Visualizing Ribbon-to-Ribbon Heterogeneity of Chemically Unzipped Wide Graphene Nanoribbons by Silver Nanowire-Based Tip-Enhanced Raman Scattering Microscopy.

Graphene nanoribbons (GNRs), a quasi-one-dimensional form of graphene, have gained tremendous attention due to their potential for next-generation nanoelectronic devices. The chemical unzipping of carbon nanotubes is one of the attractive fabrication methods to obtain single-layered GNRs (sGNRs) with simple and large-scale production.  The authors recently found that unzipping from double-walled carbon nanotubes (DWNTs), rather than single- or multi-walled, results in high-yield production of crystalline sGNRs. However, details of the resultant GNR structure, as well as the reaction mechanism, are not fully understood due to the necessity of nanoscale spectroscopy. In this regard, silver nanowire-based tip-enhanced Raman spectroscopy (TERS) is applied for single GNR analysis and investigated ribbon-to-ribbon heterogeneity in terms of defect density and edge structure generated through the unzipping process.  The authors found that sGNRs originated from the inner walls of DWNTs showed lower defect densities than those from the outer walls. Furthermore, TERS spectra of sGNRs exhibit a large variety in graphitic Raman parameters, indicating a large variation in edge structures. This work at the single GNR level reveals, for the first time, ribbon-to-ribbon heterogeneity that can never be observed by diffraction-limited techniques and provides deeper insights into unzipped GNR structure as well as the DWNT unzipping reaction mechanism.

From One-Dimensional Disordered Racemate to Ordered Racemic Conglomerates through Metal-Coordination-Driven Self-Assembly at the Liquid-Solid Interface.

In recent years, there has been significant focus on investigating and controlling chiral self-assembly, specifically in the context of enantiomeric separation. This study explores the self-assembly behavior of 4-dodecyl-3,6-di(2-pyridyl)pyridazine (DPP-C12) at the interface between heptanoic acid (HA) and highly oriented pyrolytic graphite (HOPG) using a combination of scanning tunneling microscopy (STM) and multiscale molecular modeling. The self-assembled monolayer structure formed by DPP-C12 is periodic in one direction, but aperiodic in the direction orthogonal to it. These structures resemble 1D disordered racemic compounds. Upon introducing palladium [Pd(II)] ions, complexing with DPP-C12, these 1D disordered racemic compounds spontaneously transform into 2D racemic conglomerates, which is rationalized with the assistance of force-field simulations. Our findings provide insights into the regulation of two-dimensional chirality.

Adsorption of Prochiral Solvent Molecules by Surface-Confined Chiral Supramolecular Assemblies: How Solvent Impacts on-Surface Chirality.

The understanding of supramolecular chirality in self-assembled molecular networks (SAMNs) on surfaces generates a lot of interest because of its relation to the production of chiral sensors, reactors, and catalysts. We herein report the adsorption of a prochiral solvent molecule in porous SAMNs formed by a chiral dehydrobenzo[12]annulene (cDBA) derivative. Through the prochirality recognition of a solvent molecule, the supramolecular chirality of the SAMN is switched: the cDBA exclusively forms a counter-clockwise pore through co-adsorption of the solvent molecule in prochiral 1,2,4-trichlorobenzene, while in 1-phenyloctane it produces the opposite chiral, clockwise pore. The prochirality recognition of the solvent molecule in the chiral SAMN pores is attributed to the adaptable conformational changes of the chiral chains of the cDBA molecule.

Highly Ordered Co-Assembly of Bisurea Functionalized Molecular Switches at the Solid-Liquid Interface.

Immobilization of stimulus-responsive systems on solid surfaces is beneficial for controlled signal transmission and adaptive behavior while allowing the characterization of the functional interface with high sensitivity and high spatial resolution. Positioning of the stimuli-responsive units with nanometer-scale precision across the adaptive surface remains one of the bottlenecks in the extraction of cooperative function. Nanoscale organization, cooperativity, and amplification remain key challenges in bridging the molecular and the macroscopic worlds. Here we report on the design, synthesis, and scanning tunneling microscopy (STM) characterization of overcrowded alkene photoswitches merged in self-assembled networks physisorbed at the solid-liquid interface. A detailed anchoring strategy that ensures appropriate orientation of the switches with respect to the solid surface through the use of bis-urea groups is presented. We implement a co-assembly strategy that enables the merging of the photoswitches within physisorbed monolayers of structurally similar 'spacer' molecules. The self-assembly of the individual components and the co-assemblies was examined in detail using (sub)molecular resolution STM which confirms the robust immobilization and controlled orientation of the photoswitches within the spacer monolayers. The experimental STM data is supported by detailed molecular mechanics (MM) simulations. Different designs of the switches and the spacers were investigated which allowed us to formulate guidelines that enable the precise organization of the photoswitches in crystalline physisorbed self-assembled molecular networks.

