A. Gomathi

MoS2 and WS2 analogues of graphene.

Following the discovery of fullerenes in 1985, it was soon recognized that inorganic layered materials such as MoS2 and WS2 can also form fullerene-like structures. [2] After the discovery of carbon nanotubes, inorganic nanotubes analogous to carbon nanotubes were prepared and characterized, nanotubes of MoS2 and WS2 being archetypal examples. [4] With the discovery and characterization of graphene, that is, two-dimensional nanocarbon, which has created great interest in last few years, it would seem natural to explore the synthesis of graphene analogues of layered inorganic materials such as dichalcogenides of molybdenum and tungsten. We aim to prepare graphene-like MoS2 and WS2, which are quasi-two-dimensional compounds in which the atoms within the layer are held together by strong covalent forces while van der Waals interaction enables stacking of the layers. Synthesis of crystals of MoS2 containing several molecular layers by micromechanical cleavage has been reported, and optical absorption and photoconductivity of these films have been studied. There is also a report on the intercalation of alkali metals with layered metal dichalcogenide crystals with controlled stoichiometry, but the products of exfoliation were not examined in this study. There is an early report on graphene-like MoS2 prepared by lithium intercalation and exfoliation, but the material was characterized only by X-ray diffraction, which is not sufficient to determine the exact nature and number of layers. Attempts were made to prepare single layers of WS2 by lithium intercalation and exfoliation as well, 12] but here again the product was only characterized on the basis of the (002) reflection in the X-ray diffraction pattern. Schumacher et al. and Gordon et al. prepared MoS2 samples by lithium intercalation followed by exfoliation and characterized the products by means of scanning force microscopy and X-ray absorption fine structure spectroscopy. Yang et al. report that the exfoliated MoS2 forms aqueous suspensions of single layers wherein sulfur atoms are bonded with molybdenum in an octahedral arrangement with 2a0 superlattice. Suspensions of layered chalcogenides have also been used to prepare inclusion compounds of various organic molecules and to fabricate light-emitting diodes. Since even MoS2 and WS2 containing five layers do not exhibit the (002) reflection prominently, layered MoS2 and WS2 produced by lithium intercalation and exfoliation must be investigated by transmission electron microscopy and other techniques. Furthermore, it seems desirable to explore alternative syntheses of these graphene-like materials. To this end, we employed three different methods to synthesize graphenelike MoS2 and WS2. In Method 1, bulk MoS2 and WS2 were intercalated with lithium and exfoliated in water. The reaction between lithium-intercalated MoS2 and WS2 and water forms lithium hydroxide and hydrogen gas and leads to separation of the sulfide layers and loss of periodicity along the c axis. In Method 2, molybdic acid and tungstic acid were treated with an excess of thiourea in an N2 atmosphere at 773 K. Method 3 involved the reaction between MoO3 and KSCN under hydrothermal conditions. The products of these reactions were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), Raman spectroscopy, and X-ray diffraction (XRD). The XRD patterns of the molybdenum sulfide samples obtained by the three methods do not exhibit the (002) reflection (Figure 1a). Energy-dispersive analysis of X-rays (EDAX) shows the products to be stoichiometric MoS2. The TEM and AFM images of the products show the presence of one or a few layers of MoS2 (Figures 2 and 3). Figure 2 a and b show graphene-like MoS2 layers obtained by methods 2 and 3 with a layer separation in the range of 0.65–0.7 nm. The highresolution image in Figure 2c shows the hexagonal structure formed by Mo and S atoms with an Mo S distance of 2.30 . The AFM images and height profiles of the products also confirm the formation of few-layer MoS2 (Figure 3a). Figure 4a compares the Raman spectra of graphene-like MoS2 samples with that of bulk MoS2. The bulk sample shows bands at 406.5 and 381.2 cm 1 due to the A1g and E2g modes with fullwidths at half maximum (FWHM) of 2.7 and 3.1 cm , respectively. Interestingly, few-layered MoS2 prepared by lithium intercalation exhibits corresponding bands at 404.7 and 379.7 cm . The sample obtained by Method 2 show these bands at 404.7 and 377.4 cm . The A1g and E2g modes in the graphene analogues of MoS2 are clearly softened. Furthermore, the FWHM values are larger in the graphene-like samples (10–16 cm 1 vs. ca. 3 cm 1 in the bulk sample). Broadening of the Raman bands is considered to be due to phonon confinement, and also suggests that the lateral dimensions of these layers are in the nanoregime. We also prepared graphene-like MoS2 by micromechanical cleavage of a MoS2 single crystal using the Scotch-tape technique. Raman spectra of these samples show progressive softening of the A1g and E2g bands with decreasing number of layers. [*] H. S. S. Ramakrishna Matte, A. Gomathi, A. K. Manna, Dr. D. J. Late, Dr. R. Datta, Prof. Dr. S. K. Pati, Prof. Dr. C. N. R. Rao Chemistry and Physics of Materials Unit, Theoretical Science Unit and International Centre for Materials Science Jawaharlal Nehru Centre for Advanced Scientific Research Jakkur P.O., Bangalore 560 064 (India) Fax: (+ 91)80-2208-2760 E-mail: cnrrao@jncasr.ac.in Angewandte Chemie

