Preparation of highly ordered nanoporous Co membranes assembled by small quantum-sized Co particles

Preparation of highly ordered nanoporous Co membranes assembledby small quantum-sized Co particles

Y. Lei,a) C. H. Liang, Y. C. Wu, and L. D. ZhangInstitute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031,People’s Republic of China

Y. Q. MaoNanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008,People’s Republic of China

~Received 9 August 2000; accepted 16 April 2001!

Highly ordered nanoporous Co membranes were fabricated by a two-step replication from thehoneycomb structure of porous anodic alumina. These metal membranes are confirmed to have twosubstructures: first, the Co membrane consists of very fine quantum-sized particles with diametersof about 2–5 nm; second, the very fine Co particles are assembled in a superstructure; i.e., fine anduniform channels 50 nm in diameter, more than 16 mm in thickness, having a pore density of about1010 cm22. New techniques were introduced into the two-step replication process, resulting in newfeatures of the replicated metal membrane: high aspect ratio~more than 320:1!, highly ordered porearrays, and narrow size distributions of the pore diameters. These new techniques also lead tosimplification of the fabrication process of metal membranes. Double-sided and single-sided Comembranes can be achieved simply by adjusting the electroless deposition time. ©2001 AmericanVacuum Society.@DOI: 10.1116/1.1378011#

ngtrtrgsuitneo

gneeehaCrptinrfed

re

s

oor

s inilityan-Ma-

bys inatioblyteo-

termde

ver,erra-s,

a-

helu-

si-war-

atiozeuesheheby

I. INTRODUCTION

Many processes exist for metal film deposition, includisputtering, evaporation, chemical-vapor deposition, eleclytic deposition, and electroless deposition. Of these, elecless deposition1 is particularly attractive in manufacturinbecause it offers the capability to metallize nonplanar, inlating substrates with low-temperature processes; also,readily controllable, capable of continuous operation, adoes not need expensive and energy-consuming equipmAs soft magnetic materials, many kinds of electroless Cbased films have been studied extensively2–6 for their excel-lent soft magnetic properties, such as high saturation matization and low coercivity.2,3 Their magnetic properties arassociated with the nanoscale features in thmorphologies,4,5 such as the strain anisotropies of Co-bascrystallites. According to available literature, no researchbeen reported on preparing highly ordered nanoporousfilms with fine and uniform channels, high aspect ratio poarray, and high pore density. In the present work, nanorous Co membranes were fabricated by electroless plaDetails of their nanostructures are also discussed. The pestructure of the Co films is copied from a typical self-ordernanochannel material—porous anodic alumina.

Porous anodic alumina has been studied for more thanyears.7 Because of their relatively regular structure with narow size distributions of the pore diameters and high aspratio,8 porous alumina membranes are used as templatefabricating many kinds of nanowires9,10 and nanotubes.11,12

Recently, self-organization during pore growth, leading tdensely packed hexagonal pore structure, has been repfor anodization in both oxalic13 and sulfuric14 acid solutions.

a!Electronic mail: [email protected]

1109 J. Vac. Sci. Technol. B 19 „4…, Jul ÕAug 2001 1071-1023 Õ200

o-o-

-isdnt.-

e-

irdso

eo-g.ect

40-ctfor

ated

However, porous alumina films have some disadvantageactual use, such as insufficient chemical and thermal staband low mechanical strength. To overcome these disadvtages, a two-step replication process was developed bysudaet al., and some kinds of microporous metal15,16 andsemiconductor17 membranes were successfully preparedthis method. However, there are still some disadvantagetheir replication process and products. First, the aspect rof their products never exceeded 100. This is most probadue to the lower aspect ratio of polymethyl methacryla~PMMA! cylinder arrays. Second, the regularity of the nanhole array in most of Masudaet al.’s experiments was novery satisfactory,15,17 especially on the upper surface of threplicated membranes. This is partly due to the nonunifodistribution of the metal, which acts as a catalyst or electroon the surface of the anodic alumina membrane. Moreothe MMA monomer was injected into the pores undvacuum conditions and was polymerized by ultraviolet irdiation. So, it is difficult to accomplish this two-step procesand it is not convenient to apply it to real industrial fabriction.

