Structure and resistivity of bismuth nanobelts in situ synthesized on silicon wafer through an ethanol-thermal method

Article abstract of DOI:10.1016/j.jssc.2011.10.010

Bismuth nanobelts in situ grown on a silicon wafer were synthesized through an ethanol-thermal method without any capping agent. The structure of the bismuth beltsilicon composite nanostructure was characterized by scanning electron microscope, energy-dis

Full text of DOI:10.1016/j.jssc.2011.10.010

Journal of Solid State Chemistry 184 (2011) 3257–3261
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Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Structure and resistivity of bismuth nanobelts in situ synthesized on silicon
wafer through an ethanol-thermal method
Zheng Gao, Haiming Qin, Tao Yan, Hong Liu n, Jiyang Wang
State Key Laboratory of Crystal Materials, Bio-Micro/Nano Functional Materials Center, Shandong University, Jinan 250100, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Bismuth nanobelts in situ grown on a silicon wafer were synthesized through an ethanol-thermal
method without any capping agent. The structure of the bismuth belt–silicon composite nanostructure
was characterized by scanning electron microscope, energy-dispersive X-ray spectroscopy, and high
resolution transmission electron microscope. The nanobelt is a multilayered structure 100–800 nm in
Received 7 April 2011
Received in revised form
4 October 2011
Accepted 9 October 2011
Available online 15 October 2011
width and over 50 mm in length. One layer has a thickness of about 50 nm. A unique sword-like
nanostructure is observed as the initial structure of the nanobelts. From these observations, a possible
growth mechanism of the nanobelt is proposed. Current–voltage property measurements indicate that
the resistivity of the nanobelts is slightly larger than that of the bulk bismuth material.
& 2011 Elsevier Inc. All rights reserved.
Keywords:
Bismuth nanobelts
Ethanol-thermal method
Silicon wafer
Growth mechanism
1. Introduction
reaction mechanism is based on the reaction of bismuth hydro-
xide and silicon at higher temperature and higher pressure [14].
Bismuth, a typical semimetal, has unique electric properties
because of its highly anisotropic Fermi surface, low carrier concen-
trations, and small effective carrier mass [1,2]. Recently, bismuth
nanostructures have attracted much attention because of their
quantum transport and finite-size effects and potential application
in magnetic field sensors, optical devices, and thermoelectric coolers
or power generators [3–5]. Groups have synthesized bismuth thin
films [6], nanowires [7], nanotubes [8], nanoplates [9], and nanoballs
[10] through different synthesis routes. It is well known that
nanobelts have different properties than nanowires and nanoparti-
cles. Significant developments of bismuth nanostructures and a lot
of essential bismuth chemistry have been made recently [11–13]. In
2005, we developed a hydrothermal method to in situ synthesize
bismuth macro or nanoballs on the surface of silicon wafers by a
hydrothermal route from bismuth hydroxide [14]. In this paper, we
report an ethanol-thermal method for in situ synthesis of bismuth
nanobelts on silicon wafers without any organic capping agent. The
nanobelts grow along the surface of silicon wafer and form a novel
nanostructure. This nanocomposite has potential application in
electron nanodevices.
Typically, synthesis of in situ grown bismuth nanobelts on
silicon wafer comprised three steps:
(1) Cleaning the silicon wafers ((001) direction, 10 mm ꢀ 15 mm ꢀ
1 mm) sequentially using H2SO4/H2O2 [H2SO4 (97%)þH2O2
(30%)] (90 1C, 30 min), de-ionized water (ultrasonic bath
10 min, 4–6 times, to pH¼7), and ethanol (ultrasonic bath
10 min).
(2) Preparing 0.01 mol/L bismuth hydroxide suspension: 0.0005 mol
of analytical grade Bi(NO3)3 ꢁ 5H2O (98%, Alfa) was dissolved in
50 ml de-ionized water to get bismuth nitric solution. Excessive
ammonia hydroxide solution was dropped into bismuth nitric
solution to get bismuth hydroxide precipitate. The precipitate
was filtrated and washed by de-ionized water several times until
pH reached 7. Then, 0.01 mol/L bismuth hydroxide suspension
was prepared by dispersing all of the precipitate in de-ionized
water in an ultrasonic bath for 60 min, and then adding de-
ionized water to 50 ml.
(3) The hydrothermal process was performed by placing the
above silicon wafer vertically into a 22 ml Teflon-lined reac-
tion chamber. 0.5 ml of the bismuth hydroxide suspension
and 18 ml of ethanol (99.5%, Sigma-Aldrich) were added into
the chamber. The chamber was sealed and put into a furnace,
which was preheated to 180 1C. After heating for various
times, the chamber was removed and cooled down to room
temperature. The silicon wafer was removed and washed
with de-ionized water and then with alcohol.
2. Experimental section
Bismuth hydroxide suspension was used as a raw material for
synthesis of bismuth nanobelts on surface of silicon wafer. The
n Correspondence to: State Key Laboratory of Crystal Materials, Shandong
University, 27 Shanda Nanlu, Jinan 250100, PR China. Fax: þ86 531 88362807.
E-mail address: hongliu@sdu.edu.cn (H. Liu).
Scanning electron microscopy (FE-SEM) (Field emission LEO1530
at 5–10 K), high resolution transmission electron microscope
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.10.010

