Key Figures

  • 29 articles in high-impact journals
  • 2 cover pages
  • 5 hot topics
  • 2 society volumes
  • 1 special issue
  • 1 hot paper
  • 1 very important paper
  • 120+ citations
  • 20 press releases & highlights
  • 13+ average impact factor
  • 85+% collaborative publications

Two-Dimensional Gallium Sulfide Nanoflakes for UV-Selective Photoelectrochemical-type Photodetectors

M. I. Zappia, G. Bianca, S. Bellani, N. Curreli, Z. Sofer, M. Serri, L. Najafi, M. Piccinni, R. Oropesa-Nuñez, P. Marvan, V. Pellegrini, I. Kriegel, M. Prato, A. Cupolillo, F. Bonaccorso

J. Phys. Chem. C, 2021, 125, 11857–11866. Link to article (open access).

Two-dimensional (2D) transition-metal monochalcogenides have been recently predicted to be potential photo(electro)catalysts for water splitting and photoelectrochemical (PEC) reactions. Differently from the most established InSe, GaSe, GeSe, and many other monochalcogenides, bulk GaS has a large band gap of ∼2.5 eV, which increases up to more than 3.0 eV with decreasing its thickness due to quantum confinement effects. Therefore, 2D GaS fills the void between 2D small-band-gap semiconductors and insulators, resulting of interest for the realization of van der Waals type-I heterojunctions in photocatalysis, as well as the development of UV light-emitting diodes, quantum wells, and other optoelectronic devices. Based on theoretical calculations of the electronic structure of GaS as a function of layer number reported in the literature, we experimentally demonstrate, for the first time, the PEC properties of liquid-phase exfoliated GaS nanoflakes. Our results indicate that solution-processed 2D GaS-based PEC-type photodetectors outperform the corresponding solid-state photodetectors. In fact, the 2D morphology of the GaS flakes intrinsically minimizes the distance between the photogenerated charges and the surface area at which the redox reactions occur, limiting electron–hole recombination losses. The latter are instead deleterious for standard solid-state configurations. Consequently, PEC-type 2D GaS photodetectors display a relevant UV-selective photoresponse. In particular, they attain responsivities of 1.8 mA W–1 in 1 M H2SO4 [at 0.8 V vs reversible hydrogen electrode (RHE)], 4.6 mA W–1 in 1 M Na2SO4 (at 0.9 V vs RHE), and 6.8 mA W–1 in 1 M KOH (at 1.1. V vs RHE) under 275 nm illumination wavelength with an intensity of 1.3 mW cm–2. Beyond the photodetector application, 2D GaS-based PEC-type devices may find application in tandem solar PEC cells in combination with other visible-sensitive low-band-gap materials, including transition-metal monochalcogenides recently established for PEC solar energy conversion applications.

Graphene-Based Electrodes in a Vanadium Redox Flow Battery Produced by Rapid Low-Pressure Combined Gas Plasma Treatments

S. Bellani, L. Najafi, M. Prato, R. Oropesa-Nuñez, B. Martín-García, L. Gagliani, E. Mantero, L. Marasco, G. Bianca, M. I. Zappia, C. Demirci, S. Olivotto, G. Mariucci, V. Pellegrini, M. Schiavetti, F. Bonaccorso

Chem. Mater., 2021, 33, 4106–4121. Link to article (open access).

The development of high-power density vanadium redox flow batteries (VRFBs) with high energy efficiencies (EEs) is crucial for the widespread dissemination of this energy storage technology. In this work, we report the production of novel hierarchical carbonaceous nanomaterials for VRFB electrodes with high catalytic activity toward the vanadium redox reactions (VO2+/VO2+ and V2+/V3+). The electrode materials are produced through a rapid (minute timescale) low-pressure combined gas plasma treatment of graphite felts (GFs) in an inductively coupled radio frequency reactor. By systematically studying the effects of either pure gases (O2 and N2) or their combination at different gas plasma pressures, the electrodes are optimized to reduce their kinetic polarization for the VRFB redox reactions. To further enhance the catalytic surface area of the electrodes, single-/few-layer graphene, produced by highly scalable wet-jet milling exfoliation of graphite, is incorporated into the GFs through an infiltration method in the presence of a polymeric binder. Depending on the thickness of the proton-exchange membrane (Nafion 115 or Nafion XL), our optimized VRFB configurations can efficiently operate within a wide range of charge/discharge current densities, exhibiting energy efficiencies up to 93.9%, 90.8%, 88.3%, 85.6%, 77.6%, and 69.5% at 25, 50, 75, 100, 200, and 300 mA cm–2, respectively. Our technology is cost-competitive when compared to commercial ones (additional electrode costs < 100 € m–2) and shows EEs rivalling the record-high values reported for efficient systems to date. Our work remarks on the importance to study modified plasma conditions or plasma methods alternative to those reported previously (e.g., atmospheric plasmas) to improve further the electrode performances of the current VRFB systems.

Multiscale Modeling Strategy of 2D Covalent Organic Frameworks Confined at an Air–Water Interface

A. Ortega-Guerrero, H. Sahabudeen, A. Croy, A. Dianat, R. Dong, X. Feng, G. Cuniberti

ACS Appl. Mater. Interfaces, 2021, 13, 26411–26420. Link to article.

Two-dimensional covalent organic frameworks (2D COFs) have attracted attention as versatile active materials in many applications. Recent advances have demonstrated the synthesis of monolayer 2D COF via an air–water interface. However, the interfacial 2D polymerization mechanism has been elusive. In this work, we have used a multiscale modeling strategy to study dimethylmethylene-bridged triphenylamine building blocks confined at the air–water interface to form a 2D COF via Schiff-base reaction. A synergy between the computational investigations and experiments allowed the synthesis of a 2D-COF with one of the linkers considered. Our simulations complement the experimental characterization and show the preference of the building blocks to be at the interface with a favorable orientation for the polymerization. The air–water interface is shown to be a key factor to stabilize a flat conformation when a dimer molecule is considered. The structural and electronic properties of the monolayer COFs based on the two monomers are calculated and show a semiconducting nature with direct bandgaps. Our strategy provides a first step toward the in silico polymerization of 2D COFs at air–water interfaces capturing the initial steps of the synthesis up to the prediction of electronic properties of the 2D material.

