106 research outputs found
Angstromâconfined Electrochemical Synthesis of Subâunit Cell non van der Waals 2D Metal Oxides
Bottom-up electrochemical synthesis of atomically thin materials is desirable yet challenging, especially for non-van der Waals (vdW) materials. Thicknesses below few nm have not been reported yet, posing the question how thin can non-vdW materials be electrochemically synthesized? This is important as materials with (sub-) unit cell thickness often show remarkably different properties compared to their bulk form or thin films of several nm thickness. Here, we introduce a straightforward electrochemical method utilizing the angstrom-confinement of laminar reduced graphene oxide (rGO) nanochannels to obtain a centimeter-scale network of atomically thin (< 4.3 Ă
) 2D-transition metal oxides (2D-TMO). The angstrom-confinement provides a thickness limitation, forcing sub-unit cell growth of 2D-TMO with oxygen and metal vacancies. We showcase that Cr2O3, a material without significant catalytic activity for OER in bulk form, can be activated as a high-performing catalyst if synthesized in the 2D sub-unit cell form. Our method displays the high activity of sub-unit cell form while retaining the stability of bulk form, promising to yield unexplored fundamental science and applications. We show that while retaining the advantages of bottom-up electrochemical synthesis like simplicity, high yield, and mild conditions, the thickness of TMO can be limited to sub-unit cell dimensions
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Heterostructures based on inorganic and organic van der Waals systems
The two-dimensional limit of layered materials has recently been realized through the use of van der Waals (vdW) heterostructures composed of weakly interacting layers. In this paper, we describe two different classes of vdW heterostructures: inorganic vdW heterostructures prepared by co-lamination and restacking; and organicinorganic hetero-epitaxy created by physical vapor deposition of organic molecule crystals on an inorganic vdW substrate. Both types of heterostructures exhibit atomically clean vdW interfaces. Employing such vdW heterostructures, we have demonstrated various novel devices, including graphene/hexagonal boron nitride (hBN) and MoS2 heterostructures for memory devices; graphene/MoS2/WSe2/graphene vertical p-n junctions for photovoltaic devices, and organic crystals on hBN with graphene electrodes for high-performance transistorsPhysic
Enhanced graphitic domains of unreduced graphene oxide and the interplay of hydration behaviour and catalytic activity
Previous studies indicate that the properties of graphene oxide (GO) can be
significantly improved by enhancing its graphitic domain size through thermal
diffusion and clustering of functional groups. Remarkably, this transition
takes place below the decomposition temperature of the functional groups and
thus allows fine-tuning of graphitic domains without compromising with the
functionality of GO. By studying the transformation of GO under mild thermal
treatment, we directly observe this size enhancement of graphitic domains from
originally 40 nm2 to 200 nm2 through an extensive transmission electron
microscopy (TEM) study. Additionally, we confirm the integrity of the
functional groups during this process by comprehensive chemical analysis. A
closer look into the process confirms the theoretically predicted relevance for
the room temperature stability of GO. We further investigate the influence of
enlarged graphitic domains on the hydration behaviour of GO and catalytic
performance of single-atom catalysts supported by GO. Surprisingly, both, the
water transport and catalytic activity are damped by the heat treatment. This
allows us to reveal the critical role of water transport in laminated 2D
materials as catalysts
Atomically thin pân junctions with van der Waals heterointerfaces
Semiconductor pân junctions are essential building blocks for electronic and optoelectronic devices. In conventional pân junctions, regions depleted of free charge carriers form on either side of the junction, generating built-in potentials associated with uncompensated dopant atoms. Carrier transport across the junction occurs by diffusion and drift processes influenced by the spatial extent of this depletion region. With the advent of atomically thin van der Waals materials and their heterostructures, it is now possible to realize a pân junction at the ultimate thickness limit3, 4, 5, 6, 7, 8, 9, 10. Van der Waals junctions composed of p- and n-type semiconductorsâeach just one unit cell thickâare predicted to exhibit completely different charge transport characteristics than bulk heterojunctions10, 11, 12. Here, we report the characterization of the electronic and optoelectronic properties of atomically thin pân heterojunctions fabricated using van der Waals assembly of transition-metal dichalcogenides. We observe gate-tunable diode-like current rectification and a photovoltaic response across the pân interface. We find that the tunnelling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. Sandwiching an atomic pân junction between graphene layers enhances the collection of the photoexcited carriers. The atomically scaled van der Waals pân heterostructures presented here constitute the ultimate functional unit for nanoscale electronic and optoelectronic devices.Physic
Atomic-layer-confined multiple quantum wells enabled by monolithic bandgap engineering of transition metal dichalcogenides
Quantum wells (QWs), enabling effective exciton confinement and strong light-matter interaction, form an essential building block for quantum optoelectronics. For two-dimensional (2D) semiconductors, however, constructing the QWs is still challenging because suitable materials and fabrication techniques are lacking for bandgap engineering and indirect bandgap transitions occur at the multilayer. Here, we demonstrate an unexplored approach to fabricate atomic-layer-confined multiple QWs (MQWs) via monolithic bandgap engineering of transition metal dichalcogenides and van der Waals stacking. The WOX/WSe2 hetero-bilayer formed by monolithic oxidation of the WSe2 bilayer exhibited the type I band alignment, facilitating as a building block for MQWs. A superlinear enhancement of photoluminescence with increasing the number of QWs was achieved. Furthermore, quantum-confined radiative recombination in MQWs was verified by a large exciton binding energy of 193 meV and a short exciton lifetime of 170 ps. This work paves the way toward monolithic integration of band-engineered hetero-structures for 2D quantum optoelectronics
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