Jiuxiang Dai

4D scanning transmitting electron microscopy (STEM) allows for super-resolution techniques and provides quantitative phase-based information in STEM imaging, but it lacks the ability to acquire chemical and bonding information simultaneously. This study demonstrates that the atomic structure of specimens can still be reconstructed using unconventional detectors such as hollow, asymmetric, or defective ones.

Abstract

With the recent development of high-acquisition-speed pixelated detectors, 4D scanning transmission electron microscopy (4D-STEM) is becoming routinely available in high-resolution electron microscopy. 4D-STEM acts as a “universal” method that provides local information on materials that is challenging to extract from bulk techniques. It extends conventional STEM imaging to include super-resolution techniques and to provide quantitative phase-based information, such as differential phase contrast, ptychography, or Bloch wave phase retrieval. However, an important missing factor is the chemical and bonding information provided by electron energy loss spectroscopy (EELS). 4D-STEM and EELS cannot currently be acquired simultaneously due to the overlapping geometry of the detectors. Here, the feasibility of modifying the detector geometry to overcome this challenge for bulk specimens is demonstrated, and the use of a partial or defective detector for ptycholgaphic structural imaging is explored. Results show that structural information beyond the diffraction-limit and chemical information from the material can be extracted together, resulting in simultaneous multi-modal measurements, adding the additional dimensions of spectral information to 4D datasets.

A universal fabrication strategy based on the ultrathin encapsulation-assisted photolithography and etching is proposed, which yields a large-scale perovskite photodetectors array (48 × 48) with a high resolution of 317 ppi. The device exhibits exceptional sensitivity to weak light sensing with high responsivity and excellent stability. Furthermore, it can capture the clear images under different light intensities and imaging masks.

Abstract

Metal halide perovskite photodetector arrays have demonstrated great potential applications in the field of integrated systems, optical communications, and health monitoring. However, the fabrication of large-scale and high-resolution device is still challenging due to their incompatibility with the polar solvents. Here, a universal fabrication strategy that utilizes ultrathin encapsulation-assisted photolithography and etching to create high-resolution photodetectors array with vertical crossbar structure is reported. This approach yields a 48 × 48 photodetector array with a resolution of 317 ppi. The device shows good imaging capability with a high on/off ratio of 3.3 × 10⁵ and long-term working stability over 12 h. Furthermore, this strategy can be applied to five different material systems, and is fully compatible with the existing photolithography and etching techniques, which are expected to have potential applications in the other high-density and solvent-sensitive devices array, including perovskite- or organic semiconductor-based memristor, light emitting diode displays, and transistors.

In this review, the current states of progress on the development of low-dimensional such as zero-, one- and two-dimensional nanomaterial-based photodetectors are presented. Particularly, the elemental combinations for material design and device applications such as photodiode, photoconductors, and phototransistors are discussed. Subsequently, imaging and neuromorphic devices, and flexible/wearable photodetector applications as well as possible challenges/opportunities are investigated.

Abstract

New emerging low-dimensional such as 0D, 1D, and 2D nanomaterials have attracted tremendous research interests in various fields of state-of-the-art electronics, optoelectronics, and photonic applications due to their unique structural features and associated electronic, mechanical, and optical properties as well as high-throughput fabrication for large-area and low-cost production and integration. Particularly, photodetectors which transform light to electrical signals are one of the key components in modern optical communication and developed imaging technologies for whole application spectrum in the daily lives, including X-rays and ultraviolet biomedical imaging, visible light camera, and infrared night vision and spectroscopy. Today, diverse photodetector technologies are growing in terms of functionality and performance beyond the conventional silicon semiconductor, and low-dimensional nanomaterials have been demonstrated as promising potential platforms. In this review, the current states of progress on the development of these nanomaterials and their applications in the field of photodetectors are summarized. From the elemental combination for material design and lattice structure to the essential investigations of hybrid device architectures, various devices and recent developments including wearable photodetectors and neuromorphic applications are fully introduced. Finally, the future perspectives and challenges of the low-dimensional nanomaterials based photodetectors are also discussed.

