Two-dimensional exciton properties in monolayer semiconducting phosphorus allotropes

Two-dimensional bilayer blue phosphorus Dirac-like material: a multi-orbital tight-binding investigation

This study presents a theoretical examination of the electronic band structure of AA (AB) stacked bilayer blue phosphorus system within the fifth intralayer (5NN) and second interlayer nearest-neighbor (2NN) multi-orbital tight-binding (MOTB) approach. The variation of energy levels has been investigated through the symmetrical tensile strain of the low-buckled honeycomb lattice. Here, the primary objective is to examine the existence of Dirac electronic features in hexagonal stacked bilayer geometry. Our theoretical calculations predict that the AA bilayer is a new hexagonal two-dimensional material with px,y-orbital Dirac-like states at the high-symmetry K point. Consequently, these systems can host massless (massive) Dirac fermions. In particular, the AA bilayer exhibits zero-gap Dirac-like properties and manifests distinguishable Dirac-like cones in the presence of weak spin-orbit coupling when a modest stretch of 2.30% is achieved with a remarkably high Fermi velocity of approximately vf ≈ 0.12 × 105 m s-1. The behavior of the dispersion bands aligns reasonably well with recent experimental observations. Moreover, a stretch of 7.17% breaks some of the sublattice equivalence and enhances the spin-orbit interaction, resulting in the emergence of an electronic band gap of approximately ≈ 0.27 eV in the proximity of the high-symmetry K point. Furthermore, the tiny gap induced by the spin-orbit interaction implies topological nontriviality in the electronic state (quantum anomalous Hall state) of the honeycomb lattice. These findings categorize the AA bilayer as a rare two-dimensional Dirac-like material. This work provides, to the extent of our knowledge, a pioneering investigation into the existence of Dirac electronic properties in bilayer blue phosphorus. In addition, we present the first derivation of the MOTB model. However, the identified electronic characteristics designate this two-dimensional system as an ideal candidate for high-performance nanoelectronic devices.

Functionalized hexagonal boron nitride bilayers: desirable electro-optical properties for optoelectronic applications

Structural, electronic, and optical properties of functionalized hexagonal boron nitride (h-BN) bilayer were deeply explored by carrying out the PBE + G0W0 + BSE calculations. Hydrogenation/hydrofluorination/fluorination can cause the planar h-BN bilayer to form a novel diamane-like monolayer by interfacial sp3 atom bonding. These functionalized h-BN bilayers are estimated to be stable dynamically due to their phonon dispersions. The functionalization on h-BN bilayer can induce its electronic nature to be transformed from an indirect wide-gap insulator to direct narrow-gap semiconductor, which is desirable for its application in optoelectronics. In particular, hydrogenated and hydrofluorinated h-BN bilayers have strong absorbance coefficients for the near-infrared and visible part of the incident sunlight (larger than 105 cm-1). More interestingly, the binding energy of the observed first bright exciton can achieve a value beyond 1 eV, which can effectively reduce the recombination of photogenerated electron-hole pairs. These results are potentially important for extending the applications of the h-BN bilayer in optoelectronic devices.

Monolayer Ge2Te2P4 as a promising photocatalyst for solar driven water-splitting: a DFT study

The buckling hexagonal structure of Ge2Te2P4 was studied by first-principles calculations. The newly proposed structure was proven to be stable by analyzing its cohesive energy, phonon dispersion, elastic constants and AIMD results. Poisson's ratio of the Ge2Te2P4 monolayer is in the range 0.16-0.18, and Young's modulus is in the range 40.16-43.74 N m-1. The substituted Te atoms enhance the sp2 orbitals which strengthen the σ-bonds and therefore the thickness of the Ge2Te2P4 monolayer is smaller than that of monolayer GeP3. The Ge2Te2P4 monolayer has an indirect band gap of 1.85 eV, which can be narrowed by strains. The compressive band gaps from -2% to -4% change the electronic structure from the indirect band gap into the direct band gap. Strains can also increase the light absorption rate α(ω) in the visible region, which is 2-3 × 105 cm-1 at equilibrium. The Ge2Te2P4 monolayer has a suitable band gap and an appropriate VBM and CBM position for hydrogen generation. Under strain rate of 4% and higher, the VBM and CBM remain at suitable positions for hydrogen production. Another advantage of the Ge2Te2P4 monolayer is that its charge carrier mobilities are really high. The highest electron mobility is 1301.47 cm2 V-1 s-1, and the highest hole mobility is 28627.24 cm2 V-1 s-1, which are much higher than the mobility in monolayer GeP3. The Ge2Te2P4 monolayer has advantages for photocatalytic applications and it is necessary to perform further study on the material.

