Browsing by Author "Mensah, S.Y."
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Item Acoustoelectric direct current density in fluorine doped single-walled carbon nanotubes due to harmonic mixing of bichromatic fields with commensurate frequencies(Diamond & Related Materials, 2023) Sekyi-Arthur, D.; Mensah, S.Y.; Amewode, E.K.; Arthur, R.; Adams, R.R.Herein, we theoretically report on the acoustoelectric direct current (ADC) generation in a non-degenerate fluorine doped single-walled carbon nanotubes (FSWCNTs), due to mixing of waves with commensurate harmonics in the hypersound regime, qℓ≫1 (where q is the acoustic wavenumber and ℓ is the carrier mean free path). The only restriction of the theory on the sound intensity was that, the interaction energy between the carrier and the acoustic phonons must be small in comparison with the characteristic carrier energy. It was observed that in this situation, the higher harmonics of the effective field of the acoustic wave can be neglected; the origin of the nonlinearity was due to the distortion of the distribution function for carriers moving in phase with the phonons, as a result of interaction with the acoustic wave; the nonlinear effects can then be very important. The ADC generated was highly nonlinear and non-ohmic and depended on the amplitude of the bichromatic fields (i.e., pump and probe field), overlapping integral for jumps (Δs and Δz), carrier concentration (no), Bloch frequency (Ω), photon frequency (ω) and acoustic phonon frequency (ωq). The strong nonlinearity and non-ohmicity of the I-V characteristic of the FSWCNTs may be associated with a number of nonlinear phenomena including the non-parabolic band relation, carrier heating due to distortion in carrier distribution function, Stark component, and Bloch oscillations of intraminiband carriers. It was possible to alter the magnitude and direction of the rectified ADC by adjusting the phase of the fields, and the generation of ADC corresponded to even instability zones in the FSWCNTs. Thus, based on the high ADC obtained, we propose FSWCNTs for ADC generation under bichromatic fields with commensurate harmonics.Item Differential thermopower of a CNT chiral carbon nanotube(Journal of Physics Condensed Matter, 2001-05) Mensah, S.Y.; Allotey, F.K.A.; Mensah, N.G.; Nkrumah, G.The differential thermopower of a chiral carbon nanotube (CNT) was calculated using a tractable analytical approach. We obtained an expression for the electrical conductivity σ and the thermopower α. The results were numerically analysed. We noted from the values of α that the material can behave as a semimetal, n-type semiconductor and a p-type semiconductor, depending on the parameters of the CNT. We propose the use of the CNT as a thermoelement.Item Effect of band-gap tuning on absorption of phonons and acoustoelectric current in graphene nanoribbon(Physica E: Low-dimensional Systems and Nanostructures, 2023) Dompreh, K.A.; Sekyi-Arthur, D.; Mensah, S.Y.; Adu, K.W.; Edziah, R.; Amekpewu, M.We report the use of Boltzmann Transport Equation (BTE) in the hypersound regime for investigating the gen eration of acoustoelectric (AE) current in an armchair graphene nanoribbon (AGNR). The AE current obtained was deduced from the absorption due to the Landau damping of quantized sound waves (LD-QSW) energy. By stimulating the AGNR with a non-quantized electric field, the absorption and the AE current were analysed against the frequency, the wavenumber, the width of the bandgap and the drift velocity. The absorption was observed to switch to amplification as the drift velocity was varied. At high drift velocities, the amplitude of the AE current decreases as the band-gap (that depends on the applied electric field) was increased, suggesting a potential use of AGNR in the design of tunable acoustoelectric current devices.Item Giant electrical power factor in single-walled chiral carbon nanotube(Superlattices and Microstructures, 2003-03) Mensah, S.Y.; Allotey, F.K.A.; Mensah, N.G.; Nkrumah, G.The thermoelectrical properties of single walled chiral carbon nanotubes were studied. The use of semiclassical approach resulted in the prediction of electric power factor. The predicted electric power factor was then utilized in proposing the usage of carbon nanotubes as thermoelements for refrigeration.Item Ground state energy of a polaron in a superlattice(Superlattices and Microstructures, 2001-05) Mensah, S.Y.; Allotey, F.K.A.; Nkrumah, G.; Mensah, N.G.The ground state energy of a polaron in a superlattice was calculated using the double-time Green functions. The effective mass of the polaron along the planes perpendicular to the superlattice axis was also calculated. The dependence of the ground state energy and the effective mass along the planes perpendicular to the superlattice axis on the electron-phonon coupling constant α and on the superlattice parameters (i.e. the superlattice period d and the bandwidth Δ) were studied. It was observed that if an infinite square-well potential is assumed, the ground state energy of the polaron decreases (i.e. becomes more negative) with increasing α and d, but increases with increasing Δ. For small values of α, the polaron ground state energy varies slowly with Δ, becoming approximately constant for large Δ. The effective mass along the planes perpendicular to the superlattice axis was found to be approximately