Nanoscale colloidal particles: Monolayer organization and patterningT. Sato, D. G. Hasko, and H. Ahmed Citation: Journal of Vacuum Science & Technology B 15, 45 (1997); doi: 10.1116/1.589253 View online: http://dx.doi.org/10.1116/1.589253 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/15/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Electrode modification by electron-induced patterning of self-assembled monolayers J. Vac. Sci. Technol. B 20, 2734 (2002); 10.1116/1.1523026 Direct patterning of self-assembled nanocrystal monolayers by electron beams Appl. Phys. Lett. 78, 1915 (2001); 10.1063/1.1358363 Nanoscale patterning of self-assembled monolayers with electrons J. Vac. Sci. Technol. B 18, 3414 (2000); 10.1116/1.1319711 Patterning of silicon nanopillars formed with a colloidal gold etch mask J. Vac. Sci. Technol. B 17, 3239 (1999); 10.1116/1.590988 Single electron transistor using a molecularly linked gold colloidal particle chain J. Appl. Phys. 82, 696 (1997); 10.1063/1.365600

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Nanoscale colloidal particles: Monolayer organization and patterning *T. Satoa)Hitachi Cambridge Laboratory, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE,United Kingdom

D. G. Hasko and H. AhmedMicroelectronics Research Centre, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE,United Kingdom

~Received 18 June 1996; accepted 22 November 1996!

The monolayer deposition of nanoscale colloidal particles~Au citrate sols! was demonstrated byemploying an aminofunctional silane@3-~2-Aminoethlyamino! propyltrimethoxysilane# as acoupling agent. The compatibility of this colloidal Au deposition method with conventional electronbeam lithography techniques was examined, and the two-dimensional patterning of the Au colloidalmonolayer was demonstrated. Using this fabrication method, a proposal for a single electron devicestructure based on nanoscale~2 nm diameter! gold colloidal particles was made. ©1997 AmericanVacuum Society.@S0734-211X~97!02601-2#

I. INTRODUCTION

Consider a single-electron device consisting of a metal orsemiconductor island surrounded by dielectric tunneling bar-riers. In order to realize the operation of the device at el-evated temperatures, the island should be sufficiently smallso that the charging energy of the island becomes greaterthan the thermal energy. This requires the island size to beless than; 10 nm, which is a size that is less than thefabrication limit of conventional electron beam lithography.Previous authors who have realized elevated temperature op-eration (.77 K! have overcome this limitation by employingself-organizing structures to define the island sizes. For ex-ample, Chenet al.1 employed an ionized beam depositionmethod to form AuPd metal islands of 2–3 nm. Also, Taka-hashiet al. used stress-induced oxidation inhomogeneity toform a 5-nm-wide constriction in a Si quantum wire.2

Gold colloidal particles~or citrate Au sol!3 are known tohave sizes that can be in the nanometer range. Such nanos-cale colloidal particles, if they can be properly incorporatedin a single electron device structure, may provide an alterna-tive method for forming small islands. Unlike other granularmetal films, such as those prepared by metal vapor deposi-tion, ion beam and cluster beam deposition methods, the ci-trate Au sol exhibits good size controllability with well-defined size distributions.3 For instance, the size distributionof nominally 10 nm Au colloid has a standard deviation of 1nm (;10%!. Since the self-capacitance plays a dominantrole in determining the charging energy of an island in thissize range, the improved size uniformity of the colloidal par-ticles can give a more uniform Coulomb gap in colloid-basedsingle electron devices. As a preliminary attempt at incorpo-rating the nanoscale colloidal particles in a single electrondevice, the method of forming a monolayer Au colloidal par-ticle is presented in this article.

