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  • 65 articles in high-impact journals
  • 54 joint publications
  • 5 hot papers
  • 1 very important paper
  • 6 cover pages
  • 83 press releases & highlights
  • >355 citations
  • Average impact factor >10

Fluorescence commutation and surface photopatterning with porphyrin tetra-dithienylethene switches

T. Biellmann, A. Galanti, J. Boixel, J. A. Wytko, V. Guerchais, P. Samorì, J. Weiss

Chem. Eur. J., 2017, in press.

Four tetra-dithienylethene (DTE) substituted porphyrins, the free base 1H₂ and three metal derivatives (1Zn, 1Co, 1Ni), were synthesized and studied. These dyads, in which the DTE units are connected to the porphyrin's meso positions via a meta-phenyl spacer, exhibit reversible photochromic properties in all cases with conversion to the PSS up to 88%, as confirmed by absorption and NMR spectroscopies. Compounds 1H₂ and 1Zn are fluorescent in solution and display a red emission. Upon irradiation with UV light to trigger the closing of the DTEs, the fluorescence of both the free base and zinc porphyrin was very efficiently quenched in solution. The reversible, photo-triggered switching of luminescence was retained in a tetra-DTE free base porphyrin-doped polystyrene film, for which photo-patterning was demonstrated by confocal scanning microscopy. The tunable fluorescent properties of this multi-DTE framework render this compound of interest as a photo-rewritable fluorescent ink.


Edge Functionalization of Structurally Defined Graphene Nanoribbons for Modulating the Self-Assembled Structures

A. Keerthi, B. Radha, D. Rizzo, H. Lu, V. Diez Cabanes, I. C.-Y. Hou, D. Beljonne, J. Cornil, C. Casiraghi, M. Baumgarten, K. Müllen, A. Narita

J. Am. Chem. Soc., 2017, in press.

Edge functionalization of bottom-up synthesized graphene nanoribbons (GNRs) with anthraquinone and naphthalene/perylene monoimide units has been achieved through a Suzuki coupling of polyphenylene precursors bearing bromo groups, prior to the intramolecular oxidative cyclo-dehydrogenation. High efficiency of the substitution has been validated by MALDI-TOF MS analysis of the functionalized precursors and FT-IR, Raman, and XPS analyses of the resulting GNRs. Moreover, AFM measurements demonstrated the modulation of the self-assembling behavior of the edge-functionalized GNRs, revealing that GNR-PMI formed an intriguing rectangular network. This result suggests the possibility of programming the supramolecular architecture of GNRs by tuning the functional units.


Carbon-Rich Monolayers on ITO as Highly Sensitive Platforms for Detecting Polycyclic Aromatic Hydrocarbons in Water: The Case of Pyrene

J. Muñoz, N. Crivillers, M. Mas-Torrent

Chem. Eur. J., 2017, 23, 15289–15293. Link to article. NANOMOL press release.

The determination of polycyclic aromatic hydrocarbons (PAHs) in water at low levels is a current challenge given their great impact on the health and safety of the public. Here, a novel pyrene-based self-assembled monolayer (SAM) platform is exploited as an electrochemical sensing recognition device. Interestingly, the formation of π–π sandwich complexes between PAHs and the recognition element switches the surface electron transfer capability. The unique supramolecular interaction between identical aromatic molecules provides a highly sensitive and selective sensor for pyrene in the order of part per trillion. Accordingly, and using pyrene as a proof-of-concept, this work presents the basis for an “at-point-of-use” impedimetric sensor focused on a highly sensitive carbon-rich SAM for PAHs determination in water at ultra-trace levels.

 


Light-Induced Contraction/Expansion of 1D Photoswitchable Metallopolymer Monitored at the Solid–Liquid Interface

M. El Garah, E. Borré, A. Ciesielski, A. Dianat, R. Gutierrez, G. Cuniberti, S. Bellemin-Laponnaz, M. Mauro, P. Samorì

Small, 2017, 13, 1701790. Link to article and accepted manuscript (open access).

The use of a bottom-up approach to the fabrication of nanopatterned functional surfaces, which are capable to respond to external stimuli, is of great current interest. Herein, the preparation of light-responsive, linear supramolecular metallopolymers constituted by the ideally infinite repetition of a ditopic ligand bearing an azoaryl moiety and Co(II) coordination nodes is described. The supramolecular polymerization process is followed by optical spectroscopy in dimethylformamide solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy (STM) study of the in situ reversible trans-to-cis photoisomerization of a photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto a highly oriented pyrolytic graphite surface is achieved for the first time. The STM analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed species onto a graphene slab before and after irradiation by means of density functional theory calculation. Significantly, switching of the monolayers consisting of supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern based on cis isomers can be triggered by UV light and reversed back to the trans conformer by using visible light, thereby restoring the trans-based supramolecular 2D packing. These findings represent a step forward toward the design and preparation of photoresponsive “smart” surfaces organized with an atomic precision.


Supramolecular Self-Assembly in a Sub-micrometer Electrodic Cavity: Fabrication of Heat-Reversible π-Gel Memristor

L. Zhang, S. Li, M. A. Squillaci, X. Zhong, Y. Yao, E. Orgiu, P. Samorì

J. Am. Chem. Soc., 2017, 139, 14406–14411. Link to article and accepted manuscript (open access). COVER PAGE.

The use of biomimetic approaches toward the production of nonsolid yet functional architectures holds potential for the emergence of novel device concepts. Gels, in particular those obtained via self-assembly of π-conjugated molecules, are dynamic materials possessing unique (opto)electronic properties. Their adaptive nature imparts unprecedented responsivity to various stimuli. Hitherto, a viable device platform to electrically probe in situ a sol–gel transition is still lacking. Here we describe the fabrication of a sub-micrometer electrodic cavity, which enables low-voltage electrical operation of π-gels. Thanks to the in situ supramolecular self-assembly of the π-gelator occurring within the cavity, we conceived a novel gel-based memristor whose sol–gel transition is reversible and can be controlled via heating and dc bias. This work opens perspectives toward the fabrication of a novel generation of nonsolid multiresponsive devices.


Tetrathiafulvalene–Polychlorotriphenylmethyl Dyads: Influence of Bridge and Open-Shell Characteristics on Linear and Nonlinear Optical Properties

M. Souto, J. Calbo, I. Ratera, E. Ortí, J. Veciana

Chem. Eur. J., 2017, 23, 11067–11075. Link to article.

Three conjugated donor-π-acceptor radical systems (1 a1 c) were prepared by bridging a tetrathiafulvalene (TTF) electron-donor unit to a polychlorotriphenylmethyl (PTM) electron-acceptor radical through vinylene units of different lengths. The dependence of the intramolecular charge transfer on the length of the conjugated bridge has been analyzed by different electrochemical and spectroscopic techniques. Linear optical properties and the second-order nonlinear optical (NLO) response of these derivatives have been computed by comparing systems 1 a1 c with the non-radical analogues (2 a2 c). Interestingly, an enhanced NLO response is predicted for dyads 1 a1 c with PTM in the radical form and for compounds with longer vinylene bridges. Calculations confirm the active role the bridge plays for electronic communication between the donor TTF and the acceptor PTM units.


Reversible, Fast, and Wide-Range Oxygen Sensor Based on Nanostructured Organometal Halide Perovskite

M.-A. Stoeckel, M. Gobbi, S. Bonacchi, F. Liscio, L. Ferlauto, E. Orgiu, P. Samorì

Adv. Mater., 2017, 29, 1702469. Link to article and accepted manuscript (open access). COVER PAGE. ISIS press release.

Nanostructured materials characterized by high surface–volume ratio hold the promise to constitute the active materials for next-generation sensors. Solution-processed hybrid organohalide perovskites, which have been extensively used in the last few years for optoelectronic applications, are characterized by a self-assembled nanostructured morphology, which makes them an ideal candidate for gas sensing. Hitherto, detailed studies of the dependence of their electrical characteristics on the environmental atmosphere have not been performed, and even the effect of a ubiquitous gas such as O2 has been widely overlooked. Here, the electrical response of organohalide perovskites to oxygen is studied. Surprisingly, a colossal increase (3000-fold) in the resistance of perovskite-based lateral devices is found when measured in a full oxygen atmosphere, which is ascribed to a trap healing mechanism originating from an O2-mediated iodine vacancies filling. A variation as small as 70 ppm in the oxygen concentration can be detected. The effect is fast (<400 ms) and fully reversible, making organohalide perovskites ideal active materials for oxygen sensing. The effect of oxygen on the electrical characteristics of organohalide perovskites must be taken into deep consideration for the design and optimization of any other perovskite-based (opto-) electronic device working in ambient conditions.


Asymmetric Injection in Organic Transistors via Direct SAM Functionalization of Source and Drain Electrodes

T. Mosciatti, P. Greco, T. Leydecker, M. Eredia, F. Biscarini, P. Samorì

ACS Omega20172, 3502–3508. Link to article (open access).

The fabrication of organic optoelectronic devices integrating asymmetric electrodes enables optimal charge injection/extraction at each individual metal/semiconductor interface. This is key for applications in devices such as solar cells, light-emitting transistors, photodetectors, inverters, and sensors. Here, we describe a new method for the asymmetric functionalization of gold electrodes with different thiolated molecules as a viable route to obtain two electrodes with drastically different work function values. The process involves an ad hoc design of electrode geometry and the use of a polymeric mask to protect one electrode during the first functionalization step. Photoelectron yield ambient spectroscopy and X-ray photoelectron spectrometry were used to characterize the energetic properties and the composition of the asymmetrically functionalized electrodes. Finally, we used poly(3-hexylthiophene)-based organic thin-film transistors to show that the asymmetric electronic response stems from the different electronic structures of the functionalized electrodes.


