General electrical conductivity Resistance or conductance of insulating materials ASTM D and moderately conductive materials ASTM D , volume conductivity IEC Handling of electronics In connection with manufacture of electronics, the requirements outlined in the following standards should be followed; IEC regarding the protection of electronic devices from electrostatic phenomena and as well as associated standards such as IEC on footwear and flooring.
Risk of fire and explosion The area of danger of fire and explosion caused by static electricity is covered by the ATEX Directive. Clothing Within protective clothing, the standard EN outlines the requirements, and tests can be performed in accordance with EN surface resistance or EN method 2 induction charging.
When measuring textiles, we can draw on the expertise of DTI's textiles department. Combination with other services Properties of brand new products as well as existing products that have been exposed to various weather conditions are important and need to be determined. As a general rule, an increased amount of humidity will increase conductivity, which is why the relative humidity has significant influence on the outcome.
It is also possible to adjust the temperature in the climatic test cabinet.
Even, the co-doping changes the electronic properties at the Fermi level, which is highly desired in two-dimensional materials that have semi-metallic behavior. Additionally, its physical and chemical properties are dependent on the thickness or number of layers of two-dimensional material used in the design [ , ]. By reducing the thickness of the two-dimensional material, it is even possible to modify the size of the forbidden band or even modify the type of forbidden band from indirect to direct or vice versa according to the type of two-dimensional material used as sensing material.
In very thin two-dimensional materials there are no quantum confinement effects and the electronic structure is dominated by surface states near the Fermi level. Since the selectivity of a gas sensor is related to its ability to respond to a gas in the presence of others, then the design of a gas sensor implies that the gas to be detected must be adsorbed by the sensing material, exclusively, and this is achieved, if the necessary catalysts are added to this material [ 32 ].
The catalytic materials must be based on metallic nanoparticles. These materials modify the band gap and, in this way, only those gases with ionization energy like the band can be adsorbed. The adsorption energy E a of the gases is calculated using the following mathematical expression [ 49 ]:.
Since the sensitivity of a gas sensor expresses the change in the output signal per unit gas concentration, it is necessary to design a material capable of detecting levels up to below parts per billion of gas present in the sensing material [ 17 ]. Two-dimensional materials have a high sensitivity due to their high surface-to-volume ratio and their semiconductor properties.
Both the dimensions of the sensing material and the semiconductor properties of the material can be tuned; in this way, the sensitivity of a gas sensor based on two-dimensional materials can be designed. The band gap of the sensing material plays a major role in adjusting the sensitivity of the gas sensor. Besides, the doped two-dimensional materials show higher selectivity and sensitivity toward gas molecules compared to pure two-dimensional materials [ 49 ].
The sensitivity and selectivity of a semiconductor two-dimensional material based gas sensor depend on any change in its electronic properties. Another of the important performance parameters of gas sensors is reversibility, which implies that the sensor must be capable of being used during a cyclic operation without qualitatively or quantitatively modifying its response to the target gas [ 17 ]. This is committed to selectivity, since when the latter is high; the reversibility is low due to the high bond energies involved [ 93 ]. Therefore, complete reversibility is achieved when weak interactions between the gas and the sensing material are present [ ].
Then, a two-dimensional material with a bandgap without catalysts may have better reversibility.
This can be appreciated when there are no intermediate bands or levels between the valence band and the conduction band. Gas sensors require a response time to react to the presence of the target gas whose concentration changes from zero to a certain concentration value [ 17 ].
This response time is directly related to the type of band gap that the two-dimensional material has, since being a direct band gap would be expected to have a shorter response time than a material with an indirect band gap. Although the path between the valence band and the conduction band that the electron should travel energetically was straight, the magnitude of the band gap should be short to reduce the response time. A small response time is directly associated with a high sensitivity of the gas sensor, which is linked to a reduced band gap [ ]. In any physicochemical process such as gas sensing, stability represents the ability of the sensor to keep reproducible its performance, in a specific period of time, before various physicochemical variables [ 17 ].
Since gas sensors are susceptible to being modified by various variables such as temperature, then the bandgap of the two-dimensional material must remain stable before any event presented. Some gas molecules in contact with the sensing material tend to dissociate and chemically absorb into it. Other gases tend to be physically absorbed stably on the sensing material, which leads to different interaction strengths.
These latter molecules induce different modifications in the forbidden band of the material, which can be observed in the energy band diagram [ 52 ]. Key performance scores for gas sensors based on two-dimensional materials are illustrated in Figure A more exhaustive study on optimizing the performance parameters of a gas sensor such as sensitivity, selectivity, stability, response time and operating temperature is found in [ 17 ].
Key performance scores spider chart comparing the performance of gas sensors based on two-dimensional materials and those based on other materials.
Despite the great advances in the application of two-dimensional materials in gas sensing, the study of the fundamental properties of these materials as well as the development of mathematical models to predict the electronic band structure is still in its infancy. There is a wide variety of 2D materials that can be used to sense both oxidizing gases and reducing gases.
