Ayyakkannu Manivannan

Molecular orientation of vacuum‐deposited thin films of zincnaphthalocyanine

Zinc 2,3‐naphthalocyanine (ZnNc) was vacuum evaporated onto glass, NaCl, and highly oriented pyrolytic graphite (HOPG) substrates, and the molecular orientation was investigated by x‐ray‐diffraction, Fourier‐transform‐infrared–reflection‐absorption spectroscopy, electronic spectroscopy, transmission electron microscopy, and scanning electron microscopy observations. Three types of molecular orientations were assigned for the deposited thin films depending on the different substrates kept at 250u2009°C. ZnNc molecules deposited on glass formed columnar crystals taking N orientation, in which molecular planes oriented perpendicularly to the substrate surface. Unidirectional epitaxic growth was observed in the film deposited on NaCl. ZnNc molecules seemed to be stacked in parallel having P orientation, where the molecular planes were parallel to the NaCl(001) surface. The ZnNc square lattice made an angle of 45° to the NaCl[100] direction (4×4‐type orientation). The film deposited on HOPG exhibited I orientation...

Scanning tunneling microscopic and transmission electron microscopic studies of cytochrome c551 denaturation at the air–water interface

The morphologies of pseudomonas‐cytochrome c551 in directly deposited film, mixed Langmuir–Blodgett (LB) film with an amphiphilic azobenzene derivative, and unfolded LB film have been studied using the scanning tunneling microscopy (STM) and transmission electron microscopy. In the first two cases, the protein molecule shows a nearly globular structure with an external diameter of ∼5 nm. Occasionally dimer‐ and trimerlike structures are also observed in such films. In contrast, the third case gives a rodlike structure whose dimension in length falls into a range of 21.5–29.1 nm, indicating the remarkable unfolding of cytochrome c551 at the air–water interface. Both tertiary and secondary structures of this protein have been virtually lost in such unfolding process. The polypeptide chains of cytochrome c551 molecules in the unfolded LB film are predominantly nearly fully extended, and highly aggregated in a local region, forming a crystalline or a twisted ropelike structure. Based on the high‐resolution ST...

Scanning Probe Microscopic Investigation of Epitaxially Grown C60 Film on MoS2

C60 thin films have been fabricated on MoS2 surface by the molecular-beam epitaxy (MBE) and their monolayer coverages on this substrate have been investigated by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM study indicates that the C60 molecules pack either in a square lattice with lattice parameter of 11±1 A or a hexagonal close packed structure with lattice parameter of 10±1 A. The AFM images show column like structures similar to the square lattice pattern of STM images. Both STM and AFM techniques have been used to make a comparative study of C60 film grown on MoS2. The substrate lattice has also been imaged together with C60 molecular contours by STM in order to determine the epitaxial nature of the film.

Scanning tunneling microscopy and spectroscopy of MoS2 thin films prepared by an intercalation–exfoliation method

Exfoliated MoS2 thin films on titanium substrates have been investigated by scanning tunneling microscopy (STM) and spectroscopy (STS). Single layers of MoS2 were placed on titanium substrates by an intercalation–exfoliation method. These layers consist of several hexagonal islands of surface atoms on the basal planes as well as axial planes with dangling bonds. The STS measurements showed a surface band gap value of 0.3 eV, which is lower than the value obtained on a single crystal of MoS2 (0.8 eV). Thus the differential conductivity measurements indicate that these films have a high density of states and conductivity due to the presence of a large number of axial planes at the interface.

Electroluminescence at n-SiC/electrolyte interface under cathodic polarization. Observation of EL transients in a short time scale and further evidence for a donor-acceptor transition

Electroluminescence (EL) at electrolyte junction was studied under cathodic polarization. Various redox electrolytes including current doubling reagents like persulfate were employed to understand the nature of EL. When the cathodic bias is higher than the flatband potential of electrode, species like persulfate are reduced by the conduction band electrons to form highly oxidizing intermediates which inject holes into the valence band to produce light . In the case of redox electrolytes like , , etc., the EL emission is weak and occurs at a higher cathodic bias, i.e., when the cathodic bias exceeds the bandgap energy of . Here, an electron transfer occurs from the valence band to the redox electrolyte. This indicates that the minority carrier injection (hole) is an important process in obtaining high EL intensities. The emission appears greenish‐yellow and the peak energy of the spectrum is smaller by 1.1 eV than the bandgap of (3.2 eV, calculated from photoresponse measurements), suggesting that the radiative recombination occurs through the impurity luminescent centers. Even under continuous polarization the EL intensity is steady for longer time duration and the electrodes are highly stable. The spectral distribution and increase in the EL intensity with different cathodic pulsed bias potentials were observed and explained by a donor‐acceptor (D‐A) mechanism. The charge transfer at the interface and the radiative recombination of electrons and holes inside the semiconductor are explained from the characteristics of EL and current intensities vs. time obtained under various pulsed polarized conditions. Moreover, the electrodes are highly stable under cathodic steady‐state polarization for several hours. Time resolved measurements (using our present system) seem to indicate the EL transients are limited by the current flow.

