EFFECTS OF SURFACE STATES, DEFECTS AND DOPANTS ON THE OPTICAL AND MAGNETIC PROPERTIES OF LOW-DIMENSIONAL MATERIALS

Nanomaterials have attracted the attention of researchers from various fields due to their unique features (that are otherwise absent in the bulk) such as quantum confinement, high surface to volume ratio, ability for surface modification etc. Since the discovery of fullerenes and carbon nanotubes, several synthesis techniques have been developed for nanomaterial growth. However, different control parameters in different synthesis techniques often result in nanostructures with varying defects that may alter their fundamental behavior. Such defects or disorder in the crystal lattice can lead to the disruption of lattice symmetry. The defect-induced symmetry lowering (or breaking) effects play a vital role in the determination of fundamental material characteristics. Thus, it is very important to characterize the defects in order to understand their effects on the nanomaterial properties. This thesis describes the effects of defects in low dimesional systems such as ZnO nanowires, graphene and carbon nanotubes are studied. Firstly, it describes the synthesis and characterization of ZnO nanostructures and discusses the effects of surface states, defects and dopants on their optical and magnetic properties. An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpoint its origin and mechanism were lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab-initio calculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions. For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations. In the case of grapheme, we have observed new combination modes in the range from 1650 to 2300 cm−1 in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. A peak at 1860 cm−1 (iTALO−) is observed due to a combination of the in-plane transverse acoustic (iTA) and the longitudinal optical (LO) phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent for bulk graphite. The overtone of the out-of-plane transverse optical (oTO) phonon at 1750 cm−1, also called the M band, is suppressed for both SLG and IBLG. In addition, two previously unidentified modes at 2200 and 1880 cm−1 are observed in SLG. The 2220 cm−1 (1880 cm−1) mode is tentatively assigned to the combination mode of in-plane transverse optical (iTO) and TA phonons (oTO+LO phonons) around the K point in the graphene Brillouin zone. Finally, the peak frequency of the 1880 (2220) cm−1 mode is observed to increase (decrease) linearly with increasing graphene layers. Finally, we find that the high curvature in sub-nm SWCNTs leads to (i) an unusual S-like dispersion of the G-band frequency due to perturbations caused by the strong electron-phonon coupling, (ii) an activation of diameter-selective intermediate frequency modes that are as intense as the radial breathing modes (RBMs), and (iii) a clear observation of the IR modes

Ultrasensitive Immunosensing Platform Based on Analyte Induced Disruption of Luminescence Quenching (AIDLuQ)

In this study, we design an extremely fast and sensitive immunosensing platform using graphene as the sensing platform. A solution containing a mixture of graphene nanoplatelets and gold nanoparticles was coated on to a copier paper using a spray gun to form a uniform coating. Fluorescent quantum dots (QDs) functionalized with antibodies (Ab) were drop casted on to this platform, whose fluorescence was quenched by the graphene on the graphene/gold paper. With the addition of the antigen to this graphene/gold-QD-Ab complex, a disruption of quenching was observed, and the fluorescence intensity increased with increasing concentration of the antigen. A detection limit of as low as 10 fM was obtained for the detection of human Immunoglobulin G (IgG)

Polymer-nanocarbon composites, methods of making composites, and energy storage devices including the composite

Embodiments of the present disclosure, in one aspect, relate to composites including a carbon nanomaterial having a redox-active material, such as a polymer containing redox groups, disposed on the carbon nanomaterial, methods of making the composite, methods of storing energy, and the like

Spectroscopic Insights into the Nano-Bio Interface

Engineered nanomaterials (ENMs) strongly interact with biomolecules due to their unique physicochemical properties. From the standpoint of nanotoxicity, it is imperative to achieve a comprehensive understanding of various nano-bio interactions to ultimately design benign ENMs that do not elicit adverse physiological responses. Spectroscopic tools are ideal for elucidating the underlying biophysical mechanisms of nano-bio interactions. In this chapter, we review spectroscopy techniques, such as Raman, infrared, circular dichroism (CD), and hyperspectral imaging, to illuminate the nano-bio interface. Particularly, we discuss the role of spectroscopic tools in gaining a fundamental understanding of the formation and influence of protein corona on ENM physiological responses

Variations in biocorona formation related to defects in the structure of single walled carbon nanotubes and the hyperlipidemic disease state

Ball-milling utilizes mechanical stress to modify properties of carbon nanotubes (CNTs) including size, capping, and functionalization. Ball-milling, however, may introduce structural defects resulting in altered CNT-biomolecule interactions. Nanomaterial-biomolecule interactions result in the formation of the biocorona (BC), which alters nanomaterial properties, function, and biological responses. The formation of the BC is governed by the nanomaterial physicochemical properties and the physiological environment. Underlying disease states such as cardiovascular disease can alter the biological milieu possibly leading to unique BC identities. In this ex vivo study, we evaluated variations in the formation of the BC on single-walled CNTs (SWCNTs) due to physicochemical alterations in structure resulting from ball-milling and variations in the environment due to the high-cholesterol disease state. Increased ball-milling time of SWCNTs resulted in enhanced structural defects. Following incubation in normal mouse serum, label-free quantitative proteomics identified differences in the biomolecular content of the BC due to the ball-milling process. Further, incubation in cholesterol-rich mouse serum resulted in the formation of unique BCs compared to SWCNTs incubated in normal serum. Our study demonstrates that the BC is modified due to physicochemical modifications such as defects induced by ball-milling and physiological disease conditions, which may result in variable biological responses

Defect Engineered 2D Materials for Energy Applications

Two-dimensional (2D) materials display unique properties that could be useful for many applications ranging from electronics and optoelectronics to catalysis and energy storage. Entropically necessary defects are inevitably present in 2D materials in the form of vacancies and grain boundaries. Additional defects, such as dopants, may be intentionally introduced to tune the electronic structure of 2D materials. While defects are often perceived as performance limiters, the presence of defects and dopants in 2D materials results in new electronic states to endow unique functionalities that are otherwise not possible in the bulk. In this chapter, we review defect-induced phenomena in 2D materials with some examples demonstrating the relevance of defects in electronic and energy applications. In particular, we present how the (i) N-dopant configuration in graphene changes the electron-phonon interactions, (ii) zigzag defects and edges in graphene increase the quantum capacitance to improve energy density of graphene-based supercapacitors, and (iii) charged grain boundaries in exfoliated Bi2Te3 preferentially scatter low-energy electrons and holes to enhance the thermoelectric performance

Preferential scattering by interfacial charged defects for enhanced thermoelectric performance in few-layered n-type Bi2Te3

Over the past two decades several nano-structuring methods have helped improve the figure of merit (ZT) in the state-of-the art bulk thermoelectric materials. While these methods could enhance the thermoelectric performance of p-type Bi2Te3, it was frustrating to researchers that they proved ineffective for n-type Bi2Te3 due to the inevitable deterioration of its thermoelectric properties in the basal plane. Here, we describe a novel chemical-exfoliation spark-plasma-sintering (CE-SPS) nano-structuring process, which transforms the microstructure of n-type Bi2Te3 in an extraordinary manner without compromising its basal plane properties. The CE-SPS processing leads to preferential scattering of electrons at charged grain boundaries, and thereby increases the electrical conductivity despite the presence of numerous grain boundaries, and mitigates the bipolar effect via band occupancy optimization leading to an upshift (by ~ 100 K) and stabilization of the ZT peak over a broad temperature range of ~ 150 K