Low-Temperature Graphene-Based Paste for Large-Area Carbon Perovskite Solar Cells

P. Mariani, L. Najafi, G. Bianca, M. I. Zappia, L. Gabatel, A. Agresti, S. Pescetelli, A. Di Carlo, S. Bellani, F. Bonaccorso

ACS Appl. Mater. Interfaces, 2021, 13, 22368–22380. Link to article (open access).

Carbon perovskite solar cells (C-PSCs), using carbon-based counter electrodes (C-CEs), promise to mitigate instability issues while providing solution-processed and low-cost device configurations. In this work, we report the fabrication and characterization of efficient paintable C-PSCs obtained by depositing a low-temperature-processed graphene-based carbon paste atop prototypical mesoscopic and planar n–i–p structures. Small-area (0.09 cm2) mesoscopic C-PSCs reach a power conversion efficiency (PCE) of 15.81% while showing an improved thermal stability under the ISOS-D-2 protocol compared to the reference devices based on Au CEs. The proposed graphene-based C-CEs are applied to large-area (1 cm2) mesoscopic devices and low-temperature-processed planar n–i–p devices, reaching PCEs of 13.85 and 14.06%, respectively. To the best of our knowledge, these PCE values are among the highest reported for large-area C-PSCs in the absence of back-contact metallization or additional stacked conductive components or a thermally evaporated barrier layer between the charge-transporting layer and the C-CE (strategies commonly used for the record-high efficiency C-PSCs). In addition, we report a proof-of-concept of metallized miniwafer-like area C-PSCs (substrate area = 6.76 cm2, aperture area = 4.00 cm2), reaching a PCE on active area of 13.86% and a record-high PCE on aperture area of 12.10%, proving the metallization compatibility with our C-PSCs. Monolithic wafer-like area C-PSCs can be feasible all-solution-processed configurations, more reliable than prototypical perovskite solar (mini)modules based on the serial connection of subcells, since they mitigate hysteresis-induced performance losses and hot-spot-induced irreversible material damage caused by reverse biases.

Synthetic tuning of the quantum properties of open-shell radicaloids

F. Lombardi, J. Ma, D. I. Alexandropoulos, H. Komber, J. Liu, W. K. Myers, X. Feng, L. Bogani

Chem, 2021, 7, 1363–1378. Link to article.

Open-shell molecular radicaloids could constitute the key to molecular quantum information and quantum sensing technologies. The effect of their morphology on the quantum properties is anyway unknown, hampering the development of synthetic strategies. Herein, we establish the links between morphology and quantum properties, using three related radicaloids based on meta-quinodimethane. We unravel the roles of the π-conjugated backbone and those of the side groups on the spin-flip and quantum coherence times. The temperature regions are identified where different structural parts of the molecule or solvent become the dominant decoherence channel. The record quantum coherence values obtained at room temperature are still well below the intrinsic limits of radicaloids, and we discuss the directions to optimize the quantum performance.

Inverted perovskite solar cells with enhanced lifetime and thermal stability enabled by a metallic tantalum disulfide buffer layer

K. Chatzimanolis, K. Rogdakis, D. Tsikritzis, N. Tzoganakis, M. Tountas, M. Krassas, S. Bellani, L. Najafi, B. Martín-García, R. Oropesa-Nuñez, M. Prato, G. Bianca, I. Plutnarova, Z. Sofer, F. Bonaccorso, E. Kymakis

Nanoscale Adv., 2021, 3, 3124–3135. Link to article (open access).

Perovskite solar cells (PSCs) have proved their potential for delivering high power conversion efficiencies (PCE) alongside low fabrication cost and high versatility. The stability and the PCE of PSCs can readily be improved by implementing engineering approaches that entail the incorporation of two-dimensional (2D) materials across the device's layered configuration. In this work, two-dimensional (2D) 6R-TaS2 flakes were exfoliated and incorporated as a buffer layer in inverted PSCs, enhancing the device's PCE, lifetime and thermal stability. A thin buffer layer of 6R-TaS2 flakes was formed on top of the electron transport layer to facilitate electron extraction, thus improving the overall device performance. The optimized devices reach a PCE of 18.45%, representing a 12% improvement compared to the reference cell. The lifetime stability measurements of the devices under ISOS-L2, ISOS-D1, ISOS-D1I and ISOS-D2I protocols revealed that the TaS2 buffer layer retards the intrinsic, thermally activated degradation processes of the PSCs. Notably, the devices retain more than the 80% of their initial PCE over 330 h under continuous 1 Sun illumination at 65 °C.

Vinylene-Linked Two-Dimensional Covalent Organic Frameworks: Synthesis and Functions

S. Xu, M. Richter X. Feng

Acc. Mater. Res., 2021, 2, 252–265. Link to article. TUD press release.

Two-dimensional covalent organic frameworks (2D COFs) with covalently bonded repeat units and crystalline, porous framework backbones have attracted immense attention since the first 2D COFs were reported by Yaghi’s group in 2005. The extended single-layer structures of 2D COFs are also generally considered to be the 2D polymers. The precise incorporation of molecular building blocks into ordered frameworks enables the synthesis of novel organic materials with designable and predictable properties for specific applications, such as in optoelectronics, energy storage, and conversion. In particular, the 2D π-conjugated COFs (2D-c-COFs) represent a unique class of 2D conjugated polymers that have 2D molecular-periodic structures with extended in-plane π-conjugations. In the 2D-c-COFs, the conjugated skeletons and π–π stacking interactions can provide the pathways for electron transport, while the porous channel can enable the loading of active sites for catalysis and sensing. Thus far, the synthesis of 2D-c-COFs has been mostly limited to Schiff base chemistry based on the condensation reaction between amine and aldehyde/ketone monomers because the construction of 2D COFs as thermodynamically controlled products generally requires a highly reversible reaction for error-correction processes. However, the high reversibility of imine linkages would conversely endow moderate π-electron delocalization due to the polarized carbon–nitrogen bonds and poor stability against strong acids/bases.

