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1.
J Phys Chem A ; 2020 Jun 03.
Artigo em Inglês | MEDLINE | ID: mdl-32491866

RESUMO

Combining experimental and ab initio core-level photoelectron spectroscopy (periodic DFT and quantum chemistry calculations), we elucidated how ammonia molecules bond to the hydroxyls of the (H,OH)-Si(001) model surface at a temperature of 130 K. Indeed theory evaluated the magnitude and direction of the N 1s (and O 1s) chemical shifts, according to the nature (acceptor or donor) of the hydrogen bond, and, when confronted to experiment, showed unambiguously that the probe molecule makes one acceptor and one donor bond with a pair of hydroxyls. The consistency of our approach was proved by the fact that the identified adsorption geometries are precisely those that have the largest binding strength to the surface, as calculated by periodic DFT. Real-time core-level photoemission enabled the measurement of the adsorption kinetics of H-bonded ammonia and its maximum coverage (0.37 ML) under 1.5×10 9 mbar. Experimental desorption free energies were compared to the magnitude of the adsorption energies provided by periodic DFT calculations. Minority species were also detected on the surface. As in the case of H-bonded ammonia, DFT core-level calculations were instrumental to attribute these minority species to datively bonded ammonia molecules, associated to isolated dangling bonds remaining on the surface, and to dissociated ammonia molecules, resulting largely from beam damage.

2.
Nanoscale ; 11(42): 20006-20014, 2019 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-31603165

RESUMO

Thin films of an iron(ii) complex with a photochromic diarylethene-based ligand and featuring a spin-crossover behaviour have been grown by sublimation in ultra-high vacuum on highly oriented pyrolytic graphite and spectroscopically characterized through high-resolution X-ray and ultraviolet photoemission, as well as via X-ray absorption. Temperature-dependent studies demonstrated that the thermally induced spin-crossover is preserved at a sub-monolayer (0.7 ML) coverage. Although the photochromic ligand ad hoc integrated into the complex allows the photo-switching of the spin state of the complex at room temperature both in bulk and for a thick film on highly oriented pyrolytic graphite, this photomagnetic effect is not observed in sub-monolayer deposits. Ab initio calculations justify this behaviour as the result of specific adsorbate-substrate interactions leading to the stabilization of the photoinactive form of the diarylethene ligand over photoactive one on the surface.

3.
Nanomaterials (Basel) ; 9(5)2019 May 20.
Artigo em Inglês | MEDLINE | ID: mdl-31137492

RESUMO

Gold nanoparticles (AuNPs), which are strongly hydrophilic and dimensionally suitable for drug delivery, were used in loading and release studies of two different copper(I)-based antitumor complexes, namely [Cu(PTA)4]+ [BF4]- (A; PTA = 1, 3, 5-triaza-7-phosphadamantane) and [HB(pz)3Cu(PCN)] (B; HB(pz)3 = tris(pyrazolyl)borate, PCN = tris(cyanoethyl)phosphane). In the homoleptic, water-soluble compound A, the metal is tetrahedrally arranged in a cationic moiety. Compound B is instead a mixed-ligand (scorpionate/phosphane), neutral complex insoluble in water. In this work, the loading procedures and the loading efficiency of A and B complexes on the AuNPs were investigated, with the aim to improve their bioavailability and to obtain a controlled release. The non-covalent interactions of A and B with the AuNPs surface were studied by means of dynamic light scattering (DLS), UV-Vis, FT-IR and high-resolution x-ray photoelectron spectroscopy (HR-XPS) measurements. As a result, the AuNPs-A system proved to be more stable and efficient than the AuNPs-B system. In fact, for AuNPs-A the drug loading reached 90%, whereas for AuNPs-B it reached 65%. For AuNPs-A conjugated systems, a release study in water solution was performed over 4 days, showing a slow release up to 10%.

4.
Chem Sci ; 10(6): 1857-1865, 2019 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-30842854

RESUMO

The space between a metal surface and a two-dimensional cover can be regarded as a nanoreactor, where confined molecule adsorption and surface reactions may occur. In this work, we report CO intercalation and reactivity between a graphene-hexagonal boron nitride (h-BNG) heterostructure and Pt(111). By employing high resolution X-ray photoemission spectroscopy (XPS) we demonstrate the molecular intercalation of the full h-BNG overlayer and stabilization of a dense R23.4°-13CO layer on Pt(111) under ultra-high vacuum at room temperature. We provide experimental evidence of a weakened CO-metal bond due to the confinement effects of the 2D cover. Temperature-programmed XPS results reveal that CO desorption is kinetically delayed and occurs at a higher temperature than on bare Pt(111). Moreover, CO partially reacts with the h-BNG layer to form boron-oxide species, which affect repeated CO intercalation. Finally, we found that the properties of the system towards interaction with CO can be considerably recovered using high temperature treatment.

