Session R41: Carbon Nanotubes and Graphene Nanoribbons

Live

Single-walled carbon nanotubes (SWCNTs) doped with sp3 defects are a promising class of optoelectronic materials with bright tunable photoluminescence and demonstrated single-photon emission. Here we perform a density functional theory (DFT) study, complemented by experiment, of a variety of sp3 defects attached to (6,5) SWCNT, with the goal of tuning spin-orbit coupling (SOC) by introduction of a heavy atom in the defect structure. For four recently synthesized tubes containing aryl-based defects, both computation and electron spin resonance indicate that a single unpaired electron is introduced to the system. Both the DFT bandstructure and photoluminescence measurements suggest a defect-induced gap state 0.2-0.3 eV above the valence band maximum of the nanotube. Additional theoretical analysis indicates that the spin density is localized around the sp3 site with no increase of the spin-orbit splitting in the near-gap states (a measure of SOC in the excited state) even when a Pd atom-containing ligand is introduced. We then theoretically model additional metal-containing defects with decreased metal-tube separation. We determine that forming a direct sp3 carbon-metal bond is necessary for tuning SOC.   

Live

Ultraclean, undoped carbon nanotubes are always insulating, even when the gap predicted by band theory is zero. The residual, intrinsic gap is thought to have a many-body origin, associated with either a Mott¹ or an excitonic phase². Whereas the two scenarios are fundamentally different, as they are driven by the short- and long-range part of Coulomb interaction, respectively, a conclusive experiment has been missing so far.

Here we propose as a unique fingerprint of the excitonic insulator the presence of a cusp in the evolution of the gap with the axial magnetic field, close to the gap mimimum. On the contrary, the Mott phase exhibits a featureless, rounded profile. The non-analytic spike originates from the extreme sensitivity of electron-electron interactions to the Aharonov-Bohm gap modulation, as we demonstrate by combining a first-principles analysis of screening with extensive model calculations for tubes of different size and chirality.

¹Deshpande et al., Science 323, 106 (2009)
²Varsano et al., Nat. Comm. 8, 1461 (2017)   

Live

We investigate the optical response of graphene nanoribbons (GNRs) using the broadband nonlinear generation and detection capabilities of nanoscale junctions created at the LaAlO₃/SrTiO₃ interface [1]. Using the large non-resonant third-order nonlinear susceptibility in STO, strong difference frequency mixing occurs when the junction is biased, leading to an induced polarization that can also be detected at the junction [2,3]. Here we discuss an “inkwell” method we developed to deposit GNRs on the LAO surface with an atomic force microscope tip. We also review the results of experiments in which we interrogate few-GNR clusters with LAO/STO nanoscale junctions. Results show nearly 100% extinction of VIS-NIR light in the GNR clusters, agreeing with previous results on graphene [4].
[1] C. Cen, et al., Nature Mat 7, 298 (2008).
[2] Y. Ma, et al., Nano Lett 13, 2884 (2013).
[3] L. Chen, et al., Light: Science & Appl. 8, 24 (2019).
[4] E. Sheridan, et al., Nano Lett 20, 10 (2020).   

Live

Charged excitons, i.e., trions, have attracted much interest due to their nonzero charge and spin. While the trion binding energy, defined relative to the exciton energy, is on the order of a few meV to tens of meV in III-V quantum wells and transition metal dichalcogenides, that in semiconducting single-wall carbon nanotubes (SWCNTs) can be as large as 200 meV due to enhanced Coulomb interactions in one dimension, making them clearly observable even at room temperature. Here, we present electrical creation of excitons and trions in aligned films of single-chirality (6,5) SWCNTs. Highly dense SWCNT films were prepared with minimum surfactant and without polymer wrapping to provide high conductivity and enhanced electron and hole transport inside the SWCNT film. We observed that SWCNTs aligned in the direction of the current flow emit light at a lower threshold voltage than those aligned in the perpendicular direction. This increase can be explained in terms of increased conductance in the direction of alignment. Our simulations help to elaborate the nature of the electroluminescence in the aligned CNT films.   

Live

Color centers in single-walled carbon nanotubes are of interest because of their single-photon emission at room temperature in the telecom range, but the lack of vapor-phase reaction route for forming the color centers hinders the use of the excellent optical properties of air-suspended carbon nanotubes. We herein demonstrate the functionalization of air-suspended carbon nanotubes using iodobenzene as a precursor. We rationally design the chemical reaction procedure without compromising the suspended structure. Formed phenyl group serves as a color center and exhibits localized exciton emission peaks E₁₁^* and E₁₁^*- in addition to the free exciton emission peak E₁₁. We characterize representative 12 chiralities to reveal the diameter-dependent reactivity and optical property of the color centers. We quantitatively describe the reactivity, where a strain of nanotube curvature promotes the reaction. The trapping potential of E₁₁^* and E₁₁^*- excitons also shows the diameter dependence, which we discuss in the presentation.   

