Search

Nanoscale engineered materials combined with wearable wireless technologies can deliver a new level of health monitoring. A reduced graphene oxide-nylon composite material is developed and tested, demonstrating its usefulness as a material for sensors in wearable, long-term electrocardiogram (ECG) monitoring via a comparison to one of the widely used ECG sensors. The structural analysis by scanning electron microscopy and atomic force microscopy shows a limited number of defects on a macroscopic scale. Fourier transform infrared and Raman spectroscopy confirm the presence of rGOx, and the ratio of D- and G-features as a function of thickness correlates with the resistivity analysis. The negligible effect of the defects and the tunability of electrical and optical properties, together with live ECG data, demonstrate its signal transduction capability. Other potential electronic and optical sensor uses beyond ECG are possible, given the controllable nature of the heterostructures and the correlation of transport and optical properties. [ABSTRACT FROM AUTHOR]

The temperature- and bias-dependent photocurrent spectra of very long wavelength GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) are studied using spectroscopic measurements and corresponding theoretical calculations. It is found that the peak response wavelength will shift as the bias and temperature change. Aided by band structure calculations, we propose a model of the double excited states and explain the experimental observations very well. In addition, the working mechanisms of the quasi-bound state confined in the quantum well, including the processes of tunneling and thermionic emission, are also investigated in detail. We confirm that the first excited state, which belongs to the quasi-bound state, can be converted into a quasi-continuum state induced by bias and temperature. These obtained results provide a full understanding of the bound-to-quasi-bound state and the bound-to-quasi-continuum state transition, and thus allow for a better optimization of QWIPs performance. [ABSTRACT FROM AUTHOR]

Continuous-time core-level photon-stimulated desorption (PSD) spectroscopy was used to study the soft x-ray-induced reactions of CF3Br molecules adsorbed on Si(111)-7×7 near the Si(2p) edge (98-110 eV). The monochromatic synchrotron radiation was employed as a soft x-ray light source in the photon-induced reactions and also as a probe for investigating the produced fluorination states of the bonding surface Si atom in the positive-ion PSD spectroscopy. Several different surface coverages were investigated. The PSD spectra from the low-CF3Br-covered surfaces show the production of surface SiF species, while those from the high-CF3Br-covered surfaces depict the formation of surface SiF, SiF2, and SiF3 species. The photolysis cross section of the submonolayer CF3Br-covered surface is determined as ∼4.3×10-18 cm2. A comparison with the results on CF3Cl/Si(111)-7×7 surface is discussed. [ABSTRACT FROM AUTHOR]

Despite the unparalleled mechanical properties of carbon nanotubes (CNTs), experiments have revealed large scattering which could be attributed to structural defects. How two neighboring defects may interact and influence the mechanical properties of CNTs is still unclear. Here, interactions between a Stone-Wales (SW) defect pair in axially loaded single-walled carbon nanotubes (SWCNTs) are systematically studied using molecular mechanics. The defect-defect interaction is quantified by the bond with the highest energy, E, which varies in magnitude with respect to the interdefect distance, D. Defect pairs, corresponding to combinations of two types of SW defects (namely, the SW defect of A and B modes) with a different relative orientation angle, [lowercase_phi_synonym], embedded in SWCNTs of different size and chirality were studied. It is shown from the results that, in general, E varies according to defect pair, and converges to a constant at large D. It is found that the magnitude of E is regulated by the type of defect pair, and the profile of E vs D is modulated by [lowercase_phi_synonym]. In addition, E is also influenced by the tube size and chirality. From all of the cases studied, the largest indifference length, D0, beyond which two neighboring defects do not feel the existence of each other, is found to be approximately 30 Å. [ABSTRACT FROM AUTHOR]

Magnetoresistive effects were studied in ρ-type heteroepitaxial diamond films with a strip or Corbino disk structure in a magnetic field ranging from 0 to 5 T. The films were grown by microwave plasma chemical vapor deposition and boron doped by cold ion implantation and rapid thermal annealing. The experimental results show that the magnetoresistance (MGR) of ρ-type heteroepitaxial diamond films strongly depends on the geometric form of the samples and the magnetic field. Diamond films are assumed to be an isotropic isothermal solid in which conduction is by holes from light, heavy and split-off bands. Based on the Fuchs and Sondheimer thin-film theory, considering spherical energy surfaces and mixed scattering by lattice vibrations and ionized impurities and surface, a theoretical description of the magnetoresistive effect in diamond films is presented by solving the Boltzmann transport equation in the relaxation time approximation. A relationship between the MGR and the thickness of films, magnetic field, and mobility is shown. [ABSTRACT FROM AUTHOR]

