Md. Kawsar Alam

Secondary electron yield of multiwalled carbon nanotubes

Secondary electron yield from individual multiwalled carbon nanotubes is investigated for a wide range of primary beam energies (0.5–15 keV). By using a simple experimental procedure under an optical microscope, we make suspended nanotubes, which are free from interaction with the substrate during electron yield measurements. It is found that the secondary electron yield from isolated suspended nanotubes is less than unity and decreases as a function of primary electron energy.

Unusual secondary electron emission behavior in carbon nanotube forests.

Electron yield was measured from patterned carbon nanotube forests for a wide range of primary beam energies (400-20,000 eV). It was observed that secondary and backscattered electron emission behaviors in these forests are quite different than in bulk materials. This seems to be primarily because of the increased range of electrons due to the porous nature of the forests and dependent on their structural parameters, namely nanotube length, diameter and inter-nanotube spacing. In addition to providing insight into the electron microscopy of nanotubes, these results have interesting implications on designing novel secondary electron emitters based on the structural degrees of freedom of nanomaterials.

Monte Carlo simulation of electron scattering and secondary electron emission in individual multiwalled carbon nanotubes: A discrete-energy-loss approach

Electron scattering in and secondary electron emission from multiwalled carbon nanotubes are investigated using Monte Carlo simulation. The method treats energy loss in a discrete manner, resulting from individual scattering events, rather than within a continuous-slowing-down approximation. Simulation results agree fairly well with the reported experimental data. The effect of number of nanotube walls is investigated and the energy distribution of the transmitted electrons is calculated. It is found that secondary electron yield in the low-primary-energy range is more sensitive to the number of walls and is maximized for a particular number of walls. The effect is not significant in the higher-primary-energy range. The effect of core electron ionization on secondary electron emission from nanotubes is found to be negligible because of the low scattering cross-section involved. The presented framework can also be applied to other small structures such as nanowires.

High subthreshold field-emission current due to hydrogen adsorption in single-walled carbon nanotubes: A first-principles study

We investigate the effect of hydrogen adsorption on field-emission current from a single-walled carbon nanotube using first-principles calculations. The results show a new emission regime at field values around the field-emission threshold of bare nanotubes, with emission currents comparable to those of the high-field regime. This current enhancement can be explained with the surface dipole created as a result of the difference in electronegativity between carbon and hydrogen that contributes to electron extraction from the nanotube.

Proposition and computational analysis of a kesterite/kesterite tandem solar cell with enhanced efficiency

We propose a dual junction Cu2ZnSnS4/Cu2ZnSnSe4 (kesterite/kesterite) based tandem configuration and analyze its prospect and viability as a solar cell. Cu2ZnSnS4 and Cu2ZnSnSe4, both having the kesterite crystal structure, are used as the main absorbers for the top and bottom of cells, respectively. We optimize the thickness of the absorbers using optoelectronic simulations and investigate the effect of absorber thickness on short circuit current density and open circuit voltage. The optimized thicknesses for peak efficiency are found to be 200 nm and 850 nm for Cu2ZnSnS4 and Cu2ZnSnSe4, respectively. The maximum efficiency of the tandem cell is estimated to be 19.87% including recombination effects such as Shockley–Read–Hall (SRH) and radiative recombination mechanisms. We also investigate the effect of band gap on the performance of the tandem cell and show that a 21.74% efficient tandem cell can be achieved for optimized band gaps. Finally, we report that efficiency could be further enhanced by replacing the CdS buffer layer with eco-friendly ZnS buffer layer and optimizing the tandem structure. The proposition and computational analysis presented in this work may help in achieving higher efficiency kesterite solar cells.

Monte Carlo modeling of electron backscattering from carbon nanotube forests

The authors present a new Monte Carlo tool capable of simulating electron trajectories in nanotube forests, taking into account the underlying nanoscale nature of the material. The scattering angle distribution is adaptively modified at each step of the simulation according to the local environment (how the nanotubes are positioned, their diameters, and internanotube distances). This provides additional degrees of freedom in the Monte Carlo simulation that are directly related to the internal structure of the nanotube forest, allowing the model to closely match experimental data.

