Directed Assembly of Nanostructures

J.M. MacLeod    Università degli studi di Trieste, Trieste, Italy

F. Rosei    Université du Québec, Varennes, QC, Canada

1.1 Introduction

The realization that nanoscale matter often behaves differently with respect to the same materials in the bulk form has prompted a wealth of research aimed at understanding, characterizing, describing, and predicting ‘nano’ [1,2]. However, while ‘nanotechnology’ has been a buzzword for almost two decades, it has delivered fairly little so far in terms of new technologies, that is, new products that are commercialized and used by the general public.

One of the great promises of nanotechnology is the ability to do more in the same space: to advance our current technologies through miniaturization, so that each crop of electronics is smaller, faster, and more powerful than the one before. This is the manifestation of Moore’s law [3], the now-famous 1965 empirical prediction by Gordon Moore (who later went on to co-found Intel) that the number of transistors accommodated in a chip of given size doubles roughly every two years. The semiconductor industry has used this prediction as a roadmap over the last three decades. As the limits of this down-scaling approach the dimensions of single molecules and atoms, the discrepancy between nanoscale and bulk behavior has become evident. While this is detrimental in some situations, for example, in scaled-down versions of larger transistors that can exhibit problematic behaviors, such as unexpected leakiness, at nanoscale dimensions [4], it opens the door to opportunities for custom-designing new circuit architectures to exploit behaviors unique to the nanoscale. For example, quantum size effects [5,6], confinement of excitons [7,8], and high surface-to-volume ratios [9] can all impart new, unexpected, and potentially useful behavior to nanoscale systems.

To capitalize on the full potential of nanostructured materials and their properties, it is necessary to develop the ability to purpose-build nanoscale systems, a task which hinges on the precise placement of appropriate nanoscale building blocks in two and three dimensions (2D and 3D). This approach is generally referred to as ‘bottom-up’, implying the spontaneous formation of a desired architecture. This approach provides a diametric counterpoint to the ‘top-down’ techniques (typically lithographic techniques, which are very precise but must adhere to Rayleigh’s equation, and therefore cannot resolve fine nanoscale features [10]) used in the contemporary fabrication of semiconductor devices [11], and provides an intuitive mechanism for building architectures from countable numbers of atoms or molecules.

The use of molecules as the basic building blocks of nanoscale structures capitalizes on a wealth of knowledge that can be obtained from the study of biological systems [12–14]. Supramolecular chemistry [15], applied to nanoscale design [16–18], additionally benefits from the capabilities of synthetic chemists, since molecules can essentially be custom-designed for form and functionality salient to specific systems and devices [19,20].

The aim of this article is to provide an overview of the tools and techniques available for building nanoscale architectures from molecular building blocks, limiting ourselves primarily to a discussion of the geometry of molecular assemblies at surfaces, that is, structures confined to 2D. Outside of our focus will be atomic structures [21–24], clusters [25, 26], and quantum dots [27–29], all of which provide their own unique set of challenges and rewards. Our focus will be on the major experimental advances made via surface physics and chemistry over the past 25 years. The majority of the investigations that we describe have been performed with scanning probe microscopies (SPM), specifically, scanning tunneling microscopy (STM) [30–34]. The STM is a remarkably versatile instrument capable of imaging conducting and semiconducting surfaces [35], probing their electronic characteristics [36], investigating the vibrational characteristics of adsorbed molecules [37,38], interacting with the surface or adsorbates to produce new geometric and electronic configurations [39–41], or even to initiate chemical bond formation [42,43]. Many excellent books and reviews are available, describing various facets and uses of SPM [44–64].

After briefly discussing the fundamentals of directing nanoscale assembly of surfaces, as well as the most salient experimental techniques for probing these systems, we will provide an overview of significant experiments grouped by the type of interaction used to pattern the molecules: strong bonding between the molecules and the underlying surface, molecular self-assembly driven by hydrogen bonding, and metal–organic coordination, using inclusion networks to position molecules, and, finally, surface-confined polymerization for producing robust, covalently bonded structures. An emerging area that we will unfortunately neglect due to space limitations is the formation of ordered multicomponent assemblies driven by curved surfaces. We refer interested readers to the relevant literature [65–77].

1.2 Fundamentals of Directing Nanoscale Assembly at Surfaces

There are two competing types of interactions that control the formation of patterns at surfaces: (1) molecule–molecule and (2) molecule–substrate [78]. In most cases, bottom-up assemblies depend on the balance between (1) and (2); however, depending on the choice of surface (2) can be either the dominant interaction or can be almost suppressed, with various intermediate regimes. For example, graphite surfaces are essentially inert and therefore their participation in pattern formation is usually minimal besides providing regular and planar array of adsorption sites. On the other hand, reconstructed silicon surfaces are characterized by a high density of reactive unsaturated dangling bonds (DBs) that interact strongly with molecules upon adsorption, often causing the molecules to fragment as in the case of cyclo-addition reactions [79]. With respect to (1), most intermolecular interactions used so far are noncovalent in nature, that is, they may induce the formation of long-range ordered patterns, yet, are easily disrupted because of their weak bonding. This aspect has several advantages, including the ‘self-repair’ mechanisms that are well known in supramolecular chemistry: defects tend to disappear as the interactions locally break up the pattern forming a new ordered one devoid of defects. Notably, hydrogen bonding and metal–organic coordination are noncovalent interactions frequently employed to form ordered patterns in both 2D and 3D. van der Waals forces alone can usually lead to the formation of local patterns, yet their lack of directionality is usually a barrier to producing long-range patterns.

Stronger molecule–molecule interactions can lead to the formation of covalent bonds. While these are often desirable to obtain more robust structures with interesting mechanical and electronic properties, they are significantly more difficult to direct and their use for nanostructure formation at solid surfaces has been explored only in the last decade. Some elegant examples of covalent architectures, together with a discussion of their challenges and limitations, will be provided in Section 1.7.

1.2.1 Noncovalent Interactions between Molecules

1.2.1.1 Hydrogen bonding

Hydrogen bonds are formed between an electronegative atom and a hydrogen atom bonded to a second electronegative atom [80]. The strength of the hydrogen bond depends on the electronegativity of the atoms; Table 1 classifies hydrogen bonds as very strong (e.g., [F…H…F]−), strong (e.g., O—H…OC), or weak (e.g., C—H…O) depending on the bond energy, which ranges from 40 to <4 kcal mol−1, respectively. The directionality of the bond increases with strength. For crystal engineering, the ‘strong’ hydrogen bond is perhaps the most useful type [81,82]. For example, in the systems we describe in this article, hydrogen bonds between carboxylic groups (O—H…OC) are often used to drive self-assembly.