Multiscale Roughness and Stability of Superhydrophobic Biomimetic Interfaces

Michael Nosonovsky

 

Langmuir 2007, 23, 3157-3161

 

Purpose

Wettability of a solid by a liquid is characterized by the contact angle, which is the angle between the solid-air and the liquid-air interfaces. The greater the contact angle, the more hydrophobic the material. More recently there has been an increasing interest in superhydrophobic surfaces, which are surfaces with very high contact angles (>150°) and low contact angle hysteresis. This behavior is explained for the presence of air pockets in the valleys between the asperities and the water drop. According to the previous, it is of great importance Formulate a stability criterion for composite interfaces with different roughness profiles, and analyze the implications that different kind of roughness surfaces have over the superhydrophobic character.

 

Key Findings

·         Superhydrophobic surfaces should satisfy the following requirements: they should have a hydrophobic coating, high roughness factors, providing a high contact angle, and the ability to form a composite interface.

·         Superhydrophobic surfaces require having a stable composite interface, a hierarchical roughness structure with nanoscale bumps upon microscale asperities and valleys.

·         Composite interfaces are fragile, since transition to a homogeneous interface is irreversible.

·         Multiscale roughness can help to resist the destabilization, with convex surfaces pinning the interface and thus leading to a stable equilibrium and preventing the filling of gaps between the pillars even in the case of a hydrophilic material.

 

Definitions

·         Angle hysteresis is the difference between advance and receding contact angles.

·         Advance contact angle: is the angle that forms when liquid is added to a surface.

·         Receding contact angle : is the angle that forms when liquid is removed from a surface

 

 

 

Low cycle fatigue of single crystal silicon thin films

Hsien-Kuang Liu, BJ Lee, Pang-Ping Liu

Sensors and Actuators A 140 (2007) 257-265

 

Purpose: The work in this study was aimed at understanding the fatigue of single crystal silicon thin films by a cantilever microbeam.  SEM was used to observe the modes of fracture and deformation for various conditions of loading.

Methods: Fatigue testing was conducted using a tungsten probe such that the probe is held firmly against the specimen and subjected to a cyclic load. SEM imaging was used to determine the failure modes of the cantilever under various strain amplitudes. EDS (Engergy Dispersive Spectrometer) was used to analyze the chemical composition of the failed sample.

Key Findings: 

1. Single crystal silicon can fail at strains near one half of their static failure strain for cyclic loading in excess of a million cycles.

2. Dominant failure mode is cleavage at a surface flaw with striations parallel to the length direction.

3. Fatigue crack initiates from the top flat surface of the beam and propagates in the {1 1 1} plane.

4. Two sub-mechanisms are suggested for failure: 1st) fracture debris at incident point inhibits crack propgation, and 2nd) moisture enhances surface reaction resulting in crack formation and eventual failure.

 

Direct synthesis of high-quality single-walled carbon

nanotubes on silicon and quartz substrates

Yoichi Murakami, Yuhei Miyauchi, Shohei Chiashi, Shigeo Maruyama

Chemical Physics Letters 377 (2003) 49–54

Purpose

A new dip-coating technique for synthesizing high-quality SWNT’s on silicon and quartz substrates has been developed. This new method does not require support/underlayer material and is more cost effective and easier to implement than previous techniques.

Methods

Either n-type Silicon or fused quartz was submerged into a prepared metallic acetate solution for 10 min. The substrate was then dried, placed in a 400°C furnace for 5 min., and finally underwent the CVD process. The CVD process was performed at varying temperatures and used the reaction of ethanol vapor to synthesize SWNT’s on the substrate surface. The synthesized SWNTs were characterized by FE-SEM and micro-Raman scattering measurements.

 

Key Findings

·         The amount of synthesized SWNT’s was quantified by using the height ratio of the G-band to the Si-derived peak for Raman spectra. This analysis showed that the optimal temperature for SWNT synthesis was 800°C, as this is where the largest ratio occurred.

