Kelvin probe force microscopy and its application

Kelvin probe force microscopy (KPFM) is a tool that enables nanometer-scale imaging of the surface potential on a broad range of materials. KPFM measurements require an understanding of both the details of the instruments and the physics of the measurements to obtain optimal results. The first part of this review will introduce the principles of KPFM and compare KPFM to other surface work function and potential measurement tools, including the Kelvin probe (KP), photoemission spectroscopy (PES), and scanning electron microscopy (SEM) with an electron beam induced current (EBIC) measurement system. The concept of local contact potential difference (LCPD), important for understanding atomic resolution KPFM, is discussed. The second part of this review explores three applications of KPFM: metallic nanostructures, semiconductor materials, and electrical devices.     

Luminescence of phenanthrene with impurity anthracene adsorbed on zeolite NaY

Journal of Applied Spectroscopy -     

Combined SIMS,AES, and XPS investigations of tantalum oxide layers

Thick layers of tantalum oxide prepared by thermal and anodic oxidation have been studied by combined SIMS, AES, and XPS during depth profiling by 3keV Ar⁺ ion sputtering. The chemical composition of these films is revealed by the OKLL and O 1s signals and by the “lattice valence” parameter determined from the TaO _n ^± intensities. Thus the anodic film consists of a contamination layer, an oxygen-rich reactive interface and a thick homogeneous oxide layer followed by an interface to the Ta metal. The thermal oxide shows an oxygen concentration decreasing with depth and a broad oxide-metal interface. In both cases, carbon contamination (carbide) prevents the application of the valence model to the clean Ta substrate. The sputtering yield of the oxides was found to be 0.6 Ta₂O₅/ion.     

Evidence of surface active sites on NaY zeolite by a model reaction

Owing to the development of a new test reaction, namely the isomerization of 1-dodecene, it becomes possible to characterize the activity of cationic zeolites under conditions close to those of industrial adsorption and separation processes (temperature around 150-200 °C and liquid phase). Indeed, 1-dodecene is highly active and still in a liquid state at 150 °C. Furthermore, by comparing the reactivity of NaY before and after treatments applied to reduce its activity ((i) passivation of the external surface by deposition of TetraEthylOrthoSiloxane (TEOS) and (ii) washing the zeolite with a basic or neutral solution), we are able to propose a nature and localization for the residual active sites of this zeolite. Indeed, the evolution of the NaY activity in their function indicates that the active sites are located both at the external and internal surfaces of NaY and that two types of sites can be described: OH groups and structure defects.     

Study of ethylene adsorption on zeolite NaY modified with group I metal ions

The adsorption of ethylene by zeolite NaY and zeolite NaY modified by cation exchange with potassium, rubidium, and cesium ions was studied. Cation exchanges were carried out using KNO₃, RbNO₃, and CsNO₃ in the concentration ranges of 0.2-10 mM. XRD patterns and specific surface areas illustrated that modification of NaY zeolite by very dilute solutions containing K⁺, Rb⁺ and Cs⁺ did not lead to significant changes in the crystallinity. Analysis of metals content (ICP-OES) showed that Cs⁺ can replace Na⁺ better than Rb⁺ and K⁺. Particle analysis indicated slight decreases in surface area but pore volumes and pore diameters remained unchanged. Ethylene adsorption isotherms indicated that Na-Y zeolite which was modified by 5.0 mM KNO₃, 0.5 mM RbNO₃ and 1.0 mM CsNO₃ could adsorb ethylene better than zeolite Na-Y. K-NaY zeolite adsorbed up to 102.45 cm³/g ethylene, while Rb-NaY and Cs-NaY zeolites adsorbed up to 98.50 cm³/g and 90.15 cm³/g ethylene, respectively. Ethylene adsorption capacities depended on number of adsorption sites and surface interactions.     

Combined XPS and SIMS study of amino acid overlayers

Factors influencing the SIMS fragmentation patterns are studied for three simple amino acids-glycine, α-alanine, and serine-deposited onto Ag substrates from aqueous solution. Secondary ion emissions are measured for 1 keV Ar⁺ ions incident at 70° from sample normal as a function of substrate preparation and solution concentration. Studies by XPS and X-ray induced AES prior to SIMS analysis show that the amino acids adsorb in a film on the Ag surface and that the film thickness increases with solution concentration. In addition, considerable amounts of amino acid can be deposited on the surface from a water film retained during extraction from a concentrated solution. On acid etched samples, positive ion fragments of mass AgM, Ag(M ? 45), Ag, M + 1, and M ? 45 are observed, where M is the molecular, weight of the parent amino acid. With the exception of the (M + 1)⁺ fragment, these peak intensities behaved similarly for the different surface concentrations. When the adsorbed film grows too thick, the positive molecular ion emissions drop considerably; this substantiates the need for proximity between the Ag substrate and the amino acid molecule.     

