Surface and Interface Control of Polymeric Biomaterials, Conjugated

J. Phys. Chem. B 2000, 104, 1891-1915

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FEATURE ARTICLE Surface and Interface Control of Polymeric Biomaterials, Conjugated Polymers, and Carbon Nanotubes Liming Dai* and Albert W. H. Mau CSIRO Molecular Science, Bag 10, Clayton South, Victoria 3169, Australia ReceiVed: July 29, 1999; In Final Form: NoVember 11, 1999

Surface and interface play an important role in controlling the properties of a broad range of materials. With the recent development of various advanced surface analytical techniques, our understanding of material surfaces and interfaces has been significantly increased, as with our ability to control and even to tailor the surface and interfacial characteristics for specific applications. This is a very broad field which includes material synthesis, structural determination, surface/interface design and modification. In this article, we present some of the important issues concerning the surface and interface control of polymeric biomaterials, conjugated polymers, and carbon nanotubes for biomedical and optoelectronic applications and describe the use of X-ray photoelectron spectroscopy (XPS), surface force apparatus (SFA), and neutron reflectometry (NR) for elucidation of the surface and interface structures.

I. Introduction Conjugated polymers and polymeric biomaterials are examples of functional polymers which show various highly specific physicochemical and/or biological functions of practical significance.1 The use of conjugated polymers for optoelectronic devices and polymeric biomaterials for medical applications has attracted intensive investigation in recent years.2 It has been recognized that surface and/or interfacial properties are of paramount importance for most of the applications. For instance, the use of conjugated polymers in integrated circuits, field-effect transistors, optical memory storage, electroluminescent and electrochromic displays often requires that they be microlithographically patterned on various substrate surfaces.3 Likewise, it is the surface of a biomaterial that comes into contact with the physiological environment immediately after it is placed in the body or blood-stream and the initial contact regulates its subsequent performance. However, it is very rare that a polymeric material with desirable bulk properties also possesses the surface characteristics required for certain specific applications. Therefore, surface modification and interfacial engineering are essential in making sophisticated polymeric materials of good bulk and surface properties as demanded for specific applications.4 On the other hand, the recent discovery of carbon nanotubes has opened up a new era in material science and nanotechnology.5 Having a conjugated all-carbon structure, carbon nanotubes possess certain similar physicochemical characteristics to conjugated polymers. Just as conjugated polymers have widely been regarded as quasi-one-dimensional semiconductors,2,6c carbon nanotubes can be considered as quantum wires.5 The interesting electronic and photonic properties, coupled with their * To whom correspondence [email protected].

should

be

addressed.

E-mail:

unusual molecular symmetries, have made carbon nanotubes very attractive for many potential applications, including in electronic devices and field emission panel displays.5,6 The preparation of aligned and/or micropatterned carbon nanotubes is a key prerequisite for these, and many other, applications. The aim of this article is to summarize our recent studies on the surface and interface control of polymeric biomaterials, conjugated polymers, and carbon nanotubes, though reference is also made to other complementary work as appropriate. In what follows, we first describe the surface modification of polymeric biomaterials by chemical immobilization of macromolecular chains and their structural elucidation using physically adsorbed macromolecules as model systems. Then, we illustrate the importance of conjugated polymers at surfaces/interfaces in various practical applications with emphasis on polymeric light-emitting diodes (LEDs). Finally, methods for alignment and surface-micropatterning of carbon nanotubes are discussed. Since focus is given to our own work with no intention for a comprehensive literature survey of the subject, there will be no doubt that the examples to be presented in this paper do not exhaust all significant work reported in the literature. Therefore, we apologize in advance to the authors of papers not cited here. II. The Surface and Interface of Polymeric Biomaterials Generally speaking, biomaterials represent a wide range of materials, including metals, ceramics, polymers, and their composites, which are used in contact with tissue, blood, cells, and any other living substances. Among them, polymeric materials have been widely used for various medical applications ranging from artificial organs to drug formulation.7 Surface properties play an important role in biological applications of polymeric materials not only because it is the surface of an implant material that directly interfaces with biological constituents but also because a polymer surface may respond to

10.1021/jp9926793 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/12/2000

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SCHEME 1: Grafting of Poly(ethylene glycol) onto Polyurethane Surface through Diisocyanate12

SCHEME 2: Grafting Pectic Acid onto MeOH-Plasma Surfaces through Esterification Reaction8

environmental changes by changing its surface characteristics through polymer chain mobility. Furthermore, specific applications of polymeric biomaterials often require unique surface characteristics. For example, a kind of tissue-bonding surface is normally required for those medical devices which are implanted within the body (e.g., catheters, artificial bones),7 whereas polymers for ophthalmic use (e.g., as contact lens) must be oxygenated and transparent with a wettable surface so as to keep the cornea physiologically healthy.8 As mentioned earlier, for a given material it may possess suitable bulk properties but inadequate surface characteristics. Therefore, it is often necessary to optimize the surface structure of polymeric biomaterials through various surface modification methods.4 Below, we will describe several chemical grafting methods for making polymer surfaces with specific biological characteristics. 1. Bioinert and Bioactive Polymer Surfaces. a. Bioinert Polymer Surfaces. When circulating blood comes in contact with a foreign surface, blood coagulation could occur leading to thrombus formation.9 Likewise, adsorption of biological molecules such as proteins and lipids occurs rapidly upon contact between a contact lens and the ocular fluid.10 The blood coagulation and protein adsorption often limit the general use of polymers as biomaterials. Although antithrombogenicity or antifouling is a difficult problem to solve, bioinert surfaces which can reduce blood clotting or protein fouling to a certain extent have been developed. It has been recognized, for instance, that the blood coagulation and protein adsorption may be minimized by creating either a superhydrophilic surface or a superhydrophobic surface.11 The superhydrophilic surface seems to be more suitable for most of the blood-related and ophthalmic uses. One of the useful approaches for making such surfaces is to covalently immobilize hydrophilic polymer chains onto the biomaterial surface, as exemplified by the covalent attachment of poly(ethylene glycol) (PEG) chains onto a polyurethane surface shown in Scheme 1.12 In cases where covalent immobilization of polymer chains is required for chemically inert biomaterials, such as fluorineand silicon-containing polymers,8 the substrate needs to be preactivated to introduce suitable reactive surface groups. A popular approach for preparing such activated polymer surfaces is to use the radio frequency glow-discharge plasma technique for introducing either specific functional surface groups (via plasma treatment) or functional polymer layers (via plasma polymerization).13 A radio frequency glow-discharge is formed when a low pressure of gaseous monomer ( n2, then n12 < 1 and eq 5 gives

cos θi > cos θi cos θt ) n12

takes place (see section III.1). However, it is interesting to note that reflection does not disappear completely when θc is exceeded and that the reflectivity, R, defined as the intensity ratio of the reflected light to the incident light, vs the wave vector transfer, Q, should depend on the refractive index profile in the direction normal to the reflecting interface, as mentioned above. On this basis, we investigate the segment density profiles of the surface-bound polymers normal to the interface by reflectometry, choosing neutrons as the radiation because of important contrast effects which can operate at the molecular level. Since neutrons resemble waves in their interaction with matter, they may be reflected or refracted at the boundary between two media with different neutron refractive indices. They may also be totally reflected, when the incident neutron beam is at glancing angles less than the critical angle. At a particular neutron wavelength, λ, the neutron refractive index profile (relative to vacuum) of a medium of coherent scattering length, bi (i ) 1,2, . . .), is given by55b

λ2 (6)

