Single-wall Carbon Nanotube Quantum Dots Research Articles

Metallic carbon nanotube quantum dots with broken symmetries as a platform for tunable terahertz detection

Generating and detecting radiation in the technologically relevant range of the so-called terahertz gap (0.1–10 THz) is challenging because of a lack of efficient sources and detectors. Quantum dots in carbon nanotubes have shown great potential to build sensitive terahertz detectors, usually based on photon-assisted tunneling. A recently reported mechanism combining resonant quantum dot transitions and tunneling barrier asymmetries results in a narrow linewidth photocurrent response with a large signal-to-noise ratio under weak THz radiation. That device was sensitive to one frequency, corresponding to transitions between equidistant quantized states. In this work we show, using numerical simulations together with scanning tunneling spectroscopy studies of a defect-induced metallic zigzag single-walled carbon nanotube quantum dot, that breaking simultaneously various symmetries in metallic nanotube quantum dots of arbitrary chirality strongly relaxes the selection rules in the electric dipole approximation and removes energy degeneracies. This leads to a richer set of allowed optical transitions spanning frequencies from 1 THz to several tens of THz, for a ∼10 nm quantum dot. Based on these findings, we propose a terahertz detector device based on a metallic single-walled carbon nanotube quantum dot defined by artificial defects. Depending on its length and contacts transparency, the operating regimes range from a high-resolution gate-tunable terahertz sensor to a broadband terahertz detector. Our calculations indicate that the device is largely unaffected by temperatures up to 100 K, making carbon nanotube quantum dots with broken symmetries a promising platform to design tunable terahertz detectors that could operate at liquid nitrogen temperatures.

CFD Study Based on Effect of Employing the Single-Walled Carbon Nanotube (SWCNT) and Graphene Quantum Dots (GQD) Nanoparticles and a Particular Fin Configuration on the Thermal Performance in the Shell and Tube Heat Exchanger

In this research, the thermal attributes of shell and finned tube heat exchanger such as thermal efficiency, pressure drop, heat transfer rate and average temperature in the tube side of heat exchanger with using the different volume concentration of nanoparticles (SWCNT and Graphene quantum dot) at the various Reynolds number by applying either fin blades and without fin blades have been conducted numerically. In this heat exchanger the hot fluid or nanofluid flows in the tube section and cold fluid or pure water moves in the shell side. As regarding to results obtained the majority of thermal characteristics like heat transfer rate, pressure drop and effectiveness enhanced with augmentation of Reynolds number and increasing of volume concentration of nanofluids to 1% volumetric of working fluid whereas at the higher volume concentrations of nanoparticles (upper from 1% volumetric) the thermal properties of heat exchanger decreased generally. Also pressure drop intensifies with increment of Reynolds number and volume concentration of nanoparticles that at higher Reynolds number the effects of nanoparticles on the pressure drop were more noticeable. The average temperature of heat exchanger in the end section of inside tubes increased with augmentation of Reynolds number and nanoparticles. Finally, according to the results obtained in this study, most impression on the thermal attributes enhancement was found by employing of finned tubes compared to other factor which this factor increased heat transfer rate of heat exchanger by almost 188% also the effects of nanoparticles at the high levels of volume concentration especially for 5% of SWCNT nanoparticle on the pressure drop obtained about 80% compared to the base fluid.

Electron transport through double-walled carbon nanotube quantum dots

Electron transport through double-walled carbon nanotube quantum dots (DWCNT-QD) was calculated using full valence description of the electronic structure. Rolling pin model was used to measure the electron transport through both the walls of the DWCNT-QD. Three combinations of metallic (M) and zigzag semiconductor (S) nanotubes, i.e., M@S, S@M, and S@S, were studied for various diameters, and the corresponding I–V curves were obtained using the Landauer method. The transmission through the nanotubes was calculated using elastic scattering quantum chemistry (ESQC) method. A significant difference of 0.5 to 1.5 μA in the current through the two walls of the DWCNT was found for the (4, 4)@(10, 9) and (5, 4)@(10, 9). In all other cases, the behavior of both the tubes in a DWCNT-QD was very similar to that of the corresponding single-walled carbon nanotube quantum dots (SWCNT-QD). The intermixing and correlation of the electronic states of the inner and outer shells of the DWCNTs responsible for such results were analyzed and discussed.

