Landau Levels of Topologically-Protected Surface States Probed by Dual-Gated Quantum Capacitance.

Spectroscopy of discrete Landau levels (LLs) in bulk-insulating three-dimensional topological insulators (3D TIs) in perpendicular magnetic field characterizes the Dirac nature of their surface states. Despite a number of studies demonstrating the quantum Hall effect (QHE) of topological surface states, quantitative evaluation of the LL energies, which serve as fundamental electronic quantities for study of the quantum states, is still limited. In this work, we explore the density of states of LLs by measuring quantum capacitance () in a truly bulk insulating 3D TI via a van der Waals heterostructure configuration.

Spin-optoelectronic devices based on hybrid organic-inorganic trihalide perovskites.

Recently the hybrid organic-inorganic trihalide perovskites have shown remarkable performance as active layers in photovoltaic and other optoelectronic devices. However, their spin characteristic properties have not been fully studied, although due to the relatively large spin-orbit coupling these materials may show great promise for spintronic applications. Here we demonstrate spin-polarized carrier injection into methylammonium lead bromide films from metallic ferromagnetic electrodes in two spintronic-based devices: a 'spin light emitting diode' that results in circularly polarized electroluminescence emission; and a 'vertical spin valve' that shows giant magnetoresistance.

Topological Insulator-Based van der Waals Heterostructures for Effective Control of Massless and Massive Dirac Fermions.

Three dimensional (3D) topological insulators (TIs) are an important class of materials with applications in electronics, spintronics and quantum computing. With the recent development of truly bulk insulating 3D TIs, it has become possible to realize surface dominated phenomena in electrical transport measurements e.g.

Hexaaminobenzene as a building block for a Family of 2D Coordination Polymers.

A family of 2D coordination polymers were successfully synthesized through "bottom-up" techniques using Ni, Cu, Co, and hexaaminobenzene. Liquid-liquid and air-liquid interfacial reactions were used to realize thick (∼1-2 μm) and thin (<10 nm) stacked layers of nanosheet, respectively. Atomic-force microscopy and scanning electron microscopy both revealed the smooth and flat nature of the nanosheets.

Raman spectroscopy of folded and scrolled graphene.

Here we examine the effects of folding and scrolling on the Raman spectra of mechanically exfoliated single- and bilayer graphene prepared on SiO(2) substrates. We find that incommensurate folding in bilayer graphene results in a shift of the second-order G' band frequency, similar to that observed in folded single-layer graphene due to fold-induced changes in the phonon/electronic energy dispersion. Importantly, we show that the contrasting Raman shifts reported for the G' band frequency in folded graphene can be rationalized by taking into account the relative strength of fold-induced electron/phonon renormalization.

Photosensitization of carbon nanotubes using dye aggregates.

Ordered assemblies of dye molecules, dye aggregates, possess significantly larger molar optical absorptivity than dye monomers. Yet, aggregates have not been utilized for photosensitizing nanoscale electronic devices. We find that single-walled carbon nanotubes, which are cleaned down to the atomic scale, template the growth of squaraine dye aggregates and these aggregates effectively photosensitize nanotubes.

Effects of layer stacking on the combination Raman modes in graphene.

We have observed new combination modes in the range from 1650 to 2300 cm(-1) in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. A peak at ∼1860 cm(-1) (iTALO-) is observed due to a combination of the in-plane transverse acoustic (iTA) and the longitudinal optical (LO) phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent for bulk graphite.