Chemical Doping Reveals Band-like Charge Transport at Grain Boundaries in Organic Transistors

Chemical Doping Reveals Band-like Charge Transport at Grain Boundaries in Organic Transistors

Yating Li, Mengmeng Niu, Junpeng Zeng, Quan Zhou, Xu Wu, Wei Ji, Yeliang Wang, Ren Zhu, Jingsi Qiao, Jianbin Xu, Yi Shi, Xinran Wang, and Daowei He

An in-depth understanding of the electronic structure of 2H-MoTe2 at the atomic layer limit is a crucial step towards its exploitation in nanoscale devices. Here, we show that millimeter-sized monolayer (ML) MoTe2 samples, as well as smaller sized bilayer (BL) samples, can be obtained using the mechanical exfoliation technique. The electronic structure of these materials is investigated by Angle-Resolved Photoemission Spectroscopy (ARPES) for the first time and Density Functional
Theory (DFT) calculations. The comparison between experiments and theory allows us to describe ML MoTe2 as a semiconductor with direct gap at K point. This scenario is reinforced by the experimental observation of the conduction band minimum at K in Rb-doped ML MoTe2, resulting in a gap of at least 0.924 eV. In the BL MoTe2 system the maxima of the bands at Γ and K display very similar energies, thus leaving the door open to a direct gap scenario, in analogy to WSe2. The monotonic increase in the separation between spin-split bands at K while moving from ML to BL and bulk-like MoTe2 is attributed to interlayer coupling. Our findings can be considered as a reference to understand Quantum Anomalous and Fractional Quantum Anomalous Hall Effects recently discovered in ML and BL MoTe2 based moir´ e heterostructures. Organic semiconductors are highly promising as channel materials for energy-efficient, cost-effective, and flexible electronics. However, grain boundaries (GBs) can cause significant device performance variation, posing a major challenge for the development of high-performance organic circuits. In this work, we effectively passivated GB-induced traps in monolayer organic thin-film transistors (OTFTs) via p-type doping with the organic salt TrTPFB. The doping strategy broadens the mobility edge, effectively shielding GB-induced energy barriers and Coulomb scattering, and promotes deeper nonlocalized hybridization states for conduction. Consequently, the charge transport mechanism transitions from multiple trapping and release (MTR) to a more band-like behavior, even when GBs are present within the device channel. The doped OTFTs demonstrate ultralow mobility variation (1.4%) and threshold voltage variation (4.9%), as well as record-low contact resistant of RC = 0.6 Ω·cm, outperforming most single-crystal technologies. These performance metrics render doped monolayer polycrystalline films highly promising candidates for industrial-scale organic electronics.

季威研究组合作发现单层笼目材料新关联物态

季威研究组合作发现单层笼目材料新关联物态

近日,物理学院季威教授研究组与武汉大学张晨栋教授、袁声军教授、吴冯成教授等合作者组成联合团队,通过理论计算结合实验创制和测量,获得了非简单化学配比的单层笼目(kagome)晶格材料Mo33Te56,并发现了其中蕴藏的磁性和关联绝缘体等新物态。相关结果以“Ferromagnetism and correlated insulating states in monolayer Mo33Te56”为题,发表在2025年3月31日出版的《自然·通讯》[Nature Communications 16, 3084 (2025)]上。

在凝聚态物质中,处于费米能级附近平带中的电子备受关注。这些电子的动能被显著抑制,导致电子关联效应占据主导地位,从而为探索莫特(Mott)绝缘态、非常规超导态等多种新奇量子物态提供了理想的研究平台。然而,如何获得费米能级附近的电子平带则是构筑该平带的首要问题,面临着很大挑战。现有研究大多利用转角石墨烯等人工多层结构获得费米能级附近的电子平带。所获平带受限于严格的堆垛角度与复杂的界面工程,加之较小的载流子浓度,显著限制了其实际应用潜力。尽管高通量计算预测了一些天然晶体材料可能具有费米面处的平带特性,但至今未能得到明确的实验验证。因此,获得可制备、可测量且与其他能带无交叉的近费米能级平带成为当前量子材料研究中的关键科学问题之一。

