Silicon Nanograss Research Articles

CMOS-Compatible Wafer-Level Double-layer Silicon Nanograss through Reactive Ion Etching for Applications in Optics

CMOS-Compatible Wafer-Level Double-layer Silicon Nanograss through Reactive Ion Etching for Applications in Optics

Plasma‐Etched Nanograss Surface without Lithographic Patterning to Immobilize Water Droplet for Highly Sensitive Raman Sensing

AbstractThe development of reliable, cost‐effective molecular detection at the attomolar level on analyte‐immobilizing surfaces fabricated without lithographic patterning remains a major challenge in chemical sensing technology. This issue is addressed using custom‐designed adhesive superhydrophobic silicon nanograss surfaces produced via plasma etching. When applied to ultrasensitive surface‐enhanced Raman scattering, the nanograss surface enables effective immobilization of water droplets containing Ag nanoparticles and R6G target molecules. Upon water evaporation, the R6G analytes are confined at the edge of the self‐organized coffee‐ring‐like stains with the plasmonic hot spots of the Ag nanoparticles, thus providing a reliable Raman scattering platform for detecting trace analytes. Even at an ultralow concentration of 10−16 m, the corresponding relative standard deviation is 17.57%. A novel plasma‐enabled approach for precise interface nanostructuring, potentially leading to unprecedented capabilities in molecular‐level sensing technologies, is presented.

Through-drop imaging of moving contact lines and contact areas on opaque water-repellent surfaces.

A myriad of natural surfaces such as plant leaves and insect wings can repel water and remain unwetted inspiring scientists and engineers to develop water-repellent surfaces for various applications. Those natural and artificial water-repellent surfaces are typically opaque, containing micro- and nano-roughness, and their wetting properties are determined by the details at the actual liquid-solid interface. However, a generally applicable way to directly observe moving contact lines on opaque water-repellent surfaces is missing. Here, we show that the advancing and receding contact lines and corresponding contact area on micro- and nano-rough water-repellent surfaces can be readily and reproducibly quantified using a transparent droplet probe. Combined with a conventional optical microscope, we quantify the progression of the apparent contact area and apparent contact line irregularity in different types of superhydrophobic silicon nanograss surfaces. Contact angles near 180° can be determined with an uncertainty as low as 0.2°, that a conventional contact angle goniometer cannot distinguish. We also identify the pinning/depinning sequences of a pillared model surface with excellent repeatability and quantify the progression of the apparent contact interface and contact angle of natural plant leaves with irregular surface topography.

Novel technique for large area n-type black silicon solar cell by formation of silicon nanograss after diffusion process

Metal-assisted chemical etching (MACE) method is the most convenient and cost-effective nanowire fabrication method compared to other nanowire fabrication processes although a major problem arises in silicon nanowire, formed by MACE solution during n-type c-Si solar cell fabrication steps. High-temperature boron diffusion in conventional open tube furnace breaks down the nanowire resulting in a non-uniform surface pattern which is responsible to decrease overall conversion efficiency of the finished cell. In this work, this drawback is resolved by considering silicon nanowire formation after diffusion step. A slow etchant is considered for nanostructure on diffused silicon wafer to protect the diffused junction. The generated nanowire size is very less and has forage-like structure, and so termed as nanograss. Surface morphology and the characterization of the silicon nanograss structure after diffusion process on large area (156 mm × 156 mm) c-Si solar cells using MACE method have been investigated elaborately. Further, the complete solar cell has been fabricated with an efficiency of 17.20%.

AgNPs decorated 3D bionic silicon nanograss arrays pattern with high-density hot-spots for SERS sensing via green galvanic displacement without additives.

