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In this work, we have investigated the structural and electronic properties of Mg-doped GaAs(111) nanowires synthesized through a vapor–liquid–solid growth mechanism. The crystalline structure of t...

Optical nanoantennas are well-known for the confinement of light into nanoscale hot spots, suitable for emission enhancement and sensing applications. Here, we show how control of the antenna dimensions allows tuning the local optical phase, hence turning a hot spot into a cold spot. We manipulate the local intensity exploiting the interference between driving and scattered field. Using single molecules as local detectors, we experimentally show the creation of subwavelength pockets with full suppression of the driving field. Remarkably, together with the cold excitation spots, we observe inhibition of emission by the phase-tuned nanoantenna. The fluorescence lifetime of a molecule scanned in such volumes becomes longer, showing slow down of spontaneous decay. In conclusion, the spatial phase of a nanoantenna is a powerful knob to tune between enhancement and inhibition in a 3-dimensional subwavelength volume.

Cathodoluminescence can probe photonic responses of nanostructures at high spatial and energy resolution, providing a powerful tool to investigate the radiative properties under electron excitation...

Electronic interface properties and the initial growth of hexa-peri-hexabenzocoronene with a borazine core (BN-HBC) on Au(111) have been studied by using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). A weak, but non-negligible, interaction between BN-HBC and Au(111) was found at the interface. Both hexa-peri-hexabenzocoronene (HBC) and BN-HBC molecules form well-defined monolayers. The different contrast in STM images of HBC and BN-HBC at different tunneling voltages with submolecular resolution can be ascribed to differences in the local density of states (LDOS). At positive and negative tunneling voltages, STM images reproduce the distribution of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) as determined by density functional theory (DFT) calculations very well.

Covalent organic frameworks (COFs) are molecule-based 2D and 3D materials that possess a wide range of mechanical and electronic properties. We have performed a joint experimental and theoretical study of the electronic structure of boroxine-linked COFs grown under ultrahigh vacuum conditions and characterized using scanning tunneling spectroscopy on Au(111) and hBN/Cu(111) substrates. Our results show that a single hBN layer electronically decouples the COF from the metallic substrate, thus suppressing substrate-induced broadening and revealing new features in the COF electronic local density of states (LDOS). The resulting sharpening of LDOS features allows us to experimentally determine the COF band gap, bandwidths, and the electronic hopping amplitude between adjacent COF bridge sites. These experimental parameters are consistent with the results of first-principles theoretical predictions.

Fluorescent molecules are highly susceptible to their local environment. Thus, a fluorescent molecule near a plasmonic nanoparticle can experience changes in local electric field and local density of states that reshape its intrinsic emission spectrum. By avoiding ensemble averaging while simultaneously measuring the super-resolved position of the fluorophore and its emission spectrum, single-molecule hyperspectral imaging is uniquely suited to differentiate changes in the spectrum from heterogeneous ensemble effects. Thus, we uncover for the first time single-molecule fluorescence emission spectrum reshaping upon near-field coupling to individual gold nanoparticles using hyperspectral super-resolution fluorescence imaging, and we resolve this spectral reshaping as a function of the nanoparticle/dye spectral overlap and separation distance. We find that dyes bluer than the plasmon resonance maximum are red-shifted and redder dyes are blue-shifted. The primary vibronic peak transition probabilities shift to favor secondary vibronic peaks, leading to effective emission maxima shifts in excess of 50 nm, and we understand these light-matter interactions by combining super-resolution hyperspectral imaging and full-field electromagnetic simulations.

The photoluminescence quantum yield characterizes the performance of emitters for applications in optical devices, as reporters or probes in material and analytical sciences, and for sensing applications. Quantum yield measurements are challenging for luminescent molecules and nanocrystals immobilized in thin films for many sensor applications, particularly if spatially resolved quantitative luminescence information is desired. We show here that a Drexhage-type experiment, where a silver-coated millimeter-sized sphere is used to modify the local density of states, can provide an elegant approach to counter this challenge. As a representative example of the potential of this method, we measure the pH-dependent photoluminescence quantum yield of fluorescein isothiocyanate bound to a thin mesoporous silica film. The results were compared with those of the studies on the pH dependence of the same dye in solution. We found that our approach can link single fluorophore studies to ensemble measurements and pave ...

Nanoscale heterostructure engineering is the main target for the development of optoelectronic devices. In this sense, a precise knowledge of local electronic response after materials processing is...

