NANO LETTERS
Monolayer Transistor Using a Highly Ordered Conjugated Polymer as the Channel
2006 Vol. 6, No. 12 2916-2919
J. Campbell Scott,* J. D. Jeyaprakash Samuel, Jennifer H. Hou, Charles T. Rettner, and Robert D. Miller IBM Research DiVision, IBM Almaden Research Center, San Jose, California 95120 Received September 22, 2006; Revised Manuscript Received October 24, 2006
ABSTRACT Field-effect transistor structures based on polydiacetylene (PDA) derivatives have been fabricated. Monolayer channels of UV polymerized pentacosa-10,12-diynoic ethanolamide exhibit modulation of source−drain current on application of a gating voltage. Comparison of the twodimensional crystal morphology of this material with several closely related derivatives that show no gating suggests that a high degree of alignment and order in the polymer chains is necessary for the observed transistor action.
What is the minimum thickness of an organic semiconductor that can function as the channel of a field-effect transistor? Recent studies of small-molecule organic semiconductors1,2 show that a single monolayer may be sufficient. However, data on polymeric systems3 suggest that the carrier mobility in the first few layers is too low to allow the observation of transistor action. It has been shown4 that the morphology of the semiconducting polymer at the interface with the gate dielectric has a dramatic effect on carrier mobility. Therefore, we have selected a method to prepare the polymer channel that takes advantage of the prior assembly of monomers into crystalline domains, with subsequent polymerization that preserves the ordered structure. The resulting film can be described as a sheet of parallel polymer chains, in 2D domains that extend up to several hundred micrometers. In the work to be presented here, we use LangmuirBlodgett techniques. By exploiting the well-known characteristics of diacetylene amphiphiles,5,6 which can form highly ordered two-dimensional crystalline domains,7 we have prepared transistor structures where the channel is a single polymer monolayer. The most highly ordered of the materials that we have examined to date exhibits transistor action on application of a gating voltage. We believe that this is the first observation of field-effect gating in a polymer monolayer. A number of interesting issues arise in connection with the monolayer geometry. Foremost, and the primary conclusion of this paper, is that such a structure is even capable of acting as a transistor at all. Additional motivations for studying polymer monolayers are the accessibility of the top surface, which might be used for sensor functionality or as * Corresponding author. E-mail:
[email protected]. 10.1021/nl0622449 CCC: $33.50 Published on Web 11/17/2006
© 2006 American Chemical Society
Figure 1. Pressure-area isotherms for EA-PCA. The inset shows the molecular structure and illustrates the topotactic polymerization of the crystalline monolayer.
a top gate, and the possibility of bonding every polymer chain in the channel directly to the electrodes through a suitably selected end-group. The 2D nature of the transport and the role of interchain coupling are also of interest. Furthermore, the Langmuir-Blodgett technique permits the formation of well-ordered multilayers, which can be used for further studies of charge transport at the nanoscale level. Polydiacetylenes (PDAs)8 have been studied thoroughly since the late 1960s. Many diacetylene monomer derivatives form highly ordered 3D crystals that can be topotactically polymerized8 to form polymer single crystals (see Figure 1, inset). These compounds were examined initially for their interesting photoconductive properties,9,10 which suggest relatively high charge carrier mobility in the chain direction. Estimates range from 3 cm2/Vs11 to greater than 1000
cm2/Vs.12,13 Electroreflectance measurements suggest a microscopic mobility for the dicarbazolyl derivative of 2800 cm2/Vs.14 PDAs are highly anisotropic organic semiconductors, with optical band-gaps on the order of 2.1 - 2.6 eV depending on substituents.15 However, unlike in the related polyacetylene,16 it has proved impossible to dope them to metallic conductivity levels. Therefore, interest in the conductive properties of PDAs waned as other polymers proved more amenable to processing and doping. DC conductivity measurements in undoped single crystals of PDA-pTS (pTS ) p-toluenesulphonate) suggest hopping conduction with roomtemperature resistivities on the order of 108 Ohm‚cm.17 Studies of charge transport in thick undoped PDA films18,19 as well as bilayer20 and multilayers21 also show very small currents. Nevertheless, transistor behavior has been observed recently in a thick film structure.22 The bulk properties cited in the previous paragraphs are just what one might expect from dc conductivity measurements on bulk samples of a wide band gap, one-dimensional semiconductor. Because the band gap is at least 2.1 eV, the density of thermally generated carriers at room temperature is no more than n ) 109 m-3. Even if the mobility is as high as µ ) 103 cm2/Vs (0.1 m2/Vs), as suggested by the photoconductivity and electroreflectance data,12-14 the expected intrinsic conductivity, neµ, is less than 10-11 S/m, orders of magnitude smaller than many experimental values. This suggests that these samples had a much higher density of carriers due either to impurities or, at higher voltage, injection from the electrodes. This analysis leads to the conclusion that PDAs could be useful as wide-band-gap semiconductors: low intrinsic conductivity and high carrier mobility. In particular, field-effect transistors (FETs) operating in accumulation mode should be possible.22 To test these ideas for polymer monolayers, we fabricated simple FET structures as described below. In this study, various PDA derivatives were prepared, all based on the commercially available, 25-carbon atom, monomer, pentacosa-10,12-diynoic acid (PCA). By amidization of the terminal carboxylic acid via either the acid chloride or the N-hydroxysuccinamide ester, other end groups were attached: aminopropyltriethoxysilane (APTES); ethanolamine (EA);23,24 and diethanolamine (DEA). We will refer to the derivatives according to the headgroup, for example, EA-PCA. All of these monomers are either intrinsically or, in the case of APTES-PCA which requires pH less than 4.5 to promote silanol formation, latently amphiphilic. They form Langmuir films on a pure aqueous subphase and exhibit a variety of behaviors in pressure-area isotherms, ranging from highly hysteretic and prone to collapse (PCA), to wellbehaved and reversible (EA-PCA). Isotherms for the latter compound at three different temperatures are shown in Figure 1 and reveal a liquid expanded phase that extends over a wider pressure range at higher temperatures. Extrapolation of the linear part of the isotherm in the solid phase yields an area per molecule of 23.5 Å,2 the value expected for a single layer of slightly tilted monomers 30 Å in length. There is no indication of any 2D solid-phase transitions, as are seen, Nano Lett., Vol. 6, No. 12, 2006
