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- ItemMolecularly Engineered Black Phosphorus Heterostructures with Improved Ambient Stability and Enhanced Charge Carrier Mobility(Weinheim : Wiley-VCH, 2021) Shi, Huanhuan; Fu, Shuai; Liu, Yannan; Neumann, Christof; Wang, Mingchao; Dong, Haiyun; Kot, Piotr; Bonn, Mischa; Wang, Hai I.; Turchanin, Andrey; Schmidt, Oliver G.; Shaygan Nia, Ali; Yang, Sheng; Feng, XinliangOvercoming the intrinsic instability and preserving unique electronic properties are key challenges for the practical applications of black phosphorus (BP) under ambient conditions. Here, it is demonstrated that molecular heterostructures of BP and hexaazatriphenylene derivatives (BP/HATs) enable improved environmental stability and charge transport properties. The strong interfacial coupling and charge transfer between the HATs and the BP lattice decrease the surface electron density and protect BP sheets from oxidation, resulting in an excellent ambient lifetime of up to 21 d. Importantly, HATs increase the charge scattering time of BP, contributing to an improved carrier mobility of 97 cm2 V-1 s-1 , almost three times of the pristine BP films, based on noninvasive THz spectroscopic studies. The film mobility is an order of magnitude larger than previously reported values in exfoliated 2D materials. The strategy opens up new avenues for versatile applications of BP sheets and provides an effective method for tuning the physicochemical properties of other air-sensitive 2D semiconductors.
- ItemThiophene-Bridged Donor–Acceptor sp2-Carbon-Linked 2D Conjugated Polymers as Photocathodes for Water Reduction(Weinheim : Wiley-VCH, 2021) Xu, Shunqi; Sun, Hanjun; Addicoat, Matthew; Biswal, Bishnu P.; He, Fan; Park, SangWook; Paasch, Silvia; Zhang, Tao; Sheng, Wenbo; Brunner, Eike; Hou, Yang; Richter, Marcus; Feng, XinliangPhotoelectrochemical (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. © 2020 The Authors. Advanced Materials published by Wiley-VCH GmbH
- ItemInterfacial Covalent Bonds Regulated Electron-Deficient 2D Black Phosphorus for Electrocatalytic Oxygen Reactions(Weinheim : Wiley-VCH, 2021) Wang, Xia; Raghupathy, Ramya Kormath Madam; Querebillo, Christine Joy; Liao, Zhongquan; Li, Dongqi; Lin, Kui; Hantusch, Martin; Sofer, Zdeněk; Li, Baohua; Zschech, Ehrenfried; Weidinger, Inez M.; Kühne, Thomas D.; Mirhosseini, Hossein; Yu, Minghao; Feng, XinliangDeveloping resource-abundant and sustainable metal-free bifunctional oxygen electrocatalysts is essential for the practical application of zinc–air batteries (ZABs). 2D black phosphorus (BP) with fully exposed atoms and active lone pair electrons can be promising for oxygen electrocatalysts, which, however, suffers from low catalytic activity and poor electrochemical stability. Herein, guided by density functional theory (DFT) calculations, an efficient metal-free electrocatalyst is demonstrated via covalently bonding BP nanosheets with graphitic carbon nitride (denoted BP-CN-c). The polarized P-N covalent bonds in BP-CN-c can efficiently regulate the electron transfer from BP to graphitic carbon nitride and significantly promote the OOH* adsorption on phosphorus atoms. Impressively, the oxygen evolution reaction performance of BP-CN-c (overpotential of 350 mV at 10 mA cm−2, 90% retention after 10 h operation) represents the state-of-the-art among the reported BP-based metal-free catalysts. Additionally, BP-CN-c exhibits a small half-wave overpotential of 390 mV for oxygen reduction reaction, representing the first bifunctional BP-based metal-free oxygen catalyst. Moreover, ZABs are assembled incorporating BP-CN-c cathodes, delivering a substantially higher peak power density (168.3 mW cm−2) than the Pt/C+RuO2-based ZABs (101.3 mW cm−2). The acquired insights into interfacial covalent bonds pave the way for the rational design of new and affordable metal-free catalysts. © 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH
- ItemReduced Intrinsic Non-Radiative Losses Allow Room-Temperature Triplet Emission from Purely Organic Emitters(Weinheim : Wiley-VCH, 2021) Li, Yungui; Jiang, Lihui; Liu, Wenlan; Xu, Shunqi; Li, Tian-Yi; Fries, Felix; Zeika, Olaf; Zou, Yingping; Ramanan, Charusheela; Lenk, Simone; Scholz, Reinhard; Andrienko, Denis; Feng, Xinliang; Leo, Karl; Reineke, SebastianPersistent luminescence from triplet excitons in organic molecules is rare, as fast non-radiative deactivation typically dominates over radiative transitions. This work demonstrates that the substitution of a hydrogen atom in a derivative of phenanthroimidazole with an N-phenyl ring can substantially stabilize the excited state. This stabilization converts an organic material without phosphorescence emission into a molecular system exhibiting efficient and ultralong afterglow phosphorescence at room temperature. Results from systematic photophysical investigations, kinetic modeling, excited-state dynamic modeling, and single-crystal structure analysis identify that the long-lived triplets originate from a reduction of intrinsic non-radiative molecular relaxations. Further modification of the N-phenyl ring with halogen atoms affects the afterglow lifetime and quantum yield. As a proof-of-concept, an anticounterfeiting device is demonstrated with a time-dependent Morse code feature for data encryption based on these emitters. A fundamental design principle is outlined to achieve long-lived and emissive triplet states by suppressing intrinsic non-radiative relaxations in the form of molecular vibrations or rotations.
- ItemThiophene-Based Conjugated Acetylenic Polymers with Dual Active Sites for Efficient Co-Catalyst-Free Photoelectrochemical Water Reduction in Alkaline Medium(Weinheim : Wiley-VCH, 2021) Borrelli, Mino; Querebillo, Christine Joy; Pastoetter, Dominik L.; Wang, Tao; Milani, Alberto; Casari, Carlo; Khoa Ly, Hoang; He, Fan; Hou, Yang; Neumann, Christof; Turchanin, Andrey; Sun, Hanjun; Weidinger, Inez M.; Feng, XinliangAlthough being attractive materials for photoelectrochemical hydrogen evolution reaction (PEC HER) under neutral or acidic conditions, conjugated polymers still show poor PEC HER performance in alkaline medium due to the lack of water dissociation sites. Herein, we demonstrate that tailoring the polymer skeleton from poly(diethynylthieno[3,2-b]thiophene) (pDET) to poly(2,6-diethynylbenzo[1,2-b:4,5-b′]dithiophene (pBDT) and poly(diethynyldithieno[3,2-b:2′,3′-d]thiophene) (pDTT) in conjugated acetylenic polymers (CAPs) introduces highly efficient active sites for water dissociation. As a result, pDTT and pBDT, grown on Cu substrate, demonstrate benchmark photocurrent densities of 170 μA cm−2 and 120 μA cm−2 (at 0.3 V vs. RHE; pH 13), which are 4.2 and 3 times higher than that of pDET, respectively. Moreover, by combining DFT calculations and electrochemical operando resonance Raman spectroscopy, we propose that the electron-enriched Cβ of the outer thiophene rings of pDTT are the water dissociation active sites, while the −C≡C− bonds function as the active sites for hydrogen evolution. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
- ItemTwo-Dimensional Boronate Ester Covalent Organic Framework Thin Films with Large Single Crystalline Domains for a Neuromorphic Memory Device(Weinheim : Wiley-VCH, 2020) Park, SangWook; Liao, Zhongquan; Ibarlucea, Bergoi; Qi, Haoyuan; Lin, Hung-Hsuan; Becker, Daniel; Melidonie, Jason; Zhang, Tao; Sahabudeen, Hafeesudeen; Baraban, Larysa; Baek, Chang-Ki; Zheng, Zhikun; Zschech, Ehrenfried; Fery, Andreas; Heine, Thomas; Kaiser, Ute; Cuniberti, Gianaurelio; Dong, Renhao; Feng, XinliangDespite the recent progress in the synthesis of crystalline boronate ester covalent organic frameworks (BECOFs) in powder and thin-film through solvothermal method and on-solid-surface synthesis, respectively, their applications in electronics, remain less explored due to the challenges in thin-film processability and device integration associated with the control of film thickness, layer orientation, stability and crystallinity. Moreover, although the crystalline domain sizes of the powder samples can reach micrometer scale (up to ≈1.5 μm), the reported thin-film samples have so far rather small crystalline domains up to 100 nm. Here we demonstrate a general and efficient synthesis of crystalline two-dimensional (2D) BECOF films composed of porphyrin macrocycles and phenyl or naphthyl linkers (named as 2D BECOF-PP or 2D BECOF-PN) by employing a surfactant-monolayer-assisted interfacial synthesis (SMAIS) on the water surface. The achieved 2D BECOF-PP is featured as free-standing thin film with large single-crystalline domains up to ≈60 μm2 and tunable thickness from 6 to 16 nm. A hybrid memory device composed of 2D BECOF-PP film on silicon nanowire-based field-effect transistor is demonstrated as a bio-inspired system to mimic neuronal synapses, displaying a learning–erasing–forgetting memory process. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemTailoring Magnetic Features in Zigzag-Edged Nanographenes by Controlled Diels–Alder Reactions(Weinheim : Wiley-VCH, 2020) Ajayakumar, M.R.; Fu, Yubin; Liu, Fupin; Komber, Hartmut; Tkachova, Valeriya; Xu, Chi; Zhou, Shengqiang; Popov, Alexey A.; Liu, Junzhi; Feng, XinliangNanographenes (NGs) with tunable electronic and magnetic properties have attracted enormous attention in the realm of carbon-based nanoelectronics. In particular, NGs with biradical character at the ground state are promising building units for molecular spintronics. However, most of the biradicaloids are susceptible to oxidation under ambient conditions and photolytic degradation, which hamper their further applications. Herein, we demonstrated the feasibility of tuning the magnetic properties of zigzag-edged NGs in order to enhance their stability via the controlled Diels–Alder reactions of peri-tetracene (4-PA). The unstable 4-PA (y0=0.72; half-life, t1/2=3 h) was transformed into the unprecedented benzo-peri-tetracenes (BPTs) by a one-side Diels–Alder reaction, which featured a biradical character at the ground state (y0=0.60) and exhibited remarkable stability under ambient conditions for several months. In addition, the fully zigzag-edged circumanthracenes (CAs) were achieved by two-fold or stepwise Diels–Alder reactions of 4-PA, in which the magnetic properties could be controlled by employing the corresponding dienophiles. Our work reported herein opens avenues for the synthesis of novel zigzag-edged NGs with tailor-made magnetic properties. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemPersistent peri-Heptacene: Synthesis and In Situ Characterization(Weinheim : Wiley-VCH, 2021) Ajayakumar, M.R.; Ma, Ji; Lucotti, Andrea; Schellhammer, Karl Sebastian; Serra, Gianluca; Dmitrieva, Evgenia; Rosenkranz, Marco; Komber, Hartmut; Liu, Junzhi; Ortmann, Frank; Tommasini, Matteo; Feng, Xinliangn-peri-Acenes (n-PAs) have gained interest as model systems of zigzag-edged graphene nanoribbons for potential applications in nanoelectronics and spintronics. However, the synthesis of n-PAs larger than peri-tetracene remains challenging because of their intrinsic open-shell character and high reactivity. Presented here is the synthesis of a hitherto unknown n-PA, that is, peri-heptacene (7-PA), in which the reactive zigzag edges are kinetically protected with eight 4-tBu-C6H4 groups. The formation of 7-PA is validated by high-resolution mass spectrometry and in situ FT-Raman spectroscopy. 