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Visualizing local morphology and conductivity switching in interface-assembled nanoporous C thin films 60

Jean-Nicolas Tisserant, Tino Wagner, Patrick A. Reissner, Hannes Beyer, Yuriy Fedoryshyn, and Andreas Stemmer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06682 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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ACS Applied Materials & Interfaces

Visualizing

local

morphology

and

conductivity

switching in interface-assembled nanoporous C60 thin films Jean-Nicolas Tisserant,†♮* Tino Wagner,†♮ Patrick A. Reissner,† Hannes Beyer,† Yuriy Fedoryshyn, ‡ Andreas Stemmer†*

♮

These two authors share equal contribution

†

ETH Zürich, Nanotechnology Group, Säumerstrasse 4, CH-8803 Rüschlikon,

Switzerland ‡

ETH Zürich, Institute of Electromagnetic Fields, Gloriastrasse 35, CH-8092 Zurich,

Switzerland Corresponding Authors: *[email protected]. * [email protected] Keywords: Fullerene, nanostructure, self-assembly, porous material, resistive switching

Abstract

Carbon materials promise a revolution in optoelectronics, medical applications, and sensing provided that their morphology can be controlled down to the nanometer scale. Nanoporous materials are particularly appealing as they offer a drastically enlarged interfacial area compared to the corresponding planar materials. Entire fields such as organic solar cells, catalysis or sensing may profit from an enlarged interface and facilitated molecular interaction between a carbon material and the environment. Nanoporous fullerene thin films obtained by the deposition of suspended nano-clusters of fullerene were already reported but suffered from the limitation of the size of these particles to over 100 nm. We study here a complementary method based on interfacial self-assembly forcing C60 clusters to spontaneously form 2D

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percolating monolayers with most morphological features in the 5-20 nm range. Analysis of these films by means of electron microscopy and scanning probe microscopy proved their morphology to be a nano-composite of crystalline beads embedded in an amorphous matrix of fullerenes. When contacted between two gold electrodes, these films show an intrinsic conductivity switching behavior. Their electrical conductivity could be reversibly switched on by applying a threshold electrical current and switched off by exposure to oxygen. Interestingly, the on-state exhibits an astonishing conductivity of over 10-3 S/m. Kelvin probe force microscopy (KFM) was used to observe local changes in the distribution of electrical potential upon switching, on the relevant length scale of a few nanometers.

Introduction

Controlling the morphology of organic semiconductors (OSC) in general, and fullerenes in particular, is vital for their use in optoelectronic devices because key phenomena in light and charge management depend on size of interfaces as well as molecular order. Nanoporous films intrinsically offer larger interfacial area compared to bulk materials and could already find use for example in anti-reflection coatings,1 electrode material,2 or as active layers for organic solar cells.3–6 To a large extent the necessary fine-tuning of the fullerene morphology remains inaccessible to top-down approaches. Hence researchers have concentrated on bottom-up assembly methods instead. Fullerenes and their derivatives have been self-assembled in a wealth of building blocks, such as block copolymer assemblies,7 nanotubes,8 lamellae,9 monolayers,10 vesicles,11 single crystals,12,13 or nanoparticles.3,14 An appealing method to obtain nanoporous films would be by self-assembly of fullerene nanoparticles.3 The bottleneck to the direct use of fullerene nanoparticles to form nanoporous films is the reproducible production of monodisperse nanoparticles with diameters below 100 nm.3,6,15 We have developed a complementary approach based on the spontaneous formation of nanoporous films of fullerenes by interfacial self-assembly where particles of fullerene with a diameter below 100 nm aggregate to form


