Magnetocapacitance at the Ni/BiInO₃ Schottky Interface

We report the observation of a magnetocapacitance effect at the interface between Ni and epitaxial nonpolar BiInO3 thin films at room temperature. A detailed surface study using X-ray photoelectron spectroscopy (XPS) reveals the formation of an intermetallic Ni–Bi alloy at the Ni/BiInO3 interface and a shift in the Bi 4f and In 3d core levels to higher binding energies with increasing Ni thickness. The latter infers band bending in BiInO3, corresponding to the formation of a p-type Schottky barrier. The current–voltage characteristics of the Ni/BiInO3/(Ba,Sr)RuO3/NdScO3(110) heterostructure show a significant dependence on the applied magnetic field and voltage cycling, which can be attributed to voltage-controlled band bending and spin-polarized charge accumulation in the vicinity of the Ni/BiInO3 interface. The magnetocapacitance effect can be realized at room temperature without involving multiferroic materials

Electronic Structure of a Spin Crossover Molecular Adsorbate

We have investigated the occupied and unoccupied electronic structure of ultrahigh vacuum (UHV) evaporated molecular thin films of the spin crossover [Fe­(H₂B­(pz)₂)₂(bipy)] complex (with H₂B­(pz)₂ = bis­(hydrido)­bis­(1<i>H</i>-pyrazol-1-yl)­borate and bipy = 2,2′-bipyridine) by ultraviolet photoelectron spectroscopy (UPS), inverse photoemission (IPES), and X-ray absorption spectroscopy (XAS). A bandgap of 2–3 eV is deduced from combined UPS and IPES measurements of the molecular films on Au substrates. The matching Fe XAS and IPES spectra indicate that the electronic unoccupied states have a significant Fe weight. The shift of the unoccupied density of states seen in inverse photoemission is consistent with the thermally induced spin crossover transition for [Fe­(H₂B­(pz)₂)₂(bipy)] deposited on the organic ferroelectric copolymer poly­(vinylidene fluoride) with trifluoroethylene (PVDF–TrFE)

Coverage-Dependent Interactions at the Organics–Metal Interface: Quinonoid Zwitterions on Au(111)

The large intrinsic electric dipole of about 10 D of a <i>p</i>-benzoquinonemonoimine compound from the class of <i>N</i>-alkyldiaminoresorcinone (or 4,6-bisdialkylaminobenzene-1,3-diones, i.e., C₆H₂(<u>···</u> NHR)₂(<u>···</u> O)₂, where R = H) zwitterions is reduced considerably upon adsorption on Au(111) substrates. Scanning tunneling microscopy images reveal parallel alignment of adsorbed molecules within extended islands, leading to the formation of polarized domains. This is in contrast to the typical antiparallel alignment found in the bulk. High-resolution images show that the molecules form rows along the ⟨1̅01⟩ directions of the Au(111) surface, but otherwise their arrangement is only weakly perturbed by the Au(111) (23 × √3) herringbone surface reconstruction. Density functional theory calculations show that upon increasing the molecular density the strength of the interaction between the zwitterions and the Au(111) surface decreases. Thus, the charge redistribution, which occurs at the interface as a result of molecular adsorption, and therefore the interfacial dipole is coverage dependent. The weakening of the interaction at the organic–metal interface with increasing coverage is experimentally observed as a contraction of the intermolecular bond length. Moreover, it is the strong adsorbate–adsorbate interactions (and not the interactions between the adsorbate molecules and the surface) which determine the molecular arrangement within the 2D network the zwitterions form

Altering the Static Dipole on Surfaces through Chemistry: Molecular Films of Zwitterionic Quinonoids

The adsorption of molecular films made of small molecules with a large intrinsic electrical dipole has been explored. The data indicate that such dipolar molecules may be used for altering the interface dipole screening at the metal electrode interface in organic electronics. More specifically, we have investigated the surface electronic spectroscopic properties of zwitterionic molecules containing 12π electrons of the <i>p</i>-benzoquinonemonoimine type, C₆H₂(<u>···</u>NHR)₂(<u>···</u>O)₂ (R = H (<b>1</b>), <i>n</i>-C₄H₉ (<b>2</b>), C₃H₆–S–CH₃ (<b>3</b>), C₃H₆–O–CH₃ (<b>4</b>), CH₂–C₆H₅ (<b>5</b>)), adsorbed on Au. These molecules are stable zwitterions by virtue of the meta positions occupied by the nitrogen and oxygen substituents on the central ring, respectively. The structures of <b>2</b>–<b>4</b> have been determined by single crystal X-ray diffraction and indicate that in these molecules, two chemically connected but electronically not conjugated 6π electron subunits are present, which explains their strong dipolar character. We systematically observed that homogeneous molecular films with thickness as small as 1 nm were formed on Au, which fully cover the surface, even for a variety of R substituents. Preferential adsorption toward the patterned gold areas on SiO₂ substrates was found with <b>4</b>. Optimum self-assembling of <b>2</b> and <b>5</b> results in ordered close packed films, which exhibit n-type character, based on the position of the Fermi level close to the conduction band minimum, suggesting high conductivity properties. This new type of self-assembled molecular films offers interesting possibilities for engineering metal–organic interfaces, of critical importance for organic electronics

Toward Ferroelectric Control of Monolayer MoS₂

The chemical vapor deposition (CVD) of molybdenum disulfide (MoS₂) single-layer films onto periodically poled lithium niobate is possible while maintaining the substrate polarization pattern. The MoS₂ growth exhibits a preference for the ferroelectric domains polarized “up” with respect to the surface so that the MoS₂ film may be templated by the substrate ferroelectric polarization pattern without the need for further lithography. MoS₂ monolayers preserve the surface polarization of the “up” domains, while slightly quenching the surface polarization on the “down” domains as revealed by piezoresponse force microscopy. Electrical transport measurements suggest changes in the dominant carrier for CVD MoS₂ under application of an external voltage, depending on the domain orientation of the ferroelectric substrate. Such sensitivity to ferroelectric substrate polarization opens the possibility for ferroelectric nonvolatile gating of transition metal dichalcogenides in scalable devices fabricated free of exfoliation and transfer

Surface Electronic Structure of Hybrid Organo Lead Bromide Perovskite Single Crystals

The electronic structure and band dispersion of methylammonium lead bromide, CH₃NH₃PbBr₃, has been investigated through a combination of angle-resolved photoemission spectroscopy (ARPES) and inverse photoemission spectroscopy (IPES), as well as theoretical modeling based on density functional theory. The experimental band structures are consistent with the density functional calculations. The results demonstrate the presence of a dispersive valence band in MAPbBr₃ that peaks at the M̅ point of the surface Brillouin zone. The results also indicate that the surface termination of the CH₃NH₃PbBr₃ is the methylammonium bromide (CH₃NH₃Br) layer. We find our results support models that predict a heavier hole effective mass in the region of −0.23 to −0.26 m_e, along the Γ̅ (surface Brillouin center) to M̅ point of the surface Brillouin zone. The surface appears to be n-type as a result of an excess of lead in the surface region

Gold Dispersion and Activation on the Basal Plane of Single-Layer MoS₂

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 by Au 5d_<i>z</i>2 symmetry