Ruth Pearce

Standardization of surface potential measurements of graphene domains

We compare the three most commonly used scanning probe techniques to obtain a reliable value of the work function in graphene domains of different thickness. The surface potential (SP) of graphene is directly measured in Hall bar geometry via a combination of electrical functional microscopy and spectroscopy techniques, which enables calibrated work function measurements of graphene domains in ambient conditions with values Φ1LG ~4.55 ± 0.02 eV and Φ2LG ~ 4.44 ± 0.02 eV for single- and bi-layer, respectively. We demonstrate that frequency-modulated Kelvin probe force microscopy (FM-KPFM) provides more accurate measurement of the SP than amplitude-modulated (AM)-KPFM. The discrepancy between experimental results obtained by different techniques is discussed. In addition, we use FM-KPFM for contactless measurements of the specific components of the device resistance. We show a strong non-Ohmic behavior of the electrode-graphene contact resistance and extract the graphene channel resistivity.

The influence of substrate morphology on thickness uniformity and unintentional doping of epitaxial graphene on SiC

A pivotal issue for the fabrication of electronic devices on epitaxial graphene on SiC is controlling the number of layers and reducing localized thickness inhomogeneities. Of equal importance is to understand what governs the unintentional doping of the graphene from the substrate. The influence of substrate surface topography on these two issues was studied by work function measurements and local surface potential mapping. The carrier concentration and the uniformity of epitaxial graphene samples grown under identical conditions and on substrates of nominally identical orientation were both found to depend strongly on the terrace width of the SiC substrate after growth.

Recent trends in Silicon Carbide (SiC) and Graphene based gas sensors

The introduction of silicon carbide (SiC) as the semiconductorin gas sensitive field effect devices has tremendously improved this sensor platform extending the temperature range and number of dete ...

ZnO materials and surface tailoring for biosensing.

ZnO nanostructured materials, such as films and nanoparticles, could provide a suitable platform for development of high performance biosensors due to their unique fundamental material properties. This paper reviews different preparation techniques of ZnO nanocrystals and material issues like wettability, biocompatibility and toxicity, which have an important relevance to biosensor functionality. Efforts are made to summarize and analyze existing results regarding surface modification and molecular attachments for successful biofunctionalization and understanding of the mechanisms involved. A section is devoted to implementations of tailored surfaces in biosensors. We end with conclusions on the feasibility of using ZnO nanocrystals for biosensing.

On the differing sensitivity to chemical gating of single and double layer epitaxial graphene explored using scanning Kelvin probe microscopy.

Using environmental scanning Kelvin probe microscopy, we show that the position of the Fermi level of single layer graphene is more sensitive to chemical gating than that of double layer graphene. We calculate that the difference in sensitivity to chemical gating is not entirely due to the difference in band structure of 1 and 2 layer graphene. The findings are important for gas sensing where the sensitivity of the electronic properties to gas adsorption is monitored and suggest that single layer graphene could make a more sensitive gas sensor than double layer graphene. We propose that the difference in surface potential between adsorbate-free single and double layer graphene, measured using scanning kelvin probe microscopy, can be used as a noninvasive method of estimating substrate-induced doping in epitaxial graphene.

The influence of gate bias and structure on the CO sensing performance of SiC based field effect sensors

SiC based Field Effect Transistor gas sensors with Pt as gate material have previously been shown to exhibit a binary CO response, sharply switching between a small and a large value with increasing CO or decreasing O2 concentration or temperature. In this study Pt gates with different structures have been fabricated by dc magnetron sputtering at different argon pressures and subjected to various CO/O2 mixtures under various temperatures and gate bias conditions. The influence of gate bias and gate structure on the CO response switch point has been investigated. The results suggest that the more porous the gate material or smaller the bias, the lower the temperature or higher the CO concentration required in order to induce the transition between a small and a large response towards CO. These trends are suggested to reflect the adsorption, spill-over, and reaction characteristics of oxygen chemisorbed to the Pt and insulator surfaces.

New transducer material concepts for biosensors and surface functionalization

Wide bandgap materials like SiC, ZnO, AlN form a strong platform as transducers for biosensors realized as e.g. ISFET (ion selective field effect transistor) devices or resonators. We have taken two main steps towards a multifunctional biosensor transducer. First we have successfully functionalized ZnO and SiC surfaces with e.g. APTES. For example ZnO is interesting since it may be functionalized with biomolecules without any oxidation of the surface and several sensing principles are possible. Second, ISFET devises with a porous metal gate as a semi-reference electrode are being developed. Nitric oxide, NO, is a gas which participates in the metabolism. Resistivity changes in Ga doped ZnO was demonstrated as promising for NO sensing also in humid atmosphere, in order to simulate breath.

Development of FETs and Resistive Devices Based on Epitaxially Grown Single Layer Graphene on SiC for Highly Sensitive Gas Detection

Epitaxially grown single layer graphene on silicon carbide (SiC) resistive sensors were characterised for NO2 response at room and elevated temperatures, with an n-p type transition observed with increasing NO2 concentrations for all sensors. The concentration of NO2 required to cause this transition varied with different graphene samples and is attributed to varying degrees of substrate induced Fermi-level pinning above the Dirac point. The work function of a single layer device demonstrated a steady increase in work function with increasing NO2 concentration indicating no change in reaction mechanism in the concentration range measured despite a change in sensor response direction. Epitaxially grown graphene device preparation is challenging due to poor adhesion of the graphene layer to the substrate. A field effect transistor (FET) device is presented which does not require wire bonding to contacts on graphene.

Towards optimisation of epitaxially grown graphene based sensors for highly sensitive gas detection

Epitaxially grown single-layer and many-layer (10 atomic layers thick) resistive graphene devices were fabricated and compared for response towards NO2. Single-layer devices showed far greater sensitivity. The many-layer devices reduced in resistance on exposure to electron withdrawing NO2 demonstrating a majority hole carriers (p-type), whereas the single-layer device demonstrated an increase in resistance upon NO2 exposure demonstrating a majority of electron carriers (n-type) An n-p shift is observed for the single-layer device upon exposure to increasing concentrations of NO2. This shift is thought to be due to the reduction of electrons in the conduction band upon adsorption of electron-withdrawing NO2 making holes the majority carriers.

Effect of water vapour on gallium doped zinc oxide nanoparticle sensor gas response

Zinc oxide is a wide band gap (~3.4 ev) semiconductor material, making it a promising material for high temperature applications, such as exhaust and flue environments where NO and NO2 monitoring is increasingly required due to stricter emission controls. In these environments water vapour and background levels of oxygen are present and, as such, the effect of humidity on the sensing characteristics of these materials requires further study. The reaction mechanisms in the presence of water vapour are poorly understood and there is a need for deeper understanding of the principles and mechanisms of gas response of these materials. An investigation of the influence of changing water vapour (H2O) and oxygen (O2) backgrounds on the response of nanoparticulate Ga-doped ZnO resistive sensors is presented.