Low Temperature Formation of Silver and Silver-Copper Alloy Nano-Particles Using Plasma Enhanced Hydrogenation and Their Optical Properties

doi:10.4236/wjnse.2012.23018

Paper Menu >>

Journal Menu >>

World Journal of Nano Science and Engineering, 2012, 2, 142-147

http://dx.doi.org/10.4236/wjnse.2012.23018 Published Online September 2012 (http://www.SciRP.org/journal/wjnse)

Low Temperature Formation of Silver and Silver-Copper

Alloy Nano-Particles Using Plasma Enhanced

Hydrogenation and Their Optical Properties

Zeinab Kiani, Yaser Abdi, Ezatollah Arzi*

Nanophysics Laboratory, Department of Physics, University of Tehran, Tehran, Iran

Email: *[email protected]

Received November 22, 2011; revised January 12, 2012; accepted May 23, 2012

ABSTRACT

In this paper, a novel method of producing nanoparticles at low temperatures using hydrogen bombardment of thin

films, deposited on glass substrates, is introduced. Silver nanoparticles were obtained by this method in our Plasma En-

hanced Chemical Vapor Deposition system. Optical and morphological characteristics of these nanoparticles were ex-

tensively studied for various conditions of plasma treatment, such as plasma power density, temperature, duration of

hydrogen bombardment, thickness of the initial thin metallic film etc. In addition, Ag-Cu alloy nanoparticles on glass

substrates were also achieved. The process of nanoparticle formation in this method shows that several kinds of metals

and semiconductors nanoparticles can be obtained using this approach. Scanning Electron Microscopy, Atomic Force

Microscopy and Transmission Electron Microscopy were used to analyze the nanostructures.

Keywords: Nanoparticles; Plasma; Hydrogen; PECVD; Silver; Alloy

1. Introduction

Because of their linear and nonlinear optical properties,

metal nanoparticles have been widely studied in recent

years. They are suitable candidates for various devices

because of their unique optical [1], electronic [2], cataly-

sis [3], chemical [4] properties. Many different methods

were applied for producing nanoparticles such as chemi-

cal synthesis [5], exploding wire [6], and laser ablation

[7]. We have used a novel different method that produces

nanoparticles at a temperature much lower than the

melting point of their respective bulk using hydrogen

plasma in Enhanced Chemical Vapor Deposition system.

In this work, silver nanoparticles and bimetallic alloy of

silver-copper nanoparticles were produced by this plasma

treatment method. This method can also be used for the

formation of other metallic or semiconductors nanoparti-

cles by PVD method on glass or silicon or other sub-

strates, out of which two examples of nickel and silicon

nanoparticles are presented.

Optical properties of metal nanoparticles are related to

the excitation of surface plasmon. The parameters such

as size and shape [8,9], surrounding medium [10], metal-

lic species [11] of nanoparticles change the excitation

energy of surface plasmons. We have investigated the

effects of size on the absorption spectra of the nanoparti-

cles.

2. Experimental

The work started by cleaning the glass substrate in the

standard RCA1 solution, which contained NH3/

H2O2/DI-H2O with relative volume proportions of 1:1:5,

and followed by the deposition of a thin layer of silver

with the thickness of about 10 nm in a thermal evapora-

tion system at the base pressure of 5 × 10−6 torr [Forma-

tion of bimetallic alloy nanoparticles needs to evaporate

two metallic sources simultaneously in the physical va-

por deposition (PVD) system]. The silver-coated glass

was then put in the direct current plasma enhanced

chemical vapor deposition (DC-PECVD) apparatus, as

shown schematically in Figure 1. This DC-PECVD

setup has two flat electrodes, made of iron, which hy-

drogen plasma forms between them by the application of

a voltage between the electrodes. The sample was placed

on the cathode (lower electrode) and during the plasma

treatment was bombarded by hydrogen radicals. We

made an electrical connection between the metallic layer

and the cathode to prevent the build up of positive

charges on glass substrate that might repel the plasma.

