Ultrafast carrier dynamics in few-layer colloidal molybdenum disulfide probed by broadband transient absorption spectroscopy

Insights into the photophysics of molybdenum disulfide (MoS2) flakes made by exfoliation or chemical vapor deposition (CVD) have advanced the use of these materials in a broad range of applications. More recently, colloidal synthesis has been developed as an inexpensive, scalable, and highly tunable alternative to CVD for the production of MoS2 and other transition metal dichalcogenides (TMDs). Here, we present a comprehensive study on the charge-carrier relaxation in colloidal MoS2 sheets using transient absorption spectroscopy at visible and near infrared wavelengths. We show that the transient absorbance after photoexcitation originates from a reduced oscillator strength around the direct gap and a red shift of the entire absorbance and we attribute both features to state filling and band gap renormalization, respectively. In particular, the signatures of state filling exhibit a sub-picosecond decay, which reflects the trapping of hole carriers in mid-gap states. The relaxation of the band gap renormalization, on the other hand, takes several tens of picoseconds, a process that we assign to a series of charge-carrier recombination and capture events following the initial hole trapping. Since studies on CVD-grown MoS2 point toward highly similar relaxation of photogenerated charge carriers, we conclude that colloidal synthesis yields MoS2 nanosheets of comparable quality as the state-of-the-art CVD, even if both production methods involve an entirely different chemistry. This indicates that TMDs made with both approaches may benefit from similar defect passivation strategies to slow down charge-carrier trapping and enhance the exciton lifetime

A Roadmap to Decipher Ultrafast Photophysics in Two-Dimensional Nanomaterials

Atomically thin two-dimensional (2D) semiconductors are extensively investigated for opto-electronic applications that require strong light-matter interactions. In view of such applications, it is essential to understand how (photo)excitation alters the non-linear optical response of these materials under high carrier density conditions. Broadband transient absorption (TA) spectroscopy is by now a widely used tool to study semiconductor physics in such highly excited systems. However, the complex interplay between different many-body interactions in 2D materials produces highly congested spectral information and an ensuing non-trivial nonlinear photo-response, thereby masking the desired intrinsic photophysics. Herein, we outline a concise roadmap for analyzing such congested datasets based on examples of TA analysis of various 2D materials. In particular, we emphasize the synergy between an initial qualitative understanding of the transient photo-response based on line shapes and their derivatives, and a consequent quantitative spectral deconvolution backed by such insights. </jats:p

Thermodynamic perspective on the broad solvent window for liquid-phase exfoliation of two-dimensional van der Waals solids

Liquid-phase exfoliation (LPE) is a scalable method to produce colloids of two-dimensional van der Waals solids. Solution thermodynamics correctly predicts LPE as optimal when the solubility parameters of the solvent and the solid match, yet the approach does not account for the considerable tolerance on solubility parameter mismatch found experimentally. Here, we reconcile this discrepancy starting from aggregated literature studies that yield a solubility parameter window for an LPE of 9.7 +/- 1.5 MPa1/2. As this number underestimates the exfoliated particle volume by multiple orders of magnitude, we reformulate good exfoliation conditions in terms of the critical exchange parameter chi(c) = 0.5 below which exfoliated colloids are stable according to Flory-Huggins theory. We show that this approach accounts for solubility parameter matching and the experimental solubility parameter window. This new interpretation of good exfoliation conditions can help researchers find new exfoliation solvents and obtain deeper insights into the stability of exfoliated colloids

Liquid-phase exfoliation of rhenium disulfide by solubility parameter matching

In this work, we provide a detailed account of the liquid-phase exfoliation (LPE) of rhenium disulfide (ReS2), a promising new-generation two-dimensional material. By screening LPE in a wide range of solvents, we show that the most optimal solvents are characterized by similar Hildebrand or dispersive Hansen solubility parameters of 25 and 18 MPa1/2, respectively. Such values are attained by solvents such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, and 1-butanol. In line with solution thermodynamics, we interpret the conditions for high-yield exfoliation as a matching of the solvent and ReS2 solubility parameters. Using N-methyl-2-pyrrolidone as an exemplary exfoliation solvent, we undertook a detailed analysis of the exfoliated ReS2. In-depth morphological, structural, and elemental characterization outlined that the LPE procedure presented here produces few-layer, anisotropically stacked, and chemically pure ReS2 platelets with long-term stability against oxidation. These results underscore the suitability of LPE to batch-produce few-layer and pristine ReS2 in solvents that have a solubility parameter close to 25 MPa1/2

