Session H12: Chemical Doping of Graphene and Applications: Solar Cells, Sensors

I-V characteristics of orgarnic molecules coated graphene

Thin layers of organic molecules, OTS and DMF, are coated on top of graphene which was extracted by the exfoliation. I-V characteristics of the organic molecules coated graphene were investigated at low temperature by varying the gate voltage. P-doping with slight enhancement of mobility was observed with the OTS coating. On the other hand, n-doping with relatively higher enhancement of mobility was obtained with the DMF coating. CNT is nested on top of the organic molecules coated graphene by dielectrophoresis and Coulomb blockade was observed in the sandwich device.   

Investigation of Electrochemical Gate Controlled Charge Transport in Large Area Boron-Nitrogen Doped Graphene

We report on the investigation of charge transport measurements of B and N doped graphene C (B,N) under the influence of an electrochemical gate. These C (B,N) systems are expected to have unique electronic properties due to the combination of impurities including both atomistically separated B and N species, as well as hexagonal boron nitride ($h-$BN) units within the graphitic C lattice. Investigations were performed on large area BN doped graphene devices fabricated with different BN doping levels. The electrochemically gate controlled interfacial capacitance and quantum capacitance of BN doped graphene devices were measured. The effect of doping on the quantum capacitance and electron mobility will be discussed.   

Stable Chemical Doping of Graphene for Low Electrical Resistance and High Optical Transparency

Since becoming experimentally available, graphene has been used in various devices such as field effect transistors (FETs), solar cells and sensing applications. Although graphene based devices with modest characteristics have been reported, in some device geometries a lower graphene resistance with different Fermi level values is still desired. Here, we describe our use of a hydrophobic organic dopant with strong electronegativity, environmental stability and high optical transmittance which is spin cast onto CVD-grown graphene films. We observe a typical 70{\%} reduction in resistance upon chemical modification of the graphene. Magnetoresistance measurements imply that the modified graphene is hole doped, and time-dependent resistance measurements show excellent stability. Using Raman spectroscopy we confirm the doping of graphene sheets from the shifts in G and 2D peak positions and intensity ratios. We show transmittance and SEM characteristics of the graphene sheets before and after doping. These results may serve as a guide for modification of graphene's properties as desired for various applications.   

Reversible and Robust Carrier Doping in Graphene \textit{via} Mechanical Actuation of Tethered Azobenzene

machines -- molecules capable of responding to external stimuli - have gained great interest due to their applications in molecular actuation nanodevices. In this talk, we demonstrate that ultrathin graphene exhibits high-sensitivity to orientation, surface-vicinity, electronegativity, and density-of-states of interfaced molecules. This enables the realization of reversible doping of graphene \textit{via} molecular mechanics on its surface. Here, few-layer-graphene (FLG) sheets were functionalized with electronegative and isomerizable azobenzene-molecules. The optical transformation of these azo-molecules from their \textit{trans} conformation to \textit{cis} conformation dopes 7.5 X 10$^{3}$ holes/$\mu $m$^{2}$ in the underlying graphene. This corresponds to $\sim $20 azobenzene molecules producing 1 hole (hole-mobility of 301 $\mu $m$^{2}$/V/s) in the azobenzene-FLG (AFLG) device. Further, we demonstrate the facile fabrication of the AFLG device and the mechanism of electrical modulation due to molecular mechanics. We also compare the response of the AFLG device with an FLG device directly doped with electronegative perylene tetracarboxylic acid, which led to $\sim $3 fold increase in the hole density. -abstract-   

Tuning the Electrical Properties of Large Area Graphene through Boron-Nitrogen Co-Doping

We report on investigation of low temperature electrical transport measurements of B and N co-doped graphene layers C (B, N). We find that the temperature dependence of resistance (5K $<$ T $<$ 400 K) of pure graphene shows a metallic behavior, whereas the C (B, N) samples show an increasingly semiconducting behavior with increasing doping levels. Within the studied temperature range, at higher temperatures, the doped samples showed a band-gap dominated Arrhenius-like temperature dependence. At the lowest temperatures, the temperature dependence deviates from an activated behavior, and presents evidence for a conduction mechanism that is consistent with Mott's 2D-Variable Range Hopping (2D-VRH).   

