Photoluminescence and growth mechanism of amorphous silica nanowires by vapor phase transport

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Physica E 31 (2006) 218–223

Photoluminescence and growth mechanism of amorphous silica

nanowires by vapor phase transport

Y Yanga, B.K Taya, X.W Suna, , H.M Fanb, Z.X Shenc

a School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

b Department of Physics, National University of Singapore, Blk S12, 2 Science Drive 3, Singapore 117542, Singapore

c

School of Physical and Mathematical Sciences, Nanyang Technological University, Block 5, 1 Nanyang Walk, Singapore 637616, Singapore

Received 23 November 2005; received in revised form 13 December 2005; accepted 20 December 2005

Available online 21 February 2006

Abstract

Amorphous silica [SiOxð1oxo2Þ] nanowires were fabricated on silicon substrate in an acidic environment by heating the mixture of ZnCl2, and VO2powders at 1100 1C The length of SiOxnanowires ranges from micrometers to centimeters, with uniform diameters of 10–500 nm depending on substrate temperature Room-temperature photoluminescence spectra of the SiOx nanowires showed two strong luminescence peaks in the red and green region, respectively The photoluminescence was suggested to originate from nonbridging oxygen hole center (red band), and hydrogen-related species in the structure of SiOx(green band) The study on chemical reactions and growth of the SiOx nanowires revealed the formation process of silica nanowires in acidic environment was closely related to the vapor–solid–liquid mechanism

r2006 Elsevier B.V All rights reserved

PACS: 66.66.Fn; 6.37.Hk; 78.55.Hx

Keywords: Silica nanowires; Photoluminescence; Vapor phase transport

1 Introduction

One-dimensional (1D) structures with nanometer

dia-meters, such as nanotubes and nanowires, are ideal vehicles

for testing and understanding fundamental concepts about

dimensionality and size effect in, for example, optical,

electrical, and mechanical properties Their applications

range from probing tips in microscopy to interconnect in

nanoelectronics [1]

The synthesis of 1D nanostructures is of fundamental

importance to nanotechnology Nanowires are particularly

interesting as they offer the opportunity to investigate

electrical and thermal transport processes in size-confined

systems, with the possibility of providing a deep

under-standing of physics at the nano-scale Silicon and silica

nanostructures have attracted considerable attention

because of their potential applications in light-emitting

devices compatible to CMOS technology Amorphous silica nanowires (SiONWs) are promising 1D luminescence materials The photoluminescence (PL) band of bulk silica

or silica films has a peak within 1.6–7.0 eV[2–4]from both experimental measurements and theoretical calculations

Yu et al.[5]have pointed out the potential applications of silica nanowires in high-resolution optical heads of scanning near-field optical microscope or nanointerconnec-tions in future integrated optical devices Much research interest has recently been directed to synthesize these materials by various approaches, to understand their growth mechanism and to realize their controlled growth

on planar substrates

Various approaches, for instance, vapor phase transport

[6], bio-mimetic strategies [7,8], excimer laser ablation[9], physical and thermal chemical evaporation [10–16], carbothermal reduction [17], thermal chemical vapor deposition [18], thermal oxidation [19] and solution method [20,21] have been employed to fabricate the nanostructured silica with different morphologies including

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1386-9477/$ - see front matter r 2006 Elsevier B.V All rights reserved.

doi:10.1016/j.physe.2005.12.159

Corresponding author Tel.: +65 67905369; fax: +65 67920415.

E-mail address: exwsun@ntu.edu.sg (X.W Sun).

silica ‘‘nanoflowers’’ [22], radial patterns of carbonated

silica fibers [10], silica nanowire ‘‘braids’’ [6], ‘‘bundles’’

