Understanding the structure of well-defined anatase TiO 2 surfaces is critical for deciphering site-specific thermal and photoreaction mechanisms on anatase TiO 2 . Using ultrahigh vacuum scanning tunneling microscopy (STM), we have studied the atomic structure of anatase TiO 2 (001) epitaxial thin films grown by oxygen plasma assisted molecular beam epitaxy. Bright rows of the (1×4) reconstructed surface are resolved as three types of features with different sizes. High-resolution STM images taken from the same area at different bias voltages show that these individual features are originated from combinations of two basic atomic building blocks. We propose a modified added molecule model for the anatase TiO 2 (001) surface structure.

Monolayer (ML) MoS 2 has become a very promising two-dimensional material for photorelated applications, potentially serving as the basis for an ultrathin photodetector, switching device, or transistors because of its strong interaction with light in ambient conditions. Establishing the impact of individual ambient gas components on the optical properties of MoS 2 is a necessary step toward application development. By using in situ Raman microspectroscopy with an environmentcontrolled reaction cell, the photoluminescence (PL) intensity of chemical vapor deposition (CVD)-grown MoS 2 MLs is monitored at different intralayer locations under ambient and controlled gas environments, such as N 2 , O 2 , and H 2 O. This new approach enables us to monitor the optical properties of MoS 2 at different locations on the flakes and separate the role of photoreaction of various gases during laser irradiation. Upon mild photoirradiation in ambient conditions, the PL intensity in the interior of the ML MoS 2 flakes remains unchanged, while the PL intensity at the edge region increases drastically. Photoirradiation in controlled gas environments reveals that O 2 is necessary to increase the PL intensity at the MoS 2 flake edges, attributed to the charge transfer of chemisorbed O 2 . N 2 or H 2 O and N 2 environments induce decreasing PL intensity upon repetitive laser irradiation. However, the H 2 O and O 2 gas mixture, a combination designed to mimic ambient conditions, is necessary to maintain the PL intensity at the interior of the ML MoS 2 flakes. Our study demonstrates that photoreactions with the gaseous environment on the MoS 2 ML flakes should be taken into consideration even upon mild photoirradiation because they strongly impact the flakes' optical properties.

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Tip-enhanced Raman spectroscopy (TERS) has shown that detecting single molecules with a high spatial resolution is possible in ultrahigh vacuum (UHV) at low temperature with plasmonic metallic substrates. It is still challenging to probe interactions of molecules with semiconductors, which is important in biosensing, photovoltaics, and many other applications. Here we demonstrate that in ambient conditions it is possible to obtain Raman signals from submonolayer molecular islands on bulk MoS 2 using TERS. Analysis of relative Raman signal intensity ratio and Raman spectral peak position from spatial TERS mapping showed differences in the adsorbate− adsorbate and adsorbate−substrate interactions on Au and MoS 2 substrates. The Raman transition which involves the vibration of the metal center of the CuPc molecule experienced a change in the relative Raman signal intensity ratio due to the differences in the molecule−substrate charge transfer interaction. In comparison to the other vibrational modes, the vibrational modes of the surface charge transfer interacting moieties involving the metal center experienced the smallest shift in the Raman spectral peak position on both Au and MoS 2 substrates. Further, the distributions of the peak position and relative intensity were narrower at the center of the island with respect to the isolated molecules due to the adsorbate−adsorbate interaction. This study shows the contribution of charge transfer between molecules and MoS 2 in the TERS spectra.