Publication date: September 2023

Source: Progress in Materials Science, Volume 138

Author(s): Jeotikanta Mohapatra, Pramanand Joshi, J. Ping Liu

Nature Communications, Published online: 19 May 2023; doi:10.1038/s41467-023-38533-2

Disorder and device variability in hybrid superconductor-semiconductor devices pose challenges for their application in quantum technologies. Here, the authors show that Joule heating can provide a detailed fingerprint of such devices, uncovering different sources of inhomogeneities.

A single-pixel modulation-free plasmonic nanoantenna design strategy based on loss engineering is proposed. This strategy allows a single device to enhance and capture the full and complex fingerprint vibrations in the mid-infrared range. With the aid of machine learning, the absorption fingerprints of various molecules are successfully detected and distinguished.

Abstract

Tailoring light-matter interactions via plasmonic nanoantennas (PNAs) has emerged as a breakthrough technology for spectroscopic applications. The detuning between molecular vibrations and plasmonic resonances, as a fundamental and inevitable optical phenomenon in light-matter interactions, reduces the interaction efficiency, resulting in a weak molecule sensing signal at the strong detuning state. Here, it is demonstrated that the low interaction efficiency from detuning can be tackled by overcoupled PNAs (OC-PNAs) with a high ratio of the radiative to intrinsic loss rates, which can be used for ultrasensitive spectroscopy at strong plasmonic-molecular detuning. In OC-PNAs, the ultrasensitive molecule signals are achieved within a wavelength detuning range of 248 cm⁻¹, which is 173 cm⁻¹ wider than previous works. Meanwhile, the OC-PNAs are immune to the distortion of molecular signals and maintain a lineshape consistent with the molecular signature fingerprint. This strategy allows a single device to enhance and capture the full and complex fingerprint vibrations in the mid-infrared range. In the proof-of-concept demonstration, 13 kinds of molecules with some vibration fingerprints strongly detuning by the OC-PNAs are identified with 100% accuracy with the assistance of machine-learning algorithms. This work gains new insights into detuning-state nanophotonics for potential applications including spectroscopy and sensors.

Semi-floating-gate-controlled 2D lateral homojunctions based on InSe (MoS₂)/hBN/graphite van der Waals heterostructures with an atomically sharp interface on a Si substrate, can be ultrafast-programmed in ≈20 ns. The high rectification ratio of ≈10⁵ and four dynamically switchable non-volatile memory states make them work as rectifiers, non-volatile memories, and reconfigurable multi-valued logic inverters.

Abstract

The development of electrically ultrafast-programmable semiconductor homojunctions can lead to transformative multifunctional electronic devices. However, silicon-based homojunctions are not programmable so that alternative materials need to be explored. Here 2D, multi-functional, lateral homojunctions made of van der Waals heterostructures with a semi-floating-gate configuration on a p⁺⁺ Si substrate feature atomically sharp interfaces and can be electrostatically programmed in nanoseconds, more than seven orders of magnitude faster than other 2D-based homojunctions. By applying voltage pulses with different polarities, lateral p−n, n⁺−n and other types of homojunctions can be formed, varied, and reversed. The p−n homojunctions possess a high rectification ratio of up to ≈10⁵ and can be dynamically switched between four distinct conduction states with the current spanning over nine orders of magnitude, enabling them to function as logic rectifiers, memories, and multi-valued logic inverters. Built on a p⁺⁺ Si substrate, which acts as the control gate, the devices are compatible with Si technology.

We propose that maldistribution of chemical bond strength can lead to large anisotropy of κ in non-layered materials. Taking PbSnS₃, a typical non-layered orthorhombic compound, as an example, our result reveals that the maldistribution of Pb−S bonds leads to an anisotropy ratio up to 7.1 at 200 K and 5.5 at 300 K, respectively, which are among the highest values ever reported in non-layered materials and even surpass many classical layered materials.

Abstract

Currently, the efforts to find materials with high κ anisotropy ratios mainly focus on layered materials, however, the limited quantity and lower workability comparing to non-layered ones boost the exploration of non-layered materials with high κ anisotropy ratios. Here, taking PbSnS₃, a typical non-layered orthorhombic compound, as an example, we propose that maldistribution of chemical bond strength can lead to large anisotropy of κ in non-layered materials. Our result reveals that the maldistribution of Pb−S bonds lead to obvious collective vibrations of dioctahedron chain units, resulting in an anisotropy ratio up to 7.1 at 200 K and 5.5 at 300 K, respectively, which is one of the highest ever reported in non-layered materials and even surpasses many classical layered materials such as Bi₂Te₃ and SnSe. Our findings can not only broaden the horizon for exploring high anisotropic κ materials but also provide new opportunities for the application of thermal management.

