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.

Predicting biomolecule adsorption on MoS2 nanosheets with high structural fidelity

A new force field, MoSu-CHARMM, for the description of bio-interfacial structures at the aqueous MoS2 interface is developed, based on quantum chemical data. The force field describes non-covalent interactions between the MoS2 surface and a wide range of chemistries including hydrocarbon, alcohol, aldehyde, ketone, carboxylic acid, amine, thiol, and amino acid groups. Density functional theory (DFT), using the vdW-DF2 functional, is employed to create training and validation datasets, comprising 330 DFT binding energies for 21 organic compounds. Development of MoSu-CHARMM is guided by two criteria: (i) minimisation of energetic differences compared to target DFT data and (ii) preservation of the DFT energetic rankings of the different binding configurations. Force-field performance is validated against existing high-quality structural experimental data regarding adsorption of four 26-residue peptides at the aqueous MoS2 interface. Adsorption free energies for all twenty amino acids in liquid water are calculated to provide guidance for future peptide design, and interpret the properties of existing experimentally-identified MoS2-binding peptides. This force field will enable large-scale simulations of biological interactions with MoS2 surfaces in aqueous media where an emphasis on structural fidelity is prioritised.

Li-doped beryllonitrene for enhanced carbon dioxide capture

In recent years, the scientific community has given more and more attention to the issue of climate change and global warming, which is largely attributed to the massive quantity of carbon dioxide emissions. Thus, the demand for a carbon dioxide capture material is massive and continuously increasing. In this study, we perform first-principle calculations based on density functional theory to investigate the carbon dioxide capture ability of pristine and doped beryllonitrene. Our results show that carbon dioxide had an adsorption energy of -0.046 eV on pristine beryllonitrene, so it appears that beryllonitrene has extremely weak carbon dioxide adsorption ability. Pristine beryllonitrene could be effectively doped with lithium atoms, and the resulting Li-doped beryllonitrene had much stronger interactions with carbon dioxide than pristine beryllonitrene. The adsorption energy for carbon dioxide on Li-doped beryllonitrene was -0.408 eV. The adsorption of carbon dioxide on Li-doped beryllonitrene greatly changed the charge density, projected density of states, and band structure of the material, demonstrating that it was strongly adsorbed. This suggests that Li-doping is a viable way to enhance the carbon dioxide capture ability of beryllonitrene and makes it a possible candidate for an effective CO2 capture material.

Low-dimensional HfS2 as SO2 adsorbent and gas sensor: effect of water and sulfur vacancies

First-principles based on density functional theory (DFT) calculations were performed to investigate the interaction of two-dimensional (2D) HfS₂ with SO₂, a harmful gas with implications for climate change. In particular, we describe the effect of water and sulfur vacancies on such interaction. The former promotes the physisorption of SO₂, whereas the latter promotes its chemisorption with structural changes on the absorbing surface. The results show that both structures are exothermic to adsorb the SO₂ molecules, but the adsorption type is different. The reaction of the stable structure in the presence of water with the sulfur oxides is a physisorption interaction that enhances the band gap value of the isolated monolayer. However, for the defective structure, we have a chemisorption interaction type, where the adsorption of SO₂ molecules widens the band gap values. To understand this behavior, we used Bader charge calculations and the noncovalent interactions index. While the water enhances the charge transfer between the monolayer and the adsorbed gas, the results show, however, that the defective structure is a more favorable gas sensor due to the metallic edge of the active site.

