Adam L. Friedman

Graphene As a Tunnel Barrier: Graphene-Based Magnetic Tunnel Junctions

Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We report fabrication of tunnel junctions using single-layer graphene between two ferromagnetic metal layers in a fully scalable photolithographic process. The transport occurs by quantum tunneling perpendicular to the graphene plane and preserves a net spin polarization of the current from the contact so that the structures exhibit tunneling magnetoresistance to 425 K. These results demonstrate that graphene can function as an effective tunnel barrier for both charge and spin-based devices and enable realization of more complex graphene-based devices for highly functional nanoscale circuits, such as tunnel transistors, nonvolatile magnetic memory, and reprogrammable spin logic.

Quantum linear magnetoresistance in multilayer epitaxial graphene.

We report the first observation of linear magnetoresistance (LMR) in multilayer epitaxial graphene grown on SiC. We show that multilayer epitaxial graphene exhibits large LMR from 2.2 K up to room temperature and that it can be best explained by a purely quantum mechanical model. We attribute the observation of LMR to inhomogeneities in the epitaxially grown graphene film. The large magnitude of the LMR suggests potential for novel applications in areas such as high-density data storage and magnetic sensors and actuators.

Possible room-temperature ferromagnetism in hydrogenated carbon nanotubes

We find that ferromagnetism can be induced in carbon nanotubes (CNTs) by introducing hydrogen. Multiwalled CNTs grown inside porous alumina templates contain a large density of defects resulting in significant hydrogen uptake when annealed at high temperatures. This hydrogen incorporation produces H-complex and adatom magnetism which generates a sizable ferromagnetic moment and a Curie temperature near T(C)=1000 K. We studied the conditions for the incorporation of hydrogen, the temperature-dependent magnetic behavior, and the dependence of the ferromagnetism on the size of the nanotubes.

Homoepitaxial tunnel barriers with functionalized graphene-on-graphene for charge and spin transport

The coupled imperatives for reduced heat dissipation and power consumption in high-density electronics have rekindled interest in devices based on tunnelling. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, layer uniformity, interface stability and electronic states that severely complicate fabrication and compromise performance. Two-dimensional materials such as graphene obviate these issues and offer a new paradigm for tunnel barriers. Here we demonstrate a homoepitaxial tunnel barrier structure in which graphene serves as both the tunnel barrier and the high-mobility transport channel. We fluorinate the top layer of a graphene bilayer to decouple it from the bottom layer, so that it serves as a single-monolayer tunnel barrier for both charge and spin injection into the lower graphene channel. We demonstrate high spin injection efficiency with a tunnelling spin polarization >60%, lateral transport of spin currents in non-local spin-valve structures and determine spin lifetimes with the Hanle effect.

Negative index metamaterials based on metal-dielectric nanocomposites for imaging applications

Negative index metamaterials are demonstrated based on metal-dielectric nanocomposites prepared using a versatile bottom-up nanofabrication approach. The method involves the incorporation of vertically aligned metal nanowires such as Au and Ag inside dielectric aluminum oxide nanotemplates. Optical absorbance measurements show resonance peaks corresponding to the transverse and longitudinal surface plasmon modes. A quantitative model based on effective medium theory is in excellent agreement with experimental data, and points to specific composite configurations and wavelength regimes where such structures can have applications as negative refraction media for imaging.

Epitaxial Graphene Nucleation on C-Face Silicon Carbide

The initial stages of epitaxial graphene growth were studied by characterization of graphene formed in localized areas on C-face 6H-SiC substrates. The graphene areas were determined to lie below the level of the surrounding substrate and showed different morphologies based on size. Employing electron channeling contrast imaging, the presence of threading screw dislocations was indicated near the centers of each of these areas. After the graphene was removed, these dislocations were revealed to lie within the SiC substrate. These observations suggest that screw dislocations act as preferred nucleation sites for graphene growth on C-face SiC.

Roles of pH and acid type in the anodic growth of porous alumina.

Several theoretical models have been formulated to explain the growth of porous structures in anodized alumina. Using some basic assumptions, these models predict the size and shape of the pores in the anodic porous alumina as functions of pH and voltage. Additionally, they address issues of stability in the pore growth. In this work, we have carried out a systematic experimental investigation to study the stability phase diagram as a function of pH and applied voltage. We also obtain the dependence of pore dimensions on the pH, voltage, and acid type. Based on our results, and insight gained from recent chemical analysis of the porous alumina anodization process, we conclude that the models must include an appropriate weighting factor to account for the oxidation and dissolution mechanism during the pore formation.

Optical polarization and intervalley scattering in single layers of MoS2 and MoSe2.

Single layers of MoS2 and MoSe2 were optically pumped with circularly polarized light and an appreciable polarization was initialized as the pump energy was varied. The circular polarization of the emitted photoluminescence was monitored as a function of the difference between the excitation energy and the A-exciton emission at the K-point of the Brillouin zone. Our results show a threshold of twice the LA phonon energy, specific to the material, above which phonon-assisted intervalley scattering causes depolarization. In both materials this leads to almost complete depolarization within ~100 meV above the threshold energy. We identify the extra kinetic energy of the exciton (independent of whether it is neutral or charged) as the key parameter for presenting a unifying picture of the depolarization process.

Hydrogenated Graphene as a Homoepitaxial Tunnel Barrier for Spin and Charge Transport in Graphene

We demonstrate that hydrogenated graphene performs as a homoepitaxial tunnel barrier on a graphene charge/spin channel. We examine the tunneling behavior through measuring the IV curves and zero bias resistance. We also fabricate hydrogenated graphene/graphene nonlocal spin valves and measure the spin lifetimes using the Hanle effect, with spintronic nonlocal spin valve operation demonstrated up to room temperature. We show that while hydrogenated graphene indeed allows for spin transport in graphene and has many advantages over oxide tunnel barriers, it does not perform as well as similar fluorinated graphene/graphene devices, possibly due to the presence of magnetic moments in the hydrogenated graphene that act as spin scatterers.

Van der Waals screening by single-layer graphene and molybdenum disulfide

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.