Site-selective reactivity of ethylene, cyclooctyne and tetrahydrofuran on Si(001) surfaces

Im Rahmen dieser Arbeit wurde die selektive Reaktivität dreier prototypischer organischer Adsorbate (Ethen, Cyclooctin und Tetrahydrofuran) mit der Si(001)-Oberfläche mittels Rastertunnelmikroskopie untersucht. Das Rastertunnelmikroskop ermöglicht hierbei die direkte Beobachtung der Oberfläche im Realraum mit atomarer Auflösung. Auf diese Weise ist es möglich, die auftretenden Adsorptionsgeometrien relativ zur Struktur der Oberfläche zu identifizieren. Ein einzelnes Adsorbat kann dabei durchaus mehrere Adsorptionsgeometrien mit teilweise stark verschiedenen Häufigkeiten aufweisen. Durch sorgfältige, bedeckungsabhängige Experimente können diese relativen Reaktivitätsunterschiede untersucht werden. Durch Variation der Probentemperatur während der Dosierung der Adsorbate sind zudem weitere Rückschlüsse auf die zugrunde liegenden Reaktionsmechanismen der Adsorption möglich, beispielsweise die Existenz von Zwischenzuständen. Darüber hinaus wurden in dieser Arbeit gezielt lokal gestörte Adsorptionsplätze durch verschiedene Wasserstoffvorbedeckungen hergestellt und deren platzspezifische Reaktivität für die Adsorption der verschiedenen Moleküle untersucht, was ebenfalls Rückschlüsse auf den jeweiligen Reaktionsmechanismus zulässt. Ethen ist das kleinste einfach-ungesättigte organische Molekül, dessen nicht-dissoziative Adsorption an einem Dimer der Si(001)-Oberfläche bereits seit vielen Jahren intensiv untersucht wurde. Überraschenderweise konnte in dieser Arbeit eine zweite, bisher nicht identifizierte Adsorptionsgeometrie nachgewiesen werden, die über zwei Dimere erfolgt. Obwohl die neue Adsorptionsgeometrie mit einer geringeren platzspezifischen Reaktivität verbunden ist, liegen bei höheren Bedeckungen annähernd 20% der Moleküle in dieser Adsorptionsgeometrie vor. Als wichtigstes Ergebnis der Untersuchungen zu Ethen auf Si(001) ist allerdings die – im Vergleich zur Adsorption auf der sauberen Oberfläche – deutlich erhöhte platzspezifische Reaktivität von Ethen an lokal gestörten Konfigurationen des voradsorbierten Wasserstoffs hervorzuheben. Durch diese Experimente konnte somit erfolgreich gezeigt werden, dass es auch im Fall einer nicht-dissoziativen Adsorption eines organischen Moleküls auf Si(001) prinzipiell möglich ist, das Adsorptionsverhalten durch lokale Störungen stark zu beeinflussen beziehungsweise zu steuern. Die beobachteten Effekte können durch die Adsorption des Ethens über einen mobilen Precursorzustand erklärt werden, was durch Monte-Carlo-Simulationen bestätigt wurde. Innerhalb eines Precursor-Modells konnten weitergehende Simulationen auch die experimentell beobachteten Adsorbatverteilungen des Ethens auf der sauberen Si(001)-Oberfläche von kleinen bis maximalen Bedeckungen von einer Monolage zufriedenstellend beschreiben. Insgesamt konnte somit in dieser Arbeit ein schlüssiges neues Gesamtbild der Adsorption von Ethen auf der sauberen Si(001)-Oberfläche entwickelt werden. Cycloalkine sind cyclische Kohlenwasserstoffe, die sich durch eine Dreifachbindung und eine damit zusammenhängende starke Verspannung des Molekülrings auszeichnen. Der Einfluss dieser Kombination aus Dreifachbindung und zusätzlicher Ringspannung auf den Adsorptionsmechanismus wurde in dieser Arbeit am Beispiel von Cyclooctin untersucht. Als wichtigstes Ergebnis kann zunächst festgehalten werden, dass sich bei der Adsorption von Cyclooctin auf der Si(001)-Oberfläche in Raumtemperatur- und Tieftemperaturexperimenten im Wesentlichen das gleiche Adsorptionsverhalten beobachten lässt, im Gegensatz zu vielen anderen untersuchten Molekülen. Diese Beobachtung deutet auf einen direkten (barrierelosen) Adsorptionspfad ohne Precursorzustand hin. Ferner konnte gezeigt werden, dass Cyclooctin von kleinen bis