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11Journal of Siberian Federal University. Engineering & Technologies 1 (2013 6) 50-55
УДК 517.55
The Ability of Quantum Dots Formation in Thin Nanostructured Amorphous Films
Viktor S. Zhigalova*, Ludmila I. Kveglisb Riza B. Abylkalykovac and Madina S. Rakhimovac
a Kirensky Institute of Physics SB RAS, 50 Akademgorodok, Krasnoyarsk, 660036 Russia b Siberian Federal University, 79 Svobodny, Krasnoyarsk, 660041 Russia c East Kazakhstan State University, 30 Gvardeyskoy Divizii, Ust-Kamenogorsk, 070020 Kazakhstan
Received 11.02.2013, received in revised form 18.02.2013, accepted 25.02.2013
In the last years an interest in field of quantum dots devices creating has been increased. In this work the nanocrystallite with Frank-Kasper structure was examined as the quantum dot in amorphous film. An ability to create all-inorganic Quantum Dots Light Emission Device may be considered for Tb30Fe70, Co80C20, Fe86Mn13C and Co50Pd50 films.
The self-organisation of atomic structure in Tb30Fe70, Co80C20, Fe86Mn13C and Co50Pd50 films, which possess large values of perpendicular magnetic anisotropy (PMA) constant (K± ~ 106 erg/cm3), were investigated by methods of electron diffraction and transmission electron microscopy, including the method of bend contours. The crystallization of the films proceeds in an explosive way forming different dissipative structures from initial nanocrystalline state. In previous works [2, 3] it was shown that after crystallization (Tann~ 260-330 °C) the atomic structures of Tb30Fe70, Co80C20, Fe86Mn13C and Co50Pd50 films are tetrahedrally close-packed Frank-Kasper structures. In this work the structural model of thin film at mesoscale and its correlation with magnetic and optical properties is proposed.
Keywords: dissipative structures, explosive crystallization, bend contours, quantum dots.
Introduction
The creation of new materials with unique properties is a conclusive element of new technology development. The structures arising during self-organization process are known as "dissipative structures". The term "dissipative structures" was introduced by I. Prigogine [4]. These structures appear in nonlinear open systems spontaneously. The corresponding processes are the result of the considerable remoteness from equilibrium conditions. Strong internal bends of crystalline lattice in thin solid films and solid bulk materials with high concentration of elastic stresses were observed in [5, 6]. Due to the temperature gradients and pressure the hydrodynamic and thermal effects during competing create the complex structures including periodic, quasiperiodic and chaotic structures.
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* Corresponding author E-mail address: [email protected]
1. Methods and Results
We investigated the structure and magnetic properties of Tb30Fe70, Co8oC2o, FeS6Mn13C and Co50Pd50 nanocrystalline films with strong PMA [2, 7]. The films were examined in the initial state and after annealing at vacuum. The film samples were prepared by thermal explosive evaporation at vacuum (residual pressure about 10-5 Torr) and magnetron sputtering at vacuum (residual pressure about 10-6 Torr) onto different substrates (glass, crystalline, amorphous silicon, fused silica, NaCl, CaF2, MgO, and LiF). The microstructure and phase composition of the films were analysed using PREM-200 and JEM-100C transmission electron microscopes. The chemical composition of the films was checked by X-ray fluorescent analysis. PMA constant K was determined by the torsion magnetometer at room temperature in magnetic fields with strengths up to 17 kOe.
In this article we bring results gained by using electron diffraction and transmission electron microscopy methods [2, 7]. Examples of rotation effects are shown on Fig. 1. The self-organisation of atomic structure in Co50Pd50 and Cos0C20 films, possessing large values of the PMA constant (K ~ 106 erg/cm3).
On electron microscopy image of Co-Pd film (Fig. 1(a)) explosive crystallization propagated in periodic turbulent regime is observed. The presence of rate gradients during the movement of nanoparticle groups relative to each other provided aligning action on every nanoparticle. The anisotropic distribution of particles relative to their orientation in volume is established under the action of simultaneous effect of orientating flow forces and disorientating movement [8]. As a result, the phenomenon of explosive crystallization with the formation of pseudomonocrystals, both periodic and quasiperiodic, was observed.
