Frequency Modulation Kelvin Probe Force Microscopy Research Articles

Visualizing and Controlling of Photogenerated Electron-Hole Pair Separation in Monolayer WS2 Nanobubbles under Piezoelectric Field.

The piezoelectric properties of two-dimensional semiconductor nanobubbles present remarkable potential for application in flexible optoelectronic devices, and the piezoelectric field has emerged as an efficacious pathway for both the separation and migration of photogenerated electron-hole pairs, along with inhibition of recombination. However, the comprehension and control of photogenerated carrier dynamics within nanobubbles still remain inadequate. Hence, this study is dedicated to underscore the importance of in situ detection and detailed characterization of photogenerated electron-hole pairs in nanobubbles to enrich understanding and strategic manipulation in two-dimensional semiconductor materials. Utilizing frequency modulation kelvin probe force microscopy (FM-KPFM) and strain gradient distribution techniques, the existence of a piezoelectric field in monolayer WS2 nanobubbles was confirmed. Combining w/o and with illumination FM-KPFM, second-order capacitance gradient technique and in situ nanoscale tip-enhanced photoluminescence characterization techniques, the interrelationships among the piezoelectric effect, interlayer carrier transfer, and the funneling effect for photocarrier dynamics process across various nanobubble sizes were revealed. Notably, for a WS2/graphene bubble height of 15.45 nm, a 0 mV surface potential difference was recorded in the bubble region w/o and with illumination, indicating a mutual offset of piezoelectric effect, interlayer carrier transfer, and the funneling effect. This phenomenon is prevalent in transition metal dichalcogenides materials exhibiting inversion symmetry breaking. The implication of our study is profound for advancing the understanding of the dynamics of photogenerated electron-hole pair in nonuniform strain piezoelectric systems, and offers a reliable framework for the separation and modulation of photogenerated electron-hole pair in flexible optoelectronic devices and photocatalytic applications.

Measurement of distribution of charge adsorbed on Au<i><sub>x</sub></i>/Si(111)-7×7 surface on an atomic scale in ultra-high vacuum

The physicochemical properties of Au atoms adsorbed on the surface on an atomic scale play a very important role in preparing nanodevices and surface catalysis. In this paper, we use frequency modulated Kelvin probe force microscopy (FM-KPFM)to study the multi-bit adsorbed charge distribution of Au on the surface of Si(111)-(7×7) at room temperature. Firstly, the surface topography and local contact potential difference (LCPD) of Au at different adsorption sites in Si(111)-(7×7) are successfully obtained by using home-made ultra-high vacuum Kelvin probe force microscopy. Secondly, we analyze the atomic characteristics of specific atomic positions of Au/Si(111)-(7×7) by force spectroscopy and potential difference, and realize the atomic identification . The adsorption characteristics of Au/Si(111)-(7×7) surface charge transfer and Au are explained by combining differential charge density calculations. The results show that Au atom adsorption mainly is in the form of single atom and cluster . Specifically, the Au cluster is adsorbed at the three central positions of Si(111)-(7×7) in a hexagonal structure of six atoms. Individual Au atoms are adsorbed to the positions of central adatoms of Si(111)-(7×7). At the same time, through the measurement of potential difference, it is known that a single Au atom and Au cluster lose electrons, presenting a positive electrical characteristic. The results of surface differential charge density show that Au undergoes charge transfer during adsorption, losing part of the charge, which locally reduces the work function at the position of the adsorbed atom. In the range of distances where short-range forces, local contact potential energy differences and differential charge densities change, the theoretical results and experimental results are in reasonable agreement.

