Achieving large two-dimensional (2D) sheets of any metal is challenging due to their tendency to coalescence or cluster into 3D shapes. Recently, single-atom-thick gold sheets, termed goldene, was reported. Here, we ask if goldene can be extended to include multiple layers. The answer is yes, and trilayer goldene is the magic number, for reasons of electronegativity. Experiments are made to synthesize the atomically laminated phase Ti4Au3C3 through substitutional intercalation of Si layers in Ti4SiC3 for Au. Density functional theory calculations suggest that it is energetically favorable to insert three layers of Au into Ti4SiC3, compared to inserting a monolayer, a bilayer, or more than three layers. Isolated trilayer goldene sheets, ~100 nanometers wide and 6.7 angstroms thick, were obtained by chemically etching the Ti4C3 layers from Ti4Au3C3 templates. Furthermore, trilayer goldene is found in both hcp and fcc forms, where the hcp is ~50 milli–electron volts per atom more stable at room temperature from ab initio molecular dynamic simulations.

The synthesis of monolayer gold has so far been limited to free-standingseveral-atoms-thick layers, or monolayers confned on or inside templates.Here we report the exfoliation of single-atom-thick gold achieved throughwet-chemically etching away Ti3C2 from nanolaminated Ti3AuC2, initiallyformed by substituting Si in Ti3SiC2 with Au. Ti3SiC2 is a renown MAX phase,where M is a transition metal, A is a group A element, and X is C or N. Ourdeveloped synthetic route is by a facile, scalable and hydrofuoric acid-freemethod. The two-dimensional layers are termed goldene. Goldene layerswith roughly 9% lattice contraction compared to bulk gold are observedby electron microscopy. While ab initio molecular dynamics simulationsshow that two-dimensional goldene is inherently stable, experiments showsome curling and agglomeration, which can be mitigated by surfactants.X-ray photoelectron spectroscopy reveals an Au 4f binding energy increaseof 0.88 eV. Prospects for preparing goldene from other non-van der WaalsAu-intercalated phases, including developing etching schemes,are presented.

The introduction of silicon carbide as the semiconductor in gas-sensitive field effect devices has disruptively improved this sensor platform extending the operation temperature to more than 600 °C with an increased number of detectable gases. Here, we review recent progress in research and applications, starting with transducer and detection mechanisms, presenting new material combinations as sensing layers for improved selectivity and detection limits down to subparts per billion. We describe how temperature cycled operation combined with advanced data evaluation enables one sensor to act as a sensor array thereby vastly improving selectivity. Field tests require advanced packaging, which is described, and examples of possible applications like selective detection of ammonia for urea injection control in diesel exhausts and toxic volatile organic compounds for indoor air quality monitoring and control are given.

Static and dynamic responses of a silicon carbide field-effect transistor gas sensor have been investigated at two different gate biases in several test gases. Especially the dynamic effects are gas dependent and can be used for gas identification. The addition of ultraviolet light reduces internal electrical relaxation effects, but also introduces new, temperature-dependent effects.

Data from a silicon carbide based field-effect transistor were recorded over a period of nine days in a ventilated school room. For enhanced sensitivity and selectivity especially to formaldehyde, porous iridium on pulsed laser deposited tungsten trioxide was used as sensitive layer, in combination with temperature cycled operation and subsequent multivariate data processing techniques. The sensor signal was compared to reference measurements for formaldehyde concentration, CO2 concentration, temperature, and relative humidity. The results show a distinct pattern for the reference formaldehyde concentration, arising from the day/night cycle. Taking this into account, the projections of both principal component analysis and partial least squares regression lead to almost the same result concerning correlation to the reference. The sensor shows cross-sensitivity to an unidentified component of human activity, presumably breath, and, possibly, to other compounds appearing together with formaldehyde in indoor air. Nevertheless, the sensor is able to detect and partially quantify formaldehyde below 40 ppb with a correlation to the reference of 0.48 and negligible interference from ambient temperature or relative humidity.

Ruthenium(IV)oxide (RuO₂) is a material used for various purposes. It acts as a catalytic agent in several reactions, for example oxidation of carbon monoxide. Furthermore, it is used as gate material in gas sensors. In this work theoretical and computational studies were made on adsorbed molecules on RuO₂ (110) surface, in order to follow the chemistry on the molecular level. Density functional theory calculations of the reactions on the surface have been performed. The calculated reaction and activation energies have been used as input for thermodynamic and kinetics calculations. A surface phase diagram was calculated, presenting the equilibrium composition of the surface at different temperature and gas compositions. The kinetics results are in line with the experimental studies of gas sensors, where water has been produced on the surface, and hydrogen is found at the surface which is responsible for the sensor response.

Gas sensitive metal/metal-oxide field effect transistors based on silicon carbide were used to study the sensor response to benzene (C₆H₆) at the low parts per billion (ppb) concentration range. A combination of iridium and tungsten trioxide was used to develop the sensing layer. Highsensitivity to 10 ppb C₆H₆ was demonstrated during several repeated measurements at a constant temperature from 180 to 300 °C. The sensor performance was studied also as a function of the electrical operating point of the device, i.e., linear, onset of saturation, and saturation mode. Measurements performed in saturation mode gave a sensor response up to 52 % higher than those performed in linear mode.

High-temperature iridium-gated field effect transistors based on silicon carbide have been used for sensitive detection of specific volatile organic compounds (VOCs) in concentrations of health concern, for indoorair quality monitoring and control. Formaldehyde, naphthalene, and benzene were studied as hazardous VOCs at parts per billion (ppb) down to sub-ppb levels. The sensor performance and characteristics were investigated at a constant temperature of 330° C and at different levels of relative humidity up to 60 %, showing good stability and repeatability of the sensor response, and excellent detection limits in the sub-ppb range.