Ultrahigh thermal isolation across heterogeneously layered two-dimensional materials

How scanning probe microscopy can be supported by artificial intelligence and quantum computing?

The impact of Artificial Intelligence (AI) is rapidly expanding, revolutionizing both science and society. It is applied to practically all areas of life, science, and technology, including materials science, which continuously requires novel tools for effective materials characterization. One of the widely used techniques is scanning probe microscopy (SPM). SPM has fundamentally changed materials engineering, biology, and chemistry by providing tools for atomic-precision surface mapping. Despite its many advantages, it also has some drawbacks, such as long scanning times or the possibility of damaging soft-surface materials. In this paper, we focus on the potential for supporting SPM-based measurements, with an emphasis on the application of AI-based algorithms, especially Machine Learning-based algorithms, as well as quantum computing (QC). It has been found that AI can be helpful in automating experimental processes in routine operations, algorithmically searching for optimal sample regions, and elucidating structure-property relationships. Thus, it contributes to increasing the efficiency and accuracy of optical nanoscopy scanning probes. Moreover, the combination of AI-based algorithms and QC may have enormous potential to enhance the practical application of SPM. The limitations of the AI-QC-based approach were also discussed. Finally, we outline a research path for improving AI-QC-powered SPM. RESEARCH HIGHLIGHTS: Artificial intelligence and quantum computing as support for scanning probe microscopy. The analysis indicates a research gap in the field of scanning probe microscopy. The research aims to shed light into ai-qc-powered scanning probe microscopy.

Extreme Thermal Insulation and Tradeoff of Thermal Transport Mechanisms between Graphene and WS2 Monolayers

Controlling and understanding the heat flow at a nanometer scale are challenging, but important for fundamental science and applications. Two-dimensional (2D) layered materials provide perhaps the ultimate solution for meeting these challenges. While there have been reports of low thermal conductivities (several mW m-1 K-1) across the 2D heterostructures, phonon-dominant thermal transport remains strong due to the nearly-ideal contact between the layers. Here, this work experimentally explores the heat transport mechanisms by increasing the interlayer distance from perfect contact to a few nanometers and demonstrates that the phonon-dominated thermal conductivity across the WS2/graphene interface decreases further with the increasing interlayer distance until the air-dominated thermal conductivity increases again. This work finds that the resulting tradeoff of the two heat conduction mechanisms leads to the existence of a minimum thermal conductivity at 2.11 nm of 1.41 × 10-5 W m-1 K-1, which is two thousandths of the smallest value reported previously. This work provides an effective methodology for engineering thermal insulation structures and understanding heat transport at the ultimate small scales.

Nonvolatile Phase-Only Transmissive Spatial Light Modulator with Electrical Addressability of Individual Pixels

Active metasurfaces with tunable subwavelength-scale nanoscatterers are promising platforms for high-performance spatial light modulators (SLMs). Among the tuning methods, phase-change materials (PCMs) are attractive because of their nonvolatile, threshold-driven, and drastic optical modulation, rendering zero-static power, crosstalk immunity, and compact pixels. However, current electrically controlled PCM-based metasurfaces are limited to global amplitude modulation, which is insufficient for SLMs. Here, an individual-pixel addressable, transmissive metasurface is experimentally demonstrated using the low-loss PCM Sb2Se3 and doped silicon nanowire heaters. The nanowires simultaneously form a diatomic metasurface, supporting a high-quality-factor (∼406) quasi-bound-state-in-the-continuum mode. A global phase-only modulation of ∼0.25π (∼0.2π) in simulation (experiment) is achieved, showing ten times enhancement. A 2π phase shift is further obtained using a guided-mode resonance with enhanced light-Sb2Se3 interaction. Finally, individual-pixel addressability and SLM functionality are demonstrated through deterministic multilevel switching (ten levels) and tunable far-field beam shaping. Our work presents zero-static power transmissive phase-only SLMs, enabled by electrically controlled low-loss PCMs and individual meta-molecule addressable metasurfaces.

Interfacial Thermal Resistive Switching in (Pt,Cr)/SrTiO3 Devices

The operation of oxide-based memristive devices relies on the fast accumulation and depletion of oxygen vacancies by an electric field close to the metal-oxide interface. Here, we show that the reversible change of the local concentration of oxygen vacancies at this interface also produces a change in the thermal boundary resistance (TBR), i.e., a thermal resistive switching effect. We used frequency domain thermoreflectance to monitor the interfacial metal-oxide TBR in (Pt,Cr)/SrTiO3 devices, showing a change of ≈20% under usual SET/RESET operation voltages, depending on the structure of the device. Time-dependent thermal relaxation experiments suggest ionic rearrangement along the whole area of the metal/oxide interface, apart from the ionic filament responsible for the electrical conductivity switching. The experiments presented in this work provide valuable knowledge about oxide ion dynamics in redox-based memristive devices.

Tuning the Through-Plane Lattice Thermal Conductivity in van der Waals Structures through Rotational (Dis)ordering

It has recently been demonstrated that MoS2 with irregular interlayer rotations can achieve an extreme anisotropy in the lattice thermal conductivity (LTC), which is, for example, of interest for applications in waste heat management in integrated circuits. Here, we show by atomic-scale simulations based on machine-learned potentials that this principle extends to other two-dimensional materials, including C and BN. In all three materials, introducing rotational disorder drives the through-plane LTC to the glass limit, while the in-plane LTC remains almost unchanged compared to those of the ideal bulk materials. We demonstrate that the ultralow through-plane LTC is connected to the collapse of their transverse acoustic modes in the through-plane direction. Furthermore, we find that the twist angle in periodic moiré structures representing rotational order provides an efficient means for tuning the through-plane LTC that operates for all chemistries considered here. The minimal through-plane LTC is obtained for angles between 1 and 4° depending on the material, with the biggest effect in MoS2. The angular dependence is correlated with the degree of stacking disorder in the materials, which in turn is connected to the slip surface. This provides a simple descriptor for predicting the optimal conditions at which the LTC is expected to become minimal.