Among the recent achievements of sodium-ion battery (SIB) electrode materials, hybridization of two-dimensional (2D) materials is one of the most interesting appointments. In this work, we propose to use the 2D hybrid composites of SnS2 with graphene or graphene oxide (GO) layers as the SIB anode, based on the first-principles calculations of their atomic structures, sodium intercalation energetics, and electronic properties. The calculations reveal that a graphene or GO film can effectively support not only the stable formation of a heterointerface with the SnS2 layer but also the easy intercalation of a sodium atom with low migration energy and acceptable low volume change. The electronic charge-density differences and the local density of states indicate that the electrons are transferred from the graphene or GO layer to the SnS2 layer, facilitating the formation of a heterointerface and improving the electronic conductance of the semiconducting SnS2 layer. These 2D hybrid composites of SnS2/G or GO ar...

Ab initio calculations reveal that in cold field electron emission from nanostructures, the electron state of top priority not necessarily the occupied state with largest forward kinetic energy but depends on the applied field. We show that single-walled carbon nanotube emission at low fields is dominated by the states with highest forward kinetic energy. On the other hand, high field emission is dominated by states that extend deeply into the potential barrier. We find that the distribution of the local density of states is an important factor for emission, in addition to the forward kinetic energy. We have calculated the wave functions of such states for semi-infinitely long single-walled carbon nanotubes by combining time-dependent density functional theory (TDDFT) with a reactant– product decoupling method. The emission image on a distant screen is found by matching the analytical solution of the wave function calculated with TDDFT on a plane which is a few nanometers away from the apex of the nanostr...

Electron beam spectroscopy has recently attracted much attention in the modal analysis of nanophotonic and plasmonic systems. In principle, electron energy-loss spectroscopy (EELS) provides information about the electromagnetic local density of states of all sorts of electromagnetic modes as well as nonradiative modes. However, there have not been many examples related to the EELS analyses for electromagnetic Bloch modes. Herein, EELS measurements are performed for the characterization of nonradiative band-edge modes facing the first bandgap in a plasmonic crystal with a triangular lattice, which is well-known for its full bandgap formation. The obtained spectrum images clearly determine the characteristics of the lower and upper band-edge modes, compared with an analytical model based on group theory and a numerical simulation by the finite-difference time-domain method. The EELS spectra also reveal differences in the plasmonic density of states between the lower and upper band-edges, which provides info...

Electronic band structure modification of carbon nanotubes (CNTs) through Suzuki coupling has been predicted recently. Here, scanning tunneling microscopy/spectroscopy studies of a molecular junction developed through C–C coupled CNTs are conducted to probe the local density of states variation along the junction, and the distorted band structure of CNT at the junction is unraveled, in agreement with the predictions. The band structure modification aided by charge transfer, among CNTs and C6H4 in the junction, helps to achieve a more efficient *COOH adsorption in coupled CNTs (CCNTs) than in pristine CNTs, proven via density functional theory-based calculations. This indicates the possibilities of CCNT-based electrochemical CO2 reduction. Formate production at low potential (−0.9 V vs reversible hydrogen electrode) in neutral pH (6.8) is demonstrated with CCNT, while no formic acid production is observed in uncoupled CNTs. This study opens a fundamental insight into the development of novel catalysts base...

The interaction between plasmons in metal nanostructures and excitons in layered materials attracts recent interests due to its fascinating properties inherited from the two constituents, e.g., the high tunability on its spectral or spatial properties from the plasmonic component, and the large optical nonlinearity or light emitting properties from the excitonic counterpart. Here, we demonstrate light-emitting plexcitons from the coupling between the neutral excitons in monolayer WSe2 and highly confined nanocavity plasmons in the nanocube–over−mirror system. We observe, simultaneously, an anticrossing dispersion curve of the hybrid system in the dark-field scattering spectrum and a 1700 times enhancement in the photoluminescence. We attribute the large photoluminescence enhancement to the increased local density of states by both the plasmonic and excitonic constituents in the intermediate coupling regime. In addition, increasing the confinement of the hybrid systems is achieved by shrinking down the siz...

Exploring the structural transformation of nonhexagonal rings is of fundamental importance for understanding the thermal stability of nonhexagonal rings and revealing the structure–property relationships. Here, we report on the thermally induced transformation from the fused tetragon-hexagon (4–6) carbon rings to a pair of pentagon (5–5) rings in the graphene-like nanoribbons periodically embedded with tetragon and octagon (4–8–4) carbon rings. A distinct contrast among tetragon, pentagon, hexagon, and octagon carbon rings is provided by noncontact atomic force microscopy with atomic resolution. The thermally activated bond rotation with the dissociation of the shared carbon dimer between the 4–6 carbon rings is the key step for the 4–6 to 5–5 transformation. The energy barrier of the bond rotation, which results in the formation of an irregular octagon ring in the transition state, is calculated to be 1.13 eV. The 5–5 defects markedly change the electronic local density of states of the graphene-like nan...

