Rethinking Coal: Thin Films of Solution Processed Natural Carbon Nanoparticles for Electronic Devices

Abstract

Disordered carbon materials, both amorphous and with long-range order, have been used in a variety of applications, from conductive additives and contact materials to transistors and photovoltaics. Here we show a flexible solution-based method of preparing thin films with tunable electrical properties from suspensions of ball-milled coals following centrifugation. The as-prepared films retain the rich carbon chemistry of the starting coals with conductivities ranging over orders of magnitude, and thermal treatment of the resulting films further tunes the electrical conductivity in excess of 7 orders of magnitude. Optical absorption measurements demonstrate tunable optical gaps from 0 to 1.8 eV. Through low-temperature conductivity measurements and Raman spectroscopy, we demonstrate that variable range hopping controls the electrical properties in as-prepared and thermally treated films and that annealing increases the sp 2 content, localization length, and disorder. The measured hopping energies demonstrate electronic properties similar to amorphous carbon materials and reduced graphene oxide. Finally, Joule heating devices were fabricated from coal-based films, and temperatures as high as 285°C with excellent stability were achieved. KEYWORDS: Coal, amorphous carbon, organic semiconductors, thin films, solution processing C arbon has long been known as one of the most chemically versatile elements. As a result, carbonaceous materials have been of technological interest for their wide range of electronic propertiesresulting in materials ranging from low cost conducting materials such as graphite and carbon black, to semiconducting fullerenes and carbon nanotubes, and to insulating diamond and diamond-like carbon. Specifically, carbon black, carbon filaments, and other carbon materials are used in electrodes, conductivity additives, and electromagnetic reflectors. 1 Graphitic materials 2 and amorphous carbon (a-C) 3,4 are the leading and a promising candidate, respectively, for anode materials in lithium ion batteries. Amorphous carbon has also been used in the manufacturing of transistors 5,6 and photovoltaic devices, 7,8 and nanostructured graphitic materials (such as graphene and reduced graphene oxide) have gained significant attention for applications including in photovoltaics, 9−11 transparent conductive membranes, 12−16 and Joule heating devices. 17−19 Despite decades of research on synthetically processed carbon materials, facile, tunable, 20 low-temperature, solution processing methods remain elusive. Carbon black synthesis requires temperatures as high as 2000°C, 21 and a-C is typically deposited by plasma-enhanced chemical vapor deposition (CVD) 22,23 although aerosol-assisted CVD has recently been demonstrated. 8 While extensive research has been conducted to develop solution-based methods of graphene deposition, rGO films remain several orders of magnitude less conductive than CVD graphene. 16,24,25 In contrast to the widely studied world of synthetic carbonaceous materials, the optical...

Natural Carbon By‐Products for Transparent Heaters: The Case of Steam‐Cracker Tar

“…While the mass loss does not continue beyond 600 °C (see extended TGA in Figure S2, Supporting Information), the sheet resistances of films annealed at 600 °C remain above 20 kΩ □ –1 , and therefore beyond the sensitivity of available characterization equipment. As we reported in our previous work on electronic devices made from various sintered coal particles, annealing between approximately 600 and 950 °C causes a dramatic increase in carbon bond disorder and a rapid conversion to an aromatic‐rich, highly interconnected a‐C framework with significantly lower sheet resistance, with aromatic domain size increasing with temperature . As expected, SCT films annealed at 950 °C showed significantly lower sheet resistances, as low as around 1 kΩ □ –1 depending on film thickness (see Figure c).…”

“…Figure a shows the transmittance of SCT films with varying degrees of annealing. Two effects are competing here: the previously noted mass loss of the films up to 500–600 °C, reducing the total light absorption; and the well known reduction of the bandgap with increasing conjugation at increasing annealing temperatures . Figure a shows a dramatic decrease in the slope of the transmittance between unannealed films and those annealed at 400°C, indicating a decrease in bandgap according to the Tauc construction for disordered materials .…”

“…As with conventional electronic‐grade organic films, the SCT films are smooth and uniform over the whole sample, both before and after annealing, with example RMS roughnesses of 13 and 11 nm, respectively (see cross sections in Figure e,f, and representative profilometry in Figure S1, Supporting Information). This represents a significant improvement over our previous work with films produced from ball‐milled coal nanoparticles, where film performance was limited by the numerous cracks and voids present in the film after spin coating, which were then amplified by annealing …”

“…Yet, given the chemical diversity in such materials, it is of interest to explore their tunability and applicability in long‐lifetime, potentially high‐value products as opposed to single‐use burning. For example, there is growing interest in extracting or synthesizing graphene, carbon quantum dots, and other carbon nanomaterials from natural carbon, as they offer desirable electronic properties from inexpensive feedstocks, and recently, thin films of coal microparticles were found to produce highly tunable electronic grade thin films …”

“…Coal‐based thin films proved to be excellent active materials for Joule heaters, with high thermal resilience and stability while reaching sustained temperatures in excess of 300 °C . Having sheet resistances similar to those of much thicker coal‐based films, while also being transparent, STC thin films are promising candidates for transparent heating, offering the strengths of coal‐based films with simpler processing, and the transparency required in applications such as de‐icing aircraft or automobile windows, or for heating electronic devices such as LCD screens in cold environments .…”