Cadmium arsenide (Cd3As2) has recently attracted considerable interest for the presence of 3D massless Dirac fermions with ultrahigh mobility and magnetoresistance. However, its surface properties ...are currently largely unexplored both theoretically and experimentally, due to the very large unit cell and the challenging growth of single‐crystal samples, respectively. Here, by combining ab initio calculations with surface‐science spectroscopic experiments, the presence of a surface reconstruction is unveiled in centimeter‐scale (112)‐oriented Cd3As2 single‐crystal foils produced by the self‐selecting vapor growth. Outermost Cd atoms descend into the As‐sublayer with a subsequent self‐passivation of the dangling bonds with As atoms, forming the triangle lattice previously imaged by scanning tunneling microscopy. Moreover, the oxidation mechanism of this reconstructed surface, dominated by the formation of AsOCd bonds, is revealed. Interestingly, it is found that the band structure of the reconstructed surface of Cd3As2 is quite robust against surface oxidation. Both computational and experimental findings point to a successful exploitation in technology of Cd3As2 single crystals.
The (112) surface of Cd3As2 reconstructs with Cd atoms descending into the As‐sublayer with a subsequent self‐passivation of the dangling bonds with As atoms. The oxidation mechanism is dominated by the formation of AsOCd bonds. The sticking coefficient for oxygen is <10−3 at room temperature, although encapsulation is necessary for devices.
Edge engineering is important for both fundamental research and applications as the device size decreases to nanometer scale. This is especially the case for graphene because a graphene edge shows ...totally different electronic properties depending on the atomic structure and the termination. It has recently been shown that an atomically precise zigzag edge can be obtained by etching graphene and graphite using hydrogen (H) plasma. However, edge termination had not been studied directly. In this study, termination of edges created by H-plasma is studied by high-resolution electron energy loss spectroscopy to show that the edge is sp2 bonded and the edge carbon atom is terminated by only one H atom. This suggests that an ideal zigzag edge, which is not only atomically precise but also sp2 bonding, can be obtained by H-plasma etching. Etching of the graphite surface with plasma of a different isotope, deuterium (D), is also studied by scanning tunneling microscopy to show that D-plasma anisotropically etches graphite less efficiently, although it can make defects more efficiently, than H-plasma.
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•Diamond (111) exposure to MW(N2) plasma induces no surface damage.•Observed a partial nitrogen coverage (∼1/4 monolayer) after MW(N2) exposure.•HREELS and DFT modeling confirm ...formation of NH(ad) species on H-diamond (111).
We report on an experimental investigation and density functional theory (DFT) modeling of the physico-chemical properties of nitrided diamond (111) surface prepared by exposing a hydrogenated diamond (111) surface to high purity nitrogen microwave plasma (MW(N2)). Ex-situ X-ray photoelectron spectroscopic analysis showed that the maximum nitrogen coverage is ∼1/4 monolayer alongside coadsorbed hydrogen, as corroborated by high-resolution electron energy loss spectroscopy (HREELS). The observed broad peak at ∼630 °C in the N2 temperature programmed desorption spectrum suggests that nitrogen desorption substantially occurs below 700 °C. As nitride surface exhibits an increase in electron affinity/work function compared to the hydrogenated surface. Low energy electron diffraction of the nitride surface exhibits a 1×1 pattern, which confirms that the MW(N2) exposure results in low-level damage to the diamond (111) surface; these results complement well with the presence of 1st order optical phonon peak (∼300 meV), the characteristic signature of a highly ordered diamond surface, on the HREEL spectrum. DFT simulations reveal that it is facile for nitrogen to insert into the CH bond on H-diamond (111), forming NH(ad) species adsorbed over C(111), which dimerizes into NH-NH(ad) at increasing coverages. The computed modes of vibration are in qualitative agreement with the HREELS data.
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•Partial curing of induced defects is possible for low damage surfaces.•Work function of N-HOPG is tuned over a wide-range (3.9–5.1 eV) by controlled nitridation process.•Highly ...damaged N-HOPG surfaces dominated by pyridinic N sites, which leads to a higher work function.