Nontrivial magnetoresistive behavior of a single-wall carbon nanotube with an attached molecular magnet

The spin-resolved transport properties of a single-wall carbon nanotube quantum dot, with an attached single molecular magnet, are studied theoretically. With the aid of the real-time diagrammatic technique in the lowest-order perturbation expansion with respect to the tunnel coupling, the current, differential conductance, and the tunnel magnetoresistance (TMR) are determined in both the linear and nonlinear response regimes. It is shown that transport properties depend greatly on both the shell filling sequence of the carbon nanotube and the type of exchange interaction between the molecular magnet and nanotube. This results in highly nontrivial behavior of the TMR, which is especially visible in the low bias voltage regime. Depending on the gate voltage and parameters of the system, we find transport regimes where either a greatly enhanced or negative TMR develops. The mechanism leading to such behavior is associated with nonequilibrium spin accumulation, which builds up in the antiparallel magnetic configuration of the device. We show that it is crucial whether the spin accumulation occurs in the highest-weight spin states or in states with lower spin values. While in the former case it leads to enhanced TMR, in the latter case it may result in negative tunnel magnetoresistance. In addition, we analyze how the above effects depend on the magnitude of the molecular magnet's spin, and show that this dependence is generally nonmonotonic.

Charge sensitivity enhancement via mechanical oscillation in suspended carbon nanotube devices.

Single electron transistors (SETs) fabricated from single-walled carbon nanotubes (SWNTs) can be operated as highly sensitive charge detectors reaching sensitivity levels comparable to metallic radio frequency SETs (rf-SETs). Here, we demonstrate how the charge sensitivity of the device can be improved by using the mechanical oscillations of a single-walled carbon nanotube quantum dot. To optimize the charge sensitivity δQ, we drive the mechanical resonator far into the nonlinear regime and bias it to an operating point where the mechanical third order nonlinearity is canceled out. This way we enhance δQ, from 6 μe/(Hz)(1/2) for the static case to 0.97 μe/(Hz)(1/2) at a probe frequency of ∼1.3 kHz.

Spin-Dependent Transport through a Single-Wall Carbon Nanotube Quantum Dot with an S=1 Molecule

Spin-Dependent Transport through a Single-Wall Carbon Nanotube Quantum Dot with an S=1 Molecule

(Invited) Optical Emission and Control in the Single-wall Carbon Nanotube Quantum Dots

not Available.

Few-Hundred GHz Carbon Nanotube Nanoelectromechanical Systems (NEMS)

We study 23-30 nm long suspended single-wall carbon nanotube quantum dots and observe both their stretching and bending vibrational modes. We use low-temperature DC electron transport to excite and measure the tubes' bending mode by making use of a positive feedback mechanism between their vibrations and the tunneling electrons. In these nanoelectromechanical systems (NEMS), we measure fundamental bending frequencies f(bend) ≈ 75-280 GHz and extract quality factors Q ∼ 10(6). The NEMS's frequencies can be tuned by a factor of 2 with tension induced by mechanical breakjunctions actuated by an electrostatic force or tension from bent suspended electrodes.

Physiological responses induced in tomato plants by a two-component nanostructuralsystem composed of carbon nanotubes conjugated with quantum dots and its in vivomultimodal detection

Plant seedlings were exposed to single-walled carbon nanotube–quantum dot conjugates(SWCNT–QD) mixed in the growth medium in order to understand the interactionsbetween these multicomponent nanosystems and plants. A combination of fluorescent andRaman-scattering 2D mapping analysis was used to clearly monitor the presence of theSWCNT–QD conjugates in various parts of the tomato seedlings. We found that theaddition of QDs to SWCNTs dramatically changed the biological viability of the tomatoplants by significantly accelerating leaf senescence and inhibiting root formation.Although the exposure of SWCNTs only to the plants induced positive effects, thechlorophyll content decreased by 1.5-fold in leaves, and the total weight of the rootsystem decreased four times for the tomato plants exposed to SWCNT–QDs (50 µg ml − 1) compared to plants grown on regular medium as controls. Our results clearly indicatethat the exposure of plants to multicomponent nanomaterials is highly influencedby the presence and bioactivity of each component, individually. Such studiescould be the foundation for understanding how complex nanosized systems affectthe activity of various biological systems with a major impact on ecotoxicology.

