Optically switchable organic field-effect transistors based on photoresponsive gold nanoparticles blended with poly(3-hexylthiophene)

Edited by Francisco M. Raymo, University of Miami, Coral Gables, FL, and accepted by the Editorial Board June 22, 2012 (received for review March 6, 2012)
July 16, 2012
109 (31) 12375-12380


Interface tailoring represents a route for integrating complex functions in systems and materials. Although it is ubiquitous in biological systems—e.g., in membranes—synthetic attempts have not yet reached the same level of sophistication. Here, we report on the fabrication of an organic field-effect transistor featuring dual-gate response. Alongside the electric control through the gate electrode, we incorporated photoresponsive nanostructures in the polymeric semiconductor via blending, thereby providing optical switching ability to the device. In particular, we mixed poly(3-hexylthiophene) with gold nanoparticles (AuNP) coated with a chemisorbed azobenzene-based self-assembled monolayer, acting as traps for the charges in the device. The light-induced isomerization between the trans and cis states of the azobenzene molecules coating the AuNP induces a variation of the tunneling barrier, which controls the efficiency of the charge trapping/detrapping process within the semiconducting film. Our approach offers unique solutions to digital commuting between optical and electric signals.
Organic field-effect transistors (OFETs) are basic building blocks for logic applications and for the development of electronic technologies based on soft matter (14). Currently, the greatest challenges in the field are the achievement of higher device performance and the development of devices featuring uncommon and multiple functionalities (5). In this regard, hybrid organic–inorganic materials are gaining much attention because, besides their easy processing, they can take advantage of the tunability of the chemical properties of the components, and thus of the material as a whole. Nanoparticles (NPs) of different materials (gold, silver, metal oxides, etc.) can be coated with self-assembled monolayers (SAMs) of a given molecular system, thereby providing them additional functions such as a specific surface energy or optoelectronic properties (68). These features can be optimized by achieving control over the packing of SAMs on the NPs (7). When NPs are integrated in a device, they can, for example, act as charge storage sites—i.e., trapping charges centers, allowing the system to act as a memory (9, 10).
The incorporation of photochromic molecules into electronic devices to confer them a photoresponsive nature has been recently explored (1114). Among photochromic systems (1518), azobenzene derivatives are known to undergo isomerization from trans to cis form, and vice versa, under illumination at a specific wavelength, as well as from cis to trans with temperature (15). In the last few years, we have extensively investigated the thiol-substituted azo-biphenyl (AZO, Fig. 1A) when chemisorbed on gold surfaces (1922). We demonstrated that such a SAM chemisorbed on the planar source and drain electrodes of an OFET can be used to modulate optically the charge injection at the metal—semiconductor interface because of the different tunneling barrier of the cis and trans SAMs (19). However, the observed light-modulated process in the OFET displayed an in situ photo-induced current change of only 20% and the occurrence of fatigue, with degradation of the device performance upon a few successive switching cycles.
Fig. 1.
Chemical formula of (A) S-[4-[4-(phenylethynyl)phenyl]-ethynyl]benzene-thiol ester (acetyl-OPE) and (B) the azo-biphenyl with an acetyl-protected terminal thiol anchoring group (acetyl-AZO) and its isomerization reaction between the trans and the cis form. Schemes of the differently coated Au nanoparticles: (C) OPE-NP, (D) AZO-NP in its trans and cis form. (E) Scheme of the bottom-contact/bottom-gate field-effect transistor in which S (source), D (drain), and G (gate) are the three terminals and the semiconducting material is a blend of P3HT and the coated Au nanoparticles.
Here, we report on the fabrication and characterization of OFETs based on a blend of poly (3-hexylthiophene) (P3HT) and AuNPs coated with a SAM of a thiol-substituted azo-biphenyl molecule. We show that such a blend features a remarkably higher response to light stimulus as a result of the enhanced active surface, overcoming the occurrence of fatigue and simultaneously achieving an increase in the change in the current passing through the channel of the transistor upon isomerization. Differently from previous works, here we have coated the AuNP with a responsive monolayer to provide them a photo-tunable trapping capacity once blended with P3HT, thereby modulating the number of active charge carriers in the device channel.
As a prototypical p-type polymeric semiconductor we chose P3HT because it combines a good solubility in organic solvents and large field-effect mobilities in thin films (23). Spin-coated P3HT film shows a low absorption window between 300–370 nm, minimizing any possible interference with the AZO-coating trans-cis photoisomerizaton (λ = 365 nm) (24).

