Pristine Monolayer Research Articles

Adsorption of lithium, sodium, gallium, and sulfur atoms onto a GaS monolayer

Abstract Hexagonal gallium sulfide (GaS) monolayer is a very promising monochalcogenide for applications such as electronics, optoelectronics, and catalysts. The adsorption and the diffusion of lithium (Li), sodium (Na), gallium (Ga), and sulfur (S) atoms onto the 2 × 2-GaS hexagonal monolayer are investigated using density functional theory (DFT), along with atomic pseudopotentials. The values of the calculations for the adsorption energy show that the energetically most favorable site for the Li, Na, and Ga adsorbates is the H site, while the most energetically favorable site for the S adsorbate is the TS site. The values calculated for the adsorption energy at the energetically most favorable sites for the Li, Na, Ga, and S atomic adsorbates are −1.853 eV, −1.378 eV, −1.028 eV, and −1.525 eV, respectively. Analysis of the structural properties revealed that after the adsorption process, the GaS+ads system maintains its structure and geometry, since the lattice constants and the lGa-Ga, lGa-S, and lS-S bond lengths do not change significantly with respect to the pristine monolayer. The diffusion of Li, Na, Ga, and S atoms on the 2 × 2-GaS monolayer’s surface shows energy barriers of 28 meV, 40 meV, 72 meV, and 161 meV, respectively. From the total density of states (DOS), it is established that in all cases the GaS+ads monolayer system acquires metallic behavior. Finally, analysis of the Bader charge of the GaS+ads system just at the energetically most favorable sites shows that the Li and Na atoms transfer charge to the monolayer (cations), becoming ionized, while the Ga and S atoms transfer and gain charge from the monolayer, respectively, becoming partially ionized. This electronic behavior makes the GaS monolayer a promising material for use as an anode in batteries.

Programmable Magnetic Hysteresis in Orthogonally-Twisted 2D CrSBr Magnets via Stacking Engineering.

Twisting 2D van der Waals magnets allows the formation and control of different spin-textures, as skyrmions or magnetic domains. Beyond the rotation angle, different spin reversal processes can be engineered by increasing the number of magnetic layers forming the twisted van der Waals heterostructure. Here, pristine monolayers and bilayers of the A-type antiferromagnet CrSBr are considered as building blocks. By rotating 90 degrees these units, symmetric (monolayer/monolayer and bilayer/bilayer) and asymmetric (monolayer/bilayer) heterostructures are fabricated. The magneto-transport properties reveal the appearance of magnetic hysteresis, which is highly dependent upon the magnitude and direction of the applied magnetic field and is determined not only by the twist-angle but also by the number of layers forming the stack. This high tunability allows switching between volatile and non-volatile magnetic memory at zero-field and controlling the appearance of abrupt magnetic reversal processes at either negative or positive field values on demand. The phenomenology is rationalized based on the different spin-switching processes occurring in the layers, as supported by micromagnetic simulations. The results highlight the combination between twist-angle and number of layers as key elements for engineering spin-switching reversals in twisted magnets, of interest toward the miniaturization of spintronic devices and realizing novel spin textures.

Electronic and magnetic properties of the HfO2 monolayer engineered by doping with transition metals and nonmetal atoms towards spintronic applications.

Doping two-dimensional (2D) materials is a vitally important method to modulate their electronic and magnetic properties. In this work, doping with transition metals (TM = Mn and Fe) and nonmetal atoms (X = B and C) is proposed to engineer the magnetism in the HfO2 monolayer. The pristine monolayer is an intrinsically nonmagnetic insulator with a large band gap of 4.85 (6.43) eV as calculated using the PBE (HSE06) functional. Doping with Mn and Fe atoms induces monolayer magnetization with total magnetic moments of 3.00 and 4.00μ B, respectively. Herein, Mn- and Fe-3d electrons produce mainly magnetic properties and regulate the electronic nature by forming new mid-gap energy states. Similarly, the 2p orbital of impurities plays a key role in determining the electronic and magnetic properties of B- and C-doped systems. Mn and B doping leads to the emergence of magnetic semiconductor nature, while the half-metallicity is obtained by doping with Fe and C atoms. Further, the substitution of the Hf-O pair with the TM-X pair is also studied. In these cases, both TM and X impurities induce the system magnetism, exhibiting an antiparallel spin orientation. Consequently, pair-atom-doped systems have smaller total magnetic moments in comparison with single-atom-doped systems. Interestingly, doping with all four Mn-B, Mn-C, Fe-B, and Fe-C pairs induces a magnetic semiconductor nature, where spin-dependent energy gaps are determined by the doping-induced multiple mid-gap energy states. When incorporated into the HfO2 monolayer lattice, transition metals lose charge, while nonmetal impurities act as charge gainers. This study demonstrates the effectiveness of the doping method to engineer the magnetism in the HfO2 monolayer for spintronic applications.

