Showing papers by "SungWoo Nam published in 2019"

Recent Advances in Graphene Oxide Membranes for Gas Separation Applications

Abstract: Graphene oxide (GO) can dramatically enhance the gas separation performance of membrane technologies beyond the limits of conventional membrane materials in terms of both permeability and selectivity. Graphene oxide membranes can allow extremely high fluxes because of their ultimate thinness and unique layered structure. In addition, their high selectivity is due to the molecular sieving or diffusion effect resulting from their narrow pore size distribution or their unique surface chemistry. In the first part of this review, we briefly discuss different mechanisms of gas transport through membranes, with an emphasis on the proposed mechanisms for gas separation by GO membranes. In the second part, we review the methods for GO membrane preparation and characterization. In the third part, we provide a critical review of the literature on the application of different types of GO membranes for CO2, H2, and hydrocarbon separation. Finally, we provide recommendations for the development of high-performance GO membranes for gas separation applications.

High-Mobility MoS2Directly Grown on Polymer Substrate with Kinetics-Controlled Metal-Organic Chemical Vapor Deposition

Abstract: Batch growth of high-mobility (μFE > 10 cm2V–1s–1) molybdenum disulfide (MoS2) films can be achieved by means of the chemical vapor deposition (CVD) method at high temperatures (>500 °C) on rigid s...

Ultraviolet to Mid-Infrared Emissivity Control by Mechanically Reconfigurable Graphene

Abstract: Spectral emissivity control is critical for optical and thermal management in the ambient environment because solar irradiance and atmospheric transmissions occur at distinct wavelength regions. Fo...

Photonic crystallization of two-dimensional MoS2 for stretchable photodetectors.

Abstract: Low temperature synthesis of high quality two-dimensional (2D) materials directly on flexible substrates remains a fundamental limitation towards scalable realization of robust flexible electronics possessing the unique physical properties of atomically thin structures. Herein, we describe room temperature sputtering of uniform, stoichiometric amorphous MoS2 and subsequent large area (>6.25 cm2) photonic crystallization of 5 nm 2H-MoS2 films in air to enable direct, scalable fabrication of ultrathin 2D photodetectors on stretchable polydimethylsiloxane (PDMS) substrates. The lateral photodetector devices demonstrate an average responsivity of 2.52 μW A−1 and a minimum response time of 120 ms under 515.6 nm illumination. Additionally, the surface wrinkled, or buckled, PDMS substrate with conformal MoS2 retained the photoconductive behavior at tensile strains as high as 5.72% and over 1000 stretching cycles. The results indicate that the photonic crystallization method provides a significant advancement in incorporating high quality semiconducting 2D materials applied directly on polymer substrates for wearable and flexible electronic systems.

Uniaxially crumpled graphene as a platform for guided myotube formation

Abstract: Graphene, owing to its inherent chemical inertness, biocompatibility, and mechanical flexibility, has great potential in guiding cell behaviors such as adhesion and differentiation. However, due to the two-dimensional (2D) nature of graphene, the microfabrication of graphene into micro/nanoscale patterns has been widely adopted for guiding cellular assembly. In this study, we report crumpled graphene, i.e., monolithically defined graphene with a nanoscale wavy surface texture, as a tissue engineering platform that can efficiently promote aligned C2C12 mouse myoblast cell differentiation. We imparted out-of-plane, nanoscale crumpled morphologies to flat graphene via compressive strain-induced deformation. When C2C12 mouse myoblast cells were seeded on the uniaxially crumpled graphene, not only were the alignment and elongation promoted at a single-cell level but also the differentiation and maturation of myotubes were enhanced compared to that on flat graphene. These results demonstrate the utility of the crumpled graphene platform for tissue engineering and regenerative medicine for skeletal muscle tissues. A technique that introduces instabilities onto normally flat graphene can enhance the engineering of muscle fibers. Researchers have recently found that applying cyclic electric stimuli to conductive graphene films promotes the growth of tubular myoblast cells that comprise muscle tissue. SungWoo Nam at the University of Illinois at Urbana-Champaign in the United States and colleagues have now developed a ‘crumpled’ form of graphene to further control the morphology of myoblasts. The team transferred graphene film onto a pre-stretched elastomeric tape, and then gradually released the tape’s tension. Microscopy revealed graphene’s transformation into an accordion-like surface featuring nanoscale ridges and valleys. Comparisons with flat graphene showed the new material improved the assembly of myoblasts into longer myotubes by promoting growth along the direction of the crumpled valleys.

Electrical Double Layer of Supported Atomically Thin Materials.

Abstract: The electrical double layer (EDL), consisting of two parallel layers of opposite charges, is foundational to many interfacial phenomena and unique in atomically thin materials. An important but unanswered question is how the “transparency” of atomically thin materials to their substrates influences the formation of the EDL. Here, we report that the EDL of graphene is directly affected by the surface energy of the underlying substrates. Cyclic voltammetry and electrochemical impedance spectroscopy measurements demonstrate that graphene on hydrophobic substrates exhibits an anomalously low EDL capacitance, much lower than what was previously measured for highly oriented pyrolytic graphite, suggesting disturbance of the EDL (“disordered EDL”) formation due to the substrate-induced hydrophobicity to graphene. Similarly, electrostatic gating using EDL of graphene field-effect transistors shows much lower transconductance levels or even no gating for graphene on hydrophobic substrates, further supporting our hy...

