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

Abstract

Transition-metal dichalcogenide (TMD) nanosheets have become an intensively investigated topic in the field of 2D nanomaterials, especially due to the direct semiconductor nature, and the broken inversion symmetry in the odd-layer number, of some of their family members. These properties make TMDs attractive for different technological applications such as photovoltaics, optoelectronics, valleytronics, and hydrogen evolution reactions. Among them, MoX 2 (X = S and Se) are turned to direct gap when their thickness is thinned down to monolayer, and thus, efforts toward obtaining large-scale monolayer TMDs are crucial for technological applications. Colloidal synthesis of TMDs has been developed in recent years, as it provides a cost-efficient and scalable way to produce few-layer TMDs having homogeneous size and thickness, yet obtaining a monolayer has proven challenging. Here, we present a method for the colloidal synthesis of mono-and few-layer MoX 2 (X = S and Se) using elemental chalcogenide and metal chloride as precursors. Using a synthesis with slow injection of the MoCl 5 precursor under a nitrogen atmosphere, and optimizing the synthesis parameters with a design of experiments approach, we obtained a MoX 2 sample with the semiconducting (1H) phase and optical band gaps of 1.96 eV for 1H−MoS 2 and 1.67 eV for 1H−MoSe 2 , respectively, consistent with a large monolayer yield in the ensemble. Both display photoluminescence at cryogenic and room temperature, paving the way for optical spectroscopy studies and photonic applications of colloidal TMD nanosheets.

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

“…during the synthesis of MoS 2 , WS 2 and MoSe 2 , WSe 2 NSs. 31,35,[39][40][41] However, the mechanism by which the individual crystal phases are controlled and formed during the synthesis up to now lack systematic description. In order to unlock the potential of colloidal synthesis methods for TMDCs we focus on two critical properties of 2D MoS 2 NSs: The crystal phase and the morphology which we control by variation of the reaction time and concentration of the molybdenum metal precursor.…”

“…[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.…”

A colloidal route to semiconducting tungsten disulfide nanosheets with monolayer thickness

“…This expression yields the spectral position of the A exciton as a function of the input parameters X i , expressed as the difference between the parameter value and the mean value of the range used in the DSD (see Table 1), normalized to half the width of the respective range (as 3 values are taken per range, see Table 1). 28 As such, the prefactor ( e.g. 0.871 for factor X 1 ), immediately shows the impact of each factor, with larger values indicating a stronger effect.…”

“…For this, we used a design of experiment approach, which we previously successfully applied to obtain MoX 2 colloidal nanosheets. 28 Using the DOE framework, we screened the effect of the synthesis factors on the responses above, finding, in contrast to MoX 2 , the existence of multiple optimal regions in the reaction space. More specifically we found that monolayer samples (according to the position of the A exciton) are obtained combining a high reaction temperature and a high precursor injection rate or a low reaction temperature, independent of the precursor injection rate.…”

Charge Carrier Dynamics in Colloidally Synthesized Monolayer MoX2 Nanosheets

“…As displayed in Figure a, a slow hot injection synthesis protocol is used to obtain monolayered MoS 2 and MoSe 2 nanosheets. Samples prepared via this protocol show room-temperature luminescence, as shown by Pippia et al Figure b plots the linear absorbance spectrum of the MoS 2 and MoSe 2 dispersions obtained via colloidal synthesis described above. Clear exciton resonances are identified in the windows of 1.8–2.2 eV for MoS 2 and 1.5–1.9 eV for MoSe 2 , which can be assigned to the A and B exciton transitions, as depicted in Figure b, and routinely reported in the literature. ,− Subtracting the background as a result of scattering, we clearly identify the respective A and B peak positions, as listed in section S1 of the Supporting Information.…”