The crystallization of films occured by an explosive way is forming the large variety of structures from initial nanocrystalline state to dendrit, fractal, lens (with hyperbolic and elliptic ombilica morphology), Hele-Shaw cells, shock-wave fingers, Teilor vortex. The scale invariance took place in these formations. On Fig. 2 it is shown that after crystallization (Tmn~ 260-330 °C) the atomic structures of FeS6Mn13C and Co50Pd50 films are forming tetrahedrally close-packed Frank-
(a) (b)
Fig. 1. (a) The electron microscopy image of a Co-Pd film with bend contours. Periodic turbulent regime was formed under the action of electron beam of transmisiion electron microscope; (b) Briglrt-field TEM image of a Co80C-0 film with individral quantum dol in amorphous matrix with bend-contoucs aroused at explosive crystallization under the action of electron beam of transmission election microscope during investigation
nm
(it)
Fig. 2. (a) Bright-field TEM image showing the dark quantum dots on the surface of Co-Pd film after interaction with an erection beam and the picture of electron microdiOfractinn; (b) Bright-field TEM imcge showing the dark quantum dots on the suifaco of Fe-Mn)C film adter interaction with fn election beam dnd the picture of electron microdiffraction; formation of bend-contours and monocrystad inclusions are cisible
Kasper sltuc(ures [7]. Such n kind or atomic structures te usuaf for Tb-Fe alloys riut is unknawn foi1 Co-I'd clloys.
Thr co^re^]laLt:ioir ctf nano))ry ^^atl^ine fil m strocititfr w ith their magnetic characteristics in the process of transition from tht disoodeo to their regular rtructuae is researched. The othef problom conrinrned in thir work io a docroase oft saturation magnotization in ndnocrysralline films of Cd50Pn5) find Fe86Mn13C with tetrahedral close-packed Frank-Kaspef rtrucOutos.
The dependence ol1 safurotion magnetisation (Fig. 3) at anoealing temperature showt decreasing of temperaiure over the range 300O io 400°C. Probabte reason o11 saturation decrease is significant magnetostriction. The behavior of temperature coefficient of electrical resistivity indicates structural reconstruction. The optical and structural properties of these nanoobjects (quantum dots - QD) essentially differ- from the properties of initial material states (See Fig. 1 (b)). One of the directions using such properties of nanocrystalline materials is creatirn of displays of new generation on quantum dots (QD).
Baae d (en tfe bend-contours analy sis u sitg tran smission- electron micrographs the evaluations of internal tensions in the investigated films were made. This bend-contours analysis indicated also considerable effective bending of crystalline lattice (~70°/mkm). It is shown that the peeulfarities oe rhe atomic structure of the fiems sfould caute large mrgnetostricfion effects. It is supposed fhas tire; formation of large; values of PMA tonstant is defined ley fhe magnetostriction anisotropy.
The torsion angle 0 of crystal plane dhkl was determined using distances between bend contours W on electron mic rosc ope i mages[f]:
tltie electron wavelength 1 was equal to 0.037 A, the radius of curvature r was determined by equation
(1)
from [5]
sjj W a» t. HQ 0 = ™ 3D SO 11» e - *- Ml A • * - / ff A y \t > ysk-J -8.4 Z.4 J.J 2.0 -US ~ 1.6 1.4
o <4 W » W 9U № w a
b
21 — Fe-Mn_ 100mg _nemagn
20 19 taW aW«( it/c/r 1 1 pp™ 1 Mj*ui "»fl W liWI*1
18 a , 17 t 16 ^fi r^ h^'c rOef
15 14
13 900 1000 1100 Wa 1200 vteleng tC, nm 130 1400 1500
c
Fig. 3. (a) The dependence of (Ms) - saturation magnetisation and (R/Rh - relative electrical resistivity at annealing temperature (T^) in nanocrysnalline films Co50Pd50; (b) Spectrr of optical absorption and transmission from nanocrystalline film of Co-Pd. At waveletgth about 1300 nanometers aistinct peaks with excitoa nature can be observed; fc) The spectrum of absoeption from thin film Fe86Mn13C contains distinct peak on wave length of 1320 nanometers
X t
The toasion angle and curaature eadius oW crystal could be contributed to the equation:
o 360-dt/ . / e ht
0 e-OS-. arccia(-) (3)
n-r-/ 1-il
Accordiag So /8], rofation instability of atomic lattice is passibla ai ehift defoemation. In the case when we have one variable vaiue parametee and two directing, we would gec aesembling catastrophe. Ia ihe case of two caeiaWle valuo paeameCe3s andthree dioectino, tha ccreriilicr catastrophes are observed, tliat is ro-called pseudomoaocwstal sluape omWClics. Iirtlie case of others combiaations of parameters tho shapes oC tote, heitx, fractal are realized. All mentioned above shapes were observedin our experimenfi.
