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Atomic force nanolithography was used to fabricate the submicrometric catalyst patterns which were subsequently verified by selective growth of carbon nanotubes (CNTs). The results demonstrate that Atomic force microscopy (AFM) nanoindentation of a polymer film is an effective lithographic technique for the realization of very suitable catalyst nanopatterns for subsequent chemical vapor deposition (CVD) growth of multiwalled CNTs.

The intermolecular band dispersion related to the highest occupied molecular orbital of epitaxial anthracene multilayer films on single-crystalline Bi(0001) has been measured using angle-resolved ultraviolet photoelectron spectroscopy. By comparing the dispersion to that of anthracene multilayers on Cu(110) [F. Bussolotti, Y. Yamada-Takamura, and R. Friedlein, Phys. Rev. B 80, 153402 (2009)], it is shown how the transfer integrals and the difference in on-site energies depend on lattice parameters and how this, in turn, affects the band curvature along high-symmetry directions. [ABSTRACT FROM AUTHOR]

Electronic and structural properties of thin, chemisorbed adenine films on Cu(110) have been investigated by ultraviolet photoelectron spectroscopy and reflection high-energy electron diffraction. Molecular reorientation at the particular coverage of about 0.3 ML from a disordered flat-lying to a long-range ordered standing-up phase is accompanied by pronounced changes of the interfacial dipole provided by the alignment of the intrinsic dipole moment of the polar molecule. Similar hybrid interfacial states are observed for ordered flat-lying and standing-up layers, suggesting comparable atomic site-specific N–Cu interactions. [ABSTRACT FROM AUTHOR]

The electronic and structural properties of copper phthalocyanine deposited onto multiwalled carbon nanotubes were studied as a function of the deposition thickness. The valence band and core level spectra were investigated using photoemission spectroscopy. A shift of the organic highest occupied molecular level toward the Fermi level was observed for very thin film, together with a small shift of the nitrogen and copper core level peak positions. Scanning electron microscope images showed the formation of small particles around the nanotubes, forming a thin overlayer decorating the nanotube walls. An x-ray diffraction analysis reported that these particles present both the α and β crystalline phases. [ABSTRACT FROM AUTHOR]

Developing future electronics will require aggressive scaling of the channel material thickness while maintaining device performance. Two-dimensional (2D) semiconductors are promising candidates to sustain further device scaling, but despite more than two decades of intense research, experimental performance continues to lag theoretical expectations. Here, we develop an oxygen-free approach to fabricate 2D field-effect transistors and push the electrical transport toward the theoretical phonon-limited intrinsic mobility. This approach achieves record carrier mobilities of 91 and 132 cm 2 V -1 s -1 for mono- and bilayer MoS 2 transistors on silicon oxide substrates. Statistical analysis of over 60 MoS 2 and WS 2 devices confirms that oxygen-free fabrication enhances device performance by more than an order of magnitude across key figures of merit. While previous studies suggest that 2D transition metal dichalcogenides such as MoS 2 and WS 2 are relatively stable in air, we show that even short-term ambient exposure can degrade their performance. We identify oxygen as a crucial factor in limiting transistor performance through irreversible chemisorption. This study emphasizes the criticality of avoiding oxygen exposure and offers guidance for device manufacturing that impacts fundamental research and practical applications of 2D materials.

Two-dimensional van der Waals semiconductors are promising for future nanoelectronics. However, integrating high-k gate dielectrics for device applications is challenging as the inert van der Waals material surfaces hinder uniform dielectric growth. Here, we report a liquid metal oxide-assisted approach to integrate ultrathin, high-k HfO 2 dielectric on 2D semiconductors with atomically smooth interfaces. Using this approach, we fabricated 2D WS 2 top-gated transistors with subthreshold swings down to 74.5 mV/dec, gate leakage current density below 10 -6 A/cm 2 , and negligible hysteresis. We further demonstrate a one-step van der Waals integration of contacts and dielectrics on graphene. This can offer a scalable approach toward integrating entire prefabricated device stack arrays with 2D materials. Our work provides a scalable solution to address the crucial dielectric engineering challenge for 2D semiconductor-based electronics.

Structural topology and symmetry of a two-dimensional (2D) network play pivotal roles in defining its electrical properties and functionalities. Here, a binary buckled honeycomb lattice with C 3v symmetry, which naturally hosts topological Dirac fermions and out-of-plane polarity, is proposed. It is successfully achieved in a group IV-V compound, namely monolayer SiP epitaxially grown on Ag(111) surface. Combining first-principles calculations with angle-resolved photoemission spectroscopy, the degeneration of the Dirac nodal lines to points due to the broken horizonal mirror symmetry is elucidated. More interesting, the SiP monolayer manifests metallic nature, which is mutually exclusive with polarity in conventional materials. It is further found that the out-of-plane polarity is strongly suppressed by the metallic substrate. This study not only represents a breakthrough of realizing intrinsic polarity in 2D metallic material via ingenious design but also provides a comprehensive understanding of the intricate interplay of many exotic low-dimensional quantum phenomena., (© 2024 Wiley‐VCH GmbH.)

Semiconducting transition metal dichalcogenides (TMDs) are a class of two-dimensional materials with potential applications in optoelectronics, spintronics, valleytronics, and quantum information processing. Understanding their stability under ambient conditions is critical for determining their in-air processability during device fabrication and for predicting their long-term device performance stability. While the effects of environmental conditions (i.e., oxygen, moisture, and light) on TMD degradation are well-acknowledged, the role of defects in driving their oxidation remains unclear. We conducted a systematic X-ray photoelectron spectroscopy study on WS 2 single crystals with different surface S-vacancy concentrations formed via controlled argon sputtering. Oxidation primarily occurred at defect concentrations ≥ 10%, resulting in stoichiometric WO 3 formation, while a stable surface was observed at lower concentrations. Theoretical calculations informed us that single S-vacancies do not spontaneously oxidize, while defect pairing at high vacancy concentrations facilitates O 2 dissociation and subsequent oxide formation. Our XPS results also point to vacancy-related structural and electrostatic disorder as the main origin for the p-type characteristics that persists even after oxidation. Despite the complex interplay between defects and TMD oxidation processes, our work unveils scientifically informed guidance for working effectively with TMDs.

