2. Articole

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ABOUT THE EDGE LUMINESCENCE OF CADMIUM SULPHIDE THIN LAYERS GROWN ON MOLYBDENUM
(Institute of Electrical and Electronics Engineers, 1995-11-11) Raevschi, Simion; Gorceac, Leonid; Coval, Andrei; Chetruș, Petru
CdS layers on to molybdenum were synthesized from separate elements in an open flowing hydrogen set. The surface micromorphology and some electrical and photoluminescent properties were investigated. The nature of layers edge photoluminescence peaks are discussed in the frames of the excitated exciton model.
ABSORBŢIA LUMINII ŞI FOTOLUMINESCENŢA CRISTALELOR GaSe:Eu
(CEP USM, 2012) Untila, Dumitru
GaSe monocrystals doped with 0,025%, 0,068%, 0,49% at. Eu, optically transparent in wavelengths domain λ>650 nm, were grown by Bridgman method. In addition to structural defects in ...Se-Ga-Ga-Se... stratified package, introduction of the Eu impurities in GaSe leads to the formation of Eu3+ luminescence centers. On the elementary package surface of GaSe doped with 0,49% at. Eu, there are neutral Eu atoms, what by thermic treatment at 400÷450ºC are forming a Eu2O3 and Ga2O3 composite layer.
ABSORBŢIA LUMINII ŞI FOTOLUMINESCENŢA CRISTALELOR GaSe:Eu
(CEP USM, 2012) Untila, Dumitru
GaSe monocrystals doped with 0,025%, 0,068%, 0,49% at. Eu, optically transparent in wavelengths domain λ>650 nm, were grown by Bridgman method. In addition to structural defects in ...Se-Ga-Ga-Se... stratified package, introduction of the Eu impurities in GaSe leads to the formation of Eu3+ luminescence centers. On the elementary package surface of GaSe doped with 0,49% at. Eu, there are neutral Eu atoms, what by thermic treatment at 400÷450ºC are forming a Eu2O3 and Ga2O3 composite layer.
ABSORBŢIA LUMINII ŞI FOTOLUMINESCENŢA IONILOR Eu3+ ÎN CRISTALE DE CaF2
(CEP USM, 2012) Untila, Dumitru; Racoveț, Oxana; Evtodiev, Silvia; Caraman, Mihail
From optical transmittance and photoluminescence measurements of natural crystals of CaF2:Eu was established the correlation between energy states diagram of Eu3+ ion and the structure of these bands. Photon absorption in this material occurs at electronic transitions from the 7F0 level on the G, D, L configuration levels of Eu3+ ion. Because 7F1 level population is much smaller than the 7F0 fundamental level population, the photon absorption intensity which involving 7F1 level is not evident in absorption spectra at room temperature. CaF2:Eu3+ crystal photoluminescence is determined by electron radiative transitions from 5 D0 level to the low energy levels 7F0, 7F2 and, perhaps, 7F3.
ABSORBȚIA OPTICĂ ȘI FOTOLUMINESCENȚA COMPOZITULUI Ga2S3-Ga2O3
(Universitatea Tehnică din Moldova, 2018-05-24) Сaraman, Iuliana; Evtodiev, Igor; Untilă, Dumitru; Dmitroglo, Liliana; Caraman, Mihail; Evtodiev, Silvia; Palachi, Leonid
În această lucrare sunt studiate absorbția optică și fotoluminescența cristalelor de Ga2S3, obținute prin metoda CVT în atmosferă de I2, și a compozitului Ga2S3–Ga2O3, obținut prin tratament termic al monocristalelor de Ga2S3 în atmosferă normală, la temperatura 1073K. S-a determinat că în rezultatul tratamentului termic de lungă durată (12 ore) suprafața cristalelor de Ga2S3 se acoperă cu un strat granular de Ga2O3. Din măsurări ale reflexiei difuze, lățimea benzii interzise a stratului de Ga2O3 de pe suprafața monocristalului Ga2S3 a fost aproximată ca fiind egală cu 4,47 eV. La 300K, marginea benzii de absorbție a cristalelor de Ga2S3 este formată din trei sectoare în care au loc tranziții optice directe cu lățimea benzii interzise egală cu 3,020 eV, 3,178 eV și 3,312 eV, iar la 80K - cu 3,196 eV, 3,302 eV și 3,422 eV. Spectrul de FL a cristalelor de Ga2S3 conține doar o singură bandă în regiunea roșu a spectrului, ce se interpretează ca emisie radiativă a stratului de Ga2S3, iar spectrul de FL al compozitului Ga2S3–Ga2O3 pe lângă banda roșie conține și o bandă în regiunea violet–albastru a spectrului, ce se identifică ca emisie radiativă în cristalele de oxid din compozitul Ga2O3-Ga2S3.
