Academic literature on the topic 'Monomolecular films. Nanostructured materials. Gold'

The methods are presented for fabrication of new nanoscale-organized planar inorganic nanostructures, ultrathin polymeric and nanocomposite films on solid substrates with incorporated nanosized functional and structural building blocks. The methods are based on interfacial synthesis and self-assembly, DNA templating and scaffolding. Ultimately thin monomolecular and multilayer ordered stable polymeric and nanocomposite films containing incorporated ligand-stabilized gold nanoclusters, interfacially in-film grown metallic ( Au , Pd ) nanoparticles and organized low-dimensional nanostructures were formed. N-alkylated derivatives of poly(4-vinilpyridine) were synthesized and used as water-insoluble amphiphilic polycations to form organized polymeric Langmuir monolayers and novel planar DNA/amphiphilic polycation complexes at the air–aqueous DNA solution interface. The extended net-like and quasi-circular toroidal condensed conformations of deposited planar DNA/amphiphilic polycation complexes were obtained in dependence on the amphiphilic polycation monolayer state during the DNA binding. Planar DNA/amphiphilic polycation complexes were used as nanotemplates for fabrication of organized planar bio-organic–inorganic hybrid nanostructures with ordered nanophase inorganic components (quasi-one-dimensional arrays of semiconductor (CdS) and iron oxide nanoparticles and nanorods) organized in planar matrix of deposited DNA/amphiphilic polycation complex film. The formed nanostructures were characterized by atomic force microscopy and transmission electron microscopy techniques.

We have uniformly coated the cantilever of an atomic force microscope (AFM) with gold thin films. These films are nanostructured with thickness going from 19 to 62 nm. The resonance frequencies of this cantilever have been measured, before and after the Au coatings. Taking into account these frequencies and the vibrating beam theory, we determined the Young modulus of the Au films, obtaining E2 = 69.1 ± 2.6 GPa , i.e. about 12% lower than the respective bulk elastic modulus.

A structural and morphological study of nanostructured gold thin films prepared by pulsed laser deposition in the presence of several inert background gases (Ar, He, and N2) and at various pressures (from 10 mTorr to 1 Torr) and target-to-substrate distances (from 1 to 10 cm) is presented. Structural and morphological analyses were undertaken using semiquantitative x-ray diffraction, scanning tunneling microscopy, and transmission electron microscopy. For each set of deposition conditions, the kinetic energy of the neutral gold species [Au(I)] present in the plasma plume was determined by time-of-flight emission spectroscopy and used to characterize the plasma dynamics. It is shown that all films exhibit a transition from highly [111] oriented to polycrystalline as the Au(I) kinetic energy decreases. The polycrystalline phase ratio is close to 0% for Au(I) kinetic energy larger than approximately 3.0 eV/atom and approximately 86 ± 10% for Au(I) kinetic energy smaller than approximately 0.30 eV/atom, irrespective of the background gas atmosphere. The mean crystallite size of both phases and the mean roughness of the films also follow a unique relation with the Au(I) kinetic energy, independently of the nature of the background gas, and nanocrystalline films with crystallite size as small as 12 nm are obtained for Au(I) kinetic energy smaller than 0.3 eV/atom.

We have fabricated gold thin films by metal plasma ion deposition on silicon substrates. The roughness of these nanostructured films has been measured by scanning tunneling microscopy (STM) and we have determined the growth dynamics critical exponents. We have also measured the grain sizes as a function of the film thickness.

We have measured, at room temperature, the resistivity, the surface roughness and the lateral surface correlation lengths of nanostructured platinum and gold thin films. The films' thickness d, deposited by vacuum arc plasma, is in the range 1.31≤d≤11.66 nm for platinum and 1.77≤d≤10.46 nm for gold. A theoretical estimate of our experimental data has been made.

Dissertation (Ph.D.)--Worcester Polytechnic Institute.
Keywords: molecular scale devices; self-assembled monolayers; multilayered films; non-covalent self-assembly. Includes bibliographical references (leaves 402-423).

Thesis (M.S.)--Missouri University of Science and Technology, 2010.
Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed January 6, 2010) Includes bibliographical references.

