Douglas A. A. Ohlberg

Nanoscale molecular-switch crossbar circuits

Molecular electronics offer an alternative pathway to construct nanoscale circuits in which the critical dimension is naturally associated with molecular sizes. We describe the fabrication and testing of nanoscale molecular-electronic circuits that comprise a molecular monolayer of [2]rotaxanes sandwiched between metal nanowires to form an 8 × 8 crossbar within a 1 µm 2 area. The resistance at each cross point of the crossbar can be switched reversibly. By using each cross point as an active memory cell, crossbar circuits were operated as rewritable, nonvolatile memory with a density of 6. 4G bits cm −2 .B ys etting the resistances at specific cross points, two 4 × 4s ubarrays of the crossbar were configured to be a nanoscale demultiplexer and multiplexer that were used to read memory bits in a third subarray.

Nanoscale molecular-switch devices fabricated by imprint lithography

Nanoscale molecular-electronic devices comprising a single molecular monolayer of bistable [2]rotaxanes sandwiched between two 40-nm metal electrodes were fabricated using imprint lithography. Bistable current–voltage characteristics with high on–off ratios and reversible switching properties were observed. Such devices may function as basic elements for future ultradense electronic circuitry.

Self-assembled growth of epitaxial erbium disilicide nanowires on silicon (001)

By choosing a material that has an appropriate asymmetric lattice mismatch to the host substrate, in this case ErSi2 on Si(001), it is possible to grow one-dimensional epitaxial crystals. ErSi2 nanowires are less than one nanometer high, a few nanometers wide, close to a micron long, crystallographically aligned to 〈110〉Si directions, straight, and atomically regular.

A hybrid nanomemristor/transistor logic circuit capable of self-programming

Memristor crossbars were fabricated at 40 nm half-pitch, using nanoimprint lithography on the same substrate with Si metal-oxide-semiconductor field effect transistor (MOS FET) arrays to form fully integrated hybrid memory resistor (memristor)/transistor circuits. The digitally configured memristor crossbars were used to perform logic functions, to serve as a routing fabric for interconnecting the FETs and as the target for storing information. As an illustrative demonstration, the compound Boolean logic operation (A AND B) OR (C AND D) was performed with kilohertz frequency inputs, using resistor-based logic in a memristor crossbar with FET inverter/amplifier outputs. By routing the output signal of a logic operation back onto a target memristor inside the array, the crossbar was conditionally configured by setting the state of a nonvolatile switch. Such conditional programming illuminates the way for a variety of self-programmed logic arrays, and for electronic synaptic computing.

EVOLUTION OF GE ISLANDS ON SI(001) DURING ANNEALING

The evolution of the shape and size distributions of Ge islands on Si(001) during annealing after deposition has been studied at different temperatures and effective coverages. The initial distributions of square-based pyramids, elongated “hut” structures, faceted “dome-shaped” islands, and much larger “superdomes” depends on the deposition conditions. During annealing after deposition, the islands coarsen over a limited range of times and temperatures. Those pyramidal-shaped islands that grow transform to faceted, dome-shaped islands as they become larger. Initially dome-shaped islands that dissolve transform to a pyramidal shape as they become smaller during the process of dissolving. Outside of this coarsening regime, the islands can achieve a relatively stable, steady-state configuration, especially at lower temperatures. At higher temperatures, intermixing of Si into the Ge islands dominates, decreasing the strain energy and allowing larger islands to form. At lower and intermediate temperatures, the...

