Optimization of the plasmonic properties of titanium nitride films sputtered at room temperature through microstructure and thickness control.

A current approach to depositing highly plasmonic titanium nitride films using the magnetron sputtering technique assumes that the process is performed at temperatures high enough to ensure the atoms have sufficient diffusivities to form dense and highly crystalline films. In this work, we demonstrate that the plasmonic properties of TiN films can be efficiently tuned even without intentional substrate heating by influencing the details of the deposition process and entailed films' stoichiometry and microstructure. We also discuss the dependence of the deposition time/films' thickness on the optical properties, which is another degree of freedom in controlling the optical response of the refractory metal nitride films. The proposed strategy allows for robust and cost-effective production of large-scale substrates with good plasmonic properties in a CMOS technology-compatible process that can be further processed, e.g., structurized. All reported films are characterized by the maximal values of the plasmonic Figure of Merit (FoM = - ε1/ε2) ranging from 0.8 to 2.6, and the sample with the best plasmonic properties is characterized by FoM at 700 nm and 1550 nm that is equal 2.1 in both cases. These are outstanding results, considering the films' polycrystallinity and deposition at room temperature onto a non-matched substrate.

Ultrafast optical properties of stoichiometric and non-stoichiometric refractory metal nitrides TiNx, ZrNx, and HfNx.

Refractory metal nitrides have recently gained attention in various fields of modern photonics due to their cheap and robust production technology, silicon-technology compatibility, high thermal and mechanical resistance, and competitive optical characteristics in comparison to typical plasmonic materials like gold and silver. In this work, we demonstrate that by varying the stoichiometry of sputtered nitride films, both static and ultrafast optical responses of refractory metal nitrides can efficiently be controlled. We further prove that the spectral changes in ultrafast transient response are directly related to the position of the epsilon-near-zero region. At the same time, the analysis of the temporal dynamics allows us to identify three time components: the "fast" femtosecond one, the "moderate" picosecond one, and the "slow" at the nanosecond time scale. We also find out that the non-stoichiometry does not significantly decrease the recovery time of the reflectance value. Our results show the strong electron-phonon coupling and reveal the importance of both the electron and lattice temperature-induced changes in the permittivity near the ENZ region and the thermal origin of the long tail in the transient optical response of refractory nitrides.

Ab Initio Molecular Dynamics Simulations of Aqueous LiTFSI Solutions─Structure, Hydrogen Bonding, and IR Spectra.

Simulations of Density Functional Theory-based ab initio molecular dynamics (AIMD) have been performed for a series of aqueous lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) solutions with concentrations ranging from salt-in-water to water-in-salt systems. Analysis of the structure of electrolytes has revealed a preference of Li+ cations to interact with water molecules. In concentrated LiTFSI solutions, water molecules form small associates. The total number of hydrogen bonds (HBs) in the system decreases with salt concentrations, with bonds to water acceptors being only partially replaced by interactions with TFSI anions. Infrared (IR) spectra in the region of the O-H stretching frequency calculated from AIMD trajectories are in good agreement with experimental data. Statistics of oscillations of individual O-H bonds have shown correlations between vibrational frequencies and the structure of HBs formed by water. The changes in the IR spectrum have been related to the varying contributions of different local environments of the water molecules. The abundances of the three spectral components calculated from the simulations agree well with the decomposition of the experimental IR spectra reported in the literature.

All-BN distributed Bragg reflectors fabricated in a single MOCVD process.

Distributed Bragg Reflectors (DBR) are well-established photonic structures that are used in many photonic applications. However, most of the DBRs are based on different materials or require post-process etching which can hinder integration with other components in the final photonic structure. Here, we demonstrate the fabrication of DBR structures consisting only of undoped boron nitride (BN) layers with high refractive index contrast by using metal-organic chemical vapor deposition (MOCVD). This has been achieved in a single process, without the need for any post-process etching. The difference in the refractive index of the component BN layers stems from different degrees of porosity of the individual BN layers, which is a direct result of a different growth temperature. The fabricated DBR structures consist of 15.5 pairs of BN layers and exhibit a reflectance of 87 ± 1% at the maximum. The wavelength of maximum reflectance can be tuned from 500 nm up to the infrared region (IR), by simply adjusting the growth periods of subsequent BN layers. We also demonstrate that the fabricated structures can be used to create an optical microcavity. The fabricated DBRs are very promising candidates for future applications, for example in combination with single-photon emitters in h-BN, which could allow the building of a cavity-based all-BN single-photon source.

