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Origin of Ferroelectricity in Epitaxial Si-doped HfO2 Films
摘要： HfO2-based unconventional ferroelectric (FE) materials were recently discovered and have attracted a great deal of attention in both academia and industry. The growth of epitaxial Si-doped HfO2 films has opened up a route to understand the mechanism of ferroelectricity. Here, we used pulsed laser deposition (PLD) to grow epitaxial Si-doped HfO2 films in different orientations of N-type SrTiO3 substrates. Using piezoforce microscopy, polar nanodomains can be written and read, and these domains are reversibly switched with a phase change of 180o. Films with different thicknesses displayed a coercive field Ec and a remnant polarization Pr of approximately 4～5 MV/cm and 8～32 μC/cm2, respectively. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) results identified that the as-grown Si-doped HfO2 films have strained fluorite structures. The ABAB stacking mode of the Hf atomic grid observed by HRTEM clearly demonstrates that the ferroelectricity originates from the noncentrosymmetric Pca21 polar structure. Combined with soft X-ray absorption spectra (XAS), it was found that the Pca21 ferroelectric crystal structure manifested as O sublattice distortion by the effect of interface strain and Si dopant interactions, resulting in further crystal-field splitting as a nanoscaled ferroelectric ordered state.
关键词： HRTEM,PLD,XRD,Ferroelectricity,PFM,XAS,Epitaxial Si-doped HfO2 thin films,N-type SrTiO3 substrates
更新于2025-11-14 17:04:02
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Structural and spectroscopic studies of bromofullerene
摘要： Structural and spectroscopic studies on fullerene and bromofullerene molecules were investigated. The reaction with liquid bromine was used to prepare bromofullerene and it was characterized by using X-ray diffraction, ultravioletevisible, Fourier transform infrared, Fourier transform Raman, electron paramagnetic resonance, scanning electron microscopy, high resolution transmission electron and energy dispersive X-ray techniques. The XRD pattern of bromofullerene con?rms the attachment of bromine to fullerene. FT-IR and FT-Raman characterization reveal the C-Br vibrations in the brominated fullerene which con?rms the bromination. UVeVis spectral analysis endorses shifting of absorption peak. EPR line width, with g-factor values, indicates the magnetic ion sites in the samples. Scanning electron microscope and high resolution transmission electron microscope signify the changes in surface and particle size of the molecule. The particle size was calculated from Debey-Sherrer's formula and corroborated by TEM images. Energy dispersive X-ray elemental micro analysis con?rms the presence of bromine element in the molecule. The enhanced reactivity by substitution of bromine paves the way for designing the bioactive molecules which will be useful in the ?eld of carbon nano-medicine and targeted drug delivery applications.
关键词： Bromination,HRTEM,Structure properties,Fullerene
更新于2025-11-14 15:16:37
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European Microscopy Congress 2016: Proceedings || Graphite-to-diamond (13C) direct transition in a diamond anvil high-pressure cell
摘要： As the hardest material in nature, diamond is of great importance and interest for scientific studies. However, formation of a diamond is complicated process and requires extreme conditions. Bundy and Kasper (1967) for the first time synthesized a new form of carbon—hexagonal diamond – under conditions of static pressure exceeding about 13 GPa and temperature greater than about 1000°C [1]. At room temperature the crystal structure of graphite is stable up to pressure 15 GPa and loses some of the graphite features at higher pressure, forming metastable graphitic or amorphous phases [2]. Transition of polycrystalline graphite to diamond occurs after hydrostatic pressure treatment near 70 GPa [3]. The development of solid-state phase transitions, including those at the stage of nucleation and development of a new phase practically always is connected with the relaxation of elastic stress [4], and in case of graphite-diamond transformation the latter can play main role. The goal of the present work is the formation of diamond from graphite in direct phase transition in a diamond anvil high-pressure cell, where the relaxation of elastic stress can be realized by means of plastic deformation of the sample. The experiment was performed at room temperature without a catalyst.13С was subjected to the shear deformation under pressure of 25 GPa. The structure studies of the obtained material were made by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). In order to prevent the confusion of the diamond obtained in the experiment with one of the diamond anvils we used graphite composed of 13C carbon isotope atoms as a precursor. The diamond anvils consisted of conventional 12C diamond. Before TEM examination of each sample a Raman spectroscopy was used to verify that it contains only 13 C (diamond) and no 12C. TEM and EELS were carried out using JEOL JEM-2010 high-resolution transmission electron microscope. TEM analysis has shown that the samples obtained in the series of our experiments contain several phases of carbon simultaneously. After the high pressure treatment in shear diamond anvil cell (SDAC) there were observed some fragments of the sample, which contained both hexagonal and rhombohedral graphite (significant amounts of the last one), and also diamond and lonsdaleite. Fig. 1 shows the fragment, where the rhombohedral graphite presents. Fig. 2a shows the diamond structure fragment with {111}-planes composing 70o. Interplanar distances are 0.206 nm. Fig. 2b shows the EELS-spectrum which can be unambiguously attributed to a diamond. Thus, it was shown that 13С-graphite directly transforms into 13С-diamond (at least particularly) without a catalyst at room temperature after treatment in SDAC under pressure of 25 GPa.
