Evidence for Kinetic Limitations as a Controlling Factor of Ge Pyramid Formation: a Study of Structural Features of Ge/Si(001) Wetting Layer Formed by Ge Deposition at Room Temperature Followed by Annealing at 600 {\deg}C

The article presents an experimental study of an issue of whether the formation of arrays of Ge quantum dots on the Si(001) surface is an equilibrium process or it is kinetically controlled. We deposited Ge on Si(001) at the room temperature and explored crystallization of the disordered Ge film as a result of annealing at 600 {\deg}C. The experiment has demonstrated that the Ge/Si(001) film formed in the conditions of an isolated system consists of the standard patched wetting layer and large droplike clusters of Ge rather than of huts or domes which appear when a film is grown in a flux of Ge atoms arriving on its surface. We conclude that the growth of the pyramids appearing at temperatures greater than 600 {\deg}C is controlled by kinetics rather than thermodynamic equilibrium whereas the wetting layer is an equilibrium structure.Comment: Accepted for publication in Nanoscale Research Letter

In vivo imaging of the early embryonic cortex in rodents

Embryonic brain development is a highly dynamic period in human life. Any disturbances at this stage can cause life-long negative consequences, such as developmentally related diseases, including autism, schizophrenia, and bipolar spectrum disorders. During development the mother-embryo interface plays a crucial role in supplementing the growing organism with oxygen and nutrients, regulating chemical cues, and protecting it from infections and potentially hazardous compounds. However, most of the studies of embryonic brain development have utilized ex vivo systems such as slices and neuronal cultures. Despite the great value of the data obtained from ex vivo studies, they usually completely ignore the significance of the mother-embryo interaction. Thus, there is desperate need for novel preclinical models of embryonic development. In this thesis, we developed a novel technique for in vivo two-photon imaging in mouse embryos connected to the mother via umbilical cord. We developed the special chamber with polymer membrane allowing to keep embryos separately in the heated physiological solution while the umbilical connection to the anesthetized mother is preserved. We developed the protocol for stimulation of calcium activity using high-power laser radiation and studied the propagation of the resulting calcium waves in the mouse embryonic cortex in vivo under ketamine/xylazine anesthesia. We confirm the enhancing effect of caffeine on the evoked activity and the suppressing effect of the adenosine triphosphate (ATP) -receptor blockade, known from previous ex vivo studies. We analyzed the patterns of wave propagation and show the non-uniform spreading, which suggests the presence of differing connectivity patterns in the cortex already during the early stage of development. Further, we studied spontaneous calcium activity and cellular motility in the mouse embryonic cortex in vivo under light isoflurane anesthesia. We demonstrate two various patterns of ongoing activity: sporadic activation of single cells and correlated activity in the form of calcium waves. We show that blockade of N-methyl-D-aspartate (NMDA) receptors with ketamine inhibits the calcium activity in vivo, corresponding with the arrest of cellular motility. In the last part of the thesis, we studied the dynamics of the externally introduced substance to the mouse embryonic brains in vivo. We used porous silicon nanoparticles, which are a promising drug delivery platform, as they can be loaded with poorly water-soluble drugs. We show that the nanoparticles can breach the placental barrier and accumulate in the brains of the embryo. To study the dynamics of nanoparticles when already in the cortex, we injected the embryonic brains intraventricularly. Nanoparticles, including ones 3-4 um in size, were distributed in 80% of the cortex already 4 hours following the injection, thus demonstrating high motility in the brain tissue of embryos. We confirmed the motility of nanoparticles in real