skip to main content

This content will become publicly available on August 25, 2023

Title: Anomalous thermal transport behavior in graphene-like carbon nitride (C 3 N)
The success of graphene created a new era in materials science, especially for two-dimensional (2D) materials. 2D single-crystal carbon nitride (C 3 N) is the first and only crystalline, hole-free, single-layer carbon nitride and its controlled large-scale synthesis has recently attracted tremendous interest in thermal transport. Here, we performed a comparative study of thermal transport between monolayer C 3 N and the parent graphene, and focused on the effect of temperature and strain on the thermal conductivity ( κ ) of C 3 N, by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. The κ of C 3 N shows an anomalous temperature dependence, and the κ of C 3 N at high temperatures is larger than the expected value following the common trend of κ ∼ 1/ T . Moreover, the κ of C 3 N is found to be increased by applying a bilateral tensile strain, despite its similar planar honeycomb structure to graphene. The underlying mechanism is revealed by providing direct evidence for the interaction between lone-pair N-s electrons and bonding electrons from C atoms in C 3 N based on the analysis of orbital-projected electronic structures and electron localization function (ELF). Our research more » not only conduct a comprehensive study on the thermal transport in graphene-like C 3 N, but also reveal the physical origin of its anomalous properties, which would have significant implications on the future studies of nanoscale thermal transport. « less
Authors:
; ; ; ; ;
Award ID(s):
2030128
Publication Date:
NSF-PAR ID:
10358942
Journal Name:
Journal of Materials Chemistry C
Volume:
10
Issue:
33
Page Range or eLocation-ID:
12080 to 12090
ISSN:
2050-7526
Sponsoring Org:
National Science Foundation
More Like this
  1. High-throughput calculations (first-principles density functional theory and semi-empirical transport models) have the potential to guide the discovery of new thermoelectric materials. Herein we have computationally assessed the potential for thermoelectric performance of 145 complex Zintl pnictides. Of the 145 Zintl compounds assessed, 17% show promising n-type transport properties, compared with only 6% showing promising p-type transport. We predict that n-type Zintl compounds should exhibit high mobility μ n while maintaining the low thermal conductivity κ L typical of Zintl phases. Thus, not only do candidate n-type Zintls outnumber their p-type counterparts, but they may also exhibit improved thermoelectric performance. From the computational search, we have selected n-type KAlSb 4 as a promising thermoelectric material. Synthesis and characterization of polycrystalline KAlSb 4 reveals non-degenerate n-type transport. With Ba substitution, the carrier concentration is tuned between 10 18 and 10 19 e − cm −3 with a maximum Ba solubility of 0.7% on the K site. High temperature transport measurements confirm a high μ n (50 cm 2 V −1 s −1 ) coupled with a near minimum κ L (0.5 W m −1 K −1 ) at 370 °C. Together, these properties yield a zT of 0.7 at 370 °C formore »the composition K 0.99 Ba 0.01 AlSb 4 . Based on the theoretical predictions and subsequent experimental validation, we find significant motivation for the exploration of n-type thermoelectric performance in other Zintl pnictides.« less
  2. A major recent breakthrough in materials science is the emergence of intrinsic magnetism in two-dimensional (2D) crystals, which opens the door to more cutting-edge fields in the 2D family and could eventually lead to novel data-storage and information devices with further miniaturization. Herein we propose an experimentally feasible 2D material, Fe 2 I 2 , which is an intrinsic room-temperature ferromagnet exhibiting perpendicular magnetic anisotropy (PMA). Using first-principles calculations, we demonstrate that single-layer (SL) Fe 2 I 2 is a spin-gapless semiconductor with a spin-polarized Dirac cone and linear energy dispersion in one spin channel, exhibiting promising dissipation-less transport properties with a Fermi velocity up to 6.39 × 10 5 m s −1 . Our results reveal that both strain and ferroelectric polarization switching could induce an out-of- to in-plane spin reorientation in the 2D Fe 2 I 2 layer, revealing its advantage in assembling spintronic devices. In addition, spin–orbit coupling (SOC) triggers a topologically nontrivial band gap of 301 meV with a nonzero Chern number (| C | = 2), giving rise to a robust quantum anomalous Hall (QAH) state. The 2D crystal also exhibits high carrier mobilites of 0.452 × 10 3 and 0.201 × 10 3 cmmore »2 V −1 s −1 for the electrons and holes, respectively. The combination of these unique properties renders the 2D Fe 2 I 2 ferromagnet a promising platform for high efficiency multi-functional spintronic applications.« less
