Unraveling the Mechanism of Doping Borophene.

We elucidate the doping mechanism of suitable elements into borophene with first-principles density functional theory calculation. During doping with nitrogen (N), the sp2 orbitals are responsible for arranging themselves to accommodate the electron of the N atom. Doping dramatically changes structure and electronic properties from corrugated and metallic borophene to flat and insulating h-BN with 100 % N-doping. We extend the mechanism of N-doping in borophene to doping of non-metallic and metallic ad-atoms on borophene. Our findings will help to design boron-based 2D materials.

Radial-tangential mode of single-wall carbon nanotubes manifested by Landau regulation: reinterpretation of low- and intermediate-frequency Raman signals.

The low-frequency Raman signals of single-wall carbon nanotubes (SWNTs), appearing in the range of 100-300 cm-1, have been interpreted as radial-breathing mode (RBM) comprising pure radial Eigenvectors. Here, we report that most of the low-frequency and intermediate-frequency signals of SWNTs are radial-tangential modes (RTMs) coexisting radial and tangential Eigenvectors, while only the first peak at the low-frequency side is the RBM. Density functional theory simulation for SWNTs of ~ 2 nm in diameter shows that dozens of RTMs exhibit following the RBM (~ 150 cm-1) up to G-mode (~ 1592 cm-1) in order with Landau regulation. We specify the RBM and the RTM on Raman spectra obtained from SWNTs, where both appear as prominent peaks between 149 and 170 cm-1 and ripple-like peaks between 166 and 1440 cm-1, respectively. We report that the RTMs have been regarded as RBM (~ 300 cm-1) and ambiguously named as intermediate-frequency mode (300-1300 cm-1) without assignment. The RTMs gradually interlink the RBM and the G-mode resulting in the symmetric Raman spectra in intensity. We reveal high-resolution transmission microscope evidence for a helical structure of SWNTs, informing the typical diameter of commercial SWNTs to be 1.4-2 nm.

AA h BN crystal, basic structure of boron nitride nanotubes.

AA h boron nitride (BN) crystal, assigned to an orthorhombic space group (No. 31, Pm21), is reported here. This new AA h BN crystal exhibits a 'linear' morphology for high-resolution transmission electron microscopy (HRTEM) and a (non-hexagonal) 'diagonal' electron-diffraction pattern, which have been experimentally demonstrated in this article. It is also demonstrated that this new crystal is the basic structure of multi-walled BN nanotubes (BNNTs) existing in the form of a helix. The helical AA h BNNTs exist in a metastable phase owing to 〈200〉 texture growth of the orthorhombic crystal, where the energy is ∼15 meV higher than that of stable AB or AA' BN. It is shown that the typical scanning electron microscope 'fluffy cotton-like' morphology of BNNTs is due to secondary growth of diverse BN sheets (including mono-layers) on incoherently scrolled wall strands of BNNTs, providing further evidence for the helical structure with HRTEM evidence for a left-handed helix.

Modeling the early temporal dynamics of viral load in respiratory tract specimens of COVID-19 patients in Incheon, the Republic of Korea.

OBJECTIVE: To investigate the duration and peak of severe acute respiratory syndrome coronavirus 2 shedding as infectivity markers for determining the isolation period. METHODS: A total of 2,558 upper respiratory tract (URT) and lower respiratory tract (LRT) specimens from 138 patients with laboratory-confirmed coronavirus disease were analyzed. Measurements of sequential viral loads were aggregated using the cubic spline smoothing function of a generalized additive model. The time to negative conversion was compared between symptom groups using survival analysis. RESULTS: In URT samples, viral RNA levels peaked on day 4 after symptom onset and rapidly decreased until day 10 for both E and RdRp genes, whereas those in LRT samples immediately peaked from symptom onset and decreased until days 15.6 and 15.0 for E and RdRp genes, respectively. Median (interquartile range) time to negative conversion was significantly longer in symptomatic (18.0 [13.0-25.0] days) patients than in asymptomatic (13.0 [9.5-17.5] days) patients. The more types of symptoms a patient had, the longer the time to negative conversion. CONCLUSIONS: The viral load rapidly changes depending on the time after symptom onset; the viral shedding period may be longer with more clinical symptoms. Different isolation policies should be applied depending on disease severity.

AB-stacked nanosheet-based hexagonal boron nitride.

