Alexander Yulaev

Graphene Microcapsule Arrays for Combinatorial Electron Microscopy and Spectroscopy in Liquids

Atomic-scale thickness, molecular impermeability, low atomic number, and mechanical strength make graphene an ideal electron-transparent membrane for material characterization in liquids and gases with scanning electron microscopy and spectroscopy. Here, we present a novel sample platform made of an array of thousands of identical isolated graphene-capped microchannels with high aspect ratio. A combination of a global wide field of view with high resolution local imaging of the array allows for high throughput in situ studies as well as for combinatorial screening of solutions, liquid interfaces, and immersed samples. We demonstrate the capabilities of this platform by studying a pure water sample in comparison with alkali halide solutions, a model electrochemical plating process, and beam-induced crystal growth in liquid electrolyte. Spectroscopic characterization of liquid interfaces and immersed objects with Auger and X-ray fluorescence analysis through the graphene membrane are also demonstrated.

Toward clean suspended CVD graphene

The application of suspended graphene as electron transparent supporting media in electron microscopy, vacuum electronics, and micromechanical devices requires the least destructive and maximally clean transfer from their original growth substrate to the target of interest. Here, we use thermally evaporated anthracene films as the sacrificial layer for graphene transfer onto an arbitrary substrate. We show that clean suspended graphene can be achieved via desorbing the anthracene layer at temperatures in the 100 °C to 150 °C range, followed by two sequential annealing steps for the final cleaning, using Pt catalyst and activated carbon. The cleanliness of the suspended graphene membranes was analyzed employing the high surface sensitivity of low energy scanning electron microscopy and x-ray photoelectron spectroscopy. A quantitative comparison with two other commonly used transfer methods revealed the superiority of the anthracene approach to obtain larger area of clean, suspended CVD graphene. Our graphene transfer method based on anthracene paves the way for integrating cleaner graphene in various types of complex devices, including the ones that are heat and humidity sensitive.

From Microparticles to Nanowires and Back: Radical Transformations in Plated Li Metal Morphology Revealed via in Situ Scanning Electron Microscopy

Li metal is the preferred anode material for all-solid-state Li batteries. However, a stable plating and stripping of Li metal at the anode-solid electrolyte interface remains a significant challenge particularly at practically feasible current densities. This problem usually relates to high and/or inhomogeneous Li-electrode-electrolyte interfacial impedance and formation and growth of high-aspect-ratio dendritic Li deposits at the electrode-electrolyte interface, which eventually shunt the battery. To better understand details of Li metal plating, we use operando electron microscopy and Auger spectroscopy to probe nucleation, growth, and stripping of Li metal during cycling of a model solid-state Li battery as a function of current density and oxygen pressure. We find a linear correlation between the nucleation density of Li clusters and the charging rate in an ultrahigh vacuum, which agrees with a classical nucleation and growth model. Moreover, the trace amount of oxidizing gas (≈10-6 Pa of O2) promotes the Li growth in a form of nanowires due to a fine balance between the ion current density and a growth rate of a thin lithium-oxide shell on the surface of the metallic Li. Interestingly, increasing the partial pressure of O2 to 10-5 Pa resumes Li plating in a form of 3D particles. Our results demonstrate the importance of trace amounts of preexisting or ambient oxidizing species on lithiation processes in solid-state batteries.

Interfacial Electrochemistry in Liquids Probed with Photoemission Electron Microscopy

Studies of the electrified solid-liquid interfaces are crucial for understanding biological and electrochemical systems. Until recently, use of photoemission electron microscopy (PEEM) for such purposes has been hampered by incompatibility of the liquid samples with ultrahigh vacuum environment of the electron optics and detector. Here we demonstrate that the use of ultrathin electron transparent graphene membranes, which can sustain large pressure differentials and act as a working electrode, makes it possible to probe electrochemical reactions in operando in liquid environments with PEEM.

Imaging and Analysis of Encapsulated Objects through Self-Assembled Electron and Optically Transparent Graphene Oxide Membranes

We demonstrate a technique for facile encapsulation and adhesion of micro- and nano objects on arbitrary substrates, stencils, and micro structured surfaces by ultrathin graphene oxide membranes via a simple drop casting of graphene oxide solution. A self-assembled encapsulating membrane forms during the drying process at the liquid-air and liquid-solid interfaces and consists of a water-permeable quasi-2D network of overlapping graphene oxide flakes. Upon drying and interlocking between the flakes, the encapsulating coating around the object becomes mechanically robust, chemically protective, and yet highly transparent to electrons and photons in a wide energy range, enabling microscopic and spectroscopic access to encapsulated objects. The characteristic encapsulation scenarios were demonstrated on a set of representative inorganic and organic micro and nano-objects and microstructured surfaces. Different coating regimes can be achieved by controlling the pH of the supporting solution, and the hydrophobicity and morphology of interfaces. Several specific phenomena such as compression of encased objects by contracting membranes as well as hierarchical encapsulations were observed. Finally, electron as well as optical microscopy and analysis of encapsulated objects along with the membrane effect on the image contrast formation, and signal attenuation are discussed.

