Anisotropy and interplane interactions in the dielectric response of graphite

Unveiling diverse coordination-defined electronic structures of reconstructed anatase TiO2(001)-(1 × 4) surface

Transition metal oxides (TMOs) exhibit fascinating physicochemical properties, which originate from the diverse coordination structures between the transition metal and oxygen atoms. Accurate determination of such structure-property relationships of TMOs requires to correlate structural and electronic properties by capturing the global parameters with high resolution in energy, real, and momentum spaces, but it is still challenging. Herein, we report the determination of characteristic electronic structures from diverse coordination environments on the prototypical anatase-TiO2(001) with (1 × 4) reconstruction, using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/atomic force microscopy, in combination with density functional theory calculation. We unveil that the shifted positions of O 2s and 2p levels and the gap-state Ti 3p levels can sensitively characterize the O and Ti coordination environments in the (1 × 4) reconstructed surface, which show distinguishable features from those in bulk. Our findings provide a paradigm to interrogate the intricate reconstruction-relevant properties in many other TMO surfaces.

Superior mechanical flexibility, lattice thermal conductivity and electron mobility of the hexagonal honeycomb carbon nitride monolayer

Nitrogen is the nearest neighbor element of carbon and, thus, the hexagonal honeycomb carbon nitride monolayer (C_xN_y), which consists of a covalent network of carbon and nitrogen atoms, usually has attractive physical and chemical properties similar to those in graphene. Here, we systematically investigate the geometric structure, mechanical properties, thermal transport properties, and plasmon excitation of a new phase, labeled C₃N₂, and make a detailed comparison with other possible C_xN_y allotropes. All C_xN_y have a super-high layer modulus and Young's modulus. But compared with the others, C₃N₂ exhibits excellent mechanical flexibility, and can withstand a relatively high critical strain up to 20% (18%) along the X(Y) direction. Additionally, C₃N₂ also has excellent thermal and electronic transport properties, with a super-high lattice thermal conductivity of ∼110.9 W m^-1 K^-1 and electron mobility of ∼1617.52 cm² V^-1 s^-1 at 300 K. By performing time-dependent density functional theory (TDDFT), we obtain the optical absorptions of C₃N₂ and C₃N, and meanwhile analyze their Fourier transforms of induced charge densities at some resonant frequencies. The main optical absorption peaks of the C₃N₂ nanostructure are located in the ultraviolet region, and its plasmon peaks are far higher than those in C₃N. Its excellent mechanical and optical properties, the larger electronic band gap, and the higher electron mobility suggest that C₃N₂ has great potential for application in nanoelectronics and optoelectronics.

Cost-Effective Calculation of Collective Electronic Excitations in Graphite Intercalated Compounds

Graphite/graphene intercalation compounds with good and improving electrical transport properties, optical properties, magnetic properties and even superconductivity are widely used in battery, capacitors and so on. Computational simulation helps with predicting important properties and exploring unknown functions, while it is restricted by limited computing resources and insufficient precision. Here, we present a cost-effective study on graphite/graphene intercalation compounds properties with sufficient precision. The calculation of electronic collective excitations in AA-stacking graphite based on the tight-binding model within the random phase approximation framework agrees quite well with previous experimental and calculation work, such as effects of doping level, interlayer distance, and interlayer hopping on 2D π plasmon and 3D intraband plasmon modes. This cost-effective simulation method can be extended to other intercalation compounds with unlimited intercalation species.

Secondary Electron Emission by Plasmon-Induced Symmetry Breaking in Highly Oriented Pyrolytic Graphite

Two-particle spectroscopy with correlated electron pairs is used to establish the causal link between the secondary electron spectrum, the (π+σ) plasmon peak, and the unoccupied band structure of highly oriented pyrolytic graphite. The plasmon spectrum is resolved with respect to the involved interband transitions and clearly exhibits final state effects, in particular due to the energy gap between the interlayer resonances along the ΓA direction. The corresponding final state effects can also be identified in the secondary electron spectrum. Interpretation of the results is performed on the basis of density-functional theory and tight-binding calculations. Excitation of the plasmon perturbs the symmetry of the system and leads to hybridization of the interlayer resonances with atomlike σ^{*} bands along the ΓA direction. These hybrid states have a high density of states as well as sufficient mobility along the graphite c axis leading to the sharp ∼3  eV resonance in the spectrum of emitted secondary electrons reported throughout the literature.

