Browsing by Author "Donmez, I."

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Open Access
Atomic layer deposition of GaN at low temperatures
(A I P Publishing LLC, 2012) Ozgit, C.; Donmez, I.; Alevli, M.; Bıyıklı, Necmi
The authors report on the self-limiting growth of GaN thin films at low temperatures. Films were deposited on Si substrates by plasma-enhanced atomic layer deposition using trimethylgallium (TMG) and ammonia (NH 3) as the group-III and -V precursors, respectively. GaN deposition rate saturated at 185 C for NH 3 doses starting from 90 s. Atomic layer deposition temperature window was observed from 185 to ∼385 C. Deposition rate, which is constant at ∼0.51 cycle within the temperature range of 250 - 350 C, increased slightly as the temperature decreased to 185 C. In the bulk film, concentrations of Ga, N, and O were constant at ∼36.6, ∼43.9, and ∼19.5 at. , respectively. C was detected only at the surface and no C impurities were found in the bulk film. High oxygen concentration in films was attributed to the oxygen impurities present in group-V precursor. High-resolution transmission electron microscopy studies revealed a microstructure consisting of small crystallites dispersed in an amorphous matrix. © 2012 American Vacuum Society.
Open Access
Current transport mechanisms in plasma-enhanced atomic layer deposited AlN thin films
(A I P Publishing LLC, 2015) Altuntas, H.; Ozgit Akgun, C.; Donmez, I.; Bıyıklı, Necmi
Here, we report on the current transport mechanisms in AlN thin films deposited at a low temperature (i.e., 200°C) on p-type Si substrates by plasma-enhanced atomic layer deposition. Structural characterization of the deposited AlN was carried out using grazing-incidence X-ray diffraction, revealing polycrystalline films with a wurtzite (hexagonal) structure. Al/AlN/ p-Si metal-insulator-semiconductor (MIS) capacitor structures were fabricated and investigated under negative bias by performing current-voltage measurements. As a function of the applied electric field, different types of current transport mechanisms were observed; i.e., ohmic conduction (15.2-21.5 MV/m), Schottky emission (23.6-39.5 MV/m), Frenkel-Poole emission (63.8-211.8 MV/m), trap-assisted tunneling (226-280 MV/m), and Fowler-Nordheim tunneling (290-447 MV/m). Electrical properties of the insulating AlN layer and the fabricated Al/AlN/p-Si MIS capacitor structure such as dielectric constant, flat-band voltage, effective charge density, and threshold voltage were also determined from the capacitance-voltage measurements.
Open Access
Effect of film thickness on the electrical properties of AlN films prepared by plasma-enhanced atomic layer deposition
(Institute of Electrical and Electronics Engineers Inc., 2015) Altuntas, H.; Ozgit Akgun, C.; Donmez, I.; Bıyıklı, Necmi
In this paper, AlN thin films with two different thicknesses, i.e., 7 and 47 nm, were deposited at 200 °C on p-type Si substrates by plasma-enhanced atomic layer deposition using trimethylaluminum and ammonia. To investigate the electrical characteristics of these AlN films, MIS capacitor structures were fabricated and characterized using current-voltage and high-frequency (1 MHz) capacitance-voltage measurements. The results showed that the current transport mechanism under accumulation mode is strongly dependent on the applied electric field and thickness of the AlN film. Possible conduction mechanisms were analyzed, and the basic electrical parameters were extracted and compared for AlN thin films with different thicknesses. Compared with 7-nm-thick film, a 47-nm-thick AlN film showed a lower effective charge density and threshold voltage along with a higher dielectric constant.