Comment on “Ultrahigh secondary electron emission of carbon nanotubes” [Appl. Phys. Lett.96, 213113 (2010)]

Luo et al. scanned with a 1 keV electron beam an oxidized silicon substrate partially covered with single-walled carbon nanotubes SWNTs . They observed a higher average specimen current when the scanned area contained SWNTs connected to the silicon. Thence by multiplying this increase by the ratio of the length of the scanned area over the SWNT diameter, they deduced that SWNTs connected to a reservoir have a secondary electron emission coefficient SEEC of up to 123, far higher than expected based on existing reports on the interaction of electron beams with carbon nanotubes CNTs and their scanning electron microscopy. The analysis presented by the authors of Ref. 1 does not take into account the effect of the SiO2 film underneath the CNTs adequately. For example, a more likely explanation for their measured data is that the increased secondary current is due to CNTs providing a conductive path and thus enhancing secondary electron SE emission from the surrounding oxide within the electron-beam-induced current range, which is much larger than the CNT diameter even at 1 keV. A measurement of SE emission from CNTs on a substrate should feature carefully chosen conditions scan rate and current sampling, filtering, and averaging and synchronize the current sampling with the scan so as to ensure that the electrometer takes enough samples when the beam hits the CNT. No such detail was given by the authors. These issues become all the more critical given that the claimed high yield is a product of the difference of very small leakage currents only 2%–5% of the primary beam current and a very large scan length compared to the CNT diameter Eq. 1 in Ref. 1 . Ideally one should measure the SE current from a CNT that is freestanding over a region of negligible SEEC where bombarded with electrons. The authors suggested that the claimed high value of SEEC can be explained by the primary electron raising the highest occupied molecular orbital HOMO above the vacuum level. But such a drastic raise of the HOMO applies to the CNT tip in the presence of a strong external electric field. Although a similar albeit much lower raise could partially explain SE emission from CNTs, it does not support the idea of such a high intrinsic SE yield. A 1 keV primary beam perpendicular to a CNT is likely to pass through it without encountering significant scattering: on average the primary electron loses less than 50 eV going through the CNT. Since for carbon it has been reported that an average energy loss of 80–125 eV by a primary electron is needed in order to generate one SE, the 50 eV loss cannot explain an SE yield of more than 1. The authors claimed that there was no oxide surface charging when they observed zero specimen current at 1 keV primary landing energy. This clearly cannot be the case; it is well known that in the absence of leakage an insulator whose secondary emission coefficient is greater than 1 the authors quoted 1.18 1 will charge to a potential at which enough SEs return to the surface to balance the incoming and outgoing electrons. In summary, we believe that the high SEEC reported in Ref. 1 is an artifact of the analysis of the experimental data. Although the data appear to be correct, the conducted experiment was not capable of revealing the SE yield of CNTs, and the results presented do not support the claim of high SEEC.

Self-Consistent Modeling of Ultra Thin Body Double Gate MOSFET

An accurate and efficient one dimensional self-consistent numerical model of double gate MOS structure is presented based on finite element method. The model is developed using FEMLAB considering wave function penetration effect into gate oxide. Hence, penetration effect is revealed and presented for full depleted double gate MOSFET. Accuracy of the model has been verified by comparing with established results.

Rotating‐Electric‐Field‐Induced Carbon‐Nanotube‐Based Nanomotor in Water: A Molecular Dynamics Study

Using molecular dynamics simulations, it is shown that a carbon nanotube (CNT) suspended in water and subjected to a rotating electric field of proper magnitude and angular speed can be rotated with the aid of water dipole orientations. Based on this principle, a rotational nanomotor structure is designed and the system is simulated in water. Use of the fast responsiveness of electric-field-induced CNT orientation in water is employed and its operation at ultrahigh-speed (over 1011 r.p.m.) is shown. To explain the basic mechanism, the behavior of the rotational actuation, originated from the water dipole orientation, is also analyzed . The proposed nanomotor is capable of rotating an attached load (such as CNT) at a precise angle as well as nanogear-based complex structures. The findings suggest a potential way of using the electric-field-induced CNT rotation in polarizable fluids as a novel tool to operate nanodevices and systems.

Effect of angle of incidence on the performance of bulk heterojunction organic solar cells: A unified optoelectronic analytical framework

We analyze the performance of bulk heterojunction organic solar cells under oblique incidence of light. In this regard, we present an optoelectronic analytical model that describes the current-voltage characteristics of bulk heterojunction organic solar cells taking into account the effect of angle of incidence. A closed-form general expression is derived for the optical generation rate under oblique incidence employing transfer matrix formalism. The resulting expression is then incorporated in the classical drift-diffusion transport and continuity equations of charge carriers to derive a unified expression of voltage dependent current density combining optical and electrical parameters. Thus, the model is capable of determining the accurate optical absorption in the active layer for varying angles of incidence as well as predicting the corresponding wavelength dependent external quantum efficiency of the device. The results are verified by comparing with published numerical and experimental results. We s...