NanaStructured titeiials, Vol. 6. pp. 839442.1995 Copyright Q 1995 Elscvier Science Ltd Printed in the USA. All rights rexned 09659773/95 $9.50 + no
FABRICATION, CHARACTERIZATION AND OPTICAL, THEORY OF ALUMINUM NANOMETAIJNANOPOROUS MEMBRANE THIN FILM COMPOSITES G .L. Homyak, K.L.N. Pbani, D.L. Kunkel, V.P. Menon and C.R. Martin
Departmentof Chemistry,ColoradoStateUniversity,Fort Collins,CO 80523,USA Absirad - Wehm fabricated two kiru.&of aluminum nanometaL’porousthinjbn membrane composites:those with cylin&cal, possibly tubular, aluminum nanowires embeddedin tracketchpol’ter host IO p thick -I 4%porosity and with pore r&us of 100 nm; and spherical ahuninum nanoparticlesof -31 nm radius in aluminum oxi& host 30 p thick with mrage pore radius of 60 nm and -30% porosity. The nanoparticles were platedj?om a solution com&ting of 2.OM AIBr, and 0.7M KBr in rlzy toluene with 2% @+) trimethylchlorosilane. Plating potential rangedj?om 4.6V to -3.OVwith respectto an aluminumpseudo-reference electrode We emplopd Mmrwell-Gamett eflect&? me&m theory and the *ical size&pen&at version to explain spectral results. Preliminary optical characterization shows promise in &scribing metalplasmon absorptionmaximafor the aluminum nanostru~res. No samplesexhibited visible region transparenncy dire to rcVgeparticle sizes (=-Snm m&s). JNTRODUCHON
The fabrication,character&ion and optical propertiesof nanoscopicgold particles embeddedin porousaluminumoxide host templatemembraneshavebeendescribedin detail iu a recentpaper(1). Maxwell-Gametteffectivemediumtheory (MG) can be usedto model the metal plasmon absorptionmaxima of thin tihn compositescontaininginfinitely small particles(c-5 nm radius). The n&tionship is given below.
where e, e, and e,,arethe wavelengthdependentcomplexdielectricfimctionsof the resulting composite,metal inclusionand host membrane;f,, is the metal traction and K is the saeaing parameter. However,MG does not addressparticle size and a modified version,called the dynamicalMG (DMG), is used (1). The screeningparameterK is redefined as an e&dive scmeningparameterti into which scattekg lossesarecompensated(1). Gold nanoparticles, however, do not exhibit transparency throughout the visible and, for infinitely small sphericalparticles,absorbnear5 18 mn. In orderto achievetransparency throughoutthis range, other metalssuch as aluminum must be employed.Aluminum, a quasi-tieeelectronmetal, unlike gold or silver,possesses a plasmaabsorptionedge,oP,fartherinto the ultraviolet:12.04 939
HORNYAK ET AL
vs 8.89 and 9.04 eV respectively (2). MG simulations for spherical Al, Ag and Au nanocomposites are depicted in Figure 1. The host membrane material in each case is a hypothetical dielectric material assigned a n&active index, n, of 1.3 and an absorption coefllcient, k, equal to zero tbxoughoutthe spectral range. However, real aluminum oxide films, although [email protected]
transparentthmugh the visible, begin to absorb strongly in the near-W region and can obfuscate the nanometal contribution to the composite absorption. In future simulations, the experimentally determinedoptical constantsof aluminum oxide films for each pore six will be determined. It is important to note the appamnt predicted transparency throughout the visible range for the simulated aluminum composite. DMG simulations of sphericalnanoparticlesof aluminum with increasingsize are shown in Figure 2 (host: n=1.3). The electroplatingsolution consistedof 2.OM AlBr, and 0.7M KBr in dry toluenewith 2% (v/v) trimethylchlomsilane (TMCS), and all plating was done under a dry nitrogen atmosphere(3,4,5). Cathodic contactswere formed by sputteringAg on aluminum oxide films and Au on polyester films. High purity aluminum wires were used as the counter and pseudoreferenceelectrodes,and plating was conducted at potentials ranging from -0.6V to -3.OV.