Photoswitchable Micro-Supercapacitor Based on a Diarylethene-Graphene Composite Film

Z. Liu, H. I. Wang, A. Narita, Q. Chen, Z. Mics, D. Turchinovich, M. Kläui, M. Bonn, K. Müllen

J. Am. Chem. Soc.2017139, 9443–9446. Link to article.

Stimuli-responsive micro-supercapacitors (MSCs) controlled by external stimuli can enable a wide range of applications for future on-chip energy storage. Here, we report on a photoswitchable MSC based on a diarylethene-graphene composite film. The microdevice delivers an outstanding and reversible capacitance modulation of up to 20%, demonstrating a prototype photoswitchable MSC. Terahertz spectroscopy indicates that the photoswitching of the capacitance is enabled by the reversible tuning of interfacial charge injection into diarylethene molecular orbitals, as a consequence of charge transfer at the diarylethene–graphene interface upon light modulation.


Dicyanobenzothiadiazole Derivatives Possessing Switchable Dielectric Permittivities

J. Wudarczyk, G. V. Papamokos, T. Marszalek, T. Nevolianis, D. Schollmeyer, W. Pisula, G. Floudas, M. Baumgarten, K. Müllen

ACS Appl. Mater. Interfaces, 2017, 9, 20527–20535. Link to article.

Benzothiadiazoles are important electron acceptors and are frequently employed as electron-deficient components of donor–acceptor polymers. We report the effect of nitrile functionalities on the reactivity, steric hindrance, optoelectronic properties, and dielectric permittivity in dicyanobenzothioadiazole (DCNBT). Dielectric spectroscopy in the bulk and in solution assisted by DFT-calculations revealed that these molecules can be engineered to engender maximum values of the dipole moment and of dielectric permittivity due to the strong electron-withdrawing effect of the nitrile groups. The self-assembly in the bulk was investigated by X-ray scattering performed on single crystals, fibers (2D-WAXS), and thin films (GiWAXS). Combining these results, we found a switching of dielectric permittivity of the 4,7-alkylthienyl-substituted dicyanobenzothiadiazole at the transition from the liquid crystalline to the isotropic phase with values capable of competing with the best known rodlike liquid crystals. 


Graphene oxide doped polysulfone membrane adsorbers for the removal of organic contaminants from water

M. Zambianchi, M. Durso, A. Liscio, E. Treossi, C. Bettini, M. L. Capobianco, A. Aluigi, A. Kovtun, G. Ruani, F. Corticelli, M. Brucale, V. Palermo, M. L. Navacchia, M. Melucci

Chem. Eng. J., 2017, 326, 130–140. Link to article.

This work explored polysulfone (PS) – graphene oxide (GO) based porous membranes (PS-GO) as adsorber of seven selected organic contaminants of emerging concern (EOCs) including pharmaceuticals, personal care products, a dye and a surfactant from water. PS-GO was prepared by phase inversion method starting from a PS and GO mixture (5% w/w of GO). The porous PS-GO membranes showed asymmetric and highly porous micrometer sized pores on membrane top (diameter ≈20 μm) and bottom (diameter ≈2–5 μm) surfaces and tens of microns length finger like pores in the section. Nanomechanical mapping reveals patches of a stiffer material with Young modules comprised in the range 15–25 GPa, not present in PS pure membranes that are compatible with the presence of GO flakes on the membrane surfaces. PS-GO was immersed in EOCs spiked tap water and the adsorbance efficiency at different contact times and pH evaluated by HPLC analysis. Ofloxacin (OFLOX), benzophenone-3 (BP-3), rhodamine b (Rh), diclofenac (DCF) and triton X-100 (TRX) were removed with efficiency higher than 90% after 4 h treatments. Regeneration of PS-GO and reuse possibilities were demonstrated by washing with ethanol. The adsorption efficiencies toward OFLOX, Rh, DCF and carbamazepine (CBZ) were significantly higher than those of pure PS membrane. Moreover, PS-GO outperformed a commercial granular activated carbon (GAC) at low contact times and compared well at longer contact time for OFLOX, Rh, BP-3 and TRX suggesting the suitability of the newly introduced material for drinking water treatment.


Redox-Induced Gating of the Exchange Interactions in a Single Organic Diradical

R. Gaudenzi, J. de Bruijckere, D. Reta, I. d. P. R. Moreira, C. Rovira, J. Veciana, H. S. J. van der Zant, E. Burzurí

ACS Nano, 201711, 5879–5883. Link to article (open access).

Embedding a magnetic electroactive molecule in a three-terminal junction allows for the fast and local electric field control of magnetic properties desirable in spintronic devices and quantum gates. Here, we provide an example of this control through the reversible and stable charging of a single all-organic neutral diradical molecule. By means of inelastic electron tunnel spectroscopy we show that the added electron occupies a molecular orbital distinct from those containing the two radical electrons, forming a spin system with three antiferromagnetically coupled spins. Changing the redox state of the molecule therefore switches on and off a parallel exchange path between the two radical spins through the added electron. This electrically controlled gating of the intramolecular magnetic interactions constitutes an essential ingredient of a single-molecule quantum gate.


Synthesis of Dibenzo[hi,st]ovalene and Its Amplified Spontaneous Emission in a Polystyrene Matrix

G. M. Paternò, Q. Chen, X.-Y. Wang, J. Liu, S. G. Motti, A. Petrozza, X. Feng, G. Lanzani, K. Müllen, A. Narita, F. Scotognella

Angew. Chem. Int. Ed., 2017, 56, 6753–6757. Link to article.

A large number of graphene molecules, or large polycyclic aromatic hydrocarbons (PAHs), have been synthesized and display various optoelectronic properties. Nevertheless, their potential for application in photonics has remained largely unexplored. Herein, we describe the synthesis of a highly luminescent and stable graphene molecule, namely a substituted dibenzo[hi,st]ovalene (DBO 1), with zigzag edges and elucidate its promising optical-gain properties by means of ultrafast transient absorption spectroscopy. Upon incorporation of DBO into an inert polystyrene matrix, amplified stimulated emission can be observed with a relatively low power threshold (ca. 60 μJ cm−2), thus highlighting its high potential for lasing applications.


“Supertrap” at Work: Extremely Efficient Nonradiative Recombination Channels in MAPbI3 Perovskites Revealed by Luminescence Super-Resolution Imaging and Spectroscopy

A. Merdasa, Y. Tian, R. Camacho, A. Dobrovolsky, E. Debroye, E. L. Unger, J. Hofkens, V. Sundström, I. G. Scheblykin

ACS Nano, 201711, 5391–5404. Link to article.

Organo-metal halide perovskites are some of the most promising materials for the new generation of low-cost photovoltaic and light-emitting devices. Their solution processability is a beneficial trait, although it leads to a spatial inhomogeneity of perovskite films with a variation of the trap state density at the nanoscale. Comprehending their properties using traditional spectroscopy therefore becomes difficult, calling for a combination with microscopy in order to see beyond the ensemble-averaged response. We studied photoluminescence (PL) blinking of micrometer-sized individual methylammonium lead iodide (MAPbI3) perovskite polycrystals, as well as monocrystalline microrods up to 10 μm long. We correlated their PL dynamics with structure employing scanning electron and optical super-resolution microscopy. Combining super-resolution localization imaging and super-resolution optical fluctuation imaging (SOFI), we could detect and quantify preferential emitting regions in polycrystals exhibiting different types of blinking. We propose that blinking in MAPbI3 occurs by the activation/passivation of a “supertrap” which presumably is a donor–acceptor pair able to trap both electrons and holes. As such, nonradiative recombination via supertraps, in spite being present at a rather low concentrations (1012–1015 cm–3), is much more efficient than via all other defect states present in the material at higher concentrations (1016–1018cm–3). We speculate that activation/deactivation of a supertrap occurs by its temporary dissociation into free donor and acceptor impurities. We found that supertraps are most efficient in structurally homogeneous and large MAPbI3 crystals where carrier diffusion is efficient, which may therefore pose limitations on the efficiency of perovskite-based devices.


Systematic study of the correlation between surface chemistry, conductivity and electrocatalytic properties of graphene oxide nanosheets

G. Maccaferri, C. Zanardi, Z. Y. Xia, A. Kovtun, A. Liscio, F. Terzi, V. Palermo, Renato Seeber

Carbon, 2017, 120, 165–175. Link to article.

A main advantage of graphene oxide (GO) over other materials is the high tunability of its surface functional groups and of its electric conductivity. However, the complex chemical composition of GO renders difficult to unravel the correlation between structural and electric properties. Here, we use a combination of electron spectroscopy and electrochemistry to correlate the surface chemistry of GO to its electrical conductivity and electrocatalytic properties with respect to two molecules of high biological interest: β-nicotinamide adenine dinucleotide (NADH) and vitamin C. We demonstrate that the electrocatalytic properties of the material are due to hydroxyl, carbonyl and carboxyl groups residues that, even if already present on pristine GO, become electroactive only upon GO reduction.

The results of this study demonstrate the advantages in the use of GO in amperometric biosensing and in enzymatic biofuel cells: it allows the oxidation of the target molecules at low potential values, with a sensitivity >15 times higher with respect to standard, carbon-based electrode materials.

Finally, we demonstrate that the right amount of chemical groups to achieve such high performance can be obtained also by direct electrochemical exfoliation of bulk graphite, without passing through GO production, thus rendering this approach suitable for cheap, large-scale applications.