HfO2-based ferroelectrics (FE-HfO2) have shown promise as an emerging 1-T (single-transistor) non-volatile memory candidate because of its attractive features, such as CMOS compatibility, scalability, fast switching speed, low power, and long retention. The nucleation limited switching (NLS) model, which attributes the polarization switching mainly to the waiting time of nucleation of reversed domains, has been well developed to interpret experimental results for both ferroelectric switching and retention of metal-ferroelectric-metal (MFM) devices based on conventional perovskite ferroelectric materials [such as Pb(Zr, Ti)O3]. However, the applicability of this model to HfO2-based MFM capacitors is not well understood, although some recent experimental evidence has suggested so. In this study, we thoroughly investigated the switching kinetics and the retention characteristics of FE-HfO2 in the context of the NLS model. We show that (1) the NLS model agrees well with the experimental results of switching kinetics over 5 orders of pulse widths, and the extracted modeling parameters are also consistent with those predicted by the density-functional-theory analysis, indicating that the nucleation of reversed domain indeed dominates the switching speed and (2) the NLS model also agrees well with the experimental retention characteristics of HfO2-based MFM devices both at room temperature and at 85 °C. [ABSTRACT FROM AUTHOR]

A profile for the critical gradient scale length (Lc) has been measured in L-mode discharges at the Alcator C-Mod tokamak, where electrons were heated by an ion cyclotron range of frequency through minority heating with the intention of simultaneously varying the heat flux and changing the local gradient. The electron temperature gradient scale length (LTe−1 = |∇Te|/Te) profile was measured via the BT-jog technique [Houshmandyar et al., Rev. Sci. Instrum. 87, 11E101 (2016)] and it was compared with electron heat flux from power balance (TRANSP) analysis. The Te profiles were found to be very stiff and already above the critical values, however, the stiffness was found to be reduced near the q = 3/2 surface. The measured Lc profile is in agreement with electron temperature gradient (ETG) models which predict the dependence of Lc−1 on local Zeff, Te/Ti, and the ratio of the magnetic shear to the safety factor. The results from linear Gene gyrokinetic simulations suggest ETG to be the dominant mode of turbulence in the electron scale (k⊥ρs > 1), and ion temperature gradient/trapped electron mode modes in the ion scale (k⊥ρs < 1). The measured Lc profile is in agreement with the profile of ETG critical gradients deduced from Gene simulations. [ABSTRACT FROM AUTHOR]

The effects of MgO tunnel barrier thickness on the magnetic-transport characteristics of perpendicularly magnetized magnetic tunnel junction device are explored. Saturation magnetization and initial switching field of the free layer decrease with MgO barrier thickness. Increased MgO thickness that extended interaction distance and diminished the influence from the magnetized pinned layer leads to a smaller initial switching applied field required for saturation. The oscillating dependences of the high coercivity of pinned layer and the low coercivity of free layer on MgO thickness are discovered. Moreover, an out-of-phase feature between both layers’ coercivity cyclic variation is exhibited. This unique cyclic varying of coercivity with MgO barrier thickness is revealed and analogous to the recent finding of the magnetoresistance’s periodic variation with MgO thickness. [ABSTRACT FROM AUTHOR]

We demonstrate the existence of an interfacial barrier in blocked impurity band (BIB) detectors using temperature-dependent dark current and corresponding theoretical calculations. Considering the effects of the interfacial barrier, the calculated photoresponse is in good agreement with the experimental results. A dual-excitation model, including the direct excitation over the full barrier and excitation to the band minimum with subsequent tunneling into the blocking layer, is proposed to quantitatively explain the observed photoresponse extension. A concept of extended-mode detection is developed to suggest the option for some selective photoresponse in the terahertz region and open the possibility of extending BIB photoresponse to lower frequency. [ABSTRACT FROM AUTHOR]

We report electrical transport measurements made on alkylphosphonate self-assembled monolayers grown on nanometer-thin SiO2 on top of highly p-doped silicon. At small bias direct tunneling is characterized by a decay constant of β ≈ 0.7/carbon. At larger positive bias to the silicon (1.1-1.5 V) the current-voltage traces feature a prominent shoulder, reminiscent of a negative differential resistance. We attribute this feature to a significant reduction in trap-assisted tunneling, as supported by a simulation. Hence, organophosphonate monolayers are excellent model systems to study electrical transport through ordered structures; they also provide highly efficient electrical passivation of the SiO2/Si surface. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of self-assembled monolayers prepared from 11-hydroxyundecyphosphonic acid and 9,10-diphenyl-2,6-diphosphonoanthracene on n-type 6H-SiC is described. Structural and chemical properties of these monolayers were investigated through contact angle measurements, atomic force microscopy, x-ray photoelectron spectroscopy, and Fourier transformation infrared spectroscopy, which indicate covalent bonding of the phosphonates to both (0001)- and (000-1)-oriented 6H-SiC crystal faces. Electrical characterization was achieved through contact potential difference and surface photovoltage measurements, which revealed significant changes in the work functions of the substrates by monolayer formation. [ABSTRACT FROM AUTHOR]

