KBr Films Research Articles

High‐performance pure-red light‐emitting diodes based on CsPbBrxI3-x–multi-ligands–KBr composite films

Metal halide perovskites have been extensively investigated for application in light-emitting diodes (LEDs) because their optical band gap can be adjusted via the alteration of their halide-ion composition. However, the performance of metal halide perovskite LEDs (PeLEDs) is limited by halide-vacancy-induced surface defects. Therefore, mixed-halide perovskites were introduced to prevent the formation of halide vacancies on the perovskite surface. However, typical CsPbBrxI3‑x-based PeLEDs exhibit ion migration under an electrical field, which results in phase segregation and the deterioration of the device performance. In this study, to overcome these obstacles, PeLEDs based on CsPbBrxI3-x–multi-ligands–KBr films were introduced. The use of multi-ligands could facilitate the recombination of excitons and stabilization of the α-phase perovskite crystals. Moreover, the KBr not only prevented the formation of halide vacancies by interaction with excess halide ions but also improved the surface coverage. Thus, an optimized film-based—CsPbBrxI3-x–multi-ligand (70% NMABr0.7I0.3–15% N,N-dimethyl-1,3-propanediammonium bromide–15% 1,3-propanediammonium bromide)–KBr (0.03 M)—PeLED was fabricated, which had a current density of 5.1 mA/cm2, current efficiency of 3.2 cd/A, luminance of 743.2 cd/m2, and an external quantum efficiency of 10.2%. Moreover, this PeLED exhibited a low turn-on voltage (1.6 V) with pure-red emission.

Measurement and Control of the Charge Occupation of Single Adsorbed Molecules Levels by STM and Nc-AFM

Single hexa-peri-hexabenzocoronene molecules adsorbed on KBr films, 2–8 monolayers (ML) thick, deposited on Ag(111) have been characterized by scanning tunneling microscopy coupled with noncontact ...

Observation of electron two-dimensional standing-wave patterns at the surface of an insulating layer by scanning tunneling microscopy

Electron standing wave patterns around single hexa-peri-hexabenzocoronene (HBC) molecules adsorbed on the surface of thin KBr(001) films grown on Ag(111) are observed by scanning tunneling microscopy. We suggest that they result from the scattering, by the molecules, of the electrons populating the surface resonance associated with the lowest image potential state of the insulating film on a metal system. These patterns are nearly independent on the thickness of the film from two to five KBr monolayers, implying that they are localized near the surface of the insulating film. The electrons exhibit a parabolic dispersion relation which yields an effective mass ${m}^{*}=0.62\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.16em}{0ex}}{m}_{e}$.

Correlation between photoemissive and morphological properties of KBr thin film photocathodes

In the present work, the morphological properties of KBr photocathodes are correlated with their photoemissive behavior using a combination of analyzing techniques including SEM, TEM and AFM. From morphological studies, it is observed that KBr films have granular characteristics with varied average grain size and grain density. Structure and orientation of individual grains have been investigated by selected area electron diffraction technique and found to be the crystalline in nature with a face centered cubic structure. It is evident from the AFM analysis that the root mean square roughness and maximum area peak height have been decreased with the deposition of more KBr layers. The photoemission studies reveal that the resultant photocurrent is enhanced with increasing film thickness and it is directly related to the surface area coverage and grain density.

Electronic structure and STM imaging of the KBr-InSb interface

We study the properties of the InSb (001) surface covered with ultrathin KBr films, with a thickness of 1–4 ML. KBr deposition does not strongly perturb the crystallographic structure of the InSb surface and the electronic structure of the substrate also remains unaffected by the overlayer. A simple model of the studied system is proposed, in which a thin KBr layer is treated as a dielectric film, modifying potential barrier for the electrons tunneling to/from the InSb substrate. Apparent step heights on the KBr film, measured using scanning tunneling microscope (STM), agree well with the predictions of the model and the atomically-resolved STM images show the structure of the InSb-KBr interface. Our results demonstrate that STM may be used as a tool for investigations of the semiconductor–insulator interfaces.

