Mapping Hot Spots at Heterogeneities of Few-Layer Ti3C2 MXene Sheets

Spatially-Resolved Thermometry of Filamentary Nanoscale Hot Spots in TiO2 Resistive Random Access Memories to Address Device Variability

Resistive random access memories (RRAM), based on the formation and rupture of conductive nanoscale filaments, have attracted increased attention for application in neuromorphic and in-memory computing. However, this technology is, in part, limited by its variability, which originates from the stochastic formation and extreme heating of its nanoscale filaments. In this study, we used scanning thermal microscopy (SThM) to assess the effect of filament-induced heat spreading on the surface of metal oxide RRAMs with different device designs. We evaluate the variability of TiO2 RRAM devices with area sizes of 2 × 2 and 5 × 5 μm2. Electrical characterization shows that the variability indicated by the standard deviation of the forming voltage is ∼2 times larger for 5 × 5 μm2 devices than for the 2 × 2 μm2 ones. Further knowledge on the reason for this variability is gained through the SThM thermal maps. These maps show that for 2 × 2 μm2 devices the formation of one filament, i.e., hot spot at the device surface, happens reliably at the same location, while the filament location varies for the 5 × 5 μm2 devices. The thermal information, combined with the electrical, interfacial, and geometric characteristics of the device, provides additional insights into the operation and variability of RRAMs. This work suggests thermal engineering and characterization routes to optimize the efficiency and reliability of these devices.

Heterojunction Engineering Enhanced Self-Polarization of PVDF/CsPbBr3 /Ti3 C2 Tx Composite Fiber for Ultra-High Voltage Piezoelectric Nanogenerator

Piezoelectric nanogenerator (PENG) for practical application is constrained by low output and difficult polarization. In this work, a kind of flexible PENG with high output and self-polarization is fabricated by constructing CsPbBr3 -Ti3 C2 Tx heterojunctions in PVDF fiber. The polarized charges rapidly migrate to the electrodes from the Ti3 C2 Tx nanosheets by forming heterojunctions, achieving the maximum utilization of polarized charges and leading to enhanced piezoelectric output macroscopically. Optimally, PVDF/4wt%CsPbBr3 /0.6wt%Ti3 C2 Tx -PENG exhibits an excellent voltage output of 160 V under self-polarization conditions, which is higher than other self-polarized PENG previously. Further, the working principle and self-polarization mechanism are uncovered by calculating the interfacial charge and electric field using first-principles calculation. In addition, PVDF/4wt%CsPbBr3 /0.6wt%Ti3 C2 Tx -PENG exhibits better water and thermal stability attributed to the protection of PVDF. It is also evaluated in practice by harvesting the energy from human palm taps and successfully lighting up 150 LEDs and an electronic watch. This work presents a new idea of design for high-performance self-polarization PENG.

Understanding and Controlling Photothermal Responses in MXenes

MXenes have the potential for efficient light-to-heat conversion in photothermal applications. To effectively utilize MXenes in such applications, it is important to understand the underlying nonequilibrium processes, including electron-phonon and phonon-phonon couplings. Here, we use transient electron and X-ray diffraction to investigate the heating and cooling of photoexcited MXenes at femtosecond to nanosecond time scales. Our results show extremely strong electron-phonon coupling in Ti₃C₂-based MXenes, resulting in lattice heating within a few hundred femtoseconds. We also systematically study heat dissipation in MXenes with varying film thicknesses, chemical surface terminations, flake sizes, and annealing conditions. We find that the thermal boundary conductance (TBC) governs the thermal relaxation in films thinner than the optical penetration depth. We achieve a 2-fold enhancement of the TBC, reaching 20 MW m^-2 K^-1, by controlling the flake size or chemical surface termination, which is promising for engineering heat dissipation in photothermal and thermoelectric applications of the MXenes.

