Nano-sized Dots Research Articles

The roles of intermediate fluorophores on the optical properties of bottom-up synthesized carbon nanodots

Carbon nanodots (CNDs) are the latest nano-sized carbon materials having unique properties such as biocompatible, highly photoluminescent, and nontoxic which are suitable for diverse applications including lighting, sensing, bioimaging, and biochemical analyzing. CNDs could be synthesized by top-down methods in which graphite is fragmented into nano-sized graphene dots. Alternatively, CNDs could be formed by a bottom-up synthetic strategy where organic molecules are fused together via complex condensation and carbonization processes. Although a great number of organic molecules have been used successfully to prepare CNDs there are very few CNDs that exhibit the quantum size effects. The absorption and emission properties of bottom-up synthesized CNDs rely vastly on molecular-like fluorophores which are the intermediates formed during the fusion of molecular precursors and are incorporated into CNDs in the later states of carbonization processes. This review aims to demonstrate recent understandings on the formation of intermediate fluorophores and their contribution to the optical properties of CNDs

Carbide-directed enhancement of electrochemical hydrogen evolution reaction on tungsten carbide–oxide heterostructure

Carbide–oxide heterointerfaces have been known to be responsible for electrochemical activity, however, it is rarely investigated in tungsten carbide-oxide heterostructure for the hydrogen evolution reaction (HER). Furthermore, typical carbide preparation includes carbonization of oxide under gaseous carbon sources at a high temperature, resulting in sintering and collapse of the heterostructures. In this work, nano-sized carbon dots (CDs, ∼2 nm in diameter) were adopted as carbon sources and dispersed on tungsten oxide nanorods allowing abundant nucleation sites for oxide-to-carbide conversion, resulting in high density of carbide-oxide heterointerfaces. The resulting tungsten carbide-oxide heterostructure decorated with carbon dots (WO(3−x) − WCy/CDs) exhibited superior electrocatalytic activity toward the HER with a low overpotential of 65 mV at a current density of − 10 mA cm−2 in acidic media. This performance is among the best electrocatalytic activities compared to the state-of-the-art tungsten carbide-based electrocatalysts. Importantly, the single-cell test using WO(3−x) − WCy/CDs as the cathode showed a current density of 10 mA cm−2 with a cell voltage of only 1.47 V, which is very close to that for Pt/C (1.41 V). In addition, a combined analysis of the X-ray spectroscopic and electrochemical results suggested an optimal W–C ratio in the tungsten carbide-oxide composite to guarantee the high HER activity. Theoretical calculations provided more insight into the carbide-directed enhancement of the electrocatalytic activity of WO(3−x) − WCy/CDs.

Carbon-dot hydrogels as superior carbonaceous adsorbents for removing perfluorooctane sulfonate from water

Efficiently removing perfluorooctane sulfonate (PFOS) from wastewaters is a hot topic in environmental field. Herein, crosslinking the nano-sized and zero-dimensional carbon dots (CDs) with polyethylene glycol diglycidyl ether (PEG) and polypropylene glycol diglycidyl ether (PPG) resulted in CD hydrogels that were verified by TEM, SEM, FTIR and XPS. The obtained CD hydrogels could efficiently adsorb PFOS from water through their CD moieties. The kinetic curves were fitted well by the pseudo-second order model, indicating the involvement of chemisorption in the rate-limiting step. The adsorption isotherm measurements showed that the hydrogel with the highest CD content had a PFOS maximum uptake capacity of ca. 2.82 g/g (5.64 mmol/g) in the neutral water, which was much higher than the ever-reported carbonaceous materials, and also ranked the top three among the superior positively-charged adsorbents. The superior PFOS adsorption of CD hydrogels could be attributed to good dispersion of the nanoscale CDs in the PEG/PPG hydrogel matrix, leading to the substantial exposure of the binding sites of CDs towards PFOS molecules. Moreover, CD hydrogels did not show pronounced loss of adsorption capacity after experiencing five adsorption-desorption cycles. As for the real wastewaters, CD hydrogels could efficiently treat not only the high concentration of PFOS in the neutral firefighting wastewaters, but also the extremely low concentration of PFOS in the neutral environmental wastewater, where the residual PFOS concentration could be reduced less than 5 ng/L, satisfying the standard of the U.S. Environmental Protection Agency.

