New DAST-based Organic Piezoelectric Materials And The Adjustment Of Its Optical And Electrical Properties

In the past 70 years,the Information Technology has been developing rapidly towards device integration and miniaturization.Piezoelectric materials that can convert the mechanical energy to electrical energy are widely used in sensors,actuators,detectors,etc.Thus,applications of piezoelectric materials with electro-mechanical transduction have promoted and expanded the development of IT.As the development of the internet and the device miniaturization,the topics related to piezoelectric materials have been converted from the conventional actuators and sensors to wearable,self-powered and portable devices.Consequently,the piezoelectric materials with light weight,low cost,environmentally friendly(e.g.lead-free),and biophilicity are urgently required.Organic materials would be a natural fit in this context,but they have been severely limited by their low piezoelectric coefficients.In comparison to the inorganic piezoelectric counterparts,the piezoelectric coefficients d₃₃of organic piezoelectric materials are generally one order of magnitude lower.In this dissertation,new organic piezoelectric materials with excellent piezoelectric properties were systematically investigated,which promotes the development of organic piezoelectric materials and novel piezoelectric devices.1.Organic nonlinear optical 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate(DAST)single crystals were grown by a method combining slow cooling and slope nucleation.The measured second harmonic generation(SHG)effect of DAST single crystal indicates that DAST exhibits strong nonlinear optical properties.This result,combined with the non-centrosymmetry structure of DAST,implies the piezoelectric effect of DAST.The piezoelectric strain coefficient d₃₃and piezoelectric voltage coefficient g₃₃of DAST single crystal were measured by piezoresponse force microscopy(PFM).For the first time,the d₃₃of DAST single crystal was measured to reach a value as high as?150 p C/N,many times greater than those of all organic piezoelectric materials previously reported,and is comparable to that of the best known inorganic barium titanate(Ba Ti O₃).Though the measured d₃₃of DAST is still shy of that of the champion piezoelectric-lead zirconate titanate(PZT),its piezoelectric voltage coefficient g₃₃is 4-15 times greater than the latter and 11-29 times greater than that of Ba Ti O₃,thanks to the low permittivity of the organic DAST.In addition,extensive density-functional computational analysis,in support of the experimental findings,offers detailed insights to underlying mechanisms,which will be of guidance to future research of new organic piezoelectric materials with high piezoelectric performance.2.To further confirm the PFM measurement result,we quantitatively and directly measured the d₃₃of DAST optically by a laser interferometry.Results indicate that a typical‘butterfly'loop was observed from the strain-electric field(S-E)loop measurement of the crystalline DAST films,further confirming the piezoelectricity/ferroelectric of DAST.From the measured maximum strain and the maximum electric field in the S-E loop,the d₃₃of the crystalline DAST film was calculated,yielding a value of 158 pm/V,which corresponds well with that(150 p C/N)measured by PFM.3.To explore the application of organic piezoelectric DAST,novel all-organic flexible piezoelectric devices constructed by the composite films of DAST and polydimethylsiloxane(PDMS)were fabricated by a simple casting method.The influences of the DAST concentration and the parameters on the generated open-circuit voltage and short-circuit current signals of the DAST-PDMS composite film-based piezoelectric devices were systematically investigated.Results show that the output voltage and current signals of the DAST-PDMS composite film-based piezoelectric devices first increase and then decrease with the increase of the DAST concentration,and the optimized properties of the DAST-PDMS composite film were observed at 15wt%DAST.Moreover,the output open-circuit voltage and short-circuit current increase with the increase of the external compression force,and up to 6.0 V and 188n A,respectively.Particularly,the resultant output power can directly operate commercial light emitting diodes(LEDs)with no external energy source.4.In order to improve the output performance of the DAST-based piezoelectric devices,all-organic flexible piezoelectric devices based on PDMS/DAST/PDMS sandwiched multi-layer structures were further fabricated.The all-organic piezoelectric devices based on these PDMS/DAST/PDMS structures were prepared at different annealing temperatures,and their hysteresis loop,dielectric and piezoelectric properties were investigated.Results reveal that the dielectric constant,polarization,and output voltage and current of the piezoelectric devices based on the PDMS/DAST/PDMS structures increase with the increase of the annealing temperature in the range of room temperature-180 ^oC.And the maximum output piezoelectric signals were achieved at the annealing temperature of 150 ^oC.5.Furthermore,the effects of CNT or G addition on the electrical and optical properties of DAST have been systematically investigated and compared.Results indicate that the electrical and optical properties of DAST can be rationally adjusted by the addition of carbon nanotubes(CNT)or graphene(G).Particularly,DAST-CNT binary composite films exhibit excellent electrical conductivity(lower R),while DAST-G binary composite films are more sensitive to the change of temperature(higher TCR).Accordingly,new DAST-CNT-G ternary composite films were prepared,thus further enhancing the optical and electrical properties of DAST.Results show DAST-CNT-G ternary composite films not only significantly enhance the optical absorption in the terahertz region,but also combine the excellent electrical properties of both DAST-CNT and DAST-G binary composite films.Consequently,the DAST-CNT-G ternary composite films exhibit excellent optical and electrical comprehensive properties.Therefore,the as-prepared DAST-CNT-G ternary composite films hold great potential applications in uncooled terahertz detectors.