Surface Magnetic Stabilization and the Photoemission Chiral-Induced Spin-Selectivity Effect.

The spinterface mechanism was suggested as a possible origin for the chirality-induced spin-selectivity (CISS) effect and was used to explain and reproduce, with remarkable accuracy, experimental data from transport experiments showing the CISS effect. Here, we apply the spinterface mechanism to explain the appearance of magnetization at the interface between nonmagnetic metals and chiral molecules, through the stabilization of otherwise fluctuating magnetic moments. We show that the stabilization of surface magnetic moments occurs for a wide range of realistic parameters and is robust against dephasing.

Bond Breaking Kinetics in Mechanically Controlled Break Junction Experiments: A Bayesian Approach.

Break junction experiments allow investigating electronic and spintronic properties at the atomic and molecular scale. These experiments generate by their very nature broad and asymmetric distributions of the observables of interest, and thus, a full statistical interpretation is warranted. We show here that understanding the complete lifetime distribution is essential for obtaining reliable estimates.

Localized interlayer excitons in MoSe-WSe heterostructures without a moiré potential.

Interlayer excitons (IXs) in MoSe-WSe heterobilayers have generated interest as highly tunable light emitters in transition metal dichalcogenide (TMD) heterostructures. Previous reports of spectrally narrow (<1 meV) photoluminescence (PL) emission lines at low temperature have been attributed to IXs localized by the moiré potential between the TMD layers. We show that spectrally narrow IX PL lines are present even when the moiré potential is suppressed by inserting a bilayer hexagonal boron nitride (hBN) spacer between the TMD layers.

Phenalenyls as tunable excellent molecular conductors and switchable spin filters.

Phenalenyl-based radicals are stable radicals whose electronic properties can be tuned readily by heteroatom substitution. We employ density functional theory-based non-equilibrium Green's function (NEGF-DFT) calculations to show that this class of molecules exhibits tunable spin- and charge-transport properties in single molecule junctions. Our simulations identify the design principles and interplay between unusually high conductivity and strong spin-filtering.

Correlated Energy-Level Alignment Effects Determine Substituent-Tuned Single-Molecule Conductance.

The rational design of single-molecule electrical components requires a deep and predictive understanding of structure-function relationships. Here, we explore the relationship between chemical substituents and the conductance of metal-single-molecule-metal junctions, using functionalized oligophenylenevinylenes as a model system. Using a combination of mechanically controlled break-junction experiments and various levels of theory including non-equilibrium Green's functions, we demonstrate that the connection between gas-phase molecular electronic structure and in-junction molecular conductance is complicated by the involvement of multiple mutually correlated and opposing effects that contribute to energy-level alignment in the junction.

The role of initial and final states in molecular spectroscopies.

Interpreting experimental spectra of thin films of organic semiconductors is challenging, and understanding the relationship between experimental data obtained by different spectroscopic techniques requires a careful consideration of the initial and final states for each process. The discussion of spectroscopic data is frequently mired in confusion that originates in overlapping terminology with however distinct meaning in different spectroscopies. Here, we present a coherent framework that is capable of treating on equal footing most spectroscopies commonly used to investigate thin films of organic semiconductors.

van der Waals Interaction Activated Strong Electronic Coupling at the Interface between Chloro Boron-Subphthalocyanine and Cu(111).

In this article, we investigate the interface between shuttlecock-shaped chloro boron-subphthalocyanine molecules and the Cu(111) surface. We highlight how molecular planarization induced by van der Waals forces can fundamentally alter the interface properties and how it can enable a particularly strong hybridization between molecular and metal states. In our simulations, we start from a situation in which we disregard van der Waals forces and then introduce them gradually by rescaling the interaction parameter, thereby "pulling" the molecule toward the surface.

Anisotropic attosecond charge carrier dynamics and layer decoupling in quasi-2D layered SnS.

Strong quantum confinement effects lead to striking new physics in two-dimensional materials such as graphene or transition metal dichalcogenides. While spectroscopic fingerprints of such quantum confinement have been demonstrated widely, the consequences for carrier dynamics are at present less clear, particularly on ultrafast timescales. This is important for tailoring, probing, and understanding spin and electron dynamics in layered and two-dimensional materials even in cases where the desired bandgap engineering has been achieved.

Ensemble Control of Kondo Screening in Molecular Adsorbates.

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.

