Novel field emission from graphene sheets supported by CNTs arrays

A good quality graphene is transferred onto honeycomb-like CNTs arrays with inner supporting CNTs. The efficient field emission is demonstrated due to a high aspect ratio protrusions and graphene crack edges. A high efficient current density about 1.2 mA/cm2 at threshold electric field of 7.8 V/μm with a turn-on electric field of 1.8 V/μm at the current density of 10 μA/cm2 is observed due to high localized electric field. Stable field emission is tested in a vacuum chamber. The results are of significance to the development of Graphene based field emitters. © 2013 IEEE.

Optimizing the energy offset between dye and hole-transporting material in solid-state dye-sensitized solar cells

The power-conversion efficiency of solid-state dye-sensitized solar cells can be optimized by reducing the energy offset between the highest occupied molecular orbital (HOMO) levels of dye and hole-transporting material (HTM) to minimize the loss-in-potential. Here, we report a study of three novel HTMs with HOMO levels slightly above and below the one of the commonly used HTM 2,2′,7,7′- tetrakis(N,N-di-p-methoxyphenylamino)-9,9′- spirobifluorene (spiro-OMeTAD) to systematically explore this possibility. Using transient absorption spectroscopy and employing the ruthenium based dye Z907 as sensitizer, it is shown that, despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. We further demonstrate that the design of the HTM has an additional impact on the electronic density of states present at the TiO2 electrode surface and hence influences not only hole- but also electron-transfer from the sensitizer. These results provide insight into the complex influence of the HTM on charge transfer and provide guidance for the molecular design of new materials. © 2013 American Chemical Society.

Influence of film cooling hole angles and geometries on aerodynamic loss and net heat flux reduction

Turbine design engineers have to ensure that film cooling can provide sufficient protection to turbine blades from the hot mainstream gas, while keeping the losses low. Film cooling hole design parameters include inclination angle (a), compound angle (b), hole inlet geometry, and hole exit geometry. The influence of these parameters on aerodynamic loss and net heat flux reduction is investigated, with loss being the primary focus. Low-speed flat plate experiments have been conducted at momentum flux ratios of IR=0.16, 0.64, and 1.44. The film cooling aerodynamic mixing loss, generated by the mixing of mainstream and coolant, can be quantified using a three-dimensional analytical model that has been previously reported by the authors. The model suggests that for the same flow conditions, the aerodynamic mixing loss is the same for holes with different a and b but with the same angle between the mainstream and coolant flow directions (angle k). This relationship is assessed through experiments by testing two sets of cylindrical holes with different a and b: one set with k=35 deg, and another set with k=60 deg. The data confirm the stated relationship between α, β, k and the aerodynamic mixing loss. The results show that the designer should minimize k to obtain the lowest loss, but maximize b to achieve the best heat transfer performance. A suggestion on improving the loss model is also given. Five different hole geometries (α=35.0 deg, β=0 deg) were also tested: cylindrical hole, trenched hole, fan-shaped hole, D-Fan, and SD-Fan. The D-Fan and the SD-Fan have similar hole exits to the fan-shaped hole but their hole inlets are laterally expanded. The external mixing loss and the loss generated inside the hole are compared. It was found that the D-Fan and the SD-Fan have the lowest loss. This is attributed to their laterally expanded hole inlets, which lead to significant reduction in the loss generated inside the holes. As a result, the loss of these geometries is≈50% of the loss of the fan-shaped hole at IR=0.64 and 1.44. © 2013 by ASME.

Polarization switchable diffraction based on subwavelength plasmonic nanoantennas.

We prove theoretically and experimentally the concept of polarization holography by producing visible diffraction through radiation emitted by plasmonic nanoantennas. We show a methodology to selectively activate the nanoantenna emission by controlling the orientation of the electric field of a beam. Additionally, we demonstrate that it is possible to superpose two independent transverse nanoantennas in the same plane without producing interference in their radiated field. Hence, we introduce an alternative view to the traditional concept of holography where fringes (or diffractive units) are band-limited to half the wavelength.

Instantaneous frequency and bandwidth estimation using filterbank arrays

Accurate estimation of the instantaneous frequency of speech resonances is a hard problem mainly due to phase discontinuities in the speech signal associated with excitation instants. We review a variety of approaches for enhanced frequency and bandwidth estimation in the time-domain and propose a new cognitively motivated approach using filterbank arrays. We show that by filtering speech resonances using filters of different center frequency, bandwidth and shape, the ambiguity in instantaneous frequency estimation associated with amplitude envelope minima and phase discontinuities can be significantly reduced. The novel estimators are shown to perform well on synthetic speech signals with frequency and bandwidth micro-modulations (i.e., modulations within a pitch period), as well as on real speech signals. Filterbank arrays, when applied to frequency and bandwidth modulation index estimation, are shown to reduce the estimation error variance by 85% and 70% respectively. © 2013 IEEE.

