Light Emission Spectroscopy Research Articles

A Mutagen Acts as a Potent Reducing Agent of Glycated Hemoglobin: a Combined Ultrafast Electron Transfer and Computational Studies.

Glycated hemoglobin (GHb) found in mammals undergoes irreversible damage when exposed to external redox agents, which is much more vulnerable than its normal counterpart hemoglobin (Hb). Besides the oxygen regulation throughout the body, Hb plays a vital role in balancing immunological health and the redox cycle. Photoinduced ultra-fast electron transfer phenomena actively participate in regulation of various kind of homeostasis involved in such biomacromolecules. In the present study we have shown that a well-known mutagen Ethidium Bromide (EtBr) reduces GHb in femtosecond time scale (efficiently) upon photoexcitation after efficient recognition in the biomolecule. We have performed similar experiment by colocalizing EtBr and Iron (Fe(III)) on the micellar surface as Hb mimic in order to study the excited state EtBr dynamics to rationalize the time scale obtained from EtBr in GHb and Hb. While other experimental techniques including Dynamic Light Scattering (DLS), Zeta potential, absorbance and emission spectroscopy have been employed for the confirmation of structural perturbation of GHb compared to Hb, a detailed computational studies involving molecular docking and density functional theory (DFT) have been employed for the explanation of the experimental observations.

Correlation between plasma parameters and structure of thin TiO2 films deposited by conventional and pulsed magnetron sputtering methods

The aim of the work was to diagnose plasmas excited by two magnetron sputtering techniques: conventional reactive magnetron sputtering (RMS) and magnetron sputtering realized under gas impulse conditions (GIMS) with use of light emission spectroscopy and Langmuir probing. The plasmas were studied during deposition of TiO2 coatings by magnetron sputtering of Ti target in O2 atmosphere. The obtained deposits were investigated with use of infrared spectroscopy (FTIR) and X-ray diffraction (GIXRD).Two major conclusions were drawn from the investigations. One concerned the mechanism of TiO2 formation during reactive sputtering. An absence of emission lines corresponding to plain Ti atoms proves that their oxidation takes place at the surface of the Ti target without any gas phase reactions. The second conclusion concerned deposition thermodynamics. Although the diagnostics of the GIMS plasma revealed a greater plasma density and electron temperature than in the RMS one, it was the latter technique that produced more high temperature polymorph of TiO2. It appears that the pulse technique offers a way to control thermodynamic conditions of deposition in a broad range up to a conversion to the RMS mode of deposition.

Oxidation in wire arc additive manufacturing of aluminium alloys

Abstract Wire Arc Additive Manufacturing is a near-net-shape machining technology that enables low-cost production of large and customised metal parts. In the present work, oxidation effects in Wire Arc Additive Manufacturing of the aluminium alloy AW4043/AlSi5(wt%) were investigated. Two main oxidation effects, the surface oxidation on aluminium parts and the oxidation anomalies in aluminium parts were observed and analysed. The surface oxidation on aluminium parts changed its colour during Wire Arc Additive Manufacturing from transparent to white. In the present work, it was shown by high-speed imaging that this change in the surface oxidation took place in the process zone, which was covered by inert gas. Since the white surface oxidation formed in an inert gas atmosphere, it was found that the arc interacts with the existing amorphous oxide layer of the previously deposited layer and turns it into a white duplex (crystalline and amorphous) oxide layer. In addition to the analysis of the white surface oxidation, oxidation anomalies, which occur at low shielding from the environment, were investigated. It was shown by physical experiments and Computational Fluid Dynamics simulations, that these oxidation anomalies occur at inadequate gas flow rates, too big nozzle-to-work distances, process modes with too high heat input, or too high wire feed rates. Finally, a monitoring method based on light emission spectroscopy was used to detect oxidation anomalies as they create peaks in the spectral emission when they occur.

Influence of doping level and surface states in tunneling spectroscopy of an In0.53Ga0.47As quantum well grown on p -type doped InP(001)

A steplike density of states is the hallmark of a two-dimensional electron gas. Here, scanning tunneling spectroscopy is used to investigate this property in the differential conductance of an $\mathrm{I}{\mathrm{n}}_{0.53}\mathrm{G}{\mathrm{a}}_{0.47}\mathrm{As}$ quantum well grown on $p$-type doped InP(001). We show that the appearance of a steplike function in the conduction band strongly depends on both the hole concentration in the quantum well and the current injected into the quantized states. Based on four-point probe transport measurements and tunneling-induced light-emission spectroscopy, we discuss the physical mechanisms at the origin of the current and unveil the significant contribution of midgap surface states in the relaxation of the tunneling electrons. These surface states, via pinning of the Fermi level, also affect the potential landscape across the quantum well, as demonstrated by tight-binding calculations of the quantum well electronic structure.

Device for inductive heating and focusing of laser produced plasma

The design and implementation of a pulsed magnetic induction device used to heat and focus a laser produced plasma in vacuum is described. Imaging of the plasma light emission and optical spectroscopy show that the UV/visible light emission is strongly enhanced, and the degree of ionization is increased. The plasma focusing is confirmed by contraction of the deposits formed by the plasma transmitted through the induction coil. The operation of the device can be described in the framework of resistive magnetohydrodynamics, where the induced currents heat the plasma and focus the plasma via the Lorentz interaction with the magnetic field.

STM Light Emission Spectroscopy of Self-Assembled Monolayer of Alkanethiol on Au Film

In this paper, scanning tunneling microscope (STM) light emission (STM-LE) from alkanethiol self-assembled monolayer (SAM)-covered Au film has been probed in the Kretschmann configuration. The films were deposited on the smooth plane of a hemispherical glass prism. STM-LEs from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were investigated. Our experimental results showed that the tip-side emission was scarcely found, and the prism-side emissions were successfully detected due to the enhancement of Kretschmann configuration. It was also found from the experimental study that the peak intensity of STM-LE spectra become smaller, accompanying the redshift of the peak position with the rise in thickness of the alkanethiol SAM film. The main focus of this paper is to explore the behavior of electron tunneling into the SAM-covered Au film. In order to explain these phenomena, we have designed different models regarding the tip–sample gap structure. Among these models, the antenna factor successfully explains the cutoff energy shift of STM-LE spectra. According to this model, the cutoff energy shows redshift when the electronic transition occurs at the Au–S interface layer and the amount of the shift depends on the strength of the transitions.

Determining the phonon energy of highly oriented pyrolytic graphite by scanning tunneling microscope light emission spectroscopy

We have investigated the scanning tunneling microscope (STM) light emission spectra of isolated single Ag nanoparticles lying on highly oriented pyrolytic graphite (HOPG). The STM light emission spectra exhibited two types of spectral structures (step-like and periodic). Comparisons of the observed structures and theoretical predictions indicate that the phonon energy of the ZO mode of HOPG [M. Mohr et al., Phys. Rev. B 76, 035439 (2007)] can be determined from the energy difference between the cutoff of STM light emission and the step in the former structure, and from the period of the latter structure. Since the role of the Ag nanoparticles does not depend on the substrate materials, this method will enable the phonon energies of various materials to be measured by STM light emission spectroscopy. The spatial resolution is comparable to the lateral size of the individual Ag nanoparticles (that is, a few nm).

Pump–probe STM light emission spectroscopy for detection of photo-induced semiconductor–metal phase transition of VO2

We attempted to observe pump–probe scanning tunneling microscopy (STM)-light emission (LE) from a VO2 thin film grown on a rutile TiO2(0 0 1) substrate, with an Ag tip fixed over a semiconducting domain. Laser pulses from a Ti:sapphire laser (wavelength 920 nm; pulse width less than 1.5 ps) irradiated the tip–sample gap as pump and probe light sources. With a photon energy of 2.7 eV, suggesting phase transition from semiconducting monoclinic (M) to metallic rutile (R) phases in relation to the electronic band structure, faint LE was observed roughly 30 ps after the irradiation of the pump pulse, followed by retention for roughly 20 ps. The incident energy fluence of the pump pulse at the gap was five orders of magnitude lower than the threshold value for reported photo-induced M–R phase transition. The mechanism that makes it possible to reduce the threshold fluence is discussed.

Capacitance Behavior of Alkanethiol Self-Assembled Monolayer Studied by Scanning Tunneling Microscope Light Emission Spectroscopy

We have studied the capacitance behavior of alkanethiol self-assembled monolayer (SAM)-covered Au film by scanning tunneling microscope light emission (STM-LE) spectroscopy in the Kretschmann geometry. Although, the STM-LE from tip-sample gap into the vacuum (tip-side emission) is fundamentally weak and very difficult to detect, we have succeeded in detecting the STM-LE radiated into the prism (prism-side emission) by virtue of the enhancement of prism-coupled geometry. Our experimental results shows that the cutoff energy of STM-LE spectra have been redshifted with increase in thickness (chain length) of the SAM film. In order to explain the cutoff energy shift, we have designed a two-layer tunnel junction model by considering the capacitance response of the SAM film depending on the molecular chain length. It has been found from the model analysis, that the capacitance of the SAM changes with changing of the molecular thickness. Hence, it is concluded that the shift of the cutoff energy has originated from the change of the capacitance of the SAM film depending on the molecular chain length.

