Polarization-dependent Photoemission Research Articles

Polarization-dependent photoemission electron microscopy for domain imaging of inorganic and molecular materials.

Polarization-dependent photoemission electron microscopy (PD-PEEM) exploits spatial variation in the optical selection rules of materials to image domain formation and material organization on the nanoscale. In this Perspective, we discuss the mechanism of PD-PEEM that results in the observed image contrast in experiments and provide examples of a wide range of material domain structures that PD-PEEM has been able to elucidate, including molecular and polymer domains, local electronic structure and defect symmetry, (anti)ferroelectricity, and ferromagnetism. In the end, we discuss challenges and new directions that are possible with this tool for probing domain structure in materials, including investigating the formation of transient ordered states, multiferroics, and the influence of molecular and polymer order and disorder on excited state dynamics and charge transport.

Electronic structure orientation as a map of in-plane antiferroelectricity in β'-In2Se3.

Antiferroelectric (AFE) materials are excellent candidates for sensors, capacitors, and data storage due to their electrical switchability and high-energy storage capacity. However, imaging the nanoscale landscape of AFE domains is notoriously inaccessible, which has hindered development and intentional tuning of AFE materials. Here, we demonstrate that polarization-dependent photoemission electron microscopy can resolve the arrangement and orientation of in-plane AFE domains on the nanoscale, despite the absence of a net lattice polarization. Through direct determination of electronic transition orientations and analysis of domain boundary constraints, we establish that antiferroelectricity in β'-In2Se3 is a robust property from the scale of tens of nanometers to tens of micrometers. Ultimately, the method for imaging AFE domain organization presented here opens the door to investigations of the influence of domain formation and orientation on charge transport and dynamics.

Growth Dynamics of Ultrathin Films of Benzo[1,2-b:4,5-b’]dithiophene Derivatives on Au(111): A Photoelectron Spectroscopy Investigation

Ultrathin films of a stereoisomeric mixture of benzo[1,2-b:4,5-b']dithiophene derivatives were grown by thermal evaporation in vacuum on Au(111), and they were studied in situ by photoelectron spectroscopy. X-ray photons from a non-monochromatic Mg Kα conventional X-ray source and UV photons from a He I discharge lamp equipped with a linear polarizer were used. He I photoemission results were compared with density functional theory (DFT) calculations: density of states (DOS) and 3D molecular orbital density distribution. Au 4f, C 1s, O 1s, and S 2p core-level components suggest a surface rearrangement as a function of film nominal thickness, with the variation of the molecular orientation, from flat-laying at the initial deposition to tilted toward the surface normal at coverages exceeding 2 nm. Eventually, the DFT results were exploited in assigning of the valence band experimental structures. Moreover, polarization-dependent photoemission confirmed the tilted arrangement of the molecules, starting at 2 nm. A variation of the work function of 1.4 eV with respect to the clean substrate was measured, together with a valence band offset of 1.3 eV between the organic layer and gold.

Polarization dependent photoemission as a probe of the magnetic ground state in the van der Waals ferromagnet VI3

Van der Waals ferromagnets are thrilling materials from both a fundamental and technological point of view. VI3 is an interesting example, with a complex magnetism that differentiates it from the first reported Cr based layered ferromagnets. Here, we show in an indirect way through angle resolved photoemission spectroscopy experiments, the importance of spin–orbit coupling setting the electronic properties of this material. Our light polarized photoemission measurements point to a ground state with a half-filled e±′ doublet, where a gap opening is triggered by spin–orbit coupling enhanced by electronic correlations.

Nanoimaging of the Edge-Dependent Optical Polarization Anisotropy of Black Phosphorus.

The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, not only opening the possibility for manipulating material properties but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications, such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edges of BP flakes have a shift in their optical polarization anisotropy from the flake interior due to the 1D confinement and symmetry reduction at flake edges that alter the electronic charge distributions and transition dipole moments of edge electronic states, confirmed with first-principles calculations. These results uncover previously hidden modification of the polarization-dependent absorbance at the edges of BP, highlighting the opportunity for selective excitation of edge states of 2D materials with polarized light.

