Fabrication And Properties Of Photodetectors Based On Graphene And Core-shell Quantum Dots

Graphene,due to its unique band structure and excellent optoelectronic properties,such as wide absorption bandwidth,outstanding charge transfer property,mechanical strength,and flexibility,is a promising two-dimensional material for electronic and optoelectronic applications.It is widely used in fields such as biological imaging,solar cells,terahertz communication,and broadband optical detection.However,the low absorption coefficient of graphene limits the performance of graphene-based photodetectors,with a maximum response of approximately 10² A/W.Most of the work is focused on hybrid graphene semiconductor heterojunction photodetectors to improve their responsiveness,such as molybdenum disulfide,titanium dioxide,silicon,quantum dots,and perovskite.Among them,quantum dots（QDs）,as photosensitive active materials,have received great attention in solar cells,photodetectors,and light-emitting diodes due to their extremely high extinction coefficient and adjustable bandgap.In particular,lead selenide quantum dots（Pb Se QDs）are favored because of their high infrared absorption coefficient,direct band gap,small carrier effective mass and easy large-scale preparation.However,Pb Se quantum dots exhibit high oxidation sensitivity under environmental conditions.If exposed to air for a long time,their optical performance will be greatly reduced,posing a great challenge for the preparation of stable Pb Se quantum dot devices.Research has shown that the oxidizability of Pb Se comes from the suspended bonds and vacancies on its surface.Therefore,many scholars have proposed various surface passivation methods,including the coverage of ligands and core/shell heterostructures.In addition,due to the influence of lattice matching on surface passivation,it is particularly important to choose shells with similar lattice constants.The lattice constants of Pb S and Pb Se are close,and the lattice matching degree is high,which can effectively avoid the generation of interface defects,improve the performance of Pb Se quantum dots,and form type II heterostructures between the core（Pb Se quantum dots）and shell（Pb S）.In this type of heterostructure,the conduction and valence band shifts are very small,with one type of carrier confined within the core and the other delocalized throughout the entire core/shell.In this paper,graphene were grown by plasma enhanced chemical vapor deposition（PECVD）and wet transfer method,and their surface morphology,molecular structure,optical and electrical properties were characterized and analyzed by Raman spectroscopy,ultraviolet visible absorption spectroscopy,scanning electron microscopy（SEM）,and atomic force microscope（ATM）.Then,the mask method was used to form graphene phototransistor with a channel length of 40μm.Graphene phototransistor was tested for its optoelectronic performance by the Keithley 4200A-SCS parameter analyzer.The result shows that the graphene has good contact with the copper electrode,which is an ohmic contact and the carrier mobility is as high as 10³ cm²/V·s.The photocurrent of graphene phototransistor under illumination of 405 nm to 780 nm is at the microampere level,with a maximum photoresponsivity of 10² A/W.Furthermore,Pb Se QDs were introduced to form graphene-Pb Se QDs hybrid heterostructure photodetector.By comparing graphene device with graphene Pb Se QDs device,it was found that their photocurrent（increased from microampere to milliampere）,photoresponsivity（increased from 10² to10⁴ A/W）,photosensitivity（increased from 0.8%to 32%）,and spectral detection range were significantly improved.However,the lifetime of graphene-Pb Se QDs phototransistor is relatively short,about 8.5 h,which has limited its further application in photodetection.Based on this point,Pb Se/Pb S core/shell QDs were introduced to improve the stability of Pb Se QDs.The corresponding graphene-Pb Se/Pb S QDs hybrid photodetector achieved a lifetime of 72.1 h,which is nearly 10 times longer than graphene-Pb Se QDs photodector.Additionally,its photoresponsivity and photodetectivity reached10⁴ A/W and 10¹¹ Jones,respectively.