First-Principles Study Of Gas Adsorption Properties On Silicon Carbide Nanotubes

Nanotubes have attracted considerable interest because of their unique propertiesand many potential applications. One such application is their use as a chemical sensordue to their large surface area and hollow geometry. Gases can flow through the tubesbecause they are hollow. Therefore, gas can be adsorbed on the outer and inner surface.Compared with traditional gas sensors, nanotube gas sensors have some advantages,such as higher sensitivity, faster response time, and can effectively reduce the size of thesensor. Nanotube gas sensors can be used to detect small concentrations of gasmolecules with high responsiveness (ppb level). Silicon carbide nanotubes (SiCNTs) aresemiconductors independent of their helicity and radius, and have exceptional properties,such as thermal stability, chemical inertness, which make them potential candidates forgas monitoring that operate in harsh environments. By doping impurity atoms intoSiCNTs, the electrical and chemical properties will be modified, and improve theadsorption capability of SiCNTs. Studies on adsorption properties of gases on SiCNTsare not only of high theoretical value, but also of great practical significance. Theadsorption properties of the carbon-based and nitrogen-based on intrinsic andboron-doped (B-doped) SiCNTs are studied with density functional theory (DFT). Theelectronic transport properties of B-doped and NO2adsorbed SiCNTs are studied withthe method combined DFT with nonequilibrium Green’s function (NEGF) and the mainconclusions are as follows:1. Adsorption properties of carbon-based gases on SiCNTsStructure and electronic properties of intrinsic and B-doped SiCNTs with andwithout adsorption of carbon dioxide (CO2) and carbon monoxide (CO) are calculatedwith CASTEP package based density functional theory. Stable adsorptions between thegas molecules and SiCNTs are formed. After the adsorption of CO2and CO molecules,band gap of intrinsic and B-doped SiCNTs are significantly reduced. The conductivitiesof intrinsic and B-doped SiCNTs are enhanced obviously, so the detection of electricsignal is possible. Intrinsic and B-doped SiCNT are expected a potential candidate todetect the presence of CO2and CO, and our results are meaningful to the developmentof the SiCNT gas sensors.2. Adsorption properties of nitrogen-based gases on SiCNTs Structure and electronic properties of intrinsic SiCNTs with adsorption of nitrogendioxide (NO2), nitrous oxide (N2O), nitric oxide (NO) and ammonia (NH3) arecalculated based on density functional theory. And structure and electronic properties ofB-doped SiCNTs with adsorption of NO2and N2O are also studied. After the adsorptionof NO2and N2O molecules, band gap of intrinsic and B-doped SiCNTs are significantlyreduced. The conductivities of intrinsic and B-doped SiCNTs are enhanced obviously,so the detection of electric signal is possible. Although the adsorption of NO gas isphysical adsorption, but NO molecule has evidently effect on electrical properties ofSiCNT, so SiCNT have the ability to detect NO gas. The adsorption of NH3moleculehas no significantly influence on SiCNT.3. Transport properties of B-doped (8,0) single-walled SiCNTsThe transport properties of B-doped (8,0) single-walled SiCNTs are investigatedwith the method combined DFT with NEGF. The transmission coefficients near theFermi energy are nearly zero. This means that the B-doped SiCNT is a semiconductorand consistent with the results of calculations based on the first principle calculation.The current under positive bias can be divided into three parts. Along with the biasrange from0.0V to0.8V and from1.0V to2.0V, the current rises with the increase ofthe bias. And as the bias ranges from0.8V to1.0V, negative differential resistance(NDR) effect is observed. It is difficulty for electrons tunneling from one electrode toanother with the increase of localization of molecular orbital, which is the essentialreason for NDR.4. Transport properties of SiCNTs with NO2molecule adsorptionThe transport properties of SiCNTs with NO2molecule adsorption are investigatedwith the method combined DFT with NEGF. The transport property is workingmechanism of the SiCNTs NO2gas sensor. Based on the voltage current (I-V)characteristic of the adsorption system, from+1.1V under positive bias, the differencebetween the I-V curves of the sensor with and without NO2molecule begins to increase.Under the bias of+1.5V, the current of the sensor is about1.5times of that with no gasmolecule adsorbed, which is large enough for detecting the gas in application. Ourresults are meaningful to the studies on the SiC nanotube gas sensors.The studies on the adsorption properties of the carbon-based and nitrogen-based onintrinsic and B-doped SiCNTs and electronic transport properties of B-doped and NO2adsorbed SiCNTs are meaningful to the model, development and application of SiCNTgas sensors.