Resolved Spectroscopy Research Articles

Understanding polyethylene surface functionalization by an atmospheric He/O2 plasma through combined experiments and simulations

High density polyethylene surfaces were exposed to the atmospheric post-discharge of a radiofrequency plasma torch supplied in helium and oxygen. Dynamic water contact angle measurements were performed to evaluate changes in surface hydrophilicity and angle resolved x-ray photoelectron spectroscopy was carried out to identify the functional groups responsible for wettability changes and to study their subsurface depth profiles, up to 9 nm in depth. The reactions leading to the formation of C–O, C = O and O–C = O groups were simulated by molecular dynamics. These simulations demonstrate that impinging oxygen atoms do not react immediately upon impact but rather remain at or close to the surface before eventually reacting. The simulations also explain the release of gaseous species in the ambient environment as well as the ejection of low molecular weight oxidized materials from the surface.

Origin of the energetic ions at the substrate generated during high power pulsed magnetron sputtering of titanium

High power impulse magnetron sputtering (HiPIMS) plasmas generate energetic metal ions at the substrate as a major difference to conventional direct current magnetron sputtering (dcMS). The origin of these very energetic ions in HiPIMS is still an open issue, which is unravelled using two fast diagnostics: time-resolved mass spectrometry with a temporal resolution of 2 µs and phase resolved optical emission spectroscopy with a temporal resolution of 1 µs. A power scan from dcMS-like to HiPIMS plasmas was performed, with a 2 inch magnetron and a titanium target as sputter source and argon as working gas. Clear differences in the transport as well as the energetic properties of Ar+, Ar2+, Ti+ and Ti2+ were observed. For discharges with highest peak power densities a high energetic group of Ti+ and Ti2+ could be identified with energies of approximately 25 eV and of 50 eV, respectively. A cold group of ions was always present. It is found that hot ions are observed only when the plasma enters the spokes regime, which can be monitored by oscillations in the IV characteristics in the MHz range that are picked up by the used VI probes. These oscillations are correlated with the spokes phenomenon and are explained as an amplification of the Hall current inside the spokes as hot ionization zones. To explain the presence of energetic ions, we propose a double layer (DL) confining the hot plasma inside a spoke: if an atom becomes ionized inside the spokes region it is accelerated because of the DL to higher energies whereas its energy remains unchanged if it is ionized outside. In applying this DL model to our measurements the observed phenomena as well as several measurements from other groups can be explained. Only if spokes and a DL are present can the confined particles gain enough energy to leave the magnetic trap. We conclude from our findings that the spoke phenomenon represents the essence of HiPIMS plasmas, explaining their good performance for material synthesis applications.

Effect of La3+ substitution with Gd3+ on the resistive switching properties of La0.7Sr0.3MnO3 thin films

This study demonstrated that the resistive switching voltage of perovskite manganite material could be controlled by A-site cation substitution in “A” MnO3 perovskite manganite structure. A partial substitution of La3+ in La0.7Sr0.3MnO3 with smaller cation Gd3+ induced A-site vacancy of the largest Sr2+ cation with surface segregation of SrOy due to ionic size mismatch, and the induced vacancies reduced migration energy barrier. The operating voltage decreased from 3.5 V to 2.5 V due to a favorable condition for electrochemical migration and redox of oxygen ions. Moreover, surface-segregated SrOy was enhanced with Gd-substitution and the SrOy reduced Schottky-like barrier height and resistive switching ratio from the potential drop and screening effect. The relationship between A-site vacancy generation resulting in surface segregation of SrOy and resistive switching behavior was also investigated by energy resolved x-ray photoelectron spectroscopy, O 1s near edge x-ray absorption spectroscopy, and current voltage measurement.

Pulse phase-coherent timing and spectroscopy of CXOU J164710.2−45521 outbursts

We present a long-term phase-coherent timing analysis and pulse-phase resolved spectroscopy for the two outbursts observed from the transient anomalous X-ray pulsar CXOU J164710.2−455216. For the first outburst we used 11 Chandra and XMM–Newton observations between 2006 September and 2009 August, the longest baseline yet for this source. We obtain a coherent timing solution with P = 10.61065583(4) s, Ṗ = 9.72(1) × 10−13 s s−1 and P̈ = –1.05(5) × 10−20 s s−2. Under the standard assumptions this implies a surface dipolar magnetic field of ∼1014 G, confirming this source as a standard B magnetar. We also study the evolution of the pulse profile (shape, intensity and pulsed fraction) as a function of time and energy. Using the phase-coherent timing solution we perform a phase-resolved spectroscopy analysis, following the spectral evolution of pulse-phase features, which hints at the physical processes taking place on the star. The results are discussed from the perspective of magnetothermal evolution models and the untwisting magnetosphere model. Finally, we present similar analysis for the second, less intense, 2011 outburst. For the timing analysis we used Swift data together with 2 XMM–Newton and Chandra pointings. The results inferred for both outbursts are compared and briefly discussed in a more general framework.

