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1.
J Am Chem Soc ; 145(2): 967-977, 2023 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-36580274

RESUMO

The synthesis of two-dimensionally extended polycyclic heteroatomic molecules keeps attracting considerable attention. In particular, frameworks bearing planar cyclooctatetraenes (COT) moieties can display intriguing properties, including antiaromaticity. Here, we present an on-surface chemistry route to square-type porphyrin tetramers with a central COT ring, coexisting with other oligomers. This approach employing temperature-induced dehydrogenative porphyrin homocoupling in an ultrahigh vacuum environment provides access to surface-supported, unsubstituted porphyrin tetramers that are not easily achievable by conventional synthesis means. Specifically, monomeric free-base (2H-P) and Zn-metalated (Zn-P) porphines (P) were employed to form square-type free-base and Zn-functionalized tetramers on Ag(100). An atomic-level characterization by bond-resolved atomic force microscopy and scanning tunneling microscopy and spectroscopy is provided, identifying the molecular structures. Complemented by density functional theory modeling, the electronic structure is elucidated, indeed revealing antiaromaticity induced by the COT moiety. The present study thus gives access, and insights, to a porphyrin oligomer, representing both a model system for directly fused porphyrins and a potential building block for conjugated, extended two-dimensional porphyrin sheets.


Assuntos
Porfirinas , Porfirinas/química , Estrutura Molecular , Espectroscopia de Ressonância Magnética , Microscopia de Tunelamento
2.
Proc Natl Acad Sci U S A ; 119(49): e2212730119, 2022 Dec 06.
Artigo em Inglês | MEDLINE | ID: mdl-36459647

RESUMO

In BaNiS2, a Dirac nodal line band structure exists within a two-dimensional Ni square lattice system, in which significant electronic correlation effects are anticipated. Using scanning tunneling microscopy (STM), we discover signs of correlated-electron behavior, namely electronic nematicity appearing as a pair of C2-symmetry striped patterns in the local density-of-states at ∼60 meV above the Fermi energy. In observations of quasiparticle interference, as well as identifying scattering between Dirac cones, we find that the striped patterns in real space stem from a lifting of degeneracy among electron pockets at the Brillouin zone boundary. We infer a momentum-dependent energy shift with d-form factor, which we model numerically within a density wave (DW) equation framework that considers spin-fluctuation-driven nematicity. This suggests an unusual mechanism driving the nematic instability, stemming from only a small perturbation to the Fermi surface, in a system with very low density of states at the Fermi energy. The Dirac points lie at nodes of the d-form factor and are almost unaffected by it. These results highlight BaNiS2 as a unique material in which Dirac electrons and symmetry-breaking electronic correlations coexist.


Assuntos
Eletrônica , Elétrons , Microscopia de Tunelamento , Movimento (Física) , Células Fotorreceptoras Retinianas Cones
3.
Molecules ; 27(23)2022 Nov 29.
Artigo em Inglês | MEDLINE | ID: mdl-36500413

RESUMO

Self-assembled monolayers (SAMs) of terpyridine-based transition metal (ruthenium and osmium) complexes, anchored to gold substrate via tripodal anchoring groups, have been investigated as possible redox switching elements for molecular electronics. An electrochemical study was complemented by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) methods. STM was used for determination of the SAM conductance values, and computation of the attenuation factor ß from tunneling current-distance curves. We have shown that SAMs of Os-tripod molecules contain larger adlayer structures compared with SAMs of Ru-tripod molecules, which are characterized by a large number of almost evenly distributed small islands. Furthermore, upon cyclic voltammetric experimentation, Os-tripod films rearrange to form a smaller number of even larger islands, reminiscent of the Ostwald ripening process. Os-tripod SAMs displayed a higher surface concentration of molecules and lower conductance compared with Ru-tripod SAMs. The attenuation factor of Os-tripod films changed dramatically, upon electrochemical cycling, to a higher value. These observations are in accordance with previously reported electron transfer kinetics studies.


Assuntos
Ouro , Microscopia de Tunelamento , Microscopia de Força Atômica , Propriedades de Superfície , Ouro/química , Oxirredução
4.
Chem Commun (Camb) ; 59(2): 170-178, 2022 Dec 22.
Artigo em Inglês | MEDLINE | ID: mdl-36484702

RESUMO

Molecular self-assembly is driven by intermolecular interactions between the functional groups on the component molecules. Small changes in molecular structure can make large differences in extended structure, and understanding this connection will lead to predictive power and control of the self-assembly process. Scanning tunneling microscopy is used to study self-assembly in two-dimensional clusters and monolayers, and the experimental approach is to study "families" of molecules where one or more functional groups is varied in a methodical way. Studied families include indole carboxylic acids, isatin derivatives (which have the indole backbone), quinaldic acid, thioethers, and fluorenone derivatives. In these systems, a variety of intermolecular interactions drive the assembly of the molecular monolayer, including hydrogen bonds, van der Waals forces, zwitterionic interactions, surface interactions, and halogen interactions.


