Title: Chemically identifying single adatoms with single-bond sensitivity during oxidation reactions of borophene
Abstract
The chemical interrogation of individual atomic adsorbates on a surface significantly contributes to understanding the atomic-scale processes behind on-surface reactions. However, it remains highly challenging for current imaging or spectroscopic methods to achieve such a high chemical spatial resolution. Here we show that single oxygen adatoms on a boron monolayer (i.e., borophene) can be identified and mapped via ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) with ~4.8 Å spatial resolution and single bond (B–O) sensitivity. With this capability, we realize the atomically defined, chemically homogeneous, and thermally reversible oxidation of borophene via atomic oxygen in UHV. Furthermore, we reveal the propensity of borophene towards molecular oxygen activation at room temperature and phase-dependent chemical properties. In addition to offering atomic-level insights into the oxidation of borophene, this work demonstrates UHV-TERS as a powerful tool to probe the local chemistry of surface adsorbates in the atomic regime with widespread utilities in heterogeneous catalysis, on-surface molecular engineering, and low-dimensional materials.
Li, Linfei; Schultz, Jeremy F.; Mahapatra, Sayantan; Liu, Xiaolong; Zhang, Xu; Hersam, Mark C.; Jiang, Nan(
, Angewandte Chemie International Edition)
Bilayer (BL) two-dimensional boron (i.e., borophene) emerges very recently and holds promise for fascinating physical properties and a variety of electronic applications. Despite this potential, the fundamental chemical properties of BL borophene which form the critical foundation of practical applications has been unexplored. Here, we present atomic-level chemical studies of BL borophene using ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS). UHV-TERS identifies the vibrational fingerprint of BL borophene from mixed-dimensional borophene polymorphs with angstrom-scale chemical spatial resolution. The observed Raman mode is directly correlated with the vibrations of interlayer boron-boron bonds, validating the three-dimensional lattice geometry of BL borophene. By virtue of the single-bond sensitivity of UHV-TERS to oxygen adatoms, we demonstrate the enhanced chemical stability of BL borophene compared to its monolayer counterpart by exposure to controlled oxidizing atmospheres under UHV. In addition to revealing fundamental chemical insights into BL borophene, this work establishes UHV-TERS as a powerful tool to probe interlayer bonding and chemical properties of layered materials at the atomic scale.
Mahapatra, Sayantan; Li, Linfei; Schultz, Jeremy F.; Jiang, Nan(
, Journal of Raman Spectroscopy)
Abstract
Scanning tunneling microscopy‐based tip‐enhanced Raman spectroscopy (TERS) is a powerful analytical technique for surface characterization, providing both topological and chemical information with sub‐nm spatial resolution, well below the diffraction limit of light. In order to take advantage of plasmonic activity, it is necessary to use silver (Ag) probes due to their plasmonic range in the visible region. However, the Ag probe fabrication process remains challenging and is not yet standardized in practice, leading to inconsistent enhancements even for two similar types of tips prepared consecutively. In this work, we demonstrate an alternative way to reuse and recycle a plasmonic tip for distinct molecular systems inside an ultrahigh vacuum (UHV). We provide evidence of the ability to recycle tips without compromising the TERS experimental results. A long‐term preservation (>2 months) of plasmonically active probes inside UHV is demonstrated.
The drive for atom efficient catalysts with carefully controlled properties has motivated the development of single atom catalysts (SACs), aided by a variety of synthetic methods, characterization techniques, and computational modeling. The distinct capabilities of SACs for oxidation, hydrogenation, and electrocatalytic reactions have led to the optimization of activity and selectivity through composition variation. However, characterization methods such as infrared and X‐ray spectroscopy are incapable of direct observations at atomic scale. Advances in transmission electron microscopy (TEM) including aberration correction, monochromators, environmental TEM, and micro‐electro‐mechanical system based in situ holders have improved catalysis study, allowing researchers to peer into regimes previously unavailable, observing critical structural and chemical information at atomic scale. This review presents recent development and applications of TEM techniques to garner information about the location, bonding characteristics, homogeneity, and stability of SACs. Aberration corrected TEM imaging routinely achieves sub‐Ångstrom resolution to reveal the atomic structure of materials. TEM spectroscopy provides complementary information about local composition, chemical bonding, electronic properties, and atomic/molecular vibration with superior spatial resolution. In situ/operando TEM directly observe the evolution of SACs under reaction conditions. This review concludes with remarks on the challenges and opportunities for further development of TEM to study SACs.
