Optical Properties Of Monolayer Research Articles

The structural, mechanical, electronic, and optical properties of monolayer and bilayer ABC3(A [dbnd] Ga, In; B [dbnd] Si, Ge; C [dbnd] S, Se, Te)

This paper studies the structural, mechanical, optical, and electronic characteristics of monolayer and bilayer ABC3 (AGa, In; BSi, Ge; CS, Se, Te). Their characteristics are obtained using the density functional theory with van der Waals (vdW) correction. The stability is confirmed by examining Born’s criterion, cohesive energy, phonon dispersions, and COHP analysis. In addition, the stability of these monolayers against tensile strain is explored. The electronic properties are extensively studied through band structure, PDOS, and effective masses. The optical properties are investigated for possible applications in optoelectronic devices. In the following, bilayer ABC3 with three different stacking orders is investigated. Most of the monolayers and bilayers ABC3 show an indirect bandgap. The contributions of each element at the valence and conduction bands are explored through the projected density of the state. The effective mass is derived for the valence and conduction band valleys, whereas the Γ-valley shows the lowest effective mass. Additionally, both real and imaginary parts of the dielectric constant, electron energy loss spectra, and optical absorption are studied, which might lead to intriguing usages in photovoltaic and electroluminescent systems. The analysis of the optical properties reveals that the studied monolayers and bilayers have acceptable visible-range absorption characteristics. Furthermore, the monolayer and bilayer ABC3 dielectric function and absorption coefficient spectra display prominent optical peaks in the UV light range. Due to their distinct band gaps and optical characteristics, monolayer and bilayer ABC3 materials are good prospects for upcoming electrical and optoelectronic applications.

First-principle study on the photoelectric properties of monolayer h-BN under different strain types.

Two-dimensional materials are a new and promising research field in materials science. This is mainly attributed to their unique photoelectric and chemical properties. In addition to possessing unique optoelectronic and chemical properties, two-dimensional materials also have important application prospects in the field of field-effect devices. Based on density functional theory, the effects of uniaxial strain and equibiaxial strain on the mechanical properties, electronic structure, and optical properties of monolayer h-BN were studied using first principles. The results indicate that compressive strain has a significant impact on the stability of monolayer h-BN. The band gap width of monolayer h-BN decreases with increasing strain, and the optical properties of monolayer h-BN exhibit a relative trend under tensile and compressive strains. The influence of biaxial strain on the mechanical properties, electronic structure, and optical properties of monolayer h-BN is greater than that of uniaxial strain. All the calculations were done by the VASP software based on density functional theory. The interaction between atomic nuclei and electrons is described by the projected added wave pseudopotential (PAW), using the generalized gradient approximation (GGA) to exchange the Perdew-Burke-Ernzerhof (PBE) of the functional. To avoid interlayer interactions, a 15-Å vacuum layer was set up. The Brillouin zone selects the Monkhorst-Pack method to generate 9 × 9 × 1 of k-point grid, the cut off energy is set to 500eV, the energy convergence standard of the system is 1 × 10-5eV, and the interaction force between atoms is 0.01eV/Å.

Investigation of effects of interlayer interaction and biaxial strain on the phonon dispersion and dielectric response of hexagonal boron arsenide

In this study, the effects of interlayer interaction and biaxial strain on the electronic structure, phonon dispersion and optical properties of monolayer and bilayer BAs are studied, using first-principles calculations within the framework of density functional theory. The interlayer coupling in bilayer BAs causes the splitting of out-of-plane acoustic (ZA) and optical (ZO) mode. For both structures, positive phonon modes across the Brillouin zone have been observed under biaxial tensile strain from 0 to 8%, which indicate their dynamical stability under tensile strain. Also, the phonon band gap between longitudinal acoustic (LA) and longitudinal optical (LO)/transverse optical (TO) modes for monolayer and bilayer BAs decreases under tensile strain. An appreciable degree of optical anisotropy is noticeable in the materials for parallel and perpendicular polarizations, accompanied by significant absorption in the ultraviolet and visible regions. The absorption edge of bilayer BAs is at a lower energy with respect to the monolayer BAs. The results demonstrate that the phonon dispersion and optoelectronic properties of BAs sheet could as well be tuned with both interlayer interaction and biaxial strain that are promising for optoelectronic and thermoelectric applications.

