Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our p–n diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-004565.pdf

Light Generation and Harvesting in a Van der Waals Heterostructure

Tunable electronic and magnetic properties of MoSi2N4 monolayer via vacancy defects, atomic adsorption and atomic doping

Recently, a two-dimensional (2D) MoSi 2N 4 (MSN) structure has been successfully synthesized [Hong et al., Science 369(6504), 670–674 (2020)]. Motivated by this result, we investigate the structural, electronic, and optical properties of MSN monolayer (MSN-1L) and bilayer (MSN-2L) under the applied electric field (E-field) and strain using density functional theory calculations. We find that the MSN-2L is a semiconductor with an indirect bandgap of 1.60 (1.80) eV using Perdew–Burke–Ernzerhof (HSE06). The bandgap of MSN-2L decreases as the E-field increases from 0.1 to 0.6 V/A  and for larger E-field up to 1.0 V/A  the bilayer becomes metallic. As the vertical strain increases, the bandgap decreases; more interestingly, a semiconductor to a metal phase transition is observed at a strain of 12 %. Furthermore, the optical response of the MSN-2L is in the ultraviolet (UV) region of the electromagnetic spectrum. The absorption edge exhibits a blue shift by applying an E-field or a vertical compressive strain. The obtained interesting properties suggest MSN-2L as a promising material in electro-mechanical and UV opto-mechanical devices.

Effect of electric field and vertical strain on the electro-optical properties of the MoSi2N4 bilayer: A first-principles calculation

Inspired by the recent successful growth of Ti2C and Ti3C2 monolayers, here, we investigate the structural, electronic, and mechanical properties of functionalized Ti2C and Ti3C2 monolayers by means of density functional theory calculations. The results reveal that monolayers of Ti2C and Ti3C2 are dynamically stable metals. Phonon band dispersion calculations demonstrate that two-surface functionalization of Ti2C and Ti3C2via chalcogenides (S, Se, and Te), halides (F, Cl, Br, and I), and oxygen atoms results in dynamically stable novel functionalized monolayer materials. Electronic band dispersions and density of states calculations reveal that all functionalized monolayer structures preserve the metallic nature of both Ti2C and Ti3C2 except Ti2C–O2, which possesses the behavior of an indirect semiconductor via full-surface oxygen passivation. In addition, it is shown that although halide passivated Ti3C2 structures are still metallic, there exist multiple Dirac-like cones around the Fermi energy level, which indicates that semi-metallic behavior can be obtained upon external effects by tuning the energy of the Dirac cones. In addition, the computed linear-elastic parameters prove that functionalization is a powerful tool in tuning the mechanical properties of stiff monolayers of bare Ti2C and Ti3C2. Our study discloses that the electronic and structural properties of Ti2C and Ti3C2 MXene monolayers are suitable for surface modification, which is highly desirable for material property engineering and device integration.

Surface modification of titanium carbide MXene monolayers (Ti2C and Ti3C2) via chalcogenide and halogenide atoms

Pure hydrogen production via water splitting is an ideal strategy for producing clean and sustainable energy. Two-dimensional (2D) cadmium chalcogenide single-layers with a tetragonal crystal structure, namely Tetra-CdX (X = S, Se, and Te) monolayers, are theoretically predicted by means of density functional theory (DFT). Their structural stability and electronic and optical properties are investigated. We find that Tetra-CdX single-layers are thermodynamically stable. Their stability decreases as we go down the 6A group in the periodic table, i.e., from X = S to Se, and Te which also means that the electronegativity decreases. All considered novel monolayers are indirect band gap semiconductors. Using the HSE06 functional the electronic band gaps of CdS, CdSe, and CdTe monolayers are predicted to be 3.10 eV, 2.97 eV, and 2.90 eV, respectively. The impact of mechanical strain on the physical properties was studied, which indicates that compressive strain increases the band gap and tensile strain decreases the band gap. The optical properties of the Tetra-CdX monolayers show the ability of these monolayers to absorb visible light. Due to the suitable band gaps and band edge positions of Tetra-CdX, these newly discovered 2D materials are promising for photocatalytic water splitting.

Two-dimensional buckled tetragonal cadmium chalcogenides including CdS, CdSe, and CdTe monolayers as photo-catalysts for water splitting

The electronic and optical properties of vertical heterostructures (HTSs) and lateral heterojunctions (HTJs) between (B,N)-codoped graphene (dop@Gr) and graphene (Gr), C3N, BC3 and h-BN monolayers are investigated using van der Waals density functional theory calculations. We have found that all the considered HTSs are energetically and thermally feasible at room temperature, and therefore they can be synthesized experimentally. The dop@Gr/Gr, BC3/dop@Gr and BN/dop@Gr HTSs are semiconductors with direct bandgaps of 0.1 eV, 80 meV and 1.23 eV, respectively, while the C3N/dop@Gr is a metal because of the strong interaction between dop@Gr and C3N layers. On the other hand, the dop@Gr-Gr and BN-dop@Gr HTJs are semiconductors, whereas the C3N-dop@Gr and BC3-dop@Gr HTJs are metals. The proposed HTSs can enhance the absorption of light in the whole wavelength range as compared to Gr and BN monolayers. The applied electric field or pressure strain changes the bandgaps of the HTSs and HTJs, indicating that these HTSs are highly promising for application in nanoscale multifunctional devices.

