Monolayer Materials Research Articles

Janus layers and electronic structure of 1T-(TiSeS)2.

TiS2-TiSe2 is one of the most studied titanium based solid solution systems. However, so far, all research on it has only focused on its disordered phase. Here, we systematically investigate its ordered phases. Using a structure search method based on the particle swarm optimization (CALYPSO) algorithm, we identify TiSeS-156 and discover a new structure (1T-(TiSeS)2). Based on first principles theory, their phonon spectra, formation energy, mechanical, electronic, thermal, and optical properties, as well as chemical bond analysis and synthetic pathways, have been investigated. The primitive cell of TiSeS-156 has three atoms and has a space group of P3m1 (no. 156). 1T-(TiSeS)2 has six atoms and has P3̄m1 symmetry (no. 164). TiSeS-156 and 1T-(TiSeS)2 are constructed by stacking the S-Ti-Se Janus layer materials. TiSeS-156 and 1T-(TiSeS)2 are narrow-gap semiconductors. The localized nature of the Ti(3d) states of TiSeS-156 and 1T-(TiSeS)2 leads to their semiconductor properties. 1T-(TiSeS)2 and TiSeS-156 have very similar mechanical, electronic, thermal, and optical properties of 1T-TiS2 and 1T-TiSe2, and are members of the 2D hexagonal lattice transition metal dichalcogenide layered material family. However, compared with 1T-TiS2 and 1T-TiSe2, TiSeS-156 and 1T-(TiSeS)2 have a wider range of potential applications, such as photovoltaic devices and photocatalysis, due to their S-Ti-Se Janus layer structure. They also provide a pathway for the preparation of Janus TiSeS monolayer and multi-layer materials. Moreover, our findings provide crucial insights for understanding the rich and complex crystal structures of the TiS2-TiSe2 system, which have broad implications for further exploration of this class of promising materials.

Perovskenes: two-dimensional perovskite-type monolayer materials predicted by first-principles calculations.

Inspired by the successful transfer of freestanding ultrathin films of SrTiO3 and BiFeO3 onto various substrates without any thickness limitation, in this study, using density functional theory (DFT), we assessed the structural stability of a group of two-dimensional perovskite-type materials which we call perovskenes. Specifically, we analyzed the stability of 2D SrTiO3, SrZrO3, BaTiO3, and BaZrO3 monolayers. Our simulations revealed that the 2D monolayers of SrTiO3, BaTiO3, and BaZrO3 are at least meta-stable, as confirmed by cohesive energy calculations, evaluation of elastic constants, and simulation of phonon dispersion modes. With this information, we proceeded to investigate the electronic, optical, and thermoelectric properties of these perovskenes. To gain insight into their promising applications, we investigated the electronic and optical properties of these 2D materials and found that they are wide bandgap semiconductors with significant absorption and reflection in the ultraviolet (UV) region of the electromagnetic field, suggesting them as promising materials for use in UV shielding applications. In addition, evaluating their thermoelectric factors revealed that these materials become better conductors of electricity and heat as the temperature rises. They can, hence, convert temperature gradients into electrical energy and transport electrical charges, which is beneficial for efficient power generation in thermoelectric devices. This work opens a new window for designing a novel family of 2D perovskite type materials termed perovskenes. The vast variety of different perovskite compounds and their variety of applications suggest deeper studies on the perovskenes materials for use in innovative technologies.

Tunable photoelectric properties of monolayer Mo1-x W x Te2 alloys: a first-principles study.

