Study Of Enhancement Mechanism In Low-dimensional Material-based Photodetecting And Devices

Low-dimensional materials,indicating that can stably exist with at least one nano-scale dimensional,are represented by zero-dimensional quantum dots,one-dimensional carbon nanotubes,nanowires and two-dimensional layered materials,such as graphene,transition metal dichalcogenides.In photodetecting applications,low-dimensional materials possess several merits when comparing with the traditional semiconductors.First,they are naturally more compatible with high-density integrated circuits due to their merits on size.Second,their applications are more flexible.For instance,the quantum dots,being simply coated on the device surface,may equip the device shell with the capability of photodetecting.At the same time,they also have the potential to be utilized in the flexible and wearable devices.Third,they are highly electrically tunable.For the two-dimensional layered materials,such as graphene,transition metal dichalcogenides,their carrier density can be easily tuned through the gate-biases in the transistor structures.Consequently,the performances of the as-fabricated devices can be tuned.Last,they have more potential.Many new physical mechanisms and effects are discovered in low-dimensional materials,which are useful for realizing high-performance photodetecting.However,at present,the performances of low-dimensional material-based photodetectors are far from satisfactory.And significant shortages exist which greatly limit their application prospects,for instance,the low responsivities of canbon nanotubes-based devices,the low gain in quantum dots-based Schottky-type devices and the low photo-induced on-off ratio,large dark current and slow response in transition metal dichalcogenides-based phototransistors.To solve these problems,this paper is focused on the enhancement mechanism of low-dimensional material-based photodetecting.The aim is to equip the photodetectors with both excellent and balance performances,the following detailed researches are conducted in this paper.First,the ligand exchange process in PbS quantum dots and chemical doping process in WSe₂ are investigated.PbS quantum dots,with the merits of high light absorption and bandgap tunable features,are suitable for photodetecting.However,the low carrier mobility in PbS quantum dots film is the main obstacle for further improvements.In this paper,utilizing short-chain legend TBAI?Tetrabutylammonium iodide?instead of the traditional long-chain oleic acid to do the ligand exchange process,the carrier mobility in PbS quantum dots film can be significantly improved.WSe₂ has strong interactions with the light and possesses high tunabilities.However,due to its low intrinsic carrier density,high potential barriers often occur when contacting with the electrodes,which deteriorate carriers transporting.Utilizing PEI?polyethylene imine?to chemically n-type dope the WSe₂ channel,it can form Ohmic contacts with Ti electrodes.As a result,the carrier transporting efficiency can be improved.Second,a photogating-enhanced carbon nanotubes-based infrared photodetector is fabricated.The PbS quantum dots,used both as top gating layer and light absorption layer,will produce electron-hole pairs and eject the holes into the carbon nanotubes channel with the assist of built-in electric field.Under the drive of external voltage,the channel conductivity change will be reflected by the output current change,so as to form the photocurrent.The device performance can be largely tuned by the back-gate bias,and a high responsivity of 353.4 AW^-1,a specific detectivity of 7.1?10¹⁰ Jones and1.58 mS response time have been achieved.Third,a colloidal-quantum-dot photodiode with high photoconductive gain is realized.The device is based on Schottky junction,and the potential barrier,depleted region width are characterized by Mott-schottky measurements.The photo-induced carrier life in quantum dots is measured by open-circuit voltage decay methods?OCVD?,and the height of quantum dots channel is obtained by step profiler.The low potential barrier,long photo-induced carrier life time,short channel and high carrier mobility in quantum dots film cooperate together to equip the device with high photoconductive gain.As a result,the performance of the photodiode is both excellent and balance,an external quantum efficiency of?400%,a responsivity of 5.15 AW^-1,a specific detectivity of 2?10¹⁰ Jones and a response time of 110?S have been simultaneously achieved for 1550 nm illumination.Last,a WSe₂ phototransistor with high-performance photovoltaic response is realized.In the WSe₂ channel,a high-quality intramolecular p-n junction is fabricated.The n-type region and p-type region are chemically doped by polyethyleneimine?PEI?and electrically doped by the back-gate,respectively.Utilizing high-resolution in-plane photocurrent mapping measurements,it can be observed that the originations of the photovoltaic response is tuned from the contact-induced Schottky junction to the in-plane p-n junction when the back-gate bias is changed from 0 V to-40 V.The as-fabricated p-n junction possesses an ideal factor of 1.66 and an electrical rectification ratio of 10³.At the same time,under zero bias photovoltaic detecting mode,the device can simultaneously achieve the responsivity of 80 mAW^-1,the specific detectivity of10¹11 Jones,the rising time of 200?S,the falling time of 16?S,the dark current of 10^-13A and the photo-induced on-off ratio of 10⁷ for 520 nm illumination.In addition,the device also can be utilized as a microcell to drive the circuit.A large open-circuit voltage of 0.38 V and an external power conversion efficiency of 1.4%have been achieved for 520 nm light.In all,the photodetecting applications of low-dimensional materials,represented by zero-dimensional PbS colloidal quantum dots,one-dimensional carbon nanotubes and two-dimensional layered transition metal dichalcogenides WSe₂,are investigated in this paper.The emphases are put on the material modification and doping process,the novel p-n junction realizing method,the new photodetecting enhancement structres and mechanisms.Finally,the proposed devices are fabricated and characterized.The results in this paper have important guiding significance for promoting the practical application of low-dimensional materials in photodetecting.