Contact Potential Difference Measurements Research Articles

Contact Potential Difference Measurement of Adhesion Process during Micro/Meso-scale Injection Upsetting

In micro/meso (between macro and micro)-scale metal forming, the contribution of friction between the material and tool on the material deformation is higher than that in macro-scale metal forming. The friction during forming strongly depends on the adhesion process. The process was studied by measuring the contact potential difference distribution of the tool surface after an injection upsetting stroke. The effect of the annealing temperature of materials and the forming speed on the amount of adhesion was quantified by the measurements.

Orientation of molecular interface dipole on metal surface investigated by noncontact atomic force microscopy

We investigated the orientations of interface dipole moments of individual non-planar titanyl phthalocyanine (TiOPc) molecules on Cu(111) and Cu(100) substrates using scanning tunneling microscope (STM) and noncontact atomic force microscope (NC-AFM). The dipole moment orientations corresponding to two different configurations of individual TiOPc molecules were determined unambiguously. The correlation between the actual molecular structures and the corresponding STM topographies is proposed based on the sub-molecular resolution imaging and local contact potential difference (LCPD) measurements. Comparing with the pristine substrate, the LCPD shift due to the adsorption of non-planar molecule is dependent on the permanent molecular dipole, the charge transfer between the surface and the molecule, and the molecular configurations. This work would shed light on tailoring interfacial electronic properties and controlling local physical properties via polar molecule adsorption.

Scanning Kelvin probe study of photolabile silane surface modification of indium tin oxide

The scanning Kelvin nanoprobe (SKN) is an exquisitely sensitive device capable of detecting subtle changes in work function associated with alteration of surface chemistry and interfacial dipole. This instrument is highly versatile and has notably been recently used for (i) the investigation of biological interactions occurring at the interface of multiplexed microarrayed platforms and (ii) the characterization of high work function materials for application in molecular optoelectronics. Herein, we further implement the SKN to characterize, along with angle‐resolved X‐ray photoelectron spectroscopy and contact angle goniometry, the surface modification of indium tin oxide substrates with photopatternable silane adlayers. These molecular films are constructed in a straightforward and economical manner from alkyltrichlorosilane surface‐modifying molecules that possess a distal, UV‐photolabile o‐nitrobenzyl moiety. Employing a photomask, we were able to selectively pattern regions of the photoreactive silane adlayer and confirm the corresponding changes in surface potential through contact potential difference measurements. Copyright © 2013 John Wiley & Sons, Ltd.

Organic Functionalization of 3C-SiC Surfaces

We demonstrate the functionalization of n-type (100) and (111) 3C-SiC surfaces with organosilanes. Self-assembled monolayers (SAMs) of amino-propyldiethoxymethylsilane (APDEMS) and octadecyltrimethoxysilane (ODTMS) are formed via wet chemical processing techniques. Their structural, chemical, and electrical properties are investigated using static water contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy, revealing that the organic layers are smooth and densely packed. Furthermore, combined contact potential difference and surface photovoltage measurements demonstrate that the heterostructure functionality and surface potential can be tuned by utilizing different organosilane precursor molecules. Molecular dipoles are observed to significantly affect the work functions of the modified surfaces. Furthermore, the magnitude of the surface band bending is reduced following reaction of the hydroxylated surfaces with organosilanes, indicating that partial passivation of electrically active surface states is achieved. Micropatterning of organic layers is demonstrated by lithographically defined oxidation of organosilane-derived monolayers in an oxygen plasma, followed by visualization of resulting changes of the local wettability, as well as fluorescence microscopy following immobilization of fluorescently labeled BSA protein.

O2 and organic semiconductors: Electronic effects

Contact potential difference (CPD) measurements of the relative work functions of a range of organic semiconductor thin films show that oxygen causes effective p-type doping (with work functions increasing 0.1–0.3eV). This doping effect is found to be reversible by exposure to high vacuum or heating in inert atmosphere. The mechanism of doping is explained by a model, based on a reversible formation of an O-substrate charge transfer state. Conductivity measurements of p-phthalocyanine films at variable temperatures support this doping model. The oxygen doping effect is consistent with filling of tail states in the gap, as shown by the increase of activation energy of hole transport with decreased O-doping, and by the good fit between experimental data and simulations of the in-gap density of states. A model hybrid solar cell configuration also shows the effect of doping by O2 and corroborates the fact that O-doping fills the tail states in the system.

