Ather Mahmood

Production, properties and potential of graphene

This review on graphene, a one-atom thick, two-dimensional sheet of carbon atoms, starts with a general description of the graphene electronic structure as well as a basic experimental toolkit for identifying and handling this material. Owing to the versatility of graphene properties and projected applications, several production techniques are summarized, ranging from the mechanical exfoliation of high-quality graphene to the direct growth on carbides or metal substrates and from the chemical routes using graphene oxide to the newly developed approach at the molecular level. The most promising and appealing properties of graphene are summarized from an exponentially growing literature, with a particular attention to matching production methods to characteristics and to applications. In particular, we report on the high carrier mobility value in suspended and annealed samples for electronic devices, on the thickness-dependent optical transparency and, in the mechanical section, on the high robustness and full integration of graphene in sensing device applications. Finally, we emphasize on the high potential of graphene not only as a post-silicon materials for CMOS device application but more ambitiously as a platform for post-CMOS molecular architecture in electronic information processing.

Few-layer graphene on SiC, pyrolitic graphite, and graphene: A Raman scattering study

To show the similarities between exfoliated graphene and epitaxial few layer graphite (FLG) layers, we present micro-Raman scattering measurements on three different graphite-based materials: micro-structured Highly Oriented Pyrolytic Graphite (HOPG) disks with heights in the 20-2 nm range, exfoliated graphene monolayer, and FLG epitaxially grown on carbon terminated 4H-silicon carbide (4H-SiC) substrates. We show that despite the fact the FLG layers are composed of many layers, the band structure of FLG epitaxially grown on 4H-SiC substrate must be composed of simple electronic bands as witnessed by a single component, Lorentzian shaped, double resonance Raman feature.

Distance dependence of the energy transfer rate from a single semiconductor nanostructure to graphene.

The near-field Coulomb interaction between a nanoemitter and a graphene monolayer results in strong Förster-type resonant energy transfer and subsequent fluorescence quenching. Here, we investigate the distance dependence of the energy transfer rate from individual, (i) zero-dimensional CdSe/CdS nanocrystals and (ii) two-dimensional CdSe/CdS/ZnS nanoplatelets to a graphene monolayer. For increasing distances d, the energy transfer rate from individual nanocrystals to graphene decays as 1/d(4). In contrast, the distance dependence of the energy transfer rate from a two-dimensional nanoplatelet to graphene deviates from a simple power law but is well described by a theoretical model, which considers a thermal distribution of free excitons in a two-dimensional quantum well. Our results show that accurate distance measurements can be performed at the single particle level using graphene-based molecular rulers and that energy transfer allows probing dimensionality effects at the nanoscale.

Side‐Gated Transport in Focused‐Ion‐Beam‐Fabricated Multilayered Graphene Nanoribbons

In this Letter, we present the patterning, exfoliation and micromanipulation of thin graphitic discs which are subsequently connected and patterned into sub-100nm wide ribbons with a resist-free process using Focused Ion Beam (FIB) lithography and deposition. The electronic transport properties of the double side-gated nanoribbons are then investigated down to 40 K and interpreted with a simple model of 1D array of tunnelling junctions.

Multiscale investigation of graphene layers on 6H-SiC(000-1)

In this article, a multiscale investigation of few graphene layers grown on 6H-SiC(000-1) under ultrahigh vacuum (UHV) conditions is presented. At 100-μm scale, the authors show that the UHV growth yields few layer graphene (FLG) with an average thickness given by Auger spectroscopy between 1 and 2 graphene planes. At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist. This is confirmed at the nanometer scale by scanning tunneling microscopy (STM). Finally, STM (at the nm scale) and Raman spectroscopy (at the μm scale) show that the FLG stacking is turbostratic, and that the domain size of the crystallites ranges from 10 to 100 nm. The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.

Circular dichroism of magnetophonon resonance in doped graphene

The polarization-resolved Raman-scattering response due to E-2g phonons in monolayer graphene has been investigated in magnetic fields up to 29 T. The hybridization of the E-2g phonon is only observed with the fundamental inter-Landau-level excitation (involving the n = 0 Landau level) and in just one of the two configurations of the circularly cross-polarized excitation and scattered light. This polarization anisotropy of the magnetophonon resonance is shown to be inherent to relatively strongly doped graphene samples with carrier concentrations typical for graphene deposited on Si/SiO2 substrates.

Epitaxial graphene morphologies probed by weak (anti)-localization

We show how the weak field magneto-conductance can be used as a tool to characterize epitaxial graphene samples grown from the C or the Si face of silicon carbide, with mobilities ranging from 120 to 12 000 cm2/(V·s). Depending on the growth conditions, we observe anti-localization and/or localization, which can be understood in term of weak-localization related to quantum interferences. The inferred characteristic diffusion lengths are in agreement with the scanning tunneling microscopy and the theoretical model which describe the “pure” mono-layer and bilayer of graphene [MacCann et al., Phys. Rev. Lett. 97, 146805 (2006)].

Quasiparticle scattering off phase boundaries in epitaxial graphene

We investigate the electronic structure of terraces of single layer graphene (SLG) by scanning tunnelling microscopy (STM) on samples grown by thermal decomposition of 6H-SiC(0001) crystals in ultra-high vacuum. We focus on the perturbations of the local density of states (LDOS) in the vicinity of edges of SLG terraces. Armchair edges are found to favour intervalley quasiparticle scattering, leading to the (√3 x √3)R30° LDOS superstructure already reported for graphite edges and more recently for SLG on SiC(0001). Using the Fourier transform of LDOS images, we demonstrate that the intrinsic doping of SLG is responsible for a LDOS pattern at the Fermi energy which is more complex than for neutral graphene or graphite, since it combines local (√3 x √3)R30° superstructure and long range beating modulation. Although these features have already been reported by Yang et al (2010 Nano Lett. 10 943-7) we propose here an alternative interpretation based on simple arguments classically used to describe standing wave patterns in standard two-dimensional systems. Finally, we discuss the absence of intervalley scattering off other typical boundaries: zig-zag edges and SLG/bilayer graphene junctions.

Enhancing gas sensing properties of graphene by using a nanoporous substrate

Substrate engineering is shown to be a viable approach for improving the use of graphene thin films for gas sensor applications. The performance of two-terminal devices fabricated on smooth SiO2 and nanoporous anodized aluminum oxide (AAO) substrates are compared. Raman studies indicated that both types of samples exhibit similarly low point-defect densities, but the mobility values of the SiO2-supported films were found to be three times larger than those on porous AAO substrates. However, the AAO-supported graphene devices exhibit a 3-fold enhanced sensitivity to both NO2 and NH3 gases when compared to the devices supported on SiO2. We attribute this sensitivity enhancement to the inhomogeneous electrostatic potential landscape that results from the porous nature of the AAO substrate, as well as extended defects made of wrinkles or folds originated from AAO. This substrate design strategy could be extended to other semiconductor-based sensor devices.