Charge Transport in Highly Heterogeneous Natural Carbonaceous Materials

Abstract

Natural carbonaceous materials (NCMs) have recently emerged as promising organic semiconducting materials for electronics and catalysis, although the fundamental picture of charge transport within NCM systems is still incomplete. Morphologically, NCMs exhibit reminiscence of disordered organic solids, yet the experimental measurements demonstrate a transport regime that surprisingly follows Mott's formula derived for variable‐range hopping in inorganic noncrystalline materials. With ab initio and kinetic Monte Carlo simulations, a temperature scaling is revealed between the Gaussian‐defect model log(σ) ∼ T−2 typical for organic matter and the Mott‐like log(σ) ∼ T−1/4 for a wide spectrum of intermolecular connectivity. As dominant transport descriptors, energy levels and coupling strengths are screened among 30 small molecules with varying sizes, shapes, sp2/sp3 ratios, side chains, and functional groups. These analyses provide insight for the design of NCM electronics, and should also be applicable to disordered molecular materials in general.

Laser-engineered heavy hydrocarbons: Old materials with new opportunities

“…The presence of molecular bridges can greatly increase the charge transfer and electron coupling, which in turn can enable conductivities similar in magnitude to graphite-like networks. Such phenomena have been discussed extensively in previous paper (6). By tuning the laser ablation parameters, the conductivities of HH thin films can be tuned over three orders of magnitude using tar as an example.…”

“…In this work, we selected H:C ratio and aromatic content as the key structural descriptors of the feedstock, because it has been proposed that these two factors affect properties of final products under high-temperature treatment (28). We primarily focused on the chemical evolution of molecular models for coal, tar, and pitch generated by packing the most representative molecules with corresponding H:C ratio and density (6,29,30). The initial H:C ratio (1.84) in tar leads to a more substantial dehydrogenation and increase in the sp 2 content when ablated (0.66).…”

“…Tar, made of smaller noninterconnected units and with higher aliphatic content MP and coal, makes it more mobile from a rheological perspective. This offer more degrees of freedom and therefore more tunability to reordering, and the ablated tar conductivity shows a Mott-like log() ~ T −1/4 relationship, indicating intermolecular connection and extended conjugation (6). Coal and MP, on the other end, are less tunable, more structurally rigid, and with lower aliphatic content, but have larger clusters and therefore more stacking.…”

“…Unprocessed HHs are combinations of polycyclic aromatic hydrocarbons (PAHs), alkanes, and sometimes minerals at various concentrations (4,5). The broad distribution of energy levels and the lack of effective long-range, extended C═C conjugation leads to poor electrical conductivity in HHs (6), which can be compensated by bulk annealing to increase conductivity by orders of magnitude through graphitization (3). In a traditional quartz tube, annealing temperatures (~1300 K) limit the formation of highly dehydrogenated and crystallized carbon materials such as carbon fibers, graphite, and diamond (7).…”

Upgrading carbonaceous materials: Coal, tar, pitch, and beyond

“…23 The above examples of coal-derived nanomaterials have been processed primarily by thermal annealing and chemical reduction, and their differences mainly arise from aromatic content and the smallest aromatic sizes of the functional unit that can represent the properties of the coal when tailored. 31 Anthracite coals and bituminous coals can serve as a precursor for high-yield graphitic carbon material synthesis including CNTs, 32 fullerenes, 33 and large-size (>100 nm) graphene sheets, 34 due to the large aromatics. Lignite coals, which contain comparably much smaller aromatic constituents, have mainly been used to derive carbon quantum dot diameters in the few-nanometer size range.…”