Potential Broadband Photodetector Concept Based on Three-Dimensional Graphene Foam

In this work, optical and structural properties of three-dimensional graphene (GiiTM) foam have been analysed to validate the potential of this material as an active layer in broadband optoelectronic devices. The chemical structure of GiiTM foam was characterized by Raman spectroscopy carried out at different wavelengths (455, 532, 644, and 780nm) and laser powers (1 to 8mW). In this study, two types of GiiTM foam were analysed, including standard graphene (ST-Gii), and low resistance graphene (LR-Gii) whose main difference is the defects-density modulating their electric conductivity. The performance of both ST-Gii and LR-Gii was determined by examining intensity and position, of G, D, and 2D peaks as a function of light source conditions (i.e., wavelength and power). Results demonstrate that ST-Gii presents a 1.37 G/D intensity ratio, which is 0.2 lower than that observed in LR-Gii, evidencing the drastic change in the electronic properties of both materials obtained during the synthesis process. Moreover, the study of the materials as a function of the wavelength showed – in the case of ST-Gii material – a clear ‘optical switch’ behaviour from G/D of 1.37 to 0.79 when the material is irradiated with light below and above 532nm, respectively. This interesting result indicates that GiiTM could be a potential candidate for optoelectronic devices (phototransistors, photodiodes, and photodetectors), thanks to the modulation of its electronic properties by irradiating the material to different light wavelengths. To further investigate this effect, in this work, we present the concept of a photo-resistive detector based on ST-Gii

Pyrene-Appended Boronic Acids on Graphene Foam Electrodes Provide Quantum Capacitance-Based Molecular Sensors for Lactate

Molecular recognition and sensing can be coupled to interfacial capacitance changes on graphene foam surfaces linked to double layer effects and coupled to enhanced quantum capacitance. 3D graphene foam film electrodes (Gii-Sens; thickness approximately 40 μm; roughness factor approximately 100) immersed in aqueous buffer media exhibit an order of magnitude jump in electrochemical capacitance upon adsorption of a charged molecular receptor based on pyrene-appended boronic acids (here, 4-borono-1-(pyren-2-ylmethyl)pyridin-1-ium bromide, or abbreviated T1). This pyrene-appended pyridinium boronic acid receptor is employed here as a molecular receptor for lactate. In the presence of lactate and at pH 4.0 (after pH optimization), the electrochemical capacitance (determined by impedance spectroscopy) doubles again. Lactic acid binding is expressed with a Hillian binding constant (Klactate = 75 mol-1 dm3 and α = 0.8 in aqueous buffer, Klactate = 460 mol-1 dm3 and α = 0.8 in artificial sweat, and Klactate = 340 mol-1 dm3 and α = 0.65 in human serum). The result is a selective molecular probe response for lactic acid with LoD = 1.3, 1.4, and 1.8 mM in aqueous buffer media (pH 4.0), in artificial sweat (adjusted to pH 4.7), and in human serum (pH adjusted to 4.0), respectively. The role of the pyrene-appended boronic acid is discussed based on the double layer structure and quantum capacitance changes. In the future, this new type of molecular capacitance sensor could provide selective enzyme-free analysis without analyte consumption for a wider range of analytes and complex environments

Size-Selective Carbon Nanoclusters as Precursors to the Growth of Epitaxial Graphene

The nucleation and growth mechanisms of graphene on Rh(111) via temperature-programmed growth of C2H4 are studied by scanning tunneling microscopy and spectroscopy, and by density functional theory calculations. By combining our experimental and first principles approaches, we show that carbon nanoislands form in the initial stages of graphene growth, possessing an exclusive size of seven honeycomb carbon units (hereafter labeled as 7C6 ). These clusters adopt a domelike hexagonal shape indicating that bonding to the substrate is localized on the peripheral C atoms. Smoluchowski ripening is identified as the dominant mechanism leading to the formation of graphene, with the size-selective carbon islands as precursors. Control experiments and calculations, whereby coronene molecules, the hydrogenated analogues of 7C6 , are deposited on Rh(111), provide an unambiguous structural and chemical identification of the 7C6 building blocks.PostprintPeer reviewe

Coupling Epitaxy, Chemical Bonding, and Work Function at the Local Scale in Transition Metal-Supported Graphene

