Lpl

Reading

Erasing

Figure 13.14. Sketch of a molecule that can be switched between two states denoted by M and P by using circularly polarized light (CPL). The molecular switch position is read using linearly polarized (LPL), and the information can be erased using unpolarized tight (L1PL). (With permission from M. Gomez-Lopez and F. J. Stoddart, in Handbook of Nanostructured Materials and Nanotechnology, H. S. Nalwa, ed., Academic Press, San Diego, 2000, Vol. 5, Chapter 3, p. 231.)

molecular ring mechanically interlinked with another molecular ring, as shown by the example sketched in Fig. 13.16. Its two different switched states are shown in Figs. 13.16a and 13.16b. This molecule is 0.5 nm long and 1 nm wide, making it in effect a nanoswitch. For this application a monolayer of the catenane anchored with amphiphilic phospholipid counterions is sandwiched between two electrodes. The structure in Fig. 13.16a is the open switch position because this configuration does not conduct electricity as well as the structure in Fig. 13.16b. When the molecule is oxidized by applying a voltage, which removes an electron, the tetrathiafulvalene group, which contains the sulfurs, becomes positively ionized and is thus electro-

Me Me

Me Me hvj hv2 or A

Figure 13.15. Photochemical switching of spiropyran (left) to merocyanine (right) by ultraviolet light h»u where red light (fw2) or heat (A) induces the reverse-direction conformational change in the molecule. (With permission from M. G6mez-L6pez and F. J. Stoddart, in Handbook of Nanostructured Materials and Nanotechnology, H. S. Nalwa, ed., Academic Press, San Diego, 2000, Vol. 5, Chapter 3, p. 233.)

Figure 13.16. Illustration of a switchable catenane that changes its conformation when subjected to a voltage that induces oxidation {-e") from the upper ( ■) isomeric state to the lower (•) state, and reduction (+e~) for the reverse transformation. (With permission from J. F. Stodart, Chem. Eng. News 28 (Oct. 16, 2000).]

Figure 13.16. Illustration of a switchable catenane that changes its conformation when subjected to a voltage that induces oxidation {-e") from the upper ( ■) isomeric state to the lower (•) state, and reduction (+e~) for the reverse transformation. (With permission from J. F. Stodart, Chem. Eng. News 28 (Oct. 16, 2000).]

statically repelled by the cyclophane group, the ring containing the nitrogen atoms. This causes the change in structure shown in Fig. 13.16, which essentially involves a rotation of the ring on the left side of the molecule to the right side.

An interesting aspect of this procedure is the observation that some molecules can conduct electricity, although not in large amounts. The STM has been used to measure the conductivity of long chainlike molecules. In a more recent study a monolayer of octanethiol was formed on a gold surface by self-assembly. The sulfur group at die end of the molecule bonded to the surface in the manner illustrated in Figs. 10.2-10.4 (of Chapter 10). Some of the molecules were then removed using a solvent technique and replaced with 1,8-octanedithiol, which has sulfur groups at both ends of the chain. A gold-coated STM tip was scanned over the top of the monolayer to find the 1,8-octanedithiol. The tip was then put in contact with the end of the molecule, forming an electric circuit between the tip and the flat gold surface. The octanethiol molecules, which are bound only to the bottom gold electrode, serve as molecular insulators, electrically isolating the octanedithiol wires. The voltage between the tip and the bottom gold electrode is then increased and the current measured. The results yield five distinct families of curves, each an integral multiple of the fundamental curve, which is the dashed curve in the Fig. 13.17. In the figure we only show the top and bottom curve. The fundamental curve corresponds to electrical conduction through a single dithiol molecule; the other curves correspond to conduction through two or more such molecules. It should be noted that the current is quite low, and the resistance of the molecule is estimated to be 900 Mil.

Having developed the capability to measure electrical conduction through a chain molecule, researchers began to address the question of whether a molecule could be designed to switch the conductivity on and off. They used the relatively simple molecule sketched in Fig. 13.18, which contains a thiol group (SH—) that can be attached to gold by losing a hydrogen atom. The molecule, 2-amino-4-ethylnylphenyl-4-ethylnylphenylphenyl-5-nitro-l-benzenethiolate, consists of three benzene rings linked in a row by triple bonded carbon atoms. The middle ring has an

Figure 13.17. Current-voltage characteristics of an octanethiol monolayer on a gold substrate measured by STM using a gold-coated tip. Five curves are actually observed, but oniy two, the lowest (—) and the highest (—■), are shown here. The solid curve corresponds to 4 times the current of the dashed curve. [Adapted from X. D. Cui et al„ Science 294, 571 (2001).]

Figure 13.17. Current-voltage characteristics of an octanethiol monolayer on a gold substrate measured by STM using a gold-coated tip. Five curves are actually observed, but oniy two, the lowest (—) and the highest (—■), are shown here. The solid curve corresponds to 4 times the current of the dashed curve. [Adapted from X. D. Cui et al„ Science 294, 571 (2001).]

13.3. MOLECULAR AND SUPRAMOLECULAR SWITCHES 351 A molecular electronic device

Gold electrode

13.3. MOLECULAR AND SUPRAMOLECULAR SWITCHES 351 A molecular electronic device

Gold electrode

Figure 13.18. Illustration of an electronic switch made of a conducting molecule bonded at each end to gold electrodes. Initially it is nonconducting; however, when the voltage is sufficient to add an electron from the gold electrode to the molecule, it becomes conducting. A further voltage increase makes it nonconducting again with addition of a second electron. [Adapted from J. Chen, Science 286,1550 (1999).]

amino (NH2) group, wliich is an electron donor, pushing electric charge toward the ring. On the other side is an electron acceptor nitro (NO2) group, which withdraws electrons from the ring. The net result is that the center ring has a large electric dipole moment. Figure 13.19 shows the current—voltage characteristics of this molecule, which is attached to gold electrodes at each end. There is an onset of current at 1.6 V, then a pronounced increase, followed by a sudden drop at 2.1V The result was observed at 60 K but not at room temperature. The effect is called negative-differential resistance. The proposed mechanism for the effect is that the molecule is initially nonconducting, and at the voltage where a current peak is observed the molecule gains an electron, forming a radical ion, and becomes conducting. As the voltage is increased further, a second electron is added, and die molecule forms a nonconducting dianion.

Of course, demonstrating that a molecule can conduct electricity, and that the conduction can be switched on and off, is not enough to develop a computer. The molecular switches have to be connected together to form logic gates. A roxatane molecule, shown in Fig. 13.20, which can change conformation when it gains and loses an electron by rotation of the oxygen ring on the left of Fig. 13.20, similar to the changes in the catenane of Fig. 13.16, has been used to make switching devices that can be connected together. A schematic cross section of an individual switch is shown in Fig. 13.21. Each device consists of a monolayer of rotaxane molecules sandwiched between two parallel electrodes made of aluminum (Al). The upper electrode on the figure has a layer of titanium (Ti) on it, and the lower one has an alumina (AI2O3) layer that acts as a tunneling barrier.

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