Scanning Probe Microscopy Probe Research Articles

Regulation of flux-closure domain structures via oxygen vacancies and charged scanning probe microscopy

Manipulation of topological structures has become one of the most interesting topics in ferroelectrics through multiple excitations due to their prospective applications in electro-mechanical-optic devices. Scanning probe microscopy (SPM) has been developed as a powerful tool to manipulate the polar state in ferroic materials, in which the electric field induced by charged SPM enables dynamic coupling between the switching of the polar states and electromigration of oxygen vacancies, resulting in unknown influences of oxygen vacancy on the polar state in ferroelectric topological structures. Here, we regulate the polar state by considering oxygen vacancies and a non-uniform electric field under the charged SPM experiment for flux-closure domain structures in PbTiO3 thin films. The charged SPM probe can excite the growth of newly flux-closure domains to improve the density of topological states. In contrast, oxygen vacancies are found to suddenly prohibit the evolution of the flux-closure domain structure, when their concentration jumps to a threshold value. Our results might give guidelines to increase and stabilize the memory unit through co-modulating the electric field and ions injection in the information field.

Sub-micrometer plasma-enhanced chemical vapor deposition using an atmospheric pressure plasma jet localized by a nanopipette scanning probe microscope

We developed a localized plasma-enhanced chemical vapor deposition (PE-CVD) technique to deposit silicon oxide with a sub-micrometer width on a substrate using an atmospheric pressure plasma jet (APPJ) irradiated from a nanopipette nozzle. To realize fine material deposition, hexamethyldisiloxane (HMDSO) vapor was blown into the localized helium APPJ irradiated from the sub-micrometer aperture of the nanpopipette with the jet length limited to the aperture size of the nanopipette. The irradiation distance was controlled using a shear-force positioning technique using scanning probe microscopy (SPM). The proposed system successfully deposited silicon oxide dots with sub-micrometer width on a substrate. After the deposition, the topography of the deposited surface was observed by scanning the nanopipette, which can be used as an SPM probe. The localized PE-CVD properties were systematically investigated by varying the deposition parameters. The amount of deposited material could be controlled by the flow rate of the carrier gas of the HMDSO vapor, APPJ irradiation time, and nanopipette–substrate surface irradiation distance.

Capacitive Measurements of SiO2 Films of Different Thicknesses Using a MOSFET-Based SPM Probe.

We utilized scanning probe microscopy (SPM) based on a metal-oxide-silicon field-effect transistor (MOSFET) to image interdigitated electrodes covered with oxide films that were several hundred nanometers in thickness. The signal varied depending on the thickness of the silicon dioxide film covering the electrodes. We deposited a 400- or 500-nm-thick silicon dioxide film on each sample electrode. Thick oxide films are difficult to analyze using conventional probes because of their low capacitance. In addition, we evaluated linearity and performed frequency response measurements; the measured frequency response reflected the electrical characteristics of the system, including the MOSFET, conductive tip, and local sample area. Our technique facilitated analysis of the passivation layers of integrated circuits, especially those of the back-end-of-line (BEOL) process, and can be used for subsurface imaging of various dielectric layers.

A Molecular Drone for Atomic-Scale Fabrication Working under Ambient Conditions.

The direct manipulation of individual atoms has led to the advancement of exciting cutting-edge technologies in sub-nanometric fabrication, information storage and to the exploration of quantum technologies. Atom manipulation is currently performed by scanning probe microscopy (SPM), which enables an extraordinary spatial control, but provides a low throughput, requiring complex critical experimental conditions and advanced instrumentation. Here, a new paradigm is demonstrated for surface atom manipulation that overcomes the limitations of SPM techniques by replacing the SPM probe with a coordination compound that exploits surface atom complexation as a tool for atomic-scale fabrication. The coordination compound works as a "molecular drone": it lands onto a substrate, bonds to a specific atom on the surface, picks it up, and then leaves the surface along with the extracted atom, thus creating an atomic vacancy in a specific position on the surface. Remarkably, the feasibility of the process is demonstrated under electrochemical control and the stability of the fabricated pattern at room temperature, under ambient conditions.

Probing Nanoscale Electromechanical Behaviors of Relaxor Ferroelectrics in Highly Conductive Liquid Environments

Electromechanical imaging by scanning probe microscopy (SPM) in a conductive liquid environment is interesting for investigating the physics of electronic, electrochemical, or biological underwater applications. It remains challenging, though, because a conductive liquid obscures what is really going on in SPM detection. This study presents a nanoelectrode SPM probe for such environments. The experimental and theoretical insights gained in this work could provide a powerful method for $i\phantom{\rule{0}{0ex}}n$ $v\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}o$ or $i\phantom{\rule{0}{0ex}}n$ $o\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}o$ characterization of the electromechanical functionality of materials, or the performance of underwater electronics, biological piezoelectric systems, or energy converters.