5.
Phys Chem Chem Phys ; 20(41): 26161-26172, 2018 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-30311617

RESUMO

The structure and electronic properties of carbon-based nanostructures obtained by metal surface assisted synthesis is highly dependent on the nature of the precursor molecule. Here, we report on a combined scanning tunneling microscopy, soft X-ray spectroscopy and density functional theory investigation on the surface assisted polymerization of Br-corannulene at Ag(110) and on the possibility of building a mesh of π-conjugated polymers starting from buckyball shaped molecules. Indeed, the corannulene units form one-molecule-wide ribbons in which the natural concavity of the precursor molecule is maintained. These C-based nanostructures are corrugated and merge into a covalent network on the surface.

6.
Sci Rep ; 8(1): 8740, 2018 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-29880895

RESUMO

The effects of Cr on local environment and electronic structure of rutile TiO2 are studied combining theoretical and experimental approaches. Neutral and negatively charged substitutional Cr impurities CrTi0 and CrTi1- as well as Cr-oxygen vacancy complex 2CrTi + VO are studied by the density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. Experimental results based on X-Ray absorption spectroscopy (XAS) and X-Ray photoelectron spectroscopy (XPS) performed on Cr doped TiO2 at the Synchrotron facility were compared to the theoretical results. It is shown that the electrons of the oxygen vacancy tend to be localized at the t2g states of the Cr ions in order to reach the stable oxidation state of Cr3+. Effects of Cr on crystal field (CF) and structural distortions in the rutile TiO2 cell were analyzed by the DFT calculations and XAS spectra revealing that the CF and tetragonal distortions in TiO2 are very sensitive to the concentration of Cr.

7.
Chem Sci ; 9(4): 990-998, 2018 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-29629166

RESUMO

The nature of the oxygen species active in ethylene epoxidation is a long-standing question. While the structure of the oxygen species that participates in total oxidation (nucleophilic oxygen) is known the atomic structure of the selective species (electrophilic oxygen) is still debated. Here, we use both in situ and UHV X-ray Photoelectron Spectroscopy (XPS) to study the interaction of oxygen with a silver surface. We show experimental evidence that the unreconstructed adsorbed atomic oxygen (Oads) often argued to be active in epoxidation has a binding energy (BE) ≤ 528 eV, showing a core-level shift to lower BE with respect to the O-reconstructions, as previously predicted by DFT. Thus, contrary to the frequent assignment, adsorbed atomic oxygen cannot account for the electrophilic oxygen species with an O 1s BE of 530-531 eV, thought to be the active species in ethylene epoxidation. Moreover, we show that Oads is present at very low O-coverages during in situ XPS measurements and that it can be obtained at slightly higher coverages in UHV at low temperature. DFT calculations support that only low coverages of Oads are stable. The highly reactive species is titrated by background gases even at low temperature in UHV conditions. Our findings suggest that at least two different species could participate in the partial oxidation of ethylene on silver.

8.
Nanoscale ; 10(5): 2371-2379, 2018 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-29334098

RESUMO

The patterning of silicon surfaces by organic molecules emerges as an original way to fabricate innovative nanoelectronic devices. In this regard, we have studied how a diamine, N,N,N',N'-tetramethylethylenediamine (TMEDA, (CH3)2N-[CH2]2-N(CH3)2), chelates the silicon dimers of the Si(001)-2 × 1 surface. Starting from very low coverage to surface saturation (at 300 K), we used real-time scanning tunneling microscopy (STM) in a scanning-while-dosing approach. The images show that the molecules can adopt two bonding configurations: the cross-trench (CT) configuration by bridging two adjacent dimer rows, and the end-bridge (EB) configuration by chelating two adjacent dimers in the same row. However, while CT dominates over EB at low coverage, the percentage of EB adducts steadily increases, until it becomes largely dominant at high molecular coverage. Above a critical coverage θmol of ∼0.13 monolayer (ML), a sudden change in the molecular imprints is seen, likely due to a change in the tunneling conditions. The EB stapling of two adjacent dimers in a row via a dual-dative bond (as shown by XPS) is achieved efficiently by the TMEDA molecule with a very high chemical selectivity. The EB is a unique configuration in amine adsorption chemistry, as it leads to the formation of a pair of first-neighbor, doubly-occupied dangling bonds. Further reactivity of the EB site with other molecules remains to be explored, and possible reaction schemes are envisaged.

