Lanthanide near-infrared luminescence in layered semiconductor nanosheet hosts

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been investigated extensively due to their superior properties for constructing next generation opto-electric devices. However, the intrinsic photoluminescence emission is limited by their narrow bandgap, ranging from visible to the edge of near infrared. To further tune the intrinsic properties and enhance device performance of TMDs, several approaches have been adopted such as doping or alloying. Among all the attempted doping elements by theoretical calculations and experimental results, lanthanide elements are among the most promising candidates. Their abundant 4f energy levels render the ability to tune and extend the emission of TMDs host to a wide range of spectrum from ultra-violet to infrared. However, the incorporation of lanthanide ions can be challenging owing to the high melting point of their metal oxide precursors. In our previous works, the synthesis of MoS₂: Er film and WS₂:Yb/Er film have been demonstrated through chemical vapor deposition (CVD) and pulsed laser deposition (PLD) approaches, respectively. Both results exhibited efficient near infrared (NIR) emissions from Er³⁺ dopants, which is of vital importance in optical communication applications. In this thesis, the NIR emission of lanthanide Er³⁺ ions in MoS₂ single crystals was studied using an ingenious two-step CVD method. Er-doped Mo metal film was first deposited on the substrate as growing seeds, followed by a sulfurization procedure in a conventional CVD furnace. Under high sulfur vapor concentration, the growing seeds evaporated from the metal film were sulfurized and self-assembled into single crystal islands at the downstream part of the substrate. The morphology and chemical composition of pristine and Er-doped MoS₂ single crystals were compared using optical microscopy, atomic force microscopy (AFM), Raman spectroscopy, and energy dispersive X-Ray spectroscopy (EDX). The Raman spectrum of the Er-doped MoS₂ single crystal slightly shifted to the lower energy compared with their pristine counterparts, due to the distortion induced by the incorporated Er atom on MoS₂ host matrix. The NIR emission of Er dopant was characterized by steady state photoluminescence spectroscopy with an excitation wavelength of 980 nm. The luminescence of MoS₂ single crystal host was successfully extended to the NIR range at 1530 nm, corresponding to the energy transfer between ⁴I13/2 and ⁴I15/2 4f levels of Er³⁺ ions. The broad emission band consisted of two peaks centered at 1529 and 1505 nm, respectively. This energy level splitting phenomena of Er³⁺ dopant was induced by the surrounding electric field of host MoS₂ nanosheets. Besides, the concentration dependent curve of the luminescence intensity at 1530 nm was measured. The concentration quenching effect was observed with an optimal Er content of 4 mol%, attributing to the increase of non-radiative decay process between the neighboring dopant atoms favored at high Er concentration. Moreover, the stability of the doped system under different chalcogen source content was investigated using first principle calculations. The structural and electronic properties were simulated and compared for pristine and Er-doped MoS₂ monolayer. The doping energy levels of lanthanide Er³⁺ were located with respect to MoS₂ band structure by combining the density of state (DOS) calculation results with the Dieke Diagram (a complete energy level diagram documented for lanthanide 4f levels). The simulated band structure of Er-doped MoS2 was well consistence with our experimental findings, which provides an in-depth understanding of lanthanide doping in 2D layered semiconducting materials. This two-step CVD approach to synthesize doped TMDs single crystal offers a new possibility to develop doping strategies for layered atomic thin materials. This method has great potential to be generalized to the synthesis of various 2D TMDs, especially those involving doping or alloying functionalization. The developed Er-doped MoS₂ single crystals are promising in both fundamental research and potential applications in atomically thin NIR photonic and optoelectronic devices.