Modulation effects of S vacancy and Mo edge on the adsorption and dissociation behaviors of toxic gas (H2S, SO2) molecules on the MoS2 monolayer

This study focuses on the modulation effects of S vacancy and Mo edges on the adsorption and dissociation behaviors of toxic gases (H2S and SO2) on MoS2 by first-principles calculations. Both molecules are found to chemisorb at the S vacancy (SV) and pristine Mo edge and physisorb at the Mo edge with a 50% sulfur coverage (Mo-50 edge). Among them, SO2 has larger adsorption energy than H2S on both S vacancy and pristine Mo edge, which is related to a more electronegative O than S atom and electronically rich for the pristine Mo edge. The defective states of MoS2 induced by SV can be removed by forming Mo-S, Mo-O and Mo-H bonds upon the adsorption of SO2 and the dissociation of H2S, which is applicable in designing MoS2 based nano-electronics devices in the future. The dissociations of H2S and SO2 on pristine Mo edges are found to be more favorable than those on S vacancies due to the catalytically active Mo4+ states at edge sites. H2S is found to dissociate on the Mo-50 edge more easily than SO2. The adsorptions and dissociations of toxic gas on MoS2 with SV or Mo edges suggest MoS2 is a potential candidate in detecting and removal of toxic gases.

Theoretical Insight on the Biosensing Applications of 2D Materials

The research on the design of efficient, reliable, and cost-effective biosensors is expanding given its high demand in various fields such as health care, environmental surveillance, agriculture, diagnostics, industries, and so forth. In the last decade, various fascinating and interesting 2D materials with extraordinary properties have been experimentally synthesized and theoretically predicted. 2D materials have been explored for the sensing of different biomolecules because of their large surface area and strong interaction with different biomolecules. Theoretical simulations can bring important insight on the interaction of biomolecules on 2D materials, charge transfer, orbital interactions, and so forth and may play an important role in the development of efficient biosensors. Quantum simulation techniques, such as density functional theory (DFT), are very powerful and are gaining popularity especially with the advent of high-speed computing facilities. This review article provides theoretical insight regarding the interaction of various biomolecules on different 2D materials and the charge transfer between the biomolecules and 2D materials leading to electrochemical signals, which can then provide experimentalists the useful design parameters for fabrication of biosensors. It also includes an overview of quantum simulations, use of the DFT method for simulating biomolecules on 2D materials, parameters obtained from theoretical simulations and sensitivity, and limitations of computational techniques for sensing biomolecules on 2D materials. Furthermore, this review summarizes the recent work in first-principles investigation of 2D materials for the purpose of biomolecule sensing. Beyond the traditional graphene or 2D transition-metal dichalcogenides, some novel and recently proposed 2D materials such as pentagraphene, haeckelite, MXenes, and so forth which have exhibited good sensing applications have also been highlighted.

Air Pressure, Gas Exposure and Electron Beam Irradiation of 2D Transition Metal Dichalcogenides

In this study, we investigate the electrical transport properties of back-gated field-effect transistors in which the channel is realized with two-dimensional transition metal dichalcogenide nanosheets, namely palladium diselenide (PdSe2) and molybdenum disulfide (MoS2). The effects of the environment (pressure, gas type, electron beam irradiation) on the electrical properties are the subject of an intense experimental study that evidences how PdSe2-based devices can be reversibly tuned from a predominantly n-type conduction (under high vacuum) to a p-type conduction (at atmospheric pressure) by simply modifying the pressure. Similarly, we report that, in MoS2-based devices, the transport properties are affected by pressure and gas type. In particular, the observed hysteresis in the transfer characteristics is explained in terms of gas absorption on the MoS2 surface due to the presence of a large number of defects. Moreover, we demonstrate the monotonic (increasing) dependence of the width of the hysteresis on decreasing the gas adsorption energy. We also report the effects of electron beam irradiation on the transport properties of two-dimensional field-effect transistors, showing that low fluences of the order of few e-/nm2 are sufficient to cause appreciable modifications to the transport characteristics. Finally, we profit from our experimental setup, realized inside a scanning electron microscope and equipped with piezo-driven nanoprobes, to perform a field emission characterization of PdSe2 and MoS2 nanosheets at cathode–anode separation distances as small as 200 nm.

