Chemically Doped Random Network Carbon Nanotube p–n Junction Diode for Rectifier

The ability to prepare uniform and dense networks of purely semiconducting single-walled carbon nanotubes (SWNTs) has enabled the design of various (opto-)electronic devices, especially field-effect transistors (FETs) with high carrier mobilities. Further optimization of these SWNT networks is desired to surpass established solution-processable semiconductors. The average diameter and diameter distribution of nanotubes in a dense network were found to influence the overall charge carrier mobility; e.g., networks with a broad range of SWNT diameters show inferior transport properties. Here, we investigate charge transport in FETs with nanotube networks comprising polymer-sorted small diameter (6,5) SWNTs (0.76 nm) and large diameter plasma torch SWNTs (1.17-1.55 nm) in defined mixing ratios. All transistors show balanced ambipolar transport with high on/off current ratios and negligible hysteresis. While the range of bandgaps in these networks creates a highly uneven energy landscape for charge carrier hopping, the extracted hole and electron mobilities vary nonlinearly with the network composition from the lowest mobility (15 cm2 V-1 s-1) for only (6,5) SWNT to the highest mobility (30 cm2 V-1 s-1) for only plasma torch SWNTs. A comparison to numerically simulated network mobilities shows that a superposition of thermally activated hopping across SWNT-SWNT junctions and diameter-dependent intratube transport is required to reproduce the experimental data. These results also emphasize the need for monochiral large diameter nanotubes for maximum carrier mobilities in random networks.

Dispersions of purely semiconducting single-walled carbon nanotubes (SWCNTs) have enabled solution-processed SWCNT networks as active layers in field-effect transistors (FETs) with high carrier mobilities and excellent on/off current ratios. Although reproducibility has improved in recent years, reaching the level that is required for commercial large-scale processing remains a challenge. A key issue is the tendency of SWCNTs to aggregate over time, resulting in network inhomogeneities that cause large device performance variations. Based on the tailored formulation of colloidal inks by the choice of solvent and use of additives, we demonstrate the strong stabilization effect of phenanthroline additives on polymer-sorted (6,5) SWCNT using time-dependent near-infrared absorption spectroscopy as a fast and simple assessment tool for the aggregation rate. The addition of the N-heteropolycycle 1,10-phenanthroline significantly extends the stability of dispersions of polymer-wrapped nanotubes in toluene and hence improves the morphology of spin-coated networks even after ink storage for several days. Bottom-contact, top-gate FETs based on such networks show much higher charge carrier mobilities and drastically reduced device-to-device variations compared to devices based on SWCNT dispersions without phenanthroline. Nanotube ink formulations with small-molecule additives are an important step toward reproducible device parameters and are crucial for the translation of nanotube FETs from the laboratory to commercial applications.

Recent advances in the solution-phase sorting and assembly of semiconducting single-walled carbon nanotubes (SWCNTs) have enabled significant gains in the performance of field-effect transistors (FETs) constructed from dense arrays of aligned SWCNTs. However, the channel length (LCH) downscaling behaviors of these arrays, which contain some organizational disorder (i.e., rotational misalignment and non-uniform pitch), have not yet been studied in detail below LCH of 100 nm. This study compares the behaviors of individualized SWCNTs with arrays of aligned, solution-cast SWCNTs in FETs with LCH ranging from 30 to 240 nm. The on-state conductance of both individual and array SWCNTs rises with decreasing LCH. Nearly ballistic transport is observed for LCH < 40 nm in both cases, reaching a conductance of 0.82 Go per SWCNT in arrays, where Go = 2e2/h is the quantum conductance. In the off-state, the off-current and subthreshold swing of the individual SWCNTs remain nearly invariant with decreasing LCH whereas array SWCNT FETs suffer from increasing off-state current and deteriorating subthreshold swing for LCH below 100 nm. We analyze array disorder using atomic force microscopy, which shows that crossing SWCNTs that arise from misoriented alignment raise SWCNTs off of the substrate for large portions of the channel when LCH is small. Electrostatics modeling analysis indicates that these raised SWCNTs are a likely contributor to the deteriorating off-current and subthreshold characteristics of arrays. These results demonstrate that improved inter-SWCNT pitch uniformity and alignment with minimal inter-SWCNT interactions will be necessary in order for solution processed SWCNT arrays to reach subthreshold performance on par with isolated SWCNTs. These results are also promising because they show that arrays of solution-processed SWCNTs can nearly reach ballistic conductance in the on-state despite imperfections in pitch and alignment.

