Anisotropic Thermal Conductivity of Suspended Black Phosphorus Probed by Opto-Thermomechanical Resonance Spectromicroscopy

MoS2 phononic crystals for advanced thermal management

Effective thermal management of electronic devices encounters substantial challenges owing to the notable power densities involved. Here, we propose layered MoS2 phononic crystals (PnCs) that can effectively reduce thermal conductivity (κ) with relatively small disruption of electrical conductivity (σ), offering a potential thermal management solution for nanoelectronics. These layered PnCs exhibit remarkable efficiency in reducing κ, surpassing that of Si and SiC PnCs with similar periodicity by ~100-fold. Specifically, in suspended MoS2 PnCs, we measure an exceptionally low κ down to 0.1 watts per meter kelvin, below the amorphous limit while preserving the crystalline structure. These findings are supported by molecular dynamics simulations that account for the film thickness, porosity, and temperature. We demonstrate the approach efficiency by fabricating suspended heat-routing structures that effectively confine and guide heat flow in prespecified directions. This study underpins the immense potential of layered materials as directional heat spreaders, thermal insulators, and active components for thermoelectric devices.

Realizing the Accurate Measurements of Thermal Conductivity over a Wide Range by Scanning Thermal Microscopy Combined with Quantitative Prediction of Thermal Contact Resistance

Quantitative thermal performance measurements and thermal management at the micro-/nano scale are becoming increasingly important as the size of electronic components shrinks. Scanning thermal microscopy (SThM) is an emerging method with high spatial resolution that accurately reflects changes in local thermal signals based on a thermally sensitive probe. However, because of the unclear thermal resistance at the probe-sample interface, quantitative characterization of thermal conductivity for different kinds of materials still remains limited. In this paper, the heat transfer process considering the thermal contact resistance between the probe and sample surface is analyzed using finite element simulation and thermal resistance network model. On this basis, a mathematical empirical function is developed applicable to a variety of material systems, which depicts the relationship between the thermal conductivity of the sample and the probe temperature. The proposed model is verified by measuring ten materials with a wide thermal conductivity range, and then further validated by two materials with unknown thermal conductivity. In conclusion, this work provides the prospect of achieving quantitative characterization of thermal conductivity over a wide range and further enables the mapping of local thermal conductivity to microstructures or phases of materials.

Efficient modulation of thermal transport in two-dimensional materials for thermal management in device applications

With the development of chip technology, the density of transistors on integrated circuits is increasing and the size is gradually shrinking to the micro-/nanoscale, with the consequent problem of heat dissipation on chips becoming increasingly serious. For device applications, efficient heat dissipation and thermal management play a key role in ensuring device operation reliability. In this review, we summarize the thermal management applications based on 2D materials from both theoretical and experimental perspectives. The regulation approaches of thermal transport can be divided into two main types: intrinsic structure engineering (acting on the intrinsic structure) and non-structure engineering (applying external fields). On one hand, the thermal transport properties of 2D materials can be modulated by defects and disorders, size effect (including length, width, and the number of layers), heterostructures, structure regulation, doping, alloy, functionalizing, and isotope purity. On the other hand, strain engineering, electric field, and substrate can also modulate thermal transport efficiently without changing the intrinsic structure of the materials. Furthermore, we propose a perspective on the topic of using magnetism and light field to modulate the thermal transport properties of 2D materials. In short, we comprehensively review the existing thermal management modulation applications as well as the latest research progress, and conclude with a discussion and perspective on the applications of 2D materials in thermal management, which will be of great significance to the development of next-generation nanoelectronic devices.

2D-materials-integrated optoelectromechanics: recent progress and future perspectives

The discovery of two-dimensional (2D) materials has gained worldwide attention owing to their extraordinary optical, electrical, and mechanical properties. Due to their atomic layer thicknesses, the emerging 2D materials have great advantages of enhanced interaction strength, broad operating bandwidth, and ultralow power consumption for optoelectromechanical coupling. The van der Waals (vdW) epitaxy or multidimensional integration of 2D material family provides a promising platform for on-chip advanced nano-optoelectromechanical systems (NOEMS). Here, we provide a comprehensive review on the nanomechanical properties of 2D materials and the recent advances of 2D-materials-integrated nano-electromechanical systems and nano-optomechanical systems. By utilizing active nanophotonics and optoelectronics as the interface, 2D active NOEMS and their coupling effects are particularly highlighted at the 2D atomic scale. Finally, we share our viewpoints on the future perspectives and key challenges of scalable 2D-materials-integrated active NOEMS for on-chip miniaturized, lightweight, and multifunctional integration applications.

Thermal Transport in 2D Materials

In recent decades, two-dimensional materials (2D) such as graphene, black and blue phosphorenes, transition metal dichalcogenides (e.g., WS₂ and MoS₂), and h-BN have received illustrious consideration due to their promising properties. Increasingly, nanomaterial thermal properties have become a topic of research. Since nanodevices have to constantly be further miniaturized, thermal dissipation at the nanoscale has become one of the key issues in the nanotechnology field. Different techniques have been developed to measure the thermal conductivity of nanomaterials. A brief review of 2D material developments, thermal conductivity concepts, simulation methods, and recent research in heat conduction measurements is presented. Finally, recent research progress is summarized in this article.

