Search

The realization of on-chip quantum networks ideally requires lossless interfaces between photons and solid-state quantum emitters. We propose and demonstrate on-chip arrays of metallo-dielectric antennas (MDA) that are tailored toward efficient and broadband light collection from individual embedded carbon nanotube quantum emitters by trapping air gaps on chip that form cavity modes. Scalable implementation is realized by employing polymer layer dry-transfer techniques that avoid solvent incompatibility issues, as well as a planar design that avoids solid-immersion lenses. Cryogenic measurements demonstrate 7-fold enhanced exciton intensity when compared to emitters located on bare wafers, corresponding to a light collection efficiency (LCE) up to 92% in the best case (average LCE of 69%) into a narrow output cone of ±15° that enables a priori fiber-to-chip butt coupling. The demonstrated MDA arrays are directly compatible with other quantum systems, particularly 2D materials, toward enabling efficient on...

The efficiency of thin-film organic photovoltaic (OPV) devices relies heavily upon the transport of excitons to type-II heterojunction interfaces, where there is sufficient driving force for exciton dissociation and ultimately the formation of charge carriers. Semiconducting single-walled carbon nanotubes (SWCNTs) are strong near-infrared absorbers that form type-II heterojunctions with fullerenes such as C60. Although the efficiencies of SWCNT-fullerene OPV devices have climbed over the past few years, questions remain regarding the fundamental factors that currently limit their performance. In this study, we determine the exciton diffusion length in the C60 layer of SWCNT-C60 bilayer active layers using femtosecond transient absorption measurements. We demonstrate that hole transfer from photoexcited C60 molecules to SWCNTs can be tracked by the growth of narrow spectroscopic signatures of holes in the SWCNT "reporter layer". In bilayers with thick C60 layers, the SWCNT charge-related signatures display a slow rise over hundreds of picoseconds, reflecting exciton diffusion through the C60 layer to the interface. A model based on exciton diffusion with a Beer-Lambert excitation profile, as well as Monte Carlo simulations, gives the best fit to the data as a function of C60 layer thickness using an exciton diffusion length of approximately 5 nm.

In a unique interplay of excitons, phonons, and plasmons, we demonstrate plasmonic thermometry at the single-molecule level by detecting the plasmonically induced heat from SWCNT excitons coupled to plasmonic nanocavity arrays.

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising candidates as the active layer in photovoltaics (PV), particularly for niche applications where high infrared absorbance and/or semi-transparent solar cells are desirable. Most current fabrication strategies for SWCNT PV devices suffer from relatively high surface roughness and lack nanometer-scale deposition precision, both of which may hamper the reproducible production of ultrathin devices. Additionally, detailed optical models of SWCNT PV devices are lacking, due in part to a lack of well-defined optical constants for high-purity s-SWCNT thin films. Here, we present an optical model that accurately reconstructs the shape and magnitude of spectrally resolved external quantum efficiencies for ultrathin (7,5) s-SWCNT/C60 solar cells that are deposited by ultrasonic spraying. The ultrasonic spraying technique enables thickness tuning of the s-SWCNT layer with nanometer-scale precision, and consistently produces devices with low s-SWCNT film average surface roughness (Rq of

Single-walled carbon nanotubes (SWCNTs) are promising absorbers and emitters to enable novel photonic applications and devices but are also known to suffer from low optical quantum yields. Here we demonstrate SWCNT excitons coupled to plasmonic nanocavity arrays reaching deeply into the Purcell regime with Purcell factors (F P) up to F P = 180 (average F P = 57), Purcell-enhanced quantum yields of 62% (average 42%), and a photon emission rate of 15 MHz into the first lens. The cavity coupling is quasi-deterministic since the photophysical properties of every SWCNT are enhanced by at least one order of magnitude. Furthermore, the measured ultra-narrow exciton linewidth (18 μeV) reaches the radiative lifetime limit, which is promising towards generation of transform-limited single photons. To demonstrate utility beyond quantum light sources we show that nanocavity-coupled SWCNTs perform as single-molecule thermometers detecting plasmonically induced heat at cryogenic temperatures in a unique interplay of excitons, phonons, and plasmons at the nanoscale., Single-walled carbon nanotubes offer exciting optoelectronic applications but generally suffer from low quantum yields. Here, Luo et al. demonstrate that coupling nanotubes to plasmonic antennas can lead to large Purcell enhancement and corresponding increase in quantum yield as well as plasmonic thermometry at the single molecule level.

