High Mobility in Nanocrystal-Based Transparent Conducting Oxide Thin Films

Illuminating the Brain: Advances and Perspectives in Optoelectronics for Neural Activity Monitoring and Modulation

Optogenetic modulation of brain neural activity that combines optical and electrical modes in a unitary neural system has recently gained robust momentum. Controlling illumination spatial coverage, designing light-activated modulators, and developing wireless light delivery and data transmission are crucial for maximizing the use of optical neuromodulation. To this end, biocompatible electrodes with enhanced optoelectrical performance, device integration for multiplexed addressing, wireless transmission, and multimodal operation in soft systems have been developed. This review provides an outlook for uniformly illuminating large brain areas while spatiotemporally imaging the neural responses upon optoelectrical stimulation with little artifacts. Representative concepts and important breakthroughs, such as head-mounted illumination, multiple implanted optical fibers, and micro-light-delivery devices, are discussed. Examples of techniques that incorporate electrophysiological monitoring and optoelectrical stimulation are presented. Challenges and perspectives are posed for further research efforts toward high-density optoelectrical neural interface modulation, with the potential for nonpharmacological neurological disease treatments and wireless optoelectrical stimulation.

Colloidal Approaches to Zinc Oxide Nanocrystals

Zinc oxide is an extensively studied semiconductor with a wide band gap in the near-UV. Its many interesting properties have found use in optics, electronics, catalysis, sensing, as well as biomedicine and microbiology. In the nanoscale regime the functional properties of ZnO can be precisely tuned by manipulating its size, shape, chemical composition (doping), and surface states. In this review, we focus on the colloidal synthesis of ZnO nanocrystals (NCs) and provide a critical analysis of the synthetic methods currently available for preparing ZnO colloids. First, we outline key thermodynamic considerations for the nucleation and growth of colloidal nanoparticles, including an analysis of different reaction methodologies and of the role of dopant ions on nanoparticle formation. We then comprehensively review and discuss the literature on ZnO NC systems, including reactions in polar solvents that traditionally occur at low temperatures upon addition of a base, and high temperature reactions in organic, nonpolar solvents. A specific section is dedicated to doped NCs, highlighting both synthetic aspects and structure-property relationships. The versatility of these methods to achieve morphological and compositional control in ZnO is explicated. We then showcase some of the key applications of ZnO NCs, both as suspended colloids and as deposited coatings on supporting substrates. Finally, a critical analysis of the current state of the art for ZnO colloidal NCs is presented along with existing challenges and future directions for the field.

Solvothermal synthesis of soluble, surface modified anatase and transition metal doped anatase hybrid nanocrystals

Titanium dioxide, or titania, is perhaps the most well-known and widely studied photocatalytic material, with myriad applications, due to a high degree of tunability achievable through the incorporation of dopants and control of phase composition and particle size. Many of the applications of titanium dioxide require particular forms, such as gels, coatings, or thin films, making the development of hybrid solution processable nanoparticles increasingly attractive. Here we report a simple solvothermal route to highly dispersible anatase phase titanium dioxide hybrid nanoparticles from amorphous titania. Solvothermal treatment of the amorphous titania in trifluoroacetic acid leads to the formation of anatase phase nanoparticles with a high degree of size control and near complete surface functionalisation. This renders the particles highly dispersible in simple organic solvents such as acetone. Dopant ions may be readily incorporated into the amorphous precursor by co-precipitation, with no adverse effect on subsequent crystallisation and surface modification.

Magnetoplasmonics beyond Metals: Ultrahigh Sensing Performance in Transparent Conductive Oxide Nanocrystals

Active modulation of the plasmonic response is at the forefront of today's research in nano-optics. For a fast and reversible modulation, external magnetic fields are among the most promising approaches. However, fundamental limitations of metals hamper the applicability of magnetoplasmonics in real-life active devices. While improved magnetic modulation is achievable using ferromagnetic or ferromagnetic-noble metal hybrid nanostructures, these suffer from severely broadened plasmonic response, ultimately decreasing their performance. Here we propose a paradigm shift in the choice of materials, demonstrating for the first time the outstanding magnetoplasmonic performance of transparent conductive oxide nanocrystals with plasmon resonance in the near-infrared. We report the highest magneto-optical response for a nonmagnetic plasmonic material employing F- and In-codoped CdO nanocrystals, due to the low carrier effective mass and the reduced plasmon line width. The performance of state-of-the-art ferromagnetic nanostructures in magnetoplasmonic refractometric sensing experiments are exceeded, challenging current best-in-class localized plasmon-based approaches.

Aqueous transfer of colloidal metal oxide nanocrystals via base-driven ligand exchange

A general method is developed for removal of native nonpolar oleate ligands from colloidal metal oxide nanocrystals of varying morphologies and compositions. Ligand stripping occurs by phase transfer into potassium hydroxide solution, yielding stable aqueous dispersions with little nanocrystal aggregation and without significant changes to the nanomaterials' physical or chemical properties. This method enables facile fabrication of conductive films of ligand-free nanocrystals.

Contact Conductance Governs Metallicity in Conducting Metal Oxide Nanocrystal Films

Although colloidal nanoparticles hold promise for fabricating electronic components, the properties of nanoparticle-derived materials can be unpredictable. Materials made from metallic nanocrystals exhibit a variety of transport behavior ranging from insulators, with internanocrystal contacts acting as electron transport bottlenecks, to conventional metals, where phonon scattering limits electron mobility. The insulator-metal transition (IMT) in nanocrystal films is thought to be determined by contact conductance. Meanwhile, criteria are lacking to predict the characteristic transport behavior of metallic nanocrystal films beyond this threshold. Using a library of transparent conducting tin-doped indium oxide nanocrystal films with varied electron concentration, size, and contact area, we assess the IMT as it depends on contact conductance and show how contact conductance is also key to predicting the temperature-dependence of conductivity in metallic films. The results establish a phase diagram for electron transport behavior that can guide the creation of metallic conducting materials from nanocrystal building blocks.

Electrically Tunable Solution-Processed Transparent Conductive Thin Films Based on Colloidally Dispersed ITO@Ag Composite Ink

Silver (Ag) introduced colloidal Sn-doped In2O3 (ITO) ink for transparent conductive electrodes (TCEs) was prepared to overcome the limitation of colloidally prepared thin film; low density thin film, high resistance. ITO@Ag colloid ink was made by controlling the weight ratio of ITO and Ag nanoparticles through ball-milling and fabricated using spin coating. These films were dried at 220 °C and heat-treated at 450−750 °C in an air atmosphere to pyrolyze the organic ligand attached to the nanoparticles. All thin films showed high crystallinity. As the thermal treatment temperature increased, films showed a cracked surface, but as the weight percentage of silver increased, a flattened and smooth surface appeared, caused by the metallic silver filling the gap between the nano-particles. This worked as a bridge to allow electrical conduction, which decreases the resistivity over an order of magnitude, from 309 to 0.396, and 0.107 Ω·cm for the ITO-220 °C, ITO-750 °C, and ITO@Ag (7.5 wt.%)-750 °C, respectively. These films also exhibited >90% optical transparency. Lowered resistivity is caused due to the inclusion of silver, providing a sufficient number of charge carriers. Furthermore, the work function difference between ITO and silver builds an ohmic junction, allowing fluent electrical flow without any barrier.

Role of the Precursor Composition in the Synthesis of Metal Ferrite Nanoparticles

Ternary oxide nanoparticles have attracted much interest because of their intriguing properties, which are not exhibited by binary oxide nanoparticles. However, the synthesis of ternary oxide nanoparticles is not trivial and requires a fundamental understanding of the complicated precursor chemistry that governs the formation mechanism. Herein, we investigate the role of the chemical composition of precursors in the formation of ternary oxide nanoparticles via a combination of mass spectrometry, electron microscopy with elemental mapping, and thermogravimetric analysis. Mn²⁺, Co²⁺, and Ni²⁺ ions easily form bimetallic-oxo clusters with Fe³⁺ ions with a composition of MFe₂O(oleate)₆ (M = Mn, Co, Ni). The use of clusters as precursors leads to the successful synthesis of monodisperse metal ferrite nanoparticles (MFe₂O₄). On the contrary, zinc- or copper-containing complexes are formed independently from iron-oxo clusters in the precursor synthesis. The mixture of complexes without a bimetallic-oxo core yields a mixture of two different nanoparticles. This study reveals the importance of the precursor composition in the synthesis of ternary oxide nanoparticles.

