Ferromagnetism in semiconductors and oxides: prospects from a ten years' perspective

Over the last decade the search for compounds combining the resources of semiconductors and ferromagnets has evolved into an important field of materials science. This endeavour has been fuelled by continual demonstrations of remarkable low-temperature functionalities found for ferromagnetic structures of (Ga,Mn)As, p-(Cd,Mn)Te, and related compounds as well as by ample observations of ferromagnetic signatures at high temperatures in a number of non-metallic systems. In this paper, recent experimental and theoretical developments are reviewed emphasising that, from the one hand, they disentangle many controversies and puzzles accumulated over the last decade and, on the other, offer new research prospects.Comment: review, 13 pages, 8 figures, 109 reference

DNA methylation profiling to assess pathogenicity of BRCA1 unclassified variants in breast cancer

Germline pathogenic mutations in BRCA1 increase risk of developing breast cancer. Screening for mutations in BRCA1 frequently identifies sequence variants of unknown pathogenicity and recent work has aimed to develop methods for determining pathogenicity. We previously observed that tumor DNA methylation can differentiate BRCA1-mutated from BRCA1-wild type tumors. We hypothesized that we could predict pathogenicity of variants based on DNA methylation profiles of tumors that had arisen in carriers of unclassified variants. We selected 150 FFPE breast tumor DNA samples [47 BRCA1 pathogenic mutation carriers, 65 BRCAx (BRCA1-wild type), 38 BRCA1 test variants] and analyzed a subset (n=54) using the Illumina 450K methylation platform, using the remaining samples for bisulphite pyrosequencing validation. Three validated markers (BACH2, C8orf31, and LOC654342) were combined with sequence bioinformatics in a model to predict pathogenicity of 27 variants (independent test set). Predictions were compared with standard multifactorial likelihood analysis. Prediction was consistent for c.5194-12G>A (IVS 19-12 G>A) (P>0.99); 13 variants were considered not pathogenic or likely not pathogenic using both approaches. We conclude that tumor DNA methylation data alone has potential to be used in prediction of BRCA1 variant pathogenicity but is not independent of estrogen receptor status and grade, which are used in current multifactorial models to predict pathogenicity

On the Assembly of Functionalized CdSe Nanorods

High aspect ratio (AR) CdSe nanorods (NRs) of well-defined sizes were synthesized to optimize the geometries of photovoltaic devices made from these nanorods. Long-range ordering of hexagonal arrays of high AR NRs is achieved by a combination of controlled solvent evaporation and the use of an applied electric field. Regioregular P3HT chains and oligothiophene were functionalized with ligating end-groups to provide contact to the NRs. Vertically oriented assemblies of CdSe NRs functionalized with terthiophene and polythiophene are also obtained. Hexagonal arrays of these nanocomposites were characterized by transmission electron microscopy (TEM). Three types of polythiophenes: poly(3-hexylthiol thiophene), poly(3-hexylamine thiophene), and poly(3-hexylphosphonate thiophene) with thiol, amine, and phosphonate functional groups, were synthesized to anchor to the NRs. This led to a thin layer of p-type conducting polymer covering tips of the n-type inorganic NRs forming end-to-end assembly. Ternary nanocomposites of CdSe-polythiophene-graphene were obtained via ππ stacking. These oriented CdSe NRs-polythiophenes nanocomposites have potential applications in organic-inorganic hybrid bulk heterojunction photovoltaic devices

Electrical characterization of 6H crystalline silicon carbide

Crystalline silicon carbide (SiC) substrates and epilayers, undoped as well as n- and p-doped, have been electrically characterized by performing Hall effect and resistivity measurements (van der Pauw) over the temperature range of approximately 85 K to 650 K (200 K to 500 K for p-type sample). By fitting the measured temperature dependent carrier concentration data to the single activation energy theoretical model: (1) the activation energy for the nitrogen donor ranged from 0.078 eV to 0.101 eV for a doping concentration range of 10(exp 17) cm(exp -3) to 10(exp 18) cm(exp -3) and (2) the activation energy for the aluminum acceptor was 0.252 eV for a doping concentration of 4.6 x 10(exp 18) cm(exp -3). By fitting the measured temperature dependent carrier concentration data to the double activation energy level theoretical model for the nitrogen donor: (1) the activation energy for the hexagonal site was 0.056 eV and 0.093 eV corresponding to doping concentrations of 3.33 x 10 (exp 17) cm(exp -3) and 1.6 x 10(exp 18) cm(exp -3) and (2) the activation energy for the cubic site was 0.113 and 0.126 eV corresponding to doping concentrations of 4.2 x 10(exp 17) cm(exp -3) and 5.4 x 10(exp 18) cm(exp -3)

