Critical Thickness Research Articles

Sensibility in the impact of aeolian sand cover on permafrost degradation in Qinghai-Xizang Plateau

Abstract Environmental transformations and intensifying desertification across the Qinghai-Xizang Plateau significantly influence permafrost degradation, heightening risks associated with carbon emissions, thermal hazards, and infrastructural damage. However, the specific response of permafrost to desertification remains insufficiently understood. Here, we employed numerical modelling to examine the sensitivity of the impact of aeolian sand cover (ASC) on permafrost degradation. Our findings reveal that the thickness and moisture content of ASC profoundly affects permafrost degradation. Moreover, permafrost thermal stability and the rate of climatic warming modulate this degradation process. The simulation results identify two critical thickness thresholds for ASC: 20 cm and 80-120 cm. Specifically, dry ASC thinner than 20 cm accelerates permafrost degradation driven by desertification, whereas ASC thicker than 20 cm mitigates this effect. Furthermore, increased moisture in ASC extends the thickness thresholds to 80-120 cm. These results suggest that climatic variations in the Qinghai-Xizang Plateau, particularly transitions towards either warming-drying or warming-wetting, will markedly influence the response of permafrost to desertification. Notably, a warming-drying climate may reduce the potential degradation of permafrost caused by desertification. This study provides a critical reference for understanding the impact of aeolian desertification on permafrost in regions beyond the Qinghai-Xizang Plateau. It holds significant policy implications for environmental conservation and infrastructure development within the plateau.

Spin conduction and interfacial effects in La2/3Sr1/3MnO3/NiO/Pt heterostructures

In this work we report on the generation and transport of pure spin currents in La2/3Sr1/3MnO3(LSMO)/NiO/Pt heterostructures. Pure spin currents, generated by means of spin pumping in the LSMO layer, are transmitted through the NiO layer and detected by measuring the transverse voltage signal generated by the inverse spin Hall effect (ISHE) in the Pt layer. The spin current transmission is studied as a function of the NiO thickness and temperature. NiO layers grown at room temperature are polycrystalline and their microstructural and magnetic features clearly depend on thickness. Scanning transmission electron microscopy techniques revealed that homogeneous conformal coating of the LSMO layer is only achieved for NiO layer thicknesses above about 2 nm. Below this critical thickness the NiO layer is discontinuous and its main role would be to disturb the LSMO/Pt interface causing it to behave as something similar to an insulating barrier. Magnetic measurements indicate that NiO layers are paramagnetic (PM) well below room temperature. In agreement with these observations, the amplitude of the ISHE voltage signal decreases monotonically on lowering temperature and makes evident that there are two spin conduction regimes as a function of the NiO layer thickness with different spin diffusion lengths. For thin discontinuous NiO layers a spin diffusion length of λSDL≈0.8 nm is found, which is slightly larger than typical values found for insulating barriers, but definitely smaller than λSDL≈3.8 nm found for NiO layer thickness above 2 nm. The latter being similar to λSDL previously reported for epitaxial NiO layers. Our findings indicate that inserting a PM NiO insulating layer of optimal thickness improves the spin transparency of the LSMO/Pt interface. Additionally, the results suggest that spin conduction through the NiO layer is likely driven by magnetic correlations and short-range thermal magnons.

Research on the determination and influencing factors of topographic unsymmetrical-loaded tunnels based on the unsymmetrical coefficient

The accurate judgment of topographic bias tunnels is directly related to reasonable tunnel design and construction safety. In this study, numerical simulations were carried out to determine the unsymmetrical coefficient of topographic unsymmetrical-loaded tunnels, according to the critical thickness of the cover layer of the topographic bias tunnel proposed in the Chinese Code. The unsymmetrical coefficient was analyzed via control variable method and orthogonal test method. The results showed that it is appropriate to take 1.2 as the criterion for all types of topographic unsymmetrical-loaded tunnel in practical application. The influence of different topographic factors, mechanical properties of the surrounding rock, rock quality, construction methods and tunnel spans on the unsymmetrical coefficient were summarized by the control variable method. The results of multifactor analysis under an orthogonal design revealed that the influence of Uc is in the following order: slope angle, overburden thickness, construction method, surrounding rock quality, and height-to-span ratio.Article HighlightsThe Chinese code for topographic unsymmetrical-loaded tunnels presents a the determination of the corrected unsymmetry coefficient.Single-factor analysis determines the change pattern of the corrected unsymmetry coefficient under the influencing factors.Polar analysis based on orthogonal tests determines the strength of the factors influencing the corrected unsymmetry coefficient.

