Paraboloid Structured Silicon Surface for Enhanced Light Absorption: Experimental and Simulative Investigations

Khan, Firoz,Baek, Seong-Ho,Kaur, Jasmeet,Fareed, Imran,Mobin, Abdul,Kim, Jae Hyun Springer US 2015 NANOSCALE RESEARCH LETTERS Vol.10 No.1

<P>In this paper, we present an optical model that simulates the light trapping and scattering effects of a paraboloid texture surface first time. This model was experimentally verified by measuring the reflectance values of the periodically textured silicon (Si) surface with the shape of a paraboloid under different conditions. A paraboloid texture surface was obtained by electrochemical etching Si in the solution of hydrofluoric acid, dimethylsulfoxide (DMSO), and deionized (DI) water. The paraboloid texture surface has the advantage of giving a lower reflectance value than the hemispherical, random pyramidal, and regular pyramidal texture surfaces. In the case of parabola, the light can be concentrated in the direction of the Si surface compared to the hemispherical, random pyramidal, and regular pyramidal textured surfaces. Furthermore, in a paraboloid textured surface, there can be a maximum value of 4 or even more by anisotropic etching duration compared to the hemispherical or pyramidal textured surfaces which have a maximum <I>h</I>/<I>D</I> (depth and diameter of the texture) value of 0.5. The reflectance values were found to be strongly dependent on the <I>h</I>/<I>D</I> ratio of the texture surface. The measured reflectance values were well matched with the simulated ones. The minimum reflectance value of ~4 % was obtained at a wavelength of 600 nm for an <I>h</I>/<I>D</I> ratio of 3.75. The simulation results showed that the reflectance value for the <I>h</I>/<I>D</I> ratio can be reduced to ~0.5 % by reducing the separations among the textures. This periodic paraboloidal structure can be applied to the surface texturing technique by substituting with a conventional pyramid textured surface or moth-eye antireflection coating.</P>

One-step and controllable bipolar doping of reduced graphene oxide using TMAH as reducing agent and doping source for field effect transistors

Khan, Firoz,Baek, Seong-Ho,Kim, Jae Hyun Elsevier 2016 Carbon Vol.100 No.-

<P>Simultaneous reduction and doping of the graphene oxide (GO) is an important issue for low temperature processed flexible electronic devices. A low temperature method for reduction and ambipolar doping has been developed which yield the doped reduced GO with wide range of work function with a mass production using tetra-methyl ammonium hydroxide (TMAH). The doping type of obtained reduced GO is tuned with TMAH concentration. XPS analysis revealed that the graphitic N is converted to oxidized N with increase of TMAH concentration. The work function is tuned via wide range variation in the carrier concentration in neutral (rGO-A, 4.46 eV), n-type (rGO-B, 3.90 eV) and p-type (rGO-C, 5.29 eV) regimes. The obtained Dirac voltages of field effect devices are 1 V, 31 V and broken vertical bar 35 V with active layer of rGO-A, rGO-B and rGO-C, respectively. The n-type doping is due to incorporation of graphitic N, whereas, oxidized N acts as electron withdrawing group which causes p-type doping. (C) 2016 Elsevier Ltd. All rights reserved.</P>

Effect of Al Concentration on Photoluminescence Properties of Sol-Gel Derived Hydrogen Annealed ZnO

Firoz Khan,Sadia Ameen,Minwu Song,Mushahid Husain,Abdul Mobin,Hyung-Shik Shin 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.2

The effect of the Al/Zn atomic ratio on the photoluminescence properties of hydrogen annealed undoped and Al rich ZnO (AZO) films was studied. The Al/Zn atomic ratios in the AZO films were varied from 0to 40%. All the AZO films exhibited three peaks in the UV, green and red regions, whereas the undoped ZnO films had two peaks in the UV and green regions. The PL intensity in the UV and red regions increased with an increase in Al concentrations. The highest PL intensity in the UV region was observed in the 20% Al/Zn atomic ratio due to improvement in crystal quality which was also confirmed by XRD measurements. The PL emission in the red region was due to complex luminescent centers like (Vzn-Alzn)−. A blue shift was seen in the red region with the introduction of Al. The 20% AZO films obtained the strongest signal at ~420 cm−1, whereas no FTIR signal was observed at 420 cm−1 in undoped ZnO. The bond signature at ~420 cm−1 might be responsible for the highest PL intensity in NBE and red regions.

