Efros-Shklovskii variable range hopping in reduced graphene oxide sheets of varying carbon sp2 fraction

We investigate the low temperature electron transport properties of chemically reduced graphene oxide (RGO) sheets with different carbon sp2 fractions of 55 to 80 %. We show that in the low bias (Ohmic) regime, the temperature (T) dependent resistance (R) of all the devices follow Efros-Shklovskii variable range hopping (ES-VRH) R ~ exp[(T(ES)/T)^1/2] with T(ES) decreasing from 30976 to 4225 K and electron localization length increasing from 0.46 to 3.21 nm with increasing sp2 fraction. From our data, we predict that for the temperature range used in our study, Mott-VRH may not be observed even at 100 % sp2 fraction samples due to residual topological defects and structural disorders. From the localization length, we calculate a bandgap variation of our RGO from 1.43 to 0.21 eV with increasing sp2 fraction from 55 to 80 % which agrees remarkably well with theoretical prediction. We also show that, in the high bias regime, the hopping is field driven and the data follow R ~ exp[(E(0)/E)^1/2] providing further evidence of ES-VRH.Comment: 13 pages, 6 figures, 1 tabl

Coulomb Blockade and Hopping Conduction in Graphene Quantum Dots Array

We show that the low temperature electron transport properties of chemically functionalized graphene can be explained as sequential tunneling of charges through a two dimensional array of graphene quantum dots (GQD). Below 15 K, a total suppression of current due to Coulomb blockade through GQD array was observed. Temperature dependent current-gate voltage characteristics show Coulomb oscillations with energy scales of 6.2-10 meV corresponding to GQD sizes of 5-8 nm while resistance data exhibit an Efros-Shklovskii variable range hopping arising from structural and size induced disorder.Comment: The document will be appeared in Physics Review

Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets

We elucidate on the low mobility and charge traps of the chemically reduced graphene oxide (RGO) sheets by measuring and analyzing temperature dependent current-voltage characteristics. The RGO sheets were assembled between source and drain electrodes via dielectrophoresis. At low bias voltage the conduction is Ohmic while at high bias voltage and low temperatures the conduction becomes space charge limited with an exponential distribution of traps. We estimate an average trap density of 1.75x10^16 cm^-3. Quantitative information about charge traps will help develop optimization strategies of passivating defects in order to fabricate high quality solution processed graphene devices.Comment: 6 pages, 3 figures, 1 tabl

High yield fabrication of chemically reduced graphene oxide field effect transistors by dielectrophoresis

We demonstrate high yield fabrication of field effect transistors (FET) using chemically reduced graphene oxide (RGO) sheets suspended in water assembled via dielectrophoresis. The two terminal resistances of the devices were improved by an order of magnitude upon mild annealing at 200 0C in Ar/H2 environment for 1 hour. With the application of a backgate voltage, all of the devices showed FET behavior with maximum hole and electron mobilities of 4.0 and 1.5 cm2/Vs respectively. This study shows promise for scaled up fabrication of graphene based nanoelectronic devices.Comment: 8 pages, 6 figure

Application of surface thermal lens technology combined with polarization to the study of polymers

A new approach was attempted to investigate the characteristics of polymers by using photothermal technology. The Surface Thermal Lens (STL) technique was employed to study polymers because of its higher spatial resolution and greater sensitivity than the classical photothermal detection techniques (PDT). Zeonex [Cyclo-Olefin-Polymer] and acrylic [Poly–Methyl- Meth-Acrylate] were used as the samples. Polarization was applied to the STL technique. The signals of Zeonex were different from those of acrylic when the STL probe beam was polarized. Two different polarizer orientations for the probe beam, crossed and parallel, were used to observe the STL signal response to the samples. No time dependence in the STL signals of both Zeonex and acrylic was observed when the probe beam was unpolarized, but time dependence of the signals was observed when the probe beam was polarized. Zeonex showed the most significant signal changes under the crossed-polarizer conditions, and acrylic showed the most significant changes under the parallel-polarizer conditions, indicating a difference in the response of the chain-like molecules to the heating beam. Therefore, STL techniques using polarized light may provide new insight into structural changes in polymers