The manipulation of edge configurations and structures in atomically-thin transition metal dichalcogenides (TMDs) for versatile functionalization has attracted intensive interest in recent years. The chemical vapor deposition (CVD) approach has shown promise for TMD edge engineering of atomic edge configurations (1H, 1T or 1T'-zigzag or armchair edges) as well as diverse edge morphologies (1D nanoribbons, 2D dendrites, 3D spirals, etc.). These edge-rich TMD layers offer versatile candidates for probing the physical and chemical properties and exploring potential applications in electronics, optoelectronics, catalysis, sensing, and quantum technologies. In this Review, we present an overview of the current state-of-the-art in the manipulation of TMD atomic edges and edge-rich structures using CVD. We highlight the vast range of distinct properties associated with these edge configurations and structures and provide insights into the opportunities afforded by such edge-functionalized crystals. The objective of this Review is to motivate further research and development efforts to use CVD as a scalable approach to harness the benefits of such crystal-edge engineering.

Two-dimensional (2D) semiconductors are promising channel materials for continued downscaling of complementary metal-oxide-semiconductor (CMOS) logic circuits. However, their full potential continues to be limited by a lack of scalable high- k dielectrics that can achieve atomically smooth interfaces, small equivalent oxide thicknesses (EOTs), excellent gate control, and low leakage currents. Here, large-area liquid-metal-printed ultrathin Ga 2 O 3 dielectrics for 2D electronics and optoelectronics are reported. The atomically smooth Ga 2 O 3 /WS 2 interfaces enabled by the conformal nature of liquid metal printing are directly visualized. Atomic layer deposition compatibility with high- k Ga 2 O 3 /HfO 2 top-gate dielectric stacks on a chemical-vapor-deposition-grown monolayer WS 2 is demonstrated, achieving EOTs of ∼1 nm and subthreshold swings down to 84.9 mV/dec. Gate leakage currents are well within requirements for ultrascaled low-power logic circuits. These results show that liquid-metal-printed oxides can bridge a crucial gap in dielectric integration of 2D materials for next-generation nanoelectronics.

Temperature-dependent transport measurements are performed on the same set of chemical vapor deposition (CVD)-grown WS 2 single- and bilayer devices before and after atomic layer deposition (ALD) of HfO 2 . This isolates the influence of HfO 2 deposition on low-temperature carrier transport and shows that carrier mobility is not charge impurity limited as commonly thought, but due to another important but commonly overlooked factor: interface roughness. This finding is corroborated by circular dichroic photoluminescence spectroscopy, X-ray photoemission spectroscopy, cross-sectional scanning transmission electron microscopy, carrier-transport modeling, and density functional modeling. Finally, electrostatic gate-defined quantum confinement is demonstrated using a scalable approach of large-area CVD-grown bilayer WS 2 and ALD-grown HfO 2 . The high dielectric constant and low leakage current enabled by HfO 2 allows an estimated quantum dot size as small as 58 nm. The ability to lithographically define increasingly smaller devices is especially important for transition metal dichalcogenides due to their large effective masses, and should pave the way toward their use in quantum information processing applications., (© 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Making electrical contacts to semiconducting transition metal dichalcogenides (TMDCs) represents a major bottleneck for high device performance, often manifesting as strong Fermi level pinning and high contact resistance. Despite intense ongoing research, the mechanism by which lattice defects in TMDCs impact the transport properties across the contact-TMDC interface remains unsettled. Here we study the impact of S-vacancies on the electronic properties at a MoS 2 monolayer interfaced with graphite by photoemission spectroscopy, where the defect density is selectively controlled by Ar sputtering. A clear reduction of the MoS 2 core level and valence band binding energies is observed as the defect density increases. The experimental results are explained in terms of (i) gap states' energy distribution and (ii) S-vacancies' electrostatic disorder effect. Our model indicates that the Fermi level pinning at deep S-vacancy gap states is the origin of the commonly reported large electron injection barrier (∼0.5 eV) at the MoS 2 ML interface with low work function metals. At the contact with high work function electrodes, S-vacancies do not significantly affect the hole injection barrier, which is intrinsically favored by Fermi level pinning at shallow occupied gap states. Our results clarify the importance of S-vacancies and electrostatic disorder in TMDC-based electronic devices, which could lead to strategies for optimizing device performance and production.

There have been several recent conflicting reports on the ferromagnetism of clean monolayer VSe 2 . Herein, the controllable formation of 1D defect line patterns in vanadium diselenide (VSe 2 ) monolayers initiated by thermal annealing is presented. Using scanning tunneling microscopy and q-plus atomic force microscopy techniques, the 1D line features are determined to be 8-member-ring arrays, formed via a Se deficient reconstruction process. The reconstructed VSe 2 monolayer with Se-deficient line defects displays room-temperature ferromagnetism under X-ray magnetic circular dichroism and magnetic force microscopy, consistent with the density functional theory calculations. This study possibly resolves the controversy on whether ferromagnetism is intrinsic in monolayer VSe 2 , and highlights the importance of controlling and understanding the atomic structures of surface defects in 2D crystals, which could play key roles in the material properties and hence potential device applications., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)