Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth methods ...are needed to open the way to applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to electrical, optical, and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the growth of high-quality monolayer MoS2 with control over lattice orientation. We show that the monolayer film is composed of coalescing single islands with limited numbers of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment via van der Waals interaction, we can easily transfer the grown material and fabricate devices. Local potential mapping along channels in field-effect transistors shows that the single-crystal MoS2 grains in our film are well connected, with interfaces that do not degrade the electrical conductivity. This is also confirmed by the relatively large and length-independent mobility in devices with a channel length reaching 80 μm.
Surface potential of biomaterials is a key factor regulating cell responses, driving their adhesion and signaling in tissue regeneration. In this study we compared the surface and zeta potential of ...smooth and porous electrospun polycaprolactone (PCL) fibers, as well as PCL films, to evaluate their significance in bone regeneration. The ’ surface potential of the fibers was controlled by applying positive and negative voltage polarities during the electrospinning. The surface properties of the different PCL fibers and films were measured using X-ray photoelectron spectroscopy (XPS) and Kelvin probe force microscopy (KPFM), and the zeta potential was measured using the electrokinetic technique. The effect of surface potential on the morphology of bone cells was examined using advanced microcopy, including 3D reconstruction based on a scanning electron microscope with a focused ion beam (FIB-SEM). Initial cell adhesion and collagen formation were studied using fluorescence microscopy and Sirius Red assay respectively, while calcium mineralization was confirmed with energy-dispersive x-ray (EDX) and Alzarin Red staining. These studies revealed that cell adhesion is driven by both the surface potential and morphology of PCL fibers. Furthermore, the ability to tune the surface potential of electrospun PCL scaffolds provides an essential electrostatic handle to enhance cell-material interaction and cellular activity, leading to controllable morphological changes.
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•Applied voltage polarity in electrospinning tailors surface chemistry and potential of PCL fibers.•Surface potential was analyzed by KPFM and compared with zeta potential in liquid.•Cell adhesion, collagen formation and mineralization are controlled with surface potential.•Surface potential on PCL fibers can enhance the bone regeneration process.
The full understanding of the mechanisms of hydrogen embrittlement requires the knowledge of the local hydrogen distribution. One of the microstructural features where hydrogen segregates are grain ...boundaries, which is then responsible for the transition from transgranular to intergranular fracture. Furthermore, the type of grain boundary influences the segregated hydrogen and its diffusion rates. A tool to measure local hydrogen contents qualitatively, and maybe quantitatively in the future, is Scanning Kelvin Probe Force Microscopy (SKPFM). In this work, we use SKPFM to detect hydrogen at grain boundaries in situ in a permeation setup. The influence of the operation mode as well as the atmosphere on the Contact Potential Difference VCPD is investigated in an in house atmosphere chamber. The atmosphere has a strong influence on VCPD, especially the relative humidity, which is important if the method is to be used quantitatively in the future. We propose to use low oxygen contents and high relative humidities for these SKPFM measurements.
•In situ setup for measuring hydrogen with Scanning Kelvin Probe Force Microscopy.•Qualitative detection of hydrogen distribution along different grain boundaries.•Measuring the qualitative hydrogen contents under varying atmospheres.•Variation of atmospheric oxygen content and relative humidity.•Relative humidity has huge impact, atmospheric oxygen less so.
One of the central aims of the field of spintronics is the control of individual electron spins to effectively manage the transmission of quantized data. One well-known mechanism for controlling ...electronic spin transport is the chiral-induced spin-selectivity (CISS) effect in which a helical nanostructure imparts a preferential spin orientation on the electronic transport. One potential application of the CISS effect is as a transduction pathway between electronic spin and circularly polarized light within nonreciprocal photonic devices. In this work, we identify and quantify the degree of chiral-induced spin-selective electronic transport in helical polyaniline films using magnetoconductive atomic force microscopy (mcAFM). We then induce circularly polarized quantum light emission from CdSe/CdS core/shell quantum dots placed on these films, demonstrating a degree of circular polarization of up to ∼21%. Utilizing time-resolved photoluminescence microscopy, we measure the radiative lifetime difference associated with left- and right-handed circular polarizations of single emitters. These lifetime differences, in combination with Kelvin probe mapping of the variation of surface potential with magnetization of the substrate, help establish an energy level diagram describing the spin-dependent transport pathways that enable the circularly polarized photoluminescence.
We study light-induced dynamics in thin films comprising Ruddlesden–Popper phases of the layered 2D perovskite (C4H9NH3)2PbI4. We probe ionic and electronic carrier dynamics using two complementary ...scanning probe methods, time-resolved G-mode Kelvin probe force microscopy and fast free time-resolved electrostatic force microscopy, as a function of position, time, and illumination. We show that the average surface photovoltage sign is dominated by the band bending at the buried perovskite–substrate interface. However, the film exhibits substantial variations in the spatial and temporal response of the photovoltage. Under illumination, the photovoltage equilibrates over hundreds of microseconds, a time scale associated with ionic motion and trapped electronic carriers. Surprisingly, we observe that the surface photovoltage of the 2D grain centers evolves more rapidly in time than at the grain boundaries. We propose that the slower evolution at grain boundaries is due to a combination of ion migration occurring between PbI4 planes, as well as electronic carriers traversing grain boundary traps, thereby changing the time-dependent band unbending at grain boundaries. These results provide a model for the photoinduced dynamics in 2D perovskites and are a useful basis for interpreting photovoltage dynamics on hybrid 2D/3D structures.