zu fast vollständigen und wohlgeordneten Bedeckungen mit hoher Reaktivität auf der Si(001)-Oberfläche adsorbiert. Cyclooctin zeigt dabei primär zwei unterschiedliche Adsorptionsgeometrien, die symmetrisch zu einem Dimer beziehungsweise zu zwei Dimeren sind. Dies führt entlang der Dimerreihe zu wechselnden Molekülabständen von 1.5- beziehungsweise 2-fachen Dimerabständen. In guter Übereinstimmung von Experiment und ebenfalls durchgeführten Monte-Carlo-Simulationen konnte die Maximalbedeckung des Cyclooctins zu 0.58 ML bestimmt werden. In Experimenten an wasserstoffvorbedeckten Si(001)-Oberflächen konnte keine erhöhte platzspezifische Reaktivität an lokal gestörten Adsorptionsplätzen festgestellt werden. Diese Beobachtung untermauert das Vorliegen eines direkten Adsorptionsmechanismus, wodurch sich Cyclooctin sehr wahrscheinlich durch eine, verglichen mit anderen Molekülen, erhöhte chemische Selektivität für die Adsorption auf der Si(001)-Oberfläche auszeichnet. Aufgrund der Ergebnisse dieser Arbeit gilt Cyclooctin im Zusammenhang der Funktionalisierung von Halbleitern daher als vielversprechender Kandidat für den Übergang von einer Halbleiteroberfläche zu einer organischen Multilage. Die Untersuchungen zur Adsorption von Tetrahydrofuran auf der Si(001)-Oberfläche sollten unter anderem der Frage nach möglichen Reaktionen eines Lösungsmittels mit der Halbleiteroberfläche nachgehen. Trotz der Reaktionsträgheit von Tetrahydrofuran in der flüssigen Phase konnte eine unerwartete und erstaunlich komplexe Oberflächenchemie des Tetrahydrofurans auf der Si(001)-Oberfläche festgestellt werden. So werden bei unterschiedlichen Probentemperaturen grundverschiedene Adsorptionsgeometrien und außerdem eine vielschichtige Umordnung nach dem Tempern auf höhere Temperaturen beobachtet. Bei tiefen Temperaturen deuten die Ergebnisse auf einen dativ-gebundenen, metastabilen Zwischenzustand hin, der sich durch thermische Anregung irreversibel in die bei Raumtemperaturexperimenten beobachtete Adsorptionsgeometrie umwandeln lässt. Tempern der mit Tetrahydrofuran bedeckten Si(001)-Oberfläche auf Temperaturen von 700 K führt zu einer Reihe von unterschiedlichen Konfigurationen, die auf eine Zerlegung des Moleküls und insbesondere den Einbau von Sauerstoff in das Siliziumsubstrat hindeuten. Die bei thermischer Anregung beobachtete Umwandlung der Tieftemperatur-Konfiguration in die Raumtemperatur-Konfiguration kann auch durch den Tunnelprozess selbst, das heißt spitzeninduziert, hervorgerufen werden. Diese Effekte wurden eingehend untersucht und konnten auf eine elektronische Anregung zurückgeführt werden.The site-selective reactivity of three prototypic organic molecules (ethylene, cyclooctyne and tetrahydrofuran) on a Si(001) surface was studied by means of scanning tunneling microscopy (STM). This technique permits the real-space study of clean and adsorbate covered surfaces with atomic resolution; detailed information on adsorbate geometry was obtained with respect to the underlying substrate structure. As a local probe, STM furthermore allows to distinguish between different adsorption geometries of one and the same adsorbate on the surface which may occur with varying frequency due to different site-selective reactivities. By means of careful coverage dependent measurements, the reactivity of different adsorption sites was deduced for all three molecules. Details of the adsorption mechanisms were accessible by varying the surface temperature during adsorption. Precoverage of atomic hydrogen leads to locally distorted configurations on the surface. The site-selective reactivity of these configurations for the respective molecules was studied in order to get deeper insight into the adsorption mechanism of the adsorbates. Ethylene is the smallest unsaturated organic molecule. The adsorption of