Morphological selection of electroless metal deposits on silicon in aqueous fluoride solution

Article abstract of DOI:10.1016/j.electacta.2004.02.009

Well-defined silver and gold dendrites are prepared on silicon substrates in aqueous fluoride acid solution containing AgNO₃ and KAuCl ₄, respectively. In cases of electroless platinum and copper deposition in fluoride acid solution, no corresponding metal dendrites but platinum microcrystallines and copper granular-crystallines are formed. Our experiments found that the morphological selection of electroless metal deposits is related to the microstructure of etched silicon. A possible template mechanism and affecting factors for the morphological selection of electroless metal deposits are discussed.

Full text of DOI:10.1016/j.electacta.2004.02.009

Electrochimica Acta 49 (2004) 2563–2568
Morphological selection of electroless metal deposits
on silicon in aqueous fluoride solution
∗
Kuiqing Peng , Jing Zhu
Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
Received 22 November 2003; received in revised form 12 January 2004; accepted 9 February 2004
Abstract
Well-defined silver and gold dendrites are prepared on silicon substrates in aqueous fluoride acid solution containing AgNO
3
4
and KAuCl ,
respectively. In cases of electroless platinum and copper deposition in fluoride acid solution, no corresponding metal dendrites but platinum
microcrystallines and copper granular-crystallines are formed. Our experiments found that the morphological selection of electroless metal
deposits is related to the microstructure of etched silicon. A possible template mechanism and affecting factors for the morphological selection
of electroless metal deposits are discussed.
©
2004 Elsevier Ltd. All rights reserved.
Keywords: Electroless deposition; Dendrite
1
. Introduction
on the study of silicon nanostructures. In this paper, we will
report the morphological selection of electroless metal de-
posits on silicon substrates. We found that the morphologies
of electroless metal deposits are related to the microstruc-
ture of the etched silicon substrates besides the metal
species and deposition conditions. Possible formation mech-
anism and affecting factors for both dendrites formation are
discussed.
In the past years, there has been a great deal interest in the
fabrication of nanostructured metals such as nanoparticles,
nanorods, and nanowires, because of their unique electronic,
photonic, and catalytic properties [1–7], which strongly de-
pend on by their shapes and sizes. For silver, many differ-
ent shaped nanostructures including nanowires, nanocubes,
nanodisks, have been synthesized using various chemical
approaches [8–13]. Further, a wide range of studies have
addressed the preparation of dendritic silver supramolecular
nanostructures [14–20]. For example, silver dendritic nanos-
tructures have been prepared by a template method in which
the desired materials are encapsulated into the channels or
pores of a host [14]. Silver dendrites were also observed by
ultraviolet irradiation photoreduction technique using PVA
as a protecting agent [15] and by sonoelectrochemical meth-
ods [16].
2. Experimental
The typical preparation of electroless metal deposits
was carried out as follows. Analytical grade AgNO3 or
KAuCl4 was dissolved in aqueous HF solution at room
temperature to form a homogeneous solution, which was
then transferred into a Teflon-lined stainless steel autoclave.
Then a piece of p-type (1 1 1) silicon wafer which has been
cleaned with acetone, ethanol, H2SO4:H2O2 (volume ratio
Recently, we reported a novel method to large-area silicon
nanowire arrays [21–23], which could be easily obtained by
etching silicon wafers in aqueous HF solution containing
AgNO3 or KAuCl4. In previous reports, we mainly focused
3:1), and diluted HF solution, was immersed immediately
in the etchant solution, sealed and treated. In our experi-
ments, the concentrations of HF are chosen to be 4.6 mol/l.
The treating temperature and concentrations of AgNO3 or
KAuCl4 in the solution are varied to obtain metal deposits
with different shapes and sizes. After etching, the sample
is rinsed copiously in de-ionized water, and dried at room
temperature.
∗
Corresponding author. Tel.: +86-106-277-7195;
fax: +86-106-277-2507.
E-mail address: kuiqing99@mails.tsinghua.edu.cn (K. Peng).
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.02.009

