Ultrafast carrier dynamics in colloidal WS2 nanosheets obtained through a hot injection synthesis

Phonon-mediated ultrafast energy- and momentum-resolved hole dynamics in monolayer black phosphorus

The electron-phonon scattering plays a crucial role in determining the electronic, transport, optical, and thermal properties of materials. Here, we employ a non-Markovian stochastic Schrödinger equation (NMSSE) in momentum space, together with ab initio calculations for energy bands and electron-phonon interactions, to reveal the phonon-mediated ultrafast hole relaxation dynamics in the valence bands of monolayer black phosphorus. Our numerical simulations show that the hole can initially remain in the high-energy valence bands for more than 100 fs due to the weak interband scatterings, and its energy relaxation follows single-exponential decay toward the valence band maximum after scattering into low-energy valence bands. The total relaxation time of holes is much longer than that of electrons in the conduction band. This suggests that harnessing the excess energy of holes may be more effective than that of electrons. Compared to the semiclassical Boltzmann equation based on a hopping model, the NMSSE highlights the persistence of quantum coherence for a long time, which significantly impacts the relaxation dynamics. These findings complement the understanding of hot carrier relaxation dynamics in two-dimensional materials and may offer novel insights into harnessing hole energy in photocatalysis.

The Dynamic Interaction of Surfactants with Colloidal Molybdenum Disulfide Nanosheets Calls for Thermodynamic Stabilization by Solvents

Top-down liquid-phase exfoliation (LPE) and bottom-up hot-injection synthesis are scalable methods to produce colloids of two-dimensional (2D) van der Waals (vdW) solids. Generally thought off as two entirely different fields, we show that similar stabilization mechanisms apply to colloids of molybdenum disulfide (MoS₂) produced by both methods. By screening the colloidal stability of MoS₂ produced in a hot-injection synthesis in a wide range of solvents, we observe that colloidal stability can be understood based on solution thermodynamics, wherein matching the solubility parameter of solvent and nanomaterial maximizes colloidal stability. Identical to MoS₂ produced through LPE, optimal solvents to disperse MoS₂ produced from the bottom-up have similar solubility parameters of ≈22 MPa^1/2 and include aromatic solvents with polar functionalities, such as o-dichlorobenzene, and polar aprotic solvents, such as N,N-dimethylformamide. We further complemented our findings by nuclear magnetic resonance (NMR) spectrscopy, highlighting that organic surfactants, such as oleylamine and oleic acid, have a minimal affinity toward the nanocrystal surface and engage in a highly dynamic adsorption/desorption equilibrium. We thus conclude that hot injection yields MoS₂ colloids with comparable surfaces as those produced by LPE. These similarities might offer the prospect of using established procedures developed for LPE nanomaterials to postprocess colloidally synthesized dispersions of 2D colloids as processable inks.

Hot Carrier Cooling and Trapping in Atomically Thin WS2 Probed by Three-Pulse Femtosecond Spectroscopy

Transition metal dichalcogenides (TMDs) have shown outstanding semiconducting properties which make them promising materials for next-generation optoelectronic and electronic devices. These properties are imparted by fundamental carrier-carrier and carrier-phonon interactions that are foundational to hot carrier cooling. Recent transient absorption studies have reported ultrafast time scales for carrier cooling in TMDs that can be slowed at high excitation densities via a hot-phonon bottleneck (HPB) and discussed these findings in the light of optoelectronic applications. However, quantitative descriptions of the HPB in TMDs, including details of the electron-lattice coupling and how cooling is affected by the redistribution of energy between carriers, are still lacking. Here, we use femtosecond pump-push-probe spectroscopy as a single approach to systematically characterize the scattering of hot carriers with optical phonons, cold carriers, and defects in a benchmark TMD monolayer of polycrystalline WS₂. By controlling the interband pump and intraband push excitations, we observe, in real-time (i) an extremely rapid "intrinsic" cooling rate of ∼18 ± 2.7 eV/ps, which can be slowed with increasing hot carrier density, (ii) the deprecation of this HPB at elevated cold carrier densities, exposing a previously undisclosed role of the carrier-carrier interactions in mediating cooling, and (iii) the interception of high energy hot carriers on the subpicosecond time scale by lattice defects, which may account for the lower photoluminescence yield of TMDs when excited above band gap.

