Precisely Controlled Two-Dimensional Rhombic Copolymer Micelles for Sensitive Flexible Tunneling Devices

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2021Precisely Controlled Two-Dimensional Rhombic Copolymer Micelles for Sensitive Flexible Tunneling Devices Liang Han†, Hua Fan†, Yulin Zhu, Meijing Wang, Feng Pan, Dapeng Yu, Yue Zhao and Feng He Liang Han† Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 †L. Han and H. Fan contributed equally to this work.Google Scholar More articles by this author , Hua Fan† Department of Physics, Institute for Quantum Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055 †L. Han and H. Fan contributed equally to this work.Google Scholar More articles by this author , Yulin Zhu Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 Google Scholar More articles by this author , Meijing Wang Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 Google Scholar More articles by this author , Feng Pan School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055 Google Scholar More articles by this author , Dapeng Yu Department of Physics, Institute for Quantum Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 Google Scholar More articles by this author , Yue Zhao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Physics, Institute for Quantum Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 Google Scholar More articles by this author and Feng He *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055. Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000297 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Nanoscale two-dimensional (2D) organic materials have attracted significant interest on account of their unique properties, which result from their ultrathin and flat morphology. Supramolecular 2D nanomaterials prepared by bottom-up approaches have great potential in the creation of nanoscale devices with various applications, each controlled by the properties of its single-molecular components. Here, we report a uniform and controllable 2D rhombic micelles formed by block copolymers (BCPs) with poly(p-phenylenevinylene) (PPV) as core blocks. The supramolecular 2D nanostructures created in this way and driven by π–π interactions realize the precise separation of the semiconducting and insulating constituents of the constructed BCPs. With a vertical tunneling device design, the 2D rhombic micelles exhibited an on–off current ratio of >104 and a high on-state current density of 6000 A cm−2 as the insulating layer was compressed by a conductive atomic force microscopy (C-AFM) probe. The tunneling device also shows a reproducible sensitivity on a flexible substrate, opening up potential flexible pressure sensor applications for such novel supramolecular 2D nanostructures. Download figure Download PowerPoint Introduction Owing to their unique properties, nanoscale organic two-dimensional (2D) materials are used for a variety of applications in nanomedicine, biology, and semiconductor industry.1 Composed of organic and polymeric molecules, these ultrathin and flat 2D materials exhibit fascinating physical and chemical properties, including distinct mechanical stiffness,2 electronic conductivity,3 transparency,4 and flexibility.5 They are considered to be promising candidates for the nanoscale applications of biosensors,6 electronic,7,8 optoelectronic,9 and electromechanical10 devices. Due to their nonordered nature, most of these organic 2D materials display properties, which are aggregates of the individual properties of their mixed components, and this substantially limits their application. A special arrangement is crucial in nanoscales so that complex structures with distinct properties could be generated for different application purposes. Recently, bottom-up self-assembly of block copolymers (BCPs) has been established and developed as a means of preparing 2D polymer materials with crystallization-driven self-assembly (CDSA) and other approaches involving intermolecular interactions.11–13 This strategy could be used to fabricate high-quality 2D polymer materials by a simple process with controllable size, shape, and chemical functionality on a nanoscale.14–18 Compared with the bottom-up preparation of 2D inorganic materials, this requires a lower cost of energy and fewer materials. Distinct molecular compositions offer opportunities to combine various properties in one nanoarchitecture, thus realizing tunable mechanical,19 optical,20 and electrical21 properties. However, it is not clearly understood how these molecules assemble spontaneously into 2D morphology