Assuntos
Ácidos Carboxílicos , Microscopia de Tunelamento , Humanos , Propriedades de Superfície , Estrutura Molecular , Ligação de Hidrogênio
5.
J Am Chem Soc ; 144(46): 21389-21397, 2022 11 23.
Artigo em Inglês | MEDLINE | ID: mdl-36367921

RESUMO

Selective activation and controlled functionalization of C-H bonds in organic molecules is one of the most desirable processes in synthetic chemistry. Despite progress in heterogeneous catalysis using metal surfaces, this goal remains challenging due to the stability of C-H bonds and their ubiquity in precursor molecules, hampering regioselectivity. Here, we examine the interaction between 9,10-dicyanoanthracene (DCA) molecules and Au adatoms on a Ag(111) surface at room temperature (RT). Characterization via low-temperature scanning tunneling microscopy, spectroscopy, and noncontact atomic force microscopy, supported by theoretical calculations, revealed the formation of organometallic DCA-Au-DCA dimers, where C atoms at the ends of the anthracene moieties are bonded covalently to single Au atoms. The formation of this organometallic compound is initiated by a regioselective cleaving of C-H bonds at RT. Hybrid quantum mechanics/molecular mechanics calculations show that this regioselective C-H bond cleaving is enabled by an intermediate metal-organic complex which significantly reduces the dissociation barrier of a specific C-H bond. Harnessing the catalytic activity of single metal atoms, this regioselective on-surface C-H activation reaction at RT offers promising routes for future synthesis of functional organic and organometallic materials.


Assuntos
Ouro , Compostos Organometálicos , Temperatura , Compostos Organometálicos/química , Catálise , Microscopia de Tunelamento
6.
J Am Chem Soc ; 144(43): 20126-20133, 2022 11 02.
Artigo em Inglês | MEDLINE | ID: mdl-36259686

RESUMO

We report herein the in situ electrochemical scanning tunneling microscopy (ECSTM) study on the synergistic effect of Mg2+ in CO2 reduction reaction (CO2RR) catalyzed by cobalt phthalocyanine (CoPc). ECSTM measurement molecularly resolves the self-assembled CoPc monolayer on the Au(111) substrate. In the CO2 environment, high-contrast species are observed in the adlayer and assigned to the CO2 adsorption on CoPc. Furthermore, the contrast of the CO2-bound complex is higher in Mg2+-containing electrolytes than in Mg2+-free electrolytes, indicating the formation of the CoPc-CO2-Mg2+ complex. The surface coverage of adsorbed CO2 is positively correlated with the Mg2+ concentration as the additive in electrolytes up to a plateau of 30.8 ± 2.7% when c(Mg2+) > 30 mM. The potential step experiment indicates the higher CO2 adsorption dynamics in Mg2+-containing electrolytes than without Mg2+. The rate constants of CO2 adsorption and dissociation in different electrolytes are extracted from the data fitting of statistical results from in situ ECSTM experiments.


Assuntos
Microscopia de Tunelamento , Compostos Organometálicos , Microscopia de Tunelamento/métodos , Dióxido de Carbono , Ouro
7.
Nano Lett ; 22(21): 8626-8632, 2022 Nov 09.
Artigo em Inglês | MEDLINE | ID: mdl-36256878

RESUMO

Organometallic sandwich complexes are versatile molecular systems that have been recently employed for single-molecule manipulation and spin sensing experiments. Among related organometallic compounds, the mixed-sandwich S = 1/2 complex (η8-cyclooctatetraene)(η5-cyclopentadienyl)titanium, here [CpTi(cot)], has attracted interest as a spin qubit because of the long coherence time. Here the structural and chemical properties of [CpTi(cot)] on Au(111) are investigated at the monolayer level by experimental and computational methods. Scanning tunneling microscopy suggests that adsorption occurs in two molecular orientations, lying and standing, with a 3:1 ratio. XPS data evidence that a fraction of the molecules undergo partial electron transfer to gold, while our computational analysis suggests that only the standing molecules experience charge delocalization toward the surface. Such a phenomenon depends on intermolecular interactions that stabilize the molecular packing in the monolayer. This orientation-dependent molecule-surface hybridization opens exciting perspectives for selective control of the molecule-substrate spin delocalization in hybrid interfaces.


