Contact Mode Scanning Research Articles

A high-eigenfrequency differential stiffness probe for high-speed scanning probe microscopy

Microcantilever probes used for high-speed scanning probe microscopy are typically required to satisfy two mutually contradicting objectives: high eigenfrequency ( ω n ) and low flexural rigidity ( k b ). The conventional design approach shortens the microcantilever’s length to increase the eigenfrequencies (as ω n ∝ 1 / L 2 ); however, this also increases the probe’s rigidity (as k b ∝ 1 / L 3 ). This paper demonstrates—through experiments and physical modelling—the design of a novel high-eigenfrequency probe by redistributing the microcantilever’s mass along its length. A 100% increase in the probe’s eigenfrequency is achieved with this modified design, even while reducing its stiffness by 50%. Through experiments, this reduced stiffness has been found to enhance the probe’s force sensitivity . Also, the probe’s design facilitates redistributing the fluid pressure between its underside surface and the sample, thereby improving the quality factor by more than 100%. Significant reduction in ‘overshoot’ and ‘parachute’ is observed in contact mode scanning at the video rate. The improvements in the image resolution are proven for a variety of sample surfaces at very high scan rates up to 40 Hz (corresponding to a linear speed of 3.20 mm s − 1 ) upon scanning over an area of 40 × 40 μ m and are successfully demonstrated to map topographies with negligible image artefacts.

Transitions between surface force- and inertia-dominant contact dynamics regimes in high-speed scanning probe microscopy

In contact mode scanning probe microscopy (SPM), the microcantilever probe’s dynamics are governed by the (short-range) surface interaction forces, where the tip is “always-in-contact” with the sample. In intermittent contact modes such as “tapping” or bimodal SPM, on the other hand, these are governed by the frequency of the microcantilever’s own external excitation. However, when contact mode is employed with high scan speeds (for “video-rate” SPM), we see intermittent transitions—within a single oscillation cycle—between the “always-in-contact” regime and another which is dominated by the microcantilever’s inertia. We find—through experiments and physical modeling—that the fast in-plane motion of the sample relative to the probe results in a high surface excitation frequency v/λ (and its harmonics), which excite the microcantilever’s out-of-plane eigenmodes and cause it to “break free” of the surface and “overshoot” and “parachute.” The impacts of the tip that consequently occur upon the sample inject energy over a wide frequency band into the higher eigenmodes, especially when operating in a low dissipation ambient environment. The microcantilever, then, exhibits phenomena such as eigenmode switching, sidebands, and fractional and combination resonances; such behavior is not seen in, say, tapping mode SPM, since, there, energy is injected at an externally-determined temporal rate. This article investigates the transition from the dynamics of the microcantilever at low speeds to that exhibited at high speeds. The model for dynamic contact loss is validated against the experiments and can be used to propose mitigation of such dynamics in order to achieve high-resolution imaging.

Measuring Band Modulation of MoS2 with Ferroelectric Gates.

Electronic properties of two-dimensional (2D) materials can be significantly tuned by an external electric field. Ferroelectric gates can provide a strong polarization electric field. Here, we report the measurements of the band structure of few-layer MoS2 modulated by a ferroelectric P(VDF-TrFE) gate with contact-mode scanning tunneling spectroscopy. When P(VDF-TrFE) is fully polarized, an electric field up to ∼0.62 V/nm through the MoS2 layers is inferred from the measured band edges, which affects the band structure significantly. First, strong band bending in the vertical direction signifies the Franz-Keldysh effect and a large extension of the optical absorption edge. Photons with energy of half the band gap are still absorbed with 20% of the absorption probability of photons at the band gap. Second, the electric field greatly enlarges the energy separations between the quantum-well subbands. Our study intuitively demonstrates the great potential of ferroelectric gates in band structure manipulation of 2D materials.

Flexible Disk Ultramicroelectrode for High-Resolution and Substrate-Tolerable Scanning Electrochemical Microscopy Imaging.