Systematic study into the synthesis of colloidal transition metal dichalcogenide nanocrystals

Following the discovery of graphene, a vibrant research area on 2D transition metal dichalcogenides (TMDs) layered materials has emerged in recent years due to their exciting and diverse properties. The approaches to synthesize TMDs are mainly based on top-down methods such as exfoliation and chemical vapour deposition (CVD). Exfoliation methods allow the preparation of monolayer and few-layer nanosheets, but offer little control over their size and shape. CVD enables large area uniform ultrathin TMDs to be prepared, but high temperature and high vacuum usage results in high cost. To realize exquisite control over the composition, layer thickness and crystal phases of TMDs, colloidal synthesis methods have been investigated as an alternative synthesis method. The variety of published reaction protocols yielding TMDs nanosheets by colloidal synthesis indicates that this approach can produce TMDs nanosheets with different dimensions and crystal structure. However, in order to achieve a high level control over the morphology and crystal structure, a systematic study of the effect of reaction parameters on the resulting nanocrystal properties is needed. Here, we carry out a thorough investigation on the synthesis of TMDs nanocrystals using WSe2 as a model system, where we compare reactions using either carboxylic acids or alkylamines as the surfactant. Most importantly, we find that carboxylic acids yield 2H WSe2 with a flower-like morphology, whereas alkylamines yield genuine nanosheets, yet with the 1T crystal structure. Intriguingly, the former result is also obtained when no surfactants are included in the reaction. Using solution nuclear magnetic resonance spectroscopy, we show that neither surfactant exhibits a strong interaction with the synthesized nanosheets. On the other hand, we find that the use of alkylamines slows down the reaction rate as compared to reactions using carboxylic acids or no surfactants. This indicates that the typical surfactant used in colloidal synthesis have only an indirect in the case of WSe2 formation, where they influence the reaction outcome by affecting the reaction rate. This detailed investigation provides a rational basis to further explore and understand reaction chemistry/nanocrystal property relations in the hot injection synthesis of TMDs

Synthesis of colloidal tungsten diselenide (WSe2) nanocrystals by hot injection

Following the discovery of graphene, a vibrant research area on two-dimensional (2D) transition metal chalcogenides (TMDs) layered materials has emerged in recent years due to their exciting and diverse properties.1-4 The existing approaches to make TMDs are mainly exfoliation, substrate growth and colloidal synthesis. Among them, colloidal wet-chemistry methods are particularly promising as the as-synthesized colloidal dispersions are directly fit for solution-based processing, opening a gateway for a straightforward and cost-efficient introduction into various technology platforms. However, compared to the other two preparation schemes, colloidal synthesis is relatively underdeveloped with only a few examples being known.2-4 In this contribution, we describe the preparation of WSe2 nanocrystals using a hot injection colloidal synthesis in the presence of a capping ligand as the solvent. We find that the choice of capping ligands influences the shape of the nanocrystals to great extent. Nano-flowers were obtained in the presence of oleic acid as capping ligand, whereas, nanosheets were obtained in the presence of oleylamine. Moreover, modification of the reaction temperature allowed to control the size of the nanocrystals. A thorough investigation of the reaction yield by means of quantitative XRF analysis enabled us to rationalize the reaction process. This synthesis strategy might provide a versatile approach to synthesize a wide range of TMDs such as WS2 and MoSe2. [1] Novoselov, K. S., Geim, A. K., Morozov, S. V., Jiang, D., Zhang, Y., Dubonos, S. V., ... & Firsov, A. A. (2004). Electric field effect in atomically thin carbon films. Science, 306(5696), 666-669. [2] Mahler, B., Hoepfner, V., Liao, K., & Ozin, G. A. (2014). Colloidal synthesis of 1T-WS2 and 2H-WS2 nanosheets: applications for photocatalytic hydrogen evolution. Journal of the American Chemical Society, 136(40), 14121-14127. [3] Jung, W., Lee, S., Yoo, D., Jeong, S., Miró, P., Kuc, A., ... & Cheon, J. (2015). Colloidal synthesis of single-layer MSe2 (M= Mo, W) nanosheets via anisotropic solution-phase growth approach. Journal of the American Chemical Society, 137(23), 7266-7269. [4] Jin, H., Ahn, M., Jeong, S., Han, J. H., Yoo, D., Son, D. H., & Cheon, J. (2016). Colloidal single-layer quantum dots with lateral confinement effects on 2D exciton. Journal of the American Chemical Society, 138(40), 13253-13259