Open Access
Effect of post-deposition annealing on the electrical properties of B-Ga2O3 thin films grown on p-Si by plasma-enhanced atomic layer deposition
(American Vacuum Society, 2014-08) Altuntas, H.; Donmez, I.; Akgun, C. O.; Bıyıklı, Necmi
Ga2O3 dielectric thin films were deposited on (111)-oriented p-type silicon wafers by plasma-enhanced atomic layer deposition using trimethylgallium and oxygen plasma. Structural analysis of the Ga 2O3 thin films was carried out using grazing-incidence x-ray diffraction. As-deposited films were amorphous. Upon postdeposition annealing at 700, 800, and 900°C for 30min under N2 ambient, films crystallized into β-form monoclinic structure. Electrical properties of the β-Ga2O3 thin films were then investigated by fabricating and characterizing Al/β-Ga2O3/p-Si metal-oxide-semiconductor capacitors. The effect of postdeposition annealing on the leakage current densities, leakage current conduction mechanisms, dielectric constants, flat-band voltages, reverse breakdown voltages, threshold voltages, and effective oxide charges of the capacitors were presented. The effective oxide charges (Qeff) were calculated from the capacitance-voltage (C-V) curves using the flat-band voltage shift and were found as 2.6×1012, 1.9×1012, and 2.5×10 12 cm-2 for samples annealed at 700, 800, and 900°C, respectively. Effective dielectric constants of the films decreased with increasing annealing temperature. This situation was attributed to the formation of an interfacial SiO2 layer during annealing process. Leakage mechanisms in the regions where current increases gradually with voltage were well fitted by the Schottky emission model for films annealed at 700 and 900°C, and by the Frenkel-Poole emission model for film annealed at 800°C. Leakage current density was found to improve with annealing temperature. β-Ga2O3 thin film annealed at 800°C exhibited the highest reverse breakdown field value. © 2014 American Vacuum Society.
Open Access
Effect of reactor pressure on optical and electrical properties of InN films grown by high-pressure chemical vapor deposition
(Wiley - V C H Verlag GmbH & Co. KGaA, 2015) Alevli, M.; Gungor, N.; Alkis, S.; Ozgit Akgun, C.; Donmez, I.; Okyay, Ali Kemal; Gamage, S.; Senevirathna, I.; Dietz, N.; Bıyıklı, Necmi
The influences of reactor pressure on the stoichiometry, free carrier concentration, IR and Hall determined mobility, effective optical band edge, and optical phonon modes of HPCVD grown InN films have been analysed and are reported. The In 3d, and N 1s XPS spectra results revealed In-N and N-In bonding states as well as small concentrations of In-O and N-O bonds, respectively in all samples. InN layers grown at 1 bar were found to contain metallic indium, suggesting that the incorporation of nitrogen into the InN crystal structure was not efficient. The free carrier concentrations, as determined by Hall measurements, were found to decrease with increasing reactor pressure from 1.61×1021 to 8.87×1019 cm-3 and the room-temperature Hall mobility increased with reactor pressure from 21.01 to 155.18 cm2/Vs at 1 and 15 bar reactor pressures, respectively. IR reflectance spectra of all three (1, 8, and 15 bar) InN samples were modelled assuming two distinct layers of InN, having different free carrier concentration, IR mobility, and effective dielectric function values, related to a nucleation/interfacial region at the InN/sapphire, followed by a bulk InN layer. The effective optical band gap has been found to decrease from 1.19 to 0.95 eV with increasing reactor pressure. Improvement of the local structural quality with increasing reactor pressure has been further confirmed by Raman spectroscopy measurements. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Electrical characteristics of β-Ga2O3 thin films grown by PEALD
(Elsevier, 2014) Altuntas, H.; Donmez, I.; Ozgit Akgun, C.; Bıyıklı, Necmi