Slippery and Sticky Graphene in Water

Abstract: Understanding modulation of water molecule slippage along graphene surfaces is crucial for many promising applications of two-dimensional materials. Here, we examine normal and shear forces on supported single-layer graphene using atomic force microscopy and find that the electrolyte composition affects the molecular slippage of nanometer thick films of aqueous electrolytes along the graphene surface. In light of the shear-assisted thermally activated theory, water molecules along the graphene plane are very mobile when subjected to shear. However, upon addition of an electrolyte, the cations can make water stick to graphene, while ion-specific and concentration effects are present. Recognizing the tribological and tribochemical utility of graphene, we also evaluate the impact of this behavior on its frictional response in the presence of water. It appears that the addition of an electrolyte to pure water causes a reduction of the thermal activation energy and of the shear-activation length at several concentrations, both results conversely affecting the friction force. Further, this work can inspire innovation in research areas where changes of the molecular slippage through the modulation of the doping characteristics of graphene in liquid environment can be of use, including molecular sensing, lubrication, and energy storage.

Mechanically robust flexible hybrid electrode

Abstract: A mechanically robust flexible hybrid electrode comprises a polymeric substrate, one or more monolayers of a two-dimensional (2D) material on the polymeric substrate, and an electrically conductive film on the 2D material. The mechanically robust flexible hybrid electrode may exhibit a bending strain to failure of at least about 12%. A method of making a flexible hybrid electrode may comprise transferring a monolayer comprising a 2D material to a polymeric substrate. After transferring one or more of the monolayers to the polymeric substrate, an electrically conductive film may be formed on the one or more monolayers, thereby forming a mechanically robust flexible hybrid electrode.

Dynamic Radiative Thermal Management by Crumpled Graphene

Abstract: Systems and surfaces used for aerospace applications require dynamic temperature control for optimal system performance, simultaneously fulfilling the thermal needs for personal human comfort and maintaining equipment functionality, and at the same time avoiding overheating. Radiative cooling is becoming an increasingly attractive method of passive thermal management that utilizes spectral radiation characteristics in the ambient environment. This is achieved by making use of the solar spectrum to heat up surface areas with incident solar radiation (from 200 nm to $2.5\mu\mathrm{m}$ ,, visible to near-infrared wavelengths), and utilizing the atmospheric transmission window (from $8\mu\mathrm{m}$ to $14\mu\mathrm{m}$ , mid-infrared wavelengths) to cool down the surfaces via reemission of the heat to the outer space. The distinct spectral ranges enabling heating and cooling negate the use of conventional materials with fairly uniform high or low emissivity values, and the need for tunable thermoregulation rules out most existing rigid cooling surfaces due to lack of dynamic modulation over emissivity. Here we show selective mid-infrared emissivity control by mechanically reconfigurable graphene, in which mechanical stretching and releasing induces controlled morphology changes of graphene. By integrating graphene with stretchable, elastomeric substrates, we fabricate crumpled graphene with controlled pitch size optimized for radiative cooling at $10\mu\mathrm{m}$ . Our emissivity measurements based on reflectance and Fourier transform infrared spectroscopy, validated with computations based on rigorous coupled wave analysis (RCWA) and finite-difference time-domain (FDTD) methods, demonstrate that the optimized crumpled graphene attains topography-driven high emissivity values at $9.9\mu\mathrm{m}$ and $13\mu\mathrm{m}$ wavelengths. These emissivity variations are attributed to interference between adjacent crumpled features, diffraction at the graphene/air interface, and incoherence of light. The tunability of the crumpled graphene for thermoregulation is demonstrated by mechanical stretching and releasing for controlled modulation of the surface morphology. The results show reversible changes of emissivity over 30 cycles at the wavelength of $9.9\mu\mathrm{m}$ . Our thermal analysis shows that the optimally crumped graphene will achieve a net radiative cooling power of 77 Wm 2 and a surface temperature reduction 7 K below the ambient air. The spectral-selective control of emissivity governs the thermal energy exchange in the ambient environment and enables optimal control of the surface temperature without running electricity or any active components such as bulky heat exchangers. The proposed mechanically tunable crumpled graphene could potentially lead to breakthroughs in aerospace thermal management, especially for surface systems where radiative heat transfer is critical to performance and reliability.

Crumple nanostructuring of atomically thin 2D materials for flexible optoelectronic devices and plasmonic metamaterials

Abstract: Atomically-thin two-dimensional (2D) materials including graphene and transition metal dichalcogenide (TMD) atomic layers (e.g. Molybdenum disulfide, MoS2) are attractive materials for optoelectronic and plasmonic applications and devices due to their exceptional flexural strength led by atomic thickness, broadband optical absorption, and high carrier mobility. Here, we show that crumple nanostructuring of 2D materials allows the enhancement of the outstanding material properties and furthermore enables new, multi-functionalities in mechanical, optoelectronic and plasmonic properties of atomically-thin 2D materials. Crumple nanostructuring of atomically thin materials, graphene and MoS2 atomic layers are used to achieve flexible/stretchable, strain-tunable photodetector devices and plasmonic metamaterials with mechanical reconfigurability. Crumpling of graphene enhances optical absorption by more than an order of magnitude (~12.5 times), enabling enhancement of photoresponsivity by 370% to flat graphene photodetectors and ultrahigh stretchability up to 200%. Furthermore, we present a novel approach to achieve mechanically reconfigurable, strong plasmonic resonances based on crumple-nanostructured graphene. Mechanical reconfiguration of crumple nanostructured graphene allows wide-range tunability of plasmonic resonances from mid- to near-infrared wavelengths. The mechanical reconfigurability can be combined with conventional electrostatic tuning. Our approach of crumple nanostructuring has potential to be applicable for other various 2D materials to achieve strain engineering and mechanical tunability of materials properties. The new functionalities in mechanical, optoelectronic, plasmonic properties created by crumple nanostructuring have potential for emerging flexible electronics and optoelectronics as well as for biosensing technologies and applications.