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Adsorption–desorption dynamics and local chemical composition of a physisorbed co-assembly can be characterized and controlled by scanning tunneling microscopy (STM).

Two wide-band gap U-shaped polycyclic aromatic hydrocarbons with/without boron and nitrogen (BN-) doping (BN-1 and C-1) were synthesized to tune the electronic features to suit the performance requirements for organic field-effect transistor memory (OFET-NVM). The chemical structures were characterized by scanning tunneling microscopy and single-crystal diffraction. Owing to the electron-donor effect of N and the high electron affinity of B, the BN-1-based OFET-NVM displays large ambipolar memory windows and an enhanced charge storage density compared to C-1 and most reported small molecules. A novel supramolecular system formed from BN-1 and PMMA contributes to fabricating uniform films with homogeneous microstructures, which serve as a two-in-one tunnelling dielectric and charge-trapping layer to realize long-term charge retention and reliable endurance. Our results demonstrate that both BN doping and supramolecular engineering are crucial for the charge trapping of OFET-NVM. ispartof: ANGEWANDTE CHEMIE-INTERNATIONAL EDITION vol:62 issue:22 ispartof: location:Germany status: Published online

A series of zigzag-edged polycyclic aromatic hydrocarbons (PAHs) (Z1-Z3) were synthesized from 2,12-dibromo-7,14-diphenyl-benzo[m]tetraphene (9) as a versatile building block. Their structures were unambiguously confirmed by laser desorption/ionization time-of-flight mass spectrometry, 1H NMR, Raman, and Fourier-transformed infrared (FTIR) spectroscopies as well as scanning tunneling microscopy. The fingerprint vibrational modes were elucidated with theoretical support. The edge- and size-dependent optical properties were characterized by UV-Vis absorption and fluorescence spectroscopy and DFT calculations. Moreover, ultrafast transient absorption spectroscopy revealed distinct modulation of the photophysical properties upon π-extension from Z1 to Z2, the latter having a gulf edge.

We report a facile all-optical method for spatially resolved and reversible chemical modification of a graphene monolayer. A tightly focused laser on graphene under water introduces an sp

Adsorptive separation is a promising lower-energy alternative for traditional industrial separation processes. While carbon-based materials have a long history in adsorptive removal of organic contaminants from solution or gas mixtures, separation using an adsorption/desorption protocol is rarely considered. The main drawbacks are the limited control in bulk adsorption experiments, as often all organic molecules are adsorbed, and lack of desorption methods to retrieve the adsorbed molecules. Using high-resolution on-surface characterization with scanning tunneling microscopy (STM), an increased understanding of the on-surface adsorption behavior under different conditions was obtained. The insight obtained from the nanoscale experiments was used to develop a highly selective separation method using adsorption and desorption on graphite, which was tested for the separation of quinonoid zwitterions. These experiments on adsorptive separation using self-assembly on graphite show its potential and demonstrate the advantage of combining surface characterization techniques with bulk experiments to exploit different possible applications of carbon-based materials. ispartof: Chemical Science vol:13 issue:31 pages:9035-9046 ispartof: location:England status: published

Nanoscopic lateral confinement created on a graphite surface enabled the study of embryonic stages of molecular self-assembly on solid surfaces using scanning tunneling microscopy performed at the solution/solid interface.

A green multicomponent synthesis of previously unreported octahydropyrimido[4,5-g]quinazoline-5,6-diones was developed from simple building blocks. These compounds show strong propensity towards self-assembled molecular network formation on graphite.

In this contribution we demonstrate doping of graphene by uncharged dibenzyl viologen (DBV0). Deposited electrochemically on chemical vapor deposited (CVD) graphene on SiO2, DBV0 forms a water-insoluble self-assembled molecular network, in contrast to water-soluble monocationic DBV˙+. The phase formation at the molecular scale is revealed by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The doping efficiency is characterized by a combination of Raman spectroscopy and Kelvin probe force microscopy (KPFM). The Raman mapping of the G-band of the DBV0 adlayer on top of CVD graphene on SiO2 shows an apparent red shift compared to the unmodified analogue indicating an n-doping effect. This observation is in line with the KPFM results of which the measured contact potential difference (CPD) displays a positive shift compared to that of the pristine graphene.

We herein present the periodic covalent functionalization of graphite surfaces, creating a range of patterns of different symmetries and pitches at the nanoscale. Self-assembled molecular networks (SAMNs) of rhombic-shaped bis(dehydrobenzo[12]annulene) (bisDBA) derivatives having alkyl chain substituents of different lengths were used as templates for covalent grafting of electrochemically generated aryl radicals. Scanning tunneling microscopy (STM) observations at the 1,2,4-trichlorobenzene/graphite interface revealed that these molecules form a variety of networks that contain pores of different shapes and sizes. The covalently functionalized surfaces show hexagonal, oblique, and quasi-rectangular periodicities. This is attributed to the favorable aryl radical addition at the pore(s). We also confirmed the successful transmission of chirality information from the SAMNs to the alignment of the grafted aryls. In one case, the addition of a guest molecule was used to switch the SAMN symmetry and periodicity, leading to a change in the functionalized surface periodicity from oblique to hexagonal in the presence of the guest molecule. This contribution highlights the potential of SAMNs as templates for the controlled formation of nanopatterned carbon materials.

Vapor-phase film deposition of metal-organic frameworks (MOFs) would facilitate the integration of these materials into electronic devices. We studied the vapor-phase layer-by-layer deposition of zeolitic imidazolate framework 8 (ZIF-8) by consecutive, self-saturating reactions of diethyl zinc, water, and 2-methylimidazole on a substrate. Two approaches were compared: (1) Direct ZIF-8 "molecular layer deposition" (MLD), which enables a nanometer-resolution thickness control and employs only self-saturating reactions, resulting in smooth films that are crystalline as deposited, and (2) two-step ZIF-8 MLD, in which crystallization occurs during a postdeposition treatment with additional linker vapor. The latter approach resulted in a reduced deposition time and an improved MOF quality, i.e., increased crystallinity and probe molecule uptake, although the smoothness and thickness control were partially lost. Both approaches were developed in a modified atomic layer deposition reactor to ensure cleanroom compatibility.

Synergically combining their respective ultrahigh charge mobility and strong light absorption, graphene (Gr)/semiconductor heterostructures are promising building blocks for efficient optoelectronics, particularly photodetectors. Charge transfer (CT) across the heterostructure interface crucially determines device efficiency and functionality. Here, it is reported that hole-transfer processes dominate the ultrafast CT across strongly coupled double-perovskite Cs2 AgBiBr6 /graphene (DP/Gr) heterostructures following optical excitation. While holes are the primary charges flowing across interfaces, their transfer direction, as well as efficiency, show a remarkable dependence on the excitation wavelength. For excitation with photon energies below the bandgap of DPs, the photoexcited hot holes in Gr can compete with the thermalization process and inject into in-gap defect states in DPs. In contrast, above-bandgap excitation of DP reverses the hole-transfer direction, leading to hole transfer from the valence band of DPs to Gr. Experimental evidence that increasing the excitation photon energy enhances CT efficiency for both below- and above-bandgap photoexcitation regimes is further provided, unveiling the positive role of excess energy in enhancing interfacial CT. The possibility of switching the hole-transfer direction and thus the interfacial photogating field by tuning the excitation wavelength, provides a novel way to control the interfacial charge flow across a DP/Gr heterojunction. ispartof: ADVANCED MATERIALS ispartof: location:Germany status: Published online

Structurally precision graphene nanoribbons (GNRs) are promising candidates for next-generation nanoelectronics due to their intriguing and tunable electronic structures. GNRs with hybrid edge structures often confer them unique geometries associated with exotic physicochemical properties. Herein, a novel type of cove-edged GNRs with periodic short zigzag-edge segments is demonstrated. The bandgap of this GNR family can be tuned using an interplay between the length of the zigzag segments and the distance of two adjacent cove units along the opposite edges, which can be converted from semiconducting to nearly metallic. A family member with periodic cove-zigzag edges based on N = 6 zigzag-edged GNR, namely 6-CZGNR-(2,1), is successfully synthesized in solution through the Scholl reaction of a unique snakelike polymer precursor (10) that is achieved by the Yamamoto coupling of a structurally flexible S-shaped phenanthrene-based monomer (1). The efficiency of cyclodehydrogenation of polymer 10 toward 6-CZGNR-(2,1) is validated by FT-IR, Raman, and UV-vis spectroscopies, as well as by the study of two representative model compounds (2 and 3). Remarkably, the resultant 6-CZGNR-(2,1) exhibits an extended and broad absorption in the near-infrared region with a record narrow optical bandgap of 0.99 eV among the reported solution-synthesized GNRs. Moreover, 6-CZGNR-(2,1) exhibits a high macroscopic carrier mobility of ∼20 cm2 V-1 s-1 determined by terahertz spectroscopy, primarily due to the intrinsically small effective mass (m*e = m*h = 0.17 m0), rendering this GNR a promising candidate for nanoelectronics. ispartof: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY vol:144 issue:1 pages:228-235 ispartof: location:United States status: published

