Photoluminescence Peak Intensity Research Articles

Exploring the morphological and optical properties of N-doped ZnO heterojunctions

Nitrogen-doped zinc oxide (N:ZnO) thin films were deposited on glass substrates via radio frequency (RF) magnetron sputtering and subsequently annealed at 300 °C, 400 °C, 500 °C, and 600 °C to assess their viability and stability as transparent conductive oxide (TCO) materials. Structural and compositional analyses were performed using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). XRD analysis revealed preferential crystallite orientations along the (100), (002), (101), and (110) planes. Atomic force microscopy (AFM) measurements indicated particle sizes two to four times larger than those derived from XRD, suggesting a sub-granular internal structure, as XRD probes coherently diffracting domains. XPS analysis of the N 1 s spectra identified two distinct peaks at approximately 397 eV and 407.5 eV, indicating nitrogen incorporation into the ZnO matrix. Photoluminescence spectroscopy revealed that nitrogen doping induced the formation of interstitials and defects associated with oxygen and zinc vacancies. Optical measurements showed that the (N:ZnO) thin films exhibited an average optical band gap of approximately 3.1 eV, with 80% transmittance in the visible spectrum. A linear relationship was observed between the band gap energy and the tail width. Except for the film annealed at 600 °C, all annealed films showed a reduction in peak photoluminescence intensity with increasing annealing temperature. Finally, no significant changes in the electrical performance of the p-N/n-Si diode were observed as a result of annealing-induced surface modifications. The results provide valuable insights into the optimization of (N:ZnO) thin films for use in international optoelectronic and photovoltaic research, where advancements in TCOs are critical for the development of high-performance, sustainable technologies.

(Invited) On the Improvement of Ge Lasers

Electrically pumped Ge lasers were first demonstrated in 2012, which are considered a promising solution to Si-compatible on-chip lasers. Theoretical studies showed that laser structure design, stress engineering, and Ge material quality improvement are the three main methods for improving Ge laser performance. Epitaxial Ge thin films (epi-Ge) on Si have been the choice for Ge in photonic and microelectronic devices due to the suitable thickness in the micron or sub-micron scale. For Ge on-chip lasers, this thickness range is required to overcome the self-absorption, high lasing threshold, and coupling difficulty associated with larger thicknesses. However, the major problem of epi-Ge on Si is the high threading dislocation density (TDD) ranging from 106 to 1010 cm-2 despite the extensive research efforts to reduce the TDD. In contrast, bulk Ge, typically a few hundred microns thick, maintains superior crystal quality (TDD ≤ 104 cm-2).To take advantage of this ultra-high crystal quality, we successfully developed wet etching recipes to thin bulk Ge wafer pieces down to micron-scale free-standing Ge thin films with good sample integrity and surface roughness while maintaining the ultra-high crystal quality of the starting bulk-Ge samples. Two bulk Ge wafers were chosen and thinned from 535 or 405 µm to 1-2 µm. As a result, the photoluminescence (PL) peak intensity of the thinned Ge samples increased significantly. The key material parameters are shown in the table below. The specification values beneficial for more PL intensity are shaded in green, and those not desired are in red. The benchmarking epi-Ge control sample has been compared to the epi-Ge from the best Ge lasers, which have similar specifications and PL intensity. Compared with the bulk Ge or bulk-Ge-based thin films, the epi-Ge has a higher tensile strain, higher n-type doping, and lower surface roughness, which all support much higher PL intensity except for the much higher TDD. However, the samples with much increased PL peak intensities are from bulk-Ge-based thin films with non-optimized strain, doping, and surface roughness, demonstrating the TDD reduction's high effectiveness in improving Ge PL. Measurements showed much longer minority carrier lifetimes in these high PL samples due to the very low TDDs. TDD reduction in Ge relaxes the common requirements of high n-doping and stress in enhancing Ge laser performance. It thus reduces the side effects of high optical absorption, high non-radiative recombination, bandgap narrowing, and large footprints associated with these two techniques.After proving the bulk-Ge-based thin film concept, we also developed bonding, thinning, and polishing techniques to make thin Ge bonded on handle substrates and to introduce tensile strain from the bonding step. Ultra-high-quality Ge/poly-Si/SiO2 thin films on glass with sufficient bonding strength and surface smoothness were achieved. The minority lifetimes for these bonded Ge thin films range between 200 and 1000 ns, surpassing those achieved with epi-Ge on Si by at least 20 to 100 times. This is an important indication of the ultra-high quality of the bonded Ge thin films. Later, Ge microbridges were fabricated, which amplified the tensile strain introduced in the bonding step, reaching a maximum uniaxial tensile strain of 3.7%. This is crucial for reducing the difference between Ge's direct and indirect energy bands for much more efficient light emission.In summary, this work established an economical and convenient fabrication technology for producing high-crystal-quality and high-tensile-strained Ge thin films bonded on substrates — a pivotal step in exploring Ge's potential in light emission applications, especially for Ge lasers for on-chip optical interconnects. Figure 1

