Optoelectronic and Optomechanical Properties of Few-Atomic-Layer Black Phosphorus Nanoflakes as Revealed by In Situ TEM

The optoelectronic signatures of free-standing few-atomic-layer black phosphorus nanoflakes are analyzed by in situ transmission electron microscopy (TEM). As compared to other 2D materials, the band gap of black phosphorus (BP) is related directly to multiple thicknesses and can be tuned by nanoflake thickness and strain. The photocurrent measurements with the TEM show a stable response to infrared light illumination and change of nanoflakes band gap with deformation while pressing them between two electrodes assembled in the microscope. The photocurrent spectra of an 8- and a 6-layer BP nanoflake samples are comparatively measured. Density functional theory (DFT) calculations are performed to identify the band structure changes of BP during deformations. The results should help to find the best pathways for BP smart band gap engineering via tuning the number of material atomic layers and programmed deformations to promote future optoelectronic applications.

Optomechanical Properties of MoSe2 Nanosheets as Revealed by In Situ Transmission Electron Microscopy

Free-standing few-layered MoSe₂ nanosheet stacks optoelectronic signatures are analyzed by using light compatible in situ transmission electron microscopy (TEM) utilizing an optical TEM holder allowing for the simultaneous mechanical deformation, electrical probing and light illumination of a sample. Two types of deformation, namely, (i) bending of nanosheets perpendicular to their basal atomic planes and (ii) edge deformation parallel to the basal atomic planes, lead to two distinctly different optomechanical performances of the nanosheet stacks. The former deformation induces a stable but rather marginal increase in photocurrent, whereas the latter mode is prone to unstable nonsystematic photocurrent value changes and a red-shifted photocurrent spectrum. The experimental results are verified by ab initio calculations using density functional theory (DFT).

Strategic design of Fe and N co-doped hierarchically porous carbon as superior ORR catalyst: from the perspective of nanoarchitectonics

In this study, we present microporous carbon (MPC), hollow microporous carbon (HMC) and hierarchically porous carbon (HPC) to demonstrate the importance of strategical designing of nanoarchitectures in achieving advanced catalyst (or electrode) materials, especially in the context of oxygen reduction reaction (ORR). Based on the electrochemical impedance spectroscopy and ORR studies, we identify a marked structural effect depending on the porosity. Specifically, mesopores are found to have the most profound influence by significantly improving electrochemical wettability and accessibility. We also identify that macropore contributes to the rate capability of the porous carbons. The results of the rotating ring disk electrode (RRDE) method also demonstrate the advantages of strategically designed double-shelled nanoarchitecture of HPC to increase the overall electron transfer number (n) closer to four by offering a higher chance of the double two-electron pathways. Next, selective doping of highly active Fe–N_x sites on HPC is obtained by increasing the nitrogen content in HPC. As a result, the optimized Fe and N co-doped HPC demonstrate high ORR catalytic activity comparable to the commercial 20 wt% Pt/C in alkaline electrolyte. Our findings, therefore, strongly advocate the importance of a strategic design of advanced catalyst (or electrode) materials, especially in light of both structural and doping effects, from the perspective of nanoarchitectonics.

This study elucidates the role of each class of nanopore by in-depth electrochemical analysis of three types of ZIF-8-derived carbons. Also, engineered co-doping of Fe and N is found essential to selectively form Fe–N_x sites in the carbon matrix.

Young's Modulus and Tensile Strength of Ti3C2 MXene Nanosheets As Revealed by In Situ TEM Probing, AFM Nanomechanical Mapping, and Theoretical Calculations

Two-dimensional transition metal carbides, that is, MXenes and especially Ti₃C₂, attract attention due to their excellent combination of properties. Ti₃C₂ nanosheets could be the material of choice for future flexible electronics, energy storage, and electromechanical nanodevices. There has been limited information available on the mechanical properties of Ti₃C₂, which is essential for their utilization. We have fabricated Ti₃C₂ nanosheets and studied their mechanical properties using direct in situ tensile tests inside a transmission electron microscope, quantitative nanomechanical mapping, and theoretical calculations employing machine-learning derived potentials. Young's modulus in the direction perpendicular to the Ti₃C₂ basal plane was found to be 80-100 GPa. The tensile strength of Ti₃C₂ nanosheets reached up to 670 MPa for ∼40 nm thin nanoflakes, while a strong dependence of tensile strength on nanosheet thickness was demonstrated. Theoretical calculations allowed us to study mechanical characteristics of Ti₃C₂ as a function of nanosheet geometrical parameters and structural defect concentration.

Recent Progress of In Situ Transmission Electron Microscopy for Energy Materials

In situ transmission electron microscopy (TEM) is one of the most powerful approaches for revealing physical and chemical process dynamics at atomic resolutions. The most recent developments for in situ TEM techniques are summarized; in particular, how they enable visualization of various events, measure properties, and solve problems in the field of energy by revealing detailed mechanisms at the nanoscale. Related applications include rechargeable batteries such as Li-ion, Na-ion, Li-O₂ , Na-O₂ , Li-S, etc., fuel cells, thermoelectrics, photovoltaics, and photocatalysis. To promote various applications, the methods of introducing the in situ stimuli of heating, cooling, electrical biasing, light illumination, and liquid and gas environments are discussed. The progress of recent in situ TEM in energy applications should inspire future research on new energy materials in diverse energy-related areas.

