Direct growth of amorphous silica nanowires by solid state transformation of sio2 films

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

Direct growth of amorphous silica nanowires

Ki-Hong Lee a,*, Hyuck Soo Yang a, Kwang Hyeon Baik a, Jungsik Bang a,

Richard R Vanfleet b, Wolfgang Sigmund a

a

Materials Science and Engineering Department, University of Florida, Gainesville, FL 32611, USA

b Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA

Received 16 September 2003; in final form 5 November 2003

Published online: 5 December 2003

Abstract

Amorphous silica nanowires (a-SiONWs) were produced by direct solid state transformation from silica films The silica nanowires grow on TiN/Ni/SiO2/Si substrates during the annealing in H2or a H2:CH4mixture at 1050°C Titanium nitride (TiN) films were used to induce a solid state reaction with silica (SiO2) films on silicon wafers to provide silicon atoms into growing nanowires The TiN layers induce the diffusion of silicon and oxygen to the surface by a stress gradient built inside the films The nickel diffuses to the surface during the TiN deposition and acts as a nucleation site for the a-SiONWs

Ó 2003 Elsevier B.V All rights reserved

1 Introduction

Formation of a liquid phase has been an essential

factor for the growth of one-dimensional nanowires by

the vapor–liquid–solid (VLS) [1–3] or the solid–liquid–

solid (SLS) mechanism [4] The liquid phase acts as a

source for dissolution and re-precipitation of

compo-nents for the growth of nanowires Amorphous

semi-conducting materials, such as Si–C–H, can be synthesized

with various compositions, to manipulate the optical

properties in an extremely wide range [5] Amorphous

silica is widely used in silicon based integrated devices

and can also be produced as nanowires Yu et al [6]

showed that a-SiONWs emit blue light and might hence

be applied in integrated optical devices

The VLS and the SLS mechanism have been an

act-ing mechanism for the growth of silica nanowires [7,8]

In this work, a novel growth mechanism for a-SiONWs

is presented via direct solid state transformation from

silica films Titanium nitride (TiN) films were used to

induce a solid state reaction with the silica (SiO2) films

on silicon wafers to provide silicon atoms into growing nanowires The TiN layers induce the diffusion of silicon and oxygen to the surface by a stress gradient built in-side the films The nickel diffuses to the surface during the TiN deposition and acts as a nucleation site for the a-SiONWs

2 Experimental

used as substrates for the growth of SiONWs After thermally oxidizing the Si substrates, Ni films of 5 nm were deposited on the oxide layer by e-beam evapora-tion TiN films were deposited on the nickel films by laser ablation of a TiN target (99.9%) The ablation was

nitrogen gas

for silica nanowire synthesis Annealing of the substrates

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doi:10.1016/j.cplett.2003.11.056

was carried out in two conditions After annealing in a

the system for 10 min Another approach is to flow a

was increased to 80 min in this case Ar (800 sccm) was

kept flowing through the quartz tube to purge the

sys-tem during the heating and cooling

A field emission scanning electron microscope

(FE-SEM, JEOL 6335F) was used to investigate the growth

characteristics of a-SiONWs on the substrates A

transmission electron microscope (TEM, JEOL 2010F)

equipped with an energy dispersive spectroscope (EDS),

was used for structure and composition analysis

Elec-tron energy loss spectroscopy (EELS, Tecnai F30) was

carried out for further characterization of the

nano-wires Surface analysis of the substrates was carried out

by Auger electron spectroscopy (AES, Perkin–Elmer

PHI 660) at an acceleration voltage of 8 keV The

sub-strates for the AES analysis were transferred to the

system after exposing to air

3 Results and discussion

A-SiONWs were synthesized by simply annealing

mixture gas Fig 1 show FESEM photographs after

condi-tions Fig 1a shows silica nanowires grown on the

a longer process time, as shown in Fig 1b The seed

particles are attached to the top of the grown nanowires

as shown in the insets

The phase and the structure of the nanowires are identified by high resolution transmission electron mi-croscopy (HRTEM), electron dispersive spectroscopy (EDS), as well as electron energy loss spectroscopy (EELS) One structural difference of the nanowires be-tween the two gas mixture conditions is that an amor-phous carbon shell is formed around silica nanowires

a nanowire of Fig 1a in scanning TEM (STEM) mode used for the EELS analysis The EELS line scanning profiles show composition changes for silicon and car-bon across the nanowire (Fig 2b) The fine structure EELS of the silicon 2p edge from the inner phase reveals the formation of amorphous silica nanowire (Fig 2c); Carbon 1s edge band form the outside shell shows an amorphous carbon phase (Fig 2d) No titanium and nitrogen were detected by the EELS in the nanowires The carbon on the shell is supplied by thermal

mixture (not shown here)

Titanium nitride films should play an important role

on the growth of silica nanowires No nanowires were observed without TiN films on the substrates with the same annealing processes Titanium nitride is reduced to

Ti in hydrogen atmosphere at high temperature Tita-nium has a higher tendency to oxidize than silica films,

by removal of oxygen from the silica layer [9] The re-duction of TiN seems to be a critical factor in the growth kinetics of a-SiONWs The growth of silica nanowires

system to suppress the decomposition of TiN films The

Ni islands act as a nucleation site for the a-SiONW growth, and expedite the reduction of TiN by supply-ing extra hydrogen by thermal decomposition of

Fig 1 FESEM photographs of a-SiONWs nanowires grown on the substrates after annealing at 1050 °C in: (a) H 2 (200 sccm) for 10 min followed by

CH :H (10:200 sccm) for 10 min; (b) H :Ar (200:150 sccm) for 80 min The arrows represent the seed particles attached to the top of nanowires.