Electronic Transport Investigation Of Chemically Derived Reduced Graphene Oxide Sheets

Reduced graphene oxide (RGO) sheet, a chemically functionalized atomically thin carbon sheet, provides a convenient pathway for producing large quantities of graphene via solution processing. The easy processibility of RGO sheet and its composites offer interesting electronic, chemical and mechanical properties that are currently being explored for advanced electronics and energy based materials. However, a clear understanding of electron transport properties of RGO sheet is lacking which is of great significance for determining its potential application. In this dissertation, I demonstrate fabrication of high-yield solution based graphene field effects transistor (FET) using AC dielectrophoresis (DEP) and investigate the detailed electronic transport properties of the fabricated devices. The majority of the devices show ambipolar FET properties at room temperature. However, the mobility values are found to be lower than pristine graphene due to a large amount of residual defects in RGO sheets. I calculate the density of these defects by analyzing the low temperature (295 to 77K) charge transport data using space charge limited conduction (SCLC) with exponential trap distribution. At very low temperature (down to 4.2 K), I observe Coulomb blockade (CB) and Efros-Shklovskii variable range hopping (ES VRH) conduction in RGO implying that RGO can be considered as a graphene quantum dots (GQD) array, where graphene domains act like QDs while oxidized domains behave like tunnel barriers between QDs. This was further confirmed by studying RGO sheets of varying carbon sp 2 fraction from 55 – 80 % and found that both the localization length and CB can be tuned. From the localization length and using confinement effect, we estimate tunable band gap of RGO sheets with varying carbon sp 2 fraction. I then studied one dimensional RGO nanoribbon iv (RGONR) and found ES VRH and CB models are also applicable to the RGONR. However, in contrast to linear behavior of decrease in threshold voltage (Vt) with increasing temperature (T) in the RGO, sub linear dependence of Vt on T was observed in RGONR due to reduced transport pathways. Finally, I demonstrate synthesis and transport studies of RGO/nanoparticles (CdS and CeO2) composite and show that the properties of RGO can be further tuned by attaching the nanoparticles

A Highly Conductive, Flexible, and 3D-printable Carbon Nanotube-Elastomer Ink for Additive Bio-manufacturing

The synthesis of a highly conductive, flexible, 3D-printable, and biocompatible ink has been of great interest in the field of bio-based additive manufacturing. Various applications include ultra-sensitive, microscale tactile sensors, patient-customizable scaffolds for cardiac and nerve tissue regeneration, and flexible electrocardiogram (ECG) electrodes. Here, a novel elastomeric carbon nanocomposite is presented consisting of amino-functionalized carbon nanotubes (CNT-NH2) homogenously dispersed in a one-part room-temperature vulcanizing (RTV) silicone matrix. The use of acetone as a swelling solvent aids in electrical percolation through the elastomer matrix. CNT-NH2 ratios can be tuned to fit various needs; higher tensile strength is favored at lower ratios while increased electrical conductivities are observed at greater ratios. Moreover, the fabrication process is facile, inexpensive, and can be conducted at room temperature within the span of minutes, due to the use of acetone as a unique volatile solvent. The resulting ink can be extrusion-printed to yield detailed, flexible, and highly conductive microstructures with resolution up to 250 microns. Future testing of the ink’s biocompatibility is necessary, but its electrical and mechanical properties demonstrate great potential for use in the additive manufacturing of patient-customizable bioelectronics.https://scholarscompass.vcu.edu/uresposters/1404/thumbnail.jp

Schottky diode via dielectrophoretic assembly of reduced graphene oxide sheets between dissimilar metal contacts

We demonstrate the fabrication of reduced graphene oxide (RGO) Schottky diodes via dielectrophoretic (DEP) assembly of RGO between two dissimilar metal contacts. Titanium (Ti) was used to make a Schottky contact, while palladium (Pd) was used to make an Ohmic contact. From the current-voltage characteristics, we obtain rectifying behavior with a rectification ratio of up to 600. The ideality factor was high (4.9), possibly due to the presence of a large number of defects in the RGO sheets. The forward biased turn-on voltage was 1V, whereas the reverse biased breakdown voltage was -3.1 V, which improved further upon mild annealing at 200 degrees C and can be attributed to an increase in the work function of RGO due to annealing