Crystallography-Derived Young's Modulus and Tensile Strength of AlN Nanowires as Revealed by in Situ Transmission Electron Microscopy

Aluminum nitride (AlN) has a unique combination of properties, such as high chemical and thermal stability, nontoxicity, high melting point, large energy band gap, high thermal conductivity, and intensive light emission. This combination makes AlN nanowires (NWs) a prospective material for optoelectronic and field-emission nanodevices. However, there has been very limited information on mechanical properties of AlN NWs that is essential for their reliable utilization in modern technologies. Herein, we thoroughly study mechanical properties of individual AlN NWs using direct,  in situ bending and tensile tests inside a high-resolution TEM. Overall, 22 individual NWs have been tested, and a strong dependence of their Young's moduli and ultimate tensile strengths (UTS) on their growth axis crystallographic orientation is documented. The Young's modulus of NWs grown along the [101̅1] orientation is found to be in a range 160-260 GPa, whereas for those grown along the [0002] orientation it falls within a range 350-440 GPa. In situ TEM tensile tests demonstrate the UTS values up to 8.2 GPa for the [0002]-oriented NWs, which is more than 20 times larger than that of a bulk AlN compound. Such properties make AlN nanowires a highly promising material for the reinforcing applications in metal matrix and other composites. Finally, experimental results were compared and verified under a density functional theory simulation, which shows the pronounced effect of growth axis on the AlN NW mechanical behavior. The modeling reveals that with an increasing NW width the Young's modulus tends to approach the elastic constants of a bulk material.

Structural evolution of Ag/BN hybrids via a polyol-assisted fabrication process and their catalytic activity in CO oxidation

Enhanced catalytic activity of Ag/BN nanohybrids is ascribed to the formation of a thin intermediate Ag–O–B layer.

Fabrication and application of BN nanoparticles, nanosheets and their nanohybrids

Smart implementation of novel advanced nanomaterials is the key for the solution of many complex problems of modern science. In recent years, there has been a great interest in the synthesis and application of boron nitride (BN) nanotubes because of their unique physical, chemical, and mechanical properties. By contrast, the synthesis, characterization and exploration of other morphological types of BN nanostructure - BN nanoparticles and BN nanosheets - have received less attention. However, the detailed investigations on advantages of every morphological BN type for specific applications have only recently been started. One of the promising directions is the utilization of BN-based nanohybrids. This review is dedicated to the in-depth analysis of recently published works on the fabrication and application of BN nanoparticles, nanosheets, and their nanohybrids. It covers a variety of developed synthetic methods toward fabrication of such nanostructures, and their specific application potentials in catalysis, drug delivery, tribology and structural materials. Finally, the review focuses on the theoretical aspects of this quickly emerging field.

Photocatalysis with Pt-Au-ZnO and Au-ZnO Hybrids: Effect of Charge Accumulation and Discharge Properties of Metal Nanoparticles

Metal-semiconductor hybrid nanomaterials are becoming increasingly popular for photocatalytic degradation of organic pollutants. Herein, a seed-assisted photodeposition approach is put forward for the site-specific growth of Pt on Au-ZnO particles (Pt-Au-ZnO). A similar approach was also utilized to enlarge the Au nanoparticles at epitaxial Au-ZnO particles (Au@Au-ZnO). An epitaxial connection at the Au-ZnO interface was found to be critical for the site-specific deposition of Pt or Au. Light on-off photocatalysis tests, utilizing a thiazine dye (toluidine blue) as a model organic compound, were conducted and confirmed the superior photodegradation properties of Pt-Au-ZnO hybrids compared to Au-ZnO. In contrast, Au-ZnO type hybrids were more effective toward photoreduction of toluidine blue to leuco-toluidine blue. It was deemed that photoexcited electrons of Au-ZnO (Au, ∼5 nm) possessed high reducing power owing to electron accumulation and negative shift in Fermi level/redox potential; however, exciton recombination due to possible Fermi-level equilibration slowed down the complete degradation of toluidine blue. In the case of Au@Au-ZnO (Au, ∼15 nm), the photodegradation efficiency was enhanced and the photoreduction rate reduced compared to Au-ZnO. Pt-Au-ZnO hybrids showed better photodegradation and mineralization properties compared to both Au-ZnO and Au@Au-ZnO owing to a fast electron discharge (i.e. better electron-hole seperation). However, photoexcited electrons lacked the reducing power for the photoreduction of toluidine blue. The ultimate photodegradation efficiencies of Pt-Au-ZnO, Au@Au-ZnO, and Au-ZnO were 84, 66, and 39%, respectively. In the interest of effective metal-semiconductor type photocatalysts, the present study points out the importance of choosing the right metal, depending on whether a photoreduction and/or photodegradation process is desired.

