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Understanding the growth of amorphous SiO 2 nanofibers and crystalline binary nanoparticles 

produced by laser ablation 

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2012 Nanotechnology 23 035601 

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IOP PUBLISHING NANOTECHNOLOGY 

Nanotechnology 23 (2012) 035601 (8pp) doi:10.1088/0957-4484/23/3/035601 

Understanding the growth of amorphous 

SiO2 nanofibers and crystalline binary 

nanoparticles produced by laser ablation 

Maria Dimitrakopoulou 1 , Sandeep Gorantla 1 ,Jürgen Thomas 1 , 

Thomas Gemming 1 , Gianaurelio Cuniberti 1,3 , Bernd Büchner 1,2 

and Mark H Rümmeli 1,4 

1 Leibniz Institute for Solid State and Materials Research (IFW), Helmholtzstraße 20, 

D-01069 Dresden, Germany 

2 Technische Universität Dresden, D-01062, Dresden, Germany 

E-mail: m.dimitrakopoulou@ifw-dresden.de and m.ruemmeli@ifw-dresden.de 

Received 10 September 2011, in final form 4 November 2011 

Published 16 December 2011 

Online at stacks.iop.org/Nano/23/035601 

Abstract 

The pulsed-laser evaporation synthesis of silica nanofibers and crystalline binary nanoparticles 

is investigated in detail. By careful adjustment of the synthesis parameters one can tailor the 

product to form high yield nanofibers or binary nanoparticles. Some control on their diameters 

is also possible through the synthesis parameters. Oxidation of the nanofibers occurs upon 

exposure to air after the reaction. 

S Online supplementary data available from stacks.iop.org/Nano/23/035601/mmedia 

(Some figures may appear in colour only in the online journal) 

1. Introduction 

One-dimensional (1D) nanostructures have gained tremendous 

attention over the past ten years due to their exciting and useful 

properties, for example, electrical, optical, mechanical and 

chemical properties. Many of the changes in their physical 

properties arise from the confinement of electrons in two 

dimensions. The ability to change the physical properties of 

1D structures makes them promising candidates for application 

in various fields, including electronics, photonics, sensors and 

energy applications [1–4]. A variety of synthesis approaches 

including bottom-up and top-down techniques have been 

developed for their fabrication [5–10]. Most of these methods 

have focused on the synthesis of semiconductor nanowires 

such as Si, Ge [11, 12] and III–V semiconductors, e.g. GaN, 

GaAs, GaP and InAs [13–15]. Silicon oxide nanowires and 

nanofibers are also of interest. They emit blue light over 

100 times brighter than their porous Si counterparts. This 

3 Institute for Materials Science and Max Bergmann Center of Biomaterials 

& Materials Science and Nanotechnology, Technische Universität Dresden, 

D-01062, Germany. 

4 Fakultät Mathematik & Naturwissenschaften, Technische Universität 

Dresden, D-01062, Germany. 

makes them attractive for high resolution optical heads in nearfield 

optical microscopes or optical interconnects in integrated 

optical devices [16]. In addition, they were reported to 

have very high mechanical strength and can be manufactured 

with very long lengths due to their glassy nature [17]. 

Recently, films embedded with silica nanowires were shown 

to exhibit super-hydrophilic properties useful for anti-fogging 

functionality [18]. They also offer excellent bio-compatibility, 

making them enticing for use in biomedical applications [19]. 

SiOx nanowires can be synthesized in different ways, 

including chemical vapor deposition (CVD), laser ablation, 

thermal evaporation, sol–gel, thermal-carbon reduction, flame 

spray pyrolysis, ion implantation and water-assisted synthesis 

under supercritically hydrothermal conditions [20–27]. 

