Core-shell Microrods Research Articles

2D Co-N-C catalysts derived by 1D CoxZn1-xO hexagonal microrods for oxygen reduction reaction

A novel and feasible strategy to construct 2D Co-N-C catalyst from one-dimensional (1D) CoxZn1-xO microrod is reported. In the strategy, 1D CoxZn1-xO microrods are firstly transformed into CoxZn1-xO@CoZn-ZIF hexagonal core-shell microrods by regulating the reaction conditions of high-pressure vapor-solid transformation, and subsequently pyrolyzed at high temperature to form Co-N-C hexagonal microtubes, which are finally fractured along its edges under the action of thermal stress, evolving gradually to Co-N-C nanosheets. Besides, by investigating the effects of Co doping contents, a catalyst with optimal chemical and structural properties and more highly accessible Co-N4 active sites has achieved respectable activity and stability for the oxygen reduction reaction (ORR) in alkaline media. Among them, Co0.3-N-C NS exhibits superior ORR performance (Eon=1.01 V, E1/2 = 0.87 V), durability and tolerance to methanol than those of commercial Pt/C catalysts (Eon=0.99 V, E1/2 = 0.85 V) in alkaline media.

Room-temperature exciton-polariton and photonic lasing in GaN/InGaN core-shell microrods

Room-temperature exciton-polariton is observed in GaN/InGaN core–shell microrods grown by metal-organic vapor phase epitaxy. We demonstrate a large Rabi splitting in the core–shell microrod structure over 265 meV. Room-temperature lasing in core–shell microrods is confirmed by power-dependent photoluminescence spectra. The lasing in the shell layer results to a modulated lasing wavelength and takes one step further to more stable polariton lasing in MQW core–shell microrods.

Upconversion white-light emission luminescence characteristics based on single-particle NaYF<sub>4 </sub>microrod

White upconversion (UC) luminescent materials have shown incomparable advantages over other light sources in the fields of solid-state lighting, liquid crystal display, and bioimaging, and received extensive attention from researchers. In this work, a series of microcrystals doped with different ion concentrations is synthesized by hydrothermal method, such as NaYF<sub>4</sub>: Yb<sup>3+</sup>/Ho<sup>3+</sup>/Tm<sup>3+</sup> and NaYF<sub>4</sub>: Yb<sup>3+</sup>/Ho<sup>3+</sup>/Tm<sup>3+</sup>, and their corresponding micron core-shell (CS) structures are constructed based on epitaxial growth technology. The structure and morphology of the prepared microcrystals are characterised by X-ray diffractometer (XRD) and scanning electron microscope (SEM), showing that the microcrystal has a pure hexagonal-phase crystal structure with a rod-like shape. Under the excitation of 980 nm near-infrared laser, the white UC luminescence characteristics of Ho<sup>3+</sup>/Tm<sup>3+</sup> and Er<sup>3+</sup>/Tm<sup>3+</sup> co-doped single-particle NaYF<sub>4</sub> microcrystals are systematically studied by modulating the concentration of the doping ions. The study shows that in Ho<sup>3+</sup>/Tm<sup>3+</sup> co-doped NaYF<sub>4</sub> microcrystals, white UC luminescence can be easily achieved by modulating the concentration of Yb<sup>3+</sup> ions, while in the Er<sup>3+</sup>/Tm<sup>3+</sup> co-doped NaYF<sub>4</sub> microcrystal, the white UC luminescence can be effectively achieved by modulating the concentration of Er<sup>3+</sup> ions. According to the luminescence characteristics of the microncrystals in different doping systems, the physical mechanism of white light emission regulation is revealed, which is mainly due to the interaction between the doped ions, including cross relaxation (CR) process and energy back transfer (EBT) process. Meanwhile, an effective enhancement of the white UC luminescence on CS microrod is achieved by coating the NaYF<sub>4</sub> inert shell. Therefore, ion doping technique and the construction of CS structure can not only realize the white UC luminescence of microrods, but also provide important experimental reference for further enhancing the luminescence characteristics of microrods, and expand the applications of microcrystals in the fields of display, optoelectronics and anti-counterfeiting.