Gold islands are typically associated with high binding affinity to adsorbates and catalytic activity. Here we present the growth of dispersed nanoscale gold islands on single layer MoS2, prepared on an inert SiO2/Si support by chemical vapor deposition. This study offers a combination of growth process development, optical characterization, photoelectron spectroscopy at submicron spatial resolution, and advanced density functional theory modeling for detailed insight into the electronic interaction between gold and single-layer MoS2. In particular, we find the gold density of states in Au/MoS2/SiO2/Si to be far less well-defined than Au islands on other 2-dimensional materials such as graphene, for which we also provide data. We attribute this effect to the presence of heterogeneous Au adatom/MoS2-support interactions within the nanometer-scale gold cluster. Theory predicts that CO will exhibit adsorption energies in excess of 1 eV at the Au cluster edges, where the local density of states is dominated b...

The electronic properties of metal-molecule interfaces can in principle be controlled by molecular design and self-assembly, yielding great potential for future nano- and optoelectronic technologies. However, the coupling between molecular orbitals and the electronic states of the surface can significantly influence molecular states. In particular, molecules designed to create metal-organic self-assembled networks have functional groups that by necessity are designed to interact strongly with metals. Here, we investigate the adsorption interactions of a terpyridine (tpy)-based molecule on a noble metal, Ag(111), by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) together with density functional theory (DFT) calculations. By comparing the local density of states (DOS) information gained from STS for the molecule on the bare Ag(111) surface with that of the molecule decoupled from the underlying metal by a NaCl bilayer, we find that tpy-localized orbitals hybridize strongly with the metal substrate. Meanwhile, those related to the phenyl rings that link the two terminal tpy groups are less influenced by the interaction with the surface. The selective hybridization of the tpy groups provides an example of strong, orbital-specific electronic coupling between a functional group and a noble-metal surface, which may alter the intended balance of interactions and resulting electronic behavior of the molecule-metal interface.

High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such as $MoS_2$ is key to unlocking their unique electronic and optoelectronic properties for fundamental research and device applications. Despite extensive experimental and theoretical efforts reliable ohmic contact to doped TMDCs remains elusive and would benefit from a better understanding of the underlying physics of the metal-TMDC interface. Here we present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer $MoS_2$ using ultra-high vacuum scanning tunneling microscopy (UHV-STM). Our measurements reveal that the electronic properties of these junctions are dominated by 2D metal induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (L-DOS) of the $MoS_2$ within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of ~0.55 nm near mid gap to as long as 2 nm near the band edges and are nearly identical for Au, Pd and graphite contacts, indicating that it is a universal property of the monolayer semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact.

Modulation of the local density of states of single-wall carbon nanotubes (SWCNTs) is induced by the encapsulation of europium nanowires (EuNWs). The observation of these modulated density of states using scanning tunneling microscopy/spectroscopy combined with density functional theory calculations is reported. The electronic modulation of SWCNTs by encapsulation of EuNWs is revealed as a Fermi level shift, band gap reduction, and the emergence of localized states in the gap. The present results show that the electronic interaction between EuNWs and SWCNTs is much stronger than that previously reported for nanomaterials encapsulated in SWCNTs.

Using density functional theory (DFT), combined with non-equilibrium Green’s function (NEGF) method, the effect of potential change, charge distribution, voltage drop, band lineup, and evolution of the transmission spectrum under small applied bias for a memantine-functionalized gold nanogap device for DNA detection have been studied in this report. We have investigated the potential perturbation and charge distribution introduced by the electrodes and the nucleobases separately, which helps to understand the development of the potential profile throughout the molecular break junction and the effect on the transmission spectrum. The presence of electrodes is found to modify the energy levels and band lineup of the device. We have also investigated the local density of states to understand the contact- and nucleobase-modified charge distributions in the molecular states. The electronic wave functions for the HOMO transmission peak at various applied voltages are also examined to understand the physics behi...

Switching the magnetic properties of organic semiconductors on a metal surface has thus far largely been limited to molecule-by-molecule tip-induced transformations in scanned probe experiments. Here we demonstrate with molecular resolution that collective control of activated Kondo screening can be achieved in thin-films of the organic semiconductor titanyl phthalocyanine on Cu(110) to obtain tunable concentrations of Kondo impurities. Using low-temperature scanning tunneling microscopy and spectroscopy, we show that a thermally activated molecular distortion dramatically shifts surface-molecule coupling and enables ensemble-level control of Kondo screening in the interfacial spin system. This is accompanied by the formation of a temperature-dependent Abrikosov-Suhl-Kondo resonance in the local density of states of the activated molecules. This enables coverage-dependent control over activation to the Kondo screening state. Our study thus advances the versatility of molecular switching for Kondo physics and opens new avenues for scalable bottom-up tailoring of the electronic structure and magnetic texture of organic semiconductor interfaces at the nanoscale.

