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Field-Effect Modulation of the Conductance of Single Molecules

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Science  07 Dec 2001:
Vol. 294, Issue 5549, pp. 2138-2140
DOI: 10.1126/science.1066171

This article has a correction, but has also been retracted. Please see:

Abstract

Field-effect transistors based on two-component self-assembled monolayers of conjugated and insulating molecules were prepared; the conductance through them can be varied by more than three orders of magnitude by changing the applied gate bias. With very small ratios of conjugated to insulating molecules in the two-component monolayer, devices with only a few “electrically active” molecules can be achieved. At low temperatures, the peak channel conductance is quantized in units of 2e 2/h (wheree is the electron charge and h is Planck's constant). This behavior is indicative of transistor action in single molecules. On the basis of such single-molecule transistors, inverter circuits with gain are demonstrated.

Modern microelectronics and computation are advancing at an extremely fast rate. At the heart of this field has been the technology of complementary logic metal-oxide semiconductor (CMOS)–based integrated circuits. However, fundamental physical limitations as well as increasingly prohibitive cost associated with fabrication facilities might slow down further miniaturization of devices to increase computation power within the next couple of decades (1,2). To continue the trend to ever faster and cheaper computers, new schemes will be required. One such approach is toward molecular devices (3, 4), where the idea is that only a few or even just one molecule could be used to perform basic electronic functions (5). Previous work has tended to focus on two-terminal devices: for example, negative differential resistance (6), rectifiers (7), and reconfigurable switching (8). Three-terminal devices, i.e., transistors, could offer several advantages for logic operations compared with two-terminal switches, the most important of which is “gain.” As one example of that, an electromechanical amplifier that uses a single C60 molecule has been reported (9). In addition, field-effect devices based on single nanotubes have been demonstrated (10–14). However, for these devices, the active channel length was defined by the resolution of lithography rather than the molecular scale (15). Here, we report on field-effect transistor (FET) action, i.e., conductance modulation through a third electrode, in devices consisting of only several “electrically active” molecules. The channel length of 10 to 20 Å is defined by the thickness of a self-assembled monolayer (SAM) rather than lithography. Moreover, the peak conductance at low temperatures is quantized in units of 2e 2/h, indicating that the conductance of single molecules is modulated.

We prepared vertical self-assembled monolayer field-effect transistors (SAMFETs) (15) using monolayers of alkanedithiols (e.g., 1,5-pentanedithiol) mixed with 4,4′-biphenyldithiol or 5,5′-terthiophenedithiol (Fig. 1A). The alkanedithiol was chosen to have a length similar to that of the conjugated molecule. A highly doped Si substrate, which is structured with conventional lithography and anisotropic etching to define vertical steps, was used as the gate electrode. A thin layer of thermally grown SiO2 was used as a gate insulator. To fabricate a source electrode, we deposited a gold layer by thermal evaporation. A thiol-terminated monolayer was then self-assembled on the gold surface. Ellipsometry was used to confirm that only a monolayer has been deposited. Finally, the drain electrode was prepared by shallow angle shadow evaporation of gold onto a cooled substrate (∼100 K) to avoid damage to the SAM and form an active contact area of about 0.8 × 0.1 μm2. As the gated conduction in organic FETs takes place only within 50 Å from the semiconductor-insulator interface (16), only several thousand molecules will be probed in such SAMFETs, where the channel length is defined by the thickness of the monolayer. No signs of macroscopic phase separation were observed experimentally in similar two-component SAMs with data reported in the literature (17–19). We therefore assume that the conjugated molecules, i.e., 4,4′-biphenyldithiol or 5,5′-terthiophenedithiol, are homogeneously and statistically distributed within the alkanethiol matrix. However, the possibility of bundles of conjugated molecules within the SAM cannot be ruled out completely. Nevertheless, the use of two-component SAMs allows us to study transistor action in single molecules or small bundles of molecules. Control experiments reveal that samples consisting of pure alkanedithiol SAMs do not show any conductance modulation because of gate bias variation. All the data shown here are results from 4,4′-biphenyldithiol, even though similar characteristics have been observed for 5,5′-terthiophenedithiol.

Figure 1

(A) SAMFET structure. A highly doped Si substrate is used as the gate electrode, a thermally grown SiO2 layer acts as gate insulator, the gold source electrode is deposited by thermal evaporation, and the active semiconducting material is a two-component SAM of alkanedithiols mixed with 4,4′-biphenyldithiol or 5,5′-terthiophenedithiol. The drain contact is defined by shallow angle shadow evaporation of gold. The active region of the device is magnified. (B) Transfer characteristics at room temperature of two SAMFETs (drain-source voltage of −0.5 V). The control corresponds to a “pure” 4,4′-biphenyldithiol SAM, whereas the second one is based on a two-component SAM (4,4′-biphenyldithiol to alkanedithiol ratio is 1:100). The inset shows the current at a gate and drain-source voltage of −0.5 V as a function of the mixture ratio.