Noble metal (Pd, Pt)-functionalized WSe2 monolayer for adsorbing and sensing thermal runaway gases in LIBs: a first-principles investigation.

This research using the first-principles theory introduces Pd- and Pt-functionalized WSe2 monolayers as promising materials for detecting three critical gases (H2, CO, and C2H4), to evaluate the health of Li-ion battery (LIBs). Various sites on the pristine WSe2 monolayer are considered for the functionalization with Pd and Pt atoms. The adsorption performances of the determined Pd- and Pt-WSe2 monolayers upon the three gases are analyzed by the comparative highlight of the adsorption energy, bonding behavior and electron transfer. Subsequently, an examination of the electronic properties of these monolayers uncovers their semiconducting nature and sensing mechanisms, while the sensing responses are quantified based on variations in their bandgaps. Furthermore, the practical applications of these monolayers are confirmed by assessing their recovery properties. The findings in this study concretely serve the Pd-WSe2 monolayer as a promising CO and C2H4 gas sensor, and the Pt-WSe2 monolayer as an optimal for H2 gas sensor. These findings not only underscore the promising sensing potential of WSe2-based materials for indicating thermal runaway in LIBs, but also emphasize the critical importance of metal selection for surface-functionalization on the nano-surface in the gas sensing technologies.

Theoretical investigations of transition metal atom-doped MoSi2N4 monolayers as catalysts for electrochemical CO2 reduction reactions.

Following the principle of single-atom catalysts (SACs), the fourth-period transition metals (TM) were designed as active sites on a MoSi2N4 monolayer surface with N vacancy, and the catalytic mechanisms of these single-atom active sites for the conversion of CO2 to CO were investigated by first-principles calculations. Our results showed that the doped TM atoms on the MoSi2N4 surface significantly enhanced the CO2 reduction reaction (CO2RR) activity compared with the pristine MoSi2N4 monolayer. Our findings after analyzing all the doped structures in our work were as follows: (1) the Sc-, Ti-, and Mn-doped structures exhibited very low limiting potentials; (2) out of Sc-, Ti- and Mn-doped structures, the Mn@MoSi2N4-Nv structure showed the best catalytic performance with a limiting potential of only -0.16 V, exhibiting an advantage over the hydrogen evolution reaction, which is a competitive reaction of CO2RR. However, the positive binding free energy at 298.15 K of the intermediate reactant *COOH on the Mn@MoSi2N4-Nv surface indicated its unstable state, which hinders the CO generation process. This is contrary to the results derived from the adsorption/binding energy at 0 K, which indicated that the effect of temperature cannot be ignored when considering adsorption/binding energy. Our work provides insights into the effects of temperature on the catalytic mechanisms for CO2RR through TM-doped MoSi2N4 monolayers.

Electronic and magnetic properties of GeP monolayer modulated by Ge vacancies and doping with Mn and Fe transition metals.

