Heterostructure alloys with controlled structure and composition are essential for future electronics and optoelectronics. While heterostructure formation creates sharp electronic junction, alloying allows bandgap tunability. Herein, ZnS/ZnO heterostructure-alloy hexagonal micropyramids are grown on Si/SiO2 wafers for the first time by facile thermal evaporation of ZnS powder. It is found that the pyramids with a based size of few micrometers are grown along different crystallographic directions and neighbored by ZnS microrods. XRD and XPS spectra showed that ZnS, ZnO, and ZnSO wurtzite phases coexist in the as-received samples. FESEM images, in situ point, and EDS elemental mapping spectra reveal that the micropyramids are composed of Zn, S, O, but each element's ratio changes with substrate temperature and is different at different faces of a single micropyramid. Using a high-spatial CL-FESEM, the profile of CL spectra along the substrate and at different faces in a single micropyramid were in-situ measured and disclosed red-shifted of ZnS band-edge emission with increasing substrate temperature, and the formation of new emission bands in the range from ∼345 to 352 nm. This new emission band is attributed to the formation of the ZnSO alloy phase. Our results demonstrated that the optical properties of the obtained ZnS/ZnO heterostructure-alloy hexagonal micropyramids could be tunned by controlling the substrate temperature and indicates potential applications of the ZnS/ZnO heterostructure alloy in optoelectronic and photonic applications.
ZnS has great potential as a valuable material for nanoscale devices because of its rich morphologies and unique structure. Although much effort has been made, the growth of high-quality ZnS crystal is still a challenge. In this paper, high-quality optically defect-free 1D ZnS nanostructures, including nanorods, nanowires, and nanobelts, were successfully synthesized on a large scale by a modified thermal evaporation method. XRD patterns and HRTEM images indicated that the ZnS nanostructures are single phases with hexagonal structures. Under optical excitation, all the ZnS nanostructures show intense UV emissions and almost no defect emissions at room temperature. Sharp UV lasing-like peaks with the FWHM as narrow as 2–3 nm are achieved for the ZnS nanobelts and nanowires. The optical transition from free exciton A, free exciton B, and their associated LO phonon replicas were determined from the evolution of the NBE emissions. These high-quality nanostructures are envisaged to be highly promising for high-efficiency light-emitting devices and lasers in the UV region.
The development of emission-tunable structures opens an optimistic future for white light-emitting diodes (WLED) utilizing a UV pump. Herein, we report emission-tunable Mn-doped ZnS/ZnO heterostructure nanobelts synthesized by the thermal evaporation method, following by an appropriate oxidation process. Under the UV excitation of 330 nm, the as-received nanobelts show an orange-yellow light with CCT of 3056 K, the 500 and 700 °C-oxidized samples strongly exhibit a warm white light emission with CCT of 3393 K and 3551 K, respectively. Surprisingly, the Mn-doped ZnS/ZnO heterostructures oxidized at 600 °C show a white light emission with CIE chromaticity coordinate of (0.31, 0.39) and CCT value of 6290 K, which are nearly very close to that of the commercial YAG:Ce3+ phosphor. A prototype WLED with the color coordinates of (0.33, 0.45), the CCT of 5491 K, and the QE ∼ 61% was successfully fabricated using Mn-doped ZnS/ZnO heterostructure nanobelts coated on the surface of UV (370 nm) chip. The obtained Mn-doped ZnS/ZnO heterostructures are promising to be applied in UV-pumped WLED fabrication.
Our recent article entitled "Enhanced thermoelectric properties of Hf-free half-Heusler compounds prepared via highly fast process" has been accepted for publication in Journal of Alloys and Compounds.