Effects of Thermal Energy on the Formation of Lattice Strain in VO2 Thin Films Grown on TiO2(001).

Film thickness is a well-known experimental parameter for controlling lattice strain in oxide films. However, due to environmental and resource conservation considerations, films need to be as thin as possible, increasing the need to find alternative factors for strain management. Herein, we present the importance of thermal energy as a factor for the formation of lattice strain in the oxide films, specifically focusing on the effects of laser fluence during pulsed laser deposition (PLD) on the in-plane lattice strain of vanadium dioxide (VO2) thin films grown on titanium dioxide (TiO2) (001). VO2 thin films were deposited using a KrF excimer laser (λ = 248 nm) at laser fluences ranging from 0.88 to 1.70 J/cm2. The film thickness ranged from 10-15 nm, below the critical thickness. Films grown at higher laser fluences exhibited smooth surfaces and completely strained in-plane lattices. In contrast, films grown at lower laser fluences displayed numerous small islands and relaxed in-plane lattice strain. The metal-insulator transition (MIT) temperature was lower for films grown at higher laser fluencies compared to those grown at lower laser fluences. It was also revealed that Ti-V interdiffusion occurs, forming a solid solution (V1-xTixO2) near the interface. These observations suggest that the thermal energy of the particles, influenced by laser fluence, is a critical factor in the formation of lattice strain in metal oxide films and also that laser fluence in PLD is an effective experimental parameter for strain management in oxide films. Our findings enhance the understanding of lattice strain formation in metal oxides and offer insights for establishing effective methods for controlling lattice strain in metal oxide films.

Coherent to semi-coherent transition of precipitates in Van der Merwe epitaxial films

Abstract The interplay of stresses of two coherent systems, specifically a coherent precipitate within an epitaxial film (in Van der Merve growth mode), gives rise to an interesting scenario; wherein the configurational effects and the strain energy landscape are enriched. This leads to an interdependent evolutionary pathway for the coherent to semi-coherent transition. The transition of the film-substrate interface to the semi-coherent state occurs beyond a critical thickness ( t c ) which arises from energy minimization. Using the model example of the precipitation of NbH0.5 precipitate in a Nb/Sapphire epitaxial system, the critical radii ( r film ∗ ) for model geometries have been computed. 3D finite element models have been used to compute the stress state and energetics of precipitates growing in an epitaxial film. The following important observations can be made based on the simulations. (1) The epitaxial stresses can lead to the stabilization of the coherent precipitate. (2) The transition to a semi-coherent film interface, characterized by an array of interfacial misfit dislocations, can energetically favor the semi-coherent state of the precipitate over the coherent state. The magnitude of r film ∗ depends on the spatial coordinates within the film. (3) The presence of Nb–H coherent precipitates can stabilize the coherent state of the film interface.

A reassessment of ice cliff dynamics upon debris-covered glaciers

Abstract We present a new model for understanding ice cliff dynamics within a debris-covered glacier ablation zone. This simple energy-balance model incorporates a moving frame of reference, made necessary by the melt of the surrounding debris-covered ice. In so doing, this also formalises how different types of field measurements can be utilised and compared. Our predictions include showing: ice cliffs can endogenously select their own slope angles; that there should be an indifference between illuminated north- and south-facing ice cliff slopes; that ice cliffs grow steeper with thicker debris layers; that ice cliffs cannot stably exist below a certain critical debris thickness and that some modelling of ice cliffs (when not incorporating the moving frame) may incorrectly estimate ice mass losses. All of our results are produced using parametrisations from Baltoro Glacier, Karakoram.

Theoretical studies of the bipolar properties of ScAlN/AlGaN/GaN heterostructures for enhanced device applications

Abstract Here, we describe a systematic theoretical study of ScAlN/AlGaN/GaN heterostructures for enhanced high-electron-mobility transistors (HEMTs). We have investigated the carrier distributions and energy-band diagrams by solving the coupled Schrödinger and Poisson equations self-consistently. When the heterostructure is in the “off” state, the two-dimensional electron gas (2DEG) becomes increasingly depleted as the ScAlN thickness is increased to the critical thickness, at which point the two-dimensional hole gas (2DHG) appears. This critical thickness depends on the Sc content. Increasing the AlGaN thickness or Al content causes a simultaneous increase in the 2DEG and 2DHG sheet densities. In addition, when the HEMT is in the “on” state, 2DEG sheet density increases as ScAlN thickness increases, so long as the Sc content is less than 0.3; when the Sc content exceeds 0.4, this trend is reversed. For a given Sc content, double channels are produced when the ScAlN thickness exceeds the critical thickness.