(Invited) Improving Photochemical Properties of I-III-VI-Based Quantum Dots through Size and Composition Control

Semiconductor nanocrystals smaller than 10 nm, known as quantum dots (QDs), exhibit optoelectronic properties that are dependent on their size due to the quantum size effect. Recently, QDs made of I-III-VI group multinary semiconductors, such as CuInS2 and AgInS2, have attracted significant attention for their potential in highly efficient solar energy conversion systems. This is because these QDs were composed of less toxic elements and their optical properties were broadly tuned in the visible and near-IR wavelength regions through chemical composition. It was reported in our previous papers (1,2) that solid solution QDs composed of Zn-Ag-In-S and Zn-Ag-In-Se exhibited the photocatalytic activity for H2 evolution, which varied according to their size and composition. In this study, we introduce the solution-phase synthesis of less-toxic QDs composed of AgIn(1-x)GaxS2 (AIGS) and CuIn(1-x)GaxS2 (CIGS). The photocatalytic activity for H2 evolution was investigated for AIGS QDs under visible light irradiation. The photoluminescence (PL) properties of CIGS QDs were improved for the application to QD-based light-emitting diodes (QD-LEDs).AIGS QDs with different In/(In+Ga) ratios were synthesized by thermal decomposition of corresponding metal complexes and S powders in an oleylamine-dodecanethiol solution.(3) The energy gap (E g) of AIGS QDs was increased from 2.07 to 2.54 eV with a decrease in the In fraction in the QDs from 1.0 to 0.20. After the surface coating with GaSx shell, the resulting AIGS@GaSx QDs exhibited sharp band-edge PL peak, the wavelength of which was tunable from 610 to 500 nm with an increase in the E g. The photocatalytic activity of AIGS QDs was imoroved by the modification with ZnS shell of 2 nm in thickness. The resulting AIGS@ZnS QDs were dispersed in the aqueous solution containing Na2S as a hole scavenger. Visible light irradiation (λ> 350 nm) to the dispersion caused the H2 evolution, the amount of which linearly increased with the elapse of the irradiation time. The H2 evolution rate was remarkably dependent on the composition of AIGS core. A volcano-type dependence was observed between the photocatalytic activity rate and the In fraction. The highest photocatalytic activity was obtained for AIGS@ZnS QDs with In/(In+Ga)=0.4. This result can be explained by the changes of the conduction band level of AIGS cores and the amount of photon absorbed by the photocatalysts.CIGS QDs were prepared in a similar way.(4) The E g of CIGS QDs also increased from 1.74 eV to 2.77 eV with a decrease in In/(In+Ga) ratio from 1.0 to 0. The GaSx coating on CIGS QD surface enlarged the intensity of PL peak without changing its peak width: CIGS@GaSx QDs exhibited a narrow PL peak (FWHM: 0.20 eV) at 671 nm, being comparable to the PL peak of CIGS QDs (the peak wavelength of 675 nm and the FWHM of 0.23 eV). The PL QY was also increased to 27% from 8% by the GaSx coating on CIGS QDs. QD-LEDs fabricated with CIGS@GaSx QDs showed a red color with a narrow emision peak at 688 nm with a fwhm of 0.24 eV.In cnclusion, the tunabilities of optical properties and energy structure of I-III-VI-based QDs are useful for constructing efficient light energy conversion systems and luminescent devices. References T. Kameyama, et al., J. Phys. Chem. C, 2015, 119, 24740-24749.P. Y. Hsieh, et al., J. Mater. Chem. A 2020 , 8, 13142-13149.T. Kameyama, et al., ACS Applied. Mater. Interfaces 2018, 10, 42844-42855.C. Jiang, et al., J. Chem. Phys. 2023, 158, 164708.