The Purcell effect of quantum dot (QD) spontaneous emission with Ag-SiO2 multilayer metamaterial nanostructures has been demonstrated in experiment and simulation. A broadband enhanced spontaneous emission rate of QDs is observed due to large local density of states in the epsilon-near-zero and hyperbolic regions of multilayer structures. Multilayer gratings are utilized to further enhance the QD spontaneous emission as the QDs located inside the grating grooves strongly interact with high-k coupled surface plasmon polariton modes. Photoluminescence decay measurements are in good agreement with both analytical treatment with a nonlocal effect and three-dimensional finite-element simulation. Detailed studies of QD position and polarization effects on emission rate enhancement for multilayer and multilayer grating nanostructures provide important insight for understanding the coupling mechanisms of emitter–multilayer interaction and the engineering of local density of states in metamaterial nanostructures. ...

In this Perspective, we discuss the physics and potential applications of planar hyperbolic metasurfaces (MTSs), with emphasis on their in-plane and near-field responses. After revisiting the governing dispersion relation and properties of the supported surface plasmon polaritons (SPPs), we discuss the different topologies that uniaxial MTSs can implement. Particular attention is devoted to the hyperbolic regime, which exhibits unusual features, such as an ideally infinite wave confinement and local density of states. In this context, we clarify the different physical mechanisms that limit the practical implementation of these ideal concepts using materials found in nature, and we describe several approaches to realize hyperbolic MTSs, ranging from the use of novel 2D materials such as black phosphorus to artificial nanostructured composites made of graphene or silver. Some exciting phenomena and applications are then presented and discussed, including negative refraction and the routing of SPPs within th...

Scanning tunneling microscopy is utilized to investigate the local density of states of a CH3NH3PbI3–xClx perovskite in cross-sectional geometry. Two electronic phases, 10–20 nm in size, with different electronic properties inside the CH3NH3PbI3–xClx perovskite layer are observed by the dI/dV mapping and point spectra. A power law dependence of the dI/dV point spectra is revealed. In addition, the distinct electronic phases are found to have preferential orientations close to the normal direction of the film surface. Density functional theory calculations indicate that the observed electronic phases are associated with local deviation of I/Cl ratio, rather than different orientations of the electric dipole moments in the ferroelectric phases. By comparing the calculated results with experimental data we conclude that phase A (lower contrast in dI/dV mapping at −2.0 V bias) contains a lower I/Cl ratio than that in phase B (higher contrast in dI/dV).

The adsorption and hydrogenation behaviors of hydrogen cyanide to methane and ammonia formation by W(111) catalyst were systematically investigated using the density functional theory method. Based on our calculated consequences, it is found that the WHCN(T,T-μ2-C,N) is calculated to be the most stable conformer, possessing an adsorption energy of −49.8 kcal/mol, among all calculated structures of HCN/W(111) system. To comprehend the electronic property of its interaction between the adsorbate and substrate, we calculated the electron localization functions, local density of states, and Bader charges; our results were consistent and explicable. Reaction paths in all possible mechanisms were explored in detail, involving the hydrogenation on different orientations of each adsorbate and the scission of the carbon–nitrogen bond. Before forming an imine intermediate (H2CNH(a)), two adsorbed hydrogen atoms will sequentially react with the nitrogen and then carbon atoms in the first and second hydrogenation ste...

Lanthanoid series are unique in atomic elements. One reason is because they have 4f electronic states forbidding electric-dipole (ED) transitions in vacuum and another reason is because they are very useful in current-day optical technologies such as lasers and fiber-based telecommunications. Trivalent Er ions are well-known as a key atomic element supporting 1.5 μm band optical technologies and also as complex photoluminescence (PL) band deeply mixing ED and magnetic-dipole (MD) transitions. Here we show large and selective enhancement of ED and MD radiations up to 83- and 26-fold for a reference bulk state, respectively, in experiments employing plasmonic nanocavity arrays. We achieved the marked PL enhancement by use of an optimal design for electromagnetic (EM) local density of states (LDOS) and by Er-ion doping in deep subwavelength precision. We moreover clarify the quantitative contribution of ED and MD radiations to the PL band, and the magnetic Purcell effect in the PL-decay temporal measurement. This study experimentally demonstrates a new scheme of EM-LDOS engineering in plasmon-enhanced photonics, which will be a key technique to develop loss-compensated and active plasmonic devices.

In fluorescence correlation spectroscopy (FCS), one measures and correlates the fluctuations that occur as fluorophores diffuse into and out of the detection volume of a microscope. The resulting correlations are used to determine concentrations and diffusion rates of fluorescent species in liquid environments. The sensitivity of this technique is limited by the field intensity and the dimensions of the detection volume, both of which can be modified by nanostructures through geometric and plasmonic effects. In this paper we aim to establish how far noble metal Mie spheres, acting as plasmon antennas, can boost FCS. To that end, we model a realistic scenario that takes into account the exact solutions of the field near a plasmon antenna, the modified diffusion owing to the antenna excluding volume, as well as quantum efficiency and local density of states (LDOS) effects.

