Nanotube Dots Research Articles

Effects of Three Carbonaceous Nanomaterials on the Developmental Toxicity, Oxidative Stress, and Metabolic Profile in Zebrafish

As an important part of artificial nanomaterials, carbonaceous nanomaterials (CNMs) are widely applied in a plenty of areas such as energy, manufacturing and pharmaceutical industries. In the present study, the developmental toxicity, induced by three typical CNMs including graphene oxide (GO), carbon nanotube (CNT) and graphene oxide quantum dot (GOQD) was investigated in the typical model animal, zebrafish larva. The induced sub-acute toxicity at the low concentration of GO, CNT and GOQD was investigated in adult zebrafish, either. Moreover, the molecular mechanisms at the level of metabolomics were also explored. The results showed that there was a significant increase in reactive oxygen species (ROS), and mitochondrial membrane damage was caused by GO, CNT and GOQD in zebrafish larva. However, there was no significant developmental toxicity on zebrafish larva. The toxicity order in terms of the ROS increase and mitochondrial membrane damage was GOQD > CNT > GO. The chronic exposure at the typical environment-associated concentration (0.01 mg·L-1) of CNMs can induce gill and kidney cell senescence of adult zebrafish. Meanwhile, it can also inhibit total superoxide dismutase (T-SOD) activity in adult zebrafish in the subacute toxicity test (21 d) at the concentration of 0.01 mg·L-1. The metabolomics research revealed that the toxicity order at the environment-associated concentration acting on adult zebrafish was GOQD > CNT > GO; and it showed that fatty acids and proline turbulence may be responsible for one of the molecular mechanisms of T-SOD inhibition in adult zebrafish. This work can supply rationale to evaluate the potential risk of ecosystems and human health induced by the three typical CNMs.

Field-induced SU(4) to SU(2) Kondo crossover in a half-filling nanotube dot: Spectral and finite-temperature properties

We study finite-temperature properties of the Kondo effect in a carbon nanotube (CNT) quantum dot using the Wilson numerical renormalization group (NRG). In the absence of magnetic fields, four degenerate energy levels of the CNT consisting of spin and orbital degrees of freedom give rise to the SU(4) Kondo effect. We revisit the universal scaling behavior of the SU(4) conductance for quarter- and half-filling in a wide temperature range. We find that the filling dependence of the universal scaling behavior at low temperatures $T$ can be explained clearly with an extended Fermi-liquid theory. This theory clarifies that a $T^{2}$ coefficient of conductance becomes zero at quarter-filling whereas the coefficient at half-filling is finite. We also study a field-induced crossover from the SU(4) to SU(2) Kondo state observed at the half-filled CNT dot. The crossover is caused by the matching of the spin and orbital Zeeman splittings, which lock two levels among the four at the Fermi level even in magnetic fields $B$. We find that the conductance shows the SU($4$) scaling behavior at $\mu_{B}B<k_{B}T_{K}^{\mathrm{SU(4)}}$ and it exhibits the SU($2$) universality at $\mu_{B}B\gg k_{B}T_{K}^{\mathrm{SU(4)}}$, where $T_{K}^{\mathrm{SU(4)}}$ is the SU($4$) Kondo temperature. To clarify how the excited states evolve along the SU(4) to SU(2) crossover, we also calculate the spectral function. The results show that the Kondo resonance width of the two states locked at the Fermi level becomes sharper with increasing fields. The spectral peaks of the other two levels moving away from the Fermi level merge with atomic limit peaks for $\mu_{B}B \gtrsim k_{B}T_{K}^{\mathrm{SU(4)}}$.

Field-Enhanced Kondo Correlations in a Half-Filling Nanotube Dot: Evolution of an SU(N) Fermi-Liquid Fixed Point

We theoretically study an emergent SU(2) symmetry which is suggested by recent magneto-transport measurements, carried out near two electrons filling of a carbon nanotube quantum dot. It emerges in the case where the spin and orbital Zeeman splittings cancel each other out for two of the one-particle dot levels among four. Using the Wilson numerical renormalization group, we show that a crossover from the SU(4) to SU(2) Fermi-liquid behavior occurs at two impurity-electrons filling as magnetic field increases. We also find that the quasiparticles are significantly renormalized as the remaining two one-particle levels move away from the Fermi level and are frozen at high magnetic fields. In order to clarify how the ground state evolves during such a crossover, we also reexamine the SU(N) Kondo singlet state for M impurity-electrons filling in the limit of strong exchange interactions. We show that the nondegenerate Fermi-liquid fixed point of Nozi\`{e}es and Blandin can be described as a bosonic Perron-Frobenius vector for M hard-core bosons, each of which consists of one impurity-electron and one conduction hole. This interpretation can also be applied to the Fermi-liquid fixed-point without the SU(N) symmetry.

Effective mass theory of interacting electron states in semiconducting carbon nanotube quantum dots

A modified effective mass approach is developed to describe the states of a few interacting electrons in semiconducting carbon nanotube quantum dots, and the accuracy of the approximations used is examined quantitatively. The few-particle states are calculated by exact diagonalization of the modified effective mass Hamiltonian for a range of different nanotube dots. It is shown that the two-electron states are Wigner molecule-like for a large proportion of these dots and the electron correlation is always found to be important. The addition energy is calculated for up to six electrons in the dot and the analysis of experimental addition energy data is discussed.