Resonance tunneling spectroscopy and density functional theory calculations are employed to explore local variations in the electronic surface potential of a single graphene layer grown on Rh(111). A work function modulation of 220 meV is experimentally measured, indicating that the chemical bonding strength varies significantly across the supercell of the Moire pattern formed when graphene is bonded to Rh(111). In combination with high-resolution images, which provide precise knowledge of the local atomic registry at the carbon metal interface, we identify experimentally, and confirm theoretically, the atomic configuration of maximum chemical bonding to the substrate. Our observations are at odds with reported trends for other transition metal substrates. We explain why this is the case by considering the various factors that contribute to the bonding at the graphene/metal interface.PostprintPeer reviewe

New class of metal bound molecular switches involving H-tautomerism

R.S. acknowledges the financial support from the Scottish Funding Council through EaStCHEM and SRDG grant HR07003.A potential end-point in the miniaturization of electronic devices lies in the field of molecular electronics, where molecules perform the function of single components. To date, hydrogen tautomerism in unimolecular switches has been restricted to the central macrocycle of porphyrin-type molecules. The present work reveals how H-tautomerism is the mechanism for switching in substituted quinone derivatives – a novel class of molecules with a different chemical structure. We hence reveal that the previous restrictions applying to tautomeric molecular switches bound to a surface are not valid in general. The activation energy of switching in a prototypical quinone derivative is determined using inelastic electron tunneling. Through computational modeling, we show that the mechanism underlying this process is tautomerisation of protons belonging to two amino groups. This switching property is retained upon functionalization by the addition of side groups, meaning that the switch can be chemically modified to fit specific applications.PostprintPeer reviewe

Ultra-thin graphene foam based flexible piezoresistive pressure sensors for robotics

Over recent years, robotics has made a drastic impact in a variety of different markets. Although robotics provides many advantages from, safer workspace to speed and efficiency there are several drawbacks. These all range from their lack of ability to execute functions and tasks easily performed by humans. This is mainly due to their lack of ability to implement touch and haptic feedback. In this work, we show the use and applicability of ultra-thin graphene foam (GRF), with polydimethylsiloxane (PDMS) embedded into and over the structure, as an active layer in piezoresistive based pressure sensors for use in robotic touch sensing applications. It has been demonstrated in this work that thin GRF/PDMS-GRF consisting of a few layers of graphene is able to present sensitivity to pressures within the range of 0 to >100 kPa. Although pressure sensitivities are not yet comparable to those of current work, it must be noted that the GRF used in this work is much thinner in comparison, consisting of only several layers of graphene

Electrodeposition of manganese dioxide coatings onto graphene foam substrates for electrochemical capacitors

Optimisation of electrodeposition routes of birnessite manganese dioxide (MnO2) coatings onto 3D graphene foam substrates enabled greater attainable capacitances. Current pulse deposition method resulted in highest achievable areal capacitance of 530 mF/cm2 under a 10 mA/cm2 current rate, cycling performance with 91% retention after 9000 cycles, as well as improved rate capability when compared to the cyclic voltammetry or galvanostatic deposition. Introduction of oxygen functional groups to the graphene foam added initial pseudocapacitance and accelerated the rate for nucleation and growth of the MnO2 crystal grains, resulting in an areal capacitance of 410 mF/cm2 under a 10 mA/cm2 current rate. However, in this case the increase in specific capacitance was accompanied by sluggish kinetics for charge storage seen via impedance spectroscopy. The charge storage mechanism of the deposited MnO2 films was investigated using in situ Raman microscopy and analysis of peak shifts revealed expansion and contraction of birnessite MnO2, relating to exchange of Na+ and H2O at the MnO2 interface

Estimating the range of influence of point defects on Cu(110) surface states

By utilising Reflection Anisotropy Spectroscopy (RAS) and Scanning Tunnelling Microscopy (STM) measurements of the ion bombarded Cu(110) surface at low temperatures, we have developed a simple methodology for estimating the surface area over which a single atomic defect locally influences surface states such that the contribution to the intensity of the 2.1 eV RAS peak involving these states is destroyed. We estimate this area to be approximately equal to that of a circle with a radius of 0.75 nm: an area in the surface plane equivalent to that of around 19 unit cells. By employing STM to accurately determine the coverage and spatial distribution of irradiation-induced defects, we are able to develop a coherent analytical approach to model this system