Progress and Challenges of InGaN/GaN-Based Core-Shell Microrod LEDs.

LEDs based on planar InGaN/GaN heterostructures define an important standard for solid-state lighting. However, one drawback is the polarization field of the wurtzite heterostructure impacting both electron–hole overlap and emission energy. Three-dimensional core–shell microrods offer field-free sidewalls, thus improving radiative recombination rates while simultaneously increasing the light-emitting area per substrate size. Despite those promises, microrods have still not replaced planar devices. In this review, we discuss the progress in device processing and analysis of microrod LEDs and emphasize the perspectives related to the 3D device architecture from an applications point of view.

Upconversion luminescence characteristics of Ho<sup>3+</sup> ion doped single-particle fluoride micron core-chell structure

Constructing core-shell structures can effectively reduce the surface quenching effect of luminescent materials, which becomes an effective method to enhance upconversion luminescence. In this work, a series of NaLnF<sub>4</sub>@NaLnF<sub>4</sub> (Ln = Y<sup>3+</sup>, Yb<sup>3+</sup>, Ho<sup>3+</sup>) core-shell microcrystals is successfully synthesized based on epitaxial growth technology, thereby enhancing and regulating the upconversion emission of Ho<sup>3+</sup> ions. The results of the XRD and SEM indicate that the NaLnF<sub>4</sub>@NaLnF<sub>4</sub> core-shell microcrystal possesses a pure hexagonal-phase crystal structure with a rod-like shape. Meanwhile, it is found that the epitaxial growth direction of the micro-shell is not affected by the crystal characteristics in the core, but determined by the crystal characteristics of the shell. Under 980 nm near-infrared laser excitation, the upconversion luminescence properties of single microrods with different core-shell structures are investigated via a confocal microscope spectroscopy. It is found that in the NaLnF<sub>4</sub> micro-crystal, the coated NaYF<sub>4</sub> inert shell can also effectively reduce the quenching effect on the surface of the micro-crystal for enhancing upconversion emission. When the Yb<sup>3+</sup> ions are introduced into NaYF<sub>4</sub> or NaYbF<sub>4</sub> active shell that is coated, the Yb<sup>3+</sup> ions in the shell can effectively transfer excitation energy to Yb<sup>3+</sup> in the core through energy migration, and then establish new energy transfer channels, thereby realizing the Ho<sup>3+</sup> ion luminescence enhancement. For NaHoF<sub>4</sub>@NaYbF<sub>4</sub> core-shell microrods, the Yb<sup>3+</sup> in the shell can transfer more excitation energy to Ho<sup>3+</sup> ions at the adjacent interface for enhancing the overall luminescence intensity, and its higher red-green ratio is mainly due to the cross-relaxation process occurring between the Ho<sup>3+</sup> ions at high doping concentration of Ho<sup>3+</sup> in the NaHoF<sub>4</sub> core. Meanwhile, the luminescence process of the micron core-shell system is further confirmed based on the luminescence characteristics of different structures and the dynamic luminescence process. It can be seen that constructing different micron core-shell structures and introducing sensitizing ions, can not only effectively enhance the luminous intensity of the micron materials, but also adjust the output color. Therefore, this research is an important experimental reference for enhancing the luminous intensity of the micron system and the precise adjustment of luminescence, and can effectively expand the applications of micron crystals in the fields of displays, micron lasers and anti-counterfeiting.

On-chip 3D confocal optical study of an InGaN/GaN microrod LED in the low excitation regime

In this work, we studied p-i-n InGaN/GaN core-shell microrod (μrod) LEDs using confocal microscopy with a spatial resolution below 500 nm in all three dimensions. At low excitation conditions, the devices emit in the red spectral range, while green and blue emissions become more prominent with increasing driving current. 3D photoluminescence (PL) maps demonstrate that the red emission originates from the apex of the tip area, while the green emission stems from the corners between m- and r-planes and the dominant blue emission from the m-plane. Analyzing individual μrods of the LED chip in a closed circuit configuration, a pronounced photocurrent is found for quasi-resonant laser excitation, indicating charge carrier tunneling losses out of the quantum well. This hypothesis is confirmed by applying an external voltage in the forward direction, where a characteristic blueshift of the single μrod PL signal is observed due to a modified band alignment, and a nonlinear increase in the PL intensity proves suppressed tunneling losses.