Interaction of inorganic nanoparticles with graphene.

The changes in the electronic and magnetic properties of graphene induced by interaction with semiconducting oxide nanoparticles such as ZnO and TiO(2) and with magnetic nanoparticles such as Fe(3)O(4), CoFe(2)O(4), and Ni are investigated by using Raman spectroscopy, magnetic measurements, and first-principles calculations. Significant electronic and magnetic interactions between the nanoparticles and graphene are found. The findings suggest that changes in magnetization as well as the Raman shifts are directly linked to charge transfer between the deposited nanoparticles and graphene. The study thus demonstrates significant effects in tailoring the electronic structure of graphene for applications in futuristic electronic devices.

Covalent and noncovalent functionalisation and solubilisation of nanodiamond

Covalent functionalisation of nanodiamond has been carried out by employing several methods. One of them involves the reaction of acid-treated nanodiamond with thionyl chloride followed by reaction with a long-chain aliphatic amine to produce the amide derivative. The second method involves reaction of acid-treated nanodiamond with an organosilicon or organotin reagent such as hexadecyltrimethoxysilane, dibutyldimethoxytin, and perfluoro-octyltriethoxysilane. The products of covalent functionalisation produce excellent dispersions in CCl4 and toluene. SiO2–and SnO2–covered nanodiamond are obtained by heating the nanodiamond coated with the organosilane and the organotin reagents, respectively. By interaction of nanodiamond with surfactants such as sodium bis(2-ethylhexyl) sulphosuccinate (AOT), Triton X-100 (TX-100), polyvinyl alcohol (PVA), cetyltrimethylammonium bromide (CTAB), and tert-octylphenoxy poly(oxyethylene)ethanol (IGEPAL) gives good dispersions in water, the best dispersion with the lowest surfactant concentration being obtained with IGEPAL.

Functionalization and solubilization of inorganic nanostructures and carbon nanotubes by employing organosilicon and organotin reagents

Covalent functionalization of nanowires of TiO 2 , ZnO and Al 2 O 3 has been carried out by employing the organosilicon reagents aminopropyltriethoxysilane and hexadecyltrimethoxysilane (HDTMS). The presence of the organosilane coating was confirmed by electron microscopy, energy dispersive X-ray analysis (EDXA) and IR spectroscopy. HDTMS-coated oxide nanowires give stable dispersions in CCl 4 and toluene. Nanoparticles of these metal oxides as well as of CeO 2 and Fe 3 O 4 could be solubilized in non-polar solvents by functionalizing with HDTMS. Nanotubes and nanoparticles of BN could also be functionalized and solubilized with HDTMS. Organotin reagents have also been used to covalently functionalize oxide nanostructures and multi-walled carbon nanotubes, thereby producing stable dispersions in CCl 4 and toluene. The organotin reagents used were dibutyldimethoxytin and trioctyltinchloride. Covalent functionalization of nanostructures using organosilane and organotin reagents provides a general method applicable to large class of inorganic materials as well as carbon nanotubes and is likely to be useful in practice.

Functionalization and Solubilization of Carbon and Inorganic Nanostructures

A variety of nanostructures of carbon and inorganic nanomaterials possessing different dimensionalities have been synthesized and characterized in the last few years. Several of these nanostructures are found to have properties of utility with potential applications. Using the nanostructures in many situations requires their dispersions in suitable solvents. This can be done in most instances by appropriate functionalization of the nanostructures. In this contribution, we provide an account of the covalent and noncovalent methods of functionalization of carbon and inorganic nanostructures and their subsequent solubilization in nonpolar, polar, and aqueous media.