Atomic Resolution Interfacial Structure of Lead-free Ferroelectric K0.5Na0.5NbO3 Thin films Deposited on SrTiO3

Oxide interface engineering has attracted considerable attention since the discovery of its exotic properties induced by lattice strain, dislocation and composition change at the interface. In this paper, the atomic resolution structure and composition of the interface between the lead-free piezoelectric (K0.5Na0.5)NbO3 (KNN) thin films and single-crystalline SrTiO3 substrate were investigated by means of scanning transmission electron microscopy (STEM) combining with electron energy loss spectroscopy (EELS). A sharp epitaxial interface was observed to be a monolayer composed of Nb and Ti cations with a ratio of 3/1. The First-Principles Calculations indicated the interface monolayer showed different electronic structure and played the vital role in the asymmetric charge distribution of KNN thin films near the interface. We also observed the gradual relaxation process for the relatively large lattice strains near the KNN/STO interface, which remarks a good structure modulation behavior of KNN thin films via strain engineering.

rapid thermal annealing furnace. The final KNN thin films with a thickness of about 200 nm were fabricated by repeating the coating-heat treatment process 4 times.

Image acquisition and simulation
In this work, HAADF-STEM was carried out using the aberration-corrected JEOL ARM 200F operated at 200 kV with a probe size of 0.1 nm, semi-convergence angle of α=32 mrad and a collection angle interval between 80-170 mrad. Before the atomic resolution HAADF images were recorded, two-fold astigmatism A 1 , three-fold astigmatism A 2 and the axis coma B 2 were further adjusted by using the Ronchigram of anamorphous area close to the interesting area of the sample. HAADF image simulations were carried out in order to verify the contrast difference of the interface atomic columns. We used the (S)TEM simulation software WinHREM TM . Thermal diffuse scattering was taken account in Weickenmeier-Kohl Scattering Factor. We inputted crystal structure and such real experimental parameters in microscopy as aberration coefficient in Cs-corrector, U a =200kV, θ conv =32mrad, θ HAADF =  mrad. △t slice = 2Å, calculation step: 0.2Å. We just systematically varied the ratio of Ti and Nb atoms in crystal model of the interface. Figure S1. The atomic terrace at the interface.

EELS acquisition
For the EELS acquiring, 2cm camera length with 3mm EELS entrance aperture was used to maximize the intensity while achieve reasonable signal to noise ratio (SNR).
The energy dispersion of 0.5 eV/ch was adopted to contain all the relevant edges, the scanning pixels step was 0.06 nm. After background subtraction of the signals, based on extrapolation of the background prior to the relevant edges, the intensity of the Nb M 2,3 (363 eV) edge and Ti L 2,3 edge (456 eV) signals were extracted. The corresponding color-coded Ti and Nb map showed the relative position of the Ti L 2,3 (green) and Nb M 2,3 (red) signals. In the EELS, the ratio of ln(I t /I 0 ) is sensitive to thickness variation and proportional to t/λ, where I t is the total area under the spectrum, I 0 is the area under the zero-loss peak, λ is the total bulk inelastic mean free path and t is the specimen thickness. The plasmon scattering inelastic mean free path λ B can be estimated to be 123.0 nm (for STO).

Density Functional Theory Modeling
The interface of KNN/STO geometry inferred from STEM was used in density functional theory (DFT) calculations, which were carried out using CASTEP code.
The electron-electron exchange and correlation effects were described by Perdew-Burke-Ernzerhof for solids (PBEsol) in generalized gradient approximation (GGA). Ultrasoft pseudo-potentials were utilized for the electron-ion interactions. In our calculation, cut-off energy of 370 eV and a 5×5×1 k-point Monkhorst Pack mesh in the Brillouin zone were used for the geometry optimization and the electronic structure calculation. Electron correlation was taken into account with U eff = 4.0 eV and 2.5 eV for Ti 3d and Nb 4d states, respectively. All atoms were allowed to relax until the force on each atom was below 0.01 eV/Å and the displacement of each atom was below 5.0×10 -4 Å.