Comprehensive first-principles study of bulk, bilayer, and monolayer α-PtO₂ properties

Abstract

We calculated the structural stability, electronic, optical, and thermoelectric properties for α-PtO₂ structures (bulk, bilayer, and monolayer) via first-principles density functional theory calculations. The results show that there is good agreement between the calculated lattice parameters and the available experimental data for the bulk structure. The in-plane Young's modulus and Poisson's ratio were calculated in the harmonic elastic strain range, and the results show that the monolayer is harder than the bulk. GW₀ calculations on top of a semi-local generalised gradient approximation for the exchange-correlation energy predicted that the bilayer, monolayer, and the bulk α-PtO₂ are all semiconductors with indirect band gaps. The results reveal that the band gap increases from bulk to monolayer. Calculations of optical properties show that the monolayer can absorb up to 8% of incident radiation in the visible range, which is higher than the 2.3% for graphene of the same thickness. In contrast, the bulk absorbs 1.77%. Phonon calculations confirm that all structures are dynamically stable. To examine the lattice thermal conductivity, the Boltzmann transport equation calculations in conjunction with density functional theory were implemented. The in-plane calculated lattice thermal conductivity were 8.47 × 10⁻⁸Wm⁻¹K⁻¹ for bulk, 4.59 × 10⁻⁸Wm⁻¹K⁻¹ for bilayer and 1.06 × 10⁻⁸Wm⁻¹K⁻¹ per layer at 300 K. The obtained thermoelectric figure of merit (ZT) per layer were 0.11 for bulk, 0.62 for bilayer and 0.74 for monolayer, respectively. The investigation shows that the n-type in monolayer has the most promise for thermoelectric applications.

Introduction

Researching clean and renewable energy sources such as solar energy has become necessary because of finite reserves of fossil fuels and the resulting environmental impacts [1,2]. As a result, it has become a necessity to explore other sources of clean energy such as solar energy converted by photovoltaics. On the other hand, one promising energy utilisation is the recovery of waste heat and its conversion into useful electrical energy using thermoelectric devices [3]. This applies to many applications including solar cells. Understanding the heat transfer mechanisms in solar cells is important since heat dissipation is a controlling factor in the development of many devices [4]. Transition metal dichalcogenides have demonstrated the potential for use in photovoltaics. MoS₂ is a traditional photovoltaic component which has been used as an absorber in solar cells [5,6]. Studies of two- and three-dimensional materials have confirmed that the two-dimensional polymorphs can enhance the desired properties of a compound [7,8]. The bulk and monolayer of MoS₂ show a small lattice thermal conductivity [9] which is not a consistent property for all layered materials [10]. Experimental measurements have confirmed that the (in-plane) lattice thermal conductivity (per layer) for slabs decreases as the number of layers is decreased [[11], [12], [13], [14]]. The band gap tends to increase with a decreasing number of layers [15]. Lower lattice thermal conductivity is frequently found in two-dimensional rather than three-dimensional materials. X-ray measurements on α-PtO₂ showed that it has a hexagonal CdI₂-like structure with lattice parameters a₀ and c₀ of 3.10 and 4.80 Å respectively [16]. Photoelectron spectroscopy shows that the optical gap range is 1.30–1.47 eV for bulk α-PtO₂, while optical reflectance measurements show an optical band gap of ≈1.2 eV [17]. Zhensheng et al. investigated the electronic properties of α-PtO₂, and they reported it is a semiconductor with a band gap of 1.84 eV [18]. The thermoelectric efficiency of a material is determined by the dimensionless figure of merit (ZT), with ZT = (S²σT/(κ_L + κ_e)), where S is Seebeck coefficient, σ is the electrical conductivity, κ_L is the lattice thermal, κ_e is the electronic thermal conductivity, respectively, and T is the absolute temperature [19].

Our study of α-PtO₂ structures covers the structural, mechanical and dynamical stability of the structures. The electronic, optical and thermoelectric properties of the structures are also investigated and compared with the available. For comparison, the percentage absorbance was determined for bulk, bilayer and monolayer, and the calculations show that the monolayer can absorb up to 8% of incident radiation in the visible range; also, the lattice thermal conductivities of all structures were calculated per single-layer. We investigated the phonon lifetime and group velocities, directionally, and furthermore, we examined the transport coefficients for in-plane to obtain the dimensional figure of merit for α-PtO₂ structures.