Conducting Probe Atomic Force Microscopy Research Articles

Electric-Field-Assisted Contact Mode AFM-Based Nanolithography with Low Stiffness Conductive Probes

Abstract Electric-field-assisted atomic force microscope (E-AFM) nanolithography is a novel polymer-patterning technique that has diverse applications. E-AFM uses a biased AFM tip with conductive coatings to make patterns with little probe-sample interaction, which thereby avoids the tip wear that is a major issue for contact-mode AFM-based lithography, which usually requires a high probe-sample contact force to fabricate nanopatterns; however, the relatively large tip radius and large tip-sample separation limit its capacity to fabricate high-resolution nanopatterns. In this paper, we developed a contact mode E-AFM nanolithography approach to achieve high-resolution nanolithography of poly (methyl methacrylate) (PMMA) using a conductive AFM probe with a low stiffness (~0.16 N/m). The nanolithography process generates features by biasing the AFM probe across a thin polymer film on a metal substrate. A small constant force (0.5-1 nN) applied on the AFM tip helps engage the tip-film contact, which enhances nanomachining resolution. This E-AFM nanolithography approach enables high-resolution nanopatterning with feature width down to ~16 nm, which is less than one half of the nominal tip radius of the employed conductive AFM probes.

Vibration-induced nanoscale friction modulation on piezoelectric materials

Mechanical vibration, as an alternative of application of solid/liquid lubricants, has been an effective means to modulate friction at the macroscale. Recently, atomic force microscopy (AFM) experiments and model simulations also suggest a similar vibration-induced friction reduction effect for nanoscale contact interfaces, although an additional external vibration source is typically needed to excite the system. Here, by introducing a piezoelectric thin film along the contact interface, we demonstrate that friction measured by a conductive AFM probe can be significantly reduced (more than 70%) when an alternating current (AC) voltage is applied. Such real-time friction modulation is achieved owing to the localized nanoscale vibration originating from the intrinsic inverse piezoelectric effect, and is applicable for various material combinations. Assisted by analysis with the Prandtl—Tomlinson (P—T) friction model, our experimental results suggest that there exists an approximately linear correlation between the vibrational amplitude and the relative factor for perturbation of sliding energy corrugation. This work offers a viable strategy for realizing active friction modulation for small-scale interfaces without the need of additional vibration source or global excitation that may adversely impact device functionalities.

In situ Investigation of Individual Filament Growth in Conducting Bridge Memristor by Contact Scanning Capacitance Microscopy

We report on the application of Contact Scanning Capacitance Microscopy (CSCM) to trace the growth of an individual Ni filament in a ZrO2(Y) film on a Ni sublayer (together with a conductive Atomic Force Microscope probe composing a nanometer-sized virtual memristor). An increasing of the filament length in the course of electro-forming results in an increasing of the capacitance between the probe and the sample, which can be detected by CSCM technique. This way, the filament growth can be monitored in real time in situ.

Photovoltaic distribution on an amorphous-silicon solar cell in near-band-edge excitation observed by conductive-probe atomic force microscopy combined with a fine-wavelength-tunable light source.

We describe the development of a conductive-probe atomic force microscopy method combined with a fine-wavelength-tunable light source and use it to observe the photovoltaic distribution on a cross-sectional surface of an amorphous-silicon solar cell in near-band-edge excitation. The light source's wavelength resolution is dλ = 1nm, and its intensity is 1 µW/cm2 (10 mW/m2); this excitation condition is sufficiently fine and weak to investigate electrical properties in the near-band-edge wavelength range. The photovoltage is observed in the indium tin oxide (ITO) region, and the maximum photovoltage increases when we increase the excitation energy of the illumination light. However, the photovoltaic distribution parallel to the ITO layer becomes relatively localized as the excitation energy increases. This localized photovoltaic distribution suggests that the conductivity of the electric current path within the ITO layer should be inhomogeneous.

Efficient Free Triplet Generation Follows Singlet Fission in Diketo-pyrrolopyrrole Polymorphs with Goldilocks Coupling.