Contrast inversion of the h-BN nanomesh investigated by nc-AFM and Kelvin probe force microscopy

Single sheets of hexagonal boron nitride (h-BN) on transition metals provide a model system for layered insulating materials as well as a functional substrate for molecules and metal clusters. The progress in the understanding of h-BN layers on transition metals was mainly driven by scanning tunnelling microscopy (STM) and photoelectron spectroscopy (PES) measurements within the last decade, while direct measurements of mechanical and electrical properties are still rare. Our investigations of the two-dimensional (2D) h-BN nanomesh on a Rh(111) substrate by high-resolution noncontact atomic force microscopy (nc-AFM) reveal a complex surface structure including a frequently observed contrast inversion. Detailed 2D force spectroscopy measurements are revealing towards a mechanical elastic deformation of the h-BN monolayer caused by the tip-sample interaction. Furthermore, Kelvin probe force microscopy (KPFM) and spectroscopy measurements show local work function variations of the nanomesh, proving the results obtained by PES but additionally providing detailed local information.     

Characterization of photovoltage evolution of ZnO films using a scanning Kelvin probe system

Work function (WF) and surface photovoltage evolution of films can be measured using the Kelvin probe technique, and further analysis of the photoelectronic behavior can provide information on the energy level structure. In this paper, a theoretical analysis to measure surface photovoltage using Kelvin probe technique is presented. Based on this analysis, the surface photovoltage and its time-resolved evolution process as well as the energy level structure of ZnO films are determined using a scanning Kelvin probe. The present study therefore provides a simple and practical methodology for the characterization of the electronic behavior of films.     

Surface potential images of polycrystalline organic semiconductors obtained by Kelvin probe force microscopy

P-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyanina-tocopper (F₁₆CuPc) polycrystalline films were investigated by Kelvin probe force microscopy (KPFM). Topographic and corresponding surface potential images are obtained simultaneously. Surface potential images are related with the local work function of crystalline facets and potential barriers at the grain boundaries (GBs) in organic semiconductors. Based on the spatial distribution of surface potential at GBs, donor- and acceptor-like trapping states in the grain boundaries (GBs) of p-CuPc and n-F₁₆CuPc films are confirmed respectively. In view of spatial energy spectrum in micro-scale provided by KPFM, it is going to be a powerful tool to characterize the local electronic properties of organic semiconductors.     

An XPS study of the optimum loading of barium on high-silica MFI zeolite

X-ray photoelectron spectroscopy (XPS) has been applied to the characterization of barium-impregnated MFI high-silica zeolites which are used for the conversion of methanol to light alkenes. X-ray photoelectron spectroscopy provided information about the degree of the dispersion of the various barium loadings on the silicalite structure, and this information helped in elucidating the observed relationship between the activity/selectivity of the catalysts and the barium loading. The XPS results also helped in predicting that the performance of the catalyst would be optimized at 4 wt% Ba loading which was found to agree with the catalytic conversion of methanol to light alkenes.     

混合型尿路结石的XPS和XRD联合分析

为了深入了解广东省珠江三角洲尿路结石的组份,为临床的相关研究和预防提供参考,采用X-射线光电子能谱(XPS)和X-射线衍射法(XRD)对6例典型的混合型尿路结石成份和物相进行了分析。结果表明,所检尿石成份合一水草酸钙、二水草酸钙、羟基磷灰石,其中2例含有少量磷酸铵镁。随着尿石中磷酸盐的增加,一水草酸钙(COM)的含量减少,二水草酸钙(COD)含量增加,表明磷酸盐的沉积抑制了高能量的COD向低能量的COM转化。采用XPS和XRD联合分析,可以较为准确地检测尿路结石的组份和物相。     

Chemisorption of NO and NH3 on cobalt: Studies by UPS,XPS, and work-function measurements

Chemisorption of NO and NH₃ on cobalt has been studied by UV and X-ray photoelectron spectroscopy (UPS and XPS) and work-function measurements. The 25°C data for NO are consistent with dissociative chemisorption below ～5 L exposure followed by molecular chemisorption at higher exposures; complete dissociation occurs after heating to 500°C. The UPS of molecularly chemisorbed NO exhibit peaks at ～2.7, 10.2 and 14 eV below E_F, corresponding to ionization of the 2π*, (1π + 5σ), and 4σ MO's. The UPS data for NH₃ exhibit peaks at 7.9 and 11.4 eV below E_F, consistent with ionization of the 3a₁ and 7e MO's of NH₃ and/or MO's of the fragments NH₂ and/or NH.     

