Image-potential States Research Articles

Probing charge redistribution at the interface of self-assembled cyclo-P5 pentamers on Ag(111)

Phosphorus pentamers (cyclo-P5) are unstable in nature but can be synthesized at the Ag(111) surface. Unlike monolayer black phosphorous, little is known about their electronic properties when in contact with metal electrodes, although this is crucial for future applications. Here, we characterize the atomic structure of cyclo-P5 assembled on Ag(111) using atomic force microscopy with functionalized tips and density functional theory. Combining force and tunneling spectroscopy, we find that a strong charge transfer induces an inward dipole moment at the cyclo-P5/Ag interface as well as the formation of an interface state. We probe the image potential states by field-effect resonant tunneling and quantify the increase of the local change of work function of 0.46 eV at the cyclo-P5 assembly. Our experimental approach suggest that the cyclo-P5/Ag interface has the characteristic ingredients of a p-type semiconductor-metal Schottky junction with potential applications in field-effect transistors, diodes, or solar cells.

Demonstrating the Connection between the Nonvalence Correlation-Bound Anions of Polyaromatic Hydrocarbons and the Image Potential States of Graphene Using a One-Electron Model Hamiltonian.

The ground and excited state nonvalence correlation-bound (NVCB) anion states of the hexagonal polycylic aromatic hydrocarbons and of hexagonal graphene nanoflakes are characterized using a one-electron model Hamiltonian which incorporates atomic electrostatic moments up to the quadrupole, coupled inducible charges and dipoles, and atom-centered Gaussians to describe the short-range repulsive interactions. Extrapolation of the calculated electron binding energies of the lowest energy symmetric and antisymmetric (with respect to the molecular plane) NVCB anions of both the polycylic aromatic hydrocarbons and the carbon nanoflakes to the n → ∞ limit yields binding energies that are in good agreement with those of the most stable symmetric and antisymmetric image potential states of freestanding graphene as determined from two-photon photoemission spectroscopy (2PPE) experiments.

Spin-dependent hybridization of image-potential states and overlayer states: One monolayer of Tl on Ag(111)

Spin-dependent hybridization of image-potential states and overlayer states: One monolayer of Tl on Ag(111)

Realization of large-area ultraflat chiral blue phosphorene

Blue phosphorene (BlueP), a theoretically proposed phosphorous allotrope with buckled honeycomb lattice, has attracted considerable interest due to its intriguing properties. Introducing chirality into BlueP can further enrich its physical and chemical properties, expanding its potential for applications. However, the synthesis of chiral BlueP remains elusive. Here, we demonstrate the growth of large-area BlueP films on Cu(111), with lateral size limited by the wafer dimensions. Importantly, we discovered that the BlueP is characterized by an ultraflat honeycomb lattice, rather than the prevailing buckled structure, and develops highly ordered spatial chirality plausibly resulting from the rotational stacking with the substrate and interface strain release, as further confirmed by the geometric phase analysis. Moreover, spectroscopic measurements reveal its intrinsic metallic nature and different characteristic quantum oscillations in the image-potential states, which can be exploited for a range of potential applications including polarization optics, spintronics, and chiral catalysis.

Deceptive orbital confinement at edges and pores of carbon-based 1D and 2D nanoarchitectures.

The electronic structure defines the properties of graphene-based nanomaterials. Scanning tunneling microscopy/spectroscopy (STM/STS) experiments on graphene nanoribbons (GNRs), nanographenes, and nanoporous graphene (NPG) often determine an apparent electronic orbital confinement into the edges and nanopores, leading to dubious interpretations such as image potential states or super-atom molecular orbitals. We show that these measurements are subject to a wave function decay into the vacuum that masks the undisturbed electronic orbital shape. We use Au(111)-supported semiconducting gulf-type GNRs and NPGs as model systems fostering frontier orbitals that appear confined along the edges and nanopores in STS measurements. DFT calculations confirm that these states originate from valence and conduction bands. The deceptive electronic orbital confinement observed is caused by a loss of Fourier components, corresponding to states of high momentum. This effect can be generalized to other 1D and 2D carbon-based nanoarchitectures and is important for their use in catalysis and sensing applications.