Epitaxial growth of atomically flat KBr(111) films via a thin film ionic liquid in a vacuum

The NaCl-type crystal (111) polar face is electrostatically so unstable that its atomically flat (111) surface has been difficult to obtain so far. In this work, based on our preliminary results on flat KBr(111) microcrystals on an α-Al2O3(0001) substrate grown via ionic liquid droplets by vacuum deposition, we have achieved atomically flat, epitaxial KBr(111) films grown all over an α-Al2O3(0001) substrate. The key point to this success is the great improvement in the wettability of the ionic liquid on the substrate by introducing an amorphous-like “wetting layer”. Furthermore, in situ growth monitoring by means of an optical microscope with a long-distance focus lens reveals the growth process in the ionic liquid: random nucleation occurs directly on the substrate, but not in the ionic liquid, and subsequently, the nucleated KBr islands grow more rapidly in the in-plane direction rather than in the out-of-plane direction to coalesce with each other, forming a continuous KBr film over the substrate. The lateral overgrowth mechanism is proposed to explain the epitaxial growth of KBr(111) films even on the amorphous-like wetting layer/α-Al2O3(0001) substrate on the basis of a cross-sectional TEM observation.

Linkage isomerism of CO adsorbed on alkali halides.

Variable-temperature FTIR spectra of CO adsorbed on NaCl and KBr films are shown to reveal linkage isomerism, that is, formation of C- and O-bonded adsorption complexes with the same cationic sites. Not all of the sites can be involved in that, only the three- or four-coordinated cations located on kinks, edges, or steps of microcrystals. For NaCl, these adsorption complexes account for the high-frequency (HF) C-O stretching bands at 2175 and 2160 cm(-1). Coresponding LF bands of O-bonded CO were found at 2115 and 2124 cm(-1). In the spectrum of KBr, the HF band is poorly resolved, and only one LF band can be clearly seen, near 2124 cm(-1). The value of the isomerization enthalpy for complexes that account for the bands at 2160 and 2124 cm(-1) of CO adsorbed on NaCl, estimated from the experiment, is 4.0 ± 0.2 kJ/mol. Quantum mechanical calculations by DFT methods applied to NaCl model clusters interacting with the CO molecule confirm the existence of two potential wells with adsorption and isomerization energies close to the experimentally measured values, and enable us to estimate the height of the potential barrier between the two adsorption states.

Comparison of CsBr and KBr covered Cu photocathodes: Effects of laser irradiation and work function changes

Thin films of CsBr and KBr were deposited on Cu(100) to investigate photoemission properties of these potential photocathode materials. After prolonged laser ultraviolet irradiation photoemission, quantum efficiency increases by factors of 26 and 77 for KBr/Cu(100) and CsBr/Cu(100) photocathodes, respectively. Immediately following thin film deposition, a decrease in work function is observed, compared to bare Cu, in both cases. Quantum efficiency enhancements are attributed to the decrease in photocathode work function, and photo-induced processes that introduce defect states into the alkali halide bandgap. Our results suggest that KBr, a relatively stable alkali-halide, also has potential for photocathode applications.

Determination of Uptake Coefficient for ClO Radicals on the Surface of Mineral Salts

The uptake of ClO radicals on KBr, NaCl, and NaBr dry solid films was studied at 1 Torr pressure of helium over the temperature range 290-350 K using a flow tube technique with a modulated molecular beam mass spectrometer as the detection method. A Pyrex tube with the deposited salt sample was introduced into the flow reactor along its axis. The ClO uptake coefficient on the KBr surface did not depend on temperature within the experimental accuracy of ∼20%. Chlorine oxide radicals were prepared using the reaction of Cl with ozone. It was found out that the ClO uptake coefficient strongly depended on ozone concentration. The uptake coefficients at T = 293 K and [O(3)] = 4.6 × 10(13) molecules cm(-3) were found to be (9.6 ± 5.7) × 10(-4), (3.7 ± 1.5) × 10(-4), and (12.3 ± 3.6) × 10(-4) for KBr, NaCl, and NaBr, respectively. Bromine-containing species were not observed during the interaction of ClO radicals with KBr film. The results obtained indicate that the ClO loss through heterogeneous interaction with salt surface is not sufficiently rapid to compete with gas-phase self-reaction in the atmosphere.

Atomic Friction Investigations on Ordered Superstructures

We review recent friction measurements on ordered superstructures performed by atomic force microscopy. In particular, we consider ultrathin KBr films on NaCl(001) and Cu(001) surfaces, single and bilayer graphene on SiC(0001), and the herringbone reconstruction of Au(111). Atomically resolved friction images of these systems show periodic features spanning across several unit cells. Although the physical mechanisms responsible for the formation of these superstructures are quite different, the experimental results can be interpreted within the same phenomenological framework. A comparison between experiments and modeling shows that, in the cases of KBr films on NaCl(001) and of graphene films, the tip-surface interaction is well described by a potential with the periodicity of the substrate which is modulated or, respectively, superimposed with a potential with the symmetry of the superstructure.

