Designing Nanomaterials For Electronic And Optoelectronic Devices Through Charge Carrier Control

Colloidal semiconductor nanocrystals (NCs) have been shown to be promising materials for electronic and optoelectronic device applications because of their unique size-dependent properties and low-cost solution processability. However, the integration of these materials into devices has been challenging due to a lack of available methods to: 1) accurately control charge carrier statistics, such as majority carrier type and concentration, and carrier mobilities, and 2) efficiently passivate surface defects inherent in NC materials arising from their high surface-volume ratio. In this thesis, we study the fundamental physics of charge carriers paramount for device application. Then, we introduce several measurement techniques to characterize the type, concentration, and mobility of charge carriers and the density and energy of surface states. Lastly, we propose a novel, systematic, and rational method to engineer those properties, in order to design high performance electronic and optoelectronic nanostructured devices. We develop stoichiometry control method through thermal evaporation or solution based atomic layer deposition to precisely control the electronic and optoelectronic properties of nanocrystals. We demonstrate that remote doping in nanostructured device is effective and a promising route to realizing high mobility and reducing scattering, in contrast to commonly pursued substitutional doping methods. Thermal diffusion doping process to passivate the trap states and the use of small ligands to enhance the electronic coupling are introduced. In addition, we emphasize the important role of the metal-semiconductor interface and semiconductor-gate dielectric layer, to enhance charge injection and prevent charge trapping, respectively. Through the careful engineering of the interface and junction, as well as the precise charge carrier statistics and trap states controls, we design and fabricate low cost, high performance nanocrystal thin film field-effect transistors, photodetectors, and solar cells. Finally, we introduce novel techniques, correlated scanning photocurrent microscopy and scanning confocal photoluminescence measurement system, that can explore the photoelectric and photophysical properties of semiconductor structures and devices

Photocatalytic Hydrogen Evolution from Sub-Stoichiometric Colloidal WO3-xNanowires

We report direct photocatalytic hydrogen evolution from substoichiometric highly reduced tungsten oxide (WOx) nanowires (NWs) using sacrificial alcohol. WOx NWs are synthesized via nonaqueous colloidal synthesis with a diameter of about 4 nm and an average length of about 250 nm. As-synthesized WOx NWs exhibit a broad absorption across the visible to infrared regions attributed to the presence of oxygen vacancies. The optical band gap is increased in these WOx NWs compared to stoichiometric bulk tungsten oxide (WO3) powders as a result of the Burstein\u2013Moss shift. As a consequence of this increase, we demonstrate direct photocatalytic hydrogen production from WOx NWs through alcohol photoreforming. The stable H2 evolution on platinized WOx NWs is observed under conditions in which platinized bulk WO3 and bulk WO2.9 powders either do not show activity or show very low rates, suggesting that increased surface area and specific exposed facets are key for the improved performance of WOx NWs. This work demonstrates that control of size and composition can lead to unexpected and beneficial changes in the photocatalytic properties of semiconductor materials

Bio-anthropological Studies on Human Skeletons from the 6th Century Tomb of Ancient Silla Kingdom in South Korea

In November and December 2013, unidentified human skeletal remains buried in a mokgwakmyo (a traditional wooden coffin) were unearthed while conducting an archaeological investigation near Gyeongju, which was the capital of the Silla Kingdom (57 BCE– 660 CE) of ancient Korea. The human skeletal remains were preserved in relatively intact condition. In an attempt to obtain biological information on the skeleton, physical anthropological, mitochondrial DNA, stable isotope and craniofacial analyses were carried out. The results indicated that the individual was a female from the Silla period, of 155 ± 5 cm height, who died in her late thirties. The maternal lineage belonged to the haplogroup F1b1a, typical for East Asia, and the diet had been more C3- (wheat, rice and potatoes) than C4-based (maize, millet and other tropical grains). Finally, the face of the individual was reconstructed utilizing the skull (restored from osseous fragments) and three-dimensional computerized modelling system. This study, applying multi-dimensional approaches within an overall bio-anthropological analysis, was the first attempt to collect holistic biological information on human skeletal remains dating to the Silla Kingdom period of ancient Korea

Guidelines for the use and interpretation of assays for monitoring autophagy (3rd edition)

In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. For example, a key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process versus those that measure fl ux through the autophagy pathway (i.e., the complete process including the amount and rate of cargo sequestered and degraded). In particular, a block in macroautophagy that results in autophagosome accumulation must be differentiated from stimuli that increase autophagic activity, defi ned as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (inmost higher eukaryotes and some protists such as Dictyostelium ) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the fi eld understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. It is worth emphasizing here that lysosomal digestion is a stage of autophagy and evaluating its competence is a crucial part of the evaluation of autophagic flux, or complete autophagy. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. Along these lines, because of the potential for pleiotropic effects due to blocking autophagy through genetic manipulation it is imperative to delete or knock down more than one autophagy-related gene. In addition, some individual Atg proteins, or groups of proteins, are involved in other cellular pathways so not all Atg proteins can be used as a specific marker for an autophagic process. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field

Wearable sensors based on colloidal nanocrystals

Abstract In recent times, wearable sensors have attracted significant attention in various research fields and industries. The rapid growth of the wearable sensor related research and industry has led to the development of new devices and advanced applications such as bio-integrated devices, wearable health care systems, soft robotics, and electronic skins, among others. Nanocrystals (NCs) are promising building blocks for the design of novel wearable sensors, due to their solution processability and tunable properties. In this paper, an overview of NC synthesis, NC thin film fabrication, and the functionalization of NCs for wearable applications (strain sensors, pressure sensors, and temperature sensors) are provided. The recent development of NC-based strain, pressure, and temperature sensors is reviewed, and a discussion on their strategies and operating principles is presented. Finally, the current limitations of NC-based wearable sensors are discussed, in addition to methods to overcome these limitations

Remote Doping and Schottky Barrier Formation in Strongly Quantum Confined Single PbSe Nanowire Field-Effect Transistors

We report studies of charge injection and transport in ambipolar, predominantly n-type, and unipolar p-type single, strongly quantum confined PbSe nanowire (NW) field effect transistors (FETs). The PbSe NW FETs operate as Schottky barrier FETs in which the Fermi level is pinned near midgap, consistent with the low ionicity of PbSe, and is nearly invariant with semiconductor doping. Electron and hole mobilities increase monotonically with decreasing temperature, dominated at high temperature by electron–phonon scattering with no evidence of scattering at low temperatures. Transport in NWs is consistent with their single crystalline nature. Surface oxygen used to dope the NWs acts remotely, providing a promising route to dope nanostructures

Diketopyrrolopyrrole-Based π-Bridged Donor–Acceptor Polymer for Photovoltaic Applications

We report the synthesis, properties, and photovoltaic applications of a new conjugated copolymer (C12DPP-π-BT) containing a donor group (bithiophene) and an acceptor group (2,5-didodecylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione), bridged by a phenyl group. Using cyclic voltammetry, we found the energy levels of C12DPP-π-BT are intermediate to common electron donor and acceptor photovoltaic materials, poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. Whereas P3HT and PCBM are exclusively electron donating or accepting, we predict C12DPP-π-BT may uniquely serve as either an electron donor or an acceptor when paired with PCBM or P3HT forming junctions with large built-in potentials. We confirmed the ambipolar nature of C12DPP-π-BT in space charge limited current measurements and in C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT bulk heterojunction solar cells, achieving power conversion efficiencies of 1.67% and 0.84%, respectively, under illumination of AM 1.5G (100 mW/cm²). Adding diiodooctane to C12DPP-π-BT:PCBM improved donor–acceptor inter-mixing and film uniformity, and therefore enhanced charge separation and overall device efficiency. Using higher-molecular-weight polymer C12DPP-π-BT in both C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT devices improved charge transport and hence the performance of the solar cells. In addition, we compared the structural and electronic properties of C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT blends, representing the materials classes of polymer:fullerene and polymer:polymer blends. In C12DPP-π-BT:PCBM blends, higher short circuit currents were obtained, consistent with faster charge transfer and balanced electron and hole transport, but lower open circuit voltages may be reduced by trap-assisted recombination and interfacial recombination losses. In contrast, C12DPP-π-BT:P3HT blends exhibit higher open circuit voltage, but short circuit currents were limited by charge transfer between the polymers. In conclusion, C12DPP-π-BT is a promising material with intrinsic ambipolar characteristics for organic photovoltaics and may operate as either a donor or acceptor in the design of bulk heterojunction solar cells

Effects of Sc and Be Microalloying Elements on Mechanical Properties of Al-Zn-Mg-Cu (Al7xxx) Alloy

We demonstrate via comprehensive microstructural investigation the effects of Sc and Be microalloying on the mechanical properties of Al-Zn-Mg-Cu-based alloys, where Sc microalloying enhances the tensile properties of an Al-9.0Zn-3.0Mg-3.0Cu alloy from 645 MPa (εf = ~6%) to 672 MPa (εf = ~8%). In contrast, simultaneous microalloying with Sc and Be reduces the mechanical strength of a synthesized Al alloy to 654 MPa (εf = ~8%). Comprehensive microstructural investigation revealed that Sc microalloying leads to Al grain refinement, the formation of hardening (MgZn2, Al3M) phases, and an increase in the solid solution of Al. Additional Be microalloying also enhances the formation of MgZn2 phase, while Al3M (M: Zr, Sc) type phases are restrained from forming in Al grains. Furthermore, solid solution in Al grains is reduced by the trace addition of Be microalloying, resulting in an increase in large intermetallic compounds at Al grain boundaries

Ultrasensitive Near‐Infrared InAs Colloidal Quantum Dot‐ZnON Hybrid Phototransistor Based on a Gradated Band Structure

Abstract Amorphous metal oxide semiconductor phototransistors (MOTPs) integrated with colloidal quantum dots (QDs) (QD‐MOTPs) are promising infrared photodetectors owing to their high photoconductive gain, low off‐current level, and high compatibility with pixel circuits. However, to date, the poor mobility of conventional MOTPs, such as indium gallium zinc oxide (IGZO), and the toxicity of lead (Pb)‐based QDs, such as lead sulfide and lead selenide, has limited the commercial applications of QD‐MOTPs. Herein, an ultrasensitive QD‐MOTP fabricated by integrating a high‐mobility zinc oxynitride (ZnON)–based MOTP and lead‐free indium arsenide (InAs) QDs is demonstrated. A new gradated bandgap structure is introduced in the InAs QD layer that absorbs infrared light, which prevents carriers from moving backward and effectively reduces electron–hole recombination. Chemical, optical, and structural analyses confirm the movement of the photoexcited carriers in the graded band structure. The novel QD‐MOTP exhibits an outstanding performance with a responsivity of 1.15 × 105 A W−1 and detectivity of 5.32 × 1016 Jones at a light power density of 2 µW cm−2 under illumination at 905 nm

Additional file 1 of Ink-lithographic fabrication of silver-nanocrystal-based multiaxial strain gauge sensors through the coffee-ring effect for voice recognition applications

Additional file 1: Figure S1. HRTEM images of a as-synthesized (left) and NH4Br-treated (right) Ag NCs. b UV–vis absorbance spectra, c FT-IR absorption profiles, and d XRD patterns of the as-synthesized (black) and NH4Br-treated Ag NCs with continuously (blue) and alternately printed patterns (red). Figure S2. Profile of waveform employed for inkjet printing the ligand ink. Figure S3. Plot of the AFM data corresponding to the Fig. 1c results. Figure S4. Optical image for investigating contact angle of the ligand ink on the Ag NC thin films. Figure S5. Optical images of ligand-ink-treated Ag NC line patterns. a Continuously (left) and alternately printed (right) Ag NC line patterns. b Changes in line width of the Ag NC line patterns with different micro-spacings of the jetting droplets (scale bar = 50 μm). Figure S6. a Front- and b top view optical images of the multiaxial strain gauge sensors attached to the 0.6%-strain-curved structure. Figure S7. a Schematic of films with alternately printed Ag NC patterns subjected to bending at different rotations. b Detailed schematic of alternately printed Ag NC patterns. Detailed top-view schematics of changes in the c alternately and d continuously printed Ag NC patterns with bending. Figure S8. a Gauge factor of alternately- (black dots) and continuously printed Ag NC patterns upon high bending strain. b Cycle test of alternately printed Ag NC patterns (upper = 1% strain; lower = 5% strain). Figure S9. Hysteresis plot of both Ag NC patterns with 1.0 % strain applied (filled circles or triangles) and released (vacant circles or triangles)