and how the copolymer molecules are packed in a 2D architecture. Therefore, it is significant to produce fine-controlled 2D polymer micelles from a molecular construction perspective and to tune their morphologies by crystallographic modifications further. As semiconducting materials, conjugated polymers have been investigated in various fields of photovoltaics,22 field-effect transistors,23 light-emitting devices,24 and sensors.25 They are effective core-forming blocks with strong π–π interactions, which could form one-dimensional (1D) or 2D nanomaterials,26–28 where they pack with each other in face-to-face or herringbone arrangement much like small-molecule crystals. The 1D fibers with high charge carrier mobility, measured by treating the linear aggregates that are intertwined with each other as a whole, have been obtained from π-conjugated BCPs.29 However, the performance of the individual polymer assembly nanostructures, either 1D or 2D, has not been explored thoroughly. Herein, we report the controlled preparation of colloidal stable rhombic 2D micelles using BCPs based on π-conjugated poly(p-phenylenevinylene) (PPV) blocks with a bottom-up self-assembly approach. These micelles are changed from square shape28 driven from well-defined orthorhombic forces to the monoclinic phase by controlling the molecular design to match a specific arrangement. The 2D micelles formed in this way realize the phase separation of the semiconducting layer and the insulating layer, and the thickness of the latter could be tuned. Taking advantage of a vertical tunneling device design, the 2D rhombic micelles exhibited an on–off current ratio of >104 and a high on-state tunneling current density up to 6000 A cm−2 when the insulating layer was compressed locally by a conductive atomic force microscopy (C-AFM) probe. Tunnel devices made in this way also show a reproducible sensitivity on a flexible substrate. This bottom-up approach to the preparation of 2D materials for multilayer and multifilm devices would be potentially useful in the fields of electronics, optoelectronics, and nanosensors. Experimental Methods Details of the synthesis, characterization, self-assembly, transmission electron microscopy (TEM), atomic force microscopy (AFM), helium ion microscope (HIM), fluorescence, laser scanning confocal microscopy (LSCM), UV–Vis absorption, dynamic light scattering (DLS), differential scanning calorimetry (DSC), cryo-TEM, electrochemical measurements, grazing-incidence wide-angle X-ray scattering (GIWAXS), vertical transport characterization, and flexible bending tunnel device involved in this research, could be found in the Supporting Information. Results and Discussion Design and controllable preparation of the 2D rhombic micelles Generally, π–π interactions are weaker than inorganic crystallization as a driving force in the bottom-up construction of 2D nanomaterials. With the exception of some crystalline homopolymers, polymeric building blocks usable for self-assembly are usually designed as amphiphilic diblock copolymers (diBCPs) with different polarities. This ensures that the molecules are driven together spontaneously. Herein, conjugated PPV polymers, which are solvophobic and have low polarity, were chosen as the solvophobic core-forming block. Combined with solvophilic poly(2-vinylpyridine) (P2VP), the amphiphilic diBCP targets were constructed, as shown in Figure 1a. These sheet-coil type BCPs with a fully extended conformation could make contact adequately with each other to form ordered architectures with spectacular regular shapes. Figure 1 | Design, preparation, and formation of the 2D rhombic micelles built from the diBCPs. (a) The schematic representation of the formation of the 2D rhombic micelles driven by solvophobic and π–π interactions. (b) AFM images, (c) TEM images, and (d) HIM images of the 2D rhombic micelles grown in 0.05 mg mL−1 isopropanol (i-PrOH). Solution of PP-PPV7-b-P2VP29. Download figure Download PowerPoint As we reported previously,28, the herringbone arrangement of the PPV blocks attached by ethylhexyloxy (EH-PPV) induces a square-shaped 2D architectures. In this study, i-propylpentyloxy (PP) chains were attached to the PPV blocks. The PP chain leads to a more pronounced steric hindrance, thus influencing the stacking of the PPV blocks and modifying the molecular assembly. By Siegrist polycondensation30 and quenching the anionic polymerization,31 two kinds of BCPs, PP-PPVn-b-P2VPm were synthesized, with block ratios of 1∶4 (n = 7, m = 29) and 1∶6 (n = 7, m = 40) ( Supporting Information Scheme