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PAGE Seminar ReportMoletronics- An Invisible Technology

MOLETRONICS-an invisible technology

ABSTRACT

As a scientific pursuit, the search for a viable successor to silicon computer technology has garnered considerable curiosity in the last decade. The latest idea, and one of the most intriguing, is known as molecular computers, or moletronics, in which single molecules serve as switches, "quantum wires" a few atoms thick serve as wiring, and the hardware is synthesized chemically from the bottom up.

The central thesis of moletronics is that almost any chemically stable structure that is not specifically disallowed by the laws of physics can in fact be built. The possibility of building things atom by atom was first introduced by Richard Feynman in 1959.

An "assembler", which is little more than a submicroscopic robotic arm can be built and be controlled. We can use it to secure and position compounds in order to direct the precise location at which chemical reactions occur. This general approach allows the construction of large, atomically precise objects by initiating a sequence of controlled chemical reactions. In order for this to function as we wish, each assembler requires a process for receiving and executing the instruction set that will dictate its actions. In time, molecular machines might even have onboard, high speed RAM and slower but more permanent storage. They would have communications capability and power supply.

Moletronics is expected to touch almost every aspect of our lives, right down to the water we drink and the air we breathe. Experimental work has already resulted in the production of molecular tweezers, a carbon nanotube transistor, and logic gates. Theoretical work is progressing as well. James M. Tour of Rice University is working on the construction of a molecular computer. Researchers at Zyvex have proposed an Exponential Assembly Process that might improve the creation of assemblers and products, before they are even simulated in the lab. We have even seen researchers create an artificial muscle using nanotubes, which may have medical applications in the nearer term.

Teramac computer has the capacity to perform 1012 operations in one seconds but it has 220,000 hardware defects and still has performed some tasks 100 times faster than single-processor .The defect-tolerant computer architecture and its implications for moletronics is the latest in this technology. So the very fact that this machine worked suggested that we ought to take some time and learn about it.

Such a 'defect-tolerant' architecture through moletronics could bridge the gap between the current generation of microchips and the next generation of molecular-scale computers.

Introduction

Recently, there have been some significant advances in the fabrication and demonstration of individual molecular electronic wires and diode switches. Some novel designs for several such simple molecular electronic digital logic circuits: a complete set of three fundamental logic gates: (AND, OR, and XOR gates), plus and adder function built up from the gates via the well-known combinational logic, was demonstrated. This means in coming future, this technology could be a replacement for VLSI. However, currently, this technology is only available under lab condition. How to mass product moletronics chips is still a big problem.

Currently, integrated circuits by etching silicon wafers using beam of light. It's the VLSI lithography-based technology makes mass production of Pentium III processor possible. But as the size of logic block goes to nano-scale, this technology no long available. As wavelength get too short, they tend to become X-rays and can damage the micro structure of molecules. On the other hand, the mask of lithography of Pentium III is so complex, and the shape and the dimension of its logic block varies so much. Looking at currently available integrated circuits, the transistor density of memory chip are much higher than processor chip, the reason is that the cell of memory is much more simple than circuit of processor. Because, except the decoding logic, most of the memory bit cell is the same. Could we find a way to fabricate complex logic circuit as Pentium processor using million of same logic units? The PLD(Programmable Logic Devices) is the answer. The paper is organized as following: section II presents some basic of moletronic gate circuit. section III uses PLD technology to build more complex blocks. section IV shows the nanotube can be used for interconnection wires.

To accomplish the Moletronics goal, there are three parallel tasks. The first task is the development and optimization of molecular devices such as switches, multistate molecules, and molecules exhibiting highly non-linear characteristics. The second task is learning how to build a Moletronic computer made from these molecular devices. We believe hierarchical self- assembly processes will manufacture them. Hierarchical self-assembly is a set of processes that will first assemble individual devices, then create functional circuits or blocks from those devices, and finally put together the ensemble from the blocks. It is a bottom-up manufacturing process whereas current microelectronics fabrication is based on top-down manufacturing using lithography. The third task is the development of circuit architectures. These architectures encompass how to program the molecular circuit as well as exhibit defect tolerance. Moletronic circuits will have defects. We will have to cope with them, in the vernacular, the fleas come with the dog. Approaches to quantify the level of defects that can be tolerated, the time needed to find and route around the defects, and the time to program a circuit with large numbers of devices need to be developed under the architecture umbrella. Moletronic circuit--QCA basics

We discuss an approach to computing with quantum dots, Quantum-dot Cellular Automata (QCA), which is based on encoding binary information in the charge configuration of quantum-dot cells. The interaction between cells is Coulombic, and provides the necessary computing power. No current flows between cells and no power or information is delivered to individual internal cells. Local interconnections between cells are provided by the physics of cell-cell interaction. The links below describes the QCA cell and the process of building up useful computational elements from it. The discussion is mostly qualitative and based on the intuitively clear behavior of electrons in the cell.