The phenomenon of contact electrification (CE) has been known for thousands of years, but the nature of the charge carriers and their transfer mechanisms are still under debate. Here, the CE and ...triboelectric charging process are studied for a metal–dielectric case at different thermal conditions by using atomic force microscopy and Kelvin probe force microscopy. The charge transfer process at the nanoscale is found to follow the modified thermionic‐emission model. In particular, the focus here is on the effect of a temperature difference between two contacting materials on the CE. It is revealed that hotter solids tend to receive positive triboelectric charges, while cooler solids tend to be negatively charged, which suggests that the temperature‐difference‐induced charge transfer can be attributed to the thermionic‐emission effect, in which the electrons are thermally excited and transfer from a hotter surface to a cooler one. Further, a thermionic‐emission band‐structure model is proposed to describe the electron transfer between two solids at different temperatures. The findings also suggest that CE can occur between two identical materials owing to the existence of a local temperature difference arising from the nanoscale rubbing of surfaces with different curvatures/roughness.
The effect of temperature on contact electrification (CE) is investigated at the nanoscale. It is found that hotter materials tend to be positively charged while cooler materials tend to be negatively charged in CE. Based on the results, an electron thermal‐emission model is proposed. The temperature effect provides strong evidence for electron transfer in CE.
The development of organic-inorganic hybrid perovskite solar cells requires critical understanding in the charge-carrier behaviors in the perovskite light absorbers and devices. Kelvin probe force ...microscopy (KPFM) has been applied as a powerful tool to probe the electrical potential distribution of perovskite films and devices, providing fundamental insights into their charge-carrier properties. When measuring the material photoresponses, various approaches have been employed to illuminate the samples. Here, we measured the surface potential of the layer in the regular mesoporous structure (CH
NH
PbI
/m-TiO
/c-TiO
/FTO) and inverted planar structure (CH
NH
PbI
/NiO/FTO) devices via KPFM. Effects of two representative illumination methods are compared-illumination from top, and from underneath through the transparent glass substrate. By comparing the variation in surface potential under two illumination methods, the surface potential of the perovskite-absorbing layer in a regular structure is higher than that in the inverted structure. The potential difference in two structures implies that the photogenerated charge carriers are injected to the TiO
electron-transport layer and NiO hole-transport layer, resulting in positive charges and negative charges accumulated in the perovskite-absorbing layer. We demonstrated that the illumination direction has an impact on the surface potential measurement. For the CH
NH
PbI
/TiO
structure, illumination from underneath facilitates a larger potential change. While for the CH
NH
PbI
/NiO structure with insensitive photoresponse in potential change, the illumination direction has a minor effect.
We present a study of the effect of gold nanoparticles (Au NPs) on TiO2 on charge generation and trapping during illumination with photons of energy larger than the substrate band gap. We used a ...novel characterization technique, photoassisted Kelvin probe force microscopy, to study the process at the single Au NP level. We found that the photoinduced electron transfer from TiO2 to the Au NP increases logarithmically with light intensity due to the combined contribution of electron–hole pair generation in the space charge region in the TiO2–air interface and in the metal–semiconductor junction. Our measurements on single particles provide direct evidence for electron trapping that hinders electron–hole recombination, a key factor in the enhancement of photo(electro)catalytic activity.
The critical role of grain boundaries for (CH(NH2)2PbI3)0.85(CH3NH3PbBr3)0.15 perovskite solar cells studied by Kelvin probe force microscopy under bias voltage and illumination is reported. Ion ...migration is enhanced at the grain boundaries. Under illumination, the light‐induced potential causes ion migration leading to a rearranged ion distribution. Such a distribution favors photogenerated charge‐carrier collection at the grain boundaries.
Surface properties of superprotonic (K1-х(NH4)x)3H(SO4)2 (x ≥ 0.57) single crystals and their evolution under humidity were studied by optical polarization microscopy, scanning electron microscopy ...(SEM), and energy-dispersive X-ray spectroscopy (EDXS). Traditional method of atomic force microscopy (AFM) and sophisticated methods such as Kelvin probe force microscopy (KPFM) and scanning capacitance force microscopy (SCFM) were also used. Fresh and aged cleavage surfaces perpendicular and parallel to the c axis of the trigonal superprotonic phase were examined. The effect of air humidity on the surface morphology, surface conductivity, surface potential and surface capacitance was studied. The influence of material composition and surface orientation was considered. During ageing for 430 h, the (001) sample surfaces, initially stepped, were smoothened, surface electric potential changed from negative greater than a hundred of millivolts in absolute value to positive of about 80 mV, and a modified layer containing new crystal phases was formed.
•Superprotonic phases (K1-x(NH4)x)3H(SO4)2 (x ≥ 0.57) are stabilized at room temperature and demonstrate high proton conductivity.•Their surface properties were studied by AFM. C-AFM, KPFM, SCFM, optical and electron microscopy, and structural analysis.•Local CVCs have shown that conductivity is anisotropic and is due to the composition and structure of superprotonic phases.•Based on morphological data, the stability of the surface of samples of different composition to humidity was assessed.•The formation of a modified layer with new phases on the crystal surface during prolonged exposure under humidity is shown.
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