Hf-free n-type half-Heusler with a nominal composition of Ti0.5Zr0.5NiSn0.98Sb0.02 has been reported to have a high ZT value of almost 1.2. However, the synthesis process requires a long annealing time to achieve single-phase structure, which contributes to high product costs due to energy and time consumption. Here we introduce a new route to prepare (Ti0.5Zr0.5)1−xNbxNiSn (x = 0, 0.0050, 0.0075, 0.0100, 0.0125, 0.0150, 0.0175 and 0.0200) compounds for high thermoelectric (TE) performance along with shortening time for sample preparation. The samples were prepared by a combination of arc-melting (AM) and melt-spinning (MS) followed by spark plasma sintering process (SPS). The combination of these synthetic methods produced (Ti0.5Zr0.5)1−xNbxNiSn samples with high chemical homogeneity, single-phase structure, and fine grain about 300 nm in size, which are preferred for both charge and phonon transport properties. As a result, a maximum power factor of 44.5 µW cm−1 K−2 at 817 K and a maximum ZT of 1.19 at 874 K were achieved for the sample with x = 0.015, which are comparable to the highest ZT value reported so far for the Hf-free n-type MNiSn (M = Ti, Zr) compounds. The calculated output power density Pd and efficiency η based on a single-leg device showed an excellent performance, which yields the maximum Pd of 16.2 W cm−2 and η of 12.08% at the cold side temperature TC ≈ 305 K and the hot side temperature TH ≈ 875 K for the optimized composition with x = 0.0125. Furthermore, it is noted that the synthetic process here does not require a long-annealing time and it can be easily applied to mass production.
Although it has been extensively studied for decades, the α-Al2O3:Cr3+ phosphor has rarely been investigated for horticultural lighting. In this work, for the first time, a prototype of a plant growth light-emitting diode (LED) has been fabricated by coating a deep-red-emitting α-Al2O3:Cr3+ phosphor onto a near-ultraviolet (NUV) chip. The α-Al2O3:Cr3+ phosphor, synthesized by a co-precipitation method and annealed at 1500 °C for 2 h, emits an outstanding narrow peak at 695 nm. The α-Al2O3:0.6%Cr3+ phosphor shows a high activation energy of 0.29 eV, a long lifetime of 3.4 ms, and a superior color purity of 100%. The chromatic coordinates and the QE value of the red LED obtained by coating an α-Al2O3:0.6%Cr3+ phosphor on a NUV chip are (x = 0.5650, y = 0.2429) and 87.1%, respectively.
Single-phase far-red-emitting ZnAl2O4:Cr3+ phosphor has been successfully synthesized by a sol-gel method. The phosphor can be well excited by NUV, violet, and green lights, and it strongly emits multi-peak broadband emissions in the far-red region peaking at 687 and 698 nm, matching well with the absorption band of phytochrome. The maximum PL intensity is achieved for the ZnAl2O4:0.8%Cr3+ phosphor annealed at 1400 °C. The lifetime and activation energy of the optimal ZnAl2O4:0.8%Cr3+ phosphor annealed at 1400 °C are 25.3 ms and 0.302 eV, respectively. Three types of far-red-emitting LEDs have been successfully fabricated by coating the optimal ZnAl2O4:0.8%Cr3+ phosphor on the surface of NUV (395 nm), violet (410 nm), and green (510 nm) chips. The chromatic coordinates of the corresponding LEDs are (0.2990; 0.2199), (0.2725; 0.1594), (0.2703; 0.5743), respectively. The quantum efficiency of the ZnAl2O4:Cr3+ phosphor excited by different LEDs is calculated and reported for the first time. The obtained results indicate that the ZnAl2O4:Cr3+ phosphor has a high potential for pc-converted plant growth LED application .
The development of full-visible-spectrum phosphors is essential for next-generation light-emitting devices with better light quality. Herein, we report on a novel broad-band-emitting phosphor based on single-composition Al-doped ZnO phosphors. Under the UV excitation of 325 nm, the ZnO : Al phosphor exhibits a full spectrum emission in the visible wavelength range from 400 to 800 nm with a CIE chromaticity coordinate of (0.42, 0.48), a quantum efficiency of 43%, a color rendering index (CRI) of 74, a correlated color temperature (CCT) value of 3873 K and an activation energy of 0.22 eV. A prototype of a UV-pumped warm WLED with a high CRI of 87 and a CCT of 4067 K has been achieved by using only this broad-band-emitting Al3+-doped ZnO phosphor. The obtained results indicate that the single-composition Al3+-singly doped warm white emitting phosphor is a promising candidate for UV-pump warm white light-emitting diodes.
