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MOCVD of Cobalt Oxide Using Co-Actylacetonate As Precursor: Thin Film Deposition and Study of Physical Properties

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Sangeeta Jogade
Sangeeta Jogade
Preeti Sunil Joshi at Walchand Institute of Technology
Preeti Sunil Joshi
Bashir Jamadar at Walchand College of Engineering, Sangli
Bashir Jamadar
D S Sutrave
D S Sutrave
D S Sutrave
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Metal Organic Chemical Vapor Deposition (MOCVD) is the deposition method of choice for achieving conformal uniform (composition and thickness) continuous thin films over the micron geometry topology necessary for implementing advanced devices. Thin films of cobalt oxide were prepared by MOCVD technique on alumina substrate using a cobalt acetylacetonate as precursor. The thin films of cobalt oxide were deposited on alumina substrate by MOCVD at four different temperatures viz 490 C, 515 C, 535 C, 565 C. The as deposited samples are uniform and well adherent to the substrate. Thickness of the cobalt oxide film is maximum at temperature 535 C. The crystalline and phase composition of films were examined by X-ray diffraction. The XRD reveals the crystalline nature with cubic in structure for a l l t h e s a m p l e s . T h e s u r f a c e m o r p h o l o g y o f t h e f i l m s w e r e s t u d i e d b y s c a n n i n g electron microscopy. The SEM image shows well defined closely packed grains for all the samples. The hexagonal shape of grains are observed for sample at temperature 515 C. Raman spectroscopy shows Fm3m, 225 space groups for cobalt oxide thin films deposited on alumina substrate.
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MOCVD of Cobalt Oxide Using Co-Actylacetonate As Precursor: Thin Film Deposition and Study of Physical Properti
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J. Nano- Electron. Phys.
3 (2011) No1, P.203-211 2011 SumDU
(Sumy State University)
203
PACS numbers: 47.70. – n, 68.49.Uv
MOCVD OF COBALT OXIDE USING CO-ACTYLACETONATE AS
PRECURSOR: THIN FILM DEPOSITION AND STUDY OF PHYSICAL
PROPERTIES
S.M. Jogade, P.S. Joshi, B.N. Jamadar, D.S. Sutrave
O D.B.F. Dayanand College of Arts and Science,
Solapur, 413003, Maharashtra, India
E-mail: sutravedattatray@gmail.com
Metal Organic Chemical Vapor Deposition (MOCVD) is the deposition method of
choice for achieving conformal uniform (composition and thickness) continuous thin
films over the micron geometry topology necessary for implementing advanced
devices. Thin films of cobalt oxide were prepared by MOCVD technique on alumina
substrate using a cobalt acetylacetonate as precursor. The thin films of cobalt oxide
were deposited on alumina substrate by MOCVD at four different temperatures viz
490 C, 515 C, 535 C, 565 C. The as deposited samples are uniform and well
adherent to the substrate. Thickness of the cobalt oxide film is maximum at
temperature 535 C. The crystalline and phase composition of films were examined by
X-ray diffraction. The XRD reveals the crystalline nature with cubic in structure for
all the samples. The surface morphology of the films were studied by scanning
electron microscopy. The SEM image shows well defined closely packed grains for all
the samples. The hexagonal shape of grains are observed for sample at temperature
515 C. Raman spectroscopy shows Fm3m, 225 space groups for cobalt oxide thin
films deposited on alumina substrate.
Keywords: MOCVD, THIN FILM, XRD, SEM IMAGES, RAMAN SPECTRA.
(Received 04 February 2011, in final form 04 February 2011)
1. INTRODUCTION
In response to the changing global landscape, energy has become a primary
focus of the major world powers and scientific community. There has been
great interest in developing and refining more efficient energy storage devices.
One such device, the supercapacitor, has matured significantly over the last
decade and emerged with the potential to facilitate major advances in energy
storage. Supercapacitors, also known as ultracapacitors or electrochemical
capacitors, utilize high surface area electrode materials and thin electrolytic
dielectrics to achieve capacitances several orders of magnitude larger than
conventional capacitors [1]. Electrodes for such supercapacitors are mainly
made using conductive polymers or metal oxides. The metal oxide based
supercapacitors have attracted increasingly more attention due to their high
specific capacitance, long operation time and high output.
