Zhu-Jun Wang

Observing Graphene Grow: Catalyst-Graphene Interactions during Scalable Graphene Growth on Polycrystalline Copper

Complementary in situ X-ray photoelectron spectroscopy (XPS), X-ray diffractometry, and environmental scanning electron microscopy are used to fingerprint the entire graphene chemical vapor deposition process on technologically important polycrystalline Cu catalysts to address the current lack of understanding of the underlying fundamental growth mechanisms and catalyst interactions. Graphene forms directly on metallic Cu during the high-temperature hydrocarbon exposure, whereby an upshift in the binding energies of the corresponding C1s XPS core level signatures is indicative of coupling between the Cu catalyst and the growing graphene. Minor carbon uptake into Cu can under certain conditions manifest itself as carbon precipitation upon cooling. Postgrowth, ambient air exposure even at room temperature decouples the graphene from Cu by (reversible) oxygen intercalation. The importance of these dynamic interactions is discussed for graphene growth, processing, and device integration.

Direct Observation of Graphene Growth and Associated Copper Substrate Dynamics by in Situ Scanning Electron Microscopy

This work highlights the importance of in situ experiments for an improved understanding of graphene growth on copper via metal-catalyzed chemical vapor deposition (CVD). Graphene growth inside the chamber of a modified environmental scanning electron microscope under relevant low-pressure CVD conditions allows visualizing structural dynamics of the active catalyst simultaneously with graphene nucleation and growth in an unparalleled way. It enables the observation of a complete CVD process from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time and nanometer-scale resolution without the need of sample transfer. A strong dependence of surface dynamics such as sublimation and surface premelting on grain orientation is demonstrated, and the influence of substrate dynamics on graphene nucleation and growth is presented. Insights on the growth mechanism are provided by a simultaneous observation of the growth front propagation and nucleation rate. Furthermore, the role of trace amounts of oxygen during growth is discussed and related to graphene-induced surface reconstructions during cooling. Above all, this work demonstrates the potential of the method for in situ studies of surface dynamics on active metal catalysts.

Stacking sequence and interlayer coupling in few-layer graphene revealed by in situ imaging

In the transition from graphene to graphite, the addition of each individual graphene layer modifies the electronic structure and produces a different material with unique properties. Controlled growth of few-layer graphene is therefore of fundamental interest and will provide access to materials with engineered electronic structure. Here we combine isothermal growth and etching experiments with in situ scanning electron microscopy to reveal the stacking sequence and interlayer coupling strength in few-layer graphene. The observed layer-dependent etching rates reveal the relative strength of the graphene–graphene and graphene–substrate interaction and the resulting mode of adlayer growth. Scanning tunnelling microscopy and density functional theory calculations confirm a strong coupling between graphene edge atoms and platinum. Simulated etching confirms that etching can be viewed as reversed growth. This work demonstrates that real-time imaging under controlled atmosphere is a powerful method for designing synthesis protocols for sp2 carbon nanostructures in between graphene and graphite.

A Large-Area Transferable Wide Band Gap 2D Silicon Dioxide Layer

An atomically smooth silica bilayer is transferred from the growth substrate to a new support via mechanical exfoliation at millimeter scale. The atomic structure and morphology are maintained perfectly throughout the process. A simple heating treatment results in complete removal of the transfer medium. Low-energy electron diffraction, Auger electron spectroscopy, scanning tunneling microscopy, and environmental scanning electron microscopy show the success of the transfer steps. Excellent chemical and thermal stability result from the absence of dangling bonds in the film structure. By adding this wide band gap oxide to the toolbox of 2D materials, possibilities for van der Waals heterostructures will be broadened significantly.

In Situ Formation of Crystallographically Oriented Semiconductor Nanowire Arrays via Selective Vaporization for Optoelectronic Applications.

Direct transformation of bulk crystals to single-crystalline crystallographically oriented semiconductor nanowire arrays is presented. Real-time imaging during in situ environmental scanning electron microscopy experiment clearly demonstrates that the nanowire arrays form through a selective vaporization process with respect to the crystallography of wurtzite crystals. Due to the high quality of the prepared semiconductor nanowire arrays, photodetectors constructed from them can present superior optoelectronic performances.