 

·         The high quality of the SWNT’s was confirmed by the high G/D ratio in the Raman spectra.

 

·         The diameter of the SWNT’s was estimated from the RBM Raman shift and found to be between 1.1 to 1.7 nm.

 

·         SEM images show a web-like layer of SWNT’s atop the substrate surface.

 

·         TEM images show that the SWNT are free of metal debris and multi-walled nanotubes.

 

 

RBM: Radial Breathing mode – part of the Raman spectra that can characterize certain features of SWNT’s (such as diameter).

D: disorder induces mode – part of the Raman spectra

Quenching of photoconductivity in Fe doped CdS thin films prepared by spray pyrolysis technique

Badera N, Godbole B, Srivastava SB, et al.

APPLIED SURFACE SCIENCE Vol. 254 Issue: 21 p7042-7048, 2008

Purpose

The purpose of this study was to synthesize a CdS thin film material that is Fe doped prepared using spray pyrolysis technique, and then characterize it with X-ray diffraction, Raman spectroscopy, and AFM comparing the different levels of Fe doping.

Methods Used

The Cd_1−xFe_xS film was created using a spray pyrolysis technique where the amount of iron was altered within the range of {x: 0<x<.21}, and then the ratio was confirmed by energy dispersive X-ray analysis. The characterization was done using Raman spectroscopy, an AFM in contact mode, UV–vis spectroscopy, and conductivity measurements from a specially created apparatus.

Key Findings

The first interesting finding was from the x-ray diffraction. As the concentration of iron increased the cell volume of the crystal structure decreased. This was confirmed by the AFM where the hexagonal cells slowly converted into nanorods as the concentration of iron increased. Second is that the Raman peaks did not change with the iron doping. As the concentration of iron increased, so did the size of the nanorods from nanoneedle size, which the authors attributed to Ostwald ripening. Finally as the concentration of the iron increases the photoconductivity goes to zero. This property could be useful for sensors.

Scanning Photoemission Microscopy of Graphene Sheets on Silicon Dioxide

By: Ki-jeong Kim, Hangil Lee, Jae-Hyun Choi, Young-Sang Youn, Junghun Choi, Hankoo Lee, Tai-Hee Kang, M. C. Jung, H. J. Shin, Hu-Jong Lee, Sehun Kim, and Bongsoo Kim

Published in: Adv. Mater. 2008, 20, 3589–3591

Purpose of Study

1. To use Scanning Photoemission Microscopy (SPEM) to investigate the properties of graphene flakes on SiO₂

2. To determine if there are any differences between monolayer and multilayer graphite in the Carbon 1s core-level spectra and if the layer thickness may be determined using these differences.

Methods Used

Graphene flakes (prepared by the exfoliation of bulk graphite) of monolayer thickness were selected with the aid of optical microscopy. The flakes were deposited onto the surface of a Si wafer covered with a layer of SiO₂ of 300 nm thickness. AFM was used to measure step heights. SPEM measurements were conducted under ultra high vacuum conditions. The SPEM images were obtained using binding energies between 288.2 and 281 eV of the C 1s core-level spectra. The spectra were obtained using emission and incident angles of 55° and 0° respectively. The incident photon beam size on the sample was 1µm.

Key Findings

1. The authors were able to more accurately determine layer thickness using AFM than SPEM images.
2. SPEM was used to create a chemical contrast image which was used to distinguish between single-layer and multi-layer graphene.
3. As the binding energies used were changed different regions of the graphite film became more visible. This indicated the presence of core-level-shifted features. These features represent different chemical states which depend on the thickness of the graphene flakes.
4. The core-level shift in monolayer graphene originates from graphene-SiO₂ interactions.

Definitions

SPEM- A surface is irradiated with UV photons focused to a narrow spot and the total yield (I) of electrons emitted from the surface is measured. I depends on work function and photon energy and is measured as the spot is rastered across the sample. Intensity differences are converted into an image which reflects spatial variations of the work function.