II. PRINCIPLE OF EXPERIMENT

The citrate Au sol is an aqueous solution in which Auparticles are suspended. The suspension is realized by citrateions adsorbed on the particle surface,4 which give a negativecharge to the Au particles. The incorporation of these par-ticles on a device requires the extraction of the particles fromthe solution. However, drying up a droplet of the solutiondoes not result in a monolayer coating, instead, an uncontrol-

*Published without author corrections.a!Electronic mail: sato@phy.cam. ac.uk

FIG. 1. Monolayer gold colloidal particle deposition method.~a! Hydroxylgroup terminated SiO2 surface;~b! amino functional silane deposition;~c!immobilization of gold colloidal particle.

45 45J. Vac. Sci. Technol. B 15(1), Jan/Feb 1997 0734-211X/97/15(1)/45/4/$10.00 ©1997 American Vacuum Society

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lable coagulation of particles is formed. In order to spreadthe particles on an insulating substrate surface, we have cho-sen an amino-functional silane, i.e., 3-~2-Aminoethlyamino!propyltrimethoxysilane~APTS!, as an adhesion agent for theAu particles. Unlike other colloidal Au deposition methodspublished recently,5,6 the choice of APTS gave us compat-ibility with conventional electron beam lithography tech-niques, which enables the realization of two-dimensionalpatterning of the colloidal particle monolayer.

Figure 1 is a schematic representation of the monolayerdeposition mechanism. The adsorption mechanism of the si-lane coupling agent to the SiO2 surface has been extensivelystudied.7,8 It is well known that the SiO2 surface is termi-nated by hydroxyl groups when exposed to the atmosphere,as is shown in Fig. 1~a!. The adsorbed hydroxyl group den-sity 9,10 reaches a level of 5 molecules per 1 nm2. It is thesehydroxyl-group sites to which the amino-functional silanesare coupled. The amino-functional silanes are hydrolyzed inan aqueous solution and form hydrogen bonding with thesurface assisted by the water molecules. After this physisorp-tion, the direct condensation~chemisorption! of the silanetakes place because of the catalytic effect of the aminogroup, forming siloxane bonds with the substrate Si.7 As aconsequence, APTS molecules are oriented with the freeamino groups away from the substrate as is shown in Fig.1~b!.

The affinity of these amino groups to gold immobilizesthe Au particles on the silane-coupler treated substrate~Fig.1~c!!. Because of the electric charge carried by the colloidalparticles, a repulsive force is present between the particles inthe solution and those on the substrate. This repulsive forceprevents them from piling up on top of each other. This

interparticle force is the main cause of the monolayer forma-tion.

III. EXPERIMENT

A one-micron-thick SiO2 film thermally grown on a Sisubstrate was used in this experiment. After cleaning thesubstrate using acetone and iso-propyl alcohol~IPA! withultrasonic agitation followed by oxygen plasma ashing, thesamples were immersed in a dilute water solution of APTS~0.05% ~v/v!! for 5 min. Then the samples were lifted offfrom solution, dried with a dry nitrogen gun, and baked for30 min at 120 °C in an oven. The APTS-treated sampleswere then immersed in the citrate Au sol for 3 h. After im-mersion, the samples were lifted off from the sol, dried bythe nitrogen gun to eliminate the solution, and observed by ascanning electron microscope~SEM!.

The Au citrate sol was purchased from BioCell ResearchLaboratories and used without additionalpH controls; thepH of the sol was 7. The silane coupling agent 3-~2-Aminoethlyamino! propyltrimethoxysilane was purchasedfrom Dow Corning ~Z-6020 amino-functional silane! andused as received without further purification.

IV. RESULTS AND DISCUSSIONS

Figure 2 shows the results of scanning electron micro-scope~SEM! observation performed of a sample with a 20nm diameter Au particle coating. Figure 2 indicates the coat-ing uniformity that the present deposition method canachieve as well as the uniformity in particle size. From thisobservation, the particle coverage was obtained to be131011 particles/cm2. The coating was not washed off by

FIG. 2. 20 nm Au colloidal particles deposited on APTS-treated SiO2 sur-face.

