The ozone layer protecting all living organisms from ultra-violet (UV) radiation has been destroyed because of excessive release of chlorofluorocarbons, so that reaching the UV radiation on the earth surface has been increased continuously. It has been reported that UV radiation would cause cutting DNA chain, skin cancers and immunity damage. In particular, the most common type of skin censers, such as basal cell carcinoma, squamous cell carcinoma and melanoma, are caused by sun exposure, especially overexposure to UV light [1].

Here, we developed a novel eco-friendly QD UV camera where visible-light emitting core/shell QDs were implemented on the C-MOS image sensor (CIS) pixels. QD would absorb the UV light and emit the visible light via energy-down-shift (EDS) mechanism [2], which could precisely tune the peak wavelength and full-width at half-maximum (FWHM) of the emitting visible light to maximize the photo-responsivity. Figure 1 shows the schematic structure of QD UV CIS pixel which consist of four-transistors and one-photodiode implemented with eco-friendly core/shell QDs which uniformly coated selectively on only Si photodiode region. As the UV light is absorbed the EDS-QD layer, QD would emit the blue-visible light which go through the silicon photodiode and increase the pixel intensity simultaneously. The UV photograph was shown by taking pictures, and calculating the difference pixel intensity between without QDs and with QDs.

Figure 2 shows that the core/shell QDs would absorb the light under 450-nm-wavelength and emit the blue-visible light at its peak wavelength of 434.7 nm, the quantum yield of 68.4%, and FWHM of 19.2 nm. The average diameter of core/shell QD were ~8.18 nm and it shows continuous lattice fringes and highly crystalline structure of core/shell QDs. We confirmed that the photo-responsivity of the Si photodiode implemented with the EDS-QDs was 0.78 A/W and the output voltage sensing margin (V_{out, dark-state} – V_{out, photo-state}) of CIS pixels increased 195% at 365-nm-wavelength compared to the CIS without implementing QDs.

QD UV cameras could be applied to many fields that use UV light. Figure 3 shows that the sunscreen-applied area of the human hand would reflects UV light so the pixel intensity are high. On the other hand, areas that are not applied sunscreen have low pixel intensity because human skin would absorbs UV light. We can check whether the sunscreen is well-applied on skin by simply taking a picture through the UV camera. Figure 4 shows another application of QD UV camera: skin care. The sebum on the face can be confirmed easily by taking a picture with the QD UV camera. It shows the size and distribution of sebum on the face very accurately. We will present various applications that could be realized through the eco-friendly QD UV camera in detail.

* This work was financially supported by the Brain Korea 21 Plus Program in 2018.

Reference

[1] T. Bald, et al., Nature 507, 109–113 (2014).

[2] S. W. Baek, et al., Phys. Chem. Chem. Phys. 16, 18205 (2014).