Quantum-dot cellular automata (QCA) is an emerging nanotechnology, with the potential for faster speed, smaller size, and lower power consumption than transistor-based technology. Quantum-dot cellular automata has a simple cell as the basic element. The cell is used as a building block to construct gates and wires. Previously, adder designs based on conventional designs were examined for implementation with QCA technology. That work demonstrated that the design trade-offs are very different in QCA. This paper utilizes the unique QCA characteristics to design a carry flow adder that is fast and efficient. Simulations indicate very attractive performance (i.e., complexity, area, and delay). This paper also explores the design of serial parallel multipliers. A serial parallel multiplier is designed and simulated with several different operand sizes.

Adder and Multiplier Design in Quantum-Dot Cellular Automata

Quantum-dot cellular automata (QCA) is an emerging nanotechnology for electronic circuits. Its advantages such as faster speed, smaller size, and lower power consumption are very attractive. The fundamental device, a quantum-dot cell, can be used to make gates, wires, and memories. As such it is the basic building block of nanotechnology circuits. While the physical nature of the nanoscale materials is complicated, the circuit designer can concentrate on the logical and structural design, so the design effort is reduced. Because of its novelty, the current literature shows only simple circuit structures. So this paper broadens the QCA circuit designs with larger circuits and shows analyses based on those designs. This paper proposes three kinds of adder designs in QCA. Ripple carry adders, carry lookahead adders, and conditional sum adders are designed and simulated with several different operand sizes. The designs are compared according to complexity, area, and delay

Adder Designs and Analyses for Quantum-Dot Cellular Automata

Update 1 of: Carbon in one dimension: structural analysis of the higher conjugated polyynes.

In this work we present a novel probabilistic modeling tool (QCAPro) to estimate polarization error and non-adiabatic switching power loss in Quantum-dot Cellular Automata (QCA) circuits. The tool uses a fast approximation based technique to estimate highly erroneous cells in QCA circuit design. QCAPro also provides an estimate of power loss in a QCA circuit for clocks with sharp transitions, which result in non-adiabatic operations and provides an upper bound of power expended. QCAPro can be used to estimate average power loss, maximum and minimum power loss in a QCA circuit during an input switching operation. This work will provide a good platform for researchers who wish to study polarization error and power dissipation related issues in QCA circuits.

QCAPro - An error-power estimation tool for QCA circuit design

We examine power dissipation in different clocking schemes for molecular quantum-dot cellular automata (QCA) circuits. 'Landauer clocking' involves the adiabatic transition of a molecular cell from the null state to an active state carrying data. Cell layout creates devices which allow data in cells to interact and thereby perform useful computation. We perform direct solutions of the equation of motion for the system in contact with the thermal environment and see that Landauer's Principle applies: one must dissipate an energy of at least kBT per bit only when the information is erased. The ideas of Bennett can be applied to keep copies of the bit information by echoing inputs to outputs, thus embedding any logically irreversible circuit in a logically reversible circuit, at the cost of added circuit complexity. A promising alternative which we term 'Bennett clocking' requires only altering the timing of the clocking signals so that bit information is simply held in place by the clock until a computational block is complete, then erased in the reverse order of computation. This approach results in ultralow power dissipation without additional circuit complexity. These results offer a concrete example in which to consider recent claims regarding the fundamental limits of binary logic scaling.

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Bennett clocking of quantum-dot cellular automata and the limits to binary logic scaling

The amine functionality of the linker on the dinuclear complex [trans-Ru(dppm)(2)(Ctbd1;CFc)(NCCH(2)CH(2)NH(2))][PF(6)] reacts with Si-Cl bonds of a chlorinated, highly B doped Si (111) surface to yield Si-N surface-complex bonds. The surface bound complex is constrained to a near vertical orientation by the chain length of the linker as confirmed by variable angle XPS. Oxidation of the dinuclear complex with ferrocenium ion or electrochemically generates a stable, biased Fe(III)-Ru(II) mixed-valence complex on the surface. Characterization of the array of surface bound complexes with spectroscopic as well as electrochemical techniques confirms the presence of strongly bound, chemically robust, mixed-valence complexes. Capping the flat array of complexes with a minimally perturbing mercury electrode permits the equalization of the Fe and Ru energy wells by an applied electric field. The differential capacitance of oxidized and unoxidized bound complexes is compared as a function of voltage applied between the Hg gate and the Si. The results show that electron exchange between the Fe and Ru sites of the array of dinuclear mixed-valence complexes at energy equalization generates a fluctuating dipole that produces a maximum in the capacitance versus voltage curve for each complex-counterion combination present. Passage through the capacitance maximum corresponds to switching of the molecular quantum cellular automata (QCA) cell array by the electric field from the Fe(III)-Ru(II) configuration to the Fe(II)-Ru(III) configuration, thereby confirming that molecules possess an essential property necessary for their use as elements of a QCA device.

Molecular quantum cellular automata cells. Electric field driven switching of a silicon surface bound array of vertically oriented two-dot molecular quantum cellular automata.

trans-[(H(2)NCH(2)CH(2)C triple bond N)(dppe)(2)Ru(C triple bond C)(6)Ru(dppe)(2)(N triple bond CCH(2)CH(2)NH(2))][PF(6)](2), 2[PF(6)](2), a derivative of trans-[Cl(dppe)(2)Ru(C triple bond C)(6)Ru(dppe)(2)Cl] functionalized for binding to a silicon substrate, has been prepared and characterized spectroscopically, electrochemically, and with a solid state, single-crystal structure determination. Covalent binding via reaction of one amine group to a boron-doped, smooth Si-Cl substrate is verified by XPS measurements and surface electrochemistry. Vertical orientation is demonstrated by film thickness measurements. Synthesis of the 2[PF(6)](3) mixed-valence complex on the surface is established by electrochemical techniques. Measurement of the ac capacitance of the film at 1 MHz as a function of voltage across the film with a pulse-counter pulse technique demonstrates controlled electric field generation of the two stable mixed-valence forms differing in the spatial location of one electron, that is, switching. As compared to [trans-Ru(dppm)(2)(C triple bond CFc)(NCCH(2)CH(2)NH(2))][PF(6)][Cl], 1[PF(6)][Cl], the magnitude of the capacitance signal per complex observed on switching is shown to increase with increasing distance between the metal centers. Additional experiments on 1[X][Cl] show that the potential for switching 1[X][Cl] increases in the order [X](-) = [SO(3)CF(3)](-) < [PF(6)](-) < [Cl](-). A simple electrostatic model suggests that the smaller is the counterion, the greater is the perturbation of the metal sites and the larger is the barrier for switching.

Dependence of field switched ordered arrays of dinuclear mixed-valence complexes on the distance between the redox centers and the size of the counterions.

This paper presents an overview of the electronic implementation of quantum-dot cellular automata (QCA). QCA is a computing paradigm that encodes and processes information by the position of individual electrons. This opens the possibility of dense, ultra-low power devices. Resent results are presented from QCA cells implemented using metal-dots, as well as investigations toward molecular and silicon QCA devices.

Hua Qi

Papers

Molecular quantum cellular automata cells. Electric field driven switching of a silicon surface bound array of vertically oriented two-dot molecular quantum cellular automata.

Dependence of field switched ordered arrays of dinuclear mixed-valence complexes on the distance between the redox centers and the size of the counterions.

Electronic quantum-dot cellular automata