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K. Peng, J. Zhu / Electrochimica Acta 49 (2004) 2563–2568
The samples were examined with transmission electron
3. Results and discussion
microscope (TEM; JEOL, 200CX), and scanning electron
microscopy (SEM; JSM-6301F) equipped with energy dis-
persive spectrum (EDS). Powder X-ray diffraction (XRD)
was used to characterize the products by standard reflection
mode with Cu K␣ radiation (γ = 0.154178 nm).
In case of silver deposition, the etched silicon substrates
were always wrapped with a layer of thick silver film,
which is rather loose and could be easily detached from
the surface of silicon substrates. SEM observation revealed
◦
Fig. 1. SEM images of silver and etched silicon prepared in 0.01 mol/l AgNO3 and 4.6 mol/l HF at 50 C for 60 min (a) silver dendrite; (b) surface
morphology of etched silicon showing interesting nanowire network; (c) EDS spectrum of silver dendrites.

K. Peng, J. Zhu / Electrochimica Acta 49 (2004) 2563–2568
2565
◦
Fig. 2. XRD pattern of silver dendrites prepared with 0.01 mol/l AgNO3 and 4.6 mol/l HF at 50 C for 60 min.
that the loose film is composed of fine silver dendrites.
No compact silver film was observed on the surface of
etched silicon. Fig. 1a shows the typical SEM image of
dense-branched silver deposits formed in aqueous HF solu-
network. Energy dispersive spectrum (EDS) analysis proves
that these dendrites are silver (Fig. 1c). The typical XRD
pattern of the silver dendrites prepared with 0.01 mol/l
◦
AgNO3 at 50 C is shown in Fig. 2. All these distinct
◦
tion containing 0.01 mol/l AgNO3 at 50 C. The aggregating
diffraction peaks correspond to the reflection of face cen-
tered cubic-fcc crystalline silver, with lattice constant of
α = 0.4080 nm (JCPDS cards 4-0783).
morphologies of the deposits are well-defined dendritic pat-
terns. Large-magnification SEM observation further reveals
striking quasi-periodic arrangement of nanoparticles along
the backbone or side branches of dendrites. Large numbers
of Si nanowires could be clearly seen when dismantling the
dendritic film from the surface of etched silicon. Fig. 1b
gives the SEM image of corresponding surface morphol-
ogy of etched silicon, which shows interesting nanowire
Further characterization on Si nanowires was carried out
using transmission electron microscope. To prepare a TEM
specimen, the sample was scraped using a knife, and the
scraping was collected and suspended in ethanol; then a drop
was placed on a carbon copper grid. Fig. 3 shows several
Si nanowires (SiNWs) with different diameters, inset shows
Fig. 3. TEM image of several silicon nanowires, inset shows the selected-area electron diffraction (SAED) pattern from an individual nanowire.

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K. Peng, J. Zhu / Electrochimica Acta 49 (2004) 2563–2568
the selected-area electron diffraction (SAED) pattern of an
individual nanowire, which could be indexed for the (1 1 0)
zone axis of single-crystal silicon.
deposits. Reaction temperature is also an important factor
affecting the morphology of silver deposits. Fig. 4c and d
gives the SEM images of silver deposits and etched silicon
◦
Although the growth mechanism of the silver dendrites
is not fully understood, we found that the morphologies of
silver dendrites are sensitive to the deposition conditions in-
cluding temperature and concentration of AgNO3 in etchant
solution. Fig. 4 shows the SEM images of silver deposits and
etched silicon prepared under different experimental condi-
tions. Filamentary growth of silver deposit was formed in
prepared in the 0.02 mol/l AgNO3 at 120 C, respectively.
It is clearly seen that the units of silver dendrites are larger
and less branched compared with silver deposits prepared at
low temperature; the etched silicon shows much disordered
microstructure.
Electroless gold deposition on silicon in aqueous HF so-
lution was also investigated. In case of electroless gold de-
position, large-area compact gold film was often formed
on the surface of etched silicon substrates. When dismantle
the compact gold film from the silicon substrates, porous
structures could be observed. In some area of etched silicon
substrates only with the deposition of loose gold dendrites,
small-area Si nanowire arrays could be observed. The phe-
nomena imply that the loose dendritic film could assist in
the formation of Si nanowires. Fig. 5a shows the SEM im-
age of loose gold deposits prepared in 12 M HF solution
◦
the 0.02 mol/l silver nitrate at 50 C for 60 min (Fig. 4a). The
corresponding morphology of etched silicon was given in
Fig. 4b. Oriented Si nanowire forests could be clearly seen.
As the concentration of silver nitrate increases, we found
that silver deposits become less branched. In addition, we
also found that only silver particles could be formed on the
surface of silicon substrates at low concentrations of silver
nitrate, for example 0.001 mol/l. This means that some con-
ditions must be required for the formation of branched silver
◦
Fig. 4. SEM images of as-synthesized products: (a) filamentary growth of silver deposit in the 0.02 mol/l silver nitrate at 50 C; (b) etched silicon
◦
showing nanowire forest morphology; (c) less branched silver deposits prepared at 120 C with 0.02 mol/l AgNO3 for 60 min; (d) disordered surface
microstructure of etched silicon.