High-order replica bands in monolayer FeSe/SrTiO3 revealed by polarization-dependent photoemission spectroscopy

The mechanism of the enhanced superconductivity in monolayer FeSe/SrTiO3 has been enthusiastically studied and debated over the past decade. One specific observation has been taken to be of central importance: the replica bands in the photoemission spectrum. Although suggestive of electron-phonon interaction in the material, the essence of these spectroscopic features remains highly controversial. In this work, we conduct angle-resolved photoemission spectroscopy measurements on monolayer FeSe/SrTiO3 using linearly polarized photons. This configuration enables unambiguous characterization of the valence electronic structure with a suppression of the spectral background. We consistently observe high-order replica bands derived from various Fe 3d bands, similar to those observed on bare SrTiO3. The intensity of the replica bands is unexpectedly high and different between dxy and dyz bands. Our results provide new insights on the electronic structure of this high-temperature superconductor and the physical origin of the photoemission replica bands.

Strain-gradient-induced magnetic anisotropy in straight-stripe mixed-phase bismuth ferrites: Insight into flexomagnetism

Implementation of antiferromagnetic compounds as active elements in spintronics has been hindered by their insensitive nature against external perturbations which causes difficulties in switching among different antiferromagnetic spin configurations. Electrically-controllable strain gradient can become a key parameter to tune the antiferromagnetic states of multiferroic materials. We have discovered a correlation between an electrically-written straight-stripe mixed-phase boundary and an in-plane antiferromagnetic spin axis in highly-elongated La-5%-doped BiFeO$_{3}$ thin films by performing polarization-dependent photoemission electron microscopy in conjunction with cluster model calculations. Model Hamiltonian calculation for the single-ion anisotropy including the spin-orbit interaction has been performed to figure out the physical origin of the link between the strain gradient present in the mixed phase area and its antiferromagnetic spin axis. Our findings enable estimation of the strain-gradient-induced magnetic anisotropy energy per Fe ion at around 5$\times$10$^{-12}$ eV m, and provide a new pathway towards an electric-field-induced 90$^{\circ}$ rotation of antiferromagnetic spin axis at room temperature by flexomagnetism.

Observation of an eg-derived metallic band at the Cs/SrTiO3 interface by polarization-dependent photoemission spectroscopy

A metallic band having the dz2-symmetry was evidenced at the interface between a Cs film and the SrTiO3(001) substrate. The symmetry assignment was carried out with the dipole-transition selection rule, probed by polarization-dependent photoemission spectroscopy. The eg-derived state of the SrTiO3(001) crystal is likely to be a bonding state composed of Cs s-orbital and Ti d-orbital with the eg symmetry, as predicted theoretically.

Magnetic skin layer of NiO(100) probed by polarization-dependent spectromicroscopy

Using polarization-dependent x-ray photoemission electron microscopy, we have investigated the surface effects on antiferromagnetic (AFM) domain formation. Depth-resolved information obtained from our study indicates the presence of strain-induced surface AFM domains on some of the cleaved NiO(100) crystals, which are unusually thinner than bulk AFM domain wall widths (∼150 nm). Existence of such magnetic skin layer is substantiated by exchange-coupled ferromagnetic Fe domains in Fe/NiO(100), thereby evidencing the influence of this surface AFM domains on interfacial magnetic coupling. Our observations demonstrate a depth evolution of AFM structure in presence of induced surface strain, while the surface symmetry-breaking in absence of induced strain does not modify the bulk AFM domain structure. Realization of such thin surface AFM layer will provide better microscopic understanding of the exchange bias phenomena.

Electronic structure of the hydrogen-adsorbed SrTiO3(001) surface studied by polarization-dependent photoemission spectroscopy

The metallic band structure of the hydrogen-adsorbed SrTiO${}_{3}$(001) surface is studied by polarization-dependent photoemission spectroscopy measurements. With the dipole-transition selection arguments, we attributed the metallic peaks into the Ti 3${d}_{xz}$, 3${d}_{yz}$, and 3${d}_{xy}$ bands. These ${t}_{2g}$ bands are partially filled with electrons upon H-induced downward band bending at the surface. Energy splitting of the Ti 3$d$ ${t}_{2g}$ bands is considered to be induced by a quantum confinement along the surface normal ($z$) direction. The metallic peaks are accompanied by the incoherent states induced by many-body interactions, which likely indicates that the electronic system forms a two-dimensional liquid phase. Also emerged by H exposure are an in-gap state and a $\ensuremath{\sigma}(\mathrm{O}\ensuremath{-}\mathrm{H})$ state. The possible origin and the feature of these states are discussed from the polarization dependence of the photoemission intensity.

