Ball‐Mill‐Exfoliated Graphene: Tunable Electrochemistry and Phenol Sensing

A simple wet ball‐milling method for exfoliating pristine graphite to graphene nanosheets is proposed. The surfactant of cetyltrimethyl ammonium bromide is utilized to greatly improve the exfoliation efficiency of graphene nanosheets. Variation of the ball‐milling time is an efficient way to control the size and thickness of graphene nanosheets, as well as the level of edge defects. With an increase of ball‐milling time, superior electrochemical reactivity is imparted owing to enlarged active area and increased catalytic ability. The obtained graphene nanosheets are sensitive for electrochemical oxidation of phenols (e.g., hydroquinone, p‐chlorophenol, and p‐nitrophenol), and thus qualified for the simultaneous sensing of trace level of phenols. The detection limits of simultaneous monitoring of hydroquinone, p‐chlorophenol, and p‐nitrophenol are as low as 0.017, 0.024, and 0.42 mg L^?1, respectively. The proposed strategy thus opens up a new way to tune electrochemistry of graphene materials as well as to design their new applications.     

Covalent Functionalization of Dipole‐Modulating Molecules on Trilayer Graphene: An Avenue for Graphene‐Interfaced Molecular Machines

The molecular dipole moment plays a significant role in governing important phenomena like molecular interactions, molecular configuration, and charge transfer, which are important in several electronic, electrochemical, and optoelectronic systems. Here, the effect of the change in the dipole moment of a tethered molecule on the carrier properties of (functionalized) trilayer graphene—a stack of three layers of sp²‐hybridized carbon atoms—is demonstrated. It is shown that, due to the high carrier confinement and large quantum capacitance, the trans‐to‐cis isomerisation of ‘covalently attached’ azobenzene molecules, with a change in dipole moment of 3D, leads to the generation of a high effective gating voltage. Consequently, 6 units of holes are produced per azobenzene molecule (hole density increases by 440 000 holes μm^?2). Based on Raman and X‐ray photoelectron spectroscopy data, a model is outlined for outer‐layer, azobenzene‐functionalized trilayer graphene with current modulation in the inner sp² matrix. Here, 0.097 V are applied by the isomerisation of the functionalized azobenzene. Further, the large measured quantum capacitance of 72.5 μF cm^?2 justifies the large Dirac point in the heavily doped system. The mechanism defining the effect of dipole modulation of covalently tethered molecules on graphene will enable future sensors and molecular‐machine interfaces with graphene.     

Optically Unraveling the Edge Chirality‐Dependent Band Structure and Plasmon Damping in Graphene Edges

The nontrivial topological origin and pseudospinorial character of electron wavefunctions make edge states possess unusual electronic properties. Twenty years ago, the tight‐binding model calculation predicted that zigzag termination of 2D sheets of carbon atoms have peculiar edge states, which show potential application in spintronics and modern information technologies. Although scanning probe microscopy is employed to capture this phenomenon, the experimental demonstration of its optical response remains challenging. Here, the propagating graphene plasmon provides an edge‐selective polaritonic probe to directly detect and control the electronic edge state at ambient condition. Compared with armchair, the edge‐band structure in the bandgap gives rise to additional optical absorption and strongly absorbed rim at zigzag edge. Furthermore, the optical conductivity is reconstructed and the anisotropic plasmon damping in graphene systems is revealed. The reported approach paves the way for detecting edge‐specific phenomena in other van der Waals materials and topological insulators.     

High‐ versus Low‐Quality Graphene: A Mechanistic Investigation of Electrografted Diazonium‐Based Films for Growth of Polymer Brushes

Electrografting using aryldiazonium salts provides a fast and efficient technique to functionalize commercially available 3?5 layered graphene (vapour‐deposited) on nickel. In this study, Raman spectroscopy is used to quantify the grafting efficiency of cyclic voltammetry which is one of the most versatile, yet simple, electrochemical techniques available. To a large extent the number of defects/substituents introduced to the basal plane of high‐quality graphene by this procedure can be controlled through the sweeping conditions employed. After extended electrografting the defect density reaches a saturation level (～10¹³ cm^?2) which is independent of the quality of the graphene expressed through its initial content of defects. However, it is reached within fewer voltammetric cycles for low‐quality graphene. Based on these results it is suggested that the grafting occurs (a) directly at defect sites for, in particular, low‐quality graphene, (b) directly at the basal plane for, in particular, high‐quality graphene, and/or (c) at already grafted molecules to give a mushroom‐like film growth for all films. Moreover, it is shown that a tertiary alkyl bromide can be introduced at a given surface density to serve as radical initiator for surface‐initiated atom transfer radical polymerization (SI‐ATRP). Brushes of poly(methyl methacrylate) are grown from these substrates, and the relationship between polymer thickness and sweeping conditions is studied.     

Sensing at the Surface of Graphene Field‐Effect Transistors

Recent research trends now offer new opportunities for developing the next generations of label‐free biochemical sensors using graphene and other two‐dimensional materials. While the physics of graphene transistors operated in electrolyte is well grounded, important chemical challenges still remain to be addressed, namely the impact of the chemical functionalizations of graphene on the key electrical parameters and the sensing performances. In fact, graphene – at least ideal graphene – is highly chemically inert. The functionalizations and chemical alterations of the graphene surface – both covalently and non‐covalently – are crucial steps that define the sensitivity of graphene. The presence, reactivity, adsorption of gas and ions, proteins, DNA, cells and tissues on graphene have been successfully monitored with graphene. This review aims to unify most of the work done so far on biochemical sensing at the surface of a (chemically functionalized) graphene field‐effect transistor and the challenges that lie ahead. The authors are convinced that graphene biochemical sensors hold great promise to meet the ever‐increasing demand for sensitivity, especially looking at the recent progresses suggesting that the obstacle of Debye screening can be overcome.