Metal oxides have many interesting properties that result in various
important applications [2]. Transition metal oxides (TMO), a sub group of metal
oxide are those oxides in which the cation has incompletely filled d or f
shells [3, 4]. Tremendous efforts have been devoted in recent years to study
these metal oxides as anomalous behavior observed in these materials.
204 S.M. JOGADE, P.S. JOSHI, B.N. JAMADAR, D.S. SUTRAVE
Consequently it has become increasingly important to understand them in terms
of their magnetic, electrical and optical properties. Some of the applications of
the transition metal oxides (CaO, NiO, CuO) which have generated lots of
interest among the research groups all over the world include superconductivity
in electronics, electrocromism in smart windows and electrochemical properties
in micro batteries and high density batteries [5].
Among the various metal oxides, cobalt oxides have been extensively
investigated because of their potential applications in many technological
fields, as well as those of cobalt nanoparticles films obtained by a number of
techniques [6-9]. High quality magnetic films of cobalt oxide based alloys
are currently used in magnetic heads and magnetic RAM. For example
simple binary alloys produce high quality films for magnetic recording
industry. On the other hand cobalt oxide based ceramics have attractive
magnetic properties and their films and multilayer have been studied for
decades and still motivate serious research and development efforts [10-13].
In this paper attempts are made to deposit the thin film of cobalt oxide
using metal organic chemical vapor deposition technique (MOCVD). For this
the precursor cobalt-actylacetonate is used.
2. BLOCK DESCRIPTION OF MOCVD SYSTEM
Block diagram of typical metal organic chemical vapor deposition system is
shown in fig. 1 which consists of following sub units:
Fig. 1 – Metal organic chemical vapor deposition system
2.1 Gas handling unit
The gas handling system performs the functions like, mixing and metering
of the gas that will enter into the reactor. Timing and composition of the
gas entering the reactor will determine the epilayer structure. Leak-tight of
the gas panel is essential, because the oxygen contamination will degrade the
growing films’ properties. Fast switch of valve system is very important for
thin film and abrupt interface structure growth. Accurate control of flow
rate, pressure and temperature can ensure the stable and repeat.
GAS
HANDLING
UNIT
COMPUTER
CONTROL
REACTOR
ELECTRONIC
CONTROL
SYSTEM
VACCUM &
EXHAUST UNIT
MOCVD OF COBALT OXIDE USING CO-ACTYLACETONATE… 205
2.2 Reactor
A reactor is a chamber where the deposition process is carried on. The
chamber is composed of reactor walls, a liner, gas injection units, and
temperature control units.
2.3 Vacuum and exhaust unit
Pump and pressure controller is main part of this unit which will control
the pressure growth. It is mainly used for low pressure growth. To handle
large gas load the pump must be designed in proper manner.
2.4 Electronic control system
This system contains some electronic circuits, which controls various
parameters like temperature, rate of flow of oxygen, argon gases, pressure
in the reaction chamber etc.
3. THE GROWTH OF THIN FILM BY MOCVD
The vapor pressure is an important consideration in MOCVD, since it
determines the concentration of source material in the reactor and the
deposition rate. First the metal organic sources and hydrides inject to the
reactor. The sources are mixed well inside the reactor and transfer to the
deposition area. At the deposition area, high temperature result in the
decomposition of sources and other gas-phase reaction, forming the film
precursors which are useful for film growth and by-products. Then film
precursor’s transport to the growth surface, the film precursors absorb on
the growth surface, the film precursors diffuse to the growth site. At the
surface, film atoms incorporate into the growing film through surface
reaction. The by-products of the surface reactions absorb from surface. The
by-products transport to the main gas flow region away from the deposition
area towards the reactor exit.
4. STEP BY STEP PROCESS IN DEPOSITION
4.1 Cleaning of substrates
The substrate used for depositions is alumina Al2O3. Prior to each deposition,
the substrates, the substrate holders and the reaction chamber were scrubbed
by using detergent, distilled water, trichloroethylene, acetone, ethyl alcohol
and distilled water, respectively. The substrates were washed with 1:1
Hydrochloric acid and double distilled water. Then it is cleaned by ultrasonic
cleaner for ten minutes. Again washed with double distilled water. 50 ml of
acetone was boiled, and then with the vapors of acetone substrate were
cleaned till entire acetone. After this 25 ml of trichloroethelene was boiled
and substrate was cleaned in these vapors, and kept in air-tight container.