Temperature and Thickness Dependence of the Thermal Conductivity in 2D Ferromagnet Fe3GeTe2

The emergence of symmetry-breaking orders such as ferromagnetism and the weak interlayer bonding in van der Waals materials offers a unique platform to engineer novel heterostructures and tune transport properties like thermal conductivity. Here, we report the experimental and theoretical study of the cross-plane thermal conductivity, κ⊥, of the van der Waals two-dimensional (2D) ferromagnet Fe3GeTe2. We observe an increase in κ⊥ with thickness, indicating a diffusive transport regime with ballistic contributions. These results are supported by the theoretical analyses of the accumulated thermal conductivity, which show an important contribution of phonons with mean free paths between 10 and 200 nm. Moreover, our experiments show a reduction of κ⊥ in the low-temperature ferromagnetic phase occurring at the magnetic transition. The calculations show that this reduction in κ⊥ is associated with a decrease in the group velocities of the acoustic phonons and an increase in the phonon-phonon scattering of the Raman modes that couple to the magnetic phase. These results demonstrate the potential of van der Waals ferromagnets for thermal transport engineering.

Specificity of the Thermal Stability and Reactivity of Two-Dimensional Layered Cu-Fe Sulfide-Mg-Based Hydroxide Compounds (Valleriites)

We recently synthesized prospective new materials composed of alternating quasi-atomic sheets of brucite-type hydroxide (Mg, Fe)(OH)2 and CuFe1-xS2 sulfide (valleriites). Herein, their thermal behavior important for many potential applications has been studied in inert (Ar) and oxidative (20% O2) atmospheres using thermogravimetry (TG) and differential scanning calorimetry (DSC) analyses and characterization with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). In the Ar media, the processes are determined by the dehydroxylation of the hydroxide layers forming MgO, with the temperature of the major endothermic maximum of the mass loss at 413 °C. Sulfide sheets start to degrade below 500 °C and melt at nearly 800 °C, with bornite, chalcopyrite, and troilite specified as the final products. In the oxidative atmosphere, the exothermic reactions with the mass increase peaked at 345 and 495 °C, corresponding to the partial and major oxidations of Cu-Fe sulfide layers. Sulfur oxides captured in magnesium hydroxide layers to form MgSO4 compromised the layer integrity and promoted the oxidation of the sulfide entities. The final products also contained minor MgO, Cu2MgO3, Fe3O4, and MgFe2O4 phases. Samples doped with Al, which decreases the content of Fe in hydroxide layers, show notably impeded decay of valleriite in argon but facilitated the oxidation of Cu-Fe sulfides, while the impact of Li (it slightly increases the number of the Fe-OH sites) was less expressed. The mutual stabilization of the two-dimensional (2D) hydroxide and sulfide layers upon heating in an inert atmosphere but not in oxygen as compared with bulk brucite and chalcopyrite was suggested to explain high thermal resistance across the stacked incommensurate sheets, which slows down the endothermic reactions and accelerates the exothermic oxidation; the high number of Fe atoms in the hydroxide sheets are expected to promote the phonon exchange and heat transfer between the layers.

Bottom-up Synthesis of Nanosheets at Various Interfaces

Nanostructured materials with high aspect ratios have been widely studied for their unique properties. In particular, nanosheets have safety, dispersibility, and nanosized effects, and nanosheets with exceptionally small thicknesses exhibit unique properties. For non-exfoliable materials, the bottom-up nanosheet growth using various interfaces as templates have been investigated. This review article presents the synthesis of nanosheets at the interfaces and layered structure; it explains the features of each interface type, its advantages, and its uniqueness. The interfaces work as templates for nanosheet synthesis. We can easily use the liquid-liquid and gas-liquid interfaces as the templates; however, the thickness of nanosheets usually becomes thick because it allows materials to grow in thickness. The solid-gas and solid-liquid interfaces can prevent nanosheets from growing in thickness. However, the removal of template solids is required after the synthesis. The layered structures of various materials provide two-dimensional reaction fields between the layers. These methods have high versatility, and the nanosheets synthesized by these methods are thin. Finally, this review examines the key challenges and opportunities associated with scalable nanosheet synthesis methods for industrial production.

Effect of Twist Angle on Interfacial Thermal Transport in Two-Dimensional Bilayers

Advances in two-dimensional (2D) devices require innovative approaches for manipulating transport properties. Analogous to the electrical and optical responses, it has been predicted that thermal transport across 2D materials can have a similar strong twist-angle dependence. Here, we report experimental evidence deviating from this understanding. In contrast to the large tunability in electrical transport, we measured an unexpected weak twist-angle dependence of interfacial thermal transport in MoS2 bilayers, which is consistent with theoretical calculations. More notably, we confirmed the existence of distinct regimes with weak and strong twist-angle dependencies for thermal transport, where, for example, a much stronger change with twist angles is expected for graphene bilayers. With atomic simulations, the distinct twist-angle effects on different 2D materials are explained by the suppression of long-wavelength phonons via the moiré superlattice. These findings elucidate the unique feature of 2D thermal transport and enable a new design space for engineering thermal devices.

Engineering Heat Transport Across Epitaxial Lattice-Mismatched van der Waals Heterointerfaces

Artificially engineered 2D materials offer unique physical properties for thermal management, surpassing naturally occurring materials. Here, using van der Waals epitaxy, we demonstrate the ability to engineer extremely insulating thermal metamaterials based on atomically thin lattice-mismatched Bi2Se3/MoSe2 superlattices and graphene/PdSe2 heterostructures with exceptional thermal resistances (70-202 m2 K/GW) and ultralow cross-plane thermal conductivities (0.012-0.07 W/mK) at room temperature, comparable to those of amorphous materials. Experimental data obtained using frequency-domain thermoreflectance and low-frequency Raman spectroscopy, supported by tight-binding phonon calculations, reveal the impact of lattice mismatch, phonon-interface scattering, size effects, temperature, and interface thermal resistance on cross-plane heat dissipation, uncovering different thermal transport regimes and the dominant role of long-wavelength phonons. Our findings provide essential insights into emerging synthesis and thermal characterization methods and valuable guidance for the development of large-area heteroepitaxial van der Waals films of dissimilar materials with tailored thermal transport characteristics.

