Year 2015, Volume 35

Abstract

Screen-printing technology provides a convenient method in fabricating thick-film conductive circuits and devices in the past few decades. Conventionally, piezoelectric thick-film is printed on alumina substrate with high mechanical Q-factor and resonates at higher frequency outside the range of ambient vibration. As the piezoelectric charge generation is proportional to the mechanical stress on the material, therefore the substrate can be removed in order to lower the natural frequency of the structure. In this paper, a series of piezoelectric ceramic cantilevers were fabricated in the form of free-standing cantilever. An additional layer of ceramic was also introduced to the upper and lower electrode to prevent delamination. The issue of zero resultant stress for the sandwiched piezoelectric cantilever was solved by repeating the electrode-PZTelectrode layer to form a multilayer structure. It was found from the experiment that an electrical output power of 32 μW was generated when it operates at its resonant frequency at 403 Hz.

Abstract

In contrast to the Moore’s Law exponential growth in CMOS transistor areal density, computer clock speeds have been frozen since 2003 due to excessive power dissipation. We present the development of a new digital switch, the PiezoElectronic Transistor (PET), designed to circumvent the speed and power limitations of the CMOS transistor. The PET operates on a novel principle: an electrical input is transduced into an acoustic pulse by a piezoelectric (PE) actuator, which, in turn, drives a continuous insulator-to-metal transition in a piezoresistive (PR) channel, thus switching on the device. Predictions of theory and simulation, assuming bulk materials properties can be approximately retained at scale, are that PETs can operate at one-tenth the present voltage of CMOS technology and 100 times less power, while running at multi-GHz clock speeds. CMOS-like computer architectures, such as a simulated adder, can be fully implemented. Materials development for PE and PR thin films approaching the properties of bulk single crystals, and a successful fabrication scheme, are the key to realizing this agenda. We describe progress in developing PE films (where d33 is critical) and PR films (characterized by conductance and ON/OFF ratio) of demonstration quality. A macroscopic-scale PET has been built to demonstrate PET viability over large numbers of switching cycles. The perspective for the development of pressure-driven electronics will be outlined.

Abstract

The charge density wave order in titanium diselenide is unusual in that it is characterized by a breakdown of inversion symmetry as well as translational symmetry, leading to the emergence of a chiral crystal structure. This novel state of matter was recently described theoretically in terms of the simultaneous and cooperative onset of both charge and orbital order. Here, we discuss the analysis of recent X-ray diffraction and related measurements on 1T-TiSe₂ in terms of the predictions of that theoretical model. It is show that the experimental observations agree with the expectations from an emerging combined charge and orbital ordered phase.

Abstract

Rare-earth (RE) ions-doped oxyfluoride transparent glass-ceramics containing nanocrystalline phases with very low phonon energies like LaF₃, NaLaF₄ and KLaF₄ combine the superior optical performance of low phonon energy phases with the high mechanical, chemical and thermal stability of oxide silicates. The insertion of the doping RE ions in the fluoride nanocrystals enhances the radiative optical emission processes currently focused on up- and down-conversion emissions. Thus, a key point is to identify and quantify the RE ions in the fluoride nano-crystals. In this contribution an oxyfluoride glass and the corresponding glass-ceramics of composition 55 SiO₂. 20 Al₂O₃.15 Na₂O.10 LaF₃ mol % doped with 1 mol % Tm₂O₃ is presented. The Tm³⁺ incorporation in the precipitated LaF₃ nano-crystals has been analysed by two different techniques. Firstly, TEM and Energy Dispersive X-Ray Spectroscopy (EDXS) were used to perform elemental distribution mappings of RE elements that allow to directly localising RE ions in the glass and glass-ceramics. Additionally, X-ray absorption spectroscopy (XAS) measurements were performed to analyse the distribution and first coordination sphere of Tm³⁺ ions in the glass and glass–ceramics by using Xray absorption near edge structure spectroscopy (XANES) and extended absorption X-ray fine structure spectroscopy (EXAFS), which have allow to quantify the Tm³⁺ incorporation in the LaF3 nano-crystals.

