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Next-generation devices and systems require the development and integration of advanced materials, the realization of which inevitably requires two separate processes: property engineering and patterning. Here, we report a one-step, ink-lithography technique to pattern and engineer the properties of thin films of colloidal nanocrystals that exploits their chemically addressable surface. Colloidal nanocrystals are deposited by solution-based methods to form thin films and a local chemical treatment is applied using an ink-printing technique to simultaneously modify (i) the chemical nature of the nanocrystal surface to allow thin-film patterning and (ii) the physical electronic, optical, thermal, and mechanical properties of the nanocrystal thin films. The ink-lithography technique is applied to the library of colloidal nanocrystals to engineer thin films of metals, semiconductors, and insulators on both rigid and flexible substrates and demonstrate their application in high-resolution image replications, anticounterfeit devices, multicolor filters, thin-film transistors and circuits, photoconductors, and wearable multisensors.

Wearable multi-sensors based on nanocrystals have attracted significant attention, and studies on patterning technology to implement such multi-sensors are underway. Conventional patterning processes may affect material properties based on high temperatures and harsh chemical conditions. In this study, we developed an inkjet printing technique that can overcome these drawbacks through the application of patterning processes at room temperature and atmospheric pressure. Nanocrystal-based ink is used to adjust properties efficiently. Additionally, the viscosity and surface tension of the solvents are investigated and optimized to increase patterning performance. In the patterning process, the electrical, electrothermal, and electromechanical properties of the nanocrystal pattern are controlled by the ligand exchange process. Experimental results demonstrate that a multi-sensor with a temperature coefficient of resistance of 3.82 × 10-3 K-1 and gauge factor of 30.6 can be successfully fabricated using one-step inkjet printing.

The anion exchange in lead halide perovskite is a post-synthetic process that replaces the X-site anion with the target halide without phase transition. Various methods for adjusting the anions in perovskite nanocrystals (Pe NCs) have been developed; however, the effect of the cations in halide sources on the anion exchange has rarely been discussed. Here, we investigated the suitable chemicals and tendencies according to the cation in halide sources. NH4Cl and NaI were selected as efficient anion-exchanging chemicals because they effectively provide Cl- or I- anions to Pe NCs, respectively. Additionally, we discussed the hard-soft-acid-base (HSAB) theory. We observed that the anion-exchanged Pe NCs were well-dispersed with the original surface ligands, maintaining high colloidal stability. We believe that these findings would contribute to the improvement of optoelectronic devices, such as solar cells, displays, and light-emitting diodes (LEDs).

In this study, non-temperature interference strain gauge sensors, which are only sensitive to strain but not temperature, are developed by engineering the properties and structure from a material perspective. The environmental interference from temperature fluctuations is successfully eliminated by controlling the charge transport in nanoparticles with thermally expandable polymer substrates. Notably, the negative temperature coefficient of resistance (TCR), which originates from the hopping transport in nanoparticle arrays, is compensated by the positive TCR of the effective surface thermal expansion with anchoring effects. This strategy successfully controls the TCR from negative to positive. A near-zero TCR (NZTCR), less than 1.0 × 10-6 K-1, is achieved through precisely controlled expansion. Various characterization methods and finite element and transport simulations are conducted to investigate the correlated electrical, mechanical, and thermal properties of the materials and elucidate the compensated NZTCR mechanism. With this strategy, an all-solution-processed, transparent, highly sensitive, and noninterference strain sensor is fabricated with a gauge factor higher than 5000 at 1% strain, as demonstrated by pulse and motion sensing, as well as the noninterference property under variable-temperature conditions. It is envisaged that the sensor developed herein is applicable to multifunctional wearable sensors or e-skins for artificial skin or robots.

