Quantum dots (QDs) have shown particular promise in recent years as light absorbers in solar energy conversion schemes. However, in solution junction solar devices such as QD-sensitized solar cells and photocatalytic water splitting systems, efficiencies are often limited by hole transfer from the photoexcited QD. This process is sluggish and can lead to oxidative photocorrosion of the QD material. In order to design highly efficient nanocrystal systems with hole transfer rates that outcompete these undesirable processes, a fundamental understanding of the parameters that control these rates is imperative.
We have developed a model system to study charge transfer from QDs to surface bound acceptors, to fundamentally understand the parameters of the Marcus charge transfer equation for QD systems, namely electronic coupling between the donor and acceptor and the thermodynamic driving force for the hole transfer process. Specifically, we examine hole transfer from nearly spherical CdSe-core CdS-shell QDs with photoluminescence (PL) quantum yields over 80% to ferrocene derivatives bound to the QD surface via an alkane thiol linker. In this system, we mitigate the ill-defined nonradiative charge dynamic pathways that are intrinsic to native CdSe cores, and then controllably engineer on the surface charge acceptors with well-defined oxidation potentials, spatial distribution, and quantity. By measuring the PL lifetime decay and calibrating the number of hole acceptor ligands per QD via quantitative ¹H NMR, we extracted the hole transfer rate per acceptor. This rate per acceptor could be varied over four orders of magnitude by changing the coupling between donor and acceptor through modulations in the CdS shell thickness and alkane chain length of the molecule. Furthermore, owing to the large number of acceptors on the surface, we achieve systems in which ~99% of the photoexcited holes are transferred to these well-defined mediators.
We further mapped the relationship between the thermodynamic driving force and hole transfer rate. We systematically tune the driving force over nearly 1 eV by varying the redox potentials of the ferrocene ligands through functionalization of the cyclopentadiene rings. Our results show a monotonic increase in rate as a function of increasing driving force with no observed inverted region, contrary to predictions made by the standard two-state Marcus model for charge transfer. This behavior is understood by considering the residual electron in the QD conduction band, which could exhibit intraband excitations coupled to the hole transfer, thus creating a many-state system that would eliminate the inverted region. The resulting relationship between rate and energetic driving force for hole transfer can be used to design QD-molecular systems that maximize interfacial charge transfer rates while minimizing energetic losses associated with the driving force.

Substaintial progress has been made in the synthesis of nearly monodisperse semiconductor nanocrystals suitable for optoelectronic applications such as solar cells and light emitting devices. Control of the electronic properties of assemblies of nanocrystals however has been a bottleneck to device performance. We demonstrate nanocrystal assemblies that combine spatial coherence and interparticle electronic coupling by controlled epitaxy of specific nanocrystal surface facets. We acheive this by control over the kinetics of superlattice assembly at a fluid interface followed by controlled epitaxy between neighboring nanocrystals. In addition, we show that surface termination strongly influences carrier populations and charge transport.

Functionalization of Quantum Dot (QD) surfaces with so-called “Electroactive Ligands” has emerged as a promising strategy to extend carrier wavefunctions from the QD into the ligand shell which greatly impacts the optoelectronic properties of the QD.¹ While most studies have focused on the optical features of such coupled organic-inorganic nanostructures,² first examples have appeared in which electroactive ligands are utilized to tailor carrier transport through QD ensembles.^3,4 Transport in these materials is believed to occur via suitable states in the ligand which resonate with the first excited hole or electron states in the QD. The main advantages of this novel scheme for tailoring the electrical properties of QDs include the potential for carrier-selective transport, size-tunability of the resonance and, thus, the coupling strength as well as improved long-range order in arrays of QDs due to longer interparticle spacings.
Using our recently established system of PbS QDs coupled to tetrathiafulvalene (TTF) derivatives,⁴ we now report on the structural, optical and size-dependent transport properties of this material. In contrast to QD solids functionalized with short ligands like 1,2-ethanedithiol or hydrazine, dipole-dipole effects are largely suppressed in the PbS-TTF system, such that electronic coupling can be probed directly via optical spectroscopy. We show that coupling depends greatly on QD size, which is also illustrated by size-dependent field-effect mobility measurements. Further, we demonstrate that ensembles of TTF-functionalized PbS QDs are not only conductive but also display a large degree of long-range periodicity and orientational order. This is typically not achieved with previous ligand exchange procedures and holds for versatile ways to engineer transport in QD arrays, for instance by exploiting possible miniband formation or controlled wavefunction overlap between adjacent QDs.
References:
[1] D.J. Milliron, A.P. Alivisatos, C. Pitois, C. Edder, and J.M.J. Fréchet, Adv. Mater. 15, 58 (2003).
[2] M.T. Frederick, V.A. Amin, and E.A. Weiss, J. Phys. Chem. Lett. 4, 634 (2013).
[3] K. Szendrei, D. Jarzab, M. Yarema, M. Sytnyk, S. Pichler, J.C. Hummelen, W. Heiss, and M.A. Loi, J. Mater. Chem. 20, 8470 (2010).
[4] M. Scheele, D. Hanifi, D. Zherebetskyy, S.T. Chourou, S. Axnanda, B.J. Rancatore, K. Thorkelsson, T. Xu, Z. Liu, L.-W. Wang, Y. Liu, and A.P. Alivisatos, ACS Nano 8, 2532 (2014).

We present photocurrent transient measurements as a means to investigate the carrier dynamics in solids of lead-sulfide (PbS) semiconductor nanocrystals (NCs), incorporated in solar cells [1]. In our measurements, a sub-nanosecond laser pulse generates a charge distribution within the NC film, and, with the solar cell held in reverse bias, one carrier type preferentially traverses the film, generating a displacement current. From the measured transients, the carrier mobility and the distribution of localized states below the mobility edge can be determined.
We employ this technique as a function of temperature for different NC band-gaps, NC size distributions, and NC ligands to understand the charge carrier dynamics, in order to disentangle the complex interplay of the material parameters. We explain our findings in the context of existing analytical models [2] and develop deeper insights into temperature dependent mobilities and the effect of the NC size distribution on solar cell performance.
[1] Yazdani, N. A., Bozyigit, D., Yarema, O., Yarema, M., & Wood, V. (2014). Hole Mobility in Nanocrystal Solids as a Function of Constituent Nanocrystal Size. The Journal of Physical Chemistry Letters, (5), 3522-3527.
[2] Shabaev, A., Efros, A. L., & Efros, A. L. (2013). Dark and photo-conductivity in ordered array of nanocrystals. Nano letters, 13(11), 5454-61.

PbS colloidal nanocrystals (CNCs) are promising materials on account of their prospective application in optoelectronics such as solar cells, field-effect transistors (FETs), and photodetectors. However, their use as semiconducting active materials is still limited as a consequence of their low carrier mobility. The low mobility has been attributed to trap states in the middle of band gap introduced during ligand exchange. In FETs, the number of trap states is even higher due to additional interface traps given by dangling bonds at the dielectric/active layer interface. In this work, we used hexamethyldisilazane-self-assembled monolayers (HMDS-SAMs)-treated SiO₂ and hydroxyl-free Cytop polymer as gate dielectric. The use of HMDS-SAMs passivated interface trap states and improved the organization of NC assemblies on conventional SiO₂ dielectric surface leading to improvement of mobility up to 0.07 cm²V^-1s^-1. Furthermore, when we used Cytop as gate dielectric, we demonstrated one order of magnitude improvement in carrier mobility up to 0.2 cm²V^-1s^-1. This high carrier mobility is attributed to the low density of trap states estimated in our devices at the Cytop dielectric interface (~10¹¹ cm^-2), which is almost 2 orders of magnitude lower than for samples obtained with conventional SiO₂ gate dielectric (~10¹³ cm^-2). Our results show that controlling the organization of NC assemblies and reducing trap states are effective strategies to improve performances of CNC solids.

Quantum dots (QDs) based on low-band-gap metal chalcogenides have been widely explored for next generation solar cells. QDs are of interest as solar absorbers due to their tunable band gap and high absorption coefficient. Colloidal quantum dot (CQD) solar cells made from lead sulfide (PbS) QDs have achieved a power efficiency of ~ 8%. In these devices, it is important to control the band gap as well the band position of the QDs to efficiently inject electrons and holes into their respective electrodes. We will describe experimental and theoretical studies of the effects of interface engineering through surface ligand modification on the band gap and relative band positions in lead chalcogenide (PbSe_xS_1-x) QDs. These effects were studied with UV-Vis absorption and photoelectron spectroscopies. Multilayer CQD solar cells were fabricated to investigate the effect on carrier collection of QD layers with different relative band positions. We will show that interface engineering using molecularly designed ligands can be applied to lead chalcogenide QDs in order to create a favorable band diagram and achieve enhanced photogenerated carrier collection in multilayer CQD devices.

Recent developments in the study of semiconductor nanocrystal thin film transistors give hope for solution based, low temperature fabrication of low cost, large area and high performance electronic devices. PbS quantum dots are one of the most promising materials, which have shown outstanding performances in solar cells and field effect transistors. However, the true potential of PbS quantum dots, i.e. high carrier mobility, ambipolarity and high on-off ratio are difficult to achieve in real devices. The quality and morphology of a PbS thin film, the gating performance of the dielectric layer and the properties of the interface between the gate dielectric and the active semiconductor layer are among the crucial factors that limit the overall device performance.
In our study we optimized the deposition method to fabricate EDT-crosslinked PbS transistor with high field effect mobility. Fabricated device showed ambipolar performance with linear electron mobility up to 0.8 cm²V^-1s^-1 and on-off ratio up to 10⁴ on Si substrate.
High-k relaxor ferroelectric terpolymer poly(vinylidene fluoride - trifluoroethylene - 1,1-chlorofluoroethylene) was used as a gate dielectric. P(VDF-TrFE-CFE) shows small ferroelectric hysteresis and possesses extremely high dielectric constant - up to 50, depending on annealing conditions, film thickness, etc. Devices were fabricated in double gate configuration: P(VDF-TrFE-CFE) used as the top gate dielectric and SiO₂ - as the bottom one. The use of P(VDF-TrFE-CFE) gate led to lowering of the operational voltage from 60V to 15V and the hysteresis reduced from 15V to 2V, demonstrating its high potential as a gate material especially for colloidal quantum dots.

Trap assisted recombination is one main limitations to the performance of colloidally synthesized nanocrystal (NC)-based solar cells. Using temperature dependent, transient open-circuit voltage measurements, in addition to standard photovoltaic characterization to assess the performance of the NC based solar cells and thermal admittance spectroscopy to gain insight into the carrier trap energies and densities, we investigate the recombination dynamics in lead sulfide (PbS) NC-based solids. We develop a model to explain the recombination that is self-consistent for different temperatures and NC band gaps from 0.8 ev to 1.7 eV (i.e. PbS NCs ranging in size from 6 nm to 2.5 nm). We then examine the impact of carrier blocking interfaces (e.g. MoOx) and ligands on the recombination dynamics and assess their potential role in improving device performance. The insights gain from our analysis is used to develop design guidelines for NC-based solar cell device fabrication that can drive the realization of higher efficiency devices.

Quantum Dots (QDs) have attracted a lot of attention in the last few years because of their potential in solar cell devices. It is important to understand the energetics of the various layers to construct efficient devices. Therefore, we characterize the valence and conduction band energy levels of PbS QDs using photoelectron and inverse photoelectron spectroscopy. These spectroscopies are complimentary techniques that probe the occupied (valence band) and unoccupied (conduction band) states, respectively. With these measurements, we have determined that the conduction and valence band density of states depend on the size of the QD. In addition to these studies, we have also investigated the effect of various surface treatments on PbS(e) QDs. We are able to make the PbS(e) thin films more n- or p-type depending on the surface treatment. With a better understanding of how to measure the valence and conduction bands and how to control the conductivity of the PbS(e) thin films, we will be able to construct and optimize PbS(e) solar cell devices.

Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for technological applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. With the lacking theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a sub-diffraction spatial sensitivity and a picosecond temporal resolution. This goal was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at well-defined distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of pre-dissociation hops, the rate of interparticle energy transfer, and the exciton diffusion mobility. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling.

