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arXiv2018

Analysis and Development of SiC MOSFET Boost Converter as Solar PV Pre-regulator

Renewable energy source such as photovoltaic (PV) cell generates power from the sun light by converting solar power to electrical power with no moving parts and less maintenance. A single photovoltaic cell produces voltage of low level. In order to boost up the voltage, a DC-DC boost converter is used. In order to use this DC-DC converter for high voltage and high frequency applications, Silicon Carbide (SiC) device is most preferred because of larger current carrying capability, higher voltage blocking capability, high operating temperature and less static and dynamic losses than the traditional silicon (Si) power switches. In the proposed work, the static and dynamic characteristics of SiC MOSFET for different temperatures are observed. A SiC MOSFET based boost converter is investigated which is powered by PV source. This DC - DC converter is controlled using a Pulse-Width Method (PWM) and the duty cycle d is calculated for tracking the maximum power point using incremental conductance algorithm of the PV systems implemented in FPGA. Simulation studies are carried out in MATLAB/SIMULINK.A prototype of the SiC converter is built and the results are verified experimentally. The performance parameters of the proposed converter such as output voltage ripple input current ripple and losses are computed and it is compared with the classical silicon (Si) MOSFET converter.

physics.app-ph↗
arXiv2018

High Open Circuit Voltages in pin-Type Perovskite Solar Cells through Strontium Addition

The incorporation of even small amounts of strontium (Sr) into lead-based quadruple cation hybrid perovskite solar cells results in a systematic increase of the open circuit voltage (Voc) in pin-type perovskite solar cells. We demonstrate via transient and absolute photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer and specifically at the perovskite/C60 interface. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C60 electron-transporting layer. The resulting solar cells exhibited a Voc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, bringing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high Voc values and efficiencies in Sr-containing quadruple cation perovskite pin solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer.

physics.app-ph↗
arXiv2018

Substrate induced proximity effect in superconducting niobium nanofilms

Structural and superconducting properties of high quality Niobium nanofilms with different thicknesses are investigated on silicon oxide and sapphire substrates. The role played by the different substrates and the superconducting properties of the Nb films are discussed based on the defectivity of the films and on the presence of an interfacial oxide layer between the Nb film and the substrate. The X-ray absorption spectroscopy is employed to uncover the structure of the interfacial layer. We show that this interfacial layer leads to a strong proximity effect, specially in films deposited on a SiO$_2$ substrate, altering the superconducting properties of the Nb films. Our results establish that the critical temperature is determined by an interplay between quantum-size effects, due to the reduction of the Nb film thicknesses, and proximity effects.

physics.app-ph↗
arXiv2019

The formation of epitaxial p-i-n structures on the basis of (In,Fe)Sb and (Ga,Fe)Sb layers

Multilayer structures on the basis of n-type (In,Fe)Sb and p-type (Ga,Fe)Sb diluted magnetic semiconductors (DMS) along with separate (In,Fe)Sb and (Ga,Fe)Sb layers were fabricated on GaAs substrates by pulsed laser sputtering of InSb, GaAs, GaSb, Sb and Fe targets in a vacuum. Transmission electron microscopy and energy-dispersive x-ray spectroscopy reveal a strong dependence of the phase composition of the (In,Fe)Sb compound on the growth temperature. An increase of the latter from 220C to 300C leads to a coalescence of Fe atoms and formation of a secondary crystalline phase in the (In,Fe)Sb layer with a total Fe content of ca. 10 at. %. At the same time, the Ga0.8Fe0.2Sb layers obtained at 220C and 300C are single-phase. The separate In0.8Fe0.2Sb and Ga0.8Fe0.2Sb layers grown on i-GaAs at 220C are DMS with Curie temperatures of ca. 190 K and 170 K, respectively. The three-layer p-i-n diode (In,Fe)Sb/GaAs/(Ga,Fe)Sb structure grown on a GaAs substrate at 220C with a Fe content of ca. 10 at. % in the single-phase (In,Fe)Sb and (Ga,Fe)Sb layers has a rather high crystalline quality and can be considered as a prototype of a bipolar spintronic device based on Fe-doped III-V semiconductors.

physics.app-ph↗
arXiv2019

Modulation of the Work Function by the Atomic Structure of Strong Organic Electron Acceptors on H-Si(111)