ABSORPTION AND PHOTOCONDUCTIVITY OF PbMoO4 CRYSTALS
(2011) Mușinschi, Valeriu; Caraman, Mihail; Mușinschi, Camelia
Spectral characteristics of the fundamental absorption coefficient and photoconductivity of PbMoO4 crystals with two polarizations in a temperature range of 77 to 300 K are investigated. The measurements of light transmitted in the polarized radiation for both orientations ( E|| C4 and E ┴ C4, C4 is the [001] crystallographic direction corresponding to the optical axis) of crystals are carried out. Spectral dependence of the absorption coefficient at the edge absorption (λ ≈ 0.3-0.5 μm) shows that the edge absorption is determined by indirect assisted by phonons transitions. The measured spectral dependence of the absorption coefficient at two orientations of polarization of light and the known theoretical dependence were combined and compared for determination the types of optical transitions in the fundamental absorption of PbMoO4 crystals. Limited energies Egind and Egdir for E|| C4 and E ┴ C4 were found. A detailed investigation of the fundamental absorption of PbMoO4 crystals is presented. A simple model of band structure of crystals is proposed. The photoconductivity of PbMoO4 single crystals is investigated on E|| C4 and E ┴ C4 at 300 K in the fundamental absorption spectral region of 2.8 to 5.0 eV. The spectral dependence of the photoconductivity is studied and discussed in conjunction with reported optical data. The monopolar character of the photoconductivity was confirmed by investigation time-of-flight motion of carriers. The present study reveals only the intrinsic photoconductivity in PbMoO4 pure single crystals. Spectral responses of the photocurrent through PbMoO4 crystals showed one spectral band in a range of 3.1 to 5.0 eV with maximums at 3.45 and 3.73 eV for E|| C4 and E ┴ C4, respectively. A good correlation of the photoconductivity bands at E|| C4 and E ┴ C4 with absorption bands at also polarizations has been demonstrated. The unipolar time-of flight motion of carriers was analyzed, and drift mobility of electrons and its temperature dependence were determined.
ABSORPTION SPECTRA AND EXTRINSIC PHOTOCONDUCTIVITY OF Cu AND Cd DOPED GaSe SINGLE – CRYSTAL FILMS
(2006) Cuculescu, Elmira; Evtodiev, Igor; Caraman, Mihail; Rusu, George G.
GaSe single crystal films were doped during growth process with Cu or Cd atoms with concentration ranged between 0.05 and 0.50 at. %. Single crystal films (with thickness about 0.5 μm) were obtained by mechanical splitting of bulk single crystals. Impurity concentration was determined using atomic emission spectroscopy. Spectral dependences of absorption coefficient and photoconductivity were studied in the range 1.50 eV – 3.70 eV. It was experimentally established that the absorption spectra have an additional absorption band and its corresponding energy depends on the nature (Cu or Cd) and concentration of the doping atoms. Also, independently on the presence of the dopant, other two absorption bands in the IR region are present.