Fundamental Aspects of QCA

A QCA cell consists of 4 quantum dots positioned at the vertices of a square and contains 2 extra electrons. The configuration of these electrons is used to encode binary information. The 2 electrons sitting on diagonal sites of the square from left to right and right to left are used to represent the binary "1" and "0" states respectively. For an isolated cell these 2 states will have the same energy. However for an array of cells, the state of each cell is determined by its interaction with neighboring cells through the Coulomb interaction. A schematic diagram of a four-dot QCA cell is shown in Fig. 1.

Figure: Schematic of the geometry of the basic four-site cell. The tunneling energy between two neighboring sites is designated by t, while a is the near-neighbor distance.

If the barriers between cells are sufficiently high, the electrons will be well localized on individual dots. The Coulomb repulsion between the electrons will tend to make them occupy antipodal sites in the square a shown in Fig. 2. For an isolated cell there are two energetically equivalent arrangements of the extra electrons which we denote as a cell polarization P = +1 and P = -1. The term "cell polarization" refers only to this arrangement of charge and does not imply a dipole moment for the cell. The cell polarization is used to encode binary information - P = +1 represents a binary 1 and P = -1 represents a binary 0.

Figure: Coulombic repulsion causes the electrons to occupy antipodal sites within the cell. These two bistable states result in cell polarizations of P = +1 and P = -1.

The two polarization states of the cell will not be energetically equivalent if other cells are nearby. Consider two cells close to one another as shown in the inset of Fig. 3. The figure inset illustrates the case when cell 2 has a polarization of +1. It is clear that in that case the ground-state configuration of cell 1 is also a +1 polarization. Similarly if cell 2 is in the P = -1 state, the ground state of cell 1 will match it. The figure shows the nonlinear response of the cell-cell interaction.

Figure: The cell-cell response

A Majority Gate

Fig. 4 shows the fundamental QCA logical device, a three-input majority gate, from which more complex circuits can be built. The central cell, labeled the device cell, has three fixed inputs, labeled A, B, and C. The device cell has its lowest energy state if it assumes the polarization of the majority of the three input cells. The output can be connected to other wires from the output cell. The difference between input and outputs cells in this device, and in QCA arrays in general, is simply that inputs are fixed and outputs are free to change. The inputs to a particular device can come from previous calculations or be directly fed in from array edges. The schematic symbol used to represent such a gate is also shown in Fig. 4. It is possible to "reduce" a majority logic gate by fixing one of its three inputs in the 1 or 0 state. If the fixed input is in the 1 state, the OR function is performed on the other two inputs. If it is fixed in the 0 state, the AND function is performed on the other two inputs. In this way, a reduced majority logic gate can also serve as a programmable AND/OR gate. Combined with the inverter shown above, this AND/OR functionality ensures that QCA devices provide logical completeness

Figure: The Majority Gate

Programmable Logic Devices and Field Programmable Gate Array basics

The Programmable Logic Devices(PLD) are nothing new, they have been around for almost 20 years. Since PLD device exists, it makes the life of a lot of Electronic designer's life easy. It is well known that in order to design a digital system, besides microprocessors and peripheral ICs there are needed several other devices, such as lots of logic gates to glue these chips together. This circuits make our life and our printed boards very hard and complex. It exists a way to dramatically improve this way of design digital devices that, although it is not completely different from the others, brings the desired results more efficiently: in a shorter time and with fewer expenses. The way abovementioned is Programmable logic devices (PLD), they permit the customizing of one or more logic functions on a chip in contrast to the designer being restricted to defining a logic function with specific chips. The programmability aspect permits the logic designer to spend more time on the development and validation of high level functionality. The simplest Integrated circuit of the PLD is PAL/GAL. PAL(Programmable Array Device), which was invented at Monolithic Memories in 1978 PAL consists of an AND array followed by an OR array, either (or both) of which is programmable. Inputs are fed into the AND array, which performs the desired AND functions and generates product terms. The products terms are then fed into the OR array. In OR array, the output of various product terms are combined to produced the desired output. With PAL, we can implement any combinational logic circuit. How about the sequential logic circuits? There exists another kind of customized IC: Field Programmable Gate Array. See Fig. 7.

