Nam, Sungho; de la Rosa, Victor R.; Cho, Yuljae; Hamilton, Rick; Cha, SeungNam; Hoogenboom, Richard; Bradley, Donal(Applied Physics Letters, AIP Publishing, 2019-10-01)[Article]
Thin film interlayer materials inserted at the metal/semiconductor interface provide an effective means to improve charge injection and reduce the threshold voltage for organic field-effect transistors. Here, we report the use of poly(2-alkyl-2-oxazoline) interlayers for gold electrodes within n-type poly[[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)] field-effect transistors. We specifically show that the use of poly(2-ethyl-2-oxazoline) yields a reduction in the work function from 5.07 to 4.73 eV (ΔE = 0.34 eV), an increase in the electron mobility from 0.04 to 0.15 cm2/V s (3.75 times), and a reduction in the threshold voltage from 27.5 to 16.5 V (ΔV = 11 V) relative to bare gold. The alkyl side chain of the poly(2-alkyl-2-oxazoline) has a significant influence on the film microstructure and, as a consequence, also device performance.
Ultra-sensitive solid-state magnetic sensors are in strong demand in many applications where currently available sensors are inadequate. We have used high performance magnetic tunneling junction (MTJ) sensors and pushed the magnetic sensing limit to a high level. We have incorporated double-staged magnetic flux concentrators, one on the MTJ chip level and the other on a more macroscopic level, to amplify the external field of interest. With this approach and undergoing a process of optimization on the flux concentrators, we have increased the sensitivity of the MTJ sensor by a large factor of 517 to 775.4%/Oe in terms of magnetoresistance response. The coercivity of the sensor is only 0.12 Oe. We have achieved a detectable field limit of 30 pT/Hz⎯⎯⎯⎯⎯√ at 10 kHz. We have presented the noise spectrum and the sensitivity spectrum up to a maximum frequency of 100 kHz.
Néel-type magnetic skyrmions in multilayer films have recently attracted significant attention due to their stability at room temperature and low threshold for current-driven motion, offering the potential for the construction of high-speed and high-density spintronic devices. However, to date, research studies reported in the literature have rarely examined the effect of temperature on the formation and behavior of Néel-type skyrmions. Here, we investigate the effect of the temperature on the creation of a skyrmion lattice in [Pt/Co/Ta]10 multilayer samples, using in-situ Lorentz transmission electron microscopy. By imaging the magnetization reversal process from a positive (negative) to a negative (positive) saturation, we find that the skyrmions can be created by nucleation from a ferromagnetic state and by breaking the labyrinth domains under certain external fields. More importantly, we demonstrate that the density of skyrmions in the multilayers not only depend on the external magnetic field, but also depend on the temperature and the thermal history of the materials.
We present a transmissive all-dielectric terahertz (THz) metamaterial half-wave plate with a double-working-layer structure. One layer works as a half-wave plate to enable polarization conversion of the incident THz wave, and the other layer functions as an antireflection layer to improve the transmission. The device is made of pure silicon only and can realize a high-performance polarization conversion at the designed THz frequency. Numerical simulations have been performed to show how the polarization properties of the THz wave can be adjusted by the structural parameters of the metamaterial. With appropriate structural parameters, the transmission for cross-polarization can reach 90%, and the polarization conversion rate can reach almost 100% at the designed operation frequency of 1 THz in simulation. Several samples have been fabricated and characterized, and the experimental results show a cross-polarized transmission of about 80% and a polarization conversion rate of almost 100% and agree well with the simulations.
GaN/AlGaN multiple quantum wells (MQWs) are grown on a (2⎯⎯01)-oriented β-Ga2O3 substrate. The optical and structural characteristics of the MQW structure are compared with those of a similar structure grown on sapphire. Scanning transmission electron microscopy and atomic force microscopy images show that the MQW structure exhibits higher crystalline quality of well-defined quantum wells when compared to a similar structure grown on sapphire. X-ray diffraction rocking curve and photoluminescence excitation analyses confirm the lower density of dislocation defects in the sample grown on a β-Ga2O3 substrate. A detailed analysis of time-integrated and time-resolved photoluminescence measurements shows that the MQWs grown on a β-Ga2O3 substrate are of higher optical quality. Our work indicates that the (2⎯⎯01)-oriented β-Ga2O3 substrate can be a potential candidate for UV vertical emitting devices.
