Single-wall Carbon Nanotube Quantum Dots Research Articles

Phonon runaway in carbon nanotube quantum dots

We explore electronic transport in a nanotube quantum dot strongly coupled with vibrations and weakly with leads and the thermal environment. We show that the recent observation of anomalous conductance signatures in single-walled carbon nanotube (SWCNT) quantum dots can be understood quantitatively in terms of current driven `hot phonons' that are strongly correlated with electrons. Using rate equations in the many-body configuration space for the joint electron-phonon distribution, we argue that the variations are indicative of strong electron-phonon coupling requiring an analysis beyond the traditional uncorrelated phonon-assisted transport (Tien-Gordon) approach.

Spin effects in single-electron transport through carbon nanotube quantum dots

We investigate the total spin in an individual single-wall carbon nanotube quantum dot with various numbers of electrons in a shell by using the ratio of the saturation currents of the first steps of Coulomb staircases for positive and negative biases. The current ratio reflects the total-spin transition that is increased or decreased when the dot is connected to strongly asymmetric tunnel barriers. Our results indicate that total-spin states with and without magnetic fields can be traced by this method.

Spin-dependent transport through a single-walled carbon nanotube coupled to a ferromagnetic and a nonmagnetic metal electrode

We report on transport measurements of an individual single-walled carbon nanotube (SWNT) coupled to a ferromagnetic and a nonmagnetic metal electrode. The low-temperature differential conductance shows a suppression of zero-bias conductance and Coulomb blockade oscillations, where the metallic SWNT behaves as a quantum dot. A marked hysteretic magnetoresistance is observed within the coercive region of the ferromagnetic metal film. The differential resistance has distinctly different values when the magnetization orientation is reversed. Our observed data suggest that the spin-split density of states lies within the SWNT quantum dot. We present an Anderson Hamiltonian to model the spin-polarized electron transport through the SWNT quantum dot with spin-split discrete levels, which supports the experimental observation.

Quantum response of carbon nanotube quantum dots to terahertz wave irradiation

Single-electron transport measurements have been carried out at 1.5 K on single-wall carbonnanotube (SWCNT) quantum dots (QDs) under terahertz (THz) irradiation with afrequency of 2.5 THz. The most important finding in this study was new side-peaks thatappeared only under the THz irradiation. From the detailed analysis with energy scalesassociated with the QDs, we conclude that the side-peaks originate from thephoton-assisted tunnelling (PAT) of an electron in the QD to the drain electrode. Thepresent THz-PAT is the first observation in QDs because of the larger energyscales in a SWCNT QD, compared with those of standard semiconductor QDs.

Close-conjugation of quantum dots and gold nanoparticles to sidewall functionalized single-walled carbon nanotube templates

Two types of nanoscale hybrid materials have been synthesized by conjugating CdSe–ZnS quantum dots (QDs) and gold nanoparticles (NPs) to sidewall functionalized single-walled carbon nanotube (SWNT) templates, and examined photoluminescence (PL) properties of the hybrid materials. The two types of hybrid nanoscale materials are SWNT–QD and SWNT–gold–NP conjugates. Excessive sidewall functionalization of SWNT into nitro- and amino-derivatives provided weak PL and water solubility to the SWNT derivatives. Solubility of SWNT derivatives in aqueous media was helpful for efficient conjugation of SWNTs to QDs and gold–NPs. The SWNT–QD and SWNT–gold–NP conjugates were characterized using atomic force microscopy (AFM) imaging. From AFM imaging we identified that the sidewall functionalized SWNTs assisted the formation of one-dimensional close-conjugates of QDs and gold–NPs. Although the nitro- and amino-derivatives of SWNT showed weak PL, conjugation of gold–NPs to the amino-derivative quenched the PL quantitatively. On the other hand, conjugation of luminescent QDs to SWNTs resulted in a partial (∼35%) quenching of PL from QDs. We attributed the quenching of PL from SWNTs by gold–NPs to non-radiative energy transfer between SWNTs and gold–NPs or vice versa, and the quenching of PL from QDs is due to non-radiative energy transfer from QDs to SWNTs. In the current work, we demonstrated a simple method of close-packing of QDs and gold–NPs using functionalized SWNT templates. Also, we identified that PL properties of SWNTs and QDs are affected in the close-packed structures. SWNT and NP based hybrid materials show great promise as building blocks for nanoscale devices. In this regard the preparation and understanding of the properties of SWNT–QD and SWNT–gold–NP conjugates would be useful during the design of various hybrid nanoscale materials for device applications.

