Ionic Intercalation in Two-Dimensional van der Waals Materials: In Situ Characterization and Electrochemical Control of the Anisotropic Thermal Conductivity of Black Phosphorus

A DFT Study of Band-Gap Tuning in 2D Black Phosphorus via Li+, Na+, Mg2+, and Ca2+ Ions

Black phosphorus (BP) and its two-dimensional derivative (2D-BP) have garnered significant attention as promising anode materials for electrochemical energy storage devices, including next-generation fast-charging batteries. However, the interactions between BP and light metal ions, as well as how these interactions influence BP's electronic properties, remain poorly understood. Here, we employed density functional theory (DFT) to investigate the effects of monovalent (Li+ and Na+) and divalent (Mg2+ and Ca2+) ions on the valence electronic structure of 2D-BP. Molecular orbital analysis revealed that the adsorption of divalent cations can significantly reduce the band gap, suggesting an enhancement in charge transfer rates. In contrast, the adsorption of monovalent cations had minimal impact on the band gap, suggesting the preservation of 2D-BP's intrinsic electrical properties. Energetic and charge analyses indicated that the extent of charge transfer primarily governs the ability of ions to modulate 2D-BP's electronic structure, especially under high-pressure conditions where ions are in close proximity to the 2D-BP surface. Moreover, charge polarization calculations revealed that, compared with monovalent cations, divalent cations induced greater polarization, disrupting the symmetry of the pristine 2D-BP and further influencing its electronic characteristics. These findings provide a molecular-level understanding of how ion interactions influence 2D-BP's electronic properties during ion-intercalation processes, where ions are in close proximity to the 2D-BP surface. Moreover, the calculated diffusion barrier results revealed the potential of 2D-BP as an effective anode material for lithium-ion, sodium-ion, and magnesium-ion batteries, though its performance may be limited for calcium-ion batteries. By extending our understanding of interactions between ions and 2D-BP, this work contributes to the design of efficient and reliable energy storage technologies, particularly for the next-generation fast-charging batteries.

Pressure-Induced Molding of Black Phosphorus@Ti3C2Tx Composite Electrode and Its Implications on the Lithium Storage

For the first time, an innovative pressure quenching technique is used to create the integrated electrode of the black phosphorus (BP) @Ti3C2Tx composite material, doing away with the requirement for adhesive additives and simplifying time-consuming processes. Through the formation of Ti-O-P bonds with BP, Ti3C2Tx MXenes can function as conductive additives and affect the interlayer gap. Additionally, we have found that there is a critical synthetic pressure threshold (300 kN) at which the performance of BP@Ti3C2Tx-integrated electrodes can be improved: too high of a pressure prevents lithium-ion transport because of mesopore reduction; too low of a pressure prevents Ti-O-P chemical bond formation between the two components; and suboptimal pressure does not allow for density enhancement for better electron conduction. The integrated electrode produced at 300 kN shows a discharge capacity of about 724.9 mA h/g at 0.1 A/g current density after 100 cycles, which is much larger than that obtained at 50 kN (270.2 mA h/g). Furthermore, the capacity can remain steady at 560.74 mA h/g even after 500 lengthy cycles at the high current density of 0.5 A/g. Significantly lower resistance (1.10 × 102 Ω at 300 kN; 2.02 × 103 Ω at 50 kN) and faster reaction kinetics are responsible for this improvement. This study offers a new, straightforward, and broadly useful technique for creating integrated electrodes and BP-based composite materials.

Reversible two-way tuning of thermal conductivity in an end-linked star-shaped thermoset

Polymeric thermal switches that can reversibly tune and significantly enhance their thermal conductivities are desirable for diverse applications in electronics, aerospace, automotives, and medicine; however, they are rarely achieved. Here, we report a polymer-based thermal switch consisting of an end-linked star-shaped thermoset with two independent thermal conductivity tuning mechanisms-strain and temperature modulation-that rapidly, reversibly, and cyclically modulate thermal conductivity. The end-linked star-shaped thermoset exhibits a strain-modulated thermal conductivity enhancement up to 11.5 at a fixed temperature of 60 °C (increasing from 0.15 to 2.1 W m-1 K-1). Additionally, it demonstrates a temperature-modulated thermal conductivity tuning ratio up to 2.3 at a fixed stretch of 2.5 (increasing from 0.17 to 0.39 W m-1 K-1). When combined, these two effects collectively enable the end-linked star-shaped thermoset to achieve a thermal conductivity tuning ratio up to 14.2. Moreover, the end-linked star-shaped thermoset demonstrates reversible tuning for over 1000 cycles. The reversible two-way tuning of thermal conductivity is attributed to the synergy of aligned amorphous chains, oriented crystalline domains, and increased crystallinity by elastically deforming the end-linked star-shaped thermoset.