Two-dimensional (2D) chirality has been actively studied in view of numerous applications of chiral surfaces such as in chiral resolutions and enantioselective catalysis. Here, we report on the expression and amplification of chirality in hybrid 2D metallosupramolecular networks formed by a nucleobase derivative. Self-assembly of a guanine derivative appended with a pyridyl node was studied at the solution-graphite interface in the presence and absence of coordinating metal ions. In the absence of coordinating metal ions, a monolayer that is representative of a racemic compound was obtained. This system underwent spontaneous resolution upon addition of a coordinating ion and led to the formation of a racemic conglomerate. The spontaneous resolution could also be achieved upon addition of a suitable guest molecule. The mirror symmetry observed in the formation of the metallosupramolecular networks could be broken

Tetrathiafulvalene is among the most well-known building block in molecular electronics due to its outstanding electron-donating and redox properties. Among its derivatives, dithiophene-tetrathiafulvalene (DT-TTF) received a lot of interest for organic electronics due to its high charge mobility. Herein we report the direct C-H arylation of DT-TTF (1) to synthesise mono- (2, 3) and tetraarylated (4, 5) derivatives bearing different functional groups. The incorporation of electron-donor or electron-withdrawing groups into the DT-TTF core allows the fine-tuning of its electronic structure as demonstrated by cyclic voltammetry, UV-vis spectroscopy and DFT calculations. The self-assembly of DT-TTF-tetrabenzoic acid (5) derivative was studied using scanning tunneling microscopy which revealed the formation of ordered, densely packed 2D hydrogen-bonded networks at the graphite/liquid interface. Importantly, molecule 5 can reach a planar geometry on the graphite surface due to van der Waals interactions with the surface and H-bonding with neighbouring molecules. This work demonstrates a simple method for synthesising arylated DT-TTF derivatives with tuneable molecular orbital energy levels and the formation of 2D networks on the surface, allowing the construction of extended electroactive frameworks based on DT-TTF building blocks.

ispartof: JOURNAL OF PHYSICAL CHEMISTRY C vol:125 issue:39 pages:21624-21634 status: published

ispartof: JOURNAL OF PHYSICAL CHEMISTRY C vol:126 issue:25 pages:10494-10505 status: published

A green multicomponent synthesis of previously unreported octahydropyrimido[4,5

A novel fluorinated soft thermoplastic elastomer (sTPE) for microfluidics is presented. It allows the rapid fabrication of microfluidic devices through a 30‐second hot embossing cycle at 220°C followed by self‐sealing through simple conformal contact at room temperature, or with baking. The material shows high chemical resistance, particularly in comparison to polydimethylsiloxane (PDMS), to many common organic solvents and can be rapidly micropatterned with high fidelity using a variety of microfluidic master molds thanks to its low mechanical stiffness. Self‐sealing of the material is reversible and withstands pressures of up to 2.8 bar with room temperature sealing and four bar with baking at 185°C for 2 hours. The elastomeric, transparent sTPE exhibits material characteristics that make it suited for use as a microreactor, such as low absorption, surface roughness and oxygen permeability, while also allowing a facile and scalable fabrication process. Modular microfluidic devices, leveraging the fast and reversible room temperature self‐sealing, are demonstrated for the generation of water droplets in a toluene continuous phase using T‐junctions of variable size. The sTPE offers an alternative to common microfluidic materials, overcoming some of their key drawbacks, and giving scope for low‐cost and high‐throughput devices for flow chemistry applications.

Controlled chemical functionalization of graphite’s outer surface layers into photocatalytically oxidized graphite (POG) is reported. POG can be easily prepared via a UV-driven process using titanium dioxide photocatalyst in molecular oxygen and water vapor at relatively low temperature. The photo-oxidation offers a mild and highly controllable process, providing a chemically tailored POG surface with moderate oxidation degree. Raman spectroscopy is used to directly track the transformation of planar sp2-hybridized carbon to oxygenated sp3-hybridized chemical functions. Covalent carbon-oxygen bonding at the graphite surface upon oxidation is confirmed by XPS. This vapor-phase process in absence of hazardous liquid chemicals causes oxygen-functionalized and thermally stable POG with an oxygen content up to 14 wt%, suggesting it to be useful for electrocatalysis. Oxidation can be performed with the photocatalyst in direct contact with graphite or mounted at a distance (remote mode). This titanium dioxide-based photocatalytic oxidation is also effective on refractory highly oriented pyrolytic graphite (HOPG). HRSEM and AFM reveal the existence of differently sized surface blisters on natural graphite and HOPG following oxidation. Addition of nitric oxide to the gas mixture accelerates the photoreaction and promptly leads to graphite surface pits and edge erosion due to excessive oxidative strength.

