Gagik G. Gurzadyan

The Origin of Fluorescence from Graphene Oxide

Time-resolved fluorescence measurements of graphene oxide in water show multiexponential decay kinetics ranging from 1 ps to 2 ns. Electron-hole recombination from the bottom of the conduction band and nearby localized states to wide-range valance band is suggested as origin of the fluorescence. Excitation wavelength dependence of the fluorescence was caused by relative intensity changes of few emission species. By introducing the molecular orbital concept, the dominant fluorescence was found to originate from the electronic transitions among/between the non-oxidized carbon regions and the boundary of oxidized carbon atom regions, where all three kinds of functionalized groups C-O, C = O and O = C-OH were participating. In the visible spectral range, the ultrafast fluorescence of graphene oxide was observed for the first time.

Time-resolved fluorescence spectroscopy of high-lying electronic states of Zn-tetraphenylporphyrin

Depopulation of the S2 excited electronic state of the Zn-tetraphenylporphyrin (ZnTPP) was monitored by measuring the decay of S2→S0 and the rise of S1→S0 fluorescence using the up-conversion fluorescence technique with a time-resolution of 120 fs. The lifetime of the S2 electronic state, measured for ZnTPP in ethanol (τS2=2.35 ps) correlates with the risetime of S1→S0 fluorescence. This result demonstrates the depopulation of S2 to S1 via (vibrational) states with lifetimes much shorter than that of S2. The rise time of S2 fluorescence (τv=60–90 fs) was attributed to vibrational relaxation (in S2). Fluorescence anisotropy decay of the S2 state was also studied by measuring the parallel and perpendicular fluorescence components. The high initial anisotropy of r⩾0.7 is interpreted as due to the existence of a degenerate excited electronic state S2 and the corresponding fast decay time τ1=0.2 ps to the electronic dephasing of the degenerate level pair. The long component of the anisotropy decay (τ2≫10 ps) i...

Optical and Excitonic Properties of Crystalline ZnS Nanowires: Toward Efficient Ultraviolet Emission at Room Temperature

A systematic investigation into the excitonic properties of wurtzite ZnS nanowires (NWs) is presented. Under optical excitation, the ZnS NWs exhibit strong ultraviolet (UV) emission. Optical transition from free exciton A, free exciton B, and shallow level emission are observed and analyzed through power-dependent and temperature-dependent photoluminescence spectroscopy measurements performed from 10 to 300 K. The excitonic transition and coupling strength of exciton-longitudinal optical phonon were directly determined from the evolution of exciton peak energy and peak width broadening. Our studies indicate that free excitons in ZnS nanowires are very stable, suggesting a great promise for high-efficiency light-emitting devices and lasers in the UV region. Finally, the carrier dynamics of the ZnS NWs were measured and analyzed for the first time by ultrafast spectroscopy.

Elucidating the role of disorder and free-carrier recombination kinetics in CH3NH3PbI3 perovskite films.

Apart from broadband absorption of solar radiation, the performance of photovoltaic devices is governed by the density and mobility of photogenerated charge carriers. The latter parameters indicate how many free carriers move away from their origin, and how fast, before loss mechanisms such as carrier recombination occur. However, only lower bounds of these parameters are usually obtained. Here we independently determine both density and mobility of charge carriers in a perovskite film by the use of time-resolved terahertz spectroscopy. Our data reveal the modification of the free carrier response by strong backscattering expected from these heavily disordered perovskite films. The results for different phases and different temperatures show a change of kinetics from two-body recombination at room temperature to three-body recombination at low temperatures. Our results suggest that perovskite-based solar cells can perform well even at low temperatures as long as the three-body recombination has not become predominant.

Eccentric Loading of Fluorogen with Aggregation-Induced Emission in PLGA Matrix Increases Nanoparticle Fluorescence Quantum Yield for Targeted Cellular Imaging

A simple strategy is developed to prepare eccentrically or homogeneously loaded nanoparticles (NPs) using poly (DL-lactide-co-glycolide) (PLGA) as the encapsulation matrix in the presence of different amounts of polyvinyl alcohol (PVA) as the emulsifier. Using 2,3-bis(4-(phenyl(4-(1,2,2-triphenylvinyl)-phenyl)amino)-phenyl)-fumaronitrile (TPETPAFN), a fluorogen with aggregation-induced emission (AIE) characteristics, as an example, the eccentrically loaded PLGA NPs show increased fluorescence quantum yields (QYs) as compared to the homogeneously loaded ones. Field emission transmission electron microscopy and fluorescence lifetime measurements reveal that the higher QY of the eccentrically loaded NPs is due to the more compact aggregation of AIE fluorogens that restricts intramolecular rotations of phenyl rings, which is able to more effectively block the non-radiative decay pathways. The eccentrically loaded NPs show far red/near infrared emission with a high fluorescence QY of 34% in aqueous media. In addition, by using poly([lactide-co-glycolide]-b-folate [ethylene glycol]) (PLGA-PEG-folate) as the co-encapsulation matrix, the obtained NPs are born with surface folic acid groups, which are successfully applied for targeted cellular imaging with good photostability and low cytotoxicity. Moreover, the developed strategy is also demonstrated for inorganic-component eccentrically or homogeneously loaded PLGA NPs, which facilitates the synthesis of polymer NPs with controlled internal architectures.