9.
J Colloid Interface Sci ; 513: 10-19, 2018 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-29128618

RESUMO

Gold nanoparticles with an average diameter of 10 nm, functionalized by the dye molecule rhodamine B isothiocyanate, have been synthesized. The resulting material has been extensively characterized both chemically, to investigate the bonding between the dye molecules and the nanoparticles, and physically, to understand the details of the aggregation induced by interaction between dye molecules on different nanoparticles. The plasmonic response of the system has been further characterized by measurement and theoretical simulation of the static UV-Vis extinction spectra of the aggregates produced following different synthesis procedures. The model parameters used in the simulation gave further useful information on the aggregation and its relationship to the plasmonic response. Finally, we investigated the time dependence of the plasmonic effects of the nanoparticles and fluorescence of the dye molecule using an ultrafast pump-probe optical method. By modulating the quantity of dye molecules on the surface of the nanoparticles it was possible to exert fine control over the plasmonic response of nanoparticles.

10.
Phys Chem Chem Phys ; 19(47): 32079-32085, 2017 Dec 06.
Artigo em Inglês | MEDLINE | ID: mdl-29182175

RESUMO

Well-defined sized (5-10 nm) metallic iron nanoparticles (NPs) with body-centered cubic structure encapsulated inside the tip of millimeter-long vertically aligned carbon nanotubes (VACNTs) of uniform length have been investigated with high-resolution transmission electron microscopy and soft X-ray spectroscopy techniques. Surface-sensitive and chemically-selective measurements have been used to evaluate the magnetic properties of the encapsulated NPs. The encapsulated Fe NPs display magnetic remanence up to room temperature, low coercivity, high chemical stability and no significant anisotropy. Our surface-sensitive measurements combined with the specific morphology of the studied VACNTs allow us to pinpoint the contribution of the surface oxidized or hydroxidized iron catalysts present at the VACNT-substrate interface.

11.
Phys Chem Chem Phys ; 19(39): 26672-26678, 2017 Oct 11.
Artigo em Inglês | MEDLINE | ID: mdl-28967026

RESUMO

We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concentration) for 0 ≤ x ≤ 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-δ states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.

12.
J Cataract Refract Surg ; 43(8): 1107-1114, 2017 08.
Artigo em Inglês | MEDLINE | ID: mdl-28917414

RESUMO

PURPOSE: To evaluate the surface chemical composition and roughness of different phaco tips before and after their use during cataract surgery. SETTING: Eye Clinic, University of Trieste, Trieste, Italy. DESIGN: Experimental study. METHODS: Of the 66 tips studied, 33 (15 new, 15 after single use, and 3 after multiple uses) were studied with X-ray photoemission spectroscopy and 33 (15 new, 15 after single use, and 3 after multiple uses) were examined using scanning electron microscopy with energy dispersive X-ray spectroscopy analysis and contact profilometry. All analyses were performed on the far end of the tip. RESULTS: Used phaco tips showed signs of wear at the end of the tip, with the deposition of debris. The cutting edge appeared to be rounded and irregular. After surgery, an increase in the surface roughness was detected. The chemical analyses showed modification of the superficial alloy composition and the biological origin of some debris deposited after surgery. The deterioration and wear observed were more remarkable after multiple surgical procedures. CONCLUSIONS: Used phaco tips showed relevant signs of deterioration and deposition of biological material, mostly involving the cutting edge. Reusable tips might release remnants of previous procedures. The adoption of single-use disposable phaco tips seems to be highly advisable.


