Blueshift Of Peak Research Articles

Luminescent behaviors of bipyridine proline-grafted hybrid bimodal mesoporous silica and its catalytic performance in asymmetric aldol reaction

Hybrid materials (Z1-BMMs) were prepared through grafting (2S, 2′S)-N,N’-([2,2′-bipyridine]-3,3′-diyl)bis(pyrrolidine-2-carboxamide) (Z1) onto the surface of bimodal mesoporous silicas (BMMs). Their structural properties and textural parameters were characterized with XRD, SEM, N2 sorption, UV-vis, FT-IR, TGA, elemental analysis and fluorescence emission. Fluorescence profiles of the obtained Z1-BMMs showed the remarkable blue-shift of characteristic peak in emission spectra compared to that of pure Z1, while their fluorescence intensity and peak position strongly depended on mass ratios of Z1 to BMMs. Meanwhile, their catalytic performances for asymmetric aldol reaction presented good activities (yield up to 75%) and high stereoselectivities (ee up to 82%), however, Z1-BMMs, if reused more than 2 times, showed rapid catalytic deactivation.

Effect of Sb and As spray on emission characteristics of InAs quantum dots with AlAs capping layer

We report on the influence of an Sb/As combined spray on the physical and optical characteristics of AlAs-capped InAs/GaAs quantum dots grown by Molecular Beam Epitaxy. Photoluminescence emission from the quantum dots shows a significant peak position shift under different Sb/As spray sequences. A blue-shifted quantum dot emission peak with an initial Sb rest indicates a large-to-small quantum dots transition process, with a bi-modal quantum dot size distribution inferred. High-resolution Transmission Electron Microscopy results reveal a large density of small quantum dots when the Sb spray is treated first. Furthermore, defect passivation in the vicinity of the quantum dots by use of Sb spray was detected.

Room-Temperature Spin Polariton Diode Laser.

A spin-polarized laser offers inherent control of the output circular polarization. We have investigated the output polarization characteristics of a bulk GaN-based microcavity polariton diode laser at room temperature with electrical injection of spin-polarized electrons via a FeCo/MgO spin injector. Polariton laser operation with a spin-polarized current is characterized by a threshold of ∼69 A/cm^{2} in the light-current characteristics, a significant reduction of the electroluminescence linewidth and blueshift of the emission peak. A degree of output circular polarization of ∼25% is recorded under remanent magnetization. A second threshold, due to conventional photon lasing, is observed at an injection of ∼7.2 kA/cm^{2}. The variation of output circular and linear polarization with spin-polarized injection current has been analyzed with the carrier and exciton rate equations and the Gross-Pitaevskii equations for the condensate and there is good agreement between measured and calculated data.

Defect-Based Modulation of Optoelectronic Properties for Biofunctionalized Hexagonal Boron Nitride Nanosheets.

Defect engineering potentially allows for dramatic tuning of the optoelectronic properties of two-dimensional materials. With the help of DFT calculations, a systematic study of DNA nucleobases adsorbed on hexagonal boron-nitride nanoflakes (h-BNNFs) with boron (VB ) and nitrogen (VN ) monovacancies is presented. The presence of VN and VB defects increases the binding strength of nucleobases by 9 and 34 kcal mol-1 , respectively (h-BNNF-VB >h-BNNF-VN >h-BNNF). A more negative electrostatic potential at the VB site makes the h-BNNF-VB surface more reactive than that of h-BNNF-VN , enabling H-bonding interactions with nucleobases. This binding energy difference affects the recovery time-a significant factor for developing DNA biosensors-of the surfaces in the order h-BNNF-VB >h-BNNF-VN >h-BNNF. The presence of VB and VN defect sites increases the electrical conductivity of the h-BNNF surface, VN defects being more favorable than VB sites. The blueshift of absorption peaks of the h-BNNF-VB -nucleobase complexes, in contrast to the redshift observed for h-BNNF-VN -nucleobase complexes, is attributed to their observed differences in binding energies, the HOMO-LUMO energy gap and other optoelectronic properties. Time-dependent DFT calculations reveal that the monovacant boron-nitride-sheet-nucleobase composites absorb visible light in the range 300-800 nm, thus making them suitable for light-emitting devices and sensing nucleobases in the visible region.

