1
ScIentIfIc RepoRTS | (2018) 8:4452 | DOI:10.1038/s41598-018-22509-0
www.nature.com/scientificreports
Thermal conductivity reduction in
silicon shbone nanowires
Jeremie Maire
1,2
, Roman Anufriev
1
, Takuma Hori
3
, Junichiro Shiomi
3,4
, Sebastian Volz
1,2
&
Masahiro Nomura
1,5
Semiconductor nanowires are potential building blocks for future thermoelectrics because of their low
thermal conductivity. Recent theoretical works suggest that thermal conductivity of nanowires can
be further reduced by additional constrictions, pillars or wings. Here, we experimentally study heat
conduction in silicon nanowires with periodic wings, called shbone nanowires. We nd that like in
pristine nanowires, the nanowire cross-section controls thermal conductivity of shbone nanowires.
However, the periodic wings further reduce the thermal conductivity. Whereas an increase in the wing
width only slightly aects the thermal conductivity, an increase in the wing depth clearly reduces
thermal conductivity, and this reduction is stronger in the structures with narrower nanowires. Our
experimental data is supported by the Callaway-Holland model, nite element modelling and phonon
transport simulations.
Thermal transport in low dimensional and nanostructured materials has attracted high attention over the
past decades, in particular with regards to promising prospects in thermoelectric energy generation
1
, includ-
ing the possibility of using the wave properties of phonons, which can be relevant at cryogenic temperatures
2,3
.
Nonetheless, the main impact of semiconductor nanostructures on thermal transport comes from scattering of
the heat carriers — phonons. In that regard, semiconductor nanowires (NWs) are the focus of much attention
4–6
and remain to date one of the most promising building blocks for thermoelectric
6–9
and other microelectronic
devices. Generally, the thermal conductivity of NWs depends on the diameter
4,10–14
and surface properties
4,7,14–18
,
because heat conduction in nanostructures is suppressed by diuse scattering of phonons on the surfaces
19,20
.
For example, a few experimental works
21,22
have demonstrated a reduction of thermal conductivity in corrugated
silicon NWs due to the limited phonon mean free path
21,22
. To further enhance this surface scattering, theoretical
works
23–26
proposed various diameter-modulated NWs and found that heat conduction is strongly suppressed in
these structures. Not only is it possible to reduce thermal conductivity proportionally to the ratio between the
corrugation and the central constriction, but this reduction can be larger than an order of magnitude at room
temperature for structures of a couple of nanometers in width
25
. Despite the dierence in scales, lattice dynam-
ics
25
, Monte-Carlo simulations
23,27
, and mixed calculations
24
agree that reducing the width of the central constric-
tion or increasing the depth of the corrugation reduces thermal conductivity. us, modication of the sidewall
shape of NWs is a promising approach to further thermal conductivity reduction.
In this work, we systematically study heat conduction in NWs with periodic wings, called hereaer shbone
NWs, which havefeatures of both NWs and phononic crystals. First, we nd that thermal conductivity is reduced
as the central part — the neck — becomes smaller. Next, we demonstrate that wing size in the direction parallel
to heat ux does not strongly aect heat conduction, whereas wing size in the direction perpendicular to the heat
ux can signicantly reduce thermal conductivity. We explain this reduction by the trapping and backscattering
of phonons in the wings. Overall, we experimentally demonstrate that the transient behaviour of the shbone
NWs follow the mass contrast, and that thermal conductivity and thermal relaxation rates can be reduced at room
temperature by more than 20% and 35%, respectively.
1
Institute of Industrial Science, The University of Tokyo, Tokyo, 153-8505, Japan.
2
Laboratory for Integrated Micro
Mechatronic Systems/National Center for Scientic Research-Institute of Industrial Science (LIMMS/CNRS-IIS),
The University of Tokyo, Tokyo, 153-8505, Japan.
3
Department of Mechanical Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo, Tokyo, 113-8656, Japan.
4
Center for Materials Research by Information Integration, National
Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan.
5
PRESTO, Japan Science and
Technology Agency, Saitama, 332-0012, Japan. Correspondence and requests for materials should be addressed to
J.M. (email: jmaire@iis.u-tokyo.ac.jp) or M.N. (email: nomura@iis.u-tokyo.ac.jp)
Received: 30 November 2017
Accepted: 22 February 2018
Published: xx xx xxxx
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