In this study, a self-sorting sensor was developed with the ability to distinguish between different pressure regimes and translate the pressure to electrical signals. Specifically, the self-sorting sensor can distinguish between soft and hard pressure like the human skin, without any software assistance and complicated circuits. To achieve the self-sorting property, Janus-like jagged structures were prepared via an all-solution process of spontaneous chemical patterning; they comprised electrically semi-insulating vertices and highly conductive valleys. This unique structure facilitates the detection and determination of the intensities and types of pressure by providing a significant gap between the current levels of two types of states, similar to the function of fibers in the human tactile system. The fabricated sensors also exhibit high sensitivity and durability as well as low power consumption, as demonstrated by the electronic skin and ternary Morse signal applications. Compared with conventional wearable pressure sensors, this sensor can detect signals without additional programming; thus, it is highly suitable for delay-sensitive, energy-efficient sensor applications such as driverless vehicles, autonomous artificial intelligence technology, and prosthetic devices.

Post-synthesis engineering methods that employ oriented attachment to precisely control the size and dimensionality (0D/1D/2D) of as-synthesized CsPbBr3 nanocrystals (NCs) are demonstrated. We investigated the chemical effects of the properties of polar solvents, including their immiscibility, polarity, and boiling point, on the surfaces of NCs, as well as their effect on the structural and optical properties of NCs. Appropriate exploitation of the solvent properties made it possible to use a polar solvent to mildly affect the NCs indirectly such that they discarded their ligands and became attached to proximal NCs without being destroyed. Based on our observations, we developed a method whereby a solution of the NCs in a non-polar solvent is mixed with a polar solvent to form an immiscible phase to induce epitaxial growth of CsPbBr3 NCs. The method enables the size of NCs to be easily regulated from 5 to 50 nm by controlling the engineering time. Taking advantage of the minimal effect of a mild solvent, we also developed a self-assembly method that operates at the liquid–air interface to systematically control the dimensionality. At this interface, the NCs self-assemble in the horizontal direction and grow into micron-sized, single-crystalline, defect-free nanowires (1D) and nanoplates (2D) via oriented attachment. Finally, we discuss the origin of the non-destructive oriented attachment phenomenon and the surface chemistry of a perovskite NC using density functional theory (DFT) simulations and a proposed model system.

We report a simple, solution-based, and postsynthetic process for simultaneous ligand exchange, surface passivation, and doping of CdSe nanocrystals (NCs) for the design of high-performance field-e...

Efficient methods for the controlled syntheses of metal nanocrystals are important for emerging electronics and catalysis applications. In this report, we present a solution-based method for the fabrication of Pt-coated Ag nanocrystal (NC) thin films through the use of ligand-exchange and electroless deposition. We successfully prepared uniform or bumpy Ag@Pt core/shell structures, as well as Ag–Pt alloy structures, through control of the reducing agents. We first report the Ag–Pt alloy structures can be synthesized by low-temperature and solution-based process of post treatment that exhibited high catalytic activity toward the hydrogen evolution reaction.

We investigated the chemical effects of halide ligands on the electromechanical properties of Ag nanocrystal (NC) thin films for potential uses in wearable devices and sensors. The halide treatment...

In this study, we introduce a strategy to introduce solution-processed interfacial layers in nanocrystal (NC) thin films to fabricate high-performance strain sensors. SiO2 interfacial layers are chemically introduced in ligand-exchanged Ag NC thin films to increase the tunneling gap or inter-particle distance between each Ag NC. In this way, the charge-transport mechanism is manipulated, leading to unique electromechanical properties with a high gauge factor. All solution-processed strain gauge sensors with high stability, durability, and sensitivity are fabricated. The sensor successfully measured delicate movements such as finger motions, demonstrating its possible application in electronic skin.

Ligand exchange processes have been attracting tremendous interest and are necessary when fabricating nanocrystal (NC) thin films for various applications. As ligand exchange processes are based on...

Highly transparent and sensitive pressure sensors with a wide detection range were developed by a simple process using silver nanowires and ZnO nanocrystals (NCs) without lithography. The open mesh structured spacers were sucessfully designed by one-step chemical treatment involving a ligand exchange process on the ZnO NC thin films, leading to improved sensitivity, responsivity and transparency. The device performance analyses along with chemical, structural, and electronic characterization studies reveal that the chemically designed pressure sensor has a record-breaking sensitivity of 3.23 × 103 kPa−1, a large detection range of up to 75 kPa, excellent reliability, and high transparency of 85% in the visible-light region. The origin of improved sensitivity was explained with the contact area variation theory. We also demonstrate that the pressure sensor fabricated by the all-solution-based facile process can be employed in various applications such as wearable sensors for measurement of the human pulse and attachable electronics for an electronic skin with a wide sensing range of pressures.

We developed a simple and systematic method to fabricate optically tunable and thermally and chemically stable Au-Ag nanocrystal-based plasmonic metamaterials. An Ag nanocrystal-based metamaterial with desirable optical properties was fabricated via nanoimprinting and ligand-exchange process. Its optical properties were controlled by selectively substituting Ag atoms with Au atoms through a spontaneous galvanic replacement reaction. The developed Au-Ag-based metamaterials provide excellent tunable plasmonic properties required for various applications in the visible and near-infrared regions by controlling the Au-Ag composition according to the conditions of the galvanic displacement. Furthermore, their thermal and chemical stabilities significantly improved because of the protective Au thin layer on the surface. Using this developed process, chemically and thermally stable and flexible plasmonic metamaterials were successfully fabricated on a flexible polyester terephthalate substrate.

In this study, a highly stretchable white-light electroluminescent device is fabricated using chemically designed perovskite nanocrystals (NCs). To incorporate perovskite into a stretchable polymer matrix without any degradation, gel-type silica-coated CsPbBr3 and CsPbBrxI3-x NC inks are prepared using (3-aminopropyl) triethoxysilane (APTES) with a sacrificial solvent of hexane. The chemical, optical, and structural properties of powdered and gel-type perovskite NCs with and without APTES are investigated. Silica shell with APTES treatment improves the stability of perovskite NCs, and gel-type inks improve the dispersivity in the polymer matrix. These enable the fabrication of stretchable and luminescent NC/polymer matrices with enhanced and uniform luminescence properties. By utilizing these materials, we propose a stretchable electroluminescent device emitting strong white light that is close to the ideal white position in Commission Internationale de l'eclairage (CIE) with optical simulation. Herein, we fabricate a multi-color filter with high color reproducibility on stretchable blue electroluminescent devices for white-light generation. Furthermore, all-solution-processed, color-filterless, stretchable, white-light electroluminescence devices with excellent optical and mechanical properties are fabricated.

The popularity of colloidal quantum dot (CQD) solar cells has increased owing to their tunable bandgap, multiple exciton generation, and low-cost solution processes. ZnO nanoparticle (NP) layers are generally employed as electron transport layers in CQD solar cells to efficiently extract the electrons. However, trap sites and the unfavorable band structure of the as-synthesized ZnO NPs have hindered their potential performance. Herein, we introduce a facile method of ZnO NP annealing in the colloidal state. Electrical, structural, and optical analyses demonstrated that the colloidal-annealing of ZnO NPs effectively passivated the defects and simultaneously shifted their band diagram; therefore, colloidal-annealing is a more favorable method as compared to conventional film-annealing. These CQD solar cells based on colloidal-annealed ZnO NPs exhibited efficient charge extraction, reduced recombination and achieved an enhanced power conversion efficiency (PCE) of 9.29%, whereas the CQD solar cells based on ZnO NPs without annealing had a PCE of 8.05%. Moreover, the CQD solar cells using colloidal-annealed ZnO NPs exhibited an improved air stability with 98% retention after 120 days, as compared to that of CQD solar cells using non-annealed ZnO NPs with 84% retention.

Herein, we introduce a unique nanomaterial chemical transformation (NCT) process whereby selenium (Se) anion is introduced to colloidal silver (Ag) nanocrystals (NCs) to create nanostructured silver selenide (Ag2Se). We present a complete suite of material characterizations, including chemical, structural, optical, and electrical characterizations to understand the details of the transformation process. The Ag2Se thin-films obtained through the NCT process exhibit degenerately-doped n-type semiconductor characteristics with high carrier mobility. We discuss the chemical mechanism that drives the material transformation and elucidates the origin of doping in these semiconducting thin-films. We also demonstrate the robustness of Ag2Se thin-films toward mechanical strain and temperature cycling stress for flexible device applications. We believed that the NCT process demonstrated here is of wide applicability to other material systems which can open up new avenues for solid-state chemistry and device engineering research.