Adsorption of gas molecules on monolayer MoS2 and effect of applied electric field

A negative photoconductivity photodetector based on two-dimensional Nb3Cl8

Negative photoconductivity (NPC)-based photodetectors offer a new direction for energy-efficient photodetection technologies, featuring low energy consumption and high responsivity. Two-dimensional (2D) materials are particularly promising for implementing NPC due to their large surface area, abundant surface states, and tunable bandgap properties. In this context, 2D Nb3Cl8, with its unique kagome lattice structure and broad absorption spectrum, has attracted considerable interest. Notably, metal halides such as Nb3Cl8 demonstrate significant potential as NPC materials due to their low anionic and cationic bonding strength, which allows for the formation of vacancy defects with high probability. However, the NPC characteristics of Nb3Cl8 have not been thoroughly investigated. In this study, we fabricated field-effect transistors (FETs) using Nb3Cl8 single crystals synthesized via chemical vapor transport (CVT). These devices exhibited an electron mobility of 4.24 × 10-3 cm2 V-1 s-1 and a high Ion/Ioff ratio of 1.42 × 104. Notably, Nb3Cl8-based photodetectors demonstrated consistent NPC behavior across a wide wavelength range of 400-1050 nm, with a high responsivity of 156.82 mA W-1 at 400 nm. We propose that the trapping effect due to defect levels within the bandgap is the primary cause of this NPC phenomenon. The present findings reveal the unique photodetector properties of Nb3Cl8 and highlight its promise in energy-efficient photodetectors and various optoelectronic applications.

Theoretical Calculation of Dissolved Gas in Transformer Oil Using the Gas Sensitive Properties of Sc- and Ti-Modified ZrS2

To guarantee the secure functioning of the complete power system and minimize the risks associated with oil-filled transformers during their operation, it is greatly important to carry out gas sensing studies of dissolved gases in transformer research. Through the utilization of first-principles density functional theory calculations, the adsorption energy, electronic characteristics, and recuperation duration of ZrS2 modified with Sc and Ti were examined. The results show that compared to those of the initial ZrS2 material, the doping of TM atoms Sc and Ti significantly improved the adsorption properties of the material, and the adsorption of CO and C2H4 showed chemisorption. The adsorption capacity for gases decrease in the following order: C2H4 > CO > H2. The calculated recovery times indicate that Sc-ZrS2 and Ti-ZrS2 were ideal carbon monoxide sensing materials under the specific conditions. The results of this work can establish a fundamental rationale for the use of ZrS2 in sensing the conditions of oil-immersed transformers.

Notable synthesis, properties and chemical gas sensing trends on molybdenum disulphides and diselenides two-dimensional nanostructures: A critical review

Evaluation of synthesis methods, notable properties, and chemical gas sensing properties of molybdenum disulphides and diselenides two-dimensional nanosheets is unfold. This is motivated by the fact that the two dichalcogenides have good sensitivity and selectivity to different harmful gases at ambient temperatures. Synthesis methods explored include exceptional top-down and bottom-up approaches, which consider physical and chemical compositional inceptions. Mechanical exfoliation in both molybdenum disulphides and diselenides nanosheets demonstrate good crystalline purity with structural alterations under varying stacking conditions. These chalcogenides exhibit low energy band gaps of ±1.80 eV for MoS2 and ±1.60 eV for MoSe2, which reduces with introduction of impurities. Thus, upon doping with other metal elements, a transformation from either n-type or p-type semiconductors is normally observed, leading to tuneable electronic properties. Thus, different gases such as methane, ethanol, toluene, ammonia, nitrogen oxide have been systematically detected using molybdenum disulphide and diselenide based thin films as sensing platforms. This review highlights structural, electronic and morphological characteristics of the two dichalcogenides which influences the sensitivity and selectivity ability for a couple of gases at ambient temperatures. The strategies for enhancing the selectivity by introducing defects, impurities and interfacing with other composites expanding the choice of these gases wider is also discussed in details. The review also provides overviews of challenges and limitations that open new research avenues to further enriching both chalcogenides as flexible, stable and cost effective state-of-the-art chemical gas sensors.

First-Principles Study on Bi2Te2S Monolayer for Adsorption Performance and Sensing Capability

In this study, a comprehensive investigation into the gas sensing capabilities of the two-dimensional (2D) Bi2Te2S was conducted using first-principles calculations based on density functional theory. A wide array of gas molecules, including CH4, Cl2, CO, CO2, H2, H2O, H2S, N2, NH3, NO, NO2, O2, and SO2, was encompassed in this work. Through the strategic placement of these gas molecules at different locations on the Bi2Te2S monolayer and taking into account a range of configurations, the adsorption process was thoroughly investigated, with a particular emphasis on the structures that are most thermodynamically stable. It was revealed that Cl2, O2, NO, and NO2 molecules exhibit a pronounced affinity for the Bi2Te2S monolayer. Notably, it was found that the Cl2@Bi2Te2S, O2@Bi2Te2S, and NO2@Bi2Te2S systems' gas adsorption capabilities are greatly enhanced by the introduction of an external electric field. Moreover, the addition of horizontal biaxial strain significantly impacts the gas adsorption properties of the O2@Bi2Te2S system, underscoring the tunability of the Bi2Te2S monolayer's sensing capabilities. In light of these theoretical results, the Bi2Te2S monolayer is proposed to have great potential as an extremely sensitive and selective gas sensing material, especially for identifying Cl2, O2, NO, and NO2. This study clarifies the intrinsic gas sensing capabilities of the Bi2Te2S monolayer, while highlighting how its performance can be tailored in response to external stimuli, setting the stage for the advancement of more sophisticated gas sensing devices.

Electronic and adsorption properties of halogen molecule X2 (X=F, Cl) adsorbed arsenene: First-principles study

The geometry, electronic structure, and adsorption properties of halogen molecule X2(X = F, Cl) on arsenene were investigated using first-principles calculations. The adsorption of molecules was considered at various sites and in various orientations on the pristine arsenene (p-As) surface. Both molecules show chemisorption and the crystal orbital Hamiltonian population (COHP) analysis reveals the formation of strong X-As bonds. In particular, the adsorbed molecules spontaneously dissociate into atomic halogen atoms, with a diffusion barrier of 1.91 (1.72) eV for F2(Cl2). The adsorbed X2 molecules induced distortions in the local geometry due to strong interaction with arsenene. Importantly, the formation of X-As bonding remarkably changed the electronic properties, evidenced by the decrease of the actual band gap due to the emergence of defect states within the band gap. For instance, the F2 adsorbed arsenene system (F2-As) exhibited an average band gap of 1.17 eV, and Cl2 adsorbed arsenene (Cl2-As) showed an average band gap of 0.83 eV. In particular, indirect to direct band gap transition was observed for some adsorption configurations. The reduction in band gap resulted in the enhancement of electrical conductivity. These findings suggest that the electronic properties of arsenene can be tuned by halogen decoration.

