Liquid-Phase Electron Microscopy with Controllable Liquid Thickness.

Liquid-phase electron microscopy (LPEM) is capable of imaging nanostructures and processes in a liquid environment. The spatial resolution achieved with LPEM critically depends on the thickness of the liquid layer surrounding the object of interest. An excessively thick liquid results in broadening of the electron beam and a high background signal that decreases the resolution and contrast of the object in an image. The liquid thickness in a standard liquid cell, consisting of two liquid enclosing membranes separated by spacers, is mainly defined by the deformation of the SiN membrane windows toward the vacuum side, and the effective thickness may differ from the spacer height. Here, we present a method involving a pressure controller setup to balance the pressure difference over the membrane windows, thus manipulating the shape profiles of the used silicon nitride membrane windows. Electron energy loss spectroscopy (EELS) measurements to determine the liquid thickness showed that it is possible to control the thickness precisely during an LPEM experiment by regulating the interior pressure of the liquid cell. We demonstrated atomic resolution on gold nanoparticles and the phase contrast using silica nanoparticles in liquid with controlled thickness.

Strategy for optimizing experimental settings for studying low atomic number colloidal assemblies using liquid phase scanning transmission electron microscopy.

Observing processes of nanoscale materials of low atomic number is possible using liquid phase electron microscopy (LP-EM). However, the achievable spatial resolution (d) is limited by radiation damage. Here, we examine a strategy for optimizing LP-EM experiments based on an analytical model and experimental measurements, and develop a method for quantifying image quality at ultra low electron dose De using scanning transmission electron microscopy (STEM). As experimental test case we study the formation of a colloidal binary system containing 30 nm diameter SiO2 nanoparticles (SiONPs), and 100 nm diameter polystyrene microspheres (PMs). We show that annular dark field (DF) STEM is preferred over bright field (BF) STEM for practical reasons. Precise knowledge of the material's density is crucial for the calculations in order to match experimental data. To calculate the detectability of nano-objects in an image, the Rose criterion for single pixels is expanded to a model of the signal to noise ratio obtained for multiple pixels spanning the image of an object. Using optimized settings, it is possible to visualize the radiation-sensitive, hierarchical low-Z binary structures, and identify both components.

Low-force spectroscopy on graphene membranes by scanning tunneling microscopy.

Two-dimensional atomically flat sheets with a high mechanical flexibility are very attractive as ultrathin membranes but are also inherently challenging for microscopic investigations. We report on a method using Scanning Tunneling Microscopy (STM) under ultra-high vacuum conditions for non-indenting low-force spectroscopy on micrometer-sized freestanding graphene membranes. The method is based on applying quasi-static voltage ramps with active feedback at low tunneling currents and ultimately relies on the attractive electrostatic force between the tip and the membrane. As a result a bulge-test scenario can be established. The convenience and simplicity of the method relies on the fact that the loading force and the membrane deflection detection are both provided simultaneously by the STM. This permits the continuous measurement of the stress-strain relation. Electrostatic forces applied are typically below 1 nN and the membrane deflection is detected at sub-nanometer resolution. Experiments on single-layer graphene membranes with a strain of 0.1% reveal a two-dimensional elastic modulus E2D = 220 N m-1.