Research Of Magnetoresistance Effect And Regulation In Graphdiyne

Since the discovery of magnetoresistance（MR）,magnetoresistance has been deeply studied and widely applied in various fields,such as magnetic sensor,readout head and magnetic random access memory.Due to the potential value of magnetoresistance,it will continue to be the subject of scientific attention for a long time to come.Research on magnetoresistance in carbon materials has also never stopped.Carbon materials with sp and sp² hybridization in allotrope of carbon have been widely applied due to their unique properties.Graphdiyne（GDY）,as a novel two-dimensional carbon material,awaits exploration of its magnetoresistance properties.Based on this,we have undertaken a series of works on the intrinsic magnetoresistance of GDY and the modulation of its magnetoresistance.By improving the experimental method for synthesizing GDY,we have prepared large-area,high-quality,non-transferable GDY films.Growing the film directly on the substrate avoids problems such as damage and wrinkles that may occur during the transfer process,which is beneficial for studying transport properties.We directly measured the magnetoresistance of the prepared GDY film.In addition,we investigated the modulation mechanism of magnetoresistance in GDY films,and found that the introduction of magnetic moments,modulation of carrier concentration,and migration rate can directly modulate the magnetoresistance of GDY films.The specific details and achievements obtained are as follows:（1）Firstly,we conducted an investigation into the intrinsic magnetoresistance of GDY films.In the first section,we improved the substrate for GDY growth by employing a special growth process,which enabled the synthesis of large-area GDY films on Si O₂/Si substrates.This modification eliminated the need for transferring the film to a new substrate,thereby avoiding issues such as damage and breakage during the transfer process.We directly used the GDY films for magnetoresistance measurements,and the results showed that the GDY films exhibited negative magnetoresistance（NMR）at 5-19 K and positive magnetoresistance（PMR）at 19-21 K.As the magnetic field increased,the magnetoresistance signal changed from positive to negative at 19 K.The MR signal disappeared at temperatures above 25 K,and the maximum PMR value at 20 K was approximately 150%,while the maximum NMR value at 5 K was also around 150%.To explain this significant magnetoresistance phenomenon,we combined the Hall effect measurements taken at corresponding temperatures.The nonlinear Hall effect curve showed the existence of two-carrier phenomena.Furthermore,using the two-band model,we performed an excellent fit,and analyzed the carrier concentrations and mobility.The large PMR was attributed to additional carrier scattering induced by Lorentz force,while NMR was considered to result from the superposition of magnetic polaron model（MPM）and weak localized（WL）mechanisms.（2）We conducted a study on the impact of introducing magnetic moments on the magnetoresistance of GDY films.We investigated the effects of introducing p-electron magnetic moments and d-electron magnetic moments through the introduction of hydroxyl and 3d transition metal magnetic moments,respectively.For the introduction of p-electron magnetic moments,we used a simple annealing process to directly introduce hydroxyl groups.By studying the hydroxyl magnetic moment,we were able to investigate the effect of p-electron magnetic moments on the magnetoresistance of GDY.In the experiment,three samples were heated to different temperatures（300℃,400℃,and 500℃）in an argon environment to adjust the hydroxyl content.The results showed that the C-O content was highest at 400℃,indicating the highest hydroxyl content.Magnetic characterization of the samples showed that the coercive force and residual magnetization were highest when the hydroxyl concentration was highest.This suggests that the magnetic properties of GDY films can be effectively controlled by introducing hydroxyl groups.Furthermore,the results of the transport study of magnetoresistance showed that low concentration hydroxyl introduction exhibited NMR under low magnetic fields,while under high magnetic field conditions it showed PMR.All samples exhibited significant increases in PMR under high concentration hydroxyl introduction.This indicates that p-electron magnetic moments can effectively regulate the magnetoresistance of GDY.For the introduction of d-electron magnetic moments,the most direct method is to dope GDY with 3d transition metals.We used an electrochemical deposition method to prepare Ni-anchored GDY films（Ni-GDY）and successfully introduced 3d electrons into the GDY film.The Ni content in GDY can be easily controlled by the electrochemical deposition time.Magnetic characterization results showed that Ni-GDY doped with low concentration（Ni₂-GDY）exhibited paramagnetism,while Ni-GDY doped with high concentration（Ni₈-GDY）exhibited ferromagnetism.Magnetoresistance measurements of Ni₂-GDY and Ni₈-GDY samples at 2 K showed a large negative magnetoresistance in Ni₂-GDY and a large positive magnetoresistance in Ni₈-GDY,indicating that the magnetoresistance is greatly affected by the magnetic moment.Further analysis revealed that at low doping concentrations,the distance between magnetic atoms is too far to form bound polarons,and magnetic exchange coupling does not occur,resulting in paramagnetism.As the Ni doping concentration increases,the distance between Ni clusters decreases,and the spin center is mediated by the p-orbitals of GDY’s C,and they exchange-couple,forming long-range ferromagnetism.This ultimately affects the type of magnetoresistance exhibited by the GDY film.（3）The study investigates the influence of carrier concentration and mobility on the magnetoresistance of GDY.In the study of intrinsic magnetoresistance of GDY,the two-band model was found to be capable of describing the behavior of magnetoresistance in GDY well,with the size of carrier concentration,type,and mobility directly affecting the magnetoresistance.Therefore,we further control the magnetoresistance of GDY by regulating the size of carrier concentration and mobility.Regarding the regulation of carrier concentration,nitrogen doping can be used to regulate the carrier concentration of GDY.In experiments,different nitrogen-doped GDY samples with various nitrogen contents were successfully prepared by simple NH₃annealing treatment.It was found that in N₃₀-GDY samples with low nitrogen content,the magnetoresistance showed a similar negative magnetoresistance phenomenon as the initial magnetoresistance and appeared at low temperatures.As the nitrogen content increased,the magnetoresistance showed a positive magnetoresistance trend,especially in N₆₀-GDY samples where the magnetoresistance exhibited a significant increase.Experimental characterization and data analysis showed that nitrogen doping led to an increase in carrier concentration,resulting in an increase in magnetoresistance.Regarding the regulation of carrier mobility,fluorine doping can effectively change the mobility of GDY.In experiments,we obtained fluorine-doped GDY samples F-GDY through simple fluorination process.By studying the magnetoresistance of GDY doped with different concentrations of fluorine,it was found that low-concentration fluorine-doped GDY showed a large negative magnetoresistance effect at low temperatures,while high-concentration fluorine-doped GDY showed a positive magnetoresistance phenomenon.Experimental characterization and data analysis showed that the change in magnetoresistance was due to the change in carrier mobility.