Single-Step Aerosol Synthesis and Deposition of Au Nanoparticles with Controlled Size and Separation Distributions

Immobilized noble metal nanoparticles are being explored for a variety of applications where control over the particle size and separation distance on the substrate is important for performance. A proof of concept is presented that Au nanoparticles can be deposited in a single step with control over the size and separation distributions using an aerosol process. Samples were deposited with mean particle diameters in the range from 15 to 43 nm, and mean separation distances from 11 to 39 nm. Depending on the separation distance, particles exhibited localized surface plasmon resonance dominated by either intra- or interparticle resonances, as determined by ultraviolet–visible extinction spectroscopy. Ultrathin TiO₂ shells of different thicknesses, in the range from 0 to 24 nm, were deposited on the Au nanoparticles by atomic layer deposition to determine the sensing distance into the surrounding dielectric medium for these materials, which was estimated to be 10 nm

Contact Radius and the Insulator–Metal Transition in Films Comprised of Touching Semiconductor Nanocrystals

Nanocrystal assemblies are being explored for a number of optoelectronic applications such as transparent conductors, photovoltaic solar cells, and electrochromic windows. Majority carrier transport is important for these applications, yet it remains relatively poorly understood in films comprised of touching nanocrystals. Specifically, the underlying structural parameters expected to determine the transport mechanism have not been fully elucidated. In this report, we demonstrate experimentally that the contact radius, between touching heavily doped ZnO nanocrystals, controls the electron transport mechanism. Spherical nanocrystals are considered, which are connected by a circular area. The radius of this circular area is the contact radius. For nanocrystals that have local majority carrier concentration above the Mott transition, there is a critical contact radius. If the contact radius between nanocrystals is less than the critical value, then the transport mechanism is variable range hopping. If the contact radius is greater than the critical value, the films display behavior consistent with metallic electron transport

Enthalpy of Formation for Cu–Zn–Sn–S (CZTS) Calculated from Surface Binding Energies Experimentally Measured by Ion Sputtering

Herein, we report an analytical procedure to calculate the enthalpy of formation for thin film multinary compounds from sputtering rates measured during ion bombardment. The method is based on Sigmund’s sputtering theory and the Born–Haber cycle. Using this procedure, an enthalpy of formation for a CZTS film of the composition Cu_1.9Zn_1.5Sn_0.8S₄ was measured as −930 ± 98 kJ mol^–1. This value is much more negative than the sum of the enthalpies of formation for the constituent binary compounds, meaning the multinary formation reaction is predicted to be exothermic. The measured enthalpy of formation was used to estimate the temperature dependence of the Gibb’s free energy of reaction, which appears consistent with many experimental reports in the CZTS processing literature

Visualizing Current Flow at the Mesoscale in Disordered Assemblies of Touching Semiconductor Nanocrystals

The transport of electrons through assemblies of nanocrystals is important to performance in optoelectronic applications for these materials. Previous work has primarily focused on single nanocrystals or transitions between pairs of nanocrystals. There is a gap in knowledge of how large numbers of nanocrystals in an assembly behave collectively and how this collective behavior manifests at the mesoscale. In this work, the variable range hopping (VRH) transport of electrons in disordered assemblies of touching, heavily doped ZnO nanocrystals was visualized at the mesoscale as a function of temperature both theoretically, using the model of Skinner, Chen, and Shklovskii (SCS), and experimentally, with conductive atomic force microscopy on ultrathin films only a few particle layers thick. Agreement was obtained between the model and experiments, with a few notable exceptions. The SCS model predicts that a single network within the nanocrystal assembly, composed of sites connected by small resistances, dominates conduction, namely, the optimum band from variable range hopping theory. However, our experiments revealed that in addition to the optimum band there are subnetworks that appear as additional peaks in the resistance histogram of conductive atomic force microscopy (CAFM) maps. Furthermore, the connections of these subnetworks to the optimum band change in time, such that some subnetworks become connected to the optimum band while others become disconnected and isolated from the optimum band; this observation appears to be an experimental manifestation of the “blinking” phenomenon in our images of mesoscale transport