CH4, which explains the faster growth of a-SiONWs

with CH4

Nickel atoms diffuse out to the surface during the deposition of TiN The AES profile of the substrate surface right after the laser ablation reveals the existence

of nickel on the surface, as shown in Fig 3 Silicon at-oms diffuse out to the surface and form the nanowires

shows the appearance of silicon at the surface Silica nanowires nucleate on the nickel islands and grow on the surface by silicon diffusion from the under layer SiO2 films Fig 4 shows a nanowire grown on the substrate and EDS spectra showing compositions in each layer in the structure at the same process condition with Fig 3

As shown in the TEM photographs, the nanowires begin

to grow at this stage even though they are not observed using the FESEM

The growth behavior of the nanowires and the EDS spectra of the layers in Fig 4 show several facts which would not be observed by the VLS or the SLS mecha-nism As shown in Fig 4a (also can be seen in Fig 1), the metal particles are attached to the end of the nanowires, supporting the top growth mode Catalyst

Fig 3 AES profiles from the substrates after the deposition of TiN

films, and after annealing in the H 2 :Ar (200:150 sccm) mixture for

20 min The insets show the Ni (LMM) transition peak.

Fig 2 EELS spectrum profiles of a nanowire synthesized in the CH4:H2mixture: (a) a STEM photograph of the nanowire on a TEM grid used for the EELS analysis The arrow shows a carbon wire grown from the surface amorphous carbon by a focused electron beam scanning cross the wire; (b) intensity profiles of Si and C cross the wire showing the composition variation cross the wire Oxygen has a similar profile with Si (not shown here); (c) a Si 2p EELS profile from inside the nanowire indicating the formation of an amorphous silica phase; (d) a carbon 1s profile from the outside shell indicating a amorphous carbon phase.

materials should be at the surface of the substrate in

order that source atoms dissolve and precipitate as a

nanowire on a catalyst liquid droplet, called Ôbase

growthÕ, as in the SLS mechanism The existence of

metal particles on the top of the nanowires illustrates

that the growth is not established by the SLS

mecha-nism Silicon is present after annealing in the original

TiN layer; otherwise, nickel is not present in the layer

(Fig 4d) The particle is composed of nickel mainly

(Fig 4b), which is unlikely to form a liquid phase at the

process temperature Silicon was not detected in the

particles attached to the nanowires by the EELS (not

shown here) As a result, there is little possibility to form

a liquid phase either in the supporting layers (the TiN

films) or in the seed particles In addition, silicon sources

were not incorporated into the system directly from

vapor phase These facts show that it is unlikely for the

nanowires to grow by the VLS mechanism or the SLS

The under layer silicon oxide film is the only Si source

for the growth of a-SiONWs in the system Silicon

should be supplied to growing silica nanowires by solid

state diffusion through the TiN layer The TiN layers

decompose into Ti and form an oxide by the

substitu-tion reacsubstitu-tion with silica Reduced silicon can form a

nickel silicide with Ni remaining in the interface (Fig 4e)

or diffuse out to the surface to form the silica nanowires

(Fig 4c) The Si diffusion can be derived by a stress

variation built in the TiN layer during the annealing

processes The substitutional reaction initiates at the

interface between the TiN and the silica, thereby, it

builds a compressive stress in the interface region and a

decomposition of TiN, as a result, it limits the growth of a-SiONWs nanowires by the mechanism Oxygen seems

Fig 4 (a) A TEM photograph of a cross section of the substrate with the same treatment condition as Fig 3; (b)–(f) show EDS data from each layer indicated in (a) It is difficult to define the existence of nitrogen in the TiN layer since it is close to the oxygen and the Ti(L) peak However, nitrogen is considered to be present in the layer at this annealing stage by the AES in Fig 3.

Fig 5 A schematic diagram showing a growth mechanism of silica nanowires by the solid–solid transformation.

to be supplied either from the silica or the vapor

phase Fig 5 shows a schematic diagram showing the

growth mechanism of a-SiONWs in our experimental

condition

4 Conclusion

In summary, silica nanowires were synthesized by

solid state diffusion of silicon from the silica films The

growth mechanism could be explained by direct solid to

solid phase transformation, so called, the SS

mecha-nism The TiN films react with the silica films to produce

a silicon source for the nanowires and cause the silicon

diffusion by the internal stress Our result suggests a

novel growth mechanism for growth of nanowires, and

can be applied to the synthesis of other kind of

nano-wires

Acknowledgements

This work was supported by DARPA/Army

Re-search Office under Grant No DAAD19-00-1-0002

through the center for materials in sensors and actuators (MINSA) The authors thank Kerry Siebein (of the Major Analytical Instrumentation Center at the Uni-versity of Florida) for the TEM and the EDS analysis

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