B010 Measurement of contact potential difference and material distribution by using an SEFM

This paper presents a novel method for the measurement of the contact potential difference (CPD) and material distribution over the sample surface using scanning electrostatic force microscope (SEFM). Since the intensity of electrostatic force generated between the probe and the sample surface is relied on the differences of the CPD, the CPD can be calculated by using the frequency shifts of the probe oscillation when two different bias voltages are applied between tip and the sample. The detection sensitivity corresponding to the experimental conditions has been confirmed by simulation. In addition, the basic characteristic of the CPD measurement system has been evaluated.

Research Progress in Atomic Resolution Microscopy

Tremendous progress has been made in non-contact atomic force microscopy (NC-AFM) recently. The spatial resolution of NC-AFM imaging and spectroscopy of individual molecules on surfaces has reached true atomic resolution and bond differentiation level. Combination of NC-AFM with other scanning probe techniques can open a new way for materials, physics, chemistry, and biochemistry studies. In this review, we first introduce the basic principle of NC-AFM and qPlus sensor. The interaction force at atomic scale and precise measurement of short-range force are discussed. We summarize the recent advances in structural determination of organic molecules, chemical identification, electronic structure, and atomic manipulation at the atomic scale. In addition, we also discuss the application of Kelvin probe force microscopy (KPFM) in measurement of local contact potential difference (LCPD). Finally, perspectives and challenges in NC-AFM techniques are presented.

Gas Effect On The Surface Photovoltage Of Porphyrin Functionalized ZnO Nanorods

Recent studies suggest that the gas sensitivity of porphyrin-functionalized ZnO nanorods can be activated under visible light illumination. Then the use of properly coloured light tuned to the absorbance spectra of individual porphyrins could enable a control of sensors sensitivity. The effect of light on the sensitivity to gases is critically governed by the transport phenomena of electronic charge across the interfaces of organic and inorganic structures. Therefore, accurate measurements of energy levels and contact potential differences in porphyrins functionalized ZnO nanorods are important to intepret the sensing properties of such hybrid materials. For the scope, Kelvin probe measurements of porphyrin-ZnO structures were performed exposing the material in dark and visible light and to organic vapours. Results provide an experimental basis to understand the mutual effects of gas adsorption and illumination to the device conductivity. Copyright © 2012 VBRI press.

Correlation of Photodegradation Efficiency with Surface Potential of Silver-TiO2 Nanocomposite Thin Films

The present work focuses on photocatalytic performance of Ag-TiO2 systems. Nanocrystalline thin films of TiO2-and Ag-doped TiO2 are grown by sol–gel method, followed by spin-coating technique on Si ⟨100⟩ surface. The crystallinity and crystal size were measured from X-ray diffraction and transmission electron microscopy studies. Lateral distribution of work function (WF) was examined through contact potential difference measurement done by scanning Kelvin probe microscopy. Local WFs of Ag-TiO2 thin films were found to be smaller than that of TiO2, and the minimum WF expected from the polynomial curve fitting was that of pure Ag. The photocatalytic efficiency of these thin films is estimated from photodegradation of methyl orange analyzed by UV–vis spectrophotometer. The photodegradation efficiency of Ag-TiO2 nanocrystalline thin films increases up to a certain dopant concentration of silver, beyond which it decreases. The changes in the photodegradation efficiency of these films are correlated with variation in contact potential difference.