Experimental evidence of α → β phase transformation in SiC quantum dots and their size-dependent luminescence

Phase transformation can occur among different SiC polytypes under extreme conditions such as high pressure or temperature. It remains unknown whether phase transformation can occur under normal conditions. We demonstrate that the α → β phase transformation can occur at ambient temperature and pressure in nanoscale SiC. The microstructural characterization and light absorption and emission spectroscopy demonstrate the occurrence of this phase transformation. It is found that the quantum-confinement luminescence dominates in larger SiC quantum dots (QDs) and the surface-defect luminescence dominates in ultrasmall SiC QDs. The rare phenomenon of multiple-phonon-assisted light absorption is observed in the SiC QDs.

Measurement of phonon energy of Sb2Te3 by scanning tunneling microscope light-emission spectroscopy

We have measured the energy of the A2u phonon mode of Sb2Te3 by scanning tunneling microscope light-emission (STM-LE) spectroscopy combined with ps-pulsed laser irradiation of the STM tip-sample gap. It is known that the STM-LE from Sb2Te3 is excited by electronic transitions at a photon energy of around 1.6eV, i.e, a single peak with a narrow spectral width appears at the photon energy in the STM-LE spectrum. For STM-LE occurring with the STM tip-sample gap irradiated by ps laser pulses, several supplementary peaks appear in the STM-LE spectra (in addition to the single original peak). The difference in energy between adjacent peaks gives the phonon energy of the A2u mode of Sb2Te3.

Hybrid plasmonic-photonic modes in diffractive arrays of nanoparticles coupled to light-emitting optical waveguides

We study the hybridized plasmonic-photonic modes supported by two-dimensional arrays of metallic nanoparticles coupled to light-emitting optical waveguides. Localized surface plasmon polaritons in the metallic nanoparticles can couple to guided modes in the underlying waveguide, forming quasi-guided hybrid modes, or to diffracted orders in the plane of the array, forming surface lattice resonances. We consider three kinds of samples: one sustains quasi-guided modes only, another sustains surface lattice resonances only, and a third sample sustains both modes. This third sample constitutes the first demonstration of simultaneous coupling of localized surface plasmons to guided modes and diffracted orders. The dispersive properties of the modes in the samples are investigated through light extinction and emission spectroscopy. We elucidate the conditions that lead to the coexistence of surface lattice resonances and quasi-guided hybrid modes, and assess their potential for enhancing the luminescence of emitters embedded in the coupled waveguide. We find the largest increase in emission intensity for the surface lattice resonances, reaching up to a factor of 20.

Nanoscale properties of single atoms and molecules investigated by scanning tunneling microscope light emission spectroscopy

Nanoscale properties of single atoms and molecules investigated by scanning tunneling microscope light emission spectroscopy

Vibrational Excitation of a Single Benzene Molecule Adsorbed on Cu(110) Studied by Scanning Tunneling Microscope Light Emission Spectroscopy

We have investigated the vibrational excitation of a single benzene molecule adsorbed on Cu(110) using scanning tunneling microscope light emission (STM-LE) spectroscopy. The STM-LE spectrum of the benzene molecule has a broad light emission peak and is similar to that of the Cu substrate, indicating that the STM-LE is radiated via the excitation of localized surface plasmons (LSP) confined in the tip−sample gap. However, careful analysis revealed that the STM-LE spectrum of the benzene molecule exhibits a steplike structure appearing at an energy near the quantum cutoff. Isotope-labeled experiments revealed that the fine structure is associated with the vibrational excitation of the benzene molecule, that is, the CH out-of-plane bending mode. We concluded that the vibrational excitation of benzene observed in the present study is not caused by the tunneling electrons through the inelastic tunneling process but by the Raman process due to the strong electric field generated just under the STM tip.

Prism-Coupled Scanning Tunneling Microscope Light Emission Spectroscopy of Au Film Covered with Self-Assembled Alkanethiol Monolayer

We have investigated scanning tunneling microscope (STM) light emission from bare and alkanethiol self-assembled monolayer (SAM)-covered Au films in the Kretschmann geometry. The films were deposited on the flat bottom of a hemispherical prism, and the STM light emissions from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were measured. The prism-side emission was much stronger than the tip-side emission for the bare Au film. Theoretical analysis revealed that this enhancement of emission intensity is caused by the fact that surface plasmon polaritons (SPPs) localized at the Au surface become radiative on the prism side. This geometry was applied to the investigations of STM light emission from the Au film covered with an alkanethiol SAM. The prism-side emission was successfully detected by virtue of the enhancement of STM light emission.

Electron trapping in misfit dislocations of MgO thin films

Misfit dislocations in a thin MgO/Mo(001) film have been investigated by conductance and light-emission spectroscopy using scanning tunneling microscopy and electron-paramagnetic resonance (EPR) spectroscopy. The line defects exhibit a higher work function than the pristine MgO, being explained by their ability to trap electrons. The electron traps are associated with a nonstoichiometric defect composition in thin oxide films and attractive pockets in the Madelung potential in thicker ones. The latter traps can be reproducibly filled by the adsorption of atomic hydrogen, which gives rise to a free-electronlike signal in EPR spectroscopy.