Fast sensitive amplifier for two-probe conductance measurements in single molecule break junctions.

We demonstrate an amplifier based on the Wheatstone bridge designed specifically for use in single molecule break junctions. This amplifier exhibits superior performance due to its large bandwidth, flat frequency response, and high sensitivity. The amplifier is capable of measuring conductance values from 10 to 10G (G = 2e/h), while maintaining a bandwidth in excess of 20 kHz, and shows remarkable resolution in the molecular conductance regime of 10 to 10 G.

Sticking with the Pointy End? Molecular Configuration of Chloro Boron-Subphthalocyanine on Cu(111).

In this combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study, we investigate self-assembly of the dipolar nonplanar organic semiconductor chloro boron-subphthalocyanine (ClB-SubPc) on Cu(111). We observe multiple distinct adsorption configurations and demonstrate that these can only be understood by taking surface-catalyzed dechlorination into account. A detailed investigation of possible adsorption configurations and the comparison of experimental and computational STM images demonstrates that the configurations correspond to "Cl-up" molecules with the B-Cl moiety pointing toward the vacuum side of the interface, and dechlorinated molecules.

Spectroscopy and control of near-surface defects in conductive thin film ZnO.

The electronic structure of inorganic semiconductor interfaces functionalized with extended π-conjugated organic molecules can be strongly influenced by localized gap states or point defects, often present at low concentrations and hard to identify spectroscopically. At the same time, in transparent conductive oxides such as ZnO, the presence of these gap states conveys the desirable high conductivity necessary for function as electron-selective interlayer or electron collection electrode in organic optoelectronic devices. Here, we report on the direct spectroscopic detection of a donor state within the band gap of highly conductive zinc oxide by two-photon photoemission spectroscopy.

Hybridization-Induced Carrier Localization at the C60 /ZnO Interface.

Electronic coupling and ground-state charge transfer at the C60 /ZnO hybrid interface is shown to localize carriers in the C60 phase. This effect, revealed by resonant X-ray photoemission, arises from interfacial hybridization between C60 and ZnO. Such localization at carrier-selective electrodes and interlayers may lead to severely reduced carrier harvesting efficiencies and increased recombination rates in organic electronic devices.

Disrupted Attosecond Charge Carrier Delocalization at a Hybrid Organic/Inorganic Semiconductor Interface.

Despite significant interest in hybrid organic/inorganic semiconductor interfaces, little is known regarding the fate of charge carriers at metal oxide interfaces, particularly on ultrafast time scales. Using core-hole clock spectroscopy, we investigate the ultrafast charge carrier dynamics of conductive ZnO films at a hybrid interface with an organic semiconductor. The adsorption of C60 on the ZnO surface strongly suppresses the ultrafast carrier delocalization and increases the charge carrier residence time from 400 attoseconds to nearly 30 fs.

Controlling the Spin Texture of Topological Insulators by Rational Design of Organic Molecules.

We present a rational design approach to customize the spin texture of surface states of a topological insulator. This approach relies on the extreme multifunctionality of organic molecules that are used to functionalize the surface of the prototypical topological insulator (TI) Bi2Se3. For the rational design we use theoretical calculations to guide the choice and chemical synthesis of appropriate molecules that customize the spin texture of Bi2Se3.

Tin disulfide-an emerging layered metal dichalcogenide semiconductor: materials properties and device characteristics.

Layered metal dichalcogenides have attracted significant interest as a family of single- and few-layer materials that show new physics and are of interest for device applications. Here, we report a comprehensive characterization of the properties of tin disulfide (SnS2), an emerging semiconducting metal dichalcogenide, down to the monolayer limit. Using flakes exfoliated from layered bulk crystals, we establish the characteristics of single- and few-layer SnS2 in optical and atomic force microscopy, Raman spectroscopy and transmission electron microscopy.

Defect-driven interfacial electronic structures at an organic/metal-oxide semiconductor heterojunction.

The electronic structure of the hybrid interface between ZnO and the prototypical organic semiconductor PTCDI is investigated via a combination of ultraviolet and X-ray photoelectron spectroscopy (UPS/XPS) and density functional theory (DFT) calculations. The interfacial electronic interactions lead to a large interface dipole due to substantial charge transfer from ZnO to 3,4,9,10-perylenetetracarboxylicdiimide (PTCDI), which can be properly described only when accounting for surface defects that confer ZnO its n-type properties.

