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Influence of catalysts on the electronic properties of gallium nitride nanomaterials
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The effects of atomic-scale disorder and charge (de)localization hold significant importance, and they provide essential insights to unravel the role that strong and weak correlations play in condensed matter systems. In the case of perovskite oxide heterostructures, while disorders introduced via various external stimuli have strong influences over the (de)localization of interfacial two-dimensional (2D) electrons, these factors alone could not fully account for the system's charge dynamics where interfacial hybridization holds very strong influence. Here, we determine that the displaced 2D free electrons have been localized in the specific hybridized states of the LaAlO 3 /SrTiO 3 interface. This experimental study combines both transport measurements and temperature-dependent x-ray absorption spectroscopy and suggests that the localization of 2D electrons can be induced via temperature reduction or ionic liquid gating. Furthermore, this localization effect is found to be applicable to both amorphous and crystalline interfacial systems. In particular, we demonstrate that interfacial hybridization plays a pivotal role in regulating the 2D electron localization effects. Our study resolves the location where the 2D electrons are localized not only does it highlight the importance of interfacial hybridization but it also opens a new avenue for device fabrication in amorphous film systems where charge localization can be done at much great ease as compared to epitaxial crystalline heterostructures.
The shrinking of transistors has hit a wall of material degradation and the specialized electronic applications for complex scenarios have raised challenges in heterostructures integration. Intriguingly, two-dimensional (2D) materials have excellent performance even at monolayer. The rich band structures and the lattice-mismatch-free heterostructures can further develop specific mechanisms to meet the demands of various electronic systems. Here we review the progress of 2D semiconductors to develop specific electronic applications from devices to systems. Focusing on the ultra-thin high-performance nanosheets for transistor channels, we consider channel optimization, contact characteristics, dielectric integration. Then we examined 2D semiconductors for specific electronic functions including computing, memory and sense. Finally, we discuss the specific applications of functionalized arrays aiming at problems that are difficult to solve with bulk materials, like the fusion of memory and computation and the all-in-one system.
Carbon nanomaterials with a different character of the chemical bond—graphene (sp2) and nanodiamond (sp3)—are the building bricks for a new class of all-carbon hybrid nanomaterials, where the two different carbon networks with sp3 and sp2 hybridization coexist, interacting and even transforming into one another. The extraordinary physiochemical properties defined by the unique electronic band structure of the two border nanoallotropes ensure the immense application potential and versatility of these all-carbon nanomaterials. The review summarizes the status quo of sp2 – sp3 nanomaterials, including graphene/graphene-oxide—nanodiamond composites and hybrids, graphene/graphene-oxide—diamond heterojunctions, and other sp2–sp3 nanocarbon hybrids for sensing, electronic, and other emergent applications. Novel sp2–sp3 transitional nanocarbon phases and architectures are also discussed. Furthermore, the two-way sp2 (graphene) to sp3 (diamond surface and nanodiamond) transformations at the nanoscale, essential for innovative fabrication, and stability and chemical reactivity assessment are discussed based on extensive theoretical, computational and experimental studies.
The performance of functional materials is dictated by chemical and structural properties of individual atomic sites. In catalysts, for example, the thermodynamic stability of constituting atomic sites is a key descriptor from which more complex properties, such as molecular adsorption energies and reaction rates, can be derived. In this study, we present a widely applicable machine learning (ML) approach to instantaneously compute the stability of individual atomic sites in structurally and electronically complex nano-materials. Conventionally, we determine such site stabilities using computationally intensive first-principles calculations. With our approach, we predict the stability of atomic sites in sub-nanometer metal clusters of 3-55 atoms with mean absolute errors in the range of 0.11-0.14 eV. To extract physical insights from the ML model, we introduce a genetic algorithm (GA) for feature selection. This algorithm distills the key structural and chemical properties governing the stability of atomic sites in size-selected nanoparticles, allowing for physical interpretability of the models and revealing structure-property relationships. The results of the GA are generally model and materials specific. In the limit of large nanoparticles, the GA identifies features consistent with physics-based models for metal-metal interactions. By combining the ML model with the physics-based model, we predict atomic site stabilities in real time for structures ranging from sub-nanometer metal clusters (3-55 atom) to larger nanoparticles (147 to 309 atoms) to extended surfaces using a physically interpretable framework. Finally, we present a proof of principle showcasing how our approach can determine stable and active nanocatalysts across a generic materials space of structure and composition.
Intensive effort to tailor photophysics of lead-free perovskites is appealing in recent years. However, their combined electronic and optical property elucidations remain elusive. Here, we report spectroscopic observations of the coexistence Zhang-Rice singlet state and exotic electronic transitions in two-dimensional copper-based perovskites. Herein, several per-ovskites with different alkylammonium spacers are investigated to unravel their correlated electronic systems and optical responses. Namely, methylammonium, ethylammonium, phenylmethylammonium and phenethylammonium. Using temperature dependent high-resolution X-ray absorption spectroscopy, we observe distinct electronic features highlighting the impact of short spacer chains compared to long-conjugated ligands, demonstrating a pronounced 3d 9 and 3d 9 L signature linewidth variation. Corroborated by density functional theory calculations, the transient dynamics evolution of copper-based hybrid perovskites is influenced by the strong interplay of electron-phonon interactions and geometric constrictions. This finding sheds light on tuning the electronic and optical properties of hybrid per-ovskites towards efficient photoactive-based devices.
