Quantum confinement effect in nanocomposites
TL;DR: In this paper, optical absorption data when analyzed with Mie's scattering formula give results which indicate a metal-insulator type transition in the case of silver particles having diameters around 3nm.
About: This article is published in Applied Surface Science.The article was published on 2001-10-22. It has received 4 citations till now. The article focuses on the topics: Quantum dot & Potential well.
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TL;DR: The solvent effects of electro-oxidative deposition of octyl thiolate-stabilized gold nanoparticles with 2.3 ± 0.5 nm core diameter modified with biferrocene-terminated alkanethiolates on their surface (Aun-BFc) were investigated using cyclic voltammetry, UV−vis spectroscopy, scanning tunneling microscopy (STM), and electrochemical quartz crystal microbalance (EQCM) techniques as mentioned in this paper.
Abstract: The solvent effects of electro-oxidative deposition of octyl thiolate-stabilized gold nanoparticles with 2.3 ± 0.5 nm core diameter modified with biferrocene-terminated alkanethiolates on their surface (Aun-BFc) were investigated using cyclic voltammetry, UV−vis spectroscopy, scanning tunneling microscopy (STM), and electrochemical quartz crystal microbalance (EQCM) techniques. Consecutive potential scans causing two-step one-electron oxidation of the biferrocene units of Aun-BFc in 0.1 M Bu4NClO4−organic solvent [CH2Cl2, tetrahydrofuran (THF), toluene + acetonitrile (toluene/MeCN; 2:1 v/v)] solution under Ar produced the adhesive Aun-BFc film on an electrode. The deposition rate was higher in the order THF > toluene/MeCN > CH2Cl2. The STM images indicated that the films prepared in CH2Cl2 or THF were likely to form domains with ∼80 and 170 nm diameters of the assembled Aun-BFc's, respectively, as contrast particles were randomly connected in the film deposited in toluene/MeCN. The experimental results al...
35 citations
TL;DR: In this paper, the authors used a combination of co-deposition and MeV ion bombardment to nucleate nanoclusters, which have a periodic structure consisting of alternating layers where each layer is 10nm thick.
Abstract: We prepared 50 periodic nano-layers of SiO 2 /Ag x SiO 2(1 − x ) with Au layer deposited on both sides as metal contacts. The deposited multi-layer films have a periodic structure consisting of alternating layers where each layer is 10 nm thick. The purpose of this research is to generate nano-layers of nanocrystals of Ag with SiO 2 as host and as buffer layer using a combination of co-deposition and MeV ion bombardment taking advantage of energy deposited in the MeV ion track to nucleate nanoclusters. Our previous work showed that these nanoclusters have crystallinity similar to the bulk material. Nanocrystals of Ag in silica produce an optical absorption band at about 420 nm. Due to the interaction of nanocrystals between sequential nanolayers there is widening of the absorption band. The electrical and thermal properties of the layered structures were studied before and after 5 MeV Si ions bombardment at various fluences to form nanocrystals in layers of SiO 2 containing few percent of Ag. Rutherford Backscattering Spectrometry (RBS) was used to monitor the stoichiometry before and after MeV bombardments.
10 citations
Dissertation•
01 Jan 2012
TL;DR: In this article, a new technique based on electrolysis of alcogels has been employed for the synthesis of various metals (Ni, Co & Fe) and alloys (Ni-Fe, Ni-Co, Fe-Co and Ni-Zn) nanoparticles in the pores of silica gel.