关键词： EELS,high pressure,shear diamond anvil cell,HRTEM
更新于2025-09-23 15:23:52
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Influence of 4H-SiC substrate miscut on the epitaxy and microstructure of AlGaN/GaN heterostructures
摘要： AlGaN/GaN heterostructures were grown on “on-axis” and 2° off (0001) 4H-SiC substrates by metalorganic vapor phase epitaxy (MOVPE). Structural characterization was performed by transmission electron microscopy. The dislocation density, being greater in the on-axis case, is gradually reduced in the GaN layer and is forming dislocation loops in the lower region. Steps aligned along [11?00] in the off-axis case give rise to simultaneous defect formation. In the on-axis case, an almost zero density of steps is observed, with the main origin of defects probably being the orientation mismatch at the grain boundaries between the small not fully coalesced AlN grains. V-shaped formations are observed in the AlN nucleation layer, but are more frequent in the off-axis case, probably enhanced by the presence of steps. These V-shaped formations are completely overgrown by the GaN layer, during the subsequent deposition, presenting AlGaN areas in the walls of the defect, indicating an inter-diffusion between the layers. Finally, at the AlGaN/GaN heterostructure surface in the on-axis case, V-shapes are observed, with the AlN spacer and AlGaN (21% Al) thickness on relaxed GaN exceeding the critical thickness for relaxation. On the other hand, no relaxation in the form of V-shape creation is observed in the off-axis case, probably due to the smaller AlGaN thickness (less than 21% Al). The AlN spacer layer, grown in between the heterostructure, presents a uniform thickness and clear interfaces.
关键词： TEM,HEMT,SiC substrate,Heterostructure,AlGaN/GaN,HRTEM
更新于2025-09-23 15:21:21
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Atomic and electronic structure of ferroelectric La-doped HfO<sub>2</sub> films
摘要： The atomic structure and optical properties of ferroelectric La-doped hafnium oxide (La:HfO2) thin films grown by the plasma-assisted atomic layer deposition were investigated. Using high resolution transmission electron microscopy, it was shown that the studied La:HfO2 film has a orthorhombic polar structure with the P mn21 space group. It was found that the film exhibits ferroelectric properties. By means of X-ray photoelectron spectroscopy and specroellipsometry it was established that La:HfO2 consists of the HfO2 and La2O3 phases mixture. The specroellipsometry analysis with the Bruggeman effective medium approximation showed that the investigated La:HfO2 consists of 88% HfO2 and 12% La2O3. It is shown that etching La:HfO2 with argon ions leads to the oxygen vacancies generation in the near-surface region. These vacancies are generated mainly due to the knocking out of oxygen atoms to the interstitial positions, and the following annealing at 700 ℃ in vacuum for 1 hour leads to the annihilation of that Frenkel defects.
