Douglas R. Strachan

Crystallographic Etching of Few-Layer Graphene

We demonstrate a method by which few-layer graphene samples can be etched along crystallographic axes by thermally activated metallic nanoparticles. The technique results in long (>1 microm) crystallographic edges etched through to the insulating substrate, making the process potentially useful for atomically precise graphene device fabrication. This advance could enable atomically precise construction of integrated circuits from single graphene sheets with a wide range of technological applications.

Controlled fabrication of nanogaps in ambient environment for molecular electronics

We have developed a controlled and highly reproducible method of making nanometer-spaced electrodes using electromigration in ambient lab conditions. This advance will make feasible single molecule measurements of macromolecules with tertiary and quaternary structures that do not survive the liquid-helium temperatures at which electromigration is typically performed. A second advance is that it yields gaps of desired tunneling resistance, as opposed to the random formation at liquid-helium temperatures. Nanogap formation occurs through three regimes: First it evolves through a bulk-neck regime where electromigration is triggered at constant temperature, then to a few-atom regime characterized by conductance quantum plateaus and jumps, and finally to a tunneling regime across the nanogap once the conductance falls below the conductance quantum.

High-on/off-ratio graphene nanoconstriction field-effect transistor.

A method is reported to pattern monolayer graphene nanoconstriction field-effect transistors (NCFETs) with critical dimensions below 10 nm. NCFET fabrication is enabled by the use of feedback-controlled electromigration (FCE) to form a constriction in a gold etch mask that is first patterned using conventional lithographic techniques. The use of FCE allows the etch mask to be patterned on size scales below the limit of conventional nanolithography. The opening of a confinement-induced energy gap is observed as the NCFET width is reduced, as evidenced by a sharp increase in the NCFET on/off ratio. The on/off ratios obtained with this procedure can be larger than 1000 at room temperature for the narrowest devices; this is the first report of such large room-temperature on/off ratios for patterned graphene FETs.

Preparation of atomically flat SrTiO3 surfaces using a deionized-water leaching and thermal annealing procedure

We report that a deionized water leaching and thermal annealing technique can be effective for preparing atomically flat and singly terminated surfaces of single crystalline SrTiO3 substrates. After a two-step thermal-annealing and deionized-water leaching procedure, topography measured by atomic force microscopy shows the evolution of substrates from a rough to step-terraced surface structure. Lateral force microscopy confirms that the atomically flat surfaces are singly terminated. Moreover, this technique can be used to remove excessive strontium oxide or hydroxide composites segregated on the SrTiO3 surface. This acid-etchant-free technique facilitates the preparation of atomically aligned SrTiO3 substrates, which promotes studies on two-dimensional physics of complex oxide interfaces.

Memristive switching of single-component metallic nanowires

Memristors have recently generated significant interest due to their potential use in nanoscale logic and memory devices. Of the four passive circuit elements, the memristor (a two-terminal hysteretic switch) has so far proved hard to fabricate out of a single material. Here we employ electromigration to create a reversible passive electrical switch, a memristive device, from a single-component metallic nanowire. To achieve resistive switching in a single-component structure we introduce a new class of memristors, devices in which the state variable of resistance is the systems physical geometry. By exploiting electromigration to reversibly alter the geometry, we repeatedly switch the resistance of single-component metallic nanowires between low and high states over many cycles. The reversible electromigration causes the nanowire to be cyclically narrowed to approximately 10 nm in width, resulting in a change in resistance by a factor of two. As a result, this work represents a potential route to the creation of nanoscale circuits from a single metallic element.

Crystallographically-oriented carbon nanotubes grown on few-layer graphene films

Carbon nanotubes are grown on few-layer graphene films using chemical vapor deposition without a carbon feedstock gas. We find that the nanotubes show a striking alignment to specific crystal orientations of the few-layer graphene films. The nanotubes are oriented predominantly at 60 degree intervals and are offset 30 degrees from crystallographically oriented etch tracks, indicating alignment to the armchair axes of the few-layer graphene films. Nanotubes grown on various thicknesses of few-layer graphene under identical process conditions show that the thinnest films, in the sub-6 atomic layer regime, demonstrate significantly improved crystallographic alignment. Intricate crystallographic patterns are also observed having sharp kinks with bending radii less than the ∼10 nm lateral resolution of the electron and atomic force microscopy used to image them. Some of these kinks occur independently without interactions between nanotubes while others result when two nanotubes intersect. These intersections can trap nanotubes between two parallel nanotubes resulting in crystallographic back and forth zigzag geometries. These interactions suggest a tip-growth mechanism such that the catalyst particles remain within several nanometers of the few-layer graphene surface as they move leaving a nanotube in their wake.

Do superconductors have zero resistance in a magnetic field

We show that DC voltage versus current measurements of a YBCO micro-bridge in a magnetic field can be collapsed onto scaling functions proposed by Fisher, Fisher, and Huse, as is widely reported in the literature. We find, however, that good data collapse is achieved for a wide range of critical exponents and temperatures. These results strongly suggest that agreement with scaling alone does not prove the existence of a phase transition. We propose a criterion to determine if the data collapse is valid, and thus if a phase transition occurs. To our knowledge, none of the data reported in the literature meet our criterion.

Gate coupling to nanoscale electronics

The realization of single-molecule electronic devices, in which a nanometer-scale molecule is connected to macroscopic leads, requires the reproducible production of highly ordered nanoscale gaps in which a molecule of interest is electrostatically coupled to nearby gate electrodes. Understanding how the molecule-gate coupling depends on key parameters is crucial for the development of high-performance devices. Here we directly address this, presenting two- and three-dimensional finite-element electrostatic simulations of the electrode geometries formed using emerging fabrication techniques. We quantify the gate coupling intrinsic to these devices, exploring the roles of parameters believed to be relevant to such devices. These include the thickness and nature of the dielectric used, and the gate screening due to different device geometries. On the single-molecule

Increased power factors of organic–inorganic nanocomposite thermoelectric materials and the role of energy filtering

(\ensuremath{\sim}1\text{ }\text{nm})

Striped nanoscale friction and edge rigidity of MoS2 layers

scale, we find that device geometry plays a greater role in the gate coupling than the dielectric constant or the thickness of the insulator. Compared to the typical uniform nanogap electrode geometry envisioned, we find that nonuniform tapered electrodes yield a significant 3 orders of magnitude improvement in gate coupling. We also find that in the tapered geometry the polarizability of a molecular channel works to enhance the gate coupling.