Nanoelectronics Using Metal-Insulator Transition

Electric-Field-Induced Metal-Insulator Transition for Low-Power and Ultrafast Nanoelectronics

We present here a comprehensive review of various classes of electric-field-induced reversible Mott metal-insulator materials, which have many applications in ultrafast switches, reconfigurable high-frequency devices up to THz, and photonics. Various types of Mott transistors are analyzed, and their applications are discussed. This paper introduces new materials that demonstrate the Mott transition at very low DC voltage levels, induced by an external electric field. The final section of the paper examines ferroelectric Mott transistors and these innovative ferroelectric Mott materials.

Ultrasensitive photoelectric detection with room temperature extremum

Room-temperature photodetection holds pivotal significance in diverse applications such as sensing, imaging, telecommunications, and environmental remote sensing due to its simplicity, versatility, and indispensability. Although different kinds of photon and thermal detectors have been realized, high sensitivity of photodetection with room temperature extremum is not reported until now. Herein, we find evident peaks in the photoelectric response originated from the anomalous excitonic insulator phase transition in tantalum nickel selenide (Ta2NiSe5) for room-temperature optimized photodetection from visible light to terahertz ranges. Extreme sensitivity of photoconductive detector with specific detectivity (D*) of 5.3 × 1011 cm·Hz1/2·W-1 and electrical bandwidth of 360 kHz is reached in the terahertz range, which is one to two orders of magnitude improvement compared to that of the state-of-the-art room-temperature terahertz detectors. The van der Waals heterostructure of Ta2NiSe5/WS2 is further constructed to suppress the dark current at room temperature with much improved ambient D* of 4.1 × 1012 cm·Hz1/2·W-1 in the visible wavelength, rivaling that of the typical photodetectors, and superior photoelectric performance in the terahertz range compared to the photoconductor device. Our results open a new avenue for optoelectronics via excitonic insulator phase transition in broad wavelength bands and pave the way for applications in sensitive environmental and remote sensing at room temperature.

Wafer-Scale Transfer and Integration of Tungsten-Doped Vanadium Dioxide Films

Modern optoelectronic devices trend toward greater flexibility, wearability, and multifunctionality, demanding higher standards for fabrication and operation temperatures. Vanadium dioxide (VO2), with its metal-insulator transition (MIT) at 68 °C, serves as a crucial functional layer in many optoelectronic devices. However, VO2 usually needs to grow at >450 °C in an oxygen-containing atmosphere and to function across its MIT temperature, leading to low compatibility with most optoelectronic devices, especially on flexible substrates. In this work, we report a layer-by-layer transfer method of wafer-scale tungsten-doped VO2 films, which enables sequential integration of the VO2 films with low MIT temperatures (down to 40 °C) onto arbitrary substrates. Notably, by stacking multiple VO2 films with different doped levels, a quasi-gradient-doped VO2 architecture can be achieved, effectively broadening the MIT temperature window and reducing the hysteresis of VO2. These integrated VO2 films find a wide scope of applications in flexible temperature indicator strips, infrared camouflage devices, nonreciprocal ultrafast light modulators, and smart photoactuators. Our work promotes the development of more flexible and tunable optoelectronic devices integrated with VO2.

Percolative phase transition in few-layered MoSe2 field-effect transistors using Co and Cr contacts

The metal-to-insulator phase transition (MIT) in two-dimensional (2D) materials under the influence of a gating electric field has revealed interesting electronic behavior and the need for a deeper fundamental understanding of electron transport processes, while attracting much interest in the development of next-generation electronic and optoelectronic devices. Although the mechanism of the MIT in 2D semiconductors is a topic under debate in condensed matter physics, our work demonstrates the tunable percolative phase transition in few-layered MoSe2 field-effect transistors (FETs) using different metallic contact materials. Here, we attempted to understand the MIT through temperature-dependent electronic transport measurements by tuning the carrier density in a MoSe2 channel under the influence of an applied gate voltage. In particular, we have examined this phenomenon using the conventional chromium (Cr) and ferromagnetic cobalt (Co) as two metal contacts. For both Cr and Co, our devices demonstrated n-type behavior with a room-temperature field-effect mobility of 16 cm2 V-1 s-1 for the device with Cr-contacts and 92 cm2 V-1 s-1 for the device with Co-contacts, respectively. With low temperature measurements at 50 K, the mobilities increased significantly to 65 cm2 V-1 s-1 for the device with Cr and 394 cm2 V-1 s-1 for the device with Co-contacts. By fitting our experimental data to the percolative phase transition theory, the temperature-dependent conductivity data show a transition from an insulating-to-metallic behavior at a bias of ∼28 V for Cr-contacts and ∼20 V for Co-contacts. This cross-over of the conductivity can be attributed to an increase in carrier density as a function of the gate bias in temperature-dependent transfer characteristics. By extracting the critical exponents, we find that the transport behavior in the device with Co-contacts aligns closely with the 2D percolation theory. In contrast, the devices with Cr-contacts deviate significantly from the 2D limit at low temperatures.