A simple and universal strategy for fabricating flexible 25 μm platinum (Pt) disk ultramicroelectrodes (UMEs) was proposed, where a pulled borosilicate glass micropipette acted as a mold for shaping the flexible tip with flexible epoxy resin. The whole preparation procedure was highly efficient, enabling 10 or more probes to be manually fabricated within 10 h. Intriguingly, this technique permits an adjustable RG ratio, tip length, and stiffness, which could be tuned according to varying experimental demands. Besides, the electroactive area of the probe could be exposed and made renewable with a thin blade, allowing its reuse in multiple experiments. The flexibility characterization was then employed to optimize the resin/hardener mass ratio of epoxy resin and the tip position during HF etching in the fabrication process, suggesting that more hardener, a larger RG value, or a longer tip length obtained stronger deformation resistance. Subsequently, the as-prepared probe was examined by optical microscopy, cyclic voltammetry, and SECM approach curves. The results demonstrated the probe possessed good geometry with a small RG ratio of less than 3 and exceptional electrochemical properties, and its insulating sheath remained undeformed after blade cutting. Owing to the tip's flexibility, it could be operated in contactless mode with an extremely low working distance and even in contact mode scanning to achieve high spatial resolution and high sensitivity while guaranteeing that the tip and samples would suffer minimal damage if the tip crashed. Finally, the flexible probe was successfully employed in three scanning scenarios where tilted and 3D structured PDMS microchips, a latent fingerprint deposited on the stiff copper sheet, and soft egg white were included. In all, the flexible probe encompasses the advantages of traditional disk UMEs and circumvents their principal drawbacks of tip crash and causing sample scratches, which is thus more compatible with large specimens of 3D structured, stiff, or even soft topography.

Visualizing Band Profiles of Gate-Tunable Junctions in MoS2/WSe2 Heterostructure Transistors.

Heterostructure devices based on two-dimensional materials have been under intensive study due to their intriguing electrical and optical properties. One key factor in understanding these devices is their nanometer-scale band profiles, which is challenging to obtain in devices. Here, we use a technique named contact-mode scanning tunneling spectroscopy to directly visualize the band profiles of MoS2/WSe2 heterostructure devices at different gate voltages with nanometer resolution. The long-held view of a conventional p-n junction in the MoS2/WSe2 heterostructure is reexamined. Due to strong inter- and intralayer charge transfer, the MoS2 layer in contact with WSe2 is found to convert from n-type to p-type, and a series of gate-tunable p-n and p-p+ junctions are developed in the devices. Highly conductive edges are also discovered which could strongly affect the device properties.

Nanotribological Printing: A Nanoscale Additive Manufacturing Method.

Additive manufacturing methods are transforming the way components and devices are fabricated, which in turn is opening up completely new vistas for conceiving and designing products and engineered systems. Small-scale (submicrometer) additive manufacturing methods are largely in their infancy. While a number of methods exist, a particular challenge lies in finding methods that can produce a range of materials while obtaining sufficiently robust mechanical properties. In this paper, we describe a novel nanoscale additive manufacturing technique deemed "Nanotribological Printing" (NTP), which creates structures through tribomechanical and tribochemical surface interactions at the contact between a substrate and an atomic force microscope probe, where material pattern formation is driven by normal and shear contact stresses. The "ink" consists of nanoparticles or molecules dispersed in a carrier fluid surrounding the atomic force microscope (AFM) probe, which are entrained into the contact during sliding. Being stress-driven, patterning only occurs locally within regions which experience contact and sufficiently high stresses. Thus, imaging and measurement to characterize the morphology and properties of the deposited structures can be conducted in situ during the manufacturing process. Moreover, using local mechanical energy as the kinetic driver activating the solidification process, the method is compact and does not require application of a bias voltage or laser exposure and can be performed at ambient temperatures. We demonstrate (1) control of pattern dimensions with sub-100 nm lateral and sub-5 nm thickness control through variations in contact size and applied stress, (2) creation of amorphous, polycrystalline, and nanocomposite structures including sequential multimaterial deposition, and (3) formation of manufactured structures which exhibit mechanical properties approaching those of bulk counterparts. The ability to create nanoscale patterns using standard AFM cantilever probes and operation modes (contact mode scanning in fluid) with commercial AFM instruments, independent of substrate, establishes NTP as a versatile and easily accessible method for nanoscale additive manufacturing.

