Enhancing higher-order modal response in multifrequency atomic force microscopy with a coupled cantilever system.

Multifrequency atomic force microscopy (AFM) utilizes the multimode operation of cantilevers to achieve rapid high-resolution imaging and extract multiple properties. However, the higher-order modal response of traditional rectangular cantilever is weaker in air, which affects the sensitivity of multifrequency AFM detection. To address this issue, we previously proposed a bridge/cantilever coupled system model to enhance the higher-order modal response of the cantilever. This model is simpler and less costly than other enhancement methods, making it easier to be widely used. However, previous studies were limited to theoretical analysis and preliminary simulations regarding ideal conditions. In this paper, we undertake a more comprehensive investigation of the coupled system, taking into account the influence of probe and excitation surface sizes on the modal response. To facilitate the exploration of the effectiveness and optimal conditions for the coupled system in practical applications, a macroscale experimental platform is established. By conducting finite element analysis and experiments, we compare the performance of the coupled system with that of traditional cantilevers and quantify the enhancement in higher-order modal response. Also, the optimal conditions for the enhancement of macroscale cantilever modal response are explored. Additionally, we also supplement the characteristics of this model, including increasing the modal frequency of the original cantilever and generating additional resonance peaks, demonstrating the significant potential of the coupled system in various fields of AFM.

Current and future control of the wood-boring pest Anoplophora glabripennis.

The Asian longhorn beetle (ALB) Anoplophora glabripennis is one of the most successful and most feared invasive insect species worldwide. This review covers recent research concerning the distribution of and damage caused by ALB, as well as major efforts to control and manage ALB in China. The distribution and destruction range of ALB have continued to expand over the past decade worldwide, and the number of interceptions has remained high. Detection and monitoring methods for the early discovery of ALB have diversified, with advances in semiochemical research and using satellite remote sensing in China. Ecological control of ALB in China involves planting mixtures of preferred and resistant tree species, and this practice can prevent outbreaks. In addition, strategies for chemical and biological control of ALB have achieved promising results during the last decade in China, especially the development of insecticides targeting different stages of ALB, and applying Dastarcus helophoroides and Dendrocopos major as biocontrol agents. Finally, we analyze recommendations for ALB prevention and management strategies based on native range and invasive area research. This information will hopefully help some invaded areas where the target is containment of ALB.

Additions to The Knowledge of Tubakia (Tubakiaceae, Diaporthales) in China.

The species of Tubakia (Tubakiaceae, Diaporthales, Sordariomycetes) have often been reported as endophytes and pathogens on woody plants. During the investigation of Tubakia species from Fagaceae trees in China, 46 isolates were obtained from diseased leaves and seeds. The characterization of these isolates was based on the observation of morphological characters, the effect of temperature on mycelial growth rate, as well as the combined genes of ITS, tef1 and tub2. As a result, six species were identified: Tubakia americana, T. cyclobalanopsidis sp. nov., T. dryinoides, T. koreana, T. paradryinoides and T. quercicola sp. nov. Among these, T. koreana and T. paradryinoides were firstly discovered in China. Pathogenicity tests were conducted using the conidial suspension on young, excised leaves for these six species, which showed that they were mildly pathogenic to four Fagacece hosts: C. mollissima, Q. acutissima, Q. aliena var. acutiserrata and Q. variabilis.

Gut Lignocellulose Activity and Microbiota in Asian Longhorned Beetle and Their Predicted Contribution to Larval Nutrition.