DISCUSSION A transmission electron micrograph (IEM) (Figme 3) of a cross section of the 60 mn radius pol’e aluminum oxide composite revealedhighly dispersedparticles with an average radiusof31+11mnsca&mdwithina2-3pmlayernearthecathodecontactamaoftheoxide. Severalparticles appearedto occupy the same pore channel,a phenomenonnot consistent with electroplating.In the past, we have managedto plate gold cylindrical particles with semi-minor axes strictly defined by the pore channel (l), but the dispersion evident in the aluminum nanometal flhns may be caused by the nirtic acid treatment following plating to remove the
e . Alumimun
Wavelength in Microns
Wavelength in Microns
Figure 1. MG simulations of spherical Al, Figure 2. DMG simulations of spherical Al Ag and Au nanoparticlesin aluminum oxide. nanocompositeswith increasing particle radius.
ALUMINUMNANOMETALINANOFOROUS MEMBRANETHIN FILMCOMPOSITES
Figure 3. TEM of cross section of Al nanopatticles in aluminum oxide.
Figure 4. TEM of planar section of Al cylinders and tubules in polyester.
silver cathode layer. The resultant composite consisted of metal nanoparticlesnot completely surrounded by host anodic oxide and much smaller than the pore channel diameter. An experimental spectrum of this composite (Figure 5), nonetheless,shows an absorption at 322 nm. The simulated absorption obtainedfor the spherical Al particles of radius 30 mn depicted in Figure 2 was 320 nm with n=1.3 for the hypothetical dielectric host material. A simulated absorptionof 360 run is obtained by changing the host r&active index to 1.5, closer to the true x&-active index of anodic oxides (Figure 6). This suggeststhat the particles am not completely stmotmded by the host anodic oxide in the experimentalspecimenbut by materials with a lower xeIi-&ive index. Better correlations will be expected when input parameters based on experimentally determinedphysical characterizationsoff, optical layer thickness,particle shape (related to K) and exact optical constantsn and k of the host oxide are detetmined. Aluminum nanowims, some of which appearto be tubules (Figure 4) were formed in the pores of the 100 nm radius polyester material. Electrical conductancetests proved to be positive through the thickness of the flhn and the presenceof al~inum was again verified by ICP and EDX. Experimental absorption spectra (not shown) displayed very broad bands with high absorption and will be the subject of a f&ue paper. Polyester films with such relatively large diameter pores are optically densein the visible range due to significant scattering.
CONCLUSION For this preliminary investigation, we have demonstratedthe feasibility of fabricating aluminum metal nanoparticleswithin the pores of two types of host dielectric materials by elec-
0.900 a.800 0.100 l-4
0.000 -I---,.~-__ MO.00
Wavelength in Nanometem
Wavelength in Microns
Figure 5. Experimental spectrum of highly disperse Al nanopatticles.
Figure 6. Simulated absorption spectrum of30 nm radius particles.
tmplating in a toluene,AlBr,, KBr, TMCS solution. The ahunimun nanoparticlesproduced in the aluminum oxide host were highly dispersein nature while those produced in the polyester were contiguous wims of uniform length. The masons for the the surprising tubular structure formation in the pores of the polyester fihn need to be investigated further, but we have generated such stmctums in similar tracketch type polymeric membranes, which seem to possesspropensity for tubular structure formation, by means of electrolessplating of gold. Iu the case of the aluminum oxide, baking and subsequentsilanization of the fihn would enhance its compatibility with the moisture sensitiveplating solution, thereby eliminating some problems. Removal of the silver cathodecontact by means other than nitric acid will also be investigated. Maxwell-Gamett effective medium theory pmdicts transparencythroughoutthe visible range for aluminum nanoparticleswhich reside within the infinite wavelength limit (<5 rnn radius). Obviously, the particle sizes encounteredin this study far exceedthat specified domain, but the dynamical form of the theory predicted the red shift in absorption spectra as particles acquire larger dimensions. The correlation between simulated and experimental spectra,as we have successfully demonstrated in the past with gold, will improve as particle uniformity improves and once the optical constants of the porous films are ascertained. With respect to future applications, the definitive electrical conductivity through the thickness of the plated polyester membranesdemonstratedpromise in that composites could be fabricated which eventually possessboth electrical conductivity and optical transparency.
of Naval Research,
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