Intrinsic electronic and optical properties of monolayer and Bilayer CuI under many-body effects

Using the combination of density functional theory and many-body perturbation theory, we comprehensively investigate the electronic and optical properties of monolayer and bilayer CuI. For the bilayer, we build three high-symmetry stacking configurations i.e. AA, AB, and AC. The results reveal that monolayer CuI is a semiconductor with a direct band gap of 4.03 eV while the band gap of bilayer CuI is 1.62, 1.69, and 1.62 eV for AA, AB, and AC stacking configurations, respectively. The optical responses achieved from the Bethe–Salpeter equation are equal for light polarized along the x- and y-directions. The exciton binding energy of monolayer CuI is calculated to be 0.99 eV while it is 0.57, 0.59, and 0.56 eV for AA, AB, and AC stacking configurations, respectively. The maximum extinction coefficient of monolayer CuI is located in the ultraviolet area. Although, for the bilayers, the maximum extinction coefficient appears in the middle of the visible area. Overall, it is understood that the optoelectronic performance of the CuI binary compound improves from monolayer to bilayer. Because the AA configuration is more stable than the other two configurations, it is predicted to be the most suitable candidate for optoelectronic applications such as solar cells, photodetectors, and light-emitting diodes.

Effects of post-transfer annealing and substrate interactions on the photoluminescence of 2D/3D monolayer WS2/Ge heterostructures.

The ultraflat and dangling bond-free features of two-dimensional (2D) transition metal dichalcogenides (TMDs) endow them with great potential to be integrated with arbitrary three-dimensional (3D) substrates, forming mixed-dimensional 2D/3D heterostructures. As examples, 2D/3D heterostructures based on monolayer TMDs (e.g., WS2) and bulk germanium (Ge) have become emerging candidates for optoelectronic applications, such as ultrasensitive photodetectors that are capable of detecting broadband light from the mid-infrared (IR) to visible range. Currently, the study of WS2/Ge(100) heterostructures is in its infancy and it remains largely unexplored how sample preparation conditions and different substrates affect their photoluminescence (PL) and other optoelectronic properties. In this report, we investigated the PL quenching effect in monolayer WS2/Ge heterostructures prepared via a wet transfer process, and employed PL spectroscopy and atomic force microscopy (AFM) to demonstrate that post-transfer low-pressure annealing improves the interface quality and homogenizes the PL signal. We further studied and compared the temperature-dependent PL emissions of WS2/Ge with those of as-grown WS2 and WS2/graphene/Ge heterostructures. The results demonstrate that the integration of WS2 on Ge significantly quenches the PL intensity (from room temperature down to 80 K), and the PL quenching effect becomes even more prominent in WS2/graphene/Ge heterostructures, which is likely due to synergistic PL quenching effects induced by graphene and Ge. Density functional theory (DFT) and Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations show that the interaction of WS2 and Ge is stronger than in adjacent layers of bulk WS2, thus changing the electronic band structure and making the direct band gap of monolayer WS2 less accessible. By understanding the impact of post-transfer annealing and substrate interactions on the optical properties of monolayer TMD/Ge heterostructures, this study contributes to the exploration of the processing-properties relationship and may guide the future design and fabrication of optoelectronic devices based on 2D/3D heterostructures of TMDs/Ge.

Monolayer Fullerene Networks as Photocatalysts for Overall Water Splitting.

Photocatalytic water splitting can produce hydrogen in an environmentally friendly way and provide alternative energy sources to reduce global carbon emissions. Recently, monolayer fullerene networks have been successfully synthesized [Hou et al. Nature2022, 606, 507], offering new material candidates for photocatalysis because of their large surface area with abundant active sites, feasibility to be combined with other 2D materials to form heterojunctions, and the C60 cages for potential hydrogen storage. However, efficient photocatalysts need a combination of a suitable band gap and appropriate positions of the band edges with sufficient driving force for water splitting. In this study, I employ semilocal density functional theory and hybrid functional calculations to investigate the electronic structures of monolayer fullerene networks. I find that only the weakly screened hybrid functional, combined with time-dependent Hartree-Fock calculations to include the exciton binding energy, can reproduce the experimentally obtained optical band gap of monolayer C60. All the phases of monolayer fullerene networks have suitable band gaps with high carrier mobility and appropriate band edges to thermodynamically drive overall water splitting. In addition, the optical properties of monolayer C60 are studied, and different phases of fullerene networks exhibit distinct absorption and recombination behavior, providing unique advantages either as an electron acceptor or as an electron donor in photocatalysis.

