Room Temperature Micro-Photoluminescence Studies of Colloidal WS₂ Nanosheets

Abstract

Wet-chemical syntheses for quasi two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as promising methods for straightforward solution-processing of these materials.However, photoluminescence properties of colloidal TMDs are virtually unexplored due to the typically non-emitting synthesis products. In this work, we demonstrate room temperature microphotoluminescence spectroscopy on delicate ultrathin colloidal WS 2 nanosheets synthesized from WCl 6 and elemental sulfur in oleic acid and oleylamine at 320 °C for the first time. Both, monoand multilayer photoluminescence is observed, revealing comparable characteristics to exfoliated TMD monolayers and underpinning the high quality of colloidal WS 2 nanosheets. In addition, a promising long-term air-stability of colloidal WS 2 nanosheets is observed and the control of their photodegradation under laser excitation is identified as a challenge for further advancing nanosheet monolayers. Our results render colloidal TMDs as easily synthesized and highly promising 2D semiconductors with optical properties fully competitive with conventionally fabricated ultrathin TMDs.

Untangling the Intertwined: Metallic to Semiconducting Phase Transition in Colloidal MoS2 Nanoplatelets and Nanosheets

“…44 The TEM analysis mainly shows monolayers of MoS 2 (see Figure 2a, 2c, and S2). 10,14 Partly we also find few-layered 2D MoS 2 with a molybdenum to molybdenum layer distance of 7 Å. This value is increased with respect to the bulk layer distance of 6.15 Å, 45,46 due to the colloidal structures being curved.…”

“…Typically, strongly bound excitons in TMDCs lead to high quantum yields and narrow emission [7][8][9][10] making the materials interesting for room temperature single photon emisison. Due to a strong spin-orbit coupling and the lack of inversion symmetry, group-VI TMDCs exhibit band-splitting, which causes a spin-dependent occupation of the bands and consequently spin-dependent excitons (A and B excitons).…”

“…[24][25][26][27][28][29][30] Here, we focus on a bottom-up colloidal chemistry route as emerging fabrication method for 2D TMDC NPLs and NSs which offers synthesis product control by adjustable precursor and ligand chemistry. 10,31 The molybdenum precursor MoCl 5 is dispersed in a 10:1 mixture of oleylamine (OlAm) and oleic acid (OA) forming an acid-base equilibrium. 32 The sulfur precursor is dissolved in OlAm and activated by hexamethyldisilazane (HMDS) at 320 °C, which leads to the in situ formation of highly reactive S 4 N 4 rings.…”

“…With this approach, we can tune the intrinsic properties of colloidal 2D MoS 2 NPLs and NSs including the lateral size, and the crystal phase. 10,34,35 A TMDC monolayer is composed of an atomic transition metal layer sandwiched between two layers of chalcogen atoms with the two stacking patterns shown in Figure 1. The 2H phase is thermodynamically stable, semiconducting, and the allotrope of interest for optical applications.…”

Colloidal Continuous Injection Synthesis of Fluorescent MoX2 (X = S, Se) Nanosheets as a First Step Toward Photonic Applications

“…Samples obtained via the LPE method are usually able to show after extensive separation processes (to increase the monolayer fraction) 69 and different treatments such as annealing (used to restore the 2H phase) 40 or chemical treatments to suppress defects. 70 Literature reports about fluorescent colloidal TMDs include pristine 71 and chemically treated colloidal WS 2 nanosheets, 72 as well as quantum dots (QDs). 73−76 Samples with the highest optical quality require encapsulating mechanically cleaved single-layer TMD flakes between fewnm thick hexagonal boron nitride (hBN) crystals, an insulator.…”