纳米线阵列中热电优值的提升
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"这篇研究论文探讨了在横向耦合纳米线阵列中增强热电优值（ZT）的可能性。通过应用有效质量包络函数理论，考虑了量子限制和电子的耦合效应，并且考虑了声子在边界上的散射。这种结构的优势在于其大的塞贝克系数、显著降低的晶格热导率以及在耦合纳米线的微带中保持的电子电导率，从而提高了ZT值。"
文章《Enhancement of thermoelectric figure-of-merit in laterally-coupled nanowire arrays》发表于Physics Letters A 375 (2011) 2728–2732，由Yiqun Zhang, Yi Shi, Lin Pu, Junzhuan Wang, Lijia Pan 和 Youdou Zheng等人撰写，他们均隶属于南京大学电子科学与工程学院的光子与电子材料国家重点实验室。
热电材料是一种能够将热能转化为电能或反之亦然的特殊材料，其性能通常用热电优值（ZT）来衡量，该值是塞贝克系数、电导率和热导率的乘积与绝对温度的比值。高ZT值意味着更高的能源转换效率。在本研究中，研究人员聚焦于横向耦合的纳米线阵列，这是一种新型的热电材料结构。
在纳米尺度下，量子限制和电子之间的横向耦合作用成为主导因素。这种效应使得电子的行为受到限制，导致了电子能带结构的变化，进而影响材料的电学和热学性质。有效质量包络函数理论是理解这些现象的重要工具，它允许研究人员在宏观尺度上描述微观量子效应。
在考虑了声子边界散射后，研究发现晶格热导率显著降低。声子是固体中的热载流子，其散射可以阻碍热量的传播，从而降低热导率，这对提高热电材料的ZT值至关重要。同时，由于耦合纳米线的微带结构，电子电导率得以保持，这有助于提高器件的整体性能。
论文指出，这些优势共同作用，使后来耦合的纳米线阵列预测出高的ZT值，这预示着在热电转换领域有潜力实现更高的效率。这对于开发新型的热电器件，如热电发电机和热电制冷器，具有重要的意义。此外，这样的设计可能还有助于推动纳米技术在能源领域的应用，特别是在能量回收和温差发电系统中。
Physics Letters A 375 (2011) 2728–2732
Contents lists available at ScienceDirect
Physics Letters A
www.elsevier.com/locate/pla
Enhancement of thermoelectric figure-of-merit in laterally-coupled
nanowire arrays
Yiqun Zhang, Yi Shi
∗
, Lin Pu, Junzhuan Wang, Lijia Pan, Youdou Zheng
Key Laboratory of Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China
article info abstract
Article history:
Received 13 February 2011
Received in revised form 5 May 2011
Accepted 30 May 2011
Available online 1 June 2011
Communicated by R. Wu
Keywords:
Thermoelectric
Nanowire
Lateral transport
Thermal conductivity
A high ZT value is predicted in laterally-coupled nanowire arrays. The quantum confinement and coupling
of electrons are considered in the framework of effective-mass envelope-function theory. The boundary
scattering on phonons is also taken into account. The thermoelectric properties benefit from the large
Seebeck coefficient and dramatically reduced lattice thermal conductivity, as well as the preserved
electronic conductivity in the minibands of the coupling nanowires. The enhancement of ZT to more
than 10-fold is achieved in the n-type Si nanowires/Ge host material. Results suggest that the laterally-
coupled nanowire arrays can be designed for high-performance thermoelectric devices.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Recent progress has been made in the fabrication and modifica-
tion of the nanowire arrays [1–3]. These nanowires exhibit dramat-
ically unusual physical properties and have great potential in var-
ious applications, such as photovoltaic devices [4], photodetectors
[5], field-effect transistors [3] and nanoscale sensors [6]. Recently,
much attention has been paid on their thermoelectric properties
for the purpose of energy harvesting as well as refrigeration [7–9].
The efficiency of thermoelectric material is determined by the di-
mensionless thermoelectric figure-of-merit ZT
= σ S
2
T /(κ
e
+ κ
L
),
where S is the Seebeck coefficient,
σ is the electrical conductivity,
κ
e
is the electronic thermal conductivity and κ
L
is the lattice ther-
mal conductivity. Recent experiment studies have suggested that
the nanostructures could lead to remarkable progress in improving
the figure-of-merit [9–11]. In particular, the boundary scattering of
phonons, which occur in great abundance in nanostructures, can
reduce the lattice thermal conductivity to enhance the ZT value
[9–12]. Additionally, it is proposed that at the nanoscale, the quan-
tum confinement leads to the sharp features in the density of
states, from which the Seebeck coefficient can benefit [12–14].
Because the lateral direction contains an interface density larger
than that in the longitudinal direction, a dramatic reduction in lat-
tice thermal conductivity in the lateral direction is expected to
*
Corresponding author at: School of Electronic Science and Technology, Nanjing
University, 22 Hankou Road, Nanjing 210093, China. Tel.: +86 25 83685381; fax: +86
25 83621220.
E-mail address: yshi@nju.edu.cn (Y. Shi).
enhance the ZT value—a result that has been confirmed in bulk
nanostructures [11]. Additionally, a large Seebeck coefficient can be
realized because of the sharp features of DOS. The key point is pre-
serving electron conductivity while the phonons are strongly scat-
tered on the interfaces. Although considerable attention has been
paid to nanowire arrays, few models that consider the balance
between electrons and phonons exist. We investigate the thermo-
electric properties of the closed-packed nanowire array structure
in the lateral direction using a combined model that considers
both electrons and phonons. Effective-mass envelope-function the-
ory is applied to investigate the modification of the electronic
band structure. An analytical method is presented to evaluate the
boundary scattering of phonons. In a detailed calculation of the n-
type Si nanowires/Ge host system, an enhanced ZT value is derived
as the nanowire diameter and the volume fraction are optimized.
2. Theoretical model
In Fig. 1(b), d and l denote the diameter of the nanowire and
the period length of the structure, respectively. In this Letter, the
diameter of the nanowire is assumed to be between 2–10 nm so
that the quantum confinement can be exhibited. The unit cell of
the structure is shown as the hexagon enclosed with dashed lines,
and the nanowire volume fraction is f
= πd
2
/(2
√
3l
2
).Effective-
mass envelope-function theory, which has been widely applied in
studies on the band structure of nanostructures [15,16],isalso
adopted here to evaluate the effects of the band offset and the
effective mass derivation between the nanowires and the host
materials. Disregarding the second- and higher-order terms, the
Schrödinger equation of electrons can be written as
0375-9601/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physleta.2011.05.063
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