S1, Table S1, and Figures S1–S6), respectively. The amphiphilic BCPs, based on PP-PPV that were obtained in this manner, were assembled by typically dissolving–cooling–aging approaches. The PP-PPV7-b-P2VP29 was dissolved in i-propanol by heating for 1 h at 90 °C; subsequently, the solutions were cooled down slowly to 25 °C, followed by aging for 24 h. The uniform 2D rhombic platelets of PP-PPV7-b-P2VP29 appeared in colloidally stable solutions (Figure 1). The 2D rhombic structures of PP-PPVn-b-P2VPm prepared in this way showed an apparent concentration dependence. The longer diagonal length (Dl) of the rhombic platelet was used as the benchmark of the scale statistics. In different concentrations of i-propanol solutions, PP-PPV7-b-P2VP29 could assemble into regular and uniform rhombic platelets (Figure 2 and Supporting Information Figures S7 and S8). The number-average diagonal length (Dln, the subscripts refer to the number-average length of the longer diagonal) of the rhombic platelets in the solutions of 0.1, 0.05, and 0.02 mg mL−1 had good dispersity, calculated as 3429, 2619, and 1361 nm, respectively ( Supporting Information Table S2). The scale of the formed 2D rhombic micelles decreased with the concentration of the solutions. In addition to the solution concentration, the block ratio of PP-PPV/P2VP was also a factor controlling the scales of the prepared 2D rhombic micelles. When the P2VP block was longer, the scales of the formed 2D micelles decreased. For example, the Dln values of the formed rhombic platelets constructed by PP-PPV7-b-P2VP40, which also exhibited concentration dependence, were significantly smaller than those formed by PP-PPV7-b-P2VP29 in solution at the same concentrations ( Supporting Information Figures S9 and S10). This arose because the longer P2VP blocks were more likely to cover the solvophobic edges of the formed 2D micelles, which otherwise could inhibit the growth of the blocks. Similar differences in the dimensions of 2D rhombic micelles caused by concentrations and block ratios were also observed by AFM (Figures 2d–2f and Supporting Information Figure S11). From the AFM results, it was found that the height of the 2D rhombic micelles increased in proportion to the length of the P2VP blocks, leading to the height of the 2D PP-PPV7-b-P2VP40 being ∼2 nm larger than that of PP-PPV7-b-P2VP29. Another noteworthy phenomenon was that the shape of the formed 2D rhombic micelles was not affected by the growth conditions. Independent of the block ratio of PP-PPVn-b-P2VPm and the concentration of growth solution, the sharp acute angle of the rhombic was always between 75° and 80°, and the average acute angle (θn) was essentially unchanged ( Supporting Information Figure S12 and Table S3). Figure 2 | Morphological characterizations and concentration dependence of the 2D rhombic micelles. (a–c) TEM images, (d–f) AFM images in height mode, (g–i) LSCM photos of 2D rhombic micelles grown from PP-PPV7-b-P2VP29 in i-propanol solutions at different concentrations: (a, d, and g) 0.1 mg mL−1, (b, e, h) 0.05 mg mL−1, and (c, f, and i) 0.02 mg mL−1. Download figure Download PowerPoint In the presence of the π-conjugated PPV blocks, the 2D rhombic platelets of PP-PPVn-b-P2VPm displayed strong fluorescence in colloidally stable solutions ( Supporting Information Figure S13). It could be deduced from the change in the photoluminescence (PL) spectra that the process of self-assembly of BCPs was initiated and maintained on cooling the solution. These phenomena, whereby the fluorescence intensity increased and the fluorescence peak narrowed, could be attributable to the ordered aggregation of the BCP molecules in solution, which resulted in a reduced nonradiative transition and the restricted relaxation effect of the excited states due to the limited motion and blocked changes in the configuration of the molecules.32,33 With the aid of their fluorescent properties, the platelet 2D rhombic micelles could be observed directly in the colloidal solutions in situ by LSCM (Figures 2g–2i and Supporting Information Video S1), which indicated that the formation of the 2D rhombic micelles occurs in solution rather than during the evaporation of the solvent. Assembly process and molecular arrangement of the 2D rhombic nanosheets Various optical techniques were used to monitor the assembly of the 2D nanosheets. As the temperature of the solution of PP-PPV7-b-P2VP29 was reduced from 90 to 30 °C, the intensity of the absorption increased continuously with a simultaneous redshift of 12 nm (Figure 3a). These changes indicated that