K. Peng, J. Zhu / Electrochimica Acta 49 (2004) 2563–2568
2567
Fig. 5. (a) Gold dendrites; (b) etched silicon substrate showing nanowire forest morphology; (c) platinum microcrystallines; (d) copper granular
microcrystallines.
◦
containing 0.01 mol/l KAuCl4 at 50 C. The aggregating
morphology of the deposits shows typical dendritic pattern.
The gold dendrites are of larger size and ill-defined com-
pared with fine silver dendrites. We suggest that the reduc-
tion of complex silicon microstructures. In other words, few
branched metal dendritic structures often formed on uni-
formly etched silicon substrates. Therefore, we suggest that
complex surface microstructure of etched silicon may play
an important role in the formation of well-defined metal den-
drites, such as silver and gold dendrites. However, to date
the microscopic mechanism for the formation of metal den-
drites is not clearly known.
It is well known that metal deposition on silicon from
HF-based solutions is initiated by electrochemical reduc-
tion of metal ions, a process which is driven by the differ-
ence between the electron quasi Fermi energy in the silicon
and the redox energy level of the metallic ions in solution.
For silver, the reaction could be described by the following
formula:
+
tion of Au ions to metallic Au is faster than that of Ag to
metallic Ag, and this fast reduction behavior will lead to the
more rapid nucleation and growth of gold clusters. The mor-
phology of etched silicon was also given in Fig. 5b, oriented
Si nanowire forests could clearly be observed.
In order to explore the factors affecting the formation of
silver and gold dendrites, other electroless metal deposition
processes including platinum and copper have also been in-
vestigated. However, in cases of platinum and copper, no
corresponding metal dendrites but platinum microcrystals
and copper granular-crystallines are formed on the surface
of etched silicon (Fig. 5c and d, respectively). Further SEM
observation found that the silicon substrates were relatively
uniform etched as compared with that in HF–AgNO3 or
HF–KAuCl4 system, and no complex microstructures, such
as nanowire forests and porous structure were observed in
both cases.
+
0
−
2−
4
Ag (aq) + Si (s) + 6F (aq) → 4Ag(s) + SiF (aq)
6
However, this chemical reaction could not give us detailed
information about the mechanism of the metal deposition
and pattern formation process.
Xiao et al. [14] have reported a template method to pal-
ladium and silver dendritic nanostructures; the desired ma-
terials are encapsulated into the channels or pores of a host
Further experimental results revealed that the formation
of metal dendrites was always accompanied by the forma-