In this work, Ge vacancies and doping with transition metals (Mn and Fe) are proposed to modulate the electronic and magnetic properties of GeP monolayers. A pristine GeP monolayer is a non-magnetic two-dimensional (2D) material, exhibiting indirect gap semiconductor behavior with an energy gap of 1.34(2.04) eV obtained from PBE(HSE06)-based calculations. Single Ge vacancy (VaGe) and pair Ge vacancies (pVaGe) magnetize the monolayer significantly with total magnetic moments of 2.00 and 2.02 μ B, respectively. Herein, P atoms around the defect sites are the main contributors to the system magnetism. Similarly, the monolayer magnetization is induced by doping with Mn (MnGe) and Fe (FeGe) atoms. In these cases, total magnetic moments of 3.00 and 4.00 μ B are obtained, respectively, and the system magnetism originates mainly from transition metal impurities. The calculated band structures assert the diluted magnetic semiconductor nature of VaGe and FeGe systems, while pVaGe and MnGe systems can be classified as 2D half-metallic materials. Further, the spin orientation in Mn- and Fe-doped GeP monolayers is studied. Results indicate the antiferromagnetic state in the case of doping with pair transition metal atoms. Regardless of the interatomic distance between dopant atoms, Mn-doped systems exhibit ferromagnetic half-metallicity, where the parallel spin orientation is energetically more favorable than the antiparallel configuration. In contrast, the antiparallel spin orientation is stable in Fe-doped systems, which show the antiferromagnetic semiconductor nature. Results presented herein may introduce new prospective 2D spintronic materials made from non-magnetic GeP monolayers.

Comparative Study of Different Volatile Organic Compound Sensing in Pristine and Defective PtS2 Monolayer- An Ab-initio Calculation-based Theoretical Perspective

Comparative Study of Different Volatile Organic Compound Sensing in Pristine and Defective PtS2 Monolayer- An Ab-initio Calculation-based Theoretical Perspective

Antiferromagnetic semiconductor nature in a GeS2 monolayer doped with Mn and Fe transition metals.

The absence of intrinsic magnetism in two-dimensional (2D) materials demands functionalization as necessary for broadening their applications. In this work, doping with transition metals (Mn and Fe) is proposed to modify the electronic and magnetic properties of a GeS2 monolayer. A pristine monolayer is an indirect gap semiconductor with an energy gap of 0.73(1.47) eV computed by using the PBE(HSE06) functional. Significant magnetism with a total magnetic moment of 1.18μB emerges in the GeS2 monolayer upon creating a single Ge vacancy, which is produced mainly by six nearest neighboring S atoms. In this case, the monolayer is metallized with S-px,y states responsible. Similarly, the magnetization of the GeS2 monolayer is also achieved by doping with Mn and Fe atoms with total magnetic moments of 3.00 and 3.78μB, respectively. The calculated band structures imply that the magnetic semiconductor nature with a spin-up/spin-down gap of 0.72/0.53 eV is induced by Mn impurity, while doping with Fe atoms leads to monolayer metallization. Being surrounded by more electronegative S atoms, Mn and Fe impurities lose charge amounts of 1.19 and 1.11e, respectively. Further investigations on spin coupling indicate the antiferromagnetic semiconductor nature in Mn- and Fe-doped systems, regardless of the distance between impurities. Our results provide important insights into the effects of doping into the GeS2 monolayer, which demonstrate that the doped systems hold promise for spintronic applications.

Improving the thermoelectric performance of the SnS monolayer by Na doping: A theoretical study

Aiming at exploring new thermoelectric (TE) materials of high performance, we theoretically calculate the TE transport properties including the Seebeck coefficient, electrical conductance, thermal conductance, power factor, and figure of merit ZT of the SnS monolayer. Different from the heavy-metal doping, the economical and environment-friendly Na doping is adopted to improve the ZT of the monolayer SnS. It is shown that the Na doping can increase the maximum ZT along the armchair and zigzag directions, respectively, and the highest ZT peak is moved to the proximity of chemical potential μ=0eV, which indicates that the corresponding TE device can work at a low bias voltage. As the temperature increases from 300 K to 800 K, the maximum ZT of the pristine SnS monolayer is increased from 0.89 to 2.26 (from 1.33 to 2.89) along the armchair (zigzag) direction, and the maximum ZT of the Na-doped one is increased from 1.24 to 2.45 (from 1.44 to 2.86) along the armchair (zigzag) direction. This implies that the Na-doped SnS monolayer can be utilized to design promising TE devices working in a broad temperature scope and at a lower bias voltage.