time using the in vivo two-photon imaging of embryos connected to the mother under ketamine/xylazine anesthesia. The results emphasize the susceptibility of the embryonic cortex at the early stage of development to external particles, which should be taken into account in nanomedicine development. In summary, the developed in vivo imaging technique allowed functional studies in the embryonic cortex in real time. This will allow preclinical pharmacological investigations of the compounds while maintaining the physiological mother-embryo interface.Sikiöiden aivojen kehittyminen on erittäin dynaaminen vaihe ihmiselämässä. Mikä tahansa häiriö tässä vaiheessa voi aiheuttaa elinikäisiä negatiivisia seurauksia, kuten kehitykseen liittyviä sairauksia, sisältäen autismin, skitsofrenian ja bipolaarihäiriöt. Kehityksen aikana äidin ja sikiön välisellä liitynnällä on kriittinen rooli hapen ja ravintoaineiden välittämisessä kasvavalle organismille, kemikaalien säännöstelyssä ja infektioilta ja mahdollisesti haitallisilta aineilta suojaamisessa. Kuitenkin suurin osa sikiön aivojen kehityksen tutkimuksista ovat hyödyntäneet ex vivo systeemejä, kuten leikkeitä ja neuronien kulttureita. Huolimatta ex vivo tutkimuksella kerätyn datan suuresta arvosta ne yleensä jättävät kokonaan huomiotta äidin ja sikiön välisen vuorovaikutuksen merkityksen. Täten uusille prekliinisille sikiön kehityksen malleille on välttämätön tarve. Tässä väitöskirjassa kehitettimme uusi tekniikka in vivo tuplafotonikuvantamiselle hiirisikiöissä, jotka ovat yhteydessä äitiin napanuoran kautta. Kehitimme erityisen polymeerikalvokammion, joka mahdollistaa sikiöiden pitämisen erillään lämmitetyssä fysiologisessa solutionissa sillä välin kun napanuorayhteys nukutettuun äitiin säilyyn. Kehitimme protokollan kalsiumaktiviteetin stimuloimiseen käyttäen suurteholaserin säteilyä ja tutkimme syntyvien kalsiumaaltojen etenemistä hiiren aivokuoressa in vivo ketamiini/xylasiini anestesiassa. Me vahvistamme kofeiinin edistävän vaikutuksen synnytettyyn aktiviteettiin, ja adenosiini trifosfaatti (ATP) reseptorin sulun heikentävän vaikutuksen, mitkä ovat tunnettuja aiemmista ex vivo tutkimuksista. Analysoimme aallon etenemistapoja ja näytämme epäyhtenäisen leviämisen, mikä viittaa siihen, että aivokuoressa on erilaistuneet yhteydet jo kehityksen aikaisessa vaiheessa. Lisäksi tutkimme spontaania kalsiumaktiviteettia ja solujen liikettä hiirisikiön aivokuoressa in vivo kevyen isofluraani anestesian alaisuudessa. Näytämme kaksi erilaista kaavaa olemassa olevassa aktiviteetissa: yksittäisten solujen saatunnainen aktiviteetti ja kalsiumaaltojen muodossa oleva korreloitunut aktiviteetti. Me näytämme, että N-metyyli-D-aspartaatin (NMDA) estoreseptorit yhdessä ketamiinin kanssa estää kalsiumaktiviteetin in vivo, mikä vastaa estoa soluliikkuvuudessa. Väitöskirjan viimeisessä osassa tutkimme ulkoisesti tuodun aineen dynamiikka hiirisikiöiden aivoissa in vivo. Käytimme huokoista silikonia, joka on lupaava lääkettä luovuttava alusta, sillä niihin voi kuormata lääkeitä, jotka liukenevat huonosti veteen. Näytämme että nanopartikkelit voivat mennä istukkaesteen läpi ja kerääntyä sikiöiden aivoihin. Tutkiaksemme nanopartikkelien dynamiikkaa aivokuoressa, me injektoimme sikiöiden aivoja intraventrikulaarisesti. Nanopartikkelit,sisältäen 3-4 um kokoisia partikkeleita, olivat hajautuneet 80 prosenttiin aivokuoresta jo neljä tuntia injektoinnn jälken, mikä demonstroi korkeaa nanopartikkeleiden liikkuvuutta sikiöiden aivoissa. Vahvistimme nanopartikkeleiden liikkuvuuden reaaliajassa in vivo tupla-fotonikuvantamisella sikiöillä, jotka ovat yhdistetty äiteihin ketamiini/ksylatsiini anestesiassa. Tulokset korostavat sikiön aivokuoren alttiutta ulkoisille hiukkasille, mikä tulisi ottaa huomioon nanolääkkeitä kehitettäessä. Yhteenvetona, kehitetty in vivo kuvaantamisen malli sallii toiminnallisten tutkimusten teon sikiöiden aivokuoressa reaaliajassa. Tämä mahdollistaa yhdisteiden prekliinisen farmakologisen tutkimisen samalla kun fysiologinen äiti-sikiö yhtymäkohta säilyy

Investigation of Optical Properties and Radiation Stability of TiO2 Powders before and after Modification by Nanopowders of Various Oxides