Abstract: Nanomaterials may be defined as the materials with, at least, one structural dimension in the range of 1 - 100 nm. Nanocomposites are a special class of nanomaterials and are of interest because they exhibit interesting mechanical, electrical, optical and magnetic properties in addition to high catalytic activity. Although nanomaterials can be synthesized by many methods but wet synthesis methods, often offers better control over shape, composition and structure. Wet synthesis include thermal decomposition, pyrolysis, polyol process, hydrothermal/solvothermal, sol-gel, electrochemical, chemical / borohydride reduction and, co-precipitation, etc.However, sol-gel is one of the methods, which offers better control over chemistry and composition.Consequently, sol-gel is a technique most widely used for the manufacturing and synthesis of metal/inert ceramic nanocomposites. Generally, it is difficult to prepare metallic nanoparticles in ceramic matrix, directly through sol-gel method employed for the preparation of nanocomposites, and a subsequent pyrolysis and / or hydrogen reduction treatment becomes almost essential. The metallic ions may also be reduced chemically, but is usually often accompanied by difficulty in controlling reaction conditions and composition and further demonstrated suitable for surface deposition only.Metallic species in the sol-gel ceramic could be reduced by radiations also, but this method is accompanied by inherited safety issues, and found more effective in thin films or solid sections only.Electrolysis is another very simple and often room temperature technique, that can be efficiently applied to reduce the metallic ionic species present in solution phase to their corresponding metallic state. However to get electrodepositable gel, researchers in the past opted for either long duration for gelation and /or high temperature treatments to get aged gels. In some cases gelation time was in weeks, in other case temperatures employed were high such as; >500oC.Often the technique has been limited to thin gel films for ease in soaking and shorter electrolytic conducting paths. If these limitations are overcome, this combination may possibly the simplest, most versatile, fast enough and cost-effective for the formation of metallic nanoparticles in the oxide matrices. Presently emphasis has been laid on the development of a synthesis technique based on sol-gel and electrodeposition by overcoming all the above observed problems.A new technique based on electrolysis of alcogels has been employed for the synthesis of various metals (Ni, Co & Fe) and alloys (Ni-Fe, Ni-Co, Fe-Co, Fe-Zn and Ni-Zn) nanoparticles in the pores of silica gel. Chloride(s) of respective metal(s) were used as metal precursor and introduced into the alcogel during sol formation step.The as synthesized alcogels without subsequent heat treatments were immediately subjected to electrochemical reduction,consequently forming metal and alloy nanoparticles into the pores of silica alcogel.Electrolysis of as generated alcogels (i.e., without any subsequent treatment) resulted in the formation of nickel and alloy nanoparticles within reasonable depth of the gel.The method employed, does not require high temperatures or long durations to form electrodepositable gel.This technique is simple and cost effective. Further it can produce nanomaterials in bulk and in a single go.The nanoparticles were characterized by XRD, TEM, surface area, Resistance measurements, BET, ACSusceptibility, SQUID, VSM, MA¶ssbauer and M-TGA measurements etc. From XRD analysis size of FCC Ni, Ni(Fe), Ni(Co), Ni(Zn) nanoparticles ware around 17-20 nm, 8-15 nm, 11-16 nm and 9-14 nm respectively.The FCC phase in most case was also accompanied by surface oxide; tetragonal nickel.The sample with only iron chloride in alcogel does not revealed presence of any significant amount of BCC phase, this may probably due to oxidation of iron as a consequent of small particle size.The spinel iron oxide phase had size around 8 nm.Addition of even small quantity of cobalt or zinc along with iron, resulted in the formation of BCC phase.The BCC Fe(Co) particles were around 9-12 nm, while BCC Fe(Zn) nanoparticles were around 6-11 nm. The particle size appeared to decrease with the increase in the concentration of alloying elements.However in case of Fe(Co) alloys size seems independent of alloying element concentration.In gels containing only cobalt chloride, about18 nm cobalt nanoparticles were formed.The formation of small size of nanoparticles was further confirmed from TEM studies.Resistance measurement was carried to further understand the structure of samples.Composites having more metal-oxide content such as in samples with high iron, cobalt or zinc as alloying element, resulted in increased resistance such as up to order of MK at a load of 100 kg. This is due to the formation of higher quantity of oxides between the interconnected necks of nanoparticles. However, complete metallic contact at low load was observed in FCC Ni and FCC nickel alloys, having low alloying concentrations of iron or cobalt. Besides XRD, the formation of spinel iron oxide in iron containing samples was confirmed from the presence of superparamagnetic doublet appearing in MA¶ssbauer spectra.This corresponds to iron in high spin Fe3+ state.The formation of Ni(Fe) and Fe(Co) was also confirmed by MA¶ssbauer analysis, showing presence of ferromagnetic sextets, having hyperfine field of the order of 260kOe and 340kOe respectively.The VSM of composites indicated formation of soft magnetic metal and alloy nanoparticles.The coercivity measured for nickel samples comes out around 100 Oe. While for Ni(Fe) it lies between 50 to 100Oe, with low being associated to more iron alloying.Coercivity of Ni(Co) samples lied in the range of 150 to 250Oe with higher being associated to higher concentration of cobalt in the gel. However coercivity of Fe(Co) samples decreased slightly with the cobalt addition from around 160Oe to 120Oe but resulted in increased magnetization. M-TGA studies were also performed to magnetically characterize samples. Presence of exchange coupling was observed in the samples due to ferromagnetic– antiferromagnetic interaction at the surface of nanoparticles. Consequently ferromagnetic nanoparticles remained blocked up to Curie temperature of FCC nickel in case of nickel containing samples and up to Curie temperature of spinel ferrite in case of Fe(Co) samples.The formation of alloy was further confirmed by the change in Curie transition of various samples.The Curie temperature of nickel increased from 620 K to 630 K by iron addition, and it increased to ~ 900 K in case of cobalt addition. In Fe(Co) samples, Curie transition associated with metallic phase was only observed but in samples with higher concentration of cobalt.This probably is due to oxidation of nanoparticles during M-TGA studies.From XRD and M-TGA quantity of alloying can be estimated, such as; up to 20 % Fe in Ni(Fe), up to 30% Co in Ni(Co) and up to ~30-50% Co in case of Fe(Co) samples was estimated.The present technique has proven its versatility by depositing variety of nanoparticles, and having soft magnetic properties, with high resistance.Therefore, if further characterized, these materials could stand potential candidates for high frequency applications.Since surface area of most of the samples was ~100m2/gm, besides high well dispersed metallic load (e.g.; 55% Ni in Ni/Silica samples), therefore this technique can produce potential catalytic composites too.