Thesis (M. S.)--Materials Science and Engineering, Georgia Institute of Technology, 2008.
Committee Chair: Tsukruk, Vladimir; Committee Member: Kalaitzidou, Kyriaki; Committee Member: Valeria Milam.

Water drops that nucleate and grow over an evaporating polymer solution exposed to a current of moist air remain noncoalescent and self-assemble into close packed arrays. The hexagonally close packed, nearly monodisperse drops, eventually evaporate away, leaving a polymer film, with ordered array of pores. Meanwhile, typical breath figures or dew that form when moist air contacts cold surfaces involve coalescence-assisted growth of highly polydisperse, disordered array of water drops. This dissertation provides the first quantitative attempt aimed at the elucidation of the mechanism of the breath figure templated assembly of the ordered arrays of pores in polymer films. The creation and evolution of a population of close packed drops occur in response to the heat and mass fluxes involved in water droplet condensation and solvent evaporation. The dynamics of drop nucleation, growth, noncoalescence and self-assembly are modeled by accounting for various transport and thermodynamic processes. The theoretical results for the rate and extent of evaporative cooling and growth are compared with experiments. Further, the dissertation describes a rich array of experimental observations about water droplet growth, noncoalescence, assembly and drying that have not been reported in the published literature so far. The theoretical framework developed in this study allows one to rationalize and predict the structure and size of pores formed in different polymer-solvent systems under given air flow conditions. While the ordered arrays of water drops present an example of dynamics, growth and assembly of spherical particles, the study on colloidal gold nanorods focuses on the behavior of rodlike particles. A comprehensive set of theoretical arguments based on the shape dependent hydrodynamics of rods were developed and used for centrifugation-assisted separation of rodlike particles from nanospheres that are typical byproducts of seed mediated growth of nanorods. Since the efficiency of shape separation is assessed using UV-Vis-NIR spectroscopy and transmission electron microscopy (TEM), the present dissertation elucidates the shape dependent parameters that affect the optical response and phase behavior of colloidal gold nanorods. The drying of a drop of colloidal gold nanorods on glass slides creates coffee ring like deposits near the contact line, which is preceded by the formation of a liquid crystalline phase. The assemblies of rods on TEM grids are shown to be the result of equilibrium and non-equilibrium processes, and the ordered phases are compared with two dimensional liquid crystals. The methodology of pattern characterization developed in this dissertation is then used to analyze the structure of the exocuticle of iridescent beetle Chrysina gloriosa. The patterns were characterized using Voronoi analysis and the effect of curvature on the fractions on hexagonal order of tiles was determined. Further, these patterns were found to be analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. In summary, the dissertation provides the crucial understanding required for the widespread use of breath figure templated assembly as a method for manufacturing porous films, that requires only a drop of polymer solution (dilute) and a whiff of breath! Further, the dissertation establishes the physical basis and methodology for separating and characterizing colloidal gold nanorods. The dissertation also suggests the basis for the formation and structure of tiles that decorate the exoskeleton of an iridescent beetle Chrysina gloriosa.

Since the successful isolation of graphene in 2004, two-dimensional (2D) materials have become one of the hottest research fields in material science. My research is about two kinds of popular 2D materials--graphene and transition metal dichalcogenides (TMDCs). Making graphene into nanoribbons has been predicted and demonstrated to be an effective way to open a bandgap in this pristinely zero-bandgap 2D material. But the rough edge condition of etched graphene nanoribbons has always been a big issue adversely affecting electron transport performance. The electron mean free path of this kind of devices is usually way below the channel width. By using a dual-gate structure based on bilayer graphene/hexagonal boron nitride heterostructure, we found a way to form 300nm-wide conducting channels with high aspect ratio (>15) that can achieve ballistic transport, indicating perfect edge conditions. As the first star member of TMDCs family, monolayer MoS2 is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers. We conduct the first experimental study of the piezoelectric properties of two-dimensional MoS2 and show that cyclic stretching and releasing of thin MoS2 flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90 degrees. Transport measurements show a strong piezotronic effect in single-layer MoS2, but not in bilayer and bulk MoS2. Monolayer WSe2, another popular TMDC, has also attracted much recent attention. In contrast to the initial understanding, the minima of the conduction band are predicted to be spin split. Because of this splitting and the spin-polarized character of the valence bands, the lowest-lying excitonic states in WSe2 are expected to be spin-forbidden and optically dark. We show how an in-plane magnetic field can brighten the dark excitonic states and allow their properties to be revealed experimentally in monolayer WSe2. In particular, precise energy levels for both the neutral and charged dark excitons were obtained. Greatly increased emission and valley lifetimes were observed for the brightened dark states as a result of their spin configuration.

