Phonon Lifetime Research Articles

Two-dimensional hexagonal LaOF with ultrawide bandgap, large exciton energy, and low lattice thermal conductivity

Two-dimensional (2D) LaOF monolayer is predicted based on density functional theory. It demonstrates robust stability as verified by the cleavage energy, phonon spectrum, elastic constants, and molecular dynamics simulation. The monolayer is a semiconductor with an indirect ultrawide bandgap (UWBG) of 5.234 eV, and it exhibits considerably strong optical absorption in the wavelength window of 130–250 nm. The exciton in this monolayer is strongly bound and thermally stable with a binding energy of 0.618 eV. The monolayer has an low lattice thermal conductivity kL of 3.02 Wm−1K−1 at 300 K due to the small group velocity, low Debye temperature, and short phonon lifetime. The kL descends steeply down to 0.256 Wm−1K−1 at the tensile strain of 9%. The UWBG, large exciton binding energy, and low and tunable kL of the 2D material demonstrate its potential applications in deep ultraviolet and thermoelectric devices.

Effect of crystallinity and thickness on thermal transport in layered PtSe2

We present a comparative investigation of the influence of crystallinity and film thickness on the acoustic and thermal properties of layered PtSe2 films of varying thickness (1–40 layers) using frequency-domain thermo-reflectance, low-frequency Raman, and pump-probe coherent phonon spectroscopy. We find ballistic cross-plane heat transport up to ~30 layers PtSe2 and a 35% reduction in the cross-plane thermal conductivity of polycrystalline films with thickness larger than 20 layers compared to the crystalline films of the same thickness. First-principles calculations further reveal a high degree of thermal conductivity anisotropy and a remarkable large contribution of the optical phonons to the thermal conductivity in bulk (~20%) and thin PtSe2 films (~30%). Moreover, we show strong interlayer interactions in PtSe2, short acoustic phonon lifetimes in the range of picoseconds, an out-of-plane elastic constant of 31.8 GPa, and a layer-dependent group velocity ranging from 1340 ms−1 in bilayer to 1873 ms−1 in eight layers of PtSe2. The potential of tuning the lattice thermal conductivity of layered materials with the level of crystallinity and the real-time observation of coherent phonon dynamics open a new playground for research in 2D thermoelectric devices and provides guidelines for thermal management in 2D electronics.

Enhancing spatial resolution of BOTDR sensors using image deconvolution.

We propose to employ the image deconvolution technique for Brillouin optical time domain reflectometry (BOTDR) systems to achieve a flexible and enhanced spatial resolution with pump pulses longer than phonon lifetime. By taking the measured Brillouin gain spectrum (BGS) distribution as an image blurred by a point spread function (PSF), the image deconvolution algorithm based on the two-dimensional Wiener filtering can mitigate the ambiguity effect on the Brillouin response. The deconvoluted BGS distribution reveals detailed sensing information within shorter fiber segments, improving the inferior spatial resolution and simultaneously maintaining other sensing performance parameters. Thanks to the proposed technique, a typical BOTDR sensor with 40 ns pump pulses reaches a submetric spatial resolution as high as 10 cm. Compared to the differential-spectrum-based BOTDR retrieving the same spatial resolution, the image deconvolution technique shows advantages in system complexity and measurement uncertainty. Moreover, the proposed technique is promising to improve the spatial resolution of other distributed optical fiber sensing (DOFS) techniques such as BOTDR systems with complex pump modulation methods.

Tunning lattice thermal conductivity of bilayer and trilayer molybdenum disulfide thermoelectric materials through twist angles

Exploring approaches to reduce lattice thermal conductivity is effective to improve the performance of molybdenum disulfide thermoelectric materials, and tuning lattice thermal conductivity via twist angles attracts increasing attention. However, few studies focused on the effects of twist angle on lattice thermal conductivity of molybdenum disulfide thermoelectric materials, particularly, the coupling effects of two twist angles on lattice thermal conductivity of tri-layer molybdenum disulfide. In this work, the effects of twist angles on lattice thermal conductivity of twisted bilayer and trilayer molybdenum disulfide were investigated by molecular dynamics simulation. Phonon dispersion was calculated through density functional theory to provide a deeper understanding of thermal conduction. The results indicated that for the twisted bilayer sample, the relationship between thermal conductivity and twist angle exhibits “W-shaped”, and the minimum thermal conductivity can be obtained with the twist angle of 15° The phonon analyses indicated that the twist angle has a significant effect on ZA branches, and the effects of the twist angle on group velocities and phonon lifetime of LA and TA branches contribute to the change of thermal conductivity. As for the twisted tri-layer sample, the minimum thermal conductivity can be obtained when the sum of two twist angles is an odd multiple of 15°, which is recommended for thermoelectric materials design.

