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Growth and Field-Emission Properties ofVertically Aligned Cobalt NanowireArrays

Laurent Vila, Pascal Vincent, Laurence Dauginet-De Pra, Gilles Pirio,

Eric Minoux, Laurent Gangloff, Sophie Demoustier-Champagne,

Nicolas Sarazin, Etienne Ferain, Roger Legras, Luc Piraux,*, andPierre Legagneux

Unite de Physico-Chimie et de Physique des Materiaux, Place Croix du Sud 1,

B-1348 LouVain-la-NeuVe, Belgium, AdVanced Analysis and Nanostructures

Laboratory, THALES Research and Technology France, Domaine de CorbeVille,

91404 Orsay Cedex, France, and Unite de Chimie et de Physique des Hauts

Polymeres, Place Croix du Sud 1, B-1348 LouVain-la-NeuVe, Belgium

Received January 14, 2004; Revised Manuscript Received January 29, 2004

ABSTRACT

We present the fabrication of vertically aligned cobalt nanowire arrays on planar surfaces as well as preliminary field-emission (FE) experiments

using them as cold electron cathodes. These arrays are obtained by electrodeposition into nanoporous templates on Au/Ti/Si substrates at

very low temperature (

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supported on various substrates (e.g., ITO glass and siliconwafers). The thickness of these new templates can be moni-tored between 100 nm and several micrometers. Pore density,diameter, and shape can be adjusted during the different stepsof the template preparation.

In this letter, we first describe the synthesis and field-emission properties of vertically aligned cobalt nanowirearrays obtained by electrodeposition into nanopores of track-etched polymer layers deposited on silicon substrates. Field-emission measurements on such arrays are then described;these show the expected metallic behavior and good emissionhomogeneity and reproducibility. Finally, we discuss thepotential of this technique in the realization of field-emissionarrays.

Aligned, free-standing, perpendicular nanowires on a rigidsubstrate have been prepared using a supported track-etchedtemplate (Figure 1). For this purpose, track-etched templatessupported on Au(200 )/Ti(10 )/Si(n-doped, 500-m)samples with a thickness of 2 m, pore size of 20 nm, andpore density of 1 107 pore/cm2 have been preparedfollowing a procedure shown in Figure 1 and described inmore detail elsewhere.11 Arrays of vertical, free-standingnanowires are then obtained by the electrodeposition of cobaltinto the pores of the template (Figure 1c) and the subsequentdissolution of the polymer template (Figure 1d). Theelectrolyte bath consists of 0.8 M cobalt sulfate together with0.4 M boric acid. The electrodeposition of cobalt is initiated

on the Au layer at the bottom of the pores, which serves asa cathode to fill the pores. The deposition is monitored byan EG&G model 263 potentiostat/galvanostat apparatus. Toensure good homogeneity of the growth rate and thus of thewire length, the deposit is performed in a pulse mode. Whenthe first nanowires emerge at the surface of the nanoporoustemplate, a sudden increase in the electroplating current isobserved, and the growth is immediately stopped to avoidthe formation of caps at the extremities of the wires. SEMpictures of the fabricated free-standing cobalt nanowire arraysare shown in Figure 2.

Figure 2a shows the final overall structure of the arrayobserved on a cleaved sample. The cobalt nanowires arealigned perpendicularly to the substrate and are very uniformin size. High-resolution observations (Figure 2b) show thatthe nanowires are 2.1 m long for a diameter at the apexequal to about 20 nm whereas their diameter at their base isabout twice as large, giving rise to better mechanical stability.Because the nanowire shape is very close to the columnarshape, the geometrical field amplification factor defined bythe ratio L/r is therefore 210 for the thinnest nanowires, andthe density of pores was chosen to be around 10 7/cm2 toavoid mutual screening between emitters.4

The field-emission measurements were carried out in an

ultrahigh vacuum system (10-10

Torr) using a triodeconfiguration. A phosphor screen was used as the anode toobserve the emission patterns and to measure the current.The extraction grid consisted of a pierced 100-m-thicksilicon wafer held at a precise distance of 110 m from thesample by a silica spacer. With this thick extraction grid,the cathode-to-grid distance was maintained even for fieldsup to 50 V/m. Figure 3a presents typical I(E) characteristicsmeasured for the grid and anode currents. The ratio of anode/grid current was around a few percent (corresponding to thegrid transparency) and is constant. The characteristics werereproducible and followed the metallic behavior expectedfor our cobalt nanowires. The emission current was given

by the Fowler-Nordheim relationship:

where A and B are two constants, E is the applied field inthe absence of the nanowires given by E) V/d where V isthe applied voltage and d is the cathode-grid distance, isthe field-enhancement factor due to the sharp nanowires, and is the work function of the emitter (5 eV for cobalt). Thefield-emission threshold is 12 V/m for our minimum current