Modeling of Si-B-N Sheets and Derivatives as a Potential Sorbent Material for the Adsorption of Li+ Ion and CO2 Gas Molecule

In the present exploration, a few Si-B-N derivatives are derived to adsorb Li ions and CO₂ gas molecules for the potential application of metal-air batteries. The newly derived structure's bond lengths are as follows: Si=Si, 2.2 Å; Si-B, 1.9 Å; Si-N, 1.7 Å; and B-N, 1.4 Å, consistent with the experimental results of relevant structures. The stability of the newly derived structures is examined by the atom-centered density propagation study by varying the temperature from 270 to 400 K, and no structural variations are observed throughout the dynamics. Li adsorption on the Si₄B₂ ring has the maximum binding energy of -3.9 eV, and the result is consistent with the previous results. The rings with the 2:1 silicon-boron ratio provide strong adsorption for Li atoms. The calculated maximum electromotive force of the newly derived sheets is 0.56 V with the maximum theoretical density of 783 Wh/kg. Similarly, the maximum adsorption of CO₂ on the sheet is -0.106 eV, which is considerably higher than that on graphene and its derivatives. CO₂ adsorption has been carried out in the presence of water molecules to investigate the change in CO₂ adsorption with the moisture (water) content, and the results show no significant change in the adsorption of CO₂ with moisture. However, water has a strong interaction with the maximum interaction energy of -0.72 eV. Further, to explore the potential ability of the sheets, each sheet's edges are examined as hydrogen storage expedient and the surface as an artificial photosynthesis platform. The Si₄B₂ ring is more favorable for the adsorption of H atom with the chemisorption of -7.138 eV. Similarly, all of the major UV-absorption spectral peaks fall between 450 and 800 nm, which shows that the sheet can be used as an artificial photosynthesis platform.

Two-Dimensional Vanadium Carbide MXene for Gas Sensors with Ultrahigh Sensitivity Toward Nonpolar Gases

The sensitive detection of explosive and flammable gases is an extremely important safety consideration in today's industry. Identification of trace amounts of nonpolar analytes at ambient temperatures, however, is still a challenge because of their weak adsorption, and very few studies have been able to achieve it via a chemiresistive mechanism. Herein, we demonstrate the high performance of 2D vanadium carbide MXene (V₂CT _x) gas sensors with ultrahigh sensitivity toward nonpolar gases. The fabricated 2D V₂CT _x sensor devices consisting of single-/few-layer 2D V₂CT _x on polyimide film were able to detect both polar and nonpolar chemical species including hydrogen and methane with a very low limit of detection of 2 and 25 ppm, respectively, at room temperature (23 °C). The performance of the fabricated V₂CT _x gas sensors in detection of nonpolar gases surpasses that of previously reported state-of-the-art gas sensors based on other 2D materials.

Highly selective and reversible NO2 gas sensor using vertically aligned MoS2 flake networks

We demonstrate a highly selective and reversible NO₂ resistive gas sensor using vertically aligned MoS₂ (VA-MoS₂) flake networks. We synthesized horizontally and vertically aligned MoS₂ flakes on SiO₂/Si substrate using a kinetically controlled rapid growth CVD process. Uniformly interconnected MoS₂ flakes and their orientation were confirmed by scanning electron microscopy, x-ray diffraction, Raman spectroscopy and x-ray photoelectron spectroscopy. The VA-MoS₂ gas sensor showed two times higher response to NO₂ compared to horizontally aligned MoS₂ at room temperature. Moreover, the sensors exhibited a dramatically improved complete recovery upon NO₂ exposure at its low optimum operating temperatures (100 °C). In addition, the sensing performance of the sensors was investigated with exposure to various gases such as NH₃, CO₂, H₂, CH₄ and H₂S. It was observed that high response to gas directly correlates with the strong interaction of gas molecules on edge sites of the VA-MoS₂. The VA-MoS₂ gas sensor exhibited high response with good reversibility and selectivity towards NO₂ as a result of the high aspect ratio as well as high adsorption energy on exposed edge sites.