Microcrystal electron diffraction, grazing incidence wide-angle scattering, and UV-Vis spectroscopy were used to determine the unit cell structure and the relative composition of dimethylated diketopyrrolopyrrole (MeDPP) H- and J-polymorphs within thin films subjected to vapor solvent annealing (VSA) for different times. Electronic structure and excited state deactivation pathways of the different polymorphs were examined by transient absorption spectroscopy, conductive probe atomic force microscopy, and molecular modeling. We find VSA initially converts amorphous films into mixtures of H- and J-polymorphs and promotes further conversion from H to J with longer VSA times. Though both polymorphs exhibit efficient SF to form coupled triplets, free triplet yields are higher in J-polymorph films compared to mixed films because coupling in J-aggregates is lower, and, in turn, more favorable for triplet decoupling.

Influence of oxygen ion elementary diffusion jumps on the electron current through the conductive filament in yttria stabilized zirconia nanometer-sized memristor

• Nanometer-sized virtual memristors based on an yttria stabilized zirconia thin film. • Low-frequency flicker noise in the electron current through the conducting filament. • The random motion (drift/diffusion) of oxygen ions/vacancies effect in this current. • Estimates of the root-mean-square value of the current jumps caused by this motion. • The determination of elementary electronic processes in memristor systems. The structure of the electron current through an individual filament of a nanometer-sized virtual memristor consisting of a contact of a conductive atomic force microscope probe to an yttria stabilized zirconia (YSZ) thin film deposited on a conductive substrate is investigated. Usually, such investigation is performed by the analysis of the waveform of this current with the aim to extract the random telegraph noise (RTN). Here, we suggest a new indirect method, which is based on the measurement of the spectrum of the low-frequency flicker noise in this current without extracting the RTN, taking into account the geometrical parameters of the filament. We propose that the flicker noise is caused by the motion (drift/diffusion) of oxygen ions via oxygen vacancies within and around the filament. This allows us to estimate the root mean square magnitude i 0 of the current jumps, which are caused by random jumps of oxygen ions, and the number M of these ions. This is fundamental for understanding the elementary mechanisms of electron current flowing through the filament and resistive switching in YSZ–based memristor devices.

Effects of electron beam irradiation on a Ag/AsS2 bilayer using conductive atomic force microscopy

Metal-chalcogenide systems are used in a variety of fields, including microelectromechanical systems, plasmonics, photonics and the fabrication of X-ray masks. The development of fabrication methods for micro- and nanostructures in metal-chalcogenide bilayers is of significant interest for these fields. This study investigates the effects of electron beam irradiation in an Ag/AsS2 bilayer using conductive atomic force microscopy. An unusual distribution of conductivity is observed, which suggests the presence of at least two competing mechanisms of Ag-ion migration within the amorphous chalcogenide during electron beam irradiation. The surface relief of the Ag/AsS2 bilayer is modified by an electric field from the conductive atomic force microscopy probe.

Self-assembled monolayers of oligophenylenes stiffer than steel and silicon, possibly even stiffer than Si[formula omitted]N[formula omitted

To quantify charge transport through molecular junctions fabricated using the conducting probe atomic force microscopy (CP-AFM) platform, information on the number of molecules N per junction is absolutely necessary. N can be currently obtained only via contact mechanics, and the Young’s modulus E of the self-assembled monolayer (SAM) utilized in a key quantity for this approach. The experimental determination of E for SAMs of CP-AFM junctions fabricated using oligophenylene dithiols (OPDn, 1≤n≤4) and gold electrodes turned out to be too challenging. Recent measurements (Z. Xie et al, J. Am. Chem. Soc. 139 (2017) 5696) merely succeeded to provide a low bound estimate (E≈58 GPa). Supplying this missing experimental information is the aim of the present theoretical investigation. Our microscopic calculations yield values E≈240±6 GPa for the OPDn SAMs of the aforementioned experimental study, which are larger than those of steel (E≈180−200 GPa) and silicon (E≈130−185 GPa). The fact that the presently computed E is much larger than the aforementioned experimental lower bound explain why experimentally measuring E of OPDn SAM’s is so challenging. Having E≈337±8 GPa, OPDn SAMs with herringbone arrangement adsorbed on fcc (111)Au are even stiffer than Si3N4 (E≈160−290 GPa).

