Jiuxiang Dai

Chemical vapor deposition growth of metal carbides is of great interest as this method provides large area growth of MXenes. This growth is mainly done using a melted diffusion based process; however, different morphologies in growth process is not well understood. In this work, we report deterministic synthesis of layered (non-uniform c -axis growth) and planar (uniform c -axis growth) of molybdenum carbide (Mo 2 C) using a diffusion-mediated growth. Mo-diffusion limited growth mechanism is proposed where the competition between Mo and C adatoms determines the morphology of grown crystals. Difference in thickness of catalyst at the edge and center lead to enhanced Mo diffusion which plays a vital role in determining the structure of Mo 2 C. The layered structures exhibit an expansion in the lattice confirmed by the presence of strain. Density functional theory shows consistent presence of strain which is dependent upon Mo diffusion during growth. T...

The preparation of single-crystal two-dimensional materials is of great significance for their practical applications. Here, four epitaxy modes of graphene, hexagonal boron nitride and transition metal dichalcogenide on various substrates, are summarized and discussed systematically, which may provide a deeper insight into the epitaxial growth and potential opportunities to realize large-scale production of 2D single crystals.

Abstract

Two-dimensional (2D) materials exhibit unique electronic, optical, magnetic, mechanical, and thermal properties due to their special crystal structure and thus have promising potential in many fields, such as in electronics and optoelectronics. To realize their real applications, especially in integrated devices, the growth of large-size single crystal is a prerequisite. Up to now, the most feasible way to achieve 2D single crystal growth is the epitaxy: growth of 2D materials of one or more specific orientations with single-crystal substrate. Only when the 2D domains have the same orientation, they can stitch together seamlessly and single-crystal 2D films can be obtained. In this view, four different epitaxy modes of 2D materials on various substrates are presented, including van der Waals epitaxy, edge epitaxy, step-guided epitaxy, and in-plane epitaxy focusing on the growth of graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenide (TMDC). The lattice symmetry relation and the interaction between 2D materials and the substrate are the key factors determining the epitaxy behaviors and thus are systematically discussed. Finally, the opportunities and challenges about the epitaxy of 2D single crystals in the future are summarized.

npj 2D Materials and Applications, Published online: 17 January 2022; doi:10.1038/s41699-021-00281-6

On electrically tunable stacking domains and ferroelectricity in moiré superlattices

Nature Nanotechnology, Published online: 17 January 2022; doi:10.1038/s41565-021-01059-z

Rhombohedral stacking of two identical non-ferroelectric monolayer transition metal dichalcogenides enables the observation of interfacial ferroelectricity.

Nature Nanotechnology, Published online: 17 January 2022; doi:10.1038/s41565-021-01045-5

This Review discusses the progresses in label-free nanophotonic biosensors based on photonic or dielectric surfaces and metasurfaces and highlights the challenges and benefits of their applications in the fields of public health, well-being and biosafety.

Ultra-smooth, low-stress patterned gallium nitride (GaN) films and high-quality flexible GaN-based light-emitting diodes is achieved in a one-step process by low-energy UV ultrafast laser controlled lift-off, which has dramatic potential for the fabrication of GaN-based flexible electronic devices, as well as, novel wearable electronics.

Abstract

A one-step laser lift-off (LLO) for patterned gallium nitride (GaN) film and GaN-based light-emitting diode (LED) device is achieved using 355 nm picosecond laser irradiation in this research. The laser fluence required for separation is 0.09–0.13 J cm⁻², which is much lower than that for the currently reported LLO methods. The separated GaN film is intact with only 0.04 GPa of residual stress. The ultra-smooth separated surface with root mean square roughness of only 5.2 nm is attributed to the interconnection of microcrack-free flat cavities formed by the combination of high photon energy-induced intrinsic absorption and subsequent plasma generation. The flat cavity with a depth-to-width ratio of 1:4000 limits the delamination region to a few nanometers at the GaN/sapphire interface. GaN-based LED is transferred with perfect electroluminescence (EL) by the strategy. The stable EL spectral peak positions and intensity independent of the bending state prove that the presented low-energy ultrafast LLO technique ensured the flexibility of the separated LED device without affecting the performance. This research provides a promising strategy to achieve the LLO of GaN devices with low energy consumption, high controllability, and high efficiency, which is significant for the industrial fabrication of flexible GaN-based electronics.

Special Issue

Welcome to the world of ANFF. Things are made small here, but they make a massive difference. This special issue illustrates the breadth of activity ANFF enables, ranging from medical devices, quantum technologies, to space exploration and beyond. It's all done by providing open access to world-class micro/nanofabrication equipment and expertise.

By leveraging on the synergetic effect of ferroelectric polarization and charge trapping behavior in a hybrid system of 2D MoTe₂ on ferroelectric Hf_0.5Zr_0.5O₂ thin film, the device exhibits multifunctional capability, including reliable memory device, tunable artificial synapse, and reconfigurable photodetector. The results pave the way for the fabrication of high-density data process systems with various functions.

