Study On Optoelectronic Properties Of Graphitic Carbon Nitride Films Under High Pressure

As a carbon-based two-dimensional layered material,graphitic carbon nitride has attracted enormous attention,owing to its appealing electronic structure,excellent physicochemical stability,metal-free characteristic and easy surface functionality.At present,graphitic carbon nitride has been extensively researched in the fields of photocatalysis and sensing,whereas its progress in the field of semiconductor optoelectronic devices is slow,which is mainly due to the following reasons.Firstly,the present graphitic carbon nitride is mostly in form of powder and bulk,whose high interface defects and uneven surface morphology greatly hinder the transport of carriers,and lacking high-quality graphite carbon nitride films.Secondly,graphitic carbon nitride suffers from rapid recombination of photoexcited charge carriers and limited visible light absorption originated from its intrinsic band gap.Previous studies have shown that the band gap of graphitic carbon nitride can be regulated by heteroatoms doping,nanostructuring,and heterostructure engineering,consequently improving its optoelectronic properties.Nevertheless,these methods have disadvantages such as inhomogeneous distribution and complicated synthesis and purification process.Therefore,it is of great research significance to explore a facile strategy to modify the electronic structure of graphitic carbon nitride for more accessible control of its photoelectric performances.As an important thermodynamic parameter,pressure is a crucial measure to reveal the variation of structure and properties of materials,which can continuously change the lattice constants and electronic structure of materials without introducing defects or impurities,so as to modulate the properties of materials.However,there are few researches to study graphitic carbon nitride under pressure until now,especially regulating its optoelectronic properties via pressure.To solve this problem,combining experiments and theoretical calculations,the optoelectronic properties of high-quality graphitic carbon nitride films under high pressure have been comprehensively investigated in this paper.The main research contents and results are as follows.(1)The optical properties of high-quality graphitic carbon nitride films were investigated under pressure.It can be found that the PL intensity of the films is decreased greatly upon compression,indicating the efficient suppression of the recombination of the photogenerated carriers.What’s more,it can be seen that the PL peaks undergo an anomalous shift as pressure increases,showing a first gradual blueshift in the range of 0 ～1.2 GPa,and followed by a continuous redshift above 1.2GPa.The unusual PL behaviors may be associated with the distribution of π electrons in aromatic rings.According to UV-vis absorption spectra and theoretical results,it’s noted that the interlayer distance and band gap continuously decreased with pressure increasing,which indicates that pressure plays an essential role in regulating the optical properties,thus changing the interlayer distance of graphitic carbon nitride films.This result provides a new band gap engineering route for graphitic carbon nitride.(2)The in-situ photocurrent measurements of high-quality graphitic carbon nitride films were further performed under pressure.It can be shown that the photocurrents of graphitic carbon nitride films gradually increase upon initial compression,whereas above 3.6 GPa,the decrease of the photocurrent probably because of the structural distortion,leading to carrier scattering and additional recombination channels.Furthermore,as the important parameters for optoelectronic performance,responsivity(R),external quantum efficiency(EQE)and specific detectivity(D*),were all significantly improved upon compression.The band structures and density of states were analyzed by theoretical simulation.It was observed that the continuous decrease of band gap makes the valence electrons easier to transition into conduction band,and also promotes the electron-hole effective separation and transfer.In addition,highquality graphitic carbon nitride sample will reduce the number of recombination centers of charge carriers and accelerate them separation and migration,thus improving their charge separation efficiency.It suggests that the improvement of the optoelectronic properties of graphitic carbon nitride films regulated by pressure is helpful to promote its possible optoelectronic applications,and provides a viable way to design functional materials with improved optoelectronic properties.