Large Figure Of Merit Research Articles

Simulation of Multilayered Heterojunction-Based Chalcogenide Fiber SPR Sensor With Ultrahigh Figure of Merit in Near Infrared

The dynamic nature of radiation damping in plasmonic structures can have immense potential in relation to significant improvement in plasmonic sensor performance. In this paper, we have explored the careful tuning of dynamic radiation damping in Si-Ag-graphene-Al2O3 heterojunction-based fiber optic plasmonic sensor structure. Low-refractive index rare-earth doped chalcogenide glass is considered as fiber material in near infrared spectral region. The simultaneous tuning of Ag and Al2O3 layer thicknesses with wavelength in appropriate broad ranges is investigated to enhance the sensor’s performance as much as possible. As another interesting step, the presence/absence of graphene layer on sensor’s performance is also examined. The results indicate that significantly large figure of merit (FOM) of magnitude 19212.67 RIU $^{-1}$ at $\lambda = 969$ nm for 5.2-nm-thick Al2O3 layer, and 36.6-nm-thick Ag layer is achievable from the proposed sensor with graphene. The above FOM is substantially larger than the existing (reported) plasmonic sensors claiming large FOM values. The findings of this paper can be crucial for developing surface plasmon reasonance biosensors with highly enhanced performance.

Simulation study on comprehensive sensing enhancement of BlueP/MoS2- and BlueP/WS2-based fluoride fiber surface plasmon resonance sensors: analysis founded on damping, field, and optical power

Two-dimensional (2D) heterostructure-based fluoride fiber surface plasmon resonance (SPR) sensor designs are simulated and analyzed while emphasizing figure of merit (FOM) enhancement in the near-infrared (NIR) spectral region. Through simultaneous optimization of NIR wavelength and Ag layer thickness, exceptionally large FOM values of 15,650.75 RIU-1 and 12,409.30 RIU-1 are achieved for BlueP/MoS2-based and BlueP/WS2-based fiber SPR sensors, respectively. The results are explained in terms of tunable radiation damping, power loss, and corresponding field enhancement. These FOM values are significantly greater than recently reported sensors. The BlueP/MoS2-based sensor with 48.8nm Ag film and at 738.4nm wavelength provides an all-round large FOM.

High-performance metamaterial sensors based on strong coupling between surface plasmon polaritons and magnetic plasmon resonances

Recently, there has been increasing interest in sensing by magnetic plasmon resonances (MPRs) in metamaterials. However, due to their fast radiative damping, MPRs usually have a broad bandwidth and thus a weak enhancement of electromagnetic fields, which is disadvantageous to sensing applications. In this work, we will numerically study near-infrared metamaterial sensors consisting of one-dimensional (1D) periodic array of nanogrooves drilled in Ag substrate. It is found that an ultra-narrow hybridized mode, whose full width at half maximum (FWHW) is only about 10 nm, can be formed by coupling MPRs confined within the nanogrooves to surface plasmon polaritons (SPPs) propagating on the substrate surface. When the hybridized mode is excited, the incident light is nearly completely absorbed, and thus the electric and magnetic fields in the nanogrooves are greatly enhanced. Moreover, the resonance wavelength of the hybridized mode will have a noticeable shift, even if the refractive index of the environment medium has a slight change. Thanks to these properties, the investigated metamaterial sensors have a high sensitivity (S) of more than 1500 nm/RIU and a large figure of merit (FOM) reaching to 150, and so may have potential applications in label-free biosensing.

Large optical nonlinearity of ITO/Ag/ITO sandwiches based on Z-scan measurement.

