In situ investigation of the mechanical properties of nanomaterials by transmission electron microscopy

With the progress of modern transmission elec- tron microscopy （TEM） and development of dedicated func- tional TEM specimen holders, people can now manipulate a nano-object with nanometer-range precision and simulta- neously acquire mechanical data together with atomic-scale structural information. This advanced methodology is play- ing an increasingly important role in nanomechanics. The present review summarizes relevant studies on the in situ in- vestigation of mechanical properties of various nanomateri- als over the past decades. These works enrich our knowledge not only on nanomaterials （such as carbon nanotubes, car- bon onions, boron nitride nanotubes, silicon nanowires and graphene, etc.） but also on mechanics at the nanoscate.

Influence of Width of Left Well on Intersubband Transitions in AlxGa1-x N/GaN Double Quantum Wells

Influence of width of left well in AlxGa1-xN/GaN double quantum wells （DQWs） on absorption eoefficients and wavelengths of the intersubband transitions （ISBTs） is investigated by solving the Sehroedinger and Poisson equations self-consistently. When the width of left well is 1.79 nm, three-energy-level DQWs are realized. The ISBT between the first odd and second odd order subbands （the lodd-2odd ISBT） has a comparable absorption eoefficient with the lodd-2even ISBT. Their wavelengths are located at 1.3 and 1.55 μm, respectively. When the width of left well is 1.48nm, a four-energy-level DQWs is realized. The calculated results have a possible application to ultrafast two-eolour optoeleetronie devices operating within the optical communication wavelength range.

Effect of （2 ×1） Surface Reconstruction on Elasticity of a Silicon Nano-Plate

A semi-continuum approach is developed to describe the effect of （2 ×1） surface reconstruction on the elastic modulus of the silicon nano-plate. Young＇s moduli of a （001） silicon nano-plate along the high-symmetry [100] direction are obtained with and without considering （2 × 1） surface reconstruction. The approach predicts that the nano-plate with unreconstructed （001） surface is elastically softer than the bulk while it exhibits the opposite behaviour with （2 × 1） reconstructed surface. On the （001） surface, the （2 × 1） reconstructed surface dominates the plate as the thickness of the plate scaling decreases to several tens of nanometre. Whether the nano-plate is softer or stiffer depends on bond loss, bond saturation and direction of bond alignment, which have major impacts on the mechanics of the nano-plate.

Thermodynamic and Kinetic Analysis of Low-temperature Thermal Reduction of Graphene Oxide

The thermodynamic state and kinetic process of low-temperature deoxygenation reaction of graphene oxide(GO) have been investigated for better understanding on the reduction mechanism by using Differential Scanning Calorimetry(DSC), Thermogravimetry-Mass Spectrometry(TG-MS), and X-ray Photoelectron Spectroscopy(XPS). It is found that the thermal reduction reaction of GO is exothermic with degassing of CO_2, CO and H_2O. Graphene is thermodynamically more stable than GO. The deoxygenation reaction of GO is kinetically controlled and the activation energy for GO is calculated to be 167 k J/mol(1.73 e V/atom).

Theoretical study of electromechanical property in a p-type silicon nanoplate for mechanical sensors

Electromechanical property of a p-type single-crystal silicon nanoplate is modelled by a microscopic approach where the hole quantization effect and the spin-orbit coupling effect are taken into account. The visible anisotropic subband structures are calculated by solving self-consistently the stress-dependent 6×6 k.p Schrodinger equation with the Poisson equation. The strong mixing among heavy, light, and split-off holes is quantitatively assessed. The influences of the thickness and the temperature on the piezoresistive coefficient are quantitatively investigated by using the hole concentrations and the effective masses from the complex dispersion structure of the valence band with and without stresses. Our results show that the stress determines the extent to which the band is mixed. The hole quantization effect increases as the thickness decreases, and therefore the valence band is strongly reshaped, resulting in the size-dependent piezoresistivity of the silicon nanoplate. The piezoresistive coefficient increases almost 4 times as the thickness reduces from the bulk to 3 nm, exhibiting a promising application in mechanical sensors.