Results and Discussion

We prepared two types of photoresponsive devices by blending P3HT with the AuNPs fully coated with the photochromic AZO-SAM being either in its trans or cis form (Fig. 1D). The isomerization state of the SAM has been shown capable to modulate the AuNP aggregation within the blend, thus the density and dispersion of the particles in the polymeric host (25). In the following, we name the set of devices prepared starting from the trans isomer “a-AZONPs” (in which “a” stands for aggregated), and those starting with the cis isomer “s-AZONPs” (with “s” meaning solvated). For the sake of comparison, we extended our studies to AuNPs coated with nonphotoresponsive SAMs made from a mixture of dodecanthiol and S-[4-[4-(phenylethynyl)phenyl]-ethynyl]benzene-thiol (OPE) (Fig. 1C) in ratio 1.9∶1 (extracted by 1H NMR; Fig. S1). The NP synthesis procedures are reported in SI Text. Transmission electron microscopy (TEM) provided evidence for the dissimilar aggregation propensity of the differently coated AuNPs deposited on electron microscopy grids (Fig. S2).
We studied the device performances as a function of: (i) the relative amount of NPs in the film; (ii) the type of organic layer coating NPs; and (iii) the homogeneity of the film morphology that can be varied using specific sample-preparation methods (SI Text). In order to determine the suitable concentration of NPs in transistors showing a detectable field-effect response, over 150 samples varying the percentage of NPs, ranging from 0.2% to 40%, were prepared. It was found that the devices incorporating differently coated AuNP exhibited a gate effect up to a given threshold concentration of NPs in the blend (CTH). Above that concentration the aggregation propensity of the NPs increases, resulting in a decrease in the source-drain current (ID). This is likely caused by the disruption of the conjugation in the π–π stacked architectures of P3HT, which largely affects the charge transport via interchain hopping of the polarons as well as leads to a more pronounced contribution of trapping charge centers, perturbing or even preventing charge transport in the channel. Fig. S3 displays such effect on an a-AZONPs/P3HT—based device. Thus, to allow data comparison for different binary systems, we prepared devices using the highest amount of NPs that, once blended with the P3HT, showed a gate response. The CTH amounted to 18 wt% for s-AZONPs and only to 1 wt% for a-AZONPs because of its markedly high propensity to aggregate (25). For the case of OPE-NPs, 18 wt% was used to allow optimal comparison with the s-AZONPs devices.
To gather direct evidence over NPs aggregation, the surface morphology of the blend films spin coated on the prepatterned gold electrodes/SiO2 was visualized by atomic force microscopy (AFM) in intermittent contact mode and by TEM (Fig. S2). The root-mean-square roughness (RRMS) estimated on a 16-μm2 area is reported in Table 1. Similar RRMS was observed for the s-AZONPs—and OPE-NPs—based devices, validating their use as comparative systems. Considering the 18-times-smaller percentage of NPs in the a-AZONPs device, the markedly high RRMS is further evidence of their tendency to aggregate within the film. TEM images of the blends (Fig. S2) revealed AuNPs aggregates indicating a similar dispersion of the s-AZONPs and OPE-NPs in the blend (see Table 1 for the aggregates diameter). In a dry film, no morphological modification is expected to take place during the photoisomerization because of the lowest probability of percolation of 1 nm AuNP in a solid (26, 27).
Table 1.
Comparison between CTH of AuNPs in the blend, RRMS, and aggregates size evaluated by AFM
RRMS, AFM (16-μm2 area) [nm]0.61 ± 0.171.11 ± 0.284.41 ± 1.020.83 ± 0.21
Aggregates diameter, AFM [nm]/3.0 ± 2.05.5 ± 4.02.5 ± 0.5
μmean [cm2 V-1 s-1]3.9E-3 ± 1.6E-39.2E-4 ± 4.8E-51.2E-6 ± 8.4E-71.1E-3 ± 9.2E-4
μmax [cm2 V-1 s-1]6E-39.9E-42.2E-62.6E-3
VTH [V]5.7 ± 4.94.1 ± 4.4−0.1 ± 16.75−10.0 ± 7.8
Hysteresis, [V]Dark0.4 ± 0.31.2 ± 0.55.5 ± 4.53.4 ± 1.3
UV3.6 ± 3.410.7 ± 0.820.5 ± 9.512.2 ± 4.6
Δ3.2 ± 3.79.5 ± 1.315.0 ± 14.08.8 ± 5.9
Max number of irradiation cycles> 25> 253–5> 25

