STM tip-enhanced photoluminescence from porphyrin film

Tunneling electrons-induced molecular fluorescence in organic film is enhanced by the surface plasmons. The plasmon enhancement can be expected not only by the plasmons of the substrate but also by the noble metal tip of scanning tunneling microscope (STM). In this report we investigate the tip effect in photoluminescence of meso-tetrakis(3,5-di-tertiarybutyl-phenyl)porphyrin (H₂TBPP) film on indium tin oxide (ITO) combined with a STM. The experimental result shows the PL of molecules is enhanced by an Ag tip. This enhancement factor is evaluated larger than 2000.     

Alternating current characterization of nano-Pt(Ⅱ)octaethylporphyrin(PtOEP) thin film as a new organic semiconductor

Alternating current(AC) conductivity and dielectric properties of thermally evaporated Au/Pt OEP/Au thin films are investigated each as a function of temperature(303 K–473 K) and frequency(50 Hz–5 MHz).The frequency dependence of AC conductivity follows the Jonscher universal dynamic law.The AC-activation energies are determined at different frequencies.It is found that the correlated barrier hopping(CBH) model is the dominant conduction mechanism.The variation of the frequency exponent s with temperature is analyzed in terms of the CBH model.Coulombic barrier height Wm,hopping distance Rω,and the density of localized states N(EF) are valued at different frequencies.Dielectric constant ε_1(ω,T) and dielectric loss ε_2(ω,T) are discussed in terms of the dielectric polarization process.The dielectric modulus shows the non-Debye relaxation in the material.The extracted relaxation time by using the imaginary part of modulus(M')is found to follow the Arrhenius law.     

Photoluminescent properties of heteroleptic cyclometalated platinum(II) complexes bearing 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate as an ancillary ligand

A new series of heteroleptic cyclometalated platinum(II) complexes Pt-1a-f was synthesized, employing 2-arylpyridine (or 1-arylisoquinoline) (HC^∧N-1) and 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dione (HO^∧O-1) for cyclometalation and as ancillary ligands, respectively, and photoluminescent properties were investigated. Focusing on red-shifted phosphorescence, C^∧N ligands containing π-extended aromatics and electron-rich heterocycles were examined. All obtained complexes exhibited photoluminescence at ambient temperature, and the emission maxima ranged from green (λ_PL=518 nm) to far red (λ_PL=708 nm). The large Stokes shifts of more than 100 nm and sub-microsecond or microsecond emission lifetimes revealed that these complexes are phosphorescent emissive. The quantum yield of Pt-1 ranged from 0.02 to 0.59 at ambient temperature and decreased as the emission maximum was red-shifted. In comparison with the reference platinum(II) complexes, Pt-2 bearing an aliphatic ancillary ligand, such as 2,2,6,6-tetramethylheptane-3,5-dionate (O^∧O-2), the ligand O^∧O-1 did not significantly affect the photoluminescence emission maxima, indicating that the energy gap between the singlet ground state and the triplet level was predominantly dependent on the C^∧N ligand.     

Electronic states and phosphorescence of dendron functionalized platinum(II) acetylides

The photophysical properties of bis((4-(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine) platinum(II) with 2,2-bis(methylol)propionic acid (bis-MPA) dendritic substituents were studied. The fluorescence emission decay upon excitation in the UV (typically 350-380 nm) was rapid, in the order of 1 ns or shorter. In oxygen-saturated tetrahydrofuran solvent, the phosphorescence decay time was in the order of 200 ns. Bright phosphorescence at 530 nm was found for dendrimers under certain conditions. The associated phosphorescence decay time considerably increased to above 100-200 μs at higher concentrations (30-100 μM), and in oxygen-evacuated samples. Thus, it was clarified that the strongest triplet quenching was caused by oxygen dissolved in the sample, since it was possible to reversibly go between the bright and quenched phosphorescent state by freeze-thaw pumping cycles. The bright phosphorescence formed spontaneously for the cases with the larger dendritic substituents is implying a chromophore protecting effect. From time-dependent density functional calculations, the electronic structure of a few low-lying singlet and triplet states are discussed. A new mechanism for efficient triplet state formation and phosphorescence of Pt-ethynyls is proposed. Here, a fast relaxation via internal conversion takes the excited population of the dominant π→π* excitation into a lower singlet state of ligand-to-metal charge transfer character of πσ* type. This allows an efficient inter system crossing to the triplet state manifold.     

