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1.
Science ; 377(6603): 315-319, 2022 07 15.
Artigo em Inglês | MEDLINE | ID: mdl-35857595

RESUMO

The nature of hydrated proton on solid surfaces is of vital importance in electrochemistry, proton channels, and hydrogen fuel cells but remains unclear because of the lack of atomic-scale characterization. We directly visualized Eigen- and Zundel-type hydrated protons within the hydrogen bonding water network on Au(111) and Pt(111) surfaces, using cryogenic qPlus-based atomic force microscopy under ultrahigh vacuum. We found that the Eigen cations self-assembled into monolayer structures with local order, and the Zundel cations formed long-range ordered structures stabilized by nuclear quantum effects. Two Eigen cations could combine into one Zundel cation accompanied with a simultaneous proton transfer to the surface. Moreover, we revealed that the Zundel configuration was preferred over the Eigen on Pt(111), and such a preference was absent on Au(111).

2.
Nano Lett ; 22(12): 4661-4668, 2022 Jun 22.
Artigo em Inglês | MEDLINE | ID: mdl-35640103

RESUMO

Confined nanospaces provide a new platform to promote catalytic reactions. However, the mechanism of catalytic enhancement in the nanospace still requires insightful exploration due to the lack of direct visualization. Here, we report operando investigations on the etching and growth of graphene in a two-dimensional (2D) confined space between graphene and a Cu substrate. We observed that the graphene layer between the Cu and top graphene layer was surprisingly very active in etching (more than 10 times faster than the etching of the top graphene layer). More strikingly, at a relatively low temperature (∼530 °C), the etched carbon radicals dissociated from the bottom layer, in turn feeding the growth of the top graphene layer with a very high efficiency. Our findings reveal the in situ dynamics of the anomalous confined catalytic processes in 2D confined spaces and thus pave the way for the design of high-efficiency catalysts.

3.
Adv Mater ; : e2201120, 2022 Apr 25.
Artigo em Inglês | MEDLINE | ID: mdl-35470492

RESUMO

2D metal carbides and nitrides (MXene) are promising candidates for electromagnetic (EM) shielding, saturable absorption, thermal therapy, and photocatalysis owing to their excellent EM absorption. The plasmon resonances in metallic MXene micro/nanostructures may play an important role in enhancing the EM absorption; however, their contribution has not been determined due to the lack of a precise understanding of its plasmon behavior. Here, the use of high-spatial-resolution electron energy-loss spectroscopy to measure the plasmon dispersion of MXene films with different thicknesses is reported, enabling accurate analysis of the EM absorption of complex MXene structures in a wide frequency range via a theoretical model. The EM absorption of MXene can be excited at the desired frequency by controlling the momentum (e.g., the sizes of the nanoflakes for EM excitation) as the strength can be enhanced by increasing the layer number and the interlayer distance in MXene. For example, a 3 nm interlayer distance can nearly double the plasmon-enhanced EM absorption in MXene nanostructures. These findings can guide the design of advanced ultrathin EM absorption materials for a broad range of applications.

4.
Nano Lett ; 22(7): 2725-2733, 2022 Apr 13.
Artigo em Inglês | MEDLINE | ID: mdl-35293751

RESUMO

In van der Waals (vdW) heterostructures, the interlayer electron-phonon coupling (EPC) provides one unique channel to nonlocally engineer these elementary particles. However, limited by the stringent occurrence conditions, the efficient engineering of interlayer EPC remains elusive. Here we report a multitier engineering of interlayer EPC in WS2/boron nitride (BN) heterostructures, including isotope enrichments of BN substrates, temperature, and high-pressure tuning. The hyperfine isotope dependence of Raman intensities was unambiguously revealed. In combination with theoretical calculations, we anticipate that WS2/BN supercells could induce Brillouin-zone-folded phonons that contribute to the interlayer coupling, leading to a complex nature of broad Raman peaks. We further demonstrate the significance of a previously unexplored parameter, the interlayer spacing. By varying the temperature and high pressure, we effectively manipulated the strengths of EPC with on/off capabilities, indicating critical thresholds of the layer-layer spacing for activating and strengthening interlayer EPC. Our findings provide new opportunities to engineer vdW heterostructures with controlled interlayer coupling.

