Marcelo A. Kuroda

Chemical doping of large-area stacked graphene films for use as transparent, conducting electrodes

Graphene is considered a leading candidate to replace conventional transparent conducting electrodes because of its high transparency and exceptional transport properties. The effect of chemical p-type doping on graphene stacks was studied in order to reduce the sheet resistance of graphene films to values approaching those of conventional transparent conducting oxides. In this report, we show that large-area, stacked graphene films are effectively p-doped with nitric acid. The doping decreases the sheet resistance by a factor of 3, yielding films comprising eight stacked layers with a sheet resistance of 90 Omega/(square) at a transmittance of 80%. The films were doped either after all of the layers were stacked (last-layer-doped) or after each layer was added (interlayer-doped). A theoretical model that accurately describes the stacked graphene film system as a resistor network was developed. The model defines a characteristic transfer length where all the channels in the graphene films actively contribute to electrical transport. The experimental data shows a linear increase in conductivity with the number of graphene layers, indicating that each layer provides an additional transport channel, in good agreement with the theoretical model.

Electrical and optical properties of graphene mono- and multi-layers; towards graphene-based optoelectronics

Graphene, a newly discovered material with unusual electrical and optical properties, has attracted interest for a number of potential applications. One of the most actively pursued applications is using graphene as a transparent conducting electrode for use in solar cells, displays or touch screens. In this work, two studies are pursued in parallel to explore the electrical and optical properties of graphene. Graphene was prepared on copper by the standard chemical vapor deposition (CVD) method [1], the preparation procedure and conditions are described in [2]. The effect of chemical p-type doping on graphene stacks was studied in order to reduce the sheet resistance of graphene. The doping decreases the sheet resistance by a factor of 3, yielding films comprised of eight stacked layers with a sheet resistance of 90 Ω/ at a transmittance of 80% [2]. A theoretical model that accurately describes the stacked graphene film system as a resistor network was also developed [2]. The experimental data shows a linear increase in conductivity with the number of graphene layers, indicating that each layer provides an additional transport channel, in good agreement with the theoretical model (Fig. 1).

High-response piezoelectricity modeled quantitatively near a phase boundary

Interconversion of mechanical and electrical energy via the piezoelectric effect is fundamental to a wide range of technologies. The discovery in the 1990s of giant piezoelectric responses in certain materials has therefore opened new application spaces, but the origin of these properties remains a challenge to our understanding. A key role is played by the presence of a structural instability in these materials at compositions near the “morphotropic phase boundary” (MPB) where the crystal structure changes abruptly and the electromechanical responses are maximal. Here we formulate a simple, unified theoretical description which accounts for extreme piezoelectric response, its observation at compositions near the MPB, accompanied by ultrahigh dielectric constant and mechanical compliances with rather large anisotropies. The resulting model, based upon a Landau free energy expression, is capable of treating the important domain engineered materials and is found to be predictive while maintaining simplicity...

The PiezoElectronic Switch: A Path to High Speed, Low Energy Electronics

In contrast to the Moore’s Law exponential growth in CMOS transistor areal density, computer clock speeds have been frozen since 2003 due to excessive power dissipation. We present the development of a new digital switch, the PiezoElectronic Transistor (PET), designed to circumvent the speed and power limitations of the CMOS transistor. The PET operates on a novel principle: an electrical input is transduced into an acoustic pulse by a piezoelectric (PE) actuator, which, in turn, drives a continuous insulator-to-metal transition in a piezoresistive (PR) channel, thus switching on the device. Predictions of theory and simulation, assuming bulk materials properties can be approximately retained at scale, are that PETs can operate at one-tenth the present voltage of CMOS technology and 100 times less power, while running at multi-GHz clock speeds. CMOS-like computer architectures, such as a simulated adder, can be fully implemented. Materials development for PE and PR thin films approaching the properties of bulk single crystals, and a successful fabrication scheme, are the key to realizing this agenda. We describe progress in developing PE films (where d33 is critical) and PR films (characterized by conductance and ON/OFF ratio) of demonstration quality. A macroscopic-scale PET has been built to demonstrate PET viability over large numbers of switching cycles. The perspective for the development of pressure-driven electronics will be outlined.