The efficiency and stability of perovskite solar cells are largely determined by the interfaces between different functional layers in the device. Here, we investigate the atomic scale structural and electronic properties of C60 on thin films of CsPbBr3 on Au(001) by scanning tunneling microscopy and spectroscopy (STM/STS). By varying the PbBr2 content of the film, we control the surface termination, switching between CsBr and PbBr2, with each exhibiting distinct reconstructions. Investigating the self-assembly of C60 in the submonolayer regime, we find that the molecule-substrate interaction is enhanced on the PbBr2-terminated perovskite film. By STS, we determine the electronic energy-level alignment of the C60 frontier orbitals, revealing that regardless of surface termination, a C60 film functions as an electron transport layer in a device. However, different surface terminations induce a significant shift in the molecular energy levels by 0.4 eV, with implications for electron mobility and recombination losses in applications.

In a recent publication [2D Materials, 8, 045033 (2021), arXiv:2103.11403], it was reported that the growth of a monolayer PdTe_2 in ultra-high vacuum could be achieved by deposition of tellurium on a palladium (111) crystal surface and subsequent thermal annealing. By means of low-energy electron diffraction intensity (LEED-IV) structural analysis, we show that the obtained (√(3)×√(3))R30^∘ superstructure is in fact a TePd_2 surface alloy. Attempts to produce a PdTe_2 layer in ultra-high vacuum by increasing the Te content on the surface were not successful.

In a recent publication [2D Materials, 8, 045033 (2021)], it was reported that the growth of a monolayer PdTe2 in ultra-high vacuum could be achieved by deposition of tellurium on a palladium (111) crystal surface and subsequent thermal annealing. By means of low-energy electron diffraction intensity (LEED-IV) structural analysis, we show that the obtained 3×3R30° superstructure is in fact a TePd2 surface alloy. Attempts to produce a PdTe2 layer in ultra-high vacuum by increasing the Te content on the surface were not successful.

For the production of transition-metal dichalcogenides by molecular beam epitaxy, an understanding of the interaction between chalcogenide atoms and metal surfaces is of fundamental interest. Here, we describe the occurrence of stable surface telluride phases when reacting submonolayer amounts of tellurium with a Pt(111) surface. We find that when approaching a Te amount of 0.44 monolayers from below, a disordered Te adsorbate phase is converted into a long-range ordered Pt(111)-(3 x 3)-4Te surface telluride, which is stable against loss of Te up to 890 K. Adding further Te, heavy domain walls develop that condense into a well-ordered domain structure with (10 x 10) periodicity. It hosts 49 Te atoms per unit cell and is thermally stable up to 770 K. These two phases are the only existing Te-induced surface reconstructions in the submonolayer regime. The atomic structure of the two phases is determined using low-energy electron diffraction intensity analysis, scanning tunneling microscopy, and density functional theory. The resulting complex surface structures are revealed with picometer accuracy and a great agreement between the employed methods. In particular, the analysis of the (10 x 10) structure demonstrates the currently achievable state-of-the-art for low-energy electron diffraction structural analyses in terms of experimental surface preparation and data collection but also of computational methods, and it leads the way to building up a structural database for two-dimensional materials and their interfaces.

Metal-halide perovskite (MHP) thin films for next-generation solar cells are typically fabricated by wet -chemical synthesis in which surface properties such as the orientation of surface facets and their termination cannot be controlled. MHP device efficiencies depend critically on those surface properties. We demonstrate the epitaxial growth of several nanometer thick purely (001)-orientated films of CsPbBr3 and CsSnBr3 MHPs on Au(001) by molecular beam epitaxy. The epitaxial films are in a cubic phase aligned with the substrate. Their surfaces are singly terminated and can be modified for the first time. While the CsBr and PbBr2 surface terminations differ in terms of their atomic structure and defect content, the films are intrinsic semiconductors irrespective of the termination. The work function of the PbBr2-terminated surface is increased by 0.7 eV, which has drastic implications for the level alignment at MHP interfaces.

The adsorption properties of free base 5,10,15,20-tetrakis(p-cyanophenyl)porphyrin (2H-TCNPP) on thin films of rock salt (rs) CoO(100) on Au(111) was studied in ultra-high vacuum (UHV) by a combination of low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) and density functional theory (DFT). Films of rs-CoO(100) on Au(111) are prepared with excellent quality in a suitable thickness range. Particularly, we found that films of only 1 nm thickness show a semiconducting energy gap of Eg=(2.5±0.2)eV. Upon deposition at 300 K, 2H-TCNPP adsorbs flat-lying and self-assembles in a long-range ordered superstructure that is stable at 80 K. The adsorption geometry of the molecules on the surface and within the self-assembly is analyzed by DFT. We find that the self-assemblies are stabilized by hydrogen bridge bonding via the functional cyano groups. Our STS data shows molecular states within the fundamental gap of the CoO. By comparison with the calculated DOS we determine the energetic positions of the frontier orbitals and find that the first three LUMO states 2H-TCNPP are located within the band gap, whereas the HOMO is shifted 1 eV below the CoO conduction band edge. Upon annealing to 420 K the molecules change their appearance in STM images and a new prominent electronic state located at the center of the molecule is formed. We interpret this changed configuration as Co-TCNPP created by self-metalation on the oxide surface.

The surface atomic and electronic structure after deposition of 1/3 monolayer (ML) Te on Cu(111) was determined using a combination of low-energy electron diffraction (LEED), scanning tunneling microscopy and spectroscopy (STM/STS), angle-resolved single and two-photon photoelectron spectroscopy (ARPES /AR-2PPE) and density functional theory (DFT) calculations. Contrary to the current state in literature Te does not create a two-dimensional surface alloy but forms Cu$_2$Te$_2$ adsorbate chains in a $\left(2\sqrt{3} \times \sqrt{3}\right)\textrm{R30}^\circ$ superstructure. We establish this by a high-precision LEED-IV structural analysis with Pendry $R$ factor of $R = 0.099$ and corroborating DFT and STM results. The electronic structure of the surface phase is dominated by an anisotropic downward dispersing state at the Fermi energy $E_F$ and a more isotropic upward dispersing unoccupied state at $E-E_F = + 1.43\,\textrm{eV}$. Both states coexist with bulk states of the projected band structure and are therefore surface resonances.