Strain- and electric field-induced band gap modulation in nitride nanomembranes

Large Negative Poisson's Ratio and Anisotropic Mechanics in New Penta-PBN Monolayer

The scarce negative Poisson's ratio (NPR) in a two-dimensional (2D) material is an exceptional auxetic property that offers an opportunity to develop nanoscale futuristic multi-functional devices and has been drawing extensive research interest. Inspired by the buckled pentagonal iso-structures that often expose NPR, we employ state-of-the-art first-principles density functional theory calculations and analyses to predict a new 2D metallic ternary auxetic penta-phosphorus boron nitride (p-PBN) with a high value of NPR. The new p-PBN is stable structurally, mechanically, and dynamically and sustainable at room temperature, with experimental feasibility. The short and strong quasi sp³-hybridized B-N bond and unique bond variation and geometrical reconstruction with an applied strain allow p-PBN to inherit a high value of NPR (-0.236) along the (010) direction, the highest among any other ternary penta iso-structures reported to date. Despite having a small elastic strength, the highly asymmetric Young's modulus and Poisson's ratio along the (100) and (010) directions indicate large anisotropic mechanics, which are crucial for potential applications in nanomechanics and nanoauxetics.

Strain tailored thermodynamic stability, electronic transitions, and optoelectronic properties of III (In, Ga and Al)-nitride monolayers

The thermodynamic stability of III-nitride monolayers is calculated using the phonon band structure. Electronic properties are computed using the generalized gradient approximation-Perdew-Burke-Ernzerhof exchange-correlation potentials, which show the semiconducting behavior with bandgap 0.59 eV, 2.034 eV, and 2.906 eV for InN, GaN, and AlN monolayers, respectively. The biaxial tensile and compressive strains are used as external stimuli to understand their impact on the optoelectronic properties of these monolayers. The thermodynamic stability of strained monolayers is investigated to explore the maximum possible strains, i.e. flexibility limit, these monolayers can sustain. These monolayers are more sensitive to compressive strains, showing thermodynamic instability even at 1% compressive strain for all the considered monolayers. Further, the III-nitride monolayers are more robust with the tensile strain. InN, GaN, and AlN monolayers can sustain up to 4%, 16%, and 18% tensile strain, respectively. More interestingly, the electronic transitions, such as direct to indirect and semiconducting to metallic, are noticed with strain in the considered monolayers. The optical properties also exhibit strong strain dependency at the different transition points.

Strain-induced dimensional phase change of graphene-like boron nitride monolayers

We investigate the coupling between the electronic bandgap and mechanical loading of graphene-like boron nitride (h-BN ) monolayers up to failure strains and beyond by means of first-principles calculations. We reveal that the kinks in the bandgap-strain curve are coincident with the ultimate tensile strains, indicating a phase change. When the armchair strain is beyond the ultimate tensile strain, h-BN fails with a phase transformation from 2D honeycomb to 1D chain structure, characterized by the 'V'-shape bandgap-strain curve. Large biaxial strains can break the 2D honeycomb structures into 0D individual atoms and the bandgap closes.

Double Charged Surface Layers in Lead Halide Perovskite Crystals

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Nano-structured interface of graphene and h-BN for sensing applications

The atomically-precise controlled synthesis of graphene stripes embedded in hexagonal boron nitride opens up new possibilities for the construction of nanodevices with applications in sensing. Here, we explore properties related to the electronic structure and quantum transport of a graphene nanoroad embedded in hexagonal boron nitride, using a combination of density functional theory and the non-equilibrium Green's functions method to calculate the electric conductance. We find that the graphene nanoribbon signature is preserved in the transmission spectra and that the local current is mainly confined to the graphene domain. When a properly sized nanopore is created in the graphene part of the system, the electronic current becomes restricted to a carbon chain running along the border with hexagonal boron nitride. This circumstance could allow the hypothetical nanodevice to become highly sensitive to the electronic nature of molecules passing through the nanopore, thus opening up ways to detect gas molecules, amino acids, or even DNA sequences based on a measurement of the real-time conductance modulation in the graphene nanoroad.

Hybridization effects on the out-of-plane electron tunneling properties of monolayers: is h-BN more conductive than graphene?

Electron transport properties through multilayers of hexagonal boron nitride (h-BN) sandwiched between gold electrodes is investigated by density functional theory together with the non-equilibrium Green's function method. The calculated results find that despite graphene being a gapless semimetal and h-BN two-dimensional layer being an insulator, the transmission function perpendicular to the atomic layer plane in both systems is nearly identical. The out-of-plane tunnel current is found to be strongly dependent on the interaction at the interface of the device. As a consequence, single layer h-BN coupled with atomically flat weakly interacting metals such as gold may not work as a good dielectric material, but the absence of sharp resonances would probably lead to more stable out-of-plane electronic transport properties compared to graphene.

Tunable band gaps in graphene/GaN van der Waals heterostructures

Van der Waals (vdW) heterostructures consisting of graphene and other two-dimensional materials provide good opportunities for achieving desired electronic and optoelectronic properties. Here, we focus on vdW heterostructures composed of graphene and gallium nitride (GaN). Using density functional theory, we perform a systematic study on the structural and electronic properties of heterostructures consisting of graphene and GaN. Small band gaps are opened up at or near the Γ point of the Brillouin zone for all of the heterostructures. We also investigate the effect of the stacking sequence and electric fields on their electronic properties. Our results show that the tunability of the band gap is sensitive to the stacking sequence in bilayer-graphene-based heterostructures. In particular, in the case of graphene/graphene/GaN, a band gap of up to 334 meV is obtained under a perpendicular electric field. The band gap of bilayer graphene between GaN sheets (GaN/graphene/graphene/GaN) shows similar tunability, and increases to 217 meV with the perpendicular electric field reaching 0.8 V Å(-1).

Stacking and electric field effects in atomically thin layers of GaN

Atomically thin layers of nitrides are a subject of interest due to their novel applications. In this paper, we focus on GaN multilayers, investigating their stability and the effects of stacking and electric fields on their electronic properties in the framework of density functional theory. Both bilayers and trilayers prefer a planar configuration rather than a buckled bulk-like configuration. The application of an external perpendicular electric field induces distinct stacking-dependent features in the electronic properties of nitride multilayers: the band gap of a monolayer does not change whereas that of a trilayer is significantly reduced. Such a stacking-dependent tunability of the band gap in the presence of an applied field suggests that multilayer GaN is a good candidate material for next generation devices at the nanoscale.