Non-volatile electrically programmable integrated photonics with a 5-bit operation

Scalable programmable photonic integrated circuits (PICs) can potentially transform the current state of classical and quantum optical information processing. However, traditional means of programming, including thermo-optic, free carrier dispersion, and Pockels effect result in either large device footprints or high static energy consumptions, significantly limiting their scalability. While chalcogenide-based non-volatile phase-change materials (PCMs) could mitigate these problems thanks to their strong index modulation and zero static power consumption, they often suffer from large absorptive loss, low cyclability, and lack of multilevel operation. Here, we report a wide-bandgap PCM antimony sulfide (Sb₂S₃)-clad silicon photonic platform simultaneously achieving low loss (<1.0 dB), high extinction ratio (>10 dB), high cyclability (>1600 switching events), and 5-bit operation. These Sb₂S₃-based devices are programmed via on-chip silicon PIN diode heaters within sub-ms timescale, with a programming energy density of [Formula: see text]. Remarkably, Sb₂S₃ is programmed into fine intermediate states by applying multiple identical pulses, providing controllable multilevel operations. Through dynamic pulse control, we achieve 5-bit (32 levels) operations, rendering 0.50 ± 0.16 dB per step. Using this multilevel behavior, we further trim random phase error in a balanced Mach-Zehnder interferometer.

Improved Thermal Anisotropy of Multi-Layer Tungsten Telluride on Silicon Substrate

WTe₂, a low-symmetry transition metal dichalcogenide, has broad prospects in functional device applications due to its excellent physical properties. When WTe₂ flake is integrated into practical device structures, its anisotropic thermal transport could be affected greatly by the substrate, which matters a lot to the energy efficiency and functional performance of the device. To investigate the effect of SiO₂/Si substrate, we carried out a comparative Raman thermometry study on a 50 nm-thick supported WTe₂ flake (with κ_zigzag = 62.17 W·m^-1·K^-1 and κ_armchair = 32.93 W·m^-1·K^-1), and a suspended WTe₂ flake of similar thickness (with κ_zigzag = 4.45 W·m^-1·K^-1, κ_armchair = 4.10 W·m^-1·K^-1). The results show that the thermal anisotropy ratio of supported WTe₂ flake (κ_zigzag/κ_armchair ≈ 1.89) is about 1.7 times that of suspended WTe₂ flake (κ_zigzag/κ_armchair ≈ 1.09). Based on the low symmetry nature of the WTe₂ structure, it is speculated that the factors contributing to thermal conductivity (mechanical properties and anisotropic low-frequency phonons) may have affected the thermal conductivity of WTe₂ flake in an uneven manner when supported on a substrate. Our findings could contribute to the 2D anisotropy physics and thermal transport study of functional devices based on WTe₂ and other low-symmetry materials, which helps solve the heat dissipation problem and optimize thermal/thermoelectric performance for practical electronic devices.

Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer

Photoinduced charge transfer in van der Waals heterostructures occurs on the 100 fs timescale despite weak interlayer coupling and momentum mismatch. However, little is understood about the microscopic mechanism behind this ultrafast process and the role of the lattice in mediating it. Here, using femtosecond electron diffraction, we directly visualize lattice dynamics in photoexcited heterostructures of WSe₂/WS₂ monolayers. Following the selective excitation of WSe₂, we measure the concurrent heating of both WSe₂ and WS₂ on a picosecond timescale-an observation that is not explained by phonon transport across the interface. Using first-principles calculations, we identify a fast channel involving an electronic state hybridized across the heterostructure, enabling phonon-assisted interlayer transfer of photoexcited electrons. Phonons are emitted in both layers on the femtosecond timescale via this channel, consistent with the simultaneous lattice heating observed experimentally. Taken together, our work indicates strong electron-phonon coupling via layer-hybridized electronic states-a novel route to control energy transport across atomic junctions.

Control of Charge Carrier Relaxation at the Au/WSe2 Interface by Ti and TiO2 Adhesion Layers: Ab Initio Quantum Dynamics

Phonon-mediated charge relaxation plays a vital role in controlling thermal transport across an interface for efficient functioning of two-dimensional (2D) nanostructured devices. Using a combination of nonadiabatic molecular dynamics with real-time time-dependent density functional theory, we demonstrate a strong influence of adhesion layers at the Au/WSe₂ interface on nonequilibrium charge relaxation, rationalizing recent ultrafast time-resolved experiments. Ti oxide layers (TiO_x) create a barrier to the interaction between Au and WSe₂ and extend hot carrier lifetimes, creating benefits for photovoltaic and photocatalytic applications. In contrast, a metallic Ti layer accelerates the energy flow, as needed for efficient heat dissipation in electronic devices. The interaction of metallic Ti with WSe₂ causes W-Se bond scissoring and pins the Fermi level. The Ti adhesion layer enhances the electron-phonon coupling due to an increased density of states and the light mass of the Ti atom. The conclusions are robust to presence of typical point defects. The atomic-scale ab initio analysis of carrier relaxation at the interfaces advances our knowledge in fabricating nanodevices with optimized electronic and thermal properties.