Effects of Me-Solvent Interactions on the Structure and Infrared Spectra of MeTFSI (Me = Li, Na) Solutions in Carbonate Solvents-A Test of the GFN2-xTB Approach in Molecular Dynamics Simulations.

We investigated the performance of the computationally effective GFN2-xTB approach in molecular dynamics (MD) simulations of liquid electrolytes for lithium/sodium batteries. The studied systems were LiTFSI and NaTFSI solutions in ethylene carbonate or fluoroethylene carbonate and the neat solvents. We focused on the structure of the electrolytes and on the manifestations of ion-solvent interactions in the vibrational spectra. The IR spectra were calculated from MD trajectories as Fourier transforms of the dipole moment. The results were compared to the data obtained from ab initio MD. The spectral shifts of the carbonyl stretching mode calculated from the GFN2-xTB simulations were in satisfactory agreement with the ab initio MD data and the experimental results for similar systems. The performance in the region of molecular ring vibrations was significantly worse. We also found some differences in structural data, suggesting that the GFN2-xTB overestimates interactions of Me ions with TFSI anions and Na+ binding to solvent molecules. We conclude that the GFN2-xTB method is an alternative worth considering for MD simulations of liquids, but it requires testing of its applicability for new systems.

Structural, Optical, and Electrical Properties of Hafnium-Aluminum-Zinc-Oxide Films Grown by Atomic Layer Deposition for TCO Applications.

ZnO is a widely studied material that exhibits versatile doping possibilities. Most research presents singly doped ZnO, leaving the potential of codoping unexplored. Within this study, hafnium-aluminum codoped zinc oxide (HAZO) thin films were grown on a glass substrate using the atomic layer deposition technique at 200 °C. A comprehensive analysis of the surface morphology and electrical and optical properties of the samples was conducted for varying the Al/Hf doping ratio. X-ray diffraction studies showed that the obtained films are polycrystalline, exhibiting a preferential growth direction along the (1 0 0) plane without any detectable precipitates. Moreover, the electrical measurements of HAZO films revealed that they exhibit lower resistivity (∼9.5 × 10-4 Ωcm) than the commonly used aluminum zinc oxide films (AZO). This improvement can be primarily attributed to the promotion of the n-type carrier concentration to 4.45 × 1020 cm-3 while maintaining a mobility value equal to 14.7 cm2/Vs. The doping also influences the optical properties of the material by widening the band gap and changing the refractive index, as observed by spectroscopy and ellipsometry studies. These findings highlight the potential of proposed HAZO thin films for future applications in electronic devices utilizing transparent conducting oxides.

Block Copolymer-Templated, Single-Step Synthesis of Transition Metal Oxide Nanostructures for Sensing Applications.

The synthesis of transition metal oxide nanostructures, thanks to their high surface-to-volume ratio and the resulting large fraction of surface atoms with high catalytic activity, is of prime importance for the development of new sensors and catalytic materials. Here, we report an economical, time-efficient, and easily scalable method of fabricating nanowires composed of vanadium, chromium, manganese, iron, and cobalt oxides by employing simultaneous block copolymer (BCP) self-assembly and selective sequestration of metal-organic acetylacetonate complexes within one of the BCP blocks. We discuss the mechanism and the primary factors that are responsible for the sequestration and conformal replication of the BCP template by the inorganic material that is obtained after the polymer template is removed. X-ray photoelectron spectroscopy (XPS) and powder X-ray diffraction (PXRD) studies indicate that the metal oxidation state in the nanowires produced by plasma ashing the BCP template closely matches that of the precursor complex and that their structure is amorphous, thus requiring high-temperature annealing in order to sinter them into a crystalline form. Finally, we demonstrate how the developed nanowire array fabrication scheme can be used to rapidly pattern a multilayered iron oxide nanomesh, which we then used to construct a prototype volatile organic compound sensor.

Hydrogen Bonding and Infrared Spectra of Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide/Water Mixtures: A View from Molecular Dynamics Simulations.

