Study On Gas-molecule Adsorption And Desorption Of Single-molecule Devices

Molecular sensor is one of significant molecular functional devices.With the continuous development of the single-molecule experimental technologies and theoretical methods,deeper and deeper insight into the performance machanism and potential applications of molecular gas sensors have been achieved successively.At the same time,lots of attentions have been paied to the design and investigation of molecular gas sensor devices.In recent decades,many groups devoted themselvies to the studies on single-molecule gas-sensor devices.They forcused not only on finding excellent gas-sensor material that is more sensitive,more stable and more energy-efficient,but also on increasing the recycle rate of gas sensors.Therefore,based on the density functional theory?DFT?and the non-equilibrium Green's function?NEGF?method,the small-molecule-adsorption effects on the electrical properties of single-molecule devices are investigated systematically.The adsorption and desorption process of gas molecules on functional molecules are simulated,and the intrinsic mechanism of gas-molecule adsorption and desorption processes are further analyzed.The p-s-p type molecule?denoted as TADHA?,which consists of two identical thiolated arylethynylene connected by a non-conjugated segment,possesses interesting NDC behavior at low bias regime.Based on the NEGF method and ab initio calculations,we find that the NDC behavior of the molecules originated from the bias-induced degeneration destroying and localization of the highest occupied molecular orbital?HOMO?and HOMO-1.The addition reaction of H₂,F₂,or Br₂ on the C-C triple bond of TADHA molecules increases the current,conductance,and NDC value of the molecular system by about one order of magnitude,while the addition of Cl₂ increases by about 3 times.More interestingly,the occurring bias of the NDC behavior is significantly lowered when the TADHA molecule addition reacted with Cl₂ or Br₂.In addition,the addition reactions with H₂ or halogen molecules?F₂,Cl₂ and Br₂?on the C-C triple bond of TADHA molecule lead the molecular orbitals more delocalized,which heightens and broadens the transmission peaks of the molecular systems,and consequently enhances the NDC behaviors of the molecular systems.Due to the small size,single-molecule device may be sensitive to the ambient molecules.Thus,it is significant for fabricating single-molecule sensors to understand the influence of ambient molecule on molecular device.The adsorption effects of H₂ O,CO₂ and NO₂ molecule on the pyrene-1,8-dithiol molecular junctions are studied systematically.The numerical results show that,the influence of H₂ O or CO₂ molecule on the pyrene-1,8-dithiol molecular junction is very slight when they are adsorbed on the pyrene-1,8-dithiol molecules,which attributes to the closed-shell ground states of these two molecules.Unlike the H₂ O and CO₂ molecule,as radical molecule,NO₂ molecule has obvious influence on the electronic transport of pyrene-1,8-dithiol molecular junctions,which can be embodied specifically that the system with NO₂ adsorbate is more conductive in the positive and lower negative bias regime than those of the other two molecular systems,which is due to the evident coupling between the states of NO₂ molecule and pyrene-1,8-dithiol molecule.It is significant for the reuse of gas sensor devices to desorb the gas molecule which adsorbed on the devices.Besides the heat-annealing-desorption method,the field-aided desorption method is also an attractive strategy to desorb gas molecules from functional device.The field-aided desorption process of NO₂ molecules on the surface of the graphene snippets are simulated on the bases of ab initio calculations.The graphene snippet systems consist of 3×3,4×4,5×5 or 6×6 graphene supercells respectively.The numerical results show that the most probable adsorption distance from NO₂ to the graphene snippets are fall in the range of 0.31⁰.35 nm.Generally,the O atoms of NO₂ seem likely closer to the graphene snippets.When the NO₂ adsorbed on the graphene snippets,a little amount of negative charges are transfered from graphene snippets to the NO₂ molecules.In the desorption simulation,when the electric field intensity reaches a certain level,the NO₂ gradually goes away from the surface of the functional graphene snippets.The energies of the systems are decreased with the departure of NO₂ from graphene snippets.In addition,the bigger of the graphene snippet sizes results in the weaker of the coupling strengthes between NO₂ molecule and the functional graphene snippets.Consequently,the electric field intensity which is required for NO₂ being desorbed from the surface of the functional graphene snippets is smaller.The minimum fields that needed to desorb from the graphene snippets are approximately proportional to the coupling strengthes between NO₂ molecule and the functional graphene snippets.The thesis is divided into six chapters as follows.The first chapter systematically introduces the development and present situation of molecular electronics,and the research tendency and applications of molecular gas sensors.The second chapter briefly describes the theoretical methods,which include DFT and NEGF method,for the computational simulation in this paper.In the third chapter,the effects of hydrogen or halogen dopings on the electronic transport properties and the negative differential conductance behavior of TADHA molecular device are investigrated.The adsorption effects of several common environmental molecules on the electrical transport of the pyrene-1,8-dithiol?PDT?molecular junctions are discussed in the fourth chapter.Besides,in the fifth chapter,we study the intrinsic mechanism of adsorption and desorption of NO₂ on graphene snippets.The conclusions of the whole thesis and outlook for our works are illuminated in the last chapter.