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Fabrication of Amorphous Silver Nanowires by Helium Ion Beam Irradiation

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Chinese Physics Letters
Authors:
Shehla Honey at University of Okara
Shehla Honey
  • University of Okara
Aamir Ali at University of Sargodha
Aamir Ali
Sidiki Zongo at iThemba Labs - Laboratory for Accelerator Based Science
Sidiki Zongo
I. Javed
I. Javed
I. Javed
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Abstract

Amorphous silver nanowires (a-Ag NWs) are fabricated from crystalline Ag NWs by using 5MeV helium (He+) ion beam irradiation. At low dose (5 × 1015 ion/cm2), few defects are created in Ag NWs. As dose increases, more damage to the crystalline structure of Ag NWs is observed. Finally at high dose (8 × 1016 ion/cm2), the face-centered cubic structure of Ag NWs is transformed into the amorphous structure with similar morphology as Ag NWs. Phase transformation of crystalline Ag NWs upon irradiation with 5MeV He+ ions is observed through high resolution transmission electron microscopy. Synthesis of large scale amorphous metal nanowires and metal nanowire alloy systems are discussed.
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Fabrication of Amorphous Silver Nanowires by Helium Ion Beam Irradiation
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2015 Chinese Phys. Lett. 32 096101
(http://iopscience.iop.org/0256-307X/32/9/096101)
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CHIN. PHYS. LETT. Vol. 32, No. 9 (2015) 096101
Fabrication of Amorphous Silver Nanowires by Helium Ion Beam Irradiation
Shehla H.1,2,3,4, Ali A.1
,Zongo S.3,4
,Javed I.5
,Ishaq A.1,3,4**
,Khizar H.6
,Naseem S.2
,Maaza M.3,4
1National Center for Physics, Quaid-i-Azam University, Islamabad 44000, Pakistan
2Center of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan
3UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies,
University of South Africa, Muckleneuk ridge, Pretoria-South Africa
4Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, 1 OldFaure road,
Somerset West 7129, South Africa
5Laboratory of Nanoscience and Technology, Department of Physics,
International Islamic University Islamabad, Pakistan
6Department of Physics, Mirpur University of Science and Technology, Mirpur, Azad Jammu & Kashmire
(Received 25 May 2015)
Amorphous silver nanowires (a-Ag NWs) are fabricated from crystalline Ag NWs by using 5 MeV helium (He+)
ion beam irradiation. At low dose (5×1015 ion/cm2), few defects are created in Ag NWs. As dose increases,
more damage to the crystalline structure of Ag NWs is observed. Finally at high dose (8×1016 ion/cm2), the
face-centered cubic structure of Ag NWs is transformed into the amorphous structure with similar morphology as
Ag NWs. Phase transformation of crystalline Ag NWs upon irradiation with 5 MeV He+ions is observed through
high resolution transmission electron microscopy. Synthesis of large scale amorphous metal nanowires and metal
nanowire alloy systems are discussed.
PACS: 61.80.−x,61.46.Km DOI: 10.1088/0256-307X/32/9/096101
Low-dimensional metal nanowires have gained
much attention due to their novel properties, and have
been widely applied in areas such as optoelectronic
devices including photovoltaic systems.[1−3]Recently,
another class of 0D, 1D and 2D materials, namely
amorphous nanowires, has attracted a great deal of
attention, due to their unique magnetic, corrosion
resistance catalytic and electrical properties for new
emerging applications.[4,5]Therefore, there is an in-
terest in exploring the methods of their synthesis and
properties.
Several methods including liquid metal micro
drops,[6]pulse sonoelectrochemical technique and ion
irradiation have been used to synthesize amorphous
metals.[7−9]Li et al. have synthesized the amor-
phous CoPt nanowires through direct current electro-
deposition.[10]The synthesis of amorphous metal
nanowires and their assembly into well-defined struc-
tures is currently a major challenge.
In this Letter, we report the result of success-
ful synthesis of amorphous silver nanowires using the
ion beam method. This method will also be applica-
ble to other metal nanowires or metal alloy systems.