Study of high–low KPFM on a pn-patterned Si surface

Comparative measurements between frequency modulation Kelvin probe force microscopy (FM-KPFM) using low frequency bias voltage and heterodyne FM-KPFM using high frequency bias voltage were performed on the surface potential measurement. A silicon substrate patterned with p- and n-type impurities was used as a quantitative sample. The multi-pass scanning method in the measurements of FM-KPFM and heterodyne FM-KPFM was used to eliminate the effect of the tip–sample distance dependence. The measured surface potentials become lower in the order of the p-type region, n-type region and n+-type region by both FM-KPFM and heterodyne FM-KPFM, which are in good agreement with the order of the work functions of the pn-patterned Si sample. We observed the difference in the surface potentials due to the surface band bending measured by FM-KPFM and heterodyne FM-KPFM. The difference is due to the fact that the charge transfer between the surface and bulk levels may or may not respond to AC bias voltage.

In-depth microscopic characterisation of the weld faying interface revealing stress-induced metallurgical transformations during friction stir spot welding

Friction stir spot welding (FSSW) is a solid-state welding process, wherein the properties of a weld joint are influenced by the state of friction and localised thermodynamic conditions at the tool-workpiece interface. An issue well-known about FSSW joints is their lack of reliability since they abruptly delaminate at the weld-faying interface (WFI). This study explores the origins of the delamination of multiple lap welded aluminium alloy (AA 5754-H111) sheets joined by FSSW at different rotational speeds typically used in industry. Experimental techniques such as the small punch test (SPT), Vickers hardness test, Scanning Electron Microscopy (SEM), Scanning Acoustic Microscope (SAM), Transmission Electron Microscopy (TEM), Energy-dispersive X-ray spectroscopy (EDX) and Frequency-Modulated Kelvin Probe Force Microscopy (FM-KPFM) were employed. The experimental results revealed that a complex interplay of stress-assisted metallurgical transformations at the intersection of WFI and the recrystallised stir zone (RSZ) can trigger dynamic precipitation leading to the formation of Al3Mg2 intermetallic phase, while metallic oxides and nanopits remain entrapped in the WFI. These metallurgical transformations surrounded by pits, precipitates and oxides induces process instability which in turn paves way for fast fracture to become responsible for delamination.

Surface potential measurement by heterodyne frequency modulation Kelvin probe force microscopy in MHz range

The chemical and physical processes on surfaces are significantly influenced by the surface potential of materials. When using the frequency modulation Kelvin probe force microscopy (FM-KPFM), which has been widely used for measuring the surface potential distribution with high spatial resolution, it is very difficult to distinguish the surface potential due to the surface state from that due to the bulk state, because the charge transfer between the surface and bulk states occurs at a low-frequency ac bias voltage in the kHz range. Here, we propose a heterodyne FM-KPFM method using a high-frequency ac bias voltage in the MHz range to distinguish the surface and bulk states. This method is based on the heterodyne effect between the mechanical cantilever oscillation and the oscillating electrostatic force. For the first time, we succeeded in achieving the atomic-resolution imaging of the surface potential on an O-rich TiO2(110) surface using the electrostatic interaction in the MHz range. Furthermore, we could measure the upward and downward band bending on the surface at the atomic scale.

Resolving the 3D Orientation of Terphenylthiol Molecules on Noble Metals with Kelvin Probe Force Microscopy

The local work function is an invaluable feature for the specific analysis of the influences of atomic and molecular nanostructures on each other as well as the underlying surface. Adsorbate molecules can modify this parameter by introducing an electrical dipole moment, which affects the local contact potential. This can be accessed by Kelvin probe force microscopy (KPFM). In this paper, we demonstrate, by combining highly resolved topographic atomic force microscopy (AFM) data with the simultaneously acquired local work function signal, how each of these channels yield one angular coordinate, resulting in the three-dimensional determination of surface molecular dipole orientations. We studied the adsorption of terphenylthiol (TPT) self-assembled monolayers on Au(111) and Ag(111), as it is relevant in the light of electron radiation-induced transformation to carbon nanomembranes. We present noncontact AFM data combined with frequency-modulated KPFM in ultrahigh vacuum at room temperature without any kind of deliberate tip functionalization. Our results show a surface coverage-dependent Langmuir-like evolution of phases with domains of flat lying as well as with upright molecular arrangements. Whereas we found an almost complete vertical orientation on silver, the orientation on gold was found to be tilted, corresponding to sp- and sp3-hybridized bond angles, respectively.

