Aashir Waleed
Hong Kong University of Science and Technology
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Publication
Featured researches published by Aashir Waleed.
Advanced Materials | 2016
Leilei Gu; Mohammad Mahdi Tavakoli; Daquan Zhang; Qianpeng Zhang; Aashir Waleed; Yiqun Xiao; Kwong Hoi Tsui; Yuanjing Lin; Lei Liao; Jiannong Wang; Zhiyong Fan
Large-scale and highly ordered 3D perov-skite nanowire (NW) arrays are achieved in nanoengineering templates by a unique vapor-solid-solid reaction process. The excellent material properties, in conjunction with the high integration density of the NW arrays, make them promising for 3D integrated nanoelectronics/optoelectronics. Image sensors with 1024 pixels are assembled and characterized to demonstrate the technological potency.
Nano Letters | 2017
Aashir Waleed; Mohammad Mahdi Tavakoli; Leilei Gu; Zi-Yi Wang; Daquan Zhang; Arumugam Manikandan; Qianpeng Zhang; Rong-Jun Zhang; Yu-Lun Chueh; Zhiyong Fan
Organometal halide perovskite materials have triggered enormous attention for a wide range of high-performance optoelectronic devices. However, their stability and toxicity are major bottleneck challenges for practical applications. Substituting toxic heavy metal, that is, lead (Pb), with other environmentally benign elements, for example, tin (Sn), could be a potential solution to address the toxicity issue. Nevertheless, even worse stability of Sn-based perovskite material than Pb-based perovskite poses a great challenge for further device fabrication. In this work, for the first time, three-dimensional CH3NH3SnI3 perovskite nanowire arrays were fabricated in nanoengineering templates, which can address nanowire integration and stability issues at the same time. Also, nanowire photodetectors have been fabricated and characterized. Intriguingly, it was discovered that as the nanowires are embedded in mechanically and chemically robust templates, the material decay process has been dramatically slowed down by up to 840 times, as compared with a planar thin film. This significant improvement on stability can be attributed to the effective blockage of diffusion of water and oxygen molecules within the templates. These results clearly demonstrate a new and alternative strategy to address the stability issue of perovskite materials, which is the major roadblock for high-performance optoelectronics.
Nano Letters | 2017
Aashir Waleed; Mohammad Mahdi Tavakoli; Leilei Gu; Shabeeb Hussain; Daquan Zhang; Swapnadeep Poddar; Zi-Yi Wang; Rong-Jun Zhang; Zhiyong Fan
Alluring optical and electronic properties have made organometallic halide perovskites attractive candidates for optoelectronics. Among all perovskite materials, inorganic CsPbX3 (X is halide) in black cubic phase has triggered enormous attention recently owing to its comparable photovoltaic performance and high stability as compared to organic and hybrid perovskites. However, cubic phase stabilization at room temperature for CsPbI3 still survives as a challenge. Herein we report all inorganic three-dimensional vertical CsPbI3 perovskite nanowires (NWs) synthesized inside anodic alumina membrane (AAM) by chemical vapor deposition (CVD) method. It was discovered that the as-grown NWs have stable cubic phase at room temperature. This significant improvement on phase stability can be attributed to the effective encapsulation of NWs by AAM and large specific area of these NWs. To demonstrate device application of these NWs, photodetectors based on these high density CsPbI3 NWs were fabricated demonstrating decent performance. Our discovery suggests a novel and practical approach to stabilize the cubic phase of CsPbI3 material, which will have broad applications for optoelectronics in the visible wavelength range.
ACS Applied Materials & Interfaces | 2016
Shabeeb Hussain; Shazia Hussain; Aashir Waleed; Mohammad Mahdi Tavakoli; Zilong Wang; Shihe Yang; Zhiyong Fan; Muhammad Arif Nadeem
Recently, photoelectrochemical conversion (PEC) of water into fuel is attracting great attention of researchers due to its outstanding benefits. Herein, a systematic study on PEC of water using CuFe2O4/ α-Fe2O3 composite thin films is presented. CuFe2O4/ α-Fe2O3 composite thin films were deposited on two different substrates; (1) planner FTO glass and (2) 3-dimensional nanospike (NSP). The films on both substrates were characterized and tested as anode material for photoelectrochemical water splitting reaction. During PEC studies, it was observed that the ratio between two components of composite is crucial and highest PEC activity results were achieved by 1:1 component ratio (CF-1) of CuFe2O4 and α-Fe2O3. The CF-1 ratio sample deposited on planar FTO substrate provided a photocurrent density of 1.22 mA/cm2 at 1.23 VRHE which is 1.9 times higher than bare α-Fe2O3 sample. A significant PEC activity outperformance was observed when CF-1 ratio composite thin films were deposited on 3D NSP. The highest photocurrent density of 2.26 mA/cm2 at 1.23 VRHE was achieved for 3D NSP sample which is around 3.6 times higher than photocurrent density generated by α-Fe2O3 thin film only. The higher photocurrent densities of 3D nanostructured devices compared to planar one are attributed to the enhanced light trapping and increased surface area for photoelectrochemical water oxidation on the surface. The difference between valence and conduction bands of CuFe2O4 and α-Fe2O3 allows better separation of photogenerated electrons and holes at the CuFe2O4/ α-Fe2O3 interface which makes it more active for photoelectrochemical water splitting.
Langmuir | 2018
Shabeeb Hussain; Mohammad Mahdi Tavakoli; Aashir Waleed; Umar Siddique Virk; Shihe Yang; Amir Waseem; Zhiyong Fan; Muhammad Arif Nadeem
We demonstrate for the first time the application of p-NiFe2O4/n-Fe2O3 composite thin films as anode materials for light-assisted electrolysis of water. The p-NiFe2O4/n-Fe2O3 composite thin films were deposited on planar fluorinated tin oxide (FTO)-coated glass as well as on 3D array of nanospike (NSP) substrates. The effect of substrate (planar FTO and 3D-NSP) and percentage change of each component (i.e., NiFe2O4 and Fe2O3) of composite was studied on photoelectrochemical (PEC) water oxidation reaction. This work also includes the performance comparison of p-NiFe2O4/n-Fe2O3 composite (planar and NSP) devices with pure hematite for PEC water oxidation. Overall, the nanostructured p-NiFe2O4/n-Fe2O3 device with equal molar 1:1 ratio of NiFe2O4 and Fe2O3 was found to be highly efficient for PEC water oxidation as compared with pure hematite, 1:2 and 1:3 molar ratios of composite. The photocurrent density of 1:1 composite thin film on planar substrate was equal to 1.07 mA/cm2 at 1.23 VRHE, which was 1.7 times higher current density as compared with pure hematite device (0.63 mA/cm2 at 1.23 VRHE). The performance of p-NiFe2O4/n-Fe2O3 composites in PEC water oxidation was further enhanced by their deposition over 3D-NSP substrate. The highest photocurrent density of 2.1 mA/cm2 at 1.23 VRHE was obtained for the 1:1 molar ratio p-NiFe2O4/n-Fe2O3 composite on NSP (NF1-NSP), which was 3.3 times more photocurrent density than pure hematite. The measured applied bias photon-to-current efficiency (ABPE) value of NF1-NSP (0.206%) was found to be 1.87 times higher than that of NF1-P (0.11%) and 4.7 times higher than that of pure hematite deposited on FTO-coated glass (0.044%). The higher PEC water oxidation activity of p-NiFe2O4/n-Fe2O3 composite thin film as compared with pure hematite is attributed to the Z-path scheme and better separation of electrons and holes. The increased surface area and greater light absorption capabilities of 3D-NSP devices result in further improvement in catalytic activities.
