Education & Training

  • Ph.D. 2019

    Semiconducting Nanomaterials

    University of Technology Sydney

  • M.Phil. 2011

    Solid State Physics

    Bangladesh University of Engineering and Technology

  • M.Sc. 2008

    Solid State Physics

    University of Rajshahi

  • B.Sc. 2006

    Physics

    University of Rajshahi

Honors, Awards and Grants

  • 2017
    Australia Research Training Program (RTP) Scholarship
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    The International Postgraduate Research Scholarship (IPRS) was replaced by the Research Training Program (RTP) Scholarship from 1 January 2017.

  • 2015
    International Postgraduate Research Scholarship (IPRS)
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    This scholarship is funded by the Department of Education, the Australian Government for the international PhD student.

  • 2015
    Australian Postgraduate Award (APA)
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    This scholarship is funded by the Australian Government for the living expense of international PhD student.

  • 2010
    NSICT fellowship
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    This fellowship is funded by the Ministry of Science and Technology, Government of Bangladesh for a local research student.

  • 2009
    Best presentation Award
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    Bangladesh Physical Society

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Cluster in ZnO films grown by pulsed laser deposition

Zilan Wang, Caiqin Luo, W. Anwand, A. Wagner, M. Butterling, M. Azizar Rahman, Matthew R. Phillips, Cuong Ton-That, M. Younas, Shichen Su & Francis Chi-Chung Ling

Abstract

Undoped and Ga-doped ZnO films were grown on c-sapphire using pulsed laser deposition (PLD) at the substrate temperature of 600oC. Positron annihilation spectroscopy study (PAS) shows that the dominant VZn-related defect in the as-grown undoped ZnO grown with relative low oxygen pressure P(O2) is a vacancy cluster (most likely a VZn-nVOcomplex with n = 2, 3) rather than the isolated VZn which has a lower formation energy. Annealing these samples at 900C induces out-diffusion of Zn from the ZnO film into the sapphire creating the VZn at the film/sapphire interface, which favors the formation of vacancy cluster containing relatively more VZn. Increasing the P(O2) during growth also lead to the formation of the vacancy cluster with relatively more VZn. For Ga-doped ZnO films, the oxygen pressure during growth has significant influence on the electron concentration and the microstructure of the VZn-related defect. Green luminescence (GL) and yellow luminescence (YL) were identified in the cathodoluminescence study (CL) study, and both emission bands were quenched after hydrogen plasma treatment. The origin of the GL is discussed.

Surface polarity control in ZnO films deposited by pulsed laser deposition

Cai-Qin Luo, Francis Chi-Chung Ling, M. Azizar Rahman, Matthew Phillips, Cuong Ton-That, Changzhong Liao, Kaimin Shih, Jingyang Lin, Ho Won Tam, Aleksandra B. Djurišić, Shuang-Peng Wang

Abstract

We demonstrate a simple and inexpensive method of surface polarity control of ZnO 
grown by pulsed laser deposition (PLD). The polarity control is achieved in a  
straightforward way by changing the thickness of MgO buffer layer. The Zn- and 
O-polar ZnO films possess very distinct growth rate, electron concentration and 
mobility as well as different defect structures. These different structural and electronic 
properties result in significant differences in surface reactivity and device 
performance. For example, Pd Schottky diodes fabricated onto the O-polar ZnO film 
exhibit lower barrier height and ideality factor compared with the equivalent Zn-polar 
devices, while methylammonium lead iodide perovskite films are readily formed on 
O-terminated and rapidly decompose on Zn-terminated surfaces. This can be 
attributed to higher photocatalytic activity of Zn-terminated surface, as well as higher 
surface coverage of adsorbed hydroxyl groups. Consequently, our results indicate that 
polarity engineering to obtain favorable O-terminated surface can result in improved 
performance of ZnO-containing optoelectronic devices, while Zn-terminated surfaces 
could be of interest for photocatalytic and sensing applications. 

A facile method for bright, colour-tunable light-emitting diodes based on Ga-doped ZnO nanorods

M. Azizar Rahman, John A. Scott, A. Gentle, Matthew R. Phillips, Cuong Ton-That

Abstract

Bottom-up fabrication of nanowire-based devices is highly attractive for oxide photonic devices because of high light extraction efficiency; however, unsatisfactory electrical injection into ZnO and poor carrier transport properties of nanowires severely limit their practical applications. Here, we demonstrate that ZnO nanorods doped with Ga donors by in situ dopant incorporation during vapour-solid growth exhibit superior optoelectronic properties that exceed those currently synthesised by chemical vapour deposition, and accordingly can be electrically integrated into Si-based photonic devices. Significantly, the doping method was found to improve the nanorod quality by decreasing the concentration of point defects. Light-emitting diodes (LEDs) fabricated from the Ga-doped ZnO nanorod/p-Si heterojunction display bright and colour-tunable electroluminescence (EL). These nanorod LEDs possess a dramatically enhanced performance and an order of magnitude higher EL compared with equivalent devices fabricated with undoped nanorods. These results point to an effective route for large-scale fabrication of conductive, single-crystalline ZnO nanorods for photonic and optoelectronic applications.