Nanoscale thermometry under ambient conditions via scanning thermal microscopy with 3D scanning differential method

Local temperature measurement with high resolution and accuracy is a key challenge in nowadays science and technologies at nanoscale. Quantitative characterization on temperature with sub-100 nm resolution is of significance for understanding the physical mechanisms of phonon transport and energy dissipation in nanoelectronics, optoelectronics, and thermoelectric devices. Scanning thermal microscopy (SThM) has been proved to be a versatile method for nanoscale thermometry. In particular, 2D profiling of the temperature field on the order of 10 nm and 10 mK has already been achieved by SThM with modulation techniques in ultrahigh vacuum to exclude the parasitic heat flow between air and the cantilever. However, few attempts have been made to truly realize 2D profiling of temperature quantitatively under ambient conditions, which is more relevant to realistic applications. Here, a 3D scanning differential method is developed to map the 2D temperature field of an operating nanodevice under ambient environment. Our method suppresses the thermal drift and the parasitic heat flow between air and the cantilever by consecutively measuring the temperatures in thermal contact and nonthermal contact scenarios rather than in a double-scan manner. The local 2D temperature field of a self-heating metal line with current crowding by a narrowing channel is mapped quantitatively by a sectional calibration with a statistic null-point method and a pixel-by-pixel correction with iterative calculation. Furthermore, we propose a figure of merit to evaluate the performance of thermocouple probes on temperature field profiling. The development of nanoscale thermometry under ambient environment would facilitate thermal manipulation on nanomaterials and nanodevices under practical conditions.

Ultralarge Flakes of Ti3C2Tx MXene via Soft Delamination

Two-dimensional (2D) titanium carbide MXene (Ti₃C₂T_x) has attracted significant attention due to its combination of properties and great promise for various applications. The size of the 2D sheets is a critical parameter affecting multiple properties of assembled films, fibers and 3D structures. The increased lateral size of MXene flakes can benefit not only their assemblies by improving the interflake contacts and alignment but also fundamental studies at the individual flake level, allowing for facile patterning and investigation of intrinsic physical properties of MXenes. Increasing the average size of the parent MAX phase is one of the strategies previously used to increase the flake size of the resultant MXene. Here, we show that the protocol used for the next step of the synthesis procedure, delamination of multilayer MXene into individual nanosheets, significantly affects the lateral size of the resultant flakes. We developed a soft delamination approach, which prevents fracture of flakes and preserves their size. Combining this approach with the large-grain Ti₃AlC₂ MAX phase precursor, we achieved individual flakes of up to 40 μm in lateral size. These flakes can be used for patterning multiple contacts and fabrication of field-effect transistors for multiprobe electrical characterization and other measurements. These findings indicate the importance of controlling the delamination process in order to achieve large MXene flakes and improve properties of MXene-based materials and devices.

Flexible and Transparent Electrode of Hybrid Ti3C2TX MXene-Silver Nanowires for High-Performance Quantum Dot Light-Emitting Diodes

The development of electrodes with high conductivity, optical transparency, and reliable mechanical flexibility and stability is important for numerous solution-processed photoelectronic applications. Although transparent Ti₃C₂T_X MXene electrodes with high conductivity are promising, their suitability for displays remains limited because of the high sheet resistance, which is caused by undesirable flake junctions and surface roughness. Herein, a flexible and transparent electrode has been fabricated that is suitable for a full-solution-processed quantum dot light-emitting diode (QLED). An MXene-silver nanowire (AgNW) hybrid electrode (MXAg) consists of a highly conductive AgNW network mixed with solution-processed MXene flakes. Efficient welding of wire-to-wire junctions with MXene flakes yields an electrode with a low sheet resistance and a high transparency of approximately 13.9 Ω sq^-1 and 83.8%, respectively. By employing a thin polymer buffer layer of poly(methyl methacrylate) (PMMA), followed by mild thermal treatment, a hybrid PMMA-based MXene-AgNW (MXAg@PMMA) electrode in which the work function of an MXAg hybrid FTE physically embedded in PMMA (MXAg@PMMA) can be tuned by controlling the amount of MXene in the hybrid film facilitates the development of a high-performance solution-processed QLED that exhibits maximum external quantum and current efficiencies of approximately 9.88% and 25.8 cd/A, respectively, with excellent bending stability. This work function-tunable flexible transparent electrode based on solution-processed nanoconductors provides a way to develop emerging high-performance, wearable, cost-effective, and soft electroluminescent devices.