Nanowires of four epitaxial hexagonal silicides grown on Si(001)

Epitaxial self-assembled silicide nanowires can be grown on Si (001) if the magnitude of the lattice mismatch between epilayer and substrate is large along one crystal axis and small along the perpendicular axis. This phenomenon is illustrated with four examples: ScSi2, ErSi2, DySi2, and GdSi2, which have lattice mismatches of −4.6%, 6.3%, 7.6%, and 8.9%, respectively, along one of the Si 〈110〉 directions and mismatches of 0.8%, −1.6%, −0.1%, and 0.8%, respectively, along the perpendicular Si〈110〉 direction. The resulting self-assembled nanowires have widths and heights in the range of 3–11 and 0.2–3 nm, depending on the lattice mismatches. The average lengths of the nanowires are in the range 150–450 nm, and are determined primarily by kinetic issues. The epitaxial growth of silicide nanowires should prove interesting to those studying quasi-one- dimensional systems.

Engineering nonlinearity into memristors for passive crossbar applications

Although TaOx memristors have demonstrated encouraging write/erase endurance and nanosecond switching speeds, the linear current-voltage (I-V) characteristic in the low resistance state limits their applications in large passive crossbar arrays. We demonstrate here that a TiO2-x/TaOx oxide heterostructure incorporated into a 50 nm× 50 nm memristor displays a very large nonlinearity such that I(V/2) ≈ I(V)/100 for V ≈ 1 volt, which is caused by current-controlled negative differential resistance in the device.

Patterning, characterization, and chemical sensing applications of graphene nanoribbon arrays down to 5 nm using helium ion beam lithography.

Bandgap engineering of graphene is an essential step toward employing graphene in electronic and sensing applications. Recently, graphene nanoribbons (GNRs) were used to create a bandgap in graphene and function as a semiconducting switch. Although GNRs with widths of <10 nm have been achieved, problems like GNR alignment, width control, uniformity, high aspect ratios, and edge roughness must be resolved in order to introduce GNRs as a robust alternative technology. Here we report patterning, characterization, and superior chemical sensing of ultranarrow aligned GNR arrays down to 5 nm width using helium ion beam lithography (HIBL) for the first time. The patterned GNR arrays possess narrow and adjustable widths, high aspect ratios, and relatively high quality. Field-effect transistors were fabricated on such GNR arrays and temperature-dependent transport measurements show the thermally activated carrier transport in the GNR array structure. Furthermore, we have demonstrated exceptional NO2 gas sensitivity of the 5 nm GNR array devices down to parts per billion (ppb) levels. The results show the potential of HIBL fabricated GNRs for the electronic and sensing applications.

Study of Molecular Trapping Inside Gold Nanofinger Arrays on Surface-Enhanced Raman Substrates

The binding of trans-1,2-bis(4-pyridyl)-ethylene (BPE) molecules on substrates arrayed with flexible gold nanofingers has been studied by surface-enhanced Raman spectroscopy (SERS) and angle-resolved X-ray photoelectron spectroscopy (AR-XPS). On the basis of the SERS and XPS results, BPE molecules are found to interact with the gold nanofingers through the lone pair electrons of pyridyl nitrogens, not through delocalized π electrons. Furthermore, after comparing the AR-XPS spectra of finger arrays preclosed before exposure to BPE with the spectra of arrays that closed after exposure to BPE, we observed in the latter case, at grazing takeoff angles, an increase in the component of the nitrogen photoelectron peak associated with pyridyl nitrogen atoms residing on bridging sites. These results demonstrate that a small percentage of BPE molecules was trapped between the neighboring gold finger tips during the finger closing process. However, because these trapped BPE molecules coincidently resided in the hot spots formed among the touching finger tips, the substantial increase in the observed SERS signal was dominated by the contribution from this small minority of BPE molecules.

Diffusion of Adhesion Layer Metals Controls Nanoscale Memristive Switching

First prominent more than 40 years ago, [ 1 ] electrical resistance switching in conductor/insulator/conductor structures has regained signifi cant attention in the last decade, [ 2–16 ] motivated by the search for alternatives to conventional semiconductor electronics. [ 17 ] Recent results have shown promising device behaviors, such as reversible, non-volatile, fast ( < 10 ns), lowpower ( ∼ 1 pJ/operation) and multiple-state switching, [ 18–26 ]