Optical absorption and excitation spectra of monolayer blue phosphorene

We study the optical absorption and excitation spectra of monolayer blue phosphorene with two approaches. The first is based on the [Formula: see text] approximation in conjunction with the Bethe-Salpeter equation theory. The second is based on the time-dependent density-functional theory in the adiabatic local density approximation and the random phase approximation. The spectra from the two approaches are quite similar. The optical absorption spectrum is dominated by a single peak at 4.2 eV, which originates from direct interband transitions at the [Formula: see text] point of the Brillouin zone. The excitation spectrum is dominated by a plasmon peak at 9.2 eV, which arises from collective excitations of valence electrons. The plasmon shows a positive dispersion at finite momentum transfer. The in-plane electron is responsible for the optical absorption, whereas the out-of-plane electron is responsible for the plasmon dispersion. Monolayer blue phosphorene has an indirect band gap of 2.98 eV and an exciton binding energy of 1.03 eV.

Complex momentum behavior of electronic excitations in β-CuPc

The electronic excitation spectrum of β-CuPc has been investigated using electron energy-loss spectroscopy in transmission. Our results demonstrate a rather strong momentum dependence of the lowest exciton features. Both main components show a negative dispersion, and the momentum dependence indicates that this negative dispersion is parallel to the molecular stacks in β-CuPc. In addition, the spectral shape also varies upon increasing momentum transfer indicating a particular momentum dependence of the inter-molecular interactions.

Local Plasmon Engineering in Doped Graphene

Single-atom B or N substitutional doping in single-layer suspended graphene, realized by low-energy ion implantation, is shown to induce a dampening or enhancement of the characteristic interband π plasmon of graphene through a high-resolution electron energy loss spectroscopy study using scanning transmission electron microscopy. A relative 16% decrease or 20% increase in the π plasmon quality factor is attributed to the presence of a single substitutional B or N atom dopant, respectively. This modification is in both cases shown to be relatively localized, with data suggesting the plasmonic response tailoring can no longer be detected within experimental uncertainties beyond a distance of approximately 1 nm from the dopant. Ab initio calculations confirm the trends observed experimentally. Our results directly confirm the possibility of tailoring the plasmonic properties of graphene in the ultraviolet waveband at the atomic scale, a crucial step in the quest for utilizing graphene's properties toward the development of plasmonic and optoelectronic devices operating at ultraviolet frequencies.

Structures of defects on anatase TiO2(001) surfaces

Defects on oxide surfaces play a crucial role in surface reactivity and thus it is crucial to understand their atomic and electronic structures. The defects on anatase TiO₂(001)-(1 × 4) surfaces are found to be highly reactive; however, due to the surface reconstruction, the defects exhibit a complicated character in different experiments that make it very challenging to determine their atomic structures. Here we present a systematic first-principles investigation of the defects on anatase TiO₂(001)-(1 × 4) surfaces based on a global-search adaptive genetic algorithm (AGA) and density functional theory (DFT). For different Ti-O ratios, we identify the low energy defect structures, investigate their electronic structure using a hybrid functional, and map their regions of stability under realistic conditions. We successfully find novel oxygen vacancy (O_V) and Ti interstitial (Ti_ini) structures that are different from the conventional ones in terms of their charge localization, magnetic state, and their scanning-tunneling-microscopy bright-dark image signature. This provides an insight into the complex geometric and electronic structure of the surface defects, and resolves several experimental discrepancies.

