Stabilization of the VO2(M2) Phase and Change in Lattice Parameters at the Phase Transition Temperature of WXV1-XO2 Thin Films.

Various methods have been used to fabricate vanadium dioxide (VO2) thin films exhibiting polymorph phases and an identical chemical formula suited to different applications. Most fabrication techniques require post-annealing to convert the amorphous VO2 thin film into the VO2 (M1) phase. In this study, we provide a temperature-dependent XRD analysis that confirms the change in lattice parameters responsible for the metal-to-insulator transition as the structure undergoes a monoclinic to the tetragonal phase transition. In our study, we deposited VO2 and W-doped VO2 thin films onto silica substrates using a high repetition rate (10 kHz) fs-PLD deposition without post-annealing. The XRD patterns measured at room temperature revealed stabilization of the monoclinic M2 phase by W6+ doping VO2. We developed an alternative approach to determine the phase transition temperatures using temperature-dependent X-ray diffraction measurements to evaluate the a and b lattice parameters for the monoclinic and rutile phases. The a and b lattice parameters versus temperature revealed phase transition temperature reduction from ∼66 to 38 °C when the W6+ concentration increases. This study provides a novel unorthodox technique to characterize and evaluate the structural phase transitions seen on VO2 thin films.

Surface modified hexagonal upconversion nanoparticles for the development of competitive assay for biodetection.

Up-conversion nanoparticles (UCNPs) of sodium yttrium fluoride with ytterbium and erbium ions as sensitizer and activator (ß-NaYF4/Yb3+/Er3+) have been synthesised by a solvothermal method. The synthesised particles were found to be highly uniform in size (~50 nm) and of hexagonal crystal phase producing strong up-conversion luminescence dominated in the green wavelength region. During the synthesis, photoluminescence properties of the reaction mixture were monitored at regular intervals to ensure the required particle size distribution and luminescence efficiency. The hydrophobic particles thus obtained were modified by coating with silica, yielding particles that were stable in aqueous media. The silica coated UCNPs were further modified with maleimide-polyethylene glycol-silane (mal-PEG-silane) to provide thiol reactive surface groups. The silanized, maleimide-bearing UCNPs were effective for conjugating to reductively-cleaved half antibodies against ofloxacin, a veterinary antibiotic, to produce photoluminescent nanobiosensors for its detection and quantification. The speed and minimum detection concentration (~10 nM) that we report for a competitive assay of ofloxacin in this study is promising for developing sensors for this and other biomolecules.

Infrared optical properties modulation of VO2 thin film fabricated by ultrafast pulsed laser deposition for thermochromic smart window applications.

Over the years, vanadium dioxide, (VO2(M1)), has been extensively utilised to fabricate thermochromic thin films with the focus on using external stimuli, such as heat, to modulate the visible through near-infrared transmittance for energy efficiency of buildings and indoor comfort. It is thus valuable to extend the study of thermochromic materials into the mid-infrared (MIR) wavelengths for applications such as smart radiative devices. On top of this, there are numerous challenges with synthesising pure VO2 (M1) thin films, as most fabrication techniques require the post-annealing of a deposited thin film to convert amorphous VO2 into a crystalline phase. Here, we present a direct method to fabricate thicker VO2(M1) thin films onto hot silica substrates (at substrate temperatures of 400 °C and 700 °C) from vanadium pentoxide (V2O5) precursor material. A high repetition rate (10 kHz) femtosecond laser is used to deposit the V2O5 leading to the formation of VO2 (M1) without any post-annealing steps. Surface morphology, structural properties, and UV-visible optical properties, including optical band gap and complex refractive index, as a function of the substrate temperature, were studied and reported below. The transmission electron microscopic (TEM) and X-ray diffraction studies confirm that VO2 (M1) thin films deposited at 700 °C are dominated by a highly texturized polycrystalline monoclinic crystalline structure. The thermochromic characteristics in the mid-infrared (MIR) at a wavelength range of 2.5-5.0 µm are presented using temperature-dependent transmittance measurements. The first-order phase transition from metal-to-semiconductor and the hysteresis bandwidth of the transition were confirmed to be 64.4 °C and 12.6 °C respectively, for a sample fabricated at 700 °C. Thermo-optical emissivity properties indicate that these VO2 (M1) thin films fabricated with femtosecond laser deposition have strong potential for both radiative thermal management or control via active energy-saving windows for buildings, and satellites and spacecraft.