Piezoelectricity of atomically thin WSe2 via laterally excited scanning probe microscopy

Lattices of odd-layered two-dimensional (2D) transition metal dichalcogenides (TMDs) such as WSe2 are non-centrosymmetric, and thus could possess linear piezoelectricity that is attractive for applications such as nanoelectromechanical systems (NEMS) and nanogenerators. Measuring the electromechanical coupling of 2D TMDs, however, is rather challenging, since its D3h point group symmetry makes conventional piezoresponse force microscopy (PFM) inapplicable. Here we develop a lateral excitation scanning probe microscopy (SPM) technique that enables mapping of the piezoelectric response of atomically thin WSe2 directly on a substrate with high spatial resolution. Planar electrodes are used to excite piezoelectric vibrations while imposing anisotropic in-plane mechanical constraint to the WSe2, resulting in an out-of-plane deformation due to Poisson's effect that can be measured by an SPM probe. Using this technique, we show that WSe2 monolayer and trilayers exhibit strong electromechanical response linear to the applied excitation biases that is distinct from the substrate, while WSe2 bilayers show negligible electromechanical response as expected. The effective piezoelectric coefficient is estimated to be 5.2 pm/V from the measurement, consistent with theoretical predictions. This method can be conveniently applied to a wide range of 2D materials with similar symmetry.

The intrinsic piezoresponse in piezoelectric medium under contact-mode piezoresponse force microscopy

The rapid development of the piezoresponse force microscopy (PFM) makes the investigation of the electromechanical coupling phenomena at the nanoscale to become possible in various non-centrosymmetric polar materials. However, it still remains challenging in accurately and quantitatively determinate the intrinsic piezoresponses from the detected displacement responses in PFM experiments. Here, we develop the fully coupled theory to quantitatively analyze the piezoresponses in transversely isotropic piezoelectric materials under contact-mode measurements by using the Hankel transform method, considering the profiles of the conductive scanning probe microscopy (SPM) probe. Based on this fully coupled method, we investigate the electroelastic fields in piezoelectric materials. Furthermore, the effective piezoelectric coefficients are discussed under mechanical and electrical loadings. We find out that the effective piezoelectric coefficients are influenced by external loadings and geometries of SPM probe under contact-mode measurements. These results remind us that it should be extremely careful on the quantitative investigation of the piezoelectric materials via contact-mode measurements in PFM experiments.

Scanning photoelectron microscopy using a pointed capillary probe

The possibilities of a new type of scanning probe microscopy (SPM) for two different samples are experimentally demonstrated. The method is based on the use of a pointed capillary, which can simultaneously act as a ‘classical’ SPM probe and also as a controlled thin channel for transporting charged particles emitted by the surface to the detector. In the experiment, photoelectrons pass through a dielectric hollow cone probe with an aperture radius of 1 μm and detected by microchannel plates at different points of the investigated conducting surface irradiated by the second harmonic of a femtosecond Ti : sapphire laser. As a result, the sample’s surface profile is visualised with a subwavelength spatial resolution. This method makes it possible to control spatially localised beams of electrons, ions, neutral atoms (molecules) and soft X-ray radiation, as well as opens a possibility for research in the field of nanoscale photodesorption of molecular ions.

Nanosculpting of complex oxides by massive ionic transfer

Scanning probe microscopy (SPM)-based approaches have been extensively studied as methods to control the structure and properties of materials on the nanoscale. In many cases, the SPM probe is physically utilized to control structure and properties. In addition to physical modulation, it has been reported that voltage can be effectively used to modulate electrochemical phenomena on the sample surface. These studies suggest that electrochemical modulation of the structure and properties is possible by applying a voltage. Herein, in order to demonstrate voltage induced modulation of surface structure, we explored surface nanosculpting by creating electrochemically induced pits on the surface of TiO2 thin films through the application of voltage using the atomic force microscope tip. Using a unipolar negative voltage sweep, pits were successfully generated. Further, the electric potential distribution was simulated to unravel the relationship between the pit volume and the magnitude of the applied voltage. Finally, surface protrusion induced by positive voltage sweep was also observed to elucidate the complete process of electrochemically induced surface modulation. These results can offer fundamental information for understanding how surface structure can be modulated by electrochemical phenomena.

