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1.
ACS Appl Mater Interfaces ; 14(27): 31044-31053, 2022 Jul 13.
Artigo em Inglês | MEDLINE | ID: mdl-35776551

RESUMO

We present the prototype of a ferroelectric tunnel junction (FTJ), which is based on a self-assembled monolayer (SAM) of small, functional molecules. These molecules have a structure similar to those of liquid crystals, and they are embedded between two solid-state electrodes. The SAM, which is deposited through a short sequence of simple fabrication steps, is extremely thin (3.4 ± 0.5 nm) and highly uniform. The functionality of the FTJ is ingrained in the chemical structure of the SAM components: a conformationally flexible dipole that can be reversibly reoriented in an electrical field. Thus, the SAM acts as an electrically switchable tunnel barrier. Fabricated stacks of Al/Al2O3/SAM/Pb/Ag with such a polar SAM show pronounced hysteretic, reversible conductance switching at voltages in the range of ±2-3 V, with a conductance ratio of the low and the high resistive states of up to 100. The switching mechanism is analyzed using a combination of quantum chemical, molecular dynamics, and tunneling resistance calculation methods. In contrast to more common, inorganic material-based FTJs, our approach using SAMs of small organic molecules allows for a high degree of functional complexity and diversity to be integrated by synthetic standard methods, while keeping the actual device fabrication process robust and simple. We expect that this technology can be further developed toward a level that would then allow its application in the field of information storage and processing, in particular for in-memory and neuromorphic computing architectures.

2.
ACS Appl Mater Interfaces ; 14(14): 16830-16838, 2022 Apr 13.
Artigo em Inglês | MEDLINE | ID: mdl-35352935

RESUMO

Crystalline organic semiconducting thin films from the benchmark molecule C8-BTBT-C8 were obtained using physical vapor deposition and various solution-based methods. Utilizing atomic force microscopy and X-ray spectromicroscopy, we illustrate the influence of the underlying growth mechanism and determine the highly preparation-dependent orientation of the thiophene backbone. We observe a continuous trend for crystalline C8-BTBT-C8 thin film domains to extend into the square millimeter-range under near-equilibrium growth conditions. For such well-defined systems, electron diffraction tomography allows us to precisely determine the unit cell directly after film deposition and to reveal an 8° molecular tilt angle with respect to the surface normal. This finding is in almost perfect accordance with the values derived from near-edge X-ray absorption fine structure linear dichroism. Within this work, we shine a light on both the successes and challenges connected to the realization of potent, thiophene-based semiconducting films, paving the way toward square centimeter-sized ultrathin organic crystals and their application in organic circuitry.

4.
Adv Mater ; 33(35): e2103287, 2021 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-34291511

RESUMO

Liquid-phase, quasi-epitaxial growth is used to stack asymmetric, dipolar organic compounds on inorganic substrates, permitting porous, crystalline molecular materials that lack inversion symmetry. This allows material fabrication with built-in electric fields. A new programmed assembly strategy based on metal-organic frameworks (MOFs) is described that facilitates crystalline, noncentrosymmetric space groups for achiral compounds. Electric fields are integrated into crystalline, porous thin films with an orientation normal to the substrate. Changes in electrostatic potential are detected via core-level shifts of marker atoms on the MOF thin films and agree with theoretical results. The integration of built-in electric fields into organic, crystalline, and porous materials creates possibilities for band structure engineering to control the alignment of electronic levels in organic molecules. Built-in electric fields may also be used to tune the transfer of charges from donors loaded via programmed assembly into MOF pores. Applications include organic electronics, photonics, and nonlinear optics, since the absence of inversion symmetry results in a clear second-harmonic generation signal.

5.
Phys Chem Chem Phys ; 23(13): 7961-7973, 2021 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-33459737

RESUMO

The diffusion of hydrogen adsorbed inside layered MoS2 crystals has been studied by means of quasi-elastic neutron scattering, neutron spin-echo spectroscopy, nuclear reaction analysis, and X-ray photoelectron spectroscopy. The neutron time-of-flight and neutron spin-echo measurements demonstrate fast diffusion of hydrogen molecules parallel to the basal planes of the two dimensional crystal planes. At room temperature and above, this intra-layer diffusion is of a similar speed to the surface diffusion that has been observed in earlier studies for hydrogen atoms on Pt surfaces. A significantly slower hydrogen diffusion was observed perpendicular to the basal planes using nuclear reaction analysis.

