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12Journal of Siberian Federal University. Chemistry 3 (2013 6) 230-240
УДК 537.533.35 / 537.533.73
In situ Transmission Electron Microscopy and Electron Diffraction Investigation of Solid-State Reactions and Atomic Ordering in Cu/Au Bilayer Nanofilms
Sergey M. Zharkovabc* , Evgeny T. Moiseenkoab, Roman R. Altuninab and Galina M. Zeerb
aL. V. Kirensky Institute of Physics, Russian Academy of Sciences, 50-38 Akademgorodok, Krasnoyarsk, 660036 Russia
bSiberian Federal University, 79 Svobodny, Krasnoyarsk, 660041 Russia cSiberian State Aerospace University, 31 Krasnoyarsky Rabochy, Krasnoyarsk, 660014 Russia
Received 04.08.2013, received in revised form 18.08.2013, accepted 06.09.2013
Solid-state reaction processes and atomic ordering in Cu/Au bilayer nanofilms (with the atomic ratio Cu:Au~50:50) have been studied in situ by the methods of transmission electron microscopy and electron diffraction in the process of heating from room temperature up to 700 °С at a heating rate of 4-8 °C/min. The solid-state reaction between the nanolayers of copper and gold has been established to begin at 180 °С. The process of atomic ordering has been shown to start simultaneously with the process of the formation of the disordered phase of Cu50Au50 at 245 °С. The formation processes of the ordered phases of: CuAul (L10 superstructure) and CuAull (long period superstructure) have been studied, as well as the phase transition processes: disorder - order (the transition of the disordered structure into the ordered one) and order - disorder (the transition of the ordered structure into the disordered one).
Keywords: Cu/Au nanofilm, Intermetallics, Solid state reaction, Phase transition, Atomic ordering, Superstructure.
1. Introduction
Certain interest to the experimental and theoretical investigations of Cu-Au alloys has been caused by the unique character of this system. Cu-Au alloys demonstrate a wide range of interesting phenomena, such as the formation of a number of ordered structures: Cu3Au, CuAul, CuAu3, the
© Siberian Federal University. All rights reserved Corresponding author E-mail address: zharkov@iph.krasn.ru
*
formation of a long period modulated structure CuAuII, phase transitions of the type order-disorder etc. [1-^5]. The system Cu-Au is classical for the investigation of the behavior of different properties of metallic alloys and checking the suggested theoretical models.
Of special interest are in situ investigations of solid-state reaction processes on the interface between the copper and gold layers, as well as the processes of atomic ordering in Cu-Au alloys. The authors [6] reported the investigation of a solid-state reaction in bilayer thin films of Cu/Au with the gold content of 40^60 at. %. The initiation temperature of the solid-state synthesis process was shown to be 247-267 °C, furthermore, the synthesis initiation temperature did not depend on the thickness of the copper and gold layers. In the work by Malis et al. [7] the detailed ordering kinetics in equiatomic CuAu was studied using in situ time-resolved x-ray scattering, a subtle competition between the modulated CuAu II phase and the simple ordered CuAu I phase was found to occur across the CuAu I/ CuAu II phase boundary. In the work by Bonneaux et al. [8] in-situ temperature observations were performed by transmission electron microscopy on a stoichiometric AuCu alloy. This alloy, at heating, undergoes a series of transitions: AuCu I (L10)^AuCu II (long-period) ^AuCu (disordered). However, in spite of numerous experimental and theoretical investigations of Cu-Au alloys the processes of solid-state reaction and atomic ordering in nanosystems still remain not completely understood. Also, the origin of the modulated phase is still under debate [9] and most theoretical models encounter difficulties in predicting the correct CuAuII stability in a narrow temperature range [7].
It is well known that in situ transmission electron microscopy (TEM) allows a direct observation of the dynamic process through imaging and selected area electron diffraction (SAED) is an ideal approach for the investigation of microstructure evolution. In this work we report the results of the in situ TEM and SAED investigation of the processes of solid-state reactions followed by atomic ordering and order-disorder transition in Cu/Au bilayer nanofilms with the atomic proportion Cu:Au«50:50.
