Conductive Atomic Force Microscopy Measurements Research Articles

Strong Covalent Coupling in Vertically Layered SnSe2/PTAA Heterojunctions Enabled High Performance Inorganic-Organic Hybrid Photodetectors.

Controllable large-scale integration of two-dimensional (2D) materials with organic semiconductors and the realization of strong coupling between them still remain challenging. Herein, we demonstrate a wafer-scale, vertically layered SnSe2/PTAA heterojunction array with high light-trapping ability via a low-temperature molecular beam epitaxy method and a facile spin-coating process. Conductive probe atomic force microscopy (CP-AFM) measurements reveal strong rectification and photoresponse behavior in the individual SnSe2 nanosheet/PTAA heterojunction. Theoretical analysis demonstrates that vertically layered SnSe2/PTAA heterojunctions exhibit stronger C-Se covalent coupling than that of the conventional tiled type, which could facilitate more efficient charge transfer. Benefiting from these advantages, the SnSe2/PTAA heterojunction photodetectors with an optimized PTAA concentration show high performance, including a responsivity of 41.02 A/W, an external quantum efficiency of 1.31 × 104%, and high uniformity. The proposed approach for constructing large-scale 2D inorganic-organic heterostructures represents an effective route to fabricate high-performance broadband photodetectors for integrated optoelectronic systems.

Toward coupling across inorganic/organic hybrid interfaces: polyaniline-coated gold nanoparticles with 4-aminothiophenol as gold-anchoring moieties

AbstractThe chemical binding between metal nanoparticles and (semi-)conductive polymer layers is essential to control the (opto-)electronic properties of such hybrid materials. Current approaches that achieve a conjugated binding of organic (semi-)conductive ligands to metal nanoparticles demonstrated promising functional properties, but are based on tedious multi-step organic synthesis to incorporate the required binding moieties at the chain ends of targeted macromolecular species. Herein, we explore the pre-functionalization of gold nanoparticles with p-aminothiophenol and subsequent surfactant-assisted formation of a poly(aniline) (PANI) shell as a means to access gold/PANI core–shell-type nanoparticles with enhanced conductive properties. Controlled surface deposition of these hybrid nanoparticles is achieved via template-assisted self-assembly. For these surface-deposited nanoparticles, charge transport properties are characterized at the nanoscale by conductive atomic force microscopy measurements and show a significant conductivity increase of our core–shell particles as compared to reference particles formed by conventional surfactant-assisted PANI-shell formation. Graphical Abstract

Evaluating the optoelectronic properties of individual CsPbBr3 microcrystals by electric AFM techniques

All-inorganic perovskite materials are attracting the attention of researchers owing to the good performance that they exhibit in optoelectronic device applications. In particular, cesium lead halide single crystals are of interest for their electric conductivity. However, the conductive properties of these materials are still far from being fully understood. In this work, we used several scanning probe microscopy techniques to investigate different properties of CsPbBr3 single crystals, namely their topography, electric conductivity, electric surface potential and ferroelectricity. We observed heterogeneity in conductance and work function of the different facets by using conductive atomic force microscopy and Kelvin probe force microscopy measurements. Additionally, piezo-response force microscopy was carried out to gain insight on the origin of the hysteresis in our microcrystals. Our results show the absence of ferroelectric domains and we attribute the I–V hysteresis to ionic migration within the crystal.

Atomic Defect Quantification by Lateral Force Microscopy.

Atomic defects in two-dimensional (2D) materials impact electronic and optoelectronic properties, such as doping and single photon emission. An understanding of defect-property relationships is essential for optimizing material performance. However, progress in understanding these critical relationships is hindered by a lack of straightforward approaches for accurate, precise, and reliable defect quantification on the nanoscale, especially for insulating materials. Here, we demonstrate that lateral force microscopy (LFM), a mechanical technique, can observe atomic defects in semiconducting and insulating 2D materials under ambient conditions. We first improve the sensitivity of LFM through consideration of cantilever mechanics. With the improved sensitivity, we use LFM to locate atomic-scale point defects on the surface of bulk MoSe2. By directly comparing LFM and conductive atomic force microscopy (CAFM) measurements on bulk MoSe2, we demonstrate that point defects observed with LFM are atomic defects in the crystal. As a mechanical technique, LFM does not require a conductive pathway, which allows defect characterization on insulating materials, such as hexagonal boron nitride (hBN). We demonstrate the ability to observe intrinsic defects in hBN and defects introduced by annealing. Our demonstration of LFM as a mechanical defect characterization technique applicable to both conductive and insulating 2D materials will enable routine defect-property determination and accelerate materials research.