6.
Phys Chem Chem Phys ; 23(13): 7696-7702, 2021 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-32643740

RESUMO

Detailed information on structural, chemical, and physical properties of natural cleaved (10.4) calcite surfaces was obtained by a combined atomic force microscopy (AFM) and infrared (IR) study using CO as a probe molecule under ultrahigh vacuum (UHV) conditions. The structural quality of the surfaces was determined using non-contact AFM (NC-AFM), which also allowed assigning the adsorption site of CO molecules. Vibrational frequencies of adsorbed CO species were determined by polarization-resolved infrared reflection absorption spectroscopy (IRRAS). At low exposures, adsorption of CO on the freshly cleaved (10.4) calcite surface at a temperature of 62 K led to the occurrence of a single C-O vibrational band located at 2175.8 cm-1, blue-shifted with respect to the gas phase value. For larger exposures, a slight, coverage-induced redshift was observed, leading to a frequency of 2173.4 cm-1 for a full monolayer. The width of the vibrational bands is extremely small, providing strong evidence that the cleaved calcite surface is well-defined with only one CO adsorption site. A quantitative analysis of the IRRA spectra recorded at different surface temperatures revealed a CO binding energy of -0.31 eV. NC-AFM data acquired at 5 K for sub-monolayer CO coverage reveal single molecules imaged as depressions at the position of the protruding surface features, in agreement with the IRRAS results. Since there are no previous experimental data of this type, the interpretation of the results was aided by employing density functional theory calculations to determine adsorption geometries, binding energies, and vibrational frequencies of carbon monoxide on the (10.4) calcite surface. It was found that the preferred geometry of CO on this surface is adsorption on top of calcium in a slightly tilted orientation. With increased coverage, the binding energy shows a small decrease, revealing the presence of repulsive adsorbate-adsorbate interactions.

7.
Chemphyschem ; 21(23): 2553-2564, 2020 Dec 02.
Artigo em Inglês | MEDLINE | ID: mdl-33118300

RESUMO

The growth of ZnO clusters supported by ZnO-bilayers on Ag(111) and the interaction of these oxide nanostructures with water have been studied by a multi-technique approach combining temperature-dependent infrared reflection absorption spectroscopy (IRRAS), grazing-emission X-ray photoelectron spectroscopy, and density functional theory calculations. Our results reveal that the ZnO bilayers exhibiting graphite-like structure are chemically inactive for water dissociation, whereas small ZnO clusters formed on top of these well-defined, yet chemically passive supports show extremely high reactivity - water is dissociated without an apparent activation barrier. Systematic isotopic substitution experiments using H2 16 O/D2 16 O/D2 18 O allow identification of various types of acidic hydroxyl groups. We demonstrate that a reliable characterization of these OH-species is possible via co-adsorption of CO, which leads to a red shift of the OD frequency due to the weak interaction via hydrogen bonding. The theoretical results provide atomic-level insight into the surface structure and chemical activity of the supported ZnO clusters and allow identification of the presence of under-coordinated Zn and O atoms at the edges and corners of the ZnO clusters as the active sites for H2 O dissociation.

8.
J Phys Chem Lett ; 11(18): 7925-7931, 2020 Sep 17.
Artigo em Inglês | MEDLINE | ID: mdl-32870002

RESUMO

Polar surfaces of solid oxides are intrinsically unstable and tend to reconstruct due to the diverging electrostatic energy and thus often exhibit unique physical and chemical properties. However, a quantitative description of the restructuring mechanism of these polar surfaces remains challenging. Here we provide an atomic-level picture of the refaceting process that governs the surface polarity compensation of cubic ceria nanoparticles based on the accurate reference data acquired from the well-defined model systems. The combined results from advanced infrared spectroscopy, atomic-resolved transmission electron microscopy, and density functional theory calculations identify a two-step scenario where an initial O-terminated (2 × 2) reconstruction is followed by a severe refaceting via massive mass transport at elevated temperatures to yield {111}-dominated nanopyramids. This significant surface restructuring promotes the redox properties of ceria nanocubes, which account for the enhanced catalytic activity for CO oxidation.

