Near‐Infrared Persistent Phosphor‐Mediated Smart Sensing Light‐Emitting Diodes for Advanced Driver Assistance Systems

Compared with conventional phosphor‐converted near‐infrared light‐emitting diodes (NIR pc‐LEDs), near‐infrared persistent phosphor‐converted LEDs (NIR ppc‐LEDs) are applicable not only in conventional systems but also novel devices in the fields of intelligent security, driverless vehicle technology, and virtual reality. However, NIR ppc‐LEDs have not been extensively investigated. Herein, a novel NIR ppc‐LEDs with the prepared SrAl12O19: Fe3+, Mg2+, Ti4+ NIR persistent phosphors is designed. In relation to pc‐LEDs, the new ppc‐LED exhibits full‐spectral responsive sensing ability during the photostimulated mediation of lattice defects. Furthermore, using the ppc‐LED as a smart sensing LED in a digital NIR imaging system, optical feedback could be converted to a digital signal in a self‐regulated visible‐to‐NIR operation mode. These results imply that the designed ppc‐LED, as a smart sensing LED with full‐spectral responsive ability, greatly enhances the sensitivity of digital NIR imaging, particularly low‐light digital NIR imaging, which has recently drawn considerable attention in the fields of advanced driver assistance systems and VR/AR technology.

designed NIR ppc-LEDs possess full-spectral responsive sensing ability during the photostimulated mediation of lattice defects. This allows the direct control of optical sensing and response in small-volume, low-cost devices. [18] Furthermore, using ppc-LEDs as the core sensing device in a digital NIR imaging system, optical-to-digital signal conversion feedback in a self-regulated visible-to-NIR operation mode is successfully achieved. Over the conventional Cr 3þ -or Eu 2þ -doped NIR phosphor-converted LED devices, [19,20] our designed NIR ppc-LEDs have a significant advantage as a non-visible light source because their excitation and emission spectra fall within the UV and NIR regions, respectively. Thus, ppc-LEDs can serve as smart sensors in ADAS applications toward the detection of car-to-car distance.

Materials and Synthesis
Here, it was anticipated to enhance the persistent performance of SrAl 12 19 sample was taken as an example. The synthesis process of SrAl 11.99 Fe 0.01 O 19 was described in detail as follows: 1.476g SrCO 3 , 6.113g Al 2 O 3 and 3.6 mg Fe 2 O 3 were precisely weighed and then fully mixed. After thoroughly ground in an agate mortar, the blended raw materials were placed into the corundum crucible and then calcined at 1500°C for 10 h in air. Eventually, the target samples were cooled to room temperature and ground once again.

Characterization
Powder XRD patterns were carried out by the known X-ray diffractometer (Bruker D 8 Advance, Germany) with the Cu Kα radiation (λ = 1.5406 Å). The working voltage and current of X-ray diffractometer were set as 50 kV and 40 mA, respectively. Room-temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectra were performed by a FLS980 spectrofluorometer (Edinburgh Instruments, UK) armed with a continuous xenon lamp (500 W) as the excitation source. Room-temperature persistent luminescence (PersL) spectra, spectral-response signals and decay curves of Fe 3þ -doped SrAl 12 O 19 NIR phosphor were also implemented with the FLS980 spectrofluorometer. To ensure the accuracy of the spectral-response signals, the synthetic powders (2 g) were pressed into the disc-shaped wafer with a 10 mm diameter using a uniaxial hydraulic press and the stop-excitation-to-test interval was strictly set as 3 min. To acquire the spectral response of SrAl 12 O 19 :Fe 3þ samples, xenon lamps (500 W) was employed as the radiant source and then the spectral-response signals were recorded under excitation with the external semiconductor lasers (the central emission wavelength: 405, 520, 655, 980 nm). With a heating rate of 5°C s À1 from room temperature to 350°C, thermoluminescence (TL) curves were carried out by utilizing a TL spectrometer (SL08-L model, Guangzhou Radiation Science and Technology, China). Before obtaining afterglow curves and TL curves, our prepared samples were radiated by a 254 nm illumination (15 W) for 20 min.

