Spectromicroscopic observation of a live single cell in a biocompatible liquid-enclosing graphene system.

On-the-spot visualization of biochemical responses of intact live cells is vital for a clear understanding of cell biology. The main obstacles for instant visualization of biochemical responses of living cells arise from the lack of a sophisticated detecting technique which can simultaneously provide chemical analysis tools and the biocompatible wet conditions. Here we introduce scanning transmission X-ray microscopy (STXM) combined with a liquid-enclosing graphene system (LGS), offering biocompatible conditions and improved X-ray absorption spectra to probe the chemical responses of live cells under wet conditions. This set-up enables us to probe a subtle change in absorption spectra depending on the oxidation state of a miniscule amount of oxygen in the functional groups present in each cell and its surroundings containing a minimal amount of liquid water. As an example of in situ biochemical responses of wet cells, chemical responses of a single Colo 205 cell are visualized and analyzed using X-ray absorption near the oxygen K-edge. This spectromicroscopic method using LGS can be applied to diverse biological samples under wet conditions for the analysis of their biochemical responses.

Graphene and Graphene Analogs toward Optical, Electronic, Spintronic, Green-Chemical, Energy-Material, Sensing, and Medical Applications.

This spotlight discusses intriguing properties and diverse applications of graphene (Gr) and Gr analogs. Gr has brought us two-dimensional (2D) chemistry with its exotic 2D features of density of states. Yet, some of the 2D or 2D-like features can be seen on surfaces and at interfaces of bulk materials. The substrate on Gr and functionalization of Gr (including metal decoration, intercalation, doping, and hybridization) modify the unique 2D features of Gr. Despite abundant literature on physical properties and well-known applications of Gr, spotlight works based on the conceptual understanding of the 2D physical and chemical nature of Gr toward vast-ranging applications are hardly found. Here we focus on applications of Gr, based on conceptual understanding of 2D phenomena toward 2D chemistry. Thus, 2D features, defects, edges, and substrate effects of Gr are discussed first. Then, to pattern Gr electronic circuits, insight into differentiating conducting and nonconducting regions is introduced. By utilizing the unique ballistic electron transport properties and edge spin states of Gr, Gr nanoribbons (GNRs) are exploited for the design of ultrasensitive molecular sensing electronic devices (including molecular fingerprinting) and spintronic devices. The highly stable nature of Gr can be utilized for protection of corrosive metals, moisture-sensitive perovskite solar cells, and highly environment-susceptible topological insulators (TIs). Gr analogs have become new types of 2D materials having novel features such as half-metals, TIs, and nonlinear optical properties. The key insights into the functionalized Gr hybrid materials lead to the applications for not only energy storage and electrochemical catalysis, green chemistry, and electronic/spintronic devices but also biosensing and medical applications. All these topics are discussed here with the focus on conceptual understanding. Further possible applications of Gr, GNRs, and Gr analogs are also addressed in a section on outlook and future challenges.

Proximity Effect Induced Electronic Properties of Graphene on Bi2Te2Se.

We report that the π-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material. We have grown epitaxial Bi2Te2Se (BTS) films on a chemical vapor deposition (CVD) graphene. We observe two linear surface bands from both the CVD graphene notably flattened and BTS coexisting with their DPs separated by 0.53 eV in the photoemission data measured with synchrotron photons. We further demonstrate that the separation between the two DPs, Δ(D-D), can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as Δ(D-D) = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows the opening of a spin-orbit gap of ∼20 meV in the π-band, enhanced by 3 orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semimetallic to a quantum spin Hall phase when Δ(D-D) ≤ 0.20 eV. We thus present a practical means of spin-polarizing the π-band of graphene, which can be pivotal to advance graphene-based spintronics.

A Facile Route for Patterned Growth of Metal-Insulator Carbon Lateral Junction through One-Pot Synthesis.