Moreover, amorphous nanowires assembly into a well-
defined structure could be solved and is discussed here
also. In this method we irradiate silver nanowires (Ag
NWs) with He+ions in a controlled manner. The
ion beam induced phase transition from crystalline to
amorphous is therefore studied.
The silver nanowires used in this work were pur-
chased from the Advance Chemical Suppliers (ACS)
Material (Product ID: Agnws-120). The average di-
ameter of the Ag NWs was comprised between 120nm
and 20 µm in length. Ag NWs were dispersed into
a copper grid and thereafter irradiated with He+
beam at different doses varying from 1×1015 to
8×1016 ion/cm2in a 5UDH-Pelletron accelerator.
He+ion beam irradiation energy, current and sub-
strate temperature were 5 MeV, 50nA and room tem-
perature, respectively. A TRIM code was utilized to
avoid any implantation of ions into the nanowires.[11]
The structure and morphology of both un-irradiated
and irradiated Ag NWs were characterized by using
scanning electron microscopy (SEM) and high resolu-
tion transmission electron microscopy (HRTEM).
The SEM and HRTEM images of the as-grown Ag
NWs (un-irradiated) are shown in Figs. 1(a) and 1(b),
respectively. It can be seen that the NWs are highly
crystalline. From Fig. 2, it can be observed that a
growth of amorphous clusters on Ag NWs is due to
the loss of crystalline ordering induced by ion beam
after He+ion beam bombardment at the fluence of
5×1015 ion/cm2, whereas the crystalline ordering de-
creases with increasing the irradiation dose, no mor-
phological change of Ag NWs was observed. How-
ever, the crystalline face-centered cubic (FCC) struc-
ture was transformed into an amorphous structure.
By controlling the irradiation beam current at low in-
tensity (≃50 nA), the He+beam induced heating was
minimized and no distortion of the Ag NWs morphol-
ogy was observed. This indicates that the low current
is an important factor in transforming Ag NWs into
an amorphous structure without distorting their mor-
phology. Recently, we reported the damage of the Ag
NWs with C ions under high current and found it to
change the morphology of the NWs.[13]Slicing, cut-
ting of Ag NWs was also observed under high C ion
current.[13]With a further increase in the irradiation
**Corresponding author. Email: ishaq@ncp.edu.pk
©2015 Chinese Physical Society and IOP Publishing Ltd
096101-1
CHIN. PHYS. LETT. Vol. 32, No. 9 (2015) 096101
dose to 1×1016 ion/cm2, the amorphous grains be-
come larger and the crystalline phase is reduced as
shown in Fig. 3.
200
nm 10
nm
(a) (b)
Fig. 1. The un-irradiation silver nanowires: (a) morphol-
ogy, (b) structure. Inset: the SAED image showing the
quality of crystal structure.
10
nm
140
nm
Fig. 2. He+ion beam irradiation at the dose of 5×
1015 ion/cm2. Amorphouse clusters are formed on the
Ag NWs lattice (green circles). Inset: the SAED image
showing the quality of crystal structure after ion beam
irradiation.
5
nm
100
nm
Fig. 3. He+ion beam irradiation at the dose of 1×
1016 ion/cm2. Amorphous clusters start to grow to form
larger amorphous grains.