Controlling Surface Potential of Graphene Using dc Electric Field

<p>In this work, we study surface potential of graphite deposited on SiO<sub>2</sub>/Si substrate using Kelvin Probe Force Microscopy (KPFM) and Electric Force Microscopy (EFM). The amplitude modulated KPFM (AM-KPFM) shows that the graphene layer work function is 4.69±0.02 eV, whereas frequency modulated KPFM (FM-KPFM) revealed 4.50±0.02 eV. The work function indifference of 0.19±0.02 eV was attributed to the superior resolution of FM-KPFM and higher detection sensitivity of AM-KPFM. Subsequent EFM mapping suggests that the phase monotonically increases with increasing applied <em>dc</em> bias voltage in the range from -5 V to 5 V. This phase shift is ascribed to the induced charge polarization at tip-graphene surface due to interatomic interactions induced by <em>dc</em> field effects.</p>

Local Time-Dependent Charging in a Perovskite Solar Cell.

Efficient charge extraction within solar cells explicitly depends on the optimization of the internal interfaces. Potential barriers, unbalanced charge extraction, and interfacial trap states can prevent cells from reaching high power conversion efficiencies. In the case of perovskite solar cells, slow processes happening on time scales of seconds cause hysteresis in the current-voltage characteristics. In this work, we localized and investigated these slow processes using frequency-modulation Kelvin probe force microscopy (FM-KPFM) on cross sections of planar methylammonium lead iodide (MAPI) perovskite solar cells. FM-KPFM can map the charge density distribution and its dynamics at internal interfaces. Upon illumination, space charge layers formed at the interfaces of the selective contacts with the MAPI layer within several seconds. We observed distinct differences in the charging dynamics at the interfaces of MAPI with adjacent layers. Our results indicate that more than one process is involved in hysteresis. This finding is in agreement with recent simulation studies claiming that a combination of ion migration and interfacial trap states causes the hysteresis in perovskite solar cells. Such differences in the charging rates at different interfaces cannot be separated by conventional device measurements.

Trap-clearing Spectroscopy in Perylene Diimide Derivatives

Perylene diimide (PDI) derivatives are important components of organic circuits and photovoltaic systems, but the electron trapping mechanism(s) in higher-LUMO PDIs (−3 to −4 eV, where radical anions are expected to react with atmospheric water and oxygen) are not well characterized. Here, we examine the spatial distribution of traps in transistors made from PDIs with these higher LUMO levels and measure trap-clearing spectra in n-type organic materials using time- and wavelength-resolved frequency-modulated Kelvin probe force microscopy (FM-KPFM). We find that the rate of trap-clearing under visible light does not follow a LUMO-level trend, and we observe spectral evidence for different trapping mechanisms on bare versus HMDS-treated SiO2.

Kelvin probe force microscopy for local characterisation of active nanoelectronic devices.

Frequency modulated Kelvin probe force microscopy (FM-KFM) is the method of choice for high resolution measurements of local surface potentials, yet on coarse topographic structures most researchers revert to amplitude modulated lift-mode techniques for better stability. This approach inevitably translates into lower lateral resolution and pronounced capacitive averaging of the locally measured contact potential difference. Furthermore, local changes in the strength of the electrostatic interaction between tip and surface easily lead to topography crosstalk seen in the surface potential. To take full advantage of the superior resolution of FM-KFM while maintaining robust topography feedback and minimal crosstalk, we introduce a novel FM-KFM controller based on a Kalman filter and direct demodulation of sidebands. We discuss the origin of sidebands in FM-KFM irrespective of the cantilever quality factor and how direct sideband demodulation enables robust amplitude modulated topography feedback. Finally, we demonstrate our single-scan FM-KFM technique on an active nanoelectronic device consisting of a 70 nm diameter InAs nanowire contacted by a pair of 120 nm thick electrodes.