In this work, 7.5 nm Ga2O3 dielectric thin films have been deposited on p-type (1 1 1) silicon wafer using plasma enhanced atomic layer deposition (PEALD) technique. After the deposition, Ga2O 3 thin films were annealed under N2 ambient at 600, 700, and 800 C to obtain β-phase. The structure and microstructure of the β-Ga2O3 thin films was carried out by using grazing-incidence X-ray diffraction (GIXRD). To show effect of annealing temperature on the microstructure of β-Ga2O3 thin films, average crystallite size was obtained from the full width at half maximum (FWHM) of Bragg lines using the Scherrer formula. It was found that crystallite size increased with increasing annealing temperature and changed from 0.8 nm to 9.1 nm with annealing. In order to perform electrical characterization on the deposited films, Al/β-Ga2O3/p-Si metal-oxide- semiconductor (MOS) type Schottky barrier diodes (SBDs) were fabricated using the β-Ga2O3 thin films were annealed at 800 C. The main electrical parameters such as leakage current level, reverse breakdown voltage, series resistance (RS), ideality factor (n), zero-bias barrier height (Bo), and interface states (NSS) were obtained from the current-voltage (I-V) and capacitance-voltage (C-V) measurements at room temperature. The RS values were calculated by using Cheung methods. The energy density distribution profile of the interface states as a function of (ESS-EV) was obtained from the forward bias I-V measurements by taking bias dependence of ideality factor, effective barrier height (e), and RS into account. Also using the Norde function and C-V technique, e values were calculated and cross-checked. Results show that β-Ga2O3 thin films deposited by PEALD technique at low temperatures can be used as oxide layer for MOS devices and electrical properties of these devices are influenced by some important parameters such as NSS, RS, and β-Ga2O3 oxide layer.
Open Access
Fabrication of hafnia hollow nanofibers by atomic layer deposition using electrospun nanofiber templates
(Elsevier, 2013) Donmez, I.; Kayaci, F.; Akgun, C. O.; Uyar, Tamer; Bıyıklı, Necmi
Hafnia (HfO2) hollow nanofibers (HNs) were synthesized by atomic layer deposition (ALD) using electrospun nylon 6,6 nanofibers as templates. HfO2 layers were deposited on polymeric nanofibers at 200 °C by alternating reactant exposures of tetrakis(dimethylamido)hafnium and water. Polymeric nanofiber templates were subsequently removed by an ex situ calcination process at 500 °C under air ambient. Morphological and structural characterizations of the HN samples were conducted by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Freestanding network of HfO2 HNs was found to be polycrystalline with a monoclinic crystal structure. Elemental composition and chemical bonding states of the resulting HfO2 HNs were studied by using X-ray photoelectron spectroscopy. The presence of HfO2 was evidenced by high resolution scans of Hf 4f and O 1s with binding energies of 16.3-17.9 and 529.6 eV, respectively. Combination of electrospinning and ALD processes provided an opportunity to precisely control both diameter and wall thickness of the synthesized HfO2 HNs. © 2013 Elsevier B.V. All rights reserved.
Open Access
The influence of N2/H2 and ammonia N source materials on optical and structural properties of AlN films grown by plasma enhanced atomic layer deposition
(2011) Alevli, M.; Ozgit, C.; Donmez, I.; Bıyıklı, Necmi
The influence of N2/H2 and ammonia as N source materials on the properties of AlN films grown by plasma enhanced atomic layer deposition using trimethylaluminum as metal source has been studied. The -2Θ grazing-incidence X-ray diffraction, high resolution transmission electron microscopy, and spectroscopic ellipsometry results on AlN films grown using either NH3 or N2/H2 plasma revealed polycrystalline and wurtzite AlN layers. The AlN growth rate per cycle was decreased from 0.84 to 0.54 Å/cycle when the N source was changed from NH3 to N2/H2. Growth rate of AlN remained constant within 100200 °C for both N precursors, confirming the self-limiting growth mode in the ALD window. AlAl bond was detected only near the surface in the AlN film grown with NH3 plasma. AFM analysis showed that the RMS roughness values for AlN films grown on Si(100) substrates using NH3 and N2/H2 plasma sources were 1.33 nm and 1.18 nm, respectively. The refractive indices of both AlN films are similar except for a slight difference in the optical band edge and position of optical phonon modes. The optical band edges of the grown AlN films are observed at 5.83 and 5.92 eV for ammonia and N2/H2 plasma, respectively. According to the FTIR data for both AlN films on sapphire substrates, the E1(TO) phonon mode position shifted from 671 cm -1 to 675 cm-1 when the plasma source was changed from NH3 to N2/H2. © 2011 Elsevier B.V. All Rights Reserved.
