Ivan Vlassiouk’s research while affiliated with Oak Ridge National Laboratory and other places

The difficulty of achieving ohmic contacts is a long‐standing challenge for the development and integration of devices based on 2D materials, due to the large mismatch between their electronic properties and those of both traditional metal‐based and van der Waals (vdWs) electrodes. Research has focused primarily on the electronic energy band alignment, while the effects of momentum mismatch on carrier transport across the vdWs gaps are largely neglected. Graphene‐silicon junctions are utilized to demonstrate that electron momentum distribution can dominate the electronic properties of vdWs contacts. By judiciously introducing scattering centers at the interface that provide additional momentum to compensate the momentum mismatch, the junction conductivity is enhanced by more than three orders of magnitude, enabling the formation of high‐quality ohmic contacts. The study establishes the framework for the design of high‐performance ohmic vdWs contacts based on both energy and momentum matching, which can facilitate efficient heterogeneous integration of 2D–3D systems and the development of post‐CMOS architectures.

There has been a great amount of interest in nanopores as the basis for sensors and templates for preparation of biomimetic channels as well as model systems to understand transport...

While hexagonal boron nitride (hBN) has been widely used as a buffer or encapsulation layer for emerging electronic devices, interest in utilizing it for large‐area chemical barrier coating has somewhat faded. A chemical vapor deposition process is reported here for the conformal growth of hBN on large surfaces of various alloys and steels, regardless of their complex shapes. In contrast to the previously reported very limited protection by hBN against corrosion and oxidation, protection of steels against 10% HCl and oxidation resistance at 850 °C in air is demonstrated. Furthermore, an order of magnitude reduction in the friction coefficient of the hBN coated steels is shown. The growth mechanism is revealed in experiments on thin metal films, where the tunable growth of single‐crystal hBN with a selected number of layers is demonstrated. The key distinction of the process is the use of N 2 gas, which gets activated exclusively on the catalyst's surface and eliminates adverse gas‐phase reactions. This rate‐limiting step allowed independent control of activated nitrogen along with boron coming from a solid source (like elemental boron). Using abundant and benign precursors, this approach can be readily adopted for large‐scale hBN synthesis in applications where cost, production volume, and process safety are essential.

Reduced graphene oxide (rGO) has attracted significant attention as an active electrode material for sensors and flexible energy devices due to its high electric conductivity and large surface area. rGO is usually obtained from graphene oxide(GO) through a thermal, chemical, photochemical, and electrochemical reduction process. Compared to the other reduction strategy, laser reduction is a single-step, precise, low cost, and chemical-free processing that can be directly applied on the GO membrane at room temperature in an ambient condition.The laser irradiation parameter, such as laser source, power, speed, and frequency, plays an important role in reduction optimization. In general, the quality of the rGO is not as high as that of pristine graphene because of incomplete reduction, and oxygenated defects involved in the reduction process.The not fully removed oxygenated function group not only influence the sensitivity and selectivity of the electrode, but also influences response time and recovery time during sensing and the electrochemical window of the supercapacitor made with those rGO electrode. This study aims to develop reduced graphene oxide (rGO) through laser irradiation for application in supercapacitors and electrochemical sensors as electrodes. GO membranes were fabricated with a layer-by-layer assembly process first then further reduced through conventional CO2 laser reduction. The laser irradiation parameters including intensity, frequency, and scan speed will be optimized to achieve the reduction up to maximi, C/O ratio. The effects of the reduction will be characterized and evaluated using scanning electron microscope (SEM), Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD)spectroscopy, and four-point probe station. The influence of the humidity on the impedance of the rGO electrode and the capacitance of the planar supercapacitor made with rGO electrode will also be analyzed for potential application of as-prepared rGO electrodes in supercapacitors and humidity sensors.

Selective proton permeation through atomically thin graphene while maintaining impermeability to even small gas atoms i.e. He or hydrated ions, presents potential for advancing proton exchange membranes (PEMs) across a range of energy conversion and storage applications. The incorporation of graphene into state-of-the-art proton conducting polymers e.g. Nafion can enable improvements in PEM selectivity as well as mitigate reactant crossover. The development of facile integration approaches are hence imperative. Here, we systematically study the parameters influencing the integration of monolayer graphene synthesized via scalable chemical vapor deposition (CVD) on polycrystalline Cu foils with a model proton conducting polymer (Nafion) via a facile hot-press process. The hot-press time (t), temperature (T) and pressure (P) are found to not only influence the quality of graphene transfer but can also introduce additional defects in the CVD graphene. Graphene transfers to Nafion performed below the optimum temperature (Topt ∼ 115 °C) remain patchy with ruptures, while transfers above Topt showed defect features, and transfers near Topt show minimal ruptures and defect features. We demonstrate Nafion|graphene|Nafion sandwich membranes using the optimal transfer conditions that allow for ∼50% reduction in hydrogen crossover (∼0.17 mA cm⁻²) in comparison to Nafion control membranes (∼0.33 mA cm⁻²) while maintaining comparable proton area specific resistance < 0.25 Ω cm² (areal conductance ∼ 4–5 S cm⁻²), that are adequate to enable practical PEM applications such as fuel cells, redox flow batteries, and beyond.

The two main technical limitations of direct methanol fuel cells (DMFCs) are the slow kinetic reactions of the methanol oxidation reaction (MOR) in the anode and the crossing over of unreacted methanol through the proton exchange membrane (PEM). It is common practice to use Nafion membranes as PEMs, which have high ion exchange capacity. However, Nafion-based membranes also have high fuel permeability, decreasing fuel utilization and reducing the potential power density. This manuscript focuses on using graphene-coated (Gr-coated) PEMs to reduce fuel crossover. Protons can permeate across graphene and thus it can be employed in various devices as a proton conductive membrane. Here we report efficiency of Gr-coated Nafions. We tested performance and crossover at three different temperatures with four different fuel concentrations and compared to a Nafion PEM that underwent that same test conditions. We found that the adhesion of Gr on to PEMs is not sufficient for prolong fuel cell operation resulting in Gr delamination at high temperatures leading to a higher fuel crossover values compared to lower temperature testing. The results for 7.5M methanol fuel show a reduction of up to 25% in methanol crossover, translating to a peak power density that increases from 3.9 to 9.5 mW/cm2 when using a Gr-Coated PEM compared to a Nafion PEM at 30°C.