Operando Investigation of WS2 Gas Sensors: Simultaneous Ambient Pressure X-ray Photoelectron Spectroscopy and Electrical Characterization in Unveiling Sensing Mechanisms during Toxic Gas Exposure

Ambient pressure X-ray photoelectron spectroscopy (APXPS) is combined with simultaneous electrical measurements and supported by density functional theory calculations to investigate the sensing mechanism of tungsten disulfide (WS2)-based gas sensors in an operando dynamic experiment. This approach allows for the direct correlation between changes in the surface potential and the resistivity of the WS2 sensing active layer under realistic operating conditions. Focusing on the toxic gases NO2 and NH3, we concurrently demonstrate the distinct chemical interactions between oxidizing or reducing agents and the WS2 active layer and their effect on the sensor response. The experimental setup mimics standard electrical measurements on chemiresistors, exposing the sample to dry air and introducing the target gas analyte at different concentrations. This methodology applied to NH3 concentrations of 100, 230, and 760 and 14 ppm of NO2 establishes a benchmark for future APXPS studies on sensing devices, providing fast acquisition times and a 1:1 correlation between electrical response and spectroscopy data in operando conditions. Our findings contribute to a deeper understanding of the sensing mechanism in 2D transition metal dichalcogenides, paving the way for optimizing chemiresistor sensors for various industrial applications and wireless platforms with low energy consumption.

Formation of oxygen protective layer on monolayer MoS2 via low energy electron irradiation

Monolayer molybdenum disulfide (MoS2) semiconductors are the new generation of two-dimensional materials that possess several advantages compared to graphene due to their tunable bandgap and high electron mobility. Several approaches have been used to modify their physical properties for optical device applications. Here, we report a facile and non-destructive surface modification method for monolayer MoS2 via electron irradiation at a low, 5 kV accelerating voltage. After electron irradiation, the results of Raman and photoluminescence spectroscopy confirmed that the structure remains unchanged. However, when the modified surface was illuminated with a 532 nm laser for a prolonged period, the PL intensity was quenched as a result of oxygen desorption. Interestingly, the PL intensity can be recovered when left in ambient conditions for 10 h. The analysis of the PL spectrum revealed a decrease of trion, which is consistent with the readsorbed O2 molecules on the surface that deplete electrons and lead to PL recovery. We attribute this effect to the enhancement of the n-type character of monolayer MoS2 after electron irradiation. The sensitive nature of the modified surface to oxygen suggests that this approach may be used as a tool for the fabrication of MoS2 oxygen sensors.

New Insights into Gas-Solid Interactions of NO2/MoS2 Monolayers: a Comparative Study with MoSe2 and MoTe2 Monolayers

Understanding the microscopic electronic structure determines the macroscopic properties of the materials. Sufficient sampling has the same foundational importance in understanding the interactions. The NO2/MoS2 interaction is well known, but there are still many inconsistencies in the basic data, and the source of the NO2 direct dissociation activity has not been revealed. Based on a large-scale sampling density functional theory (DFT) study, the optimal adsorption of the NO2/MoS2 monolayer system is determined. The impurity state on the top of the valence band of the S-vacancy monolayer (MoS2-VS) was determined by cross-analysis of the band structure and density of states, which has been neglected for a long time. This provides a reasonable explanation for the direct dissociation of NO2 on the MoX2 monolayers. Further atomic structure analysis reveals that the impurity state originates from the not-fully occupied valence orbitals. This also corroborates the fact that the Mo material has dissociation activity, while the W material does not. There is no impurity state on the top of the valence band of the X-vacancy WS2 and WSe2 monolayers. Interestingly, NO2 dissociation did not occur in the MoTe2-VTe monolayer. This may be related to the 6s inert electron pair effect of the Te atom. The double-oriented adsorption behavior of NO2is also revealed. In contrast to the MoSe2 and MoTe2 monolayers, NO2-oriented adsorption on the MoS2 perfect monolayer deviates obviously, which is speculated to be related to space limitation and larger electronegativity of the S atom. The oriented adsorption ability of the MoX2 monolayers followed the order MoTe2 (64.4%) > MoSe2 (44.8%) > MoS2 (42.7%), according to the directed proportion. Renewed insights into the adsorption basic data and the understanding of the electronic structure of NO2/MoX2 (X = S, Se, Te) monolayer systems provide a basic understanding of the gas-surface interactions and various future surface-related advanced applications.

Realization of efficient and selective NO and NO2 detection via surface functionalized h-B2S2 monolayer

In the ever-growing field of two-dimensional (2D) materials, the boron-sulfide (B2S2) monolayer is a promising new addition to MoS2-like 2D materials, with the boron (a lighter element) pair (B2 pair) having similar valence electrons to Mo. Herein, we have functionalized the h-phase boron sulfide monolayer by introducing oxygen atoms (Oh-B2S2) to widen its application scope as a gas sensor. The charge carrier mobilities of this system were found to be 790 × 102 cm2 V-1 s-1 and 32 × 102 cm2 V-1 s-1 for electrons and holes, respectively, which are much higher than the mobilities of the MoS2 monolayer. The potential application of the 2D Oh-B2S2 monolayer in the realm of gas sensing was evaluated using a combination of density functional theory (DFT), ab initio molecular dynamics (AIMD), and non-equilibrium Green's function (NEGF) based simulations. Our results imply that the Oh-B2S2 monolayer outperforms graphene and MoS2 in NO and NO2 selective sensing with higher adsorption energies (-0.56 and -0.16 eV) and charge transfer values (0.34 and 0.13e). Furthermore, the current-voltage characteristics show that the Oh-B2S2 monolayer may selectively detect NO and NO2 gases after bias 1.4 V, providing a greater possibility for the development of boron-based gas-sensing devices for future nanoelectronics.

Investigation of the electronic and optical properties of bilayer CdS as a gas sensor: first-principles calculations

We utilised first-principles computations based on density functional theory to investigate the optical and electronic properties of bilayer CdS before and after the adsorption of gas molecules. Initially, we examined four candidate adsorption sites to determine the best site for adsorbing CO, CO2, SO2, H2S, and SO. In order to achieve the optimal adsorption configurations, we analysed the adsorption energy, distance, and total charge. Our findings reveal that the CdS bilayer forms a unique connection between the O and Cd atoms, as well as the S and Cd atoms, which renders it sensitive to SO2, H2S, and SO through chemical adsorption, and CO and CO2 through strong physical adsorption. The adsorption of gas molecules enhances the optical properties of the CdS bilayer. Consequently, the CdS bilayer proves to be a highly efficient gas sensor for SO2, H2S, and SO gases.

