A chemisorbed interfacial layer for seeding atomic layer deposition on graphite.

The integration of graphene, and more broadly two-dimensional materials, into devices and hybrid materials often requires the deposition of thin films on their usually inert surface. As a result, strategies for the introduction of surface reactive sites have been developed but currently pose a dilemma between robustness and preservation of the graphene properties. A method is reported here for covalently modifying graphitic surfaces, introducing functional groups that act as reactive sites for the growth of high quality dielectric layers. Aryl diazonium species containing tri-methoxy groups are covalently bonded (grafted) to highly oriented pyrolytic graphite (HOPG) and graphene, acting as seeding species for atomic layer deposition (ALD) of Al2O3, a high-κ dielectric material. A smooth and uniform dielectric film growth is confirmed by scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrical measurements. Raman spectroscopy showed that the aryl groups gradually detach from the graphitic surface during the Al2O3 ALD process at 150 °C, with the surface reverting back to the original sp2-hybridized state and without damaging the dielectric layer. Thus, the grafted aryl groups can act as a sacrificial seeding layer after healing the defects of the graphitic surface with annealing treatment.

Detection and Stabilization of a Previously Unknown Two-Dimensional (Pseudo)polymorph using Lateral Nanoconfinement.

We report on the detection and stabilization of a previously unknown two-dimensional (2D) pseudopolymorph of an alkoxy isophthalic acid using lateral nanoconfinement. The self-assembled molecular networks formed by the isophthalic acid derivative were studied at the interface between covalently modified graphite and an organic solvent. When self-assembled on graphite with moderate surface coverage of covalently bound aryl groups, a previously unknown metastable pseudopolymorph was detected. This pseudopolymorph, which was presumably "trapped" in between the surface bound aryl groups, underwent a time-dependent phase transition to the stable polymorph typically observed on pristine graphite. The stabilization of the pseudopolymorph was then achieved by using an alternative nanoconfinement strategy, where the domains of the pseudopolymorph could be formed and stabilized by restricting the self-assembly in nanometer-sized shallow compartments produced by STM-based nanolithography carried out on a graphite surface with a high density of covalently bound aryl groups. These experimental results are supported by molecular mechanics and molecular dynamics simulations, which not only provide important insight into the relative stabilities of the different structures, but also shed light onto the mechanism of the formation and stabilization of the pseudopolymorph under nanoscopic lateral confinement.

Porous Self-Assembled Molecular Networks as Templates for Chiral-Position-Controlled Chemical Functionalization of Graphitic Surfaces.

Controlled covalent functionalization of graphitic surfaces with molecular scale precision is crucial for tailored modulation of the chemical and physical properties of carbon materials. We herein present that porous self-assembled molecular networks (SAMNs) act as nanometer scale template for the covalent electrochemical functionalization of graphite using an aryldiazonium salt. Hexagonally aligned achiral grafted species with lateral periodicity of 2.3, 2.7, and 3.0 nm were achieved utilizing SAMNs having different pore-to-pore distances. The unit cell vectors of the grafted pattern match those of the SAMN. After the covalent grafting, the template SAMNs can be removed by simple washing with a common organic solvent. We briefly discuss the mechanism of the observed pattern transfer. The unit cell vectors of the grafted pattern align along nonsymmetry axes of graphite, leading to mirror image grafted domains, in accordance with the domain-specific chirality of the template. In the case in which a homochiral building block is used for SAMN formation, one of the 2D mirror image grafted patterns is canceled. This is the first example of a nearly crystalline one-sided or supratopic covalent chemical functionalization. In addition, the positional control imposed by the SAMN renders the functionalized surface (homo)chiral reaching a novel level of control for the functionalization of carbon surfaces, including surface-supported graphene.

Graphite and Graphene Fairy Circles: A Bottom-Up Approach for the Formation of Nanocorrals.

A convenient covalent functionalization approach and nanopatterning method of graphite and graphene is developed. In contrast to expectations, electrochemically activated dediazotization of a mixture of two aryl diazonium compounds in aqueous media leads to a spatially inhomogeneous functionalization of graphitic surfaces, creating covalently modified surfaces with quasi-uniform spaced islands of pristine graphite or graphene, coined nanocorrals. Cyclic voltammetry and chronoamperometry approaches are compared. The average diameter (45-130 nm) and surface density (20-125 corrals/µm2) of these nanocorrals are tunable. These chemically modified nanostructured graphitic (CMNG) surfaces are characterized by atomic force microscopy, scanning tunneling microscopy, Raman spectroscopy and microscopy, and X-ray photoelectron spectroscopy. Mechanisms leading to the formation of these CMNG surfaces are discussed. The potential of these surfaces to investigate supramolecular self-assembly and on-surface reactions under nanoconfinement conditions is demonstrated.

