[Show abstract][Hide abstract] ABSTRACT: In this report, we synthesize and characterize the structural and optical properties of novel heterostructures composed of (i) semiconducting nanocrystalline CdSe quantum dots (QDs) coupled with (ii) both one- and zero-dimensional (1D and 0D) motifs of self-activated luminescent CaWO4 metal oxides. Specifically, ∼4 nm CdSe QDs have been anchored onto (i) high-aspect ratio 1D nanowires, measuring ∼230 nm in diameter and ∼3 μm in length, as well as onto (ii) crystalline 0D nanoparticles (possessing an average diameter of ∼80 nm) of CaWO4 through the mediation of 3-mercaptopropionic acid (MPA) as a connecting linker. Composite formation was confirmed by complementary electron microscopy and spectroscopy (i.e., IR and Raman) data. In terms of luminescent properties, our results show that our 1D and 0D heterostructures evince photoluminescence (PL) quenching and shortened PL lifetimes of CaWO4 as compared with unbound CaWO4. We propose that a photoinduced electron transfer process occurs from CaWO4 to CdSe QDs, a scenario which has been confirmed by NEXAFS measurements and which highlights a decrease in the number of unoccupied orbitals in the conduction bands of CdSe QDs. By contrast, the PL signature and lifetimes of MPA-capped CdSe QDs within these heterostructures do not exhibit noticeable changes as compared with unbound MPA-capped CdSe QDs. The striking difference in optical behavior between CaWO4 nanostructures and CdSe QDs within our heterostructures can be correlated with the relative positions of their conduction and valence energy band levels. In addition, the PL quenching behaviors for CaWO4 within the heterostructure configuration were examined by systematically varying (i) the quantities and coverage densities of immobilized CdSe QDs as well as (ii) the intrinsic morphology (and by extension, the inherent crystallite size) of CaWO4 itself.
Chemistry of Materials 01/2015; 27(3):150130095854002. DOI:10.1021/cm503611q · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Double-walled carbon nanotube (DWNT)-CdSe heterostructures with the individual nanoscale building blocks linked together by 4-aminothiophenol (4-ATP) have been successfully synthesized using two different and complementary routes, i.e. covalent attachment and non-covalent π-π stacking. Specifically, using a number of characterization methods, we have probed the effects of these differential synthetic coupling approaches on the resulting CdSe quantum dot (QD) coverage on the underlying nanotube template as well as the degree of charge transfer between the CdSe QDs and the DWNTs. In general, based on microscopy and spectroscopy data collectively, we noted that heterostructures generated by non-covalent π-π stacking interactions evinced not only higher QD coverage density but also possibly more efficient charge transfer behavior as compared with their counterparts produced using covalent linker-mediated protocols.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate that a non-perturbative framework for the treatment of the
excitations of single walled carbon nanotubes based upon a field theoretic
reduction is able to accurately describe experiment observations of the
absolute values of excitonic energies. This theoretical framework yields a
simple scaling function from which the excitonic energies can be read off. This
scaling function is primarily determined by a single parameter, the charge
Luttinger parameter of the tube, which is in turn a function of the tube
chirality, dielectric environment, and the tube's dimensions, thus expressing
disparate influences on the excitonic energies in a unified fashion. We test
this theory explicitly on the data reported in NanoLetters 5, 2314 (2005) and
Phys. Rev. B. 82, 195424 (2010) and so demonstrate the method works over a wide
range of reported excitonic spectra.
Physical Review B 03/2014; 91(7). DOI:10.1103/PhysRevB.91.075417 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: One means of combining the unique physical and chemical properties of both carbon nanotubes and complementary material motifs (such as metal sulfide quantum dots (QDs), metal oxide nanostructures, and polymers) can be achieved by generating carbon nanotube (CNT)-based heterostructures. These materials can be subsequently utilized as novel and interesting constituent building blocks for the assembly of functional light energy harvesting devices and because of their architectural and functional flexibility, can potentially open up novel means of using and taking advantage of existing renewable energy sources. In this review, we present the reliable and reproducible synthesis of several unique model CNT-based heterostructured systems as well as include an accompanying discussion about the charge transfer and energy flow properties of these materials for their potential incorporation into a range of practical solar energy conversion devices.
Chemical Society Reviews 07/2013; 42(20). DOI:10.1039/c3cs60088b · 33.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report unidirectional charge transfer in multidimensional nanohybrids, consisting of a quantum dot, an electronically active molecular linker, and a carbon nanotube. After covalent attachment to the nanotube, only emission consistent with the negatively charged quantum dot exciton ion rather than the neutral exciton is observed, showing nearly monoexponential recombination kinetics and an average lifetime of 3.5 ns. Using kinetic models, we explain how charge transfer is biased at the expense of other decay pathways.
[Show abstract][Hide abstract] ABSTRACT: Graphene presents many distinctive optical properties that complement its attractive electronic and mechanical characteristics. We review some of the recent progress in understanding the electronic transitions and ultrafast dynamics in single and few-layer graphene crystals.