Hexagonal Enhanced Porous GaN with Delayed Integrated Pulse Electrochemical (iPEC) Etching

This present study investigates the effect of time delay (Td) on the formation of porous GaN (P-GaN) using integrated pulse electrochemical (iPEC) etching. Porous GaN (P-GaN) was formed by etching an N-type GaN wafer with a 4% KOH electrolyte for 60 minutes under an ultraviolet (UV) lamp at a current density of 80 mA/cm2. A Td of 120 minutes was applied before electrochemically etching the P-GaN sample. The top view image of the field emission scanning electron microscopy (FESEM) revealed a significant difference when a Td was applied. A dense and uniform hexagonal P-GaN was obtained from the Td iPEC sample, while the non-Td sample exhibited a multi-layered hexagonal porous structure with unfinished pore-etched areas. Higher porosity and deeper pores were observed in the Td sample. Intense high-resolution X-ray diffraction (HR-XRD) peak intensity was observed in the Td iPEC sample with a lower full width half maximum (FWHM), indicating that the sample had better crystallinity. The Raman spectra of the sample anodized with a Td exhibited higher Raman peak intensity and a slight shift to a higher frequency concerning as-grown GaN, indicating better crystallinity and a tensile stress relaxation of 0.24 GPa. Post etching, a blue shift of the photoluminescence (PL) peak, from 364 nm (as-grown GaN) to 363 nm (P-GaN), was observed, and a small PL peak started to form around 385 nm compared to the as-grown GaN due to the relaxation of the tensile stress, which modified the bandgap. The PL peak intensity of the Td sample was higher than the non-Td sample, indicating that the porosity and uniformity allowed more light interaction with the material, resulting in more efficient photon absorption and emission. The results indicated that potentially efficient optoelectronics devices can be fabricated on a P-GaN using a combination of electroless and electrochemical etching of the GaN epitaxial layer.

Effects of off-axis angles of 4H-SiC substrates on properties of β-Ga2O3 films grown by low-pressure chemical vapor deposition

In this work, β-Ga2O3 films with (−201) preferred orientation were heteroepitaxially grown on SiC substrates with off-axis angles of 0°, 4° and 8° by low-pressure chemical vapor deposition (LPCVD). It is found that the crystal quality, surface morphology, electrical properties of the β-Ga2O3 films of the film are significantly sensitive to the off-axis angle of SiC substrates. The crystal quality and surface morphology of the film were significantly improved when the 4° off-axis substrate was used, of which the FWHM (Full width at half maximum) was as low as 0.169° and the Ra (Roughness Average) of the smooth surface was 1.98 nm. Due to the improvement of crystal quality, the Hall mobility of the films deposited on 4° off-axis substrates was increased to 50.88 cm2/V·s. The photoluminescence peak intensity of the film increases first and then decreases with the increase of the off-axis angle of the substrate. However, the use of the off-axis substrate has no obvious effect on the oxygen vacancy ratio of the films. The results of this study demonstrate the significance for the preparation of high quality heteroepitaxial β-Ga2O3 films on SiC substrate for the promising applications in power electronic and optoelectronic devices.

Development of photovoltaic and photodetection characteristics in CdS/P3HT devices through Al-doping

CdS thin films were deposited by chemical bath deposition onto FTO substrates with Al concentrations of 0, 1, 3, 5 and 6 %. X-ray diffraction revealed introduction of 1 % Al-doping reduced dislocation density and enhanced the crystal quality of CdS. Scanning electron microscopy confirmed a reduction in grain size in Al-doped CdS films compared to CdS. N3 and P3HT layers were spin-coated onto the prepared substrates, respectively. The fabrication of the solar cells was completed using Ag silver paste for the top contact. The lowest photoluminescence peak intensity was achieved for CdS (3 %Al):P3HT solar cell, indicating efficient exciton dissociation. 3 % Al-doped CdS-based device exhibited the highest efficiency at 0.210 %, nearly seven times that of reference device. CdS (3 %Al):P3HT device demonstrated the best photodetection characteristics, with a responsivity of 2.2 × 10-2 A/W, detectivity of 3.3 × 108 Jones, response time of 13 ms, and recovery time of 12 ms at zero bias voltage.