Josephson effect for SU(4) carbon-nanotube quantum dots

We present the theory of the Josephson effect in nanotube dots where an SU(4) symmetry can be realized. We find a remarkably rich phase diagram that significantly differs from the SU(2) case. In particular, \pi-junction behavior is largely suppressed. We analytically obtain the Josephson current in various parameter regions: (i) in the Kondo regime, covering the full crossover from SU(4) to SU(2), (ii) for weak tunnel couplings, and (iii) for large BCS gap. The transition between these regions is studied numerically.

2-electron Wigner molecules in carbon nanotube quantum dots: Excited states

The states of two electrons in a semiconducting nanotube dot are computed by exact diagonalisation of a 2-band effective mass Hamiltonian and compared to the states obtained from a molecular model. The molecular model is found to be accurate for the ground and excited states in a wide range of nanotube dots with different physical properties and this demonstrates that the quantum states are Wigner molecule-like and the electrons are strongly correlated. The first 97 excited states are examined for a physically realistic dot in the strongly correlated regime and, for these states, the molecular model is accurate to within 4%.

Effect of micro and nanoparticle inorganic fillers on the field emission characteristics of photosensitive carbon nanotube pastes

We successfully fabricated field emitter arrays of carbon nanotube (CNT) dots of 10 μm diameter with excellent field emission properties by using photosensitive CNT paste. The CNT paste was investigated in terms of morphologies, current –voltage properties, and luminous uniformities by varying the mixing ratios of micro and nanoparticle inorganic fillers and the amount of CNTs added into the paste. The 3:1 mixing of micro and nanoparticle fillers and the addition of 5% CNTs in the paste brought about the best field emission characteristics of dot-patterned CNT field emitter arrays.

Theory of nonequilibrium transport in theSU(N)Kondo regime

Using a Fermi-liquid approach, we provide a comprehensive treatment of the current and current noise through a quantum dot whose low-energy behavior corresponds to an $\text{SU}(N)$ Kondo model, focusing on the case $N=4$ relevant to carbon nanotube dots. We show that for general $N$, one needs to consider the effects of higher-order Fermi-liquid corrections even to describe low-voltage current and noise. We also show that the noise exhibits complex behavior due to the interplay between coherent shot noise, and noise arising from interaction-induced scattering events. We also treat various imperfections relevant to experiments, such as the effects of asymmetric dot-lead couplings.

Tunneling transport and spectroscopy in carbon nanotube quantum dots

This paper provides a theoretical description of sequential tunneling transport and spectroscopy, in carbon nanotube quantum dots weakly tunnel coupled to metallic leads under a voltage bias. The effects of Coulomb blockade charging, spin-orbit fine structure, and orbital- and spin-Zeeman effects arising from coupling to applied magnetic fields are considered; and the dependence of the conductance upon applied gate voltage, bias voltage, and magnetic fields is determined. The work is motivated by recent experiments on ultraclean carbon nanotube dots [Kuemmeth et al., Nature (London) 452, 448 (2008)], to which comparison is made.

Effects of Fermi Liquid Interactions on the Shot Noise of anSU(N)Kondo Quantum Dot

We study shot noise in the current of quantum dots whose low-energy behavior corresponds to an SU(N) Kondo model, focusing on the case N=4 relevant to carbon nanotube dots. For general N, two-particle Fermi-liquid interactions have two distinct effects: they can enhance the noise via backscattering processes with an N-dependent effective charge, and can also modify the coherent partition noise already present without interactions. For N=4, in contrast with the SU(2) case, interactions enhance shot noise solely through an enhancement of partition noise. This leads to a nontrivial prediction for experiment.

Interacting electrons in a semiconducting carbon nanotube dot: A 2-band approach

A new 2-band effective mass theory for the interacting electron states in a carbon nanotube quantum dot is outlined. The states of two interacting electrons are calculated by exact diagonalisation of the 2-band Hamiltonian. A range of different nanotube and dot parameters are investigated and, interestingly, the 2-electron ground states are found to be ferromagnetic.

Charge sensing in carbon-nanotube quantum dots on microsecond timescales

We report fast, simultaneous charge sensing and transport measurements of gate-defined carbon nanotube quantum dots. Aluminum radio frequency single electron transistors (rf-SETs) capacitively coupled to the nanotube dot provide single-electron charge sensing on microsecond timescales. Simultaneously, rf reflectometry allows fast measurement of transport through the nanotube dot. Charge stability diagrams for the nanotube dot in the Coulomb blockade regime show extended Coulomb diamonds into the high-bias regime, as well as even-odd filling effects, revealed in charge sensing data.

Shell filling in closed single-wall carbon nanotube quantum dots.

We observe twofold shell filling in the spectra of closed one-dimensional quantum dots formed in single-wall carbon nanotubes. Its signatures include a bimodal distribution of addition energies, correlations in the excitation spectra for different electron number, and alternation of the spins of the added electrons. This provides a contrast with quantum dots in higher dimensions, where such spin pairing is absent. We also see indications of an additional fourfold periodicity indicative of K-K' subband shells. Our results suggest that the absence of shell filling in most isolated nanotube dots results from disorder or nonuniformity.