The morphology and dimension of the conductive filament formed in a memristive device are strongly influenced by the thickness of its switching medium layer. Aggressive scaling of this active layer thickness is critical toward reducing the operating current, voltage, and energy consumption in filamentary-type memristors. Previously, the thickness of this filament layer has been limited to above a few nanometers due to processing constraints, making it challenging to further suppress the on-state current and the switching voltage. Here, the formation of conductive filaments in a material medium with sub-nanometer thickness formed through the oxidation of atomically thin two-dimensional boron nitride is studied. The resulting memristive device exhibits sub-nanometer filamentary switching with sub-pA operation current and femtojoule per bit energy consumption. Furthermore, by confining the filament to the atomic scale, current switching characteristics are observed that are distinct from that in thicker medium due to the profoundly different atomic kinetics. The filament morphology in such an aggressively scaled memristive device is also theoretically explored. These ultralow energy devices are promising for realizing femtojoule and sub-femtojoule electronic computation, which can be attractive for applications in a wide range of electronics systems that desire ultralow power operation.

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Excitatory and inhibitory postsynaptic potentials are the two fundamental categories of synaptic responses underlying the diverse functionalities of the mammalian nervous system. Recent advances in neuroscience have revealed the co-release of both glutamate and GABA neurotransmitters from a single axon terminal in neurons at the ventral tegmental area that can result in the reconfiguration of the postsynaptic potentials between excitatory and inhibitory effects. The ability to mimic such features of the biological synapses in semiconductor devices, which is lacking in the conventional field effect transistor-type and memristor-type artificial synaptic devices, can enhance the functionalities and versatility of neuromorphic electronic systems in performing tasks such as image recognition, learning, and cognition. Here, we demonstrate an artificial synaptic device concept, an ambipolar junction synaptic devices, which utilizes the tunable electronic properties of the heterojunction between two layered semiconductor materials black phosphorus and tin selenide to mimic the different states of the synaptic connection and, hence, realize the dynamic reconfigurability between excitatory and inhibitory postsynaptic effects. The resulting device relies only on the electrical biases at either the presynaptic or the postsynaptic terminal to facilitate such dynamic reconfigurability. It is distinctively different from the conventional heterosynaptic device in terms of both its operational characteristics and biological equivalence. Key properties of the synapses such as potentiation and depression and spike-timing-dependent plasticity are mimicked in the device for both the excitatory and inhibitory response modes. The device offers reconfiguration properties with the potential to enable useful functionalities in hardware-based artificial neural network.

Nonvolatile charge-trap memory plays an important role in many modern electronics technologies, from portable electronic systems to large-scale data centers. Conventional charge-trap memory devices typically work with fixed channel carrier polarity and device characteristics. However, many emerging applications in reconfigurable electronics and neuromorphic computing require dynamically tunable properties in their electronic device components that can lead to enhanced circuit versatility and system functionalities. Here, we demonstrate an ambipolar black phosphorus (BP) charge-trap memory device with dynamically reconfigurable and polarity-reversible memory behavior. This BP memory device shows versatile memory properties subject to electrostatic bias. Not only the programmed/erased state current ratio can be continuously tuned by the back-gate bias, but also the polarity of the carriers in the BP channel can be reversibly switched between electron- and hole-dominated conductions, resulting in the erased and programmed states exhibiting interchangeable high and low current levels. The BP memory also shows four different memory states and, hence, 2-bit per cell data storage for both n-type and p-type channel conductions, demonstrating the multilevel cell storage capability in a layered material based memory device. The BP memory device with a high mobility and tunable programmed/erased state current ratio and highly reconfigurable device characteristics can offer adaptable memory device properties for many emerging applications in electronics technology, such as neuromorphic computing, data-adaptive energy efficient memory, and dynamically reconfigurable digital circuits.

IntroductionIn the past decade, the research community has seen intense research efforts in the field of two-dimensional (2D) materials. Many of the common 2D materials, such as graphene, hexagonal boron nitride (hBN) and molybdenum disulfide (MoS2), have relatively symmetrical 2D crystal lattices, resulting in mostly isotropic in-plane physical properties. The electrical, optical and phonon properties of these materials are similar along the different in-plane crystal directions of their 2D lattice. Having similar properties along different crystal orientations are favorable for many conventional electronic and photonic applications. For example, transistors built along different crystal directions on the wafer will have the same characteristics, which is beneficial for controlling the variability and uniformity of the device performance across a large wafer. On the other hand, there is a selected group of emerging 2D layered materials that possess highly asymmetrical in-plane crystal structure. Such layered materials include black phosphorus (BP) and its arsenic alloys, compounds with isoelectronic structure as BP including the monochalcogenides of group IV elements like Ge and Sn, as well as a class of low-symmetry transition metal dichalcogenide (LS-TMDC) materials such as rhenium disulfide (ReS2) and rhenium diselenide (ReSe2) (Figure 1, upper panel). Due to their reduced crystal symmetry, they have distinct electrical, optical, thermal and mechanical characteristics along different in-plane crystal directions. This new degree of freedom in their physical properties, together with other unique features arising from their low symmetry crystal lattice, can provide previously unexplored tunability on the characteristics of electrical, optical, thermal and mechanical devices (lower panels of Figure 1), hence opening the door to a wide range of opportunities for developing conceptually new semiconductor device applications (Figure 1, lower panel).In this review, we will discuss the recent progresses in studying the fundamental properties and novel device applications of low-symmetry 2D crystals.We will also offer our perspectives on the promising future research directions related to this emerging class of materials.Black Phosphorus is a low-symmetry layered material that has recently received much attention. Similar to graphite, bulk black phosphorus is an elemental layered material and is the most stable allotrope of phosphorus due to its unique puckered orthorhombic crystal structure. Similar to the carbon atoms in graphene, each phosphorus atom in BP is connected to three adjacent phosphorus atoms to form a stable, linked ring structure, with each ring consisting of six phosphorus atoms (Figure 1, upper panel). The adjacent layer spacing is around 0.53 nm and the lattice constant in this orthorhombic system along the z-direction is 1.05 nm. The crystal structure of BP belongs to space group Cmca (a=3.31, b=10.54, c=4.36). Within each layer of BP, the phosphorus atoms stay on two separate planes with shorter i...