Analyais of the bend extinction coetouss in the films was perforated according to the procedure described in [5]. For presented studied films, the elastic stress is estimated at ~1011N/m2. In the case
when the stress in the film does not exceed the elastic limit, PMA constant is approximately equal to ~107 erg/cm3. However, dark-field electron microscopic research of these films revealed a plastic flow, which indicates a rotational effect, i.e. rotation of film regions with 1 ^m size about. Consequently, stresses arising during the formation of the crystal structure substantially exceed the elastic limit and can make a considerable contribution to PMA due to magnetostriction effects. The possibility of elastically-magnetic recording of information was shown in [9] and it can be the example of Fe-Tb film. The recording was possible because the temperature gradient during recording by laser beam caused the pressure gradient in the film. The authors [9] didn't take into consideration changing of viscosity, assuming that heating is negligible. However, it was shown in [10] that shear components of tension provided strong influence on the distribution of uniaxial anisotropy along a thin magnetic film. The intensification of magnetostriction effects by the substrate bending during evaporation of film was reported in [10]. The bending was not more than 5°/^m. With the help of bend contour analysis it was shown that in our experiment the inner bend in film material could be 10 times more. The magnetostriction constant of Fe-Tb is equal to 2i0-3, that is 1000 times more than in [11]. Therefore, the effects related to varying of anisotropy field depending on rotation shifts would be much higher in our films.
2. Conclusions
In this work the nanocrystalline films with Frank-Kasper structure were examined as the quantum dots. An ability to create all-inorganic Quantum Dots may be considered for Co80C20, Tb30Fe70, Fe86Mn13C and Co50Pd50 films. Advantages of all-inorganic Quantum Dots representing in [12].
In our opinion, there are sufficient reasons to conclude that the features of explosive crystallization processes in metal films with a nonequilibrium structure can be described in the framework of the modern STZ theory based on the excited-atom model [11].
The uniaxial magnetic anisotropy in film materials having large constant of magnetostriction is able to change its sign and value due to inner stress gradients created by dissipative structures. The data summarized in this paper have demonstrated that in nanocrystalline films of metal alloys exists possibility of the quantum dots creation in the form of nanocrystallites with Frank-Kasper structure. The appearance of this quantum dots one can connecting with spectrum of absorption, which shows visible distinct peaks at wavelength in the area of 1300 nanometers, apparently, connecting with exciton optical transition.
The work was supported by INTAS (Grant No. 00-100) and RFBR (No. 03-02-16052u). The authors thank the laboratory «IRGETAS» of the East Kazakhstan Institute of Technology for his help.
References
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Возможность формирования квантовых точек в тонких наноструктурированных аморфных пленках
В.С. Жигалова, Л.И. Квеглисб, Р.Б. Абылкалыковав, М.С. Рахимовав
а Институт физики им. Л.В. Киренского СО РАН Россия 660036, Красноярск, Академгородок 50 б Сибирский федеральный университет Россия 660041, Красноярск, пр. Свободный, 79 в Восточно-Казахстанский государственный университет Казахстан 070020, Усть-Каменогорск, ул. Гвардейской дивизии, 30
В последние годы большой интерес привлекают исследования, связанные с устройствами, работающими на квантовых точках. В данной статье нанокристаллиты со структурами Франка-Каспера исследованы как квантовые точки в аморфных пленках. Возможность создания эмиссионных устройств на полностью неорганических квантовых точках может быть рассмотрена для Co80C20, Tb30Fe70, Fe86Mn13C и Co50Pd50 пленок.
Самоорганизация атомной структуры Co80C20, Tb30Fe70, Fe86Mn13Cи Co50Pd50в пленках, которые обладают высокими значениями константы, перпендикулярной магнитной анизотропии (ПМА) K± ~ 106 эрг/см3, исследованы методами электронной дифракции и просвечивающей электронной микроскопии, включая метод изгибных контуров. Процессы взрывной кристаллизации аморфных пленок формируют различные диссипативные структуры из нанокристаллических зародышей. В предыдущих работах [2, 3] было показано, что после кристаллизации (Тотжига ~ 260-330 °C) атомная структура Tb30Fe70, Co80C20, Fe86Mn13C и Co50Pd50 была определена как тетраэдрически плотно упакованная структура Франка-Каспера. В этих работах структурные модели тонких пленок, созданные для микро- и мезомасштабов связываются с магнитными и оптическими свойствами пленок.
Ключевые слова: диссипативные структуры, взрывная кристаллизация, изгибные контуры, квантовые точки.