Temperature-Dependent Structural and Optoelectronic Properties of the Layered Perovskite 2-Thiophenemethylammonium Lead Iodide.

Improved knowledge of the influence of temperature upon layered perovskites is essential to enable perovskite-based devices to operate over a broad temperature range and to elucidate the impact of structural changes upon the optoelectronic properties. We examined the Ruddlesden-Popper layered perovskite 2-thiophenemethylammonium lead iodide (ThMA2PbI4) and observed a structural phase transition between a high- and a low-temperature phase at 220 K using temperature-dependent X-ray diffraction, UV-visible absorption, and photoluminescence (PL) spectroscopy. The structural phase transition altered the tilt pattern of the inorganic octahedra layer, modifying the absorption and PL spectra. Further, we found a narrow and intense additional PL peak in the low-temperature phase, which we assigned to radiative emission from a defect-bound exciton state. In both phases we determined the thermal expansion coefficient and found values similar to those of cubic 3D perovskites, i.e., larger than those of typical substrates such as glass. These results demonstrate that the organic spacer plays a critical role in controlling the temperature-dependent structural and optoelectronic properties of layered perovskites and suggests more widely that strain management strategies may be needed to fully utilize layered perovskites in device applications.

Wavelength-tunable photoluminescence of CsPbBr3 microcrystal via in situ halogen doping

Lead halide perovskite semiconductors have received significant attention in recent years as promising candidates for optoelectronic applications. Their controllable bandgap in nanostructures (nanocrystals, nanowires) has been widely and successfully investigated. Intentionally manipulating the atomic behavior with dopants in their macroscopic structure with high quality and stability is still challenging. We studied the crystal structure and photoluminescence (PL) behavior of the anion (Cl) doped CsPbBr3 microcrystals. The doped microcrystal with a low doping concentration has shown a large wavelength-tunable PL emission from 535 nm to 457 nm. Two separate emission peaks (blue and green) are carefully studied in the excitation power dependent PL spectra. The doping and excitation endow the perovskite microcrystal adjustable PL emission band and peak intensities, which may serve as a new degree of manipulation freedom in the color-tunable display and functional optoelectronics.

InGaAs/AlGaAs MQWs grown by MBE: Optimizing GaAs insertion layer thickness to enhance interface quality and luminescent property

The refinement of interface structure in InGaAs/AlGaAs multiple quantum wells (MQWs) through molecular beam epitaxy (MBE) is crucial for enhancing their luminescence performance. The influencing mechanisms of the GaAs insertion layer (ISL) thicknesses on the interface structure and luminescence properties of InGaAs/AlGaAs MQWs were investigated by varying the GaAs ISL thickness variations and optimizing growth parameters. In our designed InGaAs/AlGaAs MQWs structures, GaAs ISL with thickness variables of 0 nm, 0.5 nm, 1 nm, 2 nm, and 4 nm were inserted between the interfaces of the InGaAs well layer and the AlGaAs barrier layer. Through experimental characterizations. The introduction of the GaAs ISL resulted in a gradual transition of the MQWs surface morphology from a three-dimensional (3D) island structure to a typical step-flow surface morphology, leading to the reduction in surface roughness and enhancement of hetero-interfaces quality. Moreover, the GaAs ISL with thickness exceeding 2 nm effectively inhibited the segregation of indium atoms, promoting the ordering of interface alloys and decreasing the polarization degrees from 3.76 % to 2.09 %. Temperature-dependent PL measurements indicated that the GaAs ISL could reduce interface defects and the density of non-radiative recombination centers, thereby enhancing the radiative recombination efficiency of MQWs and ultimately increasing the peak intensity of photoluminescence by 3.4 times. This work is significance for understanding the luminescent properties of InGaAs/AlGaAs MQWs, which provides reference highly efficient optoelectronic devices development.