This process was used to dislodge the dirt on the glass, and ensure that hydro-
carbon and grease were removed from the substrate and also to ensure that
the substrate surfaces were free from surface contamination and defects [4].
4.2 Preparation of precursor Co-acetylacetonate
The 2.4-pentanedione (acetylacetonate) 40 ml was added slowly to a solution
of 16.0 gm of sodium hydroxide in 150 ml of water and kept at a temperature
206 S.M. JOGADE, P.S. JOSHI, B.N. JAMADAR, D.S. SUTRAVE
bellow 40 C The yellow solution was added drop wise to a solution of 47.6 gm
of cobalt (II) chloride hydrate (CoCl2.6H2O) in 250 ml of water and stirred
vigorously. The resulting orange precipitate was filtered in a large Buchner
funnel and washed with about 500 ml of water until the washing was colorless
The moist solid was then dissolved in hot mixture of 400 ml ethanol and
250 ml of chloroform. The red solution was allowed to cool slowly to room
temperature and then further cooled in ice. The orange needles were suction
filtered and washed with cold 95 % ethanol and air dried [4]. The reactions
are given bellow:
HC5H7O2 + NaOH NaC
5
H
7
O
2
+ H2O
2NaC5H7O2 + CoCl2 + 6H2O Co(C
5
H
7
O
2
)
2
+ 2NaCl + 6H2O
4.3 Deposition conditions
The thin films of cobalt oxide were deposited on alumina substrate by MOCVD
technique. The thin films of cobalt oxides are deposited at four different
temperatures viz. 490 C, 515 C, 535 C, 565 C. The different deposition
conditions and parameters are as shown in table 1.
Table 1 – Deposition parameters and conditions
Sr.No. Parameters Conditions
1 Precursor used for
deposition Cobalt acetylacetonate
2 Substrate used for
deposition Al2O3
3 Purging gas Argon
4 Purging time 30 min
5 Reacting gas Oxygen
6 Time of reaction 60 min
7 Base pressure 0.06 T
8 Purging gas pressure 0.21 T
9 Deposition pressure 10.00 T
10 Temperature of Vaporizer 185 C
11 Line temperature 200 C
12 Temperature of substrate 515 C
13 Carrier gas (Argon) flow
rate 9 %
14 Reacting gas (O2) flow rate 5 %
5. PHYSICAL PROPERTIES OF THE SAMPLE
The thin films of cobalt oxides are deposited at four different temperatures
viz. 490 C, 515 C, 535 C, 565 C. All the samples deposited on the alumina
substrate are well adherent and grayish in color. The thicknesses of the
samples were calculated by weight difference method. The table 2 shows the
variation of thickness with temperature. Thickness of the cobalt oxide film is
maximum at temperature 535 C.
MOCVD OF COBALT OXIDE USING CO-ACTYLACETONATE… 207
Table 2 – Variations of thickness of cobalt oxide thin film at various
temperatures
Sr. No. Temperature, C Thickness, mkm
1 490 0.04205
2 515 0.05600
3 535 0.11120
4 565 0.01905
6. STRUCTURAL ANALYSIS BY XRD
The X-ray diffraction patterns were obtained for all these samples by using
Bruker D8 advanced instrument with source CuK
1
with
1.5406. The
angle-2
is varied in the range between 10 to 90. The fig. 2 shows number
of peaks observed for various temperatures. All the samples are crystalline in
nature with cubic in structure. The as deposited samples show dominating
peaks these data are compared with the JCPDs-ICDD data no. 78 - 1970 and
JCPDs-ICDD data no.78-0431 [14]. The cobalt oxide films obtained on alumina
substrate shows more number of highly intensed peaks. The peak
corresponding to a plane (311) of Co3O4 is most prominent which is observed
for all the four samples and same is confirmed with JCPDS-ICDD data no. 78-
1970 for Co3O4 [14] Few peaks corresponding to the CoO are also observed.
The intensity patterns are more or less similar in all the cases. The peak
corresponding to plane (222) shows less intensity as compared to other peaks
and same is confirmed with JCPDS-ICDD data no. 78-0431 for CoO [14]. The
table 3, shows comparison between intensity of no of peaks observed at
different temperatures for angle-2
with ASTM values. Similar results are
obtained by D.Barreca et al. [15].