Multi-Color excitonic emissions in chemical dip-coated organolead mixed-halide perovskite

Al Momin M. Tanveer Karim, M. Azizar Rahman, M. S. Hossain, M. K. R. Khan, M. Mozibur Rahman, M. Kamruzzaman, and Cuong Ton-That

Abstract

Organolead mixed‐halide perovskites capable of emitting multiple colors not only allow the fabrication of compact devices but also improves the functionality of light emitting devices beyond conventional displays. Here we demonstrate a cost‐effective chemical dip‐coating technique to grow highly reproducible methylammonium‐lead‐halide perovskite film, which exhibits highly efficient multi‐color emissions of red, green and ultraviolet. The power‐ and temperature‐resolved cathodoluminescence analyses reveal that these emissions are attributed to localized excitions with thermal activation energies of 27, 29 and 57 meV. The non‐linear fitting of these emission band widths within the frame work of Boson model reveals that excitons are strongly coupled to the lattice with energy of 28 meV, which is consistent with the Raman measurement. This work presents an effective method for the deposition of high‐quality and large‐area perovskite film, which could be useful for high‐performance multi‐color display devices.

Charge state switching of Cu acceptors in ZnO nanorods

M. Azizar Rahman, Mika T. Westerhausen, Christian Nenstiel, Sumin Choi, Axel Hoffmann, Angus Gentle, Matthew R. Phillips, Cuong Ton-That

Abstract

Undoped and Ga-doped ZnO nanorods both exhibit an intense green luminescence (GL) band centered at 2.4 eV. Unlike the defect-related GL in undoped nanorods, the GL band in Ga-doped nanorods displays a periodic fine structure separated by 72 meV, which consists of doublets with an energy spacing of 30 meV. The emergence of the structured GL is due to the Cu+ state being stabilized by the rise in the Fermi level above the 0/-(Cu2+/Cu+) charge transfer level as a result of Ga donor incorporation. From a combination of optical characterization and simulation using the Brownian oscillator model, the doublet fine structures are shown to originate from two hole transitions with the Cu+ state located at 390 meV above the valence band.

Efficient multi-coloured Li-doped ZnO thin films fabricated by spray Pyrolysis

M. Azizar Rahman, Matthew R. Phillips, Cuong Ton-That

Abstract

Undoped and Li-doped ZnO films with bright visible luminescence have been fabricated by the spray pyrolysis technique at 400 C. The pyrolytic films exhibit multi-coloured emissions of yellow, green and blue, which can be tuned by varying the Li concentration. Simulation of the cathodoluminescence spectra from the Li-doped films using the Huang-Rhys model reveals the energies of the luminescence centres and their electron-phonon coupling strength. These centres are attributable to either VZn or LiZnacceptor states. This work presents a practical route to fabricate inexpensive multi-coloured light emitting coatings on any substrate.

Effect of Co doping on structural, optical, electrical and thermal properties of nanostructured ZnO thin films

Sonet Kumar Saha, M. Azizar Rahman*, Rakibul Hasan, M. Shahjahan, M.K.R. Khan

Abstract

Nanocrystalline Zn1-xCoxO (where x varies from 0 to 0.04 in steps of 0.01) thin films were deposited onto glass substrate by the spray pyrolysis technique at a substrate temperature of 350 °C. The X-ray diffraction patterns confirm the formation of hexagonal wurtzite structure. The crystal grain size of these films was found to be in the range of 11& 36 nm. The scanning electron micrographs show a highly crystalline nanostructure with different morphologies including rope-like morphology for undoped ZnO and nanowalls and semispherical morphology for Co-doped ZnO. The transmittance increases with increasing Co doping. The optical absorption edge is observed in the transmittance spectra from 530 to 692 nm, which is due to the Co2+ absorption bands corresponding to intraionic d-d* shifts. The direct and indirect optical band gap energies decrease from 3.05 to 2.75 eV and 3.18 to 3.00 eV, respectively for 4 mol% Co doping. The electrical conductivity increases with increasing both the Co doping and temperature, indicating the semiconducting nature of these films. The temperature dependence thermal electromotive force measurement indicates that both undoped and Co-doped ZnO thin films show p-type semiconducting behavior near room temperature. This behavior dies out beyond 313 K and they become n-type semiconductors.

