First Single Molecule Transistor example essay topic

We live in the technology age. Nearly everyone in America has a computer or at least access to one. How big are the computers you are used to? Most are about 7' by 17' by 17'. That's a lot of space.

These cumbersome units will soon be replaced by something smaller. Much smaller, we " re talking about computers based on lone molecules. As far off as this sounds, scientists are already making significant into researching the feasibility of this. Our present technology is composed of solid-state microelectronics based upon semiconductors. In the past few years, scientists have made moment us discoveries. These advances were in molecular scale electronics, which is based on the idea that molecules can be made into transistors, diodes, conductors, and other components of microcircuits.

(Scientific American) Last July, researchers from Hewlett-Packard and the University of California at Los Angeles announced that they had made an electronic switch of a layer of several million molecules and. 'Rotax ane is a. A is a compound consisting of cyclic moles threaded by a linear molecule. It also has no covalent interaction. In, there are bulky blocking groups at each end of the threaded molecule.

' (Scientific American) The researchers linked many of these switches and came up with a rudimentary AND gate. An AND gate is a device which preforms a basic logic function. As much of an achievement as this was, it was only a baby step. This million-molecule d switch was too large to be useful and could only be used once. In 1999, researchers at Yale University created molecular memory out of just one molecule.

This is thought to be the 'last step down in size' of technology because smaller units are not economical. The memory was created through a process called 'self-assembly'. 'Self-assembly' is where computer engineers 'grow' parts and interconnections with chemicals. (Physics News Update, 1999) This single molecule memory is better than the conventional silicon memory (DRAM) because the it live around one million times longer. ' 'With the single molecule memory, all a general-purpose ultimate molecular computer needs now is a reversible single molecule switch,' says Reed (the head researcher of the team.) 'I anticipate we will see a demonstration of one very soon. ' ' (Yale, 1999) Reed was correct.

Within a year, Cees Dekker and his colleagues at Delft University of Technology in the Netherlands had produced the first single molecule transistor. Dekker won an innovation award from Discover magazine for the switch which was also built from a lone molecule. The molecule they used was the carbon nanotube. It's composition is of a lattice of carbon atoms rolled up into a long, narrow tube, one billionth of a meter wide. These can conduct electricity or, depending on how the tube is twisted, they can semiconductors. The semiconducting nanotube is the only active element in the transistor.

The transistor works like it's silicon relatives, but in much less space. Dekker did, however, emphasize that they had made only a prototype. 'Although it is 'a technologically usable device,' he says, there's still a long way to go. The next steps include finding ways to place the nanotube's at the right locations in an electronic circuit, probably by attaching chemical guides that bind only to certain metals. ' (Discover) From there, we go back to Yale where efforts were being put forth to make a better switch. Mark Reed and his colleagues were at work on a different class of molecules.

To make a switch the inserted regions into the molecules, that when made subject to certain voltages, trapped electrons. If the voltage was varied, they could continuously change the state of the molecules from nonconducting to conducting, the requirements of a basic switch. Their device was composed of 1,000 nitro mine benzene thiol molecules in between metal contacts. One interesting development was to find that these micros witches indeed followed Moore's Law. Moore's Law says that each transistor chip contains approximately twice the memory of it's predecessor. The chips also come out 18-24 months later.

This demonstrates a rising exponential curve in the development of transistors. Engineers can now put millions of transistors on a sliver of silicon just a few square centimeters. Moore's Law does show that even technology has it's limits, as it can get only so small and stay economically possible. (Physics News Update) Free electrons can take on energy levels from a continuous range of possibilities. But in atoms or molecules, electrons have energy levels that are quantized: they can only be any one of a number of discrete values, like rungs on a ladder. This series of discrete energy values is a consequence of quantum theory and is true for any system in which the electrons are confined to an infinitesimal space.

In molecules, electrons arrange themselves as bonds among atoms that resemble dispersed 'clouds,' called orbitals. The shape of the orbital is determined by the type and geometry of the constituent atoms. Each orbital is a single, discrete energy level for the electrons. Even the smallest conventional micro transistors in an integrated circuit are still far too large to quantize the electrons within them. In these devices the movement of electrons is governed by physical characteristics -- known as band structures -- of their constituent silicon atoms. What that means is that the electrons are moving in the material within a band of allowable energy levels that is quite large relative to the energy levels permitted in a single atom or molecule.