Monolayer MoTe2 and WTe2 within the two-dimensional transition metal dichalcogenides (TMDCs) material family exhibit broad potential for application in optoelectronic devices owing to their direct band gap characteristics. In this work, upon alloying these materials into a monolayer system denoted as Mo1-x W x Te2, intriguing alterations are observed in the electronic and optoelectronic properties. The photoelectric attributes of these alloys can be tailored by manipulating the respective ratios of molybdenum to tungsten (Mo/W). This investigation employs first-principles calculations based on density functional theory (DFT) to assess physical traits of two-dimensional monolayered structures composed from varying compositions of Mo1-x W x Te2. Our findings reveal that while maintaining a direct band gap characteristic across all compositions studied, there is also a reduction observed in electron effective mass near the Fermi level. Moreover, changing in the Mo/W ratio allows gradual adjustments in electronic properties such as density of states (DOS), work function, dielectric function, absorptivity, and reflectivity. Phonon dispersion curves further demonstrate the stability of Mo1-x W x Te2 systems. Notably, Mo0.5W0.5Te2 exhibits lower polarizability and reduced band gap when compared against MoTe2 and WTe2 counterparts. This research underscores how alloying processes enable customizable modifications in the electronic and optoelectronic properties of Mo1-x W x Te2 monolayer materials which is essential for enhancing nanoscale electronic and optoelectronic device design.

Two-dimensional fullerene-based monolayer materials assembled by C80 and Sc3N@C80.

Construction of two-dimensional (2D) materials using fullerenes as building blocks has attracted particular attention, primarily due to their ability to integrate desired functionalities into devices. However, realization of stable 2D phases of polymerized fullerenes remains a big challenge. Here, we propose two stable 2D monolayer phases with covalently bridged C80 cages, namely α-C80-2D and β-C80-2D, which are semiconductors with strong absorption in the long wave range and appreciable carrier mobility, respectively. The high stability originates from the bond energy released by the [2+2] cycloaddition polymerization of C80 is greater than the deformation energy of a cage. Starting from α-C80-2D, endohedral incorporation of the Sc3N molecule into each C80 cage leads to 2D semiconductors of α-Sc3N@C80-2D and α'-Sc3N@C80-2D, which possess exceptional stability and diverse physical properties, including unique electronic band structures, strong optical absorption in the visible (VIS) to near-infrared (NIR) regime, and anisotropic optical characteristics. Remarkably, a temperature-induced order-disorder transition in the α-Sc3N@C80-2D phase has been observed at elevated temperatures above 600 K. These findings expand the family of 2D carbon materials and provide useful clue for the potential applications of fullerene-assembled monolayer networks.

Multiferroic properties and giant piezoelectric effect of a 2D Janus WO3F monolayer.

Materials possessing both ferroelectricity and ferromagnetism are regarded as ideal candidates for electronic devices, such as nonvolatile memories. Based on first-principles calculations, we systematically studied the crystal structure, electronic structure as well as magnetic, piezoelectric and ferroelectric properties of a two-dimensional van der Waals WO3F monolayer material. The WO3F monolayer was found to possess a robust square crystal structure, exhibiting exceptional stability and mechanical resilience. Magnetic characterization revealed that the material displayed a ferromagnetic state with a magnetic moment of 1μB with negligible magnetic anisotropy. In terms of ferroelectric properties, the WO3F monolayer demonstrated pronounced in-plane polarization, which is in stark contrast to its relatively weak out-of-plane polarization and indicative of anisotropic polarization behavior. Additionally, the material's piezoelectric response could be modulated through strain engineering, with its piezoelectric coefficient (d11) at 4% tensile strain, which exceeds that of the vast majority of known 2D piezoelectric materials, thus underscoring its potential for versatile multifunctional applications in diverse fields, including sensing, energy harvesting, and actuator technologies.

Dual-Use Self-Assembled Monolayer Controlling Charge Carrier Extraction in Organic Solar Cells.

The development and use of interface materials are essential to the continued advancement of organic solar cells (OSCs) performance. Self-assembled monolayer (SAM) materials have drawn attention because of their simple structure and affordable price. Due to their unique properties, they may be used in inverted devices as a modification layer for modifying ZnO or as a hole transport layer (HTL) in place of typical poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in conventional devices. In this work, zinc oxide (ZnO) is modified using five structurally similar SAM materials. This resulted in a smoother surface, a decrease in work function, a suppression of charge recombination, and an increase in device efficiency and photostability. In addition, they can introduced asfor hole extraction layer between the active layer and MoO3, enabling the use of the same material at several functional layers in the same device. Through systematic orthogonal evaluation, it is shown that some SAM/active layer/SAM combinations still offered device efficiencies comparable to ZnO/SAM, but with improved device' photostability. This study may provide recommendations for future SAM material's design and development as well as a strategy for boosting device performance by using the same material across both sides of the photoactive layer in OSCs.