Thermal Properties of MXenes and Relevant Applications

The properties and applications of MXenes (a family of layered transition metal carbides, nitrides, and carbonitrides) have aroused enormous research interests for a decade since the successful synthesis of few-layer transition metal carbides in 2011. Though MXenes, as the building blocks, have already been applied in various fields (such as wearable electronics) owing to the distinctive optical, mechanical and electrical properties, their thermal stability and intrinsic thermal properties were less thoroughly investigated compared to other characteristics in early reports. The pioneering theoretical prediction of the thermoelectric nature of MXenes was performed in 2013 while the first experiment-based report concerning the degradation behavior of the 2D structure at elevated temperatures in a controlled atmosphere was published in 2015, followed by numerous discoveries regarding the thermal properties of MXenes. Herein, after a brief description of the synthesis, this Review summarized the latest insights into the thermal stability and thermophysical properties of MXenes, and further associated these unique properties with relevant applications by multiple examples. Finally, current hurdles and challenges in this field were provided along with some advices on potential research directions in the future.

An Assessment of MXenes through Scanning Probe Microscopy

Recently, exploring the unique properties of 2D materials has constituted a new wave of research, which lead these materials to enormous applications ranging from optoelectronics to healthcare systems. Due to the profusion of surface terminated functionalities, MXenes have become an emerging class of 2D materials that can be easily integrated with other materials. The versatility of MXenes allows to tune their finest material properties for further device applications. This review initiates with the classification of preparation methods of MXenes, where the authors elaborate on the significance of top-down approaches including the exfoliation of solid layers. Next, the focus is diverted toward the materials analysis of MXenes including their terminations analysis as well as their intriguing electrical and mechanical behaviors through scanning probe microscopy. Finally, critical challenges and perspectives for MXenes analysis and applications are explored and discussed. Therefore, this comprehensive review can encourage researchers, and offer a precise direction to employ MXenes in various applications.

Characterization of phonon thermal transport of Ti3C2TxMXene thin film

Two-dimensional MXene materials with high electrotonic conductivity, good chemical stability, and unique laminar structure show great potential in the field of electrochemistry. In contrast to the widely concerned electrical properties, studies on the thermal properties of MXene materials are very limited. This paper presents a comprehensive analysis of the thermal properties of Ti₃C₂T_xMXene thin film. Thermal diffusivity and thermal conductivity of Ti₃C₂T_xfilms are characterized by the transient electro-thermal technique. The experimental results show a 16% enhancement in thermal conductivity when the temperature is increased from 307 K to 352 K. The phonon transport contributes substantially to thermal conductivity compared with electron transport. Molecular dynamic simulation is employed to further investigate the role of phonon thermal transport of Ti₃C₂layer. It is found that the combined effect of specific heat capacity, stacking structure and internal stress states is responsible for the thermal transport performance of Ti₃C₂T_xMXene thin film.

The structural, magnetic, Raman and electrical transport properties of Mn intercalated Ti3C2TX

The electronic and magnetic properties of the two-dimensional Ti₃C₂MXenes have attracted a lot of interests due to its potential applications. In this paper, Ti₃C₂T_xMXenes and Mn-doped Ti₃C₂T_xMXenes are synthesized and investigated. The experimental data shows that Mn²⁺ions are homogeneously and randomly intercalated between Ti₃C₂sheets as function terminals, which increase the interlayer distance between Ti₃C₂sheets and offer a mass of uncoupled magnetic moment. The temperature dependence of the electric resistivity of both samples show similar complex behavior although the resistivity increases dramatically as Mn doping. The inter-flake variable range hopping (VRH) dominates the low temperature electric transport behavior, while the inter-flake thermally activated hopping as well as the metallic intra-flake transport competing with the inter-flake VRH play important roles at high temperature. The increasing resistivity of the Mn-doped sample could be attributed to the increase of the interlayer distance and the enhancement of the localization of the transport electrons after Mn²⁺ions intercalation.

