Session B28: Focus Session: Carbon Nanotube Optoelectronics

Excited states and electro-optics of carbon nanotubes

We will discuss experimental and theoretical results on nanotube excited state production and luminescence through photoexcitation, electron-hole recombination and hot carrier impact excitation. The effects of an external electric field, as well as environmental effects on the absorption and emission spectra will be examined. Finally, nanotube photoconductivity and photovoltage and the role of the substrate and defects on these processes will be analyzed.   

Electrically driven thermal light emission from individual single-walled carbon nanotubes

Light emission from carbon nanotubes offer unique opportunities in nano-optoelectronics, because of their chirality dependent electronic structure, availability of high quality electrical contact, and very high aspect ratio. We study electrically- driven light emission from individual single-walled carbon nanotubes, including both quasi-metallic and semiconducting species.\footnote{D. Mann et al., submitted for publication (2006).} Our field effect transistor structure utilizes a clean, as-grown nanotube suspended across a trench, allowing for low contact resistance and good isolation from the substrate. The spectra from quasi-metallic nanotubes reveal pronounced peaks in the visible and infrared corresponding to E$_{11}$ and E$_{22}$ transitions. The emission rates show strong correlation with electrical power dissipated in the devices, consistent with thermally excited emission due to resistive heating. We observe similar behavior for the semiconducting devices, although electroluminescence in these nanotubes has been explained by either carrier injection or impact excitation.   

Electroluminescence Properties of Carbon Nanotube Network Transistors

Carbon nanotubes network transistors (CNNT) open a promising route for the integration of nanotubes in electronics for that they circumvent major issues related to their fabrication. [1] They also reduce device-to-device discrepancies because they combine the properties of an ensemble of nanotube species. Here, we investigated the optoelectronic properties of the CNNT fabricated from different nanotube sources and found bright electroluminescent (EL) emission. The EL is specific to the nanotube source and can be linked using absorption spectra to their diameter distribution. (1) E.S. Snow, P.M. Campbell, M.G. Ancona, Appl. Phys. Lett., 2005, 86, 033105.   

Localized Photoresponse and Raman Spectra of Long Carbon Nanotube FET's

The spatially resolved photoresponse, and Raman spectra of CVD grown carbon nanotube field effect transistors with channel lengths between 2 and 50$\lambda $m have been measured using conventional imaging techniques at photon energies between 1.4 and 2.7eV. A strong localized photoresponse including both the short circuit photocurrent and the high impedance photovoltage is observed even at zero bias with spatially resolvable contributions from the Schottky barriers, from observable inhomogeneities and from fluctuations all along the device. The magnitude of the photoresponse from defects such as tube crossings and fluctuations in the tube environment can be comparable to or stronger than that arising from the charge separation at the Schottky barriers. The Raman spectra show high quality CNTs with some correlation between the spatial positions of weak D lines, when observed, and changes in the photoresponse. Comparisons of the Raman spectra and the intensity of the photoresponse show the presence of significant potential fluctuations on the micron length scale along these devices   

Interplay Between Transport and Optical Properties in Carbon Nanotube $p-n$ Diodes

The $p-n$ junction diode is the basis for nearly all-modern semiconductor electronics. It is the basis for transistors and optical devices. For any new material system, therefore, a proper characterization of the $p-n$ junction is crucial for their development into electronic devices. In this talk, I will demonstrate the formation of $p-n$ junction diodes along individual single-walled carbon nanotubes (SWNTs). The $p-n$ junction is formed using a novel electrostatic doping technique using a pair of split gate electrodes, and can exhibit \textit{ideal diode} behavior, the theoretical limit of performance for any diode. The low background leakage currents coupled with a built-in electric field region to transport the quasi-particles makes these diodes ideal for studying the optical response of SWNTs. I will show that the photocurrent spectroscopy of these diodes is able to provide a comprehensive probe of the excited states in SWNTs. A series of narrow excitonic resonant peaks is observed over a wide spectral range, including the first exciton peak ($E_{11})$, which defines the optical gap. At an intermediate energy the onset of continuum (electronic band gap) is observed and demonstrate large exciton binding energies. Because of the large exciton binding energies, the large photocurrent derived from $E_{11}$ excitons is not expected. Here, I will describe several characteristics related to these peaks such as the origin of the quasi-particle (electron and hole) currents, quantum efficiency, and the role of many-body effects in determining the dark (ideal diode) and excited (excitonic) states of SWNTs.   

Impact Excitation by Hot Carriers in Carbon Nanotubes

We find in Ref. 1 and 2, that the impact excitation processes in nanoscale devices are much more efficient than in conventional bulk semiconductors due to the enhanced Coulomb interaction in low dimensions. In semiconducting carbon nanotubes, we calculate the impact excitation rates to be 4-5 orders of magnitude larger than in bulk semiconductors [2]. The impact excitation rate is much higher in nanotubes than the impact ionization, which neglects electron-hole interaction of the produced electron-hole pair, while their difference is negligible in bulk materials. The angular momentum conservation law plays a crucial role in determining the threshold energy of the impact excitation. The spectra of the produced excitons depends strongly on the bias and not constrained by the dipole selection rule as in the photoluminescence. The triplet excitons have approximately equal probability to be produced, unlike 1/4 statistical fraction for the independently injected electrons and holes. [1] J. Chen, V. Perebeinos, M. Freitag, J. Tsang, Q. Fu, J. Liu, Ph. Avouris, Science 310, 1171, 2005. [2] V. Perebeinos and Ph. Avouris, Phys. Rev. B. 74, 121410(R), 2006.   