Size Control and Photoluminescence Characteristics of InSb Quantum Dots on GaSb Substrates Grown by Molecular Beam Epitaxy

InSb self‐assembled quantum dots (QDs) have the possibility to increase the luminescence intensity of light‐emitting diodes operating in the midinfrared region. As a first step toward the midinfrared luminescence, the single‐layer InSb QDs are grown on GaSb (100) substrates by molecular beam epitaxy and the effects of V/III ratio and InSb coverage on size and density of InSb QDs and photoluminescence (PL) characteristics are investigated. As the V/III ratio increases, the distribution of QD height broadens and goes from monomodal to multimodal. The increase in the number of large QDs and their multimodal distribution are thought to be due to the ripening process. The PL intensity is maximum at a V/III ratio of 2 with PL wavelength of 1.76 μm. As the InSb coverage increases at a V/III ratio of 2, QD size and density increase while maintaining a monomodal distribution in QD size. However, the PL peak intensity decreases with increasing InSb coverage due to the increase of nonradiative QDs. The highest PL peak intensity at an InSb coverage of 2.0 monolayer and a V/III ratio of 2 with PL wavelength of 1.69 μm are obtained. The present single‐layer InSb QD shows the emission in the near‐infrared region.

High temperature synthesis, surface characterization, and photoluminescence property of boron carbon nitride

The present study demonstrates the synthesis of boron carbon nitride (BCN) by molten salt method which utilizes melamine (source of carbon and nitrogen), boric acid (source of boron) and potassium chloride as the salt. The synthesis is performed at 915 °C for 4 h under N2-atmosphere. The synthesized material is further acid-treated. The physicochemical characterization data for as-synthesized and acid-treated materials confirm successful synthesis of BCN. SEM and AFM data display that synthesized BCN pre and post acid-treatment exhibit sheet-type morphology. Thermogravimetric analysis data confirm that the acid-treated material is thermally stable up to 800 °C, with only 2 % mass-loss is observed till 800 °C. Water contact angle measurement study demonstrates that acid-treatment significantly improves the surface hydrophilicity. Additionally, the low photoluminescence peak intensity of the acid-treated material indicates low electron-hole pair recombination rate. The synthesized material may be useful for the photocatalytic degradation of potentially toxic chemicals from water.

Disentangling doping and strain effects at defects of grown MoS2 monolayers with nano-optical spectroscopy.

The role of defects in two-dimensional semiconductors and how they affect the intrinsic properties of these materials have been a widely researched topic over the past few decades. Optical characterization techniques such as photoluminescence and Raman spectroscopies are important tools to probe the physical properties of semiconductors and the impact of defects. However, confocal optical techniques present a spatial resolution limitation lying in a μm-scale, which can be overcome by the use of near-field optical measurements. Here, we use tip-enhanced photoluminescence and Raman spectroscopies to unveil the nanoscale optical properties of grown MoS2 monolayers, revealing that the impact of doping and strain can be disentangled by the combination of both techniques. A noticeable enhancement of the exciton peak intensity corresponding to trion emission quenching is observed at narrow regions down to a width of 47 nm at grain boundaries related to doping effects. Besides, localized strain fields inside the sample lead to non-uniformities in the intensity and energy position of photoluminescence peaks. Finally, two distinct MoS2 samples present different nano-optical responses at their edges associated with opposite strains. The edge of the first sample shows a photoluminescence intensity enhancement and energy blueshift corresponding to a frequency blueshift for E2g and 2LA Raman modes. In contrast, the other sample displays a photoluminescence energy redshift and frequency red shifts for E2g and 2LA Raman modes at their edges. Our work highlights the potential of combining tip-enhanced photoluminescence and Raman spectroscopies to probe localized strain fields and doping effects related to defects in two-dimensional materials.