Surface potential imaging with atomic resolution by frequency-modulation Kelvin probe force microscopy without bias voltage feedback

We investigated the capability of obtaining atomic resolution surface potential images by frequency-modulation Kelvin probe force microscopy (FM-KPFM) without bias voltage feedback. We theoretically derived equations representing the relationship between the contact potential difference and the frequency shift (Δf) of an oscillating cantilever. For the first time, we obtained atomic resolution images and site-dependent spectroscopic curves for Δf and VLCPD on a Si (111)-7 × 7 surface. FM-KPFM without bias voltage feedback does not involve the influence of the FM-KPFM controller because it has no deviation from a parabolic dependence of Δf on the dc-bias voltage. It is particularly suitable for investigation on molecular electronics and organic photovoltaics, because electron or ion movement induced by dc bias is avoided and the electrochemical reactions are inhibited.

Reconstruction of surface potential from Kelvin probe force microscopy images

We present an algorithm for reconstructing a sample surface potential from its Kelvin probe force microscopy (KPFM) image. The measured KPFM image is a weighted average of the surface potential underneath the tip apex due to the long-range electrostatic forces. We model the KPFM measurement by a linear shift-invariant system where the impulse response is the point spread function (PSF). By calculating the PSF of the KPFM probe (tip+cantilever) and using the measured noise statistics, we deconvolve the measured KPFM image to obtain the surface potential of the sample.The reconstruction algorithm is applied to measurements of CdS-PbS nanorods measured in amplitude modulation KPFM (AM-KPFM) and to graphene layers measured in frequency modulation KPFM (FM-KPFM). We show that in the AM-KPFM measurements the averaging effect is substantial, whereas in the FM-KPFM measurements the averaging effect is negligible.

Nanoscale Electrostatic Domains Induced by Cholesterol in Model Lipid Membranes Relate to Amyloid Binding and Toxicity

Kelvin Probe Force Microscopy (KPFM) is a type of scanning probe microscopy which specifically addresses electrostatic properties of materials and has a great potential to bring new discoveries in biomedical research, but currently has limited biological applications. In this work, we report one of a very few applications of Kelvin probe force microscopy (KPFM) to study complex structures of lipid films and lipid-protein interactions. Molecular arrangement of lipids and proteins gives rise to complex film morphology as well as distinct electrical surface potentials, which may rule many biological processes and diseases. Using Frequency Modulated - KPFM (FM-KPFM) we discovered an intriguing nanoscale electrostatic effect of cholesterol that may be crucial for understanding the mechanism of amyloid toxicity in relation to Alzheimer's disease (1). Earlier we observed similar electrostatic domains induced by cholesterol in pulmonary surfactant (2,3). Here we show that this electrostatic effect of cholesterol is not specific to only pulmonary surfactant films, but is also present in model lipid systems and plays an important role in amyloid-lipid interactions. We postulate that this previously unknown nanoscale electrostatic effect of cholesterol is a fundamental property, which may greatly influence the interactions of lipid membranes with other charged molecules. 1. E.Drolle, R.M.Gaikwad, Z.Leonenko, Nanoscale electrostatic domains in cholesterol-laden lipid membranes create a target for amyloid binding. Biophysical Journal, Letters, 2012, 103(4), L27-L29 2. E.Finot, Y.Leonenko, B.Moores, L.M.Eng, M.Amrein, Z.Leonenko. Effect of cholesterol on electrostatics in lipid-protein films of a lung surfactant, Langmuir, 2010, 26 (3), 1929-1935. 3. B.Moores, F.Hane, L.M.Eng, Z.Leonenko, Kelvin probe force microscopy in application to biomolecular films: Frequency modulation, amplitude modulation, and lift mode, Ultramicroscopy, 2010, 110(6), 708-711.

