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Zishu
Wang
,
Kai
Qian
,
Murat Anil
Öner
,
Peter S.
Deimel
,
Yan
Wang
,
Shuai
Zhang
,
Xiaoxi
Zhang
,
Vishal
Gupta
,
Juan
Li
,
Hong-jun
Gao
,
David A.
Duncan
,
Johannes V.
Barth
,
Xiao
Lin
,
Francesco
Allegretti
,
Shixuan
Du
,
Carlos-andres
Palma
Abstract: Precisely layered molecular heterostructures are promising but still largely unexplored materials, with the potential to complement and enhance the scope of two-dimensional heterostructures. The controlled epitaxial growth of vertically stacked molecular layers connected through tailored linkers, can lead to significant development in the field. Here, we demonstrate that sequential assembly of prototypical iron porphyrins and axial ligands can be steered via temperature-programmed desorption, and monitored by mass spectrometry and by high-resolution atomic force microscopy under ultrahigh vacuum conditions. Complementary photoelectron spectroscopy analysis delivers chemical insight into the formation of layer-by-layer nanoarchitectures. Our temperature-directed methodology outlines a promising strategy for the in vacuo fabrication of precisely stacked, multicomponent (metal–organic) molecular heterostructures.
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Nov 2020
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Abstract: By exploiting an established on-surface metallation strategy, we address the ability of the corrolic macrocycle to stabilise transition metal ions in high-valent (III) oxidation states in metal-supported molecular layers. This approach offers a route to engineer adsorbed metal complexes that cannot be easily fabricated by organic synthesis methods and bear a vacant axial coordination site for catalytic conversions.
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Aug 2020
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I09-Surface and Interface Structural Analysis
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P. T. P.
Ryan
,
P. L.
Lalaguna
,
F.
Haag
,
M. M.
Braim
,
P.
Ding
,
D. J.
Payne
,
J. V.
Barth
,
Tien-lin
Lee
,
D. P.
Woodruff
,
F.
Allegretti
,
D. A.
Duncan
Diamond Proposal Number(s):
[24113]
Open Access
Abstract: Utilising normal incidence X-ray standing waves we rigourously scrutinise the “inverted model” as the adsorption structure of free-base tetraphenyl porphyrin on Cu(111). We demonstrate that the iminic N atoms are anchored at near-bridge adsorption sites on the surface displaced laterally by 1.1 ± 0.2 Å in excellent agreement with previously published calculations.
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Mar 2020
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David A.
Duncan
,
Peter S.
Deimel
,
Alissa
Wiengarten
,
Mateusz
Paszkiewicz
,
Pablo
Casado Aguilar
,
Robert G.
Acres
,
Florian
Klappenberger
,
Willi
Auwärter
,
Ari Paavo
Seitsonen
,
Johannes V.
Barth
,
Francesco
Allegretti
Abstract: In situ preparation of oxotitanium tetraphenylporphyrin (TiO-TPP) on Ag(111) under ultra-high vacuum conditions was achieved in a multi-step procedure starting from adsorbed free-base tetraphenylporphyrin (2H-TPP). The final product as well as the intermediate titanium tetraphenylporphyrin (Ti-TPP) were characterized by a suite of surface-sensitive spectroscopic tools combined with scanning tunneling microscopy and density functional theory (DFT), and compared against the parent 2H-TPP species. Facile oxidation of Ti-TPP with molecular oxygen was observed at 300 K, with X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) from the Ti 2p core levels supporting a change in the oxidation state from Ti2+ to Ti4+. N K-edge and Ti L-edge NEXAFS suggest that the tetrapyrrole macrocycle conformation is modified upon binding to oxygen, in agreement with DFT calculations that predict a marked change of the local environment of the Ti centers upon oxygen attachment. O K-edge NEXAFS and O 1s energy-scanned photoelectron diffraction from the resulting TiO-TPP monolayer provide strong evidence for the presence of a titanium-oxygen double bond, with the latter technique yielding a bond length of 1.56 ± 0.02 Å. The majority of adsorbed TiO-TPP species have the oxo group pointing away from the surface rather than towards it, and thus the oxygen atom can potentially interact coordinatively with external species. Both the highly reactive, intermediate Ti TPP species and the final product TiO-TPP are of great interest for catalytic applications.