Therefore, a real value for θt is only obtained when (cos θi)/n12 e 1 (i.e., cos θi e n12). The particular value of θc given by cos θc ) n12 is called the critical angle. When the glancing angle is less than the critical angle, so-called total external reflection

n)1-

∑i bini(z) 2π

)1-

λ2F(z) 2π

(7)

where ni(z) is the number density of nuclei in the medium and F(z) the coherent scattering length density profile. In the kinematic or Born approximation,56 the reflectivity, R, is given

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by

R(Q) )

16π2 |F′(Q)|2 Q4

(8)

where F′(Q) is the Fourier transform of the gradient of the scattering length density profile normal to the interface. Exact calculations of the reflectivity profile for a given density profile can be made using optical matrix methods (see section III.1).57 For terminally anchored polymer chains in a good solvent, two regimes are anticipated. If the grafting density is low, the tethered chains do not interact with each other and exist as separate “mushrooms”. If the grafting density is high, with a distance between anchor points less than the Flory radius of the chains (i.e., s < RF), the polymers take up a more extended configuration to form a semidilute “brush”.58 Polymer Mushrooms. In the mushroom regime, each chain may be thought of as occupying roughly a hemisphere, with a radius comparable to the Flory radius for a polymer coil in a good solvent.59 The density profile was demonstrated to be given by the Schultz function or a skew Gaussian function,58

φ(z) ) Azn exp(-Rzβ) φ(z) ) φo

exp[-(z -

- exp[-(1/2η2i )] exp[-(1/2η2i )]

z0) /2δ2i ]

1-

(9)

2

(10)

Equation 9 becomes a Schultz function when β ) 1. Equation 10 was constructed by joining two Gaussians of different widths of δ1 and δ2 centered at z0, where z0 ) δ/η1. The parameter, ηi, allows the density profile to shift parallel to the φ(z)-axis and some of the tail of the Gaussian to be removed so that the profile can look very much like the Schultz function. Figure 4A shows the schematic diagram and model scattering length density profiles used to fit the reflectivity data of PS577PVP283 end-adsorbed chains to eqs 9 and 10, respectively. The typical reflectivity profile, together with the model fitting, is given in Figure 4B, which shows the reflectivity multiplied by Q4, as a function of Q, as this gives a better indication of the fit quality and the asymptotic limit of the reflectivity at high Q. Also included in the inset of Figure 4B is the reflectivity vs Q. The Schultz function and Gaussian density profiles for this particular system are almost identical in shape. These data could not be fitted to the parabolic profile expected for polymer brushes, which is discussed below. Polymer Brushes. Apart from the scaling theory discussed earlier, various mean field calculations of the configuration of adsorbed chains have been reported.36c,60 The main approaches to calculating polymer density profiles of end-attached polymer brushes use essentially a self-consistent mean field (SCF) theory. In particular, Milner et al.60a made use of the fact that the chains in the brush are strongly stretched (i.e., in the semidilute regime) and may be assumed to behave classically. Relating the mean field, which is the effective chemical potential, to the free energy in the classical limit gives a parabolic density profile, rather than the step function assumed by Alexander and de Gennes in their scaling calculations.46,47 In view of the parabolic density profile predicted by theory, we used the following density or volume fraction profiles to fit our neutron reflectivity data:58

φ(z) ) φ0[1 - (z/L0)n]

(11)

φ(z) ) (φ0/2){1 - erf[(z - L0)/(2δz)]}

(12)

where erf is the error function and δz is the roughness of the

layer. The value of n in the polynomial profile defined by eq 11 determines the shape of the profile. n is 1 for a linear profile, 2 for a parabolic profile, and tends to infinity in the case of a step function. For 0 < n < 1, the profile becomes concave upward. The parameter φ0 defines the volume fraction at the liquid/solid interface, and L0 defines the thickness of the adsorbed layer where φ(L0) ) 0. A representative fitting curve for the reflectivity data from PS1700-PEO167 is shown in Figure 5a. In the inset of Figure 5a, the reflectivity multiplied by Q4 is plotted as a function of Q. The density profiles correspond to eqs 11 and 12 are given in Figure 5b, which shows that the two density profiles are almost identical in the region close to the surface. In the region at the outer extremity of the brush, however, the error function type profile consists of a parabolic shape with an approximately Gaussian tail, indicating that the classical parabolic density profile60a requires a correction due to the effects of the finite chain length and/or its polydispersity.58b Overall, our reflectivity data indicate that the polymer density profile normal to the substrate is well described by either the parabolic or the error function, which is in good agreement with the theoretical prediction for semidilute polymer brushes. Consistent with our results, Cosgrove et al.61a and Mansfield et al.61b reported good fits of a parabolic polymer density profile to their neutron reflectivity data from end-adsorbed polymers. Our recent neutron reflectivity measurements on telechelic triblock copolymer chains have demonstrated that,62 to a very good approximation, ABA type end-tethered chains behave in a manner equivalent to their AB-type counterparts of half their molecular weight and they adsorb predominantly in a loop conformation for copolymers with sufficiently strong sticking A blocks, consistent with the SFA measurements discussed above.38,51 III. Conjugated Polymers at Surface and Interface Just as the surface-bound macromolecules at the liquid/solid interface have played an important role in regulating the surface characteristics of polymeric biomaterials, conjugated polymers at gas/solid and/or solid/solid interfaces are of fundamental and practical significance in organic optoelectronic devices. As spincasting is a preferable method for fabrication of most semiconductor devices based on conjugated polymers, structural knowledge of conjugated polymers at the air/liquid interface is essential for a detailed understanding of the drying dynamics at the molecular level.63 On the other hand, interfacial control of conjugated polymers is of particular importance to most multilayer polymeric optoelectronic devices,2,3 as we shall see later. During the past 20 years or so, various conjugated polymers have been synthesized with unusual electrical,64 magnetic,65 and optical properties66 owing to the substantial π-electron delocalization along their backbones (Table 2). Because of its simple conjugated molecular structure and fascinating electronic properties, polyacetylene has been widely studied as a prototype for other conducting polymers.67 As seen in Table 2, certain conducting polymers already show sufficiently high conductivities to be attractive for a number of potential applications.68 However, the scope for using them has been hampered by their poor processability and environmental stability. Before the early 1980s no conducting polymer had been shown to be soluble in any solvent without decomposition, due to backbone rigidity associate with the delocalized conjugated structure. Furthermore, most unfunctionalized conjugated polymers were found to be unstable in air.69 Since then, a number of techniques have been

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Figure 4. (A) Model scattering length density profile used to fit the reflectivity data to (a) a Schultz function defined by eq 9 and (b) a skew Gaussian function defined by eq 10. (c) Schematic diagram representing PS-PVP block copolymer chains adsorbed at the solid-liquid interface from a selective solvent. Also included is the corresponding polymer density profile.58a (B) Neutron reflectivity data of a PS577-PVP283 diblock copolymer and model fit based on the Schultz function defined by eq 9.58a

developed to overcome these problems. Relatively stable conducting polymeric materials have been made, for example, by physically blending the conjugated polymers with certain

nonconjugated macromolecules69 or chemically synthesizing conducting polymer colloids70 and conjugated oligomers.71 Meanwhile, it now appears that intractable conjugated polymers

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Figure 5. (a) Neutron reflectivity profile of an end-adsorbed PS1700-PEO167 from toluene-d8 onto quartz fitted with eq 11. The inset shows the same profile as (a) plotted as RQ4 vs Q.58b (b) Error function (____) and parabolic (- - -) density with the parameters φ0 ) 0.035, t ) 500 Å, and δ ) 170 Å.58b

may be solubilized by substitution or copolymerization with nonconjugated soluble components.2 In this context, we have synthesized polyisoprene-polyacetylene (PI-PA) diblock copolymers, which show good electrical properties after doping,72a via the so-called anionic-to-Ziegler-Natta route using CoCl2 as a cocatalyst (Scheme 5).72b