Exciton Annihilation Processes in Individual Single-Wall Nanotubes

Field enhanced photocurrent measurements of individual single-wall nanotubes show that bound exciton dissociation occurs through two distinct processes. At low fields, the barrier to field ionization is not surmounted but bound carriers can still dissociate by tunneling into the free carrier states. At high fields (approximated by the binding energy divided by the Bohr radius, or,$E_b /r)$ the bound excitonic state is destroyed. We measure the photocurrent of a SWNT capacitor, in which the nanotubes lie on a 100 nm oxide dielectric on doped silicon substrate. This allows us to apply extremely large electric fields across the nanotube. Excitons do not contribute to the photocurrent unless dissociation into free carrier states occurs. At fields below $1\times 10^8V/m$ the exciton peak increases according to Fowler-Nordheim field dependence. At a field of approximately $1.2\times 10^8V/m$the photocurrent rapidly increases by more than an order of magnitude suggesting a huge increase in the exciton dissociation rate. This corresponds to the predicted field required for exciton annihilation to occur.   

The role of intrinsic regions in nanotube photodiodes

In this work, we consider the impact of intrinsic regions on transport in carbon nanotube diodes. Recently nanotube diodes have been fabricated in a split-gate geometry where a central intrinsic region separates two regions gated p-type and n-type, respectively. These devices show near ideal diode behavior and can also act as photodetectors. We use a self-consistent non-equilibrium Green's function approach to examine how the central intrinsic region affects the properties of a nanotube p-i-n photodiode. The charge and potential along the diode are determined self-consistently for systems with different intrinsic layer lengths. We find that the intrinsic region has little effect on the dark current in the device. However, as the size of the intrinsic region increases, the photocurrent grows as well. The presence of a central intrinsic region also leads to greater power conversion efficiency in nanotube photodiodes. These changes in the photoresponse can be related to charge redistribution caused by the introduction of the intrinsic layer. This leads to a reduction of the flat band regions near the leads, while unmasking the van Hove singularities in the central intrinsic region that enhance the photoresponse for higher photon energies. This effect is quite general and may be observed in similar p-n junctions (i.e. nanowires) where the density of states is quasi-one dimensional.   

Dependence of Raman-active modes on the external voltage in single-wall carbon nanotube thin films

We report on Raman measurements under the application of an external voltage in gap-cell devices made by transparent and conducting single-wall carbon nanotube (SWNT) thin films [1] Two different Raman excitation wavelengths (785 and 633 nm) were used. Application of voltage results in downshifts of the D and G modes and in reduction of their intensity. The intensities of the radial breathing modes increase with voltage in metallic SWNTs, while decreasing in semiconducting SWNTs. A model explaining the phenomenon in terms of both direct and indirect (Joule heating) effects of the field is proposed. Our work rules out the elimination of large amounts of metallic SWNTs in thin film transistors using high field pulses. Our results support the existence of Kohn anomalies in the Raman-active optical branches of metallic graphitic materials. Additional Raman measurements in SWNT thin film transistors at varying source-drain voltage and gate voltage will be presented as well. [1] G Fanchini, et al, submitted [2] S.Piscanec et al, PRL 93 (2004) 185503   

Absolute Absorptivity of Single-walled Carbon Nanotubes Employing a Pyroelectric Detector

Optical properties are important for determining fundamental characteristics of carbon single-walled nanotube (SWNT) samples including purity, chirality, and tube diameter. Previously, we have estimated the volume fraction of metallic versus semiconducting tubes for highly purified SWNT bucky-paper on a pyroelectric detector from spectral responsivity measurements and an effective medium approximation to determine the dielectric function (1). Pyroelectric detector-based measurements are based on the thermalization of photons within the SWNT coating and provide a robust technique for measuring absolute absorptivity at normal incidence. Alternatively, we perform transmissivity measurements of SWNTs by employing a gold-black coated pyroelectric detector. Spectral responsivity measurements are made by direct substitution against a NIST calibrated detector such that quantitative changes in the volume fraction and purity of SWNT samples are revealed. These results will be compared to specular transmissivity measurements made by UV-VIS spectrometry. Raman spectroscopy will also serve to verify nanotube properties. (1) K.E.H. Gilbert, J.H. Lehman, A.C. Dillon and J.L. Blackburn Appl. Phys. Lett. 88, 143122 (2006).   

Photoresponse of Suspended Carbon Nanotube Networks: Single-Walled Carbon Nanotube Infrared Bolometer

The photoresponse of a single-walled carbon nanotube (SWNT) film is dramatically enhanced when the nanotube film is suspended between electrical contacts in vacuum. We show that the change in electrical conductivity is bolometric (caused by heating of the SWNT network). Electron-phonon interactions lead to ultrafast relaxation of the photoexcited carriers and the energy of the incident infrared radiation is efficiently transferred to the crystal lattice. The photoinduced changes in resistance occur as result of temperature changes rather than by photoexcited holes and electrons and we consider the implications of this result for the band and exciton models in carbon nanotubes. We show that the infrared photoresponse of suspended SWNT films is sufficiently high that they may function as the sensitive element of an infrared bolometric detector. M.E.Itkis, F.Borondics, A.Yu, R.C.Haddon, \textit{Science} \textbf{312}, 413 (2006)