Controlling photoluminescence of silicon quantum dots using pristine-nanostates formation

This study investigates the effective control of photoluminescence (PL) of silicon nanocrystals (Si–NCs) embedded in a SiO2 matrix, with a specific focus on tuning the luminescent wavelength profile and intensity for silicon-based optoelectronic applications. By manipulating the composition ratio (x) of SiOx, the well-known simultaneous changes in PL peak position and intensity were observed after annealing. To preserve the desired wavelength characteristics and enhance the luminesce intensity, we propose a novel approach to generate pristine nanostates (PNs) for Si-NC formation using proton irradiation (PI). The application of proton irradiation was found to be successful in significantly increasing PL intensity while strongly suppressing the peak position shifts. Through extensive spectroscopic analysis including PL, X-ray photoelectron spectroscopy, and absorption spectroscopy, we investigate specific defect states in the SiOx matrix, and identified as PNs. We interpret that these PNs consist of self-trapped exciton, E′ center, non-bridging oxygen hole center, and oxygen-deficiency center states. In the as-grown sample, the PNs promote appropriate phase separation and play a significant role in stabilizing the structure during the subsequent annealing processes. This study elucidates the Si-NC formation mechanism by enhancing O-diffusion-induced phase separation, a process accelerated in SiOx with controlled PNs. The proposed PI-induced PNs offer a novel pathway for developing Si-based optoelectronic devices with desired characteristics in the operating wavelength range. These findings open up promising possibilities for advanced silicon-based optoelectronic applications.

Multi-colour emission from ZnO[SO4]:Eu2+ nanophosphors: Effects of SO42- and Eu2+ doping concentration

Blue-emitting ZnO-SO4:Eu2+ powder phosphors were synthesized via solid state reaction method. Based on the X-ray diffraction (XRD) data, hexagonal wurtzite structures of ZnO were formed. Further structural elucidation via Raman spectroscopy showed a peak at 439 cm−1 associated with E2hgh optical mode that is a distinctive characteristic band of hexagonal wurtzite ZnO phase. The ZnO powders were composed of distorted spherical nanoparticles, which were agglomerated together as confirmed from scanning electron microscopy images. After incorporation of SO42- and Eu2+ into the ZnO lattices sites, the particle shapes became irregular. When excited at 351 nm with Xenon lamp, our samples emitted UV and blue-green visible light associated, respectively, with excitonic recombination and intrinsic structural lattice defects in ZnO. Both the photoluminescence (PL) peak intensity and positions of these emissions were influenced by SO42- and Eu2+ ions. The blue emission was dependent on the Eu2+ content with the largest intensity measured from the sample doped with 1.0 mol% of Eu2+ ions. Mechanisms of energy transfer and concentration quenching effect are discussed.

Sheaf-like Manganese-Doped Zinc Silicate with Enhanced Photoluminescence Performance

Sheaf-like manganese-doped zinc silicate (Mn-doped Zn2SiO4) was successfully synthesized without surfactant by hydrothermal route using manganese acetate, zinc nitrate, and sodium silicate as precursors. The structure, morphology, and optical properties were well investigated by various analytical techniques, such as X-ray diffraction (XRD), a scanning electron microscope (SEM), a transmission electron microscope (TEM), and photoluminescence (PL). The results showed the enhancement of crystallinity and an increase in the length of the as-prepared sample, which was achieved by prolonging the hydrothermal time. Based on the analysis of the XRD pattern, it can be stated that the sheaf-like Mn-doped Zn2SiO4 possesses a large lattice distortion compared to pure Zn2SiO4. Moreover, it was observed that hydrothermal times played a crucial role in the PL property. The PL peak intensity of samples located at 522 nm generally increased with the increase in reaction time in the range of 12–48 h. However, when the treating time reached 72 h, the property of PL decreased. The results of the PL spectra showed that Mn-doped Zn2SiO4 obtained by a hydrothermal time of 48 h displayed an efficient luminescent performance. The key to the high PL property mainly lies in the sheaf-like structure and large lattice distortion.

Correlative Spatial Mapping of Optoelectronic Properties in Large Area 2D MoS2 Phototransistors

Abstract2D materials‐based device performance is significantly affected by film non‐uniformity, especially for large area devices. Here, it investigates the dependence of large area 2D MoS2 phototransistor performance on film morphology through correlative mapping. Monolayer MoS2 films are quazi‐epitaxially synthesized on C‐plane sapphire (Al2O3 ) substrates by chemical vapor deposition, and the growth time and molybdenum trioxide MoO3 precursor volume are varied to obtain variations in film morphology. Raman, photoluminescence, transmittance, and photocurrent maps are generated and compared with each other to obtain a holistic understanding of large area 2D optoelectronic device performance. For example, it shows that the photoluminescence peak shift and intensity can be used to investigate strain and other defects across multiple film morphologies, giving insight into their effects on the photogenerated current in these devices. It also combines photocurrent and absorption maps to generate large area high‐resolution external quantum efficiency and internal quantum efficiency maps for the devices. This study demonstrates the benefit of correlative mapping in the understanding and advancement of large area 2D material‐based electronic and optoelectronic devices.