Femtosecond Laser Deposition of Germanium Selenide onto Silicon Platform at Different Substrate Temperatures.

Germanium selenide (GeSe) thin films were fabricated by employing femtosecond pulsed-laser deposition (fs-PLD) on silicon (100) substrates at various substrate temperatures, ranging from 25 °C to 600 °C. The thin films' surface morphology qualities and optical properties were studied by utilising transmission electron microscopy (TEM) and X-ray diffraction (XRD). The X-ray diffraction result signifies that the thin films deposited on the silicon at a substrate temperature below 400 °C were amorphous Ge-Se. In contrast, those grown at 400 °C and above exhibited crystallised peaks of Ge-Se orthorhombic and tetragonal structures. The deposition growth rate of the thin films was also found to decrease substantially with increasing substrate temperature. These results show that the fs-PLD process has great potential for fabricating good quality Ge-Se thin film. This technique could enable the manufacture of modern optoelectronic devices for applications in optical communication, sensing, and ovonic threshold switching for the high-density crossbar memory array.

Effect of Substrate Temperature on Morphological, Structural, and Optical Properties of Doped Layer on SiO2-on-Silicon and Si3N4-on-Silicon Substrate.

A high concentration of Er3+ without clustering issues is essential in an Er-doped waveguide amplifier as it is needed to produce a high gain and low noise signal. Ultrafast laser plasma doping is a technique that facilitates the blending of femtosecond laser-produced plasma from an Er-doped TeO2 glass with a substrate to form a high Er3+ concentration layer. The influence of substrate temperature on the morphological, structural, and optical properties was studied and reported in this paper. Analysis of the doped substrates using scanning electron microscopy (SEM) confirmed that temperatures up to approximately 400 °C are insufficient for the incoming plasma plume to modify the strong covalent bonds of silica (SiO2), and the doping process could not take place. The higher temperature used caused the materials from Er-doped tellurite glass to diffuse deeper (except Te with smaller concentration) into silica, which created a thicker film. SEM images showed that Er-doped tellurite glass was successfully diffused in the Si3N4. However, the doping was not as homogeneous as in silica.

Probing the Tribochemical Impact on Wear Rate Dynamics of Hydrogenated Amorphous Carbon via Raman-Based Profilometry.

Solid-liquid lubricating systems have received significant attention as a promising way for energy saving and emission control. For deeply understanding their tribological behaviors, it is necessary to study interaction mechanisms between solid and liquid lubricants from the tribochemical viewpoint, as tribofilms formed by tribochemical products on contact surfaces critically affect the whole tribological process. Continually or periodically monitoring tribofilm formation and evolution can contribute significantly to clarifying its dominating role in tribological behavior under boundary lubrication. However, detecting tribofilms in situ remains a big challenge for conventional surface analytical approaches, mainly due to their limitations in accessing tribofilms or low signal intensities of thin tribofilms. In this study, highly sensitive Raman-based profilometry with in situ potential has been developed for detecting molybdenum dialkyldithiocarbamate (MoDTC)-derived tribofilms and exploring their effect on a-C:H wear over time. The optical properties of tribochemical products formed on the coating surface in different wear stages could result in extra attenuation of Raman signal intensities in the form of measurement deviations in wear depth. By monitoring the deviations, key information of tribofilm compositions was obtained and a two-stage wear progression mechanism was proposed for the first time to clarify the detrimental effect of MoDTC-derived tribofilms on a-C:H wear by combining detailed structure and composition analyses.