Improving the electrical performance of solution processed oligothiophene thin-film transistors via structural similarity blending

T. Leydecker, L. Favaretto, D. T. Duong, G. Zappalà, K. Börjesson, A. Licciardello, A. Salleo, M. Melucci, E. Orgiu, P. Samorì

J. Mater. Chem. C, 2017, 5, 5048–5054. Link to article and accepted manuscript (open access).

Here we show that the blending of structurally similar oligothiophene molecules is an effective approach to improve the field-effect mobility and Ion/Ioff as compared to single component based transistors. The effect of addition of each component is studied extensively using a wide array of methods such as X-ray diffraction, ToF-SIMS, and ambient UPS correlated with the electrical characterization. 

 


Generation of Low-Dimensional Architectures through the Self-Assembly of Pyromellitic Diimide Derivatives

C. Musumeci, M. Wałęsa-Chorab, A. Gorczyński, G. Markiewicz, A. Bogucki, R. Świetlik, Z. Hnatejko, W. Jankowski, M. Hoffmann, E. Orgiu, A. R. Stefankiewicz, V. Patroniak, A. Ciesielski, P. Samorì

ACS Omega, 2017, 2, 1672–1678. Link to article (open access).

Small π-conjugated molecules can be designed and synthesized to undergo controlled self-assembly forming low-dimensional architectures, with programmed order at the supramolecular level. Such order is of paramount importance because it defines the property of the obtained material. Here, we have focused our attention to four pyromellitic diimide derivatives exposing different types of side chains. The joint effect of different noncovalent interactions including π–π stacking, H-bonding, and van der Waals forces on the four derivatives yielded different self-assembled architectures. Atomic force microscopy studies, corroborated with infrared and nuclear magnetic resonance spectroscopic measurements, provided complementary multiscale insight into these assemblies.


Photoisomerisation and light-induced morphological switching of a polyoxometalate–azobenzene hybrid

G. Markiewicz, D. Pakulski, A. Galanti, V. Patroniak, A. Ciesielski, A. R. Stefankiewicz, P. Samorì

Chem. Commun.201753, 7278–7281. Link to article and accepted manuscript (open access).

The functionalization of a spherical Keplerate-type polyoxometalate {Mo72V30} with a cationic azobenzene surfactant has been achieved through ionic self-assembly. The photoisomerisation reaction of this complex, which emerges in a light-triggered aggregation–disaggregation process, has been followed by 1H NMR spectroscopy, dynamic light scattering, absorption spectroscopy and scanning electron microscopy analyses.

 


Direct covalent grafting of an organic radical core on gold and silver

M. R. Ajayakumar, I. Alcón, S. T. Bromley, J. Veciana, C. Rovira, M. Mas-Torrent

RSC Adv., 2017, 7, 20076–20083. Link to article (open access).

The functionalisation of surfaces with organic radicals, such as perchlorotriphenylmethyl (PTM) radicals or tris(2,4,6-trichloro-phenyl)methyl (TTM) radicals, is appealing for the development of molecular spintronic devices. Conventionally, organic radicals are chemisorbed to metal substrates by using long alkyl or aromatic spacers resulting in a weak spin–electron coupling between the radical and the substrate. To circumvent this problem, here we have employed a new design strategy for the fabrication of radical self-assembled monolayers (r-SAMs). This newly designed radical–anchor (R–A) molecule, a TTM based radical disulfide (1), can be easily synthesized and it was here characterized by electron spin resonance (ESR), cyclic voltammetry (CV) and superconducting quantum interference device magnetometry (SQUID). We have succeeded in fabricating TTM based r-SAMs by using thiolate bonds (Au–S and Ag–S) where the TTM cores are only one-atom distance from the metal surface for the first time. The resultant robust 1/Au and 1/Ag r-SAMs were well characterized, and the electrochemical and the magnetic properties were unambiguously confirmed, proving the persistence of the molecular spin. 


Estimation of π–π Electronic Couplings from Current Measurements

J. Trasobares, J. Rech, T. Jonckheere, T. Martin, O. Aleveque, E. Levillain, V. Diez-Cabanes, Y. Olivier, J. Cornil, J. P. Nys, R. Sivakumarasamy, K. Smaali, P. Leclere, A. Fujiwara, D. Théron, D. Vuillaume, N. Clément

Nano Lett., 2017, 17, 3215–3224. Link to article and accepted manuscript (open access).

The π–π interactions between organic molecules are among the most important parameters for optimizing the transport and optical properties of organic transistors, light-emitting diodes, and (bio-) molecular devices. Despite substantial theoretical progress, direct experimental measurement of the π–π electronic coupling energy parameter t has remained an old challenge due to molecular structural variability and the large number of parameters that affect the charge transport. Here, we propose a study of π–π interactions from electrochemical and current measurements on a large array of ferrocene-thiolated gold nanocrystals. We confirm the theoretical prediction that t can be assessed from a statistical analysis of current histograms. The extracted value of t ≈35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the subangström level. The present work establishes a direct bridge between quantum chemistry, electrochemistry, organic electronics, and mesoscopic physics, all of which were used to discuss results and perspectives in a quantitative manner.


Rheological and physical characterization of PEDOT:PSS/graphene oxide nanocomposites for perovskite solar cells

A. Giuri, S. Masi, S. Colella, A. Listorti, A. Rizzo, A. Kovtun, S. Dell'Elce, A. Liscio, C. E. Corcione

Polym. Eng. Sci., 2017, 57, 546–552. Link to article.

In this work, the influence of graphene oxide (GO) doped poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin nanocomposite on an indium–tin-oxide (ITO) anode, as hole transport layer (HTL) in perovskite solar cells, was investigated. Different concentrations of GO were added into the PEDOT:PSS in order to enhance its conductivity. In particular, the influence of GO content on the rheological and thermal properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/GO nanocomposites was initially examined. The GO filler was prepared by using modified Hummers method and dispersed into PEDOT:PSS in different quantity (ranging from 0.05 to 0.25%wt/wt). The obtained nanocomposite solutions were analyzed by rheological characterizations in order to evaluate the influence of the GO filler on the viscosity of the PEDOT:PSS matrix. The wettability of solutions was evaluated by Contact Angle (CA) measurements. The quality of GO dispersion into the polymer matrix was studied using Scanning electron microscopy (SEM) and X-ray diffraction (XRD). Thermal characterizations (DSC and TGA) were, finally, applied on nanocomposite films in order to evaluate thermal stability of the films as well as to indirectly comprehend the GO influence on PEDOT:PSS-water links.


Tuning the Rectification Ratio by Changing the Electronic Nature (Open-Shell and Closed-Shell) in Donor–Acceptor Self-Assembled Monolayers

M. Souto, L. Yuan, D. C. Morales, L. Jiang, I. Ratera, C. A. Nijhuis, J. Veciana

J. Am. Chem. Soc., 2017, 139, 4262–4265. Link to article and accepted manuscript (open access). ICMAB-CSIC and NANOMOL press releases.

This Communication describes the mechanism of charge transport across self-assembled monolayers (SAMs) of two donor–acceptor systems consisting of a polychlorotriphenylmethyl (PTM) electron-acceptor moiety linked to an electron-donor ferrocene (Fc) unit supported by ultraflat template-stripped Au and contacted by a eutectic alloy of gallium and indium top contacts. The electronic and supramolecular structures of these SAMs were well characterized. The PTM unit can be switched between the nonradical and radical forms, which influences the rectification behavior of the junction. Junctions with nonradical units rectify currents via the highest occupied molecular orbital (HOMO) with a rectification ratio R = 99, but junctions with radical units have a new accessible state, a single-unoccupied molecular orbital (SUMO), which turns rectification off and drops R to 6.


Study of the EZ stilbene isomerisation in perchlorotriphenyl-methane (PTM) derivatives

F. Bejarano, I. Alcon, N. Crivillers, M. Mas-Torrent, S. T. Bromley, J. Veciana, C. Rovira

RSC Adv., 2017, 7, 15278–15283. Link to article (open access).

The EZ isomerisation of two perchlorotriphenylmethane derivatives containing stilbene units has been investigated, both thermally and photochemically. The irreversibility of the EZ isomerisation in both compounds is experimentally demonstrated and supported by density functional calculations.

 

 

 

 


Electronic Properties of Optically Switchable Photochromic Diarylethene Molecules at the Interface with Organic Semiconductors

Q. Wang, J. Frisch, M. Herder, S. Hecht, N. Koch

ChemPhysChem, 2017, 18, 722–727. Link to article. COVER PAGE. Cover Profile. Selected as a Very Important Paper.

Light-switching-induced changes in the electronic properties of photochromic diarylethene, i.e., 1,2-bis(2-methyl-5-p-tolylthiophen-3-yl)cyclopent-1-ene (DAE1), thin films at interfaces to a gold electrode and two polymer semiconductors are investigated by direct and inverse photoelectron spectroscopy. The photoisomerization is achieved by in situ irradiation of ultraviolet and visible light. Efficient and reversible switching between the open and closed isomers of DAE1 is evidenced at all interfaces, with profound impact on the energy-level alignment. The frontier occupied level of DAE1 changes by 0.8 eV with respect to the Au Fermi level upon switching. Corresponding sizable changes in the electron and transport level offsets between the two polymers and DAE1 in its open and closed form are determined. This gives rise to fundamentally different functionality of these interfaces in terms of charge transport. Our study proves the viability of light-controlled energy-level manipulation at various interfaces in photoswitchable opto-electronic devices.