Figure: The Architecture of Field Programmable Gate Array, a combination of PLD and Masked Programmed Gate Array(MPGA).

Unlike the traditional fully customised VLSI circuits, Field Programmable Gate Array(FPGAs) represent a technical breakthrough in the corresponding industry. Before they were introduced, an electronic designer had only a few options for implementing digital logic. These options included discrete logic devices (VLSI or SSI); programmable devices (PALs or PLDs); and Masked Programmed Gate Arrays(MPGA) or Cell-Based ASICs. A discrete device can be used to implement a small amount of logic. A programmable device is a general-purpose device capable of implementing the logic of tens or hundreds of discrete devices. Users at their site using programming hardware program it.

The size of a PLD is limited by the power consumption and time delay. In order to implement designs with thousands or tens of thousands of gates on a single IC, MPGA can be used. An MPGA consists of a base of pre-designed transistors with customized wiring for each design. The wiring is built during the manufacturing process, so each design requires custom masks for the wiring. The cost of mask-making is expensive and the turnaround time is long (typically four to six weeks). The availability of FPGAs offer the benefits of both PLD and MPGA. FPGAs can implement thousand of gates of logic in a single IC and it can be programmed by users at their site in a few seconds or less depending on the type device used. The risk is low and the development time is short. These advantages have made FPGAs very popular for prototype development, custom computing, digital signal processing, and logic emulation. From the architecture of PLD and FPGA, we could see repeated logic cell. Thus, density of this kind of chip increased very quickly. Just a few years ago, a high-density FPGA consisted of 50K gates and was used for glue logic. Today's FPGA are multi-million system gate devices at the heart of electronic systems in some of the fastest growing high-tech markets. There is a lot of computer around the world using FPGA processors.

Interconnection: nanotube

Today, one way to pack transistors more densely on a chip is to make the already microscopic wires smaller and thinner. But the wires are approaching the thickness of a few hundred atoms. Once wires get down to only several atoms thick, says IBM researcher Phaedon Avouris, they blow up when you try to send electrical signals through them. Nanotubes don't. IBM and others are racing to use nanotubes to make the first carbon chips, perhaps the successor to silicon chips, though the program is only in the earliest stages. A carbon nanotube is a tubular form of carbon with a diameter as smaller as 1 nm. The length can be from a few nanometers to several microns. (1 micron is equal to 1,000 nanometers.) It is made of only carbo atoms. To understand the CNT's structure, it helps to imagine folding a two-dimensional graphene sheet. Depending on the dimensions of he sheet and how it is folded, several variations of nanotubes can arise. Also, just like the singel or the multilayer nature of graphene sheets, the resulting tubes may be a single- or a multiwall type. The tube's orientation is denoted by a roll-up vector(See Fig.8) . Along this vector, the graphene sheet is rolled into a tubular from. The and are vectors defining a unit cell in the planer graphene sheet. n and m are integers, and is the angle. A variety of tubes-based on the orientations of the benzyne rings on the graphene tube-are possible. If the orientation is parallel to the tube axis, then the resulting "zigzag" tubes are semiconductors. When the orientation is perpendicular to the tube axis, the corresponding "arm chair" tubes are metallic. In between the two extremes, when (n-m)/3 is an integer, the nanotubes are semimetallic. The two key parameters, the diameter d and the chiral angle , are related to (n,m) by ,. For example, a(10,10) nanotube is 1.35 nm in diameter whereas a (10,10) tube is 0.78nm in diameter. Carbon nanotubes exhibit extraordinary mechanical properties as will. For example, the Young's modulus is typically over 1 Tera Pascal. Also, the nanotube along the axis is as stiff as a diamond. The estimated tensile strength is about 200 Gpa, which is an order of magnitude higher than that of any other material. Here we are mainly interested in carbon nanotube's electronic behavior and applications. The metallic and semiconducting nature described previously has given rise to the possibilities of metal-semiconductor or semiconductor junctions. These junctions may form nanoelectronic devices based entirely on single atomic species such as carbon.