Growth and local electronic properties of Cobalt nanodots underneath graphene on SiC(0001)

The coupling of graphene with a ferromagnetic material opens opportunities for technological innovations in spintronics. To obtain this coupling it is necessary to control the elaboration of interfaces at the atomic scale. Here, we present results on cobalt intercalation between graphene and a buffer layer supported on a SiC(0001) substrate. As a result, we obtain cobalt islands covered by graphene whose local electronic properties are measured by scanning tunneling microscopy and spectroscopy. These islands reveal two very distinct shapes and properties. Small-islands with atomic height and very narrow size distribution and, more interestingly, flat cobalt nanodots lower than one nanometer high, that are encapsulated by graphene. Compared to a graphene monolayer on SiC, those nanodots exhibit very different spectroscopic signatures. Using dI/dV local differential conductance spectra together with an analysis of image potential surface states measured thanks to dz/dV spectra, we show that graphene on the nanodots is neutrally charged. Moreover, its 4.65 eV work function is surprisingly larger than the predicted value of 3.8 eV for graphene on Co. First principle calculations show that those Co nanodots can be seen as cobalt bilayer sandwiched between two carbon planes.

Formation of an Extended Quantum Dot Array Driven and Autoprotected by an Atom-Thick h-BN Layer.

Engineering quantum phenomena of two-dimensional nearly free electron states has been at the forefront of nanoscience studies ever since the first creation of a quantum corral. Common strategies to fabricate confining nanoarchitectures rely on manipulation or on applying supramolecular chemistry principles. The resulting nanostructures do not protect the engineered electronic states against external influences, hampering the potential for future applications. These restrictions could be overcome by passivating the nanostructures with a chemically inert layer. To this end we report a scalable segregation-based growth approach forming extended quasi-hexagonal nanoporous CuS networks on Cu(111) whose assembly is driven by an autoprotecting h-BN overlayer. We further demonstrate that by this architecture both the Cu(111) surface state and image potential states of the h-BN/CuS heterostructure are confined within the nanopores, effectively forming an extended array of quantum dots. Semiempirical electron-plane-wave-expansion simulations shed light on the scattering potential landscape responsible for the modulation of the electronic properties. The protective properties of the h-BN capping are tested under various conditions, representing an important step toward the realization of robust surface state based electronic devices.

Fast fabrication and judgement of tip-enhanced Raman spectroscopy-active tips

The quality of the scanning tip is crucial for tip-enhanced Raman spectroscopy (TERS) experiments towards large signal enhancement and high spatial resolution. In this work, we report a controllable fabrication method to prepare TERS-active tips by modifying the tip apex at the atomic scale, and propose two important criteria to in-situ judge the tip's TERS activity for tip-enhanced Raman measurements. One criterion is based on the downshift of the first image potential state to monitor the coupling between the far-field incident laser and near-field plasmon; the other is based on the appearance of the low-wavenumber Raman peaks associated with an atomistic protrusion at the tip apex to judge the coupling efficiency of emissions from the near field to the far field. This work provides an effective method to quickly fabricate and judge TERS-active tips before real TERS experiments on target molecules and other materials, which is believed to be instrumental for the development of TERS and other tip-enhanced spectroscopic techniques.

Hybridization-Induced Image Potential States with Large Effective Mass in Lead Phthalocyanine Overlayers on Graphene

The structural and electronic properties of lead phthalocyanine (PbPc) adsorbed on graphene are studied theoretically using the van der Waals density functional method. It is revealed that an extended state analogous to an image potential state (IPS) emerges in a close-packed PbPc monolayer, hybridizing with the lowest IPS of graphene to form bonding and antibonding states at the PbPc–graphene interface. The impact of the PbPc adsorption manifests itself as increased effective masses of the hybrid IPSs as compared with those on pristine graphene. In particular, the bonding IPS exhibits a marked anisotropy in the effective mass, reflecting the uniaxially elongated unit cell adopted in the present calculations, whereas the antibonding one has an almost isotropic effective mass, which is comparable to experimental results obtained with two-photon photoemission spectroscopy of PbPc on highly oriented pyrolytic graphite and on single crystalline graphite. It is also shown that the increase in the effective masses is triggered by the hybridization with unoccupied molecular orbitals of PbPc that is localized at the Pb atom.