2. Experimental details
Cu/Au bilayer nanofilms were obtained by the method of electron beam deposition in high vacuum with the help of a high vacuum installation MED-020 (Bal-Tec). The base vacuum was 5*10-5 Pa. The films were obtained by the successive deposition of gold and copper layers onto a substrate [10]. For the evaporation the materials of a high level of purity were used: copper (ADVENT, 99.99 %); gold (ADVENT, 99.99 %) [11]. A fresh cleaved single crystal of NaCl was used as a substrate. The temperature of the substrate during the deposition was equal to room temperature. The deposition rate and the film thickness during the deposition process were controlled with the help of a quartz resonator. The deposition rate was 0.2-0.3 nm/s. The structure and the local element composition of the obtained samples were studied with a transmission electron microscope JEM-2100 (JEOL), equipped with an energy-dispersive spectrometer Oxford Inca x-sight, at the accelerating voltage of 200 kV, also the research was made using a scanning electron microscope JSM-7001F (JEOL), equipped with an energy-dispersive spectrometer Oxford Inca PentaFetx3 and a wave-dispersive spectrometer Oxford Inca Wave 500. The given electron microscopy investigations were carried out by the authors of this work in the Laboratory of electron microscopy of the Siberian Federal University [12, 13]. The films under study were Cu/Au films with the individual layer thickness of: Cu=20±2 nm; Au=28±3 nm. The thickness of the gold and copper layers was selected in such a way that the ratio between these elements amounted to «50:50 at. %. The local element composition of the obtained films was controlled by the
method of energy dispersion spectroscopy. In the thin film samples under study the deviation from the atomic ratio Cu:Au=50:50 did not exceed ±2 at. %.
The obtained samples were in situ investigated by the method of transmission electron microscopy and electron diffraction with the aim of obtaining the phase composition change in the Cu/Au films in the process of thermal heating. The heating of the film samples was carried out directly in the column of the transmission electron microscope JEM-2100 (the vacuum was 1*10-6 Pa) with the help of a special sample holder (Gatan Model 652 Double Tilt Heating Holder), which allows controlled sample heating from room temperature up to as high as +1000 °C. Cu/Au bilayer nanofilms were separated from the substrate in room temperature distilled water, then the films were put on a molybdenum TEM grid and heated up ho 700 °C. The heating mate was from 4 to 8 °C/min. Also, the sample annealing was carried (tut; at a fixed temperature. Smultaneousty witf hhe heating SAED patterns waae registered and synchronous sample temperature measurements were carried out. The registration o0 the SAED patterns was pe rforme d at a rate of 4 hramts per mtnute, thus the heating rate of (4 °C/min) 1 frame corresponded ro thee sample temperatu/e change of 1 °C. The iMterpretation of the electron diffraction patterns was made using the software D igitalMicrogaaph tGatan), and crystal stauctute databases: ICDD PDF 4+ [14], Pearson's Crystfl Data [t5f
3. Resu lts and discussion
In the initial atate Cu/Au Mayea nanofilmr consisted of crystallites with the size of «10-20 nm (Fig. 1a). The diffraction reflections in the electron diffraction pattern (Fig. 1b), obtained by the method of selected area electron diffraction from the area with a diameter of «1 ^m, have a polycrystalline view. The interpretation of the diUfraction reflections showed the ptesence of 2 phases with face-centered fabic (FCC) lattice,: Cu (tire space group Fm-3mf the ktticn parameter a=3.62 A) [r6], and, Au (the space group Fm-3m, the lattice parameter a=4:0[ A) [a7].
The in sin elecaton diffraction investigations of the phase composition changa of the Cu/Au films of in the paocess of thermal heating caaried out at a rcte of 4-8 °C/min, showed that at 180 °C the first
(a) (b)
Fig. 1. TEM image (a) and SAED pattern (b), of a Cu/Au film in the initial state
Cu (200) Au(UL)
An 1220)
(a) (b)
Fig. 2. SAED patterns of a Cu-Au film at 225 °C (a) and 285 °C (b
features of a solid-st te r actio were marked in the SAED pattern. The dif raction ing blurring began which indicated the b eginning of the reaction at the interface of the Cu and Au nanolayers and upon reaching the temperature of 22t °C (Fig. 2at tire diffraction reflections wert blurred, namely the ones which were closer to the center of the SAED pattern: Au(111), Au(200), Cu(1lf), Cu(200). The maximum reflection burring in the SAED pattern was observedupon reaching the tempefature of 240 °C. It ts woeth nreting that at 240°C not all thee diftraction reflertioni teJr^]tii:;i>]l:ia.ss€;s Cu [16] Au [17] were blurred. So, the diffractio n reflertion of Au(220) was b lurred only upon reaching the tempe rature of 285 °C (Frg. 2b). At present there is no explanation of the observed phenomenon.