Synthesis of fluoromodified carbon rich electron acceptors and exploration of their structural, electronic, and device properties

Includes bibliographical references.2020 Summer.The electronic and structural characterization of fluoro-modified carbon-rich compounds is critical to the successful implementation of these materials by physicists, biochemists, materials scientists, medicinal chemists, and most significantly for this work, organic electronics chemists. By adding powerful electron-withdrawing groups, the electron acceptor and solid-state structural properties of carbon rich substrates such as polyaromatic hydrocarbons (PAHs) and fullerenes can be improved, making these derivatives attractive semiconductor materials for organic electronics applications. This work will discuss research which has focused on expanding the library of electron acceptor compounds, elucidating the electronic and structural properties of those compounds, and exploring their physicochemical properties, focusing on properties that are important for the performance of organic electronic devices. This was accomplished by exploring reaction conditions which had not been previously reported at pressures and temperatures exceeding the operational limits of conventional reactors, developing purification methods that allow for chromatographic separation of constitutional isomers, and structural characterization of those purified materials by mass spectrometry, NMR, and most importantly X-ray crystallography. As a complement to this research, the stability of organic electronic active layers was studied to better understand how organic semiconductor active layer's degradation affects device performance over time and to better inform which active layer material properties should be pursued. Based on those findings and literature precedent, one family of compounds, C60 and C70 fauxhawk fullerenes, found to have favorable characteristics were then utilized in OFET devices as n-type semiconductors resulting in record-setting charge carrier mobilities

Upconverting Phosphor Technology: Exceptional Photoluminescent Properties Light Up Homogeneous Bioanalytical Assays

The aim of the present study was to demonstrate the wide applicability of the novel photoluminescent labels called upconverting phosphors (UCPs) in proximity-based bioanalytical assays. The exceptional features of the lanthanide-doped inorganic UCP compounds stem from their capability for photon upconversion resulting in anti-Stokes photoluminescence at visible wavelengths under near-infrared (NIR) excitation. Major limitations related to conventional photoluminescent labels are avoided, rendering the UCPs a competitive next-generation label technology. First, the background luminescence is minimized due to total elimination of autofluorescence. Consequently, improvements in detectability are expected. Second, at the long wavelengths (>600 nm) used for exciting and detecting the UCPs, the transmittance of sample matrixes is significantly greater in comparison with shorter wavelengths. Colored samples are no longer an obstacle to the luminescence measurement, and more flexibility is allowed even in homogeneous assay concepts, where the sample matrix remains present during the entire analysis procedure, including label detection. To transform a UCP particle into a biocompatible label suitable for bioanalytical assays, it must be colloidal in an aqueous environment and covered with biomolecules capable of recognizing the analyte molecule. At the beginning of this study, only UCP bulk material was available, and it was necessary to process the material to submicrometer-sized particles prior to use. Later, the ground UCPs, with irregular shape, wide size-distribution and heterogeneous luminescence properties, were substituted by a smaller-sized spherical UCP material. The surface functionalization of the UCPs was realized by producing a thin hydrophilic coating. Polymer adsorption on the UCP surface is a simple way to introduce functional groups for bioconjugation purposes, but possible stability issues encouraged us to optimize an optional silica-encapsulation method which produces a coating that is not detached in storage or assay conditions. An extremely thin monolayer around the UCPs was pursued due to their intended use as short-distance energy donors, and much attention was paid to controlling the thickness of the coating. The performance of the UCP technology was evaluated in three different homogeneous resonance energy transfer-based bioanalytical assays: a competitive ligand binding assay, a hybridization assay for nucleic acid detection and an enzyme activity assay. To complete the list, a competitive immunoassay has been published previously. Our systematic investigation showed that a nonradiative energy transfer mechanism is indeed involved, when a UCP and an acceptor fluorophore are brought into close proximity in aqueous suspension. This process is the basis for the above-mentioned homogeneous assays, in which the distance between the fluorescent species depends on a specific biomolecular binding event. According to the studies, the submicrometer-sized UCP labels allow versatile proximity-based bioanalysis with low detection limits (a low-nanomolar concentration for biotin, 0.01 U for benzonase enzyme, 0.35 nM for target DNA sequence).Siirretty Doriast