Phase selectivity triggered by nanoconfinement: the impact of corral dimensions.

By using a novel protocol to spatially confine molecules in well-defined small 2D areas, the so-called nanocorrals, we show using scanning tunneling microscopy (STM) how this kind of confinement affects self-assembled molecular network (SAMN) formation at a liquid-solid interface. The 2D lateral confinement, imposed by the size of the nanocorrals, has a clear impact on the phase selectivity of a molecule that can form both low-density and high-density SAMNs, the high-density phase being promoted by the confinement.

Self-Assembled Monolayers as Templates for Linearly Nanopatterned Covalent Chemical Functionalization of Graphite and Graphene Surfaces.

An approach for nanoscale covalent functionalization of graphite surfaces employing self-assembled molecular monolayers of n-alkanes as templating masks is presented. Linearly aligned aryl groups with a lateral periodicity of 5 or 7 nm are demonstrated utilizing molecular templates of different lengths. The key feature of this approach is the use of a phase separated solution double layer consisting of a thin organic layer containing template molecules topped by an aqueous layer containing aryldiazonium molecules capable of electrochemical reduction to generate aryl radicals which bring about surface grafting. Upon sweeping of the potential, lateral displacement dynamics at the n-alkane terminal edges acts in conjunction with electrochemical diffusion to result in templated covalent bond formation in a linear fashion. This protocol was demonstrated to be applicable to linear grafting of graphene. The present processing described herein is useful for the realization of rationally designed nanoscale materials.

Unidirectional supramolecular self-assembly inside nanocorrals via in situ STM nanoshaving.

Self-assembly of an alkylated diacetylene derivative is spatially confined via in situ scanning tunneling microscopy (STM) nanoshaving inside covalently modified highly ordered pyrolytic graphite (CM-HOPG). In contrast to unconstrained self-assembly that occurs randomly along three thermodynamically equivalent surface lattice directions, spatially confined assemblies are shown to form along chosen substrate orientations. Experimental statistics suggest two mechanisms for breaking the rotational degeneracy of the surface. First, the assembly orientation is biased via lateral confinement inside nanocorrals that do not match the substrate symmetry. Second, an interaction between the assembling molecules and the STM tip during nanoshaving guides 2D crystal nucleation and growth. The results presented here open new possibilities to regulate and orient self-assembled architectures via in situ nanomechanical manipulation techniques and provide mechanistic insights into the process.

Biasing Enantiomorph Formation via Geometric Confinement: Nanocorrals for Chiral Induction at the Liquid-Solid Interface.

Nanocorrals created by scanning probe lithography on covalently modified graphite surfaces are used to induce a chiral bias in the enantiomorphic assembly of a prochiral molecule at the liquid/graphite interface. By controlling the orientation of the nanocorrals with respect to the underlying graphite surface, the nanocorral handedness can be freely chosen and thus a chiral bias in molecular self-assembly is created at an achiral surface solely by the scanning probe lithography process.

Amphiphile self-assembly dynamics at the solution-solid interface reveal asymmetry in head/tail desorption.

Amphiphilic alkoxybenzonitriles (ABNs) of varying chain length are studied at the solution/graphite interface to analyze dynamics of assembly. Competitive self-assembly between ABNs and alkanoic acid solvent is shown by scanning tunneling microscopy (STM) to be controlled by concentration and molecular size. Molecular dynamics (MD) simulations reveal key roles of the sub-nanosecond fundamental steps of desorption, adsorption, and on-surface motion. We discovered asymmetry in desorption-adsorption steps. Desorption starting from alkyl chain detachment from the surface is favored due to dynamic occlusion by neighbouring chains. Even though the nitrile head has a strong solvent affinity, it more frequently re-adsorbs following a detachment event.

Self-Assembled Polystyrene Beads for Templated Covalent Functionalization of Graphitic Substrates Using Diazonium Chemistry.