due to the driving force of the solvophobic interactions of the conjugated core and the side alkyl chains, the copolymer molecules conglomerated spontaneously in the solution during the cooling process. When the solution was cooled down to room temperature (RT) and aged for 48 h, a blueshift of the absorption peak at 16 nm was observed (Figure 3b), characteristic of the formation of H-aggregates of the PPV blocks. The UV–vis spectra of PP-PPV7-b-P2VP40 exhibited a similar tendency ( Supporting Information Figures S14a and S14b). When the growth of the 2D rhombic micelles was monitored by DLS during the aging time, it was found that the apparent hydrodynamic diameter (Dh) of the 2D micelles, obtained by regarding the 2D micelles as sphere particles,34 increased after aging for 24 h (Figure 3c and Supporting Information Figure S14c). The process of PP-PPVn-b-P2VPm aggregation was longer, and the absorption blueshift originating from the H-aggregation of PP-PPV blocks was smaller than those observed in the assembly process of platelet 2D square micelles of EH-PPVn-b-P2VPm.28 This was attributed to the higher solvophobic interactions of PP-PPV blocks and the weaker π–π intermolecular driven forces caused by the increased steric hindrance effects of the attached PP alkyl chains. DSC measurements were also carried out to characterize the change of the molecular states ( Supporting Information Figure S15). From the DSC results, PP-PPV7-CHO showed a glass transition temperature (Tg) at 77 °C and crystalline transition temperature (Tc) at 133 °C. Compared with PP-PPV7-CHO, the BCPs of PP-PPVn-b-P2VPm showed the same Tg (77 °C) while Tc disappeared. This indicated that the crystallinity of PP-PPVn-b-P2VPm decreased, and the formation of the micelles seemed to have suffered a phase transition, dominated by PP-PPV7 blocks. During the self-assembly process, the aggregation of the BCPs molecules should occur after the solution temperature reached below 70 °C, which was consistent with the observation of the transformation trend of PL and UV–Vis spectra with changes in time and temperature. Figure 3 | The characterization of the assembly process and the molecular arrangement of the 2D rhombic micelles. (a and b) The varying UV–vis absorption spectra of PP-PPV7-b-P2VP29 in 0.05 mg mL−1i-PrOH solutions, (a) the sample solutions were heated at 90 °C for 30 min, then naturally cooled down to RT followed by (b) aging for a further 48 h. (c) Changes of Dh of 2D micelles of PP-PPV7-b-P2VP29 in 0.05 mg mL−1i-PrOH solutions during the aging times. (d) The photographs (inset) and the SAED pattern of 2D rhombic micelles formed in 0.1 mg mL−1 solution in i-PrOH of PP-PPV7-b-P2VP29 taken by cryo-TEM. (e) 2D GIWAXS patterns and (f) GIWAXS profiles along the in-plane direction of platelet 2D rhombic micelles obtained from PP-PPV7-b-P2VP29 in 0.1 mg ml−1i-PrOH solution. Download figure Download PowerPoint The stronger steric hindrance effects of PP alkyl chains also influenced the molecular packing modes, leading to a change in the morphologies of the 2D platelet assemblies from a square to a rhombus. With the aid of the cryo-TEM, at ultralow temperature, the results of the selected area electron diffraction (SAED) on the 2D rhombic platelets were simply a halation pattern (Figure 3d), and only the irregular surface morphology could be observed, with no clear crystal lattice in the high-resolution TEM photographs ( Supporting Information Figure S16). The d spacing calculated from the inner amorphous halo in reciprocal space is 2.77 Å. The GIWAXS results from the 2D rhombic micelles revealed internal molecular arrangements. The 2D rhombic micelles originated from PP-PPVn-b-P2VPm exhibit nearly the same GIWAXS patterns, and there were no periodic reflection peaks observed along the qz axes (Figure 3e and Supporting Information Figure S17). This indicated that the BCP molecules favored an end-on configuration in the 2D rhombic micelles.35 The diffraction peaks along the in-plane direction were indexed (Figure 3f), and the molecular arrangement was found to be monoclinic. The packing distances were calculated as 9.31 and 11.17 Å, corresponding to reflection (100) peak q = 0.676 Å−1 and (020) peak q = 1.125 Å−1, respectively, with an included angle between the two packing directions of 104° ( Supporting Information Table S4 and Figure S18). The d spacing calculated from the reflection (040) peak corresponded well with the result from the inner halo of the SAED pattern. As mentioned earlier, the PP alkyl chains show a stronger hindrance effect than the EH