We present a method for substitutional p-type doping in monolayer (1L) and few-layer (FL) WS using highly reactive nitrogen atoms. We demonstrate that the nitrogen-induced lattice distortion in atomically thin WS is negligible due to its low kinetic energy. The electrical characteristics of 1L/FL WS field-effect transistors (FETs) clearly show an n-channel to p-channel conversion with nitrogen incorporation. We investigate the defect formation energy and the origin of p-type conduction using first-principles calculations. We reveal that a defect state appears near the Fermi level, leading to a shallow acceptor level at 0.24 eV above the valence band maximum in nitrogen-doped 1L/FL WS. This doping strategy enables a substitutional p-type doping in intrinsically n-type 1L/FL transition metal dichalcogenides (TMDCs) with tunable control of dopants, offering a method for realizing complementary metal-oxide-semiconductor FETs and optoelectronic devices on 1L/FL TMDCs by overcoming one of the major limits of TMDCs, that is, their n-type unipolar conduction.

Photovoltaics and photocatalysis are two significant applications of clean and sustainable solar energy, albeit constrained by their inability to harvest the infrared spectrum of solar radiation. Lanthanide-doped materials are particularly promising in this regard, with tunable absorption in the infrared region and the ability to convert the long-wavelength excitation into shorter-wavelength light output through an upconversion process. In this review, we highlight the emerging applications of lanthanide-doped upconversion materials in the areas of photovoltaics and photocatalysis. We attempt to elucidate the fundamental physical principles that govern the energy conversion by the upconversion materials. In addition, we intend to draw attention to recent technologies in upconversion nanomaterials integrated with photovoltaic and photocatalytic devices. This review also provides a useful guide to materials synthesis and optoelectronic device fabrication based on lanthanide-doped upconversion materials.

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