Silicon Nanochannels Research Articles

Efficiency improvement of a nanostructured polymer solar cell employing atomic layer deposited Al2O3 as a passivation layer

We report a hybrid heterojunction solar cell based on silicon nanochannel (SiNC) arrays and poly(3,4-ethylenedioxythiophane):poly-styrenesulfonate (PEDOT:PSS) with a power conversion efficiency (PCE) in excess of 9.50%. The described device is produced by spin coating the organic polymer PEDOT:PSS on SiNC arrays fabricated employing metal assisted electroless chemical etching methods. We utilized an ultrathin Atomic Layer Deposited (ALD) aluminum oxide (Al2O3) interface passivation layer between the SiNC arrays and PEDOT:PSS layer and its influence on the photovoltaic performance of the device is analyzed herein. An open circuit voltage (VOC) and a short circuit current density (JSC) as high as 565mV and 28.4mA/cm2, respectively, were achieved, which lead to a PCE of 9.52% for the optimized SiNC depth of 1.35μm even without an antireflection coating and back surface field enhancement. The electrical performance of the described device is also analyzed in terms of the capacitance voltage (C–V) measurements collected at room temperature.

Thermotropic orientational order of discotic liquid crystals in nanochannels: an optical polarimetry study and a Landau-de Gennes analysis.

Optical polarimetry measurements of the orientational order of a discotic liquid crystal based on a pyrene derivative confined in parallelly aligned nanochannels of monolithic, mesoporous alumina, silica, and silicon as a function of temperature, channel radius (3-22 nm) and surface chemistry reveal a competition of radial and axial columnar orders. The evolution of the orientational order parameter of the confined systems is continuous, in contrast to the discontinuous transition in the bulk. For channel radii larger than 10 nm we suggest several, alternative defect structures, which are compatible both with the optical experiments on the collective molecular orientation presented here and with a translational, radial columnar order reported in previous diffraction studies. For smaller channel radii our observations can semi-quantitatively be described by a Landau-de Gennes model with a nematic shell of radially ordered columns (affected by elastic splay deformations) that coexists with an orientationally disordered, isotropic core. For these structures, the cylindrical phase boundaries are predicted to move from the channel walls to the channel centres upon cooling, and vice-versa upon heating, in accord with the pronounced cooling/heating hystereses observed and the scaling behavior of the transition temperatures with the channel diameter. The absence of experimental hints of a paranematic state is consistent with a biquadratic coupling of the splay deformations to the order parameter.

Ion specificity in NaCl solution confined in silicon nanochannels

Ion specificity of Na+ and Cl− ions for NaCl solution confined in silicon nanochannels is investigated with molecular dynamics (MD) simulations. The MD simulation results demonstrate that ion specificity for Na+ and Cl− ions exhibits clearly in nanochannels with high surface charge density. The two types of ions show different density distributions perpendicular to the channel surface due to the ion specificity when they act as counterions near negatively and positively charged surfaces, respectively. Both the two counterion distributions cannot be predicted by Poisson-Boltzmann equation within 0.75 nm near the surface. In addition, the ion specificity is also demonstrated through affecting the water density distributions. In the nanochannel with negatively charged surfaces, the presence of the Na+ ions reduces the number of water peaks in water density distribution profile. In comparison, when the Cl− ions act as counterions near positively charged surfaces, they do not affect the number of the water peaks. Besides the influence on the water density distribution, ion specificity also exhibits through affecting the water molecule orientation in the adsorbed layer. It is found that Cl− ions make the water molecules in the adsorbed layer align more orderly than Na+ ions do when the two types of ions act as the counterions near the positively and negatively charged surfaces with the same surface charge density.

Electrical Biomolecule Detection Using Nanopatterned Silicon via Block Copolymer Lithography

An electrical biosensor exploiting a nanostructured semiconductor is a promising technology for the highly sensitive, label-free detection of biomolecules via a straightforward electronic signal. The facile and scalable production of a nanopatterned electrical silicon biosensor by block copolymer (BCP) nano-lithography is reported. A cost-effective and large-area nanofabrication, based on BCP self-assembly and single-step dry etching, is developed for the hexagonal nanohole patterning of thin silicon films. The resultant nanopatterned electrical channel modified with biotin molecules successfully detects the two proteins, streptavidin and avidin, down to nanoscale molarities (≈1 nm). The nanoscale pattern comparable to the Debye screening length and the large surface area of the three-dimensional silicon nanochannel enable excellent sensitivity and stability. A device simulation confirms that the nanopatterned structure used in this work is effective for biomolecule detection. This approach relying on the scalable self-assembly principle offers a high-throughput manufacturing process for clinical lab-on-a-chip diagnoses and relevant biomolecular studies.