Charge localization is critical to the control of charge dynamics in systems such as perovskite solar cells, organic‐, and nanostructure‐based photovoltaics. However, the precise control of charge localization via electronic transport or defect engineering is challenging due to the complexity in reaction pathways and environmental factors. Here, charge localization in optimal‐doped La1.85Sr0.15CuO4 thin‐film on SrTiO3 substrate (LSCO/STO) is investigated, and also a high‐energy plasmon is observed. Charge localization manifests as a near‐infrared mid‐gap state in LSCO/STO. This is ascribed to the interfacial hybridization between the Ti3d‐orbitals of the substrate and O2p‐orbitals of the film. The interfacial effect leads to significant changes in the many‐body correlations and local‐field effect. The local‐field effect results in an inhomogeneous charge distribution, and due to perturbation by an external field, the high polarizability of this nonuniform charge system eventually generates the high‐energy plasmon. Transformation of the electronic correlations in LSCO/STO is further demonstrated via temperature‐dependent spectral‐weight transfer. This study of charge localization in cuprates and interfacial hybridization provides important clues to their electronic structures and superconductive properties.
The exciton, a quasi-particle which creates a bound state of an electron and a hole, is typically found in semiconductors. It has attracted major attention in the context of both fundamental science and practical applications. Transition metal dichalcogenides (TMDs) are a new class of two-dimensional materials that include direct band-gap semiconductors with strong spin-orbit coupling and many-body interaction. Manipulating new excitons in semiconducting TMDs could generate novel means of applications in nano-devices. Here, we report the observation of high-energy excitonic peaks in the monolayer-MoS2 on SrTiO3 heterointerface generated by a new complex mechanism, based on a comprehensive study that comprises temperature-dependent optical spectroscopies and first-principles calculations. The appearance of these excitons is attributed to the change in many-body interactions that occurs alongside the interfacial orbital-hybridization and spin-orbit coupling brought about by the excitonic effect propagated from the substrate. This has further led to the formation of a fermi-surface feature at the interface. The results provide atomic-scale understanding of heterointerface between monolayer-TMDs and perovskite oxide and highlight the importance of spin-orbit-charge-lattice coupling on the intrinsic properties of atomic-layer heterostructures, which open up a way in manipulating the excitonic effects in monolayer TMDs via an interfacial system.
Nanostructures have attracted a huge interest as a rapidly growing class of materials for many applications. Several techniques have been used to characterize the size, crystal structure, elemental composition and a variety of other physical properties of nanoparticles. In several cases, there are physical properties that can be evaluated by more than one techniques. Different strengths and limitations of each technique complicate the choice of the most suitable method, while often a combinatorial characterization approach is needed. In addition, given that the significance of nanoparticles in basic research and applications is constantly increasing, it is necessary that researchers from separate fields overcome the challenges in the reproducible and reliable characterization of the nanomaterials, after their synthesis and further process (e.g. annealing) stages. The principal objective of this Review is to summarize the present knowledge on the use, the advances, the advantages and the weaknesses of a large number of experimental techniques that are available for the characterization of nanoparticles. Different characterization techniques are classified according to the concept/group of the technique used, the information they can provide, or the materials that they are destined for. We describe the main characteristics of the techniques, their operation principles and we give various examples on their use, presenting them in a comparative mode, when possible, in relation to the property studied in each case.
In this paper we account for the physics behind the exciton peak shift in GaN nanorods (NRs) due to hydrogenation. GaN NRs were selectively grown on a patterned Ti/Si(111) substrate using plasma-assisted molecular beam epitaxy, and the effect of hydrogenation on their optical properties was investigated in detail using low-temperature photoluminescence measurements. Due to hydrogenation, the emissions corresponding to the donor–acceptor pair and yellow luminescence in GaN NRs were strongly suppressed, while the emission corresponding to the neutral to donor bound exciton (D⁰X) exhibited red-shift. Thermal annealing of hydrogenated GaN NRs demonstrated the recovery of the D⁰X and deep level emission. To determine the nature of the D⁰X peak shift due to hydrogenation, comparative studies were carried out on various diameters of GaN NRs, which can be controlled by different growth conditions and wet-etching times. Our experimental results reveal that the D⁰X shift depends on the diameter of the GaN NRs after hydrogenation. The results clearly demonstrate that the hydrogenation leads to band bending of GaN NRs as compensated by hydrogen ions, which causes a red-shift in the D⁰X emission.
The low pressure chemical vapor deposition (LPCVD) method is used to synthesize GaN nanowires. It is an alternative technique to the more conventional molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE). Nanowires grown by LPCVD are shown to have a single-crystal Wurtzite structure and present a strong luminescence at a near-band-gap energy. A sub-band-gap defect-related luminescence is also observed in the visible range. Identified as yellow (YL) and green (GL) luminescences, these emissions are very similar to the one reported from bulk and thin 2D GaN film samples and may be attributed to comparable defects. In this work, using photoluminescence (PL) spectroscopy with an above-band-gap excitation and a time-resolved PL with a below-band-gap excitation, we investigate whether these luminescences originate from bulk and/or surface defects. We demonstrate that the YL defect-related band can be significantly suppressed by 88% after passivating the surface of nanowires with aluminum oxide. This suppression is in favor of the localized surface defects responsible for the yellow luminescence, while the green luminescence band originates from deeper bulk defects.