Abstract

The “Internet-of-Things”-based information society requires the devices to possess high scaling capability as well as rich functionalities. Hybrid systems coupling 2D semiconductors and functional ferroelectrics are attracting increasing attention as complementary devices to the existing silicon systems due to their outstanding electronic and optoelectronic performances. In this work, interfacial states are introduced on the ferroelectric Hf_0.5Zr_0.5O₂ thin film during the annealing process. Utilizing the synergetic effect of ferroelectric polarization and charge trapping behavior, a multifunctional 2D Fe-FET is demonstrated, exhibiting reliable memory properties, tunable synaptic functions, and reconfigurable photodetection behaviors in one single device. Among them, multiple storage levels are achieved with a long retention time. Flexible plasticity including short-term plasticity (STP)/long-term plasticity (LTP) is emulated successfully, and the excellent emulated synaptic behaviors also contribute to the pattern recognition accuracy of ≈81% in artificial neural network simulations. Furthermore, the ferroelectric polarization-dependent optoelectronic response is observed, making it promising for the optoelectronic logic device application. The results pave the way for the fabrication of high-density data process systems with various functions.

A polarization sensitive solar-blind ultraviolet (UV) photodetector is constructed based on the chemical vapor deposition grown ultrathin KNb₃O₈ nanobelts. The high crystallinity, ultrawide bandgap, and intrinsic structural anisotropy of the KNb₃O₈ nanobelt offer its detector high responsivity to solar-blind UV light with large linear dichroism. This work opens up new prospects for developing future multifunctional solar-blind UV optoelectronic devices.

Abstract

Low-dimensional ultrawide bandgap semiconductors demonstrate great potential in fabricating solar-blind ultraviolet photodetectors. However, the widespread use of detectors is still limited by the low responsivity, large noise, and dark current, and especially few detectors can fulfill the solar-blind ultraviolet detection and the polarization dependence simultaneously. Herein, a polarization sensitive solar-blind ultraviolet photodetector based on ultrathin KNb₃O₈ nanobelts synthesized via chemical vapor deposition growth, is reported. By selecting suitable substrate and tuning the growth temperature, the nonlayered KNb₃O₈ crystal is grown into the quasi-1D ultrathin nanobelt with thickness in the range of 4.8–120 nm. Density functional theory calculations and experimental results indicate that the ultrathin KNb₃O₈ nanobelt possesses an ultrawide bandgap (4.15 eV) as well as unusual in-plane structural anisotropy. Benefiting from the above features, the ultrathin KNb₃O₈ nanobelt-based device exhibits superior photodetection performances with high responsivity (30 A W⁻¹), high detectivity (5.95 × 10¹¹ Jones), and ultralow dark current (7.1 × 10⁻¹⁵ A) in the solar-blind ultraviolet region (230–280 nm). In addition, the KNb₃O₈ photodetector displays strong polarization sensitive photoresponse with a linear dichroic ratio of 1.62 at 254 nm. With these remarkable features, the ultrathin KNb₃O₈ nanobelt provides great opportunities for designing the next-generation multifunctional solar-blind ultraviolet optoelectronic devices.

Plasmonic Biosensors

Metal nanostructures exhibit tremendous potentialities in biosensing applications relying on fluorescence, Raman spectroscopy, infrared absorption, and colorimetry thanks to unique plasmonic effects. In particular, nanostructured surfaces can be smartly employed for realizing chip-based bioassays with remarkable sensing performance. In article number 2101133, Antonio Minopoli, Adriano Acunzo, Bartolomeo Della Ventura, and Raffaele Velotta review the recent developments in plasmonic biosensors.

Layered spin-crossover metal–organic frameworks are deterministically assembled with other 2D materials, such as graphene or WSe₂, forming van der Waals heterostructures. The strain concomitant to the spin transition clearly switches the electrical and optical properties of the 2D material. Thus, spin-crossover van der Waals heterostructures represent a new route for band engineering in low-dimensional materials.

Abstract

Van der Waals heterostructures (vdWHs) provide the possibility of engineering new materials with emergent functionalities that are not accessible in another way. These heterostructures are formed by assembling layers of different materials used as building blocks. Beyond inorganic 2D crystals, layered molecular materials remain still rather unexplored, with only few examples regarding their isolation as atomically thin layers. Here, the family of van der Waals heterostructures is enlarged by introducing a molecular building block able to produce strain: the so-called spin-crossover (SCO). In these metal–organic materials, a spin transition can be induced by applying external stimuli like light, temperature, pressure, or an electric field. In particular, smart vdWHs are prepared in which the electronic and optical properties of the 2D material (graphene and WSe₂) are clearly switched by the strain concomitant to the spin transition. These molecular/inorganic vdWHs represent the deterministic incorporation of bistable molecular layers with other 2D crystals of interest in the emergent fields of straintronics and band engineering in low-dimensional materials.