Figure 4shows that amorphous structure contin-
uously grows under He+ion beam irradiation. The
crystallinity decreases and amorphous phase is dom-
inated. There are nano-crystallites left behind and
most of the nanowires become amorphous. This
appears as an alternate and indirect process for
nanocrystals formation in an amorphous matrix. The
appearance of the amorphous phase or defective Ag
NWs may originate from a direct knock-on atom dis-
placement. Due to the low irradiation beam current,
the irradiative heating could be excluded as being
responsible for the phase changes. The amorphiza-
tion process under He+ion bombardment at very
low current was almost instantaneous, suggesting that
the irradiation induced local heating does not con-
tribute to the amorphous phase transformation. In
our previous in situ experiment on ZnO NWs, a high
current focused electron beam was utilized to create
nanoholes in ZnO NWs due to local irradiation in-
duced heating.[18]Moreover, it was observed that at
a high electron current, the crystal structure remains
stable around the nanohole.[18]Similarly, coalescence
of Ag NWs was observed recently by proton beam
induced local heating produced into Ag NWs while
the crystal structure remained stable.[19]Ion beam in-
duced local heating causes rapid restructuring of Ag
NWs and finally becomes a crystalline structure while
low current irradiation causes damage in the Ag NW
system. Therefore, a low He+ion current was uti-
lized to avoid local heating for amorphization of Ag
NWs. With a further increase in irradiation dose of
8×1016 ion/cm2, the amorphous structure continu-
ously grows, leading to a complete transformation of
the FCC Ag NW structure into the amorphous struc-
ture as shown in Fig. 5. The structure transforma-
tion under He ion beam irradiation is a typical non-
equilibrium process where a thermodynamically stable
Ag NW structure was continuously transformed into
a meta-stable structure.[14,15]
5
nm
100
nm
Fig. 4. He+ion beam irradiation at the dose of 5×
1016 ion/cm2. Few crystalline planes exist and the rest
of the NWs became amorphous. Inset: the SAED image
showing the quality of crystal structure after ion beam
irradiation.
100
nm
5
nm
Fig. 5. TEM image of the ZnO NWs after irradiation with
H+beam with an intensity of 8×1016 ion/cm2. Inset: the
SAED image confirming the amorphous phase of Ag NWs
after He+ion beam bombardment.
Medium dose He+ ion irradiation
Amorphous structure
Crystalline structure
Low dose He+ ion irradiation
High dose He+ ion irradiation
Un
-
irradiated Ag
-
NWs
Fig. 6. Schematic diagram showing the phase transform
of Ag NWs to an amorphous one.
096101-2
CHIN. PHYS. LETT. Vol. 32, No. 9 (2015) 096101
Based on the above-mentioned facts, two basic
mechanisms can be proposed for the Ag NWs phase
transformation under the irradiation process. The
first mechanism could be related to the bond-breaking
process in which the irradiation process induces free
atoms set near to the surface. These atoms are there-
fore agglomerated, leading to the formation of amor-
phous structures on the NW and through an ion beam
induced Ag atom migration and re-absorption process
on the surface and inside NWs, amorphous clusters
are formed. Secondly, interstitial defects in the lattice
are produced since the high energy He+ions tend to
break the bonds in the lattice. Thus nano amorphous
grains are formed within the Ag NWs. The growth of
amorphous grains after increasing He+ion beam irra-
diation at the dose of 8×1016 ion/cm2will transform
the whole crystalline NW phase into an amorphous
phase. Another possible reason for ion beam induced
amorphization is collision cascade of atomic displace-
ments, causing the creation of dangling bonds.[10]This
was attributed by Terrones et al. to the removal of
the carbon atoms from carbon nanotubes by knock-
on displacements.[16]In the present work, a similar
explanation can be proposed for Ag NWs. The He+
ion beam displaced the Ag atoms in the Ag NWs or
can either migrate as interstitials into the NWs. At a
high He+ion dose, the incident ions create vacancies
in the Ag system due to the collision cascade effect
causing instability in the Ag NWs structure. This fi-
nally leads to the change of the structure crystalline
phase into the amorphous phase. The model in Fig. 6
shows the amorphization process of Ag NWs. At a
low irradiation dose, few amorphous zones are created
as shown in Fig. 6as black dotted circles. At medium
dose, amorphous zones grow into a larger one and fi-
nally at higher dose, whole nanowires are transformed
into an amorphous structure. Moreover, a similar ex-
periment was carried out by us in ZnO NWs by using
a proton beam and the amorphous phase transforma-
tion is observed.[17]Similar experiments could be per-
formed on other metal nanowires or metal alloys to
transform the whole system into an amorphous one
for future advance applications.
In summary, ion beam induced amorphization of
Ag NWs has been successfully achieved under He+
ion beam irradiation. It is concluded that only a low
current of He+ion beams is suitable for amorphiza-
tion Ag NWs while a high current may cause slicing
or cutting of NWs. Collision cascade effects are the
main contribution for amorphization of Ag NWs as
shown in Fig. 6. The major effect of the He+irradia-
tion is found to originate from displacement damage
by direct or indirect knock-on collisions.