Mobility (μ) estimated from the linear regime; threshold voltage (VTH) extracted for the different blend-based devices before irradiation; maximum hysteresis within irradiation (ΔVmax) extracted from the difference between the backward (-40 to 40 V) and forward (40 to -40 V) transfer characteristics acquired in the dark and under UV illumination; and maximum number of irradiation cycles allowed for each device (L = 5 μm).

The electrical characterization of the transistors and their response to UV light was investigated on the chosen devices featuring a well-defined NPs/P3HT ratio. The devices were characterized and exposed to several irradiation cycles, each cycle consisting of three steps: (i) initial electrical characterization of the device; (ii) exposure to UV irradiation for 45 min (trans to cis isomerization) and measurement in the dark; and (iii) electrical characterization after 24 h in the dark (allowing thermal back reaction from cis to trans).
For the case of s-AZONP/P3HT (i.e., the blend prepared by mixing the P3HT with AZO-coated NPs in their cis state), after step (i) the device was kept for 24 h in the dark to allow relaxation to the trans form followed by further electrical characterization, before passing to steps (ii) and (iii). Three cycles were performed for each of the four samples. For all the devices, the output and transfer characteristics obtained from the third cycle are shown in Fig. 2 and Fig. S4, respectively. The third cycle was chosen to guarantee the comparison of devices under the same conditions. Indeed, the s-AZONPs devices were prepared under UV light exposure, being a possible cause of devices overstressing.
Fig. 2.
Output characteristics of (A) P3HT, (B) OPE-NP/P3HT, (C) a-AZONP/P3HT, and (D) s-AZONP/P3HT recorded at different VG (-40, -20, 0, 20, and 40 V). Black indicates the initial state (after keeping the device in the dark for 24 h); red, after 45 min of UV irradiation; and grey, after a further 24 h in dark. Channel length (L). 5 μm.
The samples are prepared following two methods, depending on the AuNP employed: method 1 (M1), in dark, for the a-AZONPs/P3HT devices; and method 2 (M2), under UV illumination, for the s-AZONPs/P3HT and OPE-NPs/P3HT. The modulable OFETs output characteristics upon light exposure are shown in Fig. 2. The electrical performance of the bare P3HT did not show significant variation if prepared with method M1 or M2. All measurements were performed in the dark.
Although some of the devices did not reach saturation in the range of source-drain bias applied (between 0 and -30 V), this voltage limit was not overcome because of the occurrence of bias stress (Fig. S5). The incorporation of AuNPs in the film is accompanied by a decrease of the drain currents, including the ID, max measured at the highest VD and VG, of up to three orders of magnitude when compared to monocomponent P3HT-based FETs. After irradiation with UV light, the ID in s-AZONP/P3HT devices is increased by a factor of seven compared to the OPE-NP/P3HT devices. Such high current in the s-AZONP/P3HT devices may be attributed to both the conjugated nature of the SAM and to its reduced thickness (21), leading to a lower tunneling barrier (28). Table 1 summarizes the average FET parameters extracted from at least four different devices for each system before irradiation. The mean charge carrier mobility (μmean) values are in trend with the morphology of the film, showing the largest μmean for the bare P3HT and the lowest for the a-AZONPs/P3HT. The s-AZONPs/P3HT—and OPE-NPs—based devices revealed similar magnitudes of μmean. Fig. 3 displays the percentage of variation of the characteristic FET parameters (ID, max, μ, and threshold voltage VTH) caused by the irradiation of the devices, extracted from the transfer curves (Fig. S4). Upon UV illumination, the VTH shifted to higher values in the cases of the AZONPs, which, at the voltage applied, translate to a higher amount of charges in the channel, leading to an increased μ and ID, max. The device based on OPE-NPs displayed a slight decrease of the three values compared to the initial state, probably as a result of the long exposure of the film to the UV light. Conversely, the P3HT device showed almost no change in VTH, ID, and μ upon UV irradiation. This is likely caused by the fast charge recombination in bare P3HT films. The a-AZONPs/P3HT transistor revealed the largest parameter variation upon irradiation, but unfortunately the device’s response vanished after a few cycles—i.e., between two and five depending on the sample (Fig. S5). The large fatigue observed and the low reproducibility of the results are attributed to the great tendency of the a-AZONPs to aggregate.
Fig. 3.
Maximum percentage of variation during the irradiation cycles for the (A) VTH, (B) mobility (μ), and (C) ID, max (extracted in the linear regime, VSD = -10 V), for the five different devices. Blue, P3HT; red, s-AZONPs; green, a-AZONPs; magenta, OPE-NPs.
