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[10] A. van Blaaderen, R. Ruel, P. Wiltzius, Nature 1997, 385, 321.[11] A. L. Rogach, A. Susha, F. Caruso, G. Sukhorukov, A. Kornowshi, S. Ker-

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Y. Yin, Y. Liu, Adv. Mater. 2000, 12, 693. c) K. M. Kulinowski, P. Jiang,H. Vaswani, V. L. Colvin, Adv. Mater. 2000, 12, 833. d) O. D. Velev, P. M.Tessier, A. M. Lenhoff, E. W. Kaler, Nature 1999, 401, 548. e) B. T. Hol-land, C. F. Blanford, A. Stein, Science 1998, 281, 538. f) J. E. G. J. Wijn-

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[28] The preparation of well-ordered crystals from colloid particles demandsthat the particles remain as individual entities in solution prior to self-as-sembly. For all samples investigated, transmission electron microscopy(TEM) images showed that the coated PS spheres were uniformly coatedand non-aggregated [18c,27b].

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Synthesis of Large-Area Silicon Nanowire Arraysvia Self-Assembling Nanoelectrochemistry**

By Kui-Qing Peng, Yun-Jie Yan, Shang-Peng Gao,

and Jing Zhu*

In recent years, silicon nanowires have received extensive

interest due to their importance in the field of functional

nanoscale electronic devices. They can be prepared by chemi-

cal physical deposition, laser ablation, thermal evaporation,

and other methods.[14] These growth mechanisms have some

limitations, however, as they generally need high temperature

or a high vacuum, templates and complex equipment, or they

employ hazardous silicon precursors. The synthesis of ordered

silicon nanowire arrays is also a focus of research due to its

potential applications in modern devices; the template meth-

od has been used to prepare silicon nanowire arrays. [5]

Electroless metal deposition (EMD) on a silicon substrate

in ionic metal HF solution is one technique that is widely usedin the microelectronics and metal coating industry. Pt, Au, Pd,

Cu, and Ni depositions on silicon wafers in HF solution have

been extensively studied.[69] It is generally accepted that met-

al deposition from HF solution is a localized micro-electro-

chemical redox reaction process in which both anodic and

cathodic processes occur simultaneously at the silicon surface.

More specifically, the metallic atoms depositing on the silicon

surface could form nuclei that behave as a cathode, and the

area surrounding these nuclei behaves as an anode and will be

etched away and dissolved into the solution. Therefore, we

expect that the growth of silicon nanowire arrays on a silicon

wafer in ionic metal HF solution by a selective etching of the

silicon wafer will be based on the principle of above-men-tioned micro-electrochemical redox reaction. After many

efforts, we have successfully prepared large-area growth of

ordered silicon nanowire arrays on silicon wafers without the

use of a template in an aqueous HF solution containing silver

nitrate near room temperature.

The synthesis of silicon nanowire arrays was conducted in a

conventional Teflon-lined stainless steel autoclave containing

the etching HF solution containing silver nitrate at 50 C. The

substrates used in this study were p-type, B-doped silicon

(111) (26 X cm) wafers. We found that the etched silicon

wafers were always wrapped with a thick silver film, which is

rather loose and could be easily detached from the surface of

the silicon wafers. Samples were characterized using scanning

electron microscopy (SEM), and a JEOL 2010F microscope.

1164 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2002 0935-9648/02/1608-1164 $ 17.50+.50/0 Adv. Mater. 2002, 14, No. 16, August 16

______________________

[*] Prof. J. Zhu, Prof. Y. J. Yan, Dr. K.-Q. Peng, Dr. S.-P. GaoDepartment of Materials Sciences and Engineering, Tsinghua UniversityBeijing, 100084 (P. R. China)E-mail: jzhu@mail.tsinghua.edu.cn

[**] This work was supported by the National Nature Science Foundation,National 973 Project of the Republic of China, the Nation AdvancedMaterials Committee of China, and 985 Project of Tsinghua University.We thank Mr. Richard Randolph (Motorola, Inc. Libertyville (USA)) forEnglish correction.

