Izaac Mitchell

Van der Waals encapsulation of two-dimensional materials within hexagonal boron nitride (h-BN) stacks has proven to be a promising way to create ultrahigh-performance electronic devices. However, contemporary approaches for achieving van der Waals encapsulation, which involve artificial layer stacking using mechanical transfer techniques, are difficult to control, prone to contamination, and unscalable. Here, we report on the transfer-free direct growth of high-quality graphene nanoribbons (GNRs) within h-BN stacks. The as-grown embedded GNRs exhibit highly desirable features being ultralong (up to 0.25 mm), ultranarrow ( < 5 nm), and homochiral with zigzag edges. Our atomistic simulations reveal that the mechanism underlying the embedded growth involves ultralow GNR friction when sliding between AA'-stacked h-BN layers. Using the grown structures, we demonstrate the transfer-free fabrication of embedded GNR field-effect devices that exhibit excellent performance at room temperature with mobilities of up to 4,600 and on-off ratios of up to . This paves the way to the bottom-up fabrication of high-performance electronic devices based on embedded layered materials.

Van der Waals encapsulation of two-dimensional materials in hexagonal boron nitride (hBN) stacks is a promising way to create ultrahigh-performance electronic devices 1, 2, 3, 4. However, contemporary approaches for achieving van der Waals encapsulation, which involve artificial layer stacking using mechanical transfer techniques, are difficult to control, prone to contamination and unscalable. Here we report the transfer-free direct growth of high-quality graphene nanoribbons (GNRs) in hBN stacks. The as-grown embedded GNRs exhibit highly desirable features being ultralong (up to 0.25 mm), ultranarrow (< 5 nm) and homochiral with zigzag edges. Our atomistic simulations show that the mechanism underlying the embedded growth involves ultralow GNR friction when sliding between AA′-stacked hBN layers. Using the grown structures, we demonstrate the transfer-free fabrication of embedded GNR field …

In chemistry, theory of aromaticity or π bond resonance plays a central role in intuitively understanding the stability and properties of organic molecules. Here we present an analogue theory for σ bond resonance in flat boron materials, which allows us to determine the distribution of two-center two-electron and three-center two-electron bonds without quantum calculations. Based on this theory, three rules are proposed to draw the Kekulé-like bonding configurations for flat boron materials and to explore their properties intuitively. As an application of the theory, a simple explanation of why neutral borophene with ~1/9 hole has the highest stability and the effect of charge doping on borophene’s optimal hole concentration is provided with the assumption of σ and π orbital occupation balance. Like the aromaticity theory for carbon materials, this theory greatly deepens our understanding on boron materials and paves …

Fullerenes are used extensively in organic electronics as electron acceptors among other uses; however, there are still several key mysteries regarding their formation such as the importance of graphitic intermediates and the thermokinetics of initial cage formation. To this end, we have conducted density functional tight binding molecular dynamics (DFTB-MD) calculations on disintegrated Ih-C60 to investigate the formation mechanisms of fullerenes at high temperature conditions. From the results of these DFTB-MD calculations we were able to develop a thermokinetic model to describe the free energies and kinetics of fullerene formation at a range of temperatures. Direct observation of the mechanism revealed fullerenes readily forming in nanosecond times between 2000 and 3000 K but were hindered above this temperature window. Analysis revealed temperature dependent formation mechanisms where at low …

The controllable growth of multilayer graphene is a challenging research topic. Prior results show graphene adlayers can grow beneath pre-existing graphene layers on a Cu(111). The conventional inverted-wedding-cake (IWC) model used to describe this incurs an energy disadvantage due to deformation in the overlying graphene. We propose an alternative theoretical model, the sunk growth mode, for understanding multilayer graphene growth on Cu substrates. Extensive density functional theory (DFT) calculations show that multilayer graphene grown via this sunk mode is energetically favourable compared to the on-terrace growth mode for Cu(111). These results reveal that graphene underlayers tend to grow in a sunk growth mode, minimizing deformation in the overlayers, reducing deformation energy. Further density functional tight binding-molecular dynamic (DFTB-MD) simulations on Cu(111) substrates …

