Thermoelectric amplification of phonons in graphene

Amplification of acoustic in-plane phonons due to an external temperature gradient (∇T) in single-layer graphene (SLG) was studied theoretically.

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Published in:Физика низких температур
Date:2016
Main Authors: Dompreh, K.A., Mensah, N.G., Mensah, S.Y., Fosuhene, S.K.
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Cite this:Thermoelectric amplification of phonons in graphene / K.A. Dompreh, N.G. Mensah, S.Y. Mensah, S.K. Fosuhene // Физика низких температур. — 2016. — Т. 42, № 6. — С. 596-599. — Бібліогр.: 29 назв. — англ.

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Digital Library of Periodicals of National Academy of Sciences of Ukraine
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author Dompreh, K.A.
Mensah, N.G.
Mensah, S.Y.
Fosuhene, S.K.
author_facet Dompreh, K.A.
Mensah, N.G.
Mensah, S.Y.
Fosuhene, S.K.
citation_txt Thermoelectric amplification of phonons in graphene / K.A. Dompreh, N.G. Mensah, S.Y. Mensah, S.K. Fosuhene // Физика низких температур. — 2016. — Т. 42, № 6. — С. 596-599. — Бібліогр.: 29 назв. — англ.
collection DSpace DC
container_title Физика низких температур
description Amplification of acoustic in-plane phonons due to an external temperature gradient (∇T) in single-layer graphene (SLG) was studied theoretically.
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fulltext Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6, pp. 596–599 Thermoelectric amplification of phonons in graphene K.A. Dompreh1, N.G. Mensah2, S.Y. Mensah1, and S.K. Fosuhene3 1Department of Physics, College of Agriculture and Natural Sciences, University of Cape Coast, Ghana 2Department of Mathematics, College of Agriculture and Natural Sciences, University of Cape Coast, Ghana 3Ghana Space Science and Technology Institute, Ghana Atomic Energy Commission, Ghana E-mail: kwadwo.dompreh@ucc.edu.gh Received October 28, 2015, revised February 2, 2016, published online April 25, 2016 Amplification of acoustic in-plane phonons due to an external temperature gradient ( T∇ ) in single-layer graphene (SLG) was studied theoretically. The threshold temperature gradient 0( )gT∇ and the threshold voltage 0( )g TV in SLG were evaluated. For T = 77 K, the calculated value for 0( ) =gT∇ 746.8 K/cm and 0( ) = 6.6 mV.g TV The calculation was done in the hypersound regime. Further, the dependence of the normalized amplification ( 0/Γ Γ ) on the fre- quency qω and /T T∇ were evaluated numerically and presented graphically. The calculated threshold tempera- ture gradient 0( )gT∇ for SLG was higher than that obtained for homogeneous semiconductors (n-InSb) hom 0( )T∇ ≈ 103 K/cm, superlattices 0( ) =SLT∇ 384 K/cm, and cylindrical quantum wire 0( )cqwT∇ ≈ 102 K/cm. This makes SLG a much better material for thermoelectric phonon amplification. PACS: 73.22.Pr Electronic structure of graphene; 79.10.