Two- to one-dimensional crossover in graphene quantum dot arrays observed in reduced graphene oxide nanoribbons

We investigate how the electron transport properties of graphene quantum dot (GQD) arrays transition from two dimensions (2D) to one dimension (1D) in lithographically defined reduced graphene oxide nanoribbons (RGONRs). From the low-temperature electron transport measurements of 200-, 100-, and 50-nm-wide RGONRs, we find that the energy barrier for charge transport increases with decreasing RGONR width in both the Coulomb blockade and the variable-range hopping regime. Different charge transport parameters for 200-nm RGONR are in agreement with 2D transport while these parameters show a gradual transition to 1D transport in 50-nm RGONR

Finite Element Analysis of 3D-printed PCL Scaffolds

Finite Element Analysis of 3D-printed PCL Scaffolds for Synergizing Cellular Micro-Environment and Mechanical Stimuli to Enhance Engineered Tissue Growth in Vitro Ireolu Orenuga,1 Phillip Glass,2 Daeha Joung,2 Joao S. Soares1 Department of Mechanical and Nuclear Engineering, College of Engineering, Virginia Commonwealth University Department of Physics, College of Humanities & Sciences, Virginia Commonwealth University Introduction: Tissue engineering aims to create viable and functional engineered tissues via biodegradable scaffolds and autologous cells. Scaffolds play an essential part in organizing the architecture of developing tissues and aid in the proper function of implants acutely by serving as mechanical support and long-term by degrading and undergoing absorption as de novo tissue is produced. Polycaprolactone (PCL) is a commonly used biodegradable and biocompatible material. PCL scaffolds are typically electrospun or nonwoven, which produces random microstructures without a very robust control. 3D printed PCL scaffolds allow for the design of a structured and controlled cellular micro-environment and mechanical properties for cells to grow in vitro. However, the translation of mechanical training at the tissue level happening in bioreactors during in vitro culture to cellular stimulation is poorly understood. Finite Element Analysis (FEA) of 3d-printed PCL scaffolds may elucidate the different roles of microstructural parameters in cellular mechanotransduction and provide insight on how to effectively engineer better tissues. Methods: SolidWorks was used to make four models of 3D-printed PCL fibers with 0-90 and 45-45 orientations in two configurations, i.e. aligned and staggered fibers. The completed 0-90 and 45-45 orientations staggered model was exported as an .stl file to 3D-print. SEM imaging analyzing the 3d-printing results were conducted to obtain realistic microstructures of the 3D-printed PCL scaffolds. These 3D-models were imported into FEBio for FEA analysis of the deformations involved with in vitro mechanical training up to a uniaxial strain of 50% (aligned with the 0-degree orientation). Additionally, we have performed exploratory cell seeding experiments on some scaffolds to analyze seeding efficiency and mechanical testing of virgin scaffolds to validate the FEA results. Results: The 3D-printed models produced were a 0-90 and a 45-45 staggered configuration scaffold each with dimensions of 8 x 20 x 0.45mm thickness with 16 layers. PCL fibers were printed as ribbons with 10 by 85 microns and spaced apart creating pores of 235 by 235 microns. Meshing both configurations was successful and FEA simulations showed that scaffolds can undergo strains of up to 33% before pores start collapsing. Mechanical tests showed the scaffold was able to undergo strains of up to 10% in the elastic regime without appreciating damage. Drip seeding experiments showed that RVSMCs are able to survive and grow on the 3D printed PCL scaffolds. Conclusions: FEA has proven to be useful for predicting mechanical behavior of 3D printed PCL scaffolds. Future experiments will focus on conducting mechanical testing on more scaffolds and in vitro engineered tissue culture under mechanical stimulation.https://scholarscompass.vcu.edu/uresposters/1462/thumbnail.jp