RESUMEN
The surface quality of lead halide perovskite crystals can extremely influence their optoelectronic properties and device performance. Here, we report a surface engineering crystallization technique in which we in situ grow a polycrystalline methylammonium lead tribromide (MAPbBr3) film on top of bulk mm-sized single crystals. Such MAPbBr3 crystals with a MAPbBr3 passivating film display intense green emission under UV light. X-ray photoelectron spectroscopy demonstrates that these crystals with emissive surfaces are compositionally different from typical MAPbBr3 crystals that show no emission under UV light. Time-resolved photoluminescence and electrical measurements indicate that the MAPbBr3 film/MAPbBr3 crystals possess less surface defects compared to the bare MAPbBr3 crystals. Therefore, X-ray detectors fabricated using the surface-engineered MAPbBr3 crystals provide an almost 5 times improved sensitivity to X-rays and a more stable baseline drift with respect to the typical MAPbBr3 crystals.
RESUMEN
This report presents results of research on electronic structure of three interfaces composed of organic layers of Alq3, Gaq3, or Erq3 deposited on GaN semiconductor. The formation of the interfaces and their characterization have been performed in situ under ultrahigh vacuum conditions. Thin layers have been vapor-deposited onto p-type GaN(0001) surfaces. Ultraviolet photoelectron spectroscopy (UPS) assisted by X-ray photoelectron spectroscopy (XPS) has been employed to construct the band energy diagrams of the substrate and interfaces. The highest occupied molecular orbitals (HOMOs) are found to be at 1.2, 1.7, and 2.2 eV for Alq3, Gaq3, and Erq3 layers, respectively. Alq3 layer does not change the position of the vacuum level of the substrate, in contrast to the other layers, which lower it by 0.8 eV (Gaq3) and 1.3 eV (Erq3). Interface dipoles at the phase boundaries are found to be -0.2, -0.9, -1.2 eV, respectively, for Alq3, Gaq3, Erq3 layers on GaN(0001) surfaces.
RESUMEN
This paper concerns research on magnesium oxide layers in terms of their potential use as a gate material for SiC MOSFET structures. The two basic systems of MgO/SiC(0001) and MgO/graphite/SiC(0001) were deeply investigated in situ under ultrahigh vacuum (UHV). In both cases, the MgO layers were obtained by a reactive evaporation method. Graphite layers terminating the SiC(0001) surface were formed by thermal annealing in UHV. The physicochemical properties of the deposited MgO layers and the systems formed with their participation were determined using X-ray and UV photoelectron spectroscopy (XPS, UPS). The results confirmed the formation of MgO compounds. Energy level diagrams were constructed for both systems. The valence band maximum of MgO layers was embedded deeper on the graphitized surface than on the SiC(0001).
RESUMEN
To fulfil the requirements of operating at low temperature in a harsh environment, the investigation on optical thermometers plays an increasingly important role. In this work, the influence of vanadium concentration on the capability of temperature readout by La2MgTiO6:V5+,Cr3+ luminescent thermometers was investigated for the first time. The presence of V3+ and V5+ was verified by XPS and absorption measurements. In the emission spectra, a blue-green emission region was assigned to both host and V5+ emission. Moreover, a spin-forbidden emission of Cr3+ ions was also detected. Vanadium ions in the +3 oxidation state do not exhibit luminescence, but play a role as a charge compensator. The highest emission intensity was obtained from the sample doped with 0.1% V. Besides, with increasing vanadium concentration, a redshift in the maximum position of the spectrum was observed corresponding to a movement from the greenish blue to yellowish green region in the CIE1931. It was shown that the relative sensitivity (Sr) and the temperature operating range can be easily modified by changing the concentration of vanadium ions. In particular, the outstanding relative sensitivities of 1.71% K-1 (at 187 K) and 1.96% K-1 (at 165 K) obtained from La2MgTiO6:0.1%V5+,Cr3+ and La2MgTiO6:0.05%V5+,Cr3+ demonstrated the enormous potential of this material for thermal sensing application.
RESUMEN
The surface of quasi-hexagonal reconstructed Au(100) is used as the template for monolayer pentacene (PEN) self-assembly. The system is characterized by means of scanning tunneling microscopy at room temperature and under an ultra-high vacuum. A new modulated pattern of molecules with long molecular axes (MA) arranged along hex stripes is found. The characteristic features of the hex reconstruction are preserved herein. The assembly with MA across the hex rows leads to an unmodulated structure, where the molecular layer does not recreate the buckled hex phase. The presence of the molecules partly lifts the reconstruction-i.e., the gold hex phase is transformed into a (1×1) phase. The arrangement of PEN on the gold (1×1) structure is the same as that of the surrounding molecular domain on the reconstructed surface. The apparent height difference between phases allows for the distinction of the state of the underlying gold surface.