Hexagonal boron nitride (h-BN) has been generally interpreted as having an AA stacking sequence. Evidence is presented in this article indicating that typical commercial h-BN platelets (∼10-500 nm in thickness) exhibit stacks of parallel nanosheets (∼10 nm in thickness) predominantly in the AB sequence. The AB-stacked nanosheet occurs as a metastable phase of h-BN resulting from the preferred texture and lateral growth of armchair (110) planes. It appears as an independent nanosheet or unit for h-BN platelets. The analysis is supported by simulation of thin AB films (2-20 layers), which explains the unique X-ray diffraction pattern of h-BN. With this analysis and the role of pressure in commercial high-pressure high-temperature sintering (driving nucleation and parallelizing the in-plane crystalline growth of the nuclei), a growth mechanism is proposed for 2D h-BN (on a substrate) as `substrate-induced 2D growth', where the substrate plays the role of pressure.

Raman Radial Mode Revealed from Curved Graphene.

One of the unsolved fundamental issues of graphene is establishing an appropriate way to discern layers of graphene structures. We report a simple methodology to analyze graphene structures using Raman signals in the range of ∼100 to ∼500 cm-1 comprising clear 118 or 175 cm-1 peaks. We demonstrate that the low-energy signals on Raman spectra of plasma-seeded grown graphene sheets originated from nanocurvature (c) of mono- (175 and 325-500 cm-1 signals) (c ≈ 1 nm) and bilayer (118 cm-1 peak) (c ≈ 2 nm) graphene with Raman simulations, based on Raman radial mode (RM) Eigen vectors. Our RM model provides a standard way of identifying and evaluating graphene structures.

The Nature of Metastable AA' Graphite: Low Dimensional Nano- and Single-Crystalline Forms.

Over the history of carbon, it is generally acknowledged that Bernal AB stacking of the sp2 carbon layers is the unique crystalline form of graphite. The universal graphite structure is synthesized at 2,600~3,000 °C and exhibits a micro-polycrystalline feature. In this paper, we provide evidence for a metastable form of graphite with an AA' structure. The non-Bernal AA' allotrope of graphite is synthesized by the thermal- and plasma-treatment of graphene nanopowders at ~1,500 °C. The formation of AA' bilayer graphene nuclei facilitates the preferred texture growth and results in single-crystal AA' graphite in the form of nanoribbons (1D) or microplates (2D) of a few nm in thickness. Kinetically controlled AA' graphite exhibits unique nano- and single-crystalline feature and shows quasi-linear behavior near the K-point of the electronic band structure resulting in anomalous optical and acoustic phonon behavior.

The seeded growth of graphene.

In this paper, we demonstrate the seeded growth of graphene under a plasma chemical vapor deposition condition. First, we fabricate graphene nanopowders (~5 nm) by ball-milling commercial multi-wall carbon nanotubes. The graphene nanoparticles were subsequently subject to a direct current plasma generated in a 100 Torr 10%CH4 - 90%H2 gas mixture. The plasma growth enlarged, over one hour, the nuclei to graphene sheets larger than one hundred nm(2) in area. Characterization by electron and X-ray diffraction, high-resolution transmission electron microscopy images provide evidence for the presence of monolayer graphene sheets.

Structure of single-wall carbon nanotubes: a graphene helix.

Evidence is presented in this paper that certain single-wall carbon nanotubes are not seamless tubes, but rather adopt a graphene helix resulting from the spiral growth of a nano-graphene ribbon. The residual traces of the helices are confirmed by high-resolution transmission electron microscopy and atomic force microscopy. The analysis also shows that the tubular graphene material may exhibit a unique armchair structure and the chirality is not a necessary condition for the growth of carbon nanotubes. The description of the structure of the helical carbon nanomaterials is generalized using the plane indices of hexagonal space groups instead of using chiral vectors. It is also proposed that the growth model, via a graphene helix, results in a ubiquitous structure of single-wall carbon nanotubes.

The growth of AA graphite on (111) diamond.

Stacked AA graphite has been synthesized using a high-density dc plasma in hydrogen-methane mixtures. Graphene layers have been grown epitaxially with 2-1 registration between the AA graphitic edges and the (111) surface of diamond. In addition, a new graphite crystal structure containing AA(') graphene layers, where alternate planes are translated by half the hexagon width, is formed by 1-1 registry. The resulting interplanar distances of the AA graphite at the interface range from 2.20 A for the 1-1 registration to 4.40 A for the 2-1 registration and have been measured directly by high-resolution transmission electron microscopy (TEM). The appearance of the characteristic d-spacings, 3.55, 2.15, 1.80, 1.75 (not fully resolved), and 1.25 A in the selective area diffraction patterns from the TEM, are consistent with reflections from the (001), (100), (102), (002), and (110) planes of the AA graphite. Simulation of the diffraction patterns, employing the structural factors of graphene, confirms the existence of AA graphite.