Silicon Micropillars Research Articles

Robustness of amorphous silicon during the initial lithiation/delithiation cycle

Recent research on the electrochemical lithiation of amorphous silicon nanoparticles shows that amorphous silicon is more fracture resistant than crystalline silicon during lithiation. Nanoparticles of amorphous silicon can be lithiated and delithiated without any fracture at all. To fully exploit the potential of using amorphous silicon as electrodes for lithium ion batteries it is important to determine if larger, micron-sized, amorphous silicon structures can be lithiated and delithiated without fracture. Here we study the morphologies of initially amorphous silicon micropillars (∼2.3 μm tall) both before and after electrochemical lithiation and delithiation. No internal or external cohesive cracking is detected in lithiated pillars for any of the pillar sizes studied. Delithiated pillars exhibit some delamination at the interface between the pillar and the underlying nickel substrate. For larger diameter pillars, the initiated interfacial crack is driven upward into the delithiated pillar as the crack propagates radially inward. However, no cohesive fracture unrelated to interfacial cracking is seen in even the largest delithiated pillars. Finite element modeling provides support for the observation that the cohesive fracture resistance of amorphous silicon micropillars is representative of the fracture resistance of amorphous silicon microparticles of comparable dimensions.

Selective on site separation and detection of molecules in diluted solutions with super-hydrophobic clusters of plasmonic nanoparticles.

Super-hydrophobic surfaces are bio-inspired interfaces with a superficial texture that, in its most common evolution, is formed by a periodic lattice of silicon micro-pillars. Similar surfaces reveal superior properties compared to conventional flat surfaces, including very low friction coefficients. In this work, we modified meso-porous silicon micro-pillars to incorporate networks of metal nano-particles into the porous matrix. In doing so, we obtained a multifunctional-hierarchical system in which (i) at a larger micrometric scale, the super-hydrophobic pillars bring the molecules dissolved in an ultralow-concentration droplet to the active sites of the device, (ii) at an intermediate meso-scale, the meso-porous silicon film adsorbs the low molecular weight content of the solution and, (iii) at a smaller nanometric scale, the aggregates of silver nano-particles would measure the target molecules with unprecedented sensitivity. In the results, we demonstrated how this scheme can be utilized to isolate and detect small molecules in a diluted solution in very low abundance ranges. The presented platform, coupled to Raman or other spectroscopy techniques, is a realistic candidate for the protein expression profiling of biological fluids.

Cardiac Specific Knockout of P65 Mice Resist to Cardiac Ischemia- Reperfusion Injury

Superhydrophobicity refers to surfaces on which drops assume a quasi-spherical shape and a high contact angle (more than 150°). This well-known phenomenon occurs in nature and we can take advantage of this principle fabricating bio-inspired superhydrophobic surfaces constituted of ordinate arrays of silicon micro-pillars.

Enhanced lithiation and fracture behavior of silicon mesoscale pillars via atomic layer coatings and geometry design

Crystalline silicon nanostructures are commonly known to exhibit anisotropic expansion behavior during the lithiation that leads to grooving and fracture. Here we report surprisingly relatively uniform volume expansion behavior of large aspect-ratio (∼25), well-patterned, n-type (100) silicon micropillars (∼2 μm diameter) during the initial lithiation. The comparison results with and without atomic layer metal oxides (Al2O3 and TiO2) coatings reveal drastically enhanced solid electrolyte interphase (SEI) formation, higher volume expansion, and increased anisotropy. Square-pillars are found to exhibit nearly twice volume expansion without fracture compared to circular-pillars. Models are invoked to qualitatively address these beneficial or detrimental properties of silicon for lithium ion battery. Our experiments and computer simulations point at the critical relevance of SEI and pristine geometry in regulating volume expansion and failure. ALD-coated ultrathin metal oxides can act as an ion channel gate that helps promote fast Li+ transport into the bulk by changing the surface kinetics, suggesting new ways of designing electrodes for high-performance lithium ion battery applications.

High Surface Capacity Li-Ion All Solid State 3D Microbattery Based on Anatase TiO2 Deposited by ALD on Silicon Microstructures

The proposed study deals with the design, the fabrication and the characterization of 3D silicon microstructures decorated with step conformal and pin hole free anatase TiO2 negative electrode deposited by atomic layer deposition. A high aspect ratio and dense array of 3D silicon micropillars or microtubes are fabricated on silicon wafer. The obtained surface capacity of the TiO2 layer (150nm) deposited on 3D microtubes (area enlargement factor =25) reaches 0.15 mAh/cm2 at C/4. The reported 3D topologies enhance the capacity of standard TiO2 2D thin film electrode.

