High-energy Vibrational Modes Research Articles

Highly uniform core/shell structures AuR/Ag and AuR/Ag@BSA with various shell thicknesses for surface-enhanced Raman scattering

This work indicates the synthesis of uniform core/shell nanostructures (AuR/Ag) with different thicknesses of Ag shell by the double seed method. This method consists of two self-sufficient progresses, one seed for the formation of gold nanorods (AuRs) and one for the formation of the Ag shell for the gold nanorods to form the AuR/Ag. Acid ascorbic (L-AA) acts as a weak reducing agent and hexadecyltrimethylammonium chloride (CTAC) acts as a surfactant for Ag shell. The formation and growth of the Ag shell were carefully investigated by changing the reaction factors such as temperature, time, and concentration of AgNO3. The greater the concentration of AgNO3 shelling precursor, the thicker the shell and therefore the more high-energy vibrational modes appear in the near-ultraviolet region. In survey of surface-enhanced Raman scattering effect of AuRs and AuR/Ag with Rhodamine B (RB) detector, the results show that AuR/Ag has the ability to enhance Raman signal much better than AuRs. At the same time, the thicker the Ag shell, the better the Raman signal enhancement ability.

Phonon-mediated excitation energy transfer in a detuned multi-sites system

Based on a ring-shaped arrangement of interacting two-level systems, we show the important role of the phonon-mediated quantum interference in excitation energy transfer, mimicking light-harvesting antenna in natural photosynthetic systems. The pigments in a ring-shaped photosynthetic system interact with the high-energy intramolecular vibrational mode, which arises from the vibrational motion of the scaffold of the system, with different coupling phases according to the position of each pigment respect to the vibrational motion. By investigating the model systems, we demonstrate that in the presence of large detuning between donor pigments and acceptor pigments, the efficiency of excitation transfer depends directly on the relative coupling phase between two adjacent pigments. Our model system, containing more pigments, shows a better robustness of the phonon-mediated excitation energy transfer. The present results are not only helpful in understanding natural photosynthesis, but also offer an optimal design principle for artificial photosynthesis.

Vibration-assisted coherent excitation energy transfer in a detuned dimer**Project supported by the National Natural Science Foundation of China (Grant No. 11174233).

The important role of high-energy intramolecular vibrational modes for excitation energy transfer in the detuned photosynthetic systems is studied. Based on a basic dimer model which consists of two two-level systems (pigments) coupled to high-energy vibrational modes, we find that the high-energy intramolecular vibrational modes can enhance the energy transfer with new coherent transfer channels being opened when the phonon energy matches the detuning between the two pigments. As a result, the energy can be effectively transferred into the acceptor. The effective Hamiltonian is obtained to reveal the strong coherent energy exchange among the donor, the acceptor, and the high-energy intramolecular. A semi-classical explanation of the phonon-assisted mechanism is also shown.

Phonon-assisted excitation energy transfer in photosynthetic systems**Project supported by the National Natural Science Foundation of China (Grant No. 11174233).

The phonon-assisted process of energy transfer aiming at exploring the newly emerging frontier between biology and physics is an issue of central interest. This article shows the important role of the intramolecular vibrational modes for excitation energy transfer in the photosynthetic systems. Based on a dimer system consisting of a donor and an acceptor modeled by two two-level systems, in which one of them is coupled to a high-energy vibrational mode, we derive an effective Hamiltonian describing the vibration-assisted coherent energy transfer process in the polaron frame. The effective Hamiltonian reveals in the case that the vibrational mode dynamically matches the energy detuning between the donor and the acceptor, the original detuned energy transfer becomes resonant energy transfer. In addition, the population dynamics and coherence dynamics of the dimer system with and without vibration-assistance are investigated numerically. It is found that, the energy transfer efficiency and the transfer time depend heavily on the interaction strength of the donor and the high-energy vibrational mode, as well as the vibrational frequency. The numerical results also indicate that the initial state and dissipation rate of the vibrational mode have little influence on the dynamics of the dimer system. Results obtained in this article are not only helpful to understand the natural photosynthesis, but also offer an optimal design principle for artificial photosynthesis.

Confined lattice dynamics of single and quadruple SnSe bilayers in [(SnSe)(1.04)](m)[MoSe2](n) ferecrystals.

