The wetting behavior of 2D materials, MXenes in particular, is presented.
Owing to rich chemistry, MXenes have great potentials to be employed in various composite/hybrid systems.
Hydrophilicity and superior physical properties make the MXenes a great reinforcing agent.S
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Two distinct requirements of various nanomaterials, including from a single semiconductor or with van der Waals heterostructures, are summarized, including where strain must be avoided, such as solutions to produce strain-insensitive devices, and where strain is required, such as pressure-sensitive outcomes.
2D materials with dangling-bond-free surfaces and atomically thin layers have been shown to be capable of being incorporated into flexible electronic devices. The electronic and optical properties of 2D materials can be tuned or controlled in other ways by using the intriguing strain engineering method. The latest and encouraging techniques in regard to creating flexible 2D nanoelectronics are condensed in this review. These techniques have the potential to be used in a wider range of applications in the near and long term. It is possible to use ultrathin 2D materials (graphene, BP, WTe2, VSe2 etc.) and 2D transition metal dichalcogenides (2D TMDs) in order to enable the electrical behavior of the devices to be studied. A category of materials is produced on smaller scales by exfoliating bulk materials, whereas chemical vapor deposition (CVD) and epitaxial growth are employed on larger scales. This overview highlights two distinct requirements, which include from a single semiconductor or with van der Waals heterostructures of various nanomaterials. They include where strain must be avoided and where it is required, such as solutions to produce strain-insensitive devices, and such as pressure-sensitive outcomes, respectively. Finally, points-of-view about the current difficulties and possibilities in regard to using 2D materials in flexible electronics are provided.
Work function difference induces spontaneous charge donation and band bending at the interfaces of superlattices, which is the origin of modulation doping and energy filtering, leading to remarkably enhanced thermoelectric performances. This optimizing strategy is successfully elucidated in 1T′-MoTe2/Bi2Te3 superlattices. Meanwhile, the rational manipulation of interfacial band bending via tuning the Te/Bi flux ratio further promotes thermoelectric performances.
Interfacial charge effects, such as band bending, modulation doping, and energy filtering, are critical for improving electronic transport properties of superlattice films. However, effectively manipulating interfacial band bending has proven challenging in previous studies. In this study, (1T′-MoTe2) x (Bi2Te3) y superlattice films with symmetry-mismatch were successfully fabricated via the molecular beam epitaxy. This enables to manipulate the interfacial band bending, thereby optimizing the corresponding thermoelectric performance. These results demonstrate that the increase of Te/Bi flux ratio (R) effectively tailored interfacial band bending, resulting in a reduction of the interfacial electric potential from ≈127 meV at R = 16 to ≈73 meV at R = 8. It is further verified that a smaller interfacial electric potential is more beneficial for optimizing the electronic transport properties of (1T′-MoTe2) x (Bi2Te3) y . Especially, the (1T′-MoTe2)1(Bi2Te3)12 superlattice film displays the highest thermoelectric power factor of 2.72 mW m−1 K−2 among all films, due to the synergy of modulation doping, energy filtering, and the manipulation of band bending. Moreover, the lattice thermal conductivity of the superlattice films is significantly reduced. This work provides valuable guidance to manipulate the interfacial band bending and further enhance the thermoelectric performances of superlattice films.
Room temperature spin-phonon coupling in nanocrystals of antiferromagnetic Cr2O3 is investigated via Raman spectroscopy in combination with magnetic measurements. This study demonstrates the interplay between the crystalline and magnetic structures in these 3D antiferromagnets when varying the surface-to-volume ratio and helps establish the fundamentals of coupling phononic excitations with the magnetization dynamics at room temperature, offering a highly prospective nanomaterial for the design of novel magnonic devices.
In this study, nanocrystals of antiferromagnetic Cr2O3 are shown via Raman spectroscopy to display peculiar lattice dynamics in terms of phonon softening and the occurrence of an exceptionally strong spin-phonon coupling. This effect, which is observed to persist well above the onset of the antiferromagnetic ordering temperature, is ascribed to locally correlated spin fluctuations due to the modulation of the magnetic exchange interactions as the chromium atoms oscillate about their equilibrium position. It is found that the spin-phonon coupling strength is governed by the competing antiferromagnetic and ferromagnetic interactions, where changes in the surface spin configuration can also play a crucial role. Overall, this work proves the size dependence of the interplay between the crystalline and magnetic structures in 3D antiferromagnets varying the surface-to-volume ratio and helps establish the fundamentals for a spin-phonon coupling engineering at the nanoscale via a simple route in a very stable and easy to synthesize material. More importantly, it demonstrates the possibility of coupling phononic excitations with the magnetization dynamics at room temperature, offering a highly prospective nanomaterial for the design of novel magnonic devices.