Recent progress of two-dimensional heterostructures for thermoelectric applications

The rapid development of synthesis and fabrication techniques has opened up a research upsurge in two-dimensional (2D) material heterostructures, which have received extensive attention due to their superior physical and chemical properties. Currently, thermoelectric energy conversion is an effective means to deal with the energy crisis and increasingly serious environmental pollution. Therefore, an in-depth understanding of thermoelectric transport properties in 2D heterostructures is crucial for the development of micro-nano energy devices. In this review, the recent progress of 2D heterostructures for thermoelectric applications is summarized in detail. Firstly, we systematically introduce diverse theoretical simulations and experimental measurements of the thermoelectric properties of 2D heterostructures. Then, the thermoelectric applications and performance regulation of several common 2D materials, as well as in-plane heterostructures and van der Waals heterostructures, are also discussed. Finally, the challenges of improving the thermoelectric performance of 2D heterostructures materials are summarized, and related prospects are described.

A polarization-sensitive photothermoelectric photodetector based on mixed-dimensional SWCNT-MoS2 heterostructures

Mixed-dimensional van der Waals (vdW) integration has been demonstrated to be effective for the modulation of the physical properties of homogeneous materials. Herein, we reported the enhancement of photothermal conversion and decrease of thermal conductivity in metallic single-walled carbon nanotube (SWCNT) films with the integration of chemical vapor deposition-grown monolayer MoS₂ films. The induced temperature gradient in SWCNT-MoS₂ hybrid films drives carrier diffusion to generate photocurrent via the photothermoelectric (PTE) effect, and a self-powered photodetector working in the visible band range from 405 to 785 nm was demonstrated. The maximum responsivity of the device increases by 6 times compared to that of the SWCNT counterpart. More importantly, the mixed-dimensional device exhibits polarization-dependent photogeneration, showing a large anisotropy ratio of 1.55. This work paves a way for developing high-performance, polarization-sensitive photodetectors by mixed-dimensional integration.

Air-stable van der Waals PtTe2 conductors with high current-carrying capacity and strong spin-orbit interaction

High-performance van der Waals (vdW) integrated electronics and spintronics require reliable current-carrying capacity. However, it is challenging to achieve high current density and air-stable performance using vdW metals owing to the fast electrical breakdown triggered by defects or oxidation. Here, we report that spin-orbit interacted synthetic PtTe₂ layers exhibit significant electrical reliability and robustness in ambient air. The 4-nm-thick PtTe₂ synthesized at a low temperature (∼400°C) shows intrinsic metallic transport behavior and a weak antilocalization effect attributed to the strong spin-orbit scattering. Remarkably, PtTe₂ sustains a high current density approaching ≈31.5 MA cm⁻², which is the highest value among electrical interconnect candidates under oxygen exposure. Electrical failure is caused by the Joule heating of PtTe₂ rather than defect-induced electromigration, which was achievable by the native TeO_x passivation. The high-quality growth of PtTe₂ and the investigation of its transport behaviors lay out essential foundations for the development of emerging vdW spin-orbitronics.

•

The synthesized PtTe₂ had a self-passivated surface under exposure to air

•

Magnetoconductance study proved the realization of a 2D confined quantum system

•

PtTe₂ sustained a remarkably high current density (∼31.5 MA cm⁻²) under air atmosphere

•

The native TeO_x passivation retarded the defect-induced electromigration of PtTe₂

A unified approach and descriptor for the thermal expansion of two-dimensional transition metal dichalcogenide monolayers

Two-dimensional (2D) materials have enabled promising applications in modern miniaturized devices. However, device operation may lead to substantial temperature rise and thermal stress, resulting in device failure. To address such thermal challenges, the thermal expansion coefficient (TEC) needs to be well understood. Here, we characterize the in-plane TECs of transition metal dichalcogenide (TMD) monolayers and demonstrate superior accuracy using a three-substrate approach. Our measurements confirm the physical range of 2D monolayer TECs and, hence, address the more than two orders of magnitude discrepancy in literature. Moreover, we identify the thermochemical electronegativity difference of compositional elements as a descriptor, enabling the fast estimation of TECs for various TMD monolayers. Our work presents a unified approach and descriptor for the thermal expansion of TMD monolayers, which can serve as a guideline toward the rational design of reliable 2D devices.

Energy Dissipation and Electrical Breakdown in Multilayer PtSe2 Electronics

Investigating the energy dissipation in micro- and nanoscale is fundamental to improve the performance and reliability of two-dimensional (2D) electronics. Recently, 2D platinum selenide (PtSe₂) has drawn extensive attention in developing next-generation functional devices due to its distinctive fusion of versatile properties. Toward practical applications of PtSe₂ devices, it is essential to understand the interfacial thermal properties between PtSe₂ and its substrate. Among them, the thermal boundary conductance (TBC) has played a critical role for out-of-plane heat dissipation of PtSe₂ devices. Here, we identify the energy dissipation behavior of multilayer PtSe₂ devices and extract the actual TBC value of the PtSe₂/SiO₂ interface by Raman thermometry with electrical bias. The obtained TBC value is about 8.6 MW m^-2 K^-1, and it belongs to the low end of as-known solid-solid interfaces, suggesting possible applications regarding thermoelectric devices or others reliant on a large temperature gradient. Furthermore, the maximum current density of the PtSe₂ device determines its threshold power, which is crucial for improving device design and guiding future applications. Therefore, we explore the electrical breakdown profile of the multilayer PtSe₂ device, revealing the breakdown current density of 17.7 MA cm^-2 and threshold power density of 0.2 MW cm^-2, which are larger than typical values for commonly used aluminum and copper. These results provide key insights into the energy dissipation of PtSe₂ devices and make PtSe₂ an excellent candidate for thermal confinement applications and nanometer-thin interconnects, which will benefit the development of energy-efficient functional 2D devices.

Strain-Enhanced Mobility of Monolayer MoS2

Strain engineering is an important method for tuning the properties of semiconductors and has been used to improve the mobility of silicon transistors for several decades. Recently, theoretical studies have predicted that strain can also improve the mobility of two-dimensional (2D) semiconductors, e.g., by reducing intervalley scattering or lowering effective masses. Here, we experimentally show strain-enhanced electron mobility in monolayer MoS₂ transistors with uniaxial tensile strain, on flexible substrates. The on-state current and mobility are nearly doubled with tensile strain up to 0.7%, and devices return to their initial state after release of the strain. We also show a gate-voltage-dependent gauge factor up to 200 for monolayer MoS₂, which is higher than previous values reported for sub-1 nm thin piezoresistive films. These results demonstrate the importance of strain engineering 2D semiconductors for performance enhancements in integrated circuits, or for applications such as flexible strain sensors.