Transparent Conductive Oxide Nanocrystals Coated with Insulators by Atomic Layer Deposition

Thin films comprised of transparent conductive oxide (TCO) nanocrystals are attractive for a number of optoelectronic applications. However, it is often observed that the conductivity of such films is very low when they are in contact with air. It has recently been demonstrated, somewhat surprisingly, that filling in initially insulating films comprised of TCO nanocrystals with another insulator by atomic layer deposition (ALD) dramatically increases the conductivity by many orders of magnitude. This work aims to elucidate the mechanism by which the ALD coating increases conductivity. We examined the effect of removing two adsorbed oxygen species (physisorbed molecular water and chemisorbed hydroxide) on sheet resistance and compared this result to the results with thin films comprised of ZnO nanocrystals coated with Al₂O₃ and also HfO₂ by ALD. Although both insulating infills decrease the sheet resistance and increase the stability of the films, there is a stark discrepancy between the two. From the <i>in situ</i> measurements, it was found that coating with Al₂O₃ removes both physisorbed water and chemisorbed hydroxide, resulting in a net reduction of the ZnO nanocrystals. Coating with HfO₂ removes only physisorbed water, which was confirmed by Fourier transform infrared spectroscopy. A similar phenomenon was observed when thin films comprised of Sn-doped In₂O₃ nanocrystals were coated, suggesting Al₂O₃ can be used to reduce and stabilize metal oxide nanocrystals in general

Energy Levels, Electronic Properties, and Rectification in Ultrathin p‑NiO Films Synthesized by Atomic Layer Deposition

NiO is an attractive p-type transparent semiconductor that is being explored for a variety of applications. We report a systematic study of the electronic properties, relevant to hole-transporting materials in solar energy conversion applications, of NiO synthesized by atomic layer deposition (ALD). The acceptor concentration, flat band potential, and valence band position were determined by electrochemical Mott–Schottky analysis of impedance data in aqueous electrolytes for films less than 100 nm in thickness on F:SnO₂ (FTO)-coated glass substrates. The effects of postdeposition annealing and film thickness were studied. Oxidation of the NiO was observed at temperatures as low as 300 °C in 1 atm of oxygen. Films annealed at 400 °C and above in Ar exhibited signs of thermal decomposition. Thinner films were found to have a higher carrier concentration. F:SnO₂ and thermally evaporated Ag were both observed to form ohmic contact to ALD-synthesized TiO₂ and NiO. A p/n heterojunction diode was fabricated from the transparent ALD TiO₂ and NiO layers with the structure FTO/NiO/TiO₂/Ag that showed rectification

Synthesis and Characterization of High-Photoactivity Electrodeposited Cu₂O Solar Absorber by Photoelectrochemistry and Ultrafast Spectroscopy

We present a systematic study on the effects of electrodeposition parameters on the photoelectrochemical properties of Cu₂O. The influence of deposition variables (temperature, pH, and deposition current density) on conductivity has been widely explored in the past for this semiconductor, but the optimization of the electrodeposition process for the photoelectrochemical response in aqueous solutions under AM 1.5 illumination has received far less attention. In this work, we analyze the photoactivity of Cu₂O films deposited at different conditions and correlate the photoresponse to morphology, film orientation, and electrical properties. The photoelectrochemical response was measured by linear sweep voltammetry under chopped simulated AM 1.5 illumination. The highest photocurrent obtained was −2.4 mA cm^–2 at 0.25 V vs RHE for a film thickness of 1.3 μm. This is the highest reported value reached so far for this material in an aqueous electrolyte under AM 1.5 illumination. The optical and electrical properties of the most photoactive electrode were investigated by UV–vis spectroscopy and electrochemical impedance, while the minority carrier lifetime and diffusion length were measured by optical-pump THz-probe spectroscopy