Impact of molecular distribution on unoccupied image potential states of PbPc/HOPG surfaces

Two-photon photoemission (2PPE) spectroscopy revealed modified image potential states (IPSs) of sub-monolayer lead phthalocyanine (PbPc) films on graphite (HOPG). Angle-resolved (AR) 2PPE for room temperature films showed unique energy/momentum maps in which the inside of the parabolic dispersion curve of IPS on HOPG area is filled up to a specific energy. For cooled films, AR maps showed two parabolic curves originating from IPSs on HOPG areas and PbPc islands. The changes are explained with simple models of electron scattering by molecular islands. IPS feature is an indicator of nano-scale superstructures, which are not easily assessed by other experiments.

Ultrafast Ferroelectric Ordering on the Surface of a Topological Semimetal MoTe2.

Transient tuning of material properties by light usually requires intense laser fields in the nonlinear excitation regime. Here, we report ultrafast ferroelectric ordering on the surface of a paraelectric topological semimetal 1T'-MoTe2 in the linear excitation regime, with the order parameter directly proportional to the excitation intensity. The ferroelectric ordering, driven by a transient electric field created by electrons trapped ångstroms away from the surface in the image potential state (IPS), is evidenced in two-photon photoemission spectroscopy showing the energy relaxation rate proportional to IPS electron density, but with negligible change in the free-electron-like parallel dispersion. First-principles calculations reveal an improper ferroelectric ordering associated with an anharmonic interlayer shearing mode. Our findings demonstrate an ultrafast charge-based pathway for creating transient polarization orders.

Negative electron affinity opens quantum well in MgO layers on Ag(100)

Epitaxial MgO films on Ag(100) were studied by photoelectron spectroscopy. From the low-energy part of the spectra we obtain a negative electron affinity of about −0.9 eV. Even though electrons in the lowest conduction band are not confined by a potential barrier at the surface, quantum-well resonances are observed. The dispersion of the conduction band is determined in good agreement with theoretical calculations. Aspects of observing image-potential states predicted by theory on MgO films are discussed.

A theoretical investigation of quantum spin Hall state in ordered M′2M″2C3 MXenes (M′ = V, Nb, Ta and M″ = Ti, Zr, Hf)

MXenes have attracted lots of attention because of the potential applications in electronic devices and energy storage. A variety of transition metals in MXenes give rise to distinct properties and trigger more interests. Depending on the exfoliation processes from the MAX phase, the surfaces of MXenes can be terminated by O, F, Cl, OH groups. Theoretical calculations reveal that the electronic properties of MXenes can be tuned by different surface terminations. For example, some F and O terminated MXenes are predicted to be topological insulators with the quantum spin hall states. In OH terminated MXene multilayers, the image potential states are close to the Fermi level. The energies of these states are sensitive to the interlayer distances. Consequently, the topology of the energy bands can be modulated. Here, based on the density functional theory, we study the electronic structures of the ordered double transition metal MXenes M′2M″2C3T2 (where M′ = V, Nb, Ta, M″ = Ti, Zr, Hf and T = F, Cl). We propose that these materials are topologically nontrivial insulators or semimetals. The topological index is 1 and the presence of the conducting helical edge states is demonstrated. Their dynamical stabilities are confirmed by the phonon spectra. We expect that our prediction can facilitate the future application of MXenes as the topological insulating devices.

Screening in Graphene: Response to External Static Electric Field and an Image-Potential Problem.