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K. Peng, J. Zhu / Electrochimica Acta 49 (2004) 2563–2568
(Raney nickel); the host works as the template and reduc-
Acknowledgements
ing agent with the assistance of ultrasonic waves. Compar-
ing our experiments with this method, we suggest that an
analogy could be drawn between two cases. In our ex-
periments, the nanowire network or forests also could be
looked upon as the template, which acting as reducing agents
with the assistance of HF. The difference between both
methods is that the template of nanowire network or for-
est in our method is in situ prepared. We also presume
that the formation of silver and gold dendrites may be cre-
ated via diffusion-limited aggregation (DLA) model [24];
which involves the cluster formation by the adhesion of
a particle with random path to a selected seed on contact
and allows the particle to diffuse and stick to the growing
structure.
It is the fact that above discussion is only our supposi-
tion. Therefore, to give more experimental evidences, sys-
tematic studies are required, including SPM characterization
as a function of the deposition/etching time and intrinsic
factors at the atomic level, such as silicon/metal interfacial
bonding.
This work was financially supported by the National Nat-
ural Science Foundation of China, National 973 Project of
China and 985 Project of Tsinghua University.
References
[1] A.P. Alivisatos, Science 271 (1996) 933.
[2] J.H. Fendler, F.C. Meldrum, Adv. Mater. 7 (1995) 607.
[3] T. Teranishi, M. Miyake, Chem. Mater. 10 (1998) 594.
[4] B.R. Martin, D.J. Dermody, B.D. Reiss, M.M. Fang, L.A. Lyon, M.J.
Natan, T.E. Mallouk, Adv. Mater. 11 (1999) 1021.
[5] F. Kim, J.H. Song, P.D. Yang, J. Am. Chem. Soc. 124 (2002) 14316.
[6] Y. Kondo, K. Takayanagi, Science 289 (2000) 606.
[
[
7] K.B. Lee, S.M. Lee, J. Cheon, Adv. Mater. 13 (2001) 517.
8] J. Sloan, D.M. Wright, H.G. Woo, S. Bailey, G. Brown, A.P.E. York,
K.S. Coleman, J.L. Hutchison, M.L.H. Green, Chem. Commun. 8
(1999) 699.
[9] S. Bhattacharyya, S.K. Saha, D. Chakravorty, Appl. Phys. Lett. 77
(2000) 3770.
[
[
[
[
10] Y.G. Sun, Y.N. Xia, Adv. Mater. 14 (2002) 833.
11] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176.
12] M. Maillard, S. Giorgio, M.P. Pileni, Adv. Mater. 14 (2002) 1084.
13] E.C. Hao, K.L. Kelly, J.T. Hupp, G.C. Schatz, J. Am. Chem. Soc.
124 (2002) 15182.
4
. Conclusions
[
[
[
[
14] J.P. Xiao, Y. Xie, R. Tang, M. Chen, X.B. Tian, Adv. Mater. 13
(2001) 1887.
In summary, we have investigated the morphological
15] Y. Zhou, S.H. Yu, C.Y. Wang, X.G. Li, Y.R. Zhu, Z.Y. Chen, Adv.
Mater. 11 (1999) 850.
16] J.J. Zhu, S.W. Liu, O. Palchik, Y. Koltypin, A. Gedanken, Langmuir
16 (2000) 6396.
selection of electroless metal deposits on silicon in aqueous
HF solution. Complex surface microstructures generally
play an important role in the morphological selection of
electroless metal deposits. We suggest that the complex
silicon microstructures may play the roles of template and
reducing agent with the assistance of HF. In addition, it is
quite possible that some other mechanism may contribute
or even lead to the formation of dendrites. Therefore, more
extended work including experiments and theoretical cal-
culations is needed to clarify the origins of morphological
selection of electroless metal deposits on silicon in HF
solution.
17] X.Q. Wang, K. Naka, H. Itoh, S. Park, Y. Chujo, Chem. Commun.
12 (2002) 1300.
[18] G.D. Wei, Y. Deng, C.W. Nan, Chem. Phys. Lett. 367 (2003) 512.
[19] J.J. Zhu, X.H. Liao, H.Y. Chen, Mater. Res. Bull. 36 (2001) 1687.
[20] R. He, X.F. Qian, J. Yin, Z.K. Zhu, Chem. Phys. Lett. 369 (2003)
454.
[21] K.Q. Peng, Y.J. Yan, S.P. Gao, J. Zhu, Adv. Mater. 14 (2002) 1164.
[22] K.Q. Peng, J. Zhu, J. Electroanal. Chem. 558 (2003) 35.
[23] K.Q. Peng, Y.J. Yan, S.P. Gao, J. Zhu, Adv. Funct. Mates. 13 (2003)
127.
[24] T.A. Witten, L.M. Sander, Phys. Rev. Lett. 47 (1981) 1400.