High pressure induced formation of carbon nanorods from tetracosane

Morphology control of carbon structures is essential for improving their performance in many applications. Direct pyrolysis of organic precursors, however, usually yields bulk amorphous carbon. Therefore, traditional methods for controlling the morphology of carbon structures involve multistep processes and complex precursor molecules. While various methods have been developed under ambient pressure, the impact of pressure on the morphology of the resulting carbon structures remains unexplored. Herein, we present the synthesis of carbon nanorods by direct pyrolysis of the low-cost aliphatic hydrocarbon tetracosane under high pressure conditions. The diameters of the carbon nanorods are adjusted by simply varying the synthetic pressures. High pressure allows controlling both the nanorod morphology as well as the degree of order, and local conductivity of the thus prepared nanorods has been confirmed by conductive atomic force microscopy measurements. Our method promises a convenient strategy to synthesize carbon structures with controlled morphology and high ordered chemical structure, which opens opportunities for potential electronic and electrochemical applications.

Multifunctional chemical anchors achieve a boosted fill factor and mitigate ion migration of high-stability perovskite solar cells.

To date, it is urgent to produce perovskite films with comparative or even better morphologies in an open-air environment. Unfortunately, a substantial number of trap states on the grain surface, especially the grain boundaries (GBs) of a perovskite layer, can bring about significant deterioration in the performance of PSCs. Trap-induced carrier recombination directly exerts a detrimental influence on the carrier collection efficiency and electronic properties of a perovskite active film. Herein, 4(5)-iodoimidazole (4II), a small organic molecule agent, was introduced to passivate the surface and bulk traps of the active film, which resulted in a controlled morphology, improved carrier extraction and suppressed ion migration for the devices fabricated in a relatively humid and O2-containing environment. Conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM) measurements were applied to study trap passivation and suppression of ion migration across the GBs of perovskite films. The results manifest that the -CN group preferably bonds with the less-coordinated Pb2+ and the -NH- group favorably forms hydrogen bonds with the uncoordinated I-. As a result, the champion device delivered a significantly boosted power conversion efficiency from 17.22% to 20.95%, with an improved fill factor (FF) from 70.54% to 80.40%, and improved ambient stability of the unencapsulated device. This study may probe research insight into the design of passivators with synergistic effects for morphology control and reduction of carrier recombination loss for equally efficient perovskite photovoltaics fabricated in ambient air.

A multi-resistance wide-range calibration sample for conductive probe atomic force microscopy measurements.

Measuring resistances at the nanoscale has attracted recent attention for developing microelectronic components, memory devices, molecular electronics, and two-dimensional materials. Despite the decisive contribution of scanning probe microscopy in imaging resistance and current variations, measurements have remained restricted to qualitative comparisons. Reference resistance calibration samples are key to advancing the research-to-manufacturing process of nanoscale devices and materials through calibrated, reliable, and comparable measurements. No such calibration reference samples have been proposed so far. In this work, we demonstrate the development of a multi-resistance reference sample for calibrating resistance measurements in conductive probe atomic force microscopy (C-AFM) covering the range from 100 Ω to 100 GΩ. We present a comprehensive protocol for in situ calibration of the whole measurement circuit encompassing the tip, the current sensing device, and the system controller. Furthermore, we show that our developed resistance reference enables the calibration of C-AFM with a combined relative uncertainty (given at one standard deviation) lower than 2.5% over an extended range from 10 kΩ to 100 GΩ and lower than 1% for a reduced range from 1 MΩ to 50 GΩ. Our findings break through the long-standing bottleneck in C-AFM measurements, providing a universal means for adopting calibrated resistance measurements at the nanoscale in the industrial and academic research and development sectors.