Capacity Contribution Mechanism of rGO for SnO_(2)/rGO Composite as Anode of Lithium-ion Batteries

Compared with ordinary graphite anode,SnO_(2) possesses higher theoretical specifc capacity,rich raw materials and low price.While the severe volume expansion of SnO_(2) during lithium-ion extraction/intercalation limits its further application.To solve this problem,in this work the reduced graphene oxide(rGO)was introduced as volume bufer matrix of SnO_(2).Herein,SnO_(2)/rGO composite is obtained through one-step hydrothermal method.Three-dimensional structure of rGO could efectively hinder the polymerization of SnO_(2) nanoparticles and provide more lithium storage sites attributed to high specifc surface area and density defects.The initial discharge capacity of the composite cathode is 959 mA·h·g^(-1) and the capacity remained at 300 mA·h·g^(-1) after 1000 cycles at 1 C.It proved that the rGO added in the anode has a capacity contribution to the lithium-ion battery.It changes the capacity contribution mechanism from difusion process dominance to surface driven capacitive contribution.Due to the addition of rGO,the anode material gains stable structure and great conductivity.

Etching of quartz crystals in liquid phase environment:A review

Quartz crystals are the most widely used material in resonant sensors,owing to their excellent piezoelectric and mechanical properties.With the development of portable and wearable devices,higher processing efficiency and geometrical precision are required.Wet etching has been proven to be the most efficient etching method for large-scale production of quartz devices,and many wet etching approaches have been developed over the years.However,until now,there has been no systematic review of quartz crystal etching in liquid phase environments.Therefore,this article provides a comprehensive review of the development of wet etching processes and the achievements of the latest research in thisfield,covering conventional wet etching,additive etching,laser-induced backside wet etching,electrochemical etching,and electrochemical discharge machining.For each technique,a brief overview of its characteristics is provided,associated problems are described,and possible solutions are discussed.This review should provide an essential reference and guidance for the future development of processing strategies for the manufacture of quartz crystal devices.

Anisotropic metal–insulator transition in strained VO_(2)(B) single crystal

Mechanical strain can induce noteworthy structural and electronic changes in vanadium dioxide, imparting substantial scientific importance to both the exploration of phase transitions and the development of potential technological applications. Unlike the traditional rutile(R) phase, bronze-phase vanadium dioxide [VO_(2)(B)] exhibits an in-plane anisotropic structure. When subjected to stretching along distinct crystallographic axes, VO_(2)(B) may further manifest the axial dependence in lattice–electron interactions, which is beneficial for gaining insights into the anisotropy of electronic transport.Here, we report an anisotropic room-temperature metal–insulator transition in single-crystal VO_(2)(B) by applying in-situ uniaxial tensile strain. This material exhibits significantly different electromechanical responses along two anisotropic axes.We reveal that such an anisotropic electromechanical response mainly arises from the preferential arrangement of a straininduced unidirectional stripe state in the conductive channel. This insulating stripe state could be attributed to the in-plane dimerization within the distorted zigzag chains of vanadium atoms, evidenced by strain-modulated Raman spectra. Our work may open up a promising avenue for exploiting the anisotropy of metal–insulator transition in vanadium dioxide for potential technological applications.

Aligning Fe_(2)O_(3)photo-sheets on TiO_(2)nanofibers with hydrophilic and aerophobic surface for boosting photoelectrochemical performance

Photoelectrochemical(PEC)nanomaterials are critical to producing clean oxygenation or value-added chemical production by utilizing sustainable solar energy,but are always limited by simultaneous integration of architectural engineering and electronic regulation in one structure.Directed by density functional theory(DFT)calculations and finite element analysis(FEA),the bioinspired ivy-like Fe_(2)O_(3)heterostructures with enriched oxygen defects on TiO_(2)nanofibers are designed for boosting PEC performances.Ivy-like Fe_(2)O_(3)photo-sheets remarkably enhanced the light harvesting by multiple light-mater interactions.The oxygen vacancies on Fe_(2)O_(3)photo-sheets could aid the photons catching and promote the reactivity at active sites.More importantly,demonstrated by a well-designed dynamic observation,the abundant tip-edges within ivy-like Fe_(2)O_(3)photo-sheets enabled the surface of heterostructure with hydrophilic and aerophobic properties.The functionalized surface allowed the rapid desorption of produced bubbles and thus ensured a high density of unoccupied active sites for electrolyte accessing.Featured by these attributes,the Fe_(2)O_(3)@TiO_(2)nanofibers delivered an excellent photocurrent of 40.8 mA/mg,high donor density(1.2×10^(18)cm^(−3)),and rapid oxygen production rate(1 mmol/(L∙h)).This work demonstrates a new strategy on nano-structural design for enhancing light-harvesting and making a hydrophilic/aerophobic surface on low-dimensional oxide nanomaterial,holding great potential on designing high-performance PEC devices for producing survival source gas,carbon-neutral fuel,and valuedchemicals.