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Low - temperature self - limiting growth of III - nitride thin films by plasma - enhanced atomic layer deposition
(American Scientific Publishers, 2012) Bıyıklı, Necmi; Ozgit, C.; Donmez, I.
We report on the low-temperature self-limiting growth and characterization of III-Nitride thin films. AlN and GaN films were deposited by plasma-enhanced atomic layer deposition (PEALD) on various substrates using trimethylaluminum (TMA), trimethylgallium (TMG) and triethylgallium (TEG) as group-III, and ammonia (NH3) as nitrogen precursor materials. Self-limiting growth behavior, which is the major characteristic of an ALD process, was achieved for both nitride films at temperatures below 200 °C. AlN deposition rate saturated around 0.86 Å/cycle for TMA and NH3 doses starting from 0.05 and 40 s, respectively, whereas GaN growth rate saturated at a lower value of 0.56 Å/cycle and 0.48 Å/cycle for TMG and TEG doses 0.015 s and 1 s, respectively. The saturation dose for NH3 was measured as 90 s and 120 s, for TMG and TEG experiments, respectively. Within the self-limiting growth temperature range (ALD window), film thicknesses increased linearly with the number of deposition cycles. At higher temperatures (≥225 °C and ≥350 °C for AlN and GaN respectively), deposition rate became temperature-dependent, with increasing growth rates. Chemical composition and bonding states of the films deposited within the self-limiting growth regime were investigated by X-ray photoelectron spectroscopy (XPS). GaN films exhibited high oxygen concentrations regardless of the precursors choice, either TMG or TEG, whereas low-oxygen incorporation in AlN films was confirmed by high resolution Al 2p and N 1s spectra of AlN films. AlN films were polycrystalline with a hexagonal wurtzite structure regardless of the substrate selection as determined by grazing incidence X-ray diffraction (GIXRD). GaN films showed amorphous-like XRD signature, confirming the highly defective layers. High-resolution transmission electron microscopy (HR-TEM) images of the AlN thin films revealed a microstructure consisting of several-nanometer sized crystallites, whereas GaN films exhibited sub-nm small crystallites dispersed in an amorphous matrix.
Open Access
Low temperature atomic layer deposited ZnO photo thin film transistors
(AVS Science and Technology Society, 2014) Oruc, F. B.; Aygun, L. E.; Donmez, I.; Bıyıklı, Necmi; Okyay, Ali Kemal; Yu, H. Y.
ZnO thin film transistors (TFTs) are fabricated on Si substrates using atomic layer deposition technique. The growth temperature of ZnO channel layers are selected as 80, 100, 120, 130, and 250°C. Material characteristics of ZnO films are examined using x-ray photoelectron spectroscopy and x-ray diffraction methods. Stoichiometry analyses showed that the amount of both oxygen vacancies and interstitial zinc decrease with decreasing growth temperature. Electrical characteristics improve with decreasing growth temperature. Best results are obtained with ZnO channels deposited at 80°C; Ion/Ioff ratio is extracted as 7.8 × 109 and subthreshold slope is extracted as 0.116 V/dec. Flexible ZnO TFT devices are also fabricated using films grown at 80°C. ID-VGS characterization results showed that devices fabricated on different substrates (Si and polyethylene terephthalate) show similar electrical characteristics. Sub-bandgap photo sensing properties of ZnO based TFTs are investigated; it is shown that visible light absorption of ZnO based TFTs can be actively controlled by external gate bias. © 2014 American Vacuum Society.