OPTICAL CHARACTERIZATION OF INHOMOGENEITIES IN BLUE-EMITTING INGAN/GAN MQWS

The growth of blue-emitting InGaN/GaN MQWs, the system setup of a low temperature PL/EL/IV system for temperature dependent PL/EL/IV spectroscopy, and the system setup of a CLSM with nanometer-scale spectrum measurement and TRPL measurement abilities are described. A range of temperature-dependent PL experimental work, CLSM imaging experimental work and TRPL experimental work on blue-emitting InGaN/GaN MQWs are presented. In temperature-dependent PL measurements, the decreasing of spectrum- integrated PL intensity with increasing temperature is explained with a two-nonradiative- channel model, in which the two nonradiative channels correspond to the thermal activation of carriers out of the strongly localized states and the weakly localized states, respectively. The ‘S-shaped’ red-blue-red shift of PL peak energy and the ‘inverse S- shaped’ change of PL FWHM when temperature increases from 10 K to 300 K are explained with carrier localization and carrier dynamics. CLSM imaging and nanometer-scale PL spectral measurements show that the PL intensity fluctuates in micrometer scale, and that the bandgap energy in bright region is tens of meV smaller than that in dark region. The small-bandgap-energy regions are localization centers which limit the diffusion of the carriers and prevent carriers from diffusing to the NRRCs. Nanometer-scale TRPL measurements are conducted on blue-emitting InGaN/GaN MQWs for the first time, as far as the author knows. The measurements show that both bright region and dark region are characterized by two lifetimes: fast decay lifetime t1 is smaller than 3 ns and slow decay lifetime t2 is longer than 10 ns. The fast decay with shorter lifetime t1 corresponds to the carrier localization in weakly localized states, where the radiative recombination is more quenched by NRRCs and also competes with carrier transfer intro strongly localized states. And the slow decay with longer lifetime t2 corresponds carrier localization in strongly localized states. The fact that both fast decay and slow decay exist in both bright region and dark region indicates that both bright region and dark region has small bandgap energy fluctuation in themselves. Measurements show that the slow decay lifetime t2 in bright region is longer than that in dark region, indicating a higher probability of nonradiative recombination in dark region or carrier transporting from dark region to bright region. Measurements show that larger bandgap energy difference between small- bandgap-energy regions and large-bandgap-energy regions provides stronger carrier localization effect, via the presence of higher CLSM image average intensity, larger PL intensity ratio and longer smaller-bandgap-energy slow decay lifetime t2 when larger bandgap energy difference occurs. The effect of MOCVD growth parameters on MQW bandgap energy fluctuations and average intensity was analyzed. It was found out that by increasing growth pressure, decreasing growth rate, increasing growth temperature, increasing effective V/III ratio, and increasing gas speed, the bandgap energy difference between bright region and dark region increases, leading to higher average PL intensity

μLEDs for optogenetics

Optogenetics is unfolding new ways for us to study the nervous system and could one day be a standard approach to treat neurological diseases like epilepsy. To selectively study the effects on a subcellular level, microscopic light sources are needed. Nanostructure, light-emitting diodes (LEDs) can realize this criteria but processing to connect and protect them is necessary before any fruitful optogenetic tests can be conducted. In this work, micron sized, III-nitride, LED light sources were created using microfabrication techniques such as lithography, etching and thin film deposition. Experimental biointegration and passivation schemes were then used to build a prototype optogenetic device for stimulation of primary neurons grown [i]in vitro[/i] onto the device, in close proximity to the light emitters. Favorable electrical and optical characteristics were obtained for the individual nanostructure LEDs, lighting up brightly at a wavelength around 470 nm. However, larger devices revealed process related and uniformity challenges to overcome. Additionally, the biointegration design would prove too complex and in need of further improvement. This effort, while not outputting a fully functioning device, has contributed to development of the utilized nanostructure LED technology so that we may see more of it in the future.Imagine if I said there was a way to control brain cells with light. You might first think of the scary mind control applications but would you also consider the potential to one day eradicate neurological diseases like epilepsy? Optogenetics is a fairly new technique in medical science and it is still a long way away from fulfilling either of these scenarios but that makes it no less interesting.[/b] Today, optogenetics allow researchers to control nerve impulses by simply shining a light on cells that have been genetically modified with light sensitive properties of fluorescent algae. A common practice in optogenetics is to make cells sensitive to blue light and as luck would have it, blue light-emitting diodes, or LEDs for short, are relatively mature and straight forward to make with high quality. However, to study optogenetic effects subcellulary, for example how stimulation affects individual synapses, light sources would have to be microscopically tiny and this is where we come in. By using tapered hexagonal platelet, gallium nitride μLEDs, less than 1 μm in diameter, situated on a small sapphire chip, we set out to make a prototype device for high resolution optogenetics. LEDs were processed in Lund Nano Lab using microfabrication equipment for lithography, etching and thin film deposition before being characterized in a probe station rig. As we also wanted to be able to test actual nerve cell stimulation, we attempted to package the LEDs and passivate them for a biological environment with conducting fluids and sensitive nerve cells, which would have been grown directly onto the device, in close proximity too the LEDs. Initial testing of the single platelet LEDs showed very promising electrical properties such as the clearly rectifying diode behavior in addition to a rather extraordinary visible light output for such small light source. Continued testing though, revealed short circuiting issues for larger LEDs with several platelets being coupled together in parallel. These issues could be explained by minute variations in original platelet height and be amended with future processing tweaks. Furthermore, actual optogenetic testing had to be abandoned as the complex packaging scheme, featuring thin film oxide passivated, wire bonds, would end up malfunctioning, suggesting a redesign is needed to remove unnecessary points of failure. While we did not fully actualize the very ambitious goals we set out to achieve, our findings have undoubtedly aided in the understanding and fixing of issues with the platelet μLED technique so that development of it can progress. In a broader perspective, the technologies we explored are still highly interesting, combined and individually. Development of smaller LEDs and their use in more and more impressive optogenetic studies are published on a regular basis and inorganic μLED products are even starting to find their way onto the consumer electronics market in direct emitting, high resolution displays. To conclude, I am certain that even if this short text would have been the first time you heard about these topics, it will definitely not be the last