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percolating 2D superstructures.6 Here we observe in situ the growth of such 2D thin films of fullerenes on different interfacial geometries and present conditions to obtain mainly morphological features in the range between 5 and 20 nm. The morphology of the films, forming 2D percolating networks of crystalline C60 nanoparticles linked together by an amorphous matrix of C60 is explored using scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), transmission electron microscopy (TEM), and scanning probe microscopy (SPM). The formation of such composite semi-crystalline thin films within seconds on a liquid/air interface opens a whole new field towards their use in optoelectronics,3,4,6 and sensing.16 For example, nanoporous thin films of fullerenes could find application as organic field-effect transistors17 (OFET) for gas sensing.18 Because charge transport occurs mainly in the first few monolayers of molecules near the gate dielectric,19 the sensitivity of devices is often limited by the diffusion of the analyte through the film. Measures to tackle this issue comprise reducing the thickness of the active layer,20 or using nanoporous films that allow a better transport of the analyte to the active area of the device.21 The first step towards using such nanoporous fullerene films in sensors is to understand their electronic properties. We measure here their current-voltage (I-V) characteristics and show KFM scans22 of films contacted between two pre-patterned gold electrodes. C60 films are particularly sensitive to the molecular intercalation of a guest molecule such as oxygen as shown by spectroscopic studies.23 This alters their conductivity upon exposure to the guest molecule,24,25 or upon annealing to remove this guest.25–27 Owing to their porous nanostructure, our films show a reversible conductivity switching behavior without applying external heat. The devices show an on/off ratio of over three orders of magnitude upon exposure to oxygen, and in the on-state exhibit conductivity values of 3.1x10-3 S/m, comparable to highly crystalline thin films measured at high temperature,27 and better than the highest values measured for C60 single crystals in literature. 26,28,29



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Results & discussion When a drop of chloroform saturated with C60 is deposited on a convex water/air interface, a thin nanoporous layer forms,6 that is visible by eye. In a first set of experiments, we show the macroscopic film growth behavior before linking it to the film nanostructure. Figure 1 a) shows optical recordings of the sequential growth of a C60 film taken after deposition of each drop of solution on an 8 cm2 interface. Figure 1 b) shows a linear growth behavior for four reactors having different lengths (4, 3, and 2 cm respectively). Full videos of the film growth from which the data in Figure 1 was extracted are available in the Supporting Information. For all reactor sizes, we observed that directly after deposition of each drop of solution, the floating film is compacted at the opposite side of the reactor,30 then relaxes over the whole interface. This suggests that the cohesion forces in the film are sufficiently loose31 to allow aggregates of C60 nanoparticles to diffuse at the water/air interface. These free-floating sheets32 serve as building blocks for the consecutive growth of the film. Because the C60 solution is saturated,33 the previously formed sheets do not dissolve at each consecutive growth step and the amount of free interface diminishes proportionally to the number of steps. The sheets formed by the addition of consecutive drops of solution merge upon compaction of the film due to hydrophobic interaction34,35 and capillary forces.36,37 The influence of both the reactor geometry and surface area on the film morphology are shown in Figure S1, Supporting Information. It was found that larger reactors allow reaching smaller features for a fixed volume of solution and rectangular reactors allow reaching smaller features compared to circular ones.


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Figure 1. Sequential growth of C60 thin films on an 8 cm2 water/air interface. (a) Photographs of sequential deposition of 8 µL drops on an 8 cm2 interface, the brown surface serves as a guide to the eye for the C60 film, image taken at an imaging angle of ca. 45°, the drops were added from the top side of the image; (b) evolution of the surface coverage as a function of the volume added on a water interface of 2 cm2 (grey), 4 cm2 (green), 6 cm2 (blue), 8 cm2 (red), error bars indicate standard deviation for triplicate measurements.

The two extreme morphologies depending on the growth conditions are illustrated with SEM images in Figure 2 a) and b), for a 2 cm2 and an 8 cm2 rectangular geometry, respectively. An SPM image of a film with most morphological features between 5 and 20 nm is shown in Figure 2 c). The height histogram of this film is shown as an inset in Figure 2 c). The height of connected threads is smaller than 10 nm as shown in Figure 2 d). The question of film homogeneity across areas of cm2 is discussed in detail in S2, Supporting Information. Further characterization of these morphologies was carried out by high-resolution electron microscopy. The semi-crystalline nature of these nanoassemblies can be observed in Figure 2 e) where crystalline grains (bottom and top) are linked by a 4-nm-wide amorphous thread of C60. A crystalline grain



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where the packing of C60 molecules can be observed is shown in Figure 2 f). An electron diffraction pattern obtained on a 100 x 100 nm2 semi-crystalline surface is shown in the inset of Figure 2 f). The two rings observed in this image are the signature of the amorphous C60 matrix while the diffraction spots correspond to crystalline order. The random orientation of C60 crystallites with respect to the substrate is shown in Figure S3, Supporting Information. While particles in the 50-nm range and above show a high degree of crystallinity, assemblies of smaller dimensions tend to become more and more amorphous (see Figure S3, Supporting Information). This phenomenon can be explained by a competition between particle nucleation-growth responsible for the formation of crystalline nanoparticles, and the final dewetting of the solvent, that results in the formation of holes in an amorphous matrix,38 and amorphous threads linking the nanoparticles.39 It cannot be excluded that nanoparticles of C60 pre-exist in the chloroform solution,40 and participate as building blocks to self-assembly. However, UV-Vis. characterization of the solution rather suggests well solvated C60 molecules (see Figure S4, Supporting Information) and our imaging methods reveal that if any, the particles present in the solution have diameters below 10 nm. Forming similar films from solvents in which C60 has a higher solubility yielded highly crystalline films with particles in the range of hundreds of nanometers to micrometers (Figure S4, Supporting Information), suggesting that the formation of particles occurs mainly during solvent evaporation.


