Narrow Height Distribution Research Articles

Wafer-scale integration of stretchable semiconducting polymer microstructures via capillary gradient

Organic semiconducting polymers have opened a new paradigm for soft electronics due to their intrinsic flexibility and solution processibility. However, the contradiction between the mechanical stretchability and electronic performances restricts the implementation of high-mobility polymers with rigid molecular backbone in deformable devices. Here, we report the realization of high mobility and stretchability on curvilinear polymer microstructures fabricated by capillary-gradient assembly method. Curvilinear polymer microstructure arrays are fabricated with highly ordered molecular packing, controllable pattern, and wafer-scale homogeneity, leading to hole mobilities of 4.3 and 2.6 cm2 V−1 s−1 under zero and 100% strain, respectively. Fully stretchable field-effect transistors and logic circuits can be integrated in solution process. Long-range homogeneity is demonstrated with the narrow distribution of height, width, mobility, on-off ratio and threshold voltage across a four-inch wafer. This solution-assembly method provides a platform for wafer-scale and reproducible integration of high-performance soft electronic devices and circuits based on organic semiconductors.

THE ROLE OF DEPOSITION TEMPERATURE ON PARTICLE SIZE, ROUGHNESS PARAMETERS, MAGNETIC AND STRUCTURAL FEATURES OF ELECTROCHEMICALLY GROWN Ni–Fe/ITO SAMPLES

In this work, the impact of the deposition temperature (DT) on the particle size, roughness parameters, coercive force, squareness and structural features of the Ni–Fe samples electrochemically deposited onto glasses covered with indium tin oxide (ITO) was researched. An increase in the DT from 20 to 40 °C led to a very slight increase in the Fe concentration of the samples, revealing that the effect of the DT on the chemical composition was insignificant. The crystal structure was a face–centered cubic (fcc) and the preferred growth orientation was in the [111] direction irrespective of the DT. The crystallization of the samples improved, the size of the crystallites increased and the strength of the [111] growth orientation diminished with the DT. An increment in the DT resulted in a strong enhancement in the particle size and surface roughness. Further surface analysis indicated that the sample surface had a well spread out height distribution at the DT of 40 °C, while the sample surfaces at lower DTs 20 and 30 °C had narrow height distributions. In addition to that, the peaks were found to be predominant on the sample surfaces at lower DTs (20 and 30 °C), whereas the deep valleys were predominant on the sample surface at the DT of 40 °C. Magnetic measurements confirmed the existence of a semi–hard magnetic property in all samples. Compared to other samples produced at lower DTs (20 and 30 °C), the Ni–Fe sample electroplated at the DT of 40 °C possessed the highest coercive force and squareness values. Moreover, the magnetic characteristics of the samples were compatible with their morphological and structural characteristics.

THE ROLE OF DEPOSITION TEMPERATURE ON PARTICLE SIZE, ROUGHNESS PARAMETERS, MAGNETIC AND STRUCTURAL FEATURES OF ELECTROCHEMICALLY GROWN Ni–Fe/ITO SAMPLES

In this work, the impact of the deposition temperature (DT) on the particle size, roughness parameters, coercive force, squareness and structural features of the Ni–Fe samples electrochemically deposited onto glasses covered with indium tin oxide (ITO) was researched. An increase in the DT from 20 to 40 °C led to a very slight increase in the Fe concentration of the samples, revealing that the effect of the DT on the chemical composition was insignificant. The crystal structure was a face–centered cubic (fcc) and the preferred growth orientation was in the [111] direction irrespective of the DT. The crystallization of the samples improved, the size of the crystallites increased and the strength of the [111] growth orientation diminished with the DT. An increment in the DT resulted in a strong enhancement in the particle size and surface roughness. Further surface analysis indicated that the sample surface had a well spread out height distribution at the DT of 40 °C, while the sample surfaces at lower DTs 20 and 30 °C had narrow height distributions. In addition to that, the peaks were found to be predominant on the sample surfaces at lower DTs (20 and 30 °C), whereas the deep valleys were predominant on the sample surface at the DT of 40 °C. Magnetic measurements confirmed the existence of a semi–hard magnetic property in all samples. Compared to other samples produced at lower DTs (20 and 30 °C), the Ni–Fe sample electroplated at the DT of 40 °C possessed the highest coercive force and squareness values. Moreover, the magnetic characteristics of the samples were compatible with their morphological and structural characteristics.

