Rajagopal Ramasubramaniam

Homogeneous carbon nanotube/polymer composites for electrical applications

Homogeneous carbon nanotube/polymer composites were fabricated using noncovalently functionalized, soluble single-walled carbon nanotubes (SWNTs). These composites showed dramatic improvements in the electrical conductivity with very low percolation threshold (0.05–0.1 wt % of SWNT loading). By significantly improving the dispersion of SWNTs in commercial polymers, we show that only very low SWNT loading is needed to achieve the conductivity levels required for various electrical applications without compromising the host polymer’s other preferred physical properties and processability. In contrast to previous techniques, our method is applicable to various host polymers and does not require lengthy sonication.

Operation of a quantum-dot cellular automata (QCA) shift register and analysis of errors

Quantum-dot cellular automata (QCA) is a digital logic architecture that uses single electrons in arrays of quantum dots to perform binary operations. A QCA latch is an elementary building block which can be used to build shift registers and logic devices for clocked QCA architectures. We discuss the operation of a QCA latch and a shift register and present an analysis of the types and properties of errors encountered in their operation.

Experimental demonstration of clocked single-electron switching in quantum-dot cellular automata

A device representing a basic building block for clocked quantum-dot cellular automata architecture is reported. Our device consists of three floating micron-size metal islands connected in series by two small tunnel junctions where the location of an excess electron is defined by electrostatic potentials on gates capacitively coupled to the islands. In this configuration, the middle dot acts as an adjustable Coulomb barrier allowing clocked control of the charge state of the device. Charging diagrams of the device show the existence of several operational modes, in good agreement with theory. The clocked switching of a single electron is experimentally demonstrated and advantages of this architecture are discussed.

Power gain in a quantum-dot cellular automata latch

We present an experimental demonstration of power gain in quantum-dot cellular automata (QCA) devices. Power gain is necessary in all practical electronic circuits where power dissipation leads to decay of logic levels. In QCA devices, charge configurations in quantum dots are used to encode and process binary information. The energy required to restore logic levels in QCA devices is drawn from the clock signal. We measure the energy flow through a clocked QCA latch and show that power gain is achieved.

Clocked quantum-dot cellular automata shift register

The quantum-dot cellular automata (QCA) computational paradigm provides a means to achieve ultimately low limits of power dissipation by replacing binary coding in currents and voltages with single-electron switching within arrays of quantum dots (‘‘cells’’). Clocked control over the cells allows the realization of power gain, memory and pipelining in QCA circuits. We present an experimental demonstration of a clocked QCA two-stage shift register (SR) and use it to mimic the operation of a multi-stage SR. Error-bit rates for binary switching operations in a metal tunnel junction device are experimentally investigated, and discussed for future molecular QCAs. 2003 Elsevier Science B.V. All rights reserved.

Experimental demonstration of a latch in clocked quantum-dot cellular automata

We present an experimental demonstration of a latch in a clocked quantum-dot cellular automata (QCA) device. The device consists of three floating micron-size metal dots, connected in series by multiple tunnel junctions and controlled by capacitively coupled gates. The middle dot acts as an adjustable barrier to control single-electron tunneling between end dots. The position of a switching electron in the half cell is detected by a single-electron electrometer. We demonstrate “latching” of a single electron in the end dots controlled by the gate connected to the middle dot. This ability to lock an electron in a controllable way enables pipelining, power gain and reduced power dissipation in QCA arrays.

Clocked quantum-dot cellular automata devices: experimental studies

Presents an experimental demonstration of two novel clocked QCA devices - a QCA latch and a QCA shift register. We demonstrate the operation of the devices, and discuss sources and methods of lowering the digital errors in QCA clocked devices.

Quantum-dot cellular automata: introduction and experimental overview

An overview is given of the QCA architecture, along with a summary of experimental demonstrations of QCA devices. Quantum-dot cellular automata (QCA) is a transistorless computation paradigm that addresses such challenging issues as device and power density. The basic building blocks of the QCA architecture, such as AND, OR gates and clocked cells have been demonstrated and are presented. The quantum dots used in the experiments are metal islands that are coupled by capacitors and tunnel junctions. An improved design of the cell is presented in which all four dots of the cell are coupled by tunnel junctions. A noninvasive electrometer is presented which improves the sensitivity and linearity of dot potential measurements. The operation of this basic cell is confirmed by an externally controlled polarization change of the cell.

Experimental studies of clocked quantum-dot cellular automata devices

Devices based on the quantum-dot cellular automata (QCA) computational approach (Lent et al, 1993) use interacting quantum dots to encode and process binary information. In this transistorless approach to computation, logic levels are represented by the configurations of single electrons in coupled quantum-dot systems. In the last few years, significant progress has been made towards the realization of basic QCA elements. However, in these devices, power gain needed for the operation of large QCA arrays was not possible since the only source of energy was the signal input. Recent theoretical work (Lent and Tougaw, 1997) proposed clocked control of the QCA circuitry. Clocked controlled QCA systems have many advantages such as power gain, reduced power dissipation, and pipelined architectures. The original theoretical work applied only to semiconductor implementation of clocked QCA arrays, but recently a scheme for clocked control of metallic QCA cells was proposed (Toth and Lent, 1999; Korotkov and Likharev, 1998). Here an extra dot placed between the two dots of the QCA half-cell acts as a tunable barrier controlled by the clock signal. We present the experimental demonstration of a clocked QCA cell. The device consists of two capacitively coupled half-cells, where each half-cell consists of three micron-size Al islands separated by tunnel junctions, and four electrometers to measure the charge state of the half-cells. The half-cells are leadless, with no DC connection to the environment.

Experimental demonstration of a QCA shift register and analysis of errors

Quantum-dot Cellular Automata (QCA) is a device architecture that uses the position of electrons in quantum-dot arrays to implement digital logic. We present the experimental demonstration of a two-stage QCA shift register and an analysis of errors encountered in its operation.