Effect of probe pulse duration in picosecond ultrasonics

Picosecond ultrasonics is a powerful tool for nanoscale metrology, giving access to dimensions and mechanical, thermal, and optical properties of nanomaterials. By monitoring the temporal evolution of the interaction of light with coherent acoustic phonons, also known as Brillouin oscillations, phonon lifetime and optical absorption can be measured. However, the extraction of these quantities can be inaccurate due to the common assumption of the infinite coherence length of probe pulses. Here, we demonstrate the effect of probe pulse duration on picosecond ultrasonic measurements numerically and experimentally. We establish a model that shows how the probe coherence length affects the measured signal loss and how we can overcome this limitation and measure an upper limit of the acoustic attenuation factor. The model is verified experimentally on a GaAs bulk substrate by varying the probe pulse duration, showing a strong effect for sub-100 fs pulses. Finally, we applied to CH3NH3PbBr3, where we reveal a high acoustic attenuation factor, which is in line with recent claims of strong anharmonicity in halide perovskites.

The structural, electronic and thermal transport properties of pentagonal MS2 (M = Zn, Cd) monolayers: A first-principles study

Two-dimensional (2D) pentagonal materials have attracted tremendous interest due to their multifunctional applications in catalytic, optoelectronic and thermoelectric fields. In the work, we examine the dynamic and thermal stabilities for the pentagonal MS2 (M = Zn, Cd) monolayers by phonon spectra and ab initio molecular dynamics simulations. The structural, electronic and thermal transport properties of the pentagonal MS2 (M = Zn, Cd) monolayers are investigated by employing first-principles calculations coupled with Boltzmann transport theory. The pentagonal MS2 (M = Zn, Cd) monolayers exhibit the semiconductor character with indirect band gaps. The high carrier mobility with 553 cm2 V−1 s−1 of CdS2 monolayer is obtained along the y direction. The calculated anisotropic lattice thermal conductivities (Kl) are 2.23 (1.83) and 5.69 (5.27) W/m K along x (y) direction for the pentagonal ZnS2 and CdS2 monolayers at 300 K, respectively. The heat capacity, phonon group velocity and phonon lifetime are also obtained. It is found that the lower Kl along the same direction (x or y) can be attributed to the stronger phonon anharmonicity. A similar anisotropy of power factor (PF) for pentagonal MS2 (M = Zn, Cd) is discussed in detail. Additionally, the zT values of 1.09 and 0.96 are predicted for the pentagonal ZnS2 and CdS2 at the n-type doping along the y direction at 300 K, suggesting the potential applications in thermoelectric fields of pentagonal MS2 (M = Zn, Cd) monolayers.

Investigation of Eu3+ Doped Oxy-Fluoride Phosphate Glass for Red Laser Gain Medium Application

The oxyfluoride phosphate glass doped with europium oxide was synthesized by melt quenching technique. The structural and spectroscopic analysis has been carried out by Raman, absorption, photoluminescence, phonon side band and lifetime analysis. The absorption spectrum revealed the characteristic transitions between the 7F0 ground state and 7F1 first excited state to the upper-lying electronic states. The several J-O parameters: oscillator strength, radiative transition probability, stimulated emission cross-section and branching ratio. The local structure around the Eu3+ ions and the phonon energy of oxyfluoride phosphate glass have been confirmed on the basis of PSB associated with the 7F0→5D2 transition. The highest excitation spectrum showed at 395 nm for transition 7F0→5L6. The emission spectrum showed two major peaks corresponding to 591 nm orang emission (5D0→7F1) and 613 nm red emission (5D0→7F2). This work confirmed that 1 mol% Eu2O3 doped oxyfluoride phosphate glass performs a high potential for using as efficient luminescence materials for solid-state lighting application, especially for reddish-orange light-emitting glass.