2D Materials for Gas Sensing Applications: A Review on Graphene Oxide, MoS₂, WS₂ and Phosphorene

After the synthesis of graphene, in the first year of this century, a wide research field on two-dimensional materials opens. 2D materials are characterized by an intrinsic high surface to volume ratio, due to their heights of few atoms, and, differently from graphene, which is a semimetal with zero or near zero bandgap, they usually have a semiconductive nature. These two characteristics make them promising candidate for a new generation of gas sensing devices. Graphene oxide, being an intermediate product of graphene fabrication, has been the first graphene-like material studied and used to detect target gases, followed by MoS₂, in the first years of 2010s. Along with MoS₂, which is now experiencing a new birth, after its use as a lubricant, other sulfides and selenides (like WS₂, WSe₂, MoSe₂, etc.) have been used for the fabrication of nanoelectronic devices and for gas sensing applications. All these materials show a bandgap, tunable with the number of layers. On the other hand, 2D materials constituted by one atomic species have been synthetized, like phosphorene (one layer of black phosphorous), germanene (one atom thick layer of germanium) and silicone (one atom thick layer of silicon). In this paper, a comprehensive review of 2D materials-based gas sensor is reported, mainly focused on the recent developments of graphene oxide, exfoliated MoS₂ and WS₂ and phosphorene, for gas detection applications. We will report on their use as sensitive materials for conductometric, capacitive and optical gas sensors, the state of the art and future perspectives.

Ca-Embedded C2N: an efficient adsorbent for CO2 capture

Carbon dioxide as a greenhouse gas causes severe impacts on the environment, whereas it is also a necessary chemical feedstock that can be converted into carbon-based fuels via electrochemical reduction. To efficiently and reversibly capture CO₂, it is important to find novel materials for a good balance between adsorption and desorption. In this study, we performed first-principles calculations and grand canonical Monte Carlo (GCMC) simulations, to systematically study metal-embedded carbon nitride (C₂N) nanosheets for CO₂ capture. Our first-principles results indicated that Ca atoms can be uniformly trapped in the cavity center of C₂N structure, while the transition metals (Sc, Ti, V, Cr, Mn, Fe, Co) are favorably embedded in the sites off the center of the cavity. The determined maximum number of CO₂ molecules with strong physisorption showed that Ca-embedded C₂N monolayer is the most promising CO₂ adsorbent among all considered metal-embedded materials. Moreover, GCMC simulations revealed that at room temperature the gravimetric density for CO₂ adsorbed on Ca-embedded C₂N reached 50 wt% at 30 bar and 23 wt% at 1 bar, higher than other layered materials, thus providing a satisfactory system for the CO₂ capture and utilization.

Toward breath analysis on a chip for disease diagnosis using semiconductor-based chemiresistors: recent progress and future perspectives

Semiconductor gas sensors using metal oxides, carbon nanotubes, graphene-based materials, and metal chalcogenides have been reviewed from the viewpoint of the sensitive, selective, and reliable detection of exhaled biomarker gases, and perspectives/strategies to realize breath analysis on a chip for disease diagnosis are discussed based on the concurrent design of high-performance sensing materials and miniaturized pretreatment components. Carbon-based sensing materials that show relatively high responses to NO and NH₃ at low or mildly raised temperatures can be applied to the diagnosis of asthma and renal disease. Halitosis can be diagnosed by employing sensing or additive materials such as CuO and Mo that have high chemical affinities for H₂S, while catalyst-loaded metal oxide nanostructure sensors or their arrays have been used to diagnose diabetes via the selective detection of acetone or by pattern recognition of sensor signals. For the ultimate miniaturization of a breath-analysis system into a tiny chip, preconditioning that includes preconcentration, dehumidification, and flow sensing needs to be either improved through the design of gas/moisture adsorbents or removed/simplified through the design of highly sensitive sensing materials that are less impervious to interference from humidity and temperature. Moreover, an abundant sensing library needs to be provided for the diagnosis of diseases (e.g. lung cancer) that are associated with multiple biomarker gases and for finding new methods to diagnose other diseases. For this aim, p-type oxide semiconductors with high catalytic activities, as well as combinatorial approaches, can be considered for the development of sensing materials that detect less-reactive large molecules, and high-throughput screening, respectively. Selectivity for a specific biomarker gas will simplify the system further. Breath analysis on a tiny chip using semiconductor chemiresistors with ultralow power consumption that is connected to the 'Internet of Things' will pave new roads for disease diagnosis and patient monitoring.

