Ln₁₀S₁₄O (Ln = La, Pr, Nd, Sm) Oxysulfides: A Series of Direct n-Type Semiconductors

Abstract

Lanthanoid oxysulfides are promising materials for technological applications owing to their magnetic, photoluminescent, catalytic, and optoelectronic properties. Herein, we report the solid-state synthesis and structural characterization of Ln 10 S 14 O (Ln = La, Ce, Pr, Nd, Sm) oxysulfides. Then, we present a thorough discussion on their electronic and photophysical properties. Through Tauc plot analysis and the derivation of the absorption spectrum fitting method (DASF), we determine that all oxysulfides have direct band gaps with energies of 2.84 eV (La), 2.02 eV (Ce), 2.56 eV (Pr), 2.64 eV (Nd), and 2.41 eV (Sm). Furthermore, surface photovoltage spectroscopy (SPS) shows photovoltage (ΔCPD) values of −0.4 to −1.1 V for La-, Pr-, Nd-, and Sm-containing compounds when illuminated near the optical band gap, indicating that these oxysulfides are n-type semiconductors, which is consistent with Mott−Schottky analysis. Photovoltages under sub-band gap illumination energy and photovoltage decay data suggest mid-band gap states possibly arising from the lanthanoid 4f orbitals and/or defects within the crystal structure or at the particle surfaces. These photophysical properties suggest possible applications of the oxysulfides in photoelectrochemical and photovoltaic energy conversion.

BiVO4–Liquid Junction Photovoltaic Cell with 0.2% Solar Energy Conversion Efficiency

“…Transient photovoltage signals form quickly (15 s under 23 mW cm –2 illumination) and decay more slowly (61 s) after the light is turned off. Slower time scales for the SPV signal decay are frequently observed in transient SPV data for semiconductor films. − We attribute this to several factors, including a decreased charge carrier concentration in the dark, a change in the charge carrier transport mechanism from drift under illumination to diffusion in the dark, and trapping of minority carriers in deep surface states. The transient SPV data for the Mo:BiVO 4 film is shown in Figure d.…”

From Ln2O2S to Ln10OS14: exploring the sulphur spectrum of trivalent lanthanoid oxysulphides

“…Although the cause may be a combination of structural defects and lanthanoid 4f states. 14 Nevertheless, given that the substitution of O 2− anions for softer S 2− changes the chemical environment and symmetry of the lanthanoids’ coordination sphere, it would be of great interest to expand on these computational and spectroscopic studies to the full spectrum of lanthanoid oxysulphides and understand the degree of participation of the 4f orbitals in their electronic structures.…”

“…29,43 However, it is yet unclear what determines the intrinsic distinction in cerium oxysulphides’ optical properties compared with its congeners, in the absence of chemical changes. One such example is the similar band gap energies between Ce 2 O 2 S, 29 and Ce 10 OS 14 , 14 irrespective of the sulphur content. It is possible that the absorption onset is dominated by the dipole allowed 4f → 5d transitions in Ce 3+ that appear as broad features centred at 500 nm.…”

“…H 2 S or CS 2 ) if the synthesis is carried under flow. 10,14 Ln 2 O 2 S oxysulphides have been synthesized for the majority of the lanthanoids. However, reports on the properties of the pure-phase heavier members (Tb-Lu) are rare.…”

“…[6][7][8][9] While the compositional space is large, Ln 2 O 2 S has been the most widely explored stoichiometry, [10][11][12] and considerably less information is available on its remaining congeners. 13,14 The Ln 2 O 2 S 2 crystal structure was only recently understood owing to the disorder present in the disulphide layer. 15 Similarly, little is known about the mixed lanthanoid and solid solution ranges in the Ln 10 S 15−x O x system and the impact the substitution of oxygen has on its crystal and electronic structures.…”