In situ atomic-scale observation of size-dependent (de) potassiation and reversible phase transformation in tetragonal FeSe anodes

Potassium-ion batteries(PIBs)are considered promising alternatives to lithium-ion batteries owing to cost-effective potassium resources and a suitable redox potential of-2.93 V(vs.-3.04 V for Li+/Li).However,the exploration of appro-priate electrode materials with the correct size for reversibly accommodating large K+ions presents a significant challenge.In addition,the reaction mecha-nisms and origins of enhanced performance remain elusive.Here,tetragonal FeSe nanoflakes of different sizes are designed to serve as an anode for PIBs,and their live and atomic-scale potassiation/depotassiation mechanisms are revealed for the first time through in situ high-resolution transmission electron micros-copy.We found that FeSe undergoes two distinct structural evolutions,sequen-tially characterized by intercalation and conversion reactions,and the initial intercalation behavior is size-dependent.Apparent expansion induced by the intercalation of K+ions is observed in small-sized FeSe nanoflakes,whereas unexpected cracks are formed along the direction of ionic diffusion in large-sized nanoflakes.The significant stress generation and crack extension originating from the combined effect of mechanical and electrochemical interactions are elucidated by geometric phase analysis and finite-element analysis.Despite the different intercalation behaviors,the formed products of Fe and K_(2)Se after full potassiation can be converted back into the original FeSe phase upon depotassiation.In particular,small-sized nanoflakes exhibit better cycling perfor-mance with well-maintained structural integrity.This article presents the first successful demonstration of atomic-scale visualization that can reveal size-dependent potassiation dynamics.Moreover,it provides valuable guidelines for optimizing the dimensions of electrode materials for advanced PIBs.

Thermal stability of Te nanowires and their crystallographydetermined surface evolution at elevated temperatures

Nanowires are fantastic nanostructures for designing new functional devices because of their extraordinary properties.However,nanowires usually suffer pronounced size and surface effects with decreasing diameter size.Whether their structure and thermal stability can still fill the requirements of practical applications is a critical issue to be figured out.Herein,Te nanowires with diameters ranging from sub-10 to over 80 nm are used as samples to probe into this issue.In situ heating experiments are performed on these Te nanowires using an aberration-corrected transmission electron microscopy combined with a chip-based heating holder.It is found that Te nanowires suffer sublimation at elevated temperatures rather than melting,showing sizedependent sublimation scenarios.The Te nanowires with diameter smaller than 20 nm sublimate below 205℃,while the larger ones with diameter around 85 nm require a higher temperature of around 225℃.During sublimation-induced shape evolution,the interfacial wetting equilibrium and crystal orientations play critical roles,leading to the formation of spherical surfaces or featured facets at the free surfaces.A mean contact angle of 107.5°is determined at the C-Te interface when the crystalline Te nanowires stay in a quasi-liquid equilibrium state.However,once the crystalline feature is overwhelming,e.g.,at moderate temperatures,the(1011),(1120),and(1010)facets govern the free surface,despite the wetting condition at the interfaces.

Design and fabrication of a terminating type MEMS microwave power sensor

A terminating type MEMS microwave power sensor based on the Seebeck effect and compatible with the GaAs MMIC process is presented. An electrothermal model is introduced to simulate the heat transfer behavior and temperature distribution. The sensor measured the microwave power from –20 to 20 dBm up to 20 GHz. The sensitivity of the sensor is 0.27 mV/mW at 20 GHz, and the input return loss is less than –26 dB over the entire experiment frequency range. In order to improve the sensitivity, four different types of coplanar waveguide （CPW） were designed and the sensitivity was significantly increased by about a factor of 2.

Sensitivity of MEMS microwave power sensor with the length of thermopile based on Fourier equivalent model

A Fourier equivalent model is introduced to research the thermal transfer behavior of a terminating-type MEMS microwave power sensor.The fabrication of this MEMS microwave power sensor is compatible with the GaAs MMIC process.Based on the Fourier equivalent model,the relationship between the sensitivity of a MEMS microwave power sensor and the length of thermopile is studied in particular.The power sensor is measured with an input power from 1 to 100 mW at 10 GHz,and the measurement results show that the power sensor has good input match characteristics and high linearity.The sensitivity calculated from a Fourier equivalent model is about 0.12,0.20 and 0.29 mV/mW with the length at 40,70 and 100μm,respectively,while the sensitivity of the measurement results is about 0.10,0.22 and 0.30 mV/mW,respectively,and the differences are below 0.02 mV/mW. The sensitivity expression based on the Fourier equivalent model is verified by the measurement results.