Coexisting Bi and Se surface terminations of cleaved Bi2Se3 single crystals

Evidence for the coexistence of both Bi and Se terminations of the topological insulator Bi2Se3 is presented that is connected with details of sample storage and cleaving procedures. X-ray photoelectron spectroscopy of the Bi 4f core levels show a lower binding energy component indicative of metallic Bi near the sample surface. Single crystals stored and cleaved in high vacuum predominantly show the usual Se surface termination while those stored in air for long periods of time have a high probability for Bi termination. The different terminations have very different electronic structures as measured by angle resolved photoelectron spectroscopy. Our photoemission studies show the Se-terminated electronic structure can be recovered after annealing at 400 °C.

High-throughput Toroidal Grating Beamline for Photoelectron Spectroscopy at CAMD

A 5 meter toroidal grating (5m-TGM) beamline has been commissioned to deliver 28 mrad of bending magnet radiation to an ultrahigh vacuum endstation chamber to facilitate angle resolved photoelectron spectroscopy. The 5m-TGM beamline is equipped with Au-coated gratings with 300, 600 and 1200 lines/mm providing monochromatized synchrotron radiation in the energy ranges 25-70 eV, 50-120 eV and 100-240 eV, respectively. The beamline delivers excellent flux (~1014-1017 photons/sec/100mA) and a combined energy resolution of 189 meV for the beamline (at 1.0 mm slit opening) and HA-50 hemispherical analyzer was obtained at the Fermi level of polycrystalline gold crystal. Our preliminary photoelectron spectroscopy results of phenol adsorption on TiO2 (110) surface reveals the metal ion (Ti) oxidation.

Spectroscopic localization by simultaneous acquisition of the double‐spin and stimulated echoes

To design a proton MR spectroscopy ((1) H-MRS) localization sequence that combines the signal-to-noise-ratio (SNR) benefits of point resolved spectroscopy (PRESS) with the high pulse bandwidths, low chemical shift displacements (CSD), low specific absorption rates (SAR), short echo times (TE), and superior radiofrequency transmit field (B1+) immunity of stimulated echo acquisition mode (STEAM), by simultaneously refocusing and acquiring both the double-spin and stimulated echo coherence pathways from the volume of interest. We propose a family of (1)H-MRS sequences comprising three orthogonal spatially selective pulses with flip angles 90° < α, β, γ < 128°. The stimulated and double-spin echo are refocused in-phase simultaneously by altering the pulses' phases, flip angles and timing, as well as the interpulse gradient spoiling moments. The ≈ 90° nutations of α, β, γ provide STEAM-like advantages (lower SAR, in-plane CSD and TE; greater B1+ immunity), but with SNRs comparable with PRESS. Phantom and in vivo brain experiments show that 83-100% of the PRESS SNR (metabolite-dependent) is achieved at under 75% of the SAR and 66% lower in-plane CSD. The advantages of STEAM can be augmented with the higher SNR of PRESS by combining the spin and stimulated echoes. Quantification, especially of J-coupled resonances and intermediate and long TEs, must be carefully considered.

MORPHOLOGICAL AND COMPOSITIONAL STUDY AT THE Si/Fe INTERFACE OF (Fe/Si) MULTILAYER

Diffusion and reaction of elements at the interfaces of nanostructured systems play an important role in controlling their physical and chemical properties for subsequent applications. ( Fe / Si ) nanolayers were prepared by thermal evaporation under ultrahigh vacuum onto a Si (100) substrate. A morphological characterization of these films was performed by combination of scanning transmission electron microscopy (STEM) and X-ray reflectivity (XRR). The compositional depth profile of the ( Fe / Si ) structures was obtained by angle resolved X-ray photoelectron spectroscopy (ARXPS) and hard X-ray photoelectron spectroscopy (HAXPES). Moreover, determination of the stable phases formed at the Si on Fe interfaces was performed using conversion electron Mössbauer spectroscopy. The Si / Fe interface thickness and roughness were determined to be 1.4 nm and 0.6 nm, respectively. A large fraction of the interface is composed of c- Fe 1-x Si paramagnetic phase, though a minoritary ferromagnetic Fe rich silicide phase is also present.

