The key role of interband transitions in hot-electron-modulated TiN films

Titanium nitride (TiN) is an emerging new material in the field of plasmonics, both for its linear and nonlinear optical properties. Similarly to noble metals, like, e.g., gold (Au), the giant third-order optical nonlinearity of TiN following excitation with fs-laser pulses has been attributed to the generation of hot electrons. Here we provide a numerical study of the Fermi smearing mechanism associated with photogenerated hot carriers and subsequent interband transitions modulation in TiN films. A detailed comparison with Au films is also provided, and saturation effects of the permittivity modulation for increasing pump fluence are discussed

A transcriptomic snapshot of early molecular communication between Pasteuria penetrans and Meloidogyne incognita

© The Author(s). 2018Background: Southern root-knot nematode Meloidogyne incognita (Kofoid and White, 1919), Chitwood, 1949 is a key pest of agricultural crops. Pasteuria penetrans is a hyperparasitic bacterium capable of suppressing the nematode reproduction, and represents a typical coevolved pathogen-hyperparasite system. Attachment of Pasteuria endospores to the cuticle of second-stage nematode juveniles is the first and pivotal step in the bacterial infection. RNA-Seq was used to understand the early transcriptional response of the root-knot nematode at 8 h post Pasteuria endospore attachment. Results: A total of 52,485 transcripts were assembled from the high quality (HQ) reads, out of which 582 transcripts were found differentially expressed in the Pasteuria endospore encumbered J2 s, of which 229 were up-regulated and 353 were down-regulated. Pasteuria infection caused a suppression of the protein synthesis machinery of the nematode. Several of the differentially expressed transcripts were putatively involved in nematode innate immunity, signaling, stress responses, endospore attachment process and post-attachment behavioral modification of the juveniles. The expression profiles of fifteen selected transcripts were validated to be true by the qRT PCR. RNAi based silencing of transcripts coding for fructose bisphosphate aldolase and glucosyl transferase caused a reduction in endospore attachment as compared to the controls, whereas, silencing of aspartic protease and ubiquitin coding transcripts resulted in higher incidence of endospore attachment on the nematode cuticle. Conclusions: Here we provide evidence of an early transcriptional response by the nematode upon infection by Pasteuria prior to root invasion. We found that adhesion of Pasteuria endospores to the cuticle induced a down-regulated protein response in the nematode. In addition, we show that fructose bisphosphate aldolase, glucosyl transferase, aspartic protease and ubiquitin coding transcripts are involved in modulating the endospore attachment on the nematode cuticle. Our results add new and significant information to the existing knowledge on early molecular interaction between M. incognita and P. penetrans.Peer reviewedFinal Published versio

The key role of interband transitions in hot-electron–modulated TiN films

Titanium nitride (TiN) is an emerging new material in the field of plasmonics, both for its linear and nonlinear optical properties. Similarly to noble metals, like, e.g., gold (Au), the giant third-order optical nonlinearity of TiN following excitation with fs-laser pulses has been attributed to the generation of hot electrons. Here we provide a numerical study of the Fermi smearing mechanism associated with photogenerated hot carriers and subsequent interband transitions modulation in TiN films. A detailed comparison with Au films is also provided, and saturation effects of the permittivity modulation for increasing pump fluence are discussed

Tunable optical and plasmonic response of Au nanoparticles embedded in Ta-doped TiO2 transparent conducting films

Localised Surface Plasmon Resonances (LSPR) are fascinating optical phenomena occurring in metal nanostructures, like gold nanoparticles (Au NPs). Plasmonic excitation can be tailored efficiently in the visible range by acting on size, shape and NP surrounding, whereas carrier density is fixed, thus restricting the LSPR modulation. Transparent Conductive Oxides (TCOs), on the other hand, are gaining increasing interest for their transparency, charge carrier tunability and plasmonic features in the infrared. The combination of these two materials into a metal-TCO nanocomposite can give access to unique electrical and optical characteristics, to be tailored in view of the desired optoelectronic application. In this study Au NPs and Ta-doped TiO2 TCO films have been merged with the aim to master the Au plasmon resonance by acting on the dielectric properties of the surrounding TCO. Morphology, structure and electrical properties have been investigated as well, in order to understand the optical response of the nano-systems. The role of the embedding geometry has been explored, revealing that the largest LSPR shift (550-760 nm) occurs when the nanoparticles are sandwiched in the middle of the film, and not at the “bottom” of the film (substrate/film interface). Ta doping in the TCO has been varied (5-10% at. and bare TiO2) to induce a permittivity change of the matrix. As a result, Au LSPR is clearly blue-shifted when decreasing the dielectric permittivity at higher Ta content in the sandwich configuration. Despite the non-optimal electrical performance caused by defectivity of the films, Au-Ta:TiO2 multifunctional nanocomposites are promising candidates for their optical behavior as highly tunable plasmonic conductive metamaterials for advanced light management

