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The morphology and electronic structure of vapor deposited 4,4'-biphenyldiisocyanide (BPDI) on a Au(111) surface were investigated using variable-temperature scanning tunneling microscopy. When deposited at room temperature, BPDI molecules form one-dimensional molecular chains similar to that recently observed for the structurally related 1,4-phenyl diisocyanide (PDI). Compared to PDI, the longer periodicity for the BPDI molecular chains is consistent with the addition of a second phenyl ring and supports a structural model in which the BPDI molecules lie parallel to the surface and interconnected by Au-adatoms. The molecular chains are mostly aligned along the [110] direction of the Au(111) substrate, but exhibit frequent changes in angle that are consistent with directions between fcc and hcp three-fold hollow sites. Dispersion-corrected density functional theory calculations for one-dimensional chains of BPDI molecules bound end-to-end via their isocyanide groups to Au-adatoms reproduce the observed periodicity of the chains and show that this morphology is energetically favored over upright binding with one free --NC group. The spatially resolved conductance (dI/dV) map for BPDI on Au(111) exhibits a feature centered at -0.67 eV below the Fermi level which are delocalized along the chain with maxima at the Au-adatom and biphenyl positions. This occupied resonant feature is close to that previously observed for the PDI in both photoemission and conductance measurements and is attributed to an occupied interfacial state resulting from BPDI-Au interactions. [ABSTRACT FROM AUTHOR]

We use low-temperature scanning tunneling microscopy in combination with angle-resolved ultraviolet and two-photon photoemission spectroscopy to investigate the interfacial electronic structure of titanyl phthalocyanine (TiOPc) on Cu(110). We show that the presence of two unique molecular adsorption configurations is crucial for a molecular-level analysis of the hybridized interfacial electronic structure. Specifically, thermally induced self-assembly exposes marked adsorbate-configuration-specific contributions to the interfacial electronic structure. The results of this work demonstrate an avenue towards understanding and controlling interfacial electronic structure in chemisorbed films even for the case of complex film structure. [ABSTRACT FROM AUTHOR]

Black phosphorus (BP) has attracted significant interest as a monolayer or few-layer material with extraordinary electrical and optoelectronic properties. Chemical reactions with different ambient species, notably oxygen and water, are important as they govern key properties such as stability in air, electronic structure and charge transport, wetting by aqueous solutions, and so on. Here, we report experiments combined with ab initio calculations that address the effects of oxygen and water in contact with BP. Our results show that the reaction with oxygen is primarily responsible for changing properties of BP. Oxidation involving the dissociative chemisorption of O2 causes the decomposition of BP and continuously lowers the conductance of BP field-effect transistors (FETs). In contrast, BP is stable in contact with deaerated (i.e., O2 depleted) water and the carrier mobility in BP FETs gated by H2O increases significantly due to efficient dielectric screening of scattering centers by the high-k dielectric. Isotope labeling experiments, contact angle measurements, and calculations show that the pristine BP surface is hydrophobic but is turned progressively hydrophilic by oxidation. Our results open new avenues for exploring applications that require contact of BP with aqueous solutions including solution gating, electrochemistry, and solution-phase approaches for exfoliation, dispersion, and delivery of BP. [ABSTRACT FROM AUTHOR]

Using angle-resolved photoemission on micrometer-scale sample areas, we directly measure the interlayer twist angle-dependent electronic band structure of bilayer molybdenum-disulfide (MoS2). Our measurements, performed on arbitrarily stacked bilayer MoS2 flakes prepared by chemical vapor deposition, provide direct evidence for a downshift of the quasiparticle energy of the valence band at the Brillouin zone center (Γ point) with the interlayer twist angle, up to a maximum of 120 meV at a twist angle of ~40°. Our direct measurements of the valence band structure enable the extraction of the hole effective mass as a function of the interlayer twist angle. While our results at Γ agree with recently published photoluminescence data, our measurements of the quasiparticle spectrum over the full 2D Brillouin zone reveal a richer and more complicated change in the electronic structure than previously theoretically predicted. The electronic structure measurements reported here, including the evolution of the effective mass with twist-angle, provide new insight into the physics of twisted transition-metal dichalcogenide bilayers and serve as a guide for the practical design of MoS2 optoelectronic and spin-/valley-tronic devices. [ABSTRACT FROM AUTHOR]

We report angle-resolved photoemission spectroscopic measurements of the evolution of the thicknessdependent electronic band structure of the atomically heavy two-dimensional layered dichalcogenide, tungsten diselenide (WSe2). Our data, taken on mechanically exfoliated WSe2 single crystals, provide direct evidence for shifting of the valence-band maximum from Γ (multilayer WSe2) to K (single-layer WSe2). Further, our measurements also set a lower bound on the energy of the direct band gap and provide direct measurement of the hole effective mass. [ABSTRACT FROM AUTHOR]

We report the observation and molecular-scale scanning probe electronic structure (dl/dV) mapping of hydrogen-bonded cyclic water clusters nucleated on an oxide surface. The measurements are made on a new type of cyclic water cluster that is characterized by simultaneous and cooperative bonding interactions among molecules as well as with both metal and oxygen sites of an oxide surface. Density functional theory + U + D calculations confirm the stability of these clusters and are used to discuss other potential water-oxide bonding scenarios. The calculations show that the spatial distributions of electronic states in the system are similar in character to those of the lowest unoccupied molecular orbitals of hydrogen-bonded water molecules. On the partially oxidized Cu(111) investigated here, experiment and theory together suggest that Cu vacancies in the growing islands of cuprous oxide inhibit water adsorption in the centers of the islands (which have reached thermodynamic equilibrium). A stoichiometric, less stable cuprous oxide likely exists at island edges (the growth front) and selectively binds these water clusters. [ABSTRACT FROM AUTHOR]