Xuema Li

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.

Memristor-CMOS hybrid integrated circuits for reconfigurable logic

Hybrid reconfigurable logic circuits were fabricated by integrating memristor-based crossbars onto a foundry-built CMOS (complementary metal-oxide-semiconductor) platform using nanoimprint lithography, as well as materials and processes that were compatible with the CMOS. Titanium dioxide thin-film memristors served as the configuration bits and switches in a data routing network and were connected to gate-level CMOS components that acted as logic elements, in a manner similar to a field programmable gate array. We analyzed the chips using a purpose-built testing system, and demonstrated the ability to configure individual devices, use them to wire up various logic gates and a flip-flop, and then reconfigure devices.

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.

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.

Gold Nanofingers for Molecule Trapping and Detection

Here we demonstrate a molecular trap structure that can be formed to capture analyte molecules in solution for detection and identification. The structure is based on gold-coated nanoscale polymer fingers made by nanoimprinting technique. The nanofingers are flexible and their tips can be brought together to trap molecules, while at the same time the gold-coated fingertips form a reliable Raman hot spot for molecule detection and identification based on surface enhanced Raman spectroscopy (SERS). The molecule self-limiting gap size control between fingertips ensures ultimate SERS enhancement for sensitive molecule detection. Furthermore, these type of structures, resulting from top-down meeting self-assembly, can be generalized for other applications, such as plasmonics, meta-materials, and other nanophotonic systems.

Hot-Spot Engineering in Polygonal Nanofinger Assemblies for Surface Enhanced Raman Spectroscopy

Multiparticle assemblies of nanoscale structures are the fundamental building blocks for powerful plasmonic devices. Here we show the controlled formation of polygonal metal nanostructure assemblies, including digon, trigon, tetragon, pentagon, and hexagon arrays, which were formed on top of predefined flexible polymer pillars that undergo self-coalescence, analogous to finger closing, with the aid of microcapillary forces. This hybrid approach of combining top-down fabrication with self-assembly enables the formation of complex nanoplasmonic structures with sub-nanometer gaps between gold nanoparticles. On comparison of the polygon-shaped assemblies, the symmetry dependence of the nanoplasmonic structures was determined for application to surface enhanced Raman spectroscopy (SERS), with the pentagonal assembly having the largest Raman enhancement for the tested molecules. Electromagnetic simulations of the polygonal structures were performed to visualize the field enhancements of the hot spots so as to guide the rational design of optimal SERS structures.

Self-Aligned Memristor Cross-Point Arrays Fabricated with One Nanoimprint Lithography Step

We demonstrate a technique to fabricate memristor cross-point arrays using a self-aligned, one step nanoimprint lithography process that simultaneously patterns the bottom electrode, switching material film and the top electrode. Since this process does not require overlay alignment, the fabrication complexity is greatly reduced and the throughput is significantly increased. The critical interfaces are exposed to much less contamination and thus under better chemical control. With this technique, we fabricated arrays of TiO(2)-based memristive devices (junction area 100 nm by 100 nm) that did not require electrical forming and were operated with nanoampere currents.

Hybrid CMOS/memristor circuits

This is a brief review of recent work on the prospective hybrid CMOS/memristor circuits. Such hybrids combine the flexibility, reliability and high functionality of the CMOS subsystem with very high density of nanoscale thin film resistance switching devices operating on different physical principles. Simulation and initial experimental results demonstrate that performance of CMOS/memristor circuits for several important applications is well beyond scaling limits of conventional VLSI paradigm.

Impact of geometry on the performance of memristive nanodevices

We examined the influence of memristor geometry on switching endurance by comparing ribbed and planar TiO(2)-based cross-point devices with 50 nm × 50 nm lateral dimensions. We observed that planar devices exhibited a factor of over four improvement in median endurance value over ribbed structures for otherwise identical structures. Our simulations indicated that the corners in the upper wires of the ribbed devices experienced higher current density and more heating during device forming and switching, and hence a shorter life time.

Thermal stability of Ti-catalyzed Si nanowires

The stability of long, narrow Si nanowires grown by catalytic decomposition of a Si-containing gas during annealing was investigated because their use in devices will depend on their stability during subsequent processing. The nanowires can be annealed without visible change well above 900 °C in N2 after air exposure. However, annealing in a H2 ambient more readily degrades their structure. Nanowires without a native surface oxide are stable up to 850 °C, but change significantly at 900 °C. Surface diffusion appears to be sufficient to allow constant-diameter nanowire regions to form.