[Show abstract][Hide abstract] ABSTRACT: 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 both from 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, ΔD-D, can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as ΔD-D = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows a spin-orbit gap of ~20 meV in the π-band enhanced by three orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semi-metallic to a quantum spin Hall phase when ΔD-D ≤ 0.20 eV. We thus present a practical means of spin-polarizing the π-band of graphene, which can be pivotal to advance the graphene-based spintronics.
[Show abstract][Hide abstract] ABSTRACT: 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 multi-step 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-crosslinking process. Graphene is successfully grown from neat polystyrene regions, while patterned crosslinked 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 two 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.
[Show abstract][Hide abstract] ABSTRACT: 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 defects, 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.
[Show abstract][Hide abstract] ABSTRACT: 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)/Vs, remains intact from the graphene underneath without degradation. Given that the graphene was pre-patterned 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · May 2013 · Chemical Communications
[Show abstract][Hide abstract] ABSTRACT: 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. Caliximidazolium senses fluoride selectively even in aqueous solutions. Caliximidazolium 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Nov 2011 · Chemical Communications