Single-wall carbon nanotubes (SWCNTs) could be employed in organic photovoltaic (OPV) devices as a replacement or additive for currently used fullerene derivatives, but significant research remains to explain fundamental aspects of charge generation. Electron paramagnetic resonance (EPR) spectroscopy, which is sensitive only to unpaired electrons, was applied to explore charge separation in P3HT:SWCNT blends. The EPR signal of the P3HT positive polaron increases as the concentration of SWCNT acceptors in a photoexcited P3HT:SWCNT blend is increased, demonstrating long-lived charge separation induced by electron transfer from P3HT to SWCNTs. An EPR signal from reduced SWCNTs was not identified in blends due to the free and fast-relaxing nature of unpaired SWCNT electrons as well as spectral overlap of this EPR signal with the signal from positive P3HT polarons. However, a weak EPR signal was observed in chemically reduced SWNTs, and the g values of this signal are close to those of C70-PCBM anion radical. The anisotropic line shape indicates that these unpaired electrons are not free but instead localized.

Organic thermoelectric materials are emerging as low-cost, versatile alternatives to more established inorganic ones. Avery et al. report carbon nanotube-based materials with selected properties that exhibit enhanced thermoelectric performance.

In this report, we investigate the electrical and optical properties of thin conducting films of SWNTs after treatment with small molecule and polymeric amines. Among those tested, we find hydrazine to be the most effective n-type dopant. We use absorbance, Raman, X-ray photoelectron, and nuclear magnetic resonance spectroscopies on thin conducting films and opaque buckypapers treated with hydrazine to study fundamental properties and spectroscopic signatures of n-type SWNTs and compare them to SWNTs treated with nitric acid, a well-characterized p-type dopant. We find that hydrazine physisorbs to the surface of semiconducting and metallic SWNTs and injects large electron concentrations, raising the Fermi level as much as 0.7 eV above that of intrinsic SWNTs. Hydrazine-treated transparent SWNT films display sheet resistances nearly as low as p-type nitric-acid-treated films at similar optical transmittances, demonstrating their potential for use in photovoltaic devices as low work function transparent electron-collecting electrodes.

Understanding the kinetics and energetics of interfacial electron transfer in molecular systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems and energy storage. The Marcus formulation for electron transfer relates the thermodynamic driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation. For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system. The small reorganization energies (about 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes.

Single-walled carbon nanotube (SWCNT)-fullerene (${\mathrm{C}}_{60}$) bilayers represent an attractive ``donor-acceptor'' binary system for solar photoconversion, where the kinetics of photoinduced processes depend critically on the properties of the interface between the two materials. Using photoconductivity measurements we identify the kinetic scheme that describes the free carrier kinetics in such bilayers where the dominant SWCNT species is the (7,5) semiconducting nanotube. Following charge separation, the carrier kinetics, covering up to four orders of magnitude in volumetric hole density, are described by a recombination process that is limited by capture and emission at traps or states at the SWCNT-${\mathrm{C}}_{60}$ interface. The high-frequency mobility of holes in the (7,5) SWCNT phase is lower than in multichiral films, potentially due to differences in SWCNT defect density for nanotubes that have been purified more aggressively. The results obtained here provide fundamental insights into the transport and recombination of both charges and excitons within SWCNT thin films and bilayers, and point to several potential ways to improve SWCNT-${\mathrm{C}}_{60}$ photovoltaic devices.