Correlation Between Reflectance and Photoluminescent Properties of Al-rich ZnO Nano-Structures

Firoz Khan,백성호,Nafis Ahmad,이건희,서태훈,서은경,김재현 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.3

Al rich zinc oxide nano-structured films were synthesized using spin coating sol-gel technique. The films were annealed in oxygen ambient in the temperature range of 200-700 °C. The structural, optical, and photoluminescence (PL) properties of the films were studied at various annealing temperatures using X-ray diffraction spectroscopy, field emission scanning electron microscopy, photoluminescence emission spectra measurement, and Raman and UV-Vis spectroscopy. The optical band gap was found to decrease with the increase of the annealing temperature following the Gauss Amp function due to the confinement of the exciton. The PL peak intensity in the near band region (INBE) was found to increase with the increase of the annealing temperature up to 600 °C, then to decrease fast to a lower value for the annealing temperature of 700 °C due to crystalline quality. The Raman peak of E2 (low) was red shifted from 118 cm-1 to 126 cm-1 with the increase of the annealing temperature. The intensity of the second order phonon (TA+LO) at 674 cm-1 was found to decrease with the increase of the annealing temperature. The normalized values of the reflectance and the PL intensity in the NBE region were highest for the annealing temperature of 600 °C. A special correlation was found between the reflectance at λ = 1000 nm and the normalized PL intensity in the green region due to scattering due to presence of grains.

Work function tuning and fluorescence enhancement of hydrogen annealed Ag-doped Al-rich zinc oxide nanostructures using a sol–gel process

Khan, Firoz,Baek, Seong-Ho,Lee, Jae Young,Kim, Jae Hyun Elsevier 2015 Journal of alloys and compounds Vol.647 No.-