9.
Phys Chem Chem Phys ; 22(15): 8088-8095, 2020 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-32242590

RESUMO

Biorepulsivity of oligo(ethylene glycol) (OEG) substituted self-assembled monolayers (SAMs), serving as model systems for analogous polymeric surfaces, is generally ascribed to the hydration effect. In this context, we applied temperature-programmed desorption to study interaction of water (D2O) with a series of OH-terminated, OEG-substituted alkanethiolate SAMs with variable length of the OEG strand, defining their biorepulsion behavior. Along with the ice overlayer (wetting phase), growing also on the surface of the analogous non-substituted films, a hydration phase, corresponding to the adsorption of D2O into the OEG matrix, was observed, with a higher desorption energy (12.4 kcal mol-1vs. 10.4 kcal mol-1) and a weight correlating with the length of the OEG strand and, consequently, with biorepulsivity. The formation of hydration phase was found to occur over an activation barrier, presumably by temperature-promoted diffusion from the wetting phase, with this process being additionally enforced by a pre-desorption annealing.

10.
Angew Chem Int Ed Engl ; 59(26): 10514-10518, 2020 Jun 22.
Artigo em Inglês | MEDLINE | ID: mdl-32196128

RESUMO

In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal-organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms. Using defect-free HKUST-1 MOF thin films, we demonstrate that Cu+ /Cu2+ dimer defects, created in a controlled fashion by reducing the pristine Cu2+ /Cu2+ pairs of the intact framework, account for the high catalytic activity in low-temperature CO oxidation. Combining advanced IR spectroscopy and density functional theory we propose a new reaction mechanism where the key intermediate is an uncharged O2 species, weakly bound to Cu+ /Cu2+ . Our results reveal a complex interplay between electronic and steric effects at defect sites in MOFs and provide important guidelines for tailoring and exploiting the catalytic activity of single metal atom sites.

11.
Phys Rev Lett ; 125(25): 256101, 2020 Dec 18.
Artigo em Inglês | MEDLINE | ID: mdl-33416353

RESUMO

In ceria-based catalysis, the shape of the catalyst particle, which determines the exposed crystal facets, profoundly affects its reactivity. The vibrational frequency of adsorbed carbon monoxide (CO) can be used as a sensitive probe to identify the exposed surface facets, provided reference data on well-defined single crystal surfaces together with a definitive theoretical assignment exist. We investigate the adsorption of CO on the CeO_{2}(110) and (111) surfaces and show that the commonly applied DFT(PBE)+U method does not provide reliable CO vibrational frequencies by comparing with state-of-the-art infrared spectroscopy experiments for monocrystalline CeO_{2} surfaces. Good agreement requires the hybrid DFT approach with the HSE06 functional. The failure of conventional density-functional theory (DFT) is explained in terms of its inability to accurately describe the facet- and configuration-specific donation and backdonation effects that control the changes in the C─O bond length upon CO adsorption and the CO force constant. Our findings thus provide a theoretical basis for the detailed interpretation of experiments and open up the path to characterize more complex scenarios, including oxygen vacancies and metal adatoms.

12.
Angew Chem Int Ed Engl ; 58(49): 17751-17757, 2019 Dec 02.
Artigo em Inglês | MEDLINE | ID: mdl-31637780

RESUMO

The surface chemistry of water on zinc oxides is an important topic in catalysis and photocatalysis. Interaction of D2 O with anisotropic ZnO(10 1 ‾ 0) surfaces was studied by IR reflection absorption spectroscopy using s- and p-polarized light incident along different directions. Interpretation of the experimental data is aided using isotopologues and DFT calculations. The presence of numerous species is revealed: intact monomers, a mixed 2D D2 O/OD adlayer, an anisotropic bilayer, and H-bonded 3D structures. The isolated water monomers are identified unambiguously at low temperatures. The thermally induced diffusion of water monomers occurs at elevated temperatures, forming dimers that undergo autocatalytic dissociation via proton transfer. Polarization- and azimuth-resolved IR data provide information on the orientation and strength of H-bonds within the 2D and 3D structures. Ab initio molecular dynamics simulations reveal strong anharmonic couplings within the H-bond network.

