Optical
2020/11
Ashish Chhaganlal Gandhi, Chi-Yuan Lai, Kuan-Ting Wu, P. V. R. K. Ramacharyulu, Valmiki B. Koli, Chia-Liang Cheng, Shyue-Chu Ke and Sheng Yun Wu
Nanoscale, 2020, Advance Article
We report the grain growth from the nanoscale to microscale and a transformation sequence from Bi → β-Bi2O3 → γ-Bi2O3 → α-Bi2O3 with the increase of annealing temperature. The room temperature (RT) stabilization of β-Bi2O3 nanoparticles (NPs) was attributed to the effect of reduced surface energy due to adsorbed carbon species, and oxygen vacancy defects may have played a significant role in the RT stabilization of γ-Bi2O3 NPs. An enhanced red emission band was evident from all the samples attributed to oxygen-vacancy defects formed during the growth process in contrast with the observed white emission band from the air annealed Bi ingots. Based on our experimental findings, the air annealing induced oxidation of Bi NPs and transformation mechanism within various Bi2O3 nano-polymorphs are presented. The outcome of this study suggests that oxygen vacancy defects at the nanoscale play a significant role in both structural stabilization and phase transformation within various Bi2O3 nano-polymorphs, which is significant from theoretical consideration.
Full Paper: https://pubs.rsc.org/en/content/articlehtml/2020/nr/d0nr06552h
2020/09
Ashish Chhaganlal Gandhi, Hsin-Hao Chiu, Kuan-Ting Wu, Chia-Liang Cheng, and Sheng Yun Wu
Appl. Surf. Sci. ,147856, 2020
We report a possible route to achieve room temperature (RT) magnetic memory effect (MME) simply by thermal treatment from barely 1 % Fe-substituted NiO nanoparticles. The post-annealing of Ni0.99Fe0.01O nanoparticles leads to the formation of a Fe-rich NiO surface, resulting in an enhanced collective spin behavior because of the increase in interparticle interaction. This surface-spin driven collective spin behavior provided additional anisotropic energy leading to the occurrence of RT MME. This study demonstrates an approach to tune the magnitude of MME simply by controlling the post-annealing temperature for 1 % Fe-dopant concentration. The proposed method opens a route for implementing RT spintronic devices and a deeper understanding of their fundamental physics.
Full Paper: https://www.sciencedirect.com/science/article/abs/pii/S0169433220326131
2020/07
Ashish Chhaganlal Gandhi, Chia-Liang Cheng, and Sheng Yun Wu
Nanomaterial 10(6), 1023, 2020
We report the synthesis of room temperature (RT) stabilized γ–Bi2O3 nanoparticles (NPs) at the expense of metallic Bi NPs through annealing in an ambient atmosphere. RT stability of the metastable γ–Bi2O3 NPs is confirmed using synchrotron radiation powder X-ray diffraction and Raman spectroscopy. γ–Bi2O3 NPs exhibited a strong red-band emission peaking at ~701 nm, covering 81% integrated intensity of photoluminescence spectra. Our findings suggest that the RT stabilization and enhanced red-band emission of γ‒Bi2O3 is mediated by excess oxygen ion vacancies generated at the octahedral O(2) sites during the annealing process.
Full Paper: https://www.mdpi.com/2079-4991/10/6/1023
2019/01
Ashish Chhaganlal Gandhi, Ching-Hao Liao, Wei-Li yeh, Yue-Lin Huang
RSC Advacnes 7(3), 031111, 2019
The size dependence of room-temperature photoluminescence (PL) accompanied with near-band-edge emission (NBE) and defect-associated green emission (GE) was investigated using high-quality crystalline nanostructured ZnO thin films with grain sizes varying from 29 nm to 2 nm. The size dependence of correlated intensities of the PL bands was pursued in correlation with structural and defect evolution revealed by X-ray photoelectron spectroscopy (XPS) and previous studies of XRD and Raman scattering. In contrast to the influence of thermally activated reconstruction and changes in defect densities, quantum size effects emerging at grain sizes below a critical value, i.e., dc ∼ 10 nm were inspected in relation to the observed blueshift in the bandgap and correlated variations in the size dependence of the intensity of NBE and GE. Taking into account the geometry-modelled relative emission efficiency, (i) the observed overall linear size dependence of the relative intensity INBE/IGE was consistent with assuming a 1.05 nm-thick GE-active surface layer, and (ii) a local maximum of INBE/IGE emerging near grain radius R ≈ 4 nm was identified in relation to the theoretically predicted local minimum in size-dependent exciton radiative lifetime due to the intrinsic quantum nature of excitons confined in ZnO. Our results have provided new insights into non-monotonous size-dependent PL of ZnO, which can benefit future photoelectronic device design by taking advantage of the size-controlled stability of confined excitons in nanostructured thin films and luminescent quantum dots.
Full Paper: https://pubs.rsc.org/ko/content/articlehtml/2019/ra/c8ra09370a
2018/08
Ashish Chhaganlal Gandhi, Wei-Shan Yeoh, Ming-An Wu, Ching-Hao Liao, Dai-Yao Chiu, Wei-Li Yeh, Yue-Lin. Huang
Nanomaterials 8(8), 632, 2018
High-quality crystalline nanostructured ZnO thin films were grown on sapphire substrates by reactive sputtering. As-grown and post-annealed films (in air) with various grain sizes (2 to 29 nm) were investigated by scanning electron microscopy, X-ray diffraction, and Raman scattering. The electron–phonon coupling (EPC) strength, deduced from the ratio of the second-to the first-order Raman scattering intensity, diminished by reducing the ZnO grain size, which mainly relates to the Fröhlich interactions. Our finding suggests that in the spatially quantum-confined system the low polar nature leads to weak EPC. The outcome of this study is important for the development of nanoscale high-performance optoelectronic devices. View Full-Text
Full Paper: https://www.mdpi.com/2079-4991/8/8/632
2018/01
Swati R. Gawali, Ashish Chhaganlal Gandhi, Shrikrushna Shivaji Gaikwad, Jayashree Pant, Ting-Shan Chan, Chia-Liang Cheng, Yuan-Ron Ma & Sheng Yun Wu*
Scientific Reports 8(1), 249, 2018
We report the phonon and magnetic properties of various well-stabilized Co3O4 nanoparticles. The net valence in cobalt (II)/(III) cation can be obtained by subtracting the Co2+ ions in tetrahedral interstices and Co3+ ions in the octahedral interstices, respectively, which will possess spatial inhomogeneity of its magnetic moment via Co2+ in tetrahedra and Co3+ in octahedral configurations in the normal spinel structure. Furthermore, the distribution of Co2+/Co3+ governed by various external (magnetic field and temperature) and internal (particle size and slightly distorted CoO6 octahedra) sources, have led to phenomena such as a large redshift of phonon-phonon interaction and short-range magnetic correlation in the inverse spinel structure. The outcome of our study is important in terms of the future development of magnetic semiconductor spintronic devices of Co3O4.
Full Paper: https://www.nature.com/articles/s41598-017-18563-9
2017/08
Ashish Chhaganlal Gandhi and Sheng Yun Wu
Nanomaterials 7(8), 231, 2017
Nickel oxide is one of the highly promising semiconducting materials, but its large band gap (3.7 to 4 eV) limits its use in practical applications. Here we report the effect of nickel/oxygen vacancies and interstitial defects on the near-band-edge (NBE) and deep-level-emission (DLE) in various sizes of nickel oxide (NiO) nanoparticles. The ultraviolet (UV) emission originated from excitonic recombination corresponding near-band-edge (NBE) transition of NiO, while deep-level-emission (DLE) in the visible region due to various structural defects such as oxygen vacancies and interstitial defects. We found that the NiO nanoparticles exhibit a strong green band emission around~ 2.37 eV in all samples, covering 80% integrated intensity of PL spectra. This apparently anomalous phenomenon is attributed to photogenerated holes trapped in the deep level oxygen vacancy recombining with the electrons trapped in a shallow level located just below the conducting band. View Full-Text .