ACID-BASE PROPERTIES OF THE SURFACE OF ZINC OXIDE POWDERS SUBJECTED TO MILLING IN THE ATTRITOR
(IOP Publishing Ltd, 2020) Korotcenkov, Ghenadii
Using the method of acid-base indicators adsorption, the influence of mechanical stresses and defects, arising in the milling process, on the surface properties of ZnO powders is analyzed. An explanation of the changes in acid-base properties is given.
ACŢIUNEA FACTORILOR FIZICI ASUPRA PROCESULUI DE GERMINARE ŞI A DINAMICII CREŞTERII CULTURILOR AGRICOLE
(CEP USM, 2013-09-26) Lozovanu, Petru; Moşneaga, Alisa; Livinţi, Petru; Lozovanu, Dan
ADVANCED COMPUTATIONAL METHOD FOR STUDYING MOLECULAR VIBRATIONS AND SPECTRA FOR SYMMETRICAL SYSTEMS WITH MANY DEGREES OF FREEDOM, AND ITS APPLICATION TO FULLERENE
(Springer Verlag, 2017) Bogush, Igor; Ciobu, Victor; Paladi, Florentin
A computational method for studying molecular vibrations and spectra for symmetrical systems with many degrees of freedom was developed. The algorithm allows overcoming difficulties on the automation of calculus related to the symmetry determination of such oscillations in complex systems with many degrees of freedom. One can find symmetrized displacements and, consequently, obtain and classify normal oscillations and their frequencies. The problem is therefore reduced to the determination of eigenvectors by common numerical methods, and the algorithm simplifies the procedure of symmetry determination for normal oscillations. The proposed method was applied to studying molecular vibrations and spectra of the fullerene molecule C60, and the comparison of theoretical results with experimental data is drawn. The computational method can be further extended to other problems of group theory in physics with applications in clusters and nanostructured materials.
AERO-Ga2O3 NANOMATERIAL ELECTROMAGNETICALLY TRANSPARENT FROM MICROWAVES TO TERAHERTZ FOR INTERNET OF THINGS APPLICATIONS
(2020) Braniște, Tudor; Dragoman, Mircea; Jucov, Serghei; Aldrigo, Martino; Ciobanu, Vladimir; Iordănescu, Sergiu; Alîabîeva, Liudmila; Fumagalli, Francesco; Ceccone, Giacomo; Raevschi, Simion; Schűtt, Fabian; Adelung, Rainer; Colpo, Pascal; Gorșunov, Boris; Tighineanu, Ion
In this paper, fabrication of a new material is reported, the so-called Aero-Ga2O3 or Aerogallox, which represents an ultra-porous and ultra-lightweight three-dimensional architecture made from interconnected microtubes of gallium oxide with nanometer thin walls. The material is fabricated using epitaxial growth of an ultrathin layer of gallium nitride on zinc oxide microtetrapods fabricated using epitaxial growth of an ultrathin layer of gallium nitride on zinc oxide microtetrapods followed by decomposition of sacrificial ZnO and oxidation of GaN which according to the results of X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) characterizations, is transformed gradually in β-Ga2O3 with almost stoichiometric composition. The investigations show that the developed ultra-porous Aerogallox exhibits extremely low reflectivity and high transmissivity in an ultrabroadband electromagnetic spectrum ranging from X-band (8–12 GHz) to several terahertz which opens possibilities for quite new applications of gallium oxide, previously not anticipated.
AERO-TiO2 PREPARED ON THE BASIS OF NETWORKS OF ZnO TETRAPODS
(2022) Ciobanu, Vladimir; Ursachi, Veaceslav; Lehmann, Sebastian; Braniște, Tudor; Raevschi, Simion; Zamalai, Victor V.; Monaico, Eduard V.; Colpo, Pascal; Nielsch, Kornelius; Tighineanu, Ion
In this paper, new aeromaterials are proposed on the basis of titania thin films deposited using atomic layer deposition (ALD) on a sacrificial network of ZnO microtetrapods. The technol-ogy consists of two technological steps applied after ALD, namely, thermal treatment at different temperatures and etching of the sacrificial template. Two procedures are applied for etching, one of which is wet etching in a citric acid aqua solution, while the other one is etching in a hydride vapor phase epitaxy (HVPE) system with HCl and hydrogen chemicals. The morphology, composition, and crystal structure of the produced aeromaterials are investigated depending on the temperature of annealing and the sequence of the technological steps. The performed photoluminescence analysis suggests that the developed aeromaterials are potential candidates for photocatalytic applications.
