Gold Nanoparticulate Thin Films Fabricated by the Electrostatic Self-Assembly Process

Vol.17 No. 1Journal of Wuhan University of Technology - Mater. Sci. Ed.Mar. 2002Gold Nanoparticulate Thin Films Fabricated by theElectrostatic Self- Assembly ProcessYU Hai-hu JIANG De-shengWuhan University of Technology( Received :Feb.16 2001 )Abstract : Gold olloids uvere prepared by citrate- induced reduction of hydrogen tetrachlorourate , andgold nanoparicles were etrosacally sef-assembled with poly( dillydimehylammonium chloride ) into muli-layer thin films on silicon and quartz substrates. The particulate thin films were characterised by UV-vis spec-troscopy，suface enhanced Raman scattering , atomic force microscopy and resistiity measurements. Due to theinterparticle coupling betueen individual gold particles ,an obvious ollctve particle plasmon resonance rwas ob-sered on UV-vis spectra , and the particulate thin films exhibited a strong SERS efect. For mulilayer thin filmnswith a high particle coverage on substrates , resistivity of the order of 10-4 Q cm 1was yielded.Key words: gold colloid ; nanopartice ; electrostatic self-assembly ; thin film1 Introductionhave been extensively investigated for fundamental interestand technological importance. In recent years , studies onThe nanotechnology will play an important role in thegold particles as building units for the preparation of func-new century. Arising from the small sizes in the nanome-tional surfaces via chemical approaches were reported byter regime , nanoscaled or nanostructured materials possessdifferent authors5 34].In the present paper , the preparation and characteri-many unique properties distinctly different from those oftheir bulk counterparts. In order to synthesize novelzation of gold nanoparticulate thin films were described.nanostructured materials ,scientists all the world have ex-ploited many methodologies ，one category of which are2 Experimentalwet" chemical approaches. Through wet chemical ap-Ultrapure water of about 18. 0MO cm was preparedproaches , it is possible to prepare nanoparticles , build upsuperlattices and assembly novel devices in an altermativeby using a Barnstead Nanopure water purification unit.Substrates for thin film deposition were silicon wafers andway. Moreover , these approaches can offer many advan-quartz slides . The substrates were cleaned in d piranha"tages over conventional methods including low cost of in-bath( a mixed solution of 30% aqueous H2O2 and 98%strumentation and convenience in use. Electrostatic self-H2SO4 in a volume ratio of 3:7 ), rinsed with ultrapureassembly multilayer technique ，a wet chemical methodwater , then cleaned in a bath of 5:1:1 H20/30% aque-presented by Decher and his co- workens 1 , is character-ous H2O2/ 25% aqueous NH4OH and rinsed with ultra-istic of the spontaneous in situ assembly of precursorpure water again. The cleaned substrates were dried withmolecules from solutions to form ultrathin multilayera nitrogen stream .films. This novel technique provides new opportunities fonThe gold precursor was hydrogen tetrachloroauratethe development of nanocomposite layered materials 2 J.trihydrate( HAuCl; 3H2O , Aldrich ) , and the reducingDue to the great potential for applications in elec-agent was trisodium citrate( Na3 _ eitrate , Aldrich ). Col-tronic and optic devices ，metal and semiconductorloidal gold was prepared by using the chemical reductionnanoparticles are being increasingly exploited. Colloidalstrategy described by Frens 5. The preparative proceduregold particles exhibit particular diversity in optical anwas br中国煤化工0. 01wt% aqueouselectronic properties under different environments andHAuCl4YHCNMHGNa3 - citrate were pre-pared first. T0UmL ot aqueous HAuCl4 was heated to boil-YU Hai-hua( 余海湖) :Bom in 1963 ;Assoc. Prof. ;Fiber OpticSensing Technology Center ，Wuhan University of Technology ,ing on a hotplate ，and to this solution a proper amount ofWuhan 430070 , Chinaaqueous Na3- citrate was added with vigorous stirring.* This research was financially supported by China Scholar-Heating and stirring were continued until the change 0ship Councijapt数据atural Science Foundation of Hubei Province( Prjet 2000the color of the solution was completed. The obtained col-Vol.17 No. 1YU Hai-hu et al :Gold Nanoparticulate Thin Films Fabricated by...39loids were stored inside a refrigerator until needed .the dispersion. The resulted colloids remained clear andPoly( diallyldimethylammonium chloride )( PDDA ,no obvious precipitation could be observed after they wereAldrich) with average molecular weight of 400 000 -stored in a refrigerator for several days to several weeks .500000 was used without further purification. For thinIn the colloids no special stabilizer was added. On thefilm assemblies ,0. 01mol L- 1 aqueous solution of PDDAgold particles ，weekly coordinating ions such as citratewas prepared by diluting the commercial 20wt % aqueousand chloride ions form " ligand" shells. The electrostaticalPDDA with ultrapure water to the designed concentrationrepulsion between the particles prevents the colloidal par-on base of the monomeric repeating unit.ticles from aggregation.Gold particulate thin films were prepared under thefollowing procedure. The cleaned substrates were im-mersed into the aqueous solution of 0. 