Two-dimensional (2D) materials, particularly transition metal dichalcogenides (TMDs), have gathered significant attention due to their interesting electrical and optical properties. Among TMDs, monolayers of WSe2exhibit a direct band gap and high exciton binding energy, which enhances photon emission and absorption even at room temperature. This study investigates the electronic and optical properties of WSe2monolayers when they are mechanically transferred to indium tin oxide (ITO) substrates. ITO is a transparent conducting electrode (TCE) used in many industrial optoelectronic applications. Samples were mechanically transferred under ambient conditions, consequently trapping an adsorbate layer of atmospheric molecules unintentionally between the monolayer and the substrate. To reduce the amount of adsorbates, some samples were thermally annealed. Atomic force microscopy confirmed the presence of the adsorbate layer under the TMD and its partial removal after annealing. X-ray photoelectron spectroscopy confirmed the presence of carbon species among the adsorbates even after annealing. Photoluminescence measurements show that WSe2remains optically active on ITO even after annealing. Moreover, the luminescence intensity and energy are affected by the amount of adsorbates under the WSe2monolayer. Scanning tunnelling spectroscopy reveals that the TMD monolayer is n-doped, and that its band edges form a type I band alignment with ITO. Surface potential measurements show a polarity change after annealing, indicating that polar molecules, most likely water, are being removed. This comprehensive study shows that a TCE does not quench WSe2luminescence even after a prolonged thermal annealing, although its optical and electronic properties are affected by unintentional adsorbates. These findings provide insights for better understanding, controlling, and design of 2D material heterostructures on TCEs.

On-surface synthesis is a powerful tool for engineering two-dimensional (2D) organic nanostructures by controlling intermolecular interactions between the building blocks. Herein, we explore the role of Cl adatoms in the synthesis and characterization of self-assembled 1,3,5-tris[4-(pyridin)-[1,1'-biphenyl]benzene (TPyPPB) networks on Ag(111), by combining scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). In the absence of Cl, upon deposition at room temperature (RT), TPyPPB molecules form a highly ordered porous supramolecular network with triangular packing, stabilized by hydrogen bonds (N--H). In the presence of Cl adatoms, sublimated onto the surface using dichloro-(1,10phenanthrolin)-platin(II) (Cl2PhPt) a second molecular precursor, we observe a so-called mixed phase or inverted packing, depending on the applied growth procedure. The mixed phase is characterized by a nonperiodic structure stabilized by intermolecular interactions between TPyPPB, Cl2PhPt, and Cl. In contrast, when only Cl adatoms and TPyPPB are present on the Ag(111) surface, a non-porous supramolecular arrangement is obtained, stabilized by C-H--Cl hydrogen bonds.

We have investigated the adsorption and self-metalation of free-base tetraphenyltransdibenzoporphyrin (2H-TPtdBP) on Cu(111) as a function of coverage and temperature using scanning tunneling microscopy, x-ray photoelectron spectroscopy, temperature programmed desorption, and density-functional theory calculations. At low coverages (<0.16 molecules nm-2), we observe isolated individual molecules with an inverted conformation and no self-metalation up to 363 K. At higher coverages, both the formation of ordered islands and self-metalation are observed over time already at room temperature, and accelerate upon heating to 363 K. At 423 K, complete self-metalation occurs for all coverages up to the completed first layer. By comparing our results for 2H-TPtdBP to the existing literature on other tetraphenyl-based porphyrins, we demonstrate how adsorption and self-metalation can be tailored by the choice of substituents.

Large domains of two-dimensional supramolecular porous nanostructures are interesting for various applications from electronics to biology. Here, we investigate the formation of Cu-coordinated networks on Cu(111) using scanning tunneling microscopy and density functional theory (DFT). We consider two molecules with three pyridyl end groups connected to a central benzene ring by either one or two phenyl groups, namely 1,3,5-tris[4-(pyridin)phenyl]benzene (TPyPB) and 1,3,5-tris[4-(pyridin)-[1,1’-biphenyl]benzene (TPyPPB), respectively. Upon deposition of TPyPB at room temperature, a honeycomb nanostructure forms, which is stabilized by Cu adatoms, as previously seen. Upon deposition at 400 K, the growth dynamics change, and molecules become trapped in the hexagonal pores. In contrast, deposition of TPyPPB at room temperature leads to vitreous structures, which rearrange at 400 K forming a low-defect and extended ordered honeycomb phase, which is also stabilized only in the presence of Cu adatoms. The DFT calculations for both honeycomb phases show an impressive agreement with the experimental results, considering the size of such structures. After annealing at 420 K, a complex flower-like structure composed of a mix of two- and three-fold coordinated Cu centers emerges. Further annealing to above 420 K leads to another new phase composed of a high molecular density motif, the so-called diamond phase.