Elevated temperature, nano-mechanical testing in situ in the scanning electron microscope

A general nano-mechanical test platform capable of performing variable temperature and variable strain rate testing in situ in the scanning electron microscope is described. A variety of test geometries are possible in combination with focused ion beam machining or other fabrication techniques: indentation, micro-compression, cantilever bending, and scratch testing. The system is intrinsically displacement-controlled, which allows it to function directly as a micro-scale thermomechanical test frame. Stable, elevated temperature indentation∕micro-compression requires the indenter tip and the sample to be in thermal equilibrium to prevent thermal displacement drift due to thermal expansion. This is achieved through independent heating and temperature monitoring of both the indenter tip and sample. Furthermore, the apex temperature of the indenter tip is calibrated, which allows it to act as a referenced surface temperature probe during contact. A full description of the system is provided, and the effects of indenter geometry and of radiation on imaging conditions are discussed. The stabilization time and temperature distribution throughout the system as a function of temperature is characterized. The advantages of temperature monitoring and thermal calibration of the indenter tip are illustrated, which include the possibility of local thermal conductivity measurement. Finally, validation results using nanoindentation on fused silica and micro-compression of [100] silicon micro-pillars as a function of temperature up to 500 °C are presented, and procedures and considerations taken for these measurements are discussed. A brittle to ductile transition from fracture to splitting then plastic deformation is directly observed in the SEM for silicon as a function of temperature.

In-Flow DNA Extraction Using on-Chip Microfluidic Amino-Coated Silicon Micropillar Array Filter

A silicon micropillar filter (μPF) chip was fabricated, glass-bonded, coated with 3-aminopropyltriethoxysilane (APTES) then tested for its ability to bind and release pre-purified DNA in continuous flow mode. APTES coated μPF was tested over different DNA initial loading amounts and flow rates. Simple and effective washing and eluting steps were used, which allowed direct detection of obtained DNA using UV Nano-Droplet. Both the capture efficiency and elution % were inversely proportional to flow rate, with such relation being more profound at higher DNA concentration. DNA elution recovery %, as high as 80%, was achieved under flow rate of 5 μl /min and an initial DNA loading of 250 ng. The capture capacity per surface area of the APTES-coated μPFis ~186 ng/cm2, which mounts to ~3.7 times the capacity of an identical uncoated filter. These results demonstrate the potential for integrating such amino-coated μPF in a lab-on-a-chip system for the extraction of nucleic acids directly from clinical samples in a continuous flow mode.

Characterization of the state of a droplet on a micro-textured silicon wafer using ultrasound

In this work, we propose acoustic characterization as a new method to probe wetting states on a superhydrophobic surface. The analysis of the multiple reflections of a longitudinal acoustic wave from solid-liquid and solid-vapor interfaces enables to distinguish between the two well known Cassie-Baxter and Wenzel wetting configurations. The phenomenon is investigated experimentally on silicon micro-pillars superhydrophobic surfaces and numerically using a finite difference time domain method. Numerical calculations of reflection coefficients show a good agreement with experimental measurements, and the method appears as a promising alternative to optical measurement methods.

Template-free fabrication of silicon micropillar/nanowire composite structure by one-step etching

A template-free fabrication method for silicon nanostructures, such as silicon micropillar (MP)/nanowire (NW) composite structure is presented. Utilizing an improved metal-assisted electroless etching (MAEE) of silicon in KMnO4/AgNO3/HF solution and silicon composite nanostructure of the long MPs erected in the short NWs arrays were generated on the silicon substrate. The morphology evolution of the MP/NW composite nanostructure and the role of self-growing K2SiF6 particles as the templates during the MAEE process were investigated in detail. Meanwhile, a fabrication mechanism based on the etching of silver nanoparticles (catalyzed) and the masking of K2SiF6 particles is proposed, which gives guidance for fabricating different silicon nanostructures, such as NW and MP arrays. This one-step method provides a simple and cost-effective way to fabricate silicon nanostructures.

Experimental Study of Design Parameters in Silicon Micropillar Array Solar Cells Produced by Soft Lithography and Metal-Assisted Chemical Etching

Solar cells, consisting of core-shell p-n junction silicon micropillars on a thin membrane fabricated using soft lithography and metal-assisted chemical etching, are studied as a function of geometrical designs. Significant enhancement in absorption rate is found without much dependence on the pillar diameters in the range of 0.5-2 μm. However, the short-circuit current increases continuously with diameter, which is inversely proportional to the total surface area for a fixed diameter/pitch pillar array. This study provides unambiguous evidence that surface recombination is the dominant loss mechanism in nanowire- or micropillar-based solar cells.