π-Plasmon absorption of carbon nanotubes for the selective and sensitive detection of Fe3+ ions

Inspired by the remarkable electronic and optical properties of single walled carbon nanotubes (SWNTs), various molecular sensing devices with sensitivity down to the single molecule level have been developed. However, most sensing approaches such as field effect transistors or near infrared (NIR) fluorescence require the rigorous debundling and separation of metallic tubes and semiconducting tubes in order to reach the desired high sensitivity. Interestingly, all carbon nanomaterials including carbon nanotubes, graphite, graphene, and even amorphous carbon exhibit extremely strong π-plasmon absorption in the ultraviolet region. This strong absorption has been studied as an undesired optical background for applications based on visible and NIR absorptions. For the first time, we found that the strong π-plasmon absorption of SWNTs in the ultraviolet region is extremely sensitive to ion binding. It is even much more sensitive than the absorption in the visible and NIR regions. Herein, we present our first exploration into using the extremely strong plasmon absorption of SWNTs to develop a new sensing platform for the detection of metallic ions. The detection selectivity is realized by modifying the surface of SWNTs with molecular ligands that have a high specificity for metal ions. As a demonstration, the new method is applied to selectively detect iron ions (Fe3+) by modifying the surface of the SWNTs with deferoxamine (DFO), a natural bacterial siderophore, which has a high specificity and affinity for Fe3+. The selective detection of Fe3+ in both aqueous solution and complex rain water is achieved with a pM level of sensitivity and detection limit. In situ resonant Raman spectroscopy demonstrated that the sensitive detection possibly involves electron transfer between the formed Fe-DFO complexes and the SWNTs. We envisage that it can be used to detect other metal ions when a specific binding chelator is attached to the carbon nanotube surface.

Collective electronic excitations in the ultra violet regime in 2-D and 1-D carbon nanostructures achieved by the addition of foreign atoms

Plasmons in the visible/UV energy regime have attracted great attention, especially in nano-materials, with regards to applications in opto-electronics and light harvesting; tailored enhancement of such plasmons is of particular interest for prospects in nano-plasmonics. This work demonstrates that it is possible, by adequate doping, to create excitations in the visible/UV regime in nano-carbon materials, i.e., carbon nanotubes and graphene, with choice of suitable ad-atoms and dopants, which are introduced directly into the lattice by low energy ion implantation or added via deposition by evaporation. Investigations as to whether these excitations are of collective nature, i.e., have plasmonic character, are carried out via DFT calculations and experiment-based extraction of the dielectric function. They give evidence of collective excitation behaviour for a number of the introduced impurity species, including K, Ag, B, N, and Pd. It is furthermore demonstrated that such excitations can be concentrated at nano-features, e.g., along nano-holes in graphene through metal atoms adhering to the edges of these holes.

Dielectric response of pentagonal defects in multilayer graphene nano-cones

The dielectric response of pentagonal defects in multilayer graphene nano-cones has been studied by electron energy loss spectroscopy and ab initio simulations. At the cone apex, a strong modification of the dielectric response is observed below the energy of the π plasmon resonance. This is attributed to π → π* interband transitions induced by topology-specific resonant π bonding states as well as π*-σ* hybridization. It is concluded that pentagonal defects strongly affect the local electronic structure in such a way that multi-walled graphene nano-cones should show great promise as field emitters.

π and π + σ plasmon localization in single-walled carbon nanotube meta-materials

We have investigated the plasmon excitations and interband transitions in nematic nanotube meta-materials with different nanotube mean diameters. While the characteristic optical low-energy transitions give testimony to the differing mean diameters, the full momentum resolved π as well as π + σ plasmon response of the meta-materials is found to be entirely invariant with the nanotube diameters 1 nm < d < 2 nm. We hence demonstrate that plasmons on single-walled carbon nanotubes are fully channeled to be one dimensional in the typical range of nanotube diameters. The internal morphology of the meta-material offers a knob to uniformly tune the plasmon resonances.

Nonlocal screening of plasmons in graphene by semiconducting and metallic substrates: first-principles calculations

We investigate the role of substrates on the collective excitations of graphene by using a first-principles implementation of the density response function within the random-phase approximation. Specifically, we consider graphene adsorbed on SiC(0001) and Al(111) as representative examples of a semiconducting and metallic substrate. On SiC(0001), the long wavelength π plasmons are significantly damped although their energies remain almost unaltered. On Al(111), the long wavelength π plasmons are completely quenched due to the coupling to the metal surface plasmon. The strong damping of the plasmon excitations occurs despite the fact that the single-particle band structure of graphene is completely unaffected by the substrates illustrating the nonlocal nature of the effect.

