Molecular QCA Research Articles

Edge effects in quantum cellular automata binary wire

The molecular quantum cellular automata approach provides a paradigm for molecular electronics, where the charge configurations at reduction–oxidation centers of molecules represent binary information. The QCA cells are coupled by electrostatic fields and enable implementing devices for digital data transmission and logic operation at the molecular scale. Designing inputs for the desired operation of molecular QCA devices has been a challenging task, where the edge effects of electrodes may reduce the reliability of the QCA devices. We calculate the fringing fields of a pair of electrodes implemented on a model platform for the operation of a molecular QCA binary wire. The calculated nanogap and fringing fields are used in a simple quantum mechanical model to analyze the edge effects on the polarization propagation and polarization loss throughout the wire. Discussions on the stability of the wire in the presence of the fringing fields using the excited state of the wire and polarization uncertainties are presented.

Dipole Charge Detection: Toward the Readout of Bistable Charge States in Molecular QCA

Dipole Charge Detection: Toward the Readout of Bistable Charge States in Molecular QCA

In Quest of Molecular Materials for Quantum Cellular Automata: Exploration of the Double Exchange in the Two-Mode Vibronic Model of a Dimeric Mixed Valence Cell

In this article, we apply the two-mode vibronic model to the study of the dimeric molecular mixed-valence cell for quantum cellular automata. As such, we consider a multielectron mixed valence binuclear d2−d1–type cluster, in which the double exchange, as well as the Heisenberg-Dirac-Van Vleck exchange interactions are operative, and also the local (“breathing”) and intercenter vibrational modes are taken into account. The calculations of spin-vibronic energy spectra and the “cell-cell”-response function are carried out using quantum-mechanical two-mode vibronic approach based on the numerical solution of the dynamic vibronic problem. The obtained results demonstrate a possibility of combining the function of molecular QCA with that of spin switching in one electronic device and are expected to be useful from the point of view of the rational design of such multifunctional molecular electronic devices.

Quantum-dot Cellular Automata Latches for Reversible Logic Using Wave Clocking Scheme

The quantum-dot cellular automata computing paradigm shows particular promise in terms of density and power and also enabling a new approach to implement reversible computing. In this paper, we explore the efficient designs of various latches based on reversible logic for quantum-dot cellular automata building blocks, while applying the cell placement constraints introduced by the use of wave clock. Compared to previous literature, the proposed solutions show smaller cell count while using a potentially implementable clocking scheme for molecular QCA wave clocking. The functional correctness of the proposed latches are presented and verified using QCADesigner and HDLQ. Furthermore, we present detailed characterization and analysis, considering different cost metrics of the proposed latches. It is shown that, compared to previous literature, critical paths are decreased by 50%, 33%, 50%, 25% respectively for D-Latch, T-Latch, JK-Latch, and SR latch. There is also an improvement in timing is about 50% for D-Latch and JK-Latch, and 25% for SR-Latch.

Novel conservative reversible error control circuits based on molecular QCA

Novel conservative reversible error control circuits based on molecular QCA

Novel conservative reversible error control circuits based on molecular QCA

Quantum-dot cellular automata are a prominent part of the nanoscale regime. They use a quantum cellular based architecture which enables rapid information process with high device density. This paper targets the two kinds of novel error control circuits such as Hamming code, parity generator and checker. To design the HG-PP (HG = Hamming gate, PP = parity preserving), NG-PP (NG = new gate) are proposed for optimising the circuits. Based on the proposed gates and a few existing gates, the Hamming code and parity generator and checker circuits are constructed. The proposed gates have been framed and verified in QCA. The simulation outcomes signify that their framed circuits are faultless. In addition to verification, physical reversible is done. The reversible major metrics such as gate count, quantum cost, unit delay, and garbage outputs, uses best optimisation results compared to counterparts. They can be utilised as a low power error control circuit applied in future communication systems.

Effect of a Clock System on Bis-Ferrocene Molecular QCA

Molecular quantum-dot cellular automata (mQCA) is found to be the most promising among all emerging technologies. It is expected to show remarkable operating frequencies (THz), high device densities, noncryogenic working temperature, and reduced power consumption. The computation relies on a new paradigm based on the interaction between nearby molecular QCA cells. This computation requires the aid of an external signal normally referred to as clock that enables/inhibits the molecular activity. The influence of clock on realistic molecules has never been deeply studied. In this paper, we performed a thorough analysis of the clock signal added to the molecular QCA cell based on an ad hoc synthesized bisferrocene molecule. Ab-initio simulations and further postprocessing of data have been used for characterizing the performance of bisferrocene molecule under the influence of a clock signal. Quantitative results on the molecule in terms of newly defined figures of merits, i.e., aggregated charge , equivalent voltage, and Vin-Vout trans-characteristic have been shown. Meanwhile, we demonstrate when and how much the presence of clock signal enhances or hinders the interactions between QCA molecules. These unprecedented data give a fundamental improvement to the knowledge on how information can be propagated through QCA devices. The results suggest directions to chemists, technologists, and engineers on how to proceed in the next steps for this promising technology.