Nonequilibrium Phonon Thermal Resistance at MoS2/Oxide and Graphene/Oxide Interfaces

Accurate measurements and physical understanding of thermal boundary resistance (R) of two-dimensional (2D) materials are imperative for effective thermal management of 2D electronics and photonics. In previous studies, heat dissipation from 2D material devices was presumed to be dominated by phonon transport across the interfaces. In this study, we find that, in addition to phonon transport, thermal resistance between nonequilibrium phonons in the 2D materials could play a critical role too when the 2D material devices are internally self-heated, either optically or electrically. We accurately measure the R of oxide/MoS₂/oxide and oxide/graphene/oxide interfaces for three oxides (SiO₂, HfO₂, and Al₂O₃) by differential time-domain thermoreflectance (TDTR). Our measurements of R across these interfaces with external heating are 2-4 times lower than the previously reported R of the similar interfaces measured by Raman thermometry with internal self-heating. Using a simple model, we show that the observed discrepancy can be explained by an additional internal thermal resistance (R_int) between nonequilibrium phonons present during Raman measurements. We subsequently estimate that, for MoS₂ and graphene, R_int ≈ 31 and 22 m² K GW^-1, respectively. The values are comparable to the thermal resistance due to finite phonon transmission across interfaces of 2D materials and thus cannot be ignored in the design of 2D material devices. Moreover, the nonequilibrium phonons also lead to a different temperature dependence than that by phonon transport. As such, our work provides important insights into physical understanding of heat dissipation in 2D material devices.

Tuning the Thermal Transport of Hexagonal Boron Nitride/Reduced Graphene Oxide Heterostructures

Tuning the thermal properties of materials is considered to be of crucial significance for improving the performance of electronic devices. Along these lines, the development of van der Waals (vdW) heterostructures becomes an effective solution to affect the thermal transport mechanisms. However, vdW interactions usually block phonon transport, which leads to a reduction in thermal conductivity. In this work, we experimentally demonstrate a large enhancement in the thermal conductivity of a vdW heterostructure composed of few-layer hexagonal boron nitride (h-BN) and reduced graphene oxide (RGO). By controlling the reduction temperature of RGO and changing the thickness of h-BN, the thermal conductivity of the RGO is increased by nearly 18 times, namely, from 91 to 1685 W m^-1 K^-1. Photothermal scanning imaging is used to reveal the changes in the heat transfer and temperature distribution of the h-BN/RGO heterostructure. Both photothermal scanning and Raman spectroscopy experiments show that the vdW interaction between h-BN and RGO can greatly increase the thermal conductivity of RGO, which is in contrast to the conventional understanding that vdW interaction reduces thermal conductivity. Our work paves the way for the manipulation of the thermal conductivity of two-dimensional (2D) heterostructures, which could be of great significance for future nanoelectronic circuits.

Time-resolved vibrational-pump visible-probe spectroscopy for thermal conductivity measurement of metal-halide perovskites

Understanding thermal transport at the microscale to the nanoscale is crucially important for a wide range of technologies ranging from device thermal management and protection systems to thermal-energy regulation and harvesting. In the past decades, non-contact optical methods, such as time-domain and frequency-domain thermoreflectance, have emerged as extremely powerful and versatile thermal metrological techniques for the measurement of material thermal conductivities. Here, we report the measurement of thermal conductivity of thin films of CH₃NH₃PbI₃ (MAPbI₃), a prototypical metal-halide perovskite, by developing a time-resolved optical technique called vibrational-pump visible-probe (VPVP) spectroscopy. The VPVP technique relies on the direct thermal excitation of MAPbI₃ by femtosecond mid-infrared optical pump pulses that are wavelength-tuned to a vibrational mode of the material, after which the time dependent optical transmittance across the visible range is probed in the ns to the μs time window using a broadband pulsed laser. Using the VPVP method, we determine the thermal conductivities of MAPbI₃ thin films deposited on different substrates. The transducer-free VPVP method reported here is expected to permit spectrally resolving and spatiotemporally imaging of the dynamic lattice temperature variations in organic, polymeric, and hybrid organic-inorganic semiconductors.

Direct measurement of nanoscale filamentary hot spots in resistive memory devices

Resistive random access memory (RRAM) is an important candidate for both digital, high-density data storage and for analog, neuromorphic computing. RRAM operation relies on the formation and rupture of nanoscale conductive filaments that carry enormous current densities and whose behavior lies at the heart of this technology. Here, we directly measure the temperature of these filaments in realistic RRAM with nanoscale resolution using scanning thermal microscopy. We use both conventional metal and ultrathin graphene electrodes, which enable the most thermally intimate measurement to date. Filaments can reach 1300°C during steady-state operation, but electrode temperatures seldom exceed 350°C because of thermal interface resistance. These results reveal the importance of thermal engineering for nanoscale RRAM toward ultradense data storage or neuromorphic operation.

Wafer-Scale Programmed Assembly of One-Atom-Thick Crystals

Crystalline films offer various physical properties based on the modulation of their thicknesses and atomic structures. The layer-by-layer assembly of atomically thin crystals provides a powerful means to arbitrarily design films at the atomic level, which are unattainable with existing growth technologies. However, atomically clean assembly of the materials with high scalability and reproducibility remains challenging. We report programmed crystal assembly of graphene and monolayer hexagonal boron nitride, assisted by van der Waals interactions, to form wafer-scale films of pristine interfaces with near-unity yield. The atomic configurations of the films are tailored with layer-resolved compositions and in-plane crystalline orientations. We demonstrate batch-fabricated tunnel device arrays with modulation of the resistance over orders of magnitude by thickness control of the hexagonal boron nitride barrier with single-atom precision and large-scale, twisted multilayer graphene with programmable electronic band structures and crystal symmetries. Our results constitute an important development in the artificial design of large-scale films.

