Date of Award
Open Access Dissertation
Graphene, a two-dimensional material with very high charge carrier concentration, has drawn large research interest for sensing chemical species based upon charge exchange. Atomically thin 2-dimensional arrangement of carbon in hexagonal fashion in graphene, where each carbon atom is attached to 3 neighboring carbon atoms, and presence of π* and π bands imparts it many amazing properties. Some of these properties such as very high mobility, low 1/f and thermal noise, modulation of carrier concentration and Fermi level by electrical, optical, and chemical means, and very high surface to volume ratio make graphene very promising sensing material. In order to exploit these amazing properties for practical applications a reliable synthesis of high quality, large area graphene is needed. Chemical vapor deposition (CVD) based synthesis offers reliable, scalable, and inexpensive method to make low defect, uniform, large area, good quality, thinner graphene with the ability to transfer graphene on any desirable substrate. In this work, high quality single layer graphene has been synthesized by CVD for sensing applications. The growth process was optimized to yield good quality monolayer graphene, which uses CH4 and H2 as precursor gases for the growth at 1035 °C, as characterized by Raman spectroscopy.
Widely employed transduction mechanism in graphene chemical sensors or chemiresistor is conductance change due to charge exchange between graphene and vi adsorbed molecules. The reported sensitivities have been fairly low and selectivity is difficult to observe without functionalization. This work aims at improving the sensitivity of graphene sensors by three different approaches. In the first approach, the use of a global back-gate in graphene chem-FET devices has shown improvement in sensitivity and imparts selectivity as well. These devices exploit the back-gate induced Fermi level movement of graphene relative to defect level of analytes such as electron accepting NO2 and electron withdrawing NH3 molecules. In the second approach, the defect density in graphene has been used to show sensitivity enhancement.
In these two approaches the sensitivity enhancement has the limitation of linear dependence of conductivity change to that of numbers of adsorbed molecules. In the third approach the use a graphene/Si heterostructure based Schottky device or chemi-diode, has been proposed for improving sensitivity many folds. Since graphene work function can be varied electrically or chemically, the Schottky barrier height (SBH) at graphene/Si interface also varies accordingly affecting the carrier transport across the Schottky barrier. These devices take advantage of graphene’s atomically thin nature, which enables molecular adsorption on its surface to directly alter graphene/Si SBH, thus affecting the junction current exponentially when operated in reverse bias and resulting in very high sensitivity. The sensing mechanism based on SBH change has also been confirmed by capacitance-voltage measurements. By operating the devices in reverse bias, the work function of graphene, and hence SBH of the chemi-diode, can be controlled by the bias magnitude, leading to a wide tunability of the molecular detection sensitivity towards NO2 and NH3 with very low power consumption. Optimized sensor design to detect particular analyte is also possible by careful selection of graphene/Si SBH. The use of Pd and Pt nano-particles on top of graphene as a functionalization layer serves to increase the capability of these chemi-diodes in sensing analytes such as H2 which have very weak interaction with graphene. Therefore CVD graphene based sensors have been found to be very promising for practical applications in chemical sensing in ambient conditions which shows much improved sensitivity, and even selectivity towards hazardous gases.
Singh, A. K.(2014). TUNABLE GRAPHENE CHEM-FET AND GRAPHENE/SI HETEROJUNCTION CHEMI-DIODE SENSORS. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/2879