Nanoporous MoS2 monolayer as a promising membrane for purifying hydrogen and enriching methane

We present a theoretical prediction of a highly efficient membrane for hydrogen purification and natural gas upgrading, i.e. laminar MoS₂ material with triangular sulfur-edged nanopores. We calculated from first principles the diffusion barriers of H₂ and CO₂ across monolayer MoS₂ to be, respectively, 0.07 eV and 0.17 eV, which are low enough to warrant their great permeability. The permeance values for H₂ and CO₂ far exceed the industrially accepted standard. Meanwhile, such a porous MoS₂ membrane shows excellent selectivity in terms of H₂/CO, H₂/N₂, H₂/CH₄, and CO₂/CH₄ separation (>10³, >  10³, >  10⁶, and  >  10⁴, respectively) at room temperature. We expect that the findings in this work will expedite theoretical or experimental exploration on gas separation membranes based on transition metal dichalcogenides.

A theoretical simulation of small-molecules sensing on an S-vacancy SnS2 monolayer

Using first-principle atomistic simulations, we focused on the electronic structures of small gas molecules (CO, H₂O, NH₃, NO, and NO₂) adsorbed on the S-vacancy SnS₂ monolayer.

Adsorption of small inorganic molecules on a defective MoS2monolayer

Different molecules physisorbed, chemisorbed or dissociated on a defective MoS₂layer.

Structural stability and intriguing electronic properties of two-dimensional transition metal dichalcogenide alloys

Novel heterolayer and alternating structures were proposed for transition metal dichalcogenide alloys with intriguing electronic properties.

Sensor based on chemical vapour deposition-grown molybdenum disulphide for gas sensing application

The gas sensing property of CVD-grown MoS₂ for O₂ and CO₂ which acted as charge donors was examined.

Germanene: a new electronic gas sensing material

The structural stability and electronic properties of the adsorption characteristics of several toxic gas molecules (NH₃, SO₂ and NO₂) on a germanene monolayer were investigated using density functional theory (DFT) based on an ab initio method.

Controlled Doping of Vacancy-Containing Few-Layer MoS2 via Highly Stable Thiol-Based Molecular Chemisorption

MoS2 is considered a promising two-dimensional active channel material for future nanoelectronics. However, the development of a facile, reliable, and controllable doping methodology is still critical for extending the applicability of MoS2. Here, we report surface charge transfer doping via thiol-based binding chemistry for modulating the electrical properties of vacancy-containing MoS2 (v-MoS2). Although vacancies present in 2D materials are generally regarded as undesirable components, we show that the electrical properties of MoS2 can be systematically engineered by exploiting the tight binding between the thiol group and sulfur vacancies and by choosing different functional groups. For example, we demonstrate that NH2-containing thiol molecules with lone electron pairs can serve as an n-dopant and achieve a substantial increase of electron density (Δn = 3.7 × 10(12) cm(-2)). On the other hand, fluorine-rich molecules can provide a p-doping effect (Δn = -7.0 × 10(11) cm(-2)) due to its high electronegativity. Moreover, the n- and p-doping effects were systematically evaluated by photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electrical measurement results. The excellent binding stability of thiol molecules and recovery properties by thermal annealing will enable broader applicability of ultrathin MoS2 to various devices.