Effect of Trapped Charges on Local Potential Measurement of Carbon Nanotubes Using Frequency-Modulation Kelvin-Probe Force Microscopy

The performance of field effect transistors based on single-walled carbon nanotubes (SWNTs) is determined by their chirality the potential barrier at the SWNT/electrode interface, and defects in the SWNTs. Kelvin-probe force microscopy (KFM) is a powerful technique that can measure local surface potential (SP) on SWNTs for investigating local electrical properties at the interface and defects. However, the electrical potential measurement of SWNTs is often hindered by the surrounding trapped charges on the silicon oxide surface. We performed KFM imaging on an SWNT aligned between the electrodes on a silicon oxide surface by KFM using the frequency modulation (FM) method. We investigated the effect of these surface charges on the electrical potential of the SWNT measured by FM-KFM, especially in terms of the tip-sample distance. We introduced a novel two-dimensional (2D) SP mapping method in which the tip is scanned in a plane normal to the sample surface while recording the SP. The 2D SP map showed that the SP measured on the SWNT was significantly affected by the surrounding trapped charges, even when the tip-sample distance was minimized, and the electrical potential of the SWNT could not be measured accurately. [DOI: 10.1380/ejssnt.2011.210]

Visualization of anisotropic conductance in polydiacetylene crystal by dual-probe frequency-modulation atomic force microscopy/Kelvin-probe force microscopy

The authors recently developed a dual-probe atomic force microscope (DP-AFM) system as a powerful measurement or fabrication tool in the fields of nanoelectronics and nanobiology. In this study, they performed frequency-modulation Kelvin-probe force microscopy (FM-KFM) experiments on a polydiacetylene (PDA) single crystal using the DP-AFM system. While a bias voltage was locally applied to the PDA surface with one probe, the surface potential on the surrounding area was mapped with the other probe by FM-KFM. The surface potential images showed anisotropic distributions, which are explained by the anisotropic conductance of the PDA crystal due to the quasi-one-dimensional electronic band structure along the diacetylene main chain. They also discuss the mechanisms of charge injection from a probe to the PDA crystal and the difference in the anisotropic conductance ratio for electrons and holes.

Kelvin probe force microscopy in application to biomolecular films: Frequency modulation, amplitude modulation, and lift mode

Kelvin probe force microscopy (KPFM) is a powerful technique to visualize the differences of work function in metals and lateral surface potential distribution in thin organic films. Earlier we have shown that frequency modulated-Kelvin probe force microscopy has significant advantages in both sensitivity and resolution when applied to metal and inorganic interfaces in vacuum. High resolution, high sensitivity, and performance in ambient conditions are required in order to study biologically relevant samples. In this work we compared the resolution of frequency modulation (FM-KPFM), amplitude modulation (AM-KPFM), and lift modes KPFM for imaging the local electrical surface potential of complex biomolecular films and demonstrated that FM-KPFM mode has superior resolution for biological applications. The power of the method was illustrated on pulmonary surfactant films, revealing nm spatial resolution and mV potential sensitivity in ambient air. At this level of resolution this method can provide critical insight into the molecular arrangement and function of complex biosystems in a way that other KPFM modes cannot do. Based on the observed changes in the local surface potential we discovered that excess cholesterol produces nm size electrostatic domains and change the electric fields.

Effect of Cholesterol on Electrostatics in Lipid−Protein Films of a Pulmonary Surfactant

We report the changes in the electrical properties of the lipid-protein film of pulmonary surfactant produced by excess cholesterol. Pulmonary surfactant (PS) is a complex lipid-protein mixture that forms a molecular film at the interface of the lung's epithelia. The defined molecular arrangement of the lipids and proteins of the surfactant film gives rise to the locally highly variable electrical surface potential of the interface, which becomes considerably altered in the presence of cholesterol. With frequency modulation Kelvin probe force microscopy (FM-KPFM) and force measurements, complemented by theoretical analysis, we showed that excess cholesterol significantly changes the electric field around a PS film because of the presence of nanometer-sized electrostatic domains and affects the electrostatic interaction of an AFM probe with a PS film. These changes in the local electrical field would greatly alter the interaction of the surfactant film with charged species and would immediately impact the manner in which inhaled (often charged) airborne nanoparticles and fibers might interact with the lung interface.

