Abhijit Ganguly

Band Gap Engineering of Chemical Vapor Deposited Graphene by in-situ BN Doping

Band gap opening and engineering is one of the high priority goals in the development of graphene electronics. Here, we report on the opening and scaling of band gap in BN doped graphene (BNG) films grown by low-pressure chemical vapor deposition method. High resolution transmission electron microscopy is employed to resolve the graphene and h-BN domain formation in great detail. X-ray photoelectron, micro-Raman, and UV-vis spectroscopy studies revealed a distinct structural and phase evolution in BNG films at low BN concentration. Synchrotron radiation based XAS-XES measurements concluded a gap opening in BNG films, which is also confirmed by field effect transistor measurements. For the first time, a significant band gap as high as 600 meV is observed for low BN concentrations and is attributed to the opening of the π-π* band gap of graphene due to isoelectronic BN doping. As-grown films exhibit structural evolution from homogeneously dispersed small BN clusters to large sized BN domains with embedded diminutive graphene domains. The evolution is described in terms of competitive growth among h-BN and graphene domains with increasing BN concentration. The present results pave way for the development of band gap engineered BN doped graphene-based devices.

Highly Efficient Visible Light Photocatalytic Reduction of CO2 to Hydrocarbon Fuels by Cu-Nanoparticle Decorated Graphene Oxide

The production of renewable solar fuel through CO2 photoreduction, namely artificial photosynthesis, has gained tremendous attention in recent times due to the limited availability of fossil-fuel resources and global climate change caused by rising anthropogenic CO2 in the atmosphere. In this study, graphene oxide (GO) decorated with copper nanoparticles (Cu-NPs), hereafter referred to as Cu/GO, has been used to enhance photocatalytic CO2 reduction under visible-light. A rapid one-pot microwave process was used to prepare the Cu/GO hybrids with various Cu contents. The attributes of metallic copper nanoparticles (∼4-5 nm in size) in the GO hybrid are shown to significantly enhance the photocatalytic activity of GO, primarily through the suppression of electron-hole pair recombination, further reduction of GOs bandgap, and modification of its work function. X-ray photoemission spectroscopy studies indicate a charge transfer from GO to Cu. A strong interaction is observed between the metal content of the Cu/GO hybrids and the rates of formation and selectivity of the products. A factor of greater than 60 times enhancement in CO2 to fuel catalytic efficiency has been demonstrated using Cu/GO-2 (10 wt % Cu) compared with that using pristine GO.

Anomalous blueshift in emission spectra of ZnO nanorods with sizes beyond quantum confinement regime

Cathodoluminescence (CL) spectroscopy has been employed to study the electronic and optical properties of well-aligned ZnO nanorods with diameters ranging from 50to180nm. Single-nanorod CL studies reveal that the emission peak moves toward higher energy as the diameter of the ZnO nanorod decreases, despite that their sizes are far beyond the quantum confinement regime. Blueshift of several tens of meV in the CL peak of these nanorods has been observed. Moreover, this anomalous energy shift shows a linear relation with the inverse of the rod diameter. Possible existence of a surface resonance band is suggested and an empirical formula for this surface effect is proposed to explain the size dependence of the CL data.

One-Dimensional Group III-Nitrides: Growth, Properties, and Applications in Nanosensing and Nano-Optoelectronics

This review will give a brief introduction to the growth and characterization methods of both binary and ternary compounds, in particular those exhibiting one-dimensionality, of the family to orient the readers about the material system to be discussed. A section will deal with the size and shape selection in group III nitride nanomaterials with a stress on intriguing morphologies such as nanowires, nanotips, and nanobelts. Complex structures, such as hierarchical and core-shell structures, will be introduced. Optical, electrical, and mechanical property, such as hardness, will be discussed in a greater detail, distinguishing the bulk from the nano wherever possible. Available models of electrical conduction and photoconduction in nanomaterials and their dependence on the actual size of the objects will be presented and compared. Optical properties of ensemble and single nanostructures, wherever possible, will be addressed in detail. The section on application will focus mainly on the sensor applications, including chemical sensors, gas sensors, and biosensors, with a thrust on DNA sensing. Because popular applications such as light-emitting diodes (LEDs) and field effect transistors (FETs) have already been reviewed extensively, only major contributions to this field—for example, nano-LEDs—will be discussed. Some recent advances in the group III-nitride materials family will be presented that will indicate future directions of research in this area.

Thermal stability study of nitrogen functionalities in a graphene network.

Catalyst-free vertically aligned graphene nanoflakes possessing a large amount of high density edge planes were functionalized using nitrogen species in a low energy N(+) ion bombardment process to achieve pyridinic, cyanide and nitrogen substitution in hexagonal graphitic coordinated units. The evolution of the electronic structure of the functionalized graphene nanoflakes over the temperature range 20-800 °C was investigated in situ, using high resolution x-ray photoemission spectroscopy. We demonstrate that low energy irradiation is a useful tool for achieving nitrogen doping levels up to 9.6 at.%. Pyridinic configurations are found to be predominant at room temperature, while at 800 °C graphitic nitrogen configurations become the dominant ones. The findings have helped to provide an understanding of the thermal stability of nitrogen functionalities in graphene, and offer prospects for controllable tuning of nitrogen doping in device applications.

Direct voltammetric sensing of l-Cysteine at pristine GaN nanowires electrode

The study demonstrates an electrochemical approach for direct sensing of L-Cysteine at gallium nitride nanowires (GaNNWs), a wide band gap semiconductor possessing 1-dimensional nanomaterial-specific high surface-sensitivity and unusually high surface-conductivity. Pristine GaNNWs can respond to L-Cysteine oxidation without any surface-modification: a unique advantage compared with other common electrodes. Cyclic voltammetric investigations on the effects of pH and potential-scan rate reveal an electrocatalytic oxidation of L-Cysteine controlled by the electroactive L-CyS(-) species. Advantages of direct L-Cysteine oxidation at surface-dominated GaNNWs electrodes can achieve an optimum sensitivity of 42 nA/μM with an experimental detection limit of 0.5 μM, over 0.5-75 μM dynamic range, under physiological condition (pH=7.4).

Functionalized GaN nanowire-based electrode for direct label-free voltammetric detection of DNA hybridization

This study demonstrates the utility of functionalized GaN nanowires (GaNNWs) for electrochemical detection of nucleic acids, in aqueous solution, using cyclic voltammetry. In order to link probe DNA to the NW surface, we employed an organosulfur compound, 3-mercaptopropyl trimethoxysilane (MPTS), to functionalize the GaNNW surface. Interestingly, the MPTS-modified GaNNWs exhibited a potential window of 4.5 V, the widest reported to date, with very low background current, which provides an advantage for sensing DNA immobilization/hybridization, down to sub-pM concentration, via monitoring adenine and guanine oxidation. The oxidation of guanine was characterized by its peak potential and peak current, where the former serves as a fingerprint for DNA hybridization and the latter as a parameter for the extent of hybridization. Moreover, the GaNNW-based sensor exhibited excellent consistency in hybridization-dehybridization-rehybridization cycles.

Multi-porous Co3O4 nanoflakes @ sponge-like few-layer partially reduced graphene oxide hybrids: towards highly stable asymmetric supercapacitors

The controlled growth of metal oxide nanostructures within hierarchically porous conductive carbon-based frameworks is critically important to achieving high volumetric performance and appropriate channel size for energy storage applications. Herein, we grow cobalt oxide (Co3O4) nanoflakes, using a sequential-electrodeposition process, into spherically porous sponge-like few-layer partially reduced graphene oxide (SrGO) synthesized by template-directed ordered assembly. Maximum specific/volumetric capacitances of 1112 F gCo3O4−1 (at 3.3 A gCo3O4−1), 178 F cm−3 (at 2.6 A cm−2), and 406 F gtotal−1 (at 1 A gtotal−1) and sensible rate capability (80% retention by increasing the charge/discharge current from 1 A g−1 to 16 A g−1) are obtained for the Co3O4 nanoflakes@SrGO hybrid electrodes. Besides, an asymmetric supercapacitor is made with the Co3O4[63%]@SrGO[37%] hybrid and activated carbon as a positive and a negative electrode, respectively. Electrochemical results indicate an energy density of 23.3 W h kg−1 at a high power density of 2300 W kg−1 (discharge time of about 42 s) and 62% retention even at a remarkable power density of 36 600 W kg−1 (discharge time of 1.6 s). Moreover, it shows excellent cycling stability with no decay after 20 000 charge/discharge cycles. This performance is attributed to the unique pore-sizes for an ion to channel into the porous structures, conductivity, and mechanical stability of the SrGO framework, which makes it promising for next-generation supercapacitors.

Field emission effects of nitrogenated carbon nanotubes on chlorination and oxidation

With reference to our recent reports [Appl. Phys. Lett. 90, 192107 (2007); Appl. Phys. Lett. 91, 202102 (2007)] about the electronic structure of chlorine treated and oxygen-plasma treated nitrogenatcd carbon nanotubes (N-CNTs), here we studied the electron field emission effects on chlorination (N-CNT:Cl) and oxidation (N-CNT:O) of N-CNT. A high current density (J) of 15.0 mA/cm(2) has been achieved on chlorination, whereas low J of 0.0052 mA/cm(2) is observed on oxidation compared to J=1.3 mA/cm(2) for untreated N-CNT at an applied electric field E-A of similar to 1.9 V/mu m. The turn-on electric field (E-TO) was similar to 0.875. The 1.25 V/mu m was achieved for N-CNT:C1 and N-CNT:O, respectively, with respect to E-TO= 1.0 V/mu n for untreated one. These findings are due to the formation of different bonds with carbon and nitrogen in the N-CNT during the process of chlorine (oxygen)-plasma treatment by the charge transfer, or else that changes the density of free charge carriers and hence enhances (reduces) the field emission properties of N-CNTs:C1 (N-CNTs:O)

Optical properties of functionalized GaN nanowires

The evolution of the optical properties of GaN nanowires (NWs) with respect to a sequence of surface functionalization processes is reported; from pristine hydroxylated to finally, 3-mercaptopropyltrimethoxysilane (MPTMS) functionalized GaN NWs. Photoluminescence, Raman, stationary, and time-resolved photoluminescence measurements were applied to investigate the GaN NWs with different surface conditions. A documented surface passivation effect of the GaN NWs induced by the MPTMS functionalization is determined based on our characterization results. A hypothesis associated with the surface band bending and the defect levels near the band edges is proposed to explain the observed experimental results.