for example, in some acidic monomers.25 Brewster-angle microscopy of this monomer film (details of which will be described in a separate publication) reveals crystalline domains up to one millimeter in size. The various monomers are readily polymerized at the airwater interface by exposure to a low-power (∼1 mW/cm2) mercury UV lamp for about a minute. The short wavelength component (254 nm) of the radiation is critical in achieving rapid polymerization. During polymerization the surface pressure drops by about 20-30%, and generally exposure is stopped when the pressure stabilizes. The polymerized monolayer film is then transferred to solid substrates by the standard Langmuir-Blodgett method, namely by lifting the substrate, which was immersed during compression and polymerization, vertically through the surface. The rate of substrate removal was 1 mm/min. We have characterized these films by optical spectroscopy, polarization microscopy, AFM, ellipsometry, and ellipsometric imaging. This analysis reveals widely disparate behavior for the different derivatives. The acid form, PCA, forms highly crystalline, but multilayered, polymer films as a result of collapse during transfer. The highly birefringent crystalline domains of PCA are typically a few tens of micrometers in lateral extent. APTESPCA, at pH 4.5, yields a monolayer polymer film, but the crystallinity is poor, with domain sizes of only a few nanometers as revealed by AFM. The hydrolysis of the silyl head-groups opens a competing polymerization reaction, namely silanol condensation, which is probably responsible for the lack of extended order. DEA-PCA forms a good monolayer film on the water surface, but does not photopolymerize, presumably because the bulkier hydrophilic sidegroups of the monomer prevent suitable packing of the diacetylene units. EA-PCA retains much of its crystalline order after transfer of the polymer to solid substrates. The optical absorption spectrum, obtained using quartz substrates, is identical to that of “red-phase” PCA24 and other PDAs,26 provided that the temperature during compression and polymerization is above about 20 °C. Domain sizes observed by polarization microscopy7 range up to several hundred micrometers, and AFM thickness profiling (Figure 2) confirms that the molecular monolayer formed at the air-water interface was transferred, essentially intact, to the solid substrate. The UV polymerized films are insoluble, and therefore we have no direct information on the molecular weight or dispersity. However, the well-resolved structure in the absorption spectrum and the large 2D domain size suggests that the number of chain-terminating defects is probably quite low. We also note that STM-induced polymerization of a similar diacetylene monomer on a solid substrate propagates up to about 100 nm from the tip, and terminates only at deliberately introduced defects.27 Electrical measurements were attempted on devices made using all of the materials discussed, but only EA-PCA exhibited the behavior that we now describe. FETs were fabricated on silicon wafer substrates, with 100-nm-thick thermal oxide. A pattern of interdigitated gold electrodes (10 nm of gold deposited on a 5 nm Cr adhesion layer) was 2917
Figure 2. AFM and cross-section analysis of a film of EA-PCA. The height at either side of a (rare) gap in the film is 1.6-1.8 nm, corresponding to a single monolayer. The image shows two domains on either side of the gap, oriented slightly differently. The striated grooves within each domain are in the polymer chain direction, but are typically more than 10 nm apart, much more than the chain width of 0.5 nm.
Figure 3. AFM images of a multilayer PCA film (the acid precursor to the ethanolamine derivative) on the interdigitated electrodes. The height image (left) shows the monolayer steps (∼2.4 nm) in film thickness. The very bright (high) regions are where the contact pads overlap the interdigitated pattern. The right image (phase) reveals the edges of the crystalline layers more clearly.
created using electron-beam lithography and liftoff (see Figure 3). The gap between source and drain electrodes (the channel length) varied from 20 to 500 nm, and the effective channel width was approximately 2 micrometers. The channel of the transistor was created by Langmuir-Blodgett transfer of a monolayer film of EA-PCA, single domains of which are large enough conceivably to bridge from source to drain, increasing the probability of uninterrupted conjugation in the channel. Electrical measurements were performed using two Keithley source-measure units, to apply drain and gate voltages with the source grounded, and to measure the corresponding currents. The (leakage) current between gate and source was typically less than 2 pA for all values of drain and gate voltage. The drain current as a function of drain voltage for various gate voltages is shown, for one device, in Figure 4. (The data have not been corrected for a small offset current in the electronics.) There is a clear gating effect with current flowing in the channel between source and drain only for negative gate voltages. Thus, this device is a 2918
Figure 4. Drain current as a function of drain voltage for various values of the gate voltage between -10 V and +10 V. The source is grounded.
p-channel field-effect transistor. Because both the source and drain are gold, the only real distinction is which electrode is grounded. In this device the “drain” appears to be a better hole injector than the “source”, resulting in higher currents for positive source-drain voltage. The on-off ratio is as high as 100. To date, similar behavior has been seen on over 40 devices on six different substrates/films. When no EA-PCA film is present, the current is less than 1 pA for all drain voltages |Vd| < 1 V and all gate voltages |Vg| < 10 V. The currents in the devices fabricated to date are relatively low (