DoE-Assisted Development of a 2H-MoS2 -Catalyzed Approach for the Production of Indole Derivatives

2H-MoS2 is an appealing semiconductor because of its Earth-abundant nature, cheapness, and low toxicity. This material has shown promising catalytic activity for various energy-related processes, but its use in catalysis for C-C bond forming reactions towards useful organic compounds is still largely unexplored. The lack of examples in organic synthesis is mainly due to the intrinsic difficulties of using bulk 2H-MoS2 (e. g., low surface area), which implies the reliance on high catalytic loadings for obtaining acceptable yields. This makes the optimization process more expensive and tedious. Here, we report the development of a 2H-MoS2 -mediated synthesis of valuable bis(indolyl)methane derivatives, using indoles and benzaldehydes as starting materials. Exploiting the Design of Experiments (DoE) method, we identified the critical parameters affecting the catalytic performance of commercial 2H-MoS2 powder and optimized the reaction conditions. Lastly, we demonstrated that the catalytic system has versatility and good tolerance towards functional group variations of the reagents.

Graphene-like emerging 2D materials: recent progress, challenges and future outlook

Owing to the unique physical and chemical properties of 2D materials and the great success of graphene in various applications, the scientific community has been influenced to explore a new class of graphene-like 2D materials for next-generation technological applications. Consequently, many alternative layered and non-layered 2D materials, including h-BN, TMDs, and MXenes, have been synthesized recently for applications related to the 4th industrial revolution. In this review, recent progress in state-of-the-art research on 2D materials, including their synthesis routes, characterization and application-oriented properties, has been highlighted. The evolving applications of 2D materials in the areas of electronics, optoelectronics, spintronic devices, sensors, high-performance and transparent electrodes, energy conversion and storage, electromagnetic interference shielding, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nanocomposites are discussed. In particular, the state-of-the-art applications, challenges, and outlook of every class of 2D material are also presented as concluding remarks to guide this fast-progressing class of 2D materials beyond graphene for scientific research into next-generation materials.

Efficient exploration of transition-metal decorated MXene for carbon monoxide sensing using integrated active learning and density functional theory

The urgent demand for chemical safety necessitates the real-time detection of carbon monoxide (CO), a highly toxic gas. MXene, a 2D material, has shown potential for gas sensing applications (e.g., NH3, NO, SO2, CO2) due to its high surface accessibility, electrical conductivity, stability, and flexibility in surface functionalization. However, the pristine MXene generally exhibits poor interaction with CO; still, transition metal decoration can strengthen the interaction between CO and MXene. This study presents a high-throughput screening of 450 combinations of transition-metal (TM) decorated MXene (TM@MXene) for CO sensing applications using an integrated active learning (AL) and density functional theory (DFT) screening pipeline. Our AL pipeline, adopting a crystal graph convolutional neural network (CGCNN) as a surrogate model, successfully accelerates the screening of CO sensor candidates with minimal computational resources. This study identifies Sc@Zr3C2O2 and Y@Zr3C2O2 as the optimal TM@MXene candidates with promising CO sensing performance regarding the screening criteria of recovery time, surface stability, charge transfer, and sensitivity to CO. The proposed AL framework can be extended for property finetuning in the combinatorial chemical space.

MoTe2/InN van der Waals heterostructures for gas sensors: a DFT study

Vertical van der Waals (vdW) heterostructures have shown potential for gas sensing owing to their remarkable sensitivity. However, the optimization process for achieving the best gas sensing performance is complicated by the heterostructure's reliance on both physical and electrical characteristics. This study employs density functional theory (DFT) to analyse the structural and electronic parameters of a MoTe2/InN vdW heterostructure. The findings of this study indicate that the vdW heterostructure has a type-II band alignment with higher adsorption energy towards NH3, NO2, and SO2 than the individual monolayers. In specific, the heterostructure is well suited for NO2 detection but has limitations in reliably detecting NH3 and SO2 due to longer recovery times. We find significant hybridization between the adsorbate and interacting surfaces' orbitals and a notable presence of NO2 molecular orbitals in proximity to the Fermi level. Additionally, dielectric and work function modulations offer a viable means to develop optical-based gas sensors that can selectively detect NO2. Our research provides valuable insights into vdW heterostructure design for high-performance gas sensors.

Enhancement strategies of poly(ether-block-amide) copolymer membranes for CO2 separation: A review

Poly(ether-block-amide) (Pebax) membranes have become the preferred CO2 separation membrane because of their excellent CO2 affinity and robust mechanical resistance. Nevertheless, their development must be considered to overcome the typical obstacles in polymeric membranes, including the perm-selectivity trade-off, plasticization, and physical aging. This article discusses the recent enhancement strategies as a guideline for designing and developing Pebax membranes. Five strategies were developed in the past few years to improve Pebax gas transport properties, including crosslinking, mobile carrier attachment, polymer blending, filler incorporation, and the hybrid technique. Among them, filler incorporation and the hybrid technique were most favorable for boosting CO2/N2 and CO2/CH4 separation performance with a trade-off-free profile. On the other hand, modified Pebax membranes must deal with two latent issues, mechanical strength loss, and perm-selectivity off-balance. Therefore, exploring novel materials with unique structures and surface properties will be promising for further research. In addition, seeking eco-friendly additives has become worthwhile for establishing Pebax membrane sustainable development for gas separation.

Recent developments in 2D MXene-based materials for next generation room temperature NO2 gas sensors

MXenes with distinctive structures, good electrical conductivity and abundant functional groups have shown great potential in the fabrication of high performance gas sensors. Since the sensing mechanism of MXene-based gas sensors often involves a surface-dominant process, they can work at room temperature. In this regard, a significant amount of research has been carried out on MXene-based room temperature gas sensors and they can be viewed as one of the possible materials for NO2 sensing applications in the future. In this review, we focus on the most recent research and improvements in pure MXenes and their nanocomposites for NO2 gas sensing applications. First, we have explored the mechanisms involved in MXenes for NO2 gas sensing. Following that, other ways to tune the MXene sensing performance are investigated, including nanocomposite formation with metal oxides, polymers, and other 2D materials. A comparative analysis of the RT NO2 sensor performance based on MXenes and their hybrids is provided. We also discuss the major challenges of using MXene-related materials and the areas that can further advance in the future for the development of high-performance room temperature NO2 gas sensors.