Fabrication and Performance Measurement of Near-Infrared ppc-Light-Emitting Diodes
NIR ppc-LEDs were packaged by combination of the as-prepared SrAl 12 O 19 : Fe 3þ , Mg 2þ , Ti 4þ NIR phosphor and a UV LED chip (λ em = 280 nm). The mass ratio of the polydimethylsiloxane (PDMS) to SrAl 12 O 19 : Fe 3þ , Mg 2þ , Ti 4þ NIR phosphor was set to 1:1. Electroluminescence (EL) spectra of the ppc-LED device were obtained by utilizing the FLS980 spectrometer. Photos of the lighted ppc-LEDs were taken by the known industrial night-vision camera (MVCA050-20GN, Hikvision, China), and the NIR-emitting image was taken by employing a 780 nm long-pass filter to remove all the visible illumination. To determine the spectral response of the as-synthesized ppc-LEDs, our homebuilt system was composed of a photomultiplier comprehensive test instrument (JC503-42, Beijing), an illumination source system, a photodiode, and an oscilloscope (Tektronix, TDS2002B). The illumination source contained the NIR ppc-LED lighting assembly, a white LED lamp (15 W) and singlewavelength output lasers (405, 450, 520, 655 and 980 nm). An optical power meter (Coherent FieldMax II) was applied to identify the optical power density. The oscilloscope was employed for measuring the steady-state and transient photoelectric signals as well as ultrafast photoelectric response.

A Self-Regulated Visible-To-Near-Infrared Operation Mode
The excitation wavelength and optical power density are crucial factors for achieving the conversion of the feedback from an optical to an electric signal and then to a digital signal. Figure 1a shows the design of an optoelectronic testing platform consisting of three components: an NIR ppc-LED sensor, a photomultiplier, and an oscilloscope. First, the EL intensity of a NIR ppc-LED device was obtained using a photodiode detector based on excitation wavelength and optical power density. A variable (ΔI) of PL intensity was then converted into an electrical signal by a combined system of a photomultiplier tester and oscilloscope. As shown in Figure 1b, the electrical signal strength was proportional to the peak voltage (ΔV ) of the oscilloscope's output waveform. Thus, we successfully achieved the conversion feedback from optical to electric and then to digital signal. This ppc-LED-mediated digital NIR sensor exhibited optical signal modulation ranging from visible to NIR light and easily achieved an optically dependent response of smart optoelectronic devices to the environment (see the following section).