01mol L- 1 PDDAfor 10- 20 min and vigorously rinsed with ultrapure wa-ter. Then the PDDA-monolayer -masked substrates wereimmersed into a gold colloid for 10 min to 24 hours andvigorously rinsed with ultrapure water again. Multilayergold particulate thin films were formed by alternately repeating these two immersions. An immersion in aqueousPDDA was always performed before an immersion in a col-loid was done .Transmission electron microscopy ( JEM- 100CXII ,Fig. 1 TEM micrograph of 21nm gold colloidJEOL ) was used to determine the particle sizes of the gold0.16colloids and operated at 80kV accelerating voltage. UVvis spectroscopy( U-2010，Hitachi ) was used to charac-i 0.12terize the gold colloids and the assembled particulate thin0.08films. Tapping-mode AFM images of the self- assembledthin films were obtained by using a scanning probe micro-0.04scope system( Nanoscope IHIa , Digital Instruments ). Sur-face enhanced Raman scattering( SERS ) spectra of aque-250400550 70085ous pyridine on the colloidal gold surfaces were collectedwith He-Ne 632. 8nm excitation on a laser Raman spec-trometer system ( MK1-1000 , Renishaw ). MeasurementsFig.2 UV-vis absorption spectrum of 21nm gold colloidon the resistivities of the self-assembled thin films werePDDA is a strong electrolyte and ，in a wide pHcaried out with a 4-probe resistivity meter ( Lroesta-MPrange , will be strongly charged due to dissociation. WhenT-350 , Mitsubshi Chemical ) under ambient conditions.a cleaned substrate is immersed into aqueous PDDA , amonolayer of PDDA will form on the substrate. On the3 Results and DiscussionPDDA-monolayer-masked substrate ，the rest positivelycharged quatemnary ammonium groups of PDDA form a netGold colloids with different diameters were preparedpositively charged distribution on the outmost surface .by adding a proper amount of 1wt% aqueous Na3. citrateWhen the PDDA-monolayer -masked substrate is immersedto the boiling aqueous HAuCl. For example ,12nm goldinto a gold colloid , as a result of the strong electrostaticcolloid was prepared by adding 4.0mL of aqueous Na;attraction between the opposite charges of the gold parti-citrate to 100mL of aqueous HAuCl4 , 21nm gold colloidcles and the quatermary ammonium groups , gold particlesby adding 1. 8mL , and 40nm gold colloid by adding 1.0will be driven toward the positive surfaces and formn a laymL , respectively. Particle sizes were determined by anal-er distrib中国煤化工Fig.3 are the opticalysis of the TEM micrographs. In Fig.1 is the TEM micro-absorptioY片CNMHGfiImnsonquartz.Be-graph of the colloidal gold prepared by adding 1 .8mL ,cause the PDDA film is transparent over the whole wave-and the average size of the gold particles is 21.3nm. Inlength range , the optical absorption attributes to the sur-Fig.2 is the UV - vis absorption spectrum of 21nm col-face plasmon resonance of the gold particles. The spectraloidal gold. The absorption band near 520nm atributes toclearly show that the optical absorption behavior of goldthe surfacd p据resonance of isolated gold particles inparticlate films is rather different from that of the isolated40Joumal of Wuhan University of Technology - Mater. Sci. Ed.Mar. 20020.35bled onto the substrate , more obvious red shift and broad-028ening of the absorption band could be observed. These3 layerschanges indicate that the interparticle coupling is en-0.21hanced further after more gold particles are assembled on-0.14、2 layersto the substrate as the assemblies give rise to an increasedparticle density in the film.0.07 t1 layersIn Fig. 4 are the Tapping-mode AFM images of an 8-layer film of 21nm gold particles on a silicon wafer. The300 440 580 720 860 1000images reveal that the particulate film is statistically uni-WavelengUh/umform and that there is obvious interpenetration or overlapof particles between adjacent layers within the film. WhenFig.3 UV-vis absorption spectra of self- asembleda layer of gold particles is assembled from the gold colloidgold particulate thin filmsonto a PDDA-monolayer-masked substrate , there are al-particles in the dispersion. On the spectra of gold parti-ways voids left unoccupied within the layer due to the in-clate films，red shift and broadening of the absorptionterparticle repulsion , and gold particles of the next layerband are evident. The generation of the red shift featuremay either fill the voids or just locate at the tops of goldin the absorption spectra results from the interparticle cou-particles of the previous layer. As a result , an intertex-pling of individual colloidal gold within the assembledture - mode structure was formed. It should be notedfilms. When the spacing between gold particles is smallerthat, due to both the convolution of the actual particlethan the wavelength of the incident light , the interparticlewidth by the AFM tip and the presence of the polycationcoupling will be enhanced and a collective plasmon reso-shell on the gold particles , the particle sizes appear to benance at a longer wavelength will occur. When the secondmuch lager than the results yielded with TEM.layer and the third layer of the gold particles were assem-1.000.