Understanding and controlling exciton-phonon interactions in carbon nanotubes has important implications for producing efficient nanophotonic devices. Here we show that laser vaporization-grown carbon nanotubes display ultranarrow luminescence line widths (120 μeV) and well-resolved acoustic phonon sidebands at low temperatures when dispersed with a polyfluorene copolymer. Remarkably, we do not observe a correlation of the zero-phonon line width with (13)C atomic concentration, as would be expected for pure dephasing of excitons with acoustic phonons. We demonstrate that the ultranarrow and phonon sideband-resolved emission spectra can be fully described by a model assuming extrinsic acoustic phonon localization at the nanoscale, which holds down to 6-fold narrower spectral line width compared to previous work. Interestingly, both exciton and acoustic phonon wave functions are strongly spatially localized within 5 nm, possibly mediated by the copolymer backbone, opening future opportunities to engineer dephasing and optical bandwidth for applications in quantum photonics and cavity optomechanics.

The time scales for interfacial charge separation and recombination play crucial roles in determining efficiencies of excitonic photovoltaics. Near-infrared photons are harvested efficiently by semiconducting single-walled carbon nanotubes (SWCNTs) paired with appropriate electron acceptors, such as fullerenes (e.g., C60). However, little is known about crucial photochemical events that occur on femtosecond to nanosecond time scales at such heterojunctions. Here, we present transient absorbance measurements that utilize a distinct spectroscopic signature of charges within SWCNTs, the absorbance of a trion quasiparticle, to measure both the ultrafast photoinduced electron transfer time (τpet) and yield (ϕpet) in photoexcited SWCNT–C60 bilayer films. The rise time of the trion-induced absorbance enables the determination of the photoinduced electron transfer (PET) time of τpet ≤ 120 fs, while an experimentally determined trion absorbance cross section reveals the yield of charge transfer (ϕpet ≈ 38 ± 3%). The extremely fast electron transfer times observed here are on par with some of the best donor:acceptor pairs in excitonic photovoltaics and underscore the potential for efficient energy harvesting in SWCNT-based devices.

Here, we report a thorough study on the ability of fluorene-based semiconducting polymers to disperse large-diameter (average diameter ⟨d⟩ ≈ 1.3 nm) laser vaporization (LV) single-walled carbon nanotubes (SWCNTs). We demonstrate the ability to select purely semiconducting species using poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-{2,2'-bipyridine})] (PFO-BPy) and poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(9,10-anthracene)] (PFH-A), producing samples with narrow and bright excitonic emission relative to comparable aqueous dispersions. Rapid processing and high yields offer the ability to easily incorporate these semiconducting SWCNTs into commercially scalable applications, as demonstrated by large-area thin films prepared by ultrasonic spraying. By modifying the growth temperature of the LV synthesis, we demonstrate the ability to tune the range of diameters and chiralities within dispersions by exerting synthetic control over the composition of the starting material. This synthetic control allows us to show that PFH-A preferentially disperses near-armchair semiconducting SWCNTs over a large range of diameters (0.8 nm < d < 1.4 nm) and induces unique solvatochromic shifts for the excitonic transitions of nanotubes with particular chiral indices.