Strain-Modulated Electronic and Optical Properties of Monolayer and Bilayer CdS: A DFT Study

Strain-Modulated Electronic and Optical Properties of Monolayer and Bilayer CdS: A DFT Study

Engineering the band gap and optical properties of a two-dimensional molybdenum carbon fluoride MXene

Using first-principles density functional theory, we investigated the electronic and optical properties of monolayer and multilayer nanosheets of molybdenum carbon fluoride (Mo2CF2), a two-dimensional (2D) transition-metal carbide MXene. The indirect band gap of the Mo2CF2 semiconductor can be engineered by controlling the number of layers where the band gap energy changes from 0.278 eV for the monolayer to 0.249 eV for the trilayer nanosheet. The decrease in band gap energy in the multilayer is due to interlayer coupling, which splits the bands according to the number of layers. Mo2CF2 behaves as a metal with an anomalous dispersion and high optical conductivity at incident photon energies of 0.68–2.19, 3.49–6.68 and 7.30–8.31 eV. It has a relatively low reflectivity and is absorbing over a broad range of photon energies from about 0.429 (2890), 0.387 (3204) and 0.345 eV (3594 nm) for the monolayer, bilayer and trilayer nanosheets, respectively, achieving peak absorption in the vacuum ultraviolet region at about 7.9 eV (157 nm). The optical properties of Mo2CF2 can likewise be tuned by varying the number of layers. The unique behavior of its optical properties along with the ability to control its electronic and optical properties enhances the potential of 2D Mo2CF2 for various applications in the fields of electronics and energy storage.

Passivation of Transition Metal Dichalcogenides Monolayers with a Surface‐Confined Atomically Thick Sulfur Layer

Surface passivation can eliminate the charge doping of monolayer transition metal dichalcogenides (TMDs) during the device fabrication, which is important for the large‐scale production of ultra‐stable materials and high‐performance devices. The uniformity and atomical thickness of the passivating layers with a low dielectric constant (κ) are essentials to preserving the intrinsic properties of monolayer TMDs in harsh environments. Herein, a surface‐confined mechanism is developed to encapsulate TMDs monolayers by atomically thin sulfur layers with high spatial homogeneity (named S–MX2). The bottom bilayer S atoms are strongly confined by the upper S monolayers when the low‐κ S reaches three layers on the surface of TMDs, which spontaneously renders the uniform distribution on a large scale. The intrinsic electrical and optical properties of monolayer S–MX2 are well maintained and show excellent long‐term stability under harsh environments. Herein this work, a way to eliminate surface doping of monolayer TMDs for their practical application in large‐area‐integrated circuits is provided.

Monoelemental two-dimensional iodinene nanosheets: a first-principles study of the electronic and optical properties

Very recently, two-dimensional (2D) iodinene, a novel layered and buckled structure has been successfully fabricated (Qian et al 2020 Adv. Mater. 32 2004835). Motivated by this latest experimental accomplishment, for the first time we conduct density functional theory, first-principles calculations to explore the structural, electronic, and optical properties of monolayer, few-layer and bulk iodinene. Unlike the majority of monoelemental 2D lattices, iodinene is predicted to be an intrinsic semiconductor. On the basis of calculations using the generalized gradient approximation of Perdew–Burke–Ernzerhof for the exchange-correlation functional and the Heyd-Scuseria-Ernzerhof (HSE06) functional, it is shown that the electronic bandgap of iodinene decreases with increasing the number of atomic layers. Our HSE06 results reveal that the bandgap of iodinene decreases from 2.08 to 1.28 eV as the number of atomic layers change from one to five, highlighting the finely tunable bandgap. The optical study shows the monolayer has the ability to absorb a wide range of ultraviolet light, more than multilayers and bulk iodinene. As the number of layers increases, the absorption spectra exhibits a blue shift relative to monolayer iodinene. This study confirms the remarkable prospect for the application of iodinene in nanoelectronics and optoelectronics owing to its intrinsic semiconducting nature.

