Scanning Kelvin Probe Microscopy Research Articles

Electrode Dependence of Local Electrical Properties of Chemical-Solution-Deposition-Derived BiFeO3 Thin Films

The nanoscale electrical properties of chemical-solution-deposition (CSD)-derived BiFeO3 grown on pulsed-laser-ablated La0.67Sr0.33MnO3//SrTiO3 (001) thin film heterostructures are investigated using a host of scanning probe microscopy techniques, including electrostatic force microscopy (EFM), scanning Kelvin probe microscopy (SKPM), piezoresponse force microscopy (PFM), and conductive AFM (CAFM). EFM and SKPM confirm the p-type nature of the CSD-derived BFO thin films as well as charge accumulation at the film surface after electrical bias. For BiFeO3 films of a fixed thickness (∼35 nm), the local current–voltage (I–V) behavior obtained by CAFM is strongly dependent on the La0.67Sr0.33MnO3 bottom electrode thickness. BiFeO3 films on a 20 nm thick La0.67Sr0.33MnO3 demonstrate the typical switchable diode behavior governed by polarization orientation. However, when the thickness of La0.67Sr0.33MnO3 is reduced to less than 5 nm, the BiFeO3 films show only forward diode behaviors regardless of polarization ...

Two-dimensional semiconducting and single-crystalline antimony trioxide directly-grown on monolayer graphene.

The first successful synthesis and characterization of single-crystalline two-dimensional (2D) semiconducting antimony tri oxide (Sb2O3) by direct chemical vapor deposition (CVD) growth on monolayer graphene is presented herein. The Sb2O3 flakes on graphene are regular triangles with smooth edges and a thickness of ∼1.45 nm. The thickness dependence of the Raman spectra and scanning Kelvin probe microscopy (SKPM) were systematically studied.

Investigation of local charge accumulation in yttria stabilized zirconia films with Au nanoparticles by Scanning Kelvin Probe Microscopy

The time dynamics of the spatial distribution of the potential induced by the electrons locally injected from an atomic force microscope (AFM) probe into the ultrathin (< 10 nm thick) yttria stabilized zirconia (YSZ) ZrO2(Y) films with embedded Au nanoparticles (NPs) on Si substrates was studied using Scanning Kelvin Probe Microscopy (SKPM). The SKPM images and profiles of surface potential induced by the electrons confined inside the Au NPs subject to the time elapsed after the injection have been measured and analyzed. The parameters of the charge relaxation in the YSZ:NP-Au films were determined.

Structural and Electrical Properties of Nb3I8 Layered Crystal

Centimeter‐size single‐crystalline Nb3I8, a new family of two‐dimensional materials, is first synthesized by a vapor transport reaction of niobium and iodine. Through the mechanical exfoliation method, the bulk Nb3I8 crystal is cleaved to monolayer and multi‐layered flakes. It is confirmed to consist of numerous monolayers of Nb3I8 bonded by weak van der Waals interaction. By utilizing atomic force microscopy (AFM) and scanning Kelvin probe microscopy (SKPM), the information on structure and electrical properties is obtained at the same time. The analysis results show that the surface potential difference and the work function tend to increase as the Nb3I8 becomes thicker due to the interlayer screening effect. The results for the work function variation dependence on thickness are useful for selecting a suitable metal electrode for future Nb3I8‐based nano‐devices which can have a significant impact on performance.

Study on electrochemical behavior of 690 alloy with corrosion products in simulated PWR primary water environment

PurposeThe purpose of this paper is to study the electrochemical behavior of 690 alloy with corrosion products in simulated pressurized water reactor (PWR) primary water environment.Design/methodology/approachThis paper opted for a laboratory study using simulation of high temperature and high pressure environment immersion testing. The electrochemical behavior was studied by potentiodynamic polarization, electrochemical impedance spectroscopy, scanning Kelvin probe microscopy (SKP). Moreover, the corrosion products were analyzed by X-ray photoelectron spectroscopy.FindingsThe results demonstrated that the particle majority in the 690 alloy corrosion products subsequent to high temperature and high pressure immersion testing were mainly oxides of Fe and Ni, which protected the matrix. As the immersion testing duration increased, the corrosion potential of the 690 alloy apparently increased, and the corrosion current density de'creased, while the corrosion resistance Rf increased gradually along with the density. The SKP demonstrated that the EKP increased by nearly 400 mV from −0.42 to −0.03 V following the immersion testing, indicating that the corrosion product film played an apparent protective role on the substrate.Originality/valueThis paper provides a theoretical basis for the corrosion behavior and inhibition mechanism of 690 alloy in PWR primary water environment.