5.
Nat Commun ; 13(1): 1007, 2022 Feb 23.
Artigo em Inglês | MEDLINE | ID: mdl-35197463

RESUMO

The precise precursor supply is a precondition for controllable growth of two-dimensional (2D) transition metal dichalcogenides (TMDs). Although great efforts have been devoted to modulating the transition metal supply, few effective methods of chalcogen feeding control were developed. Here we report a strategy of using active chalcogen monomer supply to grow high-quality TMDs in a robust and controllable manner, e.g., MoS2 monolayers perform representative photoluminescent circular helicity of ~92% and electronic mobility of ~42 cm2V-1s-1. Meanwhile, a uniform quaternary TMD alloy with three different anions, i.e., MoS2(1-x-y)Se2xTe2y, was accomplished. Our mechanism study revealed that the active chalcogen monomers can bind and diffuse freely on a TMD surface, which enables the effective nucleation, reaction, vacancy healing and alloy formation during the growth. Our work offers a degree of freedom for the controllable synthesis of 2D compounds and their alloys, benefiting the development of high-end devices with desired 2D materials.

6.
Nano Lett ; 22(5): 1858-1865, 2022 Mar 09.
Artigo em Inglês | MEDLINE | ID: mdl-35174707

RESUMO

Understanding the origin of charge-density wave (CDW) instability is important for manipulating novel collective electronic states. Many layered transition metal dichalcogenides (TMDs) share similarity in the structural and electronic instability, giving rise to diverse CDW phases and superconductivity. It is still puzzling that even isostructural and isoelectronic TMDs show distinct CDW features. For instance, bulk NbSe2 exhibits CDW order at low temperature, while bulk NbS2 displays no CDW instability. The CDW transitions in single-layer NbS2 and NbSe2 are also different. In the classic limit, we investigate the electron correlation effects on the dimensionality dependence of the CDW ordering. By performing ab initio path integral molecular dynamics simulations and comparative analyses, we further revealed significant nuclear quantum effects in these systems. Specifically, the quantum motion of sulfur anions significantly reduces the CDW transition temperature in both bulk and single-layer NbS2, resulting in distinct CDW features in the NbS2 and NbSe2 systems.

7.
Nat Nanotechnol ; 17(1): 33-38, 2022 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-34782776

RESUMO

The growth of wafer-scale single-crystal two-dimensional transition metal dichalcogenides (TMDs) on insulating substrates is critically important for a variety of high-end applications1-4. Although the epitaxial growth of wafer-scale graphene and hexagonal boron nitride on metal surfaces has been reported5-8, these techniques are not applicable for growing TMDs on insulating substrates because of substantial differences in growth kinetics. Thus, despite great efforts9-20, the direct growth of wafer-scale single-crystal TMDs on insulating substrates is yet to be realized. Here we report the successful epitaxial growth of two-inch single-crystal WS2 monolayer films on vicinal a-plane sapphire surfaces. In-depth characterizations and theoretical calculations reveal that the epitaxy is driven by a dual-coupling-guided mechanism, where the sapphire plane-WS2 interaction leads to two preferred antiparallel orientations of the WS2 crystal, and sapphire step edge-WS2 interaction breaks the symmetry of the antiparallel orientations. These two interactions result in the unidirectional alignment of nearly all the WS2 islands. The unidirectional alignment and seamless stitching of WS2 islands are illustrated via multiscale characterization techniques; the high quality of WS2 monolayers is further evidenced by a photoluminescent circular helicity of ~55%, comparable to that of exfoliated WS2 flakes. Our findings offer the opportunity to boost the production of wafer-scale single crystals of a broad range of two-dimensional materials on insulators, paving the way to applications in integrated devices.