Poly(3-hexylthiophene) Nanoparticles Containing Thiophene-S,S-dioxide: Tuning of Dimensions, Optical and Redox Properties, and Charge Separation under Illumination

F. Di Maria, A. Zanelli, A. Liscio, A. Kovtun, E. Salatelli, R. Mazzaro, V. Morandi, G. Bergamini, A. Shaffer, S. Rozen

ACS Nano, 2017, 11, 1991–1999. Link to article.

We describe the preparation of poly(3-hexylthiophene-S,S-dioxide) nanoparticles using Rozen’s reagent, HOF·CH3CN, either on poly(3-hexylthiophene) (P3HT) or on preformed P3HT nanoparticles (P3HT-NPs). In the latter case, core–shell nanoparticles (P3HT@PTDO-NPs) are formed, as confirmed by X-ray photoelectron spectroscopy measurements, indicating the presence of oxygen on the outer shell. The different preparation modalities lead to a fine-tuning of the chemical–physical properties of the nanoparticles. We show that absorption and photoluminescence features, electrochemical properties, size, and stability of colloidal solutions can be finely modulated by controlling the amount of oxygen present. Atomic force microscopy measurements on the nanoparticles obtained by a nanoprecipitation method from preoxidized P3HT (PTDO-NPs) display spherical morphology and dimensions down to 5 nm. Finally, Kelvin probe measurements show that the coexistence of p- and n-type charge carriers in all types of oxygenated nanoparticles makes them capable of generating and separating charge under illumination. Furthermore, in core–shell nanoparticles, the nanosegregation of the two materials, in different regions of the nanoparticles, allows a more efficient charge separation.


Evolution of the size and shape of 2D nanosheets during ultrasonic fragmentation

A. Liscio, K. Kouroupis-Agalou, X. Diez Betriu, A. Kovtun, E. Treossi, N. M. Pugno, G. De Luca, L. Giorgini, V. Palermo

2D Mater., 2017, 4, 025017. Link to article.

2-dimensional (2D) nanosheets such as graphene, graphene oxide, boron nitride or transition metal dichalcogenides can be produced on a large scale by exfoliation techniques. The lateral shape of these 2D materials is typically considered random and irregular, and their average size is often estimated using techniques characterized by strong approximations or poor statistical significance. Here we measure in a quantitative, objective way the size and shape of 2D monoatomic nanosheets using a combination of optical, electronic and scanning probe techniques. We measure, one by one, the size and shape of thousands of sheets of graphene oxide as they undergo a standard ultrasonication treatment. Using automatic image processing and statistical modelling we identify two different fragmentation processes in 2D at the nanoscale, related to two populations of nanosheets described by gamma and exponential size distributions respectively. The two populations of sheets coexist during the fragmentation process, each one retaining its average size and shape. Our results explain the size reduction commonly observed in nanosheets upon sonication as an effect of changes in the respective weights of the two populations of nanosheets present in the material.


High, Anisotropic, and Substrate-Independent Mobility in Polymer Field-Effect Transistors Based on Preassembled Semiconducting Nanofibrils

S. Bonacchi, M. Gobbi, L. Ferlauto, M.-A. Stoeckel, F. Liscio, S. Milita, E. Orgiu, P. Samorì

ACS Nano, 2017, 11, 2000–2007. Link to article and accepted manuscript (open access). CNRS, CNRS Alsace, FRC and SCF press releases.

Achieving nanoscale control over the crystalline structure and morphology of electroactive polymer films and the possibility to transfer them onto any solid substrate are important tasks for the fabrication of high-performance organic/polymeric field-effect transistors (FETs). In this work, we demonstrate that ultrathin active layers preassembled at the water/air interface can possess high, anisotropic, and substrate-independent mobility in polymer FETs. By exploiting a modified approach to the Langmuir–Schaeffer technique, we self-assemble conjugated polymers in fibrillar structures possessing controlled thickness, nanoscale structure, and morphology; these highly ordered nanofibrils can be transferred unaltered onto any arbitrary substrate. We show that FETs based on these films possess high and anisotropic hole mobility approaching 1 cm2 V–1 s–1 along the nanofibrils, being over 1 order of magnitude beyond the state-of-the-art for Langmuir–Schaefer polymer FETs. Significantly, we demonstrate that the FET performances are independent of the chemical nature and dielectric permittivity of the substrate, overcoming a critical limit in the field of polymer FETs. Our method allows the fabrication of ultrathin films for low-cost, high-performance, transparent, and flexible devices supported on any dielectric substrate.


Fast-Response Photonic Device Based on Organic-Crystal Heterojunctions Assembled into a Vertical-Yet-Open Asymmetric Architecture

L. Zhang, E. Pavlica, X. Zhong, F. Liscio, S. Li, G. Bratina, E. Orgiu, P. Samorì

Adv. Mater., 2017, 29, 1605760. Link to article and accepted manuscript (open access).

Crystalline dioctyl-3,4,9,10-perylenedicarboximide nanowires and 6,13-bis(triisopropylsilylethynyl) pentacene microplates are integrated into a vertical-yet-open asymmetrical heterojunction for the realization of a high-performance organic photovoltaic detector, which shows fast photoresponse, ultrahigh signal-to-noise ratio, and high sensitivity to weak light.

 

 

 

 

 

 

 


A four-state capacitance molecular switch based on a redox active tetrathiafulvalene self-assembled monolayer

E. Marchante, M. S. Maglione, N. Crivillers, C. Rovira, M. Mas-Torrent

RSC Adv., 2017, 7, 5636–5641. Link to article (open access).

An electroactive tetrathiafulvalene (TTF) self-assembled monolayer (SAM) on gold has been prepared and fully characterised by electrochemical impedance spectroscopy. Transfer rates of the same order were found for the two redox processes. Remarkably, the TTF SAM was successfully exploited as a 4-state electrochemical switch using the capacitance of the SAM as output signal.

 

 

 


Light-Controlled Reversible Modulation of Frontier Molecular Orbital Energy Levels in Trifluoromethylated Diarylethenes

M. Herder, F. Eisenreich, A. Bonasera, A. Grafl, L. Grubert, M. Pätzel, J. Schwarz, S. Hecht

Chem. Eur. J., 2017, 23, 3743–3754. Link to article and accepted manuscript (open access). Selected as a Hot Paper.

Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their π-electronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/reduction potentials. The electronic modulation can be utilized to remote-control charge- as well as energy-transfer processes and it can be transduced to functional entities adjacent to the DAE core, thereby regulating their properties. In order to exploit such photoswitchable systems it is important to precisely adjust the absolute position of their HOMO and LUMO levels and to maximize the extent of the photoinduced shifts of these energy levels. Here, we present a comprehensive study detailing how variation of the substitution pattern of DAE compounds, in particular using strongly electron-accepting and chemically stable trifluoromethyl groups either in the periphery or at the reactive carbon atoms, allows for the precise tuning of frontier molecular orbital levels over a broad energy range and the generation of photoinduced shifts of more than 1 eV. Furthermore, the effect of different DAE architectures on the transduction of these shifts to an adjacent functional group is discussed. Whereas substitution in the periphery of the DAE motif has only minor implications on the photochemistry, trifluoromethylation at the reactive carbon atoms strongly disturbs the isomerization efficiency. However, this can be overcome by using a nonsymmetrical substitution pattern or by combination with donor groups, rendering the resulting photoswitches attractive candidates for the construction of remote-controlled functional systems.


Facile Morphology-Controlled Synthesis of Organolead Iodide Perovskite Nanocrystals Using Binary Capping Agents

E. Debroye, H. Yuan, E. Bladt, W. Baekelant, M. Van der Auweraer, J. Hofkens, S. Bals, M. B. J. Roeffaers

ChemNanoMat, 2017, 3, 223–227. Link to article (open access).

Controlling the morphology of organolead halide perovskite crystals is crucial to a fundamental understanding of the materials and to tune their properties for device applications. Here, we report a facile solution-based method for morphology-controlled synthesis of rod-like and plate-like organolead halide perovskite nanocrystals using binary capping agents. The morphology control is likely due to an interplay between surface binding kinetics of the two capping agents at different crystal facets. By high-resolution scanning transmission electron microscopy, we show that the obtained nanocrystals are monocrystalline. Moreover, long photoluminescence decay times of the nanocrystals indicate long charge diffusion lengths and low trap/defect densities. Our results pave the way for large-scale solution synthesis of organolead halide perovskite nanocrystals with controlled morphology for future device applications.

 

 


Light-Activated Sensitive Probes for Amine Detection

V. Valderrey, A. Bonasera, S. Fredrich, S. Hecht

Angew. Chem. Int. Ed.2017, 56, 1914–1918. Link to article and accepted manuscript (open access).

Our new, simple, and accurate colorimetric method is based on diarylethenes (DAEs) for the rapid detection of a wide range of primary and secondary amines. The probes consist of aldehyde- or ketone-substituted diarylethenes, which undergo an amine-induced decoloration reaction, selectively to give the ring-closed isomer. Thus, these probes can be activated at the desired moment by light irradiation, with a sensitivity that allows the detection of amines at concentrations as low as 10−6 M in solution. In addition, the practical immobilization of DAEs on paper makes it possible to detect biogenic amines, such as cadaverine, in the gas phase above a threshold of 12 ppbv within 30 seconds.

 


Covalent Modification of Highly Ordered Pyrolytic Graphite with a Stable Organic Free Radical by Using Diazonium Chemistry

G. Seber, A. V. Rudnev, A. Droghetti, I. Rungger, J. Veciana, M. Mas-Torrent, C. Rovira, N. Crivillers

Chem. Eur. J., 201723, 1415–1421. Link to article and accepted manuscript (open access). ICMAB-CSIC press release.

