Solar-blind ultraviolet-C persistent luminescence phosphors.

Visible-light and infrared-light persistent phosphors are extensively studied and are being used as self-sustained glowing tags in darkness. In contrast, persistent phosphors for higher-energy, solar-blind ultraviolet-C wavelengths (200-280 nm) are lacking. Also, persistent tags working in bright environments are not available. Here we report five types of Pr3+-doped silicates (melilite, cyclosilicate, silicate garnet, oxyorthosilicate, and orthosilicate) ultraviolet-C persistent phosphors that can act as self-sustained glowing tags in bright environments. These ultraviolet-C persistent phosphors can be effectively charged by a standard 254 nm lamp and emit intense, long-lasting afterglow at 265-270 nm, which can be clearly monitored and imaged by a corona camera in daylight and room light. Besides thermal-stimulation, in bright environments, photo-stimulation also contributes to the afterglow emission and its contribution can be dominant when ambient light is strong. This study expands persistent luminescence research to the ultraviolet-C wavelengths and brings persistent luminescence applications to light.


Supplementary Note 1. Estimation and determination of persistent luminescence power intensity of Ca 2 Al 2 SiO 7 :Pr 3+ persistent phosphor
We used a Newport 2936-R optical power and energy meter and a Newport 918D-UV-OD3R UV enhanced silicon photodetector to measure the persistent luminescence intensities of a Ca2Al2SiO7:Pr 3+ persistent phosphor disc at different decay instants in the initial decay period of 10 s to 120 s. The measurement system has a minimum measurable power of 20 pW. Based on the measured intensities, we used a semi-sphere irradiation geometry model to estimate and determine the UVC persistent luminescence power intensities of the Ca2Al2SiO7:Pr 3+ disc in various initial decay instants, e.g., 1 s, 10 s, 30 s and 60 s, in the absolute unit of mW m 2 .
Supplementary Figure 2a shows the experimental setup for the measurement of persistent luminescence intensities of a Ca2Al2SiO7:Pr 3+ disc. A carton (size: L × W × H = 24 × 22 × 16.5 mm) was used as a small dark chamber. The UV silicon photodetector was adhered onto the bottom of the carton with the sensor facing upward. The diameter of the sensor area is 10 mm. A hole with diameter of 14 mm was made on the top board of the carton with its center aligned with the sensor center. The distance between the surface of the sensor and the surface of the top board is 150 mm. A Ca2Al2SiO7:Pr 3+ disc with diameter of 15 mm was irradiated using a 4 W 254 nm UV lamp for 2 min. After the lamp was ceased, the charged disc was immediately placed on the hole and the readout on the Newport 2936-R meter was recorded from 10 s after the excitation was ceased and last to 120 s in 10 s steps, as shown in Supplementary Table 1. During the excitation and measurement, the light in the lab was turned off, making the measurement inside the carton chamber in a completely dark condition.
The grey dash curve in Supplementary Fig. 2b is the decay curve of Ca2Al2SiO7:Pr 3+ persistent phosphor in the initial decay period (10-300 s) monitored at 268 nm emission. We normalized the decay curve at the 10 s decay instant and obtained the relationship between the persistent luminescence intensity (I) and the decay time (t), i.e., I = 0.03286 + 2.08578exp(t/8.81733) + 0.35426exp(t/54.67284) The red curve in Supplementary Fig. 2b is the fitting relationship of I-t. The measured persistent luminescence intensities were also normalized at 10 s and they fit the I-t plot very well, as shown by the green dots in Supplementary Fig. 2b.
According to the fitting I-t relationship, the intensity of persistent luminescence at t = 1 s is 2.24 times of that at 10 s. Therefore, basing on Supplementary Table 1, we deduced the power measured by the UV photodetector at t = 1 s instant would be 2.03 nW.
Supplementary Figure 2c shows the persistent luminescence emission spectrum of Ca2Al2SiO7:Pr 3+ persistent phosphor acquired at 10 s decay. Besides UVC emission at 250-280 nm (region I), the spectrum also contains emission in the 280-400 nm range (region II). We integrated the areas of region I and region II and found that region I accounted for 45.7% of the whole emission area. Therefore, to calculate the UVC persistent luminescence power intensity, the intensity measured by the UV photodetector needs to be multiplied by 0.457. Finally, we assumed the UVC persistent luminescence at the position of disc was semi-sphere irradiation and the intensity at each position on the disc was the same as that at the center position. We calculated the UVC persistent luminescence intensity at the disc position using equation, I = 0.457×Im×2π×h 2 /(π×r 2 ), where Im is the measured intensity by the power meter (Supplementary  Table 1), h is the distance between the surface of the sensor and the surface of the top board, which is 150 mm, and r is the radius of sensor, which is 5 mm. The UVC persistent luminescence power intensity was then obtained by dividing the intensity (I) with the area of the hole (= π×(7 mm) 2 =153.94 mm 2 ).