This large range of allowable energy levels permits electrons to gain enough energy to leak from one device to the next. And when these conventional devices approach the scale of a few hundred nanometers, it becomes extremely difficult to prevent the minute electric currents that represent information from leaking from one device to an adjacent one. In effect, the transistors leak the electrons that represent information, making it difficult for them to stay in the 'off's tate. The standard methods of chemical synthesis allow researchers to design and produce molecules with specific atoms, geometries and orbital arrangements. Moreover, enormous quantities of these molecules are created at the same time, all of them absolutely identical and flawless. Such uniformity is extremely difficult and expensive to achieve in other batch-fabrication processes, such as the lithography-based process used to produce the millions of transistors on an integrated circuit.

The methods used to produce molecular devices are the same as those of the pharmaceutical industry. Chemists start with a compound and then gradually transform it by adding prescribed reagents whose molecules are known to bond to others at specific sites. The procedure may take many steps, but gradually the pieces come together to form a new potential molecular device with a desired orbital structure. After the molecules are made, we use analytical technologies such as infrared spectroscopy, nuclear magnetic resonance and mass spectrometry to determine or confirm the structure of the molecules. The various technologies contribute different pieces of information about the molecule, including its molecular weight and the connection point or angle of a certain fragment.

(Physics) By combining the information, we determine the structure after each step as the new molecule is synthesized. 'Once the assembly process has been set in motion, it proceeds on its own to some desired end [see 'Self- Assembling Materials,' by George M. White sides; Scientific American, September 1995]. In our research {Reed} we use self-assembly to attach extremely large numbers of molecules to a surface, typically a metal one [see illustration on self- assembly]. When attached, the molecules, which are often elongated in shape, protrude up from the surface, like a vast forest with identical trees spaced out in a perfect array. ' (Scientific American) Handy though it is, self-assembly alone will not suffice to produce useful molecular-computing systems, at least not initially.

For some time, they will have to combine self-assembly with fabrication methods, such as photolithography, borrowed from conventional semiconductor manufacturing. In photolithography, light or some other form of electromagnetic radiation is projected through a stencil-like mask to create patterns of metal and semiconductor on the surface of a semiconducting wafer. In their research they use photolithography to generate layers of metal interconnections and also holes in deposited insulating material. In the holes, they create the electrical contacts and selected spots where molecules are constrained to self-assemble.

The final system consists of regions of self-assembled molecules attached by a maze like network of metal interconnections. The molecular equivalent of a transistor that can both switch and amplify current is yet to be found. But researchers have taken the first steps by constructing switches, such as the twisting switch described earlier. In fact, Jia Chen, a graduate student in Reed's Yale group, observed impressive switching characteristics, such as an on / off ratio greater than 1,000, as measured by the current flow in the two different states. For comparison, the device in the solid-state world, called a resonant tunneling diode, has an on / off ratio of around 100.

(Yale Bulletin) 'Foremost among them is the challenge of making a molecular device that operates analogously to a transistor. A transistor has three terminals, one of which controls the current flow between the other two. Effective though it was, our twisting switch had only two terminals, with the current flow controlled by an electrical field. In a field-effect transistor, the type in an integrated circuit, the current is also controlled by an electrical field. But the field is set up when a voltage is applied to the third terminal. ' (Scientific American) Another problem with molecular switches is the thermodynamics.

A microprocessor with 10 million transistor gives and a clock cycle of half a gigahertz gives off 100 watts, which is much hotter than a stove top. Finding the minimum amount of heat that a single molecular device emits would help limit the number of devices we could put on a chip or substrate of some kind. Operating at room temperature and at today's speeds, this fundamental limit of a molecule is about 50 pico watts (50 millionths of a millionth of a watt). That suggests an upper limit to the number of molecular devices we can closely utilize: it is about 100,000 times more that what is possible now with silicon micro transistors on a chip. That may seem like a vast improvement, it is far below the density that would be possible if we did not have to worry about heat. The smaller you get, the more problems you also come across.

We are already in countering many problems in the fabrication of silicon based chips. These problem will become worse with each step down in size until they are no longer useful (or until they no longer function.) The bigger problem is that this is bound to occur before computer science is able to achieve it's primary goal of creating a viable working 'brain. ' This means that the possibility of creating artificial life forms, or 'androids,' is slim at this point due to the expected impasse in technology.

Bibliography

Reed, Mark and James Tour. 'Comparing with Molecules. ' Scientific American. July; 2000.
Scheme, Philip and Stein, Ben. 'Physics News Update: Implementation of Molecular Switches. ' The American Institute of Physics Bulletin of Physics News. July 16; 1999.
Unknown Author. 'Yale Research Team First to Describe Molecular-Sized Memory -- Discover Has Implications for Drastically Reducing Cost of Computer Memory. ' November 1, 1999.