We present a detailed first-principles investigation of the response of a free-standing graphene sheet to an external perpendicular static electric field E. The charge density distribution in the vicinity of the graphene monolayer that is caused by E was determined using the pseudopotential density-functional theory approach. Different geometries were considered. The centroid of this extra density induced by an external electric field was determined as = 1.048 Å at vanishing E, and its dependence on E has been obtained. The thus determined was employed to construct the hybrid one-electron potential which generates a new set of energies for the image-potential states.

A theoretical investigation of topological phase modulation in carbide MXenes: Role of image potential states

MXenes have triggered great interests because of their potential applications. Depending on the exfoliation, MXenes can be terminated by O, F and OH groups. The properties of MXenes can be designed. Some F and O-terminated MXenes are predicted to be topological insulators (TIs). In OH-MXenes, the work-functions are significantly reduced. The image potential states (IPS), whose energies can be modulated by the interlayer distances, appear near the Fermi level. Based on the density functional theory, we study the electronic structures of the ordered double transition metal carbides multilayers M′2M”2C3(OH)2 (where M’ = V, Nb, Ta and M” = Ti, Zr, Hf) and investigate the role played by the IPS in the determination of the energy bands topology. For example, V2Ti2C3(OH)2 monolayer is a trivial semimetal. However, the layered V2Ti2C3(OH)2 becomes a strong TI by reducing the interlayer distance. In other words, the topological properties can be modulated. A novel energy band conversion involving IPS is proposed. Although the successful synthesis of M′2M”2C3(OH)2 MXenes faces plenty of challenges, the revealing topological transition mechanism will be valid for other materials whose IPS are near the Fermi levels. We expect that our prediction can advance the applications of low work-function multilayers as the controllable TI devices.

Lateral Distribution of Optical Excitation at Boundaries around Rubrene Islands Visualized by Microspot Two-Photon Photoemission Spectroscopy

The lateral distribution of optical excitation occurring at the edge part of rubrene (5,6,11,12-tetraphenyltetracene) molecular islands formed on highly oriented pyrolytic graphite (HOPG) substrate has been visualized with microspot two-photon photoemission (2PPE) spectroscopy of 0.4 µm resolution. As we reported previously [T. Ueba, et al., J. Phys. Chem. C 117 (2013) 20098–20103], an unoccupied peak labeled Ln is strongly enhanced in 2PPE spectroscopy by a resonant optical excitation between a diffuse unoccupied Ln molecular orbital and the HOMO. The strong enhancement is due to the hybridization of Ln with the image potential state (IPS) on the HOPG substrate. In this study, using the spatially-resolved microspot 2PPE method, we obtained the real space image of the area where the optical excitation occurs. One-photon photoelectron emission microscope (1PPE-PEEM) with 50 nm resolution is used to compare with the microspot 2PPE. The 1PPE-PEEM images are homogeneous for films below and above 1 monolayer (ML) coverage, indicating that the molecular density of rubrene is homogeneous within the lateral resolution. On the other hand, microspot 2PPE images for sub-ML films show inhomogeneous areas of several µm sizes. The images depend on the detected unoccupied levels of interest. The inhomogeneity is very significant when the image is recorded at the energy of Ln peak. The different appearance between 1PPE-PEEM and microspot 2PPE images is explained on the basis that the Ln excitation occurs at the edges of islands smaller than the resolution of the PEEM. Though the microspot 2PPE achieves only moderate lateral resolution, it is sensitive to the resonant excitation occurring at nm-scale boundaries of the molecular islands. The distribution of the nm-scale islands is the origin of the µm-scale lateral inhomogeneity. The microspot 2PPE images become homogeneous for films thicker than 1 ML, reflecting the quenching of the resonant excitation. These results demonstrate a possibility that spatially- and energy-resolved microspot 2PPE can be a powerful tool to clarify nanoscale geometric and electronic structures at the molecule/substrate interfaces.