Narrow-band red-emitting phosphor Cr3+-doped BaMgAl10O17 (BAM) was successfully synthesized via a sol – gel method. The obtained Cr3+-doped BaMgAl10O17 phosphor with average particle size of about 1 μm emits an intense red light at around 695 nm with two broad excitation bands at 405 and 560 nm. The highest PL intensity is achieved at the Cr3+ doping concentration of 1 mol% and the annealing temperature of 1400 °C. The Tanabe-Sugano diagram reveals that Cr3+ ions are affected by the strong crystal field with the Dq/B ratio value of ~2.7, and the peak at 695 nm is attributed to the 2E→4A2 transitions of Cr3+ ions. A purple LED with colour coordinates of (0.263, 0.148) has been fabricated by coating the Cr3+-doped BaMgAl10O17 phosphor on a violet LED (410 nm) chip. The results indicate that BaMgAl10O17:Cr3+ phosphor could be promising red phosphor for plant growth LEDs.
Our latest article entitled "Pd80Co20 Nanohole Arrays Coated with Poly(methyl methacrylate) for High-Speed Hydrogen Sensing with a Part-per-Billion Detection Limit" has been accepted for publication in ACS Applied Nano Materials.
As hydrogen gas increasingly becomes critical as a carbon-free energy carrier, the demand for robust hydrogen sensors for leak detection and concentration monitor will continue to rise. However, to date, there are no lightweight sensors capable of meeting the required performance metrics for the safe handling of hydrogen. Here, we report an electrical hydrogen gas sensor platform based on a resistance nanonetwork derived from Pd-Co composite hole arrays (CHAs) on a glass substrate, which meets or exceeds these metrics. In optimal nanofabrication conditions, a single poly(methyl methacrylate)(PMMA)-coated CHA nanosensor exhibits a response time (t80) of 1.0 s at the lower flammability limit of H2 (40 mbar), incredible sensor accuracy (<1% across 5 decades of H2 pressure), and an extremely low limit of detection (LOD) of <10 ppb at room temperature. Remarkably, these nanosensors are extremely inert against CO and O2 gas interference and display robust long-term stability in air, suffering no loss of performance over 2 months. Additionally, we demonstrate that the unique nanomorphology renders the sensors insensitive to operation voltage/current with diminutive power requirement (∼2 nW) and applied magnetic field (up to 3 kOe), a crucial metric for leak detection and concentration control.
Our latest article entitled "Non-rare-earth dual green and red-emitting Mn-doped ZnAl2O4 phosphors for potential application in plan-growth LEDs" has been accepted for publication in Journal of Alloys and Compounds.
Non-rare-earth dual green and red-emitting Mn-doped ZnAl2O4 phosphors were successfully synthesized by a co-precipitation method. XRD and XPS spectra reveal that Mn ions in two valence states Mn2+ and Mn4+ are co-existed in the ZnAl2O4 host lattice. The ZnAl2O4:(Mn2+, Mn4+) phosphors exhibit two intense emission bands in the green and red spectral regions, peaking at 510 and 679 nm. It is demonstrated that the PL intensity ratio of green emission and red emission (IGreen/Ired) can simply be controlled by adjusting the annealing temperature and found to vary from 2.78 to 0.48 when increasing the temperature from 1200 to 1400 °C. A prototype red-emitting LED was fabricated using a 310-nm UV LED combined with the ZnAl2O4:0.5% (Mn2+, Mn4+) phosphor. The CIE color coordinates are x = 0.5643 and y = 0.2798, while the red-emitting LED is driven by 60 mA current. The quantum efficiency (QE) of the ZnAl2O4:0.5%Mn phosphor is estimated to be about 27.3%. To the best of our knowledge, this is the first time that the QE of Mn-doped ZnAl2O4 phosphor is reported.
Our latest article entitled "Excellent thermal stability and high quantum efficiency orange-red-emitting AlPO4:Eu3+ phosphors for WLED application" has been accepted for publication in Journal of Alloys and Compounds.
