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Switching the polyacetylene soliton
MATERIALS SClENCE& ElGlWEERlNG ELSEVIER
MaterialsScience and Engineering: C 3 (1995) 181-185
C
Switching the polyacetylene soliton Michael P. Groves a, Chris F. Carvalho b, Rolf H. Prager ’ aDepartment of Computer Science, The Flinders University of South Australia, GPO Box 2100, Adelaide 5001, Australia b School of Chemistry, Queensland University of Technology, GPO Box 2434, Brisbane 4001, Australia ’ Department of Chemistry, The Flinders University of South Australia, GPO Box 2100, Adelaide 5001, Australia
Abstract Molecular electronic device:s offer a way to make electronic circuits thousands of times smaller by building them up from the chemical realm, using small groups of atoms to form electrical switches. Single strands, only a few atoms thick, of the electrically conductive plastic polyacetylene could be used to interconnect these switches. Electrons are considered to travel down polyacetylene strands as solitons. This paper presents a number of simple chemical structures for the soliton switch. The advent of these simple structures makes it reasonable to consider making soliton switches, which is the first step towards soliton circuits. Designs for simple logic gates and a memory cell based on the soliton switch are also presented here, to illustrate the general usefulness of soliton switches in forming digital electronic devices. Keywords: Molecular
electronic devices; Solitons; Switches;Polyactylene;Soliton circuits;Digital electronics
1. Introduction Every two or three years, researchers have worked out how to make the transistors on silicon chips smaller to the extent of fitting twice as many on a given chip. More transistors means quicker, more powerful and more complex circuits. That the number of transistors per chip has been increasing exponentially for the last 40 years is truly remarkable. However, this reduction in size cannot go on forever; indeed, the problems of making silicon circuits smaller is rapidly approaching some practical and theoretical limits. On the other hand, if we build up chemically from the molecular level, it would be possible to make circuits thousands of times smaller. These molecular circuits would use chemical molecules as electronic switches and be interconnected by some sort of ultra-fine conducting wires. One interesting possibility for these conductors was proposed by Carter [l] and is to use single strands of the electrically conductive plastic polyacetylene. Electrons are thought to travel along polyacetylene in little packets called solitons. Hence molecular scale electronic devices constructed from molecular switches and polyacetylene chains are called soliton circuits. A key element of soliton circuits is the soliton switch [ 21. The soliton switch is a device with two polyacetylene chains running through it. These chains are not electrically connected, but are physically connected within the switch in such a way that the passage of solitons on one chain can either enable or disable the conduction of solitons on the other chain. 0928-4931/95/$09.50 0 1995 SSo10928-4931(95)00081-X
Elsevier Science S.A. All rights reserved
The era of molecular electronic devices being just theoretical speculation is coming to an end with the recent synthesis and possible demonstration [3] of the molecular rectifier proposed by Aviram and Ratner [4] in 1974. There is also the practical work done towards molecular machines by Stoddart [ 5,6]. Recent attempts to make the soliton switch have led to simpler switch structures and it is a major aim of this paper to advertise those structures in an attempt to interest other synthetic organic chemists in making them. To this end, chemical structures are used for all the devices and electronic symbols have been avoided. It will be shown how two soliton switches can be connected in series to give a structure with a basic AND property, in which both the switches (one AND the other) must be enabled before conduction is possible on the chain through them. Two switches can also be connected in parallel to give a simple OR structure. The AND and OR operations are fundamental to Boolean gates. Boolean gates can be used to construct most of the devices in digital electronics. Finally, two switches will be combined in a different manner to form a simple memory cell.
2. Solitons in polyacetylene As depicted in Fig. 1, polyacetylene consists of a chain of carbon atoms held together by alternating double and single bonds. Each carbon atom is also bonded to a hydrogen atom.
M.P. Groves et al. /Mareriuls Science and Engineering: C3 (1995) 181-185
182
Fig. 1. Solitons
in polyacetylene. Fig. 2. The soliton junction.