In this study, we investigate the chemical and electronic properties of nitrogen incorporated highly oriented pyrolytic graphite (N–HOPG) surface. The N–HOPG surfaces were prepared by three methods: radio frequency N2 (RF(N2)) plasma, microwave N2 (MW(N2)) plasma and N2+ ion implantation. The surface chemical states of carbon–nitrogen bonds on the N–HOPG surfaces, evolution of defects and thermal stability were investigated by X-ray photoelectron spectroscopy, high-resolution electron energy loss spectroscopy (HREELS) and ultraviolet photoelectron spectroscopy. Nitrogen exists in graphitic–N bonding state predominantly on the low damage N–HOPG surfaces (RF(N2) plasma at 9.3 Pa and 200 eV N2+ ion implantation) and in pyridine–N bonding state on high damage N–HOPG surfaces (RF(N2) plasma at 4.0 Pa and 500 eV N2+ ion implantation). Partial structural recovery is possible for low damage N–HOPG surfaces upon annealing. HREELS analysis revealed preferential bonding of hydrogen to defect sites and retention of graphite lattice structure for N–HOPG surfaces with low-level of surface damage. Furthermore, low damage N–HOPG surfaces exhibited low work function due to incorporation of nitrogen at graphitic sites. HCN temperature programmed desorption spectra showed a broad peak positioned at 550 °C which is associated with desorption of pyrrolic–N content.
Plasmons are self-sustained collective excitations of electron liquid, which have received increasing attention since its proposal by David Pines at 1960s. For the great potential in applications, ...the researches on plasmons make great advances all the way from semiconductors, metals, semimetals, to monolayer graphene. With the fast development of the field of topological materials, the research of plasmons has been extended into topological insulators, generating many exciting discoveries related to the topologically protected surface states. Topological semimetals, exhibiting various fantastic properties different from topological insulators, have become another research focus in condensed matter. Recently the plasmons in topological semimetals, providing a new perspective to further understand and utilize the topological states, have been attracting more and more attention. In this article, we review the recent theoretical and experimental investigations on the plasmons of topological semimetals, including the Dirac, Weyl and nodal line semimetals. In theoretical aspects, main different behaviors between the plasmons of topological semimetals and traditional metals are reviewed, such as the quantum nature, unusual dependence on temperature and charge carrier density, and the properties related to the chiral anomaly and Fermi arcs. The experimental studies are less reported, and the review is mainly focused on the measurements of optical conductivity and electron energy loss spectra in several typical real materials. Finally, the prospects of the future of the plasmons in topological semimetals in theories and experiments are outlooked.
•The acoustic plasmon of graphene is revisited by 2D HREELS.•The scattering geometric factor is visually characterized by 2D HREELS.•Loss peaks from the scattering geometric factor can be ...misinterpreted as the features of an acoustic plasmon.•A general regulation for future HREELS studies is proposed.
High-resolution electron energy loss spectroscopy (HREELS) is one of the most powerful methods to detect the dispersion of plasmons. However, we find that in the HREELS measurement, the scattering geometric configuration will seriously affect the identification of plasmons. Here, taking graphene as an example, using the HREELS capable of two-dimensional energy-momentum mapping combined with the intensity distribution calculations, we visually display the intensity distribution of the scattering geometric factor. We demonstrate that the energy loss peaks from the scattering geometric effect may be misinterpreted as the features of an acoustic plasmon. In any HREELS measurement, it is necessary to evaluate the effect of the scattering geometry quantitatively to identify the intrinsic surface excitations.
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Spin–orbit coupling (SOC) is a fundamental physical interaction, which describes how the electrons’ spin couples to their orbital motion. It is the source of a vast variety of fascinating phenomena ...in nanostructures. Although in most theoretical descriptions of high-temperature superconductivity SOC has been neglected, including this interaction can, in principle, revise the microscopic picture. Here by preforming energy-, momentum-, and spin-resolved spectroscopy experiments we demonstrate that while probing the dynamic charge response of the FeSe monolayer on strontium titanate, a prototype two-dimensional high-temperature superconductor using electrons, the scattering cross-section is spin dependent. We unravel the origin of the observed phenomenon and show that SOC in this two-dimensional superconductor is strong. We anticipate that such a strong SOC can have several consequences on the electronic structures and may compete with other pairing scenarios and be crucial for the mechanism of superconductivity.