Quantum Dots and Nanostructures: Growth, Characterization, and Modeling XV | 2018
Mohammad Mahdi Tavakoli; Daquan Zhang; Zhiyong Fan; Leilei Gu; Aashir Waleed; Qianpeng Zhang
Metal halide perovskite materials are emerging as highly promising materials for high performance optoelectronic devices thus triggered broad attention. Nanostructured perovskite materials have wide applications in nanoelectronics and nano-optoelectronics. Due to incompatibility of metal halide perovskite materials with conventional lithography techniques, it is preferable to achieve nano-perovskite material growth and assembly at the same time for further device applications. In our work, we have developed a chemical vapor deposition (CVD) process to grow ordered three-dimensional (3-D) metal halide nanowire (NW) arrays in nanoengineering templates. This unique CVD process utilizes metal nanoclusters at the bottom of vertical nanochannels to initiate high quality NW growth. As the nanochannels have largely controllable geometrical factors, namely, periodicity, diameter and depth, NW geometry can also be precisely nanoengineered. As the result, the ordered 3-D NW arrays can achieve ultra-high NW density in the range of ~109/cm2. The 3-D NW arrays are conspicuously promising for 3-D integrated nano-electronics/optoelectronics. To further demonstrate the technological potency of the perovskite NW arrays, they have been fabricated into photodetectors and proof-of-concept image sensors. Each image sensor consists of 1,024 photodiode pixels made of vertical perovskite NWs, and the imaging functionality has been verified by recognizing various optical patterns projected on the sensor. In addition, we have also discovered that the chemically and mechanically robust template can effectively protect perovskite NWs from water and oxygen invasion thus the material stability is significantly better than planar perovskite films confirmed by photoluminescence and photoelectric measurements.
ACS Applied Materials & Interfaces | 2017
Ligang Wang; Yuan Huang; Aashir Waleed; Ke Wu; Cong Lin; Zhengxu Wang; Guanhaojie Zheng; Zhiyong Fan; Junliang Sun; Huanping Zhou; Ling-Dong Sun; Chun-Hua Yan
Among the various building blocks beyond polycrystalline thin films, perovskite wires have attracted extensive attention for potential applications including nanolasers, waveguides, field-effect transistors, and more. In this work, millimeter-scale lead iodine-based perovskite wires employing various A-site substitutions, namely, Cs, methylammonium (MA), and formamidinium (FA), have been synthesized via a new type solution method with nearly 100% yield. All of the three millimeter scale perovskite wires (MPWs) compositions exhibit relatively high quality, and CsPbI3 is proven to be monocrystalline along its entire length. Furthermore, the growth thermodynamics of the APbI3 MPWs with respect to A-site cation effect were studied thoroughly by various characterization techniques. Finally, single MPW photodetectors have been fabricated utilizing the APbI3 MPWs for studying the photoconductive properties, which show different sensitivities under illumination. This systematic synthesis method of solution-processed APbI3 (Cs, MA, and FA) MPWs reveals a wide spectrum of additives with different coordination capability that mediates perovskite materials growth. It proved to serve as a new parameter that further aids in the rational process of the polycrystalline organic/inorganic hybrids materials. These MPWs also have the potential to open up new opportunities for integrated nanoelectronics ranging from the nanometer through millimeter length scales.
Advanced Materials Interfaces | 2016
Mohammad Mahdi Tavakoli; Rouhollah Tavakoli; Zahra Nourbakhsh; Aashir Waleed; Umar Siddique Virk; Zhiyong Fan
Nanoscale | 2017
Mohammad Mahdi Tavakoli; Aashir Waleed; Leilei Gu; Daquan Zhang; Rouhollah Tavakoli; Bingbing Lei; Wenjun Su; Fang Fang; Zhiyong Fan
Chinese Science Bulletin | 2016
Aashir Waleed; Qianpeng Zhang; Mohammad Mahdi Tavakoli; Siu-Fung Leung; Leilei Gu; Jin He; Xiaoliang Mo; Zhiyong Fan