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The initial micro-galvanic corrosion behavior of Mg‒30wt%Ca alloy only containing Mg2Ca and α-Mg was studied by immersion testing in a 0.9% NaCl solution at 37oC. The quasi-in situ SEM and TEM results show that Mg2Ca corroded easier than α-Mg, indicating that Mg2Ca acted as an anode. The work function (Φ) for Mg2Ca calculated by first-principles is significantly lower compared to that for α-Mg. The Volta potential measured by a scanning Kelvin probe force microscope reveals that the Mg2Ca had a relatively low Volta potential (ψ) value. The lower Φ and ψ values for Mg2Ca indicate a lower electrochemical nobility, which is consistent with the experimental phenomenon.

The corrosion inhibition behavior of Mg-8Li-3Al alloy in NaCl solution with sodium dodecyl sulfate (SDS) was investigated by hydrogen analysis, scanning electron microscopy (SEM), electrochemical test, scanning Kelvin probe force microscopy (SKPFM) and computational methods. Results showed that the corrosion resistance of Mg-8Li-3Al alloy in NaCl solution was effectively improved with SDS. The SEM and SKPFM results confirmed a dense, 200 nm-thick SDS-adsorbed layer had formed on the alloy surface. The separation energy ∆Egap and adsorption energy Eads of SDS on the Mg surface were calculated by density functional theory and molecular dynamics simulations, respectively. And the corrosion inhibition mechanism was hypothesized and described.

Effect of the second phase in the micro-galvanic corrosion of a commercial Mg alloy containing rare earth elements, cast WE43 alloy, was investigated in 0.6 M NaCl solution and 0.6 M Na2SO4 solution by scanning electron microscopy (SEM) observations, scanning Kelvin probe force microscopy (SKPFM) analysis, hydrogen evolution, weight loss measurement, and electrochemical techniques. It is confirmed that the second phase of cast WE43 alloy is more active than Mg matrix and exhibits an anodic role in the micro-galvanic corrosion with α-Mg matrix as cathode and dissolves preferentially in Na2SO4 solution, in contrast to the situation in NaCl solution. The corrosion rate of cast WE43 alloy in Na2SO4 solution is much higher than that in NaCl solution, which is different from the conventional wisdom and could be attributed to the different role of the second phase in the micro-galvanic corrosion in two solutions.

The triple cation mixed perovskites (CsFAMA) are known as one of the most efficient candidates for perovskite solar cells (PSCs). It is found that the power conversion efficiency (PCE) of triple-cation based devices would increase with the test time extending, and the maximum efficiency is normally obtained after several days aging storage. Here, the relationship between enhanced device performance, excess PbI2 and its evolution in triple cation perovskite films of initial days was systematically explored. The CsFAMA-PSCs are prepared by two-step methods under two environmental conditions, including in the glove box and the ambient air (30% humidity). After 7 days testing, the maximum PCE of PSCs under two conditions dramatically increased 12.4% and 12.2%, reached 21.68% and 21.89%, respectively. At initial days, the XRD peak intensities of perovskite phase gradually decreased and those corresponding to PbI2 increased. Along with time-resolved photoluminescence (TRPL) and kelvin probe force microscopy (KPFM), it was found that the defects were passivated with the evolution of PbI2. This work reveals the excess PbI2 and its evolution in perovskite film, which can further supplement the understanding of PbI2 defect passivation.

We present a study of the effect of gold nanoparticles (Au NPs) on TiO2 on charge generation and trapping during illumination with photons of energy larger than the substrate band gap. We used a novel characterization technique, photoassisted Kelvin probe force microscopy, to study the process at the single Au NP level. We found that the photoinduced electron transfer from TiO2 to the Au NP increases logarithmically with light intensity due to the combined contribution of electron-hole pair generation in the space charge region in the TiO2-air interface and in the metal-semiconductor junction. Our measurements on single particles provide direct evidence for electron trapping that hinders electron-hole recombination, a key factor in the enhancement of photo(electro)catalytic activity., This work was supported by the Office of Basic Energy Sciences (BES) of the U.S. Department of Energy (DOE) under contract DE-AC02-05CH11231 through the Structure and Dynamics of Materials Interfaces Program (FWP KC31SM) and the Molecular Foundry. M.L. acknowledges funds from Comunidad de Madrid (P2018/EMT-4308), a Fulbright grant PRX16/00564, and the MCIU-AEI-FEDERUE (RTI2018-096937-B-C22 and MAT2014-59772-C2-1-P). J.C. acknowledges financial support from Ministerio de Ciencia e Innovación (MICINN) and the European Union through the project PID2019-104272RB-C52. Also, Y.H. acknowledges financial support from MCIU through MAT2014-59772-C2-2- P and L.M. from EC through ERC-2013-SYG-610256. V.A.P.O. and M.B. acknowledge the financial support from EC through ERC CoG HyMAP 648319, MINECO PID2019- 106315RB-I00 and ENE2017-89170-R, ″Comunidad de Madrid″ and European Structural Funds (FotoArt-CM project S2018/NMT-4367) and Fundación Ramón Areces (Art-Leaf project). M.B. also thanks the Juan de la Cierva Incorporación contract (IJC2019042430-I). X.Z. was supported by the NSF-BSF 359 grant number 1906014. The authors thank Prof. Eran Edri, María Ujué González Sagardoy, and Judit Meseguer- Oliver for fruitful discussions and Asylum customer support for help with modifications of the AFM.