Immobilization and encapsulation of micro-and nano-objects with electron transparent graphene oxide membranes

The encapsulation of objects with protective layers is employed in anticorrosive coatings of the metals [1], conformal coatings in microelectronics [2], micro encapsulation of chemicals for drug delivery [3] and etc. In many research and histological practices the microscopy access to the coated sample is desirable. We report here on facile methodology for immobilization and encapsulation of objects at microand nanoscales by means of nanometer thin and electronically transparent graphene oxide (GO) membranes. There are several features that make GO membranes advantageous for microscopy friendly encapsulation: (i) Low Z number for GO and, therefore, its high electron transparency, (ii) facile formation of the GO membranes at liquid-gas-solid interfaces coupled with simple and cost-effective wet protocols; (iii) its mechanical stiffness upon drying [5].

Photonic waveguide to free-space Gaussian beam extreme mode converter

Integration of photonic chips with millimeter-scale atomic, micromechanical, chemical, and biological systems can advance science and enable new miniaturized hybrid devices and technology. Optical interaction via small evanescent volumes restricts performance in applications such as gas spectroscopy, and a general ability to photonically access optical fields in large free-space volumes is desired. However, conventional inverse tapers and grating couplers do not directly scale to create wide, high-quality collimated beams for low-loss diffraction-free propagation over many millimeters in free space, necessitating additional bulky collimating optics and expensive alignment. Here, we develop an extreme mode converter, which is a compact planar photonic structure that efficiently couples a 300 nm × 250 nm silicon nitride high-index single-mode waveguide to a well-collimated near surface-normal Gaussian beam with an ≈160 µm waist, which corresponds to an increase in the modal area by a factor of >105. The beam quality is thoroughly characterized, and propagation over 4 mm in free space and coupling back into a single-mode photonic waveguide with low loss via a separate identical mode converter is demonstrated. To achieve low phase error over a beam area that is >100× larger than that of a typical grating coupler, our approach separates the two-dimensional mode expansion into two sequential separately optimized stages, which create a fully expanded and well-collimated Gaussian slab mode before out-coupling it into free space. Developed at 780 nm for integration with chip-scale atomic vapor cell cavities, our design can be adapted for visible, telecommunication, or other wavelengths. The technique can be expanded to more arbitrary phase and intensity control of both large-diameter, free-space optical beams and wide photonic slab modes.Photonic chips: Converting the lightA procedure for converting light beams between widely differing modes will help fulfill the potential of photonic chips - devices that use light for data processing rather than the electrical circuits of conventional microchips. Developing photonic chip technology requires reliable interconversion between the nanometer-scale light beams traveling within the chips and wider beams that can serve as useful output from or input to the devices. Vladimir Aksyuk and colleagues at the National Institute of Standards and Technology, in Gaithersburg, Maryland, USA, have developed a ‘mode converter’ that interconverts the light beams in a photonic chip and external beams that are more than 100,000 times as wide. This reliable and precisely controlled interconversion should assist the development of wide range of applications for photonic chips, including their use in computation, optical analysis and military technologies.

Combinatorial Microscopy in Liquids with Low Energy Electrons

The modern ambient pressure or in-liquid electron microscopy of energy and catalysis related materials is based on microfluidic/closed cells equipped with a few tens of nanometers thick Si3N4 (or SiO2) windows that are highly transparent for high energy (>100 keV) electrons [1]. Scanning electron microscopy (SEM) with true secondary electrons, Low Voltage SEM [2] or Photoemission Electron Microscopy (PEEM) [3] rely on detection of slow electrons emitted from very few surface layers and therefore have unique surface sensitive contrast mechanisms. However, ca. 1 nm to 3 nm short electron mean free path of the low energy elections in condensed matter makes it impossible to apply these powerful techniques to probe liquid-solid or gas-solid interfaces using the standard Si3N4 membranes. We have recently shown, that this restriction can be lifted by filling of graphene-based membranes as electron transparent molecularly-impermeable windows separating the liquid (or gaseous) sample from the high vacuum of the microscope [4, 5].

SEM and Auger Electron Spectroscopy of Liquid Water through Graphene Membrane

Scanning electron microscopy (SEM) and Auger spectroscopy (AES) are important techniques to characterize the surface morphology and composition of materials. However, their applications for observation and characterization of liquids or aqueous samples is limited due to very short mean free path of low energy electrons in a condense phase or dense gases. Therefore, such experiments are currently conducted at synchrotron radiation facilities using sophisticated differentially pumped instrumentation [1, 2, 3]. Few years ago we proposed to use 2D materials as electron transparent membranes which separate high vacuum environment of the electron spectrometer from a liquid sample [4, 5]. Recently, we developed a new graphene liquid cell platform based on multichannel capillary array (MCA) [6]. An electrochemical version of the graphene liquid cell has been fabricated via coating the back side of the MCA with Pt counter electrode using atomic layer deposition (ALD).

Li Diffusion in All-Solid-State Batteries Imaged Through Optically and Electron Transparent Electrodes | NIST

1. Center for Nanoscale Science and Technology, NIST, Gaithersburg, MD, USA. 2. Department of Materials Science and Engineering, University of Maryland, College Park, MD, USA. 3. Maryland NanoCenter, University of Maryland, College Park, MD, USA. 4. Sandia National Laboratories, Livermore, CA, USA 5. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, USA.