KBr superstructure templates self-assembled on reconstructed A IIIB V semiconductor surfaces

In this report we present results of combined high resolution LT-STM and LEED studies of ultrathin epitaxial KBr films grown on an InSb(0 0 1) c(8 × 2) substrate. Based on atomically resolved STM maps and LEED diffractograms the film structure as well as superimposed electronic effects are explained. The origin of additional 4 × 4 superstructure observed on ultrathin films is discussed. Possible application of the KBr/InSb system as a template for growing organic molecule structures is suggested.

On the optical properties of amorphous Ge–Ga–Se–KBr films prepared by pulsed laser deposition

Amorphous thin films (1 − x)(4GeSe 2–Ga 2Se 3)– xKBr ( x = 0, 0.1, 0.2, 0.3) were prepared by the pulsed laser deposition (PLD) technique. The optical parameters were calculated using the Swanepoel method from the optical transmission spectra. The optical band gap ( E g opt ) of the studied films increased while the index of refraction decreased when increased the content of KBr. The Tauc slopes were discussed as an indicator of the degree of structural randomness of amorphous semiconductors. The index of refraction decreased and E g opt increased after annealing of as-deposited films below the glass transition temperature. The thermal-bleaching and thermal- contraction effects were observed, which are discussed in relation to the reduction in the density of homopolar bonds confirmed by the Raman spectra analysis and the decreased amount of fragments of the as-deposited films, respectively.

Molecular Assemblies on Insulating Ultrathin Films Analyzed by NC‐AFM and KPFM

AbstractRecently porphyrin‐based molecules have proven to be promising candidates to form self‐assembled structures on insulating surfaces. Their close similarity to hemoglobin and chlorophyll‐based molecules makes them promising candidates for functional building parts in molecular electronics. To analyze their opto‐electronic activity, new measurement methods have to be applied. We investigated the formation of such molecular assemblies on ultrathin insulating films by means of a room temperature non‐contact atomic force microscope (NC‐AFM) in combination with a Kelvin probe force microscope (KPFM). As a substrate we used a Cu(111) surface, where either ultrathin KBr or NaCl films are deposited. On those surfaces the evaporated cyano‐porphyrins are forming one‐ and two‐dimensional structures that are comparable to the ones observed on bulk alkali halides. The simultaneously measured surface potential clearly shows a material contrast between the molecules and the insulating films. Finally, we succeeded in obtaining variations of the surface potential induced by the molecular assemblies at the molecular level.

Atomic-scale friction modulated by a buried interface: Combined atomic and friction force microscopy experiments

Combined atomic and friction force microscopy reveals a significant modulation of atomic-scale friction related to the small periodic rumpling induced at the interface between heteroepitaxial films of KBr on NaCl(100). Transitions from dissipative atomic-scale stick slip to smooth sliding with ultralow friction are observed within the $6\ifmmode\times\else\texttimes\fi{}6$ surface unit cell of the underlying superstructure. Scanning across atomic-scale defects confirms the high-resolution capabilities of friction force microscopy close to the ultralow friction state. Strong variations of the tip-surface interaction energy across the superstructure demonstrate that subsurface chemical and size inhomogeneities dramatically change the frictional properties of the surface probed by the microscope tip.

Atomic structure and friction of ultrathin films of KBr on Cu(100)

The frictional properties of ultrathin films of KBr on a Cu(100) substrate have been studied on the atomic scale by means of friction force microscopy. Films as thin as two atomic layers support the sliding tip and exhibit atomic stick-slip friction similar to cleaved surfaces of bulk KBr. Steps on the film are prone to wear, while substrate steps overgrown by the film are found to be stable. The interpretation of these results is supported by a detailed analysis of the structure of the films by means of atomic-resolution noncontact force microscopy. The films are found to be compressively strained by $1.0\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.2em}{0ex}}%$. They exhibit a regular superstructure originating from the interference of lattice constants at the substrate-film interface, which also causes a weak modulation of the frictional response. Monolayer films of KBr are subject to significant wear when imaged in contact-mode force microscopy. Noncontact imaging confirms that these monolayers are rich in defects.