This article presents a torsional harmonic Kelvin probe force microscopy (TH-KPFM) working in amplitude modulation (AM) mode for high-sensitivity mapping of surface potential (SP). Compared with frequency modulation KPFM with higher spatial resolution, AM-KPFM has higher potential sensitivity and scanning speed. However, the traditional AM-KPFM is usually limited in the crosstalk from topography measurement and cantilever homogenization effect which causes the measured SP to be seriously affected by the substrate. TH-KPFM can effectively suppress the artifacts induced by the cantilever homogenization effect, and the coupling crosstalk caused by the topography measurement. The torsional harmonic cantilever suitable for TH-KPFM is fabricated and calibrated using the nanorobotic system first. Then, this technique is applied to determine the SP of the tobacco mosaic virus (TMV), and detect surface contaminants of a silicon wafer. Experimental results show that TH-KPFM has a 3 $\sim$ 4 mV energy resolution and sub-20 nm spatial resolution. Compared with the proposed TH-KPFM, the energy level of TMV obtained by the traditional AM-KPFM is pulled down by 80 meV. TH-KPFM is a powerful and useful technique to study the SP on the nanoscale, and greatly simplifies the difficulty of high-sensitivity measurement of the actual SP of many important nanoscale systems.

In this research, core−shell electrospun fibers loaded with the shell of cellulose acetate and the core of oleic acid and alkyd varnish resin were synthesized and used within poly(dimethyl siloxane) (PDMS) to prepare self-healing and pH-responsive coatings for a steel substrate. The morphology of the electrospun fibers was characterized by scanning electron microscopy, transmission electron microscopy and confocal fluorescence microscopy. Thermo gravimetric analysis and Fourier transform infrared spectroscopy revealed that the self-healing agents were loaded successfully with a loading rate of 2.9%. The properties of the fiber-PDMS composite coating were characterized by water contact angle measurements, mechanical tests, electrochemical impedance spectroscopy, and scanning Kelvin probe. Results show that the maximum self-healing efficiencies of the fiber-PDMS coating in alkaline and acidic solution are 95.96% and 97.04%, respectively. The composition of the self-healing agents at the damaged part of the coating was verified by an infrared mapping test and using an energy dispersive spectrometer. In addition, the sandpaper abrasion test shows the hydrophobic effect of fiber-PDMS coating remains above 88.2% and decreases slightly through the addition of abrasion cycles. This research can pave the way for the industrial applications of pH-responsive self-healing coatings.

The conversion efficiency of energy-harvesting devices can be increased by utilizing hot-carriers (HCs). However, due to ultrafast carrier-carrier scattering and the lack of carrier injection dynamics, HC-based devices have low efficiencies. In the present work, we report the effective utilization of HCs at the nanoscale and their transfer dynamics from a non-noble metal to a metal oxide interface by means of real-space photocurrent mapping by using local probe techniques and conducting femtosecond transient absorption (TA) measurements. The photocurrent maps obtained under white light unambiguously show that the HCs are injected into the metal oxide layer from the TiN layer, as also confirmed by conductive atomic force microscopy. In addition, the increased photocurrent in the bilayer structure indicates the injection of HCs from both layers due to the broadband absorption efficiency of TiN layer, passivation of the surface states by the top TiN layer, and smaller barrier height of the interfaces. Furthermore, electrostatic force microscopy and Kelvin probe force microscopy provide direct evidence of charge injection from TiN to the MoOx film at the nanoscale. The TA absorption spectra show a strong photo-bleaching signal over wide spectral range and ultrafast decaying behavior at the picosecond time scale, which indicate efficient electron transfer from TiN to MoOx. Thus, our simple and effective approach can facilitate HC collection under white light, thereby achieving high conversion efficiency for optoelectronic devices.

Molecules of 12-o-carboranyldodecylphosphonic acid form a novel self-assembled monolayer (SAM) on alumina, which can effectively tune charge carriers in organic field effect transistors (OFETs) with the assembled dipoles of o-carborane at the semiconductor–dielectric interface. This SAM has not only realized p-channel OFETs of pentacene with both low threshold voltage and high field effect mobility (up to 5.2 cm2 V−1 s−1), but also enabled ambipolar charge transport in the solution-processed thin films of a chlorinated tetraazapentacene, which is a typical n-type organic semiconductor. As measured with Kelvin probe force microscopy, the SAM of CBPA has a surface electrostatic potential of −0.40 ± 0.10 V versus gold, which is in agreement with the observed shift of threshold voltage in the OFETs.

In this contribution we demonstrate doping of graphene by uncharged dibenzyl viologen (DBV0). Deposited electrochemically on chemical vapor deposited (CVD) graphene on SiO2, DBV0 forms a water-insoluble self-assembled molecular network, in contrast to water-soluble monocationic DBV˙+. The phase formation at the molecular scale is revealed by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The doping efficiency is characterized by a combination of Raman spectroscopy and Kelvin probe force microscopy (KPFM). The Raman mapping of the G-band of the DBV0 adlayer on top of CVD graphene on SiO2 shows an apparent red shift compared to the unmodified analogue indicating an n-doping effect. This observation is in line with the KPFM results of which the measured contact potential difference (CPD) displays a positive shift compared to that of the pristine graphene.

The initial corrosion behavior and mechanism of 7B04 aluminum alloy under acid immersion and salt spray environments (pH = 3.5) are studied by Scanning Electron Microscope (SEM), optical microscope, Fourier Transform Infrared Spectroscopy (FT-IR), X-Ray Diffraction (XRD), potentiodynamic polarization, Electrochemical Impedance Spectroscopy (EIS), and Scanning Kelvin Probe (SKP). The results show that pitting corrosion occurs at the initial corrosion stage, and the potential difference between the second phase particles is the main cause of pitting. Pitting pits on different locations gradually expand and coalesce with the proceeding of corrosion. The main components of corrosion products are Al2O3, Al(OH)3, and AlCl3, and the generation rate of the corrosion product layer under the salt spray environment is larger than that under acid immersion environment. Under both environments, the Volta potential distribution first disperses and then concentrates, while the charge transfer resistance first decreases and then increases with the corrosion time. The Volta potential gradually shifts in a positive direction, indicating that corrosion products have an inhibitory effect on corrosion. After the same corrosion time, the corrosion product layer resistance and the expectation of the Volta potential of the salt spray sample are higher than those of the immersion sample. Comparatively, the corrosion current density for the salt spray sample is significantly lower than the immersion sample, which indicates that the thicker the corrosion product layer, the stronger the inhibition of corrosion reaction.

In this study, the pitting corrosion behavior of 13Cr4Ni martensitic stainless steel (BASE) and that modified with rare earth (REM) in 0.1 mol/L NaCl solution were characterized. Techniques such as automatic secondary electron microscope (ASPEX PSEM detector), scanning electron microscope (SEM), transmission electron microscope (TEM), scanning Kelvin probe force microscope (SKP), potentiodynamic and potentiostatic polarizations were employed. The results obtained indicate that BASE steel contains Al2O3/MnS, Al2O3 and MnS inclusions, while REM steels contain (La, Ce, Cr, Fe)-O and (La, Ce, Cr, Fe)-O-S inclusions. Compared with BASE steel, REM steel is more susceptible to induce the metastable pitting nucleation and repassivation, whereas it restrains the transition from metastable pitting to stable pitting. Adding 0.021% rare earth element to BASE steel can reduce the number and area of inclusions, while that of 0.058% can increase the number and enlarged the size of inclusions, which is also the reason that pitting corrosion resistance of 58REM steel is slightly lower than that of 21REM steel. In the process of pitting corrosion induced by Al2O3/MnS inclusions, MnS is preferentially anodic dissolved, and also the matrix contacted with Al2O3 is subsequently anodic dissolved. For REM steels, anodic dissolution preferentially occurs at the boundary between inclusions and matrix, while (La, Ce, Cr, Fe)-O inclusions chemically dissolve in local acidic environment or are separated from steel matrix. The chemically dissolved substance (La3+ and Ce3+) of (La, Ce, Cr, Fe)-O inclusions are concentrated in pitting pits, which inhibits its continuous growth.

Polymers used for the exteriors of spacecraft are always exposed to risks such as atomic oxygen (AO) or electrostatic discharge (ESD) degradation. In this work, an AlxTiyO/NiCr coating with excellent mechanical stability, AO durability, and electrostatic dissipative properties was deposited via ion implantation (IIP), filter cathode vacuum arc (FCVA), and high-power impulse magnetron sputtering (HiPIMS) on a flexible Kapton substrate. Scratch and cycle folding tests indicated good adhesion and toughness of the AlxTiyO/NiCr-coated Kapton, which were due to the gradient structure fabricated by the multitechnology combination. AO exposure tests demonstrated an extremely low erosion yield (Ey = 5.15 × 10-26 cm3 atom-1) of the AlxTiyO/NiCr-coated Kapton, only 1.72% of that observed for pristine Kapton. Moreover, Rutherford backscattering spectrometry (RBS) and Kelvin probe force microscopy (KPFM) results showed that the AlxTiyO/NiCr-coated Kapton has elevated surface electrostatic dissipative properties and sufficient conductivity. The multitechnology combination offers great flexibility for customizing the gradient structure to realize a comprehensive performance improvement. In addition, such a coating has great prospects for aerospace applications.

The design of photocatalysts with hierarchical pore sizes is an effective method to improve mass transport, enhance light absorption, and increase specific surface area. Moreover, the construction of a heterojunction at the interface of two semiconductor photocatalysts with suitable band positions plays a crucial role in separating and transporting charge carriers. Herein, ZIF-8 and urea are used as precursors to prepare hierarchically porous ZnO/g-C3N4 S-scheme heterojunction photocatalysts through a two-step calcination method. This S-scheme heterojunction photocatalyst shows high activity toward photocatalytic H2O2 production, which is 3.4 and 5.0 times higher than that of pure g-C3N4 and ZnO, respectively. The mechanism of charge transfer and separation within the S-scheme heterojunction is studied by Kelvin probe, in situ irradiated X-ray photoelectron spectroscopy (ISI-XPS), and electron paramagnetic resonance (EPR). This research provides an idea of designing S-scheme heterojunction photocatalysts with hierarchical pores in efficient photocatalytic hydrogen peroxide production.

2D heterostructures offer new opportunities for harnessing a wider range of the solar spectrum in high-performance photovoltaic devices. Here, we explore a type-II p–n heterojunction, by exploiting air-stable tellurene (Te) in combination with MoS2, to study its charge transfer for photovoltaic applications. The charge transfer of MoS2/Te heterojunction is confirmed by photoluminescence spectroscopy, Raman spectroscopy and Kelvin probe force microscopy. The exciton binding energy for MoS2/Te heterojunction is estimated to be around 10 meV, which is much lower than that for monolayer MoS2. This strategy can be exploited to develop next-generation intrinsically ultrathin light-harvesting devices.

The urgent need in lightweight of vehicle body promoted the application of multi materials with high specific strength, and this involves the reliable welding of dissimilar materials. In this study, AA5052 aluminum alloy and HC420LA steel were welded by magnetic pulse welding. The mechanical degradation of the dissimilar Al–Fe joint after different salt spray corrosion duration was investigated. The atomic force microscopy-scanning Kelvin probe force microscopy (AFM-SKPFM) and immersion tests were also performed. The fracture of the Al–Fe joint was observed by optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The ultimate shear strength of the Al–Fe joint only declines for 10% after three weeks' corrosion. Crevice corrosion is found in the overlapping region, and galvanic corrosion and stress corrosion occur in the weld region. The corrosion in the flat welding interface is mainly galvanic corrosion due to the high corrosion driving force, while the corrosion in the wave welding interface is mainly stress corrosion.

The initial corrosion process of dual phase (DP) steel produced by the continuous annealing process (CAP) in NaCl solution was systematically studied in this paper. The microstructure, nobility of the phases, and micro-galvanic corrosion behavior were characterized by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) technique, scanning Kelvin probe force (SKPFM), and electrochemical impedance spectroscopy (EIS). Results indicate that there are two types of martensite, namely fresh martensite (FM) and tempered martensite (TM), embedded in the ferrite matrix. The two types of martensite are surrounded by dislocations and connect to each other as in a chain-like network at the ferrite grain boundary. Both FM and TM can act as a micro-cathode to promote the anodic dissolution of adjacent ferrite matrix since FM is up to 15 mV noble to the ferrite matrix and TM is to 30 mV noble. The initial corrosion process of DP steel can be divided into two stages. The first stage involves an increasing corrosion rate of the steel due to the successive emergence of cathodic TM and FM, while the corrosion rate decreases in the second stage because of the separation of martensite.

Electrochemical reduction in aqueous graphene oxide (GO) dispersion has emerged as an alternative route to producing a reduced GO (rGO) membrane on Au mesh. Under scanning electron microscopy, an interesting pattern formed by distinct differences was discovered from the deoxidization evolution. Scanning transmission X-ray microscopy shows the chemical composition coordination mixing of C–OH, C–O–C, HO–C O, and C O bonds at nanoscale resolution. The electrochemical reduction of C–OH, new bonding of C–O–C, and structure recovery of C C were obtained from GO transformation into the rGO membrane. In Kelvin probe force microscopy, the same pattern of rGO was also observed for the diversity of work functions ranging from 5.55 to 5.70 eV compared with the uniform distribution of GO of 5.78 eV. Density functional theory calculations predicted that the work function variation originated from the dependence of O atom number and functional group species. A high (low) diversity in work function values was ascribed to the C–O–C (HO–C O) bond even with increasing oxygen numbers, accounting for the peak variation. Controlling the work function holds great significance for photovoltaic behavior and band alignment in photoelectric devices. Thus, growing large-area rGO membranes offers a new route to obtaining membranes for applications requiring transparent materials.

Chalcogenide phase change materials reversibly switch between non-volatile states with vastly different optical properties, enabling novel active nanophotonic devices. However, a fundamental understanding of their laser-switching behavior is lacking and the resulting local optical properties are unclear at the nanoscale. Here, we combine infrared scattering-type scanning near-field optical microscopy (SNOM) and Kelvin probe force microscopy (KPFM) to investigate four states of laser-switched Ge3Sb2Te6 (as-deposited amorphous, crystallized, reamorphized, and recrystallized) with nanometer lateral resolution. We find SNOM to be especially sensitive to differences between crystalline and amorphous states, while KPFM has higher sensitivity to changes introduced by melt-quenching. Using illumination from a free-electron laser, we use the higher sensitivity to free charge carriers of far-infrared (THz) SNOM compared to mid-infrared SNOM and find evidence that the local conductivity of crystalline states depends on the switching process. This insight into the local switching of optical properties is essential for developing active nanophotonic devices.

