Effect of the Content and Ordering of the sp2 Free Carbon Phase on the Charge Carrier Transport in Polymer-Derived Silicon Oxycarbides

Abstract

The present work elaborates on the correlation between the amount and ordering of the free carbon phase in silicon oxycarbides and their charge carrier transport behavior. Thus, silicon oxycarbides possessing free carbon contents from 0 to ca. 58 vol.% (SiOC/C) were synthesized and exposed to temperatures from 1100 to 1800 °C. The prepared samples were extensively analyzed concerning the thermal evolution of the sp2 carbon phase by means of Raman spectroscopy. Additionally, electrical conductivity and Hall measurements were performed and correlated with the structural information obtained from the Raman spectroscopic investigation. It is shown that the percolation threshold in SiOC/C samples depends on the temperature of their thermal treatment, varying from ca. 20 vol.% in the samples prepared at 1100 °C to ca. 6 vol.% for the samples annealed at 1600 °C. Moreover, three different conduction regimes are identified in SiOC/C, depending on its sp2 carbon content: (i) at low carbon contents (i.e., <1 vol.%), the silicon oxycarbide glassy matrix dominates the charge carrier transport, which exhibits an activation energy of ca. 1 eV and occurs within localized states, presumably dangling bonds; (ii) near the percolation threshold, tunneling or hopping of charge carriers between spatially separated sp2 carbon precipitates appear to be responsible for the electrical conductivity; (iii) whereas above the percolation threshold, the charge carrier transport is only weakly activated (Ea = 0.03 eV) and is realized through the (continuous) carbon phase. Hall measurements on SiOC/C samples above the percolation threshold indicate p-type carriers mainly contributing to conduction. Their density is shown to vary with the sp2 carbon content in the range from 1014 to 1019 cm−3; whereas their mobility (ca. 3 cm2/V) seems to not depend on the sp2 carbon content.

Electrically conductive silicon oxycarbide thin films prepared from preceramic polymers

“…Comparing the calculated L D in this work with that of Cançado in 2011, our thin film samples obtained at 1100°C show that L D < r A , which is categorized as a highly defective region. Also in agreement with the work of Rosenburg, as the heat treatment temperature is increased, the defect density decreases (Figure 7C), and L a and L D increase 10 . Looking at the calculated L eq of the C17‐1400 samples (Figure 7D), the value calculated for the C‐rich dispersed particles of the thin film is double in size as compared to that of the monolithic SiOC; while the L eq of the matrix of the thin film is less than half of the value calculated for the SiOC monolith.…”

“…Also in agreement with the work of Rosenburg, as the heat treatment temperature is increased, the defect density decreases (Figure 7C), and L a and L D increase. 10 Looking at the calculated L eq of the C17-1400 samples (Figure 7D), the value calculated for the C-rich dispersed particles of the thin film is double in size as compared to that of the monolithic SiOC; while the L eq of the matrix of the thin film is less than half of the value calculated for the SiOC monolith. This finding clearly indicates that the C-rich and O-depleted precipitates in the prepared SiOC-based thin films possess a significantly higher degree of graphitization than that of the matrix.…”

“…[8][9][10] Rosenburg et al showed the logarithmic dependence of the SiOC conductivity on the content of segregated carbon of the sample. 10 The dependence showed attribution to the nature of the percolating system which is reliant on the aspect ratio of the free carbon phase. 11 The ordering of the free carbon phase leads to a higher aspect ratio of the conductive phase within the system thus decreasing the percolation threshold.…”

“…These values are two to three magnitudes larger than that of the electrical conductivities measured for monolithic samples with a similar chemical composition. 10,[80][81][82] Another advantage of the tunneling AFM is the simultaneous mapping of the mechanical and electrical property of sample. 83 TUNA is known to have higher current sensitivity than that of the conductive AFM mode which can measure currents down to sub-picoampere range through a highly resistive sample.…”

Structure and Connectivity in an Amorphous Silicon Oxycarbide Polymer-Derived Ceramic: Results from 2D 29Si NMR Spectroscopy

“…PDCs in Si–B–C–O and Si–B–C–N systems are technologically promising due to their remarkable thermomechanical stability that can be combined with an intriguingly wide range of attributes, including high electrical conductivity, bioactivity, and optoelectronic properties. − A fundamental understanding of the relationship between these properties and the atomic structure of PDCs is of key importance in the engineering of these materials for specific applications. In this regard, the ternary Si–O–C and Si–C–N PDCs serve as model systems, with their structures characterized by a network of corner-sharing SiO x C 4– x and SiN x C 4– x (0 ≤ x ≤ 4) tetrahedra, respectively, that coexist with nanodomains of sp 2 -hybridized amorphous carbon. − ,− A wide variety of characterization techniques have been used over the last two decades to investigate the structure of these PDCs at various length scales. − In particular, 29 Si and 13 C magic angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy have played an important role in the identification and quantitation of the mixed-bonded SiO x C 4– x or SiN x C 4– x units and C in the sp 2 or sp 3 bonding mode in the structure. − ,,, The relative fractions of these mixed-bonded tetrahedral units, at least in PDCs pyrolyzed at temperatures above 1000 °C, suggest a partial avoidance of the bonding between the oxygen (nitrogen)-rich and carbon-rich SiO x C 4– x (SiN x C 4– x ) tetrahedral units, and consequently a spatial segregation of these units, in the SiOC (SiCN) tetrahedral networks. The avoidance of bonding between C and O (N) atoms in these PDCs implies that the SiO 4 (SiN 4 ) and SiC 4 tetrahedra cannot be directly connected to each other.…”