Nature Communications, Published online: 28 April 2023; doi:10.1038/s41467-023-37889-9
The authors unveil the many-particle processes underpinning the formation of bound charge transfer excitons at the interface of hBN-encapsulated lateral MoSe2-WSe2 heterostructures. The excitons can be tuned via interface (i.e. high quality lateral junction) and dielectric (i.e. hBN encapsulation) engineering.Two-dimensional (2D) layered materials have been considered promising candidates for next-generation optoelectronics. However, the performance of 2D photodetectors still has much room for improvement due to weak light absorption of planar 2D materials and lack of high-quality heterojunction preparation technology. Notably, 2D materials integrating with mature bulk semiconductors are a promising pathway to overcome this limitation and promote the practical application on optoelectronics. In this work, we present the patterned assembly of MoSe2/pyramid Si mixed-dimensional van der Waals (vdW) heterojunction arrays for broadband photodetection and imaging. Benefited from the light trapping effect induced enhanced optical absorption and high-quality vdW heterojunction, the photodetector demonstrates a wide spectral response range from 265 to 1550 nm, large responsivity up to 0.67 A·W−1, high specific detectivity of 1.84 × 1013 Jones, and ultrafast response time of 0.34/5.6 μs at 0 V. Moreover, the photodetector array exhibits outstanding broadband image sensing capability. This study offers a novel development route for high-performance and broadband photodetector array by MoSe2/pyramid Si mixed-dimensional heterojunction.
How thin can non-vanderWaals (non–vdW) materials be electrochemically synthesized? In this work, a bottom-up electrochemical synthesis of a (sub-)unit-cell-thick non-vdW material is presented. By utilizing the angstrom confinement within reduced graphene oxide stacks, 2D α-Cr2O3 and 2D α-Fe2O3 are grown with a straightforward electrochemical method obtaining a polycrystalline network of atomically thin 2Dtransition–metal oxides.
Bottom-up electrochemical synthesis of atomically thin materials is desirable yet challenging, especially for non-van der Waals (non-vdW) materials. Thicknesses below a few nanometers 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 nanometers thickness. Here, a straightforward electrochemical method utilizing the angstrom-confinement of laminar reduced graphene oxide (rGO) nanochannels is introduced to obtain a centimeter-scale network of atomically thin (<4.3 Å) 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. It is showcased that Cr2O3, a material without significant catalytic activity for the oxygen evolution reaction (OER) in bulk form, can be activated as a high-performing catalyst if synthesized in the 2D sub-unit-cell form. This 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. It is shown 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.
A direct optical writing approach is utilized to deterministically realize polymorphic 2D materials by locally inducing metallic 1T′-MoTe2 on the semiconducting 2H-MoTe2 host layer. A seven-fold enhancement in third harmonic generation intensity in 1T′-MoTe2 compared to 2H-MoTe2 is observed with telecom-band ultrafast pump laser. A Schottky photodiode with high optoelectronic performance is realized with polymorphic MoTe2.
Developing selective and coherent polymorphic crystals at the nanoscale offers a novel strategy for designing integrated architectures for photonic and optoelectronic applications such as metasurfaces, optical gratings, photodetectors, and image sensors. Here, a direct optical writing approach is demonstrated to deterministically create polymorphic 2D materials by locally inducing metallic 1T′-MoTe2 on the semiconducting 2H-MoTe2 host layer. In the polymorphic-engineered MoTe2, 2H- and 1T′- crystalline phases exhibit strong optical contrast from near-infrared to telecom-band ranges (1–1.5 µm), due to the change in the band structure and increase in surface roughness. Sevenfold enhancement of third harmonic generation intensity is realized with conversion efficiency (susceptibility) of ≈1.7 × 10−7 (1.1 × 10−19 m2 V−2) and ≈1.7 × 10−8 (0.3 × 10−19 m2 V−2) for 1T′ and 2H-MoTe2, respectively at telecom-band ultrafast pump laser. Lastly, based on polymorphic engineering on MoTe2, a Schottky photodiode with a high photoresponsivity of 90 AW−1 is demonstrated. This study proposes facile polymorphic engineered structures that will greatly benefit realizing integrated photonics and optoelectronic circuits.
