0000000001203304
AUTHOR
Hai I. Wang
Reversible Photochemical Control of Doping Levels in Supported Graphene
Controlling the type and density of charge carriers in graphene is vital for a wide range of applications of this material in electronics and optoelectronics. To date, chemical doping and electrostatic gating have served as the two most established means to manipulate the carrier density in graphene. Although highly effective, these two approaches require sophisticated graphene growth or complex device fabrication processes to achieve both the desired nature and the doping densities with generally limited dynamic tunability and spatial control. Here, we report a convenient and tunable optical approach to tune the steady-state carrier density and Fermi energy in graphene by photochemically c…
Bottom-Up, On-Surface-Synthesized Armchair Graphene Nanoribbons for Ultra-High-Power Micro-Supercapacitors
Bottom-up-synthesized graphene nanoribbons (GNRs) with excellent electronic properties are promising materials for energy storage systems. Herein, we report bottom-up-synthesized GNR films employed as electrode materials for micro-supercapacitors (MSCs). The micro-device delivers an excellent volumetric capacitance and an ultra-high power density. The electrochemical performance of MSCs could be correlated with the charge carrier mobility within the differently employed GNRs, as determined by pump–probe terahertz spectroscopy studies.
Photoswitchable Micro-Supercapacitor Based on a Diarylethene-Graphene Composite Film
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.
The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies
The ultrafast dynamics and conductivity of photoexcited graphene can be explained using solely electronic effects.
Enhanced kinetics of hole transfer and electrocatalysis during photocatalytic oxygen evolution by cocatalyst tuning
Understanding photophysical and electrocatalytic processes during photocatalysis in a powder suspension system is crucial for developing efficient solar energy conversion systems. We report a substantial enhancement by a factor of 3 in photocatalytic efficiency for the oxygen evolution reaction (OER) by adding trace amounts (∼0.05 wt %) of noble metals (Rh and Ru) to a 2 wt % cobalt oxide modified Ta3N5 photocatalyst particulate. The optimized system exhibited high quantum efficiencies (QEs) of up to 28 and 8.4% at 500 and 600 nm in 0.1 M Na2S2O8 at pH 14. By isolation of the electrochemical components to generate doped cobalt oxide electrodes, the electrocatalytic activity of cobalt oxide …
Chemical Vapor Deposition Synthesis and Terahertz Photoconductivity of Low-Band-Gap N = 9 Armchair Graphene Nanoribbons.
Recent advances in bottom-up synthesis of atomically defined graphene nanoribbons (GNRs) with various microstructures and properties have demonstrated their promise in electronic and optoelectronic devices. Here we synthesized N = 9 armchair graphene nanoribbons (9-AGNRs) with a low optical band gap of ∼1.0 eV and extended absorption into the infrared range by an efficient chemical vapor deposition process. Time-resolved terahertz spectroscopy was employed to characterize the photoconductivity in 9-AGNRs and revealed their high intrinsic charge-carrier mobility of approximately 350 cm2·V-1·s-1.
Experimental Observation of Strong Exciton Effects in Graphene Nanoribbons
Graphene nanoribbons (GNRs) with atomically precise width and edge structures are a promising class of nanomaterials for optoelectronics, thanks to their semiconducting nature and high mobility of charge carriers. Understanding the fundamental static optical properties and ultrafast dynamics of charge carrier generation in GNRs is essential for optoelectronic applications. Combining THz spectroscopy and theoretical calculations, we report a strong exciton effect with binding energy up to 700 meV in liquid-phase-dispersed GNRs with a width of 1.7 nm and an optical bandgap of 1.6 eV, illustrating the intrinsically strong Coulomb interactions between photogenerated electrons and holes. By trac…
Kinetic Ionic Permeation and Interfacial Doping of Supported Graphene
Due to its outstanding electrical properties and chemical stability, graphene finds widespread use in various electrochemical applications. Although the presence of electrolytes strongly affects its electrical conductivity, the underlying mechanism has remained elusive. Here, we employ terahertz spectroscopy as a contact-free means to investigate the impact of ubiquitous cations (Li+, Na+, K+, and Ca2+) in aqueous solution on the electronic properties of SiO2-supported graphene. We find that, without applying any external potential, cations can shift the Fermi energy of initially hole-doped graphene by ∼200 meV up to the Dirac point, thus counteracting the initial substrate-induced hole dop…
Tunable Superstructures of Dendronized Graphene Nanoribbons in Liquid Phase