Polyacetylene has two stable states, which differ in the position of the double bonds with respect to the carbon atoms. A soliton is a moving wave which causes conversion between the two states of polyacetylene. It effectively picks up one arrangement of bonds and lays down the other. The effect of solitons on the state of a polyacetylene chain is shown by the arrows in Fig. 1. The small curved arrows represent the movement of a pair of electrons. The combination of all these movements is the passage of a soliton, which is shown by the large wavy arrows at the top of Fig. 1. Because there are only two states for polyacetylene, a soliton in either direction along the chain will always cause the same change in state. That is, had the left-hand side of Fig. 1 shown a soliton moving from right to left, the result would still have been the state depicted on the right-hand side of the figure. It should be noted that two successive solitons (in the same or opposite directions) will leave the state of a polyacetylene chain unchanged. Polyacetylene is not the only conducting polymer, but it is one of the few with two equal energy ground states, which makes it very suitable for binary calculations. The mechanism for electrical conduction in polyacetylene and other conducting polymers is discussed by Rubner [7]. These polymers have many interesting properties that enable them to be used to make flexible light emitting diodes [ 81 and even to create transistors [ 91.
3. Soliton junctions The soliton junction, which is shown in Fig. 2, was introduced by Carter [ 11. The junction has three polyacetylene chains joined to a central carbon atom. Junctions allow the branching and interconnection of polyacetylene chains. The three states of a soliton junction are each characterized by the position of the double bond on the central carbon atom. Solitons passing through the junction cause its state to swap between the three states shown in Fig. 2. Only alternating double and single bonded paths can transmit solitons. Hence no solitons can pass between the two chains connected to the central carbon by single bonds, as the path between these chains includes two consecutive single bonds. Alternatively, it can be said that all solitons must pass through the central carbon’s double bond.
the two soliton paths which run through it. A possible structure for a soliton switch is given in two of its possible stable states in Fig. 3. This structure has a central four-membered carbon ring, which is drawn to indicate that it is roughly perpendicular to the plane of the paper. Solitons can at times flow along either the two polyacetylene chains connected to the top of the switch or through the chains connected to the bottom of the switch. On the lefthand side of the figure, this switch structure is shown in its clear state. In this state, the left-hand and right-hand ends of the switch can pivot in the plane of the paper around the central ring. The nitrogen atoms can form a single bond to carbon (as shown on the right-hand side of Fig. 3), but only when they are within some distance of each other. When a soliton is sent along either of the top or bottom paths through the switch and the nitrogen-carbon bond forms, the other path cannot pass a soliton, as the opposite nitrogen atom is held firmly outside its bonding distance. A soliton along the path through the top of the switch will change the state of the bonds through which it passes and give the state shown on the right-hand side of the figure. In this state there can be no conduction along the lower path because the lower nitrogen atom is held firmly outside its bonding distance. Another soliton sent on the top path would clear the switch, by restoring the switch structure to the situation shown on the left-hand side of Fig. 3. In this manner, solitons along either of the paths through a switch can stop conduction of solitons along the other path. Conduction on the second path can only be restarted by another soliton along the first path. So we have a simple electrical switch, in which the passage of a soliton along either path through the switch can control conduction on the other path. Note that there is no electrical connection between the two paths through a switch, but there is a physical connection through movement about the central carbon ring. There are many possible variations in structure for the soliton switch. In the left-hand side of Fig. 4 for example, the nitrogen atoms have been replaced by oxygen, though it would probably be much more difficult to open this switch, as the closed form would be a fairly stable ester. Many other
w \N
4. Soliton switches The other component to be discussed is called a soliton switch, because it can control the movement of solitons on
R’
Fig. 3. The soliton switch.
k
7
I
/\ JX
M. P. Groves et al. /Materials Science and Engineering: C 3 (1995) 181-185
183
structures to external electrical devices. At that point, prototype soliton circuits could be made, and the behaviour of the circuits and components could be thoroughly tested and fully verified.