Patterning of silicon nitride for CMOS gate spacer technology. III. Investigation of synchronously pulsed CH3F/O2/He plasmas

Si3N4 spacer etching processes are one of the most critical steps of transistor fabrication technologies since they must be at the same time very anisotropic to generate straight spacer profiles and extremely selective to silicon in order to minimize the silicon consumption in source/drain regions. Minimizing the silicon recess and ion-induced damages to silicon surfaces are key criterions for fully depleted silicon on insulator technologies in order to ensure a high surface quality for the subsequent step of silicon epitaxy. In this work, the authors investigate synchronously pulsed CH3F/O2/He plasmas for the etching of Si3N4 spacers selectively toward Si (the selectivity typically relies on the oxidation of the silicon layer). First, they compare the Si3N4 and silicon etch rates measured in continuous wave (CW) plasmas, while varying the [CH3F]/[O2] ratio, to the etch rates measured using pulsed plasmas. Using angle resolved x-ray photoelectron spectroscopy and scanning transmission electron microscopy (STEM) cross sections, they show that the silicon thickness oxidized during the Si3N4 etching decreases from 1.5 nm in CW to 0.5 using a plasma pulsed at 1 kHz and 10% duty cycle, and the percentage of carbon implanted into the silicon substrate is reduced by a factor 2 using pulsed plasma conditions. Moreover, STEM cross sections show an improvement of the spacer profile when the plasma is pulsed at 1 kHz and low duty cycles, with a rounded spacer top leading to a better gate encapsulation, instead of the faceted spacer obtained using CW plasma conditions.

Low dimensional GaAs/air vertical microcavity lasers

We report on the fabrication of gallium arsenide (GaAs)/air distributed Bragg reflector microresonators with indium gallium arsenide quantum wells. The structures are studied via momentum resolved photoluminescence spectroscopy which allows us to investigate a pronounced optical mode quantization of the photonic dispersion. We can extract a length parameter from these quantized states whose upper limit can be connected to the lateral physical extension of the microcavity via analytical calculations. Laser emission from our microcavity under optical pumping is observed in power dependent investigations.

Electronic and atomic structures of the Ag induced [formula omitted] superstructure on Ge(111)

The Ag/Ge(111) 3×3 surface together with Ag/Si(111) 3×3 constitutes a set of surfaces that is ideally suited for fundamental studies related to low dimensional physics. We here focus on the atomic and electronic structures of the two-dimensional 3×3 superstructure induced by Ag on Ge(111), a surface that is significantly less studied than the Si counterpart. Extensive information on the surface band structure obtained by angle resolved photoelectron spectroscopy (ARPES) is presented, complemented by atomic information from scanning tunneling microscopy (STM). The results reveal new findings that are important for the understanding of the Ag induced 3×3 structure, acting as a prototype for semiconductor/metal interfaces. i) We have identified a new occupied surface band near the M¯-point of the 3×3 surface Brillouin zone. ii) The Ag/Ge(111) 3×3 surface exhibits a partially occupied surface band, S1, with a parabolic-like shape at Γ¯. At low temperature (LT) this band splits into two bands, S1U and S1D. The identification of two bands is significantly different from the case of Ag/Si(111) 3×3 for which just one band has been reported. Besides these specific results, our extensive ARPES study reveals four surface bands at room temperature (RT), while five surface bands were identified at ≈100K (LT). Room temperature empty state STM images show, depending on the tunneling bias, both honeycomb and hexagonal periodicities which are consistent with the honeycomb chained trimer and the in-equivalent trimer models, respectively.

Polariton lasing in a ZnO microwire above 450 K

Exciton-polariton lasing in a one-dimensional ZnO microcavity is demonstrated at high temperature of 455 K. The massive occupation of the polariton ground state above a distinct pump power threshold is clearly demonstrated by using the angular resolved spectroscopy under non-resonant excitation. The temperature dependence of the polariton lasing threshold is well interpreted by two competing mechanisms, i.e., the thermodynamic and kinetic mechanisms. Michelson interference measurements are performed to investigate the temporal and spatial coherence of polariton laser, with the coherence time and coherence length being τc∼0.97 ps and rc∼0.72μm at 440 K and 400 K, respectively.

