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la104499bThickness, Surface Morphology, and Optical Properties of Porphyrin

ARTICLE

http://www.wendangku.net/doc/bacadf9b10a6f524ccbf85a1.html/Langmuir Thickness,Surface Morphology,and Optical Properties of Porphyrin Multilayer Thin Films Assembled on Si(100)Using Copper(I)-Catalyzed Azide-Alkyne Cycloaddition

Peter K.B.Palomaki,Alexandra Krawicz,and Peter H.Dinolfo*

Department of Chemistry and Chemical Biology and The Baruch’60Center for Biochemical Solar Energy Research, Rensselaer Polytechnic Institute,1108th Street,Troy,New York12180,United States

b Supporting Information

ABSTRACT:We report the structure,optical properties and

surface morphology of Si(100)supported molecular multi-

layers resulting from a layer-by-layer(LbL)fabrication method

utilizing copper(I)-catalyzed azide-alkyne cycloaddition

(CuAAC),also known as“click”chemistry.Molecular based

multilayer?lms comprised of5,10,15,20-tetra(4-ethynyl-

phenyl)porphyrinzinc(II)(1)and either1,3,5-tris(azidomethyl)-

benzene(2)or4,40-diazido-2,20-stilbenedisulfonic acid disodium

salt(3)as a linker layer,displayed linear growth properties up to

19bilayers.With a high degree of linearity,specular X-ray re?ectivity(XRR)measurements yield an average thickness of1.87nm/bilayer for multilayers of1and2and2.41nm/bilayer for multilayers of1and3.Surface roughnesses as determined by XRR data?tting were found to increase with the number of layers and generally were around12%of the?lm thickness.Tapping mode AFM measurements con?rm the continuous nature of the thin?lms with roughness values slightly larger than those determined from XRR.Spectroscopic ellipsometry measurements utilizing a Cauchy model mirror the XRR data for multilayer growth but with a slightly higher thickness per bilayer.Modeling of the ellipsometric data over the full visible region using an oscillator model produces an absorption pro?le closely resembling that of a multilayer grown on silica http://www.wendangku.net/doc/bacadf9b10a6f524ccbf85a1.htmlparing intramolecular distances from DFT modeling with experimental?lm thicknesses,the average molecular growth angles were estimated between40°and70°with respect to the substrate surface depending on the bonding con?guration.

’INTRODUCTION

The fabrication of thin?lms utilizing layer-by-layer(LbL) assembly techniques provides a straightforward and?exible method for the creation of nanostructured materials with excellent precision.The LbL approach allows for facile manip-ulation of physical,electronic,photophysical,and chemical properties of the thin?lms.Films with well-de?ned structures have successfully been made utilizing the LbL technique and their properties are generally dictated by the polymeric or molecular components used in the assembly process.This ?exibility in thin?lm design has signi?cant implications in the development of advanced materials for nonlinear optics, photovoltaics,memory devices,sensors,molecular electronics, etc.1-3

LbL based thin?lm structures are unique among self-assem-bled systems in that they provide control over the nanoscopic ordering of the materials with respect to a macroscopic orienta-tion,i.e.,the axis normal to substrate surface.The most common variety of LbL assemblies are those based on the sequential deposition of polyanions and polycations on a charged substrate surface.These materials,initially described by Decher,4-6rely on the electrostatic interactions between the opposite charges on the di?erent polyelectrolytes and have resulted in the construc-tion of a wide range of nanostructured thin?lm architectures.1-3,7

Numerous variations of the LbL assembly scheme have also been developed utilizing discrete molecular species as the layer components.Molecular based LbL methodologies,also known as molecular layer deposition(MLD),typically employ self-limiting reactions between symmetric multifunctional molecules and linkers that yield single monolayers deposited at each step. Sagiv was the?rst to describe molecular-based LbL assemblies using siloxane couplings between layers of alkane chains.8,9 Following these reports,Mallouk described the use of zirconium phosphonates as an e?cient coupling group for the creation of mixed inorganic-organic multilayers with e?cient growth over several multilayers.10-14Since these?rst reports of molecular based LbL assemblies,others have expanded on the siloxane15-21 and zirconium(or hafnium)phosphonates,22-26as well as using additional organic1,27-31and inorganic32-35coupling chemis-tries as a means of multilayer formation.Generally,these types of reaction sequences have been shown to produce well ordered, multilayer thin?lms that are amenable to various molecular

la104499bThickness, Surface Morphology, and Optical Properties of Porphyrin

components.

Received:November11,2010

Revised:February14,2011

Published:March16,2011