Indium tin oxide (ITO)-based sandwich structures with the insertion of silver (ITO/Ag/ITO) show large nonlinear optical enhancement of both nonlinear refraction and saturable absorption. Here optical nonlinearity is measured using a Z-scan experiment with a 1310nm pulsed laser at normal incidence. The nonlinear refractive index (n2=15.43×10-16 m2/W) for an ITO/Ag/ITO sandwich with a 14nm silver interlayer and the nonlinear absorption coefficient (β=-648×10-11 m/W) for an ITO/Ag/ITO sandwich with a 10nm silver interlayer are about 18 and 16 times greater than that of a single-layer ITO, respectively. Meanwhile, a large figure of merit and modulation depth values hinted that ITO/Ag/ITO sandwiches are promising saturable absorber materials to switch continuous laser waves into laser pulses. These novel nonlinear optical properties make ITO/Ag/ITO sandwiches a promising candidate for all-optical modulation devices at optical communication wavelengths.

(Invited) Halide Vapor Phase Epitaxy for b- and a-Ga2O3 Power Electronics Devices

Monoclinic b-Ga2O3 and rhombohedral a-Ga2O3 have recently emerged as promising materials for “ultrawide” bandgap electronics for next-generation high voltage lateral and vertical power switching devices, thanks to their large bandgaps (4.9eV and 5.3eV, respectively), high expected critical breakdown fields (8MV/cm and 10MV/cm, respectively), and their correspondingly large high voltage and high frequency Baliga figures of merit (FOMs). These large FOMs ultimately translate to higher efficiencies for high power (>1kW) switching devices, which will enable more compact, higher performing power electronics systems. In order to realize the high voltages required for the most demanding applications, further development of epitaxial growth techniques which can produce thick Ga2O3 films is required. Halide vapor phase epitaxy (HVPE) is a scalable and inexpensive growth technique which currently appears to be well positioned to address such a need. Kyma Technologies has developed a low-cost HVPE reactor for the growth of thick Ga2O3 films which boasts high growth rates and smoothness while simultaneously being able to be lightly and controllably doped with Si and free of carbon. The reactor has been configured to grow both b-Ga2O3 and a-Ga2O3, and in this talk we will outline our recent growth results from both. Effects of substrate preparation, epi-readiness, and growth conditions on epilayer quality as well as on the corresponding Schottky barrier diode device performance will be presented.

Possible Routes for Efficient Thermo-Electric Energy Conversion in a Molecular Junction.

In the context of designing an efficient thermoelectric energy-conversion device at nanoscale level, we suggest several important tuning parameters to enhance the performance of thermoelectric converters. We consider a simple molecular junction, which is always helpful to understand the basic mechanisms in a deeper way, where a benzene molecule is coupled to two external baths having unequal temperatures. The key component responsible for achieving better performance is associated with the asymmetric nature of transmission function, and in the present work, we show that it can be implemented in different ways by regulating the physical parameters involving the system. Employing a tight-binding framework we calculate electrical and thermal conductances, thermopower, and figure of merit (FOM) by using Landauer integrals, and thoroughly examine the critical roles played by molecule-to-lead (ML) interface geometry, magnetic field, chemical substituent group, ML coupling, and the direct coupling between the two leads. Our results show that a reasonably large FOM (≫1) can be obtained and lead to a possibility of regulating the efficiency by selectively tuning the physical parameters. We believe that the present analysis will enhance the understanding of designing efficient thermoelectric devices, and can be verified in a laboratory.

Charge transfer in graphene/WS2 enhancing the saturable absorption in mixed heterostructure films

Improving the nonlinear optical (NLO) property of two-dimensional (2D) materials is a challenging research problem for the nonlinear optical applications in optoelectronics. Taking an advantage of the van der Waals interactions, graphene/WS2 heterostructure films have been prepared by a facile liquid phase exfoliation method combined with a filtration method. The NLO properties of these graphene/WS2 films have been enhanced compared with that of WS2 and graphene, however with no evident dependency on thickness and pump fluence. The imaginary part of the third-order nonlinear optical susceptibility (Imχ(3)) and figure of merit (FOM) for graphene/WS2 are approximately four times larger than that for WS2 and two times larger than that for graphene. We have explained the mechanism of enhancement based on the rate equation models along with Runge-Kutta algorithm. Our results suggest the enhancement could be attributed to the charge transfer from WS2 to graphene due to the existence of donor-acceptor structure. First-principles calculations confirm that the van der Waals bonding at interface could accelerate charge transfer and electron-hole pair separation efficiently after the heterostructure formation. Our findings provide a way for designing NLO properties of 2D materials with large FOM and Im χ(3) for nonlinear photonic devices such as mode locking or Q-switching in ultrafast lasers.

