Eden
Steven
*a,
Victor
Lebedev
b,
Elena
Laukhina
*bc,
Concepció
Rovira
bc,
Vladimir
Laukhin
bcd,
James S.
Brooks
a and
Jaume
Veciana
bc
aDepartment of Physics, National High Magnetic Field Laboratory Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, USA
bInstitut de Ciéncia de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra, 08193, Spain
cCIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Campus UAB, Bellaterra, 08193, Spain
dInstitució Catalana de Recerca i Estudis Avançats (ICREA), ICMAB-CSIC, Campus UAB, Bellaterra, 08193, Spain. E-mail: esteven@magnet.fsu.edu; laukhina@icmab.es
First published on 3rd June 2014
Since their discovery, organic conductors have attracted fundamental and device physics interest due to their diverse physical properties. However, conventional electrochemical growth methods produce millimeter-sized crystals that do not translate to the fabrication of large-scale thin-film devices. Of late a chemical-vapor annealing method has been proved to be capable of growing a conductive polycrystalline layer of (BEDT-TTF)2I3 molecular conductor on the surface of soluble polycarbonate (PC) thin films in a bilayer configuration. (Here BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene.) This has resulted in efficient piezoresistive organic molecular sensors. Conversely, solubility and other incompatibilities limit the direct application of the crystallite growth method to other substrates with arbitrary shape and composition. Here we report methods to circumvent these limitations. Specifically, we demonstrate the transfer of the active layer of a PC/(BEDT-TTF)2I3 bilayer film from the non-porous parent PC substrate to porous and humidity-dependent Bombyx mori silk target substrates. SEM analysis, temperature dependent resistance, and electromechanical measurements show no significant damage to the transferred (BEDT-TTF)2I3 layer. The silk/(BEDT-TTF)2I3 bilayer films exhibit additional functions that can be used for humidity sensing, electric current-driven actuators, and strain detection. Of particular significance is the piezoresistive function of the porous silk bilayer structure that allows the investigation of multi-stage diffusion processes.
Conceptual insightsA polycrystalline layer of piezoresistive (BEDT-TTF)2I3-based molecular conductor is transferred from the parent polycarbonate/(BEDT-TTF)2I3 bilayers to silk and other substrates with various shapes and compositions. The transferred layer is robust as confirmed by SEM, temperature dependent resistance, and electromechanical measurements. The silk/(BEDT-TTF)2I3 bilayer films are sensitive to moisture, allowing their use for humidity sensing, electrical current driven actuators, and the study of complex moisture diffusion processes. |
Unfortunately, the chemical growth method is compatible only with solvent soluble polymer substrates, prohibiting its direct use to cover substrates of interest with variable (sculpted) shapes and compositions. For example, it is very difficult to infuse the BEDT-TTF molecules into silk materials homogeneously prior to the oxidation due to either solvent incompatibilities or the tendencies of the BEDT-TTF molecules to crystallize in silk environments prior to the oxidation process (see ESI, Fig. S1†). The purpose of the present work is to show how the functional thin polycrystalline layers can be transferred from a non-porous PC substrate to a substrate that has both shape and humidity-dependent functions, setting the stage for future applications of molecular conductors. Here we demonstrate methods of transferring the active layer of highly piezoresistive PC/(BEDT-TTF)2I3 (pristine) bilayer films from the parent PC substrate to Bombyx mori silk film target substrates.
The (BEDT-TTF)2I3 salt can be obtained in five different crystal phases, two of which are more common, α- and β-(BEDT-TTF)2I3, where the semimetal α-phase and superconducting β-phase are semiconducting and metallic as polycrystalline layers, respectively.15 The transferred (BEDT-TTF)2I3 layers (for both α- and β-phases) are robust, exhibiting similar surface and electrical properties to those of the pristine bilayers as confirmed by SEM, temperature dependent resistance, and electromechanical measurements. In comparison to the pristine bilayer films, the silk/(BEDT-TTF)2I3 bilayer films exhibit additional functions, allowing electronic monitoring of humidity and electrical current actuated functions (see ESI, Movie S1†). A unique aspect of this novel silk bilayer structure is that the humidity dependent resistance provides new ways to address complex diffusion effects in porous substrates such as the Bombyx mori silk film. By correlating moisture-induced changes in the silk structure to the piezoresistive properties of the (BEDT-TTF)2I3 layer, a non-Fickian, multi-stage moisture absorption process in the silk layer is revealed. In addition to silk substrates, the layer transfer can also be directly applied to rubber, silica, glass, sculpted plastic products, paper, and substrates with various shapes and geometry.
