Scanning tunneling microscopy investigation of 2H-MoS_2: A layered semiconducting transition‐metal dichalcogenide

Scanning tunneling microscopy (STM) has been enormously successful in solving several important problems in the geometric and electronic structure of homogeneous metallic and semiconducting surfaces. A central question which remains to be answered with respect to the study of compound surfaces, however, is the extent to which the chemical identity of constituent atoms may be established. Recently, progress in this area was made by Feenstra et al. who succeeded in selectively imaging either Ga or As atoms on the GaAs (110) surface. So far this is the only case where such selectivity has been achieved. In an effort to add to our understanding of compound surface imaging we have undertaken a vacuum STM study of 2H-MoS_2, a material which has two structurally and electronically different atomic species at its surface

Educational work as a method of implementation of plans for the development of a training group in medical universities

The article deals with the principles of educational work, with the help of which it is possible to implement the program of group development, training and formation of a qualified specialist, which is the main goal of the work of teachers of higher educational institutions. Preliminary planning and high-quality organization of educational work can help students to acquire professional knowledge and form the necessary qualities for the physician such as morality, intelligence, compassion for the neighbor, teamwork, responsibility, and others. The article emphasizes the need for cooperation in the educational work of various staff and units of the university: from the lecturer of the department, administration of the faculty and the university to public organizations and associations. The authors emphasize the continuity of educational work, the necessity of conducting it both in classrooms in classes, and in extracurricular time

Tunneling microscopy of 2H-MoS2: A compound semiconductor surface

Molybdenum disulfide, a layered semiconductor, is an interesting material to study with the tunneling microscope because two structurally and electronically different atomic species may be probed at its surface. We report on a vacuum scanning tunneling microscopy study of 2H-MoS2. Atomic resolution topographs and current images show the symmetry of the surface unit cell and clearly reveal two distinct atomic sites in agreement with the well-known x-ray crystal structure