Growth of Highly Oriented Deuterium Crystals in Silicon Nanochannels

The structure of solid deuterium confined in 9 nm wide tubular silicon nanochannels has been studied by means of elastic neutron scattering techniques. As a result we report the formation of fcc D(2) as the stable solid phase in confinement in contrast to the hcp bulk structure. Further, a preferred alignment of D(2) nanocrystals with respect to the surrounding crystalline silicon matrix is discussed in terms of heteroepitaxial growth of solid D(2) on crystalline pore walls.

Fabrication of High Aspect Ratio Silicon Nanochannel Arrays

We report a method of fabricating an array of high aspect ratio silicon nanochannels which is not dependent on nanolithography techniques and equipment. The method comprises etching of silicon micro-channels in an inductively coupled plasma system (ICP), followed by a thermal oxidation step where silicon is consumed during the process to further shrink the channel to nano dimensions. For the micro-channel formation, silicon dioxide is used as the etch mask during the ICP process where a high etch selectivity of 70:1 between silicon and silicon dioxide was achieved. By thermally oxidizing the etched silicon channels, a uniform array of nanochannels with lateral dimensions down to 40 nm was achieved with significantly high aspect ratio value of at least 65. The grown thermal oxide uniformly covers all surfaces of the silicon nanochannels.

Nanochannel system fabricated by MEMS microfabrication and atomic force microscopy

A silicon nanochannel system with integrated transverse electrodes was designed and fabricated by combining micro-electro-mechanical systems (MEMS) micromachining and atomic force microscopy (AFM)-based nanolithography. The fabrication process began with the patterning of microscale reservoirs and electrodes on an oxidised silicon chip using conventional MEMS techniques. A nanochannel, approximately 30 [micro sign]m long with a small semi-circular cross-sectional area of 20 nm × 200 nm, was then mechanically machined on the oxide surface between the micro reservoirs by applying AFM nanolithography with an all-diamond probe. Anodic bonding was used to seal off the nanochannel with a matching Pyrex cover. Continuous flow in the nanochannel was verified by pressurising a solution of fluorescein isothiocyanate in ethanol through the nanochannel in a vacuum chamber. It was further demonstrated by translocating negatively charged nanobeads (diameter approximately 20 nm) through the nanochannel by using an external DC electric field. The passage of the nanobeads caused a sharp increase in the transverse electrical conductivity of the nanochannel.

Gated and Near-Surface Diffusion of Charged Fullerenes in Nanochannels

Nanoparticles and their derivatives have engendered significant recent interest. Despite considerable advances in nanofluidic physics, control over nanoparticle diffusive transport, requisite for a host of innovative applications, has yet to be demonstrated. In this study, we performed diffusion experiments for negatively and positively charged fullerene derivatives (dendritic fullerene-1, DF-1, and amino fullerene, AC60) in 5.7 and 13 nm silicon nanochannels in solutions with different ionic strengths. With DF-1, we demonstrated a gated diffusion whereby precise and reproducible control of the dynamics of the release profile was achieved by tuning the gradient of the ionic strength within the nanochannels. With AC60, we observed a near-surface diffusive transport that produced release rates that were independent of the size of the nanochannels within the range of our experiments. Finally, through theoretical analysis we were able to elucidate the relative importance of physical nanoconfinement, electrostatic interactions, and ionic strength heterogeneity with respect to these gated and near-surface diffusive transport phenomena. These results are significant for multiple applications, including the controlled administration of targeted nanovectors for therapeutics.

Efficient prototyping of large-scale pdms and silicon nanofluidic devices using pdms-based phase-shift lithography

In this study, we explore the potential of poly-dimethylsiloxane (PDMS)-based phase-shift lithography (PPSL) for the fabrication of nanofluidic devices. We establish that this technology, which was already shown to allow for the generation of 100 nm linear or punctual features over squared centimeter surfaces with conventional photolithography systems, is readily adequate to produce some of the most popular nanofluidic systems, namely nanochannels and nanoposts arrays. We also demonstrate that PPSL technology enables to generate PDMS and silicon nanofluidic systems. This technological achievement allows us to perform single DNA molecule manipulation experiments in PDMS and silicon nanochannels, and we observe an unexpectedly slow migration of DNA in PDMS devices, which is independent on salt or pH conditions. Our data in fact hint to the existence of an anomalous response of DNA in PDMS nanofluidic devices, which is likely associated to transient nonspecific interactions of DNA with PDMS walls. Overall, our work demonstrates the efficiency and the performances of PPSL for prototyping nanofluidic systems.