Stability and reversibility of organic molecule modifications of CVD-synthesized monolayer MoS2

We investigated the stability of monolayer MoS2samples synthesized using chemical vapor deposition and subsequently modified with organic molecules under ambient conditions. By analyzing the optical signatures of the samples using photoluminescence spectroscopy, Raman spectroscopy, and surface quality using atomic force microscopy, we observed that this modification of monolayer MoS2with organic molecules is stable and retains its optical signature over time under ambient conditions. Furthermore, we show the reversibility of the effects induced by the organic molecules, as heating the modified samples restores their original optical signatures, indicating the re-establishment of the optical properties of the pristine monolayer MoS2.

Effect of Doped Carbon Atoms on Electronic Structure and Optical Properties of Monolayer 1T-ZrS2

In this paper, a method based on density functional theory is used to replace varying numbers of sulfur atoms with carbon atoms in the original monolayer ZrS2 system, resulting in a new doped system. The theory is based on the First Principle. The photovoltaic properties of the new carbon-atom doping systems have been calculated and investigated. The pristine system and the carbon-atom doped systems were structurally optimized using the automatic optimization method. It was found that the stability of the structure decreases as the number of carbon atoms increases. Pristine monolayer 1T-ZrS2 is an indirect bandgap material. The results show that after doping carbon atoms in monolayer 1T-ZrS2, the p-type conductivity of the system increases and exhibits metallicity. The density of states analysis shows that the conduction band consists mainly of S-3p, Zr-4d, Zr-4p, Zr-5s and C-2p orbitals, while the valence band consists mainly of S-3p, S-3s, Zr-4d, C-2p and C-2s orbitals. It is concluded that strong hybridization between Zr-d and S-p orbitals is exhibited by both the pristine and doped systems. The analysis of the optical properties shows that the peak absorption coefficient and reflectivity peaks are blue-shifted in the doped system, and these peaks are lower than in the pristine system. The peaks of the real and imaginary parts of the dielectric function are also blue shifted. With the increase of doping concentration, the system’s energy loss decreases, indicating that proper doping can effectively reduce the system’s energy loss. The above studies provide theoretical support for applying ZrS2 in nano-optoelectronics.

Defect dependent electronic properties of WTe2: a first-principles study

Abstract Tungsten ditelluride (WTe2) possesses fascinating electronic structures and exceptional properties that make it highly suitable for use in cutting-edge devices. Defects in WTe2 can have a significant influence on its properties, both in advantageous and disadvantageous ways. Thus, a precise classification is crucial to fully comprehend the potential impacts. Here we report a thorough investigation of the electronic characteristics of intrinsic defects, including point defects, in monolayer WTe2 using first-principles calculations based on density functional theory. Our research suggests that the presence of point defects can cause a notable shift in electronic properties, resulting in a metallic behaviour. This is due to the interesting phenomenon of Fermi-level changing near the band edges. Our research findings indicate that the energy required to form a vacancy in a Te atom is lower compared to that of a vacancy in a W atom. Based on the findings, it appears that Te atom vacancies are more likely to be generated during the synthesis process. Defects like the Te vacancy and Mo substitution in the pristine monolayer of WTe2 result in a subtle reduction in the band gap, while still maintaining its characteristics as a direct band gap semiconductor. Our study reveals that the electronic properties of monolayer WTe2 can be significantly altered by the presence of vacancy defects. This discovery highlights the exciting potential of WTe2 as a promising platform for various electronic applications. Our research is anticipated to have a beneficial impact on the comprehension and control of the characteristics of WTe2, thus expediting the development of nanomaterials in various fields.

Strain effects on electronic structure and optics of al-Doped monolayer GaS

This comprehensive study utilises rigorous first-principles calculations to meticulously investigate the influence of uniaxial tensile strain on the electronic configuration and optical behaviours of aluminium-doped gallium sulphide. The study highlights the modulation of electrical properties and optical responses in Al-doped monolayer GaS through the application of tensile strain. Notably, with increasing tensile strain, the bandgap value of the pristine GaS exhibits a pronounced decline, whereas the Al-doped variant maintains a more stable bandgap. A deeper exploration of the state density reveals the emergence of novel electronic states and energy levels in both the pristine and Al-doped monolayer GaS systems as tensile strain is applied. Crucially, the stabilising effect of the doped Al atoms minimises variations in the state density profiles of the Al-doped monolayer GaS systems, in contrast to their pristine counterparts. This result implies that fine control of the electrical structure and optical characteristics of Al-doped monolayer GaS may be achieved by the application of uniaxial tensile strain. The optoelectronic characteristics of GaS and its doped systems are explained in detail by these discoveries, which also offer a crucial theoretical foundation and direction for the use of these materials in optoelectronic devices.