(Invited) Micro- and Nano-Structure p-n Junction Based Organophotochatalyst Film

The present paper describes organic semiconductor based photocatalyst with p-n junction. A typical material combination is phthalocyanine (H2Pc) and 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) without transition metal element. It responds full spectrum of visible light and reaching to near infrared region. The film type photocatalyst was prepared by vapor deposition technique with the thickness around 10 to 100 nm, which is relating to absorbance. Typical applicable reactions are downhill redox reaction such as mineralization of volatile organic molecules. The engineering quantum efficiency per incident photon number is high >10 %. When the combination is modified with noble metal nanoparticles or transition metal ion into phthalocyanine, water reduction and oxidation occurred in the presence of bias potential under visible light illumination. Due to the film structure, layering of the film is possible for a practical application, and it is useful to utilise natural sunlight. Under the weak light illumination, internal quantum efficiency becomes high which is merit for the layering use. By changing microstructure and nanostucture of p-n junction, interesting enhancements were observed, such as near infrared region response maybe due to charge transfer complex formation between H2Pc and PTCBI. A terrace bilayer (TB) of p-n junction exhibited potential structure at the interface and it was observed by the use of scanning Kelvin probe microscopy (SKPM). 1) K. Nagai et al. ChemSusChem, 4 (6), 727-730, (2011). 2) K. Nagai et al. ACS Sustainable Chem. Eng., 1, (8), 1033-1039, (2013). 3) M.-F. Ahmad et al. Electrochemistry ., 86, in press, (2018).

Enhanced oxidation power in photoelectrocatalysis based on a micrometer-localized positive potential in a terrace hetero p\u2013n junction

Generally, p–n junction-based solar energy conversion has the disadvantage of a loss in potential gain in comparison with the photon energy. In this study, we found a more positive potential for a lateral domain interface of p–n junction than for a conventional p–n junction. A terrace bilayer (TB) p–n junction of phthalocyanine (H2Pc) and 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) was studied using scanning Kelvin probe microscopy (SKPM), and its electronic properties were analyzed using the contact potential difference (VCPD) data. The analysis of VCPD in the single layer region and the bilayer region (BLR) indicated a vacuum level shift through the electron transfer from PTCBI into indium tin oxide (ITO), from H2Pc into ITO and from H2Pc into PTCBI. Furthermore, the comparison of these VCPD data indicated a micrometer-localized positive potential in the boundary region (BDR) of the terrace bilayer structure of p-type on n-type. The gain difference of the VCPD reached +0.1 V in comparison with the BLR. The phenomena can be explained as a lateral dipole at the p–n junction. Similar phenomena were observed in TB-H2Pc/C60/ITO and TB-H2Pc/PTCBI/Au. The gain was extracted as oxidation power in photoelectrochemistry; i.e., at −0.2 V vs. Ag/AgCl a greater anodic current was observed for a patterned terrace bilayer electrode. Additionally, as a photocatalyst film (i.e., a H2Pc (dot)/PTCBI/PTFE membrane filter), the p–n dot terrace structure showed a higher quantum efficiency (5.1%) than that of the bilayer (3.2%) for the decomposition of acetic acid. The present design and method were utilized to obtain an efficient photocatalyst, especially through the mitigation of potential loss from the photon energy to redox powers without changing the molecular component.

Scanning Kelvin Probe Microscopy: Challenges and Perspectives towards Increased Application on Biomaterials and Biological Samples.

We report and comment on the possible increase of application of scanning Kelvin probe microscopy (SKPM) for biomaterials, biological substrates, and biological samples. First, the fundamental concepts and the practical limitations of SKPM are presented, pointing out the difficulties in proper probe calibration. Then, the most relevant literature on the use of SKPM on biological substrates and samples is briefly reviewed. We report first about biocompatible surfaces used as substrates for subsequent biological applications, such as cultures of living cells. Then, we briefly review the SKPM measurements made on proteins, DNA, and similar biomolecular systems. Finally, some considerations about the perspectives for the use of SKPM in the field of life sciences are made. This work does not pretend to provide a comprehensive view of this emerging scenario, yet we believe that it is time to put these types of application of SKPM under focus, and to face the related challenges, such as measuring in liquid and quantitative comparison with other techniques for the electrical potential readout.