The substrate temperature was varied between 200˚C and

400˚C and the hydrogen pressure was maintained at 0.4 -

2 torr during the Plasma treatment in the DC-PECVD

*Corresponding author.

Z. KIANI ET AL. 143

Figure 1. Schematic diagram of DC-PECVD chamber showing tw o electrodes being conne cted to a voltage supplier. The rest

of the apparatus including the heater, the temperature control, the inlets and the outlets etc are not shown.

system. The flow rate of hydrogen was kept between 50

to 70 Sccm. The power density of the DC-PECVD reac-

tor during the hydrogen bombardment step was varied

between 100 and 800 mW/cm2. The subsequent anneal-

ing step was conducted in situ at a substrate temperature

of 50˚C higher than what had been used for the bom-

bardment step. These successive plasma treatment/an-

nealing steps were carried out for each sample. Based on

the investigation conducted in this study, the bombard-

ment step served to etch the deposited layer, forming

nano-clusters and nucleation sites for subsequent forma-

tion of the nanoparticles. However, it was observed that

if the duration of the hydrogen bombardment step ex-

ceeded a certain time, the entire deposited layer was re-

moved. During the annealing period, the hydrogen

trapped in the layer was ejected out and some energy was

imparted to the nano-structures, enhancing the chance of

formation of the nanoparticles. It should be mentioned

that this step has been performed in the absence of hy-

drogen plasma. On the other hand, it has been observed

that if the annealing step is carried out for longer periods,

the small grains of nanoparticles merge, forming larger

grains. The sequence of consecutive hydrogen bom-

bardment and annealing steps has been found to be an

optimal condition for the evolution of nanoparticles.

Duration and temperature of annealing and bombard-

ment steps, plasma power, and the thickness of the initial

thin metallic film influence the size of nanoparticles. We

have investigated the effects of such parameters on

nanoparticles size and the absorption spectra. Scanning

electron microscope and atomic force microscope were

used to analyze the surface morphology of the as pre-

pared samples.

3. Results and Discussion

Figure 2 includes four atomic force microscope (AFM)

and two scanning electron microscope (SEM) images of

as-prepared silver and silver-copper nanoparticles dem-

onstrating surface morphology and nanostructures of the

samples prepared at different condition of nanoparticles

formation. As shown in this figure well-isolated nanopar-

ticles with different diameter size were obtained. The

preparation condition and the final average size of each

sample are given in Table 1. The thickness of the initial

layer for Ag samples (a-d) was about 10 nm and for

Ag/Cu alloy samples (e & f) was about 16 nm, which

contained 70% Cu and 30% Ag by weight. During the

plasma treatment the deposited layer was transformed to

nano-islands. This reshaping (from flat layer to spherical

particles) leads to have nanoparticles with diameters lar-

ger than the layer thickness. It is worth mentioning that

the total volumes of the deposited material before and

after the plasma treatment are the same.

For investigating the effects of temperature, plasma

power and other parameters of plasma treatment on the

structure and optical properties of nanoparticles, we have

prepared quite a number of samples, the results of which

are as follows. Figure 3 shows AFM images and absorp-

tion spectra of two Ag samples prepared at different

temperatures of bombardment step, i.e. 300˚C and 380˚C.

It can be clearly seen that the higher the temperature, the

larger the nanoparticles. In addition, Figure 3(c) shows

that higher temperature shifts absorption spectra peak

towards longer wavelengths. It is worth mentioning that

the duration and plasma power density have been main-

tained at 20 minutes and 100 mW/cm2 during the prepa-

ration step.