A carbon nanotube-assisted exfoliation approach is proposed to enhance the exfoliation yield of large MXene flakes and convert centrifugation residues into small MXene flakes. Furthermore, planar interdigital micro-supercapacitors are assembled to showcase the synergistic effect of combining large- and small-flake MXene, leading to a remarkable volumetric capacitance and energy density.

Abstract

The exceptional properties of 2D transition metal carbides and nitrides (MXene) have led to numerous promising applications. However, the performance of MXene is often dependent on the size of the flakes, and obtaining large-sized, high-quality MXene flakes remains challenging. Herein, a carbon nanotube (CNT)-assisted exfoliation strategy is introduced, which significantly improves the yield of large-flake MXene while entirely transforming the centrifugation residues into small-flake MXene, resulting in waste-free synthesis of MXene. The average size of the obtained MXene flakes can reach over 10 and 0.7 µm for the two populations, respectively. Additionally, the capacitive performance of MXene can be greatly enhanced with a small amount of CNTs. As a proof of concept, a planar interdigital micro-supercapacitor with heterogeneous layers assembled by large- and small-flake MXene simultaneously exhibits outstanding synergistic enhancement in volumetric capacitance (1072 F cm⁻³) and energy density (53.5 mWh cm⁻³). This CNT-assisted method provides explicit directions for achieving the ideal utilization of MXene, improving the yield of large flakes while making judicious use of waste, to meet specific application needs.

Nature Nanotechnology, Published online: 17 May 2023; doi:10.1038/s41565-023-01411-5

The passing of Gordon Moore, an Intel co-founder, is a good time to reflect on the achievements of the semiconductors industry and how nanomaterials could allow Moore’s law to outlive its formulator.

Nature Communications, Published online: 17 May 2023; doi:10.1038/s41467-023-38288-w

Graphite consists of individual layers of graphene stacked vertically and held together via van der Waals forces. Here, Markus et al studied the spin relaxation in graphite, and find a giant anisotropy between spin-relaxation time in graphite, with magnetic field aligned perpendicular to the graphene planes exhibiting a factor of 10 longer spin-relaxation time.

A machine learning attack-resilient ferroelectric true random number generator is demonstrated, unambiguously showing the enhanced stochasticity with near-ideal randomness and reliable endurance against temperature variations. The attack-resilient feature is confirmed using Fourier regressive model and long-short-term-memory approach. Furthermore, the generated cryptographic keys pass the NIST statistical test suite, highlighting the potential of integrating ferroelectric and 2D materials for data encryption.

Abstract

By harnessing the physically unclonable properties, true random number generators (TRNGs) offer significant promises to alleviate security concerns by generating random bitstreams that are cryptographically secured. However, fundamental challenges remain as conventional hardware often requires complex circuitry design, showing a predictable pattern that is susceptible to machine learning attacks. Here, a low-power self-corrected TRNG is presented by exploiting the stochastic ferroelectric switching and charge trapping in molybdenum disulfide (MoS₂) ferroelectric field-effect transistors (Fe-FET) based on hafnium oxide complex. The proposed TRNG exhibits enhanced stochastic variability with near-ideal entropy of ≈1.0, Hamming distance of ≈50%, independent autocorrelation function, and reliable endurance cycle against temperature variations. Furthermore, its unpredictable feature is systematically examined by machine learning attacks, namely the predictive regression model and the long-short-term-memory (LSTM) approach, where nondeterministic predictions can be concluded. Moreover, the generated cryptographic keys from the circuitry successfully pass the National Institute of Standards and Technology (NIST) 800–20 statistical test suite. The potential of integrating ferroelectric and 2D materials is highlighted for advanced data encryption, offering a novel alternative to generate truly random numbers.

Nature Reviews Methods Primers, Published online: 18 May 2023; doi:10.1038/s43586-023-00230-1

This PrimeView highlights how triboelectric nanogenerators can be used to harvest energy from the natural environment.

