Swapan K. Pati

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

Intrinsic half-metallicity in modified graphene nanoribbons.

We perform first-principles calculations based on density functional theory to study quasi-one-dimensional edge-passivated (with hydrogen) zigzag graphene nanoribbons of various widths with chemical dopants, boron and nitrogen, keeping the whole system isoelectronic. The gradual increase in doping concentration takes the system finally to zigzag boron nitride nanoribbons (ZBNNRs). Our study reveals that for all doping concentrations the systems stabilize in antiferromagnetic ground states. Doping concentrations and dopant positions regulate the electronic structure of the nanoribbons, exhibiting both semiconducting and half-metallic behaviors as a response to the external electric field. Interestingly, our results show that ZBNNRs with a terminating polyacene unit exhibit half-metallicity irrespective of the ribbon width as well as applied electric field, opening a huge possibility in spintronics device applications.

Novel properties of graphene nanoribbons: a review

Low-dimensional materials are of great interest to both theorists and experimentalists, owing to their novel electronic properties which arise mainly because of a host of quantum confinement effects. Recent experimental findings of graphene have provided a new platform to explore the interesting electronic properties in strictly two dimensions. In this feature article, we review the novel properties of an interesting class of quasi one dimensional materials, known as graphene nanoribbons, which can be obtained by finite termination of graphene sheet with smooth edges. Recent experimental sophistications provide various physical and chemical ways to materialize these systems. Two different edge geometries, namely zigzag and armchair, arising from the finite termination of graphene, control the electronic properties of graphene nanoribbons. Here we attempt to give an overview of their interesting electronic, magnetic, optical, conduction properties and explore possible ways of enhancing their device applicability by a number of ways including external perturbations, doping and chemical modifications.

Dipolar interactions and hydrogen bonding in supramolecular aggregates: understanding cooperative phenomena for 1st hyperpolarizability

Weak intermolecular forces like dipolar interactions and hydrogen-bonding lead to a variety of different packing arrangements of molecules in crystals and self-assemblies. Such differences in the arrangements change the extent of excitonic splitting and excitation spectra in the multichromophore aggregates. In this tutorial review, the role of such interactions in fine tuning the linear and 1st non-linear optical (NLO) responses in molecular aggregates are discussed. The non-additivity of these optical properties arise specifically due to such cooperative interactions. Calculations performed on dimers, trimers and higher aggregates for model systems provide insights into the interaction mechanisms and strategies to enhance the 1st hyperpolarizabilities of pi-conjugated molecular assemblies. Flexible dipole orientations in the alkane bridged chromophores show odd-even variations in their second-harmonic responses that are explained through their dipolar interactions in different conformations. Parameters for the optical applications of molecules arranged in constrained geometry, like in Calix[n]arene, have been elucidated. We also highlight the recent developments in this field of research together with their future prospects.

Visible-near-infrared and fluorescent copper sensors based on julolidine conjugates: selective detection and fluorescence imaging in living cells.

We present novel Schiff base ligands julolidine-carbonohydrazone 1 and julolidine-thiocarbonohydrazone 2 for selective detection of Cu(2+) in aqueous medium. The planar julolidine-based ligands can sense Cu(2+) colorimetrically with characteristic absorbance in the near-infrared (NIR, 700-1000 nm) region. Employing molecular probes 1 and 2 for detection of Cu(2+) not only allowed detection by the naked eye, but also detection of varying micromolar concentrations of Cu(2+) due to the appearance of distinct coloration. Moreover, Cu(2+) selectively quenches the fluorescence of julolidine-thiocarbonohydrazone 2 among all other metal ions, which increases the sensitivity of the probe. Furthermore, quenched fluorescence of the ligand 2 in the presence of Cu(2+) was restored by adjusting the complexation ability of the ligand. Hence, by treatment with ethylenediaminetetraacetic acid (EDTA), thus enabling reversibility and dual-check signaling, julolidine-thiocarbonohydrazone (2) can be used as a fluorescent molecular probe for the sensitive detection of Cu(2+) in biological systems. The ligands 1 and 2 can be utilized to monitor Cu(2+) in aqueous solution over a wide pH range. We have investigated the structural, electronic, and optical properties of the ligands using ab initio density functional theory (DFT) combined with time-dependent density functional theory (TDDFT) calculations. The observed absorption band in the NIR region is attributed to the formation of a charge-transfer complex between Cu(2+) and the ligand. The fluorescence-quenching behavior can be accounted for primarily due to the excited-state ligand 2 to metal (Cu(2+)) charge-transfer (LMCT) processes. Thus, experimentally observed characteristic NIR and fluorescence optical responses of the ligands upon binding to Cu(2+) are well supported by the theoretical calculations. Subsequently, we have employed julolidine-thiocarbonohydrazone 2 for reversible fluorescence sensing of intracellular Cu(2+) in cultured HEK293T cells.