Oxidative modification of graphene-based materials is an attractive route to functional materials. The use of strong oxidants to achieve surface modification of the carbon often leads to poor uniformity and limited reproducibility. Photocatalytic oxidation is a milder method. In this work, graphite is surface oxidized using an anatase thin photocatalytic film with a thickness of 22.5 nm facing the graphite surface to be oxidized. The high UV light transparency (95%) of the supported titania film simplified the experimental setup. The titania thin film was prepared using atomic layer deposition (ALD). Excellent oxidation uniformity and reaction reproducibility are achieved. By avoiding the use of chemical reagents there is no chemical contamination. Graphite photo-oxidation with the titania film prepared with ALD is much faster than with a powdery photocatalyst. The photocatalytic process runs efficiently with air containing water vapor. High resolution scanning electron microscopy (HRSEM), Raman and X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) confirmed the formation of hydroxyl, epoxy and carbonyl functional groups. ALD is a suitable method for thin film preparation rendering this photocatalytic process suitable for scale-up and automation. Photocatalytically oxidized graphite (POG) electrodes are a potential application.

Carbocatalysis holds a privileged position as a sustainable alternative to metal-based catalysis. While the focus in solution-based catalytic processes generally lies on how the heterogeneous catalyst affects the solution composition, more attention has recently been given to the analysis of the carbon material itself. Various outstanding surface characterisation techniques, efficient in assessing the catalyst on-surface composition, are now available. These include high-resolution imaging tools such as scanning tunneling microscopy (STM), capable of bringing new insights into the processes determining rate and selectivity effects induced by carbocatalysts. In this regard, the use of self-assembly on graphite as a strategy to direct the outcome of chemical reactions has already shown great potential. This promising approach gives the scientific community the exciting prospect of rationalising selectivity in carbocatalysis with pristine graphite by linking in-solution and on-surface composition.

Chirality in two dimensions (2D) has attracted increasing attention with regard to interesting fundamental aspects as well as potential applications. This article reports several aspects of supramolecular chirality control as exemplified by self-assembled monolayer networks (SAMNs) formed by a class of chiral building blocks consisting of a triangular conjugated core and alkoxy chains on the periphery. It highlights 2D chirality induction phenomena through a classic "sergeants-and-soldiers" mechanism, in which the inducer is incorporated into a network component, as well as through a "supramolecular host-guest" mechanism, in which the inducer is entrapped in the porous space, leading to counterintuitive chirality reversal. Stereochemical control can be extended to three dimensions too, based on interlayer hydrogen bonding of the same class of building blocks bearing hydroxy groups, exhibiting diastereospecific bilayer formation at both single molecule level and supramolecular level arising from orientation between the top and bottom layers. Finally, we showcase that homochiral SAMNs can also be used as templates for the grafting of in situ generated aryl radicals, by covalent bond formation to the basal graphitic surface, thereby yielding topologically chiral functionalized graphite, and thus extending the potential of chiral SAMNs.

ispartof: JOURNAL OF PHYSICAL CHEMISTRY C vol:126 issue:14 pages:6413-6419 status: published

The quality of crystalline two-dimensional (2D) polymers1-6 is intimately related to the elusive polymerization and crystallization processes. Understanding the mechanism of such processes at the (sub)molecular level is crucial to improve predictive synthesis and to tailor material properties for applications in catalysis7-10 and (opto)electronics11,12, among others13-18. We characterize a model boroxine 2D dynamic covalent polymer, by using in situ scanning tunnelling microscopy, to unveil both qualitative and quantitative details of the nucleation-elongation processes in real time and under ambient conditions. Sequential data analysis enables observation of the amorphous-to-crystalline transition, the time-dependent evolution of nuclei, the existence of 'non-classical' crystallization pathways and, importantly, the experimental determination of essential crystallization parameters with excellent accuracy, including critical nucleus size, nucleation rate and growth rate. The experimental data have been further rationalized by atomistic computer models, which, taken together, provide a detailed picture of the dynamic on-surface polymerization process. Furthermore, we show how 2D crystal growth can be affected by abnormal grain growth. This finding provides support for the use of abnormal grain growth (a typical phenomenon in metallic and ceramic systems) to convert a polycrystalline structure into a single crystal in organic and 2D material systems. ispartof: NATURE vol:603 issue:7903 pages:835-+ ispartof: location:England status: published

In this report, the influence of heterogeneity on the chiral expression of star-shaped conjugated polymers is investigated. For this purpose, two heterogeneous star-shaped conjugated polymers, i.e....