International Quantum Electronics Conference; 08/2011
[Show abstract][Hide abstract] ABSTRACT: Multiple exciton generation (MEG) is under intense investigation as potential third-generation solar photovoltaics with efficiencies beyond the Shockley-Queisser limit. We examine PbS nanocrystals, dispersed and vigorously stirred in TCE solution, by means of supercontinuum femtosecond transient absorption (TA). TA spectra show the presence of first and second order bleaches for the 1Sh-Se and 1Ph-Pe excitonic transition while photoinduced absorption for the 1Sh,e-Ph,e transitions. We found evidence of carrier multiplication (MEG for single absorbed photon) from the analysis of the first and second order bleaches, in the limit of low number of absorbed photons (Nabs˜0.01), for energy three times and four times the Energy gap. The MEG efficiency, derived from the ratio between early-time to long-time TA signal, presents a strongly dispersive behavior with maximum red shifted respect the first absorption peak. Analysis of population dynamics shows that in presence of biexciton, the 1Sh-Se bleach peak is red-shifted indicating a positive binding energy. MEG efficiency estimation will be discussed with regards to spectral integration, correlated higher-order and first excitonic transitions, as well as the nanocrystal morphologies.
[Show abstract][Hide abstract] ABSTRACT: A ferroelectric field effect transistor with an oxide channel layer and a lead zirconate titanate gate oxide has been fabricated. The channel is a strontium ruthenate/titanate solid solution with n type semiconducting behavior, which has sufficient OFF-state free carrier concentration to provide proper balancing charge for ferroelectric stability. The dependence of channel resistance with gate voltage at room temperature yields a hysteresis curve with two state at zero volts with a ΔR/R of 75% and a coercive voltage of 3 volts. The device was subjected to more than 1010 cycles with no degradation and was also operated at 60° C with a only a slight reduction in the switching ratio.
[Show abstract][Hide abstract] ABSTRACT: We have been investigating the potential for a channel transistor which utilizes a perovskite oxide capable of undergoing the Mort metal-insulator transition as the channel material. Our experiments have identified three limitations to the performance of the oxide devices: contact resistance to the channel, mobility limitations due to polycrystalline channels, and inadequate field induced surface charge density. In this paper we review progress we have made in oxide electrodes and in improving channel growth conditions which have mitigated the limitations due to contact resistance and polycrystalline channels. We conclude with an outline of our approach to improving the field induced surface charge density.
[Show abstract][Hide abstract] ABSTRACT: We present proof-of-concept experiments for developing a highly-sensitive and fast-response miniaturized single-walled carbon nanotube field-effect transistor (SWNT-FET) biosensor for electrically detecting adenovirus using ligand-receptor-protein specificity. SWNTs are mildly oxidized to form carboxylic groups on the surfaces without compromising the electronic integrity of the nanotubes. Then the human coxsackievirus and adenovirus receptor (CAR) is covalently functionalized onto the nanotube surface via diimide-activated amidation process. Upon exposure of the device to adenovirus protein, Ad12 Knob (Knob), specific binding of Knob to CAR decreases the current that flows through the SWNT-FET device. For control experiment, the CAR-SWNT device is exposed to YieF, which is a virus protein that does not bind specifically to CAR, and no current change is observed. The biological activity of the CAR and Knob proteins that are immobilized on SWNTs has been confirmed by previous fluorescence studies . AFM analysis is done to show height increase of a few nanometers at specific spots where the CAR-Knob complex are covalently linked to the nanotube surface. Therefore, our results show that the human receptor protein CAR does immobilize on SWNT surface while fully retains its biological activity. Moreover, the specific binding of CAR to its complementary adenovirus Knob can be electrically detected using individual SWNT-FET devices. These findings suggest that CAR-functionalized SWNT-FETs can ably serve as biosensors for detection of environmental adenoviruses.
[Show abstract][Hide abstract] ABSTRACT: In this paper we describe the fabrication of oxide based devices similar in architecture to a conventional FRT with source, drain, and gate electrodes and a channel. This distinctive characteristic of our device is the use of a channel material capable of undergoing a field-induced Molt insulator-metal transition at room temperature. Lithographic techniques developed for oxide materials have been combined with pulsed laser deposition of perovskite materials onto single-crystal strontium titanate (STO) substrates to fabricate these devices. Materials chosen for j the Mott transition channel include La2CuO4 (LCO) and YBCO, p-type; and Nd:CuO.i, n-type.
[Show abstract][Hide abstract] ABSTRACT: In this paper we describe the fabrication of oxide based electrodes that allow epitaxial growth of multilayer structures used to fabricate buried oxide-channel field effect transistors. The distinct characteristic of our buried electrodes is that they provide an etch stop layer which allow the opening of vias through the gate oxide using chemical etching. They can be Patterned to define 1 μm channel lengths and exhibit low contact resistance with channel materials such as YxPr1-xBa2Cu3O7-δ (YPBCO) or YBa2Cu3O7-δ (YBCO).
[Show abstract][Hide abstract] ABSTRACT: We have measured the low-energy excitonic transitions of chiral assigned individual large-diameter semiconducting single-walled nanotubes using a high-resolution Fourier transform photoconductivity technique. When photoconductivity is complemented by Rayleigh scattering spectroscopy, as many as five optical transitions can be identified on the same individual nanotube over an energy range of 0.3–2.7 eV. We find that well-established energy scaling relations developed for nanotubes of smaller diameter are not consistent with the measured low-energy transitions in large (1.8–2.3 nm) diameter nanotubes.
[Show abstract][Hide abstract] ABSTRACT: The massless Dirac spectrum of electrons in single-layer graphene has been thoroughly studied both theoretically and experimentally. Although a subject of considerable theoretical interest, experimental investigations of the richer electronic structure of few-layer graphene (FLG) have been limited. Here we examine FLG graphene crystals with Bernal stacking of layer thicknesses N = 1,2,3,...8 prepared using the mechanical exfoliation technique. For each layer thickness N, infrared conductivity measurements over the spectral range of 0.2-1.0 eV have been performed and reveal a distinctive band structure, with different conductivity peaks present below 0.5 eV and a relatively flat spectrum at higher photon energies. The principal transitions exhibit a systematic energy-scaling behavior with N. These observations are explained within a unified zone-folding scheme that generates the electronic states for all FLG materials from that of the bulk 3D graphite crystal through imposition of appropriate boundary conditions. Using the Kubo formula, we find that the complete infrared conductivity spectra for the different FLG crystals can be reproduced reasonably well within the framework a tight-binding model.
Proceedings of the National Academy of Sciences 08/2010; 107(34):14999-5004. DOI:10.1073/pnas.1004595107 · 9.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The unique electronic structure and optical properties of double-walled carbon nanotubes (DWNTs) have made them a key focus material of research in recent years. However, the incorporation of DWNTs with quantum dots (QDs) into nanocomposites via a covalent chemical approach as well as the optical properties of the composites have rarely been explored. In particular, we have been interested in this model system to investigate whether nanomaterial heterostructures can provide efficient pathways for charge separation relative to loss mechanisms such as recombination. In this specific work, the synthesis of DWNT-CdSe QD heterostructures obtained by using a conventional covalent protocol has been demonstrated. CdSe QDs with terminal amino groups have been conjugated onto the surfaces of oxidized DWNTs by the formation of amide bonds. The observed trap emission of CdSe is thought to arise from the presence of 2-aminoethanethiol capping ligands and is effectively quenched upon conjugation with the DWNT surface because of the charge transfer from CdSe to DWNTs.
The Journal of Physical Chemistry C 04/2010; 114(19). DOI:10.1021/jp100580h · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We experimentally study the optical properties of double-wall carbon nanotube and quantum dot (QD) composites. The two materials are covalently linked by an aminoethanethiol ligand (AET), which, when in complex with the QD, gives a characteristic emission in the NIR originating from trap states. The magnitude of this NIR emission peak relative to the QD exciton peak is directly proportional to the quantity of linker in the solution. Studies of the AET ligand-exchanged QD alone show that it poorly passivates the surface of the QD, leading to a short and complex multiexponential exciton lifetime, characteristic of the existence of randomly distributed surface traps. In contrast, upon linking with the DWNT complex, the defect related emission disappears, leaving only exciton emission. More striking, the exciton emission recovers a nearly monoexponential behavior of ˜ 2.8 ns.
[Show abstract][Hide abstract] ABSTRACT: While preserving many of the unusual features of single-layer graphene, few-layer graphene (FLG) provides a richness and flexibility of electronic structure that render this set of materials of great interest for both fundamental studies and applications. Essential for progress, however, is an understanding of the evolution of the electronic structure of these materials with increasing layer number. In this report, the evolution of the electronic structure of FLG, for N = 1 - 8 atomic layers, has been characterized by measurements of the optical conductivity spectra. For each layer thickness N, distinctive peaks are found in the infrared range, with positions obeying a simple scaling relation. The observations are explained by a unified zone-folding scheme that generates the electronic structure for all FLG materials from that of bulk graphite. Comment: 15 pages, 5 figures
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: The importance of generating carbon nanotube-nanoparticle heterostructures is that these composites ought to take advantage of and combine the unique physical and chemical properties of both carbon nanotubes and nanoparticles in one discrete structure. These materials have potential applicability in a range of diverse fields spanning heterogeneous catalysis to optoelectronic device development, of importance to chemists, physicists, materials scientists, and engineers. In this critical review, we present a host of diverse, complementary strategies for the reliable synthesis of carbon nanotube-nanoparticle heterostructures using both covalent as well as non-covalent protocols, incorporating not only single-walled and multi-walled carbon nanotubes but also diverse classes of metallic and semiconducting nanoparticles (221 references).
Chemical Society Reviews 05/2009; 38(4):1076-98. DOI:10.1039/b811424m · 33.38 Impact Factor