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The interplay between intra-band motion and inter-band transitions in photoexcited germanium is investigated by attosecond transient reflection spectroscopy. Advanced theoretical simulations interpret the sub-femtosecond optical response as charge carrier dynamics in the band structure.

We introduce a simple scheme in order to account for the coupling with longitudinal phonons within the first-principles Bethe-Salpeter approach based on many-body perturbation theory. This allows one to evaluate the reduction of exciton binding energies observed in polar semiconductors. The electron-phonon coupling is modeled from the macroscopic dielectric response in the infrared which, in turn, is calculated through density functional perturbation theory. In this way, the additional computational cost determined by our method is negligible. We first illustrate our approach in the case of bulk ZnS. Then, we discuss the limit of the Bethe-Salpeter equation in the case of strongly delocalized excitons and we show how the excitonic series of bulk ${\mathrm{Cu}}_{2}\mathrm{O}$ is well reproduced.

Two-dimensional boron is promising as a tunable monolayer metal for nano-optoelectronics. We study the optoelectronic properties of two likely allotropes of two-dimensional boron, β12 and δ6, using first-principles density functional theory and many-body perturbation theory. We find that both systems are anisotropic metals, with strong energy- and thickness-dependent optical transparency and a weak (

Two-dimensional boron (borophene) is a promising, newly synthesized monolayer metal with promising electronic and optical properties. Borophene has only been recently synthesized on silver substrates, and displays a variety of crystal structures and substrate-induced strains depending on the growth conditions and surface orientation. Here, we present an extensive first-principles study of the structural and optoelectronic properties of the two proposed structures of borophene, β12 and δ6, under strain. With a density functional theory analysis, we determine that the optical absorbance and electronic band structure are continuously tunable upon application of few percent of strain. Although both structures remain metallic with moderate strains of up to 6% applied, key features of the band structure, as well as the inplane anisotropy of the complex dielectric function and optical absorption, can be significantly modified.

Metal halide perovskite quantum wells (PQWs) are quantum and dielectrically confined materials exhibiting strongly bound excitons. The exciton transition dipole moment dictates absorption strength and influences interwell coupling in dipole-mediated energy transfer, a process that influences the performance of PQW optoelectronic devices. Here we use transient reflectance spectroscopy with circularly polarized laser pulses to investigate the optical Stark effect in dimensionally pure single crystals of

Metal halide perovskite quantum wells (PQWs) are quantum and dielectrically confined materials exhibiting strongly bound excitons. The exciton transition dipole moment dictates absorption strength and influences interwell coupling in dipole-mediated energy transfer, a process that influences the performance of PQW optoelectronic devices. Here we use transient reflectance spectroscopy with circularly polarized laser pulses to investigate the optical Stark effect in dimensionally pure single crystals of n = 1, 2, and 3 Ruddlesden–Popper PQWs. From these measurements, we extract in-plane transition dipole moments of 11.1 (±0.4), 9.6 (±0.6) and 13.0 (±0.8) D for n = 1, 2 and 3, respectively. We corroborate our experimental results with density functional and many-body perturbation theory calculations, finding that the nature of band edge orbitals and exciton wave function delocalization depends on the PQW “odd–even” symmetry. This accounts for the nonmonotonic relationship between transition dipole moment and PQW dimensionality in the n = 1–3 range.

We report valence band electronic structure evolution of graphene oxide (GO) upon its thermal reduction. Degree of oxygen functionalization was controlled by annealing temperatures, and an electronic structure evolution was monitored using real-time ultraviolet photoelectron spectroscopy. We observed a drastic increase in density of states around the Fermi level upon thermal annealing at ~600 oC. The result indicates that while there is an apparent band gap for GO prior to a thermal reduction, the gap closes after an annealing around that temperature. This trend of band gap closure was correlated with electrical, chemical, and structural properties to determine a set of GO material properties that is optimal for optoelectronics. The results revealed that annealing at a temperature of ~500 oC leads to the desired properties, demonstrated by a uniform and an order of magnitude enhanced photocurrent map of an individual GO sheet compared to as-synthesized counterpart., Comment: 9 pages, 4 figures in physica status solidi (a) 2016

By separating the photons from slow and fast decays of single and multi-exciton states in a time gated 2nd order photon correlation experiment, we show that solitary oxygen dopant states of single-walled carbon nanotubes (SWCNTs) allow emission of photon pairs with efficiencies as high as 44% of single exciton emission. Our pump dependent time resolved photoluminescence (PL) studies further reveal diffusion-limited exciton–exciton annihilation as the key process that limits the emission of multi-excitons at high pump fluences. We further postulate that creation of additional permanent exciton quenching sites occurring under intense laser irradiation leads to permanent PL quenching. With this work, we bring out multi-excitonic processes of solitary dopant states as a new area to be explored for potential applications in lasing and entangled photon generation.

