Nunzio Motta

Ge-Si intermixing in Ge quantum dots on Si(001) and Si(111)

Exploiting Ge K-edge x-ray absorption spectroscopy we provide direct evidence of Si–Ge intermixing in self-organized strained and unstrained Ge quantum dots on Si, and provide a quantitative measurement of the average composition. For Ge/Si(001) dots with equivalent thickness in the range 5.8–38 nm and morphology ranging from that typical of coherently strained to that associated with relaxed dots we find that the average Si composition is approximately 30%. For Ge/Si(111), we find that the wetting layer has a Si composition near 50%. We discuss these results in terms of the energetics of dot formation and argue that strain-enhanced diffusion of Si into Ge should be considered as an important factor in minimizing the strain energy of the system.

EXAFS measurements on FeB metallic glasses: Asymmetry of the radial distribution function

Abstract The EXAFS structure measured above the K-edge of Fe in the disordered Fe80B20 alloy is reported. Complete agreement is obtained with X-ray diffraction results in the position of the first neighbours coordination shell when the asymmetry of the Radial Distribution Function (R.D.F.) into the EXAFS formula is included. A Finney-like R.D.F. is used to describe the pair distribution of these metallic glasses. The weak temperature dependence of the spectra is discussed in terms of the structural properties of these amorphous alloys.

Electrochemically exfoliated graphene for electrode films : effect of graphene flake thickness on the sheet resistance and capacitive properties

We present an electrochemical exfoliation method to produce controlled thickness graphene flakes by ultrasound assistance. Bilayer graphene flakes are dominant in the final product by using sonication during the electrochemical exfoliation process, while without sonication the product contains a larger percentage of four-layer graphene flakes. Graphene sheets prepared by using the two procedures are processed into films to measure their respective sheet resistance and optical transmittance. Solid-state electrolyte supercapacitors are made using the two types of graphene films. Our study reveals that films with a higher content of multilayer graphene flakes are more conductive, and their resistance is more easily reduced by thermal annealing, making them suitable as transparent conducting films. The film with higher content of bilayer graphene flakes shows instead higher capacitance when used as electrode in a supercapacitor.

Copper phthalocyanine on Si(111)-7 × 7 and Si(001)-2 × 1: an XPS/AES and STM study

Abstract Silicon surfaces with different reconstruction, Si (111)-7 × 7 and Si (001)-2 × 1, are exposed to a molecular beam of copper phthalocyanine. On Si(111) the adsorbate-substrate interaction is strong, as suggested by a partial reduction of copper shown in the XPS spectra and by the evidence provided by STM of a chemical reaction involving some of the Si adatoms. As a consequence, the molecules are not imaged at atomic scale by STM. At variance with this behaviour, on the single-domain Si (001)-2 × 1 face, the adsorbed molecules are clearly imaged by STM, suggesting an adsorption state more weakly bonded. This is the first observation of Cu phthalocyanine molecules imaged on a silicon surface, and it further illustrates the potential of STM for surface chemistry.

Porphyrin thin films coated quartz crystal microbalances prepared by electropolymerization technique

Quartz crystal microbalances (QMB) have been coated by thin films of different metal complexes of protoporphyrin IX dimethyl ester (M-PPIX), deposited by electropolymerization technique. The structure of these films has been characterized by scanning tunnelling microscopy (STM). The sensory behaviour of the obtained QMB have been studied in the presence of four volatile organic compounds (VOC).

Preparation of graphene oxide/epoxy nanocomposites with significantly improved mechanical properties

The effect of graphene oxide (GO) on the mechanical properties and the curing reaction of Diglycidyl Ether of Bisphenol A/F and Triethylenetetramine epoxy system was investigated. GO was prepared by oxidation of graphite flakes and characterized by spectroscopic and microscopic techniques. Epoxy nanocomposites were fabricated with different GO loading by solution mixing technique. It was found that incorporation of small amount of GO into the epoxy matrix significantly enhanced the mechanical properties of the epoxy. In particular, model I fracture toughness was increased by nearly 50% with the addition of 0.1 wt. % GO to epoxy. The toughening mechanism was understood by fractography analysis of the tested samples. The more irregular, coarse, and multi-plane fracture surfaces of the epoxy/GO nanocomposites were observed. This implies that the two-dimensional GO sheets effectively disturbed and deflected the crack propagation. At 0.5 wt. % GO, elastic modulus was ∼35% greater than neat epoxy. Differential ...

Regioregular poly(3-hexyl-thiophene) helical self-organization on carbon nanotubes

Mixtures of regioregular poly(3-hexyl-thiophene) (rrP3HT) and multiwall carbon nanotubes have been investigated by scanning tunneling microscopy in ultrahigh vacuum. Carbon nanotubes covered by rrP3HT have been imaged and analyzed, providing a clear evidence that this polymer self-assembles on the nanotube surface following geometrical constraints and adapting its equilibrium chain-to-chain distance. Largely spaced covered nanotubes have been analyzed to investigate the role played by nanotube chirality in the polymer wrapping, evidencing strong rrP3HT interactions along well defined directions.

Self-assembling and ordering of Ge/Si(111) quantum dots: scanning microscopy probe studies

In order to exploit the unique electronic properties of semiconductor quantum dots (QDs) in novel quantum effect devices, the lateral dimensions of these structures have to be reduced to the order of tens of nanometres, which is the range of the de Broglie wavelength of electrons inside these materials. Moreover, millions of QDs should be packed in an orderly fashion in dense arrays to achieve the necessary active volume. So far, the most promising quantum structures have been fabricated using techniques based on direct crystal growth. This process has become very important due to its potential for creating damage-free and coherent nanocrystals. New scanning microscopy probes, such as scanning tunnelling microscopy and atomic force microscopy, are actually the most important analytical tools for checking the properties of these nanostructures. In the present paper we will review some of the results recently obtained on the Ge/Si system thanks to these techniques. Ge/Si is really appealing for future applications, in view of the integration of nanostructures with the present microelectronic technologies, but it is also at the origin of many general studies regarding atomic diffusion, intermixing, and island shape stabilization. The origin of the islands, their size and shape evolution, and the intermixing at the interface will be analysed starting from SPM data. The ordering of QDs on naturally or artificially nanostructured surfaces is one of the most recent objectives in Ge/Si research. For example, terraces created from the step bunching process on the Si(111) surface could be a way of controlling the spacing of the islands. Other possibilities are now opened by the increasing precision of electron-beam lithography and by new focused-ion-beam machines. These aspects, along with problems and drawbacks of self-assembling technology, will also be discussed.

Self-ordering of Ge islands on step-bunched Si(111) surfaces

By using step-bunched Si(111) surfaces as templates, we demonstrate the self-assembly of an ordered distribution of Ge islands without lithographic patterning. Initially, islands nucleate and evolve at step edges, up to complete ripening, forming long ribbons. Subsequently, island nucleation takes place at the center of flat terraces. Ge islands appear to be regularly spaced in scanning tunneling microscope images. The exploitation of this effect provides a possible route to grow ordered arrays of semiconducting nanostructures.

Ge-Si intermixing in Ge quantum dots on Si

We have provided direct evidence for the presence of considerable Si-Ge intermixing in strained and unstrained Ge quantum dots deposited on Si(001) and Si(111). The local structure around Ge was probed by using Ge K-edge X-ray absorption spectroscopy; complementary evidence for intermixing was provided by AFM and STM studies. These results implied that the strain energy in the dots was reduced by Si atoms diffusing into the dots, resulting in a modified form of Stranski-Krastanov growth.