Understanding Interfacial Electronic Structure and Charge Transfer: An Electrostatic Perspective.

The challenge of understanding electronic structure and dynamics at organic semiconductor interfaces arises from the richness and importance of weak interactions in thin films of extended π-conjugated molecules. In this Perspective, I discuss a conceptually simple electrostatic approach toward a molecular-level description of the electronic structure and dynamics at a subset of such interfaces. Self-assembled monolayers of oriented dipolar molecules physisorbed on metal surfaces generate sizable collective electric fields, and electrostatics determines the key factors for energy level alignment and molecular electronic structure.

Resonance and localization effects at a dipolar organic semiconductor interface.

The image state manifold of the dipolar organic semiconductor vanadyl naphthalocyanine (VONc) on highly oriented pyrolytic graphite is investigated by angle-resolved two-photon photoemission (AR-TPPE) spectroscopy in the 0-1 monolayer regimes. Interfacial charge-transfer from the image potential state of clean graphite populates a near-resonant VONc anion level, identifiable by the graphite image potential state by its distinct momentum dispersion obtained from AR-TPPE. This affinity level is subject to depolarization by the neighboring molecules, resulting in stabilization of this state with coverage.

Magnetic self-assembly of gold nanoparticle chains using dipolar core-shell colloids.

The preparation of gold nanoparticle (AuNP) assemblies was conducted by the synthesis and dipolar assembly of ferromagnetic core-shell nanoparticles composed of AuNP cores and cobalt NP shells. Dissolution of metallic Co phases with mineral acids afforded self-assembled AuNP chains and bracelets.

Image states at the interface with a dipolar organic semiconductor.

Image states of the dipolar organic semiconductor vanadyl naphthalocyanine on highly oriented pyrolytic graphite are investigated in the submonolayer to few monolayer regime. The presence of a significant molecular dipole in the organized thin films leads to a strong modification of the image states with coverage. In the 0-1 ML regime, we observe successive stabilization of the image state with increasing coverage.

Influence of electrostatic fields on molecular electronic structure: insights for interfacial charge transfer.

Molecular and interfacial electronic structure at organic semiconductor interfaces shows a rich and subtle dependence on short- and long-range electrostatic interactions. Interface dipoles can be controlled making use of the anisotropic charge distribution at the interface, often with direct consequences also for the molecular electronic structure. In this Perspective, we will discuss the emerging understanding of how local and collective electrostatic effects control energy level alignment and molecular electronic structure at organic semiconductor interfaces and highlight some of the ramifications for interfacial charge-transfer dynamics.

Synthesis and colloidal polymerization of ferromagnetic Au-Co nanoparticles into Au-Co3O4 nanowires.

The preparation of cobalt oxide nanowires with gold nanoparticle (AuNP) inclusions (Au-Co(3)O(4) nanowires) via colloidal polymerization of dipolar core-shell NPs is reported. Polystyrene-coated ferromagnetic NPs composed of a dipolar metallic cobalt shell and a gold NP core (PS-AuCoNPs) were synthesized by thermolysis of octacarbonyldicobalt [Co(2)(CO)(8)] in the presence of AuNP seeds and polymeric ligands. The colloidal polymerization process of these dipolar PS-AuCoNPs comprises dipolar nanoparticle assembly and solution oxidation of preorganized NPs to form interconnected cobalt oxide nanowires via the nanoscale Kirkendall effect, with AuNP inclusions in every repeating unit of the one-dimensional mesostructure.

Confocal single molecule fluorescence spectroscopy in ultrahigh vacuum.

We have constructed an ultrahigh vacuum confocal fluorescence microscope with the purpose of performing single molecule spectroscopy under highly defined conditions. The microscope is designed for high stability while affording the capability of sample preparation, sample transfer, and optical detection in ultrahigh vacuum. It achieves near-diffraction-limited performance and allows the observation of single molecule fluorescence dynamics over extended periods of time.

Single molecule power-law behavior on a crystalline surface.

Single perylene bisimide molecules deposited onto Al(2)O(3) (0001) and investigated under controlled ultrahigh vacuum conditions display fluorescence intermittency behavior characteristic of an interfacial charge transfer process. Remarkably, even though the molecules are deposited on a crystalline surface with reduced disorder, power-law-distributed bright and dark periods are observed. These data can be understood based on activated formation of localized small polaron states in Al(2)O(3) (0001).