Covalently modified carbon surfaces remain central to a number of applications. Surface modification is often achieved using aryldiazonium chemistry, which has become the gold standard for the functionalization of carbon-based surfaces. However, the higher reactivity and thus the lower stability of diazonium precursors at carbon interfaces have prompted a search for alternative chemistries. Diaryliodonium salts have emerged as stable alternatives to the conventional aryldiazonium precursors. In this contribution, we provide a detailed comparison between the two types of chemistries by carrying out electrochemical covalent grafting of graphite substrates using nitrophenyl groups generated from the corresponding aryldiazonium and diaryliodonium salts under identical experimental conditions. The electrochemical process was studied using cyclic voltammetry, whereas the covalently grafted substrates were characterized using a battery of surface analytical techniques spanning multiple length scales. Analysis of the modified substrates using Raman spectroscopy revealed that the efficiency of covalent grafting is higher for iodonium chemistry than that for aryldiazonium chemistry. This observation was further corroborated by detailed morphological characterization of the covalent films using atomic force microscopy and molecular-resolution scanning tunneling microscopy, which revealed higher surface coverage for iodonium modified substrates. The chemical composition of the films was probed using X-ray photoelectron spectroscopy. The detailed and systematic comparison presented here clearly demonstrates that diaryliodonium chemistry presents a robust and reliable alternative to the widely used aryldiazonium chemistry.

Reported here is a molecular dipole that self‐assembles into highly ordered patterns at the liquid‐solid interface, and it can be switched at room temperature between a bright and a dark state at the single‐molecule level. Using a scanning tunneling microscope (STM) under suitable bias conditions, binary information can be written at a density of up to 41 Tb cm−2 (256 Tb/in2). The written information is stable during reading at room temperature, but it can also be erased at will, instantly, by proper choice of tunneling conditions. DFT calculations indicate that the contrast and switching mechanism originate from the stacking sequence of the molecular dipole, which is reoriented by the electric field between the tip and substrate.

Silver nanowires have attracted considerable attention as subdiffraction limited diameter waveguides in a variety of applications including cell endoscopy and photonic integrated circuitry. Optical signal transport occurs by coupling light into propagating surface plasmons, which scatter back into light further along the wire. However, these interconversions only occur efficiently at wire ends, or at defects along the wire, which are not controlled during synthesis. Here, we overcome this limitation, demonstrating the visible laser light-induced fabrication of gold nanostructures at desired positions on silver nanowires, and their utility as efficient in/out coupling points for light. The gold nanostructures grow via plasmon-induced reduction of Au(III) and are shown to be excellent "hotspots" for surface-enhanced Raman scattering. ispartof: NANO LETTERS vol:20 issue:4 pages:2460-2467 ispartof: location:United States status: published

Synthetic two-dimensional polymers (2DPs) obtained from well-defined monomers via bottom-up fabrication strategies are promising materials that can extend the realm of inorganic 2D materials. The on-surface synthesis of such 2DPs is particularly popular, however the pathway complexity in the growth of such films formed on solid surfaces is poorly understood. In this contribution, we present a straightforward experimental protocol which allows the synthesis of large-area, defect-free 2DPs based on boroxine linkages at room temperature. We focus on unravelling the multiple pathways available to the polymerizing system for the spatial extension of the covalent bonds. Besides the anticipated 2DP, the system can evolve into self-assembled monolayers of partially fused monodisperse reaction products that are difficult to isolate by conventional synthetic methods or remain in the monomeric state. The access to each pathway can be controlled via monomer concentration and the choice of the solvent. Most importantly, the unpolymerized systems do not evolve into the corresponding 2DP upon annealing, indicating the presence of strong kinetic traps. Using high-resolution scanning tunneling microscopy, we show reversibility in the polymerization process where the attachment and the detachment of monomers to 2DP crystallites could be monitored as a function of time. Finally, we show that the way the 2DP grows depends on the choice of the solvent. Using UV-vis absorption and emission spectroscopy, we reveal that the dominant pathway for 2DP growth is via in-plane self-condensation of the monomers, whereas in the case of an aprotic solvent, the favored growth mode is via π stacking of the monomers. ispartof: ACS NANO vol:14 issue:2 pages:2354-2365 ispartof: location:United States status: published