Surface modification of cellulose fibers using dielectric barrier discharge

U teorijskom delu doktorske disertacije sistematizovani su najznačajniji literaturni podaci vezani za strukturu i reaktivnost celuloze i celuloznih vlakana. Posebna paţnja je posvećena mogućnostima modifikacije celuloznih vlakana sa akcentom na sve više zastupljeno korišćenje netermalne plazme u obradi celuloznih tekstilnih materijala. Dat je uporedni pregled karakteristika ureĎaja za dobijanje netermalne plazme kao i karakteristika same plazme dobijene u različitim konfiguracijama i tehničkim rešenjima. Posebna paţnja je posvećena dielektričnom barijernom praţnjenju (DBD), kao trenutno najperspektivnijoj tehnici dobijanja plazme na atmosferskom pritisku imajući u vidu potencijalnu primenu u obradi tekstilnih materijala. Proučeni su literaturni navodi i sistematizovani rezultati drugih istraţivačkih grupa dobijeni proučavanjem interakcije tekstilnih materijala sa DBD-em u različitim gasovima (vazduh, azot i kiseonik). U eksperimentalnom delu doktorske disertacije proučavan je uticaj DBD-a na strukturu, svojstva i reaktivnost prirodnih i regenerisanih celuloznih vlakana, odnosno pamuka i viskoze. Kao izvor plazme, korišćena su dva ureĎaja, prvi u kome se kao radni gas koristi vazduh, uz dodatak zeolita u radno polje praţnjenja radi kontrole vlaţnosti prilikom praţnjenja i u kome razmak izmeĎu elektroda iznosi 3 mm; drugi koji pruţa mogućnost obrade u različitim gasovima, od kojih su korišćeni vazduh, azot i kiseonik i gde je razmak izmeĎu elektroda podešen na 0,5 mm u cilju dobijanja homogenijeg praţnjenja. Nakon modifikovanja DBD-em ispitivane su morfološke promene (SEM, AFM), promene u strukturi uzorka (GPC-MALLS), promene u strukturi površine uzorka (ATR-FTIR, XPS), promene u sorpcionim svojstvima (brzina kvašenja, sadrţaj vlage, sposobnost zadrţavanja vode). Na odabranim uzorcima vršena je sorpcija srebra, bakra i cinka, u cilju dobijanja biološki aktivne celuloze. UtvrĎeno je da predtretman DBD-em poboljšava sorpciju pomenutih jona u uzorcima, shodno čemu je i antimikrobna aktivnost poboljšana. TakoĎe, uvoĎenjem jona Ag+, Cu2+ i Zn2+ sniţava se specifična zapreminska električna otpornost uzoraka, što je takoĎe eksperimentalno utvrĎeno. Starenje uzoraka nakon tretiranja plazmom dovodi do povećane sorpcije jona metala što moţe imati značajnu ulogu u industrijskoj primeni. Pored povećane sorpcije, utvrĎena je i selektivnost u antimikrobnoj aktivnosti prema odreĎenim patogenima u iii zavisnosti od vremena starenja nakon tretiranja plazmom koje prethodi sorpciji metala iz vodenih rastvora. Kao posledica delimičnog uklanjanja primarnog zida sa pamučnih vlakana tokom obrade plazmom u kiseoniku, došlo je do značajnog poboljšanja rastvorljivosti pamučnih vlakana u sistemu DMAc/LiCl. Istovremeno, modifikovan je postupak rastvaranja vlakana viskoze u pomenutom sistemu čime se pruţa mogućnost nesmetanog rastvaranja vlakana viskoze bez destrukcije celuloznih lanaca. Veoma značajnom se pokazala obrada celuloze u DBD-u sa azotom kao radnim gasom, pri čemu su dobijena multifunkcionalna svojstva ispitivanih celuloznih vlakana. Visokofunkcionalizovana površina vlakana utiče na redukovanje jona srebra te dolazi do in situ formiranja čestica nanometarskih dimenzija nakon sorpcije jona srebra na modifikovanim uzorcima. TakoĎe, uz obradu jonima bakra, na površini vlakana viskoze nakon obrade DBD plazmom u N2 i nakon sorpcije jona kalcijuma, dolazi do formiranja mikročestica bakra zvezdastog oblika. Mikročestice su nastale spajanjem mikropločica bakra i vezane za funkcionalizovanu površinu vlakana pokazale su odličnu stabilnost pri pranju. Celulozna vlakna, nakon obrade plazmom u N2, pokazuju smanjenu hidrofilnost i smanjenu moć sorpcije metalnih jona što je direktna posledica ablacije hidroksilnih grupa sa površine materijala pomoću plazme, bez narušavanja drugih značajnih sorpcionih svojstava (sposobnosti zadrţavanja vode, sadrţaja vlage). Uprkos smanjenoj sorpciji jona metala, antimikrobna aktivnost uzoraka obraĎenih u DBD plazmi u N2 ostaje skoro nepromenjena. Posebno je značajno što se kod uzoraka viskoze nakon tretiranja pomoću N2 DBD i jonima bakra javlja takozvani efekat „latice ruţe‖ (eng. pettal effect), gde se kapi vode zadrţavaju na površini materijala bez kvašenja. Predloţeni način obrade omogućava dobijanje hidrofobnog materijala korišćenjem netoksičnih hemikalija tokom jednostavnog postupka modifikovanja.In the theoretical part of doctoral dissertation, the most significant literature data concerning cellulose structure and properties have been systematically reviewed. Special attention has been given to the possibilities of cellulose fibers modification with an emphasis on more and more popular use of non-thermal plasma for cellulosic textile materials treatment. Comparative overview of various devices and characteristics of non thermal plasmas, obtained in different device configuration and technical characteristics, is given. Having in mind potential industrial application, special attention is devoted to the most perspective technique for obtaining atmospheric pressure plasma, so called dielectric barrier discharge (DBD). Literature data and results from other research groups regarding interaction of textile materials with DBD obtained in various gasses (air, nitrogen and oxygen) were studied systematically. The influence of DBD treatment on the structure, properties and reactivity of natural and regenerated cellulose fibers, i.e. cotton and viscose, was studied in the experimental part of doctoral dissertation. Two types of devices were used as plasma source; first device was operating in atmospheric air, with addition of zeolite granules in the field of discharge for the humidity control during the discharge and in which electrode gap was 3 mm; second device offers possibility of using different gasses for treatment, of which air, nitrogen and oxygen were used and electrode gap was 0.5 mm for obtaining more homogenous discharge. After DBD treatment, morphological changes (SEM, AFM), changes in the samples structure (GPC-MALLS), surface changes (ATR-FTIR, XPS), changes in sorption properties (wettability, moisture sorption, water retention value) have been investigated. On the chosen samples, a sorption of silver, copper and zinc has been performed in order to obtain biologically active cellulose. It was found that DBD pretreatment significantly improves sorption of aforementioned ions, and consequently, the antimicrobial activity is also improved. Also, addition of Ag+, Cu2+ and Zn2+ ions to the cellulose leads to a decrease of volume resistivity of samples which was also measured. The ageing of samples after plasma treatment leads to improved sorption of metal ions and this can have serious impact in industrial application. Beside improved sorption of ions, selectivity of antimicrobial activity against specific pathogens has been determined and found to be v dependent on days of ageing after the plasma treatment which precedes sorption of metals form aqueous solution. As a consequence of partial removal of primary wall in cotton fibers during plasma treatment in oxygen, significant improvement of solubility was achieved in DMAc/LiCl solvent. At the same time, the standard procedure of viscose dissolution in the above mentioned solvent has been modified which provided easier dissolution of viscose fibers without destruction of cellulose chains. Cellulose treatment in nitrogen DBD proved to be a very significant for obtaining multifunctional properties of cellulose fibers. Highly functionalized surface of fibers induces reduction of silver ions on modified samples and in situ formation of silver particles of nanometer size. Also, treatment with copper ions, after N2 DBD plasma treatment and calcium sorption, leads to the formation of star-shaped microparticles of copper on the viscose fibers surface. Microparticles were formed by assembling of trigonal microplates of copper and bonded to the functionalized surface of fibers exhibited excellent stability toward washing. Cellulose fibers, after nitrogen plasma treatment, show reduced hydrophilicity and reduced ability to bind metal ions from solution which is direct consequence of etching and ablation of hydroxyl groups from the surface of fibers using plasma, without significant deterioration of other sorption properties (water retention value, moisture sorption). However, antimicrobial activity of samples treated in N2 DBD plasma, despite decreased metal ions sorption, remains almost unchanged, with a much lower consumption of ions. Especially significant result is the appearance of so called ―petal effect‖ after N2 DBD treatment of viscose with subsequent sorption of copper, when during wetting of modified viscose, a drop of water sticks to the surface. Proposed procedure enables obtaining hydrophobic material using non-toxic chemicals during simple process of modification