Gallium nitride (GaN) is an III-V semiconductor with a direct band-gap of 3 . 4 e V . GaN has important potentials in white light-emitting diodes, blue lasers, and field effect transistors because of its super thermal stability and excellent optical properties, playing main roles in future lighting to reduce energy cost and sensors to resist radiations. GaN nanomaterials inherit bulk properties of the compound while possess novel photoelectric properties of nanomaterials. The review focuses on self-assemblies of GaN nanoparticles without templates, growth mechanisms of self-assemblies, and potential applications of the assembled nanostructures on renewable energy.
Synthesis of GaN nanoparticles (NPs) by a novel chemical co-precipitation method and the effect of nitridation temperature on the structural, optical and electrical properties have been reported. X-ray diffraction and high resolution transmission electron microscopy show the hexagonal wurtzite structure and highly crystalline nature of the GaN NPs. A strong blue luminescence was observed for all the GaN NPs at room temperature photoluminescence studies. Nevertheless, red and yellow luminescence were absent at the nitridation temperature of 1000 °C. The phonon frequency mode at the k-point of the Brillouin zone symmetry was observed at 271-273 cm−1 for GaN NPs by micro-Raman spectroscopy, which is normally absent for bulk GaN. The carrier concentration and mobility of GaN NPs synthesized at higher temperature were calculated to be 1.36 × 1017 cm−3 and 433 cm2 V−1 s−1, respectively, by the Raman line shape analysis of the longitudinal-optical-phonon-plasmon coupled mode.
Nitrogen (N) and metal (Al, Ga, and In) K-edge x-ray absorption near-edge structure (XANES), x-ray emission spectroscopy (XES), and Raman scattering measurements were performed to elucidate the electronic structures of group-III–nitride nanorods and thin films of AlN, GaN, and InN. XANES spectra show slight increase of the numbers of unoccupied N p states in GaN and AlN nanorods, which may be attributed to a slight increase of the degree of localization of conduction band states. The band gaps of AlN, GaN, and InN nanorods are determined by an overlay of XES and XANES spectra to be 6.2, 3.5, and 1.9 eV, respectively, which are close to those of AlN and GaN bulk/films and InN polycrystals.
A comprehensive study of the electronic structure of group-III nitrides (AlN, GaN, InN, and BN) crystallizing in the wurtzite, zinc-blende, and graphitelike hexagonal (BN) structures is presented. A large set of the x-ray emission and absorption spectra was collected at the several synchrotron radiation facilities at installations offering the highest possible energy resolution. By taking advantage of the linear polarization of the synchrotron radiation and making careful crystallographic orientation of the samples, the bonds along c axis ({pi}) and ''in plane'' ({sigma}) in the wurtzite structure could be separately examined. Particularly for AlN we found pronounced anisotropy of the studied bonds. The experimental spectra are compared directly with ab initio calculations of the partial density of states projected on the cation and anion atomic sites. For the GaN, AlN, and InN the agreement between structures observed in the calculated density of states (DOS) and structures observed in the experimental spectra is very good. In the case of hexagonal BN we have found an important influence of insufficient core screening in the x-ray spectra that influences the DOS distribution. The ionicity of the considered nitrides is also discussed. (c) 2000 The American Physical Society.
We report on the observation of broad photoluminescence wavelength tunability from n-type gallium nitride nanoparticles (GaN NPs) fabricated using the ultraviolet metal-assisted electroless etching method. Transmission and scanning electron microscopy measurements performed on the nanoparticles revealed large size dispersion ranging from 10 to 100 nm. Nanoparticles with broad tunable emission wavelength from 362 to 440 nm have been achieved by exciting the samples using the excitation power-dependent method. We attribute this large wavelength tunability to the localized potential fluctuations present within the GaN matrix and to vacancy-related surface states. Our results show that GaN NPs fabricated using this technique are promising for tunable-color-temperature white light-emitting diode applications.
We present a detailed study of the electronic structure of europium nitride (EuN), comparing spectroscopic data to the results of advanced electronic structure calculations. We demonstrate the epitaxial growth of EuN films, and show that in contrast to other rare-earth nitrides successful growth of EuN requires an activated nitrogen source. Synchrotron-based x-ray spectroscopy shows that the samples contain predominantly Eu3+, but with a small and varying quantity of Eu2+ that we associate with defects, most likely nitrogen vacancies. X-ray absorption and x-ray emission spectroscopies (XAS and XES) at the nitrogen K edge are compared to several different theoretical models, namely, local spin density functional theory with Hubbard U corrections (LSDA+U), dynamic mean field theory (DMFT) in the Hubbard-I approximation, and quasiparticle self-consistent GW (QSGW) calculations. The DMFT and QSGW models capture the density of conduction band states better than does LSDA+U. Only the Hubbard-I model contains a correct description of the Eu 4f atomic multiplets and locates their energies relative to the band states, and we see some evidence in XAS for hybridization between the conduction band and the lowest-lying 8S multiplet. The Hubbard-I model is also in good agreement with purely atomic multiplet calculations for the Eu M-edge XAS. LSDA+U and DMFT calculations find a metallic ground state, while QSGW results predict a direct band gap at X for EuN of about 0.9 eV that matches closely an absorption edge seen in optical transmittance at 0.9 eV, and a smaller indirect gap. Overall, the combination of theoretical methods and spectroscopies provides insights into the complex nature of the electronic structure of this material. The results imply that EuN is a narrow-band-gap semiconductor that lies close to the metal-insulator boundary, where the close proximity to the Fermi level of an empty Eu 4f multiplet raises the possibility of tuning both the magnetic and electronic states in this system.