Fluoride Fiber Sensor With Huge Performance Enhancement via Optimum Radiative Damping at Ag–Al2O3–Graphene Heterojunction on Silicon

Surface plasmon resonance-based fiber optic sensor with multilayer heterojunction is simulated and analyzed. The constituent materials are ZBLAN fluoride core, NaF clad, amorphous Si layer, Ag layer, Al2O3 interlayer, and graphene monolayer. The main idea behind the study is to optimize the radiative damping ( i.e. , optimum radiative damping, ORD) at the Ag–Al2O3–graphene heterojunction in order to enhance the sensor's figure of merit (FOM) as much as possible. The effect of graphene monolayer's presence on sensor's FOM is also examined. Multiple occurrences of ORD may be achieved by coordinated variation of Ag and Al2O3 layer thicknesses along with light wavelength. Among the several prominent ORD conditions, the combination of 45.3-nm-thick Ag layer, 11-nm-thick Al2O3 layer, and 938.7-nm wavelength leads to a massively large FOM of 31806.65 RIU–1. The above FOM of the FOSPR sensor is nearly six times that for the corresponding prism-based SPR sensor ( i.e. , 5500 RIU–1) reported earlier with Al2O3 interlayer and MoS2 monolayer at λ = 1200 nm. Further, the proposed sensor provides substantially greater FOM compared to existing prism-based and FOSPR sensors.

Monolithic nonlinear optical chromophores with extended conjugate bridge: Large refractive index, high thermal and electro-optic stability

Monolithic electro-optic (EO) molecular glasses are promising materials for optical waveguide device owing to their high chromophore loading density, high refractive indices and large EO coefficients (r33 values) for the optimization of in-device figure of merit (n3r33). However, the stability of EO molecular glasses including the poling induced r33 stability, chemical stability under both high temperature and high voltage in poling is still an obstruction for their real application. Herein, we used the chemical structural optimization, by means of introducing a large conjugated electron-bridge indacenodithiophene (IDT), to synthesize a monolithic EO molecular glass IDTC. Comprehensive properties of excellent glass-forming ability, high thermal stability (Td = 315 °C), and high glass transition temperature (Tg = 137 °C) was achieved in monolithic IDTC thin films. More significantly, refractive index of monolithic film was exceptionally high (n = 1.8294 @ 1310 nm and n = 1.7645 @1550 nm). The poled IDTC film exhibited the r33 value of 87.6 pm/V with large figure of merit (n3r33: 536 pm/V @1310 nm), and the EO activity can be retained over 85% of the initial value after 300 h annealing at 85 °C. Meanwhile, it was found that there is no decomposition of IDTC in poled films after high temperature and high voltage poling as well as long term annealing. The poled chromophores are recyclable EO materials for the sustainable application in low-cost EO devices.

Understanding the behavior of electronic and phonon transports in germanium based two dimensional chalcogenides

Electronic and phonon transport properties of buckled GeTe and GeSe monolayers were investigated by combining density functional theory with lattice dynamics approach. For accurate prediction of electronic bandgaps, the PBE0 hybrid functional was employed, and the bandgap values were found to be 2.33 eV and 3.55 eV for GeTe and GeSe monolayers, respectively. Electronic transport coefficients were calculated using Boltzmann transport equations implemented in the BOLTZTRAP code. The Seebeck coefficients of GeTe (2680.94 μV/K) and GeSe (2981.81 μV/K) monolayers were found to be quite higher than those of their other allotropes. The out of plane ZA mode exhibits a quadratic nature near the Γ point of the Brillouin zone, which is attributed to the flexural phonon modes in both GeTe and GeSe monolayers. Strong anharmonicity found in the GeTe monolayer compared to the GeSe monolayer leads to lower lattice thermal conductivity in the GeTe monolayer. The room temperature lattice thermal conductivity of both monolayers was found to be quite low. A comprehensive analysis of group velocity for all phonon modes shows that the ZA mode contributes less to the lattice thermal conductivity of the GeTe monolayer than to that of the GeSe monolayer. An analysis of three-phonon scattering reveals that more scattering channels are available for phonon scattering in GeTe, which leads to lower thermal conductivity in the GeTe monolayer. The GeSe monolayer has a larger figure of merit than the GeTe monolayer, but it may have low output power because of its low electrical conductivity.