Anoplophora glabripennis (Asian longhorned beetle) is a wood-boring pest that can inhabit a wide range of healthy deciduous host trees in native and invaded areas. The gut microbiota plays important roles in the acquisition of nutrients for the growth and development of A. glabripennis larvae. Herein, we investigated the larval gut structure and studied the lignocellulose activity and microbial communities of the larval gut following feeding on different host trees. The larval gut was divided into foregut, midgut, and hindgut, of which the midgut is the longest, forming a single loop under itself. Microbial community composition and lignocellulose activity in larval gut extracts were correlated with host tree species. A. glabripennis larvae fed on the preferred host (Populus gansuensis) had higher lignocellulose activity and microbial diversity than larvae reared on either a secondary host (Salix babylonica) or a resistant host (Populus alba var. pyramidalis). Wolbachia was the most dominant bacteria in the gut of larvae fed on S. babylonica and P. alba var. pyramidalis, while Enterococcus and Gibbsiella were the most dominant in larvae fed on P. gansuensis, followed by Wolbachia. The lignocellulose-degrading fungus Fusarium solani was dominant in the larval gut fed on different host trees. Functional predictions of microbial communities in the larval gut fed on different resistant host trees suggested that they all play a role in degrading lignocellulose, detoxification, and fixing nitrogen, which likely contribute to the ability of these larvae to thrive in a broad range of host tree species.

Atomic-Scale Mechanism of Spontaneous Polarity Inversion in AlN on Nonpolar Sapphire Substrate Grown by MOCVD.

The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metal-organic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.

Enhancing higher-order eigenmodes of AFM using bridge/cantilever coupled system.

The wide application of multi-frequency atomic force microscopy (AFM) places higher demands on the higher-order modes response of the cantilever. The response of the higher modes however is generally weaker than that of the fundamental mode in air. Researchers have proposed many methods, most of which involve cantilever modification, to enhance higher-order eigenmodes response. These previous results are proved to be effective, but the microfabrication is expensive. In this article, we propose a novel model based on bridge/cantilever coupled system to enhance the higher-order modes response of AFM cantilever. The segmented beam model provides a new thinking to explain the appearance of undesired peaks in mode analysis of cantilever. Through theoretical analysis and simulation, we find that higher resonance modes are enhanced by tuning the bridge to match the high resonances of the single clamped cantilever. The length, thickness of the coupled system and the location of excitation can affect the enhancement. In summary, this model provides a new way to improve higher mode response for multi-frequency and other high bandwidth applications of AFM.

Reducing molecular simulation time for AFM images based on super-resolution methods.

Atomic force microscopy (AFM) has been an important tool for nanoscale imaging and characterization with atomic and subatomic resolution. Theoretical investigations are getting highly important for the interpretation of AFM images. Researchers have used molecular simulation to examine the AFM imaging mechanism. With a recent flurry of researches applying machine learning to AFM, AFM images obtained from molecular simulation have also been used as training data. However, the simulation is incredibly time consuming. In this paper, we apply super-resolution methods, including compressed sensing and deep learning methods, to reconstruct simulated images and to reduce simulation time. Several molecular simulation energy maps under different conditions are presented to demonstrate the performance of reconstruction algorithms. Through the analysis of reconstructed results, we find that both presented algorithms could complete the reconstruction with good quality and greatly reduce simulation time. Moreover, the super-resolution methods can be used to speed up the generation of training data and vary simulation resolution for AFM machine learning.

Quasi-2D Growth of Aluminum Nitride Film on Graphene for Boosting Deep Ultraviolet Light-Emitting Diodes.

Efficient and low-cost production of high-quality aluminum nitride (AlN) films during heteroepitaxy is the key for the development of deep ultraviolet light-emitting diodes (DUV-LEDs). Here, the quasi-2D growth of high-quality AlN film with low strain and low dislocation density on graphene (Gr) is presented and a high-performance 272 nm DUV-LED is demonstrated. Guided by first-principles calculations, it is found that AlN grown on Gr prefers lateral growth both energetically and kinetically, thereby resulting in a Gr-driven quasi-2D growth mode. The strong lateral growth mode enables most of dislocations to annihilate each other at the AlN/Gr interface, and therefore the AlN epilayer can quickly coalesce and flatten the nanopatterned sapphire substrate. Based on the high quality and low strain of AlN film grown on Gr, the as-fabricated 272 nm DUV-LED shows a 22% enhancement of output power than that with low-temperature AlN buffer, following a negligible wavelength shift under high current. This facile strategy opens a pathway to drastically improve the performance of DUV-LEDs.

