Exploring Thermo-Responsive Electroluminescence Driven by Alternating Current

Temperature is one of the most crucial physical quantities that are associated with copious practical fields from industry to living environment. The burgeoning frontier technologies such as smart manufacturing, biomedicine, intracellular sensing, microelectronics, and nanoscience have promoted growing demands for remote, accurate, high-resolution, and real-time temperature detection. The unusual working conditions have also brought forward harsh requirements that cannot be fulfilled with traditional thermometric means. In this context, luminescent materials have experienced fast development during the last decades toward the highway to novel temperature sensing techniques by virtue of various thermal responsive optical parameters. Astonishing achievements have been accomplished in this field by digging out all the aspects of material luminescent properties that are potentially sensitive to temperature change. To date, luminescence emission/excitation peak position, spectral shape, decay time, and anisotropy have been explored and utilized as intrinsic temperature-dependent parameters for materials from bulk to quantum dots. They have been demonstrated as progressive means to conquer the abovementioned pain points. Rare-earth and transition metal doping strategy is intensively investigated as an easy but powerful tool to manually introduce luminescent centers to host materials. Based on the modulation of dopant energy levels and energy transfer among them, their temperature-sensing performance can be deliberately designed. It’s also worthwhile to explore the potential of special host materials that can harness other novel types of excitations, such as mechanical forces, electric field, and ultrasound, to unlock the capability of luminescence thermometry in more scenarios.

Electroluminescence under the excitation of an alternating current (AC) electric field has attracted the attention of researchers in recent years owing to its fascinating applications in displays, electronics, wearables, robotics, etc. Alternating current electroluminescence (ACEL) devices typically comprise an emissive layer (e.g., ZnS) and a dielectric layer (e.g., BaTiO₃) that are sandwiched between two electrodes. To further excavate the potential of ACEL in the field of optical ratiometric thermometry, we propose to engineer the optical layers through doping of lanthanide and transition metal ions.

This thesis begins with a survey of the fundamental principles of temperature sensing techniques. Current optical thermometry techniques are introduced in detail regarding the underlying mechanism and various evaluation indices, compared with traditional temperature sensing techniques. Progress is summarized concerning the construction of optical thermometers realized by doping strategy. The principles, properties, and current studies of ACEL are also presented with additional emphasis on the related host materials.

In Chapter 4, we investigate luminescent rare earth ion Eu³⁺ doping in BaTiO₃. The photoluminescence (PL) of Eu³⁺ originates from its direct excitation due to the lack of sensitization mechanism by BaTiO₃ host. We take advantage of the temperature dependence of the host absorption edge to affect the near-edge excitation process of Eu³⁺. Hereby, we have constructed an excitation intensity-based luminescence thermometer by monitoring merely one emission wavelength of Eu³⁺ by virtue of the thermal characteristics of the host material. This type of excitation intensity ratiometric (EIR) thermometer only monitors the luminescence intensity at a single wavelength, thereby eliminating the need for high-quality photon detectors to resolve the spectral structure. Then, we study the influence of doping at different cation sites on thermometric performance, whereby we can optimize the doping strategy to intentionally manage the doping site of Eu³⁺ towards better thermometric performance. Our findings offer a new perspective in designing EIR optical thermometers by virtue of the thermal responsive optical band gap of host materials.

In Chapter 5, we discuss the possibility of luminescence band manipulation in ZnS. ZnS is a unique multifunctional luminescent material with multiple broadband emissions activated by different dopants. However, manipulation of these bands is extremely tricky because they are mechanistically coupled with each other and deeply correlated with the intrinsic defects of ZnS. We find that Cu/Pb co-doped ZnS can generate both blue and red emission bands with an adjustable ratio by changing the doping concentration of Cu and Pb. Moreover, the two bands can be reversibly converted to each other by changing the temperature to set up a ratiometric thermometer with a visually significant luminescence color-switching phenomenon.

In Chapter 6, we attempt to develop ratiometric optical thermometry in powder-type ACEL. Cu/Cl co-activated ZnS powders are present to exhibit both PL and ACEL ranging from blue band to green band by changing dopant concentrations. Furthermore, both PL and ACEL of ZnS:Cu/Cl display temperature dependence of the green/blue emission ratio. The concept of ACEL thermometry is thereby successfully accomplished by fine-tuning the doping concentrations to maximize the spectral response to temperature. We also found that Co co-doping can enhance the relative sensitivity of the thermometer and further propose a mechanism in which the electron trapping/de-trapping at the Co defect is subjected to temperature change.