Sukumar Dey

Magnetic-field-assisted layer-by-layer electrostatic assembly of ferromagnetic nanoparticles

We report that layer-by-layer (LbL) electrostatic assembly of Fe(3)O(4) nanoparticles can be supplemented by orienting magnetic domains of the nanoparticles. With the oriented domains of ionic-capped nanoparticles, both magnetic and electrostatic forces of attraction become operative during the LbL deposition process. The magnetic-field-assisted LbL adsorption process has been evidenced by increased electronic absorbance of the films. While atomic force microscopy studies rule out formation of multiple layers during a single adsorption process, magnetic force microscopy images evidence oriented domains in the LbL films. The results show a novel route for LbL deposition of ferromagnetic nanoparticles with oriented magnetic domains in the thin films.

Band mapping across a pn-junction in a nanorod by scanning tunneling microscopy

We map band-edges across a pn-junction that was formed in a nanorod. We form a single junction between p-type Cu2S and n-type CdS through a controlled cationic exchange process of CdS nanorods. We characterize nanorods of the individual materials and the single junction in a nanorod with an ultrahigh vacuum scanning tunneling microscope (UHV-STM) at 77 K. From scanning tunneling spectroscopy and correspondingly the density of states (DOS) spectra, we determine the conduction and valence band-edges at different points across the junction and the individual nanorods. We could map the band-diagram of nanorod-junctions to bring out the salient features of a diode, such as p- and n-sections, band-bending, depletion region, albeit interestingly in the nanoscale.

Bromination of Graphene: A New Route to Making High Performance Transparent Conducting Electrodes with Low Optical Losses

The unique optical and electrical properties of graphene have triggered great interest in its application as a transparent conducting electrode material and significant effort has been invested in achieving high conductivity while maintaining high transparency. Doping of graphene has been a popular route for reducing its sheet resistance, but this has typically come at a significant loss in optical transmittance. We demonstrate doping of few layers graphene (FLG) with bromine as a means of enhancing the conductivity via intercalation without major optical losses. Our results demonstrate the encapsulation of bromine within the FLG, leading to air-stable transparent conducting electrodes with 5-fold improvement of sheet resistance reaching ∼180 Ω/□ at the cost of only 2-3% loss of optical transmittance. The remarkably low trade-off in optical transparency leads to the highest enhancements in the figure of merit reported thus far for FLG. Furthermore, we tune the work function by up to 0.3 eV by tuning the bromine content. These results should help pave the way for further development of graphene as a potential substitute to transparent conducting polymers and metal oxides used in optoelectronics, photovoltaics, and beyond.

Layer-by-Layer Electrostatic Assembly with a Control over Orientation of Molecules: Anisotropy of Electrical Conductivity and Dielectric Properties

While forming layer-by-layer (LbL) electrostatic assembly of a magnetic organic molecule, namely, nickel phthalocyanine (NiPc), we apply a magnetic field. The field orients the magnetic moment of the molecules on a monolayer along the direction of magnetic field. Such an orientation of the molecules is then electrostatically immobilized with a monolayer of a polycation. By repeating the dipping cycle, we form LbL films with planar NiPc molecules facing a particular direction. With NiPcs moment perpendicular to the molecular plane, two types of LbL films were formed: (a) NiPcs molecular plane parallel to the substrate (moment is perpendicular) and (b) molecules perpendicular to the substrate and facing one particular direction, the direction of magnetic field. Such films, with the molecules lying either (a) parallel or (b) perpendicular to the substrate, provide unique systems to study anisotropy of optical, dielectric, and electrical characteristics in these planar organic molecules. The latter film responds to the polarization of incident beam in electronic absorption spectroscopy. Here we show methods to obtain an orientation of molecules in LbL films and study anisotropy of dielectric constant and conductivity of the molecules in ultrathin films.

Double Charged Surface Layers in Lead Halide Perovskite Crystals

Understanding defect chemistry, particularly ion migration, and its significant effect on the surfaces optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Orientation of organic molecules in a monolayer vis-à-vis their molecular orbitals and transport gap

We form a monolayer of magnetic organic molecules with its plane parallel or perpendicular to the substrate. The molecules in a monolayer are oriented with an external magnetic field followed by immobilization though an electrostatic binding. In this work, from scanning tunneling microscopy (STM) measurements, we show that conductivity, molecular orbitals, and transport gap of the molecules in a monolayer depend on its orientation. From measurements carried out with different tip-to-molecule distances, we observe that the STM tip also influences molecular orbitals and transport-gap of molecules.