Orange-red-emitting AlPO4:Eu3+ phosphors have been systematically synthesized by a co-precipitation method. Under the excitation at 394 nm these phosphor shows strong orange-red emissions with the strongest peaks at 588, 594, 685 and 700 nm. The highest PL intensity is achieved at the dopant concentration of 3 mol% and the annealing temperature of 1000 °C. Additionally, 84% of the room-temperature emission intensity is still maintained at 160 °C, indicating a good thermal stability and practicality. By a Judd-Ofelt (JO) analysis, it is demonstrated that the symmetry sites surrounding Eu3+ ions are influenced by the dopant level. The lowest value of Ω2/Ω4 ratio of 0.27 reveals the highest symmetry site at the Eu3+ concentration of 3 mol%. The CIE 1931 color chromaticity coordinates (x, y) and the activation energy of the AlPO4:3%Eu3+ phosphor are (0.5602; 0.3862) and 0.28 eV, respectively. The color coordinates of the phosphor coated on NUV LED chip and its quantum efficiency (QE) are (0.5573, 0.3253) and 38.7%, respectively. To the best of our knowledge, this is the first time that a calculated QE of AlPO4:Eu3+-based orange-red-emitting LED is reported. The obtained results show a great potential of using orange-red-emitting AlPO4:Eu3+ phosphor for WLED application.
Metal chalcogenides have attracted attention as potential thermoelectric materials due to their intrinsically low thermal conductivity arising from their layered structure with weak van der Waals atomic bonding. InSe, one of the post transition metal chalcogenides, also has low thermal conductivity and doping of InSe with elements such as Sn, Si, and As is known to improve the electronic transport properties. Herein, we investigated Te doping in Si-doped InSe (In0.9Si0.1Se) and report enhanced thermoelectric properties, mainly the increased Seebeck coefficient due to the increase in effective mass. Due to the increase in effective mass, the magnitude of the Seebeck coefficient systematically increased with Te doping from 234 µV/K to 405 µV/K. Eventually, the zT at 700 K was enhanced from 0.040 for the pristine sample to 0.069–0.096 for the Te-doped samples
Congratulations! Khuất Thị Thư (2nd student) - a member of our group has recently won the second prize in Viet Nam's National Scientific Research Contest 2020 for Students for her project entitled "Synthesis and Investigation on Optical Properties of one-dimensional ZnS/MoS2 materials".
Khuat Thi Thu is a second-year student at the Faculty of Materials Science and Engineering. She joined our group when she was a freshman. She won the top prize in Phenikaa's Scientific Research Contest 2020 for Students and was selected as one of the best students among the university to participate in Vietnam's National Scientific Research Contest 2020. This research project entitled: "Synthesis and Investigation on Optical Properties of one-dimensional ZnS/MoS2 materials" was supervised by Prof. Dr. Pham Thanh Huy.
The awards ceremony for the contest took place in Ho Chi Minh on 27 November. The annual event is co-organised by the Ministry of Education and Training, the Central Committee of the Ho Chi Minh Communist Youth Union, the Ministry of Science and Technology, and the Viet Nam Union of Science and Technology Associations. The competition is to help Vietnamese students satisfy their passion for scientific research, and turn the theory they learn in their schools into prototypes for technological solutions to real-world problems.
Quantum Resistive pressure Sensors (pQRS) can be integrated into flexible electronics, smart textiles, robotics etc. Our last research on hybrid nanocomposites by 3D spraying, highlights the positive input of graphene combined with carbon nanotubes to build a robust hierarchical conducting architecture. Our best formulation of pQRS transducers, thermoplastic polyurethane (TPU) functionalized pG2%/CNT4% hybrid, exhibited a linear response from 0 to 4 MPa, the highest range ever obtained with a sensitivity as high as 11.29 × 10−5 kPa−1. Graphene allowed to multiply by three the piezo-resistive response to compression (Ar) of TPU-CNT up to 60% and improved significantly sensors’ stability. More strikingly, hybrid pQRS can convert the classical double peak of the signal resulting from the poisson’s effect at high compression, into a single peak. This performance is an exciting result ascribed to the hybridization of carbon nanotubes (CNT) with pristine graphene (pG) into an architecture keeping contact whatever the direction of solicitation. Hybrid pQRS had also a more stable piezo-resistive behavior whatever the speed of compression, and the mechanical history. Finally, the proof of concept of pressure monitoring and mapping, with a flexible and integrable array of four hybrid pQRS has demonstrated a promising potential for real time sensing.
