Zhihong Zhang, Linwei Zhou, Zhaoxi Chen* , Antonín Jaroš, Miroslav Kolíbal, Petr Bábor, Quanzhen Zhang, Changlin Yan, Ruixi Qiao, Qing Zhang, Teng Zhang, Wei Wei, Yi Cui, Jingsi Qiao, Liwei Liu, Lihong Bao, Haitao Yang, Zhihai Cheng, Yeliang Wang, Enge Wang, Zhi Liu, Marc Willinger, Hong-Jun Gao, Kaihui Liu*, Wei Ji*, and Zhu-Jun Wang*
Abstract:
Direct growth of large-area vertically stacked two-dimensional (2D) van der Waal (vdW) materials is a prerequisite for their high-end applications in integrated electronics, optoelectronics and photovoltaics. Currently, centimetre- to even metre-scale monolayers of single-crystal graphene (MLG) and hexagonal boron nitride (h-BN) have been achieved by epitaxial growth on various single-crystalline substrates. However, in principle, this success in monolayer epitaxy seems extremely difficult to be replicated to bi- or few-layer growth, as the full coverage of the first layer was believed to terminate the reactivity of those adopting catalytic metal surfaces. Here, we report an exceptional layer-by-layer chemical vapour deposition (CVD) growth of large size bi-layer graphene single-crystals, enabled by proximity catalytic activity from platinum (Pt) surfaces to the outermost graphene layers. In-situ growth and real-time surveillance experiments, under well-controlled environments, unambiguously verify that the growth does follow the layer-by-layer mode on open surfaces of MLG/Pt(111). First-principles calculations indicate that the transmittal of catalytic activity is allowed by an appreciable electronic hybridisation between graphene overlayers and Pt surfaces, enabling catalytic dissociation of hydrocarbons and subsequently direct graphitisation of their radicals on the outermost sp2 carbon surface. This proximity catalytic activity is also proven to be robust for tube-furnace CVD in fabricating single-crystalline graphene bi-, tri- and tetra-layers, as well as h-BN few-layers. Our findings offer an exceptional strategy for potential controllable, layer-by-layer and wafer-scale growth of vertically stacked few-layered 2D single crystals.
Many intriguing quantum states of matter, such as unconventional superconductivity, magnetic phases, and fractional quantum Hall physics, emerge from the spatially correlated localized electrons in the flat bands of solid materials. By using scanning tunneling microscopy and spectroscopy (STM/STS), we report on the real-space investigation of correlated electrons in the flat band of superlattice 4𝐻𝑏−TaSe𝑥S2−𝑥. In contrast with the pristine 4𝐻𝑏−TaS2, the selenium (Se) substitutions significantly affect the interfacial transfer of correlated electrons between the charge density wave (CDW) states of 1𝑇- and 1𝐻−TaS2 layers and contribute the real-space fractional electron-filling configurations with the distributed electron-filled and void Star of David (SoD) clusters of the 1𝑇 layer. The site-specific STS spectra directly reveal their respective prominent spectra weight above 𝐸F and symmetric Mott-like spectra. In addition, the spatial distributions of these electron-filled SoDs in the 1𝑇 layer of 4𝐻𝑏−TaSe0.7S1.3 demonstrate different local short-range order, clearly indicating the complex neighboring interactions among the localized electrons in the flat band of the 1𝑇 layer. Our results not only provide in-depth insight into correlated electrons in the flat CDW band but also provide a simple platform to manipulate the electron-correlation-related quantum states.
ZhiHao Zhang, Linlu Wu, Mao-Peng Miao, Hao-Jun Qin, Gan Chen, Min Cai, Lixin Liu, Lan-Fang Zhu, Wen-Hao Zhang, Tianyou Zhai, Wei Ji, and Ying Shuang Fu
Abstract:
Manipulating single electrons at the atomic scale is vital for mastering complex surface processes governed by the transfer of individual electrons. Polarons, comprised of electrons stabilized by electron-phonon coupling, offer a pivotal medium for such manipulation. Here, using scanning tunneling microscopy and spectroscopy (STM/STS) and density functional theory (DFT) calculations, we report the identification and manipulation of a new type of polaron, dubbed van der Waals (vdW) polaron, within mono- to tri-layer ultrathin films composed of Sb2O3 molecules that are bonded via vdW attractions. The Sb2O3 films were grown on a graphene-covered SiC(0001) substrate via molecular beam epitaxy. Unlike prior molecular polarons, STM imaging observed polarons at the interstitial sites of the molecular film, presenting unique electronic states and localized band bending. DFT calculations revealed the lowest conduction band as an intermolecular bonding state, capable of ensnaring an extra electron through locally diminished intermolecular distances, thereby forming an intermolecular vdW polaron. We also demonstrated the ability to generate, move, and erase such vdW polarons using an STM tip. Our work uncovers a new type of polaron stabilized by coupling with intermolecular vibrations where vdW interactions dominate, paving the way for designing atomic-scale electron transfer processes, and enabling precise tailoring of electron-related properties and functionalities.
Nanshu Liu, Cong Wang, Changlin Yan, Changsong Xu, Jun Hu, Yanning Zhang, and Wei Ji
Abstract:
Interlayer magnetism was tuned by many interlayer means, e.g., stacking, distance, and external fields in two-dimensional (2D) magnets. As an exception, the interlayer magnetism of CrSBr few layers was, however, experimentally changed by applied intralayer strains [Nat. Nanotechnol. 17, 256 (2022)], the mechanism of which is yet to be unveiled. Here, we uncovered its mechanism by investigating in-plane strained bilayer CrSBr using density functional theory calculations. Under in-plane tensile strain, wavefunction overlaps are strengthened for Br p electrons within each CrSBr layer, which delocalizes intralayer electrons and, as a consequence, promotes interlayer electron hopping. A negative interlayer Poisson’s ratio also enlarges interlayer spacing for bilayer CrSBr, which reduces the interlayer Pauli repulsion. This joint effect, further verified by examining interlayer sliding and interfacial element substitution, leads to an interlayer antiferromagnetic to ferromagnetic transition, consistent with the previous experimental observation. This mechanism enables a route to tune interlayer magnetism by modifying intralayer electron localization in 2D magnets.
Nanshu Liu, Cong Wang, Changlin Yan, Changsong Xu, Jun Hu, Yanning Zhang, and Wei Ji
Abstract:
A recent experiment reported type-II multiferroicity in monolayer (ML) NiI2 based on a presumed spiral magnetic configuration (Spiral-B), which is, as we found here, under debate in the ML limit. Freestanding ML NiI2 breaks its C3 symmetry, as it prefers a striped antiferromagnetic order (AABB-AFM) along with an intralayer antiferroelectric (AFE) order. However, substrate confinement may preserve the C3 symmetry and/or apply tensile strain to the ML. This leads to another spiral magnetic order (SpiralIVX), while 2L shows a different order (SpiralVX) and Spiral-B dominates in thicker layers. Thus, three multiferroic phases, namely, SpiralB+FE, Spiral-IVX +FE, Spiral-VX+FE, and an anti-multiferroic AABB-AFM+AFE one, show layer-thickness-dependent and geometry-dependent dominance, ascribed to competitions among thickness-dependent Kitaev, biquadratic, and Heisenberg spin–exchange interactions and single-ion magnetic anisotropy. Our theoretical results clarify the debate on the multiferroicity of ML NiI2 and shed light on the role of layer-stacking-induced changes in noncollinear spin–exchange interactions and magnetic anisotropy in thickness-dependent magnetism.