Based on the above analysis and the data in Supplementary  The measurement was carried out in a carton dark chamber. A Newport 918D-UV-OD3R UV enhanced silicon photodetector was adhered onto the bottom of the carton with the sensor facing upward. The diameter of the sensor area is 10 mm. A hole with diameter of 14 mm was made on the top board of the carton with its center aligned with the sensor center. The distance between the surface of the sensor and the surface of the top board is 150 mm. The diameter of the disc sample is 15 mm. For the measurement, the disc was irradiated by a 254 nm lamp for 2 min and was then immediately placed on top of the hole. The measured persistent luminescence intensities are shown in Supplementary Table 1. (b) Persistent luminescence decay curve (grey dash curve) monitored at 268 nm. The decay curve was normalized at the 10 s decay instant and the red line curve is the thus obtained fitting relationship between the persistent luminescence intensity (I) and the decay time (t). The measured persistent luminescence intensities were also normalized at the 10 s decay instant and were displayed as green dots in the figure. The measured intensities fit the It plot very well. (c) Persistent luminescence emission spectrum acquired at 10 s decay. Region I represents the UVC portion of the persistent luminescence, which accounts for 45.7% of the whole emission area and was used to calculated the UVC persistent luminescence power intensity. Source data for (b) and (c) are provided as a Source Data file.
Supplementary Figure 3. Thermoluminescence curves of Ca 2 Al 2 SiO 7 :Pr 3+ persistent phosphor discs. The discs were pre-irradiated by a 254 nm lamp for 2 min and underwent 1 min to 24 h decay in darkness before measurement. The discs were heated from 25 to 280 C with a heating rate of 4 C s 1 . The monitoring wavelength is 268 nm. The black dots-line curve in the bottom panel was acquired on a 12 h decayed disc after stimulation by a white LED flashlight for 20 s. Compared with the 12 h decayed disc but without stimulation (grey curve), the LED flashlight stimulation causes the redistribution of electrons in the energy traps; that is, some electrons are transferred from the deep traps to the shallow traps, resulting in enhanced persistent luminescence signal, i.e., PSPL signal, as shown in Figure 4 of the main text. Source data are provided as a Source Data file. (e,f) One disc was placed in bush. (g,h) Two discs were adhered on a brick wall. In these imaging experiments, the samples were irradiated by a 254 nm lamp for 2 min. The UVC images were taken by a corona camera at about 2 min after ceasing the lamp. The camera was located about 10 m for (a-d) and 5 m for (e-h) away from the samples. The UVC radiation is represented by red or yellow color. Figure 5. UVC radiation images of 50 mm diameter Ca 2 Al 2 SiO 7 :Pr 3+ discs immersed in water in direct sunlight. (a,b) A disc immersed in a swimming pool at about 10 cm depth. (c,d) A disc immersed in a pond at about 5 cm depth. Before immersing in water, the samples were irradiated by a battery-powered 4-W 254 nm lamp for 2 min. The charged discs were then immersed into water and the images were taken by a corona camera at about 2 min after ceasing the lamp. The distance between the sample and camera is about 10 m for in swimming pool and 3 m for in pond. The UVC radiation is represented by red color. Figure 6. Imaging of UVC radiation of 50 mm diameter Ca 2 Al 2 SiO 7 :Pr 3+ discs from long distance in direct sunlight. (a) A cell phone image showing the screen of a corona camera that displays the UVC radiation of two glowing discs held by a person. The distance between the discs and camera is about 20 m. Inset is an enlarged image of the camera screen. The discs were irradiated by a 254 nm lamp for 2 min and the UVC radiation was imaged at about 2 min after ceasing the lamp. (b) A glowing disc held by a person from about 50 m away in an open area. (c) A glowing disc held by a person standing on a wood trestle bridge in a lake from about 50 m away. In (b) and (c), the discs were irradiated by a battery-powered 4-W 254 nm lamp for 2 min and the UVC radiation was imaged at about 2 min after ceasing the lamp. Also in (b) and (c), the UVC radiation spot (in red) does not overlay onto the disc (in the visible image), because the focal planes of the UVC sensor and visible sensor are not precisely overlaid. In these imaging experiments, care was taken to avoid the operators from being exposed to the UVC radiation from the glowing samples. Figure 7. UVC radiation images of Ca 2 Al 2 SiO 7 :Pr 3+ discs and paint taken by a corona camera in room light and in darkness. (a) Visible image of letters "U", "V" and "C" made of 20 mm diameter discs on a white wood board. The disc letters were then irradiated by a 254 nm lamp for 2 min. (b) UVC image of the disc letters taken at 2 min decay in room light. (c) UVC image of the disc letters taken at 2 min decay in darkness. (d) Visible image of hollow letters "U", "V" and "C" written using UVC paint on a white wood board. The paint letters were then irradiated by a 254 nm lamp for 2 min. (e) UVC image of the three paint letters taken at 2 min decay in room light. (f) UVC image of the three paint letters taken at 2 min decay in darkness. For in room light, the UVC radiation is represented by purple color; for in darkness, the UVC radiation is represented by red color.