The quest for smaller and faster logical devices has persisted since
the invention of the classical transistor. A novel idea of using single
organic molecules as electronic circuit components has been proposed back
in 1974 (Aviram and Radner, 1974). However, the difficulty to connect a
single molecule to external leads prevented experimental verification of
this idea until recently, when molecular junctions (acting as quantum dots)
(Joachim et al., 1995; Reed et al., 1997), and a carbon nanotube field-effect
transistor (Tans et al., 1998) have been fabricated.
Here we propose a new approach to make logical devices from molecules,
which is based on our understanding (Hermon et al., 1998) that chemical
bonds can act as tunnel junctions in the Coulomb blockade regime (Grabert
and Devoret, 1992), and the technical ability to coat a DNA strand (and
other molecules) with metal, thus forming a conductive wire with self assembly
property (Braun et al., 1998). Combining the two, we suggest to utilise
the chemical bonds in DNA (or other molecules) to build logical devices.
These will be literally in the nano-meter scale and will be able to operate
at room temperature. The operation principle of the proposed devices is
the single electron effect, which makes them extremely fast. Since they
are made from specific molecules, the devices will be completely identical
to each other. They will also be highly stable due to the stability of
the chemical bonds. The devices will inherit the self assembly property,
which can be used to create complicated networks consisting of many elements.
First, we show how to build a single electron tunneling (SET) transistor from DNA (see Fig. 1). A DNA strand is made of units (or grains), composed of a sugar and a base. The grains are connected by phosphorus bridges (P-bonds), while complementary bases in different strands are connected by hydrogen bonds (H-bonds). In order to build a SET transistor, one should start with two strands (a main strand and a gate strand), and connect the end base of the gate strand to a complementary base in the middle of the main strand. Both strands should be metal-coated, except the grain in the main strand which is connected to the gate strand, and its two adjacent P-bonds. The connective H-bond should be uncoated as well. To do this, the method presented by Braun et al., has to be generalized to enable selective coating. We expect it to be feasible if artificially made strands are used, so that the coated and uncoated parts are composed of different bases. The metallic coated ends of the main strand can be now connected to a voltage source ,V, and the end of the gate strand to another voltage source, VG, which acts as a gate voltage. We propose that this DNA-made device has the properties of a SET transistor (Grabert and Devoret, 1992). This proposal is based on our understanding that a P-bond forms a tunnel junction for a net charge (Hermon et al., 1998). (By 'net' charge we mean the deviation from the charge distribution of the unperturbed DNA.) The tunneling is either stochastic (like a normal tunnel junction) or coherent (like a mesoscopic Josephson junction), according to the coupling to the environmental degrees of freedom. The origin of this tunnel junction are the two oxygen atoms transversely connected to the phosphorus atom (see Fig. 2). These oxygens share three electrons with the phosphorus, giving rise to two sigma bonds and one pi bond. As the pi electron can be shared with both oxygens, it resembles an electron in a double well potential and occupies the lowest level. When an additional electron approaches the well, it encounters a barrier due to the energy gap to the next level of the well. However, since this barrier is narrow and not very high, the approaching electron can tunnel through it.
The H-bonds have a capacitive property. The proton in the H-bond can effectively screen a net charge density on either side of the bond by shifting its position towards this side. As a result, the net charge accumulates on the sides of the H-bond, and the bond can be viewed as a capacitor. The grains themselves have inductive properties, stemming from the hopping of additional electrons. The notations are shown in Fig. 1.
As we can see, the equivalent circuit is similar to that of a SET transistor composed of normal tunnel junctions, but with the additional inductors coupled serially to each capacitor. We have analyzed the circuit, and found that in the limit of strong coupling with the environment, the I-V characteristics of this DNA SET transistor are the same as those of the normal metal SET transistor.
As a second example, we discuss a possible realization of a quantum bit, which is the fundamental element needed for quantum computation (Barenco, 1996). Several systems which can act as quantum bits have been recently proposed. They include trapped ions (Cirac and Zoller, 1995) and Josephson junctions (Shnirman et al., 1997). Since the main problem in this field is maintaining quantum coherence over long periods of time, DNA-made devices can be used for quantum computation when the interaction with the environment is weak. In Fig. 3 we show how to build a quantum bit using three DNA strands. Due to their nano-scale dimensions, conformity and availability, DNA-made devices have the advantage over other realizations of quantum computation elements.
Aviram, A.; and Radner, M. A. (1974) Chem Phys Lett, 29, pages 277-283.
Joachim, C.; Gimzewski, j. K.; Schittler, R. R.; and Chavy, C. (1995) Phys
Rev Lett, 74, 2102-2105.
Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; and Tour, J. M. (1997)
Science, 278, 252-254.
Tans, S. J.; Verschueren, R. M.; and Dekker, C. (1998) Nature, 393, 49-59.
Hermon, Z.; Caspi, S.; and Ben-Jacob, E., (1998) Europhys Lett (in press),
Prediction of charge and dipole solitons in DNA molecules based on the
behaviour of phosphate bridges as tunnel elements
Grabert, H.; and Devoret, M. H., editors (1992) Single Electron Tunneling,
Plenum Press, New York.
Braun, E.; Eichen, Y.; Sivan, U.; and Ben-Yoseph, G. (1998) Nature, 391,
Barenco, A. (1996) Contemp Phys, 37, 357-389.
Cirac, J. I.; and Zoller, P. Phys Rev Lett, 74, 4091-4094.
Shnirman, A.; Schön, G.; and Hermon, Z. (1997) Phys Rev Lett, 79, 2371-2374.
Fig 1. A schematic image of a DNA SET transistor (upper), and the equivalent electrical circuit (lower). P denotes the P-bonds between the sugars, and H denotes the H-bond between the bases. V and VG are the external and the gate voltages, respectively. C and ET are the capacitance and tunneling energy of the P-bond, C0 is the capacitance of the H-bond, and L and L0 are the longitudinal and lateral inductances, respectively.
Fig 2. A schematic image of two 'grains' in the DNA connected by a P-bond. The dark circles represent carbon atoms and the white circles oxygen atoms.
Fig 3. A qubit made of one short DNA strand, attached to two
long strands by two H-bonds. The long strands are metal-coated and connected
to an external voltage source, V, via resistance, R, and inductance, L.
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