Self-Assembly of Discrete Oligomers of Naphthalenediimides in Bulk and on Surfaces.

Here, we report on the synthesis of discrete oligomers of alkyl-bridged naphthalenediimides (NDIs) and study their molecular nanostructures both in bulk, in solution, and at the liquid-solid interface. Via an iterative synthesis method, multiple NDI cores were bridged with short and saturated alkyl-diamines (C3 and C12 ) or long and unsaturated alkyl-diamines (u2 C33 to u8 C100 ) at their imide termini. The strong intermolecular interaction between the NDI cores was observed by probing their photophysical properties in solution. In bulk, the discrete NDI oligomers preferentially ordered in lamellar morphologies, irrespective of whether a saturated or unsaturated spacer was employed. Moreover, both the molecular architecture as well as the crystallization conditions play a significant role in the nanoscale ordering. The long unsaturated alkyl chains lead preferably to folded-chain conformations while their saturated analogues form stretched arrangements. At the solution-solid interface, well-defined lamellar regions were observed. These results show that precision in chemical structure alone is not sufficient to reach well-defined structures of discrete oligomers, but that it must be combined with precision in processing conditions.

Chiral Solvent-Induced Homochiral Hierarchical Molecular Assemblies at the Liquid/Solid Interface.

We herein investigate the formation of homochiral hierarchical self-assembled molecular networks (SAMNs) via chirality induction by the coadsorption of a chiral solvent at the liquid/graphite interface by means of scanning tunneling microscopy (STM). In a mixture of achiral solvents, 1-hexanoic acid, and 1,2,4-trichlorobenzene, an achiral dehydrobenzo[12]annulene (DBA) derivative with three alkoxy and three hydroxy groups in an alternating manner forms chiral hierarchical triangular cluster structures through dynamic self-sorting. Enantiomorphous domains appear in equal probability. On the other hand, in chiral 2-methyl-1-hexanoic acid as a solvent, this molecule produces (i) homochiral small triangular clusters at a low solute concentration, (ii) a chirality-biased hierarchical structure consisting of triangular cluster structures with different cluster sizes at a medium concentration, and (iii) a dense structure with no chirality bias at a high concentration. We attribute the concentration-dependent degree of the chirality transmission to the number of coadsorbed solvent molecules in the SAMNs and to the difference in nucleus structure and size in the initial stage of the SAMN formation.

Metal Ion and Guest-Mediated Spontaneous Resolution and Solvent-Induced Chiral Symmetry Breaking in Guanine-Based Metallosupramolecular Networks.

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 via the use of an enantiopure solvent, which led to the formation of a globally homochiral surface.

Solvent Mediated Nanoscale Quasi-Periodic Chirality Reversal in Self-Assembled Molecular Networks Featuring Mirror Twin Boundaries.

Grain boundaries in polycrystals have a prominent impact on the properties of a material, therefore stimulating the research on grain boundary engineering. Structure determination of grain boundaries of molecule-based polycrystals with submolecular resolution remains elusive. Reducing the complexity to monolayers has the potential to simplify grain boundary engineering and may offer real-space imaging with submolecular resolution using scanning tunneling microscopy (STM). Herein, the authors report the observation of quasi-periodic nanoscale chirality switching in self-assembled molecular networks, in combination with twinning, as revealed by STM at the liquid/solid interface. The width of the chiral domain structure peaks at 12-19 nm. Adjacent domains having opposite chirality are connected continuously through interdigitated alkoxy chains forming a 1D defect-free domain border, reflecting a mirror twin boundary. Solvent co-adsorption and the inherent conformational adaptability of the alkoxy chains turn out to be crucial factors in shaping grain boundaries. Moreover, the epitaxial interaction with the substrate plays a role in the nanoscale chirality reversal as well.

Machine-Learning-Aided NO2 Discrimination with an Array of Graphene Chemiresistors Covalently Functionalized by Diazonium Chemistry.

Boosted by the emerging need for highly integrated gas sensors in the internet of things (IoT) ecosystems, electronic noses (e-noses) are gaining interest for the detection of specific molecules over a background of interfering gases. The sensing of nitrogen dioxide is particularly relevant for applications in environmental monitoring and precision medicine. Here we present an easy and efficient functionalization procedure to covalently modify graphene layers, taking advantage of diazonium chemistry. Separate graphene layers were functionalized with one of three different aryl rings: 4-nitrophenyl, 4-carboxyphenyl and 4-bromophenyl. The distinct modified graphene layers were assembled with a pristine layer into an e-nose for NO2 discrimination. A remarkable sensitivity to NO2 was demonstrated through exposure to gaseous solutions with NO2 concentrations in the 1-10 ppm range at room temperature. Then, the discrimination capability of the sensor array was tested by carrying out exposure to several interfering gases and analyzing the data through multivariate statistical analysis. This analysis showed that the e-nose can discriminate NO2 among all the interfering gases in a two-dimensional principal component analysis space. Finally, the e-nose was trained to accurately recognize NO2 contributions with a linear discriminant analysis approach, thus providing a metric for discrimination assessment with a prediction accuracy above 95 %.