The titanium dioxide powders are widely used as a pigment for coatings and paints, the important characteristics of which are reflectivity and stability to irradiation. The results of investigations of the optical properties and radiation stability of titanium dioxide powders before and after high-temperature modification with nanopowders are presented in this chapter. The diffuse reflection spectra of various titanium dioxide powders in the UV, visible, and near-IR ranges, and their change during irradiation by electrons with 30 keV energy and a different fluence in vacuum in situ were investigated: (1) TiO2 powders with particle size in the range 60–240 nm; (2) Microsized TiO2 powder (240 nm) modified by Al2O3, ZrO2, SiO2, TiO2, ZnO, MgO nanoparticles with grain size from 30 up to 60 nm; (3) Microsized TiO2 powder (260 nm) modified by SiO2 with the grain size of 12–14 nm at the temperature of 150, 400, and 800°C. The reduction in reflectivity in entire spectrum with decrease in grain sizes of TiO2 nanopowders was established. Nanopowder with the grain size of 80 nm possesses the highest stability to irradiation. It was shown that the average grain size and specific surface of introduced nanoparticles effect noticeably on the radiation stability increase of titanium dioxide powders modified with nanoparticles of various oxides. The micro-sized TiO2 powder heating at temperature of 800оС is the factor which positively influences on the radiation stability

In vivo two-photon imaging of the embryonic cortex reveals spontaneous ketamine-sensitive calcium activity

Prior to sensory experience spontaneous activity appears to play a fundamental role in the correct formation of prominent functional features of different cortical regions. The use of anaesthesia during pregnancy such as ketamine is largely considered to negatively affect neuronal development by interfering with synaptic transmission. Interestingly, the characteristics of spontaneous activity as well as the acute functional effects of maternal anaesthesia remain largely untested in the embryonic cortex in vivo. In the present work, we performed in vivo imaging of spontaneous calcium activity and cell motility in the marginal zone of the cortex of E14-15 embryos connected to the mother. We made use of a preparation where the blood circulation from the mother through the umbilical cord is preserved and fluctuations in intracellular calcium in the embryonic frontal cortex are acquired using two-photon imaging. We found that spontaneous transients were either sporadic or correlated in clusters of neuronal ensembles at this age. These events were not sensitive to maternal isoflurane anaesthesia but were strongly inhibited by acute in situ or maternal application of low concentration of the anaesthetic ketamine (a non-competitive antagonist of NMDA receptors). Moreover, simultaneous imaging of cell motility revealed a correlated strong sensitivity to ketamine. These results show that anaesthetic compounds can differ significantly in their impact on spontaneous early cortical activity as well as motility of cells in the marginal zone. The effects found in this study may be relevant in the etiology of heightened vulnerability to cerebral dysfunction associated with the use of ketamine during pregnancy.Peer reviewe

Evolution of Ge wetting layers growing on smooth and rough Si (001) surfaces: isolated {105} facets as a kinetic factor of stress relaxation

The results of STM and RHEED studies of a thin Ge film grown on the Si/Si(001) epitaxial layers with different surface relief are presented. Process of the partial stress relaxation was accompanied by changes in the surface structure of the Ge wetting layer. Besides the well-known sequence of surface reconstructions ($2 \times 1 \rightarrow 2 \times N \rightarrow M \times N$ patches) and hut clusters faceted with {105} planes, the formation of isolated {105} planes, which faceted the edges of $M \times N$ patches, has been observed owing to the deposition of Ge on a rough Si/Si (001) surface. A model of the isolated {105} facet formation has been proposed based on the assumption that the mutual arrangement of the monoatomic steps on the initial Si surface promotes the wetting layer formation with the inhomogeneously distributed thickness that results in the appearance of $M \times N$ patches partially surrounded by deeper trenches than those observed in the usual Ge wetting layer grown on the smooth Si(001) surface. Isolated {105} facets are an inherent part of the Ge wetting layer structure and their formation decreases the surface energy of the Ge wetting layer.Comment: 27 pages, 8 figure

Peculiarities and evolution of Raman spectra of multilayer Ge/Si(001) heterostructures containing arrays of low-temperature MBE-grown Ge quantum dots of different size and number density: Experimental studies and numerical simulations