A microfluidic approach for the detection of uric acid through electrical measurement using an atomically thin MoS2 field-effect transistor

There is a demand for biosensors working under in vivo conditions, which requires significant device size and endurance miniaturization in solution environments. We demonstrated the detection of uric acid (UA) molecules, a marker of diseases like gout, whose continuous monitoring is required in medical diagnosis. We used a field effect transistor (FET) composed of an atomically thin transition metal dichalcogenide (TMD) channel. The sensor detection was carried out in a solution environment, for which we protected the electrodes of the source and drain from the solution. A microfluidic channel controls the solution flow that can realize evaporation-free conditions and provide an accurate concentration and precise measurement. We detected a systematic change of the drain current with the concentration of the UA in isopropyl alcohol (IPA) solvent with a detection limit of 60 nM. The sensor behavior is reversible, and the drain current returns to its original value when the channel is washed with pure solvent. The results demonstrate the feasibility of applying the MoS2-FET device to UA detection in solution, suggesting its possible use in the solution environment.

Unusual Selective Monitoring of N,N-Dimethylformamide in a Two-Dimensional Material Field-Effect Transistor

N,N-Dimethylformamide (DMF) is an essential solvent in industries and pharmaceutics. Its market size range was estimated to be 2 billion U.S. dollars in 2022. Monitoring DMF in solution environments in real time is significant because of its toxicity. However, DMF is not a redox-active molecule; therefore, selective monitoring of DMF in solutions, especially in polar aqueous solutions, in real time is extremely difficult. In this paper, we propose a selective DMF sensor using a molybdenum disulfide (MoS2) field-effect transistor (FET). The sensor responds to DMF molecules but not to similar molecules of formamide, N,N-diethylformamide, and N,N-dimethylacetamide. The plausible atomic mechanism is the oxygen substitution sites on MoS2, on which the DMF molecule shows an exceptional orientation. The thin structure of MoS2-FET can be incorporated into a microfluidic chamber, which leads to DMF monitoring in real time by exchanging solutions subsequently. The designed device shows DMF monitoring in NaCl ionic solutions from 1 to 200 μL/mL. This work proposes the concept of selectively monitoring redox-inactive molecules based on the nonideal atomic affinity site on the surface of two-dimensional semiconductors.

Heteroatom-Doped Molybdenum Disulfide Nanomaterials for Gas Sensors, Alkali Metal-Ion Batteries and Supercapacitors

Molybdenum disulfide (MoS2) is the second two-dimensional material after graphene that received a lot of attention from the research community. Strong S-Mo-S bonds make the sandwich-like layer mechanically and chemically stable, while the abundance of precursors and several developed synthesis methods allow obtaining various MoS2 architectures, including those in combinations with a carbon component. Doping of MoS2 with heteroatom substituents can occur by replacing Mo and S with other cations and anions. This creates active sites on the basal plane, which is important for the adsorption of reactive species. Adsorption is a key step in the gas detection and electrochemical energy storage processes discussed in this review. The literature data were analyzed in the light of the influence of a substitutional heteroatom on the interaction of MoS2 with gas molecules and electrolyte ions. Theory predicts that the binding energy of molecules to a MoS2 surface increases in the presence of heteroatoms, and experiments showed that such surfaces are more sensitive to certain gases. The best electrochemical performance of MoS2-based nanomaterials is usually achieved by including foreign metals. Heteroatoms improve the electrical conductivity of MoS2, which is a semiconductor in a thermodynamically stable hexagonal form, increase the distance between layers, and cause lattice deformation and electronic density redistribution. An analysis of literature data showed that co-doping with various elements is most attractive for improving the performance of MoS2 in sensor and electrochemical applications. This is the first comprehensive review on the influence of foreign elements inserted into MoS2 lattice on the performance of a nanomaterial in chemiresistive gas sensors, lithium-, sodium-, and potassium-ion batteries, and supercapacitors. The collected data can serve as a guide to determine which elements and combinations of elements can be used to obtain a MoS2-based nanomaterial with the properties required for a particular application.

First-Principles Study of χ3-Borophene as a Candidate for Gas Sensing and the Removal of Harmful Gases

The potential application of borophene as a sensing material for gas-sensing devices is investigated in this work. We utilize density functional theory (DFT) to systematically study the adsorption mechanism and sensing performance of χ₃-borophene to search for high-sensitivity sensors for minor pollutant gases. We compare the results to those for two Pmmn borophenes. The first-principles calculations are used to analyze the sensing performance of the three different borophenes (2 Pmmn borophene, 8 Pmmn borophene, and χ₃-borophene) on five leading harmful gases (CO, NH₃, SO₂, H₂S, and NO₂). The adsorption configuration, adsorption energy, and electronic properties of χ₃-borophene are investigated. Our results indicate that the mechanism of adsorption on χ₃-borophene is chemisorption for NO₂ and physisorption for SO₂ and H₂S. The mode of adsorption of CO and NH₃ on χ₃-borophene can be both physisorption and chemisorption, depending on the initially selected sites. Analyses of the charge transfer and density of states show that χ₃-borophene is selective toward the adsorption of harmful gases and that N and O atoms form covalent bonds when chemisorbed on the surface of χ₃-borophene. An interesting phenomenon is that when 8 Pmmn borophene adsorbs SO₂, the gas molecules are dismembered and strongly adsorb on the surface of 8 Pmmn borophene, which provides a way of generating O₂ while adsorbing harmful substances. Overall, the results of this work demonstrate the potential applications of borophene as a sensing material for harmful gas sensing or removal.

Magnetic ε-Phosphorene for Sensing Greenhouse Gas Molecules

It is critical for gas sensors that sense greenhouse gas molecules to have both good sensitivity and selectivity for water molecules in the ambient environment. Here, we study the charge transfer, IV curves, and electric field tuning of vanadium-doped monolayer ϵ-phosphorene as a sensor for NO, NO₂, and H₂O gas molecules via first-principle and transport calculations. We find that the paramagnetic toxic molecules of NO and NO₂ have a high adsorption energy on V-ϵ-phosphorene, which originates from a large amount of charge transfer driven by the hybridisation of the localised spin states of the host with the molecular frontier orbital. Using the non-equilibrium Green's function, we investigate the IV responses with respect to the adsorption of different molecules to study the performance of gas molecule sensors. Our IV curves show a larger amount of changes in resistance of the paramagnetic NO and NO₂ than nonmagnetic H₂O gas molecules, suggesting both sensitivity and selectivity. Moreover, our calculations show that an applied external electric field (gate voltage) can effectively tune the amount of charge transfer. More charge transfer makes the sensor more sensitive to the molecule, while less charge transfer can reduce the adsorption energy and remove the adsorbed molecules, allowing for the repeated use of the sensor.

