Selecting semiconducting single-walled carbon nanotubes by polymer wrapping:Mechanism and performances

Enkelwandige koolstofnanobuizen (SWNT’s) behoren tot de meest veelbelovende materialen die silicium kunnen vervangen in toekomstige elektronica. Dit is toe te schrijven aan de buitengewoon hoge mobiliteit van de ladingsdragers en de hoge mechanische, thermische en chemische stabiliteit van het materiaal. De toepassing van deze nanobuizen in elektronica is echter nog een grote uitdaging omdat tijdens het groeien van de SWNT’s halfgeleidende en metaalachtige nanobuizen ontstaan. Het doel van dit proefschrift is het onderzoeken van de selectie van de halfgeleidende SWNT’s met behulp van de zogenaamde polymeer-inwikkelmethode. Verschillende parameters, zoals de polymeerstructuur, de diëlektrische constante van het oplosmiddel, de temperatuur tijdens de selectie en de diameters van de nanobuizen, blijken een belangrijke rol te spelen in het verkrijgen van halfgeleidende SWNT’s met een hoge zuiverheidsgraad. Optische spectroscopie, in de vorm van absorptie, fotoluminescentie en tijdsafhankelijke spectroscopie, is gebruikt om de zuiverheid van de dispersies met nanobuizen te karakteriseren en om een beter beeld te krijgen van de interactie tussen polymeren en SWNT’s. De analyse van deze interactie is ondersteund door simulaties gebaseerd op moleculaire dynamica. Oplossingen van halfgeleidende SWNT’s met een hoge zuiverheidsgraad zijn gebruikt om hoogwaardige veldeffecttransistoren te maken. De mobiliteit van deze transistoren bereikte waardes tot 33 cm²/V s, met een aan-uit-verhouding van 10⁶. Verder wordt in dit proefschrift ook beschreven welke mogelijkheden er zijn tot verhogen van de prestaties van de transistoren, door de hechting en de richting van de nanobuizen op de substraten te verbeteren door gebruik te maken van zelfassemblage

Selecting semiconducting single-walled carbon nanotubes by polymer wrapping:Mechanism and performances

Enkelwandige koolstofnanobuizen (SWNT’s) behoren tot de meest veelbelovende materialen die silicium kunnen vervangen in toekomstige elektronica. Dit is toe te schrijven aan de buitengewoon hoge mobiliteit van de ladingsdragers en de hoge mechanische, thermische en chemische stabiliteit van het materiaal. De toepassing van deze nanobuizen in elektronica is echter nog een grote uitdaging omdat tijdens het groeien van de SWNT’s halfgeleidende en metaalachtige nanobuizen ontstaan. Het doel van dit proefschrift is het onderzoeken van de selectie van de halfgeleidende SWNT’s met behulp van de zogenaamde polymeer-inwikkelmethode. Verschillende parameters, zoals de polymeerstructuur, de diëlektrische constante van het oplosmiddel, de temperatuur tijdens de selectie en de diameters van de nanobuizen, blijken een belangrijke rol te spelen in het verkrijgen van halfgeleidende SWNT’s met een hoge zuiverheidsgraad. Optische spectroscopie, in de vorm van absorptie, fotoluminescentie en tijdsafhankelijke spectroscopie, is gebruikt om de zuiverheid van de dispersies met nanobuizen te karakteriseren en om een beter beeld te krijgen van de interactie tussen polymeren en SWNT’s. De analyse van deze interactie is ondersteund door simulaties gebaseerd op moleculaire dynamica. Oplossingen van halfgeleidende SWNT’s met een hoge zuiverheidsgraad zijn gebruikt om hoogwaardige veldeffecttransistoren te maken. De mobiliteit van deze transistoren bereikte waardes tot 33 cm²/V s, met een aan-uit-verhouding van 10⁶. Verder wordt in dit proefschrift ook beschreven welke mogelijkheden er zijn tot verhogen van de prestaties van de transistoren, door de hechting en de richting van de nanobuizen op de substraten te verbeteren door gebruik te maken van zelfassemblage