Ge/Si(001) multilayer heterostructures containing arrays of low-temperature self-assembled Ge quantum dots and very thin Si$_x$Ge$_{1-x}$ layers of varying composition and complex geometry have been studied using Raman spectroscopy and scanning tunneling microscopy. The dependence of Raman spectra on the effective thickness of deposited Ge layers has been investigated in detail in the range from 4 to 18 \r{A}. The position and shape of both Ge and SiGe vibrational modes are of great interest since they are closely related to the strain and composition of the material that plays a role of active component in perspective optoelectronic devices based on such structures. In this work, we present an explanation for some peculiar features of Raman spectra, which makes it possible to control the quality of the grown heterostructures more effectively. A dramatic increase of intensity of both the Ge$-$Ge and Si$-$Ge bands for the structure containing Ge layers of 10 \r{A} and anomalous shift and broadening of the Si$-$Ge band for structures comprising Ge layers of 8 and 9 \r{A} thick were observed. In our model, the anomalous behavior of the Raman spectra with the change of thickness of deposited Ge is connected with the flatness of Ge layers as well as transitional SiGe domains formed via the stress-induced diffusion from {105} facets of quantum dots. The conclusions are supported by the STM studies and the numerical calculations.Comment: 17 pages, 11 figure

In vivo Calcium Imaging of Evoked Calcium Waves in the Embryonic Cortex

The dynamics of intracellular calcium fluxes are instrumental in the proliferation, differentiation, and migration of neuronal cells. Knowledge thus far of the relationship between these calcium changes and physiological processes in the developing brain has derived principally from ex vivo and in vitro experiments. Here, we present a new method to image intracellular calcium flux in the cerebral cortex of live rodent embryos, whilst attached to the dam through the umbilical cord. Using this approach we demonstrate induction of calcium waves by laser stimulation. These waves are sensitive to ATP-receptor blockade and are significantly increased by pharmacological facilitation of intracellular-calcium release. This approach is the closest to physiological conditions yet achieved for imaging of calcium in the embryonic brain and as such opens new avenues for the study of prenatal brain development. Furthermore, the developed method could open the possibilities of preclinical translational studies in embryos particularly important for developmentally related diseases such as schizophrenia and autism.Peer reviewe

Multi-locus transcranial magnetic stimulation system for electronically targeted brain stimulation

Background: Transcranial magnetic stimulation (TMS) allows non-invasive stimulation of the cortex. In multi-locus TMS (mTMS), the stimulating electric field (E-field) is controlled electronically without coil movement by adjusting currents in the coils of a transducer. Objective: To develop an mTMS system that allows adjusting the location and orientation of the E-field maximum within a cortical region. Methods: We designed and manufactured a planar 5-coil mTMS transducer to allow controlling the maximum of the induced E-field within a cortical region approximately 30 mm in diameter. We developed electronics with a design consisting of independently controlled H-bridge circuits to drive up to six TMS coils. To control the hardware, we programmed software that runs on a field-programmable gate array and a computer. To induce the desired E-field in the cortex, we developed an optimization method to calculate the currents needed in the coils. We characterized the mTMS system and conducted a proof-of-concept motor-mapping experiment on a healthy volunteer. In the motor mapping, we kept the transducer placement fixed while electronically shifting the E-field maximum on the precentral gyrus and measuring electromyography from the contralateral hand. Results: The transducer consists of an oval coil, two figure-of-eight coils, and two four-leaf-clover coils stacked on top of each other. The technical characterization indicated that the mTMS system performs as designed. The measured motor evoked potential amplitudes varied consistently as a function of the location of the E-field maximum. Conclusion: The developed mTMS system enables electronically targeted brain stimulation within a cortical region. (c) 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer reviewe

STM and RHEED study of the Si(001)-c(8x8) surface

The Si(001) surface deoxidized by short annealing at T~925C in the ultrahigh vacuum molecular beam epitaxy chamber has been in situ investigated by high resolution scanning tunnelling microscopy (STM) and reflected high energy electron diffraction (RHEED). RHEED patterns corresponding to (2x1) and (4x4) structures were observed during sample treatment. The (4x4) reconstruction arose at T<600C after annealing. The reconstruction was observed to be reversible: the (4x4) structure turned into the (2x1) one at T>600C, the (4x4) structure appeared again at recurring cooling. The c(8x8) reconstruction was revealed by STM at room temperature on the same samples. A fraction of the surface area covered by the c(8x8) structure decreased as the sample cooling rate was reduced. The (2x1) structure was observed on the surface free of the c(8x8) one. The c(8x8) structure has been evidenced to manifest itself as the (4x4) one in the RHEED patterns. A model of the c(8x8) structure formation has been built on the basis of the STM data. Origin of the high-order structure on the Si(001) surface and its connection with the epinucleation phenomenon are discussed.Comment: 26 pages, 12 figure