Generation of a High-Intensity Temporal Step Waveform Based on Stimulated Brillouin Scattering

This paper proposes a method based on stimulated Brillouin scattering (SBS) for reshaping a temporal Gaussian waveform into a temporal high-intensity step waveform. The theoretical analysis showed that the reshaped temporal waveform depended on the phonon lifetime, the Brillouin gain coefficient, the interaction length between the Stokes and the pump pulse, and the pump energy. It further showed the dynamic evolution of the reshaped temporal waveform with these four parameters. By optimizing these parameters, a temporal step waveform with an intensity of 36.92 MW/cm2 was obtained in the experiment.

Zintl Phase Compounds Mg3Sb2−xBix (x = 0, 1, and 2) Monolayers: Electronic, Phonon and Thermoelectric Properties From ab Initio Calculations

The Mg3Sb2−xBix family has emerged as the potential candidates for thermoelectric applications due to their ultra-low lattice thermal conductivity (κL) at room temperature (RT) and structural complexity. Here, using ab initio calculations of the electron-phonon averaged (EPA) approximation coupled with Boltzmann transport equation (BTE), we have studied electronic, phonon and thermoelectric properties of Mg3Sb2−xBix (x = 0, 1, and 2) monolayers. In violation of common mass-trend expectations, increasing Bi element content with heavier Zintl phase compounds yields an abnormal change in κL in two-dimensional Mg3Sb2−xBix crystals at RT (∼0.51, 1.86, and 0.25 W/mK for Mg3Sb2, Mg3SbBi, and Mg3Bi2). The κL trend was detailedly analyzed via the phonon heat capacity, group velocity and lifetime parameters. Based on quantitative electronic band structures, the electronic bonding through the crystal orbital Hamilton population (COHP) and electron local function analysis we reveal the underlying mechanism for the semiconductor-semimetallic transition of Mg3Sb2-−xBix compounds, and these electronic transport properties (Seebeck coefficient, electrical conductivity, and electronic thermal conductivity) were calculated. We demonstrate that the highest dimensionless figure of merit ZT of Mg3Sb2−xBix compounds with increasing Bi content can reach ∼1.6, 0.2, and 0.6 at 700 K, respectively. Our results can indicate that replacing heavier anion element in Zintl phase Mg3Sb2−xBix materials go beyond common expectations (a heavier atom always lead to a lower κL from Slack’s theory), which provide a novel insight for regulating thermoelectric performance without restricting conventional heavy atomic mass approach.

Abnormal in-plane thermal conductivity anisotropy in bilayer α-phase tellurene

In layered materials, it is well known that the cross-plane or cross-chain (CC) thermal conductivity is lower than that of the in-plane or along-chain direction due to weaker bonding in the cross-plane or CC directions. In this work, surprisingly, we predict using first-principles that the CC thermal conductivity κ⊥ is higher than the along-chain thermal conductivity κ∥ with a κ⊥/κ∥ ratio of 1.174 in bilayer α-phase tellurene (2L-αTe), an emerging 2D material that has recently drawn extensive interest due to its intriguing electronic, thermoelectronic, and piezoelectronic properties. Also, the room-temperature κ⊥ and κ∥ of 2L-αTe are 5.69 W/mK and 4.85 W/mK, which are 251% and 31% enhanced from that of bulk Te, respectively. A detailed analysis of lattice structure, phonon spectra, and electron properties of 2L-αTe and bulk Te is provided. We find that the larger lattice shrink in CC direction and the change in band angle lead to stiffened phonons and weaker anharmonicity, which in turn yield higher phonon group velocity and longer phonon lifetime in CC direction for 2L-αTe, resulting in the enhanced thermal conductivity and the anomolus thermal anisotropy. Crystal orbital Hamiltan population (COHP) and electron localization function (ELF) analysis demonstrate the enhancement of the strength of “covalent-like quasi-bonding” (CLQB) in CC direction of 2L-αTe, which is accompanied by the lattice shrink in CC direction and the change of band angle. Our study identifies a case of unexpected anisotropic thermal transport in 2D materials, and sheds light on the improvement of thermal conductivity isotropy in 2D materials for the thermal management of electronic devices.

