Jeffrey L. Blackburn

Nanostructured Fe3O4/SWNT Electrode: Binder‐Free and High‐Rate Li‐Ion Anode

Rechargeable Li-ion batteries are currently being explored for high-power applications such as electric vehicles. However, in order to deploy Li-ion batteries in next-generation vehicles, it is essential to develop electrodes made from durable, nontoxic, and inexpensive materials with a high charge/discharge rate and a high reversible capacity. Transition metal oxides such as Fe3O4, Fe2O3, MoO3, and Co3O4 [1–5] are capable of Liþ insertion/ extraction in excess of 6 Liþ per formula unit, resulting in a significantly larger reversible capacity than commercially employed graphite. In contrast to the intercalation mechanism that occurs for graphite, the transitionmetal oxides are reduced in a conversion reaction to small metal clusters with the oxygen reacting with Liþ to form Li2O. [1,2,6] This usually leads to large volume expansion and destruction of the structure upon electrochemical cycling, especially at high rate. Hence, optimizing particle size and mixing the particles with various carbon additives have been employed to improve the reversible capacity and rate capability of metal oxide electrodes. Among the transition metal oxides, Fe3O4 is both nontoxic and abundant (inexpensive) and is thus considered one of the most promising electrode materials. However, a truly durable high-rate capability and a high capacity for metal oxide based electrodes including Fe3O4 have not yet been achieved. To achieve high-rate capability and high capacity using metal oxide nanoparticles mixed with carbon materials, there are three key issues that must be considered: i) the size of the nanoparticles must be optimized such that rapid Li-ion diffusion and reaction with metal oxide nanoparticles are achieved, ii) an optimized carbon matrix must be developed that ensures both electrical conductivity and good thermal conductivity (to improve heatdissipation), and iii) the conductive additive must maintain a flexible and strong matrix that accommodates large volume changes. In most conventional electrodes, metal oxide nanoparticles are directly mixed with a carbon additive and a binder to help maintain electrical conductivity, and the large volume expansion then results inmechanical degradation of the electrode when cycled at high rate. Here we employ the unique properties of highly crystalline and long single-walled carbon nanotubes (SWNTs) to simultaneously address all of the three key issues with a simple two-step process to synthesize Fe3O4 nanoparticles embedded uniformly in an interconnected ‘‘SWNT net.’’ Furthermore, no polymer binder is required to maintain electrical conductivity. The electrodes contain 95wt% active material with only 5wt% SWNTs as the conductive additive (typical electrodes contain 80wt% active material and 20wt% conductive and binder additives). Most importantly, by using these binder-free electrodes, we have demonstrated a high reversible capacity of 1000mAhg 1 ( 2000mAh cm ) at C rate as well as high-rate capability and stable capacities of 800mAhg 1 at 5C (both for over 100 deep charge/discharge cycles) and 600mAhg 1 at 10C. Raman spectroscopy suggests that this remarkable rate capability is achieved because the Fe3O4 nanoparticles are actually bound to the flexible nanotube net. We also believe that this fabrication method may be employed for other active materials to achieve a binder-free, high-rate, and durable electrode. The FeOOH nanorods, employed as a precursor in the electrode fabrication process, have a width of 50 nm, length of 250 nm, and thickness of 20 nm and are formed with a simple hydrothermal process. X-ray diffraction (XRD) spectra of the as-prepared nanorods and reference a-FeOOH phase (goethite, JCPDS 81-0463) are shown in Figure 1a. All of the reflection peaks can be indexed to the tetragonal a-FeOOH phase. Next we created Fe3O4 nanoparticles embedded in an interconnected SWNTnetwork using FeOOHnanostructures and SWNTs as precursors for a vacuum-filtration and subsequent annealing process. We found that annealing the FeOOH nanorods without SWNTs to 450 8C in an argon atmosphere leads to a mixture of a-Fe2O3 (hematite) and Fe3O4 (magnetite) as indicated by the XRD patterns in Figure 1b. The peaks marked with * are indexed to the Fe3O4 phase (JCPDS 88-0315) and the remainder of the diffraction peaks are indexed to a-Fe2O3, (JCPDS 33-0664). In contrast, annealing FeOOHnanorods mixed with 5wt% SWNTs at 450 8C in an argon atmosphere leads to the complete reduction of FeOOH to Fe3O4, as indicated in Figure 1c. It is therefore evident that the SWNTs actually facilitate the formation of Fe3O4 nanoparticles, enabling excellent Fe3O4 nanoparticle/SWNT electronic and mechanical contact, which is further confirmed by the Raman spectroscopy analysis discussed later. The elegant morphology of the Fe3O4 nanorods embedded uniformly in the SWNT net is clearly depicted in the scanning electronmicroscope (SEM) image of Figure 2a. Figure 2b displays

Transparent Conductive Single-Walled Carbon Nanotube Networks with Precisely Tunable Ratios of Semiconducting and Metallic Nanotubes

We present a comprehensive study of the optical and electrical properties of transparent conductive films made from precisely tuned ratios of metallic and semiconducting single-wall carbon nanotubes. The conductivity and transparency of the SWNT films are controlled by an interplay between localized and delocalized carriers, as determined by the SWNT electronic structure, tube-tube junctions, and intentional and unintentional redox dopants. The results suggest that the main resistance in the SWNT thin films is the resistance associated with tube-tube junctions. Redox dopants are found to increase the delocalized carrier density and transmission probability through intertube junctions more effectively for semiconductor-enriched films than for metal-enriched films. As a result, redox-doped semiconductor-enriched films are more conductive than either intrinsic or redox-doped metal-enriched films.

Carbon nanotube network electrodes enabling efficient organic solar cells without a hole transport layer

We report on the effects of replacing both In2O3:Sn (ITO) and the hole transport layer (HTL) in organic photovoltaic (OPV) cells with single-walled carbon nanotube (SWNT) network transparent electrodes. We have produced an OPV device without an HTL exhibiting an NREL-certified efficiency of 2.65% and a short-circuit current density of 11.2 mA/cm2. Our results demonstrate that SWNT networks can be used to replace both ITO and the HTL in efficient OPV devices and that the HTL serves distinctly different roles in ITO- and SWNT-based devices.

Prolonging Charge Separation in P3HT−SWNT Composites Using Highly Enriched Semiconducting Nanotubes

Single-walled carbon nanotubes (SWNTs) have potential as electron acceptors in organic photovoltaics (OPVs), but the currently low-power conversion efficiencies of devices remain largely unexplained. We demonstrate effective redispersion of isolated, highly enriched semiconducting and metallic SWNTs into poly(3-hexylthiophene) (P3HT). We use these enriched blends to provide the first experimental evidence of the negative impact of metallic nanotubes. Time-resolved microwave conductivity reveals that the long-lived carrier population can be significantly increased by incorporating highly enriched semiconducting SWNTs into semiconducting polymer composites.

Reversibility, Dopant Desorption, and Tunneling in the Temperature-Dependent Conductivity of Type-Separated, Conductive Carbon Nanotube Networks

We present a comprehensive study of the effects of doping and temperature on the conductivity of single-walled carbon nanotube (SWNT) networks. We investigated nearly type-pure networks as well as networks comprising precisely tuned mixtures of metallic and semiconducting tubes. Networks were studied in their as-produced state and after treatments with nitric acid, thionyl chloride, and hydrazine to explore the effects of both intentional and adventitious doping. For intentionally and adventitiously doped networks, the sheet resistance (R(s)) exhibits an irreversible increase with temperature above approximately 350 K. Dopant desorption is shown to be the main cause of this increase and the observed hysteresis in the temperature-dependent resistivity. Both thermal and chemical dedoping produced networks free of hysteresis. Temperature-programmed desorption data showed that dopants are most strongly bound to the metallic tubes and that networks consisting of metallic tubes exhibit the best thermal stability. At temperatures below the dopant desorption threshold, conductivity in the networks is primarily controlled by thermally assisted tunneling through barriers at the intertube or interbundle junctions.

