The Impact of Functionalization on the Stability, Work Function, and Photoluminescence of Reduced Graphene Oxide

Reduced graphene oxide (rGO) is a promising material for a variety of thin-film optoelectronic applications. Two main barriers to its widespread use are the lack of (1) fabrication protocols leading to tailored functionalization of the graphene sheet with oxygen-containing chemical groups, and (2) understanding of the impact of such functional groups on the stability and on the optical and electronic properties of rGO. We carry out classical molecular dynamics and density functional theory calculations on a large set of realistic rGO structures to decompose the effects of different functional groups on the stability, work function, and photoluminescence. Our calculations indicate the metastable nature of carbonyl-rich rGO and its favorable transformation to hydroxyl-rich rGO at room temperature <i>via</i> carbonyl-to-hydroxyl conversion reactions near carbon vacancies and holes. We demonstrate a significant tunability in the work function of rGO up to 2.5 eV by altering the composition of oxygen-containing functional groups for a fixed oxygen concentration, and of the photoluminescence emission by modulating the fraction of epoxy and carbonyl groups. Taken together, our results guide the application of tailored rGO structures in devices for optoelectronics and renewable energy

Graphene Oxide as a Promising Hole Injection Layer for MoS₂‑Based Electronic Devices

The excellent physical and semiconducting properties of transition metal dichalcogenide (TMDC) monolayers make them promising materials for many applications. The TMDC monolayer MoS₂ has gained significant attention as a channel material for next-generation transistors. However, while n-type single-layer MoS₂ devices can be made with relative ease, fabrication of p-type transistors remains a challenge as the Fermi-level of elemental metals used as contacts are pinned close to the conduction band leading to large p-type Schottky barrier heights (SBH). Here, we propose the utilization of graphene oxide (GO) as an efficient hole injection layer for single-layer MoS₂-based electronic and optoelectronic devices. Using first-principles computations, we demonstrate that GO forms a p-type contact with monolayer MoS₂, and that the p-type SBH can be made smaller by increasing the oxygen concentration and the fraction of epoxy functional groups in GO. Our analysis shows that this is possible due to the high work function of GO and the relatively weak Fermi-level pinning at the MoS₂/GO interfaces compared to traditional MoS₂/metal systems (common metals are Ag, Al, Au, Ir, Pd, Pt). The combination of easy-to-fabricate and inexpensive GO with MoS₂ could be promising for the development of hybrid all-2D p-type electronic and optoelectronic devices on flexible substrates

Nanocarbon-Based Photovoltaics

Carbon materials are excellent candidates for photovoltaic solar cells: they are Earth-abundant, possess high optical absorption, and maintain superior thermal and photostability. Here we report on solar cells with active layers made solely of carbon nanomaterials that present the same advantages of conjugated polymer-based solar cells, namely, solution processable, potentially flexible, and chemically tunable, but with increased photostability and the possibility to revert photodegradation. The device active layer composition is optimized using <i>ab initio</i> density functional theory calculations to predict type-II band alignment and Schottky barrier formation. The best device fabricated is composed of PC₇₀BM fullerene, semiconducting single-walled carbon nanotubes, and reduced graphene oxide. This active-layer composition achieves a power conversion efficiency of 1.3%a record for solar cells based on carbon as the active materialand we calculate efficiency limits of up to 13% for the devices fabricated in this work, comparable to those predicted for polymer solar cells employing PCBM as the acceptor. There is great promise for improving carbon-based solar cells considering the novelty of this type of device, the high photostability, and the availability of a large number of carbon materials with yet untapped potential for photovoltaics. Our results indicate a new strategy for efficient carbon-based, solution-processable, thin film, photostable solar cells

Controlling Platinum Active Sites on Silver Nanoparticles for Hydrogen Evolution Reaction

Growing Pt on Ag nanoparticles is a promising approach to forming catalysts that present active sites with both Pt and Ag available to improve the activity for the hydrogen evolution reaction (HER). By carefully controlling the concentration of a Pt precursor, the amount of Pt-decorated particles onto the Ag nanoparticles could be controlled to grow Pt islands between 0.6 and 1.5 nm. As a result, the relative amounts of the Ag–Pt active sites could be tuned. The smallest, 0.6 nm Pt islands on the Ag nanoparticle, with the highest ratio of Ag–Pt to Pt–Pt sites was found to have the highest activity and an accelerated Volmer step. DFT modeling showed that the improved performance was due to increased electron density on Pt from electron donation from neighboring Ag that leads to weaker Pt–H binding and the presence of more oxophilic Ag that can bind −OH on the active site that accelerates the water splitting

Enhanced Osteogenic Differentiation of Stem Cells on Phase-Engineered Graphene Oxide

