| Graphene,a two-dimensional system composed of a single layer of carbon atoms arranged in a honeycomb lattice,has a linear dispersion relationship and a unique electronic band structure,which has made it a major concern since its first successful preparation in 2004.In terms of electricity,graphene exhibits extremely high carrier mobility;in terms of optics,graphene has high transmittance in a wide spectrum range from visible to infrared bands,and its transmittance does not change with wavelength.At the same time,due to the covalent bonding between carbon and carbon,graphene is one of the materials with the highest mechanical strength.Due to its exceptional properties,graphene has enormous application potential in photoelectric,micro-nano,flexible materials,and other fields,and it is expected to spark a revolution in modern optoelectronic technology.The application of graphene must be based on the graphene/substrate structure.Despite the fact that numerous related studies have been published in recent years,these studies have primarily focused on the graphene/SiO2(Si)structure.At the moment,a small number of researchers have turned the substrate target to optical crystals,and among those with,the combination of graphene and lithium niobate(LiNbO3,LN)is one of the most promising structures for development.As a representative ferroelectric material,LiNbO3 is known as"optical silicon" due to its comprehensive properties.Scholars have been studying the composite properties of graphene/LiNbO3 structures since around 2010,primarily using the spontaneous polarization properties of LiNbO3 at room temperature to study its influence on the chemical potential of graphene,and then adjusting graphene properties such as the formation and propagation of graphene surface plasmon plasmons(SPP).Liu et al.of our research group recently transferred monolayer graphene obtained by chemical vapor deposition(CVD)onto a thin film LiNbO3 substrate and discovered that the coverage of monolayer graphene had a good limiting effect on the guided modes in LiNbO3 thin film.At the present,research on the properties of graphene/LiNbO3 combination is in its early stages,some excellent properties have been discovered,and there is still a huge research potential in the future.To study the properties of the combination of graphene and LiNbO3,the first thing is the preparation of graphene/LiNbO3 structure.At present,the main graphene preparation processes include micro-mechanical peeling,pyrolysis of SiC,reduction of graphene oxide and CVD method.Due to its advantages in preparing high-quality graphene over a large area and realizing graphene transfer to a variety of other substrate materials,the CVD method has become the most commonly used method in commerce.However,the CVD method has inherent flaws such as the need for the host material’s carbon solubility,the inability to precisely control the number of graphene layers,low utilization of carbon,the requirement for a high heating temperature,and so on.Therefore,some preparation methods that compensate or improve the CVD method are constantly being tried,including the reports on the preparation of graphene by ion implantation in recent years.Although the ion implantation method for producing graphene is far from mature when compared to the CVD method,it has the inherent advantages of not requiring carbon solubility of the material being implanted,being able to precisely control the implantation dose to achieve control over the number of synthetic graphene layers,and having a high carbon utilization rate.The ion implantation method currently used is mainly to implant carbon ions into Ni foil and Cu foil or Ni film and Cu film deposited on SiO2/Si substrate,graphene films with good quality can be produced under the optimized experimental conditions.However,the direct preparation of graphene on the photoelectric material LiNbO3 substrate has not been reported yet.In view of this research status,first,we conducted a series of exploratory studies centered on the direct synthesis of graphene on LiNbO3 substrates by carbon ion implantation.On the one hand,the carbon ion implantation method has inherent benefits in theory.And it is worth noting that this method can define the shape of graphene using patterns of Ni or Cu films,making it particularly suitable for the preparation of graphene-LiNbO3 devices.On the other hand,the direct synthesis avoids the introduction of graphene defects caused by the transfer process.In addition,using the wet transfer method,we prepared graphene-LiNbO3 composite structures.We simulated and analyzed the light conduction properties of this new composite structure using the preliminary experimental investigation of its light conduction characteristics,and suggested a fresh idea for the potential application of the graphene-LiNbO3 composite structure.This paper mainly carries out the following four aspects of work:1.The thermal diffusion behavior of carbon implanted by ion implantation in LiNbO3 was studied,this provides a theoretical basis for the direct synthesis of graphene on the LiNbO3 surface by carbon ion implantation(Carbon ions are directly implanted into the target substrate,and then annealed to precipitate carbon into bonds),as well as a foundation for the selection and determination of subsequent experimental parameters.2.Carbon ions were implanted into the sample,which consisted of a deposited catalytic metal layer and the LiNbO3 substrate,the implanted carbon ions were distributed near the interface between the metal film and LiNbO3 substrate by optimizing the implantation parameters.And the sample was then thermally annealed before the metal film was removed.Using the method described above,whether Ni or Cu was used as the catalytic metal,the graphene film was finally directly synthesized on the surface of LiNbO3.Among them,multilayer graphene with an area of hundreds of square microns was directly synthesized on the surface of LiNbO3 by means of Cu coating.For the first time,graphene has been synthesized directly on the surface of LiNbO3 without transfer.This method is compatible with traditional semiconductor processes.Additionally,since the distribution of the prepared graphene is limited by the shape of the metal film,if combined with patterned metal film,it is expected that graphene of any shape can be prepared directly on LiNbO3 or other substrates,which creates extremely favorable conditions for the preparation of graphene-LiNbO3 devices.3.The stability of the bonding between the metal layer and the LiNbO3 substrate is a necessary and prerequisite condition for obtaining graphene films with uniform distribution and regular shape on LiNbO3 substrate.The experiment revealed that after thermal annealing,a partial separation of the metal layer occurred in the LiNbO3 sample with a Ni metal layer deposited on top.The changes in adsorption energy between the LiNbO3 substrate and the deposited Ni layer during the heat treatment process were therefore simulated and studied at various target annealing temperatures.Finally,the optimal annealing temperature range was determined.Within this range,the metal layer is firmly bonded to the LiNbO3 substrate,and local separation is difficult to occur,so it can provide a uniform space for the precipitation and aggregation of graphene,which is conducive to the formation of high-quality graphene.4.By transferring graphene with various layers grown by CVD onto LiNbO3 substrate using wet transfer method,a variety of graphene-LiNbO3 composite waveguide structures were formed.According to experimental and simulation results,the light limiting and guiding ability of graphene-LiNbO3 composite structure increases with the number of graphene layers.This discovery has far-reaching implications for the integration and miniaturization of graphenebased photoelectric structures. |
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