Morphology and Electric Conductance Change Induced by Voltage Pulse Excitation in (GeTe)2/Sb2Te3 Superlattices.

Chalcogenide superlattice (SL) phase-change memory materials are leading candidates for non-volatile, energy-efficient electric memory where the electric conductance switching is caused by the atom repositioning in the constituent layers. Here, we study the time evolution of the electric conductance in [(GeTe)2/(Sb2Te3)1]4 SLs upon the application of an external pulsed electric field by analysing the structural and electrical responses of the SL films with scanning probe microscopy (SPM) and scanning probe lithography (SPL). At a low pulse voltage (1.6–2.3 V), a conductance switching delay of a few seconds was observed in some SL areas, where the switch to the high conductance state (HCS) is accompanied with an SL expansion under the strong electric field of the SPM probe. At a high pulse voltage (2.5–3.0 V), the HCS current was unstable and decayed in a few seconds; this is ascribed to the degradation of the HCS crystal phase under excessive heating. The reversible conductance change under a pulse voltage of opposite polarity emphasised the role of the electric field in the phase-transition mechanism.

SPM system for semiconductor device applications.

Recently, scanning probe microscopy (SPM) is widely used for development of semiconductor devices. One of the important functions of SPM is high resolution topography, such as shape of the nanoscale devices and surface roughness of the films. Additionally, SPM can measure the electronic structure of the nanoscale-devices. SPM system for thin films was developed to characterize the thin films for device applications.First, SPM system which can be apply short pulses to the sample holder is constructed to evaluate the electronic response of the thin film without using complex patterning on the Si wafer as shown in Fig.1. Current design rule of the semiconductor devices is around 20 nm. The dimension of the devices are close to the probe radius of conductive SPM probes. The instrument was designed to characterize not only the static properties of nanoscale devices, but also the dynamic electronic properties. Shortest pulses which can be applied to the sample without destroying waveform were less than 50 nS. Time response of the current amplifier is ranging from 50 nS to 200 nS depending on the trans-impedance gains. The conditions (time and dimension) are similar to the active devices on the chip in the circuit. Thus, dynamic electronic properties of the thin films can be tested on a film without fabricating to the nanoscale devices. It is very helpful to optimizing the depositing conditions, such as sputtering parameters, of the thin film for semiconductor devices. For example, the system is used to optimize the film qualities for resistive memories [1].jmicro;63/suppl_1/i13-a/DFU091F1F1DFU091F1Fig. 1.Conductive probe microscopy, which is compatible to the pulse signals ranging to 50nS. The second function of the SPM system is the reproducible roughness measurement. Roughness of the film is also important for optimizing the depositing conditions of the thin film. Virtual reference probe method was developed for removing the variations of the SPM probes [2]. One of the biggest problems of SPM roughness measurement is the huge variations of the SPM probe apexes. The method is to normalizing the probe to the largest probe used in the measurements, after characterizing the probe shape with suitable reference artefact [3,4]. Image reconstruction, such as erosion and dilation process is used for the analysis.In this presentation, we will introduce the SPM system developed for semiconductor device applications. The SPM system also includes function to characterize the nanoscale contaminants on the Si wafer. AcknowledgmentI would like to thanks Ms. Ito and Dr. C.M. Wang in AIST for discussions. This study was partially supported from MEXT.

Advanced fabrication process for combined atomic force-scanning electrochemical microscopy (AFM-SECM) probes

An advanced software-controlled focused ion beam (FIB) patterning process for the fabrication of combined atomic force-scanning electrochemical microscopy (AFM-SECM) probes is reported. FIB milling is a standard process in scanning probe microscopy (SPM) for specialized SPM probe fabrication. For AFM-SECM, milling of bifunctional probes usually requires several milling steps. Milling such complex multi-layer/multi-material structures using a single milling routine leads to significantly reduced fabrication times and costs. Based on an advanced patterning routine, a semi-automated FIB milling routine for fabricating combined AFM-SECM probes with high reproducibility is presented with future potential for processing at a wafer level. The fabricated bifunctional probes were electrochemically characterized using cyclic voltammetry, and their performance for AFM-SECM imaging experiments was tested. Different insulation materials (Parylene-C and SixNy) have been evaluated with respect to facilitating the overall milling process, the influence on the electrochemical behavior and the long-term stability of the obtained probes. Furthermore, the influence of material composition and layer sequence to the overall shape and properties of the combined probes were evaluated.

