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IN THIS ISSUE

### Editorial

J. Nanotechnol. Eng. Med. 2014;5(2):020201-020201-2. doi:10.1115/1.4028352.
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Nanotechnology has seen rapid progress in recent years, with the emergence of advanced capabilities to synthesize and characterize precisely engineered materials that point toward disruptive new performance regimes of relevance for diverse application areas. Understanding how the atomic, electronic, mechanical, and magnetic structures/properties of materials relate to their performance across multiple length-scales is, thus, of growing importance. This special topic titled “Spectroscopy, Scattering, and Imaging Techniques for Nanostructured Materials” focuses on understanding these fundamental processes, which occur within material systems in the atomic or nanoscopic regime, using advanced tools such as time-of-flight secondary ion mass spectrometry, scanning electron microscopy (SEM), X-ray diffraction (XRD), helium ion microscopy (HIM), atomic force microscopy, Raman thermometry, and in situ imaging techniques. This understanding is being leveraged by the scientific community to deliver new knowledge that has the potential to improve the performance of different material systems: lithium-ion battery materials, biological materials, nanostructured materials for energy applications, carbon nanofiber, nanoparticles, nanowires (NW), silicon microcantilevers, etc. The special topic brings together a wide variety of excellent contributions from the scientific community showcasing the depth and breadth in this vibrant topical area within nanotechnology. The collection of papers exemplifies how the current state-of-the-art of imaging and spectroscopic techniques provides new insights into these exciting nano and biological materials with unprecedented resolution.