Simulations of ab initio molecular dynamics have been performed for mixtures of ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) ionic liquid and water. Statistics of donors and acceptors of hydrogen bonds has revealed that with increasing water content, hydrogen bonds between EMIM cations and TFSI anions are replaced by bonds to water molecules. In the mixture of liquids, the total number of bonds (from EMIM cations or water molecules) formed by TFSI acceptors increases. IR spectra obtained from ab initio molecular dynamics trajectories are in good agreement with literature data for ionic liquid/water systems. Analysis of oscillations of individual C-H and O-H bonds has shown correlations between vibrational frequencies and hydrogen bonds formed by an EMIM cation or water molecule and has indicated that the changes in the IR spectrum result from the decreased number of water-water hydrogen bonds in the mixture. The tests of DFTB methodology with tailored parameterizations have yielded reasonably good description of the IR spectrum of bulk water, whereas available parameterizations have failed in satisfactory reproduction of the IR spectrum of EMIM-TFSI/water mixtures in the region above 3000 cm-1.

Explicit and Hybrid Solvent Models for Estimates of Parameters Relevant to the Reduction Potential of Ethylene Carbonate.

Using ethylene carbonate as a sample solvent, we investigated two molecular parameters used to estimate the reduction potential of the solvent: electron affinity, and the energy of the lowest unoccupied molecular orbital (LUMO). The results showed that the values of these parameters are inconsistent for a single ethylene carbonate molecule in vacuum calculations and in the continuous effective solvent. We performed a series of calculations employing explicit or hybrid (explicit/continuous) solvent models for aggregates of solvent molecules or solvated salt ions. In the hybrid solvent model, values of the two estimates extrapolated to an infinite system size converged to one common value, whereas the difference of 1 eV was calculated in the purely explicit solvent. The values of the gap between the highest occupied molecular orbital (HOMO) and the LUMO obtained in the hybrid model were significantly larger than those resulting from the explicit solvent calculations. We related these differences to the differences in frontier orbitals and changes of electron density obtained in the two solvent models. In the hybrid solvent model, the location of the additional electron in the reduced system usually corresponds to the LUMO orbital of the oxidized system. The presence of salt ions in the solvent affects the extrapolated values of the electron affinity and LUMO energy.

Single pixel imaging at high pixel resolutions.

The usually reported pixel resolution of single pixel imaging (SPI) varies between 32 × 32 and 256 × 256 pixels falling far below imaging standards with classical methods. Low resolution results from the trade-off between the acceptable compression ratio, the limited DMD modulation frequency, and reasonable reconstruction time, and has not improved significantly during the decade of intensive research on SPI. In this paper we show that image measurement at the full resolution of the DMD, which lasts only a fraction of a second, is possible for sparse images or in a situation when the field of view is limited but is a priori unknown. We propose the sampling and reconstruction strategies that enable us to reconstruct sparse images at the resolution of 1024 × 768 within the time of 0.3s. Non-sparse images are reconstructed with less details. The compression ratio is on the order of 0.4% which corresponds to an acquisition frequency of 7Hz. Sampling is differential, binary, and non-adaptive, and includes information on multiple partitioning of the image which later allows us to determine the actual field of view. Reconstruction is based on the differential Fourier domain regularized inversion (D-FDRI). The proposed SPI framework is an alternative to both adaptive SPI, which is challenging to implement in real time, and to classical compressive sensing image recovery methods, which are very slow at high resolutions.

Optical Constants of Several Multilayer Transition Metal Dichalcogenides Measured by Spectroscopic Ellipsometry in the 300-1700 nm Range: High Index, Anisotropy, and Hyperbolicity.

Transition metal dichalcogenides (TMDs) attract significant attention due to their remarkable optical and excitonic properties. It was understood already in the 1960s and recently rediscovered that many TMDs possess a high refractive index and optical anisotropy, which make them attractive for nanophotonic applications. However, accurate analysis and predictions of nanooptical phenomena require knowledge of dielectric constants along both in- and out-of-plane directions and over a broad spectral range, information that is often inaccessible or incomplete. Here, we present an experimental study of optical constants from several exfoliated TMD multilayers obtained using spectroscopic ellipsometry in the broad range of 300-1700 nm. The specific materials studied include semiconducting WS2, WSe2, MoS2, MoSe2, and MoTe2, as well as in-plane anisotropic ReS2 and WTe2 and metallic TaS2, TaSe2, and NbSe2. The extracted parameters demonstrate a high index (n up to ∼4.84 for MoTe2), significant anisotropy (n ∥ - n ⊥ ≈ 1.54 for MoTe2), and low absorption in the near-infrared region. Moreover, metallic TMDs show potential for combined plasmonic-dielectric behavior and hyperbolicity, as their plasma frequency occurs at around ∼1000-1300 nm depending on the material. The knowledge of optical constants of these materials opens new experimental and computational possibilities for further development of all-TMD nanophotonics.