Rf-electrometer using a carbon nanotube resonant tunneling transistor

We have studied resonant tunneling transistors (RTT) made of single-walled carbon nanotube quantum dots in the Fabry–Pérot regime. We show sensitivity to input charge as high as 5×10−6 e/Hz1/2 with a carrier frequency of 719 MHz at 4.2 K. This result is comparable to the best values of charge sensitivity so far reported for radio frequency single electron transistors (rf-SET). Unlike SETs, whose operating temperature is limited as Coulomb blockade vanishes as 1/T, a RTT can operate at higher temperatures, since the dephasing length lϕ∝1/T2/3.

Carbon nanotube–quantum dot nanocomposites as new fluorescence nanoparticles for the determination of trace levels of PAHs in water

Amplification of fluorescence is a nanoscale phenomenon which is particularly pronounced in close proximity to metal nanostructures. We have demonstrated for first time that fluorescence amplification can also be produced by single-walled carbon nanotube–quantum dot nanocomposites (SWCNT–QDs). Concretely we exploit the adsorption capabilities of SWCNTs to facilitate the interaction of analytes with the nanoparticle. The fluorescence amplification mechanism is discussed in the paper. The analytical potential of these nanocomposites has been demonstrated for the detection of trace levels of polycyclic aromatic compounds (PAHs) in river water samples. Compared with QDs nanoparticles, the fluorescence enhancement achieved with the SWCNT–QDs nanocomposites was 3.6–5.5 times higher.

Inelastic cotunneling mediated singlet-triplet transition in carbon nanotubes

We investigate electronic transport through single-wall carbon-nanotube quantum dots weakly coupled to metallic leads in the Coulomb-blockade regime. While sequential tunneling is suppressed, the transport, due to correlated tunneling of two electrons cotunneling, is possible. We report on a pronounced current peak for the singlet-triplet transition mediated by inelastic cotunneling processes that allows for their separation from the elastic components. Using the second-order perturbation theory in the tunnel-coupling strength we are able to fit it into the experimental data, explain the details of the line shape, and extract values of parameters that describe pseudospin asymmetry.

Exchange effects in spin-polarized transport through carbon nanotube quantum dots

We investigate linear and nonlinear transport across single-walled carbon nanotube quantum dots weakly coupled to spin-polarized leads. We consider tubes of finite length and small diameter, where not only forward scattering contributions of the Coulomb potential, but also short-ranged processes play an important role. In particular, they induce exchange effects leading for electron fillings 4n+2 either to a non-degenerate groundstate of spin S=0 or to a triplet groundstate. In the linear regime we present analytical results for the conductance - for both the S=0 and the triplet groundstate - and demonstrate that an external magnetic field is crucial to reveal the spin nature of the groundstates. In the nonlinear regime we show stability diagrams that clearly distinguish between the different groundstates. We observe a negative differential conductance (NDC) effect in the S=0 groundstate for antiparallel lead magnetization. In presence of an external magnetic field spin blockade effects can be detected, again leading to NDC effects for both groundstates.

Multi-Back-Gate Control of Carbon Nanotube Double-Quantum Dot

We have fabricated single-walled carbon nanotube (SWNT) quantum dot device with local multi-back gates, in which a SWNT is not surrounded by an insulator or gate electrodes. The charge states of multi-quantum dots, which are separated by an intrinsic defect of a SWNT, can be independently controlled by applying two local back gates. The charge stability diagram changes depending on the gate voltage range, which changes the interdot coupling between two dots. Furthermore, a honeycomb charge stability diagram corresponding to an intermediately coupled double-quantum dot is also observed.