Process Variability and Electrostatic Analysis of Molecular QCA

Molecular quantum-dot cellular automata (mQCA) is an emerging paradigm for nanoscale computation. Its revolutionary features are the expected operating frequencies (THz), the high device densities, the noncryogenic working temperature, and, above all, the limited power densities. The main drawback of this technology is a consequence of one of its very main advantages, that is, the extremely small size of a single molecule. Device prototyping and the fabrication of a simple circuit are limited by lack of control in the technological process [Pulimeno et al. 2013a]. Moreover, high defectivity might strongly impact the correct behavior of mQCA devices. Another challenging point is the lack of a solid method for analyzing and simulating mQCA behavior and performance, either in ideal or defective conditions. Our contribution in this article is threefold: (i) We identify a methodology based on both ab-initio simulations and post-processing of data for analyzing an mQCA system adopting an electronic point of view (we baptized this method as “MoSQuiTo”); (ii) we assess the performance of an mQCA device (in this case, a bis- ferrocene molecule) working in nonideal conditions, using as a reference the information on fabrication-critical issues and on the possible defects that we are obtaining while conducting our own ongoing experiments on mQCA: (iii) we determine and assess the electrostatic energy stored in a bis-ferrocene molecule both in an oxidized and reduced form. Results presented here consist of quantitative information for an mQCA device working in manifold driving conditions and subjected to defects. This information is given in terms of: (a) output voltage; (b) safe operating area (SOA); (c) electrostatic energy; and (d) relation between SOA and energy, that is, possible energy reduction subject to reliability and functionality constraints. The whole analysis is a first fundamental step toward the study of a complex mQCA circuit. It gives important suggestions on possible improvements of the technological processes. Moreover, it starts an interesting assessment on the energy of an mQCA, one of the most promising features of this technology.

Electron-Beam Lithography and Molecular Liftoff for Directed Attachment of DNA Nanostructures on Silicon: Top-down Meets Bottom-up

CONSPECTUS: Our work on lithographic patterning of DNA nanostructures was inspired by a collaboration on molecular electronic devices known as quantum-dot cellular automata or QCA. QCA is a paradigm for computation in which information is transmitted and processed through the interaction of coupled electrical charges or magnetic dipoles. We began to explore the idea of molecular scale QCA and found that ab initio methods, a thermodynamic Ising model, and larger scale circuit design work suggested that circuits that did computationally interesting things could function at room temperature if made from molecular QCA cells of chemically reasonable design. But how could the QCA cells be patterned to form the complex arrays needed for computationally interesting circuitry, and how could those arrays of molecular circuitry be integrated with conventional electronic inputs and outputs? Top-down methods lacked the spatial resolution and high level of parallelism needed to make molecular circuits. Bottom-up chemical synthesis lacked the ability to fabricate arbitrary and heterogeneous structures tens to hundreds of nanometers in size. Chemical self-assembly at the time could produce structures in the right size scale, but was limited to homogeneous arrays. A potential solution to this conundrum was just being demonstrated in the late 1990s and early 2000s: DNA nanostructures self-assembled from oligonucleotides, whose high information density could handle the creation of arbitrary structures and chemical inhomogeneity. Our group became interested in whether DNA nanostructures could function as self-assembling circuit boards for electrical or magnetic QCA systems. This Account focuses on what we learned about the interactions of DNA nanostructures with silicon substrates and, particularly, on how electron-beam lithography could be used to direct the binding of DNA nanostructures on a variety of functional substrates.

Modeling of Electrostatic QCA Wires

This paper presents a yield analysis of molecular scale electrostatic QCA wires in the presence of a variety of manufacturing defects. Within this analysis, we compare wires of varying lengths and widths as thicker wires are frequently projected to be more tolerant to manufacturing defects. Additionally, we compare the simulation results of long wires to the yield rates predicted via probabilistic transfer matrix (PTM) modeling. This comparison demonstrates that PTM modeling is best used when the short wire segments used to estimate the yields of long wires have high yields.