Surface-enhanced Raman scattering (SERS) sensing has always been considered as a kind of high-efficiency analysis technique in different areas. Herein, we report a AgNPs decorated 3D bionic silicon (Si) nanograss SERS substrate with higher sensitivity and specificity by green galvanic displacement. The Si nanograss arrays are directly grown on a Si substrate via catalyst-assisted vapor–liquid–solid (VLS) growth and subsequent plasma interaction. AgNPs were rapidly immobilized on Si nanograss arrays without any organic reagents, and avoiding the interference signal of additives. The AgNPs decorated 3D bionic silicon nanograss arrays not only possess a larger specific surface area (loading more reporter molecules), but also provide a potential distribution and arrangement for plentiful hot spots. Using Rhodamine 6G (R6G) as a probe molecule, the prepared SERS substrates exhibited great potential for high-sensitivity SERS sensing, and pushed the limit of detection (LOD) down to 0.1 pM. A higher Raman analytical enhancement factor (AEF, 3.3 × 107) was obtained, which was two magnitudes higher than our previous Ag micro–nano structures. Additionally, the practicality and reliability of our 3D bionic SERS substrates were confirmed by quantitative analysis of the spiked Sudan I in environmental water, with a wide linear range (from 10−10 M to 10−6 M) and low detection limit (0.1 nM).

Fabrication of low cost nano-grass n-type C-Si solar cell with sol–gel Al2O3 passivation

Silicon nano-grass Solar cell has the advantage of enhancing efficiency with cost effective method. Highly ordered silicon nano-grass is formed by chemical etching process. During this chemical etching unwanted nanograss is formed on both side of the wafer, resulting considerable transmission loss from rear side. This results in ultimate reduction of efficiency. The detrimental effect of nano-grass formation on both side of the wafer is briefly demonstrated here by simulation methods. Adopting the idea by simulation method this paper has reported silicon front sided nano-grass n-type c-Si solar cell by spin on diffusion source and rapid thermal annealing. The sheet resistance of the nano-grass silicon wafer is varied with respect to different spin-on diffusion profile. Later front surface has been passivated by sol–gel Al2O3 passivation. Eventually all the spin doped silicon nano-grass solar cell with sol–gel Al2O3 passivation provides a very promising route of cost effective high efficiency silicon solar cell technology. Broadband reflection between 300 and 1100 nm wavelengths has been suppressed to 2% for nano-grass Solar cell and maximum efficiency of 16.57% is achieved.

Photolithographic nanoseeding method for selective synthesis of metal-catalysed nanostructures

In this work we present a general method for the selective synthesis by photolithography of localised nanostructures in planar geometries. The methodology relies on the previous concept of photo-patternable metallic nanoparticle (NP)/polymer nanocomposites, which can provide a range of NP sizes, polydispersity and densities. First, a photoresist containing metallic ions is patterned by photolithography. Silver NPs are synthesised in situ after the exposure and development of the patterned thin film via the thermal-induced reduction of ions embedded in its structure. Gentle plasma ashing is used to selectively remove the polymer, which leaves NPs on the patterned areas. These NPs are used as seeds for subsequent processes. In order to demonstrate the flexibility of the method, its use to selectively produce localised nanostructures through different processes is shown here. Following a top-down approach, high aspect-ratio silicon nanograss has been produced by reactive ion etching and masking by the NPs. In a bottom-up approach, 280 nm copper clusters have been selectively grown in arrays. This method can be easily extrapolated to other metals and it provides a quick way to selectively generate hierarchical nanostructures in large planar areas that can be used for different applications, such as the fabrication of nanostructured sensor arrays.

Electrochemical approach for monitoring the effect of anti tubulin drugs on breast cancer cells based on silicon nanograss electrodes

One of the most interested molecular research in the field of cancer detection is the mechanism of drug effect on cancer cells. Translating molecular evidence into electrochemical profiles would open new opportunities in cancer research. In this manner, applying nanostructures with anomalous physical and chemical properties as well as biocompatibility would be a suitable choice for the cell based electrochemical sensing. Silicon based nanostructure are the most interested nanomaterials used in electrochemical biosensors because of their compatibility with electronic fabrication process and well engineering in size and electrical properties. Here we apply silicon nanograss (SiNG) probing electrodes produced by reactive ion etching (RIE) on silicon wafer to electrochemically diagnose the effect of anticancer drugs on breast tumor cells. Paclitaxel (PTX) and mebendazole (MBZ) drugs have been used as polymerizing and depolymerizing agents of microtubules. PTX would perturb the anodic/cathodic responses of the cell-covered biosensor by binding phosphate groups to deformed proteins due to extracellular signal-regulated kinase (ERK1/2) pathway. MBZ induces accumulation of Cytochrome C in cytoplasm. Reduction of the mentioned agents in cytosol would change the ionic state of the cells monitored by silicon nanograss working electrodes (SiNGWEs). By extending the contacts with cancer cells, SiNGWEs can detect minor signal transduction and bio recognition events, resulting in precise biosensing. Effects of MBZ and PTX drugs, (with the concentrations of 2 nM and 0.1 nM, respectively) on electrochemical activity of MCF-7 cells are successfully recorded which are corroborated by confocal and flow cytometry assays.

Bactericidal effects of plasma-modified surface chemistry of silicon nanograss

The surface chemistry and topography of biomaterials regulate the adhesion and growth of microorganisms in ways that are still poorly understood. Silicon nanograss structures prepared via inductively coupled plasma etching were coated with plasma deposited nanometer-thin polymeric films to produce substrates with controlled topography and defined surface chemistry. The influence of surface properties on Staphylococcus aureus proliferation is demonstrated and explained in terms of nanograss substrate wetting behaviour. With the combination of the nanograss topography; hydrophilic plasma polymer coatings enhanced antimicrobial activity while hydrophobic coatings reduced it. This study advances the understanding of the effects of surface wettability on the bactericidal properties of reactive nano-engineered surfaces.

Nanoelectromechanical Chip (NELMEC) Combination of Nanoelectronics and Microfluidics to Diagnose Epithelial and Mesenchymal Circulating Tumor Cells from Leukocytes.

An integrated nano-electromechanical chip (NELMEC) has been developed for the label-free distinguishing of both epithelial and mesenchymal circulating tumor cells (ECTCs and MCTCs, respectively) from white blood cells (WBCs). This nanoelectronic microfluidic chip fabricated by silicon micromachining can trap large single cells (>12 µm) at the opening of the analysis microchannel arrays. The nature of the captured cells is detected using silicon nanograss (SiNG) electrodes patterned at the entrance of the channels. There is an observable difference between the membrane capacitance of the ECTCs and MCTCs and that of WBCs (measured using SiNG electrodes), which is the key indication for our diagnosis. The NELMEC chip not only solves the problem of the size overlap between CTCs and WBCs but also detects MCTCs without the need for any markers or tagging processes, which has been an important problem in previously reported CTC detection systems. The great conductivity of the gold-coated SiNG nanocontacts as well as their safe penetration into the membrane of captured cells, facilitate a precise and direct signal extraction to distinguish the type of captured cell. The results achieved from epithelial (MCF-7) and mesenchymal (MDA-MB231) breast cancer cells circulated in unprocessed blood suggest the significant applications for these diagnostic abilities of NELMEC.

Black poly-silicon: A nanostructured seed layer for sensor applications

Nanostructured silicon surfaces like black-silicon (b-Si) are of great interest in current sensor technology. This paper presents an alternative method to fabricate b-Si in poly-silicon thin film prepared on pre-deposited insulation layer in order to open new door to the integration of silicon nanograss in biosensor application as sensing or seed layer. In our experiment, black poly-silicon (BPS) is formed in LPCVD deposited poly-silicon thin film by deep reactive ion etching (DRIE) at cryogenic temperature in SF6+O2 plasma. Etching parameters like temperature, O2 flow and RF power is varied and morphology of the resultant thin film is analyzed by scanning electron microscopy. The fabricated samples are subjected to a comparative investigation, which contained pillar density, directionality, etch rate and loading effects. The effect of the grain size of poly-silicon layer is analyzed and compared to samples micromachined in single-crystalline silicon (c-Si). We found that fabrication parameters of BPS morphology significantly differ from that of conventional b-Si realized in c-Si substrate. A simple application example of our BPS layer for increasing specific surface area of potential sensors is also demonstrated. As far as we know, this is the first demonstration and systematic study of b-Si fabrication in poly-silicon thin film.