Aggregation-Induced Fluorogens in Bio-Detection, Tumor Imaging, and Therapy: A Review

Aggregation-Induced Fluorogens in Bio-Detection, Tumor Imaging, and Therapy: A Review

Swift heavy silver (Ag+7) ion irradiation induced self assembled nanodots on MBE grown Gallium antimonide (GaSb)

100 MeV swift heavy silver (Ag+7) ion irradiation at normal incidence results in the formation of dense and ordered nanosized dots on molecular beam epitaxy (MBE) grown GaSb epitaxial layers. The shape, size and density of the nanodots are found to be strongly dependent on the ion fluence. Ripening of the dots with change in shape from cone to dome, and finally smoothening of the surface is observed with increasing ion fluence. The dot formation mechanism is explained on the basis of a surface instability introduced by combination of sputtering, step or kink Ehrlich-Schwoebel barrier effect and surface diffusion of mobile adatoms. Raman spectroscopy and high resolution X-ray diffraction results suggest preservation of crystallinity in the irradiated epilayers with presence of small tensile stress. Prevention of amorphisation in the irradiated samples is explained on the basis of a dynamic self annealing process during irradiation.

Microstructural evolution and hardening of GH3535 alloy under energetic Xe ion irradiation at room temperature and 650 °C

The GH3535 alloy was irradiated with 7 MeV Xe26+ ions to a dose of 10 dpa at room temperature (RT) and 650 °C, and subsequently examined using Transmission Electron Microscopy (TEM) and nanoindentation. High numbers of nano-sized black dots, identified as dislocation loops were observed in both irradiated samples. The dislocation loops detected at the high temperature irradiated sample (size/number density: 9.5 nm/1.9 × 1021 m−3) were found to be larger in size but less in amount as compared to that of the case of RT irradiation (6.9 nm/18.7 × 1021 m−3). In addition, the large-sized Mo-Cr rich precipitates (16.4 nm/3.7 × 1021 m−3) were observed in the sample irradiated at 650 °C. Moreover, the Xe bubbles, with smaller size (2.9 nm) but higher number density (77.8 × 1021 m−3) among the irradiated induced defects, were also detected in the case of high temperature irradiated sample via the diffusion and aggregation of Xe atoms. Nanoindentaion measurements showed a hardening phenomenon for the irradiated sample, and the hardness increment is higher in the case of high temperature irradiated sample. Dispersed barrier-hardening (DBH) model was applied to predict the hardening produced from the irradiation induced defects. The yield strength increment calculated based on TEM observations and the nanohardness increment measured using nanoindentation are in excellent agreement.

Perpendicularly magnetized L1-FePt nanodots exchange-coupled with soft magnetic Ni81Fe19

We studied exchange-coupled bilayers consisting of a perpendicularly magnetized L10-FePt layer and a soft magnetic Ni81Fe19 (Permalloy; Py) layer, where the Py layer was thick enough to form spatially twisted magnetic structures. The Py layer showed in-plane magnetization in the case of unpatterned thin film because of its demagnetizing field even though Py was exchange-coupled to the perpendicularly magnetized L10-FePt layer at the interface. After patterning the FePt/Py bilayer into nanosized dots with cylindrical shape, the effective perpendicular magnetic anisotropy was induced. Exchange-coupling behavior, i.e., spring back behavior, was observed when the minor magnetization curves were measured. In addition to this behavior, the magnetic domain observation suggested that a spatially twisted magnetic structure was formed with sweeping the perpendicular magnetic field.

The Influence of $\hbox{CeO}_{2}$ Nano-Dots Decoration on Substrates on Flux Pinning Strength in $\hbox{Tl}_{2} \hbox{Ba}_{2}\hbox{CaCu}_{2}\hbox{O}_{8}$ Thin Films

Nano-sized CeO2 dots were deposited on (001) LaAlO3 substrate by rf magnetron sputtering. After the processing of the artificial decoration defects on the substrate, 100-nm, 150-nm, 200-nm, and 400-nm-thick Tl2Ba2CaCu2O8 (Tl-2212) thin films were grown on it, using dc magnetron sputtering and post annealing process. AFM images showed that the CeO2 nano-dots were distributed well on the substrate surface with 70 nm in diameter and 5 nm in height, and the density of nano-dots was 25±3 dots/μm2. SEM images were used to observe the variations on film surface. Transport critical current density Jc was measured both on Tl-2212 film with nano-dots decorated substrate and untreated single crystal LaAlO3 substrate. From the results, Jc values of Tl-2212 films with nano-dots decorated substrate were lifted significantly, and the pinning strength was improved on Tl-2212 films by nanostructure defects induced from nano-dots.

Frequency tuning the ultrafast magnetization dynamics of FexPt100-x alloys

We report frequency tuning of magnetic oscillations in both planar films and individual patterned nano-sized dots by varying the iron content of disordered FexPt100-x alloys from 42 at. % to 100 at. %. It is seen that as the Fe content is increased, the magnetic oscillation frequency rises by up to 50% in both the films and individual dots due to the change in the saturation magnetization. Micromagnetic simulations and analytic calculations are in agreement with the data. These tunable alloys could be used in microwave-assisted magnetic recording and especially for developing a 3D storage technology where layers of different resonance frequency are switched individually.