The Dynamics of Active Metal Catalysts Revealed by In-Situ Electron Microscopy

Conventional high-resolution imaging by electron microscopy plays an important role in the structural and compositional analysis of catalysts. However, since the observations are generally performed under vacuum and close to room temperature, the obtained atomistic details concern an equilibrium state that is of limited value when the active state of a catalyst is in the focus of the investigation. Since the early attempts of Ruska in 1942 [1], in situ microscopy has demonstrated its potential and, with the recent availability of commercial tools and instruments, led to a shift of the focus from ultimate spatial resolution towards observation of relevant dynamics. [2]

What Drives Metal-Surface Step Bunching in Graphene Chemical Vapor Deposition?

Compressive strain relaxation of a chemical vapor deposition (CVD) grown graphene overlayer has been considered to be the main driving force behind metal surface step bunching (SB) in CVD graphene growth. Here, by combining theoretical studies with experimental observations, we prove that the SB can occur even in the absence of a compressive strain, is enabled by the rapid diffusion of metal adatoms beneath the graphene and is driven by the release of the bending energy of the graphene overlayer in the vicinity of steps. Based on this new understanding, we explain a number of experimental observations such as the temperature dependence of SB, and how SB depends on the thickness of the graphene film. This study also shows that SB is a general phenomenon that can occur in all substrates covered by films of two-dimensional (2D) materials.

Initiation of Carbon Nanofiber Growth on Polycrystalline Nickel Foam under Low Ethylene Pressure

The initiation of carbon nanofiber (CNF) growth on polycrystalline Ni foam was investigated by using a combination of ex situ and in situ methods, which include SEM, XRD, and Raman spectroscopy. Experiments were performed at a low hydrocarbon partial pressure to slow down the initiation process. Very little to no CNFs were observed on reduced samples, which is caused by diffusion of C to the bulk of the Ni foam. This prevents the formation of Ni3C as a precursor of Ni nanoparticles at a low hydrocarbon partial pressure from acting as active particles for CNF formation. CNF growth was significant on oxidized samples and the initiation was slowed down by using an extremely low ethylene pressure. Ni nanoparticles are able to catalyze CNF growth, provided these are isolated from the Ni bulk by unreduced NiO, which results from the incomplete reduction of the NiO layer.

Growth Dynamics, Stacking Sequence and Interlayer Coupling in Few-Layer Graphene Revealed by in Situ SEM

We have modified the set-up of a conventional scanning electron microscope (SEM) in order to enable the observation of catalyst surface dynamics under controlled atmosphere and temperature. Using this instrument, we perform in situ investigations on chemical vapor deposition (CVD) growth of graphene on different metal catalysts. Since the experiments are performed in the chamber of a microscope, it is possible to observe complete CVD processes starting from substrate annealing through graphene nucleation and growth and, finally, substrate cooling in real time at nanometer-scale resolution without the need of sample transfer. The nucleation and growth of single layer graphene can be investigated at temperatures of up to 1000°C, while at the same time, surface dynamics of the active metal catalyst can be studied. Growth on polycrystalline substrates reveals grain orientation dependent growth dynamics and catalytic activity [1]. Due to the high sensitivity of the secondary electron signal to changes in the work function and charge transfer at the surface, we are able to visualize different degrees of graphene-substrate coupling [2].

Combining in-Situ SEM with High Sensitivity Analytical TEM for Understanding the Degradation of Metallic Interconnects in SOFC

Among the critical components of SOFC, interconnects are coated steel plates. Their main roles are to collect current, separate the gases and to ensure mechanical stability of the stack. The main problem is that at 700-800°C (SOFC working temperature), the chromium present in the steel to prevent its oxidation tends to evaporate and contaminate other parts of the stack [1]. Hence coatings are necessary to protect chromium from evaporation. Several SOFC metallic interconnect coatings are based on cobalt, which is efficient to prevent chromium evaporation and to maintain the contact resistance low after prolonged exposure [2]. The addition of a very thin cerium layer beneath the Co coating seems to reinforce its properties [3]. The oxidation of such coated SOFC FeCr steels, which contain a considerable number of alloy additives, becomes difficult to interpret.