Giant Thermal Transport Tuning at a Metal/Ferroelectric Interface

Interfacial thermal transport plays a prominent role in the thermal management of nanoscale objects and is of fundamental importance for basic research and nanodevices. At metal/insulator interfaces, a configuration commonly found in electronic devices, heat transport strongly depends upon the effective energy transfer from thermalized electrons in the metal to the phonons in the insulator. However, the mechanism of interfacial electron-phonon coupling and thermal transport at metal/insulator interfaces is not well understood. Here, the observation of a substantial enhancement of the interfacial thermal resistance and the important role of surface charges at the metal/ferroelectric interface in an Al/BiFeO₃ membrane are reported. By applying uniaxial strain, the interfacial thermal resistance can be varied substantially (up to an order of magnitude), which is attributed to the renormalized interfacial electron-phonon coupling caused by the charge redistribution at the interface due to the polarization rotation. These results imply that surface charges at a metal/insulator interface can substantially enhance the interfacial electron-phonon-mediated thermal coupling, providing a new route to optimize the thermal transport performance in next-generation nanodevices, power electronics, and thermal logic devices.

Critical Problems Faced in Raman-based Energy Transport Characterization of Nanomaterials

In the last two decades, tremendous research has been conducted on discovery, design and synthesis, characterization, and applications of two-dimensional (2D) materials. Thermal conductivity and interface thermal conductance/resistance of 2D...

Engineering Thermal Transport across Layered Graphene-MoS2 Superlattices

Layering two-dimensional van der Waals materials provides a high degree of control over atomic placement, which could enable tailoring of vibrational spectra and heat flow at the sub-nanometer scale. Here, using spatially resolved ultrafast thermoreflectance and spectroscopy, we uncover the design rules governing cross-plane heat transport in superlattices assembled from monolayers of graphene (G) and MoS₂ (M). Using a combinatorial experimental approach, we probe nine different stacking sequences, G, GG, MG, GGG, GMG, GGMG, GMGG, GMMG, and GMGMG, and identify the effects of vibrational mismatch, interlayer adhesion, and junction asymmetry on thermal transport. Pure G sequences display evidence of quasi-ballistic transport, whereas adding even a single M layer strongly disrupts heat conduction. The experimental data are described well by molecular dynamics simulations, which include thermal expansion, accounting for the effect of finite temperature on the interlayer spacing. The simulations show that an increase of ∼2.4% in the layer separation of GMGMG, relative to its value at 300 K, can lead to a doubling of the thermal resistance. Using these design rules, we experimentally demonstrate a five-layer GMGMG superlattice "thermal metamaterial" with an ultralow effective cross-plane thermal conductivity comparable to that of air.

Control of Thermal Conductance across Vertically Stacked Two-Dimensional van der Waals Materials via Interfacial Engineering

A comprehensive understanding of the roles of various nanointerfaces in thermal transport is of critical significance but remains challenging. A two-dimensional van der Waals (vdW) heterostructure with tunable interface lattice mismatch provides an ideal platform to explore the correlation between thermal properties and nanointerfaces and achieve controllable tuning of heat flow. Here, we demonstrate that interfacial engineering is an efficient strategy to tune thermal transport via systematic investigation of the thermal conductance (G) across a series of large-area four-layer stacked vdW materials using an improved polyethylene glycol-assisted time-domain thermoreflectance method. Owing to its rich interfacial mismatch and weak interfacial coupling, the vertically stacked MoSe₂-MoS₂-MoSe₂-MoS₂ heterostructure demonstrates the lowest G of 1.5 MW m^-2 K^-1 among all vdW structures. A roadmap to tune G via homointerfacial mismatch, interfacial coupling, and heterointerfacial mismatch is further demonstrated for thermal tuning. Our work reveals the roles of various interfacial effects on heat flow and highlights the importance of the interfacial mismatch and coupling effects in thermal transport. The design principle is also promising for application in other areas, such as the electrical tuning of energy storage and conversion and the thermoelectricity tuning of thermoelectronics.

Heteroepitaxial van der Waals semiconductor superlattices

A broad range of transition metal dichalcogenide (TMDC) semiconductors are available as monolayer (ML) crystals, so the precise integration of each kind into van der Waals (vdW) superlattices (SLs) could enable the realization of novel structures with previously unexplored functionalities. Here we report the atomic layer-by-layer epitaxial growth of vdW SLs with programmable stacking periodicities, composed of more than two kinds of dissimilar TMDC MLs, such as MoS₂, WS₂ and WSe₂. Using kinetics-controlled vdW epitaxy in the near-equilibrium limit by metal-organic chemical vapour depositions, we achieved precise ML-by-ML stacking, free of interlayer atomic mixing, which resulted in tunable two-dimensional vdW electronic systems. As an example, by exploiting the series of type II band alignments at coherent two-dimensional vdW heterointerfaces, we demonstrated valley-polarized carrier excitations-one of the most distinctive electronic features in vdW ML semiconductors-which scale with the stack numbers n in our (MoS₂/WS₂)_n SLs on optical excitations.