Remote Non-Invasive Fabry-Pérot Cavity Spectroscopy for Label-Free Sensing.

One way of optically monitoring molecule concentrations is to utilise the high sensitivity of the transmission and reflection rates of Fabry-Pérot cavities to changes of their optical properties. Up to now, intrinsic and extrinsic Fabry-Pérot cavity sensors have been considered with analytes either being placed inside the resonator or coupled to evanescent fields on the outside. Here we demonstrate that Fabry-Pérot cavities can also be used to monitor molecule concentrations non-invasively and remotely, since the reflection of light from the target molecules back into the Fabry-Pérot cavity adds upwards peaks to the minima of its overall reflection rate. Detecting the amplitude of these peaks reveals information about molecule concentrations. By using an array of optical cavities, a wide range of frequencies can be probed at once and a unique optical fingerprint can be obtained.

Barium yttrium fluoride based upconversion nanoparticles as dual mode image contrast agents.

Dual labeled contrast agents could provide better complementary information for bioimaging than available solely from a single modality. In this paper we investigate the suitability of Yb3+ and Er3+-doped BaYF5 upconversion nanoparticles (UCNPs) as both optical and X-ray micro computed tomography (µCT) contrast agents. Stable, aqueous UCNP dispersions were synthesised using a hydrothermal method with the addition of polyethyleneimine (PEI). UCNPs were single crystal and had a truncated cuboidal and/or truncated octahedral morphology, with average particle size of 47 ±9 nm from transmission electron microscopy which was further used to characterize the structure and composition in detail. A zeta potential value of +51 mV was measured for the aqueous nanoparticle dispersions which is beneficial for cell permeability. The outer hydrated PEI layer is also advantageous for the attachment of proteins for targeted delivery in biological systems. The prepared UCNPs were proven to be non-toxic to endothelial cells up to a concentration of 3.5 mg/mL, when assessed using an MTT assay. The particles showed intense green upconversion photoluminescence when excited at a wavelength of 976 nm using a diode laser. Quantitative X-ray µCT contrast imaging confirmed the potential of these UCNPs as X-ray contrast agents and confirming their dual modality for bioimaging.

Affimer-Based Europium Chelates Allow Sensitive Optical Biosensing in a Range of Human Disease Biomarkers.

The protein biomarker measurement has been well-established using ELISA (enzyme-linked immunosorbent assay), which offers good sensitivity and specificity, but remains slow and expensive. Certain clinical conditions, where rapid measurement or immediate confirmation of a biomarker is paramount for treatment, necessitate more rapid analysis. Biosensors offer the prospect of reagent-less, processing-free measurements at the patient's bedside. Here, we report a platform for biosensing based on chelated Eu3+ against a range of proteins including biomarkers of cardiac injury (human myoglobin), stroke (glial fibrillary acidic protein (GFAP)), inflammation (C-reactive protein (CRP)) and colorectal cancer (carcinoembryonic antigen (CEA)). The Eu3+ ions are chelated by modified synthetic binding proteins (Affimers), which offer an alternative targeting strategy to existing antibodies. The fluorescence characteristics of the Eu3+ complex with modified Affimers against human myoglobin, GFAP, CRP and CEA were measured in human serum using λex = 395 nm, λem = 590 and 615 nm. The Eu3+-Affimer based complex allowed sensitive detection of human myoglobin, GFAP, CRP and CEA proteins as low as 100 fM in (100-fold) diluted human serum samples. The unique dependence on Eu3+ fluorescence in the visible region (590 and 615 nm) was exploited in this study to allow rapid measurement of the analyte concentration, with measurements in 2 to 3 min. These data demonstrate that the Affimer based Eu3+ complexes can function as nanobiosensors with potential analytical and diagnostic applications.

