Room-temperature electrically driven phase transition of two-dimensional 1T-TaS2 layers

In Situ TEM Characterization and Modulation for Phase Engineering of Nanomaterials

Solid-state phase transformation is an intriguing phenomenon in crystalline or noncrystalline solids due to the distinct physical and chemical properties that can be obtained and modified by phase engineering. Compared to bulk solids, nanomaterials exhibit enhanced capability for phase engineering due to their small sizes and high surface-to-volume ratios, facilitating various emerging applications. To establish a comprehensive atomistic understanding of phase engineering, in situ transmission electron microscopy (TEM) techniques have emerged as powerful tools, providing unprecedented atomic-resolution imaging, multiple characterization and stimulation mechanisms, and real-time integrations with various external fields. In this Review, we present a comprehensive overview of recent advances in in situ TEM studies to characterize and modulate nanomaterials for phase transformations under different stimuli, including mechanical, thermal, electrical, environmental, optical, and magnetic factors. We briefly introduce crystalline structures and polymorphism and then summarize phase stability and phase transformation models. The advanced experimental setups of in situ techniques are outlined and the advantages of in situ TEM phase engineering are highlighted, as demonstrated via several representative examples. Besides, the distinctive properties that can be obtained from in situ phase engineering are presented. Finally, current challenges and future research opportunities, along with their potential applications, are suggested.

Out-of-Plane Transport of 1T-TaS2/Graphene-Based van der Waals Heterostructures

Due to their anisotropy, layered materials are excellent candidates for studying the interplay between the in-plane and out-of-plane entanglement in strongly correlated systems. A relevant example is provided by 1T-TaS₂, which exhibits a multifaceted electronic and magnetic scenario due to the existence of several charge density wave (CDW) configurations. It includes quantum hidden phases, superconductivity and exotic quantum spin liquid (QSL) states, which are highly dependent on the out-of-plane stacking of the CDW. In this system, the interlayer stacking of the CDW is crucial for interpreting the underlying electronic and magnetic phase diagram. Here, atomically thin-layers of 1T-TaS₂ are integrated in vertical van der Waals heterostructures based on few-layers graphene contacts and their electrical transport properties are measured. Different activation energies in the conductance and a gap at the Fermi level are clearly observed. Our experimental findings are supported by fully self-consistent DFT+U calculations, which evidence the presence of an energy gap in the few-layer limit, not necessarily coming from the formation of out-of-plane spin-paired bilayers at low temperatures, as previously proposed for the bulk. These results highlight dimensionality as a key effect for understanding quantum materials as 1T-TaS₂, enabling the possible experimental realization of low-dimensional QSLs.

Large Optical Tunability from Charge Density Waves in 1T-TaS2 under Incoherent Illumination

Strongly correlated materials possess a complex energy landscape and host many interesting physical phenomena, including charge density waves (CDWs). CDWs have been observed and extensively studied in many materials since their first discovery in 1972. Yet they present ample opportunities for discovery. Here, we report a large tunability in the optical response of a quasi-2D CDW material, 1T-TaS₂, upon incoherent light illumination at room temperature. We hypothesize that the observed tunability is a consequence of light-induced rearrangement of CDW stacking across the layers of 1T-TaS₂. Our model, based on this hypothesis, agrees reasonably well with experiments suggesting that the interdomain CDW interaction is a vital potentially knob to control the phase of strongly correlated materials.

Photocurrent Imaging of Multi-Memristive Charge Density Wave Switching in Two-Dimensional 1T-TaS2

Transport studies of atomically thin 1T-TaS₂ have demonstrated the presence of intermediate resistance states across the nearly commensurate (NC) to commensurate (C) charge density wave (CDW) transition, which can be further switched electrically. While this presents exciting opportunities for memristor applications, the switching mechanism could be potentially attributed to the formation of inhomogeneous C and NC domains. Here, we present combined electrical driving and photocurrent imaging of ultrathin 1T-TaS₂ in a heterostructure geometry. While micron-sized CDW domains are seen upon cooling, electrically driven transitions are largely uniform, indicating that the latter likely induces true metastable CDW states, which we then explain by a free energy analysis. Additionally, we are able to perform repeatable and bidirectional switching across the intermediate states without changing sample temperature, demonstrating that atomically thin 1T-TaS₂ can be further used as a robust and reversible multimemristor material for the first time.

Phase Transition Photodetection in Charge Density Wave Tantalum Disulfide

The charge density wave (CDW) phase is a macroscopic quantum state with periodic charge density modulation accompanied by periodic lattice distortion in low-dimensional metals. External fields, such as an electric field and optical excitation, can trigger the transitions among different CDW states, leaving an under-explored mechanism and attracting great interest toward optoelectronic applications. Here, we explore a photoinduced phase transition in 1T-TaS₂ under an electrical field. By analyzing the phase transition probability, we obtained a linear dependence of the phase transition barrier on the electric field and laser energy density. Additionally, the threshold laser energy for the phase transition decreases linearly with an increasing applied electrical field. Finally, picojoule photodetection was realized in the visible and near-infrared ranges near the CDW transition edge. Our work will promote the understanding of the CDW phase transition mechanism as well as open pathways for optoelectronic applications.