Molecular dynamics simulation of bimodal atomic force microscopy.

Bimodal atomic force microscopy (AFM) is an important branch of multi-frequency AFM, which can simultaneously obtain the surface morphology and properties of samples. However, the atomic-scale phenomena in the vibration process of bimodal AFM have not been observed due to the absence of atomic-scale model. In this paper, the molecular dynamics (MD) simulations are used to model bimodal AFM. A double springs oscillator model is used to describe the first two vibration mode of the AFM cantilever. By applying dual-frequencies excitation, the dynamics of the model tip and the tip-substrate interactions are observed. The amplitude, phase shift and the average force change of the tip obtained in the simulation were found to be consistent with the continuum simulation results. The effect of different amplitude ratios on the vibration response of the tip is analyzed and validated by experiments. This novel model makes it possible to simulate two vibration modes of cantilever at atomic scale in bimodal AFM.

Atomic mechanism of strong interactions at the graphene/sapphire interface.

For atomically thin two-dimensional materials, interfacial effects may dominate the entire response of devices, because most of the atoms are in the interface/surface. Graphene/sapphire has great application in electronic devices and semiconductor thin-film growth, but the nature of this interface is largely unknown. Here we find that the sapphire surface has a strong interaction with some of the carbon atoms in graphene to form a C-O-Al configuration, indicating that the interface interaction is no longer a simple van der Waals interaction. In addition, the structural relaxation of sapphire near the interface is significantly suppressed and very different from that of a bare sapphire surface. Such an interfacial C-O-Al bond is formed during graphene growth at high temperature. Our study provides valuable insights into understanding the electronic structures of graphene on sapphire and remote control of epitaxy growth of thin films by using a graphene-sapphire substrate.

Enhancement of Heat Dissipation in Ultraviolet Light-Emitting Diodes by a Vertically Oriented Graphene Nanowall Buffer Layer.

For III-nitride-based devices, such as high-brightness light-emitting diodes (LEDs), the poor heat dissipation of the sapphire substrate is deleterious to the energy efficiency and restricts many of their applications. Herein, the role of vertically oriented graphene (VG) nanowalls as a buffer layer for improving the heat dissipation in AlN films on sapphire substrates is studied. It is found that VG nanowalls can effectively enhance the heat dissipation between an AlN film and a sapphire substrate in the longitudinal direction because of their unique vertical structure and good thermal conductivity. Thus, an LED fabricated on a VG-sapphire substrate shows a 37% improved light output power under a high injection current (350 mA) with an effective 3.8% temperature reduction. Moreover, the introduction of VG nanowalls does not degrade the quality of the AlN film, but instead promotes AlN nucleation and significantly reduces the epilayer strain that is generated during the cooling process. These findings suggest that the VG nanowalls can be a good buffer layer candidate in III-nitride semiconductor devices, especially for improving the heat dissipation in high-brightness LEDs.

Tracking sodium migration in TiS2 using in situ TEM.

For alkali-metal ion batteries, revealing the phase transformation and the ion migration dynamics in the electrodes is vital for understanding how the electrodes work and thereby how we can improve them. Here, using in situ transmission electron microscopy, we track the structural evolution and migration dynamics during sodium insertion into TiS2 nanostructures with the lattice fringe resolution. We find that the sodiation process of TiS2 is initiated by an intercalation reaction and followed by a conversion reaction. From the same reaction event, the velocity of intercalation/conversion phase boundary migration is found to be ∼1.0-1.7 nm s-1, while the pristine/intercalation phase boundary migrates at a velocity of ∼2.5 nm s-1. The sodium migration leads to structural fracture to form nanometer-sized domains (∼3 nm) with volume expansion. During migration, Na prefers to transport along specific directions. Furthermore, a superstructured Na0.25TiS2 intermediate phase with ordered Na ions occupied within the (0001) plane is formed at the reaction front, which is different from the common staging phase. These findings help us understand the working principle and the failure mechanism of the sodium ion battery and also provide useful insights into the general ionic doping of transition metal dichalcogenides.