Two Janus Ga2STe monolayers and their electronic, optical, and photocatalytic properties

Recently, two-dimensional Janus materials have attracted increasing interest due to their unique structure and novel properties. Based on density-functional and many-body perturbation theories (i.e. DFT + G₀W₀ + BSE methods), the electronic, optical, and photocatalytic properties of Janus Ga₂STe monolayers with two configurations are explored systematically. It is found that the two Janus Ga₂STe monolayers exhibit high dynamical and thermal stabilities and have desirable direct gaps of about 2 eV at the G₀W₀ level. Their optical absorption spectra are dominated by the enhanced excitonic effects, in which bright bound excitons possess moderate binding energies of about 0.6 eV. Most interestingly, Janus Ga₂STe monolayers show high light absorption coefficients (larger than 10⁶ cm^-1) in the visible light region, effective spatial separation of photoexcited carriers, and suitable band edge positions, which make them potential candidates for photoelectronic and photocatalytic devices. These observed findings enrich the deep understanding of the properties of Janus Ga₂STe monolayers.

Two-dimensional ZnO/BlueP van der Waals heterostructure used for visible-light driven water splitting: A first-principles study

Solar driven water splitting for hydrogen generation has been considered as an important method for collecting clean energy. Herein, based on first-principles calculations, we propose that ZnO/BlueP van der Waals heterostructure can realize overall water splitting reaction for hydrogen generation. Strikingly, the band-gap of 1.83 eV is appropriate, and band alignments straddle the water redox potentials, ensuring the occurrence of hydrogenevolutionreaction and oxygen evolution reaction. Charge density distribution and carrier mobility exhibit significant charge separation and transfer. Visible-light response is improved compared with those of the isolated monolayers. Moreover, hydrogenevolutionreaction is actually realized on the ZnO layer, while oxygen evolution reaction is implemented on the BlueP layer. Through the investigation of the adsorption and dissociation reactions of H₂O, we observe that two neighboring H*s prefer to combine to form H₂ by overcoming a lowered energy barrier of 0.75 eV. Strain effect indicates that the lateral compressive strain of -4% to 0% and the vertical tensile strain of 0% to +6% can effectively tune band-gap and band alignments. The results indicate that ZnO/BlueP vdW heterostructure is probable highly efficient photoelectric material used for visible-light driven water splitting for hydrogen generation.

Prediction of Strong Transversal s(TE) Exciton–Polaritons in C60 Thin Crystalline Films

If an exciton and a photon can change each other’s properties, indicating that the regime of their strong bond is achieved, it usually happens in standard microcavity devices, where the large overlap between the ’confined’ cavity photons and the 2D excitons enable the hybridization and the band gap opening in the parabolic photonic branch (as clear evidence of the strong exciton–photon coupling). Here, we show that the strong light–matter coupling can occur beyond the microcavity device setup, i.e., between the ’free’ s(TE) photons and excitons. The s(TE) exciton–polariton is a polarization mode, which (contrary to the p(TM) mode) appears only as a coexistence of a photon and an exciton, i.e., it vanishes in the non-retarded limit (c→∞). We show that a thin fullerene C60 crystalline film (consisting of N C60 single layers) deposited on an Al2O3 dielectric surface supports strong evanescent s(TE)-polarized exciton–polariton. The calculated Rabi splitting is more than Ω=500 meV for N=10, with a tendency to increase with N, indicating a very strong photonic character of the exciton–polariton.

Low-Temperature Induced Enhancement of Photoelectric Performance in Semiconducting Nanomaterials

The development of light-electricity conversion in nanomaterials has drawn intensive attention to the topic of achieving high efficiency and environmentally adaptive photoelectric technologies. Besides traditional improving methods, we noted that low-temperature cooling possesses advantages in applicability, stability and nondamaging characteristics. Because of the temperature-related physical properties of nanoscale materials, the working mechanism of cooling originates from intrinsic characteristics, such as crystal structure, carrier motion and carrier or trap density. Here, emerging advances in cooling-enhanced photoelectric performance are reviewed, including aspects of materials, performance and mechanisms. Finally, potential applications and existing issues are also summarized. These investigations on low-temperature cooling unveil it as an innovative strategy to further realize improvement to photoelectric conversion without damaging intrinsic components and foresee high-performance applications in extreme conditions.

