Synthesis and characterization of copper nanoparticles and copper-polymer nanocomposites for plasmonic photovoltaic applications

Deposition techniques for the fabrication of metal nanostructures influence their morphological properties, which in turn control their optical behavior. Here, copper nanoparticles (np-Cu’s) were grown using a deposition system that was specifically set up during this work, and is based on a radio frequency (RF) sputtering source that can operate at high temperature and under bias voltage. The effect of deposition conditions (RF power, chamber pressure and substrate bias voltage) on RF sputtered np-Cu’s using RF sputtering has been studied. The study included a comparison between the morphological and optical properties of as-grown np-Cu’s and thermally treated samples. The characterization of np-Cu’s is carried out by atomic force microscopy, UV-visible transmission spectrophotometry, scanning electron microscopy and scanning near field optical microscopy (SNOM) techniques. The results of the experiment showed that the combined effects of low RF power (25 W – 75 W), high chamber pressure (17 Pa – 23 Pa) and substrate DC bias voltage (300 V – 400 V) are required for obtaining dispersed np-Cu’s. Under these conditions, copper nanoparticles grow by aggregation of initial island nuclei due to a reduction in sputtering rate. Significantly, higher dispersed np-Cu’s are obtained when a set of samples grown at 25 W and 33 W RF power is subjected to thermal treatment in an oxygen-free glove box. Optical properties of np-Cu’s show improvement in the visible region (535nm – 580 nm) related to transmission enhancement in as-deposited samples and plasmonic enhancement in thermally treated ones. Furthermore, an approach to determine the position of the np-Cu induced scattered wave was explored using SNOM (x, z) measurements. In bare np-Cu’s the path length of the scattered wave is further from the np surface, measured orthogonally. We demonstrated experimentally a method that uses an SiO2 thin film as a spacer to broaden the scattered wave up to 500 nm from the np-Cu/SiO2 composite surface. The study provides an improved insight that helps to understand the physical mechanisms that may hinder the expected performance in plasmonic solar cells. With these results, the potential of incorporating np-Cu’s in plasmonic thin film solar cell structures looks very promising

Nanoscale Thermal and Electronic Properties of Thin Films of Graphene and Organic Polyradicals

Ultrathin film materials have attracted significant attention in light of their potential applications in very large scale integrated electronics and data storage. For instance, the amount of data that can be addressed and stored in a memory device scales inversely with the thinness of the active layer of these components. In our thesis, we have developed a suite of scanning-probe and nano-optical techniques focused on understanding the electronic surface properties and the thermal conductivity of ultrathin materials. We discuss a few specific examples in which we applied these techniques towards improved performance of thin films of graphene and organic polyradicals towards specific applications. A new nano-optical technique, near field scanning thermoreflectance imaging (NeSTRI) has been invented and implemented by us for contactless imaging the thermal properties of graphene thin films and poly-[1,5-diisopropyl-3-(cis-5-norbornene-exo-2,3-dicarboxiimide)-6-oxoverdazyl] (P6OV). We utilized Kelvin-probe force microscopy for understanding the surface properties of copper nanoparticle decorated graphene thin films with superior electrical conductivity, and to design energy level matched flash memory devices from P6OV. Our work has led to deeper understanding of the nanoscale thermal and electronic properties of thin films of graphene and organic polyradicals and the interplay between their performance and fabrication parameters

A contactless scanning near-field optical dilatometer imaging the thermal expansivity of inhomogeneous 2D materials and thin films at the nanoscale

To date, there are very few experimental techniques, if any, that are suitable for the purpose of acquiring, with nanoscale lateral resolution, quantitative maps of the thermal expansivity of 2D materials and thin films, despite huge demand for nanoscale thermal management, for example in designing integrated circuitry for power electronics. Besides, contactless analytical tools for determining the thermal expansion coefficient (TEC) are highly desirable, because probes in contact with the sample significantly perturb any thermal measurements. Here, we introduce {\omega}-2{\omega} near-field thermoreflectance imaging, as an all-optical and contactless approach to map the TEC at the nanoscale with precision. Testing of our technique is performed on nanogranular films of gold and multilayer graphene (ML-G) platelets. Our method demonstrates that the TEC of Au is higher at the metal-insulator interface, with an average of (17.12 +/- 2.30)x10-6 K-1 in agreement with macroscopic techniques. For ML-G, the average TEC was (-5.77 +/- 3.79)x10-6 K-1 and is assigned to in-plane vibrational bending modes. A vibrational-thermal transition from graphene to graphite is observed, where the TEC becomes positive as the ML thickness increases. Our nanoscale method demonstrates results in excellent agreement with its macroscopic counterparts, as well as superior capabilities to probe 2D materials and interfaces

Graphene Thin Films and Graphene Decorated with Metal Nanoparticles

The electronic, thermal, and optical properties of graphene-based materials depend strongly on the fabrication method used and can be further manipulated through the use of metal nanoparticles deposited on the graphene surface. Metals that strongly interact with graphene such as Co and Ni can form strong chemical bonds which may significantly alter the band structure of graphene near the Dirac point. Weakly interacting metals such as Au and Cu can be used to induce shifts in the graphene Fermi energy, resulting in doping without significant alteration to the graphene band structure. The deposition and nucleation conditions such as deposition rate, annealing temperature and time, and annealing atmosphere can be used to control the size and distribution of metal nanoparticles. Under ideal conditions, self-assembled arrays of nanoparticles can be obtained on graphene-based films for use in new types of nano-devices such as evanescent waveguides

A Review of Three-Dimensional Scanning Near-Field Optical Microscopy (3D-SNOM) and Its Applications in Nanoscale Light Management

In this article, we present an overview of aperture and apertureless type scanning near-field optical microscopy (SNOM) techniques that have been developed, with a focus on three-dimensional (3D) SNOM methods. 3D SNOM has been undertaken to image the local distribution (within ~100 nm of the surface) of the electromagnetic radiation scattered by random and deterministic arrays of metal nanostructures or photonic crystal waveguides. Individual metal nanoparticles and metal nanoparticle arrays exhibit unique effects under light illumination, including plasmon resonance and waveguiding properties, which can be directly investigated using 3D-SNOM. In the second part of this article, we will review a few applications in which 3D-SNOM has proven to be useful for designing and understanding specific nano-optoelectronic structures. Examples include the analysis of the nano-optical response phonetic crystal waveguides, aperture antennae and metal nanoparticle arrays, as well as the design of plasmonic solar cells incorporating random arrays of copper nanoparticles as an optical absorption enhancement layer, and the use of 3D-SNOM to probe multiple components of the electric and magnetic near-fields without requiring specially designed probe tips. A common denominator of these examples is the added value provided by 3D-SNOM in predicting the properties-performance relationship of nanostructured systems