<P><B>Abstract</B></P> <P>Effect of incorporation of Ag on the structural, optical, electrical, and fluorescence properties of sol–gel derived Al-rich zinc oxide (ZnO:Al:Ag) nanostructured films was studied. The E<SUB>g</SUB> of the film slightly decreased to a minimal value with Ag doping, and was found to be about 3.65 eV for R<SUB>Ag/Zn</SUB> = 1% from its initial value of 3.72 eV (R<SUB>Ag/Zn</SUB> = 0%). The WF sudden increased to a maximal value of 5.12 eV with Ag doping (for R<SUB>Ag/Zn</SUB> = 1%) from its initial value of 4.73 eV for R<SUB>Ag/Zn</SUB> = 0% due to substitution of Ag into Zn sites until saturation was achieved (R<SUB>Ag/Zn</SUB> = 1%). After more Ag doping, WF started to decrease and finally, reached a value of 4.81 eV for R<SUB>Ag/Zn</SUB> = 3% because of the formation of an impurity-defect energy level below the intrinsic Fermi level of ZnO. With Ag-doping, the current increased up to R<SUB>Ag/Zn</SUB> = 1% due to the increase in carrier density. For R<SUB>Ag/Zn</SUB> = 3% doping, the current density started to increase due to the influence of metallic Ag. The defective peak position was blue shifted, with increased Ag-doping, from 536 nm (R<SUB>Ag/Zn</SUB> = 1%) to 527 nm for R<SUB>Ag/Zn</SUB> = 2% due to the sizes of the Ag<SUP>+</SUP> and Zn<SUP>2+</SUP> ions. The FL defective peak intensity (I<SUB>D</SUB>) in the green region increased with the concentration of Ag used for doping, up to R<SUB>Ag/Zn</SUB> = 2%. The enhancement in the I<SUB>D</SUB> may be due to charge difference between the Zn<SUP>2+</SUP> ions, caused by Ag<SUP>+</SUP> ions.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Structural, optical, electrical, and fluorescence properties are studied. </LI> <LI> Various types of nanostructure are formed using heat treatment under hydrogen. </LI> <LI> The work function and optical band-gap are tuned via Ag doping. </LI> <LI> A 30 fold enhancement fluorescence is obtained with Ag doping. </LI> <LI> The shifting of the FL positions is due to the size of Ag<SUP>+</SUP> and Zn<SUP>2+</SUP> ions. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>The effect of incorporation of Ag doping on the structural, optical, electrical, and fluorescence properties of sol–gel derived Al-rich zinc oxide (ZnO:Al:Ag) nanostructured films was studied. By Ag-doping, the lowest R<SUB>λ</SUB> is blue shifted to R<SUB>Ag/Zn</SUB> = 2% and finally red shifted for R<SUB>Ag/Zn</SUB> = 3% due to variation of optical thickness of the film. The E<SUB>g</SUB> of the film slightly decreased to a minimal value with Ag doping, and was found to be about 3.65 eV for R<SUB>Ag/Zn</SUB> = 1% from its initial value of 3.72 eV (R<SUB>Ag/Zn</SUB> = 0%). A maximal value of 5.12 eV with Ag doping for R<SUB>Ag/Zn</SUB> = 1% from its initial value of 4.73 eV for R<SUB>Ag/Zn</SUB> = 0%. With The defective peak position was blue shifted, with increased Ag-doping, from 536 nm (R<SUB>Ag/Zn</SUB> = 1%) to 527 nm for R<SUB>Ag/Zn</SUB> = 2% due to the sizes of the Ag<SUP>+</SUP> and Zn<SUP>2+</SUP> ions. The FL defective peak intensity (I<SUB>D</SUB>) in the green region increased with the concentration of Ag used for doping, up to R<SUB>Ag/Zn</SUB> = 2%. The enhancement in the I<SUB>D</SUB> may be due to charge difference between the Zn<SUP>2+</SUP> ions, caused by Ag<SUP>+</SUP> ions.</P> <P>[DISPLAY OMISSION]</P>

Influence of oxygen vacancies on surface charge potential and transportation properties of Al-doped ZnO nanostructures produced via atomic layer deposition

Khan, Firoz,Baek, Seong-Ho,Kim, Jae Hyun Elsevier 2017 Journal of alloys and compounds Vol.709 No.-