Crystal step edges can trap electrons on the surfaces of n-type organic semiconductors

Understanding relationships between microstructure and electrical transport is an important goal for the materials science of organic semiconductors. Combining high-resolution surface potential mapping by scanning Kelvin probe microscopy (SKPM) with systematic field effect transport measurements, we show that step edges can trap electrons on the surfaces of single crystal organic semiconductors. n-type organic semiconductor crystals exhibiting positive step edge surface potentials display threshold voltages that increase and carrier mobilities that decrease with increasing step density, characteristic of trapping, whereas crystals that do not have positive step edge surface potentials do not have strongly step density dependent transport. A device model and microelectrostatics calculations suggest that trapping can be intrinsic to step edges for crystals of molecules with polar substituents. The results provide a unique example of a specific microstructure–charge trapping relationship and highlight the utility of surface potential imaging in combination with transport measurements as a productive strategy for uncovering microscopic structure–property relationships in organic semiconductors.

Effect of relative humidity on the frictional properties of graphene at atomic-scale steps

The atomic-scale steps are ubiquitous on graphene. The frictional behavior of graphene at atomic-scale steps under different relative humidity (RH) is very important for application of graphene as solid lubricant in complicated environment. Frictional properties at atomic-scale graphene steps were investigated using calibrated atomic force microscopy (AFM) in various RH. Lateral force at uncovered atomic step decreased with the increase of RH, while lateral force at covered step were independent of RH. Uncovered steps possessed higher work function than graphene plane from the scanning Kelvin probe microscopy (SKPM) measurement due to the existence of dangling bonds and oxygen-containing functional groups. Then the adsorption of water molecule on hydrophilic oxygen containing functional groups at uncovered step decreased the work function and reduced the lateral force under high RH. Also, lateral force at steps increased even fast than plane after plasma treatment, because dangling bonds is easier to be oxide functionalization. The studies provide a further understanding of frictional properties at atomic scale steps and could be helpful in the applications of graphene as solid lubricant in humid environment.

Direct Observation of Surface Potential Distribution in Insulation Resistance Degraded Acceptor-Doped BaTiO3 Multilayered Ceramic Capacitors

Insulation resistance (IR) degradation in BaTiO3 is a key issue for developing miniaturized multilayer ceramic capacitors (MLCCs) with high capacity. Despite rapid progress in BaTiO3-based MLCCs, the mechanism of IR degradation is still controversial. In this study, we demonstrate the Al doping effect on IR degradation behavior of BaTiO3 MLCCs by electrical measurements and scanning Kelvin probe microscopy (SKPM). As the Al doping concentration in BaTiO3 increases, IR degradation of MLCCs seems to be suppressed from electrical characterization results. However, SKPM results reveal that the conductive regions near the cathode become lager with Al doping after IR degradation. The formation of conducting regions is attributed to the migration of oxygen vacancies, which is the origin of IR degradation in BaTiO3, in dielectric layers. These results imply that acceptor doping in BaTiO3 solely cannot suppress the IR degradation in MLCC even though less asymmetric IR characteristics and IR degradation in MLCCs with higher Al doping concentration are observed from electrical characterization. Our results strongly suggest that observing the surface potential distribution in IR degraded dielectric layers using SKPM is an effective method to unravel the mechanism of IR degradation in MLCCs.

Perovskite-Perovskite Homojunctions via Compositional Doping.

One of the most important properties of semiconductors is the possibility of controlling their electronic behavior via intentional doping. Despite the unprecedented progress in the understanding of hybrid metal halide perovskites, extrinsic doping of perovskite remains nearly unexplored and perovskite-perovskite homojunctions have not been reported. Here we present a perovskite-perovskite homojunction obtained by vacuum deposition of stoichiometrically tuned methylammonium lead iodide (MAPI) films. Doping is realized by adjusting the relative deposition rates of MAI and PbI2, obtaining p-type (MAI excess) and n-type (MAI defect) MAPI. The successful stoichiometry change in the thin films is confirmed by infrared spectroscopy, which allows us to determine the MA content in the films. We analyzed the resulting thin-film junction by cross-sectional scanning Kelvin probe microscopy (SKPM) and found a contact potential difference (CPD) of 250 mV between the two differently doped perovskite layers. Planar diodes built with the perovskite-perovskite homojunction show the feasibility of our approach for implementation in devices.

