Nitrogen-doped graphene (NG) has attracted increasing attention because its properties are significantly different to pristine graphene, making it useful for various applications in physics, chemistry, biology, and materials science. However, the NGs that can currently be fabricated using most experimental methods always have low N concentrations and a mixture of N dopants, which limits the desirable physical and chemical properties. In this work, first principles calculations combined with the local particle-swarm optimization algorithm method were applied to explore possible stable structures of 2D carbon nitrides (C1−xNx) with various C/N ratios. It is predicted that C1−xNx structures with low N-doping concentration contain both graphitic and pyridinic N based on their calculated formation energies, which explains the experimentally observed coexistence of graphitic and pyridinic N in NG. However, pyridinic N is predominant in C1−xNx when the N concentration is above 0.25. In addition, C1−xNx structures with low N-doping concentration were found to have considerably lower formation energies than those with a high N concentration, which means synthesized NGs with low N-doping concentration are favorable. Moreover, we found the restrictions of mixed doping and low N concentration can be circumvented by using different C and N feedstocks, and by growing NG at lower temperatures.
Traditional methods
to prepare two-dimensional (2D) B–C–N
ternary materials (BC
x
N), such as chemical
vapor deposition (CVD), require sophisticated experimental conditions
such as high temperature, delicate control of precursors, and postgrowth
transfer from catalytic substrates, and the products are generally
thick or bulky films without the atomically mixed phase of B–C–N,
hampering practical applications of these materials. Here, for the
first time, we develop a temperature-dependent plasma-enhanced chemical
vapor deposition (PECVD) method to grow 2D BC
x
N materials directly on noncatalytic dielectrics at low temperature
with high controllability. The C, N, and B compositions can be tuned
by simply changing the growth temperature. Thus, the properties of
the as-made materials including band gap and conductivity are modulated,
which is hardly achieved by other methods. A 2D hybridized BC2N film with a mixed BC2N phase is produced, for
the first time, with a band gap of about 2.3 eV. The growth temperature
is 580–620 °C, much lower than that of traditional catalytic
CVD for growing BC
x
N. The product has
a p-type conducting property and can be directly applied in field-effect
transistors and sensors without postgrowth transfer, showing great
promise for this method in future applications.
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