Synthesis of group-IV ternary and binary semiconductors using epitaxy of GeH3Cl and SnH4

Ge1−x−ySixSny alloys were grown on Ge buffers via reactions of SnH4 and GeH3Cl. The latter is a new CVD source designed for epitaxial development of group-IV semiconductors under low thermal budgets and CMOS-compatible conditions. The Ge1−x−ySixSny films were produced at very low temperatures between 160 and 200 °C with 3%–5% Si and ∼5%–11% Sn. The films were characterized using an array of structural probes that include Rutherford backscattering, x-ray photoelectron spectroscopy, high-resolution x-ray diffraction, scanning transmission electron microscopy, and atomic force microscopy. These studies indicate that the films are strained to Ge and exhibit defect-free microstructures, flat surfaces, homogeneous compositions, and sharp interfaces. Raman was used to determine the compositional dependence of the vibrational modes indicating atomic distributions indistinguishable from those obtained when using high-order Ge hydrides. For a better understanding of the growth mechanisms, a parallel study was conducted to investigate the GeH3Cl applicability for synthesis of binary Ge1−ySny films. These grew strained to Ge, but with reduced Sn compositions and lower thicknesses relative to Ge1−x−ySixSny. Bypassing the Ge buffers led to Ge1−ySny-on-Si films with compositions and thicknesses comparable to Ge1−ySny-on-Ge; but their strains were mostly relaxed. Efforts to increase the concentration and thickness of Ge1−ySny-on-Si resulted in multiphase materials containing large amounts of interstitial Sn. These outcomes suggest that the incorporation of even small Si amounts in Ge1−x−ySixSny might compensate for the large Ge–Sn mismatch by lowering bond strains. Such an effect reduces strain energy, enhances stability, promotes higher Sn incorporation, and increases critical thickness.

In situ monitoring of quantum dot growth using a magnification inferred curvature method

The growth of InAs on a GaAs (001) substrate follows the Stranski–Krastanov (S–K) growth mode. Initially, the stress due to the lattice constant difference is small, resulting in two-dimensional growth. However, as the thickness of the growth layer increases, this stress accumulates, and upon reaching a critical film thickness, the growth transitions to three-dimensional, facilitating stress relaxation. Strain changes during crystal growth can be observed through variations in substrate curvature. However, in InAs/GaAs quantum dots (QDs), these changes are minimal, making observation challenging. Previously, the curvature of the substrate during InAs/GaAs QD formation could only be estimated with low precision, necessitating the use of very thin substrates with cantilever structures to achieve higher curvature. In this study, we applied the magnification inferred curvature (MIC) method, which allows for high-precision estimation of substrate curvature during molecular beam epitaxy growth. This method enabled us to observe strain changes during the formation and relaxation processes of QDs on standard-thickness GaAs (001) substrates. The results highlight the potential of the MIC method in investigating the complex interplay of strain and stress in semiconductor growth processes, emphasizing its suitability for fabricating next-generation QD devices.

Experiment and Numerical Prediction on Shock Sensitivity of HMX–Based Booster Explosive with Small Scale Gap Test at Low and Elevated Temperatures

In order to analyze the effect of temperature changes on the shock initiation performance of HMX–based booster explosive, which consists of 95% HMX and 5% FPM2602 by weight, a temperature calibration test of acceptor was designed. The temperature changes in the booster at low and elevated temperatures under the constraint of steel sleeve were obtained. Based on the temperature calibration results, polymethyl methacrylate (PMMA) was selected as gap material to conduct a small scale gap test (SSGT) of HMX–based booster under different temperature conditions. The corresponding critical gap thickness was tested. Based on SSGT results at different temperatures, the shock initiation processes were simulated by adjusting parameters of ignition and growth reactive rate model. The critical gap thickness and critical initiation pressure of HMX–based booster at different temperatures were numerically predicted. By combining SSGT experimental data and simulation results, the attenuation law and fitting prediction formula of the critical initiation pressure of HMX–based booster were proposed. The mechanism of temperature effect on the shock sensitivity of HMX–based booster explosive was analyzed. The research results indicate that the critical gap thickness of HMX–based booster gradually increases with the rise in temperature, and the critical initiation pressure gradually decreases during the shock initiation process under the heating temperature conditions. In addition, the simulation results show that the heated HMX–based booster under steel constraints becomes more sensitive at high temperatures (>120 °C), while the cooled booster is more insensitive, but its critical initial pressure does not change significantly between 88 °C and 120 °C. The experimental and numerical prediction results are important for the shock initiation safety and design of an insensitive booster explosive.