8.
Nature ; 599(7885): 399-403, 2021 11.
Artigo em Inglês | MEDLINE | ID: mdl-34789901

RESUMO

The breakdown of translational symmetry at heterointerfaces leads to the emergence of new phonon modes localized at the interface1. These modes have an essential role in thermal and electrical transport properties in devices, especially in miniature ones wherein the interface may dominate the entire response of the device2. Although related theoretical work began decades ago1,3-5, experimental research is totally absent owing to challenges in achieving the combined spatial, momentum and spectral resolutions required to probe localized modes. Here, using the four-dimensional electron energy-loss spectroscopy technique, we directly measure both the local vibrational spectra and the interface phonon dispersion relation for an epitaxial cubic boron nitride/diamond heterointerface. In addition to bulk phonon modes, we observe modes localized at the interface and modes isolated from the interface. These features appear only within approximately one nanometre around the interface. The localized modes observed here are predicted to substantially affect the interface thermal conductance and electron mobility. Our findings provide insights into lattice dynamics at heterointerfaces, and the demonstrated experimental technique should be useful in thermal management, electrical engineering and topological phononics.

9.
Nano Lett ; 21(15): 6449-6455, 2021 Aug 11.
Artigo em Inglês | MEDLINE | ID: mdl-34279962

RESUMO

To elucidate the nature of light-driven photocatalytic water splitting, a polymeric semiconductor-graphitic carbon nitride (g-C3N4)-has been chosen as a prototype substrate for studying atomistic water spitting processes in realistic environments. Our nonadiabatic quantum dynamics simulations based on real-time time-dependent density functional theory reveal explicitly the transport channel of photogenerated charge carriers at the g-C3N4/water interface, which shows a strong correlation to bond re-forming. A three-step photoreaction mechanism is proposed, whereas the key roles of hole-driven hydrogen transfer and interfacial water configurations were identified. Immediately following photocatalytic water splitting, atomic pathways for the two dissociated hydrogen atoms approaching each other and forming the H2 gas molecule are demonstrated, while the remanent OH radicals may form intermediate products (e.g., H2O2). These results provide critical new insights for the characterization and further development of efficient water-splitting photocatalysts from a dynamic perspective.

10.
Adv Mater ; 33(29): e2006836, 2021 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-34096113

RESUMO

Macromolecular films are crucial functional materials widely used in the fields of mechanics, electronics, optoelectronics, and biology, due to their superior properties of chemical stability, small density, high flexibility, and solution-processing ability. Their electronic and mechanical properties, however, are typically much lower than those of crystalline materials, as the macromolecular films have no long-range structural ordering. The state-of-the-art for producing highly ordered macromolecular films is still facing a great challenge due to the complex interactions between adjacent macromolecules. Here, the growth of textured macromolecular films on a designed graphene/high-index copper (Cu) surface is demonstrated. This successful growth is driven by a patterned potential that originates from the different amounts of charge transfer between the graphene and Cu surfaces with, alternately, terraces and step edges. The textured films exhibit a remarkable improvement in remnant ferroelectric polarization and fracture strength. It is also demonstrated that this growth mechanism is universal for different macromolecules. As meter-scale graphene/high-index Cu substrates have recently become available, the results open a new regime for the production and applications of highly ordered macromolecular films with obvious merits of high production and low cost.