Graphene oxide (GO) has attracted significant interest as a template material for multiple applications due to its two-dimensional nature and established functionalization chemistries. However, for applications toward stem cell culture and differentiation, GO is often reduced to form reduced graphene oxide, resulting in a loss of oxygen content. Here, we induce a phase transformation in GO and demonstrate its benefits for enhanced stem cell culture and differentiation while conserving the oxygen content. The transformation results in the clustering of oxygen atoms on the GO surface, which greatly improves its ability toward substance adherence and results in enhanced differentiation of human mesenchymal stem cells toward the osteogenic lineage. Moreover, the conjugating ability of modified GO strengthened, which was examined by auxiliary osteogenic growth peptide conjugation. Overall, our work demonstrates GO’s potential for stem cell applications while maintaining its oxygen content, which could enable further functionalization and fabrication of novel nano-biointerfaces

Enhanced Cell Capture on Functionalized Graphene Oxide Nanosheets through Oxygen Clustering

With the global rise in incidence of cancer and infectious diseases, there is a need for the development of techniques to diagnose, treat, and monitor these conditions. The ability to efficiently capture and isolate cells and other biomolecules from peripheral whole blood for downstream analyses is a necessary requirement. Graphene oxide (GO) is an attractive template nanomaterial for such biosensing applications. Favorable properties include its two-dimensional architecture and wide range of functionalization chemistries, offering significant potential to tailor affinity toward aromatic functional groups expressed in biomolecules of interest. However, a limitation of current techniques is that as-synthesized GO nanosheets are used directly in sensing applications, and the benefits of their structural modification on the device performance have remained unexplored. Here, we report a microfluidic-free, sensitive, planar device on treated GO substrates to enable quick and efficient capture of Class-II MHC-positive cells from murine whole blood. We achieve this by using a mild thermal annealing treatment on the GO substrates, which drives a phase transformation through oxygen clustering. Using a combination of experimental observations and MD simulations, we demonstrate that this process leads to improved reactivity and density of functionalization of cell capture agents, resulting in an enhanced cell capture efficiency of 92 ± 7% at room temperature, almost double the efficiency afforded by devices made using as-synthesized GO (54 ± 3%). Our work highlights a scalable, cost-effective, general approach to improve the functionalization of GO, which creates diverse opportunities for various next-generation device applications

Interface-Controlled Phase Separation of Liquid Metal-Based Eutectic Ternary Alloys

Liquid metals (LMs) are immiscible in many common electrolytic solutions and, when immersed in them, establish phase boundaries that display intriguing interfacial characteristics. The application of a cathodic potential to such interfaces may trigger phase separation of solute elements out of the LMs. Here, we investigate this possibility in two of the most researched and industrially used eutectic ternary LMs of Galinstan (Ga-In-Sn) and Field’s metal (FM, In–Bi–Sn). We observe that upon surface perturbation by an applied electric potential, solute elements compete to segregate out of the LM alloys according to their energy levels. The nature of the electrolytic solutions plays a key role in the separation process as they dictate whether solute metals are expelled selectively in their pure form or as binary compounds. For example, in a phosphate-based aqueous electrolyte, nano-sized Sn-based entities are selectively expelled from Galinstan, while only Bi-based structures leave the surface of FM. In contrast, in a non-aqueous electrolyte, nano-sized binary compounds of Sn–In and Bi–Sn are separated from the surfaces of Galinstan and FM, respectively. We show that selectivity in the surface separation process, achieved by the alteration of the electrolytic solutions, is due to the interplay between the electrodynamic interactions and the electrocapillary effect. This study presents two key findings: (a) it is essential to carefully consider the possibility of component separation in electrochemical systems based on LMs and (b) it demonstrates interfacial metallurgical pathways to process alloys for refining metals into specific purities, component ratios, and dimensions

Liquid-Metal Solvents for Designing Hierarchical Nanoporous Metals at Low Temperatures

Metallic nanoarchitectures hold immense value as functional materials across diverse applications. However, major challenges lie in effectively engineering their hierarchical porosity while achieving scalable fabrication at low processing temperatures. Here we present a liquid-metal solvent-based method for the nanoarchitecting and transformation of solid metals. This was achieved by reacting liquid gallium with solid metals to form crystalline entities. Nanoporous features were then created by selectively removing the less noble and comparatively softer gallium from the intermetallic crystals. By controlling the crystal growth and dealloying conditions, we realized the effective tuning of the micro-/nanoscale porosities. Proof-of-concept examples were shown by applying liquid gallium to solid copper, silver, gold, palladium, and platinum, while the strategy can be extended to a wider range of metals. This metallic-solvent-based route enables low-temperature fabrication of metallic nanoarchitectures with tailored porosity. By demonstrating large-surface-area and scalable hierarchical nanoporous metals, our work addresses the pressing demand for these materials in various sectors