Contact Potential Measurements Research Articles

Contact Potential Measurement of Carbon Nanotube by Kelvin Probe Force Microscopy

The contact potential of multiwalled carbon nanotubes (MWCNTs) has been successfully measured by Kelvin probe force microscopy (KFM) using a CNT tip as a potential probe. Spatial resolution was improved using a CNT tip compared with the case of using a conventional Au-coated Si tip. The contact potential along the MWCNT placed on the source and drain electrodes with bias voltage was also measured. The potential drop was observed along the MWCNT indicating that the present MWCNTs are diffusive conductors.

Contact potential measurement of cleaved mirror surface of 1.3 µm buried heterostructure laser diode by Kelvin probe force microscopy

Contact potential images of a cleaved mirror surface of a 1.3 /spl mu/m buried heterostructure laser diode by Kelvin probe force microscopy have been successfully obtained. The embedded current blocking layer and mesa-etched active region were clearly recognised.

Contact potential measurements with a local Kelvin probe

The Kelvin probe technique can be used as a very accurate tool to measure contact potential differences (CPDs). However, the experimentally observed CPD dependence on the probe-sample distance, the so-called stray capacitance effect, is a parasitic effect not explained in the simple Kelvin probe theory. It can be especially disturbing when using a local Kelvin probe or Kelvin probe microscopy as it hinders quantitative use of CPD measurements. We show that this effect can be described without the expedient of stray capacitors by taking into account the electrostatic induction of metal objects located in the vicinity of the probe-sample system. This model leads to excellent agreement with the experimental variations in the apparent value of CPD with the probe-sample distance and allows us to recover the exact CPD value.

AFM-based Electrical Characterization of Nano-structures

ABSTRACTCarbon nanotubes have the potential of being used as interconnects and active semiconducting material in future electronic circuits. It is necessary to study such nano-scale circuits with probes that can make measurements with molecular precision. We describe results using two nanoprobe techniques, namely scanning surface potential microscopy (SSPM), and conductive tip atomic force microscopy (CT-AFM), in the investigation of electrical properties of nanotube circuits. Vertical arrays of multi-walled nanotubes, grown in a porous alumina template with a metal back contact were analyzed. Current mapping confirmed that the nanotubes were electrically connected to the back contact. Isolated single-walled nanotube bundles deposited on an oxidized silicon wafer, and contacted electrically through chromium electrodes were also studied. Contact potential differences between the metal and nanotubes, and the current in some connected nanotubes were measured. Measurements of contact potential with different metals, and the nature of microscopic transport is crucial. Contact potential measurements can also provide fast and reliable characterization of junctions between metallic and semiconducting nanotubes and metals electrodes.

AFM-based Electrical Characterization of Nano-structures

ABSTRACTCarbon nanotubes have the potential of being used as interconnects and active semiconducting material in future electronic circuits. It is necessary to study such nano-scale circuits with probes that can make measurements with molecular precision. We describe results using two nanoprobe techniques, namely scanning surface potential microscopy (SSPM), and conductive tip atomic force microscopy (CT-AFM), in the investigation of electrical properties of nanotube circuits. Vertical arrays of multi-walled nanotubes, grown in a porous alumina template with a metal back contact were analyzed. Current mapping confirmed that the nanotubes were electrically connected to the back contact. Isolated single-walled nanotube bundles deposited on an oxidized silicon wafer, and contacted electrically through chromium electrodes were also studied. Contact potential differences between the metal and nanotubes, and the current in some connected nanotubes were measured. Measurements of contact potential with different metals, and the nature of microscopic transport is crucial. Contact potential measurements can also provide fast and reliable characterization of junctions between metallic and semiconducting nanotubes and metals electrodes.

Surface State Analysis of Tribocharging Data: Effects of Carbon Blacks

Recently the surface state model of tribocharging has been used to determine the relative work functions of toners. The approach consists of examining the correlation between charge-to-mass data for a series of toners measured using several carriers of differing charging ability and the charge-to-mass of a reference toner measured using the same carriers. In this article this method has been used to analyze published data on the charging behavior of toners containing carbon blacks with different contact potentials. An excellent correlation was found between the relative work functions estimated from the triboelectric data and direct contact potential measurements. The results confirm that relative work functions of toners can be accurately estimated using the surface state model. The analysis also shows that in general these materials follow the high-density limit of the surface state model.

