Xiangjun Liu

Phonon thermal conductivity of monolayer MoS2 sheet and nanoribbons

We investigated the thermal conduction of monolayer MoS2 sheet and nanoribbons using molecular dynamics simulations. Room temperature thermal conductivity of monolayer MoS2 is found to be 1.35 W/mK, which is three orders of magnitude lower than that of graphene. In contrast to the remarkable size effect observed in graphene nanoribbons, the thermal conductivity of MoS2 nanoribbons is insensitive to width (3–16 nm), length (4–111 nm), and the type of edge, which are explained by the local heat flux analysis and phonon scattering mechanisms. The low thermal conductivity together with reported high Seebeck coefficient opens up the possibility to realize MoS2-based two-dimensional thermoelectric devices.

Topological Defects at the Graphene/h-BN interface Abnormally Enhance Its Thermal Conductance

Low thermal conductance across interface is often the limiting factor in managing heat in many advanced device applications. The most commonly used approach to enhance the thermal conductance is to reduce/eliminate the interfacial structural defects. Using a graphene/h-BN (Gr/h-BN) interface, we show surprisingly that topological defects are able to enhance the thermal conductance across the interface. It is found that the phonon transmission across the Gr/h-BN interface with 5|7 defects is higher than that of the pristine interface, which is in strong contrast to the common notion that interface defects promote phonon scattering. By analyzing the strain distribution and phonon vibrational spectra, we find that this abnormal enhancement in interfacial thermal conductance originates from the localization of the stress fields arising from misfit dislocations and their out-of-plane deformations at the interface. In the presence of the defects, the overall mismatch strain is reduced. In addition, the out-of-plane deformations screen the long-ranged dislocation strain fields, resulting in the stress fields to be localized only at the cores of the defects. This abnormal mechanism provides a new dimension to enhance the interfacial thermal conductance in two-dimensional heterostructures.

Nanotube-terminated zigzag edges of phosphorene formed by self-rolling reconstruction

The edge atomic configuration often plays an important role in dictating the properties of finite-sized two-dimensional (2D) materials. By performing ab initio calculations, we identify a highly stable zigzag edge of phosphorene, which is the most stable one among all the considered edges. Surprisingly, this highly stable edge exhibits a novel nanotube-like structure, which is topologically distinctively different from any previously reported edge reconstruction. We further show that this new edge type can form easily, with an energy barrier of only 0.234 eV. It may be the dominant edge type at room temperature under vacuum conditions or even under low hydrogen gas pressure. The calculated band structure reveals that the reconstructed edge possesses a bandgap of 1.23 eV. It is expected that this newly found edge structure may stimulate more studies in uncovering other novel edge types and further exploring their practical applications.Edge atomic configuration often plays an important role in dictating the properties of finite-sized two-dimensional (2D) materials. By performing ab initio calculations, we identify a highly stable zigzag edge of phosphorene, which is the most stable one among all the considered edges. Surprisingly, this highly stable edge exhibits a novel nanotube-like structure, which is topologically distinctively different from any previously reported edge reconstruction. We further show that this new edge type can form easily, with an energy barrier of only 0.234 eV. It may be the dominant edge type at room temperature in vacuum condition or even under low hydrogen gas pressure. The calculated band structure reveals that the reconstructed edge possesses a bandgap of 1.23 eV. It is expected that this newly found edge structure may stimulate more studies in uncovering other novel edge types and further exploring their practical applications.

Thermal transport behavior of polycrystalline graphene: A molecular dynamics study

The thermal transport behavior of polycrystalline graphene is studied using molecular dynamics simulations, with focus on the effects of grain size, tensile strain, and temperature on the thermal conductivity. All the simulation samples have the same overall dimensions of 30 × 30 nm with average grain sizes ranging from 2.5 to 12.5 nm. It is found that polycrystalline graphene exhibits a significant reduction in thermal conductivity compared to single-crystalline graphene, and the smaller the grain size is, the more the thermal conductivity drops. The thermal conductivity of polycrystalline graphene with average grain size of 2.5 nm is only about 20% of single-crystalline graphene. However, the thermal conductivity of polycrystalline graphene is less sensitive to both the applied strain and temperature than that of single-crystalline graphene. The underlying mechanisms for the differences in thermal behavior are examined and discussed. These findings are important for the thermal management of graphene-ba...

Thermal conduction across the one-dimensional interface between a MoS2 monolayer and metal electrode

The thermal conductance across the one-dimensional (1D) interface between a MoS2 monolayer and Au electrode (edge-contact) has been investigated using molecular dynamics simulations. Although the thermal conductivity of monolayer MoS2 is 2–3 orders of magnitude lower than that of graphene, the covalent bonds formed at the interface enable interfacial thermal conductance (ITC) that is comparable to that of a graphene–metal interface. Each covalent bond at the interface serves as an independent channel for thermal conduction, allowing ITC to be tuned linearly by changing the interfacial bond density (controlling S vacancies). In addition, different Au surfaces form different bonding configurations, causing large ITC variations. Interestingly, the S vacancies in the central region of MoS2 only slightly affect the ITC, which can be explained by a mismatch of the phonon vibration spectra. Further, at room temperature, ITC is primarily dominated by phonon transport, and electron–phonon coupling plays a negligible role. These results not only shed light on the phonon transport mechanisms across 1D metal–MoS2 interfaces, but also provide guidelines for the design and optimization of such interfaces for thermal management in MoS2-based electronic devices.

Graphene-based thermal modulators

The quest for materials and devices that are capable of controlling heat flux continues to fuel research on thermal controlling devices. In this letter, using molecular dynamics simulations, we demonstrate that a partially clamped single-layer graphene can serve as a thermal modulator. The mismatch in phonon dispersion between the unclamped and clamped graphene sections results in phonon interface scattering, and the strength of interface scattering is tunable by controlling the clamp-graphene distance via applying the external pressure. Owing to the ultra-thin structure of graphene and its highly sensitive phonon dispersion to external physical interaction, the modulation efficiency—which is defined as the ratio of the highest to lowest heat flux—can reach as high as 150% at a moderate pressure of 50 GPa. This modulation efficiency can be further enhanced by arranging a number of clamps in series along the direction of the heat flux.

MoS2-graphene in-plane contact for high interfacial thermal conduction

Recent studies have indicated that two-dimensional (2D) MoS2 exhibits low in-plane and inter-plane thermal conductivities. This poses a significant challenge to heat management in MoS2-based electronic devices. To address this challenge, we have designed MoS2-graphene interfaces that fully utilize graphene, a 2D material that exhibits very high thermal conductivity. First, we performed ab initio atomistic simulations to understand bonding and structural stability at the interfaces. The interfaces that we designed, which were connected via strong covalent bonds between Mo and C atoms, were energetically stable. We then performed molecular dynamics simulations to investigate interfacial thermal conductance in these materials. Surprisingly, the interfacial thermal conductance was high and comparable to those of covalently bonded graphene-metal interfaces. Importantly, each interfacial Mo–C bond served as an independent thermal channel, enabling modulation of the interfacial thermal conductance by controlling the Mo vacancy concentration at the interface. The present work provides a viable heat management strategy for MoS2-based electronic devices.