At the sample heating up to 245 °C one observed the appearance of the diffraction reflections corresponding to a new phase, FCC lattice (the space group Fm3m), with the lattice parameter a=3.87±0.02 A, which corresponds to the disordered phase Cu50Au50 [18]. In the SAED pattern, along with the reflections corresponding to the FCC lattice with the parameter a=3.87±0.02 A one observed a weak superstructure reflection (the relative reflection intensity being Irel.«5 %), corresponding to the atomic interplanar spacing of d=2.8±0.1 A. Upon reaching 280 °C in the SAED pattern it was possible to see one more weak superstructure reflection, corresponding to d=3.7±0.1 A. In the process of further heating up to as high as 350 °C (Fig. 3a), the intensity increase of the superstructure reflections (d=2.8 A, 3.7 A) was observed with respect to the intensity of the FCC lattice reflections.
The atom ordering in the alloys which have the composition close to that of Cu50Au50 is known to lead to the formation of CuAuI phase [19], which is an ordered L10 superstructure. The interpretation of the diffraction reflections (Fig. 3a) showed that the reflections corresponding to the interplanar spacings 2.8±0.1 A and 3.7±0.1 A, could be identified as the ones corresponding to the interplanar spacings of the ordered CuAuI phase: dn0=2.804 A and d00i=3.670 A [20]. Given in Fig. 3b is a scheme of the atom location in the ordered structure CuAuI. It is worth noting that there exist two descriptions of the unit cell of the phase CuAuI (Fig. 3b): a classical description, with the space group being P4/ mmm and the following lattice parameters: a=b=3.966 A, c=3.670 A [20]; a modern description, with the space group being P4/mmm, and the lattice parameters, such as a=b=2.806 A, c=3.67 A [21] in
(a) (b)
Fig. 3. SAED pattern of the Cu-Au film at 350 °C (a) and the atom location model in the ordered superstructure CuAuI (b). The black circles indicate the eopper atoms, the hollow ones are uaed to indicate the gold atoms. The solid lines show two unit cells of the phase CuAuI in the classical description [220], the dnsh-anC-dot lines show one unit cell of CuAuI in the modsrn description [21]
Fig. 3b this cell is shown byfhe dash-and-dot line. In the present work the classical description [20] of the unit cell of CuAud is used to identify the reflections of the phase CuAut. In the SAED pattern (Fig. 3a) one observes a compleSe set of reflections coaeesponding to the (CuAuI phase, besides the reflection intensity correrponds to the tabular vatues. In the process of further heating up to 39)0 °C, no phase compositiog changes occurred.
Upon reaching the temperature of 390 °C this phase tranaition otder-disorder began, i.e. the ordered CuAuI phase; began to ttansfer into the disordered phase Cu50Au50. Beginning from 390 °C the SAED pattern showed the intensity decrease ot the diffraction reflection of CuAuI d0Si=3.70 A, and at 396 °C thera appeared the inteneity decrease of the reflection of CuAuI d110=2.81 A. At 420 °C all the superstructure reflections completely disappeared and in the electron diffraction paetern (Fig. 4a) only the diffraction reflections characteristic aethe disordered phare Cu50Au50 are present [18]. Consequently, at 420 °C, in the film undet study, the ordered CuAul pliase completely transferred into the disordered phase Cu50Au50. However, it is worth noting that the diffraction reflection intensities in the SAED patterns of the Cu-Au films upon heating up to 420 °C and higher (one carried out experiments with the sample heating up to as high as 700 °C) showed the "abnormal" intensity distribution, which is not characteristic of the FCC lattice. Thus, the reflection corresponding to d111=2.24 A had an extraordinarily weak intensity, this reflection should have the relative intensity Irel=100 %. In this connection, a relatively weak reflection d220= 1.36 A (I rel. 22 %) [18], had the intensity Irel=100 % in the SAED pattern (Fig. 4a). When the sample was rapidly cooled (the cooling rate was ~300 °C/min) down to the temperature which was almost equal to room temperature, the ratio of the reflection intensities in the SAED pattern did not qualitatively change. In the SAED pattern (Fig. 4b), obtained when the sample was tilted by 20° with the help of a goniometer, the intensities of the main reflections become characteristic of the standard FCC structure. Based on this fact it is possible to make a conclusion about the preferred crystallite orientation at the formation of the disordered phase Cu50Au50 in the process of
(a) (b)
Fig. 4. Saved patterns of the Cu-Au films at 420 °C: without the sample tilt (a),with the sample tilt by 20° (b)
heating the Cu-Au films up tea 420 °C find higher. The crystallites Cu50Au50 were formed in such a way that the {220} atomic planes wereoriented normally to the film plane.