We then focused our attention on the role of the charging/retention/discharging mechanism of the gate-induced charge carriers and the photogenerated charges on the transfer characteristics, recording cycles of forward and reverse bias at a specific gate voltage range. In particular, we explored the effect of the different AuNP organic coating, considering as well the isomerization state of the AZOs. Fig. 4 displays the representative graphs. The efficiency of charging and discharging is reflected in the observed relative hysteresis, which can be expected to be larger in presence of more retained (trapped) charges or when the discharging process is longer (29). The values of the maximum hysteresis extracted from the difference between the backward and forward transfer curves are reported in Table 1. As expected, the P3HT device exhibits the lowest hysteresis. Such light response can be ascribed to an increased number of trapped charge carriers in the active layer, either at the P3HT-SiO2 interface or in the SiO2 (30), and to the effect of vacancies on the electrical hysteresis (31). Nanoparticles-based devices show higher hysteresis because of their intrinsic charge retention (i.e., storage, capability, and the inhomogeneous nature of the film). For AuNPs of a given size, a difference in this ability might rise from: (i) the different coating, giving a different electron-tunneling barrier for the charge to reach the nanoparticles core; and (ii) the AuNPs aggregation within the polymeric matrix that translates both in the disruption of the long-range crystalline structure of the P3HT film and in the increase of the charge-retention effect of the nanoparticles. The greatest hysteresis is observed in the a-AZONPs-based device, an effect that can be attributed to the strong aggregation of AuNPs. On the other hand, s-AZONPs and OPE-NPs showed a similar hysteresis because of their more homogeneous morphology, featuring a reduced aggregation propensity. Among them, the OPE-NPs revealed a smaller hysteresis determined by the dodecanethiol’s capacity to disperse well the AuNP in P3HT (32). In addition, the hydrophobic nature of the alkanethiol SAMs can enhance the crystallinity of the film, which will promote less trapping centers in the P3HT film, yielding a lower hysteresis. The hysteresis observed in the s-AZONPs/P3HT—and OPE-NP—based devices can also be determined by the mechanism governing the charge transport between the semiconductor and the NPs, which can proceed by either hopping or tunneling transport (33, 34). The different nature of the SAM chemisorbed on the AuNPs (alkanethiolated SAM or a fully conjugated aromatic molecule or AZO) will also influence the tunneling barrier. The larger tunneling barrier associated with thicker SAMs slows down the charging and trapping processes but also helps charges to remain trapped. The OPE-NPs lead to a higher contribution of the tunneling resistance to the charge trapping process, if compared to the AZO molecule, as a result of the saturated carbons in the dodecanethiol. We believe that the above factors can explain the slight increase of the absolute extracted hysteresis for the s-AZONPs with respect to the OPE-NPs, amounting to 2.2 V in the dark.
Fig. 4.
Transfer characteristics (VD = -30 V, L = 5 μm) obtained for the four different devices: black, when the device is in dark; red, when the device is under UV (the measurement started to be recorded after 5 min of irradiation). (A) P3HT, (B) OPE-NPs/P3HT, (C) a-AZONPs/P3HT, and (D) s-AZONPs/P3HT.
Under irradiation, the isomerization of the AZO-NPs has to be considered to describe the process of trapping/releasing of charges. It has been shown by conducting atomic force microscopy and Hg drop that the resistance of the molecular junction (i.e., the tunneling barrier) is higher for the trans-AZO SAM (20, 21). This is attributed to the decrease in thickness when the SAM switches to the cis form. Although on the timescale of the measurements the trans-AZO molecules forming the SAM in the s-AZONPs are only partially isomerized upon UV irradiation, the molecules converted into cis state feature a lower resistance of the organic barrier, which is translated into a higher efficiency of the charge trapping/detrapping process. This phenomenon is evident when comparing the hysteresis difference between dark and UV conditions in s-AZONPs and OPE-NPs. As predicted, the latter shows a lower increase under irradiation.
To further differentiate the two systems, the work function (WF) values of the drop-cast films of OPE-NPs and s-AZONPs on freshly cleaved mica were determined by means of photoelectron spectroscopy in air, being 4.44 ± 0.05 eV and 4.53 ± 0.07 eV, respectively. In both cases, the energetic barrier difference (mismatch of energetic levels) between the functionalized AuNPs and the HOMO of the P3HT (4.8 eV) slows down the charge detrapping because the lower AuNPs’ WF can facilitate the transport (and trapping) of holes from the semiconductor to the NPs, but disfavors the injection of the holes back to the P3HT.
