Lateral Force Microscopy of Interfacial Nanobubbles: Friction Reduction and Novel Frictional Behavior.

Atomic force microscopy is used to conduct single-asperity friction measurements at a water-graphite interface. Local mapping of the frictional force, which is based on the degree of the cantilever twisting, shows nearly friction-free when a tip scans over a nanobubble. Surprisingly, apart from being gapless, the associated friction loop exhibits a tilt in the cantilever twisting versus the tip's lateral displacement with the slope depending on the loading force. The sign of the slope reverses at around zero loading force. In addition, the measured normal and lateral tip-sample interactions exhibit unison versus tip-sample separation. Theoretical analysis, based on the balance of forces on the tip originated from the capillary force of the nanobubble and the torsion of the cantilever, offers quantitative explanations for both the tilted friction loop and the unison of force curves. The analysis may well apply in a wider context to the lateral force characterization on cap-shaped fluid structures such as liquid droplets on a solid substrate. This study further points to a new direction for friction reduction between solids in a liquid medium.

High-Resolution Characterization of Preferential Gas Adsorption at the Graphene-Water Interface.

The contact of water with graphene is of fundamental importance and of great interest for numerous promising applications, but how graphene interacts with water remains unclear. Here we used atomic force microscopy (AFM) to investigate hydrophilic mica substrates with some regions covered by mechanically exfoliated graphene layers in water. In water containing air gas close to the saturation concentration (within ∼40%), cap-shaped nanostructures (or interfacial nanobubbles) and ordered-stripe domains were observed on graphene-covered regions but not on pure mica regions. These structures did not appear on graphene when samples were immersed in highly degassed water, indicating that their formation was caused by the adsorption of gas dissolved in water. Thus, atomically thin graphene, even at a narrow width of 20 nm, changes the local surface chemistry of a highly hydrophilic substrate. Furthermore, surface hydrophobicity significantly affects gas adsorption, which has broad implications for diverse phenomena in water.

Nucleation processes of nanobubbles at a solid/water interface.

Experimental investigations of hydrophobic/water interfaces often return controversial results, possibly due to the unknown role of gas accumulation at the interfaces. Here, during advanced atomic force microscopy of the initial evolution of gas-containing structures at a highly ordered pyrolytic graphite/water interface, a fluid phase first appeared as a circular wetting layer ~0.3 nm in thickness and was later transformed into a cap-shaped nanostructure (an interfacial nanobubble). Two-dimensional ordered domains were nucleated and grew over time outside or at the perimeter of the fluid regions, eventually confining growth of the fluid regions to the vertical direction. We determined that interfacial nanobubbles and fluid layers have very similar mechanical properties, suggesting low interfacial tension with water and a liquid-like nature, explaining their high stability and their roles in boundary slip and bubble nucleation. These ordered domains may be the interfacial hydrophilic gas hydrates and/or the long-sought chemical surface heterogeneities responsible for contact line pinning and contact angle hysteresis. The gradual nucleation and growth of hydrophilic ordered domains renders the original homogeneous hydrophobic/water interface more heterogeneous over time, which would have great consequence for interfacial properties that affect diverse phenomena, including interactions in water, chemical reactions, and the self-assembly and function of biological molecules.

Mobile Charge-Induced Fluorescence Intermittency in Methylammonium Lead Bromide Perovskite.

Organic-inorganic halide perovskite has emerged as a very promising material for solar cells due to its excellent photovoltaic enabling properties resulting in rapid increase in device efficiency over the last 3 years. Extensive knowledge and in-depth physical understanding in the excited state carrier dynamics are urgently required. Here we investigate the fluorescence intermittency (also known as blinking) in vapor-assisted fabricated CH3NH3PbBr3 perovskite. The evident fluorescence blinking is observed in a dense CH3NH3PbBr3 perovskite film that is composed of nanoparticles in close contact with each other. In the case of an isolated nanoparticle no fluorescence blinking is observed. The "ON" probability of fluorescence is dependent on the excitation intensity and exhibits a similar power rule to semiconductor quantum dots at higher excitation intensity. As the vapor-assisted fabricated CH3NH3PbBr3 perovskite film is a cluster of nanoparticles forming a dense film, it facilitates mobile charge migration between the nanoparticles and charge accumulation at the surface or at the boundary of the nanoparticles. This leads to enhanced Auger-like nonradiative recombination contributing to the fluorescence intermittency observed. This finding provides unique insight into the charge accumulation and migration and thus is of crucial importance for device design and improvement.

Interface-induced ordering of gas molecules confined in a small space.

The thermodynamic properties of gases have been understood primarily through phase diagrams of bulk gases. However, observations of gases confined in a nanometer space have posed a challenge to the principles of classical thermodynamics. Here, we investigated interfacial structures comprising either O2 or N2 between water and a hydrophobic solid surface by using advanced atomic force microscopy techniques. Ordered epitaxial layers and cap-shaped nanostructures were observed. In addition, pancake-shaped disordered layers that had grown on top of the epitaxial base layers were observed in oxygen-supersaturated water. We propose that hydrophobic solid surfaces provide low-chemical-potential sites at which gas molecules dissolved in water can be adsorbed. The structures are further stabilized by interfacial water. Here we show that gas molecules can agglomerate into a condensed form when confined in a sufficiently small space under ambient conditions. The crystalline solid surface may even induce a solid-gas state when the gas-substrate interaction is significantly stronger than the gas-gas interaction. The ordering and thermodynamic properties of the confined gases are determined primarily according to interfacial interactions.

Probing and controlling fluorescence blinking of single semiconductor nanoparticles.

In this review we present an overview of the experimental and theoretical development on fluorescence intermittency (blinking) and the roles of electron transfer in semiconductor crystalline nanoparticles. Blinking is a very interesting phenomenon commonly observed in single molecule/particle experiments. Under continuous laser illumination, the fluorescence time trace of these single nanoparticles exhibit random light and dark periods. Since its first observation in the mid-1990s, this intriguing phenomenon has attracted wide attention among researchers from many disciplines. We will first present the historical background of the discovery and the observation of unusual inverse power-law dependence for the waiting time distributions of light and dark periods. Then, we will describe our theoretical modeling efforts to elucidate the causes for the power-law behavior, to probe the roles of electron transfer in blinking, and eventually to control blinking and to achieve complete suppression of the blinking, which is an annoying feature in many applications of quantum dots as light sources and fluorescence labels for biomedical imaging.