Compressive properties of hollow BN nanoparticles: theoretical modeling and testing using a high-resolution transmission electron microscope

Due to their excellent mechanical properties, nanoparticles have great potential as reinforcing phases in composite materials, friction modifiers in liquid lubricants, catalysts and drug-delivery agents. In the present study, the mechanical analysis of individual spherical hollow BN nanoparticles (BNNPs) using a combination of in situ compression tests inside a high-resolution transmission electron microscope (TEM) and theoretical modelling was conducted. It was found that BNNPs display high mechanical stiffness and a large value of elastic recovery. This enables the hollow BNNPs to exhibit considerably large cyclic deformation (up to 30% of the sphere's original external diameter) and to accumulate plastic deformation of approximately 30% of the total compression strain. Theoretical simulations allowed for elucidation of BNNPs' structural changes under compression at the atomic level and explained the origin of their high stiffness and large critical deformation values.

BN nanoparticle/Ag hybrids with enhanced catalytic activity: theory and experiments

BNNPs/Ag nanohybrids with an optimal amount of B₂O₃ demonstrated a higher catalytic activity.

BN/Ag hybrid nanomaterials with petal-like surfaces as catalysts and antibacterial agents

BN/Ag hybrid nanomaterials (HNMs) and their possible applications as novel active catalysts and antibacterial agents are investigated. BN/Ag nanoparticle (NP) hybrids were fabricated using two methods: (i) chemical vapour deposition (CVD) of BN NPs in the presence of Ag vapours, and (ii) ultraviolet (UV) decomposition of AgNO₃ in a suspension of BN NPs. The hybrid microstructures were studied by high-resolution transmission electron microscopy (HRTEM), high-angular dark field scanning TEM imaging paired with energy dispersion X-ray (EDX) mapping, X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (FTIR). They were also characterized in terms of thermal stability, Ag⁺ ion release, catalytic and antibacterial activities. The materials synthesized via UV decomposition of AgNO₃ demonstrated a much better catalytic activity in comparison to those prepared using the CVD method. The best catalytic characteristics (100% methanol conversion at 350 °C) were achieved using the UV BN/Ag HNMs without preliminary annealing at 600 °C in an oxidizing atmosphere. Both types of the BN/Ag HNMs possess a profound antibacterial effect against Escherichia coli K-261 bacteria.

Optical and Optoelectronic Property Analysis of Nanomaterials inside Transmission Electron Microscope

In situ transmission electron microscopy (TEM) allows one to investigate nanostructures at high spatial resolution in response to external stimuli, such as heat, electrical current, mechanical force and light. This review exclusively focuses on the optical, optoelectronic and photocatalytic studies inside TEM. With the development of TEMs and specialized TEM holders that include in situ illumination and light collection optics, it is possible to perform optical spectroscopies and diverse optoelectronic experiments inside TEM with simultaneous high resolution imaging of nanostructures. Optical TEM holders combining the capability of a scanning tunneling microscopy probe have enabled nanomaterial bending/stretching and electrical measurements in tandem with illumination. Hence, deep insights into the optoelectronic property versus true structure and its dynamics could be established at the nanometer-range precision thus evaluating the suitability of a nanostructure for advanced light driven technologies. This report highlights systems for in situ illumination of TEM samples and recent research work based on the relevant methods, including nanomaterial cathodoluminescence, photoluminescence, photocatalysis, photodeposition, photoconductivity and piezophototronics.

Effect of BN Nanoparticles Loaded with Doxorubicin on Tumor Cells with Multiple Drug Resistance

Herein we study the effect of doxorubicin-loaded BN nanoparticles (DOX-BNNPs) on cell lines that differ in the multidrug resistance (MDR), namely KB-3-1 and MDR KB-8-5 cervical carcinoma lines, and K562 and MDR i-S9 leukemia lines. We aim at revealing the possible differences in the cytotoxic effect of free DOX and DOX-BNNP nanoconjugates on these types of cells. The spectrophotometric measurements have demonstrated that the maximum amount of DOX in the DOX-BNNPs is obtained after saturation in alkaline solution (pH 8.4), indicating the high efficiency of BNNPs saturation with DOX. DOX release from DOX-BNNPs is a pH-dependent and DOX is more effectively released in acid medium (pH 4.0-5.0). Confocal laser scanning microscopy has shown that the DOX-BNNPs are internalized by neoplastic cells using endocytic pathway and distributed in cell cytoplasm near the nucleus. The cytotoxic studies have demonstrated a higher sensitivity of the leukemia lines to DOX-BNNPs compared with the carcinoma lines: IC₅₀(DOX-BNNPs) is 1.13, 4.68, 0.025, and 0.14 μg/mL for the KB-3-1, MDR KB-8-5, K562, and MDR i-S9 cell lines, respectively. To uncover the mechanism of cytotoxic effect of nanocarriers on MDR cells, DOX distribution in both the nucleus and cytoplasm has been studied. The results indicate that the DOX-BNNP nanoconjugates significantly change the dynamics of DOX accumulation in the nuclei of both KB-3-1 and KB-8-5 cells. Unlike free DOX, the utilization of DOX-BNNPs nanoconjugates allows for maintaining a high and stable level of DOX in the nucleus of MDR KB-8-5 cells.