Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude larger than those due to one-dimensional (perpendicular to the layer planes) confinement emphasizing the two-dimensional nature of the structure and bonding; (4) the observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size, a feature that reflects quantum confinement as well as possible changes in the degree of hybridization of the electronic orbitals which make up the states at these points; and (5) the observation of photoluminescence due to both direct and surface recombination. Several of these features bode well for the potential of these materials for solar photocatalysis.

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The dimension R in Fig. 8 is obtained from TEM measurements. Generally, we deposit MoS2 on holey carbon grids directly from solution, rapidly wicking away the solvent using a piece of underlying filter paper. The resulting clusters appear to preferentially align themselves with their c axis normal to the grid (based upon the absence of certain diffraction lines in small angle diffraction). Thus, we measure the lateral extent of the cluster, i.e., d or 2R. As a subsequent discussion will confirm, we believe this dimension controls the quantum confinement effect observed in MoS2 nanoclusters. We have only indirect evidence regarding the thickness of the MoS2 clusters perpendicular to the grid. We also measure a size R, which corresponds to an equivalent sphere radius, by measuring the relaxation time for concentration fluctuations due to translational diffusion of the MoS2 nanoclusters in solution. We use dynamic light scattering (DLS) to obtain this time. The calculated equivalent sphere size is typically slightly larger than found by TEM, [e.g., we find R (DLS)=3.2 nm for clusters which measure R=2.8 nm by TEM] which argues that the thickness is roughly comparable to the cross-sectional dimension for the smaller clusters. For the larger R=4.5 nm clusters the equivalent sphere size is actually slightly smaller [e.g., R (DLS) ∼4.0 nm] than the TEM cross-sectional area, which argues that their thickness is smaller than their cross-sectional TEM size. It is important to realize that for very small clusters with R<3 nm there is at least a 10% measurement uncertainty in determining R, so this is the largest source of uncertainty when comparing to theory.
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