Nanostructured organic semiconductors (OSCs), including single-walled carbon nanotubes (SWCNTs), offer a number of intriguing technological characteristics for thermoelectric applications, such as earth-abundant raw materials, low-cost deposition, and flexible form factors. We will initially present a series of experiments focused on understanding the thermoelectric performance of enriched semiconducting SWCNT networks dispersed in a wide bandgap semiconducting polymer matrix, followed by more recent work aimed at understanding the role that the semiconducting polymer plays in the observed transport properties. Rational choice of the starting SWCNT material and the semiconducting polymer allows us to sensitively tune the s-SWCNT diameter and band gap distributions within the composites. Consistent with theoretical calculations that consider the density of electronic states in individual s-SWCNTs, we observe a distinct dependence of the thermopower and thermoelectric power factor on the bandgap (or diameter) of the carbon nanotubes. We have measured large thermopower values (as high as ~2,500 µV/K for s-SWCNT networks with very low electrical conductivity) and impressive thermoelectric power factor as large as ~350 µW/m·K2. By varying the carrier density injected into the s-SWCNT networks by a stable charge-transfer dopant, we are able to probe the relationship between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks as a function of the carrier density and position of the Fermi energy. For carbon nanotubes prepared by high-pressure carbon monoxide (HiPCO) conversion, as we tune the carrier density, we are able to maintain a thermopower above 100 µV/K over almost the entire range of hole densities, corresponding to conductivities up to 40,000 S/m, resulting in a thermoelectric power factor of >300 µW/m·K2. These studies suggest that the low dimensionality of the SWCNTs has a stronger impact on the electrical conductivity than the thermopower, implying that they are less strongly coupled in these systems than is observed for compound inorganic semiconductors. By modifying an approach that allows us to strip the dispersing polymer from the s-SWCNTs we were able to demonstrate that the polymer appears to play no role in modifying the barriers to electrical transport present at tube-tube junctions, and simply controls the extent of nanotube bundling and thereby the surface area available to the charge-transfer dopant. This study also indicates that densification of the s-SWCNT network results in a two-fold enhancement of the thermoelectric power factor, suggesting that careful control of the amount and nature of the matrix material is required for high-performance s-SWCNT thermoelectric materials. Finally, we present a data from sensitive transport measurement technique, based on a microfabricated silicon nitride thermal isolation platform, to probe transport in the s-SWCNT networks, showing that the thermoelectric figure of merit (zT) is positively correlated with the measurement temperature, increasing by a factor of ~2.5 from 300 K to 350 K. These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by controlling the composition of the s-SWCNT network and tuning the carrier density (i.e., Fermi energy), and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

Single-walled carbon nanotubes (SWCNTs) are a versatile electronic material being explored as cost-effective, high-performance active materials in a variety of renewable energy applications such as transparent conducting or light-harvesting layers in photovoltaics and inclusions in thermoelectric composites. We present a series of experiments focused on understanding the thermoelectric performance of enriched semiconducting SWCNT networks dispersed in a semiconducting polymer matrix. Rational choice of the semiconducting polymer allows us to sensitively tune the s-SWCNT diameter and band gap distributions within the composites. We use a stable charge-transfer dopant to control the density of carriers in the s-SWCNT network, as determined by the bleach of the absorption corresponding to the S11 excitonic transition. The performance of these transparent conducting s-SWCNT composite networks is comparable to neat p-type and n-type s-SWCNT networks doped by either nitric acid or hydrazine treatments. By varying the carrier density we are able to probe the relationship between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks as a function of the carrier density and position of the Fermi energy. Although the electrical conductivity of the s-SWCNT networks is poor at very low carrier densities we have measured a colossal thermopower as high as ~2,500 µV/K, which is more than an order of magnitude larger than has been previously reported for SWCNT-based material systems and is consistent with theoretical calculations that consider the density of electronic states in individual s-SWCNTs. As we tune the carrier density, we are able to maintain a thermopower above 200 µV/K over almost the entire range of hole densities, corresponding to conductivities up to 1885 S/m, resulting in a thermoelectric power factor of ~100 µW/m·K2. These studies suggest that the low dimensionality of the SWCNTs has a stronger impact on the electrical conductivity than the thermopower, implying that they are less strongly coupled in these systems than is observed for compound inorganic semiconductors. These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by tuning the carrier density and/or Fermi energy, and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

In order to optimize next-generation thermoelectrics for use in converting waste heat into useable energy, the close correlation between the thermal conductivity, electrical conductivity, and Seebeck coefficient or thermopower in these materials must be decoupled. As promising alternatives to traditional inorganic semiconductor materials, nanocomposites constructed of conducting polymers with organic inclusions such as single-walled carbon nanotubes (SWCNTs) offer simple fabrication techniques, reduced cost, and low toxicity. They also offer the possibility of decoupling the thermal and electrical transport pathways which would enable more efficient organic composites for thermoelectric conversion. However, in order to develop high-performance organic thermoelectric devices, it is necessary to develop a detailed fundamental understanding of the factors governing directionally dependent thermal and electrical transport through these materials in reduced geometries such as thin films. In this talk, we describe our suspended membrane technique for directly measuring the in-plane thermal and electrical transport through thin films and present results for several different thin film samples. We present our approach to developing p-type materials with tunable transport behavior, by fabricating composites consisting of SWCNTs dispersed in a polymer matrix. Finally, we discuss fabrication and post-fabrication treatments of the SWCNT thin films and the benefits offered by nanostructuring these architectures to optimize the thermoelectric dimensionless figure-of-merit, ZT.