It is also well known that the photoexcitation hole density within the film contributes to the drain current (ID) and increases the VTH to larger positive values (35). Indeed, this is what is observed for the four different devices, although with different magnitude of VTH shift (ΔVTH). The values of charge carrier mobilities and threshold voltage, expressed as a range including all the results for the different devices tested, are reported in Table S1. The easier charge trapping and release at the NPs coated with azobenzene in the cis form will make the overall collected charge carriers (i.e., measured ID) higher when compared to the trans-based device. This difference between the two states will enlarge the change in ID upon sample illumination. It is expected that, in the nonphotoresponsive devices, the modulation of current is mainly induced by the photogenerated charges in the P3HT matrix.
Because of the low reproducibility, large fatigue of the switch, and poor electrical performance obtained in a-AZONPs/P3HT transistors, these devices were not studied further. The photoswitching capacity and reversible nature of the AZO-based devices is illustrated in Fig. 5. To determine the suitable irradiation times employed to gauge these two parameters, a s-AZONP/P3HT film was illuminated with UV while collecting the ID over time. We observed that ID increased and reached saturation within 5 min, and after 10 min the bias stress started to play a role, inducing a decrease in the ID. For this reason, to study the response to UV light and the cyclability of the process (at VG - VTH = -4 V and VD = -10 V), the samples were irradiated for 5 min at 365 nm followed by 10 min in the dark to allow the systems to relax. This procedure was iterated up to 25 times (Fig. 5). OPE-NPs/P3HT—and bare P3HT-based transistors were measured with the same parameters and conditions. The ID increased upon UV illumination and decreased in the dark for all the systems. A markedly higher variation was detected for the AZO-based device. We believe that the observed increase of the ID,UV/ID,dark ratio for the s-AZONPs with time can be attributed to a higher percentage of AZO molecules converted to the cis form and a modulation of the device threshold voltage with the UV light. The higher amount of free charges caused by the presence of AZO molecules in their cis conformation is accompanied by a VTH shift to more positive voltages (which implies higher amount of holes contributing to the collected ID at the applied voltage). The VTH variation during the irradiation cycles (at the same VG as in the cyclability experiments) and the amount of current variation at that voltage were investigated to understand the contribution of the VTH in the observed ID modification (Fig. S6). It was determined that the modulation of the current could not be ascribed only to the VTH shift, whose contribution is marginal compared to the observed ID modulation. Significantly, both the bare P3HT and the OPE-NPs devices showed opposite trends over time compared to the AZO-NPs because the bias stress is not compensated by an increase of current. Interestingly, this decrease in current in the case of the P3HT device is much higher than in the AuNP. This may be attributed to charging/discharging processes caused by the presence of NPs that are responsible for more constant active charge carriers present in the semiconductor. A deeper study of the dynamics involved in this particular process is underway. Based on the experimental results, a possible charge-transport mechanism and the pathways involved in the device (incorporating the blend of P3HT/AZO-AuNPs) are portrayed in Fig. S7. The charge moves from the source (A) to the drain electrode (H) following the shown path. Every letter indicates a different “node” in the mechanism and symbolizes a change in charge-transport mechanism.
Fig. 5.
Photoresponse cycles over time of s-AZONPs/P3HT (in black), OPE-NPs/P3HT (in red), and P3HT (in blue). For the latter, the ID/ID, min was multiplied by 100 to allow a better comparison. (A) Four cycles, (B) 10 cycles, and (C) 25 cycles. VG - VTH = -4 V, VD = -10 V, and L = 5 μm.
In summary, the light modulation of the current in an OFET based on AuNPs blended with P3HT has been demonstrated to be greatly enhanced when the AuNPs are functionalized with a photoresponsive azobenzene-based SAM. The light-induced isomerization between the trans and cis states of the molecules coating the AuNPs implies a variation of the tunneling barrier (decreasing from trans to cis isomer), which plays a crucial role in the efficiency of the charge trapping/detrapping process within the film. Thus, this approach makes it possible to confer a dual functionality to an organic transistor because it provides a means to gate the source-drain current through the channel both electrically (through gate control), as in a conventional transistor, and optically (through photochemical control). It also represents an innovative approach to digital commuting between optical and electric signals. Finally, such a modulable transistor might be useful for applications in UV sensing.

