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Radiative cooling forms an emerging direction in which objects are passively cooled via thermal radiation to cold space. Cooling materials should provide high thermal emissivity (infrared absorptance) and low solar absorptance, making cellulose an ideal and sustainable candidate. Broadband solar-reflective or transparent coolers are not the only systems of interest, but also more pleasingly looking colored systems. However, solutions based on wavelength-selective absorption generate not only color but also heat and thereby counteract the cooling function. Intended as coatings for solar cells, we demonstrate a transreflective cellulose material with minimal solar absorption that generates color by wavelength-selective reflection, while it transmits other parts of the solar spectrum. Our solution takes advantage of the ability of cellulose nanocrystals to self-assemble into helical periodic structures, providing nonabsorptive films with structurally colored reflection. Application of violet-blue, green, and red cellulose films on silicon substrates reduced the temperature by up to 9 degrees C under solar illumination, as result of a combination of radiative cooling and reduced solar absorption due to the wavelength-selective reflection by the colored coating. The present work establishes self-assembled cellulose nanocrystal photonic films as a scalable photonic platform for colored radiative cooling. Funding Agencies|Knut and Alice Wallenberg foundation; Linkoping University; Wallenberg Wood Science Center; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Foundation for Strategic Research; Wenner-Gren Foundations; Olle Engkvist Foundation [194-0679]; Swedish Research Council

Radiative cooling passively removes heat from objects via emission of thermal radiation to cold space. Suitable radiative cooling materials absorb infrared light while they avoid solar heating by either reflecting or transmitting solar radiation, depending on the application. Here, we demonstrate a reflective radiative cooler and a transparent radiative cooler solely based on cellulose derivatives manufactured via electrospinning and casting, respectively. By modifying the microstructure of cellulose materials, we control the solar light interaction from highly reflective (> 90%, porous structure) to highly transparent (approximate to 90%, homogenous structure). Both cellulose materials show high thermal emissivity and minimal solar absorption, making them suitable for daytime radiative cooling. Used as coatings on silicon samples exposed to sun light at daytime, the reflective and transparent cellulose coolers could passively reduce sample temperatures by up to 15 degrees C and 5 degrees C, respectively. Funding Agencies|Linkoping University

Dynamic control of structural colors across the visible spectrum with high brightness has proven to be a difficult challenge. Here, this is addressed with a tuneable reflective nano-optical cavity that uses an electroactive conducting polymer (poly(thieno[3,4-b]thiophene)) as spacer layer. Electrochemical doping and dedoping of the polymer spacer layer provides reversible tuning of the cavitys structural color throughout the entire visible range and beyond. Furthermore, the cavity provides high peak reflectance that varies only slightly between the reduced and oxidized states of the polymer. The results indicate that the polymer undergoes large reversible thickness changes upon redox tuning, aided by changes in optical properties and low visible absorption. The electroactive cavity concept may find particular use in reflective displays, by opening for tuneable monopixels that eliminate limitations in brightness of traditional subpixel-based systems. Funding Agencies|Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation; Swedish Research Council (VR)Swedish Research Council; Wenner-Gren Foundations; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

The currently dominant thermoelectric (TE) materials used in low to medium temperature range contain Tellurium that is rare and mild-toxic. Silicon is earth abundant and environment friendly, but it is characterized by a poor TE efficiency with a low figure of merit, ZT . In this work, we report that ZT of amorphous silicon (a-Si) thin films can be enhanced by 7 orders of magnitude, reaching ∼0.64 ± 0.13 at room temperature, by means of arsenic ion implantation followed by low-temperature dopant activation. The dopant introduction employed represents a highly controllable doping technique used in standard silicon technology. It is found that the significant enhancement of ZT achieved is primarily due to a significant improvement of electrical conductivity by doping without crystallization so as to maintain the thermal conductivity and Seebeck coefficient at the level determined by the amorphous state of the silicon films. Our results open up a new route towards enabling a-Si as a prominent TE material for cost-efficient and environment-friendly TE applications at room temperature.