The chemical and physical structure of semiconducting single-walled carbon nanotubes (SWCNTs) results in optical and electronic properties of promise for a wide variety of applications. Until quite recently the presence of metallic SWCNT impurities has hampered efforts to gain a deeper understanding of their true potential, with the additional complication that most commercially available materials contain tens of different chiral species. Significant effort has been devoted to elegant enrichment strategies aimed at extracting tailored semiconducting SWCNT species from the raw soot, from the use of subtly tunable surfactant interactions to the exploitation of specific DNA sequences. However, conjugated polymers, typically based on the fluorene moiety, appear to show the greatest promise with regards to their high selectivity and viability for scalable manufacturing approaches. Unfortunately, the van de Waals forces between the pi-electron systems of the polymer and SWCNT that enable the selective extraction of semiconducting SWCNTs with high purity also make removal of the polymer difficult. Since these polymers typically have a wide bandgap they act as an insulating coating on the surface of the individual SWCNTs within functional networks, inhibiting the transport of energy in the form of excitons and/or charge carriers. Here we demonstrate approaches aimed at replacing the strongly-bound polymers with variants that can be removed using simple solution-based chemical strategies, resulting in networks with vastly improved energy transport properties. We show that removal of the polymer results in a significant enhancement of the charge carrier mobility and electrical conductivity in the SWCNT networks, allowing optimization of the thermoelectric properties for both p-type and n-type transport. Finally, we extend the approach to samples strongly enriched in a single chiral SWCNT species, which allows us to employ transient spectroscopic techniques to probe enhanced exciton transport through the SWCNT network with high spectral fidelity. Our studies highlight a methodology by which high-performance SWCNT thin films can be prepared that could realize their potential for electronic and optoelectronic applications.

In this paper, the fabrication of carbon nanotubes field effect transistors by chemical self-assembly of semiconducting single walled carbon nanotubes (s-SWNTs) on prepatterned substrates is demonstrated. Polyfluorenes derivatives have been demonstrated to be effective in selecting s-SWNTs from raw mixtures. In this work the authors functionalized the polymer with side chains containing thiols, to obtain chemical self-assembly of the selected s-SWNTs on substrates with prepatterned gold electrodes. The authors show that the full side functionalization of the conjugated polymer with thiol groups partially disrupts the s-SWNTs selection, with the presence of metallic tubes in the dispersion. However, the authors determine that the selectivity can be recovered either by tuning the number of thiol groups in the polymer, or by modulating the polymer/SWNTs proportions. As demonstrated by optical and electrical measurements, the polymer containing 2.5% of thiol groups gives the best s-SWNT purity. Field-effect transistors with various channel lengths, using networks of SWNTs and individual tubes, are fabricated by direct chemical self-assembly of the SWNTs/thiolated-polyfluorenes on substrates with lithographically defined electrodes. The network devices show superior performance (mobility up to 24 cm2 V-1 s-1 ), while SWNTs devices based on individual tubes show an unprecedented (100%) yield for working devices. Importantly, the SWNTs assembled by mean of the thiol groups are stably anchored to the substrate and are resistant to external perturbation as sonication in organic solvents.

Tuberculosis (TB) is still threatening millions of people's lives, especially in developing countries. One of the major factors contributing to the ongoing epidemic of TB is the lack of a fast, efficient, and inexpensive diagnostic strategy. In this work, we developed a semiconducting single-walled carbon nanotube (SWCNT)-based field-effect transistor (FET) device functionalized with anti-Mycobacterium tuberculosis antigen 85B antibody (Ab85B) to detect the major M. tuberculosis-secreted antigen 85B (Ag85B). Through optimizing the device fabrication process by evaluating the mass of the antibody and the concentration of the gating electrolyte, our Ab85B-SWCNT FET devices achieved the detection of the Ag85B spiked in phosphate-buffered saline (calibration samples) with a limit of detection (LOD) of 0.05 fg/mL. This SWCNT FET biosensor also showed good sensing performance in biological matrices including artificial sputum and can identify Ag85B in serum after introducing bovine serum albumin (BSA) into the blocking layer. Furthermore, our BSA-blocked Ab85B-SWCNT FET devices can distinguish between TB-positive and -negative clinical samples, promising the application of SWCNT FET devices in point-of-care TB diagnostics. Moreover, the robustness of this SWCNT-based biosensor to the TB diagnosis in blood serum was enhanced by blocking SWCNT devices directly with a glutaraldehyde cross-linked BSA layer, enabling future applications of these SWCNT-based biosensors in clinical testing.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are attractive absorbers for use in solar energy harvesting schemes because of their strong and energetically tunable optical absorption, and high charge carrier mobilities due to the delocalized nature of the π-electron system. Beyond their technological potential, s-SWCNTs offer attractive properties for fundamental studies of charge generation in strongly confined nanoscale systems and photoinduced electron transfer (PET) processes. For example, strong quantum confinement and low dielectric screening impart single-walled carbon nanotubes with exciton-binding energies substantially exceeding kBT at room temperature. Additionally, the energetically narrow and distinct spectroscopic signatures for excitons and charges within s-SWCNT thin films enables the unambiguous temporal tracking of fundamental photophysical processes occuring at important photoactive heterojunctions designed for charge separation. In this presentation, I will discuss recent studies that probe the generation and recombination of long-lived charges in samples consisting of heterojunctions between s-SWCNTs and various electron acceptors. I will focus on the roles of important interfacial properties that can influence the kinetics and efficiency of the interfacial PET process and recombination of the resulting separated charges. These properties include the energetic energetic driving force for exciton dissociation, the amount of residual wrapping polymer on the highly enriched s-SWCNTs, the local dielectric enrivonrment, the presence or absence of an electric field, and geometrical factors of the interface that can be modified via structural properties of the acceptors. Better fundamental understanding of these model interfaces can inform the design of more efficient solar energy harvesting systems, photodetectors, and other opto-electronic devices.