In this article, new techniques were introduced into ttwo-step replication process: immersion of the anodic amina membrane into a methyl methacrylate~MMA ! mono-mer, pre-polymerization of MMA, and four separate depotions of Pd catalyst in different directions. These netechniques result in an almost ideally arranged nanoholeray of Co membranes with new features: high aspect r~more than 320!, highly ordered pore arrays, and narrow sidistributions of the pore diameters. These new techniqalso lead to simplification of the fabrication process: tMMA monomer gets into the pores by simply immersing tanodic alumina membrane in it, and MMA is polymerized

11091Õ19„4…Õ1109Õ6Õ$18.00 ©2001 American Vacuum Society

-:

f

o-

1110 Lei et al. : Preparation of highly ordered nanoporous Co membranes 1110

FIG. 1. Schematic diagram of the fabrication of nanoporous Co membrane~a!–~d! fabrication of a through-holealumina membrane,~e! deposition ofthe Pd catalyst layer on the surface oan alumina membrane,~f! immersionof alumina membrane into MMA andpre-polymerization of MMA,~g! poly-merization of MMA ~forming aPMMA cylindrical array! and removalof the bottom thin PMMA layer,~h! ~i!removal of Al and Al2O3 and~ii ! elec-troless metal deposition into thePMMA cylindrical array, ~i!–~j!double-sided and single-sided nanoprous Co membranes.

me

a

stnaty

andoof

ar

t

te

rstedm-d

rf

e,a

mal

weec-s,

rec-dPd

thethe

the pre-polymerization method. So, it is very easy to accoplish this two-step replication process, which no longneeds expensive and energy-consuming equipment.

II. EXPERIMENT

Figure 1 shows our preparation procedure in a schemmanner. It gives a cross-sectional view@right column of~a!–~e! and ~f!–~j!#, an upper surface view@left column of ~a!–~d!#, and a three-dimensional~3D! view @left column of~e!#.The anodic alumina membranes were prepared in a two-anodizing process13 and have a densely packed hexagopore structure with notable regularity. High-puri~99.999%! aluminum foil ~circularly shaped! was used as thestarting material. Before anodization, the aluminum foil wfirst degreased with acetone, then annealed at 400 °C uvacuum conditions to remove mechanical stresses and tcrystallize; finally, it was electropolished in a solutionperchloric acid and ethanol@HClO4:CH3CH2OH51:9 ~vol!,current density of 0.3 A/cm2#. As shown in Figs. 1~a!–1~e!~left column!, anodizations were only carried out in a circularea ~about half the diameter that of the Al foil! on onesurface of the Al foil, leaving the other surface connectedthe electrode. The first anodization was carried out at 40 V0.3 M oxalic acid solution at 17 °C for 5 h@Fig. 1~a!#. Be-cause the upper part of the anodic oxide layer was distorit was removed in a mixture of H3PO4 ~6 wt %! and H2CrO4

J. Vac. Sci. Technol. B, Vol. 19, No. 4, Jul ÕAug 2001

-r

tic

epl

ser

re-

oin

d,

~1.8 wt %! at 60 °C for 10 h@Fig. 1~b!#. This specimen wasanodized again for 2 h under the same conditions as the fistep@Fig. 1~c!#. Then, the Al layer was removed in saturatHgCl2 solution. The bottom part of the anodic alumina mebranes was removed, and the pore diameter was adjuste13,18

in 5 wt % H3PO4 solution at 32 °C for 60 min, forming athrough-hole~double-sided! membrane with a pore diameteof about 50 nm@Fig. 1~d!#. The thickness and pore density othis anodic alumina membranes are about 20mm and1010 cm22, respectively.

After fabrication of the through-hole alumina membranwe curled up the brim of the outer Al annulus, creatingcylindrical vessel@left column of Figs. 1~d!–1~e!#. Then, avery thin Pd layer~about 10 nm thick! was deposited on thesurface of the through-hole alumina membrane by therevaporation under vacuum conditions of 131025 Torr toform the catalyst layer@Fig. 1~e!#. To avoid nonuniform dis-tribution of the Pd layer on the surface of the membrane,carried out four separate Pd depositions in different dirtions ~two of them were carried out in converse directionwhereas the others were perpendicular to them; four ditions form a cross formation!. Each evaporation was carrieout at an incident angle of 30° to avoid the deposition ofinto the micropores of the membrane@Fig. 1~e!#. A uniformdistribution of the Pd layer on the surface area amongpores of the alumina membrane is very important to

m

ad

edse

ehherryhei

ac

Hst

e

nhiO

de

-are

caop

ca

cal-e

m

oma

boe

ethest of


regularity of the nanopore array of the replicated Co mebrane.