关键词： ferroelectric,XPS,electronic structure,plasma-assisted atomic layer deposition,atomic structure,La-doped HfO2,specroellipsometry,HRTEM
更新于2025-09-23 15:21:01
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Boron-doped graphene synthesis by pulsed laser co-deposition of carbon and boron
摘要： Incorporating dopants, such as boron, in graphene, is crucial for many applications in electrochemistry, sensors, photovoltaics, and catalysis. Many routes have been investigated for the preparation of B-doped graphene (BG) films, including chemical processes. A different way to obtain boron-doped layers to better control the concentration of boron in the doped graphene film, is pulsed laser co-ablation of C and B solid sources followed by rapid thermal heating of the B-doped carbon film deposited on a metal catalyst. Amorphous a-C:B films, containing 2%at. boron, are synthetized by pulse laser deposition onto a nickel film catalyst. Rapid thermal annealing at 1100°C leads to the formation of boron-doped graphene films, characterized by Raman, XPS, FEG-SEM, HRTEM and AFM. The results confirm the production of 1-4 layer boron doped graphene films, with a similar 2 at.% boron concentration to that of the a-C:B used as the graphene solid precursor. Boron doping does not modify the nano-architecture of graphene, but increases the concentration of defects in the films. Our results pave a new way for boron doped graphene synthesis using laser processing in a controlled and reproducible way, in particular to achieve designed electrical and chemical properties in various electronic and electrochemical applications.
关键词： Raman spectroscopy,Boron-doped graphene,Pulsed laser deposition,Rapid thermal annealing,HRTEM,XPS,AFM
更新于2025-09-23 15:19:57
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Trace level toxic ammonia gas sensing of single-walled carbon nanotubes wrapped polyaniline nanofibers
摘要： This paper presents a two-step enhancement and a comprehensive analysis of single-walled carbon nanotubes (SWCNTs) wrapped polyaniline nanofiber (NPANI) ammonia (NH3) gas sensor at room temperature. SWCNT-PANI composites are successfully synthesized using an efficient and cost-effective rapid in situ chemical polymerization method. The structural morphology and modification of the samples are characterized using field-emission scanning electron microscopy and HRTEM. FTIR and Raman spectroscopic studies are also performed to gain a better insight into the chemical environmental interaction in the as-prepared nanocomposite. The analysis confirms the successful formation of the nanocomposite. The observed NH3 gas-sensing response at 10 ppm of SWCNT, f-SWCNT (functionalised SWCNT), and SWCNT-PANI composite sensors are 5%–6%, 18%–20%, and 24%–25%, respectively. The SWCNT-PANI composite sensors have shown higher repeatability, selectivity, long-term stability, and fast response-recovery characteristics as compared to f-SWCNTs and pristine SWCNT sensors. Concentration and temperature dependent gas-sensing studies are also analyzed. The sensor response also shows a linear relationship with NH3 gas concentration and an inverse relationship with increasing temperature.
关键词： Raman spectroscopy,rapid in situ chemical polymerization,ammonia gas sensor,polyaniline nanofiber,field-emission scanning electron microscopy,FTIR,HRTEM,room temperature,single-walled carbon nanotubes
更新于2025-09-19 17:13:59
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A multi-level optical storage scheme via two-step picosecond laser irradiations: time/space modulations of microstructure and its optical property
摘要： China has a considerable lignite deposit in the Xinjiang province, but surface and underground coal seam fires pose a threat to the resources, environment, economy, and health. As the chemical and physical structure impacts coal fire propensity and behavior, two raw Xinjiang lignites were examined from the Shaerhu (SEH) and Piliqing (PLQ) Collieries. The SEH lignite is more prone to spontaneous combustion (from regional observations). These coals were of the same geological age and were both inertinite-rich 69.8 and 95.6% for SEH and PLQ, respectively (by the point counting approach). The more spontaneous combustion prone lignite had a significantly higher micropore contribution and surface area (92.4% and 195.6 m2/g), while both coals had similar micro- and mesopore size distribution ranges. Despite being classified as lignite by the mean vitrinite random reflectance (Rr, 0.30 and 0.26%) the inertinite contribution displaces some of the structural properties to be similar to the vitrinite-rich, high-volatile bituminous rank. The lignite coals had similar carbon content (74.6 and 78.4 wt % for SEH and PLQ) and vitrinite reflectance but differed in ash, volatile matter, total moisture, and fixed carbon yields. The crystallite parameters [from X-ray diffraction (XRD)], the interlayer spacing, crystallite height, crystallite diameter, and the numbers of aromatic layer indicated the more inertinite-rich coal (PLQ) was slightly less ordered. Both coals were highly aromatic for lignites (fa values of 0.72 and 0.71 for SEH and PLQ), as the inertinite-rich components are more aromatic and polycondensed than huminite-rich (a low rank counterpart of vitrinite) coals of similar reflectance values. Image analysis of high-resolution transmission electron microscopy lattice fringes determined the aromatic fringe length distributions, with contributions between 0.25 and 0.50 nm accounting for 37.1% for PLQ, while SEH had a lower contribution of 28.0% but also a greater contribution of longer fringes (larger polycyclic aromatic hydrocarbon cluster sizes). Image analysis of the lattice fringe orientation and XRD analysis also indicated that the SEH lignite was more ordered. These two coals are potential coals for further exploration of spontaneous combustion behavior for inertinite-rich coals.