A novel integrated charge exchange recombination spectroscopy (CXRS)/beam emission spectroscopy (BES) system is proposed for C-Mod, in which both measurements are taken from a shared viewing geometry. The supplementary BES system serves to quantify local beam densities and supplants the common calculation of beam attenuation. The new system employs two optical viewing arrays, 20 poloidal and 22 toroidal channels. A dichroic filter splits the light between two spectrometers operating at different wavelengths for impurity ion and beam neutrals emission. In this arrangement, the impurity density is inferred from the electron density, measured BES and CXRS spectral radiances, and atomic emission rates. [ABSTRACT FROM AUTHOR]

We report that depositing Al2O3 on InGaAs in an H-containing ambient (e.g., in forming gas) results in significant reduction of interface-trap density and significantly suppressed frequency dispersion of accumulation capacitance. The results of the inelastic electron tunneling spectroscopy study reveal that strong trap features at the Al2O3/InGaAs interface in the InGaAs band gap are largely removed by depositing Al2O3 in an H-containing ambient. Transmission electron microscopy images and x-ray photoelectron spectroscopy data shed some light on the role of hydrogen in improving interface properties of the Al2O3/In0.53Ga0.47As gate stack. [ABSTRACT FROM AUTHOR]

We present electron transport phenomena in a single electron transistor based on an individual indium nitride nanowire. Meticulous Coulomb oscillations are observed at low temperatures. While the device shows single period Coulomb oscillation at high temperatures or at high bias voltages, additional satellite peaks along with the main Coulomb peak appear at low temperatures and low bias voltages. The quasiperiodic structure is attributed to the mixing of dissimilar Coulomb oscillations arising from two serially coupled islands embedded inadvertently in the surface metallic states of the nanowire. The proposed model is numerically simulated with good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

A simple analytical model with two high-electric-field regions, instead of only one shifted high-electric-field region (the Kirk effect), is proposed to explain the unusual substrate-current behavior in very short high voltage n-channel metal-oxide-semiconductor transistors. This model agrees well with the experimental data in terms of the substrate-current dependence on VGS, VDS, process, and structures. [ABSTRACT FROM AUTHOR]

The intrinsic oxygen-vacancies and the extrinsic dopants are two major fundamental free-carrier sources for the extrinsic conducting oxides, such as Sn-doped In2O3. Yet, the individual contributions of the above two free-carrier sources to the total carrier concentrations have never been unraveled. A carrier-concentration separation model is derived in this work, which can define the individual contributions to the total carrier concentration from the intrinsic oxygen-vacancies and the extrinsic dopants, separately. The individual contributions obtained from the present carrier-concentration separation model are verified by the two-state trapping model, photoluminescence, and positron annihilation lifetime (PAL) spectroscopy. In addition, the oxygen-vacancy formation energy of the Sn:In2O3 thin film is determined to be 0.25 eV by PAL spectroscopy. [ABSTRACT FROM AUTHOR]

In this work, fatigue failure mechanisms of single-walled carbon nanotube (SWCNT) bundles embedded in epoxy matrix under repeated tensile load were studied. Observed damage and failure modes include: (1) splitting of SWCNT bundles, (2) kink formation and subsequent failure in SWCNTs, and (3) fracture of SWCNT bundles. Patterns of crack propagation under tension in SWCNTs were studied by molecular mechanics simulations, where defect-free SWCNTs and SWCNTs with two different modes of Stone–Wales defects were studied. It is demonstrated by the results of molecular mechanics simulation that the observed fracture surfaces of SWCNT can be reproduced reasonably well, suggesting possible fatigue failure mechanisms of SWCNT in the composite. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We introduce an ac-transconductance method to profile the gate oxide traps in a HfO2 gated AlGaN/GaN Metal-Oxide-Semiconductor High-Electron-Mobility Transistors (MOS-HEMTs) that can exchange carriers with metal gates, which in turn causes changes in analog and pulsed channel currents. The method extracts energy and spacial distributions of the oxide and interface traps under the gate from the frequency dependence of ac transconductance. We demonstrate the method using MOS-HEMTs with gate oxides that were annealed at different temperatures. [ABSTRACT FROM AUTHOR]

