Nanoparticle-Induced Anomalous Hall Effect in Graphene.

Schemes for introducing magnetic properties into graphene are of fundamental interest and could enable the development of electrically controlled magnetic devices, thereby extending graphene's applications from conventional electronics to spintronics. Proximity-induced ferromagnetism (PIFM) has been reported for graphene coupled to adjacent ferromagnetic insulators (FMIs). PIFM from an FMI preserves graphene's high carrier mobility and does not introduce a parallel current path. However, few FMIs other than yttrium-iron-garnet are suitable for practical applications due to difficulties in their growth and deposition and to their typically low Curie temperatures. Furthermore, it is difficult to obtain a high-quality FMI/graphene interface by graphene transfer methods, which are essential for obtaining the required interfacial exchange coupling. Here, we report the observation of the anomalous Hall effect (AHE) in graphene proximity coupled to an array of magnetic nanoparticles. This observation of AHE in graphene in proximity to a discontinuous magnetic structure opens the door to realizing magnetic properties in graphene from a greatly expanded range of materials and offers new possibilities for realizing patterned spintronic devices and circuitry.

Nanoscale Probing of Local Electrical Characteristics on MBE-Grown Bi2Te3 Surfaces under Ambient Conditions.

The local electrical characteristics on the surface of MBE-grown Bi2Te3 are probed under ambient conditions by conductive atomic force microscopy. Nanoscale mapping reveals a 10-100× enhancement in current at step-edges compared to that on terraces. Analysis of the local current-voltage characteristics indicates that the transport mechanism is similar for step-edges and terraces. Comparison of the results with those for control samples shows that the current enhancement is not a measurement artifact but instead is due to local differences in electronic properties. The likelihood of various possible mechanisms is discussed. The absence of enhancement at the step-edges for graphite terraces is consistent with the intriguing possibility that spin-orbit coupling and topological effects play a significant role in the step-edge current enhancement in Bi2Te3.

Characterization of buried metal-molecule-metal junctions using Fourier transform infrared microspectroscopy.

We have devised an infrared spectromicroscopy based experimental configuration to enable structural characterization of buried molecular junctions. Our design utilizes a small mercury drop at the focal point of an infrared microscope to act as a mirror in studying metal-molecule-metal (MmM) junctions. An organic molecular monolayer is formed either directly on the mercury drop or on a thin, infrared (IR) semi-transparent layer of Au deposited onto an IR transparent, undoped silicon substrate. Following the formation of the monolayer, films on either metal can be examined independently using specular reflection spectroscopy. Furthermore, by bringing together the two monolayers, a buried molecular bilayer within the MmM junction can be characterized. Independent examination of each half of the junction prior to junction formation also allows probing any structural and/or conformational changes that occur as a result of forming the bilayer. Because our approach allows assembling and disassembling microscopic junctions by forming and withdrawing Hg drops onto the monolayer covered metal, spatial mapping of junctions can be performed simply by translating the location of the derivatized silicon wafer. Finally, the applicability of this technique for the longer-term studies of changes in molecular structure in the presence of electrical bias is discussed.

Evolution of conformational order during self-assembly of n-alkanethiols on Hg droplets: an infrared spectromicroscopy study.

This Letter describes Fourier-transform infrared spectroscopy evidence for the evolution of conformational order and coverage during the formation of n-alkanethiol monolayers on microdroplets of mercury from the solution phase. At the highest coverages obtained by self-assembly, the monolayer is characterized by predominantly all-trans conformational order. For partial monolayers obtained at arbitrarily quenched incubation periods, we find a continuous evolution of the chain conformational order with monolayer coverage. Analyzing these results in light of previously reported models from X-ray scattering reveals a complex self-assembly process in which the density-dependent evolution of the chain conformational order is coupled with that of molecular orientation and density.

Two-dimensional nanoparticle arrays show the organizational power of robust DNA motifs.

The bottom-up spatial organization of potential nanoelectronic components is a key intermediate step in the development of molecular electronics. We describe robust three-space-spanning DNA motifs that are used to organize nanoparticles in two dimensions. One strand of the motif ends in a gold nanoparticle; only one DNA strand is attached to the particle. By using two of the directions of the motif to produce a two-dimensional crystalline array, one direction is free to bind gold nanoparticles. Identical motifs, tailed in different sticky ends, enable the two-dimensional periodic ordering of 5 and 10 nm diameter gold nanoparticles.

Sequence-encoded self-assembly of multiple-nanocomponent arrays by 2D DNA scaffolding.

Regular 2D arrays of multiple types of nanocomponents were constructed by self-assembly to DNA scaffolding with alternating rows of sequence-encoded hybridization sites. Different-sized Au particles coated with DNA complementary to one of the sites were bound to the scaffolding, producing alternating rows of the two nanocomponents with a 32-nm inter-row spacing. These results demonstrate the potential for using DNA to self-assemble complex arrays of components with nanometer-scale precision.