Orientation Of Films Research Articles

Controlling orientation, polymorphism, and crystallinity in thin films of poly(lactic‐acid) homopolymer and stereocomplex aligned by high temperature rubbing

AbstractHigh temperature rubbing is a fast and low‐cost method that can align semi‐crystalline polymers such as poly(lactic acid) (PLA) by application of a shear‐force via a rotating cylinder covered with a microfiber cloth on a thin film maintained at high temperature. A multi technique approach uncovers the role of rubbing temperature and film thickness on the PLA thin film structure. Both poly(l‐lactic acid) (PLLA) and the stereo‐complex (SC‐PLA) can be readily aligned by high temperature rubbing. The rubbing temperature TR and the film thickness are the key parameters that control polymorphism, in‐plane orientation, crystallinity and contact plane of the polymer crystals. Rubbing PLLA films in the range 80–140°C affords different aligned polymorphs (mesophase, α′, α′ + α mixtures and pure α phase) depending on the rubbing temperature. Pure α form of PLLA with 53% crystallinity is obtained at TR = 140°C whereas pure oriented crystalline SC‐PLA films are obtained by rubbing at 200°C.

Investigation of the stability and efficiency of MA-doped FAPbBr3 thin films for solar cells

Perovskite solar cells are becoming one of the most attractive solar cell technologies. They have achieved a power conversion efficiency (PCE) of 22.7% in 10 years. They show excellent efficiency with low stability. The degradation process of perovskite devices has been associated with mobile ion migration. The MAPbBr3 perovskite materials are significantly more stable under ambient conditions but have a larger band gap (2.3 eV) than the FAPbBr3 perovskite materials, which have an interesting band gap (1.6 eV).Here we use a mixture of MAPbBr3 and FAPbBr3 materials with a high proportion of FAPbBr3 to obtain a stable active layer with a good band gap by the one-step spin-coating method on the FTO. The X-ray diffraction (XRD) results show four peaks for all five samples, with the main one located at 26.84° for the (111) crystallographic plane. This (111) plane gives the preferred crystallographic orientation of the thin films we prepared. The surface images of the films show that the surfaces are mainly well coated with varying grain sizes depending on the layer. The absorption coefficient of the 10% MA is higher compared to the others films, The band gaps of the x% MA: FAPbBr3 films are from 2.39eV to 2.68eV with the lowest corresponding to the 10% MA doped layer. The degradation study revealed that the absorption of aged films decreased after 4 weeks of exposure to the ambient environment.for 10% of MA-doped FAPbBr3. We found intermediate properties as expected, notably the stability as well as the band gap whose value is 2.40 eV. Indeed, the degradation process of the prepared thin film is slower than that of pure FAPbBr3. The absorption of the mixite MA and FA is good to be used as an absorber layer for solar cells.

Unraveling nodal-line semimetallic phase in SrIrO3 by terahertz quasiparticle dynamics

We demonstrate a rare nonsymmorphic crystalline symmetry-protected nodal-line semimetallic phase in ${\text{SrIrO}}_{3}$ by acquiring a spatial dependence of the terahertz spectral structure. An extremely low momentum-relaxation scattering rate of $\ensuremath{\sim}8.32 {\text{cm}}^{\ensuremath{-}1}$ emerging from high-mobility Dirac electrons, diversity in the carrier dynamics controlled by the orientation and symmetry of epitaxial thin films, and a group theory analysis corroborate the formation of this phase. The presence of Dirac electrons only along selected crystal orientations of this three-dimensional semimetal uniquely interrelates the symmetry-enforced band crossings, a template that can explore the prevalence of this superior topological phase in complex systems.

Vertical aligned solid‐state electrolyte templated by nanostructured “upright” cellulose film layers for advanced cell performance

AbstractIn this work, an “upright” cellulose layered template, which can be precisely controlled by the fiber orientation and thickness of anisotropic cellulose film, was first developed to prepare vertically aligned Li7La3Zr2O12 (LLZO) ceramic membrane with low tortuosity and uniform porosity. The flexible composite solid‐state electrolytes (CSEs) were synthesized by incorporating poly(ethylene oxide) (PEO) polymer into LLZO architecture, and exhibited distinguished mechanical strength, elevated ionic conductivity (2.1 × 10−4 S cm−1) and excellent interface compatibility with lithium anodes. The assembled LiFePO4/lithium full cells assembled with as‐obtained CSEs can reach large discharge capacities of 172.3 mAh g−1 with coulombic efficiencies of above 99% and capacity retention of 93.1% even after 200 cycles. These results clearly demonstrate that the sophisticated design of vertical aligned cellulose layered template with fine‐scale size and exquisite architecture is a promising and practical approach in fabricating flexible LLZO@PEO‐LiTFSI CSEs with high‐safety and high‐performance.image

Flexible Torsional Photoactuators Based on MXene–Carbon Nanotube–Paraffin Wax Films

A flexible actuator, which can convert external stimuli to mechanical motion, is an essential component of every soft robot and determines its performance. As a novel two-dimensional material, MXene has been used to fabricate flexible actuators due to its excellent physical properties. Although MXene-based actuators exhibit excellent actuation performance, their bending deformation is solely due to the in-plane isotropy of the MXene film, and an MXene torsional actuator has not been reported. This study presents a flexible torsional actuator based on an MXene-carbon nanotube (CNT)-paraffin wax (PW) film. In this actuator, the MXene thin film acts as a light absorption layer with wavelength selectivity, superaligned CNT provides structural anisotropy for the composite film, and PW acts as the active layer. The chirality and helical structure of the actuator could be tuned by the orientation of the CNT film. Such an actuator delivers excellent actuation performance, including high work density (∼1.2 J/cm3), low triggering power (77 mW/cm2), high rotational speed (320°/s), long lifetime (30,000 cycles), and wavelength selectivity. Inspired by vines, we used the torsional actuator as a spiral grabber, which lifted an object that weighs 20 times more than the actuator. The high-performance torsional actuator would be potentially used as a noncontact sensor, rotary motor, and grabbing tool in the soft robot system.

Unraveling the Defect-Dominated Broadband Emission Mechanisms in (001)-Preferred Two-Dimensional Layered Antimony-Halide Perovskite Film

By adding molar-controlled SbCl3 in a Cs3Sb2Cl9 precursor, we employed a low-temperature solution-processed approach to prepare high-quality (001)-preferred Cs3Sb2Cl9 thin film, which demonstrates a stable defect-dominated broadband emission at room temperature. Density functional theory calculations reveal that the defect emission originates from the donor-acceptor pair (DAP) recombination between chlorine vacancy (VCl) and cesium vacancy (VCs). Furthermore, VCl + VCs DAP is more stable on the (001) surface. The improved film quality and the more stable VCl + VCs DAP increase the activation energy related to defect states, resulting in an enhancement of the defect emission for the high-quality (001)-preferred film. This work provides deep insight into the key role of the (001) surface in defect emission and a feasible strategy to enhance the defect emission in 2D halide perovskites A3B2X9 (A = CH3NH3, Cs, Rb; B = Bi, Sb; X = Cl, Br, I) by control of the thin film preferred orientation.

A liquid medium annealing strategy for highly [041]/[141]-oriented planar antimony sulfide solar cells with 7.23% efficiency

Antimony sulfide (Sb2S3) is regarded as one of candidate materials for tandem top cells with silicon based solar cells due to its non-toxicity, appropriate bandgap, high absorption coefficient and good stability. However, restricted by the difficulty of carrier transportation caused by unfavorable orientations of Sb2S3, the power conversion efficiency (PCE) is still relatively low. In this work, a new annealing method, called liquid medium annealing (LMA), is developed to versatilely regulate the orientation of Sb2S3 films by controlling the concentration of Se22--OAm. Compared with conventional vapor-assisted annealing, a highly [041]/[141] oriented Sb2S3 film with unique title angles can be skillfully prepared by LMA with top-down model, which facilitates the photo-induced carrier separation along (Sb4S6)n ribbon. Meanwhile, such a LMA tactic can effectively tailor the band alignment at back contact by raises the valence band maximum (VBM) of Sb2S3, and then reducing hole collection barrier. Finally, a high PCE of 7.23% is achieved and considered the first echelon in planar Sb2S3 solar cells.

Facile synthesis of ZnO films with anisotropic preferred orientations: An effective strategy for controllable surface and optical property

Although one-dimensional ZnO nanodevices have already been widely used in practical applications, there still exists limitation of inevitable growth along polar orientation for wurtzite ZnO. Herein, ZnO films grown along diverse preferred orientations including the non-polar, semi-polar and polar directions could be acquired by a facile and reproducible low-temperature aqueous solution deposition method. The non-epitaxial growth pattern was realized by introducing moderate amount of sodium citrate and ammonia into the non-polar solution without any surfactant or passivated agent. Preferential growth mechanism and surface properties of as-prepared anisotropic ZnO films were investigated. Transmission spectra of ZnO films with anisotropic preferred orientations exhibited an isotropic transmittance. Visible emission spectra indicated multifarious defect types and quantities related to diverse preferred orientations of anisotropic ZnO films. This work could not merely enrich the crystallographic growth of ZnO films along anisotropic orientations, but also benefit for designing ZnO film-based nanodevices with desirable performances from the perspective of regulating anisotropic growth of ZnO.

Sputtering Al2O3 as an effective interface layer to improve open-circuit voltage and device performance of Sb2Se3 thin-film solar cells

Sb2Se3 as a promising photovoltaic absorber material is attracting increasing attention. However, high open-circuit voltage (VOC) loss, especially for superstrate structured Sb2Se3 solar cells, is one of the main bottlenecks for improving device efficiency. Here, sputtering aluminum oxide (Al2O3) as an interface layer between CdS and Sb2Se3 was applied in superstrate structured Sb2Se3 solar cells. We proved that the sputtered Al2O3 layer is not fully continuous on the rough CdS film even its thickness has reached 8.8 nm. Then we systematically investigated the influence of the Al2O3 layer on Sb2Se3 material properties and device performances. It is found that the Al2O3 layer can not only effectively inhibit [hk0]-oriented crystal growth and promote [hk1] growth orientation of Sb2Se3 films but also obviously weaken the disordered growth of prominent petal-like shape Sb2Se3 grains and reduce the surface roughness. Further, the Al2O3 layer can also availably passivate interfacial defects at the CdS/Sb2Se3 interface. Consequently, the average power conversion efficiency (PCE) of Sb2Se3 solar cells with an optimal Al2O3 layer thickness is 17.86% higher than that without the Al2O3 layer. Noteworthily, the VOC of Sb2Se3 solar cells is boosted to 0.462 V, which is the highest value in superstrate structured Sb2Se3 solar cells with absorber prepared by conventional physical methods. Finally, a champion efficiency of 6.25% has been achieved in a low-cost Sb2Se3 thin-film solar cell with FTO/CdS/Al2O3/Sb2Se3/Carbon device configuration.

Direct In Situ Conversion of Lead Iodide to a Highly Oriented and Crystallized Perovskite Thin Film via Sequential Deposition for 23.48% Efficient and Stable Photovoltaic Devices.

In the sequential deposition method of perovskite films, the crystallinity and microstructure of PbI2 are often sacrificed to solve the problem of an incomplete reaction between organic halide and lead halide. As a result, the crystal orientation of the perovskite film prepared by the sequential deposition method is generally worse than that of the perovskite film prepared by a one-step antisolvent method. Here, we preplaced formamidine formate (FAFa) on the buried interface to regulate the formation mechanism from PbI2 to perovskite. As shown by the XPS measurement of the perovskite buried interface, the HCOO- anion of FAFa first partially replaces I- to coordinate with Pb2+. With the subsequent annealing process, some HCOO- anions were released and migrated upward, which promoted the recrystallization of PbI2, obtaining a PbI2 film with enhanced crystallinity and orientation. Additionally, the lift-off process proves that the HCOO- anions suppress the anion vacancy defects enriched at the buried interface and promote charge transport because the HCOO- anions are small enough to adapt to the iodide vacancy. Grazing incidence wide-angle X-ray scattering and X-ray diffraction measurements show that the in situ conversion mechanism is responsible for the PbI2-to-perovskite process, resulting in the highly oriented perovskite film without increasing the residual PbI2 content in the perovskite film. As a result, our strategies enabled a champion power conversion efficiency of 23.48% with improved storage stability and photostability. This work provides a new strategy to improve the crystallinity of sequential deposition perovskites without destabilizing the device due to more PbI2 residues.