The Sn specific densities of phonon states in the SnSe subunits of [(SnSe)1.04]m[MoSe2]n ferecrystals with (m,n) = (1,1), (4,1) and in bulk SnSe were derived from nuclear inelastic scattering by the (119)Sn Mössbauer resonance. Using different measurement configurations, phonons with polarization parallel and perpendicular to the ferecrystal plane were specifically probed. Vibrational properties and phonon spectral weight are found to strongly depend on the phonon polarization and layer count m. A highly peculiar feature of these ferecrystal densities of phonon states is the emergence of rather sharp high energy vibrational modes polarized perpendicular to the ferecrystal plane, which contrasts with usual findings in thin layered structures and nanostructured materials in general, and a depletion of modes with a gap appearing between acoustic and high energy modes. The spectral weight of these phonons depends on the overall SnSe content, m, but cannot be unambiguously attributed to SnSe-MoSe2 interfaces. Considering the low energy part of lattice dynamics, ferecrystals exhibit rather low average phonon group velocities as compared to the speed of sound in the long wavelength limit. For the (1,1) ferecrystal, this effect is most pronounced for vibrations polarized in the ferecrystal plane. Thus, an experimental microscopic origin for the vibrational and bonding anisotropy in subunits of ferecrystals is provided.

Temperature dependence of O2 singlet photoluminescence in silica nanoparticles

The near infrared singlet emission and photoluminescence lifetime of O2 molecules embedded in silica nanoparticles are studied from room temperature down to 10K. The area of the photoluminescence band under infrared excitation decreases for temperature above 100K and the lifetime is shortened. These observations provide evidence of a thermally activated relaxation channel with activation energy of about 40meV. This relaxation mechanism adds to the already known temperature independent electronic-to-vibrational coupling involving high energy vibrational modes of the host matrix or its impurities. The thermally activated process is suggested to consist in the breakage of the O2 molecule weak bonds with walls of the matrix interstices with ensuing molecular motion and collisional exchange of energy.

Photoluminescence and diffusion properties of O2 molecules in amorphous SiO2 nanoparticles

AbstractAn experimental study by Raman and Photoluminescence (PL) spectroscopies on the emission and diffusion properties of O2 molecules in amorphous SiO2 nanoparticles of commercial origin with diameters from 14 to 40 nm is reported. Stationary and time resolved PL measurements have been carried out to characterize the Near Infrared (NIR) emission at 1272 nm of O2. Emission features similar to those of bulk silica systems with a sharp PL band and excitation channels in the NIR, at 1070 nm, and in the visible, at 765 and 690 nm are found, with peculiarities arising from embedding O2 in nanostructures. The study of the NIR PL lifetime as a function of temperature down to 10 K enabled to reveal the presence of non radiative processes competitive to the radiative channel and to the non‐radiative one already highlighted and related to high energy vibrational modes of matrix or of impurity sites. By detecting the O2 content of nanoparticles, the dynamics of diffusion in the low temperature range from 98°C up to 177°C has been characterized. The applicability of Fick's theory has been tested, determining the diffusion coefficients and evidencing some peculiarities detectable in the nanosystem. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamics of ultrafast photoinduced heterogeneous electron transfer, implications for recent solar energy conversion scenarios

The general case of a heterogeneous electron transfer reaction is realized by ultrafast electron transfer from a photo-excited molecule to a wide continuum of electronic acceptor states. Two different theoretical model calculations addressing the injection dynamics have recently been presented. The first scenario predicts a wide energy distribution for the injected electron via excitations of high-energy vibrational modes in the ionized molecule, whereas the second scenario ascribes the width to thermal fluctuations. We present experimental data at different temperatures and identify the valid injection scenario for perylene/TiO2 systems. The results are discussed in view of recent solar energy conversion scenarios.

Ultrafast Photoinduced Electron Transfer at Electrodes: The General Case of a Heterogeneous Electron‐Transfer Reaction

The general case of a heterogeneous electron-transfer reaction is realized by ultrafast electron transfer from a light-absorbing molecule to a wide continuum of electronic acceptor states, realizing the so-called wide band limit. Experimental data obtained for perylene dye/TiO(2) systems confirm the predictions of fully quantum mechanical model calculations of the dynamics. The energy distribution of the injected electron shows an energy loss due to excitations of high-energy (quantum) vibrational modes in the ionized perylene moiety. The electron-transfer mechanism is non-adiabatic and the reaction is ultrafast, for example, with a time constant of 9 fs for the COOH anchor-bridge group. The underlying strong coupling of the electronic states to high-energy vibrational modes is a characteristic feature of sensitizer molecules.