Z. KIANI ET AL.

144

Figure 4 is a comparison between the samples prepared

at two different plasma power densities of 100 and 600

mW/cm2. As shown in this figure, at lower plasma power

density during the bombardment, smaller nanoparticles

Figure 2. AFM and SEM images of silver nanoparticles

(a)-(d) and Ag/Cu alloy samples ((e) & (f)) formed on glass

substrates using hydrogen plasma treatment, showing that

different conditions of sample preparation lead to different

sizes of nanoparticles.

are achieved and correspondingly, the absorption peak

appears at a wavelength shorter than that of the sample

prepared at higher plasma power density. Thus, we see

that changes in morphology and sizing of nanoparticles

caused by temperature and plasma power density lead to

variations in their optical properties. It seems that smaller

particles have their absorption peak at shorter wave-

lengths range. We have also investigated the effects of

other ranges of plasma power density on our samples. It

is observed that raising the power density to values above

800 mW/cm2 will lead to total etching of the deposited

layer. On the other hand, no grain formation was ob-

served at power densities below 100 mW/cm2, as con-

firmed by SEM and AFM results.

Comparison between two samples, one of them bom-

barded for 20 minutes and the other one for 45 minutes

but both at the same temperature of 300˚C with the same

plasma power density of 150 mW/cm2, is represented in

Figure 5. This comparison demonstrates that longer du-

ration of bombardment results in smaller nanoparticles

and hence the absorption spectrum gets blue shifted.

Further experiments revealed that the thickness of the

initial deposited layer is very important. It is found that

increasing the layer thickness above 50 nm leads to ob-

serve no grain formation. Figure 6 shows SEM images

of three samples having silver layer thickness of 70, 40

and 10 nm prepared at the same condition of plasma

treatment and annealing step, confirming the above men-

tioned conclusion. In addition, Figure 6(d) shows ab-

sorption spectra of these samples showing no absorption

peak in the spectrum of the sample with 70 nm initial

layer thickness.

We have used this approach not only to produce silver

nanoparticles, but also for other materials such as nickel

and silicon nanoparticles, their SEM images are shown in

Figure 7. Thus we believe that this method is suitable for

producing nanoparticles of various metallic and semi-

conductor materials.

Table 1. Sample preparation conditions for Figure 2.

Bombardment condition

Average particle size (nm) Plasma power density (mW/cm2)

Temperature (˚C) Duration (minutes)

Sample

75 200 300 15 a

50 180 300 20 b

65 230 300 20

70 300 380 20 d

145 120 220 20 e

55 130 300 15 f

It was then annealed at 400˚C for 20 minutes.

Z. KIANI ET AL. 145

(a) (b)

(c)

Figure 3. AFM images of two Ag samples prepared at dif-

ferent temperature of the bombardment step, (a) 300˚C and

(b) 380˚C; (c) Absorption spectra of the two above-mention-

ed samples.

(a)

(b)

Figure 4. (a) SEM images of the above-mentioned samples,

formed at plasma power density of 100 mW/cm2 (left) and

600 mW/cm2 (right). These SEM images and the corre-

sponding absorption spectra show that lower power density

of plasma during bombardment leads to the formation of

smaller nanoparticles; (b) Absorption spectra of the Ag

samples that have been formed at the plasma power density

of 100 and 650 mW/cm2.

Figure 5. Absorption spectra of samples having bombarded

for different periods: 20 and 45 minutes. Comparison be-

tween them shows that longer time of bombardment leads

to form smaller nanoparticles.

(a) (b) (c)

(d)

Figure 6. SEM images of silver samples prepared on silicon

substrates at the same condition of bombardment treat-

ment, except that the initial thickness of deposited layer was

(a) 10 nm; (b) 40 nm and (c) 70 nm; (d) Absorption spectra

of the above mentioned samples.

(a) (b)

Figure 7. SEM images of (a) semiconductor silicon nano-

particles; (b) metallic nickel nanoparticles, produced by the

proposed hydrogen tre atment method.