Coexisting Phases of Individual VO2 Nanoparticles for Multilevel Nanoscale Memory

Vanadium dioxide (VO2) has received significant interest in the context of nanophotonic metamaterials and memories owing to its reversible insulator-metal transition associated with significant changes in its optical and electronic properties. The phase transition of VO2 has been extensively studied for several decades, and the ways how to control its hysteresis characteristics relevant for memory applications have significantly improved. However, the hysteresis dynamics and stability of coexisting phases during the transition have not been studied on the level of individual single-crystal VO2 nanoparticles (NPs), although they represent the fundamental component of ordinary polycrystalline films and can also act like nanoscale memory units on their own. Here, employing transmission electron microscopy techniques, we investigate phase transitions of single VO2 NPs in real time. Our analysis reveals the statistical distribution of the transition temperature and steepness and how they differ during forward (heating) and backward (cooling) transitions. We evaluate the stability of coexisting phases in individual NPs and prove the persistent multilevel memory at near room temperatures using only a few VO2 NPs. Our findings unveil the physical mechanisms that govern the hysteresis of VO2 at the nanoscale and establish VO2 NPs as a promising component of optoelectronic and memory devices with enhanced functionalities.

Memristive Artificial Synapses Based on Brownmillerite for Endurable Weight Modulation

Exploring a computing paradigm that blends memory and computation functions is essential for artificial synapses. While memristors for artificial synapses are widely studied due to their energy-efficient structures, random filament conduction in general memristors makes them less preferred for endurability in long-term synaptic modulation. Herein, the topotactic phase transition (TPT) in brownmillerite-phased (110)-SrCoO2.5 (SCO2.5) is harnessed to enhance the reversibility of oxygen ion migration through 1-D oxygen vacancy channels. By employing a heteroepitaxial structured 2-terminal configuration of Au/SCO2.5/SrRuO3/SrTiO3, the brownmillerite SCO2.5-based synapse artificial synapses are exploited. Demonstration of the TPT behavior is corroborated by comparing oxygen migration energy by density-functional theory calculations and experimental results, and by monitoring the voltage pulse-induced peak shift in the Raman spectra of SCO2.5. With the voltage pulse-driven TPT behaviors, it is reliably characterized by linear, symmetric, and endurable long-term potentiation and depression performances. Notably, the durability of the TPT-based weight control mechanism is demonstrated by achieving consistent and noise-free weight updates over 32 000 iterations across 640 cycles. Furthermore, learning performances based on deep neural networks and convolutional neural networks on various image datasets yielded very high recognition accuracy. The work offers valuable insights into designing memristive synapses that enable reliable weight updates in neural networks.

Two-Terminal Neuromorphic Devices for Spiking Neural Networks: Neurons, Synapses, and Array Integration

The ever-increasing volume of complex data poses significant challenges to conventional sequential global processing methods, highlighting their inherent limitations. This computational burden has catalyzed interest in neuromorphic computing, particularly within artificial neural networks (ANNs). In pursuit of advancing neuromorphic hardware, researchers are focusing on developing computation strategies and constructing high-density crossbar arrays utilizing history-dependent, multistate nonvolatile memories tailored for multiply-accumulate (MAC) operations. However, the real-time collection and processing of massive, dynamic data sets require an innovative computational paradigm akin to that of the human brain. Spiking neural networks (SNNs), representing the third generation of ANNs, are emerging as a promising solution for real-time spatiotemporal information processing due to their event-based spatiotemporal capabilities. The ideal hardware supporting SNN operations comprises artificial neurons, artificial synapses, and their integrated arrays. Currently, the structural complexity of SNNs and spike-based methodologies requires hardware components with biomimetic behaviors that are distinct from those of conventional memristors used in deep neural networks. These distinctive characteristics required for neuron and synapses devices pose significant challenges. Developing effective building blocks for SNNs, therefore, necessitates leveraging the intrinsic properties of the materials constituting each unit and overcoming the integration barriers. This review focuses on the progress toward memristor-based spiking neural network neuromorphic hardware, emphasizing the role of individual components such as memristor-based neurons, synapses, and array integration along with relevant biological insights. We aim to provide valuable perspectives to researchers working on the next generation of brain-like computing systems based on these foundational elements.