ethylene on Si(001) surfaces has been studied extensively over the last years. It is well established that ethylene adsorbs non-dissociatively on one silicon dimer. In the present study, a second adsorption geometry was identified with an ethylene molecule adsorbed on two dimers. Although this second reaction path exhibits much lower reactivity on the clean Si(001) surface, nearly 20% of the molecules adsorb via this two-dimer pathway at high ethylene coverage. Preadsorption of atomic hydrogen is found to increase the site-selective reactivity of ethylene at locally distorted dangling bond configurations in comparison to the clean surface. Thus, also in the case of non-dissociative adsorption of an organic molecule, the site-selective reactivity can be controlled by changing the local electronic structure. This can be explained by a precursor mediated chemisorption process which was corroborated by means of Monte Carlo simulations. In the framework of this precursor model, the adsorbate distributions for different coverages were simulated and good agreement with the experimental results was obtained. In summary, a conclusive and advanced understanding of the adsorption process of ethylene on Si(001) has been achieved. Cycloalkynes are cyclic hydrocarbons with one triple bond leading to high ringstrain, especially in the case of smaller rings. The influence of these characteristics on the adsorption mechanism was studied for the adsorption of cyclooctyne on Si(001). Interestingly, the same adsorption behavior was found at low and at room temperatures, in contrast to most other organic adsorbates. This observation indicates a direct (barrierless) adsorption pathway. Two adsorption geometries were identified with the molecule situated symmetrically above one and two surface dimers, respectively. This leads to varying nearest neighbor distances of the adsorbed molecules of 1.5 and 2 dimer distances. At high coverage, cyclooctyne forms a well-ordered first layer on Si(001). Both the experimental results and Monte Carlo simulations yield a maximum coverage of 0.58 ML. Preadsorption of atomic hydrogen does not influence the site-selective reactivity of cyclooctyne at locally distorted dangling bond configurations. This result supports the interpretation of a direct adsorption mechanism. Based on these results, cyclooctyne is likely to show a higher chemical selectivity for the reaction with Si(001) than other molecules. As a consequence, cyclooctyne is a promising candidate for the synthesis of semiconductor-organic interfaces. The adsorption of tetrahydrofuran on Si(001) was studied in order to investigate possible reactions of organic solvents with semiconductor surfaces. Despite of the inert behavior of tetrahydrofuran in the liquid phase, an unexpectedly rich surface chemistry of tetrahydrofuran on Si(001) was observed. Entirely different adsorption geometries were identified at low temperature and at room temperature. Furthermore, at elevated surface temperatures, a complex reorganization occurs. The adsorption geometry at low temperature is likely to be a dative-bonded intermediate state. An irreversible conversion into the room temperature configuration is possible by thermal excitation of the low-temperature configuration. Further heating of the tetrahydrofuran covered surface to temperatures above room temperature leads to several new configurations which indicate decomposition of the molecule and in particular the insertion of oxygen into the substrate. In addition to the thermal excitation, a tip-induced conversion from the low temperature configuration into the room temperature adsorption geometry was identified at low temperature. Based on detailed investigations of its voltage and current dependence, this effect could be attributed to an electronic excitation