According to the phase diagram of the rystem Cu-Ah (Fig. 5) the stability area of the ordered CuAuI phast is spreae from room temperature up to as high as 3885 °C, in rhs tempebature range trom 3585 °C to 410 °C thore is an area of stability of tho ordered structure CuAulI, anO, above 410 °C 0he disordered phase Cu50Au50 with the FCC laetice is stable [19]. In this connection it becomes clear why at 39(0 °C the destruction of the phase CuAuI began eeh at 420 °C )he phase transition into the seructurallytdrsordered FCC lattice occurred. Howeve r, in tlais work, in rae procrss of heating in the
33.72 A
dWWWvVvV
o
0—o—o—o—c>
19.B6 A
O
O
O
19.86
Anti-phaSe
domain boundary
An Li-phase domain boundary
Anti-phase domain boundary
Fig. 6. Model oftheatom location in the ordered rupersfeucOupe CuAuII [24]. Ahh^tblacilr;circl^h indicate the copper atoms, the hollow ones show the gold atoms
temperature range of 385-410 °C no formation of the CuAuII phase was observed. It is quite clear that it occurred due to the lack of time for the formation of the phase CuAuII. At a heating rate of 4-8 °C/ min, the sample heating from 385 to 410 °C goes within only 3-6 min. According to paper [22] for the formation of the phase CuAuII, heating for, at least, 30-45 min at 390 °C is necessary.
The phase CuAuII, is a long period ordered structure, the space group being Imcm, the lattice parameters are: a=3.96 A, b=39.72 A, c=3.68 A [23]. Fig. 6 shows the scheme of the atom location in the ordered structure CuAuII. The unit cell of the structure CuAuII (see Fig. 6) can be presented as a sequence of ten tetragonal CuAuI cells [20] along the direction b, moreover, within one half of the unit cell all the planes (001) are filled with gold atoms and in the other half- with copper atoms. Thus, an anti-phase boundary is formed on a half of the long unit cell translation [24].
For the purpose of the formation of the ordered structure CuAuII from the structurally disordered state a Cu/Au bilayer nanofilm was heated to 500 °C, and then, successive annealing in the temperature range from 420 to 370 °C with a step of 10 °C was carried out followed by cooling down to a temperature which was close to room temperature at a rate of ~300 °C/min. The annealing time at each fixed temperature was 20 min. During the annealing process continuous registration of SAED patterns was carried out at a rate of 4 frames per min.
After the annealing at 400 °C first changes of the SAED patterns were observed - there appeared slightly broadened low intensity reflections which were superstructural for the disordered phase Cu50Au50. The appearance of these reflections indicates the beginning of the formation of an ordered structure, i.e. the beginning of the phase transition disorder-order. After the annealing at 380 °C the intensity of the superstructural reflections in the SAED patterns increased and they became more distinct. After the annealing at 370 °C, as well as at the sample cooling to room temperature no distinct changes of the diffraction reflections occurred in the SAED patterns.
Fig. 7a demonstrates a TEM image of a Cu-Au film after a series of annealings in the temperature range of 420-370 °C. One observed the increase of the size of Cu-Au crystallites from 10-20 nm in the initial state to 70-100 nm after this series of annealing procedures. The interpretation of the reflections in the SAED pattern (Fig. 7b), obtained after a series of annealings allows one to make a conclusion that both the phase CuAuII, and the phase CuAuI are present in the film. The phase composition interpretation is complicated by the fact that most of the atomic interplanar spacings of the ordered phases CuAuI and CuAuII practically coincide. Consequently, the major part of rather
intensive reflections will overlap in the electron diffraction pattern. The reflection corresponding to big interplanar spacings which are characteristic of the long period ordered phase CuAull, and are not present in the structure CuAuI, such as: d020=19.86 A; d040 = 9.93 A; d060= 6.62 A; d080=4.965 A; but they have the relative intensity of the order or considerably lower than 1 %, thus they are not observed in the SAED pattern (Fig. 7b).
It is worth noting that since the unit cell of CuAuII consists of repeated unit cells of CuAuI (Fig. 6), then, in the case of any structure defects occurring in the process of the formation of the phase CuAuII, the reflections characteristic of the phase CuAuI will be observed in an electron diffraction pattern. The most intensive reflection which allows one to determine the presence of the phase CuAuI in the sample is the reflection d110=2.800 A with the relative intensity of 24 % [20], and the reflection
(a)
(b)
(c) (d)
Fig. 7. TEM images (a, c), obtained at different magnifications; and the corresponding SAED patterns (b, d), of the Cu-Au film after a series of annealings in the temperature range of4220-370 °C (the sample temperature during the imaging was equal to room temperature)
with a closer interplanar spacing in the CuAuII phase - d091=2.824 A has the relative intensity of only 0.1 % [23].