Semiconducting single-walled carbon nanotubes (SWCNTs) are an interesting material for strong-light matter coupling due to their stable excitons, narrow emission in the near-infrared region, and high charge carrier mobilities. Furthermore, they have emerged as quantum light sources as a result of the controlled introduction of luminescent quantum defects (sp3 defects) with red-shifted transitions that enable single-photon emission. The complex photophysics of SWCNTs and the overall goal of polariton condensation pose the question of how exciton–polaritons are populated and how the process might be optimized. The contributions of possible relaxation processes, i.e., scattering with acoustic phonons, vibrationally assisted scattering, and radiative pumping, are investigated using angle-resolved reflectivity and time-resolved photoluminescence measurements on microcavities with a wide range of detunings. We show that the predominant population mechanism for SWCNT exciton–polaritons in planar microcavities is radiative pumping. Consequently, the limitation of polariton population due to the low photoluminescence quantum yield of nanotubes can be overcome by luminescent sp3 defects. Without changing the polariton branch structure, radiative pumping through these emissive defects leads to an up to 10-fold increase of the polariton population for detunings with a large photon fraction. Thus, the controlled and tunable functionalization of SWCNTs with sp3 defects presents a viable route toward bright and efficient polariton devices.

Germanium

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are considered as a replacement for silicon in field-effect transistors (FETs), solar cells, logic circuits, and so forth, because of their outstanding electronic, optical, and mechanical properties. Herein, we have studied the reaction of pristine SWCNTs dispersed in a pluronic F-68 (PF-68) polymer solution with para-amino diphenylamine diazonium sulfate (PADDS) to separate nanotubes based on their metallicity. The preferential selectivity of the reactions was monitored by changes in the semiconducting (S22 and S33) and metallic (M11) bands by ultraviolet–visible–near infrared spectroscopy. Metallic selectivity depended on the concentrations of PADDS, reaction time, and the solution pH. Furthermore, separation of pure s-SWCNTs was confirmed by Raman spectroscopy and Fourier-transform infrared spectroscopy. After the removal of metallic SWCNTs, direct current electric field was applied to the pure s-SWCNT solution, which effectively directed the nanotubes to align in one direction as nanotube arrays with a longer length and high density. After that, electrically aligned s-SWCNT solution was cast on a silicon substrate, and the length of the nanotube arrays was measured as ∼2 to ∼14 μm with an areal density of ∼2 to ∼20 tubes/μm of s-SWCNTs. Next, electrically aligned s-SWCNT arrays were deposited on the channel of the FET device by drop-casting. Field-emission scanning electron microscopy and electrical measurements have been carried out to test the performance of the aligned s-SWCNTs/FETs. The fabricated FETs with a channel length of 10 μm showed stable electrical properties with a field-effect mobility of 30.4 cm2/Vs and a log10 (Ion/Ioff) current ratio of 3.96. We envisage that this new chemical-based separation method and electric field-assisted alignment could be useful to obtain a high-purity and aligned s-SWCNT array network for the fabrication of high-performance FETs to use in digital and analog electronics.

Electrochemical biosensor for methyl parathion based on single-walled carbon nanotube/glutaraldehyde crosslinked acetylcholinesterase-wrapped bovine serum albumin nanocomposites

We investigated carbon nanotube field effect transistors (CNT FET) utilizing semiconducting single-walled carbon nanotubes (SWCNTs). Multiple devices, each of different metal source and drain contacts, were fabricated on long (~11 micron) SWCNT. Large contact resistances around four MOmega were observed at room temperature. Low temperature measurements yielded varying contact resistances for these same devices. Transport properties of the devices follow to the Schottky contact and Poole-Frenkel model with measured 25-41 meV barriers for the devices.

