Growth and Field-Emission Properties ofVertically Aligned Cobalt NanowireArraysLaurent 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 Mate´riaux, Place Croix du Sud 1,B-1348 LouVain-la-NeuVe, Belgium, AdVanced Analysis and NanostructuresLaboratory, THALES Research and Technology France, Domaine de CorbeVille,91404 Orsay Cedex, France, and Unite´ de Chimie et de Physique des HautsPolymeres, 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) experimentsusing them as cold electron cathodes. These arrays are obtained by electrodeposition into nanoporous templates on Au/Ti/Si substrates atvery low temperature (<100 °C). After the removal of the template, the arrays consist of statistically positioned vertical free-standing nanowireswith high aspect ratios, uniform dimensions, and predetermined densities. The electron field-emission measurements show metallic andreproducible characteristics. Emission is found to be reasonably homogeneous over the whole sample area, and a current density of 1mA/cm2 has been obtained. We show that the growth process permits us to predetermine the field amplification factor precisely, which is ofprimary importance for FE sources. A large variety of FE applications can be envisaged, including FE displays and microwave amplifiers.

Intensive research has been carried out over the last severaldecades on field-emission arrays (FEA) in view of numerousapplications such as flat-panel displays1 or microwave am-plifiers.2 Among the main characteristics needed for thesearrays are a low field-emission threshold, a high currentdensity, and good emission homogeneity. For several reasons,particularly cost, FEA based on the Spindt cathodes did notcreate a significant breakthrough in large-scale technologicalapplications. A new and promising direction that has gen-erated new energy in this research community has been thesynthesis of various carbon nanotubes (CNTs) or nanowiresthat present high aspect ratios and excellent field-emission(FE) properties on planar substrates. For CNTs, the develop-ment of techniques such as plasma-enhanced chemical vapordeposition (PECVD) led to the localized growth of straightand perpendicularly aligned CNT arrays on surfaces3. Forbrevity, we use the term array to include ordered or dis-ordered perpendicular CNTs and nanowires. In this case, ithas been shown that controlling the density and the geo-

metrical aspect ratio of the emitters is key to controlling thefield-emission threshold and the optimal current density.4,5

The realization of well-defined CNT arrays requires, in thefirst stage, a precise deposition of the catalyst particles fol-lowed by high-temperature growth (above 500°C) that limitsthe choice of the substrates. Other semiconducting nanowiressuch as copper sulfide or silicon carbide grown on planarsurfaces have also been successfully used for field emissionbut without controlling the emitter density.6,7 A new routethat we explore in this article is arrays of metallic nanowires.These can be prepared by template synthesis using nanopo-rous track-etched templates that give still better and simplercontrol of density and uniformity.

The track-etching process consists of irradiating a polymerby energetic ions creating linear damage tracks, followedby a chemical etching of these tracks. This permits theformation of a known number of pores in polymer layerscharacterized by a well-defined and uniform size and shape.8

Such templates can be used for the synthesis of arrays ofmetallic9 and organic nanowires or nanotubes10 having inter-esting properties mainly linked to their nanoscopic dimen-sions and to their extremely large aspect ratio. Recently, anew generation of track-etched templates has been devel-oped.11 These templates consist of thin nanoporous films

* Corresponding author. E-mail: piraux@pcpm.ucl.ac.be. Tel:+3210473566. Fax:+3210473452.

† Unite de Physico-Chimie et de Physique des Mate´riaux.‡ THALES Research and Technology France.§ Unite de Chimie et de Physique des Hauts Polyme`res.

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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 Å)/Ti(10 Å)/Si(n-doped, 500-µm)samples with a thickness of 2µm, 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 initiatedon 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 2.1µm 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 ratioL/r is therefore 210 for the thinnest nanowires, andthe density of pores was chosen to be around 107/cm2 toavoid mutual screening between emitters.4

The field-emission measurements were carried out in anultrahigh 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-µm-thicksilicon wafer held at a precise distance of 110µm from thesample by a silica spacer. With this thick extraction grid,the cathode-to-grid distance was maintained even for fieldsup to 50 V/µm. Figure 3a presents typicalI(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 givenby the Fowler-Nordheim relationship:

whereA andB are two constants,E is the applied field inthe absence of the nanowires given byE ) V/d whereV isthe applied voltage andd is the cathode-grid distance,â isthe field-enhancement factor due to the sharp nanowires, andφ is the work function of the emitter (5 eV for cobalt). Thefield-emission threshold is 12 V/µm 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 2.1µm.

I(E) ) A(âE)2 exp(- Bφ3/2

âE ) (1)

522 Nano Lett., Vol. 4, No. 3, 2004

sensitivity of 0.1 pA, and the fit of the experimental datausing eq 1 gives a field-amplification factor ofâ ) 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 MΩ,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 rewrittenE ) (V - RI)/dbecause of the resistive voltage drop. However, the maximumcurrent obtained in this run was 203µA 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 anodeI(E) characteristic is presented in the

inset of Figure 3a, and the field-emission threshold and thefield-amplification factor were respectively 14 V/µm andâ) 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 (<100°C). Thisis particularly interesting for the integration of these field-emission arrays into integrated devices that do not supporthigh-temperature synthesis or require low cost or supplesubstrates. Second, it can be easily scalable to much largersurfaces, which is more difficult for PECVD and obviouslyis an advantage for many applications and manufacturingprocesses. Third, a wide variety of materials and structurescan be electrodeposited, including metals, alloys, and oxides.Multilayers and finishing layers can be easily produced. Thisoffers the possibility to use low-work-function metals ormaterials, to incorporate resistive layers along the nanowireto limit the current for each nanowire, or to deposit magneticmaterials at the apex for spin-polarized emission. Fourth,the shape and geometrical aspect of the nanowires can becontrolled by the etching process of the tracks in the polymerlayer. As we saw, this permits us to control the operatingvoltage precisely. Furthermore, this offers the possibility tooptimize the mechanical contact between the nanowire andsubstrate and to limit the resistive heating process that hasbeen observed for CNTs12-14 and that limits high-currentoperation. Thus, applications (e.g., microwave amplification)requiring high current densities (g 1 A/cm2) are envisaged.Finally, the design of the final array can be achieved bycontrolling both the irradiation dose for the emitter densityand the UV exposition step for the definition of FED pixels,for instance.

In conclusion, we have shown that electrodeposition insupported nanoporous track-etched templates constitutes aversatile and attractive process for the achievement ofvertically aligned metallic nanowire arrays. First, arrays ofvertically aligned cobalt nanowires exhibit a homogeneousand reproducible electron emission with a current densityof up to 1 mA/cm2. This low-temperature, scalable processallows for the close tailoring of pre-specified field-emissioncharacteristics and therefore presents a new attractive alterna-tive for field-emission arrays and devices.

Acknowledgment. The work was partially supported bythe Belgian Science Policy through the InteruniversityAttraction Pole Program PAI (P5/1/1).

References

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Figure 3. (a) Typical grid and anode current characteristicsobtained from a Co nanowire array (lines with black squares); thefit to the Fowler-Nordheim equation (solid line) givesâ )211, and the threshold field for emission in the pA range is 12V/µm. The apparent deviation is due to an undesired resistance inseries (R ) 4 MΩ, dashed line) in the experimental setup. (b)Emission images versus field obtained on the phosphor screenshowing the good emission homogeneity with numerous emitters.(The corresponding anode current characteristic is presented in theinset of a.)

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