Charge Carrier Dynamics in Colloidally Synthesized Monolayer MoX2 Nanosheets

Transition metal dichalcogenides (TMDs) are nanostructured semiconductors with prospects in optoelectronics and photocatalysis. Several bottom-up procedures to synthesize such materials have been developed yielding colloidal transition metal dichalcogenides (c-TMDs). Where such methods initially yielded multilayered sheets with indirect band gaps, recently, also the formation of monolayered c-TMDs became possible. Despite these advances, no clear picture on the charge carrier dynamics in monolayer c-TMDs exists to date. Here, we show through broadband and multiresonant pump-probe spectroscopy, that the carrier dynamics in monolayer c-TMDs are dominated by a fast electron trapping mechanism, universal to both MoS₂ and MoSe₂, contrasting hole-dominated trapping in their multilayered counterparts. Through a detailed hyperspectral fitting procedure, sizable exciton red shifts are found and assigned to static shifts originating from both interactions with the trapped electron population and lattice heating. Our results pave the way to optimizing monolayer c-TMDs via passivation of predominantly the electron-trap sites.

A roadmap to decipher ultrafast photophysics in two-dimensional nanomaterials

Atomically thin two-dimensional (2D) semiconductors are extensively investigated for optoelectronic 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 the 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 non-linear 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.

Understanding the Photoluminescence Quenching of Liquid Exfoliated WS2 Monolayers

Monolayer transition metal dichalcogenides (TMDs) are being investigated as active materials in optoelectronic devices due to their strong excitonic effects. While mechanical exfoliation (ME) of monolayer TMDs is limited to small areas, these materials can also be exfoliated from their parent layered materials via high-volume liquid phase exfoliation (LPE). However, it is currently considered that LPE-synthesized materials show poor optoelectronic performance compared to ME materials, such as poor photoluminescence quantum efficiencies (PLQEs). Here we evaluate the photophysical properties of monolayer-enriched LPE WS₂ dispersions via steady-state and time-resolved optical spectroscopy and benchmark these materials against untreated and chemically treated ME WS₂ monolayers. We show that the LPE materials show features of high-quality semiconducting materials such as very small Stokes shift, smaller photoluminescence line widths, and longer exciton lifetimes than ME WS₂. We reveal that the energy transfer between the direct-gap monolayers and in-direct gap few-layers in LPE WS₂ dispersions is a major reason for their quenched PL. Our results suggest that LPE TMDs are not inherently highly defective and could have a high potential for optoelectronic device applications if improved strategies to purify the LPE materials and reduce aggregation could be implemented.

WS2 Nanorod as a Remarkable Acetone Sensor for Monitoring Work/Public Places

Here, we report the synthesis of the WS₂ nanorods (NRs) using an eco-friendly and facile hydrothermal method for an acetone-sensing application. This study explores the acetone gas-sensing characteristics of the WS₂ nanorod sensor for 5, 10, and 15 ppm concentrations at 25 °C, 50 °C, 75 °C, and 100 °C. The WS₂ nanorod sensor shows the highest sensitivity of 94.5% at 100 °C for the 15 ppm acetone concentration. The WS₂ nanorod sensor also reveals the outstanding selectivity of acetone compared to other gases, such as ammonia, ethanol, acetaldehyde, methanol, and xylene at 100 °C with a 15 ppm concentration. The estimated selectivity coefficient indicates that the selectivity of the WS₂ nanorod acetone sensor is 7.1, 4.5, 3.7, 2.9, and 2.0 times higher than xylene, acetaldehyde, ammonia, methanol, and ethanol, respectively. In addition, the WS₂ nanorod sensor also divulges remarkable stability of 98.5% during the 20 days of study. Therefore, it is concluded that the WS₂ nanorod can be an excellent nanomaterial for developing acetone sensors for monitoring work/public places.