Formation of nanodots on GaAs by 50 keV Ar+ ion irradiation

Nanopatterning of semi-insulating GaAs (100) by 50keV Ar+ ion irradiation at an angle of 50° with respect to surface normal is investigated systematically. The samples were analyzed using atomic force microscopy and Raman spectroscopy as a function of fluence. Irradiation with fluence of 1×1017ions/cm2 results in the formation of nano-sized dots on the surface of GaAs which ripens at a fluence of 3×1017ions/cm2 and shows fragmentations at a fluence of 7×1017ions/cm2. The average diameter of dots varies between 20 and 35nm in the fluence range. It is also observed that the density of dots first decreases with increase in fluence and later increases due to fragmentation of dots. Non-linear behavior of surface roughness is also observed with increase in fluence. Raman spectroscopy shows that the ion irradiation leads to amorphization of the irradiated surfaces in this range of fluence and relaxations and smoothing of the surface via decreased strain.

Patterned nano-sized gold dots within FET channel: from fabrication to alignment of single walled carbon nanotube networks

We have introduced patterned gold dots between the electrodes of single walled carbon nanotube (SWNT) network films, to effectively guide the path of randomly assembled nanotube networks. Direct visualization results show well-aligned SWNT networks across the gold dots, generating improved SWNT–FET performance with a high on/off ratio of 104 and 85 cm2 V−1s−1 mobility, a remarkable enhancement compared to SWNT–FET in the absence of patterned gold dots. Unlike previously reported alignment techniques used, the proposed method affords the selective alignment of SWNTs within electrodes at room temperature without any direct growth on the device surface, and without the need for further alignment procedures. Thus, this gold-dot pattern technique within FET channel has proven to be truly simple, reliable and cost-effective. We discuss the effect of the voltage, channel size and feature dimensions of the gold nanopatterns in field effect transistor (FET) structures.

Fabrication and transfer of nanoporous alumina thin films for templating applications: Metal dots array deposition and porous ZnO film growth

An approach to fabricate nanoporous alumina thin films on aluminum foils followed by their transfer to desired substrates via chemical lift-off and Van der Waals bonding is presented. By employing a black wax technique, we demonstrate a crack free (within the area of 2 × 2 cm 2) lift-off and bonding of a nanoporous alumina film with thickness of as small as ~ 300 nm onto a substrate. The liftoff-and-bonding process changes neither the morphology nor the structure of the nanopores in the alumina film. Templating applications of the bonded alumina thin films are demonstrated and discussed. The results reveal that the alumina nanopores, combined with the liftoff-and-bonding technique, have great potential for templating applications in both nano-sized dots array deposition and functional materials growth.

Profile Uniformity of Overlapped Oxide Dots Induced by Atomic Force Microscopy

The technique for assembling a uniform oxide line by overlapping a series of nanosized oxide dots induced by atomic force microscopy is analytically and experimentally investigated. In addition to the normal continuous (static) pulses, the oxide growth rates under various discontinuous (modulated) pluses are studied to quantify the overlapping effect under multiple pulses used by the assembling technique. In the analysis of the assembling technique, the superposition principle is used to predict the assembled profiles and to define the uniformity criteria. Experiments have been performed to demonstrate the analytical prediction, including the threshold or minimum pitch for forming uniform lines, and the onset pitch for the overlapping effect to be considered. Indeed, by following the uniformity criteria developed, uniform and reliable oxide lines can be obtained by overlapping oxide dots.

Nano peel-off fabrication pattern with polymer overcoat layer on glassy carbon electrodes for Pt electrodeposition

This article describes a new technique for fabricating an electrocatalyst model in which the particle size and interparticle distance are controlled independently. We designed a uniform insulating polymer layer as a mask on an electroconductive glassy carbon substrate and then peeled off a part of the layer in nano-sized dots by scratching the overcoat layer using an atomic force microscope (AFM) cantilever. Pt particles electrodeposited only on the peeled off area of the glassy carbon. To peel-off a small area on the glassy carbon, a 29 ± 2 nm thick insulating polymer overcoat layer and a cantilever operating area of 10 nm × 10 nm were used, and the smallest peel-off area obtained was 30 nm × 30 nm. Thereafter, we performed the peel-off procedure on the polymer overcoat layer of the glassy carbon substrate having a cantilever operating area of 80 nm × 80 nm. Pt deposition of 100–150 nm in diameter was successfully achieved by adjusting the interparticle distance.