An ellipsometric analysis to model the order-disorder transition in Au-SiO2 nano-granular thin films induced by thermal annealing

We present an ellipsometric analysis to model the order-disorder transition in nano-granular gold embedded in a silica matrix (Au-SiO2). The optical measurements were performed by means of Reflection Spectroscopic Ellipsometry (RSE) over the spectral wavelength range 300–2500 nm. Upon thermal annealing of Au-SiO2 in the temperature range of 200 to 800 °C, a clear order-disorder textural transition in the film is exhibited. Analyzing the surface roughness by atomic force microscopy (AFM), the as-prepared film shows a narrow height distribution peak around ~3 nm, which reveals a good uniformity of the particles size. When the annealing temperature increases, the particles increasingly tend to coalesce into bigger sizes leading to non-homogeneous films showing increasingly larger surface roughness. We systematically determined the evolution of the film surface roughness parameters by using the AFM cumulative height distribution, which was introduced in RSE in order to evaluate the effective optical constants of the Au–SiO2 films, and to numerically fit the recorded Ψ, Δ and T spectra using a multiple Gaussian oscillators model. As a result, a reliable determination of the Au-SiO2 effective thicknesses and optical constants was achieved. Additionally, we fitted the experimental data to the Maxwell-Garnett (MG) effective medium approximation (EMA). It is shown that MG-EMA does not fit for the lower annealing temperature due to the particle self-organization. However, when the particles disorder takes place, as a consequence of the annealing temperature increase, the MG-EMA fit is increasingly improved. All in all, a clear evidence of the order-disorder transition in the Au-SiO2 nano-granular system induced by thermal annealing is provided.

Catalyst free growth of highly crystalline ZnO nanopillars on c-GaN/sapphire templates by chemical vapor deposition technique

The catalyst free growth of a large density of highly oriented ZnO nanopillars on c-GaN/sapphire templates has been demonstrated through a chemical vapor deposition route. The pillars are found to have a narrow distribution of height and diameter. It has been observed that the morphology of the growth can be varied from entirely continuous epitaxial layer to oriented nanopillars by controlling the growth temperature and growth duration. These parameters can also be adjusted to control the average diameter and height of these pillars. The authors attribute the formation of nanopillars to a two dimension to three dimension transition of the growth mode as a result of an abrupt change of Zn to oxygen flux ratio upon the closure of oxygen flow into the reactor at the end of the growth.

Molecular configuration of gelatin–water suspensions at low concentration

Direct assessment of gelatin molecular configuration has been difficult due the complexity of the molecule and limitations of analytical techniques. The objective of this work was the molecular characterization of bovine gelatin as a function of temperature, concentration and time. Diluted suspensions were prepared at different concentrations (7.5 × 10−5 to 1.5 g/l) and kept at temperatures above (S1) (40 °C) and below (S2) (5 °C) the gelling point as determined by Differential Scanning Calorimetry. Circular dichroism measurements showed the secondary structure of a polyproline II like spectra similar to native collagen at S1 and a denaturated configuration at S2 conditioning. Atomic Force Microscopy (AFM) at S1, using a HOPG substrate, showed single strands segments with two marked distributions in heights, ∼0.6 nm and ∼1.6 nm, indicating possible helical configurations even at high temperatures. At S2, AFM showed only one height distribution in the range of ∼0.9 nm but wider (0.3–1.6 nm range). At increasing gelatin concentration (12 g/l) and annealing time (48 h), a well-defined network was detected with narrow height distribution (∼1.0 nm) featuring aggregates and highly ordered structure zones. An analysis of gelatin strand interactions, showed a network linked by knots with a coordination number Z = 3 (strands) with a bond length of ∼50 nm. The gel network formation and aggregation was consistent with molecular size increase observed by Dynamic Light Scattering, showing variations in hydrodynamic dimensions from ∼10 nm (S1) to ∼100 nm (S2). This experimental approach has allowed to pinpoint differences in molecular configuration of gelatin, which may be applied in the study the structuring pathway of other biopolymers and the association kinetics during storage for a wide range of temperatures.

Self‐Assembly: I‐Motif Nanospheres: Unusual Self‐Assembly of Long Cytosine Strands (Small 8/2011)

AbstractThe synthesis and characterization of novel DNA structures based on tetraplex cytosine (C) arrangements, known as i‐motifs or i‐tetraplexes, is reported. Atomic force microscopy (AFM) investigation shows that long C‐strands in mild acidic conditions form compact spherically shaped nanostructures. The DNA nanospheres are characterized by a typical uniform shape and narrow height distribution. Electrostatic force microscopy (EFM) measurements performed on the i‐motif spheres clearly show their electrical polarizability. Further investigations by scanning tunneling microscopy (STM) at ultrahigh vacuum reveals that the structures exhibit an average voltage gap of 1.9 eV, which is narrower than the voltage gap previously measured for poly(dG)–poly(dC) molecules in similar conditions.

