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Efficient thermoelectric energy conversion on quasi-localized electron states in diameter modulated nanowires.

Zianni X - Nanoscale Res Lett (2011)

Bottom Line: It is known that the thermoelectric efficiency of nanowires increases when their diameter decreases.We showed that the electron thermoelectric properties depend strongly on the geometry of the diameter modulation.It is demonstrated that quasi-localized states can be formed that are prosperous for efficient thermoelectric energy conversion.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Applied Sciences, Technological Institution of Chalkida, Psachna, 34400 Evia, Greece. xzianni@teihal.gr.

ABSTRACT
It is known that the thermoelectric efficiency of nanowires increases when their diameter decreases. Recently, we proposed that increase of the thermoelectric efficiency could be achieved by modulating the diameter of the nanowires. We showed that the electron thermoelectric properties depend strongly on the geometry of the diameter modulation. Moreover, it has been shown by another group that the phonon conductivity decreases in nanowires when they are modulated by dots. Here, the thermoelectric efficiency of diameter modulated nanowires is estimated, in the ballistic regime, by taking into account the electron and phonon transmission properties. It is demonstrated that quasi-localized states can be formed that are prosperous for efficient thermoelectric energy conversion.

No MeSH data available.


Related in: MedlinePlus

Sketch diagram of the diameter modulated nanowire. Two-dimensional GaAs wires have been used for illustration. The dimensions of the diameter modulations are: (a) the thick parts are 100 nm long and 50 nm thick, (b) the thin constrictions are 5 nm long and 10 nm thick, and (c) the dots are 20 nm long and 35 nm thick.
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Figure 1: Sketch diagram of the diameter modulated nanowire. Two-dimensional GaAs wires have been used for illustration. The dimensions of the diameter modulations are: (a) the thick parts are 100 nm long and 50 nm thick, (b) the thin constrictions are 5 nm long and 10 nm thick, and (c) the dots are 20 nm long and 35 nm thick.

Mentions: The formalism of the previous section explicitly shows that the transport properties of an electron propagating through a wire are sensitive to the energy dependence of the transmission coefficient. Energy selectivity is provided for electrons by their Fermi distribution at the electrodes. The electron Fermi energy, EF, varies depending on the electrode material and/or doping. It can also be varied electrostatically by an external gate in a gated-wire configuration. The transmission coefficient of an electron travelling ballistically through a uniform straight wire is a step-like function of its Fermi energy, EF [10]. If we now consider a wire with diameter modulation by units that assume discrete energy spectra, e.g. quantum dots (Figure 1), the transmission coefficient will have transmission resonances, transmission bands and transmission gaps. The shape of T(E) is sensitive to the geometry of the modulation and to the relative dimensions of the modulating parts of the wire. In Figure 2a, the transmission coefficient is shown for wire diameter modulation by one dot attached with two narrow constrictions. For the dimensions chosen here for illustration (Figure 1), the narrow constrictions have a propagation threshold of approximately 56 meV. Transmission resonances (R in Figure 2) occur in T(E) at electron energies at which no-propagating waves can exist in the narrow constrictions and which correspond to quasi-bound states of the dots. In this case, electronic transport is based on evanescent-mode coupling as in tunnelling phenomena in heterojunctions. Coupling between propagation resonances with small energy separation, result in the formation of narrow propagation bands (NB in Figure 2) in the transmission gap of the constrictions. Electron propagation states within the transmission gap of the constrictions can be interpreted as quasi-localized states.


Efficient thermoelectric energy conversion on quasi-localized electron states in diameter modulated nanowires.

Zianni X - Nanoscale Res Lett (2011)

Sketch diagram of the diameter modulated nanowire. Two-dimensional GaAs wires have been used for illustration. The dimensions of the diameter modulations are: (a) the thick parts are 100 nm long and 50 nm thick, (b) the thin constrictions are 5 nm long and 10 nm thick, and (c) the dots are 20 nm long and 35 nm thick.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211351&req=5

Figure 1: Sketch diagram of the diameter modulated nanowire. Two-dimensional GaAs wires have been used for illustration. The dimensions of the diameter modulations are: (a) the thick parts are 100 nm long and 50 nm thick, (b) the thin constrictions are 5 nm long and 10 nm thick, and (c) the dots are 20 nm long and 35 nm thick.
Mentions: The formalism of the previous section explicitly shows that the transport properties of an electron propagating through a wire are sensitive to the energy dependence of the transmission coefficient. Energy selectivity is provided for electrons by their Fermi distribution at the electrodes. The electron Fermi energy, EF, varies depending on the electrode material and/or doping. It can also be varied electrostatically by an external gate in a gated-wire configuration. The transmission coefficient of an electron travelling ballistically through a uniform straight wire is a step-like function of its Fermi energy, EF [10]. If we now consider a wire with diameter modulation by units that assume discrete energy spectra, e.g. quantum dots (Figure 1), the transmission coefficient will have transmission resonances, transmission bands and transmission gaps. The shape of T(E) is sensitive to the geometry of the modulation and to the relative dimensions of the modulating parts of the wire. In Figure 2a, the transmission coefficient is shown for wire diameter modulation by one dot attached with two narrow constrictions. For the dimensions chosen here for illustration (Figure 1), the narrow constrictions have a propagation threshold of approximately 56 meV. Transmission resonances (R in Figure 2) occur in T(E) at electron energies at which no-propagating waves can exist in the narrow constrictions and which correspond to quasi-bound states of the dots. In this case, electronic transport is based on evanescent-mode coupling as in tunnelling phenomena in heterojunctions. Coupling between propagation resonances with small energy separation, result in the formation of narrow propagation bands (NB in Figure 2) in the transmission gap of the constrictions. Electron propagation states within the transmission gap of the constrictions can be interpreted as quasi-localized states.

Bottom Line: It is known that the thermoelectric efficiency of nanowires increases when their diameter decreases.We showed that the electron thermoelectric properties depend strongly on the geometry of the diameter modulation.It is demonstrated that quasi-localized states can be formed that are prosperous for efficient thermoelectric energy conversion.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Applied Sciences, Technological Institution of Chalkida, Psachna, 34400 Evia, Greece. xzianni@teihal.gr.

ABSTRACT
It is known that the thermoelectric efficiency of nanowires increases when their diameter decreases. Recently, we proposed that increase of the thermoelectric efficiency could be achieved by modulating the diameter of the nanowires. We showed that the electron thermoelectric properties depend strongly on the geometry of the diameter modulation. Moreover, it has been shown by another group that the phonon conductivity decreases in nanowires when they are modulated by dots. Here, the thermoelectric efficiency of diameter modulated nanowires is estimated, in the ballistic regime, by taking into account the electron and phonon transmission properties. It is demonstrated that quasi-localized states can be formed that are prosperous for efficient thermoelectric energy conversion.

No MeSH data available.


Related in: MedlinePlus