Thesis (Ph. D.)--Florida State University, 2005.
Advisor: Dr. Peng Xiong, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Sept. 19, 2005). Document formatted into pages; contains xvii, 119 pages. Includes bibliographical references.

Atomically thin two-dimensional materials, such as graphene and semiconductor transition metal dichalcogenides (TMDCs), exhibit remarkable and desirable optical and electronic properties. This dissertation focuses on the excitonic properties of monolayer TMDCs taken first in isolation and then in contact with another material. We begin with a study of the exciton binding energy in two monolayer TMDCs, WS₂ and MoS₂. We observe excited states of the exciton by two different optical spectroscopy techniques: reflectance contrast and photoluminescence excitation (PLE) spectroscopy. We fit a hydrogenic model to the energies associated with the excited states and infer a binding energy, which is an order of magnitude higher than the bulk material. In the second half of this work, we study two types of two-dimensional vertical heterostructures. First, we investigate heterostructures composed of monolayer WS₂ partially capped with graphene one to four layers thick. Using reflectance contrast to measure the spectral broadening of the excitonic features, we measure the decrease in the coherence lifetime of the exciton in WS₂ due to charge and energy transfer when in contact with graphene. We then compare our results with the exciton lifetime in MoS₂/WS₂ and MoSe₂/WSe₂ heterostructures. In TMDC/TMDC heterostructures, the decrease in exciton lifetime is twice that in WS₂/graphene heterostructures and due predominantly to charge transfer between the layers. Finally, we probe the band alignment in MoS₂/WS₂ heterostructures using scanning tunneling microscopy (STM) and spectroscopy (STS).We confirm the monolayer band gaps and the predicted type II band alignment in the heterostructure. Drawing from all the research presented, we arrive at a favorable conclusion about the viability of TMDC based devices.

Nano-materials are interesting material category with a single unit size between 1 and 1000 nanometers and possess unique mechanical, electrical, optical, and other physical properties that make them stand out from ordinary materials. With increasing demand for reduced size of electronic devices and integrated micro/nano-electro-mechanical systems (MEMS / NEMS), there is a high driving force in scientific research and technological advancement in nanotechnology. My research is about two popular novel nanomaterials: carbon nanotubes (1-dimensional material) and thin-layer transition metal dichalcogenides (2-dimensional materials). My first research direction is about the characterization of electrical properties of carbon nanotubes and using them as bio-sensors. Carbon nanotubes (CNTs), in general, are a material of great interest for many applications since their first discovery in 1991 [1], due to their unique structure, extraordinary electrical and mechanical properties, and unusual chemical properties. High-throughput fabrication of carbon nanotube field effect transistors (CNTFETs) with uniform properties has been a challenge since they were first fabricated in 1998. We invent a novel fabrication method to produce a 1×1 cm2 chip with over 700 CNTFETs fabricated around one single carbon nanotube. This large number of devices allows us to study the stability and uniformity of CNTFET properties. We grow flow-aligned CNTs on a SiO2/Si substrate by chemical vapor deposition and locate a single long CNT (as long as 1 cm) by scanning electron microscopy. Two photolithography steps are then used, first to pattern contacts and bonding pads, and next to define a mask to ‘burn’ away additional nanotubes by oxygen plasma etch. A fabrication yield of ~72% is achieved. The authors present statistics of the transport properties of these devices, which indicates that all the CNTFETs share the same threshold voltage, and similar on-state conductance. These devices are then used to measure DNA conductance by connecting DNA molecule of varying lengths to lithographically cut CNTFETs. While one single carbon nanotube is considered 1-dimensional material because it only has one side with “non-nano” length, the thin-layer transition metal dichalcogenides (TMDCs) are called the 2-dimensional materials since they have two sides of normal lengths and the other side of atomic size. Atomically thin materials such as graphene and semiconducting transition metal dichalcogenides have attracted extensive interests in recent years, motivating investigation into multiple properties. We use a refined version of the optothermal Raman technique [2][3] to measure the thermal transport properties of two TMDC materials, MoS2 and MoSe2, in single-layer (1L) and bi-layer (2L) forms. This new version incorporates two crucial improvements over previous implementations. First, we utilize more direct measurements of the optical absorption of the suspended samples under study and find values ~40% lower than previously assumed. Second, by comparing the response of fully supported and suspended samples using different laser spot sizes, we are able to independently measure the interfacial thermal conductance to the substrate and the lateral thermal conductivity of the supported and suspended materials. The approach is validated by examining the response of a suspended film illuminated in different positions in radial direction. For 1L MoS2 and MoSe2, the room-temperature thermal conductivities are (80±17) W/mK and (55±18) W/mK, respectively. For 2L MoS2 and MoSe2, we obtain values of (73±25) W/mK and (39±13) W/mK. Crucially, the interfacial thermal conductance is found to be of order 0.1-1 MW/m2K, substantially smaller than previously assumed, a finding that has important implications for design and modeling of electronic devices.