Photoelectric structure and magnetic changes caused by niobium disulfide adsorbing (non)-metal atoms under defects

CONTEXT

The property transition between metal and semiconductor is the key to improving the properties of transition metal dichalcogenides (TMDCs). The adsorption of the NbS₂ compound in the defect state was adjusted for the first time. The hybrid system overwrites the original surface mechanism of NbS₂ and induces indirect band gaps. This modulation mode makes NbS₂ convert into a semiconductor and effectively improves the catalytic activity of the material in the system. In addition, the original local magnetic moment of the compound is concentrated in the vacancy region and is improved. The optical properties of the adsorption system indicate that NbS₂ compounds can be effectively applied in visible and low-frequency ultraviolet regions. This provides a new idea for the design of the NbS₂ compound as a two-dimensional photoelectric material.

METHODS

In the study, we assume that only one atom is adsorbed on the NbS₂ supercell of the defect, and the distance between the two adjacent atoms exceeds 12.74 Å, so the interaction between atoms is ignored in the study. Adsorbed atoms include nonmetallic elements (H, B, C, N, O, F), metallic elements (Fe, Co), and noble metal elements (Pt, Au, Ag). The density functional theory (DFT) was used in the experiment. The non-conservative pseudopotential method was used in the calculation to optimize the crystal structure geometrically. The approximate functional is Heyd-Scuseria-Ernzerhof (HSE06). The calculation method includes the spin-orbit coupling (SOC) effect. The crystal relaxation optimization uses a 7 × 7 × 1 k point grid to calculate niobium disulfide's photoelectric and magnetic properties. A vacuum space of 15Å is introduced in the direction outside the plane, and the free boundary condition is adopted to avoid the interaction between atomic layers. For the convergence parameter setting, the interatomic force of all composite systems is less than 0.03 eV/Å, and the lattice stress is less than 0.05 Gpa.

Deposition of Ultrathin MgB2 Films from a Suspension Using Cosolvent Marangoni Flow

Magnesium diboride (MgB₂) has demonstrated, theoretically and experimentally, promise as a candidate material for hydrogen storage and has thus attracted much contemporary research interest. To study hydrogen gas adsorption on MgB₂ thin films using a quartz crystal microbalance (QCM)─a workhorse apparatus for this specific experiment─MgB₂ must be deposited uniformly on the active surface of the QCM without damaging the quartz's performance. In work presented here, a wet-chemistry colloid synthesis and deposition process of a MgB₂ thin film on a gold (Au) surface was established to avoid the extreme conditions of conventional physical deposition methods. This process also counteracts the unwanted phenomena of drying droplets on a solid surface, particularly the coffee-ring effect. To verify the normal function of the QCM after MgB₂ deposition and its ability to obtain meaningful data, simple gas adsorption tests were conducted on the QCM, and the MgB₂ film on the QCM was characterized with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) for elemental analysis and surface roughness, respectively. To obtain information about the thickness and the involvement of the coffee-ring effect, the same synthesis route was applied on a similar gold substrate─an evaporated Au film on glass. XPS characterization of the film and its precursor suspension shows the potential existence of both MgB₂ and its oxide forms. The film's thickness on evaporated Au was measured by scanning transmission electron microscopy (STEM) to be 3.9 nm. The resulting samples show mitigation of the coffee-ring effect through roughness measurements with AFM at two scan sizes of 50 × 50 and 1 × 1 μm².

NO2 Physical-to-Chemical Adsorption Transition on Janus WSSe Monolayers Realized by Defect Introduction

As is well known, NO₂ adsorption plays an important role in gas sensing and treatment because it expands the residence time of compounds to be treated in plasma-catalyst combination. In this work, the adsorption behaviors and mechanism of NO₂ over pristine and Se-vacancy defect-engineered WSSe monolayers have been systematically investigated using density functional theory (DFT). The adsorption energy calculation reveals that introducing Se vacancy acould result in a physical-to-chemical adsorption transition for the system. The Se vacancy, the most possible point defect, could work as the optimum adsorption site, and it dramatically raises the transferred-electron quantities at the interface, creating an obviously electronic orbital hybridization between the adsorbate and substrate and greatly improving the chemical activity and sensing sensitivity of the WSSe monolayer. The physical-to-chemical adsorption transition could meet different acquirements of gas collection and gas treatment. Our work broadens the application filed of the Janus WSSe as NO₂-gas-sensitive materials. In addition, it is found that both keeping the S-rich synthetic environments and applying compression strain could make the introduction of Se vacancy easier, which provides a promising path for industrial synthesis of Janus WSSe monolayer with Se vacancy.

Cluster Formation Effect of Water on Pristine and Defective MoS2 Monolayers

The structure and electronic properties of the molybdenum disulfide (MoS₂) monolayer upon water cluster adsorption are studied using density functional theory and the optical properties are further analyzed with the Bethe-Salpeter equation (BSE). Our results reveal that the water clusters are electron acceptors, and the acceptor tendency tends to increase with the size of the water cluster. The electronic band gap of both pristine and defective MoS₂ is rather insensitive to water cluster adsorbates, as all the clusters are weakly bound to the MoS₂ surface. However, our calculations on the BSE level show that the adsorption of the water cluster can dramatically redshift the optical absorption for both pristine and defective MoS₂ monolayers. The binding energy of the excitons of MoS₂ is greatly enhanced with the increasing size of the water cluster and finally converges to a value of approximately 1.16 eV and 1.09 eV for the pristine and defective MoS₂ monolayers, respectively. This illustrates that the presence of the water cluster could localize the excitons of MoS₂, thereby greatly enhance the excitonic binding energy.

A first-principles study of the adsorption mechanism of NO2 on monolayer antimonide phosphide: a highly sensitive and selective gas sensor

A NO2/SbP adsorption system with high adsorption energy (−0.876 eV) and charge transfer value (−0.83 e) is reported.

Hybrid G/BN@2H-MoS2 Nanomaterial Composites: Structural, Electronic and Molecular Adsorption Properties

Hybrid structures often possess superior properties to those of their component materials. This arises from changes in the structural or physical properties of the new materials. Here, we investigate the structural, electronic, and gas-adsorption properties of hybrid structures made from graphene/hexagonal boron nitride and 2H-molybdenum disulfide (G/BN@MoS₂) monolayers. We consider hybrid systems in which the G/BN patch is at the Mo plane (model I) and the S plane (model II). We find that the implanted hexagon of G or BN in MoS₂ alters its electronic properties: G@MoS₂ (I,II) are metallic, while BN@MoS₂ (I) is an n-type conducting and BN@MoS₂ (II) is semiconducting. We study the molecular adsorption of some diatomic gases (H₂, OH, N₂, NO, CO), triatomic gases (CO₂, NO₂, H₂S, SO₂), and polyatomic gases (COOH, CH₄, and NH₃) on our hybrid structures while considering multiple initial adsorption sites. Our results suggest that the hybrid systems may be suitable materials for some applications: G@MOS₂ (I) for oxygen reduction reactions, BN@MoS₂ (I,II) for NH₃-based hydrogen production, and G@MoS₂ (I) and BN@MoS₂ (I,II) for filtration of No, Co, SO₂, H₂S, and NO₂.

