Tag: sensors

An innovative platform for NO₂ detection for cleaner air and safer cities

An innovative platform for NO₂ detection for cleaner air and safer cities

An international collaboration involving scientists from Italy, China, Czech Republic, Romania, Taiwan has highlighted how indium sulfide (InS), with its moderate band gap and layered structure, holds great promise for NO2 gas sensing.

Nitrogen dioxide, a harmful gas linked to respiratory and cardiovascular issues, is particularly challenging to detect due to the need for sensors that combine high sensitivity, precise selectivity, and stability under diverse conditions. Traditional materials, such as metal-oxide semiconductors, are widely used but often lack the required sensitivity and selectivity, especially for NO2 detection. In contrast, InS meets these requirements, also showing an evolution of its surface under oxidative conditions (e.g., in the air), implying chemical transformations that improve sensing performance. The sub-stoichiometric metal oxide formed upon oxidation results to be ideal for gas adsorption, with the ultimate obtainment of an ultrasensitive NO2 detection. Moreover, when exfoliated into nanosheets, 2D InS gets an intrinsically higher amount of active sites that enhances interaction with gases, making it particularly suitable for selective detection of NO2 in real-time air quality monitoring applications, as demonstrated by gas-sensing tests carried out with an operation temperature of 350°C.

To investigate chemical transformations in InS under oxidative conditions, Scanning Photoemission Microscopy (SPEM) at the ESCA Microscopy beamline of Elettra allowed real-time observation of the material as it actively interacted with NO2. Under these operando conditions, the surface of InS develops an oxygen-deficient In2O3-x layer, with nanometric thickness detected by transmission electron microscopy, through a sulfur abstraction process. This reaction, which removes sulfur atoms from the structure, creates highly active sites on the InS surface. The high spatial resolution of SPEM enabled direct observation of these nanoscale chemical changes on the surface of InS nanosheets, providing real-time visualizations of active sites as they formed.

Read more on Elettra website

Molecular movie of gold nanoparticle oscillations driven by displaced electrons

Molecular movie of gold nanoparticle oscillations driven by displaced electrons

Photocatalysis, sensors, solar cells: Plasmons promise a variety of applications if the processes triggered by optical excitation in the nanoparticles can be controlled. A research team from Hamburg and Berlin reports experimental observations of a so-called molecular movie that cannot be explained by established models in Nano Letters. The team including researchers from DESY provides a new theoretical model that explains the dynamics of excited gold nanoparticles observed in their experiments.

Plasmons are collective electron oscillations associated with highly localised fields. The decay of these oscillations after optical excitation is currently the subject of intense debate. Researchers assume that very energetic “hot” electrons are generated in the process which lose their energy by electron-electron scattering into a “warm” electron gas. The gas heats up the particle which eventually releases the excess energy into the environment. The efficiency of the energy transfer between the “hot electron”, “warm electron”, and “warm particle” stages is important for applications wanting to make use of these processes. In particular, the energy transfer from the warm electron gas to the nanoparticle appears to be so efficient that the particle is heated extremely quickly. In the process, it expands explosively, causing it to oscillate collectively, like a breathing sphere. However, so far direct experimental studies resolving the breathing oscillation have been missing.

For their study, researchers from the Departments of Physics and Chemistry at Universität Hamburg, the Max Planck Institute for the Structure and Dynamics of Matter (MPSD), the CFEL at DESY, and TU Berlin joined forces. Led by Holger Lange, Jochen Küpper, and Kartik Ayyer, who all conduct research in the Cluster of Excellence “CUI: Advanced Imaging of Matter”, and Andreas Knorr from Berlin, the team combined theory and experiment for an accurate description of the dynamics of excited gold nanoparticles.

Using single-particle X-ray diffractive imaging (SPI), performed at DESY’s FLASH facility, and transient absorption spectroscopy (TA), the researchers determined both the structural size and the electron temperature of the nanoparticles after optical excitation as a function of time. They observed that the particles already expanded with the optical excitation pulse, much faster than previously assumed. This observation directly proved the need for an immediate excitation source other than the temperature rise and associated expansion of the particle.

Read more on the DESY website

Image: Optical excitation of gold nanoparticles directly sets the particle into an oscillatory motion in which the particle periodically expands and contracts.

Credit: Univ. Hamburg/H. Lange