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.
Xuanhao Wu, Mengmeng Niu, Xin Tian, Xiaoyan Peng, Pio John S. Buenconsej, Xu Wu, Yeliang Wang, Wei Ji, Yi Li, Jingsi Qiao,Jifang Tao, Mingming Zhang, Song Xiaof and Hongye Yuan
Abstract:
Developing sensitive metal–organic framework (MOF) systems to overcome the ubiquitous trade-off between porosity and conductivity remains a formidable yet sought-after endeavor. This pursuit is of great significance for the development of MOF-based chemiresistive sensors with enhanced sensitivity and selectivity. Herein, we present an innovative template assisted strategy that utilizes the two-dimensional properties and good conductivity of MXene nanosheets, as well as lattice matching between MXene (Nb2C) and selected Ni3(HITP)2, to achieve controllable self-assembly of Ni3(HITP)2 on MXene sheets. This results in Ni3(HITP)2/MXene (HITP: 2,3,6,7,10,11-hexaaminotriphenylene) heterostructures with considerable conductivity, porosity, and solution processability. The powder and film electrical conductivity are 4.8 × 103 and 5.3 × 105 S m−1, respectively, and the BET specific surface area can reach 797.8 m2 g−1. It is worth noting that excellent solution processability helps to prepare large-area films (23 cm × 9 cm) with good uniformity. Gas sensors based on Ni3(HITP)2/MXene heterostructures exhibit high sensitivity (LOD ∼ 5 ppb) and selectivity towards ultratrace ethanol at room temperature, setting a new benchmark. Such sensing behavior stems from the strong coupling of Ni3(HITP)2/MXene heterostructures and their enhanced interaction with ethanol, evidenced by experimental results and theoretical calculations. Real-time respiratory sensing assessments underscore their practicality in healthcare monitoring. This straightforward approach simplifies the integration of MOF-related materials on miniaturized devices with outstanding performance.
Hafnia-based ferroelectric materials, like Hf0.5Zr0.5O2 (HZO), have received tremendous attention owing to their potentials for building ultra-thin ferroelectric devices. The orthorhombic(O)-phase of HZO is ferroelectric but metastable in its bulk form under ambient conditions, which poses a considerable challenge to maintaining the operation performance of HZO-based ferroelectric devices. Here, we theoretically addressed this issue that provides parameter spaces for stabilizing the O-phase of HZO thin-films under various conditions. Three mechanisms were found to be capable of lowering the relative energy of the O-phase, namely, more significant surface-bulk portion of (111) surfaces, compressive caxis strain, and positive electric fields. Considering these mechanisms, we plotted two ternary phase diagrams for HZO thin-films where the strain was applied along the in-plane uniaxial and biaxial, respectively. These diagrams indicate the O-phase could be stabilized by solely shrinking the film-thickness below 12.26 nm, ascribed to its lower surface energies. All these results shed considerable light on designing more robust and higher-performance ferroelectric devices.
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.
The intrinsic superlattice magnetic topological insulators of MnBi2Te4(Bi2Te3)𝑛 (𝑛=0,1,2…) provides a promising material platform for the realization of diverse exotic topological quantum states, such as quantum anomalous Hall effect and axion-insulator state. All these quantum states are sensitively dependent on the complex interplay and intertwinement of their band topology, magnetism, and defective structural details. Here, we report a comprehensive real-space investigation on the magnetic ordering states of MnBi2Te4(Bi2Te3)𝑛 using cryogenic magnetic force microscopy. The MnBi2Te4(Bi2Te3)𝑛 crystals exhibit a distinctive magnetic evolution from A-type antiferromagnetic to ferromagnetic states via the increased Bi2Te3 intercalation layers. The magnetic field- and temperature-dependent phase evolution behaviors of MnBi6Te10 and MnBi8Te13 are comparatively investigated to obtain the complete 𝐻−𝑇 phase diagrams. The combination impact of the intrinsic and defect-mediated interlayer coupling on their magnetic states were further discussed. Our results pave a possible way to realize more exotic quantum states via the tunable magnetic configurations in the artificial-stacking MnBi2Te4(Bi2Te3)𝑛 multilayers.