Direct Optical Lithography of Colloidal Metal Oxide Nanomaterials for Diffractive Optical Elements with 2π Phase Control

Spatially patterned dielectric materials are ubiquitous in electronic, photonic, and optoelectronic devices. These patterns are typically made by subtractive or additive approaches utilizing vapor-phase reagents. On the other hand, recent advances in solution-phase synthesis of oxide nanomaterials have unlocked a materials library with greater compositional, microstructural, and interfacial tunability. However, methods to pattern and integrate these nanomaterials in real-world devices are less established. In this work, we directly optically pattern oxide nanoparticles (NPs) by mixing them with photosensitive diazo-2-naphthol-4-sulfonic acid and irradiating with widely available 405 nm light. We demonstrate the direct optical lithography of ZrO₂, TiO₂, HfO₂, and ITO NPs and investigate the chemical and physical changes responsible for this photoinduced decrease in solubility. Micron-thick layers of amorphous ZrO₂ NPs were patterned with micron resolution and shown to allow 2π phase control of visible light. We also show multilayer patterning and use it to fabricate features with different thicknesses and distinct structural colors. Upon annealing at 400 °C, the deposited ZrO₂ structures have excellent optical transparency across a wide wavelength range (0.3-10 μm), a high refractive index (n = 1.84 at 633 nm), and are optically smooth. We then fabricate diffractive optical elements, such as binary phase diffraction gratings, that show efficient diffractive behavior and good thermal stability. Different oxide NPs can also be mixed prior to patterning, providing a high level of material tunability. This work demonstrates a general patterning approach that harnesses the processability and diversity of colloidal oxide nanomaterials for use in photonic applications.

All solution-processed silver nanowires composite silica nanospheres antireflection structure with synergetic optoelectronic performance

The silver nanowires/SNSs AR composite TCFs have demonstrated the synergetic effect on optoelectronic performance via a facile solution method, reaching sheet resistance of 49.43 Ω sq−1 dropped by 8.66% and transmittance of 99.84% increased by 6.94%.

Electron Beam Infrared Nano-Ellipsometry of Individual Indium Tin Oxide Nanocrystals

Leveraging recent advances in electron energy monochromation and aberration correction, we record the spatially resolved infrared plasmon spectrum of individual tin-doped indium oxide nanocrystals using electron energy-loss spectroscopy (EELS). Both surface and bulk plasmon responses are measured as a function of tin doping concentration from 1-10 atomic percent. These results are compared to theoretical models, which elucidate the spectral detuning of the same surface plasmon resonance feature when measured from aloof and penetrating probe geometries. We additionally demonstrate a unique approach to retrieving the fundamental dielectric parameters of individual semiconductor nanocrystals via EELS. This method, devoid from ensemble averaging, illustrates the potential for electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

Interfacial control and design of conductive nanomaterials for transparent nanocomposite electrodes

A few critical issues in preparing transparent conductive electrodes (TCEs) based on solution-processable conductive nanomaterials are their low electrical conductivity and the unfavorable trade-off between electrical conductivity and optical transparency, which are closely related to the organic ligands bound to the nanomaterial surface. In particular, bulky/insulating organic ligands bound to the surface of conductive nanomaterials unavoidably act as high contact resistance sites at the interfaces between neighboring nanomaterials, which adversely affects the charge transfer kinetics of the resultant TCEs. This article reviews the latest research status of various interfacial control approaches for solution-processable TCEs. We describe how these approaches can be effectively applied to conductive nanomaterials and how interface-controlled conductive nanomaterials can be employed to improve the electrical and/or electrochemical performance of various transparent nanocomposite electrodes, including TCEs, energy storage electrodes, and electrochromic electrodes. Last, we provide perspectives on the development direction for next-generation transparent nanocomposite electrodes and breakthroughs for significantly mitigating the complex trade-off between their electrical/electrochemical performance and optical transparency.