Research on the performance of high-strength explosive ink and application of direct writing technology in micro-explosive network

With the widespread application of 3D direct writing technology in the field of energetic materials, it has become a current research topic to design an explosive ink with excellent micro-size detonation performance and high-strength mechanical properties and combining it with explosion network technology to improve detonation synchronization has become a research hotspot. This article uses FEVE-F2311 and PF as the composite bonding system, submicron CL-20 as the main explosive, and adds an appropriate amount of TDI as the curing agent to prepare an explosive ink with excellent micro-size detonation performance and high-strength mechanical properties. Research shows that the explosive ink has a detonation velocity of 7322 m·s−1 at a charge density of 1.621 g·cm−3, a critical propagation size of 0.3×0.3 mm, a critical propagation thickness of 1×0.040 mm, and a maximum detonation Corners can reach 160°. The error of detonation synchronization in the "one in and six out" micro-explosion network is within 150 ns, showing excellent detonation synchronization. The nano-indentation test shows that the average elastic modulus of CL-20-based explosive ink is 6.991 GPa and the average hardness is 0.200 GPa, showing excellent mechanical properties; the compressive stress-strain curve shows that its average compressive stress is 7.62 MPa, showing excellent Compressive strength. It not only greatly improves the mechanical properties of explosive ink, but also provides a method to improve the synchronization performance of micro-explosive networks.

Study on the critical conditions for ductile-brittle transition in ultrasonic-assisted grinding of SiC particle-reinforced Al-MMCs

High volume fraction (45 %) silicon carbide particle-reinforced aluminum matrix composites (SiCp/Al-MMCs) play a significant role in various engineering fields due to their outstanding performance. However, damages characterized by SiC particle fracture during machining lead to poor surface integrity, which severely affects the fatigue performance of SiCp/Al composite structural components. Ultrasonic-assisted grinding (UAG) is acknowledged as beneficial for promoting ductile grinding in hard and brittle materials. This study focuses on the critical conditions for ductile-brittle transition in ultrasonic-assisted grinding of SiCp/Al composites and develops a mechanism and data driven model of critical undeformed chip thickness (UCT) for ductile grinding to accurately predict the material removal mode. The model thoroughly integrates considerations of chip morphology, removal mode transition mechanism, and grinding surface damage characteristics. Response surface methodology (RSM) and genetic algorithms (GA) were utilized to correct the impact of processing parameters on the removal regime. Experiments on ultrasonic-assisted side and end grinding were conducted to thoroughly discuss the effects of different undeformed chip thicknesses, grinding speeds and ultrasonic vibration amplitude on surface damage and the critical conditions for the ductile-brittle transition. The findings corroborate the experimental data with the predicted values. Ultrasonic vibration can effectively reduce the brittle fracture of SiC particles in SiCp/Al composites. Appropriately increasing the grinding speed and reducing the chip thickness can enhance the critical chip thickness for ductile grinding and decrease the proportion of brittle surfaces, thereby achieving a balance between surface integrity and grinding efficiency. This research provides guidance for high-performance machining of SiCp/Al composites.

Real-time study of the interplay between phase formation and stress evolution at the W1−xSix/a−Si interface

Nanoscale W1−xSix layers with different Si content were deposited by magnetron sputtering on amorphous silicon layers. The structure and stress evolution during deposition were monitored and in real time. Thus, it was possible to disentangle different origins of stress built-up (interface formation, phase formation, grain growth, and texture) on the nanoscale. It was found that the bcc phase with a composition-dependent texture forms at low Si content (less than 10% Si), but only above a critical thickness. At intermediate Si content (13.9% and 16.5% Si), or low Si content and low thicknesses (≲4.5 nm), the β phase with A15 structure and coexisting phases (bcc or amorphous) form, while a single, amorphous phase is observed at high Si content (≳22% Si). An A15 formation driven by impurities (O,C,N) was excluded by combined x-ray photoelectron spectroscopy and x-ray diffraction measurements on a second sample series. calculations of substitutional bcc and A15 W1−xSix alloys support the formation of A15 at higher Si content. The A15-containing interlayer should be taken into account when discussing the superconductivity of W/Si multilayers. The stress evolution during deposition of W1−xSix was correlated with the microstructure evolution, and compared to similar observations for Mo1−xSix. Due to the crystalline phase (bcc/A15) and bcc texture ([111] and [110]) competition, the structure formation of W1−xSix shows a higher complexity. This explains the wide range of stress states (from tensile to compressive) observed after deposition of W1−xSix. Nanoscale W-Si based stress-compensation layers could be employed for tailoring the stress state of nonepitaxial semiconductor devices. Published by the American Physical Society 2024

