Structure and growth of self-assembling monolayers

Abstract

The structural phases and the growth of self-assembled monolayers (SAMs) are reviewed from a surface science perspective, with emphasis on simple model systems. The concept of self-assembly is explained, and different self-assembling materials are briefly discussed. A summary of the techniques used for the study of SAMs is given. Different general scenarios for structures obtained by self-assembly are described. Thiols on Au(1 1 1) surfaces are used as an archetypal system to investigate in detail the structural phase diagram as a function of temperature and coverage, the specific structural features on a molecular level, and the effect of changes of the molecular backbone and the end group on the structure of the SAM. Temperature effects including phase transitions are discussed. Concepts for the preparation of more complex structures such as multi-component SAMs, laterally structured SAMs, and heterostructures, also with inorganic materials, are outlined. The growth and ways to control it are discussed in detail. Solution and gas phase deposition and the impact of various parameters such as temperature, concentration (in solution) or partial pressure (in the gas phase) are described. The kinetics and the energetics of self-assembly are analyzed. Several more complex issues of the film formation process including non-equilibrium issues are discussed. Some general conclusions are drawn concerning the impact of various molecular features on the growth behavior and concerning the relationship between growth and structural phase diagram. Finally, the potential of self-assembly as a route for the preparation of monolayers with pre-designed properties and SAMs as building blocks in heterostructures as well as application strategies are discussed.

Introduction

For a multitude of reasons, organic thin films have attracted considerable attention over the last years, although the subject has fairly old roots. More than 200 years ago, Franklin observed the calming influence of oil on water surfaces [1]. In the 19th century, Pockels prepared monolayers at the air–water interface [2], [3], [4], [5], followed by the works of Rayleigh [6], Hardy [7], Devaux [8], and others. Later, monolayers of amphiphilic molecules on the water surface were named after Langmuir [9], [10].

On solid substrates, Blodgett did the first study on the deposition of long-chain carboxylic acids [11], [12]. Around that time, amphiphilic monolayers were already used to control the wetting behavior of metal condenser plates in steam engines [13], [14], [15]. Systematic research on systems related to self-assembled monolayers (SAMs) was performed later by Zisman [16] and Blackman and Dewar [17]. For further account of the history of organic thin films, we refer to [18], [19].

In these earlier studies, in which structures and processes on the molecular level remained unexplored due to the lack of appropriate tools, much of the interest centered around macroscopic properties such as surface tension and wetting properties. With the microscopic tools available today, one can attempt to correlate macroscopic to microscopic properties, e.g., the change in surface energy to a change in molecular structure. In fact, the field of wetting and surface modification has undergone a revival, and the great potential of organic thin films for wetting control is recognized [20], [21], [22].

In addition, these materials often exhibit optical, electrical, optoelectronical, mechanical, chemical, or other properties interesting from the applications point of view, which are not accessible with inorganic materials. Besides applications in “classical” areas of technology, organic thin films can play an important role in interfacing bio-technological devices.

One deeper reason why organic materials are attractive in such diverse fields is probably what might be called the “modular concept of organic chemistry”, i.e., the tunability of the properties of these materials by selectively modifying specific functional groups while leaving the rest of the molecule unchanged. A good example for this is the change from a hydrophobic to a hydrophilic surface by changing just the endgroup of alkylthiol-based monolayers from –CH₃ to –OH.

In view of the several million organic compounds known, and a correspondingly wide variety of molecular properties, it is not surprising that there are different routes for the preparation of organic thin films. For thin polymer films, e.g., spin-coating is a very popular preparation method [23]. For the group of crystalline films of relatively small molecules, Fig. 1 schematically shows the most common methods.

• Langmuir films consist of amphiphilic molecules spread on a liquid surface like water [18], [24]. The hydrophilic headgroup has an affinity to the water while the hydrophobic endgroup sticks out on the other side.

• Langmuir–Blodgett (LB) films are prepared by transferring Langmuir films onto a solid substrate [19]. Multilayers are prepared by repeated (periodic) dipping of the substrate in appropriate solutions.

• Organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE) is very similar to evaporation techniques in ultrahigh vacuum (UHV) for inorganic materials. For example, aromatic molecules such as perylene-derivatives, which form molecular crystals in the bulk, are typical systems for OMBD [25], [26]. In OMBD, similar to inorganic MBE, not only the two-dimensional epitaxy of monolayers, but also the behavior along the normal, when thicker films are grown, is an important issue [25], [27].

• SAMs grown from solution or from the gas-phase, represent a further class of organic thin films. The defining feature is the chemisorption (or, generally, strong interaction) of the headgroup with a specific affinity to the substrate (Fig. 2). Since SAMs are the subject of this review, the concept of self-assembly is discussed in more detail in the next section.