Abstract

We report on the fabrication and on the assessment of the properties of two glass-based photonic crystals (PhCs) obtained using alternative approaches such as rf-sputtering and sol-gel techniques. (i) By means of rf-sputtering a one-dimensional dielectric photonic crystal constituted by an Er³⁺-doped SiO₂ active layer inserted between two Bragg reflectors consisting of 10 pairs of SiO₂⁄TiO₂ layers has been realized. Near infrared transmittance spectra evidenced the presence of a stop band from 1500 to 2000 nm with a cavity resonance centered at 1749 nm. Intensity enhancement and narrowing of the emission band of Er³⁺ ions, due to the cavity effect have been observed and a cavity quality factor of 890 has been achieved. (ii) Through chemical route a 3D colloidal crystal based on polystyrene (PS) nanoparticles (NPs) embedded in elastomeric matrix has been realized. In the specific has been shown that the structure can produce a variation of its color applying different organic solvents that can be also easily observed by the naked eye. Optical measurements have evidenced a red shift of the diffraction peak as a function of the solvents applied. This feature has been exploited in order to create a sensitive material showing high sensitivity and reversibility of the signal change.

Abstract

A novel phosphor, copper doped hydronium alunite ((H₃O)Al₃(SO₄)₂(OH)₆:Cu), exhibiting a blue photoluminescence peak at a wavelength of 420 nm was successfully synthesized from aluminum and copper sulfates solution under hydrothermal condition (240°C, 60 min). The measurement of XRD revealed that the obtained products were single phase with a crystal structure of (H₃O)Al₃(SO₄)₂(OH)₆. Luminescence intensity of (H₃O)Al₃(SO₄)₂(OH)₆:Cu synthesized from sulfates solution was 6.2 times higher than that from an aluminum nitrate solution mixed with an elemental sulfur and a copper nitrate solution. The increase of luminescence intensity was resulted from an improvement of the crystallinity of (H₃O)Al₃(SO₄)₂(OH)₆.

Abstract

The Akermanite type alkaline earth silicate Ca₂MgSi₂O₇ activated by different types of rare earths was prepared by the conventional solid state reaction method under weak reductive atmosphere. The phase formation, particle size distribution, particle morphologies and photoluminescence properties of the samples have been investigated respectively. The comparative results of SEM and laser particle size analysis revealed that the relatively regular morphology, smaller particle size distribution could be achieved for the phosphors synthesized by the solid state reaction method including dry-ground after which powders were sieved below 170 meshes. The effects of rare earth oxides; Nd₂O₃, Pr₆O₁₁, Ce₂O₃ and Sm₂O₃ on the luminescence properties of the host material, Ca₂MgSi₂O₇, were studied. Remarkable enhancement and novel color emitting including white in luminescence characteristics of host material were observed as a result of doping the mentioned rare-earths were doped.

Abstract

Oxynitrides compounds are interesting for white LED industry because more covalent matrix's bonds in comparison to oxides enable to obtain higher color temperature of wLED. The Sialon's luminescence material doped by rare earth ions with high mechanical and chemical resistance and a broad emission band may become an alternative for garnet phosphors. The goal of the study was to investigate influence of divalent stabilizing cations on luminescence properties of M-α-Sialon doped with Eu²⁺ in order to estimate its potential for YAG:Ce replacement in WLEDs. The compound of general formula Eu_xM_1-xSi_9,6Al_2,4O_0,8N_15,2, where M=Ca, Ba, Sr, was prepared by the solid state reaction method from the mixture of the relevant oxides, nitrides and carbonates. Synthesis was carried out at the temperature of 1650°C for 4 h in a reduction atmosphere (N₂+CO). The structure and morphology of obtained powders were analyzed by XRD and SEM/EDS methods, respectively. Optical properties were investigated by excitation, emission and reflection spectra and compared to photoluminescence properties of standard YAG:Ce. The obtained specimens show the significant effect of synthesis parameters on the phase composition and intensity of emission of M-α-Sialon:Eu²⁺ powder.