Relaxation mechanism of (x) Mn0.45Ni0.05Zn0.50Fe2O4+(1-x) BaZr0.52Ti0.48O3 multiferroic composites

M. Azizar Rahman and A. K. M. Akther Hossain

Abstract

Polycrystalline (x) Mn0.45Ni0.05Zn0.50Fe2O4+(1-x) BaZr0.52Ti0.48O3 (with 0.2 >x>0.8) multiferroic materials were prepared by the standard solid state reaction technique and samples prepared from these composites were sintered at 1200oC. The impedance, electrical modulus, ac conductivity and dielectric permittivity were investigated over a wide range of frequencies (20 Hz–1 MHz) and at various temperatures (room temperature to 600o C) to understand the relaxation phenomenon in these materials. X-ray diffraction patterns confirm the presence of a simple cubic spinel structure for the ferromagnetic phase and tetragonal peroveskite for the ferroelectric phase. Frequency-independent conductivity was observed in the low-frequency region, which shifts to a higher frequency and dominates over a wide range of frequency (up to 1 MHZ) at higher temperature (600o C). The transition temperature (~675oC) of these composites is higher than that of ferrite and ferroelectric phases. The frequency response electric modulus graphs for some composites show two maxima in the relaxation process. The first relaxation process appears at lower temperature and higher frequency with a lower value of activation energy for the composites containing more than 20% ferrimagnetic phase. This relaxation process is due to the first ionization energy of oxygen vacancies. The second relaxation process appears at all temperatures and at lower frequency, which shifts to higher frequency with increasing temperature, possessing a comparatively higher value of activation energy. This relaxation process is attributed to the Maxwell-Wagner-Sillars relaxation phenomenon. The frequency-dependent impedance and modulus plots exhibit a non-coincidence of relaxation peaks, indicating the deviation from the Debye-type relaxation process.

Structural and electrical properties of (x) Mn0.45Ni0.05Zn0.50Fe2O4+ (1-x) BaZr0.52Ti0.48O3 multiferroic materials

M. Azizar Rahman, Sonet Kumar Saha and A. K. M. Akther Hossain

Abstract

Multiferroic (x)Mn0.45Ni0.05Zn0.50Fe2O4+(1-x)BaZr0.52Ti0.48O3materials (where x varies from 0.0 to 0.8 in steps of 0.20) were prepared by the standard solid state reaction method. X-ray diffraction patterns verify the development of tetragonal perovskite structure for ferroelectric and cubic spinel structure for ferrite phase. The frequency and temperature dependent electrical parameters have been investigated to understand the conduction mechanism in these multiferroic materials. The grain effect contributes to the conduction mechanism for the sample containing 0–60% ferrite and both the grain and grain boundary effect contribute to the conduction mechanism for the composite containing 80% ferrite at high temperature. The ac conductivity increases with increasing frequency for the sample x=0. This is due to the small polaron hopping. For the sample containing 20% ferrite, the frequency independent dc conductivity is observed at low temperature and dominates over a wide frequency range at high temperature region for the sample containing higher percentage of ferrite. The frequency independent dc conductivity is shifted to the frequency dependent ac conductivity, indicating the beginning of the conductivity relaxation phenomenon. This is attributed to the translation of long range polaron hopping to the small range charge carriers. The temperature dependence conductivity indicates that the impurities present in these multiferroic materials are almost minimized and polaron hopping type of conduction mechanism is valid.

Effect of Cu2+ substitution on structural, magnetic and transport properties of Fe2.5Zn0.5-xCuxO4

M. Arifur Rahman, M. Azizar Rahman and A.K.M. Akther Hossain

Abstract

Polycrystalline Fe2.5Zn0.5−xCuxO4 ferrites (where y varies from 0 to 0.45 in steps of 0.05) were prepared by the standard solid state reaction technique. The X-ray diffraction patterns consist of major cubic spinel Fe2.5Zn0.5−xCuxO4 phase with minor impurity phases (Fe2O3 and CuO). The lattice constant decreases slightly with increasing Cu content. The bulk density and average grain size increases with Cu content up to x=0.30 due to the lattice diffusion. The initial permeability and relative quality factor increases with increasing Cu content up to x=0.30. It is attributed to the migration of Fe3+ in the octahedral sublattice and a decrease beyond this content is due to the spin canting effect. The dielectric constant and loss tangent decrease with increasing frequency of the applied alternating electric field because of dielectric polarization, which is similar to the conduction phenomenon. The ac conductivity increases with increasing frequency for the samples x≤0.15 due to the small polaron hopping conduction. The frequency-independent conductivity is found below a certain frequency for the samples x≥0.20 and dominated over a wide frequency range (up to 0.5 MHz) for the sample containing x=0.30 due to band conduction.