11.
J Chem Phys ; 154(16): 164302, 2021 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-33940817

RESUMO

Accurate ab initio calculations of 3d transition metal monoxide molecules have attracted extensive attention because of their relevance in physical and chemical science as well as theoretical challenges in treating strong electron correlation. Meanwhile, recent years have witnessed the rapid development of the full configuration interaction quantum Monte Carlo (FCIQMC) method to tackle electron correlation. In this study, we carry out FCIQMC simulations to ScO, TiO, and VO molecules and obtain accurate descriptions of 13 low-lying electronic states (ScO 2Σ+, 2Δ, 2Π; TiO 3Δ, 1Δ, 1Σ+, 3Π, 3Φ; VO 4Σ-, 4Φ, 4Π, 2Γ, 2Δ), including states that have significant multi-configurational character. The FCIQMC results are used to assess the performance of several other wave function theory and density functional theory methods. Our study highlights the challenging nature of the electronic structure of transition metal oxides and demonstrates FCIQMC as a promising technique going forward to treat more complex transition metal oxide molecules and materials.

12.
Phys Rev Lett ; 126(13): 136001, 2021 Apr 02.
Artigo em Inglês | MEDLINE | ID: mdl-33861106

RESUMO

The fundamental understanding of crystallization, in terms of microscopic kinetic and thermodynamic details, remains a key challenge in the physical sciences. Here, by using in situ graphene liquid cell transmission electron microscopy, we reveal the atomistic mechanism of NaCl crystallization from solutions confined within graphene cells. We find that rock salt NaCl forms with a peculiar hexagonal morphology. We also see the emergence of a transitory graphitelike phase, which may act as an intermediate in a two-step pathway. With the aid of density functional theory calculations, we propose that these observations result from a delicate balance between the substrate-solute interaction and thermodynamics under confinement. Our results highlight the impact of confinement on both the kinetics and thermodynamics of crystallization, offering new insights into heterogeneous crystallization theory and a potential avenue for materials design.

13.
Rev Sci Instrum ; 92(1): 013704, 2021 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-33514196

RESUMO

Exploring the corresponding relation between structural and physical properties of materials at the atomic scale remains the fundamental problem in science. With the development of the aberration-corrected transmission electron microscopy (AC-TEM) and the ultrafast optical spectroscopy technique, sub-angstrom-scale spatial resolution and femtosecond-scale temporal resolution can be achieved, respectively. However, the attempt to combine both their advantages is still a great challenge. Here, we develop in situ optical spectroscopy with high temporal resolution in AC-TEM by utilizing a self-designed and manufactured TEM specimen holder, which has the capacity of sub-angstrom-scale spatial resolution and femtosecond-scale temporal resolution. The key and unique design of our apparatus is the use of the fiber bundle, which enables the delivery of focused pulse beams into TEM and collection of optical response simultaneously. The generated focused spot has a size less than 2 µm and can be scanned in plane with an area larger than 75 × 75 µm2. Most importantly, the positive group-velocity dispersion caused by glass fiber is compensated by a pair of diffraction gratings, thus resulting in the generation of pulse beams with a pulse width of about 300 fs (@ 3 mW) in TEM. The in situ experiment, observing the atomic structure of CdSe/ZnS quantum dots in AC-TEM and obtaining the photoluminescence lifetime (∼4.3 ns) in the meantime, has been realized. Further ultrafast optical spectroscopy with femtosecond-scale temporal resolution could be performed in TEM by utilizing this apparatus.

14.
Nat Mater ; 20(1): 43-48, 2021 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-32807920

RESUMO

Phonon polaritons enable light confinement at deep subwavelength scales, with potential technological applications, such as subdiffraction imaging, sensing and engineering of spontaneous emission. However, the trade-off between the degree of confinement and the excitation efficiency of phonon polaritons prevents direct observation of these modes in monolayer hexagonal boron nitride (h-BN), where they are expected to reach ultrahigh confinement. Here, we use monochromatic electron energy-loss spectroscopy (about 7.5 meV energy resolution) in a scanning transmission electron microscope to measure phonon polaritons in monolayer h-BN, directly demonstrating the existence of these modes as the phonon Reststrahlen band (RS) disappears. We find phonon polaritons in monolayer h-BN to exhibit high confinement (>487 times smaller wavelength than that of light in free space) and ultraslow group velocity down to about 10-5c. The large momentum compensation provided by electron beams additionally allows us to excite phonon polaritons over nearly the entire RS band of multilayer h-BN. These results open up a broad range of opportunities for the engineering of metasurfaces and strongly enhanced light-matter interactions.