In this Letter, we exploit recent breakthroughs in monochromated aberration-corrected scanning transmission electron microscopy (STEM) to resolve infrared plasmonic Fano antiresonances in individual ...nanofabricated disk-rod dimers. Using a combination of electron energy-loss spectroscopy and theoretical modeling, we investigate and characterize a subspace of the weak coupling regime between quasidiscrete and quasicontinuum localized surface plasmon resonances where infrared plasmonic Fano antiresonances appear. This work illustrates the capability of STEM instrumentation to experimentally observe nanoscale plasmonic responses that were previously the domain only of higher-resolution infrared spectroscopies.
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•Annealing of highly-damaged surfaces promotes nitrogen incorporation into graphitic sites.•Surface damage enhances hydrogen incorporation into the N delta layer.•Etching of the ...disordered layer occurs during the initial stages of diamond overgrowth.•NRR efficiency depends on the crystallinity of the nitrogen terminated surface and bonding.
We investigate the influence of surface chemical and structural properties of initial nitrogen terminated diamond surfaces on the nitrogen and hydrogen distribution in the N delta layers fabricated onto diamond (100) surfaces by different surface nitridation methods followed by a diamond layer overgrowth. Surface analysis shows that annealing of highly-damaged surfaces results in graphitization and promotes nitrogen incorporation into graphitic sites whereas lower damaged surfaces result in recovery of the near surface diamond structure alongside with nitrogen bonded mostly in a single configuration. Surface damage enhances hydrogen incorporation into the N delta layer. The etching of the disordered layer (formed during the nitrogen termination) occurs during the initial stages of diamond over growth. The nitrogen retention ratio efficiency in the delta layer depends on the crystallinity of the nitrogen terminated surface and bonding: is higher when nitrogen is bonded in the sub-surface region. Hydrogen diffusion into the N delta layer occurs following the nitrogen distribution most likely bonding to nitrogen and defects. At optimal conditions, the FWHM of N delta layer in diamond (100) is ~3 nm with a maximum nitrogen and hydrogen concentration of ~1.2×1020 atoms.cm−3 and ~2.0×1021 atoms.cm−3, respectively. The overgrown layers possess high crystallinity.
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•Phase fractions & microstructure solved via XRD & HR-EELS, uncommon in catalysis.•A 2 wt% rutile fraction is able to red-shift the optical Eg of the mixture.•PC activity tested in ...gas- and liquid-solid phase using purely visible-light.•A rutile fraction ≥10 wt% granted vis-light absorption & less charge recombination.•A rutile fraction ≥10 wt% gave real vis-light PC activity.
Titanium dioxide is by far the most used semiconductor material for photocatalytic applications. Still, it is transparent to visible-light. Recently, it has been proved that a type-II band alignment for the rutile − anatase mixture would improve visible-light absorption. In this research paper we thoroughly characterised the real crystalline and amorphous phases of synthesised titanias – thermally treated at different temperatures to get distinct ratios of anatase-rutile-amorphous fraction – as well as that of three commercially available photocatalytic nano-TiO2.
Optical spectroscopy showed that even a small fraction of rutile (2 wt%) is able to shift to lower energies the apparent optical band gap of an anatase-rutile mixed phase. But is this enough to attain a real photocatalytic activity promoted by merely visible-light? We tried to give an answer to that question.
Photocatalytic activity was assessed in the liquid- and gas-solid phase (employing rhodamine B and 4-chlorophenol, and isopropanol, respectively, as the organic substances to degrade) using a light source irradiating exclusively in the visible-range.
Photocatalytic activity results in the liquid-solid phase showed that a high surface hydroxylation led to specimen with superior visible light-promoted catalytic activity – i.e. dye and ligand-to-metal charge transfer complexes sensitisation effects, not photocatalysis sensu-strictu.
On the other hand, the gas-solid phase results showed that a higher amount of the absolute rutile fraction (around 10 wt%), together with less recombination of the charge carriers, were more effective for both visible-light absorption and a “real” visible-light promoted photocatalytic oxidation of isopropanol.
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