Boosting the performance of NO2 gas sensors based on n-n type mesoporous ZnO@In2O3 heterojunction nanowires: in situ conducting probe atomic force microscopic elucidation of room temperature local electron transport

Herein, n-n type one dimensional ZnO@In₂O₃ heterojunction nanowires have been developed and their local electron transport properties during trace-level NO₂ gas sensing process have been probed at room-temperature using conducting probe atomic microscopy. Solvothermally synthesized 1D ZnO@In₂O₃ heterojunction nanowires have been characterized by various spectroscopic and microscopic techniques, which revealed the mesoporous structure indicating their enhanced sensing properties. The dangling bonds and fraction of metal ions to oxygen ions existing on the exposed crystal facets of the heterojunction nanowires have been visualized by employing crystallographic simulations with TEM analysis, which aided in forecasting the nature of surface adsorption of NO₂ gas species. In situ electrical characteristics and Scanning Spreading Resistance Microscopic (SSRM) imaging of single ZnO@In₂O₃ heterojunction nanowires revealed the local charge transport properties in n-n type ZnO@In₂O₃ heterojunction nanowires. Moreover, the ZnO@In₂O₃ heterojunction nanowires based sensor exhibited excellent sensitivity (S = 274%), a fast response (4-6 s) and high selectivity towards trace-level concentration (500 ppb) of NO₂ gas under ambient conditions with low power consumption. Spatially resolved surface potential (SP) variations in ZnO@In₂O₃ heterojunction nanowires have been visualized using in situ Scanning Kelvin Probe Force Microscopy (SKPM) under NO₂ gas environment at room temperature, which was further correlated with its energy band structure. The work functions of the material evaluated by SKPM reveal considerable changes in the energy band structure owing to the local electron transport between ZnO and In₂O₃ at the heterojunctions upon exposure to NO₂ gas indicating the charge carrier recombination. A plausible mechanism has been proposed based on the experimental evidences. The results suggest that new insights into complex sensing mechanisms deduced from the present investigation on n-n type MOS based heterojunction nanowires under ambient conditions can pave the way for the novel design and development of affordable and superior real-time gas sensors.

Radio-frequency and optically transparent radome de-icing materials: fluorine-doped tin oxide

Ice-elimination systems are very common in radio-frequency (RF) structures like radomes. For a radome application, the de-icing materials must be predominantly transparent to broadband RF radiation and have an adequate heating performance to remove the ice. The current development of high-performance radome de-icing materials is limited with a trade-off between the sheet resistance and RF transmission because one cannot be improved without sacrificing the other. We report for the first time a transparent conductive oxide (TCO) film as a lightweight and high optically transparent radome de-icing material. In this research, we prepared fluorine-doped tin oxide (FTO) films by horizontal ultrasonic spray pyrolysis (USP) deposition and found that the sheet resistance varied from 9 to 5000 Ω sq⁻¹ with 0.219 to 90.0% RF transmission. Dassault CST software was used to validate the RF transmission at the X-band (8.2 to 12.4 GHz) region. The FTO films also exhibited sufficient optical transparency with efficient voltage-induced heating performance. With optimized electrical properties and RF transparency, FTO films will be good candidates for next-generation radome de-icing materials.

Ice-elimination systems are very common in radio-frequency (RF) structures like radomes.

Instant Postsynthesis Aqueous Dispersion of Sb-Doped SnO2 Nanocrystals: The Synergy between Small-Molecule Amine and Sb Dopant Ratio

Direct printing of transparent conducting oxide (TCO) nanocrystal dispersions holds great promise in solution-processed optoelectronics due to its advantages of low material waste and direct patterning on substrates. An essential prerequisite for printable TCO colloidal solutions is the effective stabilization of TCO nanocrystals to prevent their strong aggregation. In situ stabilization uses long-chain ligands to provide interparticle steric repulsion between TCO nanocrystals during the growth of TCO nanocrystals. In sharp contrast, the postsynthesis dispersion of TCO nanocrystals is particularly challenging since the agglomeration already occurs, especially for TCO nanocrystals synthesized without protection by any organic species. Herein, we propose an instant postsynthesis strategy for aqueous colloidal dispersions of Sb-doped SnO₂ (ATO) nanocrystals using small-molecule amines of propylamine, ethylenediamine, monoethanolamine, and triethylamine. The average size of ATO secondary particles in aqueous dispersions can be instantly reduced from around 400 to about 25 nm using these amines. The increased Sb dopant ratio also plays a synergistic role in the dispersion effect. The small-molecule amines are found to be preferably adsorbed onto the Sb sites exposed on ATO nanocrystal surface. A higher Sb dopant ratio would facilitate the adsorption of more amines and induce stronger surface charge repulsion that benefits the stable dispersion of ATO nanocrystals. TCO films fabricated with the ATO nanocrystal dispersions have a high transparency of 80.6% and low sheet resistance of 492 Ω/sq, showing promising application in electrochromic devices.

Spectrally tunable infrared plasmonic F,Sn:In2O3 nanocrystal cubes

A synthetic challenge in faceted metal oxide nanocrystals (NCs) is realizing tunable localized surface plasmon resonance (LSPR) near-field response in the infrared (IR). Cube-shaped nanoparticles of noble metals exhibit LSPR spectral tunability limited to visible spectral range. Here, we describe the colloidal synthesis of fluorine, tin codoped indium oxide (F,Sn:In₂O₃) NC cubes with tunable IR range LSPR for around 10 nm particle sizes. Free carrier concentration is tuned through controlled Sn dopant incorporation, where Sn is an aliovalent n-type dopant in the In₂O₃ lattice. F shapes the NC morphology into cubes by functioning as a surfactant on the {100} crystallographic facets. Cube shaped F,Sn:In₂O₃ NCs exhibit narrow, shape-dependent multimodal LSPR due to corner, edge, and face centered modes. Monolayer NC arrays are fabricated through a liquid-air interface assembly, further demonstrating tunable LSPR response as NC film nanocavities that can heighten near-field enhancement (NFE). The tunable F,Sn:In₂O₃ NC near-field is coupled with PbS quantum dots, via the Purcell effect. The detuning frequency between the nanocavity and exciton is varied, resulting in IR near-field dependent enhanced exciton lifetime decay. LSPR near-field tunability is directly visualized through IR range scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). STEM-EELS mapping of the spatially confined near-field in the F,Sn:In₂O₃ NC array interparticle gap demonstrates elevated NFE tunability in the arrays.

Advances in nanomaterials for electrochromic devices

This review article systematically highlights the recent advances regarding the design, preparation, performance and application of new and unique nanomaterials for electrochromic devices.

Integrating Polar and Conductive Fe2O3-Fe3C Interface with Rapid Polysulfide Diffusion and Conversion for High-Performance Lithium-Sulfur Batteries

A low-cost in situ formed Fe₂O₃-Fe₃C heterostructure highly dispersed in carbon nanofiber was delicately designed via a facile one-pot electrospinning method. The intense anchoring (by Fe₂O₃) and rapid electron transfer (by Fe₃C) for lithium polysulfides transformation can be simultaneously achieved on the Fe₂O₃-Fe₃C heterostructure interface, thus preventing the amassment of lithium polysulfides benefiting from its excellent interfacial contact and improving sulfur utilization. Experimental characterizations and DFT calculations confirmed the restrained polysulfides shuttle and enhanced redox kinetics. Therefore, the battery containing optimal Fe₂O₃-Fe₃C heterostructure delivered a high capacity of 776.2 mA h g^-1 after 300 cycles at 1 C at a low fading rate of 0.037% per cycle. Even at a high sulfur loading of 3.5 and 4.5 mg cm^-2, high capacities of 773.6 and 533.6 mA h g^-1 at 0.5 C can be achieved with capacity retentions of 91.7 and 94.2%, respectively. This distinctive heterostructure proposes an effective design of high-performance lithium-sulfur batteries contributing to the excellent electrochemical performance, which can synergize the virtues of effectively adsorptive metal oxides and appealing conductive metal carbides.

Tunable Near-Infrared Localized Surface Plasmon Resonance of F, In-Codoped CdO Nanocrystals

Nanocrystals (NCs) of transparent conducting oxides with a localized surface plasmon resonance (LSPR) in the near-infrared (NIR) spectral region show promising electrochromic properties for the development of a new generation of dynamic "smart windows". In this regard, we exploit thin films of F, In-codoped CdO (FICO) NCs as active coatings for electrochromic devices. The control over the dopants concentration in FICO NCs results in fine tuning of their LSPR across the NIR region of the electromagnetic spectrum. Highly transparent mesoporous electrodes were prepared from colloidal FICO NCs by in situ ligand exchange of the pristine organic capping ligands. This approach preserves the optical and electrical properties of native NCs and delivers highly homogeneous, nonscattering films with a good electronic coupling between the NCs. We achieved a dynamic control over the LSPR frequency by reversible electrochemical doping, hence a spectrally selective modulation of the optical transmittance in the NIR region of the solar spectrum, which carries nearly 50% of the whole solar heat. Spectroelectrochemical characterization, coloration efficiency, and switching kinetics results indicate that thin film based on FICO NCs are potential candidates for plasmonic electrochromic applications. Moreover, the high electron mobility and wide optical bandgap of FICO makes NCs of this material suitable for large-area devices capable of dynamically controlling the heat load coming from the solar infrared radiation, without affecting the visible light transmittance.

ITO@SiO2 and ITO@{M6Br12}@SiO2 (M = Nb, Ta) nanocomposite films for ultraviolet-near infrared shielding

Transparent optical thin films for energy saving applications have recently gained substantial prominence for functional window processes. In this study, highly visible transparent nanocomposite films with ultraviolet (UV) and near-infrared (NIR) blocking capabilities are reported. Such nanocomposite films, prepared by electrophoretic deposition on ITO-coated glass, are composed of indium tin oxide (ITO) nanocrystals (9 nm) and octahedral metal atom clusters (1 nm, Nb₆ or Ta₆) embedded into silica nanoparticles (∼80 nm). The functional silica nanoparticles were prepared by a reverse microemulsion process. The microstructural characterization proved that ITO nanocrystals are centered in the silica nanoparticles, whereas the metal atom clusters are homogeneously distributed in the silica matrix. The optical absorption spectra of these transparent nanocomposite films exhibit distinct and complementary contributions from their ITO nanoparticles and metal atom clusters (absorption in the UV range) and from the ITO layer on silica.

Metal-insulator transition in a semiconductor nanocrystal network

Many envisioned applications of semiconductor nanocrystals (NCs), such as thermoelectric generators and transparent conductors, require metallic (nonactivated) charge transport across an NC network. Although encouraging signs of metallic or near-metallic transport have been reported, a thorough demonstration of nonzero conductivity, σ, in the 0 K limit has been elusive. Here, we examine the temperature dependence of σ of ZnO NC networks. Attaining both higher σ and lower temperature than in previous studies of ZnO NCs (T as low as 50 mK), we observe a clear transition from the variable-range hopping regime to the metallic regime. The critical point of the transition is distinctly marked by an unusual power law close to σ ∝ T ^1/5. We analyze the critical conductivity data within a quantum critical scaling framework and estimate the metal-insulator transition (MIT) criterion in terms of the free electron density, n, and interparticle contact radius, ρ.

Nanomechanical and Material Properties of Fluorine-Doped Tin Oxide Thin Films Prepared by Ultrasonic Spray Pyrolysis: Effects of F-Doping

Fluorine-doped tin oxide (FTO) thin films were deposited on glass substrates using ultrasonic spray pyrolysis (USP) at a fixed substrate temperature of 400 °C and various Fluorine/Tin (F/Sn) atomic ratios of 0, 0.1, 0.5, and 1.0. Effects of F/Sn atomic ratios on structural-morphological, compositional, electrical, optical, and nanomechanical properties of the FTO thin films were systematically studied. The FTO films exhibited a tetragonal structure with preferred orientations of (110), (200), and (211), and polycrystalline morphology with spear-like or coconut shell-like particles on the surfaces. The presence of F-doping was confirmed by XPS results with clear F1s peaks, and F-concentration was determined to be 0.7% for F/Sn = 0.1 and 5.1% for F/Sn = 0.5. Moreover, the resistivity of FTO films reduced remarkably from 4.1 mΩcm at F/Sn = 0 to 0.7 mΩcm at F/Sn = 1, primarily due to the corresponding increase of carrier concentration from 2 × 10²⁰ cm⁻³ to 1.2 × 10²¹ cm⁻³. The average optical transmittance of the films prepared at F/Sn of 0–0.5 was over 90%, and it decreased to 84.4% for the film prepared at F/Sn = 1. The hardness (H) and Young’s modulus (E) of the FTO films increased when the F/Sn ratios increased from 0 to 0.5, reaching maximum values of H = 12.3 ± 0.4 GPa, E = 131.7 ± 8.0 GPa at F/Sn = 0.5. Meanwhile, the H and E reduced considerably when the F/Sn ratio further increased to 1.0, following the inverse Hall-Petch effect approximately, suggesting that the grain boundary effect played a primary role in manipulating the nanomechanical properties of the FTO films. Furthermore, favorable mechanical properties with large H/E_f and H3/Ef2 ratios were found for the FTO film prepared at F/Sn = 0.5, which possessed high crystallinity, large grain size, and compact morphology.

Rational design and observation of the tight interface between graphene and ligand protected nanocrystals

Heterostructures constructed of graphene and colloidal nanocrystals provide a unique way to exploit the coupled physical properties of the two functional building blocks. Studying the interface structure between the two constituent materials is important to understand the formation mechanism and the resulting physical and chemical properties. Along with ab initio calculations, we elucidate that the bending rigidity and the strong van der Waals interaction of graphene to the metal surface guide the formation of a tight and conformal interface. Using theoretical foundations, we construct colloidal nanocrystal-graphene heterostructures with controlled interfacial structures and directly investigate the cross-sectional structures of them at high resolution by using aberration-corrected transmission electron microscopy. The experimental method and observations we present here will link the empirical methods for the formation of nanocrystal-graphene heterostructures to the mechanistic understanding of their properties.

Controlling Morphology in Polycrystalline Films by Nucleation and Growth from Metastable Nanocrystals

Solution processing of polycrystalline compound semiconductor thin film using nanocrystals as a precursor is considered one of the most promising and economically viable routes for future large-area manufacturing. However, in polycrystalline compound semiconductor films such as Cu₂ZnSnS₄ (CZTS), grain size, and the respective grain boundaries play a key role in dictating the optoelectronic properties. Various strategies have been employed previously in tailoring the grain size and boundaries (such as ligand exchange) but most require postdeposition thermal annealing at high temperature in the presence of grain growth directing agents (selenium or sulfur vapor with/without Na, K, etc.) to enlarge the grains through sintering. Here, we show a different strategy of controlling grain size by tuning the kinetics of nucleation and the subsequent grain growth in CZTS nanocrystal thin films during a crystalline phase transition. We demonstrate that the activation energy for the phase transition can be varied by utilizing different shapes (spherical and nanorod) of nanocrystals with similar size, composition, and surface chemistry leading to different densities of nucleation sites and, thereby, different grain sizes in the films. Additionally, exchanging the native organic ligands for inorganic surface ligands changes the activation energy for the phase change and substantially changes the grain growth dynamics, while also compositionally modifying the resulting film. This combined approach of using nucleation and growth dynamics and surface chemistry enables us to tune the grain size of polycrystalline CZTS films and customize their electronic properties by compositional engineering.