Capillary Filling in Nanochannels—Modeling, Fabrication, and Experiments

While capillary filling in channels of micrometers scale is experimentally verified to obey Washburn's law well, the speed of capillary filling in nanochannels is noticeably lower than described by Washburn's formula. This article reports the theoretical and experimental results on capillary filling in open-end and closed-end nanochannels. Nanochannels of 45 nm and 80 nm depth, 10 μm width, were etched in silicon and bonded to a glass cover. Experiments on filling of non-electrolytic liquid in silicon nanochannels were carried out. The filling processes were observed and recorded. To estimate the influence of electrokinetics, a mathematical model to calculate the electroviscous effect was established. This model shows that the contribution of the electroviscous effect in the reduction of filling speed is small. This result also agrees well with previous theoretical work on the electroviscous effect. That means that besides the electroviscous effect, there are other phenomena that contribute to the reduction of capillary filling speed in a nanochannel, such as air bubbles formation. Experimental investigation of capillary filling in open-end and closed-end nanochannels with different lengths was performed. The filling processes of ethanol and isopropanol and the behavior of the trapped air were recorded and evaluated. Analytical models based on the continuum assumption were used to evaluate the experimental data. We observed that the filling process consists of two stages. At the initial stage, experimental data agree well with the theoretical model, but with a higher apparent viscosity. In the final stage, condensation of the liquid phase and dissolution of the gas phase lead to total filling of the nanochannel. The observed phenomena are important for understanding the behavior of multiphase systems in nanochannels.

A Long-Range Electric Field Solver for Molecular Dynamics Based on Atomistic-to-Continuum Modeling.

Understanding charge transport processes at a molecular level is currently hindered by a lack of appropriate models for incorporating nonperiodic, anisotropic electric fields in molecular dynamics (MD) simulations. In this work, we develop a model for including electric fields in MD using an atomistic-to-continuum framework. This framework provides the mathematical and the algorithmic infrastructure to couple finite element (FE) representations of continuous data with atomic data. Our model represents the electric potential on a FE mesh satisfying a Poisson equation with source terms determined by the distribution of the atomic charges. Boundary conditions can be imposed naturally using the FE description of the potential, which then propagate to each atom through modified forces. The method is verified using simulations where analytical solutions are known or comparisons can be made to existing techniques. In addition, a calculation of a salt water solution in a silicon nanochannel is performed to demonstrate the method in a target scientific application in which ions are attracted to charged surfaces in the presence of electric fields and interfering media.

Design of numerical algorithms for the problem of charge transport in a 2D silicon MOSFET transistor with a silicon oxide nanochannel

We are concerned with the problem of charge transport in a 2D silicon MOSFET transistor occupying a domain Ω with a silicon oxide nanochannel occupying a domain ΩG. After proposing an additional boundary condition for the electric potential on the common boundary of the domains Ω and ΩG we design two numerical algorithms for funding approximate solutions of this problem. The first algorithm is a new one and uses interpolation polynomials of spline-collocation and the sweep method. The second algorithm is based on the well-known longitudinal-transverse sweep (l.t.s.) method. By using these algorithms we obtain graphs of stationary solutions of our problem. We also compare the workability and efficiency of the proposed algorithms for various values of parameters.

On calculating the electric potential for 2D silicon transistor with a silicon oxide nanochannel

The problem of calculating the electric potential for a 2D silicon transistor occupying a domain Ω with an adjoining silicon oxide nanochannel, i.e., the domain ΩG, is studied. The initial problem of calculating the potential in a domain Ω ∪ ΩG is reduced to that of a confined domain Ω; this procedure is justified with computing experiments.

Formation of Nanoparticle-Containing Multilayers in Nanochannels via Layer-by-Layer Assembly

Arrays of silicon nanochannels were used to investigate the layer-by-layer (LbL) assembly of nanoparticle-containing multilayers in confined geometries. LbL assembly of poly(vinylsulfonic acid)/titania (PVS/TiO2) and poly(diallyldimethylammonium chloride)/silica (PDAC/SiO2) resulted in conformal, uniform thickness multilayers throughout the high-aspect-ratio nanochannels. These multilayers were also calcinated to form nanoporous coatings within the nanochannels. Confined and unconfined multilayers were compared using scanning electron microscopy (SEM), showing significantly lower bilayer thicknesses when layers were assembled in confined geometries. Deposition was found to essentially stop at a nanochannel gap size of 55 nm for the PVS/TiO2 system and at 210 nm for the PDAC/SiO2 system, even though the channels were not plugged. These results provide further evidence of surface charge-induced depletion of unadsorbed species during LbL assembly, which occurs because of the closely spaced walls with like su...