Symmetry lowering of pentacene molecular states interacting with a Cu surface

Pentacene adsorbed on the Cu(119) vicinal surface forms long-range ordered chain structures. Photoemission spectroscopy measurements and ab initio density functional theory simulations provide consistent evidences that pentacene molecular orbitals mix with the copper bands, giving rise to interaction states localized at the interface. Angular-resolved and polarization dependent photoemission spectroscopy shows that most of the pentacene derived intensity is strongly dichroic. The symmetry of the molecular states of the free pentacene molecules is reduced upon adsorption on Cu(119), as a consequence of the molecule-metal interaction. Theoretical results show a redistribution of the charge density in $\ensuremath{\pi}$ molecular states close to the Fermi level, consistent with the photoemission intensities (density of states) and polarization dependence (orbital symmetry).

Morphology and electronic structure of thin 3,4,9,10-perylenetetracarboxylic dianhydride layers on Si(001)

The growth of 3,4,9,10-perylenetetracarboxylic dianhydride on Si(001) was examined in the light of varying flux of impinging molecules. Using atomic force microscopy and synchrotron radiation photoelectron spectroscopy Vollmer–Weber growth mode was observed on a wide range of growth rates. The island size initially decreases rapidly with growth rate, for the low growth rate reaches a minimum, and then gradually increases. Polarization dependent photoemission indicates that the orientation of the molecules within the islands remains flat on the substrate.

Electronic structure and fluctuation effects in the spin-1/2 quantum magnet TiOCl

We present a joint experimental and theoreticalstudy of the spin-1/2 quantum magnet TiOCl. From polarization-dependent momentum-resolved photoemission spectroscopy we can exclude sizable phonon-induced orbital fluctuations as origin for the non-canonical phenomenology of the spin-Peierls transition in this compound. While our data does confirm the quasi-one-dimensional character of the electronic structure along the crystallographic b-axis as inferred from previous LDA+U calculations these as well as other band and model calculations apparently fail to describe the detailed behavior of the quasi-one-dimensional dispersions in the normal state of this material.

Epitaxial growth of Ag on W(110)

Epitaxial growth of Ag on W(110) at room temperature was studied by scanning tunneling microscopy (STM) and polarization-dependent photoemission. At coverages far below one monolayer Ag atoms populate bcc sites of the substrate and form close-packed islands of monolayer thickness. With increasing coverage geometrical misfit between Ag(111)-like layers and W(110) generates surface stress along $\mathrm{W}[1\overline{1}0]$. This is released by formation of domain walls parallel W[001] which are observed with a distance between about $25\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, depending on the details of the growth process. At one monolayer coverage most of the Ag atoms still reside in or very near to bcc substrate positions, but now the strain release pattern is changed: solitons aligned along $\mathrm{W}[\overline{1}12]$ are formed at an average distance between $35\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and $50\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The details of the soliton arrangement depend critically on the degree of equilibration and the presence of holes in the monolayer film which allow an additional stress release. This is evident from a comparison with results of STM studies performed at the closed and carefully annealed Ag monolayer [Kim et al., Phys. Rev. B 67, 223401 (2003)]. Further deposition of Ag starts growth of a second monolayer by formation of islands which increase in size with coverage. At a nominal coverage of 1.5 monolayers the strain relieve pattern changes again: some corrugation lines are oriented along W[001] as in the submonolayers, but other orientations related to Ag(111) directions appear as well. This indicates that several possibilities are available at similar energy costs and that the transition from the W substrate potential to a Ag potential seen by the second layer is very soft. Finally at a nominal coverage of several monolayers, Stranski-Krastanov growth is observed producing Ag(111)-like terraces with one of the dense-packed Ag rows oriented parallel to $\mathrm{W}[1\overline{1}1]$.

Metal hybridization and electronic structure of Tris(8-hydroxyquinolato)aluminum (Alq 3)

The metal-organic complex Tris(8-hydroxyquinolato)aluminum (Alq 3) has been studied by energy and light polarization dependent photoemission. Resonant photoemission was used to identify the molecular orbitals involved in metal chelation. When adsorbed on cobalt and gold surfaces, marked differences in the Alq 3 metal-to-ligand bonds were observed. The results indicate intramolecular aluminum-to-ligand bonding through the oxygen heteroatom when Alq 3 is adsorbed on gold, but through the nitrogen heteroatom when on cobalt. These results indicate that substrate interfacial complex formation plays an important role in the Alq 3 molecular configuration and intramolecular bonding.