Fig. 2 – XRD of cobalt oxide thin film deposited on alumina substrate
208 S.M. JOGADE, P.S. JOSHI, B.N. JAMADAR, D.S. SUTRAVE
Table 3 – XRD Analysis of cobalt oxide thin films deposited at various
temperatures
Observed data ASTM data
Peak No.
Angle-2
Intensity Angle-2
Intensity Plane
Temperature 490 C
1 26.00 47.2 -- -- --
2 36.25 96.5 36.84 999 (311)
3 38.50 33.2 38.54 080 (222)
4 44.22 68.4 44.80 173 (400)
5 53.35 45.0 -- -- --
6 58.00 88.6 59.34 210 (511)
7 66.50 33.3 65.22 305 (440)
8 68.60 34.0 68.61 002 (531)
9 77.54 45.0 77.52 108 *(222)
Temperature 515 C
1 25.75 45.5 -- -- --
2 36.60. 94.5 36.84 999 (311)
3 38.00 31.0 38.54 080 (222)
4 43.25 61.5 42.38 999 *(200)
5 53.00 43.3 -- -- --
6 58.10 77.2 59.34 210 (511)
7 67.30 37.5 68.61 002 (531)
8 68.50 31.5 68.61 002 (531)
9 77.50 52.0 77.52 108 *(222)
Temperature 535 C
1 26.40 45.5 -- -- --
2 36.65 97.5 36.84 999 (311)
3 38.50 31.0 38.54 080 (222)
4 44.00 62.0 44.80 173 (400)
5 52.50 39.2 -- -- --
6 58.50 77.7 59.34 210 (511)
7 67.25 33.2 68.61 002 (531)
8 69.00 32.0 69.73 001 (442)
9 78.00 46.0 78.39 032 (622)
Temperature 565 C
1 26.25 48.0 -- -- --
2 36.28 97.5 36.84 999 (311)
3 37.75 33.0 38.54 080 (222)
4 44.00 74.8 44.80 173 (400)
5 53.00 43.5 -- -- --
6 58.00 88.0 59.34 210 (511)
7 67.00 33.5 68.61 002 (531)
8 68.50 35.0 68.61 002 (531)
9 77.50 44.8 77.52 108 *(222)
MOCVD OF COBALT OXIDE USING CO-ACTYLACETONATE… 209
7. MORPHOLOGICAL CHARACTERISTICS
The SEM images were obtained from ESEM Quonta 200 instrument. The SEM
images obtained for cobalt oxide thin films on alumina substrate shows more
packed grains with increase in size of grain. The hexagonal shape of grains
are observed for sample at temperature 515 C. Above this temperature the
grain size found to be decreased. At temperature 565 C the different shapes
of grains are observed. The patterns are shown in fig. 3. The table 4 shows
average grain size of deposited thin films observed from observed from SEM
images. This is confirmed by Mordi et al. and Nygirnyi et al. [4, 16].
Fig. 3 – SEM images of cobalt oxide thin film deposited on alumina substrate at
T 490 C (a), T 515 C (b), T 535 C (c) and T 565 C (d)
8. RAMAN SPECTRA ANALYSIS
The RAMAN Spectra for the cobalt oxide thin films are obtained with the
help of “NSOM” instrument. The similar characteristic is observed for all the
samples deposited on alumina substrate. Raman spectroscopy shows Fm3m,
225 space groups for cobalt oxide thin films deposited on alumina substrate.
The Fig. 4 shows RAMAN Spectra for the cobalt oxide thin films deposited on
alumina substrate.
a b
d
c
210 S.M. JOGADE, P.S. JOSHI, B.N. JAMADAR, D.S. SUTRAVE
Table 4 – Average grain size observed from SEM images for various tempe-
ratures
Sr.No. Temperature, C Average grain size, mkm
1 490 2.82
2 515 2.35
3 535 3.26
4 565 2.55
Fig. 4 – Raman spectra of cobalt oxide thin film deposited on glass substrate
9. CONCLUSION
The MOCVD technique is most suitable to deposit good quality thin films of
cobalt oxide from a cobalt acetylacetonate precursor. The as-deposited samples
are well adherent to the substrates. The samples are a crystalline in nature
with cubic in structure. The SEM images shows well developed packed grains
of cobalt oxide. Raman spectroscopy shows Fm3m, 225 space groups.
