The interaction of oxygen on Cu(110) in the temperature range between 10 and $500\mathrm{K}$ has been investigated by reflectance difference spectroscopy, scanning tunneling microscopy, and low energy electron diffraction. The results show that the dissociation of molecular oxygen is complete below $45\mathrm{K}$. Atomic oxygen induces the expulsion of Cu atoms from the terraces in a temperature range between 70 and $200\mathrm{K}$, indicating a low energetic barrier for this process. Further annealing to temperature above $250\mathrm{K}$ leads to the formation and ordering of a $(2\times 1)\mathrm{O}$ stripe phase.

Real-time monitoring is essential for understanding and precisely controlling of growth of two-dimensional transition metal dichalcogenide (2D TMDC) materials. However, it is very challenging to carry out such studies during chemical vapor deposition (CVD). Here, we report the first, real time, in situ study of the CVD growth of 2D TMDCs. More specifically, the CVD growth of a molybdenum disulfide (MoS₂) monolayer on sapphire substrates has been monitored in situ using differential transmittance spectroscopy (DTS). The growth of the MoS₂ monolayer can be precisely followed by observation of the evolution of the characteristic optical features. Consequently, a strong correlation between the growth rate of the MoS₂ monolayer and the temperature distribution in the CVD reactor has been revealed. Our results demonstrate the great potential of real time, in situ optical spectroscopy to assist the precisely controlled growth of 2D semiconductor materials.

An atomically thin MoSe ₂ layer has been synthesized on mica using molecular beam epitaxy (MBE). The polymorphous of the MoSe ₂ layer depends on the coverage and the growth temperature. At low coverages and low growth temperature, 1T-MoSe ₂ forms in addition to a comparable quantity of 2H-MoSe ₂ . The metastable 1T-MoSe ₂ transfers gradually to the stable 2H-MoSe ₂ before the completion of the first monolayer. The current result sheds some light on the complexity of the nucleation and growth of transition metal dichalcogenide (TMDC) monolayers and implies a possible route for a phase selective synthesis using MBE.

We report a combined reflectance difference spectroscopy and scanning tunneling microscopy study of ultrathin α-sexithiophene (6T) films deposited on the Cu(110)-(2×1)O surface. The correlation between the layer resolved crystalline structure and the corresponding optical spectra data reveals a highly sensitive dependence of the excitonic optical properties on the layer thickness and crystalline structure of the 6T film.

The thickness dependent optical and electronic structure of para-sexiphenyl thin films grown on TiO₂(110) at around 400 K reveals that the substrate is first wet by one monolayer of molecules lying with their long axis parallel to the [001] direction of the substrate, while the molecules in subsequent layers are almost standing upright. Whilst ultraviolet photoemission spectroscopy (UPS) is sensitive to the molecules in the outermost layer, reflection difference spectroscopy (RDS) shows that the molecules at the buried interface do not dewet and maintain the orientation of the original wetting monolayer.

Thermally activated layer inversion of ultrathin pentacene/para-sexiphenyl organic heterostructures is observed using a combination of reflectance difference spectroscopy and scanning tunneling microscopy. The heterostructures are formed by deposition of sub-monolayer pentacene (PEN) on top of well ordered para-sexiphenyl (p-6P) layers on Cu(110) at 15 K. When the sample temperature is raised, these heterostructures invert, with pentacene molecules diffusing through the para-sexiphenyl buffer layer and getting in direct contact with the substrate. The observed irreversible inversion demonstrates that the p-6P/PEN/Cu(110) is energetically preferred over PEN/p-6P/Cu(110). Furthermore, the onset temperature of the inversion increases with the layer thickness of para-sexiphenyl indicating a corresponding increase of the kinetic barrier for the inversion. Our results demonstrate the strong influence of the configuration of organic heterostructures on their thermal stability, especially for the very thin layers.