Room-temperature highly sensitive and selective NH3gas sensor using vertically aligned WS2nanosheets

Here, we report the room temperature (35 °C) NH₃gas sensor device made from WS₂nanosheets obtained via a facile and low-cost probe sonication method. The gas-sensing properties of devices made from these nanosheets were examined for various analytes such as ammonia, ethanol, methanol, formaldehyde, acetone, chloroform, and benzene. The fabricated gas sensor is selective towards NH₃and exhibits excellent sensitivity, faster response, and recovery time in comparison to previously reported values. The device can detect NH₃down to 5 ppm, much below the maximum allowed workspace NH₃level (20 ppm), and have a sensing response of the order of 112% with a response and recovery time of 54 s and 66 s, respectively. On the other hand, a sensor made from nanostructures has a bit longer recovery time than a device made from nanosheets. This was attributed to the fact that NH₃molecules adsorbed on the surface site and those trapped in between WS₂layers may have different adsorption energies . In the latter case, desorption becomes difficult and may give rise to slower recovery as noticed. Further, stiffened Raman modes upon exposure to NH₃reveal strong electron-phonon interaction between NH₃and the WS₂channel. The present work highlights the potential use of scaled two-dimensional nanosheets in sensing devices and particularly when used with inter-digitized electrodes, may offer enhanced performance.

Application of Two-Dimensional Materials towards CMOS-Integrated Gas Sensors

During the last few decades, the microelectronics industry has actively been investigating the potential for the functional integration of semiconductor-based devices beyond digital logic and memory, which includes RF and analog circuits, biochips, and sensors, on the same chip. In the case of gas sensor integration, it is necessary that future devices can be manufactured using a fabrication technology which is also compatible with the processes applied to digital logic transistors. This will likely involve adopting the mature complementary metal oxide semiconductor (CMOS) fabrication technique or a technique which is compatible with CMOS due to the inherent low costs, scalability, and potential for mass production that this technology provides. While chemiresistive semiconductor metal oxide (SMO) gas sensors have been the principal semiconductor-based gas sensor technology investigated in the past, resulting in their eventual commercialization, they need high-temperature operation to provide sufficient energies for the surface chemical reactions essential for the molecular detection of gases in the ambient. Therefore, the integration of a microheater in a MEMS structure is a requirement, which can be quite complex. This is, therefore, undesirable and room temperature, or at least near-room temperature, solutions are readily being investigated and sought after. Room-temperature SMO operation has been achieved using UV illumination, but this further complicates CMOS integration. Recent studies suggest that two-dimensional (2D) materials may offer a solution to this problem since they have a high likelihood for integration with sophisticated CMOS fabrication while also providing a high sensitivity towards a plethora of gases of interest, even at room temperature. This review discusses many types of promising 2D materials which show high potential for integration as channel materials for digital logic field effect transistors (FETs) as well as chemiresistive and FET-based sensing films, due to the presence of a sufficiently wide band gap. This excludes graphene from this review, while recent achievements in gas sensing with graphene oxide, reduced graphene oxide, transition metal dichalcogenides (TMDs), phosphorene, and MXenes are examined.

Magnetically Doped Molybdenum Disulfide Layers for Enhanced Carbon Dioxide Capture

Carbon capture and storage (CCS) technologies have the potential for reducing greenhouse gas emissions and creating clean energy solutions. One of the major aspects of the CCS technology is designing energy-efficient adsorbent materials for carbon dioxide capture. In this research, using a combination of first-principles theory, synthesis, and property measurements, we explore the CO₂ gas adsorption capacity of MoS₂ sheets via doping with iron, cobalt, and nickel. We show that substitutional dopants act as active sites for CO₂ adsorption. The adsorption performance is determined to be dependent on the type of dopant species as well as its concentration. Nickel-doped MoS₂ is found to be the best adsorbent for carbon capture with a relatively high gas adsorption capacity compared to pure MoS₂ and iron- and cobalt-doped MoS₂. Specifically, Brunauer-Emmett-Teller (BET) measurements show that 8 atom % Ni-MoS₂ has the highest surface area (51 m²/g), indicating the highest CO₂ uptake relative to the other concentrations and other dopants. Furthermore, we report that doping could lead to different magnetic solutions with changing electronic structures where narrow band gaps and the semimetallic tendency of the substrate are observed and can have an influence on the CO₂ adsorption ability of MoS₂. Our results provide a key strategy to the characteristic tendencies for designing highly active and optimized MoS₂-based adsorbent materials utilizing the least volume of catalysts for CO₂ capture and conversion.

Tabletop Fabrication of High-Performance MoS2 Field-Effect Transistors

A simple way to prepare field-effect transistors (FETs) using MoS₂ on tabletop is presented. Conductive silver paste was applied onto chemical vapor deposition (CVD)-grown MoS₂ as Ohmic-contact electrodes. Heating the device in vacuum further enhances the performance without damage. The final performance is comparable to that of the SiO₂-backgated devices prepared by lithography and metal evaporators. The role of the silver paste and heat treatment in vacuum is investigated by device and spectroscopic analysis.

Imaging Spatial Distribution of Photogenerated Carriers in Monolayer MoS2 with Kelvin Probe Force Microscopy

The spatial distribution of photogenerated carriers in atomically thin MoS₂ flakes is investigated by measuring surface potential changes under light illumination using Kelvin probe force microscopy (KPFM). It is demonstrated that the vertical redistribution of photogenerated carriers, which is responsible for photocurrent generation in MoS₂ photodetectors, can be imaged as surface potential changes with KPFM. The polarity of surface potential changes points to the trapping of photogenerated holes at the interface between MoS₂ and the substrate as a major mechanism for the photoresponse in monolayer MoS₂. The temporal response of the surface potential changes is compatible with the time constant of MoS₂ photodetectors. The spatial inhomogeneity in the surface potential changes at the low light intensity that is related to the defect distribution in MoS₂ is also investigated.

