ββββ-SiC/SiO 2 core-shell nanowires studied by TEM and...
Transcript of ββββ-SiC/SiO 2 core-shell nanowires studied by TEM and...
ββββ-SiC/SiO2 core-shell nanowires studied by TEM and SEM-CL
F.Rossi1, F. Fabbri1, G.Attolini1, M.Bosi1, B.E.Watts1, G. Salviati1, B. Dierre2, N. Fukata3, and T. Sekiguchi2
1 IMEM-CNR Institute, viale Usberti 37/A, 43100 Parma (Italy)2 Nano device characterization group, Advanced Electronic Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba,Ibaraki, 305-0044 (Japan)
3 International Center for Materials Nanoarchitectonics, National Institute for Materials Science & PRESTO JST, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044 (Japan)
INTRODUCTIONThe combination of the distinctive physical and chemical properties of SiC and uniqueadvantages of nanowires (NWs) opens promising near-future perspectives for the designand fabrication of nano-scale devices. The main interests are addressed to nano-electronic devices (e.g. nano field-effect transistors) and nano-electromechanicalsystems able to operate even in harsh environments, and to nano-sensors exploiting theSiC NWs as biocompatible nanoprobes for biological systems.
GROWTH PROCESSβ-SiC NWs are grown by a CVD process on n-type Si (001) substrates, using carbonmonoxide as the carbon source and nickel nitrate as the catalyst. The synthesis, performed attemperatures between 1050 and 1100 °C in an open-tube, is based on the carbothermalreduction of silica present as the native oxide on the substrate surface, where the reaction iscatalyzed by Ni.Very long (hundreds µm) and thin wires in a dense network can be obtained.
STRUCTURAL PROPERTIES: µ-RAMAN
we estimate ∆a/a0 of the order of -0.02% and -0.07% respectively, corresponding to -∆a of the order of 10-3 Å. The longitudinal variation is slightly higher than the transverse,
consistently with the NW geometry. Due to lattice imperfections, some broadening (e.g. TO-FWHM ∼ 4 cm-1) of the SiC phonon modes is observed.
*ref.: S. Nakashima and H. Harima, phys. stat. sol. (a) 162, 39 (1997)
The TO and LO phonon modes of the β-SiC core are detected at 797.3 and 976.2cm-1 respectively in Raman spectra. Othervibrational modes are due to the shell.From the relations*:
OPTICAL PROPERTIES: CATHODOLUMINESCENCE
The RT CL spectrum of the NWs bundle shows a broad peak centred at about 2.34 eV, related to the indirect band gap transition in β-SiC, and an intense blue band at about 2.68 eV
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2.68 eV(SiO
2)
CL
Int
ensi
ty (
a.u.
)
2.34 eV
T=300 K
STRUCTURAL PROPERTIES: TEMThe NWs have a core-shell structure, with a β-SiC crystalline core (≤50 nm) coated by anamorphous SiO2 shell .Elemental mapping (energy-filtering: in-column Ω filter in a JEM 2200FS JEOLmicroscope at 200 kV) highlights the C and O complementary distributions.
The core has a 3C structure with some stacking faults and rotational twins mainly on(111) planes perpendicular to the growth axis, as common in β-SiC. The SFs canoriginate very local segments of different polytypes (e.g. 2H, 4H, 6H).HRTEM (zone axis [110]) and HAADF studies of defects:
Zero loss image
O map (edge K)
Si map (edge L)
C map (edge K) colour-coded map
C (violet) O (blue)
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9
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related to the oxide shell
3C-SiC LO3C-SiC TO
Inte
nsity
(a.
u.)
Raman Shift (cm)-1)
---- standard values for β-SiC TO (796 cm-1) LO (973 cm-1)
∆−⋅+=
0
37345.796)(a
aTOω
∆−⋅+=
0
4532973)(a
aLOω
SiC, and an intense blue band at about 2.68 eV
Blue luminescence from cubic SiC whiskers and nanocrystallites has been reported in theliterature, but the contribution of an SiO2 related emission in this spectral region should alsobe considered. In our samples, size-dependent CL studies on single NWs do not evidencequantum-confinement effects.
Electron beam irradiation effects were studied. The SiC-related emission stays almostconstant, while the blue luminescence increases till saturation as a function of theirradiation time.
The samples were etched in HF. After treatment, the luminescence is stable against e-beamirradiation.
On the basis of these results and of literature data, the blue band is attributed to oxygen-deficient centres in the shell.The UV band observed in some NW at about 3.85 eV can be possibly due to anincorporation of nitrogen (from the growth process) in the silicon dioxide near the core.
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Energy (eV)
2.34 eV3C-SiC
SUMMARY and future perspectivesThe growth of core-shell NWs with a 3C-SiC core showing adequate structural and opticalproperties has been demonstrated. Future experiments will be aimed to study the application of i)
the oxidised NWs, as cylindrical nano FETs and of ii) the oxide-free SiC core, as nanoprobe tobe functionalised with specific organic molecules (e.g. porphyrins) for a new-class of biosensorsbased on molecular recognition. The choice of porphyrins to create a suitable inorganic/organicsystem is also supported by a good match between the optical emission of the SiC NWs at 2.34eV and the porphyrin absorption Q band. This could allow an efficient inorganic/organic energytransfer, exploitable for instance to realize electro-optical sensing devices.
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single NW etched in HF (1:3) 40 sec
t=0 t= sec
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streaky pattern → SFs twin spots
Ni catalyst particles are observed at the tip of the NWs (likely VLS growth mechanism).
Z contrast
EDX
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single NW analysis vs diameter Point 1 Point 2 Point 3 Point 4 Point 5
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nsity
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.)
Energy (eV)
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single NW as-grownelectron beam irradiation E
B=15 keV, I
B=2 nA
t = 0 t = 3600 sec
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oxide related
SiC