关键词： inertinite,lignite,spontaneous combustion,LDIMS,HRTEM,solid-state 13C NMR,XRD
更新于2025-09-19 17:13:59
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Synthesis and Characterization of Aluminum Nanoparticles Prepared in Vinegar Using a Pulsed Laser Ablation Technique
摘要： The second harmonic wavelength of a neodymium-doped yttrium-aluminum-garnet (Nd-YAG) laser (λ = 532 nm) was used in a pulsed laser ablation technique (PLAL) to synthesize aluminum nanoparticles suspended in white vinegar from an aluminum target. The nanoparticles were characterized by HRTEM and UV-Vis spectrophotometry. They were found to range in size between 2 and 50 nm in diameter, with an average diameter of 12 ± 9 nm. The nanoparticles had a maximum absorption peak at 237 nm and were found to exhibit a core-shell structure with an Al core coated by a thin layer of an amorphous material which could be attributed to amorphous carbon. HRTEM results revealed that the small nanoparticles (<20 nm) had an fcc phase of aluminum crystalline structure, where the larger particles represented alumina (γ-Al2O3) nanoparticles. Such observation suggests that the use of white vinegar as an ablation medium could facilitate the synthesis of aluminum nanoparticles with minimal evidence of the existence of aluminum oxide nanoparticles in the resultant suspension.
关键词： pulsed laser ablation in liquid (PLAL),UV-Vis spectrophotometry,white vinegar,HRTEM,Aluminum nanoparticles
更新于2025-09-19 17:13:59
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Development of BMG-B2 nanocomposite structure in HAZ during laser surface processing of ZrCuNiAlTi bulk metallic glasses
摘要： In this study, the laser scanning speed used during laser surface processing of a Zr-based bulk metallic glass (BMG) was optimised to obtain amorphous structure in the melt zone and nanocomposite structure in the heat affected zone (HAZ). For this aim, Zr65.7Ti3.3Al3.7Ni11.7Cu15.6 (wt.%) BMG foils with a 100 μm thickness were laser surface treated at the different speeds. The microstructural characteristics of the melt zone and HAZ of the different samples were studied using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and high-resolution transmission electron microscope (HRTEM). Nanoindentation testing was employed to evaluate the mechanical properties. In addition, molecular dynamic (MD) simulation was used to discover the deformation mechanisms. The results showed that at a laser speed of 22.6 mm/s, the amorphous structure remained, and an improvement in mechanical properties was obtained in the HAZ. The HRTEM images revealed that in the HAZ of the optimum sample, a BMG-B2 nanocomposite structure was formed that improved the mechanical response of the HAZ, considerably. The MD results showed that the existence of B2 phase avoid stress reduction after starting of plastic deformation and even a higher stress during plastic deformation was recorded.
关键词： Laser processing,HRTEM,Zr-bases BMGs,Microstructure
更新于2025-09-12 10:27:22