A network of self-assembled polystyrene beads was employed as a lithographic mask during covalent functionalization reactions on graphitic surfaces to create nanocorrals for confined molecular self-assembly studies. The beads were initially assembled into hexagonal arrays at the air-liquid interface and then transferred to the substrate surface. Subsequent electrochemical grafting reactions involving aryl diazonium molecules created covalently bound molecular units that were localized in the void space between the nanospheres. Removal of the bead template exposed hexagonally arranged circular nanocorrals separated by regions of chemisorbed molecules. Small molecule self-assembly was then investigated inside the resultant nanocorrals using scanning tunneling microscopy to highlight localized confinement effects. Overall, this work illustrates the utility of self-assembly principles to transcend length scale gaps in the development of hierarchically patterned molecular materials.

Physical and chemical model of ion stability and movement within the dynamic and voltage-gated STM tip-surface tunneling junction.

The interaction and mobility of ions in complex systems are fundamental to processes throughout chemistry, biology, and physics. However, nanoscale characterization of ion stability and migration remains poorly understood. Here, we examine ion movements to and from physisorbed molecular receptors at solution-graphite interfaces by developing a theoretical model alongside experimental scanning tunneling microscopy (STM) results. The model includes van der Waals forces and electrostatic interactions originating from the surface, tip, and physisorbed receptors, as well as a tip-surface electric field arising from the STM bias voltage (Vb). Our model reveals how both the electric field and tip-surface distance, dtip, can influence anion stability at the receptor binding sites on the surface or at the STM tip, as well as the size of the barrier for anion transitions between those locations. These predictions agree well with prior and new STM results from the interactions of anions with aryl-triazole receptors that order into functional monolayers on graphite. Scanning produces clear resolution at large magnitude negative surface biases (-0.8 V) while resolution degrades at small negative surface biases (-0.4 V). The loss in resolution arises from frequent tip retractions assigned to anion migration within the tip-surface tunneling region. This experimental evidence in combination with support from the model demonstrates a local voltage gating of anions with the STM tip inside physisorbed receptors. This generalized model and experimental evidence may help to provide a basis to understand the nanoscale details of related chemical transformations and their underlying thermodynamic and kinetic preferences.

Ion-Pair Oligomerization of Chromogenic Triangulenium Cations with Cyanostar-Modified Anions That Controls Emission in Hierarchical Materials.

The hierarchical assembly of colored cationic molecules with receptor-modified counteranions can be used to control optical properties in materials. However, our knowledge of when the optical properties emerge in the hierarchical organization and the variety of cation-anion salts that are available to create these materials is limited. In this work, we extend the salts from small halides to large inorganic anions and determine how the structure coevolves with the emission properties using solution assemblies. We study the chromogenic trioxatriangulenium (TOTA+) cation and its coassembly with cyanostar (CS) macrocycles selected to modify tetrafluoroborate (BF4-) counteranions through formation of 2:1 sandwich complexes. In the solid state, the TOTA+ cation stacks in an alternating manner with the sandwich complexes producing new red-shifted emission and absorption bands. Critical to assigning the structural origin of the new optical features across the four levels of organization (1° → 4°) is the selection of specific solvents to produce and characterize different assemblies present in the hierarchical structure. A key species is the electrostatically stabilized ion pair between the TOTA+ cation and sandwich complex. The red-shifted features only emerge when the ion pairs oligomerize together into larger (TOTA·[CS2BF4])n assemblies. New electronic states emerge as a result of multiple copies of the TOTA+ making π-contact with cyanostar-anion complexes. Our findings and the ease with which the materials can be prepared as crystals and films by mixing the salt with a receptor provide a strong platform for the de novo design of new optical materials.

Confined polydiacetylene polymerization reactions for programmed length control.

Polydiacetylene polymers of defined lengths are formed from self-assembled precursors inside nanocorrals created within grafted graphite substrates. A scanning tunneling microscope tip is used to nanoshave corrals at the liquid-solid interface allowing orientationally controlled supramolecular self-assembly of linear diacetylene molecules. Electrical pulses trigger topological one-dimensional polymerization reactions that are confined by the nanocorral template dimensions.

Self-Assembly under Confinement: Nanocorrals for Understanding Fundamentals of 2D Crystallization.

Nanocorrals with different size, shape, and orientation are created on covalently modified highly oriented pyrolytic graphite surfaces using scanning probe nanolithography, i.e., nanoshaving. Alkylated diacetylene molecules undergo laterally confined supramolecular self-assembly within these corrals. When nanoshaving is performed in situ, at the liquid-solid interface, the orientation of the supramolecular lamellae structure is directionally influenced by the gradual graphite surface exposure. Careful choice of the nanoshaving direction with respect to the substrate symmetry axes promotes alignment of the supramolecular lamellae within the corral. Self-assembly occurring inside corrals of different size and shape reveals the importance of geometric and kinetic constraints controlled by the nanoshaving process. Finally, seed-mediated crystallization studies demonstrate confinement control over nucleation and growth principles.