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Figure 2. Nanoporous films of C60 having most morphological features in the 520 nm range. (a) Morphology obtained by depositing 8 µL of C60 solution CHCl3 on a 2.0 cm2 rectangular interface, similar to published results;6 (b) morphology obtained by depositing 8.0 µL of C60 in CHCl3 on a rectangular interface of 8.0 cm2; (c) SPM image of a film similar to (b), with corresponding height histogram as inset; (d) SPM zoom showing a film thickness below 10 nm; (e) TEM image showing crystalline grains bound by a 4 nm wide amorphous thread, inset: zoom on a crystalline grain, scale represents 20 nm, red arrows point to 3 crystallites with 3 different orientations, the central one is magnified in the inset; (f) high resolution TEM image of a crystalline C60 domain; inset: TEM diffraction pattern measured on 100 x 100 nm2 surface, scale represents 2.5 nm-1.

Such nanoporous morphologies appear to be ideal to measure a modulation of current upon exposure to guest molecules,18,21 first because of the large amount of interface available between the free-standing grains of fullerene and second because the porous films could facilitate access of the guest analyte



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to the active layer of a sensor.18 A first step towards such applications is to understand the electronic behavior of these films. To this end, nanoporous films of C60 with most morphological features below 20 nm were transferred onto 50 nm thick gold electrodes separated by a gap of 200 nm over a total device length of 40 μm. The pristine film of C60 is not conducting, even in high vacuum (< 5x107

mbar) as shown in Figure 3 (a). The presence of trapped oxygen limits the

mobility of charge carriers in C60.25,26,29 In a previous report thermal annealing removed trapped oxygen and rendered the films more conductive by over five orders of magnitudes.25 Figure 3 (a) shows a similar switching-on behavior for a nanoporous film exposed to a bidirectional sweep with a 5 μA current compliance and a rate of 1 V/s. A clear hysteretic behavior was observed during this sweep (Figure 3 (b)). Instead of applying external heating the confined nanostructure described here allows driving sufficient currents through the device to remove the oxygen trapped inside, causing a switch to higher device conductivity. Both before and after switching-on, the device exhibits symmetric ambipolar41 non-linear I-V characteristics due to Schottky barriers at the interfaces with the gold electrodes.26 The device was switched to lower conductivity again by venting the chamber with air, the I-V curves as a function of time recorded upon switching are shown in Figure S5, Supporting Information. For control, the chamber was flooded with nitrogen and no change of conductivity was measured (Figure S5, Supporting Information). Devices were imaged in the off-state and in the on-state under 5 V bias using KFM. With this technique, we could probe locally the changes in surface potential upon switching. Figure 3 (c) shows representative profiles of two paths through the film. The local potential gradients along these two paths are shown in Figure S6, Supporting Information. Potential maps are shown for the off-state, the on-state, and as a potential difference (Uon-Uoff) in Figure 3 (d), (e), and (f), respectively. It is worth noting that most changes in the conductivity are not located at the gold/C60 interface, but rather in the middle of the gap. The global increase in conductance suggests that on average the internal resistances have decreased upon switching. The maps and profiles of surface potential, however, point to local changes of the resistance distribution as not all junctions along the individual paths are affected in the same manner by the switching current. After


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switching, C60 branches have a lower resistance to either the negatively-biased electrode (profile 1), or the positively-biased electrode (profile 2).

Figure 3. Electronic characterization of nanoporous thin films of fullerenes. (a) IV characteristics of a 200 nm wide, 40 μm long device; red and inset: off-state measured in vacuum, black: on-state obtained after applying the voltage sweep shown in (b); (b) switching-on under 60 V, 5 μA compliance; (c) evolution of the potential per C60 particle for two characteristic current paths, arrows show the evolution upon switching; (d) KFM image of the off-state; (e) KFM image of the same area in the on-state; (f) map of the difference between on-state and offstate.