Raman studies of ultra-clean graphene on hexagonal boron nitride with controlled doping

Graphene prepared by exfoliation on hexagonal boron nitride (h-BN) provides an ideal platform for studies of the intrinsic properties of Dirac electrons because of its unprecedented charge homogeneity. With this system, many of the fascinating phenomena hidden by charge inhomogeneity in conventional graphene samples have recently been revealed. Here we describe progress in examining both electron-phonon and electron-electron interactions by means of Raman scattering by the G- and the 2D-modes. In our studies, we were able to observe a symmetric energy shift in the Raman 2D peak at low doping levels when the Fermi level was tuned from the electron side to the hole side. This shift is understood as a change in the double-resonance condition induced by the renormalization of the electron and phonon band structures. At the same time, the 2D peak is broadened under electron or hole doping. Additionally, we observed very weak doping dependence of the G peak (both position and width) at Fermi energies less than half of the phonon energy and the subsequent usual removal of non-adiabatic Kohn anomaly with increased doping, which reflects again the extremely homogeneous charge distribution in our samples.   

Scanning Tunneling Microscope (STM) Study of B-doped Monolayer Graphene

Chemical doping is a promising technique to tailor the electronic properties of graphene. Here we focus on an atomic scale characterization of Boron-doped monolayer graphene sheets using primarily STM, assisted by Raman Spectroscopy and X-ray Absorption Spectroscopy (XAS). We will show in topography that there are two major structures that result from B-doping, and in spectroscopy that each of these structures plays different roles in modifying the electronic properties of graphene. Raman Spectroscopy and XAS provide complementary information about the nature of the B-C bonds in the sample.   

Substituent Effects for the Control of Covalent Electronic Doping of CVD Graphene

Controlling the electrical properties of graphene is of particular interest for future electronic, optoelectronics and sensing applications. We investigate the doping effect of different chemical functional groups covalently attached to CVD-grown graphene devices with a polymer electrolyte top-gate. The covalent reaction is based on a diazonium chemistry, specifically the type and degree of doping for a diazonium salt with a nitro group, a bromo group or an alkyne group attached are investigated. We use three different approaches to inspect various aspect of the doping in graphene: Gaussian calculations, Raman measurements and transfer-characteristics. The transfer curves show that nitro groups induce p-doping while the bromo- and alkyne groups induce n-doping. A new model that takes into account both coulombic and resonant scattering as well as a asymmetric electron and hole transport is developed to fit the transfer-curves. The graphene transistors are very robust and reproducible, suggesting this is a simple and facile way to control the electronic properties of graphene.   

Charge transfer and density of states modifications of graphene upon molecular adsorption - Implications for gas and molecular sensors

The adsorption of molecules on single layer graphene can result in significant modifications to the band structure and density of states near the Dirac point and can result in the introduction of scattering centres which can modify the carrier mobility. Understanding how the competing interactions of increased carrier density and density of scattering centres is therefore an important consideration in the description of the properties of graphene. We have used ab initio methods to explore the degree of charge transfer, modification to the band structure and density of states associated with the adsorption of a range of open and closed shell molecules, organometallic molecules and planar organic molecules. We show how the charge transfer can be related to the position of the molecule related energy levels on adsorption relative to the Dirac point. We find low levels ($<$0.05e) of charge transfer for NH$_{3}$, NO and NO$_{2}$ molecules but larger values for cobaltocene (n-type, 0.31 e/molecule) and about 0.3 e/molecule for the organic molecules TDAE (n-type) and DDQ (p-type) respectively. These molecules open up ways to dope graphene to high levels and are important considerations in sensing. We also discuss the factors that control the charge transfer.   

Biochemical Sensing in Solution Gated Graphene Field Effect Transistors

Epitaxial graphene is a promising material for the construction of label-free chemical and biochemical sensors. In this work, graphene of few-layers is used as a sensor for pH and ionic strength in a solution gated field effect transistor (SGFET). In order to improve the sensitivity of the SGFET to pH and ionic changes the transistor is connected in a four-point (van der Pauw) configuration. Results for the shift in the Dirac point when the pH or ionic strength is changed are shown.   