Surface immobilized biochemical macromolecules studied by scanning Kelvin microprobe.

The measurement of work function is a particularly effective method for the characterization of surfaces because of the sensitivity of the parameter to interfacial structure, modification and overall chemistry. Accordingly, techniques for the analysis of work function offer a powerful tool for monitoring surface chemical changes, especially for situations involving the immobilization of new moieties at the interface. In the present paper, we describe the performance of a new, modified scanning Kelvin microprobe which is capable of the tandem measurement of contact potential and surface topography with resolutions of 1 mV and 10 nm, respectively. The lateral resolution is 1 micron. The instrument has been applied to the study of substrates modified by the attachment of biochemical macromolecules such as oligonucleotides and DNA. This preliminary work confirms the great potential of the technique in the study of biocompatibility, macromolecular structure and microarray devices.

Measurement of Contact Potential of GaAs pn Junctions by Kelvin Probe Force Microscopy

The contact potential of GaAs pn junctions was measured by Kelvin probe force microscopy (KFM). The contact potential profile of the pn junctions was not clear in the case of no illumination. When the sample was illuminated with a microscope light, the potential profile reflecting pn junctions became clear. The observed surface potential was explained by taking the surface band bending due to surface states into account.

Determination of the mobility gap of microcrystalline silicon and of the band discontinuities at the amorphous/microcrystalline silicon interface using in situ Kelvin probe technique

A method to determine the mobility gap of thin films and the band discontinuities in heterojunctions is presented. It combines in situ contact potential measurements with dark conductivity activation energy measurements. The method is applied to determine the mobility gap of microcrystalline silicon (μc-Si:H) and the band discontinuities at the μc-Si:H/amorphous silicon (a-Si:H) interface. The mobility gap of μc-Si:H depends on its crystalline volume fraction and varies between 1.48 and 1.55 eV. The main band discontinuity occurs at the valence band side. The consequences of the band discontinuities on a-Si:H based solar cells using μc-Si:H doped layers are discussed.

Universal Mechanism for Gas Adsorption and Electron Trapping on Oxidized Silicon

We report that common gases (such as He, Ar, ${\mathrm{H}}_{2}$, ${\mathrm{O}}_{2}$, ${\mathrm{N}}_{2}$, CO) experience adsorption at oxidized silicon surfaces at 300 K via electrostatic coupling. This is deduced using contact potential measurements of the work function for gas pressure in the range ${10}^{\ensuremath{-}3}<P<{10}^{2}\mathrm{Torr}$. The adsorption can be enhanced through surface charging via internal photoemission of electrons leading to mutual electron-gas transient trapping. A simple electrostatic model based on monopole-dipole coupling results in an isotherm in agreement with the data.

Scanning Kelvin microprobe in the tandem analysis of surface topography and chemistry

This article presents the principles of operation, performance and applications of a scanning Kelvin microprobe for combined contact potential and surface topographical measurements. The new instrument uses a miniaturized vibrating probe for exploring, point by point, the surface of a sample, through measurement of the current emerging from the local ‘capacitor’ formed between the vibrating tip and the surface. In particular, the instrument is capable of simultaneously imaging the contact potential difference and topography of a scanned surface. This tandem characterization provides unique information regarding material properties. The lateral resolution is 1 µm, the topographic resolution is in the nanometer range and contact potential sensitivity is in the millivolt range. We also present preliminary studies of graphite, silicon, mica, metal and polymer surfaces.

Contact corrosion measurements on the pair UO 2+ x and carbon steel 1.0330 in brines and bentonite porewater with respect to direct waste disposal

Contact corrosion between carbon steel and UO 2 was studied in the MgCl 2 rich Q-brine, in bentonite porewater and in saturated NaCl solution by use of contact potential and contact current measurements. In all solutions the carbon steel dominates the contact potential, so that this potential is near to the rest potential of the carbon steel. Only in solutions without precipitation of iron corrosion products, the presence of metallic iron slightly reduces the UO 2 corrosion rate. If iron corrosion products precipitate, the relevant adsorption of the uranium species will be more effective than any direct cathodic corrosion protection.