A novel, persistent, electrochemically active perchlorinated triphenylmethyl (PTM) radical with a diazonium functionality has been covalently attached to highly ordered pyrolytic graphite (HOPG) by electrografting in a single-step process. Electrochemical scanning tunneling microscopy (EC-STM) and Raman spectroscopy measurements revealed that PTM molecules had a higher tendency to covalently react at the HOPG step edges. The cross-section profiles from EC-STM images showed that there was current enhancement at the functionalized areas, which could be explained by redox-mediated electron tunneling through surface-confined redox-active molecules. Cyclic voltammetry clearly demonstrated that the intrinsic properties of the organic radical were preserved upon grafting and DFT calculations also revealed that the magnetic character of the PTM radical was preserved.


TTF–PTM dyads: from switched molecular self assembly in solution to radical conductors in solid state

M. Souto, C. Rovira, I. Ratera, J. Veciana

CrystEngComm, 2017, 19, 197–206. Link to article and accepted manuscript (open access). COVER PAGEICMAB-CSIC and NANOMOL press releases.

Organic donor–acceptor (D–A) systems formed by the electron-donor tetrathiafulvalene (TTF) linked to the electron-acceptor perchlorotriphenylmethyl (PTM) radical through different π-conjugated bridges exhibit interesting physical properties such as bistability in solution or conductivity in solid state. Understanding the interplay between intra- and intermolecular charge transfer processes in solution is of high interest in order to rationalize the self-assembling ability and conducting properties of such dyads in solid state. In this Highlight we examine the self-assembling properties of different TTF–π–PTM radical dyads that have potential applications as molecular switches or conductors in the field of molecular electronics.


Field-Controlled Charge Separation in a Conductive Matrix at the Single-Molecule Level: Toward Controlling Single-Molecule Fluorescence Intermittency

K Kennes, P. Dedecker, J. A. Hutchison, E. Fron, H. Uji-i, J. Hofkens, M. Van der Auweraer

ACS Omega, 2016, 1, 1383–1392. Link to article (open access).

The fluorescence intermittency or “blinking” of single molecules of ATTO647N (ATTO) in the conductive matrix polyvinylcarbazole (PVK) is described in the presence of an external applied electric field. It is shown that due to the energy distribution of the highest occupied molecular orbital (HOMO) level of PVK, which is energetically close to the HOMO of ATTO, sporadic electron transfer occurs. As a result, the on/off dynamics of blinking can be influenced by the electric field. This field will, depending on the respective position and orientation of the dye/polymer system with respect to those of the electrodes, either enhance or suppress electron transfer from PVK to ATTO as well as the back electron transfer from reduced ATTO to PVK. After the charge-transfer step, the applied field will pull the hole in PVK away from the dye, increasing the overall time the dye resides in a dark state.


Synthesis and Characterization of Ethylenedithio-MPTTF-PTM Radical Dyad as a Potential Neutral Radical Conductor

M. Souto, D. Bendixen, M. Jensen, V. Díez-Cabanes, J. Cornil, J. O. Jeppesen, I. Ratera, C. Rovira, J. Veciana

Magnetochemistry, 2016, 2, 46. Link to article (open access).

During the last years there has been a high interest in the development of new purely-organic single-component conductors. Very recently, we have reported a new neutral radical conductor based on the perchlorotriphenylmethyl (PTM) radical moiety linked to a monopyrrolo-tetrathiafulvalene (MPTTF) unit by a π-conjugated bridge (1) that behaves as a semiconductor under high pressure. With the aim of developing a new material with improved conducting properties, we have designed and synthesized the radical dyad 2 which was functionalized with an ethylenedithio (EDT) group in order to improve the intermolecular interactions of the tetrathiafulvalene (TTF) subunits. The physical properties of the new radical dyad 2 were studied in detail in solution to further analyze its electronic structure.


Understanding the Influence of the Electronic Structure on the Crystal Structure of a TTF-PTM Radical Dyad

S. Vela, M. Souto, I. Ratera, C. Rovira, J. Veciana

J. Phys. Chem. A, 2016120, 10297–10303. Link to article and accepted manuscript (open access).

The understanding of the crystal structure of organic compounds, and its relationship to their physical properties, have become essential to design new advanced molecular materials. In this context, we present a computational study devoted to rationalize the different crystal packing displayed by two closely related organic systems based on the TTF-PTM dyad (TTF = tetrathiafulvalene, PTM = polychlorotriphenylmethane) with almost the same molecular structure but a different electronic one. The radical species (1), with an enhanced electronic donor–acceptor character, exhibits a herringbone packing, whereas the nonradical protonated analogue (2) is organized forming dimers. The stability of the possible polymorphs is analyzed in terms of the cohesion energy of the unit cell, intermolecular interactions between pairs, and molecular flexibility of the dyad molecules. It is observed that the higher electron delocalization in radical compound 1 has a direct influence on the geometry of the molecule, which seems to dictate its preferential crystal structure.


Modifying the Size of Ultrasound-Induced Liquid-Phase Exfoliated Graphene: from Nanosheets to Nanodots

A. Ciesielski, S. Haar, A. Aliprandi, M. El Garah, G. Tregnago, G. F. Cotella, M. El Gemayel, F. Richard, H. Sun, F. Cacialli, F. Bonaccorso, P. Samorì

ACS Nano, 201610, 10768–10777. Link to article and accepted manuscript (open access).

Ultrasound-induced liquid-phase exfoliation (UILPE) is an established method to produce single- (SLG) and few-layer (FLG) graphene nanosheets starting from graphite as a precursor. In this paper we investigate the effect of the ultrasonication power in the UILPE process carried out in either N-methyl-2-pyrrolidone (NMP) or ortho-dichlorobenzene (o-DCB). Our experimental results reveal that while the SLGs/FLGs concentration of the NMP dispersions is independent of the power of the ultrasonic bath during the UILPE process, in o-DCB it decreases as the ultrasonication power increases. Moreover, the ultrasonication power has a strong influence on the lateral size of the exfoliated SLGs/FLGs nanosheets in o-DCB. In particular, when UILPE is carried out at ∼600 W, we obtain dispersions composed of graphene nanosheets with a lateral size of 180 nm, whereas at higher power (∼1000 W) we produce graphene nanodots (GNDs) with an average diameter of ∼17 nm. The latter nanostructures exhibit a strong and almost excitation-independent photoluminescence emission in the UV/deep-blue region of the electromagnetic spectrum arising from the GNDs’ intrinsic states and a less intense (and strongly excitation wavelength dependent) emission in the green/red region attributed to defect states. Notably, we also observe visible emission with near-infrared excitation at 850 and 900 nm, a fingerprint of the presence of up-conversion processes. Overall, our results highlight the crucial importance of the solvent choice for the UILPE process, which under controlled experimental conditions allows the fine-tuning of the morphological properties, such as lateral size and thickness, of the graphene nanosheets toward the realization of luminescent GNDs.


A redox-active radical as an effective nanoelectronic component: stability and electrochemical tunnelling spectroscopy in ionic liquids

A. V. Rudnev, C. Franco, N. Crivillers, G. Seber, A. Droghetti, I. Rungger, I. V. Pobelov, J. Veciana, M. Mas-Torrent, C. Rovira

Phys. Chem. Chem. Phys., 2016, 18, 27733–27737. Link to article (open access).

A redox-active persistent perchlorotriphenylmethyl (PTM) radical chemically linked to gold exhibits stable electrochemical activity in ionic liquids. Electrochemical tunnelling spectroscopy in this medium demonstrates that the PTM radical shows a highly effective redox-mediated current enhancement, demonstrating its applicability as an active nanometer-scale electronic component.

 

 


Photoluminescence Blinking of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods Induced by Surface Traps

H. Yuan, E. Debroye, G. Caliandro, K. P. F. Janssen, J. van Loon, C. E. A. Kirschhock, J. A. Martens, J. Hofkens, M. B. J. Roeffaers

ACS Omega, 2016, 1, 148–159. Link to article (open access).

Photoluminescence (PL) of organometal halide perovskite materials reflects the charge dynamics inside of the material and thus contains important information for understanding the electro-optical properties of the material. Interpretation of PL blinking of methylammonium lead iodide (MAPbI3) nanostructures observed on polycrystalline samples remains puzzling owing to their intrinsic disordered nature. Here, we report a novel method for the synthesis of high-quality single-crystal MAPbI3 nanorods and demonstrate a single-crystal study on MAPbI3 PL blinking. At low excitation power densities, two-state blinking was found on individual nanorods with dimensions of several hundred nanometers. A super-resolution localization study on the blinking of individual nanorods showed that single crystals of several hundred nanometers emit and blink as a whole, without showing changes in the localization center over the crystal. Moreover, both the blinking ON and OFF times showed power-law distributions, indicating trapping–detrapping processes. This is further supported by the PL decay times of the individual nanorods, which were found to correlate with the ON/OFF states. Furthermore, a strong environmental dependence of the nanorod PL blinking was revealed by comparing the measurements in vacuum, nitrogen, and air, implying that traps locate close to crystal surfaces. We explain our observations by proposing surface charge traps that are likely related to under-coordinated lead ions and methylammonium vacancies to result in the PL blinking observed here.


Chemical control over the energy-level alignment in a two-terminal junction

L. Yuan, C. Franco, N. Crivillers, M. Mas-Torrent, L. Cao, C. S. S. Sangeeth, C. Rovira, J. Veciana, C. A. Nijhuis

Nat. Commun., 20167, 12066. Link to article (open access). ICMAB-CSIC and NANOMOL press releases.