Batch fabrication of insulated conductive scanning probe microscopy probes with reduced capacitive coupling

We report a novel fabrication process for the batch fabrication of insulated conductive scanning probe microscopy (SPM) probes for electrical and topographic characterization of soft samples in liquid media at the nanoscale. The whole SPM probe structure is insulated with a dielectric material except at the very tip end and at the contact pad area to minimize the leakage current in liquid. Additionally, the geometry of the conducting layer in the probe cantilever and substrate is engineered to reduce the parasitic capacitance coupling with the sample. The electrical characterization of the probes has shown that parasitic capacitances are significantly reduced as compared to fully metallized cantilevers.

The controlled fabrication of “Tip-On-Tip” TERS probes

Tip-enhanced near-field Raman spectroscopy has exhibited a great ability to detect in situ chemical and structural information on a sample surface with the highest lateral resolution combined with a high sensitivity at the same time. A key challenge in the TERS field is the development and improvement of the fabrication of reproducible metal tip geometries with a sharper apex. In this study, one novel apertureless “Tip-On-Tip” TERS probe with a special plasmonic nanostructure is designed, and a simple Ar+-ion sputtering route has been developed to fabricate silver nanoneedle arrays on the scanning probe microscopy (SPM) cantilevers for tip-enhanced Raman spectroscopy (TERS) probes. These silver nanoneedles possess a very sharp apex with an apex diameter of 15 nm and an apex angle of 20°. The novel TERS probes exhibit enhanced near-field Raman signals compared to an Ag-coated SPM probe, which are attributed to the intense electromagnetic field around the apexes of the Ag nanoneedles and the periodic structure of the Ag nanoneedles.

SPM nanolithography of hydroxy-silicates

Bio-nanopatterning of surfaces is becoming a crucial technique with applications ranging from molecular and cell biology to medicine. Scanning probe microscopy (SPM) is one of the most useful tools for nanopatterning of flat surfaces. However, these patterns are usually built on homogeneous surfaces and require chemical functionalization to ensure specific affinity. Layered magnesium–aluminum hydroxide–silicates have already shown unique self-assembly properties on DNA molecules, due to their peculiar crystal chemistry based on alternating positive and negative crystal layers. However, patterns on these surfaces tend to be randomly organized. Here we show etching and oxidation at the nanometer scale of magnesium–aluminum hydroxide–silicates using the same SPM probe for the creation of organized nanopatterns. In particular, it is possible to produce three-dimensional structures in a reproducible way, with a depth resolution of 0.4 nm, lateral resolution of tens of nm, and a speed of about 10 μm s−1. We report, as an example, the construction of an atomically flat charged pattern, designed to guide DNA deposition along predetermined directions without the need of any chemical functionalization of the surface.

Ageing effect of plasma‐treated wool

Atmospheric pressure plasma treatment of wool fabric, with a relatively short exposure time, effectively removed the covalently bonded lipid layer from the wool surface. The plasma‐treated fabric showed increased wettability and the fibres showed greater roughness. X‐ray photoelectron spectroscopy (XPS) analysis showed a much more hydrophilic surface with significant increases in oxygen and nitrogen concentrations and a decrease in carbon concentration. Adhesion, as measured by scanning probe microscopy (SPM) force volume analysis, also increased, consistent with the more hydrophilic surface leading to a greater meniscus force on the SPM probe. The ageing of fibres from the plasma‐treated fabric was assessed over a period of 28 days. While no physical changes were observed, the chemical nature of the surface changed significantly. XPS showed a decrease in the hydrophilic nature of the surface with time, consistent with the measured decrease in wettability. This change is proposed to be due to the reorientation of proteolipid chains. SPM adhesion studies also showed the surface to be changing with time. After ageing for 28 days, the plasma‐treated surface was relatively stable and still dramatically different from the untreated fibre, suggesting that the oxidation of the surface and modification or removal of the lipid layer were permanent.

Conductive polymer patterned media fabricated by diblock copolymer lithography forscanning multiprobe data storage

A conductive polymer dot pattern has been fabricated as a patterned medium usingdiblock copolymer lithography (DCL) for scanning multiprobe data storage systems(SMDSSs). DCL can easily provide a higher dots pattern density than that obtainedusing electron beam lithography. For DCL, the microphase-separated structure ofpolystyrene-block-polymethylmethacrylate is utilized. Then, the closed dot pattern ofpolyaniline (PANI) with a center to center distance of adjacent dots of 30 nm is fabricatedby DCL. Electrical modification experiments of the fabricated PANI dots are demonstratedusing scanning probe microscopy (SPM). As a result, the conductivities of themodified dots are selectively changed by applying modification voltages with the tipof the SPM probe. Recording on the conductive polymer with 30 nm pitch atthe minimum can be demonstrated, which corresponds to a recording density of∼700 Gbits inch−2. These results show that the conductive polymer patterned medium has the potentialability to achieve high-density recording for SMDSSs.