Comparative study by DFT method of structural, electronic and optical properties of monolayer, bilayer and bulk CdS

Comparative study by DFT method of structural, electronic and optical properties of monolayer, bilayer and bulk CdS

Two-dimensional MoS2 2H, 1T, and 1T' crystalline phases with incorporated adatoms: theoretical investigation of electronic and optical properties.

Although there has been progress in studying the electronic and optical properties of monolayer and near-monolayer (two-dimensional, 2D) MoS2 upon adatom adsorption and intercalation, understanding the underlying atomic-level behavior is lacking, particularly as related to the optical response. Alkali atom intercalation in 2D transition metal dichalcogenides (TMDs) is relevant to chemical exfoliation methods that are expected to enable large scale production. In this work, focusing on prototypical 2D MoS2, the adsorption and intercalation of Li, Na, K, and Ca adatoms were investigated for the 2H, 1T, and 1T' phases of the TMD by the first principles density functional theory in comparison to experimental characterization of 2H and 1T 2D MoS2 films. Our electronic structure calculations demonstrate significant charge transfer, influencing work function reductions of 1-1.5 eV. Furthermore, electrical conductivity calculations confirm the semiconducting versus metallic behavior. Calculations of the optical spectra, including excitonic effects using a many-body theoretical approach, indicate enhancement of the optical transmission upon phase change. Encouragingly, this is corroborated, in part, by the experimental measurements for the 2H and 1T phases having semiconducting and metallic behavior, respectively, thus motivating further experimental exploration. Overall, our calculations emphasize the potential impact of synthesis-relevant adatom incorporation in 2D MoS2 on the electronic and optical responses that comprise important considerations toward the development of devices such as photodetectors or the miniaturization of electroabsorption modulator components.

Strain-tunable electronic structure of two-dimensional monolayer SiP

The 2D monolayer [Formula: see text]-SiP has a honeycomb lattice and an intrinsic indirect band gap. Herein, the density functional theory calculations are performed to modulate the electronic structure of 2D monolayer [Formula: see text]-SiP by applying strains. The band gap of monolayer [Formula: see text]-SiP is monotonously reduced by the strains. More interestingly, a direct band gap is more likely to be achieved by applying strains along the armchair direction than along the zigzag direction. Finally, 2D monolayer [Formula: see text]-SiP can possess a tunable direct band gap of 1.57–0.73 eV (HSE06) and considerable visible light absorption index, by applying compression strains of −6–−10% along the armchair direction. The work provides a route of modulating the electronic and optical properties of monolayer [Formula: see text]-SiP, which extends its application range for various fields such as electronic devices and solar energy conversion.

Novel properties of transition metal dichalcogenides monolayers and nanoribbons ([formula omitted], where M = Cr, Mo, W and X = S, Se): A spin resolved study

Discovery of two dimensional materials have drawn considerable attention to 2D transition metal dichalcogenides. We have systematically elucidated the tunable spin-resolved electronic and optical properties of monolayers and nanoribbons (armchair and zigzag) of MX2 (where M = Cr, Mo, W and X = S, Se). This study reveals that monolayers and armchair nanoribbons are non-magnetic semiconductors except CrS2 and CrSe2 nanoribbons. The armchair nanoribbons of CrS2 and CrSe2 and all zigzag nanoribbons show magnetic nature. The 7-ACrS2NR, 5-ZCrS2NR and 5-ZWS2NR systems are found to be antiferromagnetic while all other magnetic MX2 TMDs are ferromagnetic. In contrast to all ZMX2NR, the 5-ZCrS2NR shows characteristics of Huesler alloy. In optical properties, there are multiple plasmon frequencies for semiconductor monolayer and metallic nanoribbons. The metallic, ZMX2NR (↓) show transparent behaviour beyond infrared region whereas ZMX2NR(↑), reflect extreme ultraviolet radiations up to ωp. These findings underscore their importance for future applications in spintronics, photovoltaic devices, optoelectronics etc.

