Zero-bias conductance in carbon nanotube quantum dots

We present numerical renormalization group calculations for the zero-bias conductance of quantum dots made from semiconducting carbon nanotubes. These explain and reproduce the thermal evolution of the conductance for different groups of orbitals, as the dot-lead tunnel coupling is varied and the system evolves from correlated Kondo behavior to more weakly correlated regimes. For integer fillings N=1, 2, 3 of an SU(4) model, we find universal scaling behavior of the conductance that is distinct from the standard SU(2) universal conductance, and concurs quantitatively with experiment. Our results also agree qualitatively with experimental differential conductance maps.     

碳纳米管量子点的电子输运

在紧束缚近似下,利用常量相互作用模型和Landauer-Bütticker公式,计算了扶手椅型和金属锯齿型碳纳米管量子点的电导。发现,根据碳纳米管量子点的长度的不同,扶手椅型碳纳米管量子点的电导可以具有两电子或四电子的壳层结构。而锯齿型碳纳米管量子点的电导却仅有四电子的壳层结构,与长度无关;这些理论结果与之前的实验结果符合的很好。     

Adiabatic charge pumping in carbon nanotube quantum dots

We investigate charge pumping in carbon nanotube quantum dots driven by the electric field of a surface acoustic wave. We find that, at small driving amplitudes, the pumped current reverses polarity as the conductance is tuned through a Coulomb blockade peak using a gate electrode. We study the behavior as a function of wave amplitude, frequency, and direction and develop a model in which our results can be understood as resulting from adiabatic charge redistribution between the leads and quantum dots on the nanotube.     

Correlated tunneling in intramolecular carbon nanotube quantum dots

We investigate correlated electronic transport in single-walled carbon nanotubes with two intramolecular tunneling barriers. We suggest that below a characteristic temperature the long-range nature of the Coulomb interaction becomes crucial to determine the temperature dependence of the maximum G(max) of the conductance peak. Correlated sequential tunneling dominates transport yielding the power law G(max) proportional, variant T(alpha(end-end)-1), typical for tunneling between the ends of two Luttinger liquids. Our predictions are in agreement with recent measurements.     

Size effect of quantum conductance in single-walled carbon nanotube quantum dots

The quantum conductance of two kinds of carbon nanotube quantum dots (CNQD) composed of (5,5) and (10,0) tubes, namely (10,0)/(5,5)/(10,0) and (5,5)/(10,0)/(5,5) with different quantum sizes, are calculated. It is shown that for (10,0)/(5,5)/(10,0) CNQD, one on-resonant peak at the Fermi energy exists only for special QD sizes, and the width of the conductance gap increases from 1.0 eV to 3.2 eV with the increase of size. The positions of peaks around the Fermi energy are obtained by the electronic structure of individual finite (5,5) tubes. We also find that the (5,5)/(10,0)/(5,5) CNQDs behave as a quantum dot, and its localized QD states are different from that of the former CNQD because of the existence of the interface states between (5,5)/(10,0) junctions. For (5,5)/(10,0)/(5,5) CNQD, there is no conductance gap with QDs size smaller than 7 layers, and the conductance peak around the interface quasilocalized state -0.26 eV disappears with QD sizes larger than 23 layers. In addition, for the (5,5)/(10,0)/(5,5) CNQD, the connection method can change the degree of electronic localization of intermediate (10,0) tube.Received: 8 August 2003, Published online: 23 December 2003PACS: 61.48. + c Fullerenes and fullerene-related materials - 71.20.Tx Fullerenes and related materials; intercalation compounds - 72.80.Rj Fullerenes and related materials - 68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.     

Evolution of transport regimes in carbon nanotube quantum dots

We study the evolution of conductance regimes in carbon nanotubes with doubly degenerate orbitals (shells) by controlling the contact transparency within the same sample. For sufficiently open contacts, Kondo behavior is observed for 1, 2, and 3 electrons in the topmost shell. As the contacts are opened more, the sample enters the "mixed valence" regime, where different charge states are strongly hybridized by electron tunneling. Here, the conductance as a function of gate voltage shows pronounced modulations with a period of four electrons, and all single-electron features are washed away at low temperature. We successfully describe this behavior by a simple formula with no fitting parameters. Finally, we find a surprisingly small energy scale that controls the temperature evolution of conductance and the tunneling density of states in the mixed valence regime.     

