Mikhail Shekhirev

Bottom-up solution synthesis of narrow nitrogen-doped graphene nanoribbons.

Large quantities of narrow graphene nanoribbons with edge-incorporated nitrogen atoms can be synthesized via Yamamoto coupling of molecular precursors containing nitrogen atoms followed by cyclodehydrogenation using Scholl reaction.

Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon-Based Two-Dimensional and Three-Dimensional Metamaterials

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties.

Graphene substrate for inducing neurite outgrowth.

A few recent studies demonstrated that graphene may have cytocompatibility with several cell types. However, when assessing cell behavior on graphene, there has been no precise control over the quality of graphene, number of graphene layers, and substrate surface coverage by graphene. In this study, using well-controlled monolayer graphene film substrates we tested the cytocompatibility of graphene for human neuroblastoma (SH-SY5Y) cell culture. A large-scale monolayer graphene film grown on Cu foils by chemical vapor deposition (CVD) could be successfully transferred onto glass substrates by wet transfer technique. We observed that graphene substrate could induce enhanced neurite outgrowth, both in neurite length and number, compared with control glass substrate. Interestingly, the positive stimulatory effect by graphene was achieved even in the absence of soluble neurogenic factor, retinoic acid (RA). Key genes relevant to cell neurogenesis, e.g., neurofilament light chain (NFL), were also upregulated on graphene. Inhibitor studies suggested that the graphene stimulation of cellular neurogenesis may be achieved through focal adhesion kinase (FAK) and p38 mitogen-activated protein kinase (MAPK) cascades. Our data indicate that graphene may be exploited as a platform for neural regenerative medicine, and the suggested molecular mechanism may provide an insight into the graphene control of neural cells.

Interfacial Self-Assembly of Atomically Precise Graphene Nanoribbons into Uniform Thin Films for Electronics Applications

Because of their intriguing electronic and optical properties, atomically precise graphene nanoribbons (GNRs) are considered to be promising materials for electronics and photovoltaics. However, significant aggregation and low solubility of GNRs in conventional solvents result in their poor processability for materials characterization and device studies. In this paper, we demonstrate a new fabrication approach for large-scale uniform thin films of nonfunctionalized atomically precise chevron-type GNRs. The method is based on (1) the exceptional solubility of graphitic materials in chlorosulfonic acid and (2) the original interfacial self-assembly approach by which uniform films that are single-GNR (∼2 nm) thick can be routinely prepared. These films can be transferred to various substrates including Si/SiO2 and used for the streamlined fabrication of arrays of GNR-based devices. The described self-assembly approach should be applicable to other types of solution-synthesized atomically precise GNRs as well as large polyaromatic hydrocarbon (PAH) molecules and therefore should facilitate and streamline their device characterization.

Oxidative peeling of carbon black nanoparticles

We demonstrate that layered carbon black nanoparticles can be oxidatively peeled via the reaction with potassium permanganate in sulfuric acid. As a result of this reaction, outer layers of carbon nanoparticles “peel” off due to high levels of oxidation while the less oxidized inner cores, though they exhibit remarkable solubility in water, remain mostly intact.

Inkjet printable-photoactive all inorganic perovskite films with long effective photocarrier lifetimes

Photoactive perovskite quantum dot films, deposited via an inkjet printer, have been characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The crystal structure and bonding environment are consistent with CsPbBr3 perovskite quantum dots. The current-voltage (I-V) and capacitance-voltage (C-V) transport measurements indicate that the photo-carrier drift lifetime can exceed 1 ms for some printed perovskite films. This far exceeds the dark drift carrier lifetime, which is below 50 ns. The printed films show a photocarrier density 109 greater than the dark carrier density, making these printed films ideal candidates for application in photodetectors. The successful printing of photoactive-perovskite quantum dot films of CsPbBr3, indicates that the rapid prototyping of various perovskite inks and multilayers is realizable.

Phenyl Functionalization of Atomically Precise Graphene Nanoribbons for Engineering Inter-ribbon Interactions and Graphene Nanopores

Graphene nanoribbons (GNRs) attract much attention from researchers due to their tunable physical properties and potential for becoming nanoscale building blocks of electronic devices. GNRs can be synthesized with atomic precision by on-surface approaches from specially designed molecular precursors. While a considerable number of ribbons with very diverse structures and properties have been demonstrated in recent years, there have been only limited examples of on-surface synthesized GNRs modified with functional groups. In this study, we designed a nanoribbon, in which the chevron GNR backbone is decorated with phenyl functionalities, and demonstrate the on-surface synthesis of these GNRs on Au(111). We show that the phenyl modification affects the assembly of the GNR polymer precursors through π-π interactions. Scanning tunneling spectroscopy of the modified GNRs on Au(111) revealed that they have a band gap of 2.50 ± 0.02 eV, which is comparable to that of the parent chevron GNR. The phenyl functionalization leads to a shift of the band edges to lower energies, suggesting that it could be a useful tool for the GNR band structure engineering. We also investigated lateral fusion of the phenyl-modified GNRs and demonstrate that it could be used to engineer different kinds of atomically precise graphene nanopores. A similar functionalization approach could be potentially applied to other GNRs to affect their on-surface assembly, modify their electronic properties, and realize graphene nanopores with a variety of structures.

Aggregation of atomically precise graphene nanoribbons

Solution bottom-up approaches can be used to prepare bulk quantities of narrow atomically precise graphene nanoribbons (GNRs) with various widths and geometries. These GNRs are often considered as promising materials for electronic and optoelectronic applications. However, the handling and processing of nanoribbons for practical applications can be difficult because of their entanglement and aggregation, and thus poor solubility in conventional solvents. In this work, we studied the aggregation-dependent properties of solution-synthesized chevron GNRs in a variety of solvents. We demonstrate that the spectroscopic features observed in the experimentally measured absorbance spectra of chevron GNRs are in a good agreement with the theoretically predicted excitionic transitions. We also show that the absorbance spectra of GNRs evolve with aggregation time, which is important to consider for the spectroscopic determination of optical bandgaps of nanoribbons. We discuss two types of GNR assemblies: bulk aggregates of π–π stacked nanoribbons that form in a solution and rather long one-dimensional (1D) structures that were observed on a variety of surfaces, such as Au(111), mica and Si/SiO2. We demonstrate that the few-μm-long 1D GNR structures can be conveniently visualized by conventional microscopy techniques and used for the fabrication of electronic devices.

Solution Synthesis of Atomically Precise Graphene Nanoribbons

Abstract Bottom-up fabrication of narrow strips of graphene, also known as graphene nanoribbons or GNRs, is an attractive way to open a bandgap in semimetallic graphene. In this chapter, we review recent progress in solution-based synthesis of GNRs with atomically precise structures. We discuss a variety of atomically precise GNRs and highlight theoretical and practical aspects of their structural design and solution synthesis. These GNRs are typically synthesized through a polymerization of rationally designed molecular precursors followed by a planarization through a cyclodehydrogenation reaction. We discuss various synthetic techniques for polymerization and planarization steps, possible approaches for chemical modification of GNRs, and compare the properties of GNRs that could be achieved by different synthetic methods. We also discuss the importance of the rational design of molecular precursors to avoid isomerization during the synthesis and achieve GNRs that have only one possible structure. Significant attention in this chapter is paid to the methods of material characterization of solution-synthesized GNRs. The chapter is concluded with the discussion of the most significant challenges in the field and the future outlook.