To achieve robust and highly conjugated 2D-c-COFs, a series of synthesis strategies have been developed, including a one-step reversible reaction with a bond-forming–bond braking–bond reforming function, a quasi-reversible reaction combing reversible and irreversible processes, and postmodifications converting labile bonds to a robust linkage. Among all of the reported 2D-c-COFs, vinylene-linked (also sp2-carbon-linked) 2D covalent organic frameworks (V-2D-COFs) with high in-plane π-conjugation have attracted increasing interest after we reported the first V-2D-COFs via a Knoevenagel polycondensation in 2016. Although C═C bonds have low reversibility, making the synthesis of V-2D-COFs quite challenging, there have been around 40 V-2D-COFs reported over the past 5 years, which demonstrated the merits of V-2D-COFs combining with unique optoelectronic, redox, and magnetic properties.

In this Account, we will summarize the development of V-2D-COFs, covering the important aspects of synthesis methods, design strategies, unique physical properties, and functions. First, the solvothermal synthesis of V-2D-COFs using different reaction methodologies and design principles will be presented, including Knoevenagel polycondensation, other aldol-type polycondensations, and Horner–Wadsworth–Emmons (HWE) polycondensation. Second, we will discuss the optoelectronic and magnetic properties of V-2D-COFs. Finally, the promising applications of V-2D-COF in the fields of sensing, photocatalysis, energy storage, and conversion will be demonstrated, which benefit from their robust vinylene-linked skeleton, full in-plane π-conjugation, and tailorable structures. We anticipate that this Account will provide an intensive understanding of the synthesis of V-2D-COFs and inspire the further development of this emerging class of conjugated organic crystalline materials with unique physicochemical properties and applications across different areas.

Sulfur-Doped Nanographenes Containing Multiple Subhelicenes

W. Niu, Y. Fu, H. Komber, J. Ma, X. Feng, Y. Mai, J. Liu

Org. Lett., 2021, 23, 2069–2073. Link to article.

In this work, we describe the synthesis and characterization of three novel sulfur-doped nanographenes (NGs) (1–3) containing multiple subhelicenes, including carbo[4]helicenes, thieno[4]helicenes, carbo[5]helicenes, and thieno[5]helicenes. Density functional theory calculations reveal that the helicene substructures in 1–3 possess dihedral angles from 15° to 34°. The optical energy gaps of 1–3 are estimated to be 2.67, 2.45, and 2.30 eV, respectively. These three sulfur-doped NGs show enlarged energy gaps compared to those of their pristine carbon analogues.

Two‐Step Thermal Annealing: An Effective Route for 15 % Efficient Quasi‐2D Perovskite Solar Cells

V. Romano, L. Najafi, A. A. Sutanto, G. Schileo, V. Queloz, S. Bellani, M. Prato, S. Marras, M. K. Nazeeruddin, G. D’Angelo, F. Bonaccorso, G. Grancini

ChemPlusChem, 2021, in press. Featured in Hot Topic: Solar Cells, Society Volumes: Italy and Society Volumes: Switzerland.

Low‐dimensional perovskites (LDP) are nowadays recognized as promising materials for the realization of highly performing photovoltaic cells. However, issues related to film morphology, composition, crystal quality and material homogeneity limit the device performances and reproducibility. In this work, we implement a robust method for the deposition of a LDP mixing methylammonium (MA) and phenylethylammonium (PEA) cations to create the mixed system (PEA)2MA39Pb40I121 by using a two‐step thermal annealing treatment (at 60 and 100 °C). Our approach results in LDP films with high crystal quality and enhanced carrier lifetime, which double the power conversion efficiency of reference devices, reaching up to 15 %.

Chemical Conversion and Locking of the Imine Linkage: Enhancing the Functionality of Covalent Organic Frameworks

L. Cusin, H. Peng, A. Ciesielski, P. Samorì

Angew. Chem. Int. Ed., 2021, in press.

Imine‐based covalent organic frameworks (COFs) are a widely studied class of functional, crystalline, and porous nanostructures which combine a relatively facile crystallization with tuneable compositions and porosities. However, the imine linkage constitutes an intrinsic limitation due to its reduced stability in harsh chemical conditions and its unsuitability for in‐plane π‐conjugation in COFs. Urgent solutions are therefore required in order to exploit the full potential of these materials, thereby enabling their technological application in electronics, sensing, and energy storage devices. In this context, the advent of a new generation of linkages derived from the chemical conversion and locking of the imine bond represents a cornerstone for the synthesis of new COFs. A marked increase in the framework robustness is in fact often combined with the incorporation of novel functionalities including, for some of these reactions, an extension of the in‐plane π‐conjugation. This Minireview describes the most enlightening examples of one‐pot reactions and post‐synthetic modifications towards the chemical locking of the imine bond in COFs.

