Ultra High Vacuum Transmission Electron Microscopy

By controlling the environment around the sample, the electron microscope becomes a powerful tool for examining processes such as materials growth, catalysis, oxidation, phase transformations and defect formation. This movie shows an example of crystal growth using a nanoscale catalyst. It was recorded in the Environmental TEM at Brookhaven National Laboratory. The hemispherical droplet is a liquid catalyst, AuSi. We see it at work as it converts a reactive gas, disilane, into a solid nanowire made of silicon. Disilane molecules arrive at the droplet surface, stick there and break up. The silicon atoms are released, diffuse through the liquid and end up at the top of the existing Si crystal. Every few seconds a complete layer of Si atoms is added to the nanowire. Measuring the details of this self-assembly process allows us to visualize the pathways by which the atoms add to the nanostructure, develop models for growth, identify new growth modes, and generate strategies for precise control of the growth process.

Video 1: A Si nanowire grows when a gas (disilane, 2×10^-5 torr) reacts at the surface of a liquid catalyst (AuSi) at 550 ^oC. Every few seconds a complete layer of atoms is added

By redesigning the microscope hardware it is possible not just to grow but to measure the properties of individual nanostructures. In this example, nanowires grow between two cantilevers to form a tiny bridge. Both the structure and the electrical transport properties of a single nanowire can be correlated. This detailed view of structure-property relationships helps to design nanostructures with specific electronic or mechanical characteristics

Image 1: In situ UHV experiments on nanowire growth provide guidance for the formation of complex structures through catalyst design (left), information on phase transformations in small volumes (center) and methods for the control of growth using parameters such as electric field (right)

Image 2: Catalytic growth of germanium from gold at 200oC. Both crystal lattices are aligned with the substrate, hexagonal boron nitride

Liquid Cell Transmission Electron Microscopy

Since its development over fifty years ago, transmission electron microscopy was generally used for thin, solid samples: liquid samples were incompatible with the vacuum inside the microscope. But now we can use microfabrication techniques to enable electron microscopy to provide high spatial and temporal resolution in liquids. The messy device shown here provided an early electron microscopic view of a materials growth process in water.

Image 1: Early liquid cell experimental set-up

More recent liquid cells consist of two ultrathin SiN_x membranes that sandwich the liquid when clamped together in a specially designed sample holder. In situ liquid cell experiments can be applied to study a wide range of phenomena. Seeing every step of a reaction gives us a much better chance of understanding what is going on. Our ultimate goal is to understand materials growth and structure in liquids well enough to control the properties of the materials.

Image 2: Schematic of a typical liquid cell enclosure formed from two window chips

Early liquid cell experiments involved measurements of the electrochemical deposition of copper, the process used to fabricate conductive lines in microelectronic circuits. In the image sequence below, copper (dark areas) grows when a voltage is applied and the electrochemical characteristics (here, voltage and current vs. time) are measured simultaneously.

Image 3: Image series recorded during electrochemical deposition of copper at constant potential. The current during deposition is measured simultaneously.

Liquid cell TEM provides a unique window into many other materials reactions. We find electrochemical reactions particularly interesting: batteries as they charge and discharge, corrosion, and growth of compact layers and dendrites. We are also interested in nanobubble dynamics in water and nanoparticle formation from metal ions in solution. To help quantify observations made in the microscope, we also model the effects on the water itself due to irradiation by the high energy electrons.

Image 4: Liquid cell images of copper electrodeposition showing the onset of instability in the growth front, with a “heat map” showing the growth rate at each time and position

Image 5: Nucleation and growth of bubbles over a 2 second time interval. Bubble dynamics are relevant to catalysis, cavitation and corrosion

Project highlight: Metal nanoparticle dissolution

Inside a liquid cell, we can observe both growth and etching of nanoscale materials. We are particularly interested in the dissolution of alloy metal nanoparticles inside the liquid cell. The figure below shows TEM images of several bimetallic nanoparticles that we prepare using a wet chemical synthesis method then transfer into an electrochemical liquid cell. We can selectively etch away one component by tuning the electrochemical potential of the cell. We are also developing new liquid cell chips with additional capabilities which will open up more opportunities for future in situ studies.

Image 6: TEM images of several bimetallic nanoparticles illustrating selective etching

UHV Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) provides key information on the structure and electronic properties of the surfaces of materials. Our UHV STM is a versatile instrument that combines STM with other ways to probe or measure the sample. UHV conditions allow clean surfaces to be prepared. A side chamber enables deposition of metals onto the sample within the same vacuum system. The STM itself is a four-probe system. Scanning tunneling microscopy and spectroscopy can be carried out with any one of the four tips, and another tip can be used to contact samples such as isolated flakes of 2D materials. Furthermore, measurements are possible such as surface potential mapping, where two tips are placed on the sample, current flows between them, and a third tip measures local changes in surface potential. A scanning electron microscope is integrated within the system so that probe positions and voltage contrast can be measured. Finally, a mass-filtered focused ion beam source placed above the sample allows patterning of the surface with ions such as Si⁺, Si²⁺, Au³⁺ and Ge⁺ while under vacuum.

Image 1: Photograph of the interior of the STM chamber showing the geometry of the sample and four probes

Image 2: One probe contacting a flake of a 2D material and a second scanning probe, imaged using the SEM within the STM chamber

Image 3: STM topography of native defects in few-layer MoS2, recorded at constant current and at room temperature. The most common defect is the single S vacancy; the defects that show a larger wave function extent may be charged states of the same defect [J. Klein et al., Appl. Phys. Lett. 115, 261603 (2019)]

Image 4: Defects formed in a thin Si foil by first irradiating the sample with a focused beam of Si²⁺ ions then annealing. The implantation was carried out using the mass-filtered FIB within the STM chamber and the annealing and imaging was carried out subsequently in a TEM