The energy-level alignment of molecular transistors can be controlled by external gating to move molecular orbitals with respect to the Fermi levels of the source and drain electrodes. Two-terminal molecular tunnelling junctions, however, lack a gate electrode and suffer from Fermi-level pinning, making it difficult to control the energy-level alignment of the system. Here we report an enhancement of 2 orders of magnitude of the tunnelling current in a two-terminal junction via chemical molecular orbital control, changing chemically the molecular component between a stable radical and its non-radical form without altering the supramolecular structure of the junction. Our findings demonstrate that the energy-level alignment in self-assembled monolayer-based junctions can be regulated by purely chemical modifications, which seems an attractive alternative to control the electrical properties of two-terminal junctions.

 

 


Hexa-peri-hexabenzocoronene with Different Acceptor Units for Tuning Optoelectronic Properties

A. Keerthi, I. C.-Y. Hou, T. Marszalek, W. Pisula, M. Baumgarten, A. Narita

Chem. Asian J., 2016, 11, 2710–2714. Link to article and accepted manuscript (open access). Featured in Chemistry – An Asian Journal's top 15 most accessed articles in August 2016.

Hexa-peri-hexabenzocoronene (HBC)-based donor–acceptor dyads were synthesized with three different acceptor units, through two pathways: 1) “pre-functionalization” of monobromo-substituted hexaphenylbenzene prior to the cyclodehydrogenation; and 2) “post-functionalization” of monobromo-substituted HBC after the cyclodehydrogenation. The HBC–acceptor dyads demonstrated varying degrees of intramolecular charge-transfer interactions, depending on the attached acceptor units, which allowed tuning of their photophysical and optoelectronic properties, including the energy gaps. The two synthetic pathways described here can be complementary and potentially be applied for the synthesis of nanographene–acceptor dyads with larger aromatic cores, including one-dimensionally extended graphene nanoribbons.


A nanomesh scaffold for supramolecular nanowire optoelectronic devices

L. Zhang, X. Zhong, E. Pavlica, S. Li, A. Klekachev, G. Bratina, T. W. Ebbesen, E. Orgiu, P. Samorì

Nat. Nanotech., 2016, 11, 900–906. Link to article and accepted manuscript (open access). CNRS, CNRS Alsace, SCF, ISISFRC, LabEx CSCChemistryViews and IEEE Spectrum press releases. Highlighted in Nature NanotechnologyScience, L’Actualité Chimique and CNRS Alsace Activity Report 2016. Featured in Nature Nanotechnology's top 50 most read articles.

Supramolecular organic nanowires are ideal nanostructures for optoelectronics because they exhibit both efficient exciton generation as a result of their high absorption coefficient and remarkable light sensitivity due to the low number of grain boundaries and high surface-to-volume ratio. To harvest photocurrent directly from supramolecular nanowires it is necessary to wire them up with nanoelectrodes that possess different work functions. However, devising strategies that can connect multiple nanowires at the same time has been challenging. Here, we report a general approach to simultaneously integrate hundreds of supramolecular nanowires of N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) in a hexagonal nanomesh scaffold with asymmetric nanoelectrodes. Optimized PTCDI-C8 nanowire photovoltaic devices exhibit a signal-to-noise ratio approaching 107, a photoresponse time as fast as 10 ns and an external quantum efficiency >55%. This nanomesh scaffold can also be used to investigate the fundamental mechanism of photoelectrical conversion in other low-dimensional semiconducting nanostructures.


Flexible non-volatile optical memory thin-film transistor device with over 256 distinct levels based on an organic bicomponent blend

T. Leydecker, M. Herder, E. Pavlica, G. Bratina, S. Hecht, E. Orgiu, P. Samorì

Nat. Nanotech., 2016, 11, 769–775. Link to article and accepted manuscript (open access). CNRS, CNRS Alsace, SCFISIS, FRCIRIS AdlershofNanowerkResearch in GermanyChemistryViewsIEEE Spectrum and Alsace.info press releases. Highlighted in Le FigaroNature Photonics, C&EN, CNRS Activity Report 2016 (available in English and French) and CNRS Alsace Activity Report 2016. Featured in Nature Nanotechnology's top 50 most read articles.

Organic nanomaterials are attracting a great deal of interest for use in flexible electronic applications such as logic circuits, displays and solar cells. These technologies have already demonstrated good performances, but flexible organic memories are yet to deliver on all their promise in terms of volatility, operational voltage, write/erase speed, as well as the number of distinct attainable levels. Here, we report a multilevel non-volatile flexible optical memory thin-film transistor based on a blend of a reference polymer semiconductor, namely poly(3-hexylthiophene), and a photochromic diarylethene, switched with ultraviolet and green light irradiation. A three-terminal device featuring over 256 (8 bit storage) distinct current levels was fabricated, the memory states of which could be switched with 3 ns laser pulses. We also report robustness over 70 write–erase cycles and non-volatility exceeding 500 days. The device was implemented on a flexible polyethylene terephthalate substrate, validating the concept for integration into wearable electronics and smart nanodevices.


Electrochemical Functionalization of Graphene at the Nanoscale with Self-Assembling Diazonium Salts

Z. Xia, F. Leonardi, M. Gobbi, Y. Liu, V. Bellani, A. Liscio, A. Kovtun, R. Li, X. Feng, E. Orgiu, P. Samorì, E. Treossi, V. Palermo

ACS Nano, 201610, 7125–7134. Link to article and accepted manuscript (open access).

We describe a fast and versatile method to functionalize high-quality graphene with organic molecules by exploiting the synergistic effect of supramolecular and covalent chemistry. With this goal, we designed and synthesized molecules comprising a long aliphatic chain and an aryl diazonium salt. Thanks to the long chain, these molecules physisorb from solution onto CVD graphene or bulk graphite, self-assembling in an ordered monolayer. The sample is successively transferred into an aqueous electrolyte, to block any reorganization or desorption of the monolayer. An electrochemical impulse is used to transform the diazonium group into a radical capable of grafting covalently to the substrate and transforming the physisorption into a covalent chemisorption. During covalent grafting in water, the molecules retain the ordered packing formed upon self-assembly. Our two-step approach is characterized by the independent control over the processes of immobilization of molecules on the substrate and their covalent tethering, enabling fast (t < 10 s) covalent functionalization of graphene. This strategy is highly versatile and works with many carbon-based materials including graphene deposited on silicon, plastic, and quartz as well as highly oriented pyrolytic graphite.


Pressure-Induced Conductivity in a Neutral Nonplanar Spin-Localized Radical

M. Souto, H. Cui, M. Peña-Álvarez, V. G. Baonza, H. O. Jeschke, M. Tomic, R. Valentí, D. Blasi, I. Ratera, C. Rovira, J. Veciana

J. Am. Chem. Soc., 2016, 138, 11517–11525. Link to article and accepted manuscript (open access). ICMAB-CSIC and NANOMOL press releases.

There is a growing interest in the development of single-component molecular conductors based on neutral organic radicals that are mainly formed by delocalized planar radicals, such as phenalenyl or thiazolyl radicals. However, there are no examples of systems based on nonplanar and spin-localized C-centered radicals exhibiting electrical conductivity due to their large Coulomb energy (U) repulsion and narrow electronic bandwidth (W) that give rise to a Mott insulator behavior. Here we present a new type of nonplanar neutral radical conductor attained by linking a tetrathiafulvalene (TTF) donor unit to a neutral polychlorotriphenylmethyl radical (PTM) with the important feature that the TTF unit enhances the overlap between the radical molecules as a consequence of short intermolecular S···S interactions. This system becomes semiconducting upon the application of high pressure thanks to increased electronic bandwidth and charge reorganization opening the way to develop a new family of neutral radical conductors.


Tuning the energetics and tailoring the optical properties of silver clusters confined in zeolites

O. Fenwick, E. Coutiño-Gonzalez, D. Grandjean, W. Baekelant, F. Richard, S. Bonacchi, D. De Vos, P. Lievens, M. Roeffaers, J. Hofkens, P. Samorì

Nat. Mater., 2016, 15, 1017–1022. Link to article and accepted manuscript (open access). CNRS, CNRS Alsace, SCFISIS, FRCQMUL, Phys.orgNANOR&DECN, AlphaGalileo, EurekAlert!NanowerkScienceDailyMaterials Today, KU Leuven, ECNEurekAlert!, NanowerkScienceDaily, Electronics WeeklyAzoOptics, EC and Madri+d press releases. Highlighted in Nature Materials. Featured in Nature Materials' top 50 most read articles.

The integration of metal atoms and clusters in well-defined dielectric cavities is a powerful strategy to impart new properties to them that depend on the size and geometry of the confined space as well as on metal–host electrostatic interactions. Here, we unravel the dependence of the electronic properties of metal clusters on space confinement by studying the ionization potential of silver clusters embedded in four different zeolite environments over a range of silver concentrations. Extensive characterization reveals a strong influence of silver loading and host environment on the cluster ionization potential, which is also correlated to the clusters optical and structural properties. Through fine-tuning of the zeolite host environment, we demonstrate photoluminescence quantum yields approaching unity. This work extends our understanding of structure–property relationships of small metal clusters and applies this understanding to develop highly photoluminescent materials with potential applications in optoelectronics and bioimaging.


Light-Modulation of the Charge Injection in a Polymer Thin-Film Transistor by Functionalizing the Electrodes with Bistable Photochromic Self-Assembled Monolayers

T. Mosciatti, M. G. del Rosso, M. Herder, J. Frisch, N. Koch, S. Hecht, E. Orgiu, P. Samorì

Adv. Mater., 201628, 6606–6611. Link to article and accepted manuscript (open access).