We report on an efficient and highly thermally stable doping with iodine on ink-jet printed graphene films. The films consist of pristine few-layer graphene nano-platelates (p-GNPs) that are randomly stacked. With iodine doping simply by soaking in aqueous iodine solution, the printed p-GNPs films are enhanced in electrical conductivity by up to around 2 times. The doping effect exhibits excellent thermal stability up to 500 °C under high vacuum condition (10 −6 mBar) evidenced by electrical and spectroscopic means. Furthermore, the doping of iodine leads to a slight increment of work function by 0.07 eV. Using depth profile measurements, it is found that iodine species diffuse deeply into the films and likely intercalate between two adjacent p-GNPs which interpret the aforementioned efficient enhancement and thermal stability of the doping effect. The reported doping scheme offers a viable low-temperature optimization method for conductive electrodes with p-GNPs in the application of printed devices.

In this letter, we report the theoretical study on phonon transport in monocrystalline silicon thin-films having unfilled or metal-filled circular holes (i.e., phononic crystals, PnCs) and show that the thermal conductivity, κ, at 1 K can be maximally reduced by using a multiscale structure, which allows us control over the porosity of the structure. The circular scatterers are placed in the square (SQ) and hexagonal (HX) pattern with a fixed 100 nm interhole spacing, and the pit diameter is varied between 10 and 90 nm. Each of the corresponding silicon PnCs shows reduced κ compared to the unpatterned film. The SQ-PnC having tungsten-filled pits shows the greatest reduction in κ when we consider only the effects of coherent scattering. Furthermore, we have computed κ for the PnC where the unit cell, of 100 nm and 500 nm sizes, comprises the Sierpinski gasket (SG) with circular holes of different diameters (depending on the fractal order) in the same cell. It is observed that the κ for the 2nd (100 nm cell) and 3rd order (500 nm cell) SG-PnC is comparable to the SQ- and HX-PnC with a pit diameter of 90 nm. When we add the effect of the diffuse boundary scattering in κ, there is a lowering in κ compared to that when only the coherent effects are considered. The additional κ-reduction due to boundary scattering for the SQ-PnC and HX-PnC (both with 90 nm diam) as well as the 2nd and 3rd order SG-PnCs is 47%, 40%, 80%, and 60%, respectively.In this letter, we report the theoretical study on phonon transport in monocrystalline silicon thin-films having unfilled or metal-filled circular holes (i.e., phononic crystals, PnCs) and show that the thermal conductivity, κ, at 1 K can be maximally reduced by using a multiscale structure, which allows us control over the porosity of the structure. The circular scatterers are placed in the square (SQ) and hexagonal (HX) pattern with a fixed 100 nm interhole spacing, and the pit diameter is varied between 10 and 90 nm. Each of the corresponding silicon PnCs shows reduced κ compared to the unpatterned film. The SQ-PnC having tungsten-filled pits shows the greatest reduction in κ when we consider only the effects of coherent scattering. Furthermore, we have computed κ for the PnC where the unit cell, of 100 nm and 500 nm sizes, comprises the Sierpinski gasket (SG) with circular holes of different diameters (depending on the fractal order) in the same cell. It is observed that the κ for the 2nd (1...

We have demonstrated that deep level traps created by B implantation can reduce the contact resistance by forming an additional path for electron transport via trap assisted tunneling in GaN and AlGaN/GaN heterostructures. B implantation by plasma-immersion ion implantation creates deep level traps 0.36 eV below the conduction band edge to a shallow depth (10–25 nm) in the structure. These traps act as efficient percolation path for electrons between the TiN Ohmic contact and the active region, which can be bulk GaN or a two-dimensional electron gas (2DEG) formed at the AlGaN/GaN heterostructure. The improved Ohmic behavior is manifested as reduced specific contact resistivity and normalized contact resistance. The specific contact resistance and the normalized contact resistance are found to decrease by 59% (30%) and 86% (51%), respectively, for bulk GaN (AlGaN/GaN). The effect of B implantation on the sheet resistance of the 2DEG is insignificant when the implantation energy and post-implantation annealing time are controlled optimally.