On-surface coupling reactions and molecular conformation are essential processes for building tailored functional molecular nanostructures. Here, we study the thermal debromination and reactivity of free-base tetra(4-bromophenyl) porphyrin (H2TBrPP) on Cu(111) as a function of the substrate temperature. It has been previously reported in the literature that C-Br bonds remain intact at room temperature (RT) and that the Br center dot center dot center dot Cu(111) interaction induces a drastic surface reconstruction around the molecule periphery and a distortion in the adsorbate itself. However, based on a combination of STM and XPS experiments, supported by density functional theory (DFT) calculations, we instead demonstrate that debromination readily occurs at RT, leading to a new interpretation of both the adsorption behavior and the molecular coupling of H2TBrPP on Cu(111). For the molecules deposited on the metallic substrate held above RT, our STM measurements show the growth of ordered 2D metal-organic frameworks (MOFs).

Monolayer hexagonal boron-nitrogen-carbon (h-BNC) is considered a prominent candidate for the next generation of semiconductor electronic devices. Nevertheless, experimental evidence of h-BNC formation is limited, including a detailed study of its morphological and electronic properties. Here, successful growth of h-BNC from an unexplored single molecular precursor (hexamethyl borazine, C6H18B3N3) using a conventional CVD approach on Ir(111) is reported. The conformation structure of the monolayer and its correlation with the local electronic properties are discussed based on scanning tunneling microscopy/spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) results. The results show an h-BNC structure that can be described as BN-doped graphene since the moire ' lattice parameter is preserved along with the alloy. This BN-doped cluster, renamed as h-BN "nanodonuts" according to the electronic density exhibited in STM images, have a tendency to place specific positions within the moire superstructure, and it is constituted by at least (BN)(8) units arranged in a 6-fold BN rings conformation, as evidenced by simulation of STM images based on density functional theory (DFT). For a BN concentration of about 17%, a band gap between 1.4 and 1.6 eV was determined. The versatility of the novel molecular precursor is proven by the growth of a high-quality h-BN monolayer on Rh(111).

Metal adatoms play a key role in surface diffusion, adsorption conformation, and self-assembly of porphyrin molecules on metal surfaces. Herein, we study the specific influence of coadsorption of Fe, Co, and Pd atoms on the behavior of 2H-tetiakis(p-cyano)phenylporphyrin (2H-TCNPP) on Cu(111) using scanning tunneling microscopy. Upon codeposition of Fe and Co, the molecules form one-dimensional (1D) linear chains after mild annealing on Cu(111) driven by the interaction of its cyano groups with metal adatoms. A similar behavior has been observed previously on Cu(111), mediated by Cu adatoms, where the functional CN groups were also found to lower the reaction rate of the so-called porphyrin self-metalation reaction with Cu atoms significantly, in comparison to the non-cyano-functionalized porphyrin. Upon co-deposition of Pd and mild annealing, we find a remarkably different behavior, that is, a massive reorganization from 1D molecular chains to a peculiar rectangular 2D (two-dimensional) network. The molecular appearance changes to a clover shape, which is attributed to a Pd-induced dehydrogenation and subsequent ring closure reaction of the phenyl and pyrrole groups.

Surface diffusion, molecular conformation, and on-surface coupling reactions are key processes for building tailored molecular nanostructures such as graphene nanoribbons, polycyclic aromatic hydrocarbons, and one-dimensional/two-dimensional (2D) polymers. Here, we study the surface diffusion and coupling in situ of a chlorinated porphyrin, namely 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (Cl4TPP), using a combined scanning tunneling microscopy (STM), density functional theory (DFT), and X-ray photoelectron spectroscopy approach. Using STM, we obtain surface migration and rotation barriers Delta E of 0.77 +/- 0.09 and 0.93 +/- 0.28 eV, respectively, indicative of covalent binding to the surface. In fact, we find that the precursors as well as all the reaction species exclusively (approximate to 100%) adopt a peculiar "inverted" conformation covalently bonded to Cu(111). Using DFT, we have mapped two coupling reaction pathways: direct dechlorination and Cu adatom-mediated Ullmann coupling. We find that the latter is essentially barrierless, whereas the former faces a barrier of about 0.9 eV for inverted Cl4TPP on Cu(111). Our STM measurements show that C-Cu-C organometallic species are the main final products in the presence of Cu adatoms, which is explained by our DFT reaction profile when heat dissipation to the substrate is taken into account. This work not only highlights the relevance of surface adatoms in selecting the reaction pathway but also opens the possibility of precisely tailoring 2D molecular assemblies by controlling the supply of Cu adatoms.

Resumo Neste projeto de IC estudamos a coordenação molecular – formação de nanofios moleculares de 2H-TCNPP na superfície de Cu(111) mediados por átomos de Fe ou Pd, além de analisar a sua variação com a temperatura. O estudo envolveu a preparação deste particular substrato, evaporação das moléculas, aquecimento da amostra e sua investigação pelas técnicas de microscopia de tunelamento eletrônico (STM) e espectroscopia de elétrons excitados por raios X (XPS). No trabalho procuramos desvendar o papel do Fe ou Pd na formação dos nanoestruturas moleculares.