Quantum oscillations of nitrogen atoms in uranium nitride

The vibrational excitations of crystalline solids corresponding to acoustic or optic one-phonon modes appear as sharp features in measurements such as neutron spectroscopy. In contrast, many-phonon excitations generally produce a complicated, weak and featureless response. Here we present time-of-flight neutron scattering measurements for the binary solid uranium nitride, showing well-defined, equally spaced, high-energy vibrational modes in addition to the usual phonons. The spectrum is that of a single atom, isotropic quantum harmonic oscillator and characterizes independent motions of light nitrogen atoms, each found in an octahedral cage of heavy uranium atoms. This is an unexpected and beautiful experimental realization of one of the fundamental, exactly solvable problems in quantum mechanics. There are also practical implications, as the oscillator modes must be accounted for in the design of generation IV nuclear reactors that plan to use uranium nitride as a fuel.

The separation of overlapping transitions in [formula omitted]-carotene with broadband 2D electronic spectroscopy

Broadband 2D electronic spectroscopy is applied to β-carotene, revealing new insight into the excited state dynamics of carotenoids by exploring the full energetic range encompassing the S0→S2 and S1→S1n transitions at 77K. Multiple signals are observed in the regime associated with the proposed S∗ state and isolated through separate analysis of rephasing and nonrephasing contributions. Peaks in rephasing pathways display dynamic lineshapes characteristic of coupling to high energy vibrational modes, and simulation with a simple model supports their assignment to impulsive stimulated Raman scattering. A signal persisting beyond 10ps in the nonrephasing spectra is still under investigation.

Surface Modification of Upconverting NaYF4 Nanoparticles with PEG−Phosphate Ligands for NIR (800 nm) Biolabeling within the Biological Window

We present a technique for the replacement of oleate with a PEG-phosphate ligand [PEG = poly(ethylene glycol)] as an efficient method for the generation of water-dispersible NaYF(4) nanoparticles (NPs). The PEG-phosphate ligands are shown to exchange with the original oleate ligands on the surface of the NPs, resulting in water-dispersible NPs. The upconversion intensity of the NPs in aqueous environments was found to be severely quenched when compared to the original NPs in organic solvents. This is attributed to an increase in the multiphonon relaxations of the lanthanide excited state in aqueous environments due to high energy vibrational modes of water molecules. This problem could be overcome partially by the synthesis of core/shell NPs which demonstrated improved photophysical properties in water over the original core NPs. The PEG-phosphate coated upconverting NPs were then used to image a line of ovarian cancer cells (CaOV3) to demonstrate their promise in biological application.

Comparison of Raman spectra and vibrational density of states between graphene nanoribbons with different edges

Vibrational properties of graphene nanoribbons are examined with density functional based tight-binding method and non-resonant bond polarization theory. We show that the recently discovered reconstructed zigzag edge can be identified from the emergence of high-energy vibrational mode due to strong triple bonds at the edges. This mode is visible also in the Raman spectrum. Total vibrational density of states of the reconstructed zigzag edge is observed to resemble the vibrational density of states of armchair, rather than zigzag, graphene nanoribbon. Edge-related vibrational states increase in energy which corroborates increased rigidity of the reconstructed zigzag edge.

Strong electron-phonon coupling of the high-energy modes of carbon nanotubes

We present ab initio calculations of electron-phonon coupling matrix elements of the totally symmetric high-energy vibrational modes of carbon nanotubes. The matrix elements depend on nanotube family $({n}_{1}\ensuremath{-}{n}_{2})\mathrm{mod}\phantom{\rule{0.2em}{0ex}}3$, chiral angle, and the particular optical transition, similarly to the radial-breathing mode. The strength of the matrix elements of the high-energy mode is up to 6 times higher than for the radial breathing mode. We discuss the implications of our results for the Raman spectrum of nanotubes and for charge carrier relaxation.

Electrochemical switching of the Peierls-like transition in metallic single-walled carbon nanotubes

The high-energy vibrational modes of metallic carbon nanotubes are believed to be softened compared to the semiconducting ones by a Peierls-like transition. The Raman modes, when excited with a red laser to enhance the metallic tubes, were found to exhibit an exceptionally high sensitivity to electrochemical doping. Our data may be interpreted as controlling the Peierls-like instability in metallic tubes with the applied potential. We also discuss the limits of applicability of the double-layer charging model and show Raman evidence for a hysteretic transition to a chemical doping regime.

Optical Properties of as-Grown and Proton Irradiated ZnO

Conventional photoluminescence spectroscopy (PL) shows several high energy optical transitions within the region 3.072 to 3.4345 eV from as-grown ZnO excited with a 257 nm UV line. Performing inelastic light scattering experiments (ILS), we probed some previously reported modes. Proton implantation caused drastic changes in the excitonic region and generates a broad band centered at about 650 nm (1.9eV) In addition, we observed from both sides of the ZnO sample implantation-induced high-energy vibrational modes centered at about 1350 and 1600 cm -1 (0.167 and 0.198 eV, respectively).