Electronic Structure Characterization of Hydrogen Terminated n-type Silicon Passivated by Benzoquinone-Methanol Solutions

The electrical passivation mechanism of benzoquinone-methanol solutions on silicon has been examined through the study of the silicon surface electronic structure. Surface photovoltage (SPV) measurements using both X-ray photoelectron spectroscopy (XPS) and scanning Kelvin probe microscopy (SKPM) indicate a downward band bending of H-Si and benzoquinone (BQ) and methanol (ME) treated samples. This suggests the creation of an accumulation layer of majority carriers near the surface, with a significant field-effect contribution to the observed surface passivation. The highest SPV values recorded for the ME-Si and BQ-Si samples of about −220 mV are approaching the Fermi level—conduction band crossover. Density functional theory (DFT) calculations show that a dipole is formed upon bonding of BQ radicals on the surface, decreasing the surface electron affinity and work function. Considering the 0.07 eV shift due to the dipole and the 0.17 eV downward band bending, the work function of BQ-Si is found to be 4.08 eV. Both the dipole and downward band bending contribute to the formation of surface electron accumulation, and decrease the minority carrier density of n-Si passivated by BQ.

Study of Topography and Distribution State of the Nanoscale Passivation Film on a Rough Tinplate Surface

Topography observation of the nanoscale passivation film on tinplate using a scanning electron microscope and an electro-optical surface profilometer showed that it was difficult to observe the true topography because of the high surface roughness of the tinplate. Topography observation using a profilometer on bright tin plating with low surface roughness and work function measurement on tinplate surface using a Scanning Kelvin Probe Microscope (SKPM) were then carried out to solve the problem. The results indicated that there was a thin chemical passivation film distributed uniformly in different areas of microscopic bulges and valleys on the tinplate surface, and the film became more uniform with longer passivation time. In comparison, the electrolytic passivation film was thick, and the distribution uniformity was poorer. Specifically, the film was thicker on microscopic bulges and thinner in microscopic valleys, and this was worse with longer passivation time. Thus, the difference in performances of the corrosion and paint adhesion of the tinplate treated with different passivation processes can be explained in terms of the topography and distribution state of the passivation film.

SKPM study on organic-inorganic perovskite materials

We report Atomic Force Microscopy (AFM) and Scanning Kelvin Probe Microscopy (SKPM) studies on the surface morphology and surface potential properties of CH3NH3PbI3, CH3NH3PbI3-xClx, CH3NH3PbI3-xBrx and CH3NH3PbBr3-xClx, respectively. For CH3NH3PbI3 rod structure, its surface potential is independent of the precursor concentration, suggesting a robust electronic feature. Surface potential studies of CH3NH3PbI3 particle reveal that the Fermi level within CH3NH3PbI3 is strongly influenced by the substrate. In the case of CH3NH3PbI3-xClx, its surface potential depends on precursor concentrations and we suspect that chlorine concentrated solutions might lead to more chlorine incorporation in the final products, thus lowering its Fermi level. Also, we studied the surface potentials of CH3NH3PbI3-xBrx and CH3NH3PbBr3-xClxwith specified halide ratios. The surface potential differences between different samples are related to their work function variations. These results are helpful to the understanding of the structural and electronic properties of perovskite materials.

The effect of Langmuir arachidic acid layers on surface morphology and electrical properties of a polycrystalline CdS film

The effect of number of arachidic acid (AA) Langmuir layers and pH values on morphology and local electrical properties of polycrystalline cadmium sulphide (CdS) film was investigated by atomic force microscopy, electric force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM). Atomic force microscopy (AFM) surface analysis showed the polycrystalline relief of CdS surface with the roughness value about 8.5 ± 0.3 nm and the drastic increases up to 25 ± 0.9 nm for semiconductor surface modified by 1, 3 and 5 AA Langmuir layers at both pH value (pH 3.6 and 8.6). Surface potential (SP) values were estimated by EFM method and showed the same sequence of changes and were equal to -1 V for CdS film and to 2.4 V and 3.5 V for 3 AA layers at pH 3.6 and 8.6, respectively. SP of 1 and 5 AA Langmuir layers was about 2 V at both pH values. Furthermore, the electrical characteristics and parameters of samples calculated by EFM quantitative method were compared with SKPM method. EFM analysis showed more pronounced SP value changes than SKPM measuring, due to mathematical value processing of the established Coulomb interaction between the probe and the sample surface.

Work function and temperature dependence of electron tunneling through an N-type perylene diimide molecular junction with isocyanide surface linkers.