'N
N
R’
Fig. 4. Other structures for the soliton switch.
6. Combining switches
atoms, such as sulphur, phosphorous and even silicon could be tried in place of the nitrogen atoms. Also, on the righthand side of Fig. 4, the central four-carbon ring has been replaced by a six-carbon ring. This structure would probably operate more slowly, as a soliton could not pass until the key carbon and nitrogen atoms draw together. Almost any difference between single and double bonds could be the basis for designing a soliton switch. In addition to the hinge type discussed here, there are also possibilities based on the stereochemical requirements of double bonds and the electron-rich nature of double bonds.
5. Testing the switches The properties of soliton switches will not be fully known until they have been made and connected to potential differences and electronic measuring devices. However, if a switch is made with electron-donating groups (EDG) and electronwithdrawing groups (EWG), as shown in Fig. 5, the basic operation of the switch can be tested by chemical means. The switch structure shown on the left-hand side of Fig. 5 is symmetrical about the dashed line through its centre. If the electron donating and withdrawing groups are correctly activated, they can cause a pair of electrons to move either along the top of the switch, as shown by the arrows, or, by symmetry, along the bottom. However, if the switch works correctly, once an electron pair has passed down one side of the switch, it should not be possible for electrons to pass down the other side. So the only product that should be obtained from the above reaction is the switch closed on one side only. Then the electron-donating and withdrawing groups could be suitably modified and the conditions changed so as to reverse the reaction and open the switch up again. Once a soliton switch is made and tested, the next step would be to connect a few together to form Boolean gates and memory cells, as described in the following sections. With the correct electron-d’onating and withdrawing groups, these structures could also be tested chemically. They could then be connected together to form larger and larger structures. At some point, it should be possible to connect these
R’
Fig. 5. Testing the operation of the soliton switch.
By using soliton switches it is not difficult to implement some of the basic logic operations of digital circuits. Connecting two switches in series, as shown in Fig. 6, will give an AND operation. In the structure shown, conduction will only be possible along the bottom path through both switches if both the left-hand AND the right-hand switches are in their clear states. That is, it only takes a soliton along one of the upper paths, through either of the switches, to stop conduction along the bottom path. Connecting two switches in parallel, as shown in Fig. 7, will give an OR operation. In the structure shown, conduction will be possible along the main path from left to right if either the top switch OR the bottom switch is in its clear state. That is, it requires solitons along the outside paths of both of these switches to stop conduction along the main path through the device. The properties of these structures fall short of Boolean gates, in that these structures do not offer ways to drive the input of one gate from the output of another. The inputs require single solitons and the outputs enable the passage of any number of solitons. Also, there is the question of where these solitons come from. Both of these problems are solved by an additional device called a soliton capacitor or SID. The soliton capacitor can store a single electronic charge. Soliton capacitors can be combined with soliton switches to form all the different types of Boolean gate [ 21.
Fig. 6. The AND operation
EWG
Fig. 7. The OR operation.
184
M.P. Groves et al. /Materials Science und Engineering: C3 (1995) 181-185
Given Boolean gates, it is possible the functions required to make digital be used to form memories; however, the polyacetylene chain offers a better
to perform nearly all of circuits. Gates can even the two-state nature of way to form memories.