Advances in hybrid organic/inorganic architectures for optoelectronics can be achieved by understanding how the atomic and electronic degrees of freedom cooperate or compete to yield the desired functional properties. Here we show how work-function changes are modulated by the structure of the organic components in model hybrid systems. We consider two cyano-quinodimethane derivatives (F4-TCNQ and F6-TCNNQ), which are strong electron-acceptor molecules, adsorbed on H-Si(111). From systematic structure searches employing range-separated hybrid HSE06 functional including many body van der Waals contributions, we predict that despite their similar composition, these molecules adsorb with significantly different densely-packed geometries in the first layer, due to strong intermolecular interaction. F6-TCNNQ shows a much stronger intralayer interaction (primarily due to van der Waals contributions) than F4-TCNQ in multilayered structures. The densely-packed geometries induce a large interface-charge rearrangement that result in a work-function increase of 1.11 and 1.76 eV for F4-TCNQ and F6-TCNNQ, respectively. Nuclear fluctuations at room temperature produce a wide distribution of work-function values, well modeled by a normal distribution with σ=0.17 eV. We corroborate our findings with experimental evidence of pronounced island formation for F6-TCNNQ on H-Si(111) and with the agreement of trends between predicted and measured work-function changes.

cond-mat.mtrl-sci↗
arXiv2018

Valley selective optical control of excitons in 2D semiconductors using Chiral metasurface

Recent advances in condensed matter physics have shown that the valley degree of freedom of electrons in 2D materials with hexagonal symmetry, such as graphene, h-BN, and TMDs, can be efficiently exploited, leading to the emergent field of valleytronics, which offers unique opportunities for efficient data transfer, computing and storage. The ability to couple the valley degree of freedom of electrons with light can further expand the ways one manipulate this degree of freedom, thus envisioning a new class of solid-state-photonic interfaces and devices. Besides this expansion of control of valley by light-waves, coupling of photons with valley-polarized electrons can dramatically expand the landscape of available optical responses, which may bring new means of controlling light in photonic devices. In this work we design such hybrid solid-state photonic metasurface integrating 2D TMD and photonic all-dielectric metasurface. While TMD is naturally endowed with the property of valley to optical-polarization coupling, the photonic metasurface is designed to produce chiral field which selectively couples to the valley degree of freedom of solid-state TMD component. We experimentally demonstrate that such coupling leads to controlled valley polarization due to the coupling of 2D materials with the chiral photonic metasurface. The measured emission from valley excitons in this hybrid system yields the preferential emission of specific helicity.

physics.optics↗
arXiv2018

Direct observation of the CVD growth of monolayer MoS$_2$ using $in-situ$ optical spectroscopy

Real-time monitoring is essential for understanding and eventually precise controlling of the growth of two dimensional transition-metal dichalcogenides (2D TMDCs). However, it is very challenging to carry out such kind of studies on chemical vapor deposition (CVD). Here, we report the first real time $in-situ$ study on the CVD growth of the 2D TMDCs. More specifically, CVD growth of molybdenum disulfide (MoS$_2$) monolayer on sapphire substrates has been monitored $in-situ$ using differential transmittance spectroscopy (DTS). The growth of the MoS$_2$ monolayer can be precisely followed by looking at the evolution of the characteristic optical features. Consequently, a strong correlation between the growth rate of MoS$_2$ monolayer and the temperature distribution in the CVD reactor has been revealed. Our result demonstrates the great potential of the real time $in-situ$ optical spectroscopy for the realization of the precisely controlled growth of 2D semiconductor materials.

cond-mat.mtrl-sci↗
arXiv2018

Effect of sputter deposited ZnO thin layers on magnetoimpedance response and field sensitivity of Co-based micro-wires

Semiconductive ZnO has shown to be a fascinating element for application in high-performance sensors. ZnO with various thicknesses (87-500 nm) was deposited on the surface of Co-based amorphous micro-wire (Co68.15Fe4.35Si12.5B15) using magnetron sputtering technique and the magnetoimpedance (MI) response was evaluated. The MI% increases monotonously up to the 240 nm thickness of the ZnO layer. By further increase in the ZnO layer thickness the MI% decreases. MI response and field sensitivity increased from 227% and 1.7 %/Oe for bare micro-wire up to 406% and 4.99 %/Oe for ZnO deposited micro-wire with a thickness of 240 nm. The x-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the structural grown ZnO layers. Vibrating sample magnetometry (VSM) was used to reveal the role of the ZnO thin layer in the MI response. The same trend of increase and decrease in the MI% by thickness was observed for the transverse permeability of the samples determined by VSM. The obtained results address a simple way to achieve high MI response and sensitivity with selective surface sensing.