and silica ‘‘nanobrushes’’ [23] In the vapor transport

process, catalyst such as In2O3, Al, Ni, Ga and Sn, and

transport gases such as O2, and H2, are often introduced

into the reaction In this paper, we shall report a

catalyst-assisted synthesis of amorphous silica nanowires by vapor

phase transport method in an acidic environment without

transport gas

2 Experiment

Nanostructural SiONWs have been prepared by a simple

vapor-phase transport method using high-temperature

tube furnace, which has been reported in our previous

work [24] In brief, the mixture of hydrous zinc chloride

(ZnCl2
nH2O) and vanadium dioxide (VO2) powder was

placed at the end of a slender one-end sealed quartz tube A

p-type silicon trip with (1 0 0) orientation was also inserted

into the quartz tube at downstream area with lower

temperature (1090–700 1C) as the source of silicon as well

as substrate Then the small quartz tube was placed into a

bigger quartz tube and pushed into the tube furnace The

furnace was heated from room temperature to 1100 1C and

kept at this temperature for 30 min When temperature

reached 975 1C, an extra mechanical pump was used

to collect the white fog due to the hydrolyzation and

oxidation of ZnCl2, and maintain a pressure of 2–0.3 Pa in

the tube

The morphology, size, and crystal structure, of the

SiONWs were determined using a cold cathode field

emission scanning electron microscope (SEM) from Jeol

(model JSM-6340F), and a transmission electron

micro-scope (TEM) from Jeol (model JEM-2010F) too The

chemical composition analysis was carried out using energy

dispersive X-ray spectroscopy (EDX) which was attached

to the SEM PL measurements were carried out at room

temperature using a Micro-PL system from Renishaw The

excitation line used was 325 nm and the average power was

10 mW Laser beam was focused into a spot diameter

below 1 mm on the specimen in the PL measurement.

3 Results and discussion

Figs 1(a) and (b)show the SEM images of the nanowires

with high aspect ratio (length/diameter) Fig 1(c) shows

the photograph of the sample, with positions (a) and (b)

labeled, the SEM images inFigs 1(a) and (b)were taken,

respectively It can be seen from Fig 1(c) that bulk

quantity of cotton-like nanostructures was formed The

cotton-like SiONWs were formed on silicon wafer with

temperature ranged from 1050 to 720 1C due to a

temperature gradient The average diameter of SiOx

nanowires varied from 10 too100 nm when the substrate

temperature decreased from 1050 to 1000 1C (region A

which is near the powder source),Fig 1(a), and it suddenly

increased from o100 to 500 nm when the substrate

temperature decreased from 1000 to 720 1C (region B which is further away from the source),Fig 1(b).Fig 1(a)

is a typical SEM image of product found in region A It can

be seen that the products synthesized consist of a large number of curved SiOxnanowires with length of a few tens

of micrometers.Fig 1(b)shows that the products in region

B generally align in one direction with length up to a few hundreds of micrometers The diameters measured are around 200 nm Some SiONW nanowires grown in 1000–720 1C region were up to 1 cm in length, which was observed in SEM by tracking the nanowires in their growth direction Similar cotton-like nanowires has been obtained

by Lee et al [25] grown on TiN/Ni/Si and TiN/Ni/SiO2 substrates

Fig 1 Low magnification SEM images of the amorphous silica nanowires deposited at two different temperature regions: (a) region A of 1050–1000 1C, 1020 1C (the average diameter is 30–50 nm); (b) region B

of 1000–720 1C, 900 1C (the average diameter is around 200 nm); (c) photo

of a typical sample growing on a silicon trip.

Fig 2(a) shows a TEM image of the SiOx nanowires

obtained near 1000 1C temperature region The inset is the

corresponding selected-area electron diffraction (SAED)

pattern recorded from the nanowires The as-deposited

SiONWs is of amorphous phase, indicated by the highly

diffusive SAED ring pattern We can see that, the

nanowires are remarkably clean and smooth Fig 2(b)

shows the high-magnification TEM image of a catalyst tip,

and Figs 2(c) and (d) are the corresponding SAED and

high-resolution TEM image of the catalyst, respectively

The catalyst is crystallized and surrounded by amorphous

silica, indicating that the growth process of amorphous

silica nanowires is catalyst-assisted

EDX was applied to examine the chemical composition

of the as-grown nanowires Fig 3(a) shows the EDX

spectrum of the round tip on top of a nanowire (Fig 2(b)),

andFig 3(b)shows the EDX spectrum from long bundles

of tangled silica nanowires in Fig 1(b), where the

nanowires are so long that almost no tip was in the area

examined Analyzing Figs 3(a) and (b) using the EDX

spectrometer’s own computer program, the chemical

compositions are 48.77 at% of O, 41.87 at% of Si,

8.66 at% of V, 0.74 at% of Zn, and no Cl, for the catalyst

tip in Fig 2(b), and 60.28 at% of O, 39.40 at% of Si,

0.32 at% of V, and no Zn or Cl, for the long nanowires in

Fig 1(b) Thus, it is confirmed that, V acted as a catalyst in

the growth of silica nanowires; however, most of Zn (ZnOx

or ZnCl2) with a low-melting point, was probably pumped

out from the tube It is worth mentioning that, the

composition obtained here serves only as an evidence for

our argument, as in general, there is about 1% error in the

composition analysis from EDX

Fig 4shows the room-temperature PL spectra of the

as-grown SiONW sample obtained under excitation of 325 nm

(3.8 eV) light from He–Cd laser For the first time, both

distinct PL bands corresponding to red and green light emissions are observed around 770 nm (1.61 eV) and

550 nm (2.25 eV), respectively By fitting the curves by two Gaussian functions, two peak energies E (red

Fig 2 (a) TEM image (the inset shows the SAED pattern) of the

as-grown amorphous SiO x nanowires in the temperature region of 1000 1C;

(b) TEM image of a catalyst tip, and the corresponding (c) SAED and (d)

high-resolution TEM image.

