Waste Material Classification: A Short-Wave Infrared Discrete-Light-Source Approach Based on Light-Emitting Diodes.

Waste material classification is a challenging yet important task in waste management. The realization of low-cost waste classification systems and methods is critical to meet the ever-increasing demand for efficient waste management and recycling. In this paper, we demonstrate a simple, compact and low-cost classification system based on optical reflectance measurements in the short-wave infrared for the segregation of waste materials such as plastics, paper, glass, and aluminium. The system comprises a small set of LEDs and one single broadband photodetector. All devices are controlled through low-cost and low-power electronics, and data are gathered and managed via a computer interface. The proposed system reaches accuracy levels as high as 94.3% when considering seven distinct materials and 97.0% when excluding the most difficult to classify, thus representing a valuable proof-of-concept for future system developments.

Material classification based on a SWIR discrete spectroscopy approach.

A crucial yet difficult task for waste management is the identification of raw materials like plastic, glass, aluminum, and paper. Most previous studies use the diffused reflection spectroscopy for classification purposes. Despite the benefits in terms of speed and simplicity offered by modern compact spectrometers, their cost and the need for an external, wide-spectrum source of illumination create complications. To address this issue, the present paper proposes a discrete spectroscopy method that utilizes short-wave infrared (SWIR) reflectance to identify waste materials, exploiting a small set of selected wavelengths. This approach reduces the complexity of the classification data analysis and offers a more practical alternative to the conventional method. The proposed system comprises a single germanium photodetector and 10 different light emitting diodes (LEDs). The LED wavelengths are selected to maximize the system sensitivity towards a set of seven different waste materials. Using a classification strategy relying on support vector machines, the proposed methodology reaches a classification accuracy up to 98%.

Optical gas sensor based on the combination of a QD photoluminescent probe and a QD photodetector.

We report on a sensor architecture for detection of hazardous gases. The proposed device is based on the integration of a solid-state quantum dot (QD) photoluminescent probe with a QD photodetector on the same substrate. The effectiveness of the approach is demonstrated by developing a compact optical sensor for trace detection of explosives in air. The proposed architecture is very simple and consists of a silicon substrate with both surfaces coated with QD films. The upper layer acts as photoluminescent probe, pumped by a blue LED. The change of photoluminescence intensity associated to the interaction between the QDs and the target analyte is measured by the QD photodetector fabricated on the opposite side of the substrate. The sensor is mounted into a small chamber provided with the LED and the front-end electronics. The device is characterized by using nitrobenzene as representative nitroaromatic compound. Extremely low concentrations (down to 0.1 ppm) can be detected by the proposed device, with a theoretical detection limit estimated to be as low as 2 ppb. Results are repeatable and no ageing effect is observed over a 70 d period. The proposed architecture may provide a promising solution for explosive detection in air as well as other sensing applications, thanks to its sensitivity, simple fabrication process, practical usability and cost effectiveness.

Increasing responsivity and air stability of PbS colloidal quantum dot photoconductors with iodine surface ligands.

PbS colloidal quantum dots (QDs) are a promising material for the realization of low-cost, high-responsivity near-infrared photodetectors. Previously reported attempts showed high responsivity but a fast performance decay in air-exposed devices, demanding encapsulation of the photodetectors. Conversely, devices with very high air stability have been demonstrated but the low trap-state density hinders photoconductive gain and reduces overall responsivity. In this paper, photoconductive devices prepared with partially tetrabutylammonium iodide exchanged PbS QDs are presented with enhanced air stability and high responsivity at low voltage, low optical power.

Colloidal Quantum Dots for Explosive Detection: Trends and Perspectives.

Sensitive, accurate, and reliable detection of explosives has become one of the major needs for international security and environmental protection. Colloidal quantum dots, because of their unique chemical, optical, and electrical properties, as well as easy synthesis route and functionalization, have demonstrated high potential to meet the requirements for the development of suitable sensors, boosting the research in the field of explosive detection. Here, we critically review the most relevant research works, highlighting three different mechanisms for explosive detection based on colloidal quantum dots, namely photoluminescence, electrochemical, and chemoresistive sensing. We provide a comprehensive overview and an extensive discussion and comparison in terms of the most relevant sensor parameters. We highlight advantages, limitations, and challenges of quantum dot-based explosive sensors and outline future research directions for the advancement of knowledge in this surging research field.

Narrowband colloidal quantum dot photodetectors for wavelength measurement applications.

High performance photodetectors based on colloidal quantum dots have been demonstrated in a wide spectral range spanning from the visible to the mid infrared. Quantum dot photodetectors typically show a low-pass type spectral response with a tunable cutoff wavelength. In this paper, we propose a method for the realization of narrowband photodetectors based on the combination of photoconductors and optical filters, both realized with colloidal PbS quantum dots. We demonstrate that an array of such narrowband photodetectors can be effectively employed for the realization of a compact wavemeter operating in the visible and near-infrared spectral range.

Lead sulphide colloidal quantum dots for room temperature NO2 gas sensors.

Colloidal quantum dots (CQDs) have been recently investigated as promising building blocks for low-cost and high-performance gas sensors due to their large effective surface-to-volume ratio and their suitability for versatile functionalization through surface chemistry. In this work we report on lead sulphide CQDs based sensors for room temperature NO2 detection. The sensor response has been measured for different pollutant gases including NO2, CH4, CO and CO2 and for different concentrations in the 2.8-100 ppm range. For the first time, the influence of the QDs film thickness on the sensor response has been investigated and optimized. Upon 30 ppm NO2 release, the best room temperature gas response is about 14 Ω/Ω, with response and recovery time of 12 s and 26 min, respectively. A detection limit of about 0.15 ppb was estimated from the slope of the sensor response and its electric noise. The gas sensors exhibit high sensitivity to NO2, remarkable selectivity, repeatability and full recovery after exposure.

PbS Colloidal Quantum Dot Photodetectors operating in the near infrared.

Colloidal quantum dots have recently attracted lot of interest in the fabrication of optoelectronic devices due to their unique optical properties and their simple and low cost fabrication. PbS nanocrystals emerged as the most advanced colloidal material for near infrared photodetectors. In this work we report on the fabrication and characterization of PbS colloidal quantum dot photoconductors. In order to make devices suitable for the monolithic integration with silicon electronics, we propose a simple and low cost process for the fabrication of photodetectors and investigate their operation at very low voltage bias. Our photoconductors feature high responsivity and detectivity at 1.3 µm and 1 V bias with maximum values of 30 A/W and 2·1010 cmHz1/2W-1, respectively. Detectivity close to 1011 cmHz1/2W-1 has been obtained resorting to bridge sensor readout.