We report on the kinetics and ground‐state thermodynamics associated with electrochemically driven molecular mechanical switching of three bistable 2rotaxanes in acetonitrile solution, polymer ...electrolyte gels, and molecular‐switch tunnel junctions (MSTJs). For all rotaxanes a π‐electron‐deficient cyclobis(paraquat‐p‐phenylene) (CBPQT4+) ring component encircles one of two recognition sites within a dumbbell component. Two rotaxanes (RATTF4+ and RTTF4+) contain tetrathiafulvalene (TTF) and 1,5‐dioxynaphthalene (DNP) recognition units, but different hydrophilic stoppers. For these rotaxanes, the CBPQT4+ ring encircles predominantly (>90 %) the TTF unit at equilibrium, and this equilibrium is relatively temperature independent. In the third rotaxane (RBPTTF4+), the TTF unit is replaced by a π‐extended analogue (a bispyrrolotetrathiafulvalene (BPTTF) unit), and the CBPQT4+ ring encircles almost equally both recognition sites at equilibrium. This equilibrium exhibits strong temperature dependence. These thermodynamic differences were rationalized by reference to binding constants obtained by isothermal titration calorimetry for the complexation of model guests by the CBPQT4+ host in acetonitrile. For all bistable rotaxanes, oxidation of the TTF (BPTTF) unit is accompanied by movement of the CBPQT4+ ring to the DNP site. Reduction back to TTF0 (BPTTF0) is followed by relaxation to the equilibrium distribution of translational isomers. The relaxation kinetics are strongly environmentally dependent, yet consistent with a single electromechanical‐switching mechanism in acetonitrile, polymer electrolyte gels, and MSTJs. The ground‐state equilibrium properties of all three bistable 2rotaxanes were reflective of molecular structure in all environments. These results provide direct evidence for the control by molecular structure of the electronic properties exhibited by the MSTJs.
Thermodynamics to the rescue: the nuances of a molecular switch, perched in its lower‐conducting ground state on the brink between being ON and OFF, and after WRITING in its fully ON higher‐conducting metastable state, lend credence to the postulated mechanism of action of such switches. No surprises: the amplitude of switching in a molecular‐switch tunnel junction is controlled by a finely balanced bistable rotaxane (B), fulfilling the role of such an indecisive molecular switch that is extremely sensitive (A) to changes in temperature on READING.
Langmuir−Blodgett monolayers of a bistable 2rotaxane were prepared at packing densities of 118, 73, and 54 Å2/molecule. The monolayers were both characterized via infrared spectroscopy before and ...after evaporation of a 2 nm film of titanium and incorporated into molecular switch tunnel junction devices. The study suggests that the evaporation process primarily affects portions of the molecule exposed to the metal atom source. Thus, in tightly packed monolayers (73 and 54 Å2/molecule), only the portions of the 2rotaxane that are present at the molecule/air interface are clearly affected, leaving key functionality necessary for switching intact. Monolayers transferred at a lower pressure (118 Å2/molecule) exhibit nonspecific damage and poor switching behavior following Ti deposition. These results indicate that tightly packed monolayers and sacrificial functionality displayed at the molecule/air interface are important design principles for molecular electronic devices.
We describe our research into building integrated molecular electronics circuitry for a diverse set of functions, and with a focus on the fundamental scientific issues that surround this project. In ...particular, we discuss experiments aimed at understanding the function of bistable rotaxane molecular electronic switches by correlating the switching kinetics and ground state thermodynamic properties of those switches in various environments, ranging from the solution phase to a Langmuir monolayer of the switching molecules sandwiched between two electrodes. We discuss various devices, low bit-density memory circuits, and ultra-high density memory circuits that utilize the electrochemical switching characteristics of these molecules in conjunction with novel patterning methods. We also discuss interconnect schemes that are capable of bridging the micrometre to submicrometre length scales of conventional patterning approaches to the near-molecular length scales of the ultra-dense memory circuits. Finally, we discuss some of the challenges associated with fabricated ultra-dense molecular electronic integrated circuits.
The primary metric for gauging progress in the various semiconductor integrated circuit technologies is the spacing, or pitch, between the most closely spaced wires within a dynamic random access ...memory (DRAM) circuit. Modern DRAM circuits have 140 nm pitch wires and a memory cell size of 0.0408 mum(2). Improving integrated circuit technology will require that these dimensions decrease over time. However, at present a large fraction of the patterning and materials requirements that we expect to need for the construction of new integrated circuit technologies in 2013 have 'no known solution'. Promising ingredients for advances in integrated circuit technology are nanowires, molecular electronics and defect-tolerant architectures, as demonstrated by reports of single devices and small circuits. Methods of extending these approaches to large-scale, high-density circuitry are largely undeveloped. Here we describe a 160,000-bit molecular electronic memory circuit, fabricated at a density of 10(11) bits cm(-2) (pitch 33 nm; memory cell size 0.0011 microm2), that is, roughly analogous to the dimensions of a DRAM circuit projected to be available by 2020. A monolayer of bistable, 2rotaxane molecules served as the data storage elements. Although the circuit has large numbers of defects, those defects could be readily identified through electronic testing and isolated using software coding. The working bits were then configured to form a fully functional random access memory circuit for storing and retrieving information.
