Glassy Graphene Research Articles

Atomistic insights into the effect of cooling rates on mechanical properties of glassy graphene

Glassy graphene is obtained via molecular dynamics simulation, and we investigated the effect of cooling rates on the forming and mechanical properties of glassy graphene (GG). The system interactions are modeled using the adaptive intermolecular reactive bond order (AIREBO) potential. The results of our simulations are in good agreement with experimental ones in the case of crystalline graphene. We calculated the radial distribution function (RDF) to characterize the structural properties of the forming glass and used the Wendt-Abraham parameter to determine the glass temperature transition (Tg). As a result, we found that as the cooling rate increased, the rigidity of the glassy graphene decreased. The calculated Young's modulus equals 70.33 GPa, 29.76 GPa, 12.42 GPa, and 2.85 GPa for cooling rates equal to 0.1 K/ps, 1 K/ps, 10 K/ps, and 100 K/ps, respectively.

Multifunctional two-dimensional glassy graphene devices for vis-NIR photodetection and volatile organic compound sensing

Multifunctional devices are of great interest for integration and miniaturization on the same platform, but simple addition of functionalities would lead to excessively large devices. Here, the photodetection and chemical sensing device is developed based on two-dimensional (2D) glassy-graphene that meets similar property requirements for the two functionalities. An appropriate bandgap arising from the distorted lattice structure enables glassy graphene to exhibit comparable or even improved photodetection and chemical sensing capability, compared with pristine graphene. Due to strong interactions between glassy graphene and the ambient atmosphere, the devices are less sensitive to photoinduced desorption than the ones based on graphene. Consequently, the few-layer glassy graphene device delivers positive photoresponse, with a responsivity of 0.22 A W−1 and specific detectivity reaching ~1010 Jones under 405 nm illumination. Moreover, the intrinsic defects and strain in glassy graphene can enhance the adsorption of analytes, leading to high chemical sensing performance. Specifically, the extracted signal-to-noise-ratio of the glassy graphene device for detecting acetone is 48, representing more than 50% improvement over the device based on graphene. Additionally, bias-voltage- and thickness-dependent volatile organic compound (VOC) sensing features are identified, indicating the few-layer glassy graphene is more sensitive. This study successfully demonstrates the potential of glassy graphene for integrated photodetection and chemical sensing, providing a promising solution for multifunctional applications further beyond.

Graphene Oxide-Based Silico-Phosphate Composite Films for Optical Limiting of Ultrashort Near-Infrared Laser Pulses.

The development of graphene-based materials for optical limiting functionality is an active field of research. Optical limiting for femtosecond laser pulses in the infrared-B (IR-B) (1.4–3 μm) spectral domain has been investigated to a lesser extent than that for nanosecond, picosecond and femtosecond laser pulses at wavelengths up to 1.1 μm. Novel nonlinear optical materials, glassy graphene oxide (GO)-based silico-phosphate composites, were prepared, for the first time to our knowledge, by a convenient and low cost sol-gel method, as described in the paper, using tetraethyl orthosilicate (TEOS), H3PO4 and GO/reduced GO (rGO) as precursors. The characterisation of the GO/rGO silico-phosphate composite films was performed by spectroscopy (Fourier-transform infrared (FTIR), Ultraviolet–Visible-Near Infrared (UV-VIS-NIR) and Raman) and microscopy (atomic force microscopy (AFM) and scanning electron microscopy (SEM)) techniques. H3PO4 was found to reduce the rGO dispersed in the precursor’s solution with the formation of vertically agglomerated rGO sheets, uniformly distributed on the substrate surface. The capability of these novel graphene oxide-based materials for the optical limiting of femtosecond laser pulses at 1550 nm wavelength was demonstrated by intensity-scan experiments. The GO or rGO presence in the film, their concentrations, the composite films glassy matrix, and the film substrate influence the optical limiting performance of these novel materials and are discussed accordingly.

Structural relaxation time and dynamic shear modulus of glassy graphene

We theoretically investigate glass transition behaviors of the glassy graphene in a wide range of temperature, where this amorphous graphene is described as a hard-sphere fluid. The dynamic arrest of a particle is assumingly caused by interactions with its nearest neighbors and surrounding fluid particles. The assumption allows us to analyze roles of local and collective particle mobility. We calculate the temperature dependence of structural relaxation time and dynamic shear modulus, the dynamic fragility, and the glass transition temperature. In addition, correlations between these physical quantities are comprehensively discussed. Our theoretical calculations agree quantitatively well with recent simulations and Dyre’s shoving model.

Evidence of a two-dimensional glass transition in graphene: Insights from molecular simulations

Liquids exhibit a sudden increase in viscosity when cooled fast enough, avoiding thermodynamically predicted route of crystallization. This phenomenon, known as glass transition, leads to the formation of non-periodic structures known as glasses. Extensive studies have been conducted on model materials to understand glass transition in two dimensions. However, despite the synthesis of disordered/amorphous single-atom thick structures of carbon, little attention has been given to glass transition in realistic two-dimensional materials such as graphene. Herein, using molecular dynamics simulation, we demonstrate the existence of glass transition in graphene leading to a realistic two-dimensional glassy structure, namely glassy graphene. We show that the resulting glassy structure exhibits excellent agreement with experimentally realized disordered graphene. Interestingly, this glassy graphene exhibits a wrinkled but stable structure, with reduced thermal vibration in comparison to its crystalline counterpart. We suggest that the topological disorder induced by glass transition governs the unique properties of this structure.

Ultra-smooth glassy graphene thin films for flexible transparent circuits.

Large-area graphene thin films are prized in flexible and transparent devices. We report on a type of glassy graphene that is in an intermediate state between glassy carbon and graphene and that has high crystallinity but curly lattice planes. A polymer-assisted approach is introduced to grow an ultra-smooth (roughness, <0.7 nm) glassy graphene thin film at the inch scale. Owing to the advantages inherited by the glassy graphene thin film from graphene and glassy carbon, the glassy graphene thin film exhibits conductivity, transparency, and flexibility comparable to those of graphene, as well as glassy carbon-like mechanical and chemical stability. Moreover, glassy graphene-based circuits are fabricated using a laser direct writing approach. The circuits are transferred to flexible substrates and are shown to perform reliably. The glassy graphene thin film should stimulate the application of flexible transparent conductive materials in integrated circuits.