The PI-PA diblock copolymers thus prepared present an unusual situation where the stiffness characteristic of insoluble conjugated polyacetylene chains and the high flexibility and solubility characteristic of polyisoprene chains are combined into one macromolecule. Because of the large difference in solubility between the two blocks, PI-PA diblock copolymers are

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TABLE 2: Some Conjugated Conducting Polymersa

a Dates in the parentheses are the year in which the particular conducting polymer was discovered, and the conductivities listed range from original reported values to the highest values obtained to date.2c

expected to be amphiphilic and behave like surfactants.74a In fact, we have noticed that dilute solutions of PI-PA in most organic solvents foam when shaken, indicating surface-active behavior. We have investigated the amphiphilic behavior of PIPA at the air/liquid interface by neutron reflectivity measurements.74b 1. Conjugated Polymers at Air/Liquid Interface. In general, a surface excess almost always exists at the gas/surfactant solution interface, since a fresh surfactant solution surface free from adsorbed surfactant molecules is not an equilibrium state.73 By analogy with surfactant solutions, this suggests that there will be an associated surface excess at the air/PI-PA copolymer solution interface. In fact, we have successfully detected this by neutron reflectivity measurements.74b As mentioned earlier, the reflection profile from an interface may be calculated analytically75 as may that from a series of thin layers of different refractive indices using the optical matrix methods.57 The latter case approximates our air/PI-PA solution interface, here we model the reflectivity profile with a minimum number of thin slices. Since the neutron’s potential energy at nucleus i, whose coherent scattering length is bi, can be written as

2πp2bi V(ri) ) m

(13)

(where m is the neutron mass and p ) h/2π), the modified neutron kinetic energy (in units of p2) per unit volume is 2 2πbi q2zi qz0 ) 2m 2m mVm

(14)

where qzo is the component of the neutron’s momentum perpendicular to the surface, in a vacuum, which is given by

qz0 )

2π sin θ λ

(15)

For a medium containing different nuclei (e.g., a molecular substance or alloy) we must sum over the nuclei and use the molecular volume, Vm. 2 q2zi ) qz0

4π Vm

∑i bi ) qz02 - 4πF

(16)

The sign of F determines whether the neutron refractive index

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SCHEME 5: Diblock Copolymerization of Isoprene and Acetylene via the Anionic to Ziegler-Natta Route Using the Cobalt Catalyst72b

TABLE 3: Parameters Used for the Best Fit to the Neutron Reflectivity Profile of PI117-PA(D)71 in Toluene-d8 sample

thickness (Å)

scattering density (×10-5 Å-2)

d1 d2 d3 d4 F1 F2 F3 F4 PI-PA(D) in toluene-d8 8.59 11.66 18.63 39.05 0.69 0.41 0.51 0.54

is increased or decreased in the medium and hence the total (external or internal) reflection condition (see eqs 5-7) and Figure 6S of the Supporting Information). Here we use this critical condition as a means of probing the sharpness of the interfacial structure. The reflectivity can be calculated for a system composed of a histogram of layers.76 At the boundary between the ith and (i + 1)th layers it is given by

ri,i+1 )

qz,i - qz,i+1 qz,i + qz,i+1

(17)

For an air/layer 1/layer 2 system with layer 2 much thicker than layer 1 and real qz,o, the reflectivity has the simple form

R)

r201 + r212 + 2r01r12 cos(2qz1d1) 1 + r201r212 + 2r01r12 cos(2qz1d1)

(18)

where r01 is the reflectivity of the air/layer 1 interface, r12 that of layer 1/layer 2 interface and d1 is the thickness of layer 1 (cf. Figure 6Sb of the Supporting Information). If the interface is not perfectly sharp, the diffusive “roughness” may cause the reflectivity to fall increasingly below the calculation value (eq 18) as the momentum transfer increases. For an interface which is rough over a mean-squared thickness 〈Z2〉 with a Gaussian distribution, the ideal reflectivity need to be modified by77

(ri,i+1)effective ) (ri,i+1)ideal exp(-2qiqi+1〈Z2〉)

(19)

In the case of PI-PA(deutero), the effective reflectivity should be further corrected by a wavelength-dependent background due to the incoherent scattering from the protoncontaining component of the copolymer,77 i.e.,

background ) Bλ

(20)

Equations 15-20 enable us to test our neutron reflectivity data, recorded from the air/solution interface of PI117-PA(D)71 copolymers in toluene-d8 (deuteriotoluene) at low resolution in the range 0.05 < Q/Å-1 < 0.65, against the theory. Figure 7Sa of the Supporting Information shows the best fit to the reflectivity profile in this range of the model with a PI/toluene-

d8 layer (F ) 0.496 × 10-5 Å-2, d ) 40 Å) between air and the bulk toluene-d8. By observing the reflectivity profile under a higher resolution, an indication of how the PA is arranged relative to the PI at the interface is obtained. The full reflection profile for PI117-PA(D)71 in toluene-d8 has been fitted with a four layer model (see Figure 7Sb of the Supporting Information and Table 3). In view of the scattering length densities for the principal components of our PI-PA(D) solutions (i.e., FPA(D) ) 0.71 × 10-5 Å-2, FPI ) 0.27 × 10-5 Å-2, Ftoluene-d8 ) 0.57 × 10-5 Å-2),74 the parameters listed in Table 3 for the best fit to the neutron reflectivity profile from PI-PA(D) solutions show that there is a surface excess of the copolymer at the air/toluene interface. As mentioned above, this may be described as a layer about 40 Å thick at low resolution. The higher resolution reflectivity profile is consistent with formation of a polyacetylene layer at the interface and the polyisoprene tails remaining in the toluene solution in a manner analogous to surfactants at air/water interfaces.73 2. Conjugated Polymers at Solid/Solid Interfaces. Semiconductor devices based on conjugated polymers may be divided into two categories depending on whether a current or light is produced. The former includes thin film transistors,78 photoconductors,79 and photovoltaic cells,80 while the latter encompasses light-emitting diodes (LEDs)81 and optically/electrically pumped lasers.82 For all of them, the charge transport process across interfaces between the semiconducting polymers and the metal electrodes is of vital importance. Therefore, knowledge of polymer/electrode and polymer/polymer interfaces forms the basis for understanding and improving the performance of these devices. Because of the space limitation, we choose polymeric LEDs as an example to illustrate the importance of the polymer/ metal (electrode) and polymer/polymer interfaces in regulating the device performance of organic optoelectronic devices. a. Polymeric LEDs. The development of electroluminescent devices based on inorganic semiconductors and conjugated organic molecules has for decades been an active research area.83 The discovery of electroluminescence (EL) in poly(p-phenylenevinylene), PPV,84 in 1990 has further stimulated activity in this research area and opened up new possibilities. Polymeric electroluminescent materials offer many competitive advantages, including their versatility for fabrication (especially over a large area), flexibility, low operating voltage, and the wide tunability of emission wavelength,81 which make polymer LEDs very attractive for large-area display applications at a low cost. Figure 8SA of the Supporting Information schematically shows a typical structure of the simplest single layer LED devices, in which a thin layer of electroluminescent (EL) conjugated polymer (e.g., PPV) is sandwiched between two electrodes (an anode and a cathode). Among the two electrodes, at least one should be transparent or semitransparent. In practice, a transparent indium-tin-oxide (ITO) coated glass is often used as the anode and a layer of low work function metal (e.g., aluminum, calcium) acts as the cathode. Upon application of an electrical voltage onto the EL polymer layer in a LED device, electrons from the low work function cathode (i.e., Al in Figure 8SA) and holes from the high work function anode (i.e., ITO

