Nearly-freestanding supramolecular assembly with tunable structural properties

Abstract

The synthesis and design of two-dimensional supramolecular assemblies with specific functionalities is one of the principal goals of the emerging field of molecule-based electronics, which is relevant for many technological applications. Although a large number of molecular assemblies have been already investigated, engineering uniform and highly ordered two-dimensional molecular assemblies is still a challenge. Here we report on a novel approach to prepare wide highly crystalline molecular assemblies with tunable structural properties. We make use of the high-reactivity of the carboxylic acid functional moiety and of the predictable structural features of non-polar alkane chains to synthesize 2D supramolecular assemblies of 4-(decyloxy)benzoic acid (4DBA;C[Formula: see text]H[Formula: see text]O[Formula: see text]) on a Au(111) surface. By means of scanning tunneling microscopy, density functional theory calculations and photoemission spectroscopy, we demonstrate that these molecules form a self-limited highly ordered and defect-free two-dimensional single-layer film of micrometer-size, which exhibits a nearly-freestanding character. We prove that by changing the length of the alkoxy chain it is possible to modify in a controlled way the molecular density of the "floating" overlayer without affecting the molecular assembly. This system is especially suitable for engineering molecular assemblies because it represents one of the few 2D molecular arrays with specific functionality where the structural properties can be tuned in a controlled way, while preserving the molecular pattern.

Figure 1

(a) Sketch of the 4DBA molecule (carbon atoms are drawn in dark grey, oxygen atoms in red and hydrogen atoms in white). Large scale (b) and close-up (c) STM images of the molecular assembly of 4DBA on Au(111) (V${}_b$=1.6 V; I${}_t$=100 pA; T=300 K); (c) Represents area marked by the white box (b). (d) Constant-current STM image of 4DBA/Au(111) (V${}_b$=1.3 V; I${}_t$=100 pA; T=300 K) displaying both the supramolecular pattern and the herringbone reconstruction.

Figure 2

(a,c) Constant-current STM images (V${}_b$=1.1 V; I${}_t$=100 pA; T=300 K) displaying two different supramolecular assemblies observed for 4DBA/Au(111); The black arrows represent the unit cell vectors. (b) and (d) are the Fourier transforms of panels (a) and (c), respectively; The blue arrows identify the unit cell vectors in the reciprocal space. (e) Top panel: STM image of 4DBA/Au(111) (V${}_b$=1.1 V; I${}_t$=100 pA; T=300 K); Bottom panel: line profiles along the green and blue lines of the top panel marking the length (l) and benzoic head width (w) of the 4DBA molecule.

Figure 3

Constant-current STM images of (a) 4TBA/Au(111) ( V${}_b$=0.22 V; I${}_t$=160 pA; T=300 K) and (b) 4HBA/Au(111) (V${}_b$=0.50 V; I${}_t$=500 pA; T=300 K). White vectors identify the unit cell vectors. The insets of panels (a) and (b) shows the corresponding Fourier transform; The blue vectors in the insets mark the unit cell vectors in the reciprocal space.

Figure 4

(a,b) Simulated 4DBA molecular building blocks having opposite supramolecular chirality. The red arrows mark the 4DBA molecules in a different conformation with respect to the others. The green box highlights the double hydrogen bonds within one dimer, whereas the blue boxes mark the electrostatic C–H$\cdots$O interaction between dimers. (c,d) The optimized supramolecular assemblies from the building blocks presented in panel (a) and (b), respectively. The purple box encloses the C–H$\cdots$O interaction between adjacent building blocks. (e,f) The optimized supramolecular structures of panels (c,d) are overlaid to the STM images of the two experimental assemblies (V${}_b$=1.1 V; I${}_t$=100 pA; T=300 K).

Figure 5

(a) Photoemission spectrum of the C 1s core level region of 4DBA/Au(111) measured at normal emission with photon energy of 350 eV. (b) Simulated C 1s photoemission line for 4DBA in gas phase. The energy positions of the 1s state of the different C atoms are marked by different color bars; A sketch of the 4DBA molecule is shown in the inset. A Voigt shape was adopted for each state. (c) Valence band for bare Au(111) (dotted gray curve), and 4DBA/Au(111) (full black curve) measured at normal emission with photon energy of 45 eV. ARPES map displaying the Shockley surface state of (d) Au(111), and (e) 4DBA/Au(111) measured with photon energy of 45 eV.