A Gas Proportional Scintillation Counter With KI and KBr-Covered Microstrip Plate as Photosensor: Experimental Results

The performance of a xenon-filled gas proportional scintillation counter instrumented with a microstrip plate covered with KI and KBr films as photocathodes to detect the VUV Xe scintillation light is reported. The results for its energy resolution and detector gain for 5.9 keV X-rays are presented and compared with previous results obtained with a CsI film deposited on the microstrip plate under the same conditions. Its performance was studied as a function of the halide film thickness and the scintillation region length. The best energy resolutions obtained reached values of 11.4% and 14.9% for KI and KBr, respectively.

Asymmetry in the reciprocal epitaxy of NaCl and KBr

Ultrathin well-ordered films of KBr on NaCl(100) and of NaCl on KBr(100) have been grown. The films were imaged by means of noncontact atomic force microscopy with atomic resolution under ultrahigh vacuum conditions. An extreme asymmetry in the structure of the interface was found for the two systems. The first layer of KBr on NaCl(100) grows with the lattice constant of bulk KBr, while the first layer of NaCl on KBr(100) stretches to the bulk lattice constant of KBr. The KBr films exhibit a superstructure with the periodicity of the least common multiple of the NaCl and KBr lattice constants, while the stretched NaCl films grow flat. We discuss the anharmonicity of the ionic bond as origin of the dependence of the interface structure on the growth sequence.

Ordered growth and local workfunction measurements of tris(8-hydroxyquinoline)aluminium on ultrathin KBr films

In this work we investigate the growth of tris(8-hydroxyquinoline) aluminium(Alq3) on single-crystal Ag(111) substrates partially covered by an ultrathin KBr film. Noncontactatomic force microscopy is used to determine the molecular ordering of 0.8 monolayerAlq3 evaporated onto these substrates. The simultaneous measurement of the local surface potentialby means of Kelvin probe force microscopy yields the local workfunction difference betweenthe pure Ag(111) surface and the one covered by an ultrathin KBr film, by pureAlq3, or byboth (KBr|Alq3). The molecular ordering and the interface dipole formation are discussed with respect toexperiments described in the literature in which electron diffraction and photoelectronspectroscopy were used, respectively.

Scanning-tunneling/atomic-force microscopy study of the growth of KBr films on InSb( [formula omitted

Thin epitaxial films of KBr have been grown on InSb(0 0 1) substrate. Scanning tunneling microscopy and non-contact atomic-force microscopy in ultra-high vacuum have been used to study the film surfaces for coverages ranging from 0.3 to 120 ML. It has been found that initially islands of monatomic thickness are formed. These islands are often cut along 〈1 1 0〉 crystallographic direction and the distribution of these islands on the substrate surface is anisotropic which reflects the anisotropic diffusion of KBr molecules during growth. At 1–1.5 ML coverage a wetting single-atomic KBr film is formed and the deposited material in excess of 1 ML forms rectangular islands with edges oriented along 〈1 0 0〉 and 〈0 1 0〉 crystallographic directions. The KBr/InSb interface is likely stabilized by a bond between the halide ion and A III atoms arranged in chains on c(8×2) InSb. For high (multilayer) KBr coverages the growth is basically a layer-by-layer type but, due to slow diffusion of KBr molecules down across steps, the ( n+1)th layer starts to grow before the completion of nth layer. In result, pyramidal structures of rectangular bases are formed on the surface. These rough films can be, with thermal annealing, converted to flat films exposing large (0 0 1) terraces.

Composition dependence of the four A bands for thin-solid films of KBr 1– xI x:Tl +

Thin crystalline solid films of KBr 1– x I x :Tl + were prepared from a melt by a cell method and impurity absorption spectra were investigated at 11 K for the whole composition range. In the middle composition range no spectral traces of phase separation were observed, indicating that the films were in a state equivalent to that of a supersaturated solid solution. The A band of KBr:Tl + and that of KI:Tl + were persistent with each other and two additional bands attributable to the A bands of the mixed crystals were apparent in the range of 0.01≤ x≤0.5. According to the results of Gaussian fitting of the bands, it was evident that two of them have threshold compositions with increasing I − ion composition x. In addition, the I − ion concentration around the impurities was found to be greater than the I − ion composition of the host materials. These findings are discussed in terms of a model, according to which the bands are associated with various I − ion configurations around the impurities.