因其独特的几何特性,笼目晶格(kagome lattice)可导致电子波函数干涉相消,是实现平带和狄拉克能带的理想晶格之一。现有的笼目材料多为块体材料,难以保证获得近费米能级平带且不与体能带交叉。为了获得与其他能带无交叉的近费米能级平带,研究团队聚焦在二维极限下构筑笼目晶格材料,创新性地提出了化学势控制孪晶界尺寸和排列的新策略,利用孪晶界超晶格创制出非简单化学计量比的笼目单层材料——Mo33Te56单层材料(下图左上)。

通过密度泛函理论计算和紧束缚近似模拟,结合扫描隧道显微镜/谱(下图中上)、原子力显微镜(下图左下)等先进实验技术,研究团队确认该材料具备扭转笼目晶格的几何对称性,在费米能级附近形成了两套纯净的笼目能带(1平带+2狄拉克带,下图中下)。更为重要的是,自旋极化STM测量和密度泛函理论计算证实了Mo33Te56是首个单层极限下的笼目磁体,其在0.35 K温度下展现出显著的铁磁回滞特性,矫顽力场达到0.1 T(下图右上)。其扫描隧道显微谱还呈现出位于费米能级处约15 meV宽的显著电子关联能隙,并在磁场下展现出极化塞曼效应(下图右下)。该研究工作实现了二维极限下平带驱动的强关联电子态及自发磁性, 为基于笼目晶格的强关联量子材料的设计提供了新的路径,并为平带驱动的量子物态研究提供了更加清晰的物理平台。

值得一提的是,2023年,物理学院程志海研究组(实验)和季威研究组(计算)利用类似策略创制了首个染色三角形单层材料Mo5Te8,获得近费米能级平带的特征证据[Nature Communications 14, 6320 (2023)]。本工作则更近一步,在该类材料的另一个稳定相(Mo33Te56)中发现了磁性和关联绝缘体态,并促使联合团队从理论计算的角度重新审视这类材料,揭示了四种在不同化学势下的稳定结构,更加系统地描述了他们的几何和电子结构特性[arXiv:2408.14285 (2024)]。

中国人民大学博士生戴佳琦、武汉大学博士生潘泽敏、博士后熊文奇博士和张辉博士为论文共同第一作者。物理学院季威教授、武汉大学张晨栋教授、袁声军教授、吴冯成教授为该论文的共同通讯作者。该工作的主要理论计算由戴佳琦和季威教授完成,其余部分由合作单位完成,并得到了国家自然科学基金、科技部、教育部等的支持。相关计算在中国人民大学计算云平台和物理学院高性能计算实验室等处完成。

Ferromagnetism and correlated insulating states in monolayer Mo33Te56

Ferromagnetism and correlated insulating states in monolayer Mo33Te56

Zemin Pan#, Wenqi Xiong#, Jiaqi Dai#, Hui Zhang#, Yunhua Wang, Tao Jian, Xingxia Cui, Jinghao Deng, Xiaoyu Lin, Zhengbo Cheng, Yusong Bai, Chao Zhu, Da Huo, Geng Li, Min Feng, Jun He, Wei Ji*, Shengjun Yuan*, Fengcheng Wu*, Chendong Zhang*, and Hong-Jun Gao

Although the kagome model is fundamentally two-dimensional, the essential kagome physics, i.e., the kagome-bands-driven emergent electronic states, has yet to be explored in the monolayer limit. Here, we present the experimental realization of kagome physics in monolayer Mo33Te56, showcasing both ferromagnetic ordering and a correlated insulating state with an energy gap of up to 15 meV. Using a combination of scanning tunnelling microscopy and theoretical calculations, we find a structural phase of the monolayer Mo-Te compound, which forms a mirror-twin boundary loop superlattice exhibiting kagome geometry and multiple sets of kagome bands. The partial occupancy of these nearly flat bands results in Fermi surface instability, counteracted by the emergence of ferromagnetic order (with a coercive field ~0.1 T, as observed by spin-polarized STM) and the opening of a correlated hard gap. Our work establishes a robust framework featuring well-defined atomic and band structures, alongside the intrinsic two-dimensional nature, essential for the rigorous examination of kagome physics.