The correlation of XPS analysis with electrochemical history in aqueous corrosion

The response of the surface composition of aluminium brass to electrochemical polarization is sea water has been studied by XPS in an attempt to elucidate the behaviour of condenser tube surfaces in marine environments. Individual specimens were polarized at 100 mV intervals between +100 and ?1200 mV (SHE) for a charge transfer of 500 mA s. The rather complex spectra were normalised using Jorgenson's factors to permit meaningful discussion of the surface chemistry. In addition an ion etching was used to remove the high surface coverage of carbon molecules picked up from solution. The validity of each of these techniques, necessary for examination of samples removed from the aqueous phase, is discussed. In the present case they permit the observation of a strong correlation between the Mg⁺⁺/Cl^? ratio and the surface potential. The movement of the ratio gives a nice demonstration of cathodic basicity and anodic acidity in accord with electrochemical concepts. This result may be of practical use in ascertaining whether corrosion pits on industrial plant are still active.     

The influence of adsorbed molecules on Na-sites in NaY zeolite investigated by triple-quantum MAS NMR spectroscopy

The effect of hydration and benzene adsorption on resonance and the quadrupolar interaction in NaY zeolites is studied by triple-quantum MAS NMR spectroscopy. In the case of a C₆D₆/NaY system, the results show that with an increase in benzene loading, there is an up-field trend in isotropic chemical shift (δ_CS) and a decreasing second order quadrupolar effect (χ_s) for the site II sodium ions. It was found that adsorbed benzene molecules have a slight effect on the environment of sodium ions on site I. All the sodium sites in NaY are influenced upon hydration. The up-field shift of the sodium δ_CS reflects the effect of coordination of oxygen atoms on sodium cations due to hydration. The magnitude of χ_s for hydrated sodium sites increases and then falls off with water loading. The increase in χ_s is due to the initial hydration among SI-, SI′- and SII-sodium ions, while the decrease is the result of approaching the final stage of saturated hydration.     

129Xe NMR study of the homogeneity and the size distribution of small sodium metal particles in NaY zeolite

NaY zeolite samples loaded with sodium metal by vapor phase deposition have been investigated using¹²⁹Xe NMR spectroscopy. At low sodium concentration, the¹²⁹Xe NMR spectrum showed three resonance lines which clearly indicate the existence of distinct domains in the zeolite sample. Such an observation suggests that the diffusion of the xenon atoms into each domain only occurs with respect to the NMR time scale (2.9 ms). As the sodium concentration increases, observation of a single broad line indicate a macroscopic homogenization of the system. The shift of this line is explained in part due to a paramagnetic interaction between the xenon atoms and the unpaired electrons of particles containing an odd number of sodium atoms. The linewidth is due to the distribution of the local magnetic fields partially averaged by the rapid motion of the xenon atoms and to the statistical distribution of the sodium particles in the supercage cavities. The paramagnetic interaction vanishes with the oxidation of the sample leading to a narrowing and a shift of the line to higher magnetic fields.     

FTIR和XPS对沸石合成特性及Cr(Ⅲ)去除机制的谱学表征

以粉煤灰为原料,采用优化的水热晶化法合成沸石,使用XRD,SEM和ζ电位分析沸石产品的组成特性,借助FTIR和XPS等揭示废水中Cr(Ⅲ)的去除机制。合成产品主要为NaP1型沸石,在pH值8～12区间内,ζ电位由-8.72mV降到-24.46mV。准二级方程和Langmuir等温线对试验的拟合效果更好,理论饱和吸附量为33.557 0mg.g-1。FTIR图谱表明—OH和Si—O类官能团在反应过程中有重要贡献。XPS全谱发现:结合能576.45eV为Cr(2p3/2)的特征峰,揭示了吸附过程的有效性。吸附Cr(Ⅲ)后,沸石中Si—Si和Si—O对应的结合能增加了0.25和0.60eV,含Si官能团可能与Cr(Ⅲ)发生了配位反应。O(1s)的结合能在反应后变得更低。这些证据表明:沸石对Cr(Ⅲ)的去除过程是物理吸附和化学吸附共同作用的结果。     

A new approach to modelling Kelvin probe force microscopy of hetero-structures in the dark and under illumination

A numerical method is proposed to model Kelvin probe force microscopy of hetero-structures in the dark and under illumination. It is applied to FTO/TiO₂ and FTO/TiO₂/MAPbI₃ structures. The presence of surface states on the top of the TiO₂ layers are revealed by combining theoretical computation and experimental results. Basic features of Kelvin probe force microscopy under illumination, namely surface photovoltage, are simulated as well. The method paves the way toward further investigations of more complicated optoelectronic devices.