Moiré Pattern Controlled Phonon Polarizer Based on Twisted Graphene

Twisted van der Waals materials featuring Moiré patterns present new design possibilities and demonstrate unconventional behaviors in electrical, optical, spintronic, and superconducting properties. However, experimental exploration of thermal transport across Moiré patterns has not been as extensive, despite its critical role in nanoelectronics, thermal management, and energy technologies. Here, the first experimental study is conducted on thermal transport across twisted graphene, demonstrating a phonon polarizer concept from the rotational misalignment between stacked layers. The direct thermal and acoustic measurements, structural characterizations, and atomistic modeling, reveal a modulation up to 631% in thermal conductance with various Moiré angles, while maintaining a high acoustic transmission. By comparing experiments with density functional theory and molecular dynamics simulations, mode-dependent phonon transmissions are quantified based on the angle alignment of graphene band structures and attributed to the coupling among flexural phonon modes. The agreement confirms the dominant tuning mechanisms in adjusting phonon transmission from high-frequency thermal modes while having negligible effects on low-frequency acoustic modes near Brillouin zone center. This study offers crucial insights into the fundamental thermal transport in Moiré structures, opening avenues for the invention of quantum thermal devices and new design methodologies based on manipulations of vibrational band structures and phonon spectra.

Intercalation in 2D materials and in situ studies

Intercalation of atoms, ions and molecules is a powerful tool for altering or tuning the properties - interlayer interactions, in-plane bonding configurations, Fermi-level energies, electronic band structures and spin-orbit coupling - of 2D materials. Intercalation can induce property changes in materials related to photonics, electronics, optoelectronics, thermoelectricity, magnetism, catalysis and energy storage, unlocking or improving the potential of 2D materials in present and future applications. In situ imaging and spectroscopy technologies are used to visualize and trace intercalation processes. These techniques provide the opportunity for deciphering important and often elusive intercalation dynamics, chemomechanics and mechanisms, such as the intercalation pathways, reversibility, uniformity and speed. In this Review, we discuss intercalation in 2D materials, beginning with a brief introduction of the intercalation strategies, then we look into the atomic and intrinsic effects of intercalation, followed by an overview of their in situ studies, and finally provide our outlook.

Optically Gated Electrostatic Field-Effect Thermal Transistor

Dynamic tuning of thermal transport in solids is scientifically intriguing with wide applications for thermal transport control in electronic devices. In this work, we demonstrate a thermal transistor, a device in which heat flow can be regulated using external control, realized in a topological insulator (TI) through the topological surface states. The tuning of thermal transport is achieved by using optical gating of a thin dielectric layer deposited on the TI film. The gate-dependent thermal conductivity is measured using micro-Raman thermometry. The transistor has a large ON/OFF ratio of 2.8 at room temperature and can be continuously and repetitively switched in tens of seconds by optical gating and potentially much faster by electrical gating. Such thermal transistors with a large ON/OFF ratio and fast switching times offer the possibilities of smart thermal devices for active thermal management and control in future electronic systems.

Machine-Learning-Driven High-Throughput Screening of Transition-Metal Atom Intercalated g-C3N4/MX2 (M = Mo, W; X = S, Se, Te) Heterostructures for the Hydrogen Evolution Reaction

Rising global energy demand, accompanied by environmental concerns linked to conventional fossil fuels, necessitates a shift toward cleaner and sustainable alternatives. This study focuses on the machine-learning (ML)-driven high-throughput screening of transition-metal (TM) atom intercalated g-C3N4/MX2 (M = Mo, W; X = S, Se, Te) heterostructures to unravel the rich landscape of possibilities for enhancing the hydrogen evolution reaction (HER) activity. The stability of the heterostructures and the intercalation within the substrates are verified through adhesion and binding energies, showcasing the significant impact of chalcogenide selection on the interaction properties. Based on hydrogen adsorption Gibbs free energy (ΔGH) computed via density functional theory (DFT) calculations, several ML models were evaluated, particularly random forest regression (RFR) emerges as a robust tool in predicting HER activity with a low mean absolute error (MAE) of 0.118 eV, thereby paving the way for accelerated catalyst screening. The Shapley Additive exPlanation (SHAP) analysis elucidates pivotal descriptors that influence the HER activity, including hydrogen adsorption on the C site (HC), MX layer (HMX), S site (HS), and intercalation of TM atoms at the N site (IN). Overall, our integrated approach utilizing DFT and ML effectively identifies hydrogen adsorption on the N site (site-3) of g-C3N4 as a pivotal active site, showcasing exceptional HER activity in heterostructures intercalated with Sc and Ti, underscoring their potential for advancing catalytic performance.

Electromechanical Origin of Phonon Dynamics Exhibiting Tunable Anisotropic Heat Transport in Layered Nanostructures

Owing to the structural characteristics of 2D layered nanomaterials, anisotropic thermal conductivity is considered an attractive design factor for constructing efficient heat-transfer pathways. In this study, the electromechanical origin of anisotropic thermal conduction in Ti3 C2 O2 M (M = Li, Na, K) is investigated at the atomic scale using theoretical multiscale analysis. The results demonstrate that the acoustic and optical phonon modes drive interlayer and intralayer heat conduction, respectively. Further, the lower the atomic number of the alkali ions intercalated in the Ti3 C2 O2 layer, the more immediately it responds to externally applied oscillations owing to its low inertia and high electrostatic force. The Li-ion layer exhibits an instantaneous response to vibrational excitations from an external source, making it transparent to higher phonon modes under interlayer and intralayer thermal conduction. The electromechanical modulation properties of the ion layer are further elucidated, providing practical insights into the design of anisotropic thermal paths.

