We combine CdSe semiconductor nanocrystals (or quantum dots) and single-crystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell. An array of ZnO nanowires was grown vertically from a fluorine-doped tin oxide conducting substrate. CdSe quantum dots, capped with mercaptopropionic acid, were attached to the surface of the nanowires. When illuminated with visible light, the excited CdSe quantum dots injected electrons across the quantum dot-nanowire interface. The morphology of the nanowires then provided the photoinjected electrons with a direct electrical pathway to the photoanode. With a liquid electrolyte as the hole transport medium, quantum-dot-sensitized nanowire solar cells exhibited short-circuit currents ranging from 1 to 2 mA/cm2 and open-circuit voltages of 0.5-0.6 V when illuminated with 100 mW/cm2 simulated AM1.5 spectrum. Internal quantum efficiencies as high as 50-60% were also obtained.

We report a new type of excitonic solar cell based on planar heterojunctions between PbSe semiconductor nanocrystals and thin ZnO films. These solar cells generate large photocurrents and higher photovoltages compared to Schottky cells assembled with similar nanocrystal films. When illuminated with 100 mW/cm(2) simulated AM1.5 spectrum, these solar cells exhibit short-circuit currents between 12 and 15 mA/cm(2), open-circuit voltages up to 0.45 V, and a power conversion efficiency of 1.6%. The photovoltage depends on the size of the nanocrystals, increasing linearly with their effective band gap energy.

Thin films of colloidal PbSe quantum dots can exhibit very high carrier mobilities when the surface ligands are removed or replaced by small molecules, such as hydrazine. Charge transport in such films is governed by the electronic exchange coupling energy (beta) between quantum dots. Here we show that two-dimensional quantum dot arrays assembled on a surface provide a powerful system for studying this electronic coupling. We combine optical spectroscopy with atomic force microscopy to examine the chemical, structural, and electronic changes that occur when a submonolayer of PbSe QDs is exposed to hydrazine. We find that this treatment leads to strong and tunable electronic coupling, with the beta value as large as 13 meV, which is 1 order of magnitude greater than that previously achieved in 3D QD solids with the same chemical treatment. We attribute this much enhanced electronic coupling to reduced geometric frustration in 2D films. The strongly coupled quantum dot assemblies serve as both charge and energy sinks. The existence of such coupling has serious implications for electronic devices, such as photovoltaic cells, that utilize quantum dots.

We report the effects of N2 and O2 on the electrical properties of PbSe quantum-dot (QD) films treated with 1,2-ethanedithiol (EDT) by measuring the changes in the current−voltage characteristics of QD field-effect transistors (FETs). EDT-treated PbSe QD films at a base pressure of ∼10−5 Torr exhibit ambipolar transport. Exposing these films to N2 shifts the transfer characteristics toward negative gate-voltage values and increases the electron mobility. These changes could be reversed entirely by removing the N2 gas over the FET and returning to base pressure. Oxygen exposure shifts the transfer characteristics in the opposite direction toward positive gate-voltage values. Moreover, oxygen exposure reduces charge mobility but increases film conductivity. For exposures up to ∼108 langmuir, these O2-induced changes could be reversed completely by removing the O2 gas over the sample and returning to base pressure. However, after ∼1010 langmuir of O2 exposure, the changes are irreversible. The QD films then permanently become p-type and the decrease in charge mobility remains even after returning to base pressure.

A key issue governing efficient electron transfer between two semiconductors is interfacial electronic energy alignment. We address this issue in a model system relevant to quantum-dot-sensitized solar cells, cadmium selenide (CdSe) quantum dots adsorbed on a single crystal zinc oxide (ZnO) (101̅0) surface via 3-mercaptopropionic acid linkers, using ultraviolet photoelectron spectroscopy. The valence band maximum (VBM) of the CdSe quantum dots is found to be located at 1.1 ± 0.1 eV above the VBM of ZnO, nearly independent of the size of the quantum dots (2.1−4.2 nm). This finding suggests that, upon adsorption, there is direct electronic interaction between CdSe quantum dots and the ZnO surface involving CdSe valance bands. Such electronic interaction pins the CdSe valence band to the Fermi level. As a result, varying the quantum dot size mainly tunes the alignment of the conduction band minimum of CdSe with respect to that of the ZnO surface.

External quantum efficiency in solar cells based on junctions between PbSe quantum dots (QDs) and thin ZnO films is increased by replacing the ZnO films with a vertically oriented array of single-crystalline ZnO nanowires, and infiltrating this array with colloidal QDs. When illuminated with 100 mW/cm2 of simulated solar light, QD-nanowire solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with thin-film ZnO devices constructed with the same amount of QDs. Significant photocurrent and power conversion improvement with increasing nanowire length is consistent with higher exciton and charge collection efficiencies.

This paper describes a facile method for coating Ag nanowires with uniform, ferromagnetic sheaths made of polycrystalline Ni. A typical sample of these core/sheath nanowires had a saturation magnetization around 33 emu g(-1). We also demonstrated the use of this magnetic property to align the nanowires by simply placing a suspension of the nanowires on a substrate in a magnetic field and allowing the solvent to evaporate. The electrical conductivity of these core/sheath nanowires (2 × 10(3) S cm(-1)) was two orders of magnitude lower than that of bulk Ag (6.3 × 10(5) S cm(-1)) and Ni (1.4 × 10(5) S cm(-1)). This is likely caused by the transfer of electrons from the Ag core to the Ni sheath due to the difference in work function between the two metals. The electrons are expected to experience an increased resistance due to spin-dependent scattering caused by the randomized magnetic domains in the polycrystalline, ferromagnetic Ni sheath. Studies on the structural changes to the Ni coating over time under different storage conditions show that storage of the nanowires on a substrate under ambient conditions leads to very little Ni oxidation after 6 months. These Ag/Ni core/sheath nanowires show promise in areas such as electronics, spintronics, and displays.

Collapse of nanostructures with high aspect ratio and/or low mechanical strength during drying step in wet process has become a critical issue in semiconductor industry. Even current workable drying practices are facing steeply rising challenges from rapid device scaling advancements. The objective of this work, following evaluation of multiple advanced drying technique, is to develop a sustainable non-stiction drying solution incorporating supercritical fluids. Leaning-free performance has been consistently demonstrated with our supercritical drying sequence on 2x NAND STI (trench aspect ratio ~20) and low-k trench structures, where pattern collapses were observed under conventional solvent-assisted drying or even with advanced self-assembled monolayer approach. Release of sporadic stiction and complete recovery from global pattern leaning were both achieved with our integrated supercritical drying process. Particle performance < 50 adders @0.09μm in average was attained with background particles subtracted. Metallic contamination analysis with TXRF showed all metal components interested were below detection limit.