Abstract:We study the optical properties of carbon nanotubes within the single-band tight-binding approximation. The great number of one-dimensional-like subbands of the nanotubes lead to typical van Hove singularities on the local density of states. When a tube is under the influence of a laser beam, optical transitions are allowed between Van Hove singularities and they can be observed experimentally in absorption spectra. External magnetic and electric fields modify the energy spectrum of carbon nanotubes inducing c… Show more
“…In contrast, the bandgap reduction of the semiconducting part of the junction in Figure is from 0.86 to 0.18 eV when the laser is on. A realistic Stark coefficient of 1.5 eV/(V/nm) 2 for the S 11 state based on the theoretical literature ,, cannot account for the full shift by only the tip electric field. Instead we invoke laser excitation as an additional source of bandgap shift.…”
Section: Resultsmentioning
confidence: 99%
“…Computational studies have predicted that fields on the order of V/nm can induce Stark effects that strongly perturb the electronic structure of semiconducting and metallic carbon nanotubes. 11,17 Such calculations also show how electronic structure varies across a semi-conductingÀmetallic junction. 5 We showed that we can tune the bandgaps of the two halves of a single semiconducting-metallic CNT junction with an applied electric field.…”
Section: Discussionmentioning
confidence: 95%
“…Stark effect-tuning of CNT bandgaps in the presence of strong static and optical electric fields, but away from junctions, has been studied extensively. − To a first approximation, tight binding models describe the band edges as van Hove singularities, although more sophisticated models − and experiments − highlight the electron–hole bound character of the optically brightest excited states, with exciton dimeters of 1.5–1.8 nm calculated and measured. The conclusion from various levels of theory is that static and optical fields on the order of several V/nm can cause semiconducting tubes to acquire metallic character (close the bandgap), and vice versa for metallic tubes. − …”
Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting-metallic junction. We image topography, laser absorption at 532 nm, and measure I-V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting-metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.
“…In contrast, the bandgap reduction of the semiconducting part of the junction in Figure is from 0.86 to 0.18 eV when the laser is on. A realistic Stark coefficient of 1.5 eV/(V/nm) 2 for the S 11 state based on the theoretical literature ,, cannot account for the full shift by only the tip electric field. Instead we invoke laser excitation as an additional source of bandgap shift.…”
Section: Resultsmentioning
confidence: 99%
“…Computational studies have predicted that fields on the order of V/nm can induce Stark effects that strongly perturb the electronic structure of semiconducting and metallic carbon nanotubes. 11,17 Such calculations also show how electronic structure varies across a semi-conductingÀmetallic junction. 5 We showed that we can tune the bandgaps of the two halves of a single semiconducting-metallic CNT junction with an applied electric field.…”
Section: Discussionmentioning
confidence: 95%
“…Stark effect-tuning of CNT bandgaps in the presence of strong static and optical electric fields, but away from junctions, has been studied extensively. − To a first approximation, tight binding models describe the band edges as van Hove singularities, although more sophisticated models − and experiments − highlight the electron–hole bound character of the optically brightest excited states, with exciton dimeters of 1.5–1.8 nm calculated and measured. The conclusion from various levels of theory is that static and optical fields on the order of several V/nm can cause semiconducting tubes to acquire metallic character (close the bandgap), and vice versa for metallic tubes. − …”
Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting-metallic junction. We image topography, laser absorption at 532 nm, and measure I-V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting-metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.
“…To capture the effect of a QD neighboring the CNT, the onsite energy of each carbon atom in the tight-binding unit cell was modified by the electric potential of the QD at each carbon position . The excited QD was approximated as a point dipole of magnitude μ.…”
Efficient
heat dissipation and large gate capacitance have made
carbon nanotube field-effect transistors (CNT FETs) devices of interest
for over 20 years. The mechanism of CNT FETs involves localization
of the electronic structure due to a transverse electric field, yet
little is known about the localization effect, nor has the electronic
polarization been visualized directly. Here, we co-deposit PbS quantum
dots (QDs) with CNTs and optically excite the QD so its excited-state
dipolar field biases the local environment of a CNT. Using single-molecule
absorption scanning tunneling microscopy, we show that the electronic
states of the CNT become transversely localized. By nudging QDs to
different distances from the CNT, the magnitude of the localization
can be controlled. Different bias voltages probe the degree of localization
in different CNT excited states. A simple tight-binding model for
the CNT in an electrostatic field provides a semiquantitative model
for the observed behavior.
“…The field is applied perpendicularly to the tube axis, and the intensities are within the range of 10 -2 -10 -1 V/Å, as used in previous calculations. [16][17][18][19][20][21][22][23] The applied field is always parallel to the COOH radical, as shown in Figure 1.…”
Carboxylated semiconductor and metallic carbon nanotubes under transverse electrical fields are investigated through density functional theory based on first-principles calculations. The external field polarizes the system, resulting in an induced electric dipole moment toward the incident field with the modulus directly dependent on the field strength. The structural and electronic properties of the resulting system due to the orbital hybridization between the nanotube and COOH states are shown to be affected by the applied field. These results open new perspectives for different potential uses, such as to enhance the capacity of the composite to bind and characterize other substances, especially polar molecules, and as mechanisms to monitor the bound substances or control electron injection or detection, by varying the external field through a controlled application.
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