The open-loop (OL) variant of Kelvin probe force microscopy (KPFM) provides access to the voltage response of the electrostatic interaction between a conductive atomic force microscopy (AFM) probe and the investigated sample. The measured response can be analyzed a posteriori, modeled, and interpreted to include various contributions from the probe geometry and imaged features of the sample. In contrast to this, the currently implemented closed-loop (CL) variants of KPFM, either amplitude-modulation (AM) or frequency-modulation (FM), solely report on their final product in terms of the tip–sample contact potential difference. In ambient atmosphere, both CL AM-KPFM and CL FM-KPFM work at their best during the lift part of a two-pass scanning mode to avoid the direct contact with the surface of the sample. In this work, a new OL AM-KPFM mode was implemented in the single-pass scan of the PeakForce Tapping (PFT) mode. The topographical and electrical components were combined in a single pass by applying the electrical modulation only in between the PFT tip–sample contacts, when the AFM probe separates from the sample. In this way, any contact and tunneling discharges are avoided and, yet, the location of the measured electrical tip–sample interaction is directly affixed to the topography rendered by the mechanical PFT modulation at each tap. Furthermore, because the detailed response of the cantilever to the bias stimulation was recorded, it was possible to analyze and separate an average contribution of the cantilever to the determined local contact potential difference between the AFM probe and the imaged sample. The removal of this unwanted contribution greatly improved the accuracy of the AM-KPFM measurements to the level of the FM-KPFM counterpart.

We have developed a scanning probe microscope to evaluate the device with high spatial resolution in a power semiconductor device with a voltage applied. Specifically, we observed the cross sectional structure exposed SiC planar MOS-FET using a multifunctional probe microscope, which combines atomic force microscopy, Kelvin probe force microscopy, and scanning capacitance force microscopy. By observing the channel formation and carrier concentration change in response to the applied voltage, we were able to visualize the internal state of the SiC power MOSFET.

Mg-Sn-Y alloys with different Sn contents (wt%) were assessed as anode candidates for Mg-air batteries. The relationship between microstructure (including the second phase, grain size, and texture) and discharge properties of the Mg-Sn-Y alloys was examined using microstructure observation, electrochemical measurements, and galvanostatic discharge tests. The Mg-0.7Sn-1.4Y alloy had a high steady discharge voltage of 1.5225 V and a high anodic efficiency of 46.6% at 2.5 mA·cm−2. These good properties were related to its microstructure: small grain size of 3.8 µm, uniform distribution of small second phase particles of 0.6 µm, and a high content (vol%) of ( $$11\bar 20$$ )/( $$10\bar 10$$ ) orientated grains. The scanning Kelvin probe force microscopy (SKPFM) indicated that the Sn3Y5 and MgSnY phases were effective cathodes causing micro-galvanic corrosion which promoted the dissolution of Mg matrix during the discharge process.

Probing CO at a specific site on a metal oxide surface is essential for characterizing various applications such as CO oxidation, hydrogenation, and water-gas shift reaction. Herein, we use atomic force microscopy and Kelvin probe force microscopy to probe the CO on a rutile TiO2(110) surface. Our results indicate that CO can be manipulated along the Ti row by the repulsive lateral force of “pushing” mode. Furthermore, the joint combination of precise manipulation and the distance dependence of local contact potential difference allow us to resolve the interatomic dipole moment and charge state of CO at atomic resolution. Therefore, we found that the negatively charged CO with the dipole moment of negative pole down on the rutile TiO2(110) surface. Our results suppose that both the charge state as well as the on-surface dipole interaction are very effective for CO reaction on rutile TiO2(110) surface.

Reducing the dimensions of ferroelectric materials down to the nanoscale has strong implications on the ferroelectric polarization pattern and on the ability to switch the polarization. As the size of ferroelectric domains shrinks to the nanometer scale, the heterogeneity of the polarization pattern becomes increasingly pronounced, enabling a large variety of possible polar textures in nanocrystalline and nanocomposite materials. Critical to the understanding of fundamental physics of such materials and hence their applications in electronic nanodevices is the ability to investigate their ferroelectric polarization at the nanoscale in a nondestructive way. We show that contact Kelvin probe force microscopy cKPFM combined with a k means response clustering algorithm enables to measure the ferroelectric response at a mapping resolution of 8 nm. In a BaTiO3 thin film on silicon composed of tetragonal and hexagonal nanocrystals, we determine a nanoscale lateral distribution of discrete ferroelectric response clusters, fully consistent with the nanostructure determined by transmission electron microscopy. Moreover, we apply this data clustering method to the cKPFM responses measured at different temperatures, which allows us to follow the corresponding change in the polarization pattern as the Curie temperature is approached and across the phase transition. This work opens up perspectives for mapping complex ferroelectric polarization textures such as curled swirled polar textures that can be stabilized in epitaxial heterostructures and more generally for mapping the polar domain distribution of any spatially highly heterogeneous ferroelectric materials

Two-dimensional semiconductors exhibit strong light emission under optical or electrical pumping due to quantum confinement and large exciton binding energies. The regulation of the light emission shows great application potential in next-generation optoelectronic devices. Herein, by the physical vapor deposition strategy, we synthesize monolayer hexagonal-shaped WS2, and its photoluminescence intensity mapping show three-fold symmetric patterns with alternating bright and dark regions. Regardless of the length of the edges, all domains with S-terminated edges show lower photoluminescence intensity, while all regions with W-terminated edges exhibit higher photoluminescence intensity. The photoluminescence segmentation mechanism is studied in detail by employing Raman spectroscopy, atomic force microscopy, high-resolution transmission electron microscopy, and Kelvin probe force microscopy, and it is found to originate from different strain distributions in the S-terminated region and the W-terminated region. The optical band gap determined by the photoluminescence in the dark region is ∼2 meV lower than that in the bright region, implying that more strain is stored in the S-terminated region than in the W-terminated region. The photoluminescence segmentation vanishes in transferred hexagonal-shaped WS2 from the initial substrate to a fresh silicon substrate, further confirming the physical mechanism. Our results provide guidance for tuning the optical properties of two-dimensional semiconductors by controllable strain engineering.