The transfer characteristics of the SAMFETs scale with the ratio of conjugated to insulating molecules in the two-component SAM (Fig. 1B), suggesting that the properties of individual molecules are responsible for the conductance modulation rather than interactions of molecules with their nearest neighbors. However, it is difficult to explain the strong conductance modulation solely on the basis of quantum-mechanical treatment of simple coherent transport through a monolayer as first-principle calculations indicate a modulation of more than one order of magnitude for p-phenylenedithiol (20). However, it is particularly noteworthy that resonances and strong modulation have been observed and studied for single-molecule junctions (21). Additional effects could include a gate modulation of the potential distribution along the channel length, a suppression of the barrier at the contacts (22–24), or a conformational change of the molecule (25). At low temperatures (below 200 K), the transfer characteristics show a well-pronounced peak of the conductance as a function of gate voltage (Fig. 2A). Although quite broad, this peak might be related to resonant electron transfer. However, more detailed investigations will be needed to develop a better understanding of the microscopic processes underlying the transistor action (Fig. 2B) in these devices. For very “dilute” (ratio 1:5000) two-component SAMs, the peak conductance of the different investigated devices is quantized in units of 2e 2/h (Fig. 3A). Assuming a homogeneous distribution of the conjugated molecules in the SAM, we should find about two molecules in the active area of the device. However, in some devices, one or also more than two molecules might assemble in this region. This is in accordance with the results depicted in Fig. 3A, assuming that the maximum conductance through a single molecule is given by 2e 2/h. We found that the value of the gate voltage leading to the conductance peak can differ from device to device and is ascribed to different positions of the active molecule with respect to the gate insulator. Consequently, the effective field experienced by the molecule varies for different SAMFETs depending on its position relative to the gate insulator (vertical step). The peak conductance quantization is also observed for two-component SAMs with lower ratios of alkanedithiols compared with the conjugated species (Fig. 3B).

Figure 2

(A) Transfer characteristics at 4 K of a two-component SAMFETs (ratio 1:5000). (B) Transistor characteristics at 4 K of a two-component SAMFETs (ratio 1:5000).

Figure 3

Histogram of the peak conductance at 4 K of various two-component SAMFETs for a ratio 1:5000 (A) and 1:500 (B) revealing quantization in units of 2e 2/h.

These results indicate that the conductance through a single molecule can be modulated by the field effect, leading to transistor action in a single molecule or small cluster. As the off-current of the SAMFET is limited by the leakage through the alkanedithiol matrix, the actual conductance modulation should be many orders of magnitude larger than 103, which is observed for the devices reported here. From the transfer characteristics, a transconductance (differential change of the drain current with the gate voltage for a constant drain voltage) in the range of 150 μA/V is estimated for a single molecular SAMFET at low temperature (4 K), decreasing to about 10 μA/V at room temperature. One obvious way of increasing this value would be to decrease the gate insulator thickness. The control of placement of single molecules though more complexed self-assembly techniques (26) may lead to further improvements in device performance as well as simpler device design. Nevertheless, a small signal voltage gain, necessary for amplification, was observed for inverters consisting of two SAMFETs, which exhibited switching characteristics of one single molecule in the active channel. Figure 4 shows the static switching characteristics of such an inverter circuit, which is a basic building block for more complex logic circuits. Here, the output switches from logic “1” (−2 V) to logic “0” (0 V) when the input switches from logic 0 to logic 1. A gain up to 10 is obtained (see inset in Fig. 4), demonstrating the possibility of switching and amplification based on single molecular devices.

Figure 4

Output characteristics at 4 K of an inverter with two two-component SAMFETs (ratio of 1:5000). The small signal gain is plotted in the inset revealing the possibility of amplification and switching of SAMFETs based on single molecular operation.V s is the supply voltage of –2 V.

Although many challenges remain, the demonstration of FET action with a single electroactive molecule can be regarded as an important step toward “molecular electronics.” Moreover, the use of two-component SAMs will allow the study of gating effects on various molecules. With molecular heterostructures and molecular wires, it might be possible to combine the insulating and semiconducting properties as well as the contacts within a single molecule or supramolecular architecture.

  • * To whom correspondence should be addressed. E-mail: hendrik{at}lucent.com

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