Photoluminescence Property of Cu(In,Ga)S2 Quantum Dots Depending on Their Composition and Nanostructure

Multinary quantum dots (QDs) composed of less toxic elements, such as CuInS2 and AgInS2, have been intensively investigated for the application to highly efficient solar energy conversion systems and novel luminescent devices, because their electronic energy structure and optical properties can be controlled by the size and chemical composition of QDs. Recently we have successfully prepared alloy QDs composed of Ag–In–Ga–S and Ag–In–Ga–Se semiconductors, the Eg of which was tunable in the visible and near-IR wavelength regions, respectively [1, 2]. These QDs exhibited a narrow PL peak assignable to the band-edge emission and their peak wavelengths were widely tunable by changing their Eg. However, there have been few investigations to prepare Cu-based multinary QDs with a narrow PL peak. Thus, in this study, we prepare spherical QDs composed of less-toxic Cu–In–Ga–S (CIGS) alloy semiconductor and investigate their composition-dependent optical properties.The synthesis of CIGS QDs was carried out by our previous heating-up method with a slight modification [1]. Powders of Cu(OAc)2, In(acac)3, Ga(acac)3, elemental sulfur were dispersed in a mixture solution of oleylamine and dodecanethiol, followed by the heat treatment at 300 oC. The resulting solution was subjected to centrifugation to remove large precipitates, and QDs were isolated from the supernatant by adding methanol as a non-solvent. Thus-obtained QDs were used as a core to prepare cores-shell-structured particles, in which the surface of CIGS core was coated by thin shell composed of ZnS or GaSx.The chemical composition of obtained QDs was controlled by the fractions of individual metal salts used as precursors. The Cu/(In+Ga) and In/(In+Ga) of obtained QDs roughly agreed with those used in preparation. The obtained QDs were spherical and their average diameter decreased from 5.4 nm to 3.4 nm with an increase in the In fraction. The absorption spectra of CIGS QDs were varied depending on both the fractions of Cu and In. The onset wavelength of QDs was blue-shifted from 840 nm to 540 nm with a decrease in the Cu/(In+Ga) in preparation from 0.7 to 0.1 when the QDs were prepared with the fixed ratio of In/(In+Ga)= 0.7. On the other hand, the QDs exhibited a blue shift of absorption onset from ca. 750 nm to 480 nm with a decrease in In/(In+Ga) ratio from 1.0 to 0 under the fixed Cu fraction in preparation, Cu/(In+Ga)= 0.3. The obtained Eg monotonously decreased from 2.77 eV to 1.74 eV with an increase in In/(In+Ga) ratio from 0 to 1.0, and these values were larger than those of corresponding bulk semiconductors due to the quantum size effect. From the ionization energy of the QDs evaluated from the onset energy of the photoelectron yield spectra in air, we determined the electronic energy structure of CIGS QDs. With a decrease in the Eg of QDs, the conduction band minimum level shifted to a lower level from ca. –2.3 eV to –3.4 eV, while the valence band maximum level was almost constant to ca. 5.1–5.2 eV.Surface coating of CIGS QDs with other semiconductor shells increased the photoluminescence (PL) intensity. The ZnS shell coating to form CIGS@ZnS core-shell QDs produced a PL peak at 649 nm (FWHM: 0.47 eV), being much broader than that of CIGS cores (FWHM: 0.23 eV) because of alloying at the interface between ZnS shell and CIGS core, though the PL quantum yield (QY) increased to 30% from 8%. In contrast, the GaSx coating on CIGS QD surface enlarged the intensity of PL peak without changing its peak width: CIGS@GaSx QDs exhibited a narrow PL peak (FWHM: 0.20 eV) at 671 nm, being comparable to the PL peak of CIGS QDs (the peak wavelength of 675 nm and the FWHM of 0.23 eV). The PL QY was also increased to 27% from 8% by the GaSx coating on CIGS QDs. The tunabilities of electronic energy structure and PL property of CIGS QDs are useful for constructing efficient light energy conversion systems and luminescent devices.[1] T. Kameyama, et al., ACS Applied. Mater. Interfaces 2018, 10, 42844-42855.[2] T. Kameyama, et al., ACS Appl. Nano Mater. 2020, 3, 3275-3287.