AERO-ZNS ARCHITECTURES WITH DUAL HYDROPHILIC–HYDROPHOBIC PROPERTIES FOR MICROFLUIDIC APPLICATIONS
(2020) Pleșco, Irina; Braniște, Tudor; Wolff, Niklas; Gorceac, Leonid; Duppel, Viola; Cinic, Boris; Mishra, Yogendra Kumar; Sarua, Andrei; Adelung, Rainer; Kienle, Lorenz; Tighineanu, Ion
Here, we report on a new aero-material, called aero-ZnS, representing self-organized architectures made of ZnS hollow micro-tetrapod struc- tures with nanoscale thin walls. The fabrication process is based on the hydride vapor phase epitaxy of CdS on sacrificial micro-tetrapods of ZnO with simultaneous or subsequent transformation of CdS into ZnS and removal of the sacrificial ZnO crystals. The nanostructure of the obtained ZnS hollow micro-tetrapods exhibits the polytypic intergrowth of wurtzite- and sphalerite-type phases perpendicular to their close packed planes. The inner surface of the micro-tetrapod walls preserves oxygen sites, as demonstrated by imaging based on electron energy- loss filtering. The self-organized aero-ZnS architecture proves to be hydrophilic under tension and hydrophobic when compressed against water. Self-propelled liquid marbles assembled using ZnS hollow micro-tetrapod structures are demonstrated.
ALGORITMUL DE OBŢINERE A DEPLASĂRILOR SIMETRIZATE ÎN SISTEMELE SIMETRICE COMPLEXE PRIN REDUCEREA LA PROBLEMA VECTORILOR PROPRII
(CEP USM, 2015) Boguș, Igor; Ciobu, Victor; Paladi, Florentin
În articol este propus un algoritm modificat de obţinere a oscilaţiilor simetrizate în sistemele simetrice complexe. În teoria grupurilor anterior au fost dezvoltate metode de determinare a unor astfel de oscilaţii. Însă, aceste metode sunt dificil de algoritmizat pentru si stemele simetrice complexe care deţin multe grade de libertate. Algoritmul modificat propus este elaborat pentru a fi capabil să găsească deplasările simetrizate în astfel de sisteme.şi prin urmare, de a obţine şi clasifica oscilaţiile normale şi frecvenţele acestora. Metoda respectivă reduce această problemă la problema determinării vectorilor proprii, pentru care deja există metode numerice răspândite pe larg.
ANALIZA ROENTGEN-STRUCTURALĂ A ALIAJELOR METALICE
(CEP USM, 2022) Ghilețchii, Gheorghe
ANALIZA STRUCTURALĂ PRIN DIFRACŢIE DE RAZE X ŞI STUDIUL PROPRIETĂŢILOR OPTICE ALE STRATURILOR SUBŢIRI DE ZnTe
(CEP USM, 2013) Mîrzac, Alexandra
Structural and optical properties of ZnTe thin films fabricated by close space sublimation method with different thicknesses are investigated. It was evidenced a relatively small variation of the lattice parameter with increasing the thickness of the layers, starting from the value of 6.1052 Å (0.83 μm) to 6.1047 Å (2.59 μm). With the varying of the thickness the value of the band gap is changed from 2.237 eV up to 2. 243 eV. The photovoltaic devices ZnSe/ZnTe with efficiency of 1.69 % was obtained.