Magneto-Coulomb effect in carbon nanotube quantum dots filled with magnetic nanoparticles

Electrical transport measurements of carbon nanotubes filled with magnetic iron nanoparticles are reported. Low-temperature (40 mK) magnetoresistance measurements showed conductance hysteresis with sharp jumps at the switching fields of the nanoparticles. Depending on the gate voltage, positive or negative hysteresis was observed. The results are explained in terms of a magneto-Coulomb effect: The spin flip of the iron island at a nonzero magnetic field causes a shift of the chemical potential induced by the change of Zeeman energy; i.e., an effective charge variation is detected by the nanotube quantum dot.     

Linear and nonlinear transport across carbon nanotube quantum dots

We present a low energy-theory for non-linear transport in finite-size interacting single-wall carbon nanotubes. It is based on a microscopic model for the interacting p_z electrons and successive bosonization. We consider weak coupling to the leads and derive equations of motion for the reduced density matrix. We focus on the case of large-diameter nanotubes where exchange effects can be neglected. In this situation the energy spectrum is highly degenerate. Due to the multiple degeneracy, diagonal as well as off-diagonal (coherences) elements of the density matrix contribute to the nonlinear transport. At low bias, a four-electron periodicity with a characteristic ratio between adjacent peaks is predicted. Our results are in quantitative agreement with recent experiments.     

Disorder-mediated electron valley resonance in carbon nanotube quantum dots

We propose a scheme for coherent rotation of the valley isospin of a single electron confined in a carbon nanotube quantum dot. The scheme exploits the ubiquitous atomic disorder of the nanotube crystal lattice, which induces time-dependent valley mixing as the confined electron is pushed back and forth along the nanotube axis by an applied ac electric field. Using experimentally determined values for the disorder strength we estimate that valley Rabi oscillations with a period on the nanosecond time scale are feasible. The valley resonance effect can be detected in the electric current through a double quantum dot in the single-electron transport regime.     

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.     

High-quality CdS quantum dots sensitized ZnO nanotube array films for superior photoelectrochemical performance

The surface characteristics of ZnO were synthetically optimized by a self-designed simultaneous etching and W-doping hydrothermal method utilizing as-prepared ZnO nanorod (NR) array films as the template. Benefiting from the etching and regrowth process and the different structural stabilities of the various faces of ZnO NRs, the uniquely etched and W-doped ZnO (EWZ) nanotube (NT) array films with larger surface area, more active sites and better energy band structure were used to improve the photoelectrochemical (PEC) performance and the loading quality of CdS quantum dots (QDs). On the basis of their better surface characteristics, the CdS QDs were uniformly loaded on EWZ NT array film with a good coverage ratio and interface connection; this effectively improved the light-harvesting ability, charge transportation and separation as well as charge injection efficiency during the PEC reaction. Therefore, all the CdS QD-sensitized EWZ NT array films exhibited significantly enhanced PEC performance. The CdS/EWZ-7 composite films exhibited the optimal photocurrent density with a value of 12 mA· cm^-2, 2.5 times higher than that of conventional CdS/ZnO-7 composite films under the same sensitization times with CdS QDs. The corresponding etching and optimizing mechanisms were also discussed.     

Magnetic-field dependence of tunnel couplings in carbon nanotube quantum dots

By means of sequential and cotunneling spectroscopy, we study the tunnel couplings between metallic leads and individual levels in a carbon nanotube quantum dot. The levels are ordered in shells consisting of two doublets with strong- and weak-tunnel couplings, leading to gate-dependent level renormalization. By comparison to a one- and two-shell model, this is shown to be a consequence of disorder-induced valley mixing in the nanotube. Moreover, a parallel magnetic field is shown to reduce this mixing and thus suppress the effects of tunnel renormalization.     