Adsorption Properties of Metal Atom (Co, V, W, Zr)-Modified MoTe2 for CO, CH3CHO, and C6H6 Gases: A DFT Study.

This study investigates the adsorption characteristics of the pristine MoTe2 monolayer and the metal atom (Co, V, W, Zr)-modified MoTe2 monolayer on the hazardous gases CO, CH3CHO, and C6H6 based on the density functional theory. The adsorption mechanism was studied from the perspectives of molecular density differences, band structures, molecular orbitals, and the density of states. Research analysis showed that the changes in conductivity caused by the adsorption of different gases on the substrate were significantly different, which can be used to prepare gas sensing materials with selective sensitivity for CO, CH3CHO, and C6H6. This study lays a reliable theoretical foundation for the gas sensing analysis of toxic and hazardous gases using metal atom-modified MoTe2 materials.

Thermal transport and spin–phonon interaction in magnetic Janus Cr2X3S3 (X = Br, I) monolayers

Comprehending the relationship between spin and phonons is essential to regulate the lattice thermal conductivity in 2D magnetic materials. Lattice thermal conductivity is a relevant part to consider in magnetic data storage and the working of spin-based devices. In this article, we examined the origin and effect of spin–phonon coupling (SPC) on the lattice thermal conductivity of pristine CrI3 monolayer-based Janus monolayers Cr2X3S3 (X=Br,I). We find a high SPC in these Janus monolayers due to in-plane Cr–S atomic vibrations. We observe a reduction in the lattice thermal conductivity at Janus monolayer magnetic states (∼52.3% in Cr2Br3S3 and ∼63.9% in Cr2I3S3) compared to its paramagnetic states. The analysis is also conducted to determine which magnetic state has more anharmonicity using potential energy wells. A wide range of 2D magnetic materials can benefit from our results in the future development of spin-based thermal devices.

Theoretical insights of 2D Fe3GeTe2/In2Se3 multiferroic vdW heterostructure based gas sensors for high-performance NO detecting

The impact of the harmful gas NO on the atmospheric environment cannot be ignored, so it is necessary to develop efficient gas sensors to detect and absorb NO at room temperature. In this work, we systematically explored the electron transport properties and gas sensing properties of Fe3GeTe2, In2Se3 monolayers and its multiferroic van der Waals (vdW) heterostructures of Fe3GeTe2/In2Se3 for detecting NO by using density functional theory and nonequilibrium Green's function methods. The calculated adsorption energies, electron localisation functions, band structures and density of states indicate that its a chemisorption for the adsorption of NO on Fe3GeTe2 and a physisorption on In2Se3, while the specific adsorption style is highly dependent on the stacking and molecular adsorption sites of different Fe3GeTe2/In2Se3 structures. In addition, the pristine Fe3GeTe2 monolayer and Fe3GeTe2/In2Se3 based nanodevices realized a significant anisotropy for electron transport along the zigzag and armchair directions, and their anisotropic maximum current ratios in the zigzag to armchair directions were 1.90 and 1.89. Meanwhile, the NO sensitivity ratio of Fe3GeTe2 and Fe3GeTe2/In2Se3 based gas sensors can be up to 79 % and 107 %, respectively. This highlights a notably superior gas-sensitive performance of the Fe3GeTe2/In2Se3 heterostructure compared to the Fe3GeTe2 monolayer gas devices. These findings provide valuable references for the application of multiferroic heterostructures in the field of gas sensors.