Functionalized metallic transition metal dichalcogenide (TaS2) for nanocomposite membranes in direct methanol fuel cells

H. Beydaghi, L. Najafi, S. Bellani, A. Bagheri, B. Martín-García, P. Salarizadeh, K. Hooshyari, S. Naderizadeh, M. Serri, L. Pasquale, B. Wu, R. Oropesa-Nuñez, Z. Sofer, V. Pellegrini, F. Bonaccorso

J. Mater. Chem. A, 2021, 9, 6368–6381. Link to article.

In this work, we designed a novel nanocomposite proton-exchange membrane (PEM) based on sulfonated poly(ether ether ketone) (SPEEK) and tantalum disulfide functionalized with terminal sulfonate groups (S-TaS2). The PEMs are prepared through a solution-casting method and exploited in direct methanol fuel cells (DMFCs). Two-dimensional S-TaS2 nanoflakes were prepared as a functional additive to produce the novel nanocomposite membrane for DMFCs due to their potential as a fuel barrier and an excellent proton conductor. To optimize the degree of sulfonation (DS) of SPEEK and the weight percentage (wt%) of S-TaS2 nanoflakes in PEMs, we used the central composite design of the response surface method. The optimum PEM was obtained for SPEEK DS of 1.9% and a weight fraction (wt%) of S-TaS2 nanoflakes of 70.2%. The optimized membrane shows a water uptake of 45.72%, a membrane swelling of 9.64%, a proton conductivity of 96.24 mS cm−1, a methanol permeability of 2.66 × 10−7 cm2 s−1, and a selectivity of 36.18 × 104 S s cm−3. Moreover, SPEEK/S-TaS2 membranes show superior thermal and chemical stabilities compared to those of pristine SPEEK. The DMFC fabricated with the SPEEK/S-TaS2 membrane has reached the maximum power densities of 64.55 mW cm−2 and 161.18 mW cm−2 at 30 °C and 80 °C, respectively, which are ∼78% higher than the values obtained with the pristine SPEEK membrane. Our results demonstrate that SPEEK/S-TaS2 membranes have a great potential for DMFC applications.

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs): chemistry and function for MOFtronics

M. Wang, R. Dong, X. Feng

Chem. Soc. Rev., 2021, 50, 2764–2793. Link to article (open access).

The 21st century has seen a reinvention of how modern electronics impact our daily lives; silicon-electronics and organic electronics are currently at the core of modern electronics. Recent advances have demonstrated that conductive metal–organic frameworks (MOFs), as another unique class of electronic materials, are emerging to provide additional possibility for multifunctional electronic devices that brings us “MOFtronics”. Typically, two-dimensional conjugated MOFs (2D c-MOFs) are a novel class of layer-stacked MOFs with in-plane extended π-conjugation that exhibit unique properties such as intrinsic porosity, crystallinity, stability, and electrical conductivity as well as tailorable band gaps. Benefiting from their unique features and high conductivity, 2D c-MOFs have displayed great potential for multiple high-performance (opto)electronic, magnetic, and energy devices. In this review article, we introduce the chemical and synthetic methodologies of 2D c-MOFs, intrinsic influences on their electronic structures and charge transport properties, as well as multifunctional applications of this class of materials for MOFtronics and potential power sources for MOFtronics. We highlight the benefits and limitations of thus-far developed 2D c-MOFs from synthesis to function and offer our perspectives in regard to the challenges to be addressed.

Predicting the bulk modulus of single-layer covalent organic frameworks with square-lattice topology from molecular building-block properties

A. Raptakis, A. Dianat, A. Croy, G. Cuniberti

Nanoscale, 2021, 13, 1077–1085. Link to article (open access).

Two-dimensional Covalent Organic Frameworks (2D COFs) have attracted a lot of interest because of their potential for a broad range of applications. Different combinations of their molecular building blocks can lead to new materials with different physical and chemical properties. In this study, the elasticity of different single-layer tetrabenzoporphyrin (H2-TBPor) and phthalocyanine (H2-Pc) based 2D COFs is numerically investigated using a density-functional based tight-binding approach. Specifically, we calculate the 2D bulk modulus and the equivalent spring constants of the respective molecular building-blocks. Using a spring network model we are able to predict the 2D bulk modulus based on the properties of the isolated molecules. This provides a path to optimize elastic properties of 2D COFs.

High-Mobility Semiconducting Two-Dimensional Conjugated Covalent Organic Frameworks with p-Type Doping

M. Wang, M. Wang, H.-H. Lin, M. Ballabio, H. Zhong, M. Bonn, S. Zhou, T. Heine, E. Cánovas, R. Dong, X. Feng

J. Am. Chem. Soc., 2020, 142, 21622–21627. Link to article. TUD press release.

Two-dimensional conjugated covalent organic frameworks (2D c-COFs) are emerging as a unique class of semiconducting 2D conjugated polymers for (opto)electronics and energy storage. Doping is one of the common, reliable strategies to control the charge carrier transport properties, but the precise mechanism underlying COF doping has remained largely unexplored. Here we demonstrate molecular iodine doping of a metal–phthalocyanine-based pyrazine-linked 2D c-COF. The resultant 2D c-COF ZnPc-pz-I2 maintains its structural integrity and displays enhanced conductivity by 3 orders of magnitude, which is the result of elevated carrier concentrations. Remarkably, Hall effect measurements reveal enhanced carrier mobility reaching ∼22 cm2 V–1 s–1 for ZnPc-pz-I2, which represents a record value for 2D c-COFs in both the direct-current and alternating-current limits. This unique transport phenomenon with largely increased mobility upon doping can be traced to increased scattering time for free charge carriers, indicating that scattering mechanisms limiting the mobility are mitigated by doping. Our work provides a guideline on how to assess doping effects in COFs and highlights the potential of 2D c-COFs to display high conductivities and mobilities toward novel (opto)electronic devices.

Microwave‐Induced Structural Engineering and Pt Trapping in 6R‐TaS2 for the Hydrogen Evolution Reaction

L. Najafi, S. Bellani, R. Oropesa‐Nuñez, R. Brescia, M. Prato, L. Pasquale, C. Demirci, F. Drago, B. Martín‐García, J. Luxa, L. Manna, Z. Sofer, F. Bonaccorso

Small, 2020, 16, 2003372. Link to article (open access). Featured in Hot Topic: Water Splitting.