Bis-Ferrocene Molecular QCA Wire: Ab Initio Simulations of Fabrication Driven Fault Tolerance

Molecular quantum dot cellular automata (MQCA) are among the most promising emerging technologies for the expected theoretical operating frequencies (THz), the high device densities, and the noncryogenic working temperature. Due to the small size of an MQCA cell, based on one or two molecules, the device prototyping and even a simple circuit fabrication are limited by the lack of control on the technological process. In this paper, we performed an analysis of the possible fabrication defects of a molecular QCA wire built with ad hoc synthesized bis-ferrocene molecules. We evaluated the fault tolerance of a real QCA device and assessed its performance in nonideal conditions due to the fabrication criticalities as we faced in our experiments. We achieved these results by defining a new methodology for the fault analysis in the MQCA technology, based both on ab initio simulations and theoretical computations. The obtained results give quantitative information on the safe operating area (SOA) of a bis-ferrocene molecular wire, and represented an important feedback to improve the technological process for the final experimental set-up.

Radius of effect in molecular quantum-dot cellular automata

Molecular quantum-dot cellular automata (mQCA) is a promising technology for building molecular computers, where binary information is represented by the charge configuration within molecular redox centres. Molecular QCA devices are coupled by electric fields and no current flows in the circuits, in theory. The Coulomb forces, which cause electrons to move from one redox centre to another within a cell, may result in faults in neighbouring cells as well. A simple electrostatic model is presented that describes the interactions of double-dot molecules within an effective radius-of-effect (RoE). The proposed model can be applied to approximate the RoE in device simulation, as well as determining a reliable cell-spacing in mQCA circuits as to avoid faults. Compared with full quantum chemistry calculations, which are of high computational cost, this simple model provides a rapid and straightforward tool for application in reliable mQCA device and circuit design.

Towards a molecular QCA wire: simulation of write-in and read-out systems

Among emerging beyond CMOS technologies Molecular Quantum Dot Cellular Automata (MQCA) are estimated as extremely promising for computational purposes. The elementary nanoelectronic devices are molecular systems in which the binary encoding is provided by the charge localization within a molecule. As a consequence, there is no current flowing among the cells and power dissipation is dramatically reduced.We study a new real molecule that was synthesized ad hoc for this technology. Differently with respect to previous contributions, this study has the aim of assessing the realistic properties of this molecule in a perspective experimental system based on a molecular wire principle. We use a combination of ab initio calculations and molecular dynamics simulations and analyze the molecule behavior when specific electric fields are applied to move the electrons inside the molecule in order to force a logic state. Our results allowed us (i) to asses the molecule behavior and to explore the working points of our experimental system for the write-in, (ii) to introduce in this scenario new metrics for studying and using these new devices from an electronic point of view, (iii) to give a perspective and to define design constraints for possible experimental solutions eligible for issue of molecule state read-out.

P-QCA

Quantum-dot cellular automata is an interesting computation fabric with many never-seen-before properties. However, no programmable fabric scheme has utilized all these properties effectively. We propose an architecture for a programmable device using molecular QCA which exploits all the specialities of the fabric. The architecture taps the flexibility provided by the clocking system of molecular QCA to build a simple tile-based programmable device with the 3-input Majority gate as the fundamental logic element. Observing how a QCA structure can behave as either an interconnect or a logic gate depending on clocking, the proposed architecture merges routing and logic elements, thus drastically changing how programmable fabrics have been designed.

The synthesis and characterization of a side-by-side iron phthalocyanine dimer

The QCA paradigm is one of the approaches to decrease the size scale of computing devices. When molecules are used as QCA cells, they may be able to perform computing at room temperature. This paper describes a novel molecular QCA cell candidate which is a side-by-side iron phthalocyanine dimer, and an investigation of its optical and redox properties. The new dodeca(pentyloxy) substituted side-by-side iron phthalocyanine dimer, along with the octa(pentyloxy) iron phthalocyanine monomer, are soluble in non-polar organic solvents. These compounds were isolated by gel permeation chromatography (GPC) and high-performance liquid chromatography (HPLC) to final purities of 98% and 99%, respectively. The NMR spectra of both compounds in CDCl3 are broad due to aggregation, but become well resolved after the addition of the coordinating solvent pyridine-d5. Addition of pyridine also gives changes in the UV-vis spectra and electrochemical peaks of both monomer and dimer in dichloromethane indicative of axial iron coordination. The electrochemical data indicates the loss of pyridine ligands from the oxidized products of both monomer and dimer. The comproportionation constant of side-by-side phthalocyanine dimer shows that its oxidized and reduced mixed-valence complexes are fairly stable. The dimer is thus a candidate for molecular QCA systems.

