7 research outputs found
Etching of Surfactant from Solution-Processed, Type-Separated Carbon Nanotubes and Impact on Device Behavior
Semiconducting single-walled carbon nanotubes (SWCNTs) have great potential for use in electronic and optoelectronic devices. However, methods for synthesizing SWCNTs produce a mixture of metallic and semiconducting materials, which require additional processing to separate by electronic type. Purification and enrichment of the semiconducting fraction is readily achieved by using the centrifugation of aqueous suspensions of SWCNTs with the help of surfactants, but this leaves residual surfactant on the SWCNT surface that can impact their electronic and optical properties. Here, we present a detailed study of the sodium taurodeoxycholate (STDC) surfactant removal process during vacuum annealing, showing that it occurs through fragmentation of the surfactant, and that complete removal requires exceedingly high temperatures, which indicates strong binding to the SWCNTs. We then present an approach based on air oxidation and mild annealing to completely remove the surfactant while maintaining the SWCNT properties. Using this approach, we compare single SWCNT electronic devices with and without STDC and show that, despite the very strong surfactant binding, it does not affect device performance substantially
Color Detection Using Chromophore-Nanotube Hybrid Devices
We present a nanoscale color detector based on a single-walled carbon nanotube functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrate the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggest that upon photoabsorption, the chromophores isomerize from the ground state trans configuration to the excited state cis configuration, accompanied by a large change in dipole moment, changing the electrostatic environment of the nanotube. All-electron ab<i></i> initio calculations are used to study the chromophore-nanotube hybrids and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments support the notion of dipole changes as the optical detection mechanism
Functionalization of Single-Wall Carbon Nanotubes with Chromophores of Opposite Internal Dipole Orientation
We report the functionalization of
carbon nanotubes with two azobenzene-based
chromophores with large internal dipole moments and opposite dipole
orientations. The molecules are attached to the nanotubes noncovalently
via a pyrene tether. A combination of characterization techniques
shows uniform molecular coverage on the nanotubes, with minimal aggregation
of excess chromophores on the substrate. The large on/off ratios and
the subthreshold swings of the nanotube-based field-effect transistors
(FETs) are preserved after functionalization, and different shifts
in threshold voltage are observed for each chromophore. Ab initio
calculations verify the properties of the synthesized chromophores
and indicate very small charge transfer, confirming a strong, noncovalent
functionalization
Figure of Merit for Carbon Nanotube Photothermoelectric Detectors
Carbon nanotubes (CNTs) have emerged as promising materials for visible, infrared, and terahertz photodetectors. Further development of these photodetectors requires a fundamental understanding of the mechanisms that govern their behavior as well as the establishment of figures of merit for technology applications. Recently, a number of CNT detectors have been shown to operate based on the photothermoelectric effect. Here we present a figure of merit for these detectors, which includes the properties of the material and the device. In addition, we use a suite of experimental characterization methods for the thorough analysis of the electrical, thermoelectric, electrothermal, and photothermal properties of the CNT thin-film devices. Our measurements determine the quantities that enter the figure of merit and allow us to establish a path toward future performance improvements
Photothermoelectric p–n Junction Photodetector with Intrinsic Broadband Polarimetry Based on Macroscopic Carbon Nanotube Films
Light polarization is used in the animal kingdom for communication, navigation, and enhanced scene interpretation and also plays an important role in astronomy, remote sensing, and military applications. To date, there have been few photodetector materials demonstrated to have direct polarization sensitivity, as is usually the case in nature. Here, we report the realization of a carbon-based broadband photodetector, where the polarimetry is intrinsic to the active photodetector material. The detector is based on p–n junctions formed between two macroscopic films of single-wall carbon nanotubes. A responsivity up to ∼1 V/W was observed in these devices, with a broadband spectral response spanning the visible to the mid-infrared. This responsivity is about 35 times larger than previous devices without p–n junctions. A combination of experiment and theory is used to demonstrate the photothermoelectric origin of the responsivity and to discuss the performance attributes of such devices
Carbon Nanotube Terahertz Detector
Terahertz (THz) technologies are
promising for diverse areas such
as medicine, bioengineering, astronomy, environmental monitoring,
and communications. However, despite decades of worldwide efforts,
the THz region of the electromagnetic spectrum still continues to
be elusive for solid state technology. Here, we report on the development
of a powerless, compact, broadband, flexible, large-area, and polarization-sensitive
carbon nanotube THz detector that works at room temperature. The detector
is sensitive throughout the entire range of the THz technology gap,
with responsivities as high as ∼2.5 V/W and polarization ratios
as high as ∼5:1. Complete thermoelectric and opto-thermal characterization
together unambiguously reveal the photothermoelectric origin of the
THz photosignal, triggered by plasmonic absorption and collective
antenna effects, and suggest that judicious design of thermal management
and quantum engineering of Seebeck coefficients will lead to further
enhancement of device performance
Superlinear Composition-Dependent Photocurrent in CVD-Grown Monolayer MoS<sub>2(1–<i>x</i>)</sub>Se<sub>2<i>x</i></sub> Alloy Devices
Transition
metal dichalcogenides (TMDs) have emerged as a new class of two-dimensional
materials that are promising for electronics and photonics. To date,
optoelectronic measurements in these materials have shown the conventional
behavior expected from photoconductors such as a linear or sublinear
dependence of the photocurrent on light intensity. Here, we report
the observation of a new regime of operation where the photocurrent
depends superlinearly on light intensity. We use spatially resolved
photocurrent measurements on devices consisting of CVD-grown monolayers
of TMD alloys spanning MoS<sub>2</sub> to MoSe<sub>2</sub> to show
the photoconductive nature of the photoresponse, with the photocurrent
dominated by recombination and field-induced carrier separation in
the channel. Time-dependent photoconductivity measurements show the
presence of persistent photoconductivity for the S-rich alloys, while
photocurrent measurements at fixed wavelength for devices of different
alloy compositions show a systematic decrease of the responsivity
with increasing Se content associated with increased linearity of
the current–voltage characteristics. A model based on the presence
of different types of recombination centers is presented to explain
the origin of the superlinear dependence on light intensity, which
emerges when the nonequilibrium occupancy of initially empty fast
recombination centers becomes comparable to that of slow recombination
centers