Since the discovery of two-dimensional (2D) graphene[1], 2D materials have been widely investigated due to their intriguing physical/chemical property and outstanding optoelectronic performance[1−3]. Generally, 2D materials can be divided into isotropy and anisotropy according to crystal structures. For isotropic 2D materials like graphene and hexagonal boron nitride, they have obvious lattice symmetry. Whereas, anisotropic 2D materials possess strong anisotropic crystal structure (Fig. 1(a)), providing new degree of freedom for exploring 2D materials. Currently, 2D materials with high anisotropy, due to their anisotropic electrical, optical, thermal, and phonon properties, are finding applications in polarizationsensitive photodetection, neural network construction, spinpolarized transport and other emerging fields[3−6]. In layered 2D materials, the in-plane atoms are held together by strong covalent bonds, while different layers are stacked by weak van der Waals (vdW) forces[7]. 2D materials can be easily exfoliated from layered crystals, and present a large surface-to-volume ratio. Interestingly, anisotropic 2D materials possess excellent polarized photodetection abilities because of their intrinsic sensitivity to polarized light[8]. A novel polarization-sensitive photodetector based on anisotropic 2D materials has recently been proposed (Fig. 1(b)). In such devices, the laser is set to pass through a polarizer, and then the direction of incident light can be controlled by a halfwave plate. 2D anisotropy phototransistor can convert polarization characteristics into electrical signals. Such polarizationsensitive photodetectors will find applications in optical communication, near-field imaging, navigation, and military fields[7−9]. In 2020, Pi et al.[4] reported a highly sensitive polarized photodetector based on 2D palladium diselenide (PdSe2). PdSe2 not only possesses high room-temperature mobility and high air stability, but also has a puckered pentagonal structure with highly anisotropic properties, which gifts it advantages in polarization detection. The strong anisotropic photoelectric properties of 2D PdSe2 were revealed by azimuthdependent reflectance difference microscopy. The polarized photodetector presented excellent performance with dichroic ratios as high as ~2.2 at 369 nm and ~1.8 at 532 nm, respectively (Fig. 1(c)). Moreover, their primary polarization orientations differed by 90°, which is due to the characteristic difference between a-axis and b-axis. This phenomenon was ascribed to the inherent linear dichroism of PdSe2. Very recently, Li et al.[5] also demonstrated that PdSe2 photodetector possesses excellent polarization sensitivity and high stability. The phonon anisotropy of PdSe2 was confirmed by polarization-dependent Raman. Such polarization-sensitive photoresponse based on photothermoelectric effect was also demonstrated. These results indicate that anisotropic 2D materials can promote the development of next-generation high-performance polarized photodetection systems. Compared with traditional 2D materials, anisotropic 2D materials like black phosphorus, ReS2, and PdSe2 show strong anisotropy in their crystal structure and photoelectric property[4−8]. The related devices can offer a high intrinsic heterogeneity for information transmission. In biological synapses, the heterogeneity of synaptic connections is the basis for the diversity of neural activity[6]. The realization of heterogeneity in synaptic plasticity is very crucial for constructing high-complexity neural networks. Therefore, the setup based on anisotropic 2D materials can bring intrinsic heterogeneity into new-generation neuromorphic electronics. Tian et al.[6] realized the first anisotropic neuronal transistor based on black phosphorus. They used structure anisotropy of 2D materials to imitate the heterogeneity of synaptic behavior. The key characteristics of biological synapses were successfully emulated, such as long-term plasticity and spike-timing-dependent plasticity. More importantly, a simple axon-multi-synapse heterogeneity network was demonstrated. These devices can be regarded as the building blocks for future neuromorphic systems. Recently, Qin et al.[9] used 2D trigonal selenium (t-Se) nanosheet to make an anisotropic electrolyte-gated synaptic transistor (EGT) (Fig. 1(d)). t-Se is a one-dimensional vdW material, where Se atoms are covalently bonded along c-axis direction while stacked together along the perpendicular plane via vdW interactions. 2D t-Se nanosheets tend to crystallize in an irregular quadrilateral. In the EGT devices based on t-Se nanosheets, the synaptic weight variation and temporal filtering capability showed a strong anisotropic response because of the inherent heterogeneity of channel conductance. Moreover, the complex axon-multi-synapse system with heterogeneous signal-transmission ability was also realized in multiterminal EGT devices (Fig. 1(e)). For the same stimulus input, the system exhibited an anisotropic filtering behavior (Fig. 1(f)). The devices based on 2D anisotropic materials can be used to build heterogeneous artificial neural networks. Some isotropic 2D materials with special structures may