Highly efficient sputtered Ni-doped Cu2O photoelectrodes for solar hydrogen generation from water-splitting

Hydrogen gas can be converted to electricity through fuel cells and is considered as a friendly energy source. Herein, pure Cu2O and Ni-doped Cu2O thin films were deposited on glass substrates using the RF/DC-sputtering technique for hydrogen production via the photoelectrochemical (PEC) water-splitting process. The preferred orientation for pure and Ni-doped Cu2O films was (111) crystallographic plane. The average nanograins size was decreased from 32.17 nm for pure to 10.40 nm through the doping process with Ni content. Field-emission scanning electron microscopy (FE-SEM) and ImageJ analysis showed that the pure Cu2O and Ni-doped Cu2O were composed of normal distribution of nanograins in a regular form. The optical bandgap of the Cu2O film was decreased from 2.35 eV to 1.9 eV after doping with 2.6 wt% of Ni-dopants. The photoluminescence (PL) spectra for all the sputtered films were recorded at room temperature to examine the effect of Ni-dopants in the Cu2O lattice. Pure and Ni-doped Cu2O films were applied for PEC water splitting for hydrogen (H2) production under white light and monochromatic illumination. The PEC studies displayed that increasing the Ni content up to 2.6 wt% in the pure Cu2O films led to an increase in the photocurrent density to reach −5.72 mA/cm2. The optimum photoelectrode was studied for reproducibility, stability, and electrochemical impedance. The incident photon to current conversion efficiency (IPCE%) was 16.35% at 490 nm, and the applied bias photon to current conversion efficiency (ABPE%) was 0.90% at 0.65 V. Consequently, Ni-doped Cu2O photoelectrodes are efficient and low-cost for practical and industrial solar H2 production.

Unraveling the Implications of Macromolecular Orientation on the Planar and Vertical Charge Transport in Organic Electronic Devices

The role of film morphology of conjugated polymers (CPs) and their macromolecular conformation with respect to the substrate in governing charge transport and overall device performances has been a matter of debate. The nature of the macromolecular orientation of CPs in thin films has a pivotal role in controlling the performance of planar and sandwich devices. Thin-film characterizations unveiled that spin-coated films are isotropic with mixed edge-on/face-on conformation, whereas unidirectional floating film transfer method (UFTM) films are anisotropic with edge-on-oriented micrometer-sized ordered domains. To unravel the explicit role of molecular orientation on device performance, two types of devices, organic Schottky diodes (OSDs) for out-of-plane carrier transport and organic field-effect transistors (OFETs) for in-plane charge transport, were fabricated and characterized. Isotropic and relatively low crystalline spin-coated films exhibited superior transport characteristics in the out-of-plane direction than UFTM films irrespective of the crystallinity. On the other hand, edge-on-oriented UFTM thin films demonstrated remarkably enhanced in-plane charge transport with more than two orders of magnitude of enhancement in the field-effect mobility. The nature of molecular orientation of different films is significantly correlated with the performance of the OSDs and OFETs.

ITO films with different preferred orientations prepared by DC magnetron sputtering

The preparation process of ITO film has been fully matured, but the precise control of preferred orientation has always been a difficult problem. ITO films with different preferred orientations were prepared on glass substrates by DC sputtering with a ceramic target, and the precise control of ITO films with (211), (222), (400), and (440) high preferred orientations was achieved. The effect of technological conditions on the preferred orientation of ITO film was systematically studied, and the growth mechanism of the preferred orientation of ITO film was analyzed by surface energy calculation. (400) preferred film has excellent electric properties (ρ = 2.633 × 10−4 Ω cm) and an average transmittance of visible light (87.1%). Compared with the other three preferred oriented films, (400) preferred film has the highest quality factor and surface energy, which is attributed to the abundance of the surface dangling-bonds of Indium atoms on the end surface of (400) preferred film, which provides higher carrier transfer capacity for ITO film. (400) Preferentially oriented films are of great significance for the application of transparent electrodes.

The Preparation of High Saturation Magnetization and Low Coercivity Feco Soft Magnetic Thin Films via Controlling the Thickness and Deposition Temperature.