Ternary two-dimentional (2D) materials exhibit diverse physical properties depending on their composition, structure, and thickness. Through forming heterostructures with other binary materials that show similar structure, there can be numerous potential applications of these ternary 2D materials. In this work, we reported the structure of few-layer CrPS4 by X-ray diffraction, transmission electron microscope, and electron-density distribution calculation. We also demonstrated a new application of the CrPS4/MoS2 heterobilayer: visible-infrared photodetectors with type-II staggered band alignment at room temperature. The response of the heterostructure to infrared light results from a strong interlayer coupling that reduces the energy interval in the junction area. Since the intrinsic bandgap of individual components determines wavelengths, the decrease in energy interval allows better detection of light that has a longer wavelength. We used photoluminescence (PL) spectroscopy, Kelvin probe force microscopy (KPFM) under illumination, and electrical transport measurements to verify the photoinduced charge separation between the CrPS4/MoS2 heterostructures. At forward bias, the device functioned as a highly sensitive photodetector, as the wavelength-dependent photocurrent measurement achieved the observation of optical excitation from 532 to 1,450 nm wavelength. Moreover, the photocurrent caused by interlayer exciton reached around 1.2 nA at 1,095 nm wavelength. Our demonstration of the strong interlayer coupling in the CrPS4/MoS2 heterostructure may further the understanding of the essential physics behind binary-ternary transition metal chalcogenides heterostructure and pave a way for their potential applications in visible-infrared devices.

Combining WO3 with suitable materials to form heterojunction is essential to overcome the limitations of WO3 to enhance its photoelectrochemical (PEC) water splitting activity. Moreover, a clear understanding of photo-response and charge behavior of materials could lead to the rational design of efficient photoelectrodes. Given this, an efficient strategy is applied to fabricate WO3 heterojunction with nitrogen-doped carbon dots (NCDs) and in-depth characterization to investigate the surface charge dynamics using nano imaging in a relation to the enhanced PEC water splitting activity. The optimized NCDs loading to the WO3 NRs exhibited the enhanced photocurrent density of 1.54 mA cm−2 at 1.23V vs RHE under AM 1.5 G illumination, highest IPCE of ~82 % (at 308.32 nm). The Kelvin probe force microscopy and electrostatic force microscopy reveal that after loading NCDs to the WO3, a relatively smooth charge transport has been observed, which improves the PEC. Furthermore, this work demonstrates the effect of photogenerated charges caused by the NCDs that assist in enhancing the increased photocurrent, hydrogen production efficiency, and stability of the PEC water splitting system. Significantly, the nano imaging characterization utilized in this work could be extended to various photoanodes to study the surface charge dynamics.

MnOx thin films on various types of substrates (glass, quartz, alumina, and Pt/Al2O3/Pt) were prepared by a combination of the sol–gel method and pen plotter printing, using the hydrolytically active heteroligand complex [Mn(C5H7O2)2 – x(C4H9O)x] as the precursor. The effect of synthesis parameters and printing modes on the microstructure, optical and electrophysical properties of the manufactured planar nanomaterials was studied. The electron work function of the MnOx film surface was assessed by Kelvin probe force microscopy (KPFM). Impedance spectroscopy was used to determine the temperature-dependent electrical conductivity of the prepared films in the temperature range 325–500°С. The proposed synthetic method and printing technology show promise for the formation of thin-film manganese oxide nanostructures for supercapacitor electrodes and optical devices.

Traditional P-i-N perovskite solar cells have achieved high efficiency over 25%, but the device structure is relatively complex, and generally requires expensive low-conductivity p-type organic semiconductors as hole transport materials. As a typical semiconductor, it is very important to realize the bipolar doping of methylamine lead iodine (CH3NH3PbI3) that is the light-absorbing layer of perovskite solar cell. Also, it is significant to develop perovskite cells with new device structure. Here, we chose lithium iodide (LiI) as the dopant and successfully grow p-type Li+ doped CH3NH3PbI3 (MAPbI3:Li) perovskite films with low resistivity (103 Ω•cm). The temperature-dependent photoluminescence, Hall effect and Kelvin probe force microscope (KPFM) results strongly confirmed that Li+ doping promotes the transformation of pristine weak n-type MAPbI3 to intentionally doped p-type film. Subsequently, the p-type MAPbI3 film is used as both the light absorption layer and hole transport layer, and form a PN heterojunction with n-type titanium dioxide (TiO2), which is used as the electron transport layer, to construct the heterostructural perovskite cell with a photoelectric conversion efficiency (PCE) of 10.43%. The electron beam induced current (EBIC) signal also strongly confirms the existence of the PN heterojunction and the new working mechanism of such new solar cells.