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in Figure 8SA) are injected respectively into the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the polymer. The injected charges combine in the band gap of the EL polymer to form singlet excitons, which decay radiatively to emit light. The wavelength (and hence the color) of the photons thus produced depends on the energy gap of the organic light-emitting material, coupled with a Stokes shift.81 Figure 8SB of the Supporting Information shows the absorption (curve a) and emission (curves b and c) spectra of PPV. The similarity of the emission spectra generated by photoexcitation (curve b of Figure 8SB) and by charge injection (curve c of Figure 8SB) suggests that the same excited state is responsible for both photoluminescence (PL) and EL.81 The theoretical maximum EL efficiency, however, may only reach one-quarter of the best PL efficiency (27% for PPV), as electron-hole capture is expected to result in the loss of 75% of the electron-hole pairs to triplet excitons, which do not decay radiatively with high efficiency.81a For comparison purposes, it is worthwhile to mention here that the first LED device with the Al/PPV/ITO structure reported by Burroughes et al.84 had an efficiency of only 0.05%. A few years later, this group obtained a very high internal efficiency (ca. 4%) from both a single active layer device based on the PPV-dimethoxysubstituted PPV copolymer85a and a PPV/cyano-substituted PPV bilayer device.85b-d Although the synthesis of EL copolymers with alternating conjugated and nonconjugated segments has since been demonstrated to be a useful method for improving the EL efficiency through exciton confinement,86 the work functions of the cathode and anode need to be matched with the corresponding LUMO and HOMO levels of the EL polymer in a particular LED in order to achieve efficient emission. The internal quantum efficiency, ηint, defined as the ratio of the number of photons produced within an EL device to the number of electrons flowing through the external circuit,81a is given by

ηint ) grstq

(21)

where g is the number of excitons formed per electron flowing through the external circuit, rst is the fraction of excitons formed as singlets, and q is the efficiency of radiative decay of these singlet excitons. Therefore, to improve the EL efficiencies, various issues concerning the formation of excitons and their decay need to be considered. The process of exciton formation, in turn, involves carrier injection, carrier transport, and carrier combination. As we shall see later, the polymer/electrode interface has an important impact on the carrier injection and carrier transport. Balanced injection and transport are prerequisites for LEDs with a high quantum efficiency.81 Balanced injection may be achieved using two polymer/electrode interfaces with equal barriers, which must be small for low operating voltages.81 The work functions for some of the commonly used electrode materials are given in Table 4. It can be seen that the Ca/PPV/ ITO provides a better match in the barriers for electron and hole injection than Al/PPV/ITO. Consequently, the quantum efficiency of a PPV/ITO based LED was significantly improved when calcium was used in place of aluminum.87 However, metals of a low work function are generally oxygen reactive. The use of highly reactive metals with a low work function (e.g., calcium) often requires careful encapsulation which makes this method difficult, if not impossible, for practical applications. To circumvent this difficulty, modifications of the electrode(s)88 and the charge injection characteristics89 by surface and interface

TABLE 4: The Work Function of Electrode Materials and Barriers to Injection of Electrons and Holesa electrode

work function (eV)b

barrier to injection (eV)

Al Ca Mg Au In Cu Cr ITO

4.2 2.9 3.7 5.3 4.1 4.7 4.5 4.6

1.6 0.3 1.1 2.7 1.5 2.1 1.9 0.5

a The schematic energy level diagram below shows the ionization potential (IP) and electron affinity (EA) of the light-emitting polymer, the work function of the anode (Φa) and cathode (Φc), and the barriers to injection of electrons (∆Ee) and holes (∆Eh). By taking the IP and the band gap (i.e., LUMO-HOMO) of PPV as 5.1 and 2.5 eV, respectively,87a we should have EA ) 5.1 - 2.5 ) 2.6 eV for PPV, ∆Ee ) Φc - 2.6 (eV), and (∆Eh) ) IP - Φa.

b

See the following: Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 77th ed.; CRC Press: Boca Raton, 1996.

control of the polymer/metal (electrode) and/or polymer/polymer interfaces have been exploited as alternative avenues for improving the EL efficiencies. b. Polymer/Metal Interface. Chemical DeriVatization of the Metal Electrodes. Many recent experimental/theoretical investigations have shown that metal may interact, either physically or chemically, with EL polymers in polymeric LEDs.90 For instance, it was reported that calcium atoms, like other metal atoms (e.g., Al, K, Rb, or Na),91 diffused from the cathode into the near surface region of a PPV film to form a 20-40 Å thick interface of the p-doped PPV,92 leading to emission quenching.93 Aluminum was even found to chemically break the conjugation sequences of PPV by covalently bonding onto the vinylene units.94 Furthermore, ITO electrodes were shown to not only chemically interact with conjugated polymers95 but also with the HCl released from a sulfonium precursor polymer of PPV upon the thermal conversion (Scheme 6).84,96,97 We have followed the thermal conversion shown in Scheme 6 for oligo(ethylene oxide) grafted PPVs by XPS measurements.97 Figure 6 shows the XPS survey spectra of the freshly prepared EO1PPV precursor polymer (i.e., product II of Scheme 6, R ) OCH2CH2OCH3) spin-cast on a quartz plate before and after the thermal conversion. Also included in the inset of Figure 6 are the corresponding high-resolution C(1s) spectra. As expected, Figure 6a shows the presence of C, O, Cl, and S in the EO1PPV precursor polymer. However, XPS analyses on the precursor polymer after the thermal transformation gave a survey spectrum with no signals attributable to Cl and S (Figure 6b). The large decrease in the aliphatic carbon peak region at 285 eV seen for the C(1s) spectrum of the EO1-PPV precursor polymer upon thermal treatment (inset of Figure 6) arises from

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Figure 6. XPS survey spectra of (a) EO1-PPV precursor polymer and (b) EO1-PPV. The inset shows the high-resolution C(1s) spectra of EO1-PPV precursor polymer (‚‚‚) and EO1-PPV (s).97

SCHEME 6: Sulfonium Precursor Route to PPV and Its Derivatives (R ) H or O(CH2CH2O)mCH3 ) EOm, m ) 1, 2, 3)97

the concomitant loss of the tetrahydrothiophene units. The calculated carbon atomic ratio of ca. 8 to 6 from the signals characteristic of the ether carbons at ca. 286.6 eV and carbons of the conjugated backbone at ca. 284.7 eV is consistent with the structure of the monomer unit of EO1-PPV (i.e., product IV of Scheme 6, R ) OCH2CH2OCH3). A separate XPS study on a thin film of the sulfonium precursor polymer of PPV on an ITO substrate, however, indicated the formation of InCl3 during the thermal conversion.98 In this study, the indium ions in the uppermost surface layer of ITO were partially substituted by gallium ions in order to detect the subtle chemical interactions at the ITO/precursor-PPV interface. Given the chemical similarity between gallium and indium, the chemical reactivity of a Ga-substituted ITO surface is taken to be the same as a pristine ITO substrate. The Ga(2p) spectra, together with the curve fits, for the pristine and sulfonium precursor-coated Ga-substituted ITO substrate are