Increase of photoluminescence from fullerenes-doped poly (alkyl methacrylate) under laser irradiation

Increase of photoluminescence (PL) from fullerenes (C₆₀ and C₇₀)-doped poly(alkyl methacrylate), such as poly(ethyl methacrylate) (PEMA), poly(isopropyl methacrylate) (PiPMA) and poly(isobutyl methacrylate) (PiBMA), have been studied under laser irradiation with wavelength of in air. After laser irradiation, PL peaks of all fullerenes doped-polymers are broadened and blue-shifted. This PL increase depends on the fullerene concentrations. By comparing with fullerenes-doped PMMA, fullerenes-doped PEMA have the greatest PL increase among the four kinds of polymers, including PEMA, PiPMA, PiBMA and PMMA. PL intensity of C₇₀-doped polymers increases much more quickly than the corresponded C₆₀-doped polymers at the initial stage of laser irradiation. Great change on their UV-visible absorption spectra before and after laser irradiation indicate some great variation on chemical structure of fullerene molecules dispersed in polymer matrix under laser irradiation. This great PL increase may be attributed to formation of fullerene oxide-polymer and oxidized fullerene-polymer adducts due to laser-induced photochemical reactions among fullerene, oxygen and polymer.     

Fluorescent aggregates in naphthalene containing poly(urethane-urea)s

A series of segmented poly(urethane-urea)s containing naphthalene in the hard block and hexamethylene spacers in the soft block was prepared. The hard to soft segment ratio was varied systematically, to afford a series of polymers with various chromophore concentrations and a constant length of the chromophoric block, using a three-step synthetic procedure.The absorption, fluorescence and fluorescence-excitation spectra of solutions and films of the block copolymers provide strong evidence for aggregation. A red-shifted fluorescence spectrum peaking at gains in intensity as the naphthalene concentration is increased. The excitation spectrum of this new emission is well to the red of the normal naphthalene absorption spectrum, consistent with the UV spectrum. Formation of a fluorescent ground state dimer (or higher aggregate) is proposed to account for these observations.     

Femtosecond laser induced photoluminescence in poly(methyl methacrylate) and three-dimensional optical storage

We report on a femtosecond-laser induced photoluminescence (PL) in poly(methyl methacrylate) and its potential application to three-dimensional optical storage. Irradiation with a focused 800 nm, 1 kHz, 100 fs pulsed laser induced a strong PL change in UV-visible region. Absorption spectra and Fourier-transform infrared spectra before and after laser irradiation indicate the PL may result from the emissive oxidized products of photo-degradation reaction of PMMA. This makes it possible to read out the stored data by detecting the PL change. The pulse energy threshold of the light-induced PL change of PMMA is found to be at ∼2 μJ/pulse and the optimal recording energy is ∼3 μJ/pulse. A ten-layer pattern inside the bulk sample recorded by tightly focusing a pulsed laser beam was read out by a reflection-type fluorescent confocal microscope, which detected the emission in visible range as the signal. High-contract fluorescent images with a much higher signal-to-noise ratio were obtained without crosstalk in comparison with the ordinary reflection mode.     

Two distinct photoluminescence responses of CdTe quantum dots to Ag (I)

Four sizes of water-soluble thiol-capped CdTe quantum dots (QDs) have been synthesized and used to investigate the photoluminescence (PL) responses to Ag⁺ ions. For small particles, the CdTe QDs exhibit PL enhancement in the presence of lower concentration of Ag⁺ but show obvious quenching with the further increase of Ag⁺; for larger particles, however, PL of CdTe QDs is quenched all the time with the Ag⁺ addition, no PL enhancement is observed. Mechanism study shows that small QDs with more traps on the particle surface are effectively passivated by initial adsorbed Ag⁺, which accounts for the PL enhancement observed; after the initial traps are saturated, the excess Ag⁺ facilitates nonradiative recombination, resulting in PL quenching. For larger particles, the nonradiative recombination dominates the whole process even for the lower concentration of Ag⁺, due to the fewer traps on the QD surface. Compared with larger particles, the small CdTe QDs are more suitable for sensing Ag⁺ because of the more sensitive and selective PL response. To our best knowledge, this is the first systematical study on the interaction of Ag⁺ with different-sized CdTe QDs.     

Structural control of photoluminescence of four poly(urethane-urea-co-1,3,5-triazine)s: Synthesis and characterization

A series of segmented poly(urethane-urea)s containing 1,3,5 triazine in the hard block and hexamethylene spacers in the soft block was prepared. The hard to soft segment ratio was varied systematically, to afford a series of polymers in which the chromophore concentration varied from 4.2% to 18.1%. Although triazine emission is located in the UV region, the films with higher content of the chromophore emitted a visible blue light (425 nm) when excited at the very red-edge of the absorption band. The photophysical properties of the materials were strongly dependent on the relative amount of triazine moieties along the main chain. Isolated moieties emit in copolymers with small amount of triazine groups, indicating that even though in solid state, these moieties tend to be apart. Two photophysical consequences were observed when the amount of triazine increases: there is some energy transfer process involving isolated moieties with consequent decrease of the lifetime and an additional red-edge emission attributed to aggregated lumophores. The mono-exponential decay observed for the isolated form is substituted by a bi-exponential decay of the aggregated species. The materials were not strong emitters, but since the N-containing triazine moieties are good electron transport groups, the polymers have potential application as electron transport enhancers in various applications.     