After deposition of Pd, the cylindrical vessel~aluminamembrane and aluminum! was immersed into the MMAmonomer, which contained 2 wt % benzoyl peroxide asinitiator for polymerization. The MMA was pre-polymerizewith a heat treatment of 85 °C for 16 min@Fig. 1~f!#, then thevessel was removed from the MMA. After that, we droppthe MMA into the vessel using a dropper, forming a baplate of the resulting PMMA cylindrical array@Fig. 1~g!#.Polymerization of the pre-polymerized MMA in the vesswas carried out with a heat treatment of 40 °C for 24Because the MMA in the pores dropped slightly during tpolymerization process and because the vessel was immein the MMA during the pre-polymerization process, a vethin PMMA layer also formed at the bottom surface of tcylindrical vessel. This layer was removed just by pullingout with a pair of tweezers, and the bottom vessel surfwas polished slightly@Fig. 1~g!#. Finally, the alumina mem-brane and aluminum were dissolved with 10 wt % NaOresulting in a PMMA cylindrical array with the Pd catalylocated at the root of the PMMA cylinders@Fig. 1~h! ~i!#.This PMMA cylindrical array is a full replica of the porarray of the alumina membrane.

The electroless Co deposition was carried out@Fig. 1~h!~ii !# by immersing the PMMA cylindrical array into aelectroless deposition solution at 70 °C for 3 and 5 h. Telectroless deposition solution is composed of 29 g/l CoS4•

7H2O, 20 g/l NaH2PO2 •H2O, 66 g/l~NH4!2SO4, and 106 g/lNa4P2O7•10H2O, using NH3•H2O to adjust thepH to 10.Then, the PMMA was removed with acetone@Fig. 1~j!#. Bychanging the deposition time, we fabricated a double-siCo membrane@left column of Figs. 1~i! and 1~j!# and asingle-sided Co membrane@right column of Figs. 1~i! and1~j!#. The deposition times were 3 h for double-sided membranes and 5 h for single-sided ones. The resulting film istrue image of the original alumina membrane. The structuof the porous alumina membrane and the PMMA cylindriarray were observed using an atomic force microsc~AFM! ~Park Scientific Instruments, Autoprobe CP!. Thestructures of the Co membrane were observed using a sning electron microscope~SEM! ~JEOL JSM-6300! and atransmission electron microscope~TEM! ~JEOL 200CX!.

III. RESULTS

Figure 2 shows AFM micrographs of the upper surfaand the bottom surface of the porous alumina membranethe bottom surface of the PMMA cylindrical array. An amost perfect hexagonally arranged pore array with an idtical pore diameter of 50 nm can be seen from Figs. 2~a! and2~b!. Figure 2~c! shows that the PMMA cylindrical arrayobtained is an almost ideally arranged array with the sacylinder diameter as that of the alumina membrane.

SEM micrographs of the upper surface and the bottsurface of the double-sided nanoporous Co membraneshown in Fig. 3. It can be seen that the pore diameter is a50 nm, the pore interval is about 100 nm, and the pore d

JVST B - Microelectronics and Nanometer Structures

-

n

-

l.

sed

te

,

s

d

sle

n-

end

n-

e

reutn-

sity is about 1010 cm22. Moreover, the regularity of the porarrangement is very high and is almost identical to that oforiginal alumina membrane@Fig. 2~a!#. Because the Co wafirst grown at the site of the Pd catalyst located at the roo

FIG. 2. AFM images of the~a! upper surface~1.25 mm31.25 mm imagefield! and the ~b! bottom surface~0.7 mm30.7 mm image field! of theporous alumina membrane, and the~c! bottom surface of the PMMA cylin-drical array~2 mm32 mm image field!.

e

ee

ory

oshe

ssm

icata-

eneioth

thglthh

th

m-

as-eenmatA

on

in-ntal

po-


the PMMA cylinders, the regularity of the Pd layer on thsurface of the alumina membrane~area among the pores! orat the root of the PMMA cylinders will greatly affect thregularity of the pore arrangement of the Co membrane,pecially on the surface area. The high regularity of the parrangement at the surface~s! of the Co membrane not onlconfirms the high regularity of the Pd layer on the surfacethe alumina membrane but also confirms the usefulnesthe fourfold Pd deposition method to the regularity of treplicated membrane.