Z. KIANI ET AL.

146

To form alloy nanoparticles, silver and copper were

simultaneously evaporated by PVD system as two com-

ponents of bimetallic alloy on glass substrate. AFM im-

ages of plasma treated silver/copper nanoparticles are

represented in Figure 8. Figure 9 shows TEM images of

silver/copper alloy nanoparticles that were produced by

this novel hydrogen bombardment method, which clearly

shows the formation of alloy nanoparticles. By looking

carefully at this image, one can see that some parts of

some grains in the TEM image are darker, which means

that these grains contain the two component materials

and thus the alloy of silver/copper was formed in this

sample. Therefore, the method is capable of producing

alloys. Another method for depicting the structure is the

optical analysis [12]. Optical results of the as-pre- pared

samples depict two different structures for these nanos-

tructures. Figure 10 shows the absorption spectra of two

Ag/Cu nanoparticles prepared at two different tempera-

tures of 200˚C and 300˚C during the bombardment step.

As shown in this figure, the sample prepared at the lower

temperature has two isolated peaks corresponding to the

absorption peaks of pure silver and pure copper nanopar-

ticles whilst the other one (dashed curve) has a single

(a) (b)

Figure 8. AFM images of Ag/Cu alloy nanoparticles with

initial layer thickness of about 15 nm that have been pre-

pared in different condition, (a) contains 30% Ag and 70%

Cu by weight, bombarded for 20 minutes at plasma power

density of 100 mW/cm2 at 300˚C and (b) contains 50% Ag

and 50% Cu by weight, bombarded for 20 minutes at

plasma power density of 140 mW/cm2 at 370˚C.

Figure 9. TEM images of silver/copper alloy nanoparticles

showing that the grains were formed by this method. The

darker color of some parts of the grains shows that the

grains contain more than one material, and thus the sil-

ver/copper alloy nanoparticles were formed.

peak at a wavelength between the two above-mentioned

peaks. It depicts that the sample with two peaks is just a

mixture of Ag nanoparticles and Cu nanoparticles but the

other one is an alloy of Ag/Cu nanoparticles. Figure 11

shows the absorption spectra of the alloys with different

mixing ratio. As shown in this figure, shifting the ab-

sorption peak towards the blue or red regions depends on

the mass percentage of silver or copper components in

the alloy.

Figure 10. Absorption spectra of samples having different

temperature in their bombardment steps. The sample that

exhibits two peaks in its absorption spectrum (solid line),

bombarded at 200˚C, shows the formation of separated Ag

and Cu nanoparticles, not alloy. The other sample , which is

bombarded at 300˚C, shows one peak in its spectrum

(dashed line) that can be attributed to alloy for mation.

Figure 11. Comparison between the absorption spectra of

Ag/Cu alloy nanoparticles having different weight ratio of

Cu; all prepared at the same condition. It shows that by

increasing the Cu ratio in the nanoparticles, the absorption

spectrum gets a red shift.

Z. KIANI ET AL.

147

The results of the visible light spectroscopy presented

in this paper show the wavelength dependent absorption

in the fabricated nanostructures. It can be explained by

the Langmuir theory describing the collective oscillations

of surface charges. In metal particles with subwavelength

dimensions there are in fact dipole electron oscillations

bounded by the nanoscopic particle. From quantum me-

chanical point of view, the charge oscillations are easily

excited by the incident photons with energy of ћωp,

where ωp is the resonance frequency of the charge oscil-

lations depends on the size, material and shape of the

nanoparticle. The source of absorption peaks in the pre-

sented spectra is the light induced quantum excitation of

such oscillations.

4. Conclusion

In summary, we have successfully produced the silver

and silver-copper alloy nanoparticles using a low tem-

perature plasma bombardment method, well below the

melting point of the sample constituents. The method can

be used for any other materials to produce nanoparticles

of them on any arbitrary substrates (even flexible sub-

strates). It is easy to make a patterned structure of

nanoparticles by this method. We believe that the pro-

posed method can produce clusters of nanoparticles and

may be the produced films can be used in device fabrica-

tions such as transistors, single electron transistors,

nanoparticles-based gas sensors etc. Optical measure-

ments of as-prepared nanoparticles confirm the quantum

behavior of the samples arising from decreasing the size

and confinement.

5. Acknowledgements

We would like to thank the research council of the Uni-

versity of Tehran for partial financial support.