Screening Nitrides with High Debye Temperatures as Nonlinear Optical Materials

Nitrides, in which P in pnictides is substituted by N, are proposed as nonlinear optical (NLO) materials. Herein, 34 asymmetric nitrides collected in the Inorganic Crystal Structure Database (ICSD) are evaluated and predicted by first-principles calculations. As a result, three Si-nitrides possessing wide band gaps and large NLO coefficients are screened out: BeSiN2 (6.66 eV, d24 = −1.09 pm/V), LiSi2N3 (6.51 eV, d15 = −1.38 pm/V), and LaSi3N5 (4.73 eV, d14 = −0.98 pm/V). In addition, LiGaS2-like structure MoZn3N4 is a promising infrared NLO material with a large NLO coefficient d24 = −16.70 pm/V, which is 1.2 times that of the infrared benchmark AgGaS2 (d36 = 13.4 pm/V); meanwhile, its band gap of 3.05 eV is better than that of AgGaS2 (2.73 eV). Moreover, MoZn3N4 with a higher Debye temperature of 795.5 K, which suggests higher sound speeds and longer phonon lifetimes, thus should exhibit higher thermal conductivity. These nitrides represent a new exploration family of NLO materials.

The in-depth description of phonon transport mechanisms for XC (X=Si, Ge) under hydrostatic pressure: Considering pressure-induced phase transitions

In this paper, the effect of hydrostatic pressure on the thermal transport properties of silicon carbide (SiC) and germanium carbide (GeC) with zinc blende (ZB) and rock salt (RS) phases at room temperature is investigated by solving the phonon Boltzmann heat transport equation. Studies show that the thermal conductivities of the two materials increase with the raising pressure before the phase transition, however, the enhancement of GeC (40%) is much smaller than that of SiC (100%). Combined with phonon analysis, it is found that this phenomenon originates from the abnormal enhanced phonon scattering rate under high pressure, which partly offsets the increase of phonon group velocity and further suppresses the increase of thermal conductivity. Next, when the phase transition occurs, the thermal conductivities of SiC and GeC decrease sharply by 76% and 86%, respectively, which is mainly caused by the increase of the anharmonic characteristic related to phonon scattering. Finally, after the phase transition, the thermal conductivities of the two materials still show increasing trends with the raising pressure, however, the phonon group velocity of GeC does not change significantly and the variation of its thermal conductivity is mainly determined by the phonon lifetime, which is opposite to the case before phase transition, whose enhancement of thermal conductivity is mainly caused by the increase of phonon group velocity. The results indicate that GeC has a more complex phonon transport mechanism compared with SiC, i.e., its thermal conductivity before phase transition is determined by harmonic characteristic and that after phase transition shows a closer relationship with the anharmonic characteristic. This study provides data and mechanism support for the regulation of thermal conductivity from the perspective of pressure-induced phase transition, and as well as supplies some guiding suggestions for engineering applications in the field of thermal transport regulation.

Strong four-phonon scattering in monolayer and hydrogenated bilayer BAs with horizontal mirror symmetry

Recently, the important role of high-order anharmonic phonon–phonon interactions has been revealed in several materials, such as cubic boron arsenide (BAs), in which the wide phononic energy gap is found to be a critical factor causing the importance of four-phonon scattering. In this work, by solving the Boltzmann transport equation, we show that the four-phonon scattering has a significant impact on the thermal transport in honeycomb structured monolayer BAs (m-BAs) and its hydrogenated bilayer counterparts (bi-BAs). The lattice thermal conductivity (κL) values of all these structures are reduced after considering four-phonon scattering. Particularly, a huge drop in κL as large as 80% is observed for m-BAs compared to the case without four-phonon scattering, which is mainly caused by the suppression of phonon lifetimes. More interestingly, as opposed to the case of graphene, κL of m-BAs is abnormally lower than its bi-BAs counterparts, which is attributed to the much larger phonon scattering rate in m-BAs compared to that in bi-BAs. By further comparing BAs sheets with and without horizontal mirror symmetry, it is found that the contribution of flexural acoustic phonon exhibits most significant reduction in both mi-BAs and bi-BAs with horizontal mirror symmetry after including four-phonon scattering. This work provides physical understanding of the role of mirror symmetry and high-order phonon scattering on the thermal transport in two-dimensional materials.