Assuntos
Cristalino , Facoemulsificação , Catarata , Humanos , Teste de Materiais , Microscopia Eletrônica de Varredura , Facoemulsificação/métodos
13.
J Am Chem Soc ; 139(26): 8960-8970, 2017 07 05.
Artigo em Inglês | MEDLINE | ID: mdl-28598604

RESUMO

Photoelectrochemical water splitting is a promising approach for renewable production of hydrogen from solar energy and requires interfacing advanced water-splitting catalysts with semiconductors. Understanding the mechanism of function of such electrocatalysts at the atomic scale and under realistic working conditions is a challenging, yet important, task for advancing efficient and stable function. This is particularly true for the case of oxygen evolution catalysts and, here, we study a highly active Co3O4/Co(OH)2 biphasic electrocatalyst on Si by means of operando ambient-pressure X-ray photoelectron spectroscopy performed at the solid/liquid electrified interface. Spectral simulation and multiplet fitting reveal that the catalyst undergoes chemical-structural transformations as a function of the applied anodic potential, with complete conversion of the Co(OH)2 and partial conversion of the spinel Co3O4 phases to CoO(OH) under precatalytic electrochemical conditions. Furthermore, we observe new spectral features in both Co 2p and O 1s core-level regions to emerge under oxygen evolution reaction conditions on CoO(OH). The operando photoelectron spectra support assignment of these newly observed features to highly active Co4+ centers under catalytic conditions. Comparison of these results to those from a pure phase spinel Co3O4 catalyst supports this interpretation and reveals that the presence of Co(OH)2 enhances catalytic activity by promoting transformations to CoO(OH). The direct investigation of electrified interfaces presented in this work can be extended to different materials under realistic catalytic conditions, thereby providing a powerful tool for mechanism discovery and an enabling capability for catalyst design.

14.
Nanoscale ; 9(18): 6056-6067, 2017 May 11.
Artigo em Inglês | MEDLINE | ID: mdl-28443889

RESUMO

Achieving control of the surface chemistry of colloidal quantum dots (CQDs) is essential to fully exploit their properties in solar cells, but direct measurement of the chemistry and electronic structure in the outermost atomic layers is challenging. Here we probe the surface oxidation and passivation of cation-exchanged PbS/CdS core/shell CQDs with sub nm-scale precision using synchrotron-radiation-excited depth-profiling photoemission. We investigate the surface composition of the topmost 1-2.5 nm of the CQDs as a function of depth, for CQDs of varying CdS shell thickness, and examine how the surface changes after prolonged air exposure. We demonstrate that the Cd is localized at the surface of the CQDs. The surface-localized products of oxidation are identified, and the extent of oxidation quantified. We show that oxidised sulfur species are progressively eliminated as Cd replaces Pb at the surface. A sub-monolayer surface 'decoration' of Cd is found to be effective in passivating the CQDs. We show that the measured energy-level alignments at PbS/CdS colloidal quantum dot surfaces differ from those expected on the basis of bulk band offsets, and are strongly affected by the oxidation products. We develop a model for the passivating action of Cd. The optimum shell thickness (of around 0.1 nm, previously found to give maximised power conversion efficiency in PbS/CdS solar cells) is found to correspond to a trade-off between the rate of oxidation and the introduction of a surface barrier to charge transport.

15.
Phys Chem Chem Phys ; 18(48): 33233-33239, 2016 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-27892576

RESUMO

In this paper, we study the magnetic and chemical properties of Fe/graphene vertically stacked ultrathin films by means of X-ray magnetic circular dichroism and X-ray photoelectron spectroscopy. We compare two systems: an iron layer deposited directly on top of the Pt(111) surface, and an intercalated Fe film sandwiched between graphene and Pt(111). The system composed of a submonolayer Fe deposited directly on Pt(111) maintains an out-of-plane easy magnetization axis, even if it has been covered by graphene that quenches effectively the magnetic orbital moment of Fe. However, when the Fe coverage is increased above 1 ML the easy magnetization axis flips in the in-plane direction.

16.
Nanoscale ; 8(41): 17843-17853, 2016 Oct 20.
Artigo em Inglês | MEDLINE | ID: mdl-27714142

RESUMO

By a combination of scanning tunneling microscopy, X-ray spectroscopic techniques and density functional theory calculations, we prove the formation of extended patterns of parallel, graphene nanoribbons with alternate zig-zag and armchair edges and selected width by surface-assisted Ullmann coupling polymerization and dehydrogenation of 1,6-dibromopyrene (C16H8Br2). Besides the relevance of these nanostructures for their possible application in nanodevices, we demonstrate the peculiarity of halogenated pyrene derivatives for the formation of nanoribbons, in particular on Ag(110). These results open the possibility of tuning the shape and dimension of nanoribbons (and hence the correlated electronic properties) by choosing suitably tailored or on-purpose designed molecular precursors.