Electrically gated molecular thermal switch

Controlling heat flow is a key challenge for applications ranging from thermal management in electronics to energy systems, industrial processing, and thermal therapy. However, progress has generally been limited by slow response times and low tunability in thermal conductance. In this work, we demonstrate an electronically gated solid-state thermal switch using self-assembled molecular junctions to achieve excellent performance at room temperature. In this three-terminal device, heat flow is continuously and reversibly modulated by an electric field through carefully controlled chemical bonding and charge distributions within the molecular interface. The devices have ultrahigh switching speeds above 1 megahertz, have on/off ratios in thermal conductance greater than 1300%, and can be switched more than 1 million times. We anticipate that these advances will generate opportunities in molecular engineering for thermal management systems and thermal circuit design.

Electrochemical sensor for single-cell determination of bacteria based on target-triggered click chemistry and fast scan voltammetry

Herein, an electrochemical sensor for single-cell determination of bacteria was developed based on target-triggered click chemistry and fast scan voltammetry (FSV). In it, bacteria not only are the detection target, but also can use their own metabolism to achieve first-level signal amplification. More electrochemical labels were immobilized on functionalized 2D nanomaterials to achieve second-level signal amplification. At 400 V/s, FSV can achieve third-level signal amplification. The linear range and limit of quantification (LOQ) are 1 ∼ 10⁸ CFU/mL and 1 CFU/mL, respectively. When the reaction time of E. coli-instructed Cu²⁺ reduction is extended to 120 min, PCR-free single-cell determination of E. coli was achieved by electrochemical method first time. The feasibility of the sensor was verified by analysis of E. coli in seawater and milk samples with recoveries ranging from 94% to 110%. This detection principle is widely applicable, providing a new path for the establishment of single-cell detection strategy for bacteria.

Effect of 3d Transition Metal Atom Intercalation Concentration on the Electronic and Magnetic Properties of Graphene/MoS2 Heterostructure: A First-Principles Study

The electronic and magnetic properties of graphene/MoS2 heterostructures intercalated with 3d transition metal (TM) atoms at different concentrations have been systematically investigated by first principles calculations. The results showed that all the studied systems are thermodynamically stable with large binding energies of about 3.72 eV-6.86 eV. Interestingly, all the TM-intercalated graphene/MoS2 heterostructures are ferromagnetic and their total magnetic moments increase with TM concentration. Furthermore, TM concentration-dependent spin polarization is obtained for the graphene layer and MoS2 layer due to the charge transfer between TM atoms and the layers. A significant band gap is opened for graphene in these TM-intercalated graphene/MoS2 heterostructures (around 0.094 eV-0.37 eV). With the TM concentration increasing, the band gap of graphene is reduced due to the enhanced spin polarization of graphene. Our study suggests a research direction for the manipulation of the properties of 2D materials through control of the intercalation concentration of TM atoms.

Interlayer Doping of Cu on Bilayer Black Phosphorus for Enhanced Charge Transfer and Transport Properties

Metal doping between black phosphorus (BP) layers has great advantages in modulating electronic properties. Here, the effects of Cu intercalation on charge transfer and carrier dynamics are investigated by theoretical calculations. Relative to the pristine bilayer BP, Cu suppresses the nonradiative electron-hole recombination, reducing the major pathways of energy and current loss. Furthermore, we investigate a novel pn homogeneous junction based on the Cu-doped bilayer BP, which shows enhanced transport properties and Ohmic contact characteristics. This is because doping leads to the transformation of BP from p-type to n-type, charge accumulation on conduction bands allows electrons to be easily transferred to the p-type bilayer BP, and associated electrical properties can be modulated by the doping concentration. This study has fundamental importance for understanding structure-property relationships in metal intercalation, which is an important guidance for integration and interlayer engineering for two-dimensional materials.

Anomalous thermal transport under high pressure in boron arsenide

High pressure represents extreme environments and provides opportunities for materials discovery^1-8. Thermal transport under high hydrostatic pressure has been investigated for more than 100 years and all measurements of crystals so far have indicated a monotonically increasing lattice thermal conductivity. Here we report in situ thermal transport measurements in the newly discovered semiconductor crystal boron arsenide, and observe an anomalous pressure dependence of the thermal conductivity. We use ultrafast optics, Raman spectroscopy and inelastic X-ray scattering measurements to examine the phonon bandstructure evolution of the optical and acoustic branches, as well as thermal conductivity under varied temperatures and pressures up to 32 gigapascals. Using atomistic theory, we attribute the anomalous high-pressure behaviour to competitive heat conduction channels from interactive high-order anharmonicity physics inherent to the unique phonon bandstructure. Our study verifies ab initio theory calculations and we show that the phonon dynamics-resulting from competing three-phonon and four-phonon scattering processes-are beyond those expected from classical models and seen in common materials. This work uses high-pressure spectroscopy combined with atomistic theory as a powerful approach to probe complex phonon physics and provide fundamental insights for understanding microscopic energy transport in materials of extreme properties.