Nature Physics, Published online: 18 May 2023; doi:10.1038/s41567-023-02061-z

A moiré potential may play a role in determining the magnetic properties of a two-dimensional homo or heterostructure. Now, non-collinear spin structures are observed in twisted double bilayer CrI3, providing a platform to engineer unusual magnetic textures.

Perfectly square SnO₂ nanotubes are synthesized using mist chemical vapor deposition on Au nanoparticle-coated sapphire, silicon, and quartz substrates. Controllable n-type doping with antimony allows the fabrication of a range of single-nanotube electronic devices. The surface of the nanotubes contains an unusually strong and thermally resilient 2D electron gas that shall may prove useful in catalytic and gas sensing applications of these remarkable structures.

Abstract

Nanotechnology has delivered an amazing range of new materials such as nanowires, tubes, ribbons, belts, cages, flowers, and sheets. However, these are usually circular, cylindrical, or hexagonal in nature, while nanostructures with square geometries are comparatively rare. Here, a highly scalable method is reported for producing vertically aligned Sb-doped SnO₂ nanotubes with perfectly-square geometries on Au nanoparticle covered m-plane sapphire using mist chemical vapor deposition. Their inclination can be varied using r- and a-plane sapphire, while unaligned square nanotubes of the same high structural quality can be grown on silicon and quartz. X-ray diffraction measurements and transmission electron microscopy show that they adopt the rutile structure growing in the [001] direction with (110) sidewalls, while synchrotron X-ray photoelectron spectroscopy reveals the presence of an unusually strong and thermally resilient 2D surface electron gas. This is created by donor-like states produced by the hydroxylation of the surface and is sustained at temperatures above 400 °C by the formation of in-plane oxygen vacancies. This persistent high surface electron density is expected to prove useful in gas sensing and catalytic applications of these remarkable structures. To illustrate their device potential, square SnO₂ nanotube Schottky diodes and field effect transistors with excellent performance characteristics are fabricated.

Two switchable 3D surface topographies are created in LC elastomer (LCE) coatings using a two-step imprint lithography process. The resulting LCE coatings display reversible surface switching between the two programmed 3D states.

Abstract

While dynamic surface topographies are fabricated using liquid crystal (LC) polymers, switching between two distinct 3D topographies remains challenging. In this work, two switchable 3D surface topographies are created in LC elastomer (LCE) coatings using a two-step imprint lithography process. A first imprinting creates a surface microstructure on the LCE coating which is polymerized by a base catalyzed partial thiol-acrylate crosslinking step. The structured coating is then imprinted with a second mold to program the second topography, which is subsequently fully polymerized by light. The resulting LCE coatings display reversible surface switching between the two programmed 3D states. By varying the molds used during the two imprinting steps, diverse dynamic topographies can be achieved. For example, by using grating and rough molds sequentially, switchable surface topographies between a random scatterer and an ordered diffractor are achieved. Additionally, by using negative and positive triangular prism molds consecutively, dynamic surface topographies switching between two 3D structural states are achieved, driven by differential order/disorder transitions in the different areas of the film. It is anticipated that this platform of dynamic 3D topological switching can be used for many applications, including antifouling and biomedical surfaces, switchable friction elements, tunable optics, and beyond.

MgFe₂O₄ films are deposited through magnetic field-assisted chemical vapor decomposition using a single-source [MgFe₂(O^tBu)₈] under varying magnetic fields. Morphological and optical analyses indicate that the magnetic field impacts both the grain distribution and absorbance spectra. The X-ray absorption spectra demonstrate that the magnetic field promotes the cation distribution in tetrahedral/octahedral holes, facilitating the degree of inversion in MgFe₂O₄ deposits.