A new calculation is made of the noise (excluding excess noise) in p-n junction diodes and bipolar transistors. The theory is based on an analysis of the current waveforms caused by thermal motion of individual carriers and by individual recombination events in a one-dimensional model; unlike that of van der Ziel (1955) it does not rely on arbitrary assumptions about thermal fluctuations in carrier concentration and does not involve a transmission-line analogy. When depletion-layer recombination effects are neglected, the results are found to be identical to those obtained by van der Ziel, although one of his postulates was invalid. The theory of van der Ziel and Becking (1958) which also leads to the same results does not appear to be consistent with the new treatment.

We have studied the effect of thermal annealing periods on the optoelectrical properties of Mg2Si pn ‐junction diodes fabricated by a conventional thermal diffusion process of Ag acceptor. The Au/Ag/n ‐Mg2Si (carrier concentration = 2 × 1015cm‐3, resistivity = 5‐10 Ωcm) substrate was annealed at 550 °C in the Ar‐atmosphere using a rapid thermal annealing (RTA) furnace. The annealing periods were varied between 2 and 10 minutes. Clear rectifying behavior of J ‐V characteristics was observed for the diodes annealed for more than 2 minutes. Spectral responsivity from the photon energy threshold of approximately 0.6 eV was demonstrated with the Mg2Si pn ‐junction diodes under the zero bias condition at room temperature. The maximum intensity of spectral responsivity was increased with decrease the annealing period. The spectral responsivity of the diode with the RTA periods of 4 minutes was approximately 4 times higher than that of 10 minutes annealed one. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Donor-acceptor conjugated polymers have been widely investigated in recent years. By optimization of donor and acceptor structures, the conjugated copolymers can exhibit broad absorption from visible region to near-infrared range for electronic and optoelectronic applications, such as light-emitting diodes, photovoltaic cells, and thin film transistor. However, the electronic and optoelectronic properties of thienopyrazine-based conjugated polymers have not been fully explored yet. The goal of this thesis is to investigate the effects of donor-acceptor or donor-acceptor-donor structures on the electronic properties of thienopyrazine based conjugated polymers. In chapter 2, the optical, electrochemical, and field effect charge transport properties of the new thienopyrazine-based alternating donor-acceptor conjugated copolymers were explored. The new copolymers, Poly[5-(2,5-bis(decyloxy)phenyl)-2,3- bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine] (PDPTP), Poly[5-(9,9-dioctyl- 9H-fluoren-2-yl)-2,3-bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine] (PFPTP), Poly[5-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-2,3-bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine] (PCPTP) and Poly[2,3-bis(4-(2-ethylhexyloxy)phenyl)-5-(thiophen-2-yl) thieno[3,4-b]pyrazine] (PTPTP) had excellent solubility and broad optical absorption bands with absorption maxima at 540-950nm in thin film. The PTPTP exhibits the smallest band gap (0.98eV), indicating a stronger intramolecular charge transfer. The four polymers show the low ionization potentials (4.57-4.99 eV) and high electron affinity (3.26-3.49eV). The hole mobilities of PDPTP, PCPTP, and PTPTP are 2.62×10-6, 2.74×10-5, and 2.00×10-4 cm2V-1s-1. PTPTP has the highest hole mobility due to low band gap and strong intramolecular charge transfer.The AFM topographic images of the copolymers show smooth and amorphous phases. From these results, intramolecular charge transfer might be main impact on hole mobility. In chapter 3, thienopyrazine-based donor-acceptor-donor alternating conjugated copolymers were synthesized. The copolymers including Poly[5-(5-(2,5-bis(decyloxy)-4-methylphenyl)thiophen-2-yl)-2,3-bis(4-(2-ethylhexyloxy)phenyl)-7-(5-methylthiophen-2-yl)thieno[3,4-b]pyrazine] (PDDTTP), Poly[5-(5-(9,9- dioctyl-9H-fluoren-2-yl)thiophen-2-yl)-2,3-bis(4-(2-ethylhexyloxy)phenyl)-7-(thiophen-2-yl)thieno[3,4-b]pyrazine] (PFDTTP), and Poly[2,3-bis(4-(2-ethylhexyloxy)phenyl)-5 ,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine] (PDTTP) exhibit broad optical absorption bands (662-816nm) and small optical band gaps (1.15-1.57eV). All polymers exhibit reversible oxidation and reduction and low ionization potential (4.62-4.98 eV). The hole mobility of PDDTTP, PFDTTP, and PDTTP are 7.24×10-4, 1.61×10-3 and 1.16×10-3cm2V-1s-1. The hole mobility of PFDTTP is the highest probably due to the relatively smooth and amorphous phase in thin film. According to these results, thienopyrazine-based donor-acceptor-donor copolymers combine small band gap and high carrier mobility. Such polymers may have potential optoelectronic device applications, such as thin film transistor and photovoltaic cells.