Laser and Electric Arc Synthesis of Nanocrystalline Scintillators

Two new methods of preparation of nanocrystalline scintillators are described. Laser ablation of microscopic powders immersed in optically transparent liquid was used to produce spherical nanoparticles, which preserved the initial compositions. Electric arc discharge between electrodes of definite metals immersed in water solutions of different salts produces a vast variety of scintillating compounds with nanoscopic dimensions and morphologies having crystallographic symmetry of the corresponding equilibrium phases. A wide range of different compositions and structures of tungsten oxides are obtained during one synthesis process, which is due to variety of temperatures and other conditions around the arc channel. It was found that the light emission spectroscopy of the discharge is a rather informative method of diagnostics of the process of the nanoparticle synthesis inside the discharge chamber. The synthesis of nanoscintillators by arc discharge turned out to be rather efficient and capable to create nanocrystalline scintillators of easily regulated compositions. Hydrogen injection into nanoparticles of tungsten oxide is detected by light emission and infrared absorption spectroscopy. Hydrogenated nanoscintillators obtained by this method are interesting for registration of fast neutrons.

Local Density of States of Partially Oxidized Ag(110) Surfaces Observed Using Scanning Tunneling Microscope Light-Emission Spectroscopy

We have observed the scanning tunneling microscope (STM) light-emission spectra of partially oxidized Ag(110) surfaces with atomic spatial resolution. The STM image visualized Ag–O chains along the [001] direction as light rows, separated by a distance n ×a from adjacent light rows. Here, n is an integer between 2 and 7, and a is the surface lattice constant of Ag in the [110] direction. The STM light-emission spectra were measured for tip locations over the light rows and the midpoints of the darkly imaged areas sandwiched by two adjacent light rows. While the spectral shapes of the STM light-emissions were identical, their amplitudes showed atomic-site dependence, i.e., the STM light-emission intensities showed atomic-site dependence without any spectral differences. The STM light-emission intensities at the midpoints of the dark areas increased with n. The STM light-emission intensities of the light rows were independent of the widths of the adjacent dark areas. The experimental results were analyzed by the dielectric theory of STM light-emission, and it was found that the observed atomic-site dependence is caused by electronic states localized along the light rows. These states are identified as the py and pz surface bands induced by adsorbed oxygen.

Nanoscale coupling of photons to vibrational excitation of Ag nanoparticle 2D array studied by scanning tunneling microscope light emission spectroscopy

Scanning tunneling microscope light emission (STM-LE) spectroscopy has been utilized to elucidate the luminescence phenomena of Ag nanoparticles capped with myristate (myristate-capped AgNP) and 2-methyl-1-propanethiolate (C(4)S-capped AgNP) on the dodecanethiol-precovered Au substrate. The STM imaging revealed that myristate-capped AgNPs form an ordered hexagonal array whereas C(4)S-capped AgNPs show imperfect ordering, indicating that a shorter alkyl chain of C(4)S-capped AgNP is not sufficient to form rigid interdigitation. It should be noted that such a nanoparticle ordering affects the luminescence properties of the Ag nanoparticle. We found that the STM-LE is only detected from the Ag nanoparticles forming the two-dimensional superlattice. This indicates that the STM-LE of the Ag nanoparticle is radiated via the collective excitation of the local surface plasmon resonance (LSPR) spread over the Ag nanoparticles. Note that the STM-LE spectra of the Ag nanoparticles exhibit spike-like peaks superimposed on the broad light emission peak. Using Raman spectroscopy, we concluded that the spike-like structure appearing in the STM-LE spectra is associated with the vibrational excitation of the molecule embedded between Ag nanoparticles.

Structural and electronic characterization of the MgO/Mo(001) interface using STM

The nucleation and growth of ultrathin MgO films on Mo(0 0 1) have been investigated with scanning tunneling microscopy and spectroscopy. In the initial growth stage, the MgO forms rather uniform islands with rectangular shapes and defined orientation. This behavior reflects a preferential binding of the oxide O ions to the top positions in the Mo support, which can be realized only in confined areas due to the MgO/Mo lattice mismatch. At monolayer coverage, a characteristic square pattern becomes visible in the STM, indicating the formation of an MgO/Mo coincidence lattice. In the coincidence cell, the interface registry alternates between O and Mg ions being in Mo top positions. The resulting imaging contrast in the STM is dominated by a work-function modulation and not by a topographic effect, as demonstrated with STM-conductance and light-emission spectroscopy. The modulated work function in the coincidence cell is assigned to a small buckling of the oxide film with either O or Mg ions being closer to the Mo surface.