Light Confinement in Twisted Single-Layer 2D+ Moiré Photonic Crystals and Bilayer Moiré Photonic Crystals

Twisted photonic crystals are photonic analogs of twisted monolayer materials such as graphene and their optical property studies are still in their infancy. This paper reports optical properties of twisted single-layer 2D+ moiré photonic crystals where there is a weak modulation in z direction, and bilayer moiré-overlapping-moiré photonic crystals. In weak-coupling bilayer moiré-overlapping-moiré photonic crystals, the light source is less localized with an increasing twist angle, similar to the results reported by the Harvard research group in References 37 and 38 on twisted bilayer photonic crystals, although there is a gradient pattern in the former case. In a strong-coupling case, however, the light source is tightly localized in AA-stacked region in bilayer PhCs with a large twist angle. For single-layer 2D+ moiré photonic crystals, the light source in Ex polarization can be localized and forms resonance modes when the single-layer 2D+ moiré photonic crystal is integrated on a glass substrate. This study leads to a potential application of 2D+ moiré photonic crystal in future on-chip optoelectronic integration.

Controlling the Charge Density Wave Transition in Single-Layer TiTe2xSe2(1-x) Alloys by Band Gap Engineering.

Closing the band gap of a semiconductor into a semimetallic state gives a powerful potential route to tune the electronic energy gains that drive collective phases like charge density waves (CDWs) and excitonic insulator states. We explore this approach for the controversial CDW material monolayer (ML) TiSe2 by engineering its narrow band gap to the semimetallic limit of ML-TiTe2. Using molecular beam epitaxy, we demonstrate the growth of ML-TiTe2xSe2(1-x) alloys across the entire compositional range and unveil how the (2 × 2) CDW instability evolves through the normal state semiconductor-semimetal transition via in situ angle-resolved photoemission spectroscopy. Through model electronic structure calculations, we identify how this tunes the relative strength of excitonic and Peierls-like coupling, demonstrating band gap engineering as a powerful method for controlling the microscopic mechanisms underpinning the formation of collective states in two-dimensional materials.

Chemical Bonding and Activity of Atomically Dispersed Silicon in Two- and Three-Dimensional Materials.

On the basis of the especially tunable electronic property of Si, several kinds of nanomaterials with atomically dispersed Si were constructed and characterized by extensive first-principles calculations and ab initio molecular dynamics (AIMD) simulations. The new-type Si(X≡Y)n wide-bandgap semiconductors featuring through-space d-π* hyperconjugation exhibit unique properties in photoelectric conversion, photoconductivity, structural mechanics, etc. The SiC8 siligraphene with the planar tetracoordinate Si (ptSi) has a high lithium-storage capacity and comparably facile surface migration behaviors of both Li and Li+, making it a promising anode material for high-performance Li-ion batteries. The atomically dispersed Si sites of 2D monolayer materials, such as ptSi and three- and four-coordinated Si atoms, generally exhibit remarkable catalytic activity toward CO2 activation with different electron mechanisms, resulting in different scaling relations between the activity and the p-band center. The computational findings enrich the understanding of structural and chemical properties of silicon and open up avenues for developing Si-based functional materials.

Theoretical prediction of stable WB4 monolayer as a high-capacity anode material for alkali-metal ion batteries

As a rapidly evolving 2D monolayer material, tungsten tetraboride MBene exhibits innate advantages in electrochemical applications because of its unique graphene-like structure and metallic properties. In this study, we utilized first-principles calculations to identify the potential applications of WB4 as an anode material in rechargeable alkali-metal-ion batteries. The findings imply that the Li, Na, and K adsorption on the WB4 surface results in exceptionally high conductivity, and the WB4 monolayer can effectively adsorb these ions with significant adsorption energies of −2.516 eV, −2.356 eV, and −2.941 eV for Li, Na, and K ions, respectively. The results indicate that WB4 exhibits excellent stability during lithiation, sodiation, and potassiation. Notably, we propose high storage capacities for LIBs (708mAh/g), SIBs (472mAh/g), and KIBs (177 mAh/g), with maintained structural stability for the adsorption of these metal ions. The measured average open circuit voltages for WB4 were found to be 0.77 V (Li), 0.73 V (Na), and 0.80 V (K), and metallicity remain good maintained within the entire adsorption process. The calculated migration energy barriers for Li, Na, and K were 0.47 eV,0.21 eV and 0.13 eV respectively. The notable properties of WB4 make it an advantageous electrode material for alkali-metal ion batteries.