Zigzag-Edged Polycyclic Aromatic Hydrocarbons from Benzo[m]tetraphene Precursors.

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, 1 H 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.

Direct C-H Arylation of Dithiophene-Tetrathiafulvalene: Tuneable Electronic Properties and 2D Self-Assembled Molecular Networks at the Solid/Liquid Interface.

Tetrathiafulvalene is among the best known building blocks in molecular electronics due to its outstanding electron-donating and redox properties. Among its derivatives, dithiophene-tetrathiafulvalene (DT-TTF) has attracted considerable interest in organic electronics, owing to its high field-effect mobility. Herein, we report the direct C-H arylation of DT-TTF to synthesise mono- and tetraarylated derivatives functionalised with electron-withdrawing and electron-donating groups in order to evaluate their influence on the electronic properties by cyclic voltammetry, UV-vis spectroscopy and theoretical calculations. Self-assembly of the DT-TTF-tetrabenzoic acid derivative was studied by using scanning tunnelling microscopy (STM) which revealed the formation of ordered, densely packed 2D hydrogen-bonded networks at the graphite/liquid interface. The tetrabenzoic acid derivative can attain a planar geometry on the graphite surface due to van der Waals interactions with the surface and H-bonding with neighbouring molecules. This study demonstrates a simple method for the synthesis of arylated DT-TTF derivatives towards the design and construction of novel π-extended electroactive frameworks.

Direct C-H Arylation of Dithiophene-Tetrathiafulvalene: Tuneable Electronic Properties and 2D Self-Assembled Molecular Networks at the Solid/Liquid Interface.

Invited for the cover of this issue is the group of Manuel Souto and co-workers at the University of Aveiro and CICECO-Aveiro Institute of Materials. The image depicts the direct C-H arylation of dithiophene-tetrathiafulvalene (DT-TTF) and the self-assembly of DT-TTF-tetrabenzoic acid studied by using scanning tunnelling microscopy. Read the full text of the article at 10.1002/chem.202300572.

Formation of Supramolecular Heterostacks at the Liquid-Solid Interface: Impact of Symmetry Mismatching on Structural Growth.

The construction of intricate three-dimensional (3D) nanoarchitectures on surfaces through molecular self-assembly attracts attention not only from a crystal engineering viewpoint but also because of its potential in a range of applications, given the current interest in van der Waals heterostructures. We herein report the formation of porous structures on alkane buffer layers on graphite. A dehydrobenzo[12]annulene (DBA) derivative having six decyloxy chains forms hexagonal structures on n-pentacontane and n-hexacontane buffer layers through van der Waals interactions at the 1-octanoic acid/graphite interface. The structural features are very similar to those on the graphite surface, except for the slight structural distortion, which is attributed to the p2 symmetry of the buffer layer underneath. Moreover, based on the observation of small clusters of the DBA molecules, we discussed the nucleation and structural growth of the DBA network on a buffer layer. Finally, a hierarchical multicomponent structure was formed through the coadsorption of a heteromolecular cluster formed by a hydrogen-bonded isophthalic acid cyclic hexamer hosting a coronene molecule on the buffer layer. This study on supramolecular heterostacks provides insights into the construction of intricate 3D nanoarchitectures using self-assembly at interfaces.

Spatially Controlled Aryl Radical Grafting of Graphite Surfaces Guided by Self-Assembled Molecular Networks of Linear Alkane Derivatives: The Importance of Conformational Dynamics.

The covalent functionalization of carbon surfaces with nanometer-scale precision is of interest because of its potential in a range of applications. We herein report the controlled grafting of graphite surfaces using electrochemically generated aryl radicals templated by self-assembled molecular networks (SAMNs) of bisalkylurea derivatives. A bisalkylurea derivative having two butoxy units acts as a template for the covalent functionalization of aryl groups in between self-assembled rows of this molecule. In contrast, grafting occurs without a spatial order when an SAMN of bis(tetradecyl)urea was used as a template. This indicates that a degree of dynamics at the alkyl termini is required to favor controlled covalent attachment, a situation that is suppressed by strong intrarow intermolecular interactions resulting from the hydrogen bonding of the urea groups, but favored by terminal short alkoxy groups. The present information is useful for understanding the mechanism of the template-guided aryl radical grafting and the molecular design of new generations of template molecules.

A BN-Doped U-Shaped Heteroacene as a Molecular Floating Gate for Ambipolar Charge Trapping Memory.