Off-resonance intermittent contact mode multi-harmonic scanning force microscopy

A robust off-resonance intermittent contact mode scanning force microscopy technique suitable for operation under different environmental conditions is presented. The technique relies on a multi-channel lock-in amplifier to measure multiple high harmonic magnitudes and phases. For distance control, the fundamental harmonic magnitude is used. With this, high intermittent contact frequencies become feasible even with older atomic force microscope data acquisition systems with limited measurement bandwidths, provided high frequency tip-sample distance actuation techniques are used. Suitable higher harmonic magnitude images provide a qualitative materials' contrast. If a sufficiently high number of high harmonic magnitudes and phases are recorded, force-distance curves at all imaged points can be reconstructed. From fitting models of the contact mechanics to force versus tip-sample penetration data, the elastic modulus of the sample can be obtained.

Intermittent impact dynamics of a cantilever scanning a surface at high speed

In contact-mode scanning probe microscopy (SPM), the cantilever's dynamics are conventionally investigated by assuming the tip to be always in contact with the sample's surface. At high scanning speeds, however, the cantilever's inertia becomes dominant and the tip can therefore completely detach from the surface and start impacting upon it. Experiments at the macro-scale cannot emulate the complex micro-scale dynamics, as the system exhibits negligible effects due to meniscus forces and the surrounding medium's squeeze film damping; however, they can provide qualitative insight into the cantilever's dynamics at high frequencies, corresponding to those likely to be excited during video-rate SPM imaging. This paper investigates such intermittent impact dynamics for an upscaled cantilever, analytically, numerically, and experimentally. In contact-mode scanning, a critical scan speed exists beyond which the cantilever's tip loses contact with the sample's surface; a closed-form expression for this contact loss frequency is derived. At high scan speeds, impacts cause the cantilever to switch between different contact regimes: in-, off-, and grazing-contact; within each regime, the system's modal configuration is different. Experimentally-obtained Poincare maps indicate quasi-periodic behaviour at frequencies for which the response is repetitive, as is also predicted by the model. Intermittent impacts excite the sub- and super-harmonics of the excitation frequency, which are related to the natural frequencies of different system configurations based on the “effective” tip-end boundary conditions. The cantilever's response exhibits several phenomena, such as modal transition, beating, grazing, and possible chaotic behaviour, depending upon the relation between the excitation harmonics and the natural frequencies.

A low-noise measurement system for scanning thermal microscopy resistive nanoprobes based on a transformer ratio-arm bridge

Atomic force microscopy (AFM) is a widely used technology for the investigation and characterization of nanomaterials. Its functionality can be easily expanded by applying dedicated extension modules, which can measure the electrical conductivity or temperature of a sample. In this paper, we introduce a transformer ratio-arm bridge setup dedicated to AFM-based thermal imaging. One of the key features of the thermal module is the use of a low-power driving signal that prevents undesirable tip heating during resistance measurement, while the other is the sensor location in a ratio-arm transformer bridge working in the audio frequency range and ensuring galvanic isolation of the tip, enabling contact-mode scanning of electronic circuits. The proposed expansion module is compact and it can be integrated onto the AFM head close to the cantilever. The calibration process and the resolution of 11 mK of the proposed setup are shown.

Soft Probe Scanning Electrochemical Microscopy with Spider Array for Visualizing Biomarkers and Redox Active Proteins in Animal Tissues

We present soft spider array for gentle contact mode scanning electrochemical microscopy (SECM) imaging of tissues. The spider array contains up to eight completely individual, parallel, and flexible carbon microelectrodes designed to scan square centimeter sized areas on reasonable time scales while avoiding sample aging and electrode fouling. The contact mode enables the scanning of largely curved samples at a constant distance. The spider array is applied for the SECM imaging of the distribution of certain biomarkers, such as tyrosinase, in thin tumor tissues as well as thick tumor blocks. Furthermore, the spider array can be applied for mapping the antioxidant distribution in an apple peel.

Mapping the antioxidant activity of apple peels with soft probe scanning electrochemical microscopy

We present a non-invasive electrochemical strategy for mapping the antioxidant (AO) activity of apple peels, which counterbalances oxidative stress caused by various external effectors. Soft carbon microelectrodes were used for soft probe scanning electrochemical microscopy (SECM) enabling the gentle and scratch-free contact mode scanning of rough and delicate apple peels in an electrolyte solution. The SECM feedback mode was applied using ferrocene methanol (FcMeOH) as redox mediator that gets electrochemically oxidized at the soft probe and diffuses towards the apple peel where it gets regenerated by certain AOs leading to a redox mediator recycling and increased current signal. The global AO activity in the apple peel including lenticels and regions with artificially degraded AOs were mapped using the soft microelectrodes. Finally, in an apple cross-section the higher and homogeneous AO concentration in the peel with a heterogeneously decaying AO gradient towards the apple inward was visualized, demonstrating the adequate micrometer resolution of the SECM probe and the possibility to get information of the interior AO activity of the apple.