The electrocatalytic properties of Cu, Ni, and Cu0.75Ni0.25 alloy are investigated for CO and CO2 reduction to methane by density functional calculations. We show that, as the Ni content increases in Cu(1–x)Nix (211) surfaces (x = 0, 0.25, and 1), the binding energies (ΔEs) of the adsorbates involved in the reaction mechanism decrease. Linear scaling relations are known to exist between ΔEs of adsorbates binding via a C (O) atom over pure transition metal surfaces. However, we find that alloying Cu and Ni has the potential for breaking these relations for certain pairs of adsorbates. The decrease in the repulsive Coulombic interaction between the adsorbate and the charges induced on the Cu–Ni alloy surface explains the adsorption site preference. The ΔE shift with respect to pure Cu is larger for species binding through C than O. Various trends exhibited by the binding energies are understood by analyzing the chemical bonding through local density of states and charge density isosurfaces of the bare and a...

We analyze the electronic structures and optical absorption spectrum of Au133(SPh-tBu)52, particularly in terms of interface electronic properties between the Au core and the thiolate ligands. Computations are performed by using the time-dependent density functional theory approach in real-time and real-space that has recently been developed by our group. Local density of states analysis reveals a relationship between an icosahedral Au core and a thiolate-protected Au cluster; Au atoms associated with the edge or surface sites of the icosahedral core anchor the ligands to the core, whereas Au atoms associated with the apex sites bridge two thiolates forming −S–Au–S– bonds. We compare the optical absorption spectrum of Au133(SPh-tBu)52 with that of an icosahedral Au146 bare cluster to clarify effects of the ligands on the optical absorption. The absorption intensity for Au133(SPh-tBu)52 is obviously higher than that for the bare cluster. The significant increase in the optical absorption of Au133(SPh-tBu)5...

High-temperature-deposited rotated graphene (Gr) on Ni(111) has been investigated by in situ scanning tunneling microscopy and spectroscopy at room temperature. The rotated Gr exhibits weak bonding to the Ni(111) surface, which facilitates formation of Ni2C or a second Gr layer underneath via bulk carbon segregation. Areas of rotated Gr present a bias voltage dependence of the apparent amplitude of Gr superlattice corrugations. We find that Ni2C underneath rotated Gr introduces additional electronic features that vary with the gap resistance, which could be related to an orientation-dependent interaction between Ni2C and Gr. Furthermore, the exposure to oxygen has a significant influence on the local density of states of Gr/Ni2C, other than on Ni(111) covered with nonrotated Gr.

We present a combined experimental and theoretical study of electron confinement in graphene nanoislands (GNs) grown on a Pt(111) substrate using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. We observed standing wave patterns in the STM images of GNs, and the bias dependency of the standing wave pattern was reproduced by considering free electrons with an effective mass of m* ≈ (0.27 ± 0.03)me. Because the effective mass of Pt is m* = 0.28me, our results reveal that the electron confinement is due to the effect of the Pt substrate rather than the massless Dirac electrons of graphene. Our calculated maps of the local density of states (LDOS) for the GNs confirm that the electronic properties of the confinement may be described in terms of electrons with an effective mass. The DFT-calculated charge distribution for graphene on the Pt system also shows a clear hybridization between the pz orbitals of both the first layer of the Pt substrate and the carbon atoms.

Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) have proved during the past few years to be tremendous tools to study surface plasmons in metallic nanoparticles, thanks to an extremely high spatial resolution combined with a broad spectral range. Despite their apparent close resemblance, qualitative differences between EELS and CL have been theoretically as well as experimentally pinpointed. We demonstrate that these differences are recovered when comparing the full electromagnetic local density of states (EMLDOS) and the radiative EMLDOS. Following the known relation established between EELS and the projection along the electron trajectory of the full EMLDOS, we introduce a formalism based on the Maxwell electric Green tensor to draw a link between CL and the projection along the electron trajectory of the radiative EMLDOS. We discuss in simple terms the differences between EELS (projected full EMLDOS) and CL (projected radiative EMLDOS) through modal decompositions obtained in the ...

Evanescent waves induced by thermal fluctuations can tunnel through nanoscale gap spacing, leading to super-Planckian thermal radiation. However, investigations of near-field thermal radiation of macroscopic objects have been limited to simple planar or effectively planar geometries until recently. Based on exact formulations including the scattering theory and Green’s function method, patterning thin films into 1D and 2D metasurfaces is found to increase the radiative heat flux by more than 1 order of magnitude in a certain range of thicknesses. The underlying mechanism of this counterintuitive phenomenon lies in the excitation of hyperbolic modes supporting high local density of states for broad frequency and k-space regimes. The radiative heat flux of a 2D metasurface increases monotonically with the thickness, while the heat flux of a 1D metasurface is not so sensitive to the thickness and is surprisingly higher than that of its 2D counterparts. The stark difference is attributed to the rapid-decay su...