Impact of strain relaxation on the growth rate of heteroepitaxial germanium tin binary alloy

The growth of high-quality germanium tin (Ge1–y Sn y ) binary alloys on a Si substrate using chemical vapor deposition (CVD) techniques holds immense potential for advancing electronics and optoelectronics applications, including the development of efficient and low-cost mid-infrared detectors and light sources. However, achieving precise control over the Sn concentration and strain relaxation of the Ge1–y Sn y epilayer, which directly influence its optical and electrical properties, remain a significant challenge. In this research, the effect of strain relaxation on the growth rate of Ge1–y Sn y epilayers, with Sn concentration >11at.%, is investigated. It is successfully demonstrated that the growth rate slows down by ~55% due to strain relaxation after passing its critical thickness, which suggests a reduction in the incorporation of Ge into Ge1–y Sn y growing layers. Despite the increase in Sn concentration as a result of the decrease in the growth rate, it has been found that the Sn incorporation rate into Ge1–y Sn y growing layers has also decreased due to strain relaxation. Such valuable insights could offer a foundation for the development of innovative growth techniques aimed at achieving high-quality Ge1–y Sn y epilayers with tuned Sn concentration and strain relaxation.

Infrequent trigonal crystal system CoNb thin films: investigation of growth behavior, microstructure and magnetic properties

Magnetic films with excellent initial permeability, high resonance frequency, suitable anisotropy fields, and well compatibility are highly desirable for on-chip inductors and transformers. Here, Co90Nb10 thin films with different thicknesses are grown on Si substrates of different orientations using an electron beam evaporation technique. A rare trigonal crystal structure of the CoNb films was observed and was virtually unaffected by substrate orientation. Films grown exhibit thickness-dependent magnetic behavior: coercivity initially decreases and then increases, while the in-plane anisotropy field progressively diminishes. Meanwhile, the initial permeability increases initially before decreasing, and the resonance frequency continuously decreases as the thickness increases. The resultant Co90Nb10/Si (111)-50 nm film displays a high μi of 496 and a great fr of 2.1GHz. The improved magnetic performance originates from the smaller crystallite size, smoother surface roughness, and suitable in-plane anisotropy field. Moreover, the in-plane anisotropy transition to the out-of-plane anisotropy in the Co90Nb10 films whose thickness exceeds 70nm, which is attributed to the growth of the coarse columnar grains once the critical thickness is exceeded. This study demonstrates the correlation between the growth behavior, microstructure, and magnetic properties of CoNb thin films, presenting a new potential for utilizing Co-based thin films in high-frequency applications.

Longitudinal torsional ultrasonic vibratory assistant milling of borosilicate glass material removal mechanism based on the thickness of undeformed chips

Borosilicate glass is an ideal material for microfluidic chip substrates due to its excellent light transmittance and stable chemical properties. However, it is also prone to brittle fracture during processing, making it a typical difficult-to-machine material. As an emerging processing technology, longitudinal torsional ultrasonic assistant milling (LTUVAM) can effectively reduce the surface damage of glass materials and improve the proportion of plastic removal of materials. The mechanism of LTUVAM of borosilicate glass was studied in this paper. Critical undeformed chip thickness (hc) is a key factor in achieving plastic material removal. At present, the influence of LTUVAM on the hc is not clear. Therefore, in this paper, the material-specific cutting energy model in the LTUVAM process was established to predict the hc. Firstly, the motion path of LTUVAM was analyzed, and the undeformed chip thickness model and the tool-workpiece separation rate model were established. Then, based on the above model, the material-specific cutting energy model was established according to the material removal principle. Finally, an LTUVAM orthogonal experiment was carried out to study the surface quality of the material. The experimental results showed that with the increase of ultrasonic power, the roughness and side damage area increased first and then decreased. With the increase of spindle speed, it showed that the roughness and side damage area decreased first and then increased. With the increase of feed per tooth, it showed that the roughness increased first and then decreased, and then increased, and the side damage area increased first and then decreased. Through the analysis of the experimental and model results, the increase of the separation rate between the tool and the workpiece contributes to the increase of the hc and the improvement of the processing quality.