Scanning Probe-Based in-Situ High Temperature Electrical and Electrochemical Studies

Scanning Probe Microscopy (SPM) is arguably the only tool that is available for direct nanoscale electrical and electrochemical characterization since electrical measurement through a highly localized area necessitates a nanoscale electrode. However, conventional SPM systems are functional only up to ~250°C under atmospheric pressure because significant heat convection from the heating stage to thermally-sensitive system modules disturbs a stable operation1,2. In operando observations of non-vacuum processes necessitating a much higher temperature, e.g. solid oxide-based electrochemical reactions and phase changes, have been largely limited. To address these problems, we have developed micro-thermal stage (MTS)3 in which the heating element is on the micrometer scale with an area roughly a million times smaller than that of a conventional SPM heating stage. We also prepared SPM probes custom-made to suppress thermal degradation during high temperature operation. At high temperature, metal-coated SPM tips tend to degrade at a faster rate, resulting in erroneous observations. The modified SPM setup was applied to study thermally activated bonding formation and highly localized oxygen reduction reaction kinetics on a solid oxide electrolyte. For the MTS, Pt is micro-patterned on a silicon chip as the heating element and a thin film-based layer of study are deposited on top of the heating element after electrical passivation. The temperature of the Pt heater can be determined by the measured electrical resistance of the heater because the resistance of a metal linearly increases with temperature4. A four-probe measurement scheme is integrated onto the chip for sensing the resistance and corresponding temperature of the heating element. The temperature of the heater is feedback controlled. Due to the small size of the heating element and small sample thickness, the desired temperature can be reached and controlled in milliseconds. To circumvent the degradation issue of Pt-coated tip, an all-metal tip is custom-made from Pt-Ir wire by electrochemical etching method, from which tip radii less than 100 nm can be routinely achieved. The non-etched part of the wire is flattened and polished to mount on an SPM system. After verifying the functionality of the Pt-Ir SPM tip through routine SPM contact mode scanning, we used the modified setup to study localized in-situ oxygen reduction catalysis and ionic transport on a platinum/yttria-stabilized ceria (Pt/YSZ) half cell. The aforementioned modified SPM setup has potential to be used in many other applications, such as in-situ observation of changes in mechanical, electrical and thermal properties at the tip-sample interfaces under high-temperature and high-pressure conditions. Research supported by the AFOSR (Grant # FA9550-15-1-0256), USA and MoE AcRF Tier 2 (Grant # ARC20/14), Singapore. 1 J. Broekmaat, A. Brinkman, D.H.A. Blank, and G. Rijnders, Appl. Phys. Lett. 92, (2008). 2 K. V Hansen, T. Jacobsen, A.M. Norgaard, N. Ohmer, and M. Mogensen, Electrochem. Solid State Lett. 12, B144 (2009). 3 J. Lee, S. Kim, R. Jeyasingh, M. Asheghi, H.-S.P. Wong, and K.E. Goodson, IEEE Electron Device Lett. 32, 952 (2011). 4 R.B. Belser and W.H. Hicklin, J. Appl. Phys. 30, 313 (1959). Figure 1

Micro-Thermal Stage and Custom-Made Pt-Ir Tips for High Temperature Electrical and Electrochemical Studies By Scanning Probe Microscopy

