Self-assembled monolayer

Self-assembled monolayers (SAM) are assemblies of organic molecules that form spontaneously on surfaces by <a href="/facts/Adsorption/ARH9peYM">adsorption</a> and organize themselves into more or less distinct domains (head group, chain/backbone, and tail/end group). In some cases, molecules that form the monolayer do not interact strongly with the substrate. This is the case for <a href="/facts/Porphyrins/lKNtdXp1">porphyrins</a> on <a href="/facts/Highly_oriented_pyrolytic_graphite/jxXtbSsX">HOPG</a> and two-dimensional supramolecular networks of <a href="/facts/Perylenetetracarboxylic_dianhydride/q2Y7elYa">PTCDA</a> on <a href="/facts/Gold/1znknojp">gold</a>. In other cases, the head group has a strong affinity for the substrate and anchors the molecule. Such an SAM consisting of a head group, chain (labeled "tail"), and functional end group is depicted in Figure 1. Common head groups include <a href="/facts/Thiols/VemKEDP9">thiols</a>, <a href="/facts/Silanes/1t4s6Icu">silanes</a>, and <a href="/facts/Phosphonates/8mg6nNRP">phosphonates</a>.

SAMs are created by the <a href="/facts/Chemisorption/QW4hL1RR">chemisorption</a> of head groups onto a substrate from either the vapor or liquid phase followed by a slower organization of "tail groups". Initially, at small molecular density on the surface, adsorbate molecules form either a disordered mass of molecules or an ordered two-dimensional "lying down phase". At higher molecular coverage, adsorbates can begin to form three-dimensional crystalline or semicrystalline structures on the substrate surface over a period of minutes to hours. The head groups assemble on the substrate, while the tail groups assemble far from the substrate. Areas of close-packed molecules nucleate and grow until the surface of the substrate is covered in a single monolayer.
Adsorbate molecules adsorb readily because they lower the surface free-energy of the substrate and are stable due to the strong chemisorption of the head groups. These bonds create monolayers that are more stable than the physisorbed bonds of <a href="/facts/Langmuir%E2%80%93Blodgett_film/bBFS3ruV">Langmuir–Blodgett films</a>. For example, the <a href="/facts/Trichlorosilane/nEYfkAeP">trichlorosilane</a> head group of an <a href="/facts/Perfluorodecyltrichlorosilane/BwMHfUKg">FDTS</a> molecule reacts with a <a href="/facts/Hydroxyl/1Hs2QAEv">hydroxyl</a> group on a substrate to form a very stable covalent bond [R-Si-O-substrate] with an <a href="/facts/Bond_energy/q4be3EeR">energy</a> of 452 kJ/mol. Thiol-metal bonds are on the order of 100 kJ/mol, making them fairly stable in a variety of temperatures, solvents, and potentials. Monolayers pack tightly due to <a href="/facts/Van_der_Waals_interactions/jyGU8Vh9">van der Waals interactions</a>, thereby reducing their own free energy. The adsorption can be described by the <a href="/facts/Langmuir_adsorption_isotherm/U7eDnqIv">Langmuir adsorption isotherm</a> if lateral interactions are neglected. If they cannot be neglected, the adsorption is better described by the <a href="/facts/Alexander_Frumkin/PaKExeC9">Frumkin</a> isotherm.

Self-assembled monolayer open-in-new

Self-assembled monolayer