physics.app-ph↗
arXiv2018

Efficient Volumetric Absorption Solar Thermal Platforms Employing Thermally Stable - Solar Selective Nanofluids Engineered from Used Engine Oil

We report a low cost and scalable method to synthesize solar selective nanofluids from 'used engine oil'. The as-prepared nanofluids exhibit excellent long-term stability and photo-thermal conversion efficiency. Moreover, these were found to retain their stability and functional characteristics even after extended periods of high temperature (300°C) heating, ultra violet light exposure and thermal cyclic loading. Building upon it, we have been able to successfully engineer an efficient volumetric absorption solar thermal platform that employs the as-prepared nanofluids and achieves higher steady state temperatures (approximately 5% higher) relative to the conventional surface absorption based solar thermal system under the sun. The developed volumetric absorption solar thermal platform could prove to be significant step in the evolution of efficient solar thermal systems which could potentially be deployed for host of applications ranging from solar driven heating, air-conditioning, and desalination units to solar energy electricity generation systems.

physics.app-ph↗
arXiv2018

Advances in Modelling and Simulation of Halide Perovskites for Solar Cell Applications

Perovskite solar cells (PSCs) are attracting great attention as the most promising candidate for the next generation solar cells. This is due to their low cost and high power conversion efficiency in spite of their relatively short period of development. Key components of PSCs are a variety of halide perovskites with ABX3 stoichiometry used as a photoabsorber, which brought the factual breakthrough in the field of photovoltaic (PV) technology with their outstanding optoelectronic properties. To commercialize PSCs in the near future, however, these materials need to be further improved for a better performance, represented by high efficiency and high stability. As in other materials development, atomistic modelling and simulation can play a significant role in finding new functional halide perovskites as well as revealing the underlying mechanisms of their material processes and properties. In this sense, computational works for the halide perovskites, mostly focusing on first-principles works, are reviewed with an eye looking for an answer how to improve the performance of PSCs. Specific modelling and simulation techniques to quantify material properties of the halide perovskites are also presented. Finally, the outlook for the challenges and future research directions in this field is provided.

physics.app-ph↗
arXiv2018

Communicating Two States in Perovskite Revealed by Time-Resolved Photoluminescence Spectroscopy

Organic-inorganic perovskite as a promising candidate for solar energy harvesting has attracted immense interest for its low-cost preparation and extremely high quantum efficiency. However, the fundamental understanding of the photophysics in perovskite remains elusive. In this work, we have revealed two distinct states in MAPbI3 thin films at low temperature through time-resolved photoluminescence spectroscopy (TRPL). In particular, we observe a photo-induced carrier injection from the high energy (HE) state to the low energy (LE) state which has a longer lifetime. The strong interaction between the two states, evidenced by the injection kinetics, can be sensitively controlled through the excitation power. Understanding the interacting two-states not only sheds light on the long PL lifetime in perovskite but also helps to understand the different behavior of perovskite in response to different excitation power. Further efforts in modifying the low energy state could significantly improve the quantum efficiency and lead to novel application in optoelectronics based on perovskite.

physics.app-ph↗
arXiv2018

Spatially Separated Self-Assembled Silver Dendrites for Highly Sensitive SERS Applications

Even after more than four decades of rapid-pace advances, the adoption of Surface-Enhanced Raman Scattering (SERS) remains limited, due to the difficulties in fabrication of effective and affordable nanostructured surfaces or templates. From this point-of-view, self-assembly techniques of SERS substrates formation allowing fabrication of highly branched dendritic silver nanostructures, which are very promising for biosensor applications. However, these techniques suffer from the lack of control over the resulting structures. Herein, we propose an electroless wet-chemical approach for highly reproducible fabrication of spatially separated plasmonic active silver dendrites in a porous matrix of Si/SiO2 template via controlled self-assembly of silver in a limited volume. The proposed approach allows fabrication of spatially separated silver dendrites with sub-10-nm gaps between neighboring branches, which possessing strong electric field enhancement in a wide spectral range. The fabricated silver dendritic structures enable a SERS enhancement factor of ~10^8 and the analyte detection limit of ~10^(-15) M, which is at the level of single molecule sensitivity. The comparison of simulation results with SERS experiments allows to distinguish the presence of electromagnetic and chemical contributions, which have a different effect at various analyte concentrations.