The photoluminescence spectra of mono- and bilayer WS, gated by the ionic liquid, were systematically studied at 77 K. Interesting phenomena, such as a redshift of the exciton peaks and a change in the spectral weight of the exciton, trion, and biexciton peaks, were observed at intermediate doping levels. By increasing the doping level, all the exciton, trion, and biexciton peaks vanished, which is attributed to the phase-space filling effect and the Coulomb screening effect. The variation in the band structure, which was induced by the quantum-confined Stark effect in both the mono- and bilayer WS, was also studied using first-principle calculations.
We report investigations of magnetoresistance (MR) in epitaxial and polycrystalline Fe3O4 films. MR in epitaxial Fe3O4 films exhibits a local maximum at TV and a large value of −20% at 60 K. Based on a 1D half infinite spin chain model, the fitting parameter, which depends on the volume fraction of electronic scattering boundaries, sharply increases below TV with the decreased temperature. We suppose that the twin boundaries formed below TV facilitate the increase in MR and can act as antiphase boundaries (APBs) where the magnetic moments across twin boundaries are coupled antiferromagnetically. Similar MR behavior in Fe3O4(100) and (111) epitaxial films manifests the independence of MR on the spatial distribution of APBs. The outline of normalized MR in the epitaxial films shows a distinct temperature dependence. The temperature dependence may result from the different electronic transport mechanisms in Fe3O4 films. In a polycrystalline Fe3O4 film, MR comes from the disordered distribution of magnetic moments at grain boundaries. The effects of APBs, twin boundaries, and grain boundaries on MR are discussed in detail.
Kirmani, Ahmad R.; Mansour, A. E.; Saidaminov, Makhsud I.; Cui, X.; Shi, D.; Alofi, A.; Losovyj, Ya. B.; Gurung, G.; Paudel, T. R.; Yost, A. J.; Dowben, P. A.; Tsymbal, E. Y.; Amassian, Aram; Katsiev, Khabiboulakh(Applied Physics Letters, AIP Publishing, 2018-07-09)[Article]
The electronic structure of methylammonium lead bromide (CH3NH3PbBr3) single crystals has been investigated through a combination of resonant photoemission and theoretical modeling. There are Pb spectral contributions throughout the valence band. Importantly, the electronic structure at the top of the valence band is found to be dominated by the hybridized Pb-Br bands, not methylammonium bromide. The results line up with the partial density of states obtained from density functional theory and confirm that much of the valence band has some Pb spectral weight.
Magnetic skyrmions are topologically protected nanoscale spin textures exhibiting fascinating physical behaviors. Recent observations of room temperature skyrmions in sputtered multilayer films are an important step towards their use in ultra-low power devices. Such practical applications prefer skyrmions to be stable at zero magnetic fields and room temperature. Here, we report the creation of skyrmion lattices in Pt/Co/Ta multilayers by a scanning local field using magnetic force microscopy tips. We also show that those newly created skyrmion lattices are stable at both room temperature and zero fields. Lorentz transmission electron microscopy measurements reveal that the skyrmions in our films are of Néel-type. To gain a deeper understanding of the mechanism behind the creation of a skyrmion lattice by the scanning of local fields, we perform micromagnetic simulations and find the experimental results to be in agreement with our simulation data. This study opens another avenue for the creation of skyrmion lattices in thin films.
Based on first-principles calculations, the structural, electronic, and optical properties of monolayers PtS2 and PtSe2 are investigated. The bond stiffnesses and elastic moduli are determined by means of the spring constants and strain-energy relations, respectively. Dynamic stability is confirmed by calculating the phonon spectra, which shows excellent agreement with experimental reports for the frequencies of the Raman-active modes. The Heyd-Scuseria-Ernzerhof functional results in electronic bandgaps of 2.66 eV for monolayer PtS2 and 1.74 eV for monolayer PtSe2. G0W0 calculations combined with the Bethe-Salpeter equation are used to predict the optical spectra and exciton binding energies (0.78 eV for monolayer PtS2 and 0.60 eV for monolayer PtSe2). It turns out that the excitons are strongly bound and therefore very stable against external perturbations.
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