Thesis (Ph. D.)--University of Alberta, 2010.
Title from pdf file main screen (viewed on June 28, 2010). A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Department of Chemistry, University of Alberta. Includes bibliographical references.

This thesis is devoted to the investigation of mechanical properties and self-assembly process of materials at the nanoscale. Various nanostructured materials such as nanoparticles, nanotubes, nanowires and thin films are used as constituent elements of nanodevices. Hence, knowledge of the mechanical properties of materials at the nanoscale is extremely important for understanding their functionality in nanodevices. Mechanical properties of nanostructured materials may significantly differ from those of their bulk counterparts due to the high surface to volume ratio in nanostruc-tures. We particularly focus on the role of the surface region on the stiffness of nanomaterials. We have shown that the stiffness of a nanomaterial can be tuned over a wide range by introducing appropriate coating on the nanostructure surface. We have also explored the effects of the surface region on the stability of various phases in a nanostructure. In the second part of this thesis, we have described the self-assembly process of nanostructures mediated by drendrimers. Self-assembly techniques are frequently used to decorate nanostructures into specific networks. The motivation of this study is to investigate the mechanisms which control the effective interaction and the inter-particle distance between nanoparticle-dendrimer compos-ites. Control over the inter-particle separation is very important since it has a strong influence on the electronic and optical properties of the nanostructures. In the following paragraphs, we sum-marize the results of our study. We start with a brief introduction to the mechanical properties and self-assembly process of nanostructures in the first chapter. A brief review of the work done on these topics in the recent past is presented in this chapter. We discuss the results and conclusions of various experimental and numerical studies on these topics. We also mention the motivation for the studies we have carried out. At the end, we briefly describe the numerical methods (molecular dynamics (MD) and density functional theory (DFT)) which have been used in our investigations. In the second chapter, we discuss the effects of the surface region on the mechanical properties of nanostructures. We have investigated the size and growth direction dependence of the mechanical properties of ZnS nanowires and thin films as a case study. We observe that the Young’s modulus of nanowires and thin films strongly depends on their size and growth direction. This size and growth direction dependence of the stiffness of nanostructured materials can be explained in terms of their surface modifications. Since the energy of the surface region is usually higher than that of the core region in a nanostructure, the surface atoms move their positions to minimize the surface energy. As a result, bond lengths at the surface region are usually different from their bulk values. We observe that in ZnS nanowires and thin films, the average bond length at the surface region is lower than that in the core region which remains unchanged from its bulk value. This decrease in the bond length (or equivalently increase in the bond energy) increases the effective stiffness of the entire nanostructure. As the size of the nanowire/thin film increases, the effect of the surface region gradually decreases and hence the Young’s modulus value converges to the bulk value. Since the surface region has a strong influence on the mechanical properties of nanostructures, the stiffness of a nanostructure can be tuned by modifying the surface region with other materials. In chapter three, we have shown that the stiffness of ZnS nanowires can be tuned by introducing a thin CdS shell on top of the ZnS surface. In general, the stiffness of a nanostructure can be increased (decreased) by coating the surface region with a stiffer (less stiff) material. However, the stiffness of the core/shell nanostructures strongly depends on the properties of the interface between the core and the shell. We observe that the binding energy between the core and shell regions is relatively low due to the lattice mismatch at the interface region of core/shell nanostructures. This lower binding energy strongly affects the stiffness of core/shell nanostructures. We have also shown that thermal properties such as thermal conductivity and melting temperature of core/shell structures can be tuned by changing the coating material. In chapter four, we discuss the effects of the surface region on the stability of various phases in a nanostructure. The surface atoms may stabilize a particular phase in a