Yongli Gao

Yongli Gao

Professor of Physics


Department of Physics and Astronomy
University of Rochester
Rochester, NY 14627

(585) 275-8574 (O)
(585) 275-6987 (L)
(585) 273-2813 (F)
ygao@pas.rochester.edu

Research Interests

My research interests are in experimental surface physics. Specifically, I concentrate on spectroscopic studies on physical phenomena of solid surfaces and interfaces by examining the electronic and structural properties; electronic interactions and morphology of interfaces and interface formation; structural and spectroscopic properties of nanostructures; transient behavior of charge transfer across interfaces; interfaces in organic semiconductor devices; ultrafast dynamics of photoexcited electrons in solids.


Recent Research

 

Angle-resolved energy distributions curves of CH3NH3PbBr3, measured along _X and along _M. We studied the electronic structure of CH3NH3PbBr3 single crystal (001) surface with ARPES and IPES. The high quality of the smooth surface was confirmed by AFM and SEM. The elemental composition was investigated by XPS with a ratio close to the ideal value. Highly reproducible dispersive features of valence bands were observed with symmetry at the BZ center and boundaries. The four VB peaks are composed of Pb-s and Br-p orbitals as theoretical models predicted, and had different band dispersion widths. The largest dispersion came from the lowest binding energy band, being ~0.73 eV and ~0.98 eV for _X and _M directions, respectively. The measured band dispersions correspond to an effective hole mass as ~0.59 m0 and a lower limit of the hole mobility of 33.90 cm2V-1s-1 from the tight-binding fitting (C.C. Wang et al., Phys. Chem. Chem. Phys., in press).
Angle-resolved energy distributions curves of CH3NH3PbBr3, measured along _X and along _M

 

We investigated the energy level alignment and electronic structure evolution of the C60/LSMO interface. A total upward band bending of 0.72 eV was observed when the C60 film thickness increases from 0.5 to 8.0 nm, indicating an n-type to p-type transition of the film. The oxygen is proven to diffuse from LSMO to the C60 film. The n-p transition is ascribed to the diffused oxygen that acts as p-dopant of the C60 over layer. Shown in the figure are the UPS and XPS spectra of LSMO substrate. (a) Cut-off region of UPS, (b) O 1s core level, and (c) Mn 2p1/2 core level of XPS. Inset: LEED pattern of LSMO. (H.P. Xie et al. Appl. Phys. Lett. 108, 011603 (2016)).
The UPS and XPS spectra of LSMO substrate.

 

We have investigated the degradation mechanisms of CH3NH3PbI3 using XPS and XRD. CH3NH3PbI3 films with the right atomic ratio and crystal structure were successfully fabricated by co-evaporation. We find that the CH3NH3PbI3 film is not sensitive to oxygen. It is quite sensitive to moisture, and the interactions can be characterized in two stages. The first stage is for H2O exposure less than about 2 _ 1010 L, in which H2O acts as an n-dopant and the integrity of the perovskite remains intact. The first stage ends when the surface of the perovskite becomes highly n-doped. Higher exposure leads to the second stage when the perovskite quickly decomposes. The decomposition is characterized by the complete removal of N and part of I. The remnants of the film on the surface are crystalline PbI2, hydrocarbon complex, and O, the latter of which most likely from the adsorption of H2O on the surface. A model of water-catalyzed vaporization (WCAV) is proposed based on our observations (Y.Z. Li et al., J. Phys. Chem. C 119, 23996 (2015)).
Degradation mechanisms of CH3NH3PbI3 using XPS and XRD. CH3NH3PbI3 films

 

Schematic energy level alignment at the NPB/MoO3/PEN/C60/LiF/Alq3 interfaces and proposed mechanisms of holes and electrons generation. We investigated the interfacial electronic structure and the energy level alignment of MoO3 and LiFmodified PEN/C60-based CGL using UPS. As the charge generation unit, a dipole and a band bending were observed at the PEN/C60 interface. The energy level offset between the LUMO of C60 and the HOMO of PEN was measured to be 0.83 eV, which was in fact significantly smaller after considering the Urbach tail states. It was very beneficial for charges to generate and transport. High-density electrons and holes accumulated on the n-type C60 and p-type PEN, respectively, in the vicinity of the PEN/C60 junction, would be swept away from the PEN/C60 interface in opposite directions under external electric field and built-in field (X. Liu et al., Org. Elec. 17, 325 (2015)).
Schematic energy level alignment at the NPB/MoO3/PEN/C60/LiF/Alq3 interfaces

 

Citeria for MoOx to be effective with different organic materials. The horizontal dashed lines from the top to the bottom are criteria for one day air exposed, 450 _C vacuum annealed after 1 h air exposure, and as deposited MoOx, respectively. If the HOMO of a given organic material is above one of the lines, it will enjoy the full benefit for hole injection/extraction by the MoOx insertion layer treated corresponding to the lines (C.G. Wang et al., J. Vac. Sci. Tech. B32, 040801 (2014)).
Criteria for MoOx to be effective with different organic materials.