Long regarded as a matter of scientific curiosity, GaN has been continued as the most important and indispen- sable materials used among all other compound semiconductors and is also expected to play a critical role in future optoelectronic devices. We cover recent advances in growth and characterization of chemically synthesized quasi one dimensional GaN nanostructure in various forms, such as nanowires, nanorods, nanotubes, nanopipes, and nanotips and their complex heterostructures. We first briefly introduce the general scheme based on metal catalyzed vapour–liquid– solid growth mechanism for the synthesis of a broad range of nanostructures. The novelty of catalyst free growth of such nanostructures has also been highlighted. Complex structures, such as hierarchical and core-shell structures, are also touched upon.
The electrical and optoelectronic properties depend on chemical composition, doping and other physical dimensions along with the type of growth technique being used. Size sensitive optical, electrical, thermal and mechanical properties of such nanostructure have strong influence on device properties. We intend to review these properties in case of GaN nanostruc- tures. We will also explore the methods to assemble and integrate such nanostructures into large-scale functional devices for varied practical applications. Room-temperature high-performance electrical and optical devices will then be discussed at the single as well as assembly of nanowire. A section will be dedicated for detailed discussion on recent development of GaN nanostructures, including, light emitting diode, photovoltaic, photocatalysis, biosensor, gas and chemical sensor, and nanochannel. Technical challenges and scientific questions for the widespread use of GaN nanostructures, so relevant issues will also be briefly addressed.
For the past 25 years NIH Image and ImageJ software have been pioneers as open tools for the
analysis of scientific images. We discuss the origins, challenges and solutions of these two programs,
and how their history can serve to advise and inform other software projects.
X-ray absorption near edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been employed to obtain information on the electronic structures of the GaN nanowires and thin film. The comparison of the XANES spectra revealed that the nanowires have a smaller (larger) N (Ga) K edge XANES intensity than that of the thin film, which suggests an increase (decrease) of the occupation of N 2p ( Ga 4p) orbitals and an increase of the N (Ga) negative (positive) effective charge in the nanowires. The SPEM spectra showed that the Ga 3d band for the nanowires lies about 20.8 eV below the Fermi level and has a chemical shift of about -0.9 eV relative to that of the thin film. © 2003 American Institute of Physics.
We report on the direct fabrication of single-crystalline wurtzite-type hexagonal GaN nanotubes via a newly designed, controllable, and reproducible chemical thermal-evaporation process. The nanotubes are single crystalline, have one end closed, an average outer diameter of ∼300 nm , an inner diameter of ∼100 nm , and a wall thickness of ∼100 nm . The structure and morphology of the tubes are characterized using a scanning electron microscope and a transmission electron microscope. The cathodoluminescence of individual nanotubes is also investigated. The growth mechanism, formation kinetics, and crystallography of GaN nanotubes are finally discussed.
The authors report on time-integrated and time-resolved photoluminescence (PL) of GaN nanowires grown by the Ni-catalyst-assisted vapor-liquid-solid method. From PL spectra of Ni-catalyzed GaN nanowires at 10 K, several PL peaks were observed at 3.472, 3.437, and 3.266 eV, respectively. PL peaks at 3.472 and 3.266 eV are attributed to neutral-donor-bound excitons and donor-acceptor pair, respectively. Furthermore, according to the results from temperature-dependent and time-resolved PL measurements, the origin of the PL peak at 3.437 eV is also discussed. (c) 2006 American Institute of Physics.
Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The point defects include native isolated defects (vacancies, interstitial, and antisites), intentional or unintentional impurities, as well as complexes involving different combinations of the isolated defects. Further improvements in device performance and longevity hinge on an in-depth understanding of point defects and their reduction. In this review a comprehensive and critical analysis of point defects in GaN, particularly their manifestation in luminescence, is presented. In addition to a comprehensive analysis of native point defects, the signatures of intentionally and unintentionally introduced impurities are addressed. The review discusses in detail the characteristics and the origin of the major luminescence bands including the ultraviolet, blue, green, yellow, and red bands in undoped GaN. The effects of important group-II impurities, such as Zn and Mg on the photoluminescence of GaN, are treated in detail. Similarly, but to a lesser extent, the effects of other impurities, such as C, Si, H, O, Be, Mn, Cd, etc., on the luminescence properties of GaN are also reviewed. Further, atypical luminescence lines which are tentatively attributed to the surface and structural defects are discussed. The effect of surfaces and surface preparation, particularly wet and dry etching, exposure to UV light in vacuum or controlled gas ambient, annealing, and ion implantation on the characteristics of the defect-related emissions is described.
Rational design is an important approach to consider in the
development of low-dimensional hybrid organic−inorganic perovskites (HOIPs). In this study, 1-butyl-1-methyl pyrrolidinium (BMP), 1-(3-aminopropyl)imidazole (API), and 1-butyl-3-methyl imidazolium (BMI) serve as prototypical ionic liquid components in bismuth-based HOIPs. Element-sensitive X-ray absorption spectroscopy measurements of BMPBiBr4 and APIBiBr5 reveal distinct resonant excitation profiles across the N Kedges, where contrasting peak shifts are observed. These 1D-HOIPs exhibit a
large Stokes shift due to the small polaron contribution, as probed by photoluminescence spectroscopy at room temperature. Interestingly, the incorporation of a small fraction of tin (Sn) into the APIBiBr5 (Sn/Bi mole ratio of 1:3) structure demonstrates a strong spectral weight transfer accompanied by a fast decay lifetime (2.6 ns). These phenomena are the direct result of Sn-substitution in APIBiBr5, decreasing the small polaron effect. By changing the active ionic liquid, the electronic interactions and optical responses can be moderately tuned by alteration of their intermolecular interaction between the semiconducting inorganic layers and organic moieties.