Three strong solid state emissive cyclometalated platinum(II) complexes [Pt(C⁁N) (CH⁁N) (Cl)] (1) (C⁁N/CH⁁N = 2-phenylpyridine, C⁁N = bidentate and CH⁁N = monodentate), [Pt(C⁁N) (P⁁P)]Cl [P⁁P = bis(diphenylphosphino)ethane (2) and cis-1,2-bis(diphenylphosphino)ethene (3)] were reported. These were identified as ‘Aggregation Induced Emission (AIE)’ active complexes based on controlled experiments. Cytotoxicity and cell imaging have been studied for the complex 2.

A surface emitting spin-polarized light emitting diode (Spin-LED) is considered for the study of the device geometry dependence of the output circular polarization It is found that the output circular polarization is maximum at the outer region of the circular non-FM Ohmic contact to the p-region, which delineates radial extent of the active region in the intrinsic layer. We observe that the output circular polarization increase as a function of the p-contact radius when the contact size is small but for large values of the p-contact radius the reverse trend is noted. An enhancement of the spin diffusion length increases the degree of polarization of the emitted circular light. The enhancement however is observed to fall off for still higher values of spin lifetime.

The inherent network of nanopores and voids in silicon dioxide (SiO2) is generally undesirable for aspects of film quality, electrical insulation and dielectric performance. However, if we view these pores as natural nano-patterns embedded in a dielectric matrix then that opens up new vistas for exploration. The nano-pattern platform can be used to tailor electrical, optical, magnetic and mechanical properties of the carrier film. In this article we report the tunable electrical properties of thermal SiO2 thin-film achieved through utilization of the metal-nanopore network where the pores are filled with metallic Titanium (Ti). Without any intentional chemical doping, we have shown that the electrical resistivity of the oxide film can be controlled through physical filling up of the intrinsic oxide nanopores with Ti. The electrical resistivity of the composite film remains constant even after complete removal of the metal from the film surface except the pores. Careful morphological, electrical and structural analyses are carried out to establish that the presence of Ti in the nanopores play a crucial role in the observed conductive nature of the nanoporous film.

Background Antimicrobial resistance (AMR) is a major threat to global public health. Medicinal plants have long been used as remedies for infectious diseases by native cultures around the world and have the potential for providing effective treatments for antibiotic-resistant infections. Rhamnus californica (Rhamnaceae) and Umbellularia californica (Lauraceae) are two indigenous California plant species historically used by Native Americans to treat skin, respiratory and gastrointestinal infections. This study aimed to assess the in vitro antimicrobial activity of methanolic extracts of leaves and bark of R. and U. californica against methicillin-resistant Staphylococcus aureus (MRSA) and other Gram-positive and Gram-negative bacteria. Methods Methanolic extracts of leaves and bark of R. and U. californica were prepared by soxhlet extraction and evaluated for their antimicrobial activity against Bacillus cereus, Streptococcus pyogenes, Mycobacterium smegmatis, Staphylococcus aureus, MRSA, Escherichia coli and Pseudomonas aeruginosa using disc diffusion and minimal inhibitory concentration (MIC) assays. Chemical profiling of the extracts was performed using standard methods. Results All extracts inhibited the growth of MRSA and other Gram-positive bacteria with MICs of 3.3-6.0 mg/ml. Gram-negative organisms were unaffected by these extracts. U. californica extracts (leaves and bark) had the lowest MIC values. Chemical profiling detected the presence of quinones, alkaloids, flavonoids, cardenolides, tannins and saponins in these extracts. Our study is the first to report the antimicrobial properties of R. and U. californica and illustrates their promising anti-MRSA potential. Conclusions Our results give scientific credence to the traditional medicinal uses of these plants by the indigenous peoples of California. Further investigation of the secondary metabolites responsible for the antimicrobial activity of these extracts against MRSA is warranted.

We have demonstrated a type-II band-aligned heterostructure between pulsed laser deposited topological insulator bismuth telluride and metal organic-chemical-vapour deposited GaN on a sapphire substrate. The heterostructure shows a large valence band-offset of 3.27 eV as determined from x-ray photoelectron spectroscopy, which is close to the bandgap of GaN (3.4 eV). Further investigation using x-ray diffraction, Raman spectroscopy, and energy-dispersive x-ray spectrum reveals the stoichiometric and material properties of bismuth telluride on GaN. Steady state photon emission from GaN is found to be modulated by the charge transfer process due to diffusion across the junction. The time constant involved with the charge transfer process is found to be 0.6 ns by transient absorption spectroscopy. The heterostructure can be used for designing devices with different functionalities and improving the performance of the existing devices on GaN. (C) 2015 AIP Publishing LLC.