New Concept for the Facile Fabrication of Core-Shell CuO@CuFe2O4 Photocathodes for PEC Application.

The CuO@CuFe2O4 core-shell structure represents a new family of photocatalysts that can be used as photoelectrodes that are able to produce hydrogen under a broad spectrum of visible light. Herein, we report a novel approach for the production of this active film by the thermal conversion of CuFe Prussian Blue Analogues. The outstanding photoelectrochemical properties of the photocathodes of CuO@CuFe2O4 were studied with the use of combinatory photo-electrochemical instrumental techniques which proved that the electrodes were stable over the whole water photolysis run under relatively positive potentials. Their outstanding performance was explained by the coupling of two charge transfer mechanisms occurring in core-shell architectures.

Photoelectrochemical Behavior of WO3 in an Aqueous Methanesulfonic Acid Electrolyte.

N-type semiconducting WO3 is widely investigated as a photoanode operating in water and seawater splitting devices. Because of the propensity of WO3 to favor photo-oxidation of acidic electrolyte anions and, in parallel, the formation on the electrode surface of the peroxo species, the choice of the appropriate electrolyte to allow stable operation of the photoanode is of critical importance. Our results from structural and photoelectrochemical tests performed using mesoporous WO3 photoanodes exposed to 80 h long photoelectrolysis in a 1 M aq. methanesulfonic acid supporting electrolyte demonstrate the photostability of both the WO3 photomaterial and the CH3SO3H electrolyte. The reasons for the stability of aqueous solutions of CH3SO3H are discussed on the basis of earlier literature reports.

Titanium Nitride as a Plasmonic Material from Near-Ultraviolet to Very-Long-Wavelength Infrared Range.

Titanium nitride is a well-known conductive ceramic material that has recently experienced resumed attention because of its plasmonic properties comparable to metallic gold and silver. Thus, TiN is an attractive alternative for modern and future photonic applications that require compatibility with the Complementary Metal-Oxide-Semiconductor (CMOS) technology or improved resistance to temperatures or radiation. This work demonstrates that polycrystalline TiNx films sputtered on silicon at room temperature can exhibit plasmonic properties continuously from 400 nm up to 30 µm. The films' composition, expressed as nitrogen to titanium ratio x and determined in the Secondary Ion Mass Spectroscopy (SIMS) experiment to be in the range of 0.84 to 1.21, is essential for optimizing the plasmonic properties. In the visible range, the dielectric function renders the interband optical transitions. For wavelengths longer than 800 nm, the optical properties of TiNx are well described by the Drude model modified by an additional Lorentz term, which has to be included for part of the samples. The ab initio calculations support the experimental results both in the visible and infra-red ranges; particularly, the existence of a very low energy optical transition is predicted. Some other minor features in the dielectric function observed for the longest wavelengths are suspected to be of phonon origin.

How Temperature, Pressure, and Salt Concentration Affect Correlations in LiTFSI/EMIM-TFSI Electrolytes: A Molecular Dynamics Study.

Classical polarizable molecular dynamics simulations have been performed for LiTFSI solutions in the EMIM-TFSI ionic liquid. Different temperature or pressure values and salt concentrations have been examined. The structure and dynamics of the solvation shell of Li+ cations, diffusion coefficients of ions, conductivities of the electrolytes, and correlations between motions of ions have been analyzed. The results indicated that regardless of the conditions, significant correlations are present in all systems. The degree of correlations depends mainly on the salt fraction in the electrolyte and is much less affected by temperature and pressure changes. A positive correlation between motions of Li+ cations and TFSI anions, leading to the occurrence of negative Li+ transference numbers, exists for all conditions, although temperature and pressure changes affect the speed of anion exchange in Li+ solvation shells.

Differential real-time single-pixel imaging with Fourier domain regularization: applications to VIS-IR imaging and polarization imaging.

The speed and quality of single-pixel imaging (SPI) are fundamentally limited by image modulation frequency and by the levels of optical noise and compression noise. In an approach to come close to these limits, we introduce a SPI technique, which is inherently differential, and comprises a novel way of measuring the zeroth spatial frequency of images and makes use of varied thresholding of sampling patterns. With the proposed sampling, the entropy of the detection signal is increased in comparison to standard SPI protocols. Image reconstruction is obtained with a single matrix-vector product so the cost of the reconstruction method scales proportionally with the number of measured samples. A differential operator is included in the reconstruction and following the method is based on finding the generalized inversion of the modified measurement matrix with regularization in the Fourier domain. We demonstrate 256 × 256 SPI at up to 17 Hz at visible and near-infrared wavelength ranges using 2 polarization or spectral channels. A low bit-resolution data acquisition device with alternating-current-coupling can be used in the measurement indicating that the proposed method combines improved noise robustness with a differential removal of the direct current component of the signal.

