single-molecule | Ji Group@Renmin Univ.

Sub-Angstrom Imaging of Nondegenerate Kekulé Structures in a Two-Dimensional Halogen-Bonded Supramolecular Network

by JiGrp | Feb 28, 2022 | 2022, 本组论文

J. Phys. Chem. C 126, 4241–4247(2022)

Haohan Li, Mykola Telychko, Linwei Zhou, Zhi Chen, Xinnan Peng, Wei Ji, Jiong Lu & Kian Ping Loh

Abstract

Formation of a two-dimensional (2D) supramolecular self-assembly and a 2D organometallic framework derived from a brominated N-heterocyclic aromatic molecule (4Br-TAP) on Au(111) and Ag(111) substrates were studied using chemical bond-resolved scanning tunneling microscopy (STM) and noncontact atomic force microscopy (ncAFM) techniques combined with density functional theory (DFT) calculations. The 4Br-TAP-based 2D molecular framework on Au(111) is constructed by diverse Br···Br and Br···N noncovalent interactions, which are resolved with sub-angstrom resolution using combined STM and ncAFM imaging with a CO-functionalized tip and further quantified using DFT calculations. The distortion of molecular backbones, triggered by a highly nonuniform bonding environment, leads to lifting of the degeneracy of the intrinsic resonance structures of tetraazapyrene (TAP) moieties and emergence of two chiral Kekulé-like structures. In contrast, debromination of 4Br-TAP on Ag(111) leads to the formation of an ordered 2D organometallic framework linked by C–Ag–C bonds. Our results underpin the tremendous potential of the tip-functionalized ncAFM technique for microscopic identification of a complex interplay of intermolecular interactions and their associated impact on the molecular resonance structures.

Aggregation-Dependent Dielectric Permittivity in 2D Molecular Crystals

by JiGrp | Feb 17, 2022 | 2022, 本组论文

Small Methods, 6, 2101198 (2022)

Yutian Yang, Yingying Wang, Jingsi Qiao, Weiwei Zhao, Yuanfang Yu, Shaopeng Feng, Xuhong An, Jialin Zhang, Wei Ji, Xinran Wang, Junpeng Lu & Zhenhua Ni

Abstract

The functionality of 2D molecular crystal-based devices crucially depends on their intrinsic properties, such as molecular energy levels, light absorption efficiency, and dielectric permittivity, which are highly sensitive to molecular aggregation. Here, it is demonstrated that the dielectric permittivity of the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) molecular crystals on monolayer WS₂ substrates can be tuned from 4.62 in the wetting layer to 2.25 in the second layer. Its origin lies in the different molecular orientations in the wetting layer (lying-down) and in the subsequently stacked layers (standing-up), which lead to a positive Coulomb coupling (J_Coup) value (H-aggregation) and a negative J_Coup value (J-aggregation), respectively. Polarized optical contrast spectroscopy reveals that the permittivity of C₈-BTBT is anisotropic, and its direction is related to the underlying substrate. The study offers guidelines for future manipulation of the permittivity of 2D molecular crystals, which may promote their applications toward various electronic and optoelectronic devices.

Facet-Selective Dissociation and Radical-Mediated Reaction of Dibenzotetrathiafulvalene Molecules on Low-Index Copper Surfaces

by JiGrp | Jan 14, 2022 | 2022, 本组论文

J. Phys. Chem. C 126, 1281–1288 (2022)

Jun Zhang, Linwei Zhou, Pengcheng Chen, Bingkai Yuan, Zhihai Cheng, Wei Ji & Xiaohui Qiu

Abstract

Catalytic bond cleavage and formation of transient intermediates on metal substrates play an essential role in surface synthesis and heterogeneous catalysis. Previous studies usually focus on the bond-breaking process, whereas the knowledge regarding the construction of dissociative moieties that lead to the final products is limited. Here, we investigate the facet-selective dissociation of dibenzotetrathiafulvalene (DBTTF) molecules on Cu(110) and Cu(100) surfaces using low-temperature scanning tunneling microscopy and first-principles calculations. Atomic resolution images enable the identification of various intermediates and reaction pathways on different facets. The dissociation of DBTTF molecules generated 1,2-BDT–Cu complex chains on Cu(110), while phenyl diradical superstructure islands were observed on Cu(100). The various chemical species found on different Cu facets were explained in the context of their formation energies related to the spatial inhomogeneity of surface electronic states. Our results address the effects of electronic and geometrical diversities on the surface mobility of intermediate products and subsequent on-surface reaction pathways.