alkyl chains because of subtle but frustrating effects of its larger branched side chains.26–28,36,37 Therefore, PP-PPV exhibited a higher solvophobic effect than EH-PPV blocks; hence, PP-PPVn-b-P2VPm showed a longer aggregating time and looser packing, inferred from the changing UV–Vis absorption spectra. Although both of the molecules of EH-PPVn-b-P2VPm and PP-PPVn-b-P2VPm favored an end-on orientation and herringbone stacking, the steric hindrance from the side alkyl chains changed the packing mode. The dihedral angle between the adjacent PP-PPV conjugated blocks in herringbone molecular arrangement and the π–π stacking distance of the neighboring BCP molecules increased over those of EH-PPVn-b-P2VPm. These molecular packing changes induced the orthogonal molecular arrangements in 2D square platelet micelles of EH-PPVn-b-P2VPm to change to monoclinic arrays of EH-PPVn-b-P2VPm, and this drove the formation of the rhomboid shape ( Supporting Information Figure S19). The morphological transformation of 2D micelles of conjugated polymers was caused by crystallographic modifications and tuning of the intermolecular π–π interactions. High current densities performance of 2D rhombic platelet materials and preparation of a flexible pressure sensor nanodevice To understand the electrical properties of the ordered rhombic micelles, we used C-AFM to measure the topography and microscopic current distribution simultaneously over the sample surface. As shown in Figure 4b, the measurements were based on an integrated scanning probe microscope, with a thin, flexible mica sheet covered with Cr (5 nm)/Au (50 nm), and served as a conductive substrate and conductive Pt/Ir-coated silicon tips acting as the top electrode, as well as forming Au/polymer/Pt junctions. A direct current (DC) voltage was applied through the bottom electrode while the tip was grounded. Within the sensitivity of C-AFM measurement, the resistance of PP-PPV7-CHO (<50 kΩ) was as low as that of gold ( Supporting Information Table S5 and Figure S20a). In contrast, the amorphous aggregating states of PP-PPV7-b-P2VP29 spinning coated on the gold surface, which could be considered to be semiconducting PPV films doped with insulating P2VP blocks, were shown as a visible slope in the I–V curve, indicating a much larger resistance (∼2 MΩ) ( Supporting Information Figure S20b). As proposed earlier, the insulating P2VP blocks were wound on the top and bottom surfaces of the platelet 2D rhombic micelles while the semiconducting PPV blocks were sandwiched in between. The backbone of the semiconducting PPV block is rigid and ∼4 nm long according to the published calculated structure.28 Figure 4d shows the vertical conductivity measurements of the 2D rhombic micelles with different lengths of the P2VP blocks. The 2D micelles of PP-PPV7-b-P2VP14 with the thinnest insulating layer (∼1 nm) displayed remarkable conductivity similar to that of PP-PPV7-CHO, indicating a conducting vertical charge transport, potentially due to the interlayer dipole-induced bandshift near the metal–organic semiconductor interface and intermolecular hopping along the conjugated chains ( Supporting Information Figures S21a and S22a). As the thickness of the insulating layer increased, weaker current signals were detected through the 2D rhombic micelles of PP-PPV7-b-P2VP29 with a tunneling resistance of dozens of MΩ. For the longest tested insulating chains, the vertical conductivity of the 2D rhombic micelles of PP-PPV7-b-P2VP40 dropped almost to zero, indicating a complete termination of the tunneling effect due to the thick insulating separation ( Supporting Information Figures S21c and S22b). However, the uneven distribution of the P2VP chains on the surface of the 2D micelles resulted in a slight difference in the tunneling current values ( Supporting Information Figure S21b), the general decrease of the vertical conductivity as the P2VP chains increase is consistent with the proposed structure in which semiconducting PPV is sandwiched in the insulating P2VP. Figure 4 | Adjustable vertical conductivities of the 2D rhombic micelles of PP-PPV7-b-P2VPn assessed by C-AFM measurements on the gold surface. (a) AFM images of 2D rhombic micelles of PP-PPV7-b-P2VPn with different n numbers. The dots indicate the positions where the C-AFM measurements were taken in (d). The rhombic micelles of PP-PPV7-b-P2VP29 and PP-PPV7-b-P2VP40 were formed in the 0.05 mg mL−1i-PrOH solution, while those of PP-PPV7-b-P2VP14 were formed in the 0.01 mg mL−1i-PrOH solution. (b) The schematic structure of the C-AFM measurement, (c) height