Bacterial extracellular electron transfer components are spin selective.

Metal-reducing bacteria have adapted the ability to respire extracellular solid surfaces instead of soluble oxidants. This process requires an electron transport pathway that spans from the inner membrane, across the periplasm, through the outer membrane, and to an external surface. Multiheme cytochromes are the primary machinery for moving electrons through this pathway. Recent studies show that the chiral-induced spin selectivity (CISS) effect is observable in some of these proteins extracted from the model metal-reducing bacteria, Shewanella oneidensis MR-1. It was hypothesized that the CISS effect facilitates efficient electron transport in these proteins by coupling electron velocity to spin, thus reducing the probability of backscattering. However, these studies focused exclusively on the cell surface electron conduits, and thus, CISS has not been investigated in upstream electron transfer components such as the membrane-associated MtrA, or periplasmic proteins such as small tetraheme cytochrome (STC). By using conductive probe atomic force microscopy measurements of protein monolayers adsorbed onto ferromagnetic substrates, we show that electron transport is spin selective in both MtrA and STC. Moreover, we have determined the spin polarization of MtrA to be ∼77% and STC to be ∼35%. This disparity in spin polarizations could indicate that spin selectivity is length dependent in heme proteins, given that MtrA is approximately two times longer than STC. Most significantly, our study indicates that spin-dependent interactions affect the entire extracellular electron transport pathway.

In-Situ Measurement of Charge Transfer at Cocatalyst/Semiconductor Interfaces in BiVO4 Particle Photocatalyst

Semiconductor photocatalyst particles are known to transform solar energy to useful chemicals such as H2 or methanol. However, the efficiency of semiconductor photocatalysts often suffers from the quick charge carrier recombination. However, there is no real physical reasons for semiconductors to display this low performance. Multiple strategies have been proposed and applied to modify the semiconductor particles and enhance their solar-to-hydrogen efficiencies. It is often assumed that in semiconductor particles with well-defined crystal facets, the photogenerated charge carriers undergo facet-dependent separation. Thus, a common strategy is to deposit a HER electrocatalyst on ‘electron-selective’ facets and an OER electrocatalyst on ‘hole-selective’ facets. Although this strategy have showed plausible results in previous literatures, a precise understanding of how charge-carrier-selective contacts between the deposited electrocatalyst and semiconductor particles emerge and how the semiconductor photocatalysts are rationally designed is still missing.In this work, we used both ex-situ and in-situ conducting AFM experiments and a new ionomer/catalyst–semiconductor test structures to characterize the charge transfer between semiconductor and deposited electrocatalysts at individual nanoparticle scale. We showed that the presence of the water/electrolyte interface is critical to induce hole selectivity between the CoOx OER cocatalyst and the BiVO4 semiconductor light absorber. We demonstrated that the negative surface charge on the semiconductor is central to induce hole selectivity. The work also demonstrates a facile strategy to control local pH and introduce water using thin-film ionomers for conductive AFM measurements. These new insights can be broadly applied to the rational design of semiconductor photocatalysts for overall water-splitting systems to control charge-carrier selectivity and improve their performance.

SiC Doping Impact during Conducting AFM under Ambient Atmosphere.

The characterization of silicon carbide (SiC) by specific electrical atomic force microscopy (AFM) modes is highly appreciated for revealing its structure and properties at a nanoscale. However, during the conductive AFM (C-AFM) measurements, the strong electric field that builds up around and below the AFM conductive tip in ambient atmosphere may lead to a direct anodic oxidation of the SiC surface due to the formation of a water nanomeniscus. In this paper, the underlying effects of the anodization are experimentally investigated for SiC multilayers with different doping levels by studying gradual SiC epitaxial-doped layers with nitrogen (N) from 5 × 1017 to 1019 at/cm3. The presence of the water nanomeniscus is probed by the AFM and analyzed with the force-distance curve when a negative bias is applied to the AFM tip. From the water meniscus breakup distance measured without and with polarization, the water meniscus volume is increased by a factor of three under polarization. AFM experimental results are supported by electrostatic modeling to study oxide growth. By taking into account the presence of the water nanomeniscus, the surface oxide layer and the SiC doping level, a 2D-axisymmetric finite element model is developed to calculate the electric field distribution nearby the tip contact and the current distributions at the nanocontact. The results demonstrate that the anodization occurred for the conductive regime in which the current depends strongly to the doping; its threshold value is 7 × 1018 at/cm3 for anodization. Finally, the characterization of a classical planar SiC-MOSFET by C-AFM is examined. Results reveal the local oxidation mechanism of the SiC material at the surface of the MOSFET structure. AFM topographies after successive C-AFM measurements show that the local oxide created by anodization is located on both sides of the MOS channel; these areas are the locations of the highly n-type-doped zones. A selective wet chemical etching confirms that the oxide induced by local anodic oxidation is a SiOCH layer.