The effects of vacancy and heteroatoms-doping on the stability, electronic and magnetic properties of blue phosphorene

In this work, we have systematically studied the stability, electronic structure and magnetic properties of the pristine, four defect states case of blue phosphorene and the six heteroatoms doping in blue phosphorene by first-principles calculations. In our findings, both defects and heteroatoms doping can regulate the band gap of blue phosphorene and the transition from indirect to direct band gap can be dramatically tuned by DV1BP, DV2BP and Al, Si atoms substitutional doping in blue phosphorene. The presence of defects and heteroatoms doping effectively modulates the electronic properties of blue phosphorene, rendering the defect-containing phosphorene semiconducting with a tunable band gap. Spin-orbit coupling can be induced by introducing SV-, DV- defects in blue phosphorene. The results provide theoretical guidance for future bandgap regulation and magnetism, defective and substitutional doping blue phosphorene may have potential electro-optical and electromagnetic applications.

Spectrally Selective Mid-Wave Infrared Detection Using Fabry-Pérot Cavity Enhanced Black Phosphorus 2D Photodiodes

Thin two-dimensional (2D) material absorbers have the potential to reduce volume-dependent thermal noise in infrared detectors. However, any reduction in noise must be balanced against lower absorption from the thin layer, which necessitates advanced optical architectures. Such architectures can be particularly effective for applications that require detection only within a specific narrow wavelength range. This study presents a Fabry-Pérot cavity enhanced bP/MoS₂ midwave infrared (MWIR) photodiode. This simple structure enables tunable narrow-band (down to 0.42 μm full width at half-maximum) photodetection in the 2-4 μm range by adjusting the thickness of the Fabry-Pérot cavity resonator. This is achieved while maintaining room-temperature performance metrics comparable to previously reported 2D MWIR detectors. Zero bias specific detectivity and responsivity values of up to 1.7 × 10⁹ cm Hz^1/2 W^-1 and 0.11 A W^-1 at λ = 3.0 μm are measured with a response time of less than 3 ns. These results introduce a promising family of 2D detectors with applications in MWIR spectroscopy.

Enhancing electronic and optical properties of monolayer MoSe2via a MoSe2/blue phosphorene heterobilayer

Type-II heterostructures are appealing for application in optoelectronics due to their effective separation of photogenerated charge carriers.

Role of surface adsorption in tuning the properties of black phosphorus

The synergetic effect of O₂and H₂O during the oxidation of black phosphorus (BP) at the atomic level is revealed, and the effects of H₂O and/or O₂on the properties of BP are also investigated.

Mid-Wave Infrared Photoconductors Based on Black Phosphorus-Arsenic Alloys

Black phosphorus (b-P) and more recently black phosphorus-arsenic alloys (b-PAs) are candidate 2D materials for the detection of mid-wave and potentially long-wave infrared radiation. However, studies to date have utilized laser-based measurements to extract device performance and the responsivity of these detectors. As such, their performance under thermal radiation and spectral response has not been fully characterized. Here, we perform a systematic investigation of gated-photoconductors based on b-PAs alloys as a function of thickness over the composition range of 0-91% As. Infrared transmission and reflection measurements are performed to determine the bandgap of the various compositions. The spectrally resolved photoresponse for various compositions in this material system is investigated to confirm absorption measurements, and we find that the cutoff wavelength can be tuned from 3.9 to 4.6 μm over the studied compositional range. In addition, we investigated the temperature-dependent photoresponse and performed calibrated responsivity measurements using blackbody flood illumination. Notably, we find that the specific detectivity (D*) can be optimized by adjusting the thickness of the b-P/b-PAs layer to maximize absorption and minimize dark current. We obtain a peak D* of 6 × 10¹⁰ cm Hz^1/2 W^-1 and 2.4 × 10¹⁰ cm Hz^1/2 W^-1 for pure b-P and b-PAs (91% As), respectively, at room temperature, which is an order of magnitude higher than commercially available mid-wave infrared detectors operating at room temperature.