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Purpose: Nanotechnology applied to cancer treatment is a growing area of research in nanomedicine with magnetic nanoparticle-mediated anti-cancer drug delivery systems offering least possible side effects. To that end, both structural and chemical properties of commercial cobalt metal nanoparticles were studied using label-free confocal Raman spectroscopy. Materials and methods: Crystal structure and morphology of cobalt nanoparticles were studied by XRD and TEM. Magnetic properties were studied with SQUID and PPMS. Confocal Raman microscopy has high spatial resolution and compositional sensitivity. It, therefore, serves as a label-free tool to trace nanoparticles within cells and investigate the interaction between coating-free cobalt metal nanoparticles and cancer cells. The toxicity of cobalt nanoparticles against human cells was assessed by MTT assay. Results: Superparamagnetic Co metal nanoparticle uptake by MCF7 and HCT116 cancer cells and DPSC mesenchymal stem cells was investigated by confocal Raman microscopy. The Raman nanoparticle signature also allowed accurate detection of the nanoparticle within the cell without labelling. A rapid uptake of the cobalt nanoparticles followed by rapid apoptosis was observed. Their low cytotoxicity, assessed by means of MTT assay against human embryonic kidney (HEK) cells, makes them promising candidates for the development of targeted therapies. Moreover, under a laser irradiation of 20mW with a wavelength of 532nm, it is possible to bring about local heating leading to combustion of the cobalt metal nanoparticles within cells, whereupon opening new routes for cancer phototherapy. Conclusion: Label-free confocal Raman spectroscopy enables accurately localizing the Co metal nanoparticles in cellular environments. The interaction between the surfactant-free cobalt metal nanoparticles and cancer cells was investigated. The facile endocytosis in cancer cells shows that these nanoparticles have potential in engendering their apoptosis. This preliminary study demonstrates the feasibility and relevance of cobalt nanomaterials for applications in nanomedicine such as phototherapy, hyperthermia or stem cell delivery.
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In Situ Synthesis of MOF-Derived Carbon Shells for Silicon Anode with Improved Lithium-Ion Storage
Article
  • Apr 2020
Silicon (Si) has been broadly investigated as a promising anode in lithium-ion batteries (LIBs). However, there is still a problem that the alloying reactions of Si and Li often cause structural failures and rapid degradation in capacity of the electrode. Herein, an in-situ encapsulation of Si nanoparticles forming metal-organic-framework (MOF) derived carbon shells has been developed to improve the performance and retain the structural integrity of the electrode. Using this strategy, a compact and robust interface was ensured between the Si nanoparticles and carbon shell while also reducing the unnecessary exposing areas simultaneously. Moreover, the pores in the MOF-derived carbon shells offered good channels for Li-ion penetration and diffusion. The resulted composite electrode exhibited excellent electrochemical performance and delivered a capacity of 3714 mAh g⁻¹ at 200 mA g⁻¹, and an outstanding reversible capacity of 820 mAh g⁻¹ at 5000 mA g⁻¹ even after 1000 cycles, Direct comparison between the encapsulated Si and naked Si revealed the significance of the MOF-derived carbon shells and its great potential for the next-generation high capacity LIBs.
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Supercapacitors: Review of Materials and Fabrication Methods
Article
Full-text available
  • Jun 2013
Supercapacitors are considered to be promising candidates for power devices in future generations. These devices are expected to find many future applications in hybrid electric vehicles and other power devices and systems. For supercapacitors to realize their promise, it is important that their energy and power densities be maximized. An important way to address this is to develop advanced electrode materials and methods to fabricate these materials. The recent years have seen enormous interest in the research of numerous materials and methods for their synthesis for applications in supercapacitor electrode technology. In the constantly changing technological landscape, it is relevant to review the various aspects of supercapacitor devices. This review paper gives an overview of the types of supercapacitors. It describes the advanced materials and fabrication methods for these devices, including recent developments in these areas, and their implications on the future of supercapacitor technology. The paper also addresses the principal technological challenges facing the development efforts in the future.
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Intrinsic perpendicular anisotropy and exchange biasing in CoO/permalloy multilayers
Article
Full-text available
  • Feb 2005
  • J MAGN MAGN MATER
For permalloy/CoO multilayers, the exchange field and the coercivity of the perpendicular and longitudinal exchange biasing have been found to decrease with increasing temperature. After zero field cooling, perpendicular magnetic anisotropy is observed. For small permalloy layer thickness, perpendicular anisotropy film can be established at low temperatures.