Environment-Friendly Preparation of Adsorptive TiO <sub>2</sub>/SiO <sub>2</sub> Core-Shell Nanospheres and V2o5 Microrods from Spent SCR Catalysts by Ultrasonic-Coupled Alkali Leaching Process

Spent selective catalytic reduction (SCR) catalysts comprising of valuable V2O5 and TiO2 are usually disposed as hazardous waste because of the toxic V2O5 ingredients. Therefore, recovery of V2O5 and TiO2 from spent SCR catalysts has highly environmental benefits as well as economic value. In this study, an ultrasonic-coupled alkali leaching process was developed for environment-friendly preparation of adsorptive TiO2/SiO2 core-shell nanospheres and V2O5 microrods from spent SCR catalysts. The results show that the synergy of ultrasonic and alkali leaching significantly enhance the separation of V2O5 from spent SCR catalysts, and the addition of NaOH decreases by 70% compared with the traditional roasting-leaching process. The as-prepared adsorptive TiO2/SiO2 core-shell nanospheres exhibit superior adsorptive and photocatalytic performance for 2,4-dinitrophenol removal, and 99.9% of 2,4-dinitrophenol was oxidized within 120 min. In addition, high-purity V2O5 microrods with high H2 S removal rate were efficiently prepared from low-grade filtrate using a hydrothermal precipitation method, which was superior to the traditional NH4VO3 precipitation and solvent extraction process. This study provides a potential method for controlling the environmental pollution of V2O5 and recycling of valuable adsorptive TiO 2 /SiO 2 core-shell nanospheres and V2O5 microrods from spent SCR catalysts.

Enhancing upconversion emission of Er3+ in single β-NaYF4 microrod through constructing different inert and active shells with doping Yb3+ ions

The construction of core-shell structure helps in enhancing upconversion luminescence in lanthanide-doped micro/nanomaterials. This study aimed to synthesize a series of NaYF4 and NaErF4 core-shell microrods based on epitaxial growth technology so as to enhance the upconversion emission. The upconversion luminescence properties of single NaYF4:Yb3+/Er3+ and NaErF4 microrods with different core-shell structures were carefully studied using a confocal microscope spectroscopy test system under 980-nm near-infrared excitation. The upconversion emission intensity and red-to-green emission intensity ratio of Er3+ ions were remarkably enhanced in different single microrods by building different inert and active core-shell structures. The higher red-to-green ratio was mainly due to the cross-relaxation between Er3+ ions and the energy back-transfer process from Er3+ to Yb3+ ions, which were explained on the basis of spectral characteristics and rate equations. These tunable NaYF4 and NaErF4 core-shell microrods had the potential to be applied to displays, micro-lasers, and anti-counterfeiting.

Boosting the performance of ZnO microrod metal-semiconductor-metal photodetectors via surface capping of thin amorphous Al2O3 shell layer

1D ZnO nanostructures have been widely explored due to their potential applications in ultraviolet (UV) region photodetectors because of their unique structural and optoelectronic properties. However, a large number of surface defect states leading to a noticeable dark current hinders their practical applications in UV photodetection. In this work, we have shown improved ZnO/Al2O3 core–shell microrod photodetectors, whose performance is significantly enhanced by defect passivation and the introduction of trap states by atomic layer deposition grown thin amorphous Al2O3 shell layer, as evidenced by steady-state and transient photoluminescence investigations. The photodetectors demonstrated suppressed dark current and increased photocurrent after capping the Al2O3 layer. Specifically, the ZnO/Al2O3 core–shell microrod photodetector exhibited a photoresponsivity as high as 0.019 A/(W cm−2) with the dark current as low as ∼1 × 10−11 A, and a high Ilight/Idark ratio of ∼104 under relatively weak light illumination (∼10 μW cm−2). The results presented in this work provide valuable pathways to boost the performance of 1D ZnO microrod-based photodetectors for future practical applications.

Excellent microwave absorbing properties of ZnO/ZnFe2O4/Fe core-shell microrods prepared by a rapid microwave-assisted hydrothermal-chemical vapor decomposition method

To enhance the poor microwave absorption capabilities (MACs) of pure ZnO since its impedance mismatching and lack of magnetic loss ability, ZnO/ZnFe2O4/Fe core-shell microrods (CSMRs) and microflowers (CSMFs) were respectively fabricated by microwave hydrothermal-chemical vapor deposition and hydrothermal-chemical vapor deposition. The morphology, composition and resulting properties of as-prepared samples were affected by different deposition temperatures (Td) and Fe(CO)5 volumes. Due to the synergy of Fe(CO)5 adsorption and decomposition, the Fe/Zn ratio was controlled by varying Td from 300 °C to 500 °C, which conveniently modulated the static magnetic properties and MACs of ZnO/ZnFe2O4/Fe CSMRs and CSMFs. Results showed that the optimal reflection loss (RL) value was −46.88 dB at 6.88 GHz with absorber thickness as thin as 2.2 mm. When the RL was below −20 dB, 15.15 GHz was covered. The rod shaped heterostructure could be formed due to the presence of magnetic particles with Fe core/ZnFe2O4 shell embedding, promoting the orientation and interfacial polarizations, multiple relaxation and proper impedance matching. Our findings suggest that ZnO/ZnFe2O4/Fe CSMRs are a type of promising absorber material with high-performance MACs, broad bandwidths and thin thicknesses.

Micro-Raman investigation on the size effects of surface optical phonon modes in single cone-shape InGaN/GaN microrods

As one of the third generation semiconductor representative materials, nitride has been commonly used in the field of semiconductor optoelectronics. Low dimensional structures of GaN have become research hotspot. In this paper, the size effects of single InGaN/GaN core-shell microrods on the surface optical phonon mode (SO mode) are studied based on micro-Raman technology. The experimental results show that the size of the microrod would lead to the SO mode. Furthermore, the frequency of SO mode is changed with different radius of microrod. The larger the radius is, the higher the frequency of SO mode. Combined with theoretical calculation, it is found that the experimental results are in agreement with calculation. In addition, the experimental results show that the coupling of the free carrier and the longitudinal optical phonon caused the Raman shift of E1 (LO) mode. Based on this, we obtain the distribution of carrier concentration along the [0 0 0 1] growth direction.

Impact of alloy disorder on Auger recombination in single InGaN/GaN core-shell microrods

We study the influence of local inhomogeneities on carrier recombination dynamics in single InGaN/GaN core-shell microrods (MRs) by means of time-resolved microphotoluminescence (TRPL) at 10 K. At low carrier density ($\ensuremath{\sim}{10}^{11}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$), the carrier recombination in the $m$-plane quantum well is dominated by radiative processes and the recorded decay times along the MR equally amount to about 400 ps, corresponding to a bimolecular coefficient of $1.1\ifmmode\pm\else\textpm\fi{}0.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. When the excited carrier density exceeds ${10}^{12}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, both the efficiency and the decay time of the PL in the quantum well drop significantly, which indicates the onset of Auger recombination. Based on a modified ABC model, we estimate a C coefficient varying from $0.5\ifmmode\pm\else\textpm\fi{}0.2$ to $2.2\ifmmode\pm\else\textpm\fi{}0.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{4}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ from the lower to the upper part of the MR. This increase is accompanied by a rise of PL linewidth in the low excitation regime, indicating an increase of alloy disorder. Relaxation of the $k$-selection rule by alloy disorder is expected to play an important role in the observed increase of Auger coefficient. These results confirm that Auger recombination is sensitive to disorder and can be significantly enhanced in strongly disordered systems. We conclude that it is therefore crucial to minimize the degree of disorder in the active layer for high power LEDs based on core-shell MRs.

Simultaneous Dual-mode Emission and Tunable Multicolor in the Time Domain from Lanthanide-doped Core-shell Microcrystals.