Surface-confined two-dimensional mass transport and crystal growth on monolayer materials

Surface-confined two-dimensional mass transport and crystal growth on monolayer materials

Modulation of electronic and optical properties of BlueP/MoSSe heterostructures via biaxial strain and vertical electric field

Constructing van der Waals heterostructures (vdWHs) is an efficient approach for enhancing the desirable properties of two-dimensional (2D) materials and greatly expanding the range of applications of the original monolayer materials. The structure, stability, electronic and optical properties of BlueP/MoSSe heterostructures are explored by density functional theory (DFT) calculations. The different configurations of BlueP/MoSSe vdWHs are all indirect bandgap semiconductors and have similar energy band structures, with the bandgap of about 1.0 eV under the PBE method. The bandgap of the A3 (B3) configuration calculated with HSE06 method is 1.608 (1.377) eV. The A3 configuration exhibits type-II band alignment while the B3 configuration shows type-I band alignment, both of which have high stability. The bandgap and band edge of A3 (B3) configuration can be modulated effectively by biaxial strain and vertical electric field (Efield). The BlueP/MoSSe vdWHs have broader absorption range and higher absorption intensity than their monolayers. The optical absorption intensity of heterostructures is gradually improved with increasing compressive strain, and the optical absorption spectrum is red-shifted under tensile strain. We hope that our findings will provide meaningful theoretical guidance for the preparation and potential application of BlueP/MoSSe vdWHs.

Rattling-like behavior and band convergence induced ultra-low lattice thermal conductivity in MgAl2Te4 monolayer

Inspired by the excellent stability exhibited by experimentally synthesized two-dimensional (2D) MoSi2N4 layered material, the thermal and electronic transport, and thermoelectric (TE) properties of MgAl2Te4 monolayer are systematically investigated using the First-principles calculations and Boltzmann transport theory. The mechanical stability, dynamic stability, and thermal stability (900 K) of the MgAl2Te4 monolayer are demonstrated, respectively. The MgAl2Te4 monolayer exhibits a bandgap of 1.35 eV using the HSE06 functional in combination with spin-orbit coupling (SOC) effect. Band convergence in the valence band is favorable to improve the thermoelectric properties. The rattling thermal damping effect caused by the weak bonding of MgTe bonds in MgAl2Te4 monolayer leads to ultra-low lattice thermal conductivity (0.95/0.38 W/(m·K)@300 K along the x-/y-direction), which is further demonstrated by the phonon group velocities, phonon relaxation time, Grüneisen parameters, and scattering mechanisms. The optimal zT of 3.28 at 900 K is achieved for the p-type MgAl2Te4 monolayer, showing the great promising prospect for the excellent p-type thermoelectric material. Our current work not only reveals the underlying mechanisms responsible for the excellent TE properties, but also elaborates on the promising thermoelectric application of MgAl2Te4 monolayer material at high temperature.

Phosphoborane nanosheets as a sensing element for liquefied petroleum gas – A first-principles study

Nowadays, elemental monolayer materials are focused by the research communities due to their excellent selectivity, sensitivity, and stability toward air pollutants. In the present work, we deployed stable phosphoborane as a sensor to detect Liquefied Petroleum Gas (LPG) gases, namely isobutane and n-propane. Initially, the structural stability of pure phosphoborane is verified by cohesive energy. Further, the electronic characteristics of phosphoborane are studied with the support of band structure and projected density of states (PDOS) spectrum. The band gap of phosphoborane is calculated to be 2.734 eV (hybrid-GGA/B3LYP) which shows a semiconducting nature. Significantly, the adsorption properties of isobutane and n-propane molecules on phosphoborane are studied by ascertaining adsorption energy, relative band gap variation, and Mulliken charge transfer. In particular, the computed adsorption energy is noticed in the physisorption regime (−0.225 eV to −0.774 eV) and the Mulliken charge transfer is found in the range of 0.027 e to 0.223 e that infers the proposed phosphoborane is a promising sensor to detect the LPG molecules.