Fig. 7c shows a TEM image of a single crystallite, in the image the structure characteristic of the phase CuAuII [25] is observed. The period of the location of the domain walls is «20 A, which coincides with the period shown in Fig. 6. The interpretation of the reflection spots observed in the SAED pattern (Fig. 7d) of a single crystallite unambiguously confirms that this crystallite has the long period ordered structure CuAuII. In the SAED pattern one observes the reflections corresponding to d020=19.86 A [23], as well a satellite reflections which is characteristic of the electron diffraction patterns observed from the phase CuAuII [24, 25].
4. Conclusion
Cu/Au bilayer nanofilms were obtained by the method of electron beam deposition in high vacuum. The thickness of the individual layers was: Cu=20±2 nm; Au=28±3 nm (the atomic ratio Cu:Au«50:50 at. %). In situ transmission electron microscopy and electron diffraction investigations of the processes of solid-state reactions and atomic ordering in Cu/Au bilayer nanofilms were carried out. The processes were initiated by controlled sample heating in the column of a transmission electron microscope. The samples were heated from room temperature up to 700 °C. The solid-state reaction between the copper and gold nanolayers was established to begin at 180 °C. The temperature of the reaction beginning did not depend on the sample heating rate (the heating rate was from 4 to 8 °C/min). The process of the atomic ordering was shown to begin simultaneously with the formation of the disordered Cu50Au50 FCC structure at 245 °C. The investigations of various phase transitions were carried out, such as order-disorder and disorder-order. The transition of the ordered CuAuI phase (L10 superstructure) into the disordered phase Cu50Au50 was observed during the sample heating, the transition process began at 390 °C and finished at 420 °C. During the formation of the disordered phase Cu50Au50 in the process of heating of the Cu-Au films up to 420 °C and higher the crystallites of Cu50Au50 were shown to have the preferred orientation. The crystallites Cu50Au50 were formed in such a way that the {220} atomic planes were oriented normally to the film plane. The transition of the disordered Cu50Au50 phase into the ordered phases CuAuII and CuAuI was observed in the processes of several annealings carried out at the temperature decrease in the range from 420 to 370 °C. At 400 °C the process of the ordered structure formation began and after annealing at 380 °C crystallites of the ordered CuAuII and CuAuI phases were formed in the film.
Acknowledgments
The work was supported by the Presidium of RAS (project No. 24.34) and the Ministry of Education and Science of the Russian Federation (contracts ## 16.740.11.0470, 14.B37.21.0832, 14.B37.21.1646).
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In «¿«-исследования твердофазных реакций и атомного упорядочения в двухслойных нанопленках Cu/Au методами просвечивающей электронной микроскопии и дифракции электронов
С.М. Жарков3' б' в, Е.Т. Моисеенкоа' б, Р.Р. Алтунина' б, Г.М. Зеерб
а Институт физики им. Л.В. Киренского СО РАН, Россия 660036, Красноярск, Академгородок, 50-38 б Сибирский федеральный университет, Россия 660041, Красноярск, пр. Свободный, 79 в Сибирский государственный аэрокосмический университет, Россия 660014, Красноярск, пр. газ. Красноярский рабочий, 31
Методами просвечивающей электронной микроскопии и дифракции электронов проведены т &1^-исследования процессов твердофазных реакций и атомного упорядочения в двухслойных нанопленках Си/Аи (с атомным соотношением Си:Аи~50:50) при нагреве от комнатной температуры до 700 °С при скорости нагрева 4-8 °С/мин. Установлено, что твердофазная реакция между нанослоями меди и золота начинается при 180 °С. Показано, что процесс атомного упорядочения начинается одновременно с процессом формирования неупорядоченной фазы Си50Аи50 при 245 °С. Исследованы процессы формирования атомно-упорядоченных фаз: СиАи1 ^10 сверхструктура) и СиАи11 (длинно-периодическая сверхструктура), а также процессы фазовых переходов беспорядок-порядок (переход неупорядоченной структуры в атомно-упорядоченную) и порядок-беспорядок (переход атомно-упорядоченной структуры в неупорядоченную).
Ключевые слова: нанопленка Си/Аи, интерметаллиды, твердофазная реакция, фазовый переход, атомное упорядочение, сверхструктура.