Materials and Methods

P3HT was purchased from BASF Chemicals (Sepiolid P200). All the reagents, solvents included (except water), were purchased from Aldrich. The MilliQ water was provided using a MQ water filter (Millipore-Direct Q). Biphenylazo derivative was synthesized as previously described (19). All OFETs (with bottom-gate/bottom-contact configuration) were prepared starting from n ++-Si/SiO2 substrates, featuring a thermally grown 230-nm thick oxide layer, exposing prepatterned interdigitated Au source and drain electrodes (capacitance 1.5·10-8 F/cm2; Fraunhofer Institute). Before their use, they were rinsed with acetone and then isopropanol in ultrasonic bath for 15 min each to remove the photoresist-protecting layer and dried under N2 stream. Spin coating and electrical characterization were done at room temperature and in an inert environment (glove box, with water and oxygen levels below 10 ppm) by means of an electrometer (Keithley 2536) interfaced by software developed in-house.
TEM images were acquired with a Philips CM100 transmission electron microscope at 80 kV. Electron micrographs were recorded on a 2,000-by 2,000-pixel charge-coupled device camera (Veleta; Olympus). The particles were deposited by drop casting on top of a thin carbon film that spanned a perforated carbon support film covering a Au-plated Cu microscopy grid. The grid was air dried. AFM images were acquired with a Veeco Dimension 3100 AFM running with a Nanoscope IV controller. AFM images were recorded in the intermittent-contact (tapping) mode under ambient conditions. Silicon cantilevers were used (MPP-11100; Veeco) with a nominal spring constant of 40 N/m, resonance frequency of 300 kHz, and tip radius of 10 nm. Photoelectron spectroscopy (AC-2; RKI Instruments) was used to measure the work functions.