We have investigated Schottky contacts on GaN and observed that Fermi level pinning is dominant at the metal/GaN interface with a pinning factor of 0.23. A methodology to solve the problem by introducing a thin layer of MgO between the metal and the semiconductor is demonstrated here. It is observed that the insertion of a thin layer of the insulator prevents the metal wave-function penetration into the semiconductor band gap which in turn helps in the Fermi level depinning for GaN. We have particularly demonstrated the Fermi level depinning for ferromagnetic Schottky contact Fe and shown its usefulness for electrical spin injection and detection. The resistance-area product of an as deposited Fe/GaN contact is found to be too high for efficient spin injection and detection. It is improved considerably by using a 3 nm layer of MgO and the effective barrier height is reduced to 0.4 eV. We have further investigated the influence of low work function metal Gd and found it is possible to do barrier height engineering when deposited in conjunction with other metals. (C) 2013 Elsevier B.V. All rights reserved.

We have demonstrated electrical spin-injection from GaCrN dilute magnetic semiconductor (DMS) in a GaN-based spin light emitting diode (spin-LED). The remanent in-plane magnetization of the thin-film semiconducting ferromagnet has been used for introducing the spin polarized electrons into the non-magnetic InGaN quantum well. The output circular polarization obtained from the spin-LED closely follows the normalized in-plane magnetization curve of the DMS. A saturation circular polarization of similar to 2.5% is obtained at 200 K. (C) 2013 AIP Publishing LLC.

We have investigated ferromagnetic (Fe, Co and Ni) Schottky diodes on heavily n-doped GaN. We have grown, fabricated, characterized, and analyzed these diodes to extract junction parameters and ascertain transport mechanism. We have observed simple thermionic emission failed to explain the experimental characteristics. Thermionic emission with Gaussian distribution for barrier inhomogeneity can explain the data for high bias voltages. It is observed other transport mechanisms at low bias voltages are important. Direct tunneling have been found to contribute significantly for Fe/GaN Schottky diodes. However, trap assisted tunneling is found to be dominant for Co/GaN and Ni/GaN devices under same condition.

We demonstrate a method for nanowire formation by natural selection during wet anisotropic chemical etching in boiling phosphoric acid. Nanowires of sub-10 nm lateral dimensions and lengths of 700 nm or more are naturally formed during the wet etching due to the convergence of the nearby crystallographic hexagonal etch pits. These nanowires are site controlled when formed in augmentation with dry etching. Temperature and power dependent photoluminescence characterizations confirm excitonic transitions up to room temperature. The exciton confinement is enhanced by using two-dimensional confinement whereby enforcing greater overlap of the electron-hole wave-functions. The surviving nanowires have less defects and a small temperature variation of the output electroluminescent light. We have observed superluminescent behaviour of the light emitting diodes formed on these nanowires. There is no observable efficiency roll off for current densities up to 400 A/cm(2). Published by AIP Publishing.

GaN is one of the most important semiconductor materials, for high-speed and many other applications [1]. It also happens to have small spin-orbit coupling, which suggests that spintronic functionality may be effectively integrated on to it. This would require optimized ferromagnet-GaN Schottky contacts for spin injection or detection [2]. We choose iron (Fe) as a high spin polarization material and investigate for the first time the characteristics of Fe/n-GaN Schottky diodes. We have determined Schottky barrier height, barrier inhomogeneity, and ideality factor from current-voltage (I-V ) and capacitancevoltage (C-V ) characteristics. We have also determined the effect of temperature on these parameters. We have observed barrier height inhomogeneity which is found to match well with a Gaussian Distribution (GD). We have also extracted Richardson’s constant for these diodes. Though thermionic emission is found to be the dominant mode of carrier transport under forward bias condition, electron tunneling may contribute significantly under reverse biased condition. The heterostructure of the sample used in our study is shown in Fig. 1(a). An undoped GaN buffer layer of 7 μm is grown on sapphire (Al2O3) substrate by hydride vapor-phase-epitaxy (HVPE). The Si-doped (n-type) GaN layer (2 μm) is then grown by metal-organic-chemical-vapor-deposition (MOCVD) having a doping density of 1-2×10 cm−3. The sample is cleaned and native oxide is removed before depositing Ti(20 nm)/Al(150 nm)/Ni(50 nm)/Au(125 nm) metal stack by electron-beam (e-beam) evaporation and annealing at 775 C using rapid thermal annealing (RTA) for the annular low resistance ohmic contact. Fe(50 nm)/Ti (6 nm)/Au(125 nm) Schottky contacts 60 μm in diameter are then deposited at the centre by e-beam evaporation. A micro-photograph of the device is shown in Fig. 1(b).

