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In this review paper, we provide an overview of state-of-the-art Pd-based materials for optical H2 sensors. The first part of the manuscript introduces the operating principles, providing background information on the thermodynamics and the primary mechanisms of optical detection. Optical H2 sensors using thin films (i.e., films without any nanostructuring) are discussed first, followed by those employing nanostructured materials based on aggregated or isolated nanoparticles (ANPs and INPs, respectively), as well as complex nanostructured (CN) architectures. The different material types are discussed on the basis of the properties they can attribute to the resulting sensors, including their limit of detection, sensitivity, and response time. Limitations induced by cracking and the hysteresis effect, which reduce the repeatability and reliability of the sensors, as well as by CO poisoning that deteriorates their performance in the long run, are also discussed together with an overview of manufacturing approaches (e.g., tailoring the composition and/or applying functionalizing coatings) for addressing these issues.

High quality phase pure samarium sulphide (SmS) thin films with low surface roughness were prepared by electron beam evaporation using a samarium metal source in an H2S atmosphere. SmS shows a strong piezoresistive response due to a phase transition between a semiconducting and metallic state, which can be employed in piezo-electronic devices and stress sensing. Measurement of fundamental mechanical properties such as Young's modulus is thus a major objective to assess the elastic behaviour of SmS under external stress. Nanoindentation was used to measure the elastic modulus of thin semiconducting films with nominal thickness of 100 nm, 200 nm and 400 nm on silicon substrate at different loads. The indentation results were fitted to a modified King's model to exclude the effect of the substrate, giving Young's moduli of the films in the range of 79–82 GPa, consistent with measurements with a Laser Surface Acoustic Wave system (LAwave) and results from literature.

While SmS thin films show an irreversible semiconductor-metal transition upon application of pressure, the switching characteristics can be modified by alloying with other elements, such as europium. This manuscript reports on the resistance response of tri-layer SmS/EuS/SmS thin films upon applying pressure and on the correlation between the resistance response and the interdiffusion between the layers. SmS thin films were deposited by e-beam sublimation of Sm in an H2S atmosphere, while EuS was directly sublimated by e-beam from EuS. Structural properties of the separate thin films were first studied before the deposition of the final nanocomposite tri-layer system. Piezoresistance measurements demonstrated two sharp resistance drops. The first drop, at lower pressure, corresponds to the switching characteristic of SmS. The second drop, at higher pressure, is attributed to EuS, partially mixed with SmS. This behavior provides either a well-defined three or two states system, depending on the degree of mixing. Depth profiling using x-ray photoelectron spectroscopy (XPS) revealed partial diffusion between the compounds upon deposition at a substrate temperature of 400 &deg, C. Thinner tri-layer systems were also deposited to provide more interdiffusion. A higher EuS concentration led to a continuous transition as a function of pressure. This study shows that EuS-modified SmS thin films are possible systems for piezo-electronic devices, such as memory devices, RF (radio frequency) switches and piezoresistive sensors.

Samarium monosulfide (SmS) is a switchable material, showing a pressure-induced semiconductor to metal transition. As such, it can be used in different applications such as piezoresistive sensors and memory devices. In this work, we present how e-beam sublimation of samarium metal in a reactive atmosphere can be used for the deposition of semiconducting SmS thin films on 150 mm diameter silicon wafers. The deposition parameters influencing the composition and properties of the thin films are evaluated, such as the deposition rate of Sm metal, the substrate temperature and the H2S partial pressure. We then present the changes in the optical, structural and electrical properties of this compound after the pressure-induced switching to the metallic state. The back-switching and stability of SmS thin films are studied as a function of temperature and atmosphere via in-situ X-ray diffraction. The thermally induced back switching initiates at 250 &deg, C, while above 500 &deg, C, Sm2O2S is formed. Lastly, we explore the possibility to determine the valence state of the samarium ions by means of X-ray photoelectron spectroscopy.

Self-assembled nanostructures with the shape of nanospheres or nanorings were formed after annealing of ultrathin Ag films grown on glass, in a furnace with air at 460 °C. Intense localized surface plasmon resonances were recorded for these nanostructures with maxima at the green-blue light. The surface became functional in terms of enhancing the weak photoluminescence of glass between 2–400 times. This system provides an easy way of enhancing the photoluminescence emission of initially low performance materials.