Erbium-Doped Nanoparticle-Polymer Composite Thin Films for Photonic Applications: Structural and Optical Properties.

Erbium-doped nanocrystal (NC)-dispersed polymer thin films are attractive core materials for use in optical waveguides as they can provide high optical gain and enable the formation of compact waveguide amplifiers. Nonetheless, there are significant challenges associated with obtaining good dispersibility of NCs into a polymer matrix and favorable optical properties. Therefore, in this paper, we report the fabrication of Er3+-doped ceria (EGC) NCs employing the Leeds alginate process (LAP) and their incorporation into a siloxane polymer matrix. The surface morphology and compositional, structural, and optical properties of the fabricated films are evaluated to assess the NC dispersion and their suitability for the waveguide amplifier. The photoluminescence (PL) and lifetime measurements of the NCs-polymer nanocomposite thin film samples show intense, broadband PL emission of the Er3+ ions at 1534 nm (4I13/2 → 4I15/3 transition) with a full width at half-maximum (fwhm) of ∼64 nm and lifetime in the range of 2.6-3.0 ms. The inhomogeneously broadened PL spectra and improvement in lifetime of NCs in the polymer are important results that we report. The EGC NCs-polymer nanocomposite thin films also exhibit excellent transparency in the NIR wavelength range and a refractive index in the range of 1.53-1.58 in the visible wavelength. The work presented here clearly demonstrates the potential of using high-quality Er-doped nanocomposite polymer thin films for interesting applications such as compact low-cost waveguide amplifiers and lasers.

Selective cellular imaging with lanthanide-based upconversion nanoparticles.

Upconversion nanoparticles (UCNPs) with sodium yttrium fluoride, NaYF4 (host lattice) doped with Yb3+ (sensitizer) and Er3+ (activator) were synthesized via hydrothermal route incorporating polyethyleneimine (PEI) for their long-term stability in water. The cationic PEI-modified UCNPs with diameter 20 ± 4 nm showed a zeta potential value of +36.5 mV and showed an intense, visible red luminescence and low-intensity green emission with 976 nm laser excitation. The particles proven to be nontoxic to endothelial cells, with a 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay, showing 90% to 100% cell viability, across a wide range of UCNP concentrations (0.3 ng/mL-0.3 mg/mL) were used in multiphoton imaging. Multiphoton cellular imaging and emission spectroscopy data reported here prove that the UCNPs dispersed in cell culture media are predominantly concentrated in the cytoplasm than the cell nucleus. The energy transfer from PEI-coated UCNPs to surrounding media for red luminescence in the biological system is also highlighted with spectroscopic measurements. Results of this study propose that UCNPs can, therefore, be used for cytoplasm selective imaging together with multiphoton dyes (eg, 4',6-diamidino-2-phenylindole (DAPI)) that are selective to cell nucleus.

A novel dynamic multicellular co-culture system for studying individual blood-brain barrier cell types in brain diseases and cytotoxicity testing.

Blood brain barrier (BBB) cells play key roles in the physiology and pathology of the central nervous system (CNS). BBB dysfunction is implicated in many neurodegenerative diseases, including Alzheimer's disease (AD). The BBB consists of capillary endothelial cells, pericytes encircling the endothelium and surrounding astrocytes extending their processes towards it. Although there have been many attempts to develop in vitro BBB models, the complex interaction between these cell types makes it extremely difficult to determine their individual contribution to neurotoxicity in vivo. Thus, we developed and optimised an in vitro multicellular co-culture model within the Kirkstall Quasi Vivo System. The main aim was to determine the optimal environment to culture human brain primary endothelial cells, pericytes and astrocytes whilst maintaining cellular communication without formation of a barrier in order to assess the contribution of each cell type to the overall response. As a proof of concept for the present system, the effects of amyloid-beta 25-35 peptide (Aß25-35), a hallmark of AD, were explored. This multicellular system will be a valuable tool for future studies on the specific roles of individual BBB cell type (while making connection with each other through medium) in CNS disorders as well as in cytotoxicity tests.