Control of PbSe quantum dot surface chemistry and photophysics using an alkylselenide ligand.

We have synthesized alkylselenide reagents to replace the native oleate ligand on PbSe quantum dots (QDs) in order to investigate the effect of surface modification on their stoichiometry, photophysics, and air stability. The alkylselenide reagent removes all of the oleate on the QD surface and results in Se addition; however, complete Se enrichment does not occur, achieving a 53% decrease in the amount of excess Pb for 2 nm diameter QDs and a 23% decrease for 10 nm QDs. Our analysis suggests that the Se ligand preferentially binds to the {111} faces, which are more prevalent in smaller QDs. We find that attachment of the alkylselenide ligand to the QD surface enhances oxidative resistance, likely resulting from a more stable bond between surface Pb atoms and the alkylselenide ligand compared to Pb-oleate. However, binding of the alkylselenide ligand produces a separate nonradiative relaxation route that partially quenches PL, suggesting the formation of a dark hole-trap.

Fluorescence Imaging In Vivo at Wavelengths beyond 1500 nm

Compared to imaging in the visible and near-infrared regions below 900 nm, imaging in the second near-infrared window (NIR-II, 1000-1700 nm) is a promising method for deep-tissue high-resolution optical imaging in vivo mainly owing to the reduced scattering of photons traversing through biological tissues. Herein, semiconducting single-walled carbon nanotubes with large diameters were used for in vivo fluorescence imaging in the long-wavelength NIR region (1500-1700 nm, NIR-IIb). With this imaging agent, 3-4 μm wide capillary blood vessels at a depth of about 3 mm could be resolved. Meanwhile, the blood-flow speeds in multiple individual vessels could be mapped simultaneously. Furthermore, NIR-IIb tumor imaging of a live mouse was explored. NIR-IIb imaging can be generalized to a wide range of fluorophores emitting at up to 1700 nm for high-performance in vivo optical imaging.

Structural and chemical evolution of methylammonium lead halide perovskites during thermal processing from solution

Following the prominent success of CH3NH3PbI3 in photovoltaics and other optoelectronic applications, focus has been placed on better understanding perovskite crystallization from precursor and intermediate phases in order to facilitate improved crystallinity often desirable for advancing optoelectronic properties. Understanding of stability and degradation is also of critical importance as these materials seek commercial applications. In this study, we investigate the evolution of perovskites formed from targeted precursor chemistries by correlating in situ temperature-dependent X-ray diffraction, thermogravimetric analysis, and mass spectral analysis of the evolved species. This suite of analyses reveals important precursor composition-induced variations in the processes underpinning perovskite formation and degradation. The addition of Cl− leads to widely different precursor evolution and perovskite formation kinetics, and results in significant changes to the degradation mechanism, including suppression of crystalline PbI2 formation and modification of the thermal stability of the perovskite phase. This work highlights the role of perovskite precursor chemistry in both its formation and degradation.

n-Type transparent conducting films of small molecule and polymer amine doped single-walled carbon nanotubes.

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.

Phase Identification and Control of Thin Films Deposited by Co-Evaporation of Elemental Cu, Zn, Sn, and Se

Kesterite thin films [(i.e., Cu2ZnSn(S,Se)4 and related alloys] have been the subject of recent interest for use as an absorber layer for thin film photovoltaics due to their high absorption coefficient (>104 cm−1), their similarity to successful chalcopyrites (like CuInxGa1−xSe2) in structure, and their earth-abundance. The process window for growing a single-phase kesterite film is narrow. In this work, we have documented, for our 9.15%-efficient kesterite co-evaporation process, (1) how appearance of certain undesirable phases are controlled via choice of processing conditions, (2) several techniques for identification of phases in these films with resolution adequate to discern changes that are important to device performance, and (3) reference measurements for those performing such phase identification. Data from x-ray diffraction, x-ray fluorescence, Raman scattering, scanning electron microscopy, energy dispersive spectroscopy, and current-voltage characterization are presented.