Thermal and topographical characterization of polyester- and styrene/acrylate-based composite powders by scanning probe microscopy

The thermal properties of two conventional polyester-based toners and a chemically prepared styrene/acrylate toner with different thermal histories were studied by scanning probe microscopy (SPM) and differential scanning calorimetry (DSC). The thermal transition temperatures detected by SPM agreed with the results of the DSC measurements. The validity of SPM for detecting thermal transitions was further confirmed by studying two amorphous reference polymers with different glass transition points ( T g) and three crystalline reference polymers with different melting points ( T m). When the toner sample was heated by the SPM probe above the glass transition temperature of the toner powder ( T probe > T g), changes occurred in the surface topography and roughness causing different levels of local sintering of the particles. A set of roughness parameters calculated from the SPM image data were used to quantify the most essential features of toner surfaces. Environmental scanning electron microscopy (ESEM) was used to study the penetration depth of heat dissipated by the SPM probe. The probe-annealing was compared with oven-annealing in order to establish the effect of thermal history on the thermal properties of the materials.

Thermal analysis and topographical characterization of latex films by scanning probe microscopy

Scanning probe microscopy (SPM) was utilized to study the thermal behavior of latex films dried under different conditions. Using the SPM probe resonance frequency method, the resonance frequency (ω) of the oscillating probe above the sample surface was followed as a function of temperature (T). Two different approaches for detecting thermal transitions were used and compared. In the heating-by-the-sample approach, the SPM probe acts as a thermal sensor detecting the heat dissipated by the heated sample. In the heating-by-the-probe approach, the SPM probe is utilized simultaneously as an actuator and a sensor: the heated probe both dissipates heat and detects the heat reflected from the sample surface. Thermal transitions in the polymer films could be observed in the Δω-T curves. The results were consistent with those observed by differential scanning calorimetry, but also additional information about the characteristics of the transitions was obtained. The heating-by-the-probe method showed higher surface sensitivity and more localized effects than the other approach. Furthermore, the progress of the latex film formation and the effect of the different drying conditions on film topography were studied by SPM imaging and roughness analysis. Skewness and local surface curvature are demonstrated to serve as useful numerical measures with which to distinguish different levels of particle deformation and the formation of a continuous film. Preannealing of the samples at high temperature influenced not only the topography (roughness) but also the characteristics of the thermal transitions.

Liquid dispensing of ultrasmall volumes using miniaturized probes

Small-scale, high-precision liquid delivery technologies are a welcome innovation in a number of areas. For example, the increasing miniaturization of microarrays for use in genomics and proteomics makes impractical conventional dispensing systems based on pipettes, pin and ring arrangements, and inkjet printing. Microfabricated probes developed for scanning probe microscopy (SPM), on the other hand, represent unique, versatile tools for working at the nanometer scale. Several techniques exist for liquid dispensing based on SPMlike nanoprobes. The first of these, dip-pen nanolithography (DPN), is based on immersing an SPM probe into molecular solution (the ‘ink’), and then writing molecular patterns with feature sizes as small as several tens of nanometers. An alternative is bioplume technology, which allows larger amounts of ink to be deposited by charging the probe with ink along the probe shaft. Surface structures created in this way, however, have larger dimensions. Here we describe a technology called nanoscale dispensing (NADIS) that is based on apertured nanoprobes. 4 Hollow probes are loaded with ink,either manually or usinga fluidicchannelsystem. When the probe tip makes contact with the substrate, a small amount of liquid is transferredthrough theaperture to the surface: see Figure 1 (left).Tocontrol the deposition process, the surface energies of both probe and substrate must be sufficiently controlled. Figure 1 (right and inset) shows a scanning electron micrograph of a NADIS probe with an integrated channel system. This allows liquid delivery from distant macroscopic reservoirs, thus opening the way to high-volume deposition applications without having to repeatedly reload the probe. Similar developments have been reported for DPN-like systems. Figure 1. (Left) In nanoscale dispensing (NADIS), liquid is deposited by an apertured probe on contact with a substrate surface. (Right) Scanning electron microscope image of a NADIS probe with a nanochannel incorporated into the cantilever. (Inset) Cross-section of a cantilever beam shows the hollow channel structure.