The implementation of the GPU support in DFTB+, as described in Sec. III C of the original publication, 1 1. B. Hourahine, B. Aradi, V. Blum, F. Bonafé, A. Buccheri, C. Camacho, C. Cevallos, MY Deshaye, T. Dumitrică, A. Dominguez, S. Ehlert, M. Elstner, T. van der Heide, J. Hermann, S. Irle, JJ Kranz, C. Köhler, T. Kowalczyk, T. Kubař, IS Lee, V. Lutsker, RJ Maurer, SK Min, I. Mitchell, C. Negre, TA Niehaus, AMN Niklasson, AJ Page, A. Pecchia, G. Penazzi, MP Persson, J. Řezáč, CG Sánchez, M. Sternberg, M. Stöhr, F. Stuckenberg, A. Tkatchenko, VW-z. Yu, and T. Frauenheim, J. Chem. Phys. 152, 124101 (2020). https://doi. org/10.1063/1.5143190 was developed based on a previous unpublished implementation by Jacek Jakowski. In order to acknowledge his work on this first implementation, the authors of the original publication wish to include J. Jakowski as co-author. The scientific content of the original …

DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green’s functions, and many more. DFTB+ can be used as a user-friendly standalone application in addition to being embedded into other software packages as a library …

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer‐long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h‐BN) substrate through nanoparticle‐catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10 µm are synthesized. Remarkably, the as‐grown GNRs are crystallographically aligned with the h‐BN substrate, forming 1D moiré superlattices. Scanning tunneling microscopy reveals an average width of 2 nm and a typical bandgap of ≈1 eV for similar GNRs grown on conducting graphite substrates. Fully atomistic …

Alcohol chemical vapor deposition (ACVD) was established as one of the most promising methods for single-walled carbon nanotube (SWCNT) growth almost two decades ago however the mechanisms behind its success remain elusive. To unveil the mechanism of SWCNT growth via ACVD, we employed density functional tight binding molecular dynamics simulations, supplying ethanol to a Fe nanoparticle. Here we demonstrate the oxidation of the Fe catalyst with varying supply rates of ethanol and how the catalyst composition is controlled by the reaction pathways mediated by the hydroxyl OH radical. Following ethanol dissociation on Fe and subsequent O dissolution, the catalyst becomes oxidized and the mobility and availability of Fe to bond with C are reduced. However, SWCNT growth is promoted via the key reaction pathways of the hydroxyl H; controlling the catalyst composition through the formation …

The Stone–Wales bond rotation isomerization of nonicosahedral C60 (C2v-C60) into isolated-pentagon rule following icosahedral C60 (Ih-C60 or IPR-C60) is a limiting step in the synthesis of Ih-C60. However, extensive previous studies indicate that the potential energy barrier of the Stone–Wales bond rotation is between 6 and 8 eV, extremely high to allow for bond rotation at the temperatures used to produce fullerenes conventionally. This is also despite data indicating a possible fullerene road mechanism that necessitates low-temperature annealing. However, these previous investigations often have limiting factors, such as using the harmonic approximation to determine free energies at high temperatures or considering only the reverse Ih-C60 to C2v-C60 transition as a basis. Indeed, when the difference in energy between Ih-C60 and C2v-C60 is accounted for, this barrier is generally reduced by ∼1.5 eV …

In two-dimensional (2D) borophene, the structural transition from triangular lattice to hexagonal lattice with an increase in vacancy concentration is a basic principle of constructing various borophene isomers. Here, by performing an extensive structural search of 4239 borophene isomers with both hexagonal holes (HHs) and large holes (LHs), we show that the structural transformation from triangular lattice to borophene with large holes is energetically more favorable. Borophene isomers with LHs are more stable than those with only HHs at high vacancy concentrations (>20%) and are just slightly less stable than those with only HHs at low vacancy concentrations. This discovery greatly expands the family of 2D borophene and opens a route for synthesizing new borophene isomers.

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