Second Harmonic Spectroscopy of Surface Lattice Resonances.

Because of their large figures of merit, surface lattice resonances (SLRs) in metal nanoparticle arrays are very promising for chemical and biomolecular sensing in both liquid and gas media. SLRs are sensitive to refractive index changes both near the surface of the nanoparticles (surface sensitivity) and in the volume between them (bulk sensitivity). Because of its intrinsic surface-sensitivity and a power law dependence on electric fields, second harmonic generation (SHG) spectroscopy can improve upon both the surface and volume sensitivities of SLRs. In this report on SHG spectroscopy of plasmonic nanoparticles, we show that the SHG signal is greatly increased (up to 450 times) by the SLRs. We also demonstrate very narrow resonances in SHG intensity (∼5 nm fwhm). We illustrate how the SHG resonances are highly sensitive to SLRs by varying the fundamental wavelength, angle of incidence, nanoparticle material, and lattice constant of the arrays. Finally, we identify an SHG resonance (10 nm fwhm) that is electric dipole forbidden and can be attributed to higher-order multipoles, enhanced by the strong near-fields of SLRs. Our results open up new and very promising avenues for chemical and biomolecular sensing based on SHG spectroscopy of SLRs.

Anisotropic thermoelectric effect on phosphorene and bismuthene: first-principles calculations based on nonequilibrium Green’s function theory

By using the nonequilibrium Green’s function (NEGF) method, we perform first-principles calculations for phosphorene and bismuthene and evaluate their thermoelectric properties on the basis of the Landauer formalism. We find that the figures of merit strongly depend on the transport direction: the figures for the zigzag (ZZ) direction are substantially different from those for the armchair direction. These anisotropies are found to originate from the difference between the transmission functions in the AM and ZZ directions. We find that p-type bismuthene has a large figure of merit when the transport is in the ZZ direction, then we conclude that bismuthene has some advantages over phosphorene as a thermoelectric material. We also clarify that the spin–orbit coupling has a substantial effect on the thermoelectric properties of bismuthene.

Investigation of the electronic structure and lattice dynamics of the thermoelectric material Na-doped SnSe

SnSe has drawn considerable attention on a global scale due to its intrinsic low thermal conductivity and large figure of merit along the $b$ axis. In Na-doped SnSe, further enhancement of the thermoelectric performance has been reported. Using angle-resolved photoemission spectroscopy and inelastic neutron scattering, we have studied how electronic structures and lattice dynamics evolve with temperature in Na-doped SnSe. Our data show that the effective mass of the Se ${p}_{z}$ orbital along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}Z$ direction has a very weak temperature dependence, while the chemical potential shifts significantly along with the increase in the gap size evidenced by infrared absorption measurements. Inelastic neutron scattering reveals one acoustic TA and two low-lying optical (TO1 and TO2) phonon modes. Their temperature-dependent behaviors indicate that the TO1 and TA modes contribute more to the reduction of the lattice thermal conductivity with temperature increases. The estimated value of the lattice thermal conductivity based on the lattice dynamics is significantly larger than that determined by transport measurements, suggesting that extrinsic factors, such as the imperfection of the lattice, could drastically suppress the lattice thermal conductivity. Our data suggest that temperature-dependent properties of both electronic structures and phonon dynamics need to be taken into account for the investigation of the underlying physics of hole-doped SnSe.

