D. Chrobak

Deconfinement leads to changes in the nanoscale plasticity of silicon

Silicon crystals have an important role in the electronics industry, and silicon nanoparticles have applications in areas such as nanoelectromechanical systems, photonics and biotechnology. However, the elastic-plastic transition observed in silicon is not fully understood; in particular, it is not known if the plasticity of silicon is determined by dislocations or by transformations between phases. Here, based on compression experiments and molecular dynamics simulations, we show that the mechanical properties of bulk silicon and silicon nanoparticles are significantly different. We find that bulk silicon exists in a state of relative constraint, with its plasticity dominated by phase transformations, whereas silicon nanoparticles are less constrained and display dislocation-driven plasticity. This transition, which we call deconfinement, can also explain the absence of phase transformations in deformed silicon nanowedges. Furthermore, the phenomenon is in agreement with effects observed in shape-memory alloy nanopillars, and provides insight into the origin of incipient plasticity.

Two-stage martensitic transformation in a deformed and annealed NiTi alloy

Deformation and subsequent annealing of NiTi alloys offers great possibilities of controlling characteristic temperatures, sequences of transformations and shape recovery. Todoroki and Tamura showed that depending on annealing temperatures, changes in transformation sequences in deformed alloys occur during both heating and cooling. They also observed that the cooling DSC (Differential Scanning Calorimetry) curves exhibit two peaks in the range of the martensitic transformation for specimens annealed at 500 C. Referring to the paper of Monasevich and Pascal they explained this effect as the formation of two types of martensite. In this paper the authors try to prove that the appearance of two peaks in the martensitic range on the DTA (Differential Thermal Analysis) cooling curves may be due to changes of dislocation configuration caused by the low-temperature annealing.

An electric current spike linked to nanoscale plasticity

The increase in semiconductor conductivity that occurs when a hard indenter is pressed into its surface has been recognized for years, and nanoindentation experiments have provided numerous insights into the mechanical properties of materials. In particular, such experiments have revealed so called pop-in events, where the indenter suddenly enters deeper into the material without any additional force being applied; these mark the onset of the elastic-plastic transition. Here, we report the observation of a current spike--a sharp increase in electrical current followed by immediate decay to zero at the end of the elastic deformation--during the nanoscale deformation of gallium arsenide. Such a spike has not been seen in previous nanoindentation experiments on semiconductors, and our results, supported by ab initio calculations, suggest a common origin for the electrical and mechanical responses of nanodeformed gallium arsenide. This leads us to the conclusion that a phase transition is the fundamental cause of nanoscale plasticity in gallium arsenide, and the discovery calls for a revision of the current dislocation-based understanding of nanoscale plasticity.

Effect of early stages of precipitation and recovery on the multi-step transformation in deformed and annealed near-equiatomic NiTi alloy

Abstract Multi-step martensitic transformation in Ni50.6at.%–Ti alloy was studied by means of differential scanning calorimetry and electron microscopy methods. The alloy was subjected to 10% deformation after solution treatment and then annealed at 673 K for different times. The time evolution of the multi-step transformation is explained in terms of the complex alloy structure and non-homogeneity of the precipitate distribution in the matrix.

Nanoindentation experiments with different loading rate distinguish the mechanism of incipient plasticity

Recent findings in nanodeformation of semiconductors posed a dilemma whether the nanoscale plasticity starts with phase transformation or nucleation of dislocations in a stressed nanovolume. In this letter we demonstrate the results of nanoindentation experiments with different loading rate, which enable us to conclude on a mechanism of incipient plasticity. The recorded nanodeformation response of GaAs and Si contrasts that observed for either GaN or metallic Fe crystal, which supports the phase transformation nature of the GaAs incipient plasticity. The derived relationship between the energy barrier for defect nucleation and applied stress served as a verification of the obtained results.

Origin of a Nanoindentation Pop-in Event in Silicon Crystal

The Letter concerns surface nanodeformation of Si crystal using atomistic simulation. Our results account for both the occurrence and absence of pop-in events during nanoindentation. We have identified two distinct processes responsible for indentation deformation based on load-depth response, stress-induced evolution of crystalline structure and surface profile. The first, resulting in a pop-in, consists of the extrusion of the crystalline high pressure Si-III/XII phase, while the second, without a pop-in, relies on a flow of amorphized Si to the crystal surface. Of particular interest to silicon technology will be our clarification of the interplay among amorphization, crystal-to-crystal transition, and extrusion of transformed material to the surface.

Mystery of current spike: Nanoscale plasticity revisited

Abstract This paper addresses the current spike (CS) phenomenon revealed by in situ measurements of electrical response during nanoindentation. The CS is defined as the sharp initial increase in electric current through the highly compressed GaAs/metallic indenter nanocontact and its decay to zero upon termination of elastic deformation. The clarification of this new effect is justified by our ab initio analysis of the metal/semiconductor contact. The obtained results reveal the common origin of the simultaneous mechanical and electrical responses, these being the pop-in event and the CS respectively. This leads to a substantial revision of our understanding of the onset of nanoscale plasticity. Our results support the hypothesis, deduced from atomistic simulations, of the non-dislocation incipient plasticity of GaAs. They are also in accord with the fresh idea of nanoscale deconfinement driven deformation of compressed silicon nanospheres.

Structure-Dependent Mechanical Properties of ALD-Grown Nanocrystalline BiFeO3 Multiferroics

The present paper pertains to mechanical properties and structure of nanocrystalline multiferroic BeFiO3 (BFO) thin films, grown by atomic layer deposition (ALD) on the Si/SiO2/Pt substrate. The usage of sharp-tip-nanoindentation and multiple techniques of structure examination, namely, grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectrometry, enabled us to detect changes in elastic properties and hardness of BFO after stages of annealing and observe their relation to the material’s structural evolution. Our experiments point towards an increase in structural homogeneity of the samples annealed for a longer time. To our best knowledge, the present report constitutes the first disclosure of nanoindentation mechanical characteristics of ALD-fabricated BeFiO3, providing a new insight into the phenomena that accompany structure formation and development of nanocrystalline multiferroics. We believe that our systematic characterization of the BFO layers carried out at consecutive stages of their deposition provides pertinent information which is needed to control and optimize its ALD fabrication.

Investigation of electronic structures of titanium nitride layers on TiNi substrate

The electronic structure of titanium nitride layers formed on the TiNi substrates is examined by Auger electron spectroscopy and electron emission distribution methods. Spectral analysis shows that the on-top carbon layer has a graphite structure and the neighbouring layer is constituted of titanium nitride. The shape of the main valence spectra was explained by the Hubbard model. From the comparison of experiment and theory the model parameters were estimated. Besides, the existence of surface and internal plasmons verifies the layered structures with average dielectric constants.

Nanoindentation of GaAs(001) surface.A molecular dynamics study

The present paper reports molecular dynamics study of the elastic deformation of zinc- blende GaAs crystal by spherical diamond indenter acting on the (001) plane. The atomic displacements under field of elastic deformation result in generation of characteristic atomic structure with arms located along <110> directions. The phase transformation from zinc-blende to rock-salt GaAs structure was recognized in thin volume under indenter.