Effect of annealing temperature on structural, electrical and optical properties of spray pyrolytic nanocrystalline CdO thin films

M. Azizar Rahman and M. K. R. Khan

Abstract

Nanocrystalline CdO thin films were prepared onto a glass substrate at substrate temperature of 300 °C by a spray pyrolysis technique. Grown films were annealed at 250, 350, 450 and 550 °C for 2.5 h and studied by the X-ray diffraction, Hall voltage measurement, UV-spectroscopy, and scanning electron microscope. The X-ray diffraction study confirms the cubic structure of as-deposited and annealed films. The grain size increases whereas the dislocation density decreases with increasing annealing temperature. The Hall measurement confirms that CdO is an n-type semiconductor. The carrier density and mobility increase with increasing annealing temperature up to 450 °C. The temperature dependent dc resistivity of as-deposited film shows metallic behavior from room temperature to 370 K after which it is semiconducting in nature. The metallic behavior completely washed out by annealing the samples at different temperatures. Optical transmittance and band gap energy of the films are found to decrease with increasing annealing temperature and the highest transmittance is found in near infrared region. The refractive index and optical conductivity of the CdO thin films enhanced by annealing. Scanning electron microscopy confirms formation of nano-structured CdO thin films with clear grain boundary.

Relaxation and conduction mechanisms of high Tc lead-free Ba(Zr,Ti)O3 positive temperature coefficient of resistivity ceramic using impedance spectroscopy

M. Azizar Rahman, Abdul Queader and A. K. M. Akther Hossain

Abstract

High Tc lead–free Ba (Zr0.52Ti0.48)O3 positive temperature coefficient of resistivity (PTCR) ceramic was produced by the standard solid state reaction technique. X–ray diffraction pattern confirms the formation of the tetragonal perovskite structure of the ferroelectric sample. The conduction and relaxation mechanisms of the piezoelectric ceramic have been studied on the basis of activation energy. The relaxation mechanism is investigated for this sample based on the peaks of the imaginary part of electrical impedance and modulus spectra. The Cole-Cole plots indicate that the grain effect is influenced by the increase of temperature up to 200 °C and disappeared beyond this temperature. The temperature versus electrical resistivity plots show that a phase transition occurs at the Curie temperature, Tc =150 °C. The ceramic exhibits a PTCR jump of almost two orders of magnitude starting at 150 °C and ending at 275 °C with a high temperature coefficient of resistivity of ~25 % per °C. The electrical resistivity measurements also reveal that the sample shows semiconducting behavior beyond 275 °C with the value of negative temperature coefficient of resistivity of ~ 0.6% per °C.

Relaxation and conduction mechanisms of high Tc lead-free Ba(Zr,Ti)O3 positive temperature coefficient of resistivity ceramic using impedance spectroscopy

M. Azizar Rahman, Abdul Queader and A. K. M. Akther Hossain

Abstract

High Tc lead–free Ba (Zr0.52Ti0.48)O3 positive temperature coefficient of resistivity (PTCR) ceramic was produced by the standard solid state reaction technique. X–ray diffraction pattern confirms the formation of the tetragonal perovskite structure of the ferroelectric sample. The conduction and relaxation mechanisms of the piezoelectric ceramic have been studied on the basis of activation energy. The relaxation mechanism is investigated for this sample based on the peaks of the imaginary part of electrical impedance and modulus spectra. The Cole-Cole plots indicate that the grain effect is influenced by the increase of temperature up to 200 °C and disappeared beyond this temperature. The temperature versus electrical resistivity plots show that a phase transition occurs at the Curie temperature, Tc =150 °C. The ceramic exhibits a PTCR jump of almost two orders of magnitude starting at 150 °C and ending at 275 °C with a high temperature coefficient of resistivity of ~25 % per °C. The electrical resistivity measurements also reveal that the sample shows semiconducting behavior beyond 275 °C with the value of negative temperature coefficient of resistivity of ~ 0.6% per °C.

