Research On Transport Characteristics Of PbS Quantum Dots And Their Photodetection Devices

PbS Quantum dots?PbS-QDs?,combining with quantum size effect,have been attracted an everincreasing attention on both theoretical foundation and practical applications.Quantum size effects impart huge advantages for PbS-QDs,such as size tunable band gap,multiple exciton generation?MEG?capability.and high photoluminescence?PL?yield.Therefore,PbS-QDs have been exploited actively in new-generation of nano-electronic and nano-optoelectronic devices,such as solar cells,field-effect transistors?FETs?and photodetectors?PDs?.Despite PbS-QDs-based devices show impressive properties,and potential applications,these devices often show a poor response time or low electrical characteristics.PbS-QDs are generally capped by organic ligands,which employ long chains?8?18 carbons?to ensure the stability of QDs in solution.As an adverse effect,it becomes a determining factor for the high resistive nature of QDs,and impacts their charge carrier mobility and conductivity when processed into films.To form a film capable of electronic transport,QDs are usually treated using short organic ligands with characteristic functional groups?e.g.,dithiols?to exchange long hydrocarbon ligands?e.g.,oleic acid?.But the produce short barriers between QDs that also militate against efficient carrier transport when processed into films.Above these,modified the surface of the QDs,enhanced charge carrier mobility and conductivity of the PbS-QDs are the key points for the constructing high performance electronic and optoelectronic devices.In this dissertation,we have used an inorganic ligand?atomic-ligand?exchange method to modify the surface of the QDs,aiming to change the conductivity type of the QDs since atomic-ligand exchange methods offer a highly effective way of surface modification by greatly enhancing charge carrier mobility and conductivity.Then the PbS QDs-based photodetectors that was made through this method.The results suggest a great potential of PbS-QDs for construction of high-speed photodetectors,and the importance of the method in constructing high performance electronic and optoelectronic devices.The main results are as follows:1.The monodisperse and stable PbS-QDs were obtained by thermal injection using TMS as a sulfur source.The PbS-ODs shows monodispersed particles with similar shape,and the PbS-QDs size is?3 nm.Then we investigated the bottom gate FETs made by 3 run PbS-QDs treated with MPA to passivate their surfaces.The results demonstrate that FETs produced p-channel characteristics,and the tansconductance,mobility,subthreshold swing,threshold voltage are 0.075nS,0.064 cm2 V-1 S-1,102V/dec and 40V,respectively.The gate voltage has a limited control for the FETs.We used experimental semiconductor FET performance tests to evaluate the conductivity type,carrier mobility and transport properties of QDs.Unlike the theoretical calculation,we provide an experimental evaluation methods for the electrical properties of QDs.2.Inorganic?atomic?ligands were used to modify the surface of PbS-DDs,and thin-film FET was prepared to evaluate the transport performance of PbS-ODs,the results showed that after ligand exchange the electrical properties of PbS-QDs improved greatly.Then we present the observation of a unique phenomenon of converting electrical conductivity types in surface atomic-ligand exchanged of PbS-QDs,while tuning the gate voltage of PbS QD-based thin film field-effect transistors?TFTs?.The synthesized PbS-QD thin films showed different conductivity?n-or p-?types depending on the direction of gate voltage sweeping.When increasing gate voltage from negative to positive voltage,the TFT devices produced n-channel characteristics with a typical mobility on the order of ?n= 10-1 cm2 V-1 s-1.By contrast,the devices showed p-channel characteristics with similar mobility when sweeping the gate voltage from positive to negative.The studies based on density functional theory?DFT?theoretical calculations and X-ray photoelectron spectroscopy?XPS?measurements demonstrate that the band gap of ligand exchanged PbS-QDs changes upon gate voltage sweeping,affecting the surface states of the QDs due to the adhered Br-and therefore altering the conductivity type of the materials.These findings signify the carrier type switching behavior via gate voltage tuning in atomic-ligand exchange QD thin films,suggesting great application potential of QD for advanced new-generation nano-electronic devices.3.A surface atomic-ligand exchange method was applied the first time in the construction of PbS-QDs)based photodetectors?PDs?for ultra-sensitivity.The device thus produced a high photo-sensitivity to visible and near-infrared lights with photoresponsivity/gain up to 7.5×103AW-1/1.7×104 and a high stability?4 month?in air.In particular,the PbS QDs-based PDs showed the capability of following a pulse light with a frequency up to 100 kHz well at relatively fast response time/recovery time of??s/40 ?s,much faster than most of previous QDs-based PDs.Density functional theory?DFT?theoretical calculations demonstrate that the band gap,density distribution of electron,density of states of before/after ligand exchanged PbS-QDs.The results demonstrate that the short response time is attributed to modification for the surface of the PbS-QDs by atomic-ligand exchange method treatment,which effectively improves the contact between QDs and Au electrodes,leading to extract high carrier mobility.These findings show a great potential of PbS-QDs as high-speed nano-photodetectors,and more importantly,demonstrate the importance of the surface atomic-ligandexchange method in the construction of QDs-based devices.4.We fabricate self-powered photodetectors based on PbS-Quantum-Dots/Indium Schottky barrier diode successfully.These devices exhibit excellent repeatability and stability at a high frequency?up tolMHz?,and show the typical fast rise time/fall time of-0.8 ?s/3.2 ?s.They also show excellent rectification ratios up to 104 with bias from-0.5 V to +0.5 V in the dark and a pronounced photovoltaic performance under light illumination.Moreover,the devices demonstrate high sensitivity in weak light illumination detection?detectivity?approaching 1012 Jones and low noise currents<1pAHz-1/2.These findings suggest great application potential of PbS-Quantum-Dots for advanced fast response,low noise current,high detectivity and high stability photodetectors.5.We explore the optical and electrical properties of PbS-Qds and organic-inorganic hybrid perovskite CH3NH3Pbl3 composite film,and fabricate a Au/PbS-CH₃NH₃PbI₃/Au photodetectors?PDs?.The devices thus produced a high photo-sensitivity to visible and near-infrared lights.The devices showed the high conductivity?8 × 103 Scm-1?,high photoresponsivity/gain?2×107 A W-1/3×107?and high detectivity?9×1014 Jones?.The optical and electrical properties of these devices have been greatly improved compared to the Au/PbS-QD/Au photodetectors.In particular,the PDs exhibit excellent repeatability and stability at a high frequency?up to 1 MHz?,and show the typical fast rise time/fall time of?5 ?s/25 ?s.Experimental results show that the PbS-CH₃NH₃PbI₃ composite films make up for the defect of single PbS quantum dot devices.These methods brought a new way for the fabrication of new composite-type nano-devices...