In‐Plane and Out‐of‐Plane Optical Properties of Monolayer, Few‐Layer, and Thin‐Film MoS2 from 190 to 1700 nm and Their Application in Photonic Device Design

Monolayer, few‐layer, and thin‐film MoS2 is synthesized using chemical vapor deposition (CVD) and thermal vapor sulfurization (TVS) methods. The complex refractive index of these samples is assessed using variable angle spectroscopic ellipsometry (VASE) measurements over a broad spectral range between 190 and 1700 nm. The ellipsometry data are sensitive to birefringence effects in the thickest thin‐film sample. These birefringence effects are investigated, and an analysis method is developed to extract the in‐plane and out‐of‐plane optical properties. The complex refractive index is then used to calculate reflectance, transmittance, and absorption of the MoS2 films using the transfer‐matrix method (TMM) and is matched with experimentally measured transmittance of the same samples. The modeled results show that the monolayer, few‐layer, and thin‐film MoS2 absorbs 7.4%, 12.6%, and 32.4% of the incident light, respectively, between 300 and 700 nm. When normalized to per unit‐thickness absorption, they absorb 12.1%, 5.9%, and 1.1% nm−1, respectively, clearly showing superior light–matter interaction in the monolayer and few‐layer films. These new complex refractive index data are further used to design optical coatings for these films to either confine absorption in a narrow bandwidth for photodetector applications or enhance broadband absorption for photovoltaic applications.

Novel Two-Dimensional Layered MoSi2Z4 (Z = P, As): New Promising Optoelectronic Materials.

Very recently, two new two-dimensional (2D) layered semi-conducting materials MoSiN and WSiN were successfully synthesized in experiments, and a large family of these two 2D materials, namely MAZ, was also predicted theoretically (Science, 369, 670 (2020)). Motivated by this exciting family, in this work, we systematically investigate the mechanical, electronic and optical properties of monolayer and bilayer MoSiP and MoSiAs by using the first-principles calculation method. Numerical results indicate that both monolayer and bilayer MoSiZ (Z = P, As) present good structural stability, isotropic mechanical parameters, moderate bandgap, favorable carrier mobilities, remarkable optical absorption, superior photon responsivity and external quantum efficiency. Especially, due to the wave-functions of band edges dominated by d orbital of the middle-layer Mo atoms are screened effectively, the bandgap and optical absorption hardly depend on the number of layers, providing an added convenience in the experimental fabrication of few-layer MoSiZ-based electronic and optoelectronic devices. We also build a monolayer MoSiZ-based 2D optoelectronic device, and quantitatively evaluate the photocurrent as a function of energy and polarization angle of the incident light. Our investigation verifies the excellent performance of a few-layer MoSiZ and expands their potential application in nanoscale electronic and optoelectronic devices.

First-principles study of electronic structure and optical properties of monolayer defective tellurene

Monolayer tellurene is a novel two-dimensional semiconductor with excellent intrinsic properties. It is helpful in understanding doping and scattering mechanism to study the electronic structure of defective tellurene, thus it is important for the application of tellurene in electronic and photo-electronic devices. Using first-principles calculation based on the density functional theory, we investigate the effects of commonly seen point defects on the electronic structure and optical properties of monolayer <i>β</i>-Te. Seven kinds of point defects that may be present in <i>β</i>-Te are designed according to the lattice symmetry, including two single vacancies (SV-1, SV-2), two double vacancies (DV-1, DV-2) and three Stone-Wales (SW) defects (SW-1, SW-2, SW-3). It is found that the defect formation energies of these defects are 0.83–2.06 eV, which are lower than that in graphene, silicene, phosphorene and arsenene, suggesting that they are easy to introduce into monolayer <i>β</i>-Te. The two most stable defects are SV-2 and SW-1 where no dangling bond emerges after optimization. The calculated band structures show that all seven defects have little effect on the band gap width of monolayer <i>β</i>-Te, but they can introduce different numbers of impurity energy levels into the forbidden band. Among them, the SV-1, SV-2, DV-1 and SW-2 each act as deep level impurities which can be recombination centers and scattering centers of carriers, SW-1 acts as a shallow level impurity, DV-2 and SW-3 act as both deep level impurity and shallow level impurity. Besides, SW-1, SW-2 and DV-1 can change the band gap of monolayer <i>β</i>-Te from direct band gap to indirect band gap, which may result in the increase of the lifetime of carriers and decrease of photoluminescence of monolayer <i>β</i>-Te. The optical properties of monolayer <i>β</i>-Te, which are sensitive to the change in band structure, are also affected by the presence of defects. New peaks are found in the complex dielectric function and the absorption coefficient of defective monolayer <i>β</i>-Te in an energy range of 0–3 eV, of which the number and the position are dependent on the type of defect. The SV-1, DV-1, DV-2 and SW-2 can enhance the light response, polarization ability and light absorption in the low energy region of monolayer <i>β</i>-Te. This research can provide useful guidance for the applications of <i>β</i>-Te in the electronic and optoelectronic devices.