Revealing the interlayer orientations for bilayer graphene grown on hexagonal boron nitride by c-AFM measurement

Owing to its excellent properties in optical and electronic applications, twisted bilayer graphene (TBG) has attracted extensive research attention. As an encapsulation layer, hexagonal boron nitride (h-BN) is indispensable for fabricating TBG devices. Because h-BN and graphene exhibit similar lattice structures, TBG on h-BN exhibits periodic moiré patterns, which enable interlayer orientations to be revealed for bilayer graphene on h-BN. Herein, we used two-step chemical vapor deposition to synthesize TBG exhibiting multiple twist angles on h-BN. Conductive atomic force microscopy measurements revealed that the conductivity distribution was different for the graphene/h-BN and TBG moiré patterns. Using the characteristics of the moiré patterns formed by these layers, h-BN and graphene interlayer orientations were precisely determined. This provides a feasible method for investigating the turbostratic arrangement of few-layer van der Waals heterostructures.

Direct-current output of silicon–organic monolayer–platinum Schottky TENGs: Elusive friction-output relationship

Triboelectric nanogenerators (TENGs) are an emerging energy harvesting technology able to convert ubiquitous mechanical energy into electricity. Friction, static charging and flexoelectricity are all involved in the mechanism underpinning TENG operation, but their relative contribution has remained elusive. Here we used dynamic and static conductive atomic force microscopy (C-AFM) measurements on monolayer-modified silicon crystals to detect evidence of a relationship between friction and zero-bias current, and between pressure and the direction of the putative flexovoltage. We demonstrate that a static electricity-related tribovoltage is probably responsible for a friction excess, and that surprisingly this friction excess is found to be dependent on the doping level and type of the silicon substrate. Such friction excess is however no longer measurable once current is allowed to flow across the junction. This observation points to an electrostatic origin of friction in silicon-based Schottky TENGs, and suggests that the zero external bias DC current is at least in part an electronic flow to neutralize static charges. Further, the sign of the zero-bias current, but not its magnitude, is independent of the semiconductor doping type, which is again suggestive of surface statics being a main contributor to the zero-bias output rather than exclusively a space-charge effect. We also reveal the presence of a junction flexovoltage under pressures common in AFM experiments (GPa), even for negligible lateral friction. In a static Pt–monolayer–n-type Si junction the flexovoltage carries the same sign as the tribovoltage, and can reach such magnitude to overwrite external voltages as high as 2 V. The immediate implication is that the flexovoltage is likely to have i) a strong contribution to the zero-bias output of a n-Si Schottky TENG, ii) a negative effect on the output of a p-Si TENG, and iii) its detection can be straightforward, as we discovered that flexoelectricity manifests as an “inverted diode”: a n-type Si–platinum diode with negligible current even when the n-type material is negatively biased as long as the “static” diode remains under a large normal pressure.

Unambiguous Measurement of Local Hole Current in Organic Semiconductors Using Conductive Atomic Force Microscopy

Conductive atomic force microscopy (C-AFM) is a widely used tool for studying the charge transport properties of organic semiconductor films with nanoscale resolution. Local hole current is commonly measured by electrically contacting the film with a high work function C-AFM probe on top and an underlying electrode coated with a hole transport layer. The two voltage polarities, corresponding to the probe injection and substrate injection of holes, are both found in the C-AFM literature; nevertheless, there has been a lack of consideration about the possible influence of voltage polarity on image contrast and charge transport mechanisms. By analyzing local hole current maps and current–voltage curves for three organic semiconductors (a small molecule and two polymers), we find that probe and substrate injection leads to drastically different hole current maps and charge transport mechanisms. Specifically, the substrate injection of holes exhibits ohmic characteristics at low voltages and space-charge-limited current behavior at elevated voltages. Conversely, the probe injection of holes leads to injection-limited current that is sensitive to the state of the probe–sample interface. These measurements provide a blueprint for ensuring that C-AFM measurements are unambiguously probing the bulk properties of organic semiconductor films.

Disentangling Electronic Transport and Hysteresis at Individual Grain Boundaries in Hybrid Perovskites via Automated Scanning Probe Microscopy

Underlying the rapidly increasing photovoltaic efficiency and stability of metal halide perovskites (MHPs) is the advancement in the understanding of the microstructure of polycrystalline MHP thin film. Over the past decade, intense efforts have been aimed at understanding the effect of microstructures on MHP properties, including chemical heterogeneity, strain disorder, phase impurity, etc. It has been found that grain and grain boundary (GB) are tightly related to lots of microscale and nanoscale behavior in MHP thin films. Atomic force microscopy (AFM) is widely used to observe grain and boundary structures in topography and subsequently to study the correlative surface potential and conductivity of these structures. For now, most AFM measurements have been performed in imaging mode to study the static behavior; in contrast, AFM spectroscopy mode allows us to investigate the dynamic behavior of materials, e.g., conductivity under sweeping voltage. However, a major limitation of AFM spectroscopy measurements is that they require manual operation by human operators, and as such only limited data can be obtained, hindering systematic investigations of these microstructures. In this work, we designed a workflow combining the conductive AFM measurement with a machine learning (ML) algorithm to systematically investigate grain boundaries in MHPs. The trained ML model can extract GBs locations from the topography image, and the workflow drives the AFM probe to each GB location to perform a current-voltage (IV) curve automatically. Then, we are able to have IV curves at all GB locations, allowing us to systematically understand the property of GBs. Using this method, we discovered that the GB junction points are less conductive, potentially more photoactive, and can play critical roles in MHP stability, while most previous works only focused on the difference between GB and grains.

Spin Polarization of Chiral Amorphous Fe‐Ni Electrocatalysts Enabling Efficient Electrochemical Oxygen Evolution

AbstractChiral‐induced spin selectivity is recently demonstrated in a range of spin‐dependent optoelectronics and electrochemistry. Herein, a new type of amorphous chiral tartaric acid‐FeNi coordination polymer fabricated by electrodeposition methods, achieving both high spin‐polarization and high electrocatalytic activity for oxygen evolution, is reported. Circular dichroism shows signature optical activity from the coordination polymer. Conductive atomic force microscopy measurements demonstrate a high spin polarization through the chiral electrocatalyst, which significantly suppresses the formation of hydrogen peroxide byproducts compared to the achiral ones. These chiral Fe‐Ni electrocatalysts exhibit a low overpotential of 205 and 280 mV to achieve 10 and 100 mA cm−2, respectively.

Hydrocarbons in the Meniscus: Effects on Conductive Atomic Force Microscopy.

It is commonly accepted that during conductive atomic force microscopy (CAFM) measurement in ambient, a liquid meniscus can form between the tip and the sample. Such a liquid bridge, normally assumed to be composed of water, is a major factor in analyzing and understanding CAFM results. Here, we show that the adsorption of adventitious hydrocarbons from the air to a surface can greatly affect CAFM data both in imaging mode and in local spectroscopy (current-voltage or I-V curves). We propose a model to explain the phenomena whereby hydrocarbon contaminates contribute to the composition of the liquid bridge between the tip and the sample.

Nanoscale Measurements of Charge Transfer at Cocatalyst/Semiconductor Interfaces in BiVO4 Particle Photocatalysts.

Semiconductor photocatalyst particles convert solar energy to fuels like H2. The particles are often assumed to provide crystalline-facet-dependent electron-hole separation. A common strategy is to deposit a hydrogen evolution reaction (HER) electrocatalyst on electron-selective facets and an oxygen evolution reaction (OER) electrocatalyst on hole-selective facets. A precise understanding of how charge-carrier-selective contacts emerge and how they are rationally designed, however, is missing. Using a combination of ex situ and in situ conducting atomic force microscopy (AFM) experiments and new ionomer/catalyst-semiconductor test structures, we show how heterogeneity in charge-carrier selectivity can be measured at the nanoscale. We discover that the presence of the water/electrolyte interface is critical to induce hole selectivity between the CoOx water-oxidation catalyst and the BiVO4 light absorber. pH-dependent measurements suggest that negative surface charge on the semiconductor is central to inducing hole selectivity. The work also demonstrates a new approach to control local pH and introduce water using thin-film ionomers compatible with conductive AFM measurements.

Enhanced Assembly of Ag Nanoparticles for Surface-Independent Fabrication of Conductive Patterns

Polymer stamping is a reliable and cost-effective method for producing charged patterned surfaces. However, charge stability is limited, and they discharge steadily while immersed in polar solvents. Here, we applied polyelectrolytes as the stamping medium to increase this stability. Charged line patterns were fabricated by pressing a polydimethylsiloxane (PDMS) stamp covered with a polyethylenimine (PEI) solution against silicon, glass, or polystyrene. Then, the substrate was immersed in a solution of oppositely charged silver nanoparticles. Finally, silver crystallization on the deposited nanoparticle agglomerates was performed to homogenize the conductive surface. Fabricated structures were characterized by conductive AFM, SEM, and electrical measurements. Simulations of the electric field above the pattern and electrostatic deposition of nanoparticles were performed. The presented method allows for the production of high-resolution microstructures composed of parallel 45 mm paths with a width of 10 μm and a thickness below 100 nm. A conductivity of 104 S/m is high enough to keep a commercial LED on.

Reduced defect density in crystalline halide perovskite films via methylamine treatment for the application in photodetectors

Considerable efforts have been devoted to optimizing and controlling the morphology and electronic properties of lead halide perovskites. The defect density of a perovskite layer strongly depends on the processing conditions. Consequently, the fabrication process of high-quality films is often complex, and reproducibility is a challenge. In this work, we present a methylamine gas-based method to recrystallize perovskite layers of any given quality in a controlled way, leading to millimeter-sized domains. Crystallinity significantly increases upon methylamine treatment, and crystal growth follows a preferred orientation. Photoluminescence- and space-charge limited current measurements show that the trap density halves after recrystallization. Conductive atomic force microscopy measurements show a higher surface conductivity and an improved spatial homogeneity after methylamine treatment. When applied in photodetectors, the improved film quality of the recrystallized films leads to increased detectivities of ≈4 × 1011 Jones compared to 3 × 109 Jones of a reference device. The response time falls from 0.1 to 10−5 s upon methylamine treatment. Our work, thus, presents a promising route to fabricating reproducible, high-quality perovskite films through well-controllable recrystallization.

Vertical field inhomogeneity associated with threading dislocations in GaN high electron mobility transistor epitaxial stacks

A measurement technique combining Kelvin-probe force microscopy with substrate bias is developed and demonstrated on AlGaN/GaN-on-Si device structures under conditions relevant to the effect of off-state drain bias stress in transistors. For a high substrate bias, the measurements show a significantly lowered surface potential surrounding a small proportion of dislocations imaged with atomic force microscopy (AFM), laterally extending on a scale of up to a micrometer. Both the density and the size of those features increase with substrate bias; however, conductive AFM measurements under the same bias conditions showed no leakage reaching the surface associated with those features. Our model considers localized conductive paths that end a certain distance below the 2D electron gas electrically “thinning” the epitaxy and, therefore, deforming the potential and increasing the electric field under off-state stress bias. The conclusion is that the vertical electric field in the buffer is laterally highly non-uniform with an enhanced vertical field in the vicinity of those dislocations. This non-uniformity redirects the substrate bias stress from the buffer to the channel with potential consequences for breakdown.