3 citations
06 Dec 2006
TL;DR: Salmon et al. as mentioned in this paper investigated the formation and properties of self-assembled nanofibers (SANs) created by the treatment of aluminum with solutions of short chain-length alkylphosphonic acids (APAs) in ethanol.
Abstract: SALMON, MICHAEL EDWARD. The Growth and Characterization of Alkylphosphonic Acid Self-Assembled Nanofibers. (Under the direction of Dr. Phillip E. Russell.) The focus of this research was to investigate the formation and properties of novel Self-Assembled Nanofibers (SANs) created by the treatment of aluminum with solutions of short chain-length alkylphosphonic acids (APAs) in ethanol. A special emphasis was placed on the creation of APA SANs isolated from the immersed aluminum source and development of analysis techniques for artifact reduced characterization of as-grown individual SANs. Novel immersion growth techniques were devised for the reproducible creation of supported and unsupported isolated methylphosphonic acid (C1), propylphosphonic acid (C3), and pentylphosphonic acid (C5) SANs on Si3N4 and aluminum coated ProtoChips DuraSiN Si3N4 meshes respectively. Additionally, a novel biased immersion growth technique was developed, increasing growth rates as well as allowing for APA SAN deposition onto a variety of substrates including Au microelectrodes. A combination of complimentary analysis techniques including: Atomic force microscopy (AFM), Scanning Transmission Electron Microscopy (STEM), Energy Dispersive Spectrometry (EDS), X-Ray Photoelectron Spectroscopy (XPS), and Electron Energy Loss Spectroscopy (EELS) were utilized to characterize the morphology, composition and chemistry of isolated individual APA SANs. STEM and AFM revealed individual APA SANs are actually composed of layered fibril bundles. Qualitative compositional analysis showed APA SANs were primarily composed of oxygen, carbon, phosphorus, and aluminum with phosphorus:aluminum ratios determined to be between 1.5 and 4.2. Quantitative XPS and EELS analysis provided further evidence that the detected aluminum was non-metallic and likely oxidized. STEM with EELS was utilized to definitively correlate the presence of aluminum, phosphorus, oxygen, and carbon to a 5 nm region of several overlapping unsupported C1 SANs. Thermal analysis of APA SANs on Al as well as isolated on Si3N4 revealed a nearly 5X increase in thermal stability as compared to the ~ 100C-120C melting points of pure APAs. AFM nanoindentation and nanoscratching were utilized to investigate the mechanical response of individual APA SANs. Evidence of cracking and layering were observed in good agreement with the STEM fibril observations. The reduced elastic modulus, E, or stiffness, was estimated utilizing a Hertzian mechanics analysis of AFM nanoindentation data and determined to range from ~ 10GPa to 1 GPa varying inversely with chain-length. Electric Force Microscopy (EFM) of C1 SANs revealed no evidence of conductivity as compared to a control sample consisting of Focused Ion Beam (FIB) deposited platinum nanowires on Si3N4. Additionally, Current-Voltage (IV) measurements were made on individual APA SANs deposited on gold microelectrodes again with no evidence of conductivity. THE GROWTH AND CHARACTERIZATION OF ALKYLPHOSPHONIC ACID SELFASSEMBLED NANOFIBERS
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TL;DR: In this article, Heath discusses recent work reported in this issue by Murray et al. in which nanocrystals of cadmium selenide were formed into ordered assemblies of quantum dots.
Abstract: Materials with desirable properties may be created through control of crystal size and shape and assembly of such crystals into more complex arrangements. In his Perspective, Heath discusses recent work reported in this issue by Murray et al . ([p. 1335][1]) in which nanocrystals of cadmium selenide were formed into ordered assemblies of quantum dots. Such results represent a first step toward self-assembly of device structures.
[1]: /lookup/doi/10.1126/science.270.5240.1335
130 citations
TL;DR: A simulation of the deflection of a single wall nanotube under the manipulation of an atomic force microscope tip revealed the key feature characterizing the deformation at relatively small bending angles to be a reversible transition from sp(2) to sp(3) bonding configurations in the bending region, leading to a 2 orders of magnitude reduction in conductance.
Abstract: A simulation of the deflection of a single wall nanotube under the manipulation of an atomic force microscope tip revealed the key feature characterizing the deformation at relatively small bending angles to be a reversible transition from $s{p}^{2}$ to $s{p}^{3}$ bonding configurations in the bending region, leading to a 2 orders of magnitude reduction in conductance consistent with our most recent experimental observation. A local analysis elucidating the underlying physics of the findings is also discussed.
103 citations
TL;DR: In this paper, the resistivity data have been analyzed in terms of the Ziman theory of electron-phonon scattering and the effective Debye temperature Theta p has been estimated by fitting the experimental data to Ziman's equation.
Abstract: Conducting films consisting of silver particles of diameters ranging from 4 to 12 nm have been grown in glass-ceramic by subjecting the latter to a Li+ to or from Ag+ exchange followed by a suitable reduction treatment. The DC electrical resistance of these films has been measured over the temperature range 80-300 K. The resistivity data have been analysed in terms of the Ziman theory of electron-phonon scattering. The effective Debye temperature Theta p has been estimated by fitting the experimental data to Ziman's equation. Theta p is seen to vary from 98 to 192 K for silver particle sizes ranging from 4.3 to 11.0 nm. The silver particle aggregates in the present system have a fractal microstructure with fractal dimensions of around 1.6 and 1.9, respectively.
94 citations
TL;DR: In this paper, the optical absorption spectra of glass-metal nanocomposite films have been measured over the wavelength range 200 to 2000 nm, and effective medium theories of Maxwell-Garnett and Bruggeman, respectively, have been used to calculate theoretically the absorption of these materials.
Abstract: Glass-metal nanocomposites incorporating ultrafine particles of iron, nickel, cobalt and manganese, respectively, in a silica glass matrix have been prepared by heat treatment of a gel derived from a sol containing silicon tetraethoxide and a suitable metal organic compound. Metal particles in all the nanocomposites are isolated and spherical-shaped with diameters ranging from 3 to 10 nm. Films of these nanocomposites with thickness of the order of a few micrometres have been prepared on glass slides by a simple dip-and-pull technique. Optical absorption spectra of the nanocomposite films have been measured over the wavelength range 200 to 2000 nm. Effective medium theories of Maxwell-Garnett and Bruggeman, respectively, have been used to calculate theoretically the optical absorption of these materials. The Maxwell-Garnett theory gives results which are in better agreement with experimental data than those obtained from Bruggeman formalism. The filling factor f as estimated from the least-squares fit of the experimental results with the Maxwell-Garnett theory has a value in the range 1 to 4%.
92 citations
TL;DR: Nanometer-scale atom clusters of a variety of materials, including both metals and ceramics, have been synthesized by precursor evaporation and condensation in high-purity gases as discussed by the authors.
Abstract: Nanometer-scale atom clusters (with average diameters below 20 nm) of a variety of materials, including both metals and ceramics, have been synthesized by precursor evaporation and condensation in high-purity gases. The gas-entrained clusters can be collected and subsequently consolidated in situ under ultrahigh vacuum or other controlled atmosphere conditions to create bulk nanophase materials. These ultrafine-grained materials have properties that are often significantly different and considerably improved relative to those of their coarser-grained counterparts. The observed property changes relate to both their small grain sizes and the large percentage of their atoms in grain boundary environments. Since it is becoming apparent that their properties can be engineered during gas-phase synthesis and subsequent processing, nanophase materials assembled from atom clusters should have significant potential for technological development in a variety of applications. Some of the recent research on nanophase materials is reviewed.
71 citations