Figure 4 shows the SEM micrographs of the crosectional view of the double-sided nanoporous Co mebrane. It can be seen that all the nanopores are cylindrparallel to each other, and have a very high aspect r@Figs. 4~a! and 4~b!#. The thickness of the double-sided nnoporous Co membrane is about 16mm @Fig. 4~c!#. Becausea nanopore array can be seen on the bottom surface of thmembrane@Fig. 3~b!#, the nanopores in the Co membramust be the through holes. This means that the aspect ratthe nanopores of our Co membrane is at least 320:1. Wilonger Co deposition time from 3 to 5 h~about 26mm of filmthickness!, the nanopore array can no longer be seen onbottom surface of the Co membrane, indicating that a sinsided Co membrane has formed. So, it is obvious thataspect ratio of the PMMA cylindrical array is very hig@from 320 ~16 mm! to about 420~26 mm!#, and that thethickness of the Co membrane is almost equal to that ofalumina membrane~20 mm!. A conclusion can be drawn

FIG. 3. SEM images of the~a! upper surface and the~b! bottom surface ofthe double-sided nanoporous Co membrane.


s-e

fof

--

al,io

Co

ofa

ee-e

e

from the above results: the immersion of the alumina mebrane into MMA and the pre-polymerization of the MMAcan result in an almost total replication and a very highpect ratio of the Co membrane. Moreover, it can be sfrom Figs. 3~b! and 4~c! that the Co membrane has a uniforthickness, resulting from a uniform deposition rate of Coevery part of the Pd catalyst located at the root of the PMMcylinders; i.e., the Pd catalyst has a uniform distributionthe surface of the alumina membrane.

Our energy-dispersive spectroscopy~EDS! ~Kevex SigmaEDS! observation at cross-sectional areas of the sampledicates that the film is almost entirely composed of eleme

FIG. 4. SEM images of a cross-sectional view of the double-sided nanorous Co membrane:~a! partial view of the cross section,~b! surface andcross-sectional views, and~c! view of the entire cross section.

no-5o

ithe

e

t-er

ithon-

withls.

lec-ss

na.ro-rane-ch-ndof

0:1,of

etalaveon-utas-m-

ea-adti-elec-thes are

a-nce

dr-

ki,

T.

s.

c-

B.

o


Co. TEM observations were carried out to explore the nastructure of the film~Fig. 5!. The morphology of the nanopore array of the Co membrane with a diameter of aboutnm can be seen from Fig. 5~a!; it can also be seen that the Cmembrane is composed of very small particles@in the lowerright and the lower left corners of Fig. 5~a!#. The correspond-ing selected area electron diffraction~SAED! pattern in Fig.5~b! shows that the sample is polycrystalline in nature, wa hexagonal-close-packed~hcp! crystalline phase. To explordetails of the structure further, a specimen was preparedfirst grinding the Co membrane completely in a mortar, th

FIG. 5. TEM image of the~a! nanoporous Co membrane,~b! the correspond-ing SAED pattern, and~c! TEM image of the small quantum-sized Cparticles in the nanoporous Co membrane.