1.55 µm emission from a single III-nitride top-down and site-controlled nanowire quantum disk

InN/InGaN single quantum well (SQW) was fabricated on 100 nm GaN buffer layer which was deposited on GaN template by plasma assisted molecular beam epitaxy (PA-MBE). The In composition and the surface morphology were measured by x-ray diffusion (XRD) and atom force microscope (AFM), respectively. Afterwards, the sample was fabricated into site-controlled nanowires arrays by hot-embossing nano-imprint lithography (HE-NIL) and ultraviolet nanoimprint lithography (UV-NIL). The nanowires were uniform along the c-axis and aligned periodically as presented by scanning electron microscope (SEM). The single nanowire showed disk-in-a-wire structure by high angle annular dark field (HAADF) and an In-rich or Ga deficient region was observed by energy dispersive x-ray spectrum (EDXS). The optical properties of the SQW film and single nanowire were measured using micro photoluminescence (µ-PL) spectroscopy. The stimulating light wavelength was 632.8 nm which was emitted from a He-Ne laser and the detector was a liquid nitrogen cooled InGaAs detector. A blue peak shift from the film material to the nanowire was observed. This was due to the quantum confinement Stark Effect. More importantly, the 1.55 µm emission was given from the single disk-in-a-wire structure at room temperature. We believe the arrays of such nanowires may be useful for quantum communication in the future.

Influence of various complexing agents on structural, morphological, optical and electrical properties of electrochemically deposited ZnSe thin films

Electrochemical deposition (ECD) of ZnSe thin films from aqueous solution containing ZnSO4·7H2O, SeO2 and different complexing agents at bath maintained at 50 °C were investigated. The voltammetric curves were recorded in order to characterize the electrochemical behavior of Zn2+/SeO2 system with different complexing agent and to apply the appropriate range of potential for ECD. GAXRD studies confirms that as-deposited ZnSe thin films using with different complexing agents belongs to the hexagonal phase with (101) plane as the dominant peak. When EDTA, hydrazine hydrate and triethanolamine as complexing agents used, as-deposited films were shows that peaks due to the hexagonal phase along with hexagonal Se reflection observed. EDAX, AES, FESEM and UV–Visible spectrum were used to examine the compositions, morphologies and band gap of ZnSe thin films deposited on ITO glass substrate. A broad photoluminescence (PL) emission peak has been confirmed by a systematic blue-shift in emission peak due to the formation of nanocrystalline ZnSe thin films. In the Raman spectra, high intensity peak of LO phonon mode is observed at 251 cm−1 using ammonia, citric acid, ethylenediamine and polyvinyl alcohol as a complexing agents. For EDTA, hydrazine hydrate and triethanolamine as a complexing agents used, the phonon was shifted toward lower frequencies. The conductivity type, resistivity, carrier concentrations and electron mobility were measured for the as-deposited ZnSe thin films. As-deposited ZnSe thin films using all complexing agents have n-type conductivity. Electrochemical impedance spectroscopy (EIS) indicated that the ethylenediamine as a good complexing agent for ZnSe thin film deposition and the films demonstrate a less charge transfer resistance (Rct is 11 Ω) with excellent conductivity compared to other samples.

Optical gain of a triple coaxial cylindrical quantum well wires laser under the geometrical effects and magnetic fields

In this study, impacts of external magnetic fields and middle layer’s size on the optical gain of a coaxial cylindrical quantum well wire laser GaAs/AlxGa1−xAs/AlAs are investigated. By using numerical method, Schrodinger equation are solved and then its optical gain is calculated. Results show that, increasing the middle layer’s height leads to blue-shift of the optical gain, and if the middle layer has enough breadth, there is a downfall in it. Otherwise, it remains the same roughly. Also, the max of optical gain and its resonance energy depends considerably on the geometrical size. Additionally, there is a redshift in the optical gain with broadening the layer. Moreover, there are a blue-shift and increase in the optical gain peak by enhancing the magnetic field.

