Melamine self-assembly and dehydrogenation on Ag(111) studied by tip-enhanced Raman spectroscopy

Abstract

The adsorption and self-assembly structures of melamine molecules on Ag(111) surface are studied by low temperature scanning tunneling microscope (STM) combined with tip-enhanced Raman spectroscopy (TERS). Two ordered self-assembly phases of melamine molecules on Ag(111) were studied by STM and TERS, combining with first principles simulations. The α-phase consists of flat-lying melamine molecules while the β-phase consists of mixed up-standing/tilted melamine molecules. Moreover, dehydrogenation of melamine can be controlled by annealing the sample as well as by tip-enhanced photo-catalytic effect. Our work demonstrates TERS as a powerful tool not only for investigating the configuration and vibration properties of molecules on a metal surface with high spatial resolution, but also for manipulating the chemical reactions with tip and photo-induced effects.

Dehydrogenation Induced Formation of Chiral Core-Shell Arrays of Melamine on Ag(111)

“…Another stripe-like assembly predominates the lower part of the image, and its structural characteristics are consistent with the previously reported melamine 𝛽-phase (Fig.S1) [40]. Note that the 𝛽 phase consists of not only triangular-shaped species with apparent height similar to that of the 𝛼 phase, but also bright protrusions, which were verified to be the upstanding molecules due to partial dehydrogenation of the amino groups (-NH2) [31,39,41,42,45,47,53]. Annealing this sample at higher temperature (𝑇ann) induced a drastic change of the assembly patterns.…”

“…[35] Melamine has also gained a renewed interest as it is the starting compound to synthesize graphitic carbon nitride materials, g-C3N4, which are promising candidates for metal-free catalysts. [36,37] In this context, melamine adsorption on different metal surfaces [31,[38][39][40][41][42][43][44][45][46][47] as well as their covalent bonding with other molecules [48][49][50][51] were investigated. Nevertheless, synthesis of molecular precise single-layered g-C3N4 sheet still remains a great challenge.…”

“…Motivated by previous works of melamine adsorption on Ag(111), [40,47] we systematically investigate the adsorption as well as possible surface-induced transformations of the precures on Ag(111) in a wide annealing temperature range in UHV, employing low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). Although covalent linking between melamine species is not observed on Ag(111), we identify a new complex phase composed of well-ordered core-shell-type clusters assembled with handedness.…”

“…[40] Note that the 𝛽 phase consists of not only triangular-shaped species with apparent height similar to that of the 𝛼 phase, but also bright protrusions, which were verified to be the upstanding molecules due to partial dehydrogenation of the amino groups (-NH2). [31,39,41,42,45,47,53] Annealing this sample at higher temperature (𝑇ann) induced a drastic change of the assembly patterns. For 𝑇ann ≈ 376 K, 𝛼 phase vanished leaving only 𝛽 phase on the surface.…”