A photochemical approach for a fast and self-limited covalent modification of surface supported graphene with photoactive dyes.

Herein, we report a simple method for a covalent modification of surface supported graphene with photoactive dyes. Graphene was fabricated on cubic-SiC/Si(001) wafers due to their low cost and suitability for mass-production of continuous graphene fit for electronic applications on millimetre scale. Functionalisation of the graphene surface was carried out in solution via white light induced photochemical generation of phenazine radicals from phenazine diazonium salt. The resulting covalently bonded phenazine-graphene hybrid structure was characterised by scanning tunnelling microscopy (STM) and spectroscopy (STS), Raman spectroscopy and density functional theory (DFT) calculations. It was found that phenazine molecules form an overlayer, which exhibit a short range order with a rectangular unit cell on the graphene surface. DFT calculations based on STM results reveal that molecules are standing up in the overlayer with the maximum coverage of 0.25 molecules per graphene unit cell. Raman spectroscopy and STM results show that the growth is limited to one monolayer of standing molecules. STS reveals that the phenazine-graphene hybrid structure has a band gap of 0.8 eV.

The theoretical molecular weight of NaYF 4 :RE upconversion nanoparticles.

Upconversion nanoparticles (UCNPs) are utilized extensively for biomedical imaging, sensing, and therapeutic applications, yet the molecular weight of UCNPs has not previously been reported. Herein, we present a theory based upon the crystal structure of UCNPs to estimate the molecular weight of UCNPs: enabling insight into UCNP molecular weight for the first time. We estimate the theoretical molecular weight of various UCNPs reported in the literature, predicting that spherical NaYF4 UCNPs ~ 10 nm in diameter will be ~1 MDa (i.e. 106 g/mol), whereas UCNPs ~ 45 nm in diameter will be ~100 MDa (i.e. 108 g/mol). We also predict that hexagonal crystal phase UCNPs will be of greater molecular weight than cubic crystal phase UCNPs. Additionally we find that a Gaussian UCNP diameter distribution will correspond to a lognormal UCNP molecular weight distribution. Our approach could potentially be generalised to predict the molecular weight of other arbitrary crystalline nanoparticles: as such, we provide stand-alone graphic user interfaces to calculate the molecular weight both UCNPs and arbitrary crystalline nanoparticles. We expect knowledge of UCNP molecular weight to be of wide utility in biomedical applications where reporting UCNP quantity in absolute numbers or molarity will be beneficial for inter-study comparison and repeatability.

Development of Thick Superhydrophilic TiO2-ZrO2 Transparent Coatings Realized through the Inclusion of Poly(methyl methacrylate) and Pluronic-F127.

A thick coating of hierarchically porous double-templated TiO2-ZrO2-PMMA-PF127 with excellent self-cleaning properties and high transmittance has been developed for the first time on glass substrates using a simple dip-coating technique. Comparative studies of this sample with a thick and transparent coating of single-templated TiO2-ZrO2-PMMA have been performed to probe the origin of its exceptional properties. The formation of the composites, successful incorporation of the polymer into the matrix, and the porous nature of the films have been studied. The presence of Ti2+ in the double-templated samples has been confirmed, which suggest the chemisorption of water on the surface of the film. The variation in the self-cleaning properties of the samples on UV-illumination has also been studied. The double-templated film is found to possess the capability of good hydrophilic retention even 2 days after UV-irradiation.

Photoluminescence intensity ratio of Eu-conjugated lactates-A simple optical imaging technique for biomarker analysis for critical diseases.