Live

Single-wall carbon nanotubes (SWCNTs) are 1D materials that show unique properties and great potential for technological applications. As a light emitter, SWCNTs show photoluminescence quantum yields in the range of 1% - 10%, which reduce their applicability. Previous theoretical and experimental studies show that sp³defects (chemical groups or atoms attached by covalent bonds) introduce local modifications to the electronic structure, creating new photoluminescent states with red-shifted energies, in the range of 100 - 300meV. This effect is caused by exciton localization at the sp³ defects.

In this work, we calculate the quasiparticle band structure, absorption spectra and excitonic properties of SWCNTs bonded to hydrogen atoms by using ab initio calculations. We first observe a split of degenerated conduction and valence bands and the appearance of a new flat impurity band. Finally, we evaluate the absorption spectra and exciton wavefunctions and its compositions. We observed a red-shift of the E₁₁ peak, associated to pristine SWCNTS, and the appearance of new red-shifted peaks associated to the impurity band.   

Live

We report charge density and temperature-dependent field-effect mobility and on-chip gated Seebeck coeff. measurements¹ of polymer-sorted monochiral small diameter (6,5) (0.76 nm) and mixed large diameter SWCNT (1.17-1.55 nm, plasma torch, RN) networks with varied network densities and length distributions. We show that charge and thermoelectric transport in SWCNT networks can be modelled by the Boltzmann transport formalism, incorporating transport in heterogeneous media and fluctuation-induced tunneling. Considering the diameter-dep. 1D density of states (DoS) of the SWCNTs, we can simulate the Seebeck coeff.. Our simulations suggest that scattering in these networks cannot be described as 1D phonon scattering. The relaxation time is instead inversely prop. to energy, presumably pointing towards the necessity to include scattering at SWCNT junctions and the more 2D character of scattering. Observing higher power factors in trap-free, ttmgb-treated (6,5) than in the RN networks, emphasizes the importance of chirality selection to tune the width of the DoS. Hence, we propose trap-free, narrow DoS distr., large diameter SWCNT networks for electronic and thermoelectric applications.²
[1] Statz et al. Commun. Phys. 1, 16 (2018)
[2] Statz et al. ACS Nano (in review, 2020)   

Live

Isolated linear carbon chains (LCCs) encapsulated by multiwalled carbon nanotubes are studied under hydrostatic pressure (P) via resonance Raman scattering. The LCCs’ spectroscopic signature C band around 1850 cm−1 softens linearly with increasing P. A simple anharmonic force-constant model not only describes such softening but also shows that the LCCs’ Young’s modulus (E), Grüneisen parameter (γ), and strain (ε) follow universal P−1 and P2 laws, respectively. In particular, γ also presents a unified behavior for all LCCs. To the best of our knowledge, these are the first results reported on such isolated systems and the first work to explore universal P-dependent responses for LCCs’ E, ε, and γ.   

Live

Carbon dots have emerged as promising materials for numerous applications on account of their high stability, low cost, and environment-friendliness. They are promising materials for biological and biomedical applications, as well as photocatalysis, photovoltaics, chiral sensing and optoelectronics. Here we present an extensive computational study (~10¹⁰ molecular dynamics steps total) of the influence of temperature, length of polymer chain, and presence of solvents on the melting and evaporation, characterized by the dihedral distributions, probability of gauche defects and radius of gyration of the carbon dot clusters. We verified the importance of the ad hoc “scaling factor” parameter in molecular dynamics simulations [1].

[1] Firlej, L., Kuchta, B., Roth, M. W., & Wexler, C. (2010). Molecular simulations of intermediate and long alkanes adsorbed on graphite: Tuning of non-bond interactions. Journal of Molecular Modeling, 17(4), 811–816. doi:10.1007/s00894-010-0770-0   

Live

Filled carbon nanotubes (CNTs) exhibit unique physical properties arising from the synergistic effects between the carbon shells and the filling material making them attractive towards numerous applications. Although many inorganic and organic materials have been successfully encapsulated inside CNTs until now, there is still a lack of a reliable, efficient, upscaled, and economic method to achieve a complete filling of CNT cores with transition metal sulfide nanowires. We have developed a simple in situ method for the first time to synthesize CNTs filled with nickel sulfide nanowire (Ni₃S₂@CNTs) on various substrates using a chemical vapor deposition technique. Electron microscopy measurements reveal that CNTs are completely and continuously filled with single crystalline nanowires up to several micrometers in length. Raman spectroscopy suggests that the Ni₃S₂@CNTs are exceptionally well graphitized and of ultra-high quality. The I-V characteristics of individual Ni₃S₂@CNTs were studied using both two probe and four-point probe methods which reveal the metallic properties of Ni₃S₂@CNTs.   