Oxygen vacancies (OVs) are important for changing the geometric and electronic structures as well as the chemical properties of MnO2. In this study, we performed a DFT+U calculation on the electronic structure and catalytic performance of a β-MnO2 catalyst for the oxygen reduction reaction (ORR) with different numbers and extents of OVs. Comparing those results with the experimental XRD analysis, we determined that OVs produce a new crystalline phase of β-MnO2. Changes in the electronic structure (Bader charges, band structure, partial density of states, local density of states, and frontier molecular orbital), proton insertion, and oxygen adsorption in β-MnO2 (110) were investigated as a function of the bulk OVs. The results show that a moderate concentration of bulk OVs reduced the band gap, increased the Fermi and HOMO levels of the MnO2 (or MnOOH), and elongated the O–O bond of the adsorbed O2 and coadsorbed O2 with H. These changes substantially increase the conductivity of MnO2 for the catalysis of ...

We discuss the possibility of largely enhancing the nonlinear second-harmonic generation (SHG) from χ(2) nonlinear materials using metasurfaces consisting of suitably engineered optical nanoantenna arrays. The proposed setup may increase the SHG conversion efficiency by orders of magnitude, compared to a thin film of χ(2) nonlinear material of the same thickness. This enhancement is attributed to localized fields at the nanoload, due to the plasmonic resonance of noble-metal nanoantennas, combined with a suitably enhanced photonic local density of states induced by properly coupling two closely spaced metasurfaces to form a nanocavity to enhance SHG. The proposed subwavelength optical device can lead to versatile applications, such as wavelength conversion and wave mixing at the nanoscale.

Spontaneous emission by an excited molecule strongly depends upon the available density of states into which the molecule can decay. In a photonic crystal, the allowed local density of states depletes within the stop band and enhances at the band edge of the crystal. As a result, an emitter implanted in a photonic crystal is forced to redistribute its fluorescence energy within its emission spectral range, and its spontaneous emission spectrum thus gets modified. Here, we studied the influence of change in local density of states on energy transfer efficiency between a donor–acceptor pair embedded in a colloidal photonic crystal. Rhodamine-B and Rhodamine-800 dyes were chosen as the energy donor and energy acceptor, respectively. We observed an angle-dependent quenching in the emission intensity of the donor accompanied by enhancement in acceptor emission when both dyes were in the photonic crystal. This occurred owing to depletion in the allowed local density of states available to the donor. Reduction i...

In this study, we found that the local density of states and ionization energy spectrum of the valence electrons of methylnitrene (NCH3) adsorbed on Cu(110) can be calculated from molecular orbital calculations of a simple artificial isolated NCH3–Cu2 molecular cluster in which the two Cu atoms form bonds to the N atom. Such a NCH3–Cu2 cluster represents the basic structural unit of a NCH3 molecule adsorbing on a Cu double-added-row structure. This finite NCH3–Cu2 cluster structure is not optimized as a single system but is extracted directly from an optimized surface structure obtained by density functional theory with periodic boundary conditions. With this approach, we obtained excellent agreement between the measured ultraviolet photoemission spectra (UPS) and the theoretical calculation results. To further examine this minimal cluster concept, we analyzed methoxy (OCH3) adsorption on Cu(110) and found a OCH3–Cu3 cluster structure. On the basis of this structure, we calculated UPS and also obtained su...

Solid-state quantum emitters, such as artificially engineered quantum dots or naturally occurring defects in solids, are being investigated for applications ranging from quantum information science and optoelectronics to biomedical imaging. Recently, these same systems have also been studied from the perspective of nanoscale metrology. In this letter, we study the near-field optical properties of a diamond nanocrystal hosting a single nitrogen vacancy center. We find that the nitrogen vacancy center is a sensitive probe of the surrounding electromagnetic mode structure. We exploit this sensitivity to demonstrate nanoscale fluorescence lifetime imaging microscopy (FLIM) with a single nitrogen vacancy center by imaging the local density of states of an optical antenna.

In order to oxidize CO to CO2 during the process of steam pyrolysis nuclear graphite waste, the adsorption and oxidation of a CO molecule on undoped and N-doped TiO2 nanoparticles has been studied by first-principles calculations, including the adsorption energies, bond lengths, local density of states (LDOSs), and the charge density difference (CDD). In the adsorption process, two electrons transfer from CO to the particle resulting in the reduction of the Ti sites. CO2 and carbonate form during the adsorption process on the dangling oxygen atom. The CO over the undoped and N-doped nanoparticles forms CO2 by detaching the dangling oxygen atom. Less than 0.22 eV energy gained can drive the CO2 away to a distance greater than 3.01 A. Therefore, the CO molecule can be oxidized to CO2 by TiO2 nanoparticle, and the physically adsorbed CO2 molecule can spontaneously dissociate at room temperature, apart from the energies released in the adsorption process. The adsorption energy for an undoped particle is −0.93...

