Atomistic Modeling of Quantum Dots at Experimentally Relevant Scales Using Charge Equilibration.

Quantum dots (QDs) have been successfully employed within a vast array of fundamental and applied studies spanning all subdisciplines of chemistry. However, ab initio models of QD behavior are inherently limited by computational cost due to the large number of atoms within QDs of experimentally relevant size. This work builds upon the method of charge equilibration (qEQ) to account for system interactions unique to QDs (QD-qEQ) and demonstrates accuracy through calculated per-QD energies and dipole moments that agree generally with ab initio calculations and experimental observation, respectively. By forgoing electronic structure information, QD-qEQ exhibits a distinct advantage in its exceptionally low computational cost, which affords consideration of over 35,000 unique spherical wurtzite CdSe structures with radii ≤12.5 Å. A comparison of QD-qEQ calculations with experimental data relating to the phenomenon of CdSe magic size crystals (MSCs) affords statistical and structural insight into why MSCs are observed. Consideration of structures ≤12.5 Å reveals QD sizes corresponding with local minima in QD energy, correlating closely with experimentally observed MSCs. The physical origin of observed energy minima is assigned to QD structures with surfaces exhibiting large fractions of highly coordinated atoms, a physical trait postulated to yield fewer reaction sites for stepwise growth, resulting in MSC stability. The low computational cost along with the per-atom and per-structure electrostatic data afforded by QD-qEQ makes this method an enticing approach to address dynamic QD behavior and enables potential applications within a broad range of fields concomitant to those in which QD inclusion has already proven useful.

Photoinduced electron transfer from semiconductor quantum dots to metal oxide nanoparticles.

Quantum dot-metal oxide junctions are an integral part of next-generation solar cells, light emitting diodes, and nanostructured electronic arrays. Here we present a comprehensive examination of electron transfer at these junctions, using a series of CdSe quantum dot donors (sizes 2.8, 3.3, 4.0, and 4.2 nm in diameter) and metal oxide nanoparticle acceptors (SnO(2), TiO(2), and ZnO). Apparent electron transfer rate constants showed strong dependence on change in system free energy, exhibiting a sharp rise at small driving forces followed by a modest rise further away from the characteristic reorganization energy. The observed trend mimics the predicted behavior of electron transfer from a single quantum state to a continuum of electron accepting states, such as those present in the conduction band of a metal oxide nanoparticle. In contrast with dye-sensitized metal oxide electron transfer studies, our systems did not exhibit unthermalized hot-electron injection due to relatively large ratios of electron cooling rate to electron transfer rate. To investigate the implications of these findings in photovoltaic cells, quantum dot-metal oxide working electrodes were constructed in an identical fashion to the films used for the electron transfer portion of the study. Interestingly, the films which exhibited the fastest electron transfer rates (SnO(2)) were not the same as those which showed the highest photocurrent (TiO(2)). These findings suggest that, in addition to electron transfer at the quantum dot-metal oxide interface, other electron transfer reactions play key roles in the determination of overall device efficiency.

Charge transfer structure-reactivity dependence of fullerene-single-walled carbon nanotube heterojunctions.

Charge transfer at the interface between single-walled carbon nanotubes (SWCNTs) of distinct chiral vectors and fullerenes of various molecular weights is of interest both fundamentally and because of its importance in emerging photovoltaic and optoelectronic devices. One approach for generating isolated, discretized fullerene-SWCNT heterojunctions for spectroscopic investigation is to form an amphiphile, which is able to disperse the latter at the single-SWCNT level in aqueous solution. Herein, we synthesize a series of methanofullerene amphiphiles, including derivatives of C60, C70, and C84, and investigated their electron transfer with SWCNT of specific chirality, generating a structure-reactivity relationship. In the cases of two fullerene derivatives, lipid-C61-polyethylene glycol (PEG) and lipid-C71-PEG, band gap dependent, incomplete quenching was observed across all SWCNT species, indicating that the driving force for electron transfer is small. This is further supported by a variant of Marcus theory, which predicts that the energy offsets between the nanotube conduction bands and the C61 and C71 LUMO levels are less than the exciton binding energy in SWCNT. In contrast, upon interfacing nanotubes with C85 methanofullerene, a complete quenching of all semiconducting SWCNT is observed. This enhancement in quenching efficiency is consistent with the deeper LUMO level of C85 methanofullerene in comparison with the smaller fullerene adducts, and suggests its promise as for SWCNT-fullerene heterojunctions.

Single-Walled Carbon Nanotube Chiral Selectivity Exhibited by Commercially Available Hydrogels of Varying Composition.

Despite the commercial availability of many different hydrogel formulations, the effective gel-based purification of single-walled carbon nanotubes (SWNT) remains exclusive to the gel Sephacryl S-200. In this study, 12 commercially available gels and two custom-synthesized gels were investigated for their ability to effectively purify SWNT, as determined through quantification of SWNT adsorption, elution, chiral selectivity, and overall process efficiency. The ability of each gel to separate SWNT was found to correlate with physiochemical properties, such as hydrogel pore size, the presence of ionic ligands, and both polysaccharide backbone and cross-linker compositions. While Sephacryl S-200 demonstrated superior separation efficiency and chiral selectivity among the gels studied, Superose 6 was found to adsorb more SWNT than Sephacryl S-200 per cm2 of the gel surface area and exhibited a unique preference for the (7,3) and (7,5) SWNT chiralities, in contrast to the established selectivity of Sephacryl S-200 for the (6,5) chirality. Collectively, this work both identifies gels that exhibit unique SWNT chiral selectivity and provides insights into the rational design of gels tailored for SWNT purification.

Modulation of electron injection in CdSe-TiO(2) system through medium alkalinity.

Charge injection from excited CdSe quantum dots into nanostructured TiO(2) film can be modulated by varying solution pH. At increasing solution pH, the conduction band of TiO(2) shifts 59 mV/pH unit to a more negative potential, thereby decreasing the driving force and thus decreasing the rate of nonradiative electron transfer from excited CdSe. The emission yield and the average emission lifetime increase with increasing pH, thus providing a way to monitor the variation in medium pH.

Substrate driven photochemistry of CdSe quantum dot films: charge injection and irreversible transformations on oxide surfaces.

The photochemical behavior of CdSe quantum dots anchored to different surfaces was probed through their deposition on glass, SiO2, and TiO2 films. Following visible light irradiation under ambient conditions, CdSe quantum dots deposited on semiconducting TiO2 surface degraded, where no such degradation was observed when deposited on inert SiO2 surface or glass. Fluorescence decay and transient absorption experiments confirmed that charge injection from excited CdSe into TiO2 occurs with an apparent rate constant of 5.62 x 10(8) s(-1) and is the primary event responsible for photodegradation. In the presence of air, injected electrons are scavenged by surface adsorbed oxygen leaving behind reactive holes which induce anodic corrosion of CdSe quantum dots. In a vacuum environment, minimal CdSe degradation was observed as electron scavenging by oxygen is replaced with charge recombination between injected electrons and holes in CdSe nanocrystals. Spectroscopic measurements presented in this study highlight the role of both substrate and medium in dictating the photochemistry of CdSe quantum dots.

Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe-TiO2 architecture.

Different-sized CdSe quantum dots have been assembled on TiO2 films composed of particle and nanotube morphologies using a bifunctional linker molecule. Upon band-gap excitation, CdSe quantum dots inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The results presented in this study highlight two major findings: (i) ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and (ii) improvement in the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm diameter CdSe nanoparticles was 35% for particulate TiO2 and 45% for tubular TiO2 morphology. The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency

Layer number dependence of MoS2 photoconductivity using photocurrent spectral atomic force microscopic imaging.

Atomically thin MoS2 is of great interest for electronic and optoelectronic applications because of its unique two-dimensional (2D) quantum confinement; however, the scaling of optoelectronic properties of MoS2 and its junctions with metals as a function of layer number as well the spatial variation of these properties remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy (PCS-AFM) to image the current (in the dark) and photocurrent (under illumination) generated between a biased PtIr tip and MoS2 nanosheets with thickness ranging between n = 1 to 20 layers. Dark current measurements in both forward and reverse bias reveal characteristic diode behavior well-described by Fowler-Nordheim tunneling with a monolayer barrier energy of 0.61 eV and an effective barrier scaling linearly with layer number. Under illumination at 600 nm, the photocurrent response shows a marked decrease for layers up to n = 4 but increasing thereafter, which we describe using a model that accounts for the linear barrier increase at low n, but increased light absorption at larger n creating a minimum at n = 4. Comparative 2D Fourier analysis of physical height and photocurrent images shows high spatial frequency spatial variations in substrate/MoS2 contact that exceed the frequencies imposed by the underlying substrates. These results should aid in the design and understanding of optoelectronic devices based on quantum confined atomically thin MoS2.

Quantitative theory of adsorptive separation for the electronic sorting of single-walled carbon nanotubes.

Recently, several important advances in techniques for the separation of single-walled carbon nanotubes (SWNTs) by chiral index have been developed. These new methods allow for the separation of SWNTs through selective adsorption and desorption of different (n,m) chiral indices to and from a specific hydrogel. Our group has previously developed a kinetic model for the chiral elution order of separation; however, the underlying mechanism that allows for this separation remains unknown. In this work, we develop a quantitative theory that provides the first mechanistic insights for the separation order and binding kinetics of each SWNT chirality (n,m) based on the surfactant-induced, linear charge density, which we find ranges from 0.41 e(-)/nm for (7,3) SWNTs in 17 mM sodium dodecyl sulfate (SDS) to 3.32 e(-)/nm for (6,5) SWNTs in 105 mM SDS. Adsorption onto the hydrogel support is balanced by short-distance hard-surface and long-distance electrostatic repulsive SWNT/substrate forces, the latter of which we postulate is strongly dependent on surfactant concentration and ultimately leads to gel-based single-chirality semiconducting SWNT separation. These molecular-scale properties are derived using bulk-phase, forward adsorption rate constants for each SWNT chirality in accordance with our previously published model. The theory developed here quantitatively describes the experimental elution profiles of 15 unique SWNT chiralities as a function of anionic surfactant concentration between 17 and 105 mM, as well as phenomenological observations of the impact of varying preparatory conditions such as extent of ultrasonication and ultracentrifugation. We find that SWNT elution order and separation efficiency are primarily driven by the morphological change of SDS surfactant wrapping on the surface of the nanotube, mediated by SWNT chirality and the ionic strength of the surrounding medium. This work provides a foundational understanding for high-purity, preparative-scale separation of as-produced SWNT mixtures into isolated, single-chirality fractions.

A kinetic model for the deterministic prediction of gel-based single-chirality single-walled carbon nanotube separation.

We propose a kinetic model that describes the separation of single-chirality semiconducting carbon nanotubes based on the chirality-selective adsorption to specific hydrogels. Experimental elution profiles of the (7,3), (6,4), (6,5), (8,3), (8,6), (7,5), and (7,6) species are well described by an irreversible, first-order site association kinetic model with a single rate constant describing the adsorption of each SWNT to the immobile gel phase. Specifically, we find first-order binding rate constants for seven experimentally separated nanotubes normalized by the binding site molarity (M(θ)): k7,3 = 3.5 × 10⁻5 M(θ)⁻¹ s⁻¹, k6,4 = 7.7 × 10⁻8 M(θ)⁻¹ s⁻¹, k8,3 = 2.3 × 10⁻9 M(θ)⁻¹ s⁻¹, k6,5 = 3.8 × 10⁻9 M(θ)⁻¹ s⁻¹, k7,5 = 1.9 × 10⁻¹¹ M(θ)⁻¹ s⁻¹, k8,6 = 7.7 × 10⁻¹² M(θ)⁻¹ s⁻¹, and k7,6 = 3.8 × 10⁻¹² M(θ)⁻¹ s⁻¹. These results, as well as additional control experiments, unambiguously identify the separation process as a selective adsorption. Unlike certain chromatographic processes with retention time dependence, this separation procedure can be scaled to arbitrarily large volumes, as we demonstrate. This study provides a foundation for both the mechanistic understanding of gel-based SWNT separation as well as the potential industrial-scale realization of single-chirality production of carbon nanotubes.

Emergent properties of nanosensor arrays: applications for monitoring IgG affinity distributions, weakly affined hypermannosylation, and colony selection for biomanufacturing.

It is widely recognized that an array of addressable sensors can be multiplexed for the label-free detection of a library of analytes. However, such arrays have useful properties that emerge from the ensemble, even when monofunctionalized. As examples, we show that an array of nanosensors can estimate the mean and variance of the observed dissociation constant (KD), using three different examples of binding IgG with Protein A as the recognition site, including polyclonal human IgG (KD µ = 19 µM, σ(2) = 1000 mM(2)), murine IgG (KD µ = 4.3 nM, σ(2) = 3 µM(2)), and human IgG from CHO cells (KD µ = 2.5 nM, σ(2) = 0.01 µM(2)). Second, we show that an array of nanosensors can uniquely monitor weakly affined analyte interactions via the increased number of observed interactions. One application involves monitoring the metabolically induced hypermannosylation of human IgG from CHO using PSA-lectin conjugated sensor arrays where temporal glycosylation patterns are measured and compared. Finally, the array of sensors can also spatially map the local production of an analyte from cellular biosynthesis. As an example, we rank productivity of IgG-producing HEK colonies cultured directly on the array of nanosensors itself.

Polymer-free near-infrared photovoltaics with single chirality (6,5) semiconducting carbon nanotube active layers.

We demonstrate a polymer-free carbon-based photovoltaic device that relies on exciton dissociation at the SWNT/C(60) interface, as shown in the figure. Through the construction of a carbon-based photovoltaic completely free of polymeric active or transport layers, we show both the feasibility of this novel device as well as inform the mechanisms for inefficiencies in SWNTs and carbon based solar cells.