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Upconversion rare-earth nanomaterials (URENs) possess highly efficient near-infrared (NIR), e.g., 980 nm, laser absorption and unique energy upconversion capabilities. On the other hand, graphene and its derivatives, such as graphene oxide (GO), show excellent performance in optical limiting (OL); however, the wavelengths of currently used lasers for OL studies mainly focus on either 532 or 1064 nm. To design new-generation OL materials working at other optical regions, such as the NIR, a novel nanocomposites, GO-URENs, which combines the advantages of both its components, is synthesized by a one-step chemical reaction. Transmission electron microscopy, X-ray diffraction, infrared spectroscopy, and fluorescence studies prove that the α-phase URENs uniformly attach on the GO surface via covalent chemical bonding, which assures highly efficient energy transfer between URENs and GO, and also accounts for the significantly improved OL performance compared to either GO or URENs. The superior OL effect is also observed in the proof-of-concept thin-film product, suggesting immediate applications in making high-performance laser-protecting products and optoelectronic devices.

Colloidal synthesis of copper tin sulfide (CTS) nanocrystals has been of great interest due to their potential for use in solution-processed photovoltaics containing only earth-abundant, low-toxicity elements. Postsynthetic incorporation of Sn into copper sulfide is an effective means of producing CTS, and Sn content can be controlled to some extent by the amount of Sn precursor provided. However, the oxidation (valence) state of Sn before and after doping and the effectiveness of Sn­(II) vs Sn­(IV) precursors remain topics of significant debate. Here, we demonstrate a step-growth method for preparing monodisperse copper tin sulfide (CTS) nanoplatelets (NPls). We show that Sn2+ ions, but not Sn4+ ions, can convert covellite to Cu3Sn x S4 NPls (x ≤ 1) without addition of any reducing or copper-extracting agent. In this case Sn2+ reduces the disulfide bond in covellite and the crystal structure evolves from covellite to kuramite. When dodecanethiol (DDT) is added prior to Sn, only Sn4+ ions, and not Sn2+ ions, are incorporated, ultimately producing mohite Cu2SnS3. Here, DDT can reduce the disulfide bonds and extract Cu from the lattice. When djurleite Cu2–x S was used as the template, only Sn4+, and not Sn2+, could be incorporated without use of DDT, and the extent of Sn incorporation was very limited, perhaps only filling Cu vacancies and not displacing any Cu from the lattice. Together with previous studies of CTS preparation from Cu2–x S using Sn4+ and DDT, these results provide a flexible array of methods of achieving a desired final CTS composition, crystal structure, and morphology. The ability to produce two different crystal phases of CTS, with corresponding differences in band gap and other properties, from the same covellite template while retaining the template size and morphology could be particularly valuable.

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The increased utilization of high power laser sources has rendered great challenges for designing effi cient optical limiting (OL) materials to protect human eyes and various delicate optical devices. An ideal optical limiter should greatly attenuate an intense laser beam while exhibiting high transmittance for low-input optical intensity. [ 1 ] Up to now, numerous organic and inorganic materials have proved to be good candidates for optical limiters with different working mechanisms. Among them, carbon-based materials, such as fullerenes, carbon black, and carbon nanotubes, have exhibited very good OL performance. [2][3][4][5] As a typical representative, graphene, which consists of sp 2 -hybridized carbon atoms with single-atom thickness and 2D structure, has exhibited unique electronic, optical, and mechanical properties. [ 6 ] The interband optical transitions in graphene are independent of frequency over a wide range and depend only on the fi nestructure constant, thus it is naturally promising as a broadband optical limiter. [ 7 ] Another important advantage of graphene-based materials is that they can be easily functionalized with organic molecules and hybridized with inorganic nanomaterials via covalent bonding, to improve their OL performances through the synergistic effect. For example, the enhanced nonlinear optical properties have been reported in porphyrin-functionalized graphene, [ 8 ] organic dye ionic complex, [ 9 ] oligothiophene, [ 10 ] fullerene, [ 11 ] phthalocyanine, [ 12 ] and CdS quantum dots. [ 13 ] We have noticed that all the OL properties of graphenecontaining materials, usually dispersed in liquid media, have been studied only under nanosecond excitation conditions and the working wavelengths of the lasers used are mainly 532 or 1064 nm. [7][8][9][10][11][12][13] However, the studies on the OL properties of graphene-containing materials in femtosecond region and other wavelengths are still absent. To a large extent, it may be due to the saturable absorption behavior of graphene under the excitation of femtosecond laser with low frequency. [ 14 , 15 ] Though most of the materials reported have shown good OL behavior in the nanosecond region due to the dominance of nonlinear scattering (NLS), they suffer from the dramatic decrease of OL performance due to the strongly suppressed NLS effect when fabricated into solid fi lms or excited with femtosecond pulse, which remains a serious obstacle for real applications. Therefore, design and synthesis of novel graphene-based OL materials that can work in the femtosecond pulse domain and other wavelengths region still represents a signifi cant challenge.Due to the long lifetime (in the order of milliseconds) of their real metastable states, upconversion process of lanthanide (Er 3 + , Yb 3 + , and Tm 3 + ) ions doped in the lattice of NaYF 4 nanocrystals after near infra-red (NIR) laser excitation, can be nearly 10 5 times more effi cient than the typical twophoton absorption process observed in organic molecules. [ 16 ] Therefore, it is highly...