profiles for the corresponding lines drawn in AFM images, and (d) the current signals as a function of the bias voltage of the rhombic micelles of PP-PPV7-b-P2VPn: n = 14 (black), n = 29 (red), and n = 40 (blue). Download figure Download PowerPoint Since the P2VP chains are soft and could be compressed, it should be expected that the vertical tunneling could be affected greatly when the thickness of the insulating layer is changed by external pressure, which enables a tunneling-based pressure sensor application, even in a flexible configuration. The bottom-up assembly nature allows control of the thickness of the insulating barrier so that a working pressure range of such a tunneling sensor could be engineered. We used the 2D rhombic assembled sheets of PP-PPV7-b-P2VP29 as a demonstration prototype. A tunneling-based pressure sensor device was designed and examined by C-AFM. Figure 5a shows a schematic diagram of the band alignment of the Au/PP-PPV7-b-P2VP29/Pt heterojunction. With a metal–organic semiconductor interface, strong band bending and band shift could be expected due to the interfacial dipole moment,38 and this effect could be reduced as the insulating barrier increases its width. P2VP has an energy bandgap of 4.3 eV and a lowest unoccupied molecular orbital (LUMO) energy of −3.35 eV, while PPV has a bandgap of 2.3 eV, as calculated by cyclic voltammetry and optical bandgap ( Supporting Information Table S6 and Figure S23). The work functions of Pt and Au acting as the top and bottom electrodes are 5.6 and 5.1 eV, respectively. Figure 5b shows a set of current–force curves in a semilogarithmic scale for different DC bias voltages. This tunneling device showed significant “on” (current compliance 20 nA) and “off” (<1 pA) states, with a large on/off ratio (>104 and limited by our compliance settings) upon the pressure load. The on-state current density was estimated to be as high as ∼6 × 103 A cm−2, which ensured the sensitivity of the signal for the tiny scales of the nanodevice, under pressures ranging from 300 to 600 MPa by assuming an effective circled contact area with a diameter of 20 nm.39,40 When the density of the vertical PP-PPV chains in the 2D assembly was ∼2 × 1014 cm−2, it was estimated that each single PPV chain managed a current of ∼30 pA, indicating the electrical behavior of individual conjugated PPV chain. As the vertically ordered PPV chains were unchanged when compared with the elastic P2VP layers, we modeled the compression mainly from the insulating layer following Hooke’s law, and the resulting force was proportional to the change of insulating thickness ( Supporting Information “Experimental Methods” Section and Figure S24). The vertical current increased exponentially with the increasing force, suggesting the tunneling following Equation (1): I ∝ V ρ s ( 0 , E F ) e − 2 κ W , (1)where I is the tunneling current, V is the voltage bias applied at the Au electrode, EF is Fermi level of PPV,ρs(0,EF) is the local density of states (LDOS) near the Fermi level of the sample at the sample surface, e is the natural constant, W is the barrier width, and κ = 2 m ( U − E ) / ℏ . Figure 5 | Flexible tunneling-based pressure sensor device constructed by 2D rhombic micelles of PP-PPV7-b-P2VP29. (a) Schematic diagram of the band alignment of the Probe/PP-PPV7-b-P2VP29/Au tunneling junction. (b) Comparisons of current–force characteristics of the tunneling device under different tunneling biases, insets are schematic representations of the tunneling barriers under different bias voltages, which are color-coded and match the current-force curves, and different forces. The insulating barrier width decreases as the applied force increases. (c) Tunneling I–V curves under different applied forces. (d) The fatigue property of the PP-PPV7-b-P2VP29-based devices: the tunneling current at 0.5 V bias voltage recorded under a series of different applied forces. (e) A schematic of the PP-PPV7-b-P2VP29/Au/Cr/Mica tunneling devices on a flexible substrate. A quantitative measure of the bending radius derived from R=(Δh2+Δx2)/2Δh, where Δh and Δx are determined by 3D optical measurements. (f) Current–force characteristics for different bending radii, and the inset shows a photograph of the flexible device. Download figure Download PowerPoint Our tunneling device showed reproducible and symmetrical current–voltage (I–V) curves under a series of applied forces (Figure 5c) for multiple bias sweeps. This superlinear behavior at higher biases (|Vbias| > 0.25 V) attributed to the Fowler–Nordheim (FN) tunneling41 ( I F