First-principles study of a topological phase transition induced by image potential states in MXenes

MXenes, a family of two-dimensional transition metal carbides and nitrides, have various tunable physical and chemical properties. Their diverse prospective applications in electronics and energy storage devices have triggered great interests in science and technology. MXenes can be functionalized by different surface terminations. Some O and F functionalized MXenes monolayers have been predicted to be topological insulators (TIs). However, the reported OH functionalized MXenes TIs are very few and their electronic structures need to be investigated in more detail. It has been revealed that the work functions of MXenes are reduced significantly by OH termination and the image potential (IP) states move close to the Fermi level. The wave functions of these IP states are spatially extensive outside the surfaces. By stacking the OH-functionalized MXenes, the energies of the IP states can be modulated by the interlayer distances of multilayers, because the overlap and hybridization of the wave functions between the neighboring layers are significant. Therefore, these stacking layers are interacted and coupled with IP states. Here, based on first-principles calculations, we demonstrate that the stacking of two-dimensional topologically trivial OH-functionalized MXenes, such as V$_2$HfC$_2$(OH)$_2$, possibly gives rise to the topologically nontrivial energy bands. In other words, the topological properties of V$_2$HfC$_2$(OH)$_2$ multilayers can be modulated by its interlayer distance. An energy band inversion involving IP states is proposed. We expect that these results can advance the future application of MXenes or other low work function multilayer materials as controllable TI devices.

Nanoscale Probing of Image-Potential States and Electron Transfer Doping in Borophene Polymorphs.

Because synthetic 2D materials are generally stabilized by interfacial coupling to growth substrates, direct probing of interfacial phenomena is critical for understanding their nanoscale structure and properties. Using field-emission resonance spectroscopy with an ultrahigh vacuum scanning tunneling microscope, we reveal Stark-shifted image-potential states of the v1/6 and v1/5 borophene polymorphs on Ag(111) with long lifetimes, suggesting high borophene lattice and interface quality. These image-potential states allow the local work function and interfacial charge transfer of borophene to be probed at the nanoscale and test the widely employed self-doping model of borophene. Supported by apparent barrier height measurements and density functional theory calculations, electron transfer doping occurs for both borophene phases from the Ag(111) substrate. In contradiction with the self-doping model, a higher electron transfer doping level occurs for denser v1/6 borophene compared to v1/5 borophene, thus revealing the importance of substrate effects on borophene electron transfer.

Time- and momentum-resolved image-potential states of 2H-MoS2 surface.

Rydberg-like image potential states (IPSs) form special series surface states on metal and semiconducting surfaces. Here, using time-resolved and momentum-resolved multi-photon photoemission (mPPE), we measured the energy positions, band dispersion, and carrier lifetimes of IPSs at the 2H-MoS2 surface. The energy minima of the IPSs (n = 1 and 2) were located at 0.77 and 0.21 eV below the vacuum level. In addition, the effective masses of these two IPSs are close to the rest mass of the free electron, clearly showing nearly-free-electron character. These properties suggest a good screening effect in the MoS2 parallel to the surface. The multi-photon resonances between the valence band and IPS (n = 1) are observed, showing a k‖-momentum-dependent behavior. Our time-resolved mPPE measurements show that the lifetime of photoexcited electrons in the IPS (n = 1) is about 33 fs.

Impact of interface-state formation on the charge-carrier dynamics at organic-metal interfaces

We study the role of electronic interface states on the electron transfer dynamics between layers of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and the (111) and (100) surfaces of silver. For this purpose, we investigate the change of the decay dynamics of the first (n = 1) image-potential state on these surfaces upon adsorption of an ordered monolayer of PTCDA by means of time-resolved two-photon photoemission (2PPE). We find that the already short lifetime of the (n = 1)-state on Ag(111) is only slightly further reduced by PTCDA adsorption, whereas a much stronger reduction by a factor of three is observed for adsorption on Ag(100) resulting in similar lifetimes for both orientations. We show by model calculations on the basis of an analytical one-dimensional pseudo-potential that the enhanced decay for PTCDA/Ag(100) can be attributed to the opening of an additional channel for electron-electron scattering by the formation of an interface state which is derived from the Shockley-type surface resonance of Ag(100).