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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.

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.

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 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.

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.