Here we systematically investigate the formation of 

amorphous silica nanowires or silica nanofibers (NFs) using 

laser evaporation. The developed route uses Si as the feedstock 

and transition metals as the catalyst. No oxygen is provided 

during the reaction. The oxidation process of the NFs is 

shown to occur upon exposure to air. In addition, binary 

FeSi/Si nanoparticles form in the reaction. Through careful 

adjustment of the synthesis parameters one can optimize the 

system for high yield silica NF production or high yield binary 

0957-4484/12/035601+08$33.00 1 

© 2012 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 23 (2012) 035601 M Dimitrakopoulou et al 

Figure 1. (a) TEM overview of a high yield NF sample (produced at 1000 mbar, Fe content: 2 wt%, flow 0.5 slmp). (b) High yield binary NP 

sample (synthesis pressure 250 mbar, Fe content: 20 wt%, 0.5 slmp). (c) TEM micrograph typifying an NF. (d) Typical micrograph of (Fe) 

catalyst particle at the tip of an NF. The core–shell structure is clearly visible. (e) Representative TEM of a binary NP. The different contrast 

highlighting the two phases is clearly visible. (f) Magnification of a box in (e). Insets show the FFT for each side of the binary NP. All 

samples were synthesized at 1150 ◦ C. 

nanoparticle formation. The FeSi/Si nanoparticles may also 

have useful properties. β-FeSi2 is promising as a near-infrared 

detector or emitter with a bandgap of 0.83–0.87 eV [28]. In 

addition, it has a strong absorption coefficient [29] andahigh 

Seebeck coefficient [30]. 

2. Experimental methods 

The amorphous silicon dioxide nanofibers and binary 

nanoparticles were synthesized by a laser ablation system. An 

Nd:YAG laser (Model SL 803, Spectron Laser Systems, with 

an output of 1400 mJ and repletion rate of 10 Hz) was used 

to drive the ablation process. The laser spot size on the target 

was 6 mm. The reactor consisted of a 45 mm diameter quartz 

tube placed in a horizontal tube furnace (Linn High Therm 

2 

FRH-40/500/1100). At one end of the tube a gas feed system 

and laser transmission window was mounted while at the other 

(rear) end a gas exit system and Cu water-cooled cold finger 

(to collect the product) was mounted. A Mo target holder is 

then placed on the end of the cold finger so that the target sits 

at the center of the oven. The targets were prepared by mixing 

powdered catalyst metals (Fe, Ni, Co and Mo) with powdered 

Si and then pressing them in a 20 mm dye. The thickness of 

the target after pressing was approx. 5 mm. After production 

of the target pellets they were annealed for 60 min at 1250 ◦ C 

in N2 at atmospheric pressure to help sinter the pellet and thus 

form a stable target. The purities of the precursor materials was 

>99.9%. Once the targets were ready they were loaded into the 

reactor. The system was first thoroughly flushed with argon for 

at least 20 min with a flow of 1 slpm. The laser evaporation


Figure 2. (a) TEM overview of a high yield NF sample and (b) complementary EDX analysis. (c) High angle annular dark-field (HAADF) 

image of an NF and catalyst particle residing at its tip. The line indicates the local EDX scan presented in panel (d). 

reaction was run for 10 min. The temperature, gas pressure 

and flow were varied according to the experiment in progress. 

The structure and morphology of the as-grown nanostructures 

were examined using an FEI Tecnai F30 transmission 

electron microscopy (TEM) operating at 300 kV equipped 

with an electron energy dispersive x-ray analyzer (EDX). TEM 

samples were prepared by either directly pressing a small 

amount of sample onto a standard Cu grid with Al foil or 

by dispersing material in ethanol and dropping a little of the 

solution on a Cu TEM grid. 

The IR spectroscopic studies were performed using a 

Bruker IFS 113V/88 spectrometer. In this case the samples 

were dispersed on KBr crystals. Complimentary Raman 

spectroscopic investigation made using a Thermo-Fisher DXR 

Raman spectrometer (excitation lasers: 532/633/780 nm). For 

the Raman investigations a small quantity of material was 

smeared on Al foil. 

3. Results and discussion 

The employed laser evaporation route generally leads to a 

product consisting of amorphous silica nanofibers and binary 

(metal–silicide/Si) nanocrystals. We primarily focus on 

the data obtained with Fe as a catalyst. However, brief 

comparative studies with Ni, Co and Mo are also presented. 

By varying the synthesis parameters (gas pressure, gas flow 

rate and catalyst content) one can tailor the system to yield 

high yield amorphous NF samples (e.g. figure 1(a)) or high 

yield binary nanocrystal samples (e.g. figure 1(b)). The 

temperature throughout these studies remained at 1150 ◦ C. 

Closer examinations of the nanostructures clearly show the 

nanofibers are amorphous as can be seen in figure 1(c). 