<P><B>Abstract</B></P> <P>High quality Al-doped ZnO (AZO) films are advantageous for many applications. Their properties can be tuned by controlling the doping concentration and intrinsic defect density. In this work, high-quality AZO films have been synthesized using the atomic layer deposition (ALD) technique. Throughout the doping cycle and post-annealing treatments under various atmospheres, the oxygen-related vacancies were controlled. The effect of oxygen vacancies on the charge transportation and surface potential were studied. The <I>O 1s</I> X-ray photoelectron spectrometry (XPS) spectra of the AZO film were deconvoluted into three components related to the <I>O</I> <SUP> <I>2</I>−</SUP> species, corresponding to the oxygen in the ZnO lattice (<I>O</I> <SUB> <I>L</I> </SUB>); oxygen vacancies or defects (<I>O</I> <SUB> <I>V</I> </SUB>); and chemisorbed or dissociated (<I>O</I> <SUB> <I>C</I> </SUB>) oxygen species. In the case of the as-deposited films and films annealed under various atmospheres, a co-relationship between the <I>O</I> <SUB> <I>V</I> </SUB> and mobility (<I>μ</I>) can be determined. In the case of the N<SUB>2</SUB>-annealed film, the <I>O</I> <SUB> <I>V</I> </SUB> fraction is at its maximum value, while the other components are at their minimum values. Among the as-deposited films, the maximum <I>O</I> <SUB> <I>V</I> </SUB> fraction is obtained when a Al:Zn ratio (<I>R</I> <SUB> <I>Al/Zn</I> </SUB>) of 7% is used. When <I>R</I> <SUB> <I>Al/Zn</I> </SUB> = 7%, the <I>μ</I> value of the as-deposited AZO film is enhanced from 12.1 cm<SUP>2</SUP>V<SUP>−1</SUP>s<SUP>−1</SUP> (<I>R</I> <SUB> <I>Al/Zn</I> </SUB> = 3%), to 18.5 cm<SUP>2</SUP>V<SUP>−1</SUP>s<SUP>−1</SUP>. It shows its potential application as photoanode. The Hall Effect and the XPS analysis of the film reviled a co-relationship between <I>O</I> <SUB> <I>V</I> </SUB> and <I>μ</I> with the Al-doping concentration or post annealing atmosphere. Kelvin probe atomic force microscopy (KPFM) was used to evaluate the surface charge potentials of the films. The N<SUB>2</SUB>-annealed <I>AZO</I> film with <I>R</I> <SUB> <I>Al/Zn</I> </SUB> = 3% (<I>AZO-3-N</I> <SUB> <I>2</I> </SUB> <I>)</I> exhibited the maximum negative potential (−115.79 mV); however, the film with <I>R</I> <SUB> <I>Al/Zn</I> </SUB> = 7% (<I>AZO-7-N</I> <SUB> <I>2</I> </SUB> <I>)</I> exhibited the maximum positive potential (797.23 mV). Hence, the properties of these films may directly pertain to the bit readout signal and reliability of charge storage and memory applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Al-doped ZnO (AZO) films are synthesized via atomic layer deposition technique. </LI> <LI> The influence of oxygen vacancies on the surface potential is studied. </LI> <LI> <I>AZO-3-N</I> <SUB> <I>2</I> </SUB> film exhibited a maximum negative potential of −115.79 mV. </LI> <LI> <I>AZO-7-N</I> <SUB> <I>2</I> </SUB> film exhibited a maximum positive potential of 797.23 mV. </LI> <LI> A co-relationship between <I>O</I> <SUB> <I>V</I> </SUB> and <I>μ</I> is observed among as deposited or annealed films. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Significance of N-moieties in regulating the electrochemical properties of nano-porous graphene: Toward highly capacitive energy storage devices

Khan, Firoz,Kim, Jae Hyun Elsevier 2018 Journal of industrial and engineering chemistry Vol.68 No.-

<P><B>Abstract</B></P> <P>The effects of N doping concentration and dopant moieties on the electrochemical properties of nanoporous graphene and their dependence on annealing temperature are investigated. Four types of N moieties – amide, amine, graphitic-N, and oxidized-N – are obtained, which transformed into pyridinic-N and pyrrolic-N upon annealing. The diffusion coefficient (D′) of the ions in the electrode is the maximum at 400°C because of a high level of N doping, whereas the second highest D′ value is obtained at 700°C owing to a high level of reduction and N doping. The highest specific capacitance is obtained for the sample annealed at 400°C.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A novel method is developed for synthesis of N-doped nano-porous graphene. </LI> <LI> Populations of the N bonding species are established for annealing temperatures. </LI> <LI> The electrochemical properties strongly depend on N moieties type and concentration. </LI> <LI> The highest value of the diffusion coefficient is obtained at 400°C. </LI> <LI> The highest specific capacitance is obtained for 400°C (∼352F/g at 2mV/s). </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Significance of N-moieties in regulating the electrochemical properties of nano-porous graphene: Toward highly capacitive energy storage devices

Firoz Khan,김재현 한국공업화학회 2018 Journal of Industrial and Engineering Chemistry Vol.68 No.-

The effects of N doping concentration and dopant moieties on the electrochemical properties of nanoporous graphene and their dependence on annealing temperature are investigated. Four types of N moieties – amide, amine, graphitic-N, and oxidized-N – are obtained, which transformed into pyridinic-N and pyrrolic-N upon annealing. The diffusion coefficient (D′) of the ions in the electrode is the maximum at 400 °C because of a high level of N doping, whereas the second highest D′ value is obtained at 700 °C owing to a high level of reduction and N doping. The highest specific capacitance is obtained for the sample annealed at 400 °C.