15.
Phys Rev Lett ; 125(10): 106001, 2020 Sep 04.
Artigo em Inglês | MEDLINE | ID: mdl-32955332

RESUMO

Understanding the hydration and diffusion of ions in water at the molecular level is a topic of widespread importance. The ammonium ion (NH_{4}^{+}) is an exemplar system that has received attention for decades because of its complex hydration structure and relevance in industry. Here we report a study of the hydration and the rotational diffusion of NH_{4}^{+} in water using ab initio molecular dynamics simulations and quantum Monte Carlo calculations. We find that the hydration structure of NH_{4}^{+} features bifurcated hydrogen bonds, which leads to a rotational mechanism involving the simultaneous switching of a pair of bifurcated hydrogen bonds. The proposed hydration structure and rotational mechanism are supported by existing experimental measurements, and they also help to rationalize the measured fast rotation of NH_{4}^{+} in water. This study highlights how subtle changes in the electronic structure of hydrogen bonds impacts the hydration structure, which consequently affects the dynamics of ions and molecules in hydrogen bonded systems.

16.
Phys Rev Lett ; 124(20): 206801, 2020 May 22.
Artigo em Inglês | MEDLINE | ID: mdl-32501065

RESUMO

Understanding the nonequilibrium dynamics of photoexcited polarons at the atomic scale is of great importance for improving the performance of photocatalytic and solar-energy materials. Using a pulsed-laser-combined scanning tunneling microscopy and spectroscopy, here we succeeded in resolving the relaxation dynamics of single polarons bound to oxygen vacancies on the surface of a prototypical photocatalyst, rutile TiO_{2}(110). The visible-light excitation of the defect-derived polarons depletes the polaron states and leads to delocalized free electrons in the conduction band, which is further corroborated by ab initio calculations. We found that the trapping time of polarons becomes considerably shorter when the polaron is bound to two surface oxygen vacancies than that to one. In contrast, the lifetime of photogenerated free electrons is insensitive to the atomic-scale distribution of the defects but correlated with the averaged defect density within a nanometer-sized area. Those results shed new light on the photocatalytically active sites at the metal-oxide surface.

17.
J Chem Phys ; 152(23): 234301, 2020 Jun 21.
Artigo em Inglês | MEDLINE | ID: mdl-32571057

RESUMO

The hydrogen-bonding networks of water have strong intra- and intermolecular vibrational coupling which influences the energy dissipation and proton transfer in water. Disentangling and quantitative characterization of different coupling effects in water at a single-molecular level still remains a great challenge. Using tip-enhanced inelastic electron tunneling spectroscopy (IETS) based on low-temperature scanning tunneling microscopy, we report the direct quantitative assessment of the intermolecular coupling constants of the OH-stretch vibrational bands of an isolated water tetramer adsorbed on a Au(111)-supported NaCl(001) bilayer film. This is achieved by distinguishing various coupled modes of the H-bonded O-H stretching vibrations through tip-height dependent IET spectra. In contrast, such vibrational coupling is negligible in the half-deuterated water tetramer owing to the large energy mismatch between the OH and OD stretching modes. Not only do these findings advance our understanding on the effects of local environment on the intermolecular vibrational coupling in water, but also open up a new route for vibrational spectroscopic studies of extended H-bonded network at the single-molecular level.