Intra- and inter-shell Kondo effects in carbon nanotube quantum dots

The linear response transport properties of carbon nanotube quantum dot in the strongly correlated regime are discussed. The finite-U mean field slave boson approach is used to study many-body effects. Magnetic field can rebuilt Kondo correlations, which are destroyed by the effect of spin-orbit interaction or valley mixing. Apart from the field induced revivals of SU(2) Kondo effects of different types: spin, valley or spin-valley, also more exotic phenomena appear, such as SU(3) Kondo effect. Threefold degeneracy occurs due to the effective intervalley exchange induced by short-range part of Coulomb interaction or due to the intershell mixing. In narrow gap nanotubes the full spin-orbital degeneracy might be recovered in the absence of magnetic field opening the condition for a formation of SU(4) Kondo resonance.     

Two-electron and four-electron periodicity in single-wall carbon nanotube quantum dots

Single quantum dots have been fabricated in single-wall carbon nanotubes and electrical transport properties have been measured at low temperature. Two- and four-electron periodicities have been clearly observed in the same sample in different gate voltage ranges. The former is an even–odd effect which originates from the spin degeneracy, while the latter is related to the additional two-fold band degeneracy. The results are discussed with the energy scales associated with the dot, and the possibility for a single spin manipulation is suggested.     

Colloidal quantum dots for mid-IR applications

The use of colloidal material offers an interesting alternative to top down approaches for the realization of low cost infrared detectors. We demonstrate photoconduction in thin films of a colloidal material in the mid-infrared (up to 7 μm), using HgTe colloidal quantum dots. Thin films of the colloidal quantum dots have a large absorption coefficient (>10⁴ cm⁻¹), and the photoconductive response is dramatically improved by encapsulating the nanoparticle into an inorganic matrix of As₂S₃. Such devices show fast response and large detectivity (>10¹⁰ jones) at temperatures above 200 K.     

Electrical properties of multiwalled carbon nanotube film

The multiwalled carbon nanotube films examined in this study are produced by two methods: current annealing of carbon paper and dc magnetron sputtering. The conductivity and the temperature dependence of resistance of the samples are measured. The thermal conductivity of the film-substrate system is evaluated.     

Viscoelastic properties of multiwalled carbon nanotube solutions

We have prepared solutions of multiwalled carbon nanotubes in very low vapour pressure solvents (a mixture of chlorinated biphenyls). The solutions are stable and show no sign of precipitation for six months. Rheological measurements using a modified Birnboim apparatus with annular and Sogel-Pochetino geometries have been performed. Using time-temperature superposition we obtained the real and imaginary part of the complex viscosity coefficient in a frequency range covering eight orders of magnitude and a temperature range from 5 to 50 ^○C. The data shows unexpected changes in the solution with temperature: for T below 30 ^○C there appears to be some reorganization or clustering. This self-organization could result in a useful technique to improve the electronic properties of polymer/carbon nanotubes composites used in organic electronic devices.     

Electron transport in carbon nanotube quantum dots in an axial magnetic field

The quantum conductance of the quantum dots (QDs) made of two kinds of primary carbon nanotubes (CNTs), i.e., armchair and zigzag CNTs, threaded by an axial magnetic field, has been studied by using the tight binding approximation and constant interaction model. It is found that under increasing axial magnetic field, each conductance shell of the zigzag CNT-QDs could split into two groups with each group of two peaks moving up or down, respectively. And the up- and down-moving two peaks would re-group with other two peaks, down- and up-moving, in the neighboring shell, forming a new four-peak shell, and then re-splitting, re-grouping again due to the Aharonov-Bohm effect, which is in agreement with those of experiments. But, in contrast, the conductance shells of the armchair CNT-QDs do not split by the magnetic field. Our subsequent theoretical studies show further that the above phenomena, i.e., the conductance shell-splitting, re-grouping, and re-splitting again with increasing the magnetic field exist in all the CNT-QDs except for the armchair one.