Phonon Thermal Transport in Silicene/Graphene Heterobilayer Nanostructures: Effect of Interlayer Interactions

Heterostructuring, as a promising route to optimize the physical properties of 2D materials, has attracted great attention from the academic community. In this paper, we investigated the room-temperature in-plane and cross-plane phonon thermal transport in silicene/graphene van der Waals (vdW) heterostructures using molecular dynamics simulations. Our simulation results demonstrated that heat current along the graphene layer is remarkably larger than that along the silicene layer, which suggests that graphene dominates the thermal transport in silicene/graphene heterostructures. The in-plane phonon thermal conductivity of the silicene/graphene heterostructures could be a compromise between monolayer graphene and monolayer silicene. Heterostructuring can remarkably reduce the in-plane thermal conductivity of the graphene layer but increase the in-plane thermal conductivity of the silicene layer in heterobilayers compared with the freestanding monolayer counterparts because of their different structures. We also simulated the interlayer interaction strength effect on the in-plane phonon thermal conductivity and cross-plane interfacial thermal resistance of silicene/graphene heterostructures. Total in-plane phonon thermal conductivity and interfacial thermal resistance both decrease with the increase in the interlayer interaction strength in the silicene/graphene heterobilayers. In addition, the calculated interfacial thermal resistance shows the effect of the thermal transport direction across the interface. This study provides a useful reference for the thermal management regulation of 2D vdW heterostructures.

Thermal management materials for energy-efficient and sustainable future buildings

Thermal management plays a key role in improving the energy efficiency and sustainability of future building envelopes. Here, we focus on the materials perspective and discuss the fundamental needs, current status, and future opportunities for thermal management of buildings. First, we identify the primary considerations and evaluation criteria for high-performance thermal materials. Second, state-of-the-art thermal materials are reviewed, ranging from conventional thermal insulating fiberglass, mineral wool, cellulose, and foams, to aerogels and mesoporous structures, as well as multifunctional thermal management materials. Further, recent progress on passive regulation and thermal energy storage systems are discussed, including sensible heat storage, phase change materials, and radiative cooling. Moreover, we discuss the emerging materials systems with tunable thermal and other physical properties that could potentially enable dynamic and interactive thermal management solutions for future buildings. Finally, we discuss the recent progress in theory and computational design from first-principles atomistic theory, molecular dynamics, to multiscale simulations and machine learning. We expect the rational design that combines data-driven computation and multiscale experiments could bridge the materials properties from microscopic to macroscopic scales and provide new opportunities in improving energy efficiency and enabling adaptive implementation per customized demand for future buildings.

Anisotropic Angstrom-Wide Conductive Channels in Black Phosphorus by Top-down Cu Intercalation

Intercalation in black phosphorus (BP) can induce and modulate a variety of the properties including superconductivity like other two-dimensional (2D) materials. In this perspective, spatially controlled intercalation has the possibility to incorporate different properties into a single crystal of BP. We demonstrate anisotropic angstrom-wide (∼4.3 Å) Cu intercalation in BP, where Cu atoms are intercalated along a zigzag direction of BP because of its inherent anisotropy. With atomic structure, its microstructural effects, arising from the angstrom-wide Cu intercalation, were investigated and extended to relation with macrostructure. As the intercalation mechanism, it was revealed by in situ transmission electron microscopy and theoretical calculation that Cu atoms are intercalated through top-down direction of BP. The Cu intercalation anisotropically induces transition of angstrom-wide electronic channels from semiconductor to semimetal in BP. Our findings throw light on the fundamental relationship between microstructure changes and properties in intercalated BP, and tailoring anisotropic 2D materials at angstrom scale.

Anisotropic heat transfer properties of two-dimensional materials

The anisotropic heat transfer properties of two-dimensional materials play an important role in controlled heat transfer and intelligent heat management. At present, there are many references on anisotropic heat transfer of two-dimensional materials, but less systematic review of their development status, problems, and future directions. In this paper, intrinsic anisotropic heat transfer of two-dimensional materials, influencing factors and control means are introduced. The preparation methods of thin film with two-dimensional material and the influence factors of macroscopic anisotropic thermal properties are summarized. The technology of two-dimensional material oriented arrangement in matrix and the influence factors of macroscopic anisotropic thermal properties of the composite are outlined.

Tuning electrical and interfacial thermal properties of bilayer MoS2via electrochemical intercalation

Layered two-dimensional (2D) materials such as MoS₂have attracted much attention for nano- and opto-electronics. Recently, intercalation (e.g. of ions, atoms, or molecules) has emerged as an effective technique to modulate material properties of such layered 2D films reversibly. We probe both the electrical and thermal properties of Li-intercalated bilayer MoS₂nanosheets by combining electrical measurements and Raman spectroscopy. We demonstrate reversible modulation of carrier density over more than two orders of magnitude (from 0.8 × 10¹²to 1.5 × 10¹⁴cm^-2), and we simultaneously obtain the thermal boundary conductance between the bilayer and its supporting SiO₂substrate for an intercalated system for the first time. This thermal coupling can be reversibly modulated by nearly a factor of eight, from 14 ± 4.0 MW m^-2K^-1before intercalation to 1.8 ± 0.9 MW m^-2K^-1when the MoS₂is fully lithiated. These results reveal electrochemical intercalation as a reversible tool to modulate and control both electrical and thermal properties of 2D layers.