A colloidal route to semiconducting tungsten disulfide nanosheets with monolayer thickness

Transition metal dichalcogenides (TMDs) are a class of materials that have been extensively studied in the last decade, with molybdenum disulfide (MoS₂) being the main protagonist. Typically, the interesting TMD properties, e.g. a direct band gap transition, or broken inversion symmetry, are only present in monolayer thick TMDs, and in the absence of strong lateral confinement, we require different materials or alloys thereof when we want to obtain TMDs with varying (direct) band gap energies. With this in mind, tungsten disulfide (WS₂) is emerging as a direct competitor of MoS₂ due to its similar properties but larger band gap energy. While several colloidal strategies have been reported for the synthesis of WS₂, the synthesis of monolayer WS₂ and detailed studies on the effect of synthesis parameters on the synthesis outcome have remained elusive. In this work we therefore focused on a colloidal synthesis method for monolayer WS₂ using a design of experiment (DOE) approach. After optimization, we obtained nanosheets with a band gap transition consistent with the expected value for a monolayer. The thickness was further confirmed by Raman spectroscopy. While we could identify two temperature ranges where we could obtain a monolayer, sample characterization by XPS spectroscopy revealed the presence of different ratios of the metallic phase, with the sample synthesized at lower temperature displaying a lower concentration of the metallic phase.

Broadband Optical Phase Modulation by Colloidal CdSe Quantum Wells

Two-dimensional (2D) semiconductors are primed to realize a variety of photonic devices that rely on the transient properties of photogenerated charges, yet little is known on the change of the refractive index. The associated optical phase changes can be beneficial or undesired depending on the application, but require proper quantification. Measuring optical phase modulation of dilute 2D materials is, however, not trivial with common methods. Here, we demonstrate that 2D colloidal CdSe quantum wells, a useful model system, can modulate the phase of light across a broad spectrum using a femtosecond interferometry method. Next, we develop a toolbox to calculate the time-dependent refractive index of colloidal 2D materials from widely available transient absorption experiments using a modified effective medium algorithm. Our results show that the excitonic features of 2D materials result in broadband, ultrafast, and sizable phase modulation, even extending to the near infrared because of intraband transitions.

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 MPa^1/2, 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 MPa^1/2.

Colloidal WSe2 nanocrystals as anodes for lithium-ion batteries

Transition metal dichalcogenides (TMDs) are gaining increasing interest in the field of lithium ion batteries due to their unique structure. However, previous preparation methods have mainly focused on their growth from substrates or by exfoliation of the bulk materials. Considering colloidal synthesis has many advantages including precision control of morphology and crystal phases, there is significant scope for exploring this avenue for active material formation. Therefore, in this work, we explore the applicability of colloidal TMDs using WSe₂ nanocrystals for Li ion battery anodes. By employing colloidal hot-injection protocol, we first synthesize 2D nanosheets in 2H and 1T' crystal phases. After detailed structural and surface characterization, we investigate the performance of these nanosheets as anode materials. We found that 2H nanosheets outperformed 1T' nanosheets exhibiting a higher specific capacity of 498 mA h g^-1 with an overall capacity retention of 83.28%. Furthermore, to explore the role of morphology on battery performance, 3D interconnected nanoflowers in 2H crystal phase were also investigated as an anode material. It is worth noting that a specific capacity of 982 mA h g^-1 was exhibited after 100 cycles by these nanoflowers. The anode materials were characterized prior to cycling and after 1, 25, and 100 charge/discharge cycles, by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to track the effects of cycling on the material.