In situ uniaxial tensile strain is applied to a freestanding BiFeO3 film by stretching an organic substrate. A scanning nitrogen-vacancy microscopy is applied to image the nanoscale magnetic order. The strain is continuously increased to 1.5% and a spin cycloid tilting ≈12.6° is observed. A first principle calculation is processed to show that the tilting is energetically favorable.
Bismuth ferrite (BiFeO3) possesses a non-collinear spin order while the ferroelectric order breaks space inversion symmetry. This allows efficient electric-field control of magnetism and makes it a promising candidate for applications in low-power spintronic devices. Epitaxial strain effects have been intensively studied and exhibit significant modulation of the magnetic order in bismuthBiFeO3, but tuning its spin structure with continuously varied uniaxial strain is still lacking at this moment. Herein, in situ uniaxial tensile strain is applied to a freestanding BiFeO3 film by mechanically stretching an organic substrate. A scanning nitrogen-vacancy (NV) microscopy is applied to image the nanoscale magnetic order in real space. The strain is continuously increased from 0% to 1.5% and four images under different strains are acquired during this period. The images show that the spin cycloid tilts by ≈12.6° when strain approaches 1.5%. A first principle calculation is processed to show that the tilting is energetically favorable under such strain. The in situ strain applying method in combination with scanning NV microscope real-space imaging ability paves a new way in studying the coupling between magnetic order and strain in BiFeO3 films.
2D photodetectors generally suffer recombination processes, which result in the sublinear power dependence of photoresponse. Here, the article reports giant superlinear power dependence of photocurrent with power exponent reaching γ = 1.5 due to suppression of recombination channel. The photodetector is integrated into camera, showing enhanced imaging contrast due to the superlinearity.
Photodetector based on two-dimensional (2D) materials is an ongoing quest in optoelectronics. 2D photodetectors are generally efficient at low illuminating power but suffer severe recombination processes at high power, which results in the sublinear power-dependent photoresponse and lower optoelectronic efficiency. The desirable superlinear photocurrent is mostly achieved by sophisticated 2D heterostructures or device arrays, while 2D materials rarely show intrinsic superlinear photoresponse. This work reports the giant superlinear power dependence of photocurrent based on multilayer Ta2NiS5. While the fabricated photodetector exhibits good sensitivity (3.1 mS W−1per □) and fast photoresponse (31 µs), the bias-, polarization-, and spatial-resolved measurements point to an intrinsic photoconductive mechanism. By increasing the incident power density from 1.5 to 200 µW µm−2, the photocurrent power dependence varies from sublinear to superlinear. At higher illuminating conditions, prominent superlinearity is observed with a giant power exponent of γ = 1.5. The unusual photoresponse can be explained by a two-recombination-center model where density of states of the recombination centers (RC) effectively closes all recombination channels. The photodetector is integrated into camera for taking photos with enhanced contrast due to superlinearity. This work provides an effective route to enable higher optoelectronic efficiency at extreme conditions.
Nature Communications, Published online: 29 April 2023; doi:10.1038/s41467-023-38212-2
Magnetic field has been observed to promote oxygen evolution at some circumstance, however the reason for the enhancement remains unclear. Here, the authors show that enhancement is due to the disappearance of magnetic domain walls.Nature Communications, Published online: 29 April 2023; doi:10.1038/s41467-023-37917-8
Applications of van der Waals magnetic systems are typically hampered by the low Curie temperature of van der Waals magnets. Here, Wang et al use molecular beam epitaxy to grow large films of Fe4GeTe2 with Curie temperatures over 500 K, and the film’s magnetic anisotropy can be tuned arbitrarily by controlling stoichiometry.
NaNb13O33 micron-sized particles are explored as a practical Li+-storage anode material with comprehensively good electrochemical properties and broad temperature adaptability. This new niobate owns the largest interlayer spacing among the known shear ReO3-type niobates. This merit leads to very fast Li+ diffusivity, notable intercalation-pseudocapacitive behavior, and small unit-cell-volume variations, greatly benefiting the rate performance, cyclic stability, and low-temperature electrochemical properties.