–n Thermoelectronic phenomena; 63.22.Rc Phonons in graphene. Keywords: thermoelectric, graphene, acoustic phonon, amplification. Introduction The successful exfoliation of SLG sheets has attracted lots of research due to its unusual material properties such as the high carrier mobilities, unusual transport phenomena characteristic for two-dimensional Dirac fermions [1], the anomalous integer quantum Hall effect and Shubnikov–de Haas oscillations that exhibit a phase shift of π due to Ber- ry’s phase [2]. SLG is characterized by vibrations of two types of phonons: 1) in-plane vibrations with linear and longitudinal acoustic branches (LA and TA), and 2) out-of- plane vibrations known as flexural phonons (ZA and ZO) [3]. In SLG, these two forms of phonon differ in their cou- pling to charge carriers: while the coupling is conventional for in-plane phonons; reflection symmetry demands out-of- plane displacement for flexurals phonons. Moreover, scat- tering of Dirac fermions by flexural phonons requires ab- sorption (or emission) of two phonons which is quadratic in nature whilst the in-plane phonons have linear disper- sion. Due to the flexibility of SLG, its flexural mode (also called the ZA mode, bending mode, or out-of-plane trans- verse acoustic mode) can not be ignored. SLG is 2D mate- rial with an in-plane symmetry thus only in-plane phonons modes can couple linearly to electrons [4]. The amplifica- tion (absorption) of acoustic phonons in graphene [5–7] and other low-dimensional materials such as superlattices [8–11], carbon nanotubes (CNT) [12] and cylindrical quan- tum wires (CQW) [12] has attracted lots of attention re- cently. For SLG, Nunes and Fonseca [7] studied amplifica- tion of acoustic phonons and determined the drift velocity DV at which amplification occurs but Dompreh et al. [14] further showed that even at = 0DV , absorption of acoustic phonons can occur. Acoustoelectric effect (AE) involves the transfer of momentum from phonons to conducting charge carriers which leads to the generation of dc current in the sample. This has been studied both theoretically [14,15] and experimentally [16] in graphene. The interac- tion between electrons and phonons in the presence of an external temperature gradient ( T∇ ) can lead to thermo- electric effect [17–20] and thermoelectric amplification of phonons. Thermoelectric amplification of phonons has been studied in bulk [21,22] and low-dimensional materi- als such as cylindrical quantum wire (CQW) [23] and superlattices [24]. This phenomena was predicted by Gulyeav (1967) [21] but was thoroughly developed by Sharma and Singh (1974) [25] from a hydrodynamic ap- proach << 1ql (q is the acoustic wave number, l is the electron mean free path). Epstein further explained this effect for sound in the opposite limiting case, >> 1ql and showed that amplification is also possible in an electrically open-circuited sample (i.e., in the absence of an electric © K.A. Dompreh, N.G. Mensah, S.Y. Mensah, and S.K. Fosuhene, 2016 Thermoelectric amplification of phonons in graphene current) [26]. In n-InSb, Epstein calculated a threshold temperature gradient of ≈ 103 K/cm at 77 K. However, in superlattices, Mensah and Kangah (1991) [27] calculated the threshold temperature gradient necessary for amplifica- tion to occur to be hom 0 0( ) = 2.6( )SLT T∇ ∇ (where 0( )SLT∇ is the threshold temperature gradient of superlattice) but in the lowest energy mini-band ( = 0.1 eV∆ ) obtained 0( ) = 384 K/cmSLT∇ . In all these materials, the threshold temperature gradient for the amplification was found to depend on the scattering mechanism and sound frequency where the relaxation time is independent of energy [24]. Graphene differs significantly from the other low- dimensional materials. It has the highest value for thermal conductivity at room temperature (≈ 3000–5000 W/mK) [28]. This extremely high thermal conductivity opens up a variety of applications. The most interesting property of graphene is its linear energy dispersion ( ) = | |Fk V kε ± (the Fermi velocity 810 cm/sFV ≈ ) at the Fermi level with low-energy excitation. Conductivity in SLG is restricted by scattering of impurities but in the absence of extrinsic scat- tering sources, in-plane phonons constitute an intrinsic source of scattering of electrons to produce measurable temperature gradient ( T∇ ) [25]. In the works of Mariani et al. [3], the threshold temperature below which flexural phonons or above which in-plane phonons dominate was calculated to be T = 70 K. To date, there is no study of thermoelectric amplification of acoustic phonons in SLG. In this paper, thermoelectric amplification of acoustic pho- nons is theoretically studied in SLG with degenerate ener- gy dispersion. Here the threshold temperature gradient 0( )gT∇ above which amplification occur is calculated in the regime >> 1ql . The paper is organized as follows: In the theory section, the equation underlying the thermoelec- tric amplification of acoustic phonon in graphene is pre- sented. In the numerical analysis section, the final equation is analyzed and presented in a graphical form. Lastly, the conclusion is presented in Sec. 4. Theory The kinetic equation for the acoustic phonon population ( )N tq in the graphene sheet is given by 2 , , ( ) 2= | | {[ ( ) 1] (1 )s v N t g g C N t f f t ′ ′ ′ ∂ π δ + − × ∂ ∑q q k k q k k k k ( ) ( ) (1 ) ( )}.N t f f′ ′ ′× δ ε − ε + ω − − δ ε − ε − ωk k q q k k k k q  (1) Here, we have ignored other processes rather than phonon emission or absorption by electrons. The = = 2s vg g ac- count for the spin and valley degeneracy, respectively, ( )N tq represent the number of phonons with a wave vector q at time t . The factor 1N +q accounts for the presence of Nq phonons in the system when the additional phonon is emitted. The (1 )f f−k k represent the probability that the initial k state is occupied and the final electron state ′k is empty whilst the factor (1 )N f f′ −q k k is that of the boson and fermions statistics. From [29], the matrix element | |qC in Eq. (1) is given as 2 2 1 10 02 acoustic phonons, 2 | |= 2 ( )( ) optical phonons, s q q V C k k q − − ∞  Λ ρ   π ρω −  where Λ is the deformation potential constant, ρ is the crystal density, sV is the sound velocity, 0ω is the frequen- cy of an optical phonon, 1k−∞ and 1 0k− are, respectively, the low frequency and optical permeabilities of the crystal. In Eq. (1), the summation over k and k ′ can be transformed into integrals by the prescription 2 2 2 4 , , (2 )k k A d kd k ′ ′→ π ∑ ∫ where A is the area of the sample. Assuming that ( ) >> 1qN t yields = , N N t ∂ Γ ∂ q q q (2) where 2 22 3 0 0 0 0 | |= {[ ( ) ( )] (2 ) q F s A q kdk k dk d d f k f k V V ∞ ∞ π πΛ ′ ′ ′Γ ϕ θ − × π ρ ∫ ∫ ∫ ∫   1( ( ))}q F k k V ′×δ − − ω  (3) with 1= ( )q F k k V ′ − ω  , ρ is the density of the graphene sheet, ( )f k is the distribu- tion function and sV is the velocity of sound. Here the acoustic wave will be considered as phonons of frequency ( qω ) in the short-wave region. The linear approximation of the distribution function ( )f k is given as 0 1( ) = ( ( )) ( ( ))f k f k f kε + ε , (4) 1( )f k is derived from the Boltzmann transport equation as 0 1 ( ) ( ( )) = [( ( ) ) ] ( ) f pTf k k v k T ∂∇ ε τ ε − ξ ∂ε . (5) Here ( ) = ( )/v k k k∂ε ∂ is the electron velocity, ξ is the chemical potential, τ is the relaxation time and T∇ is the temperature