Structure and photoluminescence study of type-II GaAs quantum wires and dots grown on nano-faceted (3 1 1)A surface

(3 1 1)A GaAs/AlAs corrugated superlattices (CSLs) and satellite (3 1 1)B and (1 0 0) SLs were studied using Raman spectroscopy, high-resolution transmittance electron microscopy (HRTEM) and photoluminescence (PL). The thickness of GaAs layers was varied from 1 monolayer (ML) to 10 ML, the thickness of AlAs barriers was 10 ML in (3 1 1) direction. The strongest modification of the Raman spectra is found for the case of partial (<1 nm) GaAs filling of the AlAs surface. The calculated and experimental Raman spectra demonstrated a good agreement for both complete (1 nm) and partial (<1 nm) GaAs filling of the AlAs surface. According to Raman and HRTEM data, in the case of partial filling of (3 1 1)A AlAs surface, GaAs forms quantum well wires of finite length (quantum dots). A drastic difference of PL from grown side-by-side (3 1 1)A and (3 1 1)B SLs was observed. A strong room temperature PL in the green–yellow spectral region was observed in GaAs/AlAs (3 1 1)A CSLs containing GaAs type-II quantum dots.     

Dose dependence of room temperature photoluminescence from Si implanted SiO2

Photoluminescence from Si implanted silica is studied as a function of Si fluence and Si concentration profile in order to assess the effect of particle size and size distribution on emission spectra. Peaked (skewed Gaussian) concentration profiles were produced by implanting with 400 keV Si ions and uniform Si profiles were produced by a multi-energy implant sequences. Both as-implanted and annealed samples are shown to exhibit a distinct maximum in the emission intensity as a function of ion fluence, with the intensity increasing with fluence up to the maximum and then decreasing at higher fluences. Samples with a uniform Si profile are also shown to produce emission which is significantly red-shifted relative to that of samples with a peaked Si profile. This is consistent with the fact that such samples are expected to have a narrower particle size distribution (i.e. a greater fraction of larger particles).     

Investigation of photoluminescence in GaAs quantum well coupled with a ferromagnetic semiconductor layer of (Ga,Mn)As

We have investigated circular-polarized photoluminescence (CPL) from a novel quantum structure in which a ferromagnetic semiconductor (Ga,Mn)As is placed adjacent to the GaAs quantum well. By eliminating the contribution of the magneto-circular dichroism effect of the (Ga,Mn)As top layer from the observed CPL, we found a small but nonnegligible contribution of quantum mechanical coupling between the GaAs quantum well states and the spin-polarized states in (Ga,Mn)As.     

Effect of annealing temperature on the structural, photoluminescence and magnetic properties of sol-gel derived Magnetoplumbite-type (M-type) hexagonal strontium ferrite

Magnetoplumbite-type (M-type) hexagonal strontium ferrite particles were synthesized via sol-gel technique employing ethylene glycol as the gel precursor at two different calcination temperatures (800 and 1000 °C). Structural properties were systematically investigated via X-ray diffraction (XRD), field emission scanning electron microscopy, high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), photoluminescence spectrophotometry and superconducting quantum interference device magnetometer. XRD results showed that the sample synthesized at 1000 °C was of single-phase with a space group of P6₃/mmc and lattice cell parameter values of a=5.882 Å and c=23.048 Å. EDS confirmed the composition of strontium ferrite calcined at 1000 °C being mainly of M-type SrFe₁₂O₁₉ with HRTEM micrographs confirming the ferrites exhibiting M-type long range ordering along the c-axis of the crystal structure. The photoluminescence (PL) property of strontium ferrite was examined at excitation wavelengths of 260 and 270 nm with significant PL emission peaks centered at 350 nm being detected. Strontium ferrite annealed at higher temperature (1000 °C) was found to have grown into larger particle size, having higher content of oxygen vacancies and exhibited 83-85% more intense PL. Both the as-prepared strontium ferrites exhibited significant oxygen vacancies defect structures, which were verified via TGA. Higher calcination temperature turned strontium ferrite into a softer ferrite.     