Electronic and optical properties of two-dimensional heterostructures and heterojunctions between doped-graphene and C- and N-containing materials.

The combination of two-dimensional materials in the form of van der Waals (vdW) heterostructures has been shown to be an effective method for designing electronic and optoelectronic equipment. In this work, we investigated the electronic and optical properties, and photocatalytic performance of the BP–BSe vdW heterostructure using first principles calculations. We found that the BP–BSe vdW heterostructure is energetically and thermally stable at room temperature. The BP–BSe vdW heterostructure shows the type-II band alignment with a direct band gap nature. Additionally, charges are driven from the BSe to BP monolayers in the BP–BSe heterostructure. Bader charge analysis showed that about 0.23 electrons are transferred from BSe to the BP layer. Furthermore, the absorption coefficients are also calculated to understand their optical behavior. Our results show that the lowest energy transition is controlled by excitons. The photocatalytic performance of the BP–BSe vdW heterostructure shows that such a heterostructure is suitable for water splitting at pH = 0.

Electronic structures, and optical and photocatalytic properties of the BP–BSe van der Waals heterostructures

In this paper, the existence of monolayers with the chemical formula XO2, where X = Zn, Cd, and Hg with hexagonal and tetragonal lattice structures is theoretically predicted by means of first principles calculations. Through cohesive energy calculation and phonon dispersion simulation, it has been proven that the two-dimensional XO2 monolayers proposed are energetically and dynamically stable suggesting their potential experimental realization. Our detailed study demonstrates that these novel newly predicted materials are half-metals and dilute magnetic semiconductors, and they exhibit magnetism in the ground state. The half-metallic character could find many applications in electronic and spintronic devices. Research into the magnetic properties revealed here can enrich theoretical knowledge in this area and provide more potential candidates for XO2 2D-based materials and van der Waals heterostructures.

Novel two-dimensional ZnO2, CdO2 and HgO2 monolayers: a first-principles-based prediction

Chemical fertilizers, which contain a nitrogen (N) element, has been intensively used to increase the peanut productivity. However, the unstable and high cost of N fertilizer, and the great demand for N fertilizer sources have strongly increased the strategical plan of nitrogen fixation (NF). Therefore, the field research was carried out to appraise the ability of Rhizobium sp. trains and chicken manure (CM) on the quality and yield of peanuts. This research has four ratios, which valued from 0.0, 2.0, 4.0 to 6.0 t CM per ha in the combination with the Rhizobium sp. inoculum, expect control treatment (without CM and Rhizobium sp.). Different rates of CM combined with Rhizobium sp. inoculation was added by using 6.0 tons CM/ ha, which had number of the highest peanut nodules. Research results observed that the inoculant of Rhizobium sp. strain combined with CM remarkably increased the yield components per plant such as biomass, number of nodules, weight of dry nodules, weight of fill and empty pods and fresh yield of groundnut. The highest yield and quality of peanut (7.60 t/ha), oil % (50.6%), seed protein percentage (26.8%), as well as NPK content in seed (4.32, 0.912 and 0.999%, respectively) were obtained under the application of NPK+6.0 t CM/ha+ Rhizobium sp. inoculation. Co-application of 6t CM/ha and Rhizobium sp. inoculation increased by 20.5% when compared without CM application and no Rhizobium sp. inoculation. The study showed that both possibility of nitrogen fixation of peanut and nitrogen uptake of the sandy soil were raised by field inoculant with effectiveness of Rhizobium sp. with animal manure application. In really, Rhizobium sp. inoculation and CM proved a great method to increase soil nutrients for subsequent crops and it helped to enhance the taking of nitrogen from the air into the crop soil.

https://cst.kipmi.or.id/journal/article/download/1082/127

Nguyen V. Chuong

Papers

Electronic and optical properties of two-dimensional heterostructures and heterojunctions between doped-graphene and C- and N-containing materials.

Electronic structures, and optical and photocatalytic properties of the BP–BSe van der Waals heterostructures

Novel two-dimensional ZnO2, CdO2 and HgO2 monolayers: a first-principles-based prediction

Response of peanut quality and yield to chicken manure combined with Rhizobium inoculation in sandy soil