The dual-mode emission and multicolor outputs in the time domain from core-shell microcrystals are presented. The core-shell microcrystals, with NaYF4:Yb/Er as the core and NaYF4:Ce/Tb/Eu as the shell, were successfully fabricated by employing the hydrothermal method, which confines the activator ions into a separate region and minimizes the effect of surface quenching. The material is capable of both upconversion and downshifting emission, and their multicolor outputs in response to 980 nm near-infrared (NIR) excitation laser and 252 nm, and 395 nm ultraviolet (UV) excitation light have been investigated. Furthermore, the tunable color emissions by controlling the Tb3+-Eu3+ ratio in shells and the energy transfer of Ce3+→Tb3+→Eu3+ were discussed in details. In addition, color tuning of core-shell-structured microrods from green to red region in the time domain could be obtained by setting suitable delay time. Due to downshifting multicolor outputs (time-resolved and pump-wavelength-induced downshifting) coupled with the upconversion mode, the core-shell microrods can be potentially applied to displays and high-level security.

Spatially dependent carrier dynamics in single InGaN/GaN core-shell microrod by time-resolved cathodoluminescence

The optical properties of InGaN/GaN core-shell microrods are studied by time-resolved cathodoluminescence. Probing the carrier dynamics along the length of the rod from 4 to 300 K enables us to decompose radiative (τr) and non-radiative (τnr) lifetimes. At 300 K, τnr decreases from 500 at the bottom of the rod to 150 ps at its top. This variation results from an increased In-content in the upper part of the rod that causes a higher density of point defects. We further observe that thanks to the use of nonpolar m-plane growth, τr remains below 1.5 ns up to room temperature even with a thick active layer, which is promising for pushing the onset of the efficiency droop to higher current densities.

Microrod carrier dynamics map out potential ability to offset LED efficiency losses at high current

Time-resolved cathodoluminescence measurements reveal short radiative lifetimes of individual core-shell microrods, even at room temperatures, and their potential for improving LED efficiency.

Understanding GaN/InGaN core–shell growth towards high quality factor whispering gallery modes from non-polar InGaN quantum wells on GaN rods

GaN microrods are used as a basis for subsequent InGaN quantum well (QW) and quantum dot deposition by metal-organic vapor phase epitaxy. The coverage of the shell along the sidewall of rods is dependent on the rod growth time and a complete coverage is obtained for shorter rod growth times. Transmission electron microscopy measurements are performed to reveal the structural properties of the InGaN layer on the sidewall facet and on the top facet. The presence of layers in the microrod and on the microrod surface will be discussed with respect to GaN and InGaN growth. A detailed model will be presented explaining the formation of multiple SiN layers and the partial and full coverage of the shell around the core. Cathodoluminescence measurements are performed to analyze the InGaN emission properties along the microrod and to study the microresonator properties of such hexagonal core–shell structures. High quality factor whispering gallery modes with are reported for the first time in a GaN microrod/InGaN non-polar QW core–shell geometry. The GaN/InGaN core–shell microrods are expected to be promising building blocks for low-threshold laser diodes and ultra-sensitive optical sensors.

Flexibly and Repeatedly Modulating Lasing Wavelengths in a Single Core-Shell Semiconductor Microrod.

Modulating lasing wavelength flexibly and repeatedly on a single rod is essential to the practical applications of micro/nanorod lasers. In this paper, a structure that decouples the gain medium and optical cavity is proposed, where the corresponding mechanism for the lasing wavelength shift is explained. Based on the above structure, one kind of wavelength continuously variable lasers is achieved on a single GaN/InGaN core-shell microrod without modifying the geometry of the resonant cavity or cutting the microrod. By using this method, lasing wavelength can be modulated from 372 to 408 nm flexibly and repeatedly in a 10 μm facilely synthesized microrod. This approach demonstrates a big application potential in numerous fields consisting of optical telecommunication and environmental monitoring.

Structure and Composition of Isolated Core-Shell(In,Ga)N/GaNRods Based on Nanofocus X-Ray Diffraction and Scanning Transmission Electron Microscopy

Nanofocus x-ray diffraction is used to investigate the structure and local strain field of an isolated ðIn; GaÞN=GaN core-shell microrod. Because the high spatial resolution of the x-ray beam is only 80 × 90 nm2, we are able to investigate several distinct volumes on one individual side facet. Here, we find a drastic increase in thickness of the outer GaN shell along the rod height. Additionally, we performed highangle annular dark-field scanning-transmission-electron-microscopy measurements on several rods from the same sample showing that (In,Ga)N double-quantum-well and GaN barrier thicknesses also increase strongly along the height. Moreover, plastic relaxation is observed in the top part of the rod. Based on the experimentally obtained structural parameters, we simulate the strain-induced deformation using the finiteelement method, which serves as the input for subsequent kinematic scattering simulations. The simulations reveal a significant increase of elastic in-plane relaxation along the rod height. However, at a certain height, the occurrence of plastic relaxation yields a decrease of the elastic strain. Because of the experimentally obtained structural input for the finite-element simulations, we can exclude unknown structural influences on the strain distribution, and we are able to translate the elastic relaxation into an indium concentration which increases by a factor of 4 from the bottom to the height where plastic relaxation occurs.

The influence of MOVPE growth conditions on the shell of core-shell GaN microrod structures

A core-shell geometry is employed for most next-generation, three-dimensional opto-electric devices based on III–V semiconductors and grown by metal organic vapor phase epitaxy (MOVPE). Controlling the shape of the shell layers is fundamental for device optimization, however no detailed analysis of the influence of growth conditions has been published to date.We study homogeneous arrays of gallium nitride core-shell microrods with height and diameter in the micrometer range and grown in a two-step selective area MOVPE process. Changes in shell shape and homogeneity effected by deliberately altered shell growth conditions were accurately assessed by digital analysis of high-resolution scanning electron microscope images.Most notably, two temperature regimes could be established, which show a significantly different behavior with regard to material distribution. Above 900°C of wafer carrier temperature, the shell thickness along the growth axis of the rods was very homogeneous, however variations between vicinal rods increase. In contrast, below 830°C the shell thickness is higher close to the microrod tip than at the base of the rods, while the lateral homogeneity between neighboring microrods is very uniform. This temperature effect could be either amplified or attenuated by changing the remaining growth parameters such as reactor pressure, structure distance, gallium precursor, carrier gas composition and dopant materials. Possible reasons for these findings are discussed with respect to GaN decomposition as well as the surface and gas phase diffusion of growth species, leading to an improved control of the functional layers in next-generation 3D V–III devices.

Polarization-resolved micro-photoluminescence investigation of InGaN/GaN core-shell microrods

We investigate the optical emission properties of the active InGaN shell of high aspect-ratio InGaN/GaN core-shell microrods (μRods) by confocal quasi-resonant polarization-resolved and excitation density dependent micro-photoluminescence (μPL). The active shell, multiple thin InGaN/GaN quantum wells (MQWs), was deposited on GaN μRods selectively grown by metal organic vapor phase epitaxy on patterned SiO2/n-GaN/sapphire template. High spatial resolution mappings reveal a very homogeneous emission intensity along the whole μRods including the tip despite a red-shift of 30 nm from the base to the tip along the 8.6 μm-long m-plane sidewalls. Looking at the Fabry-Perot interference fringes superimposed on the μPL spectra, we get structural information on the μRods. A high degree of linear polarization (DLP) of 0.6–0.66 is measured on the lower half of the m-plane side facets with a slight decrease toward the tip. We observe the typical drop of the DLP with an excitation density caused by degenerate filling of valence bands (Fermi regime). Local internal quantum efficiencies (IQEs) of 55±11 % up to 73±7 % are estimated on the m-plane facet from measurements at low temperature. Finally, simultaneously fitting the DLP and IQE as a function of the excitation density, we determine the carrier density inside the active region and the recombination rate coefficients of the m-plane MQWs. We show that phase-space filling and the background carrier density have to be included in the recombination rate model.