Surface engineering of nanomaterials is a promising tool towards the design of new materials for conversion of solar energy into chemical energy.

Optical limiters are nonlinear devices that encompass applications from device protection to activation functionalities in neural networks. In this work, we report an optical limiter on silicon photonics based on an ultracompact VO₂/Si waveguide. Our 20-μm-long experimental device features a thermal tunable threshold power of only ∼3.5 mW while being spectrally broadband. Our work provides a new pathway to achieve integrated optical limiters for dense and low-power photonic integrated circuits.

Control of quantum-dot (QD) surface chemistry offers a direct approach for the tuning of charge-carrier dynamics in photoconductors based on strongly coupled QD solids. We investigate the effects of altering the surface chemistry of PbS QDs in such QD solids via ligand exchange using 3-mercaptopropionic acid (MPA) and tetrabutylammonium iodide (TBAI). The roll-to-roll compatible doctor-blade technique was used for the fabrication of the QD solid films as the photoactive component in photoconductors and field-effect phototransistors. The ligand exchange of the QD solid film with MPA yields superior device performance with higher photosensitivity and detectivity, which is due to less dark current and lower noise level as compared to ligand exchange with TBAI. In both cases, the mechanism responsible for photoconductivity is related to trap sensitization of the QD solid, in which traps are responsible of high photoconductive gain values, but slow response times under very low incident optical power (100 pW), where traps are filled, both MPA- and TBAI-treated photodevices exhibit similar behavior, characterized by lower responsivity and faster response time, as limited by the mobility in the QD solid.

Solution-processed QD solids are emerging as a novel concept for high-performance optoelectronic devices. In this work, doctor blading is proposed for the fabrication of strongly-coupled QD solids from a PbS nanoink for photodetection at telecom wavelengths. The key step of this procedure is the solid-state ligand exchange, which reduces the interparticle distance and increases the carrier mobility in the resulting strongly-coupled QD solid. This is accomplished by replacing the original long oleylamine molecules by shorter molecules like 3-mercaptopropionic acid, as confirmed by FTIR, TGA and XPS. Further, a detailed investigation with XPS confirms the air-stability of the QD solids and the extreme reduction of the principal oxidation product, PbSO₃, from ligand exchange times of 60 s. Finally, the QD solid is tested as an active layer for the fabrication of a Schottky NIR photodetector. The device shows a maximum responsivity of 0.26 A W⁻¹ that corresponds to an internal quantum efficiency higher than 30% at 1500 nm and detectivity around 10¹¹ jones.

Nanocrystals surface chemistry engineering offers a direct approach to tune charge carrier dynamics in nanocrystals-based photodetectors. For this purpose, we have investigated the effects of altering the surface chemistry of thin films of CsPbBr₃ perovskite nanocrystals produced by the doctor blading technique, via solid state ligand-exchange using 3-mercaptopropionic acid (MPA). The electrical and electro-optical properties of photovoltaic and photoconductor devices were improved after the MPA ligand exchange, mainly because of a mobility increase up to 5 × 10⁻³ ${\mathrm{cm}}^2/\mathrm{Vs}$ . The same technology was developed to build a tandem photovoltaic device based on a bilayer of PbS quantum dots (QDs) and CsPbBr₃ perovskite nanocrystals. Here, the ligand exchange was successfully carried out in a single step after the deposition of these two layers. The photodetector device showed responsivities around 40 and 20 mA/W at visible and near infrared wavelengths, respectively. This strategy can be of interest for future visible-NIR cameras, optical sensors, or receivers in photonic devices for future Internet-of-Things technology.