A high-resolution scanning Kelvin probe microscope for contact potential measurements on the 100 nm scale

The article describes the principles and current performance of a scanning Kelvin probe microscope for contact potential measurements with a lateral resolution on the 100 nm scale and a sensitivity in the millivolt range. Preliminary results are presented regarding the variation of the surface potential across charged grain boundaries in polycrystalline silicon.

Contact potential measurements of hard disk drive surfaces in humid environments

Contact potential difference (CPD) experiments were conducted by the vibrating probe (Kelvin) method on humidity-exposed hard disk drive surfaces. The measurements were made at various locations along the circumference of a wear track caused by the sliding contact with a spherical silicon pin, operated in a pin-on-disk configuration. The load on the pin, rotational speed of the disk, and the humidity were controlled. The CPD signals varied along the wear track and the magnitude of the CPD change increased with the load from 29 to 98 mN at a fixed relative humidity. The CPD signals also increased linearly with relative humidity ranging from 30% to 70% as the normal load was fixed at 29 and 49 mN. The CPD signal appears to saturate at a load of 98 mN where the relative humidity is 50% and higher.

Design and calibration of a scanning force microscope for friction, adhesion, and contact potential studies

We present the design and calibration of a scanning force microscope which can be used to study friction, adhesion, and contact potential differences between the cantilever tip and surface. The microscope uses a modular design where the laser, cantilever/sample holder, reflecting mirror, and detector are mounted directly on an optical table. The laser, reflecting mirror, and detector are mounted on translation and rotation stages. With this design the components can be rearranged to calibrate the Z piezo motion as a function of applied voltage. Using the detector micrometers, the detector response (voltage-to-distance relationship) can be determined after each series of measurements. The cantilever/sample holder is constructed such that the components are material matched and thermally compensated from a common reference point. This design feature minimizes thermal drift of the instrument. The instrument can be used in a contact scanning mode where both normal and lateral deflections of the cantilever are measured. In addition, the instrument can be used in frictional force studies, force curve mapping of the surface, and contact potential measurements. We present examples of each, including a detailed account of the instrument design and calibration.

Chemically-sensitive interfacial force microscopy: Contact potential measurements of self-assembling monolayer films

We report contact potential difference (CPD) measurements of n-alkanethiol self-assembling monolayers (SAMs) adsorbed to Au substrates using an organomercaptan-modified Au probe. We perform experiments by applying a triangular-sweep voltage between the sample and probe while measuring the resulting electrostatic force. The interfacial force microscope permits us to keep the probe/sample distance rigidly fixed, which allows us to directly measure theelectrostatic forceat aconstant interfacialseparation. TheCPDisdetermined by measuring the applied potential necessary to null the electric field and eliminate the interfacial force between the two We show that CPD values obtained using Au probes modified with methyl-terminated SAMs are stable and reproducible, whereas identical unmodified probes yield highly variable data. Our experimentally determined CPD values are in qualitative agreement with calculated CPDs for several different w-terminated SAMs. We report contact potential difference (CPD) measurements of n-alkanethiol self-assembling monolayers (SAMs)'J adsorbed to Au substrates using an organomercaptan-modified Au probe. Our results demonstrate for the first time the feasibility and importance of controlling the chemical properties of force microscope probe Moreover, control over the chemical characteristics of the probe provides a basis for distinguishing between chemically distinct surface features with nanometer resolution in force microscopy. We show that CPD values obtained using Au probes modified with methyl-terminated SAMs arestable and reproducible, whereas identical unmodified probes yield highly variable data. Our experimentally determined CPD values are in qualitative agreement with calculated CPDs for several different w-terminated SAMs. We have previously used the interfacial force microscope (IFM)' to obtain detailed information about the mechanical properties of methyl-terminated n-alkanethiol SAMs adsorbed to Au substrates. In the present work, we use the IFM to determine local CPDs that arise between an organomercaptanmodified Au probe and various SAM-modified Au substrates (Scheme I)? TheCPDisdefinedas theworkfunctiondifference between the modified Au surfaces, which we view as tightly bounddipolesheets that shifttbe workfunctionsofeachmodified Ausurface.lQ For example, the work functionis increased relative to bare Au for dipoles having their negative end pointing away from the modified Au surface. We perform experiments by applying a triangular-sweepvoltage between the sampleand probe, whichis heldat ground, whilemeasuring theresultingelectrostatic force. Unless otherwise noted all CPD measurements were obtained in air. The forcefeedback capability of the IFM is used tokeeptheprobe/sampledistancerigidlyfixed,whichallows us to directly measure the electrostatic force at a constant interfacial separation. The CPD is determined by measuring the applied potential necessary to null the electric field and eliminate the interfacial force between the two surfaces. Figure 1 shows forcevs substrate potential data obtained using a HS(CHZ)&H3-mdified Au probe. In these plots, negative forces are attractive, and the shape is parabolic since the electrostatic forceis proportional to thesquareof the biasvoltage. The solid line represents a second-order polynomial fit of the

Advances in Instrumentation for Human Reproduction Research

Special instrumentation technology has been conceived and clinically applied to bring in marked improvements in the management of human reproduction. From the stages prior to conception right through to the delivery of the baby, each step can be assessed by instrumentation methodologies so that corrective action can be taken at an early stage. Electrical contact potential measurement for monitoring contractions in the oviduct leading to ovum transport; ultrasound techniques in fetal development monitoring; and feedback controlled oxytocin infusion to assist in delayed delivery, are some of the advancements discussed.

Design and implementation of a Kelvin microprobe for contact potential measurements at the submicron scale

Nous presentons un instrument de mesure nouveau, alliant les techniques de microscopie en champ proche avec le principe bien connu de la sonde de Kelvin. Cet instrument effectue la mesure des variations locales du travail de sortie des electrons d'une surface par une methode non destructive (methode de Kelvin), et avec une precision de l'ordre de la dizaine de meV. Les performances de resolution spatiale actuellement atteintes (3 microns) sont assurees par l'emploi d'une sonde electrostatique gardee electriquement. L'amelioration des techniques de fabrication des sondes devrait mener sous peu a un affinement sensible de la resolution (objectif vise: 100 nm). l'instrument permet par ailleurs la detection de charges statiques de surface, et se prete ainsi a une variete d'applications: mesure de la courbure des bandes a la surface des semiconducteurs et de la densite de pieges de surface dans ces materiaux; etude de la diffusion superficielle d'impuretes dans un metal; visualisation de domaines ferroelectriques; etude de la triboelectricite, par exemple

Molecular Approach to Surface Control of Chalcogenide Semiconductors

The interaction between a chalcogenide semiconductor surface and a specific chelating ligand was studied by a number of complementary spectroscopic methods (UV/VIS, FTIR, XPS). FTIR suggests that the chelating ligand (diphenyl hydroxamic acid) complexes an In3+ ion in CuInSe2, and a Cd2+ ion in CdTe, accompanied by the loss of a proton. Contact potential measurements showed that the adsorbed ligand changes the semiconductor electron affinity without significantly influencing the band bending. On the basis of these and other results, we suggest that the molecular dipole of the chelating ligand is responsible for the change in surface potential. Because this dipole can be modified without changing the ligand's binding group, this finding opens the way to control surface potential by varying the dipole moment of the adsorbed ligand without changing the binding functional group.

Contact potential measurement: Spacing-dependence errors

We examine the causes of spacing dependence of the nulling bias voltage in the vibrating capacitor contact potential measurement technique. In addition to effects already recognized in the literature, namely, nonuniform work functions, nonparallel surfaces, fringe fields, and capacitive coupling to distant surfaces, we investigate the effects of finite gain and spurious signals in feedback loop systems. We argue that much of the spacing dependence reported in the literature may be due to microphonic signals, which are very difficult to eliminate. We also discuss the means by which existing spacing dependence can be minimized.