A Defects Controlling to SrAl 12 O 19 : Fe 3þ , Mg 2þ , Ti 4þ Persistent Phosphors
We previously discovered that the NIR persistent luminescence (PersL) of the SrAl 12 O 19 :Fe 3þ PPs was observed at 812 nm for 1 h. [21] Here, we attempted to improve the NIR afterglow performance of SrAl 12 O 19 :Fe 3þ via the nonequivalent co-substitution of [Ti 4þ þMg 2þ ] with [Al 3þ þAl 3þ ]. By substituting Ti 4þ with Al 3þ and Mg 2þ with Al 3þ , two kinds of crystal defects, Ti · Al3þ and Mg 0 Al3þ , are created, both of which serve as defect traps [22] Ti To study the effect of nonequivalent co-substitutions on the crystal structure of SrAl 12  To gain further insight into the effect of nonequivalent co-substitution, the Rietveld refinements of SrAl 12 O 19 :Fe 3þ and SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ were conducted (Figure 2b,c), which were found to be ideal. Thus, the reliability of these Rietveld refinements due to the low R-factors was confirmed (  (Figure 2d). A unit cell of SrAl 12 O 19 consists of one AlO 4 tetrahedron, an unusual AlO 5 trigonal bipyramid, and three distinct AlO 6 octahedrons. Thus, it is ideal for Fe 3þ to occupy these octahedral and tetrahedral aluminum sites, resulting in a broad NIR emission. [19c,d] Figure 2e shows the EPR spectra of SrAl 12 O 19 , SrAl 12 O 19 : Fe 3þ , and SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ . We previously investigated the signal of EPR spectra at g = 2.09 and 5.62 using concentration-dependent EPR measurements. [23] The EPR signal (g = 5.62) was attributed to Fe 3þ activators and a defect complex of the Fe 3þ -V O cluster, [21] whereas the symmetrical signal (g = 2.09) was attributed to the presence of Fe 3þ at regular octahedral sites. The EPR spectra of SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ showed broadened width and higher signals at g = 5.62 and 2.09 than those of SrAl 12 O 19 :Fe 3þ . The results showed that nonequivalent co-substitution of Ti 4þ with Al 3þ and Mg 2þ with Al 3þ produced more Fe 3þ -V O clusters and new defects (Figure 2e) [21,24] Upon excitation at 267 nm UV light, a broad NIR emission centered at 811 nm was observed owing to the typical 4 T 1 (4G) ! 6 A 1 (6S) transition of Fe 3þ . [25] Figure 3b shows the NIR afterglow curves of the SrAl 12 Figure S1, Supporting Information. Compared with other samples, the   Figure 3e were obtained by cleaning the occupied trap when excited at a different temperature. When stimulated at a higher temperature, the shallower traps were instantly bleached owing to the rising thermal energy; however, only deeper fractions of the traps were filled with a quasi-continuous trap depth distribution. [25] Figure 3e shows the shift in the TL maximum peak from low to high temperature, demonstrating the quasi-continuous trap depth distribution in SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ . Thus, the trap depths ( Figure S2, Supporting Information) were discussed in detail using the initial rise method, which is independent of the order of kinetics involved in the trapping and releasing processes. [26] Figure 3f shows that the trap depths of SrAl 12 O 19 :Fe 3þ , Ti 4þ , Mg 2þ ranged from 0.66 to 1.08 eV, which agreed well with the ideal thermal energy for trapping and releasing PerL at room temperature. [26] These results strongly confirmed the presence of a quasi-continuous trap depth distribution in the SrAl 12 O 19 : Fe 3þ , Ti 4þ , Mg 2þ phosphor, laying the foundation for its digital NIR sensing with a full spectral response ranging from 405 to 980 nm.
Based on the trap depth, density, and distribution analyses, the persistent mechanism of the SrAl 12 O 19 :Fe 3þ , Ti 4þ , Mg 2þ NIR PPs was determined. As shown in Figure 4a, under UV light irradiation, Fe 3þ electrons in the 6 A 1 (6S) ground state were pumped into the charge transfer state (CTS). Furthermore, most electrons were released nonradiatively to the first excited state, 4T 1 (4 G), resulting in a broad NIR emission owing to the 4 T 1 (4G) ! 6 A 1 (6S) transition of Fe 3þ ions. Meanwhile, a few electrons escaped into the conduction band (CB) and were possibly captured by the quasi-continuous traps. Upon thermal perturbation of the crystal lattice, the electrons trapped in the shallow traps escaped into the CB and returned to the CTS, resulting in the NIR persistent luminescence of SrAl 12 (Table S4, Supporting Information). Such NIR broad emission and persistent luminescence of Cr 3þ and Fe 3þ might be ascribed to the topological network of a magnetoplumbite-type structure. [19] The magnetoplumbite-type structure consists of two or more different lattice sites capable of triggering photon transitions of activators, resulting in NIR broad emission. [19] However, this structure contains various types of bridging oxygens that can produce oxygen vacancies capable of serving as defect traps during high-temperature synthesis. [19] In addition, we analyzed the NIR sensing performance of the SrAl 12 O 19 :Fe 3þ Mg 2þ , Ti 4þ phosphor at different excitation wavelengths. SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ was continuously