Excited-state absorption in a terpyridyl platinum(II) pentynyl complex

The singlet excited-state lifetime of a terpyridyl platinum(II) pentynyl complex was determined to be 268+/-87 ps by fitting femtosecond transient absorption data, the triplet excited-state lifetime was found to be 62 ns by fitting nanosecond transient absorption decay data, and the triplet quantum yield was measured to be 0.16. A ground-state absorption cross section of 2.5 x 10(-19) cm(2) at 532 nm was deduced from UV-vis absorption data. Excited-state absorption cross sections of 3.5 x 10(-17) cm(2) (singlet) and 4.5 x 10(-17) cm(2) (triplet) were obtained by using a five-level dynamic model to fit open-aperture Z scans at picosecond and nanosecond pulse widths and a variety of pulse energies.     

Picosecond dynamics and mechanism of the interaction of a photoexcited Cu(II)-porphyrin with a DNA-modeling polynucleotide

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 2, pp. 110–121, March–April, 1995.     

Spectral behavior of the emission around 3.31 eV (A-line) from ZnO nanocrystals

The emission at around 3.31 eV (A-line) from three types of ZnO nanocrystals with different particle sizes (10-1000 nm) was studied. The photoluminescence (PL) measurements were performed under different excitation densities and at different temperatures. The A-line emission exhibited a strong dependence on temperature and excitation power density. With increasing excitation density and temperature overlapping of the closely spaced first longitudinal optical (LO) phonon replica of free excitons by the A-line emission was observed.     

Ⅱ—Ⅵ族稀磁半导体ZnSe／Zn1—xMnxSe超晶格中的应力效应

本文通过Ⅱ－Ⅵ族稀磁半导体超晶格ＺｎＳｅ／Ｚｎ１－ｘＭｎｘＳｅ的光致发光谱的测量，对其应力效应进行了讨论。样品的组分ｘ＝０．２，０．３，０．４，测量温度为Ｔ＝１１￣３００Ｋ。结果表明：由于应力效应，ＺｎＳｅ／Ｚｎ１－ｘＭｎｘＳｅ超晶格中的激子能量随ｘ值增加而发生红移。在相同组分下，不同阱、垒宽度比使应力的分布产生明显变化，从而影响超晶格中激子能量。实验与理论计算结果相一致。超晶格中光致发光峰随温度     

Temperature-dependent photoluminescence from bicrystalline ZnS (core/shell) nanocables

The experimental study of the temperature-dependent photoluminescence properties of bicrystalline ZnS core/shell nanocables under 10 K to 300 K was reported. The results show that there are two distinct peaks situated at the UV (about 372.6 nm) and green (about 500 nm) regions, respectively, and one blue-shoulder band (about 465 nm) superimposed between them for all spectra. The UV peak shows an obvious redshift with increasing temperature. The analyses of Gaussian-fitted the blue-shoulder band and the green bands from 10 K to 300 K reveal that all spectra can be well fitted by three Gaussian peaks: blue peak (B, about 2.67 eV), two green peaks (G1, about 2.47 eV and G2, about 2.42 eV). With increasing temperature, the blue band shows blueshift. The green bands show anomalous transition: green band G1 shows blueshift-redshift transition and green band G2 shows redshift-blueshift. The origins of these bands and their temperature-dependent shifts are explained based on defect levels and strong carrier localization effect at the defect levels in addition to band-gap shrinkage.     

MOCVD法生长Ca_(0.5)In_(0.5)P外延层的近红外光致发光与温度的依赖关系

测量了用金属有机物化学气相沉积(MOCVD)方法在GaAs衬底上生长的Ga_(0.5)In_(0.5)P外延层的近红外光致发光光谱,观察到三个与深能级有关的发光带,其峰值能量分别为1.17,0.99和0.85eV.研究了这些发光带的发光强度,峰值位置和半宽度随温度的变化关系,并初步分析其来源.     

Short period (Si6Ge4)p superlattices: photoluminescence and electron microscopy study

Si–Ge heterostructures made of 6 monolayers of Si and 4 monolayers of Ge repeated p times (Si₆Ge₄)_p, strained on Si (1 0 0) substrates, have been investigated by photoluminescence measurements and electron microscopy. The films were grown at 400°C by molecular beam epitaxy, using Sb as surfactant. The photoluminescence results of the whole set of samples show similar spectra, for both the single Ge quantum well (p=1) and the thicker heterostructure (p=30). The phonon assisted transverse optical line is measured at about 40 meV far from the no-phonon one, and this corresponds to the Ge–Ge vibration. Our results demonstrate that excitonic recombination occurs mainly in the Ge layers and it is indirect in nature, whatever the repetition number (p) is. Furthermore, we evidenced a high localization of the photoluminescence process excluding any superperiodicity effect.