Open Access
Low temperature deposition of Ga2O3 thin films using trimethylgallium and oxygen plasma
(A I P Publishing, 2013) Donmez, I.; Akgun, C. O.; Bıyıklı, Necmi
Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (ALD) using trimethylgallium as the gallium precursor and oxygen plasma as the oxidant. A wide ALD temperature window was observed from 100 to 400 °C, where deposition rate was constant at ∼0.53 Å/cycle. X-ray photoelectron spectroscopy survey scans indicated the presence of gallium, oxygen, and carbon elements with concentrations of ∼36, ∼51.8, and ∼12.2 at. %, respectively. As-deposited films were amorphous; upon annealing at 900 °C under N 2 atmosphere for 30 min, polycrystalline β-Ga2O 3 phase with a monoclinic crystal structure was obtained. Refractive index and root mean square roughness of the annealed Ga2O3 film were higher than those of the as-deposited due to crystallization. © 2013 American Vacuum Society.
Open Access
Low-temperature self-limiting growth of III-Nitride thin films by plasma-enhanced atomic layer deposition
(2012) Bıyıklı, Necmi; Ozgit, C.; Donmez, I.
Open Access
Optical properties of AlN thin films grown by plasma enhanced atomic layer deposition
(A I P Publishing LLC, 2012) Alevli, M.; Ozgit, C.; Donmez, I.; Bıyıklı, Necmi
Crystalline aluminum nitride (AlN) films have been prepared by plasma enhanced atomic layer deposition within the temperature range of 100 and 500 °C. The AlN films were characterized by x-ray diffraction, spectroscopic ellipsometry, Fourier transform infrared spectroscopy, optical absorption, and photoluminescence. The authors establish a relationship between growth temperature and optical properties and in addition, the refractive indices of the AlN films were determined to be larger than 1.9 within the 300-1000 nm wavelength range. Infrared reflectance spectra confirmed the presence of E 1(TO) and A 1(LO) phonon modes at ∼660 cm -1 and 895 cm -1, respectively. Analysis of the absorption spectroscopy show an optical band edge between 5.78 and 5.84 eV and the absorption and photoluminescence emission properties of the AlN layers revealed defect centers in the range of 250 and 300 nm at room temperature. © 2012 American Vacuum Society.
Open Access
Polymer-inorganic core-shell nanofibers by electrospinning and atomic layer deposition: Flexible nylon-ZnO core-shell nanofiber mats and their photocatalytic activity
(American Chemical Society, 2012) Kayaci, F.; Akgun, C. O.; Donmez, I.; Bıyıklı, Necmi; Uyar, Tamer
Polymer-inorganic core-shell nanofibers were produced by two-step approach; electrospinning and atomic layer deposition (ALD). First, nylon 6,6 (polymeric core) nanofibers were obtained by electrospinning, and then zinc oxide (ZnO) (inorganic shell) with precise thickness control was deposited onto electrospun nylon 6,6 nanofibers using ALD technique. The bead-free and uniform nylon 6,6 nanofibers having different average fiber diameters (∼80, ∼240 and ∼650 nm) were achieved by using two different solvent systems and polymer concentrations. ZnO layer about 90 nm, having uniform thickness around the fiber structure, was successfully deposited onto the nylon 6,6 nanofibers. Because of the low deposition temperature utilized (200 °C), ALD process did not deform the polymeric fiber structure, and highly conformal ZnO layer with precise thickness and composition over a large scale were accomplished regardless of the differences in fiber diameters. ZnO shell layer was found to have a polycrystalline nature with hexagonal wurtzite structure. The core-shell nylon 6,6-ZnO nanofiber mats were flexible because of the polymeric core component. Photocatalytic activity of the core-shell nylon 6,6-ZnO nanofiber mats were tested by following the photocatalytic decomposition of rhodamine-B dye. The nylon 6,6-ZnO nanofiber mat, having thinner fiber diameter, has shown better photocatalytic efficiency due to higher surface area of this sample. These nylon 6,6-ZnO nanofiber mats have also shown structural stability and kept their photocatalytic activity for the second cycle test. Our findings suggest that core-shell nylon 6,6-ZnO nanofiber mat can be a very good candidate as a filter material for water purification and organic waste treatment because of their photocatalytic properties along with structural flexibility and stability. © 2012 American Chemical Society.