Electrical Contact Characteristics between Silicon Micropillars and Ag Nanoparticles with Controlled Mechanical Load

ABSTRACTWe report an experimental investigation on employing Ag nanoparticles to provide electrical and mechanical contacts between transfer-printed semiconductor devices in the shape of micro/nano- wires and pillars. The Ag nanoparticles have diameters ranging between 200-800nm and are assembled on a 200nm Au film deposited on glass substrates. With a customized tool, an ensemble of silicon pillars were brought into contact with the silver (Ag) nanoparticles (AgNPs) by precisely controlling the displacement and applied force (pressure). Current-voltage measurements were done at force resolution of ~0.2N. The test method aims to illuminate the pillar-particle contact mechanism using the nanoparticles as conductive fillers for the next generation of high performance heteroepitaxial device transfer-printing applications.

Resonance acoustic microbalance with naked-embedded quartz (RAMNE-Q) biosensor fabricated by microelectromechanical-system process

A resonance acoustic microbalance with a naked-embedded quartz (RAMNE-Q) is realized by a microfabrication method, aiming at broader applications of quartz-crystal microbalance (QCM) biosensors. The RAMNE-Q biosensor consists of three layers; a silicon layer with an engraved microchannel and sandwiching glass layers. The naked AT-cut quartz resonator of 9.3 or 28.5μm thick is located in the microchannel and supported by the silicon micropillars and semicircular walls without fixing, and it is encapsulated by the rigid body. Cupper antennas are used for generating and receiving electromagnetic fields to excite and detect the shear vibration of the quartz oscillator during the solution flow, thereby achieving the noncontact measurement of the resonance frequency. Because of the isolated resonator, the Q factor is high enough (about 1500 at 170–180MHz) even in the flowing solution. We succeeded in detecting 1ng/ml human immunoglobulin G in phosphate-buffered-saline solution via Staphylococcus aureus protein A immobilized nonspecifically on the quartz surfaces, demonstrating the high sensitivity and high signal-to-noise ratio of the RAMNE-Q biosensor. It does not require electrodes and is a replacement-free biosensor, and its reusability is confirmed.

Deformation of silicon – Insights from microcompression testing at 25–500 °C

The plastic deformation of silicon and other brittle materials near room temperature has conventionally been studied under high confining pressures, although it has been suggested that these may modify the dislocation core structure. Here, the possibility of using microcompression has been studied. Using this method the yield stress of silicon micropillars was measured for different pillar diameters and between 25 and 500 °C for a constant diameter of 2 μm. No pronounced effect of size on the yield stress was found, but the transition from failure by cracking to predominately plastic deformation was shown to be consistent with a previously proposed simple model for axial splitting. Deformed specimens were analysed by transmission electron microscopy to elucidate the operative dislocation mechanisms. This showed that at 500 °C deformation occurs by twinning and formation of partial dislocations, whereas at 100 °C it is associated with micro-cracking and only weakly dissociated dislocations.

Carbon nanofiber based catalyst supports to be used in microreactors: Synthesis and characterization

Carbon nanofiber (CNF) layers have been synthesized on flat fused silica and silicon substrates, as well as inside flow channels of silicon-technology based microreactors by thermal catalytic chemical vapor deposition of ethylene using nickel thin-film catalyst. These CNF layers are to be used as structured catalyst support. The influence of the ethylene concentration and addition of hydrogen to the carbon-containing gas on the morphology of CNF layers was studied. Very low amount of CNFs were produced at low ethylene concentrations (<25%) due to the restricted supply of carbon species. Addition of hydrogen during the CNF growth resulted in significant enhancement of the CNF-yield, producing thicker layers of CNFs and CNFs with smaller diameters. Channels containing silicon micropillars covered with these CNFs have a significantly enhanced surface-to-volume ratio compared to bare microreactor channels (3–4 orders of magnitude). Deposition of well-distributed platinum nanoparticles was carried out on these CNF layers, exemplifying their functionality as structured catalyst support to be used in microreactors.

Fabrication of nanocluster silicon surface with electric discharge and the application in desorption/ionization on silicon-mass spectrometry

This study presents a new, simple, and low-cost technique to fabricate a nanocluster silicon (NCSi) surface on planar silicon using a micro-scale direct current (DC) discharge under ambient conditions. The method requires no masks, chemicals, vacuum environment, or laser, but only a high-voltage supply. The NCSi surfaces, characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy, consist of oxidized silicon nanoclusters 50-200 nm in diameter, likely formed by melting due to high temperatures in the discharge. The minimum size of the NCSi spot is determined by the size of the discharge tip (approximately 90 microm). Arbitrary NCSi areas can be produced on a silicon wafer by moving the discharge needle on the surface with the help of a computer-controlled xyz stage. NCSi surfaces can also be formed on three-dimensional (3D) surfaces, as demonstrated with silicon micropillars. NCSi surfaces can be used, for example, in various analytical applications. In this study, we demonstrate their use as sample plates in the analysis of drugs and peptides with desorption/ionization on silicon-mass spectrometry (DIOS-MS).