7-PA displays a narrow optical energy gap of 1.01 eV and exhibits persistent stability (t1/2≈25 min) under inert conditions. Moreover, electron-spin resonance measurements and theoretical studies reveal that 7-PA exhibits an open-shell feature and a significant tetraradical character. This strategy could be considered a modular approach for the construction of next-generation (3 N+1)-PAs (where N≥3). © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
- ItemA Two-Dimensional Polyimide-Graphene Heterostructure with Ultra-fast Interlayer Charge Transfer(Weinheim : Wiley-VCH, 2021) Liu, Kejun; Li, Jiang; Qi, Haoyuan; Hambsch, Mike; Rawle, Jonathan; Vázquez, Adrián RomanÃ; Nia, Ali Shaygan; Pashkin, Alexej; Schneider, Harald; Polozij, Mirosllav; Heine, Thomas; Helm, Manfred; Mannsfeld, Stefan C.B.; Kaiser, Ute; Dong, Renhao; Feng, XinliangTwo-dimensional polymers (2DPs) are a class of atomically/molecularly thin crystalline organic 2D materials. They are intriguing candidates for the development of unprecedented organic–inorganic 2D van der Waals heterostructures (vdWHs) with exotic physicochemical properties. In this work, we demonstrate the on-water surface synthesis of large-area (cm2), monolayer 2D polyimide (2DPI) with 3.1-nm lattice. Such 2DPI comprises metal-free porphyrin and perylene units linked by imide bonds. We further achieve a scalable synthesis of 2DPI-graphene (2DPI-G) vdWHs via a face-to-face co-assembly of graphene and 2DPI on the water surface. Remarkably, femtosecond transient absorption spectroscopy reveals an ultra-fast interlayer charge transfer (ca. 60 fs) in the resultant 2DPI-G vdWH upon protonation by acid, which is equivalent to that of the fastest reports among inorganic 2D vdWHs. Such large interlayer electronic coupling is ascribed to the interlayer cation–π interaction between 2DP and graphene. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
- ItemHighly Crystalline and Semiconducting Imine-Based Two-Dimensional Polymers Enabled by Interfacial Synthesis(Weinheim : Wiley-VCH, 2020) Sahabudeen, Hafeesudeen; Qi, Haoyuan; Ballabio, Marco; Položij, Miroslav; Olthof, Selina; Shivhare, Rishi; Jing, Yu; Park, SangWook; Liu, Kejun; Zhang, Tao; Ma, Ji; Rellinghaus, Bernd; Mannsfeld, Stefan; Heine, Thomas; Bonn, Mischa; Cánovas, Enrique; Zheng, Zhikun; Kaiser, Ute; Dong, Renhao; Feng, XinliangSingle-layer and multi-layer 2D polyimine films have been achieved through interfacial synthesis methods. However, it remains a great challenge to achieve the maximum degree of crystallinity in the 2D polyimines, which largely limits the long-range transport properties. Here we employ a surfactant-monolayer-assisted interfacial synthesis (SMAIS) method for the successful preparation of porphyrin and triazine containing polyimine-based 2D polymer (PI-2DP) films with square and hexagonal lattices, respectively. The synthetic PI-2DP films are featured with polycrystalline multilayers with tunable thickness from 6 to 200 nm and large crystalline domains (100–150 nm in size). Intrigued by high crystallinity and the presence of electroactive porphyrin moieties, the optoelectronic properties of PI-2DP are investigated by time-resolved terahertz spectroscopy. Typically, the porphyrin-based PI-2DP 1 film exhibits a p-type semiconductor behavior with a band gap of 1.38 eV and hole mobility as high as 0.01 cm2 V−1 s−1, superior to the previously reported polyimine based materials. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.