Conducting probe atomic force microscopy (CP-AFM) was employed to examine electron tunneling in self-assembled monolayer (SAM) junctions. A 2.3 nm long perylene tetracarboxylic acid diimide (PDI) acceptor molecule equipped with isocyanide linker groups was synthesized, adsorbed onto Ag, Au and Pt substrates, and the current-voltage (I-V) properties were measured by CP-AFM. The dependence of the low-bias resistance (R) on contact work function indicates that transport is LUMO-assisted ('n-type behavior'). A single-level tunneling model combined with transition voltage spectroscopy (TVS) was employed to analyze the experimental I-V curves and to extract the effective LUMO position εl = ELUMO - EF and the effective electronic coupling (Γ) between the PDI redox core and the contacts. This analysis revealed a strong Fermi level (EF) pinning effect in all the junctions, likely due to interface dipoles that significantly increased with increasing contact work function, as revealed by scanning Kelvin probe microscopy (SKPM). Furthermore, the temperature (T) dependence of R was found to be substantial. For Pt/Pt junctions, R varied more than two orders of magnitude in the range 248 K < T < 338 K. Importantly, the R(T) data are consistent with a single step electron tunneling mechanism and allow independent determination of εl, giving values compatible with estimates of εl based on analysis of the full I-V data. Theoretical analysis revealed a general criterion to unambiguously rule out a two-step transport mechanism: namely, if measured resistance data exhibit a pronounced Arrhenius-type temperature dependence, a two-step electron transfer scenario should be excluded in cases where the activation energy depends on contact metallurgy. Overall, our results indicate (1) the generality of the Fermi level pinning phenomenon in molecular junctions, (2) the utility of employing the single level tunneling model for determining essential electronic structure parameters (εl and Γ), and (3) the importance of changing the nature of the contacts to verify transport mechanisms.

Incorporating an Electrode Modification Layer with a Vertical Phase Separated Photoactive Layer for Efficient and Stable Inverted Nonfullerene Polymer Solar Cells.

For bulk heterojunction polymer solar cells (PSCs), the donors and acceptors featuring specific phase separation and concentration distribution within the electron donor/acceptor blends crucially affect the exciton dissociation and charge transportation. Herein, efficient and stable nonfullerene inverted PSCs incorporating a phase separated photoactive layer and a titanium chelate electrode modification layer are demonstrated. Water contact angle (WCA), scanning kelvin probe microscopy (SKPM), and atomic force microscopy (AFM) techniques are implemented to characterize the morphology of photoactive layers. Compared with the control conventional device, the short-circuit current density (Jsc) is enhanced from 14.74 to 17.45 mAcm-2. The power conversion efficiency (PCE) for the inverted PSCs with a titanium (diisopropoxide)-bis-(2,4-pentanedionate) (TIPD) layer increases from 9.67% to 11.69% benefiting from the declined exciton recombination and fairly enhanced charge transportation. Furthermore, the nonencapsulated inverted device with a TIPD layer demonstrates the best long-term stability, 85% of initial PCE remaining and an almost undecayed open-circuit voltage (Voc) after 1440 h. Our results reveal that the titanium chelate is an excellent electrode modification layer to incorporate with a vertical phase separated photoactive layer for producing high-efficiency and high-stability inverted nonfullerene PSCs.

The Effect of Donor and Nonfullerene Acceptor Inhomogeneous Distribution within the Photoactive Layer on the Performance of Polymer Solar Cells with Different Device Structures.

Due to the inhomogeneous distribution of donor and acceptor materials within the photoactive layer of bulk heterojunction organic solar cells (OSCs), proper selection of a conventional or an inverted device structure is crucial for effective exciton dissociation and charge transportation. Herein, we investigate the donor and acceptor distribution within the non-fullerene photoactive layer based on PBDTTT-ET:IEICO by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) and scanning Kelvin probe microscopy (SKPM), indicating that more IEICO enriches on the surface of the photoactive layer while PBDTTT-ET distributes homogeneously within the photoactive layer. To further understand the effect of the inhomogeneous component distribution on the photovoltaic performance, both conventional and inverted OSCs were fabricated. As a result, the conventional device shows a power conversion efficiency (PCE) of 8.83% which is 41% higher than that of inverted one (6.26%). Eventually, we employed nickel oxide (NiOx) instead of PEDOT:PSS as anode buffer layer to further enhance the stability and PCE of OSCs with conventional structure, and a promising PCE of 9.12% is achieved.

Nanoscale self-recovery of resistive switching in Ar+ irradiated TiO2−x films

Nanoscale evidence of self-recovery in resistive switching (RS) behavior was found in TiO2−x film by conductive atomic force microscopy when exposed to Ar+-ions above a threshold fluence of 1 × 1016 ions cm−2. This revealed an evolution and gradual disappearance of bipolar RS-loops, followed by reappearance with increasing number of voltage sweep. This was discussed in the realm of oxygen vacancy (OV) driven formation, dissolution and reformation of conducting filaments. The presence of OVs in ion-beam irradiated TiO2−x films was evidenced by decreasing trend of work function in scanning-Kelvin probe microscopy, and was further verified by x-ray absorption near edge spectroscopy at Ti and O-K edges.