The Effect Of External Conditions On Electrical Transport Property Of Organic Molecular Devices

With the miniaturization of devices,micro devices are becoming smaller and smaller,and we have more and more extensive understanding of the microscopic world,and the studies of the functional properties of single-molecule devices have also made significant achievement.Besides,with the development of experimental techniques and theoretical methods,various kinds of molecular devices have been fabricated,such as molecular wires,molecular switches,molecular sensors,and so on.The negative differential conductance(NDC)effect is one of the most ideal functional properties of molecular devices,and has obtained more and more attention and research.In this paper,the density functional theory and the non-equilibrium Green's function are used to study the negative differential conductance effect of the single thiolated arylethynylene molecule with 9,10-dihydroanthracene core(denoted as TADHA)at low bias voltage,besides,we also studied the external factors of the NDC effect.Large negative differential conductance(NDC)at lower bias voltage is a very desirable functional property of single molecular device.And based on the study of Perrin M.L.et al.,the TADHA molecule is a non-rigid molecule which can be stretched and compressed,besides their experimental results showed that the molecular junction in the lower bias voltage showed excellent NDC effect.In order to explain the excellent NDC effect exhibited in the low bias voltage,we have studied the effect of electrode distance between the two electrodes on the negative differential conductance by stretching or compressing the TADHA molecular junction.The results show that during the stretching or compression of the molecular junction,there is an electrode distance with the lowest energy,that is to say,that electrode distance is the equilibrium distance.At the equilibrium distance,the TADHA molecular junction shows pronounced NDC behavior in lower bias regime due to the HOMO and HOMO-1 were pulled apart by the bias voltage.The conductivity and the NDC behavior of the molecular system can be enhanced by compressing the electrode distance,on the contrary,the elongating of the molecular junction will eliminate the NDC behavior of the TADHA molecular system and there will have a rectification effect.Generally,the surrounding molecules play an effects on the molecular junction,such as the aqueous solution or the water vapor can suppress the electronic transport of molecular junction dramatically,and the experiments of negative differential conductance by Perrin are carried out in an aqueous environment,if the evaporation of the molecular device is not complete,and there will be a little water still adsorbed near the molecular devices.For this reason,we simulated different number of H2O molecules adsorption on different positions of the TADHA molecule,and to study whether the adsorption of H2O has an effect on the NDC effect at lower bias.Our study shows that,the NDC behavior of TADHA molecular device presented some water-immunity character,and single or separated H2O molecule adsorbed on the TADHA molecule have a little effect on the NDC behavior of the TADHA molecular junction.However,the aggregate of several H2O molecule adsorbed on one branch of TADHA molecule can dramatically enhance the conductivity and NDC behavior of the molecular junction,besides,this also results in rectifier behavior.Moreover,we also found that different adsorption locations result in different influences on NDC effect,and the closer the adsorption position to the middle part of the molecular junction,the more obvious on the NDC effect.We also simulated the adsorption influence of F2.And the theoretical results show that the adsorption of F2 inhibits the NDC effect,and result in the reduce of the current and differential conductance.But at the same time,the adsorption of F2 results in an increase of molecular junction asymmetry,and leads to a larger rectifying ratio of the molecular junction,and the closer to the intermediate position of TADHA molecule,the more obvious about the rectification effect.This thesis consists of six chapters as follows.The first chapter includes an introduction of the main research contents and research status of the molecular electronics.In the second chapter,we briefly introduced the theoretical methods for studying the electrical transport properties of molecular devices,and main including density functional theory and non-equilibrium Green's function method.In the third chapter,we mainly discussed the effect of electrode distance on the negative differential conductance of TADHA molecular junctions,and we found that shortening the electrode separation can enhance the NDC effect which is attributed to the possible increase of coupling between the two branches of TADHA molecule.In the fifth chapter,we discussed the effect of H2O adsorption on the transport properties of TADHA molecular junctions.Besides,the fifth chapter discussed the effects of strong oxidizing gas molecule F2 on the negative differential conductance at lower bias.And the last chapter draws a conclusion for the whole work of the thesis and gives a prospect on the development of molecular devices in the future.