Detection of high-energy adsorbate vibrational modes by atom-surface scattering

It is suggested that high-frequency vibrational modes of surface adsorbates can produce very characteristic signature features in the observable inelastic intensity in experiments using atomic beams as scattering probes. These features consist of peaks in the inelastic background, due to multiquantum excitation of the high-energy vibrational modes, and these peaks are significantly broadened and shifted in the direction of energy loss by the multiphonon scattering arising from the substrate and other low-energy adsorbate modes. Calculations for the scattering of rare gases from several types of adsorbate indicate that, by selecting the probe projectile species and tuning the incident energy and incident beam angle, selected high-energy modes can be excited, and the method can be used to accurately measure the vibrational frequencies and polarizations of these modes.

Molecular dynamics simulations of vibrational lifetimes in amorphous silicon

Abstract Recent theoretical and time resolved Raman studies disagree on the lifetime of high energy vibrational modes in amorphous silicon. The latter suggests that the lifetime increases with increasing frequency and is of the order of 10 ns for the highest frequency modes, while the former predicts a picosecond timescale that follows the two-phonon density of states. Here we present the results of molecular dynamics simulations which are complementary to the perturbative calculations. At different temperatures, kinetic energy is put into selected modes of vibration in 216 and 4096 atom systems with periodic boundary conditions and the Stillinger—Weber potential. The lifetime of medium and high frequency modes is found to be of the order of 1 ops at 5, 10 and 30 K, in qualitative agreement with the perturbative calculations.

Photophysical studies of m-terphenyl-sensitized visible and near-infrared emission from organic 1∶1 lanthanide ion complexes in methanol solutions

The luminescence properties of several 11 lanthanide ion (Sm3+, Tb3+, Dy3+, Pr3+, Nd3+, Ho3+, Tm3+ and Yb3+) complexes based on the m-terphenyl-containing ligands 1–3 have been studied in methanol solutions. The organic complexes show their typical luminescence in the visible (Sm3+, Tb3+, Dy3+ and Pr3+) and in the near-infrared (Nd3+, Er3+ and Yb3+) region of the electromagnetic spectrum. The degree of shielding of the lanthanide ions from high-energy quenching modes of the solvent by the acyclic ligand 3 is less than the shielding by the macrocyclic ligands 1 and 2.Not only the high-energy vibrational modes of the solvent quench the luminescent state, but also the C–H modes of the organic ligand, and even O–D and C–D modes can contribute significantly to the quenching. In general, the high-energy vibrational O–H and C–H modes are most efficient in luminescence quenching, but the quenching is strongly dependent on the magnitude of the energy gap between the lowest luminescent state and a lower-lying state. Luminescence at longer wavelengths can be quenched relatively easily because of the smaller energy gaps, rendering all quenching pathways, especially quenching by the remaining C–H modes in the partially deuterated ligand, efficient. When the energy gap is resonant with (an overtone of) a vibrational mode, i.e. O–H, C–H, O–D or C–D, the luminescence is very efficiently quenched by these modes and can even be extinguished. For instance: Ho3+ luminescence was not observed because the 5S25F5 transition is resonant with the C–H vibrational mode, deuteration is less effective than expected for Pr3+ because the energy gap is resonant with the first overtone of the C–D vibration, and Nd3+ is efficiently quenched by the deuterated solvent because the energy gap is resonant with the first overtone of the O–D vibration.

Luminescence properties of m-terphenyl-based Eu3+ and Nd3+ complexes: visible and near-infrared emission

Two novel m-terphenyl-based organic ligands (2 and 3) have been synthesized. Photophysical studies show that the ligands form stable complexes with Eu3+, since typical Eu3+ luminescence is observed upon excitation of the ligand. The acyclic complex 2·Eu3+ shows relatively long lifetimes in methanol (e.g.τ = 0.72 ms in CH3OH). The acylic ligand 2 allows the additional coordination of two solvent molecules and their high-energy vibrational modes form the main quenching pathway for the Eu3+ luminescence. In comparison with 1·Eu3+, the more rigid dioxolane-containing complex 3·Eu3+ provides an additional donor atom, which reduces the solvent coordination of 3·Eu3+ (τ3·Eu3+ = 1.42 ms compared to τ1·Eu3+ = 0.75 ms in CH3OH). The high-energy vibrational modes of the organic ligand 3 are the most important quenchers. Typical near-infrared Nd3+ emission at 1060 and 1350 nm resulting from 3·Nd3+ has been observed. Quenching by the solvent is still operative for 3·Nd3+, because of the larger ionic radius of the Nd3+ ion.