3 

Moreover at one end of the amorphous NF a core–shell catalyst 

particle resides and its diameter closely matches that of the 

NF. A typical example is shown in figure 1(d). Lattice 

information derived from the fast Fourier transform (FFT) 

from micrographs and direct lattice fringe measurement of the 

catalyst suggest they are mostly of a pure Fe phase in the core 

with an iron oxide outer layer. In the case of the crystalline 

nanoparticles two phases are observed as shown by the clear 

difference in contrast in the two halves of the nanoparticle 

shown in figure 1(e). Closer examinations of these binary NPs 

show one side of the nanoparticle comprises a pure Si phase 

and the other an iron silicide phase. A typical example is 

presented in figure 1(f) in which the different crystalline phases 

are visibly obvious. FFT studies for each half identified lattice 

planes with spacings of 0.276 and 0.31 nm. These match 

well with (022) and (111) planes of β-FeSi2 and crystalline 

Si, respectively. The mean diameter of the produced NFs, 

depending on the growth conditions, ranges from 10 to 19 nm, 

while their length is of the order of μm. The mean diameter of 

thebinaryNPsvaryfrom25to60nm. 

The purity of the samples was evaluated by EDX. The 

high yield binary NP samples showed primarily Fe and Si 

with traces of O present; the Cu signal arises from the grid 

and the Al from the Al foil used in the transfer process 

(see supplementary information figure S1 available at stacks. 

iop.org/Nano/23/035601/mmedia). However, in the case of 

samples where NFs are present significant oxygen levels are 

detected (see figures 2(a) and (b)). In order to investigate 

this aspect in greater detail local EDX measurements in 

scanning transmission electron microscopy (STEM) mode 

were conducted. The studies showed the amorphous nanofibers 

consist of Si and O with Si:O atomic ratios close to 1:2,


Figure 3. (a) and (b) TEM overview of Co-catalyzed NFs and catalyst particle at NF tip, respectively. (c) and (d) TEM overview of 

Ni-catalyzed NFs and catalyst particle at NF tip, respectively. (e) and (f) Overview and close-up image of binary NPs formed from Mo 

catalyst. All the samples were produced at 1150 ◦ C, with a pressure of 1000 mbar and an Ar flow of 0.5 slpm Ar flow rate. 

suggesting the fibers are amorphous SiO2. With regards to the 

catalyst at the ends of the NFs, the local EDX measurements 

revealed that they consist primarily of the catalyst (Fe) and a 

little O. The oxygen probably arises from the surface oxidation 

of the catalyst particle upon exposure to air. This can explain 

the core–shell structure typically observed on catalyst particles 

at the ends of the NFs (e.g. figure 1(d)). The silicon levels are 

at the noise level indicating the Si content is minuscule or nonexistent. 

Figures 2(c) and (d) show an archetypal example of 

the local studies showing the above-discussed features. 

In addition to exploring Fe as a catalyst, we also 

investigated Ni, Co and Mo catalysts. These catalysts are able 

to form one or more silicide phases with eutectic temperatures 

above 800 ◦ C. These comparative catalyst studies were 

conducted at 1000 mbar with an Ar flow of 0.5 slpm. The 

catalyst content was 2 wt%. These conditions were chosen 

as they correspond to the optimum conditions for high yield 

NF production with Fe catalysts (discussed later). TEM 

4 

analysis showed that the produced NFs catalyzed either by Co 

or Ni were also amorphous with a polycrystalline core–shell 

catalyst particle at their ends (figures 3(a)–(d)). However, the 

Mo-catalyzed samples resulted in the formation of crystalline 

binary NPs (figures 3(e)–(f)). 

Complimentary local STEM-EDX studies (e.g. figure S2 

supplementary information available at stacks.iop.org/Nano/ 

23/035601/mmedia) were conducted. In the case of Ni and 

Co the NFs are found to consist of Si and O as found with Fe 

catalysts. The NFs are produced in high yield, slightly higher 

than for Fe (>80%). Here we refer to yield as the ratio of NFs 

to free NPs. 

We now turn to detailed vibrational spectroscopic and 

systematic statistical studies of the amorphous nanofibers and 

binary nanoparticles formed from the Fe catalyst system. We 

begin with the IR and Raman spectroscopy investigations. 