Patterning of silicon nitride for CMOS gate spacer technology. II. Impact of subsilicon surface carbon implantation on epitaxial regrowth

Molecular beam epitaxy of silicon is a key step in the complementary metal–oxide–semiconductor integration process flow as it allows the fabrication of raised source/drains for fully depleted silicon on insulator technology. One of the most important criteria is the surface state of crystalline silicon before epitaxy, which influences the preliminary stages of the epitaxial growth as well as the interface electrical properties. In this work, the authors study the effect of the Si3N4 spacer etching in CH3F/O2/He plasmas on the Si epitaxial regrowth. Using angle resolved x-ray photoelectron spectroscopy, the authors demonstrate that carbon can be implanted into the silicon substrate below the oxygen rich silicon layer that typically forms on the silicon surface during nitride spacer etching. The level of C-Si bonds is modulated by the energy of ions bombarding the silicon surface during the etching process and is correlated to the post-epitaxy silicon thickness. Using N2/H2 plasma post-treatments, the authors show the reduction of the C-Si bonds level leading to a good silicon epitaxial regrowth.

Al2O3 multi-density layer structure as a moisture permeation barrier deposited by radio frequency remote plasma atomic layer deposition

Al2O3 films deposited by remote plasma atomic layer deposition have been used for thin film encapsulation of organic light emitting diode. In this study, a multi-density layer structure consisting of two Al2O3 layers with different densities are deposited with different deposition conditions of O2 plasma reactant time. This structure improves moisture permeation barrier characteristics, as confirmed by a water vapor transmission rate (WVTR) test. The lowest WVTR of the multi-density layer structure was 4.7 × 10−5 gm−2 day−1, which is one order of magnitude less than WVTR for the reference single-density Al2O3 layer. This improvement is attributed to the location mismatch of paths for atmospheric gases, such as O2 and H2O, in the film due to different densities in the layers. This mechanism is analyzed by high resolution transmission electron microscopy, elastic recoil detection, and angle resolved X-ray photoelectron spectroscopy. These results confirmed that the multi-density layer structure exhibits very good characteristics as an encapsulation layer via location mismatch of paths for H2O and O2 between the two layers.

Polarons in a dipolar condensate

We consider a polaronic model in which impurity fermions interact with background bosons in a dipolar condensate. The polaron in this model emerges as an impurity dressed with a cloud of phonons of the dipolar condensate, which, due to the competition between the attractive and repulsive part of the dipole-dipole interaction, obey an anisotropic dispersion spectrum. We study how this anisotropy affects the Cerenkov-like emission of Bogoliubov phonon modes, which can be directly verified by experiments in which a dipolar BEC moves against an obstacle. We also study the spectral function of impurity fermions, which is directly accessible to the momentum resolved rf spectroscopy in cold atoms.

Tempo-Spatially Resolved Scattering Correlation Spectroscopy under Dark-Field Illumination and Its Application to Investigate Dynamic Behaviors of Gold Nanoparticles in Live Cells

In this study, a new tempo-spatially resolved fluctuation spectroscopy under dark-field illumination is described, named dark-field illumination-based scattering correlation spectroscopy (DFSCS). DFSCS is a single-particle method, whose principle is similar to that of fluorescence correlation spectroscopy (FCS). DFSCS correlates the fluctuations of the scattered light from single nanoparticle under dark-field illumination. We developed a theoretical model for translational diffusion of nanoparticles in DFSCS system. The results of computer simulations documented that this model was able to well describe the diffusion behaviors of nanoparticles in uniformly illuminated field. The experimental setup of DFSCS was achieved by introducing a dark-field condenser to the frequently used bright-field microscope and an electron multiplying charge-coupled device (EMCCD) as the array detector. In the optimal condition, a stack of 500 000 frames were collected simultaneously on 64 detection channels for a single measurement with acquisition rate of 0.5 ms per frame. We systematically investigated the effect of certain factors such as particle concentration, viscosity of the solution, and heterogeneity of gold nanoparticles (GNPs) samples on DFSCS measurements. The experiment data confirmed theoretical model proposed. Furthermore, this new method was successfully used for investigating dynamic behaviors of GNPs in live cells. Our preliminary results demonstrate that DFSCS is a practical and affordable tool for ordinary laboratories to investigate the dynamic information of nanoparticles in vitro as well as in vivo.