FeCo thin films with high saturation magnetization (4 πMs) can be applied in high-frequency electronic devices such as thin film inductors and microwave noise suppressors. However, due to its large magnetocrystalline anisotropy constant and magnetostrictive coefficient of FeCo, the coercivity (Hc) of FeCo films is generally high, which is detrimental to the soft magnetic properties. Meanwhile, the thickness and deposition temperature have significant effects on the coercivity and saturation magnetization of FeCo films. In this paper, FeCo thin films with different thicknesses were prepared by magnetron sputtering at different temperatures. The effects of thickness and deposition temperature on the microstructure and magnetic properties of FeCo thin films were systematically studied. When the film thickness increases from 50 nm to 800 nm, the coercivity would decrease from 309 Oe to 160 Oe. However, the saturation magnetization decreases from 22.1 kG to 15.3 kG. After that, we try to further increase the deposition temperature from room temperature (RT) to 475 °C. It is intriguing to find that the coercivity greatly decreased from 160 Oe to 3 Oe (decreased by 98%), and the saturation magnetization increased from 15.3 kG to 23.5 kG (increased by 53%) for the film with thickness of 800 nm. For the film with thickness of 50 nm, the coercivity also greatly decreased from 309 Oe to 10 Oe (decreased by 96%), but the saturation magnetization did not change significantly. It is contributed to the increase of deposition temperature, which will lead to the increase of grain size and the decrease of the number of grain boundaries. And the coercivity decreases as the number of grain boundaries decreases. Meanwhile, for the thicker films, when increasing the deposition temperature the thermal stress increases, which changes the appearance of (200) texture, and the saturation magnetization increases. Whereas, it has a negligible effect on the orientation of thin films with small thickness (50 nm). This indicates that high-temperature deposition is beneficial to the soft magnetic properties of FeCo thin films, particularly for the films with larger thickness. This FeCo thin film with high saturation magnetization and low coercivity could be an ideal candidate for high-frequency electronic devices.

Bias-assisted epitaxial Ir/YSZ (1 0 0) substrate for diamond nucleation and growth

Iridium (Ir) films were deposited on yttrium stabilized zirconia (YSZ) substrates by bias-assisted RF magnetron sputtering. The orientation of Ir films can be orientally modulated by negative substrate bias, and then used for diamond nucleation and growth by microwave plasma chemical vapor deposition technology combined with the bias-enhanced nucleation process. The morphology, microstructure and composition of Ir and diamond films were investigated by scanning electron microscope (SEM), atomic force microscope, X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy. The morphological characterizations of Ir films showed that the grain shape was more uniform and regular symmetry with the increase of substrate negative bias voltage. XRD showed that the crystal orientation of the Ir films changed from (111) texture to (200) texture, and the optimal (200) preferred orientation is achieved under a substrate bias of −150 V. SEM images of diamond films exhibited that the diamond islands gradually merge into continuous diamond films and present a high degree of epitaxy. The crystalline quality of the central region of the epitaxial diamond films is higher than that of the edge region due to the substrate temperature gradient. The Raman full width at half maximum of the central region was 5.75 cm−1.

Classification of lattice defects and the microscopic origin of p-type conductivity of Sb2Se3 solar cell absorber with varying Al2O3-layer thicknesses

Based on first-principles calculations, herein, we reveal the origin of p-type conductivity and the roles of Al and O in the enhancement of hole concentration in Sb2Se3 thin films. A new class of low-energy defects with shallow acceptor levels is presented by substituting Se with O and inducing Sb vacancy [OSe–VSb] complex. Moreover, the concentration of holes induced by the intrinsic and extrinsic defects is calculated and discussed. Our results can explain the intriguing experimental observation where the hole concentration is enhanced after Al2O3 capping and provide a new insight to the defect chemistry and physics of Sb2Se3 thin films. In addition, Al2O3 layers with varying thicknesses have been deposited on the surface of Sb2Se3 films. The crystallinity, orientation and electrical properties of Sb2Se3 films have been investigated in detail. Lastly, 6.87% efficiency of Sb2Se3 solar cells without other optimization has been obtained based on the structure of FTO/CdS (CBD)/Sb2Se3/Al2O3/Au.