Fluorescent defects have opened up exciting new opportunities to chemically tailor semiconducting carbon nanotubes for imaging, sensing, and photonics needs such as lasing, single photon emission, and photon upconversion. However, experimental measurements on the trap depths of these defects show a puzzling energy mismatch between the optical gap (difference in emission energies between the native exciton and defect trap states) and the thermal detrapping energy determined by application of the van ’t Hoff equation. To resolve this fundamentally important problem, here we synthetically incorporated a series of fluorescent aryl defects into semiconducting single-walled carbon nanotubes and experimentally determined their energy levels by temperature-dependent and chemically correlated evolution of exciton population and photoluminescence. We found that depending on the chemical nature and density of defects, the exciton detrapping energy is 14–77% smaller than the optical gap determined from photoluminesce...

The spatial extent of charged electronic states in semiconducting carbon nanotubes with indices (6,5) and (7,6) was evaluated using density functional theory. It was observed that electrons and holes self-trap along the nanotube axis on length scales of about 4 and 8 nm, respectively, which localize cations and anions on comparable length scales. Self-trapping is accompanied by local structural distortions showing periodic bond-length alternation. The average lengthening (shortening) of the bonds for anions (cations) is expected to shift the G-mode frequency to lower (higher) values. The smaller-diameter nanotube has reduced structural relaxation due to higher carbon-carbon bond strain. The reorganization energy due to charge-induced deformations in both nanotubes is found to be in the 30-60 meV range. Our results represent the first theoretical simulation of self-trapping of charge carriers in semiconducting nanotubes, and agree with available experimental data.

Covalent dopants in semiconducting single wall carbon nanotubes (SWCNTs) are becoming important as routes for introducing new photoluminescent emitting states with potential for enhanced quantum yields, new functionality, and as species capable of near-IR room-temperature single photon emission. The origin and behavior of the dopant-induced emission is thus important to understand as a key requirement for successful room-T photonics and optoelectronics applications. Here, we use direct correlated two-color photoluminescence imaging to probe how the interplay between the SWCNT bright E(11) exciton and solitary dopant sites yields the dopant-induced emission for three different dopant species: oxygen, 4-methoxybenzene, and 4-bromobenzene. We introduce a route to control dopant functionalization to a low level as a means for introducing spatially well-separated solitary dopant sites. Resolution of emission from solitary dopant sites and correlation to their impact on E(11) emission allows confirmation of dopants as trapping sites for localization of E(11) excitons following their diffusive transport to the dopant site. Imaging of the dopant emission also reveals photoluminescence intermittency (blinking), with blinking dynamics being dependent on the specific dopant. Density functional theory calculations were performed to evaluate the stability of dopants and delineate the possible mechanisms of blinking. Theoretical modeling suggests that the trapping of free charges in the potential well created by permanent dipoles introduced by dopant atoms/groups is likely responsible for the blinking, with the strongest effects being predicted and observed for oxygen-doped SWCNTs.

Here, we present an extensive first- principles study of the structural and optoelectronic properties of the two proposed structures of borophene under strain. With a density functional theory analysis, we determine that the optical absorbance and electronic band structure are continuously tunable upon application of few percent of strain. While both structures remain metallic with moderate strains of up to 6%, key features of the band structure, as well as the in-plane anisotropy of the complex dielectric function and optical absorption can be significantly modified.

Functionalized 3D nanographenes with controlled electronic properties have been synthesized through a multistep organic synthesis method and are further used as promising anode materials for lithium-ion batteries, exhibiting a much increased capacity (up to 950 mAh g-1 ), three times higher than that of the graphite anode (372 mAh g-1 ).

The conformational structure and the electronic properties of various electronic excitations in cycloparaphenylenes (CPPs) are calculated using hybrid density functional theory (DFT). The results demonstrate that wavefunctions of singlet and triplet excitons as well as the positive and negative polarons remain fully delocalized in CPPs. In contrast, these excitations in larger CPP molecules become localized on several phenyl rings, which are locally planarized, while the undeformed ground state geometry is preserved on the rest of the hoop. As evidenced by the measurements of bond-length alternation and dihedral angles, localized regions show stronger hybridization between neighboring bonds and thus enhanced electronic communication. This effect is even more significant in the smaller hoops, where phenyl rings have strong quinoid character in the ground state. Thus, upon excitation, electron–phonon coupling leads to the self-trapping of the electronic wavefunction and release of energy from fractions of an eV up to two eVs, depending on the type of excitation and the size of the hoop. The impact of such localization on electronic and optical properties of CPPs is systematically investigated and compared with the available experimental measurements.