Band alignment and interlayer hybridization in monolayer organic/WSe2 heterojunction

by JiGrp | Jan 1, 2022 | 2022, 本组论文

Nano Research 15, 1276–1281 (2022)

Yanping Guo, Linlu Wu, Jinghao Deng, Linwei Zhou, Wei Jiang, Shuangzan Lu, Da Huo, Jiamin Ji, Yusong Bai, Xiaoyu Lin, Shunping Zhang, Hongxing Xu, Wei Ji & Chendong Zhang

Abstract

Semiconducting heterojunctions (HJs), comprised of atomically thin transition metal dichalcogenides (TMDs), have shown great potentials in electronic and optoelectronic applications. Organic/TMD hybrid bilayers hold enhanced pumping efficiency of interfacial excitons, tunable electronic structures and optical properties, and other superior advantages to these inorganic HJs. Here, we report a direct probe of the interfacial electronic structures of a crystalline monolayer (ML) perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA)/ML-WSe₂ HJ using scanning tunneling microscopy, photoluminescence, and first-principle calculations. Strong PTCDA/WSe₂ interfacial interactions lead to appreciable hybridization of the WSe₂ conduction band with PTCDA unoccupied states, accompanying with a significant amount of PTCDA-to-WSe₂ charge transfer (by 0.06 e/PTCDA). A type-II band alignment was directly determined with a valence band offset of ∼ 1.69 eV, and an apparent conduction band offset of ∼ 1.57 eV. Moreover, we found that the local stacking geometry at the HJ interface differentiates the hybridized interfacial states.

A Gd@C82 single-molecule electret

by JiGrp | Oct 12, 2020 | 2016-2020, Highlighted, 本组论文

Nat. Nanotechnol. 15, 1019–1024 (2020)

Kangkang Zhang, Cong Wang, Minhao Zhang, Zhanbin Bai, Fang-Fang Xie, Yuan-Zhi Tan, Yilv Guo, Kuo-Juei Hu, Lu Cao, Shuai Zhang, Xuecou Tu, Danfeng Pan, Lin Kang, Jian Chen, Peiheng Wu, Xuefeng Wang, Jinlan Wang, Junming Liu, You Song, Guanghou Wang, Fengqi Song, Wei Ji, Su-Yuan Xie, Su-Fei Shi, Mark A Reed & Baigeng Wang

Abstract

Electrets are dielectric materials that have a quasi-permanent dipole polarization. A single-molecule electret is a long-sought-after nanoscale component because it can lead to miniaturized non-volatile memory storage devices. The signature of a single-molecule electret is the switching between two electric dipole states by an external electric field. The existence of these electrets has remained controversial because of the poor electric dipole stability in single molecules. Here we report the observation of a gate-controlled switching between two electronic states in Gd@C₈₂. The encapsulated Gd atom forms a charged centre that sets up two single-electron transport channels. A gate voltage of ±11 V (corresponding to a coercive field of ~50 mV Å^–1) switches the system between the two transport channels with a ferroelectricity-like hysteresis loop. Using density functional theory, we assign the two states to two different permanent electrical dipole orientations generated from the Gd atom being trapped at two different sites inside the C₈₂ cage. The two dipole states are separated by a transition energy barrier of 11 meV. The conductance switching is then attributed to the electric-field-driven reorientation of the individual dipole, as the coercive field provides the necessary energy to overcome the transition barrier.