H. S. S. Ramakrishna Matte

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

Hysteresis in Single-Layer MoS2 Field Effect Transistors

Field effect transistors using ultrathin molybdenum disulfide (MoS(2)) have recently been experimentally demonstrated, which show promising potential for advanced electronics. However, large variations like hysteresis, presumably due to extrinsic/environmental effects, are often observed in MoS(2) devices measured under ambient environment. Here, we report the origin of their hysteretic and transient behaviors and suggest that hysteresis of MoS(2) field effect transistors is largely due to absorption of moisture on the surface and intensified by high photosensitivity of MoS(2). Uniform encapsulation of MoS(2) transistor structures with silicon nitride grown by plasma-enhanced chemical vapor deposition is effective in minimizing the hysteresis, while the device mobility is improved by over 1 order of magnitude.

Graphene analogues of inorganic layered materials.

The discovery of graphene has created a great sensation in chemistry, physics, materials science, and related areas. The unusual properties of graphene have aroused interest in other layered materials, such as molybdenum sulfide and boron nitride. In the last few years, single- as well as few-layer as well as chalcogenides and other inorganic materials have been prepared and characterized by a variety of methods. These materials possess interesting properties, and some have potential applications. This Review provides an up-to-date account of these emerging two-dimensional nanomaterials. Not only are the synthesis and characterization covered, but also important aspects such as spectroscopic and optical properties, magnetic and electrical properties, as well as applications. Salient features of the composites formed from the layered inorganic structures with graphene and polymers are presented along with a brief description of borocarbonitrides.

GaS and GaSe Ultrathin Layer Transistors

Room-temperature, bottom-gate, field-effect transistor characteristics of 2D ultrathin layer GaS and GaSe prepared from the bulk crystals using a micromechanical cleavage technique are reported. The transistors based on active GaS and GaSe ultrathin layers demonstrate typical n-and p-type conductance transistor operation along with a good ON/OFF ratio and electron differential mobility.

A study of the synthetic methods and properties of graphenes

Abstract Graphenes with varying number of layers can be synthesized by using different strategies. Thus, single-layer graphene is prepared by micromechanical cleavage, reduction of single-layer graphene oxide, chemical vapor deposition and other methods. Few-layer graphenes are synthesized by conversion of nanodiamond, arc discharge of graphite and other methods. In this article, we briefly overview the various synthetic methods and the surface, magnetic and electrical properties of the produced graphenes. Few-layer graphenes exhibit ferromagnetic features along with antiferromagnetic properties, independent of the method of preparation. Aside from the data on electrical conductivity of graphenes and graphene-polymer composites, we also present the field-effect transistor characteristics of graphenes. Only single-layer reduced graphene oxide exhibits ambipolar properties. The interaction of electron donor and acceptor molecules with few-layer graphene samples is examined in detail.

Unusual magnetic properties of graphene and related materials

High-temperature ferromagnetism in graphene and other graphite-derived materials reported by several workers has attracted considerable interest. Magnetism in graphene and graphene nanoribbons is ascribed to defects and edge states, the latter being an essential feature of these materials. Room-temperature ferromagnetism in graphene is affected by the adsorption of molecules, especially hydrogen. Inorganic graphene analogues formed by layered materials such as BN and MoS2 also show such ferromagnetic behaviour. Magnetoresistance observed in graphene and graphene nanoribbons is of significance because of the potential applications.