Kelvin Probe Force Microscopy in Nonpolar Liquids

Work function changes of Au were measured by Kelvin probe force microscopy (KPFM) in the nonpolar liquid decane. As a proof of principle for the measurement in liquids, we investigated the work function change of an Au substrate upon hexadecanethiol chemisorption. To relate the measured contact potential difference (CPD) during the chemisorption of alkanethiols to a change of the work function, the influence of physisorbed decane must be taken into account. It is crucial that either the work function of the scanning probe microscope (SPM) tip or the sample surface remains constant throughout the reaction, since both contribute to the CPD. We describe two routes for determining the work function shift of Au coated with a monolayer of alkanethiols: In the first route, the SPM tips were taken as reference surfaces (constant tip work function). For this approach, we used Au(111) surfaces and kept the SPM tip ex situ during the adsorption process. In the second route, structured surfaces with reactive and inert parts were studied by KPFM (constant reference work function). For this route, we prepared nanometer sized Au structures by nanosphere lithography on SiO(x) substrates. Now, the SiO(x) served as the inert reference surface. The shifts in the work function after exposure to the hexadecanethiol (HDT) solution were determined to be ΔΦ(Au+HDT,decane-Au,air) = -1.33 eV ± 0.07 eV (route I) and ΔΦ(Au+HDT,decane-Au,air) = -1.46 eV ± 0.04 eV (route II). Both values are in excellent agreement with the work function shifts determined by ultraviolet photoemission spectroscopy (UPS) reported in literature. The presented procedures of measuring work function changes in decane open new ways to study local reactions at solid-liquid interfaces.

The importance of cantilever dynamics in the interpretation of Kelvin probe force microscopy

A realistic interpretation of the measured contact potential difference (CPD) in Kelvin probe force microscopy (KPFM) is crucial in order to extract meaningful information about the sample. Central to this interpretation is a method to include contributions from the macroscopic cantilever arm, as well as the cone and sharp tip of a KPFM probe. Here, three models of the electrostatic interaction between a KPFM probe and a sample are tested through an electrostatic simulation and compared with experiment. In contrast with previous studies that treat the KPFM cantilever as a rigid object, we allow the cantilever to bend and rotate; accounting for cantilever bending provides the closest agreement between theory and experiment. We demonstrate that cantilever dynamics play a major role in CPD measurements and provide a simulation technique to explore this phenomenon.

Measurement of contact potential difference in head disk interface by readback signal spectrum

The contact potential difference leads to electrostatic interaction between the slider and disk in a hard disk drive. The effect of electrostatic force on slider’s flying height and flying stability becomes more significant with the decrease of flying height. A method of measurement of contact potential difference in head disk interface by readback signal spectrum is demonstrated in this paper. When a voltage with DC and AC was applied in the head disk interface, the amplitude of readback signal at the first harmonic frequency of applied AC voltage is proportional to the sum of contact potential difference and applied DC voltage. The contact potential difference in head disk interface is equal to the negative of DC voltage when the amplitude of readback signal at the first harmonic frequency is minimal.

Surface functionalization of 6H-SiC using organophosphonate monolayers

The synthesis and characterization of self-assembled monolayers prepared from 11-hydroxyundecyphosphonic acid and 9,10-diphenyl-2,6-diphosphonoanthracene on n-type 6H-SiC is described. Structural and chemical properties of these monolayers were investigated through contact angle measurements, atomic force microscopy, x-ray photoelectron spectroscopy, and Fourier transformation infrared spectroscopy, which indicate covalent bonding of the phosphonates to both (0001)- and (000-1)-oriented 6H-SiC crystal faces. Electrical characterization was achieved through contact potential difference and surface photovoltage measurements, which revealed significant changes in the work functions of the substrates by monolayer formation.

Interaction of ytterbium nanofilms grown on tungsten substrates with oxygen

The interaction of ytterbium nanofilms evaporated on tungsten substrates with oxygen has been studied by Auger electron spectroscopy, thermal desorption spectroscopy, and contact potential difference measurements. It has been shown that at room temperature, no oxide is formed in the above interaction. In place of the oxide, a chemisorbed layer of nondissociated O2 molecules is formed on the surface of the ytterbium nanofilms. This layer modifies the ytterbium. This modification transforms ytterbium from the divalent state into the trivalent state.

Contact Potential Difference of Au/TiO<sub>2</sub>(110) Model Catalysts Measured by Kelvin Probe Force Microscopy

Contact potential difference (CPD) of Au/TiO2 model catalysts was measured by Kelvin probe force microscopy (KFM). The CPD of Au particles was lower than that on the TiO2(110) surface and the difference of CPD between them increased with the particle height. By applying the parallel disk model, the work function difference and the charge density at the Au/TiO2 interface were estimated to be 0.19 eV and 0.01 C/m2, respectively. We have previously measured local barrier height (LBH) of Au/TiO2 by scanning tunneling microscopy. From the results of CPD and LBH measurements, we found a possibility that the localized charge at the Au/TiO2 interface plays some role in the catalytic activity of Au catalysts. [DOI: 10.1380/ejssnt.2012.59]