We have demonstrated an electrically injected ultra-low threshold (8.9 nA) room temperature InGaN/GaN based lateral nanowire laser. The nanowires are triangular in shape and survived naturally after etching using boiling phosphoric acid. A polymethyl methacrylate (PMMA) and air dielectric distributed mirror provide an optical feedback, which together with one-dimensional density of states cause ultra-low threshold lasing. Finite difference eigen-mode (FDE) simulation shows that triangular nanowire cavity supports single dominant mode similar to TE01 that of a corresponding rectangular cavity with a confinement factor of 0.18. (C) 2015 AIP Publishing LLC.

We have theoretically designed and experimentally demonstrated a multiterminal lateral magnetoresistive device using electrical spin injection and detection in semiconductor, which can act as a memory array. The memory state is associated with the direction of the magnetization state. The state is read out from clear transition in current both in magnitude and direction depending on the relative magnetization direction. The current shows a plateau in varying magnetic field regions indicating stable magnetization direction due to differences in coercivity. The device characteristics are well explained in the framework of two-dimensional spin transport and spin-dependent tunneling. (C) 2013 The Japan Society of Applied Physics

We report room temperature ferromagnetism in crystalline GaCrN prepared by Cr deposition and drive-in diffusion with Curie temperature much above 300 K. The Curie temperature increases with increasing active Cr concentration. Cr doped GaN acts as an n-type material with significant increase in electron carrier concentration due to the presence of Cr. Optical property of GaCrN is found to be very similar to GaN with an additional peak at 3.29 eV due to Cr. The hysteresis measurements show that the ferromagnetic ordering is maintained up to 300 K with no significant change in saturation magnetization. Copyright 2013 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4799716]

We have studied small signal frequency response of a spin laser. We have shown that the response is characterized by two distinct resonant peaks corresponding to the two polarization modes of the spin laser. It is observed that the modulation bandwidth of a spin laser can be smaller or larger than that of a conventional laser depending upon the current bias and spin relaxation time constant. A small value for spin relaxation constant may not be detrimental for modulation bandwidth. This anomalous observation is explained by considering both the amplitude and phase response of the two polarization modes. A spin laser can act as a combination of low-pass and bandpass filters. The passband frequency range is tunable by external bias. We have also studied the evolution of resonant peaks and modulation bandwidth as a function of spin relaxation time constant.

Analytical model equations have been derived from the fundamental principles for effective relative permeability of circular multiple-inclusion magnetic structures like multiple split-ring resonators, spiral resonators and labyrinth resonators, useful for LHM applications. Tailoring of their resonant frequency, their bandwidth and their effective negative permeability is investigated. Thereafter, a suitable structure for operation at millimeter-wave frequency has been explored. When such microstructures are used in a periodic array, the dependence of resonant frequency and bandwidth on the lattice parameter has further been studied.

Computer-aided analytical studies and physical understanding of important characteristics of some commonly used left-handed material (LHM) structures (wire-array and split-ring resonator), to realize negative effective ­permittivity and permeability leading to negative refractive index over a desired frequency band have been reported in this ­paper. The transmission of electromagnetic wave through LHMs and right-handed materials (RHMs) has some ­fundamental differences. This issue of electromagnetic wave transmission through LHM has been characterized, and a ­physical ­insight of the phenomena has also been included. The LHM characterization using computer-aided ­analytical ­modeling shows good agreement with the previous numerical simulation and experimental results reported for such microstructures operating at microwave frequencies.