Light is one of the most powerful tools in our hands to study the world around us. Lasers have indeed revolutionized our daily life. Since they only work at a few fixed wavelengths, it is crucial to convert their wavelength into the required one. Nonlinear optics provides essential tools to this end. For instance, second harmonic generation is a nonlinear process in which an intense input field at frequency generates a new field at the second harmonic frequency 2. The maximum efficiency of the nonlinear process is achieved by minimising the wave vector mismatch k, i.e. the difference of the input wave vector at and the wave vector of the second harmonic at 2, achieving the so-called phase-matching condition (k=0). Birefringent phase-matching (BPM) can be obtained in anisotropic nonlinear crystals with moderate nonlinearities (few pm/V). Alternatively, quasi-phase-matching (QPM) makes use of periodically-poled nonlinear crystals with larger nonlinearities (10-20 pm/V). Typical crystals for BPM and QPM have a large efficiency but macroscopic thickness, i.e. millimeter/centimeters, and do not easily lend themselves to on-chip integration. PIONEER aims at revolutionising nonlinear optics at the nanoscale, achieving birefringent and quasi phase-matching in engineered vertical stacks of layered semiconductors, like non-centrosymmetric transition metal dichalcogenides (3R-TMDs), which possess huge nonlinearities (100-1000 pm/V) and promise to achieve the same efficiencies of bulk nonlinear crystals within micron-thicknesses. The goal of PIONEER is the development of ultra-compact on-chip integrable nonlinear devices, like entangled photon sources, nano-lasers and waveguides based on 3R-TMDs, exceeding the performances of state-of-the-art photonic resonators and nonlinear waveguides. PIONEER will initiate the field of phase-matched nonlinear optics with layered semiconductors and will trigger the next revolution of integrated nonlinear optical devices.