Arun K. Manna

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.

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.

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.

Theoretical understanding of single-stranded DNA assisted dispersion of graphene

Using atomistic molecular dynamics (MD) simulation, we investigate the structure and energetic of single-stranded DNA (ssDNA) assisted solubilisation of single-layer graphene in aqueous medium at room temperature. We choose four different ssDNA oligonucleotides of homologous base sequences; namely, ssdA12, ssdG12, ssdC12, ssdT12 (A = Adenine, G = Guanine, C = Cytosine and T = Thymine), and one mixed base-sequence ssd(AGTC)3, as the representative ssDNA for studying graphene dispersion in aqueous solution. We consider analysing different nucleobase binding modes and the role of several competing forces acting on ssDNA in contact with graphene in aqueous solution over the course of 30 ns MD simulation. Our simulation results show that there exist simultaneously two major competing forces: nucleobase-nucleobase intra-molecular π-π stacking interactions and nucleobase-graphene inter-molecular π-π stacking interactions. The former interactions help to maintain ssDNA helical geometry, whereas the latter interactions assist the ssDNA with becoming surface adsorbed on graphene. Note that both types of interactions strongly depend on the chemical nature of nucleobase and the sequence type present in various ssDNA. The calculated binding free energy strength between various ssDNA and graphene follows the order of: ssd(AGTC)3 > ssdA12 > ssdG12 > ssdC12 > ssdT12. The trend in binding free energy has been rationalized in terms of the adsorption strength of individual nucleoside over graphene, number of base-graphene π-stacks, together with the nature of nucleobase (purine: A, G and pyrimidine: C, T) present in different ssDNA sequences. We find two types of ssDNA assembly on graphene; namely, coiling and elongated networks, as was also observed in recent experiment. Interestingly, the larger extent of base-graphene π-stacking interactions found for ssd(AGTC)3 helps in forming an elongated network on graphene, and consequently, results in higher binding free energy strength. Thus, our results suggest that a mixed nucleobase sequence ssDNA, such as ssd(AGTC)3, would be potential candidate in dispersing graphene in aqueous solution than any other homologous base sequences containing ssDNA. Additionally, we also perform the electronic structure calculations for these ssDNA-graphene composites to explore the electronic properties in details using density functional theory. We believe that our simulation results together with first-principles calculations provide great microscopic detail in understanding the ssDNA assisted dispersion of graphene in aqueous environments.

Thieno analogues of RNA nucleosides: a detailed theoretical study.

We use first-principles density functional theory calculations to investigate the structural, energetic, bonding aspects, and optical properties of recently synthesized thieno-analogues of RNA nucleosides. The results are compared against the findings obtained for both the natural nucleosides as well as available experimental data. We find that the modified nucleosides form the hydrogen bonded Watson-Crick (WC) base pairing with similar H-bonding energy as obtained for the natural nucleosides. We have calculated and compared the charge transfer integrals for the H-bonded natural and thieno-modified nucleosides. We find that the thieno modification of these nucleosides strongly affects the charge transfer integrals due to the difference in extent of orbital delocalization in these two types of nucleosides. We also find that the degree of reduction of charge transfer integrals is larger for the H-bonded A-U pair than in the G-C pair. We also focus on the optical absorption properties of these thieno-modified nucleosides and their WC H-bonded base pairs in gas phase as well as with implicit water. Our calculated results show that the low energy peaks in the absorption spectra mainly arise because of the π-π* electronic transition for both the nucleosides, and the observed red shift for thieno-nucleosides compared to natural nucleosides are consistent with the calculated decrease in electronic gaps. Our results demonstrate that the thieno modification of natural nucleosides significantly modifies their electronic and optical properties, although the basic structural and bonding aspects remained the same. It also gives a microscopic understanding of the experimentally observed optical behaviors.

Discrete dinuclear complex to extended 2D compound in a Cu–azido system by controlling coligand stoichiometry: synthesis and magneto-structural correlations

This article describes syntheses, structural characterizations and magnetic studies of two different Cu(II)-azido compounds, a discrete dinuclear complex and an extended 2D network. The compounds, [Cu(μ(1,1)-N3)(N3)(Me2en)]2 (1) and [Cu3(μ(1,1,1)-N3)2(μ(1,1,3)-N3)(μ(1,1)-N3)2(μ(1,3)-N3)(Me2en)]n (2), have been synthesized by controlling the relative concentration of the blocking ligand, N,N-dimethylethylenediamine (Me2en). Compound 1 is a dinuclear compound which is formed by a doubly asymmetric μ(1,1)-N3 bridging ligand, while 2 is a rare Cu-azido system where four different types of binding modes of azide ligands are present in a single compound. Compound 2 contains a hexanuclear core, where the Cu(II) centres are connected to each other by μ(1,1,1), μ(1,1) and μ(1,1,3) bridging azide ligands. The hexanuclear core acts as a secondary building block and further assembles via μ(1,3) and μ(1,1,3) azide groups, forming a 2D network in the crystallographic ac plane. Interestingly, temperature-dependent magnetic study suggests that the dinuclear compound 1 exhibits an antiferromagnetic interaction through the μ(1,1)-N3 bridge, which has also been supported by density functional theory (DFT) calculations. In the case of 2, an overall dominant ferromagnetic interaction is observed while antiferromagnetic interaction operates between the hexanuclear cores.

Stabilization of diketo tautomer of curcumin by premicellar anionic surfactants: UV-Visible, fluorescence, tensiometric and TD-DFT evidences.

A newly observed UV band of aqueous curcumin, a biologically important molecule, in presence of anionic surfactants, viz., sodium dodecylsulfate (SDS), sodium dodecylbenzenesulfonate (SDBS), and sodium dodecylsulfonate (SDSN) in buffered aqueous solutions has been studied experimentally and theoretically. The 425 nm absorption band of curcumin disappears and a new UV-band is observed at 355 nm on addition of the surfactants in the submicellar concentration range which is reversed as the surfactant concentration approaches the critical micelle concentration (CMC). The observed spectral absorption, fluorescence intensity and surface tension behavior, under optimal experimental conditions of submicellar concentration ranges of the surfactants in the pH range of 2.00-7.00, indicate that the new band is due to the β-diketo tautomer of curcumin stabilized by interactions between curcumin and the anionic surfactants. The stabilization of the diketo tautomer by submicellar anionic surfactants described here as well as by submicellar cationic surfactant, reported recently, is unique as this is the only such behavior observed in presence of submicellar surfactants of both charge types. The experimental results are in good agreement with the theoretical calculations using ab initio density functional theory combined with time dependent density functional theory (TD-DFT) calculations.

Functional corannulene: diverse structures, enhanced charge transport, and tunable optoelectronic properties.

Chemical functionalization of various hydrocarbons, such as coronene, corannulene, and so forth, shows good promise in electronics applications because of their tunable optoelectronic properties. By using quantum chemical calculations, we have investigated the changes in the corannulene buckybowl structure, which greatly affect its electronic and optical properties when functionalized with different electron-withdrawing imide groups. We find that the chemical nature and position of functional groups strongly regulate the stacking geometry, π-stacking interactions, and electronic structure. Herein, a range of optoelectronic properties and structure-property relationships of various imide-functionalized corannulenes are explored and rationalized in detail. In terms of carrier mobility, we find that the functionalization strongly affects the reorganization energy of corannulene, while the enhanced stacking improves hopping integrals, favoring the carrier mobility of crystals of pentafluorophenylcorannulene-5-monoimide. The study shows a host of emerging optoelectronic properties and enhancements in the charge-transport characteristics of functionalized corannulene, which may find possible semiconductor and electronics applications.