Resonant activation of resistive switching in ZrO2(Y) films

Local resistive switching (RS) in ZrO2(Y) films on conductive substrates has been studied using Conductive Atomic Force Microscopy (CAFM). Switching was performed by triangle voltage pulses with superimposed a high-frequency (HF) sinusoidal signal applied to the contact of the CAFM probe to the ZrO2(Y) film (together constituting a nanometer-sized virtual memristor). Earlier, the enhancement of the RS performance has been observed when the HF signal was superimposed onto the switching pulses. The effect was attributed to the resonant activation of the migration of oxygen ions via oxygen vacancies by an external alternating electric field. In the present study, this assumption was confirmed by measuring the frequency dependence of the difference between the probe currents in the low-resistance and high-resistance states with a maximum at about 5 kHz. This frequency corresponds to the characteristic one of the jumps of oxygen ions onto adjacent oxygen vacancies in ZrO2(Y) at 300 K. The experimental results were compared with the results of simulation based on the Chua model of an ideal memristor.

Quantitative analysis of weak current rectification in molecular tunnel junctions subject to mechanical deformation reveals two different rectification mechanisms for oligophenylene thiols versus alkane thiols.

Metal-molecule-metal junctions based on alkane thiol (CnT) and oligophenylene thiol (OPTn) self-assembled monolayers (SAMs) and Au electrodes are expected to exhibit similar electrical asymmetry, as both junctions have one chemisorbed Au-S contact and one physisorbed, van der Waals contact. Asymmetry is quantified by the current rectification ratio RR apparent in the current-voltage (I-V) characteristics. Here we show that RR < 1 for CnT and RR > 1 for OPTn junctions, in contrast to expectation, and further, that RR behaves very differently for CnT and OPTn junctions under mechanical extension using the conducting probe atomic force microscopy (CP-AFM) testbed. The analysis presented in this paper, which leverages results from the previously validated single level model and ab initio quantum chemical calculations, allows us to explain the puzzling experimental findings for CnT and OPTn in terms of different current rectification mechanisms. Specifically, in CnT-based junctions the Stark effect creates the HOMO level shifting necessary for rectification, while for OPTn junctions the level shift arises from position-dependent coupling of the HOMO wavefunction with the junction electrostatic potential profile. On the basis of these mechanisms, our quantum chemical calculations allow quantitative description of the impact of mechanical deformation on the measured current rectification. Additionally, our analysis, matched to experiment, facilitates direct estimation of the impact of intramolecular electrostatic screening on the junction potential profile. Overall, our examination of current rectification in benchmark molecular tunnel junctions illuminates key physical mechanisms at play in single step tunneling through molecules, and demonstrates the quantitative agreement that can be obtained between experiment and theory in these systems.

Fabrication of layer-by-layer graphene oxide thin film on copper substrate by electrophoretic deposition

In this paper, we investigate the fabrication of graphene oxide (GO) films on copper substrates by means of an electrophoretic technique, with the aim of realizing conductive thin films that could be applied as reliable electrical contacts for use in various applications. Uniform films were fabricated, and the film thickness measured using atomic force microscopy. The graphene oxide film thickness is estimated to range from a few manometers to several tens of manometers, depending on the deposition time. Cross-sectional transmission electron microscopy images indicate that the films possess a multi-layered structure, and that the distance between layers is approximately 0.5 nm. Furthermore, the electrical contact resistance of the films was measured, using conductive probe atomic force microscopy. The prepared GO films exhibit electrical conductivity due to their layered structure, which affects their electrical properties.

Long-Range Spin-Selective Transport in Chiral Metal-Organic Crystals with Temperature-Activated Magnetization.

Room-temperature, long-range (300 nm), chirality-induced spin-selective electron conduction is found in chiral metal–organic Cu(II) phenylalanine crystals, using magnetic conductive-probe atomic force microscopy. These crystals are found to be also weakly ferromagnetic and ferroelectric. Notably, the observed ferromagnetism is thermally activated, so that the crystals are antiferromagnetic at low temperatures and become ferromagnetic above ∼50 K. Electron paramagnetic resonance measurements and density functional theory calculations suggest that these unusual magnetic properties result from indirect exchange interaction of the Cu(II) ions through the chiral lattice.

Exchange Interactions Drive Supramolecular Chiral Induction in Polyaniline (Small Methods 10/2020)

In article number 2000617, Claudio Fontanesi and co-workers study the camphor sulfonic acid induction of chirality on the supermolecular structure of electropolymerized aniline. The emergence of chirality is also associated to spin filtering properties of the chiral polymer, revealed by conducting probe atomic force microscopy in the presence of a magnetic field. Theoretical results suggest that the chiral induction is due to exchange interaction energy.