equal to the mass of an electron for all typical values of α, d and Δ.Item High electron thermal conductivity of chiral carbon nanotubes(Physica E: Low-Dimensional Systems and Nanostructures, 2004-06) Mensah, S.Y.; Allotey, F.K.A.; Nkrumah, G.; Mensah, N.G.Solving the Boltzmann kinetic equation with energy dispersion relation obtained in the tight-binding approximation, the carrier thermal conductivity κe of a chiral carbon nanotube (CCNT) was determined. The dependence of κe on temperature T, chiral geometric angle φh and overlap integrals Δz and Δs were obtained. The results were numerically analysed. Unusually high values of κe were observed suggesting that κe is nontrivial in the calculation of the thermal conductivity κ of CCNT. More interestingly we noted also that at 104 K and for Δz and Δs values of 0.020 and 0.0150 eV, respectively the κe value is about 41,000 W/mK as reported for a 99.9% pure 12C crystal. We predict that the electron thermal conductivity of CCNT should exceed 200,000 W/mK at ∼ 80 K. © 2004 Published by Elsevier B.V.Item High frequency amplification of acoustic phonons in fluorine-doped single-walled carbon nanotubes(Diamond & Related Materials, 2024) Sekyi-Arthur, D.; Mensah, S.Y.; Amewode, .K.; Jebuni-Adanu, C.; Asare, J.Herein, we report on a strong high-frequency induced amplification of coherent acoustic phonons in a non degenerate fluorine-doped single-walled carbon nanotubes (FSWCNTs) by utilising a tractable analytical approach in the hypersound regime, ql≫1 (where q is the acoustic wavenumber and l is the carrier mean free path). The acoustoelectric gain obtained is highly nonlinear and is due to stimulated Cerenkov phonon emission by electrically driven carriers undergoing intraminiband transport and capable of performing Bloch oscillations. The transport process causes the carriers to undergo population inversion leading to intraminiband phonon assisted processes. The generation rate (phonon emission) is expansive and surpasses phonon losses. The threshold field (Eo), at which attenuation switches over to amplification (gain) depends on the FSWCNT pa rameters (Δs&Δz), carrier drift velocity (vd = μEo), sound velocity (vs) and the ratio ζs,z. This result has potential for intense sources of reasonable acoustic phonons in the sub-THz regime and is vital for the generation of SASER (sound amplification by stimulated emission of acoustic radiation). The amplified phonons also have THz fre quencies with wavelengths in the nanometer range, and depends on high spatial parameters which has potential applications for phonon filters and spectrometers.Item High frequency amplification of acoustic phonons in fluorine-doped single-walled carbon nanotubes(Diamond & Related Materials, 2024) Sekyi-Arthu, D.; Mensah, S.Y.; Jebuni-Adanu, C. .; Asare, J.Herein, we report on a strong high-frequency induced amplification of coherent acoustic phonons in a non degenerate fluorine-doped single-walled carbon nanotubes (FSWCNTs) by utilising a tractable analytical approach in the hypersound regime, ql≫1 (where q is the acoustic wavenumber and l is the carrier mean free path). The acoustoelectric gain obtained is highly nonlinear and is due to stimulated Cerenkov phonon emission by electrically driven carriers undergoing intraminiband transport and capable of performing Bloch oscillations. The transport process causes the carriers to undergo population inversion leading to intraminiband phonon assisted processes. The generation rate (phonon emission) is expansive and surpasses phonon losses. The threshold field (Eo), at which attenuation switches over to amplification (gain) depends on the FSWCNT pa rameters (Δs&Δz), carrier drift velocity (vd = μEo), sound velocity (vs) and the ratio ζs,z. This result has potential for intense sources of reasonable acoustic phonons in the sub-THz regime and is vital for the generation of SASER (sound amplification by stimulated emission of acoustic radiation). The amplified phonons also have THz fre quencies with wavelengths in the nanometer range, and depends on high spatial parameters which has potential applications for phonon filters and spectrometers.Item Implementation of compensator-based intensity modulated radiotherapy with a conventional telecobalt machine using missing tissue approach(Polish Journal of Medical Physics and Engineering, 2018-12) Tagoe, S.N.A.; Mensah, S.Y.; Fletcher, J.J.Objectives: The present study aimed to generate intensity-modulated beams with compensators for a conventional telecobalt machine, based on dose distributions generated with a treatment planning system (TPS) performing forward planning, and cannot directly simulate a compensator. Materials and Methods: The following materials were selected for compensator construction: Brass, Copper and Perspex (PMMA). Boluses with varying thicknesses across the surface of a tissue-equivalent phantom were used to achieve beam intensity modulations during treatment planning with the TPS. Beam data measured for specific treatment parameters in a full scatter water phantom with a 0.125 cc cylindrical ionization chamber, with a particular compensator material in the path of beams from the telecobalt machine, and that without the compensator but the heights of water above the detector adjusted to get the same detector readings as before, were used to develop and propose a semi-empirical equation