The thermal and electrical properties of the promising semiconductor MXene Hf2CO2

Abstract

With the growing interest in low dimensional materials, MXenes have also attracted considerable attention recently. In this work, the thermal and electrical properties of oxygen-functionalized M2CO2 (M = Ti, Zr, Hf) MXenes are investigated using first-principles calculations. Hf2CO2 is determined to exhibit a thermal conductivity better than MoS2 and phosphorene. The room-temperature thermal conductivity along the armchair direction is determined to be 86.25~131.2 Wm(-1) K(-1) with a flake length of 5~100 μm. The room temperature thermal expansion coefficient of Hf2CO2 is 6.094 × 10(-6) K(-1), which is lower than that of most metals. Moreover, Hf2CO2 is determined to be a semiconductor with a band gap of 1.657 eV and to have high and anisotropic carrier mobility. At room temperature, the Hf2CO2 hole mobility in the armchair direction (in the zigzag direction) is determined to be as high as 13.5 × 10(3) cm(2)V(-1)s(-1) (17.6 × 10(3) cm(2)V(-1)s(-1)). Thus, broader utilization of Hf2CO2, such as the material for nanoelectronics, is likely. The corresponding thermal and electrical properties of Ti2CO2 and Zr2CO2 are also provided. Notably, Ti2CO2 presents relatively lower thermal conductivity but much higher carrier mobility than Hf2CO2. According to the present results, the design and application of MXene based devices are expected to be promising.

Figure 1. Structure and electronic band structure of Hf₂CO₂.

(a) Top-view of the Hf₂CO₂ structure; the hexagonal unit cell and orthorhombic cell are circled in pink and blue boxes, respectively; the x- (y-) axis corresponds to the Hf₂CO₂ zigzag (armchair) direction. (b) The Brillouin zone (BZ) of the hexagonal unit cell; the ΓΜ (ΓΚ) direction in reciprocal space corresponds to the Hf₂CO₂ armchair (zigzag) direction in real space. (c) The BZ of the Hf₂CO₂ orthorhombic cell. (d) The side-view of Hf₂CO₂. (e) The electronic band structure of Hf₂CO₂. The band gap is increased using the HSE06 correction. The Fermi level is located at 0 eV. (f) The Hf₂CO₂ electronic band structure based on the orthorhombic cell. The valance band maximum (VBM) and conduction band minimum (CBM) are denoted by colored lines.

Figure 2. Phonon dispersions and thermal conductivities along the armchair (ΓΜ) and zigzag (ΓΚ) directions.

(a) The phonon dispersion of Hf₂CO₂ along the armchair direction. The out-of-plane acoustic (ZA), longitudinal acoustic (LA) and transversal acoustic (TA) modes are denoted with black squares, red circles and blue triangles, respectively. (b) The phonon dispersion of Hf₂CO₂ along the zigzag direction. (c) The temperature dependence of the Hf₂CO₂ thermal conductivity along the armchair direction. The ZA, LA and LA mode contributions to the thermal conductivity are denoted with grey dashed, red dotted and blue dash-dotted lines, respectively. (d) The temperature dependence of the Hf₂CO₂ thermal conductivity along the zigzag direction.

Figure 3. The temperature dependence of the thermal conductivities of the M₂CO₂ (M = Ti, Zr, Hf) MXenes.

(a) The temperature dependence of the thermal conductivities of the M₂CO₂ (M = Ti, Zr, Hf) MXenes along the armchair direction. The Ti₂CO₂, Zr₂CO₂ and Hf₂CO₂ thermal conductivities are denoted in black solid, red dashed and blue dotted lines, respectively. (b) The temperature dependence of the thermal conductivities of the M₂CO₂ (M = Ti, Zr, Hf) MXenes along the zigzag direction.

Figure 4. The temperature dependence of Hf₂CO₂ thermal conductivity with varying flake lengths.

(a) The temperature dependence of the thermal conductivity with varying flake lengths in the armchair direction. The thermal conductivity for flake lengths of 1, 2, 5, 10, 20 and 100 μm are denoted by black solid, red dash-dotted, blue dashed-dotted, magenta dashed, olive dotted and navy dash-dotted lines, respectively. (b) The temperature dependence of the Hf₂CO₂ thermal conductivity with varying flake lengths in the zigzag direction.

Figure 5. The phonon dispersion, specific heat and thermal expansion coefficient of Hf₂CO₂.

(a) The phonon dispersion of Hf₂CO₂ in the BZ. (b) The temperature dependence of Hf₂CO₂ specific heat. (c) The temperature dependence of the Hf₂CO₂ thermal expansion coefficient.

Figure 6. The electronic wavefunctions of the CBM and VBM for Hf₂CO₂ based on the orthorhombic cell.

(a) CBM. (b) VBM.