Microcones and Nanograss: Toward Mechanically Robust Superhydrophobic Surfaces

We describe the generation of mechanically robust superhydrophobic surfaces, which carry a hierarchical roughness that is composed of silicon microcones and silicon nanograss. Both micro and nanostructures were fabricated using mask-free dry etching processes. The microcones were obtained utilizing a cryogenic deep reactive ion etching (DRIE) process run in the overpassivation regime. By varying process parameters, surfaces with different microcones geometries and densities were achieved. The nanograss was fabricated using a modified DRIE process with alternating etching and passivation cycles ('BOSCH process'). All surfaces were covered with a layer of a fluorinated film so that superhydrophobic structures resulted. Depending on microcone geometry and density, the advancing contact angle ranged between 170° and 180°, and roll-off angles of 10 μL drops between 30' (0.5°) and 6° were observed. The samples were exposed to varying shear loads, and the changes in the morphology were recorded by using electron microscopy. The wetting angles of the mechanically challenged surfaces were recorded and correlated with the mechanical properties of the samples.

Silicon nanograss based impedance biosensor for label free detection of rare metastatic cells among primary cancerous colon cells, suitable for more accurate cancer staging

Detection of rare metastatic cells within a benign tumor is a key challenge to diagnose the cancerous stage of the patients tested by clinical human biopsy or pap smear samples. We have fabricated and tested a nanograssed silicon based bioelectronic device with the ability of detecting a few human colon invasive cancer cells (SW48) in a mixed cell culture of primary cancerous colon cells (HT29) without any biochemical labels. A discernible impedance change was elicited after the presence of 5% metastatic cells in the whole benign sample. The electric field penetration as well as current flow to metastatic cells is different from benign ones due to their different membrane dielectric parameters. Beta dispersion as one of intrinsic bioelectrical properties of the cell membrane in blocking the stimulating current flow in the range of kHz is the specific parameter involved in our diagnosis approach. It can reflect in-depth information about the dielectric properties and the pathological condition of a cell before and after metastatic transformation. Electrically active doped silicon nanograss structures owing to their superior nanocontacts with cell membrane can detect any slight variations in current being originated from the presence of rare metastatic cells on the surface of the sensing electrode. The experimental results revealed that bare doped silicon microelectrodes are incapable of resolving different grades of attached cells.

Enhanced performance of a silicon microfabricated direct methanol fuel cell with PtRu catalysts supported on few-walled carbon nanotubes

Si-MFCs (Silicon micro fuel cells) are promising power supplies for microelectronic applications, however their development is still at early stages compared to the conventional PEM-FCs (proton exchange membrane fuel cells). There are not many published reports on the durability of Si-MFCs and those available only projected the life-time of standard Vulcan based catalysts. However, the limited durability resulting from carbon corrosion is one of the crucial issues in fuel cells. In this study, Si-MFC with an integrated silicon nanograss diffusion layer is used for the direct methanol fuel cell investigations. The long-term (3-day) performance of PtRu catalysts supported on different carbon supports, namely Vulcan, GNFs (Graphitized carbon nanofibers) and FWCNTs (Few-walled carbon nanotubes), was studied. PtRu-FWCNTs and PtRu-GNFs exhibited respectively 471% (20.0 mW cm−2) and 274% (13.1 mW cm−2) power density enhancements compared to PtRu-Vulcan (3.5 mW cm−2). After 3-day durability measurements, power density stayed at 72%, 68% and 91% of the initial value, respectively for PtRu-FWCNTs, PtRu-GNFs and PtRu-Vulcan. To evaluate the influence of carbon supports as well as the distribution and the size of the nanoparticles on the overall performance of Si-MFCs, further characterizations with Raman, BET (Brunauer–Emmett–Teller), XRD (X-ray diffraction), SEM (Scanning electron microscopy) and TEM (Transmission Electron microscopy) were performed.