Exchange Interactions Drive Supramolecular Chiral Induction in Polyaniline

AbstractThe focus of this paper is on the intermolecular interaction active between polyaniline (PANI) and 10‐camphorsulfonic acid (10CSA). Enantiopure 10CSA, present in the electropolymerization solution, promotes chiral induction in the supramolecular polyaniline polymer (cPANI). Tight integration of experimental data (circular dichroism, CD, near edge X‐ray absorption spectra, NEXAFS, conductive probe atomic force microscopy, CP‐AFM) and theoretical [density functional theory, (DFT)] results allows to unfold the nature of the electronic interaction between PANI and 10CSA and to shed light on the physical interactions inducing the chiral character to bulk pristine non‐chiral PANI: eventually yielding cPANI. The electropolymerization follows a “wet chemistry” method: electrochemical polymerization of aniline in the co‐presence in bulk solution of enantiopure 10‐camphorsulfonic acid (10CSA). The latter is exploited as chirality inductor. The method of integration between experimental results with ab‐initio theoretical calculations, strongly suggests that the chiral induction exerted by the CSA stems from exchange interaction between CSA and PANI.

High-speed digitization of the amplitude and frequency in open-loop sideband frequency-modulation Kelvin probe force microscopy

A more inclusive and detailed measurement of various physical interactions is enabled by the advance of high-speed data digitization. For surface potential characterization, this was demonstrated recently in terms of open-loop amplitude modulation Kelvin probe force microscopy (OL AM-KPFM). Its counterpart, namely open-loop frequency modulation Kelvin probe force microscopy (OL FM-KPFM), is examined here across different materials and under various bias voltages in the form of OL sideband FM-KPFM. In this implementation the changes in the amplitude and resonance frequency of the cantilever were continuously tracked as a conductive AFM probe was modulated by a 2 kHz AC bias voltage around the first eigenmode frequency of the cantilever. The contact potential difference (CPD) between the AFM probe and sample was determined from the time series analysis of the high-speed 4 MHz digitized amplitude and frequency signals of the OL sideband FM-KPFM mode. This interpretation is demonstrated to be superior to the analysis of the parabolic bias dependent response, which is more commonly used to extract the CPD in OL KPFM modes. The measured OL sideband FM-KPFM amplitude and frequency responses are directly related to the electrostatic force and force-gradient between the AFM probe and sample, respectively. As a result, clear distinction was observed for the determined CPD in each of these cases across materials of different surface potentials, with far superior spatial resolution when the force-gradient detection was used. In addition, the CPD values obtained from OL sideband FM-KPFM amplitude and frequency measurements perfectly matched those determined from their closed-loop AM-KPFM and FM-KPFM counterparts, respectively.

Observation of photovoltaic effect within locally doped silicon nanojunctions using conductive probe AFM

Abstract Localized p-doped nanojunctions (200–300 nm in diameter) were formed in n-type crystalline silicon substrates and were characterized using scanning electron microscopy (SEM) and conductive-probe atomic force microscopy (C-AFM). Localized doping was performed by diffusion through sub-micron sized holes in a silicon-oxide mask defined using self-organized polystyrene nanoparticles. After oxide removal, a significant brightness contrast in the SEM top and side view images strongly suggested the successful local doping of these areas. Furthermore, local current-voltage measurements performed by C-AFM revealed an open circuit voltage and a short-circuit current only in the areas defined as nanojunctions. This photovoltaic effect is driven by the laser used to control cantilever deflection in the AFM.

Molecule–Electrode Electronic Coupling Modulates Optoelectronics of (Bio)Molecular Junctions

The charge transport across a molecular junction formed by sandwiching molecules between two electrodes in testbed architectures depends not only on the work function of the metal electrodes and energy gap of the molecules but also on the efficacy of the molecule–electrode electronic coupling. Insights into such molecule–electrode coupling would help to understand the relation between the coupling strength and electron transport processes. With this aim, the optoelectronic modulation across bacteriorhodopsin-based molecular junctions has been studied using experimental current–voltage traces obtained by conducting-probe atomic force microscopy under various illuminations. The energy barrier $$ \left( {\varepsilon_{0} } \right) $$ , molecule–electrode coupling (Γg), and other transport parameters were determined utilizing the Landauer model with a single-Lorentzian transmission function, transition voltage spectroscopy, and the law of corresponding states in the universal tunneling model approach. The findings reveal that the optoelectronic modulation of bacteriorhodopsin molecular junctions originate from alteration of the molecule–electrode coupling, which could originate from modulation of electronic states and the electrostatic environment of retinal chromophores made of the protein under dark and green or green–blue illumination conditions.

