Ultrafast photonics has become an interdisciplinary topic of great consequence due to the spectacular progress of compact and efficient ultrafast pulse generation. Wide spectrum bandwidth is the key element for ultrafast pulse generation due to the Fourier transform limitation. Herein, monoclinic Nb2GeTe4, an emerging class of ternary narrow-gap semiconductors, was used as a real saturable absorber (SA), which manifests superior wide-range optical absorption. The crystallization form and growth mechanism of Nb2GeTe4 were revealed by a thermodynamic phase diagram. Furthermore, the Nb2GeTe4–SA showed reliable saturation intensity and larger modulation depth, ascribed to a built-in electric field driven by the asymmetric crystal architecture confirmed via X-ray diffraction, polarized Raman spectra, and scanning transmission electron microscopy. Based on the Nb2GeTe4–SA, femtosecond mode-locked operation with good overall performance was achieved by a properly designed ring cavity. These results suggest that Nb2GeTe4 shows great promise for ultrafast photonic applications and arouse interests in exploring the intriguing properties of the ternary van der Waals material family.
Two-dimensional (2D) semiconductors with high charge-carrier mobility and exotic quantum properties are of great importance for the design of outstanding electronic devices. Here we report the single crystal growth of van der Waals material Nb2GeTe4, which exhibits very high mobility and unusual transverse negative magnetoresistance (MR) from our transport-property-measurements. Nb2GeTe4 adopts a Te–Nb(Ge)–Te sandwiched layer structure and exhibits a narrow-gap semiconducting behavior. Based on ultraviolet–visible–near infrared (UV-vis-NIR) optical measurement, the optical band gap of Nb2GeTe4 is determined to be around 0.39 eV. The high electron mobility of the Nb2GeTe4 bulk crystal at room temperature, 424.37 cm2 V−1 s−1, is comparable to those of the current state-of-the-art high-mobility layered materials, making Nb2GeTe4 competitive for the design of 2D electronic devices. An anomalous negative magnetoresistance is also observed in Nb2GeTe4 below 10 K with different field dependency, where the maximum MR reaches ∼−31% at 5 K and 5 T.
Neuromorphic computing presents a promising solution for the von Neumann bottleneck, enabling energy-efficient and intelligent sensing platforms. Although 2D materials are ideal for bioinspired neuromorphic devices, achieving multifunctional synaptic operations with simple configurations and linear weight updates remains challenging. Inspired by biological axons, the in-plane anisotropy of 2D Nb 2 GeTe 4 is exploited to develop dual electronic-optical synaptic devices. The device exhibits anisotropic hole mobilities (137.97 cm 2 V −1 s −1 along the a -axis and 78.29 cm 2 V −1 s −1 along the b -axis) and a wavelength-dependent photoresponse. This enables directional synaptic plasticity under electrical-optical co-stimulation, achieving 98.3% accuracy along the a -axis and 88.3% along the b -axis in adaptive image processing. A machine vision system with 89.6% object recognition accuracy and an intelligent vehicle navigation platform with 90.2% decision-making accuracy is also demonstrated. The integration of anisotropic transport and spectrally tunable responses in a single material paves the way for compact neuromorphic hardware with multimodal sensing and parallel processing capabilities. This study advances 2D material-based neuroelectronics for edge computing, autonomous robotics, and adaptive artificial intelligence systems.
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