DNA-Hybridization Detection on Si(100) Surfaces Using Light-Activated Electrochemistry: A Comparative Study between Bovine Serum Albumin and Hexaethylene Glycol as Antifouling Layers.

Light can be used to spatially resolve electrochemical measurements on a semiconductor electrode. This phenomenon has been explored to detect DNA hybridization with light-addressable potentiometric sensors and, more recently, with light-addressable amperometric sensors based on organic-monolayer-protected Si(100). Here, a contribution to the field is presented by comparing sensing performances when bovine serum albumin (BSA) and hexaethylene glycol (OEG6) are employed as antifouling layers that resist nonspecific adsorption to the DNA-modified interface on Si(100) devices. What is observed is that both sensors based on BSA or OEG6 initially allow electrochemical distinction among complementary, noncomplementary, and mismatched DNA targets. However, only surfaces based on OEG6 can sustain electroactivity over time. Our results suggest that this relates to accelerated SiO x formation occasioned by BSA proteins adsorbing on monolayer-protected Si(100) surfaces. Therefore, DNA biosensors were analytically explored on low-doped Si(100) electrodes modified on the molecular level with OEG6 as an antifouling layer. First, light-activated electrochemical responses were recorded over a range of complementary DNA target concentrations. A linear semilog relation was obtained from 1.0 × 10-11 to 1.0 × 10-6 mol L-1 with a correlation coefficient of 0.942. Then, measurements with three independent surfaces indicated a relative standard deviation of 4.5%. Finally, selectivity tests were successfully performed in complex samples consisting of a cocktail mixture of four different DNA sequences. Together, these results indicate that reliable and stable light-activated amperometric DNA sensors can be achieved on Si(100) by employing OEG6 as an antifouling layer.

Positron annihilation on defects in silicon irradiated with 15 MeV protons.

Microstructure and thermal stability of the radiation defects in n-FZ-Si ([P] ≈ 7 × 10(15) cm(-3)) single crystals have been investigated. The radiation defects have been induced by irradiation with 15 MeV protons and studied by means of both the positron lifetime spectroscopy and low-temperature measurements of the Hall effect. At each step of the isochronal annealing over the temperature range ∼60-700 °C the positron lifetime has been measured for the temperature interval ∼30-300 K, and for samples-satellites the temperature dependences of the charge carriers and mobility have been determined over the range ∼4.2-300 K. It is argued that as-grown impurity centers influence the average positron lifetime by forming shallow (E(b) ≈ 0.013 eV) positron states. The radiation-induced defects were also found to trap positrons into weakly bound (E(b) ≤ 0.01 eV) states. These positron states are observed at cryogenic temperatures during the isochronal annealing up to T(anneal.) = 340 °C. The stages of annealing in the temperature intervals ∼60-180 °C and ∼180-260 °C reflect the disappearance of E-centers and divacancies, respectively. Besides these defects the positrons were found to be localized at deep donor centers hidden in the process of annealing up to the temperature T(anneal.) ≈ 300 °C. The annealing of the deep donors occurs over the temperature range ∼300-650 °C. At these centers positrons are estimated to be bound with energies E(b) ≈ 0.096 and 0.021 eV within the temperature intervals ∼200-270 K and ∼166-66 K, respectively. The positron trapping coefficient from these defects increases from ∼1.1 × 10(16) to ∼6.5 × 10(17) s(-1) over the temperature range ∼266-66 K, thus substantiating a cascade phonon-assisted positron trapping mechanism whose efficiency is described by ≈T(-3) law. It is argued that the value of activation energy of the isochronal annealing E(a) ≈ 0.74-0.59 eV is due to dissociation of the positron traps, which is accompanied by restoration of the electrical activity of the phosphorus atoms. The data suggest that the deep donors involve a phosphorus atom and at least two vacancies. Their energy levels are at least at E > E(c) - 0.24 eV in the investigated material.

Full surface embedding of gold clusters on silicon nanowires for efficient capture and photothermal therapy of circulating tumor cells.

We report on rapid thermal chemical vapor deposition growth of silicon nanowires (Si NWs) that contain a high density of gold nanoclusters (Au NCs) with a uniform coverage over the entire length of the nanowire sidewalls. The Au NC-coated Si NWs with an antibody-coated surface obtain the unique capability to capture breast cancer cells at twice the highest efficiency currently achievable (~88% at 40 min cell incubation time) from a nanostructured substrate. We also found that irradiation of breast cancer cells captured on Au NC-coated Si NWs with a near-infrared light resulted in a high mortality rate of these cancer cells, raising a fine prospect for simultaneous capture and plasmonic photothermal therapy for circulating tumor cells.

Bound exciton photoluminescence from ion­implanted phosphorus in thin silicon layers.

We report the observation of clear bound exciton (BE) emission from ion-implanted phosphorus. Shallow implantation and high-temperature annealing successfully introduce active donors into thin silicon layers. The BE emission at a wavelength of 1079 nm shows that a part of the implanted donors are definitely activated and isolated from each other. However, photoluminescence and electron spin resonance studies find a cluster state of the activated donors. The BE emission is suppressed by this cluster state rather than the nonradiative processes caused by ion implantation. Our results provide important information about ion implantation for doping quantum devices with phosphorus quantum bits.

Radiation damage induced dose rate non-linearity in an n-type silicon detector.

Depth dose measurements in continuous cobalt radiation and pulsed roentgen rays were performed with nonirradiated and preirradiated detectors made of n-type silicon. A change in the relative signal at 15 cm depth of 5 to 10 per cent was found in pulsed roentgen ray fields when the detector was radiation damaged. Further experiments showed that the preirradiated detector had a superlinear dose response characteristic at high dose rates. A theoretic model was worked out and the non-linearity is explained by the properties of the recombination centers created during the preirradiation. The recombination centers are also responsible for the sensitivity drop after irradiation. At low dose rates in continuous radiation the non-linearity effect is not observed, which is in accordance with the theory. The theory in conjunction with our and other experimental results suggests that a p-silicon detector will remain linear also after heavy preirradiation.