Electron Transport In Single-molecule Transistors

Individual molecules are natural zero dimensional quantum confined system,with well-defined atomic structure and electrical structure.Singe molecule transistor device is not only the ultimate limit for electrical transistor device miniaturization,but also a basic physical platform for studying different kinds of physical interactions at molecular scale.With the help of gate tuning,both the charge and spin state of single molecule can be precisely controlled in a single molecule transistor device.By changing the structure of molecules or devices,complicated quantum system can be designed to study the physical effect in limit scale.In this thesis,we use single molecule transistor to study the transport behaviors of single molecule under different conditions and explain the physical principle behind process.We use electromigration technique to fabricate single manganese phthalocyanine(Mn Pc)molecule transistor successfully.At low temperature,the transport behavior shows typically coulomb blockade behavior.When the coupling between molecule and electrodes enhanced,high order transport behavior appears.In devices with strong coupling,we observed spin 1/2 Kondo effect,as well as multi orbitals involving two-stage Kondo effect.Detailed study was performed on two-stage Kondo effect and two correlated energy scales were acquired as T_k and T~*.The universal behavior of two-stage Kondo effect with voltage,temperature and magnet field is also proved.With the assistant of gate voltage,we also observed the quantum phase transition with single Kondo channel involving.By tuning the relative position against quantum phase transition point,the relationship between the second stage Kondo temperature,T~*,and the energy difference of spin singlet and triplet is established experimentally for the first time.The coupling type and strength of two electrons in Mn Pc molecule can also be deduced.Based on single molecule transistor,we also investigated transport properity of coupled-molecule transistor.In this kind of systems,the coupling between molecules would affect the transport properties of the whole devices,which is lack of experimental results.Knowledge of coupling between molecules is a significant step towards all-molecule circuits.Two molecules show electrostatic interaction.By analyzing the interaction,we can also deduce the dynamics of single electron tunneling.When changing capacity coupling to tunneling coupling,we also observe the non-equilibrium hybridization of the orbitals from two molecules.The interaction mechanism above can also be used as a sensor at a single molecule single electron level.We use single molecule devices as a sensor to study its response to nearby molecule.When the charge state varies,the electrostatic environment around the molecular device changes accordingly,resulting a switching behavior in its transport spectrum.Detection the charge state of single molecule can be achieved by this method.Using the basic transport model of quantum dot,we can deduce the coupling between nearby molecule and the molecular device,as well as gain the charge transfer information between molecules,which contributes to the basic detection and controlling of electron transfer among molecules at single molecule level.