Asaf Salant

Quantum Dot Sensitized Solar Cells with Improved Efficiency Prepared Using Electrophoretic Deposition

Quantum dot sensitized solar cells (QDSSC) may benefit from the ability to tune the quantum dot optical properties and band gap through the manipulation of their size and composition. Moreover, the inorganic nanocrystals may provide increased stability compared to organic sensitizers. We report the facile fabrication of QDSSC by electrophoretic deposition of CdSe QDs onto conducting electrodes coated with mesoporous TiO(2). Unlike prior chemical linker-based methods, no pretreatment of the TiO(2) was needed, and deposition times as short as 2 h were sufficient for effective coating. Cross-sectional chemical analysis shows that the Cd content is nearly constant across the entire TiO(2) layer. The dependence of the deposition on size was studied and successfully applied to CdSe dots with diameters between 2.5 and 5.5 nm as well as larger CdSe quantum rods. The photovoltaic characteristics of the devices are greatly improved compared with those achieved for cells prepared with a linker approach, reaching efficiencies as high as 1.7%, under 1 sun illumination conditions, after treating the coated electrodes with ZnS. Notably, the absorbed photon to electron conversion efficiencies did not show a clear size-dependence indicating efficient electron injection even for the larger QD sizes. The electrophoretic deposition method can be easily expanded and applied for preparations of QDSSCs using diverse colloidal quantum dot and quantum rod materials for sensitization.

Highly Emissive Nano Rod-in-Rod Heterostructures with Strong Linear Polarization

We report the synthesis of CdSe/CdS rod in rod core/shell heterostructures. These rods, synthesized using a seeded-growth approach, show narrow distributions of rod diameters and lengths and exhibit high emission quantum efficiencies and highly polarized emission. The degree of polarization is controlled by the inner core rod dimensions, and it is equal or up to 1.5 times higher than the polarization of equivalent sphere in rod systems. Using the method of photoselection we measure the polarization anisotropy at different excitation wavelengths and study the interplay between electronic contribution and dielectric effects in determining the absorption and emission polarization.

Quantum rod-sensitized solar cell: nanocrystal shape effect on the photovoltaic properties.

The effect of the shape of nanocrystal sensitizers in photoelectrochemical cells is reported. CdSe quantum rods of different dimensions were effectively deposited rapidly by electrophoresis onto mesoporous TiO(2) electrodes and compared with quantum dots. Photovoltaic efficiency values of up to 2.7% were measured for the QRSSC, notably high values for TiO(2) solar cells with ex situ synthesized nanoparticle sensitizers. The quantum rod-based solar cells exhibit a red shift of the electron injection onset and charge recombination is significantly suppressed compared to dot sensitizers. The improved photoelectrochemical characteristics of the quantum rods over the dots as sensitizers is assigned to the elongated shape, allowing the build-up of a dipole moment along the rod that leads to a downward shift of the TiO(2) energy bands relative to the quantum rods, leading to improved charge injection.

Electrostatic Force Microscopy Study of Single Au−CdSe Hybrid Nanodumbbells: Evidence for Light-Induced Charge Separation

Electrostatic force microscopy is used to study light-induced charging in single hybrid Au-CdSe nanodumbbells. Upon illumination, nanodumbbells show negative charging, which is in contrast with CdSe rods and Au particles that show positive charging. This different behavior is attributed to charge separation in the nanodumbbells, where after excitation the electron is transferred to the gold tips and the hole is subsequently filled through tunneling interactions with the substrate. The process of light-induced charge separation at the metal-semiconductor interface is key for the photocatalytic activity of such hybrid metal-semiconductor nanostructures.