Impact of COVID-19 on cardiovascular testing in the United States versus the rest of the world

Objectives: This study sought to quantify and compare the decline in volumes of cardiovascular procedures between the United States and non-US institutions during the early phase of the coronavirus disease-2019 (COVID-19) pandemic. Background: The COVID-19 pandemic has disrupted the care of many non-COVID-19 illnesses. Reductions in diagnostic cardiovascular testing around the world have led to concerns over the implications of reduced testing for cardiovascular disease (CVD) morbidity and mortality. Methods: Data were submitted to the INCAPS-COVID (International Atomic Energy Agency Non-Invasive Cardiology Protocols Study of COVID-19), a multinational registry comprising 909 institutions in 108 countries (including 155 facilities in 40 U.S. states), assessing the impact of the COVID-19 pandemic on volumes of diagnostic cardiovascular procedures. Data were obtained for April 2020 and compared with volumes of baseline procedures from March 2019. We compared laboratory characteristics, practices, and procedure volumes between U.S. and non-U.S. facilities and between U.S. geographic regions and identified factors associated with volume reduction in the United States. Results: Reductions in the volumes of procedures in the United States were similar to those in non-U.S. facilities (68% vs. 63%, respectively; p = 0.237), although U.S. facilities reported greater reductions in invasive coronary angiography (69% vs. 53%, respectively; p < 0.001). Significantly more U.S. facilities reported increased use of telehealth and patient screening measures than non-U.S. facilities, such as temperature checks, symptom screenings, and COVID-19 testing. Reductions in volumes of procedures differed between U.S. regions, with larger declines observed in the Northeast (76%) and Midwest (74%) than in the South (62%) and West (44%). Prevalence of COVID-19, staff redeployments, outpatient centers, and urban centers were associated with greater reductions in volume in U.S. facilities in a multivariable analysis. Conclusions: We observed marked reductions in U.S. cardiovascular testing in the early phase of the pandemic and significant variability between U.S. regions. The association between reductions of volumes and COVID-19 prevalence in the United States highlighted the need for proactive efforts to maintain access to cardiovascular testing in areas most affected by outbreaks of COVID-19 infection

From porphyrins to pyrphyrins: adsorption study and metalation of a molecular catalyst on Au(111)

The molecular ligand pyrphyrin, a tetradentate bipyridine based macrocycle, represents an interesting but widely unexplored class of molecules. It resembles the well-known porphyrin, but consists of pyridyl subunits instead of pyrroles. Metal complexes based on pyrphyrin ligands have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting reactions. In this study, the adsorption and metalation of pyrphyrin on a single crystalline Au(111) surface is investigated in an ultrahigh vacuum by means of scanning tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy and density functional theory. Pyrphyrin coverages of approximately one monolayer and less are obtained by sublimation of the molecules on the substrate kept at room temperature. The molecules self-assemble in two distinct phases of long-range molecular ordering depending on the surface coverage. The deposition of cobalt metal and subsequent annealing lead to the formation of Co-ligated pyrphyrin molecules accompanied by a pronounced change of the molecular self-assembly. Electronic structure calculations taking the herringbone reconstruction of Au(111) into account show that the molecules are physisorbed, but preferred adsorption sites are identified where Co and the N atoms of the two terminal cyano groups are optimally coordinated to the surface Au atoms. An intermediate state of the metalation reaction is observed and the reaction steps for the Co metalation of pyrphyrin molecules on Au(111) are established in a joint experimental and computational effort

The impact of metalation on adsorption geometry, electronic level alignment and UV-stability of organic macrocycles on TiO2(110)