The infrared spectrum can be used as a fingerprint 

for identification by the comparison of the spectrum of an


Figure 4. Normalized and strapped IR absorption spectra for 

reference SiO2 powder, high yield NFs, reference Si powder and high 

yield binary NPs (from bottom to top). 

unknown sample with that of a reference spectrum. Here we 

compare our high yield binary NP and NF samples with pure 

(powdered) Si and SiO2 samples. The data are presented in 

figure 4. The spectra have been normalized and the background 

strapped. The top two spectra in figure 4 correspond to the high 

yield binary NP sample at the top and the Si reference below. 

The similarities between both are obvious and confirm the 

TEM investigations showing part of the binary NP to comprise 

crystalline Si. The spectra for these two samples show three 

major absorption bands in the range of 400–500, 770–810 and 

1000–1306 cm −1 . These three absorption peaks are ascribed 

to the Si–O–Si rocking vibrational, the Si–O–Si symmetrical 

stretching vibrational mode (SS) and the Si–O asymmetric 

stretching vibrational mode (AS), respectively. In this latter AS 

mode two longitudinal optical (LO)—transverse optical (TO) 

pairs are infrared-active. This splitting is dependent on heat 

treatment [31, 32]. Moreover, they are dependent on the oxide 

layer thickness [33, 34]. The sensitivity of the LO–TO pairs 

to the factors can explain the observed differences between the 

high yield (optimum) binary NP sample and the reference Si 

sample. The oxide modes for both these samples arise from 

surface oxide states. In this spectral region no Si crystal peaks 

are observed because the large absorption coefficient of bulk 

SiO2, even for very thin layers, masks the Si contribution [35]. 

The bottom two spectra in figure 4 correspond to the high yield 

(optimum) amorphous silica NFs and a reference silica sample 

(bottom). Both spectra are remarkably similar and confirm the 

amorphous nanofibers are indeed formed from silica. Since 

the silica samples are now much thicker, as expected the Si–O 

(AS) mode is much stronger relative to the other modes. 

Raman spectroscopy is a powerful and reliable tool for the 

characterization of Si species and can nicely compliment IR 

spectroscopy. In contrast to the IR spectra, here the dominant 

response arises from Si modes. Around 520 cm −1 a strong 

silicon peak is observed for all samples. For discussion we 

present the Raman spectra for the high yield NF and binary 

NP samples and compare them to bulk Si as shown in figure 5. 

Both the Si peaks (518 cm −1 ) for the NP and NF samples are 

5 

Figure 5. Raman spectra for reference bulk Si, high yield binary NPs 

and high yield NFs (from bottom to top). Inset: magnification of the 

Si peaks to illustrate the redshift for the binary NPs and NFs relative 

to bulk Si. 

asymmetrical, broadened and redshifted (by approx. 3 cm −1 ). 

This we attribute to size effects. In addition, the tail of these 

peaks show a weak and broad peak around 300 cm −1 which 

is not present in the signal from the bulk Si sample. This 

additional peak we attribute to a mixture of the tail from the 

asymmetrical Si peak (size effects) and a contribution from 

amorphous SiO2 [16]. 

We now turn to the systematic studies on the effect of the 

synthesis parameters on the binary NP and NF yield, and on 

their mean diameters. The explored parameters were: catalyst 

content, reactor pressure and Ar gas flow rate. The temperature 

was maintained at 1150 ◦ C. The evaluation of yield and mean 

diameter was determined from detailed statistical analysis from 

comprehensive TEM studies. A minimum of 100 particles or 

fibers were studied for each dataset. Figures 6(a) and (b) show 

3D plots of the mean diameter variation with (Fe) catalyst 

content and buffer gas pressure for the silica nanofibers and 

binary nanoparticles, respectively. The gas flow rate for 

these studies was 0.5 slpm. Comparing the two graphs, the 

most obvious differences are that the NPs are always larger 

than the NF diameters. In addition, binary NPs are always 

obtained regardless of the reaction conditions, whereas with 

low pressures (250 mbar) almost no or no NFs are obtained 

independent of the catalyst content. At pressures below 

1000 mbar, very few NFs are obtained with high catalyst 

contents. As a general behavior, both datasets show increasing 

diameters with increasing gas pressure and catalyst content. 

The maximum mean diameter of the NFs was 19 nm and 58 nm 

for the binary NPs. 