Synthesis and Selected Properties of Graphene and Graphene Mimics

Graphene has generated great excitement in the last few years because of its novel properties with potential applications. Graphene exhibits an ambipolar electric field effect, ballistic conduction of charge carriers, and the quantum Hall effect at room temperature. Some of the other interesting characteristics of graphene include high transparency toward visible light, high elasticity and thermal conductivity, unusual magnetic properties, and charge transfer interactions with molecules. In this Account, we present the highlights of some of our research on the synthesis of graphene and its properties. Since the isolation and characterization of graphene by micromechanical cleavage from graphite, several strategies have been developed for the synthesis of graphene with either a single or just a few layers. The most significant contribution from our laboratory is the synthesis of two to four layer graphene by arc-discharge of graphite in a hydrogen atmosphere. Besides providing clean graphene surfaces, this method allows for doping with boron and nitrogen. UV and laser irradiation of graphene oxide provides fairly good graphene samples, and laser unzipping of nanotubes produces graphene nanoribbons. We have exploited Raman spectroscopy to investigate the charge-transfer interactions of graphene with electron-donor and -acceptor molecules, as well as with nanoparticles of noble metals. Graphene quenches the fluorescence of aromatics because of electron transfer or energy transfer. Notable potential applications of the properties of graphene are low turn-on field emission and radiation detection. High-temperature ferromagnetism is another intriguing feature of graphene. Although incorporation of graphene improves the mechanical properties of polymers, its incorporation with nanodiamond or carbon nanotubes exhibits extraordinary synergy. The potential of graphene and its analogues as adsorbents and chemical storage materials for H(2) and CO(2) is noteworthy.

Graphene analogues of layered metal selenides

Graphene analogues of MoSe(2) and WSe(2) have been prepared by three different chemical methods and characterized by electron microscopy and other methods. Graphene analogues of these diselenides as well as of GaSe have also been obtained by liquid-phase exfoliation. Raman spectra of the graphene analogues show significant changes relative to those of the bulk samples.

Synthesis, Characterization, and Properties of Few-Layer MoO3

Nanosheets of MoO3 that consist of only a few layers have been prepared by using four methods, including the oxidation of MoS2 nanosheets, intercalation with LiBr, and ultrasonication. These nanosheets have been characterized by atomic force microscopy and other techniques. Besides showing a blue-shift of the optical absorption band compared to the bulk sample, few-layer MoO3 exhibits enhanced photocatalytic activity. In combination with a borocarbonitride, few-layer MoO3 shows good performance characteristics as a supercapacitor electrode.

Charge‐Transfer Interaction between Few‐Layer MoS2 and Tetrathiafulvalene

Graphene has emerged to be a material of great interest because of its unique electronic structure and properties associated with its two-dimensional structure. Single-layer graphene is well known for properties such as quantum Hall effect, ambipolar electric field effect, and ballistic conduction of charge carriers. Graphene exhibits significant changes in electronic structure and properties on introduction of electrons or holes by electrochemical means. Such doping is reported to stiffen the Raman G band (frequency of the Raman band increases). Electron and hole doping can also be achieved by molecular charge transfer through interaction with electron donor and acceptor molecules, respectively. Molecular charge transfer with graphene has been investigated in detail by using Raman spectroscopy and other techniques. Charge-transfer interaction with an electron donor molecule like tetrathiafulvalene (TTF) softens the G band of graphene, whereas stiffening occurs upon interaction with an electron acceptor like tetracyanoethylene (TCNE), the Raman G band frequency will increase and decrease due to interaction with TCNE and TTF, respectively. These changes in the Raman G band are different from those found with electrochemical doping. We were interested to explore the interaction of electron donor and acceptor molecules with a two-dimensional layered material such as MoS2 to explore the occurrence of charge transfer, if any. With this purpose, we have studied the interaction of few-layer MoS2 material with TTF and TCNE. It is to be noted that MoS2 layers consist of Mo atoms sandwiched between two layers of chalcogen atoms, where the adjacent sheets are stacked by van der Waals interactions. We have observed the occurrence of charge transfer of fewlayer MoS2 material with TTF, but not with TCNE. Electronic absorption spectroscopic measurements indicate the formation of TTF radical cation by the interaction of TTF with few-layer MoS2 material, accompanied by the stiffening of the A1g mode of MoS2 in the Raman spectrum. This shift in the Raman A1g mode is opposite to that found in electrochemical doping. We have carried out detailed first-principle calculations to understand the results. The XRD pattern of the few-layer MoS2 material does not exhibit the (002) reflection, thus confirming the presence of only a few layers and the graphene-like nature of the material. The AFM images and the corresponding height profiles also confirm the presence of two to three layers with an average thickness of 2.44 nm. Figure 1 shows