On-Surface Synthesis of Graphene Nanoribbons: Photoelectron Spectroscopy Reveals Impact of Substrate Reactivity

Graphene nanoribbons (GNRs) show favorable electronic and optical properties due to their excellent stability at ambient conditions and are suitable materials for nanoscale electronic devices. 10,10′-Dibromo-9,9′-bianthracene (DBBA) has proven to be a suitable precursor for on-surface synthesis of GNRs because it shows a manifold of temperature-assisted reactions like dehalogenation, debromination, or Ullman-coupling on metal surfaces. We use DBBA to conduct a thorough investigation across a wide temperature range (170–750 K) and to track the formation process of 7-graphene nanoribbons (7-GNRs) on Au(111), Ag(111), and Cu(111) substrates by utilizing X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The reaction pathways exhibit a strong dependence on the substrate reactivity: on Au(111), the reactions require annealing and 7-GNRs are formed at 560 K; on Ag(111), debromination occurs at 400 K and 7-GNRs are achieved at 695 K; and on Cu(111), the robust chemical interaction at the interface leads to the debromination upon deposition (at 170 K) and the final product is formed at 750 K. Overall, we demonstrate that DBBA serves as a valuable precursor to GNRs, while the metal substrates play a crucial role to effect the growth behavior of organic materials

Stoichiometric and Oxygen-Deficient VO₂ as Versatile Hole Injection Electrode for Organic Semiconductors

Using photoemission spectroscopy, we show that the surface electronic structure of VO₂ is determined by the temperature-dependent metal–insulator phase transition and the density of oxygen vacancies, which depends on the temperature and ultrahigh vacuum (UHV) conditions. The atomically clean and stoichiometric VO₂ surface is insulating at room temperature and features an ultrahigh work function of up to 6.7 eV. Heating in UHV just above the phase transition temperature induces the expected metallic phase, which goes in hand with the formation of oxygen defects (up to 6% in this study), but a high work function >6 eV is maintained. To demonstrate the suitability of VO₂ as hole injection contact for organic semiconductors, we investigated the energy-level alignment with the prototypical organic hole transport material <i>N</i>,<i>N</i>′-di­(1-naphthyl)-<i>N</i>,<i>N</i>′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB). Evidence for strong Fermi-level pinning and the associated energy-level bending in NPB is found, rendering an Ohmic contact for holes

HATCN-based Charge Recombination Layers as Effective Interconnectors for Tandem Organic Solar Cells

A comprehensive understanding of the energy-level alignment at the organic heterojunction interfaces is of paramount importance to optimize the performance of organic solar cells (OSCs). Here, the detailed electronic structures of organic interconnectors, consisting of cesium fluoride-doped 4,7-diphenyl-1,10-phenanthroline and hexaazatriphenylene–hexacarbonitrile (HATCN), have been investigated via in situ photoemission spectroscopy, and their impact on the charge recombination process in tandem OSCs has been identified. The experimental determination shows that the HATCN interlayer plays a significant role in the interface energetics with a dramatic decrease in the reverse built-in potential for electrons and holes from stacked subcells, which is beneficial to the charge recombination between HATCN and the adjacent layer. In accordance with the energy-level alignments, the open-circuit voltage of tandem OSC incorporating a HATCN-based interconnector is almost 2 times that of a single-cell OSC, revealing the effectiveness of the HATCN-based interconnectors in tandem organic devices

CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> under Different Fabrication Strategies: Electronic Structures and Energy-Level Alignment with an Organic Hole Transport Material

We report a photoelectron spectroscopy study on the electronic structure of CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> thin films fabricated by physical evaporation from CH₃NH₃I and PbCl₂ precursors, including (1) simultaneously evaporation and (2) sequential evaporation. The results are compared with CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> made using conventional solution chemistry (i.e., spin-coating). Depending on the fabrication method, CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> films show different chemical constituents in the near-surface region, leading to disparities in their energetic levels. The chemical identities of the surface species are revealed by an <i>in situ</i> study on the sequentially evaporated film. Moreover, air-exposure treatment also greatly alters the energetic levels of the film. Using hole transport layer of <i>N</i>,<i>N</i>′-di­(1-naphthyl)-<i>N</i>,<i>N</i>′-diphenyl­benzidine (NPB) as a model system, we find that the energy-level alignment with the spin-coated film after air exposure is most suitable for efficient hole transport