Donor- acceptor photoexcitation dynamics in organic blends investigated with a high sensitivity pump- probe system

Optical pump-probe spectroscopy is a crucial tool to investigate the excited state behaviour of materials and is especially useful to investigate the photoexcitation dynamics of bulk heterojunctions as found in organic solar cells. Most common techniques for such investigations involve pulses with an energy density in the range of several tens of J cm(-2), emitted with kHz repetition rate. Such pulse energy can entail non-linear processes due to the formation of high carrier concentrations and can furthermore severely damage the sample material. Here we introduce a softer approach with pulses of nJ cm(-2) energy density and a photon flux which is more than three orders of magnitude lower than in regular techniques. We show the capability of our low-cost and easy to make set-up by investigating two prototypical donor-acceptor polymers, C-PCPDTBT and its silicon variant Si-PCPDTBT. Given the high pulse repetition rate in the MHz regime, we are readily able to monitor sample changes of 10(-5) and find exciton lifetimes of 108 and 150 ps for C- and Si-PCPDTBT respectively

Semiconducting SWNTs sorted by polymer wrapping:How pure are they?

Short-channel field-effect transistors (FETs) prepared from semiconducting single-walled carbon nanotube (s-SWNT) dispersions sorted with poly(2,5-dimethylidynenitrilo-3,4-didodecylthienylene) are demonstrated. Electrical analysis of the FETs shows no evidence of metallic tubes out of a total number of 646 SWNTs tested, implying an estimated purity of our semiconducting SWNT solution higher than 99.85%. These findings confirm the effectiveness of the polymer-wrapping technique in selecting semiconducting SWNTs, as well as the potential of sorted nanotubes for the fabrication of short channel FETs comprising from 1 to up to 15 nanotubes without inter-nanotube junctions. Published by AIP Publishing

High performance photoelectrochemical hydrogen generation and solar cells with a double type II heterojunction

We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption of colloidal QDs on mesoporous TiO2 followed by 3-mercaptopropionic acid (MPA) ligand exchange. High efficiency photoelectrochemical hydrogen generation is demonstrated by means of these electrodes. The deposition of ZnS on TiO2/CdSe further improves the external quantum efficiency from 63% to 85% at 440 nm under -0.5 V vs. SCE. Using the same photoelectrodes, solar cells with the internal quantum efficiency approaching 100% are fabricated. The ZnS deposition increases the photocurrent and chemical stability of the electrodes. Investigation of the carrier dynamics of the solar cells shows that ZnS enhances the exciton separation rate in CdSe nanocrystals, which we ascribe to the formation of a type II heterojunction between ZnS and CdSe QDs. This finding is confirmed by the dynamics of the CdSe photoluminescence, which in the presence of ZnS becomes noticeably faster

High Performance Ambipolar Field-Effect Transistor of Random Network Carbon Nanotubes

Ambipolar field-effect transistors of random network carbon nanotubes are fabricated from an enriched dispersion utilizing a conjugated polymer as the selective purifying medium. The devices exhibit high mobility values for both holes and electrons (3 cm(2)/V.s) with a high on/off ratio (10(6)). The performance demonstrates the effectiveness of this process to purify semiconducting nanotubes and to remove the residual polymer

Carbon Nanotube Network Ambipolar Field-Effect Transistors with 10(8) On/Off Ratio

Polymer wrapping is a highly effective method of selecting semiconducting carbon nanotubes and dispersing them in solution. Semi-aligned semiconducting carbon nanotube networks are obtained by blade coating, an effective and scalable process. The field-effect transistor (FET) performance can be tuned by the choice of wrapping polymer, and the polymer concentration modifies the FET transport characteristics, leading to a record on/off ratio of 108

Understanding the Selection Mechanism of the Polymer Wrapping Technique toward Semiconducting Carbon Nanotubes