Scanning Probe Microscopy (SPM) is arguably the only tool that is available for direct nanoscale electrical and electrochemical characterization since electrical measurement through a highly localized area necessitates a nanoscale electrode. However, conventional SPM systems are functional only up to ~250°C under atmospheric pressure because significant heat convection from the heating stage to thermally-sensitive system modules disturbs a stable operation1,2. In operando observations of non-vacuum processes necessitating a much higher temperature, e.g. solid oxide-based electrochemical reactions and phase changes, have been largely limited. To address these problems, we have developed micro-thermal stage (MTS)3 in which the heating element is on the micrometer scale with an area roughly a million times smaller than that of a conventional SPM heating stage. We also prepared SPM probes custom-made to suppress thermal degradation during high temperature operation. At high temperature, metal-coated SPM tips tend to degrade at a faster rate, resulting in erroneous observations. The modified SPM setup was applied to study thermally activated bonding formation and highly localized oxygen reduction reaction kinetics on a solid oxide electrolyte. For the MTS, Pt is micro-patterned on a silicon chip as the heating element and a thin film-based layer of study are deposited on top of the heating element after electrical passivation. The temperature of the Pt heater can be determined by the measured electrical resistance of the heater because the resistance of a metal linearly increases with temperature4. A four-probe measurement scheme is integrated onto the chip for sensing the resistance and corresponding temperature of the heating element. The temperature of the heater is feedback controlled. Due to the small size of the heating element and small sample thickness, the desired temperature can be reached and controlled in milliseconds. To circumvent the degradation issue of Pt-coated tip, an all-metal tip is custom-made from Pt-Ir wire by electrochemical etching method, from which tip radii less than 100 nm can be routinely achieved. The non-etched part of the wire is flattened and polished to mount on an SPM system. After verifying the functionality of the Pt-Ir SPM tip through routine SPM contact mode scanning, we used the modified setup to study localized in-situ oxygen reduction catalysis and ionic transport on a platinum/yttria-stabilized ceria (Pt/YSZ) half cell. The aforementioned modified SPM setup has potential to be used in many other applications, such as in-situ observation of changes in mechanical, electrical and thermal properties at the tip-sample interfaces under high-temperature and high-pressure conditions. Research supported by the AFOSR (Grant # FA9550-15-1-0256), USA and MoE AcRF Tier 2 (Grant # ARC20/14), Singapore. Figure 1

Evaluation of the electrical contact area in contact-mode scanning probe microscopy

The tunneling current through an atomic force microscopy (AFM) tip is used to evaluate the effective electrical contact area, which exists between tip and sample in contact-AFM electrical measurements. A simple procedure for the evaluation of the effective electrical contact area is described using conductive atomic force microscopy (C-AFM) in combination with a thin dielectric. We characterize the electrical contact area for coated metal and doped-diamond tips operated at low force (<200 nN) in contact mode. In both cases, we observe that only a small fraction (<10 nm2) of the physical contact (∼100 nm2) is effectively contributing to the transport phenomena. Assuming this reduced area is confined to the central area of the physical contact, these results explain the sub-10 nm electrical resolution observed in C-AFM measurements.

Morphology, mechanical stability, and protective properties of ultrathin gallium oxide coatings.

Ultrathin gallium oxide layers with a thickness of 2.8 ± 0.2 nm were transferred from the surface of liquid gallium onto solid substrates, including conjugated polymer poly(3-hexylthiophene) (P3HT). The gallium oxide exhibits high mechanical stability, withstanding normal pressures of up to 1 GPa in contact mode scanning force microscopy imaging. Moreover, it lowers the rate of photodegradation of P3HT by 4 orders of magnitude, as compared to uncovered P3HT. This allows us to estimate the upper limits for oxygen and water vapor transmission rates of 0.08 cm(3) m(-2) day(-1) and 0.06 mg m(-2) day(-1), respectively. Hence, similar to other highly functional coatings such as graphene, ultrathin gallium oxide layers can be regarded as promising candidates for protective layers in flexible organic (opto-)electronics and photovoltaics because they offer permeation barrier functionalities in conjunction with high optical transparency.

Analysis of line-edge roughness due to the stick/slip motion of a contact-mode scanning probe in plasmonic lithography

In plasmonic lithography, when using a scanning probe in contact mode, degradation of the line pattern quality occasionally occurs due to variations of probe tip velocity or a stick/slip motion. To avoid poor pattern quality caused by such variations in probe tip motion, we analyzed the motion of the probe tip by using a frictional model based on conventional contact mechanics. The motion of the probe tip was numerically analyzed in terms of adhesion force and probe velocity, which are the dominant factors in micro/nanoscale motion. It was found that stick/slip spacing has a roughly positive relationship with the maximum adhesion force between the substrate and probe tip, and a negative relationship with the probe velocity. Combining the probe tip motion with the exposure model of a near-field wave, we analyzed the quality of line patterns that resulted from various stick/slip spacing in terms of line-edge roughness.

