Anomalous kinetic roughening in growth of MoS2 films under pulsed laser deposition.

Growth dynamics of thin films expressed by scaling theory is a useful tool to quantify the statistical properties of the surface morphology of the thin films. To date, the growth mechanism for 2D van der Waals materials has been rarely investigated. In this work, an experimental investigation was carried out to identify the scaling behavior as well as the growth mechanism of 2D MoS2 thin films, grown on glass substrates by pulsed laser deposition for different deposition time durations, using atomic force microscopy images. The growth of MoS2 films evolved from layer-by-layer to layer plus island with the increase in deposition time from 20 s to 15 min. The film surface exhibited anisotropic growth dynamics in the vertical and lateral directions where RMS roughness varied with deposition time as w ∼ t ß with the growth exponent ß = 0.85 ± 0.11, while the lateral correlation length ξ was ξ = t 1/z with 1/z = 0.49 ± 0.09. The films showed a local roughness exponent α loc = 0.89 ± 0.01, global roughness exponent α = 1.72 ± 0.14 and spectral roughness exponent α s = 0.85 ± 0.03, suggesting that the growth of MoS2 thin films followed intrinsic anomalous scaling behavior (α s < 1, α loc = α s ≠ α). Shadowing owing to conical incoming particle flux distribution towards the substrate during deposition has been attributed to the anomalous growth mode. The optical properties of the films, extracted from ellipsometric analysis, were also correlated with RMS roughness and cluster size variation which unveiled the important role played by surface roughness and film density.

Correlation between surface scaling behavior and surface plasmon resonance properties of semitransparent nanostructured Cu thin films deposited via PLD.

The surface scaling behavior of nanostructured Cu thin films, grown on glass substrates by the pulsed laser deposition technique, as a function of the deposition time has been studied using height-height correlation function analysis from atomic force microscopy (AFM) images. The scaling exponents α, ß, 1/z and γ of the films were determined from AFM images. The local roughness exponent, α, was found to be ∼0.86 in the early stage of growth of Cu films deposited for 10 minutes while it increased to 0.95 with a longer time of deposition of 20 minutes and beyond this, it was nearly constant. Interface width w (rms roughness) scales with depositing time (t) as ∼ t ß , with the value of the growth exponent, ß, of 1.07 ± 0.11 and lateral correlation length ξ following ξ = t 1/z and the value of 1/z = 0.70 ± 0.10. These exponent values convey that the growth dynamics of PLD Cu films can be best described by a combination of local and non-local models under a shadowing mechanism and under highly sticking substrate conditions. From the scaling exponents and power spectral density function, it is concluded that the films follow a mound like growth mechanism which becomes prominent at longer deposition times. All the Cu films exhibited SPR properties where the SPR peak shifts towards red with increasing correlation length (ξ) whereas bandwidth increases initially with ξ and thereafter decreases gradually with ξ.

Fabrication of photoluminescent nc-Si:SiO2 thin films prepared by PLD.

In the present report, the structural, compositional, morphological, and photoluminescence properties of nanostructured non-stoichiometric silicon oxide (nc-Si:SiO2 or SiOx) thin films fabricated by pulsed-laser ablation of silicon in the presence of oxygen pressure, from 10-4 to 0.5 mbar, are presented. X-ray diffraction spectra and Raman spectra confirmed the formation of nanocrystalline Si within the films while electron diffraction X-ray spectroscopy confirmed the increase in oxygen content with increasing O2 pressure. Scanning electron microscopy images of the SiOx films showed spreading of the micron-sized clusters on the otherwise uniform background, while Raman maps confirm the presence of nanocrystalline Si in these clusters embedded in a uniform matrix comprising oxidized amorphous silicon. A systematic blue shift in the band gap energy from 1.55 to 2.80 eV was observed with increasing O2 pressure in the SiOx films due to a shift in the stoichiometry of the films from x = 0.03 to 2.14 respectively. The films with higher oxygen content exhibited broad and intense PL emissions with multiple peaks originating from quantum confined (QC) Si nanocrystals as well as oxygen defects like NBOH and VO centers. The variation in PL intensity as a function of excitation intensity displays an initial linear increase followed by saturation, a characteristic feature of emissions from QC nc-Si.

Self-assembly of ketals of arjunolic acid into vesicles and fibers yielding gel-like dispersions.

Ten aliphatic and aromatic ketals of arjunolic acid, a renewable, nanosized triterpenic acid which is obtainable from Terminalia arjuna, have been synthesized upon condensation with aldehydes. Self-assembly properties of the ketals have been studied in a wide range of organic liquids. With the exception of the p-nitrobenzylidene derivative, low concentrations of the ketals self-assemble and form gel-like dispersions in many of the organic liquids examined. The morphologies of the assemblies, studied at different distance scales by optical, electron, and atomic-force microscopies, consisted of fibrillar networks and vesicles which were able to entrap 5(6)-carboxyfluorescein as a guest molecule. X-ray diffractograms indicate that the fibrillar objects are crystalline. A charge-transfer complex was formed from a 1:1 mixture of ketal derivatives with electron-donating and electron-accepting groups, and the 9-anthrylidene derivative in its fibrillar network dimerized upon irradiation. Results demonstrate that subtle changes in the ketal structures can lead to very different aggregation pathways.

Self-assembly of esters of arjunolic acid into fibrous networks and the properties of their organogels.

Nine esters of a naturally occurring triterpenoid, arjunolic acid (from Terminalia arjuna), with alkyl chains have been synthesized, and their self-assembly has been studied in organic liquids. All of the esters examined were found to be excellent gelators. No birefringence was detected in optical micrographs of the transparent toluene gels with 5% (w/w) ethyl arjunolate or 5% (w/w) p-nitrobenzyl arjunolate as the gelator, but a spherulitic-type pattern was seen for a gel of 1.2% (w/w) p-nitrobenzyl arjunolate in 1/1 (w/w) chloroform/cyclohexane. Electron microscope images revealed self-assembled fibrillar network (SAFIN) structures with right-handed helical ribbons in some gels. With increasing concentration of the gelators, the gel-to-sol transition temperature (T(gel)) increased and then approached plateau values. Differential scanning thermograms demonstrated that the heats for transition from transparent gels to sols of ethyl arjunolate or p-nitrobenzyl ajunolate in toluene are very small. Powder X-ray diffractograms revealed that the molecular packing in the SAFIN of the 5% (w/w) ethyl aijunoate in the toluene gel was amorphous and similar to the diffractogram recorded for the neat gelator. Although the diffractogram of neat p-nitrobenzyl arjunolate consisted of broad peaks, suggesting disordered packing, the low-angle peaks of the corresponding toluene gel were much sharper; these results indicate more crystalline packing in the SAFIN than in the neat gelator. The kinetics and growth of the transformation of sols of p-nitrobenzyl arjunolate in 1/1 (w/w) chloroform/cyclohexane to their gels have been investigated at different incubation temperatures by circular dichroism spectroscopy. The data have been analyzed to probe the mechanism of SAFIN formation and the relationship between the molecular structures of the esters of arjunolic acid and their abilities to function as gelators of a wide variety of organic liquids.

A simple route for renewable nano-sized arjunolic and asiatic acids and self-assembly of arjuna-bromolactone.

While separating two natural nano-sized triterpenic acids via bromolactonization, we serendipitously discovered that arjuna-bromolactone is an excellent gelator of various organic solvents. A simple and efficient method for the separation of two triterpenic acids and the gelation ability and solid state 1D-helical self-assembly of nano-sized arjuna-bromolactone are reported.