Gate- and Light-Tunable Negative Differential Resistance with High Peak Current Density in 1T-TaS2/2H-MoS2 T-Junction

Metal-based electronics is attractive for fast and radiation-hard electronic circuits and remains one of the long-standing goals for researchers. The emergence of 1T-TaS₂, a layered material exhibiting strong charge density wave (CDW)-driven resistivity switching that can be controlled by an external stimulus such as electric field and optical pulses, has triggered a renewed interest in metal-based electronics. Here we demonstrate a negative differential resistor (NDR) using electrically driven CDW phase transition in an asymmetrically designed T-junction made up of 1T-TaS₂/2H-MoS₂ van der Waals heterojunction. The principle of operation of the proposed device is governed by majority carrier transport and is distinct from usual NDR devices employing tunneling of carriers; thus it avoids the bottleneck of weak tunneling efficiency in van der Waals heterojunctions. Consequently, we achieve a peak current density in excess of 10⁵ nA μm^-2, which is about 2 orders of magnitude higher than that obtained in typical layered material based NDR implementations. The peak current density can be effectively tuned by an external gate voltage as well as photogating. The device is robust against ambiance-induced degradation, and the characteristics repeat in multiple measurements over a period of more than a month. The findings are attractive for the implementation of active metal-based functional circuits.

Lattice Discontinuities of 1T-TaS2 across First Order Charge Density Wave Phase Transitions

Transition metal dichalcogenides are lamellar materials which can exhibit unique and remarkable electronic behavior due to effects of electron-electron and electron-phonon coupling. Among these materials, 1T-tantalum disulfide (1T-TaS2) has spurred considerable interest, due to its multiple first order phase transitions between different charge density wave (CDW) states. In general, the basic effects of charge density wave formation in 1T-TaS2 can be attributed to in plane re-orientation of Ta-atoms during the phase transitions. Only in recent years, an increasing number of studies has also emphasized the role of interlayer interaction and stacking order as a crucial aspect to understand the specific electronic behavior of 1T-TaS2, especially for technological systems with a finite number of layers. Obviously, continuously monitoring the out of plane expansion of the sample can provide direct inside into the rearrangement of the layer structure during the phase transition. In this letter, we therefore investigate the c-axis lattice discontinuities of 1T-TaS2 by atomic force microscopy (AFM) method under ultra-high vacuum conditions. We find that the c-axis lattice experiences a sudden contraction across the nearly-commensurate CDW (NC-CDW) phase to commensurate CDW (C-CDW) phase transition during cooling, while an expansion is found during the transition from the C-CDW phase to a triclinic CDW phase during heating. Thereby our measurements reveal, how higher order C-CDW phase can favor a more dense stacking. Additionally, our measurements also show subtler effects like e.g. two expansion peaks at the start of the transitions, which can provide further insight into the mechanisms at the onset of CDW phase transitions.

Raman Spectroscopic and Dynamic Electrical Investigation of Multi-State Charge-Wave-Density Phase Transitions in 1 T-TaS2

Two-dimensional layered 1 T-TaS₂ exhibits rich charge-density-wave (CDW) states with distinct electronic structures and physical properties, leading to broad potential applications, such as phase-transition memories, electrical oscillators and photodetectors. Besides the various CDW ground states at different temperatures, multiple intermediate phases in 1 T-TaS₂ have been observed by applying optical and electrical stimulations. Here, we investigated the electric-field-driven multistate CDW phase transition by Raman spectroscopy and voltage oscillations in 1 T-TaS₂. Strong correlation was observed between electrical conductivity and intensity of fold-back acoustic and optical phonon modes in 1 T-TaS₂. This indicates that the multistate transitions arise from serial transitions, from the nearly commensurate (NC) CDW phase to out-of-equilibrium intermediate states, and finally to the incommensurate (IC) CDW phase. The dynamics of phase transition under an electric field was investigated. As the electrical field increased, the dwell time of different CDW states changed. At lower temperatures, the multistate oscillations disappeared because of higher-energy barriers between the intermediate phases and/or lower thermal excitation energies at lower temperatures.

Light-Tunable 1T-TaS2 Charge-Density-Wave Oscillators

External stimuli-controlled phase transitions are essential for fundamental physics and design of functional devices. Charge density wave (CDW) is a metastable collective electronic phase featured by the periodic lattice distortion. Much attention has been attracted to study the external control of CDW phases. Although much work has been done in the electric-field-induced CDW transition, the study of the role of Joule heating in the phase transition is insufficient. Here, using the Raman spectroscopy, the electric-field-driven phase transition is in situ observed in the ultrathin 1T-TaS₂. By quantitative evaluation of the Joule heating effect in the electric-field-induced CDW transition, it is shown that Joule heating plays a secondary role in the nearly commensurate (NC) to incommensurate (IC) CDW transition, while it dominants the IC-NC CDW transition, providing a better understanding of the electric field-induced phase transition. More importantly, at room temperature, light illumination can modulate the CDW phase and thus tune the frequency of the ultrathin 1T-TaS₂ oscillators. This light tunability of the CDW phase transition is promising for multifunctional device applications.

Forcing substitution of tantalum by copper in 1T-TaS2: synthesis, structure and electronic properties of 1T-Cu x Ta1-x S2

We investigated the compound 1T-Cu _x Ta_1-x S₂ with respect to its synthesis, homogeneity range, structure and electronic properties. The average structure of 1T-Cu _x Ta_1-x S₂ resembles that of the high-temperature phase of the layered transition metal dichalcogenide 1T-TaS₂ in which tantalum is partially substituted by copper. 1T-Cu _x Ta_1-x S₂ readily decomposes at elevated temperatures and can only be prepared and stabilized by a sufficiently high amount of sulfur excess. XPS and NEXAFS measurements reveal that copper has the oxidation state  +I in 1T-Cu _x Ta_1-x S₂, which is supported by quantum chemical calculations. The disorder introduced by copper doping causes an Anderson-type localization of the conduction electrons as manifested by a strong increase of the electrical resistivity and a Curie-type paramagnetism at low temperatures as in other doped systems 1T-M _x Ta_1-x S₂ with higher valent metals. Quantum chemical calculations support this interpretation.