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Magnetic Behavior of Cobalt Oxide Films Prepared by Pulsed Liquid Injection Chemical Vapor Deposition From a Metal-Organic Precursor
Article
Full-text available
  • Feb 2006
  • THIN SOLID FILMS
Thin films of cobalt oxide were prepared by the pulsed liquid injection chemical vapor deposition technique from metal-organic precursor. By using a β-diketonate complex of cobalt, namely cobalt (II) acetylacetonate (Co(acac)2) as the precursor, oxygen as the reactant and argon as the carrier gas, cobalt oxide films 100 nm in thick were deposited onto Si (100) substrates at 650 °C in about 40 min. According to the characterization by X-ray diffraction and atomic force microscopy, smooth and polycrystalline films, consisting exclusively of the Co3O4 phase, were deposited. Magnetic properties, such as saturation magnetization, the remanence, the coercivity, the squareness ratio and the switching field distribution, were extracted from the hysteresis loop. Cobalt oxide films with coercivities of 6.61 mT, squareness ratio of 0.2607 and saturation magnetization of 12.17 nA m2, corresponding to a soft magnetic material, were achieved.
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Composition and Microstructure of Cobalt Oxide Thin Films Obtained from a Novel Cobalt(II) Precursor by Chemical Vapor Deposition
Article
  • Feb 2001
The present work reports the synthesis and the characterization of cobalt oxide thin films obtained by chemical vapor deposition (CVD) on indium tin oxide (ITO) substrates, using a cobalt(II) beta -diketonate as precursor. The complex is characterized by electron impact mass spectrometry (EI-MS) and thermal analysis in order to investigate its decomposition pattern. The depositions are carried out in a cold wall reactor in the temperature range 350-500 degreesC at different oxygen pressures, to tailor film composition from CoO to CO3O4. The crystalline nanostructure is evidenced by X-ray diffraction (XRD), while the surface and in-depth chemical composition is studied by X-ray photoelectron (XPS) and X-ray excited auger electron spectroscopy (XE-AES). Atomic force microscopy (AFM) is employed to analyze the surface morphology of the films and its dependence on the synthesis conditions. Relevant results concerning the control of composition and microstructure of Co-O thin films are presented and discussed.
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Metal-organic chemical vapour deposition of thin films of cobalt on different substrates: Study of microstructure
Article
  • Jun 2002
  • THIN SOLID FILMS
We have investigated the microstructure of thin films grown by metal–organic chemical vapour deposition using a β-diketonate complex of cobalt, namely cobalt (II) acetylacetonate. Films were deposited on three different substrates: Si(100), thermally oxidised silicon [SiO2/Si(100)] and glass at the same time. As-grown films were characterised by X-ray diffraction, scanning electron microscopy, scanning tunnelling microscopy, atomic force microscopy and secondary ion mass spectrometry. Electrical resistivity was measured for all the films as a function of temperature. We found that films have very fine grains, resulting in high electrical resistivity. Further, film microstructure has a strong dependence on the nature of the substrate and there is diffusion of silicon and oxygen into cobalt from the substrate.
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Strongly oriented thin films of Co 3O 4 deposited on single-crystal MgO(1 0 0) by low-pressure, low-temperature MOCVD
Article
  • Apr 2002
  • J CRYST GROWTH
We have grown epitaxial thin films of Co3O4 on single-crystal MgO at ∼400°C by low-pressure thermal metalorganic chemical vapor deposition using cobalt(II) acetylacetonate as the precursor. X-ray phi-scan analysis shows that cube-on-cube epitaxy of Co3O4 occurs on MgO(100) in the temperature range ∼400–550°C. Films grown on MgO(100) at 410°C show a rocking curve FWHM of only 0.44°, despite a rather large lattice mismatch of 4.1%. This is interpreted to be due to the excellent match between the oxygen sublattices of Co3O4(100) and MgO(100). The strong epitaxy which occurs, leads to a low activation energy (Ea) for growth on MgO. Scanning electron microscopy analysis shows faceted grains characteristic of the cubic symmetry of Co3O4, the faceting being stronger at higher growth temperatures.