The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemical reaction enables the reduction of the thermalization losses associated with supra-band gap photons in semiconductor photoelectrochemical (PEC) cells. Through the broadband harvesting of light, hot-carrier PEC devices also produce a sensitizing effect in heterojunctions with wide-band gap metal oxide semiconductors possessing good photostability and catalytic activity but poor absorption of visible wavelength photons. There are several reports of hot electrons in Au injected over the Schottky barrier into crystalline TiO2 and subsequently utilized to drive a chemical reaction but very few reports of hot hole harvesting. In this work, we demonstrate the efficient harvesting of hot holes in Au nanoparticles (Au NPs) covered with a thin layer of amorphous TiO2 (a-TiO2). Under AM1.5G 1 sun illumination, photoanodes consisting of a single layer of ∼50 nm diameter Au NPs coated with a 10 nm shell of a-TiO2 (Au@a-TiO2) generated 2.5 mA cm-2 of photocurrent in 1 M KOH under 0.6 V external bias, rising to 3.7 mA cm-2 in the presence of a hole scavenger (methanol). The quantum yield for hot-carrier-mediated photocurrent generation was estimated to be close to unity for high-energy photons (λ < 420 nm). Au@a-TiO2 photoelectrodes produced a small positive photocurrent of 0.1 mA cm-2 even at a bias of -0.6 V indicating extraction of hot holes even at a strong negative bias. These results together with density functional theory modeling and scanning Kelvin probe force microscope data indicate fast injection of hot holes from Au NPs into a-TiO2 and light harvesting performed near-exclusively by Au NPs. For comparison, Au NPs coated with a 10 nm shell of Al2O3 (Au@Al2O3) generated 0.02 mA cm-2 of photocurrent in 1 M KOH under 0.6 V external bias. These results underscore the critical role played by a-TiO2 in the extraction of holes in Au@a-TiO2 photoanodes, which is not replicated by an ordinary dielectric shell. It is also demonstrated here that an ultrathin photoanode (

Titanium dioxide (TiO2) is of considerable interest as a photocatalyst and a catalyst support. Surface hydroxyl groups (OH) are the most common adsorbates on the TiO2 surface and are believed to play crucial roles in their applications. Although the characteristics of bridging hydroxyl (OHbr) have been well understood, the adsorption structure and charged states of terminal hydroxyl (OHt) have not yet been experimentally elucidated at an atomic scale. In this study, we have investigated an isolated OHt on the rutile TiO2(110) surface by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We found that OHt is in a negatively charged state. The unique characteristic of OHt is different from that of OHbr and involves the amphoterism and diversity of catalytic reactions of TiO2.

Here, we describe a strategy for preparing CdS/MoS2 heterostructures using initially electrodeposited MoSx on a polycrystalline gold substrate. The excess sulfur intrinsic to the electrodeposited MoS3 surface was derivatized with Cd to form spherical CdS/MoS2 particles by judicious adjustment of the medium pH and interfacial electrochemistry. The progression of this conversion was monitored by a combination of cyclic/linear sweep voltammetry coupled with electrochemical quartz crystal nanogravimetry. The electrodeposited MoSx and CdS/MoS2 films were further characterized by scanning electron microscopy, energy-dispersive X-ray analysis, laser Raman spectroscopy, and X-ray photoelectron spectroscopy. Heterojunction formation between MoS2 and CdS particles was confirmed by high-resolution transmission electron microscopy as well as via Kelvin probe measurements of the contact potential differences, with and without the presence of CdS on the MoS2 surface. The nonoptimized CdS/MoS2 heterostructures showed improved photoelectrochemical response compared with CdS or MoS2 for oxidation of sulfite species.

The microstructure and chemical compositions of the solid solution-treated Mg-3Nd-1Li-0.2Zn alloy were characterized using optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), electron probe micro-analyzer (EPMA) and X-ray photoelectron spectroscopy (XPS). The corrosion behaviour of the alloy was investigated via electrochemical polarization, electrochemical impedance spectroscopy (EIS), hydrogen evolution test and scanning Kelvin probe (SKP). The results showed that the microstructure of the as-extruded Mg-3Nd-1Li-0.2Zn alloy contained α-Mg matrix and nanometric second phase Mg41Nd5. The grain size of the alloy increased significantly with the increase in the heat-treatment duration, whereas the volume fraction of the second phase decreased after the solid solution treatment. The surface film was composed of oxides (Nd2O3, MgO, Li2O and ZnO) and carbonates (MgCO3 and Li2CO3), in addition to Nd. The as-extruded alloy exhibited the best corrosion resistance after an initial soaking of 10 min, whereas the alloy with 4h-solution-treatment possessed the lowest corrosion rate after a longer immersion (1 h). This can be attributed to the formation of Nd-containing oxide film on the alloys and a dense corrosion product layer. The dealloying corrosion of the second phase was related to the anodic Mg41Nd5 with a more negative Volta potential relative to α-Mg phase. The preferential corrosion of Mg41Nd5 is proven by in-situ observation and SEM. The solid solution treatment of Mg-3Nd-1Li-0.2Zn alloy led to a shift in corrosion type from pitting corrosion to uniform corrosion under long-term exposure.

