Thermal Stability and Ordering Study of Long- and Short-Alkyl Chain Phosphonic Acid Multilayers Muriel de Pauli, †,‡ Mariana de Castro Prado, § Matheus Josue Souza Matos, § Giselle Nogueira Fontes, ∥ Carlos Alberto Perez, † Mario Sergio Carvalho Mazzoni, § Bernardo Ruegger Almeida Neves, § and Angelo Malachias §, * † Laborató rio Nacional de Luz Síncrotron, Caixa Postal, 6192 - CEP 13083-970, Campinas, SP, Brazil ‡ Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, CEP 13083-859, Campinas, SP, Brazil § Depto. de Física, ICEx, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627 - CEP 30123-970, Belo Horizonte, MG, Brazil ∥ Divisã o de Metrologia de Materiais, Instituto Nacional de Metrologia, Normalizaç ã o e Qualidade Industrial (INMETRO) - CEP 25250-020, Duque de Caxias, RJ, Brazil ABSTRACT: Long-range order evolution of self-assembled phosphonic acid multilayers as a function of temperature is studied here for two molecules with diﬀerent alkyl chain length. By using synchrotron conventional diﬀraction, distinct order conﬁgurations are retrieved on phosphonic acid multilayers and their thermodynamic behavior monitored by energy-dispersive diﬀraction. This later technique allows us to observe the system behavior near order−disorder temper- atures, as well as to determine the most stable conﬁgurations in the range from room temperature up to 120 °C. Planar order is also addressed by wide-angle X-ray scattering (WAXS) transmission experiments. Order parameter phase diagrams are built based on the experimental results, showing the dominant conﬁguration at each temperature. The multilayer molecular long-range order retrieved from the experiments is corroborated by ﬁrst principles calculations based on the Density Functional Theory. The bulk conﬁgurations depicted in this work are produced by molecule−molecule interactions and allow for future comparisons with the behavior of ordered molecules in few-monolayers conﬁgurations, commonly used in organic devices, where the presence of surfaces and interfaces strongly aﬀects the molecule packing. ■ INTRODUCTION Amphiphilic molecules have been intensively studied in the past decades due to their wide range of applications in many areas. Their self-assembly capabilities provide the possibility of fabrication of long-range ordered structures with good coverage and layer thickness control. Surfaces modiﬁcations and functionalization can be obtained by the coating with speciﬁc organic molecules 1−3 and some of these systems have been used in the fabrication of optical 4 and electronic devices. 5−7 Phosphonic acids monolayers and few-layer structural behavior have been intensively studied 8−10 and are crucial for state-of- the-art devices. 5 However, the conﬁgurations obtained when the molecules are in contact with surfaces and interfaces can be strongly aﬀected by chemical interactions between molecules and their hosting substrates. As the number of monolayers (or bilayers) in a given system increases, the molecule−molecule interactions become extremely relevant. Knowing whether such interactions can compete or overcome the molecule−surface interactions is crucial since the precise control of their structural packing may inﬂuence device properties such as leakage currents and working temperature range. Phase transition, 11,12 structural, and morphological stud- ies 13−15 have been performed in a plethora of organic systems. Nevertheless, the detailed evolution of structural organization as a function of the temperature requires a combination of techniques and, therefore, has not been much explored in phosphonic acids, although the inﬂuence of alkyl chain length on in-plane long-range order had been addressed for phosphonic acid monolayers at room temperature. 16 In the present work, we report detailed conﬁguration and thermal stability analysis of multilayers obtained from two diﬀerent phosphonic molecules, one with a longer alkyl chain Octadecylphosphonic acid (OPA) [CH 3 (CH 2 ) 17 PO(OH) 2 ] and a shorter molecule − Octylphosphonic acid (OcPA) [CH 3 (CH 2 ) 7 PO(OH) 2 ] both of interest for device fabrication or as a model system for understanding similar molecules. 5 Studying phosphonic acid multilayers with a large number of mono- or bilayers oﬀers the possibility of observing stable conﬁgurations that are related to pure molecular interactions. Such information is crucial even to understand systems with few layers where the resulting molecule packing has weak inﬂuence from the substrate and can be simply ascribed to bulk Received: January 3, 2012 Revised: September 20, 2012 Published: September 25, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 15124 dx.doi.org/10.1021/la303087t | Langmuir 2012, 28, 15124−15133