17.
Nanoscale ; 8(5): 2832-43, 2016 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-26763792

RESUMO

The properties of ß-NaEuF4/NaGdF4 core-shell nanocrystals have been thoroughly investigated. Nanoparticles with narrow size distribution and an overall diameter of ∼22 nm have been produced with either small ß-NaEuF4 cores (∼3 nm diameter) or large ß-NaEuF4 cores (∼18 nm diameter). The structural properties and core-shell formation are investigated by X-ray diffraction, transmission electron microscopy and electron paramagnetic resonance, respectively. Optical luminescence measurements and X-ray photoelectron spectroscopy are employed to gain information about the optical emission bands and valence states of the rare earth constituents. Magnetic characterization is performed by SQUID and X-ray magnetic circular dichroism measurements at the rare earth M(4,5) edges. The characterization of the core-shell nanoparticles by means of these complementary techniques demonstrates that partial intermixing of core and shell materials takes place, and a significant fraction of europium is present in the divalent state which has significant influence on the magnetic properties. Hence, we obtained a combination of red emitting Eu(3+) ions and paramagnetic Gd(3+) ions, which may be highly valuable for potential future applications.

18.
Chemistry ; 21(15): 5826-35, 2015 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-25711882

RESUMO

Dibromotetracene molecules are deposited on the Cu(110) surface at room temperature. The complex evolution of this system has been monitored at different temperatures (i.e., 298, 523, 673, and 723 K) by means of a variety of complementary techniques that range from STM and temperature-programmed desorption (TPD) to high-resolution X-ray spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). State-of-the-art density-functional calculations were used to determine the chemical processes that take place on the surface. After deposition at room temperature, the organic molecules are transformed into organometallic monomers through debromination and carbon-radical binding to copper adatoms. Organometallic dimers, trimers, or small oligomers, which present copper-bridged molecules, are formed by increasing the temperature. Surprisingly, further heating to 673 K causes the formation of elongated chains along the Cu(110) close-packed rows as a consequence of radical-site migration to the thermodynamically more stable molecule heads. Finally, massive dehydrogenation occurs at the highest temperature followed by ring condensation to nanographenic patches. This study is a paradigmatic example of how intermolecular coupling can be modulated by the stepwise control of a simple parameter, such as temperature, through a sequence of domino reactions.

19.
Langmuir ; 31(4): 1445-53, 2015 Feb 03.
Artigo em Inglês | MEDLINE | ID: mdl-25557338

RESUMO

Nanocrystalline thin films of PbS are obtained in a straightforward reaction by precipitation at the interface between toluene (containing a Pb precursor) and water (containing Na2S). Lead thiobiuret [Pb(SON(CN(i)Pr2)2)2] and lead diethyldithiocarbamate [Pb(S2CNEt2)2] precursors are used. The films are characterized by X-ray diffraction and electron microscopy, revealing typical particle sizes of 10-40 nm and preferred (200) orientation. Synchrotron-excited depth-profiling X-ray photoelectron spectroscopy (XPS) is used to determine the depth-dependent chemical composition as a function of surface aging in air for periods of up to 9 months. The as-synthesized films show a 1:1 Pb/S composition. Initial degradation occurs to form lead hydroxide and small quantities of surface-adsorbed -SH species. A lead-deficient Pb1-xS phase is produced as the aging proceeds. Oxidation of the sulfur occurs later to form sulfite and sulfate products that are highly localized at the surface layers of the nanocrystals. These species show logarithmic growth kinetics, demonstrating that the sulfite/sulfate layer acts to passivate the nanocrystals. Our results demonstrate that the initial reaction of the PbS nanocrystals (forming lead hydroxide) is incongruent. The results are discussed in the context of the use of PbS nanocrystals as light-harvesting elements in next-generation solar technology.

20.
Small ; 11(13): 1548-54, 2015 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-25348200

RESUMO

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion. However, trap states associated with the CQD surface can produce non-radiative charge recombination that significantly reduces device performance. Here a facile post-synthetic treatment of CdTe CQDs is demonstrated that uses chloride ions to achieve near-complete suppression of surface trapping, resulting in an increase of photoluminescence (PL) quantum yield (QY) from ca. 5% to up to 97.2 ± 2.5%. The effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. This process also dramatically improves the air-stability of the CQDs: before treatment the PL is largely quenched after 1 hour of air-exposure, whilst the treated samples showed a PL QY of nearly 50% after more than 12 hours.

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