Fabrication of Periodic Arrays of Nano-sized Si and Ni dots on SiO2 Using Linearly Polarized Nd:YAG Pulsed Laser

ABSTRACTPeriodic arrays of nano-sized Si and Ni dots were fabricated by only irradiating a linearly polarized Nd:YAG pulsed laser beam to Si and Ni thin films deposited on silicon dioxide (SiO2) film. The interference between an incident beam and a scattered surface wave leads to the spatial periodicity of beam energy density distribution on the surface of the irradiated samples. A thin film was melted using a laser beam, and the molten film was split and condensed owing to its surface tensile according to the periodic energy density distribution. Then, the fine lines (line and space structure) were formed periodically. After the formation of fine lines, the sample was rotated by 90°, and the laser beam was irradiated. The periodic energy density distribution was generated on the fine lines, and the lines split and condensed according to the periodic energy density distribution. Eventually, the periodically aligned nano-sized dots were fabricated on the SiO2 film.

Tribological behavior of nano-undulated surface of diamond-like carbon films

Tribological behavior of nano-undulated diamond-like carbon (DLC) films of the surface roughness ranging from 0.6 to 13.7 nm was investigated in an ambient air of 50% relative humidity. The nano-undulated DLC films were prepared by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) using nanosized Ni dots on a Si (100) substrate. The friction coefficient between the DLC film and the steel ball was characterized by a ball-on-disk type wear rig. Auger and Raman spectroscopy analysis of the debris revealed that the tribochemical reaction with environment was significantly suppressed as the surface roughness increased. Even if the rough surface increased the wear rate of the steel ball and thus the concentration of Fe in the debris, neither the oxidation of Fe nor the graphitization of the carbon in the debris occurred on the rough surface. However, the frictional behavior was affected by several factors: the composition and the size of debris, plowing effect of the rough surface, and the presence of the transfer layer on the wear scar surface.

FABRICATION OF UNIFORM ARRAY OF NANO-DOTS AND ITS APPLICATIONS

We have developed a novel technique to fabricate extremely uniform arrays of nano-sized dots. Regular arrays of nanometer-sized metallic dots are obtianed by magnetron sputtering deposition through a nanochannel glass replica mask. The platinum replica masks are fabricated using thin film deposition on an etched nanochannel glass and contain hexagonally patterned uniform voids with diameters as small as 50 nanometers. The metallic dot density can be as high as 5×1011 per square centimeter. The magnetic properties of Ni and Co dot arrays have been studied by a Alternating Gradient Magnetometer. The magnetic dot array can be used as pinning centers for superconductors and also can be used as the discrete magnetic media for magnetic recording.

AHE measurements of very thin films and nanosized dots

In this paper we present anomalous Hall effect analysis from very thin Co (0.5nm) film, Co/Pt multilayers and large areas of nanosized dots as well as from a few magnetic dots having a diameter of 120nm. The dot array is prepared from Co/Pt multilayer by using laser interference lithography (LIL) while the Hall crosses for measuring a few dots are prepared in combination with focussed ion beam (FIB). The hysteresis loops from a few dots are showing significant Hall voltage jumps corresponding to magnetic response due to an inhomogeneous reversal mechanism because the intensity of the jumps is smaller than the expected value from a total magnetization reversal of one dot.

Nanocomposite ta-C films prepared by the filtered vacuum arc process using nanosized Ni dots on a Si substrate

Recently, structural manipulation of tetrahedral amorphous carbon (ta-C) film at the nanometer scale has attracted much attention. We demonstrate a novel method to obtain a nanocomposite film where nanoscale columns of graphitic phase are embedded in a tetrahedral amorphous carbon matrix. When using a Si substrate with nanosized Ni dots on the surface, graphitic columns grew selectively on the Ni dots, while a dense ta-C film was deposited on the bare Si surface. The growth of the graphitic columns is closely related to the nanosized Ni dots that catalyze the graphitic-carbon formation in a filtered vacuum arc deposition condition.

Molecular-beam epitaxial growth of GaAs and InGaAs/GaAs nanodot arrays using anodic Al2O3 nanohole array template masks

Highly ordered arrays of nanosized GaAs-based dots were successfully prepared on GaAs (001) substrates by molecular-beam epitaxy using selected area growth. Selected area growth employed alumina nanochannel array (NCA) templates formed by anodic oxidation, bonded to the GaAs substrates. Homogeneous GaAs dots, as well as compositionally modulated heterostructures within the nanosized dots, were demonstrated. In the latter case, multilayer InGaAs/GaAs heterostructured nanodot arrays were fabricated. Dot growth occurred only as defined by the template mask, resulting in a hexagonal lattice of dots with 100 nm period spacing, with dots retaining the circular lateral shape of the pores as determined by the NCA template pore size; dot diameters were adjustable from 45 to 85 nm for a lattice period of 100 nm. Cathodoluminescence spectra from an InGaAs/GaAs 10×10 dot array clearly showed an emission peak at 920 nm (5 K), confirming the formation of a high-quality InGaAs/GaAs quantum dot array.