Dielectric properties and crystal structure of Ba6−3x(Nd, M)8+2xTi18O54 (M = La, Bi, Y) microwave ceramics

Crystal structure and dielectric property of tungsten-bronze type microwave dielectric ceramics, i.e., BaOLa₂O₃4TiO₂ and Ba_6-3x (Nd, M)d_8+2x Ti₁₈O₅₄ (M = Y, Bi and x = 0.5, 0.7), are analysed. The optimum properties obtained in Ba(Nd_1-xBi_x)₂O₃4TiO₂ were _r = 89–92, Qf = 5855–6091 GHz, and _f = –7–+7 ppm/°C x = 0.04–0.08. The Y-substitution in BaO(Nd_1-xY_x)₂O₃4TiO₂ reduces the dielectric constant _r. Both the Y and Bi substitutions make _r positive. The relative dielectric constant _r and temperature coefficient _r are 109.5 and –180 ppm/°C in BaOLa₂O₃4TiO₂, 76 and +40 ppm/°C in BaO(Nd_0.77Y_0.23)₂O₃4TiO₂, respectively. The crystal structures were refined by Rietveld method using x-ray and neutron diffraction data. The most reliable results were obtained by refining the cation positions using the x-ray data and the oxygens from the neutron with a superlattice structure model Pnam(c-axis 7.6 Å). The refined structures show that the a/c ratios are related to the apical oxygen displacements of the Ti–O octahedra. The substitution of the small radius atom, Y, produced a structure of severely tilted and distorted Ti–O octahedra and large a/c ratio, while the large radius atom, La, small a/c ratio. Differential scanning calorimetry analysis showed heat anomaly indicating suspected phase transition in these materials. The relation between _r and octahedron tilting in tungsten-bronze type material is discussed in relation with complex perovskite structure.     

Carbon nanotube patterning with capillary micromolding of catalyst

Patterning of multi-walled carbon nanotube (MWNT) in a plasma enhanced chemical vapor deposition (PECVD) chamber has been achieved by catalyst patterning using capillary micromolding process. Iron acetate catalyst nanoparticles were dissolved in ethanol and mold was fabricated with polydimethylsiloxane (PDMS). The ethanol solution containing catalyst nanoparticles was filled into the microchannel formed between PDMS mold and Si-wafer by capillary force. The capillary action of different solvents was simulated by commercial CFD-ACE+ simulation code to determine optimal solvents. Simulated result shows that the choice of solvent was critical in this capillary filling process. After the catalyst patterning, MWNT was grown at 700 approximately 800 degrees C by PECVD process using CH4 and Ar gas in a scale of approximately 10 micro-meters in a tubular inductively coupled plasma reactor. Grown CNTs were analyzed by FE-SEM and Raman Spectroscopy.     

High frequency bandwidth measurements of micro cutting forces

The miniaturization of machine components is perceived by many as a core requirement for the future technological development of a broad spectrum of products. One of the challenges in micro engineering is the development of economical micro systems that are flexible, functional and made of appropriate engineering materials. The mechanical removal of materials using miniature tools, known as a micro machining process, has unique advantages in creating 3D components using a variety of engineering materials, when compared with photolithographic processes. Since the diameter of miniature tools is very small, excessive forces and vibrations will significantly affect the overall part and tool quality. In order to improve the part and tool quality, accurate measurement of micro cutting forces is imperative. In this paper, we focus on the development of an ultra precision micro milling system and the measurement of micro cutting forces using a three-axis miniature force sensor and accelerometers. Since the inherent dynamics of the workpiece and overall machine tool affects the frequency bandwidth, we employ the Kalman filter approach to fuse the sensor signals and compensate for unwanted dynamics, in order to increase the bandwidth of the force measurement system. Based on accurate cutting force measurement, we can come up with the optimal process parameters to maintain desired tolerances and also monitor the process to prevent failures.     

Stability of Quantum Dots,Quantum Dot Films,and Quantum Dot Light‐Emitting Diodes for Display Applications

Quantum dots (QDs) are being highlighted in display applications for their excellent optical properties, including tunable bandgaps, narrow emission bandwidth, and high efficiency. However, issues with their stability must be overcome to achieve the next level of development. QDs are utilized in display applications for their photoluminescence (PL) and electroluminescence. The PL characteristics of QDs are applied to display or lighting applications in the form of color‐conversion QD films, and the electroluminescence of QDs is utilized in quantum dot light‐emitting diodes (QLEDs). Studies on the stability of QDs and QD devices in display applications are reviewed herein. QDs can be degraded by oxygen, water, thermal heating, and UV exposure. Various approaches have been developed to protect QDs from degradation by controlling the composition of their shells and ligands. Phosphorescent QDs have been protected by bulky ligands, physical incorporation in polymer matrices, and covalent bonding with polymer matrices. The stability of electroluminescent QLEDs can be enhanced by using inorganic charge transport layers and by improving charge balance. As understanding of the degradation mechanisms of QDs increases and more stable QDs and display devices are developed, QDs are expected to play critical roles in advanced display applications.     

