N. G. Mensah

Differential thermopower of a CNT chiral carbon nanotube

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.

Nonlinear acoustoelectric effect in a semiconductor superlattice

The acoustoelectric effect in a semiconductor superlattice (SL) has been studied for a hypersound in the region ql>>1 (where q is the acoustic wave number and l is the electron mean free path). A nonlinear dependence of the acoustoelectric current jac on the constant electric field E is observed. It is noted that when the electric field is negative the current jac rises, reaches a peak and falls off. On the other hand, when the electric field is positive the current decreases, reaches a minimum and then rises. A similar observation has been noted for an acoustoelectric interaction in a multilayered structure resulting from the analysis of Si/SiO2 structure. The dominant mechanism for such a behaviour is attributed to the periodicity of the energy spectrum along the SL axis.

Amplification of acoustic phonons in a degenerate semiconductor superlattice

Amplification of acoustic phonons propagating along the axis of a semiconductor superlattice (SL) is investigated using the microscopic theory. A non-quantizing electric field is applied to the SL to produce a drift velocity vD of the charge carriers and whenever vD exceeds vs (sound velocity) amplification occurs. The threshold field E0 at which absorption switches over to amplification depends on the SL parameters and vs. It is noted that E0 in SL is by far lower than that in bulk semiconductors and that there exists the possibility of finding a field E∗ such that −Γ(E∗)⩾Γ(−E∗). This allows, in principle, the use of SL as hypersound generator.

Giant electrical power factor in single-walled chiral carbon nanotube

Using the semiclassical approach we studied the thermoelectrical properties of single-walled chiral carbon nanotubes. We predict a giant electrical power factor and hence proposed the use of carbon nanotubes as thermoelements for refrigeration.

Hot electrons injection in carbon nanotubes under the influence of quasi-static ac-field ☆

Abstract The theory of hot electrons injection in carbon nanotubes (CNTs) where both dc electric field (Ez), and a quasi-static ac field exist simultaneously (i.e. when the frequency ω of ac field is much less than the scattering frequency v ( ω ⪡ v or ω τ ⪡ 1 , v = τ − 1 ) where τ is relaxation time) is studied. The investigation is done theoretically by solving semi-classical Boltzmann transport equation with and without the presence of the hot electrons source to derive the current densities. Plots of the normalized current density versus dc field (Ez) applied along the axis of the CNTs in the presence and absence of hot electrons reveal ohmic conductivity initially and finally negative differential conductivity (NDC) provided ω τ ⪡ 1 (i.e. quasi- static case). With strong enough axial injection of the hot electrons, there is a switch from NDC to positive differential conductivity (PDC) about E z ≥ 75 kV / cm and E z ≥ 140 kV / cm for a zigzag CNT and an armchair CNT respectively. Thus, the most important tough problem for NDC region which is the space charge instabilities can be suppressed due to the switch from the NDC behaviour to the PDC behaviour predicting a potential generation of terahertz radiations whose applications are relevance in current-day technology, industry, and research.

High frequency conductivity of hot electrons in carbon nanotubes

Abstract High frequency conductivity of hot electrons in undoped single walled achiral Carbon Nanotubes (CNTs) under the influence of ac–dc driven fields was considered. We investigated semi-classically Boltzmanns transport equation with and without the presence of the hot electrons’ source by deriving the current densities in CNTs. Plots of the normalized current density versus frequency of ac-field revealed an increase in both the minimum and maximum peaks of normalized current density at lower frequencies as a result of a strong injection of hot electrons. The applied ac-field plays a twofold role of suppressing the space-charge instability in CNTs and simultaneously pumping an energy for lower frequency generation and amplification of THz radiations. These have enormous promising applications in very different areas of science and technology.

External Electric Field Effect on Electrons Transport in Carbon Nanotubes

We consider a simple model of carbon nanotubes (CNTs) subject to external electric field E(t). Using a tight-binding approximation for the description of energy bands of CNTs, together with the standard Boltzmann transport equation and constant relaxation time, we predict the effect of self-induced transparency and absolute negative conductivity. The predicted effects may be useful in diagnostics of carbon nanotubes as well as in the amplification and efficiency conversion of electromagnetic signals.

Amplification of terahertz radiation in carbon nanotubes

We investigate theoretically the feasibility of amplification of terahertz radiation in aligned achiral carbon nanotubes, a zigzag (12,0) and an armchair (10,10) in comparison with a superlattice using a combination of a constant direct current (dc) and a high-frequency alternate current (ac) electric fields. The electric current density expression is derived using the semiclassical Boltzmann transport equation with a constant relaxation time. The electric field is applied along the nanotube axis. Analysis of the current density versus electric field characteristics reveals a negative differential conductivity behavior at high frequency, as well as photon assisted peaks. The photon assisted peaks are about an order of magnitude higher in the carbon nanotubes compared to the superlattice. These strong phenomena in carbon nanotubes can be used to obtain domainless amplification of terahertz radiation at room temperature.

Laser induced amplification of hypersound in nondegenerate semiconductor superlattices

Laser induced amplification of hypersound in a nondegenerate semiconductor superlattice has been studied. It is shown that there exists a threshold laser field intensity E0 fo rw hich the hypersound attenuation coefficient Γq may change sign and thus become amplified. It is also noted that the superlattice, while amplifying, also serves as a filter, i.e. allows waves of definite frequencies.