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(HRTEM, Hitachi HF2000) were used to characterize the morphology
and microstructure of the product. Energy-dispersive X-ray spectro-
scopy (EDS) attached to the TEM was used to investigate the
chemical composition of the as-synthesized samples.
are typically 100–300 nm in width with some broad nanobelts
occurring 800 nm in breadth. These results are consistent with the
results from SEM observation. EDS spectrum (insert of Fig. 2a)
indicates that bismuth is the main component of the sample. A
small amount of silicon and oxide is present due to the amorphous
silica fragments in the sample formed from the reaction between
hydroxyl and silicon. The thickness of the nanobelt measured from
a kinked nanobelt shown on Fig. 2b is about 40 nm. In the sample,
many bismuth nanobelts are multilayered (Fig. 2c and d). The
thickness of an individual layer is about 40 nm. Fig. 2e is a very
broad single layered bismuth nanobelt about 500 nm in width. A
typical multilayered nanobelt and its electron diffraction pattern
are show in Fig. 2f and g. The diffraction pattern can be indexed
to two sets of diffraction spots of rhombohedral bismuth (JCPD-
80-0018). Both sets of diffraction spots are recorded along (100)
crystalline plane, which demonstrates that the different layers of
the nanostructure grow along the same crystal plane, {100}, and
only has some dislocation between two layers. Fig. 2h and i are a
single nanobelt and its diffraction pattern. The pattern can also be
indexed as a diffraction pattern of (100) direction of rhombohedral
bismuth structure (JCPD-80-0018), and the growth direction is
along (001). The crystal structure of the nanobelts synthesized in
this work is same as that of nanospheres and microballs in situ
3. Results and discussions
Fig. 1 shows SEM images of the nanobelts on silicon wafer
synthesized at 180 1C for 24 h. There are some wire-like nanostruc-
tures on the surface of silicon wafer. The length of most of the wires
is over 60 mm (Fig. 1a). At higher magnification, the nanowires have
belt-like nanostructures (Fig. 1b). The width of the nanobelts is
about 100–800 nm (Fig. 1c). Curved or curled nanobelts indicate
that the thickness of the nanobelts is about 50 nm (Fig. 1d). From
these observations, it is believed that one end of the nanobelts
contact the silicon wafer and form a composite structure (Fig. 1e).
Some nanobelts were integrated with the surface of the wafer and
form a special belt-wafer structure (Fig. 1f).
The nanobelts on the surface of the silicon wafer were scraped
off and dispersed in ethanol, and then dropped on a TEM grid to
investigate the nanostructure. TEM and HRTEM images of the
nanobelts are shown on Fig. 2. Fig. 2a shows that the nanobelts
Fig. 1. SEM images of bismuth nanobelts. (a), (b) Bismuth nanobelts; (c) a bundle of bismuth nanobelts parallel to the surface of the wafer cross over a bundle of nanobelts
vertical to the surface of the wafer; (d) a curved belt; the end of a belt with rectangular cross section; (e) bismuth nanobelt–silicon nanostructure, all the nanobelts started
from underneath the wafer; (f) two bismuth nanobelts integrated with the silicon wafer surface.