charged using a 500-watt Xenon lamp and then irradiated with lasers of different wavelengths. Figure 4b shows the intensity variable (ΔI) of SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ at 405, 450, 520, 655, and 980 nm. The energy density of these excitation sources was fixed at 33 mW cm À2 . Figure 4b depicts the spectral response range of SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ from NUV to NIR light, with the most prominent response at 405 nm NUV light. To quantify the response of SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ to different excitation wavelengths, its relative variable intensity was determined (Figure 4c). Based on Figure 4b,c, we can conclude that the response of SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ to 405 nm light is the most sensitive. This is because of two reasons: self-absorption and the photostimulated effect of SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ .
To demonstrate the response repeatability of SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ to 980 nm light, its PL intensity with and without 980 nm light excitation was determined (Figure 4d). Evidently, the response of SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ to 980 nm light remained constant during 10 on/off cycles over a period of 8000 s. Figure 4e presents Figure 5a shows the EL spectrum of our designed NIR ppc-LED device, and the inset shows a photograph of the SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ -mediated ppc-LEDs. Figure 5b shows the EL spectra of the NIR ppc-LED device at different driving currents (5-11 mA). The EL intensity of the NIR ppc-LED device increased with increasing working current. Compared with previously reported Cr 3þ -doped phosphor-converted NIR LEDs, the PLE and PL spectra of the Fe 3þ -doped NIR ppc-LED were invisible under working conditions, demonstrating its potential in fields such as AR, VR, machine vision, and intelligent security. We also investigated the dependence of the NIR sensing performance of the SrAl 12 Figure 5c, we concluded that the NIR ppc-LED device was most sensitive to 405 nm light, and the SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ -mediated NIR ppc-LED exhibited the optimal NIR sensing performance. Figure 5d shows the sensing performance of the SrAl 12 O 19 : Fe 3þ , Mg 2þ , Ti 4þ -mediated NIR ppc-LED as a function of the power density of a 405 nm laser. Evidently, the output photovoltage (ΔV ) of the oscilloscope increased with the increasing power density of the 405 nm laser. In theory, the output photovoltage (ΔV ) of the oscilloscope can increase indefinitely as the power density of the 405 nm laser increases. However, the maximum power density of our 405 nm laser only reached 0.4 W. To convert the ΔI of the SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ -mediated NIR ppc-LED device into an electrical signal, the output photovoltage (ΔV ) of the oscilloscope was monitored in a self-regulated visible-to-NIR operation mode. To determine the minimum threshold of the spectral response for the SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ -mediated ppc-LEDs, the minimum threshold of the laser with different wavelengths was obtained (Figure 5e). The result indicated that the device will respond only when the threshold value of 405 nm light reaches 0.93 mW cm À2 . The minimum thresholds for 450, 520, 655, and 980 nm lights were 5.13, 2.55, 6.05, and 10.08 mW cm À2 , respectively. This can avoid the impact of other illumination sources on its response performance due to the different response thresholds to the different wavelengths of light (Figure 5f ). Figure 5g shows photographs taken with traditional and NIR digital cameras using white LED and NIR ppc-LED lighting. Interestingly, NIR cameras captured nothing in the absence of NIR irradiation; however, they can capture black-and-white images when illuminated by the NIR ppc-LED device. Based on this unique feature, NIR ppc-LEDs can serve as a smart sensor that can measure the car-to-car distance of ADAS in selfregulated visible-to-NIR operation mode using our fabricated NIR ppc-LEDs. As shown in Figure 5h, when capturing the signal of the NIR ppc-LED device, the following vehicle must maintain a safe distance from the vehicle in front, allowing enough space to apply the emergency brake if necessary when multiple motor vehicles run in the same lane.

Conclusions
We successfully synthesized a novel SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ NIR PP via a simple solid-state method. Through the nonequivalent  . We achieved the conversion feedback from an optical to a digital signal in a self-regulated operation platform using a SrAl 12 O 19 :Fe 3þ , Mg 2þ , Ti 4þ -mediated ppc-LED device. In future, we will investigate the dependence of ΔV on the distance between cars (ΔL), with the goal of incorporating our NIR ppc-LEDs in automatic braking systems of automobiles. Overall, NIR-PP-mediated smart sensing LEDs can act as smart sensors in ADAS applications to detect the distance between cars.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.