Monica Samal

Vertical Pillar-Superlattice Array and Graphene Hybrid Light Emitting Diodes

We report a type of device that combines vertical arrays of one-dimensional (1D) pillar-superlattice (PSL) structures with 2D graphene sheets to yield a class of light emitting diode (LED) with interesting mechanical, optical, and electrical characteristics. In this application, graphene sheets coated with very thin metal layers exhibit good mechanical and electrical properties and an ability to mount, in a freely suspended configuration, on the PSL arrays as a top window electrode. Optical characterization demonstrates that graphene exhibits excellent optical transparency even after deposition of the thin metal films. Thermal annealing of the graphene/metal (Gr/M) contact to the GaAs decreases the contact resistance, to provide enhanced carrier injection. The resulting PSL-Gr/M LEDs exhibit bright light emission over large areas. The result suggests the utility of graphene-based materials as electrodes in devices with unusual, nonplanar 3D architectures.

A Ge inverse opal with porous walls as an anode for lithium ion batteries

Germanium holds great potential as an anode material for lithium ion batteries due to its large theoretical energy density and excellent intrinsic properties related to its kinetics associated with lithium and electrons. However, the problem related to the tremendous volume change of Ge during cycling is the dominant obstacle for its practical use. The previous research has focused on the improvement in mechanics associated with lithium without consideration of the kinetics. In this study, we demonstrate that the configuration engineering of the Ge electrode enables the improvement in kinetics as well as favorable mechanics. Two types of Ge inverse opal structures with porous walls and dense walls were prepared using a confined convective assembly method and by adjusting Ge deposition parameters in a chemical vapor deposition system. The Ge inverse opal electrode with porous walls shows much improved electrochemical performances, especially cycle performance and rate capability, than the electrode with dense walls. These improvements are attributed to a large free surface, which offers a facile strain relaxation pathway and a large lithium flux from the electrolyte to the active material.

InP/ZnS-graphene oxide and reduced graphene oxide nanocomposites as fascinating materials for potential optoelectronic applications.

Our recent studies on metal-organic nanohybrids based on alkylated graphene oxide (GO), reduced alkylated graphene oxide (RGO) and InP/ZnS core/shell quantum dots (QDs) are presented. The GO alkylated by octadecylamine (ODA) and the QD bearing a dodecane thiol (DDT) ligand are soluble in toluene. The nanocomposite alkylated-GO-QD (GOQD) is readily formed from the solution mixture. Treatment of the GOQD composite with hydrazine affords a reduced-alkylated-GO-QD (RGOQD) composite. The structure, morphology, photophysical and electrical properties of GOQDs and RGOQDs are studied. The micro-FTIR and Raman studies demonstrate evidence of the QD interaction with GO and RGO through facile intercalation of the alkyl chains. The field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) images of the GOQD composite show heaps of large QD aggregates piled underneath the GO sheet. Upon reduction to RGOQDs, the QDs become evenly distributed on the graphene bed and the size of the clusters significantly decreases. This also facilitates closer proximity of the QDs to the graphene domains by altering the optoelectronic properties of the RGOQDs. The X-ray photoelectron spectroscopy (XPS) results confirm QDs being retained in the composites, though a small elemental composition change takes place. The XPS and the fluorescence spectra show the presence of an In(Zn)P alloy while the X-ray diffraction (XRD) results show characteristics of the tetragonal indium. The photoluminescence (PL) quenching of QDs in GOQD and RGOQD films determined by the time correlated single photon counting (TCSPC) experiment demonstrates almost complete fluorescence quenching in RGOQDs. The conductance studies demonstrate the differences between GOQDs and RGOQDs. Investigation on the metal-oxide-semiconductor field-effect transistor (nMOSFET) characteristics shows the composite to exhibit p-type channel material properties. The RGOQD exhibits much superior electrical conductance as a channel material compared to the GOQD due to the close proximity of the QDs in the RGOQD to the graphene surface. The transfer characteristics, memory properties, and on/off ratios of the devices are determined. A mechanism has been proposed with reference to the Fermi energies of the composites estimated from the ultraviolet photoelectron spectroscopy (UPS) studies.

Synthesis, characterization, and antibacterial properties of silver nanoparticles-graphene and graphene oxide composites

In the field of nanotechnology, silver nanoparticles have been considered a promising antibacterial material for a century. The potential applications of graphene-based materials are increasingly recognized for their special physico-chemical and biological properties. In particular, graphene and graphene oxide as the foundation of nanocomposites have garnered much interest among researchers in many fields. In this review, we concentrate on different aspects of silver nanoparticle composites with graphene and graphene oxide, focusing on their synthesis methods, special characteristics, and antibacterial properties; we also briefly discuss limitations and future research.

Superior antibacterial activity of GlcN-AuNP-GO by ultraviolet irradiation.

A complete bacterialysis analysis of glucosamine-gold nanoparticle-graphene oxide (GlcN-AuNP-GO) and UV-irradiated GlcN-AuNP-GO was conducted. Analytical characterization of GlcN-AuNPs, GO and GlcN-AuNP-GO revealed UV-Vis absorbance peak at around 230 and 500nm. Microscopic characterization of prepared nanomaterials was performed by scanning electron microscope, atomic force microscopy, and high-resolution transmission microscopy. The results confirmed that the GlcN-AuNPs were uniformly decorated on the surface and edges of graphene sheets. In addition, potent antibacterial activity of GlcN-AuNP-GO that was UV irradiated for 10min and normal GlcN-AuNP-GO was detected, compared to the standard drug kanamycin, against both Gram-negative and positive bacteria. The minimum inhibitory concentration (MIC) and fluorescence intensity spectra results for Escherichia coli and Enterococcus faecalis showed that the UV-irradiated GlcN-AuNP-GO has better antibacterial activity than normal GlcN-AuNP-GO and kanamycin. Morphological changes were detected by AFM after treatment. These results confirmed that GlcN-AuNP-GO is a potent antibacterial agent with good potential for use in manufacturing medical instruments, pharmaceutical industries and in waste water treatment.

Tin Dioxide Nanowires: Evolution and Perspective of the Doped and Nondoped Systems

SnO2 semiconductor nanowire is an extremely important technological material for use in nanophotonic and nanoelectronic devices. These semiconductor nanowires of desirable property can be achieved through a bottom-up approach to the controlled synthesis in a pure or doped state. Each of the synthetic methods offers materials with broad range structural, morphological, optical, and electrical properties. Selective doping of the SnO2 nanowires by normal, transition or inner transition elements offer a broad variation in the optical and electrical properties and are open for further theoretical exploration of the properties as well as necessary changes possible for the improvement of the material properties. The properties of SnO2 nanowires can be tuned either in the pure state by structural modification or doping during nanowire growth or after growth to meet most of the requirements.