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Calibration of Machine Deformation for Quasistatic Fracture Toughness Testing at High Temperatures
Article
  • Jan 2009
  • MATER SCI TECH-LOND
Quasi-static J testing at elevated temperatures can be simplified to a great extent if extensometry for load line displacement measurement can be avoided. Towards this, a procedure has been developed for calibrating the non-linear elastic machine deformation as a function of load, using data for a blank specimen deforming in the elastic regime. The calibration procedure is demonstrated for ambient temperature 653 and 803 K. For a modified 9Cr-1Mo steel under the normalised and tempered condition without/with subsequent thermal aging, the viability of the calibration method for evaluating J is demonstrated for the two elevated temperatures.
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Epitaxial Growth of Co3O4 Films by Low Temperature, Low Pressure Chemical Vapor Deposition
Article
  • Sep 2001
  • J CRYST GROWTH
The growth of strongly oriented or epitaxial thin films of metal oxides generally requires relatively high growth temperatures or infusion of energy to the growth surface through means such as ion bombardment. We have grown high quality epitaxial thin films of Co3O4 on different substrates at a temperature as low as 400°C by low-pressure metalorganic chemical vapour deposition (MOCVD) using cobalt(II) acetylacetonate as the precursor. With oxygen as the reactant gas, polycrystalline Co3O4 films are formed on glass and Si (100) in the temperature range 400–550°C. Under similar conditions of growth, highly oriented films of Co3O4 are formed on SrTiO3 (100) and LaAlO3 (100). The activation energy for the growth of polycrystalline films on glass is significantly higher than that for epitaxial growth on SrTiO3 (100). The film on LaAlO3 (100) grown at 450°C shows a rocking curve FWHM of 1.61°, which reduces to 1.32° when it is annealed in oxygen at 725°C. The film on SrTiO3 (100) has a FWHM of 0.33° (as deposited) and 0.29° (after annealing at 725°C). The φ-scan analysis shows cube-on–cube epitaxy on both these substrates. The quality of epitaxy on SrTiO3 (100) is comparable to the best of the perovskite-based oxide thin films grown at significantly higher temperatures. A plausible mechanism is proposed for the observed low temperature epitaxy.
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Magnetic anisotropic properties in Fe3O4 and CoFe2O4 ferrite epitaxy thin films
Article
  • Apr 2004
  • J MAGN MAGN MATER
Epitaxial thin films of Fe3O4 and CoFe2O4 on MgO (0 0 1) substrates were grown by molecular beam epitaxy at low temperature growth process. Magnetization and hysteresis loop of both films were measured to investigate magnetic anisotropic properties at various temperatures. Anomalous magnetic properties are found to be correlated with crystalline, shape, and stress anisotropies. The Fe3O4 film below Verwey structural transition has a change in crystal structure, thus causing many anomalous magnetic properties. Crystalline anisotropy and anomalous magnetic properties are affected substantially by Co ions. The saturation magnetization of Co–ferrite film becomes much lower than that of Fe3O4 film, being very different from the bulks. It indicates that the low temperature growth process could not provide enough energy to have the lowest energy state.
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MOCVD of cobalt oxide thin films: Dependence of growth, microstructure, and optical properties on the source of oxidation
Article
  • Jul 2003
  • J CRYST GROWTH
Thin films of cobalt oxide were deposited on glass substrates by low-pressure chemical vapor deposition using cobalt (II) acetylacetonate as the precursor. The depositions were carried out using two different oxidant gases—(i) O2 and (ii) N2O, in the temperature range of 375–550°C, under otherwise identical conditions, with the expectation that oxygen radicals released by N2O may alter the CVD process. The crystallinity and phase composition of films were examined by X-ray diffraction and transmission electron microscopy. The surface morphology and microstructure of the films were studied by scanning electron microscopy and atomic force microscopy. It is found that the identity of the oxidant gas affects the phase composition, growth kinetics, uniformity, as well as microstructure, of the films. Electron microscopy shows that growth in O2 ambient results in films containing strongly faceted, uniform-sized grains of Co3O4 only, with a preference for the (1 1 1) orientation, even at relatively low temperatures. However, films grown in N2O ambient comprise poorly crystallized, randomly oriented grains of either Co3O4 or (CoO+Co3O4), depending on the deposition conditions. Optical properties of the films of the latter composition, deposited in the N2O ambient, studied by UV-visible spectrophotometry, show clearly the presence of two distinct bandgaps, corresponding, respectively, to CoO and Co3O4.
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