First Principles Study On Thermal Transport Of Two-dimensional Transition Metal Borides And Related Computational Method Development

In recent years,with the emergence of many novel two-dimensional（2D）materials,interest has grown in their physical properties and practical applications.Among them,thermal transport properties are crucial and valuable in thermal barrier coating,heat-to-electricity conversion and thermal management of electronic devices.In general,2D materials with the similar crystal structure exhibit unique thermal transport properties.Therefore,it is of significance to study the mechanisms of thermal transport in such materials and to obtain the variation pattern of lattice thermal conductivity(κ₇（6）).In particular,it can provide theoretical support and guidance for searching for materials with ultralowκ₇（6）.In this paper,based on the first-principles calculations,we study the thermal transport properties of a serious of transition metal borides,such as CrB_4,Mo B₄ and WB₄ monolayers by solving the linearized Boltzmann transport equation.The magnitude and variation ofκ₇（6）are explained by analysing their phonon properties,such as phonon dispersion,mean free path（MFP）,group velocity,absolute values of mode Grüneisen parameter（||）and scattering rate.The main results are as follows:（1）By using first-principles calculations and iteratively solving the linearized Boltzmann transport equation,we predict an ultralow in-planeκ₇（6）of 0.28W/m K at T=300 K.Further study shows that such an ultralowκ₇（6）is attributed to the WB₄ monolayer’s extraordinarily strong anharmonicity,which causes predominantly large phonon scattering rates and flat acoustic phonon dispersion.By analyzing the vibrational patterns and bonding environment,we confirm the origin of the strong anharmonicity to be tungsten atom rattling inside the framework of two boron sheets.Finally,we also discover a phonon glass-electron crystal（PGEC）mechanism whereby the loosely ionically bonded W atoms intercalated to the framework of two boron sheets scatter the heat-carrying acoustic phonons.Simultaneously,the electrons provided by W stabilize the covalently bonded boron sheets and enhance the charge transport.The PGEC mechanism is achieved by the host–guest structures with rattling modes mostly in bulk materials in previous works.Our study extends such potential to 2D systems,which pioneers a new idea for searching for 2D materials with ultralowκ₇（6）.（2）The thermal transport studies of CrB₄ and Mo B₄ monolayers show that the calculated₇（6）of CrB₄ and Mo B₄ monolayers are 31.19 and 96.59 W/m K at the temperature of 300 K.This trend is against the well-known Slack’s guideline that the same type of materials with greater atomic mass generally have lower₇（6）.Mo B₄’sκ₇（6）is three times higher than that of CrB₄ despite Mo B₄’s nearly one-and-a-half times greater mass.Mo B₄’s relatively higherκ₇（6）is mainly contributed by its in-plane transverse and longitudinal acoustic phonon modes with much longer lifetimes,i.e.,much smaller scattering rates.However,these smaller scattering rates do not correspond to smaller absolute values of||compared to CrB₄.A closer examination of the relation between||and scattering rates reveals that the widely believed positive correlation between them is ambiguous,at least here for CrB₄ and Mo B₄.The applicability of Grüneisen parameters to searching for novel materials with lowκ₇（6）is discussed from a theoretical viewpoint.From a theoretical point of view,these judging criteria are not always reliable because the inference of materials’strong anharmonicity and lowκ₇（6）by large||is not sufficient.Only accurate and complete first-principles calculations of anharmonicity andκ₇（6）can deliver reliable results.（3）Based on density functional tight-binding method（DFTB）,we combine the DFTB+package with the Sheng BTE program package to develop a calculation method for calculatingκ₇（6）of large systems.Using the new method,we calculateκ₇（6）of common semiconductor materials and compare the results with those of first-principles to verify the accuracy and applicability of the new method.