We have studied the As doping effects on the optical characteristics of GaN films by time-integrated photoluminescence and time-resolved photoluminescence. When As is incorporated into the film, the localized defect levels and donor-acceptor pair transition become less resolved. The recombination lifetime of neutral-donor-bound exciton (I[sub 2]) transition in undoped GaN increases with temperature as T[sup 1.5]. However, the I[sub 2] recombination lifetime in As-doped GaN first decreases exponentially from 98 to 41 ps between 12 and 75 K, then increases gradually to 72 ps at 250 K. Such a difference is related to the isoelectronic As impurities in GaN, which generate nearby shallow levels that dominate the recombination process. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

block2 is an open source framework to implement and perform density matrix renormalization group and matrix product state algorithms. Out-of-the-box it supports the eigenstate, time-dependent, response, and finite-temperature algorithms. In addition, it carries special optimizations for ab initio electronic structure Hamiltonians and implements many quantum chemistry extensions to the density matrix renormalization group, such as dynamical correlation theories. The code is designed with an emphasis on flexibility, extensibility, and efficiency and to support integration with external numerical packages. Here, we explain the design principles and currently supported features and present numerical examples in a range of applications., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

Electromagnetic interference (EMI) measurement is essential in wireless communications to avoid both performance degradation and serious fault failures. In this paper, we present the design of an acquisition system for a time-domain EMI measurement receiver intended for monitoring potential EMI. The platform consists of both hardware and software systems. The hardware system is constructed using a modular design method and contains a conditioning module, a data acquisition and processing module, and a clock generation module. The software system provides an interactive interface for device operation and offers an automatic sampling/processing procedure that is developed on a slot 0 controller. An additional outstanding feature of the proposed system is its high dynamic range. We propose a novel multi-stage architecture and a data reconstruction algorithm that are combined to achieve large dynamic range acquisition. The proposed acquisition architecture breaks through the dynamic range limitations of conventional analog-to-digital converters (ADCs) on a significant scale. The implemented system is able to capture an analog waveform with a 300 MHz bandwidth and a 1 GS/s sampling rate. The experimental results show that the maximum dynamic range achieved is 96.43 dB, which is far greater than the single ADC dynamic range of 52 dB.

The wideband signal transceiver module forms the core component of the radar signal target simulator and the key module in measuring the Doppler shift. In this paper, we present a design for a miniaturized wideband signal transceiver module based on a PXIe bus, with an operating frequency range of 65 MHz-3 GHz, an instantaneous measurement bandwidth of 800 MHz on a standard 3U (where 1U = 4.445 cm) board with three slots. The module adopts a zero intermediate frequency transceiver, and the baseband uses two analog to digital converters, a field programmable gate array, and two digital to analog converters as the core hardware architecture. The module combines key technologies including variable-frequency local oscillator synthesis, radio-frequency broadband signal conditioning, and high-speed high-resolution baseband signal acquisition and generation. This paper describes a novel design for the Doppler shift function. Considering the limited size and volume of the module, Doppler shift technology combining a complex signal and a real signal was adopted to achieve full band coverage with an operating frequency range of 65 MHz-3 GHz. We verified that this module could generate the Doppler shift in the frequency band from -400 Hz to 400 Hz in steps of 2 Hz and with an error of ±2 Hz.

The lithium beam is an effective diagnostic tool for investigation of stability and particle transport in the pedestal. It was used successfully to measure edge current density on DIII-D, achieving qualitative agreement with neoclassical models. Electron density profiles were also measured. Proposed upgrades will continue these measurements with higher reliability as well as explore edge current measurements using spectroscopy. The optics will be redesigned to optimize throughput and aperture broadening and to replace the photomultiplier tubes with avalanche photodiodes. The new system will yield detailed measurements of the pedestal, complementing existing diagnostics for investigating pedestal stability, edge localized mode cycle, and particle transport through the pedestal.

We present a method to correct for finite size errors in coupled cluster theory calculations of solids. The outlined technique shares similarities with electronic structure factor interpolation methods used in quantum Monte Carlo calculations. However, our approach does not require the calculation of density matrices. Furthermore we show that the proposed finite size corrections achieve chemical accuracy in the convergence of second-order Møller-Plesset perturbation and coupled cluster singles and doubles correlation energies per atom for insulating solids with two atomic unit cells using 2 × 2 × 2 and 3 × 3 × 3 k-point meshes only.

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2-OCS, ortho-H2-OCS, HD-OCS, para-D2-OCS, and ortho-D2-OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2-OCS relative to ortho-H2-OCS increases exponentially with decreasing normal H2 component in H2He gas mixtures, making the observation of para-H2-OCS in the presence of the more strongly bound ortho-H2-OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2-OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2-OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD-OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2-OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2-OCS.