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Nov 2019
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Abstract: Surface-assisted Ullmann coupling is the workhorse of on-surface synthesis. Despite its obvious relevance, many fundamental and mechanistic aspects remain elusive. In order to shed light on individual reaction steps and their progression with temperature, Temperature-Programmed X-ray Photoelectron Spectroscopy (TP-XPS) experiments are performed for a prototypical model system. The activation of the coupling by initial dehalogenation is tracked by monitoring Br 3d core levels, whereas the C 1s signature is used to follow the emergence of metastable organometallic intermediates and their conversion to the final covalent products upon heating in real time. The employed 1,3,5 tris(4 bromophenyl)benzene precursor is comparatively studied on Ag(111) vs. Au(111), whereby intermolecular bonds and network topologies are additionally characterized by Scanning Tunneling Microscopy (STM). Besides the well-comprehended differences in activation temperatures for debromination, the thermal progression shows marked differences between the two surfaces. Debromination proceeds rapidly on Ag(111), but is relatively gradual on Au(111). While on Ag(111) debromination is well explained by first-order reaction kinetics, thermodynamics prevail on Au(111), underpinned by a close agreement between experimentally deduced and Density Functional Theory (DFT) calculated reaction enthalpies. Thermodynamically controlled debromination on Au(111) over a large temperature range implies an unexpectedly long life-time of surface-stabilized radicals prior to covalent coupling, as corroborated by TP-XPS of C 1s core levels. These insights are anticipated to play an important role regarding our ability to rationally synthesize atomically precise low-dimensional covalent nanostructures on surfaces.
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Feb 2019
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I09-Surface and Interface Structural Analysis
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Martin
Schwarz
,
Manuela
Garnica
,
David A.
Duncan
,
Alejandro
Pérez Paz
,
Jacob
Ducke
,
Peter S.
Deimel
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Angel
Rubio
,
Johannes V.
Barth
,
Francesco
Allegretti
,
Willi
Auwärter
Diamond Proposal Number(s):
[14624]
Abstract: The tetrapyrrole macrocycle of porphine is the common core of all porphyrin molecules, an interesting class of π-conjugated molecules with relevance in natural and artificial systems. The functionality of porphines on a solid surface can be tailored by the central metal atom and its interaction with the substrate. In this study, we present a local adsorption geometry determination for cobalt porphine on Cu(111) by means of complementary scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy and X-ray standing wave measurements, and density functional theory calculations. Specifically, the Co center was determined to be at an adsorption height of 2.25 ± 0.04 Å occupying a bridge site. The macrocycle adopts a moderate asymmetric saddle-shape conformation, with the two pyrrole groups that are aligned perpendicular to the densely packed direction of the Cu(111) surface tilted away from the surface plane.
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Feb 2018
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I09-Surface and Interface Structural Analysis
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Martin
Schwarz
,
Alexander
Riss
,
Manuela
Garnica
,
Jacob
Ducke
,
Peter S.
Deimel
,
David A.
Duncan
,
Pardeep
Kumar Thakur
,
Tien-lin
Lee
,
Ari Paavo
Seitsonen
,
Johannes V.
Barth
,
Francesco
Allegretti
,
Willi
Auwärter
Diamond Proposal Number(s):
[14624]
Abstract: Atomically thin hexagonal boron nitride (h-BN) layers on metallic supports represent a promising platform for the selective adsorption of atoms, clusters, and molecular nanostructures. Specifically, scanning tunneling microscopy (STM) studies revealed an electronic corrugation of h-BN/Cu(111), guiding the self-assembly of molecules and their energy level alignment. A detailed characterization of the h-BN/Cu(111) interface including the spacing between the h-BN sheet and its support—elusive to STM measurements—is crucial to rationalize the interfacial interactions within these systems. To this end, we employ complementary techniques including high-resolution noncontact atomic force microscopy, STM, low-energy electron diffraction, X-ray photoelectron spectroscopy, the X-ray standing wave method, and density functional theory. Our multimethod study yields a comprehensive, quantitative structure determination including the adsorption height and the corrugation of the sp2 bonded h-BN layer on Cu(111). Based on the atomic contrast in atomic force microscopy measurements, we derive a measurable–hitherto unrecognized–geometric corrugation of the h-BN monolayer. This experimental approach allows us to spatially resolve minute height variations in low-dimensional nanostructures, thus providing a benchmark for theoretical modeling. Regarding potential applications, e.g., as a template or catalytically active support, the recognition of h-BN on Cu(111) as a weakly bonded and moderately corrugated overlayer is highly relevant.