Near-Infrared Emitting Colloidal PbS Nanoplatelets: Lateral Size Control and Optical Spectroscopy

Two dimensional (2D) colloidal PbS nanoplatelets (NPLs) with a thickness of 1.8–2.8 nm have been synthesized using a single-molecule precursor approach with lead octadecylxanthate. The lateral dimensions were tuned by varying the reaction temperature, growth time, and capping ligands. Transmission electron microscopy and X-ray diffraction reveal that the NPLs have an orthorhombic crystal structure rather than the rocksalt phase usually reported for bulk and nanostructured PbS. The 1.8 nm thickness, in combination with the tunable lateral dimensions, results in a blue-shifted absorption peak at 715–730 nm and a 48–68 nm narrow emission spectrum with a surprisingly small, 18 nm Stokes shift at room temperature. The fluorescence lifetime of these PbS NPLs is 2 orders of magnitude shorter than the typical lifetime in 0D colloidal PbS quantum dots, highlighting the advantageous properties of colloidal 2D nanostructures that combine strong transversal with weak lateral confinement.

Star Formation Activity in the Molecular Cloud G35.20–0.74: Onset of Cloud–Cloud Collision

Abstract To probe star formation (SF) processes, we present results of an analysis of the molecular cloud G35.20−0.74 (hereafter MCG35.2) using multi-frequency observations. The MCG35.2 is depicted in a velocity range of 30–40 km s−1. An almost horseshoe-like structure embedded within the MCG35.2 is evident in the infrared and millimeter images and harbors the previously known sites, ultra-compact/hyper-compact G35.20−0.74N H ii region, Ap2-1, and Mercer 14 at its base. The site, Ap2-1, is found to be excited by a radio spectral type of B0.5V star where the distribution of 20 cm and Hα emission is surrounded by the extended molecular hydrogen emission. Using the Herschel 160–500 μm and photometric 1–24 μm data analysis, several embedded clumps and clusters of young stellar objects (YSOs) are investigated within the MCG35.2, revealing the SF activities. A majority of the YSOs clusters and massive clumps (500–4250 ) are seen toward the horseshoe-like structure. The position–velocity analysis of 13CO emission shows a blueshifted peak (at 33 km s−1) and a redshifted peak (at 37 km s−1) interconnected by lower intensity intermediate velocity emission, tracing a broad bridge feature. The presence of such a broad bridge feature suggests the onset of a collision between molecular components in the MCG35.2. A noticeable change in the H-band starlight mean polarization angles has also been observed in the MCG35.2, probably tracing the interaction between molecular components. Taken together, it seems that the cloud–cloud collision process has influenced the birth of massive stars and YSOs clusters in the MCG35.2.

Paramagnetic anatase titania/carbon nanocomposites

The nanocomposites comprising of anatase titania nanoparticles uniformly distributed inside the porous carbon matrix were synthesized using furfuryl alcohol, tetrabutyltitanate, and nonionic surfactant. The characterization of the nanocomposite by scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray analysis, x-ray diffractometry, and Raman spectroscopy revealed the formation of anatase titania nanoparticles of average sizes 10 to 20 nm. The observed broadening and blueshift of the main Raman peak of titanium dioxide nanoparticles were explained using the phonon confinement effect. The nanocomposite exhibited strong paramagnetism at low temperatures and was diamagnetic at higher temperatures, as well as a small contamination by magnetic impurities was detected. The paramagnetism originated from the amorphous defect-rich carbon and oxygen vacancies in titania.