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SEM observations reveal that large quantities of silicon

nanowire arrays could be produced on the surface of the sili-

con wafer (Fig. 1A and 1B). Figure 1C shows the morphology

of the film wrapping the silicon wafer. It is clearly seen

that the film contains numerous dendrites. Energy dispersive

X-ray spectroscopy (EDS) proved that the dendrites are com-

posed of silver.

Fig. 1. SEM investigation of the sample: A) large-area silicon nanowire arraysgrown on the silicon wafer, B) high-magnification SEM image of silicon nano-wire arrays, C) silver dendrite film wrapping the silicon wafer.

We found that the morphology of silicon nanowires dramat-

ically changed depending on the etching conditions, especiallyon the concentrations of HF and AgNO3, and also the treat-

ment temperature. An increase of etching duration clearly

changed the morphology of the etched silicon from needle-

like structures to elongated fine nanowires. However, further

increase of etching time usually resulted in the removal of the

fine silicon nanowires or even disappearance of the silicon

wafer. Generally, only a few silicon nanowires were created in

cases of concentrated HF and AgNO3, or higher treatment

temperatures, such as 100 C and 170 C. So a careful tune-up

of the method is necessary to obtain more ordered silicon

nanowire arrays.

Figure 2A shows the transmission electron microscopy

(TEM) image of the silicon nanowires. The diameters of the

nanowires normally range from 30150 nm and their lengths

are 2050 lm. A typical small-angle electron diffraction

(SAED) pattern (Fig. 2A, upper left inset) of one nanowire

could be indexed for the [110] zone axis of single-crystal sili-

con. In the investigation of the TEM images, we have alsoobserved some belt-like silicon nanostructures. The typical

widths of the nanobelts are in the range of ~100200 nm.

Figure 2B shows the TEM image of a single nanobelt. The

corresponding SAED patterns of the nanobelt could also be

indexed for the [110] zone axis of single-crystal silicon. EDS

microanalysis proved that these nanowires and nanobelts are

silicon (Fig. 2C). The thickness of nanobelts determined by

the electron energy-loss spectra is in the range of~530 nm.

In order to confirm the central role of the silver dendrite in

the process of silicon nanowires growth, we have carried out

comprehensive experiments. Other oxidizing agents such as

K2PtCl6, KAuCl4, Cu(NO3)2, Fe(NO3)3, Mn(NO3)3, andCo(NO3)3, instead of AgNO3 were used in HF solution.

Porous silicon layers and corresponding metal depositions

Adv. Mater. 2002, 14, No. 16, August 16 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2002 0935-9648/02/1608-1165 $ 17.50+.50/0 1165

B

A

C

Energy (keV)1050

Si

Cu

Cu

50

40

30

20

10

0

Fig. 2. A) TEM image of silicon nanowires. The inset shows the electron diffrac-tion pattern of one nanowire. B) TEM image of a single silicon nanobelt.C) EDS analysis of a nanowire selected in Figure 2A.

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could be obtained if experimental parameters are properly

chosen for solutions with K2PtCl6, KAuCl4, and Cu(NO3)2. In

these cases, continuous metal grain films, firmly adhered to

the substrate, are always formed on the surface of the silicon

substrate. Strong luminescent porous silicon layers could also

be obtained with Fe(NO3)3, Mn(NO3)3, and Co(NO3)3, but

no corresponding metal deposition occurred. In these in-stances, numerous micrometer-sized pillar-like, cone-like, or

crater-pit-like microstructures could be observed. We also

found that a continuous micrometer-sized silver grain film

could be observed on the surface of the silicon substrate when

substituting NH4F for HF; the silicon substrate was uniformly

etched and no silicon nanowires could be created. These

experimental results confirm that the growth of silver den-

drites plays an important role in the formation of silicon nano-

wires.

It is evident that the growth of the present Si nanowires is

not determined by the vaporliquidsolid (VLS) growth

mechanisms proposed generally for the nanowires grown by acatalyst-assisted technique.[2,10] According to the investigation

of SEM and the understanding of the EMD process on silicon

substrates in ionic metal HF solution, we show a possible

schematic illustration of the formation mechanism of silicon

nanowires in Figure 3.

