Sci. Adv.: CMOS Spectral Imaging in UV Using Bioinspired Photodiode Stacking and Perovskite Downconversion

Highlights

Recently, a research team led by Viktor Gruev and Shuming Nie from University of Illinois Urbana-Champaign has published a study.

  • Bioinspired UV sensor combines perovskite nanocrystals and vertically stacked silicon photodiodes
  • Mimics tiered photoreceptor structure in Papilio xuthus butterfly eye
  • Enables wavelength-resolved UV imaging with high spatial and temporal resolution
  • Dual detection of UVB-induced fluorescence and direct UVA absorption
  • Differentiates cancer cells from normal cells via autofluorescence
  • Potential applications in medical imaging, military tracking, automation requiring enhanced UV spectral discrimination

Background

Butterflies like Papilio xuthus have specialized visual systems allowing perception of a wider range of colors and ultraviolet (UV) light compared to humans. Their eye units called ommatidia contain photoreceptors with distinct spectral sensitivities in a tiered structure. About one-third of the ommatidia have UV fluorescent pigments that absorb UV light and emit fluorescence detected by underlying photoreceptors, enabling the butterfly to discriminate small UV spectral variations. However, silicon image sensors are limited in UV sensitivity due to rapid attenuation of UV light in silicon. Here we present a bioinspired imaging sensor combining a thin layer of perovskite nanocrystals with vertically stacked silicon photodiodes to emulate the UV detection mechanisms of Papilio xuthus. The nanocrystals absorb UV light, emitting fluorescence detected by the photodiodes, while the top photodiode directly detects remaining UV light. This enables wavelength-resolved UV imaging for applications such as medical diagnostics.

Results

  • The sensor is inspired by the UV-sensitive visual system of the Papilio xuthus butterfly, which uses a tiered photoreceptor structure and fluorescent pigments to detect and discriminate UV wavelengths.
  • The sensor combines vertically stacked silicon photodiodes with a thin layer of cesium lead bromide (CsPbBr3) perovskite nanocrystals (PNCs) that act as a downconversion layer.
  • UV photons are detected via two mechanisms – direct absorption by the top photodiode, and absorption by the PNC layer which emits visible fluorescence detected by the underlying photodiodes. This allows spectral discrimination.
  • Optimization of the PNC layer thickness to 2μm provided maximum spectral discrimination across 300-400nm UV range.
  • The sensor showed 99% discrimination between UV signatures of aromatic amino acids and cancer/normal cells via autofluorescence imaging.
  • Key capabilities demonstrated are real-time, high resolution, wavelength-resolved UV imaging for biomedical applications like tumor margin detection.

Methods

  • The bioinspired imaging sensor combines a thin layer of CsPbBr3 perovskite nanocrystals (PNCs) with vertically stacked silicon photodiodes.
  • The PNCs absorb UVB light (>250nm) and emit green fluorescence detected by the photodiodes. The top photodiode directly detects remaining UVA light (>300nm).
  • The PNC layer thickness was optimized through simulations to maximize UV spectral discrimination. The optimal thickness was 2μm.
  • The sensor was fabricated using standard CMOS process. PNCs were synthesized and spin coated on the imaging sensor.

Conclusions

This paper presents a novel bioinspired CMOS imager for UV spectral imaging. The sensor uniquely combines perovskite nanocrystals with vertically stacked silicon photodiodes to enable real-time, high resolution, wavelength discrimination of UV signatures. This overcomes limitations in UV sensitivity of silicon detectors. Optimization of the nanocrystal layer allowed 99% discrimination between autofluorescence spectra of cancer and normal cells. This capability for spectral bio-imaging could have diverse applications in medical diagnostics, industrial sensing and automation. Overall, this work demonstrates a pathway to enhance UV detection in CMOS imagers through bioinspired photonic design, unlocking new possibilities for spectral imaging.

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Keywords: CMOS, photodiode

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