physics.optics↗
arXiv2018

Compact fully monolithic optomechanical accelerometer

We present a highly compact and fully monolithic optomechanical accelerometer fabricated of a single wafer of fused-silica with a total volume of less than 2cc and a total mass of approximately 4 grams. This sensor demonstrates an unprecedented mQ-product of 50kg, highlighting its unique capabilities for reaching high sensitivity in acceleration sensing. The sensor includes a monolithically co-fabricated Fabry-Perot interferometer that uses the accelerometer test mass as a mirror to measure its displacement in orders of 10-12 m/rtHz, reaching acceleration sensitivities to the order of 100 ng/rtHz - 1 ug/rtHz above 1 Hz.

physics.app-ph↗
arXiv2018

Imaging of material defects with a radio-frequency atomic magnetometer

Non-destructive eddy current testing of defects in metal plates using the magnetic resonance signal of a radio-frequency atomic magnetometer is demonstrated. The shape and amplitude of the spatial profile of signal features that correspond to defects are explored. By comparing numerical and experimental results on a series of benchmark aluminium plates we demonstrate a robust process for determining defect dimensions. In particular, we show that the observed images represent the spatial distribution of the secondary field created by eddy currents in the sample. We also demonstrate that the amplitude and phase contrast of the observed profiles enables us to reliably measure defect depth.

physics.app-ph↗
arXiv2018

The Elastic Contact of Rough Spheres Investigated Using a Deterministic Multi-Asperity Model

In this paper we use a deterministic multi-asperity model to investigate the elastic contact of rough spheres. Synthetic rough surfaces with controllable spectra were used to identify individual asperities, their locations and curvatures. The deterministic analysis enables to capture both particular deformation modes of individual rough surfaces and also statistical deformation regimes, which involve averaging over a big number of roughness realizations. Two regimes of contact area growth were identified: the Hertzian regime at light loads at the scale of a single asperity, and the linear regime at higher loads involving multiple contacting asperities. The transition between the regimes occurs at the load which depends on the second and the fourth spectral moments. It is shown that at light indentation the radius of circumference delimiting the contact area is always considerably larger than Hertzian contact radius. Therefore, it suggests that there is no scale separation in contact problems at light loads. In particular, the geometrical shape cannot be considered separately from the surface roughness at least for approaching greater than one standard roughness deviation.

physics.app-ph↗
arXiv2018

High-speed and high-responsivity hybrid silicon/black-phosphorus waveguide photodetectors at 2 μm

Silicon photonics is being extended from the near-infrared (near-IR) window of 1.3-1.5 μm for optical fiber communications to the mid-infrared (mid-IR) wavelength-band of 2 μm or longer for satisfying the increasing demands in many applications. Mid-IR waveguide photodetectors on silicon have attracted intensive attention as one of the indispensable elements for various photonic systems. Previously high-performance waveguide photodetectors on silicon were realized for the near-IR window of 1.3-1.5 μm by introducing another semiconductor material (e.g., Ge, and III-V compounds) in the active region. Unfortunately, these traditional semiconductor materials do not work well for the wavelength of ~2 μm or longer because the light absorption becomes very weak. As an alternative, two-dimensional materials provide a new and promising option for enabling active photonic devices on silicon. Here black-phosphorus (BP) thin films with optimized medium thicknesses (~40 nm) are introduced as the active material for light absorption and silicon/BP hybrid ridge waveguide photodetectors are demonstrated with a high responsivity at a low bias voltage. And up to 4.0Gbps data transmission is achieved at 2μm.

physics.app-ph↗
arXiv2018

Phase programming in coupled spintronic oscillators

Neurons in the brain behave as a network of coupled nonlinear oscillators processing information by rhythmic activity and interaction. Several technological approaches have been proposed that might enable mimicking the complex information processing of neuromorphic computing, some of them relying on nanoscale oscillators. For example, spin torque oscillators are promising building blocks for the realization of artificial high-density, low-power oscillatory networks (ON) for neuromorphic computing. The local external control and synchronization of the phase relation of oscillatory networks are among the key challenges for implementation with nanotechnologies. Here we propose a new method of phase programming in ONs by manipulation of the saturation magnetization, and consequently the resonance frequency of a single oscillator via Joule heating by a simple DC voltage input. We experimentally demonstrate this method in a pair of stray field coupled magnetic vortex oscillators. Since this method only relies on the oscillatory behavior of coupled oscillators, and the temperature dependence of the saturation magnetization, it allows for variable phase programming in a wide range of geometries and applications that can help advance the efforts of high frequency neuromorphic spintronics up to the GHz regime.

physics.app-ph↗