Live

We report a comprehensive experimental and theoretical study on the temperature-dependence of conductance in carbon nanotube networks (CNTNs). Our experimental results demonstrate up to four orders of magnitude change with temperature in electrical conductance. Some of the data can be accounted by the variable range hopping (VRH) mechanism. In particular, we report an analytical formula for the localization length as a function of the Fermi energy and CNT bandstructure, which explains the different families of the observed behavior. The localization length in the zero-gap armchair (6,6) CNT, the localization length is much longer than in other single chirality CNTs. To explain conductance temperature dependence in (6,6) tube, we employ a full-band model within the perturbation theory to account for the phonon-assisted tunneling conductance across the CNTs. The theoretical study shows that while corresponding purely electronic conductance is not-negligible, it does not depend on temperature. On the other hand, phonon-assistant tunneling accounts for the observed temperature dependence in (6,6) CNTNs sample.   

Live

Graphene nanoribbons (GNRs) are attracting great interest due to their highly tunable electronic, optical, and transport properties. On-surface synthesis has enabled realization of atomically precise GNRs. However, these bottom-up fabrication methods are based on metal-surface assisted chemical reactions, where interaction with metallic substrates screen the designer electronic properties. Here, we report a methodology for rational precursor design and direct synthesis of atomically precise GNRs on metal oxide surfaces: [1]. The thermally triggered multistep transformations rely on highly selective and sequential activations of C-Br, C-F bonds and cyclodehydrogenation. Scanning tunneling microscopy and spectroscopy (STM/S) characterization monitors in situ the formation of intermediates and GNRs revealing anticipated weak interaction between GNRs and the model rutile TiO₂(011)-(2×1) substrate.

[1] Kolmer et al., Science 369, 571–575 (2020).   

On Demand

For designing quantum devices using graphene nano-structures, there is importance in finding a general rule which controls the appearance of localized zero modes (LZMs) coexisting with the Dirac zero modes (DZMs).
Recently, we have discovered a class of graphene nano-molecular structures which we call Phenalenyl-Tessellation Molecule (PTM) with vacncies[1]. When a PTM contains atomic defects, there appear two types of special non-bonding molecular orbitals at the zero-energy level. One is LZM and the other is DZM. Considering a Vacancy-centered hexagonal Armchair Nano-Graphene (VANG) as a typical example of a PTM, one LZM and one DZM appear non-trivially, and they form a singlet[2,3].
In this presentaton, we report a general rule for the appearance of the LZMs and DZMs in a class of PTM-based poly-aromatic hydro carbon. Our rule is indispensable for designing quantum information devices using zero modes in decorated defective graphene with vacancies.
[1] N. Morishita and K. Kusakabe, J. Phys. Soc. Jpn. 88, 124707 (2019).
[2] N. Morishita, G. K. Sunnardianto, S. Miyao, andK. Kusakabe, J. Phys. Soc. Jpn. 85, 084703 (2016).
[3] S. Miyao, N. Morishita, G. K. Sunnardianto, andK. Kusakabe, J. Phys. Soc. Jpn. 86, 034802 (2017).   

On Demand

We study the damped Zaremba-Kohn model (dZK) to molecules adsorbed on a curved cylindrical conducting surface and combine this model with semilocal density functionals; i.e. Perdew-Burke-Ernzerhof (PBE) and the strongly constrained and appropriately normed (SCAN). The dZK model starts from a formula for the vdW interaction of a distant atom with a solid surface, both with known dielectric properties, damp this formula at a short-range, and then treats an adsorbed molecule or atomic layer as a collection of renormalized atoms. The vdW-dZK model has previously been successfully applied to study the physisorption of graphene on metals and graphene adsorbed on Transition metal dichalcogenide (TMD) layered materials. In this work, we put forward a derivation of vdW energy in such geometry using cylindrical green’s function and the asymptotic behavior of Bessel functions of the second kind. In general, vdW interactions are remarkably sensitive to the geometry and electronic structure of a given system. Furthermore, we compute the binding energies and equilibrium distances using our vdW model for nitrogen dioxide and ammonia molecules adsorbed on single-walled carbon nanotubes. We also compare our results with GGA and meta-GGA combined rVV10, a widely used non-local functional.   

On Demand

N-carbophenes (carbophenes) are a novel class of porous 2D hydrocarbons consisting of alternating units of (N) cyclohexatriene and (N-1) cyclobutene.¹ Carbophenes can be viewed as the 2D generalization of linear phenylenes, with structures similar to graphenylene.²

Using tight-binding density functional theory, we’ve found the ground state configurations and electronic structures of carbophenes. The energies per carbon atom needed to produce carbophenes are similar to that of graphenylene, suggesting that organic chemistry synthesis methods could produce either.² Carbophenes have interlayer configurations that are markedly different from graphenylene, suggesting that porosimetry experiments might distinguish between multilayers of the two materials. Carbophenes have band gaps that range from 1 to 2 eV. We will discuss preliminary results, which show that functionalized carbophenes may work as materials to capture atmospheric carbon (CO₂ and CH₄).

Bibliography
1. Junkermeier, C. E., Luben, J. P. & Paupitz, R. N-Carbophenes: Two-dimensional covalent organic frameworks derived from linear N-phenylenes. Mater. Res. Express 6, 115103 (2019).
2. Du, Q.-S. et al. A new type of two-dimensional carbon crystal prepared from 1,3,5-trihydroxybenzene. Sci. Rep. 7, 40796 (2017).