  3. 2D materials have attracted broad attention from researchers for their unique electronic proper-ties, which may be been further enhanced by combining 2D layers into vertically stacked van der Waals heterostructures. Among the superlative properties of 2D systems, thermoelectric energy (TE) conversion promises to enable targeted energy conversion, localized thermal management, and thermal sensing. However, TE conversion efficiency remains limited by the inherent tradeoff between conductivity and thermopower. In this paper, we use first-principles calculation to study graphene-based van der Waals heterostructures (vdWHs) composed of graphene layers and hexagonal boron nitride (h-BN). We compute the electronic band structures of heterostructured systems using Quantum Espresso and their thermoelectric (TE) properties using BoltzTrap2. Our results have shown that stacking layers of these 2D materials opens a bandgap, increasing it with the number of h-BN interlayers, which significantly improves the power factor (PF). We predict a PF of ~1.0x10 11 W/K 2 .m.s for the vdWHs, nearly double compared to 5x10 10 W/K 2 .m.s that we obtained for single-layer graphene. This study gives important information on the effect of stacking layers of 2D materials and points toward new avenues to optimize the TE properties of vdWHs.
  4. MXenes are a rapidly growing class of 2D transition metal carbides and nitrides, finding applications in fields ranging from energy storage to electromagnetic interference shielding and transparent conductive coatings. However, while more than 20 carbide MXenes have already been synthesized, Ti 4 N 3 and Ti 2 N are the only nitride MXenes reported so far. Here by ammoniation of Mo 2 CT x and V 2 CT x MXenes at 600 °C, we report on their transformation to 2D metal nitrides. Carbon atoms in the precursor MXenes are replaced with N atoms, resulting from the decomposition of ammonia molecules. The crystal structures of the resulting Mo 2 N and V 2 N were determined with transmission electron microscopy and X-ray pair distribution function analysis. Our results indicate that Mo 2 N retains the MXene structure and V 2 C transforms to a mixed layered structure of trigonal V 2 N and cubic VN. Temperature-dependent resistivity measurements of the nitrides reveal that they exhibit metallic conductivity, as opposed to semiconductor-like behavior of their parent carbides. As important, room-temperature electrical conductivity values of Mo 2 N and V 2 N are three and one order of magnitude larger than those ofmore »the Mo 2 CT x and V 2 CT x precursors, respectively. This study shows how gas treatment synthesis such as ammoniation can transform carbide MXenes into 2D nitrides with higher electrical conductivities and metallic behavior, opening a new avenue in 2D materials synthesis.« less
  5. Surface depletion field would introduce the depletion region near surface and thus could significantly alter the optical, electronic and optoelectronic properties of the materials, especially low-dimensional materials. Two-dimensional (2D) organic—inorganic hybrid perovskites with van der Waals bonds in the out-of-plane direction are expected to have less influence from the surface depletion field; nevertheless, studies on this remain elusive. Here we report on how the surface depletion field affects the structural phase transition, quantum confinement and Stark effect in 2D (BA)2PbI4 perovskite microplates by the thickness-, temperature- and power-dependent photoluminescence (PL) spectroscopy. Power dependent PL studies suggest that high-temperature phase (HTP) and low-temperature phase (LTP) can coexist in a wider temperature range depending on the thickness of the 2D perovskite microplates. With the decrease of the microplate thickness, the structural phase transition temperature first gradually decreases and then increases below 25 nm, in striking contrast to the conventional size dependent structural phase transition. Based on the thickness evolution of the emission peaks for both high-temperature phase and low-temperature phase, the anomalous size dependent phase transition could probably be ascribed to the surface depletion field and the surface energy difference between polymorphs. This explanation was further supported by the temperature dependent PLmore »studies of the suspended microplates and encapsulated microplates with graphene and boron nitride flakes. Along with the thickness dependent phase transition, the emission energies of free excitons for both HTP and LTP with thickness can be ascribed to the surface depletion induced confinement and Stark effect.« less