High fatigue resistance, bistability, and drastic property changes among isomers allow efficient modulation of the current output of organic thin-film transistors (OTFTs) to be obtained by a photogating of the charge-injection mechanism.


Direct Patterning of Organic Functional Polymers through Conventional Photolithography and Noninvasive Cross-Link Agents

M. A. Squillaci, F. Qiu, A. Aliprandi, F. Zhang, X. Feng, P. Samorì

Adv. Mater., 2016, 28, 5249–5254. Link to article and accepted manuscript (open access).

A new technique for direct patterning of functional organic polymers using commercial photolithography setups with a minimal loss of the materials' performances is reported. This result is achieved through novel cross-link agents made by boron- and fluorine-containing heterocycles that can react between themselves upon UV- and white-light exposure.

 

 

 

 

 

 

 


Design, synthesis, chemical stability, packing, cyclic voltammetry, ionisation potential, and charge transport of [1]benzothieno[3,2-b][1]benzothiophene derivatives

C. Ruzié, J. Karpinska, A. Laurent, L. Sanguinet, S. Hunter, T. D. Anthopoulos, V. Lemaur, J. Cornil, A. R. Kennedy, O. Fenwick, P. Samorì, G. Schweicher, B. Chattopadhyay, Y. H. Geerts

J. Mater. Chem. C, 2016, 4, 4863–4879. Link to article and accepted manuscript (open access). Selected as a Hot Paper.

Five new molecular semiconductors that differ from dioctylbenzothienobenzothiophene, by the introduction of ether or thioether side chains, have been synthesized and obtained in good yields. Their availability in sufficient quantities has allowed investigation of their electrochemical behaviour in solution and their electronic properties in solid state. Both ether and thioether compounds oxidise rather easily in solution, but nevertheless, they exhibit rather high ionisation potentials. This is a consequence of their crystal structure. Dioctylthioetherbenzothienobenzothiophene is rather sensitive to oxidation and degrades easily in close to ambient conditions. Dioctyletherbenzothienobenzothiophene is more stable. Its charge carrier mobility remains however rather moderate, on the order of 0.5 cm2 V−1 s−1, whereas that of dioctylbenzothienobenzothiophene reached 4 cm2 V−1 s−1, in the same conditions. The difference is explained by intrinsic factors as shown by a theoretical modelling. 


A surface confined yttrium(III) bis-phthalocyaninato complex: a colourful switch controlled by electrons

I. Alcón, M. Gonidec, M. R. Ajayakumar, M. Mas-Torrent, J. Veciana

Chem. Sci., 20167, 4940–4944. Link to article (open access). ICMAB-CSIC press release.

SAMs of a Y(III) double-decker complex on ITO have been prepared and their electrical and optical properties explored, exhibiting three accessible stable redox states with characteristic absorption bands in the visible spectra, corresponding to three complementary colors (i.e., green, blue and red). These absorption bands are exploited as output signals of this robust ternary electrochemical switch, behaving hence as an electrochromic molecular-based device.


Donor/Acceptor Mixed Self-Assembled Monolayers for Realising a Multi-Redox-State Surface

J. Casado-Montenegro, E. Marchante, N. Crivillers, C. Rovira, M. Mas-Torrent

ChemPhysChem, 2016, 17, 1810–1814. Link to article and accepted manuscript (open access).

Mixed molecular self-assembled monolayers (SAMs) on gold, based on two types of electroactive molecules, that is, electron-donor (ferrocene) and electron-acceptor (anthraquinone) molecules, are prepared as an approach to realise surfaces exhibiting multiple accessible redox states. The SAMs are investigated in different electrolyte media. The nature of these media has a strong impact on the types of redox processes that take place and on the redox potentials. Under optimised conditions, surfaces with three redox states are achieved. Such states are accessible in a relatively narrow potential window in which the SAMs on gold are stable. This communication elucidates the key challenges in fabricating bicomponent SAMs as electrochemical switches.


Coupling carbon nanomaterials with photochromic molecules for the generation of optically responsive materials

X. Zhang, L. Hou, P. Samorì

Nat. Commun., 2016, 7, 11118. Link to article (open access). CNRSCNRS Alsace, SCFFRC and LabEx CSC press releases. Highlighted in the CNRS Alsace Activity Report 2016.

Multifunctional carbon-based nanomaterials offer routes towards the realization of smart and high-performing (opto)electronic (nano)devices, sensors and logic gates. Meanwhile photochromic molecules exhibit reversible transformation between two forms, induced by the absorption of electromagnetic radiation. By combining carbon-based nanomaterials with photochromic molecules, one can achieve reversible changes in geometrical structure, electronic properties and nanoscale mechanics triggering by light. This thus enables a reversible modulation of numerous physical and chemical properties of the carbon-based nanomaterials towards the fabrication of cognitive devices. This review examines the state of the art with respect to these responsive materials, and seeks to identify future directions for investigation.

 

 

 


High-Performance Phototransistors Based on PDIF-CN2 Solution-Processed Single Fiber and Multifiber Assembly

W. Rekab, M.-A. Stoeckel, M. El Gemayel, M. Gobbi, E. Orgiu, P. Samorì

ACS Appl. Mater. Interfaces, 20168, 9829–9838. Link to article and accepted manuscript (open access).

Here we describe the fabrication of organic phototransistors based on either single or multifibers integrated in three-terminal devices. These self-assembled fibers have been produced by solvent-induced precipitation of an air stable and solution-processable perylene di-imide derivative, i.e., PDIF-CN2. The optoelectronic properties of these devices were compared to devices incorporating more disordered spin-coated PDIF-CN2 thin-films. The single-fiber devices revealed significantly higher field-effect mobilities, compared to multifiber and thin-films, exceeding 2 cm2 V–1 s–1. Such an efficient charge transport is the result of strong intermolecular coupling between closely packed PDIF-CN2 molecules and of a low density of structural defects. The improved crystallinity allows efficient collection of photogenerated Frenkel excitons, which results in the highest reported responsivity (R) for single-fiber PDI-based phototransistors, and photosensitivity (P) exceeding 2 × 103 AW–1, and 5 × 103, respectively. These findings provide unambiguous evidence for the key role played by the high degree of order at the supramolecular level to leverage the material’s properties toward the fabrication of light-sensitive organic field-effect transistors combining a good operational stability, high responsivity and photosensitivity. Our results show also that the air-stability performances are superior in devices where highly crystalline supramolecularly engineered architectures serve as the active layer.


Adding Four Extra K-Regions to Hexa-peri-hexabenzocoronene

T. Dumslaff, B. Yang, A. Maghsoumi, G. Velpula, K. S. Mali, C. Castiglioni, S. De Feyter, M. Tommasini, A. Narita, X. Feng, K. Müllen

J. Am. Chem. Soc., 2016138, 4726–4729. Link to article and accepted manuscript (open access).

A multistep synthesis of hexa-peri-hexabenzocoronene (HBC) with four additional K-regions was developed through a precursor based on two benzotetraphene units bridged with p-phenylene, featuring preinstalled zigzag moieties. Characterization by laser desorption/ionization time-of-flight mass spectrometry, Raman and IR spectroscopy, and scanning tunneling microscopy unambiguously validated the successful formation of this novel zigzag edge-rich HBC derivative. STM imaging of its monolayers revealed large-area, defect-free adlayers. The optical properties of the modified HBC were investigated by UV/visible absorption spectroscopy.


Optical Input/Electrical Output Memory Elements based on a Liquid Crystalline Azobenzene Polymer

T. Mosciatti, S. Bonacchi, M. Gobbi, L. Ferlauto, F. Liscio, L. Giorgini, E. Orgiu, P. Samorì

ACS Appl. Mater. Interfaces, 20168, 6563–6569. Link to article and accepted manuscript (open access).

Responsive polymer materials can change their properties when subjected to external stimuli. In this work, thin films of thermotropic poly(metha)acrylate/azobenzene polymers are explored as active layer in light-programmable, electrically readable memories. The memory effect is based on the reversible modifications of the film morphology induced by the photoisomerization of azobenzene mesogenic groups. When the film is in the liquid crystalline phase, the transcis isomerization induces a major surface reorganization on the mesoscopic scale that is characterized by a reduction in the effective thickness of the film. The film conductivity is measured in vertical two-terminal devices in which the polymer is sandwiched between a Au contact and a liquid compliant E-GaIn drop. We demonstrate that the transcis isomerization is accompanied by a reversible 100-fold change in the film conductance. In this way, the device can be set in a high- or low-resistance state by light irradiation at different wavelengths. This result paves the way toward the potential use of poly(metha)acrylate/azobenzene polymer films as active layer for optical input/electrical output memory elements.


Exchange Coupling Inversion in a High-Spin Organic Triradical Molecule

R. Gaudenzi, E. Burzurí, D. Reta, I. de P. R. Moreira, S. T. Bromley, C. Rovira, J. Veciana, H. S. J. van der Zant

Nano Lett., 2016, 16, 2066–2071. Link to article and accepted manuscript (open access). ICMAB-CSIC and NANOMOL press releases.