Probing the electronic structure of carbon nanotubes by nanoscale spectroscopy

Among the carbon allotropes newly discovered during the last few decades, carbon nanotubes (CNTs) have attracted enormous attention due to their structural and electronic properties with strong one dimensional character. The physical and chemical features of such systems are intrinsically rich and complex, and can only be probed by using multiple experimental and theoretical techniques. In this feature, we focus on the structural and electronic properties of CNTs that can be accessed by using transmission electron energy loss spectroscopies. The latter are complementary to optical and X-ray absorption techniques, yet allow to obtain the electronic structure with nanoscale spatial resolution. An improved understanding of the structure-electronic properties relationship of these unique 1D systems would represent a fundamental advance, and holds the promise of using CNTs in future applications.

Photonics with multiwall carbon nanotube arrays

We investigate the photonic properties of two-dimensional nanotube arrays for photon energies up to 40 eV and unveil the physics of two distinct applications: deep-UV photonic crystals and total visible absorbers. We find three main regimes: for small intertube spacing of 20-30 nm, we obtain strong Bragg scattering and photonic band gaps in the deep-UV range of 25 approximately 35 eV. For intermediate spacing of 40-100 nm, the photonic bands anticross with the graphite plasmon bands resulting into a complex photonic structure, and a generally reduced Bragg scattering. For large spacing >150 nm, the Bragg gap moves into the visible and decreases due to absorption. This leads to nanotube arrays behaving as total optical absorbers. Our results can guide the design of photonic applications in the visible and deep UV ranges.

Anomalous angular dependence of the dynamic structure factor near Bragg reflections: graphite

The electron energy-loss function of graphite is studied for momentum transfers q beyond the first Brillouin zone. We find that near Bragg reflections the spectra can change drastically for very small variations in q. The effect is investigated by means of first principle calculations in the random phase approximation and confirmed by inelastic x-ray scattering measurements of the dynamic structure factor S(q, omega). We demonstrate that this effect is governed by crystal local field effects and the stacking of graphite. It is traced back to a strong coupling between excitations at small and large momentum transfers.

Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene

We have measured a strictly linear pi plasmon dispersion along the axis of individualized single-wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single-wall carbon nanotubes. Comparative ab initio studies on graphene-based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects cause a mixing of electronic transitions, including the "Dirac cone," resulting in the observed linear dispersion.

Signatures of short-range many-body effects in the dielectric function of silicon for finite momentum transfer

We present an investigation of the dynamic structure factor and of the dielectric function epsilonM(Q,omega) of the prototypical semiconductor silicon for finite momentum transfer, combining inelastic x-ray scattering experiments and ab initio calculations. In contrast with optical spectra, for finite momentum transfer time-dependent density-functional theory in the adiabatic local-density approximation together with lifetime broadening describes the physics of valence excitations correctly. Major structures in the spectra, governed by short-range crystal and exchange-correlation local-field effects, are strongly influenced by a mixing of transitions of positive and negative energies, in striking difference to spectra for vanishing momentum transfer. This mixing gives rise to a pronounced Fano asymmetry.

Polarization dependence of the optical absorption of single-walled carbon nanotubes

Anisotropic optical absorption properties of single-walled carbon nanotubes (SWNTs) are determined from a vertically aligned SWNT film for 0.5-6 eV. Absorption peaks at 4.5 and 5.25 eV are found to exhibit remarkable polarization dependence and have relevance to optical properties of graphite. A method for determining a nematic order parameter for an aligned SWNT film based on the collinear absorption peak at 4.5 eV is presented, followed by the determination of the optical absorption cross section.

Optical and loss spectra of carbon nanotubes: depolarization effects and intertube interactions

We performed ab initio calculations of the anisotropic dielectric response of small-diameter single-walled carbon nanotubes in the framework of time-dependent density-functional theory. The calculated optical spectra are in very good agreement with experiment, both concerning absolute peak positions and anisotropy effects. The latter can only be described correctly when crystal local-field effects ("depolarization" effects) are fully taken into account. Moreover, interactions between the tubes can strongly modify their absorption and electron energy-loss spectra.