Sangeeta Sahoo

Electric field directed self-assembly of cuprous oxide nanostructures for photon sensing.

We demonstrate a novel chemical-free water-based technique to synthesize various forms of cuprous oxide nanostructures at room temperature. The self-assemblies of these nanostructures are formed by the anodic oxidation of Cu in deionized (DI) water. Direct growth of these nanostructures on SiO(2)/Si (100) substrate has been successfully achieved by tuning the bias voltage and the growth duration. A variety of nanostructures from one-dimensional nanowires to different complex two- and three-dimensional structures are successfully grown by this method. We show that the morphological evolution in the self-assembly of the structures strongly depends on the spatial electric field distribution on the substrate. Furthermore, the electrical devices made from these nanowire networks exhibit promising photon sensing characteristics under white light illumination and can be exploited for future applications in photodetection and photovoltaic studies at the nanoscale level.

Controlled Assembly of Ag Nanoparticles and Carbon Nanotube Hybrid Structures for Biosensing

Here we report a chemical-free, simple, and novel method in which a part from a silver-based anode is controllably used in a straightforward manner to produce silver nanoparticles (Ag NPs) in order to fabricate a controlled assembly of Ag NPs and single walled carbon nanotube (SWCNT) hybrid structures. The attachment and distribution of Ag NPs along SWCNTs have been investigated and characterized by field emission scanning electron microscopy (FESEM). We have achieved the decoration of SWCNTs with different densities of Ag NPs by changing the deposition time, the applied voltage, and the location of carbon nanotubes with respect to the anode. At low voltage, single silver nanoparticle is successfully attached at the open ends of SWCNTs whereas at high voltage, intermediate and full coverage densities of Ag NPs are observed. As voltage is further increased, fractals of Ag NPs along SWCNTs are observed. In addition, a device based on a Ag NPs-SWNT hybrid structure is used for the label-free detection of ssDNA molecules immobilized on it. We believe that the proposed method can be used to decorate and/or assemble metal nanoparticles or fractal patterns along SWCNTs with different novel metals such as gold, silver, and copper and can be exploited in various sensitive applications for fundamental research and nanotechnology.

ssDNA Binding Reveals the Atomic Structure of Graphene

We used AFM to investigate the interaction of polyelectrolytes such as ssDNA and dsDNA molecules with graphene as a substrate. Graphene is an appropriate substrate due to its planarity, relatively large surfaces that are detectable via an optical microscope, and straightforward identification of the number of layers. We observe that in the absence of the screening ions deposited ssDNA will bind only to the graphene and not to the SiO(2) substrate, confirming that the binding energy is mainly due to the π-π stacking interaction. Furthermore, deposited ssDNA will map the graphene underlying structure. We also quantify the π-π stacking interaction by correlating the amount of deposited DNA with the graphene layer thickness. Our findings agree with reported electrostatic force microscopy (EFM) measurements. Finally, we inspected the suitability of using a graphene as a substrate for DNA origami-based nanostructures.

Reversible separation of single-walled carbon nanotubes in bundles

We show that electrostatic charging of nanotubes and the consequent repulsion can lead to reversible separation of individual single-walled carbon nanotubes in bundles. Low-energy electron beam irradiation leads to this completely reversible phenomenon. A simple semianalytical model is used to explain the observed separation mechanism. The reversibility of the separation process is attributed to discharging and thermal-fluctuation induced motion of the nanotubes in ambient air. Further, the separation impacts the electrical conductance of small nanotube bundled devices.

Current Rectification by As-Grown Chemical Vapor Deposited Single-Walled Carbon Nanotubes

Observation of diode-like current (I) - voltage (V) characteristics of switches fabricated from chemical vapor deposited (CVD) as-grown single-walled carbon nanotube (SWNT) bundles are presented. Atomic force microscopic analysis of the device structure and surface topology of SWNT suggest the observed rectification of current to possibly result from (a) cross-tube junctions, (b) a mixture of metallic and semiconducting tubes in the SWNT bundles, and/or (c) chirality change along a single tube. The exact mechanism underlying the observed rectification could not be established. The diode-like behavior of SWNT devices discovered in this research opens up new applications of SWNTs as nanoscale AC-DC converter.