In this Communication, we report the first synthesis of structurally well-defined graphene nanoribbons (GNRs) functionalized with dendritic polymers. The resultant GNRs possess grafting ratios of 0.59-0.68 for the dendrons of different generations. Remarkably, the precise 3D branched conformation of the grafted dendrons affords the GNRs unprecedented 1D supramolecular self-assembly behavior in tetrahydrofuran (THF), yielding nanowires, helices and nanofibers depending on the dimension of the dendrons. The GNR superstructures in THF exhibit near-infrared absorption with maxima between 650 and 700 nm, yielding an optical bandgap of 1.2-1.3 eV. Ultrafast photoconductivity analyses unveil that …
Lateral Fusion of Chemical Vapor Deposited N = 5 Armchair Graphene Nanoribbons
Bottom-up synthesis of low-bandgap graphene nanoribbons with various widths is of great importance for their applications in electronic and optoelectronic devices. Here we demonstrate a synthesis of N = 5 armchair graphene nanoribbons (5-AGNRs) and their lateral fusion into wider AGNRs, by a chemical vapor deposition method. The efficient formation of 10- and 15- AGNRs is revealed by a combination of different spectroscopic methods, including Raman and UV−visnear-infrared spectroscopy as well as by scanning tunneling microscopy. The degree of fusion and thus the optical and electronic properties of the resulting GNRs can be controlled by the annealing temperature, providing GNR films with o…
Interfacial Oxide Modulated unique Exchange Bias in CrPS4/Fe3GeTe2 van der Waals heterostructures
Two-dimensional van der Waals heterostructures are an attractive platform for studying exchange bias due to their defect free and atomically flat interfaces. Chromium thiophosphate (CrPS4), an antiferromagnet, has uncompensated magnetic spins in a single layer that make it an excellent candidate for studying exchange bias. In this study, we examined the exchange bias in CrPS4/Fe3GeTe2 van der Waals heterostructures using anomalous Hall measurements. Our results show that the exchange bias strength is robust for clean interfaces, with a hysteresis loop shift of about 55 mT at 5 K for few-layer Fe3GeTe2, which is larger than that obtained in most van der Waals AFM/FM heterostructures. However…
Synthesis of Nonplanar Graphene Nanoribbon with Fjord Edges
As a new family of semiconductors, graphene nanoribbons (GNRs), nanometer-wide strips of graphene, have appeared as promising candidates for next-generation nanoelectronics. Out-of-plane deformation of π-frames in GNRs brings further opportunities for optical and electronic property tuning. Here we demonstrate a novel fjord-edged GNR (FGNR) with a nonplanar geometry obtained by regioselective cyclodehydrogenation. Triphenanthro-fused teropyrene 1 and pentaphenanthro-fused quateropyrene 2 were synthesized as model compounds, and single-crystal X-ray analysis revealed their helically twisted conformations arising from the [5]helicene substructures. The structures and photophysical properties …
Hysteresis in graphene nanoribbon field-effect devices
Hysteresis in the current response to a varying gate voltage is a common spurious effect in carbon-based field effect transistors. Here, we use electric transport measurements to probe the charge transport in networks of armchair graphene nanoribbons with a width of either 5 or 9 carbon atoms, synthesized in a bottom-up approach using chemical vapor deposition. Our systematic study on the hysteresis of such graphene nanoribbon transistors, in conjunction with temperature-dependent transport measurements shows that the hysteresis can be fully accounted for by trapping/detrapping carriers in the SiO2 layer. We extract the trap densities and depth, allowing us to identify shallow traps as the …
Efficient Hot Electron Transfer in Quantum Dot-Sensitized Mesoporous Oxides at Room Temperature
Hot carrier cooling processes represent one of the major efficiency losses in solar energy conversion. Losses associated with cooling can in principle be circumvented if hot carrier extraction toward selective contacts is faster than hot carrier cooling in the absorber (in so-called hot carrier solar cells). Previous work has demonstrated the possibility of hot electron extraction in quantum dot (QD)-sensitized systems, in particular, at low temperatures. Here we demonstrate a room-temperature hot electron transfer (HET) with up to unity quantum efficiency in strongly coupled PbS quantum dot-sensitized mesoporous SnO2. We show that the HET efficiency is determined by a kinetic competition b…
CCDC 2058017: Experimental Crystal Structure Determination
Related Article: Xuelin Yao, Wenhao Zheng, Silvio Osella, Zijie Qiu, Shuai Fu, Dieter Schollmeyer, Beate Müller, David Beljonne, Mischa Bonn, Hai I. Wang, Klaus Müllen, Akimitsu Narita|2021|J.Am.Chem.Soc.|143|5654|doi:10.1021/jacs.1c01882
CCDC 2058018: Experimental Crystal Structure Determination
Related Article: Xuelin Yao, Wenhao Zheng, Silvio Osella, Zijie Qiu, Shuai Fu, Dieter Schollmeyer, Beate Müller, David Beljonne, Mischa Bonn, Hai I. Wang, Klaus Müllen, Akimitsu Narita|2021|J.Am.Chem.Soc.|143|5654|doi:10.1021/jacs.1c01882