Quantum-dot nanodevices with carbon nanotubes

We review our recent work on quantum-dot devices with carbon nanotubes. We conclude that the single-wall carbon nanotube quantum dot is an artificial atom with two- or four-electron shell structures. Zeeman splitting of single particle levels was observed, which is advantageous for the spin based quantum computing device (spin qubit) because the single spin is generated by putting one electron in the shell. Single-electron devices such as single-electron inverter and single-electron exclusive-OR gates have been fabricated, and their performance has been demonstrated at liquid-helium temperature. Despite the expected room-temperature operation from the single-electron charging energy, the operation temperature of our devices was limited to ∼10K because of the low height of the tunnel barrier.

Pulse-Excited Current Measurements in Carbon Nanotube Quantum Dots

We present experimental results on the electrical pulse-excited transport measurements of an individual single-wall carbon nanotube (SWNT) quantum dots. By applying a square pulse signal, the pulse-excited Coulomb peaks are observed in Coulomb oscillation measurements. In the SWNT quantum dot, the even-odd effect is confirmed by the alternate change of the Coulomb diamond size in the standard dc measurement. Magnetic fields perpendicular to the tube axis have revealed the simple Zeeman splitting of single particle states, this splitting is directly observed in the pulse-excited current in even-odd regime. We find the spin relaxation time at least longer than 1 µs.

One-Dimensional Shell Structures and Excitation Spectrum in Single-Wall Carbon Nanotube Quantum Dots

We present experimental results on low-temperature transport of an individual single-wall carbon nanotube (SWNT) quantum dot. Four-electron shell structures were observed, due to the twofold subband degeneracy of the SWNT in addition to the twofold spin degeneracy. The excitation spectroscopy with a large source–drain voltage has made it possible to clearly observe the simple Zeeman splitting of single-particle states in a shell, and a textbook model of an interacting two-electron system has been demonstrated with directly distinguishable singlet and triplet states when the shell was half-filled.

Tunneling in Suspended Carbon Nanotubes Assisted by Longitudinal Phonons

Current-voltage characteristics of suspended single-wall carbon nanotube quantum dots show a series of steps equally spaced in voltage. The energy scale of this harmonic, low-energy excitation spectrum is consistent with that of the longitudinal low-k phonon mode (stretching mode) in the nanotube. Agreement is found with a Franck-Condon-based model in which the phonon-assisted tunneling process is modeled as a coupling of electronic levels to underdamped quantum harmonic oscillators. A comparison with this model indicates a rather strong electron-phonon coupling factor of order unity.

Coupling between electronic transport and longitudinal phonons in suspended nanotubes

Current–voltage characteristics of suspended single-wall carbon nanotube (NT) quantum dots show a series of steps equally spaced in voltage. The energy scale of this harmonic, low-energy excitation spectrum is consistent with that of the longitudinal low-k phonon mode in the NT. Agreement is found with a Franck–Condon-based model in which the phonon-assisted tunnelling process is modelled as a coupling of electronic levels to underdamped quantum harmonic oscillators. Comparison with this model indicates a rather strong electron–phonon coupling factor of order unity. We investigate different electron–phonon coupling mechanisms and give estimates of the coupling factor.