Observation of post-MCAO cortical inflammatory edema in rats by 7.0Tesla MRI

This study aimed to investigate inflammatory edema after cerebral ischemia through 7.0T MRI and proton magnetic resonance spectroscopy (MRS). All SD rats were randomly divided into sham operated group and middle cerebral artery occlusion (MCAO)-1 day, -3 day and -7 day groups. MRI scan of the brain was performed on a 7.0 Tesla MRI scanner. The volume of positive signals in the ischemic side was detected by using a T2 weighted spinecho multislice sequence; the changes in the height of water-peak were measured with point resolved spectroscopy (PRESS) sequences; cortical edema was detected by using wet-dry weight method; the degrees of nerve injury were evaluated by Bederson neurological score system; double-labeling immunofluorescence technique was used to explore the molecular mechanisms of post-ischemia cerebral edema. The results showed that high T2WI signals were observed in MCAO-1 day, -3 day and -7 day groups, and the water-peak height and water-peak area of MCAO groups were higher than those of sham operated group (P<0.05). Neurological score results were consistent with the degree of brain edema, and a large number of microglia accumulated in the ischemic cortex. Our results suggested that non-invasive MRI technology with the advantage of high spatial resolution and tissue resolution can comprehensively and dynamically observe inflammatory edema after cerebral ischemia from a three-dimensional space, and contribute to evaluation and treatments in clinic.

Comparison and investigation of bovine hemoglobin binding to dihydroartemisinin and 9-hydroxy-dihydroartemisinin: Spectroscopic characterization

The UV–vis absorption, steady state/time resolved fluorescence spectroscopy and synchronous fluorescence, circular dichroism (CD) spectroscopy are used to investigate the interaction mechanisms of dihydroartemisinin (DHA) and 9-hydroxy-dihydroartemisinin (9-OH DHA), respectively. The UV–vis studies present that DHA and 9-OH DHA can disturb the structure of bovine hemoglobin (BHb). Steady state/time resolved and synchronous fluorescence spectroscopy reveal that the binding constant of DHA with BHb is bigger than 9-OH DHA. CD spectra indicate DHA and 9-OH DHA can change the conformation of BHb. The comparison results suggest that the binding of BHb with DHA is more stable and stronger than 9-OH DHA.

Controlling Leakage Currents: The Role of the Binding Group and Purity of the Precursors for Self-Assembled Monolayers in the Performance of Molecular Diodes

This paper describes that the performance of molecular diodes based on self-assembled monolayers (SAMs) depends on the type of anchoring group and purity of the precursors of these SAMs. The SAMs were formed on ultrasmooth template-stripped silver (Ag(TS)) surfaces, which served as the bottom-electrode, and a eutectic alloy of gallium-indium was used as the top-electrode. When these junctions incorporate SAMs of the form S(CH2)11Fc (≡ SC11Fc) derived from HSC11Fc, they are good molecular diodes and rectify currents with rectification ratios R (≡ |J(-1.0 V)|/|J(+1.0 V)|) of ∼1.0 × 10(2). Replacing the thiol by disulfide or thioacetate functionalities in the precursor resulted in molecular diodes with values of R close to unity. Cyclic voltammetry and angle resolved X-ray photoelectron spectroscopy indicated that the SAMs derived from the disulfide or thioacetate precursors have lower surface coverages and are more defective than SAMs derived from thiols. In the junctions these defective SAMs caused defects and increased the leakage currents. The purity of the thiol-precursor is also crucial: 3 or 5% of disulfide present in the thiol caused a 28 or 61% decrease in R, respectively, and >15% of disulfide lowered R to unity, while the yield in nonshorting junctions remained unchanged. Our results show that the type of binding group, and the puritiy of the thiols, are crucial parameters in the experimental design of molecular electronic devices to ensure optimal device performance by keeping leakage currents to a minimum.

Characterization of laser induced tantalum plasma by spatio-temporal resolved optical emission spectroscopy

The spatial and temporal behavior of the tantalum plasma produced in air by third harmonic Nd:YAG laser (0.6GWcm−2) has been studied using optical emission spectroscopy. Excitation temperature and electron density have been estimated from the analysis of spectral data as well as their spatio-temporal evolutions. As the delay time increases from 400 to 2000ns, the excitation temperature has found to decrease from 10,000K to 7900K. The value of Ne decreases continuously from 4×1018 near the zero position to 5×1017 in a linear approach along with the propagation axe of the plasma plume. Laser-supported consumption wave regime has been proposed to explain the propagation of the plasma.