Fig 3 EDX spectra for (a) the catalyst tip in Fig 2(b) , and (b) tangled long silica nanowires in Fig 1(b)

0 1000 2000 3000 4000 5000 6000 7000 8000

Ered

Egreen

500 nm

100 nm

50 nm

Wavelength (nm) Fig 4 Photoluminescence spectra recorded at room temperature from SiONWs with the diameters of 50, 100, and 500 nm, respectively.

emission band) and Egreen (green emission band) can be

obtained, although the fitting was quite subjective The

results are tabulated inTable 1 The red emission band has

a relatively constant intensity, and a red-shift of about

18 nm, while the green emission band becomes weaker

compared to the red ones, and shows a blue-shift of about

14 nm, as the average diameter of nanowires increases from

50 to 500 nm, corresponding to SEM observations It is

worth mentioning that, an average diameter of 50 nm is too

thick to show any distinct quantum effect on PL Thus the

relatively small shift in emission peaks should not be

directly related to size reduction

There are several nanostructure defects related to the PL

of the SiOx system The red emission band at 1.61 eV is

attributed to bulk non-bridged oxygen hole center

(NBOHC), which is denoted as  Si2O [26,27] The

NBOHC induced band was observed in the oxygen-rich

silica and in the high –OH content silica For our

oxygen-deficient but –OH rich sample, it is possible for the

NBOHC ð Si2OÞ to be induced by the high-energy

photon (3.8 eV) excitation in our PL measurement:

Si2O2O2Si ! 2ð Si2OÞ

or

Si2OH ! Si2OþH:

The red emission band properties observed in our PL

spectra are similar to those of surface-oxidized silicon

nanocrystals, and mesoporous silica [26], without the

exhibition of green emission band

The green emission band at 2.25 eV can be attributed to

hydrogen-related species in the composites of SiONWs

[28] Defect concentration in SiONWs is related primarily

to the high surface area and the complex chemistry that

occurs during growth Thus the PL intensity is highly

related to surface area and inverse proportional to

nanowire diameters as observed in Fig 4 Considering

the width of the red (150 nm full-width half-maximum

(FWHM)) and green (100 nm FWHM) emissions in

Fig 4, the maximum peak shifts for red (18 nm) and green

(14 nm) emissions for nanowires with different diameters

are rather small (Table 1andFig 4) Obviously, the large

widths of red and green emissions indicate a large range of

energy transitions, and the emission peak should

corre-spond to the dominant transition[29] At the moment, we

cannot identify a direct link between the shift and the

nanowire diameter However, we speculate that the peak shifts for nanowires with different diameter are due to the fabrication temperature, at which these nanowires grow The temperature directly affects the chemical reactions (reaction rates) during nanowire formation, resulting in various defects with varied concentration According to Liu et al [30], the VO2 has a weak and broad emission band near 600 nm However, PL from catalyst can be ignored since it is not the major component in the area examined (Fig 3(b))

The well accepted vapor–liquid–solid (VLS) mechanism

is responsible for the catalyst-assisted amorphous SiOx

nanowires growth in our experiment[5,31] The key factor

in VLS is the formation of liquid droplets due to adding a liquid forming agent Due to the existence of a temperature gradient downstream the quartz tube, liquid droplets were formed from vapor phase, from the reaction happened in quartz tube However, there was no extra silicon source besides silicon substrate; thus, we suggest that the formation of SiOxis related to the reaction of ZnCl2and silicon Vadadium or VOx evaporated from VO2 acts as catalyst for silica nanowire to grow by VLS growth mechanism The reactions between VO2 and oxidants (i.e., O2 and Cl2) were not considered because the VO2

was not oxidized to higher oxidation state (V5+) indicated

by the color of this oxide on substrate where nanowires could be found Further investigation using V2O5instead

of VO2 revealed that neither catalyst tip nor silica nanowires could be found; with the absence of VO2, ZnO nanowires were found growing on Si substrates, which is consistent with our previous work [32] Thus, the catalyst should be related to V4+compound