Improved Epitaxy of AlN Film for Deep-Ultraviolet Light-Emitting Diodes Enabled by Graphene.

The growth of single-crystal III-nitride films with a low stress and dislocation density is crucial for the semiconductor industry. In particular, AlN-derived deep-ultraviolet light-emitting diodes (DUV-LEDs) have important applications in microelectronic technologies and environmental sciences but are still limited by large lattice and thermal mismatches between the epilayer and substrate. Here, the quasi-van der Waals epitaxial (QvdWE) growth of high-quality AlN films on graphene/sapphire substrates is reported and their application in high-performance DUV-LEDs is demonstrated. Guided by density functional theory calculations, it is found that pyrrolic nitrogen in graphene introduced by a plasma treatment greatly facilitates the AlN nucleation and enables fast growth of a mirror-smooth single-crystal film in a very short time of ≈0.5 h (≈50% decrease compared with the conventional process), thus leading to a largely reduced cost. Additionally, graphene effectively releases the biaxial stress (0.11 GPa) and reduces the dislocation density in the epilayer. The as-fabricated DUV-LED shows a low turn-on voltage, good reliability, and high output power. This study may provide a revolutionary technology for the epitaxial growth of AlN films and provide opportunities for scalable applications of graphene films.

Wavelet analysis of higher harmonics in tapping mode atomic force microscopy.

Higher harmonics have been widely used to characterize nanomechanical properties of the sample surface in tapping mode atomic force microscopy. They are usually analyzed by the Fourier transform method which provides time-averaged amplitude and phase information. In this paper, we apply the analytic wavelet transform to analyze higher harmonics. The intuitive descriptions of higher harmonics are obtained by the time-frequency analysis of the tip motion signal. The temporal evolutions of the higher harmonics are analyzed. The higher harmonics extracted by the analytic wavelet transform are closely related to the wavelet parameters. Different time and frequency features of higher harmonics can be analyzed through adjusting the wavelet parameters. Moreover, the root-mean-square amplitude and the peak amplitude obtained by the analytic wavelet transform can provide better characterization of sample properties than the amplitude obtained by the Fourier transform method.

Low Residual Carrier Concentration and High Mobility in 2D Semiconducting Bi2O2Se.

The air-stable and high-mobility two-dimensional (2D) Bi2O2Se semiconductor has emerged as a promising alternative that is complementary to graphene, MoS2, and black phosphorus for next-generation digital applications. However, the room-temperature residual charge carrier concentration of 2D Bi2O2Se nanoplates synthesized so far is as high as about 1019-1020 cm-3, which results in a poor electrostatic gate control and unsuitable threshold voltage, detrimental to the fabrication of high-performance low-power devices. Here, we first present a facile approach for synthesizing 2D Bi2O2Se single crystals with ultralow carrier concentration of ∼1016 cm-3 and high Hall mobility up to 410 cm2 V-1 s-1 simultaneously at room temperature. With optimized conditions, these high-mobility and low-carrier-concentration 2D Bi2O2Se nanoplates with domain sizes greater than 250 µm and thicknesses down to 4 layers (∼2.5 nm) were readily grown by using Se and Bi2O3 powders as coevaporation sources in a dual heating zone chemical vapor deposition (CVD) system. High-quality 2D Bi2O2Se crystals were fabricated into high-performance and low-power transistors, showing excellent current modulation of >106, robust current saturation, and low threshold voltage of -0.4 V. All these features suggest 2D Bi2O2Se as an alternative option for high-performance low-power digital applications.

Iridium-Tungsten Alloy Nanodendrites as pH-Universal Water-Splitting Electrocatalysts.