Open Access
Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control the defect density
(Royal Society of Chemistry, 2014-06-09) Kayaci, F.; Vempati S.; Donmez, I.; Bıyıklı, Necmi; Uyar, Tamer
Oxygen vacancies (VOs) in ZnO are well-known to enhance photocatalytic activity (PCA) despite various other intrinsic crystal defects. In this study, we aim to elucidate the effect of zinc interstitials (Zn i) and VOs on PCA, which has applied as well as fundamental interest. To achieve this, the major hurdle of fabricating ZnO with controlled defect density requires to be overcome, where it is acknowledged that defect level control in ZnO is significantly difficult. In the present context, we fabricated nanostructures and thoroughly characterized their morphological (SEM, TEM), structural (XRD, TEM), chemical (XPS) and optical (photoluminescence, PL) properties. To fabricate the nanostructures, we adopted atomic layer deposition (ALD), which is a powerful bottom-up approach. However, to control defects, we chose polysulfone electrospun nanofibers as a substrate on which the non-uniform adsorption of ALD precursors is inevitable because of the differences in the hydrophilic nature of the functional groups. For the first 100 cycles, Znis were predominant in ZnO quantum dots (QDs), while the presence of VOs was negligible. As the ALD cycle number increased, VOs were introduced, whereas the density of Zni remained unchanged. We employed PL spectra to identify and quantify the density of each defect for all the samples. PCA was performed on all the samples, and the percent change in the decay constant for each sample was juxtaposed with the relative densities of Znis and VOs. A logical comparison of the relative defect densities of Znis and VOs suggested that the former are less efficient than the latter because of the differences in the intrinsic nature and the physical accessibility of the defects. Other reasons for the efficiency differences were elaborated.
Open Access
Selective isolation of the electron or hole in photocatalysis: ZnO–TiO2 and TiO2–ZnO core–shell structured heterojunction nanofibers via electrospinning and atomic layer deposition
(Royal Society of Chemistry, 2014-02-06) Kayaci, F.; Vempati S.; Ozgit Akgun, C.; Donmez, I.; Bıyıklı, Necmi; Uyar, Tamer
Heterojunctions are a well-studied material combination in photocatalysis studies, the majority of which aim to improve the efficacy of the catalysts. Developing novel catalysts begs the question of which photo-generated charge carrier is more efficient in the process of catalysis and the associated mechanism. To address this issue we have fabricated core-shell heterojunction (CSHJ) nanofibers from ZnO and TiO2 in two combinations where only the 'shell' part of the heterojunction is exposed to the environment to participate in the photocatalysis. Core and shell structures were fabricated via electrospinning and atomic layer deposition, respectively which were then subjected to calcination. These CSHJs were characterized and studied for photocatalytic activity (PCA). These two combinations expose electrons or holes selectively to the environment. Under suitable illumination of the ZnO-TiO 2 CSHJ, e/h pairs are created mainly in TiO2 and the electrons take part in catalysis (i.e. reduce the organic dye) at the conduction band or oxygen vacancy sites of the 'shell', while holes migrate to the core of the structure. Conversely, holes take part in catalysis and electrons diffuse to the core in the case of a TiO2-ZnO CSHJ. The results further revealed that the TiO2-ZnO CSHJ shows ∼1.6 times faster PCA when compared to the ZnO-TiO2 CSHJ because of efficient hole capture by oxygen vacancies, and the lower mobility of holes.