High-Q Microcavity Enhanced Optical Properties of CuInS2/ZnS Colloidal Quantum Dots toward Non-Photodegradation

We report on a temporal evolution of photoluminescence (PL) spectroscopy of CuInS2/ZnS colloidal quantum dots (QDs) by drop-casting on SiO2/Si substrates and high quality factor microdisks (MDs) under different atmospheric conditions. Fast PL decay, peak blue shift, and line width broadening due to photooxidation have been observed at low excitation power. With further increasing of the excitation power, the PL peak position shows a red shift and the line width becomes narrow, which is ascribed to the enhanced Förster resonant energy transfer between different QDs by photoinduced agglomeration. The oxygen plays an important role in optically induced PL decay, which is verified by a reduced photobleaching effect under vacuum. When the QDs are drop-casted on MDs, photooxidation and photobleaching are accelerated because the excitation efficiency is greatly enhanced with coupling the pumping laser with the cavity modes. However, when the emitted photons couple with cavity modes, a PL enhancement by more than 20 times is achieved because of the increased extraction efficiency and Purcell effects of MDs at room temperature (RT) and 35 times at 20 K. The photobleaching can be avoided with a small excitation power but with a strong PL intensity by taking advantage of high quality factor cavities. The highly efficient PL emission without photodegradation is very promising for using CuInS2 QDs as highly efficient photon emitters at RT, where the photodegradation has always limited the practical applications of colloidal quantum dots.

Ligand-Stabilized ZnO Quantum Dots: Molecular Dynamics and Experimental Study

Different aminoalcohol ligands, monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA) were employed to passivate the surface of ZnO quantum dots (ZnO QDs). High-resolution transmission electron microscopy (HRTEM) imaging revealed that the higher branched aminoalcohols produced smaller sized ZnO QDs. The average size for ZnO/MEA, ZnO/DEA, and ZnO/TEA were found to be 3.2, 2.9, and 2.4 nm. TEA ligands were effective in producing stable, monodisperse ZnO QDs compared with DEA and MEA ligands. Molecular dynamics and semi-empirical calculations suggested that TEA and DEA ligands interact strongly with the partial charge of ZnO dangling bonds and have a large molar volume to hinder the diffusion of precursors through the ligands to the surface of ZnO resulting in a smaller particle size as compared with MEA ligands. As the size of ZnO QDs decreases from ZnO/MEA to ZnO/TEA, the absorption edge and emission peak maximum blue-shifts to a shorter wavelength due to the quantum size effect. The bandgap of ZnO/MEA, ZnO/DEA, and ZnO/TEA was determined to be 3.97, 4.07, and 4.23 eV, and the emission peak was found to be 472, 464, and 458 nm when excited using a 325 nm excitation wavelength, respectively.

Graphene-modulated photo-absorption in adsorbed azobenzene monolayers.

The impact of graphene on the photo-absorption properties of trans- and cis-azobenzene monolayers is studied in the framework of density-functional theory and many-body perturbation theory. We find that, despite the weak hybridization between the electronic bands of graphene and those of the azobenzene monolayers, graphene remarkably modulates the absorption spectra of the adsorbates. The excitation energies are affected via two counteracting mechanisms: substrate polarization reduces the band-gap of azobenzene, and enhanced dielectric screening weakens the attractive interaction between electrons and holes. The competition between these two effects gives rise to an overall blueshift of peaks stemming from intramolecular excitations, and a redshift of peaks from intermolecular ones. Even more interesting is that excitations corresponding to intermolecular electron-hole pairs, which are dark in the isolated monolayers, are activated by the graphene substrate. Our results demonstrate that the photoisomerization process of weakly adsorbed azobenzene undergoes notable changes on a carbon-based substrate.

Proximity-Induced H-Aggregation of Cyanine Dyes on DNA-Duplexes.