Mechanism Studies on the Superior Optical Limiting Observed in Graphene Oxide Covalently Functionalized with Upconversion NaYF4:Yb3+/Er3+ Nanoparticles

The increased utilization of high power laser sources has rendered great challenges for designing effi cient optical limiting (OL) materials to protect human eyes and various delicate optical devices. An ideal optical limiter should greatly attenuate an intense laser beam while exhibiting high transmittance for low-input optical intensity. [ 1 ] Up to now, numerous organic and inorganic materials have proved to be good candidates for optical limiters with different working mechanisms. Among them, carbon-based materials, such as fullerenes, carbon black, and carbon nanotubes, have exhibited very good OL performance. [ 2–5 ] As a typical representative, graphene, which consists of sp 2 -hybridized carbon atoms with single-atom thickness and 2D structure, has exhibited unique electronic, optical, and mechanical properties. [ 6 ] The interband optical transitions in graphene are independent of frequency over a wide range and depend only on the fi nestructure constant, thus it is naturally promising as a broadband optical limiter. [ 7 ] Another important advantage of graphene-based materials is that they can be easily functionalized with organic molecules and hybridized with inorganic nanomaterials via covalent bonding, to improve their OL performances through the synergistic effect. For example, the enhanced nonlinear optical properties have been reported in porphyrin-functionalized graphene, [ 8 ] organic dye ionic complex, [ 9 ] oligothiophene, [ 10 ] fullerene, [ 11 ] phthalocyanine, [ 12 ]

Ultrafast electron-optical phonon scattering and quasiparticle lifetime in CVD-grown graphene.

Ultrafast quasiparticle dynamics in graphene grown by chemical vapor deposition (CVD) has been studied by UV pump/white-light probe spectroscopy. Transient differential transmission spectra of monolayer graphene are observed in the visible probe range (400-650 nm). Kinetics of the quasiparticle (i.e., low-energy single-particle excitation with renormalized energy due to electron-electron Coulomb, electron-optical phonon (e-op), and optical phonon-acoustic phonon (op-ap) interactions) was monitored with 50 fs resolution. Extending the probe range to near-infrared, we find the evolution of quasiparticle relaxation channels from monoexponential e-op scattering to double exponential decay due to e-op and op-ap scattering. Moreover, quasiparticle lifetimes of mono- and randomly stacked graphene films are obtained for the probe photon energies continuously from 1.9 to 2.3 eV. Dependence of quasiparticle decay rate on the probe energy is linear for 10-layer stacked graphene films. This is due to the dominant e-op intervalley scattering and the linear density of states in the probed electronic band. A dimensionless coupling constant W is derived, which characterizes the scattering strength of quasiparticles by lattice points in graphene.

Femtosecond UV-pump/visible-probe measurements of carrier dynamics in stacked graphene films

The transient differential transmission (ΔT/T) spectra of graphene were obtained from 380 (3.3 eV) to 670 nm (1.9 eV). The intraband carrier equilibration by carrier–carrier scattering occurred within 60 fs. The subsequent carrier relaxation process was governed by carrier-optical phonon scattering and had linear dependence on the probe photon energy (Epr); lifetimes ranged from 180 to 90 fs for Epr from 2.1 to 2.8 eV. Negative ΔT/T signals in kinetic curves were discussed and assigned to thermal diffusion and shrinkage of band separation caused by lattice heating.

Metal-enhanced fluorescence of conjugated polyelectrolytes with self-assembled silver nanoparticle platforms.

We report the use of a simple Ag nanoparticle (NP) platform to enhance the fluorescence signatures of conjugated polyelectrolytes (CPEs). Ag NP platforms with different extinction intensities were fabricated by self-assembly of Ag NPs on NH(3)(+)-functionalized glass surface. Layer-by-layer (LBL) deposition of oppositely charged polyelectrolytes is used to control the Ag NP-CPE distance. The Ag NP platforms with high optical densities provide higher fluorescence enhancement factors for CPEs as compared to those with low optical densities. In addition, the CPE fluorescence enhancement is found to be directly related to the overlap between the absorption spectra of CPEs and the extinction spectra of Ag NP platforms. Both steady-state and time-resolved fluorescence spectroscopic studies reveal that the fluorescence enhancement is controlled by the increase in both absorption and radiative decay rates of the CPEs in proximity of Ag NPs. The enhanced CPE fluorescence signature is further used to detect single-stranded DNA using a Cy5 dye labeled peptide nucleic acid probe (Cy5-PNA) through Förster resonance energy transfer (FRET). We anticipate that the organic-inorganic hybrid platform will provide new opportunities for CPE application in sensing and device fabrication.

Picosecond transient absorption spectroscopy of trans-4-R-4′-nitrostilbenes with R: OMe, NH2 and NMe2

Abstract The spectroscopic properties of the trans isomer of four 4-R-4′-nitrostilbenes (I–R, R: H, OMe, NH 2 , NMe 2 ) in solution at 25°C were studied by time-resolved absorption techniques. An absorption increase within 10 ps and the subsequent decay in the visible range are attributed to the excited singlet state (S 1 →S n transition). The lifetime for this decay ( τ S ) varies for trans -I–NMe 2 from 1–3 ns in solvents of low polarity to ≅10 ps in acetonitrile. Also for trans -I–NH 2 and trans -I–OMe, τ S is sensitive to the solvent polarity. In those cases where the quantum yield of intersystem crossing is substantial, the transient absorption due to the T 1 →T n transition at an appropriate wavelength, increases with almost the same lifetime and then remains constant.