gradient. The unperturbed electron distribution function is given by the shifted Fermi–Dirac function, 1 0 ( ) = {exp ( ( ) ) 1} ,Ff k k −βε −βε + (6) Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 597 K.A. Dompreh, N.G. Mensah, S.Y. Mensah, and S.K. Fosuhene where = 1/ Bk Tβ ( Bk is the Boltzmann constant, T is the absolute temperature and Fε is the Fermi energy). At low temperatures ( << 1Bk T ), =Fε ξ, the Fermi–Dirac equilib- rium distribution function become 0 ( ( )) = exp ( ( ( ) )).f k kε −β ε − ξ (7) Inserting Eqs. (5), (6) and (7) into Eq. (4) and setting = 0ξ for intrinsic graphene, gives 2 2 3 0 | |= ( ){exp ( ( )) (2 ) q F FF s kA q k V k VV V ∞ ωΛ Γ − −β − π ρ ∫     ( ) exp( ) exp( ( ))q F F F F F TV V k V k V k T V ω∇ −β τ −β − −β − −        ( ( )) exp ( ( ))}q q F F F F F TV V k V k V T V ω ω∇ −β τ − −β −         . (8) Using standard integrals in Eq. (8) and after cumbersome calculations yield the final equation as 0= {(2 )(1 exp ( ))q qΓ Γ −β ω − −β ω −  [6(1 exp ( )) (2 exp ( ))] }F q q q q TV T ∇ −τ + β ω −β ω +β ω β ω    , (9) where 2 0 3 3 4 2 | |= 2 F s A q V V Λ Γ πβ ρ . (10) In Eq. (9), the temperature-dependent parameter is the diffusion coefficient. Increasing temperature increases the particle kinetic energy and the diffusion component of the system. The threshold temperature gradient 0( )gT∇ nec- essary for diffusion to occur is calculated by letting = 0Γ as a consequence of the laws of conservation which yields 0( ) =gT∇ (2 )(1 exp ( )) [6(1 exp ( )) (2 exp ( ))] q q F q q q q T V −β ω − −β ω = τ + β ω −β ω +β ω β ω       . (11) The 0( )gT∇ is dependent on the temperature (T ), the frequency ( qω ) and the relaxation time (τ) as well as the acoustic wavenumber (q). The threshold voltage 0( )g TV in graphene is the minimum gate-to-source voltage dif- ferential that is needed to create a conducting path be- tween the source and drain terminals. From Eq. (11), the source-to-drain voltage 0( )gV is deduced as _____________________________________________________ 0 0 [6(1 exp ( )) (2 exp ( ))] ( ) = ( ) (2 )(1 exp ( )) F q q q qg g T q q k V V T e βτ + β ω −β ω +β ω β ω ∇ −β ω − −β ω       . (12) ________________________________________________ Numerical analysis To understand the complex expressions in Eqs. (9), (11) and (12), numerical method was adopted with the follow- ing parameters used: = 9eVΛ , =sV 2.1⋅103 m/s, =τ = 5⋅10–10 s, =qω 1.5⋅1012 s–1 and 7 1= 10 mq − . At T = 77 K, the calculated values for the threshold temperature gradient is 0( ) =gT∇ 746.8 K/cm and threshold voltage 0( ) = 6.6 mV.gV Using these values, the Eq. (9) is analyzed numerically and presented graphically (see Figs. 1, 2). Figure 1(a) shows a normalized graph of 0/Γ Γ on qω for varying T∇ . For =T∇ 550 K/cm, it was observed that the amplitude of the absorption graph reduces and the absorption switches to amplification at = 10 THzqω . Interestingly, when 0> ( )gT T∇ ∇ , that is =T∇ 750 K/cm and 950 K/cm, the graph switches completely to amplification ( 0/ < 0Γ Γ ) (see Fig. 1(a)). This agrees with the theory of thermoelectric amplification of phonons. Figure 1(b) shows the graph of 0/Γ Γ on various T∇ for varying qω . The graphs increases to a maximum then de- creased with the peak shifting to the right for higher values of qω . This means the temperature gradient ( T∇ ) attains a maximum at 0/ > 0Γ Γ then decreases. A 3D graph is pre- sented to further elucidate the graphs obtained (see Fig. 2). Figure 2 shows the dependence of 0/Γ Γ on T∇ and qω . Along the T∇ axis, the graph increases to a maximum then decreases. On the qω axis, the graph attains a maximum at lower T∇ and a minimum at higher .T∇ Conclusion Thermoelectric amplification of in-plane phonons in graphenes is studied. We observed that absorption switches over to amplification at values greater than the threshold values. The threshold value calculated at T = 77 K in graphene is 0( ) =gT∇ 746.8 K/cm and threshold voltage 0( ) = 6.6 mVg TV . The threshold value is far higher than that calculated in homogeneous semiconductor using n-InSb ( =sV 2.3⋅105 cm/s at 77 K) and was found to be hom 0( )T∇ ≈ ≈ 103 K/cm [21], superlattice 0( )SLT∇ ≈ 384 K/cm for mini-band width = 0.1 eV∆ at 77 K [24], and finally for cy- lindrical quantum wires (CQW) to be 0( )CWQT∇ ≈ 102 K/cm at liquid nitrogen temperature of 77 K [23]. These makes graphene a much better material for thermoelectric phonon amplification. 598 Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 Thermoelectric amplification of phonons in graphene 1. A.C. Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81(1), 109 (2009). 2. K.S. Novoselov, E. McCann, S.V. Morozov, V.I. Fal’ko, M.I. Katsnelson, U. Zeitler, D. Jiang, F. Schedin and A.K. Geim, Nature Phys. 2, 177 (2006). 3. E. Mariani and F. von Oppen, Phys. Rev. Lett. 100, 076801 (2008). 4. K.M. Borysenko, J.T. Mullen, E.A. Barry, S. Paul, Y.G. Semenov, J.M. Zavada, M. Buongiorno Nardelli, and K.W. Kim, Phys. Rev. B 81, 121412(R) (2010). 5. K.A. Dompreh, N.G. Mensah, and S.Y. Mensah, arXiv preprint arXiv:1505.05031 (2015). 6. K.A. Dompreh, S.Y. Mensah, S.S. Abukari, F. Sam, and N.G. Mensah, arXiv preprint arXiv:1410.8064 (2014). 7. O.A.C. Nunes and A.L.A. Fonseca, J. Appl. Phys. 112, 043707 (2012). 8. G.M. Shmelev, S.Y. Mensah, and G.I. Tsurkan, J. Phys. C: Solid State Phys. 21, L1073 (1988). 9. S.Y. Mensah and G.K. Kangah, J. Phys.: Condens. Matter 3, 4105 (1991). 10. S.Y. Mensah, F.K.A. Allotey, N.G. Mensah, and V.W. Elloh, Physica E 19, 257 (2003). 11. K.A. Dompreh, S.Y. Mensah, N.G. Mensah, S.S. Abukari, F.K.A Allotey, and G.K. Nkrumah-Buandoh, arXiv preprint arXiv:1101.1854 (2011). 12. K.A. Dompreh, N.G. Mensah, S.Y. Mensah, S.S. Abukari, F. Sam, and R. Edziah, arXiv preprint arXiv:1502.07636 (2015). 13. N.Q. Hung, N.V. Nhan, and N.Q. Bau, arXiv preprint cond- mat/0204563 (2002). 14. C.X. Zhao, W. Xu, and F.M. Peeters, Appl. Phys. Lett. 102, 222101 (2013). 15. K.A. Dompreh, N.G. Mensah, and S.Y. Mensah, arXiv preprint arXiv:1503.07360 (2015). 16. L. Bandhu, L.M. Lawton, and G.R. Nash, Appl. Phys. Lett. 103, 133101 (2013). 17. S.Y. Mensah, F.K. A. Allotey, N.G. Mensah, and G. Nkrumah, J. Phys.: Condens. Matter 13, 5653 (2001). 18. S.Y. Mensah, F.K.A. Allotey, N.G. Mensah, and G. Nkrumah, Superlattices and Microstructures 33, 173 (2003). 19. S.Y. Mensah, A. Twum, N.G. Mensah, K.A. Dompreh, S.S. Abukari, and G. Nkrumah-Buandoh, arXiv preprint arXiv:1104.1913 (2011). 20. N.G. Mensah, G. Nkrumah, S.Y. Mensah, and F.K.A. Allotey, Phys. Lett. A 329, 369 (2004). 21. Yu.V. Gulayev, Phys. Lett. A 30, 260 (1969). 22. M.A. Tenan, A. Marotta, and L.C.M. Miranda, Appl. Phys. Lett. 35, 321 (1979). 23. O.A.C. Nunes, D.A. Agrello, and A.L.A. Fonseca, J. Appl. Phys. 83, 87 (1998). 24. S.Y. Mensah and G.K. Kangah, J. Phys.: Condens. Matter 4, 919 (1992). 25. S. Sharma and S.P. Singh, J. Appl Phys. 10, 46656-1 (1974). 