Synthesis and Electronic Structure of Atomically Thin 2H-MoTe2

Synthesis and Electronic Structure of Atomically Thin 2H-MoTe2

Wenjuan Zhao#, Xieyu Zhou#, Dayu Yan#, Yuan Huang*, Cong Li, Qiang Gao, Paolo Moras, Polina M. Sheverdyaeva, Hongtao Rong, Yongqing Cai, Eike F. Schwier, Xixia Zhang, Cheng Shen, Yang Wang, Yu Xu, Wei Ji, Chen Liu, Youguo Shi, Lin Zhao, Lihong Bao, Qingyan Wang, Kenya Shimada, Xutang Tao, Guangyu Zhang, Hongjun Gao, Zuyan Xu, Xingjiang Zhou*, Guodong Liu*

An in-depth understanding of the electronic structure of 2H-MoTe2 at the atomic layer limit is a crucial step towards its exploitation in nanoscale devices. Here, we show that millimeter-sized monolayer (ML) MoTe2 samples, as well as smaller sized bilayer (BL) samples, can be obtained using the mechanical exfoliation technique. The electronic structure of these materials is investigated by Angle-Resolved Photoemission Spectroscopy (ARPES) for the first time and Density Functional
Theory (DFT) calculations. The comparison between experiments and theory allows us to describe ML MoTe2 as a semiconductor with direct gap at K point. This scenario is reinforced by the experimental observation of the conduction band minimum at K in Rb-doped ML MoTe2, resulting in a gap of at least 0.924 eV. In the BL MoTe2 system the maxima of the bands at Γ and K display very similar energies, thus leaving the door open to a direct gap scenario, in analogy to WSe2. The monotonic increase in the separation between spin-split bands at K while moving from ML to BL and bulk-like MoTe2 is attributed to interlayer coupling. Our findings can be considered as a reference to understand Quantum Anomalous and Fractional Quantum Anomalous Hall Effects recently discovered in ML and BL MoTe2 based moir´ e heterostructures.

High-Throughput Discovery of Kagome Materials in Transition Metal Oxide Monolayers

High-Throughput Discovery of Kagome Materials in Transition Metal Oxide Monolayers

Renhong Wang (王人宏), Cong Wang (王聪)*, Ruixuan Li (李睿宣), Deping Guo (郭的坪), Jiaqi Dai (戴佳琦), Canbo Zong (宗灿波), Weihan Zhang (张伟 涵), and Wei Ji (季威)*

Kagome materials are known for hosting exotic quantum states, including quantum spin liquids, charge density waves, and unconventional superconductivity. The search for kagome monolayers is driven by their ability to exhibit neat and well-defined kagome bands near the Fermi level, which are more easily realized in the absence of interlayer interactions. However, this absence also destabilizes the monolayer forms of many bulk kagome materials, posing significant challenges to their discovery. In this work, we propose a strategy to address this challenge by utilizing oxygen vacancies in transition metal oxides within a “1+3” design framework. Through high-throughput computational screening of 349 candidate materials, we identified 12 thermodynamically stable kagome monolayers with diverse electronic and magnetic properties. These materials were classified into three categories based on their lattice geometry, symmetry, band gaps, and magnetic configurations. Detailed analysis of three representative monolayers revealed kagome band features near their Fermi levels, with orbital contributions varying between oxygen 2p and transition metal d states. This study demonstrates the feasibility of the “1+3” strategy, offering a promising approach to uncovering low-dimensional kagome materials and advancing the exploration of their quantum phenomena.