万0.508.2s8.20..0.258.0.7 1..Fig.4 Tapping mode AFM images of an 8- layer gold particulate thin film on siliconAs long as the interparticle spacing is substantiallycm- 1 on the SERS spectrum attribute to the vibrations ofless than the wavelength of the incident light , some roughpyridine ring. However , under the same condition , thesurfaces of metals , such as gold , silver and copper，mayvibration bands can not be observed with the same pyri-exhibit a SERS effect6]. The UV-vis spectra in Fig. 3dine solution on the PDDA monolayer surface. The resultsshow that the spacing between the gold particles is smallshow that the SERS effect displayed by the colloidal goldenough to set of interparticle coupling. So it can be ex-surface is very strong. SERS spectrum is useful for spec-pected that the self- assembled gold particulate films aretral investiatien nf rforpe nhannmena. Because it is verySERS active. In Fig.5 are the Raman spectrum of 1.0moleasy to中国煤化工flms onto substatessL-1 aqueous pyridine on PDDA-modified glass and theof any:YHC N M H G electrostatic self-as-SERS spectrum of 1x 10-'mot L 1 aqueous pyridine onsembled technique , it is possible to construct SERS-activea glass-supported 2-layer film of 21nm gold particles. Atsubstrates for Raman detection of some gases or liquids ,each measurement，5μL of aqueous pyridine was droppedwhich are difficult to be detectd by some ordinary meth-onto the seel据The bands at 1013. 1 and 1037. 8ods.Vol.17 No. 1YU Hai-hu et al :Gold Nanoparticulate Thin Films Fabricated by...413000012001000250001013.18001033.620000600150001037.8400100002005000920 950 980 1010 1040 1070 1100Raman shil0/cm 'Raman shil/rn*'(a(bFig.5 Raman spectrum of 1.0mnot L~ ! aqueous pyridine on a glass- supported PDDA monolayer( a ) and SERSspectrum of 1 x 10-*mot L 1 aqueous pyridine on a glass supprted 2-layer PDDA/ Au film( b)The conductivity of a gold particulate thin film is al-gest applications in electro-optic devices and sensors foso sensitive to the interparticle spacing. In our work , re-gold particulate films fabricated by the easy electrostaticsistivity measurements were carried out on different partic-self-assembly process.ulate films ,and a resistivity from very high level to rela-tively low level had been yielded by finely modulating theAcknowledgementcompositions and the structures of the films. Generally ,3alternative immersion cycles in aqueous PDDA and inSome work of this research was carried out at Vir21nm gold colloid could yield a resistivity detectable byginia Polytechnic Institute and State University , USA .the resistivity meter , and thin films with a low resistivityThe author would like to thank Prof. R. 0. Claus for hisof the order of 10-4 cm could be made after8~ 10 al-guidance , and thank Dr. Y. - X. Wang and Dr. Y. Liutermative immersions were done. The spacing betweenfor their helps.particles or the volume ratio of gold particles to the poly-mer is the main factor governing the conductivity of theReferencesfilms. If the spacing between particles is smaller or thevolume ratio of the gold particles to the polymer is higher，1 Decher G , Hong J D. Buildup of UInatin Mulilayer Filmns by aa multilayer thin film will have a high partice coverage onSelf- Assembly Process. II. Consecutive Adsorption of Anionicand Cationic Bipolar Amphiphiles and Polyelctrolytes onthe substrate and will be more conductive. The resultsCharged Susfaces. Ber . Bunsen Phys. Chem. ,1991 ,95:( 11)show that gold particulate films with tunable electrical1430- 1434properties can be fabricated by using the electrostatic self-2 Fendler J H. Self- Assembled Nanostructured Materials. Chem.assembly technique .Mater. , 1996 ,8 8):1616- 16243GrabarKC,FreemanRG,HonmerMB,NatanMJ.Prepa-4 Conclusionsration and Characterization of Au Colloid Monolayers. Anal .Chem. ,1995 ,674)735- 743Wang J , ZhuT ,Fu X Y ,Liu Z F. Study of Chemical En-In this paper we described that colloidal gold washancement in SERS from Au Nanoparticles Assembly. Actaprepared by citrate- induced reduction of HAuCl4 and thatPlhysico- Chimica Sinica ,1998 ,14( 6 ) 485 - 489( in Chinese )gold nanoparticulate thin films were fabricated by the5 G Frens. Contolled Nucleation for the Regulation of the Particleelectrostatic self- assembly technique. After the goldSize in Monodisperse Gold Suspension. Nature Physical Sci. ,nanoparticles were assembled onto substrates ,the collc-1973 241 20- 22tive surface plasmon resonance absorption could be ob-5 Freeman R G , Grabar K C , Allison K J ,et al . Self- Assembledserved ,and a strong SERS effect and a high conductivityMetal Colloid Monolayers An Approach to SERS substrates. Sci-0- 1637could be yielded on the assembled films. The results sug-中国煤化工YHCNMHG