Porous Si Partially Filled with Water Molecules-Crystal Structure, Energy Bands and Optical Properties from First Principles.

The paper reports the results on first-principles investigation of energy band spectrum and optical properties of bulk and nanoporous silicon. We present the evolution of energy band-gap, refractive indices and extinction coefficients going from the bulk Si of cubic symmetry to porous Si with periodically ordered square-shaped pores of 7.34, 11.26 and 15.40 Å width. We consider two natural processes observed in practice, the hydroxylation of Si pores (introduction of OH groups into pores) and the penetration of water molecules into Si pores, as well as their impact on the electronic spectrum and optical properties of Si superstructures. The penetration of OH groups into the pores of the smallest 7.34 Å width causes a disintegration of hydroxyl groups and forms non-bonded protons which might be a reason for proton conductivity of porous Si. The porosity of silicon increases the extinction coefficient, k, in the visible range of the spectrum. The water structuring in pores of various diameters is analysed in detail. By using the bond valence sum approach we demonstrate that the types and geometry of most of hydrogen bonds created within the pores manifest a structural evolution from distorted hydrogen bonds inherent to small pores (∼7 Å) to typical hydrogen bonds observed by us in larger pores (∼15 Å) which are consistent with those observed in a wide database of inorganic crystals.

Activation-relaxation processes and related effects in quantum conductance of molecular junctions.

We reveal the comparative relationship between small changes in quantum conductivity behavior for molecular junctions. We clarify the mechanisms of acquiring and losing additional thermal activation energy during average current flow in a gold-1,4 benzenediamine (BDA)-gold molecular junction and explain the quantum conductance modulation process. Small changes in working temperature lead to a change in quantum conductivity, which is reflected in random telegraph signal behavior. We demonstrate the high sensitivity of the BDA molecules to small changes in temperature. For BDA molecules, conductance thermo-sensitivity values are relatively high near to [Formula: see text] This advantage can be used to measure weak variations in the ambient temperature. We show that the additional thermal energy arising from the change in temperature can impact on the strength of the electrode-molecule coupling, on the modulation of quantum conductivity. Local changes in quantum conductance of the order of quanta or smaller are conditioned by small random changes in the working regime arising from some of the activation processes. On the basis of the modulation of conductance, we calculate the magnitude of the spring constant of the 1,4 benzenediamine molecule as [Formula: see text] at the stretching length of 0.03 nm for the Au-NH2 molecular junction.

Noise spectroscopy to study the 1D electron transport properties in InAs nanowires.

InAs nanowires (NWs) are recognized as a key material due to their unique transport properties. Despite remarkable progress in designing InAs NW device structures, there are still open questions on device variability. Here, we demonstrate that noise spectroscopy allows us to study not only the parameters of traps, but also to shed light on quantum transport in NW structures. This provides an important understanding of structural behavior as well as the background and strategy required to design NW structures with advanced properties.

Origin of noise in liquid-gated Si nanowire troponin biosensors.

Liquid-gated Si nanowire field-effect transistor (FET) biosensors are fabricated using a complementary metal-oxide-semiconductor-compatible top-down approach. The transport and noise properties of the devices reflect the high performance of the FET structures, which allows label-free detection of cardiac troponin I (cTnI) molecules. Moreover, after removing the troponin antigens the structures demonstrate the same characteristics as before cTnI detection, indicating the reusable operation of biosensors. Our results show that the additional noise is related to the troponin molecules and has characteristics which considerably differ from those usually recorded for conventional FETs without target molecules. We describe the origin of the noise and suggest that noise spectroscopy represents a powerful tool for understanding molecular dynamic processes in nanoscale FET-based biosensors.

Electronic edge-state and space-charge phenomena in long GaN nanowires and nanoribbons.

We studied space-charge-distribution phenomena in planar GaN nanowires and nanoribbons (NRs). The results obtained at low voltages demonstrate that the electron concentration changes not only at the edges of the NR, but also in the middle part of the NR. The effect is stronger with decreasing NR width. Moreover, the spatial separation of the positive and negative charges results in electric-field patterns outside the NR. This remarkable feature of electrostatic fields outside the NR may be even stronger in 2D material structures. For larger voltages the space-charge-limited current (SCLC) effect determines the main mechanism of transport in the NR samples. The onset of the SCLC effect clearly correlates with the NR width. The results are confirmed by noise spectroscopy studies of the NR transport. We found that the noise increases with decreasing NR width and the shape of the spectra changes with voltage increase with a tendency toward slope (3/2), reflecting diffusion processes due to the SCLC effect. At higher voltages noise decreases as a result of changes in the scattering mechanisms. We suggest that the features of the electric current and noise found in the NRs are of general character and will have an impact on the development of NR-based devices.

