Supplementary Information
Graphene shell on silica nanowires toward a nanostructured electrode
with controlled morphology
Byung-Sung Kim,1,3,b) Chunju Xu,2,b) Jeong-Mi Lee,1,3 Jae-Hyun Lee,1,3 Kiseok Son,1,3
Sung Woo Hwang,3,4 Jong-Jin Park,4 and Dongmok Whang1,3,a)
1
SKKU Advanced Institute of Nanotechnology, School of Advanced Materials
Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea
2
College of Materials Science and Engineering, North University of China,
Taiyuan 030051, China
3
Research Center for Time-Domain Nano-functional Devices, Samsung Advanced
Institute of Technology, Yongin 449-712, Korea
4
Samsung Advanced Institute of Technology, Yongin 449-712, Korea
a)
b)
Author to whom correspondence should be addressed. Electronic mail: dwhang@skku.edu,
Byung-Sung Kim and Chunju Xu contributed equally to this work.
Experimental Section
Growth of Au-catalyzed Si nanowires: Gold (Au, 3 nm) was deposited on a silicon wafer
((100) orientation, B-doped, 0.5−30 Ω•cm) by thermal evaporation and the substrate was then
loaded into a quartz tube of the chemical vapor deposition (CVD) system. Au-catalyzed
silicon nanowires (SiNWs) were grown by the flow of controlled amounts of silane (SiH4)
gas at 500 °C and 40 Torr for 20 min.
Preparation of SiO2 nanowires: After CVD growth, the substrate was cooled to room
temperature (RT), removed from the CVD chamber, and placed in a muffle furnace for a wet
oxidation process under ambient air conditions. The oxidation temperature was 800 °C with a
heating rate of 20 °C/min and was maintained for 4−16 h.
Direct growth of graphene on SiO2 nanowires : Nanocrystalline graphene was grown on
the thermally oxidized SiNWs at temperatures ranging from 850 to 1050 °C and at 20 Torr
using methane (CH4, 100 %) for 20−80 min. Additional hydrogen (H2, 100%) gas was
introduced to investigate the mechanism of structural control of the nanographene on SiO2
NWs.
Growth of ZnO nanorods on graphene/SiO2 NWs: The ZnO nanorod branches on
graphene/SiO2 NW backbone were prepared by electrochemical deposition from a pure zinc
nitrate hexahydrate (Zn(NO3)2•6H2O, Aldrich, 98% purity) aqueous solution in a threeelectrode cell. The deposition was carried out in a three-electrode cell at 90 oC. A saturated
calomel electrode (SCE), graphite sheet and monolayer graphene on glass (G/G) were used as
reference, counter, and working electrode, respectively. The electrolyte contained 50 mmol/L
zinc nitrate aqueous solution which was kept stable without stirring, a stable current density
of -0.5 mA/cm2 versus the SCE reference electrode was applied on the graphene electrode for
30 min.
Characterization
SEM images were acquired with a JEOL JSM-7401F field emission scanning electron
microscope (FESEM). The crystalline structure of the graphene/SiO2 and ZnO/graphene/SiO2
NWs were characterized using a JEOL JEM-2100F transmission electron microscope (TEM).
For TEM imaging, as-grown NWs suspended in ethanol were dispersed onto a copper grid
with a lacey carbon support film and imaged at an accelerating voltage of 200 kV. Raman
spectroscopy (Renishaw, RM-1000 Invia) with excitation energy of 2.41 eV (514 nm, Ar-ion
laser) was used to characterize the graphene/SiO2 NWs on 200 nm Aluminum (Al) substrates.
Transmittance spectra of the graphene were characterized by ultraviolet–visible spectroscopy
(UV-3600, Shimazdu, Japan) after transferring to a quartz plate. To evaluate the electrical
properties, the four-terminal measurement was performed from four different pairs of contact
probes (width: 4 µm/ gap: 4 µm). For metal electrodes, 20 nm nichrome (NiCr, 80% Ni and
20% Cr) and 60 nm Au were deposited by thermal evaporation
Results
Figure S1(a) shows that highly dense SiNWs were grown by the Au-catalyzed vaporliquid-solid (VLS) method. They were greater than 10 µm in length with uniform diameters
and random growth directions. As-grown SiNWs were immediately transferred into a muffle
furnace and oxidized at 800 °C for 16 h. The magnified TEM image shown in Figure S1(b)
and the diffraction rings in the selected area diffraction (SAED) pattern (insert) confirmed
that the crystalline SiNWs were fully oxidized after the wet oxidation process.
Figure S1 (a) Typical SEM image of crystalline SiNWs grown on Si (100) substrate. (b)
High resolution TEM image showing that the SiNWs were fully oxidized after wet oxidation
process. Insert of highly diffused ring-type diffraction pattern reveals that thermally oxidized
SiNWs have amorphous properties.
Figure S2 SEM images of graphene/SiO2 NWs grown at different growth temperatures: (a
and b) 1000 °C and (c and d) 1050 °C. At high temperature, the structure was changed into
multilayered leaf-like graphene/SiO2 NWs.
We also investigated the electrical and optical properties of directly-grown graphene, and
four-terminal measurement was performed from four different pairs of contact probes. It is
found that the transmittance at 550 nm and sheet resistance with different structures are
~1827.0 Ω/sq (and 97.4 %), ~1393.3 Ω/sq (and 96.7 %), ~216.1 Ω/sq (and 87.3 %) and ~182.1 Ω/sq
(and 83.2 %) at 1000 °C and 1050 °C for 20 min and 40 min, respectively. It demonstrates the good
quality of directly-grown graphene, compared with conventional one grown by metalcatalyzed CVD method
Figure S3 (a) Sheet resistance and (b) transmittance spectra of nanocrystalline graphene
grown on the SiO2 substrates at 1000 °C and 1050 °C for 20 min and 40 min, as indicated by
the corresponding colored lines in (a).
Figure S4 Scheme of the experimental process for growth of ZnO/graphene/SiO2 branched
NW heterostructures.