102,282 research outputs found
Accuracy of dielectric-dependent hybrid functionals in the prediction of optoelectronic properties of metal oxide semiconductors: a comprehensive comparison with many-body GW and experiments
No full textUnderstanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G 0 W 0 as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS)
Chemisorption of CO on defect sites of MgO
No full textChemisorption of a CO molecule on regular and defect sites of the MgO(100) surface has been investigated by means of cluster model calculations. At all sites studied, CO bonds at the cation with the C atom closest to the surface. The bonding is considerably larger at a three-coordinated corner site than for a regular five-coordinated surface site. A blueshift in the C-O stretching frequency, ωe, of adsorbed CO compared to free CO is found; the shift is much higher for a corner than for a surface site because of the larger local electric field for low-coordinated cations. Both the bond strength and the ω shift are largely due to electrostatic effects and not to the formation of a dative σ-bond with the surface. Surface relaxation effects have also been considered. © 1992
We present experimental and theoretical results for the surface core-level binding-energy shifts of Al(100), representative of an sp metal, and Cu(100), representative of a transition metal. Our analysis of these results leads to a unified interpretation for the different behavior of sp and transition metals. The d-electron contribution to smaller surface core-level binding energies is elucidated.
No full textWe present experimental and theoretical results for the surface core-level binding-energy shifts of Al(100), representative of an sp metal, and Cu(100), representative of a transition metal. Our analysis of these results leads to a unified interpretation for the different behavior of sp and transition metals. The d-electron contribution to smaller surface core-level binding energies is elucidated
FINAL-STATE EFFECTS FOR THE CORE-LEVEL XPS SPECTRA OF NIO
No full textIonization of the Ni 3s core level in NiO has been studied using ab initio wavefunctions for an NiO6 cluster model. Three important final state effects are studied: (I ) ligand to Ni 3d charge transfer; (2) exchange coupling of the ionized core level within the open 3d shell; and (3) atomic correlation effects among the metal 3s, 3p, and 3d shells. Analysis of the cluster wave-functions shows that these mechanisms are strongly coupled and must be treated on an equal footing. The ligand to metal charge transfer is often fractional, i.e. intermediate between 0 and 1
Does the Oxygen Evolution Reaction follow the classical OH*, O*, OOH* path on single atom catalysts?
No full textThe Oxygen Evolution Reaction (OER) is a key part of water splitting. On metal and oxide surfaces it usually occurs via formation of three intermediates, M(OH), M(O), and M(OOH) (also referred to as OH*, O*, and OOH* species where * indicates a surface site). The last step consists of O2 release. So far, it has been generally assumed that the same path occurs on single atom catalysts (SACs). However, the chemistry of SACs may differ substantially from that of extended surfaces and is reminiscent of that of coordination compounds. This raises the question of whether on SACs the OER follows the classical mechanism or not. Using a DFT approach, we studied a set of 30 SACs made by ten metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Pd and Pt) anchored on three widely used 2D carbon-based materials, graphene, nitrogen-doped graphene and carbon nitride. In none of the cases examined the most favourable reaction path is the conventional one. In fact, in all cases other intermediates with higher stabilities form: M(OH)2, M(O)(OH), M(O)2, and M(O2) (OH* OH*, O* OH*, O* O*, O2* according to standard nomenclature). Therefore, the common assumption that on SACs the OER proceeds via formation of OH*, O*, and OOH* intermediates is not verified. Predictions of new catalysts based on the screening of large number of potential structures can lead to completely incorrect conclusions if these additional intermediates are not taken into consideration
Decoding the Role of Adsorbates Entropy in the Reactivity of Single-Atom Catalysts
No full textSingle-atom catalysts (SACs) are rapidly gaining attention as a versatile class of materials that combine the advantages of both homogeneous and heterogeneous catalysis. A growing number of studies aim to identify potential new SACs or to describe their structure and reactivity through ab initio quantum chemical simulations. While many computational studies primarily address reactions involving small molecules, such as water splitting or CO2 reduction, the application scope of SACs is rapidly broadening to include the production of fine chemicals and the conversion of biomass-derived platform molecules, processes that involve larger, more complex reactants. Using density-functional theory (DFT) simulations, we demonstrate that, while an approximate treatment of entropy is acceptable for molecules with up to three atoms, it introduces substantial errors in reactions involving more complex molecules. Our results reveal a linear correlation between the entropy of adsorbed molecules and that of the corresponding isolated species, mirroring trends observed on extended catalytic surfaces. For the largest systems investigated in this study, the entropy of the free molecule is reduced by approximately 10-20% upon adsorption; for small molecules, this reduction can range from 50 to 70%. This disparity arises because, on SACs, the translational entropy is effectively zero, the rotational entropy is minimal, and the vibrational entropy increases with the size of the molecule. Moreover, the entropy of adsorbates scales linearly with the number of atoms in the molecule, allowing for the prediction of entropic contributions of adsorbates on SACs without additional computational cost. Using propyne hydrogenation as a test, we demonstrate that the reaction energy profile computed with current approximate approaches for estimating the entropy of adsorbates differs significantly from the profile where entropy is explicitly included. These findings highlight the importance of considering adsorbate entropy for accurately predicting the catalytic activity of SACs, particularly for reactions involving complex molecules