J. Phys. Chem. B, Vol. 104, No. 9, 2000 1905 given in Figure 9S of the Supporting Information. The Ga(2p3/2) peak from the uncoated, Ga-substituted ITO substrate located at 1118.1 eV (Figure 9Sa). The Ga(2p3/2) spectrum for the ascast precursor-PPV on the Ga-substitute ITO, however, showed a main component at 1118.6 eV and a minor component at 1118.0 eV (Figure 9Sb). The upshift of the gallium binding energy upon deposition of precursor-PPV has been taken as an indication for chemical interactions of the ITO surface with HCl to produce GaCl3 (and InCl3), as HCl was shown to be present in the precursor-PPV even at room temperature (Scheme 6).98 It seems, therefore, that some form of physicochemical interaction always occurs at the conjugated polymer/metal interface. The metal-induced emission quenching, however, could be recoverable, for example, by intentional partial oxidation of the metal surface.99 Also, the insertion of a thin layer of insulating materials (e.g., functionalized oligophenylene,100 carbon,101 or certain polymers102) between the EL polymer and metallic cathode can significantly improve the device performance.103 In particular, an oxygen or nitrogen plasma treatment of a PPV layer at the PPV/Al interface in a LED device with the Cr/PPV/Al structure has been demonstrated to cause a disappearance of the rectifying behavior, along with an increase in the current by many orders of magnitude.104 The plasma treatment is believed to be useful for creating a charge carrier confinement layer to facilitate the charge injection, as is the case with the electron-transporting layer.105 Furthermore, a few studies have reported on the use of the plasma polymerization technique for the fabrication of LED devices.106 In view of the fact that plasma treatment including plasma etching can produce a large variety of surface functionalities and surface topologies31,107 for modification of the metal electrodes,99-108 there is no doubt that the plasma technique will have potential implications for making fascinating optoelectronic devices. The ease with which micropatterns of organic materials can be made by the plasma technique (vide infra) could add additional benefits to this approach. Modification of the Charge Injection Characteristics. Yang and Heeger109 have significantly reduced the operating voltage and increased the quantum efficiency for a polymer LED based on MEH-PPV (i.e., poly[2-methoxy-5-(2′-ethylhexyloxy)-pphenylenevinylene]) by using a combination of doped polyaniline and ITO as the anode. The improvement was attributed to a higher electronegativity, and hence a smaller hole-injection barrier of polyaniline than ITO. In addition, the polyaniline layer was reported to stabilize the chemistry of the polymer/electrode interface, possibly by acting as a barrier to the passage of oxygen out of the ITO.110 LEDs with a five-layer structure, in which a polyaniline emeraldine-base film was sandwiched between the emitter and the two electrodes at each side, have also been reported.111 Due to changes in the charge injection/transport characteristics, these so-called symmetrically configured AC light-emitting (SCALE) devices are insensitive to the electrode materials used and work equally well in forward- and reversedc bias modes, indicating that they can run on ordinary household current.111,112 The change of the charge injection characteristics at the polymer-electrode interface in the SCALE devices is clearly evidenced by a decrease in the total resistance with increasing number of insulating polymer layers.113 On the other hand, light-emitting electrochemical cells (LECs) were also reported, in which a light-emitting p-n junction is formed in situ by simultaneous p-type and n-type electrochemical doping of electrolyte-containing conjugated polymer films on opposite electrodes.114 Because the carrier injection occurs through ohmic contacts into the p- and n-doped regions, stable

1906 J. Phys. Chem. B, Vol. 104, No. 9, 2000 metals can be used as the electrodes in LECs and there is no need to match the work functions of the anode and cathode to the π and π* energies of the luminescent polymer. Besides, since the color of the emitted light from an LEC is determined by the energy gap of the luminescent polymer in which the p-n junction is formed, multiccolor emissions can be achieved by region-specific control of the p-n junction in a multilayer LEC device. For instance, a two-layer LEC with the ITO/PPV + PEO(LiCF3SO3)/MEH-PPV + PEO(LiCF3SO3)/Al structure has been shown to emit green light when the induced p-n junction was completely located inside the PPV layer at one bias, whereas the same device gave the orange emission when it was biased at the opposite polarity, causing the p-n junction to be shifted into the MEH-PPV layer.115 Similar polaritydependent multicolor emissions, however, had also been obtained for certain multilayer polymer LEDs by appropriate control of the polymer/electrode and/or polymer/polymer interfaces116a-d or a single-layer LED based on polymer emitters of a small band gap symmetrically located within the work functions of the electrodes.116e c. Polymer/Polymer Interface. Unlike the single layer LED shown schematically in Figure 8SA of the Supporting Information, multilayer LED devices consist of multiple polymer layers sandwiched between two metal electrodes, in which the polymer/ polymer interfaces also play an important role in regulating the device performance. Although quantitative structural description of the polymer/polymer interfaces (e.g., in terms of interdiffusion, sharpness of the interface) in multilayer LEDs is still limited,117 the general use of neutron reflectometry for polymerpolymer interfacial characterization has been demonstrated to a significant extent during the past decade.40a-c An example of the neutron reflectivity profiles from a bilayer film of polystyrene(deutero), PS(D), and PS(H) on a glass substrate is given in Figure 10S(A) of the Supporting Information.118 In this study, all interfaces are assumed to be Gaussian, with the neutron reflectivity curves fitted by simple error functions (see eq 12). The variance δz of the error functions is then taken as a measure of the interface width. As can be seen from curve a in Figure 10SA, the significant change at larger angles upon 2 min of annealing at 120 °C indicates a fast diffusion and/or relaxation of the polymer chains on a small distance scale. Thereafter, no obvious change in the reflectivity was observed over a considerable period of annealing time (e.g., 2-910 min, curve b of Figure 10SA). Further annealing caused a significant change in the reflectivity curves at small angles (curve c of Figure 10SA), suggesting segment diffusion over large distances. The interfacial broadening due to the segment interdiffusion is clearly seen in Figure 10SB of the Supporting Information. Clearly, neutron reflectometry is a promising tool for characterizing polymer/polymer interfaces in multilayer LEDs. 3. Surface Patterning of Conducting Polymers. The microlithographic formation of conducting patterns is a key prerequisite for many potential applications of conjugated polymers (e.g., polymeric LED displays).119 With conventional conjugated polymers, this is not straightforward, although a few approaches have been recently reported.2b In view of the wide range of different conjugated polymers (cf. Table 2), it would be unrealistic to try to develop a single microlithographic technique for general purposes. Instead, conjugated polymers may be patterned to match the requirements of particular applications on a case by case basis. In what follows, we will summarize several methods which our group and others have developed for patterning various conjugated polymers. a. Photolithographic Patterning. The report,120 in 1988, that