THE ANALYSIS OF CURRENT FLOW MECHANISM IN CDS/CDTE HETEROJUNCTION
(Elsevier, 2007) Vatavu, Sergiu; Gașin, Petru
An analysis of current–voltage dependencies of CdS/CdTe heterojunction in the 78–370 K temperature range has been carried out. According to this analysis the current flow mechanism is determined by the tunneling processes through dislocations, which penetrate the heterojunction space charge region. The concentration of dislocations has been estimated as 2 · 105 cm− 2. The number of steps necessary for tunneling varies: 2.5 · 102–1.7 · 103. The characteristic energy has a weak temperature dependence (− 0.2 meV/K) and its value vary 120–200 meV. The increase of the annealing duration results in the decrease of the characteristic energy. The multistep tunneling processes through local centres, determined by impurity centres, interface states and defects in the space charge region, predominate at reverse biases. The number of tunneling steps is 1–4 · 102. The concentration of local centres (traps) in the heterojunction have been estimated as 2.37 · 105–1.63 · 106 cm− 3. The thermal annealing in the presence of CdCl2 up to 60 min does not modify the current flow mechanism in CdS/CdTe heterojunctions.
ANALYSIS OF PHASE TRANSITIONS IN THE PRESENCE OF AN INTERMEDIATE STATE. THE MODEL WITH TWO ORDER PARAMETERS
(Institutul de Fizică Aplicată, 2014) Gubceac, Ghennadii; Barsuk, A.A.; Paladi, Florentin
The main propose of this repo1t is to present the analysis of phase transitions in the presence of an intennediate state by using the model with two order parameters. The parametric modelling of phase transitions and analysis of the role of an inte1mediate liquid state in ineversible relaxation processes at low temperatures were perfonned. As an example, one can consider the systems with two stable states LI and C that means liquid and c1ystalline, respectively, and the third one - intennediate fluid state, namely L2 [1]. Such L2 state has been experimentally discovered in supercooled liquids [2]. The model includes two order parameters and three control parameters in the Landau-type kinetic general potential of 6th degree, and has been developed to study the impact of heterogeneity on phase transitions in the presence of an inte1mediate fluid state [3]. We noticed that the presence of the intennediate liquid state may indeed enhance the nucleation rate, and, fmthennore, an increase in the heterogeneity of system accelerates the transition dynamics. In the previous works, we have shown that, depending on the values of its control parameters, the potential has one, two or three possible minima, and the problem dealt with the constrnction of the equilibrilllll phase diagrams. It is also w01th mentioning that the previously obtained results are general and suggest a complete set of different transition scenarios in the entire parameter plane with two control parameters [4]. We study next the stability and bifurcation of equilibrilllll states in the kinetic processes conesponding to the first-order phase transitions in systems which can be characterized by several order parameters. Along with general analysis when the kinetic processes conesponding to phase transitions in systems characterized by r nlllllber of order parameters x1 ,x2 , ••• , X7 and m nlllllber of control parameters ai ,a2 , ... , am are described by a system of ordinaiy differential equations of the form dxr = .f,.(x1 ,x2, ••• ,x7 ;a1 ,a2, ••• ,am), where t is time, the results of specific calculations for dt systems described by the kinetic Landau-type potential with two pai·ameters are also presented. In general, the lai·gest and smallest values of order pai·aineter for the bifurcation analysis conespond to minima of free energy functional F, while the inte1mediate value conesponds to an unstable state (F has a local maximum or saddle point), and these three extrema ai·e identified with the crystalline and two liquid phases [5]. In case of a single - component glass which can be characterized in te1ms of the pressure P and volllllle V, the relation between P and V could be obtained using the equation P(V,T,x,y) = -(oF I oV)r.x.y . Note that P(V,T,x,y,) can be derived from experimental data and this equation may be fuither used to dete1mine the V-dependence of F(V,T,x,y,) Then F can be applied to get the entropy S = -(oF I oT)v.x.y, and in this way specific heats and other the1modynainic quantities for the system can be defined [ 6].
