Study On The Characteristics Of Surface Acoustic Wave Devices Based On Graphene Oxide Films

Surface acoustic wave（SAW）technology is a newly arisen interdisciplinary subject,which combines acoustics,electronics,plane technology and so on.Due to the superior properties,such as good versatility,high frequency and easy integration,SAW devices have been widely used in mobile communication,environmental monitoring,bio-medicine and other fields.Because of the large surface-to-volume ratio and oxygen-containing hydrophilic functional groups,graphene oxide（GO）films have been widely used in sensing of many substances,and have great potential in ultra-sensitive detection.In this thesis,the characteristics of SAW devices based on GO films,as well as the sensing properties,are studied.And the contents mainly include three parts:（1）The propagation characteristics of SAWs,including the Rayleigh waves excited in GO/SiO₂/IDT/128^oYX-LiNbO₃ structures and Love waves excited in GO/SiO₂/IDT/41^o YX-LiNbO₃structures,are theoretically investigated by finite element method（FEM）.The SAW properties of insertion loss,phase velocity v_p,electromechanical coupling coefficient k²,admittance and temperature coefficient of frequency（TCF）are discussed by COMSOL Multiphysics software.The simulated results show that,for the Rayleigh wave devices,the introducing of GO films has great effect on the phase velocity and electromechanical coupling coefficient.As the SiO₂ film thickness-to-wavelength ratio（h_s/λ）is 0.01 and the thicknesses of GO films(h_GO)are 0μm⁰.5μm,the phase velocity of Rayleigh waves are increased from 4008m/s to 4510m/s,meanwhile,the values of k² are increased from 5.62%to 9.32%.The GO films also have influence on the characteristics of Love wave devices.As h_s/λ<0.2,the electromechanical coupling coefficient decreases with the increasing of h_GO,while as h_s/λ>0.2,the electromechanical coupling coefficient increases.When h_s/λ=0.34,h_GO=0μm⁰.3μm,the phase velocities of Love waves are increased from 4262m/s to 4550m/s,and the values of k² are increased from 9.49%to 14.57%.In addition,the temperature compensation is realized for the two kinds of SAW devices because SiO₂ films have positive temperature coefficients of elastic parameters.（2）The gas sensing and liquid sensing characteristics of the Rayleigh wave sensors based on GO/SiO₂/IDT/128^oYX-LiNbO₃ structures are studied by using FEM.The gas sensing properties of the Rayleigh wave sensors are investigated by sensing the humidity and volatile organic gases.The results show that,as h_s/λ=0.23,h_GO=0.1μm,and RH=90%,the frequency shift of Rayleigh wave humidity sensor is-1400.8kHz,which is 11 times higher than that of Rayleigh wave humidity sensor based on ZnO/SiO₂/IDT/128^oYX-LiNbO₃ structures.Meanwhile,for the volatile organic gases of CH₂Cl₂,CHCl₃,CCl₄ and C₂Cl₄,as the gas concentration of is 500ppm and h_s/λ=0.23,h_GO=0.1μm,the frequency shifts of Rayleigh wave sensors are-0.46kHz,-24.33kHz,-60.69kHz and-79.89kHz,respectively,which are higher than that reported in references.Thus,the GO thin films have superior gas-sensing characteristics.In addition,for liquid sensing,the solid-liquid coupling between SAW devices and liquid will be occurred,which will result in the variations of sensing mechanism and sensitivity of SAW sensors.The solid-liquid coupling is simulated by using Acoustic-Structure in COMSOL,and the results show that the sensitivity of Rayleigh wave sensors based on the structures of GO/SiO₂/IDT/128^o YX-LiNbO₃ is effected by the mechanical and electrical properties of the liquid layer.As h_w=10nm,h_GO=0.1μm and h_s/λ=0.12,the maximum S_max of 188m²/kg is obtained,the sensitivity of the Rayleigh wave sensor is improved about 38m²/kg by introducing of GO thin films.It also indicates that the GO films can improve the liquid sensing performance of SAW devices.（3）The characteristics of SAW devices based on non-piezoelectric substrates are discussed by using COMSOL.The performance improvements of Rayleigh wave devices by introducing of GO films and SiO₂ films are theoretically investigated,including v_p,k² and TCF.The results show that,for the Rayleigh waves excited in ZnO/IDT/Si structures,the maximum k²_max of 2.38%is obtained as h_Z/λ=0.44 associated with v_p of 3016m/s and TCF of-32.94ppm/^oC.The k² values of Rayleigh waves in ZnO/IDT/SiO₂/Si structures are increased significantly by introducing bottom SiO₂ layer,as h_Z/λ=0.44 and h_sb/λ=0.25,the maximum coupling coefficient k²_max equals to 3.41%,with v_p and TCF are 2801m/s and-11.43ppm/^oC,respectively.The continuously introducing of top SiO₂ layer,that is the structure of SiO₂/ZnO/IDT/SiO₂/Si,results in the increasing of phase velocity,while the decreasing of the coupling coefficient.And the k² of 2.61%is obtained as h _Z/λ=0.44,h_sb/λ=0.25 and h_st/λ=0.25 associated with v_p of 3036m/s and TCF of 18.44ppm/^oC.However,the properties of Rayleigh wave devices based on GO/ZnO/IDT/SiO₂/Si structures are further improved.And as h_Z/λ=0.44,h_sb/λ=0.25,h_GO/λ=0.08,k²_max of 3.39%,v_p of 3190m/s and TCF of-8.18ppm/^oC are obtained.It can be seen that the top GO layers greatly improve the performances of Rayleigh wave devices compared with top SiO₂ layers.When there are the same thickness of GO layers and SiO₂layers,the humidity sensitivity of GO layers increase 7 times when humidity is 90%,and to volatile organic gases with the same concentration by more than 10 times.