Feature Article “I2-doping” of certain nonconjugated polydiene rubbers resulted in conductivity came as a surprise,121 as the conjugated structure has been known to be necessary for electrically conducting polymers (vide supra). We have since demonstrated that “I2doping” of 1,4-polyisoprene produces conjugated sequences of unsaturated double bonds in the polyisoprene backbone through polar addition of I2 into the isolated double bonds in the polymer chain, followed by HI elimination (Scheme 7).122 The partially conjugated polymer so formed has a chain structure characteristic of a random copolymer with conjugated sequences interspersed among polydiene sequences. It is soluble in most organic solvents and closely resembles, in electrical properties, the soluble PI-PA copolymers discussed in section III.1. Our further investigation on the I2-induced conjugation of polydienes showed that cis- and trans-1,4-polybutadienes exhibit different behavior toward the reaction of Scheme 7, although they are identical in chemical structure.123 For cis-1,4-polybutadiene, the reaction sequence given in Scheme 7 terminated, at room temperature, at product II, whereas for the trans isomer the reaction sequence proceeded toward product III and/or IV, thus leading to the formation of conjugated sequences which confer electrical conductivity. Our spectroscopic measurements and molecular orbital calculations demonstrated that it was an unfavorable combination of electronic and steric interactions within the iodinated cis-1,4-polybutadiene backbone (product II) that inhibited the elimination of hydrogen iodide through the E2 elimination mechanism (reaction (2) of Scheme 7) at room temperature, and hence halted the formation of conjugated sequences in the case of cis-1,4-polybutadiene.123 However, cis1,4-polybutadiene can be photoisomerized into the trans isomer, and the isomerized material is then amenable to “I2-doping”. On the basis of the above findings, conducting patterns have been generated in the nonconducting cis-1,4-polybutadiene film through a microlithographic method.124 As expected, only the photoisomerized regions are capable of generation of conjugated double bonds upon exposure of the entire polybutadiene film to iodine at room temperature, resulting in the formation of conducting patterns in an insulating matrix of iodinated cis1,4-polybutadiene. The conducting patterns thus formed are colored and show strong fluorescence emission, which enables visualization of the conducting polymer regions. An example of the conducting patterns thus generated is shown in Figure 7a; it is a close replication of the photomask structure. Conducting wires on a micrometer scale are clearly evident. A corresponding fluorescence microscopic image of the conducting pattern is given in Figure 7b. It shows the same features as the optical micrograph (Figure 7a) but with inverse intensities in the image. The dark regions characteristic of the “I2-doped” trans-1,4-polybutadiene in Figure 7a gave rise to bright fluorescence emission in Figure 7b, consistent with the fluorescence emission originating from the conjugated structures.2 The dark regions in Figure 7b represent nonfluorescent components associated with the cis isomer. While the conducting patterns thus prepared may find applications in certain electronic and photonic devices, the photolithographic method has also been applied to micropattern electroluminescent conjugated polymers125 including PPV.126 For instance, Cho et al.127 have discovered that the methoxysubstituted PPV precursor polymer (i.e., product III of Scheme 6) decomposed at a lower temperature (80-200 °C) after an acid-catalyzed UV irradiation than the nonirradiated precursor polymer (>220 °C). On this basis, these authors prepared PPV conducting micropatterns (10-2-10-3 S/cm) by using triphenylsulfonium salts as both an acid catalyst and photochemical

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SCHEME 7: Reactions of Polydiene with Iodine, Leading to the Formation of Conjugated Sequences122b

dopant for a selectively photoinitiated thermolysis of the methoxy-substituted PPV precursor polymer at a relatively low temperature. Similarly, PPV has also been microstructured in a nondoped form by UV interferometry,128 as the transformation from the unsubstituted PPV precursor polymer (i.e., product II of Scheme 6, R ) H) to the conjugated state (i.e., product IV of Scheme 6, R ) H) can occur not only through the conventional thermal conversion shown in Scheme 6 but also upon UV irradiation.128 Compared with the conducting PPV micropatterns, the nondoped PPV microstructures thus formed seem to be more suitable for EL applications because of the possible doping-induced fluorescence quenching.2,81 A light emitting diode array consisting of micron-sized emitting pixels has been recently constructed129 from a PPV copolymer (poly(1,4-phenylenevinylene-co-2,6-pyridylenevinylene))130 through a selective laser ablation process.131 Apart from the patterned emissions, the emission intensities increased significantly due to the much enhanced electric fields at the edges of the conducting (Al/ITO) pixels. b. Pattern Formation by SelectiVe Adsorption. Pattern formation/recognition by self-assembled surface structures has recently become a very promising technique for microstructuring organic materials.132 To mention but a few examples: (i) the selfassembled monolayer (SAM) technique has been used to pattern conducting polymers (e.g., polypyrrole, poly(3-methylthiophene), and polyaniline) through selective electropolymerization of the corresponding monomers onto photochemically patterned disulfide SAMs on gold;133 (ii) poly(thiophene-3-acetic acid) has been immobilized onto a positively charged SAM layer generated from aminopropyldimethylethoxysilane or polyethylenimine;134 and (iii) the microcontact printing (µCP) technique135a has been employed to produce hydrophobic

patterns of C18H37SiCl3 on hydrophilic glass substrates.135b,c The latter process was followed by preferentially adsorption of polypyrrole or polyaniline onto the hydrophobic regions from dilute aqueous solutions of the polymerizing monomer to form thin film patterns.135b,c The patterned conducting polymers on the hydrophobic surface were found to take extended chain conformations, leading to enhanced conductivity.136a-c Such materials have been demonstrated to be useful as electrodes in polymeric liquid crystal display devices.136a,d More recently, several groups have reported the use of plasma-patterned surfaces for region-specific deposition of soluble conjugated polymers.137,138 For example, we have developed a plasma method to produce patterns of the oligo(ethylene oxide) substituted poly(p-phenylenevinylene) synthesized via Scheme 6. In so doing, we first prepared hydrophilic (acetic acid) plasma patterns onto FEP films. The conjugated polymer patterns were then obtained by selective adsorption from a solution of EO3-PPV in chloroform through the polarpolar interaction between the oligo(ethylene oxide) side chains and the micropatterned plasma polymer.138 Figure 11S of the Supporting Information shows a typical fluorescence microscopic image of the patterned EO3-PPV on the plasma-treated FEP, which is a close replica of the TEM grid used as the mask. As can be seen in Figure 11S, the polymer absorbed regions gave fluorescence emissions characteristic of EO3-PPV, indicating potential implications for making patterned light-emitting units of use in LEDs/LECs. Furthermore, the chemical grafting of the ion conductive oligo(ethylene oxide) segments on PPV can minimize the phase separation often associated with conventional light-emitting electrochemical cells (LEC), leading to a much reduced response time and onset voltage.139 Therefore, these novel optoelectronic features, together with the above

1908 J. Phys. Chem. B, Vol. 104, No. 9, 2000

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Figure 7. (a) Optical microscopy image of a pattern obtained by “I2-doping” of the photoisomerized trans-1,4-polybutadiene regions in the iodinated cis-1,4-polybutadiene matrix. The dark areas are regions of “I2-doped” polybutadiene, and the width of the white rectangles at the bottom part of the picture is 18 µm. (b) Fluorescence micrograph of the conducting pattern.124

simple method for micropattern formation, could lead to applications of these new oligo(ethylene oxide)-substituted PPVs in various optoelectronic devices. c. Plasma Patterning. We have also developed a versatile method for obtaining patterned conducting polymers by first depositing a thin patterned nonconducting (e.g., n-hexane) plasma polymer layer onto a metal-sputtered electrode and then performing electropolymerization of monomers, such as pyrrole and aniline, within the regions not covered by the patterned plasma polymer layer (see Figure 12S of the Supporting Information).140 Figure 12Sa shows the n-hexane plasma pattern on a gold-coated mica surface, where the dark areas represent

n-hexane plasma polymer and the bright regions are associated with the plasma-polymer-free gold surface. Figure 12Sb represents a typical reflection light microscopy image of a polypyrrole pattern electrochemically polymerized onto platinumcoated mica sheets prepatterned with the freshly prepared n-hexane plasma polymer. It shows the same features as the plasma pattern of Figure 12Sa, but with inverse intensities. The bright regions characteristic of the uncovered metal surface in Figure 12Sa become dark in Figure 12Sb, due to the presence of a dark layer of the newly electropolymerized polypyrrole film. The bright regions in Figure 12Sb represent a more reflective surface associated with the n-hexane plasma polymer.

