Transition metal doped SrTiO₃ is of significance for devices in energy^1,2, information^3,4, and catalysis⁵. With the growth characterization of Ni-doped SrTiO₃ (Ni:STO), spontaneous formation of separated nano-phases is observed. In view of controlling the formation of such nano-phases and in a wider context for vertically aligned nanocolumn engineering⁶, our focus is on the growth mechanism of Ni:STO with Ni dopant enrichment in designated areas of the perovskite lattice, associated to the occasional formation of nano-phases embedded in a Ni-doped STO matrix.

Scanning Tunneling Microscopy and Spectroscopy (STM/S) are unique surface techniques for mapping both precise topography and electronic properties down to the atom scale, making them a promising instrument for studying nanostructures, even within device structures⁷.

In this study, we investigate the reflection high-energy electron diffraction-controlled pulsed laser deposition (PLD) grown 0.25 unit cell (u.c.), 0.5 u.c., and 10 u.c. Ni:STO on Nb-doped SrTiO₃ (Nb:STO) using low-temperature (LT) STM/S at 4.6 K in ultra-high vacuum (UHV). Upon in situ transfer to the LT-STM, the as-deposited samples with less than 1 u.c. show surface morphology with Ni:STO islands and bare Nb:STO spaces in between the islands, called trenches. Surprisingly, the trenches of the 0.5 u.c. sample already show specific patterns that can be corresponding to the nanocolumn patterns on a thick film, similar to vertically aligned nanostructures (VAN) morphologies observed in the literature.⁸ Film thickness of about 4.1 Å (lattice constant of STO~3.9 Å) is observed on both the 0.25 and 0.5 u.c. samples. Above the monolayer Ni:STO, homogeneously dispersed nanoparticles with diameters ranging between 2 and 3 nm exhibit low-density distribution. Local electronic properties of the nanostructures are investigated by performing STS and dI/dV, which yield spectra proportional to the local density of states. A bandgap of about 3.4 eV is seen on the Ni:STO film (tip-induced band bending takes part in the STS measurement), while the nanoparticles show a similar bandgap with extra sharp states 1.1 eV above the Fermi level, indicating increased p-doping concentration in the oxide nanoparticle. The STS on the Nb:STO substrate reveals a broader bandgap, exceeding 4 eV, attributed to the formation of Sr vacancies creating a space charge at the surface region⁹.

The study presents the very early stage formation process and the dopant distribution of the embedded nano-phase Ni:STO, allowing for improved control of such nanostructures, likely also relevant to understand the nucleation of VANs.