We are grateful to iThemba LABS, UNESCO and
TWAS.
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096101-3
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Metallic nanowires have potential applications in miniaturized metallic interconnects, sensors and heat sink materials. In this work, highly aligned amorphous copper nanowires have been prepared by electrodeposition technique using anodized aluminium oxide (AAO) templates at room temperature. FESEM analysis shows that the as-deposited nanowires are parallel to one another and have high aspect ratio with a reasonably high pore-filing factor. The electrolyte used, consisted of a bath solution containing CuCl 2 .6H 2 O buffered with H 3 BO 3 and acidified by dilute H 2 SO 4. To fabricate the working electrode, a thin film of aluminum (~ 152 nm thick) was coated on back side of AAO template by e-beam evaporation system to create electrical contact. During the electrodeposition process, the copper ions were guided to move through nano-pores where they were reduced to form one dimensional structure of pure copper nanowires. The lengths of deposited nanowires could be controlled by deposition time, applied voltage, composition of precursors. The electrodeposited nanowires have diameter varying from 15-200 nm. TEM results show that the as-deposited nanowires were dominantly amorphous with small portion of crystalline structure.
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MeV Carbon Ion Irradiation-Induced changes in the electrical conductivity of Silver Nanowire networks
Article
  • Mar 2015
  • CURR APPL PHYS
MeV carbon ion irradiation-induced changes in the electrical conductivity of Silver nanowire (Ag-NW) networks is demonstrated systematically at different C+ ion fluences ranging from 1 x 1012 to 1 x 1016 ions/cm2 at room temperature. At low C+ ion fluences, the electrical conductivity of Ag-NWs decreases and subsequently increases with increase fluence. Finally, at high C+ ion fluences, conductivity again decreases. The variation in the electrical conductivity of Ag NW network is discussed after analysis using scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The observed increase in electrical conductivity is thought to be due to ion induced coalescence of Ag-NWs at contact position, which causes reduction of wire–wire contact resistance, while the decrease in electrical conductivity may be due to defect production by C+ ions into Ag-NWs. Ion beam technology is therefore a very promising technology that is capable of fabricating highly conductive Ag-NW networks for transparent electrodes. Moreover, a method for thinning, slicing and cutting of Ag-NWs using ion beam technology is also reported.
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AMORPHIZATION OF ZnO NANOWIRES BY PROTON BEAM IRRADIATION
Article
  • Nov 2011
We report the effects of 70 keV proton (H+) irradiation on the structure of zinc oxide nanowires (ZnO NWs) for a wide range of irradiation doses at room temperature. It was found that at low dose 5 × 1015 ions/cm2 of protons, few defects were created in ZnO NWs and the defects' density was increased with an increasing proton irradiation dose. After the irradiation dose was increased to 2 × 1017 ions/cm2, the crystal structure of the ZnO NWs was almost completely damaged and the crystalline wurtzite structure of the ZnO NWs could be transformed into a disordered amorphous structure. Structural changes in the ZnO NWs upon bombardment with 70 keV protons were characterized by high resolution transmission electron microscopy (HRTEM).
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Use of high-intensity electron beam to form nanohole, induce bending and fabricate nanocontact on a ZnO nanowire
Article
This Letter presents the experiment of using a high-intensity electron beam to form nanohole, induce bending and fabricate nanocontact on a ZnO nanowire (NW). This study reveals that a nanohole with diameter of not more than 10 nm could easily been created in seconds. A lateral half-cutting on the ZnO NW could induce bending. Nanocontact of the NW could also been achieved by performing a control quasi-sectioning of NW. The electron beam performs rapid localised surface melting and vaporisation in ZnO NW. These processes could possibly be implemented for the fabrication of NW-based devices for the applications of single electron devices and also NW cutting, alignment and carving.