Determination of the local contact potential difference of PTCDA on NaCl: a comparison oftechniques

There has been increasing focus on the use of Kelvin probe force microscopy (KPFM) forthe determination of local electronic structure in recent years, especially in systems whereother methods, such as scanning tunnelling microscopy/spectroscopy, may be intractable.We have examined three methods for determining the local apparent contactpotential difference (CPD): frequency modulation KPFM (FM-KPFM), amplitudemodulation KPFM (AM-KPFM), and frequency shift–bias spectroscopy, on a testsystem of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on NaCl, anexample of an organic semiconductor on a bulk insulating substrate. We willdiscuss the influence of the bias modulation on the apparent CPD measurementby FM-KPFM compared to the DC-bias spectroscopy method, and provide acomparison of AM-KPFM, AM–slope detection KPFM and FM-KPFM imagingresolution and accuracy. We will also discuss the distance dependence of the CPD asmeasured by FM-KPFM for both the PTCDA organic deposit and the NaCl substrate.

On the relevance of the atomic-scale contact potential difference by amplitude-modulationand frequency-modulation Kelvin probe force microscopy

The influence of short-range electrostatic forces on the measured local contact potentialdifference (CPD) by means of amplitude-modulation and frequency-modulation Kelvinprobe force microscopy (AM- and FM-KPFM) is discussed on the base of numerical andanalytical descriptions of both methods. The goal of this work is to help in interpretingrecent experimental results reporting atomically resolved CPD images, in particular onbulk insulating samples. The discussion is carried out on the basis of spectroscopic curves.The expression of the bias-dependent electrostatic force is derived from a previous workand is estimated between a tip with simple geometry and the (001) facet of a perfectalkali halide single crystal. The force, with a short-range character, scales as asecond-order polynomial function of the bias voltage. It is stated that the linear term isresponsible for the occurrence of the atomic-scale CPD contrast, while the quadraticone, involving the sample polarization, accounts for the detected signal by theKPFM methods. Nevertheless, analytical and numerical approaches stress theinfluence of the linear term on the measured CPD which intrinsically hindersthe possibility to perform quantitative CPD measurements, but also makes themeasured ‘pseudo-CPD’ strongly deviating from the surface potential. Hence, in theshort-range regime, AM- or FM-KPFM measurements neither reflect the CPD nor thelocal surface potential, but rather an effective value which is convoluted by thegeometric parameters of the tip, the so-called local CPD. It is also stated that thelocal CPD measured by means of AM- or FM-KPFM differs when sub-nanometervibration amplitudes of the cantilever are used. Otherwise, AM- and FM-KPFMmeasurements should be almost similar. At last, the influence of long-range, capacitive,electrostatic forces is discussed in conjunction with the short-range ones. Thisallows us to draw conclusions regarding the distance dependence of the localCPD which then exhibits a resonant behavior as a function of the tip–surfaceseparation. This phenomenon is expected to play a role in the KPFM imaging process.

Electrical Surface Potential of Pulmonary Surfactant

Pulmonary surfactant is a mixed lipid protein substance of defined composition that self-assembles at the air-lung interface into a molecular film and thus reduces the interfacial tension to close to zero. A very low surface tension is required for maintaining the alveolar structure. The pulmonary surfactant film is also the first barrier for airborne particles entering the lung upon breathing. We explored by frequency modulation Kelvin probe force microscopy (FM-KPFM) the structure and local electrical surface potential of bovine lipid extract surfactant (BLES) films. BLES is a clinically used surfactant replacement and here served as a realistic model surfactant system. The films were distinguished by a pattern of molecular monolayer areas, separated by patches of lipid bilayer stacks. The stacks were at positive electrical potential with respect to the surrounding monolayer areas. We propose a particular molecular arrangement of the lipids and proteins in the film to explain the topographic and surface potential maps. We also discuss how this locally variable surface potential may influence the retention of charged or polar airborne particles in the lung.