The transfer of synthesized 2D MoS2 films is important for fundamental and applied research. However, it is problematic to translate the well-established transfer processes for graphene to MoS2 due to different growth mechanisms and surface properties. Here we demonstrate a surface-energy-assisted process that can perfectly transfer centimeter-scale monolayer and few-layer MoS2 films from original growth substrates onto arbitrary substrates with no observable wrinkles, cracks, and polymer residues. The unique strategies used in this process include leveraging the penetration of water between hydrophobic MoS2 films and hydrophilic growth substrates to lift off the films and dry transferring the film after the lift off. This is in stark contrast with the previous transfer process for synthesized MoS2 films, which explores the etching of the growth substrate by hot base solutions to lift off the films. Our transfer process can effectively eliminate the mechanical force caused by bubble generations, the attacks from chemical etchants, and the capillary force induced when transferring the film outside solutions as in the previous transfer process, which consists of the major causes for the previous unsatisfactory transfer. Our transfer process also benefits from using polystyrene (PS), instead of poly(methyl methacrylate) (PMMA) that was widely used previously, as the carrier polymer. PS can form more intimate interaction with MoS2 films than PMMA and is important for maintaining the integrity of the film during the transfer process. This surface-energy-assisted approach can be generally applied to the transfer of other 2D materials, such as WS2.

We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5–7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5–7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

We present a combined theoretical and experimental effort to enable strong light absorption (>70%) in atomically thin MoS2 films (≤4 layers) for either narrowband incidence with arbitrarily prespecified wavelengths or broadband incidence like solar radiation. This is achieved by integrating the films with resonant photonic structures that are deterministically designed using a unique reverse design approach based on leaky mode coupling. The design starts with identifying the properties of leaky modes necessary for the targeted strong absorption, followed by searching for the geometrical features of nanostructures to support the desired modes. This process is very intuitive and only involves a minimal amount of computation, thanks to the straightforward correlations between optical functionality and leaky modes as well as between leaky modes and the geometrical feature of nanostructures. The result may provide useful guidance for the development of high-performance atomic-scale photonic devices, such as solar cells, modulators, photodetectors, and photocatalysts.

Monolayer MoS can effectively screen the vdW interaction of underlying substrates with external systems by >90% because of the substantial increase in the separation between the substrate and external systems due to the presence of the monolayer. This substantial screening of vdW interactions by MoS monolayer is different from what reported at graphene.

We have formed noncovalent inclusion compounds (ICs) between guest poly(ε-caprolactone) (PCL) chains, with molecular weights ranging from ∼2000 to 80 000 g/mol, and host urea (U). Upon careful removal of the U host, each of the guest PCL chains were coalesced from their U-IC crystals to produce coalesced samples (c-PCLs). As previously observed for PCL and other polymer guests when coalesced from their ICs formed with host cyclodextrins (CDs), upon cooling from their melts, PCLs coalesced from their U-ICs also show enhanced abilities to crystallize, regardless of their molecular weight. Also consistent with polymer guests, including PCL, that were coalesced from their CD-ICs, c-PCLs obtained from their U-ICs retain their enhanced abilities to crystallize even after spending long times (weeks or more) in the melt. Because, unlike CD hosts, U does not thread over guest polymer chains in their ICs, we conclude that the enhanced ability of c-PCLs to crystallize from their melts upon removal of either host from their ICs is solely a consequence of their coalesced conformations/structures/morphologies, which are stable to prolonged melt-annealing. Furthermore, because c-PCLs with chain lengths well below and well above those corresponding to the entanglement molecular weight of PCL behave similarly, we conclude that their enhanced ability to crystallize from the melt is likely an exclusive consequence of the extended and unentangled arrangement of their coalesced chains. In addition, c-PCLs obtained from their U-ICs are observed to effectively act as self-nucleants, when added in small amounts to as-received PCLs, to produce nuc-PCLs, which like neat c-PCLs exhibit an enhanced ability to melt-crystallize.

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Cyclodextrins (CDs) are cyclic polysaccharides with nano-size, largely hydrophobic cavities, and exteriors covered with hydrophilic hydroxyl groups, making them water soluble. Threading and filling their cavities with polymer chains produces noncovalently bonded crystalline inclusion compounds (ICs). In this study, we formed fully covered, stoichiometric ICs between guest poly(L-lactic acid), poly(e-caprolactone), and nylon-6 chains and host a-CD. Coalesced samples of all three polymers were obtained after appropriately removing the stacked a-CD host channels from their ICs. Distinct differential scanning calorimetriy (DSC) thermograms were observed for as-received and coalesced samples, with the coalesced samples crystallizing faster at higher temperatures from their melts, and this distinction was maintained even after extensive, long-time melt-annealing (hours, days, and weeks). We believe this is due to the largely unentangled chains with extended conformations that are more densely packed in the initially coalesced samples. When small amounts ($2 wt %) of the coalesced polymers are used as self-nucleating agents for their as-received samples, the resulting self-nucleated samples show DSC thermograms similar to those of the neat coalesced polymers, including their long-time stability to melt-annealing. Coalesced polymers, whether neat or in samples they self-nucleate, may conserve their organization in the melt (largely extended and unentangled chains) for long periods, because the process of entangling the many chains influenced by a single initially extended unentangled coalesced chain, after it randomly coils, is extremely sluggish. By contrast, in melt-crystallized or solution-cast samples, polymer chains generally become fully randomly coiled, interpenetrate, and entangle after being heated and held in their melts for comparatively much shorter times. For example, we have recently observed (DSC) that ultra high molecular weight, gel-spun spectra polyethylene (PE) fibers V R did not conserve or retain any memory of their as-spun and highly drawn semicrystalline morphology even after spending as little as 2 min in the melt. As a consequence of the comparison to the behavior of coalesced polymer melts, we believe that polyethylene chains in Spectra fibers V R must be at least intimately dispersed within their crystalline regions, and likely partially coiled and entangled in their noncrystalline regions, thereby facilitating their rapid transformation into a full entanglement network of randomly coiling chains in the melt.