The development of highly efficient and durable electrocatalysts for high-performance overall water-splitting devices is crucial for clean energy conversion. However, the existing electrocatalysts still suffer from low catalytic efficiency, and need a large overpotential to drive the overall water-splitting reactions. Herein, we report an iridium-tungsten alloy with nanodendritic structure (IrW ND) as a new class of high-performance and pH-universal bifunctional electrocatalysts for hydrogen and oxygen evolution catalysis. The IrW ND catalyst presents a hydrogen generation rate ∼2 times higher than that of the commercial Pt/C catalyst in both acid and alkaline media, which is among the most active hydrogen evolution reaction (HER) catalysts yet reported. The density functional theory (DFT) calculations reveal that the high HER intrinsic catalytic activity results from the suitable hydrogen and hydroxyl binding energies, which can accelerate the rate-determining step of the HER in acid and alkaline media. Moreover, the IrW NDs show superb oxygen evolution reaction (OER) activity and much improved stability over Ir. The theoretical calculation demonstrates that alloying Ir metal with W can stabilize the formed active iridium oxide during the OER process and lower the binding energy of reaction intermediates, thus improving the Ir corrosion resistance and OER kinetics. Furthermore, the overall water-splitting devices driven by IrW NDs can work in a wide pH range and achieve a current density of 10 mA cm-2 in acid electrolyte at a low potential of 1.48 V.

Fast Growth of Strain-Free AlN on Graphene-Buffered Sapphire.

We study the roles of graphene acting as a buffer layer for growth of an AlN film on a sapphire substrate. Graphene can reduce the density of AlN nuclei but increase the growth rate for an individual nucleus at the initial growth stage. This can lead to the reduction of threading dislocations evolved at the coalescence boundaries. The graphene interlayer also weakens the interaction between AlN and sapphire and accommodates their large mismatch in the lattice and thermal expansion coefficients; thus, the compressive strain in AlN and the tensile strain in sapphire are largely relaxed. The effective relaxation of strain further leads to a low density of defects in the AlN films. These findings reveal the roles of graphene in III-nitride growth and offer valuable insights into the efficient applications of graphene in the light-emitting diode industry.

Atomic-Scale Probing of Reversible Li Migration in 1T-V1+ xSe2 and the Interactions between Interstitial V and Li.

Ionic doping and migration in solids underpins a wide range of applications including lithium ion batteries, fuel cells, resistive memories, and catalysis. Here, by in situ transmission electron microscopy technique we directly track the structural evolution during Li ions insertion and extraction in transition metal dichalcogenide 1T-V1+ xSe2 nanostructures which feature spontaneous localized superstructures due to the periodical interstitial V atoms within the van der Waals interlayers. We find that lithium ion migration destroys the cationic orderings and leads to a phase transition from superstructure to nonsuperstructure. This phase transition is reversible, that is, the superstructure returns back after extraction of lithium ion from Li yV1+ xSe2. These findings provide valuable insights into understanding and controlling the structure and properties of 2D materials by general ionic and electric doping.

Time-frequency analysis of the tip motion in liquids using the wavelet transform in dynamic atomic force microscopy.

The tip motion of the dynamic atomic force microscope in liquids shows complex transient behaviors when using a low stiffness cantilever. The second flexural mode of the cantilever is momentarily excited. Multiple impacts between the tip and the sample might occur in one oscillation cycle. However, the commonly used Fourier transform method cannot provide time-related information about these transient features. To overcome this limitation, we apply the wavelet transform to perform the time-frequency analysis of the tip motion in liquids. The momentary excitation of the second mode and the phenomenon of multiple impacts are clearly shown in the time-frequency plane of the wavelet scalogram. The instantaneous frequencies and magnitudes of the second mode are extracted by the wavelet ridge analysis, which can provide quantitative estimations of the tip motion in the second mode. Moreover, the relations of the maximum instantaneous magnitude (MIM) to the amplitude setpoint and the Young's modulus of the sample surface are investigated. The results suggest that the MIM can be used to characterize the nanomechanical property of the sample surface at high amplitude setpoints.

High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer.

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and Inx Ga1-x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs.