Probe Force Microscopy Measurements Research Articles

Optoelectronic Properties of Chemical Vapor Deposition Grown Monolayer MoS2 Nanowires

Monolayer (1L) MoS2 nanowire possesses large amount of exposed edges, resulting in unique physical properties differing to the large size flake. Preparation of high quality 1L MoS2 nanowire using chemical vapor deposition (CVD) is still challenging to the best of our knowledge. Herein, by employing space-confined CVD method using Mo foil as Mo source, 1L MoS2 nanowires with width of ~70nm can be deposited on the surface of 1L MoS2 flake. Tip-enhanced photoluminescence (TEPL) measurements indicate different PL peak position and intensity distribution across the nanowires. Photosensitive Kelvin probe force microscopy (KPFM) measurements further reveal that these nanowires exhibit lower Fermi level than the 1L MoS2 flake, resulting in tunable optosensitive charge transfer under different excitation photon energies form 1.58eV to 2.33eV.

Grain boundaries are not the source of Urbach tails in Cu(In,Ga)Se2 absorbers

The presence of Urbach tails in Cu(In,Ga)Se2 (CIGSe) absorbers has been identified as a limiting factor for the performance of the CIGSe solar cells. The tail states contribute to both radiative and non-radiative recombination processes, ultimately leading to a reduction in the open-circuit voltage and, consequently, decreasing the overall efficiency of CIGSe devices. Urbach tails result from structural and thermal disorders. The Urbach tails can be characterized by the Urbach energy, which is associated with the magnitude of the tail states. Within polycrystalline CIGSe absorbers, grain boundaries can be considered as structural disorder and, therefore, can potentially contribute to the Urbach tails. In fact, it has been proposed that the band bending at grain boundaries contribute significantly to the tail states. This study focuses on examining the correlation between Urbach tails and the band bending at the grain boundaries. The Urbach energies of the CIGSe samples are extracted from photoluminescence (PL) measurements, which reveal that the introduction of Sodium (Na) into the material can lead to a reduction in the Urbach energy, and an even further decrease can be achieved through the RbF post-deposition treatment. The band bending at the grain boundaries is investigated by Kelvin probe force microscopy measurements. A thorough statistical analysis of more than 340 grain boundaries does not show any correlation between Urbach tails and grain boundaries. We measure small band bending values at the grain boundaries, in the range of the thermal energy (26 meV at room temperature). Furthermore, our intensity dependent PL measurements indicate that Urbach tails are, at least in part, a result of electrostatic potential fluctuations. This supports the model that the introduction of alkali elements mainly decreases the magnitude of electrostatic potential fluctuations, resulting in a subsequent reduction in the Urbach energy.

Optically Triggered Emergent Mesostructures in Monolayer WS2.

The ultrahigh surface area of two-dimensional materials can drive multimodal coupling between optical, electrical, and mechanical properties that leads to emergent dynamical responses not possible in three-dimensional systems. We observed that optical excitation of the WS2 monolayer above the exciton energy creates symmetrically patterned mechanical protrusions which can be controlled by laser intensity and wavelength. This observed photostrictive behavior is attributed to lattice expansion due to the formation of polarons, which are charge carriers dressed by lattice vibrations. Scanning Kelvin probe force microscopy measurements and density functional theory calculations reveal unconventional charge transport properties such as the spatially and optical intensity-dependent conversion in the WS2 monolayer from apparent n- to p-type and the subsequent formation of effective p-n junctions at the boundaries between regions with different defect densities. The strong opto-electrical-mechanical coupling in the WS2 monolayer reveals previously unexplored properties, which can lead to new applications in optically driven ultrathin microactuators.

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.

Light-controlled stateful reconfigurable logic in a carbon dot-based optoelectronic memristor

An optoelectronic memristor can respond to both electrical and optical stimuli, which has tremendous potential to realize light-involved operations. Here, N-doped carbon dots (NCDs)-based optoelectronic memristor devices with reliable resistive switching (RS) characteristics are demonstrated. The devices possess reproducible bipolar RS behavior, good endurance, long retention time, and fast switching speed (<28 ns). Based on the light-controlled charge trapping, the SET voltage can be modulated from 1.5 to 0.2 V by varying the ultraviolet (UV) light intensity. Furthermore, the switching process can be completely triggered by external light when the intensity is more than 10 mW/cm2. The mechanism of charge trapping of the NCDs is verified via Kelvin probe force microscopy measurements. The stateful reconfigurable logic of NAND and AND operations can be achieved through the control of UV light. The results show the feasibility of light-controlled stateful logic based on NCDs memristors for in-memory computing applications.

Boosted triboelectric output performance in g-C3N4 embedded P(VDF-TrFE) composite via a coupling of photocarrier and ferroelectric dipole

Innovative material design is one of the most plausible routes to sufficiently enhance triboelectric outputs for self-powered electronic devices. Herein, we reported a g-C3N4 embedded P(VDF-TrFE) composite as a strong candidate for boosting the output power via simultaneous harvesting of mechanical and light energies. Visible light-responsive g-C3N4 nanosheets are chemically bonded to the ferroelectric P(VDF-TrFE) polymer and increase the ferroelectric polarization and dielectric constant of the composite. Kelvin probe force microscopy and photoluminescence measurements reveal that the enhanced triboelectric output is related to the increased workfunction, the reduced recombination rate of electron-hole pairs, and the band bending-induced photocarrier movement. The up-poled g-C3N4@P(VDF-TrFE) composite-based photoactive triboelectric nanogenerator (PA-TENG) generates a power (power density) of 2.0 mW (0.125 mW/cm2) under a blue light illumination, which is 10 times larger than PA-TENG under a dark condition and 54 times larger than bare P(VDF-TrFE)-based TENG. The output power is continued for at least up to 1.5 months in ambient conditions and is enough to power light-emitting diodes, a thermo-hygrometer, and a digital watch, which would be quite useful for individuals in the dark underground. This work provides a novel approach to boost the triboelectric output via a coupling of photocarrier and ferroelectric dipole and a useful hint to realize self-powered small electronics for safety issues in dark environments like caves and tunnels.

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.

Revealing bactericidal events on graphene oxide nano films deposited on metal implant surfaces.

At the time when pathogens are developing robust resistance to antibiotics, the demand for implant surfaces with microbe-killing capabilities has significantly risen. To achieve this goal, profound understanding of the underlying mechanisms is crucial. Our study demonstrates that graphene oxide (GO) nano films deposited on stainless steel (SS316L) exhibit superior antibacterial features. The physicochemical properties of GO itself play a pivotal role in influencing biological events and their diversity may account for the contradictory results reported elsewhere. However, essential properties of GO coatings, such as oxygen content and the resulting electrical conductivity, have been overlooked so far. We hypothesize that the surface potential and electrical resistance of the oxygen content in the GO-nano films may induce bacteria-killing events on conductive metallic substrates. In our study, the GO applied contains 52 wt% of oxygen, and thus exhibits insulating properties. When deposited as a nano film on an electrically conducting steel substrate, GO flakes generate a Schottky barrier at the interface. This barrier, consequently, impedes the transfer of electrons to the underlying conductive substrate. As a result, this creates reactive oxygen species (ROS), leading to bacterial death. We confirmed the presence of GO coatings and their hydrolytic stability by using X-ray photoelectron spectroscopy (XPS), μRaman spectroscopy, scanning electron microscopy (SEM), and Kelvin probe force microscopy (KPFM) measurements. The biological evaluation was performed on the MG63 osteoblast-like cell line and two selected bacteria species: S. aureus and P. aeruginosa, demonstrating both the cytocompatibility and antibacterial behavior of GO-coated SS316L substrates. We propose a two-step bactericidal mechanism: electron transfer from the bacteria membrane to the substrate, followed by ROS generation. This mechanism finds support in changes observed in contact angle, surface potential, and work function, identified as decisive factors. By addressing overlooked factors and effectively bridging the gap between understanding and practicality, we present a transformative approach for implant surfaces, combating microbial resistance, and offering new application possibilitie.

Corrosion Properties of Quenching and Partitioning-Processed Martensitic Stainless Steels

The increasing demand for lightweight and energy-efficient automobile materials has prompted consideration of the Quenching and Partitioning (Q&P) process for developing martensitic stainless steels with enhanced combinations of strength and ductility. The microstructures of Q&P-processed martensitic stainless steels primarily consist of a matrix of carbon-depleted martensite and films of retained austenite, although some high-carbon fresh martensite may be present.This research aimed to elucidate the influence of chemical composition (manganese content), heat treatment, and phase fractions (martensite and retained austenite) on the corrosion properties of Q&P-processed martensitic stainless steels.The microstructures were investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). Corrosion properties were evaluated through electrochemical measurements in a 3.5% NaCl solution, which included open-circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Scanning Kelvin probe force microscopy (SKPFM) measurements were also conducted.Q&P process ideally minimizes carbide formation, as all carbon atoms are partitioned into austenite. Nevertheless, our research demonstrates that this condition is not always met. Depending on the alloy's chemical composition and the partitioning temperature, carbides and inclusions may still form, favoring localized corrosion such as pitting and intergranular corrosion.

Characterization of band alignment at a metal–MoS2 interface by Kelvin probe force microscopy

Transition metal dichalcogenides, such as MoS2, have garnered considerable attention because of their significant potential in device applications. A limiting factor in their development is the formation of a Schottky barrier with strong Fermi-level pinning at the metal–MoS2 interface. Herein, we report Kelvin probe force microscopy (KPFM) measurements of the work function (WF) modulation at this interface. We found an increase in the WF at the metal–MoS2 interface, depending on the layer number and the contact metal used, indicating the formation of a Schottky barrier. These variations potentially arise from the layer-number-dependent strength of Fermi-level pinning in MoS2. Visualization and calculation of WF modulation at metal–MoS2 interfaces using the KPFM method can help understand the structure and properties of such interfaces.

Distribution of hydrogen atoms at metallurgical microphases of X52 pipeline steel studied by scanning Kelvin probe force microscopy and finite element modelling

The work combined scanning Kelvin probe force microscopy measurements and finite element modelling to study the diffusion and distribution of hydrogen (H) atoms at metallurgical microphases contained in X52 pipeline steel. Results show that the pearlite contained in the steel is more stable than the ferrite during electropolishing, as indicated by the measured topographic profiles and Volta potentials. The hydrogen (H)-charging enhances the electrochemical activity of both pearlite and ferrite, as shown by increased Volta potential and thus the decreased work function. As the H-charging time increases, the Volta potentials of both phases further increase, implying that their activities increase with the H-charging time. The pearlite has a greater Volta potential and thus a lower work function than the ferrite. This is associated with more H atoms accumulating at the pearlite than at the ferrite. The H atom diffusion and accumulation are affected by H diffusivity at phase boundaries, H-trap binding energy and the number of traps in the steel.

Construction of interfacial electric field via Bimetallic Mo2Ti2C3 QDs/g-C3N4 heterojunction achieves efficient photocatalytic hydrogen evolution

Exploiting photocatalysts with high interfacial charge separation efficiency remains a huge challenge for converting solar energy into chemical energy. Herein, a novel 0D/2D heterojunction is successfully constructed by using bimetallic Mo2Ti2C3 MXene Quantum Dots (Mo2Ti2C3 QDs) firmly immobilized on the surface of g-C3N4 nanosheet via an electrostatic self-assembly strategy. The Mo2Ti2C3 QDs/g-C3N4 exhibits an efficient and stable photocatalytic hydrogen production performance up to 2809 µmol g-1h−1, which is 7.96 times higher than pure g-C3N4 nanosheet, and prominently exceeding many reported photocatalysts. Besides, a prominent apparent quantum yield achieves 3.8% at 420 nm. The significant performance improvement derives from the giant interfacial electric field that formed between large interface contact areas, ensuring greatly efficient separation and transfer of the photogenerated carriers. Furthermore, the 0D/2D heterojunction possesses high-quality interfacial contact, which reduces the interfacial recombination of photoinduced electrons and holes, causing the quick electron transfer from the g-C3N4 to electron acceptor Mo2Ti2C3 QDs, thus enhancing the charge utilization. Kelvin probe force microscopy (KPFM) measurements and density functional theory (DFT) calculation comprehensively demonstrate that g-C3N4 modified by Mo2Ti2C3 QDs can modulate the electronic structure and prompt the establishment of the interfacial electric field, which consequently leads to efficient photocatalytic activity. This study adequately illustrates that constructing heterojunction interfacial electric fields based on MXene quantum dots is a prospective pathway to engineering high-performance photocatalytic platforms for solar energy conversion.

Deacetylation-controlled chitin derivative imbedded WPU nanocomposite for highly powered and robust triboelectric nanogenerator

For effective triboelectric nanogenerator, it remains a major challenge to improve the intrinsic electron-donating properties of positive materials. We report a positive triboelectric nanocomposite material based on chitin, a biopolymer, embedded in a waterborne polyurethane matrix to improve the output performance, sustainability, and robustness of triboelectric nanogenerators (TENGs). This nanocomposite can increase the output performance via electron donating control through the deacetylation of chitin nanofibers. For the deacetylation of chitin, the exposed amine group can be an attractive electron donor combined with polyurethane, resulting in a higher output characteristic (210 V and 3.2 μA/cm2), as confirmed by Kelvin probe force microscopy measurements. Moreover, this nanocomposite film showed good sustainability (V/V0 = 85%) in a humid environment (85 RH%) owing to its hydrogen bonding with water molecules. The hybridized film exhibited excellent robustness (100,000 cycles of bending test, long-term stability test over 10,000 s) in two types of harsh mechanical environments. This flexible, robust TENG with a nanocomposite film can serve as a self-powered motion sensor for monitoring various human motions, such as contact pressure and sliding speed. Combined with a homemade sodium-ion potentiometric sensor, stable operation of the dehydration monitoring system was successfully demonstrated under sweating conditions.

Site-specific optical encryption via nanoscale integration of carbon on monolayer WS2

The ability to selectively tune the optical properties of two-dimensional (2D) materials at specific sites is of great interest for the development of optoelectronic and photonic devices. We report site-specific electron-beam (e-beam) induced deposition for creating patterned carbon nanostructures on atomically thin 2D materials such as WS2 and MoS2. Various patterns ranging from microns to nanometres in size have been produced and controlled by adjusting e-beam parameters such as accelerating voltage, spot size, magnification, and irradiation time. The ultimate pattern dimensions of ∼52 nm in width and ∼2.58 nm in height is achieved. The patterned areas exhibit a quenching of fluorescence intensity of about three times, making it simple to identify the created structure from the 2D materials. Moreover, Kelvin probe force microscopy measurements indicated that the surface potential of the patterned sites is roughly 95.6 mV different from that of non-patterned WS2. Finally, we have demonstrated optical encryption by selectively integrating carbon structures to modulate the optical emission of WS2.

Bidirectional Synaptic Phototransistor Based on Two-Dimensional Ferroelectric Semiconductor for Mixed Color Pattern Recognition.

Optoelectronic synaptic devices capable of processing multiwavelength inputs are critical for neuromorphic vision hardware, which remains an important challenge. Here, we develop a bidirectional synaptic phototransistor based on a two-dimensional ferroelectric semiconductor of α-In2Se3, which exhibits bidirectional potentiated and depressed synaptic weight update under optical pulse stimulation. Importantly, the bidirectional optoelectronic synaptic behavior can be extended to multiwavelengths (blue, green, and red light), which could be used for color recognition. The mechanism underlying the bidirectional synaptic characteristics is attributed to the gate-configurable barrier heights as revealed by the Kelvin probe force microscopy measurement. The α-In2Se3 device exhibits versatile synaptic plasticity such as paired-pulse facilitation, short- and long-term potentiation, and long-term depression. The bidirectional optoelectronic synaptic weight updates under multiwavelength inputs enable a high accuracy of 97% for mixed color pattern recognition.

High-performance triboelectric nanogenerators based on TPU/mica nanofiber with enhanced tribo-positivity

As a promising energy harvesting technology, triboelectric nanogenerator (TENG) has brought a leap forward to the development of self-powered systems. Many materials, especially inorganic non-metallic materials, proved to possess excellent triboelectric property, have rarely been practically used due to their fragility and rigidity. Here, fragile and rigid mica with strong triboelectric positivity is exfoliated into 2D nanosheets and electrospun into flexible and stretchable thermoplastic polyurethane (TPU) nanofibers for high-performance TENGs. Paired with polyvinylidene fluoride/MXene (PVDF/MXene) nanofibers, the transferred charge of the TENG with TPU/mica nanofibers significantly enhanced to 82.4 nC from 38.6 nC due to the enhanced tribo-positivity. Kelvin probe force microscopy (KPFM) measurements of single pure TPU and TPU/mica nanofibers showed mica apparently enhanced the positive electrostatic surface potential (ESP) of TPU. The mean peak ESP of pure TPU nanofiber was about 194 mV, while it was increased to 218 mV on the regions of TPU/mica nanofiber without mica nanosheets aggregation and 305 mV on the regions where mica nanosheets aggregated. The power density of the TENG with TPU/mica nanofiber reached 1458 mW/m2, exhibiting a 16-fold enhancement compared with the one based on pure TPU nanofiber. A thin and flexible TENG was fabricated and conformally worn on a wrist and palm for body motion monition and object gripping sorting. This work proposed high-performance triboelectric nanogenerators based on TPU/mica nanofibers and a general approach to effectively utilize rigid and fragile materials with excellent triboelectric property for making triboelectric layers.

CuI Encapsulated within Single‐Walled Carbon Nanotube Networks with High Current Carrying Capacity and Excellent Conductivity

AbstractHigh current carrying capacity and high conductivity are two important indicators for materials used in microscale electronics and inverters. However, it is challenging to obtain high conductivity and high current carrying capacity at the same time since high conductivity requires a weakly bonded system to provide free electrons, while high current carrying capacity requires a strongly bonded system. In this paper, CuI@SWCNT networks by filling the single‐walled carbon nanotubes (SWCNTs) with CuI is ingeniously prepared. CuI@SWCNT shows good stability due to the confinement protection of SWCNTs. Through the host‐guest hybridization, CuI@SWCNT networks exhibit a current carrying capacity of 2.04 × 107 A cm−2 and a conductivity of 31.67 kS m−1. Their current carrying capacity and conductivity are significantly improved compared with SWCNT. The Kelvin probe force microscopy measurements show a drop of surface potential energy after SWCNT filled with CuI, indicating that the CuI guest molecules regulate the position of the Fermi level of SWCNTs, increasing carrier concentration, achieving high conductivity and high current carrying capacity. This study offers ideas and solutions for the regulation of high‐performance carbon tube networks, which hold great promise for future applications in carbon‐based electronic devices.

High performance composition-tailored PVDF triboelectric nanogenerator enabled by low temperature-induced phase transition

Polyvinylidene difluoride (PVDF) polymers are widely used in triboelectric nanogenerators (TENGs) due to their excellent dielectric and piezoelectric properties. To design a high-performance TENG, it is essential to investigate the individual contributions of these properties on contact electrification (CE). In this work, a range of PVDF, co- and terpolymers are used to assess the distinct contributions of dielectric and piezoelectric properties on the TENG’s output. As elucidated by a modified overlapped electron cloud model (OEC) supported with Kelvin Probe Force Microscopy (KPFM) measurements, the high dielectric constant in terpolymers hinders electron backflow after CE, while piezoelectric polarization in β-phase PVDF shifts its electron energy levels before CE. Notably, in PVDF polymers, the dielectric and piezoelectric enhancements to TENGs come at the expense of one another. To verify this phenomenon, the terpolymers are cooled to alter their phases from relaxor to regular ferroelectrics, diminishing their dielectric constants while increasing their piezoelectric polarizations. Initially, the output is high (10.2 W/m2) at room temperature during the relaxor ferroelectric phase which is due to the high dielectric constant. Upon cooling, the output decreased to its lowest during the phase transition (4.3 W/m2 at 0 °C), as a result of the tradeoff between dielectric and piezoelectric enhancement. When the temperature was further tuned down to − 40 °C, the output reaches the maximum of 16 W/m2, which is attributed mainly to the ferroelectric phase. For the first time, the phase transition is observed and evidenced by electric field-induced dielectric and ferroelectric measurements, proving the piezoelectric polarization of PVDF terpolymers. These findings therefore provide guidelines on material selection and pave the way for potential low-temperature TENG applications.

Near-Infrared-Responsive Photocatalytic CO2 Conversion via In Situ Generated Co3O4/Cu2O.

Photocatalytic CO2 conversion to fuels is a promising strategy for achieving global carbon neutrality. However, infrared light, which accounts for ∼50% of the full sunlight spectrum, has not yet been effectively utilized via photocatalysis. Here, we present an approach to directly power photocatalytic CO2 reduction using near-infrared light. This near-infrared light-responsive process occurs on an in situ generated Co3O4/Cu2O photocatalyst with a nanobranch structure. Photoassisted Kelvin probe force microscopy and relative photocatalytic measurements demonstrate the increase of surface photovoltage after illumination by near-infrared light. We also find that Cu(I) on this in situ generated Co3O4/Cu2O could facilitate the formation of a *CHO intermediate, thus enabling a high-performance CH4 production with a yield of 6.5 μmol/h and a selectivity of 99%. Moreover, we perform a practically oriented direct solar-driven photocatalytic CO2 reduction under concentrated sunlight and achieve a fuel yield of 12.5 μmol/h.

Effectively enhanced piezocatalytic activity in flower-like 2H-MoS2 with tunable S vacancy towards organic pollutant degradation

Piezocatalysis technology is emerging as a promising strategy for removing organic pollutants from water by harvesting mechanical energy from the surrounding environment. Herein, 2H-MoS2 flower-like nanosheets with tunable S vacancies was synthesized via a hydrothermal method by adjusting pH of the precursor solution, Mo and S molar ratio, reaction time and temperature. The results showed that the presence of S vacancy leads to significantly enhanced piezoelectric response in the MoS2 nanosheets, as confirmed by both theoretical calculation using first-principle based density functional theory and probe force microscopy (PFM) measurements. As a result, the as-obtained MoS2 nanoflower piezocatalyst exhibited excellent degradation efficiency for Orange Ⅱ solution (removal rate > 95 % in 1 h) under ultrasonic vibration (40 kHz and 110 W). Importantly, we demonstrated for the first time that the piezocatalytic performance was tightly related with the water level in ultrasonic cleaner. The present work not only reports a simple and effective strategy for improving the piezocatalytic degradation activity of MoS2 by introducing vacancy, but also provides new insights and deeper understanding on the underlying mechanism.