Highly anisotropic two-dimensional metal in monolayer MoOCl2

Anisotropy is a general feature in materials. Strong anisotropy could lead to interesting physical properties and useful applications. Here, based on first-principles calculations and theoretical analysis, we predict a stable two-dimensional (2D) material---the monolayer ${\mathrm{MoOCl}}_{2}$---and show that it possesses intriguing properties related to its high anisotropy. Monolayer ${\mathrm{MoOCl}}_{2}$ can be readily exfoliated from the van der Waals layered bulk, which has already been synthesized. We show that a high in-plane anisotropy manifests in the structural, phononic, mechanical, electronic, and optical properties of monolayer ${\mathrm{MoOCl}}_{2}$. The material is a metal with highly anisotropic Fermi surfaces, giving rise to open orbits at the Fermi level, which can be probed in magnetotransport. Remarkably, the combination of high anisotropy and metallic character makes monolayer ${\mathrm{MoOCl}}_{2}$ an almost ideal hyperbolic material. It has two very wide hyperbolic frequency windows from 0.41 eV (99 THz) to 2.90 eV (701 THz), and from 3.63 eV (878 THz) to 5.54 eV (1340 THz). The former window has a large overlap with the visible spectrum, and the dissipation for most of this window is very small. The window can be further tuned by the applied strain, such as that at a chosen frequency, a transition between elliptic and hyperbolic character can be induced by strain. Our work discovers a highly anisotropic 2D metal with extraordinary properties, which holds great potential for electronic and optical applications.

Understanding the influence of single metal (Li, Mg, Al, Fe, Ag) doping on the electronic and optical properties of g-C3N4: a theoretical study

ABSTRACT Semi-empirical tight-binding-based quantum chemistry method (GFN2-xTB) has been done to study the influence of single metal (Li, Mg, Al, Fe, Ag) doping on the electronic and optical properties of monolayer corrugated graphitic carbon nitride (g-C3N4). Based on the calculated formation energy and population analysis, we showed that the metal atom is chemically bound to the g-C3N4 surface. The metal doping reduces the vertical ionisation potential and raises the electron affinity and Lewis acidity of g-C3N4. The metal-doped g-C3N4 systems have lower band gap than that of pristine g-C3N4 which is attributed to the electron donating from the metal atom to g-C3N4. Analysis of the lowest unoccupied molecular orbital and the highest occupied molecular orbital shows that for the Fe- and Ag-doped g-C3N4 systems the separation of photo-generated electron–hole pairs is efficient, resulting in the enhancement of photo-catalytic activity.

Enhanced Light Transmittance of Densely Packed Metal Nanoparticle Layers

Irradiation of the metal nanoparticles causes local plasmon resonance in a specific wavelength band, which can improve the absorption and scattering properties of a structure. Since noble metal nanoparticles have better resonance effects than those of other metals, it is easy to identify plasmonic reactions and this is advantageous to find the optical tendency. Compared to having a particle gap or randomly arranged particle structures, densely and evenly packed structures can exhibit more uniform optical properties. Using the uniform properties, the structure can be applied to optical filtering applications. Therefore, in this paper, validation tests about metal nanoparticles and thin film structures are conducted for more accurate analysis. The optical properties of monolayer and bilayer noble metal nanoparticle structures with different diameters, packed in a uniform array, are investigated and their optical trends are analyzed. In addition, a thin film structure under identical conditions as metal nanoparticle structure is evaluated to confirm the improved optical characteristics.