Figure 1. Schematic of vertically aligned metallic nanowire arrayfabrication: (a) spin coating of a thin polycarbonate layer on aAu/Ti/Si(n) substrate and energetic heavy-ion irradiation of the sup-ported film; (b) track sensitization by UV exposition and pore for-mation by chemical etching of the tracks; (c) electrodeposition ofmetallic nanowires into the pores; and (d) dissolution of the poly-meric layer to obtain a perpendicular free-standing nanowire array.

Figure 2. SEM pictures of an array of cobalt nanowires obtainedby electrodeposition into nanoporous layer supported on a flatAu/Ti/Si(n) substrate. (a) Final structure of the array observed ona cleaved sample. We observe the Ti/Au layer at the silicon surfaceand the free-standing cobalt nanowires. The wire density is about107/cm2. (b) High-resolution picture of an individual cobalt nano-wire. The diameter is about 20 nm at the apex, with a total lengthequal to 2.1 m.

I(E) ) A(E)2 exp(- B3/2

E ) (1)

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sensitivity of 0.1 pA, and the fit of the experimental datausing eq 1 gives a field-amplification factor of ) 211.This value is in excellent agreement with the predictedgeometrical field-amplification factor of the nanowiresdetermined by SEM. This confirms that the FE parametersdepend directly on the geometrical aspect ratio. Thus, thechoice of the PC thickness and the nanopore fabricationprocess are two methods to grow nanowires with preciselydetermined FE properties.

For high fields, a deviation from the Fowler-Nordheimlaw of the emission current is observed. This deviation isprobably due to an undesired series resistance (R ) 4 M,

dashed line) in the experimental setup that limited theattainment of higher currents. In this case, the applied fieldE that appears in eq 1 must be rewritten E) (V- RI)/dbecause of the resistive voltage drop. However, the maximumcurrent obtained in this run was 203 A for a 0.2-cm2

emitting area that corresponds to a maximum current densityof 1 mA/cm2.

In Figure 3b, we present the electron pattern imagesobtained on the phosphor screen versus the applied field foranother sample prepared under the same conditions. Thecorresponding anode I(E) characteristic is presented in the

inset of Figure 3a, and the field-emission threshold and thefield-amplification factor were respectively 14 V/m and ) 204. The roughly square-shaped images observed on thepattern are due to the square holes of the extraction grid.Emission occurs on the whole surface of the sample withnumerous participating emitters. This good emission homo-geneity is important in many applications, including displays,and is due to the relatively good geometrical similitude ofindividual nanowires formed by this method.

Several points should be emphasized concerning thepotential of this synthesis technique. First, all of the steps inthis process are realized at low temperature (

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(3) Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.;Hasko, D. G.; Pirio, G.; Legagneux, P.; Wyczisk, F.; Pribat, D. Appl.Phys. Lett. 2001, 79, 1534-1536.

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(8) For a recent review, see Ferain, E.; Legras, R. Nucl. Instrum. MethodsPhys. Res., Sect. B 2003, 208, 115-122.

(9) For a recent review, see, for example, Fert, A.; Piraux, L. J. Magn. Magn. Mater. 1999, 200, 338-358 and references therein.

(10) Jerome, C.; Demoustier-Champagne, S.; Legras, R.; Jerome, R.Chem.sEur. J. 2000, 6, 3089-3093.

(11) Dauginet-De Pra, L.; Ferain, E.; Legras, R.; Demoustier-Champagne,S. Nucl. Instrum. Methods Phys. Res., Sect. B 2002, 196, 81-88.

(12) Purcell, S. T.; Vincent, P.; Journet, C.; Vu Thien, B. Phys. ReV. Lett.2002, 88, 105502(1-4).

(13) Vincent, P.; Purcell, S. T.; Journet, C.; Vu Thien, B. Phys. ReV. B2002, 66, 075406(1-5).

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