Structural and electronic properties of novel BeN4/MgN4 nanoribbons

Nanostructures of novel materials are capable of altering the properties of materials quite drastically. Recently discovered Dirac monolayer BeN4 has drawn much attention since this Nitrogen-based material’s electronic structure is very similar to graphene. In this work, we have analyzed various possiblities of nanoribbons structures of MN4 (M=Be, Mg) monolayer materials. We have proposed several possible geometries of BeN4 and MgN4 nanoribbon structures. More specifically, we have proposed armchair nanoribbons with three possible edge configurations as well as zigzag nanoribbons based on edge geometry. Owing to the changes in coordination environment of nanoribbon edge atoms, the resulting bonding characteristics gets altered at the edges, leading to interesting electronic properties. The electronic structures of these nanoribbons are quite different from monolayer and reveal fascinating edge states that exclusively depend on the edge configuration. Though the armchair nanoribbons are found to be metallic, the electronic structure of zigzag nanoribbons exhibits band gap which lead to huge tunability potential of the investigated nanostructures. Therefore, these nanoribbons can function both, metallic as well as semiconducting as required in device applications which can be tuned by modulating the material geometry.

Theoretical exploration of promising optoelectronic two-dimensional materials MSi2N4 (M=Cr, Mo, W)

The electronic properties of two-dimensional MSi2N4 (M = Cr, Mo, W) monolayer have been systematically investigated by strain, electric field, bilayer heterostructures, and adsorption of environmental gas molecules. The results indicate that MSi2N4 have excellent thermodynamic, chemical stability and they are semiconductor materials with perfect absorption performance in the visible and ultraviolet regions. As the tensile strain increases, the band gap of MSi2N4 gradually decreases, and these two-dimensional monolayer materials undergo a phase transition from semiconductor to spin gapless semiconductor to half-metal to metal. The bilayer heterostructures and the external electric field can effectively regulate the band gap of the MSi2N4 monolayer. Some environmental gas molecules can be effectively adsorbed, and the vast majority of adsorption models do not affect the band gap of the MSi2N4. The diverse physical properties and adjustable bandgap characteristics exhibited by MSi2N4 mean that they are promising candidate for optoelectronic functional materials.

Reducing interfacial thermal resistance between polyethylene oxide-based solid-state polymer electrolyte and lithium anode by using IVA group two-dimensional materials: A molecular dynamics study

Solid-state lithium-ion batteries, as a new generation of high energy density electrochemical energy storage devices, need to meet more stringent thermal management standards. In this work, the interfacial thermal transport phenomena between a polyethylene oxide based solid electrolyte and a lithium anode in solid-state lithium-ion batteries was investigated by using a non-equilibrium molecular dynamics simulation method. To reveal the internal heat transport mechanism of the batteries at the microscopic level and to enhance the heat dissipation in them, IVA group two-dimensional monolayer materials such as graphene, silicene, and germanene were inserted at the interface and the interfacial thermal resistances were calculated to compared with the pristine interface. It was found that interfacial interactions were enhanced and the lattice vibrations were coupled by inserting the monolayer materials. Also, compared to the pristine interface, the interfacial thermal resistance was reduced by 83.89 %, 76.05 % and 55.99 %, respectively. Thus, this work provides valuable insights into the thermal transport between electrodes and electrolytes in solid-state lithium-ion batteries, and provides methodological references for improving the heat dissipation and thermal management efficiency inside the batteries.