Flexoelectric control of beams with atomic force microscope probe excitation

Based on the converse flexoelectric effect, flexoelectric actuator is designed and used to control the dynamic displacement of cantilever beams. First, shell-type stress expression based on double-curvature shell induced by the converse flexoelectric effect is developed, which can be simplified to a flexoelectric-laminated cantilever beam by applying two Lamé parameters and beam radius of curvature. Then, the flexoelectric actuator is designed with a conductive atomic force microscope probe and a flexoelectric layer. An inhomogeneous electric field is generated when the external voltage is applied on the atomic force microscope probe and the flexoelectric layer, which leads to stress in the longitudinal direction of beam and control moment. With the flexoelectric-induced bending moment, displacement induced by the external force and flexoelectric actuator is derived. The displacement is related to many parameters, such as actuation voltage, atomic force microscope probe radius and flexoelectric layer thickness. Cases are studied to optimize the control effect with different parameters. Results show that vibration control effect is enhanced with a higher actuation voltage and a smaller atomic force microscope probe radius for each mode. Besides, the thicker flexoelectric layer enhances the control effect with a larger bending moment arm for each mode. Dynamic vibration is controlled effectively by converse flexoelectric effect.

Boosting the performance of NO2 gas sensors based on n-n type mesoporous ZnO@In2O3 heterojunction nanowires: in situ conducting probe atomic force microscopic elucidation of room temperature local electron transport.

Herein, n–n type one dimensional ZnO@In2O3 heterojunction nanowires have been developed and their local electron transport properties during trace-level NO2 gas sensing process have been probed at room-temperature using conducting probe atomic microscopy. Solvothermally synthesized 1D ZnO@In2O3 heterojunction nanowires have been characterized by various spectroscopic and microscopic techniques, which revealed the mesoporous structure indicating their enhanced sensing properties. The dangling bonds and fraction of metal ions to oxygen ions existing on the exposed crystal facets of the heterojunction nanowires have been visualized by employing crystallographic simulations with TEM analysis, which aided in forecasting the nature of surface adsorption of NO2 gas species. In situ electrical characteristics and Scanning Spreading Resistance Microscopic (SSRM) imaging of single ZnO@In2O3 heterojunction nanowires revealed the local charge transport properties in n–n type ZnO@In2O3 heterojunction nanowires. Moreover, the ZnO@In2O3 heterojunction nanowires based sensor exhibited excellent sensitivity (S = 274%), a fast response (4–6 s) and high selectivity towards trace-level concentration (500 ppb) of NO2 gas under ambient conditions with low power consumption. Spatially resolved surface potential (SP) variations in ZnO@In2O3 heterojunction nanowires have been visualized using in situ Scanning Kelvin Probe Force Microscopy (SKPM) under NO2 gas environment at room temperature, which was further correlated with its energy band structure. The work functions of the material evaluated by SKPM reveal considerable changes in the energy band structure owing to the local electron transport between ZnO and In2O3 at the heterojunctions upon exposure to NO2 gas indicating the charge carrier recombination. A plausible mechanism has been proposed based on the experimental evidences. The results suggest that new insights into complex sensing mechanisms deduced from the present investigation on n–n type MOS based heterojunction nanowires under ambient conditions can pave the way for the novel design and development of affordable and superior real-time gas sensors.

Solid-State Electron Transport via the Protein Azurin is Temperature-Independent Down to 4 K.

Solid-state electronic transport (ETp) via the electron-transfer copper protein azurin (Az) was measured in Au/Az/Au junction configurations down to 4 K, the lowest temperature for solid-state protein-based junctions. Not only does lowering the temperature help when observing fine features of electronic transport, but it also limits possible electron transport mechanisms. Practically, wire-bonded devices-on-chip, carrying Az-based microscopic junctions, were measured in liquid He, minimizing temperature gradients across the samples. Much smaller junctions, in conducting-probe atomic force microscopy measurements, served, between room temperature and the protein's denaturation temperature (∼323 K), to check that conductance behavior is independent of device configuration or contact nature and thus is a property of the protein itself. Temperature-independent currents were observed from ∼320 to 4 K. The experimental results were fitted to a single-level Landauer model to extract effective energy barrier and electrode-molecule coupling strength values and to compare data sets. Our results strongly support that quantum tunneling, rather than hopping, dominates ETp via Az.