13.
Langmuir ; 35(49): 16217-16225, 2019 Dec 10.
Artigo em Inglês | MEDLINE | ID: mdl-31664843

RESUMO

Strontium, calcium, and magnesium silicate hydrate phases are synthesized by the reaction between silica and solution of metal hydroxides. The kinetics of the reaction is recorded using a quartz crystal microbalance (QCM), continuously monitoring the change in frequency and dissipation energy. Based on QCM results, it is shown that properties of solutions like the pH-value or the type of ions play a pivotal function on the rate-determining stage of the reaction, the thickness of the diffuse layer, the formation of carbonates, as well as the kinetics of the formed phases. Further properties of the reaction products are investigated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and infrared spectroscopy (IR). With the help of thermogravimetric analysis (TGA) and temperature-dependent X-ray diffraction (XRD), we investigate how our synthesized phases can be turned into MSiO3 structures. Finally, the Goldschmidt rules for perovskites structures show that this might be an attractive way for new and nontoxic phases in the future.

14.
Front Chem ; 7: 451, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31294016

RESUMO

The precise determination of the surface structure of iron oxides (hematite and magnetite) is a vital prerequisite to understand their unique chemical and physical properties under different conditions. Here, the atomic structure evolution of the hematite (0001) surface under reducing conditions was tracked by polarization-resolved infrared reflection absorption spectroscopy (IRRAS) using carbon monoxide (CO) as a probe molecule. The frequency and intensity of the CO stretch vibration is extremely sensitive to the valence state and electronic environments of surface iron cations. Our comprehensive IRRAS results provide direct evidence that the monocrystalline, stoichiometric α-Fe2O3(0001) surface is single Fe-terminated. The initial reduction induced by annealing at elevated temperatures produces surface oxygen vacancies, where the excess electrons are localized at adjacent subsurface iron ions (5-fold coordinated). A massive surface restructuring occurs upon further reduction by exposing to atomic hydrogen followed by Ar+-sputtering and annealing under oxygen poor conditions. The restructured surface is identified as a Fe3O4(111)/Fe1-xO(111)-biphase exposing both, Fe3+ and Fe2+ surface species. Here the well-defined surface domains of Fe3O4(111) exhibit a Feoct2-termination, while the reduced Fe1-xO(111) is Fe2+(oct)-terminated. These findings are supported by reference IRRAS data acquired for CO adsorption on magnetite (111) and (001) monocrystalline surfaces.

15.
ACS Nano ; 12(8): 7571-7582, 2018 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-30004663

RESUMO

We investigate the electronic and vibrational properties of bottom-up synthesized aligned armchair graphene nanoribbons of N = 7 carbon atoms width periodically doped by substitutional boron atoms (B-7AGNRs). Using angle-resolved photoemission spectroscopy and density functional theory calculations, we find that the dopant-derived valence and conduction band states are notably hybridized with electronic states of Au substrate and spread in energy. The interaction with the substrate leaves the bands with pure carbon character rather unperturbed. This results in an identical effective mass of ≈0.2 m0 for the next-highest valence band compared with pristine 7AGNRs. We probe the phonons of B-7AGNRs by ultrahigh-vacuum (UHV) Raman spectroscopy and reveal the existence of characteristic splitting and red shifts in Raman modes due to the presence of substitutional boron atoms. Comparing the Raman spectra for three visible lasers (red, green, and blue), we find that interaction with gold suppresses the Raman signal from B-7AGNRs and the energy of the green laser (2.33 eV) is closer to the resonant E22 transition. The hybridized electronic structure of the B-7AGNR-Au interface is expected to improve electrical characteristics of contacts between graphene nanoribbon and Au. The Raman fingerprint allows the easy identification of B-7AGNRs, which is particularly useful for device fabrication.

16.
Dalton Trans ; 47(9): 2986-2991, 2018 Feb 27.
Artigo em Inglês | MEDLINE | ID: mdl-29215105

RESUMO

We report on the synthesis and spectroscopic characterization of alkali metal intercalated ZrSe2 single crystals. ZrSe2 is produced by chemical vapour transport and then Li is intercalated. Intercalation is performed from the liquid phase (via butyllithium) and from the vapour phase. Raman spectroscopy of intercalated ZrSe2 reveals phonon energy shifts of the Raman active A1g and Eg phonon modes, the disappearance of two-phonon modes and new low wavenumber Raman modes. Angle-resolved photoemission spectroscopy is used to perform a mapping of the Fermi surface revealing an electron concentration of 4.7 × 1014 cm-2. We also perform vapour phase intercalation of K and Cs into ZrSe2 and observe similar changes in the Raman modes as for the Li case.

