Dipti Ranjan Mohapatra

Dramatic enhancement of the emission current density from carbon nanotube based nanosize tips with extremely low onset fields.

Nanostructures based on multiwalled carbon nanotubes (MWNTs) are fabricated using plasma of the mixture of hydrogen and nitrogen gases. The plasma-sharpened tips of nanotubes contain only a few tubes at the apex of the structure and lead to the dramatic enhancement in the emission current density by a factor >10(6) with the onset field as low as 0.16 V/microm. We propose that the nature of the tunneling barrier changes significantly for a nanosize tip at very high local electric field and may lead to the saturation in the emission current density.

Nanotip formation on a carbon nanotube pillar array for field emission application

The field emission of a carbon nanotube (CNT) pillar array has been improved significantly by plasma treatment in a mixture of hydrogen and nitrogen gases. The plasma treatment for 30s on a pillar array decreased the turn-on electric field from 0.48to0.37V∕μm and increased the field enhancement factor from 6200 to 6900. The emission current density increased by a factor of ≈40. We report in this letter the technique of generating nanotips on CNT pillars with an enormous potential to become a tool for the control and manipulation of CNTs and nanostructures.

Tuning photoinduced terahertz conductivity in monolayer graphene: Optical-pump terahertz-probe spectroscopy

Optical-pump terahertz-probe differential transmission measurements of as-prepared single layer graphene (AG) (unintentionally hole dopedwith Fermi energy E-F at similar to -180 meV), nitrogen doping compensated graphene (NDG) with E-F similar to -10 meV, and thermally annealed doped graphene (TAG) are examined quantitatively to understand the opposite signs of photoinduced dynamic terahertz conductivity Delta sigma. It is negative for AG and TAG but positive for NDG. We show that the recently proposed mechanism of multiple generations of secondary hot carriers due to Coulomb interaction of photoexcited carriers with the existing carriers together with the intraband scattering can explain the change of photoinduced conductivity sign and its magnitude. We give a quantitative estimate of Delta sigma in terms of controlling parameters-the Fermi energy E-F and momentum relaxation time tau. Furthermore, the cooling of photoexcited carriers is analyzed using a supercollision model which involves a defect mediated collision of the hot carriers with the acoustic phonons, thus giving an estimate of the deformation potential.

Enhanced photoresponse in monolayer hydrogenated graphene photodetector.

We report the photoresponse of a hydrogenated graphene (H-graphene)-based infrared (IR) photodetector that is 4 times higher than that of pristine graphene. An enhanced photoresponse in H-graphene is attributed to the longer photoinduced carrier lifetime and hence a higher internal quantum efficiency of the device. Moreover, a variation in the angle of incidence of IR radiation demonstrated a nonlinear photoresponse of the detector, which can be attributed to the photon drag effect. However, a linear dependence of the photoresponse is revealed with different incident powers for a given angle of IR incidence. This study presents H-graphene as a tunable photodetector for advanced photoelectronic devices with higher responsivity. In addition, in situ tunability of the graphene bandgap enables achieving a cost-effective technique for developing photodetectors without involving any external treatments.

A novel structure of tungsten carbide nanowalls grown on nanocrystalline diamond film

We report the fabrication of a novel structure of tungsten carbide nanowall on the nanocrystalline diamond substrate by a simple technique. The substrate was exposed to the hydrogen plasma generated in a direct-current plasma chemical vapor deposition system using a pre-carburized tungsten cathode. The physiochemical reaction between the carburized tungsten cathode and hydrogen plasma enabled the growth of tungsten carbide nanowalls at a particular temperature of 600 °C, which has never been enabled to date. Scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, and X-ray photo-emission spectroscopy were used to investigate the structure and composition of the samples. It was found that the nanostructure was strongly affected by the substrate/cathode temperatures: the nano-grained, continuous polycrystalline film formed at a higher temperature (800 °C) while the discrete tungsten carbide nanowalls formed at a lower temperature (600 °C). Such a drastic change in the nanostructure was interpreted in terms of the change in the super-saturation of growth species according to the experimental parameters.

Healing of Broken Multiwalled Carbon Nanotubes Using Very Low Energy Electrons in SEM: A Route Toward Complete Recovery

We report the healing of electrically broken multiwalled carbon nanotubes (MWNTs) using very low energy electrons (3-10 keV) in scanning electron microscopy (SEM). Current-induced breakdown caused by Joule heating has been achieved by applying suitably high voltages. The broken tubes were examined and exposed to electrons of 3-10 keV in situ in SEM with careful maneuvering of the electron beam at the broken site, which results in the mechanical joining of the tube. Electrical recovery of the same tube has been confirmed by performing the current-voltage measurements after joining. This easy approach is directly applicable for the repairing of carbon nanotubes incorporated in ready devices, such as in on-chip horizontal interconnects or on-tip probing applications, such as in scanning tunneling microscopy.

Nonlinear optical absorption in a graphene infrared photodetector.

The photoresponse of the graphene photodetector elucidated strong dependence on several optical parameters, such as the angle of incidence and the incident power of infrared exposure at room temperature. The sinusoidal dependence of the photoresponse on incidence angle, which had not been realized before, has now been revealed. The combined effect of the photo excited charge carrier and the photon drag effect explain this nonlinear optical absorption in graphene at lower incident power. The nonlinear dependence of the charge carrier generation on the incident power revealed that this process contributed to the nonlinear photoresponse. However, a deviation is observed at a higher incident power due to the induction of thermal effects in the graphene lattice. This work demonstrates the tunability of the graphene photodetector under a systematic variation that involves both parameters.

Photoluminescence study of polycrystalline and single crystal diamond

Employing photoluminescence (PL) spectroscopy, we have carried out a detailed study of the different luminescence center in chemical-vapor- deposited (CVD) single-crystal and poly-crystalline diamond and natural diamonds. In addition to the zero phonon line, the fine structure of the PL emission spectra reveals vibronic bands corresponding to both phonon emission and absorption. At lower temperature, the emission intensity increases and peak position shifts towards higher energy band.

Ultrafast Spectral Photoresponse of Bilayer Graphene: Optical Pump–Terahertz Probe Spectroscopy

Photoinduced terahertz conductivity Δσ(ω) of Bernal stacked bilayer graphene (BLG) with different dopings is measured by time-resolved optical pump terahertz probe spectroscopy. The real part of photoconductivity Δσ(ω) (ΔσRe(ω)) is positive throughout the spectral range 0.5-2.5 THz in low-doped BLG. This is in sharp contrast to Δσ(ω) for high-doped bilayer graphene where ΔσRe(ω) is negative at low frequency and positive on the high frequency side. We use Boltzmann transport theory to understand quantitatively the frequency dependence of Δσ(ω), demanding the energy dependence of different scattering rates such as short-range impurity scattering, Coulomb scattering, carrier-acoustic phonon scattering, and substrate surface optical phonon scattering. We find that the short-range disorder scattering dominates over other processes. The calculated photoconductivity captures very well the experimental conductivity spectra as a function of lattice temperature varying from 300 to 4 K, without any empirical fitting procedures adopted so far in the literature. This helps us to understand the intraband conductivity of photoexcited hot carriers in 2D materials.

Structural Changes In Nickel Nanaorods Encapsulated In Carbon Nanotubes By 200 KV Electron Irradiation In TEM

Nickel nanorods encapsulated inside multiwalled carbon nanotubes were synthesized using microwave plasma chemical vapor deposition (MPCVD) technique. HRTEM study of these tubes exhibits long, good crystalline nickel filled CNTs with well‐graphitized walls. Different orientations of filled metal with respect to graphitic walls have been observed and the orientations were found to get affected by the electron accelerating voltages of 200 KV. We report that by deliberate e‐irradiation at specific sites the crystalline planes of nickel can change their orientation. With continuous irradiation for 10–15 minutes the nickel starts evolving out of the tube. We propose that metal at the tip of the tube may be exposed by employing e‐beam irradiation and it may be useful to study the true electrical properties of both the multiwalled carbon nanotubes and Ni nanorod inside the tube.