Dry oxidation and vacuum annealing treatments for tuning the wetting properties of carbon nanotube arrays.

In this article, we describe a simple method to reversibly tune the wetting properties of vertically aligned carbon nanotube (CNT) arrays. Here, CNT arrays are defined as densely packed multi-walled carbon nanotubes oriented perpendicular to the growth substrate as a result of a growth process by the standard thermal chemical vapor deposition (CVD) technique.(1,2) These CNT arrays are then exposed to vacuum annealing treatment to make them more hydrophobic or to dry oxidation treatment to render them more hydrophilic. The hydrophobic CNT arrays can be turned hydrophilic by exposing them to dry oxidation treatment, while the hydrophilic CNT arrays can be turned hydrophobic by exposing them to vacuum annealing treatment. Using a combination of both treatments, CNT arrays can be repeatedly switched between hydrophilic and hydrophobic.(2) Therefore, such combination show a very high potential in many industrial and consumer applications, including drug delivery system and high power density supercapacitors.(3-5) The key to vary the wettability of CNT arrays is to control the surface concentration of oxygen adsorbates. Basically oxygen adsorbates can be introduced by exposing the CNT arrays to any oxidation treatment. Here we use dry oxidation treatments, such as oxygen plasma and UV/ozone, to functionalize the surface of CNT with oxygenated functional groups. These oxygenated functional groups allow hydrogen bond between the surface of CNT and water molecules to form, rendering the CNT hydrophilic. To turn them hydrophobic, adsorbed oxygen must be removed from the surface of CNT. Here we employ vacuum annealing treatment to induce oxygen desorption process. CNT arrays with extremely low surface concentration of oxygen adsorbates exhibit a superhydrophobic behavior.

Reversible tuning of the wettability of carbon nanotube arrays: the effect of ultraviolet/ozone and vacuum pyrolysis treatments.

Among diverse types of synthetic materials, arrays of vertically aligned carbon nanotubes have attracted the most attention, mainly because of their exceptional mechanical, electrical, optical, and thermal properties. However, their wetting properties are yet to be understood. In this present study, oxygenated surface functional groups have been identified as a vital factor in controlling the wetting properties of carbon nanotube arrays. The results presented herein indeed show that a combination of ultraviolet/ozone and vacuum pyrolysis treatments can be used to vary the surface concentration of these functional groups such that the carbon nanotube array can be repeatedly switched between hydrophilic and hydrophobic.

Subwavelength plasmonics for graded-index optics on a chip.

Planar plasmonic devices are becoming attractive for myriad applications, owing to their potential compatibility with standard microelectronics technology and the capability for densely integrating a large variety of plasmonic devices on a chip. Mitigating the challenges of using plasmonics in on-chip configurations requires precise control over the properties of plasmonic modes, in particular their shape and size. Here we achieve this goal by demonstrating a planar plasmonic graded-index lens focusing surface plasmons propagating along the device. The plasmonic mode is manipulated by carving subwavelength features into a dielectric layer positioned on top of a uniform metal film, allowing the local effective index of the plasmonic mode to be controlled using a single binary lithographic step. Focusing and divergence of surface plasmons is demonstrated experimentally. The demonstrated approach can be used for manipulating the propagation of surface plasmons, e.g., for beam steering, splitting, cloaking, mode matching, and beam shaping applications.

Effect of dry oxidation on the performance of carbon nanotube arrays electrochemical capacitors

In this study, the effect of dry oxidation on the electrochemical properties of carbon nanotube arrays is investigated. Oxygenated surface functional groups were introduced to the arrays by oxygen plasma treatment, where their surface concentrations were varied by controlling the exposure time. The finding presented herein shows an augmentation of nearly thirty times in term of specific capacitance when the arrays are oxidized. Similar behavior is also observed in the non-aqueous electrolytes where the specific capacitance of the oxidized carbon nanotube arrays is measured more than three times higher than that of the pristine ones. However, overexposure to oxygen plasma treatment reverses this effect. At such high oxidation level, the damage to the graphitic structure becomes more pronounced such that the capacitive behavior of the arrays is overshadowed by their resistive behavior. These findings are important for further development of carbon nanotube based electrochemical capacitors. © 2012 Materials Research Society.

Plasmonic graded-index planar lens based on subwavelength features in the effective index regime

We experimentally demonstrate the planar focusing of Surface Plasmon Polaritons using space variant PMMA subwavelength features on top of a metallic film. Focusing is obtained by creating an effective graded refractive index profile. © 2012 OSA.

Plasmonic graded-index planar lens based on subwavelength features in the effective index regime

We experimentally demonstrate the planar focusing of Surface Plasmon Polaritons using space variant PMMA subwavelength features on top of a metallic film. Focusing is obtained by creating an effective graded refractive index profile. © OSA 2012.