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The cyclic voltammogram measurements indicate that the polypyrrole patterns thus prepared are electrochemically active.31,140 In view of the preparation of semiconducting organic films by plasma polymerization of aromatic monomers,2b such as halogenated benzene, halogenated thiophene, aniline, 1-benzothiophene, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), 3,4,9,10-perylenetetracarboxylic diimide (PTCDI), and perylene, a patterned plasma polymerization of these monomers should allow the in situ formation of semiconducting micropatterns. Therefore, plasma patterning is a very attractive method for the fabrication of future electronic and/or photonic devices with region-specific characteristics. IV. Alignment and Surface Patterning of Carbon Nanotubes The discoveries of carbon nanotubes by Iijima141 in 1991 and a method for the large-scale synthesis of nanotubes by Ebbesen and Ajayan142 in 1992 have opened up a new era in material science and nanotechnology.5,143 Since then, various nanotubes, with or without encapsulated metals and having straight, curved, planar-spiral, and single- and double-helical shapes, have been experimentally produced either by the carbon arc method or by pyrolysis of organic chemicals.5,6 These elongated nanotubes consist of carbon hexagons arranged in a concentric manner, with both ends of the tubes normally capped by fullerene-like structures containing pentagons. They usually have a diameter on the order of tens to hundreds of angstroms and a length of up to several micrometers and can exhibit semiconducting or metallic behavior depending on their diameter and helicity of the arrangement of graphitic rings in the walls.5,6 Dissimilar carbon nanotubes may be joined together, allowing the formation of molecular wires with interesting electrical, magnetic, nonlinear optical, and mechanical properties attractive for a variety of potential applications.5,6 Indeed, carbon nanotubes have been proposed as new materials for electron field emitters in panel displays,144 single-molecular transistors,145 scanning probe microscope tips,146 gas and electrochemical energy storage,147 catalyst and protein/DNA supports,148 molecular-filtration membranes,149 and artificial muscles.150 The use of a carbon nanotube as an electrode in a rectifying heterojunction with a conjugated polymer has been demonstrated to significantly reduce the onset voltage for nonlinear current injection due to an enhancement of the local field at the tip of the nanotubes,151a while LEDs based on conjugated polymer/carbon nanotube composites have been shown to exhibit lower current densities and better thermal stabilities than the corresponding pure polymer devices, as carbon nanotubes enhance the conductivity and act as nanometric heat sinks.151b For most of the above applications, however, it is highly desirable to prepare aligned/patterned carbon nanotubes. 1. Alignment of Carbon Nanotubes. Although carbon nanotubes synthesized by most of the common techniques, such as arc discharge and catalytic pyrolysis,5 often exist in a randomly entangled state (Figure 13Sa of the Supporting Information),152 aligned carbon nanotubes have been prepared either by postsynthesis fabrication153 or by synthesis-induced alignment.154 Apart from the production of nanotubes filled with guest materials,155 the template synthesis technique has recently been used to prepare aligned carbon nanotubes.154a,c,d Aligned carbon nanotubes have also been prepared by passing a nanotube dispersion through an aluminum oxide micropore filter,156a by slicing a nanotube-dispersed polymer composite,156b,c or by

Figure 8. (a) Typical SEM image of the aligned nanotubes prepared by the pyrolysis of FePc. (b) Typical HR-TEM image of the individual constituent nanotubes showing ca. 40 concentric graphitic layers.154k

rubbing a nanotube-deposited plastic surface with a thin Teflon sheet or aluminum foil.153b Recently, Kroto et al.154e,f reported a method to align carbon nanotubes by pyrolysis of 2-amino4,6-dichloro-s-triazine on a silica substrate prepatterned with cobalt catalyst via laser ablation (Figure 13Sb of the Supporting Information). More recently, Rao et al.154g have prepared aligned carbon nanotube arrays, without involving template pores or laser-etched tracks, by pyrolysis of ferrocene, which contains both the metal catalyst and carbon source required for the nanotube growth, at ca. 900 °C. In a closely related study, Ren et al.154h synthesized large arrays of well-aligned carbon nanotubes by radio frequency sputter-coating a thin nickel layer onto display glass, followed by plasma-enhanced hot filament chemical vapor deposition of acetylene in the presence of ammonia gas below 666 °C. The use of organic-metal complexes containing both the metal catalyst and carbon source for producing aligned carbon nanotubes is of particular interest. In this regard, we have recently prepared large-scale aligned carbon nanotubes perpendicular to the substrate surface by pyrolysis of iron (II) phthalocyanine, FeC32N8H16, (designated as FePc hereafter).154k The pyrolysis of FePc was performed under Ar/H2 at 8001100 °C using an appropriate substrate in a flow reactor consisting of a quartz glass tube and a dual furnace fitted with independent temperature controllers.154k As can be seen in Figure 8a, the constituent carbon nanotubes have a fairly uniform tubular length and diameter. High-resolution transmission electron microscopic (HR-TEM) study shows that most nano-

1910 J. Phys. Chem. B, Vol. 104, No. 9, 2000 tubes are well-graphitised with ca. 40 layers of graphite sheets and an outer diameter of ca. 50 nm (Figure 8b). These newlysynthesized semiconducting nanotubes of controllable diameters and lengths could open avenues for testing properties of single nanotubes and to rationalize various potential applications. 2. Surface Patterning of Carbon Nanotubes. Burghard et al.157 have recently reported the region-specific deposition of nanotubes grafted with surfactants containing negatively charged groups (e.g., sodium dodecyl sulfate) onto substrate surfaces prepatterned with positively charged functionalities (e.g., ammonium groups on silanized silica). More recently, Liu et al.158 have demonstrated that individual single-wall carbon nanotubes can be region-specifically deposited onto SAM patterned surfaces with -NH2 functionalities. Micropatterns of aligned nanotubes normal to the substrate surface have been prepared by Fan et al.159 and Huang et al.160 In particular, our method allows not only the preparation of micropatterns and substrate-free films of the perpendicularly aligned nanotubes but also their transfer onto various substrates, including those which would otherwise not be suitable for nanotube growth at high temperatures (e.g., polymer films).160 As an initial approach, we carried out the patterned growth of aligned nanotubes by placing a mask (e.g., a TEM grid consisting of hexagonal windows) on the quartz glass substrate. The partially masked quartz glass plate was then coated with aligned nanotubes by the pyrolysis of FePc, as described above. The resulting nanotube films were studied by SEM after careful removal of the TEM grid. An example of the nanotube patterns thus prepared is given in Figure 9a, which shows a close replication of the mask structure. As seen in Figure 9a, micropatterns consisting of the densely packed, perpendicularly aligned nanotubes are clearly evident; the spatial resolution of the patterns is mainly limited by the resolution of the mask used. Our further investigation on the nanotube synthesis and microfabrication showed that the as-deposited aligned nanotubes could be easily separated from the quartz glass as a substratefree, floating film simply by immersing the nanotube-deposited quartz plate into an aqueous hydrofluoric acid solution (1040% w/w).160 This finding allows not only the transfer of the nanotube films onto various other substrates of particular interest but also the simultaneous formation of both a positive and a negative image of the aligned nanotubes. The latter is done, for example, simply by lifting up a substrate-free nanotube film floating on the HF/H2O solution onto a TEM grid, followed by region specifically transferring it to another substrate (e.g., polystyrene film) through physical contact between the TEM grid and the substrate surface.160 This contact transfer process is much like the widely used microcontact printing (µCP) technology for generating self-assembled monolayer patterns (see section III.3). In this case, the nanotubes suspended over the windows were selectively transferred onto the substrate surface as a negative image (Figure 9b) while those on the grid bars remained there as a positive pattern (Figure 9c). The most important feature to note in parts b and c of Figure 9 is that the constituent nanotubes in the µCP-generated patterns are still aligned normal to the substrate surfaces. In addition to the nanotube “carpets” and nanotube patterns shown in Figures 8 and 9, nanotube “crop circles” have recently been observed in deposits formed by the pyrolysis of FePc (Figure 10a).154k These nanotube “crop circles” differ from those reported by Liu et al.161 in that the former consist of aligned multiwall nanotubes normal to the substrate surface, whereas the latter contain single-wall carbon nanotube (SWNT) ropes lying flat on the substrate. The formation of a ringlike base of