Carbon nanotube quantum dots fabricated on a GaAs∕AlGaAs two-dimensional electron gas substrate

Single-wall carbon nanotube (SWCNT) quantum dots have been fabricated on a GaAs∕AlGaAs two-dimensional electron gas (2DEG) substrate, and the single electron transport measurements have been carried out at 2.8 K and 22 mK with the 2DEG used as a gate. It was demonstrated that the gating by the 2DEG could be switched on and off by controlling a quantum point contact fabricated between the dot and the Ohmic contact to the 2DEG gate. The unique combination of the SWCNT dot and the 2DEG may open a door to realize flexible hybrid devices.

Characterization and estimation of tunnel barrier height in metallic single-wall carbon nanotube quantum dots

A tunnel barrier height has been estimated in quantum dots (QDs) formed in metallic single-wall carbon nanotubes (SWNTs), where QDs can be fabricated simply by depositing metallic contacts on top of the nanotube. Transport measurements have been carried out in a temperature range between 1.5 and 300 K, and revealed single and multi-QD behaviors in different samples at low temperatures. The Arrhenius plot gave an activation energy of ∼6 meV for the barrier formed very likely at the metal–SWNT interface for the single QDs, and two activation energies for seemingly double dots. The latter case comes from the unintentional tunnel barrier due to defects. Discussions on the QD formation and suggestions for a higher temperature operation are given.

Four-Electron Shell Structures and an Interacting Two-Electron System in Carbon-Nanotube Quantum Dots

Low-temperature transport measurements have been carried out on single-wall carbon-nanotube quantum dots in a weakly coupled regime in magnetic fields. Four-electron shell filling was observed, and the magnetic field evolution of each Coulomb peak was investigated. Excitation spectroscopy measurements have revealed Zeeman splitting of single particle states for one electron in the shell, and demonstrated singlet and triplet states with direct observation of the exchange splitting at zero-magnetic field for two electrons in the shell, the simplest example of Hund's rule.

Coherent transport between two interface states in single-walled carbon nanotube quantumdots

The resonance tunnelling of states at two interfaces are investigated through the threemodels of two-terminated carbon nanotube heterojunctions: (5, 5)/(10, 0)/(5, 5), (9, 0)/(10,0)/(9, 0) and (5, 5)/(10, 0)/(9, 0). In (5, 5)/(10, 0)/(5, 5) CNQD, we observed the existenceof interface states in LDOS, and the resonant tunnelling peak at the position ofinterface states also emergent in quantum conductance. With the increase ofQDs size, the resonant tunnelling peak gradually disappears. Comparing theirquantum conductance for different sizes, when the size of the (5, 5)/(10, 0)/(5,5) model is larger than 23 layers, there is not a quantum conductance peak at−0.26 eV. Therefore, the largest size for resonant tunnelling occurring between two interfacestates in (5, 5)/(10, 0)/(5, 5) is about 23 layers (about 45 Å).

Importance of electron–electron interactions and Zeeman splitting in single-wall carbon nanotube quantum dots

We present experimental results on the low-temperature transport of an individual single-wall carbon nanotube (SWNT) quantum dot. A variety of features in the Coulomb diamonds have been observed, such as a distortion of Coulomb diamonds as well as standard diamonds with a rectangular shape, depending on a gate voltage range. Magnetic field studies have revealed the simple Zeeman splitting of single particle states in some gate voltage ranges and a spin polarization in the dot in other gate voltage range. Our results suggest the importance of electron–electron interactions and the single particle states which move differently with a gate voltage.

Orbital motion of metallic carbon nanotubes in an axial magnetic field

It is found that the orbital effect of a single-wall carbon nanotube quantum dot with rather longer length in the axial magnetic field can become the same order as the Zeeman one or even exceed it for the states nearest to the Fermi level. We predict that the spin singlet–triplet transition can be driven almost entirely by the orbital effect in the nanotube quantum dot. Thus, a type of Kondo effect realized by coupling the singlet state to all triplet states may also be observed in the nanotubes.

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 QD’s 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.

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.

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.