Identification of surface structures on 3C-SiC nanocrystals with hydrogen and hydroxyl bonding by photoluminescence

SiC nanocrystals (NCs) exhibit unique surface chemistry and possess special properties. This provides the opportunity to design suitable surface structures by terminating the surface dangling bonds with different atoms thereby boding well for practical applications. In this article, we report the photoluminescence properties of 3C-SiC NCs in water suspensions with different pH values. Besides a blue band stemming from the quantum confinement effect, the 3C-SiC NCs show an additional photoluminescence band at 510 nm when the excitation wavelengths are longer than 350 nm. Its intensity relative to the blue band increases with the excitation wavelength. The 510 nm band appears only in acidic suspensions but not in alkaline ones. Fourier transform infrared, X-ray photoelectron spectroscopy, and X-ray absorption near-edge structure analyses clearly reveal that the 3C-SiC NCs in the water suspension have Si-H and Si-OH bonds on their surface, implying that water molecules only react with a Si-terminated surface. First-principle calculations suggest that the additional 510 nm band arises from structures induced by H(+) and OH(-) dissociated from water and attached to Si dimers on the modified (001) Si-terminated portion of the NCs. The size requirement is consistent with the observation that the 510 nm band can only be observed when the excitation wavelengths are relatively large, that is, excitation of bigger NCs.

Mn²+-bonded reduced graphene oxide with strong radiative recombination in broad visible range caused by resonant energy transfer

The photoluminescence (PL) characteristics of Mn(2+)-bonded reduced graphene oxide (rGO) are studied in details. The Mn(2+)-bonded rGO is synthesized using MnO(2)-decorated GO as the intermediate products and ideal tunable PL is obtained by enhancing the long-wavelength (450-550 nm) emission. The PL spectra excited by different wavelengths are analyzed to elucidate the mechanism, and the resonant energy transfer between Mn(2+) and sp(2) clusters of the rGO appears to be responsible for the enhanced long-wavelength emission. To examine the effect of Mn(2+) on the long-wavelength emission from the Mn(2+)-bonded rGO, the PL characteristics of Mn(2+)-bonded rGO with smaller Mn concentrations are studied and weaker emission is observed. Our theoretical calculation corroborates the experimental results.

Green light stimulates terahertz emission from mesocrystal microspheres

The discovery of efficient sources of terahertz radiation has been exploited in imaging applications, and developing a nanoscale terahertz source could lead to additional applications. High-frequency mechanical vibrations of charged nanostructures can lead to radiative emission, and vibrations at frequencies of hundreds of kilohertz have been observed from a ZnO nanobelt under the influence of an alternating electric field. Here, we observe mechanical resonance and radiative emission at ∼ 0.36 THz from core-shell ZnO mesocrystal microspheres excited by a continuous green-wavelength laser. We find that ∼ 0.016% of the incident power is converted into terahertz radiation, which corresponds to a quantum efficiency of ∼ 33%, making the ZnO microspheres competitive with existing terahertz-emitting materials. The mechanical resonance and radiation stem from the coherent photo-induced vibration of the hexagonal ZnO nanoplates that make up the microsphere shells. The ZnO microspheres are formed by means of a nonclassical, self-organized crystallization process, and represent a straightforward route to terahertz radiation at the nanoscale.

Resonant coupling of bound excitons with LO phonons in ZnO: Excitonic polaron states and Fano interference

We report on a photoluminescence observation of robust excitonic polarons due to resonant coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, resonant coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror symmetric with respect to the ZPL, strongly suggesting that they are from different coupling states of exciton and phonons. Besides these spectral features showing the quasiparticle properties of exciton-phonon coupling system, the first-order SL is found to exhibit characteristic Fano lineshape, caused by quantum interference between the LO components of excitonic polarons and the continuous phonon bath. These findings lead to a new insight into fundamental effects of exciton-phonon interactions.

Tin oxide nanoribbons with vacancy structures in luminescence-sensitive oxygen sensing

Vacancy structures in tin oxide nanoribbons fabricated via thermal evaporation and post-processing are probed by luminescence spectroscopy, and interesting properties that bode well for oxygen sensing are observed. Besides a broad 620-nm band, the fabricated tin oxide nanoribbons show a photoluminescence band at 480 nm when the measurement temperature is <100 K. The blue band appears from nanoribbons synthesized under high oxygen pressure or annealed under oxygen. The dependence suggests that the oxygen interstitial and vacancy densities determine the electronic states that produce the blue band. Calculation of the electron structures based on the density functional theory shows that decreased oxygen vacancies or increased oxygen interstitials enhance the 480-nm band but suppress the 620-nm band. The results reported here indicate that the tin oxide nanoribbons with vacancy structures have potential applications in luminescence-sensitive oxygen sensing.