In this review, we give an overview of the properties and applications of samarium monosulfide, SmS, which has gained considerable interest as a switchable material. It shows a pressure-induced phase transition from the semiconducting to the metallic state by polishing, and it switches back to the semiconducting state by heating. The material also shows a magnetic transition, from the paramagnetic state to an antiferromagnetically ordered state. The switching behavior between the semiconducting and metallic states could be exploited in several applications, such as high density optical storage and memory materials, thermovoltaic devices, infrared sensors and more. We discuss the electronic, optical and magnetic properties of SmS, its switching behavior, as well as the thin film deposition techniques which have been used, such as e-beam evaporation and sputtering. Moreover, applications and possible ideas for future work on this material are presented. Our scope is to present the properties of SmS, which were mainly measured in bulk crystals, while at the same time we describe the possible deposition methods that will push the study of SmS to nanoscale dimensions, opening an intriguing range of applications for low-dimensional, pressure-induced semiconductor–metal transition compounds.

In this work samarium sulfide thin films have been deposited by using e-beam evaporation under H2S atmosphere. Optical and structural properties were studied in several environments. The stability in several environments allows further processing into piezoelectronic devices.

Cobalt-based multilayers with excellent sequencing are grown via radiofrequency magnetron sputtering with the use of one Co target and natural oxidation. The Co layers are continuous, fully textured {111} and have the face centered cubic structure. At the end of deposition of each Co layer air is let to flow into the vacuum chamber via a fine (leak) valve. The top of Co is oxidized. The oxidized layers consist of cubic CoO crystallites. Near the film surface hexagonal Co(OH)2 is also detected. Magneto-optical Kerr effect hysteresis loops show in-plane magnetized films. The magnetic saturation field in the out-of-plane measurements is large exceeding 12 kOe. This observation supports indirectly the fact that Co is face centered cubic; if it was c-axis textured hexagonal the magnetocrystalline anisotropy would be large resulting in smaller values of the saturation field. As the Co-layer thickness decreases the in-plane loops show reduced remanence, slow approach to magnetic saturation and the out-of-plane loops show inverted hysteresis and/or crossing loop features with sizeable remanence. The effects are discussed with respect to the enhanced orbital magnetic moment of Co and the antiferromagnetic coupling between Co spins at the Co/CoO interface.

Au-doped Cu thin films are produced by co-deposition of Au and Cu via radiofrequency magnetron sputtering in a vacuum chamber with a base pressure of 1 × 10−7 mbar. After post annealing in a furnace with air, one may obtain either Au-Cu2O or Au-CuO nanocomposite thin films. The presence of Au does not have any considerable influence on the position of the optical band gap of the oxides. Only the Au-CuO system shows well-formed localized surface plasmonic resonances with Gaussian shape. We study systematically the plasmonic behavior of the nanocomposites as a function of the gold concentration, annealing time, and film thickness. The intensity of the resonances, their position, and width are intensely affected by all these parameters. The experimental results are compared with respect to rigorous theoretical calculations. The similarities and differences between experiment and theory are discussed.

Au-Ni-O thin films are produced by co-deposition of Au and Ni in a simple sputter coater with a base pressure of 2.5x10(-2)mbar. The as-deposited material is comprised of a mixture of tiny grains of crystalline gold, metallic hexagonal nickel and nickel oxide. Part of the material is still amorphous. Post-deposition annealing at 530 degrees C in a furnace in air leads to the formation of nanocomposite thin films of Au and NiO. The plasmonic behavior evolution was studied with annealing time between 2min and 30h in films with an initial thickness between 3 and 54nm. Films with thickness of up to 10nm show broad localized surface plasmon resonances already in the as-deposited state, due to the presence of nanograins of gold in the dielectric matrix of NiO. Plasmonic resonances become sharp and have a blue shift tendency as the annealing time increases. This plasmonic system produced via a cost-effective way may well be useful for a variety of technological applications.

AuxCu100−x, 4 ≤ x ≤ 12 at.%, solid solution nanocrystalline films with thickness between 1.5 and 100 nm were grown on Si(100) and Corning glass substrates at room temperature by radio frequency magnetron sputtering. After post annealing at 430 °C in a furnace with air, phase separation occurred between CuO and Au. The Au nanoparticles showed intense localized surface plasmon resonances with an amplitude increasing with Au concentration. We show the evolution of these resonances with the annealing time and correlate the structure to optical properties. The controllable tuning of position and intensity of plasmon resonances render this system a good candidate for applications.