Assuntos
Elétrons , Titânio , Propriedades de Superfície , Microscopia de Tunelamento/métodos , Adsorção
8.
Nano Lett ; 22(19): 7848-7852, 2022 10 12.
Artigo em Inglês | MEDLINE | ID: mdl-36162080

RESUMO

The phenomenon of rectification describes the emergence of a DC current from the application of an oscillating voltage. Although the origin of this effect has been associated with the nonlinearity in the current-voltage I(V) relation, a rigorous understanding of the microscopic mechanisms for this phenomenon remains challenging. Here, we show the close connection between rectification and inelastic electron tunneling spectroscopy and microscopy for single molecules with a scanning tunneling microscope. While both techniques are based on nonlinear features in the I(V) curve, comprehensive line shape analyses reveal notable differences that highlight the two complementary techniques of nonlinear conductivity spectromicroscopy for probing nanoscale systems.


Assuntos
Elétrons , Microscopia de Tunelamento , Condutividade Elétrica , Microscopia de Tunelamento/métodos , Nanotecnologia , Análise Espectral/métodos
9.
Phys Chem Chem Phys ; 24(36): 22122-22128, 2022 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-36074749

RESUMO

On-surface synthesis has been a subject of intensive research during the last decade. Various chemical reactions have been developed on surfaces to prepare compounds and carbon nanostructures, most of which are centered on the carbon-carbon bond formation. Despite the vast progress so far, the diversity of functional groups in organic chemistry has been far less explored in on-surface synthesis. Herein, we study the surface-assisted synthesis of ethers through the homocoupling of hydroxymethyl substituents on Ag(111). By using two hydroxymethyl substituent functionalized molecular precursors with different symmetries, we have achieved the formation of ether chains and rings. High-resolution scanning tunneling microscopy complemented with density functional theory calculations are used to support our findings and offer mechanistic insights into the reaction. This work expands the toolbox of on-surface reactions for the bottom-up fabrication of more sophisticated functional nanostructures.


Assuntos
Éteres , Nanoestruturas , Carbono , Éter , Microscopia de Tunelamento , Nanoestruturas/química
10.
Chemphyschem ; 23(23): e202200404, 2022 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-36166680

RESUMO

Towards the goal of covalently bound molecular wires on silicon, the adsorption of benzyne on Si(001) was studied by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional calculations (DFT). The benzyne molecule is found to adsorb preferentially via the strained triple bond on one dimer of the Si(001) surface which results in an intact π system covalently bound to the surface. With increasing coverage, the molecules primarily adsorb along the dimer rows; on stepped surfaces, these molecular wires are all oriented in the same direction.


Assuntos
Microscopia de Tunelamento , Silício , Propriedades de Superfície , Microscopia de Tunelamento/métodos , Silício/química , Semicondutores
11.
Chem Commun (Camb) ; 58(71): 9914-9917, 2022 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-35979695

RESUMO

The co-adsorption behaviours of aromatic carboxylic acids with various pyridine derivatives were investigated with scanning tunneling microscopy and density functional theory. Surprisingly, minor adjustments in the chemical structures of the pyridine derivatives, such as the relative position of the nitrogen atom or the lengths of the side chains on the backbone would evidently affect the intermolecular O-H⋯N hydrogen bonds and further form various co-adsorption structures.


Assuntos
Microscopia de Tunelamento , Piridinas , Adsorção , Ligação de Hidrogênio , Piridinas/química
12.
J Am Chem Soc ; 144(34): 15689-15697, 2022 08 31.
Artigo em Inglês | MEDLINE | ID: mdl-35930760

RESUMO

Stacking interactions are of significant importance in the fields of chemistry, biology, and material optoelectronics because they determine the efficiency of charge transfer between molecules and their quantum states. Previous studies have proven that when two monomers are π-stacked in series to form a dimer, the electrical conductance of the dimer is significantly lower than that of the monomer. Here, we present a strong opposite case that when two anthanthrene monomers are π-stacked to form a dimer in a scanning tunneling microscopic break junction, the conductance increases by as much as 25 in comparison with a monomer, which originates from a room-temperature quantum interference. Remarkably, both theory and experiment consistently reveal that this effect can be reversed by changing the connectivity of external electrodes to the monomer core. These results demonstrate that synthetic control of connectivity to molecular cores can be combined with stacking interactions between their π systems to modify and optimize charge transfer between molecules, opening up a wide variety of potential applications ranging from organic optoelectronics and photovoltaics to nanoelectronics and single-molecule electronics.