This book reviews new advances in the field of nanomaterials; their synthesis, characterization, and applications. Specific topics include nanomaterials as catalysts, photodegradation of organic pollutants, multifunctional textiles, self-healing hydrogels, nanosensors for the detection of pathogens, machine learning based prosthesis, and various applications in the sports industry, the automobile sector, the area of defense and security, pharmaceuticals, energy storage and food packaging.
The upsurge of low-dimensional Dion–Jacobson (DJ) phase perovskites brought significant interests in view of their appealing stability against harsh environmental conditions as well as their promising performance in optoelectronic applications. Few reports to date have concentrated on the fundamental relationship of fine-tuning control of diamine-based perovskite single crystals toward their electronic properties and optical behaviors. Here, we demonstrate that the cationic control is proposed to regulate the role of hydrogen bond of organic ligands with the edge-sharing [CuCl6]4- octahedra layers, leading to strong differences in the material excitonic profile and tunability of their electronic properties. Interestingly, we observe a significant reduction of photoluminescence intensity upon controlling the Cu2+/Cu+ proportion in this hybrid system. According to the photoemission measurements, the oxidation states variation of Cu cations plays a crucial role to stabilize diammonium-based perovskite geometric structure. Interestingly, we found that the electronic signatures of the singlet spin-state and high energy region transition are not influenced by thermal effect, as probed by temperature-dependent X-ray absorption spectroscopy (XAS) at elevated temperature. Density functional calculations suggest that such electronic difference originates from the hydrogen bonding reduction that altered the magnitude of the octahedral distortion within the DJ layered structure. As a result, +3NHC4H9NH3+ conformation produces a non-negligible interaction towards tuning the optical and electronic properties of DJ copper-based perovskites.
The tunable control in the inorganic octahedral framework of hybrid perovskites offers potential applications in photovoltaics, solid-state lighting, and radiation detection. However, the implication of the structure and optoelectronic properties pose challenges due to competition between organic− inorganic coupling and intraoctahedral interactions. In this study, we combine X-ray absorption spectroscopy (XAS) and Raman analysis to interpret the angular-dependent behavior and anharmonicity of manganese-based single-crystal perovskites differing by a single methylene unit. The XAS spectra of manganese-based single-crystal perovskites with 2-phenethylamine (PEA) compared to 3-phenyl-1-propylamine (PPA) as organic cations unambiguously demonstrated a 180°intensity shift as a function of the incoming photon, suggesting a pronounced structural ligand variation. The out-of-plane polarization is found to be more prominent in L 2-edge than in L 3. In addition, an accompanying shoulder peak around 643 eV was attributed to the electron excitation from Mn 2p to 3d orbitals to form d 5 L states. A decrease in terms of field strength is prominently observed that infers a low crystal field splitting energy. Raman analysis of the two hybrid perovskites displays a notable difference in the respective translational modes at 84 and 87 cm −1 , which signifies the amplified anharmonicity due to extended chain length. Based on this phenomenological approach, a longer chain promotes a rather unique octahedral deformation than anharmonicity shift that is crucially important to decoupling the nature of the active units. This effort sheds some light to implement the orientational ordering toward an efficient charge transport of hybrid perovskite semiconductors.
KEYWORDS. X-ray absorption spectroscopy; layered double perovskite; valence band; and density functional theory. ABSTRACT. The realization of lead-free all-inorganic perovskites in emergent materials requires an in-depth understanding of strongly correlated systems towards optoelectronics or spintronics applications. Herein, we report the electronic and optical variation of the <111>-oriented layered double perovskites (LDP) family with the formula of Cs4M II Bi2Br12 thin films (where M II : Cu, Mn, Pb or Sr). The element and shell-specific orbital polarization based on soft x-ray linear dichroism (XLD) spectroscopy probes the Cs M4,5-and Mn L2,3-edges of Cs4MnBi2Br12 thin films as a function of temperature. A strong reversal orbital polarization at the respective edges at 150 K indicates that a thermally induced orbital-selective rearrangement at low temperature. In 3 addition, the valence band analysis indicates different orbital admixture of Br 4p and M II d states, corroborated by the density functional theory calculations. In terms of the transient charge dynamics, we observe the photoluminescence peak maxima position trend line is shifted towards a longer wavelength. In addition, the longest average lifetime is recorded for Cs4CuBi2Br12 at 27.40 + 1 µs. As the LDPs structural integrity is a lead free, therefore these all-inorganic perovskites hold promising potentials as sustainable and green materials for photophysics applications.