中文翻译：

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用于后摩尔光电器件的各向异性二维材料

自从发现二维（2D）石墨烯[1]以来，二维材料因其有趣的物理/化学性质和出色的光电性能而被广泛研究[1-3]。一般来说，二维材料根据晶体结构可分为各向同性和各向异性。对于石墨烯和六方氮化硼等各向同性二维材料，它们具有明显的晶格对称性。而各向异性二维材料具有很强的各向异性晶体结构（图1（a）），为探索二维材料提供了新的自由度。目前，具有高各向异性的二维材料由于其各向异性的电学、光学、热学和声子特性，在偏振敏感光电探测、神经网络构建、自旋偏振输运等新兴领域得到应用[3-6]。在分层二维材料中，平面内的原子通过强共价键结合在一起，而不同的层通过弱范德华 (vdW) 力堆叠[7]。二维材料可以很容易地从层状晶体中剥离，并且具有很大的表面体积比。有趣的是，各向异性二维材料因其对偏振光的固有敏感性而具有出色的偏振光检测能力[8]。最近提出了一种基于各向异性二维材料的新型偏振敏感光电探测器（图 1（b））。在这样的装置中，激光被设置为通过偏振器，然后入射光的方向可以通过半波片来控制。二维各向异性光电晶体管可以将偏振特性转换为电信号。这种偏振敏感的光电探测器将在光通信中找到应用，近场成像、导航和军事领域[7-9]。2020年，Pi等人[4] 报道了一种基于二维二硒化钯（PdSe2）的高灵敏度偏振光电探测器。PdSe2不仅具有高室温迁移率和高空气稳定性，而且具有高度各向异性的褶皱五边形结构，使其在偏振检测方面具有优势。2D PdSe2 的强各向异性光电特性通过方位角反射差显微镜揭示。偏振光电探测器表现出优异的性能，分色比在 369 nm 处分别高达 ~2.2，在 532 nm 处高达 ~1.8（图 1（c））。此外，它们的主偏振方向相差 90°，这是由于 a 轴和 b 轴之间的特性差异。这种现象归因于 PdSe2 固有的线性二色性。最近，李等人[5] 还证明了 PdSe2 光电探测器具有出色的偏振灵敏度和高稳定性。PdSe2 的声子各向异性通过偏振相关拉曼得到证实。还展示了这种基于光热电效应的偏振敏感光响应。这些结果表明，各向异性二维材料可以促进下一代高性能偏振光电探测系统的发展。与传统二维材料相比，黑磷、ReS2、PdSe2等各向异性二维材料在晶体结构和光电性能方面表现出较强的各向异性[4-8]。相关设备可以为信息传输提供高度的内在异质性。在生物突触中，突触连接的异质性是神经活动多样性的基础[6]。突触可塑性异质性的实现对于构建高复杂度的神经网络至关重要。因此，基于各向异性二维材料的设置可以为新一代神经形态电子学带来内在的异质性。田等人[6] 实现了第一个基于黑磷的各向异性神经元晶体管。他们使用二维材料的结构各向异性来模拟突触行为的异质性。成功模拟了生物突触的关键特征，例如长期可塑性和尖峰时间依赖性可塑性。更重要的是，证明了一个简单的轴突-多突触异质性网络。这些设备可以被视为未来神经形态系统的构建块。最近，秦等人[9] 使用二维三角硒（t-Se）纳米片制造各向异性电解质门控突触晶体管（EGT）（图1（d））。t-Se 是一种一维 vdW 材料，其中 Se 原子沿 c 轴方向共价键合，同时通过 vdW 相互作用沿垂直平面堆叠在一起。2D t-Se 纳米片倾向于以不规则的四边形结晶。在基于 t-Se 纳米片的 EGT 器件中，由于通道电导的固有异质性，突触权重变化和时间过滤能力表现出强烈的各向异性响应。此外，具有异质信号传输能力的复杂轴突多突触系统也在多端EGT设备中实现（图1（e））。对于相同的刺激输入，系统表现出各向异性过滤行为（图 1（f））。基于二维各向异性材料的器件可用于构建异构人工神经网络。一些具有特殊结构的各向同性二维材料可能