Assuntos
Grafite , Condutividade Elétrica , Eletrodos , Eletrônica , Microscopia de Tunelamento , Polímeros
13.
Chemphyschem ; 23(19): e202200347, 2022 10 06.
Artigo em Inglês | MEDLINE | ID: mdl-35856831

RESUMO

Self-assembled monolayers (SAMs) of 4-fluorobenzenemethanethiol (p-FBMT) on Au(111), prepared by immersion procedure (1 mM ethanolic solution; 60 °C; 18 h), were characterized by scanning tunneling microscopy (STM). The data suggest the formation of highly ordered monolayer with a commensurate structure, described by the 2 3 × 13 R 13 ∘ unit cell. The STM appearance of this cell occurs, however, in two different forms, with either well-localized individual spots or splitting of these spots in two components. These components are assigned to the tunneling through the entire molecule or sulfur docking group only. The respective spots correspond then to the terminal fluorine atom and sulfur docking group, manifesting, thus, building-block-resolving STM imaging. The accessibility of the docking group for direct tunneling is most likely related to a specific molecular organization for one of the two possible internal structures of the unit cell. The above results represent a showcase for potential of STM for imaging of upright-arranged and densely packed molecular assemblies, such as SAMs.


Assuntos
Microscopia de Tunelamento , Compostos de Sulfidrila , Flúor , Ouro/química , Compostos de Sulfidrila/química , Enxofre
14.
Chem Commun (Camb) ; 58(64): 8938-8941, 2022 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-35851385

RESUMO

Aromatic foldamers are promising for applications such as molecular recognition and molecular machinery. For many of these, defect free, 2D-crystaline monolayers are needed. To this end, submonolayers were prepared in ultra-high vacuum (UHV) on Ag(111) via electrospray controlled ion beam deposition (ES-CIBD). On the surface, the unfolded state is unambiguously identified by real-space single-molecule imaging using scanning tunnelling microscopy (STM) and it is found to assemble in regular structures.


Assuntos
Amidas , Microscopia de Tunelamento , Amidas/química , Conformação Molecular , Nanotecnologia
15.
Nano Lett ; 22(12): 4807-4813, 2022 06 22.
Artigo em Inglês | MEDLINE | ID: mdl-35678453

RESUMO

Conventional transport methods provide quantitative information on spin, orbital, and valley states in quantum dots but lack spatial resolution. Scanning tunneling microscopy, on the other hand, provides exquisite spatial resolution at the expense of speed. Working to combine the spatial resolution and energy sensitivity of scanning probe microscopy with the speed of microwave measurements, we couple a metallic tip to a Si/SiGe double quantum dot (DQD) that is integrated with a charge detector. We first demonstrate that the dc-biased tip can be used to change the occupancy of the DQD. We then apply microwaves through the tip to drive photon-assisted tunneling (PAT). We infer the DQD level diagram from the frequency and detuning dependence of the tunneling resonances. These measurements allow the resolution of ∼65 µeV excited states, an energy consistent with valley splittings in Si/SiGe. This work demonstrates the feasibility of scanning gate experiments with Si/SiGe devices.


Assuntos
Pontos Quânticos , Microscopia , Microscopia de Tunelamento , Micro-Ondas
16.
Nano Lett ; 22(12): 4919-4924, 2022 06 22.
Artigo em Inglês | MEDLINE | ID: mdl-35640062

RESUMO

Coherent tunneling electron transport through molecular wires has been theoretically established as a temperature-independent process. Although several experimental studies have shown counter examples, robust models to describe this temperature dependence have not been thoroughly developed. Here, we demonstrate that dynamic molecular structures lead to temperature-dependent conductance within coherent tunneling regime. Using a custom-built variable-temperature scanning tunneling microscopy break-junction instrument, we find that oligo[n]phenylenes exhibit clear temperature-dependent conductance. Our calculations reveal that thermally activated dihedral rotations allow these molecular wires to have a higher probability of being in a planar conformation. As the tunneling occurs primarily through π-orbitals, enhanced coplanarization substantially increases the time-averaged tunneling probability. These calculations are consistent with the observation that more rotational pivot points in longer molecular wires leads to larger temperature-dependence on conductance. These findings reveal that molecular conductance within coherent and off-resonant electron transport regimes can be controlled by manipulating dynamic molecular structure.


