Abstract
Recently, atomically thin PdSe2 semiconductors with rare pentagonal Se–Pd–Se monolayers were synthesized and were also found to possess superior properties such as ultrahigh air stability, tunable band gap and high carrier mobility, thus offering a new family of two-dimensional (2D) materials for exploration of 2D semiconductor physics and for applications in advanced opto-electronic and nonlinear photonic devices. In this work, we systematically study the nonlinear optical (NLO) responses [namely, bulk photovoltaic effect (BPVE), second-harmonic generation (SHG) and linear electric-optic (LEO) effect] of noncentrosymmetric bilayer (BL) and four-layer PdS2 and PdSe2 by applying the first-principles density functional theory with the generalized gradient approximation plus scissors-correction. First of all, we find that these few-layer PdX 2 (X = S and Se) exhibit prominent BPVE. In particular, the calculated shift current conductivity is in the order of 130 μA V−2, being very high compared to known BPVE materials. Similarly, their injection current susceptibilities are in the order of 100 × 108 A V−2 s−1, again being large. Secondly, the calculated SHG coefficients (χ (2)) of these materials are also large, being one order higher than that of the best-known few-layer group 6B transition metal dichalcogenides. For example, the maximum magnitude of χ (2) can reach 1.4 × 103 pm V−1 for BL PdSe2 at 1.9 eV and 1.2 × 103 pm V−1 at 3.1 eV for BL PdS2. Thirdly we find significant LEO coefficients for these structures in the low photon energy. All these indicate that 2D PdX 2 semiconductors will find promising NLO applications in light signal modulators, frequency converters, electro-optical switches and photovoltaic solar cells. Fourthly, we find that the large BPVE and SHG of the few-layer PdX 2 structures are due to strong intralayer directional covalent bonding and also 2D quantum confinement. Finally, we also discuss the prominent features of these NLO spectra of these materials in terms of their electronic structure and optical dielectric functions.
Highlights
Because of their extraordinary electronic and optical properties, two-dimensional (2D) materials such as graphene, atomically thin transition metal dichalcogenides (TMDCs) and black phosphorus, have attracted an enormous amount of interest in recent years, finding diverse applications in electronic, opto-electronicLarge bulk photovoltaic effect and second-harmonic generation in few-layer pentagonal semiconductors PdS2 and PdSe22 and nonlinear photonic devices with superior performances
We focus on three principal second-order nonlinear optical (NLO) responses of few-layer PdX2 materials, namely, second-harmonic generation, linear electric-optical (LEO) effect and bulk photovoltaic effect
In the PdSe2 structures, the valence band maximum (VBM) is located in the Γ-X direction and the conduction band minimum (CBM) is located on the M-Γ line [Fig. 2(b) and Fig. S1(b)]
Summary
Because of their extraordinary electronic and optical properties, two-dimensional (2D) materials such as graphene, atomically thin (few-layer) transition metal dichalcogenides (TMDCs) and black phosphorus, have attracted an enormous amount of interest in recent years, finding diverse applications in electronic, opto-electronicLarge bulk photovoltaic effect and second-harmonic generation in few-layer pentagonal semiconductors PdS2 and PdSe22 and nonlinear photonic devices with superior performances. Group 6B TMDC semiconductors with chemical formula MX2 (M = Mo, W; X = S, Se) and each layer made up of a 2D hexagonal array of M atoms sandwiched between the similar arrays of X atoms, constitute a interesting family of 2D materials They were found to exhibit an indirect to direct band gap transition when they were thinned down to a monolayer (ML) [1]. This makes the MX2 MLs semiconductors with a direct band gap, becoming promising materials for, e.g., electro-optical devices with efficient light emission [1] and field effect transistors with high on-off ratios [2] These hexagonal 2D MX2 materials with an odd layer-number lack the spatial inversion symmetry, their bulk crystals are centrosymmetric. This broken inversion symmetry makes them exhibit novel properties of fundamental and technological interests, especially second-order nonlinear optical (NLO) responses such as second-harmonic generation (SHG) [3,4,5] and bulk photovoltaic effect (BPVE) [6]
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