Phase Diagram of Metal-Insulator Transition in System with Anderson-Hubbard Centers

The model of a strongly correlated system in which periodically spaced Anderson-Hubbard centers are introduced into narrow-band metal is considered. Besides the interactions between localized magnetic moments and strong on-site Coulomb interaction, the model takes into account the hybridization of localized and band states. To study the efect of the lattice deformation on the electrical properties of the system the phonon term and elastic energy have been taken into account. Green functions for band and localized electrons have been found. On this base, the energy spectrum has been investigated as function of model parameters, temperature and external pressure. The criterion of metal-insulator transition for integer value of electron concentration has been derived and the phase diagram of the metal-insulator transition has been built.Comment: presented at 12 International Simposium on Physics of Materials, Prague 4-8.09.201

Experimental Research Using of MQL in Metal Cutting

In this paper an effect of using of minimal quantity lubrication (MQL) technique in turning operations is presented. Experimental research was performed on carbon steel C45E. Technological parameters: depth of cut, feed rate and cutting speed were adjusted to semi-machining and roughing. Higher values ​​of feed and cutting speed were used, than recommended from literature and different types of cooling and lubrication in turning conditions were applied. As a conventional procedure and technology, lubrication with flooding was applied. As special lubrication the MQL technique was used. During research, monitoring of the cutting force, chip shape, tool wear and surface roughness was performed. Relations between parameters, material machinability and economy of process were analyzed

Ancient agricultural and pastoral landscapes on the south side of lake Issyk-Kul: preliminary surveys of the Juuku Valley and Lower Kizil Suu Valley, archaeobotanical results of three stratigraphic profiles, and GIS modeling of Iron Age in Lower Kizil Suu

The main goal of this paper is to present results of preliminary archaeological research on the south side of Lake Issyk-Kul in Kyrgyzstan. We test the hypothesis that agropastoral land use changed over four millennia from the Bronze Age through the ethnographic Kirghiz period due to economic, socio-political, and religious changes in the prehistoric and historic societies of this region. Our research objectives are to: (1) describe and analyze survey results from Lower Kizil Suu Valley; (2) discuss the results of radiometric and archaeobotanical samples taken from three stratigraphic profiles from three settlements from the Juuku Valley, including these chronological periods: the Wusun period (200 to 400 CE), the Qarakhanid period (1100 to 1200 CE), and the ethnographic Kirghiz period (1700 to 1900 CE); and (3) conduct preliminary GIS spatial analyses on the Iron Age mortuary remains (Saka and Wusun period). This research emerges out of the first archaeological surveys conducted in 2019 - 2021 and includes the Lower Kizil Suu alluvial fan; it is an initial step toward developing a model for agropastoral land use for upland valleys of the Inner Tian Shan Mountains.1. Introduction 2. Materials and Methods 2.1. Study Area 2.2. Description of Survey Methods 2.3. Stratigraphic Profiles at Juuku Valley settlements 2.4. Radiometric Dating 2.5. Archaeobotanical Methods 2.6. GIS Methods for Spatial Analysis 3. Results 3.1. Survey results 3.2. Stratigraphic Profiles 3.2.1. Profile at Site-EJS1 (Wusun Period Settlement) 3.2.2. Profile at Site-EJS2 (Qarakhanid Period Settlement) 3.3. Results of Radiometric Dating 3.4. Results of Archaeobotanical Analyses 3.4.1. Site-EJS1 (Eastern Juuku – Settlement-1) 3.4.2. Site-EJS2 (Eastern Juuku – Settlement-2) 3.4.3. Site-LJS3 (Lower Juuku – Settlement-2) 3.5. Results of the GIS Spatial Analyses 4. Discussion 5. Conclusion

Band bending and the apparent barrier height in scanning tunneling microscopy

We consider the influence of tip-induced band bending on the apparent barrier height deduced from scanning tunneling microscopy (STM) experiments at unpinned semiconductor surfaces. Any voltage applied to a probe tip appears partly in the vacuum gap as an electric field at the semiconductor surface and partly in the semiconductor interior as band bending. The fraction appearing in each region is a function of gap spacing so that modulation of the tip-sample separation inevitably modulates the induced surface potential in the semiconductor. At finite temperature, the height and shape of this barrier determine the probability that an electron will reach the semiconductor surface where it can subsequently tunnel through the vacuum gap. Since the surface potential decreases with increasing tip-sample separation, STM measurements of the tunneling barrier at unpinned semiconductor surfaces will yield unusually low values. Detailed numerical calculations of the effect for passivated n-type Si(111) show it to be of observable magnitude. This mechanism may be distinguished from other recently proposed barrier-lowering mechanisms in that it is doping dependent, potentially long range, and possesses a unique voltage signature