UC Riverside - Page 2

Researchers from the University of California, Riverside developed a graphene based transistor based on negative resistance rather than trying to open up a band gap. Negative resistance is the counterintuitive phenomenon in which a current entering a material causes the voltage across it to drop. It was shown before that graphene demonstrates negative resistance in certain circumstances.

The idea is to take a regular graphene field-effect transistor (FET) and find the circumstances in which it demonstrates negative resistance. This dip in voltage is used as a kind of switch - to perform logic. The researchers showed how several graphene FETs combined can be manipulated to produce conventional logic gates. The researchers designed such circuits that can match patterns (but they have yet to actually produce them).

The National Science Foundation (NSF) awarded a $360,000 three-year grant to three professors from the University of California, Riverside (UCR) to further study th thermal properties of graphene. The future goal of this study is to find new heat-removal approaches for electronic and optoelectronic devices.

This specific project will investigate the effect of rotation angle on the thermal conductivity of twisted bilayer graphene. The UCR team will study the possibility of suppressing the phonon coupling in twisted graphene layers, allowing for the transfer of extraordinary large heat fluxes. The phonons are quanta of crystal lattice vibrations that carry heat in graphene.

Researcherrs from the University of California (UC Riverside) have finally (after almost a century of research) managed to find out what causes the low-frequency electronic 1/f noise (also known as pink noise or flicker noise). Using graphene sheets, it was found that 1/f noise is a surface phenomenon that shows up in situations that are thinner than 2.5 nm (at least for graphene).

Graphene was essential for this work because you can test for 1/f noise using a single sheet, and then add sheet after sheet (basically adding just one-atom to the thickness of the conductive material). This cannot be done with metal films.

Researchers from IBM and University of California Riverside managed to make the slimmest graphene nanoribbon (GNR) ever - just 10 nm in width. Making one is virtually impossible, and the team created a large number of GNRs in parallel. The researchers say that the arrays cover about 50% of the prototype device channel area, which means that integrated circuits based on GNRs with the required high current densities are now possible. The narrow GNRs have a bandgap of about 0.2 eV.

The process the researchers used consists of two main steps: a top-down e-beam lithography step and a bottom-up self-assembly step involving a block copolymer template comprising alternating lamellae of the polymers PS and PMMA.

A Ph.D. student from the University of California, Riverside' won the Science as Art competition at 2012 MRS with his 0.05 millimeter nano-giraffe structure. He said that he accidentally saw this structure, and he enhanced it with Photoshop.

In his research, Amini developed a processing technique to grow single layer graphene from a molten phase. He used a process which melted a mixture of nickel, aluminum and carbon. When the mixture solidified, the nickel and aluminum formed the body of the giraffe while the carbon crystallized as a graphite cover.

The National Science Foundation (NSF) granted a four-year $1.85 million research project to UC Riverside researchers - to develop spin-based memory and logic chip. The researchers are working towards a magnetologic gate that will serve as the engine for the new technology - similar to the role of the transistor in conventional electronics.

The magnetic gate consists of graphene contacted by several magnetic electrodes. Data is stored in the magnetic state of the electrodes, similar to the way data is stored in a magnetic hard drive. For the logic operations, electrons move through the graphene and use its spin state to compare the information held in the individual magnetic electrodes.

Researchers (from University of California, Riverside) suggest a new simple method to count graphene sheets. Usually, when graphene is made by chemical vapor deposition (CVD) it results in multiple layers of sheets, and some defects and wrinkles. Current techniques to count the sheets (such as Raman and atomic force microscopy) are limited in size and need calibration.

The new method exploits the fact that graphene quenches fluorescence. The idea is to coat an area of graphene on a surface with a fluorescent polymer dye to allow visualization with a simple fluorescence microscope. The data processing is then quite straightforward.

Researchers from the University of California, Riverside successfully achieved tunneling-spin-injection into Graphene. The researchers inserted a nanometer-thick insulating layer, known as a tunnel barrier, in between the ferromagnetic electrode and the graphene layer. They found that the spin injection efficiency increased dramatically. A 30-fold increase, actually. This could lead to graphene-based Spintronic devices.

The team also made an unexpected discovery that explains short spin lifetimes of electrons in graphene that have been reported by other experimental researchers. People usually assume that the Hanle measurement accurately measures the spin lifetime, but this result shows that it severely underestimates the spin lifetime when the ferromagnet is touching the graphene, said Wei Han, the first author of the research paper and a graduate student. This is good news because it means the true spin lifetime in graphene must be longer than reported previously potentially a lot longer.

Researchers from Rice University and the University of California, Riverside (UCR) has demonstrated the first triple-mode graphene amplifier. By leveraging the ambipolarity of charge transport in graphene, the amplifier can be configured in the common-source, common-drain, or frequency multiplication mode of operation by changing the gate bias. This is the first demonstration of a single-transistor amplifier that can be tuned between different modes of operation using a single three-terminal transistor.

One of graphene's intrinsic features is ripples, similar to those seen on plastic wrap tightly pulled over a clamped edge. Induced by pre-existing strains in graphene, these ripples can strongly affect graphene's electronic properties, and not always favorably.

If the ripples can be controlled, however, they can be used to advantage in nanoscale devices and electronics, opening up a new arena in graphene engineering: strain-based devices.

UC Riverside's Chun Ning (Jeanie) Lau and colleagues now report the first direct observation and controlled creation of one- and two-dimensional ripples in graphene sheets. Using simple thermal manipulation, the researchers produced the ripples, and controlled their orientation, wavelength and amplitude.