Thermal time constant of a terminating type MEMS microwave power sensor

A terminating type MEMS microwave power sensor based on the Seebeck effect and compatible with the GaAs MMIC process is presented.An electrothermal model is introduced to simulate the thermal time constant. An analytical result,about 160 ms,of the thermal time constant from the non-stationary Fourier heat equations for the structure of the sensor is also given.The sensor measures the microwave power jumping from 15 to 20 dBm at a constant frequency 15 GHz,and the experimental thermal time constant result is 180 ms.The frequency is also changed from 20 to 10 GHz with a constant power 20 dBm,and the result is also 180 ms.Compared with the analytical and experimental results,the model is verified.

integrated structure for recognition of different joint motion states with the assistance of a deep learning algorithm

Accurate motion feature extraction and recognition provide critical information for many scientific problems.Herein,a new paradigm for a wearable seamless multimode sensor with the ability to decouple pressure and strain stimuli and recognize the different joint motion states is reported.This wearable sensor is integrated into a unique seamless structure consisting of two main parts(a resistive component and a capacitive component)to decouple the different stimuli by an independent resistance-capacitance sensing mechanism.The sensor exhibits both high strain sensitivity(GF=7.62,0-140%strain)under the resistance mechanism and high linear pressure sensitivity(S=3.4 kPa^(-1),0-14 kPa)under the capacitive mechanism.The sensor can differentiate the motion characteristics of the positions and states of different joints with precise recognition(97.13%)with the assistance of machine learning algorithms.The unique integrated seamless structure is achieved by developing a layer-bylayer casting process that is suitable for large-scale manufacturing.The proposed wearable seamless multimode sensor and the convenient process are expected to contribute significantly to developing essential components in various emerging research fields,including soft robotics,electronic skin,health care,and innovative sports systems applications.

In Situ Observation of Crystalline Silicon Growth from SiO_(2) at Atomic Scale

The growth of crystalline Si(c-Si)via direct electron beam writing shows promise for fabricating Si nanomaterials due to its ultrahigh resolution.However,to increase the writing speed is a major obstacle,due to the lack of systematic experimental explorations of the growth process and mechanisms.This paper reports a systematic experimental investigation of the beaminduced formation of c-Si nanoparticles(NPs)from amorphous SiO_(2) under a range of doses and temperatures by in situ transmission electron microscopy at the atomic scale.A three-orders-of-magnitude writing speed-up is identified under 80 keV irradiation at 600℃ compared with 300 keV irradiation at room temperature.Detailed analysis reveals that the self-organization of c-Si NPs is driven by reduction of c-Si effective free energy under electron irradiation.This study provides new insights into the formation mechanisms of c-Si NPs during direct electron beam writing and suggests methods to improve the writing speed.

Pushing the limit of thermal conductivity of MAX borides and MABs

The emergence of MAX borides as well as MAB phases attracted great attention because of the renewable developments of ternary ceramics and offering great opportunities in potential applications.However,the number of borides remains limited,and further fundamental descriptions and detailed investigations on various properties are still lacking.In this report,we employ an integrated computational scheme that combines density functional theory with the evolutional algorithm to search for the favorable structures of P-and S-glued ternary borides terminated by Nb metal.We discover that the structures of 212-type,as e.g.Nb_(2)PB_(2)and Nb_(2)SB_(2),belong to the P6m2 space group,while those of 211-type,as e.g.Nb_(2)PB and Nb_(2)SB,prefer to crystallize in the P6_(3)/mmc space group,and the corresponding carbides Nb_(2)PC and Nb_(2)SC are also considered for the sake of completeness and comparative analsys.The predicted Nb_(2)PB_(2),Nb_(2)PB,Nb_(2)SB,Nb_(2)PC and Nb_(2)SC are energetically stable,as revealed by the negative formation energies and by the proposed reaction paths with respect to the most competing phases,as well as dynamically stable,as suggested by the non-imaginary phonon spectra.The thermal conductivities of the six materials show unusual behaviors,particularly for the acoustic and optical contributions,and are accompanied by a strong anisotropy.Most importantly,Nb_(2)PB_(2) is found to be an excellent thermal conductor with a total thermal conductivity of~65 W/(m K),while Nb_(2)SC is found to be an ultra-low thermal conductor,with a total thermal conductivity of~5 W/(m K).These values are clearly outside the currently reported range of thermal conductivities,which makes Nb_(2)PB_(2)and Nb_(2)SC extremely interesting for fundamental research as well as prospective applications with the aid of artificial tunings on the almost independent MB block and the A layer.The discovery of these novel materials is expected to contribute substantially to the rapid development of ternary ceramics and to accelerate attempts in the applicability of MAX phases for heat conduction.