Niobate Li+-storage anode materials with shear ReO3 crystal structures have attracted intensive attention due to their inherent safety and large capacities. However, they generally suffer from limited rate performance, cyclic stability, and temperature adaptability, which are rooted in their insufficient interlayer spacings. Here, sodium niobate (NaNb13O33) micron-sized particles are developed as a new anode material owning the largest interlayer spacing among the known shear ReO3-type niobates. The large interlayer spacing of NaNb13O33 enables very fast Li+ diffusivity, remarkably contributing to its superior rate performance with a 2500 to 125 mA g−1 capacity percentage of 63.2%. Moreover, its large interlayer spacing increases the volume-accommodation capability during lithiation, allowing small unit-cell-volume variations (maximum 6.02%), which leads to its outstanding cyclic stability with 87.9% capacity retention after as long as 5000 cycles at 2500 mA g−1. Its cyclic stability is the best in the research field of niobate micron-sized particles, and comparable to that of “zero-strain” Li4Ti5O12. At a low temperature of −10 °C, it also exhibits high rate performance with a 1250 to 125 mA g−1 capacity percentage of 65.6%, and even better cyclic stability with 105.4% capacity retention after 5000 cycles at 1250 mA g−1. These comprehensively good electrochemical results pave the way for the practical application of NaNb13O33 in high-performance Li+ storage.
Optical memory is needed for all-optical data storage and processing, but weak optical nonlinearities make low-power optical memory states (bistability) difficult to obtain. This work shows how low-power optical bistability can be derived from mechanical nonlinearity in a nano-optomechanical structure consisting of mechanically nonlinear nanowires decorated with plasmonic metamolecules.
Metastable optically controlled devices (optical flip-flops) are needed in data storage, signal processing, and displays. Although nonvolatile memory relying on phase transitions in chalcogenide glasses has been widely used for optical data storage, beyond that, weak optical nonlinearities have hindered the development of low-power bistable devices. This work reports a new type of volatile optical bistability in a hybrid nano-optomechanical device, comprising a pair of anchored nanowires decorated with plasmonic metamolecules. The nonlinearity and bistability reside in the mechanical properties of the acoustically driven nanowires and are transduced to the optical response by reconfiguring the plasmonic metamolecules. The device can be switched between bistable optical states with microwatts of optical power and its volatile memory can be erased by removing the acoustic signal. The demonstration of hybrid nano-optomechanical bistability opens new opportunities to develop low-power optical bistable devices.
npj 2D Materials and Applications, Published online: 27 April 2023; doi:10.1038/s41699-023-00396-y
High Chern number van der Waals magnetic topological multilayers MnBi2Te4/hBNNature Electronics, Published online: 27 April 2023; doi:10.1038/s41928-023-00963-7
Memristors level upNature Materials, Published online: 27 April 2023; doi:10.1038/s41563-023-01535-y
Understanding lithium dynamics in solid-state electrolytes used for Li-ion batteries can be challenging. Using nonlinear extreme-ultraviolet spectroscopies, a direct spectral signature of surface lithium ions showing a distinct blueshift relative to the bulk absorption spectra is observed in a prototypical solid-state electrolyte.
Nature Nanotechnology, Published online: 27 April 2023; doi:10.1038/s41565-023-01375-6
Monolayer MoS2 is grown at the back end of the line of 200 mm silicon CMOS wafers at a temperature of <300 °C, and hybrid silicon CMOS/MoS2 circuits are demonstrated through heterogeneous integration.Nature Nanotechnology, Published online: 27 April 2023; doi:10.1038/s41565-023-01376-5
Intrinsic ferroelectricity in bilayer WTe2 can be used for electrical switching of the centred-rectangular moiré potential in WTe2/WSe2 heterostructures.Nature Photonics, Published online: 27 April 2023; doi:10.1038/s41566-023-01205-0
Spatial light modulator-based lithography-free programmable light transmission through optical gain medium is demonstrated for optical switching and a rudimentary photonic neural network.Nature, Published online: 26 April 2023; doi:10.1038/s41586-023-05879-y
Experimental observation and calculations show that broken reflection symmetry in graphene heterostructures allows tunable electron–flexural phonon coupling, providing a way to control quantum matter at the atomic scale.
Nature Communications, Published online: 26 April 2023; doi:10.1038/s41467-023-38005-7
Correlated electronic states in moiré matter are of great fundamental and technological interest. Here, the authors demonstrate a Josephson junction in magic-angle twisted bilayer graphene with a correlated insulator weak link, showing magnetism and programmable superconducting diode behaviour.