Interplay between Auger and Ionization Processes in Nanocrystal Quantum Dots

Robert M. Kraus, Pavlos G. Lagoudakis, Josef Muller, Andrey L. Rogach, John M. Lupton, and Jochen Feldmann, Dimitri V. Talapin and Horst Weller

J.Phys. Chem. B, Vol. 109, No.39, 2005

 

Purpose of the study

Study the interplay between Auger effects and ionization processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots. Explain that spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence decay rate on excitation density.

 

Methods

In the experiment, Time-resolved PL measurements were performed on 130-nm thick films of dispersed NQDs (average interparticle distance ~30 nm) embedded in polystyrene using time correlated single-photon counting of 1.2-ns resolution. The NQDs were excited nonresonantly with 130-fs pulses at 400nm at room temperature under vacuum conditions.

Then a systematical model is proposed in the quantized Auger regime describing these experimental observations and providing an estimate of the Auger assisted ionization rates.

 

Key findings

1.   Cooperative nonradiative effects dominate the fluorescence decay dynamics in the high-excitation-density regime. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the PL decay rate with excitation density. A kinematic analysis suggests that ionization processes, which lead to the occupation of dark, nonemissive nanocrystal states, are accelerated in the presence of Auger recombination (which occurs on much faster time scales).

2.   The distribution of NQD sizes in an ensemble impedes a quantitative interpretation of the recombination dynamics. Spectral filtering of the PL can substantially reduce the size distribution of NQDs involved in the dynamics. And Cooperative nonradiative effects, such as Auger recombination and phonon assisted tunneling dominate the PL dynamics of NQDs.

3.   Auger recombination becomes progressively faster as the number of e-h pairs N in the NQD increases.

4.   The intriguing increase in the PL decay rate above the average population of one e-h pair can be described by the limit of strong electronic confinement.

 

Useful terms: (From http://en.wikipedia.org)

Auger recombination: An electron and e-h (electron-hole pair) can recombine giving up their energy to an electron in the conduction band, increasing its energy.

Ionization: Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions.

 

Molecular Structure Analysis in a Dip-Coated Thin Film of Poly(2-perfluorooctylethyl acrylate) by Infrared Multiple-Angle Incidence Resolution Spectrometry

Masaya Matsunaga, Toshio Suzuki, Kiyoshi Yamamoto, and Takeshi Hasegawa

 

Macromolecules 2008, 41, 5780-5784

 

Purpose of the Study

 

The authors use infrared spectroscopy to determine structural and conformational details of a polymer adsorbed to a silicon substrate.

 

Methods and Procedures

 

Polymer films were prepared via dip-coating a silicon substrate with a thin film of poly(2-perfluorooctylethyl acrylate).  The samples were subjugates to infrared-multiple angle incidence resolution spectrometry (IR-MAIRS).  For comparison, the x-ray diffraction pattern of poly(2-perfluorooctylethyl acrylate) powder was also collected.  Using the diffraction patterns, infrared spectra, and appropriate models, the polymer structure and confirmation on the surface can be determined.

 

Key Results

 

• Infrared Spectroscopy can be used to determine structural and conformational properties of non-crystalline materials.
• Using IR-MAIRS, both transmittance and reflective-absorbance IR spectra can be collected on the same sample; thus, taking advantage of selectively observed parallel and normal vibrations ,respectively.
• The poly(2-perfluorooctylethyl acrylate) films investigated are oriented with the perfluoronated side-chains, with a length of 1.6 nm) normal to the surface.  The tilt angle of the carbonyl group is ~37^o.

Fundamental aspects of nano-reinforced composites

Bodo Fiedler, Florian H. Gojny, Malte H.G. Wichmann, Mathias C.M. Nolte, Karl Schulte

Composites Science and Technology, 66 (2006) 3115-3125

Purpose of Study

To understand the importance and challenges of using CNT’s as nanofillers in polymers. Particle size relations, seperation and volume content are analyzed analytically. The influence of the manufacturing process is assessed and the resulting mechanical properties are investigated.