The first procedure, referred hereafter to as the face-down layer transfer, where the active layer face of the pristine bilayer is oriented down toward the receiver substrate, is most suitable for covering various flat substrates (Fig. 1a and c). The second procedure, namely the face-up layer transfer (active layer face oriented up and away from the receiver substrate), is more universal; it allows the covering of sculpted substrates (Fig. 1b and d). In both cases, the transfer involves the separation of the (BEDT-TTF)2I3 layers from a parent PC substrate by dissolving the PC layer using a small amount of dichloromethane (DCM). Additional experimental details are given in the ESI (Fig. S2–S8†).
To cover the silk film with a (BEDT-TTF)2I3 layer using the face-down layer transfer (see ESI, Fig. S2 and S3†), the silk film is first water-annealed23 for 24 hours at 50 mbar to render it insoluble in water. Then, a thin Bombyx mori silk solution is deposited on the silk film to act as an adhesive layer. The bilayer (BL) films are laminated onto the wet silk surface by putting the top conductive (BEDT-TTF)2I3 layer of the BL film face down on the substrate. The conductive layer of the BL film can be pre-treated by argon plasma for 10 seconds to improve its homogeneous adhesion to the receiver silk film. Lamination is completed by using a commercial smooth polytetrafluoroethylene-based plate. To remove the PC layer, we drop DCM over the film at an angle to allow the solvent containing dissolved PC to flow away. The complete removal of the PC layer can be checked by either measuring the electrical resistance of the film or through direct observation under an optical microscope (see ESI, Fig. S4†).
Therefore, zone a has an average (BEDT-TTF)2I3.5 composition that is in reasonable agreement with the expected stoichiometric formula α-(BEDT-TTF)2I3. In contrast, zone b contains a very small amount of iodine: the contribution of I and S to their compositions was found to be (1.5 ± 0.5) and (98.5 ± 0.5) atomic%, respectively. Based on this observation, the minor crystallite aggregates of crystals have average (BEDT-TTF)I0.1 composition. This suggests that during the BL film preparation,13–15 un-reacted BEDT-TTF may crystallize on the back side of the conductive layer.
The cross-section of the silk/α-(BEDT-TTF)2I3 bilayer was also studied by SEM. As is evident from Fig. 2b, the transferred α-(BEDT-TTF)2I3-based layer has a good intimate contact with the receiver silk film similar to that of the pristine PC/α-(BEDT-TTF)2I3 bilayer films. Using the same film lamination principle, the face-down layer transfer may be used for covering various flat substrates including paper sheets; the paper sheets can be covered with combinations of α- and β-(BEDT-TTF)2I3 as desired to form conductive sensing paper (see ESI, Fig. S5 and S6†).
The data presented in Fig. 3c show that the electrical resistance of the transferred α-(BEDT-TTF)2I3 layer on a silk film exhibits a reversible linear response to relative strain (ε). The value of strain sensitivity (gauge factor) is 9, in good agreement with previously reported values.9 The gauge factor is defined as the ratio between the relative resistance change (ΔR/R0) and the relative strain value (ε = ΔL/L0), where ΔR = R − R0, ΔL = L − L0, R is the resistance at length L after elongation, and R0 is the resistance at the initial gauge length L0.
