Department of Physics

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AFM studies of polycrystalline calcium sulfide thin films grown by atomic layer deposition
(Elsevier, 1999-04) Dey, Srijata
Polycrystalline CaS thin films were grown on Al2O3 films deposited on Si-wafer using the atomic layer deposition (ALD) technique. The surface structure of these films was studied by AFM and compared with respective SEM images. The polycrystalline film surfaces comprise regular shaped crystallites. First report of a possible growth mechanism is presented, on studying the variation of morphological features (i.e., roughness and size of crystallites) with thickness and growth rate.
Chemical vapor deposition growth of large-area molybdenum disulphide (MoS2) dendrites
(Elsevier, 2024-12) Dey, Srijata
Molybdenum disulphide (MoS2) has emerged as a popular transition metal dichalcogenide (TMDC) in the recent decade because of its potential applications in electronic devices, optoelectronics, and fuel cells. Specifically, dendritic MoS2 has been shown to efficiently catalyse various hydrogen evolution reactions. We report the growth of dendritic MoS2 flakes on SiO2/Si wafers using a sophisticated atmospheric pressure chemical vapor deposition (APCVD) system. High-resolution optical microscopy reveals a morphology comprising different star-shaped dendrites, in addition to large MoS2 domains, which merge to form a continuous film. Our observations reveal that the dendrites originate from the nucleation centre of a monolayer MoS2 island, and their branches develop preferentially along the grain boundaries of this island. Raman spectroscopy, Atomic force microscopy (AFM), Field emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS) measurements were carried out to characterize the as-grown MoS2 dendrites and further confirm these observations.
Contrast mechanisms in photothermal scanning tunneling microscopy
(Springer, 1994-08) Dey, Srijata
By irradiation of the tunneling junction of a scanning tunneling microscope with intensity-modulated laser light a gap-width modulation due to thermal expansion of tip and sample was produced. Photothermal images were obtained by spatial mapping of the resulting modulation of the tunneling current or its logarithm. The various mechanisms responsible for the observed contrast are discussed quantitatively. In case of a highly corrugated gold film on mica the contrast arises mainly from either the current variations caused by the non-zero reaction time of the current control loop or from a geometry factor. In both cases the images reflect certain properties of the sample topography. On the other hand, for a liquid-crystal film adsorbed on graphite a contrast on a molecular scale was found which is attributed to variations of the effective barrier height.
Influence of surface treatments on nanocrystalline silicon
(Elsevier, 2004-03) Dey, Srijata
Fresh porous silicon (PS) samples show photoluminescence (PL) in visible. After a short dip in water, the PL peak shifts towards blue. However, a short dipping of PS in toluene and pentane does not change PL. The intensity of PL decreases upon exposing PS briefly to acetone. The peak position however, remains unchanged. X-Ray diffraction (XRD) and atomic force microscopy (AFM) show the presence of nanocrystals in PS. After dipping in water, XRD and AFM show structural changes. However, dipping in pentane, toluene and acetone has no effect on the XRD and AFM. Using AFM data, John–Singh model of quantum confinement explain the PL results of PS treated with water, toluene and pentane. However, the changes in PL after acetone treatment cannot be explained within this model.
Laser-Assisted Scanning Tunneling Microscopy Studies of Thin Ordered Molecular Layers
(Springer, 1995) Dey, Srijata
The irradiation of the junction of an STM with laser light may give access to the study of resonant effects. Since high laser intensities are required, an active compensation setup, designed to suppress the dominating nonresonant effects is used. Mixed films of the dye perylene-tetracarboxylic-dianhydride (PTCDA) and the liquid crystal octylcyanobiphenyl (8CB) were prepared on graphite and MoS2. STM measurements show small isolated domains of PTCDA embedded in 8CB. The islands could be imaged with high resolution even under the illumination with intense laser light. First experiments investigating the laser-induced contrast observed on the dye islands under active compensation of the nonresonant effects are discussed.
Scanning tunneling microscope-characterization of chemical vapor deposition-graphene: ripples and twisted bi-layers at multiple scales
(Wiley, 2024-09) Dey, Srijata
Despite numerous limitations, graphene characterization using scanning tunneling microscopy is an important aspect of graphene research. In the present study, an ambient, large effective field of view scanning tunneling microscope (A-LEF-STM) is used as a more practical extension of a standard STM for analyzing chemical vapor deposited (CVD)-graphene: A rigid sample stage, which allows the tip to be relocated to any point over an area of 3.5 × 3.5 mm, is attached to the latter. This simple enhancement allows rough patches spanning hundreds of nanometers on any sample to be easily circumnavigated, almost always without damaging the tip. Ripples and multi-layer regions including those with twisted bi-layers, in a graphene sheet grown on a copper foil using CVD, are located using the augmented manoeuvrability, and imaged in high-definition from micron to angström scales. Insights relating to the resolution with which surfaces of varying roughness can be imaged are developed using simulations and a careful analysis of the scanning process. The enhanced field of view is also utilized to verify the extent of graphene coverage over the entire sample area. The acquired images of single and multi-layer depositions are carefully interpreted. The applicability of this end-to-end characterization process for other samples is also discussed. Overall, this study demonstrates the potential benefits of the A-LEF-STM as an eminent characterization tool, complementary to a Raman spectrometer, for CVD-graphene and other two-dimensional materials.