The nanoengineering of the structure of transition metal dichalcogenides (TMDs) is widely pursued to develop viable catalysts for the hydrogen evolution reaction (HER) alternative to the precious metallic ones. Metallic group‐5 TMDs have been demonstrated to be effective catalysts for the HER in acidic media, making affordable real proton exchange membrane water electrolysers. Their key‐plus relies on the fact that both their basal planes and edges are catalytically active for the HER. In this work, the 6R phase of TaS2 is “rediscovered” and engineered. A liquid‐phase microwave treatment is used to modify the structural properties of the 6R‐TaS2 nanoflakes produced by liquid‐phase exfoliation. The fragmentation of the nanoflakes and their evolution from monocrystalline to partly polycrystalline structures improve the HER‐activity, lowering the overpotential at cathodic current of 10 mA cm−2 from 0.377 to 0.119 V. Furthermore, 6R‐TaS2 nanoflakes act as ideal support to firmly trap Pt species, which achieve a mass activity (MA) up 10 000 A gPt−1 at overpotential of 50 mV (20 000 A gPt−1 at overpotentials of 72 mV), representing a 20‐fold increase of the MA of Pt measured for the Pt/C reference, and approaching the state‐of‐the‐art of the Pt mass activity.

Thiophene‐Bridged Donor–Acceptor sp2‐Carbon‐Linked 2D Conjugated Polymers as Photocathodes for Water Reduction

S. Xu, H. Sun, M. Addicoat, B. P. Biswal, F. He, S. Park, S. Paasch, T. Zhang, W. Sheng, E. Brunner, Y. Hou, M. Richter, X. Feng

Adv. Mater., 2021, 33, 2006274. Link to article (open access). TUD press release.

Photoelectrochemical (PEC) water reduction, converting solar energy into environmentally friendly hydrogen fuel, requires delicate design and synthesis of semiconductors with appropriate bandgaps, suitable energy levels of the frontier orbitals, and high intrinsic charge mobility. In this work, the synthesis of a novel bithiophene‐bridged donor–acceptor‐based 2D sp2‐carbon‐linked conjugated polymer (2D CCP) is demonstrated. The Knoevenagel polymerization between the electron‐accepting building block 2,3,8,9,14,15‐hexa(4‐formylphenyl) diquinoxalino[2,3‐a:2′,3′‐c]phenazine (HATN‐6CHO) and the first electron‐donating linker 2,2′‐([2,2′‐bithiophene]‐5,5′‐diyl)diacetonitrile (ThDAN) provides the 2D CCP‐HATNThDAN (2D CCP‐Th). Compared with the corresponding biphenyl‐bridged 2D CCP‐HATN‐BDAN (2D CCP‐BD), the bithiophene‐based 2D CCP‐Th exhibits a wide light‐harvesting range (up to 674 nm), a optical energy gap (2.04 eV), and highest energy occupied molecular orbital–lowest unoccupied molecular orbital distributions for facilitated charge transfer, which make 2D CCP‐Th a promising candidate for PEC water reduction. As a result, 2D CCP‐Th presents a superb H2‐evolution photocurrent density up to ≈7.9 µA cm−2 at 0 V versus reversible hydrogen electrode, which is superior to the reported 2D covalent organic frameworks and most carbon nitride materials (0.09–6.0 µA cm−2). Density functional theory calculations identify the thiophene units and cyano substituents at the vinylene linkage as active sites for the evolution of H2.

One‐Pot Synthesis of Boron‐Doped Polycyclic Aromatic Hydrocarbons via 1,4‐Boron Migration

J.-J. Zhang, M.-C. Tang, Y. Fu, K.-H. Low, J. Ma, L. Yang, J. J. Weigand, J. Liu, V. W.-W. Yam, X. Feng

Angew. Chem. Int. Ed., 2021, 60, 2833–2838. Link to article (open access). Featured in Hot Topic: Organic Electronics. TUD press release.

Herein, we demonstrate a novel one‐pot synthetic method towards a series of boron‐doped polycyclic aromatic hydrocarbons (B‐PAHs, 1 a1 o), including hitherto unknown B‐doped zethrene derivatives, from ortho‐aryl substituted diarylalkynes with high atom efficiency and broad substrate scopes. A reaction mechanism is proposed based on the experimental investigation together with the theoretical calculations, which involves a unique 1,4‐boron migration process. The resultant benchtop‐stable B‐PAHs are thoroughly investigated by X‐ray crystallography, cyclic voltammetry, UV/Vis absorption, and fluorescence spectroscopies. The blue and green organic light‐emitting diode (OLED) devices based on 1 f and 1 k are further fabricated, demonstrating the promising application potential of B‐PAHs in organic optoelectronics.

A Curved Graphene Nanoribbon with Multi-Edge Structure and High Intrinsic Charge Carrier Mobility

W. Niu, J. Ma, P. Soltani, W. Zheng, F. Liu, A. A. Popov, J. J. Weigand, H. Komber, E. Poliani, C. Casiraghi, J. Droste, M. R. Hansen, S. Osella, D. Beljonne, M. Bonn, H. I. Wang, X. Feng, J. Liu, Y. Mai

J. Am. Chem. Soc., 2020, 142, 18293–18298. Link to article. TUD press release.

Structurally well-defined graphene nanoribbons (GNRs) have emerged as highly promising materials for the next-generation nanoelectronics. The electronic properties of GNRs critically depend on their edge topologies. Here, we demonstrate the efficient synthesis of a curved GNR (cGNR) with a combined cove, zigzag, and armchair edge structure, through bottom-up synthesis. The curvature of the cGNR is elucidated by the corresponding model compounds tetrabenzo[a,cd,j,lm]perylene (1) and diphenanthrene-fused tetrabenzo[a,cd,j,lm]perylene (2), the structures of which are unambiguously confirmed by the X-ray single-crystal analysis. The resultant multi-edged cGNR exhibits a well-resolved absorption at the near-infrared (NIR) region with a maximum peak at 850 nm, corresponding to a narrow optical energy gap of ∼1.22 eV. Employing THz spectroscopy, we disclose a long scattering time of ∼60 fs, corresponding to a record intrinsic charge carrier mobility of ∼600 cm2 V–1 s–1 for photogenerated charge carriers in cGNR.