Resistive random access memories (RRAM) can be made of a variety of materials, including metal oxides and metal-doped chalcogenides, that exhibit memristive behavior. RRAMs have been identified as a ...leading candidate to replace flash memory and may also be suitable as replacements for other types of memory, including DRAM and SRAM. When sandwiched between two electrodes, specially grown thin films can be switched with applied bias between a high (off ) and a low resistance (on) state. Since 2007, these devices have been considered as candidates for space applications due to their device properties. There is a high density of inherent defects in the as-fabricated devices, which is likely to contribute to their tolerance to displacement damage. In addition, the nanometer length scale of the active device layers allows the devices to dissipate the charge deposited from ionizing dose. However, the testing of devices over several different radiation studies has revealed potential reliability concerns, including device to device variation, device sensitivity to measurements and physical device changes that occur with cycling between states. In this paper, we will discuss these concerns and present initial results of biased radiation testing of TaO x and TiO 2 devices with ionizing radiation.
Bistable 2rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling ...motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT4+) (denoted as the ring), between a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system located along a dumbbell component. When the ring is encircling the TTF unit, this co-conformation of the rotaxane is the most stable and thus designated the ground-state co-conformer (GSCC), whereas the other co-conformation with the ring surrounding the DNP ring system is less favored and so designated the metastable-state co-conformer (MSCC). We report here the structure and properties of self-assembled monolayers (SAMs) of a bistable 2rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic molecular dynamics (MD) studies with a force field optimized from DFT calculations and we report several experiments that validate the predictions. On the basis of both the total energy per rotaxane and the calculated stress that is parallel to the surface, we find that the optimal packing density of the SAM corresponds to a surface coverage of 115 Å2/molecule (one molecule per 4 × 4 grid of surface Au atoms) for both the GSCC and MSCC, and that the former is more stable than the latter by 14 kcal/mol at the optimum packing density. We find that the SAM retains hexagonal packing, except for the case at twice the optimum packing density (65 Å2/molecule, the 3 × 3 grid). For the GSCC and MSCC, investigated at the optimum coverage, the tilt of the ring with respect to the normal is θ = 39° and 61°, respectively, while the tilt angle of the entire rotaxane is ψ = 41° and 46°, respectively. Although the tilt angle of the ring decreases with decreasing surface coverage, the tilt angle of the rotaxane has a maximum at 144 Å2/molecule (the 4 × 5 grid/molecule) of 50° and 51° for the GSCC and MSCC, respectively. The hexafluorophosphate counterions (PF6 -) stay localized around the ring during the 2 ns MD simulation. On the basis of the calculated density profile, we find that the thickness of the SAM is 40.5 Å at the optimum coverage for the GSCC and 40.0 Å for MSCC, and that the thicknesses become less with decreasing surface coverage. The calculated surface tension at the optimal packing density is 45 and 65 dyn/cm for the GSCC and MSCC, respectively. This difference suggests that the water contact angle for the GSCC is larger than for the MSCC, a prediction that is verified by experiments on Langmuir−Blodgett monolayers of amphiphilic 2rotaxanes.
Ion-strike-induced single event transients in a type of nonvolatile resistive memory known as conductive bridge resistive memory (CBRAM) are investigated. Experimental data demonstrating bit upsets ...in 1T-1R devices under heavy ion strike are presented which show evidence of transitions from not only high to low resistance states but also from low to high resistance states. This is reported for such devices here for the first time. Device and circuit level simulations performed under various bias conditions are used to analyze possible upset modes. A crossbar CBRAM architecture without transistor selectors that offers higher density is also analyzed and shown to be susceptible to multiple bit upsets unlike 1T-1R array. Susceptibility of a 256 ×256 crossbar array to strike induced transients under two different bias schemes is simulated.
Over the last several years, various research groups have shown that metal oxide-based memristors or resistive random access memory (RRAM) components are tolerant to high levels of both displacement ...damage and ionizing dose. However, above certain thresholds of either the deposited displacement damage dose or the deposited ionizing dose, devices do respond to radiation. In this paper, the sensitivity of metal oxide memristor materials to radiation damage from various types of incident energetic ions is assessed.
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