18.
Nature ; 581(7809): 406-410, 2020 05.
Artigo em Inglês | MEDLINE | ID: mdl-32461648

RESUMO

The production of large single-crystal metal foils with various facet indices has long been a pursuit in materials science owing to their potential applications in crystal epitaxy, catalysis, electronics and thermal engineering1-5. For a given metal, there are only three sets of low-index facets ({100}, {110} and {111}). In comparison, high-index facets are in principle infinite and could afford richer surface structures and properties. However, the controlled preparation of single-crystal foils with high-index facets is challenging, because they are neither thermodynamically6,7 nor kinetically3 favourable compared to low-index facets6-18. Here we report a seeded growth technique for building a library of single-crystal copper foils with sizes of about 30 × 20 square centimetres and more than 30 kinds of facet. A mild pre-oxidation of polycrystalline copper foils, followed by annealing in a reducing atmosphere, leads to the growth of high-index copper facets that cover almost the entire foil and have the potential of growing to lengths of several metres. The creation of oxide surface layers on our foils means that surface energy minimization is not a key determinant of facet selection for growth, as is usually the case. Instead, facet selection is dictated randomly by the facet of the largest grain (irrespective of its surface energy), which consumes smaller grains and eliminates grain boundaries. Our high-index foils can be used as seeds for the growth of other Cu foils along either the in-plane or the out-of-plane direction. We show that this technique is also applicable to the growth of high-index single-crystal nickel foils, and we explore the possibility of using our high-index copper foils as substrates for the epitaxial growth of two-dimensional materials. Other applications are expected in selective catalysis, low-impedance electrical conduction and heat dissipation.

19.
Nature ; 577(7788): 60-63, 2020 01.
Artigo em Inglês | MEDLINE | ID: mdl-31894149

RESUMO

The formation and growth of water-ice layers on surfaces and of low-dimensional ice under confinement are frequent occurrences1-4. This is exemplified by the extensive reporting of two-dimensional (2D) ice on metals5-11, insulating surfaces12-16, graphite and graphene17,18 and under strong confinement14,19-22. Although structured water adlayers and 2D ice have been imaged, capturing the metastable or intermediate edge structures involved in the 2D ice growth, which could reveal the underlying growth mechanisms, is extremely challenging, owing to the fragility and short lifetime of those edge structures. Here we show that noncontact atomic-force microscopy with a CO-terminated tip (used previously to image interfacial water with minimal perturbation)12, enables real-space imaging of the edge structures of 2D bilayer hexagonal ice grown on a Au(111) surface. We find that armchair-type edges coexist with the zigzag edges usually observed in 2D hexagonal crystals, and freeze these samples during growth to identify the intermediate edge structures. Combined with simulations, these experiments enable us to reconstruct the growth processes that, in the case of the zigzag edge, involve the addition of water molecules to the existing edge and a collective bridging mechanism. Armchair edge growth, by contrast, involves local seeding and edge reconstruction and thus contrasts with conventional views regarding the growth of bilayer hexagonal ices and 2D hexagonal matter in general.


Assuntos
Gelo , Microscopia de Tunelamento , Cristalização
20.
Front Chem ; 7: 626, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31572715

RESUMO

The structure and dynamics of interfacial water, determined by the water-interface interactions, are important for a wide range of applied fields and natural processes, such as water diffusion (Kim et al., 2013), electrochemistry (Markovic, 2013), heterogeneous catalysis (Over et al., 2000), and lubrication (Zilibotti et al., 2013). The precise understanding of water-interface interactions largely relies on the development of atomic-scale experimental techniques (Guo et al., 2014) and computational methods (Hapala et al., 2014b). Scanning probe microscopy has been extensively applied to probe interfacial water in many interdisciplinary fields (Ichii et al., 2012; Shiotari and Sugimoto, 2017; Peng et al., 2018a). In this perspective, we review the recent progress in the noncontact atomic force microscopy (nc-AFM) imaging and AFM simulation techniques and discuss how the newly developed techniques are applied to study the properties of interfacial water. The nc-AFM with the quadrupole-like CO-terminated tip can achieve ultrahigh-resolution imaging of the interfacial water on different surfaces, trace the reconstruction of H-bonding network and determine the intrinsic structures of the weakly bonded water clusters and even their metastable states. In the end, we present an outlook on the directions of future AFM studies of interfacial water as well as the challenges faced by this field.

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