The magnetic properties of a nanoscale system are inextricably linked to its local environment. In adatoms on surfaces and inorganic layered structures, the exchange interactions result from the relative lattice positions, layer thicknesses, and other environmental parameters. Here, we report on a sample-dependent sign inversion of the magnetic exchange coupling between the three unpaired spins of an organic triradical molecule embedded in a three-terminal device. This ferro-to-antiferromagnetic transition is due to structural distortions and results in a high-to-low spin ground-state change in a molecule traditionally considered to be a robust high-spin quartet. Moreover, the flexibility of the molecule yields an in situ electric tunability of the exchange coupling via the gate electrode. These findings open a route to the controlled reversal of the magnetic states in organic molecule-based nanodevices by mechanical means, electrical gating, or chemical tailoring.


Degradation of Methylammonium Lead Iodide Perovskite Structures through Light and Electron Beam Driven Ion Migration

H. Yuan, E. Debroye, K. Janssen, H. Naiki, C. Steuwe, G. Lu, M. Moris, E. Orgiu, H. Uji-i, F. De Schryver, P. Samorì, J. Hofkens, M. Roeffaers

J. Phys. Chem. Lett., 2016, 7, 561–566. Link to article (open access).

Organometal halide perovskites show promising features for cost-effective application in photovoltaics. The material instability remains a major obstacle to broad application because of the poorly understood degradation pathways. Here, we apply simultaneous luminescence and electron microscopy on perovskites for the first time, allowing us to monitor in situ morphology evolution and optical properties upon perovskite degradation. Interestingly, morphology, photoluminescence (PL), and cathodoluminescence of perovskite samples evolve differently upon degradation driven by electron beam (e-beam) or by light. A transversal electric current generated by a scanning electron beam leads to dramatic changes in PL and tunes the energy band gaps continuously alongside film thinning. In contrast, light-induced degradation results in material decomposition to scattered particles and shows little PL spectral shifts. The differences in degradation can be ascribed to different electric currents that drive ion migration. Moreover, solution-processed perovskite cuboids show heterogeneity in stability which is likely related to crystallinity and morphology. Our results reveal the essential role of ion migration in perovskite degradation and provide potential avenues to rationally enhance the stability of perovskite materials by reducing ion migration while improving morphology and crystallinity. It is worth noting that even moderate e-beam currents (86 pA) and acceleration voltages (10 kV) readily induce significant perovskite degradation and alter their optical properties. Therefore, attention has to be paid while characterizing such materials using scanning electron microscopy or transmission electron microscopy techniques.


Influence of the supramolecular order on the electrical properties of 1D coordination polymers based materials

C. Musumeci, S. Osella, L. Ferlauto, D. Niedzialek, L. Grisanti, S. Bonacchi, A. Jouaiti, S. Milita, A. Ciesielski, D. Beljonne, M. W. Hosseini, P. Samorì

Nanoscale, 2016, 8, 2386–2394. Link to article and accepted manuscript (open access).

The generation, under self-assembly conditions, of coordination polymers on surface based combinations of a terpyridine–antracene–pyridine based tecton and Co(II) or Pd(II) cations is primarily governed by the coordination geometry of the metal center (octahedral and square planar respectively). While the octahedral Co(II) based polymer self-assembles in insulating films exhibiting randomly oriented crystalline domains, the planarity of Pd(II) based polymers leads to the formation of conductive π–π stacked fibrillar structures exhibiting anisotropically oriented domains. In the latter case, the favorable Pd–Pd and anthracene–anthracene wavefunction overlaps along the fiber direction are responsible for the large electronic couplings between adjacent chains, whereas small electronic couplings are instead found along individual polymer chains. These results provide important guidelines for the design of conductive metal coordination polymers, highlighting the fundamental role of both intra- as well as inter-chain interactions, thus opening up new perspectives towards their application in functional devices.


Switching Diarylethenes Reliably in Both Directions with Visible Light

S. Fredrich, R. Göstl, M. Herder, L. Grubert, S. Hecht

Angew. Chem. Int. Ed., 201655, 1208–1212. Link to article and accepted manuscript (open access). Selected as a Hot Paper.

A diarylethene photoswitch was covalently connected to two small triplet sensitizer moieties in a conjugated and nonconjugated fashion and the photochromic performance of the resulting compounds was investigated. In comparison with the parent diarylethene (without sensitizers) and one featuring saturated linkages, the conjugated photoswitch offers superior fatigue resistance upon visible-light excitation due to effective triplet energy transfer from the biacetyl termini to the diarylethene core. Our design makes it possible to switch diarylethenes with visible light in both directions in a highly efficient and robust fashion based on extending π-conjugation and by-product-free ring-closure via the triplet manifold.


An Electrically Driven and Readable Molecular Monolayer Switch Based on a Solid Electrolyte

E. Marchante, N. Crivillers, M. Buhl, J. Veciana, M. Mas-Torrent

Angew. Chem. Int. Ed., 2016, 55, 368–372. Link to article and accepted manuscript (open access). Selected as a Hot Paper.

The potential application of molecular switches as active elements in information storage has been demonstrated through numerous works. Importantly, such switching capabilities have also been reported for self-assembled monolayers (SAMs). SAMs of electroactive molecules have recently been exploited as electrochemical switches. Typically, the state of these switches could be read out through their optical and/or magnetic response. These output reading processes are difficult to integrate into devices, and furthermore, there is a need to use liquid environments for switching the redox-active molecular systems. In this work, both of these challenges were overcome by using an ionic gel as the electrolyte medium, which led to an unprecedented solid-state device based on a single molecular layer. Moreover, electrochemical impedance has been successfully exploited as the output of the system.


Au nanoparticle scaffolds modulating intermolecular interactions among the conjugated azobenzenes chemisorbed on curved surfaces: tuning the kinetics of cistrans isomerisation

C. Raimondo, B. Kenens, F. Reinders, M. Mayor, H. Uji-i, P. Samorì

Nanoscale, 2015, 7, 13836–13839. Link to article (open access).

π–π Intermolecular interactions among adjacent conjugated azobenzenes chemisorbed on (non-)flat Au surfaces can be tuned by varying the curvature of the Au nanoparticles. Here we show that such interactions rule the thermal cistrans isomerization kinetics, towards a better control on the azobenzene bistability for its optimal integration as a responsive material.


Self-Assembly of an Amphiphilic π-Conjugated Dyad into Fibers: Ultrafast and Ultrasensitive Humidity Sensor

M. A. Squillaci, L. Ferlauto, Y. Zagranyarski, S. Milita, K. Müllen, P. Samorì

Adv. Mater., 2015, 27, 3170–3174. Link to article and accepted manuscript (open access). COVER PAGE. CNRSCNRS Alsace, SCFFRCLabEx CSC and Conectus Alsace press releases.

The self-assembly of an amphiphilic monomolecular electron acceptor–donor dyad into electroactive π–π stacked fibrillar structures can be triggered by irradiation with visible light. These fibers, exposing hydrophilic ethylene glycol in their external shell, show unique characteristics as resistive humidity sensors that exhibit high sensitivity and ultrafast response.

 

 

 

 

 

 

 


Optically switchable transistors comprising a hybrid photochromic molecule/n-type organic active layer

K. Börjesson, M. Herder, L. Grubert, D. T. Duong, A. Salleo, S. Hecht, E. Orgiu, P. Samorì

J. Mater. Chem. C20153, 4156–4161. Link to article (open access).

Organic semiconductors can be easily combined with other molecular building blocks in order to fabricate multifunctional devices, in which each component conveys a specific (opto)electronic function. We have fabricated photoswitchable hybrid thin-film transistors based on an active bi-component material, consisting of an n-type fullerene derivative and a photochromic diarylethene that possesses light-tunable energy levels. The devices can be gated in two independent ways by either using an electrical stimulus via the application of a voltage to the gate electrode or an optical stimulus causing interconversion of the diarylethene molecules between their two isomers. Fine control over the device output current is achieved by engineering the diarylethenes' LUMO that can act as an intra-gap state controlled by a distinct wavelength in the UV or in the visible range. Importantly, the devices based on a mixed diarylethene/fullerene active layer preserve the high mobility of the pristine semiconductor.


Modulating the charge injection in organic field-effect transistors: fluorinated oligophenyl self-assembled monolayers for high work function electrodes

O. Fenwick, C. Van Dyck, K. Murugavel, D. Cornil, F. Reinders, S. Haar, M. Mayor, J. Cornil, P. Samorì

J. Mater. Chem. C, 20153, 3007–3015. Link to article (open access). COVER PAGE. Selected as a Hot Paper.

The rapid increase in charge carrier mobility in organic field-effect transistors (OFETs) in the past few years, with a number of reports >10 cm2 V−1 s−1, calls for a simultaneous improvement in charge injection at the electrode–semiconductor interface. Chemical modification of the electrodes with self-assembled monolayers (SAMs) allows the optimization of three key properties for lowering the contact resistance, thus fine-tuning the charge injection into OFET channels: the electrode work function, the surface energy of the modified electrodes and tunnelling resistance of the SAM. Understanding of the interplay of these properties is of vital importance for organic device design. In this paper, we report a model study based on the modulation of all three of these properties via chemisorption of fluorinated mono- or biphenylthiol molecules (PFBT and PF2BT, respectively) onto gold electrodes. Density functional theory simulations confirm the higher work function of the PFBT monolayers compared to PF2BT and provide evidence that this work function difference is entirely due to differences in the bond dipole to the gold surface. This observation is of importance for the development of future SAM molecules both for organic electronics and across the field of surface chemistry. Incorporation of these SAM-modified Au surfaces as the source and drain electrodes of an OFET with prototypical polymer semiconductors exhibiting different transport levels makes it possible to unravel the role of energetic alignment as well as surface energy and tunnelling resistance on the device performance. Interestingly, our results show that it is not always the high work function PFBT-modified electrodes that give the lowest contact resistance.

 

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