A Flexible Sensor Capable Of Outputting Optical-electrical Dual Signals Under Light-thermal-mechanical Stimulation And NaCa₂GeO₄F:Mn²⁺ Material Stress Luminescence Properties Study

Skin can detect various external stimuli,such as pressure,temperature and needling,to remind human to respond in time and avoid injury.Inspired by human skin,scientists and engineers have developed flexible sensors that can detect a variety of external stimuli.In recent years,flexible sensors play an important role in the applications of bionic medicine,intelligent robots and wearable devices.However,to meet the requirements of practical application,there are still several limitations that need to be solved.The first limitation is that it is difficult for the flexible sensors to convert external optical-thermo-mechano stimulation into optical-electrical dual-signal output.Most flexible sensors based on luminescent materials can only generate optical signals under optical-thermo-mechano stimulation;while flexible sensors based on the triboelectric nanogenerator structure can only generate electrical signals under mechanical stimulation.However,there are few reports on flexible sensors that can simultaneously produce optical-electrical signals in response to optical-thermo-mechano stimulation,which greatly hinders the development of advanced flexible sensors.Another limitation is that it is difficult for the optical materials that generate the mechanoluminescence signal in flexible sensors to maintain the sustainability and stability of the mechanoluminescence signal under long delay time and cyclic mechanical stimulation.Most of the optical materials that produce mechanoluminescence signal are trap controlled luminescence materials.Under UV irradiation,the traps in the materials capture the charge carriers.After stopping irradiation,the charge carriers stored in the traps will be consumed due to the thermal stimulation of the surrounding environment.Under mechanical stimulation,the charge carriers stored in the trap will also be released to produce mechanoluminescence,which will also lead to the consumption of the charge carriers stored in the trap.These factors will lead to the problem that it is difficult for trap controlled luminescent materials to produce mechanoluminescence signal under long time delay and the mechanoluminescence signal will decay rapidly under continuous mechanical stimulation.In view of the two limitations in the development of the above-mentioned flexible sensors.On the one hand,we studied the selection of flexible sensor materials,the structure and morphology of flexible sensor,the preparation process of flexible sensor and the multiple signal response of flexible sensor,and prepared a flexible sensor which can output optical electrical dual signal under the optical-thermo-mechano stimulation.On the other hand,we prepared the mechanoluminescent material NaCa₂GeO₄F:Mn²⁺by solid-state synthesis method,and studied its photoluminescence,afterglow,mechanoluminescence,thermoluminescence and the composition of mechanoluminescence traps,and designed and demonstrated the potential applications of the synthesized material in the field of secure encryption.The main results are as follows:(1)Combining the optical properties of multi-optical functional materials with the electrical properties of single-electrode triboelectric nanogenerator,a flexible sensor strategy is proposed to output double opto-electric signals under the optical-thermo-mechano stimulation.As the proof of this concept,a flexible sensor is made up of a thermoplastic polyurethane elastic film coated with multifunctional optical particles La₂Ti₂O₇:Pr³⁺and a Nylon film coated with silver nanowires.La₂Ti₂O₇:Pr³⁺particles can generate photochromic,photoluminescence,thermoluminescence and mechanoluminescence when stimulated by optical,thermal and mechanical.The whole flexible sensor structure can be used as a single electrode triboelectric nanogenerator to generate voltage and current electrical signal output under mechanical stimulation.The repeated experiments of photoelectric signals under optical-thermo-mechano stimulation show that the flexible sensor has excellent wear resistance and signal output stability.These results are expected to promote the development of multifunctional integrated photoelectric sensors,and provide new ideas for the development of flexible sensors in the design of intelligent robots,wearable electronic devices and artificial limbs.(2)A new regenerative mechanoluminescent material NaCa₂GeO₄F:Mn²⁺has been developed.By systematic characterization of the photoluminescence,afterglow,mechanoluminescence and thermoluminescence intensity of the activated samples with different concentrations of Mn²⁺,it was determined that 0.3 mol%Mn²⁺was the best activator concentration,the sample with the concentration of 0.3 mol%Mn²⁺exhibited long delay time(up to 144 hours)and stable mechanoluminescence under short-term continuous mechanical friction(about 20 cycles).The initial rising slope method was used to estimate the depth of the material traps.It was found that the traps were distributed in two different depths,with the shallow traps ranging from 0.506-0.985 e V and the deep traps ranging from 0.99-1.09 e V.Therefore,it is inferred that the long delay time sustainable mechanoluminescence of the material is due to a long time storage capacity of the excited energy and the slow release of the captured carriers in the deep trap.By comparing the distribution of thermoluminescence traps between the developed materials and three typical materials with short-term mechanoluminescence(Ca₃Ti₂O₇:Pr³⁺,NaNb O₃:Pr³⁺,Er³⁺and La₂Ti₂O₇:Pr³⁺),it is inferred that the short-term relative stability of NaCa₂GeO₄F:Mn²⁺mechanoluminescence is the result of the supply of charge carriers from deep traps to shallow traps in the process of mechanoluminescence.In addition,the developed NaCa₂GeO₄F:Mn²⁺also shows a high image contrast,which is 135%higher than that of the commercial materials Sr Al₂O₄:Eu²⁺,Dy³⁺.These results are expected to facilitate the development of energy-storing mechanoluminescent materials and their practical applications in information encryption and electronic display.