nanostructure which is not a stable phase in the bulk material. In this chapter, we investigate the stability of the h-MgO phase, an intermediate structure found during the wurtzite to rock salt transformation, in CdSe nanostructures. We observe that this five-fold coordinated phase is more stable at lower temperatures and smaller sizes of the nanowires. The appearance of this phase has not been observed till now in experiments. We show that this phase is not stable for larger CdSe nanocrystals on which the experiments have been done. In the rest of the thesis, we have presented the results of our studies of self-assembly of nanostructures mediated by DNAs and dendrimers. First we describe in chapter five the nature of the effective interaction between two PAMAM dendrimers. Dendrimers are frequently used to coat surfaces of nanoparticles to prevent the nanoparticles from aggregation. The interaction between such nanoparticle-dendrimer composites depends strongly on the nature of the effective interac-tion between dendrimers. We have used fully atomistic MD simulations to calculate the potential of mean force (PMF) between two PAMAM dendrimers. We show that the effective interaction strongly depends on the size (generation) and protonation level of the dendrimers. The PMF profiles of nonprotonated dendrimers show a global minimum which represents the attractive nature of the interaction between the dendrimers up to a certain center-to-center distance. On the other hand, the interaction between protonated dendrimers is repulsive throughout their interaction re-gion. The PMF profiles are fitted very well by a sum of an exponential and a Gaussian function. This observation is in contradiction with some of the results of existing coarse-grained simulations which predicted the effective interaction between dendrimers to be Gaussian. Our atomistic simulation which includes all the local fluctuations is expected to give more accurate results. Information about the effective interaction between two dendrimers helps in understanding how dendrimer molecules can be used to control the interaction strength and the preferred inter-particle distance between two nanostructures. In chapter six, we discuss the effective interaction between two dendrimer grafted gold nanoparticles. We observe that dendrimer molecules can get adsorbed spontaneously on the surface of a gold nanoparticle. These grafted dendrimers significantly alter the interaction between the gold nanoparticles. We have explored the effects of proto-nation level and the density of the grafted dendrimers on the effective interaction between two gold nanoparticle-dendrimer composites. We observe that these nanoparticle-dendrimer composites at-tract each other at low grafting density. However, the interaction strength and the inter-particle distance at the minimum of the potential are much lower and higher, respectively than those between two bare gold nanoparticles. Interestingly at higher grafting density, the nature of the interaction between the nanocomposites depends on the protonation level of the grafted dendrimers. Nanoparticles grafted with nonprotonated dendrimers still attract each other but with lower inter-action strength and higher inter-particle distance compared to the values for low grafting density. On the other hand, nanocomposites grafted with protonated dendrimers repel each other at high grafting density. Thus we show that the effective interaction and the optimal inter-particle distance between the nanostructures can be tuned over a wide range by using a suitable grafting density and protonation level of the dendrimers. In the seventh chapter, we describe a strategy to assemble dendrimers with the help of sin-gle stranded DNA (ssDNA). We attach an ssDNA to one dendrimer and a complementary ssDNA to a second dendrimer. These two complementary ssDNAs bind with each other through base pair formation to assemble the dendrimers into a single structure. The complementary ssDNAs form a dsDNA which is rigid enough to maintain the inter-dendrimer distance almost the same as the length of the DNA. The inter-dendrimer distance can be tuned by changing the DNA length. However, this method strongly depends on the protonation level of the dendrimers. It works well only for nonprotonated dendrimers. Since the protonated dendrimers are positively charged, they strongly interact with the negatively charged ssDNAs through electrostatic interaction. As a result, ssDNAs wrap the dendrimer surface and hence the inter-dendrimer distance can not be controlled. We have also verified that this method works for multiple nonprotonated dendrimers as well. In the final chapter of this thesis, we summarize the main results and conclude with a brief discussion of future directions of research on the problems considered in the thesis.