**5. Conclusion**

*Practical Applications of Laser Ablation*

The sample was then air-exposed and inserted into a UV photoelectron spectroscopy chamber (UPS ESCALAB 250Xi Thermo Fisher) to evaluate the work function WF as a function of position. Although the surface contamination due to air exposure prevented to extract absolute WF values, the relative variations of WF with Sr content and STO thickness could be determined assuming a "uniform" surface contamination. UPS spectra were taken at various x content for STO thickness ranging from 0 to 9 uc. A zoom around the emission threshold of the He II UPS spectra (He II energy 40.8 V, bias 4 V) is shown in **Figure 10a** for LSMOx (tSTO = 0 uc). From the thresholds one can estimated the WF reported in **Figure 10b**. A clear continuous decrease of the work function is observed as the Sr content increases. This trend is

*(a) HeII UPS threshold for various x of air-exposed LSMOX Ubias = 4 V. (b) Corresponding extracted WF*

*(a) Schematic of the STO (sub.)/ LSMO / LSMOx / STO heterostructure. (b) UHV-AFM image. (c) Profile* 

*(a) HeII UPS threshold for various tSTO at x = 0.2 Ubias = 4 V. (b) Corresponding extracted WF versus tSTO for* 

**16**

**Figure 11.**

*various Sr content x.*

**Figure 10.**

**Figure 9.**

*from the white arrow in b).*

*versus Sr content x.*

In this chapter we reviewed the qualities and limitations of PLD for the synthesis of oxides in general and for its use in combinatorial PLD synthesis (CPLD) in particular. We listed some counter-actions to mitigate the PLD limitations together with the mandatory steps to take before attempting reliable CPLD synthesis, i.e. demonstrating the control of both thickness and composition over the whole sample surface. We then detailed a statistical characterization approach to reliably interpret results from CPLD libraries of compounds. An example of this approach is presented, regarding the exploration of lead-free Ga-doped BiFeO3 solid solution for MPB-related piezoelectric properties enhancement. Finally we described a new interface CPLD development (ICPLD) for the exploration of functional interface libraries. This combinatorial interface synthesis approach, with continuous lateral chemical modulation of a few atomic layers, is unique to the best of our knowledge. The effectiveness of ICPLD regarding the control of interface magnetism for magnetic tunnel junctions and energy band and Schottky barrier height tuning in ferroelectric tunable capacitors was demonstrated. This shows that ICPLD is a powerful tool to accelerate heterostructures functional properties enhancement.

### **Acknowledgements**

Authors would like to thanks J-L Longuet from CEA Le Ripault (France) for the WDS characterizations presented here and Xavier Wallart from IEMN, UMR CNRS 8520, Villeneuve d'Ascq (France), together with Pascal Andreazza from ICMN, UMR CNRS 7374, for UPS measurements.

This work was funded by Région Centre within the projects INTIM-C 2015 and FLEXIGEN.
