Silicon nanowires of different widths were fabricated in silicon on insulator (SOI) material using conventional process technology combined with electron-beam lithography. The aim was to analyze the size dependence of the sensitivity of such nanowires for biomolecule detection and for other sensor applications. Results from electrical characterization of the nanowires show a threshold voltage increasing with decreasing width. When immersed in an acidic buffer solution, smaller nanowires exhibit large conductance changes while larger wires remain unaffected. This behavior is also reflected in detected threshold shifts between buffer solutions of different pH, and we find that nanowires of width >150 nm are virtually insensitive to the buffer pH. The increased sensitivity for smaller sizes is ascribed to the larger surface/volume ratio for smaller wires exposing the channel to a more effective control by the local environment, similar to a surrounded gate transistor structure. Computer simulations confirm this behavior and show that sensing can be extended even down to the single charge level.
Direct electrical detection of biomolecules at high sensitivity has recently been demonstrated using semiconductor nanowires. Here we demonstrate that semiconductor nanoribbons, in this case, a thin sheet of silicon on an oxidized silicon substrate, can approach the same sensitivity extending below the picomolar concentration regime in the biotin/streptavidin case. This corresponds to less than approximately 20 analyte molecules bound to receptors on the nanoribbon surface. The micrometer-size lateral dimensions of the nanoribbon enable optical lithography to be used, resulting in a simple and high-yield fabrication process. Electrical characterization of the nanoribbons is complemented by computer simulations showing enhanced sensitivity for thin ribbons. Finally, we demonstrate that the device can be operated both in inversion as well as in accumulation mode and the measured differences in detection sensitivity are explained in terms of the distance between the channel and the receptor coated surface with respect to the Debye screening length. The nanoribbon approach opens up for large scale CMOS fabrication of highly sensitive biomolecule sensor chips for potential use in medicine and biotechnology.
We demonstrate that electrochemical size reduction can be used for precisely controlled fabrication of silicon nanowires of widths approaching the 10 nm regime. The scheme can, in principle, be applied to wires defined by optical lithography but is here demonstrated for wires of approximately 100-200 nm width, defined by electron beam lithography. As for electrochemical etching of bulk silicon, the etching can be tuned both to the pore formation regime as well as to electropolishing. By in-situ optical and electrical characterization, the process can be halted at a certain nanowire width. Further electrical characterization shows a conductance decreasing faster than dimensional scaling would predict. As an explanation, we propose that charged surface states play a more pronounced role as the nanowire cross-sectional dimensions decrease.
The influence of the proximity of a high refractive index substrate on the luminescence of Si nanocrystals was investigated by time-integrated and time-resolved photoluminescence. The luminescence yield was found to be ϳ2.5 times larger for emitters distanced from the substrate compared to those in proximity with the substrate, while luminescence decay measurements revealed only a slight increase in the luminescence lifetime ͑ϳ15% ͒. Results are discussed in terms of local density of optical modes surrounding a pointlike light emitter with important implications for the collection efficiency of luminescence and the estimation of internal quantum efficiency for a quantum dot.
The modification of the luminescence of silicon nanocrystals experiencing the effect of a photonic bandgap in a 2D photonic crystal was investigated. The time-integrated photoluminescence spectra detected in the plane of the photonic crystal revealed a dip in the light emission corresponding to the wavelength of the bandgap, whose position changes according to the geometry of the prepatterned pillar array. The calculated emission pattern for a pointlike dipole placed in such a structure suggests an inhibition of the spontaneous emission rate at certain directions as a physical reason for the observed modification of luminescence. © 2007 Optical Society of America OCIS code: 260.3800.The properties of a light emitter can be modified by the properties of the radiation field around it. As a result, it was shown that a photonic crystal could provide ways to manipulate the spontaneous emission rate for an emitter placed into such a structure [1]. In general, the presence of the bandgap in a photonic crystal affects the spontaneous emission rate of an emitter by changing the density of optical modes, which in turn governs the rate of radiative transitions following Fermi's golden rule. Such an inhibition of spontaneous emission and corresponding lifetime increase can be of interest for certain applications when realized in a controllable way. The unwanted radiative recombination channels can be suppressed, thus leading to a redistribution of the light emission into more useful collectable radiative modes [2]. Another example is a photosensitization effect, where the energy is transferred by a near-field mechanism from an entity with a high optical absorption cross section to a less susceptible one, for instance from a quantum dot to a desired atomic species [3]. An increased lifetime of the excitation in a quantum dot can, therefore, enhance the probability of such an energy transfer process. Introducing a defect into a photonic crystal bandgap structure, on the other hand, makes possible the formation of localized standing waves within the photonic lattice. If a pointlike emitter is placed into such a cavity an emitter-cavity interaction can be expected, leading to a shortening of the excitation lifetime [4]. In general, the degree of coupling depends on the proximity of emitter and cavity properties in wavelength and space domains: Q / V, where Q is a quality factor and V is the cavity volume. It was recently shown for direct bandgap nanocrystals that the highest cavity factor of any cavity type could be achieved using such photonic crystal defect structures. In the weak coupling regime, also referred to as the Purcell effect, a nearly 3 orders of magnitude increase in peak maximum intensity was reported [5]. Even a strong coupling regime was demonstrated for such systems, where the energy is continually shifted back and forth between the electromagnetic field in the cavity and the exciton in the nanocrystal, the so-called Rabi oscillations [6].Silicon nanocrystals are particularly interesting for optical applicatio...
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