REFERENCES

[1] W. H. Weber and G. W. Ford, “Propagation of Optical

Excitations by Dipolar Interactions in Metal Nanoparticle

Chains,” Physical Review B, Vol. 70, No. 12, 2004, pp.

125429.1-125429.8. doi:10.1103/PhysRevB.70.125429

[2] S. Link and M. A. El-Sayed, “Spectral Properties and

Relaxation Dynamics of Surface Plasmon Electronic Os-

cillations in Gold and Silver Nanodots and Nanorods,”

The Journal of Physical Chemistry B, Vol. 103, No. 40,

1999, pp. 8410-8426. doi:10.1021/jp9917648

[3] W. P. Zhou, A. Lewera, R. Larsen, R. I. Masel, P. S. Ba-

gus and A. Wieckowski, “Size Effects in Electronic and

Catalytic Properties of Unsupported Palladium Nanopar-

ticles in Electrooxidation of Formic Acid,” The Journal of

Physical Chemistry B, Vol. 110, No. 27, 2006, pp.

13393-13398. doi:10.1021/jp061690h

[4] P. Waszczuk, T. M. Barnard, C. Rice, R. I. Masel and A.

Wieckowski, “A Nanoparticle Catalyst with Superior Ac-

tivity for Electrooxidation of Formic Acid,” Electro-

chemistry Communications, Vol. 4, No. 7, 2003, pp. 599-

603. doi:10.1016/S1388-2481(02)00386-7

[5] B. Choi and H.-H. Lee, “Characterization of the Optical

Properties of Silver Nanoparticle Films,” Nanotechnology,

Vol. 18, No. 7, 2007, Article ID: 075706.

[6] T. K. Sindhu, R. Sarathi and S. R. Chakravarthy, “Under-

standing Nanoparticle Formation by a Wire Explosion

Process through Experimental and Modeling Studies,”

Nanotechnology, Vol. 19, No. 2, 2008, Article ID:.

025703.

[7] S. H. Ko and Y. Choi, “Nanosecond Laser Ablation of

Gold Nanoparticle Films,” Applied Physics Letters, Vol.

89, No. 14, 2006, p. 141126. doi:10.1063/1.2360241

[8] M. Valden, X. Lai and D. W. Goodman, “Onset of Cata-

lytic Activity of Gold Clusters on Titania with the Ap-

pearance of Nonmetallic Properties,” Science, Vol. 281,

No. 5383, 1998, pp. 1647-1650.

doi:10.1126/science.281.5383.1647

[9] X. Y. Xu, K. K. Caswell, E. Tucker, S. Kabisatpathy, K.

L. Brodhacker and W. A. Scrivens, “Size and Shape

Separation of Gold Nanoparticles with Preparative Gel

Electrophoresis,” Journal of Choromatography A, Vol.

1167, No. 1, 2007, pp. 35-41.

doi:10.1016/j.chroma.2007.07.056

[10] N. Nath and A. Chilkoti, “A Colorimetric Gold Nanopar-

ticle Sensor to Interrogate Biomolecular Interactions in

Real Time on a Surface,” Analytical Chemistry, Vol. 74,

No. 3, 2002, pp. 504-509. doi:10.1021/ac015657x

[11] K. Esumi, T. Matsumoto, Y. Seto and T. Yoshimura,

“Preparation of Gold-, Gold/Silver-Dendrimer Nanocom-

posites in the Presence of Benzoin in Ethanol by UV Irra-

diation,” Journal of Colloid and Interface Science, Vol.

284, No. 1, 2005, pp. 199-203.

doi:10.1016/j.jcis.2004.09.020

[12] H. J. Jiang, K. Moon and C. P. Wong, “Synthesis of

Ag-Cu Alloy Nanoparticles for Lead-Free Interconnect

Materials,” Proceedings of International Symposium on

Advanced Packaging Materials: Processes, Properties

and Interfaces, Irvine, 16-18 March 2005, pp. 173-177.

doi:10.1109/ISAPM.2005.1432072