Metal Dependence of Signal Transmission through MolecularQuantum-Dot Cellular Automata (QCA): A Theoretical Studyon Fe, Ru, and Os Mixed-Valence Complexes.

Dynamic behavior of signal transmission through metal complexes [LM-BL-ML] (M=Fe, Ru, Os, BL=pyrazine (py), 4,4’-bipyridine (bpy), L=NH), which are simplified models of the molecular quantum-dot cellular automata (molecular QCA), is discussed from the viewpoint of one-electron theory, density functional theory. It is found that for py complexes, the signal transmission time () is Fe(0.6 fs) < Os(0.7 fs) < Ru(1.1 fs) and the signal amplitude (A) is Fe(0.05 e) < Os(0.06 e) < Ru(0.10 e). For bpy complexes, and A are Fe(1.4 fs) < Os(1.7 fs) < Ru(2.5 fs) and Os(0.11 e) < Ru(0.12 e) < Fe(0.13 e), respectively. Bpy complexes generally have stronger signal amplitude, but waste longer time for signal transmission than py complexes. Among all complexes, Fe complex with bpy BL shows the best result. These results are discussed from overlap integral and energy gap of molecular orbitals.

Signal transmission through molecular quantum-dot cellular automata: a theoretical study on Creutz–Taube complexes for molecular computing

Signal transmission through Creutz-Taube complexes [(NH(3))(5)Ru-BL-Ru(NH(3))(5)](5+)(BL = pyrazine (py), 4,4'-bipyridine (bpy)), which are simplified models of the molecular quantum-dot cellular automata (molecular QCA), is discussed both statically and dynamically with a view to designing useful molecular QCA. In the static treatment, the difference between stationary states before and after the switch of the input to the molecular QCA is discussed. In the dynamic treatment, time-evolution of electronic structure after the moment of the switch is simulated, and a simple method for the simulation is also proposed. Geometric and electronic structures are obtained by density functional theory (UB3LYP) and Hartree-Fock (UHF) calculations, and discussions are based on the Mulliken charge. It is found that signal amplitude (A) is strongly dependent on the position and charge of the input to the molecular QCA, but signal period (T) is almost independent of them. These results are explained from molecular orbitals and orbital energies, and a set of large A (large overlap between orbitals) and small T (large energy gap) generally leads to a prompt signal transmission.

A model for computing and energy dissipation of molecular QCA devices and circuits

Quantum-dot Cellular Automata is an emerging technology that offers significant improvements over CMOS. Recently QCA has been advocated as a technology for implementing reversible computing. However, existing tools for QCA design and evaluation have limited capabilities. This paper presents a new mechanical-based model for computing in QCA. By avoiding a full quantum-thermodynamical calculation, it offers a classical view of the principles of QCA operation and can be used in evaluating energy dissipation for reversible computing. The proposed model is mechanically based and is applicable to six-dot (neutrally charged) QCA cells for molecular implementation. The mechanical model consists of a sleeve of changing shape; four electrically charged balls are connected by a stick that rotates around an axle in the sleeve. The sleeve acts as a clocking unit, while the angular position of the stick within the changing shape of the sleeve, identifies the phase for quasi-adiabatic switching. A thermodynamic analysis of the proposed model is presented. The behaviors of various QCA basic devices and circuits are analyzed using the proposed model. It is shown that the proposed model is capable of evaluating the energy consumption for reversible computing at device and circuit levels for molecular QCA implementation. As applicable to QCA, two clocking schemes are also analyzed for energy dissipation and performance (in terms of number of clocking zones).

High-Resolution Electron Beam Lithography and DNA Nano-Patterning for Molecular QCA

Electron beam lithography (EBL) patterning of poly(methylmethacrylate) (PMMA) is a versatile tool for defining molecular structures on the sub-10-nm scale. We demonstrate lithographic resolution to about 5 nm using a cold-development technique. Liftoff of sub-10-nm Au nanoparticles and metal lines proves that cold development completely clears the PMMA residue on the exposed areas. Molecular liftoff is performed to pattern DNA rafts with high fidelity at linewidths of about 100 nm. High-resolution EBL and molecular liftoff can be applied to pattern Creutz-Taube molecules on the scale of a few nanometers for quantum-dot cellular automata.

Characterization of Molecular QCA Cells by Q-Controlled Enhanced Amplitude Modulation Atomic Force Microscopy

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.