Although X-ray absorption spectroscopy (XAS) was conceived in the early 20th century, it took 60 years with the advent of synchrotrons for researchers to exploit its tremendous potential. Counterintuitively, researchers are now developing bench type polychromatic X-ray sources that are less brilliant to measure catalyst stability and work with toxic substances. XAS measures the absorption spectra of electrons that X-rays eject from the tightly bound core electrons to the continuum. The spectrum from 10-150 eV (kinetic energy of the photoelectrons) above the chemical potential—binding energy of core electrons—identifies oxidation state and band occupancy (X-ray absorption near edge structure, XANES), while higher energies in the spectrum relate to local atomic structure like coordination number and distance, Debye–Waller factor, and inner potential correction (extended X-ray absorption fine structure, EXAFS). Combining XAS with complementary spectroscopic techniques like Raman, Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) elucidates the nature of the chemical bonds at the catalyst surface to better understand reaction mechanisms and intermediates. Because synchrotrons continue to be the light source of choice for most researchers, the number of articles Web of Science indexes per year has grown from 1000 in 1991 to 1700 in 2020. Material scientists and physical chemists publish an order of magnitude articles more than chemical engineers. Based on a bibliometric analysis, the research comprises five clusters centred around: electronic and optical properties, oxidation and hydrogenation catalysis, complementary analytical techniques like FTIR, nanoparticles and electrocatalysis, and iron, metals, and complexes.
The methodology presented within this work is a result of years of interactions between many junior and senior X-ray Photoelectron Spectroscopy (XPS) users operating within the CasaXPS spectral processing and interpretation program framework. In particular, discussions arising from a series of workshops have been a significant source for developing the overall XPS data processing concept and are the motivation for creating this work. These workshops organized by the Institut des Matériaux Jean Rouxel (IMN), Nantes gather both experienced and novice users of XPS for a week of discourse in conceptual experiment design and the resulting data processing. However, the framework constructed and utilized within these workshops encouraged the dissemination of knowledge beyond XPS data analysis and emphasized the importance of a multi-disciplinary collaborative approach to surface analysis problem-solving. The material presented here embodies data treatment originating from data made available to the first CNRS Thematic Workshop presented at Roscoff 2013. The methodology described here has evolved over the subsequent workshops in 2016 and 2019 and currently represents the philosophy used in CasaXPS spectral data processing paradigm.
A controllable electronic manipulation in a frustrated magnetic system such as solution-based two-dimensional (2D) all-inorganic perovskites offers a possible route for their integrations with electronic and magnetic devices for their advanced applications. Here, we perform element-specific investigations of an emergent class of quasi-2D all-inorganic perovskites Cs2CuCl4-xBr
x
with (0 ≤ x ≤ 4) using a combination of synchrotron-radiation photoelectron techniques. Surface- and element-sensitive X-ray absorption spectroscopy spectra of Cu L2,3 edges indicate strong electronic transition that is largely influenced by their halogen content at room temperature. This implies that site-selective occupation largely dominates the electronic transition across the unoccupied states of these series since chlorine atoms possess a stronger electronegative character than bromine atoms. Moreover, the implication of halogen site is reflected in the valence band of Cl-rich copper perovskite in which the valence band edge is closer to Fermi energy (EF) than that of the Br-rich compound. Furthermore, X-ray magnetic circular dichroism spectra of mixed ratio and Br-rich compounds exhibit antiferromagnetism at room temperature. These site-specific magnetic-spectroscopic results are corroborated by density functional theory calculations. The strong electronic modulation and the local magnetic spectroscopy results in these solution-based and low-temperature-growth materials will pave the way toward energy- and cost-efficient perovskite devices.
Low-dimensional nanowires have received increased interest as building blocks for electronic and photonic devices. In this article, we review the recent progress made on III -nitridenanowire photonic devices by molecular beam epitaxy (MBE), with a focus on micro-LED s, deep-ultraviolet (UV)-light emitters, and solar fuel and artificial photosynthesis devices. The challenges and prospects of III -nitride nanowires for these applications are also discussed.
A new beamline and a six-circle UHV diffractometer have been constructed at the Singapore Synchrotron Light Source with a broad energy coverage from 3.5 to 1500 eV. The beamline is optimized for ultraviolet-vacuum-ultraviolet optical reflectivity and resonant soft X-ray scattering with medium energy resolution over a broad energy range, achieved by using a self-focusing monochromator consisting of a plane mirror and three variable line spacing gratings. The unique character of the diffractometer comprises 4-circles in the vertical plane and 2-circles in the horizontal plane. Thirteen motions are available inside the UHV chamber with a base pressure of 1 × 10⁻⁹ mbar. Two sample holders working independently over a temperature range of 37 K-400 K are controlled by a closed-cycle cryostat, while the bottom holder inside a high field compact pulsed magnet is available for measurements requiring a magnetic field.
The GaN-based nanoelectroceramics were produced using the sol-gel method and their variation with Ni at different atomic ratios (0%, 0.2%, 2%, 4%, 6%) was investigated. The surface morphology, structure, and optical and electrical properties of undoped and Ni doped GaN nanocomposites were characterized. The X-ray diffraction (XRD) results showed that the hexagonal Wurtzite lattice structure of the obtained samples was polycrystalline. The crystallite size of the nanopowders was calculated as 22.52, 21.36, 20.47, 18.36, and 17.96 nm for 0%, 0.2%, 2%, 4%, and 6% samples, respectively. The reflection spectra of the samples decreased with increasing Ni ratio. The optical band gap values of the samples calculated by using Kubelka-Munk function were found as 3.31, 3.29, 3.22, 2.99, and 2.88 eV for 0%, 0.2%, 2%, 4%, and 6% Ni samples, respectively. The electrical conductivity values of the samples were measured at the temperature range of 303–423 K based on the temperature and analyzed by Arrhenius equation. It was observed that Ni dopant decreased the activation energy values of GaN nanopowders. It was concluded in the light of the obtained data that the stability and the crystal structure quality of the Wurtzite-structure GaN produced in this study were high; therefore, this semiconductor material can be used in electrical and optical applications and structural and physical properties of these materials can be changed with Ni dopant.