Artificial atom and quantum terahertz response in carbon nanotube quantumdots

Artificial atom behaviours have been observed in single-wall carbon nanotube (SWCNT)quantum dots (QDs). Two-electron shell structures and the Zeeman splitting ofsingle-particle states were revealed in single-electron transport measurements in lowtemperatures. To demonstrate that the charging energy of the dot lies in a terahertz (THz)range, the THz photon-assisted tunnelling was tested, and was really observed as a satelliteCoulomb peak. Some satellite peaks moved as a frequency was changed, but other peaksdid not move. We give possible models to explain the existence of two different satellitepeaks.

Highly sensitive and broadband carbon nanotube radio-frequency single-electron transistor

We have investigated radio-frequency single-electron transistor operation of single-walled carbon nanotube quantum dots in the strong tunneling regime. At a temperature of 4.2 K and with a carrier frequency of 754.2 MHz, we reach a charge sensitivity of 2.3×10−6e/Hz over a bandwidth of 85 MHz. Our results indicate a gain-bandwidth product of 3.7×1013 Hz(3/2)/e, which is by one order of magnitude better than those for typical radio-frequency single-electron transistors.

Photon-Assisted Electron Transport for a Λ-shaped Carbon Nanotube Junction

We theoretically study the electron transport properties for two coupled single-walled carbon nanotube quantum dots connected to metallic electrodes under the irradiation of an external electromagnetic field at low temperatures. Using the standard nonequilibrium Green's function techniques, we examine the time-averaged transmission coefficient and linear conductance. It is shown that by some numerical examples, the photon-assisted inter-dot coupling causes Fano resonance and the conductance of the system is sensitive to the external field parameters. The transport dependence on the external field parameters may be used to detect the high-frequency microwave irradiation.

Gate-dependent tunneling-induced level shifts observed in carbon nanotube quantum dots

We have studied electron transport in clean single-walled carbon nanotube quantum dots. Because of the large number of the Coulomb blockade diamonds simultaneously showing both shell structure and the Kondo effect, we are able to perform a detailed analysis of tunneling renormalization effects. Thus, determining the environment induced level shifts of this artificial atom. In shells where only one of the two orbitals is strongly coupled, we observe a marked asymmetric gate dependence of the inelastic cotunneling lines together with a systematic gate dependence of the size (and shape) of the Coulomb diamonds. These effects are all given a simple explanation in terms of second-order perturbation theory in the tunnel coupling.

Even-Odd Effect in Andreev Transport through a Carbon Nanotube Quantum Dot

We have measured the current (I)-voltage (V) characteristics of a single-wall carbon nanotube quantum dot coupled to superconducting source and drain contacts in the intermediate coupling regime. Whereas the enhanced differential conductance dI/dV due to the Kondo resonance is observed in the normal state, this feature around zero-bias voltage is absent in the superconducting state. Nonetheless, a pronounced even-odd effect appears at finite bias in the dI/dV subgap structure caused by Andreev reflection. The first-order Andreev peak appearing around V=Delta/e is markedly enhanced in gate-voltage regions, in which the charge state of the quantum dot is odd. This enhancement is explained by a "hidden" Kondo resonance, pinned to one contact only. A comparison with a single-impurity Anderson model, which is solved numerically in a slave-boson mean-field approach, yields good agreement with the experiment.

Spin-flip mesoscopic transport through a carbon nanotube quantum dot system

The pure spin transport in an entire metallic single-wall carbon-nanotube (SWCN) interacting quantum dot (QD) system is investigated by using non-equilibrium Green's function (NEGF) technique. The novel spin current performance introduced by one constant and one rotating magnetic fields shows the unique four-fold degenerate electron shell structure which exists the SWCN QD sensitively. Spin transport properties can be designed by tuning the orbital and Zeeman configuration in the central resonant region, which are greatly influenced by the Coulomb interaction and the magnetic fields.