Scanning tunneling microscopy of liquid crystals, perylene‐tetracarboxylic‐dianhydride, and phthalocyanine
(AIP, 1994-05) Dey, Srijata
Scanning tunneling microscopy measurements conducted on pure liquid crystals and liquid crystals in combination with the dyes perylene‐tetracarboxylic‐dianhydride (PTCDA) and phthalocyanine (Pc), all adsorbed on highly oriented pyrolytic graphite (HOPG), are being reported on. The liquid crystal octylcyanobiphenyl (8CB) was used as a solvent for the dyes, allowing the preparation of ordered dye layers in a very simple way. A combination of vapor deposition of PTCDA and liquid deposition of 8CB provided films, where 8CB and PTCDA are embedded in each other on a small length scale. The samples were prepared in two different ways yielding either 8CB islands within a PTCDA monolayer or PTCDA islands surrounded by 8CB. The Pc films on HOPG exhibit a hexagonal molecular lattice that could be related to the lattice of the graphite substrate and for which a structural model is proposed. For dodekylcyanobiphenyl (12CB) on HOPG the ordering in unit cells containing 10 rather than 8 molecules is reported for the first time.
Scanning tunneling microscopy studies of diazo dye monolayers on HOPG
(Elsevier, 1995-05) Dey, Srijata
We report on scanning tunneling microscopy (STM) studies of monolayers of the diazo dye 4-[4-(N,N-dimethylamino)phenylazo]azobenzene (D2, summation formula C20H19N5) on the basal plane of highly oriented pyrolytic graphite (HOPG). Monolayers of the dye were prepared by vapour deposition or by dissolving the molecules in the liquid crystal octylcyanobiphenyl (8CB). The STM images show a double-row structure exhibiting different types of lattice defects and various domains. High-resolution images allow the identification of individual molecules and the observation of intramolecular contrast. The different orientations of the rows can be explained by a commensurate registry of the molecules with the substrate. A model for the unit cell is proposed.
STM studies on dye molecules embedded in ordered liquid crystal structures and an approach for laser-assisted scanning tunneling microscopy
(SPIE, 1993-06) Dey, Srijata
Scanning tunneling microscopy, although capable of yielding very high resolution on periodic structures, very often provides only moderate resolution on singular features. Our work aims at the use of laser light to improve the identification of individual molecules. We report on scanning tunneling microscopy measurements performed on dye molecules dissolved in a liquid crystal and adsorbed onto highly oriented pyrolytic graphite. Either localized perturbations of the liquid crystal structure with the size of single molecules or more or less extended ordered domains of well resolved dye molecules were reproducibly imaged for several dyes. To study light-induced resonant effects the influence of non-resonant absorption leading to thermal expansion of tip and sample has to be suppressed. Therefore, an electro- optical system was realized using an ArPLU- and a dye laser of different wavelengths power-modulated with a relative phase shift of 180 degree(s). Preliminary results obtained with this setup are presented documenting the efficiency of the compensation.
Surface Structure Of Ale-grown Polycrystalline Calcium Sulfide Thin Films Studied By Afm
(Materials Research Society of Korea, 1998) Dey, Srijata
Systematic study of CVD-growth parameters in NaCl-assisted growth of MoSe2 nanostructures: nanoribbons, dendrites and spirals
(RSC, 2025-05) Dey, Srijata
MoSe2, a two-dimensional (2D) transition metal dichalcogenide (TMDC), has garnered significant interest in recent years due to its semiconducting properties and tunable band gap with potential applications in optoelectronics, photocatalysis and atomically thin devices. In this study, we report the controlled synthesis of MoSe2 nanocrystals using a custom 12-zone atmospheric pressure chemical vapour deposition (APCVD) system. NaCl is used as a seed promoter to facilitate the growth of monolayer, bilayer, and multilayer films, often as large as 200 μm. Additionally, the morphological evolution of the MoSe2 nanostructures is controlled by tuning different growth parameters based on insights, which we discuss in detail. The growth of dominant nanostructures, which include nanoribbons, snowflakes, monolayers and multilayer domains, among others, are discussed. High-resolution optical microscopy, field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) are used to image the as-grown crystals. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) are used to verify the quality and elemental composition of our as-grown MoSe2 crystals. Our findings brighten the prospect of growing novel 1D and 2D TMDC nanostructures with sufficient control to make them suitable for advanced optoelectronic and catalytic devices.
An ultrahigh‐vacuum system for STM studies
(AIP, 1994) Dey, Srijata
An ultrahigh‐vacuum (UHV) system for STM studies is described where a home‐built STM is combined with a novel facility for in situ exchange of samples and tips. Technical details concerning the design of the sample and tip holders and the manipulation and storage equipment are given. For the preparation of organic films a compact evaporation source comprising two ovens with an excellent thermal insulation was constructed. First STM results of a coronene film on graphite and of tungsten diselenide are presented.
Zinc sulfide and terbium-doped zinc sulfide films grown by traveling wave reactor atomic layer epitaxy
(IAEA, 1998) Dey, Srijata
Zinc sulfide (ZnS) and terbium-doped ZnS (ZnS:Tb) thin films were grown by traveling wave reactor atomic layer epitaxy (ALE). In the present work, ZnCl2, H2S, and tris (2,2,6,6-tetramethyl-3,5-heptandionato) terbium (Tb(tmhd)3) were used as the precursors. The dependence of crystallinity and Cl content of ZnS films was investigated on the growth temperature. ZnS and ZnS:Tb films grown at temperatures ranging from 400 to 500 .deg. C showed a hexagonal-2H crystalline structure. The crystallinity of ZnS film was greatly enhanced as the temperature increased. At growth temperatures higher than 450.deg.C, the films showed preferred orientation with mainly (002) diffraction peak. The Cl content decreased from approximately 9 to 1 at.% with the increase in growth temperature from 400 to 500 .deg. C. The segregation of Cl near the surface region and the incorporation of O from Tb(tmhd)3 during ALE process were also observed using Auger electron spectroscopy. The ALE-grown ZnS and ZnS:Tb films revealed very uniform thickness and smooth surface morphology in the observation using atomic force microscopy and transmission electron microscopy