Full Paper: https://www.mdpi.com/2079-4991/7/8/231
2014/06
S S Gaikwad, A. C. Gandhi, S D. Pandit, J Pant, T S Chan, C L Cheng, Y R Ma, S Y Wu
Journal of Materials Chemistry C 2, 7264-7274, 2014
We report the influence of the nanosized effect on the optical properties of non-centrosymmetric ZnO nanoparticles. In this study confocal Raman scattering was employed to investigate the strain effect of the softening A1(LO) phonon mode while controlling particle sizes from 55 ± 1 to 32 ± 1 nm. The observations reveal a positive Poisson ratio between the compressive- and tensile-strain. The intensity ratio of the A1(2LO)/A1(LO) modes exhibits strong size dependence. As the particle sizes decrease further, the ratio decreases rapidly, signaling the short-range electron–phonon coupling effect which confines the electrons and holes within a smaller volume. An energy red-shift in the photoluminescence peak was observed, because of a lowering in the strain of local symmetry at O2− sites caused by excess oxygen, and by strong coupling between the electron and phonon vibration. The variation of the bandgap is very sensitive to the electron–phonon coupling and the distinct size effect of strained ZnO nanoparticles.
Full Paper:https://pubs.rsc.org/en/content/articlehtml/2014/tc/c4tc00566j
2013/08
A C Gandhi, J Pant, S D Pandit, S K Dalimbkar, T S Chan, C L Cheng, Y R Ma, S Y Wu
Journal of Physical Chemistry C 117(36), 18666-18674, 2013
Room-temperature confocal Raman scattering was used to probe the antiferromagnetic exchange energy J2 between the next-nearest-neighboring Ni ions in different sized NiO nanoparticles. NiO nanoparticles were synthesized using the sol–gel method and annealed at various temperatures ranging from 300 to 800 °C. The morphology and crystal structure of the prepared samples were characterized using SEM and XRD, revealing the uniformity and development of short-range crystallinity of NiO nanoparticles. This study shows that short spin correlation leads to an exponential dependency of particle size and the existence of Ni vacancies in two-magnon excitation. The Ni vacancy concentration plays an important role in lattice expansion, along with the size reduction effect. A direct comparison between short-range magnon excitation related to the Ni vacancy concentration and NiO nonstoichiometry has been made.
2011/12
Ashish Chhaganlal Gandhi, Chih-Yeh Huang, Chun Chuen Yang, Ting Shan Chan, Chia-Liang Cheng, Yuan-Ron Ma, Sheng Yun Wu
Nanoscale research letters 6(1), 485, 2011
The nanosized effects of short-range multimagnon excitation behavior and short-circuit diffusion in NiO nanowalls synthesized using the Ni grid thermal treatment method were observed. The energy dispersive spectroscopy mapping technique was used to characterize the growth mechanism, and confocal Raman scattering was used to probe the antiferromagnetic exchange energy J 2 between next-nearest-neighboring Ni ions in NiO nanowalls at various growth temperatures below the Neel temperature. This study shows that short spin correlation leads to an exponential dependence of the growth temperatures and the existence of nickel vacancies during the magnon excitation. Four-magnon configurations were determined from the scattering factor, revealing a lowest state and monotonic change with the growth temperature.
Full Paper:https://link.springer.com/article/10.1186/1556-276X-6-485
2010/01
Ashish C Gandhi, Hsuan-Jung Hung, Po-Hsun Shih, Chia-Liang Cheng, Yuan-Ron Ma, ShengYun Wu
Nanoscale research letters 5(3), 581-586, 2010
In this work, we succeeded in preparing in-plane zinc oxide nanowires using a Ti-grid assisted by the chemical vapor deposition method. Optical spatial mapping of the Confocal Raman spectra was used to investigate the phonon and geometric properties of a single ZnO nanowire. The local optical results reveal a red shift in the non-polar E2 high frequency mode and width broadening along the growth direction, reflecting quantum-confinement in the radial direction.
Full Paper:https://nanoscalereslett.springeropen.com/articles/10.1007/s11671-009-9509-1