We thank Drs. E. Orgiu and J. Hutchison for their comments on the manuscript. This work was supported by the European Commission Marie-Curie IEF-OPTSUFET (PIEF-GA-2009-235967) and ITN-SUPERIOR (PITN-GA-2009-238177), the European Research Council project SUPRAFUNCTION (GA-257305), the FP7 ONE-P Project No. 212311, the NanoSci-E+ project SENSORS, International Center for Frontier Research in Chemistry (icFRC), Swiss Nationals Science Foundation (SNF), and Swiss Nanoscience Institute (SNI).

Supporting Information

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Supporting Information


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Go to Proceedings of the National Academy of Sciences
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Proceedings of the National Academy of Sciences
Vol. 109 | No. 31
July 31, 2012
PubMed: 22802669


Submission history

Published online: July 16, 2012
Published in issue: July 31, 2012


  1. multifunctional material
  2. responsive material
  3. multi-gating
  4. charge transport


We thank Drs. E. Orgiu and J. Hutchison for their comments on the manuscript. This work was supported by the European Commission Marie-Curie IEF-OPTSUFET (PIEF-GA-2009-235967) and ITN-SUPERIOR (PITN-GA-2009-238177), the European Research Council project SUPRAFUNCTION (GA-257305), the FP7 ONE-P Project No. 212311, the NanoSci-E+ project SENSORS, International Center for Frontier Research in Chemistry (icFRC), Swiss Nationals Science Foundation (SNF), and Swiss Nanoscience Institute (SNI).


This article is a PNAS Direct Submission. F.M.R. is a guest editor invited by the Editorial Board.



Corinna Raimondo
Institut de Science et d'Ingénierie Supramoléculaires and International Center for Frontier Research in Chemistry, Université de Strasbourg and Centre National de la Recherche Scientifique, 8 allée Gaspard Monge, 67000 Strasbourg, France;
Núria Crivillers
Institut de Science et d'Ingénierie Supramoléculaires and International Center for Frontier Research in Chemistry, Université de Strasbourg and Centre National de la Recherche Scientifique, 8 allée Gaspard Monge, 67000 Strasbourg, France;
Federica Reinders
Department of Chemistry, University of Basel, St. Johannsring 19, 4056 Basel, Switzerland; and
Fabian Sander
Department of Chemistry, University of Basel, St. Johannsring 19, 4056 Basel, Switzerland; and
Marcel Mayor1 [email protected]
Department of Chemistry, University of Basel, St. Johannsring 19, 4056 Basel, Switzerland; and
Karlsruhe Institute of Technology, Institute for Nanotechnology, 76021 Karlsruhe, Germany
Paolo Samorì1 [email protected]
Institut de Science et d'Ingénierie Supramoléculaires and International Center for Frontier Research in Chemistry, Université de Strasbourg and Centre National de la Recherche Scientifique, 8 allée Gaspard Monge, 67000 Strasbourg, France;


To whom correspondence may be addressed. E-mail: [email protected] or [email protected].
Author contributions: C.R., N.C., and P.S. designed research; C.R. and N.C. performed research; F.R., F.S., and M.M. contributed new reagents/analytic tools; C.R., N.C., and P.S. analyzed data; and C.R. and P.S. wrote the paper.

Competing Interests

The authors declare no conflict of interest.

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