High Mobility Transistors and Flexible Optical Synapses Enabled by Wafer-Scale Chemical Transformation of Pt-Based 2D Layers.

Electronic devices employing two-dimensional (2D) van der Waals (vdW) transition-metal dichalcogenide (TMD) layers as semiconducting channels often exhibit limited performance (e.g., low carrier mobility), in part, due to their high contact resistances caused by interfacing non-vdW three-dimensional (3D) metal electrodes. Herein, we report that this intrinsic contact issue can be efficiently mitigated by forming the 2D/2D in-plane junctions of 2D semiconductor channels seamlessly interfaced with 2D metal electrodes. For this, we demonstrated the selectively patterned conversion of semiconducting 2D PtSe2 (channels) to metallic 2D PtTe2 (electrodes) layers by employing a wafer-scale low-temperature chemical vapor deposition (CVD) process. We investigated a variety of field-effect transistors (FETs) employing wafer-scale CVD-2D PtSe2/2D PtTe2 heterolayers and identified that silicon dioxide (SiO2) top-gated FETs exhibited an extremely high hole mobility of ∼120 cm2 V-1 s-1 at room temperature, significantly surpassing performances with previous wafer-scale 2D PtSe2-based FETs. The low-temperature nature of the CVD method further allowed for the direct fabrication of wafer-scale arrays of 2D PtSe2/2D PtTe2 heterolayers on polyamide (PI) substrates, which intrinsically displayed optical pulse-induced artificial synaptic behaviors. This study is believed to vastly broaden the applicability of 2D TMD layers for next-generation, high-performance electronic devices with unconventional functionalities.

Centimeter-Scale Tellurium Oxide Films for Artificial Optoelectronic Synapses with Broadband Responsiveness and Mechanical Flexibility.

Prevailing over the bottleneck of von Neumann computing has been significant attention due to the inevitableness of proceeding through enormous data volumes in current digital technologies. Inspired by the human brain's operational principle, the artificial synapse of neuromorphic computing has been explored as an emerging solution. Especially, the optoelectronic synapse is of growing interest as vision is an essential source of information in which dealing with optical stimuli is vital. Herein, flexible optoelectronic synaptic devices composed of centimeter-scale tellurium dioxide (TeO2) films detecting and exhibiting synaptic characteristics to broadband wavelengths are presented. The TeO2-based flexible devices demonstrate a comprehensive set of emulating basic optoelectronic synaptic characteristics; i.e., excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), conversion of short-term to long-term memory, and learning/forgetting. Furthermore, they feature linear and symmetric conductance synaptic weight updates at various wavelengths, which are applicable to broadband neuromorphic computations. Based on this large set of synaptic attributes, a variety of applications such as logistic functions or deep learning and image recognition as well as learning simulations are demonstrated. This work proposes a significant milestone of wafer-scale metal oxide semiconductor-based artificial synapses solely utilizing their optoelectronic features and mechanical flexibility, which is attractive toward scaled-up neuromorphic architectures.

Calcium Alginate as an Active Device Component for Light-Triggered Degradation of 2D MoS2-Based Transient Electronics.

Transient electronics technology has enabled the programmed disintegration of functional devices, paving the way for environmentally sustainable management of electronic wastes as well as facilitating the exploration of novel device concepts. While a variety of inorganic and/or organic materials have been employed as media to introduce transient characteristics in electronic devices, they have been mainly limited to function as passive device components. Herein, we report that calcium (Ca) alginate, a natural biopolymer, exhibits multifunctionalities of introducing light-triggered transient characteristics as well as constituting active components in electronic devices integrated with two-dimensional (2D) molybdenum disulfide (MoS2) layers. Ca2+ ions-based alginate electrolyte films are prepared through hydrolysis reactions and are subsequently incorporated with riboflavin, a natural photosensitizer, for the light-driven dissolution of 2D MoS2 layers. The alginate films exhibit strain-sensitive triboelectricity, confirming the presence of abundant mobile Ca2+ ions, which enables them to be active components of 2D MoS2 field-effect transistors (FETs) functioning as electrolyte top-gates. The alginate-integrated 2D MoS2 FETs display intriguing transient characteristics of spontaneous degradation upon ultraviolet-to-visible light illumination as well as water exposure. Such transient characteristics are demonstrated even in ambient conditions with natural sunlight, highlighting the versatility of the developed approach. This study emphasizes a relatively unexplored aspect of combining naturally abundant polymers with emerging near atom-thickness semiconductors toward realizing unconventional and transformative device functionalities.

Reversible Transition of Semiconducting PtSe2 and Metallic PtTe2 for Scalable All-2D Edge-Contacted FETs.

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are highly promising as field-effect transistor (FET) channels in the atomic-scale limit. However, accomplishing this superiority in scaled-up FETs remains challenging due to their van der Waals (vdW) bonding nature with respect to conventional metal electrodes. Herein, we report a scalable approach to fabricate centimeter-scale all-2D FET arrays of platinum diselenide (PtSe2) with in-plane platinum ditelluride (PtTe2) edge contacts, mitigating the aforementioned challenges. We realized a reversible transition between semiconducting PtSe2 and metallic PtTe2 via a low-temperature anion exchange reaction compatible with the back-end-of-line (BEOL) processes. All-2D PtSe2 FETs seamlessly edge-contacted with transited metallic PtTe2 exhibited significant performance improvements compared to those with surface-contacted gold electrodes, e.g., an increase of carrier mobility and on/off ratio by over an order of magnitude, achieving a maximum hole mobility of ∼50.30 cm2 V-1 s-1 at room temperature. This study opens up new opportunities toward atomically thin 2D-TMD-based circuitries with extraordinary functionalities.