Investigation of wide bandgap semiconductors for room temperature spintronic, and photovoltaic applications

Suitability of wide bandgap semiconductors for room temperature (RT) spintronic, and photovoltaic applications is investigated. Spin properties of metal-organic chemical vapor deposition (MOCVD) – grown gadolinium-doped gallium nitride (GaGdN) are studied and underlying mechanism is identified. GaGdN exhibits Anomalous Hall Effect at room temperature if it contains oxygen or carbon atoms but shows Ordinary Hall Effect in their absence. The mechanism for spin and ferromagnetism in GaGdN is a combination of intrinsic, metallic conduction, and carrier-hopping mechanisms, and is activated by oxygen or carbon centers at interstitial or similar sites. A carrier-related mechanism in MOCVD-grown GaGdN at room temperature makes it a suitable candidate for spintronic applications. Zinc oxide (ZnO) doped with transition metals such as nickel and manganese and grown by MOCVD is investigated, and bandgap tunability is studied. A bandgap reduction with transition metal doping is seen in ZnO with dilute doping of nickel or manganese. Transition metals could introduce energy states in ZnO that result in a bandgap reduction and could be tuned and controlled by growth conditions and post-growth processing such as annealing, for spintronic and photovoltaic applications”--Abstract, page iii

Theory of ferromagnetic (III,Mn)V semiconductors

The body of research on (III,Mn)V diluted magnetic semiconductors initiated during the 1990's has concentrated on three major fronts: i) the microscopic origins and fundamental physics of the ferromagnetism that occurs in these systems, ii) the materials science of growth and defects and iii) the development of spintronic devices with new functionalities. This article reviews the current status of the field, concentrating on the first two, more mature research directions. From the fundamental point of view, (Ga,Mn)As and several other (III,Mn)V DMSs are now regarded as textbook examples of a rare class of robust ferromagnets with dilute magnetic moments coupled by delocalized charge carriers. Both local moments and itinerant holes are provided by Mn, which makes the systems particularly favorable for realizing this unusual ordered state. Advances in growth and post-growth treatment techniques have played a central role in the field, often pushing the limits of dilute Mn moment densities and the uniformity and purity of materials far beyond those allowed by equilibrium thermodynamics. In (III,Mn)V compounds, material quality and magnetic properties are intimately connected. In the review we focus on the theoretical understanding of the origins of ferromagnetism and basic structural, magnetic, magneto-transport, and magneto-optical characteristics of simple (III,Mn)V epilayers, with the main emphasis on (Ga,Mn)As. The conclusions we arrive at are based on an extensive literature covering results of complementary ab initio and effective Hamiltonian computational techniques, and on comparisons between theory and experiment.Comment: 58 pages, 49 figures Version accepted for publication in Rev. Mod. Phys. Related webpage: http://unix12.fzu.cz/ms