ZnSe Quantum Dots Within CdS Nanorods: A Seeded‐Growth Type‐II System

The sizeand shape-dependent optical properties of colloidal semiconductor nanocrystals can be enhanced significantly by the growth of heterostructures of at least two materials. The potential profile of the involved materials dictates the optical properties of the final structure, where the most common typeI alignment has been extensively used in spherical core/shell nanocrystals to achieve electron and hole confinement in the core leading to high photoluminescence quantum efficiencies and enhanced stability. Type-II alignment, where the potential profile leads to charge-carrier separation, has also been reported and leads to prolonged emission lifetimes, modified multiexciton behavior, and enhanced optical-gain characteristics. Anisotropic rod heterostructures are also of great interest, and previously advantageous optical-gain studies were reported for type-I core/shell rods. Heterostructures were also grown by combining different materials into the growing nanorod. In particular, type-II nanorod heterostructures of CdSe/CdTe showing light-induced charge separation were recently reported. Exceptional optical properties and uniformity were recently reported for CdS nanorods with CdSe seeds by Talapin and Manna. A seeded-growth approach was utilized, based on earlier work on this system, which has already yielded significant insight to the electronic structure and optical properties of type-I seeded dot–rod heterostructures. Here, we expand this synthetic approach and develop CdS nanorods with embedded ZnSe quantum dots, which constitute an example of a nanorod heterostructure with a seed of zinc blende crystal structure. This structure is of high interest because of the type-II potential profile characteristics of ZnSe and CdS (see Figure 1). According to this potential profile, a charge-carrier separation should occur in the excited state of

Cobalt Growth on the Tips of CdSe Nanorods

Best of both worlds: Reduction of an organometallic Co precursor on preformed CdSe nanorods yields two distinct semiconducting-magnetic heterostructures (see picture). The selective growth of Co on the tips of CdSe first gives nanosphere-nanorod dimers, which evolve into nanorod-nanorod structures. In the hybrid objects the magnetic properties of Co remain intact, while the luminescence properties of CdSe are affected but not completely quenched.

Hierarchical Surface Patterns of Nanorods Obtained by Co‐Assembly with Block Copolymers in Ultrathin Films

The tailored integration of inorganic nanoparticles as functional building blocks in photovoltaic, optical, electronic, and sensing devices [ 1 , 2 ] requires facile bottom-up self-assembly strategies that allow control of the morphology of the nanoparticle ensemble. [ 3 ] This is of particular interest in the case of anisotropically shaped nanorods, where the collective properties of the assembly strongly depend on rod alignment and internal organization. [ 1 , 4 ] The self-organization property of block copolymers (BCPs) [ 5 ] has been successfully harnessed to direct spherical nanoparticles to preferred copolymer domains. [ 6 , 7 ] However, current knowledge of the phase behavior of rod-shaped nanoparticle/BCP assemblies is limited mostly to bulk composites, [ 8 – 11 ]

Size Dependence of Molar Absorption Coefficients of CdSe Semiconductor Quantum Rods

Fundamental properties: The molar absorption coefficients of CdSe quantum rods are determined experimentally as a function of their dimensions (see figure). Far above the band gap a simple dependence on volume is seen. The behavior at the band gap manifests a concentration of oscillator strength with decreased diameter in agreement with strong quantum confinement behavior.

Anomalous Temperature Dependent Transport through Single Colloidal Nanorods Strongly Coupled to Metallic Leads

We report wiring of individual colloidal nanorods (NRs), 30-60 nm long by 3.5-5 nm diameter. Strong electrical coupling is achieved by electron beam induced deposition (EBID) of metallic lines targeting NR tips with nanometric precision. At T = 4 K many devices exhibit smooth I(V) curves with no sharp onset features, which remarkably fit a Fowler-Nordheim tunneling model. All devices exhibit an anomalous exponential temperature dependence of the form I approximately exp(T/T(0)). This irregular behavior cannot be explained by any hopping or activation model and is interpreted by accounting for the lowering of the NR conduction band due to lattice dilation and phonon coupling.

Electrical Current Switching in Single CdSe Nanorods

Electrical current measurements through individually wired colloidal CdSe nanorods exhibit pronounced multistability. This current switching is analogous to the widely observed fluorescence intermittency in similar systems and may be associated with surface charge dynamics. Such association is quantitatively established for the case when the current is bistable, where the probability of the sojourn time t at the high or low current state follows an exponential dependence. Remarkably, this behavior can be modeled by charging dynamics of a single surface trap, whose position could be estimated from the intermittent current-voltage characteristics. The methodology presented here provides a unique route for charge dynamic sensing at the nanoscale, where the nanorod senses its own surface charge.