Tuning graphene transistors through ad hoc electrostatics induced by a nanometer-thick molecular underlayer.

We report on the modulation of the electrical properties of graphene-based transistors that mirror the properties of a few nanometers thick layer made of dipolar molecules sandwiched in between the 2D material and the SiO2 dielectric substrate. The chemical composition of the films of quinonemonoimine zwitterion molecules adsorbed onto SiO2 has been explored by means of X-ray photoemission and mass spectroscopy. Graphene-based devices are then fabricated by transferring the 2D material onto the molecular film, followed by the deposition of top source-drain electrodes. The degree of supramolecular order in disordered films of dipolar molecules was found to be partially improved as a result of the electric field at low temperatures, as revealed by the emergence of hysteresis in the transfer curves of the transistors. The use of molecules from the same family, which are suitably designed to interact with the dielectric surface, results in the disappearance of the hysteresis. DFT calculations confirm that the dressing of the molecules by an external electric field exhibits multiple minimal energy landscapes that explain the thermally stabilized capacitive coupling observed. This study demonstrates that the design and exploitation of ad hoc molecules as an interlayer between a dielectric substrate and graphene represents a powerful tool for tuning the electrical properties of the 2D material. Conversely, graphene can be used as an indicator of the stability of molecular layers, by providing insight into the energetics of ordering of dipolar molecules under the effect of electrical gating.

Simple Interface Engineering of Graphene Transistors with Hydrophobizing Stamps.

We demonstrate a simple surface engineering method for fabricating graphene transistors by using hydrophobizing stamps. By simply contact-printing hydrophobizing stamp that is made with polydimethylsiloxane (PDMS) on a standard silicon substrate for a certain contact-time, it was possible to control the contact angle of the substrate and electrical characteristics of the graphene transistors supported on the substrate. Moreover, graphene transistors supported on the engineered silicon substrate showed improved performances, including an increase in carrier mobility and loss of hysteresis. As a proof-of-concept experiment, a simple logic gate operation was demonstrated by connecting a pristine graphene device with an interface-engineered device.

Performance assessments of vertically aligned carbon nanotubes multi-electrode arrays using Cath.a-differentiated (CAD) cells.

In this work, Cath.a-differentiated (CAD) cells were used in place of primary neuronal cells to assess the performance of vertically aligned carbon nanotubes (VACNTs) multi-electrode arrays (MEA). To fabricate high-performance MEA, VACNTs were directly grown on graphene/Pt electrodes via plasma enhanced chemical deposition technique. Here, graphene served as an intermediate layer lowering contact resistance between VACNTs and Pt electrode. In order to lower the electrode impedance and to enhance the cell adhesion, VACNTs-MEAs were treated with UV-ozone for 20 min. Impedance of VACNTs electrode at 1 kHz frequency exhibits a reasonable value (110 kΩ) for extracellular signal recording, and the signal to noise ratio the is good enough to measure low signal amplitude (15.7). Spontaneous firing events from CAD cells were successfully measured with VACNTs MEAs that were also found to be surprisingly robust toward the biological interactions.

Effects of substrate conductivity on cell morphogenesis and proliferation using tailored, atomic layer deposition-grown ZnO thin films.

We demonstrate that ZnO films grown by atomic layer deposition (ALD) can be employed as a substrate to explore the effects of electrical conductivity on cell adhesion, proliferation, and morphogenesis. ZnO substrates with precisely tunable electrical conductivity were fabricated on glass substrates using ALD deposition. The electrical conductivity of the film increased linearly with increasing duration of the ZnO deposition cycle (thickness), whereas other physical characteristics, such as surface energy and roughness, tended to saturate at a certain value. Differences in conductivity dramatically affected the behavior of SF295 glioblastoma cells grown on ZnO films, with high conductivity (thick) ZnO films causing growth arrest and producing SF295 cell morphologies distinct from those cultured on insulating substrates. Based on simple electrostatic calculations, we propose that cells grown on highly conductive substrates may strongly adhere to the substrate without focal-adhesion complex formation, owing to the enhanced electrostatic interaction between cells and the substrate. Thus, the inactivation of focal adhesions leads to cell proliferation arrest. Taken together, the work presented here confirms that substrates with high conductivity disturb the cell-substrate interaction, producing cascading effects on cellular morphogenesis and disrupting proliferation, and suggests that ALD-grown ZnO offers a single-variable method for uniquely tailoring conductivity.

Novel fabrication of flexible graphene-based chemical sensors with heaters using soft lithographic patterning method.

We have fabricated graphene-based chemical sensors with flexible heaters for the highly sensitive detection of specific gases. We believe that increasing the temperature of the graphene surface significantly enhanced the electrical signal change of the graphene-based channel, and reduced the recovery time needed to obtain a normal state of equilibrium. In addition, a simple and efficient soft lithographic patterning process was developed via surface energy modification for advanced, graphene-based flexible devices, such as gas sensors. As a proof of concept, we demonstrated the high sensitivity of NO2 gas sensors based on graphene nanosheets. These devices were fabricated using a simple soft-lithographic patterning method, where flexible graphene heaters adjacent to the channel of sensing graphene were utilized to control graphene temperature.

Photoconductivity and enhanced memory effects in hybrid C60-graphene transistors.

We describe the observation of photoconductivity and enhanced memory effects in graphene devices functionalized with clusters of alkylated C(60) molecules. The alkylated C(60) clusters were adsorbed on chemical vapor deposition-grown graphene devices from an aprotic medium. The resulting alkylated C(60)-graphene hybrid devices showed reproducible photoconductive behavior originating from the electron-accepting nature of the C(60) molecules. Significantly enhanced gate hysteresis was observed upon illumination with visible light, thereby enabling the use of C(60)-graphene hybrid devices in three-terminal photo-memory applications.

Vertically aligned carbon nanotube electrodes directly grown on a glassy carbon electrode.

Three-dimensional microelectrodes were fabricated using glassy carbon electrodes combined with vertically aligned carbon nanotubes (VACNTs). VACNTs were grown on various conducting electrode patterns including a carbon electrode fabricated by pyrolysis of a negative photoresist, with plasma-enhanced chemical vapor deposition using a bilayer Fe/Al catalyst. VACNT electrodes grown on the glassy carbon showed excellent electrochemical behavior, whereas VACNT electrodes grown on Pt showed poor electrochemical performance, presumably due to the poor contact between VACNTs and the Pt electrode. Electron microscopy showed that the VACNT layer was strongly bound to the carbon electrode, while that on Pt tended to peel away. The versatility of the all-carbon microelectrodes was also tested by using them for interfacing stem cells. Their superior mechanical properties and the electrical connectivity between the carbon electrode and the VACNTs, along with the simple fabrication process, suggest that glassy carbon may be a good conducting substrate for VACNT electrodes.