As we know, ZnCl2is highly hygroscopic The powder used in our experiment was actually ZnCl2
nH2O, which behaves as a mild Lewis acid, with a pH value of around 4

[33] It is hydrolyzed to an oxychloride when hydrated forms are heated Among the solid reactant, ZnCl2has the lowest melting and boiling points (275 and 756 1C, respectively) [34] When the temperature at the source approached 800 1C or higher, the tube was heavily filled with white fog The porous silicon substrate after experi-ments, suggests that, when the furnace is heated from room temperature to 1100 1C, Si, O2(residue in air), and ZnCl2

containing moisture may react in a complex way Experi-ments without ZnCl2did not produce any silica nanowire, indicating that the ZnCl2must be a source of silicon wafer etchant In accordance with the SEM, TEM and EDX data, the main reactions that could produce SiONWs are as follows:

Hydrolyzation of ZnCl2
nH2O[35]

2ZnCl2ðlÞ þ H2OðlÞ ! Zn2OCl2ðgÞ þ 2HClðgÞ

Oxidation of ZnCl2in the melt[36]

ZnCl2ðlÞ þ 1=2O2ðgÞ ! ZnOClðgÞ þ 1=2Cl2ðgÞ and ZnCl ðlÞ þ x=2O ðgÞ ! ZnO ðlÞ þ Cl ðgÞ

Table 1

Peak emission wavelength of silica nanowires with an average diameter of

50, 100, and 500 nm, respectively, obtained by fitting the curves in Fig 4

Diameter (nm) Peak wavelength of E red

(nm)

Peak wavelength of E green

(nm)

Adsorption on substrate[37]

Cl;

Cl2þSi ! Sisurf xCl þ 1=2H2ðgÞ;

HCl:

8

>

>

Chemical reaction

Si  xCl ! SiClxðadsÞ

Product desorption

SiClxðadsÞ!SiClxðgÞ

Oxidation

SiClxðgÞ þ x=2O2ðgÞ ! SiOxðgÞ þ x=2Cl2,

2ZnðgÞ þ xO2ðgÞ ! 2ZnOxðgÞ

Hence, the white fog may contain SiOx, Cl2, HCl, ZnOx,

ZnCl2, and zinc oxychloride, with little amount of VO2

Since vapor pressure of VO2 is low even at high

temperature, and the melting point of VO2 is 1967 1C

[33], VO2 is most likely alloyed with Si or SiOx to form

liquid drops at higher temperature The bond enthalpies in

gaseous diatomic species of Si–O bond, Zn–O bond, and

Zn–Cl bond are 799.6713.4, 15974, and 228.9719.7 kJ/

mol[38], respectively; and the lattice energy in a

thermo-chemical cycle of SiO2, ZnO, and ZnCl2are calculated to

be 13125, 3971, and 2734 kJ/mol [39], respectively; while

SiCl4 in vapor phase at high temperature is known to be

unstable It is much easier to break the Si–Cl and Zn–Cl

bonds than the Si–O and Zn–O bonds; i.e it is more easily

to form SiO2 than the rest compounds during reactions

From our experiments, pumping was necessary for

synth-esis of SiONWs with controlled amount of zinc and

chloride residue By keep pumping the furnace tube, small

liquids of ZnOx and zinc oxychloride vapor could be

sucked out with the high substrate temperature ranging

from 1050 to 720 1C Meanwhile, gases containing Cl2,

and HCl could be sucked out as well to avoid excessive

etching of substrate It is worth mentioning that, although

both ZnOx and SiClx are in vapor phase, SiClx is an

intermediate phase of a series of chemical reactions, and it

oxidized quickly into SiOx with much lower vapor

pressure However, ZnOx is one of the final products of

the chemical reactions and has a much higher vapor

pressure So ZnOxcan be mostly sucked out by the pump

4 Conclusion

In conclusion, amorphous silica nanowires have been

successfully fabricated from the mixture of ZnCl2, and VO2

on silicon substrate by controlling the conditions of the

vapor-phase transport Ultra-long cotton-like nanowires

with average diameters of 50 nm show both strong PL of

red and green light originated from surface NBOHC defect

and nonstoichiometric structure The morphology of SiOx

is dependent on synthesis temperature

Acknowledgement Financial support from Research Grant Manpower Fund (RGM 21/04) of Nanyang Technological University, Singapore is gratefully acknowledged

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