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Figure 9. Typical SEM micrographs of patterned films of aligned nanotubes prepared by the pyrolysis of FePc. (a) Film produced by patterned growth using a TEM grid consisting of hexagonal windows as the mask. (b) The microcontact-printed negative image on a gold surface (Although the negative pattern could also be deposited onto various other substrates including polymers (e.g., polystyrene), gold was chosen for better SEM imaging). (c) The microcontact-printed positive image on a TEM grid used as the stamp. Note that the TEM grids used for (a) and (b, c) have grid bars of ca. 8 and 30 µm wide and hexagonal windows with a width of 50 and 80 µm, respectively.160

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J. Phys. Chem. B, Vol. 104, No. 9, 2000 1911 terns should facilitate the structural/property characterization of individual nanotubes and allow their effective incorporation into devices for practical applications (notably as electron emitters in flat panel displays). Unlike polymer brushes, however, research on the structure and property of these aligned nanotubes is still in its infancy. Continued research efforts in this embryonic field could give birth to a wide range of new nanotechnologies. V. Concluding Remarks

Figure 10. (a) Typical SEM image of the nanotube “crop circles” observed in deposits formed by the pyrolysis of FePc, showing that the “crop circles” consist of nanotubes aligned normal to the substrate surface.154k (b) Typical SEM micrograph of patterned films of aligned nanotubes prepared by the pyrolysis of FePc onto a photolithographically prepatterned quartz substrate. The slight pull-out seen for some of the carbon nanotubes was caused by the sample preparation for SEM examination.162

Fe particles has been demonstrated to be responsible for the growth of these nanotube “crop circles”.154k More recently, we have developed a novel method for photolithographic generation of the perpendicularly aligned carbon nanotube arrays with resolutions down to a micrometer scale.162 In so doing, we first photolithographically patterned a positive photoresist film onto a quartz substrate and then carried out the pyrolysis of FePc, leading to a region-specific growth of the aligned carbon nanotubes in the UV exposed regions (Figure 10b). In this case, the photolithographically patterned photoresist film, after an appropriate carbonization process, acts as a shadow mask for the patterned growth of the aligned nanotubes. This method is fully compatible with the existing photolithographic processes (see section III.3).2c This, together with the generation of perpendicularly aligned carbon nanotube arrays with an unprecedentedly high resolution (down to ca. 1 µm, see Figure 10b), represents a significant advance in the development of nanotube devices for practical applications. Just as tethered polymer chains have broadened the scope for using polymers in biomedical applications (see section II), perpendicularly aligned carbon nanotubes and their micropat-

Polymeric biomaterials, conjugated polymers, and carbon nanotubes show interesting physicochemical and optoelectronic properties which make them attractive for a wide range of potential applications. However, the surface properties of polymeric biomaterials often need to be optimized for specific applications. Likewise, conjugated polymers and carbon nanotubes need to be aligned and/or patterned, in a similar fashion as silicon or metals, for fabricating optoelectronic devices. Therefore, the surface and interface control of polymeric biomaterials, conjugated polymers, and carbon nanotubes is a key prerequisite for many practical applications. Various synthetic and chemical modification techniques have been developed for preparing polymeric biomaterials and conjugated polymers with appropriate bulk and surface properties, along with microfabrication methods for constructing conjugated polymers and carbon nanotubes into devices with desirable characteristics. We have focused our attention here on the surface and interface aspects. We have given a brief outline of the background to the field and discussed the most recent research developments. Even this brief account has revealed the versatility of surface modification and interfacial engineering for making sophisticated polymeric biomaterials and optoelectronic devices based on conjugated polymers and carbon nanotubes. Recent developments in the surface and interface control of polymeric biomaterials, conjugated polymers, and carbon nanotubes have clearly indicated the possibility of producing a wide range of new materials of exotic physicochemical properties and optoelectronic devices with novel features. Continued research in this multidisciplinary field should be very fruitful. Acknowledgment. We thank our colleagues who have helped us: Limin Dong, Thomas Gengenbach, Xiaoyi Gong, Hans J. Griesser, Weizhu He, Shaoming Huang, Xiaoyin Hong, Tom Spurling, Chris Strauss, Chris Toprakcioglu, Alfred Uhlherr, Zoran Vasic, Zhonglin Wang, John W. White, Berthold Winkler, David A. Winkler, Yangyong Yang, Paul Zientek, and many others. Supporting Information Available: Figures 1S-13S giving additional details. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) See, for example: (a) Prasad, P. N., Nigam, J. K., Eds. Frontiers of Polymer Research; Plenum Press: New York, 1991. (b) Black, J. Biological Performance of Materials: Fundamentals of Biocompatibility, 2nd ed.; Marcel Dekker: New York, 1992. (c) Galaev, I. Y. Russ. Chem. ReV. 1995, 64, 471. (d) Dai, L. Chin. J. Mater. Res. (Eng.) 1995, 9, 397. (2) See, for example: (a) Tsuruta, T., Hayashi, T., Kataoka, K., Ishihara, K., Kimura, Y., Eds. Biomedical Applications of Polymeric Materials; CRC Press: Boca Raton, 1993. (b) Nalwa, H. S., Ed. Handbook of Organic ConductiVe Molecules and Polymers; John Wiley & Sons: New York, 1997. (c) Dai, L. J. Macromol. Sci., ReV. Macromol. Chem. Phys. 1999, 39 (2), 273. (3) See, for example: (a) Dai, L.; Winkler, B.; Huang, S.; Mau, A. W. H. In Semiconducting Polymers: Applications, Properties, and Synthesis;

1912 J. Phys. Chem. B, Vol. 104, No. 9, 2000 Hsieh, B., Galvin, M., Wei, Y., Eds.; ACS Symp. Ser. 735; American Chemical Society: Washington, DC, 1999. (b) Conwell, E. M., Miller, M. R., Eds. Electroluminescent Materials, DeVices, and Large-Screen Displays; SPIE Proc. No. 1910, 1993. (4) See, for example: (a) Chan, C.-M. Polymer Surface Modification and Characterization; Carl Hanser Verlag: New York, 1994. (b) Ratner, B. D., Castner, D., Eds. Surface Modification of Polymeric Biomaterials; Plenum Press: New York, 1996. (5) For recent reviews, see: (a) Dresselhaus, M. S.; Deesselhaus, G.; Eklund, P. Science of Fullerenes and Carbon Nanotubes; Academic Press: New York, 1996. (b) Ebbsen, T. Carbon Nanotubes; CRC Press: Boca Raton, FL, 1997. (c) Saito, R.; Deesselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (d) Terrones, M.; Hsu, W.K.; Hare, J. P.; Kroto, H. W.; Terrones, H.; Walton, D. R. M. Philos. Trans. R. Soc. 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