Figure: Carbon nanotubes: their structure, properties and uses in nano-electronic devices

Fault tolerance: TeraMac

Teramac is a massively parallel experimental computer built at Hewlett-Packard Laboratories to investigate a wide range of different computational architectures. It is a true supercomputer, capable of operating 100 times faster than a high-end workstation for some configurations. Teramac also contains about 220,000 defects, any one of which could prove fatal to a more conventional machine. The architecture of Teramac, the philosophy behind its construction, and its ability to tolerate large numbers of defects have significant implications for any future nanometer-scale computational paradigm. It is not necessary to chemically synthesize perfect devices with a 100% yield and assemble them into a completely deterministic network in order to obtain a reliable and powerful system. Future computers may not have a central processing unit, but may instead be an extremely large configurable memory that is trained for specific tasks by a tutor. In this article, we will describe Teramac with particular emphasis on those aspects most relevant to scientists interested in developing computational nanotechnology. Several concepts related to the logical architecture of Teramac are graphically presented here. (A) The Cross Bar represents the heart of the configurable wiring network that makes up Teramac.

The inset shows a configuration bit (a memory element) that controls a switch. The bit is located and configured using the address lines, and its status is read using the data lines. The cross bar provides not only a means of mapping many configuration bits together into some desired sequence, but it also represents a highly redundant wiring network. Between any two configuration bits, there are a large number of pathways, which implies a high communication bandwidth within a given cross bar. Logically, this may be represented as a 'fat tree.' Such a 'fat tree' is shown in (B), where it is contrasted with a standard tree architecture. Note that both trees appear the same from the front view, but from an oblique view, the fat tree has a bandwidth that the standard tree does not. Color coded dots and a dashed box are included to show the correspondence between a given level of the fat tree and the cross bar in (A). See figure.9.

Figure: The Majority Gate

MoleComputing: Ultra-Dense Molecular Electronic Computer Processor.

The MoleApps computing thrust will develop and demonstrate a programmable nanoprocessor system. This computational system will be integrated on the molecular scale, occupy a total area of only 100 square micron, and operate at room temperature with logic and arithmetic processing capability comparable to the original Intel 4004 microprocessor. While the planned nanoprocessor will feature complexity comparable to the classic, 1971-vintage Intel 4004 microprocessor, it will occupy an area 100,000 times smaller. Accordingly, it will be 100 times as dense as systems presently promised by industry. Further, using methods of hierarchical nanofabrication and nanoassembly, the system will be much less costly to manufacture than those projected for the end of the ITRS Roadmap. MoleSensing: Ultra-Dense Molecular Electronic Sensor System. The MoleApps sensor thrust will develop and demonstrate a nanosensor system that also is integrated on the molecular scale, occupies a total area of only 100 square micron, and is capable of detecting chemical/biological agents with sensitivity and selectivity comparable to current state-of-the-art detectors. To achieve these objectives, 1,000 individual nanosensors will be placed in an ultra-dense array that occupies only one square micron.

CHARACTERISTICS OF MOLECULAR DEVICES

Nonlinear I-V Behavior

Unlike solid-state electronics, the I-V behavior of a molecular wire is nonlinear. Some molecular devices will take advantage of this nonlinearity.

Energy Dissipation

When electrons move through a molecule, some of their energy can be lost to other electrons motions and the motion of the nuclei of the molecule. The amount of energy lost depends on the electronic energy levels of the molecule and how they interact with the molecules vibrational modes. Depending on the mechanism of conductance, the energy loss can range from very small to significantly large.

Gain in Molecular Electronic Circuits

In large molecular structures deploying molecular devices with power gain, such as molecular transistors, there will be a need to restore signal loss. Gain is needed in order to achieve signal isolation, maintain signal-to-noise ratio, and to achieve fan-out.

Speeds

Energy dissipation relates closely to the speed at which a molecular electronic circuit can operate. If strong couplings cause the signal-to-noise ratio to dramatically decrease, a greater total charge flow would be needed to ensure the reading of a bit. This would require more time. Because of their scale and density, molecular electronic computers may not need to be faster than semiconductor computers to be highly important. The molecular half-added described earlier is one million times smaller than one in a Pentium processor.

Optical information technology

The ever growing demand of increased computing speed is mainly limited by memory accessing time and storage capacity. Optical storage and accessing can remove these problems, as optical speed is the ultimate speed.

Photo chromic materials show a bistable property. They undergo reversible color changes under irradiation at an appropriate wavelength. The photon absorption technique of photo chromic material, in order to build a three-dimensional optical memory, appears appropriate to build a three-dimensional optical memory. Applications of electronic materials in displays and optical filters have also been conceptualized.

With the advent of optical fiber communication an interest in components for processing optical signals has arisen. On the other hand, in order to avoid the drawbacks of conventional electronics IC technology such as problems of parasitic capacitance, inductance and resistance, less reliability and power dissipation there has arisen the need to use optical integrated circuits (OICs) in proposed all optical computers where full advantage of the fundamental speed of light is proposed to be achieved.