Electrochemical Intercalation in Atomically Thin van der Waals Materials for Structural Phase Transition and Device Applications

In van der Waals (vdWs) materials and heterostructures, the interlayers are bonded by weak vdWs interactions due to the lack of dangling bonds. The vdWs gap at the homo- or heterointerface provides great freedom to enrich the tunability of electronic structures by external intercalation of foreign ions or atoms at the interface, leading to the discovery of new physics and functionalities. Herein, the recent progress on electrochemical intercalation of foreign species into atomically thin vdWs materials for structural phase transition and device applications is reviewed and future opportunities are discussed. First, several kinds of electrochemical intercalation platforms to achieve the intercalation in vdWs materials and heterostructures are introduced. Next, the in situ characterization of electrochemical intercalation dynamics by state-of-the-art techniques is summarized, including optical techniques, scanning probe techniques, and electrical transport. Moreover, particular attention is paid on the experimentally reported phase transition and multifunctional applications of intercalated devices. Finally, future applications and challenges of intercalation in vdWs materials and heterostructures are proposed, including the intrinsic intercalation mechanism of solid ion conductors, exact identification of intercalated foreign species by near-field optical techniques, and the tunability of intercalation kinetics for ultrafast switching.

Bi-directional tuning of thermal transport in SrCoOx with electrochemically induced phase transitions

Unlike the wide-ranging dynamic control of electrical conductivity, there does not exist an analogous ability to tune thermal conductivity by means of electric potential. The traditional picture assumes that atoms inserted into a material's lattice act purely as a source of scattering for thermal carriers, which can only reduce thermal conductivity. In contrast, here we show that the electrochemical control of oxygen and proton concentration in an oxide provides a new ability to bi-directionally control thermal conductivity. On electrochemically oxygenating the brownmillerite SrCoO_2.5 to the perovskite SrCoO_3-δ, the thermal conductivity increases by a factor of 2.5, whereas protonating it to form hydrogenated SrCoO_2.5 effectively reduces the thermal conductivity by a factor of four. This bi-directional tuning of thermal conductivity across a nearly 10 ± 4-fold range at room temperature is achieved by using ionic liquid gating to trigger the 'tri-state' phase transitions in a single device. We elucidated the effects of these anionic and cationic species, and the resultant changes in lattice constants and lattice symmetry on thermal conductivity by combining chemical and structural information from X-ray absorption spectroscopy with thermoreflectance thermal conductivity measurements and ab initio calculations. This ability to control multiple ion types, multiple phase transitions and electronic conductivity that spans metallic through to insulating behaviour in oxides by electrical means provides a new framework for tuning thermal transport over a wide range.

In-plane anisotropic electronics based on low-symmetry 2D materials: progress and prospects

Low-symmetry layered materials such as black phosphorus (BP) have been revived recently due to their high intrinsic mobility and in-plane anisotropic properties, which can be used in anisotropic electronic and optoelectronic devices. Since the anisotropic properties have a close relationship with their anisotropic structural characters, especially for materials with low-symmetry, exploring new low-symmetry layered materials and investigating their anisotropic properties have inspired numerous research efforts. In this paper, we review the recent experimental progresses on low-symmetry layered materials and their corresponding anisotropic electrical transport, magneto-transport, optoelectronic, thermoelectric, ferroelectric, and piezoelectric properties. The boom of new low-symmetry layered materials with high anisotropy could open up considerable possibilities for next-generation anisotropic multifunctional electronic devices.

In Situ Probing Molecular Intercalation in Two-Dimensional Layered Semiconductors

The electrochemical molecular intercalation of two-dimensional layered materials (2DLMs) produces stable and highly tunable superlattices between monolayer 2DLMs and self-assembled molecular layers. This process allows unprecedented flexibility in integrating highly distinct materials with atomic/molecular precision to produce a new generation of organic/inorganic superlattices with tunable chemical, electronic, and optical properties. To better understand the intercalation process, we developed an on-chip platform based on MoS₂ model devices and used optical, electrochemical, and in situ electronic characterizations to resolve the intermediate stages during the intercalation process and monitor the evolution of the molecular superlattices. With sufficient charge injection, the organic cetyltrimethylammonium bromide (CTAB) intercalation induces the phase transition of MoS₂ from semiconducting 2H phase to semimetallic 1T phase, resulting in a dramatic increase of electrical conductivity. Therefore, in situ monitoring the evolution of the device conductance reveals the electrochemical intercalation dynamics with an abrupt conductivity change, signifying the onset of the molecule intercalation. In contrast, the intercalation of tetraheptylammonium bromide (THAB), a branched molecule in a larger size, resulting in a much smaller number of charges injected to avoid the 2H to 1T phase transition. Our study demonstrates a powerful platform for in situ monitoring the molecular intercalation of many 2DLMs (MoS₂, WSe₂, ReS₂, PdSe₂, TiS₂, and graphene) and systematically probing electronic, optical, and optoelectronic properties at the single-nanosheet level.