ANALYSIS OF SOLUTIONS FOR THE SYSTEMS OF NONLINEAR EQUATIONS DEPENDING ON PARAMETERS
(CEP USM, 2022-11-10) Paladi, Florentin; Barsuc, Alexandr A.
ANISOTROPY OF THE EXCITON PROCESSES IN GaSe CRYSTALS WITH LOW S AND TE CONCENTRATIONS
(American Scientific Publishers, 2009) Evtodiev, Igor
The anisotropy of the excitonical processes in the GaSe crystals and GaSe with small quantities of GaS(GaSe0.99S0.01) crystals has been studied through the optical specters (SO) and through the photoluminescence (PL) from the perpendicular surface on the symmetry axis C6 (E⊥C polarization) and from the flat surface parallel with the C6axis (E∥C and E⊥C polarization). The edge of the fundamental band of the GaSe crystals as well as of the GaSe0.99S0.01 and GaSe0.99Te0.01 crystals is formed at T = 78 K of the direct excitons' band. The width of the free excitons' band is determined by the processes of interaction between the excitons and optical and acoustic phonons. Phonons with energy of 17 meV and 27 meV participate to the formation of the edge towards small energies of the excitonic band in the GaSe crystals. The average energy of the phonons that participate to the formation of the excitonic absorbtion band in the GaSe crystals with small concentrations of S and Te equals 17 meV. Due to the mechanism of interaction of the excitons and phonons the integral absorption coefficient for the studied crystals (polarized E⊥C) is in small increase once with the temperature whilst the integral absorption coefficient in the maximum of the direct excitons' band. The n = 1 state is in diminution. For example, for the GaSe0.99Te0.01 crystals, α increases from 2700 at T = 78 K to 2025 cm−1 at 220 K. The edge towards small energies of the free excitons' band in the GaSe crystals and GaSe crystals with small quantities of S and Te is in a great concordance with Toyozowa's theory. The constant of interaction between the free excitons with phonons with an average energy of 135 cm−1 equals 0.9. Using the spectral characteristic of the reflection coefficient from the surface parallel to the C6 axis, there has been determined the refraction index placed in the center of the excitons n = 1 which equals 2.62 for GaSe and 2.58 and 2.55 respectively for the GaSe0.99S0.01 and GaSe0.99Te0.01 crystals. The shifting of the reflection specters towards big energies like ∼10 meV in a E⊥C polarization comparing to E∥C is determined by the difference of the oscillators' strength in these polarizations. The PL at T = 78 K specters from the surface parallel with the C6 axis (polarized E∥C) confirm the difference between the forces of the excitons' oscillators in the E∥C and E⊥C polarization. The intensity of PL bands, at the (001) surface as well as at the (100) surface depends on the excitation intensity by a function of a I = Ln force towards the emission bands of the direct and indirect free excitons the force factor is overlinear, and for the impurity nature bands it represents ∼0.5. The parameters that determine the width of the bands of excitonic PL is determined, considering the strong concentration of the structural faults at the (100) surface of the GaSe and GaSe0.99Te0.01 and GaSe0.99S0.01 GaSe crystals. Out of the spectral analysis I(L) the nature of the impurity bands has been determined, and from the PL specter structure there has been determined the energy of the accepting level which equals 93 meV from the maximum of the valence band of the GaSe crystals. Out of the analysis of the PL specter in a E∥C and E⊥C polarization (the (100) surface) it was stated that the process of emissional annihilation of the indirect excitons in the E∥C polarization takes place once with the emission of the phonons of a 38 meV energy whilst at the E⊥C polarization there are emitted phonons with an energy of 17 meV. GaSe with small concentrations of GaS and GaTe leads to the forming of a considerable concentration of localizing centers of the direct excitons and at the same time to the shifting towards small and big energies of the excitonic emission band (state n = 1) comparing to the GaSe crystals with a stoikiometric composition.