Photophysical comparison of liquid and mechanically exfoliated WS$_2$ monolayers

Semiconducting transition metal dichalcogenides (TMDs) are desired as active materials in optoelectronic devices due to their strong excitonic effects. They can be exfoliated from their parent layered materials with low-cost and for mass production via a liquid exfoliation method. However, the device application of TMDs prepared by liquid phase exfoliation is limited by their poor photoluminescence quantum efficiencies (PLQE). It is crucial to understand the reason to low PLQE for their practical device development. Here we evaluate the quality of monolayer-enriched liquid phase exfoliated (LPE) WS2 dispersions by systematically investigating their optical and photophysical properties and contrasting with mechanically exfoliated (ME) WS2 monolayers. An indepth understanding of the exciton dynamics is gained with ultrafast pump-probe measurements. We reveal that the energy transfer between monolayer and few-layers in LPE WS2 dispersions is a substantial reason for their quenched PL. In addition, we show that LPE WS2 is promising to build high performance optoelectronic devices with excellent optical quality. Introduction The study of transition metal dichalcogenides (TMDs) has become a vibrant area in nanomaterial science. Exfoliated TMDs have been widely used in the field of optoelectronics due to their excellent light absorptivity and semiconducting performance. To utilize TMD materials, achieving more scalable techniques is critically important, and considerable effort has been devoted to the development of cost-effective mass production methods. Although mechanical exfoliation produces the highest quality materials, its application is limited by extremely low and uncontrollable yield. In contrast, liquid exfoliation yields atomically thin TMD flakes in a liquid medium in large quantities at moderate cost. The simplest way to produce nanosheets suspended in liquid is termed liquid phase exfoliation (LPE) which relies on immersing the bulk materials into suitable solvents or aqueous surfactant solution and apply high energy, e.g. sonication, to achieve exfoliation accompanied with tearing. When appropriately chosen, the solvent or surfactant suppresses reaggregation in the liquid. This approach is widely applicable to a range of materials and takes advantage of well-established print production processes for new generation of device fabrication. Nevertheless, liquid-exfoliated (LE) TMD nanosheets have been rarely used in optoelectronics, which is often attributed to their poor quality, such as defects, impurities, nonuniformity and small size, resulting in low photoluminescence quantum efficiencies (PLQE). Hence the development of high PLQE LE TMD monolayers is of fundamental importance toward the practical implementation of optoelectronic devices with TMDs. Nowadays, the quality of the LPE TMDs is improving with the continual development of the liquid phase exfoliation methodology and subsequent size selection. Largely defect-free monolayer enriched TMD dispersions with narrow line width photoluminescence (PL), which are similar to that of mechanically-exfoliated (ME) TMDs, were demonstrated. However, the PLQE of LPE TMD dispersions remains low and exciton dynamics of TMD dispersions is only little explored. In particular, most reports focus on ensembles with low monolayer content produced either from LPE or colloidal synthesis. Recently, we achieved high quality WS2 samples by mechanical exfoliation with bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) surface treatment, showing superior PL. In this work, we evaluate the quality of monolayer enriched WS2 dispersion produced from liquid phase exfoliation by conducting a systematic study on the optical and photophysical properties of different LPE WS2 dispersions and ME WS2 monolayer samples. The LPE WS2 dispersions can achieve PL with narrow linewidth and almost no Stoke shift compared to their absorption spectra. In addition, we use ultrafast pump-probe spectroscopy to study the exciton dynamics following photoexcitation in these WS2 samples. We reveal that the energy transfer between monolayers and few-layers in LPE WS2 dispersion samples can be a major factor for their quenched PL. Results and Discussion The LPE WS2 dispersion samples used in this study were prepared as shown in Figure 1. The liquidsuspended WS2 nanosheets are generated with the aid of dip sonication and stabilized against reaggregation by the surfactant sodium cholate in water. A size selection process is followed since the as-produced dispersion is highly polydisperse displaying a low monolayer content. Size selection is achieved by liquid cascade centrifugation (LCC) with subsequently increasing rotational speeds. Heavier and multilayer nanosheets are removed in each step of the LCC process, resulting in more and more monolayer-enriched supernatants. Two size-selected nanosheet distributions are collected as sediments after 10k g and 30k g centrifugation, hereafter labelled as 5-10k g WS2/H2O sample and 10-30k g WS2/H2O sample, respectively. Isopropyl alcohol (IPA) is also known to give stable dispersions, however monolayer enrichment has not yet been demonstrated. Hence, a 10-30k g WS2/IPA sample is also prepared in comparison by replacing the water/surfactant in the 10-30k g WS2/H2O sample with IPA through a centrifugation procedure. The monolayers obtained in this way are around 50 nm as characterized by transmission electron microscopy (TEM) (Figure S1a), which is similar to what was obtained from previous work. Since mechanical exfoliation renders high quality TMD monolayers, ME WS2 samples are also prepared as reference to LPE WS2 samples. Large monolayer WS2 samples (~ 200 μm) prepared on quartz substrates with mechanical exfoliation are identified by optical microscopy (Figure S1b). As shown in Figure S1c, both LPE and ME WS2 samples on Si/SiO2 substrates are characterized by Raman spectroscopy (excitation wavelength 532 nm), confirming the monolayer with characteristic Raman modes of monolayer WS2 (e.g., the 2LA(M) at 354 cm). Figure 1. Illustration of liquid exfoliation, size selection and solvent transfer. a Schematic of the tip sonication. b Schematic of the liquid cascade centrifugation. Relative centrifugal forces of (RCF) 5k g, 10k g, and 30k g are used. The supernatant after each step is transferred to another centrifugation at higher centrifugal acceleration, while the sediments are collected. c Schematic of the dispersion solvent exchange, transferring from H2O to IPA. 30k g RCF is used to spin down the WS2 nanosheets as a pellet allowing for a solvent exchange. 2k g RCF is used to remove aggregated nanosheets as sediments. The optical absorption properties of LPE 5-10k g WS2/H2O, 10-30k g WS2/H2O, and 10-30k g WS2/IPA dispersions are characterized via UV-Vis extinction spectroscopy. The spectra depicted in Figure 2a and Figure S2a are normalized to the local minimum 290 nm, since the extinction coefficient at 290 nm is widely independent of nanosheet thickness and length. The absorption spectra are dominated by excitonic features. The fundamental A-exciton (EA ) for all LPE WS2 dispersions is analysed in more detail using the second derivative of the extinction spectra (Figure 2b and Figure S2b). Due to the previously identified exponential blueshift of the A-exciton with decreasing layer number, two components are visible in the second derivative attributed to the Aexciton of the monolayer (EA ) and the unresolvable sum of few-layers (EA ). WS2 dispersions in H2O show EA ML at 2.029 eV (611 nm), while WS2 dispersions in IPA present slightly redshifted EA ML at 2.019 eV (614 nm), which may be attributed to solvatochromism and difference of dielectric disorder. Since the contributions to the A-exciton absorbance of monoand few-layer WS2 nanosheets are differentiated, the monolayer content is estimated from the second derivative of A-exciton absorbance peak according to the previously reported method (described in SI). There is a clear increasing monolayer volume fraction (Vf) in water dispersions with increasing RCF, which c Solvent transfer RCF 30k g Sediment RCF 2k g Collect supernatant as 10-30k g WS2/IPA sample 10-30k g WS2/H2O sample a Sonication