Study Of Three-terminal Graphene Nanojunctions And Hybrid Graphene-PbS Quantum Dot Devices

As the demand for miniaturization,integration and low power consumption of electronic equipment is growing,the quantum mechanical effect of the device is increasing as well.Therefore,reducing the size of silicon-based semiconductor devices alone can not meet the needs of integrated circuits.Carbon nanostructures such as graphene become a popular material for a new generation of nanoelectronic devices with high reliability and high performance.In this thesis,graphene three-terminal ballistic junction(TBJ)devices are fabricated by using micro-nanofabrication technology.The T-shaped TBJ structures of various sizes are patterned using electron-beam lithography(EBL)followed by inductively coupled plasma etching(ICP).Ohmic contacts were defined via EBL;and a 5/90-nm Ti/Au bilayer was subsequently deposited by electron beam evaporation(EBE).The nonlinear ballistic transport properties are studied.Furthermore,the TBJ is combined with quantum point contact(QPC)to realize the lo gic function.In addition,the hybrid graphene-PbS colloidal quantum dots(CQDs)device was fabricated by directly castdropping the CQD solution,and its photoelectric response and electrical transport are studied.The main works of the thesis are summarized as follo xws:(1)We report on a study of nonlinear charge transport properties of TBJ devices made from graphene.We demonstrate that the graphene TBJs show a rectification characteristic at the room-temperature,namely,when voltages VL=V and VR=-V are applied to the left and the right terminal in a push-pull fashion,the voltage output from the central terminal Vc will always be negative in the electron transport regime(positive in the hole transport regime)and exhibits a quadratic V dependent behavior.The rectification coefficient a which describes the curvature of the parabolic output voltage curve,can be effectively modulated by a gate voltage and shows a transport carrier polarity dependence.The a is in reverse ratio to electrochemical potential ?F in the graphene at zero bias.In comparison with thermopower measurements,the nonlinear transport measurements of the graphene TBJs provide a simple,all-electrical method for investigating the electronic structures and charge states in graphene nanodevices.When two out-phase ac voltage signals(without a dc bias)are applied to the left and right branches,a symmetric TBJ device can be used for rectifier and second-harmonic generator.(2)The rectified voltage Vc of TBJs is found to be related to the derivative of the conductance at the Fermi energy.The peak-to-dip structure seen in the gate voltage V.dependence of Vc is of the same line shape as of the normalized transconductance(1/G)dG/d Vg,with a good agreement in the positive and negative extreme and the polarity reversing point.The nonlinear gate-modulation characteristics Vc-Vg with a nonuniform carrier distribution show a well-defined double peak structure.The nonlinear voltage signal are far more sensitive in detection of the inhomogeneity and the scattering mechanism than the electrical resistance measurements.These results show that the graphene TB.Js could be used as novel building blocks for nanoelectronics and as novel devices for the study of the material properties of graphene on the nanoscale.(3)We have fabricated some functional devices by integrating TBJ and QPC made from graphene.The central branch of TBJ serves as a side gate f'or QPC.In the electron transport regime,the output central branch voltage will be positive(a binary value of 1 in digital circuit)only when both of the applied dc voltages are positive.TBJ operates as a logic AND gate,when combined with QPC(an inverter),can be used to create a logic NAND gate.In the hole transport regime,TBJ operates as a OR gate,combined with QPC(an amplifier),can operate as a logic OR.The logical inversion caused by quantum oscillation is observed at low temperature.TBJ/QPC cascade devices can be used to create any logic gates modulated by gate voltages,which lays an important experimental foundation for the construction of all-carbon logic circuits.(4)We have fabricated hybrid graphene-PbS CQD photodetection devices and measured their photoelectric response characteristics.The resistance of graphene FETs decorated with PbS CQDs was observed a switching behavior in a sequence of laser illumination pulses.The photoresponse signal ?Rph,i.e.,the photoinduced change in the resistance can be modulated in both magnitude and sign with a voltage applied to the back gate,and is related to the derivative of the transfer characteristics of the graphene channel in the devices.We explain our experimental results based on a polarity-dependent carrier transfer model in which photoinduced carriers of one charge polarity in CQDs are transferred to the graphene,leading to an accumulation of carriers of the opposite charge polarity in the CQDs and thus a change in the carrier density in the graphene,i.e.,a photoinduced electric gating effect on the graphene.The electrical properties of a graphene layer can be effectively modulated using a laser beam and thus provides an additional,optical means of tuning in the study of the transport properties of graphene.(5)The low temperature magnetoconductance measurement in such hybrid devices displays robust Aharonov-Bohm(AB)type oscillations in both low and high(formation of quantum Hall states)magnetic field regimes,providing evidence for the scattering potential introduced by the CQDs.The lateral size of the scattering potential is estimated from the quantum oscillations.The coupling between CQD potential and carriers in graphene enhances intervalley scattering and weak localization effect.The conductance decreases as the temperature decreases.The temperature dependence of the conductance is logarithmic below 30 K.The carriers of graphene are sensitive to the electrostatic environment.The hybrid devices provide a flexible system for studying the quantum interference effect in 2D material systems.