Instant measurement of elevated biomarkers such as lactic acid offers the most promising approaches for early treatment and prevention of many critical diseases including cardiac arrest, stroke, septic shock, trauma, liver dysfunction, as well as for monitoring lactic acid level during intense exercise. In the present study, a unique dependence of visible photoluminescence of Eu3+ ions resulting from 5 D0 to 7 FJ(J = 0,1,2,3,4) transitions, which can be exploited for rapid detection of biomarkers, both in vitro and ex vivo, has been reported. It is observed that the integrated intensity ratio of photoluminescence signals dominating at 591 and 616 nm originating from 5 D0 to 7 F2 and 5 D0 to 7 F1 transitions in Eu3+ ions can be used as a biosensing and bioimaging tool for detection of biomarkers released at disease states. The Eu3+ integrated photoluminescence intensity ratio approach reported herein for optical detection of lactates in blood serum, plasma and confocal imaging of brain tissues has very high potential for exploitation of this technique in both in vitro monitoring and in vivo bioimaging applications for the detection of biomarkers in various diseases states.

White light induced covalent modification of graphene using a phenazine dye.

Herein, we report a novel strategy for a covalent modification of graphene nanoplatelets with photoactive dyes. The functionalization of the graphene surface was carried out using white light to photochemically generate phenazine radicals and the reaction progress was followed up spectrophotometrically. The characterization of the modified material was carried out using FTIR, XRD, UV-vis absorption, fluorescence, Raman spectroscopy and SEM imaging. The hybrid material has improved solubility, shows an optical band gap of 1.95 eV and is highly emissive in the visible wavelength region.

Ultrafast laser plasma doping of Er3+ ions in silica-on-silicon for optical waveguiding applications.

An ultrafast laser plasma doping (ULPD) technique is used for high concentration doping of erbium ions into silica-on-silicon substrate. The method uses a femtosecond laser to ablate material from TeO2-ZnO-Na2O-Er2O3 (Er-TZN) target glass. The laser-generated plasma modifies the silica network, producing a high-index-contrast optical layer suited to the production of on-chip integrated optical circuits. Cross-sectional analysis using scanning electron microscope with energy dispersive x-ray spectroscopy revealed homogeneous intermixing of the host silica with Er-TZN, which is unique to ULPD. The highly doped layer exhibits spectroscopic characteristics of erbium with photoluminescence lifetimes from 10.79 to 14.07 ms.

Target dependent femtosecond laser plasma implantation dynamics in enabling silica for high density erbium doping.

Chemical dissimilarity of tellurium oxide with silica glass increases phase separation and crystallization tendency when mixed and melted for making a glass. We report a novel technique for incorporating an Er(3+)-doped tellurite glass composition into silica substrates through a femtosecond (fs) laser generated plasma assisted process. The engineered material consequently exhibits the spectroscopic properties of Er(3+)-ions, which are unachievable in pure silica and implies this as an ideal material for integrated photonics platforms. Formation of a well-defined metastable and homogeneous glass structure with Er(3+)-ions in a silica network, modified with tellurite has been characterized using high-resolution cross-sectional transmission electron microscopy (HRTEM). The chemical and structural analyses using HRTEM, Rutherford backscattering spectrometry (RBS) and laser excitation techniques, confirm that such fs-laser plasma implanted glasses may be engineered for significantly higher concentration of Er(3+)-ions without clustering, validated by the record high lifetime-density product 0.96 × 10(19) s.cm(-3). Characterization of planar optical layers and photoluminescence emission spectra were undertaken to determine their thickness, refractive indices and photoluminescence properties, as a function of Er(3+) concentration via different target glasses. The increased Er(3+) content in the target glass enhance the refractive index and photoluminescence intensity of the modified silica layer whilst the lifetime and thickness decrease.

Femtosecond pulsed laser deposition of silicon thin films.

: Optimisation of femtosecond pulsed laser deposition parameters for the fabrication of silicon thin films is discussed. Substrate temperature, gas pressure and gas type are used to better understand the deposition process and optimise it for the fabrication of high-quality thin films designed for optical and optoelectronic applications.