ββββ-SiC/SiO 2 core-shell nanowires studied by TEM and...

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β-SiC/SiO 2 core-shell nanowires studied by TEM and SEM-CL F.Rossi 1 , F. Fabbri 1 , G.Attolini 1 , M.Bosi 1 , B.E.Watts 1 , G. Salviati 1 , B. Dierre 2 , N. Fukata 3 , and T. Sekiguchi 2 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) INTRODUCTION The combination of the distinctive physical and chemical properties of SiC and unique advantages of nanowires (NWs) opens promising near-future perspectives for the design and fabrication of nano-scale devices. The main interests are addressed to nano- electronic devices (e.g. nano field-effect transistors) and nano-electromechanical systems able to operate even in harsh environments, and to nano-sensors exploiting the SiC NWs as biocompatible nanoprobes for biological systems. GROWTH PROCESS β-SiC NWs are grown by a CVD process on n-type Si (001) substrates, using carbon monoxide as the carbon source and nickel nitrate as the catalyst. The synthesis, performed at temperatures between 1050 and 1100 °C in an open-tube, is based on the carbothermal reduction of silica present as the native oxide on the substrate surface, where the reaction is catalyzed by Ni. Very long (hundreds μm) and thin wires in a dense network can be obtained. STRUCTURAL PROPERTIES: μ-RAMAN we estimate Δa/a 0 of the order of -0.02% and -0.07% respectively, corresponding to -Δa 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 976.2 cm -1 respectively in Raman spectra. Other vibrational 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 120000 160000 200000 240000 280000 2.68 eV (SiO 2 ) ntensity (a.u.) 2.34 eV T=300 K STRUCTURAL PROPERTIES: TEM The NWs have a core-shell structure, with a β-SiC crystalline core (50 nm) coated by an amorphous SiO 2 shell . Elemental mapping (energy-filtering: in-column Ω filter in a JEM 2200FS JEOL microscope 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 can originate 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) 600 700 800 900 1000 6 9 12 15 18 related to the oxide shell 3C-SiC LO 3C-SiC TO Intensity (a.u.) Raman Shift (cm) -1 ) ---- standard values for β-SiC TO (796 cm-1) LO (973 cm-1) Δ - + = 0 3734 5 . 796 ) ( a a TO ω Δ - + = 0 4532 973 ) ( a a LO ω SiC, and an intense blue band at about 2.68 eV Blue luminescence from cubic SiC whiskers and nanocrystallites has been reported in the literature, but the contribution of an SiO 2 related emission in this spectral region should also be considered. In our samples, size-dependent CL studies on single NWs do not evidence quantum-confinement effects. Electron beam irradiation effects were studied. The SiC-related emission stays almost constant, while the blue luminescence increases till saturation as a function of the irradiation time. The samples were etched in HF. After treatment, the luminescence is stable against e-beam irradiation. 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 an incorporation of nitrogen (from the growth process) in the silicon dioxide near the core. 2.0 2.2 2.4 2.6 2.8 3.0 3.2 40000 80000 CL In Energy (eV) 2.34 eV 3C-SiC SUMMARY and future perspectives The growth of core-shell NWs with a 3C-SiC core showing adequate structural and optical properties 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 to be functionalised with specific organic molecules (e.g. porphyrins) for a new-class of biosensors based on molecular recognition. The choice of porphyrins to create a suitable inorganic/organic system is also supported by a good match between the optical emission of the SiC NWs at 2.34 eV and the porphyrin absorption Q band. This could allow an efficient inorganic/organic energy transfer, exploitable for instance to realize electro-optical sensing devices. 1,5 2,0 2,5 3,0 3,5 4,0 4,5 0 250 500 750 single NW etched in HF (1:3) 40 sec t=0 t= sec CL Intensity (a.u.) Energy (eV) streaky pattern SFs twin spots Ni catalyst particles are observed at the tip of the NWs (likely VLS growth mechanism). Z contrast EDX 3 4 5 1 2 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 single NW analysis vs diameter Point 1 Point 2 Point 3 Point 4 Point 5 CL Intensity (a.u.) Energy (eV) 1,5 2,0 2,5 3,0 3,5 4,0 4,5 0,0 0,5 1,0 1,5 2,0 2,5 single NW as-grown electron beam irradiation E B =15 keV, I B =2 nA t = 0 t = 3600 sec CL Intensity (a.u.) Energy (eV) 0 500 1000 1500 2000 2500 3000 3500 0,50 0,75 1,00 1,25 1,50 1,75 2,00 2,25 2,50 CL Intensity (a.u.) Time (sec) oxide related SiC [email protected]

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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

120000

160000

200000

240000

280000

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)

600 700 800 900 10006

9

12

15

18

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.

2.0 2.2 2.4 2.6 2.8 3.0 3.2

40000

80000

120000

CL

Int

ensi

ty (

a.u.

)

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.

1,5 2,0 2,5 3,0 3,5 4,0 4,5

0

250

500

750

single NW etched in HF (1:3) 40 sec

t=0 t= sec

CL

Int

ensi

ty (

a.u.

)

Energy (eV)

streaky pattern → SFs twin spots

Ni catalyst particles are observed at the tip of the NWs (likely VLS growth mechanism).

Z contrast

EDX

34

5

1

2

1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0

single NW analysis vs diameter Point 1 Point 2 Point 3 Point 4 Point 5

CL

Inte

nsity

(a.u

.)

Energy (eV)

1,5 2,0 2,5 3,0 3,5 4,0 4,5

0,0

0,5

1,0

1,5

2,0

2,5

single NW as-grownelectron beam irradiation E

B=15 keV, I

B=2 nA

t = 0 t = 3600 sec

CL

Int

ensi

ty (

a.u.

)

Energy (eV)0 500 1000 1500 2000 2500 3000 3500

0,50

0,75

1,00

1,25

1,50

1,75

2,00

2,25

2,50

CL

Int

ensi

ty (

a.u.

)

Time (sec)

oxide related

SiC

[email protected]