We present a method for preparation of graphene molecules (GMs), whereby a polyphenylene precursor functionalized with surface anchoring groups, preadsorbed on surface of TiO2, is oxidatively dehydrogenated in situ via a Scholl reaction. The reaction, performed at ambient conditions, yields surface adsorbed GMs structurally and electronically equivalent to those synthesized in solution. The new synthetic approach reduces the challenges associated with the tendency of GMs to aggregate and provides a convenient path for integration of GMs into optoelectronic applications. The surface synthesized GMs can be effectively reduced or oxidized via an interfacial charge transfer and can also function as sensitizers for metal oxides in light harvesting applications. Sensitized solar cells (SSCs) prepared from mesoscopic TiO2/GM films and an iodide-based liquid electrolyte show photocurrents of ∼2.5 mA/cm2, an open circuit voltage of ∼0.55 V and fill factor of ∼0.65 under AM 1.5 illumination. The observed power conversion efficiency of η=0.87% is the highest reported efficiency for the GM sensitized solar cell. The performance of the devices was reproducible and stable for a period of at least 3 weeks. We also report first external and internal quantum efficiency measurements for GM SSCs, which point to possible paths for further performance improvements.

Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron-phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes. Fil: Adamska, Lyudmyla. Los Alamos National Laboratory. Los Alamos; Estados Unidos Fil: Nayyar, Iffat. Los Alamos National Laboratory. Los Alamos; Estados Unidos Fil: Chen, Hang. Boston University; Estados Unidos Fil: Swan, Anna K.. Boston University; Estados Unidos Fil: Oldani, Andres Nicolas. Universidad Nacional de Quilmes; Argentina Fil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; Argentina Fil: Golder, Matthew R.. University of Oregon; Estados Unidos Fil: Jasti, Ramesh. University of Oregon; Estados Unidos Fil: Doorn, Stephen K.. Los Alamos National Laboratory. Los Alamos; Estados Unidos Fil: Tretiak, Sergei. Los Alamos National Laboratory. Los Alamos; Estados Unidos

Tradition graphene anodes in lithium ion batteries (LIBs) suffer significant performance loss due to the restacking of graphene layers. In this work, we have demonstrated a facile synthesis of preparing a series of graphene-derived nanomaterials. Among other potential application, these nanomaterials achieved an excellent cyclic durability with much enhanced capacity as novel anode materials in LIBs. These exceptional metrics observed within the novel assemblies is primarily due to the robust structure along with the optimal d-spacing allowing for facilitated Li adsorption/desorption and diffusion. Furthermore, the significantly enhanced rate performance was achieved after optimizing the structure of the self-assemblies. Development of nanomaterials anode suggests that the optimal design of graphene materials with robust structures is very important for next generation LIBs with high-energy storage efficiencies.

Structural, electronic, and magnetic properties of simple interfaces (graphene on top of a metallic substrate) and complex interfaces (a single metallic adlayer on a simple graphene/metal system, either on top or between the graphene and metallic substrate) have been studied using density functional theory. Two types of simple interface with strong (Ni/graphene) and weak (Cu/graphene) bonding were considered. In addition to binding energies and interface distances, which are used to quantify the strength of graphene-substrate interactions, the bonding in simple and complex interfaces was analyzed using charge density distributions and bond orders. Substantial enhancement of the metallic substrate/graphene binding was observed in complex interfaces, consisting of a Ni monolayer on top of a simple {Ni or Cu}/graphene interface. The increase of substrate-graphene bonding in such complex interfaces is accompanied by weakening of in-plane C-C bonds in graphene, as quantified by the bond orders. A weak ferrimagnetism in graphene, i.e., unequal magnetic moments $\ensuremath{-}0.04{\ensuremath{\mu}}_{B}$ and $+0.06{\ensuremath{\mu}}_{B}$ on the C atoms, is induced by a ferromagnetic Ni substrate. The strength of graphene-substrate interactions is also reflected in simulated scanning tunneling microscopy images.

A novel growth mechanism of graphene on Ni(111) has been discovered that occurs at temperatures below 460 °C. At these conditions, a surface-confined nickel−carbide phase coexists with single layer graphene. The graphene grows by in-plane transformation of the carbide along a one-dimensional phase-boundary, which is distinctively different from known growth processes on other transition metals and on Ni above 460 °C, where carbon atoms attach to “free” edges of graphene islands.

In this paper, we show that electron-plasmon interactions play a critical role in resonant charge transfer in molecular tunnel junctions (MTJs). Only when such interactions are taken into account, does it become possible to obtain the correct values of the turn-on threshold voltage in the current-voltage characteristics observed in experiment. In addition, we predict the emission of photons from MTJs with a large quantum yield $\ensuremath{\sim}{10}^{\ensuremath{-}2}$ as a result of the excitation of surface plasmons upon the application of a voltage across the junction. Finally, we discuss physical conditions, which are necessary for the experimental observation of this phenomenon.