Light management in TiO2 thin films integrated with Au plasmonic nanoparticles

The light-harvesting properties of metal oxide thin films can be remarkably increased by the introduction of èplasmonic nanostructures, leading to higher efficiencies in photovoltaic or photoelectrochemical devices. In the prototypical material combination, Au-TiO2, nano- and mesoscale porosity of TiO2 is desirable to improve not only the light-harvesting, but also the available surface area for chemical reactions. Moreover, great attention has been given to the control of size and shape of Au nanoparticles (NPs) to increase the overall optical properties of the film. In this work, we investigate the optical properties of the composite Au-TiO2 films exhibiting remarkable light scattering properties. TiO2 is characterized by a tree-like hierarchical morphology produced by pulsed laser deposition, and two different configurations for Au integration, namely Au on top and at the bottom of TiO2 are explored by varying the size of Au NPs. The hierarchical oxide morphology allow to achieve superior scattering properties after the combination with Au NPs with respect to films obtained from a commercial paste deposition. Both the Au-top and Au-bottom configurations enable to tune the plasmonic properties of Au NPs. Specifically, outstanding scattering properties are exhibited by the composite TiO2 film grown on top of large (~100 nm) Au NPs. These results show the potential interest of employing such integrated films as photoanodes in dye-sensitized or perovskite-based solar cells, or in photoelectrochemical cells for water splitting. An analogous approach can be employed for alternative materials, both considering the plasmonic structures as well as the semiconductor layer

Highly sensitive detection of estradiol by a SERS sensor based on TiO2 covered with gold nanoparticles

We propose the use of gold nanoparticles grown on the surface of nanoporous TiO2 films as Surface Enhanced Raman Scattering (SERS) sensors for the detection of 17-estradiol. Gold deposition on top of TiO2 surfaces leads to the formation of nanoparticles, which plasmonic properties fit the requirements of a SERS sensor well. The morphological and optical properties of this surface were investigated. Specifically, we demonstrated that the TiO2 background pressure during pulsed laser deposition and annealing conditions enabled the formation of a variety of Au nanoparticles with controlled size, shape and distribution thus resulting in a versatile sensor. We have exploited this surface for the detection of 17-estradiol, an emerging contaminant in environmental waters. We have found a limit of detection of 1 nM with a sensitivity allowing dynamic range of five orders of magnitude (up to 100 µM)

Optical and electronic properties of transparent conducting Ta:TiO2 thin and ultra-thin films: effect of doping and thickness

The development of low-dimensional transparent conducting systems is nowadays gaining interest in view of novel optoelectronic applications. In this paper, we investigate the evolution of optical and electronic properties of Ta-doped TiO2 films as thickness is decreased down to 5 nm and as a function of Ta doping (5-10% at.), and we correlate the observed behavior to the structural properties, showing a high degree of tunability. Ta:TiO2 polycrystalline anatase films are synthetized via pulsed laser deposition, followed by vacuum annealing. For films thick 50-200 nm, the electrical resistivity is ~8×10-4-1×10-3 Ωcm and charge carrier density increases with doping content while mobility decreases. Below a thickness of 20 nm the electrical properties partially deteriorate, but still conductive ultra-thin films can be obtained down to 5 nm. The optical response changes with Ta addition, i.e. the absorption band in the UV range blue-shifts, according to the Moss-Burstein effect, while absorption in the IR increases because of free carriers. Finally, we provide estimates of the effective mass and the plasma energy range in the IR. The fine tunability of the optoelectrical properties of Ta:TiO2 films makes them suitable for devices as transparent conductive components and for photonic or plasmonic applications in the visible and IR range

Controlling the plasmonic properties of titanium nitride thin films by radiofrequency substrate biasing in magnetron sputtering

Titanium nitride (TiN) is a promising plasmonic material alternative to gold and silver thanks to its refractory character, low resistivity (<100 μΩ cm) and compatibility with microelectronic industry processes. Extensive research is currently focusing on the development of magnetron sputtering as a large-scale technique to produce TiN thin films with low resistivity and optimized plasmonic performance. As such, more knowledge on the correlation between process parameters and the functional properties of TiN is needed. Here we report the effect of radiofrequency (RF) substrate biasing during the sputtering process on the structural, optical and electrical properties of TiN films. We employ spectroscopic ellipsometry as a sensible characterization method and we show that a moderate RF power, despite reducing the grain size, allows to achieve optimal plasmonic quality factors and a low resistivity (<100 μΩ cm). This is attributed to the introduction of a slight under-stoichiometry in the material (i.e., TiN0.85), as opposite to the films synthesized without bias or under intense bombardment conditions. RF substrate biasing during magnetron sputtering appears thus as a viable tool to prepare TiN thin films at room temperature with desired plasmonic properties