Surface Charge Transfer Doping <i>via</i> Transition Metal Oxides for Efficient p‑Type Doping of II–VI Nanostructures

Wide band gap II–VI nanostructures are important building blocks for new-generation electronic and optoelectronic devices. However, the difficulty of realizing p-type conductivity in these materials <i>via</i> conventional doping methods has severely handicapped the fabrication of p–n homojunctions and complementary circuits, which are the fundamental components for high-performance devices. Herein, by using first-principles density functional theory calculations, we demonstrated a simple yet efficient way to achieve controlled p-type doping on II–VI nanostructures <i>via</i> surface charge transfer doping (SCTD) using high work function transition metal oxides such as MoO₃, WO₃, CrO₃, and V₂O₅ as dopants. Our calculations revealed that these oxides were capable of drawing electrons from II–VI nanostructures, leading to accumulation of positive charges (holes injection) in the II–VI nanostructures. As a result, Fermi levels of the II–VI nanostructures were shifted toward the valence band regions after surface modifications, along with the large enhancement of work functions. <i>In situ</i> ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy characterizations verified the significant interfacial charge transfer between II–VI nanostructures and surface dopants. Both theoretical calculations and electrical transfer measurements on the II–VI nanostructure-based field-effect transistors clearly showed the p-type conductivity of the nanostructures after surface modifications. Strikingly, II–VI nanowires could undergo semiconductor-to-metal transition by further increasing the SCTD level. SCTD offers the possibility to create a variety of electronic and optoelectronic devices from the II–VI nanostructures <i>via</i> realization of complementary doping

On-Surface Synthesis of Rylene-Type Graphene Nanoribbons

The narrowest armchair graphene nanoribbon (AGNR) with five carbons across the width of the GNR (5-AGNR) was synthesized on Au(111) surfaces via sequential dehalogenation processes in a mild condition by using 1,4,5,8-tetrabromonaphthalene as the molecular precursor. Gold-organic hybrids were observed by using high-resolution scanning tunneling microscopy and considered as intermediate states upon AGNR formation. Scanning tunneling spectroscopy reveals an unexpectedly large band gap of Δ = 2.8 ± 0.1 eV on Au(111) surface which can be interpreted by the hybridization of the surface states and the molecular states of the 5-AGNR

Bilayer Formation vs Molecular Exchange in Organic Heterostructures: Strong Impact of Subtle Changes in Molecular Structure

Organic heterostructures are a central part of a manifold of (opto)­electronic devices and serve a variety of functions. Particularly, molecular monolayers on metal electrodes are of paramount importance for device performance as they allow tuning energy levels in a versatile way. However, this can be hampered by molecular exchange, i.e., by interlayer diffusion of molecules toward the metal surface. We show that the organic–metal interaction strength is the decisive factor for the arrangement in bilayers, which is the most fundamental version of organic–organic heterostructures. The subtle differences in molecular structure of 6,13-pentacenequinone (P2O) and 5,7,12,14-pentacenetetrone (P4O) lead to antithetic adsorption behavior on Ag(111): physisorption of P2O but chemisorption of P4O. This allows providing general indicators for organic–metal coupling based on shifts in photoelectron spectroscopy data and to show that the coupling strength of copper-phthalocyanine (CuPc) with Ag(111) is in between that of P2O and P4O. We find that, indeed, CuPc forms a bilayer when deposited on a monolayer P4O/Ag(111) but molecular exchange takes place with P2O, as shown by a combination of scanning tunneling microscopy and X-ray standing wave experiments

Epitaxial Growth of π‑Stacked Perfluoropentacene on Graphene-Coated Quartz

Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and “standing” films, we report a π-stacked arrangement of coplanar molecules in “flat-lying” films, which exhibit an exceedingly low π-stacking distance of only 3.07 Å, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired