Electronic and Optical Properties of Two-Dimensional GaN from First-Principles

Gallium nitride (GaN) is an important commercial semiconductor for solid-state lighting applications. Atomically thin GaN, a recently synthesized two-dimensional material, is of particular interest because the extreme quantum confinement enables additional control of its light-emitting properties. We performed first-principles calculations based on density functional and many-body perturbation theory to investigate the electronic, optical, and excitonic properties of monolayer and bilayer two-dimensional (2D) GaN as a function of strain. Our results demonstrate that light emission from monolayer 2D GaN is blueshifted into the deep ultraviolet range, which is promising for sterilization and water-purification applications. Light emission from bilayer 2D GaN occurs at a similar wavelength to its bulk counterpart due to the cancellation of the effect of quantum confinement on the optical gap by the quantum-confined Stark shift. Polarized light emission at room temperature is possible via uniaxial in-plane strain, which is desirable for energy-efficient display applications. We compare the electronic and optical properties of freestanding two-dimensional GaN to atomically thin GaN wells embedded within AlN barriers in order to understand how the functional properties are influenced by the presence of barriers. Our results provide microscopic understanding of the electronic and optical characteristics of GaN at the few-layer regime

Pb₇Bi₄Se₁₃: A Lillianite Homologue with Promising Thermoelectric Properties

Pb₇Bi₄Se₁₃ crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3) Å, <i>b</i> = 4.262(2) Å, <i>c</i> = 23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional structure, two NaCl-type layers A and B with respective thicknesses <i>N</i>₁ = 5 and <i>N</i>₂ = 4 [<i>N</i> = number of edge-sharing (Pb/Bi)­Se₆ octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal prisms, PbSe₇, are located on a pseudomirror plane parallel to (001). This complex atomic-scale structure results in a remarkably low thermal conductivity (∼0.33 W m^–1 K^–1 at 300 K). Electronic structure calculations and diffuse-reflectance measurements indicate that Pb₇Bi₄Se₁₃ is a narrow-gap semiconductor with an indirect band gap of 0.23 eV. Multiple peaks and valleys were observed near the band edges, suggesting that Pb₇Bi₄Se₁₃ is a promising compound for both n- and p-type doping. Electronic-transport data on the as-grown material reveal an n-type degenerate semiconducting behavior with a large thermopower (∼−160 μV K^–1 at 300 K) and a relatively low electrical resistivity. The inherently low thermal conductivity of Pb₇Bi₄Se₁₃ and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples

Pb₇Bi₄Se₁₃: A Lillianite Homologue with Promising Thermoelectric Properties

Pb₇Bi₄Se₁₃ crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3) Å, <i>b</i> = 4.262(2) Å, <i>c</i> = 23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional structure, two NaCl-type layers A and B with respective thicknesses <i>N</i>₁ = 5 and <i>N</i>₂ = 4 [<i>N</i> = number of edge-sharing (Pb/Bi)­Se₆ octahedra along the central diagonal] are arranged along the <i>c</i> axis in such a way that the bridging monocapped trigonal prisms, PbSe₇, are located on a pseudomirror plane parallel to (001). This complex atomic-scale structure results in a remarkably low thermal conductivity (∼0.33 W m^–1 K^–1 at 300 K). Electronic structure calculations and diffuse-reflectance measurements indicate that Pb₇Bi₄Se₁₃ is a narrow-gap semiconductor with an indirect band gap of 0.23 eV. Multiple peaks and valleys were observed near the band edges, suggesting that Pb₇Bi₄Se₁₃ is a promising compound for both n- and p-type doping. Electronic-transport data on the as-grown material reveal an n-type degenerate semiconducting behavior with a large thermopower (∼−160 μV K^–1 at 300 K) and a relatively low electrical resistivity. The inherently low thermal conductivity of Pb₇Bi₄Se₁₃ and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples