Silicon Nanoscale Materials : From Theoretical Simulations to Photonic Applications

Peer reviewe

Fully inorganic oxide-in-oxide ultraviolet nanocrystal light emitting devices

The development of integrated photonics and lab-on-a-chip platforms for environmental and biomedical diagnostics demands ultraviolet electroluminescent materials with high mechanical, chemical and environmental stability and almost complete compatibility with existing silicon technology. Here we report the realization of fully inorganic ultraviolet light-emitting diodes emitting at 390 nm with a maximum external quantum efficiency of ~0.3%, based on SnO(2) nanoparticles embedded in SiO(2) thin films obtained from a solution-processed method. The fabrication involves a single deposition step onto a silicon wafer followed by a thermal treatment in a controlled atmosphere. The fully inorganic architecture ensures superior mechanical robustness and optimal chemical stability in organic solvents and aqueous solutions. The versatility of the fabrication process broadens the possibility of optimizing this strategy and extending it to other nanostructured systems for designed applications, such as active components of wearable health monitors or biomedical devices

Biomimetic route to hybrid nano-Composite scaffold for tissue engineering

Hydroxyapatite-poly(vinyl) alcohol-protein composites have been prepared by a biomimetic route at ambient conditions, aged for a fortnight at 30±2°C and given a shape in the form of blocks by thermal cycling. The structural characterizations reveal a good control over the morphology mainly the size and shape of the particles. Initial mechanical studies are very encouraging. Three biocompatibility tests, i.e., hemocompatibility, cell adhesion, and toxicity have been done from Shree Chitra Tirunal, Trivandrum and the results qualify their standards. Samples are being sent for more biocompatibility tests. Optimization of the blocks in terms of hydroxyapatite and polymer composition w.r.t the applications and its affect on the mechanical strength have been initiated. Rapid prototyping and a β-tricalcium – hydroxyapatite combination in composites are in the offing

Optical and Structural Properties of Si Nanocrystals in SiO2 Films

Optical and structural properties of Si nanocrystals (Si-nc) in silica films are described. For the SiOx (x <2) films annealed above 1000 degrees C, the Raman signal of Si-nc and the absorption coefficient are proportional to the amount of elemental Si detected by X-ray photoelectron spectroscopy. A good agreement is found between the measured refractive index and the value estimated by using the effective-medium approximation. The extinction coefficient of elemental Si is found to be between the values of crystalline and amorphous Si. Thermal annealing increases the degree of Si crystallization; however, the crystallization and the Si-SiO2 phase separation are not complete after annealing at 1200 degrees C. The 1.5-eV PL quantum yield increases as the amount of elemental Si decreases; thus, this PL is probably not directly from Si-nc responsible for absorption and detected by Raman spectroscopy. Continuous-wave laser light can produce very high temperatures in the free-standing films, which changes their structural and optical properties. For relatively large laser spots, the center of the laser-annealed area is very transparent and consists of amorphous SiO2. Large Si-nc (up to ~300 nm in diameter) are observed in the ring around the central region. These Si-nc lead to high absorption and they are typically under compressive stress, which is connected with their formation from the liquid phase. By using strongly focused laser beams, the structural changes in the free-standing films can be made in submicron areas.Peer reviewe

High-resolution Observation of Nucleation and Growth Behavior of Nanomaterials on Graphene

학위논문 (석사)-- 서울대학교 대학원 : 재료공학부, 2014. 2. 김미영.Studying nucleation and growth has been one of the major goals in materials science. Fundamental understanding of initial growth is essential for fabrication of nanomaterials with desired physical properties. Consequently, atomic level investigation on as-grown nuclei and local atomic arrangements around defects is required. Such high-resolution study along with crystallographic analysis could be performed using transmission electron microscopy (TEM). Here, we report on atomic-resolution observation of initial growth behavior using TEM by growing nanomaterials directly on graphene. Graphene exhibiting excellent electron beam transparency and high mechanical strength is an ideal supporting layer for TEM measurements by minimizing the background signal from the underlying membrane. In addition to merely sustaining nanomaterials as a support, graphene can be further used as a substrate for nanomaterials growth. The crystalline nature of graphene along with its electron beam transparency ultimately enables direct imaging of nanomaterials and allows us to systematically investigate the initial growth mechanisms. Using direct growth and imaging method, we could clearly observe the initial states of Zinc oxide (ZnO) nanomaterials. This enabled the observation of the transition in crystal structure of ZnO nuclei along with their orientational relationship with graphene. Furthermore, formation of various defects during nanomaterial growth could be clearly visualized with atomic-resolution. More generally, we believe that this simple technique may be readily expanded to investigate the growth mechanisms of many other nanomaterials on various two-dimensional layered substrates.Chapter 1 Introduction 1 1.1 Importance of studying the initial growth behavior of nanomaterials 1 1.2 Materials of interest 10 1.2.1 ZnO 10 1.2.2 Graphene 12 Chapter 2 Literature Survey 17 2.1 Direct crystal growth on prefabricated thin membranes for TEM measurements 17 2.2 Graphene as a supporting layer for TEM measurements 23 Chapter 3 Experimental Method 30 3.1 Growth of CVD graphene 30 3.2 Transfer of CVD graphene onto a TEM grid 31 3.3 Growth of ZnO nanomaterials on graphene placed on a TEM grid 35 3.4 Structural characterization of ZnO nanomaterials 35 3.4.1 SEM 35 3.4.2 TEM 36 3.5 First principles calculations 39 Chapter 4 Results and Discussions 40 4.1 Feasibility of the experimental technique for ZnO growth and TEM measurements 40 4.2 Overall growth behavior of ZnO nanomaterials on graphene 43 4.2.1 Nucleation and cluster growth stages 43 4.2.2 Postnucleation stages: coalescence 45 4.2.3 Postnucleation stages: formation of epitaxial relationship 46 4.3 Crystal structure of ZnO in the initial stage of growth 51 4.3.1 Formation of rocksalt ZnO structure 51 4.3.2 Nucleation barrier in heteroepitaxial growth 52 4.4 Disorder in the atomic arrangements around defects 58 4.5 Diverse applicability of the experimental technique for the initial growth study: ZnO nanomaterials on h-BN 63 4.5.1 Hexagonal boron nitride 63 4.5.2 Initial growth behavior of ZnO nanomaterials on h-BN 64 Chapter 5 Conclusions 73 References 74 Abstract 81Maste

Ge Nanowires Anode sheathed with Amorphous Carbon for Rechargeable Lithium batteries

Interdisciplinary School of Green EnergyThe composite electrode composed of single crystalline Ge NWs sheathed with amorphous carbon showed excellent electrochemical properties of large reversible capacity, high coulombic efficiency, excellent rate capability and stable cycle performance. c-Ge NWs synthesized by using thermal decomposition of C2H2 gas at 700 °C under Ar atmosphere after SLS (solution-liquid-solid) growth were found to have good performance during cycling with Li. The rate capability for charging was shown reversible capacity of 963 mAh/g with a coulombic efficiency of 90% and 700 mAh/g at the rate of 6C (= 4800mA/g). Capacity retention after 100 cycles was 72% at the rate of 0.5C. The improved electrochemical performance of c-Ge-NWs fabricated in our experiment was attributed to the formation of amorphous Ge NWs during cycling and a homogenous carbon coating on Ge NWs. Thus, these results suggest that the use of nanowires structure can be promising for alloy anode materials in lithium ion batteries

Nanostructured Vanadium Oxides for Energy Conversion and Conservation

Increasing worldwide energy consumption has imposed strain on global natural resources. There are a number of sustainable approaches to meeting the energy needs, and many of them can be broadly classified into two main categories: energy conservation and energy conversion. Buildings consume a tremendous portion of the total energy used worldwide, and conserving some of this energy by using technological advancements, such as thermochromic films, holds promise for reducing energy footprints. There is also an urgent need to tap into renewable sources of energy. Solar energy, if harvested and stored appropriately, could easily meet the energy needs of the world’s population. One route to solar energy conversion involves conversion of solar energy to chemical energy by splitting water to produce hydrogen and oxygen; hydrogen could then be stored and combusted to release the energy. Unfortunately, this has been a challenge for the scientific community since it requires the concerted transfer of four holes and four electrons. In this work, we explore the use of vanadium oxides for both energy conservation and energy conversion applications. First, for energy conservation, vanadium dioxide (VO2) has been explored for use within a dynamically switchable thermochromic thin film. VO2 is able to block infrared light at high temperatures while allowing it to transmit at lower temperatures. Amorphous silica shells have been constituted around VO2 nanowires using a modified Stӧber method in order to embed these films onto a glass matrix. By controlling particle size, it is possible to achieve significant modulation of near-infrared wavelengths of the electromagnetic spectrum. Addressing energy conversion, a tunable platform has been developed with the intent of harvesting solar energy. Heterostructures linking CdSe/β-PbxV2O5 and CdS/β-PbxV2O5 have been synthesized through two different methods, linker-assisted assembly and successive ionic layer adsorption and reaction. Hard X-ray photoelectron spectroscopy and transient absorption spectroscopy studies show thermodynamic alignment of the energy levels and suggest hole transfer from the photoexcited semiconductor quantum dots to the mid-gap states of β-PbxV2O5. The CdS heterostructures show an improved alignment of their valence band with the mid-gap state of β-PbxV2O5. The tunable heterostructure platform holds promise for water splitting photocatalysts