Molecular engineering of low-dimensional nanostructures and regular networks
In his classic speech of 1959, Richard P. Feynman pointed out that there is ‘plenty of room at the bottom’. He predicted exciting new phenomena that might revolutionize science and technology and impact on our everyday lives—if only we gain precise control over matter, down to the molecular level.
Methodologies leading toward control of matter at the nanoscale. For top-down fabrication methods like lithography, writing, or stamping are used to define the desired features. The bottom-up techniques exploit self-processes for ordering of supramolecular or solid state architectures from the atomic to the mesosopic scale. Shown (clockwise from top) are a nanomechanical electrometer obtained by electron-beam lithography, patterned films of carbon nanotubes obtained by microcontact printing and catalytic growth, a single carbon nanotube connecting two electrodes, a regular metal-organic nanoporous network integrating iron atoms and functional molecules, and seven carbon monoxide molecules forming the letter ‘C’ positioned with the tip of a scanning tunnelling microscope. Adapted from Nature 437 (2005) 671.
With carefully designed functional molecular building blocks, weak and selective noncovalent linkages program the formation of highly organized systems with molecular-level feature control. However, the noncovalent design of molecular nanoarchitectures on solid surfaces requires the exploration of a special physicochemical stage. We explore the principles of molecular architectonic on well-defined substrates using sophisticated atomic-resolution tunneling microscopy and spectroscopy techniques in conjunction with complementary integral experimental methods and theoretical investigations. Supramolecular engineering in two dimensions opens up new avenues for novel nanostructured materials with unique functional properties and as such is a methodology of interest in the current quest for miniaturization, demanding innovative nanofabrication technologies.
Supramolecular engineering on surfaces: the crystal lattice is exposed to a beam of molecules. Decisive parameters for their spontaneous organization include adsorption (energy Ead), thermal migration and rotational motions (barriers Em and Erot), and lateral interactions (Es, Eas). Exemplary tectons with length s are shown, that self-assemble into regular chains near equilibrium conditions. The side view shows how surface atoms (a is the lattice periodicity) affect supramolecular synthon characteristics and noncovalent-bond length d. From Annu. Rev. Phys. Chem. 58 (2007) 375.
For the handling of complex molecules on metals, we need to understand adsorption, mobility, and lateral interactions, all of which depend on the substrate atomic environment, chemical nature, and symmetry. Only the accurate balancing of lateral and surface interactions allows for the emergence of supramolecular order.
Classification of basic interactions and processes, with associated energy (barrier) and typical distances relevant when functional molecular species are employed to engineer molecular architectures on metal substrates. Surface-specific aspects listed in top five rows. Characteristics of direct intermolecular noncovalent bonding (bottom four rows) are representative for 3D compounds.
|Adsorption||Ead≈0.5–10 eV||≈1.5–3 Å||Directional, site selective|
|Surface migration||Em≈0.05–3 eV||≈2.5–4 Å||1D/2D|
|Rotational motion||Erot dim (Em)||s||2D|
|Indirect substrate mediated||Es≈0.001–0.1 eV||a to nanometerrange||Oscillatory|
|Reconstruction mediated||Es 1 eV||short||Covalent|
|van der Waals||Eas≈0.02–0.1 eV||< 1 nm||Nonselective|
|Hydrogen bonding||Eas≈0.05–0.7 eV||≈1.5–3.5 Å||Selective, directional|
|Electrostatic - ionic||Eas≈0.05–2.5 eV||Long range||Nonselective|
|Metal-ligand interactions||Eas≈0.5–2 eV||≈1.5–2.5 Å|
In three dimensions, the selectivity and directionality of hydrogen bonds offer excellent means for noncovalent synthesis. Genuine H-bonded nanostructures with distinct shape can similarly be realized on surfaces. Particularly useful are planar-extended π-system tectons with peripheral functional groups. On appropriate substrates, they adsorb in flat-lying geometries favoring lateral molecular recognition. Similar with 3D hydrogen-bonded architectures, cooperativity — i.e., geometrical complementarity promoting the formation of multiple weak linkages — is an important factor in 2D supramolecular engineering..
Self-assembly of a supramolecular nanograting on the Ag(111) surface using PVBA molecules. The hydrogen bonding motif providing multiple weak linkages is shown at the right. Adapted from Angew. Chem. Int. Ed. 39 (2000) 1230.
"Engineering atomic and molecular nanostructures at surfaces"
J.V. Barth, G. Costantini and K. Kern, Nature 437, 671-679 (2005)
"Coexistence of one- and two-dimensional and supramolecular assemblies of terephthalic acid on Pd(111) due to self-limiting deprotonation" M.E. Cañas-Ventura, F. Klappenberger, S. Clair, S. Pons, K. Kern and H. Brune, T. Strunkus and Ch. Wöll, R. Fasel, J.V. Barth, J. Chem. Phys. 125, 184710 (2006)
"STM study of terephthalic acid self-assembly on Au(111) : Hydrogen-bonded sheets on an inhomogenous substrate" S. Clair, S. Pons, A.P. Seitsonen, H. Brune, K. Kern and J.V. Barth, J. Phys. Chem. B 108, 14585 (2004)
"Supramolecular assemblies with trimesic acid at a Cu(100) surface" A. Dmitriev, N. Lin, J. Weckesser, J.V. Barth and K. Kern, J. Phys. Chem. B 106, 6907 (2002).
"Stereochemical effects in supramolecular self-assembly at surfaces : 1-D vs. 2-D enantiomorphic ordering for PVBA and PEBA on Ag(111)" J.V. Barth, J. Weckesser, G. Trimarchi, M. Vladimirova, A. De Vita, C. Cai, H. Brune, P. Günter and K. Kern, J. Am. Chem. Soc. 124, 7991 (2002).
"Building supramolecular nanostructures at surfaces by hydrogen bonding" J.V. Barth, J. Weckesser, C. Cai, P. Günter, L. Bürgi, O. Jeandupeux, K. Kern, Angewandte Chemie Int. Ed. 39, 1230 (2000).