Size-dependent freezing of n-alcohols in silicon nanochannels

We present a study on the phase behavior of several linear n-alcohols (heptanol, nonanol and undecanol) in their bulk state as well as confined in mesoporous silicon. We were able to vary the mean pore radii of the nanochannels from r = 3.5 nm to 7 nm and to determine the respective temperatures of the freezing and melting transitions using infrared and dielectric spectroscopy. The smaller the chain length the lower the freezing point, both in the bulk and in the confined state. Under confinement the freezing temperature decreases by up to 28 K compared to the bulk value. In accordance with the Gibbs-Thompson model the lowering is proportional to the inverse pore radius, ΔTfr ∝ 1/r. Moreover, the ratio of freezing temperature depression to melting temperature depression is close to the theoretical value of ΔTfr/ΔTmelt = 3/2. The spectra also indicate a structural change: while the solid bulk alcohols are a polycrystalline mixture of the orthorhombic β- and monoclinic γ-form, geometrical confinement forces the alcohol-chains into the more simple orthorhombic structure. In addition, a part of the material does not crystallize. Such an additional amorphous phase seems to be a logical consequence of the size mismatch between molecular crystals and irregular shaped pores.

High Knudsen number fluid flow at near-standard temperature and pressure conditions using precision nanochannels

Gas flows over a wide range of Knudsen numbers (~0.5–10) are studied using silicon nanochannel arrays with slit-shaped pores. The pore sizes of the silicon nanochannel arrays range from micrometer to sub-10-nm scales. The flows are generated under conditions of room temperature and near-atmospheric pressure (~22°C and ~101–115 kPa) and span the continuum flow, continuum-slip flow, transition flow and free-molecular flow regimes. The measured flow rates of helium, argon and carbon dioxide are in good agreement with a theoretical model (Unified Slip Model) proposed by Beskok and Karniadakis (Nanoscale Microscale Thermophys Eng 3:43–77, 1999).

Capillary Filling in Closed End Nanochannels

We investigated the interactions between liquid, gas, and solid phases in the capillary filling process of closed-end nanochannels. This paper presents theoretical models without and with absorption and diffusion of gas molecules in the liquid. Capillary filling experiments were carried out in closed-end silicon nanochannels with different lengths. The theoretical and measured characteristics of filling length versus time are compared. The results show that the filling process consists of two stages. The first stage resembles the capillary filling process in an open-end nanochannel. However, a remarkable discrepancy between the experimental results and the theory without gas absorption is observed in the second stage. A closer investigation of the second stage reveals that the dissolution of gas in the liquid is important and can be explained by the model with gas absorption and diffusion.

Biosensors using the Si nanochannel junction-isolated from the Si bulk substrate

The biosensor using a silicon nanochannel field effect transistor has been developed on the basis of a bulk silicon substrate instead of an expensive silicon-on-insulator material, for low cost applications. “Top-down” fabricated Si nanochannels for detecting prostate specific antigen (PSA) are electrically isolated from the bulk Si substrate only by a reverse-biased p-n junction with very low reverse leakage currents. The reverse leakage current flowing through the p-n junction is small enough to be ignored, when compared with the current flowing through the p-type Si channel; roughly 100 times lower. The surface immobilization of anti-PSA has been confirmed by the specific binding test of DNA stabilized Au nanoparticles (NPs), showing 1200 NPs/μm2 and atomic force microscopy surface analysis. The injection of 10 ng/ml PSA solutions leads to a clear increase in the conductance of Si nanochannels, indicating the specific binding of PSA with the immobilized anti-PSA. The similar signal changes were observed on the injection of the 1 pg/ml PSA, very low concentration of PSA.

Simulation of nanochannel membrane formation

Monte Carlo simulations of atomic processes on the surface of silicon nanochannel membranes during molecular-beam epitaxy and subsequent thermal oxidation are performed. It is demonstrated that silicon deposition on Si(001) wafers with 1–100 nm cylindrical pores results in constriction of channel inlets. The rates of reduction of the nanochannel diameter are estimated as functions of the wafer temperature, silicon deposition rate, and initial nanochannel diameter. Optimal conditions of silicon deposition on nanochannel membranes are determined: the wafer temperature of 250–450°C and silicon flux intensity of 10−2 to 10 monolayers (ML) per second. Under these conditions, the rate of reduction of the nanochannel inlet diameter is 0.13–0.15 nm/ML, which allows membrane channel modifications over a wide range down to several nanometers. Simulations of nanochannel membrane oxidation in an oxygen flux shows that precise reduction of nanochannel inlet diameters down to complete sealing of the channel due to oxide growth is only possible for small diameters of the initial pores. For channels with large lateral sizes, the effect of reduction of the channel inlet diameter due to oxidation is insignificant. Oxidation of pores enhances their stability to subsequent high-temperature treatment.

Nanoelectromechanical system-integrated detector with silicon nanomechanical resonator and silicon nanochannel field effect transistor

We demonstrate the fabrication and operation of an integrated device containing a nanoelectromechanical system and an integrated detector. This on-chip silicon nanochannel field effect transistor is used to measure the motion of a silicon nanomechanical resonator at room temperature. Furthermore, we describe the operation of the device as a silicon-based room-temperature on-chip amplifier for improved displacement detection of nanomechanical resonators.