Photothermal Responsivity of van der Waals Material-Based Nanomechanical Resonators

Nanomechanical resonators made from van der Waals materials (vdW NMRs) provide a new tool for sensing absorbed laser power. The photothermal response of vdW NMRs, quantified from the resonant frequency shifts induced by optical absorption, is enhanced when incorporated in a Fabry–Pérot (FP) interferometer. Along with the enhancement comes the dependence of the photothermal response on NMR displacement, which lacks investigation. Here, we address the knowledge gap by studying electromotively driven niobium diselenide drumheads fabricated on highly reflective substrates. We use a FP-mediated absorptive heating model to explain the measured variations of the photothermal response. The model predicts a higher magnitude and tuning range of photothermal responses on few-layer and monolayer NbSe₂ drumheads, which outperform other clamped vdW drum-type NMRs at a laser wavelength of 532 nm. Further analysis of the model shows that both the magnitude and tuning range of NbSe₂ drumheads scale with thickness, establishing a displacement-based framework for building bolometers using FP-mediated vdW NMRs.

Analyzing Anisotropy in 2D Rhenium Disulfide Using Dichromatic Polarized Reflectance

In-plane anisotropy in 2D rhenium disulfide (ReS₂ ) offers intriguing opportunities for designing future electronic and optical devices, and toward such applications, it is crucial to identify the crystal orientation in such 2D anisotropic materials. Existing spectroscopy or electron microscopy methods for determining the crystalline orientation often require complicated sample preparing procedures and specialized equipment, which could sometimes limit their application. In this work, a dichromatic polarized reflectance method is demonstrated, which can quickly and accurately resolve the crystal orientation (Re-Re chain) in 2D ReS₂ crystals with different thicknesses. Furthermore, it can be readily extended to multi-chromatic schemes to achieve greater measurement capability and can be easily tailored to work for different 2D materials. The method offers a simple and effective approach for studying anisotropy in 2D materials.

Micro-Kelvin Resolution at Room Temperature Using Nanomechanical Thermometry

Ultrahigh sensitivity temperature measurement is becoming increasingly relevant for different scientific and technological fields from fundamental physics to high-precision engineering applications. Here, we demonstrate the use of a nanomechanical resonator-free standing silicon nitride membranes with thicknesses in the nanoscale-for room temperature thermometry reaching an unprecedented resolution of 15 μK. These devices were characterized by using an interferometric system at high vacuum, where there are only two possible mechanisms for heat transfer: thermal conductivity and radiation. While the expected behavior should be to decrease the frequency of the mechanical resonance due to the thermoelastic effect, we observe that the nanomechanical response can be both positive and negative depending on the thermal flux: a heat point source always shifts the mechanical resonance to lower frequencies, while a distributed heat source shifts the resonance to higher frequencies.

Single-Layer MoS2 Mechanical Resonant Piezo-Sensors with High Mass Sensitivity

Thin-film resonators and scanning probe microscopies (SPM) are usually used on low-frequency mechanical systems at the nanoscale or larger. Generally, off-chip approaches are applied to detect mechanical vibrations in these systems, but these methods are not much appropriate for atomic-thin-layer devices with ultrahigh characteristic frequencies and ultrathin thickness. Primarily, those mechanical devices based on atomic-layers provide highly improved properties, which are inapproachable with conventional nanoelectromechanical systems (NEMS). In this report, the assembly and manipulation of single-atomic-layer piezo-resonators as mass sensors with eigen mechanical resonances up to gigahertz are described. The resonators utilize electronic vibration transducers based on piezo-electric polarization charges, allowing direct and optimal atomic-layer sensor exports. This direct detection affords practical applications with the previously inapproachable Q-factor and sensitivity rather than photoelectric conversion. Exploration of a 2406.26 MHz membrane vibration is indicated with a thermo-noise-limited mass resolution of ∼3.0 zg (10^-21 g) in room temperature. The fabricated mass sensors are contactless and fast and can afford a method for precision measurements of the ultrasmall mass with two-dimentional materials.

Phonon-Induced Ratchet Motion of a Water Nanodroplet on a Supported Black Phosphorene

Phonons are not supposed to carry any physical momentum as lattice vibrational modes; thus, it is believed no mass transport could be induced by phonons. In this Letter, we show that a ratchet motion of a water nanodroplet could be induced on a two-dimensional puckered lattice like black phosphorene (BP) by exciting its flexural phonons through a moving substrate. The water nanodroplet exhibits a forward motion along the armchair or a backward motion along the zigzag directions on a BP lattice that is supported on a substrate possessing a relative armchair or zigzag forward motion with BP. Through the analysis of the structure and vibrational density states of BP, it is found that in-plane lattice displacement asymmetry and the in-plane vibration asymmetry are induced by the excited flexural phonons, which determine the water nanodroplet motion as an anisotropic Brownian motor. Simulations of the nanodroplet motion as functions of the substrate relative motion speed and direction and also the substrate coupling strength with BP are performed. Results of the nanodroplet ratchet motion exhibit good agreement with the theoretical predications from calculating the Brownian motor asymmetry. Our findings reveal a promising mass transport strategy and a further understanding of phonon-related interactions in crystalline solids.

Optomechanical Measurement of Thermal Transport in Two-Dimensional MoSe2 Lattices

Nanomechanical resonators have emerged as sensors with exceptional sensitivities. These sensing capabilities open new possibilities in the studies of the thermodynamic properties in condensed matter. Here, we use mechanical sensing as a novel approach to measure the thermal properties of low-dimensional materials. We measure the temperature dependence of both the thermal conductivity and the specific heat capacity of a transition metal dichalcogenide monolayer down to cryogenic temperature, something that has not been achieved thus far with a single nanoscale object. These measurements show how heat is transported by phonons in two-dimensional systems. Both the thermal conductivity and the specific heat capacity measurements are consistent with predictions based on first-principles.