Herein, we prepared CuI film by spray pyrolysis technique, and Ag nanoparticles decorated CuI thin film by photoreduction method, surface modified approach for improving the interfacial charge transfer property and work function of hole transport layer (HTL) in Perovskite Solar Cell (PSC). The concentration of the Ag was investigated by immersing the CuI thin film in the Ag solution at time intervals. The crystalline property and vibration spectra of the as-fabricated thin film were examined by an X-ray diffraction pattern (XRD) and Raman Spectroscopy. The work function of the as-fabricated thin film was analyzed by Contact Potential Difference (CPD) method as a result of Scanning Kelvin Probe (SKP). The charge transfer resistance of the working electrodes was studied, and the least dipping time sample shows the low charge transfer resistance (36.35 Ω) and less relaxation time (0.19 s).

Temperature effects on the contact electrification (CE) is of great interest. Here, different kinds of substoichiometric oxide films, such as TiO2-x, Al2O3-x, Ta2O5-x, and Cr2O3-x, are deposited and annealed at different temperatures, and the CE between the films and a Pt-coated tip is performed by using Kelvin probe force microscopy (KPFM). An intriguing finding is that the polarity on the TiO2-x surface changes from negative to positive with the increase of the sample annealing temperature in air atmosphere. Such a result is attributed to the fact that annealing under an oxidative atmosphere repairs oxygen vacancies and helps upgrade the low valency of Ti3+ to a stable high valency of Ti4+. On the contrary, after annealing occurs in an Ar/H2 atmosphere, the polarity on the TiO2-x surface reverses from positive to negative. This is mainly due to the increase of oxygen vacancies after annealing in reducing atmosphere. Through the KPFM results of Al2O3-x, Ta2O5-x, and Cr2O3-x films, the effect of oxygen vacancies is further confirmed, that is, the decrease of oxygen vacancies eases the films at capturing positive charges. Based on this, TiO2-x-based identical material triboelectric nanogenerators (IM-TENGs) are designed and prepared for the first time to control the current direction. Moreover, a surface state model for explaining the CE mechanism between the metal and annealed dielectric is proposed. This study is conducive to the development of the IM-TENGs which regulate the current direction or voltage output accurately in the future and also provides a further understanding of the dominant mechanism of electron transfer in the CE.

Exploring high conductive materials is still a challenge for high performance intermediate-temperature fuel cells. In this study, two-dimensional LiAl0.5Co0.5O2 (LACO) nanosheets coated by a compatible amorphous LiAlO2 (LAO) layer are evaluated as proton/Li+ conductor electrolyte. The fuel cell in which the LAO-LACO is used as electrolyte could deliver the maximum power output of 1120 mW cm−2 at 550 °C. The LAO coating enhances not only the ionic conductivity by modifying the space-charge regions, but also improves the LACO's chemical stability and device performance. Kelvin probe force microscopy further detected a local electric field (LEF) built in the LAO-LACO coating confines protons at the interface to transport fast. Such heterostructure with the LEF accelerating mechanism presents a novel approach for developing high-performance intermediate-temperature fuel cells.

We studied the energy-level alignment at interfaces between various transition-metal dichalcogenide (TMD) monolayers, MoS2, MoSe2, WS2, and WSe2, and metal electrodes with different work functions (WFs). TMDs were deposited on SiO2/silicon wafers by chemical vapor deposition and transferred to Al and Au substrates, with significantly different WFs to identify the metal-semiconductor junction behavior: oxide-terminated Al (natural oxidation) and Au (UV-ozone oxidation) with a WF difference of 0.8 eV. Kelvin probe force microscopy was employed for this study, based on which electronic band diagrams for each case were determined. We observed the Fermi-level pinning for MoS2, while WSe2/metal junctions behaved according to the Schottky-Mott limit. WS2 and MoSe2 exhibited intermediate behavior.

Understanding the size-dependent electronic properties of germanium nanocrystals (Ge NCs) is of fundamental importance for improving the efficiency of optoelectronic devices based on such NCs. Here, Ge NCs with a tunable size were synthesized by magnetron-sputtering cluster-beam deposition, where the size of the as-deposited Ge NCs can be finely controlled between 6 and 36 nm by helium gas flow rates and variable magnetic field configurations above the target surface. Because the size of the as-deposited Ge NCs highly depends on the nucleation process inside the plasma region, a detailed comparison between these two process parameters on the size control was formulated from the perspective of the growth kinetic mechanism. Furthermore, the local surface potential of different-sized Ge NCs deposited on n-type silicon substrates was measured by Kelvin probe force microscopy. The surface potential fluctuation of n-type Si covered by Ge NCs shows a strong size-dependent relationship with the size of the Ge NCs, whereas the surface potential fluctuation increases when their size reduced. Because the surface potential fluctuation between the intrinsic Ge NCs and the n-type silicon substrate tends to get smaller as the NCs' size decreases due to the quantum confinement effect, the number of charges transferred between the electronic bands will reduce as the size of Ge NCs decreases. The latter exactly explains the observed experimental results. Therefore, this work offers a perspective to understand the behavior of charge transfer, which plays an important role in the performance of optoelectronic devices.