As is evident in Fig. 4a, the resistance of the BL film reversibly tracks the relative humidity (RH) changes in the surrounding environment. A delay in the humidity response is observed (Fig. 4b) even though the RH is varied slowly (∼0.025% per s), indicating a diffusion-limited moisture absorption process of the Bombyx mori silk film. The humidity response can be modelled by considering the general solution to a resistor–capacitor (RC) charging circuit where the charging voltage varies with time. In this model (eqn (1)), for any specific time t, the supply voltage Vs is analogous to RH, the capacitor voltage Vc is analogous to the moisture uptake (and swelling of the silk that produces a change in the film resistance), Vr is the initial capacitor voltage (analogous to the initial moisture content) at the start of the charging process, and τ is the diffusion relaxation time constant analogous to RC. In equilibrium and/or for τ ≥ 0, the moisture uptake (∼Vc) is directly proportional to the RH (∼Vs). Since in the bilayer film we measure the piezoresistance which is analogous to the capacitor voltage, we may recast eqn (1) as eqn (2a) under the assumption in eqn (2b) that RH drives the time dependence.
Vc(t) = Vs(t)(1 − e−t/τ) + Vre−t/τ | (1) |
R(t) − R0 = Rs(t)(1 − e−t/τ) + Rre−t/τ | (2a) |
Rs(t) = ARH(t) | (2b) |
Explicitly, the analogy is that R(t) is the resistance of the BL film at any given time, R0 is the resistance of a completely dried film, Rs(t) is the resistance of the saturated film at a given RH value, Rr is the resistance due to the trapped residual moisture content at the beginning of the absorption process, and τ is the time constant of the humidity response. In Fig. 4b, R0, Rr, proportionality constant A, and τ are determined to be 6.8 kΩ, 1 kΩ, 33.4 Ω per RH%, and 645 s.
Although this model fits well to the data in Fig. 4b, the model breaks down at faster sweep rates. For the measurement obtained at a higher RH rate (0.125% per s), it is not possible to fit the data perfectly using the model (see ESI, Fig. S10†). This leads us to believe that the moisture absorption process is more complex than that described by the simple RC circuit model.
To study the moisture absorption process in more detail, a step function exposure experiment was performed. The BL film was first dried overnight at 23 °C and 30% RH, after which the film was suddenly exposed to a constant, saturated 73% RH environment. In Fig. 4c, the time dependent resistance after the sudden exposure is presented, showing features that deviate from the Fickian moisture absorption process.24,25 First, there is no clear saturation level26 that is typically observed in a Fickian process, at least within the given time scale. Instead, the resistance continues to slowly increase at longer times. Second, the normalized resistance (ΔR(t)/ΔRmax) varies with time (t) according to the following function, ΔR(t)/ΔRmax ∼ tn, where ΔR(t) = R(t) − R(at 30% RH), ΔRmax = R(at 73% RH) − R(at 30% RH), and n = 0.83 (Fig. 4c). In a Fickian process, n = 0.5 at the initial stage of absorption.24,27 These features are similar to the cases reported in the literature26 involving a dual-stage moisture diffusion process,25,26 where the first and second stages are attributed to void-filling and hydrogen bonding processes.28 As shown in Fig. 4d, there are 3 distinct stages of diffusion. The first stage can be attributed to film expansion mainly due to the filling of voids in the silk film. As the absorption transitions to the second stage, moisture continues to fill the voids but also penetrates and forms hydrogen bonds with the silk protein, swelling the protein internally. Finally in the third stage, all the voids have been filled but internal swelling continues to progress deeper into the silk structures. This result highlights the complex mechanisms of water transport processes in Bombyx mori silk.
Footnote |
† Electronic supplementary information (ESI) available: Images of formation of BEDT-TTF crystals on Bombyx mori silk, photos of different preparation stages for both face-down and face-up; as well as electronic and electromechanical properties of (BEDT-TTF)2I3-covered paper and set-up of humidity experiments and obtained data, and two movies demonstrating silk/β-(BEDT-TTF)2I3 electric current-driven actuator and deformation sensitivity test using a flexible pipette. See DOI: 10.1039/c4mh00074a |
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