Multiple exciton generation (MEG) is the phenomenon wherein the absorption of a single photon leads to the generatation multiple electron-hole pairs. This has provided exciting possibilities for improving the energy conversion efficiency of photovoltaic and photocatalytic devices. In conventional solar cell, most of the absorbed energy is wasted in the form of heat. Therefore theoretically power conversion efficiency of single p-n junction solar cell is about 33% If this wasted energy is used to create another exciton then power conversation efficiency of solar cell may be increased. This is possible in principles by exploiting MEG in different nanostructures. Thanks to their low-dimensionality, semiconductor nanocrystals can exhibit efficient carrier multiplication. However, due to the tight confinement and strong electron electron interaction, excitons in semiconductor nanocrystals can also recombine efficiently either radiatively or via Auger relaxation; these processes compete with the generation of photocurrent and the extraction of charge carriers and so far have hindered the benefits of multiple exciton generation in practical devices. However, implementing MEG in practical devices requires the extraction of multiple charge carriers before exciton-exciton annihilation and the development of materials with improved MEG efficiency.
Ultra-efficient MEG for single photon absorption in colloidal PbSe and PbS quantum dots (QDs) [1] have been previously demonstrated and therefore PbSe materials have become very attractive tp study and exploit this phenomenon for photovoltaic development.
In this work we have investigated the MEG efficiency in PbSe QDs, Nanorods (NRs) and nanosheets (NSs) employing transient absorption spectroscopy. In particular NRs and NSs have been efficiently synthetized in our lab. Here, we will present also detailed study of MEG efficiency as function exciting light polarization for all the different nanostructures.
References:
[1] R. D. Schaller, Appl. Phys. Lett. 2005, 87, 253102
[2] Paul D. Cunningham, Nano Lett. 2011,11, 3476
[3] L.D.A. Siebbeles, Nat. Comm. 4789, 2014

Lead salt quantum dots (QDs) have attracted great attention in solar-cell community lately, owing to their potential in overcoming Shockley-Queisser limit based on internal multi-exciton generation mechanism or harvest external triplet excitons generated by singlet fission. One key issue to realize such third-generation solar cells is boosting charge transport through condensed QD layer via band alignment and trap-state minimization. Particularly, the surface near-band-edge states of QDs with different surface passivations govern the patency of charge-transfer pathway. Surface states of QDs have been studied with photoinduced absorption and scanning tunneling spectroscopy which are however limited by poor energy resolution. In this contribution, we show that the alternation between trap- and core-state photoluminescence (PL) of PbS QDs is enabled by passing the QDs in solution across the solvent&’s liquid-solid transition, providing a new method to scrutinize the relevant surface trap states.
Steady-state and transient PL of PbS QDs capped with oleic acid dissolved in HMN and toluene were characterized as a function of temperature. Two Gaussian profiles are de-convolved from all the acquired PL spectra, signifying two emission channels. The PL spectra undergo a significant red shift as toluene becomes solid, suggesting a different electronic transition. Additionally, PbS QDs in solid toluene conferring a short PL lifetime (~100ns) besides the long lifetime (~1mu;s) in liquid toluene—accompanied by declined PL intensity, also supporting new radiative relaxation in solid toluene. Conversely, the PL spectrum and lifetime of PbS QDs in HMN exhibit mild variation across the liquid-solid transition. Based on these evidences, we propose one surface state (P₁) near the conduction state (1S_e) of PbS QDs and two proximate surface states (S₁ and S₂) near their valence state (1S_h), which originate from surface oleic-acid ligand. In liquid solvents, the emission channel corresponds to the transitions from 1S_e to S₁ and S₂. In solid toluene, a fast energy relaxation process transfers the 1S_e-state electrons to P₁, followed by the radiative transition to S₁ and S₂. This alternate relaxation passage would quench the PL quantum yield due to the lower transition probability between the surface states. Conversely, such P₁ state is not activated in solid HMN.
The distinct PL characteristics for toluene and HMN are understood based on their differing natures in solid phase. As solid toluene is crystallized, the rigid crystallite of toluene would force oleic acid to align with specific lattice orientation, thus begetting the surface P₁ state. In contrast, the glassy HMN allows for local structural relaxation to maintain energetically favorable bonding of the oleic acid to the surface Pb atom that inhibits the P₁ state. Our novel approach makes the revelation of such surface states—which are similarly emergent in closely-packed QDs used in solar cells—considerably easier.

The visible-light extinction of noble metal nanoparticles surpasses those of semiconductor and molecular dyes by several orders of magnitude. Such superior light-harvesting characteristics are highly attractive for photocatalytic applications. Unfortunately, the conversion of the plasmon emission into usable energy poses significant challenges. The conventional strategy relying on far-field (FF) scattering of radiation is fundamentally limited due to poor coupling of FF modes to acceptor transitions (Yablonovitch&’s limit). As a result, many photocatalytic systems developed to date suffer from significant energy losses, which negate the benefits of plasmon-enhanced absorption. In this project, we were able to resolve this difficulty by employing the near-field (NF) emission of gold (Au) nanoparticles. This task was achieved by developing a composite nanoparticle morphology where the plasmon emission of the metal component is coupled to resonant transitions in a semiconductor “acceptor” domain (CdSe). The inter-domain interfaces were designed to support a rapid conversion of the near-field energy at rates that outpace the thermal dephasing of NF plasmon modes. Without thermal losses, the NF conversion strategy is theoretically up to 15 times more efficient than the conventional FF approach. Along these lines, the project investigates the feasibility and potential benefits of the NF strategy in photocatalytic reactions. The catalytic performance of proposed hybrid materials was tested using common redox reactions, while energy transfer processes was investigated using ultrafast spectroscopy techniques.

Lead chalcogenide quantum dots (QDs), such as PbS, PbSe, are of particular interest for their potential applications in near-infrared photodetectors, light-emitting diodes (LEDs) and solar cells owing to size dependent optical absorption and emission spectra from visible to infrared and larger Bohr radii with respect to other semiconductors.^[1-3] Moreover, QDs with core shell structure allow controlling the band alignment and wave function distribution for tailing their optoelectronic properties.^[4] To access and optimize such unique properties for opto-electronic devices applications, investigations of the excitons dynamics and wave function delocalization in these inorganic QDs are critical. In the present work, we investigate excitons recombination dynamics in PbS/CdS core/shell hetero-nanostructures as a function of temperature and surface chemistry. By decreasing the temperature from 295 K to 5.4 K in PbS/CdS QDs with monolayer shell thickness, we observe a faster decay time and the appearance of a high-energy band, demonstrating that the monolayer-shell of CdS on PbS forms a quasi type-II heterostructure with a reduction of the conduction band offset at lower temperature. Furthermore, it is found that the exiton dynamics and degree of electron localization in the lowest conduction band level can be controlled by the ligand chemistry. PbS/CdS QDs capped with 3-mercaptopropionic acid (PbS/CdS-MPA) ligands show much faster exciton lifetimes (346 ps) in comparison to the PbS/CdS-OA films (~380 ns). The pronounced shortening of the overall exciton lifetime suggests that the de-excitation dynamics for MPA treated films are fundamentally different from that of the PbS/CdS-OA (caped with Oleic Acid) films. Short ligands such as MPA can effectively reduce the inter-particle spacing and increase electronic coupling between QDs. Excitons can dissociate via tunneling of charge between neighboring NCs in PbS/CdS-MPA, resulting in a shortened exciton lifetimes. These results demonstrate that controlling electron transfer and delocalization in such systems can be of pivotal importance for realizing efficient optoelectronic devices.
[1] D. Balazs, M. Nugraha, S. Bisri, M. Sytnyk, W. Heiss, M. Loi, Applied Physics Letters2014, 104, 112104.
[2] S. Z. Bisri, C. Piliego, M. Yarema, W. Heiss, M. A. Loi, Advanced Materials2013, 25, 4309.
[3] A. K. Rath, M. Bernechea, L. Martinez, F. P. G. de Arquer, J. Osmond, G. Konstantatos, Nature Photonics2012, 6, 529.
[4] L.-H. Lai, L. Protesescu, M. V. Kovalenko, M. A. Loi, Physical Chemistry Chemical Physics2014, 16, 736.

Heterostructure nanocomposites of CdSe nanocrystals (NCs) with amorphous TiO₂ (a-TiO₂) was designed for hydrogen evolution from water. Atomic defect sites in a-TiO₂ are responsible for uncontrolled, unwanted recombination of photogenerated electrons and holes and result in poor photocatalytic activity. However, the thin shell layer of a-TiO₂ onto CdSe NCs allows the photogenerated carriers to reach the surface of composite photocatalyst with alleviating charge trapping. Type II band offset in CdSe/a-TiO₂ helps the electron in the conduction band of CdSe transfer to that of a-TiO₂, which results in efficient hydrogen production. In addition, porosity of a-TiO₂ thin shell effectively makes the heterostructure more open. Both charge carriers become available for the access of water and hole scavengers. As size of CdSe NCs becomes larger, the energy difference between the conduction bands of the quantum dots (QDs) and metal oxide becomes smaller, which influences photocatalytic hydrogen evolution rate. Electron transfer from CdSe NCs to a-TiO₂ layer becomes faster with the size of QDs decreased whereas smaller NCs have larger band gap, which absorbs narrower range of sunlight. This finding provides a basis for design of heterostructure NC-based photocatalysts for efficient water splitting.

Semiconductor nanocrystal quantum dots (NQDs) have been extensively investigated due to their size-tunable energy gaps, narrow emission, and efficient carrier multiplication (CM). PbSe NQDs are particularly paid attention because PbSe has tunable band gaps in infrared (IR), a large dielectric constant (ε_m = 23), a large exciton Bohr radius (46 nm), and high CM efficiency. However oxidation-vulnerability of PbSe NQDs strongly hampers the broader utilization in optoelectronic applications such as light emitting diodes (LEDs), IR detectors, and solar cells.
Here, we present facile, but highly effective surface passivation strategies to improve the ambient stability of PbSe NQDs. Results from absorption and photoluminescence (PL) spectroscopy clearly reveal that our surface-engineered PbSe NQDs are dramatically oxidation-resistive in ambient condition. Furthermore, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations lead us to have deep insight into the origin of improved stability of PbSe NQDs. Finally, we successfully fabricated air-stable n-type field effect transistors (FETs) using our PbSe NQDs.

Photo-electrochemical cells (PECs) for water-splitting attract much attention as a means of producing renewable hydrogen energy. Especially, inorganic/organic sensitizers have been usually used to enhance the light harvesting in this system. However, the nanoscale TiO₂ porous film, most widely used n-type semiconductor, is easily clogged with the sensitizers which prevent from infiltrating into it. To figure this out, we chose the 3-D inverse opal (IO) structure TiO₂ films which have the pore size of hundreds of nanometer. Furthermore, we used tetrapod-CdSe (tp-CdSe) quantum dots (QDs), which have relatively large size while retaining the quantum confinement effect, to investigate the pore filling efficiency and the effect of morphology of QDs on PECs.
Here, tp-CdSe QDs are about 10 nm in arm length and 4 nm in arm width. Because of the interconnected macropores(~200 nm) in the IO structure, tp-CdSe infiltrated into the entire film and was deposited on the surface of TiO₂ in contrast to conventional mesoporous one. The amount of tp-CdSe deposited increased with the immersion time, evidenced by more light absorption from UV/Vis. On the other hand, the photocurrent density is not in accordance with the amount of QDs. Instead, it shows the optimum point where the electron lifetime was the lowest. As a result, we can obtain the maximum photocurrent density per Cd mass about 0.474 mA/cm², which is greater than the previous results from QD-sensitized PECs. We thus believe that macroporous IO-TiO₂ film may provide a new platform for studying a various structure of QDs in PECs.

Owing to their greater lattice covalency, reduced toxicity and size-tunable emission over the visible and near-infrared spectral range (band gap sim; 1.34 eV), indium phosphide (InP) quantum dots (QDs) have attracted interest to replace cadmium-based chalcogenides in potential technological applications. However, current state of the art InP QD emitters are not well characterized and their optical properties and photostability remain inferior to prototypical CdSe-based core/shell QDs. Recently, we have pioneered a method to correlate the photoluminescence (PL) of single QDs with their atomic structure. This precise single-QD correlation strategy allows the identification of individuals from QD ensembles with optimal luminescence properties. In this work wide-field fluorescence microscopy and atomic number contrast scanning transmission electron microscopy (Z-STEM) is utilized to correlate time-resolved photoluminescence (PL) data and atomic-level structural information from individual InP/ZnS core/shell QDs to permit selective engineering of InP QD populations with ideal structures.

Colloidal quantum dots (QD) have prompted wide-spread research activities in expectation of developing future display technologies over the past two decades. For efficient QD light emitting diodes (QLED) with low turn-on voltage, employing suitable hole transport materials attributes to better hole carrier injection between QD layer and hole transport layer (HTL). In the fabrication of QLEDs, the pre-deposited HTL must maintain its property throughout their exposure to the organic solvents, which can dissolve QDs. Here, we suggest the answer focused on solving these issues, particularly in the context of Fluorous Materials Chemistry. In general, fluorous solvents, including hydrofluoroethers and perfluorocarbons, do not interact extensively with non-fluorinated materials. Thus, fluorous QD solutions in HFE-7500 enabled the solution-casting of QD films on top of an organic hole-transporting layer without damage. It is believed that this method will provide a way of selecting organic functional materials for the construction of QD, which will lead to fabricate practicable QLEDs with RGB patterned pixels as well as white light-emitting devices for diverse applications.

Colloidal inorganic nanocrystals (NCs) are among the most exploited nanomaterials to date due to their extreme versatility. Research on NCs went through much advancement in the last fifteen years, for example in the synthesis, which opened up the possibility to control their size, shape and topology in chemical composition. An additional step forward was the creation of a wide range of superstructures from the assembly of such NCs, which can be thought of as new types of artificial solids. This, coupled with the possibility to replace the native ligands on the surface of the NCs with shorter molecules, down to single atom ligands, has conferred unique electrical features to films of NCs that make them attractive for low cost alternatives to many technologies. Progress also came from the study of chemical transformations in nanostructures, most notably via cation exchange, which involves replacement of the sublattice of cations in a crystal with a new sublattice of different cations, while the sublattice of anions remains in place. Also, a new field of study has emerged recently, aiming at investigating the transformations in colloidal synthesized nanomaterials under conditions like thermal annealing and/or irradiation. In part this research is boosted by the recent availability of microscopy tools by which one can follow the transformations on individual NCs in-situ, i.e. when such perturbations are actually applied to the sample. The present lecture will highlight the recent progress (with emphasis on the contributions from our group) in the study of chemical and structural transformations in NCs.

In the context of solar-to-chemical energy conversion, ternary and quaternary oxides are receiving increasing attention as light absorbers to drive multi-electron oxidation reactions, especially water oxidation, as they exhibit stability under oxidative and corrosion conditions. On the contrary classical semiconductors used in photovoltaics such as III-V, II-VI and silicon photocorrode in oxidizing conditions in absence of a protective layer. Monoclinic bismuth vanadate (BiVO₄) is one example of promising ternary oxide with its 2.4eV band gap.¹ Recently, first principles calculations have identified a series of ternary and quaternary oxynitrides (i.e. Ti₃O₃N₂, La₂TiO₂N₂) as particularly promising for photostable visible light-driven photocatalysts because of their predicted band gaps (around 2 eV) and band edge positions suitable for water splitting.² Nanostructuring of these oxide semiconductors provides unprecedented opportunities for spatially and temporally reconciling charge separation and extraction properties, therefore enhancing their performances in solar-to-chemical fuels conversion. However, to date, colloidal approaches for preparing complex oxide semiconductor nanocrystals are still limited.
In this talk, we present our recent results on three different semiconductor metal oxide nanocrystals absorbing visible light : N-doped TiO₂, BiVO₄ and Sb-doped BiVO₄.^3-5 Colloidal chemistry was used as a means to achieve superior structural control and to access tunable homogeneous compositions. In the case of N-doped TiO₂, we show that nitrogen dopants can be selectively positioned in substitutional or interstitial sites by properly choosing the amine type used during the synthesis. The reaction mechanism and the impact of the lattice occupational site of the nitrogen impurities on the optoelectronic properties of the TiO₂ were investigated. For doped and undoped BiVO4, key for controlling the nanocrystal composition was a one-pot seeded growth approach in which bismuth nanocrystals were reacted with the vanadium and antimony precursors in properly chosen conditions. Finally, charge carrier dynamics data and preliminary photoelectrochemical measuraments demonstrated how this atomic-level chemical manipulation provide significant opportunities to build new structure/properties relationships in light absorbing oxide nanocrystals for water oxidation.
1. Kim, T. W.; Choi, K.-S. Science 2014, 343, 990.
2. Wu, Y.; Lazic, P.; Hautier, G.; Persson, K.; Ceder, G.Energy Envion. Sci. 2013, 6, 157
3. Lynch, J.; Giannini, C.; Cooper, J.K.; Sharp, I. D.; Buonsanti, R. submitted
4. Loiudice, A.; Cooper, J. K.; Mattox, T.; Sharp, I. D.; Buonsanti, R. submitted
5. Loiudice, A.; Cooper, J. K.; Mattox, T.; Thao, T.; Drisdell, W.; Ma, J.; Wang, L-W; Yano, J.; Sharp, I. D.; Buonsanti, R. in preparation

Colloidally prepared semiconductor nanocrystals (NCs) derived from narrow band gap materials have attracted significant interest for use in solar cells because their size-tunable energy gap (E_g) both promotes maximal power conversion efficiency in single junction devices and facilitates useful physical phenomena, including the low threshold formation of multiple excitons per absorbed photon, which can potentially improve energy extraction from high energy solar photons. Compared to other narrow gap semiconductors such as PbSe, Ge, and HgTe, InSb was unexamined due to difficulties in synthesizing this material by wet chemical techniques; previous studies of this composition were limited to epitaxially prepared quantum dots, which reportedly interacted electronically with synthetically required substrates, thereby preventing strong quantum confinement. Recently developed synthetic routes to colloidal InSb NCs now permit examination of this composition under the influence of a highly confining potential.
Here, we present optical characterizations of colloidal InSb NCs using transient absorption spectroscopy. We show that InSb NCs exhibit slower intraband cooling for larger NC sizes and undergo rapid biexcitonic Auger recombination. For all samples, we observe a bleach feature that develops on ultrafast timescales, which notably moves to lower energy during the first several picoseconds following excitation. This time-dependent spectral shift persists at low temperatures and obeys Varshni relations in similarity to bulk InSb, suggesting an intrinsic origin. Given the magnitude of this red-shift and the related dynamics, we propose that the bleach red-shift arises from the transfer of population between two energetically proximal conduction band levels.

Graphene-like 2D crystals with non-zero band gaps exhibit highly interesting dimensionality-dependent properties and can potentially revolutionize electronic technologies as transistors and photodetectors [1,2].
2D crystals of the group-III metal chalcogenide InSe with a band gap of ~1.3 eV are a new kind of this material class predestined for the use in optoelectronic applications. As high responsivity photodetector covering a broad spectral range (visible to NIR), InSe outperforms other recently emerged 2D crystals including MoS₂ and GaSe[1,2] and also holds high potential in photovoltaics, where carrier multiplication and quantum efficiencies > 100 % like those seen in 2D PbS nanosheets are expected [3].
We present for the first time the combination of a colloidal synthesis route for tailoring ultrathin (~5 nm) 2D InSe nanosheets with a detailed characterization of the materials&’ optical and electrical transport properties. The results are compared with state of the art few-layered 2D InSe obtained from the bulk by Scotch tape based methods. Absorption and single sheet fluorescence measurements as well as in depth analysis of the colloidal nanosheets&’ crystal structure and stoichiometry by TEM, SEM, XPS and GISAXS complete this comprehensive study and underpin the superior potential of our wet-chemical approach.
[1] Tamalampudi, S. R.; Lu, Y.-Y.; Kumar, R. U.; Sankar, R.; Liao, C.-D.; Moorthy, K. B., Karukanara; Cheng, C.-H.; Chou, F. C.; Chen, Y.-T., Nano Lett.2014, 14, 2800-2806.
[2] Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; and Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; Mohite, A. D.; Ajayan, P. M.; ACS Nano2014, 8, 1263-1272.
[3] Aerts, M.; Bielewicz, T.; Klinke, C.; Grozema, F. C.; Houtepen, A. J.; Schins, J. M.; Siebbeles, L. D. A., Nat. Commun.2014, 5, 3789, DOI:10.1038/ncomms4789.

Semiconductor nanocrystals and nanowires have been widely explored as the basis for photocatalysts, as components of solution-processable solar cells and photodetectors, and as emitters for lighting and display applications. Such uses require good control of the interfacial chemistry of the constituent compounds. Our group has introduced preparative gel permeation chromatography (GPC) as a method to separate natively-capped nanocrystal quantum dots (QDs) from small molecules including weakly-bound ligands. The GPC method provides highly purified nanocrystals with low and consistent ligand populations that we can use to study the effects of impurities on ligand exchange and shell growth, and as a well-defined initial state from which to examine the thermodynamics of ligand binding and interactions. We have also investigated the formation of CdS shells on CdSe nanocrystals under alternating layer addition conditions to evaluate the proposed selective ionic layer adhesion and reaction (SILAR) mechanism of shell growth. Through experiments such as these we aim to identify metrics for QDs and anisotropic nanocrystals and nanowires that enable subsequent reaction chemistry and physical properties to be predicted with confidence.

Quantum-confined semiconductor nanocrystals, or “quantum dots,” are promising materials for applications in low-cost solar cells fabricated using solution-based methods. In addition to solution processability, they feature size/shape-tunable optical spectra, as well as a variety of novel physical properties that can enable fundamentally new schemes of solar energy conversion. Specifically, several recent reports have demonstrated the great potential of colloidal nanocrystals for the realization of generation-III photovoltaics by employing concepts such as hot-electron extraction or carrier multiplication.
This presentation provides an overview of fundamental and applied studies of quantum dots conducted in the context of solar energy conversion. The specific topics will include applications of “Stokes-shift-engineered” quantum dots in luminescent solar concentrators, charge transport properties of quantum dot assemblies evaluated via a novel technique of ultrafast photoconductivity, and the recent progress in understanding of carrier multiplication in quantum confined materials. The discussion of carrier multiplication will focus on spectroscopic versus photoconductive signatures of multiexcitons generated via a single-photon absorption event, the effect of structural parameters such as particle size, shape, and composition on carrier multiplication yields, and recent efforts on increasing multiexciton production by controlling the competing process of intra-band cooling.

Lead chalcogenide (PbS, PbSe and PbTe) nanocrystals (NC) posses outstanding optical properties such as broad optical absorption from the ultraviolet to the near infrared (NIR), bandgap tunability in the NIR, efficient multiple exciton generation and relatively high quantum yield luminescence. Such properties can be exploited in a variety of optoelectronic applications including biological tags, lasers, photodetectors, LEDs and photovoltaics. Thus, the electron-hole pair (or exciton) ground state electronic structure and dynamics are of particular interest. It has been reported that the luminescence of PbS NC is produced by two electronic states, a higher energy state with a lifetime of the order of tens of nanoseconds and a lower energy state with a lifetime of a few microseconds.¹ Competition between these two levels has been reported in dynamics as a function of both temperature² and NC size.¹ Furthermore, these two states have been invoked to explain the seemingly large Stokes shift dependence with NC size.¹ Although there has been a consensus on these observations, the origin of these states has been highly controversial. Explanations utilized to describe the observed structure and dynamics include: a dark-exciton state, a hybrid state consisting of a trapped electron and hole in the conduction band, a trapped exciton state, an exciton state split-off due to the intervalley interaction, and shallow trap surface states.¹
In this work, we show that both the Stokes shift and splitting between the two energy states scale as d^-3, where d is the diameter of the NC. These findings imply that these two states correspond to the singlet "bright" state and the triplet "dark" state, respectively. Furthermore we demonstrate that the Stokes shift is mainly given by the electron-hole exchange energy, and that in contrast to what was previously believed, the exchange energy is determined by the short-range interaction and not the long-range interaction. From these measurements we have also determined for the first time the bulk exchange interaction strength constant (J) for both PbSe and PbS.
[1] Ushakova E V, Litvin, A P, Parfenov P S, Fedorov A V, Artemyev M, Prudnikau A V, Rukhlenko I D and Baranov A V, 2012 ACS Nano,6, 8913.
[2] (a) Tischler J G, Kennedy T A, Glaser E R, Efros A L, Foos E E, Boercker J E, Zega T J, Stroud R M and Erwin S C, 2010 Physical Review B82; (b) Schaller R D, Crooker S A, Bussian D A, Pietryga J M, Joo J and Klimov V I., 2010 Physical Review Letters,105, 067403; (c) Kigel A, Brumer M, Maikov G I, Sashchiuk A and Lifshitz E, 2009 Small5 1675.

Absorption of photons by a semiconductor with an energy exceeding the band gap transition results in the formation of hot electron-hole pairs that quickly dissipate their excess free energy, resulting in quasi-thermalized conduction band electrons and valence band holes. For photovoltaic energy conversion, this cooling of the hot electron-hole pair is a major loss channel that restricts the maximum conversion efficiency of a single junction solar cell. Harvesting this excess free energy by hot carrier charge transfer has proven challenging due to the high cooling rates, reaching a few ps^-1 or more due to electron-phonon and/or carrier-carrier interactions. Using colloidal quantum dots (QDs), it proved possible to harvest the excess energy either by hot carrier transfer or the generation of multiple excitons (MEG), processes involving truly hot carriers with energies far above the band edge states (such as 1S, 1P). However, most studies on carrier cooling focus on changing occupation of these band edge states which only gives an idea of the rate-limiting step of the entire hot carrier cooling process. The relevant rates however are those of hot electron-hole pairs with energies far above the rate limiting 1P-1S transition.
Here, we analyzed the electron-hole pair cooling in PbX(X=S,Se) QDs after photo-excitation with high energy photons where, by means of femtosecond white light transient absorption spectroscopy, energy levels throughout the entire Brillouin zone are probed. In bulk, PbX has a direct band gap at the 4-fold degenerate L point and at higher energy a number of critical points are found, notably along the and - directions leading to discrete quantized states in QDs. We observe a transient accumulation of charge carriers at these high energy quantized states around when the pump photon energy exceeds the energy of the -states and link this cooling bottleneck, which slows down carrier cooling to a net rate of 1 ps^-1, via tight binding calculations to the energy level structure around : a specific set of longitudinal optical phonons is required to scatter hot carriers away from to the L-point absolute energy minima. We show that cooling via the -direction is the dominant pathway for hot carrier relaxation in lead-salts, indicating its importance for the study of hot carrier processes such MEG. The impact of our findings on understanding the high effiency of MEG in lead-chalcogenide QDs is discussed and implications for both theoretical and experimental work on MEG are discussed. The concept that high energy critical points slow down hot carrier cooling through a phonon scattering bottleneck is applicable to other materials than lead-chalcogenides and presents a new route to design high effiency photovoltaic materials for example through the deliberate introduction of such critical points in novel nanomaterials (e.g. twisted bilayer graphene).

Multi exciton generation (MEG), which involves creation of multiple charge carriers either from absorption of a high-energy photon or more than one low energy photons, is a very active area of research in colloidal quantum dots (QDs). The potential of MEG based solar cells to make a significant leap in photo-conversion efficiency has garnered significant interest in the study of multi-excitons in QDs.On the other hand, multiple exciton recombination, the reverse of MEG, is thought to play a key role in QD blinking and photo charging and its understanding has important implications for using QDs as reliable biomarkers, bright LEDs or single photon sources.
The fluorescent yield of biexcitons or trions is often small and their lifetimes can be comparable to fast decay components in multi-exponential exciton decays. It is therefore often challenging to reliably distinguish multi- from single-excited states using conventional time-resolved fluorescence techniques. We herein describe a novel multiple pulse photon counting technique that utilizes a pulse train containing a controllable number of 80 MHz laser pulses to probe the decay dynamics of multi-excited QDs. The technique relies on QDs&’ relatively long exciton lifetime, which allows us to generate multi-excited states using several low energy pulses. By controlling the individual pulse energy and varying the number of pulses in the pulse train, our technique allows us to controllably create and study the photo-physics of multi-excited states in colloidal QDs. We use our experimental technique (i) to resolve recombination dynamics of neutral and charged multi-excited states that are distinct from the dynamics of singly excited states in an ensemble sample of CdSe/CdS core/shell QDs and (ii) to show the effect of gold nanoparticles on multi-exciton emission from CdSe core and CdSe/CdS core/shell QDs and demonstrate the effect of plasmon coupling on the recombination dynamics of multi-excited states.

The optical and electronic properties of semiconductor nanocrystals are known to be highly sensitive to the surface chemistry of the nanocrystals. Integrating nanocrystals into functional films for optoelectronic applications requires the removal of the insulating native ligands, which frequently results in the formation of surface traps that can have a detrimental effect on the device performance. These challenges have previously been addressed via either post-synthesis or post-ligand-exchange surface passivation strategies. In this work we demonstrate simultaneous surface passivation and ligand exchange in PbSe nanocrystal films via treatment with a cadmium acetate solution. Passivation is accomplished via a kinetically limited surface cation exchange resulting in nanocrystal films that display the properties of Type I core-shell heterostructures. Both photoluminescence intensity and photoluminescence lifetime are increased following cation/ligand exchange indicating passivation of surface traps on the as synthesized nanocrystals rather than formation of additional surface traps. In addition, due to reduced interparticle spacing and limited “shell” thickness, charge transport is improved in the nanocrystal thin films. We present the concurrent cation and ligand exchange as a promising synthetic route to tailor the coupling among quantum dots in epitaxially connected confined-but-connected structures. Finally, we show that simultaneous passivation and ligand exchange occurs for a variety of cation-acetate complexes and rationalize trends via hard-soft acid-base interactions.

For over 30 years semiconductor nanocrystals (NCs) have been the subject of much interest for fundamental and applied studies. The synthesis of NCs has developed and matured over this time such that production of monodisperse, photostable NCs with close to an exact size and shape are readily achievable. However, an understanding of the chemical reaction mechanism behind the synthesis of NCs has lagged the ability to synthesize high quality nanoparticles. In this presentation, NC synthetic mechanisms that have been proposed for metal-chalcogenide (ME) semiconductor NCs, particularly CdE and PbE, will be discussed. Specifically, for PbE NCs synthesized under relatively low temperatures (< 200 0C) it was found that secondary phosphine chalcogenides are the primary reactive species that determines the formation of NC nuclei. Tertiary phosphine chalcogenides are completely unreactive at these temperatures. Small quantities of secondary phosphine impurities in the conventional tertiary phosphine based synthesis thus play an important role in determining the number of NCs formed and the rates of the reaction. Using this chemistry, it was discovered that the diameter of high-quality colloidal PbSe NCs can be easily tuned through the variation of the side-chains of a secondary phosphine precursor with the reaction also running to thermodynamic completion. Calculations of the P=Se bond strength for secondary phosphines with different side chains show that NC size tunability likely results from the ability of free Pb monomer to displace a phosphine on the NC surface. This new synthetic approach provides an effective route for IV-VI NC syntheses on the large scale while providing precise size control.
For NCs synthesized under much higher temperatures (generally > 250 0C), such as CdE NCs, several other mechanistic pathways start to dominate the reactivity. In particular, strong evidence exists for the formation of highly reactive metal-alkyl species in situ, which fundamentally bears a striking resemblance to some of the original CdSe NC syntheses developed over 20 years ago. Also to be discussed will be how an understanding of fundamental reaction mechanism can lead to improved NCs with controllable surface chemistry and with tailored optical properties.

The blinking of QDs is an intrinsic drawback for some biological and photoelectric applications based on single-dot emission. In this talk, we introduce our recent progress on the blinking suppressions of QDs and preparation of nonblinking QDs.^1-3
We successfully prepared the nonblinking QDs in optimum conditions using thiol ligands as stabilizers. We systematically studied the blinking behaviors of single QDs, investigated the effect of different surface ligand, synthetic conditions or UV irradiation etc. on the blinking suppression. We observed that blinking behaviors of QDs were able to be controlled by the structure and concentration of the thiol compounds. We found that, the suppressed blinking mechanism was mainly attributed to elimination of surface traps, formation of appropriate CdS coating on QDs, and controlling the growth dynamics of QDs. Nonblinking QDs show high quantum yield, small size, and good solubility, and will be applied to some fields that were previously limited by blinking of traditional QDs.
References:
[1] Dong C., Liu H., Zhang A., Ren J. Chem. Eur. J. 20, 1940,2014.
[2] Zan F, Dong C., Liu H., Ren J. J. Phys. Chem. C 116, 3944, 2012.
[3] Zhang A., Dong C., Liu H., Ren J. J. Phys. Chem. C 117, 24592, 2013.
[4] He H., Qian H., Dong C., Wang K., Ren J., Angew. Chem. Int. Ed. 45, 7588, 2006.

Nanocrystal quantum dots with their unique size and shape dependent properties have shown immense potential in various applications including energy harvesting and displays. However, despite much progress, the current applications of quantum dots remain limited largely due to the poor understanding of the parameters that affect their properties. For example, though it was known that surface of the nanocrystal and/or internal microstructure of the quantum dots affect the properties of the material, it is not systematically understood. Although it has been shown that the photo-physical properties like quantum yield, blinking etc are dramatically affected by the microstructure, this has not been extensively exploited. This is primarily due to the fact that though the effects of a single defect or interface on the properties of NQDs could have far-reaching consequences due to the high defect density, the control of defects in NQDs is very challenging as a result of the large number thermodynamic and kinetic factors involved. Similarly, in the case of surface modification, though solution route synthesis has opened up a possibility to passivate the surface traps using ligands, the role of various ligands is poorly understood and passivation till now has largely been carried out by chemical intuition.
In this talk, I will discuss the role of defects and surfaces in nanocrystals. I will demonstrate the role of microstructure and propose a method of tweaking the same leading to a playground to regulate the electron hole overlap at semiconductor interfaces leading to enormous potential in the field of applications taking the example of CdS as a host material in different environments.

There is currently great interest in electron transport in nanocrystal solids, driven by potential applications to electronic and optoelectronic devices. To date, most work in this area has employed disordered assemblies, and focused on enhancement of electronic coupling between nanocrystals through engineering of their ligands. A major goal of the field is to achieve band-type transport of electrons, and despite much progress in this direction, claims of band transport remain controversial.
Recently, the fabrication of quasi-two-dimensional superlattices of oriented nanocrystals with epitaxial inter-dot connections was reported [1,2]. The structures exhibit both short- and long-range order. Such “atomically-coherent” assemblies can exhibit square or honeycomb symmetry. Atomistic tight-binding calculations of the electronic states of the square superlattices of cadmium-salt and lead-salt nanocrystals reveal bandwidths that can exceed 100 meV, which imply promising transport properties [3]. Similar calculations of honeycomb superlattices additionally predict a variety of intriguing features, including Dirac cones and topological edge states [4].
We will report on the synthesis of atomically-coherent square superlattices of PbSe, along with structural characterization by x-ray diffraction and high-resolution electron microscopy. Measurements of the conductivity and mobility of the superlattices in field-effect transistor structures will be presented and compared to the results of theoretical calculations of the electronic structure of the superlattices. Prospects for achieving true band transport in these structures will be discussed.
References
1. W. H. Evers et al., Nano Lett. 13, 2317 (2013).
2. W. J. Baumgardner et al., Nano Lett. 13, 3225 (2013).
3. E. Kalesaki et al., Phys. Rev. B 88, 115431 (2013).
4. E. Kalesaki et al., Phys. Rev. X 4, 011010 (2013).

Advances in synthetic methods allow a wide range of semiconductor nanocrystals (NCs) to be tailored in size and shape and to be used as building blocks in the design of NC solids. However, the long, insulating ligands commonly employed in the synthesis of colloidal NCs inhibit strong interparticle coupling and charge transport once NCs are assembled into the solids state as NC arrays. We employ a range of short, compact ligand chemistries to exchange the long, insulating ligands used in synthesis and to increase interparticle coupling. These ligand exchange processes can have a dramatic influence on NC surface chemistry as well as NC organization in the solids, showing examples of short-range order. Synergistically, we use 1) thermal evaporation and diffusion and 2) wet-chemical methods to introduce extrinsic impurities and non-stoichiometry to passivate surface traps and dope NC solids. NC coupling and doping provide control over the density of states, the carrier statistics and the Fermi energy. Examples of strong coupling and doping in II-VI and IV-VI semiconductor NC solids will be given that yield high-mobility, high-conductivity NC solids. Temperature--dependent transport measurements of these materials are consistent with a transition from localized to extended-state charge transport. These high mobility n- and p-type materials are used as the semiconductors to construct large-area, flexible, field-effect transistors and integrated circuits and for solar photovoltaics.

Colloidal semiconductor nanocrystals, commonly known as quantum dots (QDs), provide solutions to many modern energy problems. They can be used for energy generation as active components of inexpensive solar cells and solar concentrators, but they can also enable efficient utilization of electricity for lighting. Nanocrystal structure, specifically heterostructring, size, and surface chemistry, play crucial roles in adapting their properties for efficient energy harvesting and utilization.
The good solubility of QDs in many liquids enables scalable, room-temperature, non-vacuum deposition approaches to making devices (e.g., spray deposition, dip coating). Unfortunately, most quantum dots (e.g., CdSe, PbS, and InP) are expensive and toxic, which dramatically limit their potential. Dr. McDaniel is the founder and President of UbiQD, LLC, a new company located in Los Alamos, New Mexico that is commercializing inexpensive low-toxicity I-III-VI (e.g., CuInSeS) QDs for various energy applications.
In this talk Dr. McDaniel will present recent results of research he conducted while a postdoc at Los Alamos National Laboratory that ultimately led to the founding of UbiQD. Those results include usage of CuInSeS QDs for photocatalysts, photovoltaics, transistors, light-emitting diodes, and phosphors. In particular, surface modification and cation exchange were found to be extremely useful tools for tailoring carrier dynamics for these purposes.

We report on the precision synthesis of CdSe/CdS/ZnS core-shell-shell nanocrystals using a preparative flow reactor. Experimental design is used to determine the crucial parameters and their influence on particle growth and size distribution.
In the second part of the talk, we will present applications of nanocrystals. In particular, we will report on the development of quantum dot quantum rod particles with fluorescence quantum efficiencies close to unity and applications in lighting and display technology. For biomedical applications we will present a biocompatible encapsulation technique based on amphiphilic poly(isoprene-block-ethylene oxide) (PI-b-PEO) diblock copolymers. We varied block lengths, structure and functional terminal end groups and investigated the effect on unspecific uptake. Fluorescence quenching experiments with encapsulated quantum dots show that best behavior in respect to unspecific cellular uptake is realized in those systems, in which the polymer shell yielded best protection against quenching molecules from the surrounding medium. Combination of micelle encapsulation with block copolymers and seeded emulsion polymerization finally leads to biolables for which unspecific uptake could be almost completely suppressed even under in-vivo conditions. We present various techniques for bio-conjugation with recognition molecules and show examples for specific cell and tissue targeting. In-vitro and in-vivo fluorescence and MRI data will be discussed.

Tin(II) sulfide is a p-type semiconductor with a direct band gap of 1.3 eV and an indirect band gap of 1.07 eV. Composed of earth abundant and environmentally benign elements, SnS is handled as an attractive candidate for use in various applications such as solar cells, photodetectors and transistors but also in the field of energy storage. SnS generally crystallizes in the orthorhombic Pnma structure, which favors in principle the anisotropic growth of 2D nanostructures such as platelets or sheets. However, the majority of reported chemical synthesis methods yield isotropic spherical or facetted nanoparticles. Only few examples of 2D structures exist, essentially in form of large (edge length > 200 nm) nanosheets.
Here we present a new approach for the synthesis of uniform, square SnS nanoplatelets of controlled lateral dimension (edge length: 30-100 nm) and thickness (5-15 nm). As the exciton Bohr radius of SnS is around 7 nm, the obtained thin platelets show quantum confinement in one direction of space, as evidenced by UV-vis spectroscopy. A detailed high-resolution transmission electron microscopy study combined with high angle annular dark field electron tomography, electron diffraction and comparison with simulated diffraction patterns sheds light on the growth mechanism of the anisotropic nanoparticles. Furthermore, this study indicates that the exposed (100) surfaces are sulfur-terminated, which may have important consequences on the oxidation sensitivity of the SnS nanoplatelets. The reaction mechanism has been elucidated by means of systematic control experiments. The gained knowledge allowed us determining the conditions for size- and shape-controlled growth of either spherical SnS nanocrystals or 2D nanoplatelets. Intriguingly, the same synthetic scheme using a different sulfur precursor results in small (2-4 nm) tin(IV) sulfide (SnS₂) nanocrystals. SnS₂ is a larger band gap n-type semiconductor (Eg: 2.4 eV), and promising candidate as buffer layer material in thin-film solar cells, in replacement of ubiquitously used CdS.
Sn(IV) is also constituent of the quaternary metal sulfide semiconductor Cu₂ZnSnS₄ (CZTS), under active research as low cost absorber material in thin-film solar cells (Eg: 1.5 eV). We used in situ synchrotron X-ray diffraction for studying the reaction kinetics and identifying transient crystalline phases occurring during a widely applied one-pot synthesis method of CZTS nanoparticles. Our results, backed up by EDS and TEM analyses, indicate that tin is incorporated into initially formed, off-stoichiometric CZS nanoparticles in a cation exchange reaction. This change in composition leads to an abrupt phase transformation resulting in the final kesterite phase (tetragonal space group I-4), taking place within the first few minutes after reaching the reaction temperature of 280°C.

Doped semiconductor quantum dots (QDs) play an important role in many photonics and other applications. Among the common dopants, Cu is one of the most useful and interesting due to its strong green/blue fluorescence. However, Cu(I)-doping is particularly challenging due to its low solubility and tendency of clustering. Co-doping with anions is a common strategy for improving the solubility and optical properties of the Cu(I) dopant. Recently, we have developed a new strategy to co-dope using trivalent cations such as Al(III), Ga(III) and In(III), that have been found to significantly enhance the photoluminescence (PL) properties of Cu. The enhancement is attributed to better charge balance and increased solubility of Cu(I) ions. ZnSe QDs with zinc blende crystal structure and average particle size of 6 nm were synthesized using a wet chemical route. The local structure of the Cu dopant was studied by extended X-ray absorption fine structure (EXAFS). The optical properties were studied using UV-Vis and PL spectroscopy. Exciton dynamics were investigated using time-resolved PL techniques. In addition, DFT calculations have been used to obtain the density of states of a model system to help explain the optical properties and dynamics processes observed. Our study demonstrates that co-doping using different cations with complementary oxidation states is an effective method to enhance optical properties of doped semiconductor QDs of interest for various applications. Synergistic interaction between the primary dopant and co-dopant is critical for the enhanced and tunable PL, and this interaction is likely facilitated by spatial confinement of QDs.

This talk will focus on the similarities and differences between the syntheses of group II-VI and group IV-VI nanocrystals using trialkylphosphine-based precursors. Our results showed that nucleophilic attack of a metal-activated phosphine chalcogenide is a common mechanism for the production of monomer in both material systems. The factors affecting the reactivity of the precursor pairs will also be discussed.

Chalcogenoureas with adjustable organic substituents are conveniently prepared in single steps from commercially available starting materials. These compounds react with metal surfactant complexes producing metal sulfide nanocrystals in quantitative yields. The rate of precursor conversion can be tuned by adjusting the organic substituents, allowing chalcogenoureas to be used over a wide range of reaction temperatures and together with many metal surfactant precursors. By adjusting the precursor conversion rate a desired nanocrystal size can be prepared in quantitative yield. This tunability allows us to probe the temperature dependence of nanocrystal concentration as well as the size distribution produced by the nucleation time. In particular, we have addressed the mechanisms controlling the link between precursor conversion kinetics and crystal nucleation and demonstrated that these reactions proceed by traditionally conceived homogeneous nucleation and growth pathways.

As-synthesized colloidal quantum dot samples typically or inherently contain large concentrations of molecules that could coordinate the surface. However, applications almost universally require purification and/or surface modification of as-synthesized QDs. Purification methods have frequently been seen to decrease the photoluminescence quantum yield (QY) and also to decrease ligand populations. It is essential to understand whether the changes in QY are reversible, how ensemble QY and decay profiles depend on ligand occupation, and the conditions under which surface structures that support high QY can be maintained or restored. We recently described the use of size-exclusion chromatography with a polystyrene stationary phase to separate natively capped colloidal QDs from small molecules in anhydrous solvents. This has the effect of removing impurities and weakly bound ligands, including phosphines, phosphine oxides, and primary amines, enabling the recovery of QDs with surfaces bearing a low and consistent number of metal carboxylate equivalents. In the present study, we take advantage of GPC purification of core/shell QDs to explore the role of ligand population in maintaining high QY. Our results will be discussed in the context of possible energy loss mechanisms among QDs with low ligand populations.

Colloidal quantum dots (QD) have unique electronic and optical properties such as high photoluminescence efficiency, narrow emission bandwidth and tunability of the bandgap over the wide range from UV-visible to near infrared region. In addition, they can be deposited on a large-area flexible substrate using solution processes and roll-to-roll coating. Thus, they offer promising opportunity for developing high-performance full-color light-emitting diodes, realizing next-generation flexible displays. Since the first demonstration of the QD-based light-emitting diode (QLED) in 1994, its performance has been improved significantly as a result of multilateral efforts based on development of high-quality QDs, fundamental understanding of the device physics and advanced device engineering. Here, we report our recent efforts to develop efficient QLEDs and explain the underlying mechanism for their improved performance.

Colloidal quantum dots (QDs), owing to their efficient, narrow-line, tunable emission, are rapidly emerging as promising candidates for the realization of solution processed light-emitting diodes (LEDs) with ultra-saturated colors. Despite their relatively young age, QD-LEDs have reached performances competitive to their organic counterparts, with external quantum efficiencies (EQE) as high as 18%. Over the years, the QD-LED architecture has evolved from the ‘direct&’ structure typical of OLEDs to the better performing ‘inverted&’ scheme, where electrons and holes are respectively injected from the ITO substrate coated with ZnO nanoparticles and from a thermally-evaporated organic layer deposited on top of the QD film. This additional evaporation step, however, reduces the economic and industrial advantages of using colloidal emitters and represents a process bottleneck for the transfer of QD-LEDs from research labs to the real-world. It is therefore pivotal to replace current charge injection/transport layers (CTL) with effective solution-based materials. To date, the use high mobility conjugated polymers has proven unsuccessful mainly because of their solubility in non-polar organic solvents that severely damage the QD layer.
In this talk I will demonstrate a new strategy for achieving highly efficient QD-LEDs in direct architecture using exclusively solution-processed organic CTLs. Specifically, our architecture employs novel alcohol-soluble poly(fluorene) derivatives with polar/ionic functionalities as easily processable ETL and poly-vynil-carbazole as HTL. Thin polar polymer films create a strong interfacial dipole that drastically lowers the injection barriers, effectively balancing charge injection. As a result, using deep-red-emitting (690nm) CdSe/CdS dot-in-rod QDs as emissive material, we produced all-solution-based QD-LEDs with low turn-on voltage (3V), high brightness (1200cd/m2) and record EQE of 6,1%, which is the highest ever reported for QD-LEDs with all-organic CTLs (including evaporated CTL, so-called type II QD-LEDs). Importantly, in contrast to record literature devices, our LEDs were produced using simple research equipment and no dedicated industrial facilities, which further emphasizes the great potential of the proposed strategy. Furthermore, LEDs incorporating polar CTL show enhanced stability and no droop-off of the EQE at high current densities. In order to unravel the physical mechanisms underpinning the remarkable device performances, we investigated LEDs using polymer layers with different polar and ionic functionalities also in combination with various types of hetero-nanostructures including spherical and elongated systems with different aspect ratios. The chemical flexibility of organic synthesis offers almost unlimited degrees of freedom for the realization of polar polymers matching the electronic structure of QDs and thus opens the way to a new wave of high performance organic-inorganic hybrid LEDs.

Advances in colloidal quantum dot light-emitting diodes (QD-LEDs) have led to efficiencies and brightness that rival the best organic LEDs. Apparent achievement of nearly ideal internal quantum efficiency leaves light outcoupling as the only remaining means to improve external quantum efficiency (EQE) but that might require radically different device design and re-optimization. However, the current state-of-the-art QD-LEDs are based on spherical core/shell (C/S) QDs and the effects of shape and optical anisotropy remain essentially unexplored. Here, we demonstrate a simple means to improve LED performance by using nanorod-based QD structures, the double heterojunction nanorods (DHNRs), in conjunction with enhanced hole injection via the introduction of a hole dopant to the transport layer within a conventional solution processed device structure. These DHNR-LEDs exhibit maximum brightness of 76,000 cd m-2, current efficiency of 27.5 cd A-1, and power efficiency of 34.6 lm W-1. These characteristics exceed performances of all reported QD-LEDs of similar emission wavelength. These results are especially impressive in that these DHNR-LEDs are fabricated by a simple all-solution process. More importantly, a very high EQE of 12% at photoluminescence quantum yield (PL QY) of only 40% indicates that there is a fundamentally different and higher efficiency limit to these DHNR-LEDs than other QD-LEDs. We will discuss the role of optical anisotropy of the DHNRs on light outcoupling. We will also show and discuss operational stability of DHNR-LEDs.

CdSe-based heteronanocrystals (heteroNCs) are well-established colloidal nanomaterials, mainly because of their excellent optoelectronic properties such as high optical stability, wide emission tunability, and enhanced absorption cross section in giant-shell systems. Control over their exciton recombination rate has been mostly achieved via band gap engineering of type I and quasi-type II heteroNCs. This approach typically leads to lifetimes ranging from 20-25 ns in type-I CdSe/ZnS to about 200-500 ns in quasi-type II CdSe/CdS NCs, where the conduction band offset is reduced leading to electron delocalization into the shell.
For this presentation, we used a radically different approach to control the optical properties, via strain[1] and piezo-electric potentials. We combined the shape-controlled synthesis of large (11 nm x 26 nm), wurtzite CdSe nanorods with the growth of a giant rod-like shell, in order to modify the band structure through strain engineering. This turns the CdSe/CdS rod-in-rod (RIR) into a type-II material, with an exciton emission peak at 740 nm and a lifetime of up to 4.4mu;s, while maintaining a surprisingly high PL QE of 25% due to the thick shell (lateral and axial shell thickness of 9 and 29.5 nm, resp.). Fluorescence line narrowing spectroscopy confirmed the type-II behavior of the large-core RIRs.
k.p calculations revealed that, next to a strain-induced local shift of the band structure at the CdSe/CdS interface, the wurtzite crystal structure leads to an additional internal piezo-electric field which steers the hole away from the interface-trapped electron. This is the main driving force reducing the electron hole overlap and yielding the microsecond lifetimes. However, while the shallow potential formed at the interface is sufficient to obtain strongly separated electron-hole pairs for the ground state exciton, ultrafast spectroscopy at cryogenic temperature showed that the blue shifted biexciton and excited state exciton emission have lifetimes that are again in the nanosecond regime, in agreement with k.p calculations that yield a much stronger overlap for the additional (biexciton, excited state) electrons with the holes localized in the core.
Using strain in shape- and crystal structure-controlled heteroNCs, we created a particular type of quantum dot, formed at the type-II interface of strained CdSe/CdS giant-shell RIRs. Strain engineering may therefore bring about a more complete control over NC optical properties, from spectral position of the fluorescence peak to the corresponding emission lifetime and excited-state carrier dynamics. With the unique RIR band structure in mind, we envision exciting future applications in exciton storage, or energy harvesting via solar cells or down-converting phosphors.
[1] A.M. Smith et al., Nature Nanotech.2009, 4, 56-63

We have investigated the relationship between the dimensions of core/shell heterostructured quantum dots (QDs) with the performances of light-emitting diodes (LEDs). For comparative study, we have synthesized a series of CdSe/Zn_1-XCd_XS core/shell type-I heterostructured QDs having similar optical properties (e.g., photoluminescence energy, full width at half maximum, photoluminescence quantum yield, single exciton lifetime) but varying shell thicknesses (core radius: 2.0 nm, 2.5 nm <= shell thickness <= 6.3 nm). Upon the inverted device architecture, thick-shell QDs have shown higher efficiency and operational stability along the current sweep within the actual devices. Spectroscopic analysis reveals that the suppression of energy transfer and QD charging in thick-shell QDs is indeed responsible for the improved device performances. As an ultimate achievement, deep-red QD-LEDs (lambda;_MAX = 630 nm) exhibiting peak external quantum efficiency of 7.4 % and record-high brightness above 100,000 cd/m² could be realized based on the type-I giant QDs (core radius: 2.0 nm, shell thickness: 6.3 nm).

Accurate size determination of semiconductor nanocrystals is essential for a quantitative understanding of their size dependent optical and electronic properties. While electron microscopy is the method of choice, it is time consuming and prone to user-bias since only a miniscule fraction of the sample is examined. Furthermore, for experiments involving particle diffusion or assembly into super structures, it is the hydrodynamic size which is relevant. We use analytical ultracentrifugation (AUC) to measure the sedimentation velocity of CdSe nanocrystals as a function of size [1]. From these measurements both the density and diffusion coefficient are obtained independently. These measurements can be used to determine unambiguously the size and ligand packing on the surface, provided Stokes' Law is valid. We present data showing that the sizes obtained by AUC are as precise as those obtained via electron microscopy and enable direct insight into ligand packing and orientation on nanocrystal surfaces.
In the second part of the talk, I will present initial data on the fabrication of QD LED devices fabricated using these materials. A key requirement for good device operation is high luminescence quantum yield of the semiconductor nanocrystals. High temperature synthesis of CdSe@ZnS nanocrystals results in materials, which routinely exhibit >90% quantum yield [2]. QDs prepared in the absence of oxygen and water vapour remain remarkably photostable if maintained in a pristine environment. Exposure to water vapour or to oxygen leads to slow losses in PL. As a result, we find stable LED devices require the rigorous exclusion of these elements.
[1]. Demeler et al. Analytical Chemistry 86, 7688 (2014).
[2]. Boldt et al. Chemistry of Materials 25, 4731 (2013).

Semiconductor nanocrystal quantum dots (QDs) are attractive materials for applications as laser media because of their bright, size-tunable emission and low-cost processibility. For II-VI QDs, it is has been established that due to the two-fold degeneracy of the emitting states the condition for the optical gain can be satisfied only if QDs are populated with more than one exciton. However, multiexciton population of QDs rapidly decays (within tens of ps) via nonradiative Auger recombination whereby the recombination energy of an electron-hole pair is not converted into a photon but is instead transferred to a third carrier. Consequently, fast Auger decay rates drastically limit the time available for the development of optical amplification and are responsible for high pump intensities required to excite amplified spontaneous emission (ASE) in QD media.
Here we report ASE and lasing with very low thresholds obtained using thin films made of QDs engineered in such a way as to provide suppression of Auger recombination. In this study, we use thick-shell CdSe/CdS QDs (so called “giant” QDs) that have a CdSeS alloyed layer between the CdSe core and the CdS shell. These “alloyed” QDs exhibit considerable reduction of Auger decay rates, which results in high biexciton emission quantum yields (Q_BX of ~ 12%) and extended biexciton lifetimes (tau;_BX of ~ 4ns). By using a fs laser (400 nm at 1 kHz repetition rate) as a pump source, we observed biexciton ASE emerging on the blue side of exciton emission band at the threshold intensity as low as 5 mu;J/cm². This value is about 5 times lower than the lowest ASE thresholds reported for “giant” QDs without interfacial alloying. Interestingly, we also observed biexciton random lasing from the same QD film. Lasing spectrum comprises several sharp peaks (linewidth as narrow as 0.2 nm), and the heights and the spectral positions of these peaks show strong dependence on the exact position of the excitation spot on the QD film. Further analysis on the lasing spectra indicates that whispering-gallery-mode-like microcavities with a round-trip path of ~100 mu;m are formed possibly due to the nonuniformities of the refractive index resulting from QD clustering and/or film-thickness variations. Our study suggests that further suppression of nonradiative Auger decay rates via even finer grading of the core/shell interface could lead to a further reduction in the lasing threshold and potentially realization of lasing under continuous-wave excitation.

One of the key tools for the characterization of nanoparticles has been transmission electron microscopy. No other technique can provide the size, shape, crystal structure and chemical composition of a nanocrystal. Advanced scanning transmission electron microscopy (STEM) has furthered our ability to image the chemical structure at a sub-nanometer level. Aberration-corrected Z-STEM has enabled the visualization of the chemical transition between the core and shell, accelerating the development of commercial quantum dots.¹ Further, we have shown through dynamic STEM movies the beam-induced motion of the surface atoms of nanocrystals and learned about the instability of the atomic structure of ultrasmall nanocrystals and the surface/sub-surface of large nanocrystals.² However, Z-contrast can be difficult to directly interpret due to the choice in shell material or uncertainty of the 3D morphology of large quantum dots. Recently, advancements in the detector design for performing STEM energy dispersive spectroscopy mapping (STEM-EDS) have greatly facilitated the chemical imaging of nanocrystals, enabling rapid and practical identification of their chemical structure. Specifically, a FEI Tecnai Osiris equipped with ChemiSTEM has been utilized to image the sub-nm chemical structure of a variety of emergent nanocrystal systems. For the first time, the chemical composition of an individual nanocrystal is correlated to its individual photophysics using our recently developed correlation technique.³ The fluorescence intermittency of individual giant-shelled CdSe/CdS quantum dots was first imaged using a confocal microscope, then the relative position of the core and shell was imaged using STEM-EDS. Further, STEM-EDS imaging will be presented showing development of CuInS₂/ZnS and Zn₃N₂/ZnS nanocrystals. Included in the presentation are specifics on sample preparation and the choice of beam current/spatial resolution and sample damage.
1. McBride, J.; Treadway, J.; Feldman, L.C.; Pennycook, S.J.; Rosenthal, S.J. Structural Basis for Near Unity Quantum Yield Core/Shell Nanocrystals Nano Lett.2006, 6 (7), 1496-1501.
2. McBride, J.R.; Pennycook, T.J.; Pennycook, S.J.; Rosenthal, S.J. The Possibility and Implications of Dynamic Nanoparticle Surfaces ACS Nano2013, 7 (10), 8358-8365.
3. Orfield, N.J.; McBride, J.R.; Keene, J.D.; Davis, L.M.; Rosenthal, S.J. Correlation of Atomic Structure and Photoluminescence of Single Quantum Dots Reveals Ideal, Defective, and Dark Populations Nano Lett.2014, Submitted.

Discovery of single-source broadband white-light emitters is important in the transition from traditional artificial lighting to solid-state-lighting (SSL) devices. Recent work has shown that layered hybrids comprised of corrugated lead halide sheets separated by organic molecules can radiate across the entire visible spectrum upon ultra-violet excitation [1, 2] To understand the broadband emission mechanism, we carry out both femtosecond transient absorption and optical-pump/terahertz (THz)-probe measurements on spin-coated thin film samples. THz conductivity measurements reveal that photogenerated free carriers are trapped in these materials in less than 2 ps. Light-induced defect formation associated with strong electron-phonon coupling and decay dynamics are monitored by below-gap transient absorption measurements at different wavelengths, with similar relaxation times observed compared to photoluminescence lifetime on nanosecond time-scales. On shorter time-scales, spectroscopic studies reveal complex dynamics occurring on few picosecond time-scales, turning on as the THz conductivity turns off, with different dynamics occurring for broadband and narrow-band emitting samples. These results suggest that white-light emission can be directly correlated with the formation of light-induced defects, likely in the form of self-trapped excitons in metal halide materials. This study provides a new understanding of photogenerated carrier behavior in two-dimensional perovskite materials, which in turn can provide a useful guide for engineering such layered organic-inorganic structures to achieve better performance.
[1] Dohner, E. R., Hoke, E. T., and Karunadasa, H. I. J. Am. Chem. Soc.2014, 136, 1718.
[2] Dohner, E. R., Jaffe, A., Bradshaw, L. R., and Karunadasa, H. I. J. Am. Chem. Soc. 2014, 136, 13154.

Quantum Dots (QDs) show promise for many technological applications, including photocatalysis and photovoltaics. However, their photophysical properties are sensitive to surface reactions, resulting in uncontrollable luminescence quenching. Using density functional theory (DFT) and time dependent DDFT (TDDFT), we simulate the oxidation process on the surface of Pb₁₆Se₁₆ and Pb₆₈Se₆₈ QDs and its effect on their electronic and optical properties. When oxygen substitutes for Se ions at the surafce, the electronic properties of the QD are insignificantly perturbed. In contrast, if oxygen is adsorbed on the QD surface and coordinated with two Pb ions, it introduces additional unoccupied states inside the QD&’s band gap, so called mid-gap states. Such states are slightly hybridized between the oxygen and the QD&’s surface atoms and contribute to the lowest-energy optically dark or semi-dark transitions likely resulting in quenching of the QD luminescence. In contrast, if the oxygen is coordinated with Se and Pb ions on the surface, the mid-gap states are not present and the optical transitions are similar to those of the non-oxidized QDs. Similar behavior was also found for halide radicals. However, the trap states are eliminated for halides in their ionic form or in a form of lead salts, PbCl₂ and PbI₂. Other surface ligands such as primary amines interact with the QD surface 3-4 times weaker than halides and insignificantly affect the electronic structure of QDs. However, presence of amines increases the binding energy of neighboring Cl ions, favoring their attachment to (110) surfaces, while also providing a bridging attachment to other neighboring QD. In contrast, iodine ions do not form the bridging network due to much longer Pb-I bond length as compared to the Pb-Se bond. Our results explain why Cl passivation is critical in the growth of two-dimensional nanostructures, which blocks (110) facet of QDs and induces (100) oriented attachment of QDs in PbSe nanoplatelets, as was recently observed experimentally.

The unique electronic properties of quantum-confined semiconducting nanowires hold great promise for their incorporation in next-generation transistors, circuits, and electronic devices. This reduction in dimensionality results in a dramatic change in their carrier dynamics and electronic structure, [1-2] leading to novel properties such as ballistic transport and conductance quantization. One area of particular interest is the formation and understanding of electron gases in heterostructured nanowires which provides new avenues for exploring enhanced carrier dynamics in these materials.
In order to tailor these nanostructures with the desired physical properties, we must first understand their electronic properties as a function of size and material composition. To this end, we have developed a series of self-consistent predictive computational methods [1-3] to calculate the properties of heterojunction electron gases in quantum-confined core-shell nanowires. Under certain conditions (depending on doping density and spatial geometry), we find that quasi-one-dimensional electron gases can localize at the corners of the nanowire, leading to carrier dynamics that are dramatically different than analogous bulk heterojunctions. In addition, we highlight several areas where many-body quantum effects play a significant role in these low-dimensional structures. In particular, we surprisingly find that simple theoretical approaches can (1) considerably overestimate the number of occupied electron levels, (2) overdelocalize electrons, and (3) significantly underestimate the relative energy separation between electronic subbands. Our results allow a guided understanding of electron carrier dynamics in heterostructure nanowires and further indicate that electron gases in free-standing nanoscale systems are qualitatively different from their bulk counterparts.
[1] B.M. Wong, F. Leonard, Q. Li, and G.T. Wang, Nano Letters, 11, 3074 (2011).
[2] M. Fickenscher, T. Shi, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, C. Zheng, P. Miller, J. Etheridge, B.M. Wong, Q. Gao, S. Deshpande, H.H. Tan and C. Jagadish, Nano Letters, 13, 1016 (2013).
[3] A.W. Long and B.M. Wong, AIP Advances, 2, 032173 (2012).

The efficiency of multi-exiton generation (MEG) in colloidal QDs is determined by the competition between MEG and other hot electron-cooling processes. These have characteristic times of t(MEG)~t(cooling)~1 ps in the colloidal QDs studied to date but for high efficiency t(cool)>>t(MEG) is required [1,2]. The core/shell QDs with type II band alignment offers extra degree of freedom in mediating both the optical dipoles and the Coulomb interaction between charges in such structures. To assess faithfully the effect of electron correlations on the radiative and Auger related times, theoretical methodology was established, based on an multy-band k.p Hamiltonian. Excitonic states were found using the full Configuration interaction method, that incudes explicitly the effects of Coulomb interaction, exact exchange and correlations between many-electron configurations. In setting up the full CI, particular attention was paid to accurate modeling of the dielectric constant variation through the structure and surface polarization effects. Dielectric constants of CdSe and CdTe at the transition energies are predicted using ab initio time-dependent density functional theory [3]. We map the 1S(e)nS(h) (n = 1, 2) exciton correlation energy relative to the strong confinement approximation as a function of core radius and shell thickness. The type-II confinement potentials mean that dielectric confinement has a large effect on the wave functions and exciton energies in such heterostructures, mainly increasing the correlation energy for QDs in which the corresponding single-particle hole is delocalized. We also find that correlation leads to large changes in the momentum matrix element, particularly for the lowest CdSe/CdTe QD exciton in which it is increased up to a factor of ~8 in the presence of dielectric confinement. Overall dielectric confinement affected the exciton properties in CdSe/CdTe QDs more than the inverse heterostructures due to the band alignment, which encourages holes to localize in the shell. Using the correlatedexcitonic energies and correlated optical dipoles we have predicted that the radiative times increase only for one order of magnitude (from ~20 ns to ~150 ns) by chancing the QD shell thicknesses from 0 to 2 nm. Those results are in excellent agreement with experimentally measured radiative time on similar type II QD structures [2,5]. However in the same range of shell thicknesses change, the non-radiative Auger electron cooling times increase for three orders ofmagnitude (from ~1ps to ~ 1 ns). Such dramatic slow down of the Auger electron cooling could be of the potential benefit for increased MEG efficiency.
1. A. Pandey, P. Guyot-Sionnest, Science 322 (5903), 929 (2008)
2. Z. Deutsch, et al, Phys. Chem. Chem. Phys. 13, 3210 (2011)
3. S Tomic and N Vukmirovic J. Appl Phys 110, 053710 (2011)
4. L. Bernasconi, S. Tomic et al, Phys Rev B 83, 195325 (2011)
5. K. Gong, et al, J. Phys. Chem. C 117, 20268 (2013)

Nonradiative Auger recombination is the central non-radiative relaxation process affecting all aspects of carrier dynamics in semiconductor nanocrystals. Auger processes, being significantly enhanced in quantum-confined structures, can dominate the decay of multiexcitons, facilitate fluorescence intermittency, induce the efficiency droop in nanocrystal light-emitting diodes, and limit the performance of nanocrystal lasing applications. The mechanism of the Auger rate acceleration is connected with the combined effect of spatial confinement of carriers and the presence of abrupt interfaces in the nanocrystals. These two effects admix high-momentum components into the ground state wavefunctions of the carriers, thereby relaxing the strict momentum conservation rule during the Auger recombination. Based on the experimental data, the existence of a universal material-independent scaling law of the multiexcitonic Auger recombination rate 1/a³ with the nanocrystal size, a, has been proposed [1]. A stronger size dependence, 1/a^4.5, has been measured for the Auger recombination rate of trions in CdSe quantum dots [2]. Earlier calculations of the Auger rates in CdS quantum dots [3] predicted an even stronger dependence on the nanocrystal size. Our calculations, which are based on an 8-band k.p model, of the Auger recombination rate of negatively charged trions in CdSe nanocrystals also show a much stronger 1/a⁷ average size dependence [4]. A comparison of experimental data with theoretical calculations allows us to account for differences in the experimental and theoretical size-dependence of the Auger recombination rate. The main difference arises from the common use of a single exponential approximation for the average Auger recombination rate. Due to several orders of magnitude non-monotonic changes in the Auger recombination rate with nanocrystal size within typical size dispersion of a nanocrystal ensemble this approach has very limited accuracy [4].
[1] I. Robel, R. Gresback, U. Kortshagen, R. D. Schaller, and V. I. Klimov, Phys. Rev. Lett.102, 177404 (2009).
[2] A. Cohn, J. Rinehart, A. Schimpf, A. Weaver, and D. Gamelin, Nano Lett. 14, 353 (2014).
[3] D. I. Chepic, Al. L. Efros, A. I. Ekimov, M. G. Ivanov, V. A. Kharchenko, I. A. Kudriavtsev, and T. V. Yazeva, J. Lumin.47, 113 (1990).
[4] R. Vaxenburg, A. Rodina, A. Shabaev, E. Lifshitz, and Al. L. Efros, to be published.

The dynamics of singlet and triplet states in organic molecules define their device performance for many potential applications, including organic photovoltaic cells and optoelectronics. More specifically, for electron donor-acceptor systems, undesirable charge recombination may occur before the electron approaches the interfacial contact of the device due to the relatively short lifetime of the singlet excited state. Harvesting the triplet state exciton that results from intersystem crossing (ISC) is a unique solution for significantly reducing this undesirable loss. However, to our knowledge, precise tunability and control over the ISC and triplet state lifetimes of organic or inorganic materials has never been reported in the literature. Here, we use a cationic porphyrin-CdTe QD assembly as a model system to clearly demonstrate how we can dramatically tune the ISC rate and the triplet state lifetime. The cross-talk between the positively charged pyridinium moieties in meso-tetrakis(1-methylpyridinium-4-yl) porphyrin chloride (TMPyP) and the negatively charged carboxylate groups of the thiol ligands coating the surface of the QDs causes the two systems to electrostatically approach each other to form TMPyP-CdTe QD nanoassemblies. The strength of the interaction is defined and controlled by the capping ligand and by the size of the CdTe QDs. Our time-resolved transient absorption and fluorescence results demonstrate that the ISC rate and triplet lifetime can be tuned by factors of 4 and 9 by changing the size of the QDs from 6.3 to 1.3 nm. These findings suggest the need for further studies of tunable ISC for other absorber materials, and perhaps most importantly, they open new avenues for triplet state light harvesting in solar cell applications.

The behavior of water molecules on the surfaces of the TiO₂ nanowire grown in [001] direction has been investigated by combining theoretical calculations and experiments. A first principles methodology for computing emission spectra of semiconductors is applied to TiO2 anatase nanowires. Computations predict that a photoexcitation is followed by a sequence of relaxation events resulting in photoluminescence(PL) across the gap. TiO₂ nanowires in vacuum and aqueous environment exhibit different dynamics of photoexcited charge carriers. UV-visible absorption and PL spectra for anatase TiO₂ nanowires are computed with account of (i) nonadiabatic cascade thermalization and (ii) inhomogeneous broadening.^[1,2,3] Excited state lifetimes are short due to quick cascade thermalization, leading to intense PL peaks at energies corresponding to the bandgap energy. Numerical evidence of linewidth broadening of PL is observed as a consequence of nuclear motion. Analyzing the energy fluctuations of the HOMO and LUMO states allows for a sampling of possible radiative transition energies, attributing the PL linewidth to inhomogeneous broadening. Calculated UV-visible absorption and PL reproduce the main features of the experimental spectra. Comparison of relaxation processes in TiO₂/water interfaces focusing on different surfaces and nanostructures has potential in identifying structural characteristics of TiO₂ materials important for efficient photo-electrochemical water splitting.
1. Vogel, D. J.; Kilin, D. S., MRS Proceedings 2014,1647, mrsf13-1647-gg08-07.
2. Huang, S.; Kilin, D. S., J. Chem. Theor. Computation 2014, 10 (9), 3996-4005.
3. Nelson, T.; Fernandez-Alberti, S.; Roitberg, A.E.; Tretiak, S., Acc. Chem. Res. 2014, 47 (4), 1155-1164.

Quantum dots (QDs) have a series of important properties such as high absorption coefficients, size dependence and easy tunability of their optical and electronic properties due to the quantum confinement, possibility of multiple exciton generation, which makes them a very attractive material for their applications in different technological areas, including photovoltaic devices. Colloidal quantum dots used for sensitized solar cells typically contain toxic metals, which limit the upscaling and further industrial applications of such cells. Development of “eco-friendly” quantum dots without heavy elements is thus of high importance. Ternary and quaternary nanocrystals combine the classical advantages of QDs with their non-toxicity and possibility to fine tune their properties in a larger range due to the wider choice of composition.
In a typical QD sensitized solar cell the nanocrystals are deposited onto the nanostructured electrode such as TiO2 and upon the light absorption they inject electron into the conduction band of the n-type material, while the hole is regenerated by the electrolyte. It is of high importance to develop and study the cells with the absorber injecting a hole into the valence band of a p-type material as it opens the way to tandem cells. So far the performances of such p-type cells lag far behind those of n-type ones mainly because of (i) strong recombination of typically used organic dye absorbers with p-type electrodes upon injection and (ii) lack of appropriate p-type materials which should combine good conductivity with high transparency. We have decided to act on both these limitations: (i) to use ternary QDs as sensitizers because they provide for the possibility of tuning the energy levels to minimize the recombination losses and maximize the hole injection; and (ii) to use new p-type nanostructured materials as electrodes with desired properties.
We have used CuInS₂ and CuInS_xSe_2-x QDs as light absorbers and deposited them first on mesoporous p-type NiO electrode. By fine-tuning their energy levels by varying their size, doping and surface ligands we have achieved the efficiency of 0.7%, which is comparable to the p-type DSSC record (1.3%) and to the best of our knowledge by far the best result for any p-type QD sensitized cell. Secondly, the ternary QDs were deposited on p-type CuSCN nanowires recently designed by us, which resulted in the cell with 0.028% efficiency.
We will discuss the QDs design and its influence on the hole injection efficiencies as studied by XPS/UPS and transient photoluminescence spectroscopies. Also we will discuss the deposition details and the influence of linkers and ligands of the QDs used on the PV performance and light harvesting efficiency.

Semiconductor quantum dots (QDs) are intriguing light-harvesters and excited-state charge donors for solar energy conversion, due to their size-dependent optical properties and band-edge potentials, high oscillator strengths, and the possibilities of hot-carrier extraction and multiexciton generation. Coupling of colloidal QDs within organized assemblies may afford programmable interfacial energetic offsets and tunable through-bridge distance, electronic coupling, and excited-state charge transfer reactivity. Thus, such architectures are intriguing for fundamental studies of bridge-mediated charge transfer and as candidates to harvest light and concentrate charges for solar energy conversion.
We have used carbodiimide coupling chemistry to tether thioglycolic acid-capped CdS QDs to aminothiophenol-capped CdSe QDs through the formation of amide bonds between terminal functional groups of the capping ligands. Absorption spectra of the resulting QD-molecule-QD assemblies equal the sum of the isolated CdS and CdSe QDs; thus, electronic properties of the colloidal QD components are unperturbed by carbodiimide-mediated coupling. Photoexcitation of CdS QDs within the assemblies results in dynamic quenching of emission from CdS and the growth of a broad transient absorption feature that persists vasatly longer than the excitonic bleaches of the individual, uncoupled CdS and CdSe QDs. These results suggest that photogenerated charge carriers in CdS QDs are transferred rapidly (within 10^-8 s) to CdSe QDs within the molecularly-tethered assemblies.
This presentation will highlight our recent results pertaining to the assembly of QD-molecule-QD architectures, the characterization of their molecular-level interconnectivity, and the time-resolved spectroscopic characterization of their excited-state charge-transfer reactivity.

Hybrid composites for photovoltaic application composed of conjugated polymers and colloidal semiconductor nanocrystals (NCs) such as lead sulphide (PbS) are attractive photoactive materials. The tailoring of the NC surface chemistry with shorter surface ligands is of major importance to control non-covalent and electronic interactions between the organic and inorganic components.[i] The exchange of bulky organic with short arenethiolate (ArS) ligands, as performed within this work, leads to new hybrid interfaces with a drastically altered morphology of the hybrid composite during film formation from blend solutions. After such a treatment blends of poly(3-hexylthiophene) (P3HT) and PbS NCs - a material combination that has been considered as inadequate for photovoltaics until this report - reach power conversion efficiencies of up to 3 %.[ii]
Within this paper we present a time resolved spectroscopy study to unravel the role of the hybrid interface and morphology of the blend on the photophysical scenarios occurring at the NC/polymer interface. The unique combination of transient absorption measurements in the visible (polymer) and the near infrared (PbS NCs) on the pristine materials and the treated and untreated blends allow a better understanding of the charge transfer processes at the hybrid interface. We will focus mainly on the blend P3HT-PbS. A comparison to other polymer absorber blends will provide in general a broader understanding of the role of PbS NC surface treatment with ArS on such hybrid interfaces, which is important for their photovoltaic application.
[i] Colloidal Arenethiolate-Capped PbS Quantum Dots: Optoelectronic Properties, Self-Assembly, and Application in Solution-Cast Photovoltaics, Carlo Giansante, Luigi Carbone, Cinzia Giannini, Davide Altamura, Zoobia Ameer, Giuseppe Maruccio, Anna Loiudice, Maria R. Belviso, P. Davide Cozzoli, Aurora Rizzo, and Giuseppe Gigli, J. Phys. Chem. C, 2013, 117 (25), pp 13305-13317
[ii] Molecular-Level Switching of Polymer/Nanocrystal Non-Covalent Interactions and Application in Hybrid Solar Cells, Carlo Giansante, Rosanna Mastria, Giovanni Lerario, Luca Moretti, Ilka Kriegel, Francesco Scotognella, Guglielmo Lanzani, Sonia Carallo, Marco Esposito, Mariano Biasiucci, Aurora Rizzo and Giuseppe Gigli, Adv. Funct. Mater., DOI: 10.1002/adfm.201401841

Bulk-heterojunction solar cells are promising for achieving high efficiencies since photogenerated electrons transfer from the donor to the acceptor at a very fast rate. Record photoconversion efficiency is almost 10%, but at the same time the charge separation mechanism is not well understood. In this work, we report on the carrier mobility of different polymer/fullerene combinations with varying photoconversion efficiencies. We use time-resolved terahertz spectroscopy to study frequency dependent carrier mobility. Mobility measurements at terahertz frequencies correspond to local charge transport, so our non-contact measurement technique is not influenced by carrier hopping from one polymer chain to another, where the hopping mobility is two orders of magnitude lower, directly recording the local ultrafast response[1]. We first compare regioregular poly(3-hexylthiophene) (P3HT) and regiorandom P3HT, where the side chains are oriented differently, and find the local polymer chain structure has a large impact on carrier mobility. We also compare different polymer to fullerene weight ratio and investigate the impact of fullerene loading to carrier transport in polymers. In our measurements, we avoid high photon flux since it has been shown that carrier mobility and lifetime is dramatically reduced due to exciton-exciton annihilation at high excitation densities. Accurate measurement of short-time carrier mobilities will not only help elucidate how excitons are separated into free carriers [2], it will also shed light on how carrier mobilities reflect photoconversion efficiencies of these solar cells.
[1] Noriega et al., PNAS, 110, 16315, 2013
[2] Burke and McGehee, Adv. Mater, 26, 1923, 2014

Semiconducting nanocrystals optically active in the infrared region of the electromagnetic spectrum enable exciting routes in fundamental research and novel applications compatible with the infrared transparency windows of biosystems (for instance in nanothermometry or biological and chemical optical sensing). Double color emission quantum dots (QDs) can provide self-calibrating and very accurate nanosystems.
Here, we present the preparation and the optical and structural characterization of giant core/shell/shell asymmetric QDs, having a PbS /CdS Zincblende (Zb)/CdS Wurtzite (Wz) structure with double color emission close to the red / near-infrared (NIR) region.
The giant QDs are prepared starting from PbS/CdS core-shell systems applying a successive ionic layer absorption and reaction (SILAR) technique, which allows us coating the PbS QD with a CdS shell of variable thickness. At first, the growth of this shell appears to be anisotropic, leading to tetrahedral core/shell PbS/CdS Zb QDs. Further growth of these shells results in the nucleation of a CdS Growth of Wz phase, with precise crystallographic orientation with respect to Zb phase, leading to the formation of highly asymmetric QDs. In these asymmetric QDs, the Wz phase is spatially separated from the PbS core.
We performed pump-probe experiments with ~100 fs time resolution, with a broadband visible probe and the pump tuned at various photon energies, so as to selectively excite the whole system (PbS and CdS) or only the PbS. We thus directly assessed the electron-hole dynamics in the QDs. We found that electrons are delocalized all over the QD and that the Zb CdS layer acts as hole barrier driven by the excitation condition.
The double color emission originates from quantum confined PbS core (close to the NIR region, 690 nm) and CdS Wz bulk-like (in the visible) states. The hole-blockade process produced by the Zb shell is thus responsible of the independent and simultaneous radiative exciton recombination in the PbS core and in the CdS Wz shell, respectively. These results highlight the importance of the driving force leading to preferential crystal growth in asymmetric QDs, give the route for a rational control of the synthesis of double color emitting giant QDs, and pave the way for effective exploitation of NIR transparency windows.

Arrays of nanometer-sized particles have attracted considerable attention with their potential applications in photovoltaics and optoelectronic devices. The conductivity of a nanocrystal (NC) solid can be improved by decreasing the tunneling barrier between particles, or increasing the intrinsic conductivity. Hydrazine achieves both, but the way it dopes NCs is not well understood. In addition, its effects are transient, thus precluding its use in real-world applications. Here we study the effect of hydrazine derivatives on the doping of PbSe NCs. By varying the electronegativity, affinity and stability of the hydrazine-NC adduct, we hope to engineer a NC solid that is permanently doped. This also enables characterization of the NC thin film to elucidate the mechanism of doping. The NC thin films are characterized by optical absorption, photoluminescence (PL), x-ray photoelectron and ultra-violet photoelectron spectroscopy. Effect of all treatments on conductivity is observed through Field Effect Transistor (FET) and conductivity measurements.

Separation of photogenerated electron-hole pairs in semiconductor is one of the key processes in photocatalysis. Heterostructured nanocrystals of semiconductors with a type-II bandgap offset facilitate the dissociation of excitons. Photocatalytic activity improves when the photogenerated electrons and holes become available to participate in reduction and oxidation on the surface, respectively. Therefore, prolonged electron-hole recombination time, or exciton lifetime, and heterostructures with both reduction and oxidation sites available on the surface are deemed to enhance photocatalytic activity. In this presentation, we describe the design of heterostructure nanocrystals based on Pb and Cd chalcogenides showing ultralong recombination lifetime, amorphous oxide shells that protect the nanocrystals from oxidation and enhance separation of photogenerated charge carriers. A principle that underlies the design strategy is to synthesize nano-sized junctions with electron-hole separation, with the structure of “open” geometry, such as tetrapods.
We observed noticeable increase in photocatalytic activity for methylene blue reduction as the geometry became “open”. For instance, tetrapod-shaped PbSe/CdSe/CdS core/shell/shell nanocrystals exhibited enhanced photocatalysis compared to pyramid-shaped PbSe/CdSe/CdS nanocrystals, let alone spherical structures. Our study of photocatalysis using PbSe-based heterostructure nanocrystals suggests that the geometric nature of nanocrystals should be considered in the context of the efficient conversion of photon energy into chemical reactions. In addition, amorphous TiO₂ shell turns out to enhance the photocatalytic water splitting by CdSe nanocrystals. The hydrogen production rate is comparable to the record values reported thus far, and we will discuss the possibility of expanding the nanocrystal design strategy for CO₂ reduction and other photocatalysis.

Doping, a process in which atomic impurities are intentionally added to modify the electronic properties of a semiconductor, has revolutionized our world, making computers, transistors, detectors, solar cells and other devices possible. Artificial atoms, nanocrystals (NC) with electronic properties dictated by their size and shape, have also emerged as technologically important materials. In this contribution, we introduce the concept of nanocrystal (NC) doping by merging the two above mentioned concepts. We show that, by matching the size of monodisperse gold nanocrystals (Au NCs) with that of semiconducting nanocrystals of cadmium selenide (CdSe) or lead selenide (PbSe) quantum dots (QDs), it is possible to purposely dope the semiconductor superlattices and induce novel properties in these self-assembled materials. We demonstrate that, comparably to the case of atomic doping, the Au NCs take random positions in the superlattice and their concentration can be tuned over a wide range. We show that the electronic and optical properties of the superlattices are affected by the presence of the Au dopants. We anticipate that this approach can originate a wide variety of NC-doped structures with applications in several fields including electronic materials, solar cells, sensors and catalysis.

New semiconductor materials, manufactured by colloidally synthesized semiconductor nanocrystals (NCs) are of ever growing interest for energy device applications. Although the efficiencies of NC-based LEDs and solar cells have been successfully improved,^1,2 energy efficiencies must still increase for commercial application.
Fundamentally, all processes which are considered a form of energy loss, do ultimately transform available free energy into heat, i.e. phonons carrying the vibrational energy. In electronic devices this happens through different charge carrier processes, such as thermal generation/recombination, trapping, and transport of charge carriers. All of these processes therefore depend on the available phonon modes of the NC-solid - the phonon density of states (PDOS). While the PDOS of bulk crystals is well understood, due to the high symmetry of crystals, NC-solids lack such symmetry and therefore have a more complex structure, including molecular vibrations at the surface of the NCs, localized modes within each NC, and extended modes between NCs.
Despite initial computational approaches the PDOS of NC-solids is at this point unknown.³
In this work, we present measurements of the PDOS of a set of PbS NC-solids using inelastic neutron scattering (INS). In comparison to optical techniques, such as infrared absorption and Raman scattering, INS has no selection rules and can therefore detect all vibrational modes in the system. The measurements were performed at the FOCUS time-of-flight spectrometer at Paul Scherrer Institute in Switzerland.
Our measurements show that NC-solids made from small NCs present a large excess scattering at low frequencies and a phonon spectrum that deviates significantly from the PbS bulk. For a series of larger NCs we find that the bulk phonon features are recovered at a particle size of around 16 nm. Change of the surface ligands modifies both the high and low energy spectrum.
We compare our INS measurements to IR-Absorption and Raman-Scattering measurements and ab-initio molecular dynamics simulations and discuss the implications for energy devices. In particular we discuss how the engineering of the PDOS can be used to reduce energy loss processes.
(1) Mashford, B. S. et al., Nat. Photonics2013, 7, 407-412.
(2) Chuang, C.-H. M. et al. Nat. Mater.2014.
(3) Ong, W.-L. et al. Nat. Mater.2013, 12, 410-415.

PbSe nanorods are attractive for use in next-generation optoelectronic devices due to their exceptional physical properties such as larger Stokes shifts and more efficient multiple exciton generation (MEG), relative to spherical nanocrystals.^1-3However, further development of PbSe nanorods for viable technological components requires precise control of the nanorod diameter (d) and length (l), as well as an understanding of how the nanorod dimensions affect their optoelectronic properties. In this work, we present an investigation into the nature of the PbSe nanorod synthesis, yielding a rudimentary understanding of how to independently control the nanorod diameter and length. Additionally, we have developed an elementary comprehension of how the nanorod dimensions affect their optical properties.
Previously, single-crystal, homogeneous, PbSe nanorods were synthesized using a one-pot solution method.⁴ Recently, we reported how water indirectly impacts upon the nanorod aspect ratio and yield,⁵ which through the careful drying and purification of precursors, could be varied significantly and reproducibly, albeit with no independent control of both d and l. Here we present an exploration of various synthetic parameters (e.g., temperature, reaction time, precursor concentration, etc.) which resulted in our ability to independently control the nanorod diameter and length. Aspect ratios ranging from 1 to 16 were synthesized, showing little-to-no branching (<4%) and a wide absorption energy range (~1200 nm - 2000 nm). From these optical measurements, we observe that the Stokes shifts are mainly given by the quantum mechanical effect known as Pauli repulsion, or exchange interaction, with a small correction due to potential fluctuations as in the case of nanowires. Measured potential fluctuations along the nanorods shows a relatively smooth morphology (<±15% inter-nanorod fluctuations) consistent with TEM characterization.
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[3] Sandberg R L, Padilha L A, Qazilbash M M, Bae W K, Schaller R D, Pietryga J M, Stevens M J, Baek B, Nam S W and Klimov V I 2012 ACS Nano 6 9532
[4] Koh W-K, Bartnik A C, Wise F W and Murray C B 2010 JACS 132 3909
[5] Boercker J E, Foos E E, Placencia D and Tischler J G 2013 JACS 135 15071