A wide variety of organic dyes form, under certain conditions, clusters know as J- and H-aggregates. Cyanine dyes are such a class of molecules where the spatial proximity of several dyes leads to overlapping electron orbitals and thus to the creation of a new energy landscape compared to that of the individual units. In this work, we create artificial H-aggregates of exactly two Cyanine 3 (Cy3) dyes by covalently linking them to a DNA molecule with controlled subnanometer distances. The absorption spectra of these coupled systems exhibit a blue-shifted peak, whose intensity varies depending on the distance between the dyes and the rigidity of the DNA template. Simulated vibrational resolved spectra, based on molecular orbital theory, excellently reproduce the experimentally observed features. Circular dichroism spectroscopy additionally reveals distinct signals, which indicates a chiral arrangement of the dye molecules. Molecular dynamic simulations of a Cy3-Cy3 construct including a 14-base pair DNA sequence verified chiral stacking of the dye molecules.

Structural, electrochemical and magnetic properties of codoped (Cu, Mn) CdS nanoparticles with surfactant PVP

PVP capped with (Cu2+, Mn2+) codoped CdS and undoped nanoparticles were prepared by chemical precipitation method. The XRD patterns revealed that codoped CdS nanoparticles exhibit in cubic hawleyite mixed with hexagonal greenockite phase. The functional groups of the capping agent on codoped CdS surface were identified by FTIR. The UV–visible absorption spectra show significantl blue shift (501–514nm) and second excitonic absorption peak 241nm was observed only in the presence of a narrow size distribution. The increased optical band gap confirms the presence of quantum confinement. The photoluminescence (PL) emission spectrum shows emission line in the blue region. The emission band reveals that the measured band gap of around 2.7eV is greater than that of bulk CdS (2.42eV).T The average g–value, total number of spins (Ns) and covalency parameter (α2) were calculated from EPR results. The Cyclic Voltammetry curve of cathodic and anodic peaks shifted slightly to negative and positive potentials, respectively for electrode applications.

Analysis of localization effect in blue-violet light emitting InGaN/GaN multiple quantum wells with different well widths**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0401801), the National Natural Science Foundation of China (Grant Nos. 61674138, 61674139, 61604145, 61574135, 61574134, 61474142, 61474110, 61377020, and 61376089), Science Challenge Project, China (Grant No. JCKY2016212A503), and One Hundred Person

Four blue-violet light emitting InGaN/GaN multiple quantum well (MQW) structures with different well widths are grown by metal–organic chemical vapor deposition. The carrier localization effect in these samples is investigated mainly by temperature-dependent photoluminescence measurements. It is found that the localization effect is enhanced as the well width increases from 1.8 nm to 3.6 nm in our experiments. The temperature induced PL peak blueshift and linewidth variation increase with increasing well width, implying that a greater amplitude of potential fluctuation as well as more localization states exist in wider wells. In addition, it is noted that the broadening of the PL spectra always occurs mainly on the low-energy side of the PL spectra due to the temperature-induced band-gap shrinkage, while in the case of the widest well, a large extension of the spectral curve also occurs in the high energy sides due to the existence of more shallow localized centers.

Dust masses for SN 1980K, SN1993J and Cassiopeia A from red–blue emission line asymmetries

We present Monte Carlo line transfer models that investigate the effects of dust on the very late time emission line spectra of the core collapse supernovae SN 1980K and SN 1993J and the young core collapse supernova remnant Cassiopeia A. Their blue-shifted emission peaks, resulting from the removal by dust of redshifted photons emitted from the far sides of the remnants, and the presence of extended red emission wings are used to constrain dust compositions and radii and to determine the masses of dust in the remnants. We estimate dust masses of between 0.08 - 0.15 M$_\odot$ for SN 1993J at year 16, 0.12 - 0.30 M$_\odot$ for SN 1980K at year 30 and $\sim$1.1 M$_\odot$ for Cas A at year $\sim$330. Our models for the strong oxygen forbidden lines of Cas A require the overall modelled profiles to be shifted to the red by between 700 - 1000 km s$^{-1}$, consistent with previous estimates for the shift of the dynamical centroid of this remnant.