Methods

1) Analytical methods are used to characterize fundamental aspects of particle size and separation. An average separation distance for randomly arranged particles is calculated based on volume percent of fibers.

2) CNT-polymer composites are manufactured utilizing chemical functionalization of the particle surface. This is achieved by oxidative treatment of the CNT  to enable direct bonding to the matrix. Further, dispersive techniques of sonication, stirring, and calendering are used to prevent agglomeration of fibers.

3) Fracture toughness values are measured for polymer-CNT specimens. TEM microscopy images are observed to show the phenomenon of crack bridging.

Key Findings

1) An increase in fracture toughness is observed with increased filler content. A significant enhancement is also observed in composites containing spherical nano-particles. The increase is attributed to nanofiber bridging of the matrix cracks and enhanced interfacial stresses.

2) It is seen that dispersion of nanofibers aids in increase of mechanical properties. This is due to the prevention of agglomeration which will cause weak spots inside the matrix.

3) Increase in particle content volume is seen to reduce the surface separation between spherical particles. Analytical methods show that spheres have a greater seperation compared to hexagonal fibers.

Key Terms

CNT-Carbon Nanotube

Dip-pen lithography using aqueous metal nanocrystal dispersions

John Thomas, P.. “Dip-pen lithography using aqueous metal nanocrystal dispersions.” Journal of materials chemistry 14.4 (2004):625-628.

Purpose of Study

This work seeks to fabricate patterns of chemically prepared nanocrystals using dip-pen nanolithography (DPN).  Special interest is found in the use of metallic nanocrystals due to their ability to self-assemble.  Further applications are expected in the realm of nanoelectronics; however, realizing future nanodevices will require accurate positioning of nanocrystals on a surface.

Method

Pd and Au nanocrystals were fabricated by standardized methods reported elsewhere.  Notably, each nanocrystal synthesis procedure involved a surface stabilization step.  In that manner, Pd nanocrystals were capped with polyvinylpyrrolidone (PVP) and the Au nanocrystals were stabilized with tetrakishydroxymethyl phosphonium (THP).  DPN was performed under ambient conditions using a Nanoscope IV controller attached to a Digital Instruments multimode-head set in contact mode.  Silicon-nitride cantilevers were dipped in nanocrystal dispersions and allowed to dry in air.  Patterning was carried out on mica surfaces using scan speeds of one micro-meter per second.

Key Findings

1.) Aqueous dispersions of Au and Pd nanocrystals were successfully demonstrated as nanopatterning inks using DPN.

2.) High aspect-ratio lines and rectangles of nanocrystals were fabricated. 

3.) No evidence for lateral diffusion was found when the cantilever was held to the mica surface for extended time-periods.  Instead, a small vertical height increase is observed with time.  This was shown to occur regardless of the applied force.  These results are attributed to interactions with the substrate.  Thus, this process is noted as being highly substrate dependent.

Key words:  DPN – dip pen lithography, Aspect Ratio - the proportion of an image size given in terms of the horizontal length vs. the vertical height.

Mechanochemical Preparation of Magnetite Nanoparticles by Coprecipitation

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Mechanochemical Preparation of Magnetite Nanoparticles by Coprecipitation

Purpose: This article presents a novel process for preparing magnetite nanoparticles via coprecipitation without the need for surfactant or oxidizing/reducing agents.

Method: Fe₃O₄ nanoparticles were synthesized in a ball mill by first dissolving Iron (III) Chloride in water. The solution was placed in an inert atmosphere and NaOH solution was added. The precipitate formed was then added to a ball mill which was rotated at a rate of 35 rpm. The morphology was then examied with a FE-SEM and EDS. The XRD patterns were also measured with CuKα radiation.

Key Findings: A very simple process to prepare magnetite nanoparticles with high crystallinity in a single step was shown. The ball mill technique increases the reaction rate, while inhibiting particle size. The mechanical nature of the nanoparticle formation implies the possibility of the use of these nanoparticles in the biomedical field.