Electronic charge transfer in cobalt doped fullerene thin films and effect of energetic ion impacts by x-ray absorption spectroscopy

We report on the electronic charge transfer in cobalt doped fullerene thin films by means of near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy measurement. Co-doped fullerene films were prepared by co-deposition technique and subjected to energetic ion irradiation (120 MeV Au) for possibly alignment or interconnect of randomly distributed metal particles. Polarization dependent NEXAFS spectra revealed the alignment of Co and C atoms along the irradiated ionic path. The structural changes in Co-doped as-deposited and ion irradiated fullerene films were investigated by means of Raman spectroscopy measurements. Downshift of pentagonal pinch mode A_g(2) in Raman spectroscopy indicated the electronic charge transfer from Co atom to fullerene molecules, which is further confirmed by NEXAFS at C K-edge for Co-doped fullerene films.     

Toward tunable band gap and tunable dirac point in bilayer graphene with molecular doping

The bilayer graphene has attracted considerable attention for potential applications in future electronics and optoelectronics because of the feasibility to tune its band gap with a vertical displacement field to break the inversion symmetry. Surface chemical doping in bilayer graphene can induce an additional offset voltage to fundamentally affect the vertical displacement field and the band gap opening in bilayer graphene. In this study, we investigate the effect of chemical molecular doping on band gap opening in bilayer graphene devices with single or dual gate modulation. Chemical doping with benzyl viologen molecules modulates the displacement field to allow the opening of a transport band gap and the increase of the on/off ratio in the bilayer graphene transistors. Additionally, Fermi energy level in the opened gap can be rationally controlled by the amount of molecular doping to obtain bilayer graphene transistors with tunable Dirac points, which can be readily configured into functional devices, such as complementary inverters.     

Base effects on fabrication of silver nanoparticles embedded silica nanocomposite for surface-enhanced Raman scattering (SERS)

In this paper, we studied on the effect of organic bases in the case of ethylene glycol based fabrication of silver nanoparticles embedded silica nanocomposite (Ag SNC) without heating. Considering their chemical structures, butylamine (BA), ethanolamine (EA), triethanolamine (TEA), tributylamine (TBA), octylamine (OA) and Jeffamine 500 (JA) were used as an organic base. In addition, the effect of the concentrations of AgNO3 and organic bases on the formation of Ag SNC was also examined. In conformity with the characteristics of Ag SNC, SERS signal intensity of benzenethiol on Ag SNC was measured. As a result, the SERS signal intensity of Ag SNCs was strongly dependent on the reaction conditions. Furthermore, when reacted under the best reaction condition with concentrations of AgNO3 and OA, 3 mM and 5 mM, respectively, a large-scale production of Ag SNC was possible under the mild conditions.     

In vitro anti-bacterial and cytotoxic properties of silver-containing poly(L-lactide-co-glycolide) nanofibrous scaffolds

Electrospinning has recently emerged as a leading technique for the formation of nanofibrous structures made of organic and inorganic components. In this study, nanofibrous scaffolds were prepared by electrospining a bend solution of poly(L-lactide-co-glycolide) (PLGA) and silver nanoparticles in 1,1,1,3,3,3,-hexafluoro-2-propanol (HFIP). The resulting fibers ranged from 420 to 590 nm in diameter. To evaluate the possibility of using silver-containing PLGA as a tissue engineering scaffold, experiments on cell viability and antibacterial activity were carried out. As a result, PLGA nanofibrous scaffolds having silver nanoparticles of more than 0.5 wt% showed antibacterial effect against Staphylococcus aureus and Klebsiella pneumonia. Furthermore, silver-containing PLGA nanofibrous scaffolds showed viability, indicating their possible application in the field of tissue engineering.     

Observation of room temperature negative differential resistance in multi-layer heterostructures of quantum dots and conducting polymers

Multi-layer heterostructure negative differential resistance devices based on poly-[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) conducting polymer and CdSe quantum dots is reported. The conducting polymer MEH-PPV acts as a barrier while CdSe quantum dots form the well layer. The devices exhibit negative differential resistance (NDR) at low voltages. For these devices, strong negative differential resistance is observed at room temperature. A maximum value of 51 for the peak-to-valley ratio of current is reported. Tunneling of electrons through the discrete quantum confined states in the CdSe quantum dots is believed to be responsible for the multiple peaks observed in the I-V measurement. Depending on the observed NDR signature, operating mechanisms are explored based on resonant tunneling and Coulomb blockade effects.