Hydrogen storage performance of pristine C3N3 monolayers

Hydrogen storage performance of wt = 16.13% for the lithiumless pristine monolayer of C 3 N 3 is obtained. It is found that the monolayer is capable of up to 15 hydrogen molecules with an average binding energy of 0.251 eV. 14 hydrogen molecules are adsorbed on the top and bottom of the monolayer each attaching to the seven ones. The remaining hydrogen molecule is trapped inside the monolayer corresponding hollow. The bond and charge analysis are performed to better clarify the adsorption of the H 2 molecules.

Unveiling elevated curie temperature and exceptional perpendicular magnetic anisotropy in chlorinated monolayer CrI3

Due to the finite band gap and a ferromagnetic ground state, the CrI3 monolayer is attracting great research interests. However, the low Curie temperature of 46 K is one of the major obstacles for spintronics applications. Here, we have investigated the magnetic properties of chlorinated CrI3 monolayer with varying concentration of Cl. We investigated the dynamical and thermal stability by phonon dispersion and ab-initio molecular dynamic simulation. The band gap was decreasing with increasing Cl concentration and a half-metallic state is obtained at Cl concentration of 4.22 × 1014/cm2. The pristine CrI3 monolayer had perpendicular magnetic anisotropy energy of 0.84 meV/Cr. We find that this perpendicular magnetic anisotropy energy can be even enhanced twice by chlorination. Due to the hybridization of I and Cr atom, we found the spin asymmetric density of states in the iodine atom and this strong enhancement in the magnetic anisotropy originated from asymmetric spin features in iodine atom. We propose that the chlorination can increase the Curie temperature up to 204 K and this is almost four times enhanced value. Based on these finding, we propose that the chlorinated CrI3 system can be a promising material for spintronics applications.

Assessing pristine and metal doped C2N monolayer as a nanocarriers for anticancer drug

Theoretical research has introduced two-dimensional structures of thin sheets known as the pristine C2N monolayer and the Zn-doped C2N monolayer. These sheets show promise as nanocarriers for delivering the anticancer drug purinethol (PU). Through calculations of binding energy (Eb), it was observed that both the pristine C2N monolayer (−0.505 eV) and the Zn-decorated C2N monolayer (−0.762 eV) exhibit favorable characteristics for drug delivery. Eb values fall within range of physisorption, indicating their suitability as candidates for transporting drugs. An observed charge transfer (CT) of 0.035 e occurs in the Zn-decorated C2N monolayer, leading to a depletion of charge in the Zn-doped C2N monolayer system. The primary contributor to this charge loss is the Zn atom, which experiences a charge reduction of 0.035 e. To understand the phenomenon of drug release, the binding energy was recalculated under biological conditions, specifically in an acidic environment. The results indicate a decline in Eb (−0.218 eV) as well as a short recovery time, suggesting successful release of PU within body. The theoretical predictions we have made are expected to serve as inspiration for experimental researchers in their efforts to design drug delivery systems (DDSs) based on C2N monolayers.

Doping engineering modulated adsorption and sensing performance of β-tellurene towards greenhouse gas molecules

Abstract CO2, CH4, CF4, CCl3F, CCl2F2, HCF2Cl, N2O and SF6 are well-known greenhouse gases that cause serious threat to the earth's ecological environment. To expand the application and development of two-dimensional (2D) materials in the field of greenhouse gas sensing, adsorption of the greenhouse gases on the pristine β-tellurene monolayer was investigated by first-principles calculations to estimate the potential application of β-tellurene as a monitor for greenhouse gas. The results indicate that β-tellurene exhibits favorable adsorption capabilities for greenhouse gases, especially demonstrating selective sensing potential for SF6 molecules due to the changes in electronic structures after gas exposure. The effects of noble metal atoms doping on structural, electronic and SF6 sensing properties were systematic estimated. The calculation results revealed that doping with different transition metal (TM) atom could bring diverse electronic properties to β-tellurene. Among them, doping with Os, Pd, Pt, Rh, and Ru could effectively enhance the electronic delocalization, improving the detection sensitivity for β-tellurene. In addition, TM doping could also improve the recovery time of β-tellurene by two orders of magnitude, and provided the possibility for β-tellurene as a work function type sensing material. By delving into the gas sensing properties of β-tellurene with TM doping, we provided a valuable guidance for the design of innovative tellurene- based sensing 2D materials for devices and technologies.