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Fig. 2. TEM images, EDS, and electron diffraction pattern of bismuth nanobelts. (a) Nanobelts and their EDS (inset); (b) a straight and kinked belt; (c) a belt peeled from a
multilayered nanobelt structure; (d) a bundle of the belt, the steps formed by the broken belts can be seen clearly, (e) a broad single layer nanobelt; (f) a two-layer
nanobelt and (g) its electron diffraction pattern; (h) a single layer nanobelt, and (i) its electron diffraction pattern, and (j) its high resolution TEM image.
grown on silicon wafer in our previous work [14]. HRTEM image of
the nanobelt shown in Fig. 2h is shown in Fig. 2j. The distance of
the closest neighboring lattice spots is about 0.45 and 0.59 nm,
which is consistent of d010 and d002, and agrees with the results
from electron diffraction for rhombohedral bismuth nanobelts.
The SEM images of the sample synthesized at 180 1C for 5 h give
us some indication of the growth mechanism of bismuth nanobelts.
Fig. 3a–c show SEM images of the nanostructures synthesized for
5 h. Some nanoballs 30–100 nm in diameter appeared on the silicon
surface. At the same time, small ball-belt nanostructures, or sword-
like nanostructures, can be found on the surface (Fig. 3a). SEM
observation indicates that the sword-like structure is a nanobelt
with a sharp edge on one end and a crown-like ball at the other end.
From their width, the ball-belt nanostructures can be divided into
two types: those 100 nm in width, which consistent with the
nanobelts synthesized at 180 1C for 24 h, and those 30 nm in width,
which correspond to the thickness of the nanobelts. These results
indicate that the growth plane, (100), of some nanobelts are vertical
to the wafer surface parallel to the wafer surface for others. There-
fore, the nanobelts we observed on the sample synthesized at 180 1C
for 24 h are suggested to grow in situ on the surface, not deposited
from solution in the reaction chamber. Fig. 3d–g supports this
conclusion. Fig. 3d is the initial part of the nanobelt that grows
vertical to the surface of the silicon wafer. From this figure, we can
see that the first part of the belts is buried in the wafer. The belts
grow partially out of the wafer surface and then continue growing
along the surface. Fig. 3g shows the part of the nanobelts whose
growth plane is vertical to the silicon wafer surface. Some nanobelts
also grow integrated with the surface of the wafer (Fig. 3f). This kind
of nanobelt may originate from the short nanobelts shown in Fig. 3c.
The ends of the nanobelts can also grow within the wafer (Fig. 3f).
Some nanobelts also grow along the wafer surface, then detach and
continue growing freely until they cross a nanobelt perpendicular to
their growth direction. This results in the nanobelts pilling up on the
surface of the wafer.
Based on the above experimental results, the mechanism of
in situ growth of bismuth nanobelts–silicon composite is pro-
posed. Fig. 4 illustrates the formation and growth of multilayer
bismuth nanobelts on the silicon wafer. When the bismuth
hydroxide–ethanol suspension is added into the reaction cham-
ber, some bismuth hydroxide particles adhere on the surface of
the wafer. When the system is heated, bismuth hydroxide reacts
with silicon atoms on the surface of the wafer, and forms bismuth
and silicon hydroxide
4Bi(OH)3þ3Si-3Si(OH)4þ4Bi
(1)
The bismuth atoms formed from (1) accumulate on the silicon
wafer and form bismuth nuclei and then nanospheres (Fig. 4a).
Because the pressure is high in this system, belt-structured bismuth

