Yunli Da (笪蕴力)†, Ruichun Luo (罗瑞春)†, Bao Lei (雷宝), Wei Ji (季威) and Wu Zhou (周武)*
Abstract:
The design and preparation of novel quantum materials with atomic precision are crucial for exploring new physics and for device applications. Electron irradiation has been demonstrated as an effective method for preparing novel quantum materials and quantum structures that could be challenging to obtain otherwise. It features the advantages of precise control over the patterning of such new materials and their integration with other materials with different functionalities. Here, we present a new strategy for fabricating freestanding monolayer SiC within nanopores of a graphene membrane. By regulating the energy of the incident electron beam and the in-situ heating temperature in a scanning transmission electron microscope (STEM), we can effectively control the patterning of nanopores and subsequent growth of monolayer SiC within the graphene lattice. The resultant SiC monolayers seamlessly connect with the graphene lattice, forming a planar structure distinct by a wide direct bandgap. Our in-situ STEM observations further uncover that the growth of monolayer SiC within the graphene nanopore is driven by a combination of bond rotation and atom extrusion, providing new insights into the atom-by-atom self-assembly of freestanding two-dimensional (2D) monolayers.
Nanshu Liu, Cong Wang, Changlin Yan, Changsong Xu, Jun Hu, Yanning Zhang, and Wei Ji
Abstract:
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.
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.