Magnetic moment assisted layer-by-layer film formation of a Prussian Blue analog.

We formed magnetic moment assisted layer-by-layer (LbL) films of a Prussian Blue analogue (PB). We applied an external magnetic field to each monolayer of PB to orient the magnetic moment of the compound perpendicular to the substrate. Aligned moments or orientation of the magnetic compounds themselves were immobilized in each monolayer, so that the moments could augment formation of the subsequent monolayers of LbL adsorption process. We hence could form multilayered LbL films of PB molecules with their magnetic moments oriented perpendicular to the substrate. We also formed LbL films of the compound with their moments oriented parallel to the substrate and facing one particular direction. We have measured conductivity and dielectric constant of the two types of films and compared the parameters with that of conventional LbL films deposited without orienting magnetic moments of the molecules.

Deformation stacking fault probability and dislocation microstructure of cold worked Cu–Sn–5Zn alloys by x-ray diffraction line profile analysis

Plastically deformed (hand-filed) Cu–Sn–5Zn ternary alloys with Sn concentrations 1, 2.5, and 5wt% are investigated. Microstructural parameters are studied in terms of x-ray diffraction profile fitting analysis. It is observed by Dey et al. [Acta. Mater. 53, 4635 (2005)] that the change in stacking fault probability (α) with Sn concentration for ternary Cu–Sn–5Zn alloys is similar to Cu-based binary alloy (Cu–Sn) system but behaves in a different manner from Cu–1Sn–Zn ternary alloy systems. The crystallite size distribution is broader for alloy with 1wt% Sn and becomes narrower with increasing Sn concentration. Value of dislocation density (ρ) is of the order of 1015m−2 and shows a compositional dependence. Type of dislocation is found to be predominantly screw; ⟨100⟩-type dipoles may also be present in the cold-worked alloys. The dislocation arrangement is found to be more correlated in case of 1wt% Sn compared to other alloys of higher Sn concentration. The stacking fault energy (γ) is obtained from mod...

Functional Two-Dimensional Coordination Polymeric Layer as a Charge Barrier in Li–S Batteries

Ultrathin two-dimensional (2D) polymeric layers are capable of separating gases and molecules based on the reported size exclusion mechanism. What is equally important but missing today is an exploration of the 2D layers with charge functionality, which enables applications using the charge exclusion principle. This work demonstrates a simple and scalable method of synthesizing a free-standing 2D coordination polymer Zn2(benzimidazolate)2(OH)2 at the air-water interface. The hydroxyl (-OH) groups are stoichiometrically coordinated and implement electrostatic charges in the 2D structures, providing powerful functionality as a charge barrier. Electrochemical performance of the Li-S battery shows that the Zn2(benzimidazolate)2(OH)2 coordination polymer layers efficiently mitigate the polysulfide shuttling effects and largely enhance the battery capacity and cycle performance. The synthesis of the proposed coordination polymeric layers is simple, scalable, cost saving, and promising for practical use in batteries.

Tuning of band-edges in type-I core–shell nanocrystals through band-offset engineering: selective quantum confinement effect

The addition of a shell layer onto semiconducting quantum dots is an approach to control the optical bandgap of core–shell systems. In this direction, we have grown several core–shell nanostructures having a type-I heterostructure configuration. The nature of the energy-offset has been varied by a suitable shell-material with the aim to control the conduction and the valence band-offsets separately. Confined holes or electrons could hence be relaxed selectively leading to an increase in the valence band-edge or a decrease in the conduction band-edge. In this work, after forming such core–shell systems with a control over the shell thickness, we characterized the nanostructures with scanning tunneling spectroscopy in order to determine the density of states and finally the conduction and the valence band-edges of the core–shell systems. We found that while a large band-offset strictly localizes the carriers in the core, a small perturbation indeed delocalizes the carriers up to the shell-layer shifting the relevant band-edge towards the Fermi energy and thereby decreasing the transport gap. The decrease in the transport gap was in agreement with the optical absorption spectra. The results provide a novel route to delocalize a selective type of carrier up to the shell layer of core–shell nanostructured systems.