Abstract

Ca-α-sialon doped by Eu2+ is a promising material for white LED phosphors due to the strong UV absorption and a yellow broad band emission of activator. The general formula of α sialon enables changing matrix composition by altering m (Al-N) and n (Al-O) parameters of cross- substitution. Changes of the neighboring Eu ligands in the host crystal lattice would trigger important modification of the photoluminescence properties because of different crystal field splitting and the energy of the d orbital. The aim of this study was to correlate the matrix solid solution parameters with photoluminescence spectra features. The specimens of Ca-α-sialon:Eu²⁺ with m, n parameters in the range of 0.5-2.5 and 0.25-1.25 respectively, were prepared by the solid state reaction method in the reducing atmosphere of a graphite furnace. The phase composition of the obtained specimens was characterized by XRD, microstructure and morphology by the SEM/EDS techniques. The photoluminescence spectra (excitation, emission) were also recorded. Results show that m/n parameters have a significant influence on the final optical properties.

Abstract

ZnGa₂O₄ (ZGO) is a normal spinel. When doped with Cr³⁺ ions, ZGO:Cr becomes a high brightness persistent luminescence material with an emission spectrum perfectly matching the transparency window of living tissues. It allows in vivo mouse imaging with a better signal to background ratio than classical quantum dots. The most interesting characteristic of ZGO:Cr lies in the fact that its LLP can be excited with red light, well below its band gap energy and in the transparency window of living tissues. A mechanism based on the trapping of carriers localized around a special type of Cr³⁺ ions namely Cr_N2 can explain this singularity. The antisite defects of the structure are the main responsible traps in the persistent luminescence mechanism. When located around Cr³⁺ ions, they allow, via Cr³⁺ absorption, the storage of not only UV light but also all visible light from the excitation source. Furthermore we observed long-lasting phosphorescence under 977 nm light excitation, making in vivo excitation of these spinels probes envisioned.

Abstract

Research and development in photonic micro/nano devices and structures have experienced a significant growth in recent years, fueled by their broad applications as sensors for in situ measurement of a wide variety of physical, chemical and biological quantities. Recent advancement in ultrafast and ultra-intense pulsed laser technology has opened a new window of opportunity for one-step fabrication of micro- and even nano-scale 3D structures in various solid materials. When used for fabrication, fs lasers have many unique advantages such as negligible cracks, minimal heat-affected-zone, low recast, and high precision. These advantages enable the unique opportunity to fabricate integrated sensors with unprecedented performance, enhanced functionalities and improved robustness. This paper summarizes our recent research progresses in the understanding, design, fabrication, characterization of various photonic sensors for energy, defense, environmental, biomedical and industry applications. Femtosecond laser processing/ablation of various glass materials (fused silica, doped silica, sapphire, etc.) is discussed towards the goal of one-step fabrication of novel photonic sensors and new enabling photonic devices. A number of new photonic devices and sensors are also presented.

Abstract

Glasses are very versatile materials, also because of the ease of doping them with various elements and compounds. In particular, rare-earth-doped glasses have greatly contributed to the development of optical amplifiers, lasers, active optical waveguides and white-light-emitting devices. White light emitting diodes (W-LEDs) and color LEDS obtained by the combination of an UV emitting LED, such as AlGaN-based LED, with a glass phosphor exhibit very interesting properties. In the present contribution we report the luminescence characteristics of zinc-sodiumaluminosilicates glasses variously doped, namely either singly doped with Eu³⁺, Tb³⁺ or Sm³⁺, or co-doped with Tb3+-Eu³⁺, Tb³⁺-Sm³⁺ and Tb³⁺-Ce³⁺. These glasses have also proved to be suitable for ion exchange and therefore for the production of active optical waveguides.