NaFSI and NaTFSI Solutions in Ether Solvents from Monoglyme to Poly(ethylene oxide)-A Molecular Dynamics Study.

Classical molecular dynamics simulations have been performed for a series of electrolytes based on sodium bis(fluorosulfonyl)imide or sodium bis(trifluoromethylsulfonyl)imide salts and monoglyme, tetraglyme, and poly(ethylene oxide) as solvents. Structural properties have been assessed through the analysis of coordination numbers and binding patterns. Residence times for Na-O interactions have been used to investigate the stability of solvation shells. Diffusion coefficients of ions and electrical conductivity of the electrolytes have been estimated from molecular dynamics trajectories. Contributions to the total conductivity have been analyzed in order to investigate the role of ion-ion correlations. It has been found that the anion-cation interactions are more probable in the systems with NaTFSI salts. Accordingly, the degree of correlations between ion motions is larger in NaTFSI-based electrolytes.

Development of a Novel Nanoarchitecture of the Robust Photosystem I from a Volcanic Microalga Cyanidioschyzon merolae on Single Layer Graphene for Improved Photocurrent Generation.

Here, we report the development of a novel photoactive biomolecular nanoarchitecture based on the genetically engineered extremophilic photosystem I (PSI) biophotocatalyst interfaced with a single layer graphene via pyrene-nitrilotriacetic acid self-assembled monolayer (SAM). For the oriented and stable immobilization of the PSI biophotocatalyst, an His6-tag was genetically engineered at the N-terminus of the stromal PsaD subunit of PSI, allowing for the preferential binding of this photoactive complex with its reducing side towards the graphene monolayer. This approach yielded a novel robust and ordered nanoarchitecture designed to generate an efficient direct electron transfer pathway between graphene, the metal redox center in the organic SAM and the photo-oxidized PSI biocatalyst. The nanosystem yielded an overall current output of 16.5 µA·cm-2 for the nickel- and 17.3 µA·cm-2 for the cobalt-based nanoassemblies, and was stable for at least 1 h of continuous standard illumination. The novel green nanosystem described in this work carries the high potential for future applications due to its robustness, highly ordered and simple architecture characterized by the high biophotocatalyst loading as well as simplicity of manufacturing.

MeTFSI (Me = Li, Na) Solvation in Ethylene Carbonate and Fluorinated Ethylene Carbonate: A Molecular Dynamics Study.

Classical and ab initio molecular dynamics (MD) simulations have been performed for electrolytes based on LiTFSI and NaTFSI solutions in ethylene carbonate and its mono- and difluoro derivatives. Differences between electrolytes with Li+ or Na+ ions and the effect of fluorination on the structure and transport properties have been analyzed. The observed differences are related to the strength of Me+-carbonate binding, which is weaker for the Na+ cation and/or fluorinated solvents. Infrared spectra have been computed from ab initio MD and density functional tight binding (DFTB) MD trajectories. The changes of vibrational frequencies have been related to the local structure of the electrolyte and to interactions between salt cations and solvent molecules. The frequency shifts obtained from the AIMD simulations agree with experimental data, whereas DFTB underestimates Na+-carbonate interactions.

Quantum-Chemical and Molecular Dynamics Investigations of Magnesium Chloride Complexes in Dimethoxyethane Solutions.

Quantum-chemical calculations and classical and ab initio molecular dynamics simulations have been performed to study the Mg2+-conducting electrolytes based on Mg(TFSI)2/MgCl2 solutions in dimethoxyethane. It has been shown that depending on the TFSI/Cl- ratio, the Mg2Cl2 2+ or Mg3Cl4 2+ complexes are preferred as stable ion aggregates. In the initial stages of the ion association process, MgCl+, MgCl2, and Mg2Cl3 + are formed as intermediate species. Calculations of harmonic frequencies and simulations of the IR spectrum of the electrolyte from the ab initio MD trajectories have been used to identify the spectral range of vibrations of ion aggregates found in the modeled electrolyte. The results have been discussed in the context of experimental data.