Multimodal microscopy using ‘half and half’ contact mode and ultrasonic force microscopy

Advances in the design and fabrication of multifunctional nanostructured materials require characterization techniques capable of simultaneously mapping multiple material properties with nanoscale resolution. We show that this can be achieved by combining nanomechanical information from ultrasonic force microscopy (UFM) with simultaneously acquired friction force and conductivity measurements from contact mode scanning. This utilizes a ‘half and half’ approach, where the AFM is operated alternatively in UFM and contact mode, with the switching rate sufficiently fast that simultaneous contact mode and UFM information is acquired at each pixel of an image. We demonstrate the potential of such a multimodal approach through its application to composite systems consisting of graphene islands on a copper surface, single-walled carbon nanotubes (SWNTs) on a silicon oxide substrate, and a graphene epoxy composite. The half and half approach enables the friction force to be measured without topographical cross-talk. Application to the SWNT sample reveals a further advantage; due to the superlubricity of UFM it enables standard contact mode imaging techniques to be applied to delicate samples.

Enhanced immobilization of gold nanoclusters on graphite.

The immobilization of individual biological molecules by metal nanoparticles requires that the particles themselves be immobilized. We introduce a new technique for immobilization of gold clusters based on their binding to small tunnels in a graphite support, themselves created by the implantation of small clusters. These tunnels are shown to perform as more effective cluster immobilization sites than point defects on the surface of graphite. The method is tested with atomic force microscopy (AFM) (both contact and noncontact mode) scanning. Size-selected clusters with 923, 561, 309, and 147 atoms have been immobilized and imaged with high-resolution, noncontact AFM.

In situ surface reduction of a NiO-YSZ-alumina composite using scanning probe microscopy

In situ surface reductions of NiO-YSZ-Al2O3 composites into Ni-YSZ-Al2O3 cermets were carried out at 312–525 °C in a controlled atmosphere high-temperature scanning probe microscope (CAHT-SPM) in dry and humidified 9 % H2 in N2. The reduction of NiO was followed by contact mode scanning of topography and conductance. A reproducible sequence of events was observed which included a conductance decrease upon hydrogen introduction and a reappearance of conductance after some time. It was found that this incubation time from introduction of hydrogen and until conducting Ni appeared was temperature dependent and followed the Arrhenius equation. For samples reduced in dry hydrogen, the Arrhenius plot showed two regions with different activation energies. Scanning electron microscopy confirmed a difference in microstructure between these temperature regimes. A strong retarding effect of steam (H2O) on the nucleation time of Ni particles was observed.

Nanobubbles Do Not Sit Alone at the Solid–Liquid Interface

The unexpected stability and anomalous contact angle of gaseous nanobubbles at the hydrophobic solid-liquid interface has been an issue of debate for almost two decades. In this work silicon-nitride tipped AFM cantilevers are used to probe the highly ordered pyrolytic graphite (HOPG)-water interface with and without solvent-exchange (a common nanobubble production method). Without solvent-exchange the force obtained by the single force and force mapping techniques is consistent over the HOPG atomic layers and described by DLVO theory (strong EDL repulsion). With solvent-exchange the force is non-DLVO (no EDL repulsion) and the range of the attractive jump-in (>10 nm) over the surface is grouped into circular areas of longer range, consistent with nanobubbles, and the area of shorter range. The non-DLVO nature of the area between nanobubbles suggests that the interaction is no longer between a silicon-nitride tip and HOPG. Interfacial gas enrichment (IGE) covering the entire area between nanobubbles is suggested to be responsible for the non-DLVO forces. The absence of EDL repulsion suggests that both IGE and nanobubbles are not charged. The coexistence of nanobubbles and IGE provides further evidence of nanobubble stability by dynamic equilibrium. The IGE cannot be removed by contact mode scanning of a cantilever tip in pure water, but in a surfactant (SDS) solution the mechanical action of the tip and the chemical action of the surfactant molecules can successfully remove the enrichment. Strong EDL repulsion between the tip and nanobubbles/IGE in surfactant solutions is due to the polar heads of the adsorbed surfactant molecules.