Combined transport-Scanning Probe Microscopy study of reduced graphene oxide sensors

We present an in-depth study of the sensing properties of chemically reduced graphene oxide (rGO) based devices. Graphene oxide is an electronically hybrid material that can be controllably tuned from an insulator to a semiconductor material via reduction chemistry. Due to their chemical structure and large surface to volume ratio rGO sensors can detect gas adsorption at very low concentrations. rGO devices are created by dielectrophoretic assembly of rGO platelets onto interdigitated electrode arrays, which are lithographically pre-patterned on top of SiO2/Si wafers. The gas sensing properties of these devices are characterized using novel combined transport-Scanning Kelvin Probe Microscopy and transport-Electrostatic Force Microscopy measurements in the presence of different gas analytes. These measurements show unique, very sensitive and repeatable responses to various volatile organic compounds and other gases. Maps of the electrostatic potential and charge distribution across these circuits are used to model the dynamics of electronic transport through the rGO system.   

Biomimetic graphene sensors: functionalizing graphene with peptides

Non-covalent biomimetic functionalization of graphene using peptides is one of more promising methods for producing novel sensors with high sensitivity and selectivity. Here we combine atomic force microscopy, Raman spectroscopy, and attenuated total reflection Fourier transform infrared spectroscopy to investigate peptide binding to graphene and graphite. We choose to study a dodecamer peptide identified with phage display to possess affinities for graphite and we find that the peptide forms a complex mesh-like structure upon adsorption on graphene. Moreover, optical spectroscopy reveals that the peptide binds non-covalently to graphene and possesses an optical signature of an ?-helical conformation on graphene.   

Solution-gated graphene field-effect transistors as local pH sensors in microfluidic systems

We report a study of solution-gated graphene field-effect transistors (SGGFETs) as high-performance local pH sensors in microfluidic devices. Previous experiments have shown that SGGFETs can function as pH sensors for bulk volumes of solutions, and a response of ~20 mV/(unit pH) shift in the Dirac point was typically observed. In our study, we investigated SGGFETs micro-fabricated out of CVD graphene grown on copper foil and found a robust pH sensitivity of ~50 mV/(unit pH). This value is close to the thermodynamically allowed maximum value, i.e., the Nernst value of 59 mV/(unit pH) at room temperature. We further integrated the SGGFETs into microfluidic systems for lab-on-chip applications. We found the SGGFETs are capable of real-time detection of local pH changes in microfluidic channels, thus providing reliable measurement of the local pH for small volumes of liquids (a few nL). Possible applications of this microfluidic detection system, for example, in monitoring chemical diffusion and reactions in microfluidic channels, will be discussed.   

A low cost construction method for Graphene based resistive chemical sensors

Graphene is a 2D material with distinctive properties and a large surface area that can be exposed to surface adsorbates from a target gas, making it attractive as a sensing material. This enables studies on the interaction of gas molecules with the graphene surface and subsequent changes in its properties. Due to its high electron mobility at room temperature, graphene exhibits high sensitivity, making it a good candidate for environmental and industrial sensing applications. Several models of graphene based sensors have been put forth previously based on high-resolution lithographic techniques and for individual electrode attachment to the sensing film with e-beam lithography. These techniques can produce small numbers of devices that explore the limits of molecular scale sensing, but the methods are currently impractical for large scale production of low cost sensors. We present our graphene based sensor with the focus on designing small, cost effective and reliable sensors with high sensitivity towards the target gas, detailing the assembly of graphene/acrylic devices, their characterization and investigation of their performance as resistive chemical sensors using differential voltage measurements.   

Finite temperature quantum transport in nanosensors based on graphene nanoribbons

We study finite temperature quantum conductance of nanosensors based on graphene nanoribbons exposed to carbon and nitrogen oxides. Using ab-initio-based Green's function formalism, the quantum conductance of the nanoribbon with and without adsorbed oxide molecules is calculated. We investigate the effects of molecular vibrations and electron-vibron coupling on conductance modulation. The implication of the results concerning nanosensor functionality under desired environmental temperatures, and the differences with the low-temperature cases, are discussed