Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties ...in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the 'memristor' (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron-ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance.
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.
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.
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.
Memristor crossbars were fabricated at 40 nm half-pitch, using nanoimprint lithography on the same substrate with Si metaloxide-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 (C AND D) 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.
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 TiO2-based memristive devices (junction area 100 nm by 100 nm) that did not require electrical forming and were operated with nanoampere currents.
In this review, we describe the use of nanoimprint lithography in our group to fabricate plasmonic platforms with nanometer-scale critical features that would be significantly more expensive using ...other fabrication techniques: 3-D cones that have tips with a sub-10 nm radius of curvature, active polygonal nanofingers with sub-2 nm spacing, and deterministic nanoparticle assemblies both on arbitrary substrates and in solution. These nanostructures were primarily designed to make surface-enhanced Raman Scattering a viable analytical technique for low-level chemical and biological contaminants, but the same fabrication methods should also be useful for other nanophotonic and nanoelectronic applications.
The logical relationship between two previously defined “memory resistors” is revealed by constructing and experimentally demonstrating a three‐terminal memistor equivalent circuit using two ...two‐terminal memristors. A technique is then presented, using nanoimprint lithography in combination with angle evaporation, to fabricate a single nanoscale device with a footprint of 4F2, where F is the minimum lithographic feature size, that can be operated as either a two‐terminal lateral memristor or a three‐terminal memistor inside a crossbar structure. These devices exhibit repeatable bipolar nonvolatile switching behavior with up to 103 ON/OFF conductance ratios, as well as the desired three‐terminal behavior.
A single nanoscale device that can be operated as either a two‐terminal lateral memristor or a three‐terminal memistor inside a crossbar structure is fabricated using a technique based on nanoimprint lithography and angle evaporation.
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 rotaxanes sandwiched between metal nanowires to form an 8 x 8 crossbar within a 1 micro mu2 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 6.4 Gbits cm super -2. By setting the resistances at specific cross points, two 4 x 4 subarrays of the crossbar were configured to be a nanoscale demultiplexer and multiplexer that were used to read memory bits in a third subarray.
A microneedle sensor platform with integrated silicon nanowire tip was developed for intracellular biochemical detection. Because of the virtue of miniaturized size and high sensitivity, this sensor ...has a great potential for studying individual cell or localized bioenvironment by revealing the pH level and/or enzyme activities. The fabrication of the microneedle sensor was primarily based on conventional silicon processing, where a silicon-on-insulator (SOI) wafer with 50
nm thick (1
0
0) p-type Si device layer was used as the substrate. The silicon nanowires of 50
nm height and 50–100
nm width were created by electron beam (E-beam) lithography on the tip of microneedle with good electrical connection to the contact pads for convenient electrical measurement. A three layer structure with base, support cantilever, and needle tip was designed to ensure convenient handling of sensors and minimize the invasive penetration into biological cells. In this paper, we demonstrate a preliminary assessment of this novel intracellular sensor with electrical conductance measurement under different pH levels. It is expected that this sensor with proper chemical modification will enable localized biochemical sensing within biological cells, such as neurotransmitter activities during the synaptic communication between neuron cells.
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