coordination chemistry
No full textSingle Atom Alloys (SAAs) represent one of the most promising classes of heterogeneous catalysts. They are based on isolated transition metal (TM) atoms stabilized in a host metal matrix. Among others, SAAs are widely studied for processes involving hydrogen, such as water splitting or hydrogenation reactions. On a metal surface the H-2 molecule forms either weakly physisorbed species (H-2(phys)) or dissociates with formation of chemisorbed H atoms (H*). In electrochemical processes, the adsorption free energy of the H* intermediate is normally used as a descriptor of the catalyst reactivity. Recently, it has been shown that on Single Atom Catalysts (SAC) embedded in carbon-based, sulfide, or oxide supports other species can form, where two H atoms are stably bound to the SAC, forming dihydrogen or dihydride (H*H*) surface species reminiscent of the classical Kubas's transition metal complexes in coordination chemistry. In this work we show, based on density functional theory calculations, that dihydrogen and dihydride complexes can also form on SAAs and, depending on the nature of the host metal, can be even more stable than two separated chemisorbed H* species (2H*). These new stable intermediates have not been considered so far in the analysis of the mechanism and kinetics of the reaction. The work shows that the possible formation of dihydrogen and dihydride complexes is not limited to SACs but is also valid for SAAs and needs to be considered in the study of hydrogen-based reactions on these systems
Diagnosis of biliary tract lesions by histological sectioning of brush bristles as alternative to cytological smearing.
No full textAIM:
To increase the diagnostic potential of endoscopic biliary tract brushing, we devised an approach alternative to cytological smearing, leading to the preoperative histological examination of the collected material.
METHODS:
One hundred twelve consecutive biliary brush specimens were included. All patients presented a stricture of the biliary tract, leading to a diagnostic procedure by brushing. Immediately following brushing, the endoscopist immersed the brush into methanol and sent it to the pathology laboratory. The brush was introduced into a cassette for paraffin embedding and sections parallel to the long axis of brush were cut until the metal wire was almost reached, then the block was rotated and new sections were obtained from the opposite side. Samples of the mucosa, inflammatory cell aggregates, small fragments of carcinomas, or isolated cells were observed, and displayed an optimal fixation, allowing a definite diagnosis that proved mandatory for therapy in the vast majority of cases (99.1%).
RESULTS:
The results obtained in 112 consecutive cases using such technique compared with final histological diagnosis proved: 91% sensitivity, 100% specificity, 100% positive predictive value (PPV), and 87% negative predictive value (NPV) (P < 0.001). In nonoperated patients, the clinical diagnosis after at least 6 months of follow-up showed: 95.5% sensitivity, 100% specificity, 100% PPV, and 88.2% NPV (P < 0.001).
CONCLUSION:
Such novel approach to the preoperative diagnosis of biliary tract lesions proved to be highly sensitive and specific, limiting the inadequate preoperative diagnoses to less than 1%
Modeling Hydrogen and Oxygen Evolution Reactions on Single Atom Catalysts with Density Functional Theory: Role of the Functional
No full textThe most widely used approach to predict catalytic activity is density functional theory, whose results however depend on the adopted exchange-correlation functional. In this work, the role played by the functional in predicting the activity of single atom catalysts (SAC) in the hydrogen and oxygen evolution reactions (HER and OER) is studied. 16 transition metal (TM) atoms embedded in N-doped graphene are simulated and the performance of the widely adopted Perdew–Burke–Ernzerhof (PBE) functional against the hybrid PBE0 functional is assessed. The PBE+U approach, a computationally less complex way to correct for the self-interaction error in density functional theory, is also considered. The predictions obtained with PBE have a substantial deviation from PBE0 for first row TMs, i.e., 3d systems, while smaller deviations are found for the 4d and 5d series. The PBE+U results represent an improvement with respect to PBE, although some differences from PBE0 remain. This study underlines the importance of the choice of the DFT functional in screening new catalysts and in predicting catalytic activities. The use of PBE appears acceptable for 4d and 5d metals, while in the case of 3d systems PBE+U or PBE0 approaches are recommended, in particular for magnetic ground states
Microembedding in thyroid cytology.