Adsorption of gas molecules on buckled GaAs monolayer: a first-principles study

The design of sensitive and selective gas sensors can be significantly simplified if materials that are intrinsically selective to target gas molecules can be identified. In recent years, monolayers consisting of group III-V elements have been identified as promising gas sensing materials. In this article, we investigate gas adsorption properties of buckled GaAs monolayer using first-principles calculations within the framework of density functional theory. We examine the adsorption energy, adsorption distance, charge transfer, and electron density difference to study the strength and nature of adsorption. We calculate the change in band structure, work function, conductivity, density of states, and optical reflectivity for analyzing its prospect as work function-based, chemiresistive, optical, and magnetic gas sensor applications. In this regard, we considered the adsorption of ten gas molecules, namely NH3, NO2, NO, CH4, H2, CO, SO2, HCN, H2S, and CO2, and noticed that GaAs monolayer is responsive to NO, NO2, NH3, and SO2 only. Specifically, NH3, SO2 and NO2 chemisorb on the GaAs monolayer and change the work function by more than 5%. While both NO and NO2 are found to be responsive in the far-infrared (FIR) range, NO shows better spin-splitting property and a significant change in conductivity. Moreover, the recovery time at room temperature for NO is observed to be in the sub-millisecond range suggesting selective and sensitive NO response in GaAs monolayer.

Electrostatic control of photoluminescence from A and B excitons in monolayer molybdenum disulfide

Tailoring excitonic photoluminescence (PL) in molybdenum disulfide (MoS2) is critical for its various applications. Although significant efforts have been devoted to enhancing the PL intensity of monolayer MoS2, simultaneous tailoring of emission from both A excitons and B excitons remains largely unexplored. Here, we demonstrate that both A-excitonic and B-excitonic PL of chemical vapor deposition (CVD)-grown monolayer MoS2 can be tuned by electrostatic doping in air. Our results indicate that the B-excitonic PL changed in the opposite direction compared to A-excitonic PL when a gate voltage (V g) was applied, both in S-rich and Mo-rich monolayer MoS2. Through the combination of gas adsorption and electrostatic doping, a 12-fold enhancement of the PL intensity for A excitons in Mo-rich monolayer MoS2 was achieved at V g = -40 V, and a 26-fold enhancement for the ratio of B/A excitonic PL was observed at V g = +40 V. Our results demonstrate not only the control of the conversion between A0 and A-, but also the modulation of intravalley and intervalley conversion between A excitons and B excitons. With electrostatic electron doping, the population of B excitons can be promoted due to the enhanced intravalley and intervalley transition process through electron-phonon coupling. The electrostatic control of excitonic PL has potential applications in exciton physics and valleytronics involving the B excitons.

Gate-Tuned Gas Molecule Sensitivity of a Two-Dimensional Semiconductor

In this work, we develop a gate-tunable gas sensor based on a MoS2/hBN heterostructure field effect transistor. Through experimental measurements and numerical simulations, we systematically reveal a principle that relates the concentration of the target gas and sensing signals (ΔI/I0) as a function of gate bias. Because a linear relationship between ΔI/I0 and the gas concentration guarantees reliable sensor operation, the optimal gate bias condition for linearity was investigated. Taking NO2 and NH3 as target molecules, it is clarified that the bias condition greatly depends on the electron accepting/donating nature of the gas. The effects of the bandgap and polarity of the transition metal dichalcogenides (TMDC) channel are also discussed. In order to achieve linearly increasing signals that are stable with respect to the gas concentration, a sufficiently large VBG within VBG > 0 is required. We expect this work will shed light on a way to precisely design reliable semiconducting gas sensors based on the characteristics of TMDC and target gas molecules.

Janus transition metal dichalcogenides in combination with MoS2 for high-efficiency photovoltaic applications: a DFT study

Exotic features of two-dimensional materials have been demonstrated, making them particularly appealing for both photocatalytic and photovoltaic applications. van der Waals corrected density functional theory calculations were performed on AAII-Se MoSSe, AAII-Te MoSTe, and AAII-Se WSSe heterostructures in this study. Our findings reveal that the heterostructures have high stability due to the tiny lattice mismatch and binding energy, which is extremely favorable for epitaxial growth of these heterostructures. According to the electronic band gap calculation, AAII-Se MoSSe and AAII-Se WSSe are semiconducting materials, while AAII-Te MoSTe has metallic properties. Interestingly, all three heterostructures have type II band gap alignment, which is advantageous for photovoltaic and photocatalytic applications. Furthermore, it was discovered that AAII-Se MoSSe and AAII-Se WSSe heterostructures exhibit high power conversion efficiency of up to 12.15% and 9.37%, respectively. Based on these intriguing features, the two heterostructures are excellent prospects for photovoltaic applications. The heterostructures have no appropriate band edge sites for overall water splitting at pH = 0, but they are good for the oxygen evolution process. It is feasible to alter the position of the band edges using strain resulting in improved overall water splitting by the heterostructures.

Gas sensing devices based on two-dimensional materials: a review

Gas sensors have been widely utilized penetrating every aspect of our daily lives, such as medical industry, environmental safety testing, and the food industry. In recent years, two-dimensional (2D) materials have shown promising potential and prominent advantages in gas sensing technology, due to their unique physical and chemical properties. In addition, the ultra-high surface-to-volume ratio and surface activity of the 2D materials with atomic-level thickness enables enhanced absorption and sensitivity. Till now, different gas sensing techniques have been developed to further boost the performance of 2D materials-based gas sensors, such as various surface functionalization and Van der Waals heterojunction formation. In this article, a comprehensive review of advanced gas sensing devices is provided based on 2D materials, focusing on two sensing principles of charge-exchange and surface oxygen ion adsorption. Six types of typical gas sensor devices based on 2D materials are introduced with discussion of latest research progress and future perspectives.

MoS2 Decorated Silver Nanowire-Reduced Graphene Oxide Aerogel Micro-Particle for Thermally Conductive Polymer Composites with Enhanced Flame Retardancy

Multifunctional polymer composites with efficient heat dissipation and flame retardancy are highly desirable in the electronic industry. Here, by the combination of hydrothermal reaction and in-situ fragmentation, molybdenum disulfide (MoS₂ ) decorated silver nanowire (AgNW) and 3D reduced graphene oxide (RGO) (AgNW-RGO@MoS₂ ) aerogel micro-particle (AMP) is successfully prepared. When the above AMP is introduced to epoxy (EP) resin by the simple blending method, polymer composite with continuous thermally conductive pathways and flame barrier layers is achieved. With AMP loading of 4.0 vol%, the polymer composite displays superior enhancement in thermal conductivity up to 420%. Compared to neat EP, the peak heat release rate and total heat release decrease 61.1% and 58.8%, respectively. This work provides new insights into the design and large-scale fabrication of multifunctional polymer composites for efficient thermal management materials. This article is protected by copyright. All rights reserved.