I‐Motif Nanospheres: Unusual Self‐Assembly of Long Cytosine Strands

The synthesis and characterization of novel DNA structures based on tetraplex cytosine (C) arrangements, known as i-motifs or i-tetraplexes, is reported. Atomic force microscopy (AFM) investigation shows that long C-strands in mild acidic conditions form compact spherically shaped nanostructures. The DNA nanospheres are characterized by a typical uniform shape and narrow height distribution. Electrostatic force microscopy (EFM) measurements performed on the i-motif spheres clearly show their electrical polarizability. Further investigations by scanning tunneling microscopy (STM) at ultrahigh vacuum reveals that the structures exhibit an average voltage gap of 1.9 eV, which is narrower than the voltage gap previously measured for poly(dG)-poly(dC) molecules in similar conditions.

Plasma-controlled metal catalyst saturation and the initial stage of carbon nanostructure array growth

The kinetics of the nucleation and growth of carbon nanotube and nanocone arrays on Ni catalyst nanoparticles on a silicon surface exposed to a low-temperature plasma are investigated numerically, using a complex model that includes surface diffusion and ion motion equations. It is found that the degree of ionization of the carbon flux strongly affects the kinetics of nanotube and nanocone nucleation on partially saturated catalyst patterns. The use of highly ionized carbon flux allows formation of a nanotube array with a very narrow height distribution of half-width 7 nm. Similar results are obtained for carbon nanocone arrays, with an even narrower height distribution, using a highly ionized carbon flux. As the deposition time increases, nanostructure arrays develop without widening the height distribution when the flux ionization degree is high, in contrast to the fairly broad nanostructure height distributions obtained when the degree of ionization is low.

Formation and evolution of self-assembled crystalline Si nanorings on (001) Si mediated by Au nanodots

The growth of high-density Si nanorings has been achieved on ultrathin Au films on silicon substrate. Nanorings, which are gold-free and epitaxially grown on (001) Si, had a narrow distribution of height and diameter of 1.4±0.3 and 24.9±4.0nm, with a density of 4.3×1010cm−2. In situ ultrahigh-vacuum transmission electron microscopy revealed that the formation of nanorings involves a mechanism mediated by Au nanodots and evaporation of Au-Si eutectic liquid droplets at high temperature. The process promises to be an effective nanofabrication technique to produce high-density and uniform-sized nanorings.

From large to low height dispersion for self-organized InAs quantum sticks emitting at 1.55 μm on InP (001)

We show how the height dispersion of self-organized InAs/InP(001) quantum islands emitting at 1.55 μm was reduced by optimizing the epitaxial growth parameters. Low height dispersion was obtained when the InAs deposit thickness was much greater than the critical thickness for two-dimensional/three-dimensional growth mode transition, and when adatom surface diffusion was favored by increasing the growth temperature or reducing the arsenic pressure during the InAs growth. When these growth conditions are not respected, the multicomponent photoluminescence spectrum obtained is explained through the common interpretation of island height varying with monolayer fluctuation. In optimized growth conditions, the multicomponent spectrum obtained is interpreted as emission from fundamental and excited levels of InAs islands with low height dispersion. Transmission electron microscopy (TEM) imaging shows that these InAs islands are stick-like, 50–100 nm in length and 22±1.2 nm in width. Cross-sectional TEM reveals flat islands, shaped like truncated pyramids, with a very homogeneous height measured at 2.4 nm. A fundamental level linewidth of 22 meV at 8 K is associated to this very narrow height distribution. Such low photoluminescence linewidth values are believed to be mainly due to the propensity of the InAs/InP(001) system to produce flat InAs islands with discrete height fluctuation.

Electrodeposition of stable and narrowly dispersed germanium nanoclusters from an ionic liquid

Germanium nanoclusters with a narrow height distribution have been electrodeposited from a dilute solution of GeCl4 in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate [BMIm]PF6: under the reported conditions the lateral sizes of most of the clusters range between 20 and 30 nm while their heights vary from 1 to 10 nm with most of them between 1 and 5 nm.

Self-assembled InAs islands on GaAs(1̄1̄1̄) substrates

Characterization of InAs self-assembled islands, grown on GaAs(111)B by molecular beam epitaxy, is carried out by atomic force microscopy and photoluminescence spectroscopy for different growth conditions. Islands show lateral sizes ca 40 to 300 nm long by 30 nm wide. Narrow height distributions are achieved centered ca 4 or 20 nm, depending on the growth conditions. Samples with the taller islands show the stronger luminescence. The position of the peaks is surprisingly similar to those reported for self-assembled islands on GaAs(100), despite the differences on crystal orientation and island sizes. This results suggests that the photoluminescence observed for both (100) and (111) orientation is not determined by quantum size confinement in the islands.