Synergistic effect of dual anions for efficient and stable quasi 2D perovskite solar cell

Quasi two-dimensional perovskite solar cells (Q-2D PSCs) has attracted the attention of many researchers in recent years due to improved stability compared to their three-dimensional (3D) counterparts. However, the insulation of organic cation could hinder charge transfer between the octahedral sheets, thereby decreasing photoelectric performance of Q-2D PSCs. Here, the study of the synergistic effect of MACl and GuSCN on crystal orientation and quality of films based on the 4F‐PEA 2 MA 4 Pb 5 I 16 perovskite are reported. The Q-2D perovskite film exhibits more vertical orientation growth and lower trap-state density with the effect of dual additive. As a result, the device obtained a power conversion efficiency (PCE) of 15.05% for planar p-i-n solar cell using the Q-2D perovskite film with dual additive, which retained an average of 85% of their initial PCEs after 300 h in an ambient environment with humidity of 50 ± 10%. This result indicates that the use of dual additive MACl and GuSCN is an effective way to adjust the crystal orientation and quality of perovskite films, which are key factors to influence the performance of Q-2D PSCs. • The Q-2D perovskite film 4F‐PEA 2 MA 4 Pb 5 I 16 with dual additives (MACl and GaSCN) was prepared for the first time. • With the synergistic effect, the high-quality Q-2D perovskite film with more vertical orientation was obtained. • There was a larger concentration window of GaSCN additive in the presence of MACl due to their synergistic effect. • Compared with the Q-2D PSCs without additive, the PCE of Q-2D PSCs based on dual additives increased by 1.76 times.

In Situ Observing and Tuning the Crystal Orientation of Two-Dimensional Layered Perovskite via the Chlorine Additive.

Precise control of crystal orientation in two-dimensional (2D) layered perovskites (LPs) is vital for their optoelectronic applications due to the structure-induced anisotropy in optical and electrical properties. Herein, we directly observe and control the crystal orientation of the butylammonium-based 2D LP films. Employing the synchrotron-based in situ grazing-incidence X-ray diffraction technique, we reveal the orientation modulation mechanism of the Cl additive by following the crystallization dynamics and chemical conversion pathways during film formation. Two new Cl-related intermediates are identified which serve as templates directing the orientational growth of the 2D LP films. We fine-tune the crystal orientation of 2D LP films through the Cl additive and incorporate the films with the requisite crystal orientations in solar cells and photodetectors. The optoelectronic performances of the devices show a strong correlation with the crystal orientation of the 2D LP films.

The Effect of a Topcoat with Amorphous Polymer Layers on the Mesogen Orientation and Photoalignment Behavior of Side Chain Liquid Crystalline Polymer Films

The mesogen orientations of liquid crystals are sensitive to the nature of the contacting surface. For side chain liquid crystalline polymer (SCLCP) films, most investigations have been conducted for thin films formed on a solid substrate surface such as glass, quartz and metal oxides, and little knowledge has been accumulated for SCLCP films whose top surface is covered by amorphous polymers. This work presents the effect of a topcoat with amorphous polymers placed on SCLCP films on the mesogen orientation and photoalignment behavior. When an SCLCP film that adopts a homeotropic mesogen orientation is covered with a glass plate or polymer layer, the mesogens turns to a random planar orientation. This planar orientation is favorable for efficient in-plane photoalignment by irradiation with linear polarized light. An in-plane order parameter exceeding 0.5 is readily obtained. Unexpectedly, a significant stabilization of the liquid crystal phase by over 10 °C is observed above the isotropization temperature of the SCLCP. These fundamental sets of knowledge should be significant in the fabrication of various polymer LC devices.

CoO epitaxial growth on Cu(111) by reactive PVD and plasma oxidation

Heteroepitaxial layers of ordered cobalt monoxide were prepared on single crystal Cu(111) substrate by two physical deposition methods – reactive physical vapor deposition (PVD) of Co in pure oxygen and by step-wise oxygen plasma oxidation of small Co deposits. Volume and surface-specific diffraction methods were utilized to characterize the structure and orientation of the CoO thin films, the surface morphology of the PVD grown structures was probed by scanning tunneling microscopy (STM). Photoelectron spectroscopy (XPS) was used to monitor the chemical state of the samples during preparation stage, as well as during the following thermal treatments up to 700 K. At low oxygen pressures (below 1 × 10−4 Pa), unlike for some other single crystal surfaces, on Cu(111) only cobalt monoxide phase with rocksalt structure can be established. The resulting CoO thin film exhibits a 3-dimensional mesa-like character with large flat (111) terraces aligned with the (111) face of Cu substrate and shaped by (111) and (100) side facets. The natural surface polarity of the (111) facets of CoO is suggested to be compensated primarily by a presence of aliovalent surface Co3+ cations, forming an ultrathin surface spinel-like phase, and surface-bound hydroxyls.