Scanning tunneling microscopy and local conductance mapping show spin-state coexistence in bilayer films of Fe[(H(2)Bpz(2))(2)bpy] on Au(111) that is independent of temperature between 131 and 300 K. This modification of bulk behavior is attributed in part to the unique packing constraints of the bilayer film that promote deviations from bulk behavior. The local density of states measured for different spin states shows that high-spin molecules have a smaller transport gap than low-spin molecules and are in agreement with density functional theory calculations.

We have fabricated single C(60) molecule transistors with ferromagnetic Ni leads (FM-SMTs) by using an electrical break junction method and investigated their magnetotransport. The FM-SMTs exhibited clear gate-dependent hysteretic tunnel magnetoresistance (TMR) and the TMR values reached as high as -80%. The polarity of the TMR was found to be always negative over the entire bias range studied here. Density functional theory calculations show that hybridization between the Ni substrate states and the C(60) molecular orbitals generates an antiferromagnetic configuration in the local density of states near the Fermi level, which gives a reasonable explanation for the observed negative TMR.

Pt-based core–shell (M@Pt where M stands for core element) nanoparticles (NPs) have recently been under increasing scrutiny in the fields of fuel cell and lithium air battery electrocatalysis due to their promising prospects in optimizing catalytic activity, reducing Pt loading and consequently lowering its cost. To achieve the latter, delineating spatially resolved local (surface) elemental distribution and associated variations in electronic properties under working condition (i.e., in situ) is arguably a prerequisite of fundamental importance in investigating electrocatalysis but unfortunately is still sorely missing. In this regard, in situ195Pt electrochemical NMR (EC-NMR) of Pt-based NPs is unique in terms of accessing such information, particularly the spatially resolved partition between the s- and d-like Fermi level local density of states (Ef-LDOS) modified by the core elements. In this paper, we report a comparative in situ195Pt EC-NMR investigation of Ru@Pt vs Au@Pt NPs which was complemented ...

The electronic structures, spin-states, and geometrical parameters of tetra-, penta-, and hexa-coordinated iron-porphyrins are investigated applying density functional theory (DFT) based calculations, utilizing the plane-wave pseudopotential as well as localized basis set approaches. The splitting of the spin multiplet energies are investigated applying various functionals including recently developed hybrid meta-GGA (M06 family) functionals. Almost all of the hybrid functionals accurately reproduce the experimental ground state spins of the investigated Fe-porphyrins. However, the energetic ordering of the spin-states and the energies between them are still an issue. The widely used B3LYP provides consistent results for all chosen systems. The GGA+U functionals are found to be equally competent. After assessing the performance of various functionals in spin-state calculations, the potential energy surfaces of the oxygen binding process by heme is investigated. This reveals a "double spin-crossover" feature for the lowest energy reaction path that is consistent with previous CASPT2 calculations but predicting a lowest energy singlet state. The calculations have hence captured the spin-crossover as well as spin-flip processes. These are driven by the intra-atomic orbital polarization on the central metal atom due to the atomic and orbitals rearrangements. The nature of the chemical bonding and a molecular orbital analysis are also performed for the geometrically simple but electronic structurally complicated system tetra-coordinated planar Fe porphyrin in comparison to the penta-coordinated systems. This analysis explains the observed paradoxical appearance of certain peaks in the local density of states (DOS).

We calculate the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T), deposited on graphene. We observe significant base-dependent features in the LDOS in an energy range within a few electronvolts of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases in scanning tunneling spectroscopy (STS) experiments that perform image and site dependent spectroscopy on biomolecules. Thus the fingerprints of DNA-graphene hybrid structures may provide an alternative route to DNA sequencing using STS.

While recent advances in the synthesis and crystallography of gold–thiolate nanoclusters have led to enormous progress in understanding the structure and bonding of gold thiolate nanomaterials, the changes in structure experienced by these nanomaterials in different conditions, such as low temperature and solvation, is still largely lacking. Herein, we report a temperature- and solvation-dependent extended X-ray absorption fine structure (EXAFS) study of thiolate-protected Au25 nanoclusters. The structural changes experienced by Au25 in response to low temperature and different solvation environments are illustrated in great detail using a site-specific EXAFS fitting approach consisting of one Au–S and three Au–Au shells. EXAFS simulations as well as ab initio calculations of the local density of states are used to support the EXAFS analysis and correlate the structural characteristics of Au25 with its electronic properties. These experimental and theoretical studies point out the existence of the interpl...