Electroluminescence at the SiC/electrolyte interface

Abstract Electroluminescence (EL) at n- and p-types of SiC electrodes in aqueous electrolytes was studied under cathodic polarisation during the reduction of the persulfate ions. Under these conditions electrons from the conduction band recombine radiatively with the holes injected into the valence band by SO - 4 · radicals which are produced by the reduction of persulfate ions. p-SiC shows emission in the violet-blue region with a peak at 415 nm showing the band to band transition and also giving evidence for the formation of an inversion layer. The luminescence of n-SiC appear greenish-yellow and the peak occurs at 565 nm indicating the role of dopants or impurities in the recombination process.

Direct observation of the secondary structure of unfolded pseudomonas-cytochrome c551 by scanning tunneling microscopy

Abstract The denaturation of pseudomonas-cytochrome c 551 at the air-water interface was studied using scanning tunneling microscopy (STM). The STM images indicate that the native secondary and tertiary structures of this protein has been virtually lost at such interface. The polypeptide chains of the unfolded protein are nearly fully extended, and highly aggregated in a localized region, forming a crystalline or a twisted rope-like structure. Our observation demonstrates the capability of STM in studying the unfolding of proteins, and also suggests the possibility of using STM to directly sequence the amino acid residues of a protein.

Spatial variations of the local density of states modified by CDWs in 1T-TaS2−xSex

Abstract Spatial variations of the local density of states (LDOS) near the Fermi level have been observed on the layered dichalcogenides 1T-TaS2−xSex (x = 0, 0.2, 2) for the first time. The tunneling spectra on the cleaved surfaces were measured by atomic-site tunneling (AST) spectroscopy technique at room temperature. In 1T-TaS2, the LDOS was substantially different among the three inequivalent Ta atomic sites induced by the CDW formation. However, the surface electronic structure became homogeneous, as the Se content was increased. By substituting Se for S, the minimum position of the LDOS was systematically shifted to a higher energy side above the Fermi level.

Scanning probe and transmission electron microscopy observations of cobalt naphthalocyanine molecules deposited onto a NaCl substrate

We have used scanning probe microscopy and transmission electron microscopy (TEM) to investigate the molecular orientation of cobalt naphthalocyanine (CoNc) vacuum deposited onto a NaCl substrate. Atomic force microscopy observations taken on CoNc deposited at room temperature reveal mostly amorphous grains with only few regions showing columnar structure. For CoNc films deposited at 250u2009°C, scanning tunneling microscopy and TEM showed domains of columnar structure arranged in various orientation. The periodicity of the columnar structure was determined to be 1.5 and 0.34 nm from x‐ray and electron diffraction and indicates that the molecules are standing with their planes perpendicular to the underlying NaCl substrate surface.We have used scanning probe microscopy and transmission electron microscopy (TEM) to investigate the molecular orientation of cobalt naphthalocyanine (CoNc) vacuum deposited onto a NaCl substrate. Atomic force microscopy observations taken on CoNc deposited at room temperature reveal mostly amorphous grains with only few regions showing columnar structure. For CoNc films deposited at 250u2009°C, scanning tunneling microscopy and TEM showed domains of columnar structure arranged in various orientation. The periodicity of the columnar structure was determined to be 1.5 and 0.34 nm from x‐ray and electron diffraction and indicates that the molecules are standing with their planes perpendicular to the underlying NaCl substrate surface.

Observation of vacuum‐deposited naphthalocyanine molecules using scanning tunneling microscopy

Scanning tunneling microscopy has been used to investigate the molecular orientation of iron and cobalt naphthalocyanines (Ncs). FeNc molecules deposited onto NaCl at 250u2009°C grew in a squarelike pattern with a lattice spacing of approximately 1.5 nm; CoNc deposited under the same conditions showed a similar spacing of 1.5 nm, however, the structure was more columnlike. The difference in the structure is due to the orientation of the molecular planes. In the case of FeNc, the molecular planes are parallel to the NaCl substrate, whereas the CoNc molecules are lying perpendicular to the substrate.