Single-walled carbon nanotubes (SWCNTs) have many attractive properties for conversion of sunlight into electricity or solar fuels. In solar conversion schemes where SWCNTs are used as absorptive charge donors, it is critical to understand the processes by which initially created excitons are dissociated to produce free charges. In this presentation, I’ll discuss time-resolved spectroscopy studies exploring how photogenerated excitons produce charges in both neat SWCNT films and bilayer heterojunctions designed to dissociate photogenerated SWCNT excitons. In neat SWCNT films, we find charge generation yields of several percent even in the absence of an intentional exciton dissociation interface. We use a thin film of C60 to dissociate SWCNT excitons by interfacial electron transfer from films containing a variety of SWCNT chirality combinations. In these “bilayer” PV active layers, exciton dissociation efficiency varies with the thermodynamic driving force for electron transfer, as well as a number of morphological bilayer properties. Charge mobility and recombination also vary dramatically with bilayer morphology and the diameter distribution of SWCNTs within the film. I will discuss our current understanding of the mechanisms for charge generation and recombination in both neat and bilayer films, and how this understanding helps us envision ways to improve upon SWCNT PV active layers.

Solid-state (13)C NMR spectroscopy was used to investigate the chemical shift of nanotube carbons on m- and s-SWNTs (metallic and semiconducting single-walled nanotubes) for samples with widely varying s-SWNT content, including samples highly enriched with nearly 100% m- and s-SWNTs. High-resolution (13)C NMR was found to be a sensitive probe for m- and s-SWNTs in mixed SWNT samples with diameters of approximately 1.3 nm. The two highly enriched m- and s-SWNT samples clearly exhibited features for m- and s-SNWT (13)C nuclei (approximately 123 and 122 ppm, respectively) and were successfully fit with a single Gaussian, while five mixed samples required two Gaussians for a satisfactory fit.

Single-walled carbon nanotubes (SWCNTs) have recently gained tremendous interest as a nanomaterial for next generation optoelectronics and quantum photonic devices, in particular through the demonstration of quantum light emission at room temperature in doped SWCNTs [1]. For further advances and to enable practical applications in quantum information processing light enhancement strategies are required. To this end plasmonic nanocavities such as bowtie antennas feature nanometer gaps that can support ultra-small mode volumes which are particularly attractive to enhance the exciton emission of single-walled carbon nanotubes (SWCNTs). In this talk we will discuss several technique to achieve the required spectral, spatial, and orientational resonance between SWCNT excitons and nanoplasmonic modes. Both time integrated and time-resolved measurements reveal characteristic signatures demonstrating pronounced coupling in the Purcell regime. As a test system we employ co-polymer wrapped carbon nanotubes for which we have recently shown that the polymer backbone can lead to efficient acoustic-phonon localization implying long exciton coherence times and ultra-narrow linewidth [2]. In addition, we will discuss SWCNT excitons coupled to metallo-dielectric antennas and evaluate the light collection efficiency enhancements achievable with this approach. Finally, we will discuss the prospects of achieving transform limited single photons in these systems. [1] X. Ma, N.F. Hartmann, J.K.S. Baldwin, S.K. Doorn, and H. Htoon, “Room-temperature single-photon generation from solitary dopants of carbon nanotubes”, Nature Nanotech 10, 671–675 (2015). [2] I. Sarpkaya, Ehsaneh D. Ahmadi, Gabriella D. Shepard, Kevin S. Mistry, Jeffrey L. Blackburn, and S. Strauf, “Strong Acoustic Phonon Localization in Copolymer-Wrapped Carbon Nanotubes”, ACS Nano 9, 6383 (2015).