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.

Molecular-Level Insights on Reactive Arrangement in On-Surface Photocatalytic Coupling Reactions Using Tip-Enhanced Raman Spectroscopy.

Plasmon-enhanced photocatalytic coupling reactions have been used as model systems in surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) research for decades. However, the role of reactive arrangement on efficiency of these model reactions has remained largely unknown to date often leading to conflicting interpretations of experimental results. Herein, we use an interdisciplinary toolbox of nanoscale TERS imaging in combination with molecular-resolution ambient scanning tunnelling microscopy (STM) and density functional theory (DFT) modeling to investigate the role of reactive arrangement in photocatalytic coupling of 4-nitrobenzenethiol (4-NTP) to p,p'-dimercaptoazobisbenzene on single-crystal and polycrystalline Au surfaces for the first time. TERS imaging with 3 nm resolution clearly revealed a significantly higher catalytic efficiency inside a kinetically driven disordered phase of the 4-NTP adlayer on Au compared to the thermodynamically stable ordered phase. Furthermore, molecular level details of the self-assembled structures in the disordered and ordered phases obtained using ambient high-resolution STM enabled an unambiguous structure-reactivity correlation of photocatalytic coupling. Finally, quantitative mechanistic insights obtained from DFT modeling based on the accurate parameters determined from STM imaging emphatically confirmed that a combination of steric hindrance effect and energetic barrier leads to a lower reaction efficiency in the ordered phase of the 4-NTP adlayer. This fundamental study establishes the first direct structure-reactivity correlation in photocatalytic coupling and highlights the critical role of reactive arrangement in the efficiency of on-surface coupling reactions in heterogeneous catalysis at large.

Cove-Edged Graphene Nanoribbons with Incorporation of Periodic Zigzag-Edge Segments.

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.

Direct X-ray and electron-beam lithography of halogenated zeolitic imidazolate frameworks.

Metal-organic frameworks (MOFs) offer disruptive potential in micro- and optoelectronics because of the unique properties of these microporous materials. Nanoscale patterning is a fundamental step in the implementation of MOFs in miniaturized solid-state devices. Conventional MOF patterning methods suffer from low resolution and poorly defined pattern edges. Here, we demonstrate the resist-free, direct X-ray and electron-beam lithography of MOFs. This process avoids etching damage and contamination and leaves the porosity and crystallinity of the patterned MOFs intact. The resulting high-quality patterns have excellent sub-50-nm resolution, and approach the mesopore regime. The compatibility of X-ray and electron-beam lithography with existing micro- and nanofabrication processes will facilitate the integration of MOFs in miniaturized devices.

Switching stiction and adhesion of a liquid on a solid.

When a gecko moves on a ceiling it makes use of adhesion and stiction. Stiction--static friction--is experienced on microscopic and macroscopic scales and is related to adhesion and sliding friction. Although important for most locomotive processes, the concepts of adhesion, stiction and sliding friction are often only empirically correlated. A more detailed understanding of these concepts will, for example, help to improve the design of increasingly smaller devices such as micro- and nanoelectromechanical switches. Here we show how stiction and adhesion are related for a liquid drop on a hexagonal boron nitride monolayer on rhodium, by measuring dynamic contact angles in two distinct states of the solid-liquid interface: a corrugated state in the absence of hydrogen intercalation and an intercalation-induced flat state. Stiction and adhesion can be reversibly switched by applying different electrochemical potentials to the sample, causing atomic hydrogen to be intercalated or not. We ascribe the change in adhesion to a change in lateral electric field of in-plane two-nanometre dipole rings, because it cannot be explained by the change in surface roughness known from the Wenzel model. Although the change in adhesion can be calculated for the system we study, it is not yet possible to determine the stiction at such a solid-liquid interface using ab initio methods. The inorganic hybrid of hexagonal boron nitride and rhodium is very stable and represents a new class of switchable surfaces with the potential for application in the study of adhesion, friction and lubrication.

2D Self-assembled molecular networks and on-surface reactivity under nanoscale lateral confinement.

Supramolecular self-assembly at surfaces provides a pathway for building chemically customized interfaces. Over the last three decades, research on the role of key parameters such as temperature, solute concentration, and molecular design has enabled a steady increase in the complexity of self-assembled molecular networks (SAMNs) that can thus be created. However, the structure and quality of SAMNs is often determined during the early stages of nucleation and growth. To study and influence self-assembly processes at this deterministic length scale, spatial confinement of molecular adsorbates to well-defined surface patterns with nanoscale lateral dimensions offers exciting possibilities. The aim of this tutorial review is to give an overview of the various ways in which confinement impacts SAMN formation, and how we can use that knowledge to direct assemblies towards desired structures. The possibility to exploit confinement for improved control over on-surface reactions is also contemplated.