Tuning the Magnetic Properties of Two-Dimensional MXenes by Chemical Etching

Two-dimensional materials based on transition metal carbides have been intensively studied due to their unique properties including metallic conductivity, hydrophilicity and structural diversity and have shown a great potential in several applications, for example, energy storage, sensing and optoelectronics. While MXenes based on magnetic transition elements show interesting magnetic properties, not much is known about the magnetic properties of titanium-based MXenes. Here, we measured the magnetic properties of Ti3C2Tx MXenes synthesized by different chemical etching conditions such as etching temperature and time. Our magnetic measurements were performed in a superconducting quantum interference device (SQUID) vibrating sample. These data suggest that there is a paramagnetic-antiferromagnetic (PM-AFM) phase transition and the transition temperature depends on the synthesis procedure of MXenes. Our observation indicates that the magnetic properties of these MXenes can be tuned by the extent of chemical etching, which can be beneficial for the design of MXenes-based spintronic devices.

Surface Oxidation Modulates the Interfacial and Lateral Thermal Migration of MXene (Ti3C2Tx) Flakes

The thermal management of MXene (Ti₃C₂T_x) plays a crucial role in its performance during various emerging applications. However, it is unclear how the inevitable oxidation structure of Ti₃C₂T_x influences the thermal dissipation, which might hinder its long-term performance and even create thermal damage. Here we show the thermal migration of a Ti₃C₂T_x flake with surface oxidation in film and water by combining ultrafast pump-probe technique with molecular dynamics (MD) simulations. The results demonstrate that the oxidation at the surface could facilitate interfacial thermal migration with shorter interfacial distances but would block the lateral thermal transfer. Besides, our results also identified that the slight oxidation could not obviously change the thermal decay of Ti₃C₂T_x nanosheets in water due to similar hydrogen bonds between water and interface. The research not only provides fundamental understanding of the thermal dissipation of MXene but also benefits for designing the thermal dissipation system to the MXene device.

Highly flexible few-layer Ti3C2 MXene/cellulose nanofiber heat-spreader films with enhanced thermal conductivity

Highly flexible MXene/cellulose nanofiber heat-spreader films with enhanced thermal conductivity were fabricated via simple vacuum assisted filtration.

Shape-Adaptable 2D Titanium Carbide (MXene) Heater

Prior to the advent of the next-generation heater for wearable/on-body electronic devices, various properties are required, including conductivity, transparency, mechanical reliability, and conformability. Expansion to two-dimensional (2D) structure of metallic nanowires based on network- and mesh-type geometries has been widely exploited for realizing these heaters. However, the routes led to many drawbacks such as the low-density cross-bar linking, self-aggregation of wire, and high junction resistance. Although 2D carbon nanomaterials such as graphene and reduced graphene oxide (rGO) have shown their potentials for the purpose, CVD-grown graphene with sufficiently high conductivity was limited due to its poor processability for large-area applications, while rGO fabricated with a complex reduction process involving the use of toxic chemicals suffered from a low electrical conductivity. In this study, we demonstrate a simple and robust process, utilizing electrostatic assembling of negatively charged MXene flakes on a positively treated surface of substrate, for fabricating a metal-like 2D MXene thin film heater (TFH). Our TFH showed a high optical property (>65%), low sheet resistance (215 Ω/sq), fast electrothermal response (within dozens of seconds) with an intrinsically high electrical conductivity, and mechanical flexibility (up to 180° bending). Its capability for forming a firm and stable ionic-type interface with a counterpart surface allows us to develop a shape-adaptable and patchable thread heater (TH) that can be shaped on diverse substrates even under harsh conditions of conventional sewing or weaving processes. This work suggests that our shape-adaptable MXene heaters are potentially suitable not only for wearable devices for local heating and defrosting but also for a variety of emerging applications of soft actuators and wearable/flexible healthcare monitoring and thermotherapy.