We should note that the boundaries between some of these techniques are not rigid. For example, some systems prepared from the gas phase, particularly in the monolayer regime, in principle might be considered both as OMBE-systems and as SAMs. Also, one might view the distinction between Langmuir layers and SAMs as not absolutely sharp. Langmuir layers are on liquid surfaces and are typically also weakly bound to the substrate. SAMs as we define them here are on solid substrates and chemisorbed, i.e., strongly bound. An interesting intermediate case would be molecules on a liquid substrate with strong interaction, such as thiols on liquid Hg [28]. Once the temperature is lowered and the substrate frozen, this might be considered as a transition from a Langmuir layer to a SAM.

Self-assembly, in a general sense, might be defined as the spontaneous formation of complex hierarchical structures from pre-designed building blocks, typically involving multiple energy scales and multiple degrees of freedom. Self-assembly is also a very general principle in nature, as seen in the formation of, e.g., membranes from lipid molecules, or the living cell as probably the most important paradigm.

Self-assembled monolayers are ordered molecular assemblies that are formed spontaneously by the adsorption of a surfactant with a specific affinity of its headgroup to a substrate. Fig. 2 shows a schematic, including the constituents of a SAM-molecule (headgroup, chain or backbone, endgroup). We will use the headgroup-substrate “pair” to define the individual SAM-systems.

After the historical predecessors, mentioned above, a strong activity in the area of SAMs and progress in the understanding on a microscopic level started in the 1980s. Around that time, also important experimental tools like scanning probe microscopies and grazing-incidence X-ray diffraction were developed. Thiols (R-SH, where R denotes the rest of the molecule) on Au [29] and silane-based systems on SiO₂ [30] were identified as model systems. The general interest in organic thin films was one reason for these activities. In addition, SAMs are particularly attractive for the following reasons:

• the ease of preparation;

• the tunability of surface properties via modification of molecular structure and functions;

• the use of SAMs as building blocks in more complex structures, e.g., for “docking” additional layers to a surface;

• the possibility of lateral structuring in the nanometer regime;

• the applications made possible by these features.

Several fundamental issues in the context of growth and structure require investigation, the understanding of which will also promote the design of new applications. The following questions may serve to illustrate this:

• Which types of structures and phases are formed and which parameters characterize the order?

• In which way does the order appear and disappear (e.g., as a function of coverage or temperature) and what is the nature of the phase transitions?

• In which way do the various degrees of freedom and the different constituents of the molecule (headgroup, chain or backbone, endgroup) have an impact on the growth and the structure?

• What are the driving forces of self-assembly? What determines the growth kinetics and the growth regimes? What are the “internal” (e.g., chain length or substrate orientation) and the “external” (e.g., temperature) control parameters?

This review is written from a surface science perspective, with the focus on the fundamental principles governing the growth and the structures of self-assembling monolayers. While this naturally puts the emphasis on chemically simple compounds, we outline the rich opportunities in the area of heterostructures using SAMs as building blocks, lateral patterning, chemical functionalization, and some technological applications. Some of the earlier work has already been reviewed [19], [31], [32], [33], [34], [35], [36], in some cases from a more chemical or technique-oriented perspective.

The paper is organized as follows. In Section 2, the specific techniques employed in the study of self-assembly are briefly reviewed.

Section 3 deals with the structure of SAMs. Its goal is two-fold. One is to provide an overview of various self-assembling systems, attempting to cover the breadth of the field (Section 3.2). It is clear that in this rapidly growing field this goal cannot be reached to full satisfaction, and we apologize for omissions. We try to make sure that every group is quoted at least once, so that the interested reader might obtain a more complete list of references on a given subject by computer search. The second goal is to illustrate typical scenarios and mechanisms by investigating a few systems in more detail. Particular emphasis is put on the archetypal case of alkanethiols and related compounds on Au(1 1 1) (Section 3.1). Temperature-related issues are the subject of Section 3.3. In Section 3.4, more complex systems such as multi-component SAMs and vertical heterostructures are briefly reviewed, as well as lateral structuring and surface reactions.

Section 4 deals with the growth of SAMs, its energetics, and its kinetics. The role of various growth parameters, the differences and similarities of solution and gas phase deposition as well as ways to control the growth are discussed.

A general discussion is given in Section 5. We try to outline some general principles governing the structure and the growth behavior of SAMs and address the mutual relationship of growth and phase diagram. We also discuss the theoretical approaches existing so far. Finally, we give a brief overview of the applications and conclude with an outlook and some open issues.