for converting a bolus thickness to compensator material thickness, such that any point within the phantom would receive the planned dose. Once the dimensions of a compensator had been determined, the compensator was constructed using the cubic pile method. The treatment plans generated with the TPS were replicated on the telecobalt machine with a bolus within each beam represented with its corresponding compensator mounted on the accessory holder of the telecobalt machine. Results: Dose distributions measured in the tissue-equivalent phantom with calibrated Gafchromic EBT2 films for compensators constructed based on the proposed approach, were comparable to those of the TPS with deviation less than or equal to ± 3% (mean of 2.29 ± 0.61%) of the measured doses, with resultant confidence limit value of 3.21. Conclusion: The use of the proposed approach for clinical application is recommended, and could facilitate the generation of intensity-modulated beams with limited resources using the missing tissue approach rendering encouraging results.Item The influence of external electric field on acoustoelectric effect in a superlattice(Superlattices and Microstructures, 2005-02) Mensah, S.Y.; Allotey, F.K.A.; Mensah, N.G.; Akrobotu, H.; Nkrumah, G.The de acoustoelectric current jac induced by the interaction of acoustic phonons with the conducting electrons is calculated. The calculation is done in the hypersound regime where qℓ ≫ 1 (here q is the acoustic phonon number and t is the electron mean free path). The electric field consists of dc field E0 and a low frequency alternating field E1. A nonlinear dependence of jac on E0 and E1 is observed. Depending on the direction of the dc field E0 the acoustoelectric current can either be positive or negative. At ωq = 1011 s-1 and in the absence of E0, Jac is very small and tends to zero at E1 = 1.3 × 105 V m-1. Further increase of ωq from 1011 s-1 sees a dramatic shoot "up" of Jac. Lastly the acoustoelectric currrent in the presence of E0 is modulated by E1. © 2004 Elsevier Ltd. All rights reserved.Item Telecobalt machine beam intensity modulation with aluminium compensating filter using missing tissue approach(Iranian Journal of Medical Physics, 2018-01) Tagoe, S.N.A.; Mensah, S.Y.; Fletcher, J.J.; Sasu, E.Introduction: The present study aimed to generate intensity-modulated beams with Aluminium compensating filters for a conventional telecobalt machine based on the outputs of a treatment planning system (TPS) performing forward planning and cannot simulate directly the compensating filter. Materials and Methods: In order to achieve the beam intensity modulation during treatment planning with the TPS, we used a bolus placed on the surface of a tissue-equivalent phantom. The treatment plans replicated on the telecobalt machine with the bolus were represented with compensating filters placed at a certain distance from the phantom surface. An equation was proposed for the conversion of the bolus thickness to the compensating filter thickness such that any point within the phantom would receive the planned dose. Correction factors were introduced into the proposed equation to account for the influences of field size, treatment depth, and applied bolus thickness. The proposed equation was obtained based on the analyses of empirical data measured in a full scatter water phantom with and without the compensating filter. Results: According to the results, the dosimetric verification of the proposed approach outputs in a solid water phantom with calibrated Gafchromic EBT2 films were comparable to that of the TPS with deviation of ±4.73% (mean: 2.98±1.05%). Conclusion: As the findings of the present study indicated, the discrepancy between the measured doses and TPS-estimated doses was within the tolerance of ±5%, which is recommended for dose delivery in external beam radiotherapy. Therefore, the proposed approach is recommended for clinical application.Item Temperature dependence of the thermal conductivity in chiral carbon nanotubes(Physics Letters, Section A: General, Atomic and Solid State Physics, 2004-08) Mensah, N.G.; Nkrumah, G.; Mensah, S.Y.; Allotey, F.K.A.The thermal conductivity of a chiral carbon nanotube (CCNT) is calculated using a tractable analytical approach. This is based on solving the Boltzmann kinetic equation with energy dispersion relation obtained in the tight binding approximation. The results obtained are numerically analysed. Unusually high electron thermal conductivity χ ez is observed along the tubular axis. The dependence of χ ez against temperature T was plotted for varying Δ z and a given Δ s (Δ z and Δ s are the overlapping integrals (exchange energy) for the jumps along the tubular axis and the base helix, respectively). It is noted that χ ez shows a peaking behaviour before falling off at higher temperature. As Δ z varies from 0.010 eV to 0.048 eV for a given Δ s=0.0150 eV, the peak values of χ ez shift from 40000 W/m K at 100 K to 55000 W/m K at about 300 K. Interestingly our results at 104 K which is 41000 W/m K and occurred at Δ z=0.023 eV compares very well with that reported for a 99.9% isotopically enriched 12C diamond crystal. Another interesting result obtained is the fact that the circumferential electron thermal conductivity χ ec appears to be very small. The ratio of χ ez to χ ec is of the order of 2. © 2004 Elsevier B.V. All rights reserved.