Single-phase magnesium ferrite spinel films (MgFe₂O₄) were grown by magnetic field-assisted chemical vapor deposition (mfCVD) of a mixed-metal precursor compound, [MgFe₂(O^tBu)₈]. The formation of monophasic MgFe₂O₄ deposits as a function of the applied magnetic field strength (B = 0.0, 0.5, and 1.0 T) was investigated and confirmed by X-ray diffraction and photoelectron spectroscopy analyses. Thin film cross-sectional electron microscopic analysis (FIB-SEM) exhibited higher grain growth and densification in MgFe₂O₄ films obtained under the magnetic field influence when compared to spinel samples grown under zero-field conditions. Application of an external magnetic field of varying strengths during the chemical vapor deposition process resulted in a change in the light absorption properties and crystal orientation in the MgFe₂O₄ films, evident in the decreased photoabsorbance analyzed by the UV-Vis spectra and the decrease of intensity of the (400) peak in MgFe₂O₄ films grown under magnetic field. A comprehensive analysis of X-ray diffraction and X-ray magnetic circular dichroism (XMCD) results indicated a higher degree of inversion in MgFe₂O₄ deposits grown in an external magnetic field corroborated by a larger contribution of ligand field transitions of tetrahedrally coordinated Fe(III) centers affecting the visible light absorption of MgFe₂O₄ films.

A series of molybdenum ditelluride (MoTe₂) homobilayers with precisely controlled twist-angles from 0° to 60° exhibit varying exciton-emission behaviors at low temperatures, as the twist-angle increases resulting in changes in interlayer interactions and different moiré superlattices. In a dual-gated 1.4° twisted bilayer MoTe₂, the intralayer neutral and charged excitons from the K-K valleys are identified through gate-dependent and field-dependent photoluminescence measurements.

Abstract

Layered 2H-molybdenum ditelluride (MoTe₂) is a promising near-infrared material with optical activity, which enables hybrid-integrated with silicon photonics for communication purposes. The use of various artificial hetero-stacking or twist-stacking techniques can further expand the emission bandwidth and offer more choices of optical-active materials used as building blocks in on-chip optoelectronic devices. However, while the twisting technique is an effective tool for adjusting interlayer interaction in van der Waals materials, a systematic experimental study of twisted MoTe₂ homobilayers is currently lacking. Here, a series of MoTe₂ homobilayers were prepared with precisely controlled twist-angles from 0° to 60°, with a particular focus on the small-twist region. Conducting photoluminescence measurements at low temperatures enabled observation of the evolution of exciton emission as the twist angle increases. Neutral and charged excitons were also identified in a dual-gated 1.4° twisted MoTe₂ through gate-dependent and field-dependent photoluminescence measurements. Furthermore, spatially-resolved photoluminescence measurements revealed the critical role of the interface conditions, including interlayer spacing and strain, in addition to the twist-angle, in determining the excitonic behavior of the material. This study provides compelling experimental evidence for understanding the twist-angle-dependent excitonic behaviors in atomically thin semiconductors.

A dynamically responsive hydrogel medium composed of two self-assembling materials, chitosan, and SDS, can be reversibly patterned using electronic inputs that switch chitosan between crystalline and electrostatically crosslinked supramolecular states. In addition, salt and water treatments can induce transitions in SDS's supramolecular structure that can reversibly conceal the electronically written patterns by either obscuring the pattern or making it invisible (evanescent).

Abstract

A dynamically responsive hydrogel medium is prepared from two self-assembling components, a polysaccharide (chitosan) and a surfactant (sodium dodecyl sulfate; SDS). It is shown that this medium can be patterned using an electrode “pen” to reconfigure supramolecular structure: cathodic writing induces neutral chitosan chains to form a crystalline network, while anodic writing generates cationic chitosan chains that electrostatically crosslink with anionic SDS micelles. Both supramolecular structures are re-configurable and each is stabilized by structure-induced shifts in chitosan's pKa, thus electronically written patterns can be erased, new patterns can be written, and patterns can be written in three dimensions. Further, it is shown that NaCl-induced morphological transitions of the SDS micelles allow patterns to be reversibly concealed or revealed. To demonstrate the versatility of this medium for information storage, a quick response (QR) code is electronically written and it is shown that this code can be recognized by a standard cellphone app. This QR code can be concealed by making the medium opaque (i.e., by obscuring the pattern) or by making the pattern evanescent (i.e., by making pattern invisible). Overall, this work demonstrates that a dynamically responsive medium composed of simple, safe and sustainable components can be reversibly patterned with spatial and quantitative control using top-down electronic inputs.

Nature Photonics, Published online: 15 May 2023; doi:10.1038/s41566-023-01208-x

We demonstrate an avalanche photodiode design using photon-trapping structures to enhance the quantum efficiency and minimizing the absorber thickness, yielding high quantum efficiency, suppressed dark current density and bandwidth of ~7 GHz.