A reduction in their mean diameter is observed with 

increasing Ar flow rate. Figure 6(c) shows this trend when


Figure 6. (a) and (b) Mean diameter dependence for NFs and binary NPs with respect to Fe content and pressure at a temperature of 1150 ◦ C 

and an Ar flow rate of 0.5 slpm. (c) Mean diameter dependence for NFs and binary NPs with respect to Ar flow rate (temperature = 1150 ◦ C, 

catalyst content 2 wt%). 

Figure 7. (a) Relative yield of amorphous NFs to binary NPs with respect to catalyst content and reaction pressure (temperature = 1150 ◦ C, 

Ar flow = 0.5 slpm). (b) Relative yield with respect to Ar flow rate (temperature = 1150 ◦ C, pressure = 1000 mbar). 

using a catalyst content of 2 wt% and an Ar pressure of 

1000 mbar. It is also easy to see from figure 6 the NPs are 

always larger than the NFs. One can also tailor the mean 

diameter of the NFs and binary NPs with temperature, to some 

degree (data not shown). 

The relative yield of NFs to binary NPs against (Fe) 

catalyst content and pressure is plotted in figure 7(a). The 

3D graph clearly highlights the best silica nanofiber yields 

are obtained with lower catalyst content (ideally 2 wt%) and 

higher reaction pressures (ideally 1000 mbar). Figure 7(b) 

shows the relative (NF/NP) yield with flow rate. The behavior 

is non-monotonic and shows an optimum flow occurs around 

6 

0.5 slpm. Temperature studies (not shown) show a decrease in 

yield as the temperature falls below 1000 ◦ C. 

The formation of the silica NFs and binary nanoparticles 

can be expected to form through the vapor–liquid–solid (VLS) 

mechanism first introduced by Wagner and Ellis to explain Si 

nanowire formation [36]. This is because a catalyst particle 

resides at an end of each fiber. Moreover the VLS mechanism 

is well accepted as the growth mechanism for Si nanowire 

formation using laser evaporation, similar to that used here. 

In the VLS mechanism when the laser pulse strikes the target 

(Si plus catalyst), surface material is evaporated and ejected 

into the reactor in the form of a plume. As the plume expands


and cools the material condenses, forming catalyst particles 

supersaturated with Si. As the system cools further Si begins 

to precipitate and a nanowire or nanofiber is extruded from the 

particle. It is obvious from this that the size of the catalyst 

particle has a direct influence on the nanowire or fiber diameter. 

It is not so apparent what driving mechanism differentiates 

the formation of a crystalline Si nanowire and an amorphous 

Si fiber. In our case we form amorphous Si fibers, even 

though our synthesis conditions seemingly closely match those 

from others where Si nanowires have been obtained [37, 38]. 

Aharonovich et al also obtained amorphous silica NFs via laser 

evaporation: however, they were unable to determine why they 

were amorphous and where the oxygen came from [39]. They 

were not sure if the amorphous silica formation occurred in 

the reactor or outside once the fibers were exposed to air. 

In our case we have the simultaneous formation of binary 

nanoparticles in which one part is crystalline iron silicide and 

the other is crystalline Si. This tells us the oxygen is not in the 

reaction, as we would expect. This means the laser evaporation 

reaction forms Si NFs which then oxidize upon exposure to 

the air. This is concomitant with the surface oxidation of the 

catalyst particles residing at their tips forming a core–shell 

structure. 

4. Conclusion 

The products from pulsed-laser evaporation of Si targets loaded 

with transition metals are investigated in detail. The reactions 

yield products containing amorphous silica nanofibers and 

binary crystalline iron silicide/silicon nanoparticles. By 

adjusting the reaction parameters one can tailor the product 

for either high yield silica NFs or high yield binary NPs. The 

studies show the oxidation of the fibers occurs upon exposure 

to the atmosphere. 

Acknowledgments 

MD acknowledges the Deutscher Akademischer Austausch 

Dienst (DAAD), SG thanks the ‘Pakt für Forschung und 

Innovation’ and MR thanks the European Union (ECEMP) and 

the Freistaat Sachsen for financial support. We are grateful to 

Alicja Bachmatiuk, R Schönfelder, M Ulbrich, S Leger and 

F Herold for technical assistance and help. 

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