17.
Angew Chem Int Ed Engl ; 56(51): 16399-16404, 2017 12 18.
Artigo em Inglês | MEDLINE | ID: mdl-29024254

RESUMO

An atomic-level understanding of dioxygen activation on metal oxides remains one of the major challenges in heterogeneous catalysis. By performing a thorough surface-science study of all three low-index single-crystal surfaces of ceria, probably the most important redox catalysts, we provide a direct spectroscopic characterization of reactive dioxygen species at defect sites on the reduced ceria (110) and (100) surfaces. Surprisingly, neither of these superoxo and peroxo species was found on ceria (111), the thermodynamically most stable surface of this oxide. Applying density functional theory, we could relate these apparently inconsistent findings to a sub-surface diffusion of O vacancies on (111) substrates, but not on the less-closely packed surfaces. These observations resolve a long standing debate concerning the location of O vacancies on ceria surfaces and the activation of O2 on ceria powders.

18.
Angew Chem Int Ed Engl ; 56(45): 14301-14305, 2017 11 06.
Artigo em Inglês | MEDLINE | ID: mdl-28859239

RESUMO

The photoreactivity of ceria, a photochemically inert oxide with a large band gap, can be increased to competitive values by introducing defects. This previously unexplained phenomenon has been investigated by monitoring the UV-induced decomposition of N2 O on well-defined single crystals of ceria by using infrared reflection-absorption spectroscopy (IRRAS). The IRRAS data, in conjunction with theory, provide direct evidence that reducing the ceria(110) surface yields high photoreactivity. No such effects are seen on the (111) surface. The low-temperature photodecomposition of N2 O occurs at surface O vacancies on the (110) surface, where the electron-rich cerium cations with a significantly lowered coordination number cause a local lowering of the huge band gap (ca. 6 eV). The quantum efficiency of strongly reduced ceria(110) surfaces in the photodecomposition of N2 O amounts to 0.03 %, and is thus comparable to that reported for the photooxidation of CO on rutile TiO2 (110).

19.
Chemphyschem ; 18(14): 1874-1880, 2017 Jul 19.
Artigo em Inglês | MEDLINE | ID: mdl-28472552

RESUMO

The adsorption of CO2 on the surface of a CeO2 (110) bulk single crystal was studied by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The high-quality XPS and C K-edge NEXAFS data show that CO2 adsorbs as a carbonate species on both fully oxidized CeO2 (110) and partially reduced CeO2-x (110). No evidence for the formation of a carboxylate (CO2δ- ) intermediate could be found. On the fully oxidized CeO2 (110) substrate, the carbonate decomposes upon heating to above 400 K, leading to the desorption of CO2 . The NEXAFS data reveal the presence of a minor amount of formate (or carboxylate) and bicarbonate species, which are related to reactions of CO2 with surface hydroxyl groups. In the case of reduced CeO2-x (110), the carbonate species completely disappear upon heating to temperatures above 500 K. In contrast to conclusions presented in earlier works, the oxidation state of the surface is unchanged, that is, CO2 does not re-oxidize the reduced CeO2-x (110) surface.

20.
Angew Chem Int Ed Engl ; 56(1): 375-379, 2017 01 02.
Artigo em Inglês | MEDLINE | ID: mdl-27925439

RESUMO

The surface atomic arrangement of metal oxides determines their physical and chemical properties, and the ability to control and optimize structural parameters is of crucial importance for many applications, in particular in heterogeneous catalysis and photocatalysis. Whereas the structures of macroscopic single crystals can be determined with established methods, for nanoparticles (NPs), this is a challenging task. Herein, we describe the use of CO as a probe molecule to determine the structure of the surfaces exposed by rod-shaped ceria NPs. After calibrating the CO stretching frequencies using results obtained for different ceria single-crystal surfaces, we found that the rod-shaped NPs actually restructure and expose {111} nanofacets. This finding has important consequences for understanding the controversial surface chemistry of these catalytically highly active ceria NPs and paves the way for the predictive, rational design of catalytic materials at the nanoscale.

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