Liquid-Phase Exfoliated GeSe Nanoflakes for Photoelectrochemical-Type Photodetectors and Photoelectrochemical Water Splitting

G. Bianca, M. I. Zappia, S. Bellani, Z. Sofer, M. Serri, L. Najafi, R. Oropesa-Nuñez, B. Martín-García, T. Hartman, L. Leoncino, D. Sedmidubský, V. Pellegrini, G. Chiarello, F. Bonaccorso

ACS Appl. Mater. Interfaces, 2020, 12, 48598–48613. Link to article (open access).

Photoelectrochemical (PEC) systems represent powerful tools to convert electromagnetic radiation into chemical fuels and electricity. In this context, two-dimensional (2D) materials are attracting enormous interest as potential advanced photo(electro)catalysts and, recently, 2D group-IVA metal monochalcogenides have been theoretically predicted to be water splitting photocatalysts. In this work, we use density functional theory calculations to theoretically investigate the photocatalytic activity of single-/few-layer GeSe nanoflakes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in pH conditions ranging from 0 to 14. Our simulations show that GeSe nanoflakes with different thickness can be mixed in the form of nanoporous films to act as nanoscale tandem systems, in which the flakes, depending on their thickness, can operate as HER- and/or OER photocatalysts. On the basis of theoretical predictions, we report the first experimental characterization of the photo(electro)catalytic activity of single-/few-layer GeSe flakes in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H2SO4 (pH 0.3), 1 M KCl (pH 6.5), and 1 M KOH (pH 14). The films of the GeSe nanoflakes are fabricated by spray coating GeSe nanoflakes dispersion in 2-propanol obtained through liquid-phase exfoliation of synthesized orthorhombic (Pnma) GeSe bulk crystals. The PEC properties of the GeSe nanoflakes are used to design PEC-type photodetectors, reaching a responsivity of up to 0.32 AW–1 (external quantum efficiency of 86.3%) under 455 nm excitation wavelength in acidic electrolyte. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching that of self-powered commercial UV–Vis photodetectors. The obtained results inspire the use of 2D GeSe in proof-of-concept water photoelectrolysis cells.

Synthesis of Vinylene‐Linked Two‐Dimensional Conjugated Polymers via the Horner–Wadsworth–Emmons Reaction

D. L. Pastoetter, S. Xu, M. Borrelli, M. Addicoat, B. P. Biswal, S. Paasch, A. Dianat, H. Thomas, R. Berger, S. Reineke, E. Brunner, G. Cuniberti, M. Richter, X. Feng

Angew. Chem. Int. Ed., 2020, 59, 23620–23625. Link to article (open access). TUD press release. Selected as a Hot Paper.

In this work, we demonstrate the first synthesis of vinylene‐linked 2D CPs, namely, 2D poly(phenylenequinoxalinevinylene)s 2D‐PPQV1 and 2D‐PPQV2, via the Horner–Wadsworth–Emmons (HWE) reaction of C2‐symmetric 1,4‐bis(diethylphosphonomethyl)benzene or 4,4′‐bis(diethylphosphonomethyl)biphenyl with C3‐symmetric 2,3,8,9,14,15‐hexa(4‐formylphenyl)diquinoxalino[2,3‐a:2′,3′‐c]phenazine as monomers. Density functional theory (DFT) simulations unveil the crucial role of the initial reversible C−C single bond formation for the synthesis of crystalline 2D CPs. Powder X‐ray diffraction (PXRD) studies and nitrogen adsorption‐desorption measurements demonstrate the formation of proclaimed crystalline, dual‐pore structures with surface areas of up to 440 m2 g−1. More importantly, the optoelectronic properties of the obtained 2D‐PPQV1 (Eg=2.2 eV) and 2D‐PPQV2 (Eg=2.2 eV) are compared with those of cyano‐vinylene‐linked 2D‐CN‐PPQV1 (Eg=2.4 eV) produced by the Knoevenagel reaction and imine‐linked 2D COF analog (2D‐C=N‐PPQV1, Eg=2.3 eV), unambiguously proving the superior conjugation of the vinylene‐linked 2D CPs using the HWE reaction.

Graphene transistors for real-time monitoring molecular self-assembly dynamics

M. Gobbi, A. Galanti, M.-A. Stoeckel, B. Zyska, S. Bonacchi, S. Hecht, P. Samorì

Nat. Commun., 2020, 11, 4731. Link to article (open access). UNISTRA, CNRS Alsace, ISIS and IRIS Adlershof press releases. Highlighted in Nature Research Device & Materials Engineering Community.

Mastering the dynamics of molecular assembly on surfaces enables the engineering of predictable structural motifs to bestow programmable properties upon target substrates. Yet, monitoring self-assembly in real time on technologically relevant interfaces between a substrate and a solution is challenging, due to experimental complexity of disentangling interfacial from bulk phenomena. Here, we show that graphene devices can be used as highly sensitive detectors to read out the dynamics of molecular self-assembly at the solid/liquid interface in-situ. Irradiation of a photochromic molecule is used to trigger the formation of a metastable self-assembled adlayer on graphene and the dynamics of this process are monitored by tracking the current in the device over time. In perspective, the electrical readout in graphene devices is a diagnostic and highly sensitive means to resolve molecular ensemble dynamics occurring down to the nanosecond time scale, thereby providing a practical and powerful tool to investigate molecular self-organization in 2D.

Scalable spray-coated graphene-based electrodes for high-power electrochemical double-layer capacitors operating over a wide range of temperature

M. A. Garakani, S. Bellani, V. Pellegrini, R. Oropesa-Nuñez, A. E. Del Rio Castillo, S. Abouali, L. Najafi, B. Martín-García, A. Ansaldo, P. Bondavalli, C. Demirci, V. Romano, E. Mantero, L. Marasco, M. Prato, G. Bracciale, F. Bonaccorso

Energy Storage Mater., 2021, 34, 1–11. Link to article (open access).

Advancements in electrochemical double-layer capacitor (EDLC) technology require the concomitant use of novel efficient electrode materials and viable electrode manufacturing methods. Cost-effectiveness, scalability and sustainability are key-drivers for fulfilling product development chain accepted by worldwide legislations. Herein, we report a scalable and sprayable “green” electrode material-based ink based on activated carbon and single-/few-layer graphene (SLG/FLG) flakes. We show that, contrary to commercial reduced graphene oxide, defect-free and flat SLG/FLG flakes reduce the friction of ions over the electrode films, while spray coating deposition of our ink maximises the electrolyte accessibility to the electrode surface area. Sprayed SLG/FLG flakes-based EDLCs display superior rate capability performance (e.g., specific energies of 31.5, 23.7 and 12.5 Wh kg−1 at specific powers of 150, 7500 and 30000 W kg−1, respectively) compared to both SLG/FLG flakes-free devices and commercial-like EDLCs produced by slurry-coating method. The use of SLG/FLG flakes enables our sprayed EDLCs to operate in a wide range of temperature (−40/+100°C) compatible with ionic liquid/organic solvent-based electrolytes, overcoming the specific power limits of AC-based EDLCs. A prototype EDLCs stack consisting of multiple large-area EDLCs, each one displaying a capacitance of 25 F, demonstrates the industrial potential of our technology.

Luminescent sp2-Carbon-Linked 2D Conjugated Polymers with High Photostability

Y. Li, B. P. Biswal, M. A. Addicoat, S. Paasch, P. Imbrasas, S. Park, H. Shi, E. Brunner, M. Richter, S. Lenk, S. Reineke, X. Feng

Chem. Mater., 2020, 32, 7985–7991. Link to article.

Luminescent organic materials with high photostability are essential in optoelectronics, sensor, and photocatalysis applications. However, small organic molecules are generally sensitive to UV irradiation, giving rise to chemical decompositions. In this work, we demonstrate two novel CN-substituted two-dimensional sp2-carbon-linked conjugated polymers (2D CCPs) containing a chromophore triphenylene unit. The Knoevenagel polymerization between 2,3,6,7,10,11-hexakis(4-formylphenyl)triphenylene (HFPTP) and 1,4-phenylenediacetonitrile (PDAN) or 2,2′-(biphenyl-4,4′-diyl)diacetonitrile (BDAN), provides the crystalline 2D CCP-HFPTP-PDAN (2D CCP-1) and 2D CCP-HFPTP-BDAN (2D CCP-2) with dual pore structures, respectively. 2D CCP-1 and 2D CCP-2 exhibit the photoluminescence quantum yield (PLQY) up to 24.9 and 32.3%, which are the highest values among the reported 2D conjugated polymers and π-conjugated 2D covalent organic frameworks. Furthermore, compared with the well-known emissive small molecule tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), both 2D CCPs show superior photostability under UV irradiation for 2 h, profiting from the twisted and rigid structures of the CN-substituted vinylene linkages. The present work will trigger the further explorations of novel organic emitters embedded in 2D CCPs with high PLQY and photostability, which can be useful for optoelectronic devices.

Role of Exchange Interactions in the Magnetic Response and Intermolecular Recognition of Chiral Molecules

A. Dianat, R. Gutierrez, H. Alpern, V. Mujica, A. Ziv, S. Yochelis, O. Millo, Y. Paltiel, G. Cuniberti

Nano Lett., 2020, 20, 7077–7086. Link to article and accepted manuscript (open access).

The physical origin of the so-called chirality-induced spin selectivity (CISS) effect has puzzled experimental and theoretical researchers over the past few years. Early experiments were interpreted in terms of unconventional spin–orbit interactions mediated by the helical geometry. However, more recent experimental studies have clearly revealed that electronic exchange interactions also play a key role in the magnetic response of chiral molecules in singlet states. In this investigation, we use spin-polarized closed-shell density functional theory calculations to address the influence of exchange contributions to the interaction between helical molecules as well as of helical molecules with magnetized substrates. We show that exchange effects result in differences in the interaction properties with magnetized surfaces, shedding light into the possible origin of two recent important experimental results: enantiomer separation and magnetic exchange force microscopy with AFM tips functionalized with helical peptides.

Synthesis of Robust MOFs@COFs Porous Hybrid Materials via an Aza‐Diels–Alder Reaction: Towards High‐Performance Supercapacitor Materials

H. Peng, J. Raya, F. Richard, W. Baaziz, O. Ersen, A. Ciesielski, P. Samorì

Angew. Chem. Int. Ed., 2020, 59, 19602–19609. Link to article and accepted manuscript (open access). Featured in Special Issue: Functional Porous Materials Chemistry and Hot Topic: Batteries and Supercapacitors. Selected as a Very Important Paper.

Metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have attracted enormous attention in recent years. Recently, MOF@COF are emerging as hybrid architectures combining the unique features of the individual components to enable the generation of materials displaying novel physicochemical properties. Herein we report an unprecedented use of aza‐Diels–Alder cycloaddition reaction as post‐synthetic modification of MOF@COF‐LZU1, to generate aza‐MOFs@COFs hybrid porous materials with extended π‐delocalization. A a proof‐of‐concept, the obtained aza‐MOFs@COFs is used as electrode in supercapacitors displaying specific capacitance of 20.35 μF cm−2 and high volumetric energy density of 1.16 F cm−3. Our approach of post‐synthetic modification of MOFs@COFs hybrids implement rational design for the synthesis of functional porous materials and expands the plethora of promising application of MOFs@COFs hybrid porous materials in energy storage applications.

On-surface synthesis of super-heptazethrene

S. Mishra, J. Melidonie, K. Eimre, S. Obermann, O. Gröning, C. A. Pignedoli, P. Ruffieux, X. Feng, R. Fasel

Chem. Commun., 2020, 56, 7467–7470. Link to article and accepted manuscript (open access).

Zethrenes are model diradicaloids with potential applications in spintronics and optoelectronics. Despite a rich chemistry in solution, on-surface synthesis of zethrenes has never been demonstrated. We report the on-surface synthesis of super-heptazethrene on Au(111). Scanning tunneling spectroscopy investigations reveal that super-heptazethrene exhibits an exceedingly low HOMO–LUMO gap of 230 meV and, in contrast to its open-shell singlet ground state in the solution phase and in the solid-state, likely adopts a closed-shell ground state on Au(111).

Photomodulation of Charge Transport in All‐Semiconducting 2D–1D van der Waals Heterostructures with Suppressed Persistent Photoconductivity Effect

Z. Liu, H. Qiu, C. Wang, Z. Chen, B. Zyska, A. Narita, A. Ciesielski, S. Hecht, L. Chi, K. Müllen, P. Samorì

Adv. Mater., 2020, 32, 2001268. Link to article and accepted manuscript (open access). COVER PAGE. Featured in Hot Topic: Carbon, Graphite, and Graphene.

Van der Waals heterostructures (VDWHs), obtained via the controlled assembly of 2D atomically thin crystals, exhibit unique physicochemical properties, rendering them prototypical building blocks to explore new physics and for applications in optoelectronics. As the emerging alternatives to graphene, monolayer transition metal dichalcogenides and bottom‐up synthesized graphene nanoribbons (GNRs) are promising candidates for overcoming the shortcomings of graphene, such as the absence of a bandgap in its electronic structure, which is essential in optoelectronics. Herein, VDWHs comprising GNRs onto monolayer MoS2 are fabricated. Field‐effect transistors (FETs) based on such VDWHs show an efficient suppression of the persistent photoconductivity typical of MoS2, resulting from the interfacial charge transfer process. The MoS2‐GNR FETs exhibit drastically reduced hysteresis and more stable behavior in the transfer characteristics, which is a prerequisite for the further photomodulation of charge transport behavior within the MoS2‐GNR VDWHs. The physisorption of photochromic molecules onto the MoS2‐GNR VDWHs enables reversible light‐driven control over charge transport. In particular, the drain current of the MoS2‐GNR FET can be photomodulated by 52%, without displaying significant fatigue over at least 10 cycles. Moreover, four distinguishable output current levels can be achieved, demonstrating the great potential of MoS2‐GNR VDWHs for multilevel memory devices.

Ultrathin two-dimensional conjugated metal–organic framework single-crystalline nanosheets enabled by surfactant-assisted synthesis

Z. Wang, G. Wang, H. Qi, M. Wang, M. Wang, S. Park, H. Wang, M. Yu, U. Kaiser, A. Fery, S. Zhou, R. Dong, X. Feng

Chem. Sci., 2020, 11, 7665–7671. Link to article (open access).

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs) have recently emerged for potential applications in (opto-)electronics, chemiresistive sensing, and energy storage and conversion, due to their excellent electrical conductivity, abundant active sites, and intrinsic porous structures. However, developing ultrathin 2D c-MOF nanosheets (NSs) for facile solution processing and integration into devices remains a great challenge, mostly due to unscalable synthesis, low yield, limited lateral size and low crystallinity. Here, we report a surfactant-assisted solution synthesis toward ultrathin 2D c-MOF NSs, including HHB-Cu (HHB = hexahydroxybenzene), HHB-Ni and HHTP-Cu (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene). For the first time, we achieve single-crystalline HHB-Cu(Ni) NSs featured with a thickness of 4–5 nm (∼8–10 layers) and a lateral size of 0.25–0.65 μm2, as well as single-crystalline HHTP-Cu NSs with a thickness of ∼5.1 ± 2.6 nm (∼10 layers) and a lateral size of 0.002–0.02 μm2. Benefiting from the ultrathin feature, the synthetic NSs allow fast ion diffusion and high utilization of active sites. As a proof of concept, when serving as a cathode material for Li-ion storage, HHB-Cu NSs deliver a remarkable rate capability (charge within 3 min) and long-term cycling stability (90% capacity retention after 1000 cycles), superior to the corresponding bulk materials and other reported MOF cathodes.

Demonstration of a Broadband Photodetector Based on a Two‐Dimensional Metal–Organic Framework

H. Arora, R. Dong, T. Venanzi, J. Zscharschuch, H. Schneider, M. Helm, X. Feng, E. Cánovas, A. Erbe

Adv. Mater., 2020, 32, 1907063. Link to article (open access). COVER PAGEcfaed, HZDR, EurekAlert!, NWA,, ScienceDaily, Laser Focus World, Optics & Photonics News and PhotonicsViews press releases.

Metal–organic frameworks (MOFs) are emerging as an appealing class of highly tailorable electrically conducting materials with potential applications in optoelectronics. Yet, the realization of their proof‐of‐concept devices remains a daunting challenge, attributed to their poor electrical properties. Following recent work on a semiconducting Fe3(THT)2(NH4)3 (THT: 2,3,6,7,10,11‐triphenylenehexathiol) 2D MOF with record‐high mobility and band‐like charge transport, here, an Fe3(THT)2(NH4)3 MOF‐based photodetector operating in photoconductive mode capable of detecting a broad wavelength range from UV to NIR (400–1575 nm) is demonstrated. The narrow IR bandgap of the active layer (≈0.45 eV) constrains the performance of the photodetector at room temperature by band‐to‐band thermal excitation of charge carriers. At 77 K, the device performance is significantly improved; two orders of magnitude higher voltage responsivity, lower noise equivalent power, and higher specific detectivity of 7 × 108 cm Hz1/2 W−1 are achieved under 785 nm excitation. These figures of merit are retained over the analyzed spectral region (400–1575 nm) and are commensurate to those obtained with the first demonstrations of graphene‐ and black‐phosphorus‐based photodetectors. This work demonstrates the feasibility of integrating conjugated MOFs as an active element into broadband photodetectors, thus bridging the gap between materials' synthesis and technological applications.