Open Access
Structural properties of AIN films deposited by plasma-enhanced atomic layer deposition at different growth temperatures
(Wiley, 2012) Alevli, M.; Ozgit, C.; Donmez, I.; Bıyıklı, Necmi
Crystalline aluminum nitride (AlN) films have been prepared by plasma-enhanced atomic layer deposition (PEALD) within the temperature range from 100 to 500 °C. A self-limiting, constant growth rate per cycle temperature window (100-200 °C) was established which is the major characteristic of an ALD process. At higher temperatures (>225 °C), deposition rate increased with temperature. Chemical composition, crystallinity, surface morphology, mass density, and spectral refractive index were studied for AlN films. X-ray photoelectron spectroscopy (XPS) analyses indicated that besides main Al-N bond, the films contained Al-O-N, Al-O complexes, and Al-Al metallic aluminum bonds as well. Crystalline hexagonal AlN films were obtained at remarkably low growth temperatures. The mass density increased from 2.65 to 2.96 g/cm 3 and refractive index of the films increased from 1.88 to 2.08 at 533 nm for film growth temperatures of 100 and 500 °C, respectively. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Template-based synthesis of aluminum nitride hollow nanofibers via plasma-enhanced atomic layer deposition
(Wiley, 2013) Akgun, C. O.; Kayaci, F.; Donmez, I.; Uyar, Tamer; Bıyıklı, Necmi
Aluminum nitride (AlN) hollow nanofibers were synthesized via plasma-enhanced atomic layer deposition using sacrificial electrospun polymeric nanofiber templates having different average fiber diameters (~70, ~330, and ~740 nm). Depositions were carried out at 200°C using trimethylaluminum and ammonia precursors. AlN-coated nanofibers were calcined subsequently at 500°C for 2 h to remove the sacrificial polymeric nanofiber template. SEM studies have shown that there is a critical wall thickness value depending on the template's average fiber diameter for AlN hollow nanofibers to preserve their shapes after the template has been removed by calcination. Best morphologies were observed for AlN hollow nanofibers prepared by depositing 800 cycles (corresponding to ~69 nm) on nanofiber templates having ~330 nm average fiber diameter. TEM images indicated uniform wall thicknesses of ~65 nm along the fiber axes for samples prepared using templates having ~70 and ~330 nm average fiber diameters. Synthesized AlN hollow nanofibers were polycrystalline with a hexagonal crystal structure as determined by high-resolution TEM and selected area electron diffraction. Chemical compositions of coated and calcined samples were studied using X-ray photoelectron spectroscopy (XPS). High-resolution XPS spectra confirmed the presence of AlN. © 2012 The American Ceramic Society.
Open Access
Transformation of polymer-ZnO core-shell nanofibers into ZnO hollow nanofibers: Intrinsic defect reorganization in ZnO and its influence on the photocatalysis
(Elsevier, 2015) Kayaci, F.; Vempati S.; Ozgit Akgun, C.; Donmez, I.; Bıyıklı, Necmi; Uyar, Tamer
Photocatalytic activity (PCA) on semiconductors is known to be majorly influenced by specific surface area and intrinsic lattice defects of the catalyst. In this report, we tested the efficiencies of 1D ZnO catalysts of varying fiber diameter (80. nm and 650. nm of inner diameter) in two formats, viz. core-shell and hollow nanofibers, where the former is calcined to yield the latter. These nanofibrous catalysts were produced by combining electrospinning and atomic layer deposition processes which were then subjected to thorough characterization including photoluminescence (PL) unveiling the details of intrinsic defects/densities. During the thermal treatment, intrinsic defects are reorganized and as a result a new PL band is observed apart from some significant changes in the intensities of other emissions. The densities of various intrinsic defects from PL are compared for all samples and juxtaposed with the PCA. Careful scrutiny of the various results suggested an anti-correlation between surface area and PCA; i.e., higher surface area does not necessarily imply better PCA. Beyond a limit, the most deterministic factor would be the density of surface defects rather than the specific surface area. The results of this study enable the researchers to fabricate 1D semiconductor photocatalysts while striking the balance between surface area and density of defects.