Effects of Halides on Organic Compound Degradation during Plasma Treatment of Brines

Plasma has been proposed as an alternative strategy to treat organic contaminants in brines. Chemical degradation in these systems is expected to be partially driven by halogen oxidants, which have been detected in halide-containing solutions exposed to plasma. In this study, we characterized specific mechanisms involving the formation and reactions of halogen oxidants during plasma treatment. We first demonstrated that addition of halides accelerated the degradation of a probe compound known to react quickly with halogen oxidants (i.e., para-hydroxybenzoate) but did not affect the degradation of a less reactive probe compound (i.e., benzoate). This effect was attributed to the degradation of para-hydroxybenzoate by hypohalous acids, which were produced via a mechanism involving halogen radicals as intermediates. We applied this mechanistic insight to investigate the impact of constituents in brines on reactions driven by halogen oxidants during plasma treatment. Bromide, which is expected to occur alongside chloride in brines, was required to enable halogen oxidant formation, consistent with the generation of halogen radicals from the oxidation of halides by hydroxyl radical. Other constituents typically present in brines (i.e., carbonates, organic matter) slowed the degradation of organic compounds, consistent with their ability to scavenge species involved during plasma treatment

Atomic Layer Deposition of the Quaternary Chalcogenide Cu₂ZnSnS₄

Atomic layer deposition (ALD) is a layer-by-layer synthesis method capable of depositing conformal thin films with thickness and compositional control on subnanometer length scales. While many materials have been synthesized by ALD, the technologically important metal sulfides are underexplored, and homogeneous quaternary metal sulfides are absent from the literature. We report an ALD process to synthesize Cu₂ZnSnS₄ (CZTS), a potentially low cost semiconductor being explored for photovoltaic applications. Two strategies are reported: one in which a trilayer stack of binary metal sulfides (i.e., Cu₂S, SnS₂ and ZnS) is deposited and mixed by thermal annealing, as well as a supercycle strategy that is similar to the conventional ALD procedure for forming nanolaminates. Both routes rely on the facile solid state diffusion of chalcogenides for mixing. For this ALD route to the CZTS system, the challenges are nucleation, ion-exchange between the film and the volatile chemical precursors, and phase-stability of binary SnS₂. The thin films were made with no sulfurization step. The X-ray diffraction and Raman spectra were consistent with the formation of CZTS. X-ray fluorescence measurements revealed that the films contained the expected amount of sulfur based on the target oxidation states. Photoelectrochemical measurements under simulated AM1.5 illumination using Eu³⁺ as an electron acceptor demonstrated that the films were photoactive and had an average internal quantum efficiency (IQE) of 12%

Stabilizing Cu₂S for Photovoltaics One Atomic Layer at a Time

Stabilizing Cu₂S in its ideal stoichiometric form, chalcocite, is a long-standing challenge that must be met prior to its practical use in thin-film photovoltaic (PV) devices. Significant copper deficiency, which results in degenerate p-type doping, might be avoided by limiting Cu diffusion into a readily formed surface oxide and other adjacent layers. Here, we examine the extent to which PV-relevant metal-oxide over- and underlayers may stabilize Cu₂S thin films with desirable semiconducting properties. After only 15 nm of TiO₂ coating, Hall measurements and UV–vis–NIR spectroscopy reveal a significant suppression of free charge-carrier addition that depends strongly on the choice of deposition chemistry. Remarkably, the insertion of a single atomic layer of Al₂O₃ between Cu₂S and TiO₂ further stabilizes the active layer for at least 2 weeks, even under ambient conditions. The mechanism of this remarkable enhancement is explored by in situ microbalance and conductivity measurements. Finally, photoluminescence quenching measurements point to the potential utility of these nanolaminate stacks in solar-energy harvesting applications