In the following we compare our locally measured conductivities with published values. Taking for the calculation a channel width of 200 nm, a device length of 50 μm, a film thickness of 10 nm, a contact area of α=35 %, and a resistance value fitted in the linear region of the I-V curve between -0.4 V to +0.4 V (see Figure S7, Supporting Information), we obtain for the off and on conductivities σoff=3.6x10-6 S/m and σon=3.1x10-3 S/m, respectively. The apparent conductivity of C60 films between metallic electrodes may be problematic to interpret due to n-doping of C60 by the metal,42 making a quantitative comparison between different systems difficult. However, the value for the film in the off-state is below what is reported for undoped C60 films at room temperature,43 as expected from the presence of oxygen in the film. In contrast, the value for the device in the on-state is superior to what has been



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measured on C60 single crystals, typically 99.8%) were added, the mixture was sonicated for 30 minutes at room temperature, left to settle overnight and centrifuged in a glass vial for 20 min at 13000 rpm (18900 g). C60 films were formed by adding saturated C60 solution on the convex water meniscus formed upon contact with poly(tetrafluoroethylene) (PTFE) containers having either a rectangular geometry (2 cm wide, of varying length) or a circular geometry. For sequential growth experiments, drops of 8 µL of chloroform solution were added after waiting 30 s between each drop. For KFM characterization, in order to obtain homogeneous films over several hundred µm, 50 µL of solution were deposited at once on a 6.1 cm2 circular interface and the film was left to dry in air for 5 min before transfer to substrate. 20-nm-thick bulk C60 films were fabricated by thermal evaporation at a pressure of 10-6 mbar. Transfer to substrate. The films were transferred to Si/SiO2 or on carbon-coated TEM grids by dipping the substrate vertically in the beaker and retracting it through the floating film. Films that did not cover the whole interface were mechanically compressed to cover 2-4 cm2 of plain film before transfer. 50-nm thick gold electrodes were patterned using electron beam lithography on a Si/SiO2 wafer with a 10-µm thick oxide layer.


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Characterization. In-situ records of film formation were obtained using a Fujifilm HS50 EXR camera focused on the water/air interface. Images were binarized and the contrast was enhanced using ImageJ. Scanning probe microscopy was performed on an Asylum Research Cypher with Olympus AC160TS-R3 cantilevers. Images were levelled and flattened using Gwyddion.46 SEM was performed on a Hitachi SU8000 at a typical voltage of 1.5 kV. TEM and STEM were performed on an FEI Talos microscope operated at a voltage of 200 kV. KFM was performed at room temperature in a home-built high vacuum AFM operating at pressures below 5x10-7 mbar. Topography was obtained in net-attractive mode from the frequency shift (∆f = -30 Hz, A = 10 nm, scan speed 125 nm/s). For frequency-modulated KFM, the dc bias and ac modulation (3 V amplitude at 4 kHz) were applied to tip, and the surface potential was found by a custom feedback loop and direct sideband demodulation.47 A scan pass at 5 nm lift height was added to fully eliminate crosstalk due to topography. Scans were aligned and corrected for thermally-induced drift by detecting common features in the topography using the SIFT48 feature detector in OpenCV, and by shearing and shifting the scan lines. In Figure 3, for better comparability, the surface potential was superimposed onto the topography, and gaps between C60 nanoparticles were shaded in gray. Similarly, regions not covered by C60 nanoparticles were masked black in Figure 4 c-e). Electronic characterization was performed using a Keysight B2912A precision source measure unit. Acknowledgements

The authors acknowledge the support of the BRNC cleanroom staff for SEM, the support of the ScopeM scientific centre for optical and electron microscopy of ETH Zürich for TEM and STEM measurements, Blerim Veselaj (ETH) for fabricating the PTFE containers, and Nassir Mojarad (ETH) for fruitful discussions.

Supporting Information Video recordings of film growth for a rectangular and a circular reactor, UV-Vis. spectrum and photograph of C60 in chloroform, SPM and SEM images of films obtained from toluene and dichloromethane, matrix of SEM images showing the evolution of the morphology as a function of interface geometry, SEM and SPM



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images showing the homogeneity of the films over large surfaces, TEM images of amorphous and crystalline domains, I-V characteristics of the devices upon exposure to oxygen and upon consecutive switching events, local potential gradients extracted from KFM measurements, and the I-V characteristics of devices in the on and offstate in the linear regime between -0.4 and 0.4 V, KFM images taken upon successive switching experiments, I-V characteristics and behavior of bulk C60 films upon oxygen exposure. References (1)

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