N-functionalized graphene quantum dots: Charge transporting layer for high-rate and durable Li₄Ti₅O₁₂-based Li-ion battery

Khan, Firoz,Oh, Misol,Kim, Jae Hyun Elsevier 2019 Chemical engineering journal Vol.369 No.-

<P><B>Abstract</B></P> <P>Spinel Li<SUB>4</SUB>Ti<SUB>5</SUB>O<SUB>12</SUB> can replace carbon in Li-ion battery anodes due to its high voltage, preventing decomposition of the electrolyte and formation of Li metal dendrites. However, Li<SUB>4</SUB>Ti<SUB>5</SUB>O<SUB>12</SUB> has a low electronic conductivity and Li-ion diffusion coefficient, limiting its charge/discharge properties at high rate capacities, and also suffers from gassing during cycling. Here, we used N-functionalized graphene quantum dots interfacial layer, which (1) protects Li<SUB>4</SUB>Ti<SUB>5</SUB>O<SUB>12</SUB> from ambient degradation, (2) forms a thin and smooth solid-electrolyte interphase layer on the Li<SUB>4</SUB>Ti<SUB>5</SUB>O<SUB>12</SUB> surface, (3) acts as a charge transfer layer, (4) protects the Li<SUB>4</SUB>Ti<SUB>5</SUB>O<SUB>12</SUB> electrode from reactions with the electrolyte, and (5) suppresses gassing during cycling. Consequently, the Li-ion diffusion coefficient increased by ∼19%. The effectiveness of the N-functionalized graphene quantum dots is manifested in the specific capacity of 161 mAh/g at 50C, which is improved by ∼23% compared to pure Li<SUB>4</SUB>Ti<SUB>5</SUB>O<SUB>12</SUB> electrode and maintained for over 500 cycles. Unlike graphene, N-functionalized graphene quantum dots themselves work as a stable charge transporting and protecting layer. Our strategy successfully obtained a good cycling performance and long cycling life of Li<SUB>4</SUB>Ti<SUB>5</SUB>O<SUB>12</SUB> at high C-rates.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A novel strategy was presented to enhance the performance of LTO electrode. </LI> <LI> N-GQDs were acted as protecting layer on LTO surface from interfacial reactions. </LI> <LI> The Li-ion diffusion coefficient was enhanced by ∼19%. </LI> <LI> The capacity was enhanced by ∼23% at 50C via encapsulation of LTO by N-GQDs. </LI> <LI> N-GQDs can suppress the gassing during the cycling process. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Analgesic and antidiarrhoeal activities of Treama orientalis Linn. in mice

Uddin, Sarder Nasir,Uddin, Khan Mohammad Ahsan,Ahmed, Firoz Kyung Hee Oriental Medicine Research Center 2008 Oriental pharmacy and experimental medicine Vol.8 No.2

Trema orientalis Linn. is commonly grown in many parts of Bangladesh. Its leaves have been used for analgesic and anti-diarrhoeal activity in traditional medicine. This study evaluates the potential analgesic and anti-diarrhoeal activity of methanol and aqueous extracts of leaves in experimental acetic acid induced writhing and castor oil induced diarrhoea in mice. The aqueous extract of leaves showed significant (P < 0.001) analgesic effect in acetic acid induced writhing in mice at a dose of 500 mg/kg body weight In castor oil induced antidiarrhoeal screening both extract increased latent period (P < 0.025) and decrease the number of stool (P < 0.025) at the dose of 500 mg/kg body weight comparable with that of the standard drug loperamide. The results provide a support for the use of this plant in traditional medicine and suggest its further investigation.