Scaling of directed dynamical small-world networks with random responses

A dynamical model of small-world networks, with directed links which describe various correlations in social and natural phenomena, is presented. Random responses of sites to the input message are introduced to simulate real systems. The interplay of these ingredients results in the collective dynamical evolution of a spinlike variable S(t) of the whole network. The global average spreading length <L>(s) and average spreading time <T>(s) are found to scale as p(-alpha)ln(N with different exponents. Meanwhile, S(t) behaves in a duple scaling form for N>>N(*): S approximately f(p(-beta)q(gamma)t), where p and q are rewiring and external parameters, alpha, beta, and gamma are scaling exponents, and f(t) is a universal function. Possible applications of the model are discussed.

Glycerol-bonded 3C-SiC nanocrystal solid films exhibiting broad and stable violet to blue-green emission

We have produced glycerol-bonded 3C-SiC nanocrystal (NC) films, which when excited by photons of different wavelengths, produce strong and tunable violet to blue-green (360-540 nm) emission as a result of the quantum confinement effects rendered by the 3C-SiC NCs. The emission is so intense that the emission spots are visible to the naked eyes. The light emission is very stable and even after storing in air for more than six months, no intensity degradation can be observed. X-ray photoelectron spectroscopy and absorption fine structure measurements indicate that the Si-terminated NC surfaces are completely bonded to glycerol molecules. Calculations of geometry optimization and electron structures based on the density functional theory for 3C-SiC NCs with attached glycerol molecules show that these molecules are bonded on the NCs causing strong surface structural change, while the isolated levels in the conduction band of the bare 3C-SiC NCs are replaced with quasi-continuous bands that provide continuous tunability of the emitted light by changing the frequencies of exciting laser. As an application, we demonstrate the potential of using 3C-SiC NCs to fabricate full-color emitting solid films by incorporating porous silicon.

Controlling Electron Spin Decoherence in Nd-based Complexes via Symmetry Selection

Long decoherence time is a key consideration for molecular magnets in the application of the quantum computation. Although previous studies have shown that the local symmetry of spin carriers plays a crucial part in the spin-lattice relaxation process, its role in the spin decoherence is still unclear. Herein, two nine-coordinated capped square antiprism neodymium moieties [Nd(CO₃)₄H₂O]^5– with slightly different local symmetries, C₁ versus C₄ (1 and 2), are reported, which feature in the easy-plane magnetic anisotropy as shown by the high-frequency electron paramagnetic resonance (HF-EPR) studies. Detailed analysis of the relaxation time suggests that the phonon bottleneck effect is essential to the magnetic relaxation in the crystalline samples of 1 and 2. The 240 GHz Pulsed EPR studies show that the higher symmetry results in longer decoherence times, which is supported by the first principle calculations.

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Neodymium-based complexes show the spin decoherence without the magnetic dilution

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The higher structural symmetry results in longer spin decoherence times

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The phonon bottleneck effect induces the slow magnetization relaxation behaviors

Enhancement of transport in DNA-like systems induced by backbone disorder

We report a theoretical study highlighting the fundamental effects of backbone disorder which simulates the environmental complications on charge transport properties of biological and synthetic DNA molecules. Based on effective tight-binding models of duplex DNA, the Lyapunov coefficient and current-voltage characteristics are numerically calculated by varying the backbone disorder degree. In contrast to the localization picture that the conduction of duplex DNA becomes poorer when the backbone disorder degree is increased, we find that the backbone disorder can enhance the charge transport ability of the DNA molecules when the environment-induced disorder surpasses a critical value, giving rise to a semiconducting-metallic transition. The physical origin for this is traced back to the antiresonant effects. These results provide a scenario to interpret a variety of transport behaviors observed in DNA molecules and suggest perspectives for future experiments intending to control the charge transport through DNA-based nanodevices.

Dephasing and dissipation in a source-drain model of light-harvesting systems

The energy transport process in natural-light-harvesting systems is investigated by solving the time-dependent Schrödinger equation for a source-network-drain model incorporating the effects of dephasing and dissipation, owing to coupling with the environment. In this model, the network consists of electronically coupled chromophores, which can host energy excitations (excitons) and are connected to source channels, from which the excitons are generated, thereby simulating exciton creation from sunlight. After passing through the network, excitons are captured by the reaction centers and converted into chemical energy. In addition, excitons can reradiate in green plants as photoluminescent light or be destroyed by nonphotochemical quenching (NPQ). These annihilation processes are described in the model by outgoing channels, which allow the excitons to spread to infinity. Besides the photoluminescent reflection, the NPQ processes are the main outgoing channels accompanied by energy dissipation and dephasing. From the simulation of wave-packet dynamics in a one-dimensional chain, it is found that, without dephasing, the motion remains superdiffusive or ballistic, despite the strong energy dissipation. At an increased dephasing rate, the wave-packet motion is found to switch from superdiffusive to diffusive in nature. When a steady energy flow is injected into a site of a linear chain, exciton dissipation along the chain, owing to photoluminescence and NPQ processes, is examined by using a model with coherent and incoherent outgoing channels. It is found that channel coherence leads to suppression of dissipation and multiexciton super-radiance. With this method, the effects of NPQ and dephasing on energy transfer in the Fenna-Matthews-Olson complex are investigated. The NPQ process and the photochemical reflection are found to significantly reduce the energy-transfer efficiency in the complex, whereas the dephasing process slightly enhances the efficiency. The calculated absorption spectrum reproduces the main features of the measured counterpart. As a comparison, the exciton dynamics are also studied in a linear chain of pigments and in a multiple-ring system of light-harvesting complexes II (LH2) from purple bacteria by using the Davydov D1 ansatz. It is found that the exciton transport shows superdiffusion characteristics in both the chain and the LH2 rings.