Facet-Dependent Property of Sequentially Deposited Perovskite Thin Films: Chemical Origin and Self-Annihilation.

Quantification of intergrain length scale properties of CH3NH3PbI3 (MAPbI3) can provide further understanding of material physics, leading to improved device performance. In this work, we noticed that two typical types of facets appear in sequential deposited perovskite (SDP) films: smooth and steplike morphologies. By mapping the surface potential as well as the photoluminescence (PL) peak position, we revealed the heterogeneity of SDP thin films that smooth facets are almost intrinsic with a PL peak at 775 nm, while the steplike facets are p-type-doped with 5-nm blue-shifted PL peak. Considering the reaction process, we propose that the smooth facets have well-defined crystal lattices that resulted from the interfacial reaction between MAI and PbI2 domains containing low trap states density. The steplike facets are MAI-rich originated from the grain boundaries of PbI2 film and own more trap states. Conversion of steplike facets to smooth facets can be controlled by increasing the reaction time through Ostwald ripening. The improved stability, photoresponsivity up to 0.3 A/W, on/off ratio up to 3900, and decreased photo response time to ∼160 μs show that the trap states can be annihilated effectively to improve the photoelectrical conversion with prolonged reaction time and elimination of steplike facets. Our findings demonstrate the relationship between the facet heterogeneity of SDP films and crystal growth process for the first time, and imply that the systematic control of crystal grain modification will enable amelioration of crystallinity for more-efficient perovskite photoelectrical applications.

Giant many-body effects in liquid ammonia absorption spectrum.

In the present work, we accurately calculate the absorption spectrum of liquid ammonia up to 13 eV using many-body perturbation approach. The electronic bandgap of liquid NH3 is perfectly described as the combination of density functional theory, Coulomb-hole screened exchange, and G0W0 approximation to the electronic self-energy, yielding a direct gap (Γ → Γ) of 7.71 eV, fully consistent with the experimentally measured gap from photo-emission spectroscopy. With respect to the NH3 optical properties, the entire spectrum in particular the low lying first absorption band is extremely affected by electron-hole interactions, leading to a fundamental redistribution of spectral weights of the independent-particle spectrum. Three well separated but broad main peaks are identified at 7.0, 9.8, and 11.8 eV with steadily increasing intensities in excellent agreement with the experimental data. Furthermore, we observe a giant net blue-shift of the first absorption peak of about 1.4 eV from gaseous to liquid phase as the direct consequence of many-body effects, allowing the associated liquid ammonia absorption band exciton to delocalize and feel more effectively the repulsion effects imposed by the surrounding solvent shells. Further, the spectrum is insensitive to the coupling of resonant and anti-resonant contributions. Concerning electronic response structure of liquid NH3, it is most sensitive to excitations at energies lower than its electronic gap.

Broadband full-color monolithic InGaN light-emitting diodes by self-assembled InGaN quantum dots

We have presented broadband full-color monolithic InGaN light-emitting diodes (LEDs) by self-assembled InGaN quantum dots (QDs) using metal organic chemical vapor deposition (MOCVD). The electroluminescence spectra of the InGaN QDs LEDs are extremely broad span from 410 nm to 720 nm with a line-width of 164 nm, covering entire visible wavelength range. A color temperature of 3370 K and a color rendering index of 69.3 have been achieved. Temperature-dependent photoluminescence measurements reveal a strong carriers localization effect of the InGaN QDs layer by obvious blue-shift of emission peak from 50 K to 300 K. The broadband luminescence spectrum is believed to be attributed to the injected carriers captured by the different localized states of InGaN QDs with various sizes, shapes and indium compositions, leading to a full visible color emission. The successful realization of our broadband InGaN QDs LEDs provide a convenient and practical method for the fabrication of GaN-based monolithic full-color LEDs in wafer scale.