Selective Etching of Copper Sulfide Nanoparticles and Heterostructures through Sulfur Abstraction: Phase Transformations and Optical Properties

Integrating top-down methods, such as chemical etching, for the precise removal of excess material in nanostructures with the bottom-up size and shape control of colloidal nanoparticle synthesis could greatly expand the range of accessible nanoparticle morphologies. We present mechanistic insights into an unusual reaction in which trialkylphosphines (“phosphines”), which are commonly used to protect nanoparticle surfaces as a surfactant ligand, chemically etch copper sulfide, Cu_2–<i>x</i>S, nanostructures in the presence of oxygen. Furthermore, Cu_2–<i>x</i>S is removed highly selectively from zinc sulfideCu_2–<i>x</i>S heterostructures. Structural and optical characterizations show that the addition of phosphine destabilizes the highly Cu-deficient roxbyite phase and injects Cu into the interiors of the nanoparticles, even at room temperature. Analysis of the etching products confirms that chalcogens are removed in the form of phosphine chalcogenides and shows that the removed copper is solubilized as Cu²⁺. The morphology of etched Cu_2–<i>x</i>S particles changes dramatically as the concentration of phosphine is reduced, producing anisotropically etched particles indicative of facet-selective surface chemical reactions. Additionally, ceric ammonium nitrate, another oxidizing agent, can be used to control the etching reaction; the use of this redox agent affords strictly isotropically etched particles. These results demonstrate the highly pliable structural and chemical properties of nanocrystalline Cu_2–<i>x</i>S and raise the possibility of using surface-active ligands formerly thought to be passivating to dramatically reshape as-synthesized colloidal nanostructures into more functional forms

Binder-Free and Carbon-Free Nanoparticle Batteries: A Method for Nanoparticle Electrodes without Polymeric Binders or Carbon Black

In this work, we have developed a new fabrication method for nanoparticle (NP) assemblies for Li-ion battery electrodes that require no additional support or conductive materials such as polymeric binders or carbon black. By eliminating these additives, we are able to improve the battery capacity/weight ratio. The NP film is formed by using electrophoretic deposition (EPD) of colloidally synthesized, monodisperse cobalt NPs that are transformed through the nanoscale Kirkendall effect into hollow Co₃O₄. EPD forms a network of NPs that are mechanically very robust and electrically connected, enabling them to act as the Li-ion battery anode. The morphology change through cycles indicates stable 5–10 nm NPs form after the first lithiation remained throughout the cycling process. This NP-film battery made without binders and conductive additives shows high gravimetric (>830 mAh/g) and volumetric capacities (>2100 mAh/cm³) even after 50 cycles. Because similar films made from drop-casting do not perform well under equal conditions, EPD is seen as the critical step to create good contacts between the particles and electrodes resulting in this significant improvement in battery electrode assembly. This is a promising system for colloidal nanoparticles and a template for investigating the mechanism of lithiation and delithiation of NPs

Chalcogenidometallate Clusters as Surface Ligands for PbSe Nanocrystal Field-Effect Transistors

We introduce a method to process colloidal PbSe nanocrystals (NCs) into inorganic NC thin films using chalcogenidometallate (ChaM) clusters as surface ligands, resulting in electrically coupled NC solids. NCs are first immobilized on a substrate via a self-assembled monolayer followed by chemical treatment to exchange the insulating oleate ligands with ChaM clusters. Quantum confinement in the PbSe NCs is preserved as evidenced by persistent excitonic features in the absorption spectrum. PbSe NC–ChaM composites exhibit rectification (“off” states), saturation, n-type electrical behavior, and high electron mobilities of 1.28 and 0.475 cm² V^–1 s^–1 for different composite compositions

Reconfigurable Nanorod Films: An <i>in Situ</i> Study of the Relationship between the Tunable Nanorod Orientation and the Optical Properties of Their Self-Assembled Thin Films

Understanding and controlling the self-assembly of colloidal nanostructures into ordered superstructures present scientifically interesting and technologically important research challenges. Here, we investigated the self-assembly, disordering, and reassembly of colloidal CdSe/CdS dot/rod nanorod (NR) films. We monitored the structural evolution of the NR films in real time using <i>in situ</i> grazing incidence small-angle and wide-angle X-ray scattering. In dry films, self-assembled from colloidal suspensions, NRs are oriented with the long axis normal to the substrate, but the preferred NR orientation is lost when dichlorobenzene vapor is introduced. Multiprobe optical and structural experiments allowed us to directly correlate the NR superlattice structure and optical absorption. We found that the optical absorption of the NR films is significantly enhanced in disordered NR films compared to NR arrays in which the rods are oriented normal to the plane of the substrate and parallel to the optical axis. Basic processing–structure–property relationships of NR thin films demonstrate that their structure and optical properties can be reconfigured through the adjustment of solvent vapor concentration. The phase behavior and optical properties of NRs present an interesting inorganic analogue to organic liquid crystals with potential applications in emerging optoelectronic technologies

A Generic Method for Rational Scalable Synthesis of Monodisperse Metal Sulfide Nanocrystals

A rational synthetic method is developed to produce monodisperse metal sulfide nanocrystals (NCs) in organic nonpolar solutions by using (NH₄)₂S as a sulfide precursor. (NH₄)₂S is stabilized in an organic primary amine solution and exhibits high reactivity toward metal complexes. This novel technique exhibits wide applicability for organic phase metal sulfide NC synthesis: a large variety of monodisperse NCs have been synthesized, including Cu₂S, CdS, SnS, ZnS, MnS, Ag₂S, and Bi₂S₃. The stoichiometric reactions between (NH₄)₂S and metal salts afford high conversion yields, and large-scale production of monodisperse NCs (more than 30 g) can be synthesized in a single reaction. The high reactivity of (NH₄)₂S enables low temperature (<100 °C) syntheses, and the air-stable materials (such as CdS NCs) can be produced in air. Moreover, this low-temperature technique can be used to produce small size NCs which are difficult to be synthesized by the conventional high temperature methods, such as sub-5 nm Ag₂S and Bi₂S₃ quantum dots

The Oxidation of Cobalt Nanoparticles into Kirkendall-Hollowed CoO and Co₃O₄: The Diffusion Mechanisms and Atomic Structural Transformations

We report on the atomic structural changes and diffusion processes during the chemical transformation of ε-Co nanoparticles (NPs) through oxidation in air into hollow CoO NPs and then Co₃O₄ NPs. Through XAS, XRD, TEM, and DFT calculations, the mechanisms of the transformation from ε-Co to CoO to Co₃O₄ are investigated. Our DFT calculations and experimental results suggest that a two-step diffusion process is responsible for the Kirkendall hollowing of ε-Co into CoO NPs. The first step is O in-diffusion by an indirect exchange mechanism through interstitial O and vacancies of type I Co sites of the ε-Co phase. This indirect exchange mechanism of O has a lower energy barrier than a vacancy-mediated diffusion of O through type I sites. When the CoO phase is established, the Co then diffuses outward faster than the O diffuses inward, resulting in a hollow NP. The lattice orientations during the transformation show preferential orderings after the single-crystalline ε-Co NPs are transformed to polycrystalline CoO and Co₃O₄ NPs. Our Co₃O₄ NPs possess a high ratio of {110} surface planes, which are known to have favorable catalytic activity. The Co₃O₄ NPs can be redispersed in an organic solvent by adding surfactants, thus rendering a method to create solution-processable colloidal, monodisperse Co₃O₄ NPs

Prodigious Effects of Concentration Intensification on Nanoparticle Synthesis: A High-Quality, Scalable Approach

Realizing the promise of nanoparticle-based technologies demands more efficient, robust synthesis methods (i.e., process intensification) that consistently produce large quantities of high-quality nanoparticles (NPs). We explored NP synthesis via the heat-up method in a regime of previously unexplored high concentrations near the solubility limit of the precursors. We discovered that in this highly concentrated and viscous regime the NP synthesis parameters are less sensitive to experimental variability and thereby provide a robust, scalable, and size-focusing NP synthesis. Specifically, we synthesize high-quality metal sulfide NPs (<7% relative standard deviation for Cu_2–<i>x</i>S and CdS), and demonstrate a 10–1000-fold increase in Cu_2–<i>x</i>S NP production (>200 g) relative to the current field of large-scale (0.1–5 g yields) and laboratory-scale (<0.1 g) efforts. Compared to conventional synthesis methods (hot injection with dilute precursor concentration) characterized by rapid growth and low yield, our highly concentrated NP system supplies remarkably controlled growth rates and a 10-fold increase in NP volumetric production capacity (86 g/L). The controlled growth, high yield, and robust nature of highly concentrated solutions can facilitate large-scale nanomanufacturing of NPs by relaxing the synthesis requirements to achieve monodisperse products. Mechanistically, our investigation of the thermal and rheological properties and growth rates reveals that this high concentration regime has reduced mass diffusion (a 5-fold increase in solution viscosity), is stable to thermal perturbations (∼64% increase in heat capacity), and is resistant to Ostwald ripening

Highly Conductive Cu_2–<i>x</i>S Nanoparticle Films through Room-Temperature Processing and an Order of Magnitude Enhancement of Conductivity via Electrophoretic Deposition

A facile room-temperature method for assembling colloidal copper sulfide (Cu_2–<i>x</i>S) nanoparticles into highly electrically conducting films is presented. Ammonium sulfide is utilized for connecting the nanoparticles via ligand removal, which transforms the as-deposited insulating films into highly conducting films. Electronic properties of the treated films are characterized with a combination of Hall effect measurements, field-effect transistor measurements, temperature-dependent conductivity measurements, and capacitance–voltage measurements, revealing their highly doped p-type semiconducting nature. The spin-cast nanoparticle films have carrier concentration of ∼10¹⁹ cm^–3, Hall mobilities of ∼3 to 4 cm² V^–1 s^–1, and electrical conductivities of ∼5 to 6 S·cm^–1. Our films have hole mobilities that are 1–4 orders of magnitude higher than hole mobilities previously reported for heat-treated nanoparticle films of HgTe, InSb, PbS, PbTe, and PbSe. We show that electrophoretic deposition (EPD) as a method for nanoparticle film assembly leads to an order of magnitude enhancement in film conductivity (∼75 S·cm^–1) over conventional spin-casting, creating copper sulfide nanoparticle films with conductivities comparable to bulk films formed through physical deposition methods. The X-ray diffraction patterns of the Cu_2–<i>x</i>S films, with and without ligand removal, match the Djurleite phase (Cu_1.94S) of copper sulfide and show that the nanoparticles maintain finite size after the ammonium sulfide processing. The high conductivities reported are attributed to better interparticle coupling through the ammonium sulfide treatment. This approach presents a scalable room-temperature route for fabricating highly conducting nanoparticle assemblies for large-area electronic and optoelectronic applications

Synthesis and Properties of Electrically Conductive, Ductile, Extremely Long (∼50 μm) Nanosheets of K_<i>x</i>CoO₂·<i>y</i>H₂O

Extremely long, electrically conductive, ductile, free-standing nanosheets of water-stabilized K_<i>x</i>CoO₂·<i>y</i>H₂O are synthesized using the sol–gel and electric-field induced kinetic-demixing (SGKD) process. Room temperature in-plane resistivity of the K_<i>x</i>CoO₂·<i>y</i>H₂O nanosheets is less than ∼4.7 mΩ·cm, which corresponds to one of the lowest resistivity values reported for metal oxide nanosheets. The synthesis produces tens of thousands of very high aspect ratio (50,000:50,000:1 = length/width/thickness), millimeter length nanosheets stacked into a macro-scale pellet. Free-standing nanosheets up to ∼50 μm long are readily delaminated from the stacked nanosheets. High-resolution transmission electron microscopy (HR-TEM) studies of the free-standing nanosheets indicate that the delaminated pieces consist of individual nanosheet crystals that are turbostratically stacked. X-ray diffraction (XRD) studies confirm that the nanosheets are stacked in perfect registry along their <i>c</i>-axis. Scanning electron microscopy (SEM) based statistical analysis show that the average thickness of the nanosheets is ∼13 nm. The nanosheets show ductility with a bending radius as small as ∼5 nm

Direct Measurements of Surface Scattering in Si Nanosheets Using a Microscale Phonon Spectrometer: Implications for Casimir-Limit Predicted by Ziman Theory

Thermal transport in nanostructures is strongly affected by phonon-surface interactions, which are expected to depend on the phonon’s wavelength and the surface roughness. Here we fabricate silicon nanosheets, measure their surface roughness (∼1 nm) using atomic force microscopy (AFM), and assess the phonon scattering rate in the sheets with a novel technique: a microscale phonon spectrometer. The spectrometer employs superconducting tunnel junctions (STJs) to produce and detect controllable nonthermal distributions of phonons from ∼90 to ∼870 GHz. This technique offers spectral resolution nearly 10 times better than a thermal conductance measurement. We compare measured phonon transmission rates to rates predicted by a Monte Carlo model of phonon trajectories, assuming that these trajectories are dominated by phonon-surface interactions and using the Ziman theory to predict phonon-surface scattering rates based on surface topology. Whereas theory predicts a diffuse surface scattering probability of less than 40%, our measurements are consistent with a 100% probability. Our nanosheets therefore exhibit the so-called “Casimir limit” at a much lower frequency than expected if the phonon scattering rates follow the Ziman theory for a 1 nm surface roughness. Such a result holds implications for thermal management in nanoscale electronics and the design of nanostructured thermoelectrics