Preparation And Characterization Of Gas Sensors Based On Graphene-metal Contact Structure

Graphene is widely used in gas sensors because of its excellent electrical and material properties.The construction of graphene heterojunction to improve the gas sensitivity of graphene has been proved to be an effective method.Recent experimental studies have shown that the graphene-nickel(G/Ni)contact structure has a better detection effect on ammonia gas than graphene,and the reason for this result is that graphene forms a strong chemical bond with nickel,which affects the adsorption of ammonia gas.But most metals only form physical contact with graphene.Whether and how these physical contact structures affect the gas adsorption of graphene is unknown.Therefore,the influence of graphene-metal(G/M)contact structure on the adsorption behavior of graphene gas is a problem worthy of further study.1.Theoretical calculation of gas adsorption characteristics of G/M contact structure.Five metals Au,Ag,Al,Cu and Pt were selected to contact with graphene to construct graphene-metal physical contact structure.Based on the theory of graphene-metal contact,gas adsorption was introduced into the calculation of contact structure resistivity,and the change of carrier concentration caused by gas adsorption was taken as the input variable to construct the functional relationship between graphene-metal contact structure resistivity and the change of graphene carrier concentration.The adsorption characteristics of five graphene-metal contact structures such as Au,Ag and Al on different types of gases were studied by theoretical calculation.The results show that:(1)when G/Au and G/Pt contact structures adsorb n-type gas,adsorption peaks appear on the resistivity curve;When the p-type gas is adsorbed,the resistivity of the contact structure decreases monotonously,and the resistivity change of G/Au is greater than that of G/Pt;(2)the gas adsorption characteristics of G/Cu,G/Ag and G/Al contact structures are just opposite to the contact structures of G/Au and G/Pt,that is,the resistivity of the contact structure decreases monotonously when the n-type gas is adsorbed,while the adsorption peak appears on the resistivity curve when the p-type gas is adsorbed;(3)the adsorption peak position of the contact structure is determined by the Fermi energy difference between graphene and the metal.2.Design and prepare a gas sensor based on G/M contact structureBased on the device model calculated theoretically,the device structure of the gas sensor based on G/M contact structure for gas sensitivity test is designed and prepared.The G/M contact structure device features include:(1)the contact structure is directly covered by graphene on the surface of the metal electrode,which is easy to operate;(2)graphene is exposed on the upper surface of the contact structure as a carrier for adsorbing gas;(3)in order to highlight the adsorption of contact structure and reduce the influence of channel resistance,the metal film was used to cover the channel graphene directly.Then,according to the device structure characteristics,the device preparation process is determined,and the lithographic mask is designed.Three metals,Au,Ag and Pt,were selected as electrode materials,and the gas sensor devices based on G/M contact structure were prepared by using MEMS processing technologies such as photonics,magnetron sputtering coating,plasma etching and graphene-wet etching transfer technology.Among them,graphene was selected as a monolayer prepared by copper based chemical vapor deposition.The success rate of the device is more than 95% through repeated exploration of the experimental parameters in the process of preparation.3.Gas sensitivity test of gas sensor based on G/M contact structure.Ammonia was selected as the test gas,and ammonia water and high purity ammonia were used as the gas source to test the gas sensitivity of the device.The experimental results show that:(1)the average resistance response peak value of the device based on G/Au contact structure in the humid ammonia environment is 10% to 15% higher than that in the dry ammonia environment;(2)the recovery and repetition characteristics of graphene-metal contact devices in the humid nitrogen environment are better than that in the dry ammonia environment;(3)the average response peak of G/Au contact structure resistance is higher than that of G/Ag and G/Pt contact structures in humid ammonia gas environment;However,in the dry ammonia environment,the average response peak of the resistance of G/Ag contact structure was higher than that of G/Au and G/Pt contact structures.