A recent scanning tunneling microscopy (STM) study (J. Am. Chem. Soc. 2010, 132, 3013) reported the self-assembled growth of phenylacetylene (PA) molecular lines on the H-terminated Si(100)-(2 × 1) surface. Using first-principles density functional calculations within the generalized-gradient approximation, we investigated the proposed chain reaction pathway, thermodynamic stability, and electronic structure of the PA line. We found that the growth of the PA line with sp2 hybridization at the surface is kinetically more facile than that of the styrene line with sp3 hybridization, indicating that self-assembled growth of molecular lines employing alkynes rather than alkenes will be kinetically facilitated. We also found that the thermodynamic stability of the PA line is significantly enhanced compared to that of the styrene line. Analysis of the local density of states (LDOS) shows that, in the PA line, π electrons originating from the terminal C═C bond and the benzene ring are well delocalized across the ...

We applied periodic density-functional theory to investigate the adsorption and dissociation of NO on bimetallic surfaces, including the xNi@Pt(111), NixPt4−x(111), and (4−x)Pt@Ni(111) surfaces (x = 0−4). For all bimetallic surfaces, NO is preferentially adsorbed on Ni-rich sites, and the adsorption energies increase with an increasing number of top-layer Ni atoms on the surface. When the top-layer compositions are equal (but with varied composition of inner layers), the adsorption energy of NO on these surfaces decreases in the order xNi@Pt(111) > NixPt4−x(111) > (4−x)Pt@Ni(111), whereas the NO dissociation barriers increase in the opposite order; a larger adsorption energy of NO leads to a smaller NO dissociation barrier. We employed the local density of states to investigate the inner-layer effect of the various surfaces; the inner-layer Pt atoms of the 4Ni@Pt(111) surface caused the greatest upshift of the d-band center (of top-layer Ni atoms) toward the Fermi energy, so that the 4Ni@Pt(111) surface e...

We present an approach to predicting extrinsic electron resonance widths within quantum corral nanostructures based on analogies with acoustics. Established quantum mechanical methods for calculating resonance widths, such as multiple scattering theory, build up the scattering atom by atom, ignoring the structure formed by the atoms, such as walls or enclosures. Conversely, particle-in-a-box models, assuming continuous walls, have long been successful in predicting quantum corral energy levels, but not resonance widths. In acoustics, partial reflection from walls and various enclosures has long been incorporated for determining reverberation times. Pursuing an exact analogy between the local density of states of a quantum corral and the acoustic impedance of a concert hall, we show electron lifetimes in nanoscopic structures of arbitrary convex shape are well accounted for by the Sabine formula for acoustic reverberation times. This provides a particularly compact and intuitive prescription for extrinsic finite lifetimes in a particle-in-a-box with leaky walls, including quantum corral atomic walls, given single particle scattering properties.

We report on studies of electronic properties and scanning tunneling microscopy (STM) of the most common configurations of nitrogen- or boron-doped graphene and carbon nanotubes using density functional theory. Charge transfer, shift of the Fermi level, and localized electronic states are analyzed as a function of the doping configurations and concentrations. The theoretical STM images show common fingerprints for the same doping type for graphene, and metallic or semiconducting nanotubes. In particular, nitrogen is not imaged in contrast to boron. STM patterns are mainly shaped by local density of states of the carbon atoms close to the defect. STM images are not strongly dependent on the bias voltage when scanning the defect directly. However, the scanning of the defect-free side of the tube displays a perturbation compared to the pristine tube depending on the applied bias.

The synthesis of silicon nanowires by Ga-assisted plasma enhanced chemical vapor deposition (PECVD) has been recently demonstrated. In the present work, we study in detail the structural characteristics of the synthesized nanowires. High resolution transmission electron microscopy (HRTEM) analysis reveals the existence of various types of structural defects, which can be classified mainly according to the orientation into axial twins, lateral twins, and transverse twins. We compare our results with previous studies of Si nanowires synthesized with other catalyst metals. Understanding both the origin and the effects of the observed defects is important for technological applications. The presence of twinned domains changes locally the structure of the material. As a consequence, one should find a different local density of states and band gap, which should result in a variation of the carrier transport and optical properties of the nanowires.

Water−gas shift chemistry provides a useful method for producing hydrogen from coal; however, fuel cell applications demand that this hydrogen be free of impurities. Due to their unique properties, Pd/Cu alloys represent an import class of materials used for H purification membranes and also serve as the active metals in many heterogeneous catalysts. Little is known about how Pd and Cu interact electronically in these mixed systems and there is debate in the literature over the direction of charge transfer between the two species. This study used the differential conductance (dI/dV) spectroscopy capabilities of a low-temperature scanning tunneling microscope (STM) to investigate the atomic-scale electronic structure of Pd/Cu surface alloys. dI/dV spectroscopy gives a direct measure of the local density of states of surface sites with subnanometer precision. Results from this work demonstrate that individual, isolated Pd atoms in a Cu lattice are almost electronically identical to their host atoms. Over an...