Spatially resolved electronic inhomogeneities of graphene due to subsurface charges

We probe the local inhomogeneities in the electronic properties of exfoliated graphene due to the presence of charged impurities in the SiO 2 substrate using a combined scanning tunneling and electrostatic force microscope. Contact potential difference measurements using electrostatic force microscopy permit us to obtain the average charge density but it does not provide enough resolution to identify individual charges. We find that the tunneling current decay constant, which is related to the local tunneling barrier height, enables one to probe the electronic properties of graphene distorted at the nanometer scale by individual charged impurities. We observe that such inhomogeneities do not show long-range ordering and their surface density obtained by direct counting is consistent with the value obtained by macroscopic charge density measurements. These microscopic perturbations of the carrier density significantly alter the electronic properties of graphene, and their characterization is essential for improving the performance of graphene based devices.

Surface Termination Control in Chemically Deposited PbS Films: Nucleation and Growth on GaAs(111)A and GaAs(111)B

This study addresses the question of whether chemically deposited PbS thin films grown on GaAs(111) are affected by the oppositely terminated substrate surfaces, gallium terminated GaAs(111)A and arsenic terminated GaAs(111)B. The differences in PbS film deposition pathway in both cases of substrate surface termination were investigated using X-ray photoelectron spectroscopy (XPS), Raman scattering, and contact potential difference (CPD) measurements. The morphology, microstructure, and crystallographic orientation of the films were studied using scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. XPS and CPD measurements indicated that PbS films deposited on oppositely terminated GaAs(111) surfaces possessed corresponding surface terminations, with PbS(111)B obtained on GaAs(111)B and PbS(111)A on GaAs(111)A. Subsequently, different surface oxides were detected by XPS on A and B terminated PbS(111), with lead oxide obtained on PbS(111)A and PbSO3 obtained on PbS(111)B. ...

Electronic structure of the poly(3-hexylthiophene):indene-C60 bisadduct bulk heterojunction

The electronic structure of the poly(3-hexylthiophene):indene-C60 bisadduct (P3HT:ICBA) blend is investigated with direct and inverse photoemission spectroscopy. The energy gap between the lowest unoccupied molecular orbital (LUMO) of ICBA and the highest occupied molecular orbital (HOMO) of P3HT, which is central to the performance of photovoltaic cells made with this blend, is found equal to 1.68 eV. This value is larger than the difference between the ionization energy of the donor and the electron affinity of the acceptor, implying the existence of an interface dipole barrier between these two materials. Complementary contact potential difference measurements are done with a Kelvin probe in the dark on similar donor/acceptor pairs, e.g., P3HT:C60 and copper phthalocyanine (CuPc):C60, and confirm the existence of a dipole at these interfaces. It is estimated that a 0.3–0.4 eV ground state dipole exists at the P3HT:ICBA interface.

The role of the cantilever in Kelvin probe force microscopy measurements

The role of the cantilever in quantitative Kelvin probe force microscopy (KPFM) is rigorously analyzed. We use the boundary element method to calculate the point spread function of the measuring probe: Tip and cantilever. The calculations show that the cantilever has a very strong effect on the absolute value of the measured contact potential difference even under ultra-high vacuum conditions, and we demonstrate a good agreement between our model and KPFM measurements in ultra-high vacuum of NaCl monolayers grown on Cu(111). The effect of the oscillating cantilever shape on the KPFM resolution and sensitivity has been calculated and found to be relatively small.

Electrical passivation and chemical functionalization of SiC surfaces by chlorine termination

We have developed a straightforward plasma-based method which yields chlorine-terminated n-type 6H-SiC surfaces. Atomic force microscopy shows that the surface roughness is not affected by the plasma processing. Additionally, x-ray photoelectron spectroscopy reveals a significant reduction in oxygen, and a corresponding rise of chlorine core level intensities, following halogen termination. Contact potential difference and surface photovoltage measurements show formation of negative surface dipoles and approximately flat band surface potentials after chlorine termination of (0001) n-type 6H-SiC (built-in voltage Vbi<20 meV). Starting from halogenated surfaces, we demonstrate both ultraviolet light-induced and thermally-induced functionalization with alkene-derived self-assembled organic monolayers.