Graphitic Carbon Nitride Flim-based Photodetectors

As a kind of two-dimensional(2D)layered semiconductor material,graphitic carbon nitride(g-C3N4)has a two-dimensional planar structure similar to graphene.The gC3N4 with tri-s-triazine structure is recognized as the most stable phase of carbon nitride among their allotropes.It has received extensive attention in the fields of photo/electrocatalysis and environmental energy,due to its properties of abundant reserves,non-toxicity,eco-friendly,biocompatibility,good chemical stability,thermal stability,acid/alkali resistance,and radiation resistance.In addition,its intrinsic bandgap of 2.7 eV and the conjugated π-bonds promise g-C3N4 excellent optoelectronic properties.Therefore,the g-C3N4 possesses excellent potential in the field of photodetectors.Although g-C3N4 has excellent optoelectronic properties,its research in the field of PDs is still at the start.The limited development of g-C3N4 in the field of PDs can be attributed to followed three main issues.(1)Material synthesis:Up to now,almost all prepared g-C3N4 are nanosheets or powder samples,which are very unfavourable for optoelectronic devices.(2)Absorption efficiency:Low absorption efficiency of semiconductors is a common problem for PDs.(3)Carrier recombination:The valence band and conduction band of g-C3N4 are dominated by the orbitals N2 atoms and composed of C and N2 atomic orbitals together.Therefore,the photogenerated electrons and holes cannot be separated effectively in space,resulting in fast recombination.In 2021,our group developed a universal synthesis strategy for waferscale g-C3N4 films via the vapor transport assisted condensation(VTC)method;and the obtained films have the advantages of high-quality,good homogeneity,adjustable thickness,good mechanical flexibility,and free-standing properties.Our research has broken through the synthesis barriers for large-area two-dimensional g-C3N4 and also laid the foundation for its development in the field of PDs.However,the obtained performance of g-C3N4-based PDs is not satisfactory due to the low light absorption efficiency of the films and the serious problem of carrier recombination within the films.In order to obtain high-performance g-C3N4 PDs multi-beam interference and constructing heterojunctions are introduced.The multi-beam interference is utilized to improve the optical absorption efficiency of films and heterojunctions can facilitate the separation of photogenerated carriers.The research of g-C3N4 in the field of PDs is very limited and in order to develop new structures and functions for g-C3N4 PDs,we have introduced the lateral photovoltaic effect to the applications of position-sensitive detectors(PSD).In this thesis,high-quality g-C3N4 films are used to investigate high-performance and multifunctional PDs.The details of the research are as follows:(1)Construction and performance modulation of g-C3N4-based photodetectors:We have applied multi-beam interference to study 2D material PDs.The multi-beam interference in the NiO/Au structure is used to enhance the performance of ultrathin two-dimensional materials-based photodetectors,which demonstrates that the multibeam interference effect can regulate and control the photoelectric properties of 2D material.And then,to obtain high-performance heterojunction PDs,the optimal g-C3N4 growth conditions are explored using the multi-beam interference effect.The optimal PDs show a response range of 300-1100 nm,a response/recovery time of 28/70 μs,a maximum response of 133 A/W,and a maximum detectivity of 3.3×1012 Jones.This work demonstrates that g-C3N4 has great potential for applications in the field of PDs and silicon-based integration.(2)Broadband position-sensitive detector based on g-C3N4:Previous studies have determined the optimal g-C3N4/Si heterojunction preparation conditions by multi-beam interference.To develop new structures and applications of g-C3N4 in the field of PD,the lateral photovoltaic effect is used to exploit PSD based on g-C3N4/Si heterojunction.The position sensitivity of the detector is 368 mV/mm,the nonlinearity is only 0.9%and the response/recovery time is 3.1/50 μs.In view of the excellent performance of this position-sensitive detector,it has been applied to the tracking of light trajectories and the detection of acoustic waves.The PSD can not only detect the light spot movement trajectory better,but also achieve the detection and identification of the frequency and relative amplitude of acoustic waves.This work shows that g-C3N4based PSD has an excellent performance in position detection and can be used in noncontact sensing applications.Our research uncovers the potential applications of gC3N4,which can greatly promote the development of g-C3N4 materials in the field of optoelectronics.(3)High-temperature position-sensitive detectors based on g-C3N4:Considering the excellent thermal stability,g-C3N4 can be utilized to high-temperature PSD to solve the problem that PSD is not applicable in high-temperature environments.The position sensitivity of the detector first increases with temperature and begins to decrease beyond 500 K.The sensitivity is 460 mV/mm and 315 mV/mm at 500 K and 700 K,respectively.Furthermore,the PSD still can achieve accurate angular measurements at 700 K.