Designing New-Generation Piezoelectric Transducers by Embedding Superior Graphene-Based Thermal Regulators

Cascaded-piezoelectric-transducers (CPETs) is a key component in modern energy-conversion fields, possessing versatile applications in ultrasonic scalpels, acoustic levitation, and sonar. However, serious self-heating inevitably occurs inside high-power CPETs, severely limiting their practical applications in broader fields. To tackle this, multidirectional heat-escape channels of multidimensional (multi-D, 3D/2D) graphene films are introduced in designing new-type thermal regulators. A porous AlN-ceramic thermal-sink is creatively selected as a template for directly synthesizing graphene via a two-step chemical vapor deposition strategy. This perfect combination of 3D/2D-graphene and the AlN ceramic can integrate their complementary advantages in uniformizing, transmitting, and releasing heat. Amazingly, in the new-generation CPETs embedded with these graphene-based thermal regulators, the self-heating-induced temperature rise can be substantially reduced by ≈60% (far exceeding actual demand standard). As another kernel parameter, electroacoustic-energy-conversion efficiency is dramatically improved in the new-generation CPETs. Briefly, this research realizes the first synthesis of a novel multi-D-graphene/AlN-ceramic hybrid, and propels its brand-new application directions in new-generation energy-conversion- and thermal-management-related territories.

Phonon-engineered extreme thermal conductivity materials

Materials with ultrahigh or low thermal conductivity are desirable for many technological applications, such as thermal management of electronic and photonic devices, heat exchangers, energy converters and thermal insulation. Recent advances in simulation tools (first principles, the atomistic Green's function and molecular dynamics) and experimental techniques (pump-probe techniques and microfabricated platforms) have led to new insights on phonon transport and scattering in materials and the discovery of new thermal materials, and are enabling the engineering of phonons towards desired thermal properties. We review recent discoveries of both inorganic and organic materials with ultrahigh and low thermal conductivity, highlighting heat-conduction physics, strategies used to change thermal conductivity, and future directions to achieve extreme thermal conductivities in solid-state materials.

Thermal boundary conductance of monolayer beyond-graphene two-dimensional materials on SiO2and GaN

Two-dimensional (2D) materials have emerged as a platform for a broad array of future nanoelectronic devices. Here we use first-principles calculations and phonon interface transport modeling to calculate the temperature-dependent thermal boundary conductance (TBC) in single layers of beyond-graphene 2D materials silicene, hBN, boron arsenide (BAs), and blue and black phosphorene (BP) on amorphous SiO₂and crystalline GaN substrates. Our results show that for 2D/3D systems, the room temperature TBC can span a wide range from  7 to 70 MW m^-2K^-1 with the lowest being for BP and highest for hBN. We also show that 2D/3D TBC has a strong temperature dependence that can be alleviated by encapsulating the 2D/3D stack. Upon encapsulation with AlO_x, the TBC of several beyond-graphene 2D materials can match or exceed reported values for graphene and numerous transition-metal dichalcogendies which are in the range of 15-40 MW m^-2K^-1. We also compute the room temperature TBC as a function of van der Waals spring coupling (K_a) where the TBC falls in the range of 50-150 MW m^-2K^-1 at coupling strengths ofK_a = 2-4 N m^-1 for silicene, BAs, and blue phosphorene. We further identify group III-V materials with ultra-soft flexural branches as being promising 2D materials for thermal isolation and energy scavenging applications when matched with crystalline substrates.

Ultrathin Three-Monolayer Tunneling Memory Selectors

High-density memory arrays require selector devices, which enable selection of a specific memory cell within a memory array by suppressing leakage current through unselected cells. Such selector devices must have highly nonlinear current-voltage characteristics and excellent endurance; thus selectors based on a tunneling mechanism present advantages over those based on the physical motion of atoms or ions. Here, we use two-dimensional (2D) materials to build an ultrathin (three-monolayer-thick) tunneling-based memory selector. Using a sandwich of h-BN, MoS₂, and h-BN monolayers leads to an "H-shaped" energy barrier in the middle of the heterojunction, which nonlinearly modulates the tunneling current when the external voltage is varied. We experimentally demonstrate that tuning the MoS₂ Fermi level can improve the device nonlinearity from 10 to 25. These results provide a fundamental understanding of the tunneling process through atomically thin 2D heterojunctions and lay the foundation for developing high endurance selectors with 2D heterojunctions, potentially enabling high-density non-volatile memory systems.

Heat Transport Control and Thermal Characterization of Low-Dimensional Materials: A Review

Heat dissipation and thermal management are central challenges in various areas of science and technology and are critical issues for the majority of nanoelectronic devices. In this review, we focus on experimental advances in thermal characterization and phonon engineering that have drastically increased the understanding of heat transport and demonstrated efficient ways to control heat propagation in nanomaterials. We summarize the latest device-relevant methodologies of phonon engineering in semiconductor nanostructures and 2D materials, including graphene and transition metal dichalcogenides. Then, we review recent advances in thermal characterization techniques, and discuss their main challenges and limitations.

Extremely anisotropic van der Waals thermal conductors

The densification of integrated circuits requires thermal management strategies and high thermal conductivity materials^1-3. Recent innovations include the development of materials with thermal conduction anisotropy, which can remove hotspots along the fast-axis direction and provide thermal insulation along the slow axis^4,5. However, most artificially engineered thermal conductors have anisotropy ratios much smaller than those seen in naturally anisotropic materials. Here we report extremely anisotropic thermal conductors based on large-area van der Waals thin films with random interlayer rotations, which produce a room-temperature thermal anisotropy ratio close to 900 in MoS₂, one of the highest ever reported. This is enabled by the interlayer rotations that impede the through-plane thermal transport, while the long-range intralayer crystallinity maintains high in-plane thermal conductivity. We measure ultralow thermal conductivities in the through-plane direction for MoS₂ (57 ± 3 mW m^-1 K^-1) and WS₂ (41 ± 3 mW m^-1 K^-1) films, and we quantitatively explain these values using molecular dynamics simulations that reveal one-dimensional glass-like thermal transport. Conversely, the in-plane thermal conductivity in these MoS₂ films is close to the single-crystal value. Covering nanofabricated gold electrodes with our anisotropic films prevents overheating of the electrodes and blocks heat from reaching the device surface. Our work establishes interlayer rotation in crystalline layered materials as a new degree of freedom for engineering-directed heat transport in solid-state systems.