This article studies the electronic band structure of 2D materials to optimize gas sensing through their first Brillouin zone and dispersion energy. It was shown that most two-dimensional materials used in gas sensors have a hexagonal crystalline structure, and that the tight-binding model can optimize the electronic band structure through mathematical modeling and its simulation.
It is considered important to resume the study of two-dimensional materials using the perspective of materials engineering, and to implement a wide variety of gas sensors using the different topologies of materials that have been proposed until now and propose new design options.
To finalize the material for an engineering product / application, we should have the knowledge of Electrical properties of materials. Electrical conduction. ➢ Materials are classified based on their electrical properties as conductors, semiconductors and insulators. New to this group is super.
The author also wishes to thank to your family for their support in carrying out this research. National Center for Biotechnology Information , U. Journal List Sensors Basel v. Sensors Basel.
Published online Mar Rafael Vargas-Bernal. Author information Article notes Copyright and License information Disclaimer. Received Jan 3; Accepted Mar 8. Abstract In the search for gas sensing materials, two-dimensional materials offer the possibility of designing sensors capable of tuning the electronic band structure by controlling their thickness, quantity of dopants, alloying between different materials, vertical stacking, and the presence of gases. Keywords: mathematical modeling, gas sensors, two-dimensional materials, graphene, transition metal dichalcogenides, field effect transistors, chemiresistors.
Two-Dimensional Materials Used in Gas Sensors 2D materials are referred as materials with a single layer or a few layers of material. Open in a separate window.
Figure 1. Main applications of the 2D materials in electronic industry.
Table 1 Two-dimensional materials based on one chemical element for electronic applications. Table 2 Two-dimensional semiconductor materials for electronic applications www. Figure 2. Main properties of the two-dimensional materials in gas sensing.
Table 3 Main two-dimensional materials used in gas sensing. Table 4 Examples of hybrid or composites materials based on two-dimensional materials used in gas sensing. Advantages of Two-Dimensional Materials for Gas Sensing The two-dimensional materials used as gas sensing materials are particularly interesting because [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]: 1 their different active sites of interest such as defects, vacancies and edge sites, which allow for selective molecular absorption, 2 their surface-to-volume ratios, and 3 their high yield preparations.
Figure 3. Experimentally measured values of work function for different metals. Electronic Band Structure for Two-Dimensional Materials The electrical properties of a solid are determined basically through its electronic band structure, which establishes the range of energy states that electrons may or may not have within their crystalline structure.
Figure 4. Design process of a gas sensor through the electrical properties. Figure 5. Table 5 Seven crystalline systems. First Brillouin Zone of Two-Dimensional Materials In the reciprocal space, the first Brillouin zone is the only primitive cell that can be defined. Table 6 Critical points in the first Brillouin zone in 2D materials with hexagonal lattice.
Table 7 Crystalline lattices found in 2D materials. First Brillouin Zone for materials with Hexagonal Crystalline Lattices Graphene is a two-dimensional material consisting of carbon atoms in a hexagonal lattice [ 1 , 2 , 7 , 8 , 10 , 16 , 18 , 22 , 23 , 26 , 29 , 32 , 33 , 34 , 35 , 36 , 90 , , , , ]. First Brillouin Zone for Materials with Orthorhombic Crystalline Lattices Two-dimensional materials with orthorhombic crystalline lattice are borophene striped , black phosphorus BP , germanium sulfide GeS , and dibismuth trisulphide Bi 2 S 3.
Table 10 Data of the first Brillouin zone of triclinic lattices. First Brillouin Zone for Materials with Monoclinic Crystalline Lattices Two-dimensional materials with monoclinic crystalline lattices are diarsenic tritelluride As 2 Te 3 and zirconium triselenide ZrSe 3. Table 11 Data of the first Brillouin zone of P monoclinic lattices. Tight-Binding Model for Two-Dimensional Materials To predict the electronic energy bands, the tight binding model can be used. Band Structure of Graphene Graphene is an allotrope of carbon made from a simple layer of atoms with a hexagonal crystal structure.
Figure 7. Figure 8. Figure 9. Band Structure of Silicene Silicene is the two-dimensional allotrope of silicon with a hexagonal crystalline structure like that of graphene [ 1 , 2 , 22 , 29 , 45 , 46 , 47 ]. Figure Methodology to design gas sensors based on two-dimensional materials. Correlation between Gas Sensing Characteristics and Electronic Band Structure Since the selectivity of a gas sensor is related to its ability to respond to a gas in the presence of others, then the design of a gas sensor implies that the gas to be detected must be adsorbed by the sensing material, exclusively, and this is achieved, if the necessary catalysts are added to this material [ 32 ].
Conclusions Despite the great advances in the application of two-dimensional materials in gas sensing, the study of the fundamental properties of these materials as well as the development of mathematical models to predict the electronic band structure is still in its infancy. Funding This research did not receive internal or external funds.
Conflicts of Interest The authors declare no conflict of interest. References 1. Pan Y.