Unlike the vast majority of transition metal dichalcogenides which are semiconductors, vanadium disulfide is metallic and conductive. This makes it particularly promising as an electrode material in lithium-ion batteries. However, vanadium disulfide exhibits poor stability due to large Peierls distortion during cycling. Here we report that vanadium disulfide flakes can be rendered stable in the electrochemical environment of a lithium-ion battery by conformally coating them with a ~2.5 nm thick titanium disulfide layer. Density functional theory calculations indicate that the titanium disulfide coating is far less susceptible to Peierls distortion during the lithiation-delithiation process, enabling it to stabilize the underlying vanadium disulfide material. The titanium disulfide coated vanadium disulfide cathode exhibits an operating voltage of ~2 V, high specific capacity (~180 mAh g −1 @200 mA g −1 current density) and rate capability (~70 mAh g −1 @1000 mA g −1 ), while achieving capacity retention close to 100% after 400 charge−discharge steps.

Remarkable properties of layered metal dichalcogenides and their potential applications in various fields have raised intense interest worldwide. We report tens of microns-sized ultrathin single crystal SnS 2 flakes grown on amorphous substrates using a simple one-step thermal coevaporation process. X-ray pole figure analysis reveals that a majority of flakes are oriented with the (0001) plane parallel to the substrate and a preferred fiber texture. For few-layer-thick SnS 2 , Moire patterns of 6-fold and 12-fold symmetries are observed by transmission electron microscopy imaging and diffraction. These patterns result from the relative rotation between SnS 2 layers in the ultrathin flake. The 12-fold symmetry is consistent with a known quasicrystal pattern. The photoluminescence spectrum supports that these ultrathin flakes possess a direct bandgap. Carrier lifetime measured by time resolved photoluminescence of a single flake is a few nanoseconds. These results improve our understanding of the formation and shape of ultrathin SnS 2 flakes.

In this work, we show that remote heteroepitaxy can be achieved when Cu thin film is grown on single crystal, monolayer graphene buffered sapphire(0001) substrate via a thermal evaporation process. X-ray diffraction and electron backscatter diffraction data show that the epitaxy process forms a prevailing Cu crystal domain, which is remotely registered in-plane to the sapphire crystal lattice below the monolayer graphene, with the (111) out-of-plane orientation. As a poor metal with zero density of states at its Fermi level, monolayer graphene cannot totally screen out the stronger charge transfer/metallic interactions between Cu and substrate atoms. The primary Cu domain thus has good crystal quality as manifested by a narrow crystal misorientation distribution. On the other hand, we show that graphene interface imperfections, such as bilayers/multilayers, wrinkles and interface contaminations, can effectively weaken the atomic interactions between Cu and sapphire. This results in a second Cu domain, which directly grows on and follows the graphene hexagonal lattice symmetry and orientation. Because of the weak van der Waals interaction between Cu and graphene, this domain has inferior crystal quality. The results are further confirmed using graphene buffered spinel(111) substrate, which indicates that this remote epitaxial behavior is not unique to the Cu/sapphire system.

To date, many materials have been successfully grown on substrates through van der Waals epitaxy without adhering to the constraint of lattice matching as is required for traditional chemical epitaxy. However, for elemental semiconductors such as Ge, this has been challenging and therefore it has not been achieved thus far. In this paper, we report the observation of Ge epitaxially grown on mica at a narrow substrate temperature range around 425 °C. Despite the large lattice mismatch (23%) and the lack of high in-plane symmetry in the mica surface, an epitaxial Ge film with [111] out-of-plane orientation is observed. Crystallinity and electrical properties degrade upon deviation from the ideal growth temperature, as shown by Raman spectroscopy, X-ray diffraction, and Hall effect measurements. X-ray pole figure analysis reveals that there exist multiple rotational domains in the epitaxial Ge film with dominant in-plane orientations between Ge1¯10 and mica[100] of (20n)°, where n = 0, 1, 2, 3, 4, 5. A superlattice area mismatch model was used to account for the likelihood of the in-plane orientation formation and was found to be qualitatively consistent with the observed dominant orientations. Our observation of Ge epitaxy with one out-of-plane growth direction through van der Waals forces is a step toward the growth of single crystal Ge films without the constraint in the lattice and symmetry matches with the substrates.

Atmospheric pressure chemical vapor deposition (APCVD) is employed for the synthesis of layered vanadium disulfide. By tuning several critical growth parameters, we achieve VS 2 flakes with lateral dimension over 100 μm and thickness down to monolayer (∼0.59 nm) and bilayer (∼1.17 nm), which are larger and thinner than those previously reported in the literature. Furthermore, ultrathin flakes with thicknesses of several atomic layers are directly synthesized on mica and SiO 2 substrates without the use of an exfoliation method. X-ray diffraction and high-resolution transmission electron microscopy confirm the flakes' monocrystalline quality. Raman spectra are collected and are consistent with the vibrational modes for the trigonal phase of VS 2 as determined by density functional theory calculations. Through electron backscatter diffraction pole figure analysis, transmission electron microscopy, and optical microscopy, a complex epitaxial relationship with nine preferred in-plane orientations is observed in some regions of the VS 2 /mica samples. Remarkably, this is in agreement qualitatively with a superlattice area mismatch model, providing further evidence of the interfacial interactions with mica dictating the nucleation of film atoms in van der Waals heterostructures. Finally, magnetic force microscopy measurements suggest room-temperature ferromagnetism in ultrathin VS 2 flakes, in agreement with several density functional theory calculations. The discovery of an ultrathin ferromagnetic metal such as VS 2 may have an impact on emerging fields such as spintronics and quantum computing.