Precise graphene patterning is of critical importance for tailor-made and sophisticated two-dimensional nanoelectronic and optical devices. However, graphene-based heterostructures have been grown by delicate multistep chemical vapor deposition methods, limiting preparation of versatile heterostructures. Here, we report one-pot synthesis of graphene/amorphous carbon (a-C) heterostructures from a solid source of polystyrene via selective photo-cross-linking process. Graphene is successfully grown from neat polystyrene regions, while patterned cross-linked polystyrene regions turn into a-C because of a large difference in their thermal stability. Since the electrical resistance of a-C is at least 2 orders of magnitude higher than that for graphene, the charge transport in graphene/a-C heterostructure occurs through the graphene region. Measurement of the quantum Hall effect in graphene/a-C lateral heterostructures clearly confirms the reliable quality of graphene and well-defined graphene/a-C interface. The direct synthesis of patterned graphene from polymer pattern could be further exploited to prepare versatile heterostructures.

Simultaneous visualization of graphene grain boundaries and wrinkles with structural information by gold deposition.

Although line defects such as grain boundaries (GBs) and wrinkles are unavoidable in graphene, difficulties in identification preclude studying their impact on electronic and mechanical properties. As previous methods focus on a single type of line defect, simultaneous measurements of both GBs and wrinkles with detailed structural information have not been reported. Here, we introduce effective visualization of both line defects by controlled gold deposition. Upon depositing gold on graphene, single lines and double lines of gold nanoparticles (NPs) are formed along GBs and wrinkles, respectively. Moreover, it is possible to analyze whether a GB is stitched or overlapped, whether a wrinkle is standing or folded, and the width of the standing collapsed wrinkle. Theoretical calculations show that the characteristic morphology of gold NPs is due to distinct binding energies of line defects, which are correlated to disrupting diffusion of NPs. Our approach could be further exploited to investigate the defect structures of other two-dimensional materials.

Persistent topological surface state at the interface of Bi2Se3 film grown on patterned graphene.

We employed graphene as a patternable template to protect the intrinsic surface states of thin films of topological insulators (TIs) from environment. Here we find that the graphene provides high-quality interface so that the Shubnikov de Haas (SdH) oscillation associated with a topological surface state could be observed at the interface of a metallic Bi2Se3 film with a carrier density higher than ∼ 10(19) cm(-3). Our in situ X-ray diffraction study shows that the Bi2Se3 film grows epitaxially in a quintuple layer-by-layer fashion from the bottom layer without any structural distortion by interfacial strain. The magnetotransport measurements including SdH oscillations stemming from multiple conductance channels reveal that the topological surface state, with the mobility as high as ∼ 0.5 m(2)/(V s), remains intact from the graphene underneath without degradation. Given that the graphene was prepatterned on arbitrary insulating substrates, the TI-based microelectronic design could be exploited. Our study thus provides a step forward to observe the topological surface states at the interface without degradation by tuning the interface between TI and graphene into a measurable current for device application.

Fast benchtop visualization of graphene grain boundaries using adhesive properties of defects.

Highly adhesive properties of graphene grain boundaries to permanganate lead to a very quick, easy and convenient method to visualize the grain boundaries simply using an optical microscope, which would be vital to improve specific properties of graphene.

Calix[n]imidazolium as a new class of positively charged homo-calix compounds.

Macrocycles based on neutral calixarenes and calixpyrroles have been extensively explored for ion binding, molecular assembly and related applications. Given that only these two types of calix compounds and their analogs are available, the introduction of new forms of widely usable calix macrocycles is an outstanding challenge. Here we report the quadruply/quintuply charged imidazole-based homo-calix compounds, calix[4/5]imidazolium. The noncovalent (C-H)(+)/π(+)-anion interactions of the imidazolium rings with anions inside and outside the cone are the stabilizing factors for crystal packing, resulting in self-assembled arrays of cone-shaped calix-imidazolium molecules. Calix[4]imidazolium senses fluoride selectively even in aqueous solutions. Calix[5]imidazolium recognizes neutral fullerenes through π(+)-π interactions and makes them soluble in water, which could be useful in fullerene chemistry. Not only derivatization and ring expansion of calix[n]imidazolium, but also their utilization in ionic liquids, carbene chemistry and nanographite/graphene exfoliation could be exploited.

Highly selective CO2 capture on N-doped carbon produced by chemical activation of polypyrrole functionalized graphene sheets.

N-doped porous carbon produced via chemical activation of polypyrrole functionalized graphene sheets shows selective adsorption of CO(2) (4.3 mmol g(-1)) over N(2) (0.27 mmol g(-1)) at 298 K. The potential for large scale production and facile regeneration makes this material useful for industrial applications.