Optical emission from excess Si defect centers in Si nanostructures

Four groups of Si nanostructures with and without beta-SiC nanocrystals were fabricated for clarifying the origin of a blue emission with a double-peak structure at 417 and 436 nm. Spectral analyses and microstructural observations show that the blue emission is related to the existence of excess Si atoms in these Si nanostructures. The energy levels of electrons in Si nanocrystals with vacancy defects formed from the excess Si atoms are calculated and the characteristics of the obtained density of states coincide with the observed double-peak emission. The present work provides a possible mechanism of the blue emission in various Si nanostructures.

Enhancement of coherent energy transport by disorder and temperature in light harvesting processes

We investigate the influence of static disorder and thermal excitations on excitonic energy transport in the light-harvesting apparatus of photosynthetic systems by solving the Schrödinger equation and taking into account the coherent hoppings of excitons, the rates of exciton creation and annihilation in antennas and reaction centers, and the coupling to thermally excited phonons. The antennas and reaction centers are modeled, respectively, as the sources and drains which provide the channels for creation and annihilation of excitons. Phonon modes below a maximum frequency are coupled to the excitons that are continuously created in the antennas and depleted in the reaction centers, and the phonon population in these modes obeys the Bose-Einstein distribution at a given temperature. It is found that the energy transport is not only robust against the static disorder and the thermal noise, but it can also be enhanced by increasing the randomness and temperature in most parameter regimes. Relevance of our work to the highly efficient energy transport in photosynthetic systems is discussed.

Influence of backbone on the charge transport properties of G4-DNA molecules: a model-based calculation

We put forward a model Hamiltonian to describe the influence of backbone energetics on charge transport through guanine-quadruplex DNA (G4-DNA) molecules. Our analytical results show that an energy gap can be produced in the energy spectrum of G4-DNA by hybridization effects between the backbone and the base and by on-site energy difference of the backbone from the base. The environmental effects are investigated by introducing different types of disorder into the backbone sites. Our numerical results suggest that the localization length of G4-DNA can be significantly enhanced by increasing the backbone disorder degree when the environment-induced disorder is sufficiently large. There exists a backbone disorder-induced semiconducting-metallic transition in short G4-DNA molecules, where G4-DNA behaves as a semiconductor if the backbone disorder is weak and behaves as a conductor if the backbone disorder degree surpasses a critical value.

Large-bandgap behavior in transport of electrons through individual DNA molecules caused by coupling with a two-level system

We propose a model to interpret the large-bandgap behavior in transport of electrons through an individual DNA molecule where the tunneling electrons are coupled with two-level systems (TLS). The TLS can be regarded as the simplest way to describe vibrations and inelastic scattering in the molecules if the two levels represent the low-lying phonon states. The nonlinear current-voltage curves can be derived by the use of the transfer matrices in an equivalent single-particle multichannel network. At low temperatures, the gap of the conduction band is sensitive to the strength of the coupling between the TLS and the conduction electrons. It is shown that the large-bandgap behavior similar to that of semiconductors stems from the inelastic scattering by the TLS.

Violation of the single-parameter scaling hypothesis in human chromosome 22 with charge transfer models

We investigate transport properties of DNA sequences in human chromosome 22 and compare the results with those of a random artificial DNA sequence based on the single- and double-stranded charge transfer models. The statistical quantities, including the Hurst exponent, the distribution of Lyapunov exponent (LE), the central moments, and the scaling parameter, are numerically calculated by using the transfer-matrix approach. It is found that the existence of satellite DNA segments in human chromosome 22 could result in deviations from usual Gaussian distribution of LE. Our results suggest that the presence of the satellite DNA segments, together with the long-range correlations and the base-pairing correlations could lead to the violation of single-parameter scaling hypothesis which holds for the random artificial DNA sequence although the behaviors of the averaged LEs for both DNA sequences are similar. This provides a viewpoint to analyze differences between the genomic DNA sequences and the nonliving random ones on the basis of localization properties of wave functions in the sequences.