A combined experimental-theoretical study on the temperature dependence of the X-ray absorption near-edge structure (XANES) and nuclear magnetic resonance (NMR) spectra of periclase (MgO), spinel (MgAl2O4), corundum (α-Al2O3), berlinite (α-AlPO4), stishovite and α-quartz (SiO2) is reported. Predictive calculations are presented when experimental data are not available. For these light-element oxides, both experimental techniques detect systematic effects related to quantum thermal vibrations which are well reproduced by density-functional theory simulations. In calculations, thermal fluctuations of the nuclei are included by considering nonequilibrium configurations according to finite-temperature quantum statistics at the quasiharmonic level. The influence of nuclear quantum fluctuations on XANES and NMR spectroscopies is particularly sensitive to the coordination number of the probed cation. Furthermore, the relative importance of nuclear dynamics and thermal expansion is quantified over a large range of temperatures.
The molecular orientation of organic semiconductors on a solid surface could be an indispensable factor to determine the electrical performance of organic-based devices. Despite its fundamental prominence, a clear description of the emergent two-dimensional layered material–organic interface is not fully understood yet. In this study, we reveal the molecular alignment and the electronic structure of thermally-deposited N,N'-dibutyl-3,4,9,10-perylenedicarboximide (PTCDI-C4) molecules on natural molybdenum disulphide (MoS2) using near edge X-ray absorption fine structure spectroscopy (NEXAFS). The average tilt angle determination reveals that the anisotropy in the π* symmetry transition of the carbon K-edge (284-288 eV range) is present at the submonolayer regime. Supported by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and resonant photoemission spectroscopy (RPES) measurements, we find that our spectroscopic measurements indicate a weak charge transfer that established at the PTCDI-C4/MoS2 interface. A sterical hindrance due to the C4 alkyl chain cause tilting of the molecular plane at the initial thin films deposition. Our result shows a tunable interfacial alignment of organic molecules on transition metal dichalcogenide surfaces effectively enhancing the electronic properties of hybrid organic-inorganic heterostructure devices.
We report the use of Germanium (Ge) as catalyst for Gallium Nitride (GaN) nanowires growth. High-yield growth has been achieved with Ge nanoparticles obtained by dewetting a thin layer of Ge on a Si (100) substrate. The nanowires are long and grow straight with very little curvature. The GaN nanowires are single-crystalline and show a Wurtzite structure growing along the [0001] axis. The growth follows a metal-free Vapor-Liquid-Solid (VLS) mechanism, further allowing a CMOS technology compatibility. The synthesis of nanowires has been done using an industrial Low Pressure Chemical Vapor Deposition (LPCVD) system.
A layer-structure gallium nitride (GaN) nanowires, grown on Pt-coated n-type Si (111) substrate, have been synthesized using chemical vapor deposition (CVD). The results show: (1) SEM indicates that the geometry structure is layer-structure. HRTEM indicates that GaN nanowire's preferential growth direction is along [0 0 1] direction. (2) The room temperature PL emission spectrum of the layer-structure GaN nanowires has a peak at 375 nm, which proves that GaN nanowires have potential application in light-emitting nano-devices. (3) Field-emission measurements show that the layer-structure GaN nanowires film has a low turn-on field of 4.39 V/mu m (at room temperature), which is sufficient for electron emission devices, field emission displays and vacuum nano-electronic devices. The growth mechanism for GaN nanowires has also been discussed briefly.
The mechanical properties of anisotropic nanoparticles like gold nanorods (AuNRs) and silver nanorods (AgNRs) are different from those of isotropic shapes such as nanospheres. We probed the coherent lattice oscillations of nanoparticles by following the modulation of the plasmonic band frequency using ultrafast laser spectroscopy. We found that while the frequency of the longitudinal vibration mode of AgNRs is higher than that of AuNRs of similar dimensions, similarly sized gold and silver nanospheres have similar lattice vibration frequencies. Lattice vibrations calculated by finite element modeling showed good agreement with the experimental results for both AgNRs and AuNRs. The accuracy of the calculations was improved by using actual pentagonal shapes rather than cylinders that did not agree well with the experimental results. As the plasmon energy is transferred into lattice vibrations, the temperature of the nanoparticle necessarily increases as a result of this electron-phonon relaxation process. This results in a decrease in the Young's modulus which was accounted for in the calculations. Calculations showed that the tips of the nanorods are 'softer' than the rest of the nanorod. Since the tips comprise a larger portion of the overall rod in the smaller rods, the smaller rods were more affected by the tip effects.
Semiconductor nanowires, by definition, typically have cross-sectional
dimensions that can be tuned from 2-200 nm, with lengths spanning from
hundreds of nanometres to millimetres. These subwavelength structures
represent a new class of semiconductor materials for investigating light
generation, propagation, detection, amplification and modulation. After
more than a decade of research, nanowires can now be synthesized and
assembled with specific compositions, heterojunctions and architectures.
This has led to a host of nanowire photonic devices including
photodetectors, chemical and gas sensors, waveguides, LEDs, microcavity
lasers, solar cells and nonlinear optical converters. A fully integrated
photonic platform using nanowire building blocks promises advanced
functionalities at dimensions compatible with on-chip technologies.