Criteria to Confirm Models that Simulate Deforestation and Carbon Disturbance

The Verified Carbon Standard (VCS) recommends the Figure of Merit (FOM) as a possible metric to confirm models that simulate deforestation baselines for Reducing Emissions from Deforestation and forest Degradation (REDD). The FOM ranges from 0% to 100%, where larger FOMs indicate more-accurate simulations. VCS requires that simulation models achieve a FOM greater than or equal to the percentage deforestation during the calibration period. This article analyses FOM’s mathematical properties and illustrates FOM’s empirical behavior by comparing various models that simulate deforestation and the resulting carbon disturbance in Bolivia during 2010–2014. The Total Operating Characteristic frames FOM’s mathematical properties as a function of the quantity and allocation of simulated deforestation. A leaf graph shows how deforestation’s quantity can be more influential than its allocation when simulating carbon disturbance. Results expose how current versions of the VCS methodologies could conceivably permit models that are less accurate than a random allocation of deforestation, while simultaneously prohibit models that are accurate concerning carbon disturbance. Conclusions give specific recommendations to improve the next version of the VCS methodology concerning three concepts: the simulated deforestation quantity, the required minimum FOM, and the simulated carbon disturbance.

Evolution of rough-surface geometry and crystalline structures of aligned TiO2 nanotubes for photoelectrochemical water splitting

Nowadays, increasing awareness of environment and fossil fuels protection stimulates intensive research on clean and renewable sources of energy. Production of hydrogen from water through solar-driven splitting reactions is one of the most promising approaches in the field of photoelectrochemistry (PEC). In this work we have fabricated well-aligned, highly-ordered, smooth-mouth TiO2 nanotube arrays (TNAs) in a two-step anodization process of titanium foil, which were then used as photoelectrodes for PEC water splitting. It demonstrates for the first time correspondence between non-linear component characteristics of multiscale rough surface and crystalline structure of annealed TNAs measured at various fabrication stages and their photoelectrochemical response. The as-anodized TNAs with isotropic surface (deduced from AFM and SEM images) and largest figure of merit (according to their PEC performance) were annealed at 450 °C in air. Scale-invariant descriptors of the surface structure of the deposits involved: fractal dimension, corner frequency, roughness, size of nanostructures and their dominant habits. Moreover, X-ray diffraction data processed using the Rietveld method confirmed co-existence of various oxides, for example: TiO2 in the form of anatase, TiO and Ti3O5 phases in the TNAs under study pointing that previous well-established mechanisms of the TNA growth were to certain degree incomplete.

Self-Flattened ZnO:Al Transparent Conductive Thin Films Derived by Sol–Gel Process

Self-flattened and low-resistance Al-doped ZnO (ZnO:Al) thin films were developed by sol-gel process, for providing a versatile transparent conducting oxide material that enables the growth of high-quality photoactive layers and enhancement in light absorption by intensifying light-scattering simutaneously. The electrical, optical, and structural characteristics of the sol-gel derived ZnO:Al thin film was evaluated by comparing with the B-doped ZnO (ZnO:B) fabricated by the metalorganic chemical vapor deposition method and ZnO:Al fabricated by the radio-frequency magnetron sputtering deposition method. This sol-gel derived ZnO:Al thin film shows the c-axis oriented wurtzite structure, a resistivity of 9.89 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Ω·cm, and smoother surface morphology and larger figure of merit σ/ α than the vacuum-technique derived ZnO thin film with similar thickness. This low resistivity was achieved by optimizing the Al doping concentration and annealing process. Employed as the front electrode, it enabled a better device performance of hydrogenated amorphous Si single-junction solar cells with respect to the ZnO:B thin film with similar thickness by improving the Voc and fill factor (FF). The dark J-V characteristics of these devices were analyzed to investigate the mechanism of Voc and FF improvement. These results suggest a promising candidate for the transparent electrode of superstrate-type thin-film solar cells.