Protein/Protein and Quantum Dot/Protein Organization in Sequential Monolayer Materials Studied Using Resonance Energy Transfer

Controlled junctions of proteins and nanomaterials offer multiple potential applications in the further construction of nanobiodevices. One of the possible junction types is a set of sequential monolayers of various components deposited on a given substrate. The advantage of such an organization is its high sensitivity, resulting from a huge surface covered by molecules or particles. What is more, the molecules/particles adsorbed on a substrate might be easier to handle than the assay in a cuvette. For further application, there should be crosstalk between monolayers; this is defined by the type of individuals forming a complex system. Here, we are studying, using mainly confocal microscopy and FLIM imaging, crosstalk through resonance energy transfer. The sequential monolayers of fluorescent proteins and CdTe quantum dots were deposited on a convenient substrate, a polyvinylidene difluoride membrane. First, we found that the degree of coverage is lower in the second monolayer. Hence, by manipulating the order of deposition, we obtained a system with a varied yield of resonance energy transfer with a donor excess or an acceptor excess. For a deeper understanding of the energy transfer and its limitations in this system influencing the assay pursuit, we utilized Monte Carlo computation. We found that, indeed, the distance between the monolayers, as well as the degree of coverage, is crucial. With the results of the simulation, we might estimate the relative degree of coverage in our sequential monolayers. We also found that in quantum-dots/protein-composed systems, the yield is stronger than predicted by Monte Carlo simulation. Hence, there should be protein reorientation on the nanoparticle surface, leading to such an effect. Finally, we showed that the yield of resonance energy transfer may be modulated by the external application of poly-L-lysines. These chemicals influenced QD fluorescence but not protein fluorescence and might be used, therefore, as a trigger or a switch in nanobiodevices employing those types of sequential monolayers.

An efficient GaS/ XTe2 (X=W, Mo) vdW heterstructure photocatalyst for water splitting: The first-principles study

In this paper, the electrical and optical properties of GaS/MoTe2 and GaS/WTe2 van der Waals heterostructure are investigated by first-principles methods, and their properties are modulated by applying biaxial strain and doping. It was revealed that the unstrained GaS/MoTe2 and GaS/WTe2 van der Waals heterostructures is semiconducting with an indirect bandgap of 1.16 eV and 1.28 eV using PBE methods, and the direction of the built-in electric field of the heterojunction is XTe2 pointing to GaS, which is perfect for powerful light absorption. The bandgap widths of the two heterostructure are significantly reduced compared with that of monolayer materials. Besides, the results show that the biaxial strain is an effective tool for adjusting the energy gap where the indirect energy gap under the strain effects ranged within a range of 0.33/0.34 eV at −8% to 0.361/0.464 eV at 6%. At −8% compressive strain, the interfacial charge transfer reaches a maximum for each of the two heterostructure. In addition the absorption coefficients of GaS/MoTe2 and GaS/WTe2 heterostructure increase in the visible and infrared regions and the absorption spectral range is broadened. Therefore, GaS/MoTe2 and GaS/WTe2 vdW heterostructure can be applied in photocatalytic water decomposition and other fields.

Viability of Glycolysis for the Chemical Recycling of Highly Coloured and Multi-Layered Actual PET Wastes.

The chemical recycling of poly(ethylene terephthalate) -PET- fractions, derived from actual household packaging waste streams, using solvolysis, was investigated. This recycling strategy was applied after a previous on-line automatic identification, by near-infrared spectroscopy -NIR-, and a subsequent selective sorting of the different PET materials that were present in the packaging wastes. Using this technology, it was possible to classify fractions exclusively including PET, virtually avoiding the presence of both other plastics and materials, such as paper, cardboard and wood, that are present in the packaging wastes, as they were efficiently recognised and differentiated. The simple PET fractions, including clear and monolayered materials, were adequate to be recycled by mechanical means meanwhile the complex PET fractions, containing highly coloured and multi-layered materials, were suitable candidates to be recycled by chemical routes. The depolymerisation capacity of the catalytic glycolysis, when applied to those complex PET wastes, was studied by evaluating the effect of the process parameters on the resulting formation and recovery of the monomer bis(2-hydroxyethyl) terephthalate -BHET- and the achieved quality of this reaction product. Comparable and reasonable results, in terms of monomer yield and its characteristics, were obtained independently of the type of complex PET waste that was chemically recycled.