In the molecular electronics field it is highly desirable to engineer the structure of molecules to achieve specific functions. In particular, diode (or rectification) behaviour in single molecules is an attractive device function. Here we study charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules covalently bound to two electrodes. The orientation of the diblock is controlled through a selective deprotection strategy, and a method in which the electrode-electrode distance is modulated unambiguously determines the current-voltage characteristics of the single-molecule device. The diblock molecule exhibits pronounced rectification behaviour compared with its homologous symmetric block, with current flowing from the dipyrimidinyl to the diphenyl moieties. This behaviour is interpreted in terms of localization of the wave function of the hole ground state at one end of the diblock under the applied field. At large forward current, the molecular diode becomes unstable and quantum point contacts between the electrodes form.

We study the order-from-disorder transition and reentrant magnetism in La(2-x)Sr(x)Cu(1-z)Zn(z)O(4) within the framework of a long-wavelength nonlinear sigma model that properly incorporates the Dzyaloshinskii-Moriya and XY anisotropies. Doping with nonmagnetic impurities, such as Zn, is considered according to classical percolation theory, whereas the effect of Sr, which introduces charge carriers into the CuO(2) planes, is described as a dipolar frustration of the antiferromagnetic order. We calculate several magnetic, thermodynamic, and spectral properties of the system, such as the antiferromagnetic order parameter, the Neel temperature, the spin-stiffness, and the anisotropy gaps, as well as their evolution with both Zn and Sr doping. We explain the nonmonotonic and reentrant behavior experimentally observed for T_N by Hucker et al. in Phys. Rev. B 59, R725 (1999), as resulting from the reduction, due to the nonmagnetic impurities, of the dipolar frustration induced by the charge carriers (order-from-disorder). Furthermore, we find a similar nonmonotonic and reentrant behavior for all the other observables studied. Most remarkably, our results show that while for x=2% and z=0 the Dzyaloshinskii-Moriya gap \Delta_{DM}=0, for z=15% it is approximately \Delta_{DM} = 7.5 cm^(-1). The later is larger than the lowest low-frequency cutoff for Raman spectroscopy (~ 5 cm^(-1)), and could thus be observed in one-magnon Raman scattering.

The approach based on multimode system of q-deformed oscillators and the related picture of ideal gas of q-bosons enables to effectively describe the observed non-Bose type behaviour, in experiments on heavy-ion collisions, of the intercept (or the "strength") $\lambda$ of the two-particle correlation function of identical pions or kaons. In this paper we extend main results of that approach in the two aspects: first, we derive in explicit form the intercepts of n-particle correlation functions in the case of q-Bose gas model and, second, provide their explicit two-parameter (or qp-) generalization., Comment: 10 pages, latex; v2: minor corrections; v3: minor improvement mainly to match referee's remark

First-principles density functional theory calculations were performed to investigate the effect of Sn surface alloying on the strength of interface interactions between graphene and Ni(111) or Cu(111) substrates. A substantial reduction of graphene-metal interactions was observed for the graphene/Sn-Ni(111) interface: binding energy was reduced from 0.055 eV/(C atom) to 0.015 eV/(C atom); interface distance was increased from 2.12 A to 3.52 A. The initially weak graphene/Cu(111) interface was hardly affected by Sn surface alloying. Electronic structure calculations, including local density of states and simulated scanning tunneling microscopy images, provide further details on the changing character of graphene-Ni(111) interactions upon Sn alloying.

The formation of single-layer graphene by exposure of a Ni(111) surface to ethylene at low pressure has been investigated. Two different growth regimes were found. At temperatures between 480 and 650 °C, graphene grows on a pure Ni(111) surface in the absence of a carbide. Below 480 °C, graphene growth competes with the formation of a surface Ni2C carbide. This Ni2C phase suppresses the nucleation of graphene. Destabilization of the surface carbide by the addition of Cu to the surface layer facilitates the nucleation and growth of graphene at temperatures below 480 °C. In addition to the growth of graphene on Ni substrates, the interaction between graphene and Ni was also studied. This was done both experimentally by Ni deposition on Ni-supported graphene and by density functional theory calculation of the work of adhesion between graphene and Ni. For graphene sandwiched between two Ni-layers, the work of adhesion between graphene and the Ni substrate was found to be four times as large as that for the Ni-supported graphene without a top layer. This stronger interaction may cause the destruction of graphene that is shown experimentally to occur at ~200 °C when Ni is deposited on top of Ni-supported graphene. The destruction of graphene allows the Ni deposits to merge with the substrate Ni. After the completion of this process, the graphene sheet is re-formed on top of the Ni substrate, leaving no Ni at the surface.