Multifunctional Tricarbazolo Triazolophane Macrocycles: One-Pot Preparation, Anion Binding, and Hierarchical Self-Organization of Multilayers.

Programming the synthesis and self-assembly of molecules is a compelling strategy for the bottom-up fabrication of ordered materials. To this end, shape-persistent macrocycles were designed with alternating carbazoles and triazoles to program a one-pot synthesis and to bind large anions. The macrocycles bind anions that were once considered too weak to be coordinated, such as PF6 (-) , with surprisingly high affinities (ß2 =10(11) M(-2) in 80:20 chloroform/methanol) and positive cooperativity, α=(4 K2 /K1 )=1200. We also discovered that the macrocycles assemble into ultrathin films of hierarchically ordered tubes on graphite surfaces. The remarkable surface-templated self-assembly properties, as was observed by using scanning tunneling microscopy, are attributed to the complementary pairing of alternating triazoles and carbazoles inscribed into both the co-facial and edge-sharing seams that exist between shape-persistent macrocycles. The multilayer assembly is also consistent with the high degree of molecular self-association observed in solution, with self-association constants of K=300 000 M(-1) (chloroform/methanol 80:20). Scanning tunneling microscopy data also showed that surface assemblies readily sequester iodide anions from solution, modulating their assembly. This multifunctional macrocycle provides a foundation for materials composed of hierarchically organized and nanotubular self-assemblies.

Living on the edge: Tuning supramolecular interactions to design two-dimensional organic crystals near the boundary of two stable structural phases.

One of the benefits of supramolecular assemblies that form at dynamic interfaces is the opportunity to develop condensed phase systems that respond to environmental stimuli. A prerequisite of this responsive behavior is that the supramolecular system be designed to sit very near the stability of two or more crystal structures. We have created such a bi-phasic system with aryl-triazole oligomers by investigating how phase morphology is controlled by the interplay between interactions that involve the oligomer's dipolar cores (Δµ = 3.5 debye), van der Waals contacts of their pendant alkyl chains (C4-C18), and close-contact hydrogen bonding. Scanning tunneling microscopy experiments conducted at the solution-graphite interface allow sub-molecular resolution of the ordered monolayers to unambiguously determine the packing and structure of two principle phases, α and ß. The system is balanced very near the edge of phase stability, evidenced by co-existent phases present over short time frames and by the changes in preference between the two 2D supramolecular assemblies that occur with small modifications to the molecular structure. We demonstrate that the bi-phasic behavior can be understood as a balance between electrostatic interactions and van der Waals contacts, two variables within a larger parameter space, allowing synthetic design to move this solution-surface system across the stability boundary of different condensed-phase structures. These findings are a foundation for the development of environmentally responsive 2D supramolecular arrays.

Selective anion-induced crystal switching and binding in surface monolayers modulated by electric fields from scanning probes.

Anion-selective (Br(-) and I(-)) and voltage-driven crystal switching between two differently packed phases (α ⇆ ß) was observed in 2D crystalline monolayers of aryl-triazole receptors ordered at solution-graphite interfaces. Addition of Br(-) and I(-) was found to stimulate the α → ß phase transformation and to produce ion binding to the ß phase assembly, while Cl(-) and BF4(-) addition retained the α phase. Unlike all other surface assemblies of either charged molecules or ion-templated 2D crystallization of metal-ligand or receptor-based adsorbates, the polarity of the electric field between the localized scanning tip and the graphite substrate was found to correlate with phase switching: ß â†’ α is driven at -1.5 V, while α → ß occurs at +1.1 V. Ion-pairing between the countercations and the guest anions was also observed. These observations are supported by control studies including variation of anion species, relative anion concentration, surface temperature, tip voltage, and scanning time.

Anion-induced dimerization of 5-fold symmetric cyanostars in 3D crystalline solids and 2D self-assembled crystals.

Anions stabilize stacking of C5-symmetric macrocycles, called cyanostars, into dimers in solution and herein we demonstrate this stacking in solid-state crystals. We further show that this guest binding can be applied as a route to bilayer growth at the solution-graphite interface.