Assuntos
Microscopia de Tunelamento , Transporte de Elétrons , Conformação Molecular , Estrutura Molecular , Temperatura
17.
J Am Chem Soc ; 144(23): 10282-10290, 2022 06 15.
Artigo em Inglês | MEDLINE | ID: mdl-35587810

RESUMO

Selective regulation of chemical reactions is crucial in chemistry. Oxygen, as a key reagent in ubiquitous oxidative chemistry, exhibits great potential in regulating molecular assemblies, and more importantly, chemical reactions in molecular systems supported by metal surfaces. However, the unique catalytic performance and reaction mechanisms of oxygen species remain elusive, which are essential for understanding reaction selection and regulation. In this study, by a combination of scanning tunneling microscopy (STM) imaging/manipulations and density functional theory (DFT) calculations, we showed that the on-surface reaction pathways of terminal alkynes could be steered from C-C coupling to C-H activation with high selectivity by introducing O2 into the molecular system. The catalytic performance and reaction mechanisms of oxygen species were explored in the C-H activation processes, and both molecular O2 and atomic O could efficiently steer the reaction pathways. These results would provide a fundamental understanding of interfacial catalytic reaction processes.


Assuntos
Alcinos , Oxigênio , Alcinos/química , Catálise , Metais/química , Microscopia de Tunelamento , Oxigênio/química
18.
J Phys Chem Lett ; 13(22): 4918-4923, 2022 Jun 09.
Artigo em Inglês | MEDLINE | ID: mdl-35635095

RESUMO

Kinetic analysis of surface reactions at the single molecule level is important for understanding the influence of the substrate and solvent on reaction dynamics and mechanisms, but it is difficult with current methods. Here we present a stochastic kinetic analysis of the oxygenation of cobalt octaethylporphyrin (CoOEP) at the solution/solid interface by monitoring fluctuations from equilibrium using scanning tunneling microscopy (STM) imaging. Image movies were used to monitor the oxygenated and deoxygenated state dwell times. The rate constants for CoOEP oxygenation are ka = 4.9 × 10-6 s-1·Torr-1 and kd = 0.018 s-1. This is the first use of stochastic dwell time analysis with STM to study a chemical reaction, and the results suggest that it has great potential for application to a wide range of surface reactions. Expanding these stochastic studies to further systems is key to unlocking kinetic information for surface-confined reactions at the molecular level, especially at the solution/solid interface.


Assuntos
Porfirinas , Cobalto/química , Cinética , Microscopia de Tunelamento , Porfirinas/química , Imagem Individual de Molécula
19.
Chem Asian J ; 17(15): e202200384, 2022 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-35621190

RESUMO

Electrocatalysis is the foundation of many techniques currently used to address environmental and energy problems. Therefore, understanding the electrocatalytic processes is essential to guide the rational design of electrocatalysts. Scanning tunneling microscopy (STM), developed in the 1980s, remains one of the few techniques that allow surface imaging at the atomic level, making it incredibly useful in electrocatalytic research. In this review, we introduced the basic concept and latest applications of the STM technique for in situ studies of electrocatalytic processes, particularly its capability in active site identification, species adsorption/desorption analysis, surface reconstruction imaging, and electrocatalyst dissolution detection, as well as its advantages and limitations.


Assuntos
Microscopia de Tunelamento , Adsorção , Microscopia Eletroquímica de Varredura , Microscopia de Tunelamento/métodos
20.
J Am Chem Soc ; 144(19): 8789-8796, 2022 05 18.
Artigo em Inglês | MEDLINE | ID: mdl-35503747

RESUMO

Desilylative coupling involving C-Si bond cleavage has emerged as one of the most important synthetic strategies for carbon-carbon/heteroatom bond formation in solution chemistry. However, in on-surface chemistry, C-Si bond cleavage remains a synthetic challenge. Here, we report the implementation of C(sp2)-Si bond cleavage and subsequent C-C bond formation on metal surfaces. The combination of scanning tunneling microscopy and density functional theory calculation successfully reveals that the incorporation of the C-Br group on the arylsilanes is critical to the success of this desilylative coupling reaction on metal surfaces. Our study represents a promising approach for the removal of protecting silyl groups in on-surface chemistry.


Assuntos
Carbono , Microscopia de Tunelamento , Carbono/química , Metais
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