Commentary by Dr. Valentin Fuster

### Review Article

J. Nanotechnol. Eng. Med. 2014;5(2):020801-020801-9. doi:10.1115/1.4028040.
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Differential mobility particle sizers (DMPSs) are instruments for online sizing gas-borne particles in submicrometer and nanometer diameter ranges. The aerosol charger, the differential mobility analyzer (DMA), and the particle concentration detector are three essential components in DMPSs. In the past four decades, the design of DMAs has evolved into a variety of modern versions to extend their sizing limits, especially in lower detectable size limits. The DMAs are now capable of classifying or sizing particles in the diameters down to 1.0 nm. This article gives a brief overview of state-of-the-art DMAs particularly designed for classifying particles with sizes down to sub-10 nm.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Nanotechnol. Eng. Med. 2014;5(2):021001-021001-6. doi:10.1115/1.4028045.
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This paper presents atomic force spectroscopy (AFM) results from large diameter nanowires (NWs), which range in radius from 150 nm to 300 nm, within a nano-assembled platform. The nanomechanical platform is constructed by assembling single NWs across pairs of gold nano-electrodes using dielectrophoresis and contains a short, suspended segment of the NW (in air) between the assembly electrodes. Atomic force microscope (AFM) force spectroscopy measurements are obtained by indenting the NW within this suspended segment and result in deformation of the NW involving a combination of both, bending and nano-indentation modes. This paper demonstrates the measurement technique using lithium iron phosphate NWs as a model system and presents a finite element model to extract the Young's modulus from nanomechanical data. The estimated Young's modulus of this material, which is an electrode material system of interest for next-generation lithium-ion batteries, was found to be diameter dependent and was observed to range in values between 100 MPa and 575 MPa.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2014;5(2):021002-021002-5. doi:10.1115/1.4028010.
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To develop lithium-ion batteries with a high rate-capability and low cost, the prevention of capacity loss is one of major challenges, which needs to be tackled in the lithium-ion battery industry. During electrochemical processes, lithium ions diffuse from and insert into battery electrodes accompanied with the phase transformation, whereas ionic diffusivity and concentration are keys to the resultant battery capacity. In the current study, we compare voltage versus capacity of lithium-ion batteries at different current-rates (C-rates) discharging. Larger hysteresis and voltage fluctuations are observed in higher C-rate samples. We investigate origins of voltage fluctuations by quantifying lithium-ion intensity and distribution via a time-of-flight secondary ion mass spectrometry (ToF-SIMS). The result shows that for fully discharged samples, lithium-ion intensity and distribution are not C-rate dependent, suggesting different lithium-ion insertion mechanisms at a higher C-rate discharging might be solely responsible for the observed low frequency voltage fluctuation.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2014;5(2):021003-021003-11. doi:10.1115/1.4027989.
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Biological materials such as bone have microstructure that incorporates a presence of a significant number of interfaces in a hierarchical manner that lead to a unique combination of properties such as toughness and hardness. However, studies regarding the influence of structural hierarchy in such materials on their physical properties such as thermal conductivity and its correlation with mechanical stress are limited. Such studies can point out important insights regarding the role of biological structural hierarchy in influencing multiphysical properties of materials. This work presents an analytic-experimental approach to establish stress–thermal conductivity correlation in bovine cortical bone as a function of nanomechanical compressive stress changes using Raman thermometry. Analyzes establish empirical relations between Raman shift and temperature as well as a relation between Raman shift and nanomechanical compressive stress. Analyzes verify earlier reported thermal conductivity results at 0% strain and at room temperature in the case of bovine cortical bone. In addition, measured trends and established thermal conductivity–stress relation indicates that the thermal conductivity values increase up to a threshold compressive stress value. On increasing stress beyond the threshold value, the thermal conductivity decreases as a function of increase in compressive strain. Interface reorganization versus interface related phonon wave blocking are the two competing mechanisms highlighted to affect such trend.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2014;5(2):021004-021004-9. doi:10.1115/1.4027877.
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This research reports in situ creep properties of silicon microcantilevers at temperatures ranging from 25 °C to 100 °C under uniaxial compressive stress. Results reveal that in the stress range of 50–150 MPa, the strain rate of the silicon cantilever increases linearly as a function of applied stress. The strain rate (0.2–2.5 $×10-6s-1$) was comparable to literature values for bulk silicon reported in the temperature range of 1100–1300 °C at one tenth of the reported stress level. The experiments quantify the extent of the effect of surface stress on uniaxial creep strain rate by measuring surface stress values during uniaxial temperature dependent creep.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2014;5(2):021005-021005-7. doi:10.1115/1.4028026.
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This paper presents results and discussion from a comprehensive morphological and crystallographic characterization of nickel nanowires synthesized by template-based electrodeposition method. In particular, the influence of magnetic and electric field (current density) conditions during the synthesis of nickel nanowires was studied. The structure and morphology of the synthesized nanowires were studied using Helium ion microscopy (HIM) and scanning electron microscopy (SEM) methods. The HIM provided higher quality data and resolution compared to conventional SEM imaging. The crystallographic properties of the grown nanowires were also studied using X-ray diffraction (XRD). The results clearly indicated that the morphological and crystallographic properties of synthesized nickel nanowires were strongly influenced by the applied magnetic field and current density intensity during the synthesis process.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2014;5(2):021006-021006-5. doi:10.1115/1.4028300.
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Lignin is a renewable material and it is abundantly available as low priced industrial residue. Lignin-based carbon fibers are economically attractive and sustainable. In addition, remarkably oxidized molecule of the lignin decreases the required time and temperature of the thermostabilization process compared to other carbon fiber precursors such as polyacrylonitrile (PAN); and thus, decreases the processing cost of carbon fiber production. The fraction 4 of softwood Kraft lignin (SKL-F4) was previously shown to be spinnable via electrospinning to produce carbon nanofibers. In this paper, we characterized different Kraft lignin powders through X-ray diffraction (XRD) analysis to measure the mean size of the ordered domains in different fractionations of softwood and hardwood samples. According to our results, SKL-F4 has largest ordered domains among SKLs and highest hydroxyl content according to Fourier transform infrared (FTIR) analysis. In addition, variations in the XRD patterns during carbon nanofiber formation were studied and the peak for (101) plane in graphite was observed in the carbon nanofiber carbonized at 1000 °C.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2014;5(2):021007-021007-8. doi:10.1115/1.4028284.
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Several chemical methods for the separation of nanoparticles from a colloidal mixture in a phase change material (PCM) have been developed and systematically investigated. The phase changing property of the colloidal mixture is used in energy storage applications and the mixture is labeled as the nanostructure enhanced phase change materials (NEPCM). The objective is to investigate viable methods for the separation and reclamation of the nanoparticles from the NEPCM before its disposal after its useful life. The goal is to find, design, test, and evaluate separation methods which are simple, safe, effective, and economical. The specific NEPCM considered in this study is a colloidal mixture of dodecane (C12H26) and CuO nanoparticles of 1–5% mass fraction and 5–15 nm size distribution. The nanoparticles are coated with a surfactant to maintain colloidal stability. Various methods for separating the nanoparticles from the NEPCM are explored. The identified methods are: (i) chemical destabilization of nanoparticle surfactants to facilitate gravitational precipitation, (ii) silica column chromatography, and (iii) adsorption on silica particle surface. These different methods have been pursued, tested, and analyzed; and the results are presented in this article. These methods are found to be highly efficient, simple, safe, and economical.

Commentary by Dr. Valentin Fuster
J. Nanotechnol. Eng. Med. 2014;5(2):021008-021008-7. doi:10.1115/1.4028603.
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Delivering foreign molecules into human cells is a wide and ongoing area of research. Gene therapy, or delivering nucleic acids into cells via nonviral or viral pathways, is an especially promising area for pharmaceutics. All gene therapy methods have their respective advantages and disadvantages, including limited delivery efficiency and low viability. We present an electromechanical method for delivering foreign molecules into human cells. Nanoinjection, or delivering molecules into cells using a solid lance, has proven to be highly efficient while maintaining high viability levels. This paper describes an array of solid silicon microlances that was tested to determine efficiency and viability when nanoinjecting tens of thousands of HeLa cells simultaneously. Propidium iodide (PI), a dye that fluoresces when bound to nucleic acids and does not fluoresce when unbound, was delivered into cells using the lance array. Results show that the lance array delivers PI into up to 78% of a nanoinjected HeLa cell culture, while maintaining 78–91% viability. With these results, we submit the nanoinjection method using a silicon lance array as another promising particle delivery method for mammalian culture cells.

Commentary by Dr. Valentin Fuster