Room-Temperature Possible Current-Induced Transition in Ca2RuO4 Thin Films Grown Through Intercalation-Like Cation Diffusion in the A2BO4 Ruddlesden-Popper Structure

Cation deficiency tuning is a central issue in thin-film epitaxy of functional metal oxides, as it is typically more difficult than anion deficiency tuning, as anions can be readily supplied from gas sources. Here, highly effective internal deficiency compensation of Ru cations is demonstrated for Ca2RuO4 epitaxial films based on diffusive transfer of metal cations in the A2BO4 Ruddlesden-Popper lattice from solid-phase cation sources. Through detailed structural characterization of Ca2RuO4/LaAlO3 (001) thin films grown with external cation sources by solid-phase epitaxy, the occurrence of intercalation-like, interstitial diffusion of La cations (from the substrates) in the A2BO4 structure is revealed, and that of Ru cations is also suggested. Relying on the interstitial-type diffusion, an optimized Ru deficiency compensation method, which does not induce the formation of Can +1RunO3 n +1 Ruddlesden-Popper impurity phases with higher n, is proposed for Ca2RuO4 epitaxial films. In the Ca2RuO4/LaAlO3 (001) thin films grown with Ru deficiency compensation, record-high resistivity values (102-10-1 Ω cm) and a large (more than 200 K) increase in the temperature range of the nonlinear transport properties are demonstrated by transport measurements, demonstrating the possible advantages of this method in the control of the current-induced quantum phase transition of Ca2RuO4.

Electrical Control of Magnetic Resonance in Phase Change Materials

Metal-insulator transitions (MITs) in resistive switching materials can be triggered by an electric stimulus that produces significant changes in the electrical response. When these phases have distinct magnetic characteristics, dramatic changes in the spin excitations are also expected. The transition metal oxide La0.7Sr0.3MnO3 (LSMO) is a ferromagnetic metal at low temperatures and a paramagnetic insulator above room temperature. When LSMO is in its metallic phase, a critical electrical bias has been shown to lead to an MIT that results in the formation of a paramagnetic resistive barrier transverse to the applied electric field. Using spin-transfer ferromagnetic resonance spectroscopy, we show that even for electrical biases less than the critical value that triggers the MIT, there is magnetic phase separation, with the spin-excitation resonances varying systematically with applied bias. Therefore, voltage-triggered MITs in LSMO can alter magnetic resonance characteristics, offering an effective method for tuning synaptic weights in neuromorphic circuits.

Realization of Organocerium-Based Fullerene Molecular Materials Showing Mott Insulator-Type Behavior

Electron-rich organocerium complexes (C5Me4H)3Ce and [(C5Me5)2Ce(ortho-oxa)], with redox potentials E1/2 = -0.82 V and E1/2 = -0.86 V versus Fc/Fc+, respectively, were reacted with fullerene (C60) in different stoichiometries to obtain molecular materials. Structurally characterized cocrystals: [(C5Me4H)3Ce]2·C60 (1) and [(C5Me5)2Ce(ortho-oxa)]3·C60 (2) of C60 with cerium-based, molecular rare earth precursors are reported for the first time. The extent of charge transfer in 1 and 2 was evaluated using a series of physical measurements: FT-IR, Raman, solid-state UV-vis-NIR spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and magnetic susceptibility measurements. The physical measurements indicate that 1 and 2 comprise the cerium(III) oxidation state, with formally neutral C60 as a cocrystal in both cases. Pressure-dependent periodic density functional theory calculations were performed to study the electronic structure of 1. Inclusion of a Hubbard-U parameter removes Ce f states from the Fermi level, opens up a band gap, and stabilizes FM/AFM magnetic solutions that are isoenergetic because of the large distances between the Ce(III) cations. The electronic structure of this strongly correlated Mott insulator-type system is reminiscent of the well-studied Ce2O3.

Piezo strain-controlled phase transition in single-crystalline Mott switches for threshold-manipulated leaky integrate-and-fire neurons

Electrical manipulation of the metal-insulator transition (MIT) in quantum materials has attracted considerable attention toward the development of ultracompact neuromorphic devices because of their stimuli-triggered transformations. VO2 is expected to undergo abrupt electronic phase transition by piezo strain near room temperature; however, the unrestricted integration of defect-free VO2 films on piezoelectric substrates is required to fully exploit this emerging phenomenon in oxide heterostructures. Here, we demonstrate the integration of single-crystalline VO2 films on highly lattice-mismatched PMN-PT piezoelectric substrates using a single-crystal TiO2-nanomembrane (NM) template. Using our strategy on heterogeneous integration, single-crystal-like steep transition was observed in the defect-free VO2 films on TiO2-NM-PMN-PT. Unprecedented TMI modulation (5.2 kelvin) and isothermal resistance of VO2 [ΔR/R (Eg) ≈ 18,000% at 315 kelvin] were achieved by the efficient strain transfer-induced MIT, which cannot be achieved using directly grown VO2/PMN-PT substrates. Our results provide a fundamental strategy to realize a single-crystalline artificial heterojunction for promoting the application of artificial neurons using emergent materials.