Biswas, Somnath; Baker, L. Robert(
, Accounts of chemical research)
Extreme ultraviolet (XUV) light sources based on high
harmonic generation are enabling the development of novel
spectroscopic methods to help advance the frontiers of ultrafast
science and technology. In this account we discuss
the development of XUV-RA spectroscopy at near grazing
incident reflection geometry and highlight recent applications
of this method to study ultrafast electron dynamics at
surfaces. Measuring core-to-valence transitions with broadband,
femtosecond pulses of XUV light extends the benefits
of x-ray absorption spectroscopy to a laboratory tabletop by
providing a chemical fingerprint of materials, including the
ability to resolve individual elements with sensitivity to oxidation
state, spin state, carrier polarity, and coordination
geometry. Combining this chemical state sensitivity with
femtosecond time resolution provides new insight into the
material properties that govern charge carrier dynamics in
complex materials. It is well known that surface dynamics
differ significantly from equivalent processes in bulk materials,
and that charge separation, trapping, transport, and
recombination occurring uniquely at surfaces governs the efficiency
of numerous technologically relevant processes spanning
photocatalysis, photovoltaics, and information storage
and processing. Importantly, XUV-RA spectroscopy at near
grazing angle is also surface sensitive with a probe depth of
3 nm, providing a new window into electronic and structural
dynamics at surfaces and interfaces. Here we highlight
the unique capabilities and recent applications of XUVRA
spectroscopy to study photo-induced surface dynamics
in metal oxide semiconductors, including photocatalytic oxides
(Fe2O3, Co3O4 NiO, and CuFeO2) as well as photoswitchable
magnetic oxide (CoFe2O4). We first compare the
ultrafast electron self-trapping rates via small polaron formation
at the surface and bulk of Fe2O3 where we note that
the energetics and kinetics of this process differ significantly
at the surface. Additionally, we demonstrate the ability to
systematically tune this kinetics by molecular functionalization,
thereby, providing a route to control carrier transport
at surfaces. We also measure the spectral signatures
of charge transfer excitons with site specific localization of
both electrons and holes in a series of transition metal oxide
semiconductors (Fe2O3, NiO, Co3O4). The presence of
valence band holes probed at the oxygen L1-edge confirms
a direct relationship between the metal-oxygen bond covalency
and water oxidation efficiency. For a mixed metal oxide
CuFeO2 in the layered delafossite structure, XUV-RA
reveals that the sub-picosecond hole thermalization from O
2p to Cu 3d states of CuFeO2 leads to the spatial separation
of electrons and holes, resulting in exceptional photocatalytic
performance for H2 evolution and CO2 reduction
of this material. Finally, we provide an example to show the
ability of XUV-RA to probe spin state specific dynamics in a
the photo-switchable ferrimagnet, cobalt ferrite (CoFe2O4).
This study provides a detailed understating of ultrafast spin
switching in a complex magnetic material with site-specific
resolution. In summary, the applications of XUV-RA spectroscopy
demonstrated here illustrate the current abilities
and future promise of this method to extend molecule-level
understanding from well-defined photochemical complexes
to complex materials so that charge and spin dynamics at
surfaces can be tuned with the precision of molecular photochemistry.
Schultz, Jeremy F; Li, Linfei; Mahapatra, Sayantan; Jiang, Nan(
, Journal of Physics: Condensed Matter)
Abstract Surface-bound reactions have become a viable method to develop nanoarchitectures through bottom-up assembly with near atomic precision. However, the bottom-up fabrication of nanostructures on surfaces requires careful consideration of the intrinsic properties of the precursors and substrate as well as the complex interplay of any interactions that arise in the heterogeneous two-dimensional (2D) system. Therefore, it becomes necessary to consider these systems with characterization methods sensitive to such properties with suitable spatial resolution. Here, low temperature ultrahigh vacuum scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS) were used to investigate the formation of 2D covalent networks via coupling reactions of tetra(4-bromophenyl)porphyrin (Br 4 TPP) molecules on a Ag(100) substrate. Through the combination of STM topographic imaging and TERS vibrational fingerprints, the conformation of molecular precursors on the substrate was understood. Following the thermally activated coupling reaction, STM and TERS imaging confirm the covalent nature of the 2D networks and suggest that the apparent disorder arises from molecular flexibility.
Li, Linfei, Schultz, Jeremy F., Mahapatra, Sayantan, Lu, Zhongyi, Zhang, Xu, and Jiang, Nan.
"Chemically identifying single adatoms with single-bond sensitivity during oxidation reactions of borophene". Nature Communications 13 (1). Country unknown/Code not available: Nature Publishing Group. https://doi.org/10.1038/s41467-022-29445-8.https://par.nsf.gov/biblio/10365502.
@article{osti_10365502,
place = {Country unknown/Code not available},
title = {Chemically identifying single adatoms with single-bond sensitivity during oxidation reactions of borophene},
url = {https://par.nsf.gov/biblio/10365502},
DOI = {10.1038/s41467-022-29445-8},
abstractNote = {Abstract The chemical interrogation of individual atomic adsorbates on a surface significantly contributes to understanding the atomic-scale processes behind on-surface reactions. However, it remains highly challenging for current imaging or spectroscopic methods to achieve such a high chemical spatial resolution. Here we show that single oxygen adatoms on a boron monolayer (i.e., borophene) can be identified and mapped via ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) with ~4.8 Å spatial resolution and single bond (B–O) sensitivity. With this capability, we realize the atomically defined, chemically homogeneous, and thermally reversible oxidation of borophene via atomic oxygen in UHV. Furthermore, we reveal the propensity of borophene towards molecular oxygen activation at room temperature and phase-dependent chemical properties. In addition to offering atomic-level insights into the oxidation of borophene, this work demonstrates UHV-TERS as a powerful tool to probe the local chemistry of surface adsorbates in the atomic regime with widespread utilities in heterogeneous catalysis, on-surface molecular engineering, and low-dimensional materials.},
journal = {Nature Communications},
volume = {13},
number = {1},
publisher = {Nature Publishing Group},
author = {Li, Linfei and Schultz, Jeremy F. and Mahapatra, Sayantan and Lu, Zhongyi and Zhang, Xu and Jiang, Nan},
}
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