Making Graphene Film Photoluminescent By UV Ozonation

Making graphene-based materials photoluminescent has attracted great attention in the past few years.Different from monolayer MoS₂ or WS₂with direct bandgaps,graphene,a gapless two dimensional semimetal,has to be physically or chemically modified for realizing photoluminescence?PL?.Wet chemical methods have been popularly used for this purpose with products,including graphene oxide,reduced graphene oxide,and graphene quantum dots.Besides,making luminescent nanostructures directly in graphene film by dry etching is important for its applications in photonics and opto-electronics fields.In this paper,ultraviolet ozonation?UVO?has been employed for directly etching graphene films on the Si/SiO₂ substrate.We have explored the photochemical etching conditions that can make graphene photoluminescent and studied the extraordinary PL phenomena and the underneath working mechanism.In specific,it is composed of the following three major contents.Firstly,we have studied influence of some critical factors such as the substrate temperature and the oxygen pressure,on the etching process of graphene using our home-built UV ozonation vacuum machine installed with a low-pressure mercury?Hg?lamp.Based on the optical and atomic force microscope?AFM?topographical analyses,we have observed two major trends:?1?Fixing the oxygen pressure at 96.3 kPa,the etched graphene nanostructures change from nanofilaments into nanodots with the substrate temperature varying from room temperature to 120?.The nanodot size increases upon rising of the substrate temperature with the depth of etched nanostructures increasing from 1 nm to 6 nm;?2?Keeping the substrate temperature at 100?,the etched graphene nanostructures change from a mixture of nanodots and nanoflakes into dense nanodots with an average size of about 20 nm when the oxygen pressure increases from 61.3 kPa to 81.3 kPa.The UV ozonized graphene nanostructures are testified to be mechanically stable and be enhanced in chemical reactivity.Secondly,we have made few layer graphene?FLG?photoluminescent by room-temperature UV ozonation.It has two features:?1?PL cannot be observed in mono-and bi-layer graphene nanostructures;?2?PL is localized at the edges and suspended region of FLGs with their individual morphologies of nanodots and nanofilaments,respectively.Under the excitation of 325 nm laser,the PL spectrum shows a prominent peak at⁵90 nm with the full width at half maximum?FWHM?of about 180 nm.We propose that intact carbon layers subjacent to the UV ozonized FLG and charged impurities at the surface of silicon dioxide?SiO₂?substrate may quench PL.As a result,no PL appears in the central area of etched FLGs and in the mono-and bi-layer graphene.We find that,however,the ozonized graphene on the sidewalls of FLG and at the bottom layers of the suspended FLG can effectively reduce the fluorescence quenching effect.Finally,we have made trilayer graphene photoluminescent on the whole surface by high-temperature?120??UV ozonation.The localized PL at the edges of FLG fades away upon increasing ozonation temperatures.However,weak PL can be observed in the surface of trilayer graphene that is composed of nanodot structures with a diameter of about40 nm and a height of around 6 nm.Analyses indicate that the structural agglomeration can reduce the fluorescence quenching aroused by the charged impurities at the surface of SiO₂ substrate.We have explored the working mechanism by comparing the different PL properties in room-temperature and high-temperature?120??etched graphene nanostructures.X-ray photoelectron spectroscopy?XPS?analyses indicate that the ratio of sp³ to sp² carbon atoms increases upon the UV ozonation temperature,and that the oxygen functional groups increase even though the amount of oxygen content is still much lower than that of carbon atoms.Based on the topographical,PL and surface chemical functional group analyses,we propose that PL originates from the radiative recombination of electron-hole?e-h?pairs localized in isolated sp² clusters,and that intact carbon layers and the charged impurities at the surface of silicon oxide substrate can quench PL.The PL of FLG nanostructures are stable after a long time storing,but appear instable under excitation of UV lasers due to severe quenching.In conclusion,we have for the first time made few layer graphene photoluminescent by UV ozonation both at room temperature and at high temperature.Based on the experimental analyses,we have proposed the PL mechanisms for UV ozonized FLGs.This work provides a new perspective of making graphene photoluminescent,and it may contribute to future applications in photonic and opto-electronic elements.