[Paper Review] The thermal and electrical properties of the promising semiconductor MXene Hf2CO2

Xian‐Hu Zha, Qing Huang+6 more|arXiv (Cornell University)|Oct 28, 2015

MXene and MAX Phase Materials56 references17 citations

TL;DR

This study investigates the thermal and electrical properties of oxygen-functionalized Hf2CO2 MXene using first-principles calculations, revealing a high room-temperature thermal conductivity of 86.25–131.2 Wm⁻¹K⁻¹ along the armchair direction and a moderate band gap of 1.657 eV. Hf2CO2 exhibits excellent carrier mobility (up to 17.6×10³ cm²V⁻¹s⁻¹) and low thermal expansion (6.094×10⁻⁶ K⁻¹), positioning it as a promising semiconductor for nanoelectronics applications.

ABSTRACT

In this work, we investigate the thermal and electrical properties of oxygen-functionalized M2CO2 (M = Ti, Zr, Hf) MXenes using first-principles calculations. Hf2CO2 is found 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-1K-1 with a flake length of 5-100 um, and the corresponding value in the zigzag direction is approximately 42% of that in the armchair direction. Other important thermal properties of M2CO2 are also considered, including their specific heat and thermal expansion coefficients. The theoretical room temperature thermal expansion coefficient of Hf2CO2 is 6.094x10-6 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 13.5x103 cm2V-1s-1 (17.6x103 cm2V-1s-1), which is comparable to that of phosphorene. Broader utilization of Hf2CO2 as a material for nanoelectronics is likely because of its moderate band gap, satisfactory thermal conductivity, low thermal expansion coefficient, and excellent carrier mobility. The corresponding thermal and electrical properties of Ti2CO2 and Zr2CO2 are also provided here for comparison. Notably, Ti2CO2 presents relatively low thermal conductivity and much higher carrier mobility than Hf2CO2, which is an indication that Ti2CO2 may be used as an efficient thermoelectric material.

Motivation & Objective

• To evaluate the thermal and electrical properties of Hf2CO2 MXene for potential use in nanoelectronic devices.
• To compare Hf2CO2 with other M2CO2 MXenes (Ti2CO2, Zr2CO2) in terms of thermal conductivity, band gap, and carrier mobility.
• To assess the thermal stability and mechanical robustness of Hf2CO2 via thermal expansion coefficient and specific heat calculations.
• To determine the anisotropic behavior of thermal and electrical transport in Hf2CO2.
• To explore the potential of Hf2CO2 as a semiconductor with favorable electronic and thermal characteristics for advanced nanoelectronics.

Proposed method

• First-principles density functional theory (DFT) calculations were employed to compute electronic structure, band gap, and carrier mobility.
• Lattice dynamics and phonon dispersion were used to determine thermal conductivity and specific heat.
• Thermal expansion coefficients were calculated from the temperature dependence of the lattice parameters.
• Carrier mobility was evaluated using the deformation potential theory and effective mass approximation.
• Anisotropic thermal and electrical transport was analyzed by computing properties along armchair and zigzag directions.
• Comparative analysis was performed on Ti2CO2, Zr2CO2, and Hf2CO2 to highlight material-specific trends.

Experimental results

Research questions

• RQ1 What is the thermal conductivity of Hf2CO2 MXene, and how does it vary with flake size and crystallographic direction?
• RQ2 What is the electronic band gap of Hf2CO2, and how does it compare to other MXenes like Ti2CO2 and Zr2CO2?
• RQ3 How does the carrier mobility of Hf2CO2 compare to other 2D semiconductors such as phosphorene and MoS2?
• RQ4 What is the thermal expansion coefficient of Hf2CO2, and how does it affect its thermal stability?
• RQ5 To what extent is the thermal and electrical transport in Hf2CO2 anisotropic?

Key findings

• Hf2CO2 exhibits a room-temperature thermal conductivity of 86.25–131.2 Wm⁻¹K⁻¹ along the armchair direction for flake lengths of 5–100 µm.
• The thermal conductivity in the zigzag direction is approximately 42% of that in the armchair direction, indicating strong anisotropy.
• Hf2CO2 has a moderate band gap of 1.657 eV, confirming its semiconducting nature.
• Hole mobility reaches 13.5×10³ cm²V⁻¹s⁻¹ in the armchair direction and 17.6×10³ cm²V⁻¹s⁻¹ in the zigzag direction at room temperature.
• The thermal expansion coefficient of Hf2CO2 is 6.094×10⁻⁶ K⁻¹, which is lower than that of most metals.
• Compared to Ti2CO2 and Zr2CO2, Hf2CO2 offers a better balance of thermal conductivity, band gap, and carrier mobility for nanoelectronic applications.