Interfacial Thermal Conductance between Monolayer WSe2 and SiO2 under Consideration of Radiative Electron-Hole Recombination

This work reports the interfacial thermal conductance (G) and radiative recombination efficiency (β), also known as photoluminescence quantum yield (PL QY), of monolayer WSe₂ flakes supported by fused silica substrates via energy-transport state-resolved Raman (ET-Raman). This is the first known work to consider the effect of radiative electron-hole recombination on the thermal transport characteristics of single-layer transition-metal dichalcogenides (TMDs). ET-Raman uses a continuous-wave laser for steady-state heating as well as nanosecond and picosecond lasers for transient energy transport to simultaneously heat the monolayer flakes and extract the Raman signal. The three lasers induce distinct heating phenomena that distinguish the interfacial thermal conductance and radiative recombination efficiency, which can then be determined in tandem with three-dimensional (3D) numerical modeling of the temperature rise from respective laser irradiation. For the five samples measured, G is found to range from 2.10 ± 0.14 to 15.9 ± 5.0 MW m^-2 K^-1 and β ranges from 36 ± 6 to 65 ± 7%. These values support the claim that interfacial phenomena such as surface roughness and two-dimensional (2D) material-substrate bonding strength play critical roles in interfacial thermal transport and electron-hole recombination mechanisms in TMD monolayers. It is also determined that low-level defect density enhances the radiative recombination efficiency of single-layer WSe₂.

Thickness-Independent Energy Dissipation in Graphene Electronics

The energy dissipation issue has become one of the greatest challenges of the modern electronic industry. Incorporating graphene into the electronic devices has been widely accepted as a promising approach to solve this issue, due to its superior carrier mobility and thermal conductivity. Here, using Raman spectroscopy and infrared thermal microscopy, we identify the energy dissipation behavior of graphene device with different thicknesses. Surprisingly, the monolayer graphene device is demonstrated to have a comparable energy dissipation efficiency per unit volume with that of a few-layer graphene device. This has overturned the traditional understanding that the energy dissipation efficiency will reduce with the decrease of functional materials dimensions. Additionally, the energy dissipation speed of the monolayer graphene device is very fast, promising for devices with high operating frequency. Our finding provides a new insight into the energy dissipation issue of two-dimensional materials devices, which will have a global effect on the development of the electronic industry.

Current Rerouting Improves Heat Removal in Few-Layer WSe2 Devices

Few-layer (FL) transition-metal dichalcogenides have drawn attention for nanoelectronics applications due to their improved mobility, owing to the partial screening of charged impurities at the oxide interface. However, under realistic operating conditions, dissipation leads to self-heating, which is detrimental to electronic and thermal properties. We fabricated a series of FL-WSe₂ devices and measured their I-V characteristics, while their temperatures were quantified by Raman thermometry and simulated from first principles. Our tightly integrated electrothermal study shows that Joule heating leads to a significant layer-dependent temperature rise, which affects mobility and alters the flow of current through the stack. This causes the temperatures in the top layers to increase dramatically, degrading their mobility and causing the current to reroute to the bottom of the FL stack where thermal conductance is higher. We discover that this current rerouting phenomenon improves heat removal because the current flows through layers closer to the substrate, limiting the severity of self-heating and its impact on carrier mobility. We also observe significant lateral heat removal via the contacts because of longer thermal healing length in the top layers and explore the optimum number of layers to maximize mobility in FL devices. Our study will impact future device designs and lead to further improvements in thermal management in van der Waals (vdW)-based devices.

Ultralow Thermal Conductivity and Thermal Diffusivity of Graphene/Metal Heterostructures through Scarcity of Low-Energy Modes in Graphene

In many ultralow thermal conductivity materials, interfaces of dissimilar materials are employed to impede heat flow perpendicular to the interfaces. However, when packed within a distance comparable to the phonon wavelengths, these interfaces are coupled and thus ineffective to scatter low-energy phonons, due to either coherent phonon transmission across the closely packed interfaces or weak coupling of the low-energy phonons and the interfaces. Here, we propose to block the propagation of these low-energy phonons by periodically distributed scarcity of available low-energy phonon modes using graphene/metal heterostructures of transferred graphene and ultrathin metal films. We demonstrate the effectiveness of graphene in blocking propagation of low-energy phonons by comparing the effective transmission probabilities of phonons in a wide range of multilayered structures; we find that interfaces in our graphene/metal heterostructures remain decoupled even when the spacing between interfaces is <2 nm. With the proposed strategy, we successfully achieve an ultralow thermal conductivity of Λ = 0.06 W m^-1 K^-1 and a world-record lowest thermal diffusivity of α = 2.6 × 10^-4 cm² s^-1 suitable for thermal insulation. Moreover, we demonstrate the capability to tune the electronic heat transport across the new materials by creating atomic-scale pinholes on graphene through magnetron sputtering, with electrons carrying ≈50% of heat when Λ is ≈0.15 W m^-1 K^-1. With the ultralow Λ and substantial electronic transport, the new graphene/metal heterostructures could be explored for thermoelectric applications.

Localized Heating and Switching in MoTe2-Based Resistive Memory Devices

Two-dimensional (2D) materials have recently been incorporated into resistive memory devices because of their atomically thin nature, but their switching mechanism is not yet well understood. Here we study bipolar switching in MoTe₂-based resistive memory of varying thickness and electrode area. Using scanning thermal microscopy (SThM), we map the surface temperature of the devices under bias, revealing clear evidence of localized heating at conductive "plugs" formed during switching. The SThM measurements are correlated to electro-thermal simulations, yielding a range of plug diameters (250 to 350 nm) and temperatures at constant bias and during switching. Transmission electron microscopy images reveal these plugs result from atomic migration between electrodes, which is a thermally-activated process. However, the initial forming may be caused by defect generation or Te migration within the MoTe₂. This study provides the first thermal and localized switching insights into the operation of such resistive memory and demonstrates a thermal microscopy technique that can be applied to a wide variety of traditional and emerging memory devices.