FIG. 3. Patterning of Au monolayer.~a! APTS deposition on the PMMApattern;~b! removing PMMA; ~c! Au particle deposition.

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J. Vac. Sci. Technol. B, Vol. 15, No. 1, Jan/Feb 1997

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the cleaning solvent~acetone and IPA! even with ultrasonicagitation confirming that strong adhesion of the particles tothe surface was obtained.

The use of an aqueous solution in this work, rather thanorganic solvents, gave us a satisfactory answer to the prob-lem of combining our method with conventional electronbeam lithography since most electron beam resists are notsoluble in water. We can also avoid the problems caused byresist solvent interactions~such as swelling! similar to thosecommonly observed in the development process.

The fabrication procedure is summarized in Fig. 3. APMMA mask pattern is first formed on the SiO2 surface. TheAPTS silane coupler was deposited on the window regionsof the PMMA-patterned surface~Fig. 3~a!!. Then, the

PMMA together with APTS that is not in contact with theSiO2 is removed by the immersion of the sample in an ac-etone bath for 3 h and subsequently washed in another ac-etone and IPA bath with ultrasonic agitation. This gives thepattern of the APTS layer deposited only in the windowregions~Fig. 3~b!!. Finally, the sample is immersed in the Aucitrate sols~Fig. 3~c!!.

Figures 4~a! and 4~b! demonstrate the results obtained byusing the 20 nm diameter Au sol. The rectangular windowpatterns formed on the PMMA with varying spacings werereproduced by the deposited Au particle patterns; this obser-vation indicates that APTS deposition on the previouslyPMMA coated area is negligible. These pictures show a ten-dency for the particles to stick to the edges of the windowpatterns. The cause of the edge effect is not clear at present.We note, however, that coagulation aggregates, in whichneighboring particles are separated by thin ion shells~i.e.,0.8-nm-thick citrate ion layer4 for the present case!, areknown to conduct electronic current only via tunneling.11,12

If the edge effect encourages the formation of a one-dimensional coagulation aggregate on the pattern edge, thepresent deposition method may be used to realize multiple-tunnel junctions of an Au-particle string.

Figure 5 also shows the result obtained using a 2 nm Ausol. The particles were deposited in a 50-nm-wide window.In this case, the coverage density of the order of 1012

particles/cm2 was achieved. Here, the spacings between in-dividual particles were up to about 5 nm, in which case tun-neling via the air gaps as well as over the oxide surfacebetween the particles is conceivable. Figure 5~b! indicates apossible device configuration for a lateral multijunctionsingle-electron device similar to that of Ref. 1. More pre-cisely, a stripe on which the 2 nm Au colloidal particles aredeposited is incorporated into a lateral structure of the sourceand drain together with a side gate, forming a single electrontransistor.

FIG. 4. Patterned deposition of 20 nm Au colloidal particles.

FIG. 5. Patterned deposition of 2 nm Au colloidal particles and a possiblesingle electron transistor structure based on a lateral multiple-tunnel junc-tion.

47 Sato, Hasko, and Ahmed: Nanoscale colloidal particles 47

JVST B - Microelectronics and Nanometer Structures

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V. CONCLUSIONS

We have demonstrated monolayer coating of Au colloidalparticles by employing an amino-functional silane~APTS! asa coupling agent. Combining this method with conventionalelectron beam lithography, we have deposited the Au par-ticles in two-dimensional patterns. We have also proposed anovel single electron device structure based on nanoscalegold colloidal particles.

ACKNOWLEDGMENTS

The authors wish to acknowledge the help and advicereceived from Dr. K. Murakawa~Central Research Lab., Hi-tachi Ltd.! and Dr. D. A. Williams ~Hitachi CambridgeLab.!. This work was performed under the management ofFED as a part of the MITI R&D of Industrial Science andTechnology Frontier Program~Quantum Functional Deviceproject! supported by NEDO.

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