Phonon Lifetimes in Boron‐Isotope‐Enriched Graphene‐ Hexagonal Boron Nitride Devices

Using hexagonal boron nitride (hBN) as a substrate for graphene has shown faster carrier cooling which makes it ideal for high‐power graphene‐based devices. However, the effect of using boron‐isotope‐enriched hBN has not been explored. Herein, femtosecond pump‐probe spectroscopy is utilized to measure and compare the time dynamics of photo‐excited carriers in graphene‐hBN heterostructures for hBN with the natural distribution of boron isotopes (20% 10B and 80% 11B) and hBN enriched to 100% 10B and 11B. The carriers cool down faster for systems with isotopically pure hBN substrates by a factor of ≈1.7 times. This difference in relaxation times arises from the interfacial coupling between carriers in graphene and the hBN phonon modes. The results show that the boron isotopic purity of the hBN substrate can help to reduce the hot phonon bottleneck that limits the cooling in graphene devices.

Thermal Properties of 2D Dirac Materials MN4 (M = Be and Mg): A First-Principles Study.

Recently, a novel two-dimensional (2D) Dirac material BeN4 monolayer has been fabricated experimentally through high-pressure synthesis. In this work, we investigate the thermal properties of a new class of 2D materials with a chemical formula of MN4 (M = Be and Mg) using first-principles calculations. First, the cohesive energy and phonon dispersion curve confirm the dynamical stability of BeN4 and MgN4 monolayers. Besides, BeN4 and MgN4 monolayers have the anisotropic lattice thermal conductivities of 842.75 (615.97) W m–1 K–1 and 52.66 (21.76) W m–1 K–1 along the armchair (zigzag) direction, respectively. The main contribution of the lattice thermal conductivities of BeN4 and MgN4 monolayers are from the low frequency phonon branches. Moreover, the average phonon heat capacity, phonon group velocity, and phonon lifetime of BeN4 monolayer are 3.54 × 105 J K–1 m–3, 3.61 km s–1, and 13.64 ps, which are larger than those of MgN4 monolayer (3.42 × 105 J K–1 m–3, 3.27 km s–1, and 1.70 ps), indicating the larger lattice thermal conductivities of BeN4 monolayer. Furthermore, the mode weighted accumulative Grüneisen parameters (MWGPs) of BeN4 and MgN4 monolayers are 2.84 and 5.62, which proves that MgN4 monolayer has stronger phonon scattering. This investigation will enhance an understanding of thermal properties of MN4 monolayers and drive the applications of MN4 monolayers in nanoelectronic devices.

Thermal tuning of the morphology of hydrothermally synthesized CeO2 nanotubes for photocatalytic applications

CeO2 nanotubes (NT) were synthesized via a hydrothermal method, employing two different alkaline bases, NaOH, and KOH, to obtain NTNaOH-CeO2 and NTKOH-CeO2. The as-produced NTNaOH-CeO2 and NTKOH-CeO2 were heated in air to study their morphological evolution and characterize the influence of morphology on their photocatalytic activities. Two morphological changes were observed, from NTNaOH-CeO2 and NTKOH-CeO2 nanotubes at room temperature to nanobars at temperatures starting from 375 °C, followed by a recrystallization of these at 550 °C and above. These transformations correspond to irreversible structural modifications that changed the morphological and optical properties as characterized by a high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), IR spectra, temperature-dependent Raman spectroscopy, thermo-gravimetric analysis (TGA), and photocatalytic activity. Higher temperatures produced significantly lower wavenumber shifts in the Raman band spectra of NTNaOH-CeO2 and NTKOH-CeO2. The dominant F2g phonon exhibited monotonic lower wavenumber shifts with increasing temperatures. A discontinuity on the phonon broadening for NTKOH-CeO2 was observed around 345 °C. The post-temperature HR-TEM and Raman analysis confirmed the changes in the morphology of both NTNaOH-CeO2 and NTKOH-CeO2, collapsing from nanotubes to nanobars (NB). The phonon softening's linewidths and lifetimes were analyzed using a model that considered four anharmonic processes. The study of the photocatalytic response of NTNaOH-CeO2 and NTKOH-CeO2 calcined at 850 °C (with the morphology transformation from NT to NB), which resulted in the prepared material presenting superior catalytic activity when compared with commercial non-stoichiometric CeO2, having activities up to two orders of magnitude faster than similar compounds reported in the literature. The so-prepared and calcined NTMOH-CeO2 (M = K or Na) NB has potential applications in water remediation to degrade textile dyes, which are found as contaminants in water wastage in this industry.