JVST B - Microelectronics and Nanometer Structures

-

0

byn

dispersing it in ethyl ethanol by vigorous ultrasonic vibraing; finally, it was dropped onto carbon films on coppgrids. TEM observation of the sample@Fig. 5~c!# shows thatthe film is composed of very fine quantum-sized grains wdiameters of about 2–5 nm. From the above results, we cclude the following: very small Co crystalline particles~2–5nm! assembled and form a nanoporous Co membraneabout 50-nm-diam pores and about 100 nm pore interva

IV. CONCLUSIONS

We have fabricated nanoporous Co membranes by etroless deposition following a two-step replication procefrom the honeycomb structure of porous anodic alumiNew techniques were used in the two-step replication pcess, such as the immersion of an anodic alumina membin a MMA monomer, pre-polymerization of MMA, and fourdirection evaporation of the Pd catalyst. These new teniques lead to full replication of the alumina membrane, athey result in an almost ideally arranged nanohole arrayCo membranes with high aspect ratios of more than 32highly ordered pore arrays, and narrow size distributionsthe pore diameters. The overall fabrication process of mmembranes was greatly simplified. The Co membranes htwo nanometer substructures: first, the Co membrane csists of very fine crystalline particles with diameters of abo2–5 nm, and, second, these very fine Co particles aresembled, and form the pore array structure of the Co mebrane with characteristic size of about 50 nm. The new ftures of this Co membrane should contribute to brotechnological and scientific application in many fields: opcal devices, sensor devices, magnetic recording system,trodes, and filtration. Due to their special nanostructure,magnetic properties of these nanoporous Co membraneworthy of further study.

ACKNOWLEDGMENT

This work was supported by the Key Project of the Ntional Fundamental Research of China and Natural ScieFoundation of China~Grant No. 19974055!.

1G. O. Mallory and J. B. Hadju,Electroless Plating: Fundamentals anApplications~American Electroplates and Surface Finishing Society, Olando, FL, 1990!.

2T. Osaka, T. Homma, N. Masubuchi, K. Saito, M. Yoshino, Y. Yamazaand T. Namikawa, J. Magn. Soc. Jpn.14, 309 ~1990!.

3T. Osaka, T. Homma, K. Saito, A. Tokekoshi, Y. Yamazaki, andNamikawa, J. Electrochem. Soc.139, 1311~1992!.

4A. P. Valanju, I. S. Jeong, D. Y. Kim, and R. M. Walser, J. Appl. Phy64, 5443~1988!.

5M. Mirzamaani, L. Romankiw, C. McGrath, and J. Karasinski, J. Eletrochem. Soc.135, 2813~1988!.

6T. Homma, Y. Kita, and T. Osaka, J. Electrochem. Soc.147, 160~2000!.7F. Keller, M. S. Hunter, and D. L. Robinson, J. Electrochem. Soc.100,411 ~1953!.

8C. R. Martin, Science266, 1961~1994!.9C. A. Huber, T. E. Huber, M. Sadoqi, J. A. Lubin, S. Manolis, and C.Prater, Science263, 800 ~1994!.

10S. A. Sapp, B. B. Lakshmi, and C. R. Martin, Adv. Mater.11, 402~1999!.11P. Hoyer, Adv. Mater.8, 857 ~1996!.12J. Li, C. Papadopoulos, and J. M. Xu, Appl. Phys. Lett.75, 367 ~1999!.13H. Masuda and M. Satoh, Jpn. J. Appl. Phys., Part 135, 126 ~1996!.


14H. Masuda, F. Hasegwa, and S. Ono, J. Electrochem. Soc.144, L127~1997!.

15H. Masuda, H. Tanaka, and N. Baba, Chem. Phys. Lett.1990, 621~1990!;H. Masuda, K. Nishio, and N. Baba, Thin Solid Films223, 1 ~1993!; H.Masuda, T. Mizuno, N. Baba, and T. Ohmori, J. Electroanal. Chem.368,333 ~1994!.


16H. Masuda, and K. Fukuda, Science268, 1466~1995!.17H. Masuda, K. Nishio, and N. Baba, Jpn. J. Appl. Phys., Part 231, L1775

~1992!; P. Hoyer, N. Baba, and H. Masuda, Appl. Phys. Lett.66, 2700~1995!; P. Hoyer and H. Masuda, J. Mater. Sci. Lett.15, 1228~1996!.

18A. P. Li, F. Muller, A. Birner, K. Nielsch, and U. Go¨sele, J. Vac. Sci.Technol. A17, 1 ~1999!.

Preparation of highly ordered nanoporous Co membranes assembled by small quantum-sized Co particles

Documents

Transcript of Preparation of highly ordered nanoporous Co membranes assembled by small quantum-sized Co particles