Critical-Layered MoS2 for the Enhancement of Supercontinuum Generation in Photonic Crystal Fibre.

Supercontinuum generation (SCG) from silica-based photonic crystal fibers (PCFs) is of highly technological significance from microscopy to metrology, but has been hindered by silica's relatively low intrinsic optical nonlinearity. The prevailing approaches of filling PCF with nonlinear gases or liquids can endow fibre with enhanced optical nonlinearity and boosted SCG efficiency, yet these hybrids are easily plagued by fusion complexity, environmental incompatibility or transmission mode instability. Here this work presentsa strategy of embedding solid-state 2D MoS2 atomic layers into the air-holes of PCF to efficiently enhance SCG. This work demonstrates a 4.8 times enhancement of the nonlinear coefficient and a 70% reduction of the threshold power for SCG with one octave spanning in the MoS2-PCF hybrid. Furthermore, this work finds that the SCG enhancement is highly layer-dependent, which only manifests for a real 2D regime within the thickness of five atomic layers. Theoretical calculations reveal that the critical thickness arises from the trade-off among the layer-dependent enhancement of the nonlinear coefficient, leakage of fundamental mode and redshift of zero-dispersion wavelength. This work provides significant advances toward efficient SCG, and highlights the importance of matching an appropriate atomic layer number in the design of functional 2D material optical fibers.

Predicting critical flame quenching thickness using machine learning approach with ResNet and ANN

In the study of flame quenching, quenching thickness is one of the important parameters to determine the design of a flame arrester, and often determines the flame quenching performance of the arrester. In the study, residual network (ResNet) and artificial neural network (ANN) are used to predict the critical quench thickness of combustible gas in pipelines. The critical quench thickness is influenced by fuel concentration and density, pipeline size, inert gas type and concentration, porous media porosity, and thermal conductivity. The influence of different combinations of hyper-parameters on the prediction performance of the two models is explored. The results show that the prediction performance of both models reaches the best after hyper-parameter optimization. Compared with ANN, the ResNet model shows more stable and better prediction ability, and its optimal evaluation parameters are: MAE is 1.4679, MSE is 91.7431, R2 is 0.9216. The prediction errors of the two models on the same dataset are subjected to analysis, and the impact of the use of normalized data on the performance of the two models is compared. It is determined that the ResNet model demonstrated superior robustness and generalization ability in predicting the critical quenching thickness of combustible gases. The study is helpful for the safety protection of combustible gas and the safety design of pipeline arresters.

An atomically controlled insulator-to-metal transition in iridate/manganite heterostructures.

All-insulator heterostructures with an emerging metallicity are at the forefront of material science, which typically contain at least one band insulator while it is not necessary to be. Here we show emergent phenomena in a series of all-correlated-insulator heterostructures that composed of insulating CaIrO3 and insulating La0.67Sr0.33MnO3. We observed an intriguing insulator-to-metal transition, that depends delicately on the thickness of the iridate component. The simultaneous enhancements of magnetization, electric conductivity, and magnetoresistance effect indicate a percolation-type nature of the insulator-to-metal transition, with the percolation threshold can be reached at an exceptionally low volume fraction of the iridate. Such a drastic transition is induced by an interfacial charge transfer, which interestingly alters the electronic and crystalline structures of the bulk region rather than the limited ultrathin interface. We further showcased the central role of effective correlation in modulating the insulator-to-metal transition, by demonstrating that the critical thickness of iridate for triggering the metallic state can be systematically reduced down to a single unit-cell layer.

Preserving surface strain in nanocatalysts via morphology control.

Engineering strain critically affects the properties of materials and has extensive applications in semiconductors and quantum systems. However, the deployment of strain-engineered nanocatalysts faces challenges, in particular in maintaining highly strained nanocrystals under reaction conditions. Here, we introduce a morphology-dependent effect that stabilizes surface strain even under harsh reaction conditions. Using four-dimensional scanning transmission electron microscopy (4D-STEM), we found that cube-shaped core-shell Au@Pd nanoparticles with sharp-edged morphologies sustain coherent heteroepitaxial interfaces with larger critical thicknesses than morphologies with rounded edges. This configuration inhibits dislocation nucleation due to reduced shear stress at corners, as indicated by molecular dynamics simulations. A Suzuki-type cross-coupling reaction shows that our approach achieves a fourfold increase in activity over conventional nanocatalysts, owing to the enhanced stability of surface strain. These findings contribute to advancing the development of advanced nanocatalysts and indicate broader applications for strain engineering in various fields.