26. E.M. Epstein, Fiz. Tekh. Poluprov. 8, 1584-7 (1975). 27. S.Y. Mensah and G.K. Kangah, J. Phys.: Condens. Matter 3, 4105 (1991). 28. A.A Balandin, Nat. Mat. 10, 569 (2011). 29. N.S. Sankeshwar, S.S. Kubakaddi, and B.G. Mulimani, Intech. 9, 1 (2013). Fig. 1. The normalized graph of 0/Γ Γ on qω for varying T∇ (a) and on T∇ for varying qω (b). Fig. 2. (Color online) The dependence of 0/Γ Γ on qω and T∇ . Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 6 599 http://dx.doi.org/10.1103/RevModPhys.81.109 http://dx.doi.org/10.1038/nphys245 http://dx.doi.org/10.1103/PhysRevLett.100.076801 http://dx.doi.org/10.1103/PhysRevB.81.121412 http://dx.doi.org/10.1063/1.4748174 http://dx.doi.org/10.1088/0022-3719/21/33/001 http://dx.doi.org/10.1088/0022-3719/21/33/001 http://dx.doi.org/10.1088/0953-8984/3/22/020 http://dx.doi.org/10.1016/S1386-9477(02)01121-9 http://dx.doi.org/10.1063/1.4808392 http://dx.doi.org/10.1063/1.4822121 http://dx.doi.org/10.1063/1.4822121 http://dx.doi.org/10.1088/0953-8984/13/24/310 http://dx.doi.org/10.1016/S0749-6036(03)00070-3 http://dx.doi.org/10.1016/j.physleta.2004.06.100 http://dx.doi.org/10.1016/0375-9601(69)90891-3 http://dx.doi.org/10.1063/1.91107 http://dx.doi.org/10.1063/1.91107 http://dx.doi.org/10.1063/1.366705 http://dx.doi.org/10.1063/1.366705 http://dx.doi.org/10.1088/0953-8984/4/3/031 http://dx.doi.org/10.1088/0953-8984/3/22/020 http://dx.doi.org/10.1038/nmat3064 Introduction Theory Numerical analysis Conclusion
id nasplib_isofts_kiev_ua-123456789-129136
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
issn 0132-6414
language English
last_indexed 2025-12-07T16:37:08Z
publishDate 2016
publisher Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
record_format dspace
spelling Dompreh, K.A.
Mensah, N.G.
Mensah, S.Y.
Fosuhene, S.K.
2018-01-16T15:35:11Z
2018-01-16T15:35:11Z
2016
Thermoelectric amplification of phonons in graphene / K.A. Dompreh, N.G. Mensah, S.Y. Mensah, S.K. Fosuhene // Физика низких температур. — 2016. — Т. 42, № 6. — С. 596-599. — Бібліогр.: 29 назв. — англ.
0132-6414
PACS: 73.22.Pr, 79.10.–n, 3.22.Rc
https://nasplib.isofts.kiev.ua/handle/123456789/129136
Amplification of acoustic in-plane phonons due to an external temperature gradient (∇T) in single-layer graphene (SLG) was studied theoretically.
en
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
Физика низких температур
Низкоразмерные и неупорядоченные системы
Thermoelectric amplification of phonons in graphene
Article
published earlier
spellingShingle Thermoelectric amplification of phonons in graphene
Dompreh, K.A.
Mensah, N.G.
Mensah, S.Y.
Fosuhene, S.K.
Низкоразмерные и неупорядоченные системы
title Thermoelectric amplification of phonons in graphene
title_full Thermoelectric amplification of phonons in graphene
title_fullStr Thermoelectric amplification of phonons in graphene
title_full_unstemmed Thermoelectric amplification of phonons in graphene
title_short Thermoelectric amplification of phonons in graphene
title_sort thermoelectric amplification of phonons in graphene
topic Низкоразмерные и неупорядоченные системы
topic_facet Низкоразмерные и неупорядоченные системы
url https://nasplib.isofts.kiev.ua/handle/123456789/129136
work_keys_str_mv AT domprehka thermoelectricamplificationofphononsingraphene
AT mensahng thermoelectricamplificationofphononsingraphene
AT mensahsy thermoelectricamplificationofphononsingraphene
AT fosuhenesk thermoelectricamplificationofphononsingraphene