Electron energy loss spectroscopy (EELS), in both the low loss and the core loss regions of the spectra, have been used to study local alloys composition, electronic transitions and bonding environment of different species in nanowires consisted of a GaN base-part and a superlattice upper part of InxGa1–xN nanodisks (NDs)/GaN barriers. Experimental valence (low loss) electron energy loss spectroscopy (VEELS) spectra were taken from the GaN base-part, the GaN barriers and the InxGa1–xN disks. Electron energy loss near edge structure (ELNES) spectra presenting the C K-edge, the N K-edge, the In M4,5-edge were obtained from the GaN base-part and spacer and the InxGa1–xN NDs. Variation of In concentration as well as different strain states change the intensity and result in the broadening and shifting of the edges in the spectra. The TELNES.2 program of the WIEN2k code (Blaha et al., Comp. Phys. Commun. 59, 399 (1990) [1]) was implemented in order to study the electronic properties of InxGa1–xN. The N K-edge that represents the energy loss of the electron transition from the 2p to the 1s state is unique for each structure. Details on the bonding environment of the structure were extracted from the simulations of the ELNES spectra. Moreover, the influence of the In content in the InxGa1–xN NDs on the N-K edge was interpreted. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
We report a very generic methodology to control the crystallographic orientation of GaN nanowire in a chemical vapor deposition technique employing a standard vapor-liquid-solid mechanism. Incubation time was considered as a critical parameter to control the nanowire morphology. It was found that nanowire of particular geometry, such as hexagonal, triangular, wurtzite/zinc-blende biphase, and square shaped forms could be obtained by varying the considerable length of incubation time. The perturbation on the diameter of the nanowire with respect to the size of the catalyst droplet was corroborated by a simple steady state model. Luminescence spectra recorded from the GaN NWs revealed the presence of a dominating wurtzite phase in all the as-grown samples. However, temperature independent behavior of two luminescence peaks, recorded especially from the biphase homostructure, was believed to be originated from the radiative recombination of carriers localized at potential fluctuations in the zinc-blende and wurtzite phase discretely.
We report the X-ray absorption near-edge structures (XANES) of CeAl2 thin films of various thickness, 40–120nm, at Al K-, Ce L3-, and Ce M4,5-edges. It is found that the threshold of near-edge absorption features at the Al K-edge shifts to the higher photon energy as film thickness decreases, implying that Al loses p-orbital charge and the valence of Ce increases slightly as revealed from the XANES features at Ce L3- and M4,5-edges. Above observations suggest that there is charge transfer from Al to Ce as the surface to bulk ratio is varied. This induces change in the electronic structures of CeAl2 thin films. The Ce 4f electrons are surface sensitive in nanoparticles compared to thin films. This work also shows that 4f electronic states of Ce ions are sensitive to the reduction of the coordination number induced by surface effects that would change their hybridization.
GaN nanorod films have been grown on Si(001) substrates with native silicon oxides by radio-frequency plasma-enhanced molecular beam epitaxy. GaN nanorod films are made up of single-crystalline nanorods with a so-called (0001) fiber-like texture. Each nanorod is elongated along c axis in perpendicular to the substrate surface and has no preferential axis in film plane. Excellent electron field emission characteristics were observed for the fabricated GaN nanorod films with a field emission threshold as low as 1.25 V/μm at a current density of 0.1 μA/cm2 and a field emission current density as high as 2.5 mA/cm2 at an applied field of 2.5 V/μm. These excellent characteristics are attributed to the geometrical configuration of nanorods and their good crystalline quality as well as the low electron affinity of GaN.
Photoluminescence and X-ray absorption spectroscopy (XAS) measurement were used to characterize the near-band edge (NBE) emission intensity and electronic structure of as-implanted GaN nanowires (NWs) that had been implanted with Eu ions to different fluences. The N K- and Ga L3-edge of total electron yield XAS spectra showed the formation of N interstitials (Ni) and dangling bond (Ndb) point defects and the formation of metallic Ga layers on the surface of NWs. X-ray diffraction, Ga K- and L3-edge of total fluorescence yield XAS spectra consistently revealed the wurtzite structure of the as-implanted NWs up to the highest fluence. The ratio of absorption intensity found in sp3 and sp2 environment and the NBE intensity were strongly affected by the implantation. It is suggested that the decrease of NBE intensity was closely related to the change from sp3 to sp2 environment. The absorption intensity ratio between the as-grown and as-implanted samples of Ndb and Ni was directly proportional to the fluences indicating that these defects are preferentially formed during implantation.
The current researches on GaN- and ZnO-based nanostructures are reviewed with the emphasis on fundamental growth kinetics,
characteristics, and applications. The nanostructured materials have unique properties and the devices employing the nanostructures
show superior performances when compared with the conventional devices without the nanostructures. Control of shapes and sizes
of the nanostructures has been possible by changing the synthetic methods or by processing. ZnO- and GaN-based nanostructures
can successfully be used for fabrications of various sensors, transistors as well as for photonic device applications including
light emitting diodes.
Single-crystal n-type GaN nanowires have been grown epitaxially on a Mg-doped p-type GaN substrate. Piezoelectric nanognerators based on GaN nanowires are investigated by conductive AFM, and the results showed an output power density of nearly 12.5 mW/m(2). Luminous LED modules based on n-GaN nanowires/p-GaN substrate have been fabricated. CCD images of the lighted LED and the corresponding electroluminescence spectra are recorded at a forward bias. Moreover, the GaN nanowire LED can be lighted up by the power provided by a ZnO nanowire based nanogenerator, demonstrating a self-powered LED using wurtzite-structured nanomaterials.