An in-memory photodetector is successfully designed based on a WSe₂/h-BN/SiO₂ van der Waals heterostructure, which demonstrates not only an ultrahigh photoresponsivity (337.8 A W⁻¹) but also an extremely long-term retention (>10 years). Moreover, the working mechanism with charge-storage effect of the h-BN/SiO₂ structure is proved through a novel in situ optoelectronic electron energy-loss spectroscopy analysis.

Abstract

The emerging data-intensive applications in optoelectronics are driving innovation toward the fused integration of sensing, memory, and computing to break through the restrictions of the von Neumann architecture. However, the present photodetectors with only optoelectronic conversion functions cannot satisfy the growing demands of the multifunctions required in single devices. Here, a novel route for the integration of non-volatile memory into a photodetector is proposed, with a WSe₂/h-BN van der Waals heterostructure on a Si/SiO₂ substrate to realize in-memory photodetection. This photodetector exhibits an ultrahigh readout photocurrent of 3.4 µA and photoresponsivity of 337.8 A W⁻¹ in the solar-blind wavelength region, together with an extended retention time of more than 10 years. Furthermore, the charge-storage-based non-volatile mechanism of h-BN/SiO₂ is successfully proven through a novel analysis of in situ optoelectronic electron energy-loss spectroscopy. These results represent a leap forward to future applications and insightful mechanisms of in-memory photodetection.

Employing atomically thin and photoluminescent MoS₂ monolayers, a novel 2D sensing interface is developed to detect the diffusion of probe molecules through cellular layers and tissues. In a single cellular layer, breaks of nanoscale cellular junctions less than 40 nm in length are identified that are otherwise impossible to measure with conventional microscopy.

Abstract

The permeability of cell layers plays a critical role in the life sciences and medicine. It remains a long-standing challenge to assess molecular transport through cell layers with subcellular spatial resolution. Herein, a novel sensing platform employing atomically thin and photoluminescent MoS₂ monolayers as 2D sensing interfaces is developed to detect the diffusion of probe molecules through cellular layers and tissues. The 2D form factor enables monolayer MoS₂ to serve as an array of 20 164 optical sensors covering a total area of 420 × 420 µm², being the only nanosensor interface that can spatially image molecular permeability in this way. In a single layer of human umbilical vein endothelial cells (HUVEC), regions with diffusivities ranging from 1 × 10⁻⁹ to 3 × 10⁻⁸ cm² s⁻¹ are found that are in part spatially correlated with the immunofluorescence of vascular endothelial (VE)-cadherin proteins found on the cell membrane. However, the new technique clearly identifies these locations as breaks of nanoscale cellular junctions less than 40 nm in length in intercellular clefts that are otherwise impossible to measure with conventional microscopy. With the ability to measure permeability through various tissues, this 2D sensing interface allows the measurement of biological properties to assist the development of targeted therapeutics and mechanistic models.

Nature Electronics, Published online: 12 January 2022; doi:10.1038/s41928-021-00707-5

Field-effect transistors based on heterojunctions of hydrogen-terminated diamond and hexagonal boron nitride can offer surface carrier mobilities as high as 680 cm2 V–1 s–1.

To overcome the high exciton binding energy, the efficiency of WSe₂/MoS₂ heterojunction phototransistors is improved by forming periodic arrayed nanopore structures for bandgap engineering. In particular, enhanced carrier lifetime and increased photocurrent are observed with a modulated charge carrier balance of the WSe₂/MoS₂ heterojunction phototransistor in the active area upon illumination.

Abstract

While 2D transition metal dichalcogenides (TMDs) are promising building blocks for various optoelectronic applications, limitations remain for multilayered TMD-based photodetectors: an indirect bandgap and a short carrier lifetime by strongly bound excitons. Accordingly, multilayered TMDs with a direct bandgap and an enhanced carrier lifetime are required for the development of various optoelectronic devices. Here, periodically arrayed nanopore structures (PANS) are proposed for improving the efficiency of multilayered p-WSe₂/n-MoS₂ phototransistors. Density functional theory calculations as well as photoluminescence and time-resolved photoluminescence measurements are performed to characterize the photodetector figures of merit of multilayered p-WSe₂/n-MoS₂ heterostructures with PANS. The characteristics of the heterojunction devices with PANS reveal an enhanced responsivity and detectivity measured under 405 nm laser excitation, which at 1.7 × 10⁴ A W⁻¹ and 1.7 × 10¹³ Jones are almost two orders of magnitude higher than those of pristine devices, 3.6 × 10² A W⁻¹ and 3.6 × 10¹¹ Jones, respectively. Such enhanced optical properties of WSe₂/MoS₂ heterojunctions with PANS represent a significant step toward next-generation optoelectronic applications.

npj 2D Materials and Applications, Published online: 13 January 2022; doi:10.1038/s41699-021-00280-7

Growth of high-quality semiconducting tellurium films for high-performance p-channel field-effect transistors with wafer-scale uniformity