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Sep 2017
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Abstract: The adsorption of thymine, a pyrimidine based nucleobase, was studied on the (110) termination of rutile titanium dioxide in order to understand the thermal stability and gross structural parameters of the interaction between a strongly polar adsorbate and a highly corrugated transition metal oxide surface. Near-edge X-ray absorption fine structure (NEXAFS), X-ray photoelectron spectroscopy (XPS), temperature programmed XPS and temperature programmed desorption indicated the growth of a room temperature stable bilayer, which could only be removed by annealing to 450 K. The remaining first layer was remarkably robust, surviving annealing up to 550 K before undergoing N–H bond scission. The comparison to XPS of a sub-monolayer exposure of 1-methyluracil shows that the origin of the room temperature stable bilayer is not intermolecular interactions. This discovery, alongside the deprotonation of one of the first layer's pyrimidinic nitrogen atoms at room temperature, suggests that the thymine molecules in the first layer bind to the undercoordinated surface Ti atoms, and the second layer thymine molecules coordinate with the bridging oxygen atoms which protrude above the Ti surface plane on the (110) surface. The NEXAFS results indicate an almost upright orientation of the molecules in both layers, with a 30 ± 10° tilt away from the surface normal.
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Jul 2016
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I09-Surface and Interface Structural Analysis
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Peter
Deimel
,
Reda M.
Bababrik
,
Bin
Wang
,
Philip
Blowey
,
Luke
Rochford
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Marie-laure
Bocquet
,
Johannes V.
Barth
,
Phil
Woodruff
,
David
Duncan
,
Francesco
Allegretti
Diamond Proposal Number(s):
[8940]
Open Access
Abstract: The strong parallels between coordination chemistry and adsorption on metal surfaces, with molecules and ligands forming local bonds to individual atoms within a metal surface, have been established over many years of study. The recently proposed “surface trans-effect” (STE) appears to be a further manifestation of this analogous behaviour, but so far the true nature of the modified molecule–metal surface bonding has been unclear. The STE could play an important role in determining the reactivities of surface-supported metal–organic complexes, influencing the design of systems for future applications. However, the current understanding of this effect is incomplete and lacks reliable structural parameters with which to benchmark theoretical calculations. Using X-ray standing waves, we demonstrate that ligation of ammonia and water to iron phthalocyanine (FePc) on Ag(111) increases the adsorption height of the central Fe atom; dispersion corrected density functional theory calculations accurately model this structural effect. The calculated charge redistribution in the FePc/H2O electronic structure induced by adsorption shows an accumulation of charge along the σ-bonding direction between the surface, the Fe atom and the water molecule, similar to the redistribution caused by ammonia. This apparent σ-donor nature of the observed STE on Ag(111) is shown to involve bonding to the delocalised metal surface electrons rather than local bonding to one or more surface atoms, thus indicating that this is a true surface trans-effect.
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Jun 2016
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Alissa
Wiengarten
,
Julian A.
Lloyd
,
Knud
Seufert
,
Joachim
Reichert
,
Willi
Auwärter
,
Runyuan
Han
,
David A
Duncan
,
Francesco
Allegretti
,
Sybille
Fischer
,
Seung Cheol
Oh
,
Özge
Sağlam
,
Li
Jiang
,
Saranyan
Vijayaraghavan
,
David
Écija
,
Anthoula C.
Papageorgiou
,
Johannes V.
Barth
Abstract: Selectivity in chemical reactions is a major objective in industrial processes to minimize spurious byproducts and to save scarce resources. In homogeneous catalysis the most important factor which determines selectivity is structural symmetry. However, a transfer of the symmetry concept to heterogeneous catalysis still requires a detailed comprehension of the underlying processes. Here, we investigate a ring-closing reaction in surface-confined meso-substituted porphyrin molecules by scanning tunneling microscopy, temperature-programmed desorption, and computational modeling. The identification of reaction intermediates enables us to analyze the reaction pathway and to conclude that the symmetry of the porphyrin core is of pivotal importance regarding product yields.
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Aug 2015
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