A

B

C

Fig. 3. Schematic illustration of the growth mechanism of silicon nanowires on asilicon substrate in ionic metal HF solution. ABC represents the formationprocess of silicon nanowires. A) Silicon wafer. B) Silver nanoclusters formed onthe surface of silicon wafer through electroless silver deposition. C) Formationof silicon nanowires on the surface of silicon wafer at the cost of selective etch-

ing of the silicon wafer. The film growing at the top of the nanowires is the sil-ver dendrite film.

At the start, the silicon etching and silver deposition occur

simultaneously at the silicon surface. The deposited silver

atoms form nanoclusters acting as local cathodes. The areas

surrounding these nanoclusters could act as anodes (Fig. 3B).

That is to say, numerous nanosized electrochemical cells could

be self-assembled on the surface of the silicon wafer. Gener-

ally, these nanoclusters will coalesce to a continuous grain film,

leading to uniform silicon etching. Evidently, the growth of

nanowires will be at a disadvantage if coalescence of the silver

nanoclusters occurs. Continuous metal grain films were usually

formed in the electroless deposition processes of Pt, Pd, Au,

Ni, and Cu in ionic metal HF solution. These continuous metal

grain films will lead to uniform silicon etching, therefore, no

nanowires could be created in these cases. However, the syn-

chronous growth of silver dendrites in the process of silver de-

position could consume a large quantity of superfluous depos-

ited silver atoms and hold back the coalescence of silvernanoclusters. Thus, most of the silver nanoclusters will keep

their size, and silicon nanowires capped with silver nanoclus-

ters acting as cathodes are eventually formed at the cost of

continuous etching of the surrounding anodes (Fig. 3C). The

silicon nanobelts may be caused by the growth of silver nano-

clusters in one direction along the surface of the Si wafer. That

is to say, the geometrical shapes of silicon nanostructures may

depend on the corresponding geometrical shapes of silver

nanoclusters acting as local cathodes. So silicon nanostructures

with intended geometrical shapes could be produced if we pur-

posely control the geometrical shapes of local cathodes. This is

very important for modern device technologies.In conclusion, we have demonstrated a novel method for

preparing large-area silicon nanowire arrays on a silicon wafer

without the use of a template. Silicon nanobelts were also

observed in the products. We proposed that the growth of

these silicon nanowire arrays can be ascribed to a self-assem-

bling, nanoelectrochemical process and growth of silver den-

drites occurring at the surface of silicon wafer. The growth of

these silicon nanowire arrays on the silicon wafer also supplies

visual evidence of the micro-electrochemical process

occurring at the surface of silicon wafer in ionic metal HF

solution. We also suggest that this technique may be general

and applicable to other semiconductors and metals. Further

work is under way in order to characterize the mechanismand apply it to other materials.

Experimental

The synthesis of silicon nanowire arrays was conducted in a Teflon-linedstainless steel autoclave. The autoclave was filled with etching HF solutioncontaining silver nitrate up to 8085 % of its total volume. The silicon waferwas initially cleaned with acetone and ethanol to remove organic grease. Thedegreased silicon wafers were then etched for ten minutes in diluted aqueousHF solution. Here, the etching solutions contain 5.0 mol L1 HFand 0.02 mol L1

silver nitrate. The cleaned silicon wafer was immersed into the etching solutionimmediately and treated at 50 C for 60 min. After the etching process, the sili-con wafers in the autoclave were rinsed with de-ionized water and blown dry in

air. The thick silver film wrapping the silicon wafer was detached before exam-ined the sample using SEM. Samples were characterized using an SEM instru-ment (JEOL JSM6301F) equipped with an Oxford EDS INCA 300. To preparea transmission electron microscopy specimen, the sample was scraped using aknife, and the scraping was collected and suspended in ethanol; then a drop wasplaced on a carbon copper grid and examined in a JEOL 2010F microscopeequipped with a Gatan GIF 678 system.

Received: March 7, 2002Final version: May 23, 2002

[1] J. Westwater, D. P. Gosain, S. Tomiya, S. Usui, H. Ruda, J. Vac. Sci. Tech-nol. B 1997, 15, 554.

[2] A. M. Morales, C. M. Lieber, Science 1998, 279, 208.[3] D. P. Yu, Z. G. Bai, Y. Ding, Q. L. Hang, H. Z. Zhang, J. J. Wang, Y. H.

