Article pubs.acs.org/JPCC
Light-Controlled Resistance Modulation in a Photochromic Diarylethene−Carbon Nanotube Blend Calogero Sciascia,*,† Rossella Castagna,†,‡ Maria Dekermenjian,§ Richard Martel,§ Ajay R. Srimath Kandada,∥ Fabio Di Fonzo,† Andrea Bianco,⊥ Chiara Bertarelli,†,‡ Moreno Meneghetti,# and Guglielmo Lanzani† †
Center for Nano Science and Technology @ PoliMi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Politecnico di Milano, Piazza L. da Vinci, 20133 Milano, Italy § Departement of Physics and Departement of Chemistry, Université de Montréal, Montréal QC H3T 1J4, Canada ∥ Department of Physics, Politecnico di Milano, Piazza L. da Vinci, 20133 Milano, Italy ⊥ INAF − Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy # Department of Chemical Sciences, University of Padova, Via Marzolo 1, 35131 Padova, Italy ‡
ABSTRACT: Photochromic molecules are part of a large class of materials in which light stimulus not only induces a color variation but also affects other physicochemical properties. However, the change of bulk electrical properties (e.g., electrical conductivity) via light excitation remains difficult to control because the intrinsically switchable molecules may lose their functionality when wired with conductive electrodes. In contrast with previous work based on single molecules, here we demonstrate a facile and accessible “wet-chemical” method to produce light-induced electrical switching. The electrical conductivity of a photochromic blend composed of diarylethene polymer and single-walled carbon nanotubes (SWNTs) is reversibly tuned according with UV−vis excitation. The devices present good thermal stability and remarkable fatigue resistance under ambient conditions. Supported by electrical and spectroscopic evidence, we show that the intertube electrical coupling, mediated by the light-induced electrocyclization of the diarylethene unit, is the mechanism responsible for the modulation.
1. INTRODUCTION The use of organic molecules/polymers as active elements in electronic devices has attracted in the past decade considerable attention in material science. Organic molecules have appealing optical, electrical, and mechanical properties, which make them ideal candidates for a large group of applications ranging from light emitters and thin-film transistors to photovoltaic cells.1−3 However, applications with organic-based electronics can be possible only if new functionalities are made available at low manufacturing costs.4,5 Within the large field of organic electronics, the possibility of achieving control over the electrical properties through external light stimuli is highly desirable and gives rise to a new class of active devices called optoelectronic switches for memory and sensor applications.6 Photochromic materials are good prototypes of making lighttriggered switches7,8 because not only the color but also many other physicochemical properties can be modified.6,9 Modulation of conductivity, for instance, can be classified as direct when the photochromic molecule is the electrical transport channel or indirect when the photochromic molecule is chemically attached or in proximity to another moiety involved in transport.10,11 An interesting approach for achieving electro-optical functionalities involves the use of photochromic molecules directly linked to an electrode interface having nanometric or © 2012 American Chemical Society
macroscopic dimensions. Inducing modulation of the electrical properties in such media is however not trivial. On one hand, a short conductive anchoring group inhibits photochromic operation because of steric hindrance, dipolar coupling, or fast exciton dissociation. The use of longer insulating spacers on the other hand can help to preserve the optical response, but they also greatly reduce the on-state conductivity.12 The problem of making good electrical contacts between the organic molecules and the metal electrodes is further exacerbated in hybrid photochromic layers in which conductive particles (e.g., nanoparticles) are dispersed throughout the film in order to create many active nodes in series. While such a strategy ensures the presence of an extensive “in-plane” conductive network of a large number of photoswitches in series, it requires much effort to effectively disperse the nodes throughout the conductive media. For such macroscopic systems, it is also difficult to avoid short-circuits as well as open circuits while making sure that the photoactive nodes are uniformly distributed in the network. As a solution to this problem, Molen et al. have used two-dimensionally patterned lattices of nanoparticles in order to prepare a well-dispersed Received: December 19, 2011 Revised: June 2, 2012 Published: August 10, 2012 19483
dx.doi.org/10.1021/jp212231j | J. Phys. Chem. C 2012, 116, 19483−19489
The Journal of Physical Chemistry C
Article
Figure 1. (left) Chemical structures of the photochromic polymers in their opened and closed states. (right) Vis−NIR absorption spectra of a dispersion of SWNT/photochromic polymers (weight ratio of 1:100) in N,N-dimethylformamide at different irradiation times (wood lamp λmax = 366 nm). The SWNT absorption bands in the visible (hidden by closed-form band growth) and in the NIR are indicated.