Intrinsic Low Thermal Conductivity and Phonon Renormalization Due to Strong Anharmonicity of Single-Crystal Tin Selenide

Two-dimensional (2D) van der Waals material tin selenide (SnSe) has recently attracted intensive interest due to its exceptional thermoelectric performance. However, the thermal properties and phonon transport mechanisms in its single-crystal form remain elusive. Here, we measured high-quality SnSe single crystals using nanoscale thermometry based on ultrafast optical spectroscopy and found that its intrinsic thermal conductivity is highly anisotropic in different crystallographic directions. To quantify phonon anharmonicity, we developed a new experimental approach combining picosecond ultrasonics and X-ray diffraction to enable direct measurement of temperature-dependent sound velocity, thermal expansion coefficient, and Grüneisen parameter. The measured Grüneisen parameter suggests an abnormally large temperature effect on phonon dispersion that contributes to over 90% of phonon frequency shifts. Furthermore, we performed ab initio calculations using different methods: in comparison with self-consistent phonon theory, the harmonic and quasi-harmonic models that have been widely used in current phonon calculations fail to accurately predict these important thermophysical properties at room temperature and below. Our study reveals an extremely strong intrinsic anharmonicity in SnSe that introduces phonon renormalization near room temperature. This study represents an important research benchmark in characterizing high-performance thermal energy materials and provides fundamental insight into advancing modern calculation methods for phonon transport theory.

Anisotropic Thermal Boundary Resistance across 2D Black Phosphorus: Experiment and Atomistic Modeling of Interfacial Energy Transport

Interfacial thermal boundary resistance (TBR) plays a critical role in near-junction thermal management of modern electronics. In particular, TBR can dominate heat dissipation and has become increasingly important due to the continuous emergence of novel nanomaterials with promising electronic and thermal applications. A highly anisotropic TBR across a prototype 2D material, i.e., black phosphorus, is reported through a crystal-orientation-dependent interfacial transport study. The measurements show that the metal-semiconductor TBR of the cross-plane interfaces is 241% and 327% as high as that of the armchair and zigzag direction-oriented interfaces, respectively. Atomistic ab initio calculations are conducted to analyze the anisotropic and temperature-dependent TBR using density functional theory (DFT)-derived full phonon dispersion relation and molecular dynamics simulation. The measurement and modeling work reveals that such a highly anisotropic TBR can be attributed to the intrinsic band structure and phonon spectral transmission. Furthermore, it is shown that phonon hopping between different branches is important to modulate the interfacial transport process but with directional preferences. A critical fundamental understanding of interfacial thermal transport and TBR-structure relationships is provided, which may open up new opportunities in developing advanced thermal management technology through the rational control over nanostructures and interfaces.

Ultra-low lattice thermal conductivity of monolayer penta-silicene and penta-germanene

We study the lattice thermal conductivity of two-dimensional (2D) pentagonal systems, such as penta-silicene and penta-germanene.

Mechanical properties of molybdenum diselenide revealed by molecular dynamics simulation and support vector machine

Despite the spurring interests in two-dimensional transition metal dichalcogenide (TMDC) materials, knowledge on the mechanical properties of one of their important member, i.e., molybdenum diselenide (MoSe2) is scarce and remains an open topic. In this work, the mechanical properties of h-MoSe2 and t-MoSe2 were systematically investigated using classical molecular dynamics (MD) simulations combined with machine learning (ML) techniques. The effects of chirality, temperature and strain rate on fracture strain, fracture strength and Young's modulus were characterized in both armchair and zigzag directions. For h-MoSe2, the fracture strengths were 13.6 and 13.0 GPa for armchair and zigzag chiralities, respectively, at 1 K and strain rate of 5 × 10-4 ps-1; the corresponding fracture strains were 0.23 and 0.27. The Young's moduli in armchair and zigzag directions exhibited similar values of 100.9 and 99.5 GPa, respectively. For t-MoSe2, much lower fracture strengths of 6.1 and 6.3 GPa, fracture strains of 0.13 and 0.15, and Young's moduli of 83.7 and 83.0 GPa were predicted under the same conditions. A total of 700 MD simulation cases were calculated under different impact factors and initial conditions, which were subsequently fed into the support vector machine (SVM) algorithm for ML modeling. After training, the ML model could predict the mechanical properties of both MoSe2 types given only the input features such as chirality, temperature and strain rate.

Design of phosphorene/graphene heterojunctions for high and tunable interfacial thermal conductance

Using density functional theory calculations and molecular dynamics simulations, we systematically explore various possible atomic structures of phosphorene/graphene in-plane heterojunctions and their effects on interfacial thermal conductance (ITC). Unlike the remarkable orientation-dependence of thermal conductivity in pure phosphorene, the ITC is much less orientation-dependent. In addition, the ITC is found to be high, comparable to those of graphene-MoS2 in-plane heterojunctions and chemically-bonded graphene-metal heterojunctions. Moreover, the ITC of armchair heterojunctions abnormally increases with tensile strain, while the zigzag heterojunctions simply follow the normal trend. To gain an in-depth understanding of these interesting observations, we further analyze the atomic topology and phonon vibrational spectrum and examine the nonlinear interfacial coupling in the heat transport, ITC anisotropy, and temperature effect on the ITC. Our findings suggest that phonon anharmonicity plays a critical role in the thermal transport behavior of two-dimensional in-plane heterojunctions.