Zou, W. Qian, G. C. Xiong, H. T. Zhou, S. Q. Feng, Appl. Phys. Lett.1998, 72, 3458.

1166 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2002 0935-9648/02/1608-1166 $ 17.50+.50/0 Adv. Mater. 2002, 14, No. 16, August 16

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[4] J. D. Holmes, K. P. Johnston, R. C. Doty, B. A. Korgel, Science 2000, 287,1471.

[5] X. Y. Zhang, L. D. Zhang, G. W. Meng, G. H. Li, N. Y. Jin-Phillipp,F. Phillipp, Adv. Mater. 2001, 13, 1238.

[6] G. V. Kuznetsov, V. A. Skryshevsky, T. A. Vdovenkova, A. I. Tsyganova,P. Gorostiza, F. G. Sanz, J. Electrochem. Soc. 2001, 148, C528.

[7] P. Gorostiza, R. Diaz, J. Servat, F. Sanz, J. R. Morante, J. Electrochem.Soc. 1997, 144, 909.

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Synthesis of Single Crystals of Calcitewith Complex Morphologies**

By Robert J. Park and Fiona C. Meldrum*

The range of morphologies exhibited by biominerals is truly

remarkable, as is most apparent when they are compared withtheir synthetic counterparts.[13] No better example can be

provided than the skeletal elements of sea urchins. The

CaCO3 plates forming the test of the sea urchin exhibit a

unique sponge-like, fenestrated structure, comprising continu-

ous macropores of 15 lm diameter and non-crystallographic

curved surfaces (Fig. 1a). This structure is particularly amaz-

ing when it is considered that each plate is actually a single

crystal of calcite,[4,5] the synthetic equivalent of which is a reg-

ular rhombohedron with planar faces.

The work described in this paper investigates control of

calcium carbonate morphologies and demonstrates that single

crystals of calcite with complex form can be produced in the

absence of additives, by external imposition of morphology.

Experimental conditions were selected with the aim of pro-

ducing calcite as it tends to form large crystals from solution,

and existing biominerals demonstrate that it is possible to pro-

duce calcite crystals with intricate structures. Calcium carbon-

ate was precipitated in a polymer membrane, which had an

identical morphology to a sea urchin skeletal plate. The mem-

brane structure was templated by a sea urchin plate and was

produced by dipping a plate in the polymer monomer solu-

tion, curing the polymer, and finally dissolving away the cal-

cium carbonate to generate the polymer replica.[6] As the

porous and inorganic fractions of the plate occupy equal vol-

umes, and have identical morphologies, the polymer mem-brane produced has an identical structure to the original

calcium carbonate (Fig. 1b). A schematic diagram of the poly-

mer templating process is shown in Figure 2.

Precipitation of calcium carbonate in the membrane was

achieved by placing the membrane between two half U-tube

arms, and filling them with CaCl2 and Na2CO3 solutions

respectively, in the concentration range 0.001 M to 0.800 M.

The pH of both solutions was measured before and after the

experiment, and values relevant to the particles shown in Fig-

ure 3 are given in the figure legend. The particles produced

Adv. Mater. 2002, 14, No. 16, August 16 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2002 0935-9648/02/1608-1167 $ 17.50+.50/0 1167

[*] Dr. F. C. Meldrum, R. J. ParkDepartment of Chemistry, Queen Mary College, University of LondonMile End Road, London E1 4NS (UK)E-mail: F.C.Meldrum@qmul.ac.uk

[**] We thank the EPSRC for funding a project studentship (RJP), and Prof.Mike Hursthouse and Dr. Mark Light at the EPSRC national X-ray crys-tallography service at Southampton University, for carrying out the XRDanalysis. We are also most grateful to the Department of MaterialsScience, Queen Mary for access to electron microscope facilities.

Fig. 1. a) Cross section through a sea urchin skeletal plate, showing sponge-likestructure. b) Polymer replica of sea urchin plate.

Fig. 2. Schematic diagram describing the methodology used to grow calcite crys-tals in the polymer membrane. 1) Urchin plate is dipped in polymer monomersolution and cured, 2) a thin section is cut, and 3) the calcium carbonate is dis-solved. 4) Calcium carbonate is then precipitated in the so-formed polymermembrane.