Noncovalent functionalization of single-walled carbon nanotubes (SWNTs) using π-conjugated polymers has become one of the most effective techniques to select semiconducting SWNTs (s-SWNTs). Several conjugated polymers are used, but their ability to sort metallic and semiconducting species, as well as the dispersions yields, varies as a function of their chemical structure. Here, three polymers are compared, namely, poly[2,6-(4,4-bis-(2-dodecyl)-4H-cyclopenta[2,1-b;3,4b′]dithiophene)-alt-4,7(2,1,3-ben-zothiadiazole)] (P12CPDTBT), poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PF12), and poly(3-dodecylthiophene-2,5-diyl) (P3DDT) in their ability to select two types of carbon nanotubes comprising small (≈1 nm) and large (≈1.5 nm) diameters. P12CPDTBT is a better dispersant than PF12 for small diameter nanotubes, while both polymers are good dispersants of large diameter nanotubes. However, these dispersions contain metallic species. P3DDT, instead presents the best overall performance regarding the selectivity toward semiconducting species, with a dispersion yield for s-SWNTs of 15% for small and 21% for large diameter nanotubes. These results are rationalized in terms of electronic and chemical structure showing that: (i) the binding energy is stronger when more alkyl lateral chains adsorb on the nanotube surface; (ii) the binding energy is stronger when the polymer backbone is more flex-ible; (iii) the purity of the dispersions seems to depend on a strong polymer– nanotube interaction

Conjugated Polymer-Assisted Dispersion of Single-Wall Carbon Nanotubes:The Power of Polymer Wrapping

CONSPECTUS: The future application of single-walled carbon nanotubes (SWNTs) in electronic (nano)devices is closely coupled to the availability of pure, semiconducting SWNTs and preferably, their defined positioning on suited substrates. Commercial carbon nanotube raw mixtures contain metallic as well as semiconducting tubes of different diameter and chirality. Although many techniques such as density gradient ultracentrifugation, dielectrophoresis, and dispersion by surfactants or polar biopolymers have been developed, so-called conjugated polymer wrapping is one of the most promising and powerful purification and discrimination strategies. The procedure involves debundling and dispersion of SWNTs by wrapping semiflexible conjugated polymers, such as poly(9,9-dialkylfluorene)s (PFx) or regioregular poly(3-alkylthiophene)s (P3AT), around the SWNTs, and is accompanied by SWNT discrimination by diameter and chirality. Thereby, the pi-conjugated backbone of the conjugated polymers interacts with the two-dimensional, graphene-like pi-electron surface of the nanotubes and the solubilizing alkyl side chains of optimal length support debundling and dispersion in organic solvents. Careful structural design of the conjugated polymers allows for a selective and preferential dispersion of both small and large diameter SWNTs or SWNTs of specific chirality. As an example, with polyfluorenes as dispersing agents, it was shown that alkyl chain length of eight carbons are favored for the dispersion of SWNTs with diameters of 0.8-1.2 nm and longer alkyls with 12-15 carbons can efficiently interact with nanotubes of increased diameter up to 1.5 nm. Polar side chains at the PF backbone produce dispersions with increased SWNT concentration but, unfortunately, cause reduction in selectivity. The selectivity of the dispersion process can be monitored by a combination of absorption, photoluminescence, and photoluminescence excitation spectroscopy, allowing identification of nanotubes with specific coordinates [(n,m) indices]. The polymer wrapping strategy enables the generation of SWNT dispersions containing exclusively semiconducting nanotubes. Toward the applications in electronic devices, until now most applied approach is a direct processing of such SWNT dispersions into the active layer of network-type thin film field effect transistors. However, to achieve promising transistor performance (high mobility and on-off ratio) careful removal of the wrapping polymer chains seems crucial, for example, by washing or ultracentrifugation. More defined positioning of the SWNTs can be accomplished in directed self-assembly procedures. One possible strategy uses diblock copolymers containing a conjugated polymer block as dispersing moiety and a second block for directed self-assembly, for example, a DNA block for specific interaction with complementary DNA strands. Another strategy utilizes reactive side chains for controlled anchoring onto patterned surfaces (e.g., by interaction of thiol-terminated alkyl side chains with gold surfaces). A further promising application of purified SWNT dispersions is the field of organic (all-carbon) or hybrid solar cell devices