An electrochemical thermal transistor

The ability to actively regulate heat flow at the nanoscale could be a game changer for applications in thermal management and energy harvesting. Such a breakthrough could also enable the control of heat flow using thermal circuits, in a manner analogous to electronic circuits. Here we demonstrate switchable thermal transistors with an order of magnitude thermal on/off ratio, based on reversible electrochemical lithium intercalation in MoS2 thin films. We use spatially-resolved time-domain thermoreflectance to map the lithium ion distribution during device operation, and atomic force microscopy to show that the lithiated state correlates with increased thickness and surface roughness. First principles calculations reveal that the thermal conductance modulation is due to phonon scattering by lithium rattler modes, c-axis strain, and stacking disorder. This study lays the foundation for electrochemically-driven nanoscale thermal regulators, and establishes thermal metrology as a useful probe of spatio-temporal intercalant dynamics in nanomaterials.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.

A new elliptical-beam method based on time-domain thermoreflectance (TDTR) to measure the in-plane anisotropic thermal conductivity and its comparison with the beam-offset method

Materials lacking in-plane symmetry are ubiquitous in a wide range of applications such as electronics, thermoelectrics, and high-temperature superconductors, in all of which the thermal properties of the materials play a critical part. However, very few experimental techniques can be used to measure in-plane anisotropic thermal conductivity. A beam-offset method based on time-domain thermoreflectance (TDTR) was previously proposed to measure in-plane anisotropic thermal conductivity. However, a detailed analysis of the beam-offset method is still lacking. Our analysis shows that uncertainties can be large if the laser spot size or the modulation frequency is not properly chosen. Here we propose an alternative approach based on TDTR to measure in-plane anisotropic thermal conductivity using a highly elliptical pump (heating) beam. The highly elliptical pump beam induces a quasi-one-dimensional temperature profile on the sample surface that has a fast decay along the short axis of the pump beam. The detected TDTR signal is exclusively sensitive to the in-plane thermal conductivity along the short axis of the elliptical beam. By conducting TDTR measurements as a function of delay time with the rotation of the elliptical pump beam to different orientations, the in-plane thermal conductivity tensor of the sample can be determined. In this work, we first conduct detailed signal sensitivity analyses for both techniques and provide guidelines in determining the optimal experimental conditions. We then compare the two techniques under their optimal experimental conditions by measuring the in-plane thermal conductivity tensor of a ZnO [11-20] sample. The accuracy and limitations of both methods are discussed.

Anisotropic thermal conductivity measurement using a new Asymmetric-Beam Time-Domain Thermoreflectance (AB-TDTR) method

Anisotropic thermal properties are of both fundamental and practical interests, but remain challenging to characterize using conventional methods. In this work, a new metrology based on asymmetric beam time-domain thermoreflectance (AB-TDTR) is developed to measure three-dimensional anisotropic thermal transport by extending the conventional TDTR technique. Using an elliptical laser beam with controlled elliptical ratio and spot size, the experimental signals can be exploited to be dominantly sensitive to measure thermal conductivity along the cross-plane or any specific in-plane directions. An analytic solution for a multi-layer system is derived for the AB-TDTR signal in response to the periodical pulse, elliptical laser beam, and heating geometry to extract the anisotropic thermal conductivity from experimental measurement. Examples with experimental data are given for various materials with in-plane thermal conductivity from 5 W/m K to 2000 W/m K, including isotropic materials (silicon, boron phosphide, and boron nitride), transversely isotropic materials (graphite, quartz, and sapphire), and transversely anisotropic materials (black phosphorus). Furthermore, a detailed sensitivity analysis is conducted to guide the optimal setting of experimental configurations for different materials. The developed AB-TDTR metrology provides a new approach to accurately measure anisotropic thermal phenomena for rational materials design and thermal applications.

Low-Power, Electrochemically Tunable Graphene Synapses for Neuromorphic Computing

Brain-inspired neuromorphic computing has the potential to revolutionize the current computing paradigm with its massive parallelism and potentially low power consumption. However, the existing approaches of using digital complementary metal-oxide-semiconductor devices (with "0" and "1" states) to emulate gradual/analog behaviors in the neural network are energy intensive and unsustainable; furthermore, emerging memristor devices still face challenges such as nonlinearities and large write noise. Here, an electrochemical graphene synapse, where the electrical conductance of graphene is reversibly modulated by the concentration of Li ions between the layers of graphene is presented. This fundamentally different mechanism allows to achieve a good energy efficiency (<500 fJ per switching event), analog tunability (>250 nonvolatile states), good endurance, and retention performances, and a linear and symmetric resistance response. Essential neuronal functions such as excitatory and inhibitory synapses, long-term potentiation and depression, and spike timing dependent plasticity with good repeatability are demonstrated. The scaling study suggests that this simple, two-dimensional synapse is scalable in terms of switching energy and speed.