Tuning the electronic structure of graphene by molecular charge transfer: a computational study

We have studied the modification in the electronic structure, as well as optical and transport properties of graphene induced by molecular charge transfer using ab initio density functional theory. Our results from first-principles spin-polarized calculations are compared with those of the available data from Raman spectroscopic studies of modified graphene systems. We find that electron donor and acceptor molecules adsorbed onto the graphene surface exhibit effective molecular charge transfer, giving rise to mid-gap molecular levels with tuning of the band gap region near the Dirac point. The molecular charge transfer causes the stiffening or softening of the Raman G-band frequency in graphene, and we find that it also has a significant impact on the intensity ratio of the D- to G-band, corroborating experimental findings. We suggest that these charge transfer mechanisms can be probed through the low-frequency profile of the optical conductivity.

Half-metallicity in undoped and boron doped graphene nanoribbons in the presence of semilocal exchange-correlation interactions

We perform density functional calculations on one-dimensional zigzag edge graphene nanoribbons (ZGNRs) of different widths, with and without edge doping including semilocal exchange correlations. Our study reveals that, although the ground state of edge-passivated (with hydrogen) ZGNRs prefers to be anti-ferromagnetic, the doping of both of the edges with boron atoms stabilizes the system in a ferromagnetic ground state. Both the local and semilocal exchange correlations result in half-metallicity in edge-passivated ZGNRs at a finite cross-ribbon electric field. However, the ZGNR with boron edges shows half-metallic behavior irrespective of the ribbon width even in the absence of electric field and this property sustains for any field strength, opening a huge possibility of applications in spintronics.

Dipole orientation effects on nonlinear optical properties of organic molecular aggregates

We consider a few dipolar organic molecules in several of their packing arrangements to understand the aggregation effect. We have performed an extensive semiempirical calculations based on multireference doubles configuration interactions for the dimer arrangements. This is coupled with a simple theory based on dipole–dipole interactions and hydrogen-bonding effects. We find that the best dimer configuration is the in-line head-to-tail arrangement of the monomer molecules, which gives rise to an enormous increase in nonlinear optical properties compared to its monomer counterparts, at small distances. We have also shown that such dimer configurations have an appreciable absorption intensity, and for an aggregate, the absorption appears deep in the IR region. These excitations are excitonic in character and are associated with a large dipole moment change, along the long axis of the dimer configurations. We suggest the experimental methods by which such tunings can be realized.

Synthesis, structure and properties of homogeneous BC4N nanotubes

BCN nanotube brushes have been obtained by the high temperature reaction of amorphous carbon nanotube (a-CNT) brushes with a mixture of boric acid and urea. The a-CNT brushes themselves were obtained by the pyrolysis of glucose in a polycarbonate membrane. The BCN nanotubes have been characterized by EELS, XPS, electron microscopy, Raman spectroscopy and other techniques. The composition of these nanotubes is found to be BC4N. The nanotubes, which are stable up to 900 °C, are insulating and nonmagnetic. They exhibit a selective uptake of CO2 up to 23.5 wt%. In order to understand the structure and properties, we have carried out first-principles density functional theory based calculations on (6,0), (6,6) and (8,0) nanotubes with the composition BC4N. While (8,0) BC4N nanotubes exhibit a semiconducting gap, the (6,0) BC4N nanotube remains metallic if ordered BN bonds are present in all the six-membered rings. The (6,6) BC4N nanotubes, however, exhibit a small semiconducting gap unlike the carbon nanotubes. The most stable structure is predicted to be the one where BN3 and NB3 units connected by a B–N bond are present in the graphite matrix, the structure with ordered B–N bonds in the six-membered rings of graphite being less stable. In the former structure, (6,0) nanotubes also exhibit a gap. The calculations predict BC4N nanotubes to be overall nonmagnetic, as is indeed observed.

Density-matrix renormalization-group studies of the spin-1/2 Heisenberg system with dimerization and frustration

Using the density matrix renormalization group technique, we study the ground state phase diagram and other low-energy properties of an isotropic antiferromagnetic spin-half chain with both dimerization and frustration, i.e., an alternation