Harvesting strong photoluminescence of physical vapor deposited GeSn with record high deposition temperature

In this work, strong photoluminescence (PL) from physical vapor deposited GeSn on Ge-buffered Si is harvested by pursuing high deposition temperature. High-quality Ge0.938Sn0.062 films are obtained through sputtering epitaxy at a record high temperature of 405 °C. The PL peak intensity of the sputtering-grown GeSn is enhanced by 21 times compared to that of GeSn with similar Sn content grown by molecular beam epitaxy at 150 °C. The PL intensity ratio between the sputtering-grown GeSn and Ge virtual substrate reaches a value of 18. Power-dependent and temperature-dependent PL characterizations demonstrate that band-to-band recombination dominates in the sputtering-grown GeSn film. The results indicate that high-temperature sputtering epitaxy, which has the merit of low cost and high-productivity potential, is promising for preparing high-quality GeSn films for optoelectronic applications.

Plasmon-enhanced photoluminescence and Raman spectroscopy of silver nanoparticles grown by solid state dewetting

The growth of the metallic nanoparticles (NPs) on the solid substrate with the desired shape and size is a critical issue for application of these NPs in solid-state devices. Solid state dewetting (SSD) technique is simple, low cost and can be used to fabricate the metallic NPs with control on the shape and size on different substrates. In this work, silver NPs (Ag NPs) were grown on corning glass substrate by SSD of silver precursor thin film deposited at different substrate temperatures by RF sputtering. The influence of the substrate temperature on the growth of Ag NPs and their several properties like localized surface plasmon resonance (LSPR), photoluminescence (PL), and Raman spectroscopy is studied. The size of the NPs was found to vary from 25 nm to 70 nm with the variation in substrate temperature from room temperature (RT) to . For the RT films, the LSPR peak position of Ag NPs is around 474 nm. A red shift in LSPR peak for films deposited at higher temperature is observed due to change in the particle size and interparticle separation. Photoluminescence spectra suggests the presence of two photoluminescence bands at 436 and 474 nm corresponding to Ag NPs radiative interband transition and LSPR band. An intense Raman peak was observed at 1587 cm−1. Enhancement in PL peak intensity and Raman peak intensity is found to be in accordance with the LSPR of Ag NPs.

Improved Thermoelectric Figure of Merit in Polyol Method-Prepared Cu1–xBixS (x ≤ 0.06) Nanosheets

The first thermoelectric (TE) properties of simple polyol method-synthesized Cu1–xBixS (x = 0, 0.02, 0.04, 0.06) nanosheets are reported here. Various characterizations like Rietveld-refined X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray (EDAX) data analysis confirm their single-phase nature of hexagonal crystal structure with space group P63/mmc, stoichiometric nature, valence states of the elements, and nominal elemental composition. Field emission scanning electron microscopy (FESEM) images show nanosheets (NSs) with an average thickness of 27 nm. The doping of Bi atoms in the CuS lattice has been evident from the systematic increase in its crystallite size, optical band gaps, photoluminescence peak intensities, Seebeck coefficients, resistivity, thermoelectric power factors (PFs), and thermoelectric figure of merits (ZTs) with increasing x. The selected area electron diffraction (SAED) pattern confirms that nanosheets are single crystalline in nature. Fourier transform infrared (FTIR) data confirms the absorption bands of the sulfides. The value of the Seebeck coefficient increases with increasing x without deteriorating the electrical conductivity too much due to the energy-dependent scattering of the carriers at the interfaces and the modulation doping. An enhancement of 231% in the thermoelectric power factor and a 6-fold increase in ZT, both at 300 K for x = 0.06 compared to x = 0, are found that show interesting outcomes for these toxic- and rare-earth element-free TE materials.