Post-Graphene 2D Chemistry: The Emerging Field of Molybdenum Disulfide and Black Phosphorus Functionalization

The current state of the chemical functionalization of three types of single sheet 2D materials, namely, graphene, molybdenum disulfide (MoS2 ), and black phosphorus (BP) is summarized. Such 2D sheet polymers are currently an emerging field at the interface of synthetic chemistry, physics, and materials science. Both covalent and non-covalent functionalization of sheet architectures allows a systematic modification of their properties, that is, an improvement of solubility and processability, the prevention of re-aggregation, or band-gap tuning. Next to successful functionalization concepts, fundamental challenges are also addressed. These include the insolubility and polydispersity of most 2D sheet polymers, the development of suitable characterization tools, the identification of effective binding strategies, the chemical activation of the usually rather unreactive basal planes for covalent addend binding, and the regioselectivity of plane addition reactions. Although a number of these questions remain elusive in this Review, the first promising concepts to overcome such hurdles are presented.

Straightforward measurement of anisotropic thermal properties of a Bi2Se3 single crystal

We demonstrate here a simple measurement protocol which allows the thermal properties of anisotropic crystalline materials to be determined. This protocol is validated by the measurement of Bi₂Se₃, a layered material consisting of covalently bonded sheets with weak van der Waals bonds between each layer, which has highly anisotropic thermal properties. Thermoreflectance microscopy measurements were carried out on a single-crystal Bi₂Se₃ sample, firstly on the bare sample and then after capping with a 100 nm thick gold layer. Whereas on the bare sample lateral heat diffusion is dominated by the in-plane thermal diffusivity, on the metal-capped substrate heat diffusion perpendicular to the sample surface dominates. Using a simple theoretical model, we show how this double measurement protocol allows the anisotropic thermal conductivity coefficients of bulk Bi₂Se₃ to be evaluated.

Novel double anisotropic conductive flexible composite film endued with improved luminescence

Brand-new double anisotropic conductive flexible composite films (ACFs) were firstly put forward, devised and fabricated. The flexible array composite films were constructed via electrospinning using highly aligned Janus nanoribbons as conductive and constitutive units. The Janus nanoribbon consists of two parts, which are respectively conducting side and insulating-luminescent side. The Janus nanoribbons array composite film has two layers, and the two layers are combined tightly to form a top-to-bottom structure. In the composite film, the length direction of the Janus nanoribbons (namely conducting direction) in the two layers is perpendicular, so that a composite film with double electrically conductive anisotropy is achieved. In addition, by adjusting the content of PANI, conductive anisotropy of each layer of the composite film can be tuned, and the conductance in the conducting direction is about 10⁸ times stronger than that in the insulating direction. The Janus nanoribbon array composite films also have tunable and improved luminescent properties, achieving bi-functionality of double anisotropically electrical conduction and luminescence. The proposed design concept and preparation technology will provide theoretical and technical support for the design and fabrication of novel multifunctional ACFs.

Brand-new double anisotropic conductive flexible composite films endued with tuned electrically conductive anisotropy and improved luminescence were prepared by electrospinning.

Thermal Properties and Phonon Spectral Characterization of Synthetic Boron Phosphide for High Thermal Conductivity Applications

Heat dissipation is an increasingly critical technological challenge in modern electronics and photonics as devices continue to shrink to the nanoscale. To address this challenge, high thermal conductivity materials that can efficiently dissipate heat from hot spots and improve device performance are urgently needed. Boron phosphide is a unique high thermal conductivity and refractory material with exceptional chemical inertness, hardness, and high thermal stability, which holds high promises for many practical applications. So far, however, challenges with boron phosphide synthesis and characterization have hampered the understanding of its fundamental properties and potential applications. Here, we describe a systematic thermal transport study based on a synergistic synthesis-experimental-modeling approach: we have chemically synthesized high-quality boron phosphide single crystals and measured their thermal conductivity as a record-high 460 W/mK at room temperature. Through nanoscale ballistic transport, we have, for the first time, mapped the phonon spectra of boron phosphide and experimentally measured its phonon mean free-path spectra with consideration of both natural and isotope-pure abundances. We have also measured the temperature- and size-dependent thermal conductivity and performed corresponding calculations by solving the three-dimensional and spectral-dependent phonon Boltzmann transport equation using the variance-reduced Monte Carlo method. The experimental results are in good agreement with that predicted by multiscale simulations and density functional theory, which together quantify the heat conduction through the phonon mode dependent scattering process. Our finding underscores the promise of boron phosphide as a high thermal conductivity material for a wide range of applications, including thermal management and energy regulation, and provides a detailed, microscopic-level understanding of the phonon spectra and thermal transport mechanisms of boron phosphide. The present study paves the way toward the establishment of a new framework, based on the phonon spectra-material structure relationship, for the rational design of high thermal conductivity materials and nano- to multiscale devices.

Exploring the Formation of Black Phosphorus Intercalation Compounds with Alkali Metals

Black phosphorus intercalation compounds (BPICs) with alkali metals (namely: K and Na) have been synthesized in bulk by solid‐state as well as vapor‐phase reactions. By means of a combination of in situ X‐ray diffraction, Raman spectroscopy, and DFT calculations the structural behavior of the BPICs at different intercalation stages has been demonstrated for the first time. Our results provide a glimpse into the very first steps of a new family of intercalation compounds, with a distinct behavior as compared to its graphite analogues (GICs), showing a remarkable structural complexity and a dynamic behavior.