Metal complexes of the tetradentate bipyridine based macrocycle pyrphyrin (Pyr) have recently shown promise as water reduction catalysts in homogeneous photochemical water splitting reactions. In this study, the adsorption and metalation of pyrphyrin on stoichiometric TiO2(110) is investigated in ultrahigh vacuum by means of scanning tunneling microscopy, photoelectron spectroscopy, low-energy electron diffraction, and density functional theory. In a joint experimental and computational effort, the local adsorption geometry at low coverage, the long-range molecular ordering at higher coverage and the electronic structure have been determined for both the bare ligand and the cobalt-metalated Pyr molecule on TiO2. The energy level alignment of CoPyr/TiO2 supports electron injection into TiO2 upon photoexcitation of the CoPyr complex and thus renders it a potential sensitizer dye. Importantly, Co-incorporation is found to stabilize the Pyr molecule against photo-induced degradation, while the bare ligand is decomposed rapidly under continuous UV-irradiation. This interesting phenomenon is discussed in terms of additional de-excitation channels for electronically highly excited molecular states

Functionalization and passivation of ultrathin alumina films of defined sub-nanometer thickness with self-assembled monolayers

Instability of ultrathin surface oxides on alloys under environmental conditions can limit the opportunities for applications of these systems when the thickness control of the insulating oxide film is crucial for device performance. A procedure is developed to directly deposit self-assembled monolayers (SAM) from solvent onto substrates prepared under ultra-high vacuum conditions without exposure to air. As an example, rhenium photosensitizers functionalized with carboxyl linker groups are attached to ultrathin alumina grown on NiAl(1 1 0). The thickness change of the oxide layer during the SAM deposition is quantified by x-ray photoelectron spectroscopy and can be drastically reduced to one atomic layer. The SAM acts as a capping layer, stabilizing the oxide thin film under environmental conditions. Ultraviolet photoelectron spectroscopy elucidates the band alignment in the resulting heterostructure. The method for molecule attachment presented in this manuscript can be extended to a broad class of molecules vulnerable to pyrolysis upon evaporation and presents an elegant method for attaching molecular layers on solid substrates that are sensitive to air

Centimeter-sized single-orientation monolayer hexagonal boron nitride with or without nanovoids

Large-area hexagonal boron nitride (h-BN) promises many new applications of two-dimensional materials, such as the protective packing of reactive surfaces or as membranes in liquids. However, scalable production beyond exfoliation from bulk single crystals remained a major challenge. Single-orientation monolayer h-BN nanomesh is grown on 4 in. wafer single crystalline rhodium films and transferred on arbitrary substrates such as SiO2, germanium, or transmission electron microscopy grids. The transfer process involves application of tetraoctylammonium bromide before electrochemical hydrogen delamination. The material performance is demonstrated with two applications. First, protective sealing of h-BN is shown by preserving germanium from oxidation in air at high temperatures. Second, the membrane functionality of the single h-BN layer is demonstrated in aqueous solutions. Here, we employ a growth substrate intrinsic preparation scheme to create regular 2 nm holes that serve as ion channels in liquids

Pyramidal Structure Formation at the Interface between III/V Semiconductors and Silicon

An enhancement of computer performance following Moore’s law requires the miniaturization of semiconductor devices. Presently, their dimensions reach the nanoscale. Interfaces between materials become increasingly important as the volume is reduced. It is shown here how a pyramidal interface structure is formed irrespective of the conditions applied during the growth of two semiconductors. This drastically changes the common view of interfaces, which were assumed to be either atomically abrupt or interdiffused. Especially in semiconductor heteroepitaxy, a simple surface segregation of one atomic species is often assumed. It is proven by first-principles computations and kinetic modeling that the atom mobility during growth and the chemical environment at the interface are the decisive factors in the formation of the actual structure. Gallium phosphide grown on silicon was chosen as representative, nearly unstrained material combination to study the fundamental parameters influencing the interface morphology. Beyond that, this system has significant impact for cutting-edge electronic and optoelectronic devices. The findings derived in this study can be generalized to aid the understanding of further relevant semiconductor interfaces. This knowledge is crucial to comprehend current and steer future properties of miniaturized devices