We studied spin dynamics of charge carriers in the superlattice-like Ruddlesden-Popper hybrid lead iodide perovskite semiconductors, 2D (BA)2(MA)Pb2I7 (with MA = CH3NH3, and BA = CH3(CH2)3NH3), and 3D MAPbI3 using the magnetic field effect (MFE) on conductivity and electroluminescence in their light emitting diodes (LEDs) at cryogenic temperatures. The semiconductors with distinct structural/bulk inversion symmetry breaking, when combined with colossal intrinsic spin–orbit coupling (SOC), theoretically give rise to giant Rashba-type SOC. We found that the magneto-conductance (MC) magnitude increases monotonically with the emission intensity and saturates at ≈0.05% and 0.11% for the MAPbI3 and (BA)2(MA)Pb2I7, respectively. The magneto-electroluminescence (MEL) response with similar line shapes as the MC response has a significantly larger magnitude, and essentially stays constant at ≈0.22% and ≈0.20% for MAPbI3 and (BA)2(MA)Pb2I7, respectively. The sign and magnitude of the MC and MEL responses can be quantitatively explained in the framework of the Δg-based excitonic model using rate equations. Remarkably, the width of the MEL response in those materials linearly increases with increasing the applied electric field, where the Rashba coefficient in (BA)2(MA)Pb2I7 is estimated to be about 7 times larger than that in MAPbI3. Our studies might have significant impact on future development of electrically-controlled spin logic devices via Rashba-like effects.
In this work, far-red-emitting Zn2SnO4 phosphor has been successfully synthesized by a facile modified solid-state reaction method from ZnO and SnO2 powders. FESEM images reveal that as-milled particles are nearly spherical, with size in the range of 10–30 nm. The particle size increases with increasing annealing temperature and it reaches the value of about 4 μm at 1200 °C. XRD patterns indicate that Zn2SnO4 phase starts occurring at ~700 °C with its single phase at 900 °C and the best crystalline quality of Zn2SnO4 phase is obtained at 1000 °C. UV–Vis spectra analysis reveals that ZnO phase reappears at high annealing temperature (≥1100 °C), reflecting the deterioration of the single phase of Zn2SnO4. A broad visible band centered at 684 nm, which has never been explicitly reported, is clearly observed in the PL spectrum of the single Zn2SnO4 crystals. The intensity of this new band is influenced by annealing temperatures. It reaches the maximum value at 1000 °C, corresponding to the highest quality of the Zn2SnO4 phase. By using data fitting, the PL spectrum is deconvoluted into four Gaussian peaks at ~2.20 eV (563.5 nm), ~1.98 eV (626.1 nm), ~1.80 eV (688.8 nm), and ~1.68 eV (738.0 nm). The peak at ~2.20 eV (563.5 nm) is ascribed to oxygen vacancy (Vo) while the other emission peaks are possibly associated with a significant number of trapped states due to the high concentration of deficiencies such as oxygen vacant defects, zinc interstitials, zinc/tin vacancies (VZn/VSn), Zn/Sn stoichiometry or point defects.
We develop a novel approach to create oxygen vacancies on the surface of ZnO particles and investigate their role in the photoluminescence of the ZnO. In this approach, the commercial ZnO powder is coated with a thin carbon layer using ball-milling technique, forming ZnO@C core@shell particles. By annealing the as-milled ZnO@C at a relatively low temperature (i.e., up to 600 °C) in oxygen, the surface oxygen vacancies are formed, which is confirmed by X-ray photoelectron spectroscopy analyses. The annealed powders show red photoluminescence, which largely covers the far-red region, and by varying the annealing temperature, the emission range of the ZnO can be tailored. We found that the far-red emission is associated with the surface oxygen vacancies of ZnO
On 21st, May, 2021, our group leader Prof. PHAM THANH HUY - President of the Phenikaa University shared a talk with VTV3 Program "Cafe sáng" o "Coffee Morning" about: Choosing your future career by your interest or your parents' orientation.
In frame of this talk, Prof. HUY also explained the idea and the important role of the Preparation, Goal and Self-studying in the students' life.