Improved nano-scale photodetectors
All objects emit and absorb IR radiation, 'visible' to people as heat. From military surveillance to chemical monitoring, biomedical imaging and machine vision, IR technology is used for a myriad of applications. Enhancing the sensitivity while decreasing the cost will ensure a competitive position for the EU in a large global market sector. The EU-funded project 'Colloidal quantum dot infrared photodetectors' (COQUADOT) set out to meet the challenge. Its goal was to combine the advanced properties of solution-processed colloidal quantum dots with low-cost, large-scale fabrication methods compatible with monolithic thin-film silicon technology. Quantum dots are tiny nanocrystals of semiconducting material. COQUADOT scientists explored routes to high-sensitivity optical sensing in the visible and IR regions of the EM spectrum exploiting quantum dots and plasmonics. Plasmonics is a relatively new field. It studies the enhancement in optical near-fields of sub-wavelength when the EM field interacts with conductive electrons at a metal interface or in metallic nanostructures. Integration of quantum dots with plasmonic structures including novel nano-focusing architectures such as bull's eye gratings offered significant performance enhancement. Novel hybrid photodetectors coupled colloidal quantum dots with graphene and other 2D semiconductors. These led to unprecedented performance (responsivities and sensitivities). The innovative photodetectors were further developed to operate in both the IR and visible regions. A second line of inquiry investigated a novel approach to overcome the band gap limitations of quantum dot semiconductors. Hot carrier plasmonic-based photodetectors take advantage of energetic electrons generated by the relaxation of plasmonic resonance. The latter is an excitation, a surface phenomenon at certain material interfaces in which photostimulation results in a resonant oscillation of conduction electrons. The metallic nanostructure was designed such that the spectral responsivity of the photodetector is determined by the geometry and not by the band gap of the semiconductor. The route is expected to lead to new ways of optical sensing and low-cost IR photodetectors. COQUADOT outcomes make an important contribution to a strong and growing field. Enhanced performance of IR photodetectors will not only enhance penetration of current markets but open the door to new high-tech ones as well.
Keywords
Photodetectors, infrared, colloidal quantum dot, optical sensing, plasmonics