Black silicon maskless templates for carbon nanotube forests

We present here a proof of concept for a novel fabrication method of vertically aligned carbon nanotube forests, utilizing black silicon nanograss (a forest of silicon nanometer-sized spikes created with reactive ion etching) coated with titanium tungsten diffusion barrier as a template. The method allows maskless definition of carbon nanotube forests with control of their density, nanotube diameter and height. Four nanograss reactive ion etching recipes are investigated and their wafer-to-wafer repeatability, wafer uniformity, and density control is discussed. Evaluation of carbon nanotube forests grown on the nanograss substrates is presented with discussion of their morphology, diameter distribution, and catalyst thickness influence.

Plasma enables edge-to-center-oriented graphene nanoarrays on Si nanograss

The formation of clearly separated vertical graphene nanosheets on silicon nanograss support is demonstrated. The plasma-enabled, two-stage mask-free process produced self-organized vertical graphenes of a few carbon layers (as confirmed by advanced microanalysis), prominently oriented in the substrate center–substrate edge direction. It is shown that the width of the alignment zone depends on the substrate conductivity, and thus the electric field in the vicinity of the growth surface is responsible for the graphene alignment. This finding is confirmed by the Monte Carlo simulations of the ion flux distribution in the silicon nanograss pattern.

Enhanced Piezoelectricity of Nanoimprinted Sub-20 nm Poly(vinylidene fluoride–trifluoroethylene) Copolymer Nanograss

In this paper, sub-20 nm ferroelectric PVDF–TrFE copolymer nanograss structures with aspect ratios up to 8.9 were developed. This study demonstrated sub-20 nm PVDF–TrFE nanograss structures that are nanoimprinted using a silicon nanograss mold in a single step. Vertically oriented PVDF–TrFE nanopillars were poled using the developed flip-stacking poling method. According to the PFM measurements, the piezoelectricity of flat thin films fabricated in this work reaches 14.0 pm/V. The maximum output voltage of the single PVDF–TrFE nanopillar was 526 mV, and the maximum piezoelectricity of the single PVDF–TrFE nanopillar was 210.4 pm/V. The piezoelectricity of the developed PVDF–TrFE nanograss structures was 5.19 times larger than that of the PVDF–TrFE flat thin films. The developed technique is simple, economical, and easy to fabricate. The developed ferroelectric PVDF–TrFE copolymer nanograss structures, which showed enhanced piezoelectricity compared to the PVDF–TrFE flat thin films, have potential applicat...

Formation of Ordered Silicon Nano-Textures Suitable for Field Emission Applications

We report on the use of silicon nano-grass and nano-textured structures for the field emission applications. A triode device, which can be used as a field emission transistor with gate, emitter (cathode) and a collector (anode), has been realized. Anode in this structure is a silicon substrate placed opposite to the silicon grass tips acting as the emitter. The silicon nano grass formation is based on the hydrogen assisted reactive ion etching process where a mixture of O2/H2 and SF6 gases are used in a sequential manner. The results of electrical measurements for this investigation show a sharp rise in the emission current.

Plasma-made silicon nanograss and related nanostructures

Plasma-made nanostructures show outstanding potential for applications in nanotechnology. This paper provides a concise overview on the progress of plasma-based synthesis and applications of silicon nanograss and related nanostructures. The materials described here include black silicon, Si nanotips produced using a self-masking technique as well as self-organized silicon nanocones and nanograss. The distinctive features of the Si nanograss, two-tier hierarchical and tilted nanograss structures are discussed. Specific applications based on the unique features of the silicon nanograss are also presented.

Controllable silicon nano-grass formation using a hydrogenation assisted deep reactive ion etching

Highly controllable silicon nano-grass formation is reported based on a hydrogen assisted reactive ion etching method in desirable shapes and locations. By controlling the etching parameters, one can achieve grass-free high aspect ratio vertical or three-dimensional structures on silicon substrates. On the other hand, one can program the etching procedure to arrive at grass-full surfaces and structures in pre-designed features and in desired places. The improved wetting properties of the grass-full surfaces have been investigated. In addition, the grass-full surface has been used to entrap kidney cells. Aspect ratios of the order of 40–50 and features of the size of 250 nm can be achieved.