with the FM state in the AlN and GaN hosts. The long‐ranged ferromagnetic coupling is observed along the ⋯N‐Al/Ga⋯ zigzag edge directions, which can be attributed to p–d hybridization mechanism. These results suggest that the Mn‐doped AlN and GaN monolayers are promising candidates for preparing spintronic devices at the nanoscale.

AlN interlayer to improve the epitaxial growth of SmN on GaN (0001)

An in situ study of the epitaxial growth of SmN thin films on Ga-polar GaN (0001) templates by molecular beam epitaxy is reported. Using X-ray photoelectron spectroscopy we found that Ga segregates at the surface during the first stages of growth. We showed that the problem related to Ga surface segregation can be simply suppressed by growing a few monolayers of AlN before starting the SmN growth. This results in a significant improvement of the crystallinity of SmN thin films assessed by X-ray diffraction.

Atomic structures of a liquid-phase bonded metal/nitride heterointerface.

Liquid-phase bonding is a technologically important method to fabricate high-performance metal/ceramic heterostructures used for power electronic devices. However, the atomic-scale mechanisms of how these two dissimilar crystals specifically bond at the interfaces are still not well understood. Here we analyse the atomically-resolved structure of a liquid-phase bonded heterointerface between Al alloy and AlN single crystal using aberration corrected scanning transmission electron microscopy (STEM). In addition, energy-dispersive X-ray microanalysis, using dual silicon drift X-ray detectors in STEM, was performed to analyze the local chemistry of the interface. We find that a monolayer of MgO is spontaneously formed on the AlN substrate surface and that a polarity-inverted monolayer of AlN is grown on top of it. Thus, the Al alloy is bonded with the polarity-inverted AlN monolayer, creating a complex atomic-scale layered structure, facilitating the bonding between the two dissimilar crystals during liquid-phase bonding processes. Density-functional-theory calculations confirm that the bonding stability is strongly dependent on the polarity and stacking of AlN and MgO monolayers. Understanding the spontaneous formation of layered transition structures at the heterointerface will be key in fabricating very stable Al alloy/AlN heterointerface required for high reliability power electronic devices.