Electrical transport properties of Mn-Ni-Zn Ferrite using complex impedance spectroscopy

M. Azizar Rahman and A. K. M. Akther Hossain

Abstract

Polycrystalline Mn0.45Ni0.05Zn0.50Fe2O4 was prepared by a standard solid state reaction technique. We report the electrical properties of this ferrite using ac impedance spectroscopy as a function of frequency (20 Hz–10 MHz) at different temperatures (50–350 °C). X-ray diffraction patterns reveal the formation of cubic spinel structure. Complex impedance analysis has been used to separate the grain and grain boundary resistance of this ferrite. The variation of grain and grain boundary conductivities with temperature confirms semiconducting behavior. The dielectric permittivity shows dielectric dispersion at lower frequency and reveals that it has almost the same value on the high-frequency side. The non-coincidence of peaks corresponding to modulus and impedance indicates deviation from Debye-type relaxation. A similar value of activation energy is obtained from impedance and modulus spectra, indicating that charge carriers overcome the same energy barrier during relaxation. Electron hopping is responsible for ac conduction in this ferrite. The electron hopping shifts toward higher frequency with increasing temperature, below which the conductivity is frequency independent. The frequency-independent ac conductivity has been observed at and above 300 °C in the frequency range 20 Hz–1 MHz. This frequency-independent ac conductivity is due to the long-range movement of the mobile charge carriers.

Structural, magnetic and transport properties of magnetoelectric composites

M. Azizar Rahman, M. A. Gafur and A. K. M. Akther Hossain

Abstract

Magnetoelectric composites of nominal chemical compositions (y)Mn0.45Ni0.05Zn0.50Fe2O4+(1−y)BaZr0.52Ti0.48O3 (where y varies from 0 to 1.0 in steps of 0.20) were prepared by the standard solid state reaction technique. The samples were sintered at various temperatures. X-ray diffraction patterns confirm the presence of the constituent phases. The initial permeability increases with increasing ferrite content and also with increasing sintering temperature. However, there is a slight decrease in initial permeability value for samples sintered above 1573 K. The dielectric dispersion is observed at lower frequencies (<103 Hz) due to interfacial polarization. The dielectric constant is almost independent at high frequencies (>104 Hz) for a particular composition because of the inability of electric dipoles to follow the fast variation of the alternating applied electric field. The ac electrical conductivity increases with increasing frequency, suggesting that the conduction is due to small polaron hopping. The increase in dielectric constant and dielectric loss corresponds to the increase in ac electric conductivity with increasing sintering temperature up to 1573 K. The magnetoelectric voltage coefficient of the composites decreases with increasing ferrite content and dc magnetic field.

Effect of Al-doping on optical and electrical properties of spray pyrolytic nano-crystalline CdO thin films

M. K. R. Khan , M. Azizar Rahman, M. Shahjahan, M. M. Rahman , M. A. Hakim , D. K. Saha and J. U. Khan

Abstract

CdO and Al-doped CdO nano-crystalline thin films have been prepared on glass at 300 °C substrate temperature by spray pyrolysis. The films are highly crystalline with grain size (18–32 nm) and found to be cubic structure with lattice constant averaged to 0.46877 nm. Al-doping increased the optical transmission of the film substantially. Direct band gap energy of CdO is 2.49 eV which decreased with increasing Al-doping. The refractive index and dielectric constant varies with photon energy and concentration of Al as well. The conductivity of un-doped CdO film shows metallic behavior at lower temperature region. This behavior dies out completely with doping of Al and exhibits semiconducting behavior for whole measured temperature range. Un-doped and Al-doped CdO is an n-type semiconductor having carrier concentration is of the order of ∼1021 cm−3, confirmed by Hall voltage and thermo-power measurements.

Response of Dielectric Behaviour in Piezoelectric and Magnetostrictive Composites

M. Azizar Rahman, M. A. Gafur and A. K. M. Akther Hossain

Abstract

The magnetoelectric composites consisting of ferrite and ferroelectric phases viz.(y)Mn 0.45 Ni 0.05 Zn 0.50Fe 2 O 4+ (1-y)BaZr 0.52 Ti 0.48 O 3 in which y varies from 0.20-0.80 mol% were prepared by standard solid state reaction method. X-ray analysis of these composites confirms the presence of both the phases, i.e. ferroelectric and ferrite in the composites. Detailed studies of dielectric constant, loss tangent, and ac conductivity, as a function of frequency (20 Hz - 10 MHz) were studied. The dielectric constant decreases steeply at lower frequencies and remains constant at higher frequencies, indicating dielectric dispersion. This may be attributed to polarization due to changes in the valence states of cations and space charge polarization. Results of ac conductivity show that conduction occurs by hopping of charge carriers. The static magnetoelectric voltage coefficient was measured as a function of applied dc magnetic field. It decreases with increasing dc magnetic field. These composites may be useful as transducer, magnetic sensors, filter, etc.

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