Electrowetting and droplet impalement experiments on superhydrophobic multiscale structures

The reversible actuation of droplets on superhydrophobic surfaces under ambient conditions is currently an important field of research due to its potential applicability in microfluidic lab-on-a-chip devices. We have recently shown that Si-nanowire (NW) surfaces allow for reversible actuation provided that the surface structures show certain characteristics. In particular it appears that, for such surfaces, the presence of structures with multiple specific length scales is indeed needed to have a robust reversibility of contact angle changes. Here we report on electrowetting (EW) and impalement experiments on double-scale structured surfaces prepared by a combination of silicon micropillars prepared by an association of optical lithography and silicon etching, and nanowire growth on top of these surfaces. We show that while micropillar surfaces have a low impalement threshold and irreversible EW behaviour, a surface with double-scale texture can show both a very high resistance to impalement and a limited reversibility under EW, provided that the roughness of the micro-scale is large enough--i.e. that the pillars are tall enough. The optimal performance is obtained for a space between pillars that is comparable to the height of the nanostructure.

Up, Up and Away

Materials Science![Figure][1] CREDIT: BOREYKO AND CHEN When hot vapor comes in contact with a cold surface, such as a shower wall, liquid droplets are created that quickly coalesce and form a film. This condensation process is ubiquitous in natural as well as artificial environments. In industrial settings, preventing film formation is generally desirable because liquid films are poor heat conductors. However, it can be challenging to remove the droplets more quickly than they coalesce, particularly when nonvertical sample orientations preclude help from gravity. Boreyko and Chen demonstrate the spontaneous elimination of droplets from a horizontal surface. They prepare a superhydrophobic substrate consisting of carbon nanotubes deposited on silicon micropillars (shown at left). Video imaging of the condensation of ambient moisture reveals that, after the droplets are formed, they initially coalesce without moving, then eventually reach a mobile phase where several droplets fuse and leave the surface of the sample in a dramatic out-of-plane jump. The energy for the jump is provided by the decrease in surface energy gained by coalescence; the average condensed droplet size is an order of magnitude smaller than that observed in gravitational removal. Interestingly, a similar mechanism is thought to be used by a type of mushroom to eject a spore from its sterigma. Phys. Rev. Lett. 103 , 184501 (2009). [1]: pending:yes

Electrically Controlled Wetting and Dewetting Transition on Silicon Micro-Pillar Arrays

Electrically Controlled Wetting and Dewetting Transition on Silicon Micro-Pillar Arrays

Growth of Si nanowires on micropillars for the study of their dopant distribution by atom probe tomography

This article reports on the growth of Au islands on the Si(111) surface as a function of the Au evaporation rate and the temperature of the surface in ultrahigh vacuum. By controlling the density of the Au islands and their size, it is possible to subsequently grow single vertically oriented Si nanowires on top of (111)-oriented silicon micropillar and analyze their chemical composition at the atomic scale with the femtosecond laser assisted tomographic atom probe. Three-dimensional images of the atom distribution in the nanowire, in particular, the distribution of boron impurities, are obtained and compared to the intended impurity concentration.

Suspended HOPG nanosheets for HOPG nanoresonator engineering and new carbonnanostructure synthesis

Suspended highly oriented pyrolytic graphite (HOPG) nanosheets (10–300 nm thick) werecreated by direct mechanical cleavage of a bulk HOPG crystal onto silicon micropillars andmicrotracks. We show that suspended HOPG nanosheets can be used to engineer HOPGnanoresonators such as membranes, bridges, and cantilevers as thin as 28 carbon atomlayers. We measured by Doppler laser heterodyne interferometry that the discretevibration modes of an HOPG nanosheet membrane and the resonance frequency of aFIB-created HOPG microcantilever lie in the MHz frequency regime. Moreover, a newcarbon nanostructure, named ‘nanolace’, was synthesized by focused ion beam(FIB) sputtering of suspended HOPG nanosheets. Graphite nanosheets suspendedon micropillars were eroded by a FIB to create self-oriented pseudo-periodicalripples. Additional sputtering and subsequent milling of these ripples led to theformation of honeycomb-like shaped nanolaces suspended and linked by ribbons.