Chalcogenide fiber-optic SPR chemical sensor with MoS2 monolayer, polymer clad, and polythiophene layer in NIR using selective ray launching

Abstract Surface plasmon resonance (SPR) based chalcogenide fiber-optic sensor with polymer clad and MoS2 monolayer is simulated and analyzed in near infrared (NIR) for detection of mixture of alcohols (ethanol and methanol) dissolved in water solution. The proposed fiber optic sensor is analyzed under angular interrogation method, which is based on selective ray (on-axis) launching of monochromatic light into the fiber core at varying angle followed by measuring the loss of power (in dB) after passing through the SPR probe region. The performance of the sensor is analyzed in terms of its figure of merit (FOM). The sensor’s specificity towards alcohols along with considerably larger FOM is achieved by utilizing a polythiophene (PT) layer. The results indicate that longer NIR wavelength (λ) provides superior sensing performance. The sensor’s performance is better for larger volume fraction of methanol in the water solution. The proposed fiber optic SPR sensor has the capability of providing much greater FOM compared with the previously-reported SPR sensors.

Effect of piezoelectric layer thickness and poling conditions on the performance of cantilever piezoelectric energy harvesters on Ni foils

Lead zirconate titanate (PZT) films grown on flexible Ni foils were utilized to explore the effects of thickness and poling conditions on the performance of mechanical energy harvesters. In the case of Mn doped 1 μm thick (001) oriented sol-gel PZT (52/48) films on Ni foil, the dielectric constant and |e31,f| are 390 at 10 kHz and 11.3 C/m2, respectively, after hot poling. This film has a large figure of merit (e31,f2εr), of around 0.4 C2/m4, for piezoelectric energy harvesting. Unimorph cantilever beams were easily fabricated from PZT films on Ni foil using simple mechanical cutting. The maximum power increases from 12 to 60 μW as the thickness of Nb doped PZT film increases from 1 to 3 μm at resonance frequency (∼70 Hz) at 0.5 G. The optimum poling condition (150 °C, at 3 times the coercive for 15 min) enhanced the voltage and power output of the cantilever harvester prepared using (1 μm) PZT films on Ni foil. It was found that the power performance of harvesters strongly depends on the thickness of the film with resonant harvesters of the same footprint area (0.385 cm2) using PZT films on Ni foils.

Graphene-supported plasmonic whispering-gallery mode in a metal-coated microcavity for sensing application with ultrahigh sensitivity

We propose and numerically investigate the plasmonic whispering-gallery mode (WGM) with high-quality (Q) factor (as high as 285) and ultra-small mode volume (as low as 0.04μm3) in the hybrid plasmonic microcavity with a sandwiched and electrically controlled graphene. The theoretical results present that the resonant wavelength of hybrid plasmonic microcavity dramatically changes and the corresponding intrinsic loss exhibits a distinct peak by electrically adjusting the permittivity of graphene around the epsilon-near-zero (ENZ) point. The influence of graphene with different layers on the characteristic of hybrid plasmonic microcavity is also analyzed. As a potential application, the plasmonic WGM microcavity with the sandwiched graphene could be applied for a refractometer with the sensitivity of higher than 1000 nm per refraction index unit (nm/RIU), and large figure of merit. The sensitivity can also be tuned by the electrically controlled graphene.

Extra-low thermal conductivity in unfilled CoSb3-δ skutterudite synthesized under high-pressure conditions

Thermoelectric CoSb3-δ skutterudite was synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. Structural analysis carried out from synchrotron x-ray powder diffraction data reveals a significant Sb deficiency in this material. The introduction of point defects in the form of Sb vacancies distributed at random in the structure leads to an impressive reduction (&amp;gt;50%) of the total thermal conductivity, κ, which is one of the main ingredients of good thermoelectric materials. This suggests phonon scattering effects originated in the Sb defects, which drives to a better improvement in κ than that achieved by the conventional strategy of filling the cages of the skutterudite structure with rare earths or other heavy cations. In parallel, changes in the electronic band structure caused by point variation of the stoichiometry produce an undesired increment in the electrical resistivity. Nevertheless, the low thermal conductivity combined with a high Seebeck coefficient (−434 μV K−1 at 500 K) originates a relatively large figure of merit (ZT = 0.12 at 550 K) for CoSb2.90(2).