Design Optimization of Pillar Bump Structure for Minimizing the Stress in Brittle Low K Dielectric Material Layer

Cu pillar bump offers a number of advantages for flip chip packaging,compared to the conventional solder bump.However,due to its rigidity structure,Cu pillar bump introduces a lot of stress to the chip,which causes the failure of packaging structures,especially for the advanced node devices which typically have brittle low K dielectric material.In this paper,for the first time we propose two types of Cu pillar structures to reduce the stress.The first Cu pillar structure has bigger Cu dimensions at the base.The other one is designed to add an additional Cu pad under the Cu pillar bump.Finite element analysis is used to study the stress of the both structures,and it is found that with the increase in pillar bump contact area over the chip surface,the stress decreases in both structures.Results also indicate that the Cu pillar bump undercut induces higher stress,and thin Cu6 Snss intermetallic compound has less impact on the stress during flip chip mount reflow.The study provides a novel way to improve the reliability by reducing the stress in the Cu pillar bump related packaging.

Efficient rejuvenation of heterogeneous{[(Fe_(0.5)Co_(0.5))_(0.75)B_(0.2)Si_(0.05)]_(96)Nb_(4)}_(99.9)Cu_(0.1) bulk metallic glass upon cryogenic cycling treatment

The effects of cryogenic thermal cycling on deformation behaviour and structural variation of{[(Fe_(0.5)Co_(0.5))_(0.75)B_(0.2)Si_(0.05)]_(96)Nb_(4)}_(99.9)Cu_(0.1) bulk metallic glass(BMG)were studied and compared with Cufree[(Fe_(0.5)Co_(0.5))_(0.75)B_(0.2)Si_(0.05)]_(96)Nb_(4) BMG.After thermal-cycled treatment between 393 K and cryogenic temperature,the{[(Fe_(0.5)Co_(0.5))_(0.75)B_(0.2)Si_(0.05)]_(96)Nb_(4)}_(99.9)Cu_(0.1)BMG obtained a plastic strain of 7.4%combined with a high yield strength of 4350 MPa.The excellent soft magnetic properties were maintained after CTC treatment.The minor addition of Cu element results in an initial nano-sized heterogeneity in the matrix,which facilitates the rejuvenation process during thermal cycling,and brings to a low optimal thermal temperature of 393 K,making the{[(Fe_(0.5)Co_(0.5))_(0.75)B_(0.2)Si_(0.05)]_(96)Nb_(4)}_(99.9)Cu_(0.1) BMG more attractive in industrial application.During thermal cycling,the formation of more soft regions leads to the increase of structural heterogeneities,which is beneficial to the initiation of shear transition zones and the formation of multiple shear bands,and thus results in the enhancement of plasticity.This study links the subtle variation of specific structure with macroscopic mechanical properties,and provides a new insight of composition selection for cryogenic thermal cycling treatment.

Atomic-scale insights into the formation of 2D crystals from in situ transmission electron microscopy

Two-dimensional(2D)crystals are attractive due to their intriguing structures and properties which are strongly dependent on the synthesis conditions.To achieve their superior properties,it is of critical importance to fully understand the growth processes and mechanisms for tailored design and controlled growth of 2D crystals.Due to the high spatiotemporal resolution and the capability to mimic the realistic growth conditions,in situ transmission electron microscopy(TEM)becomes an effective way to monitor the growth process in real-time at the atomic scale,which is expected to provide atomic-scale insights into the nucleation and growth of 2D crystals.Here we review the recent in situ TEM works on the formation of 2D crystals under electron irradiation,thermal excitation as well as voltage bias.The underlying mechanisms are also elucidated in detail,providing key insights into the nucleation and formation of 2D crystals.