7. Memories Two switches can be combined to form a simple memory cell, as shown in Fig. 8. The cell can only hold a single binary bit, which is represented by the value “0” or “1”. The value in this cell can be read by placing a soliton on the read input and seeing whether it is directed to the left-hand or right-hand output. The soliton being directed to the left-hand output indicates a stored value of “O”, and being directed to the right-hand output indicates that a value of “1” is stored in the cell. It is the state of the write chain, which runs through the top of both switches, that controls what is stored in this memory cell. In the state shown in Fig. 8, a soliton on the input chain cannot go to the left, as the switch on the left-hand side of the cell is closed. This soliton is therefore directed via the switch on the right-hand side of the cell to the output on the right, which signifies that the cell contains a value of “1”. The passage of this soliton will cause the bonds through which it passes to change, as indicated by the small curved arrows in Fig. 8. This gives a new state for the memory cell in which the right-hand switch is closed. The nature of this state is not important, except that another soliton along the same or opposite path will give back the state shown in Fig. 8. For this reason, the memory cell is designed to be read by pairs of solitons. The operation of writing values to this cell is accomplished by passing solitons along the write chain. In the state shown above, a soliton along the write chain would open up the lefthand switch and close the switch on the right-hand side of the figure. This would cause solitons on the read input to be directed out of the left-hand output, which would signify that the cell contained a stored value of “0”. This same structure can also be used to select one of two possible memory cells for reading. The two memory cells are. first connected to the outputs of this cell. Then the state of the write line of this cell can be used to direct solitons on the read input to the appropriate memory cell. Larger memories can then be constructed using several layers of selectors, arranged so that it is possible to select each individual cell in the memory [ lo].
8. Conclusions The brain provides ample proof that molecular memories and calculating devices are possible, but are they feasible? Given that scientific progress continues to gain momentum, this is really a question of when molecular electronic devices will become feasible. Quite simply, if the feature size of digital memories is to continue to shrink, it must get down to the molecular level, and when it does, as in the biological realm, we will be building up from molecules. It is also relevant to ask whether, at the point at which it is feasible to make molecular electronic devices, there will be some other technology which is better. For example, would an approach based more on existing biological systems [ 11,121 be better? In demonstrating how soliton circuits can perform the logic and memory functions of digital electronics, a previous paper [ 21, though quite impractical, clearly showed the great potential of soliton circuits. The much simpler structures for soliton switches and the method for their chemical verification presented here make the whole proposal of soliton circuits far more reasonable and clearly indicate the first steps towards realizing their potential. The potential of the second step, which is to combine soliton switches together, was also outlined. Soliton switches can be used much like relays and combined to perform the simple logic functions required to form Boolean gates. While using soliton circuits to construct gates probably ignores their potential to form entirely new kinds of electronic devices, the ideas for Boolean gates predate the transistor [ 131, and once some simple gates have been made they can be used to perform most of the functions required for digital electronics. A brief introduction to how soliton switches can be combined to form memory cells and the circuits required to address those memories was also given here. Given the slow speed of soliton circuits, it is unclear whether they will ever be the medium of choice for digital calculations. However, their extremely small size and their non-volatile nature make soliton circuits ideal for building memories. The problems that need to solved before molecular memories can be built are very large and challenging and, as there has been no complete design of such a memory, it is not yet feasible to make a concerted effort to build one. However, the potential benefits of molecular memories are so great that it is well worth working toward this goal by thinking about the nature of such memories and continuing to work on the chemical synthesis of soliton circuits. Acknowledgement The work towards designing the switch structures presented here was funded by Circadian Technologies Ltd. References [l] F.L. Carter, Soliton switching
Fig. 8. A memory cell.
and its implications for molecular electronics, in F.L. Carter (ed.), Molecular Electronic Devices II, Marcel Dekker, New York, 1987, p. 149.
M.P. Groves et al. /Materials Science and Engineering: C 3 (1995) 181485 [21 M.P. Groves, Dynamic circuit diagrams for some soliton switching devices, inF.L. Carter (ed.), MolecularElectronicDevices II, Marcel
Dekker, New York, 1987, p. 183. [3] A.S. Martin, J.R. Sambles and G.J. Ashwell, Molecularrectifier,Phys. Rev. Lett., 70 (1993) 218. [4] A. Aviram and M.A. Ratner, Molecular rectifiers, Chem. Phys. Z.&r., 29 (1974) 277. [5] J.F. Stoddart, Whither and thither molecular machines, Chem. Aust., 59 (1993) 576. [6] R.A. Bissell, E. Cordova, A.E. Kaifer and J.F. Stoddart, A chemically and electrochemically switchable molecular shuttle, Nature, 369 (1994) 133. [ 71 M.F. Rubner, Conjugated polymericconductors,inG.J. Ashwell (ed.), Molecular Electronics, Research Studies Press, Taunton, UK, 1992, p. 65.
185
PI D. Cleary, After years in the dark, electric plastic finally shines, Science, 263 (25 March) ( 1994) 1700. PI D. Bloor, Totally organic transistors, Nature, 349 (28 February) (1991) 738. ]lOl M.P. Groves, CF. CarvaJho, C.D. Marlin and R.H. Prager, Using soliton circuits to build molecular memories, Austral. Comput. Sci. Commun., 15 (1) Part A (1993) 37. [ 111M. Conrad, Molecular computing: the lock and key paradigm, IEEE Computer, 25 (11) (1992) 11. 1121S.R. Hameroff, J.E. Dayhoff, R. Lahoz-Beltra, A.V. Samsonovich and S. Rasmussen, Models for molecular computation: conformational automata in the cytoskeleton, IEEE Computer, 25 ( 11) ( 1992) 30. [13] R.K. Richards, Digital Computer Components and Circuits, Van Nostrand, New Jersey, 1957, Ch. 3.
MaterialsScience and Engineering: C 3 (1995) 181-185
C
Switching the polyacetylene soliton Michael P. Groves a, Chris F. Carvalho b, Rolf H. Prager ’ aDepartment of Computer Science, The Flinders University of South Australia, GPO Box 2100, Adelaide 5001, Australia b School of Chemistry, Queensland University of Technology, GPO Box 2434, Brisbane 4001, Australia ’ Department of Chemistry, The Flinders University of South Australia, GPO Box 2100, Adelaide 5001, Australia
Abstract Molecular electronic device:s offer a way to make electronic circuits thousands of times smaller by building them up from the chemical realm, using small groups of atoms to form electrical switches. Single strands, only a few atoms thick, of the electrically conductive plastic polyacetylene could be used to interconnect these switches. Electrons are considered to travel down polyacetylene strands as solitons. This paper presents a number of simple chemical structures for the soliton switch. The advent of these simple structures makes it reasonable to consider making soliton switches, which is the first step towards soliton circuits. Designs for simple logic gates and a memory cell based on the soliton switch are also presented here, to illustrate the general usefulness of soliton switches in forming digital electronic devices. Keywords: Molecular
electronic devices; Solitons; Switches;Polyactylene;Soliton circuits;Digital electronics
1. Introduction Every two or three years, researchers have worked out how to make the transistors on silicon chips smaller to the extent of fitting twice as many on a given chip. More transistors means quicker, more powerful and more complex circuits. That the number of transistors per chip has been increasing exponentially for the last 40 years is truly remarkable. However, this reduction in size cannot go on forever; indeed, the problems of making silicon circuits smaller is rapidly approaching some practical and theoretical limits. On the other hand, if we build up chemically from the molecular level, it would be possible to make circuits thousands of times smaller. These molecular circuits would use chemical molecules as electronic switches and be interconnected by some sort of ultra-fine conducting wires. One interesting possibility for these conductors was proposed by Carter [l] and is to use single strands of the electrically conductive plastic polyacetylene. Electrons are thought to travel along polyacetylene in little packets called solitons. Hence molecular scale electronic devices constructed from molecular switches and polyacetylene chains are called soliton circuits. A key element of soliton circuits is the soliton switch [ 21. The soliton switch is a device with two polyacetylene chains running through it. These chains are not electrically connected, but are physically connected within the switch in such a way that the passage of solitons on one chain can either enable or disable the conduction of solitons on the other chain. 0928-4931/95/$09.50 0 1995 SSo10928-4931(95)00081-X
Elsevier Science S.A. All rights reserved
The era of molecular electronic devices being just theoretical speculation is coming to an end with the recent synthesis and possible demonstration [3] of the molecular rectifier proposed by Aviram and Ratner [4] in 1974. There is also the practical work done towards molecular machines by Stoddart [ 5,6]. Recent attempts to make the soliton switch have led to simpler switch structures and it is a major aim of this paper to advertise those structures in an attempt to interest other synthetic organic chemists in making them. To this end, chemical structures are used for all the devices and electronic symbols have been avoided. It will be shown how two soliton switches can be connected in series to give a structure with a basic AND property, in which both the switches (one AND the other) must be enabled before conduction is possible on the chain through them. Two switches can also be connected in parallel to give a simple OR structure. The AND and OR operations are fundamental to Boolean gates. Boolean gates can be used to construct most of the devices in digital electronics. Finally, two switches will be combined in a different manner to form a simple memory cell.
2. Solitons in polyacetylene As depicted in Fig. 1, polyacetylene consists of a chain of carbon atoms held together by alternating double and single bonds. Each carbon atom is also bonded to a hydrogen atom.
M.P. Groves et al. /Mareriuls Science and Engineering: C3 (1995) 181-185
182
Fig. 1. Solitons
in polyacetylene. Fig. 2. The soliton junction.
Polyacetylene has two stable states, which differ in the position of the double bonds with respect to the carbon atoms. A soliton is a moving wave which causes conversion between the two states of polyacetylene. It effectively picks up one arrangement of bonds and lays down the other. The effect of solitons on the state of a polyacetylene chain is shown by the arrows in Fig. 1. The small curved arrows represent the movement of a pair of electrons. The combination of all these movements is the passage of a soliton, which is shown by the large wavy arrows at the top of Fig. 1. Because there are only two states for polyacetylene, a soliton in either direction along the chain will always cause the same change in state. That is, had the left-hand side of Fig. 1 shown a soliton moving from right to left, the result would still have been the state depicted on the right-hand side of the figure. It should be noted that two successive solitons (in the same or opposite directions) will leave the state of a polyacetylene chain unchanged. Polyacetylene is not the only conducting polymer, but it is one of the few with two equal energy ground states, which makes it very suitable for binary calculations. The mechanism for electrical conduction in polyacetylene and other conducting polymers is discussed by Rubner [7]. These polymers have many interesting properties that enable them to be used to make flexible light emitting diodes [ 81 and even to create transistors [ 91.
3. Soliton junctions The soliton junction, which is shown in Fig. 2, was introduced by Carter [ 11. The junction has three polyacetylene chains joined to a central carbon atom. Junctions allow the branching and interconnection of polyacetylene chains. The three states of a soliton junction are each characterized by the position of the double bond on the central carbon atom. Solitons passing through the junction cause its state to swap between the three states shown in Fig. 2. Only alternating double and single bonded paths can transmit solitons. Hence no solitons can pass between the two chains connected to the central carbon by single bonds, as the path between these chains includes two consecutive single bonds. Alternatively, it can be said that all solitons must pass through the central carbon’s double bond.
the two soliton paths which run through it. A possible structure for a soliton switch is given in two of its possible stable states in Fig. 3. This structure has a central four-membered carbon ring, which is drawn to indicate that it is roughly perpendicular to the plane of the paper. Solitons can at times flow along either the two polyacetylene chains connected to the top of the switch or through the chains connected to the bottom of the switch. On the lefthand side of the figure, this switch structure is shown in its clear state. In this state, the left-hand and right-hand ends of the switch can pivot in the plane of the paper around the central ring. The nitrogen atoms can form a single bond to carbon (as shown on the right-hand side of Fig. 3), but only when they are within some distance of each other. When a soliton is sent along either of the top or bottom paths through the switch and the nitrogen-carbon bond forms, the other path cannot pass a soliton, as the opposite nitrogen atom is held firmly outside its bonding distance. A soliton along the path through the top of the switch will change the state of the bonds through which it passes and give the state shown on the right-hand side of the figure. In this state there can be no conduction along the lower path because the lower nitrogen atom is held firmly outside its bonding distance. Another soliton sent on the top path would clear the switch, by restoring the switch structure to the situation shown on the left-hand side of Fig. 3. In this manner, solitons along either of the paths through a switch can stop conduction of solitons along the other path. Conduction on the second path can only be restarted by another soliton along the first path. So we have a simple electrical switch, in which the passage of a soliton along either path through the switch can control conduction on the other path. Note that there is no electrical connection between the two paths through a switch, but there is a physical connection through movement about the central carbon ring. There are many possible variations in structure for the soliton switch. In the left-hand side of Fig. 4 for example, the nitrogen atoms have been replaced by oxygen, though it would probably be much more difficult to open this switch, as the closed form would be a fairly stable ester. Many other
w \N
4. Soliton switches The other component to be discussed is called a soliton switch, because it can control the movement of solitons on
R’
Fig. 3. The soliton switch.
k
7
I
/\ JX
M. P. Groves et al. /Materials Science and Engineering: C 3 (1995) 181-185
183
structures to external electrical devices. At that point, prototype soliton circuits could be made, and the behaviour of the circuits and components could be thoroughly tested and fully verified.
'N
N
R’
Fig. 4. Other structures for the soliton switch.
6. Combining switches
atoms, such as sulphur, phosphorous and even silicon could be tried in place of the nitrogen atoms. Also, on the righthand side of Fig. 4, the central four-carbon ring has been replaced by a six-carbon ring. This structure would probably operate more slowly, as a soliton could not pass until the key carbon and nitrogen atoms draw together. Almost any difference between single and double bonds could be the basis for designing a soliton switch. In addition to the hinge type discussed here, there are also possibilities based on the stereochemical requirements of double bonds and the electron-rich nature of double bonds.
5. Testing the switches The properties of soliton switches will not be fully known until they have been made and connected to potential differences and electronic measuring devices. However, if a switch is made with electron-donating groups (EDG) and electronwithdrawing groups (EWG), as shown in Fig. 5, the basic operation of the switch can be tested by chemical means. The switch structure shown on the left-hand side of Fig. 5 is symmetrical about the dashed line through its centre. If the electron donating and withdrawing groups are correctly activated, they can cause a pair of electrons to move either along the top of the switch, as shown by the arrows, or, by symmetry, along the bottom. However, if the switch works correctly, once an electron pair has passed down one side of the switch, it should not be possible for electrons to pass down the other side. So the only product that should be obtained from the above reaction is the switch closed on one side only. Then the electron-donating and withdrawing groups could be suitably modified and the conditions changed so as to reverse the reaction and open the switch up again. Once a soliton switch is made and tested, the next step would be to connect a few together to form Boolean gates and memory cells, as described in the following sections. With the correct electron-d’onating and withdrawing groups, these structures could also be tested chemically. They could then be connected together to form larger and larger structures. At some point, it should be possible to connect these
R’
Fig. 5. Testing the operation of the soliton switch.
By using soliton switches it is not difficult to implement some of the basic logic operations of digital circuits. Connecting two switches in series, as shown in Fig. 6, will give an AND operation. In the structure shown, conduction will only be possible along the bottom path through both switches if both the left-hand AND the right-hand switches are in their clear states. That is, it only takes a soliton along one of the upper paths, through either of the switches, to stop conduction along the bottom path. Connecting two switches in parallel, as shown in Fig. 7, will give an OR operation. In the structure shown, conduction will be possible along the main path from left to right if either the top switch OR the bottom switch is in its clear state. That is, it requires solitons along the outside paths of both of these switches to stop conduction along the main path through the device. The properties of these structures fall short of Boolean gates, in that these structures do not offer ways to drive the input of one gate from the output of another. The inputs require single solitons and the outputs enable the passage of any number of solitons. Also, there is the question of where these solitons come from. Both of these problems are solved by an additional device called a soliton capacitor or SID. The soliton capacitor can store a single electronic charge. Soliton capacitors can be combined with soliton switches to form all the different types of Boolean gate [ 21.
Fig. 6. The AND operation
EWG
Fig. 7. The OR operation.
184
M.P. Groves et al. /Materials Science und Engineering: C3 (1995) 181-185
Given Boolean gates, it is possible the functions required to make digital be used to form memories; however, the polyacetylene chain offers a better
to perform nearly all of circuits. Gates can even the two-state nature of way to form memories.
7. Memories Two switches can be combined to form a simple memory cell, as shown in Fig. 8. The cell can only hold a single binary bit, which is represented by the value “0” or “1”. The value in this cell can be read by placing a soliton on the read input and seeing whether it is directed to the left-hand or right-hand output. The soliton being directed to the left-hand output indicates a stored value of “O”, and being directed to the right-hand output indicates that a value of “1” is stored in the cell. It is the state of the write chain, which runs through the top of both switches, that controls what is stored in this memory cell. In the state shown in Fig. 8, a soliton on the input chain cannot go to the left, as the switch on the left-hand side of the cell is closed. This soliton is therefore directed via the switch on the right-hand side of the cell to the output on the right, which signifies that the cell contains a value of “1”. The passage of this soliton will cause the bonds through which it passes to change, as indicated by the small curved arrows in Fig. 8. This gives a new state for the memory cell in which the right-hand switch is closed. The nature of this state is not important, except that another soliton along the same or opposite path will give back the state shown in Fig. 8. For this reason, the memory cell is designed to be read by pairs of solitons. The operation of writing values to this cell is accomplished by passing solitons along the write chain. In the state shown above, a soliton along the write chain would open up the lefthand switch and close the switch on the right-hand side of the figure. This would cause solitons on the read input to be directed out of the left-hand output, which would signify that the cell contained a stored value of “0”. This same structure can also be used to select one of two possible memory cells for reading. The two memory cells are. first connected to the outputs of this cell. Then the state of the write line of this cell can be used to direct solitons on the read input to the appropriate memory cell. Larger memories can then be constructed using several layers of selectors, arranged so that it is possible to select each individual cell in the memory [ lo].
8. Conclusions The brain provides ample proof that molecular memories and calculating devices are possible, but are they feasible? Given that scientific progress continues to gain momentum, this is really a question of when molecular electronic devices will become feasible. Quite simply, if the feature size of digital memories is to continue to shrink, it must get down to the molecular level, and when it does, as in the biological realm, we will be building up from molecules. It is also relevant to ask whether, at the point at which it is feasible to make molecular electronic devices, there will be some other technology which is better. For example, would an approach based more on existing biological systems [ 11,121 be better? In demonstrating how soliton circuits can perform the logic and memory functions of digital electronics, a previous paper [ 21, though quite impractical, clearly showed the great potential of soliton circuits. The much simpler structures for soliton switches and the method for their chemical verification presented here make the whole proposal of soliton circuits far more reasonable and clearly indicate the first steps towards realizing their potential. The potential of the second step, which is to combine soliton switches together, was also outlined. Soliton switches can be used much like relays and combined to perform the simple logic functions required to form Boolean gates. While using soliton circuits to construct gates probably ignores their potential to form entirely new kinds of electronic devices, the ideas for Boolean gates predate the transistor [ 131, and once some simple gates have been made they can be used to perform most of the functions required for digital electronics. A brief introduction to how soliton switches can be combined to form memory cells and the circuits required to address those memories was also given here. Given the slow speed of soliton circuits, it is unclear whether they will ever be the medium of choice for digital calculations. However, their extremely small size and their non-volatile nature make soliton circuits ideal for building memories. The problems that need to solved before molecular memories can be built are very large and challenging and, as there has been no complete design of such a memory, it is not yet feasible to make a concerted effort to build one. However, the potential benefits of molecular memories are so great that it is well worth working toward this goal by thinking about the nature of such memories and continuing to work on the chemical synthesis of soliton circuits. Acknowledgement The work towards designing the switch structures presented here was funded by Circadian Technologies Ltd. References [l] F.L. Carter, Soliton switching
Fig. 8. A memory cell.
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