No full textFine needle aspiration (FNA) of the thyroid, which is a
rapid and cost-effective procedure, has gained wide acceptance
as a valuable method for distinguishing neoplastic
from non-neoplastic nodules and identifying those patients
requiring surgery 1-4. At the present time, the thyroid gland
is the most frequently aspirated organ for triage/diagnostic
purposes.
Clinically-relevant thyroid nodules occur in 5-10% of the general
population in Italy and approximately 5% of the patients
have malignant lesions. The goal of thyroid FNA is to identify
the nodules that require surgery and decrease the number of
thyroidectomy for patients with benign disease. Overall, the
technique has a high sensitivity and specificity for the detection
of thyroid neoplasm 5-7.
Cell blocks (CBs) are often prepared on FNAs from several
organ sites as an adjunct to smears in the diagnosis of potential
lesions. However, the literature contains few reports
on their utility with regard to specific organ sites. The main
advantage of the CB is the potential to produce several sections
for special stains and other ancillary, in particular immunoistochemical,
studies 8-10. At our institution, CBs are
made routinely on thyroid FNAs since twenty years and we
have been performing about eight-hundred thyroid FNAs each
year. As a result, we have been collecting a high number of
CB of thyroid lesions, of great value for investigative and
retrospective studies.
The aspirates were procured by experienced cytopathologists,
clinicians, or radiologists. The majority of the aspirates were
performed by the radiologist or clinicians under ultrasound
guidance and each aspirate had an immediate in situ assessment
for adequacy by the cytopathologist. FNAs were performed
using a 23- or 25-gauge needle attached to a 10-ml
disposable syringe mounted to a metallic Cameco syringe
pistol (Morton Medical Ltd., London, United Kingdom). One
to 4 aspirates were performed per case. After each sampling,
air-dried Hematoxylin & Eosin-stained slides were made for
immediate assessment and alcohol-fixed slides for subsequent
Papanicolaou (Pap) staining. The residual material and the
needles were rinsed immediately in a 50% Ethanol solution
(s.c. Lysis Buffer) containing Ammonia Chloride, Potassium
Bicarbonate and EDTA, which proved effective for fixing
cells and lysing erythrocytes. Material collected by centrifugation
was embedded in paraffin.presence
of cellular fragments.
CB has greatly improved the pre-surgical diagnosis of thyroid
nodules, since small tissue specimens representative of the lesion
were detected in the great majority of cases. These “micro-biopsies”
provided morphologically-relevant data. Besides, sections
were employed for immunoistochemical and molecular tests.
Expression of galectin-3 is routinely tested on thyroid follicular
lesions (cytological cell-blocks) obtained preoperatively
by ultrasound-guided fine-needle aspiration of thyroid
nodules 11.
Moreover, CB procedure provides the basis for new molecular
and immunoistochemical studies to determine the definition
of thyroid neoplasm of indeterminate malignant behavior
(Thy3). Recently, we focused on the immunohistochemistry
of Emerin, a protein of the nuclear membrane (NM)
whose decoration best demarcates the nuclear shape of the
thyrocytes 12 and we performed this stain on a series of 82
cytological CB thyroid specimens. Emerin revealed a uniform
arrangement of the NM in non-neoplastic lesions (thyroiditis,
microfollicular goiter, follicular adenoma) and normal thyroid
as well as in follicular carcinoma. In contrast, irregular folding
of the membrane and presence of curling and invaginations,
eventually leading to the formation of nuclear pseudoinclusions,
was observed in PTC and VFPTC cells.
In conclusions, the integration of CB method with conventional
cytomorphological and clinical diagnostic procedures
represents a sensitive and reliable diagnostic approach for
preoperative identification of thyroid carcinomas. This procedure
improves the diagnostic accuracy of conventional cytology.
Moreover, the CB is useful for immunoistochemical and
molecular study of thyroid neoplas
- …