Hydrodynamically enhanced thermal transport due to strong interlayer interactions: A case study of strained bilayer graphene

Manipulating interlayer interactions in two-dimensional (2D) materials is a widely used and effective method for regulating heat transport, which is important for thermal management of electronics. In this paper, we find that thermal transport can be enhanced by strong interlayer interactions. For instance, 2D bilayer materials with interlayer bonding have larger thermal conductivity than their monolayer counterparts. To further verify the conjecture, we take bilayer graphene (BLG) as a study case to gradually strengthen the interlayer interactions in BLG by applying a series of compressive strains along the out-of-plane direction. It is found that the thermal conductivity of BLG first decreases and then anomalously increases when the strain is larger than the threshold of 12%. For the decreasing trend, it is mainly due to the strain-induced reduction of phonon group velocity and lifetime. While at larger strain (>12%), the anomalous increase of thermal conductivity is found to be caused by hydrodynamic phonon transport and weak phonon anharmonicity. Thus, enhanced interlayer interactions can enhance the hydrodynamic phonon transport and thus improve the heat transfer performance. In this paper, we not only report the hydrodynamic enhancement of thermal transport through enhanced interlayer interactions but also provide solid evidence. These findings will deepen the understanding of thermal transport in layered materials and heterostructures, which will benefit the applications in thermal management.

First-principles phonon calculations for lattice dynamics, thermal expansion and lattice thermal conductivity of CoSi in the high temperature region

This study presents the first-principles phonon calculations to understand the experimental thermal expansion () and lattice thermal conductivity of CoSi in the high temperature region. Phonon dispersion is computed using the finite displacement method and supercell approach by taking the equilibrium crystal structures obtained from DFT. The calculation of is done under quasi-harmonic approximation. The is calculated using first-principle anharmonic lattice dynamics calculations under single-mode relaxation time approximation. The calculated in the temperature range 0–1300 K gives a good match with existing experimental data. The calculated value of (∼8.0 W/mK) at 300 K is found to be in good agreement with the experimental value of ∼8.3 W/mK. The temperature dependence of phonon lifetime due to phonon-phonon interaction is calculated to understand the behaviour of . The present study suggests that ground state phonon dispersion obtained from DFT-based methods gives reasonably good explanation of experimental and .

Pressure driven phase transitions in honeycomb Fe4Nb2O9: A possible re-entrant multiferroic behavior

A detailed high-pressure investigation is carried out on Fe4Nb2O9 using angle resolved x-ray diffraction and Raman spectroscopy measurements. We find a structural transition from the ambient trigonal phase to a monoclinic phase above 8.8 GPa. The structural transition is assumed to be driven by a large distortion of Nb–O6 octahedra as seen from x-ray diffraction analysis and a large pressure dependence of an Nb–O6 octahedra breathing Raman mode. Anomalous behavior of Raman modes and an increase in the phonon lifetime at the phase transition pressure indicate a possible change in the magnetic property of the sample above 8.8 GPa. A decrease in the diffusive scattering rate of a low-frequency electronic contribution contradicts the results of a decrease in the intensity of a high-frequency electronic response and excludes the phenomenon of an insulator to metal transition. On the contrary, the enhancement of the intensity of the Raman modes up to about 8.8 GPa indicates a large change in ferroelectric polarization of the sample, indicating a possible pressure induced re-entrant multiferroic behavior in Fe4Nb2O9.

Monolayer XN2 (X[dbnd]Ti, Zr, Hf): Novel 2D materials with high stability, simultaneously high electron and hole mobilities from density functional theory

2D nitrides have attracted extensive attention because of their novel photoelectric properties and wide applications. In this work, we propose three novel 2D nitride materials, namely TiN2, ZrN2 and HfN2, and further systematically studied their stability, elastic property, electronic structure and thermal conductivity. The results show that the proposed monolayer XN2 (XTi, Zr, Hf) have high mechanical and dynamic stabilities. Furthermore, the monolayers are all direct bandgap semiconductors with band gaps of 1.87 eV, 2.00 eV and 2.69 eV, respectively. Based on deformation potential theory, it is found that the monolayers simultaneously have high hole and electron mobility at room temperature, up to 104–105 cm2 V−1 s−1. Also, due to the high group velocity and phonon lifetime, as well as volume specific heat capacity, monolayer XN2 (XTi, Zr, Hf) show high lattice thermal conductivity, as high as 41.80 W/m K, 60.22 W/m K and 56.88 W/m K, respectively. All these characteristics indicate that the proposed monolayer XN2 (XTi, Zr, Hf) have the potential to be applied in the fields of high-mobility polar semiconductors and photovoltaic devices in the future.