Contact properties to CVD-graphene on GaAs substrates for optoelectronic applications.

The optimization of contacts between graphene and metals is important for many optoelectronic applications. In this work, we evaluate the contact resistance and sheet resistance of monolayer and few-layered graphene with different metallizations using the transfer length method (TLM). Graphene was obtained by the chemical vapor deposition technique (CVD-graphene) and transferred onto GaAs and Si/SiO2 substrates. To account for the quality of large-area contacts used in a number of practical applications, a millimeter-wide TLM pattern was used for transport measurements. Different metals--namely, Ag, Pt, Cr, Au, Ni, and Ti--have been tested. The minimal contact resistance Rc obtained in this work is 11.3 kΩ µm for monolayer CVD-graphene, and 6.3 kΩ µm for a few-layered graphene. Annealing allows us to decrease the contact resistance Rc and achieve 1.7 kΩm µm for few-layered graphene on GaAs substrate with Au contacts. The minimal sheet resistance Rsh of few-layered graphene transferred to GaAs and Si/SiO2 substrates are 0.28 kΩ/□ and 0.27 kΩ/□. The Rsh value of monolayer graphene on the GaAs substrate is 8 times higher (2.3 kΩ/□), but it reduces for the monolayer graphene on Si/SiO2 (1.4 kΩ/□). For distances between the contacts below 5 µm, a considerable reduction in the resistance per unit length was observed, which is explained by the changes in doping level caused by graphene suspension at small distances between contact pads.

Advanced fabrication of Si nanowire FET structures by means of a parallel approach.

In this paper we present fabricated Si nanowires (NWs) of different dimensions with enhanced electrical characteristics. The parallel fabrication process is based on nanoimprint lithography using high-quality molds, which facilitates the realization of 50 nm-wide NW field-effect transistors (FETs). The imprint molds were fabricated by using a wet chemical anisotropic etching process. The wet chemical etch results in well-defined vertical sidewalls with edge roughness (3σ) as small as 2 nm, which is about four times better compared with the roughness usually obtained for reactive-ion etching molds. The quality of the mold was studied using atomic force microscopy and scanning electron microscopy image data. The use of the high-quality mold leads to almost 100% yield during fabrication of Si NW FETs as well as to an exceptional quality of the surfaces of the devices produced. To characterize the Si NW FETs, we used noise spectroscopy as a powerful method for evaluating device performance and the reliability of structures with nanoscale dimensions. The Hooge parameter of fabricated FET structures exhibits an average value of 1.6 × 10(-3). This value reflects the high quality of Si NW FETs fabricated by means of a parallel approach that uses a nanoimprint mold and cost-efficient technology.

Low-frequency noise in individual carbon nanotube field-effect transistors with top, side and back gate configurations: effect of gamma irradiation.

We report on the influence of low gamma irradiation (10(4) Gy) on the noise properties of individual carbon nanotube (CNT) field-effect transistors (FETs) with different gate configurations and two different dielectric layers, SiO2 and Al2O3. Before treatment, strong generation-recombination (GR) noise components are observed. These data are used to identify several charge traps related to dielectric layers of the FETs by determining their activation energy. Investigation of samples with a single SiO2 dielectric layer as well as with two dielectric layers allows us to separate traps for each of the two dielectric layers. We reveal that each charge trap level observed in the side gate operation splits into two levels in top gate operation due to a different potential profile along the CNT channel. After gamma irradiation, only reduced flicker noise is registered in the noise spectra, which indicates a decrease of the number of charge traps. The mobility, which is estimated to be larger than 2 × 10(4) cm(2) V(-1) s(-1) at room temperature, decreases only slightly after radiation treatment, demonstrating high radiation hardness of the CNTs. Finally, we study the influence of Schottky barriers at the metal-nanotube interface on the transport properties of FETs, analyzing the behavior of the flicker noise component.