Orientation and Bonding of 4,4‘-Biphenyldiisocyanide

The orientation and bonding of 4,4'-biphenyldiisocyanide (BPDI) adsorbed by solution on Au( 111) has been examined by polarization dependent photoemission and inverse photoemission. The highest occupied molecular orbital to lowest unoccupied molecular orbital gap for BPDI is about 10.8 eV, with the Fermi level nearly midgap. The electronic structure of isocyano-functionalized polyphenyls suggests that they are better dielectrics than thiol-functionalized polyphenyls.

Band structure symmetry effects in Gd multilayers grown on W(1 1 2)

We present evidence of coverage-dependent band structure symmetry effects in scanning tunneling microscopy. We find that the symmetry of the Gd induced band located at about 2 eV below the Fermi energy, at the surface Brillouin zone center, appears to be strongly dependent on the atomic structure in the bilayer regime for Gd on W(1 1 2). Light polarization dependent photoemission from this band indicates more d z 2 symmetry character for rectangular Gd structures and rather more d xz d yz character for the quasi hexagonal bilayer structure. Band symmetry changes accompanying the structural phase transition from rectangular to quasi-hexagonal Gd bilayer structures are consistent with arguments used to explain the apparent STM corrugation observed for Gd on W(1 1 2) (Surf Sci 520(2002)43).

Electric-field-induced anisotropy of excitonic optical properties in semiconductor quantum dots

We report the anisotropic excitonic optical properties (polarization dependent photoemission and photoabsorption) induced by applying a lateral electric field in a single semiconductor quantum dot. The excitonic optical polarization characteristics are examined using theoretical calculation and optical measurement. The optical properties are numerically analyzed taking into account the quantum dot potential, electric field, and electron–hole Coulomb interaction. We evaluate the polarization properties from calculated exciton wave functions. The polarization properties depend strongly on the size of the quantum dots and the spatial symmetry of the hole part of the wave function. There is large electric-field-induced anisotropy in thin quantum dots to a larger lateral extent even where the polarization properties are completely isotropic without an electric field. We compare the theoretical results with experimental results for InGaAs quantum dots obtained using the microphotoluminescence technique. Qualitative agreement between the theoretical results and experimental ones is obtained.

Electronic band structure of single-crystal and single-layerWS2:Influence of interlayer van der Waals interactions

The valence band structure of the layered transition metal dichalcogenide ${\mathrm{WS}}_{2}$ has been determined experimentally by angle resolved photoelectron spectroscopy and theoretically by augmented spherical wave band structure calculations as based on density functional theory. Good agreement between experimental and calculated band structure is observed for single crystal ${\mathrm{WS}}_{2}.$ An experimental band structure of a single layer was determined from an electronically decoupled film prepared on a single crystalline graphite substrate by metal-organic van der Waals epitaxy. The polarization dependent photoemission selection rules of the single layer film are appropriate for a free standing film. The experimental single layer band structure shows some differences compared to band structure calculations using bulk atomic positions within the layer. We conclude that relaxation of the single layer occurs as a consequence of the missing interlayer interactions leading to close agreement between electronic structure of the single layer and single crystal. As a consequence of the missing interlayer interactions the valence band maximum for the single layer is located at the K point of the Brillouin zone.

Fine-structure in the low-energy excitation spectrum of a high-Tcsuperconductor by polarization-dependent photoemission

Angle-resolved photoemission spectroscopy is performed on single crystals of the single-layer high-Tc superconductor Bi(2)Sr(2-x)La(x)CuO(6+d) at optimal doping (x=0.4) in order to study in great detail the Zhang-Rice (ZR) singlet band at the Fermi level. Besides the high crystal quality the advantages of a single-layer material are the absence of bilayer effects and the distinct reduction of thermal broadening. Due to the high energy and angle resolution and, most important, due to the controlled variation of the polarization vector of the synchrotron radiation the emission from the ZR singlet band reveals a distinct fine-structure. It consists of two maxima, the first showing only weak and the second at EF extremely strong polarization dependence. However, our observation has enormous consequences for line shape analyses and the determination of pseudo gaps by photoemission.