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Z. Gao et al. / Journal of Solid State Chemistry 184 (2011) 3257–3261
Fig. 3. Formation and in situ growth of bismuth nanobelts. (a) SEM image of the sample synthesized at 180 1C for 5 h; (b) a sword-like baby-nanobelt grown parallel to the
surface of silicon wafer; (c) a short nanobelt grown vertical to the surface of the wafer; (d) nanobelt bundle with growth surface vertical to the silicon wafer (the ends of
the belts are buried in the wafer); (e) two long nanobelts integrated with the surface of the silicon wafer; (f) multilayer nanobelt bundle with growth plane parallel to the
surface of the wafer; (g) a bundle of belts passing across another bundle.
Fig. 4. Illustration of formation and growth mechanism of multilayer bismuth nanobelts. (a) The formation of bismuth spherical nucleus; (b) formation of a multilayered
sword-like nanostructure; (c) the belts sink into the wafer and grow forward, meeting another bundle of belts perpendicular to the growth direction; (d) the belt kinks and
grows out of the surface; (e) the nucleus forms a sword-like nanostructure parallel to the surface of the wafer; (f, g) the top of the belt sinks into the surface of the wafer
and grows forward; (h) the belt finally detaches from the surface and grows freely.
nanostructures are preferentially formed, in contrast to the hydro-
thermal system mentioned in Ref. [14]. A single or a multilayered
belt-like nanocrystal grows from the nanosphere (Fig. 4b and e).
A nanosphere and a short nanobelt connect together to form the
sword-like nanostructures. The growth planes of initial nanobelts
grown from the bismuth sphere have two different directions: one is
vertical to the surface of the silicon wafer (Fig. 4b) and the other is
parallel with the surface of silicon wafer (Fig. 4f). During the growth
process, the nanobelt may ‘‘dig’’ into the wafer and form a self-
nested structure because of the ‘‘etching’’ effect of bismuth nanos-
tructure on silicon wafer [14]. For the nanobelts with growth planes
perpendicular to the silicon surface, parts of the nanobelts grow
underneath the wafer surface. These nanobelts appear integrated
with the wafer. Normally, the nanobelts can grow parallel to the
surface of the wafer in a self-nest structure (Fig. 4c and d). When
they meet another belt which is also integrated within the surface of
the wafer, the front top the belt will kink and continue to grow out
of the wafer. For the nanobelts with growth planes parallel to the
surface of the wafer (shown in Fig. 4f), the initial section of the belt
can ‘‘dig’’ into the wafer (Fig. 4g) and subsequent sections of the belt
grow out of the wafer. The large surface area of the parallel growth
nanobelt may hinder the ’’digging’’ of the nanobelts (Fig. 4g and h),
which means that when the reaction occurs over longer times
nanobelts accumulate on the surface of the wafer.
To investigate the electrical properties of the nanobelts, a three-
layer bismuth nanobelt about 800 nm in breadth, 250 nm in thick-
ness, and 8800 nm in length was used to measure I–V character-
istics. The nanobelt was aligned on a pair of gold electrodes. A layer
of gold was coated on the contact ends of the nanobelt by focus ion
beam (FIB) to lower the contact resistance between the electrodes
and the nanobelt. An I–V curve of this belt is shown in Fig. 5. The
measured electric resistance of this nanobelt is 2 ꢀ 104
O. Calculated

Z. Gao et al. / Journal of Solid State Chemistry 184 (2011) 3257–3261
3261
0.000010
0.000005
0.000000
-0.000005
-0.000010
wafer and form a special bismuth–silicon nanostructure, which
has a potential application for some nanodevices. The I–V proper-
ties of an individual multilayered nanobelt were measured and
found to have different properties that bulk bismuth materials.
Because bismuth is a semimetal with unique physical properties,
bismuth nanobelts may find application in sensitive magnetic
sensors or used to build other electronic devices.
Acknowledgment
This research was supported by NSFC (NSFDYS: 50925205,
Grant: 50990303, 50872070, IRG: 50721002), Independent Inno-
vation Foundation of Shandong University, (2009JC011) and the
Program of Introducing Talents of Discipline to Universities in
China (111 program no. b06015).
-0.2
-0.1
0.0
0.1
0.2
V (V)
Fig. 5. I–V curve measured on a nanobelt bundle (inset).
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4. Conclusions
Bismuth nanobelts in situ grown on silicon wafer were synthe-
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hydroxide as a raw material at 180 1C for 24 h. The nanobelts
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