nanotube.26 The LA SWNTs have slightly larger diameters (∼1.1−1.6 nm) and a different metallic/semiconducting ratio compared to CoMoCat nanotubes. Nevertheless, the electrical behavior of the networks is dominated in both cases by metallic tubes, even at moderate SWNT concentrations. All samples give an ohmic response and overall good electrical characteristics. SWNTs can be considered here as the conductive wires in the network: the density of the metallic SWNTs is above the percolation threshold and exhibits a metallic character. The specific photochromic moieties are part of a macromolecular chain. They are more specifically a diarylethene-based polyester with an alkyl spacer in the main chain (see chemical structure in Figure 1, on the left). The spacer is aliphatic, which guarantees a good solubility of the polymer in common organic solvents and easy processing of the material with standard solutionbased techniques. Moreover, it interrupts the extension of the π-conjugation over the backbone. Indeed, highly conjugated diarylethenes are characterized by poor ring-opening quantum yields that hamper (or even inhibit) the reversibility of the photoinduced process.27−30 The spacer is not too long to maximize the content of the active units in the material with a consequent large photoinduced response. The polymerization degree (Mn = 7500 and Mw = 12500 determined by GPC in THF at 35 °C using polystyrene as the standard) is large enough to ease the processing of the polymer for making thin and transparent films. Two different processes were developed to prepare the devices. The first was to sonicate both the SWNTs and the photochromic polymer in N,N-dimethylformamide (DMF) together in one pot.31 The fine suspensions are then centrifuged in order to remove large bundles and other metallic particles. The resulting material is deposited on a glass substrate. We prepared four specimens with SWNT nominal concentrations of 50, 10, 2, and 1% wt/wt. This procedure produced relatively good homogeneity and control over the SWNT network structure in the solid phase. The second process involves the casting of the polymer−dichloromethane (CH2Cl2) solution directly on top of a SWNT layer previously deposited on an oxide substrate containing interdigitated gold electrodes (see the Experimental Section). In both cases,
media, but their approach is elaborate and difficult to implement.13 Other authors have used conductive networks of photochromic molecules, but their conversion was slow and only poor conductance modulation has been obtained.14,15 In the present paper, we demonstrate an easy and scalable approach to produce optoelectronic switches based on a photochromic diarylethene derivative intercalated within a random network of single-walled carbon nanotubes (SWNTs).16−18 The layer forms a conductive hybrid network of SWNTs19 that can be effectively modulated through a photoinduced change of the intertube hopping rate. Thanks to the good conductivity of the SWNT network, the conductivity of the photochromic layer is much better than that of the photochromic polymers and shows well-suited characteristics for making photoswitches. The low-resistive planar structure was prepared by simple wet chemistry methods and takes advantage of the high electrical mobility, high aspect ratio, low percolation threshold, and low specific weight of SWNTs.20,21 Apart from one single exception,15 photochromic molecules and SWNTs have been used already together only at the single molecule level.22−25 This approach presents serious hurdles regarding the selection and manipulation of SWNTs, which can be readily overcome when using random distributions of the two materials. To the best of our knowledge, the only previous report on such blended systems showed a modest conductance variation (no larger than 5%) that was found to be unstable in time.15 Moreover, it involved a step of covalent functionalization of SWNTs, which is known to degrade the electrical transport properties of the nanotubes. In this paper, we demonstrate an easy and scalable way to fabricate devices based on SWNT−photochromic polymer blends that can effectively tune the electrical resistance of a layer up to 300% upon UV− visible light exposure.
2. RESULTS AND DISCUSSION In the present work, we use SWNTs grown with two different techniques: cobalt molybdenum catalyzed (CoMoCat) and laser ablated (LA) SWNTs. The former have diameters ranging from 0.7 to 1.1 nm, and the most abundant species, as evidenced by optical spectra, is the semiconducting (7,6) 19484
dx.doi.org/10.1021/jp212231j | J. Phys. Chem. C 2012, 116, 19483−19489
The Journal of Physical Chemistry C
Article
photochromism of the diarylethene-based polymer is retained in the liquid suspension as well as in the solid film. The vis− NIR absorption spectra collected at different UV irradiation times (peaked at 366 nm) in Figure 1 show a significant increase of a broad feature in the visible associated with the actinic bands of the closed form. The absorption features of the SWNTs associated with the low (Ex11) and high (Ex22) energy excitons are also seen in the regions between 1000−1400 and 800−500 nm, respectively. The Ex22 transitions are however partially masked by the fast evolving actinic bands. It is interesting to note that the position and width of the SWNT transitions appear unaffected by the state of the photochromic molecules, converted or not. This is surprising and a clear indication that the dielectric environment of the SWNTs is not modified by the state of the polymer32 and that only weak interaction is present between the polymer solution and the SWNTs. The changes in the optical spectrum and in the electrical resistance of the films (discussed below) are therefore related together. As shown in Figure 2, the photoinduced resistance changes are reversible and could be repeated many times. This behavior
Figure 3. Normalized electrical resistance upon UV exposure for four different SWNT concentrations (0, 2, 10, and 50% wt/wt).
Figure 4. Resistance of a sample with photochromic polymers casted onto a SWNT network. The left side, defined by empty blue circles, refers to the period of exposure to UV light. The right black section shows the temporal evolution of the sample resistance in the dark.
Figure 2. (A and B) Normalized resistance of the device exposed in sequence to red (λ = 635 nm, intensity ∼1.2 mW/cm2 − red traces on the graphs) and UV (λmax = 366 nm, intensity ∼2mW/cm2 − blue traces) excitations for a sample with 1% SWNTs. (C) Resistance (R) variation upon UV illumination. After 20 min, the UV exposure is interrupted and the sample stored for 12 h in the dark in order to highlight the presence of a slow (thermal) recovery.
polymer is directly involved in the electrical transport and that the difference in conductivity between the two photochromic forms modulates the overall electrical current at a given bias. In particular, the charge carriers are easily transported along the SWNTs, which act as highly conductive elements into the percolative network. This is evidenced by the data shown in Figure 5: the resistivity drops down with a power
is not detected when using SWNTs or photochromic polymers taken separately; the switching occurs only if these two elements are blended together. That is, the pristine SWNT film exhibits only a small (