Aerosol-Printed MoS2 Ink as a High Sensitivity Humidity Sensor

Molybdenum disulfide (MoS2) is attractive for use in next-generation nanoelectronic devices and exhibits great potential for humidity sensing applications. Herein, MoS2 ink was successfully prepared via a simple exfoliation method by sonication. The structural and surface morphology of a deposited ink film was analyzed by scanning electron microscopy (SEM), Raman spectroscopy, and atomic force microscopy (AFM). The aerosol-printed MoS2 ink sensor has high sensitivity, with a conductivity increase by 6 orders of magnitude upon relative humidity increase from 10 to 95% at room temperature. The sensor also has fast response/recovery times and excellent repeatability. Possible mechanisms for the water-induced conductivity increase are discussed. An analytical model that encompasses two ionic conduction regimes, with a percolation transition to an insulating state below a low humidity threshold, describes the sensor response successfully. In conclusion, our work provides a low-cost and straightforward strategy for fabricating a high-performance humidity sensor and fundamental insights into the sensing mechanism.

The Combination of Two-Dimensional Nanomaterials with Metal Oxide Nanoparticles for Gas Sensors: A Review

Metal oxide nanoparticles have been widely utilized for the fabrication of functional gas sensors to determine various flammable, explosive, toxic, and harmful gases due to their advantages of low cost, fast response, and high sensitivity. However, metal oxide-based gas sensors reveal the shortcomings of high operating temperature, high power requirement, and low selectivity, which limited their rapid development in the fabrication of high-performance gas sensors. The combination of metal oxides with two-dimensional (2D) nanomaterials to construct a heterostructure can hybridize the advantages of each other and overcome their respective shortcomings, thereby improving the sensing performance of the fabricated gas sensors. In this review, we present recent advances in the fabrication of metal oxide-, 2D nanomaterials-, as well as 2D material/metal oxide composite-based gas sensors with highly sensitive and selective functions. To achieve this aim, we firstly introduce the working principles of various gas sensors, and then discuss the factors that could affect the sensitivity of gas sensors. After that, a lot of cases on the fabrication of gas sensors by using metal oxides, 2D materials, and 2D material/metal oxide composites are demonstrated. Finally, we summarize the current development and discuss potential research directions in this promising topic. We believe in this work is helpful for the readers in multidiscipline research fields like materials science, nanotechnology, chemical engineering, environmental science, and other related aspects.

Adsorption Behavior of Environmental Gas Molecules on Pristine and Defective MoSi2N4: Possible Application as Highly Sensitive and Reusable Gas Sensors

Inspired by the recent practical application of two-dimensional (2D) nanomaterials as gas sensors, catalysts, and materials for waste gas disposal, herein, the adsorption behaviors of environmental gas molecules, including NO, CO, O2, CO2, NO2, H2O, H2S, and NH3, on the 2D pristine and defective MoSi2N4 (MSN) monolayers were systematically investigated using spin-polarized density functional theory (DFT) calculations. Our results reveal that all the gas molecules are physically adsorbed on the MSN surface with small charge transfer, but the electronic structures of NO, NO2, and O2 are obviously modified due to the in-gap states. The introduction of N vacancy on the MSN surface enhances the interaction between gas molecules and the substrate, especially for NO2 and O2. Interestingly, the adsorption type of NO and CO evolves from physisorption to chemisorption, which may be utilized in NO and CO catalytic reaction. Furthermore, the moderate adsorption strength and obvious changes in electronic properties of H2O and H2S on the defective MSN make them have promising prospects in highly sensitive and reusable gas sensors. This work offers several promising gas sensors based on the MSN monolayer and also provides a theoretical reference of other related 2D materials in the field of gas sensors, catalysts, and toxic gas disposal.

Effect of the co-adsorption of small molecules from air on the properties of penta-graphene and their proton transfer calculation

Penta-graphene has attracted considerable attention due to its unique structure and novel properties. Herein, we studied the effect of the co-adsorption of small molecules from air on the properties of penta-graphene using first-principles calculations. Our results show that oxygen molecules can be self-decomposed on the surface of penta-graphene and the process of O₂ decomposition is an exothermic reaction. On the contrary, the adsorption of H₂O or N₂ molecule on penta-graphene exhibits weak interaction characteristic. For co-adsorption systems, the adsorption of N₂ molecule has no effect on the electronic properties of penta-graphene because the N₂ molecule is more inert than other molecules. Hydrogen bonds (H-bonds) have been observed in the co-adsorption of H₂O and O₂ on penta-graphene. We find that shorter H-bonds lead to higher stability of the systems. We also explore the proton transfer process between H₂O and oxidized penta-graphene. Our results show that the proton transfer process is relatively difficult due to the high energy barrier. However, double-proton transfer is an exothermic process since the energy of the final state is 0.11 eV lower than that of the initial state. These results indicate that the configuration of oxidized penta-graphene is complicated. Our research provides a theoretical basis and important guidance for the experimental synthesis and functionalization of penta-graphene.

P-Type Metal Oxide Semiconductor Thin Films: Synthesis and Chemical Sensor Applications

This review focuses on the synthesis of p-type metal-oxide (p-type MOX) semiconductor thin films, such as CuO, NiO, Co3O4, and Cr2O3, used for chemical-sensing applications. P-type MOX thin films exhibit several advantages over n-type MOX, including a higher catalytic effect, low humidity dependence, and improved recovery speed. However, the sensing performance of CuO, NiO, Co3O4, and Cr2O3 thin films is strongly related to the intrinsic physicochemical properties of the material and the thickness of these MOX thin films. The latter is heavily dependent on synthesis techniques. Many techniques used for growing p-MOX thin films are reviewed herein. Physical vapor-deposition techniques (PVD), such as magnetron sputtering, thermal evaporation, thermal oxidation, and molecular-beam epitaxial (MBE) growth were investigated, along with chemical vapor deposition (CVD). Liquid-phase routes, including sol-gel-assisted dip-and-spin coating, spray pyrolysis, and electrodeposition, are also discussed. A review of each technique, as well as factors that affect the physicochemical properties of p-type MOX thin films, such as morphology, crystallinity, defects, and grain size, is presented. The sensing mechanism describing the surface reaction of gases with MOX is also discussed. The sensing characteristics of CuO, NiO, Co3O4, and Cr2O3 thin films, including their response, sensor kinetics, stability, selectivity, and repeatability are reviewed. Different chemical compounds, including reducing gases (such as volatile organic compounds (VOCs), H2, and NH3) and oxidizing gases, such as CO2, NO2, and O3, were analyzed. Bulk doping, surface decoration, and heterostructures are some of the strategies for improving the sensing capabilities of the suggested pristine p-type MOX thin films. Future trends to overcome the challenges of p-type MOX thin-film chemical sensors are also presented.