Nonlinear optics (NLO) is a new frontier of science and technology, multi-disciplinary in nature, which has potential applications in computer communication and information technology. Current research has made available organic NLO materials with properties superior to those of inorganic NLO materials. Discovery of laser in 1960s has given a thrust to the research of NLO materials and their applications.

Nonlinearity can be used basically in two ways for electronic devices: frequency conversion and refractive index modulation. Frequency conversion technique which is due to second order linearity, may be used for second harmonic generation, frequency mixing and parametric amplification, etc. the prime interest of second harmonic generation is for optical data storage.

Molecular Scale Electronics

The quest forever decreasing size but more complex electronic component with high speed ability gave birth to MSE. The concept that molecules may be designed to operate as self constrained devices was put forward by Carter, who proposed some molecular analogues of conventional electronic switches, gates and connections. Accordingly a molecular p-n junction gate was proposed by Aviram and Rather. MSE is a simple interpolation of IC scaling. Scaling is an attractive technology. Scaling of FET and MOS transistors is more rigorous and well defined than that of bipolar transistors.

Silicon technology has offered us SSI, LSI, VLSI and finally we have ULSI. Such technologies make even the logic gate minimization technique redundant. Today integration barrier of 2.5 million transistors on a chip is over. But there are some problems now in further scaling in silicon technology. For instance, power dissipation and quantum effect are posing problems for increasing packing density.

MSE is a remedial measure. Molecules possess great variety in the structure and properties. Therefore finding molecules and their appropriate properties for electronics, opto-electronics and bio-electronics is possible the study of a single molecule is not a problem now as we have STM (scaling tunneling microscope), AFM (atomic force microscope),L-B technique etc.

FUTURE DEVELOPMENTS

At some of the top laboratories around the country, scientists are publicly expressing beliefs that before now they would only express in private: electronics technology is on the edge of a molecular revolution where molecules will be used in place of semiconductors, creating electronics circuit small that their size will be measured in atoms not microns. They are boldly predicting that the impact on computing speed and memory resulting from circuits so small would stagger virtually all fields of technology and business.

Research teams from Rice and Yale Universities say that they have successfully created molecular size switches that can be opened and closed repeatedly. The HP/UCLA group had only reported being able to switch once, not repeatedly. Repeated switching is necessary to build functioning digital computers. These breakthroughs in the field of molecular electronics seem to be giving researches a new sense of confidence.

There are several research groups working in laboratories under top-secret conditions. They are making progress on several fronts. One of them is said to be working on molecular scale Random Access Memory (RAM). RAM, on a molecular scale, could offer incredibly huge storage capacities. Molecular methods could make it available at costs so low as to be pocket change. Because of the very small scale of such devices, it might be possible to store, for e.g., a DVD movie on something the size of a grain of rice.

The micro electronic devices on todays silicon chips have components that are 0.18 microns in size or about one thousandth the width of a human hair. They could go as small as 0.10 microns or hundred nanometers. In molecular electronics, the components could be as tiny as 1 nanometer. This would make for a new breed of super powerful chips and computers so small that could be incorporated into all manmade items.

The semiconductor world predicts it will continue to advance the silicon-based chip, making ever-smaller device, through the year 2014. But the costs involved with these advancements are enormous. Currently semiconductor chips are made in multibillion-dollar fabrication plants by etching circuitry into layers of silicon with light waves. Its a very expensive process and each new generation requires huge amounts of money to upgrade to newer fab-plants. The world of computers is in for a change.

Several computer semiconductor companies, including Sun Microsystems and Motorola have been meeting to consider forming a consortium that would look for commercial uses for molecular electronics. Researches say that this is still only the beginning in the making of molecular computers. There are still many obstacles to over come before molecular computers become reality.

Some researches believe that in order for molecular systems to work as computers, they will need to have fault tolerant architectures. Several groups are working on such devices. The progress made recently has caused a lot of excitements among researches in molecular electronics. For a long time, they have had the vision but have had few results. Now they are looking towards the future and have results that are helping to map the way for them.

Conclusion

Even a lot of approach has been proposed in Moletronic computer. But there still exists critical problem: most of the technologies are valid only in laboratory condition, and cannot be produced massively.

References

1. www.seminarsonly.com

2. www.ctcase.org

3. www.sciencewatch.com

4. www.technologyreview.com

5. www.gongoozler.com

6. www.foresight.org

7. www.nano.org.uk

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