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The atomic level understanding and control of the electron transport has important advantages for many applications, such as molecular electronics, solar cells and sensors. The target of my Ph.D. work is to investigate electronic and chemical properties of molecular nanosystems using scanning tunneling microscopy (STM) and spectroscopy (STS), which can be subdivided into two key objects. The first target is to control the electron transport of the Phthalocyanine (H2Pc) molecule deposited on the gate-tunable graphene device by applying electric filed. It is known that the electron transport properties can be controlled by the intramolecular reaction which reshapes the electronic configuration without any significant change in the conformation. The STM-induced tautomerization, i.e the interconversion between two isomers due to the migration of Hydrogen, can be observed in the form of telegraphic noise. Several methods to control the intramolecular Hydrogen-transfer by changing the chemical environment of the molecule have been developed, such as locating adatoms or introducing defects. However, these affect the molecules only in the nm range. In this work, using the gate-tunable STM, I want to demonstrate a global control of the chemical environment of the device which results in the changing of the switching rate of the Hydrogen-transfer. The second target of my Ph.D. work is to provide the atomic structure and its electronic properties of various charge traps in organic-inorganic perovskite surfaces and interfaces, particularly the surface of the CH3NH3MX3 crystal and its interface structure with TiO2, which will enable to provide new insight to design improved solar cells with better efficiency.

We explore the photoluminescence (PL) properties of hexagonal boron nitride (h-BN) quantum emitters embedded within atomically thin graphene/h-BN heterostructures fabricated by mechanical transfer. Stable light emission could be observed from h-BN emitters which due to the local presence of multilayer h-BN are not subject to fluorescence quenching by graphene. By using graphene as a top gate contact, the PL emission can be tuned by up to 24 meV per V/nm, with a high robustness of the emitters over several voltage sweep cycles. Two different types of h-BN emitters were observed, one with a quadratic and the other one with a linear Stark shift. Moreover, the vertical electric field leads to an asymmetric modulation of both the fluorescence intensity and lifetime between the negative and positive gate voltage regimes. The overall behavior can be well explained by a model involving different rates for electron and hole tunneling between the h-BN and graphene layers. Our findings suggest ultrathin van der Waals heterostructures as valuable platforms for fine tuning the optoelectronic properties of atomic defect-based quantum emitters. Published under license by AIP Publishing.

The adsorption of molecular oxygen has important implications for the catalytic activity of oxide surfaces, especially for oxidative TiO2 catalysts. In the case of anatase TiO2(101), several studies have noted the importance of surface defects which stabilize the adsorption of oxygen molecules because of charge transfer. Further oxygen adsorption via this mechanism is precluded once a stoichiometric TiO2 surface is attained. Here, the adsorption of oxygen molecules was re enabled through a simple modification of sample cleaning procedures, resulting in the formation of a well-ordered trellis arrangement of oxygen molecules. The trellis is strongly passivating and does not affect the band gap of the pristine anatase substrate. Furthermore, they induce the extension of anatase terraces up to 100 angstrom wide. The atomic and electronic structures of the trellis were investigated using scanning tunneling microscopy experimentation and density functional theory calculations.