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New technique of amorphous metal nanoparticles production by liquid drop breakup in an electron beam
Article
  • Jun 2012
We report a new technique of amorphous metal nanoparticles fabrication based on generation of liquid metal microdrops, their charging in electron flux with subsequent drops fission. Fission of charged metal drop occurs when its charge exceeds a critical value given by Rayleigh instability condition. Nanosize droplets, resulted from fission, cool down at extremely high rate to produce amorphous metal nanoparticles. Initial metal microdrops are generated at the end of an anode tip exposed to strong electric field and electron flux focused to the tip. The emitted drops then enter an electron beam where they are charged up to instability and are involved in fission process. To eliminate secondary electron emission (SEE) which can prevent drop charging we used electron beams of low energy. With beam energy properly chosen, SEE coefficient is less then unity for all drop sizes and drop charging up to unstable state is ensured. Granular films consisting of amorphous copper particles 2 nm in size are fabricated by the described technique. The results of atomic force microscopy (AFM) and transmission electron microscopy (TEM) studies of the fabricated films are presented. Possible applications of the fabricated structures related to their unusual properties are considered.
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Aligned Silver Nanorod Arrays Produce High Sensitivity Surface-Enhanced Raman Spectroscopy Substrates
Article
  • Jul 2005
Substrates consisting of silver nanorod arrays with an irregular surface lattice (i.e., random nucleation sites) and with varying rod lengths were fabricated by an oblique angle vapor deposition method. These arrays were evaluated as potential surface-enhanced Raman spectroscopy (SERS) substrates using trans-1,2-bis(4-pyridyl)ethene as a reported molecule. SERS activity was shown to depend upon the length of the nanorods. The Ag nanorods with average lengths of 508.29±44.86 nm, and having aspect ratios of 5.69±1.49 exhibited the maximum SERS enhancement factors of greater than 108. Theoretical calculations indicate that this large SERS enhancement may be partially explained by the shape, density, and lateral arrangement of the Ag nanorod arrays.
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Radiation-Induced Transformation of Graphite to Diamond
Article
  • Nov 1997
It is demonstrated theoretically that particle irradiation may lead to a destabilization of graphitic structures with respect to low-pressure growth of diamond. This is due to the large difference in the cross sections for irradiation-induced displacements of carbon atoms in diamond and graphite. A nonequilibrium phase diagram is calculated that shows the stability of graphite and diamond as a function of the displacement rate of atoms. The theoretical results are related to the experimentally observed transformation of spherical graphitic onions to diamond under electron irradiation.
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Graphite-to-amorphous structural transformation of multiwalled carbon nanotubes under proton beam irradiation
Article
  • Jul 2009
  • MATER LETT
The graphite-to-amorphous structural transformation of multiwalled carbon nanotubes (MWCNTs) was investigated under 70 keV proton beam irradiation at room temperature. It was found that under proton irradiation some amorphous structure homogeneously covers the inner tube walls with graphite structure in irradiated MWCNTs. Moreover, the amorphous structure continuously proceeds and the graphite structure is reduced during the proton irradiation until the irradiated MWCNTs become amorphous nanowires with a hollow structure. The proton irradiation induced structural transformation of MWCNTs was a unique graphite-to-amorphous structural transition from the outer walls to the inner walls of the irradiated MWCNTs. The structural evolutionary mechanism of proton-beam-irradiated MWCNTs has been discussed.
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Fabrication and magnetic properties of amorphous CoPt nanowire arrays
Article
  • Nov 2004
  • SOLID STATE COMMUN
Highly ordered Co0.71Pt0.29 alloy nanowire arrays have been fabricated successfully by direct current electro-deposition into the pores of a porous anodic aluminum oxide (AAO) template. SEM and TEM images reveal that the nanowires of array are uniform, well isolated, and parallel to one another. The aspect ratio of nanowires is over 200. XRD and EDS pattern indicates that amorphous Co0.71